The Nyquist plot is the graph of \(kG(i \omega)\). G \[G_{CL} (s) \text{ is stable } \Leftrightarrow \text{ Ind} (kG \circ \gamma, -1) = P_{G, RHP}\]. Notice that when the yellow dot is at either end of the axis its image on the Nyquist plot is close to 0. Z denotes the number of zeros of If we have time we will do the analysis. + While Nyquist is one of the most general stability tests, it is still restricted to linear, time-invariant (LTI) systems. s G entire right half plane. . By counting the resulting contour's encirclements of 1, we find the difference between the number of poles and zeros in the right-half complex plane of 0000002305 00000 n
+ Thus, for all large \(R\), \[\text{the system is stable } \Leftrightarrow \ Z_{1 + kG, \gamma_R} = 0 \ \Leftrightarow \ \text{ Ind} (kG \circ \gamma_R, -1) = P_{G, \gamma_R}\], Finally, we can let \(R\) go to infinity. There are no poles in the right half-plane. Open the Nyquist Plot applet at. P , e.g. The algebra involved in canceling the \(s + a\) term in the denominators is exactly the cancellation that makes the poles of \(G\) removable singularities in \(G_{CL}\). N Transfer Function System Order -thorder system Characteristic Equation {\displaystyle F(s)} The counterclockwise detours around the poles at s=j4 results in It can happen! as the first and second order system. Any class or book on control theory will derive it for you. s Suppose that \(G(s)\) has a finite number of zeros and poles in the right half-plane. N We know from Figure \(\PageIndex{3}\) that the closed-loop system with \(\Lambda = 18.5\) is stable, albeit weakly. Clearly, the calculation \(\mathrm{GM} \approx 1 / 0.315\) is a defective metric of stability. {\displaystyle Z} Complex Variables with Applications (Orloff), { "12.01:_Principle_of_the_Argument" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "12.02:_Nyquist_Criterion_for_Stability" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "12.03:_A_Bit_on_Negative_Feedback" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, { "00:_Front_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "01:_Complex_Algebra_and_the_Complex_Plane" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "02:_Analytic_Functions" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "03:_Multivariable_Calculus_(Review)" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "04:_Line_Integrals_and_Cauchys_Theorem" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "05:_Cauchy_Integral_Formula" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "06:_Harmonic_Functions" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "07:_Two_Dimensional_Hydrodynamics_and_Complex_Potentials" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "08:_Taylor_and_Laurent_Series" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "09:_Residue_Theorem" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10:_Definite_Integrals_Using_the_Residue_Theorem" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "11:_Conformal_Transformations" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "12:_Argument_Principle" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "13:_Laplace_Transform" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "14:_Analytic_Continuation_and_the_Gamma_Function" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "zz:_Back_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, [ "article:topic", "license:ccbyncsa", "showtoc:no", "authorname:jorloff", "Nyquist criterion", "Pole-zero Diagrams", "Nyquist plot", "program:mitocw", "licenseversion:40", "source@https://ocw.mit.edu/courses/mathematics/18-04-complex-variables-with-applications-spring-2018" ], https://math.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fmath.libretexts.org%2FBookshelves%2FAnalysis%2FComplex_Variables_with_Applications_(Orloff)%2F12%253A_Argument_Principle%2F12.02%253A_Nyquist_Criterion_for_Stability, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), Theorem \(\PageIndex{2}\) Nyquist criterion, source@https://ocw.mit.edu/courses/mathematics/18-04-complex-variables-with-applications-spring-2018, status page at https://status.libretexts.org. ) This should make sense, since with \(k = 0\), \[G_{CL} = \dfrac{G}{1 + kG} = G. \nonumber\]. {\displaystyle F} We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. We first note that they all have a single zero at the origin. A Nyquist plot is a parametric plot of a frequency response used in automatic control and signal processing. Make a mapping from the "s" domain to the "L(s)" (Actually, for \(a = 0\) the open loop is marginally stable, but it is fully stabilized by the closed loop.). Describe the Nyquist plot with gain factor \(k = 2\). If, on the other hand, we were to calculate gain margin using the other phase crossing, at about \(-0.04+j 0\), then that would lead to the exaggerated \(\mathrm{GM} \approx 25=28\) dB, which is obviously a defective metric of stability. To use this criterion, the frequency response data of a system must be presented as a polar plot in which the magnitude and the phase angle are expressed as 0 s + 0 "1+L(s)=0.". G G Nyquist plot of \(G(s) = 1/(s + 1)\), with \(k = 1\). using the Routh array, but this method is somewhat tedious. s A pole with positive real part would correspond to a mode that goes to infinity as \(t\) grows. Rule 2. encirclements of the -1+j0 point in "L(s).". ) We will be concerned with the stability of the system. s ( We can measure phase margin directly by drawing on the Nyquist diagram a circle with radius of 1 unit and centered on the origin of the complex \(OLFRF\)-plane, so that it passes through the important point \(-1+j 0\). Z does not have any pole on the imaginary axis (i.e. ) has zeros outside the open left-half-plane (commonly initialized as OLHP). F The graphical display of frequency response magnitude becoming very large as 0 is produced by the following MATLAB commands, which calculate frequency response and produce a Nyquist plot of the same numerical solution as that on Figure 17.1.3, for the neutral-stability case = n s = 40, 000 s -2: >> wb=300;coj=100;wns=sqrt (wb*coj); . . G For the Nyquist plot and criterion the curve \(\gamma\) will always be the imaginary \(s\)-axis. The Nyquist criterion is an important stability test with applications to systems, circuits, and networks [1]. \[G(s) = \dfrac{1}{(s - s_0)^n} (b_n + b_{n - 1} (s - s_0) + \ a_0 (s - s_0)^n + a_1 (s - s_0)^{n + 1} + \ ),\], \[\begin{array} {rcl} {G_{CL} (s)} & = & {\dfrac{\dfrac{1}{(s - s_0)^n} (b_n + b_{n - 1} (s - s_0) + \ a_0 (s - s_0)^n + \ )}{1 + \dfrac{k}{(s - s_0)^n} (b_n + b_{n - 1} (s - s_0) + \ a_0 (s - s_0)^n + \ )}} \\ { } & = & {\dfrac{(b_n + b_{n - 1} (s - s_0) + \ a_0 (s - s_0)^n + \ )}{(s - s_0)^n + k (b_n + b_{n - 1} (s - s_0) + \ a_0 (s - s_0)^n + \ )}} \end{array}\], which is clearly analytic at \(s_0\). This method is easily applicable even for systems with delays and other non For these values of \(k\), \(G_{CL}\) is unstable. around . ) j D \(G(s)\) has a pole in the right half-plane, so the open loop system is not stable. The condition for the stability of the system in 19.3 is assured if the zeros of 1 + L are all in the left half of the complex plane. ( . 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