Convert s in j*ω in a transfer function

Hi, in this code I want to replace the variable s with the complex j*ω

syms s;
m=100;
k=20;
c=0;
A=[m*s^2+c*s+k -s*c-k 0; -k-s*c m*s^2+s*2*c+2*k -s*c-k;0 -s*c-k m*s^2+s*c+k]
det=det(A);
AI=inv(A)

for i = 1:3
    for j = 1:3
        [nf,df] = numden(AI(i,j));
        tfn = sym2poly(nf);
        tfd = sym2poly(df);
        sys(i,j) = tf(tfn,tfd);
    end
end

AI is the stiffness matrix of a MDOF system in mechanics of vibration and I want to plot the displacement behaviour of the masses.

Thanks

[Paul]

Replace s with j*w at what part of the process? It's straightforward to do that substitution in AI, but then sys will be incorrect? Or do you want to just evaluate sys at s = j*w? Or perhaps you want to do something else?

[FEDERICO CIOL]

I want to evaluate sys(i,j) at s=j*w, but I don't know how to change s in a tf.

Answer 1

[Paul]

No need to change anything in sys. It is a tf object and either freqresp() or bode() can evaluate sys at s = j*w

doc freqresp
doc bode

[FEDERICO CIOL]

Thanks. But how can I plot my freqresp?

[Paul]

bode without output arguments will make the plots for you. The magnitude and/or phase of the output of freqresp can be plotted vs w, typically using semilog

doc abs
doc angle
doc semilog