Subject: Hi-resistance material

From: [email-masked] (Steve Vestal)
Date: Wed, 5 Jan 1994 18:47:29 GMT

This is a materials question. I'm looking for rods in the 1/8" to 1/4"
diameter range that are reasonably strong and rigid yet have a non-negligible
electrical resistance.

The project involves sensing the position of a conductive carriage that slides
along a pair of such rods with errors of a few mm.  I was hoping to sense the
position of the carriage by looking at the resistance between the ends of the
rods (the further the carriage, the higher the resistance).  The practical
problem with this is that a quick experiment indicated the sliding contact
resistance of the carriage can be as much as a few ohms and is very noisy
(i.e. varies from very small to a few ohms as it slides).  However, a quick
computation shows that even relatively high-resistance metals like stainless
or ni-cad will have resistances below 10^-5 or 10^-6 ohms/cm at these
diameters (I forget the exact numbers, but the point is it's small).

I've already considered packing thin-walled tubing with some sort of
insulating filler to add strength, but the numbers are still pretty low.  Oven
heating elements are coated with an insulating ceramic, plus I would estimate
the total end-to-end resistance is also still somewhat low (a 1KW element at
100V DC would need 10 amps, which would be 10 ohms end-to-end resistance, and
I would imagine oven elements output more than 1KW of heat).

Does anyone know of a material that is reasonably strong and rigid and
available and affordable that would have resistances of at least a few ohms/cm
of length (which would be needed to get sub-cm position accuracies).  If it
doesn't come smooth and straight, it would also need to be machineable.

Please reply by email, as my frequency of reading this newsgroup often falls
below the rate at which our site flushes old messages :-(

Steve Vestal
Mail: Honeywell Technology Center, 3660 Technology Drive, Minneapolis MN 55418 
Phone: (612) 951-7049                       Internet: [email-masked]

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From: [email-masked] (Clive Washington)
Date: Fri, 07 Jan 1994 10:51:37 +0000

In article <[email-masked]>,
[email-masked] (Steve Vestal) wrote:

> I would think that you need to use a fine wire on a support.  There are
a number of resistance wires available which I recall from old high-school
Wheatstone bridge experiments.  The rubber handbook quotes the following
names and resistances for 40-gauge wires:

Advance  0.958 ohm/cm
Climax  1.74
Eureka  0.938
Excello 1.84
Manganin 0.878
Nichrome 2.303

Some of these are trade names but nichrome is commonly available and may
suit your purpose.  A possible alternative approach to your problem would
be 
to attach the wire at one end of the carraige and allow the other end to
move 
across a fixed contact, with a weight holding the wire taut.  If you can't
get an electrical contact to the carriage, then attach the centre of the
wire to it and lead both ends away in the same direction,over sliding
contacts
to measure the resistance of the wire 'in path'.  This would also double
your precision (length of wire = 2 x carraige position) and you would have
full control over your contact resistance (which at present seems to be
a weak point).  Sorry if this is all inapplicable since I am trying to
solve
your problem remotely!  Incidentally, there is a DRO with thou precision
available in the UK which uses this principle and it costs about £100.

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From: [email-masked] (Andy Dingley)
Date: Sun, 9 Jan 1994 11:11:00 +0000

In article <[email-masked]>
           [email-masked] "Steve Vestal" writes:
> rods

Seriously, I'd forget trying to do it by resistance and use another way. It's
just *too* error prone. If you get it working (which I don't doubt) I
wouldn't expect it to last long. Sliding contacts are no fun for reliability.

How much electronics do you know ? If I were doing it, I'd use an optical
track with some (simple) electronics. Use a two track quadrature system to
drive an incremental counter (like the insides of a computer mouse). The only
problem with this is it loses position memory when you turn it off. Either
crank it to the end position when you switch on, or make the electronics
battery backed up.

I've also seen a really cheap version of this (a school project) done with a
magnetic track made of computer tape glued to perspex.

   _                              Andy Dingley
 /  ) ' ,_  _,  /   _ ) _/_       Codesmiths of Newcastle
/_ / / / / (_) /_) (_)  /_        [email-masked]
          (_)                     +44 091 230 1695

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From: [email-masked] (Michael Stein)
Date: Sun, 09 Jan 1994 10:27

>> position of the carriage by looking at the resistance between the ends of the

How fast a response do you need?  I remember seeing some national
semiconductor application note using a metal tube with a
temperature gradient along it.  The position sensor was one of
their temperature sensors sliding in the tube.

For a high vibration situation this can average out the
noise...