Snapping Legs Help Tiny Robot Hop, Swim, and Climb

Small robots have small motors by necessity, which can become a problem when they need a sudden burst of power to jump over an obstacle or cross difficult terrain. Researchers at the University of Michigan and UCLA have found a way around that limitation by letting the robot’s legs store energy and then release it all at once.

The team's palm-sized robot uses flexible rods that twist into helical shapes before rapidly snapping into a different configuration. That snap provides enough force to launch the robot forward, producing a hopping motion that works on everything from smooth wood to loose sand and grass. The researchers say the same basic mechanism could eventually be scaled down to robots just a few millimeters across.

The approach takes advantage of a mechanical instability known as snap-through. Bending and twisting a flexible rod doesn't necessarily produce the same behavior every time. Depending on its geometry and how it is loaded, the rod can either deform gradually or build up elastic energy before abruptly snapping into another shape.

To figure out how to reliably get the latter behavior, the researchers combined mathematical modeling, simulations, and robotic experiments. That allowed them to map the conditions that determine whether a helical rod will buckle smoothly or snap. They then used those results to design a leg that maximizes useful energy release while still being easy to reset for another hop.

Each leg is made from a superelastic Nitinol rod. One end is held in place while the other connects through a pushrod to a micro servo. Rather than requiring the servo to move the leg rapidly, it slowly twists the rod until enough energy has accumulated to trigger snap-through. The rod then releases that stored energy much faster than the motor could deliver it directly. Reversing the servo resets the leg for the next cycle.

Two of these legs were installed at the rear of an untethered robot measuring 11 × 8 × 4 centimeters and weighing 98.2 grams. A 7.4 V LiPo battery and custom control board ride onboard alongside the two servos. By controlling the legs independently, the robot can also steer in addition to hopping in a straight line.

In testing, it reached a peak speed of 3.21 body lengths per second. The hopping mechanism continued across surfaces including wood, cloth, acrylic, leather, grass, and sand. A nearly identical robot fitted with rigid legs averaged just 0.79 body lengths per second across those surfaces, compared with 2.46 for the snap-powered version.

In further tests, the robot climbed and descended steps, navigated sandy terrain scattered with rocks, and used light sensors to steer toward a light source. Put inside a sealed enclosure, it could even use the same snapping legs as paddles to swim at about half a body length per second.

Instead of demanding more power from tiny motors, the design effectively gives them time to save up their work and spend it in one quick burst. That could make snap-through mechanisms especially useful for future miniature robots that need to occasionally jump, swim, turn quickly, or escape terrain where conventional legs would struggle.