A dual-mode surface encoder for position measurement

A surface encoder incorporating an angle grid and two two-dimensional (2D) slope sensors is presented. The angle grid is a plate inscribed with periodic sinusoidal waves in the X and Y directions as a position reference, and the position of the plate is detected uniquely using two slope-sensor units. One of the slope sensors is fitted with a beam scanning function to probe and read the angle grid. The system provides two measurement modes: a high-speed measurement mode that measures the XYθz position at a fast sampling rate, and a multiple degree-of-freedom (DOF) measurement mode that measures the XYθxθyθz position at a lower sampling rate by probe scanning. The linearity, cross-talk and resolution of a prototype encoder system are evaluated, and the system is shown to be acceptable for application as a precise dual-mode measurement scheme for a planar stage.