Newsgroups: sci.electronics
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!csn!boulder!boulder!rainer
From: rainer@boulder.Colorado.EDU (Rainer Malzbender)
Subject: Re: X-Y detection of moving metal ball?
Message-ID: <1991Apr1.003056.15878@colorado.edu>
Sender: news@colorado.edu (The Daily Planet)
Nntp-Posting-Host: rhubarb.colorado.edu
Organization: /usr/local/lib/rn/organization
References: <1225@telesoft.com> <2642@sun13.scri.fsu.edu>
Date: Mon, 1 Apr 1991 00:30:56 GMT

I missed the original post, but I think I understood the question. I
have done an experiment which solved a similar problem. For
my thesis I did an experiment in 2D melting with lots of little steel
spheres rolling around in a hexagonal arena (~1000 particles). I used
a video camera and image processing system (Imaging Technology, Sun)
to capture sequential video frames (oh yeah, I left out the nice
optical video disk recorder) which were later analyzed. One of the
things I did was track all the particles from frame to frame. Thus, I
can verify that the suggested video solution in fact works, at least
for motions which are relatively slow compared to a video frame time.
The criterion is that the particles not move enough during a frame
time to cause confusion with neighboring particles. I just did a
local search around the previous frame's position of each particle to
find where it had moved to, and it worked. More sophisticated ideas
include minimizing the sum of the squares of all possible interparticle
distances, but this is considerably more computationally intensive.

The experiment had light sources at an angle, like this:


	Light
             \    /
              \  /
               \/          o o o (balls)
              -------------------------------------
                   (substrate = mirror)

which meant that a video camera located directly overhead only saw
light reflected from the spheres, not the lower mirror surface (we used
a mirror for flatness); the specular reflections went off to the side.

We were interested in obtaining accurate velocities, and hence looked
into doing better than video rate resolution. Our idea was to use a
laser bouncing off two piezo-controlled mirrors (x and y) which would
bounce off the sphere and be detected with a photodiode. Using
feedback one should be able to keep the beam on the sphere, and the
control signals going to the piezo mirrors give a direct readout of
position. Unfortunately we never actually got around to implementing
this, and I think the optics of the spherical reflection would be
pretty tricky, but it ought to work.

--
Rainer Malzbender                                Save a dinosaur - buy DEC.
Dept. of Physics (303)492-6829
U. of Colorado, Boulder         rainer@boulder.colorado.edu 128.138.240.246