strain measurement using strain gauges

[Engr_tech]

Hi

i am trying to measure strain using strain gauges and NI 9219.After setting up the strain gauges on the metal rod, i connected the strain gauges to NI 9129 .Note:i have glued both strain gauges in half bridge configuration and the force applied on the metal bar in horizontally(meaning the bar is streteched and contracted using proper setup).

After configuring the channel(as shown in NI white paper) and performing zero offset calibration i got some intial voltage value of -765u.

The results of strain gauges values shown in the graph (using labview and NI 9129) seems not to conform with mathematically calculated values of strain.Please share your comments i am not sure why am i not getting results closer to mathematical calculations.

also the strain values in  labview graph change alot while metal bar is stretched but its not the case in contracting.For instance with 100lbs the graph show a voltage output of 40u while in contrancting it just give an output of -10u for the same 100lbs force.

Thank you and looking forward for your positive feedback

[mikerophone]

I would first test the NI 9219 by reading in a constant voltage value with a stable voltage source to ensure that the device is reading in voltage values properly.  You can also try to self-calibrate the device in MAX to see if that helps.  A picture of your hardware setup might be helpful to determine if anything sticks out from your bridge configuration.  The article might be helpful to outline all noise considerations that might be coming into play:   http://www.ni.com/white-paper/3344/en/

[Logan_K]

If you attached the white paper you are referencing, a drawing of your connections, and maybe a copy of the VI (or screenshots if you are using a test panel), then maybe we could help narrow down the problem. I suspect there is a setup problem somewhere, but not sure. The results in SW should be in units of V/V, since the excitation voltage is not exact on this device. Not sure if this changes your assumptions in any way. You could also try to using a shunt resistor across one of the legs (i.e. 50kohm) to confirm that the deflection seen in software matches your expectations based on the circuit.

[Engr_tech]

Thank You for your  feedback

For your reference, i have attached the example VI i have made just for testing purpose.You can also see the connection diagrams and channel settings.

The strain gauges mounted on a cylindrical bar and one end of both gauges are shorted and connected to  pin 4 on the DAQ 9129,the other two ends are connected to Pin 3 and 5.i have made the connections as shown in the attached connection diagram snap.

one more thing the strain gauges have around 15" wires soldered to it having around 0.1 ohm resistance

[Engr_tech]

Here is the snaps of strain gauges installed on the metal bar.

P.S : i appolgise for low picture quality

[Logan_K]

Based on everything you've shown, I can't find any obvious issues. You could try measuring the resistance of each strain gauge directly with a high precision DMM when the load is engaged to get an idea of the delta resistance, and convert the results into strain to compare to what the 9219 is measuring.

[Engr_tech]

Thanks you for having a look at my setup and i really appreciate  your response

I want to confirm few things please if you can comment 

1. I am using strain gauges in half bridge configuration but as i mentioned the force applied on the metal bar in horizaontal i mean when the force is applied both strain gauges either expand or contract.

2. For theratical strain calculations calculations, do i need to consider only the area of the metal bar equal to the length of strain gauges or i can use the whole length of the bar(as force is applied on the whole metal bar)

Regards,

Ali

[richard98]

For this type of application, I would  suggest you use a uniaxial strain gauge which comes in different designs and patterns. This component can be implanted and it will meet your requirements. For more accuracy, you should purchase about 4 of them and make your design such that the strain gauges are placed at the highest impact stress points.Here is a place you can purchase such a strain gauge: https://tacunasystems.com/product-category/strain-gauges/