Abstract

This paper investigates the trajectory tracking control problem for an unmanned surface vehicle (USV) subject to unknown time-varying environmental disturbances and the challenge of ensuring fast convergence while avoiding singularities. A novel fixed-time nonsingular fast terminal sliding mode (NFTSM) control scheme, integrated with a fixed-time disturbance observer (DOB), is proposed to achieve high-precision and robust tracking. The control architecture consists of three key components: (i) a fixed-time virtual velocity control law designed to ensure that position errors converge to zero within a fixed time even when velocity errors vanish, thereby preventing slow convergence; (ii) a nonsingular fast terminal sliding surface that eliminates the singularity issue inherent in traditional terminal sliding mode and guarantees that the USV state converges to the desired trajectory within a fixed time independent of initial conditions; and (iii) a fixed-time DOB that accurately estimates and compensates for external disturbances, with estimation errors proven to converge to zero in a fixed time. The stability and fixed-time convergence of the closed-loop system are rigorously established using Lyapunov theory. Comparative simulations, conducted under external disturbances on the Cybership II model, demonstrate that the proposed NFTSMC strategy significantly outperforms conventional sliding mode control (SMC) and nonsingular terminal sliding mode control (NTSMC). Specifically, the proposed scheme reduces the integral of absolute error (IAE) for position tracking by over 60% and the integral of time-weighted absolute error (ITAE) by more than 30% compared to SMC, while achieving smoother control inputs and stronger disturbance rejection. These results highlight the superior convergence speed, tracking accuracy, and robustness of the proposed controller, underscoring its originality and practical value for USV autonomous navigation.

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