On Saturday 25 July 2009, Haberler Michael wrote:
>a while ago I described how I added a simple optical encoder wheel to
>my lathe. Meanwhile, I eventually got it to work with G33 threading,
>but it wasnt as straightforward as I would have believed, so I thought
>I'd describe my experience.
>
>My first attempt was to use the encoder component as described in the
>Spindle Feedback example (User Manual chapter 34). Trying that with
>the threading.ngc example failed miserably - the synced-motion
>threading part was everything but synchronized - heavy speed
>variations in the Z axis (see http://www.youtube.com/watch?
>v=efiNyzhGFbQ , starting around second 27).
>
>I traced the problem to serious noise on the encoder.0.velocity signal
>(red signal in http://mah.priv.at/gallery2/main.php?g2_itemId=69102).
>I first tried to feed it through a lowpass (white signal) but that a)
>didnt work either and b) massaging a feedback signal would be
>counterproductive to start with.
>
>I admit that the codewheel finish left to be desired - I managed to
>glue it onto the pulley with maybe 0.5mm runout (the outer diameter is
>18cm). And the disk isnt perfectly flat on the pulley - too much glue
>here and there, so the disk-sensor distance varies as well. These
>inaccuracies cause periodic fluctuations in the index signals, the
>relation to the spindle revolution can clearly be seen (note blue
>index pulse). So that introduced some phase noise on the signal, and
>that in turn causes the position estimate to fluctuate wildly, hence
>causing the strange Z move pattern.
>
>Adjusting the sensors and pulley brought some improvement but not
>sufficient. So I played around with encoder options.
>
>The fix was finally to turn off x4-mode and use encoder.0.position-
>interpolated instead of encoder.0.position to feed into motion.spindle-
>revs.
>
>The noise is pretty much gone
> (http://mah.priv.at/gallery2/main.php?g2_itemId=69105 ) - except for the
> wobble introduced by the runout of the codewheel (which btw can be clearly
> heard from the Z stepper during the
>threading run :-).
>
>The setup works fine for threading now, but I'll eventually change
>this to a more precise encoder wheel.
>
There is a bit of python code on the wiki that when fed to a 3 axis mill, will 
carve a quite good encoder wheel from a sheet of alu, say about 16 to 18 
gauge.  I took that and modified it to fit the much smaller spindle, trapped 
between the bearing adjuster nuts at the left end of the spindle of my 7x12.  
I also reduced the number of holes so the hole to space was about 50%, I think 
in half from that code.  And used a 1/16" diameter end mill and several passes 
so as not to push the light alu out of shape or out of flat.  It is on the 
lathe now, and I might have it working but when I built the interrupter opto 
assembly, I thought I would hook it up and test it while laying on the table, 
and due to a lack of docs on the opto unit, apparently hooked it up in reverse 
polarity & blew all 3 of them.  So now I need to get some fresh ones, with a 
smaller air gap, and make it again.  Bummer.  So many projects, so little time 
left.

>hal of test setup is at:
> http://mah.priv.at/cgi-bin/viewvc.cgi/emc-drehbank-test/drehbank-k12a.hal?r
>evision=1.6&root=CVS&view=markup
>
>
>out of curiosity - anybody got spindle-synced motion with *just* an
>index puls (1ppr) to work?

Work, maybe.  Reliably? Very doubtful.

>-Michael
>
>
>btw - I'm stunned by EMC2's capabilities - great piece of work, folks!

So am I, amazed at what it can do if I can just write the code, but I'm 
usually yelling for help on IRC.  Dunno what I'd do without these guys.

>
>
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