Robust pidf-pid cascade control scheme for delay-dominant stable and integrating chemical processes

Abstract

Background: Chemical processes like bioreactors, liquid storage tanks, CSTRs, and distillation columns exhibit non-self-regulating behavior, generating boundless output over a fixed input, making it challenging to control when associated with dominant time delay. Hence, a modified cascade control method (CCM) is suggested which is unified for both stable and integrating processes with delay. Methods: The lead controller is proportional–integral–derivative-filter (PIDF) type, whereas the secondary controller is a PID. Both these controllers, operating in a Smith predictor-based framework, are designed using phase margin and maximum sensitivity specifications for the same user-defined target loop. The inner and outer loop Smith predictors compensate for the delays in their respective loops, while the number of controllers in the modified CCM remains the same as that of a conventional CCM. Significant Findings: Simulation studies on benchmark models of chemical processes demonstrate that the present design provides improved regulatory and servo actions. Particularly, it is capable of enduring up to 70% variation in process delay and 60% variation in process gain while maintaining closed-loop robust stability. Practical feasibility of this design is vindicated using a two tank level control experimental loop.