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Ahem! the Nichols plot and the Nyquist plot are not the same AFAIK, contrary to what is stated here. Maybe examples of these plots would make that clear. A different engineer indeed.
Encyclops 16:08, 20 August 2006 (UTC)reply
I'm with you on that one. They are similar in that they both plot the magnitude and phase of the response with frequency as a parameter. However the Nyquist plot is done on a polar plot whereas a Nichols plot is the log of the magnitude versus phase.
I'm currently studying for my end-of-year exams... one of my exams is half control theory and so I have these plots coming out my ears. Give me a couple of weeks till the exam period is over and I'll rewrite this including examples.
Can you explain what the graph shows and where the curve has to go to represent instability? —
Omegatron 23:48, 26 March 2007 (UTC)reply
The horizontal line at Mag = 1 is the image of the unit circle in the Nyquist plot, and the vertical line at Arg=-pi is the image of the negative part of the real axis in the Nyquist plot (actually there are an infinite number of such vertical lines for Arg=-pi-2*k*pi). Their intersection is the image of -1 in the Nyquist plot. So when the loop response in the Nichols plot is higher than this point (larger magnitude), the closed-loop system is unstable (the equivalent of the full Nyquist criterion should be used when there are encirclements or unstable open loop).
Engelec 20:29, 28 March 2007 (UTC)reply
use of dB scale
On the picture the amplitude of the transfer function is not given in dB, while the article indicates 20*log10(G). So there's a mismatch, from my point of view.--
Michel192cm (
talk) 09:33, 13 August 2012 (UTC)reply