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The PID Control Algorithm:
How it works, how to tune it, and how to use it. 2nd ed.

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About the book

The heart of the process control loop is the PID algorithm. Proper understanding of controller tuning requires understanding of the PID algorithm. The PID algorithm comprises three types of responses: Proportional, Integral, and Derivative

The first part of this book will examine the PID control algorithm, as typically implemented in industry. Each of the three elements will be discussed in detail.

The book will then discuss common process responses, including dead time, process lags, and their combinations.

The amount that each of these elements contribute to the control must be adjusted to correspond to the process being controlled. This adjustment is known as "tuning" the control loop. An incorrectly tuned loop will not function correctly, providing less than the desired performance. We will first look at how to determine if a loop is properly tuned and then cover some of the typical methods of tuning loops.

The remainder of the book will discuss multiple loop control in which two or more PID control algorithms are connected together to improved the control or perform specialized control functions.

Table of Contents

Chapter 1 Introduction 1
1.1 The Control Loop 2
1.2 Role of the control algorithm 3
1.3 Auto/Manual 3
Chapter 2 The PID algorithm 5
2.1 Key concepts 5
2.2 Action 5
2.3 The PID responses 5
2.4 Proportional 6
2.5 Proportional-Output vs. Measurement 7
2.6 Proportional-Offset 7
2.7 Proportional-Eliminating offset with manual reset 8
2.8 Adding automatic reset 9
2.9 integral mode (Reset) 10
2.10 Calculation of repeat time 11
2.11 Derivative 12
2.12 Complete PID response 13
2.13 Response combinations 13
Chapter 3 Implementation Details of the PID Equation 15
3.1 Series and Parallel Integral and Derivative 15
3.2 Gain on Process Rather Than Error 16
3.3 Derivative on Process Rather Than Error 16
3.4 Derivative Filter 16
3.5 Computer code to implement the PID algorithm 17
Chapter 4 Advanced Features of the PID algorithm 20
4.1 Reset windup 20
4.2 External feedback 21
4.3 Set point Tracking 21
Chapter 5 Process responses 23
5.1 Steady State Response 23
5.2 Process dynamics 27
5.3 Measurement of Process dynamics 31
5.4 Loads and Disturbances 33
Chapter 6 Loop tuning 34
6.1 Tuning Criteria or “How do we know when its tuned” 34
6.2 Mathematical criteria-minimization of index 35
6.3 Ziegler Nichols Tuning Methods 36
6.4 Cohen-Coon 40
6.5 Lopez IAE-ISE 41
6.6 Controllability of processes 41
6.7 Flow loops 42
Chapter 7 Multiple Variable Strategies 44
Chapter 8 Cascade 45
8.1 Basics 45
8.2 Cascade structure and terminology 47
8.3 Guideline for use of cascade 47
8.4 Cascade Implementation Issues 48
8.5 Use of secondary variable as external feedback 51
8.6 Tuning Cascade Loops 52
Chapter 9 Ratio 53
9.1 Basics 53
9.2 Mode Change 54
9.3 Ratio manipulated by another control loop 54
9.4 Combustion air/fuel ratio 55
Chapter 10 Override 57
10.1 Example of Override Control 57
10.2 Reset Windup 58
10.3 Combustion Cross Limiting 59
Chapter 11 Feedforward 61
Chapter 12 Bibliography 62

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