Robust adaptive trajectory control for underactuated surface vessel

A nonlinear robust adaptive control strategy is developed to force an underactuated surface vessel to globally track a reference trajectory, despite the presence of uncertain parameters and the environmental disturbances induced by wave, wind and ocean-current. The position variables cannot be defined in the body-fixed frame. Hence, the desired motion is specified in the inertial reference frame by the surge and sway velocities and accelerations. According to the underactuated nature of the vessels, the sway axis is not directly actuated. In order to achieve the desired formation, the first-order sliding surface in terms of surge motion tracking errors and the second-order sliding surface in terms of lateral motion tracking errors were introduced. The control law is based on global sliding mode control and adaptive control. The robust adaptive tracking controller was proposed to solve the uncertain time-varying disturbance induced by harsh environment. Numerical simulations are provided to demonstrate the validity of the proposed control law.