Research On Long-range Nanometer Displacement Measurement By Dual Wavelength And Singal Grating

Displacement measuring technology by grating interferometry, which is one of the technologies that can implement displacement measurement with nanometer resolution, has the advantage of small volume, low cost, simple configuration, easy instrumentation. Compared with the traditional laser interferometer, it provides measurement benchmarks with a real object, has a relatively low request of environmental conditions, and can be stably used with a low zero excursion. Due to the above merits, it can achieve higher accuracy and resolution than geometry moiré, and has been widely used in many fields, such as bio-pharmaceuticals, ultra-precision machining, space science, microelectronics and materials science, etc.Aiming at solving the contradiction between large range and high precision in grating displacement measurement, researches of this dissertation focus on the theory of dual-wavelength single-grating displacement measurement, modeling of theory, design of light path, and signal processing of heterodyne interferometry. The concrete work and innovative ideas achieved in this dissertation are concluded as follows. 1.The dual-wavelength laser is applied into single-grating displacement measuring system for the first time. By combining both technologies, the measuring system introduces to the interference fringes changed with displacement after diffraction, a carrier, which transforms the direct-current signal of the ordinary single-grating measuring system into the alternating-current signal of dual-wavelength single-grating measuring system, and then the anti-jamming ability and stability of the measuring system are largely enhanced.2.Based on the Kirchhoff scalar diffraction theory, the theoretical model of the optical configuration of dual-wavelength single-grating measuring system is established. With an investigation on the mathematical model of the dual-wavelength Gaussian beams and the grating diffraction, the distribution function of the compositive electromagnetism field formed when the dual-wavelength laser irradiates the grating, is derived. Meanwhile, the compositive light field is simulated and analyzed through Matlab programme, and then the cosine signal of the heterodyne interference of grating diffraction is obtained. The feasibility of the method of dual-wavelength single-grating displacement measurement is proved.3.Based on the theoretical model of the optical system, the optical structure of dual-wavelength single-grating measuring system is designed, and choices of the key optics, including laser, beam splitter, polarized beam splitter, and metrologic grating, are discussed. According to the heterodyne interferometric theory, model of errors caused by non-ideal performance of lasers is built, the main factors affecting the error magnitude are analyzed, and the law of the error change is found through simulation.4 . Phase measuring method of filling pulse is studied. While combining frequency-mixing technique of phase locked loop and pulse-filling method, phase demodulation for the heterodyne signal of middle and high frequency is realized. Thus, technology for heterodyne signal processing is developed.5.A digital phase measuring method, which is based on FPGA and PLL and integrates integer and decimal phase measurement, is proposed. It is designed to realize integral period measurement and high-power subdivision of decimal phase. And by phase testing experiments, it is validated that phase measurement precision is above 0.03°and results of integral period are accurate.