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Energy-Efficient Soft Robot Swims Faster than Previous Models

Having gained inspiration from the biomechanics of the manta ray, scientists at North Carolina State University have come up with an energy-saving soft robot that has the potential to swim more than four times faster than earlier swimming soft robots.

Image Credit: NC State University.

The robots are known as “butterfly bots,” since their swimming motion matches the way a person’s arms tend to move when they swim the butterfly stroke.

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.

Jie Yin, Study Corresponding Author and Associate Professor, Mechanical and Aerospace Engineering, North Carolina State University

Yin added, “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.”

Two kinds of butterfly bots were developed by the researchers. Particularly, one was built for speed and was able to obtain average speeds of 3.74 body lengths per second. A second was developed to be highly maneuverable and capable of making acute turns to the right or left. This maneuverable prototype was capable of reaching speeds of 1.7 body lengths per second.

Researchers who study aerodynamics and biomechanics use something called a Strouhal number to assess the energy efficiency of flying and swimming animals,” stated Yinding Chi, first author of the paper and a recent Ph.D. graduate of NC State.

Chi added, “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.”

The butterfly bots gain their swimming power from their wings, which are so-called “bistable,” implying that the wings hold two stable states. The wing is like a snap hair clip: a hair clip is stable until users apply some amount of energy (by twisting it). When the amount of energy attains a critical point, the hair clip moves into various shapes and is also firm.

As far as the butterfly bots are concerned, the hair clip-inspired bistable wings are fixed to a soft and silicone body. Users have the potential to regulate the change between the two stable states in the wings by pumping air into chambers within the soft body. As those chambers inflate and deflate, the body bends up and down, thereby 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. 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.

Jie Yin, Study Corresponding Author and Associate Professor, Mechanical and Aerospace Engineering, North Carolina State University

The quicker butterfly bot has only one so-called “drive unit”—the soft body—which regulates both of its wings. This makes it very rapid, but hard to turn left or right. Basically, the maneuverable butterfly bot has two drive units, which are linked side by side. This design enables users to handle the wings on both sides or to so-called “flap” only one wing, which is what allows it to make sharp turns.

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.

Jie Yin, Study Corresponding Author and Associate Professor, Mechanical and Aerospace Engineering, North Carolina State University

The study was co-authored by Yaoye Hong, a Ph.D. student at NC State; and by Yao Zhao and Yanbin Li, who are postdoctoral researchers at NC State. The work was performed with support from the National Science Foundation under grants CMMI-2005374 and CMMI-2126072.

Journal Reference:

Chi, Y., et al. (2022) Snapping for high-speed and high-efficient butterfly stroke–like soft swimmer. Science Advances.


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