How best to plot phase difference vs. time of two signals

[afajim]

I have three channels (time, force, displacement) of about 1000 seconds of test data from a sine-wave load test (sample rate was 100Hz).  I would like to plot the phase difference between force and displacement (perhaps using a 30 second window) as a function of time.  I've tried using a few different analysis functions, but I get what looks like random noise (the phase difference between force and displacement is very small and the amplitude difference is very great---about 4 orders of magnitude).  Any suggestions would be appreciated! --Jim

[Brad_Turpin]

Hi afajim,

I would recommend that you calculate the FFT of your Force and Displacement signals individually.  The DIAdem FFT function has a checkbox in the "FFT Functions" tab for "Phase" calculation.  If you check this checkbox in both FFTs, you will get a Phase channel for your Force data and your Displacement data.  These two curves should compare quite well on a graph.

Let us know if you need further assistance,

Brad Turpin

DIAdem Product Support Engineer
National Instruments

[afajim]

Brad, thanks for the response.  Is the phase information generated by DIAdem's FFT generated in the frequency domain or the time domain?  I am able to plot it vs. either, but I'm betting only one is correct.

--Jim

[Brad_Turpin · Accepted solution]

Hi Jim,

The resulting phase channel you get with the DIAdem FFT function is in the frequency domain.  If you select a Time and Data channel in the FFT dialog, you will get a resulting Frequency channel along with the resulting Phase channel.  If instead you select only a time waveform Data channel in the FFT dialog, you will get a resulting frequency waveform Phase channel.

Brad Turpin

DIAdem Product Support Engineer

National Instruments