Process Identification and PID Control

5

Proportional–Integral–Derivative Controller Tuning

The tuning parameters of the PID controller should be set with in-depth consideration of the process dynamics. Otherwise, acceptable control performances cannot be achieved. Poor tuning parameters would result in very sluggish or unstable responses. In this chapter, simple process identification methods are introduced to obtain the process model in the form of the frequency response or low order plus time delay. Also, various tuning methods are discussed to demonstrate how to estimate the tuning parameters of the PID controller on the basis of the process model.

5.1 Trial-and-Error Tuning

Trial-and-error tuning is used to determine the tuning parameters of a PID controller by inspecting the dynamic behavior of the controlled process output. It is very important to understand the effects of the tuning parameters on the behavior of the process output for successful trial-and-error tuning. The PID controller usually shows the following dynamic behaviors with respect to the tuning parameters for the step setpoint change.

• Behavior 1. For a step setpoint change, if the process output shows a big oscillation, as shown in Figure 5.1, then the proportional gain k_c is too large. Here, the oscillation is centered on the setpoint.
• Behavior 2. For a step setpoint change, if the controlled process output shows an overdamped response, as shown in Figure 5.2, then the proportional gain k_c of the PID controller is too small.
• Behavior 3. For ...