On 09/11/2011 08:54 AM, Lloyd E. Sponenburgh wrote:
Jon fired this volley in
:

You really want to make sure these scales are DESIGNED for CNC motion
CONTROL applications, and not just DRO or static position reading use.
I just went through a mess with a CUI capacitive encoder that looked
great, but I couldn't tune the servo loop well. I finally discovered
there was a really nasty LAG in the encoder's response to
acceleration.

They guarantee these for output up to 2MHz (10m/s on the 5u scales, or
2m/s on the 1u ones). There doesn't appear to be any processing other
than signal conditioning going on in the reader heads.

I don't believe that. It would require there be magnetic stripes
recorded on the scales at a pitch of 1 um, which is pretty close!
Of course, they do this on computer disk drives, but the head rides
only a couple um above the platter there. The spacing on the read head
on a CNC measuring scale has to be bigger to allow for dust, swarf and
non-straightness. So, they really have to be interpolating the basic
scale resolution. How well they do that is up to the manufacturer.
I just wanted to throw out a warning that if the scales are made for
DRO use, only, then the latency between position changes and the
quadrature output may not be controlled. If people are using these for
closed-loop motion control, then this has probably already been dealt with.

I was pretty shocked that the CUI encoder had such a serious problem.
It is a commercial product from a major vendor, and sold specifically
for motion control use.
Besides... I can do all this at NO risk. I have the DRO, I have encoders
on my motors, and I won't be destroying any of it to have a go at this
method. All I'd lose initially would be my time to try it, and the use
of the DRO display, but I'd gain that back with the EMC display.
Well, that will work fine as a DRO, and you will just see how it works
as a closed-loop sensor. You can compare a rotary and linear encoder
on Halscope, like I did, to see if there are any anomalies.

Jon