Micro-Step Driving for Stepper Motors: A Case Study
Contents
1. procedure for driving each of them In general stepper motors are driving in full half and micro step modes In full step mode to change rotor position completely activation and deactivation signals are fed to the windings alternatively in each cycle In half step each winding can stay activated for more than one cycle Therefore the rotor motion is half of the full step and the motor resolution is two times more Micro step driving is another alternative for motor motion that has more accuracy and continuous motion Two main advantage of micro step mode is the possibility for rotor to move between full and half modes and to eliminate irregular characteristics of motor torque In this paper stepper motor driver circuit implementation process for all three modes is presented For design testing appropriate driving signal patterns are generated using PC parallel port Torque tests are applied to the driver circuit and experimental results extracted As obtained using micro step driving stepper motor rotates more continuously Also irregular torque values appeared like holes in torque curve at resonance frequencies are eliminated This paper is organized as follows in section II existing stepper motor control and driving techniques are studied In section III current control methods for stepper motors are described Full and half step implemented circuit is presented in section IV Micro step driving and its appropria2. the current to determine when the switches should be opened and when they should not This method 1s also called One Shot Feedback current control In this method when the winding current is increased greatly the winding becomes inactive for fixed time This method requires On shot controller and the current sensing modules The One Shot controller is shown at Fig 5 Fig 5 One Shot controller When the one shot output Q is low voltage across the RI caused the winding current compensated Thus one shot controller should not be too sensitive to comparator output because the chopping rate becomes variable In the Linear method when the switches were closed supply voltage completely applied and the chopping only performed when the motor current reach the threshold Vet value In some application one can use one shot controller in H Bridge structure thus the H Bridge controller will be made As shown in Fig 6 In this work combination of H Bridge and one shot controller used for motor driving and the current limiting methodologies Fig 6 One shot Controller in H Bridge structure 4 Normal driving circuit Full and Half step As shown in the previous sections stepper motor driver system is formed by signaling and switching parts One of the practical approaches for implementation of such a system is of the shelf design where existing components combined and construct the whole system on the board The main component
3. Micro Step Driving for Stepper Motors A Case Study N Sedaghati Mokhtari Graduate Student School of ECE University of Tehran Tehran Iran n sedaghati ece ut ac ir Abstract In this paper a case study for implementation of the micro step driver optimized for resonance frequencies of the stepper motors is presented First of all the motor driver is implemented in Full and Half step modes According to the irregular torque to rotation curve in these modes several optimizations are applied to the driver achieving the micro step driving circuit Driver circuit is verified for each modes of motor motion Considering obtained results torque reduction in resonance frequencies appeared as holes in torque curve at these points is eliminated with micro step driving By this optimization and in the micro step driving mode motor consumes lower power than other modes Keywords Stepper motors Micro step driving 1 Introduction Steppers are special motors that do not have any commutator to reverse flow of the windings current Stepper motors are the best alternatives for applications that high accuracy motions are required such as CNC printers and so on Because of the simple winding and control system the main applications of these motors are the motion controls On the basis of the motor structure and Operation stepper motors are divided into three types permanent magnet PM variable reluctance VR and hybrids There is special
4. e are several driving techniques for every type of the stepper motors Consider a bipolar H Bridge hybrid motor as shown in Fig2 Reversing the magnet field direction one can change the winding current flow Achieving this one can arrange the control switches like H letter called H Bridge and put the winding between them Supply Fig 2 H Bridge structure Because of four control switches H Bridge can have 16 different modes of operations Some of them are not used generally Some of the practical operation modes are as follows aj 2 t ms i ba RE aud Mad CL ee n Saf Lum ri l Ln Forward only A and D switches are closed Reverse only B and C switches are closed Fast Decay All switches are closed and the winding current decreased through the power supply and diodes path This mode doesn t damage the winding and therefore the rotor can move freely without any resistance Slow Decay In this mode the current is passed through the motor winding rotationally without any resistance Therefore 1f a few current remains in winding this operation caused the current dissipated and the rotor stopped slowly It is possible that activation of control switches in H Bridges caused short circuit between power supplies i e closing switches A and B In these cases usually logic gates used to eliminate short circuit effects Fig 3 shows the sample circuit where this technique is applied Su
5. ibit output signals of each L297 INH control corresponding enable EN lines of the L298 ENA and ENB The extra part of the minimum system shown in Fig 7 is shown in Fig 8 DACO0800 Fig 8 Extra part for micro step driving Also the implemented printed circuit board PCB of the system is shown in Fig 9 a for top and b for bottom view a Top view b Bottom view Fig 9 Printed Circuit Board PCB of the system MEM amp a 4 o s iu d primm er r LET x 4 Experimental Results In this work Oriental motor with 12v supply voltage and maximum 420 mA working current is used Torque test system is shown in Fig 10 In every round of the motor with determined rpm value the maximum calculated number in the dynamometer is recorded There are values determine the maximum motor power for appropriate rpm numbers dynamometer Motor Fig 10 Stepper motor torque test system The motor torque is calculated for each three modes of the motor motion and the extracted results represented in Fig 11 to Fig 14 It must be expressed that the motor power value is converted to the Newton unit and then comparison is done 5 Conclusion In this work implementation of the stepper motor driver in three full half and micro step modes are presented Torque test are applied to all of the driver circuits and the motor torque according to its rpm value is extracted As mentioned after optimizing driver circ
6. pply Fig 3 short circuit elimination for H Bridge structure As shown above all of the operation modes can be accessible only with a few numbers of signals This is so practical when we use microcontrollers Or microprocessors for control signal pattern generation i e X and Y signals on the above 3 Stepper Motor Current Control Small stepper motors generally have a small DC supply that control the winding current and limited with the resistance of the winding At the other hand motors with the huge torque values have a winding with small resistance Therefore they require current limitation and controlling systems as external circuits One of the simplest ways to limit the current of the winding is to put the resistance serially with the winding This is called resistive approach Another alternative is to use the mixed of the resistance and transistor for current limiting In this way rise and fall times of the current will be less than the resistive method This method also called linear approach In these two ways the current is only depends on winding reluctance and the supply voltage When the current increased the winding voltage and current decreased Therefore the current can not arrive to its maximum threshold Another current control approach for stepper motor is the open loop method At the previous approaches the motor current is automatically limited and therefore caused the power
7. s are L297 chip 3 for signaling and current control and L298 4 for switching the motor The L297 chip contains the control system for uni and bipolar motors It also contains two separate PWM chopper for current control whereas each chopper contains comparator flip flop and the external sense resistor One internal oscillator also Enable Reset Half Full Step Direction Home Sync I 5 used to generate the pulse rate The L298 chip is dual H Bridge driver that work with high voltage and current This driver converts the TTL logic levels to the signal levels that can drive relays solenoids DC and stepper motors By combining the L297 as the motor controller and current limiter and L298 as dual H Bridge driver full stepper motor driver system can be performed The step and direction data is received form the microcontroller system to the L297 and the control signals are generated for L298 These signals used to drive the stepper motor for specific steps determined with the steps number The minimum system that is performed by above components is shown in Fig 7 Eight diodes are fast diodes and hold the supply voltage level for motor windings in V value R4 and R resistors are current sensors and can enable the appropriate signals if the motor current exceeds to the threshold value When the voltage of sense resistors reaches to the Vef the winding is disabled and the diodes offload the winding current This opera
8. te implementation simulation and test results appeared in section V and the paper is concluded in section VI 2 Stepper Motor Driving and Control Generally control and driving techniques for stepper motors are categorized in open and close loop modes here open loop mode is considered In stepper motors driving is done by activation and deactivation of various windings In general stepper motor driver circuit is divided into two main parts signaling and switching Signaling part is for generating the consequent and regular pulses patterns for motor driver The switching part turns the control switches on and off according to the generated patterns A four phase stepper motor driver has two main parts as follows control signal sequencer and motor driver Fig 1 TH a mt int T Activation Currents direction commands Control Signals Fig 1 Stepper motor driving system According to the nature of the windings the current and therefore the switch state could not change suddenly except that infinite opposite voltage is applied to the winding When the control switch of the winding is closed the winding current increased gradually Thus when its state change the switch opened large voltage is applied to the winding This may damage the control switches 1f there is no care about it There are two main strategies to solve these problems using diode or capacitance in parallel with the winding Ther
9. tion continues while the internal oscillator set the stop command The chopper rate is determined with the RC network connected to the OSC input To the motor windings Ss Fig 7 Minimum system for stepper motor driver using L297 and L298 5 Micro step driving circuit In micro step mode the resolution number of steps per round is increased The motor used in this work pass 1 8 degree per rotation and has resolution equals to 200 steps After micro step driving the target becomes 800 steps per rotation 4 micro steps per each step After driver implementation for half and full step modes several parts should be changed and optimized for micro step driving One of the main parts is the reference voltage Vref Another i zi uM s 9 um optimization is to control two phase of the motor independently For this purpose driver circuit must be changed to be able to control the winding current independently to the portion of the final phase current For this one can control the current of the motor phases to be able to move between two full steps more According to this digital to analog converter DAC and operational amplifiers OpAmp are used to control the reference voltage of the motor Inputs to the ADC will determined the signal levels and the bit number can perform more accurately Two L297 devices are used to control the L298 phases separately These devices should be synchronized by each other The inh
10. to be dissipated more and more There are two solutions for this bottleneck First assume the time diagram for winding voltage characteristics though the time While the motor current is below the normal value threshold all the supply voltage is applied to the winding When the current is reached to the threshold value the voltage is dropped to the value that can keep the motor current to its normal value The second solution is to use power supply generator with two different voltage levels that applied to the winding alternatively One can use the power supply controlled with Pulse Width Modulator PWM chopper to control the current In this method while the motor current is increasing the control system applies the supply voltage to the motor When the current is reached up to the threshold the control system tries to fix the current to the ideal value by changing the supply pulse durations As shown in Fig4 For each chopper the supply pulse duration D is defined as follows D Ton Ton Tore where the To and Tor are switch activation deactivation times As shown in Fig 4 supply voltage in the chopper cycles that switches are closed on applied to the winding But the negative voltage is applied when the switches are open The main problem with this method is its open loop characteristic gt j Torn p Taft Time Fig 4 PWM operation Solution for this bottleneck is to use the feedback path and monitor
11. uit for micro step driving mode all of the irregular torque to rotation values in the curve appeared as holes in the full and half step modes eliminate and regular curve is performed After applying test to the circuits it has been determined that torque reduction in resonance frequencies is eliminated with micro step driving and motor power consumption becomes less than other modes of operations References 1 Online documents http www epanorama net 2 Online documents http www cs uiowa edu 3 L297 user manual at www alltronics com 4 L298 user manual at www st com 5 LMD18245 manual http ww national com 6 Akira Sugawara Takashi Kenjo Stepping Motors and Their Microprocessor Controls Oxford University Press 1995 Ole e cem pee gree Gps uS eus FTongue vs Frpm E k i n L 4 E b i 4 i00 200 5300 A0 500 aoo 700 B00 Bon Fig 11 Full Step mode Torque to rpm Fig 14 All modes Torque to rpm Fig 12 Half Step mode Torque to rpm Fig 13 Micro Step mode Torque to rpm
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