Contact-free planar stage using linear induction principle

The feasibility of using a linear induction motor as a driving actuator for a contact-free planar stage is investigated for various situations. In this stage, the features of an induction motor such as no cogging force, ease of workspace enlargement, and a simple metrology process may be advantageous. The induction principle has inherently limited performance. So, a novel control method and a structural modification are suggested to compensate for the poor characteristics of the induction system. First, a modified vector controller, in which the normal force is controlled with direct current (dc) current biased to three-phase current under settled d-axis flux, replaces the general dq-based controller. Secondly, a pair of linear induction motors, in which the traveling direction of the moving part changes not by a phase change of three-phase current but by a difference of thrust forces from each linear induction motor, substitutes for a single linear induction motor to compensate for discontinuity due to the phase shift of alternating current (ac). Therefore, the stage with a dynamic characteristic, such as a double integrator in the planar direction, is preloaded in the planar direction. Finally, the influence of air stiffness on the ripple of normal force due to the ac drive is analyzed for two types of air-supporting mechanisms. All the aforementioned methods are verified experimentally.