Nov 062010

Tethys floating at the sea surface in Monterey Bay (Image: Todd Walsh copyright 2010 MBARI)

When it comes to exploring the murky depths of the oceans, autonomous underwater vehicles (AUVs) have become increasingly important over the past decade.

These vehicles generally fall into one of two groups: propeller-driven vehicles such as Snookie that can travel fast and carry lots of instruments, but are limited to expeditions of just a few days, and “gliders,” which can stay at sea for weeks or even months at a time, but are slow. Engineers have combined the best of these two approaches to create a new long-range AUV (LRAUV) that can travel rapidly for hundreds of kilometers, “hover” in the water for weeks at a time, and carry a wide variety of instruments.

The engineers from the Monterey Bay Aquarium Research Institute (MBARI), which earlier this year used an AUV to study the Deepwater Horizon oil spill, spent four years designing, building and testing the new LRAUV dubbed Tethys– the name of an aquatic sea goddess from Greek mythology. The new robot spent most of October crisscrossing Monterey Bay as part of MBARI’s CANON experiment whose primary goal is to develop methods of tracking ocean features as they move through the ocean, and for studying how microscopic plankton in the water change over time.

During the October experiment, oceanographers used Tethys to track patches of microscopic algae that were carried around the bay by currents. The robot showed that it could travel fast enough to buck the currents, but could also go into “hover mode” to drift with the currents when needed. In “high-speed mode” the LRAUV can travel up to one meter per second (2.25 mph), which is about four times faster than most underwater gliders. However, it can also travel long distances at around half this speed.

Designed for efficiency

To maximize energy efficiency, Tethys’ hull, motor and propeller were computer designed and tested to minimize drag and maximize efficiency of propulsion. Like a fish, it can control its buoyancy and the angle at which it “swims” through the water. Like many laptops, the robot also incorporates sophisticated power-saving software that monitors what systems are being used and turns those systems off when they’re not in use.

Since most robotic vehicles are slightly buoyant, they will float to the surface if the robot’s system’s fail. However, the variable buoyancy systems employed in Tethys means it can make itself neutrally buoyant. If it were to go dead in that state it would remain drifting somewhere below the surface making it potentially difficult to recover. To mitigate the risk of this happening the MBARI engineers designed numerous fail-safe systems into the robot, including some with their own independent power supplies.

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