Contact Characteristics of a Laser Scanner Motor at Low Speed Region.

Abstract

This paper describes a transient contact phenomenon during start-stop operation of a polygonal mirror scanner rotor driven by a flat-type brushless DC motor and supported by a passive thrust magnetic bearing and a radial air bearing. From experimental investigations, the following characteristics have been clarified. The rotor is statically unstable at very low speed region. This instability occurs because the stiffness of the air bearing is not induced without rotation and it is small at the very low speed region, whereas the magnetic negative stiffness of the magnetic bearing is existent even at zero speed. Dry contact between the rotor and stator shaft is induced at the speed lower than the threshold, because a static displacement due to non-uniform magnetization of the magnetic bearing and forced vibration due to unbalance of the rotor exceed the air gap of the air bearing. Vertical support of the rotor results in the increase of the threshold speed, in the order of several tens per cent, compared to horizontal support. Vibration due to unbalance of the rotor scarecely affects the contact phenomena. Since the dry contact reduces the lifetime of the air bearing, it is preferable to avoid the frequent start-stop operations and to keep the rotor speed higher than the threshold. The instability is reduced to adopt the magnetic bearing composed of a pair of permanent magnets.