Optimizing Gain Margin in Matlab Bode Diagram

[reik6149]

For the Matlab bode diagram, normally when we want to obtain the gain margin, we have to cross the line from phase -180 and touch the curve. (Picture1 is what I did) however from the matlab, the gain margin crossover is start from -540 instead of -180 degree by using "Margin" function(Picture2 is what Matlab shown).

Attached is the problem's picture, may I know should I stick with -180 like the "picture1" ? or just follow what the Matlab show which is -540?

What i guess is, due to the gain margin is positive, thus the system auto change to -540 so the gain margin will be negative? Does this make any sense?

 

[uart]

I think what you're looking at there is a non-minimum phase system. As such you'd be better looking at the Nyquist stability criterion rather than the simpler Barkhausen stability criterion.

BTw. Can you post the transfer function?http://en.wikipedia.org/wiki/Nyquist_stability_criterion
http://en.wikipedia.org/wiki/Barkhausen_stability_criterion

 

[reik6149]

OK, here is the code in matlab

num=[24];
den=[11.5 1];
g=tf(num,den);
set(g,'inputdelay',2.3)
bode(g)
margin(g)...-2.3
24 . e
--------------
11.5s + 1