Predefined-Time Terminal Sliding Mode Trajectory Tracking Control for Autonomous Underwater Vehicle with Unknown External Disturbance

Considering trajectory tracking control for autonomous underwater vehicles (AUVs) with unknown external disturbances, this paper presents a novel predefined-time terminal sliding mode control methodology. To begin with, an adaptive predefined-time disturbance observer (APTDO) is developed to reconstruct unmeasured environmental disturbances online, wherein an adaptive law is formulated to estimate the unknown perturbation bounds and ensure that the observation error converges within a user-predefined time interval that is completely independent of initial conditions. Compared with the fixed-time observer, the presented observer realizes the predefined-time convergence with a predefined convergence time and simple parameter design. Furthermore, a terminal sliding mode controller is formulated to achieve predefined-time convergence while enhancing both transient response rapidity and steady-state precision. Adaptive switching mechanisms are incorporated into the sliding surface design to eliminate potential singularity problems under varying operational conditions. The proposed control architecture guarantees that the control system achieves stability in a user-specified time interval, which time could be directly prescribed by adjusting designated parameters. This paper conducts a comprehensive numerical simulation for a four-degree-of-freedom AUV model and compares it with finite-time and fixed-time controllers. Numerical simulations are presented to validate the efficacy of the control approach in this paper.