Saturation-Tolerant Adaptive Fixed-Time Control of Underactuated Surface Vessel With Improved Given Performances

In this article, a novel adaptive fixed-time terminal sliding mode control (AFiTSMC) algorithm is proposed for a class of underactuated unmanned surface vessels (USVs) with model uncertainties, external disturbances and actuator constraints. Specially, by introducing variable exponent coefficient that depends on the system states, a nonsingular terminal sliding mode (NTSM) surface based fixed-time controller is designed, which is not only beneficial to maintain continuity, non-singularity and higher robustness to lumped system uncertainties, but also ensures that the system tracking errors converge to zero in fixed time. Besides, a new time scaling function is developed realizing that the convergence ranges, rates and times of tracking errors are fully adjustable, then excellent transient-state performance can be obtained. Moreover, the calculation burden caused by utilizing approximation function is reduced effectively by devising proper parameter adaptive methods, even under unknown complex system actuator constraints. As far as we know for underactuated USVs with uncertainties/disturbances, it is the first time to guarantee transient-state and fixed-time zero-error tracking results both for actuated and unactuated states. Finally, numerical simulation results demonstrate the effectiveness of the proposed method.