STUDIES ON DISTURBANCE-REJECTION CAPABILITY OF CASCADE SYSTEM AND ROBUST CONTROL OF PARALLEL CASCADE AND PARALLEL SYSTEM

Abstract

The disturbance-rejection capability of the series cascade control structure is explored. A theoretical study of series cascade control and conventional feedback control shows that the proposed steady-state measure can indicate whether a better load response is achievable by series cascade control structure. This measure also quantifies the extent of improvement in load response due to cascade control in comparison with feedback control. When a better load response is expected from cascade control, a design procedure for both primary and secondary controllers is also proposed. The basic principle is to design primary controller for servo response and the secondary controller for load response. The simulation results show that the proposed measure is useful in deciding when to use series cascade control and the proposed design procedure is an effective and simple alternative for designing series cascade controllers. Robust design of the parallel cascade control system is studied in two aspects: secondary measurement selection and robust controller design. Two steady-state secondary measurement selection methods are proposed for this control system: Integrated Error (IE) analysis and robust performance criterion. The optimal choice for the secondary measurement can always be found using IE analysis. In contrast, such an optimal solution may not be found using the selection criterion based on robust performance requirement since it is more conservative. In robust controller design, two methods are proposed: sequential design and simultaneous design. Applications show that no significant difference in the regulatory performance is observed between these two methods. The simultaneous design method is preferred because it is easy to use. Applications of the simultaneous design method to literature examples show that the robust performance could be achieved using the simultaneous design method. Lastly, the simultaneous robust controller design method is also applied to the parallel control system. Application to two examples shows that the robust performance can also be achieved using this controller design method.