Comparative Study of Different Supervisory Control Structures

With the advent of computer control, supervisory controllers such as simple cascade control, model predictive control (MPC), dynamic matrix control (DMC), etc are increasingly being used in process industries. In this study, performance of three such controllers namely simple cascade controller, ‘MPC cascaded to PID’ and ‘PID free MPC’ are compared on a continuous stirred tank heater (CSTH) system. In the MPC cascaded structure the flow-loops are regulated by the PID controller. On top of that a DMC manipulates the set-points of the flow-loops to control tank temperature and level. The ‘PID-free MPC’ structure uses a DMC to manipulate the valve positions directly. The study reveals that the PID-free MPC structure outperforms the cascade structure in both disturbance rejection and set-point tracking. However, the PID-free MPC structure demands more control action and has more control load. Integrated square error (ISE) is used to quantify the performance.