PID Controllers Tuning for Improved Performance of Unstable Processes

The design of PID controller for unstable processes is always difficult due to instability and non-minimum phase. The design procedures for unstable processes involve compensation of unstable zeros or poles. In most of the cases PID controllers are used to control the stable processes, however less common for unstable processes. In this paper, a simple controller design rule and tuning procedure for unstable processes with delay time is discussed. The method is developed based on a -3 dB, gain crossover and phase crossover frequencies with Pade approximation for the time delay. Simulation examples are included to show the effectiveness of the proposed method.