Positioning control effectiveness of optical knife edge displacement sensor-embedded monolithic precision stage

This paper presents positioning control effectiveness of optical knife edge sensors (OKES) for compact nanopositioning stage applications. The OKES performance was characterized in terms of precision, dynamic characteristics, working range and control effectiveness. The OKES consists of the laser diode, collimation lens, beam splitter, mirror, double-side sharp knife edge, and two lensed photodiodes. The double compound notch type flexure stage was fabricated by additive manufacturing process, stereolithography, and the OKES was successfully embedded into the flexure stage to control the positioning in a compact and efficient configuration. The voice coil motor was aligned along the moving axis of the stage without offset to mitigate Abbe error, and the OKES was setup at the center perpendicular to the moving axis. The positioning of flexure stage was proportional-integral-derivative feedback controlled. As a result, the AM flexure mechanism was successfully position controlled to a margin of 50 nm within a 500 μm range. It was also found that the OKES is sensitive to torsional motion and it can be used for angular motion measurement such as parasitic motions with high sensitivity. Therefore, the OKES and voice coil actuator can be a good alternative to the capacitive sensors and piezoelectric transducers in terms of performance: measuring and actuation range, positioning accuracy, resolution, bandwidth and control effectiveness.