NXT motor: use as an encoder with removed motor?

[mattallen37]

If I were to use this method, I would apply power to the NXT motors. Maybe not 100%, but at least enough to pull their own weight plus a little.

[HaWe]

we've been discussing this already before...

thank you matt,
that indeed was what I already was thinking about too. But as usual of course it won't be as easy to do as it sounds at the first sight:
I always have to be sure that both motors will run synchronized with 10, 20, 30,...90, 100% power forward and the same backwards all the time. A little tricky if the motors are ramped up or down and rather tricky if the Tetrix and the NXT motors are to be run by PID mode to reach a target, because I can't predict at what time they will run by which power (forward, reverse, or alternating forw/rev when approximating a target in cases they have been intermediately overshooting the target).

But a good idea anyway, I'll try it and will see if the issues can be handled.

a "pure" sensor (of what kind ever) which fits to the NXT motor port would be the best way anyway, that was the point which my TO question actually was about...