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'[EE]:Force or weight sensor and PIC interface'
2001\01\27@205018 by Brian Luterbach

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I would like to be able to measure the strain in a steel blade. The strain
is created by tightening a bolt and the force the bolt applies can be
measured. The force, with a margin for error, will not exceed 200 lbs. Are
there any suitable inexpensive sensors available and what circuit could be
used to connect the sensor to a pic?

The pic will calculate the strain from the force applied.

Thank you.

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2001\01\27@231126 by Chris Eddy

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Brian, have a look through the Omega book (or web site).  They have a long
list of strain gauge styles, and even if they do not have the one that you
want, you will probably see most common styles there.

You could always make your own (probably not wise in an industry such as
elevators.. refer to insurance man and lawyers).  If you do, you could do some
neat things.  You could take a large spiral spring and a flat plate.  Then,
when the cable tightens down, the spring compresses.  Attach a strain gauge
which measures torsional stress to one spot on the spring.  Voila.

Pre-calibrate it in a vice ganged with a calibration cell with a readout of
some kind.  Then you can zero and span it in the vice.

Chris~

Brian Luterbach wrote:

{Quote hidden}

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2001\01\28@004458 by Robert Rolf

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Look for 'load cells' on the web. Lots of suppliers. Turn key solution
if you want high (1%) accuracy in your measurement. Hang a low drift
DC amplifier with high gain (strain guage amp since that what most load
cells use) and you're in business.

Chris Eddy wrote:
{Quote hidden}

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2001\01\28@135206 by Peter L. Peres

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Is the blade fixed ? Do you know its E ? (you do, since you calculate
strain). There are several load cell arrangements that will read 300lb but
none are cheap in the pic sense of the word afaik. You say nothing about
precision, accuracy, etc. If you know E then you can measure the
displacement alone and thus calculate the strain. This is likely going to
be much cheaper than a strain gauge or load cell. The displacement can be
measured optically, capacitively, mechanically, lvdt etc. I'd be tempted
to use a hollow bolt as a capacitive load cell ;-)

$0.02

Peter

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2001\01\28@180339 by Brian Luterbach

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Thanks for the responses - by searching the web for "load cells' and Omega I realized that I misunderstood how strain gages measure. I thought that they measured force not displacement. I was expecting a piezo effect from an applied force and not a change in resistance as a result of a physical displacement. Physical displacement to measure strain certainly make sense.

I was not able to find any inexpensive load cells that could measure 200 lbs. Did I miss something or are they just expensive?

Peter could you point me to some more information on using a "hollow bolt as a capacitive load cell". Accuracy is not critical I would like to be within 2-3%. The displacement of the blade cannot be measured directly but the displacement of the mechanism applying the tension can be measured. I know the modulus of the blade and the other relevant parts.

Thank you for all the help.

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2001\01\28@232147 by Russell McMahon

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I was not able to find any inexpensive load cells that could measure 200
lbs. Did I miss something or are they just expensive?

The capacity of load cells are in general terms NOT a function of the sensor
element but of the mechanical structure whos edeflection they are measuring.
A larger cell will be bending a larger or stronger piece iof material and so
MAY be more expensive.
It is possible to make your own load cells of sensibly indefinitely large
capacity by using standard strain gauge foils attached to a deflection beam
of your choice.

RM

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2001\01\30@130413 by Peter L. Peres

picon face
'Hollow bolts' are available, which have built-in strain gages or other
means to measure deformation. They are as expensive as strain gages and
have the same poblem (sensitive amplifier required). A capacitive 'hollow
bolt' sensor is a 'homebrew' type of sensor where the sensing element is a
capacitor formed between the bottom of the bolt (which is not drilled
through) and a conical piston that is mounted insulated inside the bolt
hole and does not touch the bottom of the drilled hole. It is sensed in an
AC bridge. The bolt construction is done such that the tip part and the
conical piston mount are far removed from the threads so the thread/bolt
tightening has no influence on the read value. If you have machining
capability you can make your own relatively easily. Capacitive readout
using a PIC can be done with a RC delay method (with C being the sensor)
which avoids A/D conversion. I do not know if you can reach 1% with this
type of conversion. That would be 'cheap' by me anyway ;-).

As you have discovered, there are no 'cheap' force measuring devices
available. One hack would be to use one of those $50 digital readout
micrometers and mount it on the hollow bolt or some other part to measure
displacement directly, using a prolonged measuring stick, and connect the
optional serial data output from the micrometer to the PIC or another
computer. This would give an advantage in that it has a direct readout in
place, for adjustment etc. If you have control over the modulus of the
part it measures, then you can make it read out directly in lb by
adjusting the modulus and the position where you measure the displacement.
50uM readouts are available, if that's 1% of 200 lb then 200lb => 5000um =
5mm. That's a lot of displacement depending on where it happens. I am not
a mech. eng. but I think I'd use a compression spring for this. I don't
know what compression springs do with temperature variation and aging
though.

hope it helps,

Peter

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2001\01\30@141037 by James Paul

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All,

Just a couple of tidbits for your perusal.

1.  Use static conductive foam (the kind IC's are shipped in)
    mounted between two conductive plates.  Sort of a foam
    dielectric capacitor.   But, pressure on the foam decreases
    it's resistance, which can be sensed.   This can then give an
    indication of stress or strain.

2.  Radio Shack has a bendable resistance element that changes
    resistance proportional to the amount of bend.  This might
    work if room isn't a problem.

    Anyway, just a couple of things you might want to check out.


                                        Regards,

                                          Jim




On Mon, 29 January 2001, "Peter L. Peres" wrote:

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jimspamKILLspamjpes.com

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2001\01\31@041759 by Attila Muhi

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If esthetic aspects are not important, I remember an article I read some years ago.

They used an about 8-inch loudspeaker, a bit disassembled, with a pin attached to the center of the cone. The pin was connected to a tray. On the pin, there was an index coming out, which was sensed by an infrared fork sensor. The idea was to keep the index in the same position all the time, to keep it that way you had to increase the current when anything was dropped into the tray,  thus the current in the speaker coil was proportional to the weight of what was in the tray. I remember (I hope) that this solution gave a quite good precision if it was properly tuned.

Regards

Attila Muhi - SM4RAN
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Datum: den 30 januari 2001 20:12
Ämne: Re: [EE]:Force or weight sensor and PIC interface


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2001\01\31@163300 by Chris Carr

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Attila Muhi wrote

They used an about 8-inch loudspeaker, a bit disassembled, with a pin
attached to the center of the cone. The pin was connected to a tray. On the
pin, there was an index coming out, which was sensed by an infrared fork
sensor. The idea was to keep the index in the same position all the time, to
keep it that way you had to increase the current when anything was dropped
into the tray,  thus the current in the speaker coil was proportional to the
weight of what was in the tray. I remember (I hope) that this solution gave
a quite good precision if it was properly tuned.

Now there's an idea. I do a similar modification to speakers. Attach rod to
centre of cone, invert so rod is pointing to ground, push rod into ground.
Two thin wires from speaker terminals to pre-amplifier and you have a
Geo-phone, pick up footsteps or any other (low frequency) vibration passing
through the earth, interesting on their own and useful in an array.

Regards

Chris

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'[EE]:Force or weight sensor and PIC interface'
2001\02\01@150617 by Oliver Broad
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The capacitive idea sounds like you'd have a tiny change in a tiny
capacitance, I can't help wondering how you'd measure it. Capacitive touch
sensor must be easy by comparison.

I guess an oscillator could be formed, frequency to be counted by the RTCC,
a chance to put the prescaler to good use!

Oliver.

{Original Message removed}

2001\02\01@153705 by Chris Carr

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Oliver Broad wrote


> The capacitive idea sounds like you'd have a tiny change in a tiny
> capacitance, I can't help wondering how you'd measure it. Capacitive touch
> sensor must be easy by comparison.
>
> I guess an oscillator could be formed, frequency to be counted by the
RTCC,
> a chance to put the prescaler to good use!
>
I have sort of being following this thread, but excuse me if I am missing
the point it was the word capacitive that jumped out.

I have a pair of bathroom scales that appear to operate on a capacitive
basis by moving a small metal plate between two others, not in and out but
within the gap between the fixed outer plates, from one plate to the other.
I also have a turntable motor from a microwave oven that has a weight sensor
that works on the same principle.

Sorry but I do not know the principle of operation and I really don't have
the time to rip the scales apart to find out - sorry. But I have no doubt
that someone on the list will be able to tell us.


Regards

Chris

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2001\02\01@162512 by Robert.Rolf

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RF oscillator with frequency determined by capacitance.
FM discriminator for detection.
R

Chris Carr wrote:
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2001\02\01@185011 by Eisermann, Phil [Ridg/CO]

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A few years ago, I worked on a custom force sensor that did this. I remember
getting $75 for filling out the patent information, but I don't think we
ever applied for the actual patent. Or we did, but didn't get it.

In any case, the tiny change in capacitance was measured by a PIC. The
created capacitor was in a 555 circuit, and the PIC measured the resulting
frequency to determine the capacitance/force.

The metal elements were actually formed by circular traces on two circuit
boards. Belville washers created the spring force. You can get a surprising
range if you get the air gap right.

There, that's one way to do it :) You did ask for it.


{Original Message removed}

2001\02\02@180912 by Peter L. Peres

picon face
>capacitive sensor readout method

The cheap commercial designs almost always use a AC bridge in which a
(CMOS) oscillator provides drive (5-10Vpp). The readout is simply a CMOS
input opamp used as 'perfect' rectifier. 1% can be attained like this.

Better designs use a double slope A/D in which the measuring capacitor is
a part of the A/D.

Latest designs use a switched capacitor approach and measure charge
directly (the sensor is the 1st capacitor in a C/2C A/D converter). This
is a neat and minimum part count solution.

Devices that measure delta-C down to 0.1 pF and better are easily built
using off the shelf components, even by amateurs. Oscillators followed by
demodulation are more complicated tham this and suffer from drift etc. The
only real advantage they have is, that the measured value can be
transmitted easily for telemetry in an analog system.

Peter

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2001\02\02@180915 by Peter L. Peres

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The capacitive sensor mesaures a change of up to 30% in the capacitance of
the sensor. It all depends on the spring which is the wall of the bolt in
this case. There are ways to form it such that the deformation is as large
as you wish but normally one does a very small distance capacitor so your
'very small distance change' becomes a hefty percentage of d. Also the
dielectric need not be air. 20-30pF is easily done. It is not very precise
it is very *cheap*.

Peter

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