Preface:
This book is written to help control system designers improve their processes and machines. The topics span a broad range; the most concrete of which are the techniques that improve a facet of operation such as making the system faster or more accurate. Designers need to know such techniques-when to use an integrating controller, how to tune control gains, and when to use feed-forward. Having a toolbox full of these techniques makes an engineer a valuable asset when a problem strikes, but deeper understanding is needed to avoid problems in the first place.
The most abstract topics are the methods of analysis. They bring understanding of how the system works; modeling techniques and the frequency-domain are examples of such methods. They are most useful when the system architecture is created or when substantial changes are contemplated. Here, the designer cannot test the equipment; the key measures of performance must be estimated before the system is built. A wellequipped engineer has suitable means of analysis to design the system, and a box full of tools to correct shortcomings that eventually will present themselves.
A book that claims to help its readers improve their systems must ensure that the material contained within its covers is both useful and enabling. Here, "useful" implies knowledge that makes real-world control systems better. An enabling presentation provides readers with sufficient information so that the knowledge can be applied in their own setting. Miss either mark and readers will be disappointed. The material in this book is meant to be useful, and the author has used or observed the use of most of it in one control system or another. The presentation is meant to be enabling, with, the author hopes, a generous use of text, examples, illustrations, and simulations to demonstrate key points.
Since the First Edition
In the decade since the first edition, much has changed in our industry. Digital control is cheaper and more capable; accordingly, much more of this edition is focused on digital techniques. Other changes include improvements in feedback and power devices. These changes have caused a proliferation of control systems in industrial, military, and commercial products; they allow the use of feedback control systems where open-loop control was once the only affordable option.
The software that controls engineers use has changed dramatically. Microsoft Windows tools have supplanted the DOS tools that were common ten years ago. As a result, the simulations in this book, originally based on DOS, are now written for Windows. That change required the single largest effort in this revision, the coding of ModelQ, the simulation environment that is used extensively in this book. More about ModelQ, including how every reader can obtain a copy, will be discussed in Chapter 1.
Organization of the Book
This edition is organized in three sections. Section I, Applied Principles of Controls, consists of nine chapters, and is roughly equivalent to the first six chapters of the first edition. Chapter 1, Introduction to Controls, discusses the role of controls and controls engineers in industry. Chapter 2, The Frequency Domain, reviews the sdomain, the basis of control systems. Chapter 3, Tuning a Control System, is provided to allow you to practice tuning; for many, this is the most difficult part of commissioning control systems. There is more emphasis on tuning in this edition based on the observation that universities do little to teach this craft.
Chapter 4, Delay in Digital Controllers, culls out the fundamental difference in the application of digital and analog controllers, the contribution of instability from sample delay. Chapter 5, The z-Domain, discusses z-transforms, the technique that extends the s-domain to digital control. Chapter 6, Six Types of Controllers, covers practical issues in the selection and use of six variations of PID control. Chapter 7, Disturbance Response, provides a detailed discussion of how control systems react to inputs other than the command. Chapter 8, Feed-Forward, provides techniques that can substantially improve command response. Chapter 9, Filters in Control Systems, provides discussion of the use of filters in both analog and digital controllers.
Section II, Modeling, has three chapters. Chapter 10, Introduction to Modeling, provides overviews of timeand frequency-domain modeling methods. Chapter 11, Nonlinear Behavior and Time Variation, addresses how to deal with nonlinear operation when using linear control techniques. Unfortunately, this subject is missing from most texts on controls although significant nonlinear effects are common in industrial applications. Chapter 12, Seven Steps to Developing a Model, gives a stepby-step procedure for developing models.
Section III, Motion Control, was added to the second edition. It was omitted from the first edition because the book was aimed at the general controls industry. The first two sections of this edition attempt to remain general although readers outside this discipline may feel some influence of motion control as it is the area where the author has practiced his trade for nearly two decades. This last section concentrates entirely on motion control using electric servomotors. Chapter 13, Encoders and Resolvers, discusses the most common feedback sensors used with electric servomotors. Chapter 14, Basics of the Electric Servomotor and Drive, reviews the operation of these motors. Chapter 15, Compliance and Resonance, is dedicated to one of the most widely felt problems in motion control, instability resulting from mechanical resonance. Chapter 16, Position Loops, addresses the control of position since the great majority of applications control position rather than velocity or tor! que.
