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Homework Help: Root locus varying H

  1. Nov 6, 2013 #1
    Hello guys, I need help to sketch the root locus of the system when K is varying from 0 to infinity. (K>=0)
    The system:
    http://www.freeimagehosting.net/newuploads/anspe.jpg

    I'm stuck. I really don't know how proceed.
    Thanks in advance.
     
  2. jcsd
  3. Nov 6, 2013 #2
    Plot the poles and zeroes (There are no zeroes here). Use the root locus drawing rules.
     
  4. Nov 6, 2013 #3
    yeah, that's what I'm doing... the problem is: K is not a constant. to use the root locus drawing rules the characteristic equation of the closed loop system must be written as 1 + GH = 0. All that I got in this case is
    (s+2)(s+5)(s+K) + 10K = 0.
     
  5. Nov 7, 2013 #4
    I guess I finally did it.
    my characteristic equation is (s+2)(s+5)(s+K) + 10K = 0.
    we expand to s³ + 7s² + 10s + Ks² 7Ks + 20K = 0
    the C.E. must be written as 1 + K*F(s) = 0, then

    s³ + 7s² + 10s + K(s² + 7s + 20) = 0

    1 + K(s² + 7s + 20)/( s³ + 7s² + 10s) = 0



    this is equal to

    1 + K(s + 3.5 - 2.8i)(s + 3.5 + 2.8i)/(s(s+2)(s+5)) = 0


    now we can sketch the root locus where the zeros are (z1= 3.5 + 2.8i and z2 = 3.5 - 2.8i) and the poles p1=0, p2=-2 and p3=-5.

    :)
     
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