%% Disk Gain and Phase Margins
% When you call dmplot without an argument, the resulting plot and text shows a comparison of a disk margin analysis with the classical notations of gain and phase margins. The Nyquist plot is of the loop transfer function L(s):
% L(s) = \frac{{\frac{s}{{30}} + 1}}{{(s + 1)({s^2} + 1.6s + 16)}}.

dmplot

%
% The x-axis corresponds to the gain variation in dB and the y-axis corresponds to the allowable phase variation in degrees. For a disk gain margin corresponding to 3 dB (1.414), the closed-loop system is stable for all phase and gain variations inside the blue ellipse. For example, the closed-loop system can simultaneously tolerate +/– 2 dB gain variation and +/– 14 deg phase variations. To see the allowable variations for a given disk gain margin, use the given value as an input to dmplot.

figure           % new figure window
dmplot(1.414)

% Copyright 2012 The MathWorks, Inc.