Observed-based fast fixed-time fault-tolerant trajectory tracking control with prescribed performance for unmanned underwater vehicles under constraints

This paper is concerned with the observed-based fast fixed-time fault-tolerant tracking control problem for unmanned underwater vehicles (UUVs) under constraints. Specifically, in order to attenuate the adverse effects of the lumped uncertainty that comprises model uncertainties, external disturbances and actuator faults, a novel fixed-time disturbance observer (FTDO) is developed, which stabilizes the observed errors to zero in a shorter settling time and discards the restriction on requirements for prior knowledge of lumped disturbance in comparison to existing disturbance observers. Moreover, a predefined-time prescribed function (PTPF) based on finite-time stability is designed to ensure both transient and steady-state performance behaviors, operating independently of initial condition and allowing for the selection of an upper bound of the predefined time. By integrating FTDO and PTPF into the non-singular fixed time terminal sliding mode manifold (NFTSM), a novel fast fixed-time control scheme is developed, ensuing that tracking errors converge to prescribed bounded regions. The Lyapunov stability theorem is used to prove that the closed-loop system remains bounded. Numerical simulation verifies the effectiveness of the proposed method.