matlabmapletransfer-functionnyquist

Nyquist plot from transfer function with quartic polinominal in denominator

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?

Solution

  • 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.