Zero gain margin for an stable system from both allmargin and margin

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Dave Lee 2019-10-7

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https://ww2.mathworks.cn/matlabcentral/answers/483923-zero-gain-margin-for-an-stable-system-from-both-allmargin-and-margin

Hi there,

I have the following open-loop transfer function

s=tf('s');
OL_tf = (5.782*s^2 + 1.395*s + 0.1361)/(s^3 + 4*s^2);
margin_numbers = allmargin(OL_tf);
gain_margin = margin_numbers.GainMargin
CL_tf = minreal(OL_tf/(1+OL_tf));
CL_poles = roots(CL_tf.den{1})
margin_numbers.Stable

You can see that gain_margin = 0 while CL_poles are stable, also margin_numbers.Stable = 1 means that it is stable. The closed-loop is definitely stable, what should we say about the commands in this case?

Thanks,

Dave