Re: Microstepping thru the Tulips

It is normal for drivers running a stepper motor
above the corner frequency to never reach the
specified coil current before the next step comes
along. The result is that a micro-stepping drive
will seem to act more like a full step drive at
higher speeds. This is normal and is above the
dreaded resonance ranges. When the step rate
drops to somewhere that resonance and micro-steps
matter, the relationship between the PWM or chop
rate and the step frequency once again allows
real micro-stepping currents to be established.

Regards,
Steve Stallings



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Phil Mattison"
<mattison20@...> wrote:

>
> 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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