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freqplot.py
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1760 lines (1510 loc) · 70.2 KB
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# freqplot.py - frequency domain plots for control systems
#
# Author: Richard M. Murray
# Date: 24 May 09
#
# This file contains some standard control system plots: Bode plots,
# Nyquist plots and pole-zero diagrams. The code for Nichols charts
# is in nichols.py.
#
# Copyright (c) 2010 by California Institute of Technology
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions
# are met:
#
# 1. Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
#
# 2. Redistributions in binary form must reproduce the above copyright
# notice, this list of conditions and the following disclaimer in the
# documentation and/or other materials provided with the distribution.
#
# 3. Neither the name of the California Institute of Technology nor
# the names of its contributors may be used to endorse or promote
# products derived from this software without specific prior
# written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
# FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL CALTECH
# OR THE CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
# SUCH DAMAGE.
#
# $Id$
import math
import matplotlib as mpl
import matplotlib.pyplot as plt
import numpy as np
import warnings
from math import nan
from .ctrlutil import unwrap
from .bdalg import feedback
from .margins import stability_margins
from .exception import ControlMIMONotImplemented
from .statesp import StateSpace
from .xferfcn import TransferFunction
from . import config
__all__ = ['bode_plot', 'nyquist_plot', 'gangof4_plot', 'singular_values_plot',
'bode', 'nyquist', 'gangof4']
# Default values for module parameter variables
_freqplot_defaults = {
'freqplot.feature_periphery_decades': 1,
'freqplot.number_of_samples': 1000,
'freqplot.dB': False, # Plot gain in dB
'freqplot.deg': True, # Plot phase in degrees
'freqplot.Hz': False, # Plot frequency in Hertz
'freqplot.grid': True, # Turn on grid for gain and phase
'freqplot.wrap_phase': False, # Wrap the phase plot at a given value
# deprecations
'deprecated.bode.dB': 'freqplot.dB',
'deprecated.bode.deg': 'freqplot.deg',
'deprecated.bode.Hz': 'freqplot.Hz',
'deprecated.bode.grid': 'freqplot.grid',
'deprecated.bode.wrap_phase': 'freqplot.wrap_phase',
}
#
# Main plotting functions
#
# This section of the code contains the functions for generating
# frequency domain plots
#
#
# Bode plot
#
def bode_plot(syslist, omega=None,
plot=True, omega_limits=None, omega_num=None,
margins=None, method='best', *args, **kwargs):
"""Bode plot for a system
Plots a Bode plot for the system over a (optional) frequency range.
Parameters
----------
syslist : linsys
List of linear input/output systems (single system is OK)
omega : array_like
List of frequencies in rad/sec to be used for frequency response
dB : bool
If True, plot result in dB. Default is false.
Hz : bool
If True, plot frequency in Hz (omega must be provided in rad/sec).
Default value (False) set by config.defaults['freqplot.Hz']
deg : bool
If True, plot phase in degrees (else radians). Default value (True)
config.defaults['freqplot.deg']
plot : bool
If True (default), plot magnitude and phase
omega_limits : array_like of two values
Limits of the to generate frequency vector.
If Hz=True the limits are in Hz otherwise in rad/s.
omega_num : int
Number of samples to plot. Defaults to
config.defaults['freqplot.number_of_samples'].
margins : bool
If True, plot gain and phase margin.
method : method to use in computing margins (see :func:`stability_margins`)
*args : :func:`matplotlib.pyplot.plot` positional properties, optional
Additional arguments for `matplotlib` plots (color, linestyle, etc)
**kwargs : :func:`matplotlib.pyplot.plot` keyword properties, optional
Additional keywords (passed to `matplotlib`)
Returns
-------
mag : ndarray (or list of ndarray if len(syslist) > 1))
magnitude
phase : ndarray (or list of ndarray if len(syslist) > 1))
phase in radians
omega : ndarray (or list of ndarray if len(syslist) > 1))
frequency in rad/sec
Other Parameters
----------------
grid : bool
If True, plot grid lines on gain and phase plots. Default is set by
`config.defaults['freqplot.grid']`.
initial_phase : float
Set the reference phase to use for the lowest frequency. If set, the
initial phase of the Bode plot will be set to the value closest to the
value specified. Units are in either degrees or radians, depending on
the `deg` parameter. Default is -180 if wrap_phase is False, 0 if
wrap_phase is True.
wrap_phase : bool or float
If wrap_phase is `False` (default), then the phase will be unwrapped
so that it is continuously increasing or decreasing. If wrap_phase is
`True` the phase will be restricted to the range [-180, 180) (or
[:math:`-\\pi`, :math:`\\pi`) radians). If `wrap_phase` is specified
as a float, the phase will be offset by 360 degrees if it falls below
the specified value. Default value is `False` and can be set using
config.defaults['freqplot.wrap_phase'].
The default values for Bode plot configuration parameters can be reset
using the `config.defaults` dictionary, with module name 'bode'.
Notes
-----
1. Alternatively, you may use the lower-level methods
:meth:`LTI.frequency_response` or ``sys(s)`` or ``sys(z)`` or to
generate the frequency response for a single system.
2. If a discrete time model is given, the frequency response is plotted
along the upper branch of the unit circle, using the mapping ``z =
exp(1j * omega * dt)`` where `omega` ranges from 0 to `pi/dt` and `dt`
is the discrete timebase. If timebase not specified (``dt=True``),
`dt` is set to 1.
Examples
--------
>>> sys = ss("1. -2; 3. -4", "5.; 7", "6. 8", "9.")
>>> mag, phase, omega = bode(sys)
"""
# Make a copy of the kwargs dictionary since we will modify it
kwargs = dict(kwargs)
# Check to see if legacy 'Plot' keyword was used
if 'Plot' in kwargs:
import warnings
warnings.warn("'Plot' keyword is deprecated in bode_plot; use 'plot'",
FutureWarning)
# Map 'Plot' keyword to 'plot' keyword
plot = kwargs.pop('Plot')
# Get values for params (and pop from list to allow keyword use in plot)
dB = config._get_param(
'freqplot', 'dB', kwargs, _freqplot_defaults, pop=True)
deg = config._get_param(
'freqplot', 'deg', kwargs, _freqplot_defaults, pop=True)
Hz = config._get_param(
'freqplot', 'Hz', kwargs, _freqplot_defaults, pop=True)
grid = config._get_param(
'freqplot', 'grid', kwargs, _freqplot_defaults, pop=True)
plot = config._get_param('freqplot', 'plot', plot, True)
margins = config._get_param(
'freqplot', 'margins', margins, False)
wrap_phase = config._get_param(
'freqplot', 'wrap_phase', kwargs, _freqplot_defaults, pop=True)
initial_phase = config._get_param(
'freqplot', 'initial_phase', kwargs, None, pop=True)
omega_num = config._get_param('freqplot', 'number_of_samples', omega_num)
# If argument was a singleton, turn it into a tuple
if not isinstance(syslist, (list, tuple)):
syslist = (syslist,)
omega, omega_range_given = _determine_omega_vector(
syslist, omega, omega_limits, omega_num, Hz=Hz)
if plot:
# Set up the axes with labels so that multiple calls to
# bode_plot will superimpose the data. This was implicit
# before matplotlib 2.1, but changed after that (See
# https://github.com/matplotlib/matplotlib/issues/9024).
# The code below should work on all cases.
# Get the current figure
if 'sisotool' in kwargs:
fig = kwargs.pop('fig')
ax_mag = fig.axes[0]
ax_phase = fig.axes[2]
sisotool = kwargs.pop('sisotool')
else:
fig = plt.gcf()
ax_mag = None
ax_phase = None
sisotool = False
# Get the current axes if they already exist
for ax in fig.axes:
if ax.get_label() == 'control-bode-magnitude':
ax_mag = ax
elif ax.get_label() == 'control-bode-phase':
ax_phase = ax
# If no axes present, create them from scratch
if ax_mag is None or ax_phase is None:
plt.clf()
ax_mag = plt.subplot(211, label='control-bode-magnitude')
ax_phase = plt.subplot(
212, label='control-bode-phase', sharex=ax_mag)
mags, phases, omegas, nyquistfrqs = [], [], [], []
for sys in syslist:
if not sys.issiso():
# TODO: Add MIMO bode plots.
raise ControlMIMONotImplemented(
"Bode is currently only implemented for SISO systems.")
else:
omega_sys = np.asarray(omega)
if sys.isdtime(strict=True):
nyquistfrq = math.pi / sys.dt
if not omega_range_given:
# limit up to and including nyquist frequency
omega_sys = np.hstack((
omega_sys[omega_sys < nyquistfrq], nyquistfrq))
else:
nyquistfrq = None
mag, phase, omega_sys = sys.frequency_response(omega_sys)
mag = np.atleast_1d(mag)
phase = np.atleast_1d(phase)
#
# Post-process the phase to handle initial value and wrapping
#
if initial_phase is None:
# Start phase in the range 0 to -360 w/ initial phase = -180
# If wrap_phase is true, use 0 instead (phase \in (-pi, pi])
initial_phase = -math.pi if wrap_phase is not True else 0
elif isinstance(initial_phase, (int, float)):
# Allow the user to override the default calculation
if deg:
initial_phase = initial_phase/180. * math.pi
else:
raise ValueError("initial_phase must be a number.")
# Shift the phase if needed
if abs(phase[0] - initial_phase) > math.pi:
phase -= 2*math.pi * \
round((phase[0] - initial_phase) / (2*math.pi))
# Phase wrapping
if wrap_phase is False:
phase = unwrap(phase) # unwrap the phase
elif wrap_phase is True:
pass # default calculation OK
elif isinstance(wrap_phase, (int, float)):
phase = unwrap(phase) # unwrap the phase first
if deg:
wrap_phase *= math.pi/180.
# Shift the phase if it is below the wrap_phase
phase += 2*math.pi * np.maximum(
0, np.ceil((wrap_phase - phase)/(2*math.pi)))
else:
raise ValueError("wrap_phase must be bool or float.")
mags.append(mag)
phases.append(phase)
omegas.append(omega_sys)
nyquistfrqs.append(nyquistfrq)
# Get the dimensions of the current axis, which we will divide up
# TODO: Not current implemented; just use subplot for now
if plot:
nyquistfrq_plot = None
if Hz:
omega_plot = omega_sys / (2. * math.pi)
if nyquistfrq:
nyquistfrq_plot = nyquistfrq / (2. * math.pi)
else:
omega_plot = omega_sys
if nyquistfrq:
nyquistfrq_plot = nyquistfrq
phase_plot = phase * 180. / math.pi if deg else phase
mag_plot = mag
if nyquistfrq_plot:
# append data for vertical nyquist freq indicator line.
# if this extra nyquist lime is is plotted in a single plot
# command then line order is preserved when
# creating a legend eg. legend(('sys1', 'sys2'))
omega_nyq_line = np.array(
(np.nan, nyquistfrq_plot, nyquistfrq_plot))
omega_plot = np.hstack((omega_plot, omega_nyq_line))
mag_nyq_line = np.array((
np.nan, 0.7*min(mag_plot), 1.3*max(mag_plot)))
mag_plot = np.hstack((mag_plot, mag_nyq_line))
phase_range = max(phase_plot) - min(phase_plot)
phase_nyq_line = np.array(
(np.nan,
min(phase_plot) - 0.2 * phase_range,
max(phase_plot) + 0.2 * phase_range))
phase_plot = np.hstack((phase_plot, phase_nyq_line))
#
# Magnitude plot
#
if dB:
ax_mag.semilogx(omega_plot, 20 * np.log10(mag_plot),
*args, **kwargs)
else:
ax_mag.loglog(omega_plot, mag_plot, *args, **kwargs)
# Add a grid to the plot + labeling
ax_mag.grid(grid and not margins, which='both')
ax_mag.set_ylabel("Magnitude (dB)" if dB else "Magnitude")
#
# Phase plot
#
# Plot the data
ax_phase.semilogx(omega_plot, phase_plot, *args, **kwargs)
# Show the phase and gain margins in the plot
if margins:
# Compute stability margins for the system
margin = stability_margins(sys, method=method)
gm, pm, Wcg, Wcp = (margin[i] for i in (0, 1, 3, 4))
# Figure out sign of the phase at the first gain crossing
# (needed if phase_wrap is True)
phase_at_cp = phases[0][(np.abs(omegas[0] - Wcp)).argmin()]
if phase_at_cp >= 0.:
phase_limit = 180.
else:
phase_limit = -180.
if Hz:
Wcg, Wcp = Wcg/(2*math.pi), Wcp/(2*math.pi)
# Draw lines at gain and phase limits
ax_mag.axhline(y=0 if dB else 1, color='k', linestyle=':',
zorder=-20)
ax_phase.axhline(y=phase_limit if deg else
math.radians(phase_limit),
color='k', linestyle=':', zorder=-20)
mag_ylim = ax_mag.get_ylim()
phase_ylim = ax_phase.get_ylim()
# Annotate the phase margin (if it exists)
if pm != float('inf') and Wcp != float('nan'):
if dB:
ax_mag.semilogx(
[Wcp, Wcp], [0., -1e5],
color='k', linestyle=':', zorder=-20)
else:
ax_mag.loglog(
[Wcp, Wcp], [1., 1e-8],
color='k', linestyle=':', zorder=-20)
if deg:
ax_phase.semilogx(
[Wcp, Wcp], [1e5, phase_limit + pm],
color='k', linestyle=':', zorder=-20)
ax_phase.semilogx(
[Wcp, Wcp], [phase_limit + pm, phase_limit],
color='k', zorder=-20)
else:
ax_phase.semilogx(
[Wcp, Wcp], [1e5, math.radians(phase_limit) +
math.radians(pm)],
color='k', linestyle=':', zorder=-20)
ax_phase.semilogx(
[Wcp, Wcp], [math.radians(phase_limit) +
math.radians(pm),
math.radians(phase_limit)],
color='k', zorder=-20)
# Annotate the gain margin (if it exists)
if gm != float('inf') and Wcg != float('nan'):
if dB:
ax_mag.semilogx(
[Wcg, Wcg], [-20.*np.log10(gm), -1e5],
color='k', linestyle=':', zorder=-20)
ax_mag.semilogx(
[Wcg, Wcg], [0, -20*np.log10(gm)],
color='k', zorder=-20)
else:
ax_mag.loglog(
[Wcg, Wcg], [1./gm, 1e-8], color='k',
linestyle=':', zorder=-20)
ax_mag.loglog(
[Wcg, Wcg], [1., 1./gm], color='k', zorder=-20)
if deg:
ax_phase.semilogx(
[Wcg, Wcg], [0, phase_limit],
color='k', linestyle=':', zorder=-20)
else:
ax_phase.semilogx(
[Wcg, Wcg], [0, math.radians(phase_limit)],
color='k', linestyle=':', zorder=-20)
ax_mag.set_ylim(mag_ylim)
ax_phase.set_ylim(phase_ylim)
if sisotool:
ax_mag.text(
0.04, 0.06,
'G.M.: %.2f %s\nFreq: %.2f %s' %
(20*np.log10(gm) if dB else gm,
'dB ' if dB else '',
Wcg, 'Hz' if Hz else 'rad/s'),
horizontalalignment='left',
verticalalignment='bottom',
transform=ax_mag.transAxes,
fontsize=8 if int(mpl.__version__[0]) == 1 else 6)
ax_phase.text(
0.04, 0.06,
'P.M.: %.2f %s\nFreq: %.2f %s' %
(pm if deg else math.radians(pm),
'deg' if deg else 'rad',
Wcp, 'Hz' if Hz else 'rad/s'),
horizontalalignment='left',
verticalalignment='bottom',
transform=ax_phase.transAxes,
fontsize=8 if int(mpl.__version__[0]) == 1 else 6)
else:
plt.suptitle(
"Gm = %.2f %s(at %.2f %s), "
"Pm = %.2f %s (at %.2f %s)" %
(20*np.log10(gm) if dB else gm,
'dB ' if dB else '',
Wcg, 'Hz' if Hz else 'rad/s',
pm if deg else math.radians(pm),
'deg' if deg else 'rad',
Wcp, 'Hz' if Hz else 'rad/s'))
# Add a grid to the plot + labeling
ax_phase.set_ylabel("Phase (deg)" if deg else "Phase (rad)")
def gen_zero_centered_series(val_min, val_max, period):
v1 = np.ceil(val_min / period - 0.2)
v2 = np.floor(val_max / period + 0.2)
return np.arange(v1, v2 + 1) * period
if deg:
ylim = ax_phase.get_ylim()
ax_phase.set_yticks(gen_zero_centered_series(
ylim[0], ylim[1], 45.))
ax_phase.set_yticks(gen_zero_centered_series(
ylim[0], ylim[1], 15.), minor=True)
else:
ylim = ax_phase.get_ylim()
ax_phase.set_yticks(gen_zero_centered_series(
ylim[0], ylim[1], math.pi / 4.))
ax_phase.set_yticks(gen_zero_centered_series(
ylim[0], ylim[1], math.pi / 12.), minor=True)
ax_phase.grid(grid and not margins, which='both')
# ax_mag.grid(which='minor', alpha=0.3)
# ax_mag.grid(which='major', alpha=0.9)
# ax_phase.grid(which='minor', alpha=0.3)
# ax_phase.grid(which='major', alpha=0.9)
# Label the frequency axis
ax_phase.set_xlabel("Frequency (Hz)" if Hz
else "Frequency (rad/sec)")
if len(syslist) == 1:
return mags[0], phases[0], omegas[0]
else:
return mags, phases, omegas
#
# Nyquist plot
#
# Default values for module parameter variables
_nyquist_defaults = {
'nyquist.primary_style': ['-', '-.'], # style for primary curve
'nyquist.mirror_style': ['--', ':'], # style for mirror curve
'nyquist.arrows': 2, # number of arrors around curve
'nyquist.arrow_size': 8, # pixel size for arrows
'nyquist.encirclement_threshold': 0.05, # warning threshold
'nyquist.indent_radius': 1e-4, # indentation radius
'nyquist.indent_direction': 'right', # indentation direction
'nyquist.indent_points': 50, # number of points to insert
'nyquist.max_curve_magnitude': 20, # clip large values
'nyquist.max_curve_offset': 0.02, # offset of primary/mirror
'nyquist.start_marker': 'o', # marker at start of curve
'nyquist.start_marker_size': 4, # size of the maker
}
def nyquist_plot(
syslist, omega=None, plot=True, omega_limits=None, omega_num=None,
label_freq=0, color=None, return_contour=False,
warn_encirclements=True, warn_nyquist=True, **kwargs):
"""Nyquist plot for a system
Plots a Nyquist plot for the system over a (optional) frequency range.
The curve is computed by evaluating the Nyqist segment along the positive
imaginary axis, with a mirror image generated to reflect the negative
imaginary axis. Poles on or near the imaginary axis are avoided using a
small indentation. The portion of the Nyquist contour at infinity is not
explicitly computed (since it maps to a constant value for any system with
a proper transfer function).
Parameters
----------
syslist : list of LTI
List of linear input/output systems (single system is OK). Nyquist
curves for each system are plotted on the same graph.
plot : boolean
If True, plot magnitude
omega : array_like
Set of frequencies to be evaluated, in rad/sec.
omega_limits : array_like of two values
Limits to the range of frequencies. Ignored if omega is provided, and
auto-generated if omitted.
omega_num : int
Number of frequency samples to plot. Defaults to
config.defaults['freqplot.number_of_samples'].
color : string
Used to specify the color of the line and arrowhead.
return_contour : bool, optional
If 'True', return the contour used to evaluate the Nyquist plot.
**kwargs : :func:`matplotlib.pyplot.plot` keyword properties, optional
Additional keywords (passed to `matplotlib`)
Returns
-------
count : int (or list of int if len(syslist) > 1)
Number of encirclements of the point -1 by the Nyquist curve. If
multiple systems are given, an array of counts is returned.
contour : ndarray (or list of ndarray if len(syslist) > 1)), optional
The contour used to create the primary Nyquist curve segment, returned
if `return_contour` is Tue. To obtain the Nyquist curve values,
evaluate system(s) along contour.
Additional Parameters
---------------------
arrows : int or 1D/2D array of floats, optional
Specify the number of arrows to plot on the Nyquist curve. If an
integer is passed. that number of equally spaced arrows will be
plotted on each of the primary segment and the mirror image. If a 1D
array is passed, it should consist of a sorted list of floats between
0 and 1, indicating the location along the curve to plot an arrow. If
a 2D array is passed, the first row will be used to specify arrow
locations for the primary curve and the second row will be used for
the mirror image.
arrow_size : float, optional
Arrowhead width and length (in display coordinates). Default value is
8 and can be set using config.defaults['nyquist.arrow_size'].
arrow_style : matplotlib.patches.ArrowStyle, optional
Define style used for Nyquist curve arrows (overrides `arrow_size`).
encirclement_threshold : float, optional
Define the threshold for generating a warning if the number of net
encirclements is a non-integer value. Default value is 0.05 and can
be set using config.defaults['nyquist.encirclement_threshold'].
indent_direction : str, optional
For poles on the imaginary axis, set the direction of indentation to
be 'right' (default), 'left', or 'none'.
indent_points : int, optional
Number of points to insert in the Nyquist contour around poles that
are at or near the imaginary axis.
indent_radius : float, optional
Amount to indent the Nyquist contour around poles on or near the
imaginary axis. Portions of the Nyquist plot corresponding to indented
portions of the contour are plotted using a different line style.
label_freq : int, optiona
Label every nth frequency on the plot. If not specified, no labels
are generated.
max_curve_magnitude : float, optional
Restrict the maximum magnitude of the Nyquist plot to this value.
Portions of the Nyquist plot whose magnitude is restricted are
plotted using a different line style.
max_curve_offset : float, optional
When plotting scaled portion of the Nyquist plot, increase/decrease
the magnitude by this fraction of the max_curve_magnitude to allow
any overlaps between the primary and mirror curves to be avoided.
mirror_style : [str, str] or False
Linestyles for mirror image of the Nyquist curve. The first element
is used for unscaled portions of the Nyquist curve, the second element
is used for portions that are scaled (using max_curve_magnitude). If
`False` then omit completely. Default linestyle (['--', ':']) is
determined by config.defaults['nyquist.mirror_style'].
primary_style : [str, str], optional
Linestyles for primary image of the Nyquist curve. The first
element is used for unscaled portions of the Nyquist curve,
the second element is used for portions that are scaled (using
max_curve_magnitude). Default linestyle (['-', '-.']) is
determined by config.defaults['nyquist.mirror_style'].
start_marker : str, optional
Matplotlib marker to use to mark the starting point of the Nyquist
plot. Defaults value is 'o' and can be set using
config.defaults['nyquist.start_marker'].
start_marker_size : float, optional
Start marker size (in display coordinates). Default value is
4 and can be set using config.defaults['nyquist.start_marker_size'].
warn_nyquist : bool, optional
If set to 'False', turn off warnings about frequencies above Nyquist.
warn_encirclements : bool, optional
If set to 'False', turn off warnings about number of encirclements not
meeting the Nyquist criterion.
Notes
-----
1. If a discrete time model is given, the frequency response is computed
along the upper branch of the unit circle, using the mapping ``z =
exp(1j * omega * dt)`` where `omega` ranges from 0 to `pi/dt` and `dt`
is the discrete timebase. If timebase not specified (``dt=True``),
`dt` is set to 1.
2. If a continuous-time system contains poles on or near the imaginary
axis, a small indentation will be used to avoid the pole. The radius
of the indentation is given by `indent_radius` and it is taken to the
right of stable poles and the left of unstable poles. If a pole is
exactly on the imaginary axis, the `indent_direction` parameter can be
used to set the direction of indentation. Setting `indent_direction`
to `none` will turn off indentation. If `return_contour` is True, the
exact contour used for evaluation is returned.
Examples
--------
>>> sys = ss([[1, -2], [3, -4]], [[5], [7]], [[6, 8]], [[9]])
>>> count = nyquist_plot(sys)
"""
# Check to see if legacy 'Plot' keyword was used
if 'Plot' in kwargs:
warnings.warn("'Plot' keyword is deprecated in nyquist_plot; "
"use 'plot'", FutureWarning)
# Map 'Plot' keyword to 'plot' keyword
plot = kwargs.pop('Plot')
# Check to see if legacy 'labelFreq' keyword was used
if 'labelFreq' in kwargs:
warnings.warn("'labelFreq' keyword is deprecated in nyquist_plot; "
"use 'label_freq'", FutureWarning)
# Map 'labelFreq' keyword to 'label_freq' keyword
label_freq = kwargs.pop('labelFreq')
# Check to see if legacy 'arrow_width' or 'arrow_length' were used
if 'arrow_width' in kwargs or 'arrow_length' in kwargs:
warnings.warn(
"'arrow_width' and 'arrow_length' keywords are deprecated in "
"nyquist_plot; use `arrow_size` instead", FutureWarning)
kwargs['arrow_size'] = \
(kwargs.get('arrow_width', 0) + kwargs.get('arrow_length', 0)) / 2
kwargs.pop('arrow_width', False)
kwargs.pop('arrow_length', False)
# Get values for params (and pop from list to allow keyword use in plot)
omega_num_given = omega_num is not None
omega_num = config._get_param('freqplot', 'number_of_samples', omega_num)
arrows = config._get_param(
'nyquist', 'arrows', kwargs, _nyquist_defaults, pop=True)
arrow_size = config._get_param(
'nyquist', 'arrow_size', kwargs, _nyquist_defaults, pop=True)
arrow_style = config._get_param('nyquist', 'arrow_style', kwargs, None)
indent_radius = config._get_param(
'nyquist', 'indent_radius', kwargs, _nyquist_defaults, pop=True)
encirclement_threshold = config._get_param(
'nyquist', 'encirclement_threshold', kwargs,
_nyquist_defaults, pop=True)
indent_direction = config._get_param(
'nyquist', 'indent_direction', kwargs, _nyquist_defaults, pop=True)
indent_points = config._get_param(
'nyquist', 'indent_points', kwargs, _nyquist_defaults, pop=True)
max_curve_magnitude = config._get_param(
'nyquist', 'max_curve_magnitude', kwargs, _nyquist_defaults, pop=True)
max_curve_offset = config._get_param(
'nyquist', 'max_curve_offset', kwargs, _nyquist_defaults, pop=True)
start_marker = config._get_param(
'nyquist', 'start_marker', kwargs, _nyquist_defaults, pop=True)
start_marker_size = config._get_param(
'nyquist', 'start_marker_size', kwargs, _nyquist_defaults, pop=True)
# Set line styles for the curves
def _parse_linestyle(style_name, allow_false=False):
style = config._get_param(
'nyquist', style_name, kwargs, _nyquist_defaults, pop=True)
if isinstance(style, str):
# Only one style provided, use the default for the other
style = [style, _nyquist_defaults['nyquist.' + style_name][1]]
warnings.warn(
"use of a single string for linestyle will be deprecated "
" in a future release", PendingDeprecationWarning)
if (allow_false and style is False) or \
(isinstance(style, list) and len(style) == 2):
return style
else:
raise ValueError(f"invalid '{style_name}': {style}")
primary_style = _parse_linestyle('primary_style')
mirror_style = _parse_linestyle('mirror_style', allow_false=True)
# If argument was a singleton, turn it into a tuple
if not isinstance(syslist, (list, tuple)):
syslist = (syslist,)
# Determine the range of frequencies to use, based on args/features
omega, omega_range_given = _determine_omega_vector(
syslist, omega, omega_limits, omega_num, feature_periphery_decades=2)
# If omega was not specified explicitly, start at omega = 0
if not omega_range_given:
if omega_num_given:
# Just reset the starting point
omega[0] = 0.0
else:
# Insert points between the origin and the first frequency point
omega = np.concatenate((
np.linspace(0, omega[0], indent_points), omega[1:]))
# Go through each system and keep track of the results
counts, contours = [], []
for sys in syslist:
if not sys.issiso():
# TODO: Add MIMO nyquist plots.
raise ControlMIMONotImplemented(
"Nyquist plot currently only supports SISO systems.")
# Figure out the frequency range
omega_sys = np.asarray(omega)
# Determine the contour used to evaluate the Nyquist curve
if sys.isdtime(strict=True):
# Restrict frequencies for discrete-time systems
nyquistfrq = math.pi / sys.dt
if not omega_range_given:
# limit up to and including nyquist frequency
omega_sys = np.hstack((
omega_sys[omega_sys < nyquistfrq], nyquistfrq))
# Issue a warning if we are sampling above Nyquist
if np.any(omega_sys * sys.dt > np.pi) and warn_nyquist:
warnings.warn("evaluation above Nyquist frequency")
# do indentations in s-plane where it is more convenient
splane_contour = 1j * omega_sys
# Bend the contour around any poles on/near the imaginary axis
# TODO: smarter indent radius that depends on dcgain of system
# and timebase of discrete system.
if isinstance(sys, (StateSpace, TransferFunction)) \
and indent_direction != 'none':
if sys.isctime():
splane_poles = sys.poles()
splane_cl_poles = sys.feedback().poles()
else:
# map z-plane poles to s-plane, ignoring any at the origin
# because we don't need to indent for them
zplane_poles = sys.poles()
zplane_poles = zplane_poles[~np.isclose(abs(zplane_poles), 0.)]
splane_poles = np.log(zplane_poles) / sys.dt
zplane_cl_poles = sys.feedback().poles()
zplane_cl_poles = zplane_cl_poles[
~np.isclose(abs(zplane_poles), 0.)]
splane_cl_poles = np.log(zplane_cl_poles) / sys.dt
#
# Check to make sure indent radius is small enough
#
# If there is a closed loop pole that is near the imaginary access
# at a point that is near an open loop pole, it is possible that
# indentation might skip or create an extraneous encirclement.
# We check for that situation here and generate a warning if that
# could happen.
#
for p_cl in splane_cl_poles:
# See if any closed loop poles are near the imaginary axis
if abs(p_cl.real) <= indent_radius:
# See if any open loop poles are close to closed loop poles
p_ol = splane_poles[
(np.abs(splane_poles - p_cl)).argmin()]
if abs(p_ol - p_cl) <= indent_radius and \
warn_encirclements:
warnings.warn(
"indented contour may miss closed loop pole; "
"consider reducing indent_radius to be less than "
f"{abs(p_ol - p_cl):5.2g}", stacklevel=2)
#
# See if we should add some frequency points near imaginary poles
#
for p in splane_poles:
# See if we need to process this pole (skip if on the negative
# imaginary axis or not near imaginary axis + user override)
if p.imag < 0 or abs(p.real) > indent_radius or \
omega_range_given:
continue
# Find the frequencies before the pole frequency
below_points = np.argwhere(
splane_contour.imag - abs(p.imag) < -indent_radius)
if below_points.size > 0:
first_point = below_points[-1].item()
start_freq = p.imag - indent_radius
else:
# Add the points starting at the beginning of the contour
assert splane_contour[0] == 0
first_point = 0
start_freq = 0
# Find the frequencies after the pole frequency
above_points = np.argwhere(
splane_contour.imag - abs(p.imag) > indent_radius)
last_point = above_points[0].item()
# Add points for half/quarter circle around pole frequency
# (these will get indented left or right below)
splane_contour = np.concatenate((
splane_contour[0:first_point+1],
(1j * np.linspace(
start_freq, p.imag + indent_radius, indent_points)),
splane_contour[last_point:]))
# Indent points that are too close to a pole
for i, s in enumerate(splane_contour):
# Find the nearest pole
p = splane_poles[(np.abs(splane_poles - s)).argmin()]
# See if we need to indent around it
if abs(s - p) < indent_radius:
# Figure out how much to offset (simple trigonometry)
offset = np.sqrt(indent_radius ** 2 - (s - p).imag ** 2) \
- (s - p).real
# Figure out which way to offset the contour point
if p.real < 0 or (p.real == 0 and
indent_direction == 'right'):
# Indent to the right
splane_contour[i] += offset
elif p.real > 0 or (p.real == 0 and
indent_direction == 'left'):
# Indent to the left
splane_contour[i] -= offset
else:
raise ValueError("unknown value for indent_direction")
# change contour to z-plane if necessary
if sys.isctime():
contour = splane_contour
else:
contour = np.exp(splane_contour * sys.dt)
# Compute the primary curve
resp = sys(contour)
# Compute CW encirclements of -1 by integrating the (unwrapped) angle
phase = -unwrap(np.angle(resp + 1))
encirclements = np.sum(np.diff(phase)) / np.pi
count = int(np.round(encirclements, 0))
# Let the user know if the count might not make sense
if abs(encirclements - count) > encirclement_threshold and \
warn_encirclements:
warnings.warn(
"number of encirclements was a non-integer value; this can"
" happen is contour is not closed, possibly based on a"
" frequency range that does not include zero.")
#
# Make sure that the enciriclements match the Nyquist criterion
#
# If the user specifies the frequency points to use, it is possible
# to miss enciriclements, so we check here to make sure that the
# Nyquist criterion is actually satisfied.
#
if isinstance(sys, (StateSpace, TransferFunction)):
# Count the number of open/closed loop RHP poles
if sys.isctime():
if indent_direction == 'right':
P = (sys.poles().real > 0).sum()
else:
P = (sys.poles().real >= 0).sum()
Z = (sys.feedback().poles().real >= 0).sum()
else:
if indent_direction == 'right':
P = (np.abs(sys.poles()) > 1).sum()
else:
P = (np.abs(sys.poles()) >= 1).sum()
Z = (np.abs(sys.feedback().poles()) >= 1).sum()
# Check to make sure the results make sense; warn if not
if Z != count + P and warn_encirclements:
warnings.warn(
"number of encirclements does not match Nyquist criterion;"
" check frequency range and indent radius/direction",
UserWarning, stacklevel=2)
elif indent_direction == 'none' and any(sys.poles().real == 0) and \
warn_encirclements:
warnings.warn(
"system has pure imaginary poles but indentation is"
" turned off; results may be meaningless",
RuntimeWarning, stacklevel=2)
counts.append(count)
contours.append(contour)
if plot:
# Parse the arrows keyword
if not arrows:
arrow_pos = []
elif isinstance(arrows, int):
N = arrows
# Space arrows out, starting midway along each "region"
arrow_pos = np.linspace(0.5/N, 1 + 0.5/N, N, endpoint=False)
elif isinstance(arrows, (list, np.ndarray)):
arrow_pos = np.sort(np.atleast_1d(arrows))
else:
raise ValueError("unknown or unsupported arrow location")
# Set the arrow style
if arrow_style is None:
arrow_style = mpl.patches.ArrowStyle(
'simple', head_width=arrow_size, head_length=arrow_size)
# Find the different portions of the curve (with scaled pts marked)
reg_mask = np.logical_or(
np.abs(resp) > max_curve_magnitude,
splane_contour.real != 0)
# reg_mask = np.logical_or(
# np.abs(resp.real) > max_curve_magnitude,
# np.abs(resp.imag) > max_curve_magnitude)
scale_mask = ~reg_mask \
& np.concatenate((~reg_mask[1:], ~reg_mask[-1:])) \