Enhanced control of unstable cascade systems using direct synthesis approach

Abstract

This paper presents a controller design method for the open-loop unstable process in the primary loop and the open-loop unstable, integrating and stable processes in the secondary loop of the simple cascade control configuration. The controller of the secondary loop and the primary loop is designed through a direct synthesis (DS) approach in the frequency domain for load disturbance rejection and set-point tracking, respectively. Moreover, a set-point filter is used to reduce unnecessary overshoot in servo tracking. The proposed method is applicable to a wide range of processes having unstable, integrating and stable dynamics in a unified way. It avoids the complexity of order reduction and dead time approximation of the plant. The simulation study illustrates that the present approach provides enhanced closed-loop performance as compared to the approaches recently reported in the literature.