High-resolution position encoder insensitive to misalignment

The operation of optical position encoders relies on careful mechanical alignment of the detection system relative to a scale. We present a novel optical position encoder based on a glass scale, a dedicated photodetector array and a micro- optical imaging system. The complete detection unit is small enough to fit into the housing of the detector heads of standard Moire based incremental position encoders. Operating with a scale of 20 micrometer period, our working demonstrator achieves a resolution of up to 10 nm while offering a tolerance of plus or minus 80 micrometer in the distance from scale to detection system and a high angular mounting tolerance. The interpolation error was experimentally determined to be below 0.1 micrometer for an angular misalignment of plus or minus 12 mrad. The position encoder system is equally well suited for the setup of high- resolution linear and rotary incremental encoders which are employed in precision machine tools.