Resident autonomous underwater vehicle: Underwater system for prolonged and continuous monitoring based at a seafloor station

This paper proposes a resident autonomous underwater vehicle (AUV) system for monitoring around an underwater infrastructure using an AUV and a seafloor station (SS), where the SS serves as the positioning reference and a battery charging station for the AUV. Docking to the SS is a key technology in the system. The proposed method utilizes acoustic and visual positioning based on the SS. To increase the robustness of the method, further methods for visual positioning using limited information and autonomous judging of docking success are implemented. After completing docking, the AUV charges its batteries wirelessly. The process of balancing battery consumption and the amount of charge is formulized to enable prolonged and continuous surveying. The proposed system was evaluated in sea and tank environments using the AUV Tri-TON 2 (172) and a test-bed SS. In the sea experiments, TT2 succeeded in docking to the SS under the condition of low visibility which was within 2 m and complicated sea current induced by waves which was around 0.5 m/s. To enhance the success rate of docking, a control method considering the resistance due to sea currents is formulated. It was verified that this control method can ensure a position of TT2 maintained with 0.1 m vibration throughout the control simulation using the velocity of sea currents obtained in the sea experiments. In the tank trials, TT2 succeeded in operating continuously during three days while performing autonomous docking to the SS and charging its batteries. (C) 2019 Elsevier B.V. All rights reserved.