Bypass strain indicator and measure strain with LabVIEW (SCB-68,PCI-6221)

[pd1234]

Hi,

I would like to know if I can bypass my strain indicator and directly measure the strain in LabVIEW.

I have a sealed super-mini load cell with a nominal output of 3mv/V and a recommended Excitation of 10 VDC. The load cell is attached to a dynamometer of which I wish to measure the strain.

The strain readings are currently going through a P3 strain indicator which provides a signal that is routed through the M-series SCB-68 connector board to the NI PCI-6221 data acquisition card and into LabVIEW.

I was wondering if it would be possible to bypass the P3 strain indicator and input my four wires(+exc, -exc, -out,+out) directly into the SCB-68 to the NI PCI-6221 and have a VI in LabVIEW read the strain.

The NI PCI-6221 has two analog outputs capable of supplying -10V,10V and has analog input channels with the following specifications:

Maximum voltage range: -10V,10V

Maximum voltage range Accuracy:112 micro Volts

Maximum voltage range Sensitivity: 97.6 micro Volts

Minimum voltage range: -200 mV,200mV

Minimum voltage range Accuracy: 112 micro Volts

Minimum voltage range Sensitivity: 5.2 micro Volts

Sealed super-mini load cell specifications:

Nominal output:3mV/V

Recommended Excitation: 10 VDC

Will it be possible to directly route the signal to LabVIEW with this hardware? (strain gauge->SCB-68->PCI-6221->LabVIEW)

(I'm wondering if the voltage changes will be too small to measure or if the board can even supply an excitation voltage)

If this is possible, any suggestions on implementation would be much appreciated.

Thank you,

Peter

[KyleTTU]

Hi Peter, 

You are correct that you can use the AO to provide excitation voltage if need be.  Some of our DAQ cards come with a seperate output for Excitation Voltage but since this one does not, we would only need to set up a DAQmx Write Task at the beginning of your program.  We have several examples that shipped with LabVIEW on measuring strain as well as a DAQ Assistant Express vi that will let you setup all of your scales to output the correct strain measurements.  Since the accuracy of the board is 112 uV it should be able to detect changes in strain but this might need to be something to test beforehand.  Since you say it has a nominal voltage of 3mV/V do you know how wide the fluctuation is varying with strain?  

One easy way to test to see if you are getting the right measurements is to create a Task in Measurement and Automation Explorer(MAX).  You can do this by right clicking on your PCI-6221 and selecting Create Task.. this will allow you to setup an analog input task that measures strain.  In this configuration you will still need to provide an external excitation voltage but it will be a good way to determine if the PCI-6221 is getting valid data before diving into programming.  

If you are able to setup your task and verify that you are indeed receiving valid data the programming will be very straight forwards.  Please give these steps a try and see what you come up with.

Regards,

Kyle S 

[pd1234]

Hi,

thanks for the reply,

I managed to get a strain reading with the shipped example "Cont Acq Strain Samples (with Calibration).vi" to which I added an excitation voltage which is output at the beginning of the program (I have attached my program below "straingauge.vi"). However, I have a few problems:

1) I have my strain gauge near a battery source and I get noise when I run the signal directly to LabVIEW which I do not get when I run my signal through my P3 strain indicator first. Is there a way to filter out this noise? Note: analog low pass filter does not appear to be an option for my DAQ card.

2)  I want to use the strain output for the calibration of a dynamometer. What does LabVIEW do to the incoming voltage signal to convert it to strain? (I am using Full Bridge I setting in VI)

3) When I set the excitation voltage to any number above 3.4 V (i.e. any number between 3.4 and 10 V) and I measure the voltage accross the P+ and P- leads with my multimeter, the voltage accross those leads stays at 3.4V ...Is this normal?

Thank you for any assistance you can provide,

Peter

[KyleTTU]

Hi Peter, 

1.) Would it be possibel to share a common ground with your external excitation source with the PCI-6221?  It is very possible that we are not referencing the same ground and thus have a floating signal that can contain a bunch of noise.  Also, how do you have your guage hooked up?  I have included a diagram below that shows how to connect the signal to ai0.

2.)  Labview has defined scales for measuring strain.  Since the guage will be outputting a detectable voltage that is then converted into strain, LabVIEW uses the formula related to the bridge configuration you specify to convert the voltage it is reading.  Some good documentation can be found below on bridge configurations and the method for detecting strain.

Measuring Strain with Strain Guages 

http://zone.ni.com/devzone/cda/tut/p/id/3642  

3.) Can we test the Analog Output with nothing connected to it and see if we are getting the full 10 volts when connected directly to the pins on the SCB68?  To do this, just set up the analog output task to output a constant 10v and connect your multimeter directly to the pins. 

Regards,

Kyle S

[rakilab]

Bonjour

je suis à la recherche d'une possibilite d'ouvrir  le  STRAIN GAUGE P3 sous Labview 6

est ce que qlq à une solution

merci d'avance

[a_clucker]

Hello rakilab,

NI also has a French language forum, which can be found here . Additionally, please create a new thread when you have a new question. This thread has been dormant for over three years. When you create the new thread, please include as much information about your question as possible.

Thanks,

Andrew

In French, if you prefer:

Bonjour rakilab ,

NI a également un forum en français qui peut être trouvé ici. En outre , nous vous prions de créer un nouveau fil de discussion lorsque vous avez une nouvelle question . Ce fil n’a pas été touché depuis trois ans . Lorsque vous créez le nouveau fil , s’il vous plaît fournir le plus d'information possible à propos de votre question pour que nous pouvons vous aider.

Merci ,

Andrew