DAQ USB 6008 and Measuring mV Output from a Load Cell

[redtop91]

I'm thinking my issues are stemming from not knowing Labview as intimately as I should.

Dennis:

How would I index each column when the index feature indexes by row correct? I guess I just don't understand how to use the index 0, index n-1, etc inputs on the index array VI.

Eric_L:

How does the for loop index the array? Does the Array to spreadsheet string VI have to be in the for loop? Does the point by point average belong in the loop with it? I'd like to work through this without being given the answer but I think a visual example would be beneficial at this juncture.

[Dennis_Knutson]

The Index Array function will get either rows or columns. Wire up the 2D array to the function and when you move the mouse over them, you will see an input called 'index (row)' and an input called 'disabled index (col). Just wire a constant to the col input. Drag the function down to get more outputs. No need to wire the column inputs if the indexes are sequential.

By default, a for loop will autoindex any array that is passed in.

You have a serious misunderstanding of the point-by-point function. You should use the ordinary mean and you should not be converting the output of the mean to a 2D Dbl array.

[Eric_L]

Hello Redtop,

Have a look at the following code snippet. I split the dynamic data channels then convert them to 1D arrays. The for loop will index the array to points and calculate a running average. Note that a single for loop will work if each channel has the same number of points, otherwise you will need a for loop for each channel.

Eric

[redtop91]

I think that might have done it Eric. I'll have to take a multimeter to it to verify the averaging of voltages is accurate but the VI seems to run well with this and give a clearer distribution of data. Here's your code used in context of my VI, does it look to be applied correctly?:

Dennis: I tried using that method of indexing the array also and it yielded similar results but there were only 2 or so points with data and they were the same across all three analog input channels even when voltage was only being applied to one channel.

[Dennis_Knutson]

Your use of the point-by-point still seems pointless (and slow). Why? Do you want to just save the mean for the current acquisition or an ongoing mean. The latter is what you have just done.

 

[redtop91]

The only reason for the mean is to provide a type of digital filtration. Because of the accuracy issues of the 6008 DAQ I want to know where centerline of these jagged, noise-ridden, waveforms resides and log that data. I want the centerline (or mean data) continuously logged as the voltages output from the load cell vary depending on the test that they are involved in. Irregardless of the method used, however, the data being measured isn't matching the multimeter. Which puts me on the verge of concluding I cannot programatically account for hardware limitations so pronounced.

[redtop91]

How would I increase the accuracy of the data being output to Excel to more significant figures?

[Eric_L]

Hello Redtop,

I am usure of the VI you are using to write the Excel file, but I have noticed that the format being used in the posted examples show string format as %.3f which is a 3 significant figures, floating point. You can change the format to a higher precision number (for example 12 digits, %.12f). I hope this information helps.

Eric

[redtop91]

That's exactly the info I'm looking for. Thank you for yours and Dennis' help. I have a clearer idea on the limitations of what I'm attempting and I'll just have to tinker with the acquired data until I get a scaling factor that suits the task. It might not be possible, but now I know how to make that call. Thanks tremendously.

[Time loop]

Hi Eric,

I am working on placing Time loop structure around my DAQ assistant to get my loop rate to 5KHZ. As you know if I would like to run my iteration speed to more than while loop, I need to go for Time loop .Currently after replacing while loop to time loop around my DAQ, I found that my loop is not executing also I can not stop my running VI. I have DAQ 6008 with properties set as continous sample mode with 1000 samples to read with 1K Hz of sample rate. Can you kindly let me know where am I doing mistakes ? 

Thanks,

Nick