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The laser interferometer is widely used in various fields because of its high resolution, high stability, high measurement speed and large-scale measurement capabilities, thus many research groups and equipment manufacturers have devoted time and resources to its development. This study presents an innovative symmetrical double diffraction laser encoder for precision displacement measurement. The system has the advantages of not defocusing during measurement, and can provide long range dual-axis linear displacement and rotation angle measurement. The system consists of two detection configurations, each composed of a double diffraction optical configuration, grating interferometer and phase demodulation system. The light source is passed through a non-polarized beam splitter, diffraction grating and mirror to form a grating interferometer system. The positive and negative first order beams formed from grating diffraction are reflected back through the grating by mirrors, forming a symmetrical double diffraction optical configuration to effectively enhance the system resolution. When the grating moves a corresponding phase shift will be introduced into the signal. Finally, a photodetector receives the signal and the data is analyzed with a self-developed phase demodulation program to obtain the displacement information. By comparing the displacement information of the two axes, rotation information can be obtained via trigonometric calculation. It can be inferred from the measurement principles that the theoretical resolution can be as high as 15 pm. Experimental results demonstrate that for displacement and rotation measurement, the repeatability of the symmetrical double diffraction laser encoder is 5 nm and 35 nrad, respectively. The system has excellent measurement performance, and its simple structure lends to easy setup and calibration.
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