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Homework Help: Control Systems: Frequency response Bode plots and analytical expressions

  1. Apr 7, 2010 #1
    1. The problem statement, all variables and given/known data

    Find analytical expressions for the magnitude and phase of the frequency response for each G(s) below:

    (a) [tex]G(s)\,=\,\frac{20}{s\,\left(s\,+\,5\right)\left(s\,+\,5\right)}[/tex]


    (b) [tex]G(s)\,=\,\frac{2\,\left(s\,+\,5\right)}{\left(s\,+\,1\right)\,\left(s\,+\,10\right)}[/tex]


    (c) [tex]G(s)\,=\,\frac{100}{s\left(s^2\,+\,10\,s\,+\,100\right)}[/tex]



    2. Relevant equations

    [tex]M\left(\omega\right)\,=\,\vert\,G\left(j\omega\right)\vert[/tex]

    Also need to find the angle expression.

    Complex number operations are quite intensive, this is probably why I can't figure it out!



    3. The attempt at a solution

    Prof. told us to first replace all s with jw.

    (a) [tex]G(j\omega)\,=\,\frac{20}{\left(j\omega\right)\,\left[\left(j\omega\right)\,+\,1\right]\,\left[\left(j\omega\right)\,+\,5\right]}[/tex]

    Then I do some complex number manipulation to slightly simplify the expression.

    [tex]G(j\omega)\,=\,\frac{20}{\left(-j\omega^3\,-\,6\omega^2\,+\,5j\omega\right)}[/tex]

    Does that look right? I've tried to get the magnitude expression multiple times, but it just doesn't seem right!!! I did the Bode plot for this transfer function in MATlab and it reports that at w=1 the magnitude should be 8.83dB. NONE of my magnitude expressions produce that data point. What am I doing wrong?!?!?


    EDIT:

    I figured out the magnitude part. But I still don't understand how to get the phase part.

    [tex]M(\omega)\,=\,\frac{20}{\sqrt{\left(5\omega\,-\,\omega^3\right)^2\,+\,\left(6\omega^2\right)^2}}[/tex]

    And then to get the dB magnitude...

    [tex]20\,log\left(M(\omega)\right)[/tex]

    But now how do I get the phase?
     
    Last edited: Apr 7, 2010
  2. jcsd
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