Newbie Help: Interface Load cell connected through BNC 2120 into PCI-6221

[Imran Malek]

Hi - I'm new to LabView and I'm having some trouble configuring a load cell to work properly with Labview.

The load cell is an interface SML-50, and I've linked the spec sheet below.

http://www.interfaceforce.co.uk/pdf/2000/sml.pdf

I have the load cell wired into the "AI3" port on my BNC-2120 and I have the BNC-2120 connected to a PC through a PCI-6221 card.

My problem is that I can't seem to get a zero value for the load cell when there is no loading on it. I've tried manual calbiration by measuring voltages and calculating calibration coefficients, but even then the data is too erratic. The spec on the SML-50 says that it requires a MAX excitation of 15V - I don't know how to incorporate this excitation into the BNC-2120 or the PCI-6221 - will the excitation help even out my data?

If not, what suggestions can you guys offer on this matter?

Thank you for your time, and have a great day!

Imran Malek

[JB]

The load cell is a strain gage and can't be wired directly to the BNC 2120 ! As you can see in the specifications its nominal rated output is 2mV/V --> if supplied with 15V, the output voltage will only be 30mV at full scale. You must therefore use an amplifier that will :

1. supply the load cell
2. adjust the zero balance
   
3. amplify its very low level output signal to 0-5V, 0-10V, 4-20mA...

The analog output signal of the amplifier can then be connected to the BNC 2120. Some NI hardware are able to interface strain gages without an intermediate amplifier but not yours.


Message Edité par JB le 02-27-2008 05:30 PM

[Imran Malek]

Thanks JB for the prompt reply, I will try to get my hands on an appropriate amplifier and then report my findings here!

Thanks again

[Brooks_C]

Hello Imran,

JB is correct; the BNC-2120 is not made to directly make load cell measurements.  You may want to consider getting a SCC-68 with an SCC-SG24 module (The SCC-SG24 provides 10V excitation, but we have other SCC-SG0X modules for other bridge configurations).  This setup will provide the excitation voltage and allow you to make the measurement you want.  The reason you need to apply the excitation voltage is because load cells are really just resistors.  The excitation voltage is applied to the load cell resistor bridge and then the measured output voltage is proportional to the excitation voltage--without this excitation it’s difficult to predict what you will read, but it won't be the correct value. If you’d like to speak with a technical representative about getting these signal conditioning modules you can contact National Instruments directly at www.ni.com/contact.

I believe that the load cell you are using has the full bridge internally so it is possible that you could used this with your current setup, but as JB said, the signals you’re measuring are very small so the SCC modules would be the preferred method for obtaining accurate measurements.  The 6221 has a +5 volt pin that you could use as the excitation voltage for your bridge, but you’d have to configure the measurement as a voltage task and do all of the analysis in software to convert the voltage to a strain measurement taking into account the excitation voltage.  You can try that out and see if it meets your needs, but otherwise I’d suggest looking into the signal conditioning options since they will provide much better results.

I hope this helps, and have a great afternoon!

[Bjorn_Beckman]

Hi guys!

Im borrowing this tread a little. I have a customer that uses a PCI6251+ BNC2120 to get an input signal to his computer. He puts a 3V input DC gain and records the signal to his harddrive, but the recorded signal only shows 0.01V. Do anybody have an idea what the problem might be?

Bjorn