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From: jwatts@hpihoah.cup.hp.com (Jon Watts)
Newsgroups: sci.electronics
Subject: Re: X-Y detection of moving metal ball?
Message-ID: <5910004@hpihoah.cup.hp.com>
Date: 3 Apr 91 01:10:00 GMT
References: <1225@telesoft.com>
Organization: Hewlett Packard, Cupertino
Lines: 39

/ hpihoah:sci.electronics / rlk@telesoft.com (Bob Kitzberger @sation) /  9:48 pm  Mar 29, 1991 /
>1. Array of LEDs and corresponding phototransistors, one array for the
>   X direction and one for Y:
>
>        +-L-L-L-L-L-L-L-+
>        |               |
>        L               P
>        |    B          |               L = LED
>        L               P               P = phototransistor
>        |               |               B = Ball
>        L               P
>        |               |
>        +-P-P-P-P-P-P-P-+
>
>    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?

Don't use so many LEDs.  Just use one in the northwest corner and one in the
Northeast corner and photo transistors on the west, south and east sides.  The
LED can be lighted alternately.  The NW LED should illuminate S and E sides.
The NE LED should illuminate S and W sides.

Set the LED frequency and phototransistor sample rate to met your time
resolution requirements, set the spacing of the photo transistors to met your
spatial resolution requirements.  If the accuracy degradation when the ball
approaches the LEDs is unacceptable use four LEDs one in each corner and double
your frequency.

All the position decoding can be done in software unless you need to be very
fast.

Jon Watts
Hewlett-Packard
jwatts@mothra.rose.hp.com