Re: Glass scales

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

On 8/9/02 8:22 PM, "Jon Elson" <elson-at-pico-systems.com> wrote:

> 
> 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?
Oh, don't I wish. 
I'm using an stg-I and patiently waiting for something to test.
If you have a tarball or whatever I might be able to download and compile.
As usual I'm a little short on resources. E-mail is on a Mac that can burn
CD's but I don't think I have cvs support. Can always download a tarball and
burn on CD then sneakernet to a real machine. (linux).
 Dave
> 
> Jon
> 
>