# PROGRAMMABLE LOGIC CONTROLLERS Unit code: A/601/1625 QCF level: 4 Credit value: 15 OUTCOME 3 PART 1

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1 UNIT 22: PROGRAMMABLE LOGIC CONTROLLERS Unit code: A/601/1625 QCF level: 4 Credit value: 15 OUTCOME 3 PART 1 This work covers part of outcome 3 of the Edexcel standard module: Outcome 3 is the most demanding of the outcomes and can only be affectively studied with the use of suitable hardware and/or simulation software such as PneusimPro or Bytronics simulation software. An industrial background will also be of great benefit to students. The work is continued in part 2 where more advanced programming is covered. SYLLABUS 3 Be able to apply programmable logic programming techniques Write programs: use of ladder and logic diagrams; statement lists; Boolean algebra; function diagrams; graphical programming languages; production of a PLC Advanced functions: less than; greater than; binary to BCD conversion; proportional feedback control Producing and storing text: contact labels; rung labels; programming lists; cross-referencing Test and debug programs: forcing inputs, forcing outputs; changing data; comparing files (tapes, EPROM, disc); displayed error analysis Associated elements: contacts; coils; timers; counters; override facilities; flip-flops; shift registers; sequencers Learning outcomes On successful completion of this unit a learner will: L03 Be able to apply programmable logic programming techniques Assessment criteria for pass The learner can: 3.1 identify elements associated with the preparation of a programmable logic controller program 3.2 write programs using logic functions based on relay ladder logic 3.3 evaluate the range and type of advanced functions of programmable logic controllers 3.4 use and justify methods of testing and debugging hardware and software CONTENTS 1. Introduction 2. Tags, Labels and Identifiers 3. Flip Flops and Darlington Pairs 4. Flow Charts 5. Step Controllers 6. Function Diagrams - Grafcet SFC 7. Ladder Logic Diagrams 8. Instruction Lists 9. Structured Text 10. Function Block Diagrams 11 Truth Tables 12. Boolean Algebra D.J.Dunn 1

2 1. INTRODUCTION PLCs must contain a programme to make them perform their purpose. In order to create these programmes they must be compiled, tested (simulated ), debugged, converted and transferred to the PLC s memory. Figure 1 Many programming aids have been developed using either text or graphics. There are 5 programming languages/methods defined by the international standard IEC Ladder diagram described below. Function block diagram described below. Structured text (ST), textual described below. Instruction list (IL), textual described below. Sequential Function Chart (SFC) also called Grafcet and described below. There are various suites of software that can produce programmes in one or more of these styles. In the main these are made specifically for a specific manufacturer s range of PLCs and designed to interface with them. In one or more of these methods you will come across many other terms and this should be revised next. D.J.Dunn 2

3 2. TAGS, LABELS and IDENTIFIERS Consider the simple arrangement shown with four input elements connected to the PLC terminals and four output elements. Figure 2 The input and output terminals and other internal relays used inside a PLC are identified by the manufacturer. For example on the Mitsubishi range of PLCs, input terminals are numbered with an X such as X40, X41 and so on. Output terminals start with a Y (e.g.y40). Other letters are used for internal functions such as T for timers and C for counters. The input and output devices such as switches, sensors motors and actuators and many other items are more easily recognised with labels and tags such as Start, Stop, Guard, Fill, Conveyor, and so on. When programming the PLC, you have to set up the labels and tags first so that if you allocate Start to terminal X40, then whenever you enter Start, X40 is automatically identified. How this is done depends upon the programming method being used but it makes it easier to programme. Programming software also allows you to add comments with the labels to help you remember what they represent. 3. FLIP FLOPS and DARLINGTON PAIRS Consider the circuit shown. D.J.Dunn 3 Figure 3

4 The pneumatic cylinder identifier is A. A Darlington Pair has two outputs connected to the two solenoids. The single input is connected to the PLC terminal and tagged A. If the PLC puts a high signal onto this terminal, the solenoid A+ is switched on and the cylinder extends. If the PLC puts a low signal on the terminal the solenoid A- is energised and the A+ de-energised and the cylinder retracts. In this way only one terminal on the PLC is needed to control the cylinder as either on (high) or off (low). (Note that the same object may be achieved without a Darlington pair by using one solenoid and a spring at the other end). The cylinder has two proximity switches tagged A1 and A0. The problem is that when the piston is between the sensors, neither sensor is activated and this makes programming difficult. Ideally we wish to connect to a single input terminal on the PLC to indicate whether the cylinder is out (on) or in (off). This is done with a FLIP FLOP. A flip flop has two inputs. The status of the output remains unchanged until the other input changes. The status of the feedback signal remains unchanged when the piston is in between the sensors. Hence the single signal line to the PLC is high (on) when the cylinder is out and low (off) when the cylinder is in and does not change status while moving from one to the other. 4. FLOW CHARTS Flow diagrams (also called algorithms) are widely used to explain decision making processes that arrive at a logical answer. They are particularly useful for computer programmers because most programmes can be reduced to a series of YES or NO answers to each decision that must be made. In PLC work they can be a useful tool to help produce a ladder logic diagram. The main symbols for flow diagrams are shown here. Figure 4 WORKED EXAMPLE No.1 A machine has 2 actuators A and B that must perform the sequence A+(on) B+ (on) B- (off) A- (off). Each actuator has a sensors and flip flops so that a single signal indicates when they are on or off. Each actuator is operated by a single on/off signal. Produce a flow chart for the sequence. D.J.Dunn 4

5 SOLUTION Figure 5 Step 1 When we start we must make sure both actuators are off by switching them off. Step 2 Start the cycle by switching A on. Step 3 Check that A is on and if it is not, then keep looping back until it is. Step 4 When A is on switch on B. Step 4 Check that B is on and if it is not, then keep looping back until it is. Step 5 When B is on, switch B off. Step 6 Check if B is off and if it isn t keep looping back until it is. Step 7 When B is off, switch A off. Step 8 Check if A is off and if it isn t keep looping back until it is. Step 9 Loop back to the start. D.J.Dunn 5

6 5. STEP CONTROLLERS For a sequence like the one in the last section, the best form of PLC is a dedicated STEP controller. These have input and output terminals in the normal way. They are programmed through a built in panel with programming buttons and a display to show the programme and the state of the inputs and outputs. These conduct the steps one at a time waiting for the correct feedback data before executing the next step. A typical display from a sequential controller is like this. Figure 6 The display informs us that at step 63 in the programme, OUTPUTS 1 and 4 are already switched ON and before it will advance to step 64, the sensors attached to terminals 11 OR 12 must be activated. The OP numbers are codes for logical commands and in this case OP5 is an OR. Ordinary PLCs can be programmed in this way also if they are designed for it. 6. FUNCTION DIAGRAMS - GRAFCET SFC Sequential function chart (SFC) is a graphical programming language used for programmable logic controllers. It is one of the five languages defined by IEC standard. The SFC standard is defined in IEC The method is easiest to use with machines that go through a fixed sequence of operations with feed back after the completion of each step to confirm that it has happened but it is possible to do more advanced things as well. The chart starts with an initial step shown as a double box. This is followed by a transition state and here you must enter the tag (PB1) of the input switches that must be activated in order to proceed. In this case PB1 (Push Button 1) must be pressed before you can proceed to step 2. This is followed by the next step (2) and to this is attached an action box. In the action box you enter the tag name for the output element. In this case it switches on actuator B and the tag is B+. D.J.Dunn 6 Figure 7 If you want two things to happen together you attach a second action box as shown for step 6. At the end of the programme you may loop back to the beginning to complete the sequence. You may also use logic statements such as AND, OR, NAND and so on at the transition points. The next step is not performed until the logical condition is met. This makes it a versatile system. It is also possible to branch and jump to other routines.

7 WORKED EXAMPLE No.2 Produce a sequential function diagram to control the system shown so that the actuators perform the following sequence. It starts with everything off (retracted). Closing the push button switch PB1 starts the cycle. Next B goes on (B+) and then when reaching full stroke it goes off again (B-) When B is fully retracted (off) it is switched on again. When B reaches its full stroke for the second time, A is switched on. When A reaches full stroke both cylinders are switched off together and retract together. The cycle stops until PB1 is pushed again. B+ B- B+ A+ (A- B-) simultaneously, If you have access to Automation Studio or PneusimPro simulation software, you should construct the circuit and Grafcet programme using appropriate tags and simulate the result to see if it works. Figure 8 D.J.Dunn 7

8 SOLUTION In this kind of circuit it is normal to tag the actuators A, B, C etc. Plus (+) means extend, minus (-) means retract. These tags are allocated to the solenoids that produce the required action. The sensors are located at the two positions of each cylinder and are tagged A0/B0 for the retracted position and A1/B1 for the extended position and so on for each cylinder. Figure 9 D.J.Dunn 8

9 7. LADDER LOGIC DIAGRAMS INPUT AND OUTPUT ELEMENTS This is one of the programming methods defined in the international standard IEC This was briefly covered in outcome 2. Here we will revise it and take it further. Each rung of the ladder is an imaginary circuit linking the Plus and Minus bus rails. When the circuit is made, current flows and the output element is on. We shall only use American symbols here. Consider the circuit below. The left diagram shows a motor connected through an open contact. When the contact is closed the motor is switched on. The right circuit shows the motor connected through a closed contact and will normally be on. Operating the switch will disconnect the motor and turn it off. Figure 10 Remember that output elements may be a wide range of things such as heating elements, solenoids, valves and so on. They may also be imaginary relays used for various purposes such as latches and flags. They are commonly tagged with L, M or F. LATCHES On many PLCs, the programming allows you to use an imaginary switch with the same tag as the output. The left circuit below shows that when A is operated the motor comes on and the imaginary contact tagged M will close so that if A is opened again, the motor stays on. Another way to do this is to use another imaginary output (often called internal relay) which carry identifiers such as L for Latch and use them as shown in the right diagram. The origins of this are in the equipment that used actual relays which when turned on, closed a set of contacts connected across the switch so that the switch became latched. Figure 11 D.J.Dunn 9

10 The next diagram shows how two normally open switches are used to start and stop a motor. When the start switch is operated, the programme turns the motor and the Latch on. The latch keeps the motor running even when the start switch is released. The stop switch is seen by the programme as a normally closed switch. When operated, the programme breaks the circuit, the motor stops and the latch is broken. Figure 12 Some PLCs have a function called SET. When an output is set, it stays on no matter what else happens until the command RESET is used. This is simpler than using latches. LOGIC FUNCTIONS The rungs of the ladder can be designed to produce many logical functions such as AND, OR, NAND, NOR and so on. MORE ON FLIP FLOPS Figure 13 It is possible to make the PLC perform the flip flop function. On the downside, this means that two PLC input terminals are used per cylinder instead of 1. Consider a cylinder with two proximity detectors A1 and A0 which are both normally open switches. The programme treats A0 as if it were normally closed. Figure 14 When the cylinder is in between the two positions the contact A1 is open. The circuit is not made and F4 is off. If the cylinder moves out and operates A1, then the circuit is made and F4 is turned on. F4 is latched across A1 so if the piston retracts and A1 becomes disconnected, the circuit is still complete and F4 stays on. When fully retracted, the sensor A0 is activated and breaks the circuit. F4 is then switched off. If the piston moves forward again, F4 remains off until the contact A1 is again closed. In this way the status of the cylinder is indicated by F4. The contact F4 should now be used instead of the contact A in the programme. D.J.Dunn 10

11 TIMERS AND COUNTERS The way that timers and counters work varies from one type of PLC to another. The right diagram shows a timer. When switch A is closed, the timer starts running and after 2 seconds (indicated by k2) the timer contact closes and switches on the motor. The right hand diagram shows a counter. The counter is set to 8 (the k8 term). Each time switch A is closed the counter decrements. If the switch is closed and opened repeatedly, after the eighth closure, the counter contact closes and turns on the motor. Figure 15 There are many variations on this such as counting up rather than down. Once a counter or timer has operated, they need to be reset again before they can be reused. On some PLCs the timer automatically resets when the switch A is opened. The diagrams above show that switch B is used to reset the counter and timer. Reset is an example of a command being used as an output element. TIMING DIAGRAMS Many items of equipment such as traffic lights, may work entirely off timers. In order to help produce a PLC programme a timing diagram is very useful. These are useful when using timers to control a sequence and when the same set of conditions exist more than once in a cycle. WORKED EXAMPLE No.3 Two motors (outputs M1 and M2) are to be controlled as follows. When the run switch is operated both motors must run. After 4 minutes motor 1 must stop. Motor 2 continues running for another 2 minutes and stops. At this point a lamp is switched on. After a further 90 seconds, the lamp goes off and the cycle restarts. If a stop switch is operated at any time, the system will continue to the end of the cycle and then stop. Produce a PLC programme to make it work. D.J.Dunn 11

12 SOLUTION There are many solutions to this problem and this is just one. Timers may run in parallel or series or both. Here is the timing cycle diagram. D.J.Dunn 12 Figure 16 In this solution we shall need 3 timers T1, T2 and T3. T1 and T2 are started together. Timer 1 is set to 4 minutes, timer 2 to 6 minutes. Timer 3 is started after 6 minutes and runs for 90 seconds (1.5 minutes). There are a wide variety of PLC commands for ladder logic. For example, in the Mitsibushi system timers and counters are automatically reset when the logic which starts them running becomes untrue. In other types such as Bytronics - LADSIM programmes, you need to use a reset command to reset timers and counters. The following is the solution for a Mitsubishi using MEDOC programming. When the run switch is activated all timers are off. If the cycle is turned into ladder logic, this is the result. Figure 17 RUNG 1 Timers 1 and 2 are set running by timer 3 being off (i.e. as soon as the run switch is activated). RUNG 2 Motor 1 runs as long as timer 1 is off. After 4 minutes timer 1 comes on and motor 1 stops. RUNG 3 Motor 2 runs as long as timer 2 is off. Timer 2 continues running and after 6 minutes from start it comes on and motor 2 stops. RUNG 4 The lamp is on only if timer 2 is on (Motor 2 stopped) and timer 3 is off. Hence the lamp comes on as soon as motor 2 stops and goes off after 90 seconds when timer 3 comes on. Since timer 3 OFF is in rung 1, timers 1 and 2 are automatically reset and go off causing rung 1 to be reactivated. STOP If the STOP switch is put on, timers 1 and 2 will not reset and the cycle stops at the end. An alternative for Mitsubishi PLC is to use special relay M77 to interrupt the cycle by disabling all outputs. This would be activated by the interrupt switch in series with T3 to stop the cycle when the lamp comes off.

13 WORKED EXAMPLE No.4 Components pass along a chute and interrupt a light switch which goes low (off) each time it is interrupted. Every time 6 components have been counted, an eject operation is used to remove the batch and the then it all starts again. Produce a ladder logic diagram to do this operation. The counter is designated C460. SOLUTION Again, the solution depends upon the type of PLC and programming facilities. This is a solution for a Mitsubushi. EXPLANATION Figure 18 RUNG 1 Each time the light switch is interrupted, the counter is decremented by 1. The counter is set up for 6 counts (the k6). RUNG 2 When the counter value reaches zero, the counter contact (C460) closes and operates the eject mechanism. RUNG 3 The eject output is used as an input contact so that each time the eject command is executed, the counter is reset (R C460). RUNG 4 Most PLCs require an END command as shown to stop it running on into any other programme fragments left in the memory. D.J.Dunn 13

14 8 INSTRUCTION LISTS This is one of the programming methods defined in the international standard IEC Instruction List (IL) is a low level assembler like language that is based on similar instructions list languages found in a wide range of today s PLCs. Any of the programmes so far described can be created using statement lists. These use mnemonics to describe the action. The mnemonic is also known as the OPCODE. The OPERAND is the data to be executed by the opcode. For example the mnemonic Ld X400 is an instruction to load something (the opcode) and the something is the status of input X400 (the operand). Here are the mnemonics used by Mitsubishi. Figure 19 This is the statement list for the last ladder programme. Figure 20 Instructions sets can use labels and tags in just the same way as ladder diagrams and they can be automatically produced from a ladder diagram. D.J.Dunn 14

15 9. STRUCTURED TEXT This is one of the programming methods defined in the international standard IEC Structured Text (ST) is a high level text language that encourages structured programming. It has a language structure (syntax) that strongly resembles PASCAL and supports a wide range of standard functions and operators. For example IF A>B AND B<C THEN SET D ELSE RESET D. This means if A is larger than B and B is smaller than C then set D otherwise Reset D. It is possible to use tags instead as in the following example. If Speed1 > then Flow_Rate: = Offset_A1; Else Flow_Rate: = 100.0; Steam: = ON End_If; Some PLCs may be programmed using computer programming language such as C++. Elements of this are incorporated in the SFC programming method where statements such as these may be used at the transition state point. PLCs like the Festo/Beck range use this form of programming. 10 FUNCTION BLOCK DIAGRAMS This is one of the programming methods defined in the international standard IEC A block has inputs and outputs. The outputs correspond to a defined function written for the box in terms of the inputs. A simple example is shown below. The function block is called ADD and it is defined as adding the three inputs A, B and C. The result is placed on output O. All these have to be defined in structured text. Structured Text Function Block O :=ADD(A, B, C); Figure 21 The next example shows a block named as divide and its function is to divide the input a by the input b and place the result at output c. The definition in structured text is also shown. Structured Text FUNCTION_BLOCK DIVIDE VAR_INPUT A: INT; B: INT; END_VAR VAR_OUTPUT C: INT; END_VAR IF B<>0 THEN C:=A/B ELSE C:= 0; END_IF; END_FUNCTION_BLOCK Function Block Figure 22 D.J.Dunn 15

16 11 TRUTH TABLES This was explained in Outcome 2 but we need to examine them again. A truth table is a useful tool to a programmer and it should be considered as a binary number or pattern covering every possible combination of input conditions. Suppose we have four input sensors. We need a table with 4 bits and we will put the most significant bit (MSB) on the left. Figure 23 The maximum possible value is 15 so there are 16 possible combinations of input conditions. Show each starting with a binary value of 0 and ending with 15. When the truth table has been filled in as shown, you must decide which combinations switch on which outputs. Figure 24 Suppose there are two outputs A and B and these are only to be switched on as follows. A is on when S3 OR S2 AND S1 is on. B is on when S4 or S1 is on. The truth table below only shows the conditions that switch on A and B. If the outputs are being controlled by a computer, it is useful to know the binary value required to switch the outputs on. The number that switches on A is 2, 3 and 12. The number hat switches on B are 1, 3 and 8. D.J.Dunn 16 Figure 25

17 12. BOOLEAN ALGEBRA This was partially covered in outcome 2 but we need to refresh ourselves on the subject. It is a useful tool for writing out and simplifying logic statements in short hand. A plus sign means OR and a dot means AND. Here are the four main logic gates shown as American Logic symbols along with the ladder logic diagram, truth table and Boolean expression. Remember a circle on the symbol at the input or output is a NOT gate and inverts the action. Here is another example. Figure 26 Figure 27 Boolean algebra can be used to simplify logic circuits. Consider the following circuit. Figure 28 D.J.Dunn 17

18 The output may be simplified first by removing the square brackets which makes no difference to the result. Z= (A.C) + (A.D) + (B.C) + (B.D) Next take out common A and B Z = A. (C + D) + B. (C + D) Next form a new bracket Z = (A + B). (C + D) Now redraw the circuit. Figure 29 SELF ASSESSMENT EXERCISE 1 A machine is switched on by either of two push buttons A or B. There is a safety barrier on the machine with a limit switch D at the closed position. In addition there is proximity switch C to detect if anyone is standing inside the barrier. If there is the machine must not start. All have normally open contacts. Figure 30 Write out a suitable Boolean expression. Draw a ladder logic diagram for the Boolean expression Write out the instruction set. Construct the truth Table. D.J.Dunn 18

19 ANSWERS Z = (A + B). C. D Ld A Or B Ani C And D Figure 31 SELF ASSESSMENT EXERCISE 2 The diagram shows a pneumatic system for sequencing the operation of two cylinders A and B. Using worked example No. 2 as a template, produce a SFC diagram for producing one of the sequences from the list below. A+ means A extended, A- means A retracted, B+ means B extended, B- means B retracted. SEQUENCE 1 A+, B+, B-, A-, (B+ and A+), (B- and A-) SEQUENCE 2 A+, A-, (B+ and A+), B-,A- SEQUENCE 3 A+, (A- and B+), A+, (A- and B-) SEQUENCE 4 B+, B-, (A+ and B+), A-, B- SEQUENCE 5 B+, A+, (A- and B-), (A+ and B+), A-, B- SEQUENCE 6 B+, A+, A- (A+ and B-), A- SEQUENCE 7 A+, B+, B-, (A- and B+), B- SEQUENCE 8 A+, A-, (A+ and B+), A-,B- SEQUENCE 9 A+, (A- and B+), (B- and A+), A- SEQUENCE 10 B+, B-, (B+ and A+), A-, B+ Figure 32 D.J.Dunn 19

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