diff --git a/control/lti.py b/control/lti.py
index ef5d5569a..b56c2bb44 100644
--- a/control/lti.py
+++ b/control/lti.py
@@ -159,7 +159,7 @@ def damp(self):
poles = self.pole()
if isdtime(self, strict=True):
- splane_poles = np.log(poles)/self.dt
+ splane_poles = np.log(poles.astype(complex))/self.dt
else:
splane_poles = poles
wn = absolute(splane_poles)
diff --git a/control/tests/lti_test.py b/control/tests/lti_test.py
index 9156ecb7e..7e4f0ddb4 100644
--- a/control/tests/lti_test.py
+++ b/control/tests/lti_test.py
@@ -70,6 +70,14 @@ 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 zeta.
+ p2_zplane = -0.2
+ sys_dt2 = tf(1, [1, -p2_zplane], dt)
+ wn2, zeta2, p2 = sys_dt2.damp()
+ p2_splane = -wn2 * zeta2 + 1j * wn2 * np.sqrt(1 - zeta2**2)
+ p2_zplane = np.exp(p2_splane * dt)
+ np.testing.assert_almost_equal(p2, p2_zplane)
+
def test_dcgain(self):
sys = tf(84, [1, 2])
np.testing.assert_allclose(sys.dcgain(), 42)
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