To quote myself:
I’m actually talking about only 4 total cable motors. Ideally the take-up spools could simply be spring tensioned, and one spool could take up for two motors. (take-up spool is the one with the arrow showing spring tension)
The take-up can be “dumb”, if using something like a capstan as the driver motors the take-up only needs to provide enough tension to produce enough friction for the capstan to function. Generally speaking capstans by their nature are usually designed so they can slip to allow a variable feed rate; for a Maslow a slipping driver motor obviously would be useless if counting rotations is how we are measuring distance.
However, I assume the cord being discussed here is braided, if we use cord with a single, high contrast, “flag” thread I wonder if the measurement could be done optically by watching the pattern of the cord as it is fed and taken up. Or, perhaps the cord can be measured with a pincher roller (like how they measure rope at the hardware store).
It might be better to let each component do its own job well instead of expecting one component to do several jobs. Instead of a driver motor that spools cord, measures distance, and drives let’s let the driver motors drive, let the measuring rollers measure, and let the take-up spool take-up.
Without having tested anything at all I would think that a capstan could be made to have no slip under a given tension (more wraps = more friction. Does it slip? Then add wraps.)
I can think of a few other methods of take-up but the one I just explained is probably the simplest.