A new method for obstacle avoidance of underactuated unmanned surface vessels is presented which combines trajectory planning with real-time tracking control. In this method, obstacles are approximated and enclosed by elliptic shapes which represent the stable limit cycle solution of a special class of ODEs (ordinary differential equation). The vessel trajectory at any moment is defined by the ODEs whose solution is the limit cycle defining the obstacle immediately on its path to the target. When no obstacle remains on the vessel's path, the trajectory is defined by exponentially stable ODEs whose solution is the target trajectory. The planned trajectories are tracked by the vessel through a sliding mode control law which is robust to environmental disturbances and modeling uncertainties and can be computed in real time. One advantage of the method is that it allows for dynamic (moving and rotating) obstacles as well as a moving target. Another advantage is that only the current information about the obstacles and the target are required for real-time trajectory planning. Since the vessel current position is used as feedback to redefine the limit cycle trajectories, the method is also robust to large disturbance.
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