How do I program labview to read a full bridge strain gauge positioned at a 45 degree angle to measure strain in twist?

[kw92]

Hello,

I am currently trying to use Convert Strain Gauge Reading to obtain strain readings from a full bridge configuration strain gauge set at a 45 degree angle on a hollow cyllinder. It is set at 45 degrees because we are trying to get the tensile and compressive strains that result from applying torsion, or a twisting moment, to the hollow cyllinder. Eventually we want to convert these strain readings to obtain torque. Our problem is that the full bridge options in Convert Strain Gauge appear to only be applicable to 0 or 90 degree setups. My question is how do I take my current VI and adjust it to take strain readings from a 45 degree alignment. Also any suggestions on how to add torque measurements to this VI would be appreciated. We have scoured the site, but we are very new to Labview.

Thank you!

[RavensFan]

Are the gauges 45 degrees to the cylinder, but 90 degrees to each other?  Or are they 45 degrees to each other, and some other angle relative to the cylinder?  It is not clear from your description.  The former is what you typically use to measure strain from torque.  The latter, I have no idea off the top of my head what math you would use to derive torsional strain out of that.

[kw92]

Sorry, 45 degrees to the cylinder but 90 degrees to each other. I was just confused which full bridge (I,II, or III) to use for this setup. Is Convert Strain Gauge Reading referring to the angle relative to each other, is that why only 90 and 0 degrees are available formations? If so that should answer my question, I thought it was referencing the angle on the cyllinder. I'm a Biomedical Engineering student so I'm new to using strain gauges and this sort of application for Labview, so sorry if these questions seem obvious. 

[RavensFan]

You want to use Full Bridge I  (one) since that is the one that contains 4 active gages all seeing the full strain either compressive or tensile depending on which segment of the bridge it is in.

I don't see where the Convert Strain Gage Reading VI is talking about 0 or 90 degrees in its help.  But typically, the other bridge configurations have other gages that are defined as Poisson gages where the gage is rotated 90 degrees from the principal strain in order to account for or measure the Poisson's ratio of the material.  Honestly, I've never worried about Poisson gages (except back in school when he had it in a class on measurment techniques.)

One other thing you can look at.  When you define the DAQ task, you can define it as an analog input for measuring strain, and can define all the bridge parameters in there.  That will do all the conversion and scaling for you and you won't have to worry about using the Convert Strain VI in your LabVIEW code.

[kw92]

Thank you, sorry it wasn't detailed help it was from the following page:

http://www.ni.com/white-paper/4172/en

I just got confused because the formations are said to either work for bending or axial strain and I am looking for torsional strain. I didn't think the listed bridge setups applied to my need. So to clarify, the Full Bridge I setup is appropriate for my needs.

[kw92]

Thanks for all your help on this by the way.

[RavensFan · Accepted solution]

That's not a very good example they show.  Full bridge 1 will also read torsional strain.  Full bridge 1 is for any configuration where you'll have 2 tensile gages and 2 compressive gages and you expect pretty much equal magnitudes.  With a torsional bridge, the only thing you have to do is be careful wiring it and pay attention to the gage numbering.  Just mentally think about which gages are in tension and which are in compression as you twist the shaft or tube.

If you get it wrong, you'll pretty much know right away because you'll see very little change in output when you go to twist the shaft since the tensile and compressive gages will wind up cancelling each other out in the bridge rather than magnify their effects.  To fix it, all you'll have to swap a couple wires to effectively rewire the bridge correctly.

Good luck on your project.

[kw92]

Thank you so much! Yeah, the example was confusing me a little bit.