Underwater Platform for Long-Term Observation Using a Cable-Restricted Underwater Vehicle and Its Design Concept for Trajectory Planning

Abstract

Long-term and repeated observation of shallow seafloor environments is required to evaluate temporal changes in seagrass meadows and other coastal ecosystems. In this study, we propose a cable-restricted observation system consisting of an underwater vehicle, a tether cable, and a seafloor station equipped with a reel. The vehicle can repeatedly cruise over a designated area along nearly the same trajectory using the cable constraint. A design flow was developed to determine system parameters, including the reel radius, the number of cable turns, and the thrust required for the vehicle. In addition, a vehicle position model based on an involute curve and a motion model considering cable deflection were introduced. To evaluate the proposed system and trajectory model, water tank test and a sea trial were conducted using a 2.5 m tether cable. The radii of reels were 0.084 m and 0.775 m in the water tank test and sea trial, respectively. The experimental results confirmed that the vehicle could repeatedly cruise along approximately the same trajectory by winding and unwinding the cable. In the sea trial, the maximum differences between the measured and estimated positions were 0.541 m in the x-direction and 0.643 m in the y-direction. These results demonstrate the feasibility of the proposed system for repeated observation of a specified seafloor.