Autonomous docking of hovering type AUV to seafloor charging station based on acoustic and visual sensing

Autonomous underwater vehicles (AUVs) have advantage that they do not require tether cables or human control. AUVs have recently been used for seafloor observation. However, AUVs have limitation of their batteries. AUVs must be recovered before they have run out of batteries. In order to charge batteries of AUVs, underwater charging systems have attracted a lot of attention. We have developed the seafloor charging station for hovering type AUVs. In the proposed system, AUVs dock to the seafloor station autonomously according to both acoustic and visual landmarks attached on the seafloor station. If AUVs are far away the seafloor station, AUVs approach the seafloor station by acoustic positioning device. When AUVs get close enough to observe the visual landmark, AUVs approach landing point of the seafloor station by visual localization in addition to the acoustic positioning device. The visual landmark consists of 4 LED markers. AUVs observe the LED markers by forward looking camera and calculate the position of themselves relative to the seafloor station. After reaching the landing point, AUVs dock to the seafloor station by descending. When the docking is completed, the seafloor station starts electric power transmission by our newly developed non-contact charging devices. The performance of the proposed system was verified through a series of tank and sea experiments. In this paper, the results of the autonomous docking and battery charging experiments using a hovering type AUV Tri-TON 2, are described.