This textbook, now in its sixth edition, continues to be straightforward and easy-to-read, presenting the principles of PLCs while not tying itself to one manufacturer or another. Extensive examples and chapter ending problems utilize several popular PLCs, highlighting understanding of fundamentals that can be used regardless of manufacturer. This book will help you to understand the main design characteristics, internal architecture, and operating principles of PLCs, as well as Identify safety issues and methods for fault diagnosis, testing, and debugging. New to This edition:
- A new chapter 1 with a comparison of relay-controlled systems, microprocessor-controlled systems, and the programmable logic controller, a discussion of PLC hardware and architecture, examples from various PLC manufacturers, and coverage of security, the IEC programming standard, programming devices and manufacturer's software
- More detail of programming using Sequential Function Charts
- Extended coverage of the sequencer
- More Information on fault finding, including testing inputs and outputs with an illustration of how it is done with the PLC manufacturer's software
- New case studies
- A methodical introduction, with many illustrations, describing how to program PLCs, no matter the manufacturer, and how to use internal relays, timers, counters, shift registers, sequencers, and data-handling facilities
- Consideration of the standards given by IEC 1131-3 and the programming methods of ladder, functional block diagram, instruction list, structured text, and sequential function chart
- Many worked examples, multiple-choice questions, and problems are included, with answers to all multiple-choice questions and problems given at the end of the book
Former Lecturer at Buckingham Chilterns University College, High Wycombe, UK, and now retired, William Bolton has worked in industry and academia as a senior lecturer in a college of technology, a member of the Nuffield Advanced Physics team, an adviser to a British government aid project in Brazil on technical education, as a UNESCO consultant in Argentina and Thailand, and as Head of Research and Development at the Business and Technician Education Council. He has written many engineering textbooks, including Mechatronics, 4th ed., Engineering Science, 5th ed., Higher Engineering Science, 2nd ed., Mechanical Science, 3rd ed., and Instrumentation and Control Systems.
Programmable Logic Controllers
This chapter is an introduction to control systems, starting with a discussion of relay-controlled systems before discussing the programmable logic controller (PLC) and its general function, hardware forms, and internal architecture. PLCs are widely used for a range of automation tasks in such areas as industrial processes in manufacturing. The IEC standard 61131 is outlined. This overview is followed by more detailed discussion in the following chapters.
Relay-controlled systems. Programmable logic controller
This chapter is an introduction to the programmable logic controller (PLC) and its general function, hardware forms, and internal architecture. PLCs are widely used for a range of automation tasks in areas such as industrial processes in manufacturing. This overview is followed by more detailed discussion in the following chapters. For a summary of the history, development, features, and comparison with other control systems, see the Wikipedia entry for Programmable logic controller.
What type of task might a control system handle? It might be required to control a sequence of events, maintain some variable constant, or follow some prescribed change. For example, the control system for an automatic drilling machine (Figure 1.1a) might be required to start lowering the drill when the workpiece is in position, start drilling when the drill reaches the workpiece, stop drilling when the drill has produced the required depth of hole, retract the drill, and then switch off and wait for the next workpiece to be put in position before repeating the operation. Another control system (Figure 1.1b) might be used to control the number of items moving along a conveyor belt and direct them into a packing case. The inputs to such control systems might come from switches being closed or opened; for example, the presence of the workpiece might be indicated by it moving against a switch and closing it, or other sensors such as those used for temperature or flow rates. The controller might be required to run a motor to move an object to some position or to turn a valve, or perhaps a heater, on or off. Figure 1.1
An example of a control task and some input sensors: (a) an automatic drilling machine; (b) a packing system.
What form might a controller have? For the automatic drilling machine, we could wire up electrical circuits in which the closing or opening of switches would result in motors being switched on or valves being actuated. Thus, as a result, we might have a relay (Figure 1.2) closing or opening contacts which, in turn, switches on the current to a motor and causes the drill to rotate (Figure 1.3). Another switch might be used to activate a relay and switch on the current to a pneumatic or hydraulic valve, which results in pressure being switched to drive a piston in a cylinder and so results in the workpiece being pushed into the required position. Such electrical circuits would have to be specific to the automatic drilling machine. For controlling the number of items packed into a packing case, we could likewise wire up electrical circuits involving sensors and motors. However, the controller circuits we devised for these two situations would be different. In the "traditional" form of control system, the rules governing the control system and when actions are initiated are determined by the wiring. When the rules used for the control actions are changed, the wiring has to be changed. Figure 1.2
A basic relay. Figure 1.3
A control circuit.
1.1.1 Relay-Controlled Systems
Relay-controlled systems are hard-wired systems. Figure 1.2 shows the basic elements of a simple relay. When a current is switched on to flow through the relay solenoid, normally-closed (NC) contacts open and normally-open (NO) contacts close. These contacts can be used to give control in a system. As an illustration consider a relay being used to operate a pneumatic or hydraulic valve, this then results in pressure being applied to drive a piston to move a workpiece. We can represent the situation by a control drawing. Figure 1.4 shows the standard symbols used for relays and Figure 1.5 shows the control drawing with the vertical lines representing the power rails and the horizontal lines to systems connected between them. The sequence of events is read from the top horizontal line downwards. Thus, in the top line of Figure 1.5(a), when the Off-On switch is closed, the relay is activated. This closes the contacts on the second line and so the solenoid valve is switched on. A more usual control drawing is shown in Figure 1.5(b) which has the relay switched on by a momentary NO push-button switch. This closes two sets of contacts. Contacts 1 latch the push button switch so that when the push stops there is still connection of power to the relay. Contacts 2 switch on the solenoid valve. The relay, and hence power to the solenoid valve, is switched off when the normally closed push-button switch is pressed. The control drawings are obviously only part of the control system as there will need to be further lines for when the solenoid valve has moved the workpiece the required distance so that it stops its action. Figure 1.4
Relay symbols. Figure 1.5
Relay-controlled system control drawings.
Figure 1.6 shows another example of a relay control system. When the start push button is closed, the relay coil is switched on and latches the push button switch so that the relay remains on until the stop push button is pressed. The relay closes the NO contacts and opens the NC contacts. As a result, the green light is switched on and the red light switches off. When the stop push button is pressed, the current to the relay coil is switched off. This results in the NO contacts opening and the NC contacts closing and so the green light going off and the red light comes on. The next stage in the relay circuit might be a motor that is switched on by NO contacts, so the green light indicates when the motor is running and the red light when it is off. Figure 1.6
Relay circuit to control red and green lights.
1.1.2 Microprocessor-Controlled Systems
Instead of hardwiring each control circuit for each control situation, we can use the same basic system for all situations if we use a microprocessor-based system and write a program to instruct the microprocessor how to react to each input signal from, say, switches and give the required outputs to, say, motors and valves. Thus we might have a program of the form: If switch A closes Output to motor circuit If switch B closes Output to valve circuit
By changing the instructions in the program, we can use the same microprocessor system to control a wide variety of situations.
As an illustration, the modern domestic washing machine uses a microprocessor system. Inputs to it arise from the dials used to select the required wash cycle, a switch to determine that the machine door is closed, a temperature sensor to determine the temperature of the water, and a switch to detect the level of the water. On the basis of these inputs the microprocessor is programmed to give outputs that switch on the drum motor and control its speed, open or close cold and hot water valves, switch on the drain pump, control the water heater, and control the door lock so that the machine cannot be opened until the washing cycle is completed.
1.1.3 The Programmable Logic Controller
A programmable logic controller (PLC) is a special form of microprocessor-based controller that uses programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting, and arithmetic in order to control machines and processes (Figure 1.7). It is designed to be operated by engineers with perhaps a limited knowledge of computers and computing languages. They are not designed so that only computer programmers can set up or change the programs. Thus, the designers of the PLC have preprogrammed it so that the control program can be entered using a simple, rather intuitive form of language (see Chapter 4). The term logic is used because programming is primarily concerned with implementing logic and switching operations; for example, if A or B occurs, switch on C; if A and B occurs, switch on D. Input devices (that is, sensors such as switches) and output devices (motors, valves, etc.) in the system being controlled are connected to the PLC. The operator then enters a sequence of instructions, a program, into the memory of the PLC. The controller then monitors the inputs and outputs according to this program and carries out the control rules for which it has been programmed. Figure 1.7
A programmable logic controller.
PLCs have the great advantage that the same basic controller can be used with a wide range of control systems. To modify a control system and the rules that are to be used, all that is necessary is for an operator to key in a different set of instructions. There is no need to rewire. The result is a flexible, cost-effective system that can be used with control systems, which vary...