Sufficient condition for second-order linear systems having no zeros

Some of the model independent controllers such as the virtual passive contr oller require a minimum-phase system to start with the controller design pr ocess (Morris and Juang, 1994). Recently, Lin and Juang (1995) have present ed some sufficient conditions for second-order linear systems to be minimum phase. However, if a second-order linear system has no zeros, then we can directly make a conclusion by using the definition of minimum phase (Chen, 1984), and thus we need not apply the sufficient conditions of Lin and Juan g to determine whether this second-order linear system is minimum phase. Th erefore, the purpose of this note is to propose a sufficient condition for second-order linear systems to have no zeros. Note that the work by Lin and Juang is very significant and excellent. It is hoped that the content of t his note may serve as a complement of their work.