Employing relay-based identification for improving model-based PI-PID autotuning

This paper presents and discusses a quite unusual application of the widely adopted relay-based identification technique for stable SISO processes. Besides providing a point on the process Nyquist diagram, the proposed method can obtain some quantitative information on its unit step response, namely a quite precise and highly noise-insensitive measure of the settling time. The usefulness of this capability is demonstrated by employing the proposed technique for parametrizing an approximated process model without the need of performing open-loop experiments. In addition, it is shown that the adoption of this method can improve the performance of model-based automatic tuning by applying it to the well-known IMC-PI autotuner.