diff --git a/control/dtime.py b/control/dtime.py
index 2ae482811..9b91eabd3 100644
--- a/control/dtime.py
+++ b/control/dtime.py
@@ -55,8 +55,7 @@
# Sample a continuous time system
def sample_system(sysc, Ts, method='zoh', alpha=None, prewarp_frequency=None,
name=None, copy_names=True, **kwargs):
- """
- Convert a continuous time system to discrete time by sampling.
+ """Convert a continuous time system to discrete time by sampling.
Parameters
----------
@@ -67,9 +66,9 @@ def sample_system(sysc, Ts, method='zoh', alpha=None, prewarp_frequency=None,
method : string
Method to use for conversion, e.g. 'bilinear', 'zoh' (default)
alpha : float within [0, 1]
- The generalized bilinear transformation weighting parameter, which
- should only be specified with method="gbt", and is ignored
- otherwise. See :func:`scipy.signal.cont2discrete`.
+ The generalized bilinear transformation weighting parameter, which
+ should only be specified with method="gbt", and is ignored
+ otherwise. See :func:`scipy.signal.cont2discrete`.
prewarp_frequency : float within [0, infinity)
The frequency [rad/s] at which to match with the input continuous-
time system's magnitude and phase (only valid for method='bilinear',
diff --git a/control/freqplot.py b/control/freqplot.py
index 164ec28a2..533515415 100644
--- a/control/freqplot.py
+++ b/control/freqplot.py
@@ -894,8 +894,10 @@ def gen_zero_centered_series(val_min, val_max, period):
# list of systems (e.g., "Step response for sys[1], sys[2]").
#
- # Set the initial title for the data (unique system names)
- sysnames = list(set([response.sysname for response in data]))
+ # Set the initial title for the data (unique system names, preserving order)
+ seen = set()
+ sysnames = [response.sysname for response in data \
+ if not (response.sysname in seen or seen.add(response.sysname))]
if title is None:
if data[0].title is None:
title = "Bode plot for " + ", ".join(sysnames)
diff --git a/control/tests/discrete_test.py b/control/tests/discrete_test.py
index 96777011e..cccb53708 100644
--- a/control/tests/discrete_test.py
+++ b/control/tests/discrete_test.py
@@ -5,11 +5,12 @@
import numpy as np
import pytest
+import cmath
-from control import (StateSpace, TransferFunction, bode, common_timebase,
- feedback, forced_response, impulse_response,
- isctime, isdtime, rss, c2d, sample_system, step_response,
- timebase)
+import control as ct
+from control import StateSpace, TransferFunction, bode, common_timebase, \
+ feedback, forced_response, impulse_response, isctime, isdtime, rss, \
+ c2d, sample_system, step_response, timebase
class TestDiscrete:
@@ -526,3 +527,33 @@ def test_signal_names(self, tsys):
assert sysd_newnames.find_input('u') is None
assert sysd_newnames.find_output('y') == 0
assert sysd_newnames.find_output('x') is None
+
+
+@pytest.mark.parametrize("num, den", [
+ ([1], [1, 1]),
+ ([1, 2], [1, 3]),
+ ([1, 2], [3, 4, 5])
+])
+@pytest.mark.parametrize("dt", [True, 0.1, 2])
+@pytest.mark.parametrize("method", ['zoh', 'bilinear', 'matched'])
+def test_c2d_matched(num, den, dt, method):
+ sys_ct = ct.tf(num, den)
+ sys_dt = ct.sample_system(sys_ct, dt, method=method)
+ assert sys_dt.dt == dt # make sure sampling time is OK
+ assert cmath.isclose(sys_ct(0), sys_dt(1)) # check zero frequency gain
+ assert cmath.isclose(
+ sys_ct.dcgain(), sys_dt.dcgain()) # another way to check
+
+ if method in ['zoh', 'matched']:
+ # Make sure that poles were properly matched
+ zpoles = sys_dt.poles()
+ for cpole in sys_ct.poles():
+ zpole = zpoles[(np.abs(zpoles - cmath.exp(cpole * dt))).argmin()]
+ assert cmath.isclose(cmath.exp(cpole * dt), zpole)
+
+ if method in ['matched']:
+ # Make sure that zeros were properly matched
+ zzeros = sys_dt.zeros()
+ for czero in sys_ct.zeros():
+ zzero = zzeros[(np.abs(zzeros - cmath.exp(czero * dt))).argmin()]
+ assert cmath.isclose(cmath.exp(czero * dt), zzero)
diff --git a/control/xferfcn.py b/control/xferfcn.py
index b5334b7b8..206f0bc92 100644
--- a/control/xferfcn.py
+++ b/control/xferfcn.py
@@ -1181,7 +1181,9 @@ def sample(self, Ts, method='zoh', alpha=None, prewarp_frequency=None,
if not self.issiso():
raise ControlMIMONotImplemented("Not implemented for MIMO systems")
if method == "matched":
- return _c2d_matched(self, Ts)
+ if prewarp_frequency is not None:
+ warn('prewarp_frequency ignored: incompatible conversion')
+ return _c2d_matched(self, Ts, name=name, **kwargs)
sys = (self.num[0][0], self.den[0][0])
if prewarp_frequency is not None:
if method in ('bilinear', 'tustin') or \
@@ -1284,9 +1286,12 @@ def _isstatic(self):
# c2d function contributed by Benjamin White, Oct 2012
-def _c2d_matched(sysC, Ts):
+def _c2d_matched(sysC, Ts, **kwargs):
+ if not sysC.issiso():
+ raise ControlMIMONotImplemented("Not implemented for MIMO systems")
+
# Pole-zero match method of continuous to discrete time conversion
- szeros, spoles, sgain = tf2zpk(sysC.num[0][0], sysC.den[0][0])
+ szeros, spoles, _ = tf2zpk(sysC.num[0][0], sysC.den[0][0])
zzeros = [0] * len(szeros)
zpoles = [0] * len(spoles)
pregainnum = [0] * len(szeros)
@@ -1302,9 +1307,9 @@ def _c2d_matched(sysC, Ts):
zpoles[idx] = z
pregainden[idx] = 1 - z
zgain = np.multiply.reduce(pregainnum) / np.multiply.reduce(pregainden)
- gain = sgain / zgain
+ gain = sysC.dcgain() / zgain
sysDnum, sysDden = zpk2tf(zzeros, zpoles, gain)
- return TransferFunction(sysDnum, sysDden, Ts)
+ return TransferFunction(sysDnum, sysDden, Ts, **kwargs)
# Utility function to convert a transfer function polynomial to a string
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