Measurement and Control Basics Fourth Edition

Chapter 2 - Process Control Loops

Introduction

We discussed the general concepts of process control in Chapter 1. In this
chapter, we will cover the basic principles of process control loops. Single-
loop feedback control is the most common type of control used in industrial
processes, so it will be discussed in the greatest detail. We will then
discuss other types of control loops, such as cascade, ratio, and feedforward.
Finally, we will examine several common methods used to tune control
loops.

Single-loop Feedback Control

In a feedback control loop, the variable to be manipulated is measured.
This measured process value (PV) is then compared with a set point (SP)
to generate an error signal (e = PV - SP). If a difference or error exists
between the actual value and the desired value of the process, a process
controller will take the necessary corrective action to return the process to
the desired value. A block diagram of a single-feedback control loop is
shown in Figure 2-1.

The measured process variable is sensed or measured by the appropriate
instrumentation, such as temperature, flow, level, or analytical sensors.
This measured value is then compared with the set point. The controller
uses this comparison to adjust the manipulated variable appropriately by
generating an output signal. The output signal is based in turn on whichever
control strategy or algorithm has been selected. Because in the process
industries the manipulated variable is most often flow, the output of

Figure 2-1. Feedback control loop

the controller is usually a signal to a flow control valve, as shown in
Figure 2-1.

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