Dimensional Measurements with Micro- CT -Test Procedures and Applications

Transcription

1 Dimensional Measurements with Micro- CT -Test Procedures and Applications Ulrich Neuschaefer-Rube, Markus Bartscher, Marko Neukamm, Frank Härtig PTB Physikalisch-Technische Bundesanstalt, Braunschweig und Berlin, Germany Karsten Ehrig, Andreas Staude, Jürgen Goebbels 20 mm 10 mm 1 mm BAM Bundesanstalt für Materialforschung und prüfung Berlin, Germany 2 mm

2 Content Foil 2 1. Introduction 2. Test procedures and error correction 3. Application examples 4. Conclusions

3 Introduction Foil 3 Set-up of industrial computed tomography (CT) Measurement object on rotary table between X-ray source and X-ray detector Micro-CT: Cone-beam and area detector Scale factor dependent on the distances between X-ray source, measurement object and detector

4 Introduction Foil 4 Flow chart of typical dimensional CT measurement processes measurement object CT measurement Each step contributes to the measurement uncertainty of CT voxel data threshold process reference data (e.g. CAD model) surface data actual/nominal comparison geometry fitting wall thickness analysis result report

5 Introduction Structural resolution depends on: Size of X-ray focus Minimal distance between X-ray source and measurement object Distance between measurement object and detector Pixel size of detector Foil 5 Smallest achievable voxel size (X-ray tube): approx. 1 µm (Ø object approx. 2 mm) Smallest achievable voxel size (Synchrotron): approx. 0,5 µm (Ø object approx. 2 mm) Further reduction possible with microscope set-up Image: Andrei Tkachuk et. al: Z. Kristallogr. 222 (2007)

6 Introduction Industrial CT available and manufactured in Germany (examples) Carl Zeiss Fraunhofer-Gesellschaft Werth Messtechnik RayScan Technologies GE sensing & inspection Yxlon International Wenzel Volumetric

7 Introduction - Influences on CT measurements Foil 7 detector stability / drift quantum efficiency dynamics contrast sensitivity internal scattering pixel variance, noise measurement object penetrated depth beamhardening scattered radiation multiple materials rotary table / translation axis radial movement axial movement angular-position error wobbling linear-position error environment temperature / - gradients vibrations geometry guiding errors squareness local heating intrinsic vibrations Micro CT-system of the BAM acceleration voltage: kv flat panel X-ray detector: 2048 x 2048 achievable voxel size: (3 µm 200 µm) 3 maximum object diameter: 180 mm X-ray source spectrum focus properties stability temperature opening angle software / data processing reconstruction threshold & surface generation data reduction data corrections user source current acceleration voltage integration time number of projections object orientation

8 Introduction Foil 8 CT acts as coordinate measuring machine (CMM) Advantages of CT as dimensional measurement technique Non-destructive Determination of inner and outer geometry Achieves a very high point density in relative short time Disadvantages of CT as dimensional measurement technique No accepted test procedures available so far (see following) Complex and numerous influence quantities Reduced form measurement capability due to measurement errors (artefacts) Measurement uncertainty in many cases unknown Solution Apply material standards to correct measurement errors and achieve traceability Adopt experiences from coordinate metrology to CT: Use material standards and characteristics to qualify new technique

9 Test procedures Foil 9 German standardisation activities VDI-GMA 3.33 technical committee Computed tomography in dimensional metrology Acceptance and reverification tests for dimensional CT systems are described in: VDI/VDE 2617 Sheet 13 = VDI/VDE 2630 Sheet 1.3 (Draft) Computed tomography in dimensional metrology - Guideline for the application of DIN EN ISO for coordinate measuring machines with CT sensors International standardisation activities ISO TC213, WG 10: Dimensional CT accepted as a work item

10 Test procedures Foil 10 Characteristics in VDI/VDE 2617 Sheet 13 / VDI/VDE 2630 Sheet 1.3 Global error characteristics Error of indication for length measurement (length measurement error) Material standard: length standard, e.g. step gage, ball bar, ball plate, gauge block Local error characteristics Probing error (size and form) P S, P F Material standard: reference sphere CT-specific characteristics (optional) Material- and geometry-dependent errors G S, G G, G F Material standard: step cylinder Structural resolution (additional to every specification) Material standard: e.g. small spheres (under discussion)

11 Test procedures Foil 11 Development of material standards Step gage 10 mm Mini- Probe 10 mm Ball plate Calotte cube 10 mm Material standards for conventional CMM Material standards for micro-ct systems

12 Test procedures - Material standards Foil 12 Length standards / test spheres for CT application 20 mm 10 mm 1 mm 10 mm Sphere calotte plate Sphere calotte cube Mini probe Micro tetrahedron CT-specific standards 25 mm 300 mm 10 mm Step cylinder Ring / Cylinder pair Multi-material rings

13 Test procedures and error correction Foil 13 Sphere calotte cube made from titanium Calottes manufactured by eroding (EDM) Cube features 3 facets (x, y, z) with 5 x 5 calottes Calottes: Ø 0.8 mm form deviation: nominal < 2 µm Optional: Cavity at the bottom Modified calotte grid Advantages Increased information content compared to 2D standard Traceable tactile calibration available Standard can be used with others sensors, too (multisensor)

14 Test procedures and error correction Foil 14 Application of sphere calotte cube to reduce scale errors Sphere distance deviation of a CT measurement of calotte cube before (red squares) and after (black squares) scale correction sphere distance deviation µm m m m easured length voxel size 15 µm 2775 lengths red = uncorrected black = corrected Recent result (other CT system): Max. deviation < 1 µm

15 Test procedures and error correction Foil 15 Application of sphere calotte cube Spatial distribution of the residual position errors of a corrected CT system Cone beam CT 180 kv, (18.4 µm) 3 voxel size measurement position

16 Test procedures and error correction Foil 16 Correction of geometry errors of involved CT axes applying micro tetrahedron Manufactured by PTB using micro mechanics CT measurement BAM 100 kv, (2 µm) 3 voxel size 1 mm Micro tetrahedron (Ruby balls 0,5 mm) µm Probing error form P F after before correction 2 < µm 5 µm (voxel size)

17 Example 1 Micro spur-gear study Foil 17 Parameters of micro spur-gear under study No. of teeth 14 Normal modul m n 0.12 mm Pressure angle α 0 20 Addendum modification coefficient x Base circle radius r b Helix angle β 0 0 Tip diameter d a Face width b Reference circle radius r mm 2.0 mm 1.0 mm mm CAD of micro spur-gear 2 mm Micro spur-gear made from steel

18 Example 1 Micro spur-gear study Foil 18 Micro-CMM measurements (Carl Zeiss F25) using software Zeiss Calypso and Gear Pro Tactile micro probe Probe diameter 120 µm Analysis parameters: Tooth No. 1, 5, 8, 12 Flanks Radius / length of roll foot r SAP / l αsap Radius / length of roll tip r EAP / l αeap Reference for the height of measurement Measurement range in height left and right mm / mm mm / mm mm (related to the datum face of the gear) 0.7 mm Measurements with F25 by Michael Neugebauer (PTB)

19 Example 1 Micro spur-gear study Foil 19 used BAM micro-ct system reconstructed volume constant threshold surface reconstructed volume with adaptive threshold Parameters of micro CT measurement Voltage Filtering No. of angles Time Detector size 80 kv 0.25 mm Cu h 2048 x 2048 Data binning Reconstruction Voxel size Surface extraction No. of measurements No Feldkamp (3.6 µm) 3 adaptive 6

20 Example 1 Micro spur-gear study Registration and analysis of data First: Define workpiece coordinate system with Calypso. Second: Measure the flanks with gear specific software Gear Pro. Orientation of the x-axes is unknown! Define Zeiss F25 CMM workpiece coordinate system CMM-measurement with Zeiss F25 CMM Probe radius correction Point Data Object Comparison Report Projection with µct at BAM Reconstruction Voxel data Adaptive threshold Surface Define coord. system Tooth flanks Best fit alignment Aligned flanks Best fit alignment with restricted degrees of freedom Foil 20

21 Foil 21 Example 1 Micro spur-gear study measurement error in mm Deviation of CT measurement (data-set 1 of 6) Distribution of deviations without strong outliers

22 Foil 22 Example 1 Micro spur-gear study Histogram of all 6 assessed measurements % frequency frequency frequency cumulative % 80% 60% 40% 20% cumulative percentage 0 0 0% measurement error in mm absolute value of measurement error in mm Analysis of deviations 95% of all observed absolute deviations are less mm Due to the statistics of 6 measurements (correction with t-distribution) a value of mm can be attributed as a first measure of the uncertainty of measurement

23 Example 2 Internal micro gear study Foil 23 Internal micro gear made from steel length ca. 11 mm No. of teeth 40 Normal module m n 0.12 mm Pressure angle α 0 20 Addendum modification coefficient x Base circle radius r b mm Helix angle β 0 0 Tip circle diameter d a Root circle diameter d f 4.8 mm 5.23 mm Reference measurement with micro CMM Werth VideoCheck HA 400 with tactile-optical probe Probe diameter: 61 µm Modus of operation: single point probing

24 Example 2 Internal micro gear study Foil 24 Reproducibility test of CMM measurement (7 measurements) Result of gear analysis: Profile deviations (example: tooth 10, left flank, z = -0.4 mm) Reproducibility better than 1 µm in analysis interval profile deviation in mm tooth tip analysis interval tooth root length of roll in mm

25 Foil 25 Example 2 Internal micro gear study Object Projection Reconstruction Voxel data CMM meas. Threshold Point data Gear analysis Surface Comparison Segmented slice data Gear analysis Synchrotron CT measurement at BESSY BAMLine Report Energy: 60 kev Voxel size: (2.2 µm) 3 Detector: GdOS scintillator, CCD-camera (3209 x 801 pixel) Exposure (each): 3 s No. of angles: 2500 (0-180 )

26 Example 2 Internal micro gear study Foil 26 Comparison of CT and CMM data Result of gear analysis: Profile deviations (example: tooth 10, left flank) Deviations < 2 µm in analysis interval tooth tip analysis interval tooth root profile deviation in mm CT, z = -0.4 mm CT, z = -0.6 mm CMM, z = -0.4 mm CMM, z = -0.6 mm length of roll in mm

27 Conclusions Foil 27 Complete dimensional measurements of microparts possible with micro-ct Smallest voxel size without optics approx 0.5 µm Successful application of material standards Standardization of dimensional CT: - VDI/VDE 2617 Sheet 13 = VDI/VDE 2630 Sheet 1.3 available as Draft - Work item CT in ISO TC 213 Achieving traceability of CT measurements is still a challenge First results: measurement uncertainties of several Vision in future: micrometer for small complex objects CT is a fully accepted measurement technique which is used coequal to conventional coordinate metrology

28 Foil 28 Thank you for your attention! Contact: Activities are supported by the German Research Foundation (DFG) project No. NE 757/2-2 and by the German Federal Ministry of Economics and Technology project No. AZ: II D 5 07/06