Stepper motor noise (was Re: Chopper Transient Suppression)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
<mariss92705@y...> wrote:

>
> You are probably using a "constant off-time" switching topology. The
> screeching, grunting, whistling, hissing, etc. comes for free with
> that method.:-)

I'm not sure about the terminology, so I'll explain what
I'm doing, and maybe somebody can tell me what the correct
term for the technique is.

My system turns on all three (or four if the 4th one is
plugged in) stepper moters at the same time, blanks for 4ms
(the .2uS blanking is history), and stays on until the
voltage through the current sense resistor exceeds the a
voltage provided by an 8-bit DAC. The current sense
resistor is a 2W .51 Ohm metal film resistor. This occurs
at a frequency of 25kHz. I am pulse width modulating the
L298 via the Enable pin. The IN1 and IN2 pins are held
constant for the entire width of the pulse.

The noise I am hearing is extremely high pitched and quite
loud. Loud enough that I wear ear protectors. This is
when the motors are standing still. I would guess the sound
frequency is in the 15kHz range. There is no lower frequency
beating noise. One possibility is that there is a bug in my
firmware and it oscillating at 15kHz rather than 25kHz.
I'll borrow my friends scope and see if I can measure the
distance between pulses.

Again, everybody thanks for all the help; it has been
fantastic!!!

Thanks,

-Wayne

[snip]