For a droid enthusiast like myself, one of the best parts of the new Star Wars trailers has been the new little astromech droid, the ball-like BB-8. I’ve speculated about how that design could work before, but in the context of fiction. Now we know they actually built one. So, how did those kooky Imagineers do it?

Seriously, it’s real. Just ask Luke Skywalker himself, quoted in Wired:

In the interview, Hamill said: “They never cease to amaze me with what they’re able to come up with, you know? I said, “How are you ever gonna top R2-D2, the most adorable droid in movie history?” And then they have this new one. I can’t even tell you his name, but you saw it in the trailer. And when they were demonstrating how they did this thing, live on set — because it’s not CGI, that’s a live prop — I was just amazed.”

The physical, real-world droid was shown in Anaheim at the Star Wars Celebration, where JJ Abrams let the little guy out of his droid-cage to come out onstage and wow everyone with even a passing interest in How Stuff Moves And Stays Together. The robot demonstrated a very interesting range of motions — the ball moved around omnidirectionally, while the head pivoted and moved independently of the ball, clearly rolling on the unbroken surface of the sphere.

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So how, exactly, are they pulling this off? Clearly, some sort of Juggalo-baffling magnetic force is used to keep the head in place, but how is the motion controlled? And how does the head keep from rolling off?

There’s been speculation over at Reddit, and many are pointing to this XKCD cartoon as a likely inspiration/method — and I think they’re generally right. The XKCD design, with some modification, seems very capable of what we’ve seen the droid do.

With that in mind, I made this quick sketch of what I think is going on in that ball-and-a-half:

So, let’s walk through this. I think the head is a very light, unmotorized part. I think keeping the weight (and intertia) of the head low is key to making this work well. The head has some LEDs and things inside it, and some magnetic ball rollers at the base, but that’s about it.

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Inside, everything revolves around a circular baseplate with a Segway-like electronic control system. This base plate has four motor-and-omni wheel ( like some forklifts use) assemblies which control the overall direction and speed of the base sphere.

In the center of the baseplate is an arm used to support and control the head. The Segway-style control system keeps this arm roughly vertical at all times. The top of the arm has a spherical-section top that contains magnetic rollers that engage the head’s base rollers through the shell of the ball. A rotational motor co-axial with the arm is used to rotate the head, and a servo midway through the arm, like an elbow, is used to adjust the angle of the head on the ball.

I think with this basic set of components you could get all of the motions we’ve seen so far. Of course, I’m no engineer, just a dipshit with a willingness to do crude sketches, so I’d be curious to hear the opinions of you actual engineers out there.

Overall, I think it’s a pretty smart design for a small robot that has to enclose a decent-sized volume while moving rapidly over a variety of terrain. The old R2 units, as great as they were, weren’t exactly speed demons on rough terrain or over the thick shag carpeting of the Death Star’s rec room. I’m still not sure what it does on stairs, though. Bounce, I guess? Controlled hops?

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Hopefully, we’ll get to see inside the little ball at some point, and it’ll be more than a cybernetically-enhanced gerbil. Until then, it’s fun to guess.