Brushed Motor

Lock the armature so that it cannot rotate

How would that help?

I mainly want position control of the motor using the EMF some how...

I don't think so. If the motor is well designed, the back-EMF should be relatively constant. It is possible to sense position on some types of motors using back-EMF or current (for example: switched reluctance motors), but not with brush motors. You'll need an position sensor.

You either need to use a stepper motor or gear down the dc motor and use a resolver or encoder to sense position. Back emf would sense motor speed.

Steve is right, not possible.

Yes, well, there is if you go for "brushless" dc Motors. I think the question you are asking is whether you can eliminate the brushes from a motor!

Look up "sensorless brushless dc motors" on the www and there should be a number of electronics/motor manufacturers marketing this technology.

I know about brushless motor, but that would mean you need a frequency control....What if the motor was slow 10~20rpm? wouldn't you see the EMF spikes more easily?

If you look at the spectrum of the DC current, you will see a component at the commutator bar passing frequency (we looked at this as a diagnostic tool when I was at GE Research). However at best that gives you an estimate of speed - and of brush condition - if you're interested in diagnostics :-)

http://www.spectrum-engineering.com/01/index.htm

If it is 12v or so, this out to do it.

Please read the post. He wants to sense and control the rotor position of a DC motor without using a position sensor. Spectrum doesn't seem to address this issue.

This looks like a problem that the model railway world solved many years ago by froviding feedback controllers. Check out "Gaugemaster" on the www.

I'm not sure how this is relevant. He wants to sense and control the rotor position of a DC motor without using a position sensor.

Simple . . . Integrate and scale the terminal voltage, compensate for the IR drop in the armature and include brush contact resistance, and run it really, really slowly, otherwise, the brake you must have to stop at a point in the rotation may not work well.

....which is why NOBODY does it that way!!!! You really need a position sensor for DC motors with a commutator..

EUREKA!!!

Open loop integration won't work. The integral will quickly diverge from the true position (where quickly = milliseconds, or seconds at most). We've used integration in the past, but only when there's a way to reset the integrator at very frequent intervals.

Puleeeezeee! I was only joking . . . But it works well mathematically!

Wasn't it Yogi Berra who said: "In theory, there's no difference between theory and practice."

Yeah I did try Open loop integration, but its was out 5mm every time I went forwards then backwards..lol

I was of course joking when I suggested you prevented the amature from rotating I misread your post and thought you wanted to eliminate the back EMF!

And I've been thinking of you as syphrum alone... Turns out there is a whole other side to you.

I was indisposed and called upon my alto ego to stand in for me

......sadly, using your alto ego is always a failure.......

... especially if you are a baritone

OK, shoot me.