Design and Technology of a Magnetic Levitation System for Linear Micro Actuators

A major drawback of magnetic linear micro actuators is the vertical attractive force between stator and traveler. In the case of a micro step motor, this force is typically one order of magnitude greater than the driving force itself. To compensate for this undesired vertical force and thus taking full advantage of the driving force available, a magnetic levitation system was developed and implemented as a guide. The electromagnetic field generated by the stator coils interacts with the field of permanent magnets positioned in the traveler. This way, the traveler is elevated. Since repulsive magnetic levitation systems are inherently unstable, a tribological guide was integrated on both sides of the magnetic levitation system. During motion, the combination of stationary coils in the stator and moving permanent magnets in the traveler lifts up the traveler, while the lateral tribological guide prevents the traveler from shifting sideways. Initial investigations proved the feasibility of this magnetic levitation concept (1). A complete linear micro step motor system with magnetic levitation guide consists of the micro step motor itself, the magnetic micro levitation system (including the lateral guides), and a capacitive air gap measurement system. The latter one detects the size of the air gap between stator and traveler of the micro actuator. An assembly consisting of the three components results in a linear micro actuator system with adjustable air gap. For achieving optimal working conditions of the linear micro step motor, the magnetic levitation system was designed for a nominal air gap of 8 μm at the micro step motor. While earlier work proved the feasibility of such a guide, it also indicated (i) that the levitation system has to be capable of correcting a pitch motion of the traveler and (ii) that an as high magnetic levitation force as possible is desirable. To address the first issue, the magnetic levitation system received four coils arranged in a square along the axis of motion of the micro step motor. This way, both pitch and roll may be controlled. For resolving the second issue, the number of coil layers was increased from two to four. The technology for such a four layer coil is quite challenging, particularly since every effort has to be made to minimize its building height. The challenges were resolved by creating a coil system where the lateral insulation between conductors consists of SU-8™ (a photosensitive epoxy by Micro Resist Technology), while the vertical insulation layers were formed by a thin, stress compensated Si₃N₄ film. This way, a very compact coil with a high conductor-to-insulator ratio and thus a great current conducting capability could be realized. Due to the thin Si₃N₄ insulation, it also features an excellent thermal conductivity.