ss2tf error. The TF does not match.

Dear all,

I am working in State-Sapce (SS) representation, so, when I want to obtain a Transfer Function (TF) in order to create a bode, the bode does not match with the expected frequency response. The A matrix in SS is 13x13. I tried with a simple example that I know what is its TF, and I do not obtain the expected resoult. How could I solve it? I think the problem is in the way that the determinant is calculated. It is not a problem to wait more time to obtain a more precise TF.

Thank you very much fot your time.

[Siddharth Jawahar]

Hi Alvaro,

Can you share an example of how you are doing this conversion?

Best,

Sid

[Álvaro Morales Muñoz]

This is the code:

format long

A = [

0 1 0 0 0 0 0 0 0 0 0 0 0;

0 0 1 0 0 0 0 0 0 0 0 0 0;

0 0 0 1 0 0 0 0 0 0 0 0 0;

0 0 0 0 1 0 0 0 0 0 0 0 0;

0 0 0 0 0 1 0 0 0 0 0 0 0;

0 0 0 0 0 0 1 0 0 0 0 0 0;

0 0 0 0 0 0 0 1 0 0 0 0 0;

0 0 0 0 0 0 0 0 1 0 0 0 0;

0 0 0 0 0 0 0 0 0 1 0 0 0;

0 0 0 0 0 0 0 0 0 0 1 0 0;

0 0 0 0 0 0 0 0 0 0 0 1 0;

0 0 0 0 0 0 0 0 0 0 0 0 1;

-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1];

B = [1 0 0 0 0 0 0 0 0 0 0 0 0]';

C = [1 0 0 0 0 0 0 0 0 0 0 0 0];

D = 0;

[num, den] = ss2tf(A,B,C,D)

% The denominator should be [1 1 1 1 1 1 1 1 1 1 1 1 1], but I obtain in

% den =

% Columns 1 through 7

% 1.000000000000000 1.000000000000009 1.000000000000009 1.000000000000008 1.000000000000004 0.999999999999994 0.999999999999990

% Columns 8 through 14

% 0.999999999999994 1.000000000000001 1.000000000000004 1.000000000000001 0.999999999999996 0.999999999999999 0.999999999999999

Answer 1

[Star Strider]

The bode function accepts state space models as input. See the sys documentation section for an extended discussion.

EDIT — (23 Mar 2021 at 15:50)

Try this:

sys = ss(A,B,C,D);

figure
bode(sys)
grid

.