re: probe design

Christopher C. Stratton wrote:
Date: Fri, 02 Nov 2001 00:10:18 -0500
From: ccs@...
Subject: Probe Design

>Stainless is suggested for all the contacts. Wouldn't gold be better?

Is it a good idea to use a soft metal for contacts whose position is very
critical? If there were much current flowing a soft metal would make sense,
but if everything is pretty high impedance a hard metal should work fine.

How about a hard, conductive material like tungsten carbide? Balls and rods
are available.

This could make a pretty good probe, but for really high sensitivity sensors
they use a laser reflection or a capacitance sensor. It would be easier to
make a capacitance sensor, it is easy to imagine a bridge circuit working
with a few plates built into the probe.

Capacitance sensors are used at the sub nanometer level, this should be good
enough for this application. Noise limits are on the order of
picometer/root Hertz.

They have the additional advantage of being a linear sensor, not a switch.
You'll get a nulled output, but if anything moves the output will go
positive or negative.

It is possible to make them insensitive to temperature.

There is voluminous information on capacitance sensors, a good place to
start is Foundations of Ultraprecision Mechanism Design, by Smith and
Chetwynd. There are plenty of references there that will lead you
elsewhere. Probably there are web references too.

The book referenced above also has an interesting section on the elastic
contact of a ball with a plane. The simplified analysis was done by Hertz,
and it give lots of very useful design ideas.

Elliot Burke