Re: [CAD_CAM_EDM_DRO] Re: low cost linear encoders

Ian Wright wrote:

> From: "Ian Wright" <Ian@...>
>
> Hi,
>
> At a slight tangent to these discussions, does anyone know how the
> Mitutoyo
> digital vernier works? Mine has a bit of the scale torn off at the
> very end
> and under it appears to be a length of something like printed circuit
> board
> with diagonal cuts across it. I assume they must use some form of
> quadrature
> sensing on these copper parallelograms but I can't think of either how
> they
> would sense the presence or absence of copper without touching it
> (it's
> under the thin plastic adhesive scale) or how they would get the
> required
> resolution of better than .0001" with something which seems so crudely
> made.

Yes, it is an electronic capacitive vernier. I haven't had a Mitutoyo
apart, but have had several
Chinese imports apart for cleaning. The Chinese scale has little
squares about 2 mm square of copper
on a PC board material, with the squares ocurring about every 2 cm. The
head has a bunch of thin lines
all connected to the control chip. Presumably by exciting some lines
and sensing others, it can tell where
the square is in relation to the lines. I haven't figured out the
entire scheme, there must be some
interpolation of signal strength, because the strips are only about 3 or
4 per mm, at the most.

>
> Perhaps this method could be adapted for our linear DROs? Making the
> linear
> strip would seem to be quite possible in the home shop, probably by
> making
> an indexing jig with a guide for a knife to cut the copper of thin
> PCB, and
> the fact that it is a non-contact method which is also non-optical
> should
> mean that it would be relatively immune to the mess around a machine -
>
> certainly my vernier seems to work OK in such an environment as it is
> usually lying under a pile of muck in the lathe drip tray and I have
> to wipe
> it off each time I want to use it (This one's only my rough measurer
> which
> cost me the princely sum of 5UKP at a boot sale - my other tools get
> treated
> better!)

Well, making these things with such methods would likely cause a pretty
inaccurate scale.
You'd be lucky if you got better than .25 mm accuracy even over a short
distance.

Jon