Enhanced Control of Unstable Cascade Processes with Time Delays Using a Modified Smith Predictor

This paper presents a simple cascade controller in the enhanced modified Smith predictor structure for control of open loop unstable cascade processes with/without zero. The proposed structure consists of two control loops, a secondary inner loop and a primary outer loop. The method has totally three controllers of which the secondary loop has one controller and the primary loop has two controllers. The secondary loop controller is designed using the internal model control (IMC) technique. The primary loop set point tracking and disturbance rejection controllers are designed using the direct synthesis method. The primary set point tracking controller is designed as a proportional, integral, and derivative (PID) controller with lag filter and the primary disturbance rejection controller is designed as a PID with lead-lag filter. Simulation studies have been carried out on various cascade unstable processes with/without zero. The present method gives significant disturbance rejection both in the inner and outer loops. Also, the proposed method shows significant improvement when compared to the recently reported methods in the literature.