Active robust control of uncertainty and flexibility suppression for air-breathing hypersonic vehicles

Inevitable uncertainties and cross couplings between rigid and flexible modes pose huge challenges to control system design for flexible air-breathing hypersonic vehicles. This paper addresses an active robust control scheme that can simultaneously suppress diverse uncertainties and flexible modes using active approaches rather than inherent system robustness. Frequency-domain analysis is conducted to investigate the cross couplings. A novel conclusion lies in that the most significant cross couplings exist between the flexible modes and the rigid-body phugoid modes, followed by the altitude mode. Based on the analysis, a robust control scheme is proposed which consists of a stabilizing control frame and two active control techniques: a nonlinear extended state observer (ESO) and a notch filter. The ESO estimates diverse uncertainties to form a compensation law, and the notch filter is integrated to prevent the flexible modes from being excited by some specific high-frequency signals coming from the ESO estimated values. Thus, both uncertainties and flexible modes can be simultaneously suppressed. A Lyapunov-based stability analysis is conducted for the overall closed-loop system. At last, several representative simulations are conducted to demonstrate the advantages of the proposed active control scheme.