Can I measure dynamic strain with a SCXI-1531 and how steady is the excitation current?

[CAS]

I am looking at the possibility of using an SCXI-1531 module to take measurements from a single strain gauge. I am only interested in the dynamic content of the signal and so am considering using the constant current excitation method which does not require bridge completion. Is there anyone out there that has tried this or could advise me as to whether this module is suitable?

Having read the spec of the module, it looks promising except for the data on the excitation current which states:

"Accelerometer input excitation current: 4mA +/-10%, 24V compliant."

Now, does that mean that the current will be steady at a value in the range 3.6 - 4.4mA or that it may fluctuate within this range during measurement? The latter would not be acceptable for my application as this would be indistinguishable from a variation in strain.

Thanks for any help.

CAS

[StevenH]

Hi CAS,

Essentially the excitation provided by the module will not allow you to acquire as you would like.

However, the good news is that you are able to disable the excitation provided by the module, and provide your own external excitation. You should then be able to acquire the signal using the SCXI-1531.

To disable the excitation in LabVIEW using DAQm, place down a DAQmx Create Channel.vi and then configure it for Strain measurements. Then you can create a control/constantto wire to the Bridge Information Terminal by right-clicking on the terminal and selecting control/constant. Within this cluster control/constant will be an enum element where you can specify the voltage excitation source to be external.

I have attached an example VI demonstrating this.

Hope this helps.

Steve

[CAS]

Dear Steve,

Thank you for your advice. I had considered the option of providing excitation separately and using 'any old' voltage input module but had hoped that the SCXI-1531 would do both for jobs for me. We have decided to make our own board to do this.

I'd still be interested in clearing up exactly what the specification of the accuracy of the current excitation means.

CAS

[Logan_K]

Hello,

The truth is that at the moment we don't have a product at NI that has been validated for performing strain measurements using the constant current technique that you have suggested, however if you have time I would like to collect some feedback about your application to better understand what things are important to you?

Most of our strain products have built-in quarter bridge completion, and allow you to measure strain gauges using just 2 wires, so I assume that wire count is not your primary reason for this request?

What are the reasons that existing NI strain products won't work for your application?

What range of frequencies are you looking to measure?

How do you intend to calibrate your measurements?

To answer your previous question... I believe that the 10% is just the absolute output level, because we don't calibrate the excitation circuit. The amount that this value can vary over time is captured in the other two specs: 

Current source drift ................................ –2.5 μA/°C max
Current source noise, 0.1 to 10 kHz....... 22 μArms typ, 80 μΑpp max

Thanks,

Logan K

National Instruments, R&D