Dexter Walks

26 Feb 2007

This Valentine's Day I saw one of the most exciting things I've seen in my life: Trevor Blackwell's robot finally walked. Dexter is, as far as we know, the first dynamically balancing biped robot—that is, the first robot that walks like we do.

There are of course biped robots that walk. The Honda Asimo is the best known. But the Asimo doesn't balance dynamically. Its walk is preprogrammed; if you had it walk twice across the same space, it would put its feet down in exactly the same place the second time. And of course the floor has to be hard and flat.

Dynamically balancing—the way we walk—is much harder. It looks fairly smooth when we do it, but it's really a controlled fall. At any given moment you have to think (or at least, your body does) about which direction you're falling, and put your foot down in exactly the right place to push you in the direction you want to go. Practice makes it seem easy to us, but it's a hard problem to solve. Something as tall as a human becomes irretrievably off balance very rapidly. When a robot is falling, meaning its center of gravity is not centered over the foot (or feet) on the ground, the error grows by e^(t/.5). If a robot gets more than a few centimeters off balance, it's unlikely to recover, because you just can't move the limbs fast enough to compensate.

Trevor started working on this problem in May 2001, and it was not till February 2007 that he could even make Dexter lift one foot off the ground and put it down again without falling over. In retrospect, I don't know what kept him going. I talked to him constantly about this project as he was working on it, and I know for most of those six years he had no idea how he was going to solve the problem. This isn't like software: when a robot can't walk, you can't say for sure why not.

The breakthrough, according to Trevor, was to dramatically improve the robot's sense of where its center of gravity was. None of the commercial gyroscopes were good enough, he said, so he built his own. It also helped to make the feet lighter. The original feet, wearing heavy Doc Martens, were replaced by lighter ones outfitted with Vans. (I'm not joking.)

We were very excited when Dexter could lift a foot up without falling, because it meant walking was not far behind. Y Combinator's west coast offices are within the building of Anybots, Trevor's robot company, so every Tuesday dinner we got to see a demo of whatever Trevor had made in the past week. And on February 13th we saw something really dramatic: Dexter could walk a few steps forward without falling over. I hung around the next day as well, because I had the feeling I was witnessing something historic. On the morning of Valentine's Day, Dexter was walking so well that Trevor and Scott Wiley took the front off his rolling cage (which catches him when he falls) so that he'd have more room. And by that night, Dexter was really walking. The longest walk, which Dan Miller got on video, was over six feet.

Now, a week later, Dexter is so good at walking that the limit on the length of his walks is the size of room he lives in. Next step: a cage that can operate outdoors, so Trevor can take Dexter for a walk in the park.

A dynamically balancing robot is really something to see. You can't turn away from it. It's so shockingly anthropomorphic. Because it walks like you do, you sense what it's feeling. But of course it wasn't (just) for entertainment that Trevor built this thing. Any robot for use in real world situations has to balance dynamically, because you can't predict what surface it would have to walk on.

What makes Dexter all the more impressive is that Anybots consists of just three guys and a machine shop. Basically it's PARC without Xerox. Eventually they'll have to take outside money. I tell Trevor that he should just find a big company to pay Anybots' operating costs in exchange for a license to use what they develop. Or they could go after government grants, or raise venture capital. One way or another, ten or twenty years from now you'll see robots like this walking around.


Trevor Blackwell

Dexter Jumps