my real motor question

If there's overshoot, or if there isn't enough friction, then there might be some uncertainty.

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Good point. So if you're going to use a pulley to move a big wheel, you'd want to have your encoder on the big wheel, or at the very least, geared to the big wheel. That'd give you a realistic idea of where the wheel is regardless of how much the belt's slipped.

Very true.

Very true.

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Or...
I forget who had the idea of using a hacked mouse to sense slide movement, but thanks because I'm starting to entertain that idea again. I could attach an encoder with a pulley to the main wheel, and here's how:

Since the mouse is only reliable to read displacement at speeds ol less than 1m/s, my encoder pulley will be attached to the axle of the big wheel for a ratio of 6:1 or better. The encoder pulley can be really lightweight, so it won't add a lot of extra inertia to the system and there'll be little risk of encoder's pulley slipping. In the event the encoder pulley does slip, or if the mouse occasional errs in its sensing, I could have a couple or so strategically-placed contact points on the big wheel to reset the mouse. Just keeping my options open...