Re: stepper vs. servo - Allegro and ignoring the edges....

--- 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

I guess there are people who might want to read what was said. This
does not appear to be an Allegro Technical paper, but one written and
then incluced into their references.

http://www.allegromicro.com/techpub2/stp/stp99-11.pdf

Here is the section I was referring to.


"Note: the correct mathematical calculations for microsteps #1 and #7
are: Phase A = 98.1% and Phase B = 19.5% (#1), and the reverse for #7
(19.5%; 98.1%). However, the `non-linear' DAC tolerance is +/-3%;
hence, this `9th' current level (98.1%) was deemed as unnecessary
since it is within the +/-3% window. Further, the step motor
positional accuracies are also <+/-3% (for most PM hybrid motors,
while most `canstack' steppers are >+/-5%); therefore, the 98.1%
current ratio is basically unnecessary for 1/8th-step operation, and
this `9th' current value is circumvented.

Similarly, the first subdivision with a 1/16th step format calculates
to a 99.5% ratio for Phase A. Based upon the +/-3% DAC tolerance and
the step motor accuracy =+/-3%), this 17th current ratio is also
deemed as needless for low-cost, simple, open-loop, microstepping
designs."



As I read that, it seems they are saying that even though the
calculation says the current should be 98.1% of the fla, the motor
error is 3% and this it is not worth chasing.

Dave