IMC-based control strategy for open-loop unstable cascade processes

In this paper, a new three degree-of-freedom control structure is proposed for open-loop unstable cascade processes. Enhanced regulatory capacities for both set point tracking and load disturbance rejection are obtained. By virtue of an open-loop control manner for the set point tracking, the nominal set point response is decoupled from the load disturbance response and thus can be quantitatively regulated by tuning the single adjustable parameter of the set point tracking controller. There are two closed loops for rejecting load disturbances, of which the inner loop is primarily responsible for the load disturbances that seep into the intermediate process and the outer loop for the load disturbances injected into the primary unstable process. Both of the closed-loop controllers are analytically derived by proposing the desired closed-loop complementary sensitivity functions, and meanwhile, the robust stability constraints for tuning the two closed loops are analyzed in the presence of the process multiplicative uncertainties. Simulation examples are included to demonstrate the superiority of the proposed method.