Basic and Advanced Regulatory Control - System Design and Application.pdf

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Basic and Advanced Regulatory Control: System Design and Application 2nd Edition

By Harold L. Wade

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Notice The information presented in this publication is for the general education of the reader. Because neither the author nor the publisher have any control over the use of the information by the reader, both the author and the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected to exercise sound professional judgment in using any of the information presented in a particular application. Additionally, neither the author nor the publisher have investigated or considered the affect of any patents on the ability of the reader to use any of the information in a particular application. The reader is responsible for reviewing any possible patents that may affect any particular use of the information presented. Any references to commercial products in the work are cited as examples only. Neither the author nor the publisher endorse any referenced commercial product. Any trademarks or tradenames referenced belong to the respective owner of the mark or name. Neither the author nor the publisher make any representation regarding the availability of any referenced commercial product at any time. The manufacturer’s instructions on use of any commercial product must be followed at all times, even if in conflict with the information in this publication.

Copyright © 2004 by

ISA—The Instrumentation, Systems, and Automation Society 67 Alexander Drive P.O. Box 12277 Research Triangle Park, NC 27709

All rights reserved. Printed in the United States of America. 10 9 8 7 6 5 4 3 2 ISBN 1-55617-873-5 (pbk.) No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher.

Library of Congress Cataloging-in-Publication Data Wade, Harold L. Basic and advanced regulatory control :system design and application / Harold L. Wade.-- 2nd ed. p. cm. ISBN 1-55617-873-5 (pbk.) 1. Automatic control. 2. Feedback control systems. I. Title. TJ213.W313 2004 629.8--dc22 2004001322

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DEDICATION To my mother, who provided an impetus for life-long learning, and to Mary, who has provided love, support, encouragement, and criticism when needed.

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ABOUT THE AUTHOR Harold L. Wade is president of Wade Associates, Inc., a Houston, Texas, firm specializing in process control systems consulting and training. He has 45 years’ experience in applying and installing process control systems in such industries as petroleum refining, chemical production, textiles, and water and waste treatment systems among others. He has held technical positions with Honeywell, Foxboro, and Biles & Associates. Dr. Wade received a B.S. in Mechanical Engineering from Oklahoma State University and his M.S. and Ph.D. in Systems Engineering from Case Western Reserve University. A Senior Member of ISA, he also holds membership in IEEE and AIChE and is a licensed professional engineer in Texas and Oklahoma. Dr. Wade has taught courses in process control systems design for ISA since 1986. He has presented process control and controller tuning seminars for many companies worldwide. He is also the developer of the process control training program, PC-ControLAB. Dr. Wade was a 2002 inductee into Control magazine’s Process Automation Hall of Fame.

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PREFACE This book presents a practical approach to process control for the chemical, refining, pulp and paper, utilities, and similar industries. It is the result of seminars in process control that I have presented both in the United States and abroad. A typical participant in my seminars is an engineer, currently employed by a processing company, who may have had formal training in an undergraduate process control course but who may not be able to fully relate the material from that course to his or her work experiences. This book aims to meet this need by explaining concepts in a practical way with only a minimal amount of theoretical background. The book serves both the beginning and the experienced control systems engineer. For the beginning engineer, it initially presents very simple concepts. For the experienced engineer, it develops these initial concepts so as to provide deeper understanding or new insights into familiar concepts. The purpose is to provide everyone, beginner or experienced engineer, with something they can put to beneficial use in their plant. This edition also develops a unique method of controller tuning and a novel form of decoupling control, both of which were only introduced briefly in the first edition. The impact on control strategy configuration of advances in the standardization of fieldbus communication systems for process control is discussed. The coverage of model predictive control has been expanded to reflect the wider acceptance of this technology, the development of more efficient systems, and falling prices for the supporting hardware platform. This edition also includes a new set of process control strategy application examples. Although this is intended to be a practical “how-to” book, readers should not infer that this means it is devoid of mathematical concepts. Where such concepts are utilized, however, it is their application to practical situations, rather than the theory behind the concepts, that is emphasized. A theme of the first edition—that wherever I had to choose between providing mathematical rigor or promoting intuitive understanding, I always gave preference to understandability—has been carried forward into the present edition. This practicality distinguishes this book from many academic texts. The book is organized generally into three parts. The first three chapters present background information, including a brief nonrigorous mathematical review, a discussion of symbols and terminology, and a description of general characteristics of processes and of selected types of control loops.

PREFACE — XIII

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The second part—chapters 4 through 7—deals with feedback control. The objective is to provide the reader with a thorough intuitive grasp of feedback control behavior and all its nuances. In the chapter on feedback controller tuning (chapter 6), the discussion on improving as-found tuning (also called “intelligent trial-and-error tuning”) has been considerably expanded, and supplemented by the presentation of a tuning flow chart that embodies this technique. This new tuning technique has been proven in practical applications and has been well accepted in training classes where it has been presented. In this same chapter, new material is included on tuning liquid-level control loops. The tuning of these loops, which have a completely different characteristic from most other process control loops, has in general received very little specific attention in the process control literature. The last portion of the book—chapters 8 through 16—begins by defining the “feedback penalty” that must be paid if feedback control alone is used. This leads into a discussion of advanced regulatory control techniques (chapter 9), including chapters on cascade (chapter 10), ratio (chapter 11), feedforward (chapter 12), override (chapter 13), decoupling (chapter 13), model-based (chapter 14), and model predictive control (chapter 15). The chapter on feedforward control offers expanded coverage on the application of multiplicative feedforward control. The chapter on override (selector) control includes additional application examples for this technique, as well as an assessment of the performance of several alternative techniques. The chapter on the control of multiple-input, multiple-output (MIMO) processes (chapter 15) contains additional coverage of inverted decoupling. This MIMO technique was introduced in the first edition; new material previously available only in technical journals is presented here. The chapter on model-based control in the first edition has been split into two chapters. Chapter 14, devoted primarily to dead-time compensation, covers Smith predictor control, internal model control, and Dahlin’s algorithm. The other chapter, chapter 15, contains very significantly expanded coverage of model predictive control. The concluding chapter, which is almost entirely new, covers process control application topics that do not readily fit into any of the other chapters. In addition to cross-limiting control for fired heaters, which was covered in the first edition, these new topics include floating control, techniques for increasing valve rangeability, and time proportioning control. One of the themes of this book is to emphasize control strategies that are platform independent. However, since the appearance of the first edition, FOUNDATION™ Fieldbus (FF), which permits the control strategy to be distributed directly into field devices, has grown in acceptance. The network architecture, communication, and implementation aspects of FF are briefly summarized in chapter 5. In this edition, the process control aspects of FF receive greater coverage. Moreover, the chapters on modifications to feedback control, cascade, ratio, feedforward, and override (chapters 5, 9, 10, 11, and 12) all conclude with an example in which that chapter’s strategy is implemented using FF function blocks. I would like to express gratitude to the many students who, by asking probing questions, have enabled me to revise and sharpen my presentation and come up with more meaningful exam-

XIV — PREFACE

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ples. In particular, I would like to thank the engineers at BASF–Freeport for encouraging me to develop the controller tuning flow chart, to the staff of the ISA Training Institute for their support during my seminars, and to Adrian and Ivan Susanto in Indonesia and Michael Wang in Taiwan for sponsoring courses and providing me with an opportunity for travel abroad. I would also like to express my thanks to Dr. R. Russell Rhinehart for many helpful comments and suggestions, to my longtime friend and mentor (and reviewer of this book) Greg Shinskey, as well as to John Shaw, Jonas Berge, and Bryan Griffen who have reviewed all or parts of this book. And I have special thanks for Susan Colwell, who, through humor and patience, has helped me endure the arduous task of writing.

PREFACE — XV

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TABLE OF CONTENTS PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii CHAPTER 1—INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

CHAPTER 2—MATHEMATICAL BACKGROUND, DIAGRAMS, AND TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Mathematical Foundation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Diagrams and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Direct- or Reverse-acting? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

CHAPTER 3—PROCESS AND CONTROL LOOP CHARACTERISTICS . . 27 Steady-state Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dynamic Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control Loop Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

28 35 48 54 55

CHAPTER 4—PID CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Feedback Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

CHAPTER 5—MODIFICATIONS TO STANDARD PID CONTROL . . . . . . . 79 Set Point “Softening” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Integral-only Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interactive or Noninteractive Controller. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Independent Gains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Internal Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nonlinearization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Set Point Tracking and Bumpless Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . “Bumpless” Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preventing Reset Windup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

80 85 86 89 90 92 94 96 97

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Discrete Control Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Incorporating Engineering Units in Controller Gain. . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Commercial Examples of Modifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Process Control Using FOUNDATION™ Fieldbus . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

CHAPTER 6—TUNING FEEDBACK CONTROL LOOPS . . . . . . . . . . . . . . . 119 Performance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Tuning for Self-regulating Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 Tuning Liquid-level Control Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Other Tuning Situations: Runaway Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Typical Tuning Values for Particular Types of Loops . . . . . . . . . . . . . . . . . . . . . . . . . . 170 Practical Considerations for Loop Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

CHAPTER 7—SELF-TUNING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Scheduled Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 On-demand Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Adaptive Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Tuning Aids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

CHAPTER 8—ADVANCED REGULATORY CONTROL . . . . . . . . . . . . . . . 183 CHAPTER 9—CASCADE CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Cascade Control Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Identifying Candidate Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Implementation, Operation, and Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Cascade Control Using FOUNDATION™ Fieldbus . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

CHAPTER 10—RATIO CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Ratio Control Technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Automatic Ratio Adjustment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Scaling the Ratio Control Components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Ratio Control Using FOUNDATION™ Fieldbus Function Blocks . . . . . . . . . . . . . . . . 208 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

CHAPTER 11—FEEDFORWARD CONTROL . . . . . . . . . . . . . . . . . . . . . . . . . 213 Designing Feedforward Control Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Dynamic Compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Further Considerations of the Feedback Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Feedforward: In Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 Feedforward Control Using FOUNDATION™ Fieldbus. . . . . . . . . . . . . . . . . . . . . . . . 241 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 X — TABLE OF CONTENTS

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CHAPTER 12—OVERRIDE (SELECTOR) CONTROL . . . . . . . . . . . . . . . . . 245 Override Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Methods of Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Override Control Using FOUNDATION™ Fieldbus . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

245 256 263 265

CHAPTER 13—CONTROL FOR INTERACTING PROCESS LOOPS. . . . . 267 Variable Pairing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Decoupling Application Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

268 276 286 290

CHAPTER 14—DEAD-TIME COMPENSATION AND MODEL-BASED CONTROL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Smith Predictor Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dahlin’s Algorithm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . An Overview of Z-Transform Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Internal Model Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

292 299 300 305 310

CHAPTER 15—MULTIVARIABLE MODEL PREDICTIVE CONTROL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 Real-world Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Unconstrained MPC for SISO Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Unconstrained MPC for MIMO Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Constrained MPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Variations in MPC Vendor Offerings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MPC in Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

311 313 314 315 324 329 330 332 334

CHAPTER 16—OTHER CONTROL TECHNIQUES. . . . . . . . . . . . . . . . . . . . 335 Split-range Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cross-Limiting Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Floating Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hot or Chilled Water Supply Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cooling Tower Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Increasing Valve Rangeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time Proportioning Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

335 338 339 343 344 346 349 351

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APPENDIX A—SIGNAL SCALING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353 APPENDIX B—DERIVATION OF EQUATIONS FOR INSTALLED VALVE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . 359 INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365

XII — TABLE OF CONTENTS

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1

INTRODUCTION

The term process control implies that there is a process for which there is a desired behavior and that there is some controlling function that acts to elicit that desired behavior. This broad concept can embrace everything from societal processes governed by some regulatory control authority to automated manufacturing processes. In practically all cases, however, a common thread is that some measure of the actual process behavior is compared with the desired process behavior. This feedback action then generates a control policy that acts to minimize or eliminate the deviation between desired and actual behavior. We are concerned in this book with a particular segment of automated process control—that which is applied to chemical, refining, pulp and paper, power generation, and similar types of processes. Even within this limited scope of applications, we will limit the discussion primarily to processes that are operated continuously for long periods of time and within a narrow region of the operating variables. In other words, we exclude such important operating modes as batch processing, start-ups, and grade changes. Many of the control techniques to be presented here, however, can be adapted to these other modes of operation. For the processes we focus on in this book, the process’s behavior is often characterized by measured values of such process variables as temperatures, flow rates, pressures, and the like. The desired behavior, then, is stated to be the set points of those process variables. Until fairly recent times, most applications of industrial process control used simple feedback controllers that regulated the flows, temperatures, and pressures. These controllers required a form of adjustment called tuning to match their controlling action to the unique requirements of individual processes. Occasionally, more advanced forms of control, such as ratio and cascade, could be found; even more rarely one might find a feedforward control loop. As long as most of the control systems were implemented with analog hardware, applications were limited to simple regulatory control. This was due to the cost of additional components, the additional interconnections more advanced control required, the burden of maintenance, and the vulnerability to failure of many devices in the control loop. With the advent of digital control systems, however, more sophisticated loops became feasible. Advanced regulatory control—which includes the previously mentioned ratio, cascade, and feedforward control as well as additional forms such as constraint (selector) control and decoupling—could readily be implemented simply by configuring software function blocks. With this additional capability, however, a need developed for a systematic approach toward using it. This is called control strategy design. In order to design a technically successful and CHAPTER 1 — 1

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BASIC AND ADVANCED REGULATORY CONTROL: SYSTEM DESIGN AND APPLICATION

economically viable control strategy, the control system engineer must be well grounded in the techniques of feedback control as well as the tools of advanced regulatory control. The requisite knowledge includes both how to implement and how to tune. Even before that, however, the control system engineer must be adept at recognizing when to use (and conversely, when not to use) certain control methods as well as in projecting the expected benefits. Using advanced regulatory control provides many benefits. One of the most important is simply closer control of the process. It will become very clear later in this book that with basic regulatory (i.e., feedback) control, there must be a deviation from set point before control action can occur. We will call this the “feedback penalty.” The objective of advanced regulatory control is for the control action to be taken by incurring only a minimal feedback penalty. The reduction in feedback penalty may be stated in a variety of ways, such as a reduction of the maximum deviation from set point, as a reduction of the standard deviation, or simply as a reduction in the amount of off-spec product produced. This reduction in feedback penalty can provide several forms of economic benefit, such as improvement in product quality, energy savings, increased throughput, or longer equipment life. Process control is but one part of an overall control hierarchy. It extends downward to safety controls and other directly connected process devices and upward to encompass optimization and even higher levels of business management, such as scheduling, inventory, and asset management (see Figure 1-1). Indeed, corporate profitability may be enhanced more significantly as a result of these higher-level activities than from improved process control per se. However, since each layer of the hierarchy depends upon the proper functioning of the layers beneath it, one of the primary benefits of advanced regulatory control is that it enables the higher levels, such as optimization and enterprise management and control.

™ SYMBOLS Chapter 2 discusses the graphical symbols used in control system documentation. Listed below are the mathematical symbols that are used generally throughout the book. Some symbols used in this book are used only for the discussion of a particular topic; these symbols are therefore defined in that discussion and are not listed here. Chapter 15 uses a unique set of symbols that are defined at the beginning of that chapter. The following are the symbols found throughout this book: b e E K KC KD KI KP Kp m 2 — CHAPTER 1

bias value (manual reset) on proportional-only controller output error (deviation between set point and process variable) when capitalized, refers to (Laplace) transform of error steady-state gain of first-order lag controller gain (noninteractive and interactive control algorithms) derivative gain (independent gains control algorithm) integral gain (independent gains control algorithm) proportional gain (independent gains control algorithm) process gain (change in process variable / change in controller output) manipulated variable, controller output

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BASIC AND ADVANCED REGULATORY CONTROL: SYSTEM DESIGN AND APPLICATION

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