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bode plot & stability

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keiichi451

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how to interprete bode plots

Here is the situation...

I have 4 pole, 2 zero open-loop system. The phase margin is greater than 0 due to the placement of the zeros. However, there are points where the phase goes beyond -180 degrees due to the higher number of poles. Is this system stable when place in closed-loop feedback?

I believe the answer is no, because for some points... gain > 1 and a phase shift of -180 in conjuction with negative feedback will yield positive feedback.

Please advise. Thanks in advance.
 

This is a case where the simple bode plot method is hard to interpret. Use the polar plot method and see how many times the locus wraps around -1 +j0. This will give you the number of poles minus the number of zeros in the right half plane.
 

flatulent said:
This is a case where the simple bode plot method is hard to interpret. Use the polar plot method and see how many times the locus wraps around -1 +j0. This will give you the number of poles minus the number of zeros in the right half plane.
Click to expand...

This is also called Nyquist Plot. But it takes time to learn this method and to implement it.
 

the easier method is routh-horwitz method i think...
if you have the exact location of poles and zeros of open-looped, then why not calculate the closed-loop transfer function and try this method?
 

hi
use root locus plot (rlocus in matlab) and analyse the stability.
 

first derive the closed loop transfer function ,the use the matlab instruciton bode to study the stability!
 

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