I've been down this road as well... (but only in my head:) ) As I said in
another post, the bore is 350mm long with 6mm at one end & 12mm at the
other. Any pots which would fit into this would be surface mount trimmers. I
can't see these being either repeatable nor capable of measuring to the
precision he requires (0.1mm). I may be wrong here. Having a precision pot
outside the tube means using a probe wore 350mm long! As you point out the
angle of such a wire will be a problem....

However this idea is not dead yet if only for the fact that electronically
it is simple for him to implement. The strain guage stuff requires another
look as well for the same reason.

Thanks again to all.

Joe



-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Russell McMahon
Sent: 17 December 2004 01:00
To: Microcontroller discussion list - Public.
Subject: Re: [EE]:Interesting measurement problem.


A variant: Have a spring loaded wire at a lowish angle off the shaft so it 
touches the wall at an angle. Place an outwards bend at the end and a bead 
or similar on the end so that it is certain that the bead touches the wall 
in all cases as you move it up the tube. So far it's the same as Brent's 
with a little more detail. Now use a potentiometer to measure the wire 
angle. Even easier for a non-electronic person to do. Result could be read 
on a multimeter using either resistance of pot or voltage picked off wiper. 
Probe can be inserted X units into pipe and then rotated 360 degrees to see 
if there are any variations at that point.

The lower the wire angle the less sensitive the arrangement is to diameter 
variations. A compromise should be possible. A log law pot may help 
(correctly connected). Calibration by simple measurement before use at 
different diameter settings. Rod needs to be centralised somehow - maybe a 
rod is inserted full length and arm and pot are on a slider.

When pulling back out the arm will tend to bind on the sides shark's-tooth 
style. Add a "string" to pull it flat before withdrawing (or something more 
elegant).

Use of a tiny stepper motor and arm would do similarly to above - would need

to measure current drawn each step to tell when wall was touched.

A linear rack and stepper motor may be possible. Arm then extends out at 90 
degrees. Can only handle 2:1 diameter variation.


        RM




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