Multi control scheme with modified Smith predictor for unstable first order plus time delay system

A multi control scheme is proposed for unstable first order plus time delay (UFOPTD) system. The proposed control structure consists of two controllers which are intended for two distinct roles, set point tracking and disturbance rejection. A PI controller cascaded with a first order filter is employed as servo controller and a PID controller in series with a lead/lag filter is employed for disturbance rejection. The controller parameters are derived systematically using polynomial approach. The proposed scheme decouples the servo response from the regulatory response under nominal conditions which facilitates to tune the controllers independently. Analytical rules for the tuning of controllers are proposed based on maximum sensitivity (MS) which is a measure of robust stability. Set point weighting is employed to reduce the overshoot and settling time in the servo response. Several examples are included to show the effectiveness of the proposed structure. Keywords: PID controller, Maximum sensitivity, Unstable process, Set point weighting, Smith predicto