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This paper presents a design study of a four-probe optical sensor head for a three-axis surface encoder with a mosaic scale grating. As the mosaic scale grating, multiple two-dimensional reflective-type scale gratings are aligned in a matrix on a plane so that measurement ranges of the three-axis surface encoder along the X- and Y-axes can be extended. An optical layout for a four-probe unit, which consists of a laser diode and two beam splitters, was designed based on geometrical optics so that four probes aligned in the matrix can be generated. The designed four-probe unit was integrated into the optical sensor head of the three-axis surface encoder. Interference signals generated by the superimposition of diffraction beams from the XY gratings were utilized in the three-axis surface encoder so that the translational displacement motions of the mosaic scale grating along the X-, Y- and Z-axes can be detected simultaneously by each probe in the four probes. Experiments were carried out to verify the feasibility of both the designed four-probe optical sensor head and the proposed concept of the mosaic scale grating for expanding the measurement range of the three-axis surface encoder along the X- and Y-directional primary axes of motion. Measurement sensitivity of each probe was compared, and its compensation was carried out by linear matrix calculations. Measurement resolution and nonlinearity of each probe were also investigated. In addition, XYZ-directional translational motions of a linear stage when a long-range translational motion along the X-axis was given to the mosaic scale grating were measured by stitching the measured results by the four probes.

A surface encoder, which can measure the XYZ-directional displacements, has been developed. The XY-directional measurement range depends on the size of the XY-axis scale grating. A mosaic XY-axis scale grating, which consists of multiple XY-axis grating, has been proposed to expand the XY-directional measurement range of the three-axis surface encoder. And multi-probe surface encoder, which has two-probe, for the mosaic scale grating has been proposed. In this paper, an optical sensor head, which has four-probe for the mosaic XY-axis scale grating, was developed and basic experiment was carried out to confirm the feasibility of the proposed method.

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