Nonlinear-disturbance-observer-based sliding mode backstepping control of hypersonic vehicles

A nonlinear-disturbance-observer-based sliding mode backstepping controller is designed for flexible hypersonic vehicles with the characters of nonlinearity,strong couplings and parameter uncertainties.The vehicle curve-fitted model is decomposed as velocity subsystem and altitude-related subsystem,and is expressed as strict feedback form.The actual control laws and virtual control laws are designed based on the design procedures of sliding mode control and backstepping control,respectively.Low-pass first order filter is introduced to obtain the derivatives of virtual control laws,which avoids the explosion of differentiation terms in the traditional backstepping control.A new nonlinear disturbance observer designed based on improved sliding mode differentiator is introduced to estimate and compensate the model uncertainties.Simulation results demonstrate the effectiveness of this controller in tracking velocity and altitude commands in the presence of model uncertainty and aeroelasticity.