An improved continuous sliding mode controller for flexible air-breathing hypersonic vehicle

An improved continuous sliding mode control algorithm is proposed for a flexible air-breathing hypersonic vehicle (FAHV), including nonsingular fast fixed-time sliding surface (NFFS) and dual-layer adaptive continuous twisting reaching law (DACTL). Firstly, the nonlinear control-oriented model of FAHV is processed using input/output feedback linearization method with the significant flexible effects modeling as unknown matched disturbances. Secondly, a novel NFFS is improved from conventional fixed-time sliding surface by adjusting power exponent to accelerate convergence rate. In the meanwhile, in order to avoid singularity aroused by fractional power term, an exponential convergent sliding surface is switched when tracking error approaches zero. Thirdly, a DACTL is proposed to realize finite-time convergence of sliding mode variable with higher convergence precision and less chattering. Dual-layer adaptive law is utilized to adjust the gain in DACTL based on equivalent control concept so as to enhance robustness automatically and avoid overestimation of control gain. Meanwhile, disturbances can be compensated without knowledge of Lipschitz constants. Ultimately, simulations on longitudinal control of FAHV demonstrate the control algorithm proposed is superior to conventional quasi-continuous sliding mode controller in the aspect of convergence accuracy and chattering suppression.