Re: Glass scales

[Jon Elson <elson-at-pico-systems.com>]

Dave Engvall wrote:

> 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.

Yup, this sounds like what I expected.

Are you using my PPMC cards with this?

Jon