help selecting proximity sensor for part ID

[Bob Stegmann]

I'm looking for some help/recommendations in selecting a proximity sensor
for use in detecting small parts placed on or near the sensor. I would like
to be able to detect the presence of about 12 different small items placed
on the sensor. I can modify the parts in any way--i.e. add
magnets/foil/anything to them. Can anyone recommend a particular sensor or
give me some hints on how to achieve my goal? I do not know much about
proximity sensors, so any information you can point me to would be greatly
appreciated.

Thanks,
Bob.

 

[Lewin A.R.W. Edwards]

for use in detecting small parts placed on or near the sensor. I would like
to be able to detect the presence of about 12 different small items placed
on the sensor. I can modify the parts in any way--i.e. add

This application sounds tailor-made for RFID.

 

[Bob Stegmann]

"Lewin A.R.W. Edwards" <larwe@larwe.com> wrote in message
news:608b6569.0309110853.2dda42de@posting.google.com...

for use in detecting small parts placed on or near the sensor. I would
like
to be able to detect the presence of about 12 different small items
placed
on the sensor. I can modify the parts in any way--i.e. add

This application sounds tailor-made for RFID.

Thanks for the reply. I did a little reading, and the RFID approach does
sound promising. However, my application (electronic chessboard) requires
many readers (64--one for each square), and I think the cost is going to
skyrocket. Do you think it's possible to use one reader multiplexing the 64
chessboard squares somehow? (switching between coils under each square or
something)? Any hints appreciated.

Thanks,
Bob.

 

[Lewin A.R.W. Edwards]

Hi Bob,

skyrocket. Do you think it's possible to use one reader multiplexing the 64
chessboard squares somehow? (switching between coils under each square or

It is absolutely possible to do this. I built a prototype device that
used a single 125kHz reader with 16 antennas in close proximity over
the surface of an object. The device was covered in soft velcro. There
were various objects with hard velcro on them and RFID tags inside.
The user stuck various objects on the outside of the device at various
locations. The device could identify each different article, and
identify where they were stuck onto the body.

I just used a transistor for each coil (actually an etched spiral on a
tiny PCB), switching each in turn to the antenna input of the RFID
reader. You have to fine-tune the scan rate to cope with settle time
after each switch, and you might also have to mess with attenuation to
make sure adjacent squares aren't being scanned, but it's possible. Of
course, it might take quite a few seconds to scan the board... To
improve the user experience, maybe have some flashing lights that show
where on the board you're scanning.

If you are happy with a contact solution, simply having sets of
electrodes on each square, and different resistor values inside each
chess piece, with contacts on the bottom, is a cheaper way of solving
the same problem. However you then need to worry about contact
integrity and you need to make sure the pieces are properly aligned
when set down on the board.

If you want to be cunning, mount a medium-resolution camera above the
board and recognize the chess pieces in software