Hi,

This is my first contribution on GitHub ;)

Problem

The methoddamp() seems to return a wrong result for discrete systems with negative poles

import control as ctlH = ctl.tf([1],[1,0.2],1)H.damp()

Result : RuntimeWarning: invalid value encountered in log

Investigation

The problem seems to come from the type of thepoles variable in thedampmethod of the LTI object

if isdtime(self, strict=True):     splane_poles = np.log(poles)/self.dtelse:      splane_poles = poleswn = absolute(splane_poles)Z = -real(splane_poles)/wnreturn wn, Z, poles

Solution

Thepoles variable must be cast to a numpy complex type as follows

if isdtime(self, strict=True)      splane_poles = np.log(poles.astype(complex))/self.dt...