Comments on "Magnetically levitated micro PM motors by two types of active magnetic bearings"

This note clarifies the design of proportional derivative (PD) controllers for the magnetic levitation systems of micro permanent magnet (PM) motors proposed in the original paper of Komori and Yamane (ibid., vol.6, p.43-9, 2001). It is shown that the PD controllers cannot stabilize the described levitated micro motors because it is necessary to use other values of parameters for these controllers. We present necessary and sufficient conditions for the stability of the controlled systems described in the paper.