The Heading Control Of POD-driven Ship Using Sliding Mode Control

Podded propeller system is the most widely concerned in ship electric propulsion system. Its motor and propeller connected directly, and it can achieve 360 degrees rotation in horizon. Besides, it combines the functions of rudder and traditional propeller, improving the hydrodynamic characteristics and maneuverability of the ship. The podded propeller has achieved a breakthrough development in the electric propulsion system.The function of the ship heading control system is to make the course change or stay the same with the disturbances. The steering performance and the stability on heading are the evaluation criterions of its maneuverability. The ship motion system is a nonlinear system due to the navigation speed change and the external disturbance on the voyage. At present, most of the ship heading autopilots all ignore the rudder rotation characteristics and the autopilot design is carried out by using the linear or nonlinear model. But considering the pod rotation angle changes is closely related to all directions force and moment, the rudder rotation characteristics should be taken into account in the vessel motion model in the design of ship course controller.This paper is based on the project ―Manoeuvring study of the podded propulsion system‖. Based on the MMG model, the model proposed by the Ship Manoeuvring Mathematiccal Model Group, the three degree of freedom of podded vessel has been established in this paper. Then its thrust and moment and the hydrodynamic forces and moments acting on the hull such as wind, wave and other external environment disturbing force and moment has been analyzed respectively. In order to verify the feasibility of the ship model, the speed and turning simulation was carried out, and the data have also been compared with the real ship test data. The heading model with certain uncertainty was established by decoupling aforementioned mathematical model in addition to the mathematical m odel of pod steering angle.The sliding mode controller based on the backstepping is designed for the nonlinear system in this paper. The Lyapunov stability theory is used to demonstrate and the stability on ship course is provided. The sliding mode control algorithm provides a systematic solution to diminish the uncertainty caused by modeling and keep the stability of system, based on the backstepping which has obvious advantages in the design the high-order system with uncertain parameters and structure: the system needn’t change to normal form, and the virtual controller is only connected to the subsystem. The simulation for the heading control of podded-driven ship proves that this controller has a high robustness. Then adaptive control is adopted to correct the uncertainty of the mathematical model in order to simplify the question of feedback linearization. The simulation proves that the adaptive sliding mode controller also has a good robustness.