Programmable Logic Controllers Fourth Edition (2006).pdf

Programmable Logic Controllers Programmable Logic Controllers Fourth Edition W. Bolton AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO ELSEVIER Newnes is an imprint of Elsevier Newnes Newnes is an imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP 30 Corporate Drive, Suite 400, Burlington, MA 01803 First edition 1996 Second edition 2000 Third edition 2003 Fourth edition 2006 Copyright  2006, W. Bolton. Published by Elsevier Newnes. All rights reserved The right of W. Bolton to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act 1988 No part of this publication may be reproduced, stored in a retri system or transmitted in any or by any means electronic, mechanical, photocopying, recordinh or otherwise without the prior permission of the publisher Permissions may be sought directly from Elsevier’s Science phone 44 1865 843830; fax 44 0 1865 853333; e-mail permissionselsevier.co.uk. Alternatively you can your request on-line by visiting the Elsevier web site at http and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any s, products, instructions or ideas contained in the material herin. Because of rapid advances in the medical sciences, in particular, independent verification of diganoses and drug dosages should be made British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging -in-Publication Data A catalog record for this book is available from the Library of Congress ISBN-13 978-0-7506-8112-4 ISBN-10 0-7506-8112-8 Printed and bound in the UK 06 07 08 09 10 10 9 8 7 6 5 4 3 2 1 International Sabre Foundation BOOK AID ELSEVIER | www.bookaid.org | www.sabre.org Working together to grow libraries in developing countries For ination on all Newnes publications visit our website at Contents Prefacevii 1 4 5 10 15 Controllers Hardware Internal architecture PLC systems Problems 1.1 1.2 1.3 1.4 Programmable logic controllers 1 17 30 39 41 devices Output devices Examples of applications Problems 2.1 2.2 2.3 -output devices2 44 45 47 51 52 The binary system Octal and hexadecimal Binary arithmetic PLC data Problems 3.1 3.2 3.3 3.4 Number systems3 53 59 62 69 75 76 77 /output units Signal conditioning Remote connections Networks Processing s I/O addresses Problems 4.1 4.2 4.3 4.4 4.5 4.6 I/O processing4 80 84 90 91 93 94 100 103 Ladder diagrams Logic functions Latching Multiple outputs Entering programs Function blocks Program examples Problems 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Ladder and functional block programming 5 108 115 120 124 Intruction lists Sequential function charts Structured text Problems 6.1 6.2 6.3 IL, SFC and ST programming s 6 132 133 136 Internal relays Ladder programs Battery-backed relays 7.1 7.2 7.3 Internal relays7 137 138 142 146 One-shot operation Set and reset Master control relay Problems 7.4 7.5 7.6 154 156 157 Jump Subroutines Problems 8.1 8.2 Jump and call8 159 160 163 165 166 167 Types of timers Programming timers Off-delay timers Pulse timers Programming examples Problems 9.1 9.2 9.3 9.4 9.5 Timers9 173 174 178 179 180 182 s of counter Programming Up and down counting Timers with counters Sequencer Problems 10.1 10.2 10.3 10.4 10.5 Counters10 189 190 194 Shift registers Ladder programs Problems 11.1 11.2 Shift registers11 197 198 202 203 206 Registers and bits Data handling Arithmetic functions Closed loop control Problems 12.1 12.2 12.3 12.4 Data handling12 210 214 218 220 227 248 Program development Safe systems Commissioning Fault finding System documentation Problems 13.1 13.2 13.3 13.4 13.5 Designing systems13 250 254 265 269 271 Temperature control Valve sequencing Conveyor belt control Control of a process Problems 14.1 14.2 14.3 14.4 Programs14 276 Appendix Symbols 281 288 Answers Index vi Contents Preface Technological advances in recent years have resulted in the development of the programmable logic controller and a consequential revolution of control engineering. This book is an introduction to programmable logic controllers and aims to ease the tasks of practising engineers coming first into contact with programmable logic controllers, and also provides a basic course for students on courses such as Nationals and Higher Nationals in Engineering, company training programmes and as an introduction for first year undergraduate courses in engineering. The book has been designed to provide full syllabus coverage of the BTEC National and Higher National in Engineering units Programmable Controllers and Programmable Logic Controllers from Edexcel. It addresses the problem of different programmable control manufacturers using different nomenclature and program s by describing the principles involved and illustrating them with examples from a range of manufacturers. The text includes wThe basic architecture of PLCs and the characteristics of commonly used and outputs to such systems. wA discussion of the number systems denary, binary, octal, hexadecimal and BCD. wA painstaking ical introduction, with lots of illustrations, of how to program PLCs, whatever the manufacturer, and make use of internal relays, timers, counters, shift registers, sequencers and data handling facilities. wConsideration of the standards given by IEC 1131-3 and the programming s of ladder, functional block diagram, instruction list, structured text and sequential function chart. wTo assist the reader to develop the skills necessary to write programs for programmable logic controllers, many worked examples, multi-choice questions and problems are included in the book with answers to all multi-choice questions and problems given at the end of the book. Changes from third edition The fourth edition is a complete restructuring and updating of the third edition and includes a more detailed consideration of IEC 1131-3, including all the programming s given in the standard, and the problems of safety. This includes a discussion of emergency stop relays and safety PLCs. Aims This book aims to enable the reader to wIdentify and explain the main design characteristics, internal architecture and operating principles of programmable logic controllers. wDescribe and identify the characteristics of commonly used and output devices. wExplain the processing of s and outputs by PLCs. wDescribe communication links involved with PLC systems, the protocols and networking s. wDevelop ladder programs for the logic functions AND, OR, NOR, NAND, NOT and XOR. wDevelop ladder programs involving internal relays, timers, counters, shift registers, sequencers and data handling. wDevelop functional block diagram, instruction list, structured text and sequential function chart programs. wIdentify safety issues with PLC systems. wIdentify s used for fault diagnosis, testing and debugging. Structure of the book The figure on the following page outlines the structure of the book. W. Bolton viii Preface Design and operational characteristics PLC ination and communication techniques Programming techniques Chapter 1 Programmable logic controllers Chapter 2 -output devices Chapter 4 I/O processing Chapter 5 block programming Chapter 7 Internal relays Chapter 9 Timers Chapter 10 Counters Chapter 11 Shift registers Chapter 12 Data handling Chapter 13 Designing programs Chapter 14 Programs Number systems Chapter 3 Programming s Ladder and functional Chapter 6 IL, SFC and ST programming s Chapter 8 Jump and call Preface ix 1 Programmable logic controllers This chapter is an introduction to the programmable logic controller, its general function, hardware s and internal architecture. This overview is followed up by more detailed discussion in the following chapters. 1.1 ControllersWhat type of task might a control system have It might be required to control a sequence of events or 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 s to such control systems might be from switches being closed or opened, e.g. 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. Drill Workpiece Switch contacts close when workpiece in position Switch contacts opened when drill reaches the surface of the workpiece Switch contacts opened when drill reaches required depth in workpiece Photoelectric sensor gives signal to operate deflector Deflector Deflected items Items moving along conveyor a b Figure 1.1 An example of a control task and some sensors a an automatic drilling machine, b a packing system What 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 we might have the closing of a switch activating a relay which, in turn, switches on the current to a motor and causes the drill to rotate Figure 1.2. 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’ 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. Motor Relay to switch on large current to motor Low voltage Switch Figure 1.2 A control circuit 1.1.1 Microprocessor controlled system 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 signal from, say, switches and give the required outputs to, say, motors and valves. Thus we might have a program of the 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. s 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 2 Programmable Logic Controllers a switch to detect the level of the water. On the basis of these s the microprocessor is programmed to give outputs which 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. 1.1.2 The programmable logic controller A programmable logic controller PLC is a special of micro- processor-based controller that uses a 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 1.3 and are 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 pre-programmed it so that the control program can be entered using a simple, rather intuitive, of language, see Chapter 4. The term logic is used because programming is primarily concerned with implementing logic and switching operations, e.g. if A or B occurs switch on C, if A and B occurs switch on D. devices, e.g. sensors such as switches, and output devices in the system being controlled, e.g. motors, valves, etc., are connected to the PLC. The operator then enters a sequence of instructions, i.e. a program, into the memory of the PLC. The controller then monitors the s and outputs according to this program and carries out the control rules for which it has been programmed. Program PLC s Outputs Figure 1.3 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 which can be used with control systems which vary quite widely in their nature and complexity. PLCs are similar to computers but whereas computers are optimised for calculation and display tasks, PLCs are optimised for control tasks and the industrial environment. Thus PLCs are 1Rugged and designed to withstand vibrations, temperature, humidity and noise. 2Have interfacing for s and outputs already inside the controller. Programmable logic controllers 3 3Are easily programmed and have an easily understood programming language which is primarily concerned with logic and switching operations. The first PLC was developed in 1969. They are now widely used and extend from small self-contained units for use with perhaps 20 digital s/outputs to modular systems which can be used for large numbers of s/outputs,