Nine-year-old Salto, Berkeley’s foot-tall, one-legged jumping robot, just got a new superpower: squirrel-like leaping and landing abilities. It’s an enviable achievement and the latest advance in a long collaboration between biology students in Professor Robert Full’s Poly-PEDAL Lab and engineering students in Professor Ronald Fearing’s Biomimetic Millisystems Lab.
As Full told Berkeley News, “Squirrels are nature’s best athletes.” By studying the movements of campus fox squirrels in detail using high-speed video and other sophisticated tools, his team hoped that they could help Fearing’s lab build more agile robots, ones that might someday navigate tree canopies, industrial sites, or disaster areas with equal aplomb.
While Salto (the name stands for Saltatorial Agile Locomotion on Terrain Obstacles) could already jump, it was no squirrel. It couldn’t stick a landing on anything other than flat ground. Jumping from a perch to a branch or other point was a new challenge.
So, how do squirrels do it? What Full’s team learned was that the acrobatic rodents perform a kind of handstand when landing on a branch, their front legs absorbing most of the force on impact. They then apply torque with their feet so as not to undershoot or overshoot. Their front legs also relax or tighten to adjust their inertia.
Engineers were able to use these insights to successfully modify Salto. By adjusting leg tension and running the reaction wheel in reverse to mimic the breaking motion of the squirrel’s “handstand,” they were able to get Salto to leap and balance on a branch, even without a prehensile foot.
While a gripper foot was ultimately added, researchers deliberately “decided to take the most difficult path and give the robot no ability to apply any torque on the branch with its feet. We specifically designed a passive gripper that even had very low friction to minimize that torque,” explained Justin Yim, Ph.D. ’20, a former graduate student in Fearing’s lab who co-authored a recent paper on the team’s work in Science Robotics. “In future work, I think it would be interesting to explore other more capable grippers that could drastically expand the robot’s ability to control the torque it applies to the branch and expand its ability to land. Maybe not just on branches, but on complex flat ground, too.”
Yim, now an assistant professor of mechanical science and engineering at the University of Illinois Urbana-Champaign, is currently working on a NASA-funded project to design a one-legged robot that could explore Enceladus, one of Saturn’s moons, where the gravity is so weak that the robot could jump the length of a football field in a single bound.
