# Undefined function 'hinfnorm' for input arguments of type 'tf'.

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Kashish Pilyal on 4 May 2022
Edited: Paul on 4 May 2022
I am trying to calculate the H inf norm of a transfer function. The code is
theta=0;
kp=0.2;
kd=0.7;
tau=0.1;
h=0.5;
s=tf('s');
Gamma=(exp(-theta*s)*s^2*((0.6*s)+1)+(kd*s)+kp)/((h*s+1)*(s^2*((0.1*s)+1)+(kd*s)+kp));
ninf=hinfnorm(Gamma);
I do have the control system toolbox but I do not know why am I getting this error?

Paul on 4 May 2022
Edited: Paul on 4 May 2022
Hi Kashish,
If you don't have the Robust Control Toolbox for hinfnorm(), you can probably use sigma() from the Control Systems Toolbox
Example from the Question:
theta=0;
kp=0.2;
kd=0.7;
tau=0.1;
h=0.5;
s=tf('s');
Gamma=(exp(-theta*s)*s^2*((0.6*s)+1)+(kd*s)+kp)/((h*s+1)*(s^2*((0.1*s)+1)+(kd*s)+kp));
ninf=hinfnorm(Gamma)
ninf = 1.0753
sv = sigma(Gamma);
max(sv(1,:))
ans = 1.0736
MIMO example from doc page for hinfnorm()
G = [0 tf([3 0],[1 1 10]);tf([1 1],[1 5]),tf(2,[1 6])];
ninf = hinfnorm(G)
ninf = 3.0150
sv = sigma(G);
max(sv(1,:))
ans = 2.9976
See the doc page for sigma() for more info to see if it will suit your needs.
Note that, unilke hinfnorm(),sigma() doesn't check for stability of the system.

R2021a

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