Trajectory Tracking Control for a Hybrid Underwater Vehicle in Free-Flying and Crawling Operation Modes

A hybrid underwater vehicle (HUV) equipped with thrusters and tracks has the great ability of free flying in the water and crawling on the surfaces of underwater structures, making it highly effective for inspecting underwater structures and cleaning hulls. In this article, a novel cascade control strategy that consists of a kinematic controller and a dynamic controller is proposed for trajectory tracking control of HUVs in free-flying and crawling operation modes. Based on the tracking error, a model predictive control (MPC)-based kinematic controller is designed for both free-flying and crawling modes. To improve the tracking accuracy, an improved snake optimizer is used in the optimization process of MPC to derive the expected optimal velocity. Then, the error between the expected optimal velocity and the real velocity is used as the input of the dynamic controller. To compensate for external disturbances, such as ocean currents and waves, a dynamic controller composed of a nonlinear disturbance observer and an integral sliding mode control (ISMC) is adopted to optimize the thrust force for trajectory tracking in free-flying mode. In addition, a dynamic controller composed of a radial basis function neural network and an ISMC is established to reduce the impact of slipperiness in crawling mode. The simulation results show that the proposed cascade trajectory tracking control strategy for HUVs in free-flying and crawling modes can improve the trajectory tracking accuracy and robustness to unknown dynamic factors.