Nonlinear disturbance observer based robust backstepping control for a flexible air-breathing hypersonic vehicle

This paper presents a robust backstepping approach for a flexible air-breathing hypersonic vehicle in the longitudinal plane. The control design and stability analysis of the backstepping approach are based on a control-oriented model, which treats the flexible dynamics as dynamic perturbations. The backstepping method in this study utilizes a command filter to avoid the problem of "explosion of complexity" that occurs in the traditional backstepping method. To suppress the flexible dynamics and parameter uncertainties, a nonlinear disturbance observer is proposed to estimate these uncertainties during the tracking process. By introducing a group of virtual states, a Lyapunov-based stability analysis of the closed-loop system indicates that the tracking errors vanish asymptotically. A guideline for tuning the controller is given from a physical perspective to avoid exciting the flexible modes. This approach presents a good tracking performance with respect to both the flexible modes and parameter uncertainties. Simulation results of the full nonlinear model show the effectiveness of the proposed approach.