Adaptive Fixed-Time Sliding Mode Tracking Control of Underactuated AUVs with Flexible Prescribed Performance

This paper proposes a fixed-time nonsingular terminal sliding mode control (FXTNTSMC) method for underactuated autonomous underwater vehicles (AUVs) under uncertain dynamics and flexible prescribed performance. To address the inherent limitation of conventional prescribed performance control (PPC), the rigid performance bounds that may induce system instability under large disturbances, a flexible PPC strategy is proposed. This approach incorporates a risk monitoring mechanism that proactively predicts the tendency of the tracking error to violate performance constraints and dynamically adjusts the performance boundaries, thereby preserving transient performance while circumventing controller singularity. Furthermore, a FXTNTSMC law is designed to guarantee the tracking error can achieve a bounded region within a fixed time independent of initial conditions. An improved adaptive radial basis function neural network is employed for online approximation of the lumped disturbance, which consists of both uncertain dynamics and unknown external disturbances. Finally, a simulation shows that the superiority of the FXTNTSMC scheme in terms of tracking accuracy and convergence rate.