A new 3-DOF Z-tilts micropositioning system using electromagnetic actuators and air bearings

A new 3-DOF Z-tilts (z, pitch, and roll motion) micropositioning system has been developed. It uses electromagnetic actuators and air bearings. An electromagnetic actuator produces an attraction force between the air bearing and the base plate. An air bearing has the role of suspension and guidance, with a clearance of several tens of micrometers in the z-direction. Therefore, this system has design features of guiding 3-DOF XYθ motion without limiting the plane motion and playing the role of a z-directional position actuator. With the control of current, the equilibrium position between magnetic attraction force and air bearing thrust force can be controlled with inherently infinite resolutions. The theoretical background of an electromagnetic actuator is explained. Then, an air bearing is analyzed in the point of z-directional positioning mechanism. The air bearing can be modeled as a second-order system with parameter variation—stiffness and damping vary with respect to the z-directional displacement. Therefore, a simple robust control algorithm is applied to improve the control performance. With the aid of robust control, this system provides 25 nm positioning resolution over the total range of 40 μm along the z-direction and, accordingly, 0.29 μrad resolution over the total range of 460 μrad in pitch and roll motion.