Automatic Tuning of PID Controllers for Unstable Processes

Abstract The tuning of PI and PID controllers for unstable processes is addressed. A well known technique which makes use of the closed-loop dynamics with a relay to find the controller parameters is here extended to unstable processes. The stabilizability of unstable systems with PID controllers is analysed first; then the technique is developed and tested with good results on a strongly nonlinear process which is described by a complex model that cannot be easily used for control purposes. The technique is particularly appropiate for frequent automatic retuning.