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Why this ‘butterfly bot’ can swim four times faster than other robots

How they swim resembles the way a person’s arms move when they're doing the butterfly stroke.

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By Gwyn Wright via SWNS

New robots - called "butterfly bots" - can SWIM four times faster than existing ones.

The machines get their name because the way they swim resembles the way a person’s arms move when they're doing the butterfly stroke.

The flapping gadgets are energy-efficient, soft and inspired by the biomechanics of manta ray fish, say the American researchers, who developed two types of butterfly bots.

One, which was specifically designed for speed, was able to reach average speeds of 3.74 body lengths per second.

It has only one “drive unit” – the soft body – which controls both of its wings. This makes it very fast, but difficult to turn left or right.

Another, which was highly maneuverable, was capable of making sharp turns to the right or left and was able to reach speeds of 1.7 body lengths per second.

It essentially has two drive units, which are connected side by side. This design allows users to manipulate the wings on both sides, or to “flap” only one wing, which is what enables it to make sharp turns.

Study corresponding author Dr. Jie Yin, of North Carolina State University, said: “To date, swimming soft robots have not been able to swim faster than one body length per second, but marine animals – such as manta rays – are able to swim much faster, and much more efficiently.

“We wanted to draw on the biomechanics of these animals to see if we could develop faster, more energy-efficient soft robots.

“The prototypes we’ve developed work exceptionally well.”

First author Dr. Yinding Chi added: “Researchers who study aerodynamics and biomechanics use something called a Strouhal number to assess the energy efficiency of flying and swimming animals. “

“Peak propulsive efficiency occurs when an animal swims or flies with a Strouhal number of between 0.2 and 0.4. Both of our butterfly bots had Strouhal numbers in this range.”

“Both of our butterfly bots had Strouhal numbers in this range.”

Butterfly bots get their swimming power from their wings which are similar to hair clips.

Like hair clips, the wings are “bistable”, which means they are stable until a certain amount of energy is applied to them by bending.

(Courtesy of NC State)

When the amount of energy reaches a critical point, the hair clip snaps into a different shape – which is also stable.

In the butterfly bots, the hair clip-inspired wings are attached to a soft, silicone body.

Users control the switch between the two stable states in the wings by pumping air into chambers inside the soft body.

As those chambers inflate and deflate, the body bends up and down – forcing the wings to snap back and forth with it.

Dr. Yin added: “Most previous attempts to develop flapping robots have focused on using motors to provide power directly to the wings.

“Our approach uses bistable wings that are passively driven by moving the central body.

“This is an important distinction because it allows for a simplified design, which lowers the weight.

“This work is an exciting proof of concept, but it has limitations.

“Most obviously, the current prototypes are tethered by slender tubing, which is what we use to pump air into the central bodies.

“We’re currently working to develop an untethered, autonomous version.”

The findings were published in the journal Science Advances.

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