In control theory and stability theory, the **Nyquist stability criterion** or **Strecker–Nyquist stability criterion**, independently discovered by the German electrical engineer Felix Strecker at Siemens in 1930^{ [1] }^{ [2] }^{ [3] } and the Swedish-American electrical engineer Harry Nyquist at Bell Telephone Laboratories in 1932,^{ [4] } is a graphical technique for determining the stability of a dynamical system. Because it only looks at the Nyquist plot of the open loop systems, it can be applied without explicitly computing the poles and zeros of either the closed-loop or open-loop system (although the number of each type of right-half-plane singularities must be known). As a result, it can be applied to systems defined by non-rational functions, such as systems with delays. In contrast to Bode plots, it can handle transfer functions with right half-plane singularities. In addition, there is a natural generalization to more complex systems with multiple inputs and multiple outputs, such as control systems for airplanes.

- Nyquist plot
- Background
- Cauchy's argument principle
- The Nyquist criterion
- The Nyquist criterion for systems with poles on the imaginary axis
- Mathematical derivation
- Summary
- See also
- References
- Further reading
- External links

The Nyquist criterion is widely used in electronics and control system engineering, as well as other fields, for designing and analyzing systems with feedback. While Nyquist is one of the most general stability tests, it is still restricted to linear time-invariant (LTI) systems. Non-linear systems must use more complex stability criteria, such as Lyapunov or the circle criterion. While Nyquist is a graphical technique, it only provides a limited amount of intuition for why a system is stable or unstable, or how to modify an unstable system to be stable. Techniques like Bode plots, while less general, are sometimes a more useful design tool.

A **Nyquist plot** is a parametric plot of a frequency response used in automatic control and signal processing. The most common use of Nyquist plots is for assessing the stability of a system with feedback. In Cartesian coordinates, the real part of the transfer function is plotted on the *X*-axis. The imaginary part is plotted on the *Y*-axis. The frequency is swept as a parameter, resulting in a plot per frequency. The same plot can be described using polar coordinates, where gain of the transfer function is the radial coordinate, and the phase of the transfer function is the corresponding angular coordinate. The Nyquist plot is named after Harry Nyquist, a former engineer at Bell Laboratories.

Assessment of the stability of a closed-loop negative feedback system is done by applying the Nyquist stability criterion to the Nyquist plot of the open-loop system (i.e. the same system without its feedback loop). This method is easily applicable even for systems with delays and other non-rational transfer functions, which may appear difficult to analyze with other methods. Stability is determined by looking at the number of encirclements of the point (−1, 0). The range of gains over which the system will be stable can be determined by looking at crossings of the real axis.

The Nyquist plot can provide some information about the shape of the transfer function. For instance, the plot provides information on the difference between the number of zeros and poles of the transfer function ^{ [5] } by the angle at which the curve approaches the origin.

When drawn by hand, a cartoon version of the Nyquist plot is sometimes used, which shows the linearity of the curve, but where coordinates are distorted to show more detail in regions of interest. When plotted computationally, one needs to be careful to cover all frequencies of interest. This typically means that the parameter is swept logarithmically, in order to cover a wide range of values.

We consider a system whose transfer function is ; when placed in a closed loop with negative feedback , the closed loop transfer function (CLTF) then becomes . Stability can be determined by examining the roots of the desensitivity factor polynomial , e.g. using the Routh array, but this method is somewhat tedious. Conclusions can also be reached by examining the open loop transfer function (OLTF) , using its Bode plots or, as here, its polar plot using the Nyquist criterion, as follows.

Any Laplace domain transfer function can be expressed as the ratio of two polynomials:

The roots of are called the *zeros* of , and the roots of are the *poles* of . The poles of are also said to be the roots of the *characteristic equation*.

The stability of is determined by the values of its poles: for stability, the real part of every pole must be negative. If is formed by closing a negative unity feedback loop around the open-loop transfer function , then the roots of the characteristic equation are also the zeros of , or simply the roots of .

From complex analysis, a contour drawn in the complex plane, encompassing but not passing through any number of zeros and poles of a function , can be mapped to another plane (named plane) by the function . Precisely, each complex point in the contour is mapped to the point in the new plane yielding a new contour.

The Nyquist plot of , which is the contour will encircle the point of the plane times, where by Cauchy's argument principle. Here and are, respectively, the number of zeros of and poles of inside the contour . Note that we count encirclements in the plane in the same sense as the contour and that encirclements in the opposite direction are *negative* encirclements. That is, we consider clockwise encirclements to be positive and counterclockwise encirclements to be negative.

Instead of Cauchy's argument principle, the original paper by Harry Nyquist in 1932 uses a less elegant approach. The approach explained here is similar to the approach used by Leroy MacColl (Fundamental theory of servomechanisms 1945) or by Hendrik Bode (Network analysis and feedback amplifier design 1945), both of whom also worked for Bell Laboratories. This approach appears in most modern textbooks on control theory.

We first construct **the Nyquist contour**, a contour that encompasses the right-half of the complex plane:

- a path traveling up the axis, from to .
- a semicircular arc, with radius , that starts at and travels clock-wise to .

The Nyquist contour mapped through the function yields a plot of in the complex plane. By the argument principle, the number of clockwise encirclements of the origin must be the number of zeros of in the right-half complex plane minus the number of poles of in the right-half complex plane. If instead, the contour is mapped through the open-loop transfer function , the result is the Nyquist Plot of . 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 . Recalling that the zeros of are the poles of the closed-loop system, and noting that the poles of are same as the poles of , we now state the * Nyquist Criterion*:

Given a Nyquist contour , let be the number of poles of encircled by , and be the number of zeros of encircled by . Alternatively, and more importantly, if is the number of poles of the closed loop system in the right half plane, and is the number of poles of the open-loop transfer function in the right half plane, the resultant contour in the -plane, shall encircle (clockwise) the point times such that .

If the system is originally open-loop unstable, feedback is necessary to stabilize the system. Right-half-plane (RHP) poles represent that instability. For closed-loop stability of a system, the number of closed-loop roots in the right half of the *s*-plane must be zero. Hence, the number of counter-clockwise encirclements about must be equal to the number of open-loop poles in the RHP. Any clockwise encirclements of the critical point by the open-loop frequency response (when judged from low frequency to high frequency) would indicate that the feedback control system would be destabilizing if the loop were closed. (Using RHP zeros to "cancel out" RHP poles does not remove the instability, but rather ensures that the system will remain unstable even in the presence of feedback, since the closed-loop roots travel between open-loop poles and zeros in the presence of feedback. In fact, the RHP zero can make the unstable pole unobservable and therefore not stabilizable through feedback.)

The above consideration was conducted with an assumption that the open-loop transfer function does not have any pole on the imaginary axis (i.e. poles of the form ). This results from the requirement of the argument principle that the contour cannot pass through any pole of the mapping function. The most common case are systems with integrators (poles at zero).

To be able to analyze systems with poles on the imaginary axis, the Nyquist Contour can be modified to avoid passing through the point . One way to do it is to construct a semicircular arc with radius around , that starts at and travels anticlockwise to . Such a modification implies that the phasor travels along an arc of infinite radius by , where is the multiplicity of the pole on the imaginary axis.

Our goal is to, through this process, check for the stability of the transfer function of our unity feedback system with gain *k*, which is given by

That is, we would like to check whether the characteristic equation of the above transfer function, given by

has zeros outside the open left-half-plane (commonly initialized as OLHP).

We suppose that we have a clockwise (i.e. negatively oriented) contour enclosing the right half plane, with indentations as needed to avoid passing through zeros or poles of the function . Cauchy's argument principle states that

Where denotes the number of zeros of enclosed by the contour and denotes the number of poles of by the same contour. Rearranging, we have , which is to say

We then note that has exactly the same poles as . Thus, we may find by counting the poles of that appear within the contour, that is, within the open right half plane (ORHP).

We will now rearrange the above integral via substitution. That is, setting , we have

We then make a further substitution, setting . This gives us

We now note that gives us the image of our contour under , which is to say our Nyquist plot. We may further reduce the integral

by applying Cauchy's integral formula. In fact, we find that the above integral corresponds precisely to the number of times the Nyquist plot encircles the point clockwise. Thus, we may finally state that

We thus find that as defined above corresponds to a stable unity-feedback system when , as evaluated above, is equal to 0.

- If the open-loop transfer function has a zero pole of multiplicity , then the Nyquist plot has a discontinuity at . During further analysis it should be assumed that the phasor travels times clockwise along a semicircle of infinite radius. After applying this rule, the zero poles should be neglected, i.e. if there are no other unstable poles, then the open-loop transfer function should be considered stable.
- If the open-loop transfer function is stable, then the closed-loop system is unstable for
*any*encirclement of the point −1. - If the open-loop transfer function is
*unstable*, then there must be one*counter*-clockwise encirclement of −1 for each pole of in the right-half of the complex plane. - The number of surplus encirclements (
*N*+*P*greater than 0) is exactly the number of unstable poles of the closed-loop system. - However, if the graph happens to pass through the point , then deciding upon even the marginal stability of the system becomes difficult and the only conclusion that can be drawn from the graph is that there exist zeros on the axis.

**Control theory** deals with the control of dynamical systems in engineered processes and machines. The objective is to develop a model or algorithm governing the application of system inputs to drive the system to a desired state, while minimizing any *delay*, *overshoot*, or *steady-state error* and ensuring a level of control stability; often with the aim to achieve a degree of optimality.

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In control theory and stability theory, **root locus analysis** is a graphical method for examining how the roots of a system change with variation of a certain system parameter, commonly a gain within a feedback system. This is a technique used as a stability criterion in the field of classical control theory developed by Walter R. Evans which can determine stability of the system. The root locus plots the poles of the closed loop transfer function in the complex *s*-plane as a function of a gain parameter.

In complex analysis, the **argument principle** relates the difference between the number of zeros and poles of a meromorphic function to a contour integral of the function's logarithmic derivative.

In mathematics, a **Hankel contour** is a path in the complex plane which extends from (+∞,δ), around the origin counter clockwise and back to (+∞,−δ), where δ is an arbitrarily small positive number. The contour thus remains arbitrarily close to the real axis but without crossing the real axis except for negative values of *x*. The Hankel contour can also be represented by a path that has mirror images just above and below the real axis, connected to a circle of radius ε, centered at the origin, where ε is an arbitrarily small number. The two linear portions of the contour are said to be a distance of δ from the real axis. Thus, the total distance between the linear portions of the contour is 2δ. The contour is traversed in the positively-oriented sense, meaning that the circle around the origin is traversed counter-clockwise.

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In mathematics, a **line integral** is an integral where the function to be integrated is evaluated along a curve. The terms *path integral*, *curve integral*, and *curvilinear integral* are also used; *contour integral* is used as well, although that is typically reserved for line integrals in the complex plane.

**Iso-damping** is a desirable system property referring to a state where the open-loop phase Bode plot is flat—i.e., the phase derivative with respect to the frequency is zero, at a given frequency called the "tangent frequency", . At the "tangent frequency" the Nyquist curve of the open-loop system tangentially touches the sensitivity circle and the phase Bode is locally flat which implies that the system will be more robust to gain variations. For systems that exhibit iso-damping property, the overshoots of the closed-loop step responses will remain almost constant for different values of the controller gain. This will ensure that the closed-loop system is robust to gain variations.

The **matched Z-transform method**, also called the **pole–zero mapping** or **pole–zero matching method**, and abbreviated **MPZ** or **MZT**, is a technique for converting a continuous-time filter design to a discrete-time filter design.

**Hall circles** are a graphical tool in control theory used to obtain values of a closed-loop transfer function from the Nyquist plot of the associated open-loop transfer function. Hall circles have been introduced in control theory by Albert C. Hall in his thesis.

- ↑ Reinschke, Kurt (2014). "Chapter 4.3. Das Stabilitätskriterium von Strecker-Nyquist".
*Lineare Regelungs- und Steuerungstheorie*(in German) (2 ed.). Springer-Verlag. p. 184. ISBN 978-3-64240960-8 . Retrieved 2019-06-14. - ↑ Bissell, Christopher C. (2001). "Inventing the 'black box': mathematics as a neglected enabling technology in the history of communications engineering" (PDF). Archived (PDF) from the original on 2019-06-14. Retrieved 2019-06-14.
- ↑ Strecker, Felix (1947).
*Die elektrische Selbsterregung mit einer Theorie der aktiven Netzwerke*(in German). Stuttgart, Germany: S. Hirzel Verlag . (NB. Earlier works can be found in the literature section.) - ↑ Nyquist, Harry (January 1932). "Regeneration Theory".
*Bell System Technical Journal*. USA: American Telephone and Telegraph Company (AT&T).**11**(1): 126–147. doi:10.1002/j.1538-7305.1932.tb02344.x. - ↑ Nyquist Plots Archived 2008-09-30 at the Wayback Machine

- Faulkner, E. A. (1969):
*Introduction to the Theory of Linear Systems*; Chapman & Hall; ISBN 0-412-09400-2 - Pippard, A. B. (1985):
*Response & Stability*; Cambridge University Press; ISBN 0-521-31994-3 - Gessing, R. (2004):
*Control fundamentals*; Silesian University of Technology; ISBN 83-7335-176-0 - Franklin, G. (2002):
*Feedback Control of Dynamic Systems*; Prentice Hall, ISBN 0-13-032393-4

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