Active Rudders for Ship Berthing System Design with Flexible Prescribed Performance

Automatic towing operations for non-powered ships at sea constitute a fundamental step toward automating the towing mission at harbor. This paper is devoted to solve one of the sea maneuvering problems that uses an automatic leader tug. The proposed maneuver involves implementing of robust escorting system (also called towing system) of partially non-powered towed cargo ship for an automatic berthing. To enhance the maneuverability of the towed ship, its rudder system is made active throughout the mission. To address the problem of rejecting the time-varying disturbances induced by wind, current and system parametric uncertainties, an adaptive finite-time sliding mode disturbance observer is first proposed. Then, a composite controller with flexible prescribed performance is developed based on a novel unified towing tracking error, which can ensure the geometric state variables and its corresponding velocity meet the prescribed performance constraints at the same time. Stability analysis is conducted to prove finite-time convergence of the towing tracking errors. Finally, numerical simulations validate the proposed approach.