The fact that the dots on the CD form a spiral means it is a no-no as far as I can see. Although CD players are cheap they are modern marvels of control technology in the way they track etc.
Harmonic drives are one way to get true zero backlash gearing but looking at Faro patents it does seem that they use just small and expensive encoders.
One thought I had was to run a cord from the base of one arm portion to the tip of the next and wrap this multiple times around the shaft of an encoder that was springloaded, the extension and retraction of the cord would create multiple revolutions of the encoder, however this would be messy and possibly difficult to calibrate.
Graham |