Subject: Linear encoders.

From: [email-masked] (John Dammeyer)
Date: Sun, 30 Jun 1996 18:07:38 GMT

Some of the DROs for milling machines in the various catalogues are
listed as having a resolution of 0.00015"/12".  Given that standard
slotted opto devices have a slot about 0.020" wide,  how do these more
accurate linear scales work?

I can fathom that a strip of glass has been metalized,  then a mill or
a shaper type machine scratches marks every 0.0004, in two rows to
give the biphase - quadrature output but does that mean the slot on
the sensor is also only 0.0004" wide.  Does it let any light in?  Is
the LED transmitting the signal really really bright.  What about
defraction in such a narrow slit?

Anyone had one apart to see how it works?

Thanks,

John.

Pioneers are the ones, face down in the mud,
with arrows in their backs.
Automation Artisans Inc.      Ph. 604-544-4950
6468 Loganberry Place         Fax 604-544-4954
Victoria BC CANADA V8Z 7E6

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From: Clarence Noyer <[email-masked]>
Date: 30 Jun 1996 22:52:12 GMT

How about a diffraction grating with optical diodes readin the 
interferometer patern?

"CC"

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From: [email-masked]
Date: 1 Jul 1996 02:29:02 GMT

>defraction in such a narrow slit?

The typical glass scale encoder indeed may have stripes that are on the
order of 0.0004 inches wide. The slits are photoetched in the metal,
which is typically chrome on glass. The reason that the light through
the slit is not vanishingly small is that many slits are used in
parallel. The LED light source is collimated by a lens so that it
puts out a parallel beam which is much wider than the slit spacing.
After passing through the scale rulings, it passes through another
set of "analyzer" rulings, then to the photosensor. Like passing
the fingers of two hands in front of each other. As the scale
moves a distance equal to the ruling spacing, the sensor light signal
goes from maximum to minimum and back to maximum, even though
the sensor picks up light from a large area. This not only allows
lot of light through, it also averages over any irregularities
in individual slits.

A second set of analyzer rulings is offset 1/4 of the slit spacing,
as you suggest, to give the quadrature signal. This is usually
photoetched on the same piece of glass as the other analyzer rulings,
so no special alignment is needed. Actually, four sets of rulings
are sometimes used in the analyzer, along with four photosensors,
to make the processing easier.

If the rulings are well-made, the photosensor outputs are sinusoidal
as the encoder scale is moved. This makes it possible to interpolate
electronically and get resolution about 100 times finer than the
ruling spacing.

                      ...when I sit down to write
Don Holly             my intentions are always blameless however deplorable
                      the ultimate effect of the act may turn out to be.
                                           --Joseph Conrad

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From: [email-masked] (Jonathan M. Elson)
Date: 1 Jul 1996 03:13:00 GMT

John Dammeyer ([email-masked]) wrote:
: Some of the DROs for milling machines in the various catalogues are
: listed as having a resolution of 0.00015"/12".  Given that standard
: slotted opto devices have a slot about 0.020" wide,  how do these more
: accurate linear scales work?

: I can fathom that a strip of glass has been metalized,  then a mill or
: a shaper type machine scratches marks every 0.0004, in two rows to
: give the biphase - quadrature output but does that mean the slot on
: the sensor is also only 0.0004" wide.  Does it let any light in?  Is
: the LED transmitting the signal really really bright.  What about
: defraction in such a narrow slit?

The linear encoders use a glass scale, nickel plated in a vacuum coating
machine, and then photoetched with a fine series of lines.  Since a
quadrature encoder produces 4 edges/ cycle, the .0005" encoder has line
spacing of .002" (500 lines/inch), and the .0001" scale has 2500
lines/inch.  The readout has two identical gratings, one offset by 1/4
of the scale pitch from the other.  These receive light in varying
amounts, depending on how the reader grating aligns with the scale.  You
get two sine waves, 90 degrees out of phase form the sensors behind the
gratings.  These are read by 2 comparators, to give 2 square waves 90
degrees out of phase.  Using a grating with many tiny slits that are either
in phase or out of phase with the scale makes it work.  As you suspected,
making a single slit would not work.

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From: Mike Rehmus <[email-masked]>
Date: 2 Jul 1996 17:17:19 GMT

The LEDs used are normally IR emitters which light
frequency just happens to match with the peak 
sensitivity of silicon photodetectors.  Depending
upon how good the emitter/sensor pair have been
made, you might not 'see' any light.

As I recall, the emitter/sensor pair 'look'
through a double grating.  The grating of
the scale and a second, stationary grating inside
the read head.

BTW, the grating is photetched.

Best regards,

Mike Rehmus

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From: [email-masked] (John Dammeyer)
Date: Tue, 02 Jul 1996 19:18:50 GMT

>accurate linear scales work?

Thanks all.  A very understandable description.

John.