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A Real-Time Approach to Process Control, 3rd Edition by William Svrcek, Donald Mahoney, Brent Young

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2

Feedback Control Loop Concepts

What we have to learn to do, we learn by doing.

—Aristotle

Introduction

Prior to attempting this workshop, you should review Chapter 3 in the book.

Process systems respond to various disturbances (or stimuli) in many different ways. However, certain types of responses are characteristic of specific types of processes. The characteristic response of a process can be described as its personality. Process control engineers have developed a range of terms and concepts to describe different process personalities and they use this knowledge to develop effective control systems.

Two of the most common personalities are those for first- and second-order systems. First-order systems may also be called first-order processes or first-order lags and can be mathematically modelled through the use of a first-order differential equation. Shown in Figure 2.1 is the typical step response of a first-order process. The time constant, τ, was discussed in Chapter 3 and is related to the speed of the process response; the slower the process the larger the value of τ.

Figure 2.1 First-order process response to a step change.

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Unlike first-order processes, second-order processes can have several different types of responses. Second-order processes are more complex than first order, and hence the mathematical models used to describe these processes are also more complex. ...

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