Robust fixed-time sliding mode controller for flexible air-breathing hypersonic vehicle.

An improved robust fixed-time sliding mode controller (RFSMC) is presented for flexible air-breathing hypersonic vehicle (FAHV) with actuator faults, composing of a novel fast fixed-time integral sliding surface (FFIS), a continuous fixed-time super-twisting-like reaching law (CFSTL) and a uniformly convergent observer. Firstly, the nonlinear control-oriented model of FAHV is processed via input/output feedback linearization with flexible effects and actuator faults modeling as matched Lipschitz disturbances. Secondly, a novel non-singular FFIS is established based on a fast fixed-time high-order regulator (FFTR), which is improved with two gains incorporating into standard fixed-time high-order regulator via dilation rescaling. The FFTR proposed can accelerate respond speed of system by tuning values of two gains simply without complicated parameters selection and the stability is proved strictly via Lyapunov criteria. Thirdly, a CFSTL is utilized to ensure high-precision convergence of sliding mode vector and its derivative in fixed time. Afterwards, a uniformly convergent observer is applied to estimate lumped disturbances accurately in fixed time. With the estimated values compensated into controller, RFSMC can enhance fault-tolerant performance and attenuate chattering efficiently. Finally, simulations on FAHV are performed to verify the effectiveness and superiority of the method proposed.