Glass scales

[Dave Engvall <dengvall-at-charter.net>]

Hi everyone,
When I automated my knee mill (Cincinnti contourmaster....think BP ) I used
acu-rite 0.0002" glass scales on all X, Y and W. It came with a lashup on Z
but 50 thou backlash doesn't contribute lend itself to good tolerances.

Jon speculated some time ago that with 0.004 backlash and glass scales I
might have some trouble tuning. It turns out they are tunable but the
results might not be the best.

The other day I converted the X axis to an 250 cpr encoder on the ball
screw. This was too good a chance to pass up so I hooked up the glass scale
to an Acu-rite II DRO and proceeded to compare results.
Using G0 (60 ipm) moves it tracks rather well considering the backlash.
G0 X 0 followed by G0 X n produced agreements of usually less than 0.001 and
often 0.0002. Slowing things down to my usual operating speeds of 10-15 ipm
the dynamics change and disagreements got larger but never more than 0.002.

The real change is in tuning: I needed a D of 6 to 7 with the glass scale to
get reasonable following which also produced funny noises in the servo motor
as it tried to follow the commanded position....and dithered plus or minus a
thou or so at audible frequencies producing a solid band when (commanded -
actual) was plotted with gnuplot.

With the encoder I was able to reduce the D to 1.5 and get much quieter
operation and occasional wandering of one or two encoder counts. The next
step here is to change to a higher resolution encoder and see if it gives
better control. 

Taking this one step further I may try putting the encoder on the servo
shaft. 

In retrospect: glass scales might be a good choice if one has tight ball
screws or is working in a position where static control is good enough ie G0
to a position with G61 enabled and then drill a hole or in CMM applications
but probably not the best choice in dynamic situations with older equipment.

Dave