PSN-L Email List Message

Subject: Re: Strain meters?
From: Bob Smith bobsmith5@........
Date: Sun, 21 May 2000 16:50:57 -0400

Hello all ---

Having no idea of the range of motion and
resolution/repeatability you are looking for I'm not sure
that this approach will be of any use.

In the '80s and 90's I worked several years for a client
that built medical X-Ray systems.  Such a machine is
essentially a low precision robot or industrial grade
positioning system.  An essential element of the control
system was a linear sensor to measure the position of
various arms and slides that held the X-Ray tube and imaging

We looked at a number of devices:

1. Laser interferometers (too expensive).

2. LVDTs (limited range of travel)

3. Gray coded sensor bars and optical sensors (too expensive
and clumsy to get the desired resolution).

4. Optical encoders such as used on precision milling
machines (Technically good solution, but too expensive for a
multi-axis system and very sensitive to mechanical

5. And a million other ideas, some of them pretty loony :-)

The final solution was a Rube Goldberg inspired device
affectionately known as a "string pot".  It consisted of a
grooved or threaded drum around which a small stainless
steel aircraft cable (about 0.032" diam) was wound.  The
drum and cable arrangement was pre loaded by a clock type
spring motor.  The tension was quite high, probably around
15-20 pounds force.  (The thing could produce quite a nasty
cut/burn if you let it get away while pulled well out
against the string :-( ).  Finally the drum drove a ten turn
potentiometer through a pair of gears.  The position of the
potentiometer shaft was measured by a twelve or fourteen bit
A/D converter.

Performance --

Range of motion - Typically 250 to 1000 mm depending on drum
diameter and gear ratios.
(Range of motion is virtually unlimited as it is controled
by the drum diameter and gear ratios.)

Resolution - Reliably better than 1 part per 1000 of total

Repeatability - Excellent, essentially equal to resolution.

Absolute accuracy - also 1 part per 1000 after calibration.

Suitability for this application???

1. Temperature stability was not evaluated (working in a
hospital environment has some advantages).

2. Micro controller (A/D) interfacing required.

If anyone is interested, I can provide more details on this
wacky device.

	Best wishes, Bob Smith

Jim ODonnell wrote:
> Sean Thomas et al,..
> A student at UNLV would like to measure movement across some fissures in
> Las Vegas.  The fissures are cracks in the soil (deep alluvium) about 10m
> long in an area of mapped faulting.  I was wondering if strain meters
> would do the trick, the cheaper the better,and can you recommend or
> suggest any methods or devices that would work?
> Jim O'Donnell
> Registered Geologist No. 1240, State of California, 1970
> Registered Geophysicist  No. 158, State of California, 1974
> 702 293-5664
> jimo17@........
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---------  Avoid computer viruses  --  Practice safe hex 
 * * Specializing in small, cost effective embedded control
systems * *
Robert L. (Bob) Smith			Smith Machine Works, Inc.
internet   bobsmith5@.............. Lumlay Road
landline   804/745-1065	                Richmond, Virginia

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