ADRC-Based cooperative control for unmanned towing operation by multiple DP tugboats: A comparative study of two cooperative controllers

This paper addresses the challenge of coordinated manipulation of an unactuated floating object by multiple tugboats to execute both positioning and towing operation within complex marine environments. We proposes a multi-layer cooperative control algorithm for the multi-tugboat system, including tugboat control level and cooperative control level. At the individual tugboat control level, the dynamic positioning system (DPS) has been enhanced to match the operational behavior of the tugboat, enabling it to handle cable tension and environmental disturbances during transit. The controller incorporates a simple yet effective linear active disturbance rejection control method (ADRC) to counter external disturbances and ensure accurate trajectory tracking without relying on preset models. Moving to the multi-tugboat collaboration level, two distinct control strategies are proposed for towing operations: formation-based (FB) and force-allocation-based (FAB). The FB strategy involves multiple tugboats forming a coordinated formation, with control focused on the formation center to indirectly manipulate the object. On the other hand, the FAB strategy utilizes model predictive control (MPC) for the object to allocate the necessary towing force for each cable, thereby determining the required actions for each tugboat based on the cable model and the needed towing force. Simulation results demonstrate that both control strategies can effectively maneuver the floating object with high accuracy to perform positioning and towing tasks. Nevertheless, due to the variations in their fundamental mechanisms, there are significant discrepancies in towline tension, tugboat utilization, and energy consumption.