Transient Vibration of a Laser Scanner Motor in Digital Electrophotography

This paper describes a transient vibration during start-stop operation of a polygonal mirror scanner motor in a laser printer. Two types of abnormal vibration take place in the rotor driven by a flat-type brushless DC motor and supported by a passive thrust magnetic bearing and a radial air bearing. One is an axial displacement and vibration and another is a radial dry-contact at very low speed region. From results of theoretical and experimental investigations, the following characteristics and countermeasures have been deduced: (1) Repulsive magnetic force induced between the stator coils and rotor magnet of the motor causes the axial static displacement and the axial transient vibration. Soft start-stop scheme of the motor current is effective to reduce the axial vibration. (2) The rotor is statically unstable in the radial direction 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 a zero speed. Dry contact between the rotor and the stator shaft at the air bearing is induced at the speed lower than a threshold. Since the dry contact reduces the lifetime of the air bearing, it is preferable to avoid 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.