measure initial voltage - strain gauge measurement - NI 9237 -

[Dr._Imad]

I am using the     Strain - Continuous Input.vi     (that is included in the LabVIEW examples), to read strain using NI CDAQ 9237

One of the inputs is the Initial Bridge voltage (voltage output when no strain is applied)

How to measure this voltage using the CDAQ 9237 module?

One of the forum posts suggested to configure the CDAQ 9237 for voltage and read the channel voltage, and then use it as the input (Initial Bridge voltage) in the Strain - Continuous Input.vi.

The issue here is that the CDAQ 9237 cannot be configured for voltage readouts!

Any suggestions?

[Dr._Imad]

Using LabVIEW 2021

[Dr._Imad]

Any NI application engineer ?

[Jay14159265]

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@Dr._Imad wrote:

I am using the     Strain - Continuous Input.vi     (that is included in the LabVIEW examples), to read strain using NI CDAQ 9237

One of the inputs is the Initial Bridge voltage (voltage output when no strain is applied)

How to measure this voltage using the CDAQ 9237 module?

One of the forum posts suggested to configure the CDAQ 9237 for voltage and read the channel voltage, and then use it as the input (Initial Bridge voltage) in the Strain - Continuous Input.vi.

The issue here is that the CDAQ 9237 cannot be configured for voltage readouts!

Any suggestions?

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<rant>, I will not buy another piece of NI hardware until they fix their website, It does not need to be so hard to find a data sheet, and when you do finally find it, it should be a real data sheet. NI used to have the best hardware documentation hands down, now it just some useless waste of time going to NI.com to find anything. </end rant>

I have used NI hardware to measure strain but I have never used the 9237, and I can't find the data sheet. If it is true that you can't configure the 9237 to give you a voltage ( which is crazy, I'm sure there is a way) . You can always calculate the voltage by solving the cal (y[strain]=m[strain/V]X[V] + b[strain]) for X[Volts] so take the first reading in strain, calculate the voltage using the calibration then store the initial offset voltage and use it for the input of the terminal. 

(edit) I don't see the VI you are using but if it is asking for an initial voltage offset, it is probably expecting some voltage as the continuous input as well. 

Make sure you save the initial offset,  calibration, and serial number of the strain gauge with you data, it saves on headache when doing data analysis later : ) 

[Antoniooh]

NI-9237 can be configured to read voltage, but the voltage returned is volts per volt of excitation, google:

Incorrect voltage readings on the NI 9237

and you'll find a NIs page that explains this.

[Dr._Imad]

of course there has to be a way of reading the voltage directly.  It is voltage output of a Wheatstone, bridge , scaled.

But I did not see a way of doing that with the  DAQMx vi's.

would you please provide the NI page you are referencing, or an example vi for illustration.

[Jay14159265]

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@Antoniooh wrote:

NI-9237 can be configured to read voltage, but the voltage returned is volts per volt of excitation, google:

Incorrect voltage readings on the NI 9237

and you'll find a NIs page that explains this.

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I see, in that case you need this equation to get the initial voltage

https://www.ni.com/docs/en-US/bundle/labview-wireless-sensor-network-module/page/niwsnhelp/bridgesca...

[Dr._Imad]

This is my vi for converting strain to voltage

Basically I reversed it (back-solved for voltage) , the       convert strain gauge reading .vi (that ships with LabVIEW)

However,  NI should provide a way in DaqMx of reading the voltage directly (that is configuring the NI9237 for voltage readout)

[Antoniooh]

To read the voltage place a DAQmx Create Virtual Channel.vi and configure it for "AI Custom Voltage with Excitation"

It's under

[Dr._Imad]

I was able to read the initial voltage by reverse reading the formulas for Wheatstone bridge strain vs voltage by reading the initial unstrained strain, and convert it to voltage.  The reading matched the voltage readout from another software that is currently used in the lab.

This works, and I called it good!

Note, I tried to read the voltage using the DAQmx create Channel (AI bridge), but the reading did not match the other software.