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'Load cells, thermocouples'
1997\11\11@062221 by Alberto Smulders

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Does anybody have experience with interfacing load cells and thermocouples
to microprocessors? And does anybody know where to find practical
schematics about it on the internet?

Albert Smulders
InSAD - Encarnacion, Paraguay
spam_OUTinsadTakeThisOuTspamitacom.com.py

1997\11\11@091030 by Octavio Nogueira

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I have experience in interfacing load cells with PICs.
There is no big deal about it.

Regards,

Octavio

======================================================
Octavio Nogueira  - e-mail:   .....nogueiraKILLspamspam@spam@mandic.com.br
http://www.geocities.com/~oct_nogueira
"ProPic" Production PIC Programmer Windows under US$20
======================================================
-----Mensagem original-----
De: Alberto Smulders <insadspamKILLspamITACOM.COM.PY>
Para: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Data: Tera-feira, Novembro 11, 1997 08:29
Assunto: Load cells, thermocouples


>Does anybody have experience with interfacing load cells and thermocouples
>to microprocessors? And does anybody know where to find practical
>schematics about it on the internet?
>
>Albert Smulders
>InSAD - Encarnacion, Paraguay
>insadspamspam_OUTitacom.com.py
>

1997\11\11@132945 by Dave Celsnak

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> Does anybody have experience with interfacing load cells and thermocouples
> to microprocessors? And does anybody know where to find practical
> schematics about it on the internet?

I have successfully interfaced a K-type thermocouple to my PIC.
I am using Analog Devices AD595 Thermocouple Amplifier.
http://www.analog.com

Not sure what a load cell is....
Dave Celsnak

1997\11\11@153712 by Jim Ham

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Analog Devices has a series of 24 bit A/D chips that have on-board current
generators and amplifiers for RTDs and strain gages. They are AD7701 to
AD7714 the last time I looked. These chips all have serial interfaces so
are pretty easy to use. Prices vary, but are US$25 or cheaper in singles.

Regards, Jim Ham
At 08:16 AM 11/11/97 -0400, you wrote:
>Does anybody have experience with interfacing load cells and thermocouples
>to microprocessors? And does anybody know where to find practical
>schematics about it on the internet?
>
>Albert Smulders
>InSAD - Encarnacion, Paraguay
>@spam@insadKILLspamspamitacom.com.py
>
>
Jim Ham, Porcine Associates
(415)326-2669 fax(415)326-1071
"http://www.porcine.com"

1997\11\11@160612 by Alberto Smulders

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Dave Celsnak wrote:
.....
> Not sure what a load cell is....
.....

A load cell is a piece of steel with strain gauges in wheatstone-bridge
configuration in it, normally used for weighting purposes....

As the steel (and the strain gauges glued on it) deforms, the resistance
values of the bridge change, and there develops an output voltage between
two of the legs of the bridge, normally a very small voltage (in the order
of some mV), not so easy to measure and process in an industrial
environment with lots of noise, induction on cables, sometimes very high
temperature fluctuations, and the like. Also the input voltage to the
bridge must be held very constant, or there must be some means to measure
that voltage too and compensate (software !!!) for fluctuations in it I
think.......

That's why I asked for practical schematics, specially of the input
amplifier stage (I'm very bad at analog electronics.....), I couldn't find
anything up to now (although I have some broken down PCB's (lightning !!!),
so perhaps I'll do some "reverse engineering" on them to get an idea).

Albert Smulders
InSAD - Encarnacion, Paraguay
KILLspaminsadKILLspamspamitacom.com.py

1997\11\11@185558 by Steven Keller

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Alberto,

Check out this site www.cirrus.com/products/overviews/cs5516.html
It has some application sheets at the end that might be of help.  They
cover interfacing Strain gages and thermocouples to their analog to digital
converters but much of the info applies to all ADC's.

You might want to check out Analog Devices ( http://www.analog.com/ ) ADCs
and instrumentation amplifiers.  Many of the data sheets have example
circuits.

Steve


----------
> From: Alberto Smulders <RemoveMEinsadTakeThisOuTspamITACOM.COM.PY>
> To: spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU
> Subject: Load cells, thermocouples
> Date: Tuesday, November 11, 1997 6:16 AM
>
> Does anybody have experience with interfacing load cells and
thermocouples
> to microprocessors? And does anybody know where to find practical
> schematics about it on the internet?
>
> Albert Smulders
> InSAD - Encarnacion, Paraguay
> TakeThisOuTinsadEraseMEspamspam_OUTitacom.com.py

1997\11\11@223018 by Scott Newell

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>That's why I asked for practical schematics, specially of the input
>amplifier stage (I'm very bad at analog electronics.....), I couldn't find
>anything up to now (although I have some broken down PCB's (lightning !!!),
>so perhaps I'll do some "reverse engineering" on them to get an idea).

I like the circuit descriptions in the *excellent* Linear Technology
Application Handbooks.  The one that's most applicable is App Note #43,
"Bridge Circuits: Marrying Gain and Balance".  It's a Jim Williams app
note--'nuff said.  (I'm really fond of the common-mode surpression circuit;
it works well with a single ended ADC.)

For the digital side of things, I've been reading the Crystal Semiconductor
Industrial Data Acquisition book.  A lot of it is geared toward their
converters, but there are some good suggestions about tempco, trimming, AC
excitation, etc.

The LT notes are on their web site (http://www.linear-tech.com, I think).  Don't
know about the Crystal site.

If you find any other good reference material on load cells, be sure and
let me know!


later,
newell

1997\11\11@223227 by Scott Newell

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>That's why I asked for practical schematics, specially of the input
>amplifier stage (I'm very bad at analog electronics.....), I couldn't find

Oops, missed a couple.  Keithley Instruments has a thin little book
entitled "Low Level Measurements" that might help you out.  Stuff like
Kelvin connections, guarding, etc.  I think it's free upon request from a
Keithley dealer.

The National Semiconductor Linear Applications Handbook is another book
everyone should have.  It's pretty weak on load cell interfaces (maybe 3 or
4 circuits, but a lot gets rehashed by Williams in the LT book), but there
are several good V-F type circuits.  Some of it is starting to look a
little dated, though.  I think you can still request it (for free) from
their web site.


later,
newell

1997\11\12@064011 by wwl

picon face
On Tue, 11 Nov 1997 18:04:58 -0400, you wrote:

>Dave Celsnak wrote:
>.....
>> Not sure what a load cell is....
>.....
>
>A load cell is a piece of steel with strain gauges in wheatstone-bridge
>configuration in it, normally used for weighting purposes....
>
>As the steel (and the strain gauges glued on it) deforms, the resistance
>values of the bridge change, and there develops an output voltage between
>two of the legs of the bridge, normally a very small voltage (in the order
>of some mV), not so easy to measure and process in an industrial
>environment with lots of noise, induction on cables, sometimes very high
>temperature fluctuations, and the like. Also the input voltage to the
>bridge must be held very constant, or there must be some means to measure
>that voltage too and compensate (software !!!) for fluctuations in it I
>think.......
Not necessarily - if you link the ADC reference to the bridge supply
(e.g. connect both to a stable supply rail), the errors cancel out.
You do need to be careful, though if there are significant offsets &
tempcos to deal with.  
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1997\11\12@120520 by Paul BRITTON

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This is called a ratiometric conversion, and it means that you don't need a
particularly high spec reference, you can even get away with the supply rail if
it's well smoothed and bypassed.

TTFN Paul

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