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

Michael Fagan wrote:

> The Newall system scales are available in a 5 um resolution and would
> probably work for this application (they're not optical scales, they're
> based on capacitance I think). Also, I believe that the Acu Rite MillPWR,
> their turnkey cnc system, uses servos with rotary encoders for gross
> positioning but relies on feedback from their glass scales for closed-loop
> feedback, as well as the 3-axis DRO component of the system.

Newall scales are based on steel bearing balls in a stainless
tube, and use a magnetic sensing and interpolation scheme. I
have never seen Newall scales used in a CNC application, and
therefore I worry that they may be too slow to readout for
motion control. They say "sample period = 100 us", but I really
don't know what that means. If it means that the max pulse rate
is 10 KHz (1/100 us) then that may be a problem at 5 um.
5 um * 10 KHz = 50 mm/second, which is roughly 120 IPM. If the
spec means that the pulses will not come out faster than 10 KHz,
then the servo loop will get unstable as you near that speed.
(I assume it would catch up later, no problem for a DRO,
disastrous for a servo.)

With the dual-encoder approach, what software supports that?

Jon