Decoupling two-degree-of-freedom control strategy for cascade control systems

In this paper, two new two-degree-of-freedom control structures are proposed for cascade control systems, both of which are identical in the controller design procedure. The primary controller used for setpoint tracking is derived in terms of H2 optimal performance objective. The secondary controller is responsible for rejecting load disturbances that seep into the intermediate process and therefore is called a load disturbance estimator, which is inversely figured out by proposing the desired complementary sensitivity function of the inner loop for disturbance rejection. As a result, the nominal setpoint response is decoupled with the load disturbance response of the inner loop, both of which can be optimized independently. Based on the control system stability analysis, the rule of thumb for tuning the single adjustable parameter of each controller is provided to cope with the process uncertainty in practice. Several illustrative examples are included to demonstrate the superiority of the proposed method.