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Honor explicit Nyquist indent direction near imaginary axis #1223

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Merged
murrayrm merged 1 commit into from
Jun 8, 2026
Merged

Honor explicit Nyquist indent direction near imaginary axis #1223

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19 changes: 17 additions & 2 deletions control/freqplot.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1312,6 +1312,8 @@ def nyquist_response(
# Use existing dictionary, to keep track of processed keywords
_kwargs |= kwargs

indent_direction_arg = 'indent_direction' in _kwargs

# Get values for params
omega_num_given = omega_num is not None
omega_num = config._get_param('freqplot', 'number_of_samples', omega_num)
Expand Down Expand Up @@ -1469,8 +1471,21 @@ def nyquist_response(
- (s - p).real

# Figure out which way to offset the contour point
if p.real < 0 or (p.real == 0 and
indent_direction == 'right'):
if indent_direction_arg:
if indent_direction == 'right':
# Indent to the right
splane_contour[i] += offset

elif indent_direction == 'left':
# Indent to the left
splane_contour[i] -= offset

else:
raise ValueError(
"unknown value for indent_direction")

elif p.real < 0 or (p.real == 0 and
indent_direction == 'right'):
# Indent to the right
splane_contour[i] += offset

Expand Down
23 changes: 23 additions & 0 deletions control/tests/nyquist_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -372,6 +372,29 @@ def test_nyquist_indent_im():
assert _Z(sys) == response.count + _P(sys)


def test_nyquist_indent_near_imaginary_axis():
"""Test indent direction for poles near the imaginary axis."""
sys = ct.tf([1, 11, 10], [0.01, 1, 0.01, 1])
omega = np.linspace(0, 2, 21)

_, contour_default = ct.nyquist_response(
sys, omega, indent_radius=0.1, return_contour=True,
warn_encirclements=False)
_, contour_right = ct.nyquist_response(
sys, omega, indent_radius=0.1, indent_direction='right',
return_contour=True, warn_encirclements=False)
_, contour_left = ct.nyquist_response(
sys, omega, indent_radius=0.1, indent_direction='left',
return_contour=True, warn_encirclements=False)

# The pole near +1j has a small positive real part, so the default
# behavior indents to the left. Explicit directions override this.
pole_index = np.argmin(np.abs(contour_default.imag - 1))
assert contour_default[pole_index].real < 0
assert contour_right[pole_index].real > 0
assert contour_left[pole_index].real < 0


def test_nyquist_exceptions():
# MIMO not implemented
sys = ct.rss(2, 2, 2)
Expand Down
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