the `margin` command gives incorrect gain when comparing to the actual Bode plot

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Randy Chen 2021-12-10

评论： Mathieu NOE 2021-12-13

I have a continuous-time tranfser function with delay , and I printed the gain and phase margin as following:

[Gm0,Pm0] = margin(sys*C_pidf_fast);

where C_pidf_fast is a PIDF controller. The gain margin is returned as 73.4 However, when I plotted the bode plot

bode(sys*C_pidf_fast)

Apparently, the phase margin shown on the plot is different from the value I found by using the margin command. What is going on here?

Mathieu NOE 2021-12-13

hello

I believe 73.4 is not the phase but the gain margin

this is expressed in dB when you do bode

as 20*log10(73.4) = 37.3 which is what is displayed in the bode plot

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