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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Result of the damp function for discrete time systems","articleBody":"Hi, \r\n\r\nThis is my first contribution on GitHub ;)\r\n\r\n### Problem \r\n\r\nThe method `damp()` seems to return a wrong result for discrete systems with negative poles\r\n\r\n```\r\nimport control as ctl\r\nH = ctl.tf([1],[1,0.2],1)\r\nH.damp()\r\n```\r\n\r\nResult : RuntimeWarning: invalid value encountered in log\r\n\r\n### Investigation\r\n\r\nThe problem seems to come from the type of the `poles` variable in the `damp`method of the LTI object\r\n\r\n```\r\nif isdtime(self, strict=True):\r\n     splane_poles = np.log(poles)/self.dt\r\nelse:\r\n      splane_poles = poles\r\nwn = absolute(splane_poles)\r\nZ = -real(splane_poles)/wn\r\nreturn wn, Z, poles\r\n```\r\n\r\n### Solution\r\n\r\nThe `poles` variable must be cast to a numpy complex type as follows\r\n\r\n```\r\nif isdtime(self, strict=True)\r\n      splane_poles = np.log(poles.astype(complex))/self.dt\r\n...\r\n```\r\n","author":{"url":"https://github.com/vincentchoqueuse","@type":"Person","name":"vincentchoqueuse"},"datePublished":"2021-08-17T06:48:00.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/646/python-control/issues/646"}