Fix negative real handling for tfinal of discrete-time systems

UbeenII · slivingston · commit 7d5019e0b31f · 2026-06-09T12:49:33.000-07:00
Squash-merge of #1216 Fixes #1204
diff --git a/control/tests/timeresp_test.py b/control/tests/timeresp_test.py
@@ -2,6 +2,7 @@
 
 from copy import copy
 from math import isclose
+import warnings
 
 import numpy as np
 import pytest
@@ -827,6 +828,14 @@ def test_auto_generated_time_vector_tfinal(self, tfsys, tfinal):
         T = _default_time_vector(tfsys)
         np.testing.assert_allclose(T[-1], tfinal, atol=0.5*ideal_dt)
 
+    def test_discrete_time_negative_one_settling(self):
+        #system with -1 pole
+        TF = TransferFunction([1,3,0],[1,3,2], dt=True)
+        with warnings.catch_warnings():
+            warnings.simplefilter("error")
+            impulse_response(TF)
+
+
     @pytest.mark.parametrize("wn, zeta", [(10, 0), (100, 0), (100, .1)])
     def test_auto_generated_time_vector_dt_cont1(self, wn, zeta):
         """Confirm a TF with a natural frequency of wn rad/s gets a
diff --git a/control/timeresp.py b/control/timeresp.py
@@ -2172,7 +2172,7 @@ def _ideal_tfinal_and_dt(sys, is_step=True):
         m_z = np.abs(p) < sqrt_eps
         p = p[~m_z]
         # Negative reals- treated as oscillatory mode
-        m_nr = (p.real < 0) & (np.abs(p.imag) < sqrt_eps)
+        m_nr = (p.real < 0) & (np.abs(p.imag) < sqrt_eps) & (np.abs(p.real+1) > sqrt_eps)
         p_nr, p = p[m_nr], p[~m_nr]
         if p_nr.size > 0:
             t_emp = np.max(log_decay_percent / np.abs((np.log(p_nr)/dt).real))
