fix damp method for discrete time systems with a negative real-valued pole by vincentchoqueuse · Pull Request #647 · python-control/python-control
(first pull request on GitHub)
When a discrete time system has a real-valued negative pole, the discrete to continuous conversion using the log function returns nan. Then, the damp method is not able to compute the damping and the wn values.
To avoid this problem, the poles must be cast to complex values (https://numpy.org/doc/stable/reference/generated/numpy.log.html)