diff --git a/control/freqplot.py b/control/freqplot.py
index 86b548b38..881ec93dd 100644
--- a/control/freqplot.py
+++ b/control/freqplot.py
@@ -714,41 +714,53 @@ def nyquist_plot(syslist, omega=None, plot=True, omega_limits=None,
if np.any(omega_sys * sys.dt > np.pi) and warn_nyquist:
warnings.warn("evaluation above Nyquist frequency")
- # Transform frequencies to continuous domain
- contour = np.exp(1j * omega * sys.dt)
- else:
- contour = 1j * omega_sys
+ # 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 sys.isctime() and indent_direction != 'none':
- poles = sys.pole()
- if contour[1].imag > indent_radius \
- and 0. in poles and not omega_range_given:
+ and indent_direction != 'none':
+ if sys.isctime():
+ splane_poles = sys.pole()
+ 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.pole()
+ zplane_poles = zplane_poles[~np.isclose(abs(zplane_poles), 0.)]
+ splane_poles = np.log(zplane_poles)/sys.dt
+
+ if splane_contour[1].imag > indent_radius \
+ and np.any(np.isclose(abs(splane_poles), 0)) \
+ and not omega_range_given:
# add some points for quarter circle around poles at origin
- contour = np.concatenate(
+ splane_contour = np.concatenate(
(1j * np.linspace(0., indent_radius, 50),
- contour[1:]))
- for i, s in enumerate(contour):
+ splane_contour[1:]))
+ for i, s in enumerate(splane_contour):
# Find the nearest pole
- p = poles[(np.abs(poles - s)).argmin()]
-
+ p = splane_poles[(np.abs(splane_poles - s)).argmin()]
# See if we need to indent around it
if abs(s - p) < indent_radius:
- if p.real < 0 or \
- (p.real == 0 and indent_direction == 'right'):
+ if p.real < 0 or (np.isclose(p.real, 0) \
+ and indent_direction == 'right'):
# Indent to the right
- contour[i] += \
+ splane_contour[i] += \
np.sqrt(indent_radius ** 2 - (s-p).imag ** 2)
- elif p.real > 0 or \
- (p.real == 0 and indent_direction == 'left'):
+ elif p.real > 0 or (np.isclose(p.real, 0) \
+ and indent_direction == 'left'):
# Indent to the left
- contour[i] -= \
+ splane_contour[i] -= \
np.sqrt(indent_radius ** 2 - (s-p).imag ** 2)
else:
ValueError("unknown value for indent_direction")
- # TODO: add code to indent around discrete poles on unit circle
+ # 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)
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