PROGRAMMABLE LOGIC CONTROLLERS - PLC

Transcription

1 UNESCO-NIGERIA TECHNICAL & VOCATIONAL EDUCATION REVITALISATION PROJECT-PHASE II NATIONAL DIPLOMA IN ELECTRICAL ENGINEERING TECHNOLOGY PROGRAMMABLE LOGIC CONTROLLERS - PLC COURSE CODE: EEC 246 YEAR II- SEMESTER IV THEORY Version 1: December

2 TABLE OF CONTENTS Week Introduction to PLC Solid state logical circuits Ladder Logic Week Simple Relays controllers Motor control circuit Capabilities of programmable logic controllers Week Ladder diagram Review Week Ladder Logic Programme Programming PLC connections...22 Week PLC operation PLC ladder logic input Ladder Logic Outputs A CASE STUDY(Assignment)..28 Week6..29

3 6.1 Solution (case study) Tutorials 30 Week Digital logic functions.. 33 Week Permissive and interlock circuits.41 Week Digital Logic Design Concepts The basic Idea of PLC Digital logic with feedback Week The S-R latch.50 Week The D latch..56 Week Edge-triggered latches: Flip-Flops 59 Week Monostable multivibrators..67

4 Week Monostable multivibrators.72 Week The J-K flip-flop 78

5 PROGRMMABLE LOGIC CONTROLLER PLC Week 1 General Objectives 1. Know general PLC issues 2. To be able to write simple ladder logic programs. 3. Understand the operation of a PLC. 4. Execute and run programmable controller programs with actual loads. 5. Identify and isolate common problems encountered in the control systems of automated facilities. The student should be able to: - 1) Know general PLC issues. 2) Know the construction of PLC. 3) Understand the function of PLC 4) Understand the operation of PLC. 5) Understand the ladder control action 6) Understand how to follow the ladder format (layout). 7) Writes a simple ladder logic programme. 8) Understand some troubleshooting 9) Study some applications of PLC used in electronics. 10) Study some applications of PLC used in electricity. 11) Study some applications of PLC used in machine shop. 1

6 PROGRMMABLE LOGIC CONTROLLER PLC Week INTRODUCTION Control engineering has evolved over time. In the past humans was the main method for controlling a system. More recently electricity has been used for control and early electrical control was based on relays. These relays allow power to be switched on and off without a mechanical switch. It is common to use relays to make simple logical control decisions. The development of low cost computer has brought the most recent revolution, the Programmable Logic Controller (PLC). The advent of the PLC began in the 1970s, and has become the most common choice for manufacturing controls. PLC's have been gaining popularity on the factory floor and will probably remain predominant for some time to come. Most of this is because of the advantages they offer. Cost effective for controlling complex systems. Flexible and can be reapplied to control other systems quickly and easily. Computational abilities allow more sophisticated control. Trouble shooting aids make programming easier and reduce downtime. Reliable components make these likely to operate for years before failure. 1.2 Solid state logical circuits. Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays. Relays are far from obsolete in modern design, but have been replaced in many of their former roles as logic-level control devices, relegated most often to those applications demanding high current and/or high voltage switching. Systems and processes requiring "on/off" control abound in modern commerce and industry, but such control systems are rarely built from either electromechanical relays or discrete logic gates. Instead, digital computers fill the need, which may be programmed to do a variety of logical functions. In the late 1960's an American company named Bedford Associates released a computing device they called the MODICON. As an acronym, it meant Modular Digital Controller, and later became the name of a company division devoted to the design, manufacture, and sale of these special- 2

7 PROGRMMABLE LOGIC CONTROLLER PLC Week 1 purpose control computers. Other engineering firms developed their own versions of this device, and it eventually came to be known in non-proprietary terms as a PLC, or Programmable Logic Controller. The purpose of a PLC was to directly replace electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks. A PLC has many "input" terminals, through which it interprets "high" and "low" logical states from sensors and switches. It also has many output terminals, through which it outputs "high" and "low" signals to power lights, solenoids, contactors, small motors, and other devices lending themselves to on/off control. In an effort to make PLC's easy to program, their programming language was designed to resemble ladder logic diagrams. Thus, an industrial electrician or electrical engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC to perform the same control functions. PLC's are industrial computers, and as such their input and output signals are typically 120 volts AC, just like the electromechanical control relays they were designed to replace. Although some PLC's have the ability to input and output low-level DC voltage signals of the magnitude used in logic gate circuits, this is the exception and not the rule. Signal connection and programming standards vary somewhat between different models of PLC, but they are similar enough to allow a "generic" introduction to PLC programming here. The following illustration shows a simple PLC, as it might appear from a front view figure 1.1. Two screw terminals provide connection to 120 volts AC for powering the PLC's internal circuitry, labeled L1 and L2. Six screw terminals on the left-hand side provide connection to input devices, each terminal representing a different input "channel" with its own "X" label. The lower-left screw terminal is a "Common" connection, which is generally connected to L2 (neutral) of the 120 VAC power source. 3

8 PROGRMMABLE LOGIC CONTROLLER PLC Week 1 Figure 1.1 Simple PLC view. Inside the PLC housing, connected between each input terminal and the Common terminal, is an optic-isolator device (Light-Emitting Diode) that provides an electrically isolated "high" logic signal to the computer's circuitry (a photo-transistor interprets the LED's light) when there is 120 VAC power applied between the respective input terminal and the Common terminal. An indicating LED on the front panel of the PLC gives visual indication of an "energized" input: 4

9 PROGRMMABLE LOGIC CONTROLLER PLC Week 1 Figure 1.2 PLC gives visual indication of an "energized" output: Output signals are generated by the PLC's computer circuitry activating a switching device (transistor, TRIAC, or even an electromechanical relay), connecting the "Source" terminal to any of the "Y-" labeled output terminals. The "Source" terminal, correspondingly, is usually connected to the L1 side of the 120 VAC power source. As with each input, an indicating LED on the front panel of the PLC gives visual indication of an "energized" output: Figure

10 PROGRMMABLE LOGIC CONTROLLER PLC Week 1 Figure 1.3 Output Y energized In this way, the PLC is able to interface with real-world devices such as switches and solenoids. The actual logic of the control system is established inside the PLC by means of a computer program. This program dictates which output gets energized under which input conditions. Although the program itself appears to be a ladder logic diagram, with switch and relay symbols, there are no actual switch contacts or relay coils operating inside the PLC to create the logical relationships between input and output. These are imaginary contacts and coils, if you will. The program is entered and viewed via a personal computer connected to the PLC's programming port. 6

11 PROGRMMABLE LOGIC CONTROLLER PLC Week Ladder Logic Ladder logic is the main programming method used for PLC's. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and trades people was greatly reduced. Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in See Simple Relay Layouts and Schematics. Figure (1.4) when a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed (conducting). Figure (1.4) Simple Relay Layouts and schematics 7

12 PROGRMMABLE LOGIC CONTROLLER PLC Week Simple Relay Controllers Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in See A Simple Relay Controller. Figure 2.1 in this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on. Figure 2.1 See a Simple Relay Controller The example in See: A Simple Relay Controller figure (2.1) does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. See: A PLC Illustrated With Relays Figure 2.2 shows a more complete representation of the PLC. Here there are two inputs from push buttons. We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC that will turn on a light. Note, 8

13 PROGRMMABLE LOGIC CONTROLLER PLC Week 2 in actual PLC's inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a computer program that the user can enter and change. Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. Do not think that the ladder logic in the PLC needs to match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types. Figure 2.2 A PLC Illustrated With Relays 2.2 Motor control circuits The interlock contacts installed in the previous section's motor control circuit work fine, but the motor will run only as long as each pushbutton switch is held down. If we wanted to keep the motor running even after the operator takes his or her hand off the control switch(es), we could change the circuit in a couple of different ways: we could replace the pushbutton switches with toggle switches, or we could add some more relay logic to "latch" the control circuit with a single, momentary actuation of either switch. Let's see how the second approach is implemented, since it is commonly used in industry: see circuit 2.3a and 2.3b 9

14 PROGRMMABLE LOGIC CONTROLLER PLC Week 2 Figure 2.3a Motor control circuit. When the "Forward" pushbutton is actuated, M 1 will energize, closing the normally-open auxiliary contact in parallel with that switch. When the pushbutton is released, the closed M 1 auxiliary contact will maintain current to the coil of M 1, thus latching the "Forward" circuit in the "on" state. The same sort of thing will happen when the "Reverse" pushbutton is pressed. These parallel auxiliary contacts are sometimes referred to as seal-in contacts, the word "seal" meaning essentially the same thing as the word latch. However, this creates a new problem: how to stop the motor! As the circuit exists right now, the motor will run either forward or backward once the corresponding pushbutton switch is pressed, and will continue to run as long as there is power. To stop either circuit (forward or backward), we require some means for the operator to interrupt power to the motor contactors. We'll call this new switch, Stop: 10

15 PROGRMMABLE LOGIC CONTROLLER PLC Week 2 Figure 2.3b Motor control circuit. Now, if either forward or reverse circuits are latched, they may be "unlatched" by momentarily pressing the "Stop" pushbutton, which will open either forward or reverse circuit, de-energizing the energized contactor, and returning the seal-in contact to its normal (open) state. The "Stop" switch, having normally-closed contacts, will conduct power to either forward or reverse circuits when released. 2.3 Capabilities of programmable logic controllers. This section on programmable logic controllers illustrates just a small sample of their capabilities. As computers, PLCs can perform timing functions (for the equivalent of time-delay relays), drum sequencing, and other advanced functions with far greater accuracy and reliability than what is possible using electromechanical logic devices. Most PLCs have the capacity for far more than six inputs and six outputs. The following photograph shows several input and output modules of a single Allen-Bradley PLC, see figure2.4a and 2.4b. 11

16 PROGRMMABLE LOGIC CONTROLLER PLC Week 2 Figure 2.4a View of several input and output module. With each module having sixteen "points" of either input or output, this PLC has the ability to monitor and control dozens of devices. Fit into a control cabinet, a PLC takes up little room, especially considering the equivalent space that would be needed by electromechanical relays to perform the same functions: 12

17 PROGRMMABLE LOGIC CONTROLLER PLC Week 2 Figure 2.4b View of several input and output module. One advantage of PLCs that simply cannot be duplicated by electromechanical relays is remote monitoring and control via digital computer networks. Because a PLC is nothing more than a special-purpose digital computer, it has the ability to communicate with other computers rather easily. The following photograph shows a personal computer displaying a graphic image of a real liquid-level process (a pumping, or "lift," station for a municipal wastewater treatment system) controlled by a PLC. The actual pumping station is located miles away from the personal computer display: 13

18 PROGRMMABLE LOGIC CONTROLLER PLC Week 2 Figure 2.5 Monitor (output device) 14

19 PROGRMMABLE LOGIC CONTROLLER PLC Week Ladder diagrams Ladder diagrams are specialized schematics commonly used to document industrial control logic systems. They are called "ladder" diagrams because they resemble a ladder, with two vertical rails (supply power) and as many "rungs" (horizontal lines) as there are control circuits to represent. If we wanted to draw a simple ladder diagram showing a lamp that is controlled by a hand switch, it would look like this: Figure 3.1 Figure 3.1a ladder diagram showing lamp controlled by a hand switch The "L 1 " and "L 2 " designations refer to the two poles of a 120 VAC supply, unless otherwise noted. L 1 is the "hot" conductor, and L 2 is the grounded ("neutral") conductor. These designations have nothing to do with inductors, just to make things confusing. The actual transformer or generator supplying power to the circuit of figure 3.1a is omitted for simplicity. In reality, the circuit looks something like this: Figure 3.1b Figure 3.1b ladder diagram showing lamp controlled by a hand switch 15

20 PROGRMMABLE LOGIC CONTROLLER PLC Week 3 Typically in industrial relay logic circuits, but not always, the operating voltage for the switch contacts and relay coils will be 120 volts AC. Lower voltage AC and even DC systems are sometimes built and documented according to "ladder" diagrams: See figure 3.2, Figure 3.2 Industrial relay logic circuits So long as the switch contacts and relay coils are all adequately rated, it really doesn't matter what level of voltage is chosen for the system to operate with. Note the number "1" on the wire between the switch and the lamp. In the real world, that wire would be labeled with that number, using heat shrink or adhesive tags, wherever it was convenient to identify. Wires leading to the switch would be labeled "L 1 " and "1," respectively. Wires leading to the lamp would be labeled "1" and "L 2," respectively. These wire numbers make assembly and maintenance very easy. Each conductor has its own unique wire number for the control system that it's used in. Wire numbers do not change at any junction or node, even if wire size, color, or length changes going into or out of a connection point. Of course, it is preferable to maintain consistent wire colors, but this is not always practical. What matters is that any one, electrically continuous point in a control circuit possesses the same wire number. Take this circuit section, for example, figure 3.3 with wire 25 as a single, electrically continuous point threading to many different devices: 16

21 PROGRMMABLE LOGIC CONTROLLER PLC Week 3 Figure 3.3 Circuit with continuous point threading to many different devices: In ladder diagrams, the load device (lamp, relay coil, solenoid coil, etc.) is almost always drawn at the right-hand side of the rung. While it doesn't matter electrically where the relay coil is located within the rung, it does matter which end of the ladder's power supply is grounded, for reliable operation. Take for instance this circuit: figure 3.4 Figure 3.4 Lamp located at the right-hand side. 17

22 PROGRMMABLE LOGIC CONTROLLER PLC Week 3 Here, the lamp (load) is located on the right-hand side of the rung, and so is the ground connection for the power source. This is no accident or coincidence; rather, it is a purposeful element of good design practice. Suppose that wire 1 were to accidently come in contact with ground, the insulation of that wire having been rubbed off so that the bare conductor came in contact with grounded, metal conduit. Our circuit would now function like this: Figure 3.5 Wire 1 in contact to accidental ground With both sides of the lamp connected to ground, the lamp will be "shorted out" and unable to receive power to light up. If the switch were to close, there would be a short-circuit, immediately blowing the fuse. However, consider what would happen to the circuit with the same fault (wire 1 coming in contact with ground), except this time we'll swap the positions of switch and fuse (L 2 is still grounded): see figure

23 PROGRMMABLE LOGIC CONTROLLER PLC Week 3 Figure 3.6 Wire 1 in contact to accidental ground but with now with a switch. This time the accidental grounding of wire 1 will force power to the lamp while the switch will have no effect. It is much safer to have a system that blows a fuse in the event of a ground fault than to have a system that uncontrollably energizes lamps, relays, or solenoids in the event of the same fault. For this reason, the load(s) must always be located nearest the grounded power conductor in the ladder diagram Review: Ladder diagrams (sometimes called "ladder logic") are a type of electrical notation and symbolic frequently used to illustrate how electromechanical switches and relays are interconnected. The two vertical lines are called "rails" and attach to opposite poles of a power supply, usually 120 volts AC. L 1 designates the "hot" AC wire and L 2 the "neutral" (grounded) conductor. Horizontal lines in a ladder diagram are called "rungs," each one representing a unique parallel circuit branch between the poles of the power supply. 19

24 PROGRMMABLE LOGIC CONTROLLER PLC Week 3 Typically, wires in control systems are marked with numbers and/or letters for identification. The rule is, all permanently connected (electrically common) points must bear the same label. 20

25 PROGRMMABLE LOGIC CONTROLLER PLC Week Ladder Logic Programme Many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in See a Seal-in Circuit Figure 4.1 is an example of this, it is called a seal in circuit. In this circuit the current can flow through either branch of the circuit, through the contacts labelled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on, and keep output B on even if input A goes off. After B is turned on the output B will not turn off Programming Figure 4.1 a Seal-in Circuit The first PLC's were programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a computer - but, this method has stuck and it is the most common technique for programming PLC's today. An example of ladder logic can be seen in See a Simple Ladder Logic Diagram Figure 3.2 To interpret this diagram imagine that the power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the neutral rail. In the figure there are two rungs, and on each rung there are combinations of inputs (two vertical lines) and outputs (circles). If the inputs are opened or closed in the right combination the power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail. An input can come from a sensor, switch, or any other type of sensor. An output will be some device outside the PLC that is switched on or off, such as lights or motors. In the top rung the contacts are 21

26 PROGRMMABLE LOGIC CONTROLLER PLC Week 4 normally open and normally closed. Which means if input A is on and input B is off, then power will flow through the output and activate it? Any other combination of input values will result in the output X being off. Figure 4.2 Simple Ladder Logic Diagram 4.2 PLC Connections When a process is controlled by a PLC, it uses inputs from sensors to make decisions and update outputs to drive actuators, as shown in See the Separation of Controller and Process. Figure (4.3) the process is a real process that will change over time. Actuators will drive the system to new states (or modes of operation). This means that the controller is limited by the sensors available, if an input is not available, the controller will have no way to detect a condition. 22

27 PROGRMMABLE LOGIC CONTROLLER PLC Week 4 Figure 4.3 the Separation of Controller and Process The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any computer this does not happen instantly. See The Scan Cycle of a PLC Figure (4.4) shows the basic operation cycle of a PLC. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there is an error. For example, if the PLC backup battery is low and power was lost, the memory will be corrupt and this will result in a fault. If the PLC passes the sanity checks it will then scan (read) all the inputs. After the inputs values are stored in memory the ladder logic will be scanned (solved) using the stored values - not the current values. This is done to prevent logic problems when inputs change during the ladder logic scan. When the ladder logic scan is complete the outputs will be scanned (the output values will be changed). After this the system goes back to do a sanity check, and the loop continues indefinitely. Unlike normal computers, the entire program will be run every scan. Typical times for each of the stages are in the order of milliseconds. 23

28 PROGRMMABLE LOGIC CONTROLLER PLC Week 4 Figure 4.4 the Scan Cycle of a PLC 24

29 PROGRMMABLE LOGIC CONTROLLER PLC Week PLC operation There are four basic steps in the operation of all PLCs; Input Scan, Program Scan, Output Scan, and Housekeeping. These steps continually take place in a repeating loop. Four Steps in the PLC operations 1. Input Scan Detects the state of all input devices that are connected to the PLC 2. Program Scan Executes the user created program logic 3. Output Scan Energizes or de-energize all output devices that are connected to the PLC. 4. Housekeeping This step includes communications with programming terminals, internal diagnostics, etc... Figure 5.1 These steps are continually processed in a loop. 5.2 PLC ladder logic input PLC inputs are easily represented in ladder logic. In See Ladder Logic Inputs Figure (5.2) there are three types of inputs shown. The first two are normally open and normally closed inputs, discussed previously. The IIT (Immediate Input) function allows inputs to be read after the input scan, while the ladder logic is being scanned. This allows ladder logic to examine input values more often than once every cycle. 25

30 PROGRMMABLE LOGIC CONTROLLER PLC Week 5 Figure (5.2) Ladder Logic Inputs 5.3 Ladder Logic Outputs In ladder logic there are multiple types of outputs, but these are not consistently available on all PLC's. Some of the outputs will be externally connected to devices outside the PLC, but it is also possible to use internal memory locations in the PLC. Six types of outputs are shown in Outputs. Figure (6.1) the first is a normal output, when energized the output will turn on, and energize an output. The circle with a diagonal line through is a normally on output. When energized, the output will turn off. This type of output is not available on all PLC types. When initially energized the OSR (One Shot Relay) instruction will turn on for one scan, but then be off for all scans after, until it is turned off. The L (latch) and U (unlatch) instructions can be used to lock outputs on. When an L output is energized the output will turn on indefinitely, even when the output coil is de-energized. The output can only be turned off using a U output. The last instruction is the IOT (Immediate Output) that will allow outputs to be updated without having to wait for the ladder logic scan to be completed. 26

31 PROGRMMABLE LOGIC CONTROLLER PLC Week 5 Figure (5.3) Ladder Logic Outputs 27

32 PROGRMMABLE LOGIC CONTROLLER PLC Week A CASE STUDY(Assignment) Problem: Try to develop (without looking at the solution) a relay based controller that will allow three switches in a room to control a single light. 28

33 PROGRMMABLE LOGIC CONTROLLER PLC Week Solution (case study) Fig.6.1 Switch Arrangement 29

34 PROGRMMABLE LOGIC CONTROLLER PLC Week Tutorials Summary Normally open and closed contacts. Relays and their relationship to ladder logic. PLC outputs can be inputs, as shown by the seal in circuit. Programming can be done with ladder logic, mnemonics, SFC's, and structured text. There are multiple ways to write a PLC program. PROBLEMS 1. A PLC can effectively replace a number of components. Give examples and discuss some good and bad applications of PLC's. (Ans. PLC could replace a few relays. In this case the relays might be easier to install and less expensive. To control a more complex system the controller might need timing, counting and other mathematical calculations. In this case a PLC would be a better choice._ 2. Give an example of where a PLC could be used. (Ans. to control a conveyor system) 3. Why would relays be used in place of PLC's? (Ans. for simple designs) 4. Give a concise description of a PLC. (Ans. PLC is a computer based controller that uses inputs to monitor a process, and uses outputs to control a process. A simple program is used to set the controller behaviour.) 5. List the advantages of a PLC over relays. (Ans. 1- less expensive for complex processes. 30

35 PROGRMMABLE LOGIC CONTROLLER PLC Week 6 2- Debugging tools. 3- Reliable. 4- Flexible. 5- Easy to expand. 6. Explain the trade-offs between relays and PLC's for control applications. (Ans. tradeoffs include: cost, complexity, easy of debugging, etc.) 7. Explain why ladder logic outputs are coils? (Ans. the ladder logic outputs were modelled on relay logic diagrams. The output in a relay ladder diagram is a relay coil. This is normally drawn as a circle.) 8. In the figure below, will the power for the output on the first rung normally be on or off? Would the output on the second rung normally be on or off? (Ans. off, on) 9. Write the mnemonic program for the Ladder Logic below. 31

36 PROGRMMABLE LOGIC CONTROLLER PLC Week 6 (Ans. LD 100, LD 101, OR, ST 201) Multiple program examples: 32

37 PROGRMMABLE LOGIC CONTROLLER PLC Week Digital logic functions We can construct simply logic functions for our hypothetical lamp circuit figure 7.1, using multiple contacts, and document these circuits quite easily and understandably with additional rungs to our original "ladder." If we use standard binary notation for the status of the switches and lamp (0 for not actuated or de-energized; 1 for actuated or energized), a truth table can be made to show how the logic works: Figure 7.1 hypothetical lamp circuit Now, the lamp will come on if either contact A or contact B is actuated, because all it takes for the lamp to be energized is to have at least one path for current from wire L 1 to wire 1. What we have is a simple OR logic function, implemented with nothing more than contacts and a lamp. We can mimic the AND logic function by wiring the two contacts in series instead of parallel: 33

38 PROGRMMABLE LOGIC CONTROLLER PLC Week 7 Figure 7.2 b hypothetical lamp circuit Now, the lamp energizes only if contact A and contact B are simultaneously actuated. A path exists for current from wire L 1 to the lamp (wire 2) if and only if both switch contacts are closed. The logical inversion, or NOT, function can be performed on a contact input simply by using a normally-closed contact instead of a normally-open contact: Figure 7.2c hypothetical lamp circuit Now, the lamp energizes if the contact is not actuated, and de-energizes when the contact is actuated. 34

39 PROGRMMABLE LOGIC CONTROLLER PLC Week 7 If we take our OR function and invert each "input" through the use of normally-closed contacts, we will end up with a NAND function. In a special branch of mathematics known as Boolean algebra, this effect of gate function identity changing with the inversion of input signals is described by DeMorgan's Theorem, a subject to be explored in more detail in a later chapter. Figure 7.2d hypothetical lamp circuit The lamp will be energized if either contact is unactuated. It will go out only if both contacts are actuated simultaneously. Likewise, if we take our AND function and invert each "input" through the use of normally-closed contacts, we will end up with a NOR function: 35

40 PROGRMMABLE LOGIC CONTROLLER PLC Week 7 Figure 7.2e hypothetical lamp circuit A pattern quickly reveals itself when ladder circuits are compared with their logic gate counterparts: Parallel contacts are equivalent to an OR gate. Series contacts are equivalent to an AND gate. Normally-closed contacts are equivalent to a NOT gate (inverter). We can build combinational logic functions by grouping contacts in series-parallel arrangements, as well. In the following example, we have an Exclusive-OR function built from a combination of AND, OR, and inverter (NOT) gates: 36

41 PROGRMMABLE LOGIC CONTROLLER PLC Week 7 Figure7.2f hypothetical lamp circuit The top rung (NC contact A in series with NO contact B) is the equivalent of the top NOT/AND gate combination. The bottom rung (NO contact A in series with NC contact B) is the equivalent of the bottom NOT/AND gate combination. The parallel connection between the two rungs at wire number 2 forms the equivalent of the OR gate, in allowing either rung 1 or rung 2 to energize the lamp. To make the Exclusive-OR function, we had to use two contacts per input: one for direct input and the other for "inverted" input. The two "A" contacts are physically actuated by the same mechanism, as are the two "B" contacts. The common association between contacts is denoted by the label of the contact. There is no limit to how many contacts per switch can be represented in a ladder diagram, as each new contact on any switch or relay (either normally-open or normally-closed) used in the diagram is simply marked with the same label. 37

42 PROGRMMABLE LOGIC CONTROLLER PLC Week 7 Sometimes, multiple contacts on a single switch (or relay) are designated by a compound labels, such as "A-1" and "A-2" instead of two "A" labels. This may be especially useful if you want to specifically designate which set of contacts on each switch or relay is being used for which part of a circuit. For simplicity's sake, I'll refrain from such elaborate labeling in this lesson. If you see a common label for multiple contacts, you know those contacts are all actuated by the same mechanism. If we wish to invert the output of any switch-generated logic function, we must use a relay with a normally-closed contact. For instance, if we want to energize a load based on the inverse, or NOT, of a normally-open contact, we could do this: Figure7.2g hypothetical lamp circuit We will call the relay, "control relay 1," or CR 1. When the coil of CR 1 (symbolized with the pair of parentheses on the first rung) is energized, the contact on the second rung opens, thus de-energizing the lamp. From switch A to the coil of CR 1, the logic function is noninverted. The normally-closed contact actuated by relay coil CR 1 provides a logical inverter function to drive the lamp opposite that of the switch's actuation status. 38

43 PROGRMMABLE LOGIC CONTROLLER PLC Week 7 Applying this inversion strategy to one of our inverted-input functions created earlier, such as the OR-to-NAND, we can invert the output with a relay to create a noninverted function: Figure 7.2h hypothetical lamp circuit From the switches to the coil of CR 1, the logical function is that of a NAND gate. CR 1 's normallyclosed contact provides one final inversion to turn the NAND function into an AND function. Review: Parallel contacts are logically equivalent to an OR gate. Series contacts are logically equivalent to an AND gate. Normally closed (N.C.) contacts are logically equivalent to a NOT gate. 39

44 PROGRMMABLE LOGIC CONTROLLER PLC Week 7 A relay must be used to invert the output of a logic gate function, while simple normallyclosed switch contacts are sufficient to represent inverted gate inputs. 40

45 PROGRMMABLE LOGIC CONTROLLER PLC Week Permissive and interlock circuits A practical application of switch and relay logic is in control systems where several process conditions have to be met before a piece of equipment is allowed to start. A good example of this is burner control for large combustion furnaces. In order for the burners in a large furnace to be started safely, the control system requests "permission" from several process switches, including high and low fuel pressure, air fan flow check, exhaust stack damper position, access door position, etc. Each process condition is called a permissive, and each permissive switch contact is wired in series, so that if any one of them detects an unsafe condition, the circuit will be opened: see fig.8.1 Figure 8.1 Permissive switch contact wired in series If all permissive conditions are met, CR 1 will energize and the green lamp will be lit. In real life, more than just a green lamp would be energized: usually a control relay or fuel valve solenoid would be placed in that rung of the circuit to be energized when all the permissive contacts were "good:" that is, all closed. If any one of the permissive conditions are not met, the series string of switch contacts will be broken, CR 2 will de-energize, and the red lamp will light. 41

46 PROGRMMABLE LOGIC CONTROLLER PLC Week 8 Note that the high fuel pressure contact is normally-closed. This is because we want the switch contact to open if the fuel pressure gets too high. Since the "normal" condition of any pressure switch is when zero (low) pressure is being applied to it, and we want this switch to open with excessive (high) pressure, we must choose a switch that is closed in its normal state. Another practical application of relay logic is in control systems where we want to ensure two incompatible events cannot occur at the same time. An example of this is in reversible motor control, where two motor contactors are wired to switch polarity (or phase sequence) to an electric motor, and we don't want the forward and reverse contactors energized simultaneously: see fig Figure 8.2 Control system of motor. When contactor M 1 is energized, the 3 phases (A, B, and C) are connected directly to terminals 1, 2, and 3 of the motor, respectively. However, when contactor M 2 is energized, phases A and B are reversed, A going to motor terminal 2 and B going to motor terminal 1. This reversal of phase wires results in the motor spinning the opposite direction. Let's examine the control circuit for these two contactors: see fig

47 PROGRMMABLE LOGIC CONTROLLER PLC Week 8 Figure 8.3 Motor control system. Take note of the normally-closed "OL" contact, which is the thermal overload contact activated by the "heater" elements wired in series with each phase of the AC motor. If the heaters get too hot, the contact will change from its normal (closed) state to being open, which will prevent either contactor from energizing. This control system will work fine, so long as no one pushes both buttons at the same time. If someone were to do that, phases A and B would be short-circuited together by virtue of the fact that contactor M 1 sends phases A and B straight to the motor and contactor M 2 reverses them; phase A would be shorted to phase B and vice versa. Obviously, this is a bad control system design! To prevent this occurrence from happening, we can design the circuit so that the energization of one contactor prevents the energization of the other. This is called interlocking, and it is accomplished through the use of auxiliary contacts on each contactor, as such: 43

48 PROGRMMABLE LOGIC CONTROLLER PLC Week 8 Figure 8.4 interlocking auxiliary contacts on each contactor Now, when M 1 is energized, the normally-closed auxiliary contact on the second rung will be open, thus preventing M 2 from being energized, even if the "Reverse" pushbutton is actuated. Likewise, M 1 's energization is prevented when M 2 is energized. Note, as well, how additional wire numbers (4 and 5) were added to reflect the wiring changes. It should be noted that this is not the only way to interlock contactors to prevent a short-circuit condition. Some contactors come equipped with the option of a mechanical interlock: a lever joining the armatures of two contactors together so that they are physically prevented from simultaneous closure. For additional safety, electrical interlocks may still be used, and due to the simplicity of the circuit there is no good reason not to employ them in addition to mechanical interlocks. REVIEW: Switch contacts installed in a rung of ladder logic designed to interrupt a circuit if certain physical conditions are not met are called permissive contacts, because the system requires permission from these inputs to activate. Switch contacts designed to prevent a control system from taking two incompatible actions at once (such as powering an electric motor forward and backward simultaneously) are called interlocks. 44

49 PROGRMMABLE LOGIC CONTROLLER PLC Week Digital Logic Design Concepts The basic digital logic gates include: Logic level high - 1 or one HIGH or H or +Vdd or yes or ON or True Logic level low -0 or ZERO or LOW or L or GND or no or OFF or False Definition of some terms in logic gates 1. Assert: A control signal is asserted when the action control by the signal is being done. Ex: Assume a control signal labeled en en is asserted when en is high 2. Active low signal: A signal that is asserted when it is a logic level low. 3. Active high signal: A signal that is asserted when it is a logic level high Types of digital logic circuits 1. Combinatorial Logic Circuits No Memory (or Registers) 2. Sequential Logic Circuits Memory (or Registers) 3. Asynchronous Logic Circuits No common clock 4. Synchronous Logic Circuits Common clock 9.2 The basic Idea of PLC The Basic PLC described here combines BASIC with a PLC and inherits the advantages of both systems. A Programmable Logic Control (PLC) is a simple controlling computer for universal use in measuring, steering and controlling tasks. It is usable in any industrial or 45

50 PROGRMMABLE LOGIC CONTROLLER PLC Week 9 laboratory environment or at home. As of it's programming it can be an intelligent alarm clock, the central processing unit of a heating with data acquisition from a weather station, an entry system with burglar alarm or whatever else. It is able to control everything controllable. The programming of this special PLC is very easy. It is done with the well known programming language BASIC. No special knowledge and no expensive development tools are required to program the Basic PLC. Not even a Personal Computer (PC) is necessary to program, only a spare keyboard is necessary. The combination of PLC and BASIC forms the name of the game: Basic PLC, a BASIC Programmable Logic Control. 9.3 Digital logic with feedback With simple gate and combinational logic circuits, there is a definite output state for any given input state. Take the truth table of an OR gate, for instance: Figure 9.1 OR gate with truth table 46

51 PROGRMMABLE LOGIC CONTROLLER PLC Week 9 For each of the four possible combinations of input states (0-0, 0-1, 1-0, and 1-1), there is one, definite, unambiguous output state. Whether we're dealing with a multitude of cascaded gates or a single gate, that output state is determined by the truth table(s) for the gate(s) in the circuit, and nothing else. However, if we alter this gate circuit so as to give signal feedback from the output to one of the Figure 9.1b OR gate inputs, strange things begin to happen: figure 9.1b We know that if A is 1, the output must be 1, as well. Such is the nature of an OR gate: any "high" (1) input forces the output "high" (1). If A is "low" (0), however, we cannot guarantee the logic level or state of the output in our truth table. Since the output feeds back to one of the OR gate's inputs, and we know that any 1 input to an OR gates makes the output 1, this circuit will "latch" in the 1 output state after any time that A is 1. When A is 0, the output could be either 0 or 1, depending on the circuit's prior state! The proper way to complete the above truth table would be to insert the 47

52 PROGRMMABLE LOGIC CONTROLLER PLC Week 9 word latch in place of the question mark, showing that the output maintains its last state when A is 0. Any digital circuit employing feedback is called a multivibrator. The example we just explored with the OR gate was a very simple example of what is called a bistable multivibrator. It is called "bistable" because it can hold stable in one of two possible output states, either 0 or 1. There are also monostable multi-vibrators, which have only one stable output state (that other state being momentary), which we'll explore later; and astable multi-vibrators, which have no stable state (oscillating back and forth between an output of 0 and 1). A very simple astable multivibrator is an inverter with the output fed directly back to the input: Figure 9.2 Inverter with feedback When the input is 0, the output switches to 1. That 1 output gets fed back to the input as a 1. When the input is 1, the output switches to 0. That 0 output gets fed back to the input as a 0, and the cycle repeats itself. The result is a high frequency (several megahertz) oscillator, if implemented with a solid-state (semiconductor) inverter gate: Some question and solution Why a PLC? Well, a PLC is mainly a computer or better a controller. I always wanted to have a universal controller to switch the lights at home or to acquire data from my weather station and control my 48

53 PROGRMMABLE LOGIC CONTROLLER PLC Week 9 heating. This universal controller should be easy applicable to any hardware. Therefore it should have opto-couplers on its in-ports and potential-free out-ports. That is exact what a PLC is. What is a PLC? A PLC, or Programmable Logic Controller, is a computer with a remarkable capacity for realworld I/O connectivity. It is designed for use in an industrial or laboratory environment. Normally there isn't any kind of human interface like a keyboard or a display. Usually a PLC is programmed via a PC. When it is necessary to edit or create the program running in the PLC, a computer with special programming software is necessary. You have to buy very expensive development tools, and you have to learn a special programming language. The controller I wanted to build should be programmable in an easier way. Why Basic? Do you remember the good old C64 from Commodore? That bread bin computer? When I got that thing, I assembled the hardware, put it into operation and about 20 minutes later I saw my first program running. That was really easy. Remembering that, I wanted my PLC to be as easy programmable as that old home computer. The programming language used by Commodore was a version of BASIC and so that language came into mind. 49

54 PROGRMMABLE LOGIC CONTROLLER PLC Week The S-R latch A bistable multivibrator has two stable states, as indicated by the prefix bi in its name. Typically, one state is referred to as set and the other as reset. The simplest bistable device, therefore, is known as a set-reset, or S-R, latch. To create an S-R latch, we can wire two NOR gates in such a way that the output of one feeds back to the input of another, and visa-versa, like this: Figure 10.1 S-R latch The Q and not-q outputs are supposed to be in opposite states. I say "supposed to" because making both the S and R inputs equal to 1 results in both Q and not-q being 0. For this reason, having both S and R equal to 1 is called an invalid or illegal state for the S-R multivibrator. Otherwise, making S=1 and R=0 "sets" the multivibrator so that Q=1 and not-q=0. Conversely, making R=1 and S=0 "resets" the multivibrator in the opposite state. When S and R are both equal to 0, the multivibrator's outputs "latch" in their prior states. Note how the same multivibrator function can be implemented in ladder logic, with the same results: 50

55 PROGRMMABLE LOGIC CONTROLLER PLC Week 10 Figure 10.2 multivibrator ladder logic By definition, a condition of Q=1 and not-q=0 is set. A condition of Q=0 and not-q=1 is reset. These terms are universal in describing the output states of any multivibrator circuit. The astute observer will note that the initial power-up condition of either the gate or ladder variety of S-R latch is such that both gates (coils) start in the de-energized mode. As such, one would expect that the circuit will start up in an invalid condition, with both Q and not-q outputs being in the same state. Actually, this is true! However, the invalid condition is unstable with both S and R inputs inactive, and the circuit will quickly stabilize in either the set or reset condition because one gate (or relay) is bound to react a little faster than the other. If both gates (or coils) were precisely identical, they would oscillate between high and low like an astable multivibrator upon power-up without ever reaching a point of stability! Fortunately for cases like this, such a precise match of components is a rare possibility. It must be noted that although an astable (continually oscillating) condition would be extremely rare, there will most likely be a cycle or two of oscillation in the above circuit, and the final state of the 51

56 PROGRMMABLE LOGIC CONTROLLER PLC Week 10 circuit (set or reset) after power-up would be unpredictable. The root of the problem is a race condition between the two relays CR 1 and CR 2. A race condition occurs when two mutually-exclusive events are simultaneously initiated through different circuit elements by a single cause. In this case, the circuit elements are relays CR 1 and CR 2, and their de-energized states are mutually exclusive due to the normally-closed interlocking contacts. If one relay coil is de-energized, its normally-closed contact will keep the other coil energized, thus maintaining the circuit in one of two states (set or reset). Interlocking prevents both relays from latching. However, if both relay coils start in their de-energized states (such as after the whole circuit has been de-energized and is then powered up) both relays will "race" to become latched on as they receive power (the "single cause") through the normally-closed contact of the other relay. One of those relays will inevitably reach that condition before the other, thus opening its normally-closed interlocking contact and de-energizing the other relay coil. Which relay "wins" this race is dependent on the physical characteristics of the relays and not the circuit design, so the designer cannot ensure which state the circuit will fall into after power-up. Race conditions should be avoided in circuit design primarily for the unpredictability that will be created. One way to avoid such a condition is to insert a time-delay relay into the circuit to disable one of the competing relays for a short time, giving the other one a clear advantage. In other words, by purposely slowing down the de-energization of one relay, we ensure that the other relay will always "win" and the race results will always be predictable. Here is an example of how a timedelay relay might be applied to the above circuit to avoid the race condition: 52