Re: stepper vs. servo

Mariss,

Glad to see your input on this. Perhaps the Allegro designers also
consider 98% of a design close enough to 100% <G>...

What do you use instead of "constant-off" time? I seem to recall you
sayingsomething about this a year or two ago?

ST uses constant-ON in many of their products; and they also hiss and
spit<G>. I'm assuming it's the "constant" that's the problem???

Thank you in advance,

Ballendo

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

> I think that is an interesting and confused assertion by Allegro.

The

> motor error of 3% probably refers to the non-accumulative step

error

> of a premium motor. This is a cyclic error, not a step to step one.
> Step-to-step errors are about an order of magnitude smaller, well
> under 0.5%.
>
> Calling 98% "close enough" to 100% is not good enough; it is a
> perceptible error in motor smoothness anyone can spot in a side-by-
> side comparison.
>
> Allegro also makes other, larger mistakes that affect motor
> smoothness. This is their insistance on switching between
> recirculating and non-recirculating modes while stepping. That
> introduces large offset errors at the full step locations that
> completely swamp the potential microstep accuracy.
>
> My other complaint is their continued use of a "constant off-time"
> chopper method for the current servo loops. This produces the usual
> assortment of hissing, squealing and grunting sounds.
>
> A well designed (read low distortion) drive used with an accurate
> motor (the new square ones) should completely resonance free. That
> means no perceptible vibration at all as you slowly sweep the zero

to

> 2 revs/sec speed range. Allegro doesn't even come close; they

should

> re-think their switching topology. Otherwise they're nice chips.
>
> Mariss
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
> <davemucha@j...> wrote:
> >
> > Interestingly, Allegro's paper notes that at the 1st and 16th

step,

> > the current is so close to full that the value (98.089%) is

smaller

> > than the motor error (3%) so they ignore the very end of the
> ranges.
> > I'd quote the pages, but Allegros site is not responding. If
> anyone
> > wants the reference, please let me know.
> >
> > But that opens the question of how others can get 256 microsteps

or

> > more. And THAT really is a more general motion control questions
> as
> > even though we tend to think our interests are 98% of the

universe,

> > CNC machining applications of motion control are probably more in
> the
> > 2% range. I'll bet Hewlett Packard (one supplier) sells more

Laser

> > Printers in one year using motion control than all the home brew
> CNC
> > stuff ever built.
> >
> > Dave