I need to find a 2-D postion of the object within .1 inch accuracy. 
Basically I have a table(4 feet on each side), with 7 objects(about 2
inches wide) on them that move around on the table randomly.  I'de
like to be able obtain the position of each object, and then feed that
information to my pc, where I can simulate the same thing on the
screen.  I'm wondering if there are some kind of instruments that will
allow me to do this.  My budget for this is also like under 500$. 
Maybe some kind of sensors that i can place on the objects, or some
optical tool that can measure it.

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Request for Question Clarification by omnivorous-ga on 13 May 2004 09:18 PDT

Namor --

You might want to let users know something about the objects.  A
capacitive-sensing system could make sense, but it might not sense
certain types of materials.

And your comment below implies that you'd like continual, automated,
digital sensing.  If so, you should confirm it to researchers and
indicate a type of Input/Output -- such as USB, IEEE 488 or IEEE 1394.

Best regards,

Omnivorous-GA

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Clarification of Question by namor-ga on 13 May 2004 10:29 PDT

All positions need to be taken at a point in time, on continual
basis(every 5 seconds), with some type of interface to a computer.

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Request for Question Clarification by omnivorous-ga on 13 May 2004 10:40 PDT

Namor --

The multiple objects on the table rules out laser-measuring devices,
as objects will presumably block each other.

However, I've found one magnetic solution, if that would be
acceptable.  Knowing the vendor, I suspect that it's expensive (more
than $1,000).  I'll gladly post if as an Answer, if acceptable.

Best regards,

Omnivorous-GA

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Clarification of Question by namor-ga on 13 May 2004 11:06 PDT

no, I cant afford $1000

Namor --

I was going to recommend a "laser ruler" or "laser tape" (laser tape
measure) but unfortunately the reviews of the Irwin Straitline note
that it's not particularly accurate:
http://www.amazon.com/exec/obidos/tg/detail/-/B0000ALR30/104-6058330-5571112?v=glance

Best regards,

Omnivorous-GA

Thanks, for the comment.
One other thing, I need all the positions need to be taken at a point
in time, and then continuosly repeated every 5 seconds, and fed to a
PC.

How about a simple digital imaging system (e.g., a digital camera or
videocamera) positioned above the table that takes an image and feeds
it to your computer every 5 seconds.  In order to achieve 0.1"
position resolution out of a 48" square table, you would need the
table to occupy an area of at least 480x480 pixels on the image
sensor. (In practice, you'd probably want twice this resolution.) 
Appropriate image processing on the computer should be able to
determine the center of each object.

You would probably need to correct for optical distortion when
converting from pixel coordinates to "real" coordinates, but the
necessary corrections could easily be determined by imaging a grid of
known dimensions (say 1"x1") laid on the table and comparing the
coordinates of the actual grid intersection points with their pixel
coordinates.

To follow up on 
"How about a simple digital imaging system"
Very good idea.
If you are running a Linux OS on your computer, then this project is
quite affordable.
You can position a web cam above the table, just as hfshaw-ga
suggested. Here is a list of Web cams for which there exists
a Linux driver:
http://www.smcc.demon.nl/webcam/

I suggest one of the recent cameras -
Logitech QuickCam 4000 Pro
You can get it for under $100

The "image processing" C code (it is not that involved) already
exists - check out the senior design project.
http://www.sydow.us/sdp/default.asp 
I'm sure the guys will be happy to answer specific questions.

More details:
The code is written in C.
You will want to color the top of each object with distinct colors.
Instead of the grid you could place two colored circles on the table, 
and if the physical coordinate of each is known to you. You can then
use a simple matrix calculation to transform each pixel coordinate to
real coordinate

Best regards,
Parubok

Hi Omni,

Just have to say it .. invite a different friend around each evening
to a barbecue at your place .. with tape or ruler in hand .. have them
call out the measurements which you can then enter into a simple Excel
spreadsheet for correlation.

Better still, invite 7 friends, place their name label on each of the
7 objects .. tell 'em it's a new version of Battleship. They have to
call out or record the X and Y co-ordinates of each random move in 5
second intervals. This includes of course the co-ordinates of each
extremity of the said object.

It might be slower, but much more fun especially if you provide the Cab Sav.

:)