How BB-8 Works

By now, there’s a pretty good chance that you’ve seen the new Star Wars movie — and for those who haven’t, don’t worry: no spoilers here. At long last, we’ve got a much-needed dose of Starfighters, Rebel Alliances, Lasers, and droids. BB-8, the adorable successor to R2-D2, has captured hearts and minds. As lovable as it is, and even with as much life as its creators managed to instill into it, in the end… it’s a super sophisticated prop. And now you’re wondering: how the heck does this thing work?

While JJ and Co. have kept the specifics of BB-8’s innards mostly under wraps, we can suss out the basics. Behind that trilling orange and white sphere, BB-8 is likely a set of wheels (propelling the sphere by spinning against its inner-wall) and a magnetic mast (to hold on to and control BB-8’s head). The sort of “wobbly” way BB-8 moves? It’s all inherent to the design — what might be considered a flaw if used anywhere else, here it helps to give BB-8 much of its character.

That explanation will leave a lot of you wanting for more. Want a more detailed breakdown than that? Read on, folks.

A lot of the analysis you see on the internet today has people treating BB-8 as two discrete operations, one each for the head and ball. However, one of the many patents filed by Disney and partner Sphero paints a different picture. This patent, for a “Magnetically coupled accessory for a self-propelled device,” explains how to get BB-8 to work without a separately controlled head. Combine this with Sphero’s Chief Scientist Adam Wilson telling Polygon.comthat the head isn’t articulated independently, and you can start to paint a picture of what the internals look like.

The strongest justification for this kind of system is in watching BB-8 try to lean over. It doesn’t seem to be able to maintain a constant head angle while static. This strongly implies that the head isn’t articulated independently, and is consistent with the patent.

Let’s get things rolling

The head complicates things, so let’s just think about the body for now. An image from one of the patents is an extremely helpful illustration. To make things simple, let’s think about this problem in two dimensions, looking only at forward and backward motion. The same principles apply to keep this stable; all that’s happening when BB-8 turns is the internal assembly yawing to point in a different direction.

In a nutshell, what we’ve got is a sphere with wheeled mechanism inside it. The wheels are forced down against the wall of BB-8 in some way (either spring or gravity, it doesn’t matter a huge deal). Rotating the wheels shifts the center of the system’s mass, the bulk of which is in the wheel assembly, off of the vertical line that includes the center of the ball and the contact point with the ground. Leaning generates a moment. Do this right, and the ball moves in the direction that the wheels were shifted to. If we were to picture a mast mounted perpendicularly on top of the wheel base, the ball would move in a direction opposite to the mast.

IMAGE: UNITED STATES PATENT APPLICATION

In broad strokes, this is similar to what it’s like to get a Zorb ball moving. Being the heaviest thing in the Zorb, moving forward changes your position relative to the ball’s center of mass. This, in the end, leads to rotation and forward motion. (We’re not going to go over the dynamics of this problem, but reach out if you really want to know).

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