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Controls - gain margin question

  1. Nov 24, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the Bode Plot, and the phase and gain margins for the uncompensated system.

    2. Relevant equations
    G = (3.6s+6)/(s(.1s^2+.7s+1))

    3. The attempt at a solution
    upload_2015-11-24_18-21-11.png

    I understand the phase and gain margin ideas, but the gain margin hinges on the phase crossing -180 degrees, and in this case it never crosses. So am I doing something wrong here? If not, how do I find the gain margin in this case? Is it infinite? Thanks for any help.
     
  2. jcsd
  3. Nov 24, 2015 #2

    berkeman

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    Staff: Mentor

    Phase margin is measured in degrees, and gain margin is measured in dB. What do you get for this problem for each?
     
  4. Nov 24, 2015 #3
    The phase margin is approximately 46 degrees. The gain margin, on the other hand, is the gain needed to increase gain to 0 dB when phase is equal to -180 degrees. Well in this case, the Phase is never equal to -180 degrees so how do I establish the gain margin? That is the entire point of this posting. I don't understand how to find gain margin when the phase never crosses -180 degrees.
     
  5. Nov 25, 2015 #4

    berkeman

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    If you extend the frequency plot out another decade or two, the phase should get pretty close to -180 degrees. You have a good point about what is the gain margin if the phase asymptotically approaches -180 degrees with the gain still falling...
     
  6. Nov 25, 2015 #5

    LvW

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    Yes - this leads to the intuitive - but unrealistic - answer: The gain margin is infinite.
    This applies to the given function - however, one should know that each real system will exhibit additional poles and, hence, the phase will cross the 180 deg line ar a finite frequency.
     
  7. Nov 29, 2015 #6

    Maylis

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    Gold Member

    By the way, in matlab with Bode plots you can left click the plot, press characteristics, and click show all stability margins to get your gain and phase margins.
     
  8. Dec 1, 2015 #7

    rude man

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    Doen't matter if the gain is below 0dB at that point.
     
  9. Dec 2, 2015 #8

    LvW

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    The question was about the gain margin (and not if it "matters").
     
  10. Dec 2, 2015 #9

    rude man

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    It does not "matter" so long as the gain is < 0 dB when the phase hits -180 deg or intreger multiples of +/-180 deg.
     
  11. Dec 2, 2015 #10

    rude man

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    No such thing as "infinite gain margin" unless the gain is absolute zero (-∞dB). The gain margin is the number of dB below 0 dB when the phase shift is an integer number of π. It can never be infinite unless you have a short circuit!
     
  12. Dec 2, 2015 #11

    LvW

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    rude man, if you read my answer carefully, you will notice that I spoke in my post#5 about an "unrealistic" case which never will happen.
    I am familiar with the stability criterion (general and simplified form) and the definition of the stability margins.

    But the title of this thread is "gain margin question" . Therefore, the value of the loop gain <0 dB matters because it gives the margin, OK?
     
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