Re: stepper vs. servo

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