Nyquist Stability Criterion, Part 2

An explanation of the Nyquist Stability Criterion part 2. This video steps through the how to sketch a Nyquist plot by hand, what to do if there are open loop pools on the imaginary axis, and goes through a example showing the benefit of Nyquist over Bode plots.

Errata:

Links:
Check out nyqlog.m from the mathworks website.

Professor Gopal explaining how to do a rough sketch of the Nyquist plot: Starts at 17:15.

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[ChuyFlores]

The 3 possibilities:
1.- The -1 does not get circled; the system is stable if there are no poles in the RHP.
2.- The -1 gets circled one or more times in a CCW direction; the system is stable if the # of times it gets circled is equal to the # of poles in the RHP.
3.- The -1 gets circled one or more times in a CW direction; the system is unstable.

[bmoore]

To clarify; if a zero (not a pole) is in the RHP the system is unstable, because the Transfer Function we're evaluating is actually the Denominator of the ACTUAL Transfer Function we're determining stability on. (GH is open loop, but X / (1+GH) is close loop).  Nyquist difference between GH and 1+GH is just a shift of -1, so we evaluate GH as though its our transfer function, when in reality we're evaluating the Denominator of the transfer function.  So what indicates stability, and what indicates instability, is flipped. 

I.E. a pole creates a denominator of infinity, which is stable, while a zero creates a denominator of zero, which is unstable for the full thing.

Just wanted to help make that clear for anyone that missed it.

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