Measuring very low frequency (newbie)

[hriv]

Hello

I have a very simple application to construction but I am having problems because of the low frequency invovled. I must send to a very large platform a signal that is 0.50 Vpp at 0.1 Hz. I am expecting a return signal from the platform that is exactly the same but slightly phase shifted. I have attached my starting output-side vi and you can see that I have tried using several functions to trap the readings on indicators. You will see some labels at the top of the waveform chart which is my end goal. I have experimented with settings all over the place but the end result is that the frequency is too low to trap the readings (It will work if the frequency was>= 10Hz). I need to accumulate 30 seconds worth of data to validate the platform test.

What do I do next? I am guessing I have to send the data to an array and then examine each item to determine the high peak, low peak (amp) and zero-crossing time difference (freq). Any constructive suggestions for this beginner would be greatly appreciated. My fingers so desperately want to type in text code but alas this is graphical.

Thanks

HR  

[johnsold]

The Express VIs use the FFT internally. You can see this by opening the front panel of the Express VI, which converts it to a standard VI. You can then open the block diagram. About 3-4 levels down in the subVIs you find the FFT.vi.

FFT is not very good on samples with only a few cycles of the frequency of interest. For the low frequencies you are using a time domain analysis (Look at zero crossings if the data is not too noisy) or some type of modelling (Fit the data to a sine wave and extract its parameters) tend to work better. I do not know of any off the shelf VIs which will give you those results directly. You will need to code them yourself.

Ask your typing fingers to be patient with your click and draw for a few days ane yu will find LabVIEW quite versatile and faster than typing for many programs.

Lynn