Parent-child underwater robot-based manipulation system for underwater structure maintenance

Abstract

This study proposes a parent–child robot-based underwater manipulation system for underwater structure maintenance comprising two underwater vehicles with split roles. The larger autonomous underwater vehicle (AUV) acts as the base to transport and control a tethered child robot that is deployed to perform actual manipulation tasks. Conventional AUVs face several challenges when performing manipulation missions because of their large and massive body and the complexity of dynamic control with a rigid arm manipulator in the precise hovering state. To address this problem, we proposed an underwater manipulation system that uses a large parent AUV with high-performance sensors to navigate to the desired location and a child robot with high mobility to perform manipulation tasks. In this study, a parent–child robot-based manipulation system was developed and mathematically modeled considering hydrodynamics. Based on the system model, a nonlinear time delay (TD) controller was applied to the child robot to ensure robustness of the movement, and complementary sensing methods were proposed according to the observation distance. Subsequently, we detailed the mission process such that the manipulation tasks could be performed autonomously. To implement and verify the proposed method, we conducted three-dimensional (3D) simulations, water tank experiments, and sea trials. In the 3D simulation and water tank experiments, we evaluated the position control using a TD controller during a specific disturbance and performed an operational evaluation of the parent–child system. In the water tank, we performed the operation of the child robot with the tether length adjustment of the winch system and the manipulation task. In the sea trial, the predefined mission was successfully completed, which demonstrated the feasibility of properly operating both the robots. The results of the experiments demonstrated the high potential and capabilities of the proposed manipulation system.