New long-range undersea robot goes the distance

Over the past decade, the undersea robots known as autonomous underwater vehicles (AUVs) have become increasingly important in oceanographic research. Today's AUVs fall into two groups: 1) propeller-driven vehicles that can travel fast and carry lots of instruments, but are limited to expeditions of only a few days; and 2) "gliders," which can stay at sea for weeks or even months at a time, but cannot travel very quickly. MBARI engineers recently demonstrated a new super-efficient AUV that combines the best of these two approaches. This 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 MBARI's CANON experiment. Oceanographers used Tethys to track patches of microscopic algae that were carried around the bay by currents. During this experiment, 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 miles an hour) - about four times faster than most underwater gliders. However, it can also travel long distances at around half this speed.
After spending four years designing, building, and testing Tethys, MBARI engineers were happy to see it working out in the real ocean. In fact, the AUV 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 promising initial results, the researchers hope that the little robot will eventually be able to travel from California to Hawaii using high-power disposable batteries.
Thomas Hoover and Brett Hobson work on the long-range AUV. Photo: Todd Walsh © 2010 MBARI
Tethys is the brainchild of MBARI's Chief Technologist, Jim Bellingham, who has been designing cutting-edge AUVs for almost 20 years. Bellingham says, "In designing this AUV, we were actually trying to make a fundamental change in how we do oceanography. 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 was designed 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.

Most AUVs are programmed at the surface to follow a pre-set path through the water. Some can be reprogrammed via a satellite link when they come to the surface. Tethys takes this process further, and can make some decisions without human intervention. For example, during the CANON experiment, it was programmed to follow a course that would allow it to map the vertical and horizontal extent of algal blooms beneath the ocean surface.

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