Relay autotuning: an overview and alternative approach

This paper discusses the use of relay-produced limit cycles for selecting the parameters of fixed controllers, with particular reference to the proportional−integral−derivative (PID) controller. This work shows how the current ideas relate to the early work of Ziegler and Nichols, particularly their loop cycling test. The evaluation of limit cycles in relay feedback systems is discussed, and details of their possibilities for determining parameters of an assumed plant transfer function or the value of the plant critical point are given. To conclude, it is shown how the approximate value of points on the plant frequency response locus can be determined from using the relay in parallel with the controller, which can be done without opening the control loop