A Model-Free Adaptive Positioning Control Method for Underactuated Unmanned Surface Vessels under Unknown Ocean Currents

Aiming at the problem of underactuated unmanned surface vehicles (USVs) performing fixed-point operations in the sea but without dynamic positioning control system. This paper introduces an original approach for positioning control, and the method is named as virtual anchor control method. This method is applicable in slow-varying current environments, and this approach does not need the prior knowledge of current information or vessel motion model parameters, thus offering convenient usability. This method is comprised by four steps. Firstly, a linear concise motion model with unknown disturbances is proposed. Then, the motion planning law is designed by imitating underlying principles of ship anchoring. Next, an adaptive disturbance observer is proposed for estimating uncertainties in the motion model. Last step, based on the observer, a sliding-mode method is used for heading control law designing, and thrust control law is also conducted by applying the Lyapunov method. Finally, numerical simulation experiments with significant disturbances and tidal current variations are conducted, which demonstrate that the proposed method has good control effect and robustness.