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Homework Help: Finding gain from a bode plot

  1. Dec 14, 2009 #1
    1. The problem statement, all variables and given/known data

    Transfer function:
    G(s) =[tex]\frac{k}{s^{2}+2\varsigma\omega_{n}s+\omega_{n}^{2}}[/tex]

    a) if u(t) = cos(2t), find the steady state response
    b) determine the values for k, damping ratio ([tex]\varsigma[/tex]), and the natural frequency.

    2. Relevant equations

    3. The attempt at a solution
    I know how to find the steady state response by finding G(wj) where w = 2 from the input function. However, I can't seem to find the correct values for k, damping ratio, and the natural frequency.

    first I found the cut off magnitudes from the bode plot by multiplying the largest magnitude (40) by (1/[tex]\sqrt{2}[/tex], to obtain the magnitude of the cut off frequencies to be approx. 28.28db. and found the cut off frequencies to be 8 and 10.1 rad/sec, which allows the Bandwidth to be determined by subtracting the two cut off frequencies.

    I believe the natural frequency should be 10, since it is also the center frequency, and I found the damping ration to be 0.105 by dividing the bandwidth by 2 times the natural frequency. And I think the gain should be 10.

    my transfer function
    G(s) =[tex]\frac{10}{s^{2}+2.1s+100}[/tex]

    when I find the bode plot of this function it looks similar to the provided plot, but the magnitude is off.

    Attached Files:

  2. jcsd
  3. Dec 15, 2009 #2


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    Homework Helper

    It's been a while I've done these, hopefully I'm not terribly wrong here as it's not quite my area of specialty. At least I got the image plotted in Matlab and it is indeed exactly the same.

    Anyway, setting s = 0, you immediately see that the low pass gain ought to be k/w_n^2. Yours is 1/10, which, if my math is right, gives -20 dB gain rather than 20 dB as shown in the figure.

    Phase -90 is quite special. Use that to get a second relation between the parameters. Finally I'd proceed to check what value and where |G| gets as its maximum to find the third and last relation between the parameters. You ought to get nice integers as answers (well, the damping parameter is a reciprocal of one).
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