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Matlab Gain Margin problem.

  1. Aug 25, 2011 #1
    For the Matlab bode diagram, normally when we want to obtain the gain margin, we have to cross the line from phase -180 and touch the curve. (Picture1 is what I did) however from the matlab, the gain margin crossover is start from -540 instead of -180 degree by using "Margin" function(Picture2 is what Matlab shown).

    Attached is the problem's picture, may I know should I stick with -180 like the "picture1" ? or just follow what the Matlab show which is -540?

    picture1.png

    Picture2.png

    What i guess is, due to the gain margin is positive, thus the system auto change to -540 so the gain margin will be negative? Does this make any sense?
     
  2. jcsd
  3. Aug 25, 2011 #2

    uart

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  4. Aug 25, 2011 #3
    OK, here is the code in matlab

    num=[24];
    den=[11.5 1];
    g=tf(num,den);
    set(g,'inputdelay',2.3)
    bode(g)
    margin(g)


    .........-2.3
    24 . e
    --------------
    11.5s + 1
     
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