adjusted prewarping to only apply to 'bilinear' cont-to-discrete approximation, fixed failing tests

sawyerbfuller · sawyerbfuller · commit 03c04e298b86 · 2020-06-11T12:58:09.000-07:00
diff --git a/control/statesp.py b/control/statesp.py
@@ -868,7 +868,9 @@ def sample(self, Ts, method='zoh', alpha=None, prewarp_frequency=None):
 
         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 (the gain=1 crossover frequency, 
+            for example). Should only be specified with method='bilinear' or 
+            'gbt' with alpha=0.5 and ignored otherwise.
 
         Returns
         -------
@@ -889,11 +891,12 @@ def sample(self, Ts, method='zoh', alpha=None, prewarp_frequency=None):
             raise ValueError("System must be continuous time system")
 
         sys = (self.A, self.B, self.C, self.D)
-        if prewarp_frequency is not None:
+        if (method=='bilinear' or (method=='gbt' and alpha==0.5)) and \
+                prewarp_frequency is not None:
             Twarp = 2*np.tan(prewarp_frequency*Ts/2)/prewarp_frequency
-            Ad, Bd, C, D, _ = cont2discrete(sys, Twarp, method, alpha)
-        else:
-            Ad, Bd, C, D, _ = cont2discrete(sys, Ts, method, alpha)
+        else: 
+            Twarp = Ts
+        Ad, Bd, C, D, _ = cont2discrete(sys, Twarp, method, alpha)
         return StateSpace(Ad, Bd, C, D, Ts)
 
     def dcgain(self):
diff --git a/control/tests/statesp_test.py b/control/tests/statesp_test.py
@@ -624,7 +624,7 @@ def test_sample_system_prewarping(self):
         wwarp = 50
         Ts = 0.025
         plant = StateSpace(A,B,C,0)
-        plant_d_warped = plant.sample(Ts, prewarp_frequency=wwarp)
+        plant_d_warped = plant.sample(Ts, 'bilinear', prewarp_frequency=wwarp)
         np.testing.assert_array_almost_equal(
             evalfr(plant, wwarp*1j), 
             evalfr(plant_d_warped, np.exp(wwarp*1j*Ts)), 
diff --git a/control/tests/xferfcn_test.py b/control/tests/xferfcn_test.py
@@ -868,7 +868,7 @@ def test_sample_system_prewarping(self):
         Ts = 0.025
         plant = StateSpace(A,B,C,0)
         plant = ss2tf(plant)
-        plant_d_warped = plant.sample(Ts, prewarp_frequency=wwarp)
+        plant_d_warped = plant.sample(Ts, 'bilinear', prewarp_frequency=wwarp)
         np.testing.assert_array_almost_equal(
             evalfr(plant, wwarp*1j), 
             evalfr(plant_d_warped, np.exp(wwarp*1j*Ts)), 
diff --git a/control/xferfcn.py b/control/xferfcn.py
@@ -988,7 +988,9 @@ def sample(self, Ts, method='zoh', alpha=None, prewarp_frequency=None):
         
         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 (the gain=1 crossover frequency, 
+            for example). Should only be specified with method='bilinear' or 
+            'gbt' with alpha=0.5 and ignored otherwise.
 
         Returns
         -------
@@ -1014,11 +1016,12 @@ def sample(self, Ts, method='zoh', alpha=None, prewarp_frequency=None):
         if method == "matched":
             return _c2d_matched(self, Ts)
         sys = (self.num[0][0], self.den[0][0])
-        if prewarp_frequency is not None:
+        if (method=='bilinear' or (method=='gbt' and alpha==0.5)) and \
+                prewarp_frequency is not None:
             Twarp = 2*np.tan(prewarp_frequency*Ts/2)/prewarp_frequency
-            numd, dend, _ = cont2discrete(sys, Twarp, method, alpha)
-        else:
-            numd, dend, _ = cont2discrete(sys, Ts, method, alpha)
+        else: 
+            Twarp = Ts
+        numd, dend, _ = cont2discrete(sys, Twarp, method, alpha)
         return TransferFunction(numd[0, :], dend, Ts)
 
     def dcgain(self):
