Re: stepper accuracy compared to servos...

Jon,

Never any argument from me regarding HIGH-END("real") servo systems
versus steppers...

The post and thread was about low-end step-servos versus steppers...

Ballendo

P.S. I'd still like to see a ballbar test of the step-servos. That
would pretty much put it to bed,either way, IMO. Say we take 3
sherlines/taigs; one with Geckos, one with steppers, and one with
your pico setup. Tune each up as much as possible, and then test 'em
with the ballbar (does renishaw MAKE a ballbar that small?) Maybe
follow that up with some "common" toolpaths run on each machine,
followed up by a CMM stat testing. Imention the sherline/taig because
there's a pretty good likelihood of comparing apples to apples; not
so with BP's or other candidates...

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
> Thanks, Paul, for a word of reason. Some of this stuff is starting
to
> sound like
> the OS wars of the past. I think some people think of steppers as
some
> sort of
> really solid detent, but they aren't. If you've ever seen those
dynamic
> response
> plots of a stepper arriving at the end of a move, with the line
that wiggles
> WILDLY back and forth across the center of the step 5 or more times
before
> settling, you realize the stepper motor is not very stiff at all.
Comparing
> steppers to cheap brush motors that barely qualify for servo use,
fitted
> with 128 cycle/rev encoders probably makes the steppers look really
good.
> But, comparing fine motors designed for servo use from the ground
up,
> fitted with 1000 cycle/rev encoders and a high gain velocity servo
amp
> is another story, especially if it is tuned right. I have gotten
the graphs
> from EMC, so I know what my servos do, and they stay tighter than a
> stepper could be expected to do. So many people think they can
position
> a stepper motor to 1/2000 of a rev, just because they have a
microstepping
> drive! It sometimes gets hard to fight all that.
>
> The commercial machine manufacturers have been busy, of course, and
> have continued to tighten the response of these systems. They are
now
> interpolating high resolution encoders to get 1 uM resolution from
their
> encoders!
>
> Jon