Global robust adaptive path following of underactuated ships

We propose a method for designing a global robust adaptive controller that forces an underactuated ship to follow a reference path under both constant and time-varying disturbances induced by waves, wind and ocean-currents. Both linear and nonlinear damping terms are included to cover both low- and high-speed applications. All nonlinear damping coefficients are assumed unknown but lie in a known compact set. The new results are derived using a choice of an appropriate body-fixed frame origin, a smooth approximation of nonsmooth damping terms, several nonlinear coordinate changes, the backstepping technique, and utilization of the ship dynamic structure. Experiments on a model ship illustrate the results.