Substitute s for jw in a transfer function

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Inés Bodoque
Inés Bodoque le 11 Jan 2021
Modifié(e) : Paul le 29 Nov 2024
Hi, I have a transfer function define by
G = ([1 2],[3 4 5]) (as an example)
I want to change the 's' for 'jw'. Does anyone know how to do it?
I've tried subs(G,{s},{1j*omega}) but it didn't work.

Réponse acceptée

Jon
Jon le 11 Jan 2021
Hi,
I think you mean that you define your transfer function using (you forgot the tf() in your example)
G = tf([1 2],[3 4 5]) %(as an example))
Then if you want to evaluate it at a particular frequency, that is a specific value of jw, you can use
w = 3; % for example 3 radians/sec
val = evalfr(G,j*w)
You can also use the freqresp function to evaluate it for multiple values along the jw axis
  6 commentaires
Walter Roberson
Walter Roberson le 28 Nov 2024
For 1 x 1 systems, G,
syms omega Omega
EVALFR = subs(poly2sym(G.Numerator{1}, Omega), Omega, 1j*omega) ./ ...
subs(poly2sym(G.Denominator{1}, Omega), Omega, 1j*omega)
Paul
Paul le 29 Nov 2024
Modifié(e) : Paul le 29 Nov 2024
The numeric solution using polyval from this comment should use rdivide, ./, not mrdivide, /, for the general case where w is not a scalar. And safer to use 1j instead of j, which is a common variable name that can shadow the built-in function of the same name
num = [1,2];
den = [3,4,5];
w = [3 4]; % for example 3 and 4 rad/s
val = polyval(num,j*w)/polyval(den,j*w) % wrong
val = -0.0110 - 0.1076i
val = polyval(num,1j*w)./polyval(den,1j*w)
val =
-0.0127 - 0.1433i -0.0105 - 0.0969i

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Pat Gipper
Pat Gipper le 11 Jan 2021
Matlab uses the reserved constant "i" which is set equal to sqrt(-1). Using your transfer function as defined try the following which will result in the variable "G" which will be a complex number.
num=1*i*w+2;den=3*(i*w)^2+4*i*w+5;G=num/den;
  2 commentaires
Pat Gipper
Pat Gipper le 11 Jan 2021
Modifié(e) : Pat Gipper le 11 Jan 2021
Based on some of your other questions it looks like you don't have access to the Control Systems Toolbox. So you need to do this arithmetic explicitly.
Inés Bodoque
Inés Bodoque le 11 Jan 2021
Thank you for your answer! I need this for a project from one of my subjects and I am not allowed to use the Control Systems Toolbox, that's why I have to do it this way

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