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I'm trying to make a Nyquist diagram based on experimental data obtained with frequency, output, input respectively in excel for the system.

Unfortunately I couldn't get any help with this problem so far and I haven't found anything relevant online.

I have 3 values containing the collected data (frequency in Hz, output V, input V). Is there a method to plot the Nyquist diagram using the experimental data and not by the transfer function method where Matlab can generate a Nyquist plot from my data obtained.

Nyquist plot: Nyquist plot

I dont understand why the plot has 2 graphs individually and maybe my sequence of (Frequency, output, input) in excel is wrong?

If it has to be done manually, then how should I approach this?

Any explanation, sample code, or perhaps links would be greatly appreciated.

Circuit: 1st order system

codeaviator
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Niam
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  • Try estimating the transfer function based on the data using the [`tfest`](http://mathworks.com/help/ident/ref/tfest.html) function (see [question](http://stackoverflow.com/q/18684436/3599179)) and then use it to generate the Nyquist diagram with the [`nyquist`](https://mathworks.com/help/control/ref/nyquist.html) function. – codeaviator Jan 31 '17 at 13:20
  • @Cebri Hi there, as my values come directly from a circuit design software, it's not easy for me to determine the np/nz of the system. All I've got are the input output and frequency experimental datas, is the tfest still able to generate an estimated TF ? – Niam Jan 31 '17 at 13:42
  • Can you determine the order of your circuit (`np`) based on its topology? For example, if the circuit is a second order system you would use `tfest(data, 2)`. – codeaviator Jan 31 '17 at 13:59
  • I think it is not possible to use `tfest` without knowing at least the number of poles (see [Transfer function estimation with unknown number of poles and zeros](https://mathworks.com/matlabcentral/answers/127090-transfer-function-estimation-with-unknown-number-of-poles-and-zeros)). – codeaviator Jan 31 '17 at 14:21
  • @Cebri I've added the pictures in, i believe it is a first order system. Does your (data) meaning the experimental values that I've gotten ? And is the sequence of (Frequency, Output V, Input V) the correct sequence as The graph looks kind of off. – Niam Jan 31 '17 at 14:21

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