problem in summation of two transfer function

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Hello guys,

I have defined two transfer function as below:

numerator1 = 1;

denominator1 = [2,1];

sys1 = tf(numerator1, denominator1)

sys1 =

1

-------

2 s + 1

numerator2 = 2;

denominator2 = [2,1];

sys2 = tf(numerator2, denominator2)

sys2 =

2

-------

2 s + 1

when I use

sys3 = sys1 + sys2

I get the multiple version of two TF while I expect to get normal summation

I want this:

sys3 =

3

-------

2 s + 1

PS: I already tested minreal(sys3) and sys3 = parallel(sys1, sys2), again same (multiply) result!

where am I doing wrong?

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Answer 1

Paul 2022-1-11

在 MATLAB Online 中打开

0 个投票

sys1 = tf(1,[2 1]);
sys2 = tf(2,[2 1]);
sys3 = sys2 + sys1
sys3 =
 
      6 s + 3
  ---------------
  4 s^2 + 4 s + 1
 
Continuous-time transfer function.
minreal(sys3)
ans =
 
    1.5
  -------
  s + 0.5
 
Continuous-time transfer function.
minreal(parallel(sys1,sys2))
ans =
 
    1.5
  -------
  s + 0.5
 
Continuous-time transfer function.

minreal() normalizes the denominator to have unity leading coefficient. But it does give the correct answer. Is ths not the desired/expected result?

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JAVAD GORJI 2022-1-11

Thanks Paul, yes you are right.

when I used minreal(), because the denominator was changed I thought that the result is wrong while, the answer is correct and numerator and denominator are simplified together.

cheers and thank you again

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