Building a better underwater bot

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    The robot cousin of the black ghost knifefish. Photo courtesy of Malcolm MacIver.

    The black ghost knifefish robot is not as scary as it sounds. It is neither fast nor strong, and it doesn’t have any knives (or ghosts.) But this nimble underwater robot may be our only hope the next time an oil rig explodes or a delicate coral reef is in danger, thanks to its unique ability to move vertically without the use of clunky propellers.

    The real black ghost knifefish is an Amazonian creature that hunts at night using a self-generated electric field to sense its victims. Instead of regular fins, it has one long, creepy ribbon-like fin that runs along the underside of its rigid, torpedo-like body. It can go from swimming forward and backward to vertically just by rippling its ribbon fin.

    This strange vertical movement intrigued Malcolm MacIver, associate professor of mechanical and biomedical engineering at Northwestern and former science consultant for Tron: Legacy. Current underwater robots use propellers to move around clumsily, so they can’t operate in small spaces. A robot that could swim slowly and precisely like the knifefish would be invaluable for tasks like plugging an oil pipe leak deep underwater or getting up-close data about fragile aquatic ecosystems.

    Two years ago, a graduate student in MacIver’s lab saw for the first time a knifefish rise vertically in its tank — yes, there is an aquarium full of knifefish in Tech — and puzzled over how it had managed such an un-fishy maneuver. After many hours of watching the animal skulk around the tank, the team finally figured it out.

    The knifefish ripples its fin to create a traveling wave. They already knew that when the wave moves toward the tail, the fish goes forward, and when the wave moves toward the head, it goes backward. But if the fish makes two waves, one traveling toward the tail and one toward the head, the waves meet in the middle and create a jet of water that sends the fish straight up.

    Armed with this knowledge, MacIver and his team hired Kinea Design, a design firm founded by Northwestern faculty, to build GhostBot. GhostBot is waterproof and about the length of a forearm, and, like its animal counterpart, has a rigid body with a ribbon fin on its belly (made of plastic and lycra, respectively). Thirty-two motors control 32 rods that move back and forth to mimic the wave motion. The result is not a fast, powerful machine, but instead an agile little bot that can fill in the gaps where current underwater technology is lacking.

    “We’re good at speed demons,” MacIver says. “We’re good at planes, trains and automobiles. What animals are really good at is taking vast quantities of sensory information and using that to nimbly move through their environment without colliding with anything.”

    The results of the team’s work with GhostBot were published in the Journal of the Royal Society Interface. Future generations of GhostBot will hopefully be able to navigate using the same self-generated electric field that the living knifefish uses to hunt. This way of underwater sensing is much more efficient than our current technique: boring, old-fashioned light. That’s because light doesn’t travel well underwater, and it needs a lot of energy, so bots using light need a tether to bring power from the surface. An electric field would only require an on-board battery pack.

    Building robots that can perfectly copy animal movement, whether it’s walking feet or flapping wings, is extremely difficult. But when you get it right, it’s like having a telescope into the animal world, MacIver says. And who knows: We might get a really awesome pool toy out of the deal.


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