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Last Updated: Tuesday, 31 July 2007, 16:34 GMT 17:34 UK
Robot fins to propel submarines
Robo-fin and bluegill sunfish (Image: Donna Coveney)
The robo-fin is based on the bluegill sunfish's movements

The distinctive swimming technique of a bony fish is being used to develop agile, propellerless submarines.

US researchers have created prototype mechanical fins that mimic the movements of the bluegill sunfish.

The robo-fins could recreate the fish's powerful forward thrust and its manoeuvrability, the researchers reported.

Further work is being carried out to see how other sunfish movements could help to improve underwater vehicles.

The hope is that in the future propellerless, fish-like submarines could carry out a range of tasks, such as mapping oceans, surveying shipwrecks or sweeping for mines, with more agility and speed than current autonomous underwater vehicles (AUVs) controlled by propellers.

James Tangorra, a researcher at MIT's Bio-instrumentation Laboratory and a professor of mechanical engineering at Drexel University, said: "If we could produce AUVs that can hover and turn and store energy and do all the things a fish does, they'll be much better than the remotely operated vehicles we have now."

'Cupping motion'

The bluegill sunfish (Lepomis macrochirus) moves with great efficiency; its pectoral fins are able to propel it creating very little backwards thrust.

Where most fish move by flapping their fins back and forth, the bluegill sunfish uses a "cupping and sweeping" motion.

Its fin sweeps forwards then curls at its lower and upper edges to create a cup-like shape. This produces a thrust that propels the fish with very little water resistance.

Robo-fin (MIT)
The robo-fins use a cupping and sweeping motion

To see how they could recreate the movement with a robotic fin, scientists at George Washington University broke down the pectoral fin's motion into 19 different components.

"We then worked out the parts that were most important for propulsion," said Dr Tangorra.

The researchers created several prototypes; the first two were constructed using urethane plastic webbing and electric motors to control the nylon tendons.

These prototypes were able to recreate the cupping and sweeping motion but the motors made them too bulky, if scaled up, to be used on AUVs.

So the researchers turned to a thin, flexible polymer material that could also conduct electricity for their third prototype.

This meant that the fin was able to mimic the highly controllable, shape-morphing fish fin, but through electrical signals that swept through the polymer rather than motors.

The result, said the researchers, was a powerful thrusting motion, although they added that some adjustments would be needed to increase its speed and agility.

More movements

Dr Tangorra said the team was now looking in even more detail at the bluegill sunfish to create propellerless AUVs.

He told the BBC News website that to gain all of the benefits of the fish's motion, you also had to look at how the pectoral fin interacted with the dorsal and tail fins, as well as the rest of the movements within the fish's body.

The result, he said, would be a fully flexible, fish-mimicking, underwater robot vehicle that could operate at high and low speeds, hover and have excellent manoeuvrability, in calm or rough seas.

He said a prototype of this new style AUV was expected within the next two years.

The research was carried out by the Massachusetts Institute of Technology, Harvard University and George Washington University.

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