SIMPLE PI AND PID TUNING FOR OPEN-LOOP UNSTABLE SYSTEMS

Simple tuning methods for traditional low-order schemes are lacking for open-loop unstable systems. In this paper, the limits to the applicability of proportional-integral-derivative (PID) controllers are derived for unstable processes in the presence of gain uncertainty and unmodeled dynamics (such as dead time or neglected poles). Simple IMC-based tuning rules are employed that, when combined with appropriate robustness analysis, provide a useful alternative to lengthy on-line tuning.