Prototype robot hops on snapping rods, outpacing rigid legs

Remember the “dancing pencils” trick from school, where two pencils pass through a rubber band, get twisted, and then leap apart when released? Researchers at UCLA and the University of Michigan have turned a version of that stored-tension idea into a frog-like robot that hops over grass, sand and small steps. Its limbs are not pushed slowly by a motor. They are loaded with energy and then let go in a single snap.

For a machine this small, the obstacles are everyday things. Grass, uneven ground, tiny puddles and loose objects can each stop a robot barely taller than they are. A jump gets it over them, but it needs a strong push delivered very quickly, and the small motors that power tiny robots deliver steady, continuous motion, not bursts. Springs can bridge that gap: a motor loads them slowly, then they release the energy at once. Snapping beams have also been tried, but in a three-dimensional structure they can flip violently or bend gradually, depending on their shape and how they are loaded.

The team's approach was to find out exactly what separates a sharp snap from a slow bend. They first used computer models to predict how elastic rods respond to combinations of bending and twisting, then used a robotic arm to test those predictions on physical rods. This mapped the conditions that produce a clean snap, and the robot's limbs were designed from that map. Each limb is a straight rod bent into a helix, fixed at one end and turned at the other by a small servo motor. Once the rod's current shape becomes unstable, it flips into another configuration, pushing against the ground. The motor then resets the limb, much of it while the robot is in the air. The team also had to balance force against recovery: some shapes released more energy but needed larger motor rotations between jumps, and the chosen design gives a strong push and a quick enough reset for repeated hops.

In the study published in Science Advances, the team built an 11 cm (4.3 in), 98.2 g (3.5 oz) robot with two snapping rear limbs and compared it with an identical-power version fitted with rigid legs. The snapping robot was faster on every surface, reaching 3.21 body lengths per second on wood and averaging 2.46 body lengths per second across all six surfaces, against 0.79 for the rigid-legged version. The gap was widest on cloth and grass, where the rigid-legged robot nearly stalled. The prototype also climbed small steps, did repeated backflips, and, with thin flexible fins, swam at about half a body length per second and steered around obstacles, including in wind. For now it remains a research prototype: its steering was shown with remote guidance through a sandbox of rocks and with light sensors, and the team has measured its speed only against its own rigid-legged twin.

For inspection teams and search-and-rescue crews working in debris-heavy sites, a robot that leaps over rubble rather than getting stuck could matter, and one that can also paddle through water with the same limbs widens where it can go. The researchers see a second benefit beyond navigation: the mechanics do part of the work that would otherwise need a larger motor or more complex control. Huang, one of the researchers, suggests the idea could fit a fast-closing robotic gripper. Those uses are not yet demonstrated, and they depend on the robot first proving itself against other small robots, which the study has not yet done.