Slow Log Rate Needed using cDAQ-9174

[amwrobel]

All,

I am logging strain data for a thermal sweep we are doing on one of our products.  The sweep occurs over a long period of time (48 hours) so I realistically want to log one signal per second.  I tried using a continuous sample and then subsetting it, but I can't get it to 1 sample per minute which is where I want it to be.  Does anyone have any suggestions on using the subset (or another method) to slow down the capturing of data for the log?

Thanks,

Adam

[Jacobson-ni]

Adam,

Can you simply acquire data at 16.67mHz (once per minute)?  I created a simple SignalExpress DAQmx project and set the acquisition mode to continuously read 1 sample at .01667 Hz.  After changing the timeout and configuring recording options I was able to log thermocouple data from my 9212 in a 9174 once per minute.

If you want to acquire data at faster rates but only log 1 data point a minute, that might be trickier.

[amwrobel]

Matt,

I have tried this and setting the project to read 1 sample at 16.67mHz and running it for around a minute I get a couple of hundred samples.  Can you send me the sample project you used so I can compare your setup on the timing and recording options?

Thanks!

Adam

[Jacobson-ni]

Adam,

Here is the project I used and the resulting TDMS file.  I'm not extremely familiar with SignalExpress so there may be a better way to do this but I was able to log 1 data point every minute.

[Jacobson-ni]

Adam,

Were you able to get everything working with the above project as your reference?

[OKors]

What modules are you using with the 9174 to acquire your strain data?
Some modules have minimum sample rate limitations, some do not. The ones that do will default to the minimum allowed rate regardless of what you set in software below that min.
Go ahead, ask me how I know?

NI 9237 Half/Full bridge module has a minimum rate of 1.613kS/s which I didnt realize. Got bit when a 30 second data file looked more like several minutes
TC modules on the other hand I've had good sucess using 0.01666hz which is 1 sample per minute.

Another thought to keep in mind. If you have a module with a min sample rate such as the 9237 in combination with modules that can sample lower. The 9237 takes hardware priority so ALL your devices now default to the 1.613kS/s min rate.

Come back with a list of hardware you're using and we'll go from there.

[Jacobson-ni]

The original post was made in regards to a "thermal sweep" so I just assumed that it was a TC or RTD module, although looking back it could have been any AI device if they wanted.

Did they mention they were measuring strain and I missed that?

[OKors]

"I am logging strain data for a thermal sweep we are doing on one of our products."

From what I'm gathering yes, strain is the primary data type (although it would make sense to measure temps also). In either case, if the min sample rate was true it would also affect the TC module. TC module would bump up to the min s/s rate of the bridge module.
Of course this is all assuming he's using a module with low sample limitations.

[Jacobson-ni]

I went through that post many times on many different occasions. My reading comprehension skills are at an all time low, thank you.

I also did not know that the strain modules had minimum rates either which is pretty interesting.

[amwrobel]

All,

Sorry for the delay.  To answer your questions briefly, I was not able to sample data once per minute.  To get a smaller data set I ended up creating a subset and resample.  I created a subset of 60s and resampled every 30s.

If I try to set up my system for 1 sample per minute, I end up with many data points after two minutes, not just two like it should have.

My hardware:

- NI cDAQ-9174

- (2) 9235 Analog Input Modules

- Kyowa 0-45-90 rosette strain gages, gage factor 2.05

The boards we were measuring strain on were placed in a Thermotron (temperature chamber) and cycled to -25 degrees C then back up to 70 degrees C.  The idea is to measure our thermal expansion and use that to predict the thermal fatigue life of the product.

I hope this helps!  Thanks for all the input!

Adam