Experimental study on performance verification tests for coordinate measuring systems with optical distance sensors

Transcription

1 Experimental study on performance verification tests for coordinate measuring systems with optical distance sensors Simone Carmignato University of Padova, DTG, Stradella San Nicola 3, Vicenza, Italy ABSTRACT Optical sensors are increasingly used for dimensional and geometrical metrology. However, the lack of international standards for testing optical coordinate measuring systems is currently limiting the traceability of measurements and the easy comparison of different optical systems. This paper presents an experimental investigation on artefacts and procedures for testing coordinate measuring systems equipped with optical distance sensors. The work is aimed at contributing to the standardization of testing methods. The VDI/VDE :2005 guideline, which is probably the most complete document available at the state of the art for testing systems with optical distance sensors, is examined with specific experiments. Results from the experiments are discussed, with particular reference to the tests used for determining the following characteristics: error of indication for size measurement, probing error and structural resolution. Particular attention is given to the use of artefacts alternative to gauge blocks for determining the error of indication for size measurement. Keywords: Optical distance sensors, Coordinate Metrology, Performance verification 1. INTRODUCTION The use of optical measuring systems for industrial quality control and the integration of optical sensors into Coordinate Measuring Machines (CMMs) are getting more and more common, due to several advantages of optical measuring techniques respect to contact probing systems. The main benefits of optical measuring technologies in industrial metrology concern the measurement speed and the suitability for in-process control in production. The increasingly complexity of the industrial components often require thousands or even millions of points to obtain the accurate model of a workpiece; in this case the time needed to digitalize the entire part by contact probing could grow to hours or sometimes days, while non-contact measuring systems can be considerably faster. Furthermore, the use of non-rigid materials in today s products can be a limit for traditional contact probes: in fact, measuring the surface of delicate or flexible parts with a touch probe, the part could be damaged and the measuring accuracy affected by incidental deflections. There are a large number of different types of optical distance sensors on the market with different interfaces, characteristics and performances [1]. The most common types of sensors are based on the following principles: laser triangulation (1D or 2D), holographic conoscopy, image auto-focusing, laser focusing, chromatic focusing, confocal microscopy, white light interferometry, structured light projection, stereo photogrammetry. Each type of sensor can be influenced in a different way from several different error sources. The principal influencing quantities can be classified as follows: - Environmental error sources: illumination, temperature, humidity, dust, vibrations; - Properties the object: surface finishing, colour, opacity, translucency, reflectivity, accessibility; - Measuring strategy: measuring field, calibration procedure, coordinate system alignment; - Hardware components: optical sensor, geometrical errors of positioning system, mechanical stability; - Data processing: filtering, algorithms implementations, registration of multiple views; - Extrinsic factors: operators, surface cleanliness, clamping system. One of the main obstacles to a wider use of optical sensors and to their integration on CMMs can be found in the lacking of internationally accepted specification and verification rules. International standards regarding performance verification of optical systems are still missing. The existing standards related to contact probing CMMs cannot be applied directly to optical systems, due to the different error sources influencing the measurement process. Three-Dimensional Imaging Metrology, edited by J. Angelo Beraldin, Geraldine S. Cheok, Michael McCarthy, Ulrich Neuschaefer-Rube, Proc. of SPIE-IS&T Electronic Imaging, SPIE Vol. 7239, 72390I 2009 SPIE-IS&T CCC code: X/09/$18 doi: / SPIE-IS&T/ Vol I-1

2 Internationally accepted testing procedures are essential not only to allow the users to easily compare different measuring devices, but also for taking full advantage of optical sensors capabilities [2]. Furthermore, standardization is needed to protect the manufacturers from technical and commercial risks, reducing the sensors integration costs and giving the right mean to guarantee the overall system performance [3]. In the following sections, procedures and artefacts for testing coordinate measuring systems with optical distance sensors will be discussed. Particular focus will be put on the guideline VDI/VDE [4], which is expressly dealing with performance verification of measuring systems with optical distance sensors. 2. STANDARDIZATION OF TESTING PROCEDURES The lack of internationally recognised standards for acceptance and reverification tests for optical coordinate measuring systems causes difficulties in comparing different measuring systems and in evaluating the accuracy of optical measurements. The current status of published standards and guidelines for coordinate measuring systems is shown in Table 1. As visible from the table, there are not international standards available for testing optical coordinate measuring systems. Table 1. Standards and guidelines for testing coordinate measuring systems. Subject Acceptance and reverification tests Characteristics, components of the measur. deviation Nomenclature, terms and basic definitions Standards and guidelines subdivided by type of coordinate measuring system Tactile CMMs Optical systems VDI/VDE VDI/VDE 2634 OSIS WG3 ISO * ASME B VDI/VDE , -2.3 ISO , -4, -5-6 * VDI/VDE , -3, -4 ISO * VDI/VDE VDI/VDE DIN OSIS WG3 VDI/VDE OSIS WG3 Measuring uncertainty GUM * ISO , -4 * *International standards. GUM * In its present form, the ISO series of standards provides acceptance and reverification tests that are intended for tactile CMMs. Such tests are not directly applicable to optical measuring systems, mainly because optical sensors are influenced by different error sources respect to tactile measuring techniques. The German guideline VDI/VDE [4], proposes an adaptation of the ISO [5] tests for coordinate measuring systems equipped with optical distance sensors. Such guideline is already applied by several manufacturers and users of optical measuring systems and is considered as one of the fundamental documents for the development of a future new part of the ISO series for optical sensors [6]. Another important document that is already applied by several manufacturers for performance verifications of optical measuring sensors and systems was published by OSIS WG3 [7]. Both OSIS WG3 and VDI/VDE documents are briefly discussed in the following. 2.1 OSIS WG3 Driven by the urgent need of standardization, the International Association of CMM Manufacturers IACMM [8] has carried out the cooperative Project OSIS (Optical Sensor Interface Standard [9]) with the main goal to realize the first official version of a common interface standard. OSIS is very significant for performance verification of optical systems because it takes into consideration the existing national and international standards for verification of coordinate measuring systems and integrates or adapts them where necessary [3, 10]. OSIS has been structured into three subworkgroups, dealing with different aspects of the integration of optical sensors: WG1: Mechanical / electrical interface; WG2: Data integration (Software interface); SPIE-IS&T/ Vol I-2

3 WG3: Specification, classification and performance verification. The OSIS WG3 documentation on specification and verification [7] is of particular interest for the topic of this paper. Such documentation is based on the following requirements: - A four-shells model approach is adopted for performance verification. The four-shells are four levels of verification: (1) performance verification of the Optical Probing Sensor (OPS) itself, (2) performance verification of the OPS integrated on the positioning system, (3) performance verification of the entire measuring system, (4) performance verification of the measuring system when performing specific measurements close to the actual application. The idea is to provide a systematic approach and a set of tests for all the four levels. - The standards for systems specification shall be adopted wherever possible in order to be compatible with internationally standardized or accepted procedures. In particular the series ISO for performance verification of mechanical CMMs and the German guidelines VDI/VDE 2617 have to be taken in consideration; - The additional specification and verification rules must be feature-related, in order to relax the need to be sensitive for each and every uncertainty influence; - The additional rules must cover virtually any optical sensor type, and must as well be open for future ones and for any new measurement task. The adaptation can be simply done by specifying an adequate task-related reference artefact; The testing procedures developed by OSIS WG3 have been already investigated in a previous work [11], in which each procedure was experimented with different optical sensors. Results from such experimental investigation demonstrated the significance of the four-shell approach, even though some modifications should be implemented on the procedures in order to improve them. For instance, the repeatability and reproducibility tests were found to be a valid method to compare performance of different integrated sensors; however, since outliers are not removed in the data evaluation, outliers can consistently affect the calculation of the characteristics. In addition, due to the number of repetitions and related time of execution, these tests are very time consuming and hence they can hardly be accepted in industry to test the sensors (e.g. experiments revealed that the complete test of a video sensor can take more than 20 hours). 2.2 VDI/VDE The German guideline VDI/VDE :2005, proposes an adaptation of the ISO tests for coordinate measuring systems equipped with optical distance sensors. While ISO is expressly written for CMMs with tactile probes only, the VDI/VDE guideline provides the necessary additional specifications for the use of optical sensors: - artefacts to be used as an alternative to gauge blocks; - comparability of the characteristics when different artefacts are used (e.g. ball bars or other artefacts with spherical surfaces); - comparability of the characteristics when different probing strategies are used for different sensors (e.g. different points number, differences in coverage of the elements to be probed, use of swing-and-tilt equipment or other accessories); - definition of the characteristics for different operating conditions; - regulation on how to take into account influencing variables such as environmental factors, mathematical filters, and surface properties of the artefact. According to VDI/VDE , the two principal characteristics to be tested are: (1) probing error and (2) error of indication for size measurement. Such two characteristics are briefly described in the following. 1) Probing error: According to ISO , the probing error characterises the three-dimensional errors of the entire system consisting of coordinate measuring machine, optical distance sensor and accessories (such as articulators) within a very small measurement volume. In VDI/VDE , a distinction is made between the probing errors for the form (PF) and for the size (PS), where: SPIE-IS&T/ Vol I-3

4 - PF is the range of radial deviations between the measurement points and the calculated regression sphere. This is identical to the difference between the maximum and minimum distances of probing points from the centre of the regression sphere. The regression sphere is determined according to the least-squares method, with the radius being unrestricted. - PS is the error calculated in the diameter of the test sphere. This error is obtained from the difference between the diameter of the test sphere as measured and the calibrated diameter of the test sphere. 2) Error of indication for size measurement: This characteristic evaluates the three-dimensional errors of the entire measuring system (positioning system plus measuring sensor) within the whole measurement volume. According to ISO , the error of indication for size measurement, E, is the error with which lengths on calibrated artefacts can be measured using a CMM if measurement is performed by bidirectional probing of two opposite points on nominally parallel surfaces, perpendicular to one of the two surfaces. The test length is the distance between these two points. The artefacts shall be measured along four body diagonals and three further orientations chosen by the user. In each position, five test lengths shall be measured three times each. Concerning the artefacts to be used, VDI/VDE allows not only the use of gauge block, but also of ball bars. With optical sensors, ball bars can be much more easy to use respect to gauge blocks: the distance between spheres centres is easier than the distance between parallel faces because typically it does not need articulating system. When using ball bars, it shall be noted expressly that the error of indication for size measurement differs from the sphere-distance error, SD, introduced in VDI/VDE In fact, the probing error is not included completely in the sphere-distance error; this is due to averaging effects in the calculation of the regression sphere from the large number of permissible measurement points. Therefore, the sphere-distance measurement is not comparable to bidirectional point-to-point measurements using gauge blocks. For this reason, when using ball bars, VDI/VDE states clearly that the contribution of the probing error shall be added separately to errors of indication for size measurement calculated from more than two probing points per length. An additional characteristic that should always be tested is structural resolution (lateral resolution). This characteristic is briefly described in the Appendix A of VDI/VDE Structural resolution characterises the smallest structure perpendicular to the direction of measurement that is measurable with the specified probing error. Structural resolution is not already included in the probing error test. For instance, with increasing low-pass effect of filtering, the probing error decreases while the structural resolution deteriorates. Structural resolution must, therefore, be considered and specific artefacts have to be used in order to determine this characteristic. 3. TESTING ARTEFACTS For testing optical sensors, calibrated artefacts are needed. In order to perform valid performance verification tests, the properties of the artefacts surfaces should have no significant effect on the characteristic to be determined. Surfaces with an insignificant influence on the measurement results are called cooperative surfaces. For many optical systems, including triangulation sensors, the surface of the artefacts needs to be a diffuse scattering surface (ideally a Lambertian surface). In such cases, artefacts with optically rough surfaces are needed [12]. Within this work, several artefacts have been developed for testing optical distance sensors. Figure 1, for example, show a ball-bar that has been realized for testing laser scanners (2D triangulation laser sensors); this artefact allows the determination of the error of indication for size measurement, according to VDI/VDE The spheres of the ballbar are made of ceramic and their surface have a roughness Ra of about 1µm. Several other spherical artefacts have been developed for testing the probing error. Different types of material can be used in this case, for example stainless steel with various surface treatments. A different kind of artefact is shown in Figure 2. This artefact has been developed for testing the structural resolution according to VDI/VDE The artefact is made of stainless steel, with rough and flat surfaces. It includes a 1 mm step with sharp edges. SPIE-IS&T/ Vol I-4

5 Fig. 1. Ball-bar with six ceramic spheres. The spheres have diameter of 10 mm and distance between spheres of 15 mm. Fig. 2. Artefact developed for testing the structural resolution of optical distance sensors. The artefact features a 1 mm step with sharp edges. 4. EXPERIMENTAL INVESTIGATION ON VDI/VDE An experimental study was carried out on artefacts and procedures proposed by the guideline VDI/VDE This investigation is aimed at contributing to the standardization of verification methods and determining the applicability of the proposed artefacts and procedures, with particular focus on limitations arising in the implementation of tests. The work was carried out using a 3-axis coordinate measuring system equipped with a 2D laser-triangulation sensor (laser scanner). The experiments included the implementation of the principal tests proposed in the VDI/VDE guideline: (1) test for determination of the probing error and (2) test for determination of the error of indication for size measurement. The results from such tests are briefly presented and discussed in the following, together with some consideration on verification of the structural resolution. In addition, at the end of this section, the characteristics determined by the performance verification tests are compared with the values of measurement uncertainties for specific measurement tasks that are typically performed with the measuring system under testing. 4.1 Probing Error The tests for the determination of the probing error were performed on a calibrated sphere with nominal diameter D = 22 mm, according to VDI/VDE The investigation revealed that the values obtained for the probing error are particularly influenced by two factors: - value of the maximum slope angle α (see Figure 3), which is the cone angle of the spherical bowl containing the measured points considered for determining PF; - algorithms and filtration methods employed for data elaboration. SPIE-IS&T/ Vol I-5

6 Fig. 3. Schematic illustration showing the cone angle α on a calibrated sphere. This angle delimits the area that is measured on the sphere during the test for determining the probing error. The red dots schematize the points measured by the probing sensor. Table 2 illustrates the influence of the maximum slope angle α on the probing error PF. From this and other similar tests, it can be inferred that the probing error is strongly dependent from the angle α chosen for the test. Table 2. Variation of the probing error PF depending from the chosen value of the angle α. Angle α Probing error, PF 20 deg mm 40 deg mm 60 deg mm 80 deg mm The influence of the algorithms and filtration methods employed for data elaboration is clearly visible from the comparison of Figures 4 and 5. Figure 4 shows the data obtained measuring the sphere with the laser scanner using a simple elaboration algorithm, without further filtration. Figure 5, instead, show the data obtained when using a different elaboration algorithm, including linear filtering. The probing error determined in the first case is PF 1 = 0.18 mm, while in the second case is PF 2 = 0.06 mm. In both cases the probing error PS remains well below 0.01 mm [mm] Fig. 4. Measurement data obtained using a simple elaboration algorithm, without further filtration. The colour map shows the difference from the measured data and the regression sphere recalculated according to the least-squares method. In this case the value of PF is 0.18 mm. SPIE-IS&T/ Vol I-6

7 [mm] Fig. 5. Measurement data obtained using an elaboration algorithm including linear filtering. The colour map shows the difference from the measured data and the regression sphere recalculated according to the least-squares method. In this case the value of PF is 0.06 mm. As suggested in the Appendix A of VDI/VDE , the structural resolution of the sensor shall always be determined additionally to the probing error. In general, filter methods will impair structural resolution while at the same time improving the probing error. Structural resolution can be tested by measuring a step artefact, as the one shown in Figure 2. An example result of the implementation of a structural resolution test is shown in Figure 6. In this case, from the measured profile (see Figure 6-right), a structural resolution of about 0.1 mm is determined according to the method described in VDI/VDE Further details on structural resolution tests are given in the previous paper [13] Z (pm) Z (pm) Fig. 6. Left: nominal profile for the step artefact shown in Figure 2. Right: measured profile obtained with the laser scanner; from the measured profile the actual structural resolution can be calculated as described in VDI/VDE App.A. 4.2 Error of indication for size measurement The tests for the determination of the error of indication for size measurement were performed on a calibrated ball bar (Figure 1) with 6 spheres having nominal diameter of 10 mm and distance between two contiguous spheres of 15 mm, according to VDI/VDE As suggested by the guideline, the spheres were measured using points distributed as evenly as possible over the entire surface of the spheres and as symmetrically as possible to a plane that is perpendicular to the measurement line and goes through the spheres centres. Figure 7 shows the results obtained in one of the tests performed on the laser scanner. This diagram reports the values of the so-called sphere-distance error, SD, which is defined as the difference between (1) the measured distance between two sphere centres and (2) the calibrated distance between the same two sphere centres. In the diagram, there are actually SPIE-IS&T/ Vol I-7

8 present 15 measurements, which result from the measurement of 5 lengths per 3 repetitions (as specified in VDI/VDE for testing one of the seven positions to be verified within the measurement volume). 0,02 Sphere-distance error Rotation -45 des Swing -4? deco U 2E c' ,01 0 I A a -0, L(mmj Fig. 7. Sphere-distance error, SD, determined through the measurement of five test lengths measured three times each. As mentioned in the previous section, VDI/VDE specifies that the sphere-distance error cannot be used instead of the error of indication: the averaging effect and compensation of bias errors encountered in measurements of sphere distances shall be taken into account additionally to allow comparison with measurements of individual points on opposite faces of a gauge block. In order to ensure comparability, the VDI/VDE guideline specifies that a correction must be added to SD, using one of the following two methods: (1) additional measurement of a short gauge block for each test length (2) correction obtained from the evaluation of the probing error. The first method, however, is not always applicable; for example it is typically not applicable when the measuring sensor is not equipped with an articulating system. Hence, the second method can actually be the only valid one for testing many measuring systems. Figure 8 shows the calculation of the error of indication for size measurement (E) obtained from the sphere-distance error (SD), given in Figure 7, plus the signed value of the probing error for form (PF) and the probing error for the size (PS), as specified by VDI/VDE Error of indication, E Rotation 45 leo Swiito 45 Ieo} 0,01 E E 0-0,01-0, L(mm] a Fig. 8. Error of indication for size measurement, E, determined by adding to the results given in Figure 7 the probing errors PS and PF, according to VDI/VDE Discussion of results From the results presented above it is clear that, when using the verification procedure specified in VDI/VDE , the value obtained for the error of indication for size measurement can be dominated by the influence of the probing errors. This is due to the fact that the VDI/VDE procedure requires adding the probing errors to the sphere-distance errors for taking into account a single point measurement. However, for many optical sensors actually there exists not a single point measurement: for example, for some sensors the row data is actually an image acquired from a camera and so there exist not row single points. Depending on the algorithms used for points reconstruction and filtering, probing SPIE-IS&T/ Vol I-8

9 errors can be much bigger than sphere-distance errors, so that the error of indication, E, can be completely masked by the probing error, PF. For this reason, the intrinsic test uncertainty may result too high. In conclusion, keeping the probing error and the sphere-distance error distinct could help the users to obtain more significant characteristics. 4.4 Comparison with measurement uncertainty As part of this work, a comparison was carried out between the performance verification characteristics of the measuring system under testing and the typical measuring uncertainties for specific measurement tasks of the same measuring system. The workpiece shown in Figure 9 is a typical measuring task for the laser scanner system that was tested as described above. Fig. 9. Metallic workpiece. The two cylinders A and B were measured with the laser scanner system under testing. In particular, cylinders A and B shown in Figure 9 were measured with the laser system and their diameters and cylindricities were chosen as measurands. Cylinders A and B have diameters of approximately 15 mm. The measurement uncertainties were determined by applying the substitution method, using a procedure derived from ISO [14]. The results of the uncertainty calculation are shown in Table 3. The values given in the table reveal that for the laser scanner under testing the measuring uncertainties are very different depending from the type of measurement: dimensional measurements (diameters) have lower uncertainty (U D15 = 7 µm in the measurement of a 15 mm diameter) while form measurements have higher uncertainty (U form = µm in measurements of cilinricity). Table 3. Measurement uncertainty determined according to ISO Measurand Diameter A Diameter B Cilindricity A Cilindricity B Measurement uncertainty U D15 = mm U D15 = mm U form = 0.06 mm U form = 0.07 mm This strong difference between U D15 and U form could approximately be inferred also from the difference between the characteristics SDE and PF: the laser scanner has SDE 15 = 4 µm and PF = 60 µm (where SDE 15 is the sphere-distance error corresponding to a distance of 15 mm between two spheres centres). The same inference is not possible if the characteristic E is used instead of SDE; in fact E 15 is 70 µm, which is very different from U D15 = 7 µm and from SDE 15 = 4 µm because the error of indication for size measurement, E, is completely dominated by the probing error, PF. This example confirms that keeping the probing error and the sphere-distance error distinct could be more significant, rather than putting all together in only one characteristic. SPIE-IS&T/ Vol I-9

10 5. CONCLUSIONS AND FURTHER DEVELOPMENTS Coordinate measuring systems equipped with optical distance sensors are increasingly used for dimensional and geometrical measurements, due to their rapidity and other advantages respect to contact probing CMMs. However, the lack of internationally recognised standards for acceptance and reverification tests of optical systems is currently a major problem in this field. The work presented in this paper was aimed at improving the state of the art methods that are used for testing optical coordinate measuring systems. An experimental investigation was presented on procedures and artefacts for testing specific characteristics according to the guideline VDI/VDE : 2005, with particular reference to: error of indication for size measurement, probing error and structural resolution. Based on experimental results obtained from a laser scanner on specifically developed artefacts, the main problems arising in the implementation of testing procedures were analyzed. In particular it was demonstrated that, when using ball bars, the error of indication for size measurement, E, may be completely masked by the probing error, PF. Furthermore the value of the probing error is extremely influenced by elaboration algorithms and filtration methods. As a consequence, the intrinsic test uncertainty may result too high. Keeping the probing error and the sphere-distance error distinct could be more significant. Further work is needed to extend the experimental investigation presented in this paper to other coordinate measuring systems using different optical distance sensors. 6. ACKNOWLEDGEMENTS Part of this work was financed by the European Union: Co-operative Research Project OP3MET, contract n. COOP- CT Alessandro Voltan and Raffaella Novello (University of Padova) are thanked for their valuable work in connection with laser scanner testing. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Schwenke, H., Neuschaefer-Rube, U., Kunzmann, H., Pfeifer, T., Optical methods for dimensional metrology in production engineering, CIRP Annals, vol. 51/1, , (2002). Beraldin, J.-A., Blais, F., El-Hakim, S., Cournoyer, L., Picard, M., Traceable 3D Imaging Metrology: Evaluation of 3D Digitizing Techniques in a Dedicated Metrology Laboratory, 8th Conference on Optical 3-D Measurement Techniques, Zurich, Switzerland, July 9-12, (2007). Keferstein, C. P., Züst, R., Minimizing technical and financial risk when integrating and applying optical sensors for in-process measurement, IMS International Forum, (2004). VDI/VDE : 2005, Accuracy of CMMs - Characteristics and their testing - Guideline for the application of DIN EN ISO to CMMs with optical distance sensors, VDI, Düsseldorf, (2005). ISO : 2001, GPS Acceptance tests and reverifications test for coordinate measuring machines (CMM) Part 2: CMMs used for measuring sizes, ISO, Geneva, (2001). ISO/DIS , Geometrical Product Specifications (GPS) - Acceptance and reverification tests for coordinate measuring machines (CMM) - Part 7: CMMs equipped with imaging probing systems, ISO, Geneva, (2008). OSIS WG3, Documentation on Specification, classifications and performance verification, Rel. 1.1, (2006). Website IACMM: visited in September Website OSIS: visited in September Carmignato, S., Savio, E., Verdi, M., Standardizzazione e verifica dell accuratezza dei sistemi ottici, Proceedings of Reverse Engineering: potenzialità e applicazioni, Modena, (2004). Carmignato S., Voltan A., Savio E., Investigation on testing procedures for optical coordinate measuring systems, VIII International Science Conference on Coordinate Measuring Techniques, Bielsko-Biala, Poland, 31 March - 2 April Ehrig, W., Neuschaefer-Rube, U., Artefacts with rough surfaces for verification of optical microsensors, SPIE Conference on Optical Measurement Systems for Industrial Inspection V, Munich, Germany, June 18-22, Carmignato S., Neuschaefer-Rube U., Schwenke H., Wendt K., Tests and artefacts for determining the structural resolution of optical distance sensors for coordinate measurement, Euspen International Conference, Baden bei Wien, May ISO/TS : 2004, GPS - CMMs: Technique for determining the uncertainty of measurement - Part 3: Use of calibrated workpieces or standards, International Organization for Standardization, Geneva, (2004). SPIE-IS&T/ Vol I-10