Singularity-Free Practical Fixed-Time Fault-Tolerant Control of Underwater Unmanned Vehicles With Input and Output Constraints

This article investigates the singularity-free practical fixed-time fault-tolerant control problem for unmanned underwater vehicles (UUVs) with input and output constraints. Firstly, with the aid of fuzzy logic system (FLS) and fixed-time stability, a fixed-time disturbance observer (FTDO) is developed to compensate for the lumped disturbance including uncertain dynamics, external disturbances, actuator faults, and input saturation. Moreover, to guarantee the optimal tracking accuracy of output errors and reduce consumption of control inputs, an appointed-time prescribed performance control (APPC) scheme is devised by integrating both the appointed-time performance function and unlimited-domain transformation function. Additionally, based on a novel prescribed sliding mode manifold that utilizes the precise estimation and prescribed performance, an adaptive non-singular fast terminal sliding mode controller (ANFTSMC) is proposed to stabilize tracking errors with fixed-time convergence while avoiding singularity constraints. Within this framework, an adaptive term is introduced to compensate for the unknown and time-varying input gains. Finally, Lyapunov stability analysis proves that all signals in the closed-loop system can achieve practical fixed-time stability, with all tracking errors confined strictly within prescribed regions. Numerical simulations further validate the effectiveness of the proposed scheme.