Engineers are looking to the natural world for ideas on how to make robots move
The Mini Cheetah, developed at the Massachusetts Institute of Technology, can run at speeds of up to 3.9 metres per second.Inspiration can come from anywhere. For Radhika Nagpal, it came from her honeymoon.
Nagpal was snorkelling in the Bahamas when she was approached by a school of colourful striped fish, moving as one. “They come straight at you and check you out and then move off,” says Nagpal, now a mechanical engineer at Princeton University in New Jersey. “I was like, ‘Wow, that is a collective behaviour that I’ve never seen.’”Her mind returned to those curious fish years later, when she was pondering ways to build swarms of robots that could coordinate their behaviour in challenging environments. The result is a school of robotic fish — called Bluebots — that can coordinate their activity with their fellowsNagpal’s school is small, only ten fish with limited abilities. The fish are equipped with blue LEDs so that their comrades can spot them underwater. Simple rules in their programming, such as swimming to the left when they see another Bluebot, enable them to synchronize their movement. But Nagpal hopes to eventually build larger collectives with more complex behaviours. Such robotic schools could be tasked with locating and recording data on coral reefs to help researchers to study the reefs’ health over time. Just as living fish in a school might engage in different behaviours simultaneously — some mating, some caring for young, others finding food — but suddenly move as one when a predator approaches, robotic fish would have to perform individual tasks while communicating to each other when it’s time to do something different.Credit: Berlinger, F.“The majority of what my lab really looks at is the coordination techniques — what kinds of algorithms have evolved in nature to make systems work well together?” she says. Many roboticists are looking to biology for inspiration in robot design, particularly in the area of locomotion. Although big industrial robots in vehicle factories, for instance, remain anchored in place, other robots will be more useful if they can move through the world, performing different tasks and coordinating their behaviour. Some robots can already move on wheels, but wheeled robots cannot climb stairs and are stymied by rough or shifting terrain, such as sand or gravel. By borrowing movement strategies from nature — walking, crawling, swimming, slithering, flying or leaping — robots could gain new functionality. They might perform search-and-rescue operations after an earthquake, or explore caves that are too small or unstable for people to venture into. They could carry out underwater inspections of ships and bridges. And unmanned aerial vehicles could fly more efficiently and better handle turbulence. “The basic idea is looking to nature to see how things can potentially be done differently, how we can improve our automated systems,” says Michael Tolley, a mechanical engineer who heads the Bioinspired Robotics and Design Lab at the University of California, San Diego.Perhaps the most obvious strategy for robotic motion is walking, and legged robots do exist. Spot, a low-slung, four-legged machine that looks like a headless yellow dog, can climb uphill and navigate stairs. Its developer, Boston Dynamics in Waltham, Massachusetts, markets the US$74,500 device for mobile inspection of factories, construction sites and hazardous environments. A similar-looking robot, the Mini Cheetah, has been developed at the Massachusetts Institute of Technology in Cambridge. “More than 90% of land animals are quadruped,” says Sangbae Kim, a mechanical engineer at MIT who helped to design the Mini Cheetah. “So a natural place to look at is the quadrupedal world. And the cheetah is a king of that world in terms of the speed.”The Mini Cheetah can already perform backflips, and it runs as fast as 3.9 metres per second — about one-tenth as fast as an actual cheetah, but speedy for a robot. Now Kim is developing control software that he hopes will allow the robot to move smoothly across varying surfaces. This is challenging because the rules for how best to move a limb vary depending on the friction and hardness of the surface. Currently, moving from grass to concrete, or running up a gravelly hill, can cause the robot to stumble. “It runs really ugly and awkward,” Kim says. “It doesn’t fall, but it’s not efficient.” Nevertheless, quadruped robots are one of the better options for negotiating difficult terrain, says J. Sean Humbert, a mechanical engineer who directs the Bio-Inspired Perception and Robotics Laboratory at the University of Colorado, Boulder. Last year, his group took part in the US Defense Advanced Research Projects Agency’s Subterranean Challenge, in which robots were tasked with navigating tunnels, caves and urban settings to find particular targets; the team took third place, winning $500,000. “The robots that ended up doing really well across the teams were the legged robots,” Humbert says. But faced with a sandy, uphill, rocky landscape, these robots struggled. “Even our Spot robot tipped over and slid around,” he says.One possible solution, Humbert says, is to endow robots with animals’ innate ability to sense and respond to mechanosensory information, such as pressure, strain or vibration. He’s been taking that approach with flying machines by embedding strain sensors in the wings of fixed-wing UAVs, as well as in the arms of quadrotor drones, which rely on spinning blades to fly and hover. The work grew out of studies of honey bees. When Humbert placed bees in a wind tunnel and hit them with sudden gusts of air, their flight would be momentarily disturbed. After a quick change in the pattern of their wing beats, they would right themselves. Honey bees beat their wings 251 times per second, and the animals could make these corrections in just 15 to 20 beats — about 0.08 seconds. “Our conclusion was that [that] had to be mechanosensory information,” Humbert says. “Vision is just not fast enough to correct the spins that we’re seeing.” If a drone could similarly sense a disturbance and automatically correct for it that rapidly, he says, it would be much less likely to crash or be knocked off course. Some researchers are turning to bees as inspiration for robots that can respond to mechanosensory information.Fish also respond to mechanosensory stimuli, using a system of sensory organs known as the lateral line. The structure consists of hundreds of tiny sensors spread along the head, trunk and tail fin, and it enables fish to sense changes in the motion and pressure of water caused by obstacles, such as rocks and other animals. “Fish are sensing all of that and are using that, as well as vision, to position themselves relative to each other,” Nagpal says. No comparable underwater pressure sensor exists, but her team hopes to develop one to improve the Bluebots’ navigation. In San Diego, Tolley is exploring robots built from polymers or other pliable materials that can more safely interact with humans or squeeze through tight spaces. Squishy, pliable robots could have more flexible motion than hard robots with only a few joints, but getting them to walk on soft legs is a challenge.. Pressurized air first enters one chamber, then moves to the next. This movement causes the legs to bend, then relax. By alternatively activating opposing pairs of legs, the robot trundles along like a turtle. And because it does not need electronic controls, its design could be useful even in the presence of electromagnetic interference. Hard or soft, one issue robots struggle with is falling over. If a multimillion-dollar robot trips over a rock on Mars, an entire mission could be jeopardized. Some researchers are looking to insects for solutions, particularly click beetles, which can jump up to 20 times their body length without using their legs
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