Why the order of the system is reduced while using PEM in System Identification?

Hi All,

I used the following code for System Identification using PEM and iddata. But the initial system used to generate the step response was a 4th order system and the system identified using the following commands gave a 3rd order system. Why does this disparity happen?

Regards, Raghu

clc
clear all
close all
A=[-0.6129 0 .0645 0 ; 0 .7978 0 -.4494; -6.4516 0 -.7419 0;0 0.4494 0 .8876];
B=[.0323;.8989;.1290;.2247];
C=[.96774 .1124 1.6129 .4719];
D=0;
% statespace model
sys1=ss(A,B,C,D);
sys5=c2d(sys1,.001,'zoh')
step(sys1);
%figure;
resp=step(sys5);
[m,n]=size(resp);
k=10000/2;
% dividing data into two datasets
for j=1:k
  sys2(j,1)=resp(j,1);
  sys3(j,1)=resp(k,1);
  k=k+1;
end
% modelling system
u=ones((m-1)/2,1);
sys4=iddata(sys2,u,.001);
sysid=pem(sys4);
% system parameters from the model
figure,step(sysid)
[ad bd cd dd]=ssdata(sysid)
ssfinal=ss(ad,bd,cd,dd,.001)
figure,plot(sysid)
figure,plot(sys3)

Answer 1

[Arkadiy Turevskiy]

If you follow the process described here