MATLAB: My model output is different with use of Simulink and sim function

Question

I have a NARX model with 4 input and 1 output. I use Iddata Source, Nonlinear ARX Model and Siganl to workspace bolck in Simulink. The simulation stop time is 100 and i use these code for iddata object and model initial state:

XX = iddata(out, in, 1);

[X1,U1,REPORT1] = findop(MN1,'steady',[in(1,1),in(1,2),in(1,3),in(1,4)],NaN);

After run, output from signal to workspace block contain 101 data!?. and I have to erase the first data because I don’t need it. This issue happened to me for several other problem in Simulink such as MPC or PID or other model that the block output is always have one extra data.

Next problem is that, when I use same model with same iddata and initial state with sim function, I get 100 output data, but completely different from Simulink output!?

mout11=sim(MN1,XX,'init',X1);

mout1=mout11.OutputData; %Extracting data to array matrix

Can anyone help me?

Answer 1

For iddata source, it helps to set Tstart to 0 since the simulation in Simulink starts there by default. That is, use: XX = iddata(out, in, 1, 'Tstart', 0)

For matching simulation response, you have to set the initial conditions correctly. See:

http://www.mathworks.com/matlabcentral/answers/51708-problem-with-system-identification-toolbox-and-sim-command