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A Hybrid Underwater Robot

Combining the best characteristics of earlier systems, researchers have built a new type of autonomous deep-ocean explorer that could revolutionize marine biology.

A new type of underwater robot could be better at tracking marine organisms and measuring the physical and chemical properties of the ocean than previous robot designs. The vehicle, called Tethys and developed by the Monterey Bay Aquarium Research Institute (MBARI) in California, compensates for the shortcomings of current robots by merging their best qualities into one unit.

All wet: Tethys is a new autonomous underwater vehicle developed by the Monterey Bay Aquarium Research Institute in California. It is being tested in Monterey Bay (top), and in a tank at the institute (bottom).

For decades, researchers have used underwater vehicles to study the biological processes and physical characteristics of the ocean. But such work has been constrained because there were only two types of underwater robots: gliders and propeller-driven vehicles. A glider drifts very slowly through the ocean, using a buoyancy system for propulsion. Its low speed makes it vulnerable to tides and currents, which can knock it off course. It also has a small payload capacity, but high endurance, so it can remain at sea for months at a time. In contrast, propeller-driven vehicles can zoom through the ocean like torpedoes. They can be up to 10 times the size of gliders, but they can remain at sea only for about 24 hours.

Tethys combines the speed of propeller-driven systems with the range and duration of gliders to create a new kind of robot. It uses a new propeller and body design to travel about four times as fast as a glider, but slightly slower than the cruising speed of high-powered vehicles. Tethys also has an efficient power management system, so it can spend many weeks to months in the ocean while carrying a large payload of sophisticated instrumentation.

“To understand the biological processes in the ocean, which change very quickly, you need a flexible system,” says James Bellingham, chief technologist at MBARI and project lead for Tethys. Sometimes an interesting area is far offshore, so you need a vehicle that can get there quickly and then remain, slowly following organisms for an extended period of time, he says.

The new underwater robot fills a void in the commercial market and in oceanographic research, says David Kelly, CEO of Bluefin Robotics, a company based in Cambridge, Massachusetts, that designs and develops autonomous underwater vehicles. Others have experimented with hybrids—including the company iRobot—but Tethys is the first fully developed vehicle. Kelly says his company would be interested in using the technology.

Tethys is about two meters long and weighs 110 kilograms. MBARI researchers designed the vehicle’s tube-like shape to minimize drag and optimize propulsion. The design also allows the researchers to lengthen or shorten the body to accommodate a range of payload sizes. The researchers increased the robot’s performance over previous designs by building a propeller that works at two speeds: one meter per second and half a meter per second. To make Tethys consume little power, the MBARI researchers custom-built most of the onboard electronics. They also built a system to monitor the instrumentation and turn devices on and off every fraction of a second they are not being used.

Using Tethys is like putting a laboratory in the ocean, says Bellingham. During its autonomous journeys through the water, it occasionally surfaces to send data back to the researchers via satellite. MBARI researchers have tested the robot in California’s Monterey Bay, tracking algal blooms and phytoplankton, and measuring the physical and chemical properties of the surrounding water. Eventually, the system could do much more, says Eric D’Asaro, a professor of oceanography at the University of Washington. “This could be the first vehicle to take sample materials in the water and bring them back to the lab,” he says.

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