Darwin:

It seems unlikely to me that "shining an LED and measuring the return light
(intensity?)" would be a practicle method to measure piston size.

Instead:

Can you move the piston or LED (and its detector)?

If they were moving relative to each other, then measurement
resolution/accuracy would be limited only by the positioning sensor
resolution and accuracy.

...you could time the distance from one end to the other, if speed
reasonable and consistent...

...you could scan the (emmission or return) beam with a moving or spinning
mirror - like those used for automatic price & product entry at cash
registers (though with a spinning mirror you'd have to know the distance and
do a little trig to convert rotation to length)...

...since the range you need to measure is limited, maybe you can hold the
piston against a stop and have the sensor sweep accross only slightly more
than the needed range...

...perhaps you could project the shadow of the piston onto a sensor mounted
on a linear postioner or a maybe a photo-diode sensor array...

Does it have to be non contacting?

If physical contact is allowed, then perhaps you could use and/or modify
something off the shelf such as electronic calipers, etc...

Bruce.

-----Original Message-----
From: darwin reynoso [mailto:jayian@MAILHOST.DAY.AMERITECH.NET]
Sent: Wednesday, March 12, 2003 3:48 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: (EE)How to measure a piston


I need to measure pistons from 11.00 mm to 14.00 mm
each piston increment on a five units like 11.00-11.05
I'm thinking about shining a led read the return light
with ADC and have my pic calculate the size of the piston
but there's a draw back with this method keeping the ambient
light out. any idea on how to measure this piston

thanks

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