“Squirrels are nature’s best athletes,” UC Berkeley professor of integrative biology Robert Full explained, as he detailed the impressive agility of the small mammals. Their capacity to bound across gaps, perch on thin limbs, and adjust course in mid-air has dazzled biologists for centuries. Today, their amazing versatility is leading engineers to reexamine robot design. The outcome? A revolutionary robot called Salto that replicates the biomechanics of a squirrel jumping and landing, paving the way for robots to traverse difficult terrain.
Salto or Saltatorial Agile Locomotion on Terrain Obstacles, is no ordinary robot. Created at UC Berkeley, the one-legged robot has come a long way since it was first designed in 2016 to be one of the most agile jump robots ever. Its current achievement landing on thin rods such as twigs is a robotics milestone. That was the inspiration behind the joint work of biologists and engineers, combining biomechanics with advanced robotics for unmatched agility.
Salto’s success is based on its mimicking squirrel-like movements for stability and accuracy. Squirrels, scientists learned, modify the braking effect of their legs on impact to equilibrate over- or undershot impacts to their destination. They essentially do a handstand on the branch, dispersing most of the kinetic energy using their front legs and using their feet to provide torque for balance. Drawing inspiration from these mechanisms, Salto’s design includes variable leg forces and a reaction wheel balance that allows it to stay upright on slender perches, an unprecedently seen feat by robots. “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, one of paper’s senior authors and a professor of integrative biology at UC Berkeley.
Justin Yim, a former UC Berkeley graduate student and now assistant professor at the University of Illinois, Urbana-Champaign, was responsible for bringing these biological principles to robot functionality. “If you feel like you’re going to fall over forward, then you might pinwheel your arms, but you’ll also probably stand up straight in order to keep yourself from falling over. If it feels like you’re falling backward and you might have to sit down because you’re not going to be able to quite make it, you might pinwheel your arms backward, but you’re likely also to crouch down as you do this. 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.” Yim said, describing the behavior training that replicates squirrel landings. The process not only improved Salto’s accuracy but also opened the door for its uses in disaster response and climate monitoring.
Salto’s agility is not only an engineering wonder it is a template for the way robots should be able to interact with their environment. Older robots are trapped on rugged terrain, pipes, or beams, but the potential of Salto to jump and alight on slender structures could make it priceless where mobility is needed. Picture a robot moving through the trusses of a building in construction or jumping over thick forests to track ecosystems. These capabilities are now possible, thanks to the technology derived from the squirrel.
The possibilities stretch far beyond our planet. Yim is developing a NASA-funded project to scale Salto to a size that can be applied on Enceladus, a moon of Saturn. At a gravity just one-eightieth Earth’s, the robot would have to take but one leap to span the size of a football field. “We can cover large distances, we can get over obstacles, we don’t require an atmosphere, and we don’t pollute anything,” Yim explained, calling attention to the ability of the robot to extend the space frontier. This vision, realized in the LEAP (Legged Exploration Across the Plume) mission concept, could allow Salto to sample the cryovolcano plumes of Enceladus and provide a window into Enceladus’ ocean beneath the ice.
The process of Salto moving from concept to reality has been interdisciplinary in nature. Ronald Fearing’s Biomimetic Millisystems Lab at UC Berkeley, where Salto was born, is a decades-old hub for bio-inspired robotics. “Biomimetic means that we are inspired by what nature can do,” Fearing said. Scientists, by studying how galagos and squirrels move, have developed robots that mimic their agility and efficiency. The variable mechanical advantage leg of Salto, for example, mimics the galago’s capability at power amplification in jumping to facilitate quick and high jumps.
Its development has also been made possible through advanced control systems. The initial models used motion capture cameras and off-board calculations, limiting them to laboratory settings. Later innovations, like the SHOVE orientation velocity estimator, have made Salto deployable in outdoor settings without restriction. Such developments not only make it useful in practice but also show the potential of biomimetic robotics to solve real problems.
It is not flawless, however. Yim admits the robot can be improved, particularly in the construction of its gripper. Future models may be outfitted with sturdier grippers to increase its capability to land on intricate surfaces. “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 teased, suggesting the horizon of Salto’s applications.
The union of biology and robotics represented by Salto is not just a technological feat it is a new paradigm for engineering. By replicating nature’s evolutionary solutions, researchers are designing robots that not only replicate but surpass the abilities of living things. As Full so aptly stated, “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.”
Salto’s squirrel-like agility is a vision of the robotics future, one in which machines can move about the world with the elegance and accuracy of their biological counterparts. Whether bounding between branches, climbing walls, or venturing onto distant moons, Salto is the promise of biomimetic design a promise that could redefine the way robots engage with their world and push the boundaries of human exploration.
