nyquist

Nyquist response of dynamic system

Syntax

[re,im,wout] = nyquist(sys)

[re,im,wout] = nyquist(sys,w)

[re,im,wout,sdre,sdim] = nyquist(sys,w)

nyquist(___)

Description

[re,im,wout] = nyquist(sys) returns the real and imaginary parts of the frequency response at each frequency in the vector wout. The function automatically determines frequencies in wout based on system dynamics. This syntax does not draw a plot.

[re,im,wout] = nyquist(sys,w) returns the response data at the frequencies specified by w.

• If w is a cell array of the form {wmin,wmax}, then wout contains frequencies ranging between wmin and wmax.

• If w is a vector of frequencies, then wout = w.

[re,im,wout,sdre,sdim] = nyquist(sys,w) also returns the estimated standard deviation of the real and imaginary parts of the frequency response for the identified model sys. If you omit w, then the function automatically determines frequencies in wout based on system dynamics.

nyquist(___) creates a Nyquist plot of the frequency response of sys with default plotting options for all of the previous input argument combinations. The plot displays real and imaginary parts of the system response as a function of frequency. For more plot customization options, use nyquistplot.

• To plot responses for multiple dynamic systems on the same plot, you can specify sys as a comma-separated list of models. For example, nyquist(sys1,sys2,sys3) plots the responses for three models on the same plot.

• To specify a color, line style, and marker for each system in the plot, specify a LineSpec value for each system. For example, nyquist(sys1,LineSpec1,sys2,LineSpec2) plots two models and specifies their plot style. For more information on specifying a LineSpec value, see nyquistplot.

Examples

Nyquist Plot of Dynamic System

Create the following transfer function and plot its Nyquist response.

$\mathit{H}\left(\mathit{s}\right)=\frac{2{\mathit{s}}^2+5\mathit{s}+1}{{\mathit{s}}^2+2\mathit{s}+3}$.

H = tf([2 5 1],[1 2 3]);
nyquist(H)

The nyquist function can display a grid of M-circles, which are the contours of constant closed-loop magnitude. M-circles are defined as the locus of complex numbers where the following quantity is a constant value across frequency.

$\mathit{T}\left(\mathit{j}\omega \right)=\left|\frac{\mathit{G}\left(\mathit{j}\omega \right)}{1+\mathit{G}\left(\mathit{j}\omega \right)}\right|$.

Here, ω is the frequency in radians/TimeUnit, where TimeUnit is the system time units, and G is the collection of complex numbers that satisfy the constant magnitude requirement.

To display the grid of M-circles, right-click in the plot and select Grid. Alternatively, use the grid command.

grid on

Nyquist Plot at Specified Frequencies

Create a Nyquist plot over a specified frequency range. Use this approach when you want to focus on the dynamics in a particular range of frequencies.

H = tf([-0.1,-2.4,-181,-1950],[1,3.3,990,2600]);
nyquist(H,{1,100})

The cell array {1,100} specifies a frequency range [1,100] for the positive frequency branch and [–100,–1] for the negative frequency branch in the Nyquist plot. The negative frequency branch is obtained by symmetry for models with real coefficients. When you provide frequency bounds in this way, the function selects intermediate points for frequency response data.

Alternatively, specify a vector of frequency points to use for evaluating and plotting the frequency response.

w = 1:0.1:30;
nyquist(H,w,'.-')

nyquist plots the frequency response at the specified frequencies.

Nyquist Plot of Several Dynamic Systems

Compare the frequency response of several systems on the same Nyquist plot.

Create the dynamic systems.

rng(0)
sys1 = tf(3,[1,2,1]);
sys2 = tf([2 5 1],[1 2 3]);
sys3 = rss(4);

Create a Nyquist plot that displays all systems.

nyquist(sys1,sys2,sys3)
legend('Location','southwest')

Nyquist Plot with Specified Line Attributes

Specify the line style, color, or marker for each system in a Nyquist plot using the LineSpec input argument.

sys1 = tf(3,[1,2,1]);
sys2 = tf([2 5 1],[1 2 3]);
nyquist(sys1,'o:',sys2,'g')

The first LineSpec, 'o:', specifies a dotted line with circle markers for the response of sys1. The second LineSpec, 'g', specifies a solid green line for the response of sys2.

Obtain Real and Imaginary Parts of Frequency Response

Compute the real and imaginary parts of the frequency response of a SISO system.

If you do not specify frequencies, nyquist chooses frequencies based on the system dynamics and returns them in the third output argument.

H = tf([2 5 1],[1 2 3]);
[re,im,wout] = nyquist(H);

Because H is a SISO model, the first two dimensions of re and im are both 1. The third dimension is the number of frequencies in wout.

size(re)

ans = 1×3

     1     1   141

length(wout)

ans = 
141

Thus, each entry along the third dimension of re gives the real part of the response at the corresponding frequency in wout.

Nyquist Plot of MIMO System

For this example, create a 2-output, 3-input system.

rng(0,'twister');
H = rss(4,2,3);

For this system, nyquist plots the frequency responses of each I/O channel in a separate plot in a single figure.

nyquist(H)

Compute the real and imaginary parts of these responses at 20 frequencies between 1 and 10 radians.

w = logspace(0,1,20);
[re,im] = nyquist(H,w);

re and im are three-dimensional arrays, in which the first two dimensions correspond to the output and input dimensions of H, and the third dimension is the number of frequencies. For instance, examine the dimensions of re.

size(re)

ans = 1×3

     2     3    20

Thus, for example, re(1,3,10) is the real part of the response from the third input to the first output, computed at the 10th frequency in w. Similarly, im(1,3,10) contains the imaginary part of the same response.

Create Nyquist Plot of Identified Model with Response Uncertainty

Compute the standard deviations of the real and imaginary parts of the frequency response of an identified model. Use this data to create a 3σ plot of the response uncertainty.

Load the estimation data z2.

load iddata2 z2;

Identify a transfer function model using the data. Using the tfest command requires System Identification Toolbox™ software.

sys_p = tfest(z2,2);

Obtain the standard deviations for the real and imaginary parts of the frequency response for a set of 512 frequencies, w.

w = linspace(-10*pi,10*pi,512);
[re,im,wout,sdre,sdim] = nyquist(sys_p,w);

re and im are the real and imaginary parts of the frequency response, and sdre and sdim are their standard deviations, respectively. The frequencies in wout are the same as the frequencies you specified in w.

Use the standard deviation data to create a 3σ plot corresponding to the confidence region.

re = squeeze(re);
im = squeeze(im); 
sdre = squeeze(sdre);
sdim = squeeze(sdim);
plot(re,im,'b',re+3*sdre,im+3*sdim,'k:',re-3*sdre,im-3*sdim,'k:')
xlabel('Real Axis');
ylabel('Imaginary Axis');

Nyquist Plot of Model with Complex Coefficients

Create a Nyquist plot of a model with complex coefficients and a model with real coefficients on the same plot.

rng(0)
A = [-3.50,-1.25-0.25i;2,0];
B = [1;0];
C = [-0.75-0.5i,0.625-0.125i];
D = 0.5;
Gc = ss(A,B,C,D);
Gr = rss(4);
nyquist(Gc,Gr)
legend('Complex-coefficient model','Real-coefficient model')

The Nyquist plot always shows two branches, one for positive frequencies and one for negative frequencies. The arrows indicate the direction of increasing frequency for each branch. For models with complex coefficients, the two branches are not symmetric. For models with real coefficients, the negative branch is obtained by symmetry.

Input Arguments

sys — Dynamic system

dynamic system model | model array

Dynamic system, specified as a SISO or MIMO dynamic system model or array of dynamic system models. Dynamic systems that you can use include:

• Continuous-time or discrete-time numeric LTI models, such as tf (Control System Toolbox), zpk (Control System Toolbox), or ss (Control System Toolbox) models.

• Sparse state-space models, such as sparss (Control System Toolbox) or mechss (Control System Toolbox) models. Frequency grid w must be specified for sparse models.

• Generalized or uncertain LTI models such as genss (Control System Toolbox) or uss (Robust Control Toolbox) models. Using uncertain models requires Robust Control Toolbox™ software.

  • For tunable control design blocks, the function evaluates the model at its current value to plot the response.

  • For uncertain control design blocks, the function plots the nominal value and random samples of the model.

• Frequency-response data models such as frd models. For such models, the function plots the response at the frequencies defined in the model.

• Identified LTI models, such as idtf, idss, or idproc models.

If sys is an array of models, the plot shows responses of all models in the array on the same axes.

w — Frequencies

{wmin,wmax} | vector | []

Frequencies at which to compute the response, specified as one of the following:

• Cell array of the form {wmin,wmax} — Compute the response at frequencies in the range from wmin to wmax. If wmax is greater than the Nyquist frequency of sys, the response is computed only up to the Nyquist frequency.

• Vector of frequencies — Compute the response at each specified frequency. For example, use logspace to generate a row vector with logarithmically spaced frequency values. The vector w can contain both positive and negative frequencies.

• [] — Automatically select frequencies based on system dynamics.

Specify frequencies in units of rad/TimeUnit, where TimeUnit is the TimeUnit property of the model.

Output Arguments

re — Real part of system response

3-D array

Real part of the system response, returned as a 3-D array. The dimensions of this array are (number of system outputs)-by-(number of system inputs)-by-(number of frequency points).

• For SISO systems, re(1,1,k) is the real part of the response at the kth frequency in wout. For an example, see Obtain Real and Imaginary Parts of Frequency Response.

• For MIMO systems, re(i,j,k) gives the real part of the response at the kth frequency from the jth input to the ith output. For an example, see Nyquist Plot of MIMO System.

im — Imaginary part of system response

3-D array

Imaginary part of the system response, returned as a 3-D array. The dimensions of this array are (number of system outputs)-by(number of system inputs)-by-(number of frequency points).

• For SISO systems, im(1,1,k) gives the imaginary part of the response at the kth frequency in wout. For an example, see Obtain Real and Imaginary Parts of Frequency Response.

• For MIMO systems, im(i,j,k) gives the imaginary part of the response at the kth frequency from the jth input to the ith output. For an example, see Nyquist Plot of MIMO System.

wout — Frequencies

vector

Frequencies at which the function returns the system response, returned as a column vector. The function chooses the frequency values based on the model dynamics, unless you specify frequencies using w.

wout also contains negative frequency values for models with complex coefficients.

Frequency values are in radians per TimeUnit, where TimeUnit is the value of the TimeUnit property of sys.

sdre — Standard deviation of real part

3-D array | []

Estimated standard deviation of the real part of the response for identified models, returned as a 3-D array with the same dimensions as re.

If sys is not an identified LTI model, sdre is [].

sdim — Standard deviation of imaginary part

3-D array | []

Estimated standard deviation of the imaginary part of the response for identified models, returned as a 3-D array with the same dimensions as im.

If sys is not an identified LTI model, sdim is [].

Tips

• Two zoom options that apply specifically to Nyquist plots are available from the right-click menu:

  • Full View — Clips unbounded branches of the Nyquist plot, but still includes the critical point (–1, 0).

  • Zoom on (-1,0) — Zooms around the critical point (–1, 0).

Version History

Introduced before R2006a