Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!uwm.edu!ogicse!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: X-Y detection of moving metal ball?
Message-ID: <1991Mar31.063417.28735@milton.u.washington.edu>
Date: 31 Mar 91 06:34:17 GMT
References: <1225@telesoft.com>
Organization: University of Washington, Seattle
Lines: 30

In article <1225@telesoft.com> rlk@telesoft.com (Bob Kitzberger @sation) writes:

>Problem: I want to detect the X and Y coordinates of a metal ball bearing
>(magnetized or not) in motion within a box.  The data collection must
>occur in a fashion that allows me to derive the velocity vector of the ball
>bearing in the X-Y plane.  The size of the box is approximately one foot by one
>foot.  Also, I should mention that the detection must occur in real-time.

	This is similar to the magnetic-suspension problem (solved by
Jesse W. Beams, in the era of vacuum tubes) for an ultracentrifuge.
The rotor is held up by a magnet, and it is necessary to sense the distance
to the rotor so the magnetic field can be adjusted (by a feedback loop).
	Though I don't know how relevant the solution is to your situation,
it was done by measuring the capacitance change of a pair of plates
near the rotor; a conductor inserted into a capacitor raises the capacity
(and lowers the frequency of the attached oscillator).  The position
detector was an FM detector.  To make a 2-dimensional version, you will
need to make at least two oscillators (at very different frequencies,
so the one doesn't affect the other), attaching one to the N and W
sides of your box, the other to the S and E sides.  
	Calibrating the thing will be a task.
	If you know someone with good skills at conformal mapping, you
can probably find some electrode shapes/placements that simplify the
problem.  
	
	Parenthetically, I didn't like the Hall sensor approach; the
Hall sensors are polarity-dependent, and a rolling ball will swap poles
a LOT.  Descrambling the resulting info will not be easy.  

	John Whitmore