Substitute s for jw in a transfer function

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Inés Bodoque 2021-1-11

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https://ww2.mathworks.cn/matlabcentral/answers/714228-substitute-s-for-jw-in-a-transfer-function

评论： Inés Bodoque 2021-1-11

采纳的回答： Jon

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.

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Answer 1

Jon 2021-1-11

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在 MATLAB Online 中打开

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

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Jon 2021-1-11

在 MATLAB Online 中打开

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

It worked! Thank you very much

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Answer 2

Pat Gipper 2021-1-11

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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;

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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

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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