unit tests for infinite frequency response values, warnings

murrayrm · murrayrm · commit 96e44fe425a0 · 2021-02-10T21:44:39.000-08:00
diff --git a/control/lti.py b/control/lti.py
@@ -644,8 +644,9 @@ def dcgain(sys):
     Returns
     -------
     gain : ndarray
-        The zero-frequency gain, or np.nan if the system has a pole
-        at the origin
+        The zero-frequency gain, or np.inf if the system has a pole at the
+        origin, np.nan if there is a pole/zero cancellation at the origin.
+
     """
     return sys.dcgain()
 
diff --git a/control/tests/freqresp_test.py b/control/tests/freqresp_test.py
@@ -364,3 +364,73 @@ def test_phase_wrap(TF, wrap_phase, min_phase, max_phase):
     mag, phase, omega = ctrl.bode(TF, wrap_phase=wrap_phase)
     assert(min(phase) >= min_phase)
     assert(max(phase) <= max_phase)
+
+
+def test_freqresp_warn_infinite():
+    """Test evaluation of transfer functions at the origin"""
+    sys_finite = ctrl.tf([1], [1, 0.01])
+    sys_infinite = ctrl.tf([1], [1, 0.01, 0])
+
+    # Transfer function with finite zero frequency gain
+    np.testing.assert_almost_equal(sys_finite(0), 100)
+    np.testing.assert_almost_equal(sys_finite(0, warn_infinite=False), 100)
+    np.testing.assert_almost_equal(sys_finite(0, warn_infinite=True), 100)
+
+    # Transfer function with infinite zero frequency gain
+    with pytest.warns(RuntimeWarning, match="divide by zero"):
+        np.testing.assert_almost_equal(sys_infinite(0), np.inf)
+    with pytest.warns(RuntimeWarning, match="divide by zero"):
+        np.testing.assert_almost_equal(
+            sys_infinite(0, warn_infinite=True), np.inf)
+    np.testing.assert_almost_equal(sys_infinite(0, warn_infinite=False), np.inf)
+
+    # Switch to state space
+    sys_finite = ctrl.tf2ss(sys_finite)
+    sys_infinite = ctrl.tf2ss(sys_infinite)
+
+    # State space system with finite zero frequency gain
+    np.testing.assert_almost_equal(sys_finite(0), 100)
+    np.testing.assert_almost_equal(sys_finite(0, warn_infinite=False), 100)
+    np.testing.assert_almost_equal(sys_finite(0, warn_infinite=True), 100)
+
+    # State space system with infinite zero frequency gain
+    with pytest.warns(RuntimeWarning, match="not finite"):
+        np.testing.assert_almost_equal(sys_infinite(0), np.inf)
+    with pytest.warns(RuntimeWarning, match="not finite"):
+        np.testing.assert_almost_equal(sys_infinite(0), np.inf)
+    np.testing.assert_almost_equal(sys_infinite(0, warn_infinite=True), np.inf)
+    np.testing.assert_almost_equal(sys_infinite(0, warn_infinite=False), np.inf)
+    
+
+def test_dcgain_consistency():
+    """Test to make sure that DC gain is consistently evaluated"""
+    # Set up transfer function with pole at the origin
+    sys_tf = ctrl.tf([1], [1, 0])
+    assert 0 in sys_tf.pole()
+
+    # Set up state space system with pole at the origin
+    sys_ss = ctrl.tf2ss(sys_tf)
+    assert 0 in sys_ss.pole()
+
+    # Evaluation
+    np.testing.assert_equal(sys_tf(0), np.inf + 0j)
+    np.testing.assert_equal(sys_ss(0), np.inf + 0j)
+    np.testing.assert_equal(sys_tf.dcgain(), np.inf + 0j)
+    np.testing.assert_equal(sys_ss.dcgain(), np.inf + 0j)
+
+    # Set up transfer function with pole, zero at the origin
+    sys_tf = ctrl.tf([1, 0], [1, 0])
+    assert 0 in sys_tf.pole()
+    assert 0 in sys_tf.zero()
+    
+    sys_ss = ctrl.tf2ss(ctrl.tf([1, 0], [1, 1])) * \
+        ctrl.tf2ss(ctrl.tf([1], [1, 0]))
+    assert 0 in sys_ss.pole()
+    assert 0 in sys_ss.zero()
+
+    # Pole and zero at the origin should give nan for the response
+    np.testing.assert_equal(sys_tf(0), np.nan)
+    np.testing.assert_equal(sys_tf.dcgain(), np.nan)
+    # TODO: state space cases not yet working
+    # np.testing.assert_equal(sys_ss(0), np.nan)
+    # np.testing.assert_equal(sys_ss.dcgain(), np.nan)
diff --git a/control/tests/statesp_test.py b/control/tests/statesp_test.py
@@ -523,7 +523,7 @@ def test_dc_gain_discr(self):
     @pytest.mark.parametrize("dt", [None, 0, 1, True],
                              ids=["dtNone", "c", "dt1", "dtTrue"])
     def test_dc_gain_integrator(self, outputs, inputs, dt):
-        """DC gain when eigenvalue at DC returns appropriately sized array of nan.
+        """DC gain w/ pole at origin returns appropriately sized array of inf.
 
         the SISO case is also tested in test_dc_gain_{cont,discr}
         time systems (dt=0)
