Cascade Disturbance Observer-Based Control Design for Cascaded Systems With Considerable Inner-Loop Dynamics

Cascade control permeates a wide range of industrial processes, due to its significant improvement in disturbance rejection. However, conventional cascade control turns laborious to control the sluggish outer loop and suffers from complex inner-outer-loop coupling dynamics. Combining the merits of conventional cascade control and powerful capability of disturbance observer (DOB), this paper presents a cascade disturbance observer (CDOB) based control approach for cascaded systems. The resulting cascade control system features equivalent decoupling of the inner- and outer-loop disturbance rejection (DR), which allows the relevant filters to be designed separately. For the inner-loop filter, a robust design approach, directly based on the performance specifications and robustness constraints, is proposed, which enables an intuitive trade-off between performance and robustness. A novel filter is devised to improve the sluggish outer-loop DR performance. With the feedforward controller designed based on the nominal model, a composite control strategy is finally constructed. Some simulations and experiments are presented to validate the efficacy of the proposed method, and the significant performance improvements depict application prospects in process control.Note to Practitioners—This paper focuses on cascade control design for cascaded systems and presents a CDOB based control scheme as well as its robust design approach. The DR response of the control system and detailed control design, including inner- & outer-loop filters design, compensated plant analysis and robust stability analysis, are demonstrated, which can provide a design reference for practitioners. Specifically, the inner- and outer-loop filters are constructed subject to the performance and robustness trade-offs, which can help practitioners grasp the performance and robustness of the control system and facilitate control practice. The proposed control method features equivalent decoupling of the inner- and outer-loop DR and the CDOB can also serve as a patch for the baseline controller just like the conventional DOB. Further, the CDOB structure is also promising to be extended to more general cascade systems, such as non-minimum phase processes. The simulation and experiment results demonstrate the superiority of the proposed method and show promise for practical application in other systems.