A necessary and sufficient condition for the stability of a linear closed-loop system formulated in terms of properties of the open-loop system.
Consider the linear single input-linear single output system with the following transfer function:
where it is assumed that the degree of the polynomial $M(z)$ does not exceed that of the polynomial $N(z)$ (i.e. $W(p)$ is a proper rational function). The original Nyquist criterion gives necessary and sufficient conditions for the stability of the closed-loop system with unity feedback $u=y$. This is done in terms of the complex-valued function $z=W(i\omega)$ of the real variable $\omega\in[0,\infty)$ (the amplitude-phase characteristic of the open-loop system) which describes a curve in the complex $z$-plane, known as the Nyquist diagram. Suppose that the characteristic polynomial $N(z)$ of the open-loop system has $k$, $0\leq k\leq n$, roots with positive real part and $n-k$ roots with negative real part. The Nyquist criterion is as follows: The closed-loop system is stable if and only if the Nyquist diagram encircles the point $z=-1$ in the counter-clockwise sense $k/2$ times. (An equivalent formulation is: The vector drawn from $-1$ to the point $W(i\omega)$ describes an angle $\pi k$ in the positive sense as $\omega$ goes from $0$ to $+\infty$.)
This criterion was first proposed by H. Nyquist  for feedback amplifiers; it is one of the frequency criteria for the stability of linear systems (similar, e.g., to the Mikhailov criterion, see , ). It is important to note that if the equations of some of the elements of the systems are unknown, the Nyquist diagram can be constructed experimentally, by feeding a harmonic signal of variable frequency to the input of the open feedback .
Generalizations of this criterion have since been developed for multivariable, infinite-dimensional and sampled-data systems, e.g. , , , .
|||H. Nyquist, "Regeneration theory" Bell System Techn. J. , 11 : 1 (1932) pp. 126–147|
|||B.V. Bulgakov, "Oscillations" , Moscow (1954) (In Russian)|
|||M.A. Lavrent'ev, B.V. Shabat, "Methoden der komplexen Funktionentheorie" , Deutsch. Verlag Wissenschaft. (1967) (Translated from Russian)|
|||Ya.N. Roitenberg, "Automatic control" , Moscow (1978) (In Russian)|
|||L.S. Gnoenskii, G.A. Kamenskii, L.E. El'sgol'ts, "Mathematical foundations of the theory of control systems" , Moscow (1969) (In Russian)|
For generalizations of the Nyquist criterion in various directions, see [a1].
|[a1]||C.A. Desoer, M. Vidyasagar, "Feedback systems: input-output properties" , Acad. Press (1975)|
|[a2]||C.A. Desoer, "A general formulation of the Nyquist stability criterion" IEEE Trans. Circuit Theory , CT-12 (1965) pp. 230–234|
|[a3]||C.A. Desoer, Y.T. Wang, "On the generalized Nyquist stability criterion" IEEE Trans. Autom. Control , AC-25 (1980) pp. 187–196|
|[a4]||F.M. Callier, C.A. Desoer, "On simplifying a graphical stability criterion for linear distributed feedback systems" IEEE Trans. Automat. Contr. , AC-21 (1976) pp. 128–129|
|[a5]||J.M.E. Valenca, C.J. Harris, "Nyquist criterion for input-output stability of multivariable systems" Int. J. Control , 31 (1980) pp. 917–935|
|[a6]||P. Faurre, M. Depeyrot, "Elements of system theory" , North-Holland (1977)|
Nyquist criterion. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Nyquist_criterion&oldid=32802