Snapping Elastic Rods Propel Frog-Inspired Robot Across Six Terrains

Roboticists at the UCLA Samueli School of Engineering and the University of Michigan have developed a propulsion mechanism that relies on elastic rods snapping between geometric configurations to generate high-speed motion from small, low-power motors. According to a study published in Science Advances and reported by TechXplore , the snapping architecture allows an 11-centimeter robot to hop, swim, and climb across varied terrain.

The research was co-led by Khalid Jawed, an associate professor of mechanical and aerospace engineering at UCLA Samueli and director of the Structures-Computer Interaction Lab, and Xiaonan (Sean) Huang, an assistant professor of robotics at the University of Michigan. The team examined how combining bending, twisting, and compression causes elastic rods to deform three-dimensionally, accumulating strain energy until a critical threshold triggers a rapid snap into a secondary shape.

Because the transition dynamic depends on the rod's geometry rather than motor wattage, small actuators can gradually wind a rod into a helix and release the accumulated energy instantaneously. "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," Jawed said. "This opens a promising path toward robots just a few millimeters wide, turning small motor movements into powerful bursts of motion."

The team developed computational simulations to map how rods react to specific bending and twisting combinations, verifying the models by manipulating physical rods with a robotic arm. They then built a 98.2-gram (3.5-ounce), 4.3-inch-long prototype that uses motor-wound helical rods as snap actuators. "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," said co-first author Dezhong Tong, a postdoctoral researcher at Michigan who conducted the work as a doctoral student at UCLA.

During physical trials across six surfaces—wood, cloth, acrylic, leather, grass, and sand—the snap-actuated prototype achieved an average velocity of 2.46 body lengths per second and a peak speed of 3.21 body lengths per second on wood. A rigid-legged control robot averaged 0.79 body lengths per second across the same surfaces and nearly stalled on grass and cloth. The snapping mechanism also enabled the robot to climb steps and execute aerial backflips.

When equipped with thin, flexible fins, the robot swam at approximately 0.5 body lengths per second while navigating obstacles and counteracting wind disturbances. The team also demonstrated teleoperated navigation through a rock-filled sandbox and basic autonomous steering using integrated light sensors.

"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 said. "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."

The paper was co-authored by Jiaqi Wang, a doctoral student at Michigan who served as co-first author alongside Tong; Zexiong Chen, a former graduate student at Michigan; Andy Borum, an assistant professor of mathematics and statistics at Vassar College; and Weicheng Huang, an assistant professor of mechanics and robotics at Newcastle University.