Process Identification and PID Control

12

Relay Feedback Methods

In this chapter, the conventional relay feedback method is first introduced. It is the simplest and has been the most widely used in industry for a long time. In particular, it is one of the most important process activation methods for automatic tuning of a PID controller. Next, three important relay feedback methods recently developed to overcome the several problems of the conventional relay feedback methods are introduced. The first method activates the process with a guarantee of a symmetric oscillation under the circumstance of disturbances so that the describing function analysis method provides accurate frequency response data of the process. The second method can guarantee a symmetric oscillation under the circumstance of disturbances and nonlinearity. The third method can provide estimates for the frequency responses for which the phase angle is specified a priori and also manipulate a large range of operation, possibly larger than the magnitude of the relay.

12.1 Conventional Relay Feedback Methods

The conventional relay feedback method was proposed by Åström and Hägglund (1984). It is the simplest among the various versions of relay feedback methods. Two types are available, according to the actual operations: unbiased relay and biased relay.

12.1.1 Unbiased-Relay Feedback Method

Chapter 8 in Part Three briefly explained how to activate a process using the unbiased-relay feedback method, and the MATLAB code for its implementation was introduced. ...