Robots that move typically have legs or wheels, for walking or rolling. But researchers at U.C. Berkeley have developed a pogo-like robot with just a single leg. It locomotes—up to 8 miles an hour—by hopping.

The diminutive robot is called SALTO, for Saltatorial Agile Locomotion on Terrain Obstacles. ("Saltatorial" is the scientific name for the locomotion style used by kangaroos, grasshoppers and rabbits.) The thinking behind the robot's small size and unusual movement style is that it can travel through challenging terrain. "Small robots are really great for a lot of things, like running around in places where larger robots or humans can't fit," said UC Berkeley robotics grad Justin Yim.

"For example, in a disaster scenario, where people might be trapped under rubble, robots might be really useful at finding the people in a way that is not dangerous to rescuers and might even be faster than rescuers could have done unaided. We wanted Salto to not only be small, but also able to jump really high and really quickly so that it could navigate these difficult places."

That was the robot's original mission, dating back to 2016 when development on it first started. Now, nearly ten years on, another potential application has come into sight. As Robert Sanders of U.C. Berkeley Research explains, "Yim is embarking on a NASA-funded project to design a small, one-legged robot that could explore Enceladus, a moon of Saturn, where the gravity is one-eightieth that of Earth, and a single hop could carry the robot the length of a football field."

In order to achieve a moon-ready robot, the researchers reckon SALTO must be able to land on small targets with pinpoint accuracy. They've looked to squirrels, who can leap long distances and land precisely on branches. After studying squirrels' leaping mechanics, they've modified SALTO with both a grasping claw and a propellor-like flywheel that helps the robot make attitudinal adjustments:

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As for why the robot only has one leg and not multiples, Yim explains that it's more efficient. "One leg is the best number for jumping; you can put the most power into that one leg if you don't distribute that power among multiple different devices. And the drawbacks you get from having only one leg lessen as you jump higher. When you jump many, many times the height of your legs, there's only one gait, and that is the gait in which every leg touches the ground at the same time and every leg leaves the ground at approximately the same time. So at that point, having multiple legs is kind of like having one leg. You might as well just use the one."

It's worth noting that SALTO's development was made possible by collaboration between researchers from two very different fields: Biology students from Berkeley's PolyPedal Lab, and Engineering students from the school's Biomimetic Millisystems Lab. It's a great example of how two diverse disciplines can come together to yield true innovation.