A composite robust control scheme is proposed by combining a sliding mode controller with an adaptive fuzzy control algorithm to control a 3-DOF underactuated underwater vehicle with model parameter perturbations and environmental disturbances based on the backstepping control method and the Lyapunov stability theory. The adaptive fuzzy control algorithm is employed to compensate for model parameter perturbations and the sliding mode controller is adopted to eliminate the effects of environmental disturbances and approximation errors. A horizontal dynamic model and tracking error equations are established to describe the trajectory tracking control for the underactuated underwater vehicle. The relation between sliding mode control gains and model parameter uncertainties is derived to determine the error eliminating ability of the controller. The convergence and stability of the composite robust controller are demonstrated using the Lyapunov's direct method. The proposed control scheme is simulated for a 3-DOF underactuated underwater vehicle and its efficiency in the error elimination is validated in numerical simulations. Results confirm that the composite robust control law can be used to achieve a robust and preferable control performance for the horizontal trajectory tracking control of the vehicle.

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