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From: jbm@eos.arc.nasa.gov (Jeffrey Mulligan)
Newsgroups: sci.electronics
Subject: Re: X-Y detection of moving metal ball?
Message-ID: <7992@eos.arc.nasa.gov>
Date: 2 Apr 91 03:42:31 GMT
References: <1225@telesoft.com>
Distribution: usa
Organization: NASA Ames Research Center, California
Lines: 34

rlk@telesoft.com (Bob Kitzberger @sation) writes:

[ statement of problem deleted ]

>    The idea is to detect when the ball bearing breaks the light path for
>    a L/P combination.  The timing between light path breaks allows
>    determination of the velocity vector.

>    This is my cleanest solution, but I haven't been able to find a source
>    for the LED/phototransistor pairs that won't disperse over the 
>    distance involved (one foot).  Ideas/sources?

It is not clear whether the problem with dispersion is the absolute
sensitivity of the detectors, or crosstalk between emmitter-detector
pairs.  If the latter is the case, you can solve it by modulating
the emitters at non-harmonically related (high) frequencies,
and decoding all of the frequencies at each detector.  (You could
reduce the number of detectors also.)  This would
probably be a fair amount of circuitry; you haven't said
what your budget is, or what accuracy is required.


I don't think the video camera solution is so ridiculous,
especially if you are allowed to paint the surface flat black
and the ball flat white.  There are video-based pupillometers
and pupil-based eye trackers that work simply by thresholding
the video signal and comparing with a timer.  You don't
have to digitize and number crunch pixels to use video.

-- 

	Jeff Mulligan (jbm@eos.arc.nasa.gov)
	NASA/Ames Research Ctr., Mail Stop 262-2, Moffett Field CA, 94035
	(415) 604-3745