Re: Microstepping thru the Tulips

"Chopping" is a method of regulating current in a coil by switching
the the power supply voltage across the coil to reverse the direction
of current flow.

Choppers oftentimes use a full bridge (4 MOSFETs or bipolar
transistors) just like a PWM (pulse-width modulation) does. Both have
similar looking power stages.

Choppers use the "constant off-time" method to regulate current. This
technique causes the switching frequency to vary as it regulates
current. Step motors have 2 windings so two choppers are needed.
These 2 choppers interact (phase lock and break) and that produces
the unpleasant grunting, squealing and hissing sounds one associates
with choppers.

PWM also regulates coil current by switching but the major difference
is the switching frequency remains constant. In fact the two PWMs in
the drive are clocked from the same crystal controlled oscillator. No
audible noise or motor dithering is possible.

The difference between choppers and PWMs is entirely in the control
section of the drive. PWM gives generally better motor performance at
the expense of greater circuit complexity.

Mariss







--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
<dave_mucha@...> wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
> <mariss92705@> wrote:
> >
> > 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 how 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
> >
>
> It sounds like the chopping and PWM and microstepping are all
> unrelated (isn't de-coupled a better word?), yet doing a very
> choreographed dance.
>
> In your post, you didn't describe chopping and it's part.
>
> I have always thought chopping was a separate circuit from the pulse
> rate and also the PWM.
>
> Can you elaborate a little on the chopping ?
>
> Dave
>