How to defined a transfer function which includes letters and numbers

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Halil 2016-9-14

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https://ww2.mathworks.cn/matlabcentral/answers/303095-how-to-defined-a-transfer-function-which-includes-letters-and-numbers

评论： Walter Roberson 2016-9-14

I need to draw the bode diagram of a transfer function:

L(s)=C*(1+ms)/(1+ns) where m and n are positive. My problem is that I didnt creat transfer function on MATLAB because of m and n variables. It gives me the error. How can I solve that problem. Thanks in advance

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Answer 1

Walter Roberson 2016-9-14

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https://ww2.mathworks.cn/matlabcentral/answers/303095-how-to-defined-a-transfer-function-which-includes-letters-and-numbers#answer_234763

编辑：Walter Roberson 2016-9-14

https://www.mathworks.com/matlabcentral/newsreader/view_thread/122517

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Answer 2

Halil 2016-9-14

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https://ww2.mathworks.cn/matlabcentral/answers/303095-how-to-defined-a-transfer-function-which-includes-letters-and-numbers#answer_234768

编辑：Walter Roberson 2016-9-14

在 MATLAB Online 中打开

Thank you so much for your link but I didnt find the answer. If I put this code:

MySys=tf([9 0.2 1],[1 1.2 9 0])
bode(MySys)

I get this answer:

Transfer function:
 9 s^2 + 0.2 s + 1
-------------------
s^3 + 1.2 s^2 + 9 s

that is what I need

If I put another code, I get the error:

syms C m n
MySys=tf(C*[m 1],[n 1])
bode(MySys)
??? Error using ==> tf.tf>tf.tf at 287
The values of the "num" and "den" properties must be row vectors or cell arrays of row
vectors, where each vector is nonempty and containing numeric data. Type "help tf.num"
or "help tf.den" for more information.
Error in ==> hw12 at 12
MySys=tf(K*[b 1],[a 1])

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Walter Roberson 2016-9-14

Construct the ratio symbolically, not using tf(). Then use https://www.mathworks.com/matlabcentral/fileexchange/10816-mimo-toolbox/content/Mimotools/sym2tf.m

That might not work until you have substituted in definite values for all except one of the variables, though.

You are unlikely to be able to get a frequency response plot from a transfer function that involves variables other than the one one (such as s): the closest you would be able to get would be to substitute in specific values for those variables and plot that.

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