Re: [CAD_CAM_EDM_DRO] Re: Poor Mans DRO

jethrobodine wrote:

>Another related question, how do digital calipers work ? I disassembled my foreign built calipers to see how they work and I don’t see any moving parts. If I measure something with the power off then turn them on, the measurement is accurate which makes me think it’s not a counter. Also they are really screwed up by moisture.
>
>

Yes, most of them do count, in a manner. The most common design has a
row of capacitive pads on a PC board on the back of the "read head". There
is a passive PC board behind a cosmetic tape layed into a channel in the
spar.
The PC board has square holes in an otherwise solid copper plane on the
front
surface. The capacitive sensor pads on the back of the read head detect the
position of those square holes. Some units have small copper squares
surrounded
by a square blank area in the copper. There are almost certainly other
designs, as there has been some pretty intense patent suits over these
units.
I think Mitutoyo had the patents over these capacitive systems, and kept the
Chinese units off the market for about 5 years. I'm guessing the US patent
on it must have expired about 1997 or so, as the Chinese calipers came
flooding
back in about then.

It is not surprising that water, especially with conductive ions in it,
would
mess up the relatively weak coupling you'd get through that thick crepe tape
over the scale.

By the way, these things are NOT terribly accurate, although I'm sure the
real Mitutoyo and other "name-brand" makes are fine. I've seen some be
off as far as .006" per inch, although that is not cumulative. My mechanical
dial calipers are well within .001"/inch (until I drop them, that is!)

Jon