i have simple question. On what criterion the bode plot can be used to study the stability. If it can be used for all kind of the system, why we need nyquist plots? Can someone explain me more on this. Thanks. regds, kristo

They're different ways of presenting more or less the same information. Engineers may debate which one is better, or which one they prefer to use. Both provide an indication of stability if interpreted correctly.

The only systems that either of these techniques can be appalied to are linear systems.

Both present the same information, but they do it in different ways. Often one or the other makes a particular aspect more apparent. Given a pair of Bode plots (amplitude and phase) you can construct a Nyquist plot. Given a Nyquist plot with an adequate number of frequency tics, you can construct the Bode pair.

Just about any good book on basic control theory will discuss Bode and Nyquist plots -- I certainly do in mine.

As Jerry said, they each present the same information in different ways. When I design by frequency response I usually have three plots available: open-loop Bode, open-loop Nyquist, and closed-loop Bode. I'll use the Bode plots to see how the closed-loop system is coming along, and I'll use the Nyquist plot (with a 3dB or 6dB sensitivity circle inscribed around

-1) to keep the system within it's design sensitivity limits.

The Nyquist plot can be used to apply the full Nyquist stability criteria. The bode plot is useful only for a those systems with stable open loop poles and zeros. But since this constitutes most systems, the bode plot is used most often.

Even the Bode plot is useful for systems that are open-loop unstable if you've found _some_ tuning that is stable. The Bode plot will tell you how close you are to crossing the stability boundary; if you already know you're inside of it it's a useful tool for keeping yourself there no matter what your starting point.

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