Construction of a measurement and control system for a sawyer motor-driven planar motion stage

This paper describes a precision positioning system combined with a Sawyer motor-driven stage and a surface encoder. The stage consists of a platen and forcers (linear pulse motors). The platen is a passive steel plate with teeth over the surface. Each of the forcers, which is levitated on the platen as stage moving element by air-bearing, is composed of a permanent magnet and two driving coils to generate X-directional and Y-directional thrust forces against the platen for driving the stage. A surface encoder is constructed for measurement and control of the stage movement. A sine-grid is mounted on the platen and a sensor-unit is mounted on the stage moving element. There are three optical sensors in the sensor unit for measurement of the X-and Y-positions at three separate points. The θ _z displacement of the stage can be calculated from the sensor outputs. A feedback control system is developed for controlling the movement of the stage. Measurement and feedback control experiments are carried out to verify the feasibility of the precision positioning system.