Scientists develop flexible robotic fin that does away with drag

March 2nd, 2008 - 11:39 am ICT by admin  

London, March 2 (ANI): US researchers have made a robotic fin that mimics the energy-efficient manoeuvres of an agile fish’s pectoral fins, which they believe may help shed some light on how robot submarines may be enabled to hover and turn on a dime as natural swimmers can.

James Tangorra of Drexel University pointed out that 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, and propellers can already go fast,” New Scientist magazine quoted him as saying.

He said that he and his colleagues were not interested solely in speed. According to him, fish-like fins were best used to make autonomous underwater vehicles (AUVs) more agile.

The researcher revealed that the latest prototype made by his team was modelled on the pectoral fin of the bluegill sunfish, a freshwater fish found throughout North America.

Tangorra said that though bluegills generally use all their fins and tail, they could hover, rotate, move forwards and come to a stop using pectoral fins alone.

The research teamwhich included Tangorras collaborators from Drexel, MIT, Harvard and George Washington Universitiesmade its 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, and contract 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 said that they learnt about the bluegill’s exceptional ability by tracking the flow of small particles in water around their fins with the help of digital cameras, which revealed that beating pectoral fins could produce constant thrust.

“A flapping fin will produce drag at some point during its stroke,” says Tangorra.

That is not the case with the sunfish that has an unusual “cup and sweep” motion of its fin, which produces no drag.

With a view to recreating the effect, the researchers studied one fin at a time by attaching it to a carriage mounted on rails over a water tank. The fin was dipped into the water, and could freely manoeuvre while its weight was 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 research team found that the trick was to improve the flexibility of the fin, so that it could flick 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. The fin never produces drag unless it stops moving,” Tangorra says.

The researchers are now hoping to combine multiple fins at once to see how they interact. Tangorra says that the basic design of their fin can easily be adapted to make versions of the Bluegill’s other fins.

“There’s a reason they have them all, so we need to try that too,” says the researcher.

David Lane at Heriot-Watt University, UK, who has also investigated fish-like propulsion, said: “There’s no doubt that fish are great, and they have great actuators in their fins, but our ability to build actuators and sensors is not as sophisticated as nature’s.”

He says that artificial takes on fish-style propulsion are so far unable to compete with propellers. According to him, they are not powerful enough to deal with currents and other adverse forces out in the oceans.

Lane says that much more research is needed to understand how fish fins can be so efficient and powerful. (ANI)

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