Development of a robust non-linear observer for dynamic positioning of ships

Abstract

A robust non-linear observer, utilizing the sliding mode concept, is developed for the dynamic positioning (DP) of ships. The main advantage of the proposed observer is its robustness. The observer is based on work by Fossen and Strand, providing estimates of linear velocities of the ship and bias from slowly varying environmental loads. It also filters out wave frequency motion to avoid wear of actuators and excessive fuel consumption. In particular, the observer structure with a saturation function makes the proposed observer robust against neglected non-linearities, disturbances and uncertainties. A non-linear output feedback controller is derived on the basis of the developed observer using the observer backstepping technique, and the global stability of the observer and control law is shown by Lyapunov stability theory. A set of simulations was carried out to investigate the performance of the proposed observer for dynamic positioning of a ship.