Research On Automatic Berthing Path Tracking Control For Underactuated Ships

In recent years,with the development of intelligent and unmanned ship industry,the control technology of ship automatic berthing has become a hot research topic,and the problems of ship model uncertainty caused by inaccurate hydrodynamic parameters,shore wall effects,invalid rudder effect and disturbance uncertainty in the process of berthing make the berthing control technology become a difficult point in the research.In this paper,the problem of automatic berthing of underactuated ships is studied.The berthing process is divided into two stages,that is the path tracking of the far distance and the berthing near the port.In the research,taking into account the disturbance of the external environment and the unmeasurable state of the ship’s velocity,based on the three-degree-offreedom mathematical model of underactuated ship motion on the water surface,the controllers for path tracking and berthing are designed respectively.The design idea of the controller is to take sliding mode control as the main body and combine the line of sight method,nonlinear observer,adaptive control and backstepping method.The design method is as follows:(1)For the path tracking stage of berthing,assuming that the ship’s motion state is fully measurable and the disturbance of the external environment is known,the line of sight method is used to obtain the desired heading angle from the ship’s desired path,and then the longitudinal propulsion control law and the steering torque control law of the underactuated ship are designed by sliding mode control using the desired heading angle and the given desired speed.Finally,the stability analysis and simulation experiment verify that the underactuatedship can achieve the control goal of tracking the desired path at the desired speed under the action of the designed controller.(2)On the conditions that the velocity component of the ship is unmeasurable and the upper bound of the environmental disturbance is unknown,a nonlinear observer is used to estimate the velocity state of the ship,and the sliding mode control method is used to design the longitudinal propulsion and steering torque control law of the motion system through the estimated value of the velocity,and then the adaptive control strategy is adopted to estimate the upper bound of environmental disturbance,so as to improve the longitudinal propulsion control law and the steering torque control law.Through the overall stability analysis of the system and simulation experiment,it is found that the improved controller can achieve the control goal of the first stage of berthing and make preparations for the subsequent berthing.(3)For the stage of parking in the berthing point,a state transformation is introduced to carry out differential homeomorphism transformation for the mathematical model of the motion of the underactuated ship.On the basis of the transformed mathematical model,the desired virtual control input of the system is obtained by using the backstepping method.Then the longitudinal propulsion and steering moment control law of the ship are designed by sliding mode control using the desired virtual control input.Finally,the controller is improved after considering the adaptive disturbance.According to the overall stability analysis of the system and simulation experiment,the controller can make the underactuated ship reach the berthing point smoothly,and the motion states converge stably.The work of the second stage of berthing is completed.