Systematic Errors in Dimensional X-ray Computed Tomography

Måletekniske dage

Teknologisk Institut

31.05.2012

Jochen Hiller

2 DTU Mechanical Engineering, Technical University of Denmark

Industrial X-ray CT today

Dimensional CT as a key technology in production metrology

Errors sources and a good practice in CT scanning

Conclusions and future works

Overview

3 DTU Mechanical Engineering, Technical University of Denmark

Industrial X-ray CT today

Phoenix

Zeiss

Werth Skyscan

Wenzel

4 DTU Mechanical Engineering, Technical University of Denmark

Aluminum casting

Fiber-composite

Insulin pen

Tissue Dental impression

Foam Turbine plate

Seed

Industrial X-ray CT today

5 DTU Mechanical Engineering, Technical University of Denmark

Industrial X-ray CT today

Industrial

X-ray CT

Material Analysis

Material Testing

Dimensional (geometrical) Metrology

6 DTU Mechanical Engineering, Technical University of Denmark

Dimensional CT as a key technology in production metrology

Measurement of size, form, and position CAD/CT comparison

7 DTU Mechanical Engineering, Technical University of Denmark

Dimensional CT as a key technology in production metrology

8 DTU Mechanical Engineering, Technical University of Denmark

Dimensional CT as a key technology in production metrology

We will never know the true value of a measurement

Measurement results must be repeatable and reproducible

What about systematic (effects) errors? Should be corrected!

Measurement uncertainty U:

9 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

10 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

Image artefacts

Scaling (voxel size) error

CT system limits (image blurring, noise)

Metrological data evaluation strategy

Segmentation and surface

determination errors

What are sources of systematic errors?

11 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

Only for the compensation of effects linked to geometrical scanner misalignment or beam-hardening artefacts

Calibrated masterpieces

Systematic scanning and evaluation planning to avoid high systematic errors (blunder)

Can we use calibration artefacts?

Ball-plate Ball-bar

Step-wedge

12 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

Beam-hardening Cone-beam Misalignment Undersampling

Truncation Ring artifacts Metal artifacts Noise artifacts

13 DTU Mechanical Engineering, Technical University of Denmark

X-ray tube Rotation

axis

Object

Detector

Helical sampling

z

Errors sources and good practice in CT scanning

Cone-beam artefacts:

X-ray tube

Object

Rotation

axis Detector

Circular sampling

Missing data

14 DTU Mechanical Engineering, Technical University of Denmark

Standard-CT Helical-CT

Me

asu

rem

en

t e

rro

r (µ

m)

0

5

10

15

20

25

30

35

Standard-CT Helical-CT

Me

asu

rem

en

t e

rro

r (µ

m)

0

5

10

15

20

25

Big drill hole Small drill hole

Voxel size: 105,7 µm3

Errors sources and good practice in CT scanning

Standard vs.helical CT:

15 DTU Mechanical Engineering, Technical University of Denmark

Good practice:

Tilted position of the workpiece

Errors sources and good practice in CT scanning

Sideview

Source

Detector

16 DTU Mechanical Engineering, Technical University of Denmark

Penetration length (mm)

P =

-ln

(I/I

0)

Errors sources and good practice in CT scanning

Beam-hardening:

17 DTU Mechanical Engineering, Technical University of Denmark

Penetration length (mm)

Aluminum

Iron

rela

tive

Inte

nsi

ty

Step wedge Radiography Linerization of projections with inverse function

Errors sources and good practice in CT scanning

Beam-hardening correction:

18 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

DTU beam-hardening correction GUI

19 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

Beam-hardening correction:

20 DTU Mechanical Engineering, Technical University of Denmark

uncorrected corrected 0

50

100

150

200

250

300

350

Measure

ment

err

or

(µm

)

Big drill hole

uncorrected corrected 0

50

100

150

200

Me

asu

rem

en

t e

rror

(µm

)

Small drill hole

Voxel size: (156,7 × 156,7 × 179,3) µm3

Errors sources and good practice in CT scanning

Beam-hardening correction:

21 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

Good practice:

Tilted position of the workpiece

Sideview

Source

Detector

Using a prefilter

22 DTU Mechanical Engineering, Technical University of Denmark

SRD SDD

Detector

Source Calibrated length: 8,7678 mm

v = 30 µm, Δv = 1 µm

l = 1,5 mm

Error: Δl = 50 µm

LCT

Errors sources and good practice in CT scanning

m = SDD/SRD v = a/m

23 DTU Mechanical Engineering, Technical University of Denmark

Variations are caused by

inaccuracies of the SRD

measurement (manipulator)

+

Focus drift during scanning

due to tube temperature

changes

superimposed

Errors sources and good practice in CT scanning

1 2 3 4 5 6 7 88.760

8.761

8.762

8.763

8.764

8.765

8.766

8.767

8.768

8.769

8.770

Measurement

Dis

tance (

mm

)

Reference value

24 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

Voxel size rescaling:

1 2 3 4 5 6 7 88.760

8.761

8.762

8.763

8.764

8.765

8.766

8.767

8.768

8.769

8.770

Measurement

Dis

tance (

mm

)

Reference value

25 DTU Mechanical Engineering, Technical University of Denmark

Errors sources and good practice in CT scanning

Good practice:

In particular at high magnifications: Ball-bar must be scanned together

with the workpiece

Sideview

Source

Detector

Ball-bar

Focus drift differs from scan to scan!

26 DTU Mechanical Engineering, Technical University of Denmark

Image unsharpness Noise

Object

CT

Depending on object, scanning parameters, system hard- and software

Errors sources and good practice in CT scanning

27 DTU Mechanical Engineering, Technical University of Denmark

Conclusions and future works

Importance of a consistent procedure in CT scanning planning

Variety of error sources and influence quantities in CT metrology

Possibilities to reduce systematic errors (effects)

CT as a powerful and flexible tool in production metrology

28 DTU Mechanical Engineering, Technical University of Denmark

Conclusions and future works

Material, shape, penetration lengths

Fixture, positioning, orientation

Tube voltage, current, prefilter, detector settings

Evaluation of detector images histogram analysis

Image quality (artefacts, sharpness, noise)

Voxel histogram analysis, threshold tests

Surface quality inspection

Alignment

Measurement strategy (elements, points, methods)

Reference data available, repeated measurements

Scanning preparation

29 DTU Mechanical Engineering, Technical University of Denmark

Invitation to Conference on

“Industrial Applications of CT Scanning –

Possibilities & Challenges in the Manufacturing Industry”

June 12, 2012, 10:00-16:30

DTU, Building 101, meeting room 1

2800 Kgs. Lyngby, Denmark

30 DTU Mechanical Engineering, Technical University of Denmark

Thank you very much!