Combined design of robust controller and disturbance observer in a fixed-order H∞ control framework

The disturbance observer (DOB) and H∞ based robust control methods are widely used in feedback control applications due to their disturbance rejection capability and robustness improvement. However, the simultaneous synthesizing of DOB and H∞ controller considering the real uncertain parameters and dynamic uncertainties of the models is an open problem. This paper presents a new fixed-order μ-synthesis algorithm based optimization methodology for the combined design of DOB and robust controller in the presence of mixed uncertainty. The fixed-order robust H∞ control framework is adopted to optimize structured plant inverse, so-called Q filter and feedback controller. In order to guarantee the robust stability of the main feedback loop and inner DOB loop, the overall closed-loop system is considered with uncertainties during the design procedure. The proposed approach renders the need for an explicit plant inverse unnecessary. Thanks to this advantage and generality of μ-synthesis techniques, the proposed method can be directly applied to both the non-minimum phase and multiple-input and multiple-output (MIMO) uncertain systems without any approximation. The theoretical design approach is experimentally verified on an electromechanical control actuation system of an air vehicle and numerically on a non-minimum phase system.