Sunday, February 16, 2014

Amazon Electric Ghost Knifefish inspire underwater robotics research

Northwestern has developed a number of robotic prototypes based on the Ghost Knifefish.

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

Knifefish put a small current through the water to sense their environment, and undulate a long fin to move around.

Scientists at the Neuroscience and Robotics Lab in 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 fit 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).

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.

Knifefish hunt in darkess

"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.

"The knifefish inspired GhostBot. We've built one of the most advanced fish robots in the world, with 34 degrees of freedom (the humanoid robot ASIMO has 26; the Roomba floor vacuum robot has 2), to better understand knifefish mechanics and sensorimotor coupling. "

"The video is the first where we discovered that inward counter-propagating waves generate a strong downward jet, producing vertical thrust. "

"This is a key element of knifefish maneuverability. The water is seeded with reflective particles for subsequent particle imaging velocimetry. "

You will see that 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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