How do I set strain gauge resistance on 9237

[rex1030]

Would you mind posting an example (maybe allow the gauge info to be input fields in the front panel of the FPGA) ?

Maybe even with a host vi that allows for continuous aquisition and display to a graph?

It would really help us see what you are trying to describe.

[Yi Y]

Hi rex1030,
I think I might have to correct a couple of things I said before. It turns out that the 9237 have the calibration procedure built into the drivers, so you don't have to take the AI yourself and calculate the offset.

So here's a little summary on the issue.
When you read the LSB Weight and Offset, it's reading the factory calibration data that's stored on the EEPROM and use the value to adjust the reading on the cRIO controller.  Those values are constants stored in the EEPROM and will not change until the next time you send the module into the factory for calibration.

The calibration you have to do is yourself is the null offset calibration and shunt calibration. You can also enable null offset calibration and shunt calibration. The code to do null offset and shunt calibration goes like the screenshot attached.
The procedure involves compiling this VI, load it on your FPGA and run it with no load on your strain gauge.  This will calculate the null offset and shunt calibration data, store the calibration data in the EEPROM and apply the calibration to your future measurements until the next time you run this VI.

Hope this clears things up a little bit.  Sorry about the confusion earlier, apparently the driver made the calibration procedure a lot simpler than I thought it would be :-).

Yi Y.
Applications Engineer
National Instruments
http://www.ni.com/support

[pauloeduardo@maxioncr.com.br]

Hi Yi Y,

I'm having the same problem posted here. I appreciate very much if you could explain me better how can I get the "coefficent acquired based DELTA V". This value must be multiply every loop, mustn't it? Don't you have a example (VI)?

Thank you very much!

Paulo Paraizo.

[Craig_D]

Paulo,

I think what Yi was saying in her later post is that in order to enable shunt and offset calibration, you need to follow these steps:

1. Attach your unloaded strain gauge to the module.
2. Compile and run the VI she attached
   

This will take a snapshot of what the shunt calibration and offset calibration under no load are and store these correcting values to the EEPROM.  Then, when you take measurements in future programs (like in the getting started example), these values will be applied automatically.

As for what resistance to select for, see this KnowledgeBase article.

Regards,

Craig D
Applications Engineer
National Instruments

[pauloeduardo@maxioncr.com.br]

Hi Grag, Thank you very much!

Just one more question! Can I put these 2 frames starting my FPGA VI? By this way, I suppose running the VI, it'll calibrate every time, won't it?

Once more, tk's!

Paulo.

[Craig_D]

Paulo,

If your VI will start with your strain gauge in an unloaded state, then yes, you can put these frames at the start of your VI.  As you may suspect, if the strain gauge is loaded when you enable null offset, it will equate that load point to be zero.

Regards,

Craig D
Applications Engineer
National Instruments

[rex1030]

- Is there a way to do the calibration on the fly? For instance you have your test all set up and your program has been running for a little while. Now, from the RT side, you want to press a "null&shunt calibration" button and have it do the calibration. This just seems more practical than having the FPGA calibrate when you press the run button.

- Is there a way to read the offsets so that you can throw errors if the offset is not within tolerance (usually indicating faulty hardware or setup)?


Message Edited by rex1030 on 11-28-2007 12:23 PM

Message Edited by rex1030 on 11-28-2007 12:23 PM

[rex1030]

Quote "- Is there a way to read the offsets so that you can throw errors if the offset is not within tolerance (usually indicating faulty hardware or setup)?"

Nevermind, I answered my own question here because the FPGA I/O Property Node has an output for offset so I  can  error check there.  My first question still stands  though. I would really like to calibrate on the fly.

[Craig_D]

Actually, upon further review, it does not appear that the information given in regards to the module saving data to the EEPROM is accurate.  This means that shunt calibration and offset null data must be saved manually and also applied to measurements manually.

To do calibration on the fly, this could be programmed in the FPGA and RT VIs using interrupts for synchronizing an input from you RT VI to switch to a case in your FPGA VI which will enable the calibration properties, take measurements, and send back the updated calibration information.  This is the same as including the calibration part of your setup code into the main part of your VI but only having it run when a calibrating input is asserted on the RT VI.

Regards,

Craig D

Applications Engineer

National Instruments

[rex1030]

Craig,

I really appreciate your help so far. You said, "...it does not appear that the information given in regards to the module saving data to the EEPROM is accurate.  This means that shunt calibration and offset null data must be saved manually and also applied to measurements manually."

This is contradictory to everything the other engineers have said, which is great news because what they have been telling me to do hasn't been working. Will you please post some example code of shunt callibrating by storing the data and calculating the offset manually?

Is the way i have done it (see attatched picture) not the way to do it? Can you show us how to do it right?

rex

P.S. Hey Craig I would like to email you my code and some screen shots and stuff because I would really appreciate your help with this. Can you message me your email address? Do I need to start a support ticket and request you as the replying engineer? Let me know 🙂




Message Edited by rex1030 on 12-05-2007 12:08 PM