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'[OT]: source for cheap strain guages?'
2002\03\15@035658 by dpharris

picon face
Hi All -

Does anyone know of a soucre of cheap strain gauges?

What I want to do is measure the force applied to the coupler on my
model train.  The plan is to measure the force and transmit it to a
readout station - via IR or rf.  So, I need a way of measuring the force
-- any other ideas?  (cheap is in :-)

David

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2002\03\15@175042 by Dave Dilatush

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David Harris wrote...

>Does anyone know of a soucre of cheap strain gauges?
>
>What I want to do is measure the force applied to the coupler on my
>model train.  The plan is to measure the force and transmit it to a
>readout station - via IR or rf.  So, I need a way of measuring the force
>-- any other ideas?  (cheap is in :-)

Sad to say, I don't think there is any such thing as an inexpensive
strain gauge, at least I've never come across one.

This topic comes up every so often on the PICLIST and it seems there's
never any really good solution.  There ought to be, because there
certainly is a need.

I've been working on a low-cost, high-precision system for force sensing
off and on as a "back burner" project for a while now and I'm making
good progress; but it's going to be some weeks yets before I'm ready to
post anything to the list.

Dave D.

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2002\03\15@175826 by David P. Harris

picon face
I'll wait ;-)
Can't we just use a piezocrystal -- maybe a buzzer -- stretch and measure.
David

Dave Dilatush wrote:

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2002\03\15@181809 by Dave Dilatush

picon face
David Harris wrote...

>I'll wait ;-)
>Can't we just use a piezocrystal -- maybe a buzzer -- stretch and measure.

The problem with those critters is that they only produce a momentary
output.

If you had a perfect measuring instrument, with infinite input impedance
and absolutely zero leakage current, you could conceivably get one of
those things to produce an output that would stay steady for at least a
few seconds after a force is applied.  But they will not respond
indefinitely to steady force because they themselves have internal
leakage.  After all, there's no such thing as a perfect insulator.

They're great for sensing mechanical vibration, that's for sure; and for
picking up sound as well.  But their piezoelectric nature renders them
useless for measuring constant force.

Dave D.
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2002\03\15@190922 by M. Adam Davis

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face
The cheapest expirementer's strain guage I've seen is conductive foam
with copper foil tape on two sides.  Compressed foam shows a lower
resistance than non-compressed, but the foam wears out and must be
recalibrated.

I'm trying to think in terms of your model trains though.  The space to
put a gauge in is so small.  Measuring the twisting or pull of a part
that doesn't twist or pull very much requires vey high precision sensors
or a series of levers to amplify the change.

-Adam

David Harris wrote:

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2002\03\15@192342 by Pic Dude

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Hmmm... from out in left field here, how about a small piece of
something pliable between the 2 cars.  Forces of compression
& extension would actually cause the cars to get closer/further,
though it may not be easily visible/apparent.  Now the problem
is to find a (precise) infrared or sonic distance/range detector,
which I would think should be easier than sourcing a strain gauge.



{Original Message removed}

2002\03\15@192544 by kent

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What about a capacitive measurement ?
Kent

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2002\03\15@194912 by michael brown

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> >Hi All -
> >
> >Does anyone know of a soucre of cheap strain gauges?
> >
> >What I want to do is measure the force applied to the coupler on my
> >model train.  The plan is to measure the force and transmit it to a
> >readout station - via IR or rf.  So, I need a way of measuring the force
> >-- any other ideas?  (cheap is in :-)
> >
> >David

If you want to experiment a little, I have an idea.  I didn't think this up,
but I heard about it before.  WARNING Untested Idea follows:  You can paint
a piece of flexible plastic with metallic paint (aluminum was what I heard,
but I don't see why copper wouldn't work).  The aluminum paint looks like
molten solder in the can, and it apparently contains allot of metal or some
other conductive material.  Once the home-made strain gauge dries, here
comes the tricky part.  You have to find a way to bond your wiring to the
painted surface.  You connect two wires with some separation between them.
When you flex the plastic, the resistance will vary between the contacts.  I
have never made one of these, but I can understand how it may work.  I would
guess that thousands (millions?) of micro-stress-cracks form in the surface
creating a maze like path for the electricity.  If you bend it too far, it
will become and open circuit, but should begin functioning again after
removing the stress.  Repeatability should prove to be all over the graph
and completely unpredictable.  ;-D  But, provided that you keep the range of
motion reasonable, and self-calibrate on every power up, it may do the job
for you.

Just an idea.

michael brown

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2002\03\15@195331 by michael brown

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> What about a capacitive measurement ?
> Kent

I like this idea.  Provided that actual compression force measurement is not
provided, I was thinking that a tiny magnet and hall-effect sensor may do
the trick and still be hidden.

michael

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2002\03\15@201210 by Dave Dilatush

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michael brown wrote...

>WARNING Untested Idea follows:

Ah, I like it already...  :)

>You can paint
>a piece of flexible plastic with metallic paint (aluminum was what I heard,
>but I don't see why copper wouldn't work).  The aluminum paint looks like
>molten solder in the can, and it apparently contains allot of metal or some
>other conductive material.

Years ago I recall seeing a "PC Board Repair Kit" which, among other
things like eyelets, etc. contained a metallic paint which became
conductive upon drying; it was to be used for bridging gaps in PC board
traces.  Is there still such a thing?  Maybe that could be used...

>Once the home-made strain gauge dries, here
>comes the tricky part.  You have to find a way to bond your wiring to the
>painted surface.

Seems like that might actually be the easy part: use a strip of PC board
material, with copper removed from the middle but present on the ends.
Then paint a line of the stuff between the copper lands, let it dry, and
solder your wires to the copper.

Voila.  Measure the change in resistance of the dried metallic goop as
the strip is flexed, and you're good to go.

That's probably how you'd want to operate it, too: with the applied
force acting to flex the strip, rather than acting to put the strip
under tension along its long axis.  You'd get more resistance change
that way.

{Quote hidden}

Cool.  Funky, home made, paint-on strain guages!  What the heck, try it!

Dave D.

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2002\03\15@201215 by michael brown

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> I like this idea.  Provided that actual compression force measurement is
not
> provided, I was thinking that a tiny magnet and hall-effect sensor may do

I meant to say "required" not "provided".  Not sure what I was thinking.  I
may try reading things before I send them next time.

> the trick and still be hidden.
>
> michael

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2002\03\15@201621 by Spehro Pefhany

picon face
At 12:57 AM 3/15/02 -0800, you wrote:
>Hi All -
>
>Does anyone know of a soucre of cheap strain gauges?

Strain gauges themselves are pretty cheap- maybe $5-(unlike
load cells), but you have to mount them on a suitable substrate
(matched tempco if you get fussy) and deal with rather low-level
signals. They measure strain (deformation of the substrate) so
the way you mount them determines how they work.

P.S. If you've even seen a piece of aerospace mechanics- such as
the Shuttle's robot arm (made by Spar Aerospace in Toronto), they
plaster the prototypes with strain gauges at every joint and
every other conceivable location. They have excellent
computer models, but *everything* is verified.

Best regards,



>  The plan is to measure the force and transmit it to a
>readout station - via IR or rf.  So, I need a way of measuring the force
>-- any other ideas?  (cheap is in :-)
>
>David
>
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2002\03\15@210125 by M. Adam Davis

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What about putting an integrator after the piezo?

-Adam

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2002\03\15@215537 by Dave Dilatush

picon face
Adam wrote...

>What about putting an integrator after the piezo?

That's in fact what's usually done with piezoelectric accelerometers,
and the integrator is referred to as a charge amplifier.

However, there's no such thing as an integrator with zero leakage,
either, so a piezo is still only good for measuring momentary forces.

(The reason charge amplifiers are used with piezo accelerometers is to
allow long cabling between the accelerometer and its instrumentation; if
the accelerometer were used in "voltage mode" instead of with the charge
amplifier, the cable capacitance would shunt the output and reduce the
output voltage, requiring recalibration with every setup.)

Dave D.

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2002\03\15@225328 by Vit

picon face
David,

I think you're familiar with the carbon microphone.  Why can't you use
something like that?  You will have a container full of carbon grains, and a
membrane covering the top.  The harder you push, the less the resistance,
and vice versa.

How precise do you want your strain gauge to be?

Vitaliy


> Does anyone know of a soucre of cheap strain gauges?
>
> What I want to do is measure the force applied to the coupler on my
> model train.  The plan is to measure the force and transmit it to a
> readout station - via IR or rf.  So, I need a way of measuring the force
> -- any other ideas?  (cheap is in :-)

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2002\03\15@231659 by Jim

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Permeability change (inductance change) on a small
coil (I'm think of the paper-form or plastic style
IF cans of years past) with a movable (sliding)
core against the force a small spring or other
elastic component ...

Resistance change of a small linear 'sliding'
resistive 'pot' against the force of a small
spring ...

LVDT to measure the displacement that ocurrs
against the force of a small spring ..

Pressure change on a small air-filled bladder
or piston/cylinder combo ...

Linear shaft and 'resistance spring' or other
'springy resistance component' coupled to a
rotary shaft (mouse?) encoder ...

Jim



{Original Message removed}

2002\03\15@233441 by M. Adam Davis

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"Trust...
but verify."

-Adam

Spehro Pefhany wrote:

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2002\03\16@050604 by Peter L. Peres

picon face
> What about putting an integrator after the piezo?

This works, but fyi the piezo in a sounder has very bad characteristics,
it creeps, it builds up DC potentials for no reason etc etc. I used a
section of TL074 and 1nF polypropylene feedback cap, and a switch for
reset when I tested this. The bronze backing flexes and stays flexed and
induces a lot of stress that affects the readout. Not so good.

Peter

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2002\03\16@103825 by Thomas McGahee

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One way to measure force is to measure the distortion it
produces. All the methods discussed on the PIC List so far
assume the use of this methodology.

But it isn't the only methodology available.

There is a feedback method in which an opposing force is applied
until the resultant is zero. It is easy to measure a resultant of
zero.

As an illustration of the technique I will discuss a method used
to measure weight:

The moveable iron core of a small vertical solenoid is linked to a thin
wire that connects to the object to be weighed. The top of the moveable
iron core connects to a thin metal vane. The vane is arranged so that
when it moves UP slightly, it breaks a light beam that is aimed at a
light sensor.

A physical stop mechanism is normally included so that the possible
travel of the solenoid core is restriced to a very small distance.

An electronic feedback circuit controls the current feeding the solenoid.
The solenoid position is maintained at the precise position where the
vane just begins to obscure the light beam. (simple negative feedback).

The circuit is calibrated by recording the current required to bring
the circuit to equilibrium for a variety of weights.

When I say we measure the current, we usually don't measure the current
itself, but the average ON versus OFF time of the circuit. This
is proportional to the average current, and easier to measure with
a PIC.

**********

You can dispense with the light sensor method and make an even cheaper
version by using the stops as switch contacts, with the armature of the
resulting SPDT switch being the metal piece that contacts the stops.

You wire the resulting SPDT switch contacts to the R and S inputs
of an RS flip flop. The armature goes to either + or ground, depending
on the kind of RS flip flop you are using. The R and S inputs will
need either pullups or pulldown resistors.

The output of the RS flip flop controls a transistor that provides
current to the solenoid in ON/OFF fashion. As before, we measure the
average ON/OFF ratio, which is proportional to the applied force.

Proper calibration is important.

Fr. Thomas McGahee

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2002\03\16@112818 by Paul Hutchinson

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> Years ago I recall seeing a "PC Board Repair Kit" which, among other
> things like eyelets, etc. contained a metallic paint which became
> conductive upon drying; it was to be used for bridging gaps in PC board
> traces.  Is there still such a thing?  Maybe that could be used...

GC Electronics still makes Silver and Nickel based conductive paints. They
used to make a copper conductive paint but I don't see it listed on their
web site.
http://www.gcwaldom.com

Used to use a lot of their "Silver Print" when I worked for a precision
potentiometer company in the 70's. Back then we also used to use a silver
epoxy for attaching leads to the conductive paint. Don't remember who made
the silver epoxy but I do remember the silver paint and epoxy where both big
$$$.

Paul

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2002\03\16@164156 by Dwayne Reid

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At 10:16 PM 3/15/02 -0600, Jim wrote:
>Permeability change (inductance change) on a small
>coil (I'm think of the paper-form or plastic style
>IF cans of years past) with a movable (sliding)
>core against the force a small spring or other
>elastic component ...

Even easier - measure the inductance change in a coil spring.

dwayne


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2002\03\16@165851 by David P. Harris

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Hi Dwayne -
Could this be done by including the coil into an oscillator and then
measuring teh frequency change?
David

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2002\03\16@172129 by David P. Harris

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Hi again-
Here's a circuit I found on the net at:
http://www.ee.washington.edu/circuit_archive/circuits/F_ASCII_Schem.html

CMOS Oscillator

From: TakeThisOuTmjohnsonEraseMEspamspam_OUTnetcom.Netcom.COM (Mark Johnson)

> What I am looking for is a low power oscillator (<.5 mA @ 5V)
> running at a frequency of roughly 1 MHz. However, the frequency-
> determining component should be an inductor with a value of
> approx. 75 uH


The circuit below uses a single CMOS lowspeed 74C14 inverting Schmitt
trigger chip, your 75uH inductor, and two 10K resistors. It draws about
400uA and oscillates at about 4MHz.

The oscillator period will be approximately linearly related to the
inductor value, Period ~=~ K1 + (K2 * L) [Note also that K1 will not be
zero]

                                            +-----------> Output
                                            |
                                            |
    |\            |\            |\          |     |\
    |  \          |  \          |  \        |     |  \
    |    \        |    \        |    \      |     |    \
 +--|     >O------|     >O------|     >O----+-----|     >O-----|
 |  |    /        |    /        |    /            |    /       |
 |  |  /          |  /          |  /              |  /         |
 |  |/            |/            |/                |/           |
 |                                                             |
 |                              +5.0V                          |
 |                                |                            |
 |                                \                            |
 |                                /  R1                        |
 |                      /|        \ 10K                        |
 |                    /  |        /                 L1         |
 |                  /    |        |                75 uH       |
 +----------------O<     |--------+---+----------)()()()(------+
                    \    |            |
                      \  |            /
                        \|            \  R2
                                      / 10K
                                      \
                                      |
                                     GND

"David P. Harris" wrote:

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2002\03\16@184603 by Dwayne Reid

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At 01:59 PM 3/16/02 -0800, David P. Harris wrote:
>Hi Dwayne -
>Could this be done by including the coil into an oscillator and then
>measuring teh frequency change?
>David

Exactly!

Beau Schwabe has a working circuit for one implementation on his web-site
<http://home.earthlink.net/~y2kbc/Electronics/BasicStamp/Coilread.gif> but
there are MANY possible ways of doing the same thing.

dwayne


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2002\03\16@190509 by David P. Harris

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Thanks Dwayne, this is great.
David

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2002\03\18@125351 by Pic Dude

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I recently saw these at Fry's electronics near the PCB etching supplies.
It's a conductive pen for fixing PCB probs and they also had another
pen to protect the conductive pen (sort of like a clear-coat I guess).
I'm guessing GC makes it cause it was near all the other GC products.

If that's what you're looking for, let me know and I can get some info
when I head over there next.

Cheers,
-Neil.


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