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Flexible robotic fin does away with drag

Posted on 07.03.2008 - 14:00 UTC in SCIENCE & TECH NEWS by Rons_ROV_Links

A robotic fin that mimics the energy-efficient manoeuvres of an agile fish's pectoral fins has been developed by US researchers. Working out how to use multiple versions in conjunction could allow robot submarines to hover and turn on a dime as natural swimmers can.

Researchers have been building fish and fin-like robots for over a decade. "But they have been primarily body- and tail-based swimmers aiming to improve on propeller propulsion," says James Tangorra, at Drexel University, "and propellers can already go fast."

Four parts of the motion of an earlier prototype robotic fin modelled on the pectoral fin of the bluegill sunfish (A: relaxed, B: expansion, C: curl, D: cupping). The most recent version can generate thrust at all times while moving, with no drag or recovery phase to its motion (Image: MIT)Tangorra and colleagues from Drexel, MIT, Harvard and George Washington Universities, all in the US, are not interested solely in speed. They say fish-like fins are best used to make autonomous underwater vehicles (AUVs) more agile.

Their latest prototype fin is modelled on the pectoral fin of the bluegill sunfish, a freshwater fish found throughout North America.

Flashy movers

Although they generally use all their fins and tail, bluegills can hover, rotate, move forwards and come to a stop using pectoral fins alone.

The team's artificial fin is made from elastane - also known as Spandex - and polyester webbing supported by a skeleton of flexible plastic struts called fin rays.

Each ray is controlled by conducting polymers that act like muscles, contracting when supplied with current. Using two layers that pull the fin in different directions mimics the way the fish uses opposing muscles to do the same task.

The researchers learnt about the bluegill's exceptional ability using digital cameras to track the flow of small particles in water around the fins. This revealed that beating pectoral fins can produce constant thrust.

Flick trick"A flapping fin will produce drag at some point during its stroke," says Tangorra. That is not the case with the sunfish which has an unusual "cup and sweep" motion of its fin, which produces no drag.

In their attempts to recreate the effect, the researchers trialed one fin at a time by attaching it to a carriage mounted on rails over a water tank. The fin is dipped into the water and can freely manoeuvre while its weight is supported.

Initial designs could mimic the beating "cup and sweep" motion of a bluegill fin, but they still produced drag during part of the motion.

The trick, they discovered, was to improve the flexibility of the fin, so that it flicks after being moved by the polymers at the base. "That passive tip flick lets the fin store and then release energy - that removes the drag," Tangorra says. "The fin never produces drag unless it stops moving."

Multiple fins

Tangorra and colleagues now hope to combine multiple fins at once and see how they interact. The basic design of their fin can easily be adapted to make versions of the Bluegill's other fins, says Tangorra. "There's a reason they have them all, so we need to try that too."
David Lane at Heriot-Watt University, UK, works on UAVs and has also investigated fish-like propulsion.

"There's no doubt that fish are great, and they have great actuators in their fins," he told New Scientist, "but our ability to build actuators and sensors is not as sophisticated as nature's."

So far, Lane says, artificial takes on fish-style propulsion are unable to compete with propellers. They just aren't powerful enough to deal with currents and other adverse forces out in the oceans. Much more scientific work is needed to understand how fish fins can be so efficient and powerful, he adds.

© 2008 - New Scientist Tech

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