Re: stepper vs. servo

Constant off-time choppers hiss, squeal and "spit" because you have
two free running variable frequency oscillators. A typical chopper
(20 microsecond off-time) may run at 25kHz at min current, dropping
to 15kHz at max current.

There is an unavoidable tendency for these oscillators to phase-lock
(run at the same frequency). The winding currents diverge while phase
locked so the lock has to break after a number of cycles. This
repeated locking and breaking generates frequencies in the audible
range, (lock for 9 cycles, break on the 10th generates a loud 2kHz
squeal for a chopper freq of 20kHz).

I get around this by making the current servos synchronous, and by
definition, always phase locked. A 20kHz clock rising edge causes
both coils to increase in current. Once current reaches the desired
value, the current is caused to decay until the next clock rising
edge. Both coils switch at the same constant 20kHz, no audible sub
harmonics are generated and the drive is silent.

Mariss


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "ballendo" <ballendo@y...>
wrote:

> 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