fix the return of the damp method for discrete time systems with a negative real-valued pole

vincentchoqueuse · vincentchoqueuse · commit 78b6fc255c5d · 2021-08-17T19:22:01.000+02:00
diff --git a/control/lti.py b/control/lti.py
@@ -159,6 +159,7 @@ def damp(self):
         poles = self.pole()
 
         if isdtime(self, strict=True):
+            poles = poles.astype(complex)
             splane_poles = np.log(poles)/self.dt
         else:
             splane_poles = poles
diff --git a/control/tests/lti_test.py b/control/tests/lti_test.py
@@ -70,6 +70,15 @@ def test_damp(self):
         np.testing.assert_almost_equal(sys_dt.damp(), expected_dt)
         np.testing.assert_almost_equal(damp(sys_dt), expected_dt)
 
+        #also check that for a discrete system with a negative real pole the damp function can extract wn and theta.
+        p2_zplane = -0.2
+        sys_dt2 = tf(1,[1,-p2_zplane],dt)
+        wn2, zeta2, _ = sys_dt2.damp()
+        p2 = -wn2 * zeta2 + 1j * wn2 * np.sqrt(1 - zeta2**2)
+        p2_zplane = np.exp(p2*dt)
+        np.testing.assert_almost_equal(sys_dt2.pole(),p2_zplane)
+        
+
     def test_dcgain(self):
         sys = tf(84, [1, 2])
         np.testing.assert_allclose(sys.dcgain(), 42)
