A New Analytical Design Method of Controllers in Modified Parallel Cascade Structure for Stable, Integrating and Unstable Industrial Primary Processes including Time Delay under Robustness Constraints

This paper presents an analytical method for designing controllers within a modified parallel cascade control structure for stable, integrating, and unstable primary processes considering robustness and the sensitivity to measurement noise. The secondary process is assumed to be described by the stable, non-integrating first-order plus dead-time (FOPDT) model. In the proposed design procedure, using the complementary sensitivity approach, the secondary controller H2(s) is first designed, which is then decomposed into two controllers C2(s) and F2(s). Then, the primary controller H1(s) is determined and further decomposed into two controllers C1(s) and F1(s). Finally, the high-order/non-rational transfer function of the controller C1(s) is suitably approximated to obtain a low-order controller. Alternatively, for stable primary processes with dominant time delay, the realization of the full controller C1(s) without any approximation is proposed. The trade-off between performance and robustness of the closed-loop system is achieved by two user-specified parameters λ and β for the secondary and primary control loops, respectively. The effectiveness of the proposed design method is analyzed by simulations on typical representatives of industrial processes.