Self-tuning Smith predictors for processes with long dead time

Abstract

A simple relay feedback auto-tuning method is proposed for the Smith predictor, an advanced controller for processes with long dead time. The relay feedback control gives information on one point of the Nyquist curve in terms of ultimate gain and frequency. With an additional measurement of the static gain, a reduced order process model in terms of a first- or second-order dynamics plus dead time could be computed and used to auto-tune the Smith predictor. When the process dynamics changes more frequently, a self-tuning controller is required. In this case the Fast Fourier Transform technique can be further employed to track the points on the Nyquist curve by estimating the process frequency response and then used to update the Smith predictor. Excellent performance of the auto-tuned and self-tuned Smith predictor has been substantiated by simulations.