Re: interface board

Gar Willis wrote:

> I even had a chat with one HP 'marketing droid' type about this, and
> couldn't seem to get across to him the value of a 250 (X 4 = 1000 in
> quadrature) cpi sensor. Since they make a 200 and a 360 cpi model
> presently, there's no reason they couldn't build a 250! And NO,
> unhappily, their engineering guy told me (hope springs eternal) you
> couldn't USE a 250 cpi optical strip with a 200 cpi sensor to get 0.001"
> native resolution. The sensor arrays are specific to almost exactly the
> optical resolution, so they need to MAKE the bloody thing!! 8(
>

actually, you can make the whole works yourself, if you can get the
grating. What you do is use another piece of the grating as an analyzer.
To obtain the 90 degree quadrature shift, you TILT the analyzer grating
such that the spots you pick for your photocells are skewed 1/4 of a
grating pitch with respect to itself. The only limitation on this is that
the grating needs to be wide enough so the 2 photocells can be placed
a short distance apart (1/4 to 1/2 inch) and when the analyzer grating is
tilted, both photocells can see through both the analyzer and the
measuring grating. A 1/4" wide grating would be about the minimum
width. If this isn't the case with whatever strip you plan to use, then
you need to make 2 analyzer pieces, and give one of them a position
adjustment, so you can set up the quadrature phasing. It really isn't
that hard. I was working with a .000005" version of a dial test
indicator that used the tilted analyzer grating trick. Even setting
that one up wasn't very hard, and that was a VERY precise set
of gratings.

Jon