Robotic Brooms

An autonomous robot designed at MIT (the yellow device in the collage) is designed to swim around a ship’s hull to detect small mines.

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The dolphins and sea lions the U.S. Navy has trained to search for underwater mines attached to ship hulls are neat, but far from perfect. While the animals can cover a large area in a short amount of time, they are costly to train and care for, and don’t always perform as expected.

So in the last few years, Navy scientists and research institutions around the world have begun engineering resilient robots for mine sweeping and other risky underwater missions, says Franz Hover, mechanical engineering professor at the Massachusetts Institute of Technology in Cambridge.

The ultimate goal is to design completely autonomous robots capable of navigating and mapping cloudy underwater environments and detecting mines as small as an iPod, Hover says.

Hover and his team are involved in creating algorithms to control such robots. The group has designed algorithms to program a robot called the Hovering Autonomous Underwater Vehicle. Using the group’s algorithms, a robot can swim around a ship’s hull and view complex structures such as propellers and shafts, Hover said.

The group hopes to achieve a resolution fine enough to detect a 10-cm mine attached to the side of a ship.

Researchers designed the algorithms that program the robot to swim around and then model a ship’s propeller.

“A mine this small may not sink the vessel or cause loss of life, but if it bends a shaft or damages a bearing, you still have a big problem,” Hover says. “The ability to ensure that the bottom of the boat doesn’t have a mine attached to it is really critical to vessel security today.”

The robot programmed by the algorithms can fully view a naval combat vessel, as well as all its small features, including bolts, struts, and any small mines.

“It’s not enough to just view it from a safe distance,” Hover says. “The vehicle has to go in and fly through the propellers and the rudders, trying to sweep everything, usually with short-range sensors that have a limited field of view.”

The team approached that challenge in two stages.

For the first stage, the researchers programmed the robot to approach the ship’s hull from a safe 10-m distance, swimming in a square around the structure. The vehicle’s sonar camera emits signals that boomerang back as the robot makes its way around the ship; the researchers process the sonar signals into a grainy point cloud that looks something like a mist, Hover says. At such a low resolution, one can clearly make out a ship’s large propeller, though not a small mine, he adds.

But the point cloud mist doesn’t necessarily tell a robot where a ship’s structures begin and end—crucial information for the robot to avoid colliding with a ship’s propellers. To translate the mist into a solid structure, the researchers adapted computer-graphics algorithms to their sonar data, generating a three-dimensional, mesh model.

In the second stage of their approach, the researchers programmed the robot to swim closer to the ship, navigating around the structure based on the mesh model.

The optimization algorithms program the robot to sweep across the structures while taking into account their complicated 3-D shapes, says Brendan Englot, an MIT mechanical engineering graduate student who helped develop these algorithms as part of Hover’s team.

“Over a minute or two of computation, we’re able to make tremendous improvements to the length of this path, and do so while keeping every single point in view,” Englot says. And it’s that view that ensures the hull stays mine-free as it moves through the water.

To read the latest issue of Mechanical Engineering, click here.

It’s not enough to just view it from a safe distance. The vehicle has to go in and fly through the propellers and the rudders, trying to sweep everything, usually with short-range sensors. Prof. Franz Hover, Massachusetts Institute of Technology

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