Computed Tomography & 3D Metrology · 2014-10-04 · regarding the systems metrology performance • System-Characteristics following VDI 2630-1.3 in mode “Measurement in the image“

Imagination at work.Inspection Technologies 1

ECNDT 2014 – Prague – October 6-10,

2014

Dr. Eberhard NeuserDr. Alexander Suppes

Computed Tomography & 3D MetrologyApplication of the VDI/VDE Directive 2630 and Optimization of the CT system

Copy right © 2014 General Electric Company

Inspection Technologies 2Copy right © 2014 General Electric Company

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


Typical metrology workflow with 3D X-Ray CT

Data-analysis

Surface-

extraction

Volume

Reconstruction

Acquisition

of projections

Physical

measurement

CT Data-

Processing

Analysis

Tube: focalspot diam., stabilityManipulator: accuracy, stability

Detector: Dynamic range, noise

Used algorithms:Geometry correction

Beamhardening correctionSuppression of artefacts in

Surface detection

Measurement strategy,Fit-Algorithms, Reporting

Impactin

g a

ccura

cy



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


VDI 2630, Part 1.3 – Form & Size Probing error

Form Probing error: PF = Rmax – Rmin

• 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

Rmin

RmaxPF

Da


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


Test Specimen & Measurement setup

Test Specimen Length measurement error

• 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

2448

0120 mm

8mm


Results NOT compensated

-0,016

-0,014

-0,012

-0,010

-0,008

-0,006

-0,004

-0,002

0,000

0,002

0,004

0 20 40 60 80 100 120 140

Sp

he

re D

ista

nce

Err

or

SD

, m

m

Calibrated sphere distance, mm

Distance error of sphere centers, SD [mm]

At 80µm/vox: Max. Distance error 15µm.� But, can we further improve it ?


Influence of System components

X-Ray tube: Focal spot position:

- Focal-object and - Focal-detector-Distance

Detector:Is the detector ideal

regarding its geometry ?

Manipulator:Linearity of

magnification axis


Compensation - Determination of Focal-Object- and Focal-Detector-Distance

Scan1 – low magnification

REC1

Spherefit1

REC2

Spherefit2

Scan2 – high magn.

� �� ∙ ��

�∙ ��

pixel size measured length in voxel

calibratedlength

wanted: Focal-Object- andFocal-Detector-Distance

• Determination of 2 variables from two functions (measurements)

• Robust results using more than 2

length in the ball bar


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)

12

+0,5

-0,5

Nominal position of magnification axis, mm

Po

sitio

n d

evi

atio

n, µ

m

10

5

0,0

0

Nominal position of magnification axis, mm

NON compensated compensated


Compensation – Detector flatness

11 Scans

CT Scan of cylindrical object

1) Acquisition2) Reconstruction3) Evaluation of cylinder diameter at different

cylinder heights with 3D image processing4) Determination of “detector bending”

5) Compensation by including the “real” detector shape in the reconstruction algorithm


Compensated Measurement Results

-0,016

-0,014

-0,012

-0,010

-0,008

-0,006

-0,004

-0,002

0,000

0,002

0,004

0 20 40 60 80 100 120 140


Incl. compensation

-0,016

-0,014

-0,012

-0,010

-0,008

-0,006

-0,004

-0,002

0,000

0,002

0,004

0 20 40 60 80 100 120 140

Dis

tan

ce

err

or

SD

, mm


Non compensated

Distance error of sphere centers, SD [mm]

M „metrology edition“:Threshold value for Sphere distance error - v|tome|x M „metrology edition“:SDMPE = 4µm +L/100, L: nominal length in mm


Summary & Conclusion

• Sphere distance error at 80 µm voxel sizeNON 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):

SDMPE(TS) = 4µm+L/100

PSMPE(TS) = 3µm

PFMPE(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

Visit GE at ECNDT booth 100 on floor 2

www.ge-mcs.com/x-ray

Documents

Computed Tomography & 3D Metrology · 2014-10-04 · regarding the systems metrology performance • System-Characteristics following VDI 2630-1.3 in mode “Measurement in the image“