How do they work?

I have just purchased one of the popular short scale units much like the electronic verniers etc. to fit to my lathe saddle and then with a quick
release clamp to the bed so that I can have accurate bed travel measurement for putting accurate shoulders on or parting off to length and the like and I realise looking at it that there is no apparent connection, mechanical or optical between the moving display head and the fixed printed scale bar, so how does it pick up its measurement?
I would have thought if it was a friction drive in the moving head it would be prone to error.
Such is the inquisitiveness of child like minds .....
Alan
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ISTR that it is based upon changes in capacitance. There is a special pattern of lands laid down on the fixed portion together with a different and finer pattern on the movable portion. The changes in capacitance then gives the desired measurement.
I presume that with the density of electronic storage chips available these days that there is suffucuent storage within the electronic chip to carry a calibration table within to cater for manufacturing variation in the lands.
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I resorted to string one down eventually, but this might save you the trouble: http://books.google.com/books?id=Tjd2laRnO4wC&pg=PA244&lpg=PA244&dq=Digital+caliper+capacitive+cos+sin&source=web&ots=NuW_riwE_3&sig=-iYkH9Ni7AYzS85zQXiVBcZkCiA&hl=en&sa=X&oi=book_result&resnum=6&ct=result#PPA244,M1
Scrim
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Thanks, very informative. However I am still puzzled as the vernier and scale I have just seem to have machine marks on the fixed portion of the device with no apparent accurate graticule scaling that would fit the description. maybe it is these "machine marks" that are sensed in which case I am amazed at how the accuracy is achieved.
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On Sun, 2 Nov 2008 10:34:28 -0000, "Alan Marshall"

The pcb is under the marked "tape" that you can see.
Richard
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Did'n't think of it being under the visible scale .... Doh!
Thanks
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On Sun, 2 Nov 2008 05:03:10 -0800 (PST), BobKellock

And if you really want to find out how it works then take a look at <http://books.google.com/books?id=Tjd2laRnO4wC&pg=PA244&lpg=PA244&dq=Digital+caliper+capacitive+cos+sin&source=web&ots=NuW_riwE_3&sig=-iYkH9Ni7AYzS85zQXiVBcZkCiA&hl=en&sa=X&oi=book_result&resnum=6&ct=result#PPA242,M1 page 243 onwards
Richard
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