re:CCD

To measure very small motions optically there are lots of techniques:
A really great device is the position sensing diode (PSD), made by lots of
people. These are linear diodes, with 2 or 4 outputs, depending on if it is
a one axis or two axis device. They are in essence potentiometers. The
ratio of outputs is proportional to displacement of the light centroid from
the center of the PSD. Measurements of <1 microns are very easily made.
Commonly used for metrology applications. There are several varities,
differing in sensitivity, linearity, temperature coefficients, and so on.
The really nice thing about PSD's is that they don't require a small point
of light. As long as the light beam stays on the PSD during the range of
motion it will work properly. They come in sizes to at least 10 mm.
There are also quadrant detectors, which are used in every CD player for
tracking. These are 4 detectors set up as the quadrants of a circle.
Differential amplification give beam centering information. These have a
very small linear range, but are less sensitive to temperature than PSD's.

If you need a beam of light, go with a laser diode collimator, or a LED with
a collimating lens in front of it. Some of the commercial encoders use a
slit mask in front of the LED to reduce divergence angle. Even when well
collimated, a large LED will cause appreciable beam angle. This is why
lasers have such narrow beams, they emit light from what is optically a
point.

Like David said, most of these lenses and slits are custom. I have designed
them for encoders, they are really quite simple. There are off the shelf
lenses that can do a good job of collimation, if you don't mind your package
being a little larger than a commercial one.

Most grating systems will accomodate some beam angle, the tolerable amount
depends on many things, such as grating pitch, grating spacing, detector
position. A well designed system is almost entirely insensitive to the
source (LED) size, although of the encoder gratings I've examined, most of
them seem unaware of the optical techniques for achieving the insensitivity.
It takes more than a little fiddling around to get there (apologies to any
violinists out there).
Many encoders these days incorporate the analysis grating into the detector.
There are four phases of detector, interdigitated. Put fingers from each
hand between eachother, and you'll get the point. This allows better use of
light and a smaller readout area.


Elliot Burke

From: David Howland <dhowland@...>
Subject: RE: CCD

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