Half bridge shear strain gage output is double the expected reading

[Jody123]

Hello,

I have a test setup and am trying to measure shear strain using a half bridge (two 350 ohm gages with a 90 degree offset) configuration and am having a configuration issue.

The first half bridge configuration option NI offers, NI calls it Half-Bridge Type I, is described here.  http://zone.ni.com/reference/en-XX/help/370466V-01/measfunds/half1/  I do not think this is the correct option for the type of configuration that I have so I have not messed with this option.

The second configuration option, NI calls Half-Bridge Type II, is described here http://zone.ni.com/reference/en-XX/help/370466W-01/measfunds/half2/ and appears to be the correct configuration for a bending measurement but is somewhat similar to the shear measurement that I am trying to take.  However, the shear strains that I am reading using this option are about twice what is expected.  I have a Vishey 1550A Strain Indicator calibrator that I have used to test/calibrate my NI hardware and when I use it to simulate a strain signal, the output of the NI hardware matches the signal that I am generating.  Does anyone have any suggestions as to what may be causing this factor of 2 difference?  The equation that NI uses (given in the half-bridge type II link) to convert the bridge output to strain has a 2 in the numerator that I am guessing should not be there for the shear measurement that I am taking but it would be nice to have someone confirm this.

Any feedback would be greatly appreciated!

[kevins33]

Jody123,

Can you please provide a little more information/description of your setup? You say that you have a 90 degree offset, does this mean the gauges are perpendicular? Are they on the same plane? How similar is your configuration to the setup in the link below:

http://zone.ni.com/reference/en-XX/help/370466V-01/measfunds/half2/

Thanks,

[Jody123]

The setup I am using is pictured in the attached file.  Let me know if you any additional questions.  Thanks!

[Jody123]

Hello,

Vishay has a good article (located at http://www.vishaypg.com/docs/11062/tn5121tn.pdf) about the direct measurnment of shear strain using a half bridge which is what I am trying to do.  According to this article, the total output from the half bridge should be equal to the shear strain (shear strain is the difference between the principal strains).  See Figure 4a in the article.  Based on this, it appears that the equation that NI is using to convert bridge output to strain for the Half Bridge Type II option (given in this link http://zone.ni.com/reference/en-XX/help/370466W-01/measfunds/half2/) is not the correct equation for the direct measurnment of shear.  If this is the case, (I am will do some further testing to confirm this but would love input from others too) it would be very nice if NI could add a half bridge shear selection to their MAX configuration options as these types of measurement are fairly common.  This setup could also be used to calculate the torque in a shaft using a half bridge, which is equivalent to the measurement that I am trying to make.

[Jody123]

After further study, it appears that NI's half-bridge type II configuration is the correct method to calculate shear even though it is presented as a configuration for bending.  On a side note, it does not however appear to have any built in correction for nonlinearities in the wheatstone bridge.  These errors as small at low strain levels but can be significant at higher strain levels.  See Table 1, #3 of this Vishay article (http://www.vishaypg.com/docs/11057/tn5071.pdf) for more infomation.   

[Jody123]

After some additional testing and research, I have confirmed that the half bridge type II configuration is the correct configuration to use for taking half bridge shear measurements.   The original shear predictions I was given are not accurate so, please ignore the title of this post. 

I have one suggestion that my help clarify things for future users.  It would be great if NI could add a note that the half bridge type II configuration is also the correct setup for taking half bridge shear and torque measurements (not just for bending).   Thanks for your help and sorry for the confusion.

[ico82]

Hi Jody123,

I have to measure the shear stress in a beam. I intend to use the strain gauge with two grid (+/- 45°) in the half bridge type II configuration with the NI 9237 module (see the jpg image to watch the simplification of my configuration). In LabVIEW I will use the AIStrain block to read the physical channel of the strain gauge.

What I need to know is what kind of data I will read?

It will be the pure shear strain (γ) or it will be the pure strain (ε)?

If it is not, what is the formula to calculate the pure shear strain (γ)?

Many thanks

P.S. Sorry for my poor english

[rirving]

Did you see this link: http://zone.ni.com/reference/en-XX/help/370466W-01/measfunds/half2/

Best,

Ryan

[ico82]

Yes I already saw that link, however I dont understand if I use the double strain gauge with axis at +/-45° I get the shear stress?

I need to measure the shear stress, not the strain stress or bending.

I will place the strain gauge as in figure: http://www.ni.com/cms/images/devzone/tut/a/dbc374b41052.gif where bending stress in null (in the neutral axis). What I need to know is what the output of the NI 9237 in half bridge type II configuration represent.

[Jody123]

Hello ico82,

Your bridge output should be 2 X shear strain.  To get the actual shear strain, just devied by 2.  Refer to Table 1, row 3, of the following link for more info.  http://www.vishaypg.com/docs/11057/tn5071.pdf  Of course you should always do a basic sanity check to varify what you are reading.  Hope this helps!