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Phase Margin (deg)
The phase margin is the difference, in degrees, between the phase of the open-loop response at the magnitude crossover frequency and -180 degrees. In
The phase margin measures axis stability. It also measures robustness against variations in the plant dynamics. If you increase the phase margin, the axis becomes more stable. But if you decrease the phase margin, the axis becomes less stable and can stop being stable. If you specify a Phase Margin (deg) that is less than or equal to 20 degrees, it is possible that your axis will not be stable.
A larger phase margin gives you less oscillation and a better-damped response. But that response is slower. If the open-loop response has many magnitude crossover frequencies, then the overall phase margin is the worst and lowest phase margin across the system. If you specify a Phase Margin (deg) between 30 and 60 degrees, it balances performance and robustness satisfactorily.
Figure: Margins and Axis Stability
