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An improved NMPC-NDOB scheme for trajectory tracking of unmanned surface vessel

This study investigates the trajectory-tracking problem of fully actuated unmanned surface vessels (USVs). A three-degree-of-freedom (3-DOF) dynamic model was used with three control variables: surge force, sway force, and yaw moment, where the model uncertainties, environmental disturbances, and actuator saturation were also considered. An improved nonlinear model predictive control (NMPC) method was investigated to improve the control performance in the trajectory tracking of USVs, and a nonlinear optimization problem based on the finite horizon was developed to minimize the divergence between the actual and the reference states. Moreover, a nonlinear disturbance observer (NDOB) was employed to estimate the environmental disturbance, which could enhance the robustness of the controller. Furthermore, an event-triggered mechanism was also employed to reduce the computational frequency of NMPC. Simulations of slide mode control, NMPC, and improved NMPC were performed in the trajectory-tracking control. Besides, the simulation results verified the effectiveness and robustness of the proposed improved NMPC-NDOB scheme.