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Hie all I have two systems as:

sys1 = ss(A1,B1,C1,D);

and a reduced order model as:

sys2 = ss(A2,B2,C2,D);

I want to take the error between them such as:

sys= sys1 - sys2;

I have two questions: 1)- Is this true, that i can write sys as follows?

sys=ss([A1,0;0 A2],[B1;B2],[C1 -C2],0)

2)- When I check the h infinity norm of sys, it is becoming equal to the h infinity norm of sys1 (the high order system) which is very surprising. I am using bounded real lemma to minimize the h infinity norm for this error dynamics using LMI constraints on projection matrices.

Thanks in advance.

share|improve this question
    
sys and sys1 are SISO systems, while all the system matrices have appropriate dimensions. –  Mohsin Jul 23 '12 at 10:01
    
Perhaps your reduced order model is not capturing the relevant dynamics of $sys1$? –  copper.hat Jul 23 '12 at 13:53
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