Abstract
A nonlinear six degree-of-freedom dynamic model is presented for a marine surface vessel. The formulation closely follows the current literature on ship modeling. It considers the effects of inertial forces, wave excitations, retardation forces, nonlinear restoring forces, wind and current loads along with linear viscous damping terms. The capability of the model is shown through its prediction of the ship response during a turning-circle maneuver. The ship model is used herein as a test bed to assess the performance of the proposed controller. The present study assumes that the ship is fully actuated and all state variables of the system are available through measurements. A nonlinear robust controller, based on the sliding mode methodology, has been designed based on a reduced-order version of the ship model. The latter accounts only for the surge, sway and yaw motions of the ship. The initial simulation results, generated based on the reduced-order model of the marine vessel, demonstrate robust performance and good tracking characteristics of the controller in the presence of structured uncertainties and external disturbances. Furthermore, they illustrate the adverse effects of the physical limitations of the propulsion system on the controlled response of the ship. Next, the same controller is implemented on the six degree-of-freedom model of the ship. The simulation results reveal tracking characteristics of the controller that are similar to those observed in the initial results, in spite of significantly larger modeling uncertainties.