using SPC output for 'occasional' closed loop position verification

Off list I have been discussing with DaveK
(author of TurboCNC) the possibility of using
SPC (statistical process control - ie serial
bit stream position output devices such as
quill scales and digital readout calipers)
in addition to the normal open-loop software.

The idea is that the gcode interpretation
software functions as normal, but occasionally
stops and verifies its current position
(using the SPC). If out a few steps, that
discrepancy is rectified, and off you'd go
again.

The reason why you wouldn't do it more often is
that it takes up to 1/2 second per axis to read
the SPC output. But for a 3D carving/machining
run that takes hours to complete, its a comforting
thing to know that you're not making a junk part
from some missed steps early in the cycle.

Even better, the addition of this sort of checking
is easily added to the gcode interpreter software.

To back up my allegations yesterday that Digital
Readout (digital caliper SPC style) was more
affordable inexpensive than the full fledged
DRO quadrature scales:

http://calaero.zoovy.com/DRO_Quill_Kits_and_Access/

- 6" horizontal display $50
- 6" vertical display $60
- 8" horizontal display $70
- 8" vertical display $70
- 12" horizontal display $90
- 12" vertical display $90
- 20" horizontal display $100

also
- remote display box for above $130 (which you
don't need, but it demonstrates remote SPC works).

so in my case for my 4 axis mill, I
would need a 6,8,20 & 20 (quill,Y,X,column)
costing merely 50+70+100+100= US$320

There is an additional benefit to these quill
scales, in that the visible digital outputs
are available to the machinist during manual
operations without the computer.

(disclaimer - not associated with Calaero and haven't
bought these or anything else from them).

Doug Fortune
http://www.cncKITS.com