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