Improved disturbance rejection control based on H∞ synthesis and equivalent-input-disturbance for aircraft longitudinal autopilot design

Disturbance rejection control has been developed over decades attracting wide interest and research attention. Aiming at providing a potential engineering application of disturbance rejection control to aircraft control system design following traditional frequency-domain methods, this paper presents an improved disturbance rejection control of two degrees of freedom based on H infinity synthesis and equivalent-input-disturbance for an aircraft longitudinal autopilot design. The mismatched disturbance is transformed as a "total disturbance" in the input channel for compensation through the establishment of an equivalent-input-disturbance system. The H infinity synthesis based on classical frequency-domain analysis is applied to a disturbance filter and a composite feedback controller design. In terms of the controller design, the system including the filter is considered as a whole in H infinity optimization process without separate design to guarantee the stability of the overall system. Furthermore, the proposed method is successfully implemented on the autopilot design by modeling nonlinear aircraft longitudinal dynamics as a linear equivalent-input-disturbance formulation of angle of attack. The simulation of tracking performance in comparison with existing renowned methods is conducted in the presence of aerodynamic uncertainties, gust disturbance, actuator characteristics and sensor noise. The results verify the effectiveness of the proposed method with excellent performance and practical prospects.