Computed Tomography & 3D Metrology Application of the VDI/VDE Directive 2630 and Optimization of the CT system ECNDT 2014 Prague October 6-10, 2014 Dr. Eberhard Neuser Dr. Alexander Suppes Imagination at work. Inspection Technologies 1
Content - CT vs metrology workflow - System v tome x M 300 - Short introduction to VDI 2630 - Measurement Setup & Test Specimen - NON compensated Results - Correction Method - Compensated Results - Summary & Conclusion Inspection Technologies 2
Typical metrology workflow with 3D X-Ray CT Physical measurement Acquisition of projections Tube: focalspot diam., stability Manipulator: accuracy, stability Detector: Dynamic range, noise CT Data- Processing Volume Reconstruction Surfaceextraction Used algorithms: Geometry correction Beamhardening correction Suppression of artefacts in Surface detection Impacting accuracy Analysis Data-analysis Measurement strategy, Fit-Algorithms, Reporting Inspection Technologies 3
Inspection Technologies 4
VDI 2630, Part 1.3 Motivation Definition of common performance characteristics Ability to compare different systems regarding to their basic metrology specifications Definition of test specimens Based on current standards established in the metrology world Allow user acceptance of conventional metrology equipment like CMMs NO Statement regarding measurement accuracy in specific customer applications Inspection Technologies 5
VDI 2630, Part 1.3 Form & Size Probing error Form Probing error: PF = R max R min Surface point deviation with respect to the fitted sphere Indicator for i.e. Surface noise Inaccuracies in sample rotation Size Probing error: PS = Da Dr Deviation from the calibrated diameter Accuracy of surface detection between air and material Indicator for i.e. Correct Adjustment of tube parameters (Voltage / beam filtration) Proper Beam hardening corrections Measurement requirements Sphere diameter 10-20% of measurement space diagonal 6 measurements: (top + center + bottom) x (center + periphery) At least a minimum 25 surface points to generate PF and PS 2 measurements at significant different magnifications (recommended if technically possible) Regression sphere PF Da R min R max Inspection Technologies 6
VDI 2630, Part 1.3 Length measurement error Typical approach: Measurement of sphere distances with a calibrated test specimen and comparison to the calibrated values Including of PS/PF necessary measuring sphere arrangements Indicator for overall accuracy of CT system geometry setup Measurement requirements: Measurement of 5 length, in 7 spatial directions, each 3x at minimum two different magnifications Smallest test length = 30 mm, biggest test length = 66% of measurement space diagonal Inspection Technologies 7
Test Specimen & Measurement setup Test Specimen Length measurement error 8mm 0 24 48 120 mm Ball bar CFC with Ruby spheres, DAkkS calibrated Nominal length: 24, 48, 72, 96, 120mm Test Specimen Probing error 30mm Al-Oxide sphere, DAkkS calibrated Measurement Setup and -parameter v tome x M 300/180-16 detector Focal-Detector-Distance: 800mm 200kV; 0,5mm Cu approx. 80µm / voxel approx. Ca. 25 min / measurement Quelle: Carl Zeiss Inspection Technologies 8
Results NOT compensated Distance error of sphere centers, SD [mm] 0,004 0,002 Sphere Distance Error SD, mm 0,000-0,002-0,004-0,006-0,008-0,010-0,012-0,014-0,016 0 20 40 60 80 100 120 140 Calibrated sphere distance, mm At 80µm/vox: Max. Distance error 15µm. But, can we further improve it? Inspection Technologies 9
Influence of System components X-Ray tube: Focal spot position: - Focal-object and - Focal-detector-Distance Manipulator: Linearity of magnification axis Detector: Is the detector ideal regarding its geometry? Inspection Technologies 10
Compensation - Determination of Focal-Object- and Focal-Detector-Distance pixel size measured length in voxel Scan1 low magnification Spherefit1 REC1 calibrated length wanted: Focal-Object- and Focal-Detector-Distance Determination of 2 variables from two functions (measurements) Robust results using more than 2 length in the ball bar Scan2 high magn. REC2 Spherefit 2 Inspection Technologies 11
Compensation - Adjusting linearity of the magnification axis Direct Measurement system: high accuracy and reproducibility Utilizing a Laser interferometer to linearize the axis: 1) Measuring the actual position of the magnification axis and comparing to nominal position (target position) 2) Using the measured deviations to compensate the axis error (linearizing the axis) Position deviation, µm 10 5 0 NON compensated +0,5 0,0-0,5 12 compensated Nominal position of magnification axis, mm Nominal position of magnification axis, mm Inspection Technologies 12
Compensation Detector flatness 11 Scans 1) Acquisition 2) Reconstruction 3) Evaluation of cylinder diameter at different cylinder heights with 3D image processing 4) Determination of detector bending CT Scan of cylindrical object 5) Compensation by including the real detector shape in the reconstruction algorithm Inspection Technologies 13
Compensated Measurement Results Distance error of sphere centers, SD [mm] Non compensated Incl. compensation 0,004 0,004 0,002 0,002 Distance error SD, mm 0,000-0,002-0,004-0,006-0,008-0,010-0,012 0 20 40 60 80 100 120 140 0,000-0,002-0,004-0,006-0,008-0,010-0,012 0 20 40 60 80 100 120 140-0,014-0,014-0,016 Calibrated sphere distance, mm -0,016 Calibrated sphere distance, mm : Threshold value for Sphere distance error - v tome x M metrology edition : SD MPE = 4µm +L/100, L: nominal length in mm Inspection Technologies 14
Summary & Conclusion Sphere distance error at 80 µm voxel size NON compensated 15 µm Compensated 2 µm Compensation of detector and magnification axis lead to much better results regarding the systems metrology performance System-Characteristics following VDI 2630-1.3 in mode Measurement in the image (Static): SD MPE(TS) = 4µm+L/100 PS MPE(TS) = 3µm PF MPE(TS) = 3µm Understanding the key system components like tube, detector and manipulation system in detail gives the opportunity to improve the metrology performance following VDI 2630 significantly by compensating the effects Inspection Technologies 15
Visit GE at ECNDT booth 100 on floor 2 www.ge-mcs.com/x-ray