What is the significance of a bode plot obtained by the load disturbance transfer function G(s)/(1 + G(s)C(s))? I have two control systems and have attached the bode plot of both. The time response of the one controller is better than the other one. But how to conclude this from the frequency response? (The better controller's frequency response is shown in red color). How does a bode plot justify that the red one is better than the other in disturbance rejection?
Dear Purav,
One explanation can be formulated as follows:
The magnitude Bode plot show us that the amplitude of the disturbance effect is bigger with about 40 dB for the "better controller".
So, the system error generated by the disturbance is bigger and the controller action (to reduce the disturbance effect) it is powerful.
But, an important question is about the type of the disturbances.
Dear Dan,
Many thanks for your reply. When the magnitude is high, the controller effort is increased. This is valid but if the magnitude is less then the disturbance effect is also less right? So it should perform better right?
Dear Purav,
As per my analysis, if you look into the phase plot of bode diagrams then can see the red one never cross the -180 degree line whereas green one cross -180 degree line. So obviously red one has infinite gain margin and better than green one. The gain margin is infinite means the path never crosses the real axis in the left half plane (the path goes to the origin as |s|→∞. from nyquist/polar plot perspective.
Now if you look into the phase margin the red one also have substantial amount phase margin though green one has infinite phase margin.
Overall we can say red one is better than green one.
For more better analysis you can check the bandwidth as well as sensitivity measurement as per type of the disturbance ( whether input/output/feedback path disturbance)
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