Re: Relation of a DRO to a CNC modification of a machine?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "gfull" <gafaw@m...> wrote:

> Alan,
>
> Thanks for that advice, I immediately called Shoptask and was given
> the contact @ Star-Techno that builds the DRO. They have an option
> for output (RS232 port), so I ordered the upgrade to be installed

to

> my unit.
>
> I am told that the DRO processor may not be "fast" enough to keep

up

> with CNC controller...however, I imagine I can build in pauses for
> error checking of some kind (especially when machining critical
> parts).
>
> Thanks again.
> Gary
>
>
> If the DRO is the Star-Techno normal config the scales are

precision rack and pinion. The encoder is rotary and a standard
quadrature output. There is even a way to tap into the quadrature
signal to use it with a PC. That is the good news. Now for not so
good news: The machine and DRO readout have to be really low
backlash if you want to use it as the postional feedback for a servo
system. The introduction of backlash anywhere between the motors
table will cause the servo system to try and compensate and move
fromone end of the backlash to the other and usually overshooting and
setting up an oscillation of massive proportions. If you have .010 in
backlash it will shake itself like a wet dog.

Now for stepper control, the only affordable closed loop solution
(software can figure out if there have been lost steps and
compensate) is EMC (that I am aware of).

I think the idea of using the DRO manually and to manually verify the
position on the software DRO vs the hardware DRO for specific
terminated moves would let you identify the presence of lost steps.
The solution is not to compensate for the lost steps but to find the
cause and eliminate it so it does not recur. Properly designed
stepper systems used under normal conditions should no loose steps.
Most software for PC control has backlash compensation built in so
part of your tuning might be to run out a distance and back and see
the different between the software DRO and hardware DRO and set the
backlash compensation so the two are in sync.