test prewarp in c2d and sample_system

sawyerbfuller · sawyerbfuller · commit 4c66fb1ea47f · 2021-11-08T11:04:53.000-08:00
diff --git a/control/dtime.py b/control/dtime.py
@@ -5,6 +5,7 @@
 Routines in this module:
 
 sample_system()
+c2d()
 """
 
 """Copyright (c) 2012 by California Institute of Technology
@@ -58,16 +59,19 @@ def sample_system(sysc, Ts, method='zoh', alpha=None, prewarp_frequency=None):
 
     Parameters
     ----------
-    sysc : LTI (StateSpace or TransferFunction)
+    sysc : LTI (:class:`StateSpace` or :class:`TransferFunction`)
         Continuous time system to be converted
-    Ts : real > 0
+    Ts : float > 0
         Sampling period
     method : string
         Method to use for conversion, e.g. 'bilinear', 'zoh' (default)
-
-    prewarp_frequency : real within [0, infinity)
+    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`.
+    prewarp_frequency : float within [0, infinity)
         The frequency [rad/s] at which to match with the input continuous-
-        time system's magnitude and phase
+        time system's magnitude and phase (only valid for method='bilinear')
 
     Returns
     -------
@@ -76,7 +80,7 @@ def sample_system(sysc, Ts, method='zoh', alpha=None, prewarp_frequency=None):
 
     Notes
     -----
-    See :meth:`StateSpace.sample` or :meth:`TransferFunction.sample`` for
+    See :meth:`StateSpace.sample` or :meth:`TransferFunction.sample` for
     further details.
 
     Examples
@@ -99,20 +103,19 @@ def c2d(sysc, Ts, method='zoh', prewarp_frequency=None):
 
     Parameters
     ----------
-    sysc : LTI (StateSpace or TransferFunction)
+    sysc : LTI (:class:`StateSpace` or :class:`TransferFunction`)
         Continuous time system to be converted
-    Ts : real > 0
+    Ts : float > 0
         Sampling period
     method : string
         Method to use for conversion, e.g. 'bilinear', 'zoh' (default)
-
     prewarp_frequency : real within [0, infinity)
         The frequency [rad/s] at which to match with the input continuous-
-        time system's magnitude and phase
+        time system's magnitude and phase (only valid for method='bilinear')
 
     Returns
     -------
-    sysd : linsys
+    sysd : LTI of the same class
         Discrete time system, with sampling rate Ts
 
     Notes
diff --git a/control/tests/discrete_test.py b/control/tests/discrete_test.py
@@ -7,8 +7,8 @@
 import pytest
 
 from control import (StateSpace, TransferFunction, bode, common_timebase,
-                     evalfr, feedback, forced_response, impulse_response,
-                     isctime, isdtime, rss, sample_system, step_response,
+                     feedback, forced_response, impulse_response,
+                     isctime, isdtime, rss, c2d, sample_system, step_response,
                      timebase)
 
 
@@ -382,10 +382,20 @@ def test_sample_system_prewarp(self, tsys, plantname):
         Ts = 0.025
         # test state space version
         plant = getattr(tsys, plantname)
+        plant_fr = plant(wwarp * 1j)
+
         plant_d_warped = plant.sample(Ts, 'bilinear', prewarp_frequency=wwarp)
-        plant_fr = evalfr(plant, wwarp * 1j)
         dt = plant_d_warped.dt
-        plant_d_fr = evalfr(plant_d_warped, np.exp(wwarp * 1.j * dt))
+        plant_d_fr = plant_d_warped(np.exp(wwarp * 1.j * dt))
+        np.testing.assert_array_almost_equal(plant_fr, plant_d_fr)
+
+        plant_d_warped = sample_system(plant, Ts, 'bilinear',
+                prewarp_frequency=wwarp)
+        plant_d_fr = plant_d_warped(np.exp(wwarp * 1.j * dt))
+        np.testing.assert_array_almost_equal(plant_fr, plant_d_fr)
+
+        plant_d_warped = c2d(plant, Ts, 'bilinear', prewarp_frequency=wwarp)
+        plant_d_fr = plant_d_warped(np.exp(wwarp * 1.j * dt))
         np.testing.assert_array_almost_equal(plant_fr, plant_d_fr)
 
     def test_sample_system_errors(self, tsys):
diff --git a/control/xferfcn.py b/control/xferfcn.py
@@ -1086,12 +1086,10 @@ def sample(self, Ts, method='zoh', alpha=None, prewarp_frequency=None):
             * euler: Euler (or forward difference) method ("gbt" with alpha=0)
             * backward_diff: Backwards difference ("gbt" with alpha=1.0)
             * zoh: zero-order hold (default)
-
         alpha : float within [0, 1]
             The generalized bilinear transformation weighting parameter, which
             should only be specified with method="gbt", and is ignored
-            otherwise.
-
+            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 (the gain=1 crossover frequency,
@@ -1101,7 +1099,7 @@ def sample(self, Ts, method='zoh', alpha=None, prewarp_frequency=None):
         Returns
         -------
         sysd : TransferFunction system
-            Discrete time system, with sampling rate Ts
+            Discrete time system, with sample period Ts
 
         Notes
         -----
