We humans envy birds for their seemingly effortless ability to fly, and for their ability to extract endless amounts of bread from old people in parks. But there’s a middle ground between those two states—soaring and ambling around on two feet in pursuit of crumbs—that we tend to overlook: Birds are a kind of hybrid. To walk over difficult terrain, they can flap to stabilize, while we humans trip over ourselves.
When roboticists look to nature for inspiration, they overlook this middle-ground state, developing robots that either walk or fly—but not both. “Everyone is working on drones or bipedal locomotion,” says Caltech roboticist Soon-Jo Chung. “We want to lay the groundwork for combining these two different types.”
Chung and his colleagues have debuted a strange machine called Leonardo (LEg ON Aerial Robotic DrOne, obviously), which walks on two legs but also uses thrusters attached to its torso to move like a MechWarrior. Its main mode is a kind of hover-walk. If its engineers have anything to say about it, this wild new kind of locomotion could help load injured humans into drone ambulances, or even explore the surface of Mars. Baby steps, though: The researchers are still experimenting with Leo on a tether, so it won’t be scrambling up mountains anytime soon.
At two and a half feet tall and just six pounds (thanks to carbon fiber construction), Leo looks something like a whooping crane. It can walk around on lanky legs, just as a traditional bipedal robot like Boston Dynamics’ Atlas would (minus the backflips), but unlike a traditional robot it can use its thrusters as a failsafe. Sure, Atlas can frantically stumble to regain its footing, but that’s no guarantee it’ll save itself.
“You can take a couple of steps to avoid falling, but what if that fails?” asks Northeastern University roboticist Alireza Ramezani, who helped develop Leo. “The thrusters in this scenario can make the system almost fault tolerant.” They offer a backup plan in case of trouble, allowing the robot to switch seamlessly between legs and thrusters depending on what’s most useful in the moment.
Combining flight and walking is no easy feat. Roboticists have been working toward a mastery of bipedal locomotion for decades, and the machines still don’t casually stroll among us. One challenge is that it takes a lot of energy just to get a two-legged robot to balance in place—even when it’s standing still, a bipedal robot has to constantly make corrections (you do the same, you just don’t notice it).
The promise of Leo is that it’ll be more efficient at this kind of idling. Instead of wobbling to correct itself, the biped can switch on the thrusters to become a drone on legs: The propellers won’t have to waste energy lifting the robot, per se, because the feet will still be in contact with the ground. “It can kind of defy the rules of gravity in a sense,” says Leo codeveloper Morteza Gharib, director of the Graduate Aerospace Laboratories at Caltech.
Another challenge of bipedal locomotion: The inherent instability of walking on two legs means robots struggle on uneven terrain. This became abundantly clear a few years ago during the Darpa Robotics Challenge, in which humanoid robots succeeded mostly in just falling on their faces. One wrong step, or a slight shift in the surface beneath its feet, and the bot hits the ground. That’s particularly problematic for bipedal robots because they aren’t yet dexterous enough to pick themselves back up.
Leo, on the other hand, would essentially float over difficult terrain, making missteps less likely to be catastrophic. If things get really hairy, for instance if the robot needs to get up a hill, it might be able to bound instead of scramble. “How can we actually leverage the legs to a point where you can turn on the thrusters and manage a jumping transition to flight?” asks Ramezani.
Which brings us to Mars—at least one day. Caltech has been collaborating with NASA on a helicopter for the Red Planet, which could bypass difficult terrain and scoot around more quickly than a wheeled rover. The nice thing about a rover, though, is that it can sit on the surface without draining its batteries. To hover, a drone has to use power.
“The Mars helicopter concept is going to be very limited,” says Gharib. “We realized we need to have a much longer flight time and also be able to stabilize and take samples or look at rocks.” Now Gharib and his colleagues are exploring how a robot like Leo might navigate the rough Martian landscape more efficiently than a traditional helicopter by putting its feet on the ground and lightly powering up the thrusters.
They’re also looking into how Leo might be of use here on Earth as a sort of robot companion to a flying ambulance (a small-scale version of which Caltech is testing against a wall of 1,300 computer fans, by the way), which could carry a person out of an otherwise inaccessible area. The problem, of course, is in getting an incapacitated person into the ambulance without another human on the ground helping. But teams of Leo robots may one day take up that job.
To be clear, Leo isn’t meant to be the only bipedal machine in a world that will soon be crawling with robots. It will have its use cases, as will traditional humanoid robots like Atlas. You wouldn’t want a robot like Leo to be buzzing around your home, after all. But this new class of bipeds could well find a footing on Earth—and beyond.
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