To elaborate my question, the considered MIMO system is a stirred tank reactor which has two inputs and outputs. The transfer function is as follows:

G=[tf(1,[0.7 1]) tf(5,[0.3 1]);tf(1,[0.5 1]) tf(2,[0.4 1])];

I'm looking for finding the bandwidth of this system. I searched in some references and I found two theorems.

First, In [1], it's mentioned that the closed-loop bandwidth is the frequency where |S(jw)| first crosses 0.707 (-3 db) from below which S is the sensitivity function. I tried to form S(jw) by S=inv(I+KG) but after I tried to plot the bode diagram it had multi channels and I didn't find a unique answer.

Second, in [2], it's said that the closed-loop bandwidth is the frequency where upper bound of sigma(S) crosses -6 db. I tried to plot this too but the upper bound didn't cross -6db, however, the lower bound crossed it at w=10 rad/s.

I really appreciate if you can help me to find a solution for this problem.

Thanks in advance.

Refs:

[1] Skogestad, Sigurd, and Ian Postlethwaite. Multivariable feedback control: analysis and design. Vol. 2. New York: Wiley, 2007.

[2] Albertos, Pedro, and Sala Antonio. Multivariable control systems: an engineering approach. Springer Science & Business Media, 2006.

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