3 November 2003 Novel planar laser encoder system for two-dimensional positioning with ultrahigh head-to-scale alignment tolerance
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Laser encoders overcome the fundamental resolution limit of geometrical optical encoders by cleverly converting the diffraction limit to phase coded information so as to facilitate nanometer displacement measurement. As positioning information is coded within the optical wavefront of laser encoders, interferometry principles must be adopted in the design of the laser encoders. This effect has posed a very strong alignment tolerance among various components of the whole laser encoder, which in turn imposes a serious user adaptation bottleneck. Out of all alignment tolerances, the head-to-scale alignment tolerance represents the most important hindrance for wider ap-plications. This paper presents a novel laser planar encoder, which serves as a two-dimensional position detection apparatus for precision machine applications and can provide a measuring resolution less than 1 nm. Improving the IBM laser optical encoder design by taking into consideration manufacturing tolerance of various optical components, an innovative two-dimensional laser encoder with ultra high head-to-scale tolerance is presented. It was identified that this newly proposed laser encoder design could avoid the effect of differences in polarization diffraction efficiencies for the 2-D grating scale used. Optimizing the system performance by cleverly designing the profile of the 2-D grating scale was also detailed. The effect of non-uniform temperature field within the head-to-scale range that can yield a nonzero initial phase so as to decrease the system measurement accuracy was analyzed. In addition, misalignment of the polarizers located in front the photodiodes were identified to be the main cause for imperfect Lissajous circles, which may lower the measuring resolution when traditional arctangent algorithm was adopted for circular polarization interferometers. The resolution of the newly developed laser planar encoder was verified by experiments and was found to agree well with the theoretical predictions.
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Chyan-Chyi Wu, Chyan-Chyi Wu, Calvin C. Chang, Calvin C. Chang, Ching-Fen Kao, Ching-Fen Kao, Gwo-Sheng Peng, Gwo-Sheng Peng, } "Novel planar laser encoder system for two-dimensional positioning with ultrahigh head-to-scale alignment tolerance", Proc. SPIE 5173, Current Developments in Lens Design and Optical Engineering IV, 517307 (3 November 2003); doi: 10.1117/12.504442; https://doi.org/10.1117/12.504442

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