Optical quadrature encoder, much like how mechanical mice worked. Make the ring with a series of evenly spaced holes on the inside, then use a pair of optical sensors across that track of holes to detect when it's moving and in what direction.
Hah, I'd never looked inside one before but that's exactly how I'd imagined it would work.
The optics for a quadrature encoder are very forgiving -- it's much easier to implement than the camera-based sensor that's used for position, which requires a plastic lens.
> I don't understand how to get the stepped motion of the Kensington scroll rings and most mouse wheels. Maybe it has bumps to move past.
Most mouse wheels use a mechanical rotary encoder, and the detents are inherent to that part. Some of the fancier ones use custom mechanisms.
The Expert Mouse scroll ring originally was designed with a neodynium magnet and a steel slotted ring to provide the detents. The two I currently have don't have a scroll ring detent, so presumably the magnet was removed as a cost saving and/or reliability change. (for standard mouse scroll wheels, you buy the encoder with the detents already built in.)
I just took apart a spare one. The piece with the optical slots is metal and moves past a magnet - it's not neodymium in this one, and you can barely feel the scroll steps (but this trackball is very old and worn out, too). I can only see one optical scroll wheel sensor lens, so I'm not clear on how it detects scroll direction - maybe there are two offset detectors behind the same lens.
I thought about just using a standard mouse wheel sensor, but can't think of a good way to couple it to a big ring. Gears seem like they'd have an annoying amount of play, making it feel indirect.
