Re: Microstepping thru the Tulips

Phil,

(1) Try a microstepping PWM drive, you'll like it.
(2) You'll see it works despite your insistence it can't.:-)

At a speed of 100 full-steps a second (30 RPM), 1.8 degrees of motion
takes 0.01 seconds. There are 200 PWM cycles in that 0.01 second if
the PWM frequency is 20kHz.

The reference will simply get sampled 200 times for that time
duration, it matters not if the reference is digitally generated at a
100kHz rate or if the reference is pure analog (infinite clock rate).

You can also have a full-step frequency in excess of the PWM rate. We
do it all the time by running a step motor to over 10,000 RPM or
35,000 full-steps per second. We simply reset the 19.531kHz PWM 8-bit
counter (counted down from a 5MHz crystal oscillator) on every full-
step edge. It's a sorry motor that can't reach 10,000 RPM unloaded on
a G203V.

Mariss



--- 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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