Configuring Load Cell [ NEW ] Feild Point

[itchdbz]

Hi, I'm trying to set up some Data acquisition from a load cell through my FeildPoint AI-110. The Load Cell I have is rated to 500KG and has a Out put of 2.997 mV/V. I can connect the load cell to field point and display the voltage difference in Max, but I’m struggling to use this to get any kind of scaled reading to using in Labview.

Can any one help.

Many thanks

[Matthew_Williams]

Two suggestions:

1) If you can switch to the SG-140 module instead of the AI-110 you will probably get a better measurement with much less work.

2) If you can't switch, are you also measuring the excitation voltage to your bridge?  Your algorithm should be something like:

(bridge voltage / excitation voltage) X (500 kg / 2.997 mV/V) = unknown kg

(mV / V) X (kg/(mV/V)) = kg

I haven't run the accuracy/resolution calculations from the AI-110 specs, so the above may or may not give you a
measurement that meets your needs.

Hope this is useful.

Matt

[itchdbz]

That’s certainly helpful, I’m stuck with the AI-110 I'm afraid. I should be able to find out the excitation voltage with a multi-meter reading. But what do you mean by Bridge voltage?

Thanks for the help

Rich

[Matthew_Williams]

The bridge voltage is the voltage measured across the output of the bridge.

If you take a typical bridge drawing, the excitation voltage is applied to the top and bottom pins,
the bridge output voltage is measured at the left and right pins.

For this to work, I expect you will have to wire one of the input channels of the AI-110 to the excitation voltage
and another channel to the bridge output.  In theory you don't have to actually measure the excitation voltage if
your source is known and very stable, but I expect you will get better results if you do measure it with the AI-110.

There is a good drawing of what you need to do on page 5 of the SG-140 manual, http://www.ni.com/pdf/manuals/323346a.pdf
Vin+ and Vin- represent the bridge voltage.  Just substitute two channels of the AI-110 (and your excitation source) for the
SG-140 block shown.

Matt

[itchdbz]

Hi,

Made a bit of progress, I know that the excitation voltage is 12.25 Volts. Would the bridge voltage be the figure displayed in Max?. If so I get -0.000079 Volts with no load then when I add a 1.85 Kg to the load cell the figure changes to -0.000238 Volts.

But I'm still no coming close any workable reading. Any suggestions?.

Thanks

Rich

[Matthew_Williams]

I think the attached should get you pretty close.  The top section uses just the constants you posted and
assumes a constant excitation voltage of 12.25.  The bottom section uses the actual, current excitation
voltage when the measurement is made.  I think this second approach will probably give you better long
term results.

The bridge voltage is the voltage displayed in MAX.

Matt

Message Edited by Matthew Williams on 07-04-2007 08:48 AM

[Mukul J]

Hello,

My question is probably directed to Rich, but any help would be much appreciated. Im in the same situation where Im trying to measure strain of an electrical gage in a quarter bridge arrangement and using a cFP-AI-110 module to do this. What is the way that i should connect my 3 wire strain gage to the module channel such that i have an excitation voltage to the gage alongwith the measurement in volts.

Many thanks

Mukul

[rocksolidsr]

would you be able to use a FP-AI-100 to measure the output of a load cell