The development of a laser array measurement system for three dimensional positioning testing in machine tool

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1 The development of a laser array measurement system for three dimensional ing testing in machine tool Wenyuh Jywe 1, Fong-Zhi Chen 2, Chun-Jen Chen 2, Hsin Hong Jwo 1, Jhih-Ming Pan 1 1 National Formosa University, Department of Automation Engineering 2 Instrument Technology Research Center, National Applied Research Laboratories 20, R&D Rd. Vi, Hsinchu Science Park, Hsinchu, 300, Taiwan, R.O.C. Tel.: #403 Fax: feet@itrc.org.tw Abstract In this paper, a novel laser array measurement system was established for precision machine calibration, which does not need high accuracy reference parts. The system was established by 3 sensing detector (PSD) and 27 laser diodes. During test, only a laser ray projected on a PSD in each test. The PSD and laser diodes were mounted on the spindle of machine tool and machine tool table respectively by fixtures. The advantages of this system are faster and cheaper than those conventional systems (i.e. laser interferometer and grid encoder). And it can test the 3D simultaneously in the machine tool and CMM. In this paper, the PSDs calibration had completed. And the system also was used to test the ing repeatability for machine tool. The accuracy of the laser array system was about 1 μm. The measurement range of the laser array system was based on the number of the laser diode. If larger measurement range was needed, the measurement range could be enlarged by increased the number laser diode. In this paper, 27 diode was used; the xy plane, yz plane and zx plane had 9 laser diode respectively. The measurement range of this system is 30mm 30mm 30mm. Keywords: Repeatability test, machine tool calibration, PSD 1. Introduction The performance testing of a precision machine is very important. Every company must prove the accuracy, repeatability and geometric errors of its products by performance testing. In general, the laser interferometer and planar encoder was used to test the machine tool and CMM, their accuracy are very high in measurement. However, they cannot measure three dimensional (3D) s simultaneously and they are very expensive. Beside of the laser interferometer and planar encoder, the ball plate, hole plate or step stage were also used to test the 3D ing performance of the machine tools or CMM. But the high accuracy reference parts are needed in these systems. Moreover, implementing one full test will spend much time by these conventional methods. The ISO and ANSI/AMSE also provide some test standard for the performance testing of a machine, such as ISO [1] and ANSI/ASME [2]. Tlusty [3, 4] built some systemized test procedures for a machine tool in 1959, and since then many test methods for different test items have been developed and published. The ing repeatability test, contouring test, geometric errors measurement and spindle test are frequently used by industry, but, the 3D test is not normally by a machine tool company because it is complex requires a great deal of time. In general, the coordinate measurement machine (CMM) must test its 3D ing performance (space al error). Because of the CMM frequently are used to measure the 3D profile of work piece. In 1991, Zhang and Zang [5] used a 1 dimensional (1D) ball array to calibrate a machine s geometry. In 1991, Lingard et al. [6] used the laser interferometer measurement system with a CMM to calibrate the length-bar and step-stage calibration. In 1995, Ouyang and Jawahir [7] used a gauge block to calibrate a CMM before the CMM was used to calibrate a ball array. In 1997, Ling et al. [8] developed a CCD based multi-degree-of

2 freedom measuring system for simultaneous measurement of the straightness, pitch, yaw and roll errors of a moving CMM. In 2000, Okafar and Errekin [9, 10] used a laser interferometer to characterize the accuracy of a vertical machine. In addition, Che and Ni [11] presented a ball-target-based method for the extrinsic calibration of a 3D, multiple-axis, structure-light (laser-stripe) scanning system. In 2002, Takatsuji et al. [12] analyzed the uncertainty of the calibration of a geometrical gauge. In 2002, Osawa et al. [13] calibrated a ball-plate by using a ball step-stage and a interferometric stepper. In 2002, Lei and Hsu [14] tested the accuracy of a five-axis CNC machine tool with a 3D probe-ball. In the above studies, most tests of the CMM and machine tool were based on a ball plate, step stage and hole plate etc. However, a great deal of time was needed to calibrate the reference object. Therefore, this paper presents a laser array measurement system to test the 3D performance of a machine tool. This method doesn t require the calibration of the s of each laser. Consequently, it is very easy to use for calibrating a machine tool and CMM. 2. Principle of the measurement system The Laser array measurement system include 3 PSD (UDT-SC10D), 3 signal conditioning of the PSD (On-Trak OT-301), 27 laser diodes, 1 A/D card (Adventech PCI-1716) and 1 personal computer (PC). Its photograph shows in Fig. 1. The 3 PSDs were mounted on the spindle of machine tool by a fixture. The PSD is a sensing detector which can sense the of a laser ray when it is projected within its measuring range. The output of a PSD is the optical circuit which varies with the variation of the of the laser ray. After signal processing by the signal conditioning of the PSD, the signal of the PSD becomes voltage. Therefore, the signal of a PSD can be recorded by PC through the A/D card. The relative of 3 PSD and 27 laser diodes were fixed because each PSD and laser diode was mounted on a spindle fixture and aluminum fixture separately. Each laser diode was fixed on the machine tool s table by a fixture and projected the laser ray onto each PSD. The laser diode fixture of xy plane, yz plane and xz plane has 9 laser diodes separately. But only a lasers ray projected onto a PSD, other laser ray didn t project on any PSD. When the machine tool s table or spindle was moved, the of the laser ray would change. In order to apply this measurement system to test the repeatability of the machine tools, each PSD must be calibrated. Because the laser interferometer has high accuracy ( 0.1 m ), it was used to calibrate every PSD. In the PSD calibration process, the PSD was fixed on the spindle, and each PSDs calibration process was divided into two parts. The first part was the x-axis calibration and the second part was the y-axis calibration. The 3 PSDs were calibrated separately because each PSD had one signal conditioning whose amplifier gain was not the same as the others. After calibrating each PSD, the PSD was mounted on the machine tool s spindle. The of this machine tool s table was recorded when the laser light was on the center of the PSD. The center of the ith PSD is defined as Pi ( x, y, z ).Then the recorded s were used to program the NC (numerical control) code. In this paper, the path can be written as follows: P1 P2 P3 P6 P5 P4 P7 P8 P9 The above arrangement of each laser diode is shown in Fig. 2. When the laser ray was projected to PSD, the spot of the laser ray on the PSD was recorded by the PC. During the repeatability test, the each laser light moved along the center of each PSD. The PC continually recorded the output of the PSD signal conditioning. Following the above path, the machine tool might be moved 40 times. Consequently from the 40 records, the repeatability performance of the machine tool or CMM could be evaluated easily. In the repeatability test, the number of implementations is decided by the user. In the laser array measurement system, the relative of each PSD and laser diode was not necessary to known, because the system was used to test the repeatability of the machine tool or CMM. The repeatability tests needed to show only the difference between each test, not the of the laser spot on the PSD. Ideally, the laser ray would project 2-044

3 onto the same during the repeatability test because the NC code was the same during the test. However, each test result will not be the same because every machine has errors. Fig. 1. Photograph of laser array measurement system. P1 P2 P3 P4 P5 P6 P7 P8 P9 Y X Fig. 2. Arrangement of laser diodes in repeatability tests. 3. Experiment and discussion The interferometer can only measure one dimensional movement. Therefore, only a single axis of the PSD can be calibrated at one time calibration. At first, the x-axis PSD xy was calibrated. The laser light was moved to the center of the PSD by moving the machine tool s table. Then the readout of the laser interferometer was reset to zero. The outputs of the PSD and laser interferometer were recorded for every 1000 m increment of the x-axis from m to 4000 m. In each, samples were acquired with 1 khz. From the samples, the accuracy and standard deviation could be calculated. When the calibration of the x-axis of the PSD plate was completed, the PSD had to be rotated 90 degrees along the z-axis. Then the calibration of the y-axis of the PSD was implemented one time. The complete calibration of PSD xy, PSD yz and PSD xz required 6 implementations (3 times for the x-axis and 3 times for the y-axis). Figure 3-5 show the calibration results of PSD xy PSD yz and PSD xz. In the calibration results, the linear least square method was used to fit the calibration results. They are also listed in Fig Conventionally, a repeatability test is implemented by a laser interferometer, but a laser interferometer is expensive and can only test the repeatability for one axis at a time. Also, setting up a laser interferometer requires a great deal of time. The laser array measurement system is easy to set up, low cost and can measure the three dimensional each time. In begin of a repeatability test the user of this system needs to record the each of a machine tool when the lasers are projected onto the center of each PSD. Then the user uses the recorded to program the NC code and uses the NC program to implement the repeatability test. The of the laser diodes don t need to be calibrated because this system is used to test the repeatability of the machine tool, not to calibrate the machine tool. Figure 1 shows the setup of this system on a machine tool. The repeatability test was implemented on a machine tools which had three linear encoder (for the x, y and z axis). The repeatability test report of the machine tool was about 1 m. In order to test machine tool, the xy plane was set parallel to the XY plane of machine tool, and machine tool moved along the x-axis and y-axis. After the XY plane test. The repeatability test results of machine tool are shown as Figure 6. The repeatability of the x axis, y axis and z axis were about 0.4 m, 2-045

4 0.5 m and 0.6 m respectively. These results are similar to those reported for machine tool when tested by the laser interferometer. The other repeatability results are shown as Table 1. Fig. 3. Calibration result of xy palne. Fig. 4. Calibration result of yz plane. Fig. 5. Calibration result of xz plane. Fig. 6. Results of repeatability in P1. Table 1. Results of repeatability test. Number of point X-axis Y-axis Z-axis P1(x,y,z) ±0.4 ±0.5 ±0.6 P2(x,y,z) ±0.8 ±1.2 ±0.4 P3(x,y,z) ±1.0 ±0.8 ±0.5 P4(x,y,z) ±0.6 ±0.6 ±0.4 P5(x,y,z) ±1.6 ±0.4 ±0.4 P6(x,y,z) ±0.8 ±0.3 ±0.4 P7(x,y,z) ±0.4 ±0.3 ±

5 P8(x,y,z) ±0.4 ±0.4 ±0.4 P9(x,y,z) ±0.9 ±0.7 ± Conclusion This paper describes the establishment of a novel measurement system, the calibration of the PSDs, and completion of a repeatability test. The conclusions can be listed as below: 1. Because this system was established by many laser diodes, the measurement range can be increased by increasing the numbers of the laser diode. 2. This system is easy to set up and low cost. 3. The calibration results of each PSD show the accuracy of this system is about 1 m. 4. The repeatability tests were implemented for machine tools. 5. This system can be used not only to test the repeatability one axis but also to test the 2 dimensional or 3 dimensional path repeatability of a machine tool. 5. Acknowledgements Grateful appreciation is given to the National Science Council of Taiwan for financial support under grant number (NSC E MY3). References 1. ANSI/ASME. ANSI/ASME B : Methods for Performance Evaluation of Coordinate Measuring Machines. American Society of Mechanical Engineers ISO 230-2:2006: Test code for machine tools -- Part 2: Determination of accuracy and repeatability of ing numerically controlled axes. International Organization for Standardization J. Tlusty. System and Methods for Testing Machine Tools. Microtechnic. 1959, vol. 13, pp J. Tlusty. Specification and tests of Metal Cutting Machine Tools. University of Manchester, Revell and George Ltd G.X. Zhang and Y.F. Zang. A method for machine geometry calibration using 1-D ball array. Ann. of CIRP. 1991, 40, pp P.S. Lingard, M.E. Purss, C.M. Sona and E.G. Thwaite. Length-bar and step-stage calibration using a laser measurement system with a coordinate measuring machine. Ann. of CIRP. 1991, vol. 40, No.1, pp J.F. Ouyang and I.S. Jawahir. Ball array calibration on a coordinate measuring machine using a gauge block. Measurement. 1995, 16, pp C.C. Ling, C.K. Chou, Y.C. Huang and K.C. Fan. CCD-based CMM geometrical error measurement using fourier phase shift algorithm. Int. J. Mach. Tools Manufact. 1997, 37, pp A.C. Okafor and Y.M. Errekin. Vertical machining center accuracy characterization using laser interferometer Part1. Linear al errors. Journal of Materials Processing Technology. 2000, 105, pp A.C. Okafor, Y.M. Errekin. Vertical machining center accuracy characterization using laser interferometer Part2. Angular errors. Journal of materials Processing Technology. 2000, 105, pp J. Ni and C. Che. A ball-target-based extrinsic calibration technique for high accuracy 3-D metrology using off-the-shelf laser-strip sensors. Precision Engineering. 2000, 24, pp T. Takatsuji, S. Osawa and T. Kurosawa. Uncertainty analysis of calibration of geometrical gauges. Precision Engineering. 2002, 26, pp S. Osawa, T. Takatsuji, H. Noguchi and T. Kurosawa. Development of a ball step-stage and an interferometer stepper used for ball-plate calibration. Precision Engineering. 2002, 26, pp W.T. Lei and Y.Y Hsu. Accuracy test of five-axis CNC machine tool with 3D probe-ball. International Journal of Machine Tool & Manufacture. 2002, 42, pp