The system identification and PID auto-tuning for unstable processes

PID auo-tuning is widely used to control stable processes, however, its application to unstable processes is less common. In this paper, an engineering oriented identification technique for single process is proposed, and simple formulas are derived to tune the PID controller for unstable processes with time delay to meet gain and phase margin specifications. With the proposed PID tuning method, a loop transfer function with a good shape, such as phase margin 60deg, gain margin over 3 can be obtained. This guarantees both robustness and performance. Simulation examples are given to show the performance of the method.