problem in summation of two transfer function

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?

Answer 1

[Paul]

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?

[JAVAD GORJI]

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