The butterfly bots, whose swimming motion resembles the way a person’s arms move when they are swimming the butterfly stroke, have been developed at North Carolina State University.
“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,” said Jie Yin, corresponding author of a paper on the work and an associate professor of mechanical and aerospace engineering at NC State. “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.”
The researchers developed two types of butterfly bots, one built for speed and able to reach average speeds of 3.74 body lengths per second, and a second designed to be highly manoeuvrable and capable of making sharp left or right turns. This manoeuvrable prototype was able to reach speeds of 1.7 body lengths per second, the team said.
“Researchers who study aerodynamics and biomechanics use something called a Strouhal number to assess the energy efficiency of flying and swimming animals,” said Yinding Chi, first author of the paper and a recent Ph.D. graduate of NC State. “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.”
According to NC State, the butterfly bots derive their swimming power from their bistable wings that 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.
“Most previous attempts to develop flapping robots have focused on using motors to provide power directly to the wings,” Yin said in a statement. “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.”
The faster butterfly bot has one ‘drive unit’ – the soft body – which controls both of its wings. This makes it very fast, but difficult to turn left or right. The manoeuvrable butterfly robot 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 enables it to make sharp turns.
“This work is an exciting proof of concept, but it has limitations,” said Yin. “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 team’s paper, “Snapping for high-speed and high-efficient, butterfly stroke-like soft swimmer,” was published on November. 18, 2022 in Science Advances.
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