Vol.11 no.3 2019

Research paper : Development and commercialization of laser inspection system to detect surface aws of machined holes (S. OKADA et al.)−138−Synthesiology - English edition Vol.11 No.3 (2018) National Institute of Advanced Industrial Science and Technology (AIST). In 2003, AIST Chugoku set a policy of placing importance on biomass research, the researchers for manufacturing were transferred to other centers, and we stood at a major crossroad of whether to continue or discontinue the R&D for laser defect inspection system right before its realization. When Okada et al. were seeking a solution, President Shitanaka of Sigma who was certain about the future of laser defect inspection strongly requested the continuation of the development, and he was also willing to provide research funds. After discussing with Director-General Yabe (at the time) of AIST Chugoku and the researchers who were transferred to Tsukuba, AIST agreed to continue the R&D based on the funds provided by the company after setting up a collaborative research unit that would be the base of R&D at the Industry Academia Government Collaboration Promotion Division, AIST Chugoku, and we applied for its establishment. There were many conditions for application: there must be requests from many companies; pure private company funds will be provided for three years to fulfill AIST’s rules; there must be a clear research goal, and impact on industry, and the project must be achievable by joint research; and there must be research capacity at AIST. The hurdles to clear were high, but the establishment of a collaborative research unit for laser application functional diagnosis was accepted for three years starting in 2004.The research goal was set as the R&D and product realization of a laser defect inspection system for the inner wall surfaces of automobile cylinder bores, to meet the demands from the automobile manufacturers. The eddy current and optical inspection systems for cylinder bore interior that were commercially available at the time did not satisfy the on-site demands, and a higher-performance and higher-functional inspection system was desired. Figure 11 shows the inspection system of cylinder bore interior that was developed and prototyped through one-year joint research with the basic concept provided by AIST. The key of the development is the structure in which the inspection probe rotating at 1500 rpm is lowered at a steady rate along the central axis of a hole, a semiconductor laser beam formed into a true circle of a diameter of 0.1 mm is irradiated perpendicularly onto the wall surface, the specular reection light, reected scattered light, and diffracted light from the wall surface are collected by optical bers arranged in double concentric circles, light intensity is measured by an optical sensor installed at the other end of the optical ber, and the defects are detected by light intensity change.[7][8] Since the structure of the probe tip greatly affected the performance, much time and effort were needed for repeated experiments done by changing the tip form and end face position of the optical ber. However, through the efforts of the development personnel at Sigma, the collaborative research unit was able to nd the optimal position and form. Also, the probe tip was made removable, and by employing a structure in which the tip could be slid back and forth, the device could measure a wide-range of inner diameters from 40 mm to 150 mm.Figure 12 shows the light intensity of specular reflection light measured by inner optical fibers, and Fig. 13 shows the image of the light intensity of scattered light measured by outer optical fibers, and the light intensity data of one rotation. The inspection target was a cylinder bore with an inner diameter of 60 mm, and the measurement was done in 0.2 mm intervals in both the circumferential direction and axial direction. The measurement points were about 4,000 points per rotation, 600 lines in the axial direction, and total data volume was 2,400,000 points (5 megabytes). A clear difference in light intensity distribution between the specular reection light and scattered light images can be seen.In the two figures, the figures shown at the bottom is the actual value (blue or red line) of each light intensity per rotation, the maximum and minimum of the thresholds automatically calculated using the actual values are shown in yellow and green lines, and the parts that surpass this range at the top and bottom are candidates of defects. The reason the maximum and minimum are set is because the laser beam is scattered and the light intensity is decreased below Fig. 11 Inspection system for inner wall surfaces of machined holesFixed partFixed partProbe structureProbe tipInspection probeAppearance of inspection device

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