Researchers and engineers from the University of California, Berkeley, developed a robot named Salto, inspired by squirrels, that can jump from branch to branch and land on narrow perches while maintaining balance.
"Squirrels are nature's best athletes. The way that they can maneuver and escape is unbelievable. The idea is to try to define the control strategies that give the animals a wide range of behavioral options to perform extraordinary feats and use that information to build more agile robots," said Robert Full, one of the paper's senior authors and a professor of integrative biology at UC Berkeley, according to Science Daily.
Salto, originally developed in 2016, could already hop, parkour, and stick a landing on flat ground, but it has been redesigned to navigate complex environments like construction sites. The goal of this research is to enable the robot to jump between beams and maneuver around pipes, beams, and cables.
Despite advancements in robotics, the team at UC Berkeley realized that no existing robot can match a squirrel's ability to parkour through branches, leap across gaps, and execute precise landings on flimsy branches. Biologists and engineers designed an approach to increase the chances of a robot achieving upright and balanced landings without needing great grip strength by controlling the forces along its leg.
The research team found that when squirrels land after a leap, they do a handstand on the branch, directing the force through their shoulder joint to minimize stress. This biomechanical analysis of squirrel landings represents an advancement in creating agile robots that can navigate complex environments like forests and construction sites.
"The robots we have now are OK, but how do you take it to the next level? How do you get robots to navigate a challenging environment in a disaster where you have pipes and beams and wires? Squirrels could do that, no problem. Robots can't do that," said Full.
Squirrels use pads on their feet to grasp the branch and twist to overcome excess torque that could send them over or under the branch. Inspired by this, Salto is equipped with a special gripper and a motorized flywheel to help it balance, similar to how humans use their arms to restore balance.
Scientists found that in squirrels, almost all the energy—86% of the kinetic energy—is absorbed by the front legs. "Single-legged robots may seem impractical due to the risk of falling when standing, but for jumping very high, a single leg is best," said Justin Yim, now an assistant professor at the University of Illinois, Urbana Champaign.
"The challenge was to achieve a landing after a jump on a specific point, like a narrow rod," explained Yim.
"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 of having only one leg lessen as you jump higher," elaborated Yim.
Salto achieved upright and balanced landings in two of the successful attempts and grabbed the branch and swung above or below it in the remaining attempts. The team points out that future research could improve balance control.
Biologists and engineers at UC Berkeley are studying squirrel biomechanics to improve robot agility and inform more agile robot designs. Using these strategies, Yim is working on a NASA-funded project to design a small, one-legged robot that could explore Enceladus, a moon of Saturn.
Much of Salto's ability to hop, parkour, and land is due to collaboration between biology students in Robert Full's Polypedal Lab and engineering students in Ronald Fearing's Biomimetic Millisystems Lab. The research was funded by the U.S. Army Research Office and the National Institutes of Health.
Full is now investigating the importance of the torque applied by a squirrel's foot upon landing. His team instrumented a branch with sensors to measure the force and torque applied by squirrels when landing. Yim decided to reverse the motors that launch Salto and use them to brake when landing. The biology and robotics teams worked in parallel to confirm that this approach would help Salto stick a landing.
Salto, short for Saltatorial Agile Locomotion on Terrain Obstacles, originated a decade ago in the lab of Ronald Fearing at UC Berkeley. Yim added, "In future work, I think it would be interesting to explore other more capable grippers that could 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."
"If you're a squirrel being chased by a predator, like a hawk or another squirrel, you want to have a sufficiently stable grasp, where you can parkour off a branch quickly, but not too firm a grasp. They don't have to worry about letting go, they just bounce off," said Full.
Unlike monkeys, squirrels do not have a usable thumb for a prehensile grasp, so they must palm a branch. "We 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," said Yim.
In 30 tests, Salto jumped and landed successfully 25 times and fell five times. These agile robots could leap among trusses and girders of buildings under construction or monitor environments in tangled forests or tree canopies.
While primates depend on their strong grip to hold on while swinging under branches, squirrels can jump and maintain upright balance despite lacking similar grip strength. Maintaining upright balance after jumping onto a narrow beam can be a challenge for robots, as they need to counteract a large impulse using a limited support space.
They conducted experiments in which Salto jumped from one branch to another and attempted to maintain balance, initially controlling only the force of its legs and later with the help of a wheel in its body. Salto needs to reorient its leg to prepare for landing while in the air, so it has to make steeper jumps than squirrels, which initially balance with their front limbs after shallower jumps.
Researchers have taken the unique abilities of squirrels as a model to give the one-legged jumping robot Salto new capabilities. Squirrels use skillful leg movements and body adjustments to manage their balance after jumping.
For robots, landing on a thin surface after a jump is hard because they have to quickly stop their movement with very little space to stand on. Using their analysis of squirrel landings, researchers were able to program the robot to carry out similar feats.
"If you're going to undershoot, you can generate less leg-braking force; your leg will collapse some, and your inertia will be less, swinging you back up to correct," explained Full, according to Science Daily. He added, "Whereas if you are overshooting, you want to generate more braking force so that you have bigger inertia and it slows you down for a balanced landing."
Yim and UC Berkeley undergraduate Eric Wang redesigned Salto to incorporate adjustable leg forces, supplementing the torque of the reaction wheel.
The article was written with the assistance of a news analysis system.