Robust dynamic positioning of ships with disturbances under input saturation

In the presence of unknown time-varying disturbances and input saturation, this paper develops a robust nonlinear control law for the dynamic positioning (DP) system of ships using a disturbance observer, an auxiliary dynamic system, and the dynamic surface control (DSC) technique. The disturbance observer is constructed to provide the estimates of unknown time-varying disturbances, the auxiliary dynamic system is employed to handle input saturation, and the DSC technique makes the designed DP control law be simple and easy to implement in practice. It is proved that the designed DP robust nonlinear control law can maintain ship's position and heading at desired values, while guaranteeing the uniform ultimate boundedness of all signals in the DP closed-loop control system. Finally, simulations on a supply ship are carried out to demonstrate the effectiveness of the developed DP control law.