Anti‐Unwinding Faster Fixed‐Time Attitude and Position Tracking Control for Flexible Spacecraft Proximity Operations

This paper addresses the integrated attitude and position tracking control for servicing spacecraft in proximity operations with non‐cooperative targets. The concerned servicing spacecraft is subject to flexible vibrations, asymmetric actuator saturation, and unknown external disturbances. First, the attitude‐position‐vibration coupling dynamic model of the servicing spacecraft is established using dual quaternion. Second, an adaptive fixed‐time disturbance observer (AFTDO) is designed to compensate for unknown external disturbances and flexible coupling terms. Third, a faster fixed‐time tracking controller (FFTC) is proposed based on a novel unwinding‐free nonsingular sliding mode surface. In addition, an anti‐windup compensator is developed to mitigate the adverse effects of asymmetric actuator saturation. It is demonstrated through Lyapunov stability analysis that the practical fixed‐time convergence of the attitude and position tracking errors is guaranteed, while the unwinding phenomenon caused by dual quaternion description is avoided. Numerical simulation validates the effectiveness of the proposed control scheme.