Re: [CAD_CAM_EDM_DRO] Re: Microstepping thru the Tulips

Perhaps, but a 100KHz microstep rate and a 20KHz PWM frequency implies that
the coil current is modified five times in a single cycle of the PWM, which
is absurd. In that case, the PWM (or chopper, whatever) cannot produce the
power modulation that is supposed to be the big deal with microstepping. Any
microstep rate above the PWM frequency renders it meaningless. What you end
up with in practice is a single-step driver.
--
Phil Mattison
http://www.ohmikron.com/

----- Original Message -----
From: Mariss Freimanis <mariss92705@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Sunday, August 13, 2006 12:48 PM
Subject: [CAD_CAM_EDM_DRO] Re: Microstepping thru the Tulips


> I have thought a little about the relationships you mentioned over
> the years.:-) I'll just cover the step frequency vs. PWM frequency
> thing:
>
> I think you misunderstand PWM works. Microstepping frequency and PWM
> frequency are completely unrelated. This thought experiment may help
> to straighten it out:
>
> I think you agree that 400 Hz step rate on a full-step drive results
> in a 100Hz current waveform frequency. The full-step sequence repeats
> after 4 full steps after all. This modulates OK on a 20kHz PWM for a
> motor speed of 120RPM.
>
> A 250-microstep drive needs a 100kHz step pulse frequency to get a
> speed of 120RPM. The current waveform frequency is still the same and
> it still modulates OK.
>
> 100kHz is way above the PWM frequency but the PWM never sees it. It
> only sees the resulting 100Hz reference, the same as for the full-
> step drive. This also modulates OK on a 20kHz PWM.
>
> Mariss
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
> <mattison20@...> wrote:
> >
> > I've seen a lot of comments about microstepping on this board as if
> it is
> > the best thing since sliced bread. I wonder how many people have
> thought
> > about the relationships between step rate, PWM frequency and motor
> > inductance. Since microstepping supposedly uses the PWM chopper to
> regulate
> > current into something like a sine wave to improve motor
> performance, it
> > stands to reason that if the step rate gets anywhere near the PWM
> frequency
> > that theory goes out the window. Also, the motor coil inductance
> puts an
> > upper bound on the step rate at which the PWM does anything at all.
> Exactly
> > what those limits are depends on the combination of motor
> inductance, power
> > supply voltage, chopper switching speeds and probably several other
> factors
> > with less impact. So to say that more microstepping is better than
> less is
> > like saying that blue is better than orange.
> > --
> > Phil Mattison
> > http://www.ohmikron.com/
> >
>
>
>
>
>
>
>
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