Modified Smith predictor design for unstable processes with step and periodic disturbances

Abstract

In this article, a modified Smith predictor control scheme is proposed for step and periodic disturbance rejection for unstable processes with time delay. The scheme consists of three controllers: first one for set-point tracking, second one for nonperiodic disturbance rejection, and the third one for rejecting the periodic disturbances and improving the robustness. The set-point tracking controller is designed using the classical direct synthesis method based on the process model without considering the time delay. The nonperiodic disturbance rejection controller is also designed using the direct synthesis method. For further improved disturbance attenuation, a periodic disturbance rejection controller is designed. An analysis for the selection of the closed-loop tuning parameters of the controllers is carried out with the peak value of sensitivity function ( MS) as a robustness measure. The present method gives improved nominal and robust performances for step and periodic disturbances and also improved closed-loop performances when compared to recently reported methods.