The fastest underwater soft robot yet: Butterfly bot

Diagram of butterfly bot soft robot
Butterfly bot’s design. Credit: Jie Yin, NC State University

Marine “soft robots” could help with deep ocean exploration, and cleaning up pollution such as  microplastics.

Robots made from soft material can get into places, and do tasks, that traditional, hard robot’s can’t, particularly underwater.

But they’re currently pretty slow swimmers.

Now a team of US researchers has made a soft robot that’s nearly four times faster than any other reported soft swimmer.

The researchers, who have published a description of their robot in Science Advances, based their robot design on manta rays.

“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,” says co-author Jie Yin, an associate professor of mechanical and aerospace engineering at North Carolina State University, US.

“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 robot, both called “butterfly bots” because the way they move looks like someone swimming with butterfly stroke.

One butterfly bot was fast, zipping through the water at 3.74 body lengths per second. The other can get to 1.7 body lengths per second but is more manoeuvrable, making sharp turns.

“Researchers who study aerodynamics and biomechanics use something called a Strouhal number to assess the energy efficiency of flying and swimming animals,” says first author Dr Yinding Chi, a recent PhD graduate of North Carolina State University.

“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 have soft, silicone bodies, with “bistable” wings attached. These wings can snap comfortably from one position to another and back again.

As air chambers in the butterfly bot’s body fill and deflate, the silicone bends and the wings are snapped up and down. This makes them flap, and propels the robot forward.

“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 driven by moving the central body. This is an important distinction, because it allows for a simplified design, which lowers the weight.”

The manoeuvrable bot has a drive unit connected to each wing, meaning one wing can flap at a time and the robot can turn.

The faster bot has just the one unit moving both wings, sacrificing agility for a lighter weight – and more speed.

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

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