Objective To improve the overall operational capability of autonomous underwater vehicles (AUVs) and address the critical issue of collision risks during the dynamic docking process with towed recovery docks (TRDs), this study conducts a systematic investigation into the collision mechanisms and control strategies of the docking system. Reliable docking and recovery technology is essential for extending AUV operational endurance, enhancing data transmission efficiency, and enabling long-term underwater deployment. However, in real marine environments, limitations in sensor accuracy, external disturbances, and the dynamic response of the docking system often lead to unavoidable contact or collision between AUVs and TRDs, which may result in mission failure or structural damage to the equipment. Therefore, this study aims to clarify the influence of key initial operating conditions on docking-induced collisions and to propose an effective control strategy for optimizing the dynamic docking process, thereby providing theoretical and technical support for the engineering application of AUV towed recovery systems.
Methods Based on dynamic analysis, a simulation model incorporating contact and collision dynamics was developed using the ADAMS-MATLAB co-simulation platform. First, rigid body dynamic models of AUV and TRD were constructed. The AUV model accounts for gravity, buoyancy, viscous hydrodynamic drag, inertial hydrodynamic drag, thrust, and environmental disturbances. The TRD model adopts a frame-cage structure with a bell-mouth guiding cover, and a discrete flexible body method is used to model the towing cable. Subsequently, a nonlinear contact model based on Hertz theory was employed to calculate the collision forces between AUV and TRD, which more accurately captures the transient impact characteristics of the collision process compared with the linear contact model. On this basis, the effects of initial operating conditions including eccentric angle, eccentric distance, relative initial velocity, and mother vessel acceleration on docking collisions were systematically analyzed using the control variable method. To mitigate attitude disturbances induced by collisions, a multi-stage coordinated control strategy based on PID control was proposed, which realizes active attitude adjustment of AUV by switching control modes across different docking phases.
Results The simulation results indicate that increases in eccentric angle and eccentric distance primarily prolong the docking time while exerting only a limited influence on the peak collision force, which remains within the range of 1 000–2 000 N under most working conditions. In contrast, increasing the relative initial velocity can shorten the docking time but significantly amplifies the peak collision force, showing a positive correlation between them. Further analysis of mother vessel acceleration reveals the complex, non-monotonic relationship between collision force and docking efficiency. As the mother vessel's acceleration increases, the amplitude of the TRD attitude variations intensifies, leading to greater uncertainty in the collision position, and the peak collision force reaches its maximum value when the acceleration is 0.2 m/s². Moreover, the proposed multi-stage coordinated control strategy enables effective post-collision attitude adjustment of the AUV. In the case of uniform motion of the mother vessel, the strategy reduces the peak collision force by up to 74.5% and shortens the docking time from 7.56 s to 5.93 s. Even under the complex working condition of uniform acceleration of the mother vessel, the peak collision force is reduced by 19.6%, and the docking time is shortened by 16.7%, effectively optimizing the dynamic docking process and ensuring both docking safety and efficiency.
Conclusion This study systematically clarifies the effects of key initial operating conditions on the docking collision between AUV and TRD. The research findings indicate that controlling the initial eccentric angle and eccentric distance can improve docking efficiency, whereas adjustments to the relative initial velocity and mother vessel acceleration require a careful balance between collision risk and docking speed. The proposed multi-stage coordinated control strategy can significantly reduce the peak collision force while maintaining docking efficiency, achieving reductions of 14%–74.5% under different working conditions. This strategy exhibits superior robustness and stability compared with the traditional position tracking control strategy, effectively addressing the limitations of passive control methods that rely solely on the dock structure. Overall, this study provides a reliable simulation basis and design reference for the design and stability control of AUV towed recovery systems. In addition, the research framework and methods provide guidance for the collision analysis and control in similar underwater docking systems.
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