Re: encoder head pitch, or, quit your griping.

"Ian W. Wright" wrote:

> From: "Ian W. Wright" <Ian@...>
>
> Hi,
>
> A number of measuring systems I have seen use the Moire effect - is this
> a tecnique which might be used to overcome the lack of a suitable pitch
> grating? As I remember, two gratings are used mounted at a slight angle
> to each other. this has the effect of producing strong dark bands moving
> at right angles to the lines of the grating and allowing finer
> measurement from relatively coarse gratings.

Nope, it doesn't do that. All you get is one wave when the analyzer is moved
one grating pitch. (If you had a VERY wide grating, maybe several inches
wide, then you could put enough photocells on it to do what you are
talking about, but electronic interpolation is easier and cheaper than having
glass scales several inches wide.

> The angle at which the
> gratings are set affects the width of the bands and IIRC a smaller angle
> increases resolution - or have I got it all wrong?

I'm afraid so. The angle of the analyzer does allow you to place your
photocells closer or farther apart. But, you'd probably set the spacing
of the photocells, and then adjust the analyzer to match.

Now, if you want REAL moire' gratings, you use a different spacing
grating for the analyzer, and DON'T tilt one with respect to the other,
and you get a vernier effect, but it gets real complicated.

>
> The kind of system I envisage would be easily made on the mechanical
> side and would consist of a tape with a number of straight black lines
> running the length of the machine - say ten at any even spacing - and a

Yes, but how are you going to make these gratings that have constant
spacing, accurate to, say, .001" over 24 or 36", and with no short-term
errors worse than this? This is why the scales cost $300 and up.
This is actually DARN hard, and requires VERY expensive precision
cameras, or laser systems, etc.

Jon