Weird DC motors

[Ro-Bot-X]

I just received a couple of Barber Coleman DC motors I bought from Electronic Goldmine. They have an extra coil inside that will generate AC voltage (probably proportional with speed?) on a separate pair of wires. My questions are:

Should I use that voltage (converted to DC) fed through an analog pin to determine the speed of the robot or take it off and replace it with a built in quadrature encoder?

Should I use both?

I want to use these motors for a balancing robot. I don't need to know the distance traveled (impossible to determine accurately), so I thought the voltage generated by the motors may be used instead of the encoders. I still need to determine the relation between the PWM I send to the motors and the generated voltage.

[ArcMan]

I've never heard of a motor like this, but if it generates AC voltage, I suspect that the frequency of the AC sine wave will very accurately reflect the motor's speed.  If you reduced the coil voltage (if necessary) and ran it through an opto-isolated Schmitt trigger, you should be able to get out a nice encoder-like signal.  Stick that output on your scope and see if I'm right.

[dunk]

are you sure the motor dose not just have more than one drive coil?
ie. both are supposed to be connected to power but with only one connected the other is acting like a generator?

dunk.

[airman00]

I've never heard of a motor like this, but if it generates AC voltage, I suspect that the frequency of the AC sine wave will very accurately reflect the motor's speed.  If you reduced the coil voltage (if necessary) and ran it through an opto-isolated Schmitt trigger, you should be able to get out a nice encoder-like signal.  Stick that output on your scope and see if I'm right.

maybe but if the motor turns very little RPM( or 1/4 of an RPM, etc.) i dont think it will be accurate enough

@dunk
RobotX sent me the link in a chat. It is for AC. Its a motor from electronic goldmine , but i dont have the link right now.

[Ro-Bot-X]

Didn't had a chance to test them again and unfortunately I don't have a scope yet. But my DMM has a frequency meter scale of 20kHz, I'll try that. I will also try using PWM to increase the speed of the motor to see how this influences the voltage or the frequency.

Oh, and here is the link: http://www.goldmine-elec-products.com/prodinfo.asp?number=G16477