1. Accuracy and comparability of xCT measurements
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Ville Heikkinen, Björn Hemming, Antti Lassila

2. MIKES Metrology / V. Heikkinen
Outline
VTT MIKES
Traceability
Industrial xCT
Error sources in xCT
xCT traceability
Related research activities
Conclusion
MIKES Metrology / V. Heikkinen

3. VTT MIKES Metrology National metrology institute of Finland
~65 skilled scientist & engineers
MIKES:
realizes SI units in Finland
performs top level research within the field of measurement science
develops measurement techniques for the industry and society
offers calibration services, specialist services and training

4. VTT MIKES Length Metrology
BA1172 Length metrology
Excellent medium size laboratory
~13-16 researchers & technicians
8 excellently air conditioned and vibration isolated laboratory rooms for length metrology
>150 different calibration & measurement services
Consultancy for solving measurement problems of industry & academia
R&D research projects in scientific and industrial metrology with national and international partners
Realisation of the metre
Interfero- metry
Nano- metrology
Form meas.
Angle
CMM
Compar. Meas.
MIKES Metrology / V. Heikkinen

5. Traceability: Accurate measurements require traceability
property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty
provides measurement results in correct SI-units, comparable worldwide
Metre convention
SI-unit system
National Metrology Laboratory
Accredited laboratory
Measurement laboratory
End user
Definition of the unit
Realization of the unit: National measurement standard
Calibrated secondary measurement standard
Calibrated reference measurement standard
Calibrated working measurement standard
Traceable measurement
Example of traceability chain
MIKES Metrology / V. Heikkinen

6. MIKES Metrology / V. Heikkinen
Traceability: why?
Modern industrial production is not possible if subcontractors have different scales
Scientific technical research can not be repeated if it is done without traceably calibrated instruments
MIKES Metrology / V. Heikkinen

7. Traceability: Challenges
Based on our experience even 20% scale errors are possible with several measuring instruments e.g. AFM, SEM
Difficult when one measures things that cannot be seen with other methods
When the dimensions are small
Or hidden
With industrial xCT errors of several % have been found [1] “typically the participants have difficulties both in stating correctly the maximum permissible errors of their CT systems and in evaluating appropriately the measurement uncertainty”
[1] S. 492 Carmignato / CIRP Annals - Manufacturing Technology 61 (2012) 491–494
MIKES Metrology / V. Heikkinen

8. Industrial xCT: 3rd revolution in dimensional metrology1.
1. Coordinate measuring machines (CMM)
2. Optical CMMs e.g. Laser tracker
3. Industrial xCT
xCTs have been developed level close to traditional dimensional measurements, accuracy only 1-2 orders of magnitude worse
At the same time new production techniques make xCT measurements more important to industry
Printed components with complex internal shapes
Modern cast components
Precision engineered components 2. 3.
MIKES Metrology / V. Heikkinen

9. MIKES Metrology / V. Heikkinen
Error sources in xCT
errors affecting
geometric accuracy
surface detection accuracy
MIKES Metrology / V. Heikkinen

10. Error sources in xCT: Geometric magnification
Source sample distance
Source detector distance
Source spot geometry
Geometry of the detector
Rotary table
Rotation, radial/axial wobble, tilt
xCT kinematics
Thermal expansion effects
MIKES Metrology / V. Heikkinen

11. Error sources in xCT: surface detection
Surface detection algorithms may place the surface few voxels in/out from the real surface
Sources of error:
Detector linearity
Beam hardening, flux, energy spectrum
Noise
Reflections
Artefacts due measurement geometry
Algorithm
Etc.
MIKES Metrology / V. Heikkinen

12. xCT traceability: different methods
Measurement of known reference standards
Valid only for same measurement geometry
Hybrid measurement
Referencing from dimension in measured area measured with e.g. CMM
Virtual xCT approach
Characterization of different error components of xCT for the model
Develop model of xCT to calculate errors based on different components and prop. distr.
Often best results by combining these in measurement specific way
MIKES Metrology / V. Heikkinen

13. Traceability: Artefacts for calibration
Geometry errors
Surface detection error
Geometry & surface
=> ISO 10360:11 under preparation 1. 1. 2. 3. 3.
MIKES Metrology / V. Heikkinen

14. Traceability: VTT MIKES probe forest v1
Probe forest for dimensional calibration
Calibrated by tactile CMM at MIKES
Measured with xCT
Distance and size evaluated by GOM inspect program
Geometrical error stays relatively constant
Surface error within ~few voxels
Distance error / %
Diameter error (distance corrected) / µm
Balls
Test1
Test2
Ball
A-C
-1,0
-0,8 A
-16
-17
B-C
-1,1
-0,9 B
-22
C-D C
-12
B-D D
-21
C-E E
-3,9
-38
A-E
MIKES Metrology / V. Heikkinen

15. Traceability: VTT MIKES calotte plate
For geometry and surface detection error calibration
Internal and external diameters
straightness & flatness & orthogonality
Several sizes
Materials aluminium & steel
Edge / mm
Hole / mm
Thickness / mm 8
0.8
1,6 10 1 2 20
1,5 3 50 5
MIKES Metrology / V. Heikkinen

16. MIKES Metrology / V. Heikkinen
Traceability: Hybrid measurement with length measuring machine
Ti implant length from xCT data mm
Result from length measuring machine (SIP) was mm
xCT result for same part was
0.6 % to 0.8 % smaller than by SIP
0.8 % to 1.1 % expected by probe forest test
MIKES Metrology / V. Heikkinen

17. Related research activities
Industrial xCT, 2018 – VTT internal development project
Advanced Computed Tomography for dimensional and surface measurements in industry, AdvanCT, – EMPIR Industry project
Metrology for additively manufactured medical implants, MetAMMI, – EMPIR Health project
MIKES Metrology / V. Heikkinen

18. Related research activities: Industrial xCT
Method development for industrial traceable measurements
First tests e.g. a defect causing Helium leak was discovered using xCT
MIKES Metrology / V. Heikkinen

19. Related research activities: AdvanCT
EMPIR project co-ordinated by PTB
Establishment of a full 9 degrees of freedom in situ metrology CT system
Correction of errors caused by X-ray source and detector
Determination and correction of measurement errors associated with multi-materials
Improved correction of CT image forming artefacts
Achievement of traceability by estimation of measurement uncertainty using virtual CT models and Monte-Carlo simulations
CT based measurement of surface texture and roughness
Fast CT to enable inline measurements
Standardisation regarding multi-material measurements
MIKES Metrology / V. Heikkinen

20. Related research activities: MetAMMI
Additively manufactured implant measured using XCT
Measured implant surface was +0.1 ± 0.3 mm from specification
MIKES Metrology / V. Heikkinen

21. MIKES Metrology / V. Heikkinen
Conclusion
VTT MIKES is developing industrial xCT metrology
Targeting to more reliable accurate xCT for industrial needs
Understanding of error sources and development of metrological methods
VTT MIKES will manufacture and calibrate a large range of reference standards for XCT
The goal is to achieve traceability for XCT, this includes the means for calculation of uncertainty
Interested in collaboration with FinTomo community
MIKES Metrology / V. Heikkinen

22. Thank you for your attention