>=20
> On Jun 20, 2011, at 4:57 PM, Jim wrote:
>=20
> Hi,
>=20
> I'm hoping that someone more experienced with stepper motors than I can s=
hed
> some light on a problem I'm having.  I am using a PIC18F2410 with a MAX58=
20
> DAC and a pair of LMD18245 H-bridges to drive a hybrid stepper motor with=
in
> a speed range of 2.4 RPM to 192 RPM.  The motor produces a great deal of
> audible noise between ~70 RPM and 100 RPM, which I'm attempting to reduce=
..
> The I2C interface to the MAX5820 isn't fast enough to maintain a 1/32nd
> microstep increment through the entire speed range of the motor, so I
> increase the microstep increment at the following threshholds: 1/16th
> microstepping above 45 RPM, 1/8th microstepping above 84 RPM, and then 1/=
4
> microstepping above 120 RPM.
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> Background on the circuit:
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> This is a sine-cosine microstepping chopper drive.  The MAX5820 has each =
of
> its outputs connected to the Vref pin on one of the LMD18245s, to provide
> the chopping currrent level.  The PIC sets the maximum current level at 1=
..4A
> using the LMD18245's internal DAC's at startup, and leaves them the same
> after that.  I've verified with a scope and logic analyzer that the I2C
> communications between the PIC and the MAX5820 are working, and the MAX58=
20
> is outputting 2 stepped sine waves.  I've also checked with National's te=
ch
> support, and they've told me the circuit looks good.
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> Here's the weird part:
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> When I view the CSOUT pin on one of the LMD18245s, it follows the curve o=
f
> the Vref pin for most of the step period, but as the motor speed increase=
s,
> there is a flattened region of the CSOUT waveform right before the zero
> crossing, shown in these scope images with CSOUT on the top and Vref from
> the MAX5820 on the bottom (zoomed out timescale:
> http://s1218.photobucket.com/albums/dd411/picster733/?action=3Dview&curre=
nt=3Dzoomedout.jpg)
> (zoomed-in timescale:
> http://s1218.photobucket.com/albums/dd411/picster733/?action=3Dview&curre=
nt=3Dzoomedin.jpg
> )
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> As the motor speed increases further, the flattened region changes to a
> second peak in the current.  Concerned that the LMD18245's internal
> comparator wasn't switching at the right times, I viewed the COMPOUT pin =
on
> the scope (bottom) along with the CSOUT pin (top), and during the time th=
at
> the current is rising to the second peak, COMPOUT is low, indicating that
> the low-side switch of the H-bridge should be off, forcing the current to
> recirculate through the high-side switch and the opposing high-side
> intrinsic diode.  Image here:
> http://s1218.photobucket.com/albums/dd411/picster733/?action=3Dview&curre=
nt=3D0615111856.jpg.
> This second peak is most apparent at the motor speeds that produce the
> worst audible noise, so I believe that the non-sinusoidal current wavefor=
m,
> with it's sharp drop to zero when the DIR input switches, is the cause of
> the noise.  The state of the COMPOUT pin makes sense, because the motor
> current detected on the CSOUT pin is higher than the current level set by
> Vref and the internal DAC.
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> So my question is, how can the current be increasing during this time, an=
d
> what can I do to fix this problem?
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> Note: the maximum current level was set at 1.88A for the photos I took, b=
ut
> reducing the maximum current with the internal DAC's merely reduced the
> amplitude of the CSOUT waveform proportionally, without any apparent chan=
ge
> in shape.
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> Thanks,
> Jim

One obvious thing to check is mechanical resonance issues.
Change the rotational mass of your stepper motor.  Add a shaft ring or simi=
lar.  Then check to see if the noisy sections of the RPM curve have changed=
..  See if your waveform issues have changed.
99guspuppet
--=20
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