Full-link trajectory tracking control for underwater snake robot with vector thrusters in the presence of strong time-varying disturbances

In this paper, a strategy for fixed-time trajectory tracking control based on a nonlinear disturbances observer is proposed for underwater snake robots (USR) facing model uncertainty and external disturbances. Firstly, as the foundation of this study, a USR with vector thrusters and orthogonal joints is designed and mathematically modeled. Subsequently, a fast fixed-time system is introduced, demonstrating superior convergence compared to existing systems. Leveraging this system and considering the motion characteristics of the USR, a novel decoupling control strategy is developed for the full-link trajectory tracking of the USR, effectively segregating the motion into that of the head and the other links. In response to the decoupling scenario, cascade controllers are meticulously designed for the trajectory tracking process of the full links in USR. Additionally, a nonlinear disturbance observer based on linear matrix inequalities (LMI) is constructed to estimate model uncertainty and external disturbance terms. Incorporating this estimate, a non-singular fast fixed-time terminal sliding mode controller is formulated for USR trajectory tracking, and the fast fixed-time convergence of the controller is validated using Lyapunov stability theory. Finally, simulation results are presented to substantiate the effectiveness of the proposed control scheme.