Plotting with experimental data

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Niam
Niam 2017-2-3
评论： Star Strider 2017-2-6
Hi, I have 3 columns, time(in s), output(in V), input(in V) respectively, where by using iddata, is it correct for me to determine the ['Frequency',time] in seconds ? or should i do it in frequency (Hz)
tf = iddata(data(:,2),data(:,3),'Frequency',time)
sys = tfest(tf,2)
By using the system identification app, after importing the output and input values and estimation using state space model it turns that that the order is 2. Is it right for to to key in [tfest(tf,2)] in this case ? Or doing it wrongly?
Any help is appreciated !

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Star Strider
Star Strider 2017-2-3
I would use this syntax (from the documentation):
  • data = iddata(y,u,Ts) creates an iddata object containing a time-domain output signal y and input signal u, respectively. Ts specifies the sample time of the experimental data.
Specifically with respect to your data, I would do something like this:
time = data(:,1);
Ts = mean(diff(time));
time_data = iddata(data(:,2),data(:,3),Ts);
freq_data = fft(time_data);
freq_units = freq_data.units;
Fr = freq_data.frequency;
sys = tfest(freq_data,2)
NOTE — I do not have your data so this is UNTESTED CODE. I cannot be certain it will work without modification.
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Niam
Niam 2017-2-6
Thanks again for the clear explanation ! It have benefited me in many ways for understanding ! (:

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Walter Roberson
Walter Roberson 2017-2-3
The 'Frequency' parameter should have frequencies. If you have sample times then pass them as the third parameter with no 'Frequency' option; https://www.mathworks.com/help/ident/ref/iddata.html
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Niam
Niam 2017-2-3
I understand for this part now , do you have any idea for the System identification app ?

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