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.)−134−Synthesiology - English edition Vol.11 No.3 (2018) measurement based on a coaxial linear displacement method, collaborated with Osaka University. The coaxial linear displacement method has advantages of being less likely affected by specular reflection light, relationship between displacement and output being linear, and precision being unchanging in all measurement ranges. Therefore, there was much expectation for realization, but, as shown in Fig. 1, the realization was hampered by the reduction of precision due to specklesTerm 1 that were characteristics of laser beams.[2] To solve this problem, Okada et al. used a high-density line sensor instead of an area sensor in the photoreceptor as shown in Fig. 2, and created a unique mechanism for rotating the sensor. It was demonstrated that the speckles could be reduced greatly by rotating the line sensor at 200 rpm and conducting space averaging. That is, as shown in Fig. 3, the speckles reduced due to the rotation of the line sensor and the image quality improved, and this allowed the measurement of shapes at precision within 0.1 mm in the measurement range of 150 mm. The light at the end of the tunnel for the road toward realization could be seen.[3]Next, Okada worked on the development of a noncontact 3D measurement device for mirror surface objects that were more difficult than glossy surfaces. Since mirror objects totally reflected laser beams and specular points could not be seen at all, it was extremely difficult to measure surface forms, and while measurement could be done for flat surfaces, there was no measurement device that could measure curved surfaces. Therefore, Okada devised a method for calculating the 3D coordinates of specular points based on a ray tracing method, by capturing the laser reflection light in multiple positions in 3D space by rotating several position sensitive detectors (PSD) arranged in a dome shape. Figure 4 shows the appearance of the mirror-surface object measurement device[5] that was developed and prototyped. By capturing reected laser beams in two places of the 3D space by arranging two sets of four PSDs unevenly in a vertical direction, an equation for laser beams that pass through two points in the 3D space was determined. Then, form measurement became possible by setting the intersection point with the irradiation light as a virtual reection point.[4] A patent was led for this technology and was registered as intellectual property, and was selected as a notable invention by the Agency of Science and Technology in 2000 (Patent No. 317857, 1999.2).As Okada was working on the R&D for new measurement technology to utilize semiconductor lasers in industrial measurement, Okada was consulted by a local steel sheet manufacturer about a device to inspect minute flaws and defects in rolled steel sheet surfaces with high glossiness. This launched us into the development of inspection technology using lasers. The requests from the steel sheet manufacturer were the detection of micro-defects of micron order on the surfaces of high-grade rolled steel sheets, the separation of defects and roll marks, and the distinction of detected defect types.The newly developed laser defect inspection system[5] is shown in Fig. 5. The point of development is the structure for measuring light intensity distribution of the reected scattered light and diffracted light using a planar photodetector placed at a focal position by gathering all the reflected light within the measurement range to a focal point, and using parabolic cylindrical mirrors in the photoreceptor system in addition to Fig. 1 Laser beam ring image by area sensorResting stateRotating stage15003500300025002000Number of pixels of line sensorNumber of pixels of line sensorBrightnessThresholdBrightnessThreshold15003500300025002000Rotating photoreceptor partFig. 3 Effect of line sensor rotationFig. 2 Developed and prototyped measurement system

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