Relay autotuning: a use of old ideas in a new setting

The paper discusses the relatively new approach of relay autotuning and shows how a sound understanding of the approach requires knowledge of two approaches introduced early in the search for methods to study nonlinear control systems. The two approaches - the describing function method and procedures for evaluating limit cycles exactly in relay systems - are first reviewed. After a short section on techniques for choosing parameters of fixed controllers it is shown how relay autotuning can be used for PID controllers based on either plant critical point information or the estimation of a plant model. Finally some examples are given which illustrate the controller design techniques presented.