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Load Cell: Measurement, Calibration and Tare Option using NI 9219 and FPGA

Hello All, Greetings! I am trying to measure and record weight from 6 load cells (Model: SMT1-56 and SMT2-112 lbs, MFG. Interface) in real time for research experiment using NI 9219 and EthernetRio NI 9149.


I was able to acquire the mV/V data but did not find any option to convert/measure mV/V into weight (lbf or kgf) unit. I also need help calibrating the load cells and null-offset/tare each load cell every time before acquiring the data/signal. I have attached the screenshot of the FPGA and PC vi, and module configurations setup for your information. Your help would be very much appreciated in this regard.

Thanks and regards,


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Message 1 of 7

Have you read the manual and any calibration information that came with the load cell?


Usually the calibration sheet will say something like 2.002 mV/V for the range of the load cell.  In your case, it says the picture says capacity is 11 pounds.  So you'd have a scale factor that would be something like 0.182 m/V per pound.


For taring you'll need to add into your code a state where you instruct the user to make sure nothing is on the load cell, then press a button.  The values being read in at that time will be the tare values you store in a shift register and subtract from all future readings.


Your picture of the PC shows a bunch of broken wires.  My guess is you took a screen shot at a time when you didn't have the FPGA and PC fully set up correctly in the LabVIEW project.  Without seeing your project, it is impossible to tell you what went wrong there.  Your FPGA picture shows a bunch of coercion dots.  You should clean them up.  Delete the controls and indicators.  Right click on the terminals of the remaining nodes and pick Create Indicator or Create Constant.  Those will now have the correct datatype. 



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Message 2 of 7

Hello Ravens,

Thank you very much for your prompt response. I went through the load cell manuals, it says Rated Output: 2.0 mV/V (Nominal). FYI, the capacity of load cells are 56 lbf (LC1:1, LC1:2 & LC1:3 as specified in FPGA vi) and 112 lbf (LC2:1, LC2:2, & LC2:3 as specified in FPGA vi). Based on your suggestion and reply, I did some research and revised the VI's including scale factor and taring option (only for one LC2:3 and need your suggestion how to do for all). I have attached the updated revised VI's i for your information. I am not exactly sure I am in right track or not. I will appreciate if you find sometime to go through the attached VI's and provide necessary correction/editing and suggestion regarding this.

Looking froward to hear from you.




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Message 3 of 7

You're VI's are too broken for me to try to make modifications to them.


But what you did for one load cell with the shift register, you can do it where the shift register is an array holding the tare values for all of the channels.

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Message 4 of 7

Hello Ravens,

Thanks for your reply. Sorry for too many broken wires in the VI, since I was not connected them with the FPGA while taking screenshots. I connected with FPGA and then compiled the revised VI's. Please have a look into the attached images and VI's. I think still when you open the VI you may see the broken wire since they will not be connected with NI 9149. Anyway I will appreciate if you can help to do the necessary modifications. I do not have any experience to make the shift register in array. Please let me know if you need any other information. Looking froward to hear from you.



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Message 5 of 7

If I have time today, I'll open them up.


But if you don't know how to work with arrays in LabVIEW, then I would recommend looking at the online LabVIEW tutorials
LabVIEW Introduction Course - Three Hours
Learn LabVIEW

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Message 6 of 7

Hello Ravnes,

Thank you fro your reply and the suggested links. I will go through them. I will appreciate your time to go through the VI's once you have time. By the way one quick question do you think, it is possible to measure weight accurately using NI 9219. I saw people usually use NI 9237 for measuring weight from load cell. 




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