Science & Environment

Amazon electric fish inspire underwater robotics

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Media captionProf Malcolm MacIver: "We've generated a robot that essentially duplicates the way fish sense"

Electric fish from South America are opening up new ideas in robotics.

Ghost knifefish, as they are known, put a small current through the water to sense their environment, and undulate a long fin to move around.

Scientists at Northwestern University, US, believe both features could be harnessed in a new class of autonomous underwater vehicles.

They are developing robots that will be able to swim around debris in total darkness, such as inside a sunken ship.

"Today, we don't really have underwater robots that work well in really cluttered conditions or in conditions where vision isn't useful," said Prof Malcolm MacIver.

"Just consider the sunken cruise ship. It is very dangerous to send divers into such situations where the water can be very cloudy.

"But we can learn from the electric fish. They don't use vision to hunt at night in the rivers of the Amazon basin, and their movement through the cluttered root masses and flooded forests requires incredible precision. They fill a big hole in terms of our capabilities in underwater robots."

Prof MacIver was explaining his work here at the annual meeting of the American Association for the Advancement of Science (AAAS).

Image copyright M.MacIver/Northwestern
Image caption Northwestern has developed a number of robotic prototypes based on the knifefish

He has studied knifefish for years, deciphering their sensory and locomotion systems.

The animals generate an electric field from modified neurons running along their spinal cord. When prey, such as aquatic insects, enter this field the fish measure a tiny change in voltage at the surface of their skin.

The perturbation is only one-tenth to one-hundredth of a millionth of a volt, but sufficient for the receptors to detect it.

Image copyright M.MacIver/Northwestern
Image caption Knifefish hunt in darkness

"The fish have evolved an amazing system," said Prof MacIver. "Imagine your retina stretched over your entire body and what that would be like. That's the situation that knifefish find themselves in.

"They perceive in all directions. They emit a kind of radar, but it's an electric field; and the sensory receptors scattered over their entire body surface mean they can detect things coming from all directions."

The technology in Prof MacIver's lab is now simulating this enabling a robot in a tank to react to what is around it and move accordingly.

But it is the special propulsion technique employed by knifefish that the Northwestern researcher also wants to copy.

These are the ripples sent through the long fin on the belly. Undulate one way, and the fish will move forward; undulate the other way, and the direction of travel is reversed. Use counter-propagating waves that meet in the middle, and the fish will move up.

"From all our simulations, we now have mathematical relationships between things like the frequency and amplitude of the travelling wave and how much propulsion you get," said Prof MacIver. "So now we can put that into technology and get it to work properly."

Currently, the Northwestern lab is demonstrating artificial sensory and locomotion capabilities on two separate robotic platforms. The aim now is to bring them together into a single working device.

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Media captionListen to the sound of the "knifefish choir"

In the meantime, Prof MacIver has also been having some fun with his fish by putting them in a "choir".

Every fish produces a constant electric field and every species emits at a different frequency. So, by converting these frequencies to a sound, it is possible to make some knifefish music. Prof MacIver has developed an art installation based on 12 fish tanks.

Image copyright M.MacIver/Northwestern
Image caption By sonifying the electric fields of different species, it is possible to make knifefish music and follow me on Twitter: @BBCAmos

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