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Developments in Dimensional Metrology in X-ray Computed Tomography at NPL Wenjuan Sun and Stephen Brown 10 th May 2016 1

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Page 1: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Developments in Dimensional Metrology

in X-ray Computed Tomography at NPL

Wenjuan Sun and Stephen Brown

10th May 2016

1

Page 2: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Possible factors influencing XCT measurements

2

Components Influencing variables Possible effects

X-ray source voltage

current

focal spot

Inherent filtration

(materials thickness)

Stability of focal spot

spectrum shape

stability of intensity

affect the resolution

affect the X-ray spectrum

affect projection image

Detector exposure, Image lag

pixel size, resolution

linearity and efficiency

Others(dynamic range etc.)

affect image quality

Mechanical parts alignment of the x-ray source, the detector

and the sample stage.

scale (for example, source to detector

distance, source to object distance)

deviations cause partially blurred

reconstructions

cause error in pixel value

Environmental temperature, humility, etc.

Software reconstruction algorithm

surface determination algorithm

correction algorithms: beam hardening,

scattering corrections.

affect dimensional measurements

Operators measurement and analyses approach can significantly affect the results

Page 3: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Most frequent questions: • What are the best settings? (voltage, current, average of images, etc.)

• Should we use physical filters or not? What are the differences of

having different thickness of filters?

• What are the influences of sample orientation and position?

• What is the influence of shading correction? Linear or nonlinear

correction?

• How should we evaluate the quality of measurements?

• What is the smallest feature that can be measured?

• …

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Page 4: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Influence factors: X-ray source (focal spot)

• Stability during scan

4

target

Part of X-ray

tube that

consists of X-Y

coils and focus

coil

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Page 5: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Influence factors: X-ray source (focal spot)

• Stability during scan

Material:

Tungsten carbide

Sphere diameters:

Sphere 1: 0.7954 mm

Sphere 2: 0.7951 mm

Centre to centre distance:

0.9795 mm (k = 2)

Reference sample calibrated by

traceable contact CMM at NPL.

Two-sphere reference sample

Projection images

Boundary

Page 6: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

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Cold start

(without system warm up)

Hot start

(with system warm up)

Sphere centre drift

along x-axis

Sphere centre drift

along y-axis

Sphere separation

(reflect variation of

X-ray source along

beam axis)

Influence factors: X-ray source (focal spot)

Reference:

Sun W, et al. 2015 A preliminary study of characteristics of X-ray computed tomography, Proc. of Digital

Industrial Radiology and Computed Tomography (Ghent, Belgium)

Flay N, et al. 2015 Investigation of the focal spot drift in cone-beam industrial X-ray computed tomography,

Proc. of Digital Industrial Radiology and Computed Tomography (Ghent, Belgium)

Page 7: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

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Detective quantum efficiency (DQE): describes the ability of an X-ray detector to transfer

the signal-to-noise ratio (SNR) from the radiation field to the resulting digital image

DQE results of the scintillation detector at 70 kV (top-left), 90 kV (top-right), 120 kV

(bottom-left) and 150 kV (bottom-right). lp/mm is line pair per millimetre

Influence factors: detector

Reference: Sun W, et al. 2016 Characterisation of a scintillation flat panel detector employed by an X-ray

computed tomography system: measurements of MTF, NPS and DQE, Proc. of EUSPEN (Nottingham)

Page 8: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Spatial resolution

8

0

5

10

15

0 200 400 600

Sp

ati

al

reso

luti

on

: µ

m

Current:µA

Influence of magnification Influence of current

0

5

10

15

20

0 100 200 300

Sp

ati

al

reso

luti

on

: µ

m

Voltage: kV

Influence of voltage

1

10

100

1000

1 10 100Sp

ati

al

reso

luti

on

: µ

m

Magnification

Slanted edge sample

50 100 150 2000

5

x 104

pixel

gra

y v

alu

e

-30 -20 -10 0 10 20 30-0.2

0

0.2

0.4

0.6

0.8

1

1.2

x

y

data

fitted curve

Edge response function

Line spread function

0 5 10 15 20 25 300

0.2

0.4

0.6

0.8

1

m

Modulation transfer function

(MTF)

Reference: Sun W, et al. 2015 A preliminary study of characteristics of X-ray computed

tomography, Proc. of Digital Industrial Radiology and Computed Tomography (Ghent, Belgium)

Page 9: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

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Geometrical error modeling for

coordinate measurements by XCT

Modelling volumetric errors due to detector angular misalignments

Modelling radiographic binning errors due

to detector angular misalignments

Ref: Ferrucci M, et al. 2016 Evaluating the effects of detector angular misalignments on simulated

computed tomography data Precis. Eng.

Page 10: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Other factors considered

• Scattering

• Beam hardening

• Cone beam error

• Reconstruction algorithm

• Surface determination algorithm

• More…

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Page 11: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Scale check – 1D distance verification

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• Traceability: Calibrated Ball

bar

• Scale: Distance is

independent on threshold

• Downfall: measured at a fixed

height, therefore alignment

issues not highlighted

d

Ball separation vs height

85.55

85.555

85.56

85.565

85.57

85.575

85.58

85.585

180 190 200 210 220 230 240 250 260 270 280 290 300

Y position/mm

Sp

he

re s

ep

era

tio

n/m

m

Page 12: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Ball plate – 2D distance verification

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• Ball plate manufactured from

6 mm ruby spheres, 25 in total

• Allows measurements

between spheres in multiple

directions

• Verification of system at

different scales

• Alignment issues

Page 13: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

Holeplate– investigation

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• Multi direction distances

• Bi-directional and uni-

directional measurements

of hole plate, immersed

inside NPL cam.

• Multi-machine

measurements

Bi-directional

Uni-directional

Page 14: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

XCT vs Other measurement techniques

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Advantages Disadvantages

Tactile Profile/surface

data

Can be traceable

Contact method, can damage

surface

External surface only

Angle limitation

Time consuming for large area

Optical Surface data

Noncontact

External surface only

Angle limitation

Time consuming for large area

Influence of some systematic

errors on measurement not

fully understood

XCT Volumetric data

(both external

and internal)

Noncontact

Non-invasive

Systematic errors

Large amount of data

Influence of systematic errors

on measurement not

understood

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Dimensional summary - Verification

Standards

Tactile •CMMs ISO 10360

Optical •VDI/VDE 2634

XCT •VDI/VDE 2630, (ISO

10360 Part XCT under

development)

Performance verification

External dimensions •Surface roughness may

affects all measurement

systems

•Young’s modulus affects tactile

but not optical or XCT

•Optical properties may affect

optical techniques but do not

affect tactile or XCT

Internal dimensions • Surface has to be

accessible for tactile and

optical

• XCT has the ability to

measure both surfaces and

material interfaces, through

threshold values, however,

not traceable.

The surface roughness of the AM parts, limit the ability to

make confident dimensional measurements.

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An example of NPL designed test artefact

• Corkscrew distortion

during build

• Prismatic simple

geometric structures

• Equivalent internal

structure

• Allow traceability

• Effects of beam

hardening

• AM capability

• Parallel flat side

faces

• Include cube and

cylinder

• Internal cube and

cylinder

• Tooling balls

• External cylinders of

a variety of material

• Conventional

manufactured

Manufactured from XCT compliant

materials: AlSi10Mg and Al for

conventional

Page 17: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

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Overview of dimensional tests carried

out using contact CMM • Measurements of large flat areas on the

AM surface.

• Evaluation of best fit plane

• Corkscrew distortion

• Measurement of cylinder at different

heights, above the plane ‘A’:

• (18.5, 21, 23.5, 26, 28.5) mm

• Best fit circle and standard

deviation of data

• External (and internal proposed work)

comparison measurements.

• AM allows easy to build but,

unfortunately, sometimes difficult

to measure.

A

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Dimensional measurements - Summary

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• Comparison of data from CMM,

XCT and optical.

• Sphere diameter and distance

• Circle diameters and standard

deviation (Std)

18.5

21.0 23.5

26.0 28.5

C

B

A

Position

of scans CMM Std

CMM -

Optical Std

CMM -

XCT Std

18.5 14.070 0.037 0.206 0.030 0.165 0.012

21.0 14.051 0.036 0.232 0.029 0.177 0.012

23.5 14.066 0.043 0.246 0.030 0.192 0.013

26.0 14.052 0.040 0.202 0.035 0.163 0.012

28.5 14.059 0.034 0.201 0.034 0.177 0.013

Item CMM CMM-

Optical

CMM-

XCT

A(Dia) 9.991 0.079 0.005

B(Dia) 9.991 0.188 0.006

C(Dia) 9.992 0.064 0.006

A-B 68.381 -0.063 0.014

A-C 50.769 -0.026 0.013

B-C 57.820 -0.025 0.008 All units

are in mm

Page 20: Developments in Dimensional Metrology in X-ray … · Developments in Dimensional Metrology in X-ray Computed Tomography at NPL ... traceable contact CMM at NPL. ... Advantages Disadvantages

The National Measurement System is the UK’s national infrastructure of measurement

Laboratories, which deliver world-class measurement science and technology through four

National Measurement Institutes (NMIs): LGC, NPL the National Physical Laboratory, TUV NEL

The former National Engineering Laboratory, and the National Measurement Office (NMO).

The National Measurement System delivers world-class

measurement science & technology through these organisations

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