For an underactuated unmanned surface vehicle (USV), time-varying external disturbances affect the accuracy of trajectory tracking. To ensure trajectory tracking accuracy, in this paper the reduced-order extended state observer (ESO) and the super-twisting second-order sliding mode controller are adopted. The ESO is designed to address the unknown time-varying sideslip angle in the guidance law. Additionally, the super-twisting technology contributes to a reduced chattering effect of the sliding mode. The Lyapunov method is used to analyze the stability of the tracking system and prove that the proposed controllers can ensure the convergence of the tracking errors in finite time. The simulation experiments of the proposed super-twisting sliding mode control (STSMC) and adaptive sliding mode control (ASMC) methods are compared. The results show that the STSMC method enables the USV to complete the task of the reference trajectory tracking. The chattering of the STSMC is also significantly reduced compared to that of the ASMC.

In order to improve the persistent operation capacity of autonomous underwater vehicle (AUV), autonomous recovery system draws more and more attention in recent years. A hybrid scheme combined with navigation, guidance and control is proposed to complete the function of autonomous recovery of AUV in the paper. Based on the hybrid scheme, the autonomous recovery is firstly transformed into the trajectory tracking problem, and the Dubins path planning method is adopted to build the threedimensional(3D) motion path of AUV. Secondly, the virtual target guidance strategy is introduced to guide the AUV to track the desired path. Finally, a non-singular terminal sliding mode controller is designed to control the AUV. Due to the existence of model inaccuracy of AUV and external disturbances from the environment, a finite-time disturbance observer is adopted to estimate the disturbance and uncertainty in the system. Numerical simulation is conducted to verify its performance. The simulation results show that a 3D motion path can be planed and the AUV can track the motion path well.

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