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Positive results with strain gauge at compression

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Hello there. I have done a strain gauge analysis of an aluminum bracket under loading. I used a rossette (0/45/90) gauge. I have wired my setup as a quarter bridge.

I wonder why I get a positive strain output when I apply a compression stress on the bracket. I have gone over the wiring and I believe it is correct per NI specifications.

See attached photo

Best regards Khaledpic_1 (1).PNG

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Hello Khaled,

And welcome to the forums!

Your strain gage looks a bit like an RM8. Is that correct? What hardware are you using apart from this? Something like an NI-9237?

I cannot see the markings on your photo, is the following assumption about a/b/c made from your diagram correct? Also, is the direction of compression (in regards to the gage itself) correct?

chrome_2018-12-11_09-29-27.png

 

If so, the resistance of the gage will decrease due to compression. Using the (Wheatstone) bridge, you should see a negative voltage resulting from this negative resistance change.

What were you plotting in the graph attached to your posting? It does not have dimensions on the axis. Is it resistance? Is it voltage? Also, how and where have you connected your six wires? Are you working with extinction voltage?


Ingo – LabVIEW 2013, 2014, 2015, 2016, 2017, 2018, NXG 2.0, 2.1, 3.0
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Hey Ikaiser.

Your assumptions about the a/b/c markings and the compression direction are correct. It is an aircraft wing that is bended upwards about the y axis. The wing tip is in the positive x-axis direction. I am indeed using an NI-9237. You are right, I should get a negative output at compression according to the Wheatstone theory, but its positive in my case :/
The gauge specs(attachment 1) and wire connection (attachment 2 and 4) info that you requested are attached. I am reading strain on the y axis ( attachment 3).

 

best regards Khaled :-)

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Solution
Accepted by topic author Khaled_Shniter
12-12-2018 03:46 AM

Hello Khaled,

 

Thank you for all the additional information. From what I can see, everything is correct, including your positive measurements.

Just one piece of information about your wiring is missing. As the results look correct, I assume your wiring is correct as well: I cannot tell the colors of the wires on your strain gages apart. The red wire from your NI-9945/9944 terminal 0 should go to one pad of your strain gage, the blue(?) and grey ones from terminals 1 and 2 should be both connected to the other pad of your gage.

 

The reason for the voltage having a different mathematical sign than you expect is that NI uses the other (arbitrary) orientation for defining a positive voltage.

Using Wikipedia as example: the English Wheatstone bridge article contains this picture:

Assuming R1=R2=Rx and R3>R1 (tensile stress). Yes, I intentionally use R3 as the gage, as NI uses this one. Using Rx as the variable resistor instead would change the orientation of the measurement.

 

Using R3>R1, you will have current flowing in the direction of the arrows shown. Depending on which orientation you define for VG, this can be counted as positive or negative. Or, in other words: (D-B) is a positive voltage, (B-D) is a negative one.

 

(B-D) is actually the orientation that NI uses. See e.g. Figure 1 in NI-9236's data sheet (I added letters to it according to the diagram above):

chrome_2018-12-12_11-56-41.png

This diagram assumes the changing (left) arm of the bridge as the positive side for the differential amplifier, and the other one as the negative one. In case the resistors on the right side (around D) are equal, and the external resistor is >350 Ohm (tensile stress), then it's B<D, giving a negative reading.

(Note: I use this figure from the NI-9236 as it contains all the bridge's resistors in one diagram. Using the 9944/9945 with the 9237 gives the same result, but you need to merge two diagrams to understand why.)

 

So basically the reason for you seeing a positive measurement on compression is that there are two decisions to make when defining the "direction" of a Wheatstone bridge, and all options are arbitrary. NI decided to use the variable resistor close to the positive excitation voltage, and to define this arm of the bridge as the positive one.


Ingo – LabVIEW 2013, 2014, 2015, 2016, 2017, 2018, NXG 2.0, 2.1, 3.0
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Hey Ikaiser.

Thank you for your adequate answer. I belive it solves my problem. Smiley Happy

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Thank you for reporting back and good to hear I could answer your question. I have seen questions very similar to yours several times before, so I felt digging into the topic would be beneficial not only for you (:


Ingo – LabVIEW 2013, 2014, 2015, 2016, 2017, 2018, NXG 2.0, 2.1, 3.0
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