Research On Ship Heading Detection And Dynamic Positioning Control Method

The dynamic positioning system resists against the disturbance by wind, wave and current by the use of thrust system on the ship, and then maintains the desired position and heading on the sea. This system has advantages that are not restricted by the depth of water and enter or retreat rapidly, and achieve the precise flexibility of ships. To many vessels which carry on the operation on the sea, it is necessary to use the dynamic positioning system. In order to improve the performance of dynamic positioning system, research on heading detection and dynamic positioning control method of ship are carried out in this paper.Magnetic heading measure system is a device which can measure heading angle of ship by using the earth's magnetic field magnetic. Its accuracy directly affects the control precision of ship dynamic positioning system. As the core sensor of magnetic heading measure system, the error of three axis magnetometer is a primary source of magnetic heading measure system's error. Based on the analysis of error sources on the three axis magnetometer, a linear error model of three axis magnetometer has been built up in this paper. And on the basis of the error model, three kinds of magnetic field correction algorithms are designed to be used in different environments. They are the four elements method, seven elements method and ten elements method. These methods are benefit to research on the control method of dynamic positioning system.The movement of vessels on the sea is usually divided for low frequency movement that composes of wind, current, second-order wave drift and high frequency movement that composes of first-order wave-induced disturbances. As the performance of high frequency movement is cyclical shocks and will not lead to the influence of average position, in order to avoid unnecessary energy waste and the thruster system wear, it usually isolates the low frequency signal to control from comprehensive position signal of vessels. So, filter plays a very important role in the dynamic positioning system. This paper is based on dynamic positioning ship. A simple motion mathematical model of the vessel is established and educed according to some reasonable supposition. An environment disturbance estimate model is established for the slowly varying environmental forces and moments, such as second-order wave drift, sea current, and integrates this model into dynamic positioning system model. We design a nonlinear estimate filter. The proposed estimate filter produces noise-free estimates of the low-frequency position, the velocity and the slowly varying environmental disturbances.In order to solve the point stabilization control problem of the underactuated surface vessels, a switch control method based on time-invariant state feedback is proposed. First, a global diffeomorphism transformation is introduced to decoupling the strong multivariable coupling system by transfer the ship's position to a new coordinate frame rotating with the ship. Then the dynamic properties of the new dynamic system are analyzed by using the Lyapunov direct method. The analysis result allows us to consider a low-level order nonlinear subsystem to realize the whole point stabilization control of the underactuated surface vessel, which simplifies the controller design, and a switch state-feedback control law is proposed to achieve the global asymptotically stable by using the backstepping approach. Finally, the global point asymptotic stabilization control of underactuated surface vessel is solved by time-invariant feedback switching control method. The simulation experiments have shown that controller has good effect in different sea conditions.