Squirrel-inspired robot masters the art of precision landing

When it comes to agility, squirrels are in a league of their own. These small animals can leap between branches, stick impossible landings, and move effortlessly through trees.

While robots have been designed to mimic the motion of snakes, birds, and fish, none has come close to matching a squirrel’s mastery of dynamic movement – until now.

A team of biologists and engineers from the University of California, Berkeley has designed a hopping robot that can replicate one of the squirrel’s most impressive skills: landing precisely on a narrow perch.

Their inspiration came from studying how squirrels leap, land, and adjust their bodies in mid-air to maintain balance.

Squirrel skills, robotic goals

“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 Robert Full, co-senior author of the study.

“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.”

The team’s answer to this challenge is Salto – a small, one-legged robot originally developed at UC Berkeley in 2016.

Salto was already capable of hopping and landing on flat surfaces, but researchers wanted to see if it could do something much trickier: land accurately on a narrow rod.

Teaching robots to think like squirrels

Justin Yim, a former UC Berkeley graduate student and now an assistant professor at the University of Illinois Urbana-Champaign, worked to translate what the biology team observed in squirrels into movement strategies for Salto.

“If you think about trying to jump to a point – maybe you’re doing something like playing hopscotch and you want to land your feet in a certain spot – you want to stick that landing and not take a step,” Yim explained.

The behaviors that we carry out in order not to fall over when landing on our feet are familiar to us all. For example, we may pinwheel our arms and stand up straighter to stop ourselves from falling forwards. Similarly, we may pinwheel our arms backwards and sit down to prevent ourselves from falling backwards.

“That is the same behavior that we programmed into the robot. If it’s going to be swinging under, it should crouch. If it’s going to swing over, it should extend out and stand tall,” explained Yim.

The physics-based body adjustments that squirrels make to recover from a misjudged jump are now helping robots make smarter mid-air decisions.

Yim is currently using these strategies in a NASA-funded project that is aimed at building a one-legged robot capable of hopping across the low-gravity surface of Enceladus, Saturn’s icy moon.

What squirrels teach us about balance

The robot’s improved performance is built on detailed biomechanical research. UC Berkeley scientists equipped branches with sensors and captured high-speed video to analyze just how squirrels land.

They found that when a squirrel lands after a long leap, it essentially performs a front-legged handstand. Most of the energy – about 86% – is absorbed by the front legs.

“They’re really doing front handstands onto the branch, and then the rest of it follows,” Full said. “Then their feet generate a pull-up torque, if they’re going under; if they are going to go over the top – they’re overshooting, potentially – they generate a braking torque.”

But the key to a squirrel’s success isn’t just muscle strength. It’s the ability to fine-tune that force depending on whether the jump was too short or too long.

Upgrades for Salto

To replicate these subtle adjustments, Yim and UC Berkeley undergraduate Eric Wang modified Salto’s design.

They gave it the ability to adjust its leg forces during landing and combined this with the robot’s existing reaction wheel, which helps with balance.

Despite not having grippers or any way to hold onto the perch, Salto was able to land on narrow branches several times.

“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,” Yim said.

“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.”

The advantage of no thumbs

The team also discovered that squirrels, unlike monkeys, do not rely on thumbs to grab branches. Instead, they use the entire palm of the foot to apply torque. Surprisingly, this might offer an advantage.

“If you’re a squirrel being chased by a predator like a hawk, or [by] another squirrel, you want to have a sufficiently stable grasp, where you can parkour off a branch quickly, but not too firm a grasp,” Full said. “They don’t have to worry about letting go, they just bounce off.”

Why one leg works

Although one-legged robots may seem unstable, Yim believes this design is the most efficient for high jumps.

“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,” Yim said.

Combining biology and engineering

With inspiration from nature, specifically squirrels, Salto is now better equipped to take on challenging terrains. The research shows that, by combining biology and engineering, robots can be taught to land with the finesse of an agile animal.

As this work continues, it may lead to the development of robots that are capable of exploring unstable buildings, treetops, and even alien worlds – one hop at a time.

Click here to watch the robot squirrel video…

The full study was published in the Journal of Experimental Biology.

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