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Failure while using function pem of system identification - plot compare shows step at the beginning of the signal trend

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Hulk0815
Hulk0815 2012-12-19
Hey,
I’m using the function pem to identify a process. When I plot the result with function compare the plot shows a step at the beginning. Does anybody know why this happens?
I used the code:
Ts=1;
z_mittel = iddata(y,u,Ts);
ze_mittel=z_mittel(1:1040);
zv_mittel=z_mittel(1040:2201);
P1DI_mittel=pem(ze_mittel,'P1I');
compare(z_mittel,P1DI);
And the plot result is:
Why do I get the step at the beginnig of the plot? I think as a result of this step the toolbox doesn't provide the best parameters.
Thanks for your help!

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Rajiv Singh
Rajiv Singh 2012-12-21
Difficult to say without looking at the data, but try using zero initial conditions for the comparison:
compare(z_mittel,P1DI, 'init', 'zero');
You may observe that the overall fit is even worse. But then that is just saying that you do not have a good model.

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