Variable tension motion control for a ship towing system with event and time-triggered mechanisms

The ship towing system,consisting of a tugboat,a towing cable,and a towed ship,is widely used in port navigation,emergency towing,and offshore equipment transportation. The objective of this paper is to enhance the stability and efficiency of autonomous navigation control for a Passive Steering Ship Towing System (PSSTS),in which the towed ship loses active steering capability due to steering-gear failures or operational requirements and must rely on the tug for heading control. In order to achieve this objective,first a dynamic model for the PSSTS incorporating cable elasticity based on the MMG (Maneuvering Modeling Group) framework is established. A hierarchical PID control strategy is then proposed,decomposing the overall control objectives into heading-keeping control and trajectory tracking control. Furthermore,a novel variable tension control method has been developed to mitigate speed loss caused by constant towline tension. This method is triggered by both events and time. Finally the effectiveness of the proposed method is verified by simulations on a customized platform. Results show that the proposed method improves navigation efficiency and reduces tension fluctuation,while maintaining control accuracy.