Re: [CAD_CAM_EDM_DRO] Re: stepper accuracy compared to servos...

Paul wrote:

>At first read, the original post looked like B.S., however, if the servo
>system in question is a G320/340 step/dir type drive, then yes, the dynamic
>accuacy is going to be suspect. The one point I would disagree with is the
>"you are guaranteed to have a dynamic accuracy of much better than one full
>step" statement. Your accuracy is only going to be plus or minus one step,
>never "better than". Factor in the angular inaccuracies of a typical stepper
>motor, say +/-5% (and even worse when micro stepping) along with dynamic
>errors, and steppers can not compete with TRUE servo systems.
>
> True servo systems, either full analogue or digital SERCOS, will always have
>better dynamic responses than any step/dir system. If steppers were really
>that good, everyone would be using them.
>
>Regards, Paul.
>
>

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