Thanks for the extra info. You are right that I misunderstood the issue you were pointing out, and you are correct that the documentation for the control.margin() function is incorrect.

Just for posterity, here is what is going on in a bit more detail:

In python-control, according to the documentation for control.margins(), Wcg is the gain crossover frequency (corresponding to phase margin) and Wcp is the phase crossover frequency (corresponding to gain margin). What the documentation says is the order of the return values is:
- gm = Gain margin
- pm = Phase margin (in degrees)
- Wcg = Gain crossover frequency (corresponding to phase margin)
- Wcp = Phase crossover frequency (corresponding to gain margin) (in rad/sec)
What is actually returned is a subset of the return values in the order of the control.stability_margins() function, which is
- gm = Gain margin
- pm = Phase margin
- sm = Stability margin, the minimum distance from the Nyquist plot to -1
- wg = Gain margin crossover frequency (where phase crosses -180 degrees)
- wp = Phase margin crossover frequency (where gain crosses 0 dB)
- ws = Stability margin frequency (where Nyquist plot is closest to -1)
In MATLAB, the relevant function call is [Gm,Pm,Wgm,Wpm] = margin(sys) where Wgm is 'the frequency where the gain margin is measured' (from MATLAB documentation) and Wpm is 'the frequency where the phase margin is measured'. This is the same as control.stability_margins().

So the problem is that the documentation for control.margin is incorrect.

Since I'm doing a bit of work on the code today, I'll try to post a PR to fix this soon. @repagh and @blinkminster: I think one or both of you worked on this in the past, so if you happen to be online and I am missing something in terms of what is going on, let me know.

Documentation for margin command is incorrect #195

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