Chopper methodology was Re: stepper vs. servo

Mariss,

Thank you for the reply. As I read what you wrote, it sounded
familiar; first I thought of the Hard sync of two synthesizer VCO's
(music), then thought of the ST chips, which use constant-ON time...

Here is a quote from the L297 spec sheet (under the heading circuit
operation):

"A common on-chip oscillator drives the dual chopper. It supplies
pulses at the chopper rate which set the two flip flops FF1 and FF2.
When the current in a winding reaches the programmed peak value the
voltage across the sense resistor (connected to one of the sense
inputs SENS1 or SENS2) equals Vref and the corresponding comparator
resets its flip flop, interrupting the drive current until the next
oscillator pulse arrives. The peak current for both windings is
programmed by a voltage divider on the Vref input."

This sounds exactly like what you just decribed, but I know htat
these chips spit, squeal and hiss too!

What am I missing?

Thank you in advance,

Ballendo

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

> 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