Undersea Robot Goes the Distance

Wednesday, December 8, 2010 @ 06:12 PM gHale

Autonomous underwater vehicles (AUVs) have become increasingly important in oceanographic research along with the potential to use as a security device.
A new long-range AUV (LRAUV) can travel rapidly for hundreds of kilometers, “hover” in the water for weeks at a time, and carry a wide variety of instruments.
The new robot, called Tethys, spent most of October crisscrossing Monterey Bay as part of an experiment to track and monitor patches of microscopic algae carried around the bay by currents. During this experiment, the robot showed it could travel fast enough to buck the currents, but could also go into “hover mode” to drift with the currents when needed.

Tethys  can travel fast enough to buck the currents, but also hover and monitor with the currents when needed.

Tethys can travel fast enough to buck the currents, but also hover and monitor with the currents when needed.

In “high-speed mode” the LRAUV can travel up to one meter per second (2.25 miles an hour), about four times faster than most underwater gliders. However, it can also travel long distances at half this speed.
After spending four years designing, building, and testing Tethys, Monterey Bay Aquarium Research Institute (MBARI) engineers said the robot performed even better than expected. The AUV completed a four-day science run with plenty of battery power remaining, using relatively low-power rechargeable batteries. Based on these results, the researchers hope the robot will eventually be able to travel from California to Hawaii using high-power disposable batteries.
“In designing this AUV, we were actually trying to make a fundamental change in how we do oceanography,” said MBARI’s Chief Technologist, Jim Bellingham. “In the past, if we wanted to study something like an algal bloom, we could either put out a mooring and hope that the bloom would drift past it, or we could schedule a research cruise and hope that the bloom would happen while we were out on the ship. Tethys can travel to a spot in the ocean and ‘park’ there until something interesting happens. Once a bloom occurs, Tethys can move fast enough to follow the bloom and watch it evolve, the way a biologist on land might follow and study a herd of deer.”
From the very beginning, Tethys needed to be as energy efficient as possible. Its 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. The robot also incorporates sophisticated power-saving software like that found in some laptop computers, which monitors what systems are being used, and turns off those systems that are not in use.
This quest for energy efficiency is not without its risks. For example, most robotic vehicles are slightly buoyant. This means that, if all else fails, they will float up to the surface. However, with its variable buoyancy system, Tethys can make itself neutrally buoyant. If the entire system went dead under these conditions, the robot would remain drifting somewhere below the surface, and could be very difficult to recover. To mitigate these risks, MBARI engineers designed numerous fail-safe systems into the robot, some of which have their own independent power supplies.
Tethys is able to make some decisions without human intervention.
Bellingham feels this new AUV design will be small and inexpensive enough so it can see use for a variety of ocean research and monitoring. “We did some research and found that the most used research vehicles were the smallest ones. But it’s a real challenge to balance the factors of complexity, weight, and cost. Ultimately we’d like to Tethys to be a vehicle that is broadly accessible to scientists—a vehicle that can be launched from a small boat, at relatively low cost.”

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