change of program when add strain gages to a different chassis

[jyuan]

Hello,

I have a quick question here. I have to connect strain gages to different chassis, NI SCXI-1001 and NI cDAQ 9172. In Labview, I have already programmed the part for strain data collection for NI SCXI-1001, I was wondering if I need to program another part parallel to the previous part for cDAQ 9172, or I just change the physical channels there?

Thanks! 

[User002]

You should be able to just change the physical channel input. Are you going to run these at the same time? Or just swap out which HW you will be using?

If you're running them both and you want to do any kind of synchronization you've got more work ahead of you, but if you just want code that will run either SCXI or the cDAQ, you can change which channel you are looking at each time you run the code.

[jyuan]

Hi, Mark,

I have to run them at the same time. We are going to do an impact test. Many suppliers don't have concrete strain gages in stock, only one company have them in stock, but it's 120ohm. So we need to run the code for 350 strain gages and 120 strain gages at the same time during the impact.

Yuan

[User002]

Make two DAQmx tasks and configure each. They will almost be identical besides the Physical Channel Input and the Gage Resistance.

[jyuan]

Thanks Mark. I already made another DAQmx for the 120 strain gage, which is almost identical to 350 strain gage. I used a flat structure to include "start" icon of both. But I can only got the signal from 350 strain gage, cannot figure out what's the problem. I also launched Max, seems I can get the variable data from 120 gage, does that mean my gage is fine?

By the way, when I set the sampling rate to 1000Hz and samples to read to 100, and I got the data from 350 strain gage. But when I change them to 10000Hz and 1000, I got nothing, do you have any idea why this happened?

Thanks.

Yuan

[dkfire]

What card are you using in the two chassis?

For each card there is a max sampling rate, and for most card that sampling rate is depewding on how many channel it has to sample. 350 channels is a lot, at 10kHz that is fast for so many channels. There could also be a limit on the amount of data you chassis can transfer to the PC or can hold on to in the buffer.

Do you get any errors?

[jyuan]

For NI SCXI 1001, I use NI 1521 card, I have NI SCXI 1317 on it, with 24 channels. For NI cDAQ 9172, I use NI 9237. The max sampling rate of NI 9237 might be the reason why I cannot get the signals from 120 ohms strain gage, maybe it's less than 1000Hz.

And I didn't get any errors.

Btw, 350 I mean 350ohms strain gage, sorry for the confusing.

Thanks.

[User002]

Jyuan,

You can use this example code with the 120 Ohm Strain gages to make sure they're working.

In LabVIEW select:                              Help >>  Find Examples to open the Example Finder.

In the Example Finder Select:              Hardware Input and Output >> DAQmx >> Analog Input >> Bridge - Continuous Input.vi

Set the controls on the front to correspond to your HW and strain gage properties.

[jyuan]

Hi, Mark,

I already got my 120 ohms strain gage working. I got a message from Labview that the max sampling rate is 5000Hz for 350 ohms strain gage, and I use NI SCXI-1521 card. But I want my sampling rate as 10k Hz, does that mean I can do nothing about it?

Thanks.

Yuan

[MajorTom]

Good Afternoon Yuan,

The maximum sample rate is 333kS/s as evident by the minimum scan interval of 3us. (This is a common maximum sampling rate for all SCXI modules). If you are sampling this quickly, your measurement accuracy will suffer and the signal measurement will be off by +/-0.012% in addition to the normally calculated accuracy.

With regards to the filter, the filter attenuates all signals over 10 Hz, thus the signal bandwidth is 10Hz. Since this is a strain guage module, it is meant for low frequency signals.

Unfortunately, the 1521 will not be suitable to measure stain events that have a oscillating frequency of more than 10Hz, as the filter will attenuate this signal. Using the 1120 will be a better option for this type of measurement, given that it can also measure at 333KS/s, but has configurable filters of 4hz or 10kHz. I hope this helps!