High resolution planar encoder by retro-reflection

This investigation presents a planar diffractive laser encoder system (PDLENS), which serves as a two-dimensional (2D) position detection apparatus for precision machine applications. Traditional 2D position detection utilizes a pair of linear encoders in crossed construction and so maintaining the perpendicularity between this pair of encoders is difficult. Besides, the rigorous alignment requirements among various components of the encoder system impose a serious user adaptation bottleneck. Of all alignment tolerances, the head-to-scale alignment tolerance is the most important problem for applications. In this work, a 2D grating is employed as the scale and the PDLENS is based on the retro-reflection configuration. Therefore the new encoder can provide good perpendicularity and tolerate larger alignment errors than the conventional encoder does. The grating pitch is 1.6 μm and the period of the output signal is 0.4 μm due to the double diffraction. Electronic interpolation with a factor of 400 leads to a readout resolution of 1 nm. The new encoder and a capacitive sensor were employed to simultaneously measure a circular motion with a radius of 1 μm generated by a piezo stage. Comparing the measured positions, the deviation is less than 30 nm and the repeatability is better than 8 nm