Talk:Nichols plot
Add topicLatest comment: 13 years ago by Michel192cm in topic use of dB scale
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Nichols != Nyquist
[edit]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)
- 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.
- Blair - Speak to me 08:05, 22 October 2006 (UTC)
Interpretation
[edit]Can you explain what the graph shows and where the curve has to go to represent instability? — Omegatron 23:48, 26 March 2007 (UTC)
- 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)
use of dB scale
[edit]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)