On Wed, Mar 03, 2010 at 12:36:44PM -0600, Jon Elson wrote:
> >
> What does this time constant actually mean? Is is a response to changes
> in velocity?
> That seems to be the only thing I can figure out it means. If EMC is
> sampling position at 1 ms intervals, then 400 us is 2.5 time constants,
> so the velocity error should be pretty small.
Yes, that's what I inferred; a delayed response to acceleration. Your
direct and second hand experience (quoted below) seems to confirm my
sneaking suspicions. (Drat that thinking! ;-)
Erik wrote:
> > It would be nice to be able to fit encoders to 3 axes on a
> > woodworking machine for the price of one axis with US Digital
> > encoders, but what's the stability like?
> I have put these encoders on the Keling motor, and gotten the servo loop
> to work, using my own servo amps. I DID notice the stability margin is
> less than with some other motor/encoder combinations. The Keling motors
> have VERY light rotors, and so I added just a little mass to the motor
> by mounting a small shaft extension, and it helped quite a bit. I would
> assume this is a lot less angular momentum than even a small leadscrew.
>
> I also had a customer who was experimenting with a very high
> acceleration machine, and I believe he will eventually abandon the CUI
> encoders. He was accelerating at something like 10000 rad/sec^2, which
> is really high acceleration, though. Currently, he has cut the
> acceleration rates to prevent missing counts.
Looks like you've already done the research which my snoopy mind just
thought prudent. Grateful thanks for the practical insight into the
stability limits. _Where_ it would become a problem is what I couldn't
guess from that datasheet. I take your point that too much motor and
too little load inertia is needed to create trouble. That's very
comforting for me, but perhaps your customer is/was trying to push peas
up too steep a hill.
Thanks again.
Erik
--
If you understand what you're doing, you're not learning anything.
- A. L.
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