RE: That encoder stuff.

I'll give it a try.
The light transmission through two contacted 50% duty gratings is a triangle
wave. Separate them and it approaches a sine wave, if you have a non-point
source (LED rather than laser). If spacing is increased to the Talbot
distance, and you have a LED (extended) source, you'll get the triangle
wave. Operating at half Talbot spacing gives close to sine wave.
Different types of SiPD have different linearities. CdS cells are very
nonlinear. SiPD cell are linear over at least 5-6 decades, if properly
amplified. Phototransistors are somewhere in between.

So if you want to get a high degree of interpolation, you'll want a known,
stable output waveform.
It also helps if it is easily invertable. With a trapezoidal waveform you
don't need a lookup table (or arcsin), like would be needed with a sine
wavefrom from the encoder.
Unfortunately, the waveform changes quickly from triangle as soon as the
gratings separate. The spacing sensitivity for the sine pattern is less
than for the triangle. Most designs don't want to give the space needed to
use the Talbot spacing.

At one time I did an experiment: what would be needed to get 1 microinch
linearity from a 100 lpi grating? A HP distance measuring interferometer
was use to measure grating translation. LED source and SiPD were used to
illuminate and detect signal.
A sine waveform with distortion corresponding to 2 microinch error was
obtained, but it was very fussy in terms of grating spacing.

We used rotary encoders to measure the angle of a scan mirror in weather
satellites. The entire weather satellite depended on a small light bulb,
with a spare or two. Several times in the mid 80's the weather satellites
failed, you may remember this- it was because the little light bulb failed.
We had hundreds of light bulbs in long term test facilities, but the ones in
orbit kept failing. Eventually they went to LED sources. This took a long
time because they didn't know how reliable they would be.
Other than the lightbulbs, these encoders were the same as the ones used
today.

Elliot Burke
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Message: 2
Date: Thu, 23 Sep 1999 11:45:46 +0200
From: "Arne Chr. Jorgensen" <instel@...>
Subject: That encoder stuff.

Hi,

No one has come up with any good answers to my question, - those
zig zags. So I am left with my initial thoughts about it. I just
share this with you.

Bertho says: You will have a varying light from 0 to 50%, - (for
short.)

Okay, this means that if you had a linear light sensor, then you
should have a sine output. I am not sure, it could be a
triangular shape.
The other thing is that a semiconductor photo sensor, is non linear.

So what I always have thought is that - if you calculate the
transfer function and you would like a sine, then you offset parts
of the gratings to match the transfer function. In computer terms,
it like having a lookup table. Say that you rotate the disk by 1/4
of line spacing. You would think that the amplitude should be 25%
of the max. value.

But to have a sine, that is 45 degrees. The amplitude should be (
0.5 * 2^0.5 ) ( I don't have a calculator nearby - that's why I
wrote it this awkward way - but I think it should be 0,7 something )

So I believed that the gratings was offset so that the light
intensity -> to electrical amplitude was modified by these "zig
-zags". This has just been what I thought myself, but I raised the
question because I am not sure. All I know is that it is difficult
to get a sine wave that keep it's shape independent of the angular
velocity.