Fixed-time fault-tolerant attitude control for flexible spacecraft without angular velocity sensor

This paper deals with the appealing problem of fixed -time fault -tolerant attitude control, using attitudeinformation -only, for a flexible spacecraft under the influence of inertial parametric variations, external disturbances, and multiple actuator faults while suppressing the flexible appendages' vibrations without using additional sensors or smart vibration suppression actuators. First, an adaptive fixed -time model -free observer (AFTMFO) is designed, using the attitude information, for rapid estimation of unavailable angular velocity. A novel adaptive continuous control is then synthesized based on an anti -unwinding fast fixed -time nonsingular sliding surface (AFFTNSS), utilizing variable gains in both the control law and sliding surface; that simultaneously alleviates the chattering but also improves the convergence speed when compared to existing fixed -time approaches. The proposed scheme offers superior performance characteristics such as velocity sensor -free fixed -time attitude maneuvering with high pointing accuracy, fault tolerance, vibration suppression, nonsingular and chattering -free control. The spacecraft can carry out the coveted control objective in a predeterminable time independent of the knowledge of initial states while overcoming the unwinding effect to reduce the control effort and time. The fixed -time closed -loop stability of the proposed scheme is corroborated via Lyapunov techniques. Finally, a comparative simulation analysis with the existing results elucidated the proposed scheme's efficacy.