Magnetically Levitated Linear Drive Using an Active Gravity Compensation Based on Hybrid Shape Memory Actuators

This article presents a magnetically levitated linear drive for precision engineering with integrated active gravity compensation. The gravity compensation combines permanent magnets with novel hybrid shape memory actuators. The magnetically levitated system uses a homopolar linear drive as well as nine reluctance actuators to levitate a passive armature with no mechanical connection between the stator and armature. The mechanical design is based on geometrically decoupled horizontal and vertical guidance axes. A decoupled Cartesian control scheme is developed, which compensates for the actuator and system nonlinearities in order to achieve high bandwidths. The active gravity compensation adjusts the air gap between the permanent magnets and armature to the required gravity compensation force. This helps to keep the dissipated power and temperature increase in the magnetically levitated drive low (and thus, system precision high). To avoid the generation of particles in clean room applications, the actuators positioning the permanent magnets are solely based on solid-state effects: thermal shape memory alloys actively generate motion, while passive magnetic shape memory alloy elements realize a multistable behavior based on their internal twinning force. The resulting actuator is compact, energy efficient, and abrasion-free. The implementation of the gravity compensation in the levitated linear drive is verified and the functionality of the system is demonstrated in experiments.