This paper presents a barrier Lyapunov function based nonsingular fast terminal sliding mode control with fixed-time non-recursive disturbance observer for the blended-wing-body (BWB) aircraft in the presence of mismatched disturbances and output constraints. The fixed-time non-recursive disturbance observer based nonsingular fast terminal sliding mode control is proposed for matched and mismatched disturbances attenuation. Furthermore, in order to pursue high-precision attitude tracking regarded as output tracking errors constraints before disturbance observer tracking errors converge to a small prescribed range, the barrier Lyapunov function is designed to constrain the sliding mode variables before states reach the sliding mode surface. Consequently outputs tracking errors constraints can be satisfied by the sliding variables constraints. More importantly, barrier Lyapunov function based terminal sliding mode controller ensures the finite-time convergence. Finally, the numerical simulations demonstrate the chattering attenuation and high-precision attitude tracking of the BWB aircraft in the presence of mismatched disturbances and output constraints.INDEX TERMS Barrier Lyapunov function, blended-wing-body aircraft, fixed-time non-recursive disturbance observer, mismatched disturbances, output constraints.

With the development of the space missions, there are extensive missions in the demand of the prescribed time convergence. However, it's still a difficult work to combine the prescribed time method with the sliding mode control due to the infinite gain of the prescribed time method while approaching the prescribed time and two periods of sliding mode control. In this paper, a new prescribed time sliding mode control method is proposed for general systems with matched disturbances, from the second-order system to the high-order system. A novel sliding mode variable with explicit time term is designed for achieving the prescribed time convergence. More importantly, as time approaches the prescribed time, the singularity of control input can be avoided. Finally, this paper presents a disturbance observer based prescribed time sliding mode control method for attitude tracking of spacecraft and the efficiency of this method has been verified through the numerical simulations.

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