Relay autotuning of PID controllers

This section is concerned with the relay autotuning method for setting the parameters of a fixed form controller, usually a PID controller. It is first explained how the method is an extension of a concept first discussed by Ziegler and Nichols for setting PID controller parameters based on an estimate of the gain margin, or process critical point. The theoretical approaches to analyzing the limit cycle obtained in an autotuning experiment are explained. The merits and some problems of using the describing function method of analysis for estimating the critical point are discussed. Setting of the PID controller parameters based on the critical point information is then covered and an example given. Finally a few further points are mentioned regarding practical aspects of PID control