Re: [CAD_CAM_EDM_DRO] Re: DRO

"k8zre " wrote:

>
> Doug...
>
> Shooting Star 3 axis DRO is reported to have TTL quad. output for
> about $750.00. It uses a mechanical wheel encoder on a linear track
> having 2000 counts per inch. I would like to know your thoughts on
> this system especially as it might apply to servo encoder feedback.

Yes, that (TTL quadrature output) is one of the benefits of the
Shooting Star.... (but you have to remember to order that option for
$50 or so extra). http://www.star-techno.com/index3.htm

Yes a linear scale is great for servo motor feedback, even better than
a rotary scale on the leadscrew, because the linear scale directly
feeds back the motion of the table (whereas there are various sources
of leadscrew-to-table backlash, such as loose thrust bearings and
worn leadscrew nuts).

I'm sure 2000 cpi (counts per inch) will be fine in practice, but
you must remember that the encoder resolution is not really the
system resolution on servo systems... there might be +/- 5 or more
count difference between the actual and commanded position (at
128 count difference the Gecko (for example, I think) faults). So 5 counts
is 2.5 thou and 10 counts is 5 thou. Therefore I'd prefer a
5000 cpi or even better, 10,000 cpi linear encoder for feedback
to the motor driver... although a half thou display to the human
is sufficient for manual table movement (any more, drives you nuts
with needless precision).

I notice with 8000 CPR (counts in quadrature/rev) USDigital encoders
on my cnc lathe (with 5 TPI ballscrews) that the 'zero seeking'
(while stopped) results in no movement readable on a tenths dial
indicator, and no noise from the motor (although that might be
the G340's behaviour at stopped speeds). Other drivers I've had
actually continually oscillated +/- 1 or 2 or 3 counts at stop
which is annoying.

Anyway, I'm a fan of high resolution encoders (as long as the
computer or Gecko is looking at it, instead of humans).

Doug Fortune
http://www.cncKITS.com















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