Substitute s for jw in a transfer function
显示 更早的评论
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.
采纳的回答
Jon
2021-1-11
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 个评论
Jon
2021-1-11
If as Pat suggest you don't have the Control System Toolbox, you can do this a little more neatly using
num = [1,2]
den = [3,4,5]
w = 3; % for example 3 rad/s
val = polyval(num,j*w)/polyval(den,j*w)
You can also use i*w MATLAB defines by default i and j as sqrt(-1)
Inés Bodoque
2021-1-11
kang
2024-11-28
can I use syms omega and use evalfr(G,j*omega)?
Walter Roberson
2024-11-28
No, evalfr() is part of the Control System Toolbox, and there are no functions in the Control System Toolbox that accept symbolic expressions.
Walter Roberson
2024-11-28
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)
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 = polyval(num,1j*w)./polyval(den,1j*w)
更多回答（1 个）
Pat Gipper
2021-1-11
0 个投票
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 个评论
Pat Gipper
2021-1-11
编辑：Pat Gipper
2021-1-11
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
2021-1-11
类别
在 帮助中心 和 File Exchange 中查找有关 Time-Domain Analysis 的更多信息
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!
Translated by 
