Abstract
A robust nonlinear controller has been designed to control the surge speed and the heading angle of a marine surface vessel. The control actions are carried out through the propeller and the rudder. Moreover, a nonlinear observer has been devised to accurately estimate the surge speed and the yaw angle and their time derivatives. Both the controller and the observer are designed based on a reduced-order model of the ship. However, their performances have been assessed on a six degree-of-freedom ship model, which accounts for the wave excitation, retardation forces, nonlinear restoring forces, wind and sea-current resistive loads. Furthermore, the model accounts for the physical limitations of both the rudder and the ship propulsion system. The simulation results demonstrate the capability of the integrated controller-observer system in providing a good tracking characteristic of the ship in spite of significant modeling imprecision and environmental disturbances.