FPGA NI 9237 Load Cell Input

[cuzb]

I'm trying to collect measurements from three load cells (Cooper LKCP 414-500 Load Cells) using an NI 9237 routed through a NI cRIO-9014. In the FPGA (included as a screenshot below), for the NI 9237, I have the excitation voltage set to 10 V, the data rate set to 25 kS/s, and the Enable Half-Bridge and Offset Cal set to false (I wasn't sure if the load cell was a full-bridge or half-bridge, so I was fiddling with the settings).

To connect to the 9237 we're using an RJ-50 cable spliced to the Load Cells. I have the excitation and signal I/O set up as shown in the second image attached below. 

When we excite the load cell and compress it, we can measure a change in the voltage output of the Load Cells using a DMM. But our output in the FPGA will oscillate until the load cell undergoes slight compression and the output will then rail, or the output will simply rail without any load being applied.

Any ideas why this railing might be occurring, and any suggestions of how I should proceed? Could it be the wiring of the RJ-50, or some code missing from the FPGA?

Thanks!

[Matthew-B]

Hi cuzb,

From the portion of the code I could see, your code setup looks okay with the possible exception of a loop. Are you continuously reading the voltage? Right now, it looks like you start the measurement, read a single point, then stop the measurement. If you want to read in a loop, the easiest way to do this would be to put a loop around the FPGA I/O node.

In terms of the physical setup, is the voltage you read with the DMM in the correct range? Is the output excitation voltage correct? The datasheet or load cell documentation should tell you what kind of bridge circuit you would need (if any).

I also noticed your fixed-point indicators from the FPGA I/O node have coercion dots on them. I would recommend removing the coercion dots to improve performance. You can either adjust each of the indicators' fixed-point settings to match that of the FPGA I/O node or you can delete the indicators and create new ones by right-clicking and creating an indicator.

Regards,