Nonlinear Model Predictive Control for trajectory tracking and collision avoidance of underactuated vessels with disturbances

This paper presents a combined Nonlinear Model Predictive Control (NMPC) for position and velocity tracking of underactuated surface vessels, and collision avoidance of static and dynamic objects into a single control scheme with sideslip angle compensation and environmental disturbances counteraction. A three-degree-of-freedom (3-DOF) dynamic model is used with only two control variables: namely, surge force and yaw moment. External environmental forces are considered as constant or slowly varying disturbances with respect to the inertial frame, and hence nonlinear for the body frame of the vessel. Nonlinear disturbance observer (NDO) is used to estimate these disturbances in order to be fed into the prediction model and enhance the robustness of the controller. A nonlinear optimization problem is formulated to minimize the deviation of the vessel states from a time varying reference generated over a finite horizon by a virtual vessel. Sideslip angle is considered in the cost function formulation to account for tracking error caused by the transverse external force in the absence of sway control force. Collision avoidance is embedded into the trajectory tracking control problem as a time-varying nonlinear constraint of position states to account for static and dynamic obstacles. MATLAB simulations are used to assess the validity of the proposed technique.