How to analyze this frequency response measured by labview?

[Lancelot3718]

You mean this timewave?

It is the time response of my cantilever....

The sweep sine signal is 0Hz at 0s and 500Hz at 30s, gradually increased.

Maybe I did not speak clearly.

And still, sincerely, Thank you.

Greet you from China, sichuan, where is the pandas' home.

Lance.

[CoastalMaineBird]

every period may last about 6s to 5s , while when I set the sweep signal range from 0 to 50Hz, I may only heard two turns' change, each lasts for about 15s.

If the item you're testing is very resonant (low damping), then your signal could be real.

The first time you go through a resonance, you set it to vibrating, and it keeps on vibrating at that freq.

[CoastalMaineBird]

Something to try: 

To measure freq response, we would hit the item with an impact hammer, and record for a few seconds.

If you provide NO drive, but record while striking, you should see a damped response.

How soon it decays is a measure of how damped the item is.

[RavensFan]

I think you are using the wrong analysis.

An FFT is intended for analyzing a single at a point of time assuming the signal is basically continuous before and after what you are grabbing.  But your signal is continually changing during the course of your 30 seconds of acquisition.  You are getting an "average FFT" of a long period of time where all the different resonances that occurred during that time are showing up within the FFT.

Also 500 Hz change in 30 seconds seems rather fast.  You are changing 16.6667 Hz per second.  That isn't long enough in any spot see the resonance.

When you do sweeps, you should be looking at a waterfall plot.  It is a 2-D plot that shows how the FFT changes during the course of time and each individual FFT represents a short segment of time where the waveform is relatively stable.

EDIT:  Looking at your VI, I don't see where you are ouputting your generation signal or reading your acquisition signal.

[CoastalMaineBird]

I agree with RavensFan - the signal is real but your test is not set up correctly.

Looks like your item is very resonant.

[RavensFan]

If I had to guess.  You have a resonant frequency about 100 Hz.  It shows up at 100, 200, 300, 400 Hz because you are seeing the harmonics get excited during that fraction of the 30 seconds where the waveform passes through 100 Hz,  then you see a part of the waveform get excited when the generator passes through 200 Hz.  And so on.

But it is the wrong test.  And the sweep is so fast that you don't have enough time at any resonant frequency.  It winds up spilling into multiple nearby bins.

[Bob_Schor]

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@RavensFan wrote:

EDIT:  Looking at your VI, I don't see where you are ouputting your generation signal or reading your acquisition signal.

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I think it is being done inside the Timed Loop using what look to me like Globals, AO0 and AI0.

Bob Schor

[RavensFan]

Okay.  I was looking for DAQ functions.

I see what you are saying.  Ive worked with FPGA's, but not cRIO's.  That global must be an access point to the cRIO's AO then.

I don't remember seeing in this thread that it was mentioned a cRIO was being used.

Which leads me to another question.  What is the target for this VI?  Is it running on a host, or one the cRIO's RT controller?  Or on it's FPGA?

Are there software timing issues, or is this hardware timed?

[Lancelot3718]

Hi,Ravens

Sorry, I had to go to sleep last night, you know , it's China.

The target of this vi is to use the sweep sine input to sweep out the frequency characters of the system (my cantilever), and get the response. Then put both the input and output signal (sweep sine excitation and response) into a system identification vi to estimate its transfer function.

The cantilever is not easy to resonate, only occurs at it natural frequences, and the first three orders' natural frequences are 1.6Hz 10Hz 27 Hz (also can be represented by rad/s) .And from the frequency response graph, it is easy to see them, on the left of the yellow cursor.

figure 1: for 0—500Hz input

I do not think the main reason is for the sample time is too short for 0—500Hz, because when I adjust the frequency range to 0—50Hz， it is still the same.  The strange thing is .I the highest frequence input to sitimulate my cantilever is 50Hz, but the measure  vi can still get the response in high frequency range ,like this:

figure 2: for 0—50Hz input

Maybe if we can solve this problem, we can solve all the problems. 

[Lancelot3718]

Hi Bob：

It is really nice of you to help me check my vi. 

The time loop is to sample the input and output signals, also to generate the input sweep sine signal.