STABILIZATION AND TRACKING OF UNDERACTUATED SURFACE VESSELS IN RANDOM WAVES WITH FIN BASED ON ADAPTIVE HIERARCHICAL SLIDING MODE TECHNIQUE

Trajectory tracking and roll stabilization are both vital practices in ship motion control. Trajectory tracking is a kind of low-frequency control, while roll stabilization by means of fins is a kind of high-frequency control. However, they have been studied separately previously; most tracking control of underactuated surface vessels in the previous studies do not account for roll stabilization by means of fins. In reality, however, they are an integral system. In this paper, a simple control strategy is proposed to achieve trajectory tracking and fin roll stabilization simultaneously. Four degrees of freedom derived from a six degrees of freedom mathematical model of a surface vessel is considered, including surge, sway, roll and yaw. Surge force, roll moment and yaw moment are considered as control inputs, while position, yaw angle and roll angle are controlled. The number of control inputs is fewer than the outputs to be controlled. Therefore, we are dealing with an underactuated problem. An adaptive hierarchical sliding mode control technique is employed to deal with the underactuation. Stabilization of underactuated surface vessels is studied as a special case. Random waves are applied to test the robustness of the designed controllers. Lyapunov stability theory is used to show the stability of closed-loop system. The simulation results verify the effectiveness of the proposed strategy.