Reading loud cell output with NI-9237

[JNEM]

Hi All,

I was looking to purchase for NI-9237 in order to attain the analog output of a load cell. There are 2 load cells I want to read and store the data from. A load cell that is being used in a frame and the load cell that is under test. The goal is to confirm the load cell under test outputs the same signal as the load cell in the frame (testing the DUT load cell, with the frame load cell). 

I only have access to 4 wires on the DUT load cell. Excitation +/- and signal(AI+/-). Will I still be able to use this DAQ by only using these 4 wires, supply the load the cell with 10V and the signal wires? I would be using the DSUB option. So, I would only be using EX0-, EX0+, AI0-, AI0+. All others would be no connection. 

Now, the load cell under the frame. With this load cell I only have access to the signal (AI+/-). Would this be fine? The load cell is getting its excitation from a 10v power supply that is already built into the frame. So I'd only be making a connection on the DAQ to AI1-, AI1+. 

And ultimately I would write a script in labVIEW to compare these 2 load cell outputs. Thanks all for the help.

[santo_13]

Please share the datasheet or link to the loadcell in the frame.

[JNEM]

Lbf +/- 110,000 (for my purpose I won't go over +/- 20,000lbf) 

[santo_13]

Need more detail on the wiring of your loadcell to see if there are other ways.

[Henrik_Volkers]

Sounds like the loadcell in the frame is used by the controller of the frame 😉

If the accuracy of the controller is OK, can you read that value via a digital connection from the controller?

The 9237 support an external bridge supply, so another option is to buffer the frame bridge supply and use it as an external supply. However you need to check and test first if the additional connection of the 9237 sense wires affects the frame controller. I would take that serious!  What will happen if the controller goes mad due to interference due to bias currents or other EMC from the 9237?

[JNEM]

Hi santo_13,

honestly I don't have much more details on the wiring except there's a BNC output for the signal. A lot of the frame area is kind of inaccessible. Does the 9237 need to have the excitation connection to just read the output?

[JNEM]

Hi Henrik,

Correct, it is used by a controller. The accuracy is good, but there is only an option to read the output analog output signal. It actually converts the digital signal to analog output (sigh). There is a handheld controller, but I didn't see any way to try and digitally connect, and if there is then it's likely inaccessible. 

The issue is I don't think I have access to the excitation lines; I only have access to the analog output. Do I need to have any connection other than the output signal? I know in the software you can pick to read the bridge, and that's probably where the critically of excitation voltage comes right? What if I set up the DAQ to just read the mv output and somehow convert it to lbf?

[Henrik_Volkers]

What is the voltage range of that analog output?

mV  , 1V or maybe 10V ?

(is there a link to that device/controller?)

If your DUT only has connection to the 9237 , you can measure the controller output voltage with the 6237 but some constrains have to be regarded:

voltage range: 25mV/V , if set to 10V exitation the range is +-250mV , so if the output of the controller is greater you need to build a voltage devider (2 or 3 resistors) depending of the controller output specs.

ground loops and common voltage range ....

scaling the voltage to force can be done in software , multiply the voltage with a scaling factor and add an offset.  depends on the output spec and maybe the 9237 settings (if output is volt or mV/V)

[JNEM]

Hi Henrik_Volkers,

I am not sure of that. I reached out to the supplier to see if they can provide the full scale for this load cell. This frame is decades old so the documentation wasn't accessible online. I can attach what I was sent. 

So, is the only purpose for having the frame load cell wired to excitation on the DAQ for using the bridge feature in the DAQassistant?