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'[OT]: Force Sensors or load cells'
2001\09\10@192900 by Dave King

picon face
Hi

Just received an email asking if I can build some snow pack sensors. In
other words a digital scale.
It needs to measure 0-1000grams with 0.1g resolution. They of course want
me to build 20 of these for
about $20 each. ( I assume they are joking but you never know). Anyway I'm
wondering if anyone has a few
urls for some cheapy load cells or sensors. I'm still waiting to hear back
about the actual requirements so
I can see just how cheap I can go.

Any hints help  would be appreciated.

Cheers

Dave


The only thing that helps me maintain my slender grip on reality is the
friendship I share with my collection of singing potatoes

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2001\09\11@064915 by fernteix

flavicon
face
Hi

In my past experience , many times I was asked for to build devices with a
precision that was not necessary.
Is normal that the client have not an idea about it .
Precision  and inexpensive devices? I remember only the measurement of time,
frequency,duty-cycle.

Best Regards

Fernando

{Original Message removed}

2001\09\11@143847 by Thomas McGahee

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face
A PIC can very easily measure things like frequency and period.
So, what you need is a way to convert mass to frequency or period.

One way to do this is to suspend the mass to be measured from
a spring and make it vibrate. The "spring" can be a conventional
coiled spring with the mass hung below it, or a horizontal flat
spring, or a rocker assembly in which the mass is on one side of
the fulcrum, and the spring is on the other side of the fulcrum.

There are many different ways that you can set up the "spring"
type assembly. The important thing is that the greater the mass,
the LONGER the period. A way needs to be set up to put the
mass in motion. For the cheapest method you could have the
user push a mechanical button that then transferred that motion
to the spring. Once the mass is in motion, it will oscillate
with a period that is proportional to the mass. Now, realize
that the system itself has mass, and therefore you are measuring
more than just the mass you WANT to measure. Such a device
would obviously have to be calibrated. Temperature will have an effect
on the springiness of the spring, so the BEST type of calibration
is one which is done just prior to the actual measurement.

If a small bar magnet is caused to move back and forth through a
coil of wire, it will generate a sine wave of diminishing
amplitude. Use a comparator to convert this to a square
wave, and measure the period using a PIC. You may have to use
an algorithm or a lookup table to convert the period to the
measured mass.

Disgusting Ascii diagram follows:

    S   where S=Spring
    S
    S
    B   B=Bar magnet and coil assembly
    |
    M   M=Mass to be measured



Fr. Thomas McGahee


> {Original Message removed}

2001\09\11@174137 by Dave King

picon face
At 10:00 AM 9/11/01 +0100, you wrote:
>Hi
>
>In my past experience , many times I was asked for to build devices with a
>precision that was not necessary.
>Is normal that the client have not an idea about it .
>Precision  and inexpensive devices? I remember only the measurement of time,
>frequency,duty-cycle.
>
>Fernando

In this case the precision is needed. These are evidently snow pack
monitors for
ski hills. So they need to get accurate measurements of snow samples.


Dave

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2001\09\11@214926 by Arnold Chord

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Is it possible to use force sensing resietors?
Also called fsr's

----- Original Message -----
From: "Dave King" <spam_OUTKingDWSTakeThisOuTspamHOME.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Monday, September 10, 2001 4:18 PM
Subject: [OT]: Force Sensors or load cells


{Quote hidden}

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2001\09\11@223418 by Dave King

picon face
At 06:44 PM 9/11/01 -0700, you wrote:
>Is it possible to use force sensing resietors?
>Also called fsr's

Actually that was my first thought about this. A fsr on a bending beam or
something close. I don't know if something could be made with enough accuracy
or repeatability for this. I've only worked with them a couple of time a
few ice ages ago
so I'm not at all sure they could do this. I also wonder if they would have
to be temperature
compensated and calibrated.

In a way this is a very interesting little project as it has to be
accurate, has to be pretty tough
has to function in sub zero temps and high altitudes. Of course the buggers
want em cheap ;-]

Dave

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