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'[EE]: Strain gauge amplifier reccomendations'
2006\12\12@111425 by Denny Esterline

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A colleague of mine recently had a minor car accident, he'll recover- but
I've been asked to pick up one of his projects while he's out.

I have a three axis force transducer to install in a ELD wind tunnel. Three
separate four-arm strain bridges ~900 ohms. With the recommended 10 volt
excitation I see about +-7mV at full load. The DAQ hardware that accompanied
this is a NI USB9215A with +-10 volt range and 16 bit resolution. (I'll save
you the math - 305 uV :-)

Clearly I need better resolution.

Always looking to heat up my soldering iron, my first thought was to lash up
a three channel amplifier unit with something like TIs INA129. Then I came
to my senses. This is a reasonably well funded one-off project, custom
hardware probably isn't the best choice.

So, does anyone have recommendations for a more suitable DAQ unit or a
suitable amplifier unit to tie these together?

Thanks,
-Denny

2006\12\12@124222 by Cristóvão Dalla Costa

picon face
If that's a "well funded one-off project", what's the problem with
custom hardware? I don't think a few PCBs plus parts to produce an
amplifier unit would cost you more than $100. Also I don't think you
can even take readings with useful precision from a load cell without
an instrumentation amplifier to get rid of common mode noise.

On 12/12/06, Denny Esterline <spam_OUTdesterlineTakeThisOuTspamgmail.com> wrote:
{Quote hidden}

> -

2006\12\12@132814 by Denny Esterline

picon face
Yes, a good instrumentation amplfier is needed here, but the question is:
Can I get a DAQ that includes one internaly, do I add prebuilt external
modules, or do I build one from scratch?

I was looking to avoid custom hardware for a couple reasons. This is a
measurment insrument, they will add to the complexity and uncertainty of
calibration. Custom hardware will add an ongoing service commitment I'd
rather avoid, and anything in the microvolt range requires a lot of detail
to stuff like drift and tempco. I'd like to avoid all that.

-denny


If that's a "well funded one-off project", what's the problem with
{Quote hidden}

2006\12\12@183929 by Dwayne Reid

flavicon
face
At 09:14 AM 12/12/2006, Denny Esterline wrote:

>So, does anyone have recommendations for a more suitable DAQ unit or a
>suitable amplifier unit to tie these together?

No part numbers to suggest but I do offer a warning: I've got an
otherwise nice DAQ unit downstairs that was purchased for exactly the
application you have: strain gauge measurements.  The unit we chose
was a Quatech DAQP-12H (sold by Superlogics sells it as their PCM12H)
which contains a PGA with gain up to 1000X (60dB).

It is essentially useless with the maximum gain selected: even with
the inputs shorted, the noise floor approaches the a/d range.

The reason I bring this up is that this unacceptable performance was
not detectable from the datasheets.  However, the distributor from
whom we purchased the unit told us that was as good as it got - and
NO, we could NOT return the unit as defective.

Still do use the unit to this day - but only with very low values of
PGA gain.  I use a cobbled-together diff-amp with a mux on the front
end (swaps the differential inputs) when measuring strain-gauges or
thermocouples.  The mux to swap the inputs lets me get rid of any
offset added by the diff-amp.

I guess that I'm saying that spec sheets don't tell the whole story
and perhaps its better to get whichever unit you choose on demo first
and verify its performance before plunking down your customer's hard
earned cash.

dwayne

PS - just thinking about it now - I should rip my unit apart and see
just what they used for the gain stages.  Should be able to stick in
better parts than were available when this unit was purchased.

dwayne

--
Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2006\12\12@192913 by Bill Clawson

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I looked at the national instruments website and their
units start at around $300.  If you're willing to
plunk down the money, you may want to start there.
Otherwise, there are instrumentation amplifier ICs
from places like Analog Devices, and it seems to me
that coming up with a custom circuit might not be all
that hard, albeit I haven't found one on the web just
yet.

Bill

--- Denny Esterline <desterlinespamKILLspamgmail.com> wrote:

{Quote hidden}

> --

2006\12\13@001259 by Robert Rolf

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www.omega.com also has some SGA for around the same $300 price.

And google turns up many other turn key solutions.
www.google.ca/search?hl=en&q=%22strain+gage+amplifiers%22+multi+channel&meta=
Prices vary depending on features (# channels, frequency response, stability specs,)

www.grasstechnologies.com/products/amplifiers/p122.html
Grass has been doing this for decades so they know what they're doing. Well built.

Found this helpful app note while looking.
http://www.iotech.com/catalog/techtip/TechTip_60602.pdf

There are A/D cards that are made specifically for strain gage use.
http://www.iotech.com/strain.html

www.iotech.com/catalog/daq/strainbook.html
8 channels, 16bit 1Mhz SGA+ A/D for ethernet. $6k

Don't forget that at the voltages you are measuring that thermocouple effects
can be significant sources of error.

Robert


Bill Clawson wrote:
{Quote hidden}

2006\12\13@034619 by William Chops Westfield

face picon face
>> So, does anyone have recommendations for a more suitable DAQ unit or a
>> suitable amplifier unit to tie these together?
>>
It looks like the DAQ crowd call this "signal conditioning", and they
seem to charge a lot of money for it.
http://www.dataq.com/products/startkit/di195b.htm is a $300 unit
that is apparently designed to be used with "signal conditioner
modules" that run an additional $200 each (but you can get modules
specifically aimed at strain gauges...)

www.aeroconsystems.com/electronics/amp02.htm
is a somewhat less professional instrumentation amp aimed at
load cells.

BillW

2006\12\13@040154 by Alan B. Pearce

face picon face
>Found this helpful app note while looking.
> http://www.iotech.com/catalog/techtip/TechTip_60602.pdf

It might be just semantics, for ease of explanation, but I understood one of
the tricks was to AC excite the strain gauge, then AC amplify the output so
one wasn't dealing with high DC gain and the resultant offset problems, and
synchronous detection to remove other stray hum and noise. That app note
shows it all done with DC.

I am not that familiar with strain gauges, but did have a small amount of
contact with a system that uses them about 35 years ago, and that was the
impression I got of how that system worked.

2006\12\14@125718 by Denny Esterline

picon face
>
> >So, does anyone have recommendations for a more suitable DAQ unit or a
> >suitable amplifier unit to tie these together?
>
> No part numbers to suggest but I do offer a warning: I've got an
> otherwise nice DAQ unit downstairs that was purchased for exactly the
> application you have: strain gauge measurements.  The unit we chose
> was a Quatech DAQP-12H (sold by Superlogics sells it as their PCM12H)
> which contains a PGA with gain up to 1000X (60dB).
>
> It is essentially useless with the maximum gain selected: even with
> the inputs shorted, the noise floor approaches the a/d range.


This is more the kind of info I was hoping for. Initial efforts with google
turned up many, many solutions, but it's a lot of work to separate the
marketing hype from real performance.

This list is so broad, I figured somebody would have done something similar
and would chime in with a "We use product X" answer.


-Denny

2006\12\16@015341 by tachyon 1

picon face
I recently have seen a lot of NASA surplus equipment on e-bay in in tech
surplus stores.
Almost all the listings include strain guages which I assume are probably
pretty high end models.
That and lots of re-breathers.

 {Original Message removed}

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