Tuning of industrial control systems

Tuning of Industrial Control Systems

Third Edition

By

Armando B. Corripio, Ph.D., P.E.

Chemical Engineering

Louisiana State University

and

Michael Newell

Automation Designer

Polaris Engineering

Notice

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neither the author nor the publisher has 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 has investigated or considered the effect of

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The reader is responsible for reviewing any possible patents that may affect any particular use of

the information presented.

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author nor the publisher endorses any referenced commercial product. Any trademarks or

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nor the publisher makes any representation regarding the availability of any referenced

commercial product at any time. The manufacturer’s instructions on the use of any commercial

product must be followed at all times, even if in conflict with the information in this publication.

Copyright © 2015 International Society of Automation (ISA)

All rights reserved.

Printed in the United States of America.

10 9 8 7 6 5 4 3 2

ISBN: 978-0-87664-034-0

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Library of Congress Cataloging-in-Publication Data in process

ix

Preface to the

Third Edition

This third edition of Tuning of Industrial Control Systems has been significantly

simplified from the second edition with the goal of having the discussion

more in line with modern control systems and with language that is less aca￾demic and more in tune with the vocabulary of the technicians who do the

actual tuning of control systems in industry. For example, we have eliminated

any references to first- and second-order models since these terms are highly

mathematical and may discourage some from appreciating the usefulness of

the models. We have also eliminated the distinction between series and paral￾lel PID controllers since most modern installations use the series version and

there is not much difference between the tuning of the two versions.

We have reduced the tuning strategies to just one; the quarter-decay-ratio

(QDR) formulas slightly modified by the Internal Model Control (IMC) rules

for certain process characteristics. All the tuning strategies are intended for

responses to disturbances with a discussion on how to modify these responses

to avoid sudden excessive changes of the controller output on set point

changes when such changes are undesirable.

Chapter 10 is new and deals with the auto-tuning feature that has become

standard on current process control systems. We have successfully used the

auto-tuning feature in our tuning work on oil refineries as a reference to guide

our selection of the final tuning parameters for the controllers.

We have kept the previous edition’s discussions on the problems of process

nonlinearities and reset windup, and how to compensate for them. All of the

tuning strategies are demonstrated with computer simulation examples.

v

Contents

Preface to the Third Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ix

1 – Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1-1. The Goal of Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1-2. Feedback Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1-3. Other Control Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1-4. Organization of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1-5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2 – The Feedback Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2-1. The PID Control Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2-2. Stability of the Feedback Loop. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2-3. PID Controller Tuning by the Ultimate Gain and Period Method . . . . . 21

2-4. The Need for Alternatives to Ultimate Gain Tuning . . . . . . . . . . . . . . . . 29

2-5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3 – Open-loop Characterization of Process Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 33

3-1. Open-loop Testing—Why and How . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3-2. Process Parameters from the Open-loop Test . . . . . . . . . . . . . . . . . . . . . . 36

3-3. Physical Significance of the Time Constant . . . . . . . . . . . . . . . . . . . . . . . . 41

3-4. Physical Significance of Dead Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3-5. Effect of Process Nonlinearities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3-6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

vi Tuning of Industrial Control Systems, Third Edition

4 – How to Tune Feedback Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

4-1. Tuning from Open-loop Test Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 58

4-2. Practical Controller Tuning Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4-3. Reset Windup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4-4. Processes with Inverse Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4-5. Effect of Nonlinearities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4-6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5 – Mode Selection and Tuning of Common Feedback Loops . . . . . . . . . . . . . . . . . . 77

5-1. Deciding on the Control Objective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5-2. Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5-3. Level and Pressure Control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

5-4. Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

5-5. Analyzer Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

5-6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

6 – Tuning Sampled-Data Control Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

6-1. The Discrete PID Control Algorithm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

6-2. Tuning Sampled-data Feedback Controllers . . . . . . . . . . . . . . . . . . . . . . 103

6-3. Selection of the Sampling Frequency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

6-4. Compensation for Dead Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

6-5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

7 – Tuning Cascade Control Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

7-1. When to Apply Cascade Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

7-2. Selection of Controller Modes for Cascade Control . . . . . . . . . . . . . . . . 125

7-3. Tuning of Cascade Control Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

7-4. Reset Windup in Cascade Control Systems . . . . . . . . . . . . . . . . . . . . . . . 136

7-5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

8 – Feedforward and Ratio Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

8-1. Why Feedforward Control? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

8-2. Design of Linear Feedforward Controllers. . . . . . . . . . . . . . . . . . . . . . . . 149

Contents vii

8-3. Tuning of Linear Feedforward Controllers . . . . . . . . . . . . . . . . . . . . . . . 152

8-4. Nonlinear Feedforward Compensation . . . . . . . . . . . . . . . . . . . . . . . . . . 158

8-5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

9 – Multivariable Control Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

9-1. What is Loop Interaction?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

9-2. Pairing of Controlled and Manipulated Variables . . . . . . . . . . . . . . . . . 174

9-3. Design and Tuning of Decouplers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

9-4. Tuning of Multivariable Control Systems . . . . . . . . . . . . . . . . . . . . . . . . 193

9-5. Model Reference Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

9-6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

10 – The Auto-tuner Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201

10-1. Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

10-2. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

10-3. Features and Settings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

10-4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

Appendix A – Suggested Reading and Study Materials . . . . . . . . . . . . . . . . . . . . . . 215

Appendix B – Answers to Study Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

1

1

Introduction

Automation is essential for the operation of chemical, petrochemical, and

refining processes. It is required to maintain process variables within safe

operating limits while maintaining product purity and optimum operating

conditions. Because all processes are different in their speed of response and

sensitivity to control adjustments and disturbances, the parameters of the

automatic controllers must be adjusted to match the process characteristics.

This procedure is known as tuning. The purpose of this book is to provide

you, the reader, with an understanding of the most commonly used and suc￾cessful tuning techniques for the various control strategies used in industry.

This first chapter presents a general discussion of the goal of tuning, a descrip￾tion of feedback control—the most common strategy—and a brief introduc￾tion to other common control strategies.

Learning Objectives — When you have completed this chapter, you should be

able to

A. Define the main goal of tuning a control system.

B. Understand the feedback control strategy.

C. Identify the various components of a feedback control loop.

2 Tuning of Industrial Control Systems, Third Edition

1-1. The Goal of Tuning

The goal of tuning is to produce a smoothly operating process. One common

misconception is that every process variable should be brought to its desired

value as quickly as possible and closely maintained at that value. When a con￾troller is “tightly” tuned to maintain close control of a process variable, it must

make large, fast changes in its output, which usually causes disturbances to

other variables in the process. As the controllers of these other variables take

action they, in turn, cause further disturbances that affect other variables.

Before long the entire process is in a state of continuous change, which is

undesirable and may be unsafe in some occasions. The situation worsens

when the controllers cause oscillatory process responses, because then the

process variables will be continuously changing.

The following heuristics (“rules of thumb”) may prove helpful to those just

starting in the tuning of processes:

• The variability of the controller output should not be excessive; how￾ever, keeping the output variability low must be balanced against the

precision with which the process variable is to be controlled.

• Some variables do not have to be maintained at their desired values.

The most common example of this is liquid levels, which usually only

need to be kept within a safe range.

• The controller cannot move the process variable faster than the process

can respond, so the controller speed must be matched to the speed of

response of the process. Some processes respond in a matter of min￾utes, while others may take close to an hour or longer to respond. Not

many processes respond in a matter of seconds.

One more item to keep in mind is that there is no such thing as fine-tuning a

controller, particularly a feedback controller. In most cases the tuning parame￾ters need only be adjusted to one, or at most, two significant digits. There are

two reasons for this. One is that feedback controllers are not that sensitive to

variations in the third digit of their tuning parameters. The other is that the

characteristics of most processes—that is, speed of response and sensitivity to

changes in controller output—vary with operating conditions, sometimes

slightly and other times not so slightly. This means that the controller tuning

Introduction 3

parameters are usually compromises selected to work in the range of operat￾ing conditions, and so their values are not precise.

Understanding this simplifies the task of tuning because it reduces the num￾ber of values of the tuning parameters to be tried. For example, it is a lot easier

to decide between gain values of 1.0 or 1.5 than to try to find out whether the

gain should be 1.276. In practice, all three of these values will work about the

same.

Armed with these heuristics and basic concepts, we are now ready to look at

the feedback control strategy.

1-2. Feedback Control

Feedback control is the basic strategy for the control of industrial processes. It

consists of measuring the process variable to be controlled (the controlled

variable), comparing the measurement with its desired value or set point, and

taking action based on the difference between them to reduce or eliminate the

difference—that is, to bring the controlled variable to its desired value. The

action taken results in the adjustment of a process flow, such as the steam flow

to a heater, which has a direct effect on the controlled variable, such as the

outlet process temperature. The three instrumentation components required

for feedback control are:

• A sensor/transmitter to measure the process variable and send its

value to the controller (Measurement)

• A controller to compare the value of the process variable to its desired

value, determine the required control action and send it to the final

control element (Decision)

• A final control element, usually a control valve or variable speed drive,

to vary the manipulated process flow (Action).

A fourth element of the loop is the process itself, through which the manipu￾lated flow affects the controlled variable. The controlled variable is also

known as the process variable (PV), its desired value is the set point (SP), and the

signal from the controller to the final control element is the controller output

(OP).

4 Tuning of Industrial Control Systems, Third Edition

It is important to realize that a feedback controller does not use a model of the

process to compute its output. It takes action by trial and error. Tuning the

controller is the procedure of adjusting the controller parameters to ensure

that the controller output converges quickly to its correct value.

In order to better understand the concept of feedback control, consider as an

example the process heater sketched in Figure 1-1. The process fluid flows

inside the tubes of the heater and is heated by steam condensing on the out￾side of the tubes. The objective is to control the outlet temperature T of the

process fluid in the presence of variations in process fluid flow (throughput or

load) F and in its inlet temperature Ti

. This is accomplished by manipulating

or adjusting the steam flow to the heater Fs and with it the rate at which heat is

transferred into the process fluid, thus affecting its outlet temperature.

Figure 1-1. Feedback Temperature Control of a Process Heater

TT

TC

SP

PV

OP

T

Fs

F

Ti

Process

fluid

Steam

Steam

trap

Condensate

Introduction 5

In this example, the outlet temperature T is the (controlled) process variable

PV, the steam flow Fs is the manipulated variable, and changes in the process

flow F and inlet temperature Ti

are the disturbances that cause the tempera￾ture to deviate from its desired value or set point SP. The job of the feedback

controller is to bring the temperature back to the set point by adjusting the

steam flow whenever variations in the process flow or inlet temperature cause

the outlet temperature to deviate.

In Figure 1-1 the sensor transmitter is shown as a circle with the letters TT in it

and the feedback controller is a circle with the letters TC in it. This follows the

standard ISA instrumentation notation1

in which the first letter denotes the

variable being measured or controlled, in this case “T” for temperature, and

the second letter is “T” for the transmitter and “C” for the controller. The con￾trol valve is represented by the symbol shown on the steam line to the heater.

Its purpose is to adjust the flow of steam (Fs) in response the controller output

signal (OP).

The transmitter and the control valve are located in the field while the control￾ler is located in a central control room. Today, the signals between the trans￾mitter and the controller and between the controller and the control valve are

typically digital signals transmitted through a fieldbus or by wireless transmis￾sion. The control function is carried out by a computer or distributed control

system (DCS) that receives the transmitter signal and transmits the controller

output to the control valve. The control valve is usually pneumatically oper￾ated, requiring that the controller output be converted to an air pressure sig￾nal. This is done by a current-to-pressure (I/P) transducer.

This book uses the instrumentation symbols recommended by the ISA-5.1-

1984 standard for conceptual diagrams, that is, diagrams that convey the basic

control concept without regard to the specific implementation hardware. In

these diagrams the signals are represented as percent of range. To facilitate

understanding we will deviate slightly from the standard ISA notation for sig￾nals and show the signals as arrows to indicate the direction in which the sig￾nals travel, as shown in Figure 1-1.

Figure 1-2 is a block diagram of the feedback control loop for the process

heater. It graphically shows the loop around which signals travel: a change in

outlet temperature T causes a proportional change in the signal PV to the con￾troller; the summer (circle), a part of the controller, calculates the error E or

6 Tuning of Industrial Control Systems, Third Edition

deviation of the process variable from the set point SP and acts on this error

by changing the signal OP to the control valve; the control valve position

changes, causing a change in steam flow Fs to the heater; this in turn causes a

change in the outlet temperature T which then starts a new cycle of changes

around the loop.

The signs in Figure 1-2 represent the action of the various input signals on the

output signal; that is, a positive sign means that an increase in input causes an

increase in output—direct action—while a negative sign means that an increase

in input causes a decrease in output—or reverse action. For example, the nega￾tive sign by the process flow into the heater means that an increase in flow

results in a decrease in outlet temperature. Notice that by following the sig￾nals around the loop, there is a net reverse action in the loop. This property is

known as negative feedback and is a required characteristic of a feedback loop

for the loop to be stable. In this example it means that an increase in heater

outlet temperature results in a decrease in controller output, which in turn

closes the control valve and reduces the steam flow. This results in a decrease

in outlet temperature, as desired.

To ensure this self-regulating effect the controller must act in the correct direc￾tion when the process variable changes. In this example the controller action is

reverse, that is, an increase in process variable results in a decrease in control￾Figure 1-2. Block Diagram of the Temperature Control Loop of the Process

Heater

Controller Control Valve Heater

Sensor

Transmitter

SP

PV

OP Fs

F Ti

+ T

-

E

- +

+ + +

+

Introduction 7

ler output. Other processes may require direct action, for example when a tank

level controller adjusts the flow out of the tank. In this case, an increase in liq￾uid level in the tank requires that the exit control valve open to increase the

flow out of the tank and decrease the level. Consequently, the action (direct or

reverse) of the feedback controller is its most important characteristic.

1-3. Other Control Strategies

Although feedback control is by far the most common automatic control strat￾egy, there are other strategies that have been known to enhance control per￾formance in terms of improving loop stability, preventing initial deviation of

the process variable, and allowing tighter control. This section will briefly

introduce these strategies; their details and tuning procedures will be pre￾sented in later chapters.

• Cascade Control. This strategy consists of cascading feedback controllers

in a hierarchy with each controller adjusting the set point of the control￾ler below it in the hierarchy, the controller at the top of hierarchy, or

primary, controls the primary process variable while the output of the

controller at the bottom of the hierarchy adjusts the final control ele￾ment. The controllers below the master controller, called secondaries,

control variables that have an effect on the primary controlled variable.

The basic premise is that the secondary feedback loops improve the sta￾bility of the primary controller by speeding up the overall response of

the process.

• Feedforward and Ratio Control. This strategy consists of measuring the

disturbances that affect the controlled variable and adjusting the final

control element to prevent deviation from the desired value of the con￾trolled variable. In general the scheme requires a model of the process

to determine the control adjustment in the final control element. Feed￾back control is combined with the feedforward controller to correct for

errors in the process model. Ratio control is the simplest form of feedfor￾ward control in which the manipulated flow is ratioed to the flow

which constitutes the disturbance.

• Decoupling. This strategy consists of installing decouplers between the

output signals of two or more feedback controllers to reduce the effect

of interaction between the controllers. The interaction occurs through

8 Tuning of Industrial Control Systems, Third Edition

the process when each controller output affects the process variables

controlled by the other controllers.

1-4. Organization of the Book

The details of the PID (Proportional-Integral-Derivative) controller are pre￾sented in Chapter 2, and tuning methods for feedback controllers are pre￾sented in Chapters 2, 3, and 4. How to select the controller modes for various

types of control loops is the subject of Chapter 5. Chapter 6 presents the tun￾ing of loops in which the process variable must be sampled, such as composi￾tions measured by gas chromatographs and similar analyzers. Tuning of

cascade control systems is discussed in Chapter 7, design and tuning of feed￾forward and ratio controllers in Chapter 8, and design and tuning of decou￾plers in Chapter 9. Finally Chapter 10 presents the auto-tuning algorithms

available in current computer control systems.

1-5. Summary

This first chapter has presented the goals of the tuning procedure and has

introduced the feedback control strategy. A brief description of other common

control strategies has also been presented.

References

1. ANSI/ISA-5.1-2009 - Instrumentation Symbols and Identification, Interna￾tional Society of Automation, Research Triangle Park, NC.

Review Questions

1-1. What is the main goal of controller tuning?

1-2. Which two process characteristics must be considered when tuning the

controller?

1-3. What are the three instrumentation components of a feedback control

loop?

1-4. What is the fourth element of the feedback loop?

1-5. What is the most important characteristic of a feedback control loop?

Introduction 9

1-6. A controller controls the temperature in an exothermic reactor by manip￾ulating the flow of cooling water to the jacket around the reactor. What

should be the fail position of the cooling water control valve, open or

closed? What must be the action of the controller, direct or reverse?

1-7. A controller controls the level in a stirred tank reactor by manipulating

the flow of the reactants into the reactor. Recommend the fail position of

the reactants control valve, open or close, and the controller action, direct

or reverse.

1-8. A controller controls the composition of a caustic stream by manipulat￾ing the flow of the water that dilutes the concentrated caustic stream

entering a mixer. The control valve fails closed. What must be the con￾troller action, direct or reverse?