UDE based backstepping design for ship autopilot

Designing autopilot for marine surface vessels poses challenging task as a ship is always under the influence of uncertainties which are of structured as well as unstructured in nature and external disturbances due to wave, wind, and ocean currents. In this work, to address the issues, a control strategy based on backstepping technique robustified by Uncertainty and Disturbance Estimator (UDE) is proposed. The design does not require any knowledge of bounds of uncertainties and disturbances and is able to effectively deal with the uncertainties and disturbances. It is shown by simulations how performance of autopilot is enhanced by combining these two methods in the presence of significant external disturbance. The limitation posed by rudder saturation and rudder rate is taken into consideration while designing the controller.