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So I have some very large and difficult transfer function, after some expanding and collecting by 's' in maple, i convert this to matlab and try to get nyquist plot, but this doesnt work. I am realy noob in this, why i cant take some plot. Also bode plot works fine.

There is my real code in matlab:

s = tf('s');
H1_e = tf ([-Mst_pr K3+2*K5 2*C3-C5], [Mch*Mst_pr (-K1-K5)*Mst_pr+Mch*(-K3-K5) (-C1-C3)*Mst_pr+(-K1-K5)*(-K3-K5)+Mch*(-C3+C5)-K5^2 (-C1-C3)*(-K3-K5)+(-K1-K5)*(-C3+C5)-C3*K5+K5*C5 (-C1-C3)*(-C3+C5)+C3*C5])
nyquist (H1_e);

After this my plot is just empty with real and img axis.

Also there is a real transfer function with substitution of all parameters:

H1_e =

             -17.65 s^2 + 12000 s + 7.996e09
 -------------------------------------------------------------
 352.9 s^4 - 2.553e05 s^3 - 1.558e11 s^2 + 5.13e13 s + 1.72e19

What can be wrong?

Tartachnyi
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  • Ok i have some suggestion. Is it because free radicals is too large than others coefs - 1.72e19 and 7.996e09? – Tartachnyi Apr 02 '16 at 17:43

1 Answers1

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It is not invisible, you just can't see it. Due to the fact that you have very high coefficients.
By zooming you can actually see something

enter image description here

As you can see the imaginary part is ~10^(-10) and the real part is ~10^(-3).
To avoid this problems it's usual to normalize the transfer function in such a way that the lowest order term has value 1.

AlessioX
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