Linear Motor Stages - Performance of Direct-Driven Linear Motors in Precision Positioning Applications

Typically, two types of brushless linear motors are used in magnetic direct drive technology: ironless linear motors and iron core linear motors.

With iron core motors, the active part (primary part) consists of a sheet metal stack (ferromagnetic steel) with “teeth” and the coils wound around those teeth.

Basically iron core motors provide high force and low cost, but the cyclical variation of moving force within the magnetic field (cogging) has negative side effects on the precision of the positioning device due to limited bearing stiffness. The high attraction forces between the sheet metal stack and the magnets can cause wear of the linear guides and the (moving) mass of the iron core can limit the dynamic performance of the positioner.

The basic version is equipped with an absolute encoder which provides advantages especially in automation applications: No more reference moves required and no more phase finding at power up. Absolute encoders have been improved continuously over the last decade in terms of resolution and cost, however, when it comes to highest possible resolution, incremental encoders still have the edge.

For sub-nanometer resolution performance, PI uses an in-house designed incremental encoder (PIOne).

When it comes to nanopositioning applications, smallest details in the design and assembly of the positioning devices matter. These in include, stress relieved materials, precision machining of essential surfaces / mounting interfaces after the final heat treatment (i.e. anodizing process), assembly and alignment of all mechanical components using state-of-the-art metrology equipment and last but not least, performance testing in a metrology lab with designed to provide the stability and accuracy required in the nano-world.