Simple answers to usual questions about unusual forms of the Evans' root locus plot

摘要

After a first contact with Evans' root locus plots, in an introductory course about classical control theory, students usually pose questions for which the answers are not trivially found in the usual textbooks. Examples of such questions are: Can a branch intersect itself? Can two or more branches be coincident? Can a branch intersect its asymptote? In this paper devoted to helping teaching, numerical examples and an incremental property are used for answering some questions about unusual forms of root loci related to closed-loop control systems. In some cases, answering to these questions can be fundamental for making a correct sketch of a root locus.