Re: CCD

One thing I meant to say in my last post on the possibility of using
phototransistors (or diodes) for measuring is that, if you used the
'gratings crossed at an angle' method of generating moire fringes which
greatly emphasises the distance moved and then use ,say, 9 or 11
phototransistors spaced to exactly match the distance between 10, 20 or 30
moire fringe shadows, you can use the vernier effect to increase the
measuring precision. I am not too well up on the software side but I imagine
it would be possible to use a couple of logic circuits 'and'ed' together or
whatever, to count the number of fringes which pass the first photocell and
also determine which of the other cells is most obscured by a fringe, so
giving the second part of the vernier reading. As to making the gratings,
how about using 'piano' wires stretched taught along the bed or slides
spaced apart by something like the teeth in short lengths of hacksaw blade
which are produced to quite close tolerances. The reading head could then
have a short identical set up at a suitable angle. Are these thoughts
practical or am I
descending into a state of Arne-itis?? ;o)

Ian
--
Ian W. Wright
Sheffield UK

----- Original Message -----
From: Bertho Boman <boman@...>
To: <CAD_CAM_EDM_DRO@onelist.com>
Sent: 21 September 1999 04:22
Subject: Re: [CAD_CAM_EDM_DRO] CCD


> From: Bertho Boman <boman@...>
>
> David,
> The dual gratings side by sides avoids the problem that you are discussing
with less and less signals. You have a "big" light source and a "big"
photocell and the signal to noise is independent of grating size. The light
goes from 0 to 50 % if you have 100 mil spacing, 10, 1 or what ever. That
is how it is done on many commercial products.
> Bertho
> =============================
>
> > David Howland <dhowland@...> wrote:
>
> > Note on using Photo-transistors and IR Diodes:
> > When you order devices with a magnifier lens (not flat lens), the beam
is not as narrow as you might hope. The devices sealed with flat glass,
(you can look at the die), are the ones which we have found better for
looking through narrow holes (if you can line up the die with the holes).
The devices with a small magnifier lens offer more distance between the
sender and the receiver, but they have a wider beam. We found magnifier lens
type devices better with slits. A better lens for a slit will look
different than the off the shelf devices offered.
> >
> > I have been thinking about the DRO component, and have experience with
an IR card reader. As a matter of fact, the design narrows the beam, by
passing the beam through two slits (on one side) to minimize the scatter of
light and potential for reflections ouside of zero degrees.
> >
> > I have been turning a few parts to obtain a high resolution analog
signal from 880 nm IR devices. If anyone wants to explore the concept,
here are a few details. I have bored 0.125" holes and inserted stacks of
black plastic washers into the holes. Every other washer has a 0.010" hole,
and the others between have 0.092" holes. What ever light spreads out into
the 0.092" cavaties bounces around and gets lost. After 3 sets of 0.092"
cavaties, the light which is emitted or received with the last 0.010" hole
will not spread out or be received from anything other than straight.
Naturally, one must make all the parts perfect and match drill the 0.125"
holes to assembly the "Light muffler" into.
> >
> > The concern I have is that it may fail because the signal to noise ratio
is out of reason, but if it works, by overlapping several pairs of these, a
good analog quadature signal might be obtained. I am trying this because
holes are much easier for a home type DRO than slits might be.
> >
> > As you narrow the beam width, by whatever means, there is less energy to
work with. If one goes to a lazer diode, the energy goes up and so does the
cost. FIber might be interesting, but it is difficult to cut and polish.
> >
> > I am interested in what technology exists for micro motion (0.0001") to
analog signals in general (within the reach of home fabrication). Any
thoughts?
> >
> > David Howland
> >
> > -----Original Message-----
> > From: Bertho Boman [SMTP:boman@...]
> > Sent: Monday, September 20, 1999 3:27 PM
> > To: CAD_CAM_EDM_DRO@onelist.com
> > Subject: Re: [CAD_CAM_EDM_DRO] CCD
> >
> > From: Bertho Boman <boman@...>
> >
> > For the ones that have not seen it:
> >
> > I used to think that I had to have a narrow slit on the photo transistor
or photo cell to get good encoder resolution. There is
> > a much better way:
> >
> > Have a regular photo transistor and light source but set up a second
stationary grating next to the moving one with the same
> > pitch. The light has to shine through both gratings. The detector sees
either many slots of light or the light is blocked. It
> > just average all the slots and counts light and dark cycles, not
individual slots!
> >
> > Try it using two regular combs with the same teeth spacing and you see
what I mean.
> > Bertho Boman
> > ================================
>
>
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