Tonewheels

Around music, tonewheels may well refer to optical tonewheels.

One of the better known ones will be the optigan, which is basically PCM-like in optical form.

There are various similar ideas. The Hammond organ, for example, uses gears and magnetic pickups as a (near-sine) tonewheel.

And many more experimental things http://www.umatic.nl/tonewheels_historical.html

The idea is perfectly sound as a sensor. ABS wheel speed sensors seem to often be a tonewheel plus hall sensor, presumably because they are accurate what doesn't need to touch isn't going to wear.(verify)