Chapter 1 Introuduction

1.1 Ten fundamental questions 1. What is a programmable controller?

A programmable controller is a microprocessor-based industrial controller, the functions of which are determined by a stored program. 2. What is a program?

A program is a set of instructions 'telling' the controller how to behave. It is stored in the controller's memory. 3. How does a programmable controller differ from a computer? A computer is optimized for calculation and display tasks and is programmed by specialists. A programmable controller is optimized for control and regulation tasks and can be programmed by non-specialists. It is also well adapted to the industrial environment.

4. Why are programmable controllers so common?

Because they are cost-effective and have significant advantages over traditional control systems based on relays or pneumatics.

5. Where are they used?

In virtually every industry where automation is involved, from individual machines to whole processes, in commercial, institutional and industrial premises. The newest 'microcontrollers' are so cheap and compact that they are economical even in domestic applications.

6. What are the main advantages?

A control system based on a programmable controller is flexible, reliable and compact and can be assembled at a relatively low cost.

7. Are all programmable controllers the same?

They are broadly similar in a functional sense, but they differ in size, programming detail and mechanical design. Most manufacturers offer several models with different levels of performance to cater for the wide variety of tasks to which programmable controllers can be applied.

8. What tasks does a programmable controller perform?

The control tasks previously undertaken with electrical and/or pneumatic controls, e.g. interlocking, sequencing, timing and counting. It can, in addition, perform a variety of calculation, communication and monitoring tasks far outside the competence of traditional systems.

9. Does a programmable controller eliminate contactors and valves? No, but these items are brought under the programmable controller's influence in modern control systems. 10. Are there drawbacks? Yes. Programmable controllers still do not enjoy the same trust or acceptance as traditional control techniques, even though the technology is nearly 30 years old. The natural reluctance to accept the 'new' technology is understandable; most of our current industrial staff were educated and trained before this technology became common. Some technical adaptations have to be made in implementing programmed control. These problems will be overcome with

• Education for new users and the subsequent building of experience • A measure of understanding on the part of vendors and the ‘converted’

1.2 Control System Overview There are three characteristic features of a control system, whether it is programmable or not: 1. There are certain actions to be taken (such as turning a valve ON or OFF or regulating its

position). 2. There are certain rules governing those actions—the control system should, as far as possible,

be predictable and not subject to random behavior. 3. The rules take account of certain relevant conditions in the plant (such as manual switches,

sensors for level, pressure, temperature, position). It is interesting and informative to compare a control system of traditional design with one of modem design (using a programmable controller); in the context of these three features. Figure 1.1 shows the two control systems applied to the same task, namely control of a relay K1. The action, in this case, is turning on K1. There are three switches in the plant, whose status is relevant to the control of Kl. For simplicity we shall say that these three switches account for all the conditions that have to be checked. By simply examining the illustration we can see that the 'action' content and the ‘condition’ content of the two control systems arc identical. We must therefore conclude that the main difference between the two systems is in the implementation of the rules. In the traditional control system the wiring inside the control cabinet connects the three switches in series. This means that S1 and S2 and S3 must close for K1 to be turned on — the wiring makes the rules! In the modem control system the place of the wiring is taken by a program. The instructions contained in the program must have the following effect: ‘when S1 is closed AND S2 is closed AND S3 is closed, turn on K1’. To manage this, the programmable controller must be able to check the state of the three switches; this is facilitated by connecting each switch

to an input terminal on the controller. It must also be able to take charge of the relay; this is facilitated by connecting the relay to an output terminal on the controller. 1.3 The benefits of programmable control At first sight the solitary difference between the two systems may not seem profound but consider the following results:

• Change of rules

If the rules need to be changed, a traditional system must be rewired. This may be inconvenient, expensive and protracted. A programmable controller can be reprogrammed to accommodate a change of rules—no rewiring is needed. Neither is there any need to alter drawings, since program development systems have automatic documentation!

• Extra functions

If extra functions are needed, a traditional control system must be fitted with the additional equipment—assuming the space is available for it. A programmable controller has a vast array of built-in functions such as relays, timers, counters and sequencers which are freely accessible at any time and demand no extra space in the control cabinet.

• Reliability Moving parts are subject to mechanical failure, and mechanical failure accounts for a substantial proportion of faults in the components of traditional control systems. Programmable controllers have very few, if any, moving parts. In addition they are amenable to manufacturing methods that include exhaustive, automatic test procedures. Defective or even potentially defective components can be identified and excluded. As a result, reliability is excellent.

• Communication

Traditional control systems have little or no potential for interconnection with other systems for the purposes of control, supervision or reporting. Programmable controllers are inherently suited to, and increasingly better prepared for, such a role. The availability of communications modules allows the connection of controllers to industrial networks, which facilitates data interchange on a grand scale.

1.4 Structure Figure 1.2 illustrates the structure of the programmable controller and its setting in the control environment. The key piece of equipment is the microprocessor, which is the 'decision-maker' in the scheme. It makes decisions based on the instructions stored in the

memory (the program). Nowadays, microprocessor chips are very small, reliable, powerful and being mass-produced, they are relatively cheap. The switches, detectors and sensors in the plant are connected to the input terminals of the controller, where the microprocessor can evaluate their status. These input devices send signals into the controller. The only task of the input circuits is to interface between the input devices and the processor. This means isolating them electrically and adjusting for the voltage difference between them. Thus the processor can read the input signals without having to tolerate the high plant voltages. The relays, valves and indicators in the plant are connected to the output terminals where the microprocessor can exercise its control influence. These output devices receive their signals/row the controller. The output circuits are there to provide isolation and to adjust for the voltage difference between the processor and the output devices. Thus the processor can transmit the output signals without having to encounter the (relatively) high plant voltages. The power supply serves the various parts of the controller with the right kind and rating of supply. It may also supply the voltage for the input circuits, but not the output circuits. 1.5 The processor The processor fitted in a modem programmable controller is a class of semiconductor chip known as an embedded microcontroller. All the functions of a computer are encapsulated on this chip, which typically has an active area of less than 1 cm2 and contains hundreds of thousands of transistors (Fig.1.3). With the support of other chips, it is capable of executing many millions of instructions every second. It communicates with the memory, input and output interfaces through a system of conductors called a bus, on which it may send/receive 8, 16 or 32 signals at a time. The operating voltage is typically 5 V d.c. Modern semiconductor products are subject to a rigorous design process, a manufacturing technique of extraordinary cleanliness and accuracy, and the most intensive testing. As a result, they are extremely reliable in service and have a failure rate less than 10 per cent of the rate for the next best technology.

PLCs Programmable Logic Controllers (PLCs), also referred to as programmable controllers, are in the computer family. They are used in commercial and industrial applications. A PLC monitors inputs, makes decisions based on its program, and controls outputs to automate a process or machine. This course is meant to supply you with information on the functions and configurations of PLCs.

Basic Operations PLCs consist of input modules or points, a Central Processing Unit (CPU), and output modules or points. An input accepts a variety of digital or analog signals from various field devices (sensors) and converts them into a logic signal that can be used by the CPU. The CPU makes decisions and executes control instructions based on-program instructions in memory. Output modules convert control instructions from the CPU into a digital or analog signal that can be used to control various field devices (actuators). A programming device is used to input the desired instructions. These instructions determine what the PLC will do for a specific input. An operator interface device allows process information to be displayed and new control parameters to be entered.

Pushbuttons (sensors), in this simple example, connected to PLC inputs, can be used to start and stop a motor connected to a PLC through a motor starter (actuator).

Hard Wired Control Prior to PLCs, many of these control tasks were solved with contactor or relay controls. This is often referred to as hardwired control. Circuit diagrams had to be designed, electrical components specified and installed, and wiring lists created. Electricians would then wire the components necessary to perform a specific task. If an error was made, the wires had to be reconnected correctly. A change in function or system expansion required extensive component changes and rewiring.

PLCs The same, as well as more complex tasks can be done with a PLC. Wiring between devices and relay contacts is done in the PLC program. Hard-wiring, though still required to connect field devices, is less intensive. Modifying the application and correcting errors are easier to handle. It is easier to create and change a program in a PLC than it is to wire and rewire a circuit. Advantages

• Smaller physical size than hard-wire solutions • Easier and faster to make changes • PLCs have integrated diagnostics and override functions • Diagnostics are centrally available • Applications can be immediately documented • Applications can be duplicated faster and less expensively

Inrtoduction In this introductory chapter, we will tell what a programmable logic controller (PLC) is. We will then discuss the evolution of relay logic and computer systems into the present-day programmable logic controller. We will also list and discuss some advantages and disadvantages of using a PLC over other control systems. Finally, the knowledge level required for PLC programming and operating will be evaluated. Definition Of A Programmable Logic Controller A programmable logic controller is a user-friendly electronic computer that carries out control functions of many types and levels of complexity. It can be programmed, controlled, and operated by a person unskilled in operating computers. The programmable logic controller essentially draws the lines and devices of ladder diagrams. The resulting drawing in the computer takes the place of much of the external wiring required for control of a process. The programmable logic controller will operate any system that has output devices that go on and off. It can also operate any system with variable outputs. The programmable logic controller can be operated on the input side by on/off devices or by variable input devices. Evolution To The Present PLC The first PLC systems evolved from conventional computers in the late 1960s and early 1970s. These first PLCs were mostly installed in automotive plants. Traditionally, the auto plants had to be shut down for up to a month at model changeover time. The early PLCs were used along with other new automation techniques to shorten the changeover time. One of the major time-consuming changeover procedures had been the wiring of new or revised relay and control panels. The PLC keyboard reprogramming procedure replaced the rewiring of a panel full of wires, relays, timers, and other components. The new PLCs helped reduce repro-gramming time to a matter of a few days. There was a major problem with these early-1970s computer/PLC reprogramming procedures. The programs were compiicated and required a highly trained programmer to make the changes- Through the late 1970s, improvements were made in PLC programs to make them somewhat more user friendly; in 1978, the introduction of the microprocessor chip increased computer power for all kinds of automation systems and lowered the computing cost. Robotics, automation devices, and computers of all types, including the PLC, consequently underwent many improvements. PLC programs became more understandable to more people. PLCs became more affordable, as well. In the 1980s, with more computer power per dollar available, the PLC came into exponentially increasing use. Some large electronics and computer companies and some diverse corporate electronics divisions found that the PLC had become their greatest volume product.

The market for PLCs grew from a volume of $80 million in 1978, to $1 billion dollars per year by 1990 and is still growing. Even the machine tool industry, where computer numerical controls (CNCs) have been used in the past, is using PLCs. PLCs are also used extensively in building energy and security control systems. Other non traditional uses of PLCs, such as in the home and in medical equipment, will be increasing in the 1990s. Advantages Of The PLC The following are some of the major advantages of using a programmable controller: Flexibility: In the past, each different electronically controlled production machine required its own controller; 15 machines might require 15 different controllers. Now. it is possible to use just one model of a PLC to run any one of the 15 machines. Furthermore, you would probably need fewer than 15 controllers, because one PLC can easily run many machines. Each of the 15 machines under PLC control would have its own distinct program. Implementing Changes and Correcting Errors: With a wired relay-type panel any program alterations require time for rewiring of panels and devices. When a PLC program circuit or sequence design change is made, the PLC program can be changed from a keyboard sequence in a matter of minutes. No rewiring is required for a PLC-controlled system. Also, if a programming error has to be corrected in a PLC control ladder diagram, a change can be typed in quickly. Large Quantities of Contacts: The PLC has a large number of contacts for each coil available in its programming. Suppose that a panel-wired relay has four contacts and all are in use when a design change requiring three more contacts is made. It would mean that time must be taken to procure and install a new relay or relay contact block. Using a PLC, however, would only require that three more contacts be typed in. The three contacts would be automatically available in the PLC. Indeed, a hundred contacts can be used from one relay—if sufficient computer memory is available. Lower Cost: Increased technology makes it possible to compact more functions into smaller and less expensive packages. In the 1990s you can purchase a PLC with numerous relays, timers, counters, a sequencer, and other functions for a few hundred dollars. Pilot Running: A PLC programmed circuit can be pre-run and evaluated in the office or lab. The program can be typed in, tested, observed, and modified if needed, saving valuable factory time. In contrast, conventional relay systems have been best tested on the factory floor, which can be very time consuming. Visual Observation: A PLC circuit's operation can be seen during operation directly on a CRT screen. The operation or mis-operation of a circuit can be observed as it happens. Logic paths light up on the screen as they are energized. Troubleshooting can be done quicker during visual observation.

In advanced PLC systems, an operator message can be programmed for each possible malfunction. The malfunction description appears on the screen when the malfunction is detected by the PLC logic (for example, "MOTOR # 7 IS OVERLOADED"), Advanced PLC systems also may have descriptions of the function of each circuit component. For example, input # 1 on the diagram could have "CONVEYOR LIMIT SWITCH" on the diagram as a description. Speed of Operation: Relays can take an unacceptable amount of time to actuate. The operational speed for the PLC program is very fast. The speed for the PLC logic operation is determined by scan time, which is a matter of milliseconds. Ladder or Boolean Programming Method: The PLC programming can be accomplished in the ladder mode by an electrician or technician. Alternately, a PLC programmer who works in digital or Boolean control systems can also easily perform PLC programming. Reliability: Solid state devices are more reliable, in general, than mechanical or electrical relays and timers. The PLC is made up of solid state electronic components with very high reliability rates. Simplicity of Ordering Control System Components: A PLC is one device with one delivery date. When the PLC arrives, all the counters, relays, and other components also arrive. In designing a relay panel, on the other-hand, you may have 20 different relays and timers from 12 different suppliers. Obtaining the parts on time involves various delivery dates and availabilities. With a PLC you have one product and one lead time for delivery. In a relay system, forgetting to buy one component would mean delaying the start-up of the control system until that component arrives, With the PLC, one more relay is always available, providing you ordered a PLC with enough extra computing power. Documentation: An immediate printout of the true PLC circuit is available in minutes, if required. There is no need to look for the print of the circuit in remote files. The PLC prints out the actual circuit in operation at a given moment. Often, the file prints for relay panels are not properly kept up to date. A PLC printout is the circuit at the present time; no wire tracing is needed for verification. Security: A PLC program change cannot be made unless the PLC is properly unlocked and programmed. Relay panels tend to undergo undocumented changes. People on late shifts do not always record panel alterations made when the office area is locked up for the night. Ease of Changes by Reprogramming: Since the PLC can be reprogrammed quickly, mixed production processing can be accomplished. For example, if part B comes down the assembly line while part A is still being processed, a program for part B's processing can be reprogrammed into the production machinery in a matter of seconds. These 13 items are some of the advantages of using a programmable logic controller, There will, of course, be other advantages in individual applications and industries.

Disadvantages Of The PLC

Following are some of the disadvantages of, or perhaps precautions for, using PLCs: Newer Technology: It is difficult to change some personnel's thinking from ladders and relays to the PLC computer concepts. Fixed Program Applications: Some applications are single-function applications. It doesn't pay to use a PLC that includes multiple programming capabilities if they are not needed. One example is in the use of drum controller/sequencers. Some equipment manufacturers still use a mechanical drum with pages at an overall cost advantage. Their operational sequence is seldom or never changed, so the reprogramming available with the PLC would not be necessary. Environmental Consideration: Certain process environments, such as high heat and vibration, interfere with the electronic devices in PLCs, which limits their use. Fail-Safe Operation: In relay systems, the stop button electrically disconnects the circuit; if the power fails, the system stops. Furthermore, the relay system does not automatically restart when power is restored. This, of course, can be programmed into the PLC; however, in some PLC programs, you may have to apply an input voltage to cause a device to stop. These systems are not "failsafe." This disadvantage can be overcome by adding safety relays to a PLC system, as we will see later in this text. Fixed-Circuit Operation: If the circuit in operation is never altered, a fixed control system such as a mechanical drum, for example, might be less costly than a PLC. The PLC is most effective when periodic changes in operation are made.

Knowledge Level For PLC Programming

A person knowledgeable in relay logic systems can master the major PLC function in a few hours. These functions might include coils, contacts, timers, and counters. The same is true for a person with a digital principles, however, the learning process takes more time. A person knowledge in relay logic can master advanced PLC functions in a few days with proper instruction. Company schools and operating manuals are very helpful in mastering these advanced functions. Advanced functions in order of learning might include sequence/drum controller, register bit use, and move functions.

Applications Many industries use PLCs. Even though each automation task is different, the PLC adapts optimally to the most varied jobs, whether they involve simple open loop control or complex close loop control. Present areas of applications include the following: 1 Automobile Industries Automatic drilling/assembly and test equipment, painting facilities, shock absorber test bays. 2 Plastic Industries Blow, injection and thermal molding machines, synthetic production systems, temperature &

pressure control. 3 Heavy Industries

Molding equipment, industrial furnaces, rolling mills, automatic pit shaft, temperature control systems.

4 Chemical Industries

Proportioning & mixing systems, temperature & pressure control, boiler & chiller control. 5 Food & Beverages Industries

Brewery systems, centrifuging, batch processing, temperature & pressure control, boiler & chiller control.

6 Machinery's

Packing, wood-working, machine control, machine tools, drilling mills, fault alarm centers, welding technology.

7 Building Services

Elevators, climate control, ventilation, lighting, alarm & security systems. 8 Transport Systems

Transport and sorting equipment, high bay ware-houses, conveyor and crane systems, traffic signals.

9 Energy, Gas, Water & Air

Pressure booster stations, standby power supplies, pump control, water & air treatment, filtering and gas recovery systems, emergency systems.

10 Textile Industries

AC/DC drive control, temperature control, heating/drying control, spinning, dying and color mixing.

CHAPTER 7

Relays CHAPTER OBJECTIVES

This chapter will help you to:

1. Compare the electromagnetic and solid-slate relay in terms of construction and operation.

2. Identify relay symbols used on schematic diagrams,

3. Illustrate uses of relays for industrial applications.

4. Explain how relays are rated.

5. Describe the operation of ON-delay and OFF-delay timer relays.

6. Discuss the use of relays in logic circuits.

Most applications in industry and in process control require relays as critical control elements. Relays are used primarily as switching devices in a circuit. This chapter explains the operation of different types of relays along with the advantages and limitations of each type- Relay specifications are also presented to show how to determine the correct relay type for different applications.

7-1 ELECTROMECHANICAL CONTROL RELAYS

An electromechanical relay (EMR) is a magnetic switch. It turns a load circuit ON or OFF by energizing an electromagnet, which opens or closes contacts in the circuit. The EMR has a large variety of applications in both electric and electronic circuits. For example, EMRs may be used in the control of fluid power valves and in many machine sequence controls such as drilling, boring, milling, and grinding operations.

A relay will usually have only one coil, but it may have any number of different contacts. A typical EMR is shown in Fig. 7-1. Electro-mechanical relays contain both stationary and moving contacts. The moving contacts are attached to the plunger. Contacts are referred to as normally open (NO) and normally closed (NC). When the coil is energized, it produces an electromagnetic field. Action of this field, in turn, causes the plunger to move through the coil, closing the NO contacts and opening the NC contacts. The distance that the plunger moves is generally short — about 1/4 inch or less.

Normally open contacts are open when no current flows through the coil but closed as soon as the coil conducts a current or is energized. Normally closed contacts are closed when the coil is deenergized and open when the coil is energized. Each contact is usually drawn as it would appear with the coil deenergized. Most machine control relays have some provision for changing contacts normally open to normally closed, or vice versa. It ranges from a simple flip-over contact to removing the contacts and relocating with spring location changes.

. Many EMRs contain several sets of contacts operated by a single coil. Such relays are used to control several switching operations by a single, separate current. A typical control relay used to control two pilot lights is shown in Fig. 7-2. With the switch open, coil ICR is deenergized, The circuit to the green pilot light is completed through NC contact ICR 2, so this light will be ON. At the same time, the circuit to the red pilot light is opened through NO contact ICR l, so this light will be OFF. With the switch closed, the coil is energized. The NO contact ICR 1 closes to switch the red pilot light ON.

Fig. 7-2 Relays used to control several switching operations by a single, separate current.

At the same time, the NC contact ICR 2 opens to switch the green pilot light OFF.

In general, control relays are used as auxiliary devices to switch control circuits and

loads such as small motors, solenoids, and pilot lights. The EMR can be used to control a high-vollage load circuit wish a low-voltage control circuit. This is possible because the coil and contacts of the relay are electrically insulated from each other. From a safety point of view, this circuit provides extra protection for the operator. For example, assume that you want to use a relay to control a 120-V lamp circuit with a 12-V control circuit. The lamp would he wired in series with the relay contact to the 120-V source (Fig. 7-3). The switch would be wired in series with the relay coil to the 12-V source. Operating the switch would energize or deenergize the coil. This in turn, would close or open the contacts to switch the lamp ON or OFF.

Another basic application for a relay is to

control a high-current toad circuit with a low-current control circuit. This is possible because the current that can be handled by the contacts can be much greater than what is required to operate the coil. Relay coils are capable of being controlled by low-current signals from integrated circuits and transistors, as shown in Fig. 7-4 on page 204. In this circuit, the electronic control signal switches the transistor ON or OFF, which in turn causes the relay coil to energize or deenergize. The current in the control circuit, which consists of the transistor and relay coil, is quite small. The current in the

power circuit, which consists of the contacts and small motor, is much larger in comparison.

The level of voltage at which the relay coil is energized, resulting in the contacts switching, is called the pick-up voltage. After the relay is energized, the level of voltage on the relay coil at which the contacts return to their unoperated condition is called the drop-out voltage. Relay coils are designed to not drop out until the voltage drops to a minimum of approximately 85 percent of the rated voltage. The relay coils also will not pick up (energize) until the voltage rises to 85 percent of the rated voltage- Generally, coils wilt operate continuously at 110 percent of the rated voltage without damage to the coil. Relay coils are now being made of a molded construction. This aids in reducing moisture absorption and increases mechanical strength.

There is also a difference in the current in the relay coil from the time the coil is first energized to when the contacts are completely operated. When the coil is energized, the plunger is in an out position- Because of the open gap in the magnetic path (circuit); the initial current in the coil is high. The current level at this time is known as in-rush current. As the plunger moves into the coil, closing the gap, the current level drops to a lower value. This lower value is called sealed current. The in-rush current approximates six to eight times the sealed current-Electromechanical relays are made in a variety of types for different applications. Relay coils and contacts have separate ratings. Relay coils are usually rated for type of operating current (dc or ac), normal operating voltage or current, resistance, and power. Very sensitive relay coils, rated in the low milli ampere range, are often operated from transistor or integrated circuits. Figure 7-5(a), an open-type relay, is not enclosed; the contacts, coil, and all moving

parts are exposed and thus readily visible. With the enclosed-type relay a plastic cover keeps the contacts from being exposed to corrosive environments. The plug-in type shown in Fig. 7-5(b) can be changed without disturbing the circuit wiring. When a relay is used for a particular application, one of the first steps should be to determine the control (coil) voltage at which the relay will operate. Coils are available to cover most standard voltages.

Relays differ in the number and arrangement

of contacts. Although there are some single-break contacts used in industrial relays, most of the relays used in machine tool control have double-break contacts (Fig, 7-6). All contacts bounce on closing, and in rapid-operating relays this can be a source of trouble. The use of double-break contacts reduces this problem.

The most important relay contact specifica-tion is its current rating. This indicates the maximum amount of current the contacts are capable of handling.

The three current ratings generally specified are:

• In-rush or "make contact" capacity. • Normal or continuous carrying capacity. • The opening or break capacity.

Contacts are also rated for the maximum ac-or dc-voltage level at which they can operate, Most relays are used in control circuits; therefore, their lower contact ratings (0 to 15A maximum to 600 V) show the reduced current levels at which they operate. Although control relays from various manufacturers differ in appearance and construction, they are inter-changeable in control wiring systems if their specifications are matched to the requirements of the system.

Most contacts today are made of a silver alloy rather than copper. This material is used because of the excellent conductivity of silver. Silver oxide, which forms on the contacts, is also a good conductor. Even when the contacts appear dull or tarnished, they are still capable of operating normally.

Self-Test

Answer the following questions.

1. The two basic parts of an electromechani-cal relay are the ______ and the _______.

2. Normally open contacts are denned as those contacts that are open when the coil is ______.

3. True or false? When no current flows through the coil of a relay, it is said to be energized.

4. True or false? The coil and contacts of a relay are not normally electrically insulated from each other,

5. True or false? The relay can be used to control a high-current load circuit with a low-current control circuit.

6. True or false? The pick-up and drop-out voltage of an EMR is normally the same value.

7. True or false? The in-rush current of an EMR is always greater than its sealed current.

8. True or false? It is possible for a relay coil to be rated for 6 Vdc and its contacts rated for 240 Vac.

9. True or false? Contact bounce on closing is normal in an EMR.

10. Control relays are interchangeable provided their ______ are matched.

11. Most relay contacts are made of a ______ alloy.

7-2 Solid-State Relays After performing switching tasks for several decades, the EMR is being replaced in some applications by a new type of relay, the solid state relay (SSR) (Fig. 7-7 on page 206). Although EMRs and solid-state relays are designed to perform similar functions, each accomplishes the final results in different ways. Unlike EMRs, SSRs do not have actual coils and contacts. Instead, they use semiconductor switching devices such as bipolar transistors, MOSFETs. Silicon controlled rectifiers (SCRs), or triacs. The solid-state relay has no moving parts, it is resistant to shock and vibration, and it is sealed against din and moisture.

Like EMRs, SSRs find application in isolat-ing a low-voltage control circuit from a high-power load circuit. A block diagram of an optically coupled solid-stale relay is shown in Fig. 7-8(a) on page 206. A light-emitting diode (LED) incorporated in the input circuit glows when the conditions in that circuit are correct to actuate the relay. The LED shines on a phototransistor, which then conducts, causing the trigger current to be applied to the triac. Thus, the output is isolated from the input by the simple LED and phototransistor arrangement. just as the electromagnet isolated the input from the switching contacts in the conventional EMR, Because a light beam is used as the control medium, no voltage spikes or electrical noise produced on the load side of the relay can be transmitted to the control side of the relay. Most often, the black-box approach is used to symbolize an SSR. That is a square or rectangle will be used on the schematic to represent the relay. The internal circuitry will not be shown, and only the input and output connections to the box will be given.

Solid-slate relays can be used to control ac

or dc loads (Fig, 7-9). If the relay is designed to control an ac load, a triac is used to connect the load to the line. Solid-state relays intended for use as dc controllers have a power transistor, rather than a triac, connected to the load circuit. When the input voltage turns on the LED, a photodetector connected to the base of the transistor turns the transistor ON and connects the load to the line.

The control voltage for SSRs can be direct

current or alternating current and usually ranges from 3 10 32 V for the dc versions and 80 to 280 V for the ac versions. Maximum load circuit amps of up to 50 A are possible at input line voltage ratings of 120, 240, and 480 Vac, In most applications, SSRs are used to interface between a low-voltage control circuit and a higher ac line voltage.

Many SSRs used to control ac loads have a feature known as zero switching (Fig, 7-10). Zero switching ensures that the relay is turned ON or OFF at the beginning of the ac voltage wave at the zero crossover point. Zero voltage switching is often needed to reduce in-rush current and radio frequency interference (RFI).

Also available are hybrid SSRs that incorporate a small reed relay to serve as the actuating device (Fig. 7-11). A small set of reed contacts are connected to the gate of the triac. The control circuit is connected to the coil of the reed relay. When the coil is energized by the control current, a magnetic field is produced around the coil of the relay. This magnetic field closes the reed contacts, causing the triac to turn ON. In this type of SSR, a magnetic field, rather than a light beam, is used to isolate the control circuit from the load circuit.

The SSR has several advantages over the EMR. The SSR is more reliable and has a longer life because it has no moving parts. It is compatible with transistor and IC circuitry and does not generate as much electromagnetic interference. The SSR is more resistant to shock and vibration, has a much faster response lime, and does not exhibit contact bounce.

As with every device, SSRs do have some

disadvantages. The SSR contains semiconductors that are susceptible to damage from voltage and current spikes. Unlike the EMR contacts, the SSR switching semiconductor has a significant ON-state resistance and OFF-state leakage current.

Self-Test

Answer the following questions.

12. True or false? Solid-stale and electromechanical relays perform similar functions.

13. An SSR has no ______ parts. 14. In an optically coupled SSR, the output is

isolated from the input by a(n) ______. 15. True or false? An SSR designed to control

an ac load may use a transistor to connect the load to the line.

16. True or false? The control voltage for SSRs can be direct current or alternating current.

17. ______ switching refers to a feature of SSRs that ensures the relay is turned ON or OFF at the beginning of the ac voltage wave.

18. Hybrid SSRs may incorporate a small ______ relay to serve as the actuating device.

19. True or false? Solid-state relays do not exhibit contact bounce.

20. True or false? Solid-state switching semi-conductors have a significant ON-slate resistance and OFF state leakage current.

7-3 Timing Relays There are very few industrial control systems that do not need at least one or two limed functions. Typical applications include

machines in which the start of an event must be delayed until another event has occurred. For example, a mixing machine may be delayed until the liquid has been heated, or a fan may remain OFF until a heating coil has heated the surrounding air.

Timing relays are conventional relays that are equipped with an additional hardware mechanism or circuitry to delay the opening or closing of load contacts. Timing relays are similar to other control relays in that they use a coil to control the operation of some number of contacts. The difference between a control relay and a timing relay is that the contacts of the timing relay delay changing their position when the coil is energized or deenergized.

A pneumatic (air) timing relay uses mechanical linkage and an air-bellows system to achieve its timing cycle (Fig. 7-12). The bellows design allows air to enter through a needle valve at a predetermined rate to provide the different time-delay increments and to switch a contact output. Pneumatic timing relays are popular throughout industry because they are rugged and dependable. They are adjustable over a wide range of time periods. They are relatively unaffected by temperature or voltage variations, and they have good repeat accuracy.

Some circuits require both timing contacts as well as instantaneous contacts operated by the same energized relay coil. The instantaneous contacts operate when the coil

is energized or deenergized, independent of the timing mechanism. The timing contacts can be arranged to delay after energizing or deenergizing the coil, Figure 7-13 shows the construction of an ON-delay pneumatic (air) timer with two timed and two instantaneous contacts. When the coil is energized, the timed contacts are prevented from opening or closing. When the coil is deenergized, however, the timed contacts return immediately to their normal state. The instantaneous contacts change their positions immediately when the coil is energized and change back to their normal positions immedi-ately when the coil is deenergized.

A solid-state timing relay (Fig. 7-14) uses

electronic circuitry to achieve its timing cycle. Some of these timers use a resistor- capacitor (RC) time constant to obtain the time base, and others use quartz clocks as the time base. An RC oscillator network generates a highly stable and accurate pulse that is used to pro-vide the different time delay increments and switch a contact output. The length of the time delay can be set by adjusting a control knob or potentiometer located on the front of the timer. Timing indication is provided by an LED that flashes during timing, glows steadily after timing, and is OFF when the timer is deenergized.

Time-delay relays can be classified into two basic groups: ON-delay and OFF-delay. The ON-delay relay (Fig, 7-15) is often referred to as DOE, which stands for "delay on energize." When power is connected to the coil of an ON-delay timer,

the contacts delay changing position for some period of time. For this example a time delay of 10 s is assumed. When voltage is removed and the coil is deenergized, the contacts will immediately change back to their normal positions. The contact symbols shown are standard NEMA symbols. Time-delay relays can have NO. NC, or a combination of NO and NC contacts. The OFF-delay relay (Fig. 7-16) is often referred to as DODE, which stands for

“delay on deenergize”. The operation of the

OFF-delay timer is the opposite of the operation of the ON-delay timer. When voltage is applied to the coil of the OFF-delay timer, the contacts will change position immediately. When the coil is deenergized, however, there is a time delay before the contacts change to their normal positions.

The abbreviations TO and TC are used with standard contact symbols on some control schematics to indicate a time-operated contact (Fig. 7-17). The abbreviation TO stands for

"time opening," and TC stands for "lime clos-ing."

Solid-state time-delay relays can also be

designed for interval-ON and recycle operation. Different timing functions are illustrated in Fig. 7-18.

Self-Test

Answer the following questions.

21. Timing relays are used to ______ the

opening or closing of contacts for circuit control.

22. A(n) _____ timing relay uses mechanical linkage and an air-bellows system to achieve its timing cycle.

23. The ______ contacts of a timing relay operate independently of the timing mech-anism.

24. A solid-state timing relay uses ______ circuitry to achieve its timing cycle.

25. The ON-delay timer is sometimes referred to as DOE, which represents ______.

26. An OFF-delay timing relay provides time delay when its coil is ______.

27. The abbreviation TC used with standard contact symbols represents______.

7-4 LATCHING RELAYS

Electromechanical latching relays are designed to hold the relay closed after power has been removed from the coil. Latching relays are used where it is necessary for contacts to stay open and/or closed even though the coil is

energized only momentarily. Figure 7-19(o)

shows a mechanically held latching relay that uses two coils. The latch coil is momentarily energized to set the latch and hold the relay in the latched position. The unlatch or release coil is momentarily energized to disengage the mechanical latch and return the relay to the

unlatched position.

Figure 7-19(b) shows the schematic diagram for an electromagnetic latching-type relay circuit, the contact is shown with the relay in the unlatched position. In this state, the circuit to the pilot light is open, so the light is OFF. When the ON button is momentarily actuated, the latch coil is energized to set the relay to its latched position. The contacts close, completing the circuit to the pilot light, so the light is switched ON- Note that the relay coil does not have to be continuously energized to hold the contacts closed and keep the light ON, The only way lo switch the lamp OFF is to actuate the OFF button, which will energize the unlatch coil and return the contacts to their open, unlatched

state. In cases of power loss, the relay will remain in its original latched or unlatched state when power is restored. This arrangement is often referred to as a memory relay.

The latching relay has several advantages in electrical circuit design. For example, it is common in a control circuit to have to remember when a particular event takes place and not permit certain functions once this event occurs. Running out of a part on an assembly line may signal the shutdown of the process by momentarily energizing the unlatch coil. The latch coil would then have to be momentarily energized before further operations could occur.

Another use for the latching-type relay involves power failure. Circuit continuity dur-ing power failures is often important in auto-matic processing equipment, where a sequence of operations must continue from the point of interruption after power is resumed—rather than return to the beginning of the sequence. In addition, latching relays are often used in machine tool control circuits- These relays can be latched and unlatched through the opera-lion of pilot devices such as limit switches and pushbuttons.

Self-Test Answer the following questions. 28. True or false? Latching relays are designed

to hold the relay closed after power has been removed from the coil.

29. In a latching relay that uses two coils, the coils are identified as the ______ coil and._____ coil.

30. True or false? Latching relays can provide circuit continuity during power failures.

7-5 RELAY LOGIC

A relay can be considered digital in nature because it is basically an ON/OFF, two-state device. The coil is the input and the contacts are the output. Whereas magnetic relays are single-input, multi-output devices, solid-state logic gate circuits are multi-input, single-out-put devices.

Control circuitry that requires two or more-functions to be completed as a precondition for another event to take place describes the AND

circuit. Figure 7-20 illustrates the relay equivalent circuit of a logic AND gale. This is an example of a safety interlock found on many punch presses. Both pushbuttons PB1

and PB2 must be pushed at the same time if the solenoid is to energize and operate the punch.

The controls are placed on opposite sides of the punch press in such a way that the operator must use both hands to operate the machine. This placement of the switches eliminates any possibility of injury to the operator's hands. Control circuitry in which one condition or another separate condition can cause an event to lake place describes the OR circuit. The example in Fig, 7-21 shows a circuit that can turn a light ON if the photo sensor senses dark-ness or someone turns the switch to the ON

position. Note that either the sensor or the switch can allow the light to be illuminated and that they can occur independent of each other. These are the basic criteria for the OR

circuit. The requirement of a NOT or inverting gate

is that it will produce an output when an input is not present. There are occasions when an event is taking place and some indication is desired to specify the negative or opposite indication. The example in Fig. 7-22 shows a circuit that indicates the open (OFF light) and closed (ON light) state of the pressure switch.

Memory is used in logic circuits to recall or Store past events. Figure 7-23 illustrates an example of OFF-return memory, which remembers the slate of its output until the power is

turned OFF and then always goes to the OFF

condition. The operation of the circuit is simi-lar to that of a magnetic starter with a 3-wire control circuit. Momentarily actuating PB2 turns the motor ON. It will remain ON until PB1 is momentarily actuated. This memory element

remembers the conditions of its output as long as the power remains ON.

Memory that is lost with a power failure is called volatile memory. Memory that is retained with a power loss is called nonvolatile. Nonvolatile relay memory is implemented using a latching relay that mechanically remembers its last position.

Logic control can be implemented by three different technologies- Relay control was first and it is still being used. The second tech-nology was solid-state logic modules [Fig.7-24(b) on page 214], a minor player because it led to the development of inexpensive electronic computers- The third, and latest, technology is the microcomputer, which has many important advantages over the other two, Microcomputer technology has been responsible for the programmable logic controller (PLC), which is the device of choice for many control systems [Fig. 7-24(b)].

Self-Test

Answer the following questions. 31. A relay can be considered digital in nature

because it is basically a(n) ______ slate device.

32. Control circuitry that requires two or more functions to be completed for another

event to take place describes the logic

______ circuit. 33. Control circuitry in which one condition

or another separate condition can cause an event to take place describes the logic ______ circuit.

34. The requirement of a logic SOT circuit is 1. An electromechanical relay (EMR) operates

by energizing an electromagnet, which in turn opens or closes contacts.

2. Normally open (NO) relay contacts are open when the coil is deenergized but closed as soon as the coil is energized.

3. Normally closed (NC) relay contacts are closed when the coil is deenergized but open as soon as the coil is energized.

4. Relays with multiple contacts are used to control several switching operations by a single separate current.

5. Because the coil and contacts of a relay are electrically insulated from each other, the relay can control a high-voltage load circuit with a low-voltage control circuit.

6. Because the current that can be handled by the contacts can be much greater than what is required to operate the coil, the relay can be used to control a high-current control circuit.

7. The level of voltage at which a relay coil is energized is called the pick-up voltage; it is always greater than the drop-out voltage.

8. The in-rush current of a relay coil is always greater than the sealed current.

that it will produce an output when a(n) ______ is not present.

35. Nonvolatile relay memory is implemented using a(n) ______ relay.

36. True or false? Relay logic control is the most popular choice for complex control systems.

9. Relay coils are rated for voltage, current,

resistance, and power. 10. Relay contacts may be of the single-

break or double-break type. 11. In addition to maximum operating

voltage, relay contacts are rated for in-rush current, continuous current, and opening current.

12. Most relay contacts are made of a silver alloy.

13. Electromechanical and solid-state relays are designed to perform similar functions.

14. Solid-stale relays (SSRs) use bipolar transistors. MOSFETs. SCRs. or Triacs for switching load circuits.

15. In an optically coupled SSR, the output is isolated from the input by an LED.

16. Solid-slate relays use triacs to control ac loads and iransisiors to control dc loads,

17. Zero switching is a feature of SSRs that ensures the relay is turned ON or OFF at the beginning of the ac voltage wave.

18. Hybrid SSRs incorporate a small reed relay to serve as the actuating device.

19. Advantages of SSRs over EMRs include (a) higher reliability, (b) longer life, (c) more compatibility with solid-slate circuits,

SUMMARY

(d) the generation of less electromagnetic interference, (e) more resistance to vibration, (f) faster response time, and (g) no contact bounce. 20. Unlike contacts, SSR switching

semiconductors have significant ON-state resistance and OFF-state leakage current.

21. Timing relay contacts delay changing position when a coil is energized or deenergized.

22. A pneumatic timing relay uses mechanical linkage and an air-bellows system to achieve its timing cycle.

23. A timing relay may contain both timing contacts as well as instantaneous contacts operated by the same energized relay coil.

24. Solid-state timing relays use an RC time constant or quartz clock to achieve their timing cycle.

25. The ON-delay timer is often called DOE, which means "delay on energize."

26. The OFF-delay timer is often called DODE, which means "delay on deenergize."

27. Latching relays are used where it is necessary for contacts to stay open and/or closed even though the coil is energized only momentarily.

28. A relay is digital in nature because it is basically an ON/OFF, two-stale device.

29. AND relay logic requires two or more functions to be completed as a precondition for another event to take place.

30. With OR relay logic, one condition or another separate condition can cause an event to lake place.

31. A NOT relay logic circuit will produce an output when an input is not present.

32. Memory is used in logic circuits to recall or store past events.

33. Nonvolatile relay memory is implemented using a latching relay.

Answer the following questions.

7-1. Explain the basic operating principle of an EMR. 7-2. Define the terms normally open and normally dosed contact as they apply to a relay, 7-3. List three basic ways in which control relays are put to use in electric and electronic circuits. 7-4. What are the three current ratings generally specified for control Relay contacts? 7-5. What electronic component is used to control the output of an SSR that controls a dc voltage? 7-6. What electronic component is used to control the output of an SSR that controls an ac

voltage? 7-7. Explain opto-isolation as it applies to an SSR. 7-8. Explain zero switching as it applies to an SSR.

7-9. Explain how isolation between the control and load circuit is obtained in SSRs that incorporate a reed relay.

7-10. List the advantages of SSRs over EMRs. 7-11. List three limitations of SSRs. 7-12. In what way is a timing relay different from a standard control relay? 7-13. What are instantaneous contacts? 7-14. a. What are the two basic classifications of timers?

b. Explain the operations of each. 7-15. Name two methods used by electronic timers to obtain their time base. 7-16. Describe the operation of a latching relay that uses two coils. 7-17. Why can a relay be considered digital in nature? 7-18. What type of relay logic circuit would be used to:

a. Produce an output, when an input is not present. b. Produce an output when each of several input conditions is met. c. Produce an output when any one of several input conditions is met. d. Upon failure of power, have output remain in its original state when power is restored.

CHAPTER REVIEW QUESTIONS

Basic Components For Control Circuits

Disconnecting switches A disconnecting switch isolates the motor

from the power source. It consists of 3 knife-switches and 3 line fuses enclosed in a metallic box. The knife-switches can be opened and closed simultaneously by means of an external handle. An interlocking mechanism prevents the hinged cover from opening when the switch is closed. Disconnecting switches (and their fuses) are selected to carry the nominal full-load current of the motor, and to withstand short-circuit currents for brief intervals.

Figure 20.1 Three-phase, fused disconnecting switch

rated 600V, 30 A. (Courtesy of Square D) Manual Circuit Breakers A manual circuit breaker opens and closes a

circuit, like a toggle switch. It trips (opens) automatically when the current exceeds a predetermined limit. After tripping, it can be reset manually. Manual circuit breakers are often used instead of disconnecting switches because no fuses have to be replaced.

Figure 20.2 Three-phase circuit breaker, 600 V, 100 A.

(Courtesy of Square D) Cam Switches A cam switch has a group of fixed contacts

and an equal number of moveable contacts. The contacts can be made to open and close in a preset sequence by rotating a handle or knob. Cam switches are used to control the motion and position of hoists, callenders, machine tools. and so on.

Figure 20.3 Three-phase surface-mounted cam switch,

230V, 2kW. (Courtesy of Klockner-Moeller)

Pushbuttons A pushbutton is a switch activated by finger

pressure. Two or more contacts open or close when the button is depressed. Pushbuttons are usually spring loaded so as to return to their normal position when pressure is removed.

Figure 20.4 Mechanical-interlocked pushbuttons with

NO (normally open) and NC (normally closed) contacts; rated to interrupt an ac current of 6A one million times.

(Courtesy of Siemens) Control Relays A. control relay is an electromagnetic switch

that opens and closes a set of contacts when the relay coil is energized. The relay coil produces a strong magnetic field which attracts a movable armature bearing the contacts. Control relays are mainly used in low-power circuits. They include time-delay relays whose contacts open or close after a definite time interval. Thus, a time-delay closing relay actuates its contacts after the relay coil has been energized. On the other hand, a time-delay opening relay actuates its contacts some time after the relay coil has been de-energized,

Figure 20.5 Single-phase relays: 25 A, 115/230 V and 5

A, 115V. (Courtesy of Potter and Brumfield) Thermal Relays A thermal relay (or overload relay) is a

temperature-sensitive device whose contacts open or close when the motor current exceeds a preset limit. The current flows through a small, calibrated heating element which raises the temperature of the relay. Thermal relays are inherent time-delay devices because the temperature cannot follow the instantaneous changes in current.

Figure 20.6 Three-phase thermal relay with variable

current setting, 6 A to 10 A (Courtesy of Kiockner-Moeller)

Magnetic Contactors A magnetic contactor is basically a large control relay designed to open and close a power circuit. It possesses a relay coil and a magnetic plunger, which carries a set of movable contacts. When the relay coil is energized, it attracts the magnetic plunger, causing it to rise quickly against the force of gravity. The movable contacts come in contact with a set of fixed contacts, thereby closing the power circuit. In addition to the power contacts, one or more normally open or normally closed auxiliary contacts are usually available, for control purposes. When the relay coil is de-energized, the plunger falls, thereby opening and closing the respective contacts. Magnetic contactors are used to control motors ranging from 0.5 hp to several hundred horsepower. The size, dimensions, and performance of contactors are standardized. Figure 20.7 Three-phase magnetic contactor rated 50 hp, 575 V, 60 Hz. Width: 158 mm; height: 155mm; depth: 107 mm; weight: 3.5 kg. (Courtesy of Siemens)

Pilot Lights A pilot light indicates the on/off slate of a remote component in a control system. Figure 20.8 Pilot light, 120 V, 3 W mounted in a start-stop pushbutton station. (Courtesy of Siemens)

Limit Switches And Special Switches A limit switch is a low-power snap-action device that opens or closes a contact, depending upon the position of a mechanical part. Other limit switches are sensitive to pressure, temperature, liquid level, direction of rotation, and soon. Figure 20.9a Limit switch with one NC contact; rated for ten million operations; position accuracy: 0.5 mm. (Courtesy of Square D) Figure 20.9b Liquid level switch. (Courtesy of Square D)

Chapter 3 Input/Output Devices & Circuits 3.1 General In this chapter we examine the input and output devices commonly found in automation systems, their connection to the programmable controller and a selection of the I/O modules available to the purchaser. We deal exclusively with ON-OFF signals (known also as 'binary', 'discrete', or 'digital' signals). The programmable controller's input and output modules must be compatible with the devices and supplies in use. Four points need to be considered: • kind of current (a.c. or d.c.) • rated current • rated voltage • polarity This information is best obtained from manufacturers' catalogues, data sheets or user manuals. The most commonly used supplies are: 24 V. 48 V, 115 V and 230 V (a.c. or d.c.). 3.2 Input Devices An input signal can be generated by a manual or an automatic device. For example, a pushbutton gives a manual control signal such as START or STOP. The contacts can be make (normally open) or break (normally closed). It is normal practice to obtain a START signal from a make contact and a STOP signal from a break contact (Fig. 3.1). A selector switch is a manually operated switch having two or more positions (see Fig. 3.3). Typically one contact closes for each selected position, and each contact is connected individually to an input on the programmable controller.

A thumbwheel switch is used to make a numerical selection. The switch itself has four poles (Fig-3.2) a contact may be open or closed depending on the selected position. For example at position 3, the first and second contacts are closed. The thumbwheel switch can give 10 different selections using only four inputs. By combining two such switches, we can obtain 100 selections using only 8 inputs. A limit switch is an electromechanical position switch used to detect the passage or travel of a moving part. It can be actuated by can roller or lever; it supplies an input signal through its make or break contact (Fig. 3.3). A thermostat is a heat sensor operating on the expanding liquid or bimetal principle, which acts on a make and/or break contact when a pre-set temperature is reached. Either contact may be used to switch the input signal (Fig. 3.3). A pressure switch is used for sensing fluid pressure or vacuum. The mechanism incorporates a bellows and snaps action springs which act on make and/or break contacts when the set pressure is reached. Either contact may be used for switching the input signal (Fig. 3.4).

A level switch is used for sensing liquid level. Make and/or break contacts are operated by a lever attached to a float mechanism. Either contact may carry the input signal to the programmable controller (Fig. 3.4). Level probes used in conjunction with an amplifier perform a similar function to the level switch. Recently sonar devices have begun to be used for level detection. A proximity sensor is a solid state device used to detect presence, passage or flow of parts, for positioning, end of travel, or rotation. The inductive type is able to detect a nearby metal object because of a change in the magnetic field at its sensing face. The capacitive type is able to detect a nearby non-metallic object because of a change in the electric field at its sensing face (see Fig. 3.5). Two-wire and three-wire sensors are available. The older type of two-wire sensor often has a high off-state current which makes it unsuitable for use with a programmable controller. The three-wire sensor is given its own d.c. supply which is independent of the signal wire; as a result it is much more reliable in programmable controller applications. The three-wire type (Fig. 3.5) can have either PNP or NPN switching. During operation the PNP type switches the positive to the input of the programmable controller; thus the PNP device is used with controllers expecting a positive input signal. The NPN type switches the negative to the input; thus the NPN device is used with controllers expecting a negative input signal. Photoelectric sensors are solid state devices that use infra-red light for detecting the presence, passage or movement of objects. There are two main types: through-beam and reflex (Fig. 3.6). The through-beam type has a separate transmitter and receiver between which the beam of light passes. The reflex type has a combined transmitter/receiver and is used in conjunction with a reflector. PNP and NPN types are available. The sensor is so positioned that the target object interrupts the light beam as it passes and an input signal is generated accordingly. The three-wire proximity or photoelectric sensor has a small current demand. In real automation projects, however, large numbers of sensors may be used and the accumulated current could well exceed the rated current of the power supply in the programmable controller. If this problem is foreseen at the planning stage it can be offset by choosing a controller with an adequate power supply. If it is discovered after installation, either the larger power supply must be retrofitted or a new, separate power supply must be provided to serve the sensors.

3.3 Output Devices The contactor is a switching device having a number of contacts which are operated together by its electromagnet (the ‘coil’). It is used very extensively for switching motors, heaters, etc. For automation work we connect the coil of the contactor to an output of the programmable controller. When the output is turned on, the coil is energized and the contactor connects its load to the supply (Fig. 3.7). For small contactors the coil current is modest and can be switched directly by the output of the controller. For large contactors the coil current could exceed the capacity of the output. In this case we let the controller’s output drive a relay (also-called interface relay) which in turn switches the current of the larger contactor coil.

Functionally the relay and the contactor are very similar; the main difference lies in the switching capacity of the contacts. A contactor is designed for power switching (currents up to 1600 A) whereas a relay is meant for ‘small’ currents ( ≤10 A). The coil of a relay or a contactor is highly inductive and this means it tends to produce high induced voltages on switch-off. We have to design our output circuits with due regard to this problem (see Section 3.6). A solenoid valve is another extensively used component. The coil is arranged to open or close the route between two or more ‘ports’, enabling it to control the flow of liquid or gas. The coil is connected to the controller's output; by this means hydraulic and pneumatic mechanisms can become part of an automation system (Fig. 3.8). Like the relay and contactor, the coil of a valve is highly inductive. The current drawn by small valves is extremely low (tens of mA) and may be comparable with the leakage current of some output switching devices or their internal protective circuits. This can lead to the valve being energized even when the output is off In such a case the valve coil may be shunted by a resistor which will ‘absorb’ the leakage current (Fig. 3.9); alternatively, it can be switched through the contact of an interface relay (Fig. 3.7). A signal lamp is used to indicate the status of a plant or an operating cycle. It is normally switched directly by (he programmable controller as shown in Fig. 3.10. The general-purpose indicator is fitted with an incandescent lamp which presents no difficulty for the output. In recent designs the neon lamp or the high-intensity light-emitting diode (LED) has gained in popularity. The very low current of this kind of lamp occasionally presents problems (see Section 3.6).

The trend in recent times has been to replace dedicated signal lamps with message displays or operator interfaces. These furnish the operator with a large repertoire of messages in clear text, in the local language and at a single location. There is a drastic reduction in the number of outputs used and a simplification of the wiring.

3.4 Operator Interface In projects of larger scale or complexity, the task of informing the operator takes on a greater significance. Meeting this task with traditional signal lamps can result in large display panels which can be difficult for the operator to survey and which use up great numbers of outputs on the programmable controller. One solution is the message (text) display, an example of which appears in Fig. 3.11. This device (model UCT-35P from Brodersen Control Systems of Denmark) displays messages of ordinary text, organized in two lines of 20 characters each. Thus the operator may receive

• Information DEFROST CYCLE IN PROGRESS • Alarms CONVEYOR MOTOR OVERLOAD • Instructions PRESS START TO ADVANCE • Data WATER TEMP. > 80°C The messages are loaded in the memory of the display using a personal computer or a keyboard; each message has its own recall number. The display is given its own power supply and is connected to a number of outputs on the programmable controller (nine for maximum exploitation of the model referred to). The controller activates a message by turning on an appropriate pattern of outputs, which the display interprets as a recall number. Table 3.1 shows how this works using binary code (BCD (binary coded decimal) is also possible). Each output is weighted: output 0 is worth 1, output 1 is worth 2, output 2 is worth 4, and so on. The number conveyed to the display is the sum of the values of the outputs that are on. Thus, for example, message 37 (highlighted) requires the following outputs to be on:

output 0 worth 1 output 2 worth 4 output 5 worth 32 total worth 37 A message display may be fitted with buttons or keys. In our example there are four keys, which are connected to inputs on the controller. The operator can use these keys to respond to the messages displayed, effectively opening up two-way communication with the programmable controller. A device capable of this kind of service is known as an operator interface. The operator interface, shown in Fig. 3.11 is described in literature as a parallel type and has the following advantages: • It can he used with any programmable controller, regardless of manufacture • It requires no communications port on me controller, just inputs and outputs • It is easy to set up and requires only minor programming work in the controller A popular alternative is the serial-type operator interface, shown in Fig. 3.12, for which inputs and outputs are not necessary. Instead, dialogue with the controller takes place via the programming or other port.

This demands full compatibility between the controller and interface, which is most easily assured by choosing both from the same manufacturer. The advantages are that • Inputs and outputs are not used and are available for other purposes • All parameters in the control program are fully accessible • Simple, low-cost versions are available from each manufacturer 3.5 Input Modules A programmable controller input module is selected to suit the control voltage and the input devices proposed to be connected to it. Figure 3.13 illustrates the content and function of a d.c. input module, in a simplified form. Each incoming input signal passes through a status LED (for visual indication on the module), a voltage dropping resistor and an opto-isolator (which consists of an LED and photo-transistor). In an a.c. module it also passes through a rectifier. The objective is to provide the microprocessor with a signal it can handle — 5 V d.c., free of interference (‘noise’) and electrically isolated.

Each input module will accommodate a number of input signals, e.g. eight for the Siemens SIMATIC® S5-100U and the Sprecher + Schuh SESTEP® 290. The relevant catalogue should be consulted before choosing the controller or input module, for example 1. We have 21 inputs for 24 V d.c. and we want a Mitsubishi controller; we choose an F2-40MR-ES or F2-40MS-ES or F2-40MR-DS (all accept 24 inputs at 24 V d.c.; they differ only in respect of outputs or power supply) 2. We have 14 inputs for 230 V a.c. and we want a Siemens controller: we choose two modules 6ES5-8MD11 (each accepts 8 x 230 V a.c. inputs). 3.6 Output Modules An output module of the programmable controller transfers the output signals from the microprocessor at very low voltage to the output terminals of the controller at plant voltage, typically 115/230 V a.c. Once again, opto-isolators provide electrical isolation and limit the effects of interference.

Figure 3.14 illustrates how a relay output module functions. Each outgoing signal is routed through an opto-coupler for isolation, then an LED for indication on the module, and finally turns on the coil of a sub-miniature relay. The contact of this relay connects the supply to the output device. In other modules the place of the relay is taken by a triac or a transistor (Fig. 3.14). The Relay Output • is suitable for a.c. and d.c. switching • provides real separation when the contact is open • can withstand high surge currents and severe voltage transients • is subject to mechanical failure and contact erosion The Triac Output • is strictly for a.c. switching • is silent, has no moving parts and does not suffer from contact wear • is easily destroyed by over current The Transistor Output • is strictly for d.c. switching • is silent, has no moving parts and does not suffer from contact wear • is capable of switching at very high speed • can be destroyed by over current and high reverse voltage In general the Triac output is the most prone to damage by over current, and a fuse is nearly always incorporated in the module to protect each output or group of outputs. This type of fuse is specially adapted for semiconductor protection and must be used and replaced in strict accordance with the manufacturer’s advice.

For a Triac to remain conductive, the current must exceed a minimum value known as the ‘holding current’. Some low power loads may need a shunt resistor to meet this requirement (see Fig. 3.14). The Transistor output is also prone to over current damage, but nowadays many transistor modules come equipped with built-in electronic protection (so-called short-circuit-proof outputs). An external fuse or circuit breaker may none the less be fitted to protect the power supply and the wiring. Relay outputs are the most robust and protection is straightforward: each output or group of outputs is protected with a fuse or circuit-breaker of suitable rating, fitted externally. Once again we need to refer to a catalogue to choose the correct controller or output module, for example 1. We have 19 x 230 V a.c. outputs and we want a Sprecher+Schuh controller: we choose three modules ODR-21 or ODS-21 (8 x 250 V a.c. outputs; ODR-21 has relays while ODS-21 has triacs) 2. We have 18 x 115 V a.c. outputs and we want a Telemecanique controller: we choose a controller TSX 172 3444E or a TSX 172 3428E (12 x 115 V relay outputs) and extension module TSX DSF 635 (6 x 115 V relay outputs). Relay, triac and transistor outputs can be damaged or even destroyed by over-voltage. Although protective circuits are included in the output module by the manufacturer, it is better to eliminate the problem at source. The most common sources are the coils of relays, contactors and solenoid valves, which are highly inductive. Over voltages from these sources can be suppressed by fitting ‘snubbers’ as shown in Fig. 3.14. For a.c. circuits either an RC network or a varistor is used; for d.c. circuits a reverse-biased diode is used.