Multiple-time scale smooth second order sliding mode controller design for flexible hypersonic vehicles

A nonlinear robust control strategy is proposed for flexible hypersonic vehicles with aerodynamic parameters uncertainties. Firstly, the longitude dynamics of this vehicle are divided into velocity subsystem, altitude and flight path angle subsystem, angle of attack, and pitch rate subsystem. Then the smooth second order sliding mode controller and disturbance observer are designed to ensure the finite time stability for each subsystem. Furthermore, the stability of the whole system is guaranteed via the assumption of multiple-time scale characteristic. Moreover, the effect of flexible modes on rigid-body states can be estimated exactly using the disturbance observer. In addition, two representative simulations are carried out to analyze the effect of elevator deflection and canard on the dynamics of velocity, flight path angle, and angle of attack. Finally, the Monte Carlo simulations are provided to verify the effectiveness of the proposed control strategy.