diff --git a/control/margins.py b/control/margins.py
index 392acbc9a..a98ff3936 100644
--- a/control/margins.py
+++ b/control/margins.py
@@ -108,12 +108,12 @@ def stability_margins(sysdata, returnall=False, epsw=0.0):
Linear SISO system
mag, phase, omega : sequence of array_like
Arrays of magnitudes (absolute values, not dB), phases (degrees),
- and corresponding frequencies. Crossover frequencies returned are
+ and corresponding frequencies. Crossover frequencies returned are
in the same units as those in `omega` (e.g., rad/sec or Hz).
returnall: bool, optional
- If true, return all margins found. If false (default), return only the
- minimum stability margins. For frequency data or FRD systems, only one
- margin is found and returned.
+ If true, return all margins found. If False (default), return only the
+ minimum stability margins. For frequency data or FRD systems, only
+ margins in the given frequency region can be found and returned.
epsw: float, optional
Frequencies below this value (default 0.0) are considered static gain,
and not returned as margin.
@@ -127,11 +127,11 @@ def stability_margins(sysdata, returnall=False, epsw=0.0):
sm: float or array_like
Stability margin, the minimum distance from the Nyquist plot to -1
wg: float or array_like
- Gain margin crossover frequency (where phase crosses -180 degrees)
+ Frequency for gain margin (at phase crossover, phase = -180 degrees)
wp: float or array_like
- Phase margin crossover frequency (where gain crosses 0 dB)
+ Frequency for phase margin (at gain crossover, gain = 0 dB)
ws: float or array_like
- Stability margin frequency (where Nyquist plot is closest to -1)
+ Frequency for stability margin (complex gain closest to -1)
"""
try:
@@ -340,18 +340,22 @@ def margin(*args):
Gain margin
pm : float
Phase margin (in degrees)
- Wcg : float
- Gain crossover frequency (corresponding to phase margin)
- Wcp : float
- Phase crossover frequency (corresponding to gain margin) (in rad/sec)
+ wg: float
+ Frequency for gain margin (at phase crossover, phase = -180 degrees)
+ wp: float
+ Frequency for phase margin (at gain crossover, gain = 0 dB)
- Margins are of SISO open-loop. If more than one crossover frequency is
- detected, returns the lowest corresponding margin.
+ Margins are calculated for a SISO open-loop system.
+
+ If there is more than one gain crossover, the one at the smallest
+ margin (deviation from gain=0dB), in absolute sense, is
+ returned. Likewise the smallest phase margin (in absolute sense)
+ is returned.
Examples
--------
>>> sys = tf(1, [1, 2, 1, 0])
- >>> gm, pm, Wcg, Wcp = margin(sys)
+ >>> gm, pm, wg, wp = margin(sys)
"""
if len(args) == 1:
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