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Matlab transfer function problem

  1. Oct 11, 2013 #1
    I have a transfer function

    L(s) = A(s)/B(s)

    that I call Ls.

    However, when defining

    H(s)= L(s)/(1+L(s))

    by

    Code (Text):
     

    %%
    Ls =
     
      -70.6s^2 - 2003 s + 3375
      --------------------------
       s^3 + 119.6 s^2 + 2806 s

    %%

    Hs = Ls / (1+Ls)  
    things go wild. The highest exponent power doubles or so and I get several extra poles or zeros. What I should get is

    H(s) = A(s) / ( B(s)+A(s) )

    which is the exact result. This I can achieve in matlab by

    Code (Text):
    [As,Bs] = tfdata(Ls,'v');

    Hs= tf(As,Bs+As)
    and the result from this are exactly the expected.

    However, why wont

    Code (Text):
     Hs = Ls / (1+Ls)
    work?

    Are matlabs routines that bad? Or am I doing something wrong?
     
  2. jcsd
  3. Oct 11, 2013 #2

    marcusl

    User Avatar
    Science Advisor
    Gold Member

    Is s a vector?
     
  4. Oct 14, 2013 #3
    No,

    s= tf('s');
     
  5. Oct 18, 2013 #4

    marcusl

    User Avatar
    Science Advisor
    Gold Member

     
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