Improved Output Feedback Control for Underactuated Surface Vehicles via Fractional-Order Disturbance Observer

This paper studies the output feedback control for underactuated surface vehicles with uncertainties and environmental disturbances. A novel finite-time state observer is proposed by using the three-order Levant differentiator to guarantee the Lyapunov finite-time stability of the estimation error and its first derivative. Then, a fractional-order disturbance observer is established to estimate the uncertainties and the disturbances in the model. Combining the backstepping control with the command filter techniques, the output feedback controller is developed for the system. It is shown that the tracking error of the closed-loop system asymptotically converges to zero, and the tracking performance is improved. Numerical simulation illustrates the advantages of the proposed method.