[Shop-talk] Digital Calibers

[David Scheidt]

On Jan 27, 2008 1:48 PM, Bob Nogueira <nogera at worldnet.att.net> wrote:
>   The digital calibers that have severed me well for several years started
> acting weird so, just like when I was 8 years old and my mothers clock
> stopped working, I decided to open it up and see how it works.
> Looking at the internals, I could find nothing that would give any notion as
> to how movement of the lower jaw could be translated in to a measurement.
> In my quest to find a question that can stump this group, anyone know how
> these digital calibers actually work?

Digital calipers (it's spelt with a P!  Guns have calibers!) have a
linear encoder of some sort.  I think -- though i've never taken one
all the way apart -- that they're using a variable capacitance.
There's a  printed pattern on the PCB in the sliding bit, and another,
inside the fixed part.  As you move them, the capacitance changes.  It
goes  up, reaches a peak, and then falls, and the goes up, etc.  The
little thing counts the cycles, and knows, with amazing accuracy,
where it is.  No moving parts means they're very reliable  (much more
so then dial ones, which have a rack and pinion which collects little
bits of junk that break them...).



>
> Bob Nogueira  ( who magically got them working again after reassembly,
> unlike my mothers clock)
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-- 
David Scheidt
dmscheidt at gmail.com