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Nyquist diagram

Discussion in 'Homework Help' started by eclipto, Jul 2, 2016.

  1. eclipto

    eclipto New Member

    Jul 2, 2016
    Hi, I have a question about the Nyquist diagrams.

    Say I have a loop function L and its bode plot.
    The phase of the function, say, goes from -90° to -270°, so it crosses -180°. I know the magnitude for each point (for example, I plot the bode using Matlab).
    Now, I want to use the bode plot to obtain the Nyquist diagram.
    I know the Nyquist diagram will cross the begative Re axis at some point.
    But how do I know the exact point? In particular, how do I know if it will be more negative with respect to the (-1, 0*j) critical point or if it will be more positive?
    If I look at the frequency on the bode I can read the exact magnitude in dB.
    What's next?
  2. MrAl

    MrAl Well-Known Member Most Helpful Member

    Sep 7, 2008

    Are you talking about the Nyquist criterion?
    In that case you would be concerned with the mapping of F(s)=1+P(s)
    which changed around a little is P(s)=F(s)-1
    so you are looking for the number of encirclements of the (-1,0) point
    and how they relate to the number of poles.

    Have you ever tried Root Locus, or do you have to use Nyquist?

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