This Tiny Frog-Like Robot Hops, Flips and Swims With Just Elastic Twists

The palm-sized robot turns slow motor twists into explosive hops across rough terrain.

At first glance, it doesn’t really look like mechanical frog. Assembled from a circuit board, a battery and two loops of wire, it looks like something a student whipped up in a frenzy. Then its hind limbs snap — and the tiny machine launches itself across the ground.

The 11-centimeter-long robot stores energy by twisting bent elastic rods, then releases that energy in a sudden burst in a frog-like movement. The trick lets a relatively weak motor produce powerful hops, helping the robot cross grass, sand and slippery surfaces, climb small stairs, perform backflips and even swim. Basically, it’s a different way to build small robots, using the robot’s body itself to do more of the work instead of bigger motors.

This robot stores energy

Small robots have a big challenge. Shrinking motors, batteries and electronics leaves less room for the hardware needed to generate sudden, powerful movements. The new robot attempts to sidestep that problem in much the same way that a drawn bow does: it builds energy slowly, then releases it quickly.

Khalid Jawed, an associate professor of mechanical and aerospace engineering at UCLA, and Xiaonan (Sean) Huang, an assistant professor of robotics at U-M built two flexible rods at the back of the machines. These rods are bent into loops, and small motors rotate the ends of each rod. As this happens, the rod simply stores more and more elastic energy.

Then it reaches a tipping point.

Instead of continuing to deform smoothly, the rod rapidly snaps into another shape. That sudden rearrangement releases stored energy and pushes against the ground, throwing the robot forward. The motor can then unwind the rod and start the process again.

“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,” said Huang. “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.”

Reliable snaps

There’s nothing really new about this idea, springs have been used for centuries in various projects. But the main challenge is making the snap predictable and controllable.

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Engineers call this kind of abrupt change snap-through instability. To control this instability, researchers first combined mathematical models, computer simulations and robotic-arm experiments to map which combinations of shape and loading produced each behavior.

Then, using what they learned in the models, they designed a coil-like limb that repeatedly reached the useful snapping state.

“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 U-M postdoctoral scholar Dezhong Tong, the study’s co-lead author.

Robot goes brrr

Their untethered prototype measures about 11 centimeters long and weighs 98.2 grams. It carries its own battery and motors rather than relying on an external power supply.

It did well in the lab, but researchers wanted to test it out in the real world.

The team sent it hopping over six surfaces: wood, cloth, acrylic, leather, grass and sand. It moved faster than a comparable robot fitted with rigid legs on every one, particularly on challenging surfaces like grass.

This is great news, because the real world is rarely like a laboratory floor. Small machines intended to inspect damaged buildings, move through vegetation or explore unfamiliar environments may encounter loose soil, slick surfaces and obstacles larger than their own legs.

The prototype also climbed and descended a miniature staircase. By driving its two rear limbs at different rates, the researchers made it turn. They remotely steered it through a sandbox containing rocks and added light sensors that allowed it to move toward a light source.

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A new way to replace muscles?

Modern robots often solve movement problems with stronger motors, more actuators and increasingly sophisticated software. But that gets harder and harder with smaller robots.

The new work suggests another route: design structures that use clever physics to amplify a modest input.

The mechanism itself may have room to shrink even more. Because the transition between gradual bending and rapid snapping depends strongly on the rod’s geometry, co-lead researcher M. Khalid Jawed said the same design rules could potentially apply to robots only a few millimeters across.

This is still an experimental prototype. It has shown relatively simple navigation rather than the perception and decision-making needed for a robot to independently explore a chaotic disaster site or natural environment.

For small machines starved of power, the fastest route to stronger movement may not be a better motor at all. It may be learning exactly when to snap.

The study was published in Science Advances.