Contact characteristics of a laser scanner motor in a laser printer in the low speed region

This article 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 statistically unstable in the very low speed region. This instability occurs because the stiffness of the air bearing is not induced without rotation, and it is small in the very low speed region, whereas the magnetic negative stiffness of the magnetic bearing exists even at zero speed. Dry contact between the rotor and the stator is induced at the speed lower than a threshold, because a static displacement due to the gravity and 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 percent, compared to horizontal support. Vibration due to unbalance of the rotor scarcely affects the contact phenomenon. Because 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 by adopting a magnetic bearing composed of a pair of permanent magnets.