#439849 Boots Full of Nickels Help Mini Cheetah ...As quadrupedal robots learn to do more and more dynamic tasks, they're likely to spend more and more time not on their feet. Not falling over, necessarily (although that's inevitable of course, because they're legged robots after all)—but just being in flight in one way or another. The most risky of flight phases would be a fall from a substantial height, because it's almost certain to break your very expensive robot and any payload it might have. Falls being bad is not a problem unique to robots, and it's not surprising that quadrupeds in nature have already solved it. Or at least, it's already been solved by cats, which are able to reliably land on their feet to mitigate fall damage. To teach quadrupedal robots this trick, roboticists from the University of Notre Dame have been teaching a Mini Cheetah quadruped some mid-air self-righting skills, with the aid of boots full of nickels.
If this research looks a little bit familiar, it's because we recently covered some work from ETH Zurich that looked at using legs to reorient their SpaceBok quadruped in microgravity. This work with Mini Cheetah has to contend with Earth gravity, however, which puts some fairly severe time constraints on the whole reorientation thing with the penalty for failure being a smashed-up robot rather than just a weird bounce. When we asked the ETH Zurich researchers what might improve the performance of SpaceBok, they told us that "heavy shoes would definitely help," and it looks like the folks from Notre Dame had the same idea, which they were able to implement on Mini Cheetah. Mini Cheetah's legs (like the legs of many robots) were specifically designed to be lightweight because they have to move quickly, and you want to minimize the mass that moves back and forth with every step to make the robot as efficient as possible. But for a robot to reorient itself in mid air, it's got to start swinging as much mass around as it can. Each of Mini Cheetah's legs has been modified with 3D printed boots, packed with two rolls of American nickels each, adding about 500g to each foot—enough to move the robot around like it needs to. The reason why nickel boots are important is because the only way that Mini Cheetah has of changing its orientation while falling is by flailing its legs around. When its legs move one way, its body will move the other way, and the heavier the legs are, the more force they can exert on the body. As with everything robotics, getting the hardware to do what you want it to do is only half the battle. Or sometimes much, much less than half the battle. The challenge with Mini Cheetah flipping itself over is that it has a very, very small amount of time to figure out how to do it properly. It has to detect that it's falling, figure out what orientation it's in, make a plan of how to get itself feet down, and then execute on that plan successfully. The robot doesn't have enough time to put a whole heck of a lot of thought into things as it starts to plummet, so the technique that the researchers came up with to enable it to do what it needs to do is called a "reflex" approach. Vince Kurtz, first author on the paper describing this technique, explains how it works:
This technique works quite well, but there are a few constraints, most of which wouldn't seem so bad if we weren't comparing quadrupedal robots to quadrupedal animals. Cats are just, like, super competent at what they do, says Kurtz, and being able to mimic their ability to rapidly twist themselves into a favorable landing configuration from any starting orientation is just going to be really hard for a robot to pull off:
Lastly, we asked Kurtz to talk a bit about whether it's worth exploring flexible actuated spines for quadrupedal robots. We know that such spines offer many advantages (a distant relative of Mini Cheetah had one, for example), but that they're also quite complex. So is it worth it?
Mini Cheetah, the Falling Cat: A Case Study in Machine Learning and Trajectory Optimization for Robot Acrobatics, by Vince Kurtz, He Li, Patrick M. Wensing, and Hai Lin from University of Notre Dame, is available on arXiv. |
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