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Positive Feedback in Matlab

  1. Sep 11, 2013 #1
    Hi All,

    Suppose I have the following system:

    A = 1 / (S + 1) (open loop)

    B = S / (2S + 1) (feedback path)

    I know that the positive feedback closed loop system will thus be: A / (1 - AB), and entering this manually in matlab (as well as doing it by hand), I get:


    2 s^2 + 3 s + 1
    -----------------------
    2 s^3 + 4 s^2 + 3 s + 1

    However, when I use the following command:

    sys = feedback(A,B,+1)

    I get the following:

    2 s + 1
    ---------------
    2 s^2 + 2 s + 1


    Is there something i'm missing?

    Regards,
     
  2. jcsd
  3. Sep 19, 2013 #2

    kreil

    User Avatar
    Gold Member

    [tex] \frac{2s^2+3s+1}{2s^3+4s^2+3s+1} = \frac{(2s+1)(s+1)}{(2s^2+2s+1)(s+1)} = \frac{2s+1}{2s^2+2s+1} [/tex]
     
  4. Sep 21, 2013 #3
    Thanks Kreil, realized this later on after the post. Thought it was an issue with the way i was using the feedback function.
     
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