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Research paper : High accuracy three-dimensional shape measurements for supporting manufacturing industries (S. Osawa et al.)−103−Synthesiology - English edition Vol.2 No.2 (2009) mechanical error of the tracking system by using a hemisphere for the mirror to scan the laser. Normally, the two axes for scanning the laser in horizontal and vertical directions must be adjusted accurately, and an advanced skill is required for this adjustment. In our device, three spheres are arranged at 120 ° intervals, and hemispheric mirrors are fixed to the three spheres to create a high-precision laser scanning mechanism using low-cost mechanical elements[16]. Upon comparing this laser tracker with high-precision CMM, it was confirmed that the mechanical precision of the system itself was 0.3 µm or less[17]. The geometrical error of the CMM was calculated and compared using a ball plate and a laser tracker, and the values matched within 2 µm in the measurement space of 300 mm cube[18].When the user uses the laser tracker, the ease of handling is important. Therefore, we achieved downsizing and weight reduction by using a spherical motor that was developed by the Intelligent Systems Research Institute, AIST[19]. This is a good example of a research result produced by a fusion with other disciplines. With this technological development, high-precision calibrations of large CMM and hand coordinates of industrial robots can be conducted.Currently, discussions of standardization have been started for the assessment of the laser tracker in ISO meetings, and we plan to contribute to its standardization through our knowledge and experience gained in this technological development.5.2 High-precision measurement by CMMIn industries such as mold making where highly precise manufacturing is done, assessment at higher precision than the innate precision of the measurement machine may be required occasionally. Although normally such assessment is impossible, it is possible to conduct higher precision measurement by special arrangements and procedures that will mutually cancel out the errors of the measuring machine. The inverse method used for calibration of a ball plate is one example. AIST developed such high-precision measurement technology, and we shall present the result for measuring a cylinder as an example.As shown in Fig. 15, the circularity of a cylinder is measured and assessed by conducting measurement at eight equal-interval points. After the first measurement, the workpiece is rotated 45 degrees for the second measurement. It is again rotated 45 degrees, with measurements for total eight positions. When the average of the eight measurements is calculated, geometrical error, offset error for the two styli, and effects of probe directionalities can be cancelled out. This method is called the multiple measurement method, and can be used for precise measurement of rotationally symmetrical shapes[20]. For calibration of standards, AIST is planning high-precision calibration experiments for such applications. This measurement method can be applied in sites of production, and we plan to disseminate this knowledge widely through regional public laboratories.6 Future of three-dimensional shape measurement6.1 Measurement standards and standardization for digital engineering[21]As shown in Fig. 1, in digital engineering where the entire process of design, manufacturing, and assessment is done by digital data, it is necessary to compare the CAD design data and the actual measured results. CAD data has planar information, and the volume of information from discrete measurement data of a conventional CMM with a tactile probing system is insufficient. Therefore, noncontact CMM is now being used, since it can obtain measurements for high-density multiple points in one shot. Previously, for noncontact CMM, the manufacturers were conducting precision assessments and providing precision guarantees using their own standards, and there was no system based on common assessment methods. Hence, there was no uniform index when the user was purchasing a CMM, and it was not possible to determine whether the precision of the instrument was as stated in the catalog. Therefore, Fig. 14 Laser tracking interferometric distance meter.Quartered photo diodeStepping motorLaser distance meterHemispheric mirrorProbe 2Probe 1Measurement point by probe 2Measurement point by probe 1Probe 2Probe 1Position 2Position 145°ZYXZYXPosition 4,5,6,7,812345678123456781Fig. 15 Multiple measurement method of cylinder.

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