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1. 1 0 History of PLCs A programmable logic controller PLC is an electronic device that controls machines and processes It uses a programmable memory to store instructions and execute specific functions that include On Off control timing counting sequencing arithmetic and data handling PLC development began in 1968 in response to a request from the Hydramatic Division of General Motors At the time GM frequently spent days or weeks replacing inflexible relay based control systems whenever it changed car models or made line modifications To reduce the high cost of rewiring GM s control specification called for a solid state system that had the flexibility of a computer yet could be programmed and maintained by plant engineers and technicians It also had to withstand the dirty air vibration electrical noise humidity and temperature extremes found in the industrial environment The first PLCs were installed in 1969 and quickly became a success Functioning as relay replacements even the early PLCs were more reliable than relay based systems largely due to the ruggedness of their solid state components compared with the moving parts in electromechanical relays PLCs provided material installation troubleshooting and labor cost savings na StS SS AANI An early PLC L1 by reducing wiring and associated wiring errors They took up less space than the counters timers and other control components they replaced
2. 5 4 Installation Requirements 5 5 5 0 What is a Potential Control Application Any situation where coordinated operation of electrical or electronic devices is required is a potential control application Typical devices controlled include contactors solenoid valves relays lights and motors Machines or processes that operate based on any of the following characteristics could be considered potential control applications e Repetitive operations e High speed control e Time driven operations e Requirements for data e Event driven operations acquisition manipulation Examples include conveyors form and fill operations packaging operations strapping machines palletizing and wrapping machines traffic light sequencing gate control cut to length lines semi automatic welding and painting storage and retrieval systems pump alternators car washes printing presses vending machines and many more These applications may be able to be controlled by relays PLCs or single board controllers SBCs all of which possess logic capabilities However before selecting a control system the application s requirements must be determined as they help guide the selection process Personal computers PCs are also sometimes used for control applications but always for more complex control requirements than the applications controlled by relays micro PLCs or SBCs 5 1 What are the Application s Requir ements No ma
3. Fig 3 3 3 6 Oper ating Cycle All the components of the PLC system come into play during the operating cycle which consists of a series of operations performed sequentially and repeatedly While PLCs operate in binary 1 and 0 they also use binary to convert accept and mani pulate data from other number systems These systems indude binary coded deci mal BCD hexadeci mal octal and gray code see Fig 3 3 Beginning PLC users probably do not need to know how to use these different number systems so they will not be explained further However note that they may need to be learned later as these numbering systems are valuable when working with certain types of inputs For example thumbwheel switches usually require four bits per wheel i e they communicate in BCD Thus any PLC used with a thumbwheel must be able to accept a BCD input 1 0 Wiring Pushbutton wired aay vo 1 Te 1 3 DG V4 W 5 1 6 7 7g 1 9 to input terminal 1 3 Ibe oul CoM te O BT SY O00000000 IN O POWER 8 Bal 8 POWER O00000 OUT Pilot light wired to output terminal O 2 Memory location of I O status This bit corresponds to Input word 0000 0000 0000 go00 Input terminal 1 3 j 25 Output word 0000 0000 0000 00 p dol This bit corresponds to Output terminal 0 3 Program with addresses 1 3 0 2 Figure 3 2 This figure shows the relationship between the actual I O wiring terminal location and the address of the instructi
4. RUNG 2 This is the rung that controls the actual output address 0 0 If the timer has timed for 10 seconds the timer done bit T4 0 DN is energized the output 0 0 is energized Rung 0 Wired Wired Normally Normally Closed Open Pushbutton Pushbutton Motor Run Request Stop Start 1 0 1 1 B3 0 Motor Run Request B3 0 Rung 1 Motor Run 10 Second Request Timer B3 0 TON TIMER ON DELAY EN Timer T4 0 Time Base 1 0 DN Preset 10 Rung 2 Accum 0 10 Second Timer Done T4 0 DN Motor 1 23 18 7 8 Off Delay Uses This logic turns Off a device after a programmed time delay Operation The Off delay program allows a control instruction output device to be turned Off after a preset amount of time Ladder Logic The key item in these rungs is the normally closed condition instruction programmed in series with the control instruction on the first rung RUNG 0 This is the rung that controls the actual output address O 0 It operates in the same manner as the start stop example in section 7 3 Notice that a normally closed instruction has been added in series with the output This condition instruction has the address of the timer done bit T4 0 DN from the timer in rung 1 It is the addition of this instruction that creates the Off delay operation of the rung RUNG 1 This rung contains an On delay timer with an address of T4 0 When the output O 0 from rung 0 has been energized the timer begins t
5. Device To be determined To be determined Voltage 24V dc 24V dc 24V dc 24V dc 24V dc 24V dc 24V dc Voltage 24V dc 24V dc 24V dc 24V dc 120V ac 120V ac Voltage TBD TBD Voltage 24V dc or 120V ac Speed of Operation When determining speed of operation consider these points e How fast does the process occur or machine operate e Are there time critical operations or events that must be detected e In what time frame must the fastest action occur input device detection to output device activation e Does the control system need to count pulses from an encoder or flow meter and respond quickly The control system selected needs to meet the speed demands of the process or machine so knowing these criteria is important Clearly the parking garage control system does not require a fast response Considering that PLC or SBC based control systems respond in milliseconds the relative speed of operation for many applications such as the parking garage is very slow compared to the processing speed of a PLC or SBC Operator Interfaces and Communication In order to convey information about machine or process status or to allow an operator to input data many applications require operator interfaces Traditional operator interfaces include pushbuttons thumb wheel switches pilot lights and LED numeric displays Electronic operator interface devices display messages about machine status in descriptive
6. activate at a desired liquid level Flow Switches A flow switch is inserted into a pipe or duct to sense the movement of a fluid The fluid might be air water oil or some other gas or liquid The sensing element is a valve or vane that extends into the fluid stream The vane will move and actuate electrical contacts whenever the flow is sufficient to exceed a i B preset spring tension on the vane E 6 Pressure Switches Pressure switches are used to detect a pressure level and provide digital feedback to the PLC if the level exceeds a specified amount They are typically used to notify the control system or operator that an excessive pressure condition exists Pressure switches use a spring loaded bellows mechanism to close contacts Pressure of the fluid being sensed is directed into the bellows by tubing or other means When the pressure in the bellows exceeds the preset spring tension the switch is actuated Temperature Switches Temperature switches are typically used to detect overtemperature conditions When the temperature of the object or process being monitored approaches a preset threshold the device switches Bimetallic and bulb capillary type temperature switches typically use switching contacts while thermocouple switches typically use solid state outputs Encoders An encoder is a form of sensor that changes rotary motion into high speed pulses Encoders are either incremental which track speed and di
7. diagnostics The detection and indication of errors or malfunctions digital Information presented as a discrete value 1 or 0 Compare with analog drum timer A mechanical device which controls a sequential operation by means of a drum with pegs where the presence of a peg represents a logical 1 and the absence of a peg represents a logical O Its operation is similar to that of a music box mechanism EEPROM Electrically Erasable Programmable Read Only Memory A type of PROM that is programmed and erased by electrical pulses Data stored toa EEPROM will not be erased just by interrupting power to the chip EIA Electronic Industries Association An agency which sets electrical electronic standards See also RS 232 EMI ElectroMagnetic Interference Magnetic fields generated by electrical devices execution time The time required to perform one specific instruction a series of instructions or a complete program The execution time for a given instruction may vary depending on the status of the instruction True or False and other parameters False The status of an instruction that does not provide logical continuity on a ladder rung fault Any malfunction that interferes with the normal operation of an application FET Field Effect Transistor A high performance solid state device capable of switching higher current dc loads than transistors FIFO First In First Out The order in which data is entered into and re
8. from the output on the non functioning rung and examine the logic to determine what may be preventing the output from energizing Typical logic errors include e Programming a normally open instruction instead of a normally closed instruction or vice versa e Using an incorrect address in the program Most programming software packages and HHPs have a feature called the trace search or find function Simply enter the address of the instruction to be found and the HHP searches for the first occurrence of that address If the address is found the search feature can also search for other instances of 91 the same address This lets you quickly find all occurrences of an address and verify that the logic associated with it is both correct and operating as expected no I O faults etc 6 6 Faults Fault messages are displayed on the HHP or programming software for easier problem identification Error messages coupled with information from the PLC user manual help locate the fault determine its cause and suggest corrective actions This self diagnostic capability which is not available with most other control systems greatly facilitates troubleshooting Some of the more common causes of faults include memory errors data corruption errors watchdog timer errors and momentary power problems 6 7 Safety After identifying the problem and determining the appropriate corrective measure consider the fol
9. or power lines L1 PB2 L2 FBI Start Stop Motor M1 35 Power Bus Power Bus Auxiliary Holding Contact Rung Fig 4 1 Electrical diagram of a hardwired start stop circuit with current flowing from the left bus to the right bus Each electrical circuit in the diagram is considered a rung Every rung has two key components it contains at least one device that is controlled and it contains the condition s that control the device such as power from the bus or a contact from a field device A rung is said to have electrical continuity when current flows uninterrupted from left to right across the rung i e all contacts are closed If continuity exists then the circuit is complete and the device controlled by the rung turns On see Fig 4 2 If continuity does not exist the device stays Off Electrical Continuity L1 PB1 PB2 Le Stop Start Motor Rung Power Bus Power Bus Auxiliary Holding Contact Fig 4 2 If PB1 is NOT pushed and PB is pushed the circuit will be complete Under these conditions the rung has electrical continuity and the motor will turn On 4 2 Ladder Logic Programs A PLC ladder logic program closely resembles an electrical ladder diagram Fig 4 3 On an electrical diagram the symbols represent real world devices and how they are wired A PLC program uses similar symbols but they represent ladder logic instructions for the application A ladder logic program exists only in the PL
10. 3 Pump 3 B3 2 0 2 Stage 2 Internal Bit for Pump 3 B3 5 Stage 3 Internal Bit for Pump 3 B3 8 RUNG 13 e This rung is the counter rung and controls which stage will be run next The counter increments each time all pumps are off Rung 13 Sequence Pump 1 Pump 2 Pump 3 Counter 0 0 O 1 0 2 CTU if 1 E 1 RUNG 14 e This rung resets the counter after the last stage is run and starts the entire sequence over again Rung 14 Cycle Sequence Complete Low Level Counter B3 9 1 0 C5 10 1 E 1 E RES Notice that float switch F5 is not used in this program It can be used in the program as an alarm condition that ensures all pumps are running if this float is tripped This would help prevent damage if any of the other floats malfunctioned Or it can sound an alarm to identify a problem with the tank for instance that it is about to overflow Appendices Glossary 25d oe eK ce eK ae Appendix A Input and Output DEVICES aw eho e ag K eee RS Appendix B Instruction Execution Times Appendix C Sample Program Worksheets Appendix D K w Appendix A Glossar y address A unique memory location identified by an alphanumeric character F or example 1 2 is the memory address for data located in bit 2 of the input file alphanumeric Character strings composed of any combination of letters or numbers analog A numeric value that represents
11. 6 0 Commissioning Preparing a control system for start up also called commissioning involves executing a series of tests to ensure that the PLC the ladder logic program the I O devices and associated wiring operate according to specifications Before commissioning any control system the technician must have a clear understanding of how the control system operates and how the various components interact e g sequence of operation timing and speed related issues For a PLC based system understanding the application can be accomplished by studying a printout of the current program If properly documented the printout should note addresses for I O devices and contain comments describing the operation of each program rung Assuming installation is complete and the application program has been loaded into the PLC the following checklist provides a good guide for commissioning a PLC 1 Be aware of the hazards posed by inadvertently energized outputs Before applying power to the PLC or the input devices disconnect or otherwise isolate any output device that could potentially cause damage or injury typically an output that causes movement like starting a motor opening a valve etc Apply power to the PLC and the input devices To verify that there is proper power check the PLC and input devices with a voltmeter If there is a power problem tighten connections and check for broken wiring or faulty input devices Examine the
12. And their ability to be reprogrammed dramatically increased flexibility when changing control schemes Perhaps the biggest key to industry s acceptance of the PLC was that the initial programming language was based on the ladder diagrams and electrical symbols commonly used by electricians see Fig 1 1 Most plant personnel were already L2 swo LT3 HzR Uuphfhmqppqkczz swi LT3 sw2 LT5 PA sw3 sw4 Sw5 LI7 Fig 1 1 Typical electrical ladder diagram trained in ladder logic and they easily adopted it for PLCs In fact ladder logic still plays an integral role in programming and troubleshooting even though more advanced programming languages have been developed 1 1 Why Use a PLC Should we be using a programmable logic controller During the 1970s and early 80s many engineers manufacturing managers and control system designers spent considerable time debating this issue trying to evaluate cost effectiveness Today one generally accepted rule is that PLCs become economically viable in control systems that require three to four or more relays Given that micro PLCs cost only a few hundred dollars coupled with the emphasis manufacturers place on productivity and quality the cost debate becomes almost immaterial In addition to cost savings PLCs provide many value added benefits Reli ability Once a program has been written and debugged it can be easily transferred and downloaded to other PLCs Th
13. Motor 1 1 1 2 O 3 E F O 3 Auxiliary Holding Contact Fig 4 14 An auxiliary holding contact keeps its referenced output energized even after a momentary start signal has been removed 4 5 Program Execution Before reading how the PLC executes a ladder logic program re reading Chapter 3 6 Operating Cycle may be helpful The PLC solves each rung sequentially from top to bottom of the program Even if the output of the current rung e g rung 5 affects a previous rung e g rung 2 the PLC does not go back to solve the earlier rung until the next program scan For the output of one rung to affect an instruction in another rung in the same scan it must have a lower rung number than the rung it is to affect That is the controlling rung must be programmed before the controlled rung While rungs are often ordered to show a sequence of events the top most rung is the first event and so on this is done purely for organizational convenience In both electrical diagrams and ladder logic programs rung order does not necessarily dictate the sequence of operation Remember the status of the condition instructions of each rung dictates the sequence in which outputs are controlled How to Apply a Micro PLC What is a Potential Control Application 5 0 What are the Application s Requirements 5 1 Selecting a Control Method 5 2 What are the PLC Specifications 5 3 Program Development Procedures
14. PLCs helps increase competitiveness Processes using PLCs include packaging bottling and canning material handling machining power generation HVAC building control systems security systems automated assembly paint lines and water treatment PLCs are applied in a variety of industries including food and beverage automotive chemical plastics pulp and paper pharmaceuticals and metals Virtually any application that requires electrical control can use a PLC Traditional PLC applications 6 i Wastewater treatment facility Papermaking operation The Micro PLC Development of the Micro PLC 2 0 What Makes a Micro PLC a Micro 21 Capabilities Overview 2 2 Micro PLC Applications 2 3 2 0 Development of the Micro PLC Until the introduction of the micro PLC in the mid 1980s the potential to increase automation on simple machines or less complex processes remained largely untapped This was due to the lack of attractive alternatives to hardwired relay control Though OEMs had benefitted by using PLCs to control equipment process lines or even whole plants they could not always justify using a PLC on small applications and low cost machines And if cost was not an issue size often was Sometimes even small PLCs were simply too large to fit in the space allocated for electrical controls As such the driving force behind the development of the micro P
15. deep RO daIG an input device rather than electrical noise A typical filter time is 8 ms but some PLCs have adjustable input filter response times A longer response time provides better filtering of electrical noise A shorter response time helps in applications that require high speed operation e g interrupts or counting 3 2 Outputs Connected to the output terminals of the PLC are devices such as solenoids relays contactors motor starters indicator lights valves and alarms Output circuits operate in a manner similar to input circuits signals from the CPU pass through an isolation foe y Transistor relay and triac for PLC output barrier before energizing output circuits circuitry Paper clip indicates relative size PLCs use a variety of output circuits to energize their output terminals relays transistors and triacs Relays are for either ac or dc power Traditional PLC electromagnetic relays typically handle current up to a few amps Relays can better withstand voltage spikes and they have an air gap between their con tacts which eliminates the possibility of current leakage However they are comparatively slow and subject to wear over time Transistors switch dc power are silent and have no moving parts to wear out Transistors are fast and can reduce response time but only carry loads of 0 5 amp or less Special types of transistors such as FETs Field Effect Transistors can handle more power
16. essing Whereas memory is a physical space data is information stored in that space The CPU operates just like a computer it manipulates data using binary digits or bits A bit is a discrete location within a silicon chip that either has a voltage present read as a value of 1 On or not present read as a value of 0 Off Thus data is a pattern of electrical charges that represent a numerical value A bit is the smallest unit of memory available Generally CPUs process and store data in 16 bit groups also known as words However users can still manipulate data on the bit level Each word of data has a specific physical location in the CPU called an address or a register note that the terms word address and register are often used interchangeably Every element in the user program is referenced with an address to indicate where data for that element is located When assigning addresses to I O in a program note that the address is related to the terminal where input and output devices are connected see Fig 3 2 Decimal Hexadecimal Binary BCD Octal Gray code 0 0 000 0000 0 0000 1 1 001 0001 1 0001 2 2 010 0010 2 0011 3 3 011 0011 3 0010 4 4 100 0100 4 0110 5 5 101 0101 5 0111 6 6 110 0110 6 0101 7 7 111 0111 7 0100 8 8 1000 1000 10 1100 9 9 1001 1001 11 1101 10 A 1010 12 1111 11 B 1011 13 1110 12 C 1100 14 1010 13 D 1101 15 1011 14 E 1110 16 1001 15 EF 1111 17 1000
17. for probable cause and recommended action Clear the fault Could you successfully clear the fault No The PLC is faulty Replace PLC Check power Yes Place the controller in program mode Correct the condition causing the fault Return controller to Run or any of the Test modes Fig 6 2 Troubleshooting Model Are the wire connections tight Yes Is the Power Yes LED on No Does the controller have power supplied No Yes Can you communicate with the PLC Yes PLC is functioning Check field devices Test and verify system operation Tighten the wire connections Is the Run LED N No on constantly Yes Is the PLC in Run mode Place PLC in Run d Is an input or output LED showing proper Refer to the user manual for probable cause and recommended action Application Examples Introduction AA 7 0 Basic Logic OR CIRCUIT AA 7 1 AND cirCUit eenn 7 2 Start stop circuit seee 7 3 Flip flop CIrCUit eee 7 4 Alarm circuit O 7 5 Start stop with jog 7 6 Timing and Counting UNGGOY aaa alison Panini 7 7 Off delay enerne 7 8 One minute CIOCK seeen 7 9 Up down Counting 7 10 Data Instructions Moving data seen 7 11 Comparing data 7 12 Math COMMANAS sse 7 13 Advanced Instructions Sequence oeeo 7 14 FIF O Aa AE 7 15 High speed counter 7 16 Two
18. memory 22 142 150 temperature limitations 59 80 81 throughput time 27 150 164 thumbwheel switches 19 24 31 58 142 150 154 time base 106 111 151 timer instructions 12 37 43 44 112 161 transistor 21 68 69 145 150 151 153 159 triac 21 68 69 151 153 159 twisted pair cable 59 81 UL 62 151 warning lamps 20 158 Micro Programmable Logic Controllers Small enough to fit in one hand the compact micro PLC provides a powerful solution to today s electronic control applications from the simple to the complex at an affordable price The MicroMentor reference book serves as an introduction to micro PLCs for anyone from design engineers or electrical technicians to maintenance personnel and students While some knowledge of basic electronic controls is helpful it is not required nor is previous experience with programmable logic controllers Through easy to understand text and numerous illustrations the reader will gain a practical understanding of micro PLCs MicroMentor demonstrates the advantages of micro PLCs over electromechanical controls and it helps readers evaluate the best control system for their application Topics covered include e History of PLCs e Micro PLC capabilities and operation Programming with ladder logic e How to apply a micro PLC e Commissioning and troubleshooting e Application examples N Rockwell Automation Allen Bradley a Rockwell automation business has been hel
19. on the green light Condition If the gate is up Rung 5 Gate Green GO is Up Light Control point e Count cars entering turn on full sign at 500th car Conditions If the gate has been lowered si If accumulated counter value preset value of 500 Rung 6 Number of Vehicles in Garage Lower Gate CTU Count Up Counter C5 10 Preset 500 Accum 0 Control point Turn on the Full sign Condition If accumulated counter value preset value of 500 Rung 7 C5 10 Garage is Full DN Control point Decrement the counter count departing vehicles Condition If avehicle departs the garage Rung 8 Number of Vehicle Vehicles Photo Sensor in Garage Departing Garage CTD Count Down cD Counter C5 10 DN Preset 500 Accum Control point Sound alarm Condition If the gate is obstructed Rung 9 Gate is Alarm Obstructed Sounded 1 Programming Tips e When programming condition instructions refer back to Fig 4 8 to determine if a normally open or a normally closed instruction produces the desired action e When defining the rules of operation the text should use language that helps convert the operating characteristics to ladder logic Recall from Chapter 4 that AND logic connects instructions in series on a ladder diagram rung while the OR logic connects instructions in parallel e If an output needs to remain on after the condition that originally energized it is no longer
20. or other devices can use a lower amperage output from a PLC to switch a higher current load These devices are sometimes referred to as solid state relays Contactors Contactors are relays that are able to switch high current loads gt 10A The coil voltage of a large contactor in many cases must be switched by a relay contact because the coil operating current is higher than the output current of the PLC Contactors are used for switching motors heaters etc co Motor Starters Motor starters are contactors which have the protection of an overload circuit An overload circuit protects the motor from damage if operating current is less than the starting inrush current the current rating of the fuses but greater than the current normally observed during operation Solenoids Solenoids convert electrical signals to mechanical motion An electromagnetic coil attracts a plunger or other mechanism to an alternate position when energized Spring tension or gravity is used to return the plunger to the original position when the electromagnet is de energized Solenoids are most commonly used as part of other machines or components Valves Solenoid operated valves are a very common type of output device A linear solenoid operates the valve mechanism to control the flow of materials in a process The addition of the valve allows the PLC to control pneumatic and hydraulic operations in addition to electrical and ele
21. present use an auxiliary holding contact or a latched output e A condition instruction can be used more than once in a program because it exists in the software a benefit over hardwired relays Also remember that the status of an output can be used as a condition instruction e Only program a specific output instruction once If an output instruction with the same address is programmed more than once the last occurrence of the instruction in the user program will determine the actual output state e When each I O field device is wired to a terminal on the PLC it then has a unique address which corresponds to that terminal e Follow the instruction manual Each PLC manufacturer uses slightly different terms and techniques These should be noted and followed carefully Addressing All elements of a ladder diagram are labeled with a letter numerical designation Because every PLC manufacturer has a variation of this designation be sure to follow the addressing conventions outlined in the operator s manual The parking garage example uses I to indicate inputs O for outputs All input and output terminals in this example are numbered starting with zero 0 The program for the parking garage has its inputs and outputs addressed as such Input address 1 0 Ticket request pushbutton 1 1 Ticket taken limit switch 1 2 Car cleared gate photoelectric sensor 1 3 Car departed garage photoelectric sensor 1 4 Gate obs
22. process entails monitoring the status of devi ces connected as inputs and based on a user written program controlling devi ces connected as outputs Inputs include items such as pushbuttons thumbwheel switches limit switches selector switches proximity sensors and photoelectric sensors These are all discrete devices that provide an On or Off status to the PLC While larger PLCs can directly accept analog values variable voltage or current signals such as from temperature or pressure sensors 13 R micro PLCs do not typically possess this capability The electrical signals that field devices send to the PLC are typically unfiltered 120V ac or 24V dc The input circuitry on the PLC takes this field voltage and conditions it to be usable by the PLC Conditioning is necessary because the internal components of a PLC operate on 5V dc and this minimizes the possibility of damage by shielding them from voltage spikes To electrically isolate the internal components from the input terminals PLCs employ an optical isolator which uses light to couple signals from one electrical device to another The PLC s input circuitry also filters field voltage signals to qualify them as valid such as a signal from a sensor or not valid such as high frequency electrical noise or static Input filters determine the validity of a signal by its duration they wait to confirm that a signal is a reference from aatrasan
23. processing unit program scan A part of the controller s operating cycle During the program scan the ladder logic program is executed and the output data file is updated based on the logic of the program and the status of the input data file See also communication scan input scan and output scan RAM Random Access Memory A fast volatile when power is interrupted data is lost form of memory Each bit in RAM can be stored or retrieved in the same amount of time at any time Commonly referred to as read write memory because it can be written to as well as read from This type of memory typically uses a battery or capacitor for back up power read To acquire data from a memory location For example the controller reads information from the input data file to solve the program register A temporary storage space for various types of information and data such as timer or counter values In PLCs a register is normally 16 bits wide 1 word relay An electrically operated mechanical device the contacts of which open and close based on the presence of an electrical signal relay logic A program written with relay symbols contacts and coils Relay logic is commonly referred to as contact symbology retentive data Information data stored in memory that is not lost when power is interrupted RS 232 An EIA standard that specifies electrical and mechanical characteristics for serial binary communications It is a single ended s
24. stage alternator 7 17 Three station alternator 7 18 7 0 Introduction As the parking garage example in Chapter 5 demonstrates developing a ladder logic program for a PLC consists of identifying the logic required and building the program one rung at a time While different programs can achieve the same outcome every program uses the same building blocks the micro PLC s instruction set This chapter takes some of the most commonly used instructions and demonstrates their use in control applications In addition to explaining how PLC users can apply these powerful tools the examples highlight typical micro PLC applications and how to build complex programs from the simple steps shown 7 1 OR Cir cuit Uses This type of logic is used to turn On an output device control instruction when any input device condition instruction in the rung provides logical continuity Operation Turn On an output with more than one input device condition instruction Ladder Logic The logic used in this example consists of one rung with two condition instructions programmed in parallel RUNG 0 e This rung shows that whenever input device 1 0 OR input device 1 1 is On output device 0 0 will be energized Rung 0 If the tank high level detector is activated OR the manual by pass switch is On open the tank drain valve Tank Tank High Level Drain Detector Valve 1 0 0 0 Manual By Pass S
25. text Motor 1 On display parts count and track alarms They can also be used for data input see section 3 9 for details Communication involves sharing application data or status with another electronic device such as a computer or a monitor in an Operator s station Communication can take place locally through a twisted pair wire or remotely via telephone or radio modem PLC based control systems are designed to support communication and electronic operator interfaces where relay based systems are not SBC based systems typically support communications and some operator interfaces As it has been defined the parking garage control system does not require operator interfaces beyond the ticket request pushbutton the green enter light and the alarm horn However advanced communication capabilities could provide benefits For example if a portion of the garage was being repaired and 50 parking spaces were eliminated it would be advantageous for the garage operator to change the control system parameters so that only 450 vehicles could be admitted In addition the control system could also let drivers know an area had been temporarily closed Environment Consider the environment where the control system will be located Will it be subjected to temperature extremes Water Humidity Salt Shock Dust Vibration In harsh environments house the control system in an appropriate NEMA or IP rated enclosure Also remember to cons
26. update the I O independently of the normal program scan This can substantially improve speed and performance Simplifying the program also increases performance because program length directly impacts scan time Every instruction in a program takes time to execute and reducing or simplifying the program reduces time PLC users can calculate program execution time by referring to the PLC operator s manual which should list execution times for all of the instructions See the worksheet in Appendix D for an example as well as a listing of typical instruction execution times in Appendix C 5 4 Program Development Procedures Even the simplest programs rarely go directly from the programmer s head to the PLC In fact attempting this time saving step often prolongs the process Instead begin by writing out the operation sequence both sentences and flow charts work well There are three steps to developing a sequence of operation e Define the rules of operation for each control point e Identify and label inputs and outputs e Convert the rules of operation to ladder logic Define Rules of Operation What conditions permit or prevent responses from the control system Defining these conditions is known as developing the rules of operation To begin carefully describe the control system at its most basic level Recall from section 5 1 that the parking garage control system was described like this The driver appro
27. 4 continuity Having the capability of passing a voltage logic state or any other signal unimpeded control instruction Instruction pertaining to the output portion of a rung on a ladder diagram These commands detail exchanges of data with external output devices or internal devices such as timers counters math functions or other high level instructions See also condition instruction controller A device capable of controlling other devices For example a programmable controller is used to monitor input devices implement logic and control output devices counter A device or software instruction that counts the occurrence of some event It may be pulses resulting from operations such as switch closures or other discrete events CPU Central Processing Unit The decision making section of a programmable controller that executes the instructions contained in the user program CSA Canadian Standards Association An agency which regulates the specifications and testing required of electrical devices used in Canada cycle A single sequence of operation In the PLC one full operating scan from start to finish data Within the PLC a general term for any type of information stored in memory data table The part of the PLC memory that contains I O values and files where data is monitored manipulated and changed for control purposes debug The process of locating the source of control system malfunctions and correcting the problems
28. 8 Programmer HHP The PC is used to run PLC programming software This software allows users to create edit document store and troubleshoot ladder diagrams and generate printed reports Software instructions are based on graphical symbols for various functions Using such software does not require knowledge of higher programming languages just a general understanding of standard electrical wiring diagrams While the HHP can be used to program the PLC it is more commonly used as a troubleshooting tool This is because the HHP is compact and has its own memory to store programs HHPs are invaluable for troubleshooting equipment while on the factory floor for modifying programs and transferring programs to multiple machines The language used by the HHP is a graphical form of instruction list programming based on the PLC s ladder logic instructions 3 9 Oper ator Interfaces In order to convey information about Au O ANa ELET Sannn an ren EA O DO bt o pe machine status the front panel of a micro PLC has a series of indicator lights These are for such things as power run faults or I O status To communicate with the PLC to enter data or monitor and control machine status traditional operator interfaces include pushbuttons thumbwheel switches pilot lights and LED numeric displays To improve the interface between the operator and the micro PLC a new generation of electronic operator
29. Bit Low Tevel Pulse On Pulse Off Firal Call Pump 1 1 2 1 0 B3 2 1 1 0 0 E pm Pump 1 O 0 Second Call Pump 2 F3 It Pump 1 O 0 RUNG 3 e This rung controls the operation of pump 2 O 1 If the low float 1 0 is On and the alternator bit B3 2 is On and the level in the tank has reached the First Call float F2 1 1 this pump will be the first one energized If B3 2 is Off pump 1 will be the second pump energized Rung 3 Low Level Alternator Bit First Call F1 Pulse On Pulse Off F2 Pump 2 1 0 B3 2 1 1 O 1 Pump 2 O 1 Second Call Pump 1 F3 0 0 1 2 1 E E Pump 2 O 1 One of the powerful features of PLCs is the ability to monitor and alert operators to alarm conditions You may have noticed that float switch 4 F4 is not being used in the program This float switch is an alarm condition It can be used in the program to make sure the pumps are running if this float is tripped This check operation would help minimize damage if the level 1 or level 2 floats malfunctioned It can also sound an alarm that indicates the tank is about to overflow 7 18 Three Station Alter nator Uses This example is similar in function to the example in section 7 17 except that we are adding an additional device to alternate 3 rather ka i than 2 For ease of description we will discuss three pumps that empty a tank The control system needs to be able to rotate the pump that turns On first each time a r
30. C programs are written in ladder logic than any other language The ladder logic programming language is an adaptation of an electrical relay wiring diagram also known as a ladder diagram Because ladder logic is a graphical system of symbols and terms even those not familiar with electrical relay wiring diagrams can easily learn it Other control languages occasionally used to program PLCs include BASIC C and Boolean These computer languages facilitate programs that require complex instructions and calculations too cumbersome to implement with a ladder logic program However micro PLCs that can be programmed with BASIC and C are not widely available The instructions used to program most micro PLCs are based on a combination of Boolean ladder logic and mnemonic expressions A mnemonic expression is a simple and easy to remember term which represents a complex or lengthy instruction For example TON stands for timer on Different PLCs use slightly different instructions and these can be found by consulting the user s manual 4 1 Electrical Ladder Diagr ams Ladder logic programs evolved from electrical ladder diagrams which represent how electric current flows through devices to complete an electric circuit These diagrams show the interconnection between electrical devices in an easy to read graphical format that guides the electrician when wiring see Fig 4 1 An electrical diagram consists of two vertical bus lines
31. C s software it is not the actual power bus or the flow of current through circuits Another difference is that in an electrical diagram devices are described as being open or closed Off or On In a ladder logic program instructions are either True or False however the terms are often used interchangeably Condition Instructions Control Instruction Stop Start 1 1 1 2 Rung Motor M1 Motor M1 Auxiliary Holding Contact Fig 4 3 Notice the similarity between the ladder logic program and the hardwired circuitin Fig 4 1 Each rung in a ladder logic program must contain at least one control instruction output and usually contains one or more condition instructions inputs Condition instructions are programmed to the left of the control instruction Examples of condition instructions include signals from connected input devices contacts associated with outputs and signals from timers and counters Programmed on the right side of the rung a control instruction is the operation or function that is activated de activated by the logic of the rung Examples of control instructions include output energize turn On the PLC s output circuitry to activate a field device and instructions internal to the PLC such as bit commands timers counters and math commands The control instructions are energized or de energized based on the status of the condition instructions in the rung The PLC Recall from Chapte
32. Control point e The ticket machine will provide a ticket Conditions If the driver presses the ticket request pushbutton AND the Full sign is NOT on AND the gate is lowered Rung 0 Provide Ticket Garage is Gate is Ticket Request PB Full Lowered Solenoid _ _ A A 4 Control point e Raise the gate until fully up Conditions After the driver takes the ticket AND the gate is NOT up AND the Full sign is NOT on Rung 1 Ticket has Been Taken Gate Garage is Limit Switch is Up Full Raise Gate U Raise Gate Control point e Vehicle present latch Conditions e Vehicle has been detected AND the vehicle has NOT cleared the gate Rung 2 Vehicle Photo Vehicle is Vehicle Sensor Clear of Present gate Gate Latch aasa St Vehicle Present Latch Control point e Vehicle clear of gate Conditions e Vehicle present latch is on AND a vehicle is NOT detected AND the ticket request pushbutton is NOT pressed Rung 3 Vehicle Vehicle Photo Vehicle is Present Sensor Ticket Clear of Latch gate Request PB Gate H AH SARS Ad Vehicle is Clear of Gate Control point Lower the gate until fully down Conditions If the gate is up AND the car has cleared the gate AND the gate is NOT down AND the gate is not obstructed Rung 4 Vehicle is Gate Clear of Gate is Gate is Lower is Up Gate Lowered Obstructed Gate c i i Lower Gate Control point Turn
33. Cs also open up new control possibilities with advanced functions such as four function math data comparison i e equal to greater than etc data handling such as parts sorting or fault tracking sophisticated subroutines sequencing replacing drum sequencers and other features that experienced control system designers can appreciate To demonstrate the value of these features application examples are provided in Chapter 7 Possibly the most exciting feature of micro PLCs is their high speed counting capabilities Speed the key to success for many automated applications can also cause problems if the speed of the PLC cannot keep up with the manufacturing operation For example if parts or material are moving at high speed past a proximity sensor a normal PLC counter could miss some parts This is because the parts are moving faster than the PLC scans the sensor s input However a high speed counter operates independently of the program scan This enables it to count at a much faster rate typically 2 000 to over 6 000 times per second In addition some high speed counters can energize an output immediately i e without having to wait for the normal program scan time thus substantially improving speed and performance This enables the counter to affect control operations when split second accuracy is critical 2 3 Micro PLC Applications Micro PLCs are ideal for controlling stand alone discrete machinery or proces
34. Double Divide Multiply Scale Data instruction when True 1 statement Mnemonic Time needed to execute the it is not True 0 approx psec NEQ 6 60 AND 6 78 FRD 5 52 TOD 6 78 DCD 6 78 ENC 6 78 XOR 6 92 FFL 33 67 FFU 34 90 COP 6 60 FLL 6 60 LFL 33 67 LFU 35 08 MVM 6 78 MOV 6 78 NEG 6 78 NOT 6 78 OR 6 78 HSC 21 00 HSD 7 00 HSE 7 00 HSL 7 00 RES 6 00 high speed counter RAC 6 00 OTE 7 00 high speed counter ADD 6 78 CLR 4 25 DIV 6 78 DDV 6 78 MUL 6 78 SCL 6 78 Time needed to execute asa approx psec 21 52 34 00 56 88 49 64 27 67 54 80 33 64 61 13 73 78 4 34 x position value 27 31 5 06 word 26 86 3 62 word 61 13 64 20 33 28 25 05 29 48 28 21 33 68 21 00 8 00 10 00 66 00 51 00 56 00 12 00 33 09 20 80 147 87 6 00 57 96 169 18 Memory usage user words 1 50 1 50 1 00 1 00 1 50 1 50 1 50 1 50 1 50 1 50 1 50 1 50 1 50 1 50 1 50 1 50 1 00 1 50 1 00 1 25 1 25 1 50 1 00 0 75 1 50 1 00 1 50 1 00 1 50 1 75 Instruction Type Math Math Program F Program F Program F Program F Program F Program F Program F Program F Program F Program F ow Contro ow Contro ow Contro ow Contro ow Contro ow Contro ow Contro ow Contro ow Contro ow Contro Name Square Root Subtract Immediate Input with Mask Immediate Output with Mask Jump to Label Jump to Subroutine Label Master Control Reset Retu
35. ES LESS TH AN Source A C5 10 ACC Source B 3 RUNG 3 e This rung contains an Equal instruction The E QU will turn On the control instruction O 1 whenever the data in source A the accumulated value of counter C5 10 is exactly the same as data in source B a constant 5 Rung 3 EQU O 1 EQUAL Source A C5 10 ACC 5 Source B RUNG 4 e This rung contains a Greater Than instruction The GRT will turn On the control instruction 0 2 whenever the data in source A the accumulated value of counter C5 10 is greater than the data in source B a constant 7 Rung 4 0 2 GRT GREATER THAN Source A C5 10 ACC Source B 7 RUNG 5 e This rung contains a Limit instruction The LIM will turn On the control instruction 0 3 whenever data in the Test position the accumulated value of counter C5 10 is greater than the data in Low Limit the constant 3 and is less than the data in High Limit the constant 7 Rung 5 7 13 Math Commands Uses Most PLCs on the market today offer a range of math capabilities Some examples of the use of math include combining parts counts subtracting detected defects calculating run rates and logging or counting product Operation Math operations are performed as control instructions in the rung Here we have illustrated the program from a PLC that supports function block math commands This type of math instruction is much ea
36. LC was the demand by OEMs for a PLC that was small and inexpensive enough to replace relays dedicated timers and counters and single board controllers For a 5 000 machine a small PLC control system costing 1 000 is not economical However at a few hundred dollars a micro PLC is cost effective and provides all the benefits of traditional PLC logic control PLCs have followed a product development curve similar to that of the personal computer early PLCs were large cost thousands of Shown near actual size a 16 I O micro PLC Hundreds of electromagnetic relays would be needed to obtain an equivalent level of control dollars and had relatively few features But with the evolution of microprocessors and other board level components PLCs grew in sophistication while size and cost shrank In fact advanced features that were considered strictly in the domain of medium size PLCs five years ago are now common in micro PLCs Typical Micro PLC Features e Math capabilities e Data handling instructions i i e High speed counting BCD to binary conversion routines e Drum timer and sequencer functionality e Subroutines and interrupts e Programmed with a personal computer Communication with other electronic devices 2 1 What Makes a Micro PLC a Micro Several criteria are used to categorize PLCs as micro small medium or large Criteria include functionality number of inputs and outputs see Fig 2 1 cost and physi
37. Output A W Ww OFF SEAN A w w ON L x Fie lal OFF B Y ON ai P2 B Z OFF Controlled by the condition instructions that precede it on a rung the output energize instruction OTE turns On a bit element in the output image file when rung conditions are True Output energize is the ladder logic equivalent of a relay coil on an electrical diagram When logical continuity exists on a rung the On condition binary 1 is written to the location in memory associated with the output energize instruction If the address is that of an external output device the PLC energizes the output during the output scan When the rung is False the PLC de energizes the output The output energize instruction Hardwired to Programmed Remember to make a distinction between the physical input device and its ladder logic representation and note that an instruction in a ladder program is programmed independently of how the input device is wired Therefore the status of a N O pushbutton can be tested with a N C instruction and vice versa Fig 4 8 demonstrates all the possi ble combi nations and their results Also remember that when PLC instructions change state e g make a False to True transition a normally open instruction does not change to a normally closed instruction Where electromechanical rday contacts open and close PLC instructions test a memory location for a 1 or O Lower Gate controls real world devices
38. PLC s LED status indicators If power is properly applied to the PLC the power indicator should be On and there should be no fault indication Fig 6 1 If the PLC is not powering up properly the PLC may be faulty However remember that PLCs rarely fail But if they do fail it usually happens immediately upon powering up A PLC almost always functions either as designed or not at all they are designed not to run on a fault After making sure that the PLC has power verify communication with the PLC To do this use a Hand Held Programmer HHP ora PC running the PLC programming software If communication is possible the technician can assume the PLC is functional Place the PLC in a mode that prevents it from energizing its output circuits Depending on the make of the PLC this mode may be called the disable test scan or stop mode This mode permits the PLC to monitor input devices execute the program and update the output image file while keeping the output circuits de energized One at atime manually activate each input device Verify that the PLC s input status LEDs turn On and Off as expected Using the HHP or PC monitor the associated condition instruction to verify that the input device corresponds to the correct program address and that the instruction turns On and Off as expected If they do not operate as expected see the troubleshooting section of this chapter 7 Manually
39. Understanding and Applying Micro Programmable Controllers I tome Wy e re Understanding and Applying Micro Programmable Controllers MicroMentor Understanding and Applying Micro Programmable Controllers Solid state equipment has operational characteristics differing from those of electromechanical equipment Safety Guidelines for the Application Installation and Maintenance of Solid State Controls Publication SGI 1 1 Allen Bradley Company describes some important differences between solid state equipment and hard wired electromechanical devices Because of this difference and also because of the wide variety of uses for solid state equipment all persons responsible for applying this equipment must satisfy themselves that each intended application of this equipment is acceptable In no event will the Allen Bradley Company be responsible or liable for indirect or consequential damage resulting from the use or application of this equipment The examples and diagrams in this book are included solely for illustrative purposes Because of the many variables and requirements associated with any particular installation the Allen Bradley Company cannot assume responsibility or liability for actual use based on the examples and diagrams Reproduction of the contents of this book in whole or in part without written permission of the Allen Bradley Company is prohibited 1995 Allen Bradley Company Inc Printed in USA TM a T
40. aches an automated ticket machine at a gate The driver pushes a button on the ticket machine to receive a ticket The machine should not provide a ticket if the lot is full or the gate is up Removing the ticket raises the gate and turns on a green light After the car clears the gate the gate lowers and the green light shuts off The vehicle population is known at any time If maximum capacity is reached 500 cars a Full sign is illuminated the ticket machine will not provide a ticket and the gate will not raise An alarm sounds when the gate is obstructed Outputs Inputs Provide ticket Ticket request pushbutton i R Raise gate Ticket taken limit switch Lower gate Vehicle cleared gate photo sensor Garage Full sign Car departed garage photo sensor Green enter light Gate obstructed motor overload contact Alarm Gate up proximity sensor Gate down proximity sensor To control any machine or process first identify each action or control point Ask What action is the system controlling Then create a simple description of the conditions that control each action Start with the control point and work back to define the conditions inputs that produce the desired action Notice that each control point corresponds to an output on arung of the ladder program When carefully written the rules of operation convert easily to a ladder logic program as the parking garage example shows Rules of Operation
41. ain splashing and hose directed water and external icing A NEMA 4 enclosure is also rated for both indoor and outdoor installation e Enclosures do not protect against the internal condensation that can occur with temperature fluctuations To protect against condensation as well as extreme cold below 0 C consider installing some type of heating element in the enclosure Electrical Environment e Do not mount the PLC near high voltage equipment such as motors and arc welders as electrical interference could cause errors A properly grounded steel enclosure helps reduce electrical interference If possible do not locate the PLC on the same power feed as high frequency equipment such as inverters ac drives Power filtering may be required for dirty or noisy electrical environments Using a shielded twisted pair cable with the shield connected to ground at one end between field devices and the input terminals reduces the effects of high frequency disturbances Power e Follow the manufacturer s recommended procedures for wiring the PLC e Place the main power disconnect switch where operators and maintenance personnel have quick and easy access to it If the built in disconnect switch is mounted inside an enclosure make 81 i sure to install an externally panel mounted switch Mounting e Mount the micro PLC using the manufacturer s recommendations Generally mount the PLC to the back panel or sides of a
42. alarm Ladder Logic The logic used to perform this operation uses three rungs Note the use of the internal timer 54 4 used here as the flasher RUNG 0 e This rung latches the alarm state When motor 1 input 1 0 is over its temperature limit and the alarm reset button input 1 2 has not been pressed the control instruction B3 0 is turned On RUNG 1 e This rung flashes alarm light 0 0 when an alarm is present as indicated by the condition instruction B3 0 being On In this example S4 4 is an internal PLC address that cycles On and Off at 32 second intervals RUNG 2 e When the alarm condition is acknowledged by pressing the pushbutton 1 1 control instruction B3 1 is energized This address is also turned On as a condition instruction in rung 1 bypassing the flasher at address 54 4 and changing the state of alarm light 0 0 from flashing to steady Rung 0 Motor 1 Overtemp Alarm Alarm Signal Reset Latch 1 0 1 2 B3 0 a Alarm Latch B3 0 Rung 1 Alarm Alarm Latch Flasher Light B3 0 S4 4 O 0 Alarm Acknowledged Latch B3 1 Rung 2 Alarm Alarm Alarm Acknowledged Acknowledged Latch Pushbutton Latch B3 0 1 1 B3 1 Alarm Acknowledged Latch B3 1 1 If at any time the alarm condition is corrected the motor cools down 1 0 goes Off The alarm condition will be maintained until an operator acknowledges the alarm The alarm acknowledged pushbutton 1 1 must be pressed to unlatch B3 0 This in turn de energiz
43. any of the micro PLCs on the market that are capable of HSC operation have a number of modes for specific types of operation Up Down Up Down Quadrature Quadrature With External Hold amp Reset etc In addition many also support special commands that are intended to be used with the HSC such as high speed compares resets updates etc Ladder Logic RUNG 0 e The High Speed Counter instruction HSC in this rung contains all of the parameters required to define its mode of operation These parameters are selected when the instruction is programmed A separate High Speed Counter Load instruction not shown here is required if outputs control instructions are to be controlled directly by this high speed function The operation of a high speed counter instruction is very specific to the micro PLC used Consult the controller s user manual before using the instruction Rung 0 HSC HIGH SPEED COUNTER Type Encoder Res Hid Counter C5 0 High Preset 360 Accum 7 17 Two Stage Alter nator Uses This type of logic is used to alternate devices typically pumps in applications like the emptying of wells reservoirs and vessels tanks where the rate of flow into the tank is not constant Operation In an application like this two smaller pumps are frequently used instead of one large one Alternating pump operation Pump 1 as the 129 i primary then pump 2 as the primary reduces the maintenance requir
44. ated value of a timer or counter If retaining process and modify control programs Memory data is important for an is a physical space inside the CPU where application look for a micro PLC that offers 100 percent data retention Upon power loss and manipulated this type of PLC automatically logic controllers have programmable memory that allows users to develop the program files and data files are stored Memory types fall into two categories saves process data to the nonvolatile EEPROM volatile or nonvolatile Volatile memory can be easily altered or erased and it can be written to and read from However without proper backup a power loss can cause the loss of programmed contents The best known form of volatile memory is Random Access Memory or RAM RAM is relatively fast and offers an easy means to create and store users application programs If normal power is disrupted micro PLCs with RAM memory use battery or capacitor backups to prevent program loss However note that capacitors and batteries may fail Nonvolatile memory retains its programmed contents without a battery or capacitor backup even if power is lost The EEPROM Electrically Erasable Programmable Read Only Memory isa nonvolatile memory that has the same flexibility as RAM and is programmed through application software which runs on a personal computer or through a micro PLC s Hand Held Programmer 3 5 Data Memory and Addr
45. ative cost comparison of control methods ax Moderate xxx High Space and Cost System designers usually consider physical space and cost for components the two most important issues by far Many applications especially machinery have a small finite amount of space allocated for controls If an assembled control system occupies more space than allotted it often cannot be used because too many changes to the machinery would need to be made to accommodate it Once mounted on a panel a relay based control system typically occupies much more space than the equivalent control implemented with a micro PLC or SBC With micro PLCs available in the size of a brick and smaller only the simplest relay based system takes up less space With the control system for the parking garage requiring 13 I O and a counter a micro PLC or SBC are the most space efficient control solutions Several cost factors influence the selection of a control method including control system design and development costs for components assembly space and logic implementation e Control system design and development costs are incurred in the design of the system Fora relay system these costs are not applicable as the components have already been designed and produced Fora micro PLC these costs are not applicable because the PLC has already been designed and produced For an SBC costs involve securing the services of an electronic en
46. atus of LED does not matter If the LEDs indicate The Following Probable Cause 0 O O O Error Exists No Line Power CJ POWER CI RUN N t C FAULT power or FORCE power supply Power Supply error Overloaded If the LEDs indicate The Following Probable Cause 4 4 4 4 g g y Error Exists Processor Memory EE POWER Error CC RUN ma FAULT Hardware c FORCE Loose Wiring If the LEDs indicate The aaa Following Probable Cause A 4 A A Error Exists Hardware Software EE POWER Major Fault Detected CI RUN FAULT Application FORCE fault ASG 000000 the HHP greatly simplifies and speeds troubleshooting e If the output LED is On and the output device is not On test for power at the suspected output terminal If there is power at the output terminal the PLC is functioning 1f power is not present on the PLC output terminal the PLC has failed and must be replaced e Next test for power at the non functioning output device If there is power then the device is faulty and should be fixed or replaced If there is no power at the device then there is a blown fuse in the field wiring or another wiring fault between the PLC and the device If the PLC and output devices are functional examine the program a printout will be helpful or use an HHP and look at the rung s with the non functioning output s Determine what condition instructions inputs need to be True to enable activation of the outp
47. cal size 1024 a 1 O Count Complexity and Cost Fig 2 1 I O count is the most common method of categorizing PLCs Generally speaking micro PLCs share the following characteristics e lt 32 1 0 e Cost lt 500 e 1K of memory e Small size roughly 5 127 mm long x 3 76 mm high x 3 76 mm deep 16 1 0 8 203 mm long x 3 76 mm high x 3 76 mm deep 32 1 0 Micro PLCs come as self contained units with the processor power supply and I O all in one package Because they are self contained micro PLCs are also known as packaged controllers A modular PLC is one that has separate components that interconnect The advantage of A 32 I O packaged micro controller is considerably more compact than a 32 I O modular controller Relay logic instructions examine if closed normally open contacts examine if open normally closed contacts output energize coils output latch output unlatch one shot rising Timers on delay timer off delay timer retentive timer Up and down counters High s peed counter Math add subtract divide multiply clear square root Boolean logic AND OR Exclusive OR NOT Negate Compar ison lt gt 2 limit Data handling move masked move FIFO and LIFO First In First Out Last In First Out BCD to binary conversion binary to BCD conversion Application
48. ces use e What electrical codes apply In the parking garage example safety is a primary consideration because people physically contact the ticket machine By using 24V dc power for the input and output devices hazards to the user dramatically decrease However the gate controller selected for the parking garage requires devices capable of switching 120V ac such as dry contact relays Since people do not touch the gate controller in the normal course of Operation it poses a minimal hazard to users While it may be more convenient to use one voltage application requirements often dictate the need for different voltages If this is the case as with the parking garage isolate the different voltages from each other on separate commons Summarizing the electrical requirements for a control system in a chart facilitates organization For the parking garage example it looks like this Function inputs Ticket request Ticket taken Car cleared gate Car departed garage Gate obstructed Gate in up position Gate in down position Function outputs Ticket provided Full sign Green light Alarm Gate up Gate down Advanced functions Up counter Down counter Control system To be determined Device Pushbutton Limit switch Photoelectric sensor Photoelectric sensor Motor overload contact Proximity sensor Proximity sensor Device Solenoid Light Light Horn Gate controller Gate controller
49. cially if more than one machine needs rewiring Further documenting relay wiring logic changes requires drafting a new wiring diagram Because this task is so tedious and adds cost system changes can go undocumented In fact short of tracing every wire there is no way to ensure that the latest wiring diagram actually reflects the logic being executed by the system With an SBC based control system users typically cannot communicate with the microprocessor nor is there programming software available Logic changes are not easy to implement automated documenting capabilities do not usually exist and users typically cannot upload or download programs SBC based systems are difficult to troubleshoot because they rarely have troubleshooting features built into their software Users of these systems must go to the manufacturer for support because no one else understands the SBC operation PLCs offer considerably more flexibility Programming software facilitates relatively quick logic changes and permits the new program to be easily downloaded to multiple machines The program is always up to date and documentation is accomplished with the push of a button Troubleshooting help and diagnostic functions are a standard part of the software and can be conducted with the Hand Held Programmer as well see Chapter 6 PLCs are the easiest control system to support Assistance for programming and troubleshooting is available at reasonable costs
50. ction s address is On binary 1 If the PLC detects an On condition the instruction is True and has logical continuity For example a N O pushbutton PB1 is wired to input terminal 1 3 on the PLC The ladder logic program contains the following rung Fig 4 6 1 where I 3 is programmed as a N O instruction When PB1 is pressed On that On status is written to input image memory location 1 3 during the PLC s input scan When the rung containing the N O instruction with address 1 3 is scanned that instruction is seen as True and the PLC energizes output 0 4 during its output scan Input Input Terminal Status of Device on PLC Ladder Program output Output Ka 1 3 aa ba 1 3 0 4 ol iy u ee ee True True Fig 4 6 1 Normally open instructions When PB1 is released the Off status is written to address 1 3 The N O instruction is now False and the rung lacks logical continuity 4 6 2 During the PLC s output scan output 0 4 will be de energized Input Input Terminal Status of Device on PLC Ladder Program Output Output Terminal PB1 onPLC OFF 0 4 13 1 3 014 oo if o e False False Fig 4 6 2 Normally open instructions Normally Closed Instruction t A normally closed instruction examines the PLC memory for an Off condition i e it checks to see if the bit element at the instruction s address is Off or 0 If the PLC detects an Off condition the instruction is True and has logical continu
51. ction I 0 is True since a N C pushbutton is wired to input terminal 1 0 and that pushbutton has not been pressed and N O instruction 1 1 is False When the start pushbutton is pressed N O instruction I 1 becomes True energizing output 0 0 The True status of control instruction 0 0 is reflected in N O condition instruction O 0 which is programmed in parallel with the start instruction This keeps the output On even when 1 1 is no longer true When the stop pushbutton is pressed N O instruction 1 0 becomes False and the output is de energized Rung 0 Start the motor running by pressing the Start pushbutton Keep the motor running until the Stop pushbutton is pressed Wired Wired Normally Normally Closed Open Pushbutton Pushbutton Motor Starter Stop Start 1 0 1 1 0 0 Motor Starter 7 4 Flip Flop Circuit Push On Pus h Off Uses This circuit is used to provide a single change of state each time a new condition is detected The mechanical equivalent of this function would be a push On push Off pushbutton This type of logic can be handy for a wide range of miscellaneous uses such as alternators or memory circuits Operation Turn On and maintain an output with momentary pushbutton turn the output Off the next time the same pushbutton is pressed Ladder Logic The logic used to perform this consists of three rungs that make use of special instructions The logic also takes advantage of how the PLC scans the
52. ctromechanical operations Appendix C Instruction Execution Times Typical Instruction Execution Times and Memory Usage The table below lists the execution times and memory usage for controller instructions typically shown in a PLC Instruction Type Name Mnemonic Time needed to Time needed to Memory execute the execute as a usage instruction when True 1 statement user it is not True 0 approx psec words approx psec Application Specific Bit Shift Left BSL 19 80 53 71 5 24 x 2 00 position value Application Specific Bit Shift Right BSR 19 80 53 34 3 98 x 2 00 position value Application Specific Interrupt Subroutine INT 0 99 1 45 0 50 Application Specific Selectable Timer Interrupt Disable STD 3 16 6 69 0 50 Application Specific Selectable Timer Interrupt Enable STE 3 16 10 13 0 50 Application Specific Selectable Timer Interrupt Start STS 6 78 24 59 1 25 Application Specific Sequencer Compare SQC 27 40 60 52 2 00 Application Specific Sequencer Load SQL 28 12 53 41 2 00 Application Specific Sequencer Output SQO 27 40 60 52 2 00 Basic Count Down CTD 27 22 32 19 1 00 Basic Count Up CTU 26 67 29 84 1 00 Basic Examine if Closed XIC 1 72 1 54 0 75 Basic Examine if Open XIO 1 72 1 54 0 75 Basic One Shot Rising OSR 11 48 13 02 1 00 Basic Output Energize OTE 4 43 4 43 0 75 Basic Output Latch OTL 3 16 4 97 0 75 Basic Output Unlatch OTU 3 16 4 97 0 75 Basic Reset RES 4 25 15 19 1 00 timer cou
53. e B the accumulated value of counter C5 10 with the result being placed in the Dest destination N7 2 Rung 4 1 4 E MUL MULTIPLY Source A Source B RUNG 5 e When condition instruction 1 5 is on the PLC will enable the divide DIV instruction In this example the data in source A the accumulated value of counter C5 10 will be divided by the data in source B the constant 2 with the result being placed in the Dest destination N7 3 Rung 5 7 14 Sequencers Uses Many of the micro PLCs on the market today offer a command that replaces electromechanical devices called drum sequencers or drum switches These electromechanical devices were designed for simple control systems that required specific On or Off patterns of outputs that are continuously repeated A sequencer instruction can perform the same function as a drum switch but with more flexibility It is typically used for sequencing the operation of valves solenoids or lights 123 i for many varieties of machines or processes Operation Typically these instructions take the form of a single high level instruction A memory location is designated within the PLC that forms the pattern of the outputs during the sequence The table below illustrates this architecture Fig 7 14 The bit data file B3 0 through B3 3 contains the data for each step of the sequence controlled by the sequencer instruction The bit patterns t
54. e of the position parameter of the instruction As soon as the data is transferred this position value will point to the next position in the stack The size of the stack corresponds to the value programmed as the length parameter In this example the FIFO stack is 10 words long RUNG 1 When the logic preceding the FIFO Unload instruction changes from False to True data is retrieved from the stack In other words the oldest data the first in will be transferred to the destination register N7 51 Rung 0 1 0 FFL FIFO LOAD Position Rung 1 Wy FFU FIFO UNLOAD FIFO N7 50 Dest N7 51 Control R6 0 Length Position 7 16 High Speed Counter HSC Uses Many micro PLCs on the market today have the ability to detect and control high speed operations One of the most versatile features is a full function high speed counter This feature gives a micro PLC the ability to count a high speed input signal and control the corresponding outputs based on the accumulated count independent of the processor s scan This capability allows micro PLCs to be used in applications that previously required much larger PLCs Some of the micro PLCs available today even have the ability to modify what outputs will be controlled during the HSC s operation This functionality allows the micro PLC to replace cut to length controllers rotary cam switches programmable limit switches and other mechanical devices Operation M
55. e source data here a variable into the preset value of a counter instruction Perform 3rd Data Move 1 2 RUNG 3 e This rung is simply used to clear the data from the working register Whenever condition instruction 1 3 is energized data is cleared from Destination N7 20 Rung 3 Clear all data at the Destination dest address Clear N7 10 1 3 7 12 Comparing Data Uses PLCs can monitor and take action based on numerical values Operation In many instances devices may need to be controlled when they are less than equal to or greater than other data values or set points used in the application like timer and counter values Comparison instructions are always programmed as condition instructions Ladder Logic The logic used in this application consists of 6 sample rungs RUNG 0 and RUNG 1 e Rung 0 uses pushbutton 1 0 to increment a counter C5 10 Rung 1 uses pushbutton 1 1 to reset the counter These rungs simply setup some data values to use in the following rungs Rung 0 1 0 Pushbutton ya nee nag to advance increment counter 10 the data 1 0 Rung 1 Counter 10 Reset Pushbutton is used to to clear increment counter 10 the data 1 1 C5 10 e res RUNG 2 e This rung contains a Less Than instruction The LES will turn On the control instruction 0 0 whenever the data in source A the accumulated value of counter C5 10 is less than the data in source B a constant 3 O 0 L
56. ed memory usage is only an estimate Actual memory usage can vary by 10 to 15 An example set of instruction memory usage is provided in Appendix C Index address 24 25 38 76 79 84 86 91 92 142 AND logic 12 44 46 97 142 application memory 22 142 auxiliary holding contact 47 48 76 BASIC programming language 34 142 basic instructions 161 battery backup 23 142 BCD 10 12 24 142 Boolean 12 34 143 branching instructions 46 48 bus 35 143 C programming language 34 143 communication with programming device 31 communication with operator interface 5 31 comparison instructions 44 117 161 condition instructions 42 44 46 75 76 143 contact symbology 39 143 149 control instructions 37 38 44 46 144 counter instructions 111 113 128 129 CSA 62 144 EEPROM 22 23 145 EMI 27 28 145 FET 21 68 69 145 floating point decimal 24 function blocks 22 43 44 106 108 110 120 GM 2 64 Gray code 24 hexadecimal 24 HHP fault codes 91 92 HHP use in programming 28 30 HHP use in troubleshooting 30 85 88 91 high speed counter 14 69 70 128 129 IEC 146 input device 18 19 43 84 85 90 146 153 158 input scan 22 26 40 41 146 installation 62 63 80 82 instruction set 12 14 96 161 163 integer 24 114 116 IP 59 80 146 ladder logic 4 36 44 49 70 72 146 languages programming 3 4 30 34 142 143 latched output 76 146 limit switch 19 55 57 147 155 logical continuit
57. ed on the individual devices and provides more reliable operation In addition the secondary or standby pump is available if the rate of water entering the vessel is more than the first pump can handle If this situation occurs the second pump will also turn On and assist the primary pump The triggers for these events could be analog signals or simple discrete inputs float switches etc This illustration shows a typical application with float switches in a tank Fig 7 17 130 Fig 7 17 Ladder Logic The logic used in this application consists of 4 rungs RUNG 0 AND RUNG 1 e These two rungs form a flip flop circuit as described in the example in section 7 4 Each time the fluid in the tank reaches the low level float switch F1 1 0 the alternator bit in rung 1 B3 2 changes state The status of this bit determines which pump will be the first to turn On Rung 0 Low Level Low Level F1 Pulse 1 0 B3 0 B3 1 itag Rung 1 Low Level Alternator Bit Alternator Bit Pulse Pulse On Pulse Off Pulse On Pulse Off B3 1 B3 2 B3 2 1 E Alternator Bit Low Level Pulse On Pulse Off Pulse B3 2 B3 1 RUNG 2 e This rung controls the operation of pump 1 0 0 If the low float 1 0 is On and the alternator bit B3 2 is Off and the level in the tank has reached the First Call float F2 1 1 this pump will be the first one energized If B3 2 is On pump 1 will be the second pump energized Rung 2 Alternator
58. equest is made and also to bring other pumps on line as demand increases Operation A series of five float switches are used to monitor the level of fluid in the tank Fig 7 18 The control system monitors these float switches and determines which pump is the primary pump lag pump 1 and lag pump 2 Input Device Status Pump Requirements Float Switch 1 Off All pumps off Float Switch 1 On None Float Switches 1 2 On Primary pump On Float Switches 1 2 amp 3 On Primary and Lag 1 pumps On Float Switches 1 2 3 amp 40n Primary Lag 1 and Lag 2 pumps On Float Switches 1 2 3 4 amp 5 On ALARM condition Whenever the primary pump is needed called the control system will then rotate the assignment of the primary pump This ensures even wear between all three pumps and verifies that each pump is operational As each pump is designated as the primary the remaining lag pumps will also be rotated A breakdown of priorities for each pump at any given time is included here The sequence for the running of each pump is called a stage There are three pumps and therefore three stages that operate as follows Stage Pump 1 Pump 2 Pump 3 1 Primary Lag 1 Lag 2 2 Lag 2 Primary Lag 1 3 Lag 1 Lag 2 Primary Fig 7 18 13 Ladder Logic The logic used in this application consists of 15 rungs The EQUAL TO comparison instruction at the start of the first ten rungs compares the accum
59. ergized its contacts open to de energize all application I O devices memory The part of the controller where programs and data are stored mnemonic An easy to remember term that is used to represent a complex or lengthy set of information modular controller Programmable controller in which the power supply processor and I O interfaces reside in separate units or modules Compare with packaged controller NEMA Standards Standards for the performance and construction of electrical equipment that have been agreed upon and approved by the members of the National Electrical Manufacturer s Association NEMA normally closed contact A switch or relay contact pair that is closed when the switch or the coil of the relay is not activated and open when the switch mechanism or coil is activated Compare with normally open contact normally closed instruction A ladder program symbol that will allow logical continuity flow if the referenced address is Off Compare with normally open 147 instruction E us normally open contact A switch or relay contact pair that is open when the switch or the coil of the relay is not activated and closed when the switch mechanism or coil is activated Compare with normally closed contact normally open instruction A ladder program symbol that will allow logical continuity flow if the referenced address is On Compare with normally closed instruction one shot A programming instruction that t
60. erial communication interface rung Ladder logic is comprised of a set of rungs A rung contains condition input ia p and control output instructions j iso SBC Single Board Controller A custom control solution using a proprietary electronic circuit board designed to control one specific application scan time The time required to read all inputs execute the control program and update all outputs sequencing Using a software device to initiate or terminate events in a desired sequence solenoid A device that transforms electrical current into linear mechanical motion it consists of one or more electromagnets that move a metal plunger The plunger is sometimes returned to its original position after excursion with a spring or permanent magnet solid state Circuitry designed using only integrated circuits transistors diodes etc no relays or other electromechanical devices are used software 1 The ladder logic program stored in the PLC 2 Executable programming package used to develop ladder logic programs Compare with hardware system A set of one or more PLCs that together with I O devices computers associated software peripherals terminals and communications networks provide a means of performing information processing for the control of machines or processes system memory The total memory space within the controller including the user program data and the operating system terminal A po
61. es alarm light 0 0 7 6 Start Stop with J og Program Uses Use this logic to start a device with a momentary input or to jog the device with a separate input Operation Turn On an output with a momentary input and keep it On until instructed to turn it Off Or turn On an output whenever the jog pushbutton is pressed If the jog pushbutton is released the output must turn Off Ladder Logic The logic used to perform this consists of two rungs with three conditional devices programmed in series and parallel RUNG 0 e This is the start stop rung It operates in the same manner as in the start stop example in section 7 3 except that instead of energizing an external output address internal bit B3 0 is energized when start pushbutton I 1 is pressed RUNG 1 e This is the rung that controls the actual output address 0 0 If bit B3 0 has been energized in rung 0 the output is energized If B3 0 has not been energized the output can be jogged by pushing the jog pushbutton 1 2 Every time 1 2 is pressed motor 0 0 turns On Rung 0 Wired Wired Normally Normally Closed Open Pushbutton Pushbutton Motor Run Internal Status Stop Start 1 0 1 1 B3 0 Motor Run Internal Status B3 0 Rung 1 Motor Run Internal Status Motor B3 0 PS Jog Pushbutton 1 2 7 7 On Delay Uses This logic turns On a device after a programmed time delay Operation The On delay can be programmed to delay activation of a control ins
62. etentive timers are very useful for keeping track of the amount of time a device has been On This can be very helpful for tracking device maintenance or other run time type requirements Retentive timers are reset using a separate instruction that is used to clear a timer The instruction is called reset RES and is programmed as a control instruction 7 10 Up Down Counting Uses Up Down counters are often used to monitor and track materials in conveying packaging systems An example is a bottle labeling application where the bottle making machine produces bottles at a greater rate than the labeling machine can apply labels One method for compensating for the difference in production rates is to add a buffer area where the bottles can stack up to await labeling Operation A counter is used to track how many bottles are in the buffer The counter increments its count when a bottle enters the holding area from the bottle making machine and decrements each time a bottle exits the holding area When the holding area is full a signal can be sent to the bottle making machine to stop producing bottles Ladder Logic The logic used in this application consists of 4 rungs RUNG 0 e This rung contains a count up instruction with an address of C5 0 Each time the limit switch wired to the input terminal 1 0 is activated condition instruction 1 0 changes from False to True and the counter increments by one count Rung 0 Count up Li
63. ethod is to have a holding area into which the PLC can redirect the parts As each part is sent into the area the color ID is loaded into a FIFO stack When the paint booth returns to operation the PLC will draw a part out of the holding area and track the identifier with it This i assures that the part will get the correct color of paint Operation FIFO commands typically take the form of two high level instructions FIFO Load FFL and FIFO Unload FFU These instructions are used in pairs The FFL instruction loads words into a user created group of registers called a FIFO stack The FFU instruction unloads words from the FIFO stack in the same order as they were entered Fig 7 15 FIFO Stack Position Ng 0o CU N753 J 1 Source COO N754 2 Register Na O TU N7565 4 CT Nz 5 Destination CU N58 6 Register E3 N59 af E CU Nze 8 amp 8 c E Ns 9 Fig 7 15 The unique feature of the FIFO stack is its ability to manage where the data is This is done by tracking where data is entered into the stack The FIFO instructions manage all aspects of entering and removing data from the stack Ladder Logic RUNG 0 e This rung controls the transfer of data to the FIFO stack When the logic preceding the FIFO Load instruction changes from False to True the data located in the source register N7 50 is stored in the next available location in the stack This location is designated by the current valu
64. from many sources And if a PLC fails a replacement PLC can be purchased off the shelf from the nearest industrial electrical Ka i supplier there is no need to wait for a shipment from the factory Furthermore the ruggedness of PLCs compared to SBCs gives them a definite advantage in harsh environments or when durability is a primary consideration Selecting the Micro PLC For all criteria by which control systems are evaluated cost size flexibility and supportability micro PLCs provide the user with distinct advantages over other control options for many control applications Thus a micro PLC has been selected to provide the logic control for the parking garage 5 3 What are the PLC Specifications After determining application requirements and selecting a method for providing system control the next step is to determine specifications for the control system When determining PLC specifications identifying application requirements in certain categories can be helpful Categories that typically need to be considered are e Total number of I O e Electrical requirements e Output circuits Memory requirements e Speed of operation e Communication e Operator interfaces 1 0 Total To determine a PLC s I O requirements examine the application requirements to determine how many input and output devices the PLC needs to monitor and control Reviewing the I O requirements for the parking garage a PLC for this app
65. ge is Full C5 10 O 3 F DN Rung 8 Vehicle Photo Number of Sensor Vehicles Departing Garage in Garage 1 3 CTD E Count Down CD Counter C5 10 DN Preset 500 Accum 0 R 9 Ha Gate is Alarm Obstructed Sounded 1 4 5 5 Installation Requir ements A PLC user s manual contains detailed installation instructions pertinent to that particular model and they should be followed carefully As with any product being installed proper planning assures smooth start up When installing micro PLCs consider the physical and electrical environments and requirements for power mounting and wiring The following are some suggestions for installing PLCs Physical Environment Whether the micro PLC is mounted within a machine or in a separate enclosure it requires protection against temperature extremes humidity dust shock vibration or corrosive environments e Be careful about locating the PLC in an enclosure with other heat generating sources 55 C is the maximum ambient operating temperature for most micro PLCs Ensure sufficient ventilation and space between components Install a fan to help circulate the air if necessary e Installing the PLC ina NEMA Type 12 IP 60 enclosure provides protection against dust falling dirt and dripping noncorrosive liquids A NEMA 12 enclosure is rated for both indoor and outdoor installation e Installing the PLC in a NEMA Type 4 IP 65 enclosure provides protection against windblown dust and r
66. gineer to design the board and test its viability unlike relays and PLCs SBCs are not typically available off the shelf Note Many installations require the control system to meet global industrial standards such as UL CE or CSA PLCs usually have been certified to meet those standards where relay and SBC based systems typically are not Component costs are for the control related hardware Costs also include receiving inventory and the quality control of the components Fora relay system this includes relays mechanical timers and counters Fora micro PLC all necessary hardware is packaged in the PLC Foran SBC this includes the board its components and circuitry Assembly costs cover putting the components together so they are usable For a relay system this includes mounting components on a panel and wiring the logic power Fora micro PLC the only assembly costs are for mounting the unit to a panel with screws or on a DIN rail Foran SBC this involves securing a manufacturing facility to produce it For this reason SBCs become economically viable only in high volume or very unique applications Panel space costs include the size of the panel and the enclosure needed to house the control system The larger the enclosure the greater the material costs for it Fora relay system with many components size could be prohibitive 63 R For a micro PLC size is minimal For an SBC size is usua
67. h the controller Do this by using a Hand Held Programmer HHP or aPC running the PLC programming software If communication is possible assume that the PLC is functioning properly and investigate field devices field wiring and field power 6 4 Troubleshooting 1 0 If attempts to re start the PLC fail to solve the problem and the PLC has power most technicians start troubleshooting at the outputs and work backwards This is usually the quickest and most efficient procedure Typically operators or technicians first notice a problem when an action output fails to occur Begin troubleshooting by examining the output LEDs Note Using Understanding LED Status Most micro PLCs have LEDs to indicate I O status On Off and to indicate if the controller has power is running has faulted and if a force exists Between the time power is applied to the controller and the time it has to establish communication with a connected programming device the only form of communication between the technician and the controller is through the LEDs When powered up When placed in RUN 0000000 0000000 mx POWER ma POWER CI RUN HE RUN FAULT CJ FAULT CI FORCE ZZ FORCE UUUOUU HOUUUU Refer to the following key to determine the status of the LED indicators CT Indicates the LED is OFF E Indicates the LED is FLASHING HMM Indicates the LED is ON St
68. h the same way that a bat navigates during flight They are typically used to detect the level of materials Output Devices Output devices are field devices used to carry out the control instructions for the PLC Think of them as the hands and feet of the PLC The micro PLC is capable of activating a large variety of output devices Output voltage and current characteristics of the PLC are the only limiting factors for output device application The following is a listing of the most popular output devices Lamps Lamps are used to indicate status of an operation or to warn of undesirable or dangerous conditions Lamp color can be used to differentiate functions or parameters Lamps come in several different types incandescent flourescent neon and LEDs Supply voltage and current lamp life and cost are the three variables which determine which lamp is best for a given application Audible Alarms Audible alarms are available in the form of horns buzzers bells chimes capacitive alerters and even synthesized voice modules All may be used in the control process to alert the machine operator to a condition or event Relays Electromechanical relays use a low amperage control signal to electromagnetically engage a set of contacts This set of contacts is used to switch a current that can be much higher than the original control signal In a similar fashion semiconductor devices like transistors FETs triacs
69. hat are stored in each of these locations form the output pattern that will be seen for each of the sequencer steps Bit Address gt B3 4 B3 3 B3 2 B3 1 B3 0 STEP WORD pooog o B3 1 1 e2 brod te omms eof bai _ a J Output Address gt 0 4 0 3 O 2 O 1 0 0 Fig 7 14 Ladder Logic RUNG 0 e The sequencer instruction typically looks like this A single instruction that identifies where the output pattern data is stored B3 0 the destination or address of that output data and the length or number of steps of the sequence This instruction also manages or tracks what the current sequencer position is Each time the conditional logic preceding the instruction changes from F alse to True the sequencer will increment to the next step Rung 0 1 0 FFL FIFO LOAD EN Source N7 50 DN FIFO N7 52 HEM Control R6 0 Length Position 7 15 FIFO First In First Out Uses FIFOs are part of a special set of commands that deal with storing numeric data These commands are primarily used in tracking products and materials during processes An example would be an overhead conveyor system that feeds parts into a paint booth Each part requires a different color and the color ID is tracked while the part is moved through the manufacturing process If the conveyor is running and a problem occurs in the paint booth the parts need to be stored until the paint booth is back on line One m
70. he machine for production and status reports out of spec or faulty parts count total parts count production rates and machinerun time which is valuable for periodic maintenance operations Further PLCs can communicate this data to other control equipment or to operators in remote locations Application Required Quantity Can the control method accomplish task Characteristic Relay PLC SBC Inputs Yes 7 Yes Yes Yes Outputs Yes 6 Yes Yes Yes Timers No 0 Yes Yes Yes Counters Yes 1 up down Yes Yes Yes High speed required No 0 No Yes Yes Data calculations No 0 No Yes Yes Data acquisition No 0 No Yes Yes Communications No 0 No Yes Yes Operator interfaces No 0 No Yes No typically Fig 5 1 Comparison of application requirements and control options As Fig 5 1 shows all three control methods can accomplish the task so selecting a control method cannot be based on application requirements alone However this does not mean that all three methods provide the optimum solution To differentiate between control methods evaluate the relative cost impact of each method using the following criteria Criteria Relays Micro PLCs SBCs System design and development Not applicable Not applicable ORK Control system hardware Kk kpo Panel assembly kkk Panel space Kk x Implementing logic kkk kk kkk Duplicating application ORK x Documenting logic KK kk Modifying logic ORK Kk Maintenance kkk k x Low Fig 5 2 Rel
71. he micro symbol is a trademark of Allen Bradley Company Inc a Rockwell International company Page PO CIC way van cere hy eee AA ee vii Chapter 1 Introduction to PLCs 1 0 History of PLOS tai kA kam KANG 2 1 1 Why Use a PLOT pasakan Luna duos a 4 1 2 Traditional PLC Applications 5 Chapter 2 The Micro PLC 2 0 Development of the Micro PLC 8 2 1 What Makes a Micro PLC a Micro 10 2 2 Capabilities Overview 13 2 3 Micro PLC Applications 14 Chapter 3 Micro PLC Oper ation 3 0 Components Overview 18 SMSI puta ee 18 EN Outpulppa na A 20 3 3 Central Processing Unit CPU 21 3 4 Types of Application Memory 22 3 5 Data Memory and Addressing 23 3 6 Operating Cycle 24 3 7 Power Supplies sanaaa 26 3 8 Programming Devices 28 3 9 Operator Interfaces 30 o Chapter 4 Ladder Logic Fundamentals 4 0 Programming Languages 34 4 1 Electrical Ladder Diagrams 35 4 2 Ladder Logic Programs 36 4 3 Ladder Logic Instructions 39 4 4 Combining Instructions 44 4 5 Program Execution 49 Chapter 5 How to Apply a Micro PLC 5 0 What is a Potential Control Application 52 5 1 What are the Application s Requirements 53 5 2 Selecting a Control Method 60 5 3 What are the PLC S
72. he system The power supply then instructs the processor to execute a controlled shut down which saves the user s program and data in memory Another factor affecting the function of the PLC is electromagnetic interference EMI or electrical noise While PLCs are more rugged than most electronic equipment especially the PCs or single board controllers sometimes used instead of PLCs EMI may still be a problem If so the PLC should be electrically isolated by installing an isolation transformer Until recently all micro PLCs operated on 24V dc However several micro PLC manufacturers now offer products that operate on either 120V ac 220V ac or 24V dc This gives the user the option of selecting the voltage that best suits the application For example if ac power is used on other parts of the machine actuators for example a micro PLC that can accept ac power may eliminate the need to install adc power supply 3 8 Programming Devices When entering a program into a micro PLC the two devices most commonly used are a personal computer PC and a Hand Held asa T M me a ee te 7 ik mpi oe Most users create their programs with software run on a PC Na f Plant technicians ang Cif value Hand Held shown actual size because of their E portability F Hi F NO ruggedness and Hit 105 ED ang Hi SOP troubleshooti ng capabilities OUT SET E GIAO 9800
73. ider accessibility for maintenance troubleshooting or reprogramming 59 i If the control system for the parking garage is located in the ticket machine it needs to be housed in an enclosure to protect it against moisture and dirt Considering that outdoor temperature extremes may exceed the control system operating temperature the enclosure may also need temperature and condensation controls See the section on Installation Requirements later in this chapter for further environmental considerations 5 2 Selecting a Control Method Once application requirements have been defined the next step is determining which type of control method can accomplish the task As noted at the start of this chapter system designers can select from three types of control systems relays PLCs or SBCs To help determine which control method is best suited for the task develop a chart which integrates application requirements with control methods The following chart Fig 5 1 has been filled out for the parking garage example PLC Advantages While relay based control systems can perform some advanced functions typically timing and counting with limited sequencing a wide range of higher leva instructions can only be performed by PLCs or SBCs The data acquisition and communication capabilities of PLCs also deserve special mention as they far exceed the capabilities of traditional raays PLCs can gather information from t
74. imer with a time base less than 1 second Ladder Logic The logic used in this application consists of 2 rungs RUNG 0 e This is the timing rung Notice that the condition instruction that controls the timer is the done bit of the timer address T 4 0 DN Since this condition instruction is normally closed it will have logical continuity when the timer is not done that is when the accumulated value is less than the preset value Therefore prior to the preset time being reached the timer times Once the preset value is reached the normally closed instruction becomes False and the timer resets to zero on the next scan of the program The normally closed instruction is now True and the timer begins timing from zero RUNG 1 e This rung contains a counter instruction The condition instruction that controls this counter is the done bit from the timer in the previous rung In this case it is a normally open instruction As soon as the accumulated time of the timer in rung 0 reaches 60 seconds the preset value of 60 using a time base of 1 second the done bit energizes and increments the counter Rung 0 One Minute Timer Done Timing ae T4 0 DN TON TIMER ON DELAY Timer T4 Time Base 1 Preset 6 Accum Rung 1 T4 0 DN CTU COUNT UP Counter C10 1 Preset 5 Accum Retentive Timers Timers are available that retain their time when the conditions preceding the timer instruction are False open R
75. iming Notice that the time base in the timer function block reads one second This means that the timer will time in one second increments Also notice that the preset value reads 10 This means that the timer will be done timing after 10 one second increments have passed for a total delay of ten seconds The timer done bit T4 0 DN will be energized at this point This will de energize the normally closed instruction T4 0 DN in rung O turning Off the output See the example in section 7 7 for a more thorough description of timer operation Rung 0 Wired Wired Normally Normally Closed Open Pushbutton Pushbutton 10 Second Timer Done Motor 1 Stop Start 1 0 1 1 T4 0 DN O 0 Ib A Motor 1 O 0 Rung 1 10 Second Motor 1 Timer O 0 TON TIMER ON DELAY Timer T4 0 Time Base 1 0 Preset 10 Accum 0 7 9 One Minute Clock Uses This is an example of a repetitive or free running clock Operation In this example the clock interval is set for 1 minute but any interval could be selected If a different time interval is required simply change the value in the preset location in the timer function block As discussed in the sections on On and Off Delays the resolution of the clock will be determined by its time base In this example the timer is programmed with a 1 second time base so the timer will only be capable of timing accuracies greater than or equal to 1 second If a more accurate time is required then use a t
76. int on a PLC where external I O devices such as a pushbutton or pilot light are wired throughput The amount of time it takes to sense an input and energize the corresponding output thumbwheel switch A rotary switch used to input numerical information into a controller time base The unit of time used by a timer to register events A one second time base is accurate to the nearest second Many controllers are capable of operating with 01 or 001 second time bases transistor A solid state electronic device that functions as an electrically controlled switch commonly used to control dc loads A component of dc output circuits triac A solid state electronic device that functions as an electrically controlled switch for ac loads A component of ac output circuits True The status of an instruction that provides logical continuity on a ladder rung UL Underwriters Laboratories An agency that recommends minimum specifications for the construction and operation of electrical equipment used in the United States UL also tests equipment to determine adherence to those specifications watchdog timer A timer that monitors the logical operations within the circuitry of the processor If the timer ever times out it indicates that there is a problem with the normal operation of the processor and operation is terminated word A unit of memory composed of 16 individual bits Words or portions of words are used when programming instruc
77. interface devices or peripherals can be connected These are not programming devices but graphic or alphanumeric displays and control panels that consolidate all the functions of traditional operator interface devices into a single panel These interfaces can output data and display messages about machine status in descriptive text Motor 1 On display parts count and track alarms They can also be used for data input By providing better and more easily conveyed information these interfaces decrease the need for operator training on machine operation and reduce system component and installation costs These products communicate with the PLC through an RS 232 communications port This opens up I O points which can be used for sensors and output devices and enables a micro PLC to control a more complex machine or process Ladder Logic Fundamentals Programming Languages 4 0 Electrical Ladder Diagrams 41 Ladder Logic Programs 4 2 Ladder Logic Instructions 4 3 Combining Instructions 4 4 Program Execution 4 5 4 0 Programming Languages A program is a user developed series of instructions or commands that direct the PLC to execute actions A programming language provides rules for combining the instructions so that they produce the desired actions The most commonly used language for programming PLCs is ladder logic In fact more PL
78. is reduces programming time minimizes debugging and increases reliability With all the logic existing in the PLC s memory there s no chance of making a logic wiring error The only wiring required is for power and inputs and outputs Flexibility Program modifications can be made with just a few key strokes OEMs original equipment manufacturers can easily implement system updates by sending out a new program instead of a service person End users can modify the program in the field or conversely OEMs can prevent end users from tinkering with the program an important security feature Advanced functions PLCs can perform a wide variety of control tasks from a single repetitive action to complex data manipulation Standardizing on PLCs opens many doors for designers and simplifies the job for maintenance personnel Communications Communicating with operator interfaces other PLCs or computers facilitates data collection and information exchange Speed Because some automated machines process thousands of items per minute and objects spend only a fraction of a second in front of a sensor many automation applications require the PLC s quick response capability Diagnostics The troubleshooting capability of programming devices and the diagnostics resident in the PLC allow users to easily trace and correct software and hardware problems 1 2 Traditional PLC Applications No matter what the application the use of
79. ity Input Input Terminal Status of Device on PLC Ladder Program Output Output Terminal ON PB1 on PLC 4 1 4 0 5 oe T a o o Q m oeo True True Fig 4 7 1 Normally closed instructi ons For example a N O pushbutton PB1 is wired to input terminal 1 4 on the PLC The ladder logic program contains the following rung Fig 4 7 1 where 1 4 is programmed as a N C instruction When PB is not pressed Off that Off status is written to input image memory location 1 0 during the PLC s input scan When the rung containing the N C instruction with address 1 0 is scanned that instruction is seen as True NOT On and the PLC energizes output 0 5 during the output scan When PB1 is pressed the On status is written to address 1 4 The N C instruction is now False and the rung lacks logical continuity Fig 4 7 2 During the PLC s output scan output 0 5 will be de energized 41 Input Input Terminal Status of Device on PLC Ladder Program Output Output Terminal OFF on PLC PB1 Kg 1 4 015 ve VI o _e False False Fig 4 7 2 Normally closed instructions Input Input Terminal Device on PLC PB1 5 hb N O Pushbutton Not Activated PB1 0 Oo o PB1 N O Pushbutton Activated B olo 10 N C Pushbutton Not Activated N C Pushbutton Activated Fig 4 8 Condition instructions and their results Output Energize Instruction Output Terminal Status of Ladder Program on PLC
80. lication requires seven inputs and six outputs Note When determining I O total many people add an extra 10 for unanticipated I O needs as well as future changes to the control system Electrical Requirements To determine a PLC s electrical requirements consider the voltage and current requirements for the PLC incoming power each output and the inputs Until recently micro PLCs operated on 24V dc only This limitation often necessitated installing a dc power source especially when the other control system components operated on 120V ac Newer micro PLCs however offer users standard voltage options 24V dc 120V ac or 240V ac For the parking garage a PLC using 24V dc may be the best choice for the stated safety reasons However if the PLC is not located inside the ticket machine using 120V ac may be acceptable Isolated relays Shared common relays vac vas DC DC NOT VDC 0 0 VDC 0 1 24V 0 2 0 3 0 4 0 5 Of 0 7 0 8 0 9 O 10 O 11 24V USED Output terminal strip Fig 5 3 Diagram of a micro PLC s output terminal Note that output terminals O 0 and O 1 are isolated relays For applications requiring control of different output voltages the PLC selected needs to have isolated output terminals to keep the voltages separated Note Power from different sources or of different voltages must be isolated from each other In the parking garage example the 120V ac gate controller signal must be isolated fr
81. lly minimal Logic implementation costs relate to the installation of the logic into the control system assuming costs for developing the logic are similar for all three control methods For a relay system implementing logic involves wiring the components together Each subsequent application requires the same amount of labor to assemble debug and adjust timer and counter presets Fora micro PLC costs include purchase of programming software or a Hand Held Programmer Programming a subsequent application only requires downloading the program there are no program debugging costs for duplicate applications However users still need to commission each control system see Chapter 6 Foran SBC costs involve retaining an electrical engineer to program a microprocessor Programming each subsequent application typically requires copying a memory chip there are no program debugging costs for duplicate applications Commissioning is also required Future Costs Total costs for a control system don t end after implementation After system start up it may be necessary to modify the control logic document system changes and troubleshoot the system With a relay based system re wiring costs associated with logic changes can be extraordinarily high it was just this type of situation that prompted General Motors to call for PLC development in the first place The labor involved with relays can be intensive and costly espe
82. lowing safety measures when repairing the system e Disconnect the power to the whole system while making repairs and make sure there is no chance of someone inadvertently reconnecting the power e Make sure that no system elements can be harmed if and when the system is restored to working order e Some applications require all system components field devices to be in a start position this is often due to mechanical considerations Before bringing a control system back on line know the system requirements e After making repairs ensure that the system works properly to the extent that operators and bystanders are not jeopardized by system operation This may include partial or full testing of the system 6 8 Troubleshooting Model In addition to becoming familiar with all of the troubleshooting tools and techniques available it s important to develop a troubleshooting routine The following error recovery model Fig 6 2 demonstrates a common routine for troubleshooting hardware and software problems After expending all reasonable efforts to restore the PLC to proper operation call your distributor or manufacturer Good distributors and manufacturers employ skilled technicians and engineers who can provide assistance often over the phone 94 is the error PLON No related Fault LED On Yes Using the HHP or programming software identify the error code and description Refer to the user manual
83. measurable quantities such as temperature weight pressure etc Compare with digital AND A Boolean operation that produces a True output only when all conditions are True and a False output if any condition is False application A machine or process that requires a control system for operation application memory The portion of the total system memory dedicated to storage of the application program and associated data BASIC Beginner s All Purpose Symbolic Instruction Code a versatile easy to learn computer language commonly used for simple programming tasks battery backup A battery or set of batteries that provide power to maintain the contents of processor memory in case of a system power outage Note Processors utilizing EEPROM memory typically do not require battery backup BCD Binary Coded Decimal A binary system in which each decimal digit from 0 to 9 is represented by four binary digits bits A thumbwheel switch is usually a BCD device and when connected to a programmable controller each decade or decimal place requires four wires binary A numbering system using only the digits 0 and 1 Also called base 2 bit The smallest storage location in memory A bit contains either a 1 On True or a 0 Off F alse Boolean operators Logical operators such as AND and OR that can be used singly or in combination to form logical statements or circuits These statements must have an output response which is either Tr
84. mit Switch Ko CTU RUNG 1 e This rung contains the count down instruction Notice that it has the same address as the count up instruction in rung 0 C5 0 Each time the limit switch wired to terminal 1 1 is activated condition instruction 1 is made True and the counter decrements by one count It is important to note that any number of condition instructions can be on the rung that controls a counter instruction Anytime the status of the rung goes from False to True an up counter instruction will increment and a down counter will decrement by one count Rung 1 Count Down Limit Switch 1 1 RUNG 2 e This is the rung that controls the output 0 0 When the number of counts accumulated in the counter equals or exceeds the counter s preset value the done bit C5 0 DN is energized turning On output O 0 Rung 2 Counter at Stop Preset Bottle Machine C5 0 DN 0 0 RUNG 3 e This is the reset rung When the condition instruction 1 2 comes On the accumulated value of counter C5 0 is reset to zero Rung 3 Reset Counter Counter Reset 1 2 C5 0 RES 7 11 Moving Data Uses One of the most useful and versatile features a PLC has is its ability to move and manipulate data This ability turns the PLC into a powerful processing platform capable of changing data values in integer files timers counters stacks and many other areas Moving data is done for control purposes or to simply better organize informatio
85. n Operation To move data in a PLC is a simple command Move data from point A to point B The structure is easy to understand and troubleshoot Ladder Logic The logic used in this application consists of 4 rungs the first three rungs illustrate actual move commands while the third is used to clear one of the destination registers RUNG 0 e This rung demonstrates moving a constant to an integer location Whenever condition instruction I 0 is energized the PLC will move the data 1234 in the Source location to the Dest destination location Integer location N7 10 An integer location is a specific word where the data is stored The data in the source location may be either a constant or an address internal to the PLC Rung 0 Press input 1 0 to move the source data here a constant to the Dest destination Perform 1st Data Move 1 0 RUNG 1 e This rung demonstrates the moving of data from one integer location to another Whenever condition instruction I 1 is energized the data at N7 10 Source will be moved to N7 20 Destination Rung 1 Move the source data here a variable into the destination Perform 2nd Data Move 1 1 RUNG 2 e This rung demonstrates the moving of data from an integer location to the preset value of a counter Whenever input instruction 1 2 is energized the data at N7 20 Source will be moved to the counter preset C5 10 PRE Destination Rung 2 Move th
86. n enclosure not the top or bottom using either a DIN rail or mounting screws Be sure to provide proper ventilation e Do not exceed the shock and vibration specifications published by the PLC manufacturer Avoid sources of high vibration Use cushioned mounting if necessary e Allow enough clearance between the door and the components Consider using documentation pockets which often are affixed to the inside of the door Wiring e Allow at least 2 in 50 mm between I O wiring ducts or terminal strips and the PLC for ease of access during installation and maintenance e Do not run signal or communication wiring and power wiring in the same conduit Wires with different signal characteristics should be routed along separate paths e Follow manufacturer s grounding instructions carefully e Inductive output devices such as motor starters and solenoids may require surge suppression to protect the PLC output contacts Locate the suppression device e g a varistor for an ac load a diode for dc as close as possible to the output device Commissioning and Tr oubles hooting COMMISSIONING aaa wa vo nen dean PRRwn ors 6 0 Troubleshooting Overview 6 1 Finding the Problem 6 2 Troubleshooting the PLC 6 3 Troubleshooting I O 6 4 Program Troubleshooting 6 5 Faults oaoa aaua 6 6 Safety ANNA KA GAP AA nG 6 7 Troubleshooting Model 6 8
87. nd add instruction execution times when all instructions are True ust 5 To estimate program throughput time A Without communications add sections 1 4 us B With communications add sections 1 4 and multiply by 1 05 us 6 PLC input circuit filter time us 7 PLC output circuit turn on time us 8 To estimate total throughput time for the PLC add sections 5 7 us Note This will result in the worst case or longest possi ble throughput ti me An example set of instruction execution times is provided in Appendix C Communication with devices such as a Hand Held Programmer a personal computer or an electronic operator interface Estimating Memory Usage for The Control System Once your program is written use the following worksheet to estimate memory usage To assist you typical words of memory have been provided where needed To determine actual memory usage consult your PLC user manual This is very important as the amount of memory consumed by various instructions differ between PLC manufacturers 1 Determine the total number of instruction words used by the instructions in your program and enter the result i 2 Multiply the total number of rungs by 0 75 and enter the result do not count Start of File or End of File rungs 3 Words allocated by controller 280 typically 4 Add steps 1 3 for total estimated memory usage 5 Subtract the total from 1024 to determine memory remaining Important The calculat
88. nter Basic Retentive Timer RTO 27 49 38 34 1 00 Basic Timer Off Delay TOF 31 65 39 42 1 00 Basic Timer On Delay TON 30 38 38 34 1 00 Comparison E qual EQU 6 60 21 52 1 50 Comparison Greater Than GRT 6 60 23 60 1 50 Comparison Greater Than or Equal GEQ 6 60 23 60 1 50 Comparison Less Than LES 6 60 23 60 1 50 Comparison Less Than or Equal LEQ 6 60 23 60 1 50 Comparison Limit Test LIM 7 69 36 93 1 50 161 Comparison Masked Comparison for E qual MEQ 7 69 28 39 1 50 162 Instruction Type Comparison Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling Data Handling igh Speed Counter igh Speed Counter THI I igh Speed Counter I igh Speed Counter igh Speed Counter I I igh Speed Counter I igh Speed Counter Math Math Math Math Math Math Name Not Equal And Convert from BCD Convert to BCD Decode 4 to 1 of 16 Encode 1 of 16 to 4 Exclusive Or FIFO Load FIFO Unload File Copy Fill File LIFO Load LIFO Unload Masked Move Move Negate Not Or High Speed Counter High Speed Counter Interrupt Disable High Speed Counter Interrupt Enable High Speed Counter Load High Speed Counter Reset High Speed Counter Reset Accumulator Update High Speed Counter Image Accumulator Add Clear Divide
89. nternal Bit for ung amp F1 F2 Pump 1 EQU 1 0 1 1 B3 6 EQUAL Source A C5 10 ACC Stage 3 Source B 3 Internal Bit for Pump 1 B3 6 Stage 3 Stage 3 Rung 7 Low Level Internal Bit for Second Call Internal Bit for F1 Pump 1 F2 Pump 2 EQU 1 0 B3 6 1 2 B3 7 EQUAL Source A C5 10 ACC Stage 3 Source B 3 Internal Bit for Pump 2 B3 7 137 Stage 3 Stage 3 Rung 8 Low Level Internal Bit for Third Call Internal Bit for F1 Pump 2 F4 Pump 3 EQU 1 0 B3 7 1 3 B3 8 EQUAL 1 E JE 1 E Source A C5 10 ACC 3 Stage 3 Internal Bit for Pump 3 B3 8 Source B RUNG 9 e This rung sets internal bit B3 9 when the final stage has been completed Rung 9 Stage 3 Internal Bit for Pump 1 Cycle Complete B3 6 B3 9 EQU EQUAL JE Source A C5 10 ACC Cycle Complete Source B 3 B3 9 RUNGS 10 11 12 e These three rungs link the preceding rungs to actual output terminals on the PLC Making use of internal bits for logic purposes provides an easy method of controlling an output from multiple sources within a program Rung 10 Stage 1 Internal Bit for Pump 1 Pump 1 B3 0 0 0 Stage 2 Internal Bit for Pump 1 B3 3 Stage 3 Internal Bit for Pump 1 B3 6 13 Rung 11 Stage 1 Internal Bit for Pump 2 Pump 2 B3 1 O 1 Stage 2 Internal Bit for Pump 2 B3 4 Stage 3 Internal Bit for Pump 2 B3 7 Rung 12 Stage 1 Internal Bit for Pump
90. ode defined by IEC publication number 529 specifying the level of resistance an enclosure exhibits towards penetration by objects dust or water ladder logic A PLC program written in a format resembling an electrical ladder diagram The program is used by a programmable controller to sense inputs and control output devices latch A ladder program output instruction that retains its state even though the conditions that caused it to latch On may go Off A latched output must be unlatched to turn Off A latched output will retain its last state On or Off if power is removed LED Light Emitting Diode A semiconductor diode the junction of which emits light when passing a current LEDs are used as diagnostic indicators on various PLC hardware components LIFO Last In First Out The order in which data is entered into and retrieved from a file See also FIFO First In First Out limit switch An electrical switching device that is actuated by some part and or motion of a machine or equipment logic A process of solving complex problems through the repeated use of simple functions that can be either True or False It is a general term for digital circuits and programmed instructions designed to perform decision making and computational functions Master Control Relay MCR A hard wired relay that can be de energized by one of any number of series connected emergency stop switches Whenever the master control relay is de en
91. ol instructions to be programmed in parallel in a single rung Fig 4 13 e Condition instructions programmed in parallel are the equivalent of an OR operation e Control instructions programmed in parallel are the equivalent of an AND operation Front Door Driver s Side t Front Door Passenger s Side maa Rear Door Driver s Side Rear Door Passenger s Side Key Present Dome Light Oe Bell Fig 4 13 In this example branch instructions are applied to a program controlling the dome light and door ajar bell of a 4 door sedan The light and bell multi ple outputs will turn On if ANY of the doors multiple inputs are opened while a key is present in the ignition Branch operations also provide the relay wiring equivalent of an auxiliary holding contact or memory function refer back to Fig 4 3 Auxiliary contacts keep their output energized after a momentary start signal is no longer present E As shown in Fig 4 14 an auxiliary holding contact is always programmed with the same address as its referenced output remember one of the advantages of a PLC is that an address can be used more than once Momentarily pressing start button 1 2 energizes control instruction 0 3 which turns On the motor and it also energizes condition instruction 0 3 Energizing 0 3 in the branch operation maintains the On status of the output until stop button 1 1 is pressed Stop Button Start Button
92. om the other output signals which are 24V dc Some micro PLCs now offer individually isolated outputs with other outputs on different commons Fig 5 3 A micro PLC accepts signals for all its inputs at the same voltage level usually 120V ac or 24V dc The application requirements and the power available dictate which voltage is selected Recall that for the parking garage the inputs operate on 24V dc for safety reasons The chart below summarizes the electrical requirements for the parking garage Incoming power Output voltages Input voltage 24V dc 120V ac 2 devices 24V dc 7 devices 24V dc 4 devices Output Circuits Recall from section 3 2 that micro PLCs are available with different types of outputs to suit different situations For the parking garage relay outputs will work best Relays can switch both dc and ac current have adequate response times and wear is not a significant issue In addition micro PLCs with relay outputs usually cost less than those with solid state outputs For applications requiring fast response or having a high cycle rate such as a high speed cut to length line a micro PLC with solid state output circuits transistor FET or triac might be the optimum choice These circuits respond faster and do not wear out because there are no moving parts Memory Requirements To quickly estimate the memory an application requires a general rule is to add the number of I O and then multiply by 10 whe
93. ons in the program Note The I O address format may differ depending on the PLC manufacturer The major elements of an operating cycle are 1 The input scan During the input scan the PLC examines the external input devices for a voltage present or absent i e an On or Off state The status of the inputs is temporarily stored in an PLC Operating input image memory file Cycle 2 Program scan During the program scan the PLC scans the instructions in the ladder logic program uses the input status Program 52 from the input image file and determines if an output will or will not be energized IE ean ea Ocenia res The resulting status of the outputs is written to the output image memory file 3 Output scan Based on the data in the output image file the PLC energizes or de energizes its output circuits controlling external devices 3 7 Power Supplies The power supply provides power to the controller s internal electronics converts the incoming voltage to a usable form and protects the PLC5 components from voltage spikes Speed What is the fastest action required in the control process How much time is needed to control that action Speed is one of the primary advantages of today s micro controllers Operating cycles typically take 1 to 25 milliseconds thousandths of a second When judging the speed it is important to look at total throughput time not just the operating c
94. opy of the program is required 6 2 Finding the Problem If a control system has been operating the technician should be confident of the accuracy of the program logic In this case malfunctioning field devices or loose wiring associated with the field devices cause most errors For a control system that has never worked e g just being commissioned programming errors should also be considered Before spending hours troubleshooting a system and searching for a complicated problem first rule out any obvious problem e g a broken belt or jammed machinery Then cycle power to the PLC Remember that power surges or other momentary problems may have caused the PLC to stop and it may only need to be re started i 87 6 3 Troubleshooting the PLC If the PLC is running properly its power and run LEDs should be On and there should be no fault indication refer to Fig 6 1 If the fault LED is On use a Hand Held Programmer HHP or aPC running the PLC programming software to determine the cause of the fault Then consult the user manual to determine possible causes and corrective actions refer to section 6 6 and Fig 6 2 for more details If all LEDs are Off verify that the PLC has proper power with a voltmeter If a power problem exists verify that all wiring connections are good and that there are no broken wires Check for power from the circuit breaker or fuse block After verifying PLC power check communication wit
95. or the parking garage The car approaches an automated ticket machine at a gate The driver pushes a button on the ticket machine to receive a ticket If there is space left in the garage the driver will receive a ticket The machine should not provide a ticket if the garage is full or if the gate is already up Removing the ticket raises the gate and turns on a green enter light e After the car clears the gate the gate lowers and the green light shuts off e The number of vehicles in the garage needs to be known at any time e If maximum capacity is reached a Garage Full sign is illuminated the ticket machine will not provide a ticket and the gate will not raise e An alarm must sound when the gate is obstructed Input and Output Requirements After defining the operation of the system the next step is to determine what input and output devices the system requires List the function required and identify a specific type of device Also group devices by whether they sense an event has occurred or is occurring inputs or whether they control something outputs From the description of the parking garage control system the following I O requirements can be listed Function inputs Ticket request Ticket taken Car cleared gate Car departed garage Gate obstructed Gate in up position Gate in down position Function outputs Provide ticket Garage Full sign Green light Alarm Raise ga
96. pecifications 66 5 4 Program Development Procedures 70 5 5 Installation Requirements 80 Chapter 6 Commissioning and Troubleshooting 6 0 COMMISSIONING eax gu tease c s 6 1 Troubleshooting Overview 6 2 Finding the Problem 6 3 Troubleshooting the PLC 6 4 Troubleshooting 1 O 6 5 Program Troubleshooting 6 0 Faults sirai erakoen ey eas 6 7 Safety d wets weaned Hes wwe 6 8 Troubleshooting Model Chapter 7 Application Examples 7 0 Introduction aonana tees Lapad Basic Logic oP OR Circuit casni siara 7 2 AND Circuit aaua 7 3 Start Stop Circuit 7 4 Flip F lop Circuit 7 5 Alarm Circuit auaa 7 6 Start Stop with Jog Timing and Counting 7 7 On Delay apa kdi aahh ede 106 7 8 Off DELAY mana tia ae ADRIANA 108 7 9 One Minute Clock 110 7 10 Up Down Counting 112 Data Instructions 7 11 Moving Data 114 7 12 Comparing Data 117 7 13 Math Commands 120 Advanced Instructions 7 14 Sequencers 123 aa Pao ad O AA cies wake eels 125 7 16 High Speed Counter 128 7 17 Two Stage Alternator 129 7 18 Three Station Alternator 133 Appendices Glossary Senge as Appendix A 142 Input and Output Devices Appendix B 153 Instruction Execution Times Appendix C 161 Sample Program Workshee
97. ping its customers improve productivity and quality for more than 90 years We design manufacture and support a broad Allen Br adley range of automation products worldwide They include logic processors power and motion control devices operator interfaces sensors and a variety of software Rockwell is one of the world s leading technology companies x r EA A IN j Worldwide representation 7 z Argentina Australia Austria Bahrain Belgium Brazil Bulgaria Canada Chile China PRC Colombia Costa Rica Croatia Cyprus Czech Republic Denmark Ecuador Egypt El Salvador Finland France Germany Greece Guatemala Honduras Hong Kong Hungary Iceland India Indonesia Ireland Israel Italy Jamaica Japan Jordan Korea Kuwait Lebanon Malaysia e Mexico Netherlands New Zealand Norway Pakistan Peru Philippines Poland Portugal Puerto Rico Qatar Romania Russia CIS Saudi Arabia Singapore Slovakia Slovenia South Africa Republic Spain Sweden Switzerland Taiwan Thailand Turkey United Arab Emirates United Kingdom United States Uruguay Venezuela Yugoslavia Allen Bradley Headquarters 1201 South Second Street Milwaukee WI 53204 USA Tel 1 414 382 2000 Fax 1 414 382 4444 1761 MMB September 1995 1995 Allen Bradley Company Inc Printed in USA
98. r switches use a rotary motion of the knob or other operator to accomplish switching Foot switches are used where the operator s hands need to be used to manipulate other things while operating the equipment or where repetitive hand operations of a switch might cause the operator discomfort Thumbwheel switches are a common way of entering numerical data into a control circuit Each digit or decade has a physical marking that represents a number from 0 to 9 Each decade requires four inputs to connect it to the PLC By changing the sequence of Ons and Offs BCD code the switch tells the controller what number has been entered Limit Switches Limit switches are used to sense the position of objects or materials Conveyors doors swingarms valves and many other devices use limit switches to provide control system information on the physical position of equipment The limit switch uses an actuating mechanism to make or break switch contacts Many types of actuating mechanisms are available but the most common are the roller lever the push roller the fork lever and the wobble stick Float Switches Float switches are the easiest means to monitor liquid level in a container They are typically used in wet wells tanks sumps reservoirs etc As the liquid level in the container changes the actuating mechanism moves Control of the level of liquid in the container is achieved by setting the limit switch to
99. r 3 that every element in the user program is referenced with an does this by examining a rung for logical address to indicate where data continuity i e all condition instructions are HS KANA NIC i6 UNG saba evaluated as True If logical continuity exists the PLC energizes the control instruction see Fig 4 4 If logical continuity does not exist then the PLC maintains the control instruction in the Off or de energized state Logical Continuity Stop Start Motor M1 1 1 1 2 0 1 Rung M1 O 1 Auxiliary Contact Fig 4 4 If a signal is NOT present at input terminal 1 1 and a signal is present at input terminal 1 2 the rung has logical continuity and the PLC will energize output terminal O 1 controlling the motor 4 3 Ladder Logic Instructions The most frequently used instructions in a PLC ladder logic program are the normally open N O instruction the normally closed N C instruction and the output energize instruction see Fig 4 5 These instructions are represented as symbols placed on the rungs of the program which is why PLC users may hear ladder logic described as contact symbology Normally Normally Output Closed Instruction Open Instruction Energize Instruction Normally Open Instruction Fig 4 5 Common ladder symbols Normally Open Instruction A normally open instruction examines a PLC memory location for an 39 i On condition i e it checks to see if the bit element at the instru
100. ram equivalent of AND logic Fig 4 10 For example picture a metal stamping operation where the machine activates only if the operator simultaneously pushes both a left hand start button X AND a right hand start button Y Xx Y Z Fig 4 10 With instructions programmed in series output Z will be True On only if both input X AND input Y are True On The output of an AND equation will be True only if all conditions in series are True If any condition is False then the rung does not have logical continuity and the output will be Off OR Logic Condition instructions programmed in parallel are the ladder diagram equivalent of the OR operation see Fig 4 11 For example imagine a conveyor that has two run switches one located at each end The conveyor could be configured to start if an operator pressed a start button at one end X OR the other Y H Fig 4 11 With instructi ons programmed in parallel output Z will be True On if either X OR Y are True On The output of an OR equation will be True if any condition in parallel is True If all conditions are False then the rung does not have logical continuity and the output will be False Notice that AND and OR logic w Y Z series and parallel circuits can it Bi be combi ned on a single rung as st shown in Fig 4 12 Fig 4 12 Combining series and parallel logic Branch Operations The function of a branch is to allow both condition and contr
101. re 10 is the words of memory needed per I O The parking garage control system has 13 1 0 plus one extra for expansion yielding a total of 14 14 x 10 words 140 estimated words of memory required Today nearly all micro PLCs have at least 1 2K of memory available for application programs 1 2K equals 512 words For the parking garage control system as well as most low I O count applications micro PLCs usually have more than sufficient memory Typically applications will exceed a micro controller I O capacity before its memory capacity Once the logic required for an application has been developed PLC users can calculate how much memory a program will consume by refer ring to the PLC operator s manual which typically lists memory use for all of the instructions See the worksheet in Appendix D for an example Speed of Operation If application requirements indicate the need for a PLC with high speed operation look for a PLC with the following features 69 i e Adjustable input filters see section 3 1 e Transistor FET or triac outputs not relays see section 3 2 e High speed counter high speed interrupts and immediate outputs High speed counters high speed interrupts and the ability to immediately update outputs allows PLCs to meet the demands of most high speed applications On user specified conditions high speed interrupts and immediate output instructions direct the PLC to immediately process the logic and
102. rection of motion of a shaft or absolute which track shaft position at all times The number of pulses generated corresponds to distance or degree of shaft rotation Proximity Sensors Proximity sensors are used to detect the presence or absence of an object without making contact with it Capacitive sensors sense the change in dielectric field strength as an object moves closer to and further from the sensor Inductive sensors depend upon the changes in inductance within a coil when a metallic object comes within range of the sensor Which sensor is appropriate for a given application depends on the material to be sensed Photoelectric Sensors Photoelectric sensors use a light beam to detect objects There are three basic types of photoelectric sensors e In transmitted beam sensors the object being sensed moves between a light source and a receiver module that contains the photodetector e In retroreflective type sensors the object to be sensed moves between the sensor which contains both the light source and the photodetector and a reflector e In diffuse sensors the natural reflectivity of the object being sensed causes the return signal that triggers the photodetector 157 p i 16 Ultrasonic Proximity Sensors Ultrasonic proximity sensors use the comparative strength of the return signal from a projected ultrasonic signal to sense how far an object is from the source of the sound in muc
103. rn from Subroutine Subroutine Suspend Temporary End Mnemonic SQR SUB IIM IOM JMP JSR LBL MCR RET SBR SUS TND Time needed to execute the instruction when it is not True 0 approx psec 6 78 6 78 6 78 6 78 6 78 4 25 0 99 4 07 3 16 0 99 7 87 3 16 Time needed to execute as a True 1 statement approx psec 71 25 33 52 35 72 41 59 9 04 22 24 1 45 3 98 31 11 1 45 10 85 7 78 Memory usage user words 1 25 1 50 1 50 1 50 1 00 1 00 0 50 0 50 0 50 0 50 1 50 0 50 163 a Appendix D Sample Program Wor ksheets Throughput Time Worksheet Throughput is the amount of time it takes for the PLC to sense an input and energize the corresponding output Components of throughput time include time for the PLC s input circuit to sense the signal time for the input output and program scans time for actuation of the PLC s output circuits and time for the CPU s housekeeping functions Once your program is written use the following worksheet to estimate PLC throughput time To assist you typical times have been provided where needed To determine actual throughput time consult your PLC users manual This is very important as execution times differ between PLC manufacturers Procedure Maximum Scan Time 1 Input scan time 8us typically 2 Output scan time 8 us typically 3 Housekeeping time 180us typically 4 To estimate program scan time take your program a
104. ses Many applications that are presently controlled by relays and or custom single board controls are migrating toward micro PLCs Micro PLC applications are considered in detail in Chapter 5 How to Apply a Micro PLC and Chapter 7 Application Examples Typical micro PLC applications Packaging machine Micro PLC Oper ation Components Overview 3 0 NAA APP 3 1 QUEPULS racirea tani PERA BAS 3 2 Central Processing Unit CPU 3 3 Types of Application Memory 3 4 Data Memory and Addressing 3 5 Operating Cycle 3 6 Power Supplies 3 7 Programming Devices 3 8 Operator Interfaces 3 9 3 0 Components Over view In order to learn how PLCs operate a quick overview of PLC components is necessary All PLCs from micro to very large use the same basic components and are structured in a similar fashion PLC systems consist of e Inputs e Outputs e Central processing unit CPU e Memory for program and data storage e Power supply e Programming device e Operator interfaces 3 1 Inputs The input screw terminals on a PLC form the interface by which field devices are connected to the PLC Programming Communication Devices 56 Central A2 O Processing Unit KL Optical MEMORY Optical Isolation Isolation program data Power Supply Fig 3 1 Control of a machine or
105. sier to use than one that uses an accumulator for math operations Ladder Logic The four basic math instructions are illustrated below RUNG 0 and RUNG 1 e These first two rungs make use of a counter to provide an easy method of changing a data value to be used in the math instructions to follow Condition instruction 1 0 will increment counter C5 10 each time it is energized Condition instruction I 1 will reset the accumulated value of counter C5 10 when it is energized Rung 0 1 0 CTU Rung 1 IA C5 10 RES RUNG 2 e When condition instruction 1 2 is on the PLC will enable the add ADD instruction In this example the data in source A in this case the constant 5 will be added with the data in source B the accumulated value of counter C5 10 with the result being placed in the Dest destination N7 0 Rung 2 1 2 Source A Source B Dest RUNG 3 e When condition instruction 1 3 is on the PLC will enable the subtraction SUB instruction In this example the data in source B the constant 5 will be subtracted from the data in source A the accumulated value of counter C5 10 with the result being placed in the Dest destination N7 1 SUB SUBTRACT Source A C5 10 ACC Source B RUNG 4 e When condition instruction 1 4 is on the PLC will enable the multiply MUL instruction In this example the data in source A the constant 20 will be multiplied by the data in sourc
106. solenoid valves motors lights etc or internal bit elements Higher Level Instructions While relay logic is suitable for simple On Off sensing and control many applications require more powerful instructions To allow this enhanced ladder language commands have been developed These instructions deal with numerical data beyond simple 1s or 0s by manipulating data in bytes or words Examples of higher level instructions include counters timers sequencers math comparison and other Operations that N O N C and OTE instructions cannot perform To keep the implementation of these Operations simple higher level instructions are usually represented in ladder logic programming as function blocks As shown Number of Vehicles in Garage CTU Count Up She Counter C5 10 DN Preset 500 Accum 0 Fig 4 9 Higher level instructions such as this counter are represented with function blocks in the ladder program in Fig 4 9 function blocks are literally programmed as blocks on the rung of a ladder program Depending on their operation higher level instructions can be either condition instructions e g comparison instructions or control instructions e g timer or counter instructions 4 4 Combining Instructions Two fundamental logic operations AND and OR provide the rules for governing how instructions are combined AND Logic Condition instructions programmed in series are the ladder diag
107. specific instructions sequencer bit shift Program flow subroutine MCR master control reset immediate input or output with mask selectable timed interrupt jumps Fig 2 2 Theinstruction set of a typical micro PLC a packaged controller is that the all in one package is smaller less costly and convenient to install see photo p 11 However few packaged controllers have expandable I O capabilities where all modular controllers can be expanded easily by adding more I O cards to the rack 2 2 Capabilities Over view A PLC s capabilities are determined by the type of commands a user can instruct it to execute While the instruction set and names of instructions will vary slightly among micro PLC manufacturers Fig 2 2 gives an overview of the instructions commonly available As has been noted PLCs were initially designed to function as electronic replacements for hardwired control devices primarily relay coils and contacts counters and timers Today these functions still comprise the majority of instructions used in micro PLC applications B i By way of example imagine designing a control system for a conveyor in a food packaging operation Based on the status of field devices a PLC can start a conveyor sense the presence of a box move the box forward to the desired position hold it there for a predetermined filling time and count the number of full boxes coming off the line Micro PL
108. te Lower gate Device Pushbutton Limit switch Photoelectric sensor Photoelectric sensor Motor overload contact Proximity sensor Proximity sensor Device Solenoid Light Light Horn Gear motor forward Gear motor reverse From the list of field devices the parking garage control system requires seven inputs and six outputs Advanced Function Requirements Applications often require operations beside simple discrete On Off logic These advanced functions include timing counting sequencing communications math comparison and many other operations involving data manipulation and calculation List the advanced functions required and note how they will be used From the description of the parking garage control system the following advanced function requirements can be listed Function Use Up counter Count cars entering garage Down counter Count cars leaving garage Electrical Requirements When determining the electrical requirements of a system consider three items incoming power power for the control system input device voltage and output voltage and current Because the voltage used with each device may be different making a distinction is important To decide what voltage to use consider the following e What type of power is available e g 24V dc 120 or 240V ac e How will the machine or process controlled be used e Will people come in contact with the machine e What power do the field devi
109. test each output Many technicians do this by applying power to the terminal where the output device is wired This checks the field device and its associated wiring 8 After verifying all inputs outputs and program addresses verify all preset values for counters timers etc 9 Place the PLC in the run mode and verify that the run LED is On Reconnect any output devices that were disconnected in step 1 Test all emergency stop buttons Test total system operation 6 1 Troubleshooting Over view When a control system error occurs many new PLC users first suspect the PLC is at fault Usually this assumption is unjustified as devices other than the PLC such as sensors solenoids and wiring cause the vast majority of faults It is worth repeating that PLCs are among the most rugged durable and reliable control equipment available today However faults are inevitable in any control system including PLCs Fortunately PLCs have been specifically designed to incorporate troubleshooting aides that enable users to get the application up and running quickly This is an advantage over relays SBCs and other control solutions Troubleshooting consists of three activities understanding how the application control system operates finding the problem and correcting it Before troubleshooting any control system the technician must understand how the system works and how the various components interact As with commissioning a hard c
110. tions or performing math operations write To move or copy data to a memory location For example the controller writes the information to the output data file based on the logic of the ladder program Appendix B Sample Input and Output Devices Input Devices Input devices are field devices that act as information gatherers for the PLC Think of them as the eyes and ears of the PLC Most micro PLCs need to recognize a discrete On or Off signal Input devices typically communicate with the PLC by switching current On or Off by either electromechanical or solid state contacts Solid state input devices like transistors FETs and triacs are sensitive to input wiring conditions polarity and leakage current issues Electromechanical input devices such as switches and relays close sets of contacts to allow current to pass and as such are less sensitive to those situations Check the specifications for the sensors and the PLC before making connections It is likely that the manufacturer of the sensor or switch you are using has a version of the device that is appropriate for use with your particular micro PLC 153 B j 154 Operator Manipulated Switches The pushbutton switch is one of the simplest and most commonly used forms of input control Pushbuttons are used to start and stop equipment and to initiate processes Selector switches incorporate an operator or switch mechanism that has several positions Selecto
111. trieved from a file See also LIFO Last In First Out force Software function that allows the programmer to energize or de energize an input or output independent of the program logic It is used primarily for troubleshooting hardware Includes all the physical components of the control system including the programmable controller peripherals and interconnecting wiring Compare ie with software E u IEC International Electrotechnical Commission An international association with members representing electrical manufacturers The IEC establishes standards for the construction and operation of electrical devices 1 O Inputs and Outputs Consists of devices that provide data to input and receive data from output the PLC input device A device such as a pushbutton sensor or a switch of some sort that supplies signals to the PLC input scan Part of the controller s operating cycle During the input scan the controller examines all input devices for an On or Off state This status is temporarily written to the input image memory file for use during the program scan See also communication scan program scan and output scan instruction A command defining an operation to be performed by the controller A rung in a program consists of a set of condition input instructions and control output instructions See also condition instruction and control instruction IP Ingress Protection A designation c
112. tructed motor overload contact 1 5 Gate up proximity sensor 1 6 Gate lowered proximity sensor Output address 0 0 Ticket provided solenoid O 1 Gate up motor controller 0 2 Gate down motor controller 0 3 Garage Full sign 0 4 Green light 0 5 Alarm horn Counter address The program for the parking garage also needs two counters notice k i that the counter uses an internal address C5 10 Count Up CTU for cars entering C5 10 Count Down CTD for cars departing With the addresses inserted the program for the parking garage is complete and looks like this see next page Rung 0 Provide Ticket Garage is Gate is Ticket Request PB Full Lowered Solenoid 1 0 0 3 1 6 O 0 SSS a Rung 1 Ticket has Been Taken Gate Garage is Limit Switch is Up Full Raise Gate 1 1 1 5 0 3 0 1 a Raise Gate 0 1 Rung 2 Vehicle Photo Vehicle is Vehicle Sensor Clear of Present gate Gate Latch 1 2 B 2 B 3 aE Rung 3 Vehicle 78 Vehicle Photo Vehicle is Present Sensor Ticket Clear of Latch gate Request PB Gate B 3 1 2 1 0 B 2 L dkpjp HAH Vehicle is Clear of Gate B 2 Rung 4 Vehicle is Gate Clear of Gate is Gate is Lower is Up Gate Lowered Obstructed Gate 1 5 B 2 1 6 1 4 0 2 c i Lower Gate 0 2 Rung 5 Gate Green enter is Up Light 1 5 0 4 Rung 6 Number of Vehicles Lower Gate in Garage 0 2 CTU Count Up cu Counter C5 10 DN Preset 500 Accum 0 Rung 7 Gara
113. truction output device for a preset period of time Ladder Logic The logic used in this application consists of three rungs RUNG 0 e This is the start stop rung It operates in the same manner as the start stop example in section 7 3 however instead of energizing an external output address internal bit B3 0 is energized when start pushbutton 1 0 is pressed RUNG 1 e This rung contains an On delay timer with an address of T4 0 When condition instruction B3 0 has been energized by the control instruction B3 0 in rung 0 the timer begins timing Notice that the time base in the timer function block reads one second This means that the timer will time in one second increments Also notice that the preset value reads 10 This means that the timer will be done timing after a time delay of 10 one second increments for a total of ten seconds The timer done bit T4 0 DN in rung 2 will be energized at this point If at any time rung 1 lacks logical continuity B 3 0 is Off the timer will reset to zero The length of the time delay can be adjusted by changing the preset value In addition most PLCs allow the option of changing the time base or resolution of the timer The smaller the time base selected the better the accuracy of the timer Typical time bases are 0 01 0 1 and 1 0 second The accumulated value of the timer shown as ACCUM in the function block is the number of increments the timer has accumulated since it began timing
114. ts Appendix D 164 Welcome to MicroMentor Understanding and Applying Micro Programmable Controllers In less than a decade micro PLCs have gone from a blueprint to one of the o fastest growing segments of the control products market Unfortunately scant literature exists about micro PLCs In addition many of the current PLC text books are too cumbersome for today s busy personnel With those thoughts in mind Allen Bradley produced this book as an introduction to micro PLCs for the design engineer electrical technician and maintenance person with little or no background in programmable logic controls Readers will quickly learn about the micro PLC s evolution capabilities operation and advantages over other control options Non specific to any manufacturer the text also covers basic programming instructions application examples and troubleshooting ka Written to be easily understood the MicroMentor can augment classroom material and it can serve as a supplement to the operator manuals and technical data supplied by micro PLC manufacturers The authors hope that those experienced with micro PLCs will use this book as a training aid and that MicroMentor prompts all readers to ask What is the best control solution for my application Good Luck Introduction to PLCs History of PLCS s ove no nu kaa Dee ones 1 0 Why Use a PUG i cosas kak due as 1 1 Traditional PLC Applications 1 2
115. tter what type of control system is ultimately selected the first step in approaching a control situation is to specify the application s requirements This includes determining e Input and output device requirements e The need for special operations in addition to discrete On Off logic including Timing Sequencing Counting Data acquisition High speed counting Data calculations e The electrical requirements for inputs outputs and system power e How fast the control system must operate speed of operation e If the application requires sharing data outside the process i e communication e If the system needs operator control or interaction e The physical environment in which the control system will be located To determine application requirements designers need to begin by identifying all operations the control system needs to perform and the conditions that affect the system Note If an operation specification exists for the process machine consult it before beginning If no specification exists it needs to be created at this point As an example imagine designing a control system for a parking garage with a 500 car capacity The first step is to define and describe the car parking process Note that while descriptive text is used here most people simply write out a description with pen and paper sequence of operation charts or process sheets are used too What is the desired operation f
116. typically up to 1 amp Triacs strictly switch ac power Like transistors triac outputs are silent have no moving parts to wear are fast and carry loads of 0 5 amp or less Note solid state outputs triacs and transistors can be damaged or destroyed by over voltage or over current 3 3 Central Processing Unit CPU The CPU made up of a microprocessor and a memory system forms the primary component of the PLC The CPU reads the inputs executes logic as dictated by the application program performs calculations and controls the outputs accordingly PLC users work with two areas of the CPU program files and data files Program files store a user s application program subroutine files and the error file Data files store data associated with the program such as I O status counter timer preset and accumulated values and other stored constants or variables Together these two areas are called the application memory or user memory Also within the CPU is an executive program or system memory that directs and performs operation activities such as executing the user program and coordinating input scans and output updates System memory which is programmed by the manufacturer cannot be accessed by the user 3 4 Even though EEPROM Types of Application Memory is ules Gain sahil application programs if power As the name indicates programmable is lost they do not necessarily save process data such as the accumul
117. ue 1 or False 0 branch A parallel logic path within a ladder logic rung bus 1 A group of lines used for data transmission or control 2 Power distribution conductors byte A group of adjacent bits usually operated upon as one unit such as when moving to and from memory There are eight bits in one byte A byte is capable of storing and displaying anumeric equivalent between 0 and 255 C A computer system programming language initially developed for the UNIX operating system communication scan A part of the PLC s operating cycle that manages communication with other devices such as a hand held programmer See also input scan output scan and program scan condition instruction Instruction pertaining to the input portion of a rung on a ladder diagram It is the condition or status of these instructions that determine how the control instruction is to be controlled See also control instruction contact 1 One of the conducting parts of a connector switch or relay that are engaged or disengaged to open or close an electrical pathway 2 With reference to PLC ladder logic programs a condition that provides a logical pathway continuity when True contact symbology A set of symbols used to express logic the control program using conventional relay symbols For instance indicates a normally open contact 143 i indicates a normally closed contact and indicates a relay coil or output H 14
118. ulated value of the counter in rung 13 to a constant The value of the constant designates which stage is to be run i e the operating sequence of the pumps RUNGS 0 through 8 e The first 9 rungs in the program set the priority assignment for the primary Lag 1 and Lag 2 pumps Stage 1 Rung 0 Low Level First Call Internal Bit for F1 F2 Pump 1 1 0 1 1 B3 0 Stage 1 Internal Bit for Pump 1 B3 0 Stage 1 Stage 1 Rung 1 Low Level Internal Bit for Second Call Internal Bit for F1 Pump 1 F3 Pump 2 TE NG B3 1 Stage 1 Internal Bit for Pump 2 B3 1 Stage 1 Stade 1 Rung 2 Low Level Internal Bit for Third Call Internal Bit for F1 Pump 2 F4 Pump 3 1 0 B3 1 1 3 B3 2 E E E Stage 1 Internal Bit for Pump 3 B3 2 Stage 2 R Low Level First Call Internal Bit for ung 3 F1 F2 Pump 1 EQU 1 0 1 1 B3 3 EQUAL J Source A C5 10 ACC Stage 2 Source B 2 Internal Bit for Pump 1 B3 3 Stage 2 Stage 3 Rung 4 Low Level Internal Bit for Second Call Internal Bit for F1 Pump 1 F3 Pump 2 EQU 1 0 B3 3 1 2 B3 4 EQUAL J Source A C5 10 ACC Stage 2 Source B 2 Internal Bit for Pump 2 B3 4 Stage 2 Stage 2 Rung 5 Low Level Internal Bit for Third Call Internal Bit for F1 Pump 2 F4 Pump 3 EQU 1 0 B3 4 1 3 B3 5 EQUAL 1 E 1 E E Source A C5 10 ACC Source B 2 Stage 2 Internal Bit for Pump 3 B3 5 R Low Level First Call I
119. urns On a bit for a single program scan operating voltage F or inputs the voltage range needed for the input to be in the On state For outputs the allowable range for user supplied voltage The PLC or other control system itself will have a specified range of allowable voltage for system operation OR A logical operation that produces a True output when one of any number of conditions is True and a False output if all conditions are False output device A device such as a pilot light or a motor starter coil that is controlled by the PLC output scan A part of the controller s operating cycle Using information obtained during the program scan about the status of the output devices the controller energizes or de energizes its output circuits to control output devices See also communication scan input scan and program scan packaged controller Programmable controller with the processor power supply inputs and outputs all in one package Compare with modular controller peripheral External devices that are connected via a communications port to the programmable controller usually for programming data exchange or operator interface power supply Electrical circuit that filters conditions and supplies appropriate voltages for system components and circuitry processor A central processing unit See also CPU program A set of instructions stored in memory that are executed in a predetermined order by the central
120. user program RUNG 0 e A momentary pushbutton wired to input 1 5 is in series with a one shot rising OSR instruction B3 2 that controls output B3 0 An OSR is a specialized instruction that is only energized for one processor scan This causes control instruction B3 0 to be energized for one processor scan Another way to think of this is as a leading edge triggered device RUNG 1 e The second rung detects the pulse each time the condition instruction 1 5 is energized and changes the output to the opposite state each time the pushbutton in rung 0 goes True RUNG 2 e This rung directly controls the load device wired to terminal 0 0 Rung 0 This rung generates a pulse each time the pushbutton is pressed Leading Push Edge Pulse On Button One Shot For 1 Scan 1 5 B3 2 B3 0 88 PO Rung 1 This rung detects the pulse and changes the output to the opposite state Pulse On For 1 Scan Toggle Toggle B3 0 B3 1 B3 1 Pulse On Toggle For 1 Scan B3 1 B3 0 Rung 2 This rung uses the toggle bit to turn on the load device Toggle Load B3 1 0 0 102 7 5 Alarm Circuit with Flash and Acknow ledge Uses This type of logic is used to detect hold and reset alarm events Operation e Detect the alarm condition and maintain the event e Flash an indicator to represent an alarm is present e Maintain the indication after the alarm has been acknowledged but is still present e Reset clear the
121. ut s and start tracing them to find out which conditions are not satisfied e If the input device is supposedly On but the corresponding input LED is not On use the HHP or a voltmeter to check for a signal at the input terminal If there is no signal examine wire connections between the terminal and the field device and tighten or repair wiring as necessary Check the devices for proper power and see if the field device is broken The Hand Held Programmer For the technician trouble shooting PLCs in the field or on the plant floor the value of a Hand Held Programmer cannot be overstated Features include an ability to e Identify the status On Off of any I O or bit element e Display the data located in a higher level instruction such as the accumulated value of a timer or counter e Trace or search for faulty instructions e Forceinstructions On or Off e Identify and clear faults e Download and upload programs Hand Held Programmers are also more rugged and portable than most PCs 6 5 Program Troubleshooting For a system that was working but has stopped suspect the program only after checking the PLC and verifying the integrity of the field devices and associated wiring However the same procedure is used to debug new and existing programs Start program troubleshooting by identifying which outputs operate properly and which outputs do not Then using the HHP or programming software trace back
122. witch 1 1 7 2 AND Cir cuit Uses This type of logic is used to turn On an output device control instruction when all input devices condition instructions in the rung provide logical continuity Operation Turn On an output only when all input devices condition instructions have logical continuity Ladder Logic The logic used to perform this consists of one rung with at least two condition instructions programmed in series RUNG 0 e This rung shows that whenever input devices 1 0 AND 1 1 are On output device O 0 will be energized Rung 0 If a part is present as detected by the photo eye AND it is in position as detected by the Limit Switch LS1 then operate the Clamp Part In Position Present in palag Photo Eye si Clamp Sb o 7 3 Start Stop Circuit Uses This is used to start a device with a momentary input and stop it with a second momentary input Typically the start and stop input devices are momentary pushbuttons or a similar type of device Once the start pushbutton is pressed the output energize instruction will stay On until the momentary stop pushbutton is pressed Operation Turn On an output with a momentary input and keep it On until instructed to turn it Off Ladder Logic The logic used to perform this consists of one rung Note that in this example the stop pushbutton is a normally closed switch but is programmed as a normally open instruction RUNG 0 e Before any inputs are activated N O instru
123. y 38 45 96 96 145 147 148 151 math instructions 9 12 120 122 162 163 micro PLC capabilities 13 14 micro PLC characteristics 9 10 micro PLC introduction 8 9 micro PLC typical applications 14 micro PLC typical features 8 9 mnemonic 34 35 147 modular controller 13 147 NEMA 4 80 147 NEMA 12 80 147 normally closed instruction 39 40 75 91 108 111 147 normally open instruction 39 40 43 75 91 99 111 147 octal 24 operating cycle 24 27 49 143 146 148 149 operator interfaces 5 18 30 31 58 59 148 optical isolation 19 20 OR logic 12 44 46 76 96 143 148 162 OSR one shot rising 100 161 output device 20 24 42 53 54 84 86 90 148 158 160 output image file 26 output scan 26 27 40 42 148 packaged controller 11 13 148 photoelectric sensors 19 55 57 157 PLC advantages over relays 2 61 PLC economic benefits 2 5 61 PLC history of 2 3 PLC typical applications 5 6 power supplies 11 18 26 28 148 program file 21 22 program printout 84 90 program scan 14 26 27 49 70 149 programming examples 57 61 68 72 79 96 140 proximity sensors 14 19 157 158 pushbuttons 19 31 39 41 43 58 98 154 RAM 22 23 149 register 24 149 retentive data 22 149 retentive timers 12 111 RS 232 31 149 RES reset 112 161 rung 35 46 49 72 149 SBC description 52 150 SBC typical application 60 SBC use of 62 65 solenoid 20 43 52 82 86 123 150 160 system
124. ycle Components of throughput time include time for actuation of the physical input time for PLC s input circuit to sense the signal time for input scan program scan and output scan time for actuation of the output circuit and corresponding field device and time for the CPU s housekeeping or overhead functions See throughput time worksheet in Appendix D For applications that require high speed operation advanced micro controllers offer functions such as high speed counting with direct control of outputs and immediate I O update instructions These functions enable the micro controller to detect and react quickly to changing input conditions Given that most facilities experience line voltage fluctuations PLC power supplies are designed to maintain normal operation even if the voltage varies from 10 to 15 percent Dips or surges in power are caused by natural line losses from the utility brownouts or the start up or shutdown of nearby heavy equipment such as motors or arc welders For voltage conditions that are especially unstable consider installing a constant voltage transformer between the PLC and the primary power source The PLC s power supply is designed to withstand short power losses without affecting the operation of the system A PLC can operate for several milliseconds without line power before the power supply signals the processor that it can no longer provide 27 l adequate dc power to t

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