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J.h. Kuiper
05-11-1994, 06:44 AM
About three weeks ago, I posed the following question:

> At this moment I am working on a project to build a device for
> determining the centre of gravity of a person ambulating in a
> 'SWIVEL walker'. A SWIVEL walker allows paraplegic persons who
> have no lower limb function to ambulate again in an upright
> position. For a good functioning of the SWIVEL walker it is
> important to set the centre of gravity correct.
> To find the centre of gravity I want to use three
> transducers which measure the load from the patient. This brings
> me to my question: can somebody give me information about force
> transducers or piezoelectrical devices which can measure loads up
> to 1000 Newton? Their height should be less than 15 mm, and they
> should not be too expensive because of a small budget.
These are the answers that I got, and I thank all the respondents.

Ron Suurd
Robert Jones & Agbes Hunt Orthopaedic and District Hospital
Unit for joint replacement
Oswestry
Shropshire SY10 7AG
United Kingdom
osa02@seq1.keele.ac.uk

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Hi Ron,

We made our own, if you need I can send you some of the info .
They consisted of little aluminium cylinders that fitted into the hollow
legs of the walkers we used. They protruded out by about 2 cm's , just
enough to fit the rubber bumpers that you normally see on walkers and
crutches.
Mounted on the cylinders was an array of strain gauges that allowed
triaxial force measurement. We used this to determine the centre of the
applied force relative to the walker feet, and also horizontal force
measurements allowed us to assess the patients 'balance 'and dependency
on the walker for stability(greater horizontal forces meant the patient
was leaning into the walker to achieve greater stability). A backpack
provided the signal conditioning. The circuit was not a conventional
one, I used an old bridge configuration to minimise the number of gauges
(there wasn't much space). In effect we were able to get 12 channels of
force (3 per walker foot) form a fairly small signal processing box (
roughly 10x10x10 cm's) and telemeter them to a receiver, to give the
subject freedom from tether cords, and maintain electrical safety
isolation.
If you have someone who's handy with strain gauges, this is cheap way to
go. The signal conditioners weren't very expensive (home-brew , of
course), they ran about 30-40$ worth of parts for each triaxial gauge
set. If you are planning to try to try strain gauges for the first time
it's not worth the trouble. they aren't that nice to play with,
especially for small set-ups. parts for the gauges themselves would cost
around 100-200$ per triaxial set (including spares for botched
attempts). If you include time, I'd guess that it would tale 5-10 hours
per triaxial set to make them if you have some idea what you are doing.
Telemetry is very expensive.. if you don't use it, make sure your
amplifiers are electrically isolated ( to the requirements of your
appropriate safety boards) you must be very careful about this aspect.

-Paul

Paul guy
Gait Laboratory
Dept. of Kinesiology
University of Waterloo
Waterloo, ontario
Canada
paul@gaitlab1.uwaterloo.ca

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Dear Ron,

Two companies come to mind. Transducer Techniques (Temecula, CA (909)
676 3965) and Entran (Fairfield, NJ (202)227-1002). Both companies
carry low profile load cells with the capacity that you require.

Brent Parks
Drexel/Hahnemann
Biomechanics Lab
Parksb@hal.hahnemann.edu

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Dear Ron,

With respect to your question about transducers:

In the past I have used force transducers made by Birste. They are
fairly small and low cost, starting at around 800 German Marks and going
up, of course, to over thousands of Marks. I am not sure of the exact
dimensions. Birster Prazisionsmesstechnik is a German company
(Gernsbach, Germany, I do ot have a full address) and is represented in
the Netherlands by Multitronics, P.O. Box 965, 3800 AZ Amersfoort, fax
(0)33-611385. They also sell transducers made by Sensotac Inc.(Columbus,
OHIO, USA). I am sure there are companies which sell transducers in your
area or at least in your country. I suggest checking if there is a
service at your university which offers assistance for instrumentation;
most universities here have such a service. They have Catalogues.

In general neasuring forces using strain-gage based transducers is
cheaper than using piezo electric transducers. Part of this is caused by
transducer prices, but the equipment needed to supply the energy and to
collect and represent the measured data is more expensive in the case of
piezo electric transducers. Which type you would choose depends on what
you want to measure.
In general:

-Piezo-electric transducers can measure high loads (ranging up to over
100 kN), while strain-gage transducers can measure very low forces
(ranges starting at 0.1 N or less.

-Piezo-electric transducers can not measure static forces bur are highly
suitable for dynamic forces with high frequencies (up to over 10 kHz).
Strain-gage transducers can measure static and low-dynamic forces but
are less suited for high-frequency dynamic measurements.

Sinc.

Edsko Hekman
Twente University
Fac. Werktuigbouwkunde -BW
Postbus 217
7500 AE Enschede
The Netherlands
e.e.g.hakman@wb.utwente.nl