cumulative subtraction on real time data

[netra]

Hi,

I'm new to LabVIEW and have got a basic question here. I am acquiring frequency data from a QCM which I want to process in order to display the mass change on a chart in real time!

I've got a formula which says f1-f2 = const*dm (dm is change in mass)

so I want to calculate f1-f2, f2-f3, f3-f4 and so on.. I know the concept of shift registers but am not able to implement it in my circuit

I've attached the VI. Please guide me.

Thanks in advance!

[GerdW · Accepted solution]

Hi netra,

"circuit" is called "block diagram" in LabVIEW terms! It's a programming language and no electronic circuits designer...

In your VI I don't spot any "f", "mass", "frequency", or "QCM" indicators. It's really hard to follow your VI when your description doesn't provide any useful information.

Create a shift register in your while loop. Store the current value in the shift register. Use a subtract to calculate the difference of current and previous value:

The same solution is shown using a feedback node instead...

[netra]

hello GerdW,

Thank you very much for the help. I have implemented it in the block diagram and its working fine..
After you gave such an easy solution the problem looked very silly so really thanks a lot.

I have another doubt now..

i'm getting the frequency in real time.. and there is no problem with it but i also want to display the mass i calculate (using the feedback loop you suggested) after each cycle.

i've calculated the delay needed between two mass readings (i.e. time between two cycles) and a local variable 'delay' is available. but i dont understand how to put this delay there since i want delay only between the mass readings and not freq.

i have tried putting a for loop but its not working. you see, i want values of mass only after 'delay' and frequency continuously.

Sorry if my explaination isn't clear.

I have attached the VI.

you must be finding this too basic but i could really use your help 🙂

Thanks in advance!

[netra]

Hi again GerdW,

Another problem in the VI that I've attached in the previous post is that dueto the for loop and the delay, the frequency readings have become really slow!

I tried taking the for loop out of the case structure, but the readings are still slow

Thanks in advance!

[netra]

I also tried using 'elapsed time' and it is still not working.

I have attached the VI.

Please have a look at it and guide me!

Thanks in advance!