System Identification toolbox, can't import frequency domain data above frequency of pi rad/s

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Olav W. Alstad 2015-2-24

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评论： Olav W. Alstad 2015-2-24

I'm using the System Identification toolbox to approximate a transfer function from frequency domain data. The data is numerical values of a frequency function (complex) in the range of 0.63 rad/s too 94.25 rad/s (0.1 Hz - 15 Hz). The problem is that frequency function above pi rad/s (3.14 rad/s) will not be imported. If the frequency is scaled down, such that the max frequency is lower than pi rad/s, the whole frequency function is sucsessfully imported.

It might be a simple answer to this question, but I've no idea. Thanks for helping me :)

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Answer 1

Arkadiy Turevskiy 2015-2-24

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You are forgetting to set the sample time somewhere in your code or in the app. The tool will show the data up to the Nyquist frequency. So if you leave sample time at default Ts=1 sec, that would be 0.5*(1/Ts)*2pi=pi rad/sec.

D you know the sampling time your data was collected at? Say if it was 100 samples/sec, Nyquist frequency would be 0.5*(100)*2*pi=314 rad/s, allowing you to see all your data.

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Olav W. Alstad 2015-2-24

Thank you! You are right - by changing the sample time I'm now able to import the entire frequency function. The time-dependent data was measured with resolution of 1e4 samples per seconds. Further, the frequency function were calculated from the time domain data. I guess the sampling interval then will be 1e-4? I will check out some theory regarding the Nyquist frequency as well!

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