question about DC motor behavior

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I'm not familiar with this motor, but from your description it sounds
like it's an all-in-one design and that the motor contains the servo
circuitry for speed control. If that's the case, the first place I'd
look is the control circuitry on the motor. It may be very simplistic,
for whatever application the motor was originally intended for.

But...what does "1/3rd percent of its average" mean? Do you mean 33%
variance, or 0.3% variance. If the latter, how are you confirming this?
An error rate of 0.3% doesn't sound like a lot to me...

-- Gordon

Gordon,

Some more detail...

The motor is a brush-commuted DC motor with a double-ended shaft.  The
encoder is attached to one end, a gearhead to the other:  There are no
control electronics. The motor is rated for 7.2 V, 4 Watts max.  I've
been running it with 6V from a good quality bench-top power supply. The
gearhead is a 2 stage spur gear arrangement with a 6:1 reduction
ration. Again, the encoder reads directly from the motor shaft (not
from the reduced gearhead output).  To get some sense of the motor, you
can look at one of its upscale relatives at Acroname
http://www.acroname.com/robotics/parts/R179-6V-ENC-MOTOR.html  This
particular one is Maxon package 132015 and I bought it used.  The
encoder is a  two-channel, quadrature encoder with 100 counts per
revolution.

To evaluate the speed, I take lots of samples (at 100 counts per rev,
they pile up pretty fast), performing measurements with each encoder
click.  Then I average together the 1st, 101st, 201st, 301st... 9001st,
etc, the 2nd, 202nd, 302nd, etc., etc. Thus  persistent features show
up, less persistent features go away.  Since the encoder doesn't have
an "indexing" channel, I don't know its position when the measurements
start, but visual inspection of the results from several test runs
appear to be consistent, except for a difference in phase.

Anway, looking at the data (which I didn't have with me when I posted
earlier) there is a persistent set of about 15 percent of the
measurements where the speed is larger than the rest (normally about
4050 RPM, increases to 4055 RPM.). There is also a spike where for 1
interval I measure about 4060 RPM. Again all the numbers are the
results of averaging about 90 measurements per each "encoder bin" so
they are persistent.  As far as where the 0.3 percent came...  that's
the result of doing arithmetic in my head.  I was thinking of the
(4060-4050)/4050 = about 0.0025.   The main reason that I find this
small number interesting is that it persistently happens in the same
place.   Something's going on and I wish I knew what it was.

My equipment is pretty minimal (I'm not a hardware guy and what I do
have is kind of embarrassing), but I've looked at the data a couple of
different  ways and think that my speed estimates are basically correct
and that I do,  indeed, have the data to support the kind of precision
I'm tossing around. I haven't done much analysis yet, and haven't
tested with slow speeds or loaded motors.


Gordon McComb wrote:

 >
 >>I'm not familiar with this motor, but from your description it sounds
 >>like it's an all-in-one design and that the motor contains the servo
 >>circuitry for speed control. If that's the case, the first place I'd
 >>look is the control circuitry on the motor. It may be very simplistic,
 >>for whatever application the motor was originally intended for.
 >>But...what does "1/3rd percent of its average" mean? Do you mean 33%
 >>variance, or 0.3% variance. If the latter, how are you confirming this?
 >>An error rate of 0.3% doesn't sound like a lot to me...
 >>
 >>-- Gordon
 >
 > gwlucas wrote:

Have you tried testing the motor under a mechanical load?

Is this variation causing a problem? If it is causing a problem, I would
suggest putting a flywheel on the motor shaft to smooth out the torque
ripples.

I am not sure, but my first guess on the cause would be that as the
brushes cross the boundaries of the commutator, there are periods where
the brush spans two commutator bars, powering up two sets of windings in
parallel. Worn brushes will increase this period, as they contact a
larger angular part of the commutator.

Good Luck,
Bob

Bob and Gordon,

Thank you both for your excellent advice.  After looking at the data
again, I'm pretty sure that I am looking at mechanical issues not a
glitch in the sensor. I've now got a matching motor that I can test
with for comparison and and friend has promised me a load of a flywheel
that matches the 6mm output shaft so that I can see how the motors run
under load.

                                                         Gary

As this is an open system, and unloaded, and if you're sure of the other
variables you mentioned, then the variance is more than likely
mechanical, probably due to some wear of the internal gears, or even the
normal tolerance of the gearhead and bearing surfaces. A (very) slightly
out-of-round gear within the gearhead could very possibly produce what
you're seeing, for example. As the encoder is attached to the motor
shaft that is attached to the gearhead, any wear or misalignment, even
minor, in the gearhead will directly affect the rotation speed of the
encoder wheel.

An unloaded output speed with 0.3% accuracy is pretty good. Actually
quite good.

-- Gordon


gwlucas wrote: