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'[EE]: Mechanical Force Transducers'
2002\08\14@232714 by Michael A. Powers

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Hello,

I have an application that requires the use of a mechanical force transducer.  Basically, I want to know the force that one flat surface imparts upon another.  Does anyone know of a cheap sensor that can be bought (or easily made) that is appropriate?  I saw some pressure sensors in Jameco's catalog, but they appeared to be gas pressure sensors and wouldn't be applicable here, or would they?

Thanks,
-Mike

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2002\08\15@090416 by Francisco Ares

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While reading your post, I begun thinking about those pressure sensors
when you said "cheap sensor", before reaching that part on your text
that you mention the pressure sensors! Funny, isn't it?

I guess that you can use them if you could dissassemble one and build
another type of structure for it.

Could you give more details on those flat surfaces? If one of them is
fixed and stable, maybe you could just stick the force sensing part of
that pressure sensor over or under that surface.

Force sensors measures forces indirectly, measuring some mechanical
deformation due to that force, in the range of micrometers.

Another issues are calibration, relative position independance,
temperature compensation, noise, protection, cabling, signal conditioning...

What about those bathroom weight scales? ;-)

Francisco


Michael A. Powers wrote:

{Quote hidden}

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2002\08\15@100549 by Josh Koffman

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How about some sort of airbag that is placed in between the two
surfaces. put a tube from it to the pressure sensor, and you should be
able to measure the pressure, right?

Josh
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"Michael A. Powers" wrote:
> I have an application that requires the use of a mechanical force transducer.  Basically, I want to know the force that one flat surface imparts upon another.  Does anyone know of a cheap sensor that can be bought (or easily made) that is appropriate?  I saw some pressure sensors in Jameco's catalog, but they appeared to be gas pressure sensors and wouldn't be applicable here, or would they?

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2002\08\15@121430 by Richard Mellina

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       A cheap pressure sensor can be made from conductive foam. Sandwich the
conductive foam between two sheets of copper foil. Solder a wire to each
sheet of copper. It works just like a variable resistor:  when pressure is
applied the resistance between the two plates decreases. You will probably
need to experiment with it to find the amount of pressure associated with
the resistance change.

{Original Message removed}

2002\08\16@151305 by Michael A. Powers

picon face
I was thinking of the opposite of that.  Suppose I put dielectric foam
between two conductive parallel plates.  When force is applied to the
plates, the foam will deform and cause an increase in capacitance.  The only
trick is finding foam with a coefficient of deformation and thickness that
allows for capacitance changes that are significant to the measurement
instrument. The permittivity of the foam isn't important.


-Mike


----- Original Message -----
From: "Richard Mellina" <rsillemspamKILLspamFLASH.NET>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Thursday, August 15, 2002 12:07 PM
Subject: Re: [EE]: Mechanical Force Transducers


>         A cheap pressure sensor can be made from conductive foam. Sandwich
the
> conductive foam between two sheets of copper foil. Solder a wire to each
> sheet of copper. It works just like a variable resistor:  when pressure is
> applied the resistance between the two plates decreases. You will probably
> need to experiment with it to find the amount of pressure associated with
> the resistance change.
>
> {Original Message removed}

2002\08\16@163029 by Dave Tweed

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"Michael A. Powers" <EraseMEmapowersspam_OUTspamTakeThisOuTIEEE.ORG> wrote:
> I was thinking of the opposite of that.  Suppose I put dielectric foam
> between two conductive parallel plates.  When force is applied to the
> plates, the foam will deform and cause an increase in capacitance.  The only
> trick is finding foam with a coefficient of deformation and thickness that
> allows for capacitance changes that are significant to the measurement
> instrument. The permittivity of the foam isn't important.

I think you'll be fairly disappointed with the repeatability and hysteresis
associated with using foam (conductive or dielectric) as the spring element
of your force sensor. Mechanically speaking, foams are extremely lossy.

Metal springs (leaf, coil or thin diaphrams) give the best performance.
Use a rigid dielectric to separate two metal elements and measure the
capacitance change as they deform.

Sensing small changes in resistance or capacitance is generally not a
problem if you build the appropriate (DC or AC) bridge circuit. For
example, the following circuit measures small changes in a differential
capacitor used in a tilt sensor. It was originally written up as a quiz
question for Circuit Cellar Magazine.

  Question

  The following circuit is used to read a tilt sensor that is implemented
  as a differential capacitor -- when the sensor is tilted, C1 increases
  while C2 decreases (or vice-versa). Both capacitors vary between 4 and
  30 pF.
                        1 nF
                     +---||---+---------------+----------o A
                     |        |               |
                     |     D1 v               - D3
                     |        -               ^
                     |        |               |
       +-o 10VAC o---+        +---||--+--||---+
       |   1 MHz     |        |   C1  |  C2   |
       V             |     D2 v       V       - D4
      Gnd            |        -      Gnd      ^
                     |        |               |
                     +---||---+---------------+----------o B
                        1 nF

  The voltage differential between points A and B varies with the position
  of the sensor. How would you analyze the operation of this circuit?

  Answer

  The 10V, 1MHz source causes the diodes to conduct in pairs -- D1 and
  D4 conduct on the positive peaks, and D2 and D3 conduct on the negative
  peaks. Since D1 and D2 never conduct at the same time, the net current
  through them is directly porportional to the value of C1. Similarly, the
  current through D3 and D4 is proportional to the value of C2.

  If C1 and C2 have the same value, the D1/D2 current equals the D3/D4
  current, and the average voltage difference between points A and B is
  zero (although both are swinging up and down at 1 MHz). On the other hand,
  if the sensor is unbalanced, say C1 increases and C2 decreases, more
  current will flow in D1/D2 than in D3/D4, causing the average voltage at B
  to rise relative to A.

  Note that the difference between A and B can't exceed two forward diode
  drops (about 1.5 V) in either direction, and in fact, the voltage between
  them will be related to the net current flow by the diode equation. For
  values less than 1 V, the voltage varies nearly linearly with the
  capacitance difference.

  Additional information on this topic can be found in INK #61 ConnectTime.

-- Dave Tweed

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2002\08\16@170042 by Nelson Hochberg

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How about a modification of Alexander Graham Bell's original microphone?
Carbon granules between two metal plates.

Nelson
nelsonspamspam_OUTnosuffering.com
http://www.nosuffering.com

I was thinking of the opposite of that.  Suppose I put dielectric foam
between two conductive parallel plates.  When force is applied to the
plates, the foam will deform and cause an increase in capacitance.  The only
trick is finding foam with a coefficient of deformation and thickness that
allows for capacitance changes that are significant to the measurement
instrument. The permittivity of the foam isn't important.


-Mike


----- Original Message -----
From: "Richard Mellina" <@spam@rsillemKILLspamspamFLASH.NET>
To: <KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Thursday, August 15, 2002 12:07 PM
Subject: Re: [EE]: Mechanical Force Transducers


>         A cheap pressure sensor can be made from conductive foam. Sandwich
the
> conductive foam between two sheets of copper foil. Solder a wire to each
> sheet of copper. It works just like a variable resistor:  when pressure is
> applied the resistance between the two plates decreases. You will probably
> need to experiment with it to find the amount of pressure associated with
> the resistance change.
>
> {Original Message removed}

2002\08\16@181932 by Bill & Pookie

picon face
Mr. Bell was ahead of his time, as we are just now
learning.

Have heard about the amateur radio operator who
hooked a carbon microphone across the twin lead
antenna feed lines, turned on his transmitter and
started talking.

The black antistatic material that ic's are stuck
into is conductive and resistance decreases as it
is compressed.  So maybe that could be the new
carbon granules?

Bill

----- Original Message -----
From: "Nelson Hochberg" <RemoveMEnelsonTakeThisOuTspamNOSUFFERING.COM>
To: <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
Sent: Friday, August 16, 2002 2:00 PM
Subject: Re: [EE]: Mechanical Force Transducers


> How about a modification of Alexander Graham
Bell's original microphone?
> Carbon granules between two metal plates.
>
> Nelson
> TakeThisOuTnelsonEraseMEspamspam_OUTnosuffering.com
> http://www.nosuffering.com
>
> I was thinking of the opposite of that.  Suppose
I put dielectric foam
> between two conductive parallel plates.  When
force is applied to the
> plates, the foam will deform and cause an
increase in capacitance.  The only
> trick is finding foam with a coefficient of
deformation and thickness that
> allows for capacitance changes that are
significant to the measurement
> instrument. The permittivity of the foam isn't
important.
{Quote hidden}

from conductive foam. Sandwich
> the
> > conductive foam between two sheets of copper
foil. Solder a wire to each
> > sheet of copper. It works just like a variable
resistor:  when pressure is
> > applied the resistance between the two plates
decreases. You will probably
> > need to experiment with it to find the amount
of pressure associated with
> > the resistance change.
> >
> > {Original Message removed}

2002\08\16@192210 by Richard Mellina

picon face
       Why not try something like a strain gauge or force sensors? There are a
variety of force sensors for tension measurement, compression etc. that will
surely fit your requirements. The only problem is that I am sure they must
be expensive.

Check this site:
http://www.globalspec.com/Frames?Url=http%3A//http://www.globalspec.com/ProductFind
er%3Fse%3Dgt

Good luck!!

{Original Message removed}

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