Double Closed-loop Control with Input Saturation for DP Vessels based on Disturbance Observer

To control the operating vessel by the dynamic positioning (DP) system with input saturation, a sliding mode control strategy, based on the double closed-loop controller and disturbance observer (DDSMC) for the DP vessel is proposed. The trajectory tracking of DP vessel is controlled by the position and velocity loop controller, and the integrated disturbance caused by environment and the saturated input is estimated by the disturbance observer. Based on the established three-degree-of-freedom kinematics and dynamic model of DP vessel in horizontal, the controller of position loop and loop is designed based on the integral sliding mode surface, and the stability of position loop is achieved through the fast convergence of velocity error. The observer is designed based on the fast terminal sliding surface, and the system is stabilized through compensation. The stability of the system is analyzed with Lyapunov principle, and the proposed control strategy is verified by digital simulation. The results show that the designed disturbance observer can realize the observation of the integrated disturbance, and the DDSMC control strategy can improve the dynamic performance of the system and reduce the workload of the propeller.