Strain gauge data acquisition

[Mick790]

Hi,

Carried out first test for my data acquisition project for uni today. The general basic operation is I am carrying out a test on a super duplex steel tube. A new load is applied every 30 seconds and the program should record when the change in strain occurs which is then plotted on a table in an excell file. Some slight issues that I was hoping someone could maybe explain to me...

1) get a negative value. "-2.107" unsure how to solve this. Have swapped wires around and no luck. Not sure if maybe he filter in my lab view is causing problems as I get an error with it. Problems with differential amplifier maybe?

2) I wanted my program to plot ten results before save to a file. However it only takes one value and then saves. This means to get the next value when a new load is applied I have to save to a new document.

I will post the actual program soon. Any ideas help would be much appreciated.

Cheers

Mike.

[Mick790]

Heres the software.

[Mick790]

Anybody got any ideas?

[aranS]

Hi there,

I've had a bit of a look into your code, and have modified it a bit to try better understand it. I'm not quite sure what's happening to give you negative results out of your strain analysis, but I think I may have picked up on some other issues.

I've commented the changes I've made, but in summary:

1) Removed sequence structure and enforced dataflow with error line

2) Modified the DAQ section to just a random number generator for the sake of testing.

3) Set data feed into excel section to indexing output rather than last value

4) set iteration count of the for loop to address 10 strain measurements out of the array

5) Changed text file logging to "open or create" to avoid errors which might occur if the file already exists during multiple runs - but this would depend on you wanting to write to the same file each time

Note: In your for loop, if your attempting to relate the loads in the first column to your last 10 strain measurements, then this would be an incorrect way to go about interacting with the listbox. Please see this example which demonstrates how to address items in a listbox: https://decibel.ni.com/content/docs/DOC-5808

Attached is the edited/commented vi - I hope this helps in some way and I haven't misunderstood! I'll have another think about the negative values issue..

[Mick790]

That has been a great help, thanks!

One thing though the ms toolkit is causing an error-- "The subVI is not executable. You must fix all errors in the subVI before this VI can run." Will this be because I don't have the template for it on my computer?

Cheers,

Mike.

[Mick790]

Apologies I'm tripping over my own feet here. The error with the toolkit will be because it is not available on the trial software that I am using at home. 

Many Thanks.

[Mick790]

Another question after having placed the correct listbox vi into my program how do I then go about relating it to the strain values so that the ms toolkit gathers the correct data?

[Mick790]

Tested the improved version of the program, gathers data fine using the random number generator as a simulator to check everythings working. Stores data in a single text file and saves first 10 values to excel spreadsheet. 

However, still getting negative values and cant think why this would be.

Other thing trying to connect the listbox up is causing me some problems. 

[aranS]

The example link demonstrates how you can use property nodes to access your listbox items, and then use array functions to write new values to your listbox - you can't just copy and paste the sample code section to get the functionality you need.

You'll want to create new property nodes for the listbox on your Front Panel (copying and pasting property nodes won't reference the correct object on your front panel, notice the name is "..Listbox 3"), edit the array aspects of this code to read values from the array rather than write values to the array, and remove the while loops and event structures as your just using this to access 10 items in the list.

Unfortunately I'm not sure what the issue is regarding negative values from your strain analysis - is this a case of the value being negative/positve depending on it being a tensile/compressive stress?

[Mick790]

Ok cheers.

I would expect a negative value if the matetial was under compression however it's under tension on the test.