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J

Jonathan Barnes

Jan 1, 1970
0
Mark J. said:
In (Richard Crowley):


That's not "How"... :)
There is a difference between resolution and linearity.

Glass scales are made by a photographic process, and one manufacturer points
out that his plant is underground to reduce vibration to a minimum.


--
Jonathan

Barnes's theorem; for every foolproof device
there is a fool greater than the proof.

To reply remove AT
 
"Mark J." wrote ...

I missed the post with the webby link, but most use a rotary encoder on the
motor spindle (gives many pulses per revolution), it would be linear even if you
had a mile long table.

Clive
 
T

Tom Del Rosso

Jan 1, 1970
0
In Mark Zenier typed:
The typical scale is a row of stripes, 500 per inch, (or the metric
equivalent), chrome on quartz glass. Then there's a scale in the
sensor that's a bit different pitch so that light projected through
both scales gives a moire effect. Like the vernier scale on
(non-dial) calipers. And two photosensors, set to that they read the
the fringes 1/4 of a cycle out.

Ingenious. Thanks for the clarification.
If these sensors are analog, they give you a sine and cosine signal
that can interpolated further.

Oh, so then it is sometimes a real sine and cosine. I thought that was
just a reference to the fact that digital quadrature signals are 90 deg
out of phase like a sine and cosine.
 
J

Jeff Lowe

Jan 1, 1970
0
Mark Zenier said:
The typical scale is a row of stripes, 500 per inch, (or the metric
equivalent), chrome on quartz glass. Then there's a scale in the sensor
that's a bit different pitch so that light projected through both scales
gives a moire effect. Like the vernier scale on (non-dial) calipers.
And two photosensors, set to that they read the the fringes 1/4 of a
cycle out.

If these sensors are analog, they give you a sine and cosine signal
that can interpolated further. (One system I worked on by a factor of
320, so that the resolution was 1/160000 of an inch. Using a laser
interferometer, accuracy was about 5 times worse than that, depending
on defects on the scales). The resolution limit with machine tools
is more a result of vibration than the limits of the encoder system.

Sony glass scales have a scale pitch of about 7300 lines per mm and can be
interpolated by a factor of 4096x for a final resolution of 34 picometers.
Accuracies are typically in the half micron range, but they are repeatable
and can be mapped out in software.
Jeff Lowe
Moore Nanotechnology
 
M

Mark J.

Jan 1, 1970
0
In news:[email protected] (Jeff Lowe):
Sony glass scales have a scale pitch of about 7300 lines per mm and can
be interpolated by a factor of 4096x for a final resolution of 34
picometers. Accuracies are typically in the half micron range, but they
are repeatable and can be mapped out in software.
Jeff Lowe
Moore Nanotechnology


Wow, little chance of that for the average homebrewer. :)
 
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