Recent progress on modeling and control of reluctance actuators in precision motion systems

Reluctance actuators (RA) are a type of electromagnetic actuator that offers high forces for short-range motions. The RA takes advantage of the electromagnetic reluctance force property in air gaps between the stator core and mover parts. The stator generates a magnetic flux that produces a magnetic attraction force between the stator and the mover, where the output force is dependent on the air gap displacement nonlinearly. It is demonstrated that the RA can produce a force that is effective and suitable for millimeter- range high-acceleration applications. One application for the RA is the short-stroke stage of photolithography or lithography machines, for example. The RA is available in a wide variety of configurations, such as C-Core, E-Core, Maxwell, and Plunger-type designs. The RA requires precise dynamic models and control algorithms to help linearize the RA for better control and optimization. Some nonlinear dynamics include magnetic hysteresis, flux fringing, and eddy currents. The RA is shown to have a larger force density than any other traditional actuator designs, with the main disadvantage being the nonlinear and hysteresis nonlinearities, making it difficult to control precision motion applications without proper dynamic and control models in place. This review documents currently available knowledge of the RA such as available applications, configurations, dynamic models, measurement systems, and control systems for the RA.