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Sea Grant tests AUVs' docking mechanisms

MIT Sea Grant's Autonomous Underwater Vehicles (AUV) Lab and collaborators working in Buzzards Bay successfully ran the first mission with a new fleet of AUVs.

A month of field operations with four Odyssey IIB vehicles off the coast of Woods Hole, part of the Autonomous Ocean Sampling Network (AOSN), began in mid-March. That project will combine a fleet of AUVs with a network of other tracking and communications devices to provide high-quality monitoring in coastal waters and in the deep ocean.

Along with the AUV Lab, which constructed five AUVs with funding from the Office of Naval Research, participants included the Woods Hole Oceanographic Institute, NRaD (a San Diego-based Naval Research organization), and Electronic Design Consultants (EDC), a private firm based in Chapel Hill, NC.

Deep-diving, low-cost AUVs will allow researchers to study closely areas that would otherwise be unreachable. Numerous vehicles working together could gather data over a wide area for periods of up to several months in changing tides, currents, waves and weather. Wide-ranging applications will include studies of marine populations and habitats, temperature and salinity, underwater sound and pollution.

In order to transmit data and recharge its batteries, an AUV must be able to guide itself into a docking station at a buoy far off at sea. In Buzzards Bay, MIT Sea Grant's AUV Lab and collaborators tested three different docking technologies under similar conditions. The AUV Lab also tested acoustic communication between the research ship and the vehicles.

The homing system designed by NRaD uses an optical sensor, whereby an AUV finds its docking station by honing in on a light source. The submarine swims into the station, which catches the vehicle's nose much like a catcher's mitt. With EDC's system, the docking station emits electromagnetic fields that the AUV follows. "The tests went very well," said EDC owner Mike Feezor, who first tried the submarine technology out in his backyard with an electric toy car.

The acoustic system developed by WHOI researchers uses a stationary beacon that sends out "pings." By computing its position relative to the beacons, the AUV navigates toward its docking station. Unlike EDC's and the Navy's "catcher's mitt" approach, the WHOI dock consists of a pole to which the vehicle latches.

One of the biggest challenges will be recharging the AUVs' batteries underwater because recharging in a closed compartment can trigger gas emissions and explosions. Thus, whether the researchers generate power at the docking station via wind, waves or advanced primary batteries, they will need to devise new approaches for battery management and recharging.

Findings from the Buzzards Bay tests will help the MIT Sea Grant researchers select one homing system, or perhaps a hybrid, for upcoming AOSN missions. In June, researchers will travel to the Haro Strait off the coast of British Columbia to study how tidal fronts form and dissipate. In Monterey Bay this fall, the AUV Lab will participate in studies with NRaD to demonstrate deep-water docking techniques.

A version of this article appeared in MIT Tech Talk on May 1, 1996.

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