Performance overview of relay feedback tuning of PID controller

In recent years, relay feedback method has found a new lease of life in the automatic tuning of PID controllers and in the initialization of other sophisticated adaptive controllers. This tuner is based on the approximate estimation of the critical point on the process frequency response from relay oscillations. A continuous cycling of the controlled variable is generated from a relay feedback experiment and the important process information, ultimate gain and ultimate period can be extracted directly from the experiment. This is a very efficient way, i.e., a one shot solution, to generate sustained oscillations. The success of this auto tuner is due to the fact that the identification and tuning mechanism is simple so that process operators understand how it works. Moreover, it works well even in slow and non-linear processes.