Robust adaptive path following of underactuated ships

Robust path following is an issue of vital practical importance to the ship industry. In the paper, a nonlinear robust adaptive control strategy is developed to force an underactuated surface ship to follow a predefined path at a desired speed, despite the presence of environmental disturbances induced by wave, wind and ocean-current. The proposed controller is scalable and is designed using Lyapunov´s direct method and the popular backstepping and parameter projection techniques. Whilst proving closed-loop stability, we obtain a new stability result for nonlinear cascade systems with non-vanishing uncertainties. It is shown that our developed control strategy is easily extendible to situations of practical importance such as parking and point-to-point navigation.