Strain measurement with PCI6251 and SCc-SG-01

[subodhscs]

I am new to Labview and trying to measure strain from multiple strain gages. I have a NI PCI6251 DAQ with SC-2345 carrier equipped with multiple quarter bridge strain gage modules SCC-SG-01.

Whenever I try to configure and create a strain measurement task in Lab View, I do not see any device calibration options in the setup window. There are Settings and Calibration tab but not the Device Tab. What could be the possible error in my setup?

Thank you,

[Case_in_Point]

Hi subodhscs,

The Device tab in the task configuration window doesn't exist for all devices, only devices which have specific options. For example, dedicated bridge measurement modules let you launch the bridge calibration wizard from the device tab. The PCI 6251 does not appear to have any functionality that would be in the device tab. Furthermore, calibration of the SCC modules has to be done manually, as outlined in the user manual (http://www.ni.com/pdf/manuals/371073b.pdf, page 18). Dedicated bridge measurement modules have the circuitry to make the calibration automatically, but since you're using the SCC add-on for a multifunction PCI card, the calibration has to be done manually instead. 

So the short answer is, you're doing nothing wrong. The 6251 isn't expected to have a Device tab.

Cason

Applications Engineer, NI

[subodhscs]

 Thank you @Cason_Point for the response. 
The problem with bridge calibration is that, I've connected a variable resistor as one leg of the quarter bridge in one of the Strain Gage module (SCC-SG1) with 2 wire configuration, I create a strain measurement task in NI MAX, I see 16 channels (ai0 to ai15) for PCI-6251 and 2 channels (ai0 and ai1) for SCC1Mod1 (SCC-SG01). Which one is the channel I should go with, is it one of the PIC6251 or SCC1MOd1?

I've checked the documentation regarding SCC-SG01 module and its shown to select the SCC-SC01 ai0 channel. I tried that too but as I change the resistance of the variable resistor, there's no change in the strain amplidute and stays the same. 

Attached is the screenshot that I have been using. 

Thank you for the help again,

[Case_in_Point]

Hey subodhscs,

You definitely want to address the ai0 and ai1 on SCC1Mod1. Which one depends on if you're connected to pin 6 or pin 4 (ai0 and ai1, respectively) on the SG1 breakout. Page 9 in the manual has a circuit diagram for making a 2 wire quarter bridge connection, so be sure and double-check your wiring is good, including the short to the pin with R3.

Looking at the screenshot you posted, it definitely looks like the signal is railing. The signal is below -1.65m and the lower limit is set to -1.0. What's the nominal resistance range of the potentiometer you're using? It should be pretty close to 120 ohms.

You should also try to just use a test panel in MAX to completely take task config out of the equation. Just right-click on the SCC-SG01 in Devices and Interfaces and hit Test Panel. Then we can figure out if the task config is the culprit or not.

Regards,

Cason

Applications Engineer, NI

[subodhscs]

Thank you @Cason_Point for your response.

I have been trying the same thing since the beginning as I followed the SCC Manual for quarter bridge configuration for 3-wire strain measurement.  I did get some signals when I changed the limits to 86m and 85m. Whenever I extend the beam (strain gage), there's no change in the signal and keeps being a straight line, but when I compress the beam, there's a fluctuating signal as seen in the attached screenshots for your reference. 

I have configured the carrier as instructed in the manual.I have tried the self test and self calibrate options for the PCI card too and they show no errors. I tried to use the example Strain measurement VI from LabView too, but still do not get a change in signal when there's a change in loading in the beam.

Is there anything that I am still missing? Why do not I get a continuous strain signal on labview? I have used strain gage with 120ohms, and SCC-SG01 modules. 

[Case_in_Point]

Hey subodhscs,

It looks like your connection is proper, and the compression image proves that you are reading something off of the sensor. Since we've proved that, what's probably happening now is there's a significant offset that's causing most data to saturate the circuitry and display as a flat line. Check out the Calibration instructions on page 18; sounds like you're at the phase where you can set higher limits to see actual fluctuating signal, and now you just need to adjust the potentiometer to null your offset. This is pretty common practice with strain sensors.

Remember that the actual voltage you're seeing only ranges across a few millivolts in most scenarios, so these measurements are pretty sensitive to offset error. Also, know that you can set your input ranges to much, much larger than 1 millistrain. The manual suggests looking at +/- 50,000 microstrain (+/- 50 millistrain), for instance. Set your limits higher and then you'll probably see your signal fluctuating, so you can then adjust the pot accordingly.

Cason

Applications Engr., NI