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Engineering Reference and Application Solutions

1. Ballscrew Rotating Nut wa aaananananad Transfer Machine Indexer Circuit Board Rotary Indexer Controller Drive PLC Programmable Logic Controller Engineering Reference and Application Solutions Motion amp Control Overview Introduction Motion control in its widest sense could relate to anything from a welding robot to the hydraulic system in a mobile crane In the field of Electronic Motion Control we are primarily concerned with systems falling within a limited power range typically up to about LOHP 7KW and requiring precision in one or more aspects This may involve accurate control of distance or speed very often both and sometimes other parameters such as torque or acceleration rate In the case of the two examples given the welding robot requires precise control of both speed and distance the crane hydraulic system uses the driver as the feedback system so its accuracy varies with the skill of the operator This wouldn t be considered a motion control system in the strict sense of the term Our standard motion control system consists of three basic elements Fig 1 Elements of motion control system High Level Command Commands Signals Host Indexer i 1 ndexer i i Drive eon Bc I Connelly Loe ee te el Motor b Hybrid Stepper DC Servo Brushless Servo Linear Ste
2. 90 Phase Shift Tolerance Fig 4 6 shows that for each complete square wave from channel A if channel B output is also considered during the same period four pulse edges will occur This allows the resolution of the encoder to be quadrupled by processing the A and B outputs to produce a separate pulse for each square wave edge For this process to be effective however it is important that quadrature is maintained within the necessary tolerance so that the pulses do not run into one another Square wave output encoders are generally available in a wide range of resolutions up to about 5000 lines rev and with a variety of different output configurations some of which are listed below TTL Transistor Transistor Logic This is a commonly available output compatible with TTL logic levels and normally requiring a 5 volt supply TTL outputs are also available in an open collector configuration which allows the system designer to choose from a variety of pull up resistor value A40 CMOS Complimentary M etal Oxide Semiconductor Available for compatibility with the higher logic levels it normally used with CMOS devices Line driver These are low output impedance devices designed for driving signals over a long distance and are usually used with a matched receiver Complementary outputs Outputs derived from each channel give a pair of signals 180 out of phase These are useful where maximum immunity to
3. O hM oD U iS oO io q oO oc aD oD E e Lu Digital Servo Drive Operation Fig 2 19 shows the components of a digital drive for a servo motor All the main control functions are carried out by the microprocessor which drives a D to A convertor to produce an analog torque demand signal From this point on the drive is very much like an analog servo amplifier Feedback information is derived from an encoder attached to the motor shaft The encoder generates a pulse stream from which the processor can determine the distance travelled and by calculating the pulse frequency it is possible to measure velocity The digital drive performs the same operations as its analog counterpart but does so by solving a series of equations The microprocessor is programmed with a mathematical model or algorithm of the equivalent analog system This model predicts the behavior of the system In response to a given input demand and output position It also takes into account additional information like the output velocity the rate of change of the input and the various tuning settings To solve all the equations takes a finite amount of time even with a fast processor this time is typically between 100us and 2ms During this time the torque demand must remain constant at its previously calculated value and there will be no response to a change at the input or output This upd
4. oD io q oO cc e iS oD oD E e W NY AU O00 Winding QU OD UL All these losses will contribute heat to the motor and it is this heating that will ultimately limit the motor application Other Limiting Considerations Torque ripple The requirement for constant torque output from a DC motor is that the magnetic fields due to the stator and the armature are constant in magnitude and relative orientation but this ideal is not achieved in practice As the armature rotates the relative orientation of the fields will change slightly and this will result in small changes in torque output called torque ripple Fig 1 31 Fig 1 31 Torque ripple components Torque Torque Ripple k Z Z Steady Torque O P gt Time This will not usually cause problems at high speeds since the inertia of the motor and the load will tend to smooth out the effects but problems may arise at low speeds Motors can be designed to minimize the effects of torque ripple by increasing the number of windings or the number of motor poles or by skewing the armature windings Motion amp Control A15 Motor Technologies Demagnetization The permanent magnets of a DC motor field will tend to become demagnetized whenever a current flows in the motor armature This effect is known as armature reaction and will have a negligible effect in normal use
5. Application Type Metering Dispensing Motion Linear Application Description The design requires a machine to dispense radioactive fluid into capsules After the fluid is dispensed it is inspected and the data is stored on a PC There is a requirement to increase throughput without introducing spillage Machine Requirements e Increase throughput e No spilling of radioactive fluid e Automate two axes e PC compatible system control e Low cost solution e Smooth repeatable motion Motion Control Requirements e Quick accurate moves e Multi axis controller e PC bus based motion control card Open loop stepper if possible e High resolution motor drive microstepping Top View Filling Heads Application Examples Application Solution The multi axis indexer is selected to control and synchronize both axes of motion on one card residing in the IBM PC computer An additional feature is the integral I O capability that s necessary to activate the filling process The horizontal axis carrying the tray of capsules is driven by a linear motor The simple mechanical construction of the motor makes it easy to apply and guarantees a long maintenance free life The vertical axis raises and lowers the filling head and is driven by a microstepping motor and a leadscrew assembly A linear motor was also considered for this axis but the fill head would have dropped onto the tray with a loss of power to the motor Lea
6. Ga Drive Controller A93 Motion amp Control 0D U oD io q oO cc e iS oD oD E e W Application Examples 22 Moving Positioning System Application Type Following Motion Linear Application Description In a packaging application a single conveyor of boxes rides between 2 conveyors of product The product must be accurately placed in the boxes from alternate product conveyors without stopping the center conveyor of boxes The line speed of the boxes may vary When the product is ready the controller must decide which box the product can be placed into and then move the product into alignment with the moving box The product must be moving along side of the box in time for the product to be pushed into the box Machine Requirements e Reliable product packaging on the fly Standalone operation e Multiple product infeeds e Continuous operation without stopping the box conveyor Motion Control Requirements e Programmable I O Sequence storage e Complex following capabilities e Moving positioning system functionality e Multitasking Product Application Solution A standalone multiple axis controller provides the control for this application The controller can perform motion profiling based on an external encoder that is mounted on the center conveyor of boxes The two product conveyors are driven by servo motors for high speeds and accelerat
7. Mforcer 2 0 12 0 lbs Step 2 Acceleration rate A Average velocity move distance move time 40 inches 1 0 sec 40 0 in sec B Maximum velocity Based on trapezoidal move profile Vmax 1 33 x Vavg 40 0 in sec 53 2 in sec Veloci inisee 53 2 40 Time sec 0 1 1 0 C Minimum acceleration rate A Vmax 53 2 in sec 212 8 in sec Accel time 250 sec A Minimum acceleration 212 8 in sec 386 in sec per 1 G 0 551 g s Step 3 Calculate maximum acceleration rate of L20 using constant acceleration indexer Based on the speed force curve below the L20 has 14 0 Ibs of force at 53 2 in sec Vmax Force vs Speed L L20 9 08 14 0 Ibs Force Ibs kg 20 40 60 80 100 50 8 101 6 152 4 203 2 254 0 Speed in cm Step 4 Non damped safety margin If all available force could be used the maximum calculated acceleration rate Force 14 0 Ib Mtotal 12 0 Ibs The calculated acceleration rate should be reduced by 50 100 non damped safety margin netting an acceleration rate for the L20 of 0 58 g s The application requires a 0 55 g s acceleration rate The L20 meets the requirements of this application Amax 1 16 g s Velocity Ripple Velocity ripple is most noticeable when operating near the motor s resonant frequency Rotary stepping motor s have this tendency as well but it is
8. information that is reflected Parker Motor Sizing amp Selection E through a variety of machine transmissions and File Axis Transmission Move About D reductions including leadscrews gears belts and eer od pulleys This software then produces graphs of the Motor Types ee results allowing you to select the proper motor Z from a comprehensive detailed database of more Technology Size Ww than 200 motor models g e y E Stepper Size 23 or smaller lt IBM PC compatible Motor Sizing amp Selection A t Brushless servo Size 34 or smaller software also generates a number of application specific reports including profiles and speed Compumoter Plus __ size 42 or smaller wv PS Orive Larger than size 42 _ Brush servo w aaar etails Dunaserv R Parker Motor Sizing amp Selection ow All LeadScrew Transmission pees Units Lead 0 2 infrev Screw Diameter 0 75 inches Screw Length 36 inches 5 Cdetticient Load Weight 50 pounds SELETA Parker Motor Sizing amp Selection Counterbalance 8 ounces CATER LeadScrew Transmission Thrust Load fa ounces Nid BreakAway Torque 15 oz in Units e lotor _ Reducer oe Friction Coefficient 0 15 e pre E i Incline Angle a degrees r Screw Efficiency 65 percent Es Surnu Units Units Brass Screw Inertia 5 99986 oz in a 7 Diameter 0 75 Pa Tangel Diameter ia opper Reduction Menu
9. 4 oO oc e iS oD oD E e W Glossary of Terms Absolute Positioning Refers to a motion control system employing position feedback devices absolute encoders to maintain a given mechanical location Absolute Programming A positioning coordinate referenced wherein all positions are specified relative to some reference or zero position This is different from incremental programming where distances are specified relative to the current position AC Servo A general term referring to a motor drive that generates sinusoidal shaped motor currents in a brushless motor wound as to generate sinusoidal back EMF Acceleration The change in velocity as a function of time Acceleration usually refers to increasing velocity and deceleration describes decreasing velocity Accuracy A measure of the difference between expected position and actual position of a motor or mechanical system Motor accuracy is usually specified as an angle representing the maximum deviation from expected position Ambient Temperature The temperature of the cooling medium usually air immediately surrounding the motor or another device ASCII American Standard Code for Information Interchange This code assigns a number series of electrical signals to each numeral and letter of the alphabet In this manner information can be transmitted between machines as a series of binary numbers Bandwidth A measure of
10. All digital systems have difficulty interpolating between output pulses Therefore knowledge of position will be accurate only to the grating width Fig 4 10 Fig 4 10 Encoder quantization error Feedback Devices Eccentricity This may be caused by bearing play shaft run out incorrect assembly of the disc on its hub or the hub on the shaft Eccentricity may cause a number of different error conditions a Amplitude Modulation In a sine wave encoder eccentricity will be apparent as amplitude modulation Fig 4 11 Fig 4 11 Amplitude modulation caused by eccentricity Nominal Signal Level Quantization Error gt lt Quantization error F 1 2N N of lines disk rotation Signal Amplitude b Frequency modulation As the encoder is rotated at constant speed the frequency of the output will change at a regular rate Fig 4 12 If viewed on an oscilloscope this effect will appear as jitter on the trace Fig 4 12 Encoder frequency modulation Increased Frequency fp gt gt lt Nominal Frequency f4 c Inter channel jitter If the optical detectors for the two encoder output channels are separated by an angular distance on the same radius then any jitter will appear at different times on the two channels resulting in inter channel jitter Environmental Cons
11. N 2 W Weight of load W ounces W 1 Weight G 1 W 1 ounces W 2 Weight G 2 W 2 ounces L Length L inches F Friction F BT Breakaway torque BT ounce inches Gear Drive Formulas 2 J W boad R2 N sear 2 Load 2 Load N Gear 1 or 2 TL sadP Load R4 Noear2 2 Load J Load Noear1 Ww N ear i J Gearl gt R Geant ie Gear 1 gt D D inertia oz in gm cm as seen by the motor torque oz in gm cm weight oz gm radius in cm number of gear teeth constant length in cm density oz in gm cm angular velocity radians sec motor shaft time seconds gravity constant 386 in sec Gear2 R2 J Gear2 Gear2 T 1 Total g Load J Gearl J Gear2 J Mond t a Sor zvy A64 Tangential Drives System Calculations 0 1S L w o lt e D 0 amp D LL n lt R Radius R inches W Weight include weight of belt or chain s ounces W P Weight of pulley or material W P ounces F Breakaway force F ounces V Linear velocity V inches sec CT Coupling type CT SL Side load SL Tangential Drive Formulas Problem T Vig FT ee t haet Tyce FT Total Load Pulley Belt Motor Friction Total 1 0 g Load J Pulley t J Belt J T t FT rieton W R J Load L Remember to multiply by 2 z 2 J Pulley 2 4 if there are 2 pulleys J ser WR B Tra
12. W Motor Technologies The Hybrid Servo In terms of their basic operation the step motor and the brushless servo motor are identical They each have a rotating magnet system and a wound stator The only difference is that one has more poles than the other typically two or three pole pairs in the brushless servo and 50 in the stepper You could use a brushless servo as a stepper not a very good one since the step angle would be large But by the same token you can also use a stepper as a brushless servo by fitting a feedback device to perform the commutation Hence the hybrid servo so called because it is based ona hybrid step motor Fig 1 44 These have also been dubbed stepping servos and closed loop steppers We prefer not to use the term stepper at all since such a servo exhibits none of the operating characteristics of a step motor The hybrid servo is driven in precisely the same fashion as the brushless motor A two phase drive provides sine and cosine current waveforms in response to signals from the feedback device This device may be an optical encoder or a resolver Since the motor has 50 pole pairs there will be 50 electrical cycles per revolution This conveniently permits a 50 cycle resolver to be constructed from the same rotor and stator laminations as the motor itself A hybrid servo generates approximately the same torque output as the equivalent step motor assuming the same dri
13. q oO cc e iS oD oD E e Lu How the Linear Motor Works The forcer consists of two electromagnets A and B and a strong rare earth permanent magnet The two pole faces of each electromagnet are toothed to concentrate the magnetic flux Four sets of teeth on the forcer are spaced in quadrature so that only one set at a time can be aligned with the platen teeth The magnetic flux passing between the forcer and the platen gives rise to a very strong force of attraction between the two pieces The attractive force can be up to 10 times the peak holding force of the motor requiring a bearing arrangement to maintain precise clearance between the pole faces and platen teeth Either mechanical roller bearings or air bearings are used to maintain the required clearance When current is established in a field winding the resulting magnetic field tends to reinforce permanent magnetic flux at one pole face and cancel it at the other By reversing the current the reinforcement and cancellation are exchanged Removing current divides the permanent magnetic flux equally between the pole faces By selectively applying current to phase A and B it is possible to concentrate flux at any of the forcer s four pole faces The face receiving the highest flux concentration will attempt to align its teeth with the platen Fig 1 17 shows the four primary states or full steps of the forcer The four steps result in motion of
14. the smoothness and stiffness of a microstepping system is required Motor speeds are to be low and the inertias of the valves connected to the motors are insignificant The main torque requirement is to overcome valve friction Machine Requirements Low wear e Remote operation e High reliability Motion Control Requirements e Motor velocity is low e High stiffness at standstill Slow speed smoothness e Four axes of control Homing function Application Solution Each valve is measured with a torque wrench Two valves measure at 60 oz in and the other two measure at 200 oz in Two high power and two low power microstepping motor drives systems are selected These choices provide approximately 100 torque margin and result in a conservative design The operator would like to specify each valve position as an angle between 0 and 350 Home position switches are mounted on the test rig and connected to each indexer to allow for power on home reference using the indexer s homing feature Product Solutions Indexer Drive Motor AT6400 S Drive S57 102 A standalone indexer could also be used instead of a bus based indexer refer to the Model 4000 Drive Computer Drive Indexer installed in a PC A79 Application Examples Motion amp Control oO U oD io q J cc e iS 1 J Ss e W 8 Capsule Filling Machine
15. Aligned Y ele XK A A Aligned B Aligned A10 Step Motor Characteristics There are numerous step motor performance characteristics that warrant discussion However we ll confine ourselves to those traits with the greatest practical significance Fig 1 18 illustrates the static torque curve of the hybrid step motor This relates to a motor that is energized but stationary It shows us how the restoring torque varies with rotor position as it is deflected from its stable point We re assuming that there are no frictional or other static loads on the motor As the rotor moves away from the stable position the torque steadily increases until it reaches a maximum after one full step 1 8 This maximum value is called the holding torque and it represents the largest static load that can be applied to the shaft without causing continuous rotation However it doesn t tell us the maximum running torque of the motor this is always less than the holding torque typically about 70 Fig 1 18 Static torque displacement characteristic A
16. Any closed loop servo system whether analog or digital will require some tuning This is the process of adjusting the characteristics of the servo so that it follows the input signal as closely as possible Why is tuning necessary A servo system is error driven in other words there must be a difference between the input and the output before the servo will begin moving to reduce the error The gain of the system determines how hard the servo tries to reduce the error A high gain system can produce large correcting torques when the error is very small A high gain is required if the output is to follow the input faithfully with minimal error Now a servo motor and its load both have inertia which the servo amplifier must accelerate and decelerate while attempting to follow a change at the input The presence of the inertia will tend to result in over correction with the system oscillating or ringing beyond either side of its target Fig 3 1 This ringing must be damped but too much damping will cause the response to be sluggish When we tune a Servo we are trying to achieve the fastest response with little or no overshoot Fig 3 1 System response characteristics Underdamped Output Response Critical Damping Overdamped Response Time In practice tuning a servo means adjusting potentiometers in an analog drive or changing gain values numerically in a digital system To carry out this process ef
17. Applications that require the coordination of motion to be in conjunction with an external speed or position sensor Application No Page 20 Labelling Machine sssscscrsr A92 21 Window Blind Gluing seesssscseee A93 22 Moving Positioning Systems nees A94 Injection Molding Applications where raw material is fed by gravity from a hopper into a pressure chamber die or mold The mold is filled rapidly and considerable pressure is applied to produce a molded product Application No Page 23 Plastic Injection Molding eses A95 Flying Cutoff Applications where a web of material is cut while the material is moving Typically the cutting device travels at an angle to the web and with a speed proportional to the web Application No Page 24 Rotating Tube Cutting eee A96 1 BBQ Grill Making Machine Application Type Feed to Length Motion Linear Application Description A manufactuer was using a servo motor to feed material into a machine to create barbeque grills shopping carts etc The process involves cutting steel rods and welding the rods in various configurations However feed length was inconsistent because slippage between the drive roller and the material was too frequent Knurled nip rolls could not be used because they would damage the material The machine builder needed a more accurate method of cutting the material at uniform lengths The customer used a load mounted encoder to provide feedback of t
18. No Reduction Plastic Length u 36 Hard Wood FAN ns 8 Soft Wood i ig 4 48 oz cub f Accept the information in this screen ensity n AXIS 1 Leadscrew selected out of 205 motors Parker Motor Sizing amp Selection PARKER MOTOR SIZING AXIS SUMMARY Uersion 2 Copyright c 1991 Parker Hannifin All rights reserved worldwide For Application Assistance call 808 358 9078 Outside the United States call 787 584 7558 Thu Jan 87 13 17 39 1993 Application Leadscr1 Number of defined axes 1 Report for axis 1 Description Leadscrew axis with 58 Ib load Velocity of 5 ips distance at speed of 24 inches z Okay Cance el Reduc nter to select steel Leadscrew selected out of 205 motors Parker Motor Sizing amp Selection MMARY AXIS 1 Leadscreu ofile Velocity amp distance ssion Leadscrew on No Reduction selected 1 Type S Microsteppi Move Torque Tine Margin Inertia Ratio er 5 488 141 177 a Transmission Motor s A57 Motion amp Control System Calculations Move Profile Before calculating torque requirements of an application you need to know the velocities and accelerations needed For those positioning
19. damping K is given by T T nK If the motor is coupled to a load T then at constant speed T T T nK 3 Equations 1 2 and 3 allow us to calculate the required current and drive voltage to meet given torque and speed requirements The values of K K etc are given in the motor manufacturer s data Brushless Motors Before we talk about brushless motors in detail let s clear up a few points about terminology The term brushless has become accepted as referring to a particular variety of servo motor Clearly a step motor is a brushless device as is an AC induction motor in fact the step motor can form the basis of a brushless servo motor often called a hybrid servo which is discussed later However the so called brushless motor has been designed to have a similar performance to the DC brush servo without the limitations imposed by a mechanical commutator Within the brushless category are two basic motor types trapezoidal and sine wave motors The trapezoidal motor is really a brushless DC servo whereas the sine wave motor bears a close resemblance to the AC synchronous motor To fully explain the difference between these motors we must review the evolution of the brushless motor Fig 1 35 Conventional DC brush motor Commutator A simple conventional DC brush motor Fig 1 35 consists of a wound rotor that can turn within a magnetic field provided by the stator If the coi
20. iS oD oD E e W Application Examples 16 Flute Grinder Application Type Tool Feed Motion Linear Application Description A low cost machine for grinding the flutes in twist drills requires two axes of movement one moves the drill forwards underneath the grinding wheel the other rotates the drill to produce the helical flute At the end of the cut the rotary axis has to index the drill round by 180 to be ready to grind the second flute The linear speed of the workpiece does not exceed 0 5 inches sec Machine Requirements e Two axis control e Low cost e Easy set up and change over of part programs e Smooth accurate cutting motion Motion Control Requirements e Two axis indexer e Linear interpolation between axes Nonvolatile program storage Flexible data pad input Moderate speeds e Programmable I O Grinding Wheel Application Solution This is a natural application for stepper motors since the speeds are moderate and the solution must be minimum cost The grinding process requires that the two axes move at accurately related speed so the controller must be capable of performing linear interpolation The small dynamic position error of the stepper system ensures that the two axes will track accurately at all speeds Product Solutions Operator Controller Drive Motor Interface 6200 S Drive S83 135 RP240 The Model 4000 FP has also been used to solve simila
21. may then be selected and executed from switches via the I O interface or from a separate machine controller such as a PLC A47 op U iS 0p a o 4 oO aa aD oD E e Lu Motion amp Control Control Systems Understanding Input and Output Modules Most motion controllers indexers offer programmable inputs and outputs to control and interact with other extemal devices and machine elements Programmable Output Example After indexing a table to a preset position energize a programmable output to activate a knife that will cut material on the table Programmable Input Example After indexing a table in a pick and place application the indexer waits for an input signal from a robot arm signaling the indexer that a part has been located on the table The primary reason for using I O modules is to interface 5VDC logic signals from an indexer to switches and relays on the factory floor which typically run on voltage levels ranging from 24VDC to 220VAC Solid state I O modules are essentially a relay utilizing light emitting diode LED and a transistor along with a signal conditioning circuit to activate a switch These I O modules isolate no direct connection the internal microprocessor circuitry of an indexer from oversized DC and AC voltages The lack of a physical connection between the indexer and external devices protects the indexer from hazardous voltage spikes and current
22. pitch in revs in e leadscrew efficiency F u W for horizontal surfaces where u coefficient of static friction and W is the weight of the load This friction component is often called breakaway Dynamic Friction F W is the coefficient to use for friction during a move profile However torque calculations for acceleration should use the worst case friction coefficient u 1 Accel 7 g Load go eer es J Shee T 2mpv w 7 mLpR J Load 2npF J Leadscrew 2 Where T torque 0z in angular velocity radians sec t time seconds v linear velocity in sec length inches radius inches p density ounces in g gravity constant 386 in sec The formula for load inertia converts linear inertia into the rotational equivalent as reflected to the motor shaft by the leadscrew Problem Find the torque required to accelerate a 200 Ib steel load sliding on a steel table to 2 inches per second in 100 milliseconds using a 5 thread inch steel leadscrew 36 inches long and 1 5 inches in A62 diameter Assume that the leadscrew has an Acme thread and uses a plastic nut Motor inertia is given as 6 56 oz in In this example we assume a horizontally oriented leadscrew where the force of gravity is perpendicular to the direction of motion In non horizontal orientations leadscrews will transmit varying degrees of influence from gravity to the motor depending on the angle of inclination Compumotor Sizing
23. 000 steps rev and in addition to improved current control they often have adjustments to balance offsets between each phase of the motor and to optimize the current profile for the particular motor being used Full Step and Half Step Systems Full step and half step systems do not have the resolution capability of the ministepping or microstepping systems However the drive technology is not as complex and the drives are relatively inexpensive Full step and half step systems will not have the same low speed smoothness as higher resolution systems An inherent property of a stepper motor is its low speed resonance which may de synchronize a motor and cause position loss Full step and half step drives are more prone to resonance effects and this may limit their application in low speed systems Full step and half step systems can be operated at speeds above the motor s resonant speed without loss of synchronization For this reason full step and half step systems are normally applied in high speed point to point positioning applications In these types of applications the machine designer is primarily concerned with selecting a motor drive system capable of producing the necessary power output A28 Since power is the product of torque and speed a high torque system with low speed capability may not produce as much power as a low torque high speed system Sizing the system for torque only may not provide the most cost eff
24. 1 0 Rack a MMT 1 0 to Limits AB Cylinders and 5 D m eee Som BS e Cathode Computer Foil Reel Indexer installed in a PC Paper Reel Motion amp Control A91 20 Labelling Machine Application Type Following Motion Linear Application Description Bottles on a conveyor run through a labelling mechanism that applies a label to the bottle The spacing of the bottles on the conveyor is not regulated and the conveyor can slow down speed up or stop at any time Machine Requirements e Accurately apply labels to bottles in motion Allow for variable conveyor speed e Allow for inconsistent distance between bottles e Pull label web through dispenser Smooth consistent labelling at all speeds Motion Control Requirements e Synchronization to conveyor axis e Electronic gearbox function e Registration control e High torque to overcome high friction High resolution Open loop stepper if possible Primary Axis Velocity Registration Input Application Examples Application Solution A motion controller that can accept input from an encoder mounted to the conveyor and reference all of the speeds and distances of the label roll to the encoder is required for this application A servo system is also required to provide the torque and speed to overcome the friction of the dispensing head and the inertia of the large roll of labels A photosensor connected to a programmable input on the c
25. 20 30 The Z606 motor will meet the requirements RMS torque falls within the continuous duty cycle and total torque vs velocity falls within the intermittent range How to Use a Step Motor Horsepower Curve Horsepower HP gives an indication of the motor s top usable speed The peak or hump in a horsepower curve indicates a speed that gives maximum power Choosing a speed beyond the peak of the HP curve results in no more power the power attained at higher speeds is also attainable at a lower speed Unless the speed is required for the application there is little benefit to going beyond the peak as motor wear is faster at higher speeds Applications requiring the most power the motor can generate not the most torque should use a motor speed that is just below the peak of the HP curve oz in N m 175 1 22 HP 175 Torque 140 1 98 140 105 73 105 Torque JaM0d 70 49 070 Horsepower 35 24 035 0 Speed rps Motion amp Control A59 System Calculations Leadscrew Drives Leadscrews convert rotary motion to linear motion and come in a wide variety of configurations Screws are available with different lengths diameters and thread pitches Nuts range from the simple plastic variety to precision ground versions with recirculating ball bearings that can achieve very high accuracy The combination of microstepping and a quality leadscr
26. A then as we rotate B a voltage will be induced into this winding and this voltage will vary as the cosine of the angle so that Eoc EjCos EER Fig 4 19 Resolver principle Winding B Niman Winding A A44 Referring to Fig 4 20 we can see that if we are able to measure the relative amplitudes of the two winding A amp C outputs at a particular point in the cycle these two outputs will be unique to that position Fig 4 20 Resolver output A E jCos 360 a EjSin 1 Electrical Cycle Bs gt The information output from the two phases will usually be converted from analog to digital form for use in a digital positioning system by means of a resolver to digital converter Fig 4 21 Resolutions up to 65 536 counts per revolution are typical of this type of system Fig 4 21 Resolver to digital converter Sine Multiplier ry Integrator Phase Comparitor Cosine Multiplier A Voltage Up Down Ea counten Oscillator j Integrator Digital Output Shaft Angle DC Signal Velocity In addition to position information speed and direction information may also be derived The resolver is an absolute position feedback device Within each electrical cycle Phase A and Phase B maintain a constant fixed relationship The excitation voltage E may be coupled to the rotating winding
27. Control A69 Glossary of Terms Resonance Designates the condition resulting from energizing a motor at a frequency at or close to the motor s natural frequency Lower resolution open loop systems will exhibit large oscillations from minimal input Ringing Oscillation of a system following a sudden change in state RMS Torque For an intermittent duty cycle application the RMS Torque is equal to the steady state torque that would produce the same amount of motor heating over long periods of time Tous Ti ti RMS eS Sti Where Ti Torque during interval i ti Time of interval i RS 232C A data communications standard that encodes a string of information on a single line in a time sequential format The standard specifies the proper voltage and time requirements so that different manufacturers devices are compatible Servo A system consisting of several devices which continuously monitor actual information position velocity compares those values to desired outcome and makes necessary corrections to minimize that difference Slew In motion control the portion of a move made at a constant non zero velocity Static Torque The maximum torque available at zero speed Step Angle The angle the shaft rotates upon receipt of a single step command Stiffness The ability to resist movement induced by an applied torque Is often specified as a torque displacement curve indicating the amount a motor
28. Hence the motor is actually being driven by an alternating current Fig 1 37 Brushless motor Za 7 Commutation Encoder gt Drive Going back to the conventional brush motor a rotor consisting of only one coil will exhibit a large torque variation as it rotates In fact the characteristic will be sinusoidal with maximum torque produced when the rotor field is at right angles to the stator field and zero torque at the commutation point see Fig 1 38 A practical DC motor has a large number of coils on the rotor each one connected not only to its own pair of commutator segments but to the other coils as well In this way the chief contribution to torque is made by a coil operating close to its peak torque position There is also an averaging effect produced by current flowing in all the other coils so the resulting torque ripple is very small Motion amp Control A17 oD U oD io q oO cc e iS oD oD E e W Motor Technologies Fig 1 38 3 phase brushless motor Fig 1 41 Position of rotor at commutation point A1 C1 B1 We would like to reproduce a similar situation in the brushless motor however this would require a large number of coils distributed around the stator This may be feasible but each coil would require its own individual drive circuit This is clearly c2 S prohibitive so a compromise is made A typi
29. R Most hybrid motors are 2 phase although 5 phase versions are available A recent development is the enhanced hybrid motor which uses flux focusing magnets to give a significant improvement in performance albeit at extra cost Fig 1 3 Hybrid stepper motor Non magnetic Stainless Steel Shaft Prelubricated Bearing Housing Rotor Stator The operation of the hybrid motor is most easily understood by looking at a very simple model that will produce 12 steps per rev Fig 1 4 Fig 1 4 Simple 12 step rev hybrid motor Motor Technologies The rotor of this machine consists of two pole pieces with three teeth on each In between the pole pieces is a permanent magnet that is magnetized along the axis of the rotor making one end a north pole and the other a south pole The teeth are offset at the north and south ends as shown in the diagram The stator consists of a shell having four teeth that run the full length of the rotor Coils are wound on the stator teeth and are connected together in pairs With no current flowing in any of the motor windings the rotor will take one of the positions shown in the diagrams This is because the permanent magnet in the rotor is trying to minimize the reluctance or magnetic resistance of the flux path from one end to the other This will occur when a pair of north and south pole rotor teeth are aligned with two of the stator poles The torqu
30. Software automatically calculates these torques using vector analysis 1 Calculate the torque required to overcome friction The coefficient of static friction for steel to steel lubricant contact is 0 15 The median value of efficiency for an Acme thread and plastic nut is 0 65 Therefore F pW 0 15 200 tb 2502 F 480 oz dnpe 2m 5reVx0 65 23 51 oz in rev in 480 oz Friction 2 Compute the rotational inertia of the load and the rotational inertia of the leadscrew W_ 200lb x 160z 2mpP 215F Ib in ne zLoR m 36 in 4 48 oz 0 75 in 2 in 80 16 oz in 3 The torque required to accelerate the load may now be computed since the motor inertia was given 3 24 oz in J Load Vises g toad tJ Leadscrew J on t 2r 5 tj 207 o 2n j sec 1 207r 386 in sec 4 99 80 16 6 56 0z in TEK 149 oz in T T T Pi Friction Accel 23 51 oz in 149 oz in 172 51 oz in Tiotal Directly Driven Loads There are many applications where the motion being controlled is rotary and the low speed smoothness and high resolution of a Compumotor system can be used to eliminate gear trains or other mechanical linkages In direct drive applications a motor is typically connected to the load through a System Calculations flexible or compliant coupling This coupling provides a small amount of damping and helps correct for any mechanical misalignment Direct driv
31. Under high load conditions however when motor current may be high the effect will cause a reduction in the torque constant of the motor and a consequent reduction in torque output Above a certain level of armature current the field magnets will become permanently demagnetized Therefore it is important not to exceed the maximum pulse current rating for the motor Mechanical resonances and backlash It might normally be assumed that a motor and its load including a tachometer or position encoder are all rigidly connected together This may however not be the case It is important for a bi directional drive or positioning system that the mechanics are free from backlash otherwise true positioning will present problems In high performance systems with high accelerations interconnecting shafts and couplings may deflect under the applied torque such that the various parts of the system may have different instantaneous velocities that may be in opposite directions Under certain conditions a shaft may go into torsional resonance Fig 1 32 Fig 1 32 Torsional oscillation Shaft Back emf As described previously a permanent magnet DC motor will operate as a generator As the shaft is rotated a voltage will appear across the brush terminals This voltage is called the back electromotive force emf and is generated even when the motor is driven by an applied voltage The output voltage is essentially linear with motor
32. by slip rings and brushes though this arrangement is a disadvantage when used with a brushless motor In such applications a brushless resolver may be used so that the excitation voltage is inductively coupled to the rotor winding Fig 4 22 Fig 4 22 Brushless resolver Stator Phase 1 Rotor Stator Phase 2 Machine Control Many industrial designers are concerned with controlling an entire process Motion control is one important and influential aspect of complete machine control The primary elements of machine control include Fig 5 1 Primary machine control elements Servos Steppers Hydraulics Switches Indicators Mainframes Readout Actuators Motion Control Machine Control Displays Sensors Keyboards Gauges Touchscreens Meters Data Acquisition Proportional Valves Motion Control For precise programmable load movement using a servo motor stepper motor or hydraulic actuators Feedback elements are often employed Analog and Digital I O For actuation of an external process devices such as solenoids cutters heaters valves etc Operator Interface For flexible interaction with the machine process for both setup and on line variations Touchscreens data pads and thumbwheels are examples Communications Support For process monitoring diagnostics and data transfer with peripheral systems There are many different machine control architectures that integrate these elemen
33. cancels itself over 360 of rotation and is typically distributed in a sinusoidal fashion This means the error will gradually increase decrease to zero increase in the opposite direction and finally decrease again upon reaching 360 of rotation Absolute accuracy causes the size of microsteps to vary somewhat because the full motor steps that must be traversed by a fixed number of microsteps varies The steps can be over or undersized by about 4 5 as a result of absolute accuracy errors Relative Accuracy Also referred to as step to step accuracy this specification tells how microsteps can change in size In a perfect system microsteps would all be exactly the same size but drive characteristics and the absolute accuracy of the motor cause the steps to expand and contract by an amount up to the relative accuracy figure The error is not cumulative Hysteresis The motor s tendency to resist a change in direction This is a magnetic characteristic of the motor it is not due to friction or other external factors The motor must develop torque to overcome hysteresis when it reverses direction In reversing direction a one revolution move will show hysteresis by moving the full distance less the hysteresis figure Servo amp Closed Loop Stepper Accuracy Repeatability accuracy and relative accuracy in servos and closed loop stepper systems relate as much to their feedback mechanisms as they do to the inherent characteris
34. characteristics vary with frequency and this includes phase characteristics So feedback that starts out negative at low frequencies can turn positive at high frequencies The result can be overshoot ringing or ultimately continuous oscillation We ve said that the purpose of servo tuning is to get the best possible performance from the system without running into instability We need to compensate for the characteristics of the servo components and maintain an adequate stability margin What determines whether the system will be stable or not Closed loop systems can be difficult to analyze because everything is interactive The output gets fed back to the input in antiphase and virtually cancels it out so there seems to be nothing left to measure The best way to determine what s going on is to open the loop and then see what happens Fig 3 2 Closed loop velocity servo Velocity Servo Motor Input Amplifier gt E 9 i 1 Tach Feedback Signal Fig 3 3A Measuring open loop characteristics gt Oscillator Motor Vs As Scope Measuring the open loop characteristic allows us to find out what the output and therefore the feedback signal will be in response to a particular input We need to measure the gain and phase shift at different frequencies and we can plot the results graphically For a typical servo system the results might look like this Fig 3 3B
35. degrading acceleration performance by adding further magnet sections or stacks to the same Shaft Fig 1 15 A second stack will enable twice the torque to be produced and will double the inertia so the torque to inertia ratio remains the same Hence stepper motors are produced in single two and three stack versions in each frame size Fig 1 15 Three stack hybrid stepping motor 3i h i h h l As a guideline the torque to inertia ratio reduces by a factor of two with each increase in frame size diameter So an unloaded 34 size motor can accelerate twice as rapidly as a 42 size regardless of the number of stacks gt T ip Linear Stepping Motors Fig 1 16 Linear stepping motor Forcer Permanent Magnet Phase A Electromagnet Phase B Electromagnet _ Field Windings Platen Teeth Ay A2 By Bo Pole Faces The linear stepper is essentially a conventional rotary stepper that has been unwrapped so that it operates in a Straight line The moving component is referred to as the forcer and it travels along a fixed element or platen For operational purposes the platen is equivalent to the rotor in a normal stepper although it is an entirely passive device and has no permanent magnet The magnet is incorporated in the moving forcer
36. developed to automatically inspect small parts for defects The parts are located ona small conveyor and pass through the camera s field of view The conveyor is started and stopped under computer control and the engineer wants to use a system to drive the conveyor because it is necessary for the part to pass by the camera at a constant velocity It is desired to accelerate the conveyor to a speed of 20 inches sec in 100 milliseconds A flat timing belt weighing 20 ozs is driven by a 2 inch diameter aluminum pulley 4 inches wide this requires a motor velocity of 3 2 rps The maximum weight of the parts on the pulley at any given time is 1 Ib and the load is estimated to have an inertia of 2 oz in Static friction of all mechanical components is 30 oz in The required motor toque was determined to be 50 9 oz ins refer to Direct Drive Formulas on p A63 Application Examples Machine Requirements Computer controlled system e High accuracy e Low backlash Motion Control Requirements e Accurate velocity control Linear motion e High resolution e AT bus based motion control card Application Solution A computer controls the entire inspection machine A bus based compatible indexer card was selected A microstepping motor drive system that supplied 100 oz in of static torque was also chosen to complete the application Product Solutions Indexer Drive Motor PC21 S Drive 57 83 The AT6200
37. difficult to use Fig 4 4 Fig 4 4 Encoder output voltage Output y A Voltage DC Offset Shaft Rotation In practice two photodiodes are used with two masks arranged to produce signals with 180 phase difference for each channel the two diode outputs being subtracted so as to cancel the DC offset Fig 4 5 This quasi sinusoidal output may be used unprocessed but more often it is either amplified or used to produce a square wave output Hence incremental rotary encoders may have sine wave or Square wave outputs and usually have up to three output channels Motion amp Control A39 Feedback Devices Fig 4 5 Output from dual photodiode system A WEN AAS Output 1 V4 ee __ Output 2 DC Offset O i tk ae 0 gt Rotation y y Combined Output 1 2 A two channel encoder as well as giving position of the encoder shaft can also provide information on the direction of rotation by examination of the signals to identify the leading channel This is possible since the channels are normally arranged to be in quadrature i e 90 phase shifted Fig 4 6 For most machine tool or positioning applications a third channel known as the index channel or Z channel is also included This gives a single output pulse per revolution and is used when establishing the zero position Fig 4 6 Quadrature output signals Channel A Channel B
38. g 4 Motor Steps 2 S S A 2 Max 2 Torque g 9 a Angle t a Stable Unstable Stable D x 8 ro y As the shaft is deflected beyond one full step the torque will fall until it is again at zero after two full steps However this zero point is unstable and the torque reverses immediately beyond it The next stable point is found four full steps away from the first equivalent to one tooth pitch on the rotor or 1 50 of a revolution Although this static torque characteristic isn t a great deal of use on its own it does help explain some of the effects we observe For example it indicates the static stiffness of the system i e how the shaft position changes when a torque load is applied to a stationary motor Clearly the shaft must deflect until the generated torque matches the applied load If the load varies so too will the static position Non cumulative position errors will therefore result from effects such as friction or out of balance torque loads It is important to remember that the static stiffness is not improved by using a microstepping drive a given load on the shaft will produce the same angular deflection So while microstepping increases resolution and smoothness it may not necessarily improve positioning accuracy Under dynamic conditions with the motor running the rotor must be lagging behind the stator field if it is producing torque Similarly there will be a lead situation when the torque rev
39. handle electrical noise problems is before they occur When a motion system is in the design process the designer should consider the following system wiring guidelines listed by order of importance 1 Put surge suppression components on all electrical coils resistor capacitor filters MOVs Zener and clamping diodes 2 Shield all remote connections and use twisted pairs Shields should be tied to Earth at one end 3 Put all microelectronic components in an enclosure Keep noisy devices outside Monitor internal temperature 4 Ground signal common wiring at one point Float this ground from Earth if possible 5 Tie all mechanical grounds to Earth at one point Run chassis and motor grounds to the frame frame to Earth 6 Isolate remote signals Solid state relays or opto isolators are recommended 7 Filter the power line Use common RF filters isolation transformer for worst case situations A noise problem must be identified before it can be solved The obvious way to approach a problem situation is to eliminate potential noise sources until the symptoms disappear as in the case of ground loops When this is not practical use the above guidelines to troubleshoot the installation References Information about the equipment referred to may be obtained by calling the numbers listed below e Corcom line filters 312 680 7400 e Opto 22 optically isolated relays 714 891 5861 Crydom optically isolated relays 21
40. instabilities The BLHX servo was chosen because it can switch between position control and torque control on the fly without instability or saturation and then while in torque control mode directly controls motor torque Product Solutions 0D U oD io q oO cc e iS oD oD E e W Actuator ET580 BO4LA Motor ML3450B 10 Controller Drive BLHX75BN Motion amp Control A95 Application Examples 24 Rotating Tube Cutter Application Type Flying Cutoff Motion Linear Application Description Metal tubing feeds off of a spool and needs to be cut into predetermined lengths A rotating blade mechanism is used to cut the tube and the blade mechanism must spin around the tube many times in order to complete the cut The throughput of this machine must be maximized so the tubing cannot be stopped while this cut is being made Therefore to make a clean cut on the tube the blade must move along with the tube while the cut is being performed Machine Requirements e Standalone operation e Move cutting mechanism with the tubing to make the cut without stopping e Simple user interface to set different tube lengths e High accuracy on cut Motion Control Requirements e Programmable I O Program storage e Position following e High acceleration and speed Cutting __ gt Mechanism A96 Application Solutio
41. interference is required Noise problems The control system for a machine is normally screened and protected within a metal cabinet An encoder may be similarly housed However unless suitable precautions are taken the cable connecting the two can be a source of trouble due to its picking up electrical noise This noise may result in the loss or gain of signal counts giving rise to incorrect data input and loss of position Fig 4 7 Corruption of encoder signal by noise Ve Noise Pulse un Channel A Ve Noise Pulse Fig 4 7 shows how the introduction of two noise pulses has converted a four pulse train into one of six pulses A number of techniques are available to overcome problems due to noise The most obvious resolution is to use Shielded interconnecting cables However since the signals may be at a low level 5 volts and may be generated by a high impedance source it may be necessary to go to further lengths to eliminate the problem The most effective way to resolve the problem is to use an encoder with complementary outputs Fig 4 8 and connect this to the control system by means of shielded twisted pair cable The two outputs are processed by the control circuitry so that the required signal can be reconstituted without the noise Fig 4 8 Complementary output signals Channel A Noise Spike gt If the A signal is inv
42. is applied to a stepper drive it is usual for it to energize in the zero phase state in which there is current in both sets of windings The resulting rotor position does not correspond with a natural detent position so an unloaded motor will always move by at least one half step at power on Of course if the system was turned off other than in the zero phase state or the motor is moved in the meantime a greater movement may be seen at power up Another point to remember is that for a given current pattern in the windings there are as many stable positions as there are rotor teeth 50 fora 200 step motor If a motor is de synchronized the resulting positional error will always be a whole number of rotor teeth or a multiple of 7 2 A motor cannot miss individual steps position errors of one or two steps must be due to noise spurious step pulses or a controller fault oD U oD io q oO cc e iS oD oD E e W Motion amp Control Motor Technologies Bifilar Windings Most motors are described as being bifilar wound which means there are two identical sets of windings on each pole Two lengths of wire are wound together as though they were a single coil This produces two windings that are electrically and magnetically almost identical if one coil were to be wound on top of the other even with the same number of turns the magnetic characteristics would be differe
43. motion control and intended for transmission along an RS 232C link Controllers using this language either accept real time commands from a host computer or execute stored sequences that have been previously programmed The simplicity of RS 232C communication allows the controller to be incorporated into the drive itself resulting in an integrated indexer drive package A46 X Code Programming X Code has been designed to allow motion control equipment to be programmed by users with little or no computer experience Although the language includes more than 150 commands depending on the product it is only necessary to learn a small percentage of these to write simple programs Most command codes use the initial letter of the function name which makes them easy to remember Here are some examples of frequently used commands V velocity in revs sec D distance in steps A acceleration rate in revs sec G go start the move T time delay in seconds A typical command string might look like this V10 A50 D4000 GT2G This would set the velocity to 10 revs sec acceleration to 50 revs sec and distance to 4000 steps The 4000 step move would be performed twice with a 2 second wait between moves Please refer to specifications of X Code products for a list of all the available X Code commands Single axis and Multi axis Controllers A single axis controller can as the name implies only control one motor The control
44. motor this introduces additional complications In extreme cases where personal safety is at risk it may be necessary to mechanically lock the system even at the expense of possible damage to the machine Emergency Stop Methods 1 Full torque controlled stop Applying zero velocity command to a servo amplifier will cause it to decelerate hard to zero speed in current limit in other words using the maximum available torque This will create the fastest possible deceleration to rest In the case of a digital servo with step and direction inputs cutting off the step pulses will produce the same effect The situation is different for a stepper drive The step pulse train should be decelerated to zero speed to utilize the available torque Simply cutting off the step pulses at speeds above the start stop rate will de synchronize the motor and the full decelerating torque will no longer be available The controller needs to be able to generate a rapid deceleration rate independent of the normal programmed rate to be used only for overtravel limit and emergency stop functions 2 Disconnect the motor Although this method is undoubtedly safe it is not highly recommended as a quick stop measure The time taken to stop is indeterminate since it depends on load inertia and friction and in high performance systems the friction is usually kept to a minimum Certain types of drives may be damaged by disconnecting the motor under power Th
45. one tooth interval to the right Reversing the sequence moves the forcer to the left Motion amp Control Motor Technologies Repeating the sequence in the example will cause the forcer to continue its movement When the sequence is stopped the forcer stops with the appropriate tooth set aligned At rest the forcer develops a holding force that opposes any attempt to displace it As the resting motor is displaced from equilibrium the restoring force increases until the displacement reaches one quarter of a tooth interval See Fig 1 18 Beyond this point the restoring force drops If the motor is pushed over the crest of its holding force it slips or jumps rather sharply and comes to rest at an integral number of tooth intervals away from its original location If this occurs while the forcer is travelling along the platen it is referred to as a Stall condition Fig 1 17 The four cardinal states or full steps of the forcer Phase A gt lt XIX ol Na i Direction of MMF due to electromagnet Flux Lines X X x x B
46. oz in However a low load to rotor inertia ratio was necessary to gently move the vials and fill them Machine Requirements e Smooth motion PLC control e Variable index lengths Motion Control Requirements e Smooth motion e Sequence select capability I O for sequence select e Programmable acceleration and deceleration Application Solution The index distance may be changed by the engineer who is controlling the machine with a programmable controller Move parameters will be changing and can therefore be set via BCD inputs The indexer can be buried in the machine and activated with a remote START input Product Solutions Drive Indexer Motor SX Drive Indexer 83 135 The 6200 AT6200 and Model 500 are other indexer products that have been used in these types of applications PLC Controller Programmable Logic Controller A82 Drive 11 Conveyor Application Type Indexing Conveyor Motion Linear Tangential drives consist of a pulley or pinion which when rotated exerts a force on a belt or racks to move a linear load Common tangential drives include pulleys and cables gears and toothed belts and racks and pinions Tangential drives permit a lot of flexibility in the design of drive mechanics and can be very accurate with little backlash Metal chains should be avoided since they provide little or no motor damping Application Description A machine vision system is being
47. pass to the detector Fig 4 3 Feedback Devices Fig 4 3 Principle of optical encoder a ae Collimated Grating Mask Detector Light Source An incremental encoder generates a pulse fora given increment of shaft rotation rotary encoder or a pulse for a given linear distance travelled linear encoder Total distance travelled or shaft angular rotation is determined by counting the encoder output pulses An absolute encoder has a number of output channels such that every shaft position may be described by its own unique code The higher the resolution the more output channels are required op U iS 0p a o 4 oO aa aD oD E e W The Basics of Incremental Encoders Since cost is an important factor in most industrial applications and resetting to a known zero point following power failure is seldom a problem the rotary incremental encoder is the type most favored by system designers Its main element is a shaft mounted disc carrying a grating which rotates with the grating between a light source and a masked detector The light source may be a light emitting diode or an incandescent lamp and the detector is usually a phototransistor or more commonly a photo voltaic diode Such a simple system providing a single low level output is unlikely to be frequently encountered since quite apart from its low output signal it has a DC offset that is temperature dependent making the signal
48. position information for each shaft location The location is independent of all other locations unlike the incremental encoder where a count from a reference is required to determine position Fig 4 15 Absolute disk In an absolute encoder there are several concentric tracks unlike the incremental encoder with its single track Each track has an independent light source As the light passes through a slot a high state true 1 is created If light does not pass through the disk a low state false 0 is created The position of the shaft can be identified through the pattern of 1 s and O s The tracks of an absolute encoder vary in slot size moving from smaller at the outside edge to larger toward the center The pattern of slots is also staggered with respect to preceding and succeeding tracks The number of tracks determines the amount of position information that can be derived from the encoder disk resolution For example if the disk has ten tracks the resolution of the encoder would usually be 1 024 positions per revolution or 2 For reliability it is desirable to have the disks constructed of metal rather than glass A metal disk is not as fragile and has lower inertia Feedback Devices Fig 4 17 Absolute encoder output ai saat 0 1S 1 j 0 u 1 ue vU oc g D i D 0 1011 Decimal 11 o The disk patte
49. rate of rise of current depends on the inductance and the applied voltage so a higher voltage must be applied to get the current to rise more quickly In a practical inductor possessing resistance the final current is determined by the resistance and the applied voltage Once the turbine has been accelerated up to speed stopping it again is not a simple matter The kinetic energy of the flywheel has to be dissipated and as soon as the tap is turned off the flywheel drives the turbine like a pump and tries to keep the water flowing This will set up a high pressure across the inlet and outlet pipes in the reverse direction The equivalent energy store in the inductor is the magnetic field As this field collapses it tries to maintain the current flow by generating a high reverse voltage A24 Reverse Pressure When Flow Interrupted By including a one way valve across the turbine connections the water is allowed to continue circulating when the tap is turned off The energy stored in the flywheel is now put to good use in maintaining the flow We use the same idea in the recirculating chopper drive in which a diode allows the current to recirculate after it has built up Going back to our simple unipolar drive if we look at the way the current builds up Fig 2 4 we can see that it follows an exponential shape with its final value set by the voltage and the winding resistance To get it to build up more rapidly we could
50. rotational losses These losses give rise to heat generation within the motor Motor losses can be divided into two areas Those that depend on the load and those that depend on speed Fig 1 29 Fig 1 29 Losses in a DC motor Motor losses 1 Load Speed Winding losses T T 1 lron Friction Brush Short cut losses losses losses circuit losses Winding losses These are caused by the electrical resistance of the motor windings and are equal to R where armature current and R armature resistance As the torque output of the motor increases increases which gives rise to additional losses Consideration of winding losses is very important since heating of the armature winding causes an increase in R which results in further losses and heating This process can destroy the motor if the maximum current is not limited Furthermore at higher temperatures the field magnets begin to lose their strength Hence for a required torque output the current requirement becomes greater Brush contact losses These are fairly complex to analyze since they depend upon several factors that will vary with motor operation In general brush contact resistance may represent a high proportion of the terminal resistance of the motor The result of this resistance will be increased heating due to I R losses in the brushes and contact area Iron losses Iron losses are the major factor in determining the maximum speed that may
51. servo systems It is made up of two main parts a housing containing the field magnets and a rotor made up of coils of wire wound in slots in an iron core and connected to a commutator Brushes in contact with the commutator carry current to the coils Fig 1 25 Iron cored motor Moving coil There are two principle forms of this type of motor 1 The printed motor Fig 1 26 using a disc armature 2 The shell type armature Fig 1 27 Since these types of motors have no moving iron in their magnetic field they do not suffer from iron losses Consequently higher rotational speeds can be obtained with low power inputs Fig 1 26 Disc armature printed motor Motion Permanent magnet 8 pole Fig 1 27 Shell armature motor Motion core Armature Hollow cup shaped conductor array Diagrams courtesy of Electro Craft Ltd A14 Brushless The major limiting factor in the performance of iron cored motors is internal heating This heat escapes through the shaft and bearings to the outer casing or through the airgap between the armature and field magnets and from there to the casing Both of these routes are thermally inefficient so cooling of the motor armature is very poor Fig 1 28 Brushless motor Backiron Return Path Stator Windings Lam Teeth In the brushless motor the construction of the iron cored motor is turned inside out so that the rotor becomes
52. shielded Even the worst noise problems in environments near 600 amp welders and 25kW transmitters have been solved using enclosures conduit optical isolation and single point ground techniques Ground Loops Ground Loops create the most mysterious noise problems They seem to occur most often in systems where a control computer is using RS 232C communication Garbled transmission and intermittent operation symptoms are typical The problem occurs in systems where multiple Earth ground connections exist particularly when these connections are far apart Example Suppose a Model 500 is controlling an axis and the limit switches use an external power supply The Model 500 is controlled by a computer in another room If the power supply Common is connected to Earth ground loop problems may occur most computers have their RS 232C signal common tied to Earth The loop starts at the Model 500 s limit switch ground goes to Earth through the power supply to Earth at the computer From there the loop returns to the Model 500 through RS 232C signal ground If a voltage potential exists between power supply Earth and remote computer Earth ground current will flow through the RS 232C ground creating unpredictable results The way to test for and ultimately eliminate a ground loop is to lift or cheat Earth ground connections in the system until the symptoms disappear Control Systems Defeating Noise The best time to
53. small phase imbalance that may be barely detectable in a half step drive can produce unacceptable positioning errors in a microstep system Pulse width modulation is frequently used to achieve higher accuracy than can be achieved using a simple threshold system The phase currents necessary to produce the intermediate steps follow an approximately sinusoidal profile as shown in Fig 2 12 However the same profile will not give the optimum response with all motors Some will work well with a sinusoidal shape whereas others need a more filled out or trimmed down shape Fig 2 12 Soa microstep drive intended to operate with a variety of motors needs to have provision for adjusting the current profile The intermediate current levels are uSually stored as data in an EPROM with some means of selecting alternative data sets to give different profiles The change in profile may be thought of in terms of adding or subtracting a third harmonic component to or from the basic sine wave Fig 2 12 Microstep current profile Sinewave Filled out Trimmed In the case of high resolution microstep drives producing 10 000 steps per rev or more the best performance will only be obtained with a particular type of motor This is one in which the stator teeth are ona 7 5 pitch giving 48 equal pitches in 360 In most hybrid steppers the stator teeth have the same pitch as the rotor teeth giving equal increments of 7 2 This latter arrangem
54. so that the angle now increases to 120 and it stays here during the next 60 of rotation A18 The Trapezoidal Motor With a fixed current level in the windings the use of this extended portion of the sinusoidal torque characteristic gives rise to a large degree of torque ripple We can minimize the effect by manipulating the motor design to flatten out the characteristic to make it trapezoidal Fig 1 42 In practice this is not very easy to do so some degree of non linearity will remain The effect of this tends to be a slight kick at the commutation points which can be noticeable when the motor is running very slowly Fig 1 42 Trapezoidal motor characteristic 60 7 60 Torque ripple resulting from non linearity in the torque characteristic tends to produce a velocity modulation in the load However in a system using velocity feedback the velocity loop will generally have a high gain This means that a very small increase in velocity will generate a large error signal reducing the torque demand to correct the velocity change So in practice the output current from the amplifier tends to mirror the torque characteristic Fig 1 43 so that the resulting velocity modulation is extremely small Fig 1 43 Current profile in velocity controlled SEINO Torque Current The Sine Wave Motor In the sine wave motor sometimes called an AC brushless servo no attempt is made to modify
55. speed and has a slope that is defined as the motor voltage constant K Fig 1 33 K is typically quoted in volts per 1000 rpm Fig 1 33 Back emf characteristic Output volts Shaft speed A16 Motor Equations Unlike a step motor the DC brush motor exhibits simple relationships between current voltage torque and speed It is therefore worth examining these relationships as an aid to the application of brush motors The application of a constant voltage to the terminals of a motor will result in its accelerating to attain a steady final speed n Under these conditions the voltage V applied to the motor is opposed by the back emf nK and the resultant voltage drives the motor current I through the motor armature and brush resistance R The equivalent circuit of a DC motor is shown in Fig 1 34 Fig 1 34 DC motor equivalent circuit L R motor resistance L winding inductance V back emf and R represents magnetic losses The value of R is usually large and so can be ignored as can the inductance L which is generally small If we apply a voltage V to the motor and a current I flows then V IR V but V nk so V IR nK 1 This is the electrical equation of the motor If K is the torque constant of the motor typically in oz in per Amp then the torque generated by the motor is given by T IK 2 The opposing torque due to friction T and viscous
56. speed of the conveyor This provides an acceleration such that 2 inches of the part passes by the weld head by the time the weld head reaches 100 of the conveyor velocity 3 The controller changes the speed ratio to 1 1 so the weld head maintains the speed of the conveyor for the first weld The weld takes 1 second 4 The following ratio is set to zero and the welder decelerates to zero velocity over 2 inches 5 The controller commands the linear forcer to repeat the same acceleration ramp as in step above This causes the weld head to position itself at an equal velocity with the conveyor 4 inches behind the first weld 6 Step 3 is repeated to make the second weld 7 Once the second weld is finished the controller commands the linear forcer to return the weld head to the starting position to wait for the next part to arrive 0D U oD io q oO cc e iS oD oD E e W Product Solutions Indexer Drive Motor Encoder Model500 L Drive PO L20 P54 E Microstepping Drive _ Linear Motor Spot Welds er we Encoder Mounted to Conveyor Weld Head Motion amp Control A75 Application Examples 4 Optical Scanner Application Type X Y Point to Point Motion Rotary Application Description A dye laser designer needs to precisely rotate a diffraction grating under computer control to tune the frequency of t
57. the basic sinusoidal torque characteristic Such a motor can be driven like an AC synchronous motor by applying sinusoidal currents to the motor windings These currents must have the appropriate phase displacement 120 in the case of the three phase motor We now need a much higher resolution device to control the commutation if we want smooth rotation at low speeds The drive needs to generate 3 currents that are in the correct relationship to each other at every rotor position So rather than the simple commutation encoder generating a handful of switching points we now need a resolver or high resolution optical encoder In this way it s possible to maintain a 90 torque Motor Technologies angle very accurately resulting in very smooth low speed rotation and negligible torque ripple A simplified explanation of why the sine wave motor produces constant torque is given in the next section The drive for a sine wave motor is more complex than for the trapezoidal version We need a reference table from which to generate the sinusoidal currents and these must be multiplied by the torque demand signal to determine their absolute amplitude With a star connected three phase motor it is sufficient to determine the currents in two of the windings this will automatically determine what happens in the third As previously mentioned the sine wave motor needs a high resolution feedback device However this device can also provide
58. the bobbin by a ballscrew driven arm which oscillates back and forth at constant speed The arm must reverse rapidly at the end of the move The required ballscrew speed is 60 rpm Machine Requirements e Controlled tension on monofilament Simple operator interface e High throughput Motion Control Requirements e 2 axes of coordinated motion e Linear interpolation e Constant torque from motor Bobbin Application Examples Application Solution The prime requirement of the bobbin drive is to provide a controlled tension which means operating in Torque mode rather than Velocity mode If the motor produces a constant torque the tension in the filament will be inversely proportional to the winding diameter Since the winding diameter varies by 2 1 the tension will fall by 50 from start to finish A 3 1 variation in tension is adequate so constant torque operation is acceptable To maintain constant tension torque must be increased in proportion to winding diameter This requirement leads to the use of a servo operating in torque mode the need for constant speed operation at 2000 rpm also makes a stepper unsuitable In practice a servo in Velocity mode might be recommended but with an overriding torque limit the programmed velocity would be a little more than 2000 rpm In this way the servo will normally operate as a constant torque drive However if the filament breaks the velocity would be lim
59. this increase in current and momentarily turns off all the bridge transistors Fig 2 10 There is now a path for the regenerated current back to the supply capacitor where it increases the supply voltage During this phase the current is no longer flowing through the sense resistors so the power switches must be turned on again after a short period typically 30uS for conditions to be reassessed If the current is still too high the drive retums to the regenerative state Fig 2 10 Current flow during regeneration V 1 Power Power i A dump supply circuit capacitor A small increase in supply voltage during regeneration is acceptable but if the rise is too great the switches may be damaged by over voltage rather than excessive current To resolve this problem we use a power dump circuit that dissipates the regenerated power Drive Technologies The circuit of a simple power dump is shown in Fig 2 11 A rectifier and capacitor fed with AC from the supply transformer provide a reference voltage equal to the peak value of the incoming AC Under normal conditions this will be the same as the drive supply voltage During excess regeneration the drive supply voltage will rise above this reference and this will turn on the dump transistor connecting the 33 ohm resistor across the power supply When the supply voltage has decreased sufficiently the transistor is turned ba
60. together with the coils see Fig 1 16 Motor Technologies The forcer is equipped with 4 pole pieces each having 3 teeth The teeth are staggered in pitch with respect to those on the platen so that switching the current in the coils will bring the next set of teeth into alignment A complete switching cycle 4 full steps is equivalent to one tooth pitch on the platen Like the rotary stepper the linear motor can be driven from a microstep drive In this case a typical linear resolution will be 12 500 steps per inch The linear motor is best suited for applications that require a low mass to be moved at high speed Ina leadscrew driven system the predominant inertia is usually the leadscrew rather than the load to be moved Hence most of the motor torque goes to accelerate the leadscrew and this problem becomes more severe the longer the travel required Using a linear motor all the developed force is applied directly to the load and the performance achieved is independent of the length of the move A screw driven system can develop greater linear force and better stiffness however the maximum speed may be as much as ten times higher with the equivalent linear motor For example a typical maximum speed for a linear motor is 100 in sec To achieve this with a 10 pitch ballscrew would require a rotary speed of 6 000 rpm In addition the linear motor can travel up to 12 feet using a standard platen oD U oD io
61. 1 34900 10 lt x oz in s 7 06154 107 70 6154 7 06154 10 7 20077 10 7 20077 107 72 00766 3 86089 102 1 24 13045 6 250 10 0 167573 5 20833 103 Ib in 2 92641 104 2 92641 2 92641 10 2 98411 10 2 98411 103 2 98411 16 4 14414 102 1 2 59008 103 6 94444 10 2 15840 10 Ib in s 0 112985 1 12985 10 1 12985 10 1 15213 107 1 152126 1 15213 10 6 17740 103 16 3 86088 10 1 2 681175 8 3333 10 lb ft 4 21403 10 4 21403 10 4 21403 10 4 29711 10 0 429711 4 297114 10 2 304 10 5 96755 144 0 372971 1 3 10809 102 Ib ft s slug ft 1 35583 1 35582 10 1 35582 107 0 138255 13 82551 1 38255 10 7 41289 10 192 4 63306 10 12 32 1740 il Torque Conversion Table To convert from A to B multiply by entry in Table B A N m N cm dyn cm kg m kg cm g cm oz in ft lbs in lbs N m 1 102 107 0 1019716 10 19716 1 019716 104 141 6119 0 737562 8 85074 N cm 107 1 105 1 019716 103 0 10197163 1 019712 102 1 41612 7 37562 103 8 85074 10 dyn cm 107 10 1 1 019716 10 1 01972 10 1 01972 103 1 41612 105 7 37562 10 8 85074 10 7 kg m 9 80665 9 80665 10 9 80665 10 1 10 10 1 38874 103 7 23301 86 79624 kg cm 9 80665 10 9 80665 9 80665 10 10 1 10 13 8874 7 23301 10 0 86792 g cm 9 80665 10 9 80665 10 9 80665 10 105 103 1 1 38874 10 7 23301 10 8 679624 10 4 oz in 7 06155 10 3 0 706155 7 06155 104 7 20077 10 4 7 20077 10 72 0077 1 5 20833 10 6 250 10 ft lbs 1 35582 1 35582 10 1 35582 107 0 1382548 13 82548 1 382548 10 192 1 12 in lbs 0 112085 11 2985 1 1298
62. 1 micron accuracies without high grade linear encoders It is necessary for the Compumotor Model AT6400 indexer which resides directly on the computer bus to provide full X Y Z microscope control and accept incremental encoder feedback lt Microstepping motors System Calculations perform both cuts This will be accomplished by moving a cutting tool mounted on the end of the leadscrew into the workpiece at two velocities an initial velocity for the rough cut and a much reduced final velocity for the finish cut The torque required to accelerate the load and overcome the inertia of the load and the rotational inertia of the leadscrew is determined to be 120 oz in The torque necessary to overcome friction is measured with a torque wrench and found to be 40 oz in A microstepping motor with 290 oz in of torque is selected and provides adequate torque margin Pick and Place machines Articulated arms Precision Grinder Other Leadscrew Drive Applications A bearing manufacturer is replacing some e XY Plotters equipment that finishes bearing races The old Facsimile transmission v equipment had a two stage grinding arrangement ee 2 where one motor and gearbox provided a rough cut Tool bit positioning F o and a second motor with a higher ratio gearbox Cut to length machinery o performed the finishing cut The designer would like Back gauging D to simplify the mechanics and eliminate one motor e Microscope drives H
63. 26 JE 010 0A LF 040 28 069 45 E 098 62 b 127 7F 011 OB VT 041 29 070 46 F 099 63 c 012 OC FF 042 2A 071 47 G 100 64 d 013 0D CR 043 2B 072 48 H 101 65 e 014 OE SO 044 2C 073 49 l 102 66 f 015 OF S1 045 2D 074 4A J 103 67 g 016 10 DLE 046 2E 074 4B K 104 68 h 017 11 DC1 047 2F 075 AC L 105 69 i 018 12 DC2 048 30 0 076 4D M 106 6A j 019 13 DC3 049 31 1 077 4E N 107 6B k 020 14 DC4 050 32 2 078 4F O 108 6C 021 15 NAK 051 33 3 080 50 P 109 6D m 022 16 SYN 052 34 4 081 51 Q 110 6E n 023 17 ETB 053 35 5 082 52 R 111 6F o 024 18 CAN 054 36 6 083 53 S 112 70 p 025 19 EM 055 37 7 084 54 T 113 71 q 026 1A SUB 056 38 8 085 55 U 114 72 r 027 1B ESC 057 39 9 086 56 V 115 73 S 028 1C FS 058 3A 087 57 W 116 74 t 029 1D GS Motion amp Control Control Systems NULL MODEM A simple device or set of connectors that switches the receive and transmit lines a 3 wire RS 232C connector PARITY An RS 232C error detection scheme that can detect an odd number of transmission errors SERIAL POLLING Method of checking the status of the IEEE 488 device By reading the status byte the host can determine if the device is ready to receive or send characters START BITS When using RS 232C one or two bits are added to every character to signal the end of a character TEXT ECHO ON OFF This setup allows received characters to be re transmitted back to the original sending device Echoing characters can be used to verify or close th
64. 3 322 4987 Potter Brumfield optically isolated relays 812 386 1000 e General Electric MOVs 315 456 3266 Teal power line isolation filters 800 888 8325 A53 op U iS 0p a o 4 oO aa aD oD E e Lu Motion amp Control Control Systems Stopping in an Emergency For safety reasons it is often necessary to incorporate some form of emergency stop system into machinery fitted with stepper or servo motors There are several reasons for needing to stop quickly To prevent injury to the operator if he makes a mistake or operates the machinery improperly To prevent damage to the machine or to the product as a result of a jam To guard against machine faults You should consider all the possible reasons for stopping to make sure that they are adequately covered How should you stop the system There are several ways to bring a motor to a rapid stop The choice depends partly on whether it is more important to stop in the shortest possible time or to guarantee a stop under all circumstances For instance to stop as quickly as possible means using the decelerating power of the servo system However if the servo has failed or control has been lost this may not be an option open to you In this case removing the power will guarantee that the motor stops but if the load has a high inertia this may take some time If the load is moving vertically and can back drive the
65. 5 10 1 15212 10 1 15212 1 15212 10 16 8 33333 107 1 Densities of Common Materials Calculate Horsepower Material oz in gm cm Horsepower Torque x Speed Aluminum cast or hard drawn 1 54 2 66 16 800 Brass cast or rolled 60 CU 40 Zn 4 80 8 30 Torque oz in Bronze cast 90 CU 10 Sn 4 72 8 17 Speed revolutions per second Copper castor hand drawn 5 15 8 91 The horsepower calculation uses the torque available at the specified speed Plastic 0 64 1 11 Steel hot or cold rolled 0 2 or 0 8 carbon 4 48 7 75 a a ae Hard Wood 0 46 0 80 Most tables give densities in lb ft To convert to oz in divide this value by 108 To convert lb ft to gm cm Soft Wood 0 28 0 48 divide by 62 5 The conversion from 0z in to gm cm is performed by multiplying 02 in by 1 73 Reference Elements of Strength of Materials S Timoshinko and D H Young pp 342 343 Motion amp Control A71 Application Examples Summary of Application Examples Feed to length Applications in which a continuous web strip or strand of material is being indexed to length most often with pinch rolls or some sort of gripping arrangement The index stops and some process occurs cutting stamping punching labeling etc Application No Page 1 BBQ Grill Making Machine sees A73 2 Film ADVANCE eerren A74 3 On the Fly Welder sssscccrsrserenen A75 X Y Point to point Applications that deal with parts handling mechanisms that sort route o
66. 9659 oz in 2 50 17 1807 18 4079 5 9059 oz in 5 00 274 8916 294 5267 94 4940 oz in Coefficients of Static Friction Materials Leadscrew Efficiencies Dry Contact Unless Noted uS Efficiency Steel on Steel 0 58 Type High Median Low Steel on Steel lubricated 0 15 Ball nut 95 90 85 Aluminum on Steel 0 45 Acme with metal nut 55 40 35 Copper on Steel 0 22 Acme with plastic nut 85 65 50 Brass on Steel 0 19 Since metallic nuts usually require a viscous Teflon on Steel 0 04 lubricant the coefficient of friction is both speed and temperature dependent Motion amp Control A61 System Calculations Leadscrew Drives Vertical or Horizontal Application ST Screw type ball or acme ST e Efficiency of screw e u Friction coefficient Ll L Length ofscrew L inches D Diameter of screw D inches p Pitch p threads inch W Weight of load W lbs F Breakaway force F ounces Directly coupled to the motor yes no If yes CT Coupling type If no belt amp pulley or gears Radius of pulley or gear inches Gear Number of teeth Gear 1 Number of teeth Gear 2 Weight of pulley or gear ounces Weight of belt ounces Leadscrew Formulas The torque required to drive load W using a leadscrew with pitch p and efficiency e has the following components Tiotal T T Friction Acceleration lk Friction 2mpe Where F frictional force in ounces p
67. E e W Fig 2 13 Regulated and voltage limited regions of the torque speed curve Voltage Limited Region Regulated Region Drive with Higher Supply m Voltage o f Torque R z Speed LUU AAK Average Current 1 1 J I During ema Pulse As speed increases the time taken for the current to rise becomes a significant proportion of the interval between step pulses This reduces the average current level so the torque starts to fall off As speed increases further the interval between step pulses does not allow the current time to reach a level where the chopping action can begin Under these conditions the final value of current depends only on the supply voltage If the voltage is increased the current will increase more rapidly and hence will achieve a higher value in the available time So this region of the curve is described as voltage limited as a change in the drive current setting would have no effect We can conclude that at low speeds the torque depends on the drive current setting whereas at high speeds it depends on the drive supply voltage It is clear that high speed performance is not affected by the drive current setting Reducing the current simply flattens out the torque curve without restricting the ability to run at high speeds When performance is limited by the available high speed torque there is much to be said for running at the lowest current th
68. Load 2 R R Where W the weight is known or nlp J Load 2 R4 7 R4 Where p the density is known W 2Lp R R o We igs aad Problem Calculate the motor torque required to accelerate a solid cylinder of aluminum 5 in radius and 0 25 thick from rest to 2 1 radians sec 0 33 revs sec in 0 25 seconds First calculate J using the density for aluminum of 1 54 02 in ee zLeR 7X0 25 as X 5 378 o7 in2 Assume the rotor inertia of the motor you will use is 37 8 oz in Tyrotat F Load J maor x l 2 1 396 378 37 8 x 635 9 05 oz in Motion amp Control A63 System Calculations Gear Drives Traditional gear drives are more commonly used reflected back to the motor through the gearing is with step motors The fine resolution of a divided by the square of the gear ratio microstepping motor can he sal i In this manner large inertial loads can be moved unnecessary N Many appiicauons ears geEneray while maintaining a good load inertia to rotor inertia have undesirable efficiency wear characteristics ratio less than 10 1 backlash and can be noisy ki Gears are useful however when very large inertias must be moved because the inertia of the load Gear Driven Loads R Radius R inches R 1 Radius gear 1 R 1 inches 2 Radius gear 2 R 2 inches N 1 Number of teeth G 1 N 1 N 2 Number of teeth G 2 N 2 G Gear ratio N 1 G
69. Open loop gain and phase characteristics 30 2 Gain 10 db 0 am Frequency gt 0 90 Phase Shift 180 The gain scale is in decibels dB which is a logarithmic scale a 6dB decrease corresponds to a reduction in amplitude of about 50 The OdB line represents an open loop gain of one unity so at this frequency the input and output signals will have the same amplitude The falling response in the gain characteristics is mainly due to the inertia of the motor itself The phase scale is in degrees and shows the phase lag between input and output Remember that the feedback loop is arranged to give negative feedback at low frequencies i e 180 phase difference If the additional phase lag introduced by the system components reaches 180 the feedback signal is now shifted by 360 and therefore back in phase with the input We need to make sure that at no point do we get a feedback signal larger than the original input and in phase with it This would amount to positive feedback producing an ever increasing output leading to oscillation Fortunately it is possible to predict quite accurately the gain and phase characteristics of most servo systems provided that you have the necessary mathematical expertise and sufficient data about the system So in practice it is seldom necessary to measure these characteristics unless you have a particular stability problem that persists Servo Tuning We ve sai
70. a permanent magnet and the stator becomes a wound iron core The current carrying coils are now located in the housing providing a short efficient thermal path to the outside air Cooling can further be improved by finning the outer casing and blowing air over it if necessary to effectively cool an iron cored motor it is necessary to blow air through it The ease of cooling the brushless motor allows it to produce a much higher power in relation to its size The other major advantage of brushless motors is their lack of a conventional commutator and brush gear These items are a source of wear and potential trouble and may require frequent maintenance By not having these components the brushless motor is inherently more reliable and can be used in adverse environmental conditions To achieve high torque and low inertia brushless motors do require rare earth magnets that are much more expensive than conventional ceramic magnets The electronics necessary to drive a brushless motor are also more complex than for a brush motor A more thorough explanation of brushless motors is provided on page A17 Losses in DC Motors DC motors are designed to convert electrical power into mechanical power and as a consequence of this during periods of deceleration or if externally driven will generate electrical power However all the input power is not converted into mechanical power due to the electrical resistance of the armature and other
71. al for the application s industrial environment machine shop The controller s Teach mode and sizable nonvolatile memory allows for easy entry and storage of new part programs Microstepping drives which plenty of power resolution and accuracy are selected instead of more expensive closed loop servo systems The operator utilizes the controller s jog function to position the grinding head at the proper spark off height From this point the controller takes over and finishes the part while the operator works on other critical tasks Increasing the parts repeatability and throughput of the process justified the cost of automating the machine Product Solutions Indexer Drive Motor Model 4000 S Drive S83 93 The AT6400 PC based indexer has also been used to solve similar applications F m all Z I Grinding Wheel Control Panel wA ee Indexer A86 15 Transfer Machine Application Type Tool Feed Motion Linear Application Description A stage of a transfer machine is required to drill several holes in a casting using a multi head drill The motor has to drive the drill head at high speed to within 0 1 of the workpiece and then proceed at cutting speed to the required depth The drill is then withdrawn at an intermediate speed until clear of the work then fast retracted and set for the ne
72. amping Ratio Ratio of actual damping to critical damping Less than one is an underdamped system and greater than one is an overdamped system Dead Band A range of input signals for which there is no system response Decibel A logarithmic measurement of gain If G is a system s gain ratio of output to input then 20 log G gain in decibels dB Detent Torque The minimal torque present in an unenergized motor The detent torque of a step motor is typically about 1 of its static energized torque Direct Drive Servo A high torque low speed servo motor with a high resolution encoder or resolver intended for direct connection to the load without going through a gearbox Duty Cycle For a repetitive cycle the ratio of on time to total cycle time On Time Duty cycle On Time Off Time Efficiency The ratio of power output to power input Electrical Time Constant The ratio of armature inductance to armature resistance Encoder A device that translates mechanical motion into electronic signals used for monitoring position or velocity Form Factor The ratio of the RMS value of a harmonic Signal to its average value in one half wave Friction A resistance to motion Friction can be constant with varying speed Coulomb friction or proportional to speed viscous friction Gain The ratio of system output signal to system input signal Holding Torque Sometimes called static torque it speci
73. and AT6400 are other PC based indexer products that are often used in these types of applications Computer Indexer installed in a PC A83 Motion amp Control 0D U oD io q oO cc e iS oD oD E e W Application Examples 12 Engraving Machine Application Type Contouring Motion Linear Application Description An existing engraving machine requires an upgrade for accuracy beyond 0 008 inches capability and operating environment Using a personal computer as the host processor is desirable Machine Requirements e Positional accuracy to 0 001 inches e Easy to use open loop control e CNC machining capability e Interface to digitizer pad e Compatibility with CAD systems Motion Control Requirements e High resolution e Microstepping e G Code compatibility e IBM PC compatible controller Application Solution A four axis motion controller resides on the bus of an IBM compatible computer allowing full integrated control of four axes of motion Axes 3 and 4 are synchronized to prevent table skew CompuCAM s G Code package allows the user to program in industry standard machine tool language RS274 G Code or to import CAD files with CompuCAM DXF Open loop microstepping drives with precision leadscrews give positional accuracies better than the desired 0 001 inch This simple retrofit to the existing hardware greatly improved system performance Product Solu
74. applications where only a distance X and a time S to move that distance are known the trapezoidal motion profile and formulas given below are a good starting point for determining your requirements If velocity and acceleration parameters are already known you can proceed to one of the specific application examples on the following pages Move distance X in time S Assume that 1 Distance X 4 is moved in time S 3 Acceleration 2 Distance X 2 is moved in time S 3 Run 3 Distance X 4 is moved in time S 3 Deceleration The graph would appear as follows Velocity l i l l l ERE L 0 3 S 3 8 time S 3 S 3 S 3 Common Move Profile Considerations Distance Inches of Travel The acceleration a velocity v and deceleration d may be calculated in terms of the knowns X and S R a acg 2x9 45X t S 2 S2 S2 5 S S Example You need to move 6 in 2 seconds a d 456 inches _ 75 inches 2 seconds second v 1 5 6 inches 4 5 inches 2 seconds second revolutions of motor seconds arcminutes degrees or inches arcseconds degrees or inches seconds seconds sec min hour Motor Drive Selection Based on Continuous Torque Requirements Having calculated the torque requirements for an application you can select the motor drive suited to your needs Microstepping motor systems S Series Zeta Series OEM650 Series LN Series have speed torque cu
75. are energized in the reverse sequence the motor will go round the other way Motion amp Control Motor Technologies If two coils are energized simultaneously Fig 1 6 the rotor takes up an intermediate position since it is equally attracted to two stator poles Greater torque is produced under these conditions because all the stator poles are influencing the rotor The motor can be made to take a full step simply by reversing the current in one set of windings this causes a 90 rotation of the stator field as before In fact this would be the normal way of driving the motor in the full step mode always keeping two windings energized and reversing the current in each winding alternately Fig 1 6 Full stepping two phase on By alternately energizing one winding and then two Fig 1 7 the rotor moves through only 15 at each stage and the number of steps per rev will be doubled This is called half stepping and most industrial applications make use of this stepping mode Although there is sometimes a slight loss of torque this mode results in much better smoothness at low speeds and less overshoot and ringing at the end of each step Fig 1 7 Half stepping Current Patterns in the Motor Windings When the motor is driven in its full step mode energizing two windings or phases at a time see Fig 1 8 the torque available on each step will be the same Subject to very small variations in the A6 mot
76. at a byte of data has been sent 2 The peripheral receives data and sets a bit on the bus signalling to the host that data have been received The advantage of communicating in parallel vs serial is faster communications However since parallel communications require more communication lines the cost can be higher than serial communications Parallel bus structures include IEEE 488 IBM PC VME MULTIBUS Q and STD Troubleshooting Procedure for troubleshooting parallel communication 1 Make certain the address setting of the peripheral device is configured properly 2 Confirm that multiple boards are not set to the same address and each board is sealed properly into a slot 3 Verify that peripheral subroutines to reset the board write data and read data work properly Follow the handshaking procedure outlined in the device s user manual Note Compumotor bus based indexers come complete with a diskette that includes pretested programs to verify system functions and routines for simple user program development Serial and Parallel Communications ADDRESS Multiple devices are controlled on the same bus each with a separate address or unit number This address allows the host to communicate individually to each device ASCII American Standard Code for Information Interchange This code assigns a number to each numeral and letter of the alphabet In this manner information can be transmitted between machine
77. at gives an adequate torque margin In general dissipation in motor and drive is reduced and low speed performance in particular will be smoother with less audible noise Motion amp Control A29 Drive Technologies With a bipolar drive alternative possibilities exist for the motor connections as shown in Fig 2 14 An 8 lead motor can be connected with the two halves of each winding either in series or in parallel With a 6 lead motor either one half winding or both half windings may be connected in series The alternative connection schemes produce different torque speed characteristics and also affect the motor s current rating Fig 2 14 Series amp parallel connections Q 1A 1B 2A 2B 1A 1B 2A 2B Series Parallel Fig 2 15 Series amp parallel torque speed curves Series Torque Parallel Speed Compared with using one half winding only connecting both halves in series requires the drive current to flow through twice as many turns For the same current this doubles the amp tums and produces a corresponding increase in torque In practice the torque increase is seldom as high as 100 due to the non linearity of the magnetic material Equally the same torque will be produced at half the drive current when the windings are in series A30 However having doubled the effective number of turns in the winding means that we have also increased the inductan
78. ate time therefore becomes a critical factor in the performance of a digital servo and in a high performance system it must be kept to a minimum The tuning of a digital servo is performed either by pushbuttons or by sending numerical data from a computer or terminal No potentiometer adjustments are involved The tuning data is used to set various coefficients in the servo algorithm and hence determines the behavior of the system Even if the tuning is carried out using pushbuttons the final values can be uploaded to a terminal to allow easy repetition In some applications the load inertia varies between wide limits think of an arm robot that starts off unloaded and later carries a heavy load at full extension The change in inertia may well be a factor of 20 or more and such a change requires that the drive is re tuned to maintain stable performance This is simply achieved by sending the new tuning values at the appropriate point in the operating cycle Motion amp Control A33 Drive Technologies Brushless Motor Drives The trapezoidal drive Fig 2 20 shows a simplified layout of the drive for a three phase trapezoidal motor The switch set is based on the familiar H bridge but uses three bridge legs instead of two The motor windings are connected between the three bridge legs as shown with no connection to the star point at the junction of the windings By turning on the appropriate two transistors in the bridge
79. back into the start stop range before the clock pulses are terminated Using acceleration and deceleration ramping allows much higher speeds to be achieved and in industrial applications the useful speed range extends to about 3000 rpm 10 000 full steps sec Note that continuous operation at high speeds is not normally possible with a stepper due to rotor heating but high speeds can be used successfully in positioning applications The torque available in the slew range does not depend on load inertia The torque speed curve is normally measured by accelerating the motor up to speed and then increasing the load until the motor stalls With a higher load inertia a lower acceleration rate must be used but the available torque at the final speed is unaffected A11 oD U oD io q oO cc e iS oD oD E e W Motion amp Control Motor Technologies Common Questions and Answers About Step Motors 1 Why do step motors run hot Two reasons 1 Full current flows through the motor windings at standstill 2 PWM drive designs tend to make the motor run hotter Motor construction such as lamination material and riveted rotors will also affect heating 2 What are safe operating temperatures The motors have class B insulation which is rated at 130 C Motor case temperatures of 90 C will not cause thermal breakdowns Motors should be mounted where operators cannot come into contact w
80. be attained by an iron cored motor These fall into two categories Eddy current losses are common in all conductive cored components experiencing a changing magnetic field Eddy currents are induced into the motor armature as it undergoes changes in magnetization These currents are speed dependent and have a significant heating effect at high speeds In practice eddy currents are reduced by producing the armature core as a series of thin insulated sections or laminations stacked to produce the required core length e Hysteresis losses are caused by the resistance of the core material to constant changes of magnetic orientation giving rise to additional heat generation which increases with speed Friction losses These are associated with the mechanical characteristics of the motor and arise from brush friction bearing friction and air resistance These variables will generate heat and will require additional armature current to offset this condition Motor Technologies Short circuit currents As the brushes slide over the commutator the brush is in contact with two commutator segments for a brief period During this period the brush will short out the coil connected to those segments Fig 1 30 This condition generates a torque that opposes the main driving torque and increases with motor speed Fig 1 30 Generation of short circuit currents Brush N Commutator i oD U
81. bearings magnetic components and integral feedback in a compact motor package see Fig 1 46 The motor is an outer rotor type providing direct motion of the outside housing of the motor and thus the load The cross roller bearings that support the rotor have high stiffness to allow the motor to be connected directly to the load In most cases it is not necessary to use additional bearings or connecting shafts Fig 1 46 Expanded motor view Dynaserv Model DM oD U oD io q oO cc e iS oD oD E e W Encoder Plate PDA Kit LED Kit Clamp Ring O G X N Housing Kit o AX o Retaining _ Ps Ring eS Stator Core fe J Encoder Rotor Core The torque is proportional to the square of the sum of the magnetic flux of the permanent magnet rotor and the magnetic flux of the stator windings See Fig 1 47 High torque is generated due to the following factors First the motor diameter is large The tangential forces between rotor and stator act as a large radius resulting in higher torque Secondly a large number of small rotor and stator teeth create many magnetic cycles per motor revolution More working cycles means increased torque Fig 1 47 Dynaserv magnetic circuit MCMC Excitation Coil Permanent Magnet Stator B Motion amp Control A21 Motor Technologies Direct Drive Motor Advantages High Precision Dy
82. cal P brushless motor has either two or three sets of coils ct B1 or phases see Fig 1 38 The motor shown in Fig 1 38 is a two pole three phase design The rotor usually has four or six rotor poles with a corresponding increase in the number of stator poles This doesn t increase the number of phases each phase has its turns distributed between several stator poles Fig 1 39 Position torque characteristic 4 Torque Direction of Rotor 0 90 180 gt Field Relative C2 J A2 to Stator Field C1 B1 Fig 1 40 Stator field positions for different phase currents Stator Field Stator Field y Rotation A1 N Rotor Stator Field Field Average Lag 90 The torque characteristic in Fig 1 39 indicates that maximum torque is produced when the rotor and stator fields are at 90 to each other Therefore to generate constant torque we would need to keep the stator field a constant 90 ahead of the rotor Limiting the number of phases to three means that we can only advance the stator field in increments of 60 Fig 1 40 This means we must keep the stator field in the same place during 60 of shaft rotation So we can t maintain a constant 90 torque angle but we can maintain an average of 90 by working between 60 and 120 Fig 1 41 shows the rotor position at a commutation point When the torque angle has fallen to 60 the stator field is advanced from 1 to 2
83. ce by a factor of 4 This causes the torque to drop off much more rapidly as speed is increased and as a result the series mode is most useful at low speeds The maximum shaft power obtainable in series is typically half that available in parallel using the same current setting on the drive Connecting the two half windings of an 8 lead motor in parallel allows the current to divide itself between the two coils It does not change the effective number of turns and the inductance therefore remains the same So at a given drive current the torque characteristic will be the same for two half windings in parallel as for one of the windings on its own For this reason parallel in the context of a 6 lead motor refers to the use of one half winding only As has already been mentioned the current rating of a step motor is determined by the allowable temperature rise Unless the motor manufacturer s data states otherwise the rating is a unipolar value and assumes both phases of the motor are energized simultaneously So a current rating of 5A means that the motor will accept 5A flowing in each half winding When the windings of an 8 lead motor are connected in parallel half of the total resistance is produced For the same power dissipation in the motor the current may now be increased by 40 Therefore the 5A motor will accept 7A with the windings in parallel giving a significant increase in available torque Conversely co
84. ck off Although the instantaneous current flowing through the dump resistor may be relatively high the average power dissipated is usually small since the dump period is very short In applications where the regenerated power is high perhaps caused by frequent and rapid deceleration of a high inertia a supplementary high power dump resistor may be necessary Fig 2 11 Power dump circuit O HV R6 33Q 10W TR2 A27 oD U iS oO io q oO oc aD oD E e W Motion amp Control Drive Technologies Stepper Drive Technology Overview Within the various drive technologies there is a spectrum of performance The uni polar resistance limited R L drive is a relatively simple design but it lacks shaft power performance and is very inefficient A uni polar system only uses half of the motor winding at any instant A bi polar design allows torque producing current to flow in all motor windings using the motor more efficiently but increasing the complexity of the drive A bi polar R L drive improves shaft performance but is still very inefficient generating a lot of wasted heat An alternative to resistance limiting is to control current by means of chopper regulation chopper regulator is very efficient since it does not waste power by dropping voltage through a resistor However good current control in the motor is essential to deliver optimum shaft power Pulse wi
85. ction FR Friction D lt Where T torque oz in gm cm angular velocity radians sec t time seconds weight of the load oz pulley weight oz W belt or rack weight oz F frictional force oz gm R radius in cm V linear velocity g gravity constant 386 in sec p density 0z in What torque is required to accelerate a 5 Ib load to a velocity of 20 inches per second in 10 milliseconds using a flat timing belt The motor drives a 2 inch diameter steel pulley 1 2 inch wide The timing belt weighs 12 oz Load static friction is 30 ozs Motor rotor inertia is 10 24 oz in W R 5 lb x 16 T x 1 in 80 oz in J Load 2 nLpR J Pulley 2 r x 0 5 in x 4 48 oz in 1 in 7 04 oz in W R 12 oz 1 in 12 oz in J Belt T F xR 30 oz x 1 in 30 oz in Friction V in 1 rad rad RF 20cec X Tin 29 sec 1 20 Troi 386 80 7 04 12 10 24 01 30 T _ 596 2 oz in Total Motion amp Control A65 System Calculations Linear Step Motors There are many characteristics to consider when designing selecting and installing a complete motion control system The applications data worksheet and the application considerations detailed below will help determine if a linear motor system is recommended for a given application A linear motor when properly specified will provide the optimum performance and the greatest reliability Application Data Work
86. current can be made to flow in either direction through any two motor windings At any particular time the required current path depends on rotor position and direction of rotation so the bridge transistors are selected by logic driven from the commutation encoder A PWM recirculating chopper system controls the current in the same way as in the DC brush drive described previously The required current feedback information is provided by sense resistors connected in series with two of the motor leads The voltage signals derived from these resistors must be decoded and combined to provide a useful current reference and the circuit that does this also uses the commutation encoder to determine how to interpret the information In fact this is not a simple process because the relatively small feedback voltage about 1V must be separated from the large voltage excursions generated by the chopping system 240V in the case of a typical high power drive The input stages of the brushless drive follow the same pattern as a conventional analog brush drive using a high gain velocity amplifier that generates the torque demand signal Velocity feedback can be derived in a number of ways but it is clearly desirable to use a brushless method in conjunction with a brushless motor Some motors incorporate a brushless tach generator that produces multi phase AC outputs These signals have to be processed in a similar way to the current feedback i
87. d that a problem can occur when there is a phase shift of 180 round the loop When this happens the open loop gain must be less than one 1 so that the signal fed back is smaller than the input So here is a basic requirement for a stable system The open loop gain must be less than unity when the phase shift is 180 When this condition is only just met i e the phase shift is near to180 at unity gain the system will ring after a fast change on the input Fig 3 4 Underdamped response vA Ringing at unity gain frequency Input Output Characteristics of a Practical Servo System Typical open loop gain and phase characteristics of an unloaded drive motor tach system will look something like Fig 3 5 Fig 3 5 Characteristics of a practical system Crossover frequency dB 40 300 Hz typical Shaft resonance Gain Pa pe 2 kHz typical gt dB Phase 180 360 The first thing we notice is the pronounced spike in the gain plot at a frequency of around 2kHz This is caused by shaft resonance torsional oscillation in the shaft between the motor and the tach Observe that the phase plot drive dramatically through the critical 180 line at this point This means that the loop gain at this frequency must be less than unity 0dB otherwise the system will oscillate The TIME CONSTANT control determines the frequency at which the gain of the amplifier starts to roll off You can thin
88. der step of accuracy with great dependability This is a continuous process that will respond to outside events that disturb the motor s position Motion amp Control A55 Selection Considerations Application Considerations Load characteristics performance requirements and coupling techniques need to be understood before the designer can select the best motor drive for the job While not a difficult process several factors need to be considered for an optimum solution A good designer will adjust the characteristics of the elements under his control including the motor drive and the mechanical transmission type gears lead screws etc to meet the performance requirements Some important parameters are listed below Torque Rotational force ounce inches defined as a linear force ounces multiplied by a radius inches When selecting a motor drive the torque capacity of the motor must exceed the load The torque any motor can provide varies with its speed Individual speed torque curves should be consulted by the designer for each application Inertia An object s inertia is a measure of its resistance to change in velocity The larger the inertial load the longer it takes a motor to accelerate or decelerate that load However the speed at which a motor rotates is independent of inertia For rotary motion inertia is proportional to the mass of the object being moved times the square of its distance from the ax
89. dexer the data consist of parameters such as acceleration Fig 5 8 Serial Communications Data bits l boob bee el ei l a a tt tol l l l3 l l Sis l top bits S s l Nja lt Time baud rate velocity move distance and move direction configured in ASCII characters Both communication techniques are generally bi directional allowing the host to both transmit and receive information from a peripheral device Fig 5 9 Parallel Communications 0 EEE 488 1 IBM PC o Databus VME Bus STD Bus 0 Multi Bus 0 0 0 Signals 1 A 0100 0001 1 0011 0001 Serial Serial communication transmits data one bit at a time on a single data line Single data bits are grouped together into a byte and transmitted at a predetermined interval baud rate Serial communication links can be as simple as a 3 line connection transmit Tx receive Rx and ground G This is an advantage from a cost standpoint but usually results in slower communications than parallel communications Common serial interfaces include RS 232C RS 422 RS 485 RS 423 Troubleshooting Procedure for troubleshooting 3 wire RS 232C communication 1 Verify that the transmit of the host is wired to the receive of the peripheral and receive of the host is wired to the transmit of the peripheral Note Try switching the receive and transmit wires on either the host or peripheral if you fail to ge
90. ding system since an absolute system always knows its location In many motion control applications it is difficult or impossible to find a home reference point This situation occurs in multi axis machines and on machines that can t reverse direction This feature will be particularly important in a lights out manufacturing facility Significant cost savings is realized in reduced scrap and set up time resulting from a power loss Provide Reliable Position Information in High speed Applications The counting device is often the factor limiting the use of incremental encoders in high speed applications The counter is often limited to a maximum pulse input of 100 KHz An absolute encoder does not require a counting device or continuous observation of the shaft or load location This attribute allows the absolute encoder to be applied in high speed and high resolution applications Resolvers A resolver is in principle a rotating transformer If we consider two windings A and B Fig 4 19 and if we feed winding B with a sinusoidal voltage then a voltage will be induced into winding A If we rotate winding B the induced voltage will be at maximum when the planes of A and B are parallel and will be at minimum when they are at right angles Also the voltage induced into A will vary sinusoidally at the frequency of rotation of B so that Eoa Ej Sing If we introduce a third winding C positioned at right angles to winding
91. dscrew friction and the residual torque of the step motor prevents this occurrence Product Solutions Indexer Drive Motor AT6200 Axis 1 ZETA Drive 57 51 Axis 2 ZETA Drive PO L20 P18 Empty Ca Side View Capsules A80 Computer Indexer installed in a PC 9 Indexing Table Application Type Indexing Conveyor Motion Linear Application Description A system is required to plot the response of a sensitive detector that must receive equally from all directions It is mounted on a rotary table that needs to be indexed in 3 6 steps completing each index within one second For set up purposes the table can be positioned manually at 5 rpm The table incorporates a 90 1 worm drive Machine Requirements e Low EMI system e Repeatable indexing e Remote operation Table speed of 5 rpm Motion Control Requirements e ogging capability e Sequence select functionality Capable of remote drive shutdown Source Detector Rotary Stage Radiation Application Examples Application Solution The maximum required shaft speed 450 rpm is well within the capability of a stepper which is an ideal choice in simple indexing applications Operating at a motor resolution of 400 steps rev the resolution at the table is a convenient 36 000 step rev In this application it is important that electrical noise is minimized to avoid interference with the d
92. dth modulation PWM and threshold modulation are two types of chopper regulation techniques PWM controls the average of the motor current and is very good for precise current control while threshold modulation controls current to a peak level Threshold modulation can be applied to a wider range of motors but it does suffer greater loss of performance than PWM when the motor has a large resistance or long motor cables are used Both chopper regulation techniques can use recirculating current control which improves the power dissipation in the motor and drive and overall system efficiency As system performance increases the complexity and cost of the drive increases Stepper drive technology has evolved being driven by machine builders that require more shaft power in smaller packages higher speed capability better efficiency and improved accuracy One trend of the technology is towards microstepping a technique that divides each full step of the motor into smaller steps This is achieved electronically in the drive by proportioning the current between the motor windings The higher the resolution the more precision is required in the current control circuits In its simplest form a half step system increases the resolution of a standard 1 8 full step motor to 400 steps rev Ministepping drives have more precise current control and can increase the resolution to 4 000 steps rev Microstep drives typically have resolutions of 50
93. e natural choice is a brushless servo system The speed of this axis depends on head position and will need to increase as the heads scan from the outside to the center To successfully solve this application the multi axis indexer requires variable storage the ability to perform math functions and the flexibility to change velocity on the fly The sense and burnishing heads traverse at low speed and can be driven by stepper motors Stepper motors since the sense and burnishing heads need to start and step at the same time linear interpolation is required Product Solutions Controller Drive 1 Drive 2 Drive 8 Model 4000 S Drive S Drive Z Drive Motor 1 Motor 2 Motor 3 583 93 S83 93 Z60 The AT6400 PC based indexer has also been used in these types of applications Axis 2 Burnishing Head eee ES Axis 3 Disc Drive Motor Motion amp Control A89 0D U oD io q oO cc e iS oD oD E e W 18 Monofilament Winder Application Type Winding Motion Rotary Application Description Monofilament nylon is produced by an extrusion process that results in an output of filament at a constant rate The product is wound onto a bobbin that rotates at a maximum speed of 2000 rpm The tension in the filament must be held between 0 2 lbs and 0 6 lbs to ensure that it is not stretched The winding diameter varies between 2 and 4 The filament is laid onto
94. e tending to hold the rotor in one of these positions is usually small and is called the detent torque The motor shown will have 12 possible detent positions If current is now passed through one pair of stator windings as shown in Fig 1 5 a the resulting north and south stator poles will attract teeth of the opposite polarity on each end of the rotor There are now only three stable positions for the rotor the same as the number of rotor teeth The torque required to deflect the rotor from its stable position is now much greater and is referred to as the holding torque oD U oD io q oO cc e iS oD oD E e W Fig 1 5 Full stepping one phase on By changing the current flow from the first to the second set of stator windings b the stator field rotates through 90 and attracts a new pair of rotor poles This results in the rotor turning through 30 corresponding to one full step Reverting to the first set of stator windings but energizing them in the opposite direction we rotate the stator field through another 90 and the rotor takes another 30 step c Finally the second set of windings are energized in the opposite direction d to give a third step position We can now go back to the first condition a and after these four steps the rotor will have moved through one tooth pitch This simple motor therefore performs 12 steps per rev Obviously if the coils
95. e current flowing in one half of each winding If we could utilize both sections at the same time we could get a 40 increase in amp turns for the same power dissipation in the motor To achieve high performance and high efficiency we need a bipolar drive one that can drive current in either direction through each motor coil and a better method of current control Let s look first at how we can make a bipolar drive Drive Technologies Bipolar Drive An obvious possibility is the simple circuit shown in Fig 2 6 in which two power supplies are used together with a pair of switching transistors Current can be made to flow in either direction through the motor coil by turning on one transistor or the other However there are distinct drawbacks to this scheme First we need two power supplies both of which must be capable of delivering the total current for both motor phases When all the current is coming from one supply the other is doing nothing at all so the power supply utilization is poor Second the transistors must be rated at double the voltage that can be applied across the motor requiring the use of costly components Fig 2 6 Simple bipolar drive oD U iS oO io q oO oc aD oD E e W v The standard arrangement used in bipolar motor drives is the bridge system shown in Fig 2 7 Although this uses an extra pair of switching transistors the p
96. e is attractive when mechanical simplicity is desirable and the load being driven is of moderate inertia Direct Drive Formulas op U op io 4 oO oc e iS oD oD E e Lu R Radius R inches R 1 Inner radius R 1 inches R 2 Outer radius R 2 inches L Length L inches W Weight of disc W ounces p Density M aterial p ounces inch g Gravity constant g 386 in sec Solid Cylinder oz in Where Inertia J 44 WR a angular acceleration radians sec 2 final velocity radians sec Where weight and radius are known initial velocity radians sec Inertia 0z in J 4 mpR t time for velocity change seconds 2 inertia in units of oz in Where p the material density is known Weight W zLpR Inertia may be calculated knowing either the weight and radius of the solid cylinder W and R or its density radius and length p R and L The torque required to accelerate any load is T 0z in J a 0 a 1 2n accel for Accel in rps jJ The angular acceleration equals the time rate of change of the angular velocity For loads ancela from zero 0 and a Ti g Load J Motor a T a represents the torque the motor must deliver The gravity constant g in the denominator represents acceleration due to gravity 386 in D sec and converts inertia from units of oz in to oz in sec Hollow Cylinder w J
97. e loop on a transmission XON XOFF Two ASCII characters supported in some serial communication programs If supported the receiving device transmits an XOFF character to the host when its character buffer is full The XOFF character directs the host to stop transmitting characters to the device Once the buffer empties the device will transmit an XON character to signal the host to resume transmission A51 HEX GRAPHIC Or wWwTAN lt xse lt c m rc op U iS 0p a o 4 oO aa aD oD E e Lu Control Systems Electrical Noise Sources Symptoms and Solutions Noise related difficulties can range in severity from minor positioning errors to damaged equipment from runaway motors crashing blindly through limit switches In microprocessor controlled equipment the processor is constantly retrieving instructions from memory in a controlled sequence If an electrical disturbance occurs it could cause the processor to misinterpret an instruction or access the wrong data This is likely to be catastrophic to the program requiring a processor reset Most Compumotor indexers are designed with a watchdog timer that shuts down the system if the program is interrupted This prevents the more catastrophic failures Sources of Noise Being invisible electrical noise can be very mysterious but it invariably comes from the following sources e Power line disturbances e Externally conduc
98. e wants to use a single leadscrew and exploit the Coil winders D wide speed range available with microstepping to Slides co i LL This grinder is controlled by a programmable controller PC and the environment requires that the electronics withstand a 60 C environment An indexer will provide the necessary velocities and accelerations The speed change in the middle of the grinding operation will be signaled to the PC with a limit switch and the PC will in tum program the new velocity into the indexer Additionally the indexer Stall Detect feature will be used in conjunction with an optical encoder mounted on the back of the motor to alert the PC if the mechanics become stuck Leadscrew Application Data Inertia of Leadscrews per Inch Diameter Diameter In Steel Brass Alum In Steel Brass Alum 0 25 0 0017 0 0018 0 0006 oz in 2 75 25 1543 26 9510 8 6468 oz in 0 50 0 0275 0 0295 0 0094 oz in 3 00 35 6259 38 1707 12 2464 oz in 0 75 0 1392 0 1491 0 0478 oz in 3 25 49 0699 52 5749 16 8678 oz in 1 00 0 4398 0 4712 0 1512 oz in 3 50 66 0015 70 7159 22 6880 oz in 1 25 1 0738 1 1505 0 3691 oz in 3 75 86 9774 93 1901 29 8985 oz in 1 50 2 2266 2 3857 0 7654 oz in 4 00 112 5956 120 6381 38 7047 oz in 1 75 4 1251 4 4197 1 4180 oz in 4 25 143 4951 153 7448 49 3264 oz in 2 00 7 0372 7 5399 2 4190 oz in 4 50 180 3564 193 2390 61 9975 oz in 2 25 11 2723 12 0774 3 8748 oz in 4 75 223 9009 239 8939 76
99. ective solution selecting a system based on power output will make the most efficient use of the motor and drive Step motor systems typically require the motor to accelerate to reach high speed Ifa motor was requested to run instantaneously at 3000 rpm the motor would stall immediately At slow speeds it is possible to start the motor without position loss by applying unramped step pulses The maximum speed at which synchronization will occur without ramping is called the start stop velocity The start stop velocity is inversely proportional to the square root of the total inertia The start stop capability provides a benefit for applications that require high speed point to point positioning since the acceleration to the start stop velocity is almost instantaneous the move time will be reduced No additional time is required to accelerate the motor from zero to the start stop velocity While the move time can be reduced it is generally more complicated for the controller or indexer to calculate the motion profile and implement a start stop velocity In most applications using start stop velocities will eliminate the need to run the motor at its resonant frequency and prevent de synchronization Ministep Systems Applications that require better low speed smoothness than a half step system should consider using a microstepping or ministepping solution Microstepping systems with resolutions of 50 000 steps rev can offer e
100. ects can be the same as if the rotor had been de magnetized Fig 1 14 Longitudinal section through single stack motor Fig 1 14 also shows that the rotor flux only has to cross a small air gap typically 0 1mm or 0 004 when the rotor is in position By magnetizing the rotor after assembly a high flux density is obtained that can be largely destroyed if the rotor is removed Stepper motors should therefore not be dismantled purely to satisfy curiosity since the useful life of the motor will be terminated Because the shaft of the motor passes through the center of the permanent magnet a non magnetic material must be used to avoid a magnetic short circuit Stepper shafts are therefore made of stainless steel and should be handled with care Small diameter motors are particularly vulnerable if they are dropped on the shaft end as this will invariably bend the shaft To produce a motor with a higher torque output we need to increase the strength of both the permanent magnet in the rotor and the field produced by the stator A stronger rotor magnet can be obtained by increasing the diameter giving us a larger cross sectional area However increasing the diameter will degrade the acceleration performance of the motor because the torque to inertia ratio worsens to a first approximation torque increases with diameter squared but inertia goes up by the fourth power Nevertheless we can increase torque output without
101. eed to servo which meets detect position the torque speed loss OR measure Yes f requirements actual load gt Is rapid settling Yes position to correct important for backlash No No i Try a hybrid y Use a microstepping servo with with encoder encoder feedback feedback if necessary Is low speed Yes smoothness bi important i i Yes Yes Is quiet operation Reall iet w Y No important on Really quiet Is quiet operation Yes No A N important a Q y No i Use a microstepping Use a stepper hybrid servo or direct drive servo Motion amp Control 0D U iS 09 io q oO co oD oD E e Lu Motor Technologies Stepper Motors Stepper Motor Benefits Stepper motors have the following benefits Low cost e Ruggedness e Simplicity in construction e High reliability e No maintenance e Wide acceptance e No tweaking to stabilize No feedback components are needed They work in just about any environment e Inherently more failsafe than servo motors There is virtually no conceivable failure within the stepper drive module that could cause the motor to run away Stepper motors are simple to drive and control in an open loop configuration They only require four leads They provide excellent torque at low speeds up to 5 times the continuous torque of a brush motor of the same frame size or double the torque of the equivalent brushless motor This often e
102. ent tends to give superior torque output but is less satisfactory as a microstepper since the magnetic poles are harder there is no progressive transfer of tooth alignment from one pole to the next In fact with this type of motor it can be quite difficult to find a current profile that gives good static positioning combined with smooth low speed rotation An alternative to producing a 7 5 pitch stator is to incorporate a slight skew in the rotor teeth This produces a similar effect and has the benefit of using standard 7 2 laminations throughout Skewing is also used in DC brush motors as a means of improving smoothness Drive Technologies Due to this dependence on motor type for performance it is usual for high resolution microstep systems to be supplied as a matched motor drive package The Stepper Torque Speed Curve We have seen that motor inductance is the factor that opposes rapid changes of current and therefore makes it more difficult to drive a stepper at high speeds Looking at the torque speed curve in Fig 2 13 we can see what is going on At low speeds the current has plenty of time to reach the required level and so the average current in the motor is very close to the regulated value from the drive Changing the regulated current setting or changing to a drive with a different current rating will affect the available torque accordingly oD U iS oO io q oO oc aD oD
103. epping drive is used because its linear amplifier technology produces little EMI The PC monitor is the operator interface A C language program controls the machine Machine operation begins with a display to the operator of a main menu This main menu lets the operator select between three modes Automated Test J oystick Position and Teach New Automated Test In Automated Test mode the PC displays a menu of preprogrammed test routines Each of these programs has stored positions for the different test locations This data is downloaded to the controller when a test program is selected The controller controls the axes to a home position moves to each scan position and waits for scan completion before moving to the next position 0D U oD io q oO cc e iS oD oD E e W In J oystick Position mode the controller enables the joystick allowing the operator to move in both X and Y directions using the joystick The AT6400 waits for a signal from the PC to indicate that the joystick session is over When Teach mode is selected the PC downloads a teach program to the controller written by the user After the axes are homed the controller enables the joystick and a position select joystick button The operator then jogs axes to a position and presses the position select button Each time the operator presses this position select button the motion controller reads t
104. er operates without data or other control signals from external sources A standalone unit usually incorporates a keypad for data entry as well as a display and frequently includes a main power supply It will also include some form of nonvolatile memory to allow it to store a sequence of operations Many controllers that need to be programmed from a terminal or computer can once programmed also operate in standalone mode Bus based A bus based controller is designed to accept data from a host computer using a standard communications bus Typical bus systems include STD VME and IBM PC bus The controller will uSually be a plug in card that conforms to the standards for the corresponding bus system For example a controller operating on the IBM PC bus resides within the PC plugging into an expansion slot and functioning as an intelligent peripheral PLC based A PLC based indexer is designed to accept data from a PLC in the form of I O communication Typically the I O information is in BCD format The BCD information may select a program to execute a distance to move a time delay or any other parameter requiring a number The PLC is well suited to I O actuation but poorly suited to perform complex operations such as math and complicated decision making The motion control functions are separated from the PLC s processor and thus do not burden its scan time X Code based X Code is a command language specifically developed for
105. erses during deceleration Note that the lag and lead relate only to position and not to speed From the static torque curve Fig 1 18 clearly this lag or lead cannot exceed two full steps 3 6 if the motor is to retain synchronism This limit to the position error can make the stepper an attractive option in systems where dynamic position accuracy is important When the stepper performs a single step the nature of the response is oscillatory as shown in Fig 1 19 The system can be likened to a mass that is located by a magnetic spring so the behavior resembles the classic mass spring characteristic Looking at it in simple terms the static torque curve indicates that during the step the torque is positive during the full forward movement and so is accelerating the rotor until the new stable point is reached By this time the momentum carries the rotor past the stable position and the torque now reverses Slowing the rotor down and bringing it back in the opposite direction The amplitude frequency and decay rate of this oscillation will depend on the friction and inertia in the system as well as the electrical characteristics of the motor and drive The initial overshoot also depends on step amplitude so half stepping produces less overshoot than full stepping and microstepping will be better still Fig 1 19 Single step response _ gt Angle Time gt Attempting to step the motor at its natural o
106. erted and is fed with the A signal into an OR gate whose output depends on one signal or the other being present the resultant output will be a square wave Fig 4 9 Fig 4 9 Reduction of noise in a complementary system Inverted A The simple interconnection of encoder and controller with channel outputs at low level may be satisfactory in electrically clean environments or where interconnections are very short In cases where long interconnections are necessary or where the environment is noisy complementary line driver outputs will be needed and interconnections should be made with shielded twisted pair cable Factors Affecting Accuracy Slew rate speed An incremental rotary encoder will have a maximum frequency at which it will operate typically LOOKHz and the maximum rotational speed or slew rate will be determined by this frequency Beyond this the output will become unreliable and accuracy will be affected Consider a 600 line encoder rotated at 1rpm gives an output of 10Hz If the maximum operating frequency of the encoder is 50KHz its speed will be limited to 5000 times this i e 50KHz 10Hz 5000 rpm If an encoder is rotated at speeds higher than its design maximum there may be conditions set up that will be detrimental to the mechanical components of the assembly This could damage the system and affect encoder accuracy Quantization error
107. etector Two possible solutions are to use a low EMI linear drive or to shut down the drive after each index with a stepper driving a 90 1 worm gear there is no risk of position loss during shutdown periods Product Solutions Indexer Drive Motor Model 500 LN Drive LN57 102 The SX drive indexer and PK2 drive are other products that have been used in these types of applications Indexer A81 Motion amp Control 0D U oD io q oO cc e iS oD oD E e W Application Examples 10 Rotary Indexer Application Type Indexing Conveyor Motion Rotary Application Description An engineer for a pharmaceutical company is designing a machine to fill vials and wants to replace an old style Geneva mechanism A microstepping motor will provide smooth motion and will prevent spillage The indexing wheel is aluminum and is 0 250 inch thick and 7 5 in diameter Solving the equation for the inertia of a solid cylinder indicates that the wheel has 119 3 oz in The holes in the indexing wheel reduce the inertia to 94 oz in The vials have negligible mass and may be ignored for the purposes of motor sizing The table holds 12 vials 30 apart that must index in 0 5 seconds and dwell for one second Acceleration torque is calculated to be 8 2 oz in at 1 33 rps A triangular move profile will result in a maximum velocity of 0 33 rps The actual torque requirement is less than 100
108. ew provides exceptional positioning resolution for many applications A typical 10 pitch 10 threads per inch screw attached to a 25 000 step rev motor provides a linear resolution of 0 000004 4 millionths or approximately 0 1 micron per step A flexible coupling should be used between the leadscrew and the motor to provide some damping The coupling will also prevent excessive motor bearing loading due to any misalignment _ Encoders A60 Microscope Positioning Application Type X Y Point to Point Motion Linear Description A medical research lab needs to automate their visual inspection process Each specimen has an origin imprinted on the slide with all other positions referenced from that point The system uses a PC AT Bus computer to reduce data input from the operator and determines the next data point based on previous readings Each data point must be accurate to within 0 1 microns Machine Objectives Sub micron positioning e Specimen to remain still during inspection e Low speed smoothness delicate equipment e Use PC AT Bus computer Motion Control Requirements e High resolution linear encoders Stepper zero speed stability e Microstepping e PC AT Bus controller Compumotor Solution Microstepping motors and drives in conjunction with a precision ground 40 pitch leadscrew table provide a means of sub micron positioning with zero speed stability Conventional mechanics cannot provide 0
109. face Memory ROM to Drive Outputs Processor based Controllers The flexibility offered by a microprocessor system makes it a natural choice for motion control Fig 5 2 shows the elements of a typical step and direction controller that can operate either in conjunction with a host computer or as a stand alone unit All the control functions are handled by the microprocessor whose operating program is stored in ROM This program will include an interpreter for the command language which may be X Code for example X Code commands are received from the host computer or terminal via the RS 232C communications interface These commands are simple statements that contain the required speed distance and acceleration rate etc The processor interprets these commands and uses the information to control the programmable pulse generator This in turn produces the step and direction signals that will control a stepper or servo drive The processor can also switch outputs and interrogate inputs via the I O interface Outputs can initiate other machine functions such as punching or cutting or simply activate drive panel indicators to show the program status Inputs may come from sources such as operator pushbuttons or directional limit switches When the controller is used in a standalone mode the required motion sequences are programmed from the host and stored in nonvolatile memory normally battery backed RAM These sequences
110. fectively it helps to understand what s going on in the drive Unfortunately the theory behind servo system behavior though well understood does not reveal itself to most of us without a struggle So we ll use a simplified approach to explain the tuning process in a typical analog velocity servo Bear in mind that this simplified approach does not necessarily account for all aspects of servo behavior A Brief Look at Servo Theory A servo is a closed loop system with negative feedback If you make the feedback positive you will have an oscillator So for the servo to work properly the feedback must always remain negative otherwise the servo becomes unstable In practice it s not as clear cut as this The servo can almost become an oscillator in which case it overshoots and rings following a rapid change at the input A36 So why doesn t the feedback stay negative all the time To answer this we need to clarify what we mean by negative In this context it means that the input and feedback signals are in antiphase If the input is driven with a low frequency sinewave the feedback Signal which will also be a sinewave is displaced in phase by 180 The 180 phase displacement is achieved by an inversion at the input of the amplifier In practice it s achieved simply by connecting the tach the right way round connect it the wrong way and the motor runs away The very nature of a servo system is such that its
111. fies the maximum external force or torque that can be applied to a stopped energized motor without causing the rotor to rotate continuously Home A reference position in a motion control system derived from a mechanical datum or switch Often designated as the zero position Hybrid Servo A brushless servo motor based ona conventional hybrid stepper It may use either a resolver or encoder for commutation feedback Hysteresis The difference in response of a system to an increasing or a decreasing input signal IEEE 488 A digital data communications standard popular in instrumentation electronics This parallel interface is also known as GPIB or General Purpose Interface Bus Incremental Motion A motion control term that describes a device that produces one step of motion for each step command usually a pulse received Incremental Programming A coordinate system where positions or distances are specified relative to the current position Inertia A measure of an object s resistance to a change in velocity The larger an object s inertia the larger the torque that is required to accelerate or decelerate it Inertia is a function of an object s mass and its shape Inertial Match For most efficient operation the system coupling ratio should be selected so that the reflected inertia of the load is equal to the rotor inertia of the motor Indexer See PMC I O Abbreviation of input output Refer
112. g 2 17 has a high gain so that a small velocity difference will produce a large error signal In this way the accuracy of speed control can be made very high even when there are large load changes A torque demand from the velocity amplifier amounts to a request for more current in the motor The control of current is again achieved by a feedback loop that compares the torque demand with the current in the motor This current is measured by a sense resistor R which produces a voltage proportional to motor current This inner feedback loop is frequently referred to as a torque amplifier since its purpose is to create torque in response to a demand from the velocity amplifier The torque amplifier alone may be used as the basis of a servo drive Some types of position controller generate a torque output signal rather than a velocity demand and there are also applications in which torque rather than speed is of primary interest winding material onto a drum for instance Most analog drives can be easily configured either as velocity or torque amplifiers by means of a switch or jumper links In practice the input signal is still taken to the same point but the velocity amplifier is bypassed Fig 2 19 Digital servo drive Tuning RS 232C Drive Technologies Step gt DtoA Microprocessor gt p Converter Direction gt PWM Control Amplifier gt Encoder
113. g its output or collector to a low level Fig 5 4 Typical DC output connection diagram Ground The operation of the DC output model is similar to the DC input module A 5VDC signal from an indexer is used to activate an LED The output of the module is defined as open collector Fig 5 4 represents a typical DC output schematic Note the diode across the relay coil These should always be installed to eliminate the leading inductive kick caused by the relay A typical part number for such a diode is 1N4004 Failure to provide this protection can cause noise problems or the destruction of the output device Freewheeling Diode Indicator pao coe i 5VDC f a o p a4 Saag Liar i eas i 1 Load 10 Q a AK i Amplifier ae solenoin n0 Tno S Le rp i Logic LED Photo Output i gt 24VDC 1 Transistor Transistor Screw 2 4 Amps Terminals equivalent circuit A48 AC Input and Output Modules AC modules are not polarized devices This makes Control Systems voltage to DC levels AC input modules also include it virtually impossible to install a unit backwards transient protection to filter out spikes from the AC u AC input modules operate like DC input modules line caused by lightning strikes arc welders etc 5 with the addition of a bridge rectifier to change AC Oo T oc ron Fig 5 5 Typical AC input connection diagram
114. ground or into the processor power supply and scramble the program The problem here is that control equipment often shares a common DC ground that may run to several devices such as a DC power supply programmable controller remote switches and the like When some noisy device particularly a relay or solenoid is on the DC ground it may cause disturbances within the indexer The solution for DC mechanical relays and solenoids involves connecting a diode backwards across the coil to clamp the induced voltage kick that the coil will produce The diode should be rated at 4 times the coil voltage and 10 times the coil current Using solid state relays eliminates this effect altogether Fig 5 11 Coil Suppression Methods Diode Varistor MOV AC or DC Multiple devices on the same circuit should be grounded together at a single point Furthermore power supplies and programmable controllers often have DC common tied to Earth AC power ground As a rule it is preferable to have indexer signal ground or DC common floating with respect to Earth This prevents noisy equipment that is grounded to Earth from sending noise into the indexer The Earth ground connection should be made at one point only as discussed in Ground Loops on p A53 In many cases optical isolation may be required to completely eliminate electrical contact between the indexer and a noisy environment Solid state relays provide this isolation Tran
115. gs A23 oD U iS oO io q oO oc aD oD E e W Motion amp Control Drive Technologies Inductance Water Analogy For those not familiar with the property of inductance the following water analogy may be useful Fig 2 3 An inductor behaves in the same way as a turbine connected to a flywheel When the tap is tumed on and pressure is applied to the inlet pipe the turbine will take time to accelerate due to the inertia of the flywheel The only way to increase Fig 2 3 Inductance water analogy the acceleration rate is to increase the applied pressure If there is no friction or leakage loss in the system acceleration will continue indefinitely for as long as the pressure is applied In a practical case the final speed will be determined by the applied pressure and by friction and the leakage past the turbine blades Pressure Equivalent to Applied Voltage x 7 1 Way Pdi vave ks Pe N ae cc oe Water Flow Equivalent to Current Kinetic Energy of Flywheel Equivalent to Energy Stored in Magnetic Field beet ees Higher Pressure Causes Flywheel to Accelerate More Rapidly Voltage Pressure Current Flow Applying a voltage to the terminals of an inductor produces a similar effect With a pure inductance i e no resistance the current will rise in a linear fashion for as long as the voltage is applied The
116. h the gain and phase characteristics Not only will the overall gain be reduced owing to the larger inertia but an additional gain spike will be introduced due to torsional oscillation between the motor and the load This gain spike may well be larger than the original 2kHz spike in which case the motor will start to buzz at a lower frequency when the time constant is adjusted A38 Fig 3 6 Characteristics of a system with inertial load N dB N Motor alone no load N Ss 7 vil N Motor dB plus load Gain Phase 180 360 The amplitude of this second spike will depend on the compliance or stiffness of the coupling between motor and load A springy coupling will produce a large gain spike this means having to reduce the gain to prevent oscillation resulting in poorer system stiffness and slower response So if you re after a snappy performance it s important to use a torsionally stiff coupling between the motor and the load Tachometers A permanent magnet DC motor may be used as a tachometer When driven mechanically this motor generates an output voltage that is proportional to shaft speed see Fig 4 1 The other main requirements for a tachometer are that the output voltage should be smooth over the operating range and that the output should be stabilized against temperature variations Small permanent magnet DC motors are frequently used in servo systems as speed se
117. he laser The grating must be positioned to an angular accuracy of 0 05 The high resolution of the microstepping motor and its freedom from hunting or other unwanted motion when stopped make it ideal Machine Requirements e System must precisely rotate a diffraction grating to tune the frequency of the laser PC compatible system control e Angular accuracy of 0 052 IEEE 488 interface is required Motion Control Requirements e High resolution Microstepper Little to no vibration at rest Stepper No hunting at the end of move Stepper e Limited space is available for motor small motor is required A76 Application Solution The inertia of the grating is equal to 2 of the proposed motor s rotor inertia and is therefore ignored Space is at a premium in the cavity and a small motor is a must A microstepping motor which provides ample torque for this application is selected The laser s instrumentation is controlled by a computer with an IEEE 488 interface An indexer with an IEEE 488 interface is selected It is mounted in the rack with the computer and is controlled with a simple program written in BASIC that instructs the indexer to interrupt the computer at the completion of each index Product Solutions Indexer Drive Motor Model 4000 LN Drive LN57 51 ey Model 4000 4000 5 Circuit Board Scanning Application Type X Y Poi
118. he actual amount of material fed into the cutting head Machine Objectives e Compnesate for material slippage Interface with customer s operator panel e Smooth repeatable operation e Variable length indexes Application Examples Motion Control Requirements e Accurate position control e Load mounted encoder feedback e High speed indexing e XCode language Application Solution By using the global position feedback capability of the BLHX drive the machine builder was able to close the position loop with the load mounted encoder while the velocity feedback was provided by the motor mounted encoder and signal processing The two encoder system provides improved stability and higher performance than a single load mounted encoder providing both position and velocity feedback The load mounted encoder was coupled to friction drive nip rollers close to the cut head Product Solutions e High reliability Controller Drive Motor BLHX75BN ML3450B 10 Motor and Nip Roll and BLHX150BN Drive Roll Load Mounted Servo Drive Encoder 7 Cutting Head A73 Motion amp Control 0D U oD io q oO cc e iS oD oD E e Lu Application Examples 2 Film Advance Application Type Feed to Length Motion Linear Tangential drives consist of a pulley or pinion which when rotated exerts a force on a belt or racks to move a linear load Common tangential drives incl
119. he system cause this current to slowly decay and when a pre set lower threshold is reached the top switch is turned back on and the cycle repeats The current is therefore maintained at the correct average value by switching or chopping the supply to the motor A26 This method of current control is very efficient because very little power is dissipated in the switching transistors other than during the transient switching state Power drawn from the power supply is closely related to the mechanical power delivered by the shaft unlike the R L drive which draws maximum power from the supply at standstill A variant of this circuit is the regenerative chopper In this drive the supply voltage is applied across the motor winding in alternating directions causing the current to ramp up and down at approximately equal rates This technique tends to require fewer components and is consequently lower in cost however the associated ripple current in the motor is usually greater and increases motor heating Regeneration and Power Dumping Like other rotating machines with permanent magnets the step motor will act as a generator when the shaft is driven mechanically This means that the energy imparted to the load inertia during acceleration is returned to the drive during deceleration This will increase the motor current and can damage the power switches if the extra current is excessive A threshold detector in the drive senses
120. hieved the first motor comes to an abrupt stop while a second axis places a tab The controller then initiates a cold weld pressure weld of the tab onto the paper and foil To avoid material breakage constant tension is applied to each of the six reels via air cylinders Sensors are installed on all axes so that if a break occurs the controller can stop the process A computer makes this process easy to use and set up PC AT based support software allows the user to build his controller command program The operator sets the diameter of the appropriate Capacitor the operating speed and the number of Capacitors all via the keyboard After this process the machine runs until a malfunction occurs or it has completed the job Product Solutions 0D U oD io q oO cc e iS oD oD E e W Controller Drive Motor Accessories AT6250 BL30 ML2340 E Encoder The 6250 standalone 2 axis servo controller and APEX20 APEX40 servo drives have also been used in these types of applications zal Drive Anode Cathode Tab A Tab Encoder Motor Input Output Reel i Reel a a gt Anode Tab Feeder A e e A i gt Axis Motor 0D 020 Foil Reel Spindel Axis Motor Paper and Encoder TD e aper Reel T 3 Capacitor wen lt S Wound Onto Spindle e D Paper Encoder Reel Opto
121. his position into a variable and sends this data to the PC for memory storage These new position coordinates can now be stored and recalled in Automated Test mode Product Solutions Controller Drive Motor Accessories AT6400 AUX1 LN Drive LN57 83 MO E Daedal X Y Table J oystick Motion amp Control A77 Application Examples 6 Telescope Drive Application Type Metering Dispensing Motion Rotary Traditional gear drives are more commonly used with step motors The fine resolution of a microstepping motor can make gearing unnecessary in many applications Gears generally have undesirable efficiency wear characteristics backlash and can be noisy Gears are useful however when very large inertias must be moved because the inertia of the load reflected back to the motor through the gearing is divided by the square of the gear ratio In this manner large inertial loads can be moved while maintaining a good load inertia to rotor inertia ratio less than 10 1 Application Description An astronomer building a telescope needs to track celestial events at a slow speed 15 hour and also slew quickly 15 in 1 second Machine Requirements e Smooth slow speed is required microstepper e High data intensive application bus based indexer e Future capabilities to control at least 2 axes of motion e Visual C interface Motion Control Requirements e High resolution e Very slow speed 1 25 rev
122. ical connections may be brought out through a high quality connector Modular or kit encoder Fig 4 14 These are available in a number of forms their principal advantage being that of reduced cost Fig 4 14 Modular encoder Cover Hub and Disc m 161 fi 61 lt Electronics Leads XN Body Since many servo motors have a double ended shaft it is a simple matter to fit a kit encoder onto a motor A42 The kit encoder will usually be less robust than the shaft encoder but this need not be a problem if the motor is mounted so that the encoder is protected If this cannot be done it will normally be possible to fit a suitable cover over the encoder A typical kit encoder will include a body on which will be mounted the electronic components and a hub and disc assembly for fitting to the shaft Some form of cover will also be provided mainly to exclude external light and provide some mechanical protection Linear encoder These encoders are used where it is required to make direct measurement of linear movement They comprise a linear scale which may be from a few millimeters to a meter or more in length and a read head They may be incremental or absolute and their resolution is expressed in lines per unit length normally lines inch or lines cm Basics of Absolute Encoders An absolute encoder is a position verification device that provides unique
123. iderations Like electrical noise other environmental factors should be considered before installing an optical encoder In particular temperature and humidity should be considered consult manufacturers specifications In environments contaminated with dust oil vapor or other potentially damaging substances it may be necessary to ensure that the encoder is enclosed within a sealed casing A41 Motion amp Control op U iS 0p a o 4 oO aa aD oD E e W Interconnecting Feedback Devices Mechanical Construction Shaft encoder Fig 4 13 In this type of encoder which may be either incremental or absolute the electronics are normally supported on a substantial mounting plate that houses the bearings and shaft The shaft protrudes from the bearings on the outside of the encoder for connection to the rotating system and on the inside so that the encoder disc may be mounted in the appropriate position relative to the light source and detector The internal parts are covered by an outer casing through which the interconnecting leads pass Fig 4 13 Shaft encoder Interconnecting Leads 4 Cover N Mounting Plate Shaft For use in extreme environmental or industrial conditions the whole enclosure may be specified to a more substantial standard heavy duty with sealed bearings and sealing between the mounting plate and cover Also the external electr
124. ill run at case temperatures up to 100 C 212 F temperatures hot enough to burn individuals who touch the motors To Improve Duty Cycle e Mount the drive with heatsink fins running vertically e Fan cool the motor e Fan cool the drive e Put the drive into REMOTE POWER SHUTDOWN when it isn t moving or reduce current e Reduce the peak current to the motor if possible e Use a motor large enough for the application Motor Sizing and Selection Software A wide range of applications can be solved torque curves that are based on user provided effectively by more than one motor type However information This advanced graphics package is some applications are particularly appropriate for VGA EGA compatible and allows data entry with a each motor type Compumotor s Motor Sizing and either a mouse or keypad 9 Selection Software package is designed to help Contact your local Automotive Technology Center v you easily identify the proper motor size and type D te see to obtain a copy rtd for your specific motion control application D A oc This software helps calculate load inertias and required torques
125. increase the applied voltage but this would also increase the final current level A simple way to alleviate this problem is to add a resistor in series with the motor to keep the current the same as before R L Drive The principle described in the Inductance Water Analogy p A24 is applied in the resistance limited R L drive see Fig 2 4 Using an applied voltage of 10 times the rated motor voltage the current will reach its final value in one tenth of the time If you like to think in terms of the electrical time constant this has been reduced from L R to L 10R so we ll get a useful increase in speed However we re paying a price for this extra performance Under steady state conditions there is 9 times as much power dissipated in the series resistor as in the motor itself producing a significant amount of heat Furthermore the extra power must all come from the DC power supply so this must be much larger R L drives are therefore only suited to low power applications but they do offer the benefits of simplicity robustness and low radiated interference Fig 2 4 Principle of the R L drive V m L R _l VOID VVV gt _ lt gt au R L R R l 2N 00w ama lt 2V gt L A av 2R Unipolar Drive Fig 2 5 Basic unipolar drive T 1A 1B TR1 TR2 K K TR3 m A drawback of the unipolar drive is its inability to utilize all the coils on the motor At any one time there will only b
126. ing a desired position Some types of accuracy are affected by the application For example repeatability will change with the friction and inertia of the system the motor is driving Accuracy in a rotary motor is usually defined in terms of arcminutes or arcseconds the terms System Selection Considerations arcsecond and arcminute are equivalent to second and minute respectively There are 1 296 000 seconds of arc in a circle For example an arcsecond represents 0 00291 inches of movement on a circle with a 50 foot radius This is equivalent to about the width of a human hair Stepper Accuracy There are several types of performance listed under Compumotor s motor specifications repeatability accuracy relative accuracy and hysteresis Repeatability The motor s ability to return to the same position from the same direction Usually tested by moving the motor one revolution it also applies to linear step motors moving to the same place from the same direction This measurement is made with the motor unloaded so that bearing friction is the prominent load factor It is also necessary to ensure the motor is moving to the repeat position from a distance of at least one motor pole This compensates for the motor s hysteresis A motor pole ina Compumotor is 1 50 of a revolution Accuracy Also referred to as absolute accuracy this specification defines the quality of the motor s mechanical construction The error
127. ion controllers controlling microstep drives must be able to output a much higher step frequency Fig 2 1 Stepper drive elements Stepper Drive Step Elements i Phase 1 Translator Switch EEO Motor Direction Phase 2 Step Direction The logic section of the stepper drive is often referred to as the translator Its function is to translate the step and direction signals into control waveforms for the switch set see Fig 2 1 The basic translator functions are common to most drive types although the translator is necessarily more complex in the case of a microstepping drive However the design of the switch set is the prime factor in determining drive performance so we will look at this in more detail Drive Technologies The simplest type of switch set is the unipolar arrangement shown in Fig 2 2 It is referred to as a unipolar drive because current can only flow in one direction through any particular motor terminal A bifilar wound motor must be used since reversal of the stator field is achieved by transferring current to the second coil In the case of this very simple drive the current is determined only by the motor winding resistance and the applied voltage Fig 2 2 Basic unipolar drive Lw 1A 1B 2A 2B K TR1 a L Such a drive will function perfectly well at low stepping rates but as speed is increased the torque will fall off rapidly due to the inductance of the windin
128. ions The controller looks for a product ready signal from a sensor mounted on the product infeed conveyor and then makes a move based on the status of the boxes on the box conveyor and the status of the product on the other product conveyor The controller is multitasking the control of the two product conveyors and the external encoder input as well as a sensor input to monitor the status of the boxes Thus the controller can instantaneously decide into which box the product should be placed and where that box is located The controller then accelerates the product into alignment with the appropriate box in time for the product to be completely placed in the box and continues to monitor the other rest of the product and box positions Product Solutions Controller Drive Motor Encoder Model500_ L Drive L20 E Encoder Product Box Product Product Infeed Conveyor Infeed l 0 lt Product Synchronization Controll Drive A94 23 Plastic Injection Molding Application Type Injection Molding Motion Linear Application Description A manufacturer of injection molding machines wants a system that will close a molding chamber apply pressure to the molding chamber for 5 seconds and then open the mold This action needs to be synchronized with other machine events When the molding chamber is open the mot
129. is method is particularly unsatisfactory in the case of a vertical axis since the load may fall under gravity A54 3 Remove the AC input power from the drive On drives that incorporate a power dump circuit a degree of dynamic braking is usually provided when the power is removed This is a better solution than disconnecting the motor although the power supply capacitors may take some time to decay and this will extend the stopping distance 4 Use dynamic braking A motor with permanent magnets will act as a generator when driven mechanically By applying a resistive load to the motor a braking effect is produced that is speed dependent Deceleration is therefore rapid at high speeds but falls off as the motor slows down A changeover contactor can be arranged to switch the motor connections from the drive to the resistive load This can be made failsafe by ensuring that braking occurs if the power supply fails The optimum resistor value depends on the motor but will typically lie in the 1 3 ohms range It must be chosen to avoid the risk of demagnetization at maximum speed as well as possible mechanical damage through excessive torque 5 Use a mechanical brake It is very often possible to fit a mechanical brake either directly on the motor or on some other part of the mechanism However such brakes are usually intended to prevent movement at power down and are seldom adequate to bring the system to a rapid halt partic
130. is amplifier is the most commonly used type for all but very low power requirements and the most commonly used method of output control is by pulse width modulation PWM Power dissipation is greatly reduced since the output transistors are either in an on or an off State In the off state no current is conducted and so no power is dissipated In the on state the voltage across the transistors is very low 1 2 volts so that the amount of power dissipated is small Such amplifiers are suitable for a wide range of applications including high power applications The operation of a switching or chopper amplifier is very similar to that of the chopping stepper drive already described Only one switch set is required to drive a DC brush motor making the drive simpler However the function of a DC drive is to provide a variable current into the motor to control the torque This may be achieved using either analog or digital techniques Analog and Digital Servo Drives Unlike stepper drives amplifiers for both brush and brushless servo motors are either analog or digital The analog drive has been around for many years whereas the digital drive is a relatively recent innovation Both types have their merits Drive Technologies Overview The Analog Drive In the traditional analog drive the desired motor velocity is represented by an analog input voltage uSually in the range 10 volts Full forward veloci
131. is of rotation Friction All mechanical systems exhibit some frictional force and this should be taken into account when sizing the motor as the motor must provide torque to overcome any system friction A small amount of friction is desirable since it can reduce settling time and improve performance Torque to Inertia Ratio This number is defined as a motor s rated torque divided by its rotor inertia This ratio is a measure of how quickly a motor can accelerate and decelerate its own mass Motors with similar ratings can have different torque to inertia ratios as a result of varying construction Load Inertia to Rotor Inertia Ratio For a high performance relatively fast system load inertia reflected to the motor should generally not exceed the motor inertia by more than 10 times Load inertias in excess of 10 times the rotor inertia can cause unstable system behavior Torque Margin Whenever possible a motor drive that can provide more motor torque than the application requires should be specified This torque margin accommodates mechanical wear lubricant hardening and other unexpected friction Resonance effects while dramatically reduced with the Compumotor microstepping system can cause the motor s torque to be slightly reduced at some speeds Selecting a motor drive that provides at least 50 margin above the minimum needed torque is good practice A56 Velocity Because available torque varies with veloc
132. ited to the programmed value The traversing arm can be adequately driven by a smaller servo Product Solutions Indexer Drive Motor 6250 BL30 ML2340 The AT6450 PC based servo controller and the APEX20 APEX40 servo controllers have also been used in this type of application J A90 Torque Motor Controller 19 Capacitor Winder Application Type Winding Motion Linear Application Description The customer winds aluminum electrolytic capacitors Six reels two with foil anode and cathode and four with paper are all wound together to form the capacitor After winding the material a designated number of turns the process is stopped and anode and cathode tabs are placed on the paper and foil The tabs must be placed so that when the capacitor is wound the tabs end up 90 0 1 from each other This process is repeated until the required number of tabs are placed and the capacitor reaches its appropriate diameter The previous system used a PLC conventional DC drives and counters to initiate all machine functions DIP switches were used to change and select capacitor lengths Lengthy set up and calibration procedures were required for proper operation In addition material breakage was common resulting in extensive downtime An operator had
133. ith the motor case 3 What can be done to reduce motor heating Many drives feature a reduce current at standstill command or jumper This reduces current when the motor is at rest without positional loss 4 What does the absolute accuracy specification mean This refers to inaccuracies non cumulative encountered in machining the motor 5 How can the repeatability specification be better than that of accuracy Repeatability indicates how precisely a previous position can be re obtained There are no inaccuracies in the system that affect a given position returning to that position the same inaccuracy is encountered 6 Will motor accuracy increase proportionately with the resolution No The basic absolute accuracy and hysteresis of the motor remain unchanged 7 Can luse a small motor on a large load if the torque requirement is low Yes however if the load inertia is more than ten times the rotor inertia cogging and extended ringing at the end of the move will be experienced 8 How can end of move ringing be reduced Friction in the system will help damp this oscillation Acceleration deceleration rates could be increased If start stop velocities are used lowering or eliminating them will help 9 Why does the motor stall during no load testing The motor needs inertia roughly equal to its own inertia to accelerate properly Any resonances developed in the motor are at their worst in a no load condi
134. ition Bear in mind that the reference table will only indicate relative currents in the two windings the absolute Fig 2 21 Two phase sine wave brushless drive Drive Technologies values will depend on the torque demand at the time So the processor must multiply the sine and cosine values by the torque demand to get the final value of current in each phase The resulting numbers are fed to D to A converters that produce an analog voltage proportional to demanded current This is fed to the two PWM chopper amplifiers Commutation information for a sine wave drive may also be derived from an absolute or incremental optical encoder An incremental encoder will be less expensive for the same resolution but requires some form of initialization at power up to establish the required 90 torque angle A pseudo sine wave drive using feedback from a low resolution absolute encoder can offer a cost effective alternative The pseudo sine wave drive gives superior performance to the trapezoidal drive at lower cost than the standard high resolution sine wave system D to A gt H Bri Step Converter Control eee Micro processor D to A PWM PWM H Bridge Motor oD U iS oO io q oO oc aD oD E e Lu Resolver to Digital Converter Resolver Motion amp Control A35 Servo Tuning Tuning a Servo System
135. ity motor drives must be selected with the required torque at the velocities needed by the application In some cases a change in the type of mechanical transmission used is needed to achieve the required performance Resolution The positioning resolution required by the application will have an effect on the type of transmission used and the motor resolution For instance a leadscrew with 4 revolutions per inch and a 25 000 step per revolution motor drive would give 100 000 steps per inch Each step would then be 0 00001 inches Duty Cycle Some motor drives can produce peak torque for short time intervals as long as the RMS or average torque is within the motor s continuous duty rating To take advantage of this feature the application torque requirements over various time intervals need to be examined so RMS torque can be calculated Solving Duty Cycle Limitation Problems Operating a motor beyond its recommended duty cycle results in excessive heat in the motor and drive The drive cycle may be increased by providing active cooling to the drive and the motor A fan directed across the motor and another directed across the drive s heatsink will result in increased duty cycle capability In most cases it is possible to tell if the duty cycle is being exceeded by measuring the temperature of the motor and drive Refer to the specifications for individual components for their maximum allowable temperatures Note Motors w
136. ively large step angles but their overall simplicity lends itself to economic high volume production at very low cost The axial air gap or disc motor is a variant of the permanent magnet design which achieves higher performance largely because of its very low rotor inertia However this does restrict the applications of the motor to those involving little inertia e g positioning the print wheel in a daisy wheel printer Fig 1 1 Canstack or permanent magnet motor WAAR dp A4 Variable Reluctance V R Motors There is no permanent magnet in a V R motor so the rotor spins freely without detent torque Torque output for a given frame size is restricted although the torque to inertia ratio is good and this type of motor is frequently used in small sizes for applications such as micro positioning tables V R motors are seldom used in industrial applications having no permanent magnet They are not sensitive to current polarity and require a different driving arrangement than the other motor types Fig 1 2 Variable reluctance motor Stator cup A Stator cup B Output shaft U Courtesy Airpax Corp USA Hybrid Motors The hybrid motor shown in Fig 1 3 is by far the most widely used stepper motor in industrial applications The name is derived from the fact that it combines the operating principles of the other two motor types P M amp V
137. izing both phases with equal currents produces an intermediate step position half way between the one phase on positions If the two phase currents are unequal the rotor position will be shifted towards the stronger pole This effect is utilized in the microstepping drive which subdivides the basic motor step by proportioning the current in the two windings In this way the step size is reduced and the low speed smoothness is dramatically improved High resolution microstep drives divide the full motor step into as many as 500 microsteps giving 100 000 steps per revolution In this situation the current pattern in the windings closely resembles two sine waves with a 90 phase shift between them see Fig 1 11 The motor is now being driven very much as though it is a conventional AC synchronous motor In fact the stepper motor can be driven in this way from a 60 Hz US 50Hz Europe sine wave source by including a capacitor in series with one phase It will rotate at 72 rpm Fig 1 11 Phase currents in microstep mode Ws Phase 1 Current Zero Phase 2 Current Zero a Standard 200 Step Hybrid Motor The standard stepper motor operates in the same way as our simple model but has a greater number of teeth on the rotor and stator giving a smaller basic step size The rotor is in two sections as before but has 50 teeth on each secti
138. k of it rather like the treble control on an audio amplifier When we adjust the time constant control we are changing the high frequency gain to keep the gain spike at 2kHz just below 0dB With too high a gain time constant too low the motor will whistle at about 2kHz A37 Motion amp Control op U op io 4 oO e oO op E W Servo Tuning The second point of interest is the CROSSOVER FREQUENCY which is the frequency at which the gain curve passes through OdB unity gain This frequency is typically between 40 and 300Hz On the phase plot amp beta is the phase margin at the crossover frequency If B is very small the system will overshoot and ring at the crossover frequency So amp represents the degree of damping the system will be heavily damped if amp is large The DAMPING control increases the phase margin at the crossover frequency It operates by applying lead compensation sometimes called acceleration feedback The compensation network creates a phase lead in the region of the crossover frequency which increases the phase margin and therefore improves the stability Increasing the damping also tends to reduce the gain at the 2kHz peak allowing a higher gain to be used before instability occurs Therefore the time constant should be re adjusted after the damping has been set up What s the effect of adding load inertia An external load will alter bot
139. l connections were made through slip rings this motor would behave like a step motor reversing the current in the rotor would cause it to flip through 180 By including the commutator and brushes the reversal of current is made automatically and the rotor continues to turn in the same direction Fig 1 36 Inside out DC motor Reversing Switch Motor Technologies Brushless Motor Operation To turn this motor into a brushless design we must start by eliminating the windings on the rotor This can be achieved by turning the motor inside out In other words we make the permanent magnet the rotating part and put the windings on the stator poles We still need some means of reversing the current automatically a cam operated reversing switch could be made to do this job Fig 1 36 Obviously such an arrangement with a mechanical switch is not very satisfactory but the switching capability of non contacting devices tends to be very limited However in a servo application we will use an electronic amplifier or drive which can also be used to do the commutation in response to low level signals from an optical or hall effect sensor see Fig 1 37 This component is referred to as the commutation encoder So unlike the DC brush motor the brushless version cannot be driven by simply connecting it to a source of direct current The current in the external circuit must be reversed at defined rotor positions
140. ler in an integrated indexer drive comes into this category However such units are frequently used in systems using more than one motor where the operations do not involve precise synchronization between axes A multi axis controller is designed to control more than one motor and can very often perform complex operations such as linear or circular interpolation These operations require accurate synchronization between axes which is generally easier to achieve with a central controller A variant of the multi axis controller is the multiplexed unit which can control several motors on a time shared basis A printing machine having the machine settings controlled by stepper motors could conveniently use this type of controller when the motors do not need to be moved simultaneously Hardware based Controllers Control systems designed without the use of a microprocessor have been around for many years and can be very cost effective in simpler applications They tend to lack flexibility and are therefore inappropriate where the move parameters are continually changing For this reason the hardware based controller has now given way almost exclusively to systems based ona microprocessor Fig 5 2 Processor based controller Control Systems XCode RS 232C Nonvolatile T d Communications RAM i Interface Step Programmable JUL Microprocessor Pulse Direction Generator z Inputs 1 0 Program Output Inter
141. liminates the need for a gearbox A stepper driven system is inherently stiff with known limits to the dynamic position error Stepper Motor Disadvantages Stepper motors have the following disadvantages e Resonance effects and relatively long settling times e Rough performance at low speed unless a microstep drive is used e Liability to undetected position loss as a result of operating open loop They consume current regardless of load conditions and therefore tend to run hot Losses at speed are relatively high and can cause excessive heating and they are frequently noisy especially at high speeds e They can exhibit lag lead oscillation which is difficult to damp There is a limit to their available size and positioning accuracy relies on the mechanics e g ballscrew accuracy Many of these drawbacks can be overcome by the use of a closed loop control scheme Note The Compumotor Zeta Series minimizes or reduces many of these different stepper motor disadvantages There are three main stepper motor types e Permanent Magnet P M Motors e Variable Reluctance V R Motors e Hybrid Motors Permanent Magnet P M Motors The tin can or canstack motor shown in Fig 1 1 is perhaps the most widely used type in non industrial applications It is essentially a low cost low torque low speed device ideally suited to applications in fields such as computer peripherals The motor construction results in relat
142. lity In a different application a linear motor is used to position a measuring device The size of an object can be measured by positioning the forcer to a point on the object Determining the measured value is based on the number of steps required to reach the point on the object System accuracy must be smaller than the tolerance on the desired measurement Open loop absolute accuracy of a linear step motor is typically less than a precision grade leadscrew system If a linear encoder is used in conjunction with a linear motor the accuracy will be equivalent to any other transmission system The worst case accuracy of the system is the sum of these errors Accuracy A B C D E F A Cyclic Error The error due to motor magnetics that recurs once every pole pitch as measured on the body of the motor B Unidirectional Repeatability The error measured by repeated moves to the same point from different distances in the same direction C Hysteresis The backlash of the motor when changing direction due to magnetic non linearity and mechanical friction D Cumulative Platen Error Linear error of the platen as measured on the body of the motor E Random Platen Error The non linear errors remaining in the platen after the linear is disregarded F Thermal Expansion Error The error caused by a change in temperature expanding or contracting the platen A67 Motion amp Control op U op io
143. n A single axis servo controller drive was chosen to solve this application An external encoder monitors the tube output and sends this information back to the servo system The servo system tracks the length of the tube that is being fed past the cutting blade Once the appropriate amount of material has been fed past the blade the servo accelerates the cutting device up to the speed of the tube sends an output to start the cutter and then follows the tube speed exactly Product Solutions Drive Controller Motor APEX6152 APEX610 Controller Drive
144. n In this application the controller will receive velocity and position data from an incremental encoder mounted to a roller on the conveyor belt carrying the unfastened parts The conveyor is considered the primary drive system The secondary motor drive system receives instructions from the controller based on a ratio of the velocity and position information supplied by the primary system encoder The linear motor forcer carries the weld head and is mounted on an overhead platform in line with the conveyor Linear motor technology was chosen to carry the weld head because of the length of travel The linear step motor is not subject to the same linear velocity and acceleration limitations inherent in systems converting rotary to linear motion For Indexer Application Examples example in a leadscrew system the inertia of the leadscrew frequently exceeds the inertia of the load and as the length of the screw increases so does the inertia With linear motors all the force generated by the motor is efficiently applied directly to the load thus length has no effect on system inertia This application requires a 54 inch platen to enable following of conveyor speeds over 20 in sec Application Process 1 A sensor mounted on the weld head detects the leading edge of a moving part and sends a trigger pulse to the controller 2 The controller receives the trigger signal and commands the linear motor drive to ramp up to twice the
145. n The rotor sprocket and film inertia is calculated to be 0 545 o0z in sec Solving the torque formula indicates that the motor for this application must provide 11 9 oz in to drive the film and pulley refer to Direct Drive Formulas on p A63 An indexer is selected to be connected to a BCD interface in the camera electronics Preset and Slew modes on the indexer are then controlled by the camera electronics to provide fast rewind and frame indexing Product Solutions Drive Indexer Motor SX 57 51 MO Drive Indexer 3 On the Fly Welder Application Type Feed to Length Motion Linear Description In a sheet metal fabrication process an unfastened part rides on a conveyor belt moving continuously at an unpredictable velocity Two spot welds are to be performed on each part 4 inches apart with the first weld 2 inches from the leading edge of the part A weld takes one second Machine Objectives e Standalone operation e Position welder according to position and velocity of each individual part e Welding and positioning performed without stopping the conveyor e Welding process must take 1 second to complete Motion Control Requirements e Programmable I O sequence storage Following e Motion profiling complex following e High linear acceleration and speed Application Solution This application requires a controller that can perform following or motion profiling based ona primary encoder positio
146. naserv motors eliminate the backlash or hysteresis inevitable in using any speed reducer Absolute positioning of 30 arc sec is typical with a repeatability of 2 arc sec Faster Settling Time The Dynaserv system reduces machine cycle times by decreasing settling times This result is realized because of the gearless design and sophisticated l PD control algorithm High Torque at High Speed The torque speed curve of the various Dynaserv models is very flat This results in high acceleration at high speeds 4 0 rps with good controllability Smooth Rotation The very low velocity and torque ripple of the Dynaserv contribute to its excellent speed controllability with a more than 1 1 000 speed ratio Fig 1 48 Dynaserv velocity torque ripple Optimum Tuning Dynaserv systems offer the user a tuning mode that simplifies the setting of optimum parameters for the actual load Turning on the test switch on the front panel of the drive produces a test signal Using an oscilloscope the gain settings are quickly optimized by adjusting the digital switches and potentiometers on the front panel Clean Operation The Dynaserv system is brushless and gearless which results in a maintenance free operation With preparation the Dynaserv can operate in class 10 environments Torque Ripple DM1015A Speed Ripple DM1150A 20 Conditions e Load 30 x Rotor Inertia e R
147. nductor is replaced by a large number of conductors i e a length of wire is wound into a coil the force per unit length is increased by the number of turns in the coil This is the basis of a DC motor Motor Technologies Practical Considerations The problem now is that of using this force to produce the continuous torque required ina practical motor To achieve maximum performance from the motor the maximum number of conductors must be placed in the magnetic field to obtain the greatest possible force In practice this produces a cylinder of wire with the windings running parallel to the axis of the cylinder A shaft is placed down this axis to act as a pivot and this arrangement is called the motor armature Fig 1 23 Fig 1 23 DC motor armature Resultant Field Due to Armature Shaft Current f 2 9 O D O Armature O ae OO Direction Q of Current Stator Field Into Page As the armature rotates so does the resultant magnetic field The armature will come to rest with its resultant field aligned with that of the stator field unless some provision is made to constantly change the direction of the current in the individual armature coils Commutation The force that rotates the motor armature is the result of the interaction between two magnetic fields the stator field and the armature field To produce a constant torque from the motor these two fields must remain constant in magnitude and in rela
148. nformation using additional data from a tach encoder Again this is not a particularly Straightforward process and it is difficult to obtain a smooth glitch free feedback signal A more satisfactory alternative is to use a high resolution optical encoder and convert the encoder pulse frequency to an analog voltage The encoder can also be used as the feedback device for a position controller Fig 2 20 Simplified trapezoidal brushless servo drive Velocity Amp Velocity Input Logic PWM amp Circuit Velocity Feedback Current Sense Selector Communication Encoder A34 The Sine Wave Drive Sine wave brushless motors can be two or three phase and the drive we ll look at is for the two phase version See Fig 2 21 This uses two H bridges to control current in the two motor windings and the power section of this drive closely resembles a pair of DC brush drives By contrast with the previous example this drive uses a digital processor based control section that takes its input in the form of step and direction signals We need to generate currents in the two motor windings that follow a sine and cosine pattern as the shaft rotates The drive shown in Fig 2 21 uses a brushless resolver and a resolver to digital converter RDC to detect the shaft position From this we will get a number that can be fed to a reference table to determine the instantaneous current values for that particular shaft pos
149. ng this technique are called multi tum absolute encoders This same technique can be incorporated in a rack and pinion style linear encoder resulting in long lengths of discrete absolute locations Advantages of Absolute Encoders Rotary and linear absolute encoders offer a number of significant advantages in industrial motion control and process control applications No Position Loss During Power Down or Loss of Power An absolute encoder is not a counting device like an incremental encoder because an absolute system reads actual shaft position The lack of power does not cause the encoder lose position information Motion amp Control A43 Feedback Devices Whenever power is supplied to an absolute system it can read the current position immediately In a facility where frequent power failures are common an absolute encoder is a necessity Operation in Electrically Noisy Environments Equipment such as welders and motor starters often generate electrical noise that can often look like encoder pulses to an incremental counter Electrical noise does not alter the discrete position that an absolute system reads High speed Long distance Data Transfer Use of a Serial interface such as RS 422 gives the user the option of transmitting absolute position information over as much as 4 000 feet Eliminate Go Home or Referenced Starting Point The need to find a home position or a reference point is not required with an absolute enco
150. nnecting the windings in series will double the total resistance and the current rating is reduced by a factor of 1 4 giving a safe current of 3 5A for our 5A motor in series As a general rule parallel is the preferred connection method as it produces a flatter torque curve and greater shaft power Fig 2 15 Series is useful when high torque is required at low speeds and it allows the motor to produce full torque from a lower current drive Care should be taken to avoid overheating the motor in series since its current rating is lower in this mode Series configurations also carry a greater likelihood of resonance due to the high torque produced in the low speed region DC Brush Motor Drives Linear and Switching Amplifiers Linear amplifiers this type of amplifier operates in such a way that depending on the direction of motor rotation either TR1 or TR2 will be in series with the motor and will always have a voltage V developed across it Fig 2 16 This characteristic is the primary limitation on the use of linear amplifiers since there will always be power dissipated in the output stages of the amplifier To dissipate this power large transistors and heat sinks will be required making this type of amplifier unsuitable for use in high power systems However the linear amplifier does offer the benefit of low radiated electrical noise Fig 2 16 Linear servo amplifier TR2 Ve Switching amplifiers th
151. nnot be used with a unipolar drive There is obviously no alternative connection method with a 4 lead motor but in many applications this is not a drawback and the problem of insulating unused leads is avoided Fig 1 13 Motor lead configurations OkO FO PATI 5 lead PT 8 lead 4 lead 6 lead Occasionally a 5 lead motor may be encountered These are not recommended since they cannot be used with conventional bipolar drives requiring electrical isolation between the phases Looking at the motor longitudinal section Fig 1 14 we can See the permanent magnet in the rotor and the path of the flux through the pole pieces and the stator The alternating flux produced by the stator windings flows in a plane at right angles to the page Therefore the two flux paths are at right A8 angles to each other and only interact in the rotor pole pieces This is an important feature of the hybrid motor it means that the permanent magnet in the rotor does not see the alternating field from the windings hence it does not produce a demagnetizing effect Unlike the DC servo motor it is generally impossible to de magnetize a stepper motor by applying excess current However too much current will damage the motor in other ways Excessive heating may melt the insulation or the winding formers and may soften the bonding material holding the rotor laminations If this happens and the laminations are displaced the eff
152. no extemal sensors are used to provide position or velocity correction signals Opto isolated A method of sending a signal from one piece of equipment to another without the usual requirement of common ground potentials The signal is transmitted optically with a light source usually a Light Emitting Diode and a light sensor usually a photosensitive transistor These optical components provide electrical isolation Parallel Refers to a data communication format wherein many signal lines are used to communicate more than one piece of data at the same time Phase Angle The angle at which the steady state input signal to a system leads the output signal Input Output Phase Angle Phase Margin The difference between 180 and the phase angle of a system at its crossover frequency PLC Programmable logic controller a machine controller that activates relays and other O units from a stored program Additional modules support motion control and other functions Glossary of Terms PMC Programmable motion controller primarily designed for single or multi axis motion control with I O as an auxiliary function Pole A frequency at which the transfer function of a system goes to infinity Pulse Rate The frequency of the step pulses applied to a motor driver The pulse rate multiplied by the resolution of the motor drive combination in steps per revolution yields the rotational speed in revolutions
153. nsing devices These systems usually incorporate thermistor temperature compensation and make use of a silver commutator and silver loaded brushes to improve commutation reliability at low speeds and at the low currents which are typical of this application To combine high performance and low cost DC servo motor designs often incorporate a tachometer mounted on the motor shaft and enclosed within the motor housing Fig 4 1 Fig 4 1 Tachometer output characteristics Output A nae Volts gt Shaft Speed Fig 4 2 Motor with integral tachometer Tachometer YIM il Motor Optical Encoders In servo control systems where mechanical position is required to be controlled some form of position sensing device is needed There are a number of types in use magnetic contact resistive and optical However for accurate position control the most commonly used device is the optical encoder There are two forms of this encoder absolute and incremental Optical encoders operate by means of a grating that moves between a light source and a detector When light passes through the transparent areas of the grating an output is seen from the detector For increased resolution the light source is collimated and a mask is placed between the grating and the detector The grating and the mask produce a shuttering effect so that only when their transparent sections are in alignment is light allowed to
154. nt In simple terms whereas almost all the flux from the inner coil would flow through the iron core some of the flux from the outer coil would flow through the windings of the coil underneath The origins of the bifilar winding go back to the unipolar drive See Drive Technologies section page A23 Rather than have to reverse the current in one winding the field may be reversed by transferring current to a second coil wound in the opposite direction Although the two coils are wound the same way interchanging the ends has the same effect So with a bifilar wound motor the drive can be kept simple However this requirement has now largely disappeared with the widespread availability of the more efficient bipolar drive Nevertheless the two sets of windings do give us additional flexibility and we shall see that different connection methods can be used to give alternative torque speed characteristics If all the coils in a bifilar wound motor are brought out separately there will be a total of 8 leads see Fig 1 13 This is becoming the most common configuration since it gives the greatest flexibility However there are still a number of motors produced with only 6 leads one lead serving as a common connection to each winding in a bifilar pair This arrangement limits the motor s range of application since the windings cannot be connected in parallel Some motors are made with only 4 leads these are not bifilar wound and ca
155. nt to Point Motion Linear Application Description An Original Equipment Manufacturer OEM manufactures X Ray Scanning equipment used in the quality control of printed circuit boards and wafer chips The OEM wants to replace the DC motors mechanics and analog controls with an automated PC based system to increase throughput and eliminate operator error The host computer will interact with the motion control card using a C language program The operator will have the option to manually override the system using a joystick This machine operates in an environment where PWM pulse width modulation related EMI emission is an issue Machine Requirements e 2 Axis analog joystick J oystick button e Travel limits e Encoder feedback on both axes Display Requirements X and Y position coordinates Operator Adjustable Parameters Dimensions of sample under test 0 0 position starting point Motion Control Requirements e AT based motion controller card e Replace velocity control system DC motors and mechanics with more accurate and automated positioning scheme e Manual J oystick control e Continuous display of X amp Y axis position e User friendly teach mode operations e Low EMI amplifiers drives Application Solution The solution of this application uses the existing PC by providing a PC based motion controller and Indexer Application Examples the AT6400 to control both axes A microst
156. nts and the operator interface are handled internally invisible to the user A common software language is provided to integrate the motion and I O actuation This pre tested approach allows a typical machine control application to be developed with a minimum of effort and cost The total application cost is the major consideration when selecting an integrated machine controller While the initial hardware cost is typically higher than other solutions the software investment and maintenance of a single language is an overriding and positive factor Software development and maintenance costs for any machine control application can dwarf the initial hardware expense The integrated approach offers a more economical solution Motion amp Control A45 Control Systems Control System Overview The controller is an essential part of any motion control system It determines speed direction distance and acceleration rate in fact all the parameters associated with the operation that the motor performs The output from the controller is connected to the drive s input either in the form of an analog voltage or as step and direction signals In addition to controlling one or more motors many controllers have additional inputs and outputs that allow them to monitor other functions on a machine see Machine Control p A45 Controllers can take a wide variety of forms Some examples are listed below Standalone This type of controll
157. o 0 Pushbutton Rectifying Indicator E Bridge peee Sessa iT a LED an aes me aun Pe Signal ___ vv Logic Signal lt E lt na Conditioning a Las S i Terminals 14K l n Pe Baca ase j 120 VAC IY Ground 60Hz Power Line AC output modules feature a Triac powerdevice as 1 Zero voltage turn on eliminates in rush currents its output A Triac output offers three distinct to the load advantages 2 Zero voltage turn off eliminates inductive kick problems 3 A snubber across the output Fig 5 6 Zero voltage turn on and off The module will only turn on or off at points A B or C where the voltage is zero potential problems add a parallel resistor across the load 5K 5W for 120VAC and a 10K 10W for AC output modules do have leakage current which 24OVAC may turn on small current loads To solve Fig 5 7 Typical AC output connection diagram Indicator l cetyl bloating a D gt gt W i oT gt 1 Zero i 1 D 1 Voltage 1 ence l 1 Circuit F777 R 7770 o si i i Logic LED Photo bonne l a TAG comeae eee Triac Snubber i Screw ae 4KV Isolation Barrier Terminals 120VAC 60Hz Power Line Motion amp Control A49 Control Systems Serial and Parallel Communications Serial and parallel communications are methods of transferring data from a host computer to a peripheral device such as a Compumotor indexer In the case of a Compumotor in
158. olutions per hour microstepping e AT bus based motion controller card e Dynamic Link Library DDL device driver must be provided with indexer This helps Windows programmers create Windows based applications i e Visual C to interface with the indexer A78 Application Solution A 30 1 gearbox is selected so that 30 revolutions of the motor result in 1 revolution 360 of the telescope A tracking velocity of 15 hour corresponds to a motor speed of 1 25 revs hour or about 9 steps sec on a 25 000 steps rev Moving 15 1 25 revolutions in 1 second requires a velocity of 1 25 rps The inverse square law causes the motor to see 1 900 of the telescope s rotary inertia The equations are solved and the torque required to accelerate the telescope is 455 oz in The step pulses required to drive the motor are obtained from a laboratory oscillator under the operator s control Product Solutions Indexer Drive Motor AT6200 AUX1 S Drive S 106 178 To control up to four axes refer to the AT6400 Computer Indexer installed Drive in aPC 7 Engine Test Stand Application Type Metering Dispensing Motion Rotary Application Description A jet engine manufacturer is building a test facility for making operational measurements on a jet engine The throttle and three other fuel flow controls need to be set remotely While the application only calls for a rotary resolution of 1 degree 1 360 rev
159. on Math capabilities Application Solution Controlled by a multi axis step and direction controller microstepping motors and drives are attached to four axes for smooth programmable motion at all speeds Motor Axis 4 Rotation Motor Axis 2 Chamfer ee Cutting s s Tool NY Bit Motor 4 C Axis 1 Alignment earned Application Examples e Axis 1 Alignment e Axis 2 Chamfer cutting depth e Axis 3 Traverse e Axis 4 Rotation To allow for the flexibility required to cut a bit ata desired pitch the traverse and rotation axes axes 3 and 4 are synchronized along a straight line The controller s linear interpolation allows this functionality Both the alignment and chamfer axes axes 1 and 2 remain stationary during the cutting process The controller s operator input panel and math Capabilities allow the operator to enter the bit diameter desired pitch depth and angle Using these part specifications the controller generates all motion profiles and stores them in nonvolatile battery backed RAM Programming is accomplished with the controller s menu driven language The typical process is as follows 1 Axis 1 aligns the center line of the bit to the cutting tool 2 Axis 2 lowers the cutting tool to the desired cutting depth chamfer 3 Axis 3 traverses the bit along the cutting tool 4 While axis 3 traverses axis 4 rotates the bit to create the desired
160. on The half tooth displacement between the two sections is retained The stator has 8 poles each with 5 teeth making a total of 40 teeth see Fig 1 12 Fig 1 12 200 step hybrid motor Stator Rotor Motor Technologies If we imagine that a tooth is placed in each of the gaps between the stator poles there would bea total of 48 teeth two less than the number of rotor teeth So if rotor and stator teeth are aligned at 12 o clock they will also be aligned at 6 o clock At 3 o clock and 9 o clock the teeth will be misaligned However due to the displacement between the sets of rotor teeth alignment will occur at 3 o clock and 9 o clock at the other end of the rotor The windings are arranged in sets of four and wound such that diametrically opposite poles are the same So referring to Fig 1 12 the north poles at 12 and 6 o clock attract the south pole teeth at the front of the rotor the south poles at 3 and 9 o clock attract the north pole teeth at the back By switching current to the second set of coils the stator field pattern rotates through 45 However to align with this new field the rotor only has to turn through 1 8 This is equivalent to one quarter of a tooth pitch on the rotor giving 200 full steps per revolution Note that there are as many detent positions as there are full steps per rev normally 200 The detent positions correspond with rotor teeth being fully aligned with stator teeth When power
161. ons can operate through a standard operating system DOS OS 2 OS 9 that can be used to program add on cards for I O motion and communication interfaces Flexible graphical operator interfaces remain one of the computer s major advantages Some successful examples of bus based machine control applications include gear grinding and dressing PCB placement machines hard disk manufacturing and automotive glass bending Wherever intensive communications or data processing are required the benefits of the bus structure can be realized op U iS 0p a o 4 oO aa aD oD E e W There are some disadvantages to the bus based machine control system that relate to the amount of integration between the motion and O structure Separate cards are required for each resulting in a need for software integration of different programming languages Motion control operations such as servo loops should be polled and updated on a more immediate basis than auxiliary I O or the operator interface The programmer must develop this polling hierarchy to thread the system together Integrated controllers A more integrated approach to machine control uses a stand alone architecture that builds in the same essential elements of I O motion operator interface and communication This approach uses a single software and hardware platform to control an entire machine application The polling of servo loops I O poi
162. ontrol motor velocity in response to an analog input voltage Fig 2 17 Elements of an analog servo system Velocity Torque Control Control Signal Signal gt B Drive Amplifier Torque Feedback Loop Go ca Velocity Feedback Loop Motor velocity is measured by a tach generator attached to the motor shaft This produces a voltage proportional to speed that is compared with the incoming velocity demand signal and the result of this comparison is a torque demand If the speed is too low the drive delivers more current which in turn creates torque to accelerate the load Similarly if the speed is too high or the velocity demand is reduced current flow in the motor will be reversed to produce a braking torque This type of amplifier is often referred to as a four quadrant drive This means that it can produce both acceleration and braking torque in either direction of rotation If we draw a diagram representing direction of rotation in one axis and direction of torque in the other See Fig 2 18 you will see that the motor can operate in all four quadrants By contrast a simple variable speed drive capable of running only in one direction and with uncontrolled deceleration would be described as single quadrant Fig 2 18 Four quadrant operation cw A Braking Accelerating CCW CW Accelerating Braking CCW Cw y CCW A32 The velocity amplifier in Fi
163. ontroller monitors the bottles positions on the conveyor The controller commands the label motor to accelerate to line speed by the time the first edge of the label contacts the bottle The label motor moves at line speed until the complete label is applied and then decelerates to a stop and waits for the next bottle Product Solutions Controller Motor APEX6152 APEX604 The ZXF single axis servo controller has also been used in these types of applications Secondary Axis f S Time Start Photocell Encoder Me ee oe ENEE EE es EEEE E Drive Controller A92 21 Window Blind Gluing Application Type Following Motion Linear Application Description A window blind manufacturer uses an adhesive to form a seam along the edge of the material It is critical that the glue be applied evenly to avoid flaws however the speed that the material passes beneath the dispensing head is not constant The glue needs to be dispensed at a rate proportional to the varying speed of the material Machine Requirements Allow for varying material speed Dispense glue evenly Allow for multiple blind lengths Motion Control Requirements e Synchronization to material speed e Velocity following capabilities e Sequence storage Application Solution A step and direction indexer follower and a microstepping motor drive are used to power a displacement pump The indexer follower is programmed to run
164. or and drive characteristics In the half step mode we are alternately energizing two phases and then only one as shown in Fig 1 9 Assuming the drive delivers the same winding current in each case this will cause greater torque to be produced when there are two windings energized In other words alternate steps will be strong and weak This does not represent a major deterrent to motor performance the available torque is obviously limited by the weaker step but there will be a significant improvement in low speed smoothness over the full step mode Clearly we would like to produce approximately equal torque on every step and this torque should be at the level of the stronger step We can achieve this by using a higher current level when there is only one winding energized This does not over dissipate the motor because the manufacturer s current rating assumes two phases to be energized the current rating is based on the allowable case temperature With only one phase energized the same total power will be dissipated if the current is increased by 40 Using this higher current in the one phase on state produces approximately equal torque on alternate steps see Fig 1 10 Fig 1 8 Full step current 2 phase on 1 Phase 1 Phase 2 Fig 1 9 Half step current Phase 1 Phase 2 1 2 3 4 5 6 7f8 Phase 1 Phase 2 w We have seen that energ
165. or must be parked ata designated position to allow clearance to remove the molded part The manufacturer would like an electronic solution this is the only hydraulic axis on the current machine Machine Requirements e Electronic solution e Computer controlled solution e 4000N 900lbs force Motion Control Requirements e Position and torque control e Serial link to computer and other drives e Ability to change pressure and dwell ML3450B 25 gt Motor Electric Cylinder Top Mold Chamber Bottom Mold Gene a lt HOME Position lt PARK Position Application Examples Application Solution A BLHX75BP brushless servo drive with an ML3450B 25 motor and an ETS80 BO4LA Electro Thrust Electric Cylinder were used The motor drives the rod inside the cylinder and extends retracts the top molding chamber During this portion of the machine cycle the servo drive must control the position of the motor When the top molding chamber closes on the bottom molding chamber a pressure must be applied While pressure is being applied to the mold the position of the motor is not important However the motor must control the pressure on the molding chamber by applying a torque from the motor A regular positioning servo can only apply torque by generating a position error trying to control torque through position is not very accurate and can create
166. ortional to velocity cubed Increasing velocity raises the temperature of the platen due to eddy current losses in the solid platen material In normal high speed high duty cycle operation over a small piece of platen the platen can become almost too hot to touch B Load on the forcer Load has some effect on the life expectancy of the linear motor Users are urged to adhere to the load specifications for each motor Yaw Pitch and Roll In applications such as end effector devices or where the load is located far from the motor s center of gravity the stiffness characteristics of the forcer must be considered Moment producing forces tend to deflect the forcer and if strong enough will cause the motor to stall or be removed from the platen Yaw pitch and roll specifications are used to determine the maximum torque you can apply to the forcer System Calculations Accuracy In linear positioning systems some applications require high absolute accuracy while many applications require a high degree of repeatability These two variables should be reviewed to accurately evaluate proper system performance In the teach mode a linear motor can be positioned and subsequently learn the coordinates of any given point After learning a number of points in a sequence of moves the user will be concerned with the ability of the forcer to return to the same position from the same direction This scenario describes repeatabi
167. otation CW 15 Speed Mode 15 3 15 x 147 L 10 a 2 E ac e 5 S j f So 9 TEE E W ERE SO EERON OPERE H a M i m j 0 02 04 06 08 10 12 0 Revolution rps A22 90 180 270 360 Rotational Angle degrees Stepping Motor Drives The stepper drive delivers electrical power to the motor in response to low level signals from the control system The motor is a torque producing device and this torque is generated by the interaction of magnetic fields The driving force behind the stator field is the magneto motive force MMF which is proportional to current and to the number of turns in the winding This is often referred to as the amp turns product Essentially the drive must act as a source of current The applied voltage is only significant as a means of controlling the current Input signals to the stepper drive consist of step pulses and a direction signal One step pulse is required for every step the motor is to take This is true regardless of the stepping mode So the drive may require 200 to 101 600 pulses to produce one revolution of the shaft The most commonly used stepping mode in industrial applications is the half step mode in which the motor performs 400 steps per revolution At a shaft speed of 1800 rpm this corresponds to a step pulse frequency of 20kHz The same shaft speed at 25 000 steps per rev requires a step frequency of 750 kHz so mot
168. per second PWM Pulse Width Modulation A method of controlling the average current in a motors phase windings by varying the on time duty cycle of transistor switches oD U op ta o 4 oO co oO J iS e W Ramping The acceleration and deceleration of a motor May also refer to the change in frequency of the applied step pulse train Rated Torque The torque producing capacity of a motor at a given sped This is the maximum torque the motor can deliver to a load and is usually specified with a torque speed curve Regeneration Usually refers to a circuit in a drive amplifier that accepts and drains energy produced by a rotating motor either during deceleration or free wheel shutdown Registration Move Changing the predefined move profile that is being executed to a different predefined move profile following receipt of an input or interrupt Repeatability The degree to which the positioning accuracy for a given move performanced repetitively can be duplicated Resolution The smallest positioning increment that can be achieved Frequently defined as the number of steps required for a motor s shaft to rotate one complete revolution Resolver A feedback device with a construction similar to a motor s construction stator and rotor Provides velocity and position information to a drive s microprocessor or DSP to electronically commutate the motor Motion amp
169. pitch Product Solutions Indexer Drives Motor Model 4000 S Drives 83 135 The Model AT6400 and AT6450 are other controllers that have been used in these types of applications Indexer Axis 3 Traverse A85 Motion amp Control 0D U oD io q oO cc e iS oD oD E e Lu Application Examples 14 Surface Grinding Machine Application Type Tool Feed Motion Linear Application Description A specialty machine shop is improving the efficiency of its surface grinding process The existing machine is sound mechanically but manually operated Automating the machine will free the operator for other tasks which will increase overall throughput of the machine shop Machine Requirements e Allow flexibility to machine various parts e Easy set up for new parts e Automate all three axes e Keep operator informed as to progress e Low cost solution e High resolution grinding Motion Control Requirements Nonvolatile memory for program storage e Teach mode e Multi axis controller e Interactive user configurable display Open loop stepper if possible e High resolution motor drive microstepping Application Solution A four axis motion controller with a user configurable front panel is required for this application An indexer with a sealed backlit display would be ide
170. position and velocity information for the controller Why constant torque from a sine wave motor To understand this it s easier to think in terms of a two phase motor This has just two sets of windings that are fed with sinusoidal currents at 90 to each other If we represent shaft position by an angle 9 then the currents in the two windings are of the form Isin and Icos Going back to our original motor model you ll remember that the fundamental torque characteristic of the motor is also sinusoidal So for a given current I the instantaneous torque value looks like T IK sin Where K is the motor torque constant By making the motor current sinusoidal as well and in phase with the motor torque characteristic the torque generated by one phase becomes T I sin K sin K sin 0 Similarly the torque produced by the other phase is T K cos 0 The total torque is T T IK sin 0 cos 6 but sin 9 cos 1 for any value of 0 therefore T T IK So for sinusoidal phase currents with a constant amplitude the resultant torque is also constant and independent of shaft position For this condition to remain true the drive currents must accurately follow a sine cosine relationship This can only occur with a sufficiently high resolution in the encoder or resolver used for commutation A19 Motion amp Control oD U oD io q oO cc e iS oD oD E e
171. pper The motor This may be a stepper motor either rotary or linear a DC brush motor or a brushless servo motor The motor needs to be fitted with some kind of feedback device unless itis a stepper motor Fig 2 shows a system complete with feedback to control motor speed Such a system is known as a closed loop velocity servo system Fig 2 Typical closed loop velocity servo system Controller gt Drive gt lt lt Velocity Feedback Tachometer j The drive This is an electronic power amplifier that delivers the power to operate the motor in response to low level control signals In general the drive will be specifically designed to operate with a particular motor type you can t use a stepper drive to operate a DC brush motor for instance A2 The control system The actual task performed by the motor is determined by the indexer controller it sets things like speed distance direction and acceleration rate The control function may be distributed between a host controller such as a desktop computer and a slave unit that accepts high level commands One controller may operate in conjunction with several drives and motors ina multi axis system We ll be looking at each of these system elements as well as their relationships to each other Table of Contents Motor Applications A3 Step Motor Technology A4 Linear Step Motor Technology A9 Common Questions Regarding Step Motors A12 DC Br
172. r This solution may also be dictated when maintenance free operation is necessary Low speed high smoothness applications are appropriate for microstepping or direct drive servos Applications in hazardous environments or in a vacuum may not be able to use a brush motor Either a stepper or a brushless motor is called for depending on the demands of the load Bear in mind that heat dissipation may be a problem in a vacuum when the loads are excessive Start Here Motor Technologies Will a stepper meet the torque speed requirements Will a brush servo meet the torque speed requirements Will a brushless servo meet Do you need to run continuously at speeds above 2000 rpm Yes pa No Do you need to Must the motor EITHER 1 Be maintenance free 2 Operate in any environment Y No the torque speed requirements No Higher torque speed technology Use a brushless servo control torque PP Are there any No A other brush No servos in the 2 Does the load system change rapidly gt If there are other during operation Yes brushless motors it may be better to be consistent No with this one Use a brush servo Otherwise use a brush servo y t e Is there a hybrid No Do you n
173. r applications Axis Motor X Axis Motor Operator Interface Controller A88 Axis Drive J X Axis Drive Drive 17 Disc Burnisher Application Type Tool Feed Motion Rotary Application Description Rigid computer discs need to be burnished so that they are flat to within tight tolerances A sensor and a burnishing head move together radially across the disc When a high spot is sensed both heads stop while the burnishing head removes the raised material The surface speed of the disc relative to the heads must remain constant and at the smallest diameter the required disc speed is 2400 rpm The machine operates in a clean environment and takes approximately one minute to scan an unblemished disk Machine Requirements e High speed burnishing Surface speed of disc relative to the heads must remain constant Clean environment no brushed servo motors Motion Control Requirements e Variable storage conditional branching and math capabilities e Linear interpolation between the head axes axes 1 and 2 e Change velocity on the fly e Programmable inputs Axis 1 Sensing Head eee i Multi Axis Controller 4000 Application Examples Application Solution The drive for the disc requires continuous operation at high speed and a brushless solution is desirable to help maintain clean conditions Th
174. r divert the flow of parts Application No Page 4 Optical Scanner A76 5 Circuit Board Scanning A77 Metering Dispensing Applications where controlling displacement and or velocity are required to meter or dispense a precise amount of material Application No Page 6 Telescope Drive escrire A78 7 Engine Test Stand essccsceen A79 8 Capsule Filing Machine eee A80 Indexing Conveyor Applications where a conveyor is being driven ina repetitive fashion to index parts into or out of an auxiliary process Application No Page 9 Indexing Table sen A81 10 Rotary Indexer oo eect eeeteeeeeeeees A82 11 CONVEYOT sesssesssesissiesrinsinsrnsrensrnsrenrensrenns A83 Contouring Applications where multiple axes of motion are used to create a controlled path e g linear or circular interpolation Application No Page 12 Engraving Machine essecsreccsrsren A84 13 Fluted Bit Cutting Machine A85 A72 Tool Feed Applications where motion control is used to feed a cutting or grinding tool to the proper depth Application No Page 14 Surface Grinding Machine essees A86 15 Transfer Machine ccccceccsecsseecseeereeees A87 16 Flute Grinder A88 17 Disc Burnisher nsscssscsesrrcereen A89 Winding Controlling the process of winding material around a spindle or some other object Application No Page 18 Monofilament Winder A90 19 Capacitor Winder cee eeeeeeeteeeeees A91 Following
175. rn of an absolute encoder is in m machine readable code usually binary grey code or a variety of grey The figure above represents a lt x simple binary output with four bits of information The current location is equivalent to the decimal number 11 Moving to the right from the current position the next decimal number is 10 1 0 1 0 binary Moving to the left from the current position the next position would be 12 1 1 0 0 Fig 4 18 Multi turn absolute encoders High Resolution Main Disk Bearing mer at VAN Additional Turns Seals Stages Gearing an additional absolute disk to the primary high resolution disk provides for turns counting so that unique position information is available over multiple revolutions Here is an example of how an encoder with 1 024 counts per revolution becomes an absolute device for 524 288 discrete positions The primary high resolution disk has 1 024 discrete positions per revolution A second disk with 3 tracks of information will be attached to the high resolution disk geared 8 1 The absolute encoder now has 8 complete turns of the shaft or 8 192 discrete positions Adding a third disk geared 8 1 will provide for 64 turns of absolute positions In theory additional disks could continue to be incorporated But in practice most encoders stop at or below 512 turns Encoders usi
176. roblems associated with the previous configuration are overcome Only one power supply is needed and this is fully utilized transistor voltage ratings are the same as that available for driving the motor In low power systems this arrangement can still be used with resistance limiting as shown in Fig 2 8 Fig 2 7 Bipolar bridge T Motion amp Control A25 Drive Technologies Recirculating Chopper Drive The method of current control used in most stepper drives is the recirculating chopper Fig 2 9 This approach incorporates the four transistor bridge recirculation diodes and a sense resistor The resistor is of low value typically 0 1 ohm and provides a feedback voltage proportional to the current in the motor Fig 2 9 Recirculating chopper drive gt z Vs Rs s Injection TR1 TR3 amp TR2 D1 D2 A TR4 D Y z Vs Rs Recirculation Motor current Current is injected into the winding by turning on one top switch and one bottom switch and this applies the full supply voltage across the motor Current will rise in an almost linear fashion and we can monitor this current by looking across the sense resistor When the required current level has been reached the top switch is turned off and the stored energy in the coil keeps the current circulating via the bottom switch and the diode Losses in t
177. rves based on continuous duty operation To choose a motor simply plot total torque vs velocity on the speed torque curve This point should fall under the curve and allow approximately a 50 margin for safety An S106 178 and an S83 135 curve are shown here Note When selecting a ZETA Series product a 50 torque margin is not required Example Assume the following results from load calculations TF 25 oz in Friction torque TA 175 oz in Acceleration torque TT 200 oz in Total torque V 15 rev sec Maximum velocity You can see that the total torque at the required velocity falls within the motor drive operating range for both motors by plotting T A58 600 300 Tr 200 0 10 20 30 40 50 RPS Vmax The 83 135 has approximately 250 oz in available at V max 25 more than required The 106 178 has 375 oz in available an 88 margin In this case we would select the S106 178 motor drive to assure a Sufficient torque margin to allow for changing load conditions Motor Drive Selection Based on peak torque requirements Servo based motor drives have two speed torque curves one for continuous duty operation and another for intermittent duty A servo system can be selected according to the total torque and maximum velocity indicated by the continuous duty curve However by calculating the root mean square RMS torque based on your duty cycle you may be able to take advantage of
178. s as a series of binary numbers BAUD RATE Number of bits transmitted per second Typical rates include 300 600 1 200 2 400 4 800 9 600 19 200 This means at 9 600 baud 1 character can be sent nearly every millisecond DATA BITS Since the ASCII set consists of 128 characters computers may transmit only 7 bits of data Most computers do however support an 8 bit extended ASCII character set DCE Data Communications Equipment transmits on pin 3 and receives on pin 2 DTE Data Terminal Equipment Transmits on pin 2 and receives on pin 3 FULL DUPLEX The terminal will display only received or echoed characters HALF DUPLEX In half duplex mode a terminal will display every character transmitted It may also display the received character HANDSHAKING SIGNALS RTS Request To Send DTR Data Terminal Ready CTS Clear To Send IDB Input Data Buffer DSR Data Set Ready ODB Output Data Buffer ASCII Table DEC HEX GRAPHIC DEC HEX GRAPHIC DEC HEX GRAPHIC DEC HEX GRAPHIC DEC 000 00 NUL 030 I RS 059 3B 088 58 X 117 75 001 01 SOH 031 1F US 060 3C lt 089 59 Y 118 76 002 02 STX 032 20 SPACE 061 3D 090 5A Z 119 77 003 03 ETX 033 21 062 3E gt 091 5B 120 78 004 04 EOT 034 22 k 063 3F 092 5C 121 79 005 05 ENQ 035 23 064 40 093 5D 122 7A 006 06 ACK 036 24 065 41 A 094 5E V 123 7B 007 07 BEL 037 25 066 42 B 095 5F 124 7C 008 08 BS 038 26 amp 067 43 C 096 60 125 7D 009 09 HT 039 27 j 068 44 D 097 6l a 1
179. s made in one direction and then the motor is commanded to move the same distance but in the opposite direction The move ends up short why Two factors could be influencing the results First the motor does have magnetic hysteresis that is seen on direction changes This is in the area of 0 03 Second any mechanical backlash in the system to which the motor is coupled could also cause loss of motion Why are some motors constructed as eight lead motors This allows greater flexibility The motor can be run as a six lead motor with unipolar drives With bipolar drives the windings can then be connected in either series or parallel What advantage do series or parallel connection windings give With the windings connected in series low speed torques are maximized But this also gives the most inductance so performance at higher speeds is lower than if the windings were connected in parallel Can a flat be machined on the motor shaft Yes but care must be taken to not damage the bearings The motor must not be disassembled Compumotor does not warranty the user s work How long can the motor leads be For bipolar drives 100 feet For unipolar designs 50 feet Shielded twisted pair cables are required Can specialty motors explosion proof radiation proof high temperature low temperature vacuum rated or waterproof be provided Compumotor is willing to quote on most requirements with the exception of explo
180. s price This is particularly true when the dynamic requirements are not severe such as setting type applications like positioning a guillotine back stop or a print roller High torque low speed continuous duty applications are also appropriate for step motors At low speeds it is very efficient in terms of torque output relative to both size and input power Microstepping can improve low speed applications such as a metering pump drive for very accurate flow control High torque high speed continuous duty applications suit the servo motor and in fact a step motor should be avoided in such applications because the high speed losses can cause excessive motor heating A DC motor can deliver greater continuous shaft power at high speeds than a stepper of the same frame size Short rapid repetitive moves are the natural domain of steppers or hybrid servos due to their high torque at low speeds good torque to inertia ratio and lack of commutation problems The brushes of the DC motor can limit its potential for frequent starts stops and direction changes Low friction mainly inertial loads can be efficiently handled by the DC servo provided the start stop duty requirements are not excessive This type of load requires a high ratio of peak to continuous torque and in this respect the servo motor excels Very arduous applications with a high dynamic duty cycle or requiring very high speeds may require a brushless moto
181. s to input signals from switches or sensors and output signals to relays solenoids etc Lead Compensation Algorithm A mathematical equation implemented by a computer to decrease the delay between the input and output of a system Limits Properly designed motion control systems have sensors called limits that alert the control electronics that the physical end of travel is being approached and that motion should stop Logic Ground An electrical potential to which all control signals in a particular system are referenced Mechanical Time Constant The time for an energized DC motor to reach 2 3rds of its set velocity Based on a fixed voltage applied to the windings Mid range Instability Designates the condition resulting from energizing a motor at a multiple of its natural frequency usually the third orders condition Torque loss and oscillation can occur in underdamped open loop systems Microstepping An electronic control technique that proportions the current in a step motor s windings to provide additional intermediate positions between poles Produces smooth rotation over a wide speed range and high positional resolution Open Collector A term used to describe a signal output that is performed with a transistor An open collector output acts like a switch closure with one end of the switch at ground potential and the other end of the switch accessible Open Loop Refers to a motion control system where
182. scillation frequency can cause an exaggerated response known as resonance In severe cases this can lead to the motor desynchronizing or stalling It is seldom a problem with half step drives and even less so with a microstepper The natural resonant speed is typically 100 200 full steps sec 0 5 1 rev sec Motor Technologies Under full dynamic conditions the performance of the motor is described by the torque speed curve as shown in Fig 1 20 There are two operating ranges the start stop or pull in range and the slew or pull out range Within the start stop range the motor can be started or stopped by applying index pulses at constant frequency to the drive At speeds within this range the motor has sufficient torque to accelerate its own inertia up to synchronous speed without the position lag exceeding 3 6 Clearly if an inertial load is added this speed range is reduced So the start stop speed range depends on the load inertia The upper limit to the start stop range is typically between 200 and 500 full steps sec 1 2 5 revs sec Fig 1 20 Start stop and slew curves Steps per second gt Holding Torque A Start Stop Curve Slew Curve Torque Start Stop Range To operate the motor at faster speeds it is necessary to start at a speed within the start stop range and then accelerate the motor into the slew region Similarly when stopping the motor it must be decelerated
183. shaft will rotate upon application of a known external force when stopped Synchronism A motor rotating at a speed correctly corresponding to the applied step pulse frequency is said to be in synchronism Load torques in excess of the motor s capacity rated torque will cause a loss of synchronism The condition is not damaging to a step motor Torque Force tending to produce rotation A70 Torque Constant K The torque generated in a DC motor per unit Ampere applied to its windings K 1oz in A amp Simplified for a brushless motor at 90 commutation angle Torque Ripple The cyclical variation of generated torque ata frequency given by the product of motor angular velocity and number of commutator segments or magnetic poles Torque to Inertia Ratio Defined as a motor s holding torque divided by the inertia of its rotor The higher the ratio the higher a motor s maximum acceleration capability will be Transfer Function A mathematical means of expressing the output to input relationship of a system Expressed as a function of frequency Triggers Inputs on a controller that initiate or trigger the next step in a program TTE Transistor Transistor Logic Describes a common digital logic device family that is used in most modern digital electronics TTL signals have two distinct states that are described with a voltage a logical zero or low is represented by a voltage of less than 0 8
184. sheet 1 Application Single Axis Y Multi Axis X Y Gantry Description of system operation A partis moved in and out of a machine very quickly The part comes to rest at the same point in the machine each time An operator sets this distance with a thumbwheel switch Sketch the proposed mechanical configuration 1 Motor Sizing AXIS 1 A Weight of payload lbs 10 0 B Fixed forces if any Ibs 0 C Known move distance in 40 time sec 1 0 D Angle from horizontal degrees 0 2 Total length of travel inches 40 3 Desired repeatability in 001 4 Desired resolution in 0005 5 Necessary settling time after move 100 ms to within 001 inches 6 Life expectancy Percent duty cycle 20 Estimated number of moves year 200 000 7 Is the center of gravity significantly changed no 8 What is the environment clean v dirty Specifics 9 Operating temperature range 65 to 85 F 10 Can air be available yes The Solution Actual and assumed factors that contribute to the solution are 1 Force F mass M x acceleration A Note mass units are in pounds 2 Acceleration due to gravity 1g 386 inches sec 3 L20 forcer weighs 2 0 Ib 4 Attractive force between L20 forcer and platen 200 lbs 5 Trapezoidal velocity profile Acceltime 1 0 sec 4 0 250 sec Vmax 1 33 x Vavg A66 Step 1 Total mass to be accelerated Mtotal Mload 10 0
185. sion proof What are the options if an explosion proof motor is needed Installing the motor in a purged box should be investigated DC Brush Motors The history of the DC motor can be traced back to the 1830s when Michael Faraday did extensive work with disc type machines Fig 1 21 Fig 1 21 Simple disc motor Conductive Disc LS X N S Magnet i Brush This design was quickly improved and by the end of the 19th century the design principles of DC motors had become well established About that time however AC power supply systems came into general use and the popularity of the DC motor declined in favor of the less expensive AC induction motor More recently the particular characteristics of DC motors notably high starting torque and controllability have led to their application in a wide range of systems requiring accurate control of speed and position This process has been helped by the development of sophisticated modern drive and computer control systems Principles It is well known that when a current carrying conductor is placed in a magnetic field it experiences a force Fig 1 22 Fig 1 22 Force on a conductor in a magnetic field Magnetic Field B Q bin Conductor Carrying Current 1 Into Page Force F Force on Conductor F x B The force acting on the conductor is given by F IxB where B magnetic flux density and current If this single co
186. smitted Noise Transmitted noise is picked up by external connections to the indexer and in severe cases can attack an indexer with no external connections The indexer enclosure will typically shield the electronics from this but openings in the enclosure for connection and front panel controls may leak As with all electrical equipment the indexer chassis should be scrupulously connected to Earth to minimize this effect When high current contacts open they draw an arc producing a burst of broad spectrum radio frequency noise that can be picked up on an indexer limit switch or other wiring High current and high voltage wires have an electrical field around them and may induce noise on signal wiring especially when they are tied in the same wiring bundle or conduit When this kind of problem occurs consider Shielding signal cables or isolating the signals A proper shield surrounds the signal wires to intercept electrical fields but this shield must be tied to Earth to drain the induced voltages At the very least wires should be run in twisted pairs to limit straight line antenna effects Most Compumotor cables have shields tied to Earth but in some cases the shields must be grounded at installation time Installing the indexer in a NEMA electrical enclosure ensures protection from this kind of noise unless noise producing equipment is also mounted inside the enclosure Connections external to the enclosure must be
187. surges DC Input and Output Modules As with all DC devices this is a polarized and input module Since current will flow in only one direction care must be taken to observe these polarities during installation DC input modules typically feature an input signal conditioning circuit This circuit requires the input to remain on off for a minimum of 5 milliseconds Fig 5 3 Typical DC input connection diagram before recognizing the switch This eliminates a short voltage spike or de bounce contact closure less than 5 milliseconds in duration However a 0 1 microfarad ceramic disc capacitor across the actual switching contacts is still recommended to prevent switch bounce that can be as long as 10 80 milliseconds 4KV Isolation Barrier Pei Indicating eee oe ees LED Switch 1 l ee gt gt wine L i i f oe 1 aes 10 to 24VDC i lt lt a Signal a i Logic Signal Floating Coupling Wo f Conditioning Source _ LED C lt r Photo ae 10 to 24mA lt j Transistor 1 Screw E Terminals DC Input Operational Sequence As switch 1 closes current flows through the limiting resistor 1K ohm and then into the LED The light issued by the LED due to this forward current flow in turn simulates the photo transistor Hence the tem opto or optically isolated The phototransistor then drives the base of the second transistor to a high level bringin
188. system response It is the frequency range that a control system can follow BCD Binary Coded Decimal is an encoding technique used to describe the numbers 0 through 9 with four digital on or off signal lines Popular in machine tool equipment BCD interfaces are now giving way to interfaces requiring fewer wires such as RS 232C Bit Abbreviation of Binary Digit the smallest unit of memory equal to 1 or 0 Back EMF The voltage produced across a winding of a motor due to the winding turns being cut by a magnetic field while the motor is operating This voltage is directly proportional to rotor velocity and is opposite in polarity to the applied voltage Sometimes referred to as counter EMF A68 Block Diagram A simplified schematic representing components and signal flow through a system Bode Plot A graph of system gain and phase versus input frequency which graphically illustrates the steady state characteristics of the system Break Frequency Frequency ies at which the gain changes Slope on a Bode plot break frequencies correspond to the poles and zeroes of the system Brushless DC Servo A general term referring to a motor drive that generates trapezoidal shaped motor currents in a motor wound as to generate trapezoidal Back EMF Byte A group of 8 bits treated as a whole with 256 possible combinations of one s and zero s each combination representing a unique piece of information Commuta
189. t any communication 2 Some serial ports require handshaking You can establish 3 wire communication by jumpering RTS to CTS usually pins 4 and 5 and DSR to DTR usually pins 6 and 20 3 Configure the host and peripheral to the same baud rate number of data bits number of stop bits and parity 4 If you receive double characters e g typing A and receiving AA your computer is set to half duplex mode Change to full duplex mode 5 Use DC common or signal ground as your reference NOT earth ground 6 Cable lengths should not exceed 50 ft unless you are using some form of line driver optical coupler or shield As with any control signal be sure to shield the cable to earth ground at one end only 7 To test terminal or terminal emulation software for proper 3 wire communication unhook the peripheral device and transmit a character An echoed character should not be received If a character is received you are in half duplex mode J umper the host s transmit and receive lines and send another character You should receive the echoed character If not consult the manufacturer of the host s serial interface for proper pin outs A50 Parallel Parallel communication requires handshaking and transmits data one byte 8 bits at a time When data are transferred from the host processor to a peripheral device the following steps take place 1 The host sets a bit on the bus signalling to the peripheral th
190. ted noise Transmitted noise Ground loops Some common electrical devices generate electrical noise e Coil driven devices conducted and power line noise e SCR fired heaters transmitted and power line noise Motors and motor drives transmitted and power line noise e Welders electric transmitted and power line noise Power line disturbances are usually easy to solve due to the wide availability of line filtering equipment for the industry Only the most severe situations call for an isolation transformer Line filtering equipment is required when other devices connected to the local power line are switching large amounts of current especially if the switching takes place at high frequency Corcom and Teal are two manufacturers of suitable power line filters Also any device having coils is likely to upset the line when it is switched off Surge suppressors such as MOVs General Electric can limit this kind of noise A series RC network across the coil is also effective resistance 500 to 1 000 Q capacitance 0 1 to 0 2uF Coil driven devices inductive loads include relays solenoids contactors clutches brakes and motor starters Fig 5 10 Typical RC Network R Inductive T Load C A52 Externally Conducted Noise Externally conducted noise is similar to power line noise but the disturbances are created on signal and ground wires connected to the indexer This kind of noise can get onto logic circuit
191. the higher peak torque available in the intermittent duty range _ STP ti Sti Where e Ti is the torque required over the time interval ti e Z means the sum of T RMS Example Assume the following results from your load calculations T 250z in Friction Torque T 775 oz in Acceleration Torque T 800 oz in Total Torque Vinx 20 rps Maximum Velocity Motion Profile 20 rps System Calculations Duty Cycle Index 4 revs in 0 3 seconds dwell 0 3 seconds then repeat Y If you look at the S106 178 speed torque curve T you ll see that the requirements fall outside the T curve 5 T Torque reqired to accelerate the load from zero speed to maximum speed T T 2 Torque required to keep the motor moving T once it reaches max speed T V T Torque required to decelerate from max a speed to a stop T T T Torque required while motor is sitting still at zero speed lt t Time spent accelerating the load t Time spent while motor is turning at constant speed t Time spent decelerating the load t Time spent while motor is at rest Ne Tt T t T 72 t Re t t t t 800 1 25P 1 750 1 0 3 1 1 1 3 T lt 447 02 in RMS Now plot T and T vs T ax on the speed torque curve The drawing below resembles the speed torque curve for the Z606 motor 1800 4 T4 800 600 T ams 506 J
192. the motor drive at a velocity proportional to the primary velocity of the material based on input from a rotary incremental encoder This assures a constant amount of glue along the length of the material When the start button is depressed the glue will begin dispensing and can be discontinued with the stop button If a new speed ratio is desired FOR can be changed with either the front panel pushbutton thumbwheels or with the RS 232C serial link Application Examples Program Two following commands are used FOR Sets the ratio between the secondary motor resolution and the primary encoder resolution FOL Sets the ratio of the speed between the primary and secondary motor One input will be configured to start motion a second input will be used to stop motion The motor has 10000 steps revolution The encoder that is placed on the motor pulling the material has 4000 pulses revolution It is desired to have the motor dispensing the glue turning twice as fast as the encoder sensing the material FOR2 5 Setthe motor to encoder ratio FOL2 The following speed ratio is 200 or twice as fast A1 Set acceleration to 10 rps AD1 Set deceleration to 10 rps MC The controller is placed in Continuous mode Product Solutions Drive Controller Motor SXF Drive Controller S57 102 The Model 500 single axis controller and the S Drive have also been used in these types of applications a Motor i a
193. tics of the motor and drive op U op io 4 oO oc e iS oD oD E e W Servos Compumotor servos use resolver feedback to determine their resolution and position It is essentially the resolution of the device reading the resolver position that determines the highest possible accuracy in the system Digiplan servos use encoder feedback to determine their resolution and position In this case it is the encoder s resolution that determines the system s accuracy The positional accuracy is determined by the drive s ability to move the motor to the position indicated by the resolver or encoder Changes in friction or inertial loading will adversely affect the accuracy until the system is properly tuned Closed Loop Steppers Compumotor closed loop stepper systems use an encoder to provide feedback for the control loop The encoder resolution determines the system s accuracy When enabled the controlling indexer attempts to position the motor within the specified deadband from the encoder Typically this means the motor will be positioned to within one encoder step To do this satisfactorily the motor must have more resolution than the encoder If the step size of the motor is equal to or larger than the step size of the encoder the motor will be unable to maintain a position and may become unstable In a system with adequate motor to encoder resolution the motor is able to maintain one enco
194. tion 10 Why is motor sizing important why not just go with a larger motor If the motor s rotor inertia is the majority of the load any resonances may become more pronounced Also productivity would suffer as excessive time would be required to accelerate the larger rotor inertia Smaller may be better 11 What are the options for eliminating resonance This would most likely happen with full step systems Adding inertia would lower the resonant frequency Friction would tend to A12 12 13 14 15 16 17 18 19 20 21 dampen the modulation Start stop velocities higher than the resonant point could be used Changing to half step operation would greatly help Ministepping and microstepping also greatly minimize any resonant vibrations Viscous inertial dampers may also help Why does the motor jump at times when it s turned on This is due to the rotor having 200 natural detent positions Movement can then be 3 6 in either direction Do the rotor and stator teeth actually mesh No While some designs used this type of harmonic drive in this case an air gap is very carefully maintained between the rotor and the stator Does the motor itself change if a microstepping drive is used The motor is still the standard 1 8 stepper Microstepping is accomplished by proportioning currents in the drive higher resolutions result Ensure the motor s inductance is compatible A move i
195. tion The switching sequence of drive voltage into motor phase windings necessary to assure continuous motor rotation A brushed motor relies upon brush bar contact to mechanically switch the windings A brushless motor requires a device that senses rotor rotational position feeds that information to a drive that determines the next switching sequence Closed Loop A broadly applied term relating to any system where the output is measured and compared to the input The output is then adjusted to reach the desired condition In motion control the term describes a system wherein a velocity or position or both transducer is used to generate correction signals by comparison to desired parameters Critical Damping A system is critically damped when the response to a step change in desired velocity or position is achieved in the minimum possible time with little or no overshoot Crossover Frequency The frequency at which the gain intercepts the 0 dB point on a Bode plot used in reference to the open loop gain plot Daisy Chain A term used to describe the linking of several RS 232C devices in sequence such that a single data stream flows through one device and on to the next Daisy chained devices usually are distinguished by device addresses which serve to indicate the desired destination for data in the stream Damping An indication of the rate of decay of a Signal to its steady state value Related to settling time D
196. tions Indexer Drives Motor AT6400 S Drives 83 135 The Model 4000 standalone and AT6450 are servo controller products that have also been used in these types of applications IBM PC with Indexer Digitizer Pad Drives A84 13 Fluted Bit Cutting Machine Application Type Contouring Motion Linear Application Description The customer manufactures a machine that cuts a metal cylinder into fluted cutting bits for milling machines The machine operation employed a mechanical cam follower to tie the bit s rotation speed to the traverse motion of the bit relative to the cutting tool The cut depth was manually adjusted using a hand crank This arrangement was acceptable when the company had a bit for the cam they wanted to grind Unfortunately custom prototype bits made of titanium or other high tech metals required that they make a cam before they could machine the bit or do those parts on a 10 000 CNC screw machine Both of these alternatives were too expensive for this customer Machine Requirements e Machine must be capable of making low volume custom bits as well as high volume standard bits an be economical for both processes Quick set up routine e Operator interface for part entry Motion Control Requirements e Smooth motion e Four axes of coordinated motion e 2 axes of linear interpolati
197. tive orientation Fig 1 24 Electrical arrangement of the armature Current In This is achieved by constructing the armature as a series of small sections connected in sequence to the segments of a commutator Fig 1 24 Electrical connection is made to the commutator by means of two brushes It can be seen that if the armature rotates through 1 6 of a revolution clockwise the current in coils 3 and 6 will have changed direction As successive commutator segments pass the brushes the current in the coils connected to those segments changes direction This commutation or switching effect results in a current flow in the A13 Motion amp Control oD U oD io q oO cc e iS oD oD E e W Motor Technologies armature that occupies a fixed position in space independent of the armature rotation and allows the armature to be regarded as a wound core with an axis of magnetization fixed in space This gives rise to the production of a constant torque output from the motor shaft The axis of magnetization is determined by the position of the brushes If the motor is to have similar characteristics in both directions of rotation the brush axis must be positioned to produce an axis of magnetization that is at 90 to the stator field DC Motor Types Several different types of DC motor are currently in use Iron cored Fig 1 25 This is the most common type of motor used in DC
198. to monitor the machine at all times to constantly adjust the distances for accurate tab placement Machine Requirements e Constantly monitor the linear feed length of the paper and foil and calculate the constantly changing capacitor circumference as a function of that length A complete motion control package is required to eliminate the need for a PLC and separate motion cards e Reduce time and complexity of set up too much wiring in previous system e Reduce machine downtime caused by material breakage Motion Control Requirements e Following e Two axes of coordinated motion e Math capability e AT based control card Application Examples Application Solution Precise motion control of the material feed axes demands closed loop servo commands Actuation of external cylinders and solenoids requires both analog and digital I O A flexible operator interface is needed for diagnostics and other alterations of machine function Motion I O and an operator interface should be provided with a machine controller The first motorized axis mandril pulls all six materials together and feeds an appropriate distance An encoder is placed on this motor as well as on the materials as they are fed into the mandril The controller constantly compares the two encoders to get an exact measurement of linear distance and compensates for material stretching When the linear distance is ac
199. ts Each results in varying levels of complexity and integration of both motion and non motion elements PLC based bus based and integrated solutions are all commercially available Your selection of a machine control strategy will often be based on performance total application cost and technology experience PLC based Control The PLC based architecture is utilized for I O intensive control applications Based upon banks of relays that are scanned or polled by a central processor the PLC provides a low cost option for those familiar with its ladder logic programming language Integration of the motion I O operator interface and communication are usually supported through additional cards that are plugged in its backplane The addition of scanning points decreases the polling rate of any individual point and can thus lead to lower machine response Because PLCs Control Systems have not historically concentrated on motion control plug in indexers or those that communicate over BCD are preferable Because these indexer boards often include their own microprocessor they prevent slow polling rates but incorporate a separate programming language In general this compromise is acceptable for all but the most complicated motion machine applications Bus based Systems Bus based machine control systems are common in today s industrial environment STD VME and PC AT are only a few of the numerous options Most of these opti
200. ty is represented by 10v and full reverse by 10v Zero volts represents the stationary condition and intermediate voltages represent speeds in proportion to the voltage The various adjustments needed to tune an analog drive are usually made with potentiometers With a little experience this can usually be performed quite quickly but in some difficult applications it may take longer Repeating the adjustments on subsequent units may take the same time unless there is an easy way of duplicating the poten tiometer settings For this reason some proprietary drives use a plug in personality card that may be fitted with preset components However this not only increases the cost but may preclude the possibility of fine tuning later oD U iS oO io q oO oc aD oD E e W Overview The Digital Drive An alternative to the analog system is the digitally controlled drive in which tuning is performed by sending data from a terminal or computer This leads to easy repetition between units and since such drives are invariably processor based facilitates fully automatic self tuning The input signal to such a drive may also be an analog voltage but can equally take the form of step and direction signals like a stepper drive Digital drives are used more in conjunction with brushless servo motors than with DC brush motors Such drives are almost wholly digital with the exception of the po
201. ude pulleys and cables gears and toothed belts and racks and pinions Tangential drives permit a lot of flexibility in the design of drive mechanics and can be very accurate with little backlash Metal chains should be avoided since they provide little or no motor damping Application Description A movie camera is being modified to expose each frame under computer control for the purpose of generating special effects A motor will be installed in the camera connected to a 1 2 inch diameter 2 inch long steel film drive sprocket and must index one frame in 200 milliseconds The frame spacing is 38 mm 1 5 Machine Requirements e Index one frame within 200 milliseconds e Indexer must be compatible with BCD interface e Fast rewind and frame indexing Motion Control Requirements Little to no vibration at rest Stepper e Minimum settling time e Preset and slew moves Motor A74 Application Solution In this application the move distance and time are known but the required acceleration is not known The acceleration may be derived by observing that for a trapezoidal move profile with equal acceleration slew and deceleration times 1 3 of the move time is spent accelerating and 1 3 of the total distance is travelled in that time a trapezoidal move It is determined that the acceleration required is 107 4 rps at a velocity of 7 166 rps Assume that the film weighs 1 oz and total film friction is 10 oz i
202. ularly if the drive is delivering full current at the time Brakes can introduce friction even when released and also add inertia to the system both effects will increase the drive power requirements What is the best stopping method It is clear that each of the methods outlined above has certain advantages and drawbacks This leads to the conclusion that the best solution is to use a combination of techniques ideally incorporating a short time delay We can make use of the fact that a contactor used for dynamic braking will take a finite time to drop out so it is possible to de energize the contactor coil while commanding zero speed to the drive This allows for a controlled stop to occur under full torque with the backup of dynamic braking in the event that the amplifier or controller has failed WARNING there is a risk that decelerating a servo to rest in full current limit could result in mechanical damage especially if a high ratio gearbox is used This does not necessarily ensure a safe stop be sure that the mechanism can withstand this treatment A mechanical brake should also be applied to a vertical axis to prevent subsequent movement An alternative to the electrically operated brake is the differential drag brake which will prevent the load from falling but creates negligible torque in the opposite direction Application Considerations Accuracy An accuracy specification defines the maximum error in achiev
203. ush Motor Technology A13 Brushless Motor Technology A17 Hybrid Servo Technology A20 Direct Drive Motor Technology A21 Step Motor Drive Technology A23 Microstepping Drive Technology A29 Analog and Digital Servo Drives A31 Brushless Servo Drive Technology A34 Servo Tuning A36 Feedback Devices A39 Machine Control A45 Control System Overview A46 Understanding I O Modules A48 Serial amp Parallel Communications A51 Electrical Noise Symptoms amp Solutions A52 Emergency Stop A54 System Selection Considerations A55 Motor Sizing and Selection Software A57 System Calculations Move P rofiles A58 System Calculations Leadscrew Drives A60 System Calculations Direct Drives A63 System Calculations Gear Drives A64 System Calculations Tangential Drives A65 System Calculations Linear Motors A66 Glossary of Terms A68 Technical Data A71 Application Examples A72 Application Areas of Motor Types The following section gives some idea of the applications that are particularly appropriate for each motor type together with certain applications that are best avoided It should be stressed that there is a wide range of applications that can be equally well met by more than one motor type and the choice will tend to be dictated by customer preference previous experience or compatibility with existing equipment Cost conscious applications will always be worth attempting with a stepper as it will generally be hard to beat the stepper
204. usually less noticeable due to mechanical losses in the rotary to linear transmission system which dampens the effects Velocity ripple due to resonance can be reduced with the electronic accelerometer damping option AC Platen Mounting The air gap between the forcer and the platen surface can be as small as 0 0005 inches Properly mounting the platen is extremely important When held down ona magnetic chuck the platen is flat and parallel within its specifications however in its free state slight bows and twists may cause the forcer L20 to touch the platen at several places Compumotor recommends mounting the platen using all its mounting holes on a ground flat piece of steel such as an beam U channel or tube Environment Due to the small air gap between the forcer and platen care should be taken to keep the platen clean A small amount of dirt or adhesive material such as paint can cause a reduction in motor performance When appropriate mounting the motor upside down or on its side will help keep foreign particles off the platen Protective boots that fold like an accordion as the motor travels can also be used to assist in keeping the platen clean Linear Step Motors Life Expectancy The life of a mechanical bearing motor is limited by wearing of the platen surface over which the bearings roll Factors that affect wear and life of a mechanical bearing system include A High velocities Life is inversely prop
205. ve Motors Motor Construction and Operation Direct drive systems couple the system s load directly to the motor without the use of belts or gears In some situations brushed or brushlesss servo motors may lack adequate torque or resolution to satisfy some applications needs Therefore mechanical means such as gear reduction systems to increase torque and resolution are used to meet system requirements The Dynaserv Direct Drive can provide very high torque in a modest package size and solves many of the performance issues of the gear reducer All in a system that is as easy to use as a stepping motor Fig 1 45 below shows the construction of the Dynaserv DM Series direct drive motor compared to a conventional motor with a gear reducer The gear reducer relies on large amounts of frictional contact to reduce the speed of the load This gearing effectively increases torque and resolution but sacrifices speed and accuracy The direct drive motor is brushless and gearless so it eliminates friction from its power transmission Since the feedback element is coupled directly to the load system accuracy and repeatability are greatly increased and backlash is eliminated Fig 1 45 Construction comparison Direct Drive Motor Conventional Motor DC AC Motor Rotating Element Bearing Encoder PCB Slit Plate Stator Element Encoder Rotor Core Stator Core Motor Technologies The motor contains precision
206. ve current and supply voltage However the full torque capability of the motor can be utilized since the system is operating in a closed loop with an open loop step motor it is always necessary to allow an adequate torque margin The hybrid servo system will be more expensive than the equivalent step motor systems but less costly than a brushless servo As with the step motor continuous operation at high speed is not recommended since the high pole count results in greater iron losses at high speeds A hybrid servo also tends to run quieter and cooler than its step motor counterpart since it is a true servo power is only consumed when torque is required and normally no current will flow at standstill Low speed smoothness is vastly improved over the open loop full step motor It is worth noting that the hybrid servo is entirely different from the open loop step motor operated in closed loop or position tracking mode In position tracking mode an encoder measures the load movement and final positioning is determined by encoder feedback While this technique can provide high positioning accuracy and eliminates undetected position loss it does not allow full torque utilization improve smoothness or reduce motor heating Fig 1 44 Hybrid servo motor with resolver feedback Stator Housing Two MS Style Connectors Position Feedback Device Rotor Bearing Position Feedback Device Stator A20 Direct Dri
207. volts and a logical one or high is represented by a voltage from 2 5 to 5 volts Voltage Constant K The back EMF generated by a DC motor at a defined speed Usually quoted in volts per 1000 rpm Zero A frequency at which the transfer function of a system goes to zero Rotary Inertia Conversion Table Don t confuse mass inertia with weight inertia mass inertia wt inertia Technical Data g o9 To convert from A to B multiply by entry in Table z B Ib ft s S A kg m kg cm g cm kg m sec kg cm sec g cm sec oz in oz in s Ib in Ib in s Ib ft slug ft 2 D kg m 1 10 10 0 10192 10 1972 1 01972 10 5 46745 10 1 41612 10 3 41716 10 8 850732 23 73025 0 73756 aa kg cm 104 al 10 1 01972 10 1 01972 107 1 01972 5 46745 1 41612 107 0 341716 8 85073 104 2 37303 10 7 37561 10 2 g cm 107 103 1 1 01972 10 1 01972 10 1 01972 103 5 46745 10 1 41612 10 3 41716 10 8 85073 107 2 37303 10 7 37561 10 T kg m s 9 80665 9 80665 10 9 80665 10 1 10 10 5 36174 105 1 388674 10 3 35109 10 86 79606 2 32714 10 7 23300 v kg cm s 9 80665 10 9 80665 10 9 80665 105 10 1 10 5 36174 103 13 88741 3 35109 10 0 86796 2 327143 7 23300 107 e g cm s 9 80665 10 0 980665 9 80665 102 10 103 1 5 36174 1 38874 107 0 335109 8 67961 104 2 32714 10 7 23300 10 m oz in 1 82901 10 0 182901 1 82901 10 1 86506 10 1 86506 10 0 186506 1 2 59008 107 6 250 10 1 61880 104 4 34028 10
208. wer stage that actually delivers current to the motor Velocity feedback is derived either from an encoder or resolver and again is processed as digital information It becomes logical to incorporate a position controller within such a drive so that incoming step and direction signals can be derived from a conventional stepper type indexer Equally the positioner may be controlled by simple commands using a high level motion control language see the X code products in this catalog A Comparison of Analog and Digital Drives The analog drive offers the benefit of lower cost and in the case of a drive using tach feedback very high performance The wide bandwidth of the brush tach allows high gains to be used without inducing jitter at standstill resulting in a very stiff system The digital drive while more costly is comparatively easy to set up and adjustments can be quickly repeated across several units Automatic self tuning can be a distinct advantage where the load parameters are unknown or difficult to measure The digital drive also offers the possibility of dynamic tuning sometimes vital where the load inertia changes dramatically during machine operation Such an option is clearly out of the question with a potentiometer adjusted drive Motion amp Control A31 Drive Technologies Analog DC Drive Operation The elements of an analog velocity amplifier are shown in Fig 2 17 The function of the system is to c
209. xceptional smoothness without requiring a gear reducer Ministepping systems typically do not have wave trimming capability or offset adjustment to achieve the optimum smoothness but offer a great improvement over full step and half step systems Ministepping systems have resolutions between 1 000 and 4 000 steps rev The motor is an important element in providing good smoothness Some motor designs are optimized for high torque output rather than smooth rotation Others are optimized for smoothness rather than high torque Ministepping systems are typically offered with a motor as a packaged total solution using a motor that has been selected for its premium smoothness properties Ministep systems are sometimes selected to improve positional accuracy However with an open loop system friction may prevent the theoretical unloaded accuracy from being achieved in practice Velocity Velocity Time Time Microstepping Drives As we mentioned earlier subdivision of the basic motor step is possible by proportioning the current in the two motor windings This produces a series of intermediate step positions between the one phase on points It is clearly desirable that these intermediate positions are equally spaced and produce approximately equal torque when the motor is running Accurate microstepping places increased demands on the accuracy of current control in the drive particularly at low current levels A
210. xt cycle The complete drilling cycle takes 2 2 seconds with a 0 6 second delay before the next cycle Due to the proximity of other equipment the length in the direction of travel is very restricted An additional requirement is to monitor the machine for drill wear and breakage Machine Requirements e Limited length of travel e Limited maintenance e Monitor and minimize drill damage e High speed drilling Motion Control Requirements e Packaged drive controller Complex motion profile e High speed e High duty cycle Application Solution The combined requirements of high speed high duty cycle and monitoring the drill wear all point to the use of a servo motor By checking the torque load on the motor achieved by monitoring drive current the drilling phase can be monitored an increased load during this phase indicates that the drill is broken This type of application will require a ballscrew drive to achieve high stiffness together with high speed One way of minimizing the length of the mechanism is to attach the ballscrew to the moving stage and then rotate the nut allowing the motor to be buried underneath the table Since access for maintenance will then be difficult a brushless motor should be selected Product Solutions Drive Controller Motor APEX6152 606 Motor Drive Controller A87 Application Examples Motion amp Control 0D U oD io q oO cc e

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