On the impact of probing form error on form measurement in Computed Tomography

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1 On the impact of probing form error on form measurement in Computed Tomography Jan Lasson Andreasen 1, Filippo Montanari 2, Leonardo De Chiffre 2 More info about this article: 1 Novo Nordisk A/S, Device R&D, Hilleroed, 3400, Denmark, jlsa@novonordisk.com 2 Department of Mechanical Engineering, Technical University of Denmark, Produktionstorvet 425, DK-2800 Kgs. Lyngby, Denmark, Montanari: montanari93@gmail.com, De Chiffre: ldch@mek.dtu.dk Abstract The work deals with an experimental investigation over the influence of probing form error on form measurements using Computed Tomography. In a first part of the investigation, a 2 mm sphere on a reference ball bar was scanned for probing form error (P Form ) evaluation. A study was carried out involving different scan parameter settings, i.e., magnification, image averaging, gain, number of projections, and position in the measurement volume. Scanning was performed on a Zeiss Metrotom 800 CTscanner. The results show that probing form error will decrease with increased magnification. For a fixed magnification, the probing form error can be reduced by increasing the parameters affecting the scanning time, i.e., image averaging and number of projections. In a second part of the investigation, an industrial injection moulded Cyclic Olefin Copolymer (COC) tubular component was scanned, and roundness was measured of the external and internal cylindrical surfaces showing the impact of magnification on the error of the roundness determined using CT scanning. Reference measurements on the component were carried out using a Zeiss O-Inspect CMM. Both probing form error and measured component roundness decrease with increased magnification. A conclusion from our investigation is that the effect of magnification is important in connection with measurement of form and also dimensions that are influenced by form. An overall conclusion from the investigation is that reducing the scanning time by scanning several objects together is problematic in connection with measurement of form but also in connection with dimensions which depend on form. Keywords: CT-scanning, probing form error, magnification, injection moulding 1 Introduction The work deals with an investigation over the impact of probing form error on the error of form measurements in Computed Tomography. Some publications that consider this topic can be found in [1] - [9]. The paper addresses one of the commonly applied methods to reduce the scanning time of components whereby several parts are scanned together in the same setup. While standards are currently available with guidelines concerning dimensional measurements using CT scanning, there exist no such guidelines related to form measurements [10], [11]. When tactile coordinate measuring machines (CMM), or form testers (FT), are used, the acquired data are generally mechanically filtered by the probe size, e.g., typically 0.3 mm on a CMM and mm on a FT. In CT scanning, the probing size is not constant but changes with magnification, which is connected to the source to object distance, SOD. 2 Investigation of probing form error In a first part of the investigation, a 2 mm sphere on a 14 mm reference ball bar was scanned for probing form error (PForm) evaluation. A study was carried out involving different scan parameter settings, i.e., magnification, image averaging, gain, number of projections, and position in the measurement volume. Scanning was performed on a Zeiss Metrotom 800 CT-scanner. The probing form error was estimated based on a gaussian sphere fit with points applying an outlier elimination filter (95% level of retention). A reconstructed volume of the reference object can be seen in Fig. 1. The results are shown in Fig. 2 indicating that the probing form error decreases with increased magnification and is lowest when the object is placed in the centre of the measurement volume. Probing form error is converging to the value obtained under acceptance test of the CT-scanner, which was 2.1 µm at magnification 23 (PF MPE = ± 4.0 µm). It can be seen that the MPE value underestimates the probing form error at magnifications lower than 7, which is critical because the most commonly used magnification range is between 3 and 6 for typical component sizes in medical device production. 1

2 Figure 1: The 14 mm ball bar with 2 mm spheres used for investigation of probing form error. Left image shows reconstructed ball bar. Right images show point location and variation on a sphere fit before outlier elimination. Figure 2: Probing form error versus magnification investigated for two different sphere positions in measurement volume. Solid black line shows trend line through all the experimental points. Red steepled line shows the error calculated from the MPE value. A particular investigation was carried out at magnification 7, corresponding to SOD 100 mm, on parameters expected to influence the probing form error. The results can be seen in Fig. 3. A variation of approximately 1 µm was experienced in this investigation. Better results in terms of lower probing form error were obtained when increasing number of projections and number of averaged images. The drawback of increasing projections and averaging is that the measurement time will increase from 32 minutes to 140 minutes. 2

3 4 3,5 3,3 Probing form error (µm) 3 2,5 2 1,5 1 0,5 2,9 2,9 2,2 2,3 2,5 2,4 2,2 2,0 2,0 0 Gain 8, 500 Proj, Average 1 Gain 8, 500 Proj, Average 5 Sphere placed in center Gain 2.5, 500 Proj, Average 1 Gain 2.5, 1000 Proj, Average 1 Sphere placed 7 mm off center Gain 2.5, 1000 Proj, Average 5 Figure 3: Probing form error at magnification 7 investigated for two different sphere positions in measurement volume and 5 different combinations of gain, number of projections, and image averaging. 3 Investigation of COC component In a second part of the investigation, an industrial injection moulded tube shaped container made in Cyclic Olefin Copolymer (COC) was scanned, the magnification in CT scanning was varied in the same interval from 2 to 7 as in the first investigation, and diameters and roundness were measured of the external and internal cylindrical surfaces. Reference measurements on the component were carried out using a Zeiss O-Inspect CMM with a Ø3 mm tactile probe, see Fig. 4. Figure 4: Setup for reference measurements on tubular shaped part on CMM. 3

4 a) b) c) d) Figure 5: Top: Image of reconstructed volume of tube shaped part using magnification 7 (a), and 2 (b). Bottom: Internal and external roundness in a section 5 mm from top using magnification 7 (c), and 2 (d). Inner and outer diameters were estimated based on a circle fit with 1000 points, applying an outlier elimination filter with 95% level of retention. For the inner surface, a Gaussian circle fit (GG) and a maximum inscribed circle fit (GX) were made and the roundness was estimated without spline filtering and with a 15 UPR spline filter. For the outer surface, the same parameters were estimated with the exception that here the minimum circumscribed circle fit (GN) is used instead of GX. Two reconstructed volumes of the part at high and low magnification together with roundness plots can be seen in Fig. 5. On Fig. 6 it can be seen that Gaussian diameters are less dependent on magnification than minimum circumscribed (GN) and maximum inscribed diameters (GX), where a variation of approximately 10 µm is observed. The best conformance with CMM measurements are obtained at high magnifications. On Fig. 7 the roundness is plotted versus magnification and here it can be seen that roundness is decreasing with increasing magnification. The largest decrease is in the unfiltered roundness on the inner surface which drops more than 10 µm. When the roundness is filtered with a low pass spline filter of 15 undulations per revolution (UPR) the roundness is seen to be less influenced by the magnification: for the inner surface, the drop is reduced to 5 µm. Moreover the best agreement with CMM measurements is obtained with filtered roundness. It can be directly concluded from the investigations that one of the commonly applied methods to reduce the scanning time of components, whereby several parts are scanned together in the same setup at a lower magnification, is problematic in connection with the measurement of form. 4

5 Fig. 6: Top: Outer diameter versus magnification: red curve and square markers for Gaussian fit (GG) and blue triangular markers for minimum circumscribed fit (GN) Bottom: Inner diameter versus magnification: red curve and square markers for Gaussian fit (GG) and blue circular markers for maximum inscribed fit (GX). Reference results using CMM are shown as line markers on both top and bottom graph Fig. 7: Roundness versus magnification, red curves and markers belong to the roundness of the inner surface while blue markers belong to the roundness of the outer surface. Reference results using CMM are shown as markers without line placed at magnification 1. 5

6 4 Conclusions When carrying out form measurements using CT, the error introduced by the probing system is crucial for the result. For CMM s with CT-probes, the probing form error Pf is quantifiable by measurement on a reference sphere. In the present investigation, the probing form error was measured at different scanner settings. The results showed that the probing form error decreases with increased magnification. An improvement from 12 to 2 µm has been seen by changing magnification from 2 to 7. It was also seen that increasing the measurement time is not significantly reducing the probing form error. In an investigation with measurement of diameters and roundness on a tube shaped plastic container, it was concluded that diameters based on Gaussian fitted circles are not influenced significantly by the magnification applied when scanning. However, when it comes to diameter measurements which are influenced by the form errors (in this investigation minimum circumscribed circle and maximum inscribed circle) a 10 µm variation in result is seen. This variation was also observed when measuring roundness. The application of a low pass spline filter did reduce the variation seen in roundness measurement obtained at different magnification and, moreover, it also made the results agree with CMM measurements. The paper addresses one of the commonly applied methods to reduce the scanning time of components whereby several parts are scanned together in the same setup at a lower magnification. While this approach is not problematic in connection with measurement of dimensions, it can be concluded from the investigation that this is not the case for form measurements. Acknowledgements The valuable assistance of Lorenzo Carli and Maarja-Helena Kallasse from Device Metrology, Novo Nordisk A/S, is acknowledged. References [1] J.P. Kruth, M. Bartscher, S. Carmignato, R. Schmitt, L. De Chiffre, A. Weckenmann: Computed Tomography for Dimensional Metrology, Ann CIRP, 61/2 (2011) p [2] L. De Chiffre, S. Carmignato, J.P. Kruth, R. Schmitt, A. Weckenmann, Industrial applications of computed tomography, Ann CIRP, 63/2 (2014) p [3] S. Carmignato, W. Dewulf, R. Leach (editors), Industrial X-ray Computed Tomography, Springer, [4] C. Siwen, S.Li, Q. Li, W. Li, D. Li, X. Song, S. Gao, Y. Shi, The assessment of industrial CT's probing error, Tianjin University, Proceedings of SPIE - The International Society for Optical Engineering, [5] J. Angel, A. Stolfi, L. De Chiffre, Form measurements in an industrial CT scanner investigated using a polymer step gauge, Proc. 15th Int. Euspen Conf., pp , [6] P. Müller, Coordinate Metrology by Traceable Computed Tomography, PhD thesis, Technical University of Denmark, [7] J. Angel, Quality assurance of CT scanning for industrial applications, PhD thesis, Technical University of Denmark, [8] A. Stolfi, Integrated quality control of precision assemblies using computed tomography, PhD thesis, Technical University of Denmark, [9] F. Montanari, Probing error in dimensional CT-scanning and their impact on form measurements, MSc Thesis, Mechanical Engineering Dept., Technical University of Denmark, Kgs. Lyngby, [10] VDI/VDE 2617 Blatt 13: Accuracy of coordinate measuring machines - Characteristics and their testing - Guideline for the application of DIN EN ISO for coordinate measuring machines with CT-sensors - VDI/VDE 2630 Blatt 1.3: Computed tomography in dimensional measurement - Guideline for the application of DIN EN ISO for coordinate measuring machines with CT-sensors. [11] ISO/CD Geometrical product specifications (GPS) - Acceptance and reverification tests for coordinate measuring machines (CMM) - Part 11: CMMs using the principle of computed tomography (CT) (under development by ISO TC 213). 6