Research On Control Algorithm Of Tracking For Underactuated Ships

The character of underactuated surface ships is that it has more degrees of freedom to be controlled than the number of independent actuators. Generally speaking, a surface vessel doesn't have an independent actuator in the sway, therefore it has difficulty in tracking control and dynamic positioning control, and it can not get a good performance by general method. Because the tracking control have some advantages, such as boasting traveling economy, improving the security and decreasing expenses, researchers have put more attentions on it. It is also the main problem we will study. The main works of this paper are as follow:Firstly, based on some reasonable assumptions, a 3-DOF maneuvering motion mathematic model of underactuated surface vessel has been established and analyze the control characters of underactuated surface ships. In order to guarantee correctness during designing controller, the simulation of straight and turning movement in the still water were accomplished.Sencondly, during designing tracking controller, the first step is to build the mathematic model between the virtual ship and the controlled ship. A full-states feedback controller is proposed by backstepping technology and Lypunov's direct method. In order to prove the validity of the controller, the simulation tests are carried out without environmental disturbance and with limitary environmental disturbance by software instructions.Based on the theroy of passivity, a tracking controller is proposed and comparing to the controller designed by backstepping under the same simulation conditions.Thirdly, base on cascaded theory, divide the tracking error system into two subsystems, and then design the control law to stabilize error states respectively. In order to investigate the effectiveness of the controller, the simulation by three control laws compares to each other in the same simulation conditions.Finally, the simulation tests illustrate that duing to the persistent excitation conditons of angular velocity, passivity and cascade controllers only achieve arc tracking. Although the control objectives can be achieved eventually, but the convergence rate of tracking error is slow, it needs improve the control performance. Comparatively speaking, the tracking controller designed by backstepping has a good performance and keeps the controlled vessel following reference path precisely even if existing limitary environmental disturbance. The controller has the property of robustness.