Saturated backstepping control for boat with disturbance estimator

This paper address the problem of driving an underactuated ship to following a desired path under current disturbance. Based on a practical ship, the damping force should be nonlinear which related with velocity. The solution consists of a Lyapunov-based trajectory tracking control law and disturbance estimator. The controller solves the boundedness and convergence of position tracking error, which should be approaching to equilibrium states. The estimator calculates an unknown current under an assumption that the disturbance force is constant. Regarding to a practical boat, we demonstrate how Lyapunov based techniques yield a control structure that i) works against unknown current disturbance, i) considers the saturation influence on position error and iii) computes the nonlinear damping force, torque.