Talbot ?

Hi folks,

I have a terrible "arne-itis" , and I don't have anyone that will
take me for a walk in the park :)

Elliot:
----
I am not familiar with the "nomentclure" - what is Talbot spacing
? Is there any papers on the net, which could sheed some light
into this ?

There is something you said, that I take as:

The Talbot spacing - must be the distance between the stationary and
the moving gratings.

Could this be known under another name ? What we have is two
diffraction gratings, and just a bunch of difficult stuff appears,
interference, etc.

But you hit a "nerve" - telling me something I think I recognize. I
just can't grasp it, yet.

Ray :
----

Do you think you could find where you read about it ?

Bertho:
-----

Sure it could be done with the same micro/DSP, the thing is how.


Some notes:
---------

It is this "Arne-itis" thing. I have done some things over the
years, and I can not remember the exact stuff, and my own attempts
has not been scientific.

Jon is smart, he know that an old fashion tacho generator works,
and what we are looking for is something that will behave like one.
There is some PPL, freq->dc signal tachometer cicuits like ( LM2907)
etc. But somewhere in the back of my mind, I have read something
about designs that did not match an old fashion tacho. What I would
like is something "stupid simple" - but it has to work properly. I
wish I had some better knowledge about what was wrong with my own
attempts, and Ray Henry has pointed out that there is other
attempts, - that must have experienced something similar to what I
have done.

Bertho: You say it will not change shape regardless of SPEED.
Well, my attempts has maybe been in the region above the speed of
light ! ? :)

No, I think I used a simple encoder with no stationary gratings.
It was just a photo interrupter reading a disk with holes. That is
probably the error. Most of my encoders is simple ones, i.e. -
they just generate digital square waves, not sine waves. The sine
waves would be needed for a tacho generator chip like the L290, -
that is why I have not been able to test it, but this chip is made
for these kind of applications. That is servo control, and it will
replace a dc tacho generator.

Of course the nice thing would be to generate the velocity loop in a
micro controller, using just digital input. All I know is that I got
some "servo hunting" - and were not able to get it respond as I
wished. I could not record the output, and pinpoint what parameters
I had overlooked.

But here is an idea: What if we wrote some code in RTLinux, and
we could log it in a file ? I mentioned something like this
before, using the Stg board, making everything in software. Later
this could be transferred to another processor. Or maybe we should
dig into the simulator that Fred has written ? If we could put
together something - mold it to our liking. The trouble is that we
would need some good ICE - in circuit emulation - to work with a
controller. Sure you could write some code to a chip, and see what
happens, but it would pose a lot of difficulties to tune it, check
other design options, etc.

Now this - "arne-itis" is killing me .

How is the pico servos working ? How good are they.

//ARNE