Re: [CAD_CAM_EDM_DRO] Closed-loop steppers, was: breakout boards plus - hard choice

scyvt wrote:

> Jon
>
> You seem to be assuming rotary encoders mountes directly on the stepper shaft. That still
> makes assumptions about the linearity of the mechanical drive components. What if a
> timing belt breaks? Seems more like "partially-closed-loop" to me. What about feedback
> from linear encoders reading actual position? More costly, of course.

The problem with linear encoders is that the typical resolutions
are 20 um and 10 um. Those are .000787 and .000394"
respectively. That is pretty coarse resolution for CNC control.
My Bridgeport retrofit with 5 TPI ballscrews and 1000
cycle/rev encoders directly on the leadscrew has a resolution of
40,000 counts/inch, or .000025" (25 micro-inches). This is 31
times the resolution of a 20 um linear scale!

I am using only 3/8" XL series timing belts on the Bridgeport,
and I haven't broken a timing belt in 9 years of use. And, I
dont't use steppers, only servos.

But, if you wanted to use steppers with linear scales, I'm sure
it could be made to work. A good 31" linear glass scale for a
Brisgeport X axis would cost at least $350. You might run into
the problem where the motor has more steps than the encoder has
counts, and that leads to some problems in how you manage that
discrepancy.

Jon