The twisting of bent elastic rods can produce a snapping motion that enables small robots to hop or swim, researchers have shown.
The advance offers a promising mechanism for robots with limited power, particularly miniature robots, according to the research team.
Published in Science Advances, the research was co-led by Khalid Jawed, an associate professor of mechanical and aerospace engineering at UCLA, and Xiaonan (Sean) Huang, an assistant professor of robotics the University of Michigan.
“The broader opportunity is to let the mechanics of the robot do some of the work that would otherwise require larger motors or more complicated control,” Huang says.
“By programming when an elastic structure stores and rapidly releases energy, we can give small robots access to powerful, repeatable motions without continuously demanding high output from the motor. In the future, this principle could be useful for robots that must navigate cluttered terrain, overcome obstacles, reorient quickly, or operate across both land and water.”
Perfecting the snap
When a flexible rod is bent, and its ends are rotated, it eventually reaches a point where it changes shape to release built-up tension—but this change does not always happen in the same way. Under some combinations of bending and twisting, the rod changes shape gradually. Under others, it snaps rapidly from one shape to another, with the potential to provide a strong push.
Through computer modeling and experiments, the team optimized a helical shape, like a segment of a coiled spring, that maximized the burst of energy while resetting quickly for the next snap.
“Because it’s the rod’s shape—not its size—that determines whether it snaps sharply or deforms gradually, the same design rules apply across a wide range of scales,” says Jawed, whose lab worked on the simulation and robot arm experiments.
“This opens a promising path toward robots just a few millimeters wide, turning small motor movements into powerful bursts of motion.”
Testing snap propulsion in a robot
After optimizing the propulsion mechanism, the team designed and built real robots that use the snapping rods to hop. Connected to a rotating motor, the bent rods twist until they produce the snapping motion. Then the motor unwinds the twist and proceeds to contort the rod again.
“Once we could predict when a rod would snap, we could use that sudden release of energy to turn a simple motor movement into a powerful push that sends the robot hopping forward,” says UM postdoctoral scholar Dezhong Tong, the study’s co-lead author with Jiaqi Wang, a PhD student in robotics at UM. Tong started working on the project while a graduate student in Jawed’s group at UCLA.
The frog-like prototype, with a pair of snapping rods at the rear of the device, hopped over a broad range of test surfaces—from solid materials like wood and glass to soft and slippery materials like leather. Outside, the palm-sized robot tackled sand and grass. It could climb and descend steps and, with paddle attachments, could even swim.
Moving the two snapping rods at different rates enabled the robot to turn, and the team used a remote control to maneuver the robot around a small sandbox with rock obstacles. They also automated a simple navigation method, using light sensors so that the robot would approach a light source.
The small prototype, weighing about 0.25 lbs, could move about three body lengths per second—similar to a baby loggerhead turtle making its way to the sea.
Additional study authors are from the University of Michigan; Vassar College in New York; and Newcastle University, UK.
The study was funded by the National Science Foundation.
Source: University of Michigan
