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'[EE] Back EMF and brushed DC motors.'
2007\03\31@161840 by

I'm currently doing a project (persistance of vision clock) and I'm
experimenting with a couple componets and ideas that arent really needed for
the clock, but could be useful in other applications. I will be using a
brushed DC motor to turn the clock, and I was wondering about feedback from
back EMF. Ive seen back emf being used by stopping the pwm output to the
motor and measuring how fast the motor is going by measuring the voltage
generated. But I was thinking more along the lines of something that would
work without removing power from the motor.
I was thinking something like putting an inductor in series with the motor,
and watching for spikes/dips in the motors voltage, as the brush crosses the
commutator. I dont know if the motor would momentarily go open circuit as it
crosses the joint in the commutator, or if it would halve in impedance if 2
coils got connected in parallel for a short moment. I guess that would
really depend on the motors construction. Anyone ever experiment with this
of turning off the motor output, waiting for the current to stabilize, and
then measuring the voltage across the motor, either that or stuff about
brushless DC motors.

Either that or a way to make a stepper motor turn @ 1200rpm fairly quietly,
I was thinking about a VFD (variable frequency drive) but making my own VFD
with an output that is somewhat close to a sinewave to keep the motor quiet
might be a bit much for me now.
Thanks,
Jonthan Hallameyer

Hi Johnathan,

If I understand your application properly, I would think that you need
to know absolute position in the rotational cycle, not just roational
speed. If that is truly what you need to know, then I don't see how
you are going to get it from motor back EMF or noise from the brushes.

I'd recommend using either an optical or magnetic sensor to give you a
single pulse as the motor passes some rotational reference point. By
counting the number of these pulses per second, you can get speed.
Then, you can make an estimator which resets itself to 0 deg when it
sees the pulse and then counts forward from there based on time and
the latest best estimate of rotational speed.

As for back EMF sensing, I have never tried it. For an application
such as this, I think measuring speed would be very doable because
torque will be a monotonic function of speed. This means that the
current in the motor will increase as speed increases, so that the
voltage across the motor terminals (which is the back EMF plus I*R),
will increase monotonically with speed. You could calibrate this and
then use a lookup table or equation fitted to the data and get a
pretty good estimate of speed. Assuming that your PWM is operated in
such a way that there is an "on" period and an "off" period (where the
motor is freewheeling), and that the motor inductance is high enough
to keep the current fairly constant during the "off" period, then you
could just take your measurements when you know that the PWM is in the
"off" part of the cycle.

Another possibility is that you might find that there is a very
consistent relationship between applied PWM duty cycle and rotational
speed and you might not need a measurement.

The method of counting spikes of noise when the commutation happens is
probably doable but would, as you say, depend on the motor
construction. I am under the impression that most motors never totally
break the circuit but rather short several windings together at
commutation step points.

Sean

On 3/31/07, Jonathan Hallameyer <jmhtaugmail.com> wrote:
{Quote hidden}

> -

Sean,
I was planning on using a phototransistor and LED to index the LED array
every time it went around on a rotation, and the method of pausing the PWM
would work pretty well when the rotor is on the back side of the clock, I
just wanted to see what I could do with brush noise for later robotics use.
I have a plan which I could get the clock working with, just gotta
overcomplicate it a bit and see if I can use that somewhere else.
-Jon

On 3/31/07, Sean Breheny <shb7cornell.edu> wrote:
{Quote hidden}

'[EE] Back EMF and brushed DC motors.'
2007\04\01@074647 by
Take a look at dc motor speed regulator ICs used for tape mechanisms. The ones
using negative resistance.

Peter P.

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