Load cell reading not correct with PCI6224/SCC68 using SignalExpress

[Xiuping]

I connected two load cell FUTEK MBA500(T5105) with each one has two cells Force (200lb) and Torque (200lbin) to the 4 channels of SCC68 from AI0 to AI3 (connect - signal to AI8-AI11, shielding to 4 AIGND).  I used 12 VDC Excitation, and the load cell rated output is 2mv/v. 

I used both MAX and LabView Signal Express to view the data, both read 10m to -30m for zero load, and hundres of negtive mv for some small loads. (The force is compression thus I think the signal should be positive, am I right?) Essentially the output is not correct. My load cells are new and after calibration. Could you please tell me why the problem is? Thanks.

[BrowningG]

Hello Xiuping,

Have you checked the connections for your load cell?  I was looking at the specifications sheet for a load cell similar to yours, and noticed that it had a specific wiring code for connecting the device.  Since most load cells are based on strain gauges, the components inside of the device are resistive in nature.  It is possible that either your excitation or signal connections are reversed.  Basically if you have you excitation connected properly and your signal wires connections reversed, then the output will be the reverse of what you expect.  The voltage output of the sensor depends how the manufacturer build the device.  Most likely the output should be positive for compression (assuming this is a compression sensor), but the manufacturer would have the details on this.  Either your connections are reversed, or you can use a DAQmx scale to invert your reading. 

As for the offset of your signal when you have zero load, you can calibrate your load cell for being used in this particular setup.  Depending on your device, you can either software calibrate your load cell at known loads, or you can perform offset nulling and shunt calibration.  Which data acquisition device are you using?  When you create your task, you will either have a device tab or a calibration tab.  The calibration tab will allow you to software calibration your sensor under known conditions and the device tab will allow you to perform offset or shunt calibration if you are using a strain gauge task.  If you are using a custom voltage with excitation or voltage task you can only software calibrate your device.

[Xiuping]

Thank you , BrowningG.

I have asked for a specific datasheet of that loadcell from the manufacturer. It said that compression force is negtive. And also I did not notice that there are amplifiers connected to the load cell. So the connection and the reading should  not be a probem.