Improved Cascade Control System for a Class of Unstable Processes with Time Delay

An improved cascade control scheme is proposed based on modified Smith predictor for controlling a class of unstable processes with time delay. The proposed control structure consist three controllers of which the secondary loop has one controller and the primary loop has two controllers. The secondary loop controller is designed according to Internal Model Control (IMC) principles. The primary loop set point tracking controller and disturbance rejection controller are designed using the same procedure based on modified Smith predictor control structure. An analytical method on the basis of the Internal Model Control design principle is adopted for the two controllers which reduces complexity in design. The two controllers in primary loop are designed as proportional, integral and derivative (PID) controller cascaded with a lead-lag filter. Moreover, suitable values are recommended for the three controllers based on extensive simulations. Simulations studies are carried out to demonstrate the effectiveness of the proposed method.