Re: DRO

> >
> > 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

> > having 2000 counts per inch. I would like to know your thoughts

> > this system especially as it might apply to servo encoder

>
> Yes, that (TTL quadrature output) is one of the benefits of the
> Shooting Star.... (but you have to remember to order that option

> $50 or so extra). http://www.star-techno.com/index3.htm
>
> Yes a linear scale is great for servo motor feedback, even better

> a rotary scale on the leadscrew, because the linear scale directly
> feeds back the motion of the table (whereas there are various

> 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

> 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

> 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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