Tuning of Industrial Control Systems Second Edition by Armando B Corripio, Ph.D., P.E Louisiana State University 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 © 2001 ISA—The Instrumentation, Systems, and Automation Society All rights reserved Printed in the United States of America No part of this publication may be reproduced, stored in 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 ISA 67 Alexander Drive P.O Box 12277 Research Triangle Park North Carolina 27709 Library of Congress Cataloging-in-Publication Data Corripio, Armando B Tuning of industrial control systems / Armando B Corripio. 2nd ed p cm Includes bibliographical references and index ISBN 1-55617-713-5 Process control Automation Feedback control systems I Title TS156.8 C678 2000 670.42’75 dc21 00-010127 TABLE OF CONTENTS Unit 1: Introduction and Overview 1-1 Course Coverage 1-2 Purpose 1-3 Audience and Prerequisites 1-4 Study Materials 1-5 Organization and Sequence 1-6 Course Objectives 1-7 Course Length 4 4 Unit 2: Feedback Controllers 2-1 The Feedback Control Loop 2-2 Proportional, Integral, and Derivative Modes 13 2-3 Typical Industrial Feedback Controllers 19 2-4 Stability of the Feedback Loop 22 2-5 Determining the Ultimate Gain and Period 24 2-6 Tuning for Quarter-decay Response 25 2-7 Need for Alternatives to Ultimate Gain Tuning 31 2-8 Summary 32 Unit 3: Open-Loop Characterization of Process Dynamics 3-1 Open-Loop Testing: Why and How 3-2 Process Parameters from Step Test 3-3 Estimating Time Constant and Dead Time 3-4 Physical Significance of the Time Constant 3-5 Physical Significance of the Dead Time 3-6 Effect of Process Nonlinearities 3-7 Testing Batch Processes 3-8 Summary 35 37 39 41 45 49 52 55 56 Unit 4: How to Tune Feedback Controllers 4-1 Tuning for Quarter-decay Ratio Response 4-2 A Simple Method for Tuning Feedback Controllers 4-3 Comparative Examples of Controller Tuning 4-4 Practical Controller Tuning Tips 4-5 Reset Windup 4-6 Processes with Inverse Response 4-7 Summary 59 61 64 65 74 77 78 81 Unit 5: Mode Selection and Tuning Common Feedback Loops 5-1 Deciding on the Control Objective 5-2 Flow Control 5-3 Level and Pressure Control 5-4 Temperature Control 5-5 Analyzer Control 5-6 Summary 83 85 86 88 94 96 97 Unit 6: Computer Feedback Control 99 6-1 The PID Control Algorithm 101 6-2 Tuning Computer Feedback Controllers 108 6-3 Selecting the Controller Processing Frequency 115 6-4 Compensating for Dead Time 117 6-5 Summary 121 vii viii Table of Contents Unit 7: Tuning Cascade Control Systems 7-1 When to Apply Cascade Control 7-2 Selecting Controller Modes for Cascade Control 7-3 Tuning Cascade Control Systems 7-4 Reset Windup in Cascade Control Systems 7-5 Summary 125 127 130 131 139 142 Unit 8: Feedforward and Ratio Control 8-1 Why Feedforward Control? 8-2 The Design of Linear Feedforward Controllers 8-3 Tuning Linear Feedforward Controllers 8-4 Nonlinear Feedforward Compensation 8-5 Summary 143 145 150 152 157 164 Unit 9: Multivariable Control Systems 167 9-1 What Is Loop Interaction? 169 9-2 Pairing Controlled and Manipulated Variables 173 9-3 Design and Tuning of Decouplers 183 9-4 Tuning Multivariable Control Systems 188 9-5 Model Reference Control 191 9-6 Summary 194 Unit 10: Adaptive and Self-tuning Control 10-1 When Is Adaptive Control Needed? 10-2 Adaptive Control by Preset Compensation 10-3 Adaptive Control by Pattern Recognition 10-4 Adaptive Control by Discrete Parameter Estimation 10-5 Summary 197 199 202 209 212 220 Appendix A: Suggested Reading and Study Materials 223 Appendix B: Solutions to All Exercises 227 Index 251 Unit 1: Introduction and Overview UNIT Introduction and Overview Welcome to Tuning of Industrial Control Systems The first unit of this selfstudy program provides the information you will need to take the course Learning Objectives — When you have completed this unit, you should be able to: A Understand the general organization of the course B Know the course objectives C Know how to proceed through the course 1-1 Course Coverage This book focuses on the fundamental techniques for tuning industrial control systems It covers the following topics: A The common techniques for representing and measuring the dynamic characteristics of the controlled process B The selection and tuning of the various modes of feedback control, including those of computer- and microprocessor-based controllers C The selection and tuning of advanced control techniques, such as cascade, feedforward, multivariable, and adaptive control When you finish this course you will understand how the methods for tuning industrial control systems relate to the dynamic characteristics of the controlled process By approaching the subject in this way you will gain insight into the tuning procedures rather than simply memorizing a series of recipes Because microprocessor- and computer-based controllers are now widely used in industry, this book will extend the techniques originally developed for analog instruments to digital controllers We will examine tuning techniques that have been specifically developed for digital controllers as well as those for adaptive and auto-tuning controllers No attempt is made in this book to provide an exhaustive presentation of tuning techniques In fact, we have specifically omitted techniques based on frequency response, root locus, and state space analysis because they are more applicable to electrical and aerospace systems than to industrial Unit 1: Introduction and Overview processes Such techniques are unsuitable for tuning industrial control systems because of the nonlinear nature of industrial systems and the presence of transportation lag (dead time or time delay) 1-2 Purpose The purpose of this book is to present, in easily understood terms, the principles and practice of industrial controller tuning Although this course cannot replace actual field experience, it is designed to give you the insights into the tuning problem to speed up your learning process during field training 1-3 Audience and Prerequisites The material covered will be useful to engineers, first-line supervisors, and senior technicians who are concerned with the design, installation, and operation of process control systems The course will also be helpful to students in technical schools, colleges, or universities who wish to gain some insight into the practical aspects of automatic controller tuning There are no specific prerequisites for taking this course However, you will find it helpful to have some familiarity with the basic concepts of automatic process control, whether acquired through practical experience or academic study In terms of mathematical skills, you not need to be intimately familiar with some of the mathematics used in the text in order to understand the fundamentals of tuning This book has been designed to minimize the barrier that mathematics usually presents to students’ understanding of automatic control concepts 1-4 Study Materials This textbook is the only study material required in this course It is an independent, stand-alone textbook that is uniquely and specifically designed for self-style Appendix A contains a list of suggested readings to provide you with additional reference and study materials 1-5 Organization and Sequence This book is organized into ten separate units The next three units (Units 2-4) are designed to teach you the fundamental concepts of tuning, namely, the modes of feedback control, the characterization and measurement of process dynamic response, the selection of controller Unit 1: Introduction and Overview performance, and the adjustment of the tuning parameters Unit tells you how to select controller modes and tuning parameters for some typical control loops An entire unit, Unit 6, is devoted to the specific problem of tuning computer- and microprocessor-based controllers The last four units, Units through 10, demonstrate how to tune the more advanced industrial control strategies, namely, cascade, feedforward, multivariable, and adaptive control systems As mentioned, the method of instruction used is self-study: you select the pace at which you learn best You may browse through or completely skip some units if you feel you are intimately familiar with their subject matter and devote more time to other units that contain material new to you Each unit is designed in a consistent format with a set of specific learning objectives stated at the very beginning of the unit Note these learning objectives carefully; the material in the unit will teach to these objectives Each unit also contains examples to illustrate specific concepts and exercises to test your understanding of these concepts The solutions for all of these exercises are contained in Appendix B, so you can check your own solutions against them You are encouraged to make notes in this textbook Ample white space has been provided on every page for this specific purpose 1-6 Course Objectives When you have completed this entire book, you should: • Know how to characterize the dynamic response of an industrial process • Know how to measure the dynamic parameters of a process • Know how to select performance criteria and tune feedback controllers • Know how to pick the right controller modes and tuning parameters to match the objectives of the control system • Understand the effect of sampling frequency on the performance of computer-based controllers • Know when to apply and how to tune cascade, feedforward, ratio, and multivariable control systems • Know how to apply adaptive and auto-tuning control techniques to compensate for process nonlinearities Unit 1: Introduction and Overview Besides these overall course objectives, each individual unit contains its own set of learning objectives, which will help you direct your study 1-7 Course Length The basic premise of self-study is that students learn best when they proceed at their own pace As a result, the amount of time individual students require for completion will vary substantially Most students will complete this course in thirty to forty hours, but your actual time will depend on your experience and personal aptitude Unit 2: Feedback Controllers INDEX Index Terms Links A adapter 212 217 218 76 77 199 200 209 212 217 220 algorithm 101 104 ammonia synthesis 138 94 97 20 32 adaptive control analog analog-to-digital converter (ADC) 16 96 analyzer control 96 arrow 12 averaging level control 20 102 analysis cycle auto-tuning 202 97 217 90 91 92 106 107 122 B batch process 69 bias 14 blending process 176 185 blending tank 46 47 51 block diagram 11 12 19 This page has been reformatted by Knovel to provide easier navigation Index Terms Links C capacitance 45 46 47 48 cascade control 86 127 129 132 134 135 137 138 139 140 142 cascade windup 139 141 cascade-to-ratio control 202 207 209 characteristics, of valve 52 53 202 closed-loop gain 174 175 177 closed-loop time constant 110 119 120 121 47 48 49 coarse tuning 75 comparator 12 compensation for dead time composition control 117 220 85 computer cascade control 134 142 computer-based controller 20 conductance 45 46 52 58 conductance, valve 47 control objective 85 145 157 159 160 control valve 10 11 12 13 14 controllability 24 27 controllable process 70 71 72 76 85 controlled variable 10 13 20 22 32 controller 13 14 20 22 action 17 23 computer-based 20 gain 13 24 27 15 33 panel-mounted 22 proportional-integral (PI) 19 This page has been reformatted by Knovel to provide easier navigation 32 Index Terms Links controller (Cont.) proportional-integral-derivative (PID) 19 proportional-only 14 19 single-mode 13 19 synthesis 86 118 three-mode 20 two-mode 13 correction for sample time 108 109 covariance matrix 216 218 current-to-pressure transducer 10 D Dahlin controller 117 118 119 damping parameter 210 dead band 117 dead time 37 39 40 41 42 43 45 49 50 52 57 85 96 117 119 120 121 122 156 157 163 dead time compensator 151 152 156 157 158 dead-band controller 202 208 decoupler 173 183 184 185 186 188 190 195 17 19 20 21 dead time compensation derivative action 17 factor 211 filter 103 104 kick 20 103 105 122 mode 13 17 18 19 20 24 27 28 85 86 90 103 131 132 This page has been reformatted by Knovel to provide easier navigation Index Terms Links derivative (Cont.) time unit differencing digital controller digital-to-analog converter (DAC) direct action 17 21 23 33 104 111 20 104 105 26 28 219 215 11 102 12 direct material balance control 181 182 discrete model 213 214 217 218 distillation column 169 179 194 195 distributed control systems (DCS) 11 101 distributed controller 20 disturbance 10 14 22 26 85 86 89 97 147 152 156 158 161 dynamic compensation 32 163 dynamic gain limit 104 122 dynamic interaction 182 194 160 161 E efficiency electrical system 46 electronic 10 22 energy balance control 180 182 equal percentage 203 error steady-state estimation of parameters 10 11 12 13 14 15 16 18 20 23 26 32 70 214 218 80 217 212 This page has been reformatted by Knovel to provide easier navigation Index Terms Links estimation off line 214 recursive 216 EXACT controller 209 211 212 expert system 209 212 110 112 11 31 feedback control loop 10 11 12 feedback controller 13 16 19 22 29 146 150 158 161 162 146 148 149 151 158 160 161 164 145 146 147 148 149 151 152 157 161 162 F fast process feedback control feedback trim feedforward control 163 feedforward tuning 152 feedforward-feedback control 148 149 filter parameter 103 104 fine tuning 75 first-order-plus-dead-time (FOPDT) 51 56 85 flow control 86 87 89 96 97 129 132 137 138 139 56 85 flow control response 87 FOPDT model 51 frequency 134 This page has been reformatted by Knovel to provide easier navigation Index Terms Links G gain 37 40 41 44 176 177 54 56 closed-loop 174 175 nonlinear 106 107 open-loop 173 174 177 180 181 relative 176 177 179 182 183 188 190 203 208 scheduling 202 steady-state 178 189 54 200 variation gap 117 gap controller 208 gas surge tank 46 graceful degradation 47 159 H half decoupling 185 heat exchanger 94 heat transfer 89 94 96 149 159 161 160 161 27 heater efficiency example feedforward control 162 Heaviside operator 18 higher-order system 48 hydrogen/nitrogen ratio hysteresis 138 87 This page has been reformatted by Knovel to provide easier navigation Index Terms Links I IMC instrumental variable (IV) regression 86 216 integral controller 16 86 97 integral mode 15 16 18 19 27 129 130 141 16 22 23 24 26 27 61 64 71 73 integral time 24 75 integrating process 40 interaction 76 169 171 172 173 174 176 177 179 181 183 185 188 189 190 194 interaction measure 175 intermediate level control 92 internal model control (IMC) 86 120 inverse response 78 79 80 81 172 182 185 188 193 194 134 139 141 92 93 J jacketed reactor L lead-lag compensation 162 lead-lag unit 152 162 least-squares regression 214 218 89 90 94 97 level control limiting controller output 115 linear feedforward controllers 150 91 152 This page has been reformatted by Knovel to provide easier navigation Index Terms Links liquid storage tank 46 47 169 172 173 183 194 10 24 32 145 146 148 150 151 152 157 160 161 163 164 129 130 131 132 134 138 139 140 141 142 material balance control 181 182 maximum likelihood regression 215 measured disturbance 146 148 149 157 158 159 160 161 164 loop interaction M manipulated variable master controller microprocessor controller mode 20 modes for the master controller 130 multiplexer 102 multivariable control 169 170 173 180 185 187 188 189 191 194 172 177 179 181 183 185 188 189 190 201 202 N negative feedback negative interaction 12 noise band 211 nonlinear controller gain 106 nonlinear feedforward compensation 157 164 62 200 nonlinearity This page has been reformatted by Knovel to provide easier navigation 209 Index Terms Links O off-line estimation offset on-line estimation 214 14 15 17 28 180 215 on-off control 18 open-loop gain 176 177 178 open-loop test 37 38 39 optimizing feedback loops output output pulse 33 117 87 90 103 105 output variable 10 overshoot 76 189 209 211 220 172 173 176 177 179 182 183 186 190 194 20 27 28 61 105 110 119 199 202 212 215 217 202 209 210 212 P pairing panel-mounted controller parallel paths parallel PID controller parameter estimation 22 185 218 pattern recognition 199 220 PD controller 17 percent controller output (%C.O.) 41 percent transmitter output (%T.O.) 41 perfect control performance pH control 145 86 164 87 201 208 This page has been reformatted by Knovel to provide easier navigation Index Terms Links PI controller 16 19 21 86 91 182 189 190 97 129 139 52 92 PID algorithm 101 PID controller 19 20 pneumatic 10 22 172 177 28 74 positive interaction practical tips preset compensation 202 pressure control 89 92 process dead time 41 51 process gain 37 39 40 44 53 54 57 110 process nonlinearity 37 53 209 process time constant 42 44 45 92 96 101 102 103 104 110 119 122 processing frequency 101 115 programmable logic controllers (PLC) 101 16 17 process variable proportional band 13 proportional controller 15 33 proportional kick 105 proportional mode 13 proportional-derivative (PD) controller 86 proportional-integral (PI) controller 86 proportional-integral controller 87 proportional-only controller 14 19 proportional-on-measurement 70 106 215 218 214 215 pseudo-random binary signal (PRBS) 14 91 pulse symmetric PV 101 This page has been reformatted by Knovel to provide easier navigation Index Terms Links Q QDR response 26 66 68 QDR tuning 26 27 30 33 61 66 67 68 71 73 quarter-decay ratio (QDR) response 26 R rate mode 17 rate time 17 20 145 149 164 40 127 129 131 133 134 135 137 138 141 216 218 220 ratio control reactor recursive 218 recursive estimation 212 215 regression 214 220 instrumental variable (IV) 216 maximum likelihood 215 relative gain 174 175 176 177 178 179 180 182 183 188 189 194 relative gain matrix 173 177 182 reset feedback 141 reset mode 15 reset rate 17 24 33 reset time 15 19 33 reset windup 53 61 76 77 78 81 127 132 133 140 142 resistance 45 46 48 reverse action 12 22 23 This page has been reformatted by Knovel to provide easier navigation Index Terms Links S sample time selection 64 96 104 110 111 112 113 115 116 117 64 115 sampling frequency 63 64 115 sampling period 63 108 112 saturation 52 53 self-regulating 40 sensor time constant 94 sensor/transmitter 10 series PI controller 120 series PID controller 12 20 21 27 61 66 81 104 111 10 13 14 18 20 22 24 26 27 65 66 68 71 73 75 78 81 single-mode controller 13 19 slave controller 82 127 130 131 132 134 136 138 139 140 set point 141 slave controller modes 142 slave flow loop 132 slave pressure loop 133 slave temperature loop 132 slow sampling 112 113 114 Smith Predictor 117 118 122 22 32 stability static compensation static friction 162 87 This page has been reformatted by Knovel to provide easier navigation Index Terms Links steady-state 42 compensation steam heater 44 146 14 23 27 28 32 149 159 162 163 37 39 41 45 51 54 55 56 214 215 tangent method 42 43 44 45 tangent-and-point method 43 temperature control 94 95 96 127 129 131 133 136 94 97 step test symmetric pulse T three-mode controller 20 tight control 89 93 tight tuning 22 23 Time time constant time delay trace of matrix 128 120 37 39 40 41 43 44 45 46 47 48 52 85 86 87 90 92 94 96 49 50 26 217 transducer 10 transfer function 37 transportation lag 49 50 51 tuning parameter 13 16 19 21 27 28 29 32 two-mode controller 13 two-point method 42 This page has been reformatted by Knovel to provide easier navigation Index Terms Links U ultimate gain 23 24 25 26 27 28 31 108 ultimate period 24 25 27 28 uncontrollability 62 63 65 66 71 72 73 76 108 109 203 220 115 uncontrollable process 74 76 81 unstable 22 24 40 53 202 V vacuum pan 55 valve characteristics 52 conductance 52 gain 52 hysteresis 87 position control 117 valve position control 117 variance of the estimates 216 variance-covariance matrix 216 208 218 W wait time Watt, James windup windup, of cascade system 211 11 53 140 139 142 141 This page has been reformatted by Knovel to provide easier navigation Index Terms Links Z Ziegler and Nichols 24 25 65 66 42 61 This page has been reformatted by Knovel to provide easier navigation 64 ... characteristics of the controlled process B The selection and tuning of the various modes of feedback control, including those of computer- and microprocessor-based controllers C The selection and tuning of. .. of the publisher ISA 67 Alexander Drive P.O Box 12277 Research Triangle Park North Carolina 27709 Library of Congress Cataloging-in-Publication Data Corripio, Armando B Tuning of industrial control. .. feedback control loop, which is the most common industrial control technique—in fact, it is the “bread and butter” of industrial automatic control The following procedure illustrates the concept of