How do I set strain gauge resistance on 9237

[Yi Y]

Hi Rex1030,
Sorry about the mistake earlier.  I was misinformed about the functionality of the 9237.
In any case, I started writing some code to acquire and calibrate the data, but since I currently don't have the hardware to test out the code, I'm not sure now well the code works. I'm pretty sure the offset calibration is correct, but I'm not certain about the shunt calibration part.
The formula used in the calculation of the Rscal (shunt resistor) is in http://ae/Operations/AE/public.nsf/webPreview/C0EDFB13B2D8D10E862573A900803579?OpenDocument
I'll have to dig a little deeper to see how to convert the difference of the AI voltage with shunt resister disabled and enabled into the calibrated Vexe values.
In any case, I'm attaching the code here just to get you a reference point and some space to explore. I'll post more complete and tested code when I have hardware I can test with.
Please feel free to post any updates or questions here.

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

[pauloeduardo@maxioncr.com.br]

Hi Yi Y,

Could you confirm the address about the calculation of Shunt Resistor?

http://ae/Operations/AE/public.nsf/webPreview/C0EDFB13B2D8D10E862573A900803579?OpenDocument..... is it correct?

Tks

Paulo Paraizo.

[Yi Y]

Oops, a slight mishap with copy&paste.
Here's the correct address--
http://digital.ni.com/public.nsf/allkb/9D5648C81141022B862561AA004B7383?OpenDocument

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


Message Edited by Yi Y on 12-07-2007 01:09 PM

[pauloeduardo@maxioncr.com.br]

Hi Yi,

Please, could you tell me where I can find the meaning of each variable for this formula?

Tks once more!

Paulo Paraizo.

[Yi Y]

The link to the manual is on the bottom of that page.

It's at http://www.ni.com/pdf/manuals/321924b.pdf

page 5.

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

[pauloeduardo@maxioncr.com.br]

Hi Yi.

I have already tested the code you posted, but I think it's not working as you intended. Don't you have any news about this issue?

I'm looking forward your reply.

Thank you.

Paulo Paraizo.

[CharlieM]

Hi Paulo,

I haven’t tested Yi’s code out myself but it looks like she has done some gauge specific calculations (calculations with knowledge of gauge resistances). This is something that NI has left up to the user because it is unique to each system and requires understanding of strain and bridge theory. The NI 9237 Getting Started example can give you a value of volts per excitation volts. It is up to the user to determine how that relates to strain and force as well as coding in any further shunt calibration. Yi’s code looks like it calculates a value of resistance based on some formula. This example is a long way from an application that calculates strain or force with knowledge of shunt calibration data.

[pauloeduardo@maxioncr.com.br]

Hi Charlie, thank you for your post.

The point is: What's the procedure to use the shunt calibration with 9237 module, and how can I interpret this one?

All the theory to calculate the values like strain, force, I just have in my code. My doubt is about the shunt calibration.

Could you help me solve this problem?

Thanks again!

Paulo Paraizo.

[CharlieM]

Paulo,

We don’t have a specific procedure but here is how I would use the shunt calibration feature with no load on my system:

1.      Place a precision 100 KOhm resistor in parallel with my gauge by coding “Shunt Cal Enable = True” on the FPGA

2.      Take an analog reading

3.      Remove the precision 100 KOhm resistor in parallel by coding “Shunt Cal Enable = False” on the FPGA

4.      Take an analog reading

5.      Compare the two readings and ensure that the strain values returned indicate a resistance change of having your strain gauge of some resistance shunted by 100 KOhms.

All we have done with the NI 9237 is provide the user with a precision resistor to place across the input terminals. You could do this with your own resistor but we have made it easy by building in such a resistor.

[pauloeduardo@maxioncr.com.br]

Ok Charlie. It'll help me a lot.

Just some questions again:

I'm using 9237 module with 9944 connector (to assembly quarter bridge). I think this connector has a precision resistor internally, doesn't it? Could I use this one to do the calibration? If it's possible, how can I do it?

Tk's

Paulo Paraizo.