Analog encoder

Hi again,

Bertho Boman:

I must have lost track of something. Do you mean that these dual
gratings would produce a usable sine wave ?

Here is a few notes:

Back in the mid 80's, I studied a broken encoder from BEI
electronics. If was some electronics inside it, because the outputs
were differential, - I think. The disk was made of glass, - and had
cracked.

The disc had segments, or lines about 1/2" long. Underneath was a
stationary glass plate with similar lines, but these were offset a
bit :

I will try to draw one. O !!!!!!!!!!!""""""----____!

The "O" is the center hole. The active area is between the "!" and
instead of a straight line or grating, - the lines was offset about
1/3 of line width.
Behind the stationary glass plate, - looking as I have drawn it,
was an ordinary light bulb. The sensors was large and above the
rotating disk.

I newer quite understood why the stationary pattern was made like
this, but thought it some how had to make a sine wave. But the
output was shaped as a square wave. I guess the electronic circuits
had done that. The more I think of it, - the more I start to
understand this - I think. What made me confused was the fact that
I believed it would just read one line at any one time, not as you
say - looking at many "slots" at the same time. This would also make
some sense to the use of a light bulb instead of LED's.

Do you think I am right or wrong here ?

Note: I remember the earlier note from Ian, but I didn't understand
what he meant. Sorry.

//ARNE