Positioning Control for Underactuated Unmanned Surface Vehicles to Resist Environmental Disturbances

In this paper, we present the positioning control problem for underactuated unmanned surface vehicles (USVs) in the presence of unknown external disturbances, such as the wind, waves, and currents. The three control objectives of positioning control for underactuated USVs are, firstly, to retain a predefined constant distance to a look-ahead point, secondly, to regulate the vehicle heading to point at the look-ahead point and, thirdly, to update the position of the look-ahead point to rotate the USVs. Particularly, a new guidance rule is proposed to update the look-ahead point based on the positioning error such that the predefined desired position is maintained and the vehicle heading is counter to the direction of resultant environmental force at the same time. To illustrate the stability of the positioning control system, the error dynamics driven by the component of resultant environmental force is derived and analyzed. In addition to the simulations, experiments were carried out offshore in the face of wind and waves to verify the validity and effectiveness of the proposed method.