Can you use a stepper motor as input?

I'm not too versed in EE so I apologize for my duncitude -

My thought is that you can precisely control a stepper motor, but can you inversely get precise readings out of a stepper motor?

For example: if you have an xy plane controlled by stepper motors, you can say "navigate to 0,0 by moving x motor 40 times and y motor 30 times". If you manually drug both axis in position, could you get a reading of x: 40, y: 30?

Answer

While turning a stepper motor does generate an AC output, it's not suitable for a general position sensor.

The magnitude of signal it generates is proportional to its speed. As long as the table moves at least at at a minimum speed (which will depend on the sensitivity of your measurement hardware), then you will see all the pulses. Move at a lower speed, and you will miss pulses. With a simple voltage measurement, you would not be able to tell whether the motor was stationary, or turning very slowly.

A position sensor really needs the ability to be able to tell stationary from slow. An optical encoder can do this, as can a synchro.

Some constructions of stepper may permit you to sense its position by measuring the inductance of all the windings, treating it as a sort of variable reluctance synchro. However, by the time you've done that, it would be far simpler to use an optical encoder.