Analytical design of fractional IMC filter - PID control strategy for performance enhancement of cascade control systems

The cascade control is widely used in chemical industries for disturbance attenuation as an alternative to a single feedback loop. This article presents an internal model control (IMC)-based series cascade control system for different time delay process models. The novelty of the work lies in the use of fractional order IMC filter of the first order and higher order for designing controller in the outer loop based on specific maximum sensitivity. The design also considers different approximations for the time delay term. The controller designed for the inner loop assumes the standard PID controller structure and the outer loop controller takes a fractional filter PID structure. The proposed method gives improved performance for set point tracking and disturbance rejection. Further, the proposed method gives robust performance for noise in the measured signal. Also, the robustness analysis is performed to know the insensitivity of the closed-loop system for process parameter variations. The fragility of the controller is investigated for perturbations in the controller parameters.