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

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!

Answer 1

[Rajiv Singh]

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.

[Hulk0815]

Thanks