Computer Automation in Manufacturing

Transcription

1 Computer Automation in Manufacturing

2 Computer Automation in Manufacturing An introduction Thomas O. Boucher Department of Industrial Engineering Rutgers University Piscataway NJ USA SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

3 First edition Thomas O. Boucher Originally published by Chapman & Hall in 1996 Softcover reprint of the hardcover 1st edition 1996 Typeset in 1O/12pt. Times by Thomson Press (I) Ltd., New Delhi ISBN ISBN (ebook) DOI / Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Library of Congress Catalog Card Number: eprinted on permanent acid-free text paper, manufactured in accordance with ANSI/NISO Z and ANSI/NISO Z (Permanence of Paper). [Paper = Magnum. 70 gsm]

4 To My Mother and Father

5 Contents Preface Acknowledgements Xl xiii 1 Introduction Developments in manufacturing technology for automation A hierarchical model of factory automation System requirements and automatic control technology About this book Summary 12 Further reading 12 2 The architecture of microprocessor-based systems Introduction The transistor and basic digital logic functions Gates and their I/O functions Basic properties of boolean variables Gates, boolean algebra and the design of logic networks An overview of the architecture and operations of a microprocessor-based system Number systems and low-level computer languages Assembly language programming Communications and automatic data transfer Software strategies for I/O communication Summary 80 Exercises 81 Further reading 90 3 Sensors and automatic data acquisition Introduction Discrete event sensors Continuous sensors Interfacing a digital controller with an analog world Transducers Summary 132

6 viii I I CONTENTS Exercises 132 Further reading Actuators and the performance of work Introduction Concepts of work, force, torque and power Power transmission and reflected forces Basic principles of a DC machine Operating characteristics of DC motors Speed control of DC motors Stepping motors Relay switches and solenoids Fluid actuators Summary 167 Appendix 4A: Quantities and their units 168 Exercises 168 Further reading Control theory Introduction Components of a control system Mathematical characterization and transfer functions Laplace transforms Using transforms to analyze system response Closed loop speed control Position transform of a DC motor Position control of a DC motor Characteristic responses of a closed loop control system Control strategies Typical classes of control system models Digital control Summary 229 Appendix SA: Partial fraction expansion 230 Appendix 5B: Solution of a linear differential equation with imaginary roots 231 Exercises 233 Further reading Programmable logic controllers Introduction PLC hardware Relay circuits and ladder diagrams Sequential control using relay circuits Sequential control using PLCs 251

7 CONTENTS 6.6 Scan sequence and program execution 6.7 Decoupling and program control 6.8 Counters and timers 6.9 Sequential function charts and GRAFCET 6.10 Advanced PLC programming 6.11 Summary Exercises Further reading 7 Supervisory control of manufacturing systems 7.1 Introduction 7.2 Production organization and manufacturing system design 7.3 Communication architecture and local area networks 7.4 Discrete event systems and supervisory controller software design 7.5 Some mathematical properties of ordinary Petri nets i6 Case study: Petri net representation of a machining cell controller 7.7 Software specification for a machining cell controller 7.8 Imposing priorities in Petri net models 7.9 Summary Exercises Further reading I I ix Appendix: Index Conversion tables

8 Preface The use of industrial pes and programmable controllers in manufacturing has been growing rapidly over the last twenty years. With their flexibility of programming for different products and processes, capability for real time data gathering for on-line product inspection and process control, and potential for integrating the shop floor with a factory-wide information system, it is likely that this technology will continue to evolve and be used for the forseeable future. The primary purpose of writing this book is to provide a text with a broad treatment of computer automation that can be used in the third or fourth year of an undergraduate engineering program. The book covers the technology of microprocessor based systems, emphasizing the similarities between microcomputers, microcontrollers and programmable controllers. It covers the use of this technology in automating manufacturing applications at the individual machine level, the manufacturing production line or cell level, and the shop floor level. The writing of this book was motivated by the need to cover a range of important subjects in an engineering curriculum that is oriented toward manufacturing. Some subjects are so fundamental to manufacturing automation that all engineers interested in manufacturing should have a basic level of understanding of them. Today, every engineer interested in manufacturing should have a grasp of computer architecture and how computer processors are interfaced to electromechanical processes to gather information and control machines. The theories of continuous and discrete control and their applications using imbedded controllers and programmable logic controllers are important ideas for engineers planning to work on the current generation of manufacturing processes. An understanding of the uses of information and the network technologies that support its transmission throughout the factory will be important to the engineers who design and operate the next generation of manufacturing systems. Typically, one would expect to cover these subjects in individual specialized courses. However, this is seldom done as a requirement at the undergraduate level. This book helps to overcome two problems associated with the individual courses approach. The first problem is a lack ofroom in the curriculum. In any engineering discipline, there is so much foundation material to teach that the ability to add new material oriented toward the practice of manufacturing

9 xii P_R_E_F_AC_E ----l is limited. Students may complete a degree in industrial, manufacturing, or mechanical engineering without having studied computer or controller architecture. On the other hand, some curricula may have an entire course on computer architecture and digital logic, but provide no instruction in control theory or local area networks. The second problem is that, regarding manufacturing, these subjects are part of a continuum of related ideas and should be integrated as they are taught. It is the relationship between these technologies that is important to the practising engineer in manufacturing and that relationship should be emphasized in instruction. This text was written from lecture notes developed for a senior undergraduate course in computer control in manufacturing. The course was designed to be taken by undergraduate students with a background in physics and calculus at the sophomore level. Although it is helpful to have an introductory course in linear circuit theory, it is not essential if the instructor gives a light treatment of the basic equations for passive circuits or if the physics background of the students has touched on elementary concepts of DC circuits. Due to the introductory nature of this text, I have focused on covering topics to the level required for an understanding of basic principles. Hence, each chapter is supplemented by a further reading list that provides the reader with references to a more exhaustive treatment of the subject matter of the chapter. I have found that it is possible to cover most of the material in this text in one semester even if the students have no background in computer architecture, a subject which usually consumes a third of the instruction time for the course. Instructors with a student population having a background in computer architecture may wish to supplement the remaining subjects with advanced materials from the reading lists. I have taught this material as part of a four credit course which includes a one credit laboratory that meets for three hours a week. The lab is an important component of instruction because it provides hands-on experimentation that reinforces the concepts of this book. I have built the laboratory around experiments on the inexpensive and popular Motorola 68HCll microcontroller and the Allen-Bradley 500 series programmable controller, both of which are referenced in this text. Any instructor interested in obtaining a copy of the laboratory manual can write to me at Rutgers University. A solution manual for end-of-chapter problems is also available by writing to me. Readers should note that this textbook was written for the purpose of instructing students on the principles of computer control. Throughout this book computer programs are used to illustrate control applications in manufacturing. Because of the variable requirements of specific installations, particularly safety requirements, these programs should not be copied for actual installations without appropriate modifications.

10 Acknowledgements I would like to thank Renata Joyner and Suzanne Becker for their assistance in typing several earlier versions ofthe manuscript. Thanks, also, to my colleague E. A. Elsayed, who encouraged me to develop the course on which the book is based, and to my colleague Mohsen Jafari, who provided me with inputs for Chapter 7. I am grateful to Gulgan Alpan, and Wei Zhou for reviewing portions of manuscript, to Peter Brassington and Ramgopal Venkataraman, who helped prepare several of the text drawings and Dan Diana, who provided the cover artwork. Thanks also to my teaching assistant, Ozerk Gogus, who helped me to refine end-of-chapter problems. Particular acknowledgement is made to the Allen-Bradley Corporation for permission to use material describing the Data Highway local area networks that appears in Chapter 7 (Copyright 1991 Allen-Bradley Company, Inc. Used with permission. Data Highway Plus, DH +, Data Highway 485, DH-485 are all trademarks of Allen-Bradley Company, Inc.) Thanks also to Dr Seymour Melman for permission to use Figure 1.1. I thank the professional staff of Chapman & Hall for their help. Thanks especially to my Editor, Mark Hammond, and to Helen Hodge for her meticulous work as Copy Editor. As have many authors, I found that an important person in the realization of a book is a supportive and understanding spouse. I am grateful to my wife, Unn, not only for putting up with my preoccupations while writing this book, but for her active assistance and encouragement toward the end, when I needed it.