steppers and DC motors as cheapo encoders ?

I know that the kinds of uses you are talking about are done.
Specifically, there are some DC motor controllers that stop providing
power for a brief moment so that it can measure the "back EMF".
Essentially using the fact that the DC motor will continue to spin
from its own inertia, and become a generator.  I know this is done in
some robotic wheel driver circuits.

Any DC motor can be used like a speedometer.  Just measure the voltage
the motor generates while it is being spun in generator mode. (you may
need to filter the voltage to some extent)

I am sure any stepper (or AC) motor can be used in a comparable way,
but instead of a DC voltage, you get an AC waveform.  You can use a
frequency counter to measure the speed.  It may also be possible to
rectify the AC voltage and then meaure the resulting DC voltage.

Sorry, but I have not done any of the above, only read about them. I
don't have any sources for more info.  A search for "robot" and "back
EMF" may be a good start.

Joe Dunfee

Yeah my favorite motor controller to date - the Sabertooth 2x10 - works this way
(if  I remember correctly).

Thanks !
JCD