Development of a grating based multi-degree-of-freedom laser linear encoder using diffracted light

A multi-degree-of-freedom laser linear encoder that can simultaneously measure the position, straightness, pitch, roll, and yaw errors of a linear stage is developed. The proposed laser encoder uses the phase and spatial information of diffracted light. When the light is reflected from a grating and diffracts, the relative movement makes the orders of diffracted light have both phase and spatial geometric variations. The proposed encoder design makes the ±1 order diffracted light interfere and outputs pure sine and cosine wave signals for linear displacement. The ±2 order diffracted light spots are received by two quadrant photodiode detectors and analyzed to inversely calculate the other motion errors. The experimental results show that the measurement error is ±0.6 μm for straightness, ±0.8 arcsec for angular error, and ±1.2 μm for linear displacement.