Inspired by the biomechanics of manta rays, researchers at North Carolina State University have developed an energy-efficient soft robot that can swim more than four times faster than previous soft swimming robots. The robots are called “butterfly bots” because their swimming movements resemble the movements of human arms during butterfly swimming.
“To date, soft swimming robots have not been able to swim faster than one body length per second, but marine animals such as manta rays can swim much faster and more efficiently,” says Jie Yin, corresponding author of the paper and associate professor of mechanical and aerospace engineering at NC State. “We wanted to build on the biomechanics of these animals to see if we could develop faster and more energy-efficient soft robots. The prototypes we’ve developed work exceptionally well.”
The researchers developed two types of butterfly boots. One was built specifically for speed and was able to reach an average speed of 3.74 body lengths per second. The second was designed to be highly maneuverable, able to make sharp turns to the right or left. This maneuverable prototype was able to develop a speed of 1.7 body lengths per second.
“Researchers studying aerodynamics and biomechanics use something called the Strouhal number to estimate the energy efficiency of flying and swimming animals,” says Yingjin Chi, first author of the paper and a recent Ph.D. graduate of NC State. “Peak propulsive efficiency occurs when the animal swims or flies with a Strouhal number between 0.2 and 0.4. Both of our robot butterflies had a Strouhal number in this range.”
Butterfly bots get their ability to float from their wings, which are “bistable”, meaning the wings have two stable states. The wing is like a hair clip. A hair clip is stable until you apply a certain amount of energy (bending it). When the amount of energy reaches a critical point, the hair clip takes another form that is also stable.
The butterfly boots have bistable hairpin-style wings attached to a soft silicone body. Users control the switch between two stable states in the wings by pumping air into chambers inside the soft body. As these chambers inflate and deflate, the body flexes up and down – causing 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,” says Yin. “Our approach uses bistable wings that are passively powered by moving the central body. This is an important difference because it allows for a simpler design, which reduces weight.”
The faster butterfly bot has only one “drive unit” – a soft body – that drives both of its wings. This makes it very fast, but difficult to turn left or right. The Maneuverable Butterfly Bot essentially has two actuators connected side by side. This design allows users to manipulate the wings on both sides, or to “flail” just one wing, allowing it to make sharp turns.
“This work is an exciting proof of concept, but it has limitations,” says Yin. “It’s more obvious that the current prototypes are tethered by thin tubes that we use to pump air into the center cases. We are currently working on an untethered, standalone version.”
The paper, titled “Snapping for a high-speed and high-performance butterfly-like soft swimmer,” will be published Nov. 18 in the open-access journal. Achievements of science. The paper was co-authored by Yaoe Hong, Ph.D. student at NC State; and Yao Zhao and Yanbin Li, who are doctoral students at NC State. This work was supported by the National Science Foundation under grants CMMI-2005374 and CMMI-2126072.
Video about butterfly bots can be found on https://youtu.be/Pi-2pPDWC1w.