Passive acoustic and optical guidance for underwater vehicles

Long term and intermittent monitoring of a certain area will be one of future uses of AUVs. Since AUVs have limited battery capacity, they need their batteries to be charged intermittently by docking to a charger station. In this scenario, AUVs need to have the capability to dock by themselves. Another case where docking capability will be useful is a submerged recovery by a support vessel. When AUVs are on the surface waiting to be recovered, waves and winds make the operation dangerous. AUVs might collide with a support vessel and be damaged. Thus, a docking cage that is hung from a crane of the support vessel and submerged to recover AUVs might be one of the solutions for safety. This paper proposes a docking method for hovering AUVs which usually have capabilities to move in surge, sway, heave and yaw motions. The proposed method uses both acoustic and optical systems to guide the vehicle. For far distances an acoustic positioning system is used to guide the vehicle. When the vehicle enters a certain area where the vehicle's camera can see the docking target, an optical camera positioning system is used to guide the vehicle into the docking cage. The proposed method has been implemented in a small hovering AUV and tested in a tank. In this paper, only the optical guidance system was tested using the vehicle, and the acoustic system was tested separately due to the vehicle's payload capacity. The vehicle switched its behavior automatically to aim the docking station and could enter the docking cage in several entry patterns.