dimensional measurements with micro- ct -test · pdf filewith micro- ct-test procedures and...

‘Microparts’ Interest Group Workshop28. to 29. October 2009, NPL, Teddington

Ulrich Neuschaefer-Rube, Markus Bartscher,

Marko Neukamm, Frank Härtig

PTB Physikalisch-Technische Bundesanstalt,

Braunschweig und Berlin, Germany

Karsten Ehrig, Andreas Staude, Jürgen Goebbels

BAM Bundesanstalt für

Materialforschung

und –prüfung Berlin, Germany

Dimensional Measurements

with Micro- CT

-Test Procedures and Applications20 mm

10 mm

1 mm

2 mm

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1. Introduction

2. Test procedures

and error correction

3. Application examples

4. Conclusions

Content

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Set-up of industrial computed tomography (CT)Measurement object on rotary table between X-ray source and X-ray detector

Micro-CT: Cone-beam and area detector

Scale factor dependent on the distances between X-ray source, measurement object

and detector

Introduction

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geometry fittingwall thickness

analysis

result report

measurement object

threshold process

CT measurement

voxel data

surface datareference data

(e.g. CAD model)

actual/nominalcomparison

Introduction

Flow chart of typicaldimensional CTmeasurement processes

�Each step contributes

to the measurementuncertainty of CT

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Introduction

Structural resolution depends on:Size of X-ray focus

Minimal distance between X-ray source and measurement object

Distance between measurement object and detector

Pixel size of detector

…

Smallest achievable voxel size (X-ray tube): approx. 1 µm (Ø object approx. 2 mm)

Smallest achievable voxel size (Synchrotron): approx. 0,5 µm (Ø object approx. 2 mm)

Further reduction possible with microscope set-up

Image:Andrei Tkachuk et. al:Z. Kristallogr. 222 (2007) 650–655


Carl Zeiss Fraunhofer-Gesellschaft Werth Messtechnik

RayScanTechnologies

GE sensing& inspection

YxlonInternational

Introduction

Industrial CT available and manufactured in Germany (examples)

WenzelVolumetric

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� stability / drift� quantum efficiency � dynamics� contrast sensitivity � internal scattering� pixel variance, noise

detector

� spectrum� focus properties� stability� temperature� opening angle

X-ray source

� radial movement� axial movement� angular-position error� wobbling� linear-position error

rotary table / translation axis

� reconstruction� threshold & surface

generation � data reduction � data corrections

software / data processing

� penetrated depth� beamhardening� scattered radiation � multiple materials

measurement object

Micro CT-system of the BAMacceleration voltage: 30 - 225 kVflat panel X-ray detector: 2048 x 2048achievable voxel size: (3 µm – 200 µm)3

maximum object diameter: 180 mm

� source current� acceleration voltage� integration time� number of projections

� object orientation

user

� temperature / - gradients

� vibrations

environment

� guiding errors� squareness� local heating� intrinsic vibrations

geometry

Introduction - Influences on CT measurements

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Advantages of CT as dimensional measurement technique

� Non-destructive

� Determination of inner and outer geometry

� Achieves a very high point density in relative short time

Disadvantages of CT as dimensional measurement technique

� No accepted test procedures available so far (see following)

� Complex and numerous influence quantities

� Reduced form measurement capability due to measurement errors (artefacts)

� Measurement uncertainty in many cases unknown

Solution

� Apply material standards to correct measurement errors and achieve traceability

� Adopt experiences from coordinate metrology to CT:Use material standards and characteristics to qualify new technique

CT acts as coordinate measuring machine (CMM)

Introduction

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Test procedures

German standardisation activitiesVDI-GMA 3.33 technical committee

“Computed tomography in dimensional metrology”

Acceptance and reverification tests for

dimensional CT systems are described in:

VDI/VDE 2617 Sheet 13

= VDI/VDE 2630 Sheet 1.3 (Draft)

“Computed tomography in dimensional metrology -

Guideline for the application of DIN EN ISO 10360

for coordinate measuring machines with CT sensors”

International standardisation activitiesISO TC213, WG 10:

Dimensional CT accepted as a work item

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Test procedures

Characteristics in VDI/VDE 2617 Sheet 13 / VDI/VDE 2630 Sheet 1.3

Global error characteristics

Error of indication for length measurement (length measurement error)

Material standard: length standard, e.g. step gage, ball bar, ball plate, gauge block

Local error characteristics

Probing error (size and form) PS, PF

Material standard: reference sphere

CT-specific characteristics (optional)

Material- and geometry-dependent errors GS, GG, GF

Material standard: step cylinder

Structural resolution (additional to every specification)

Material standard: e.g. small spheres (under discussion)

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Material standardsfor conventional CMM

Material standardsfor micro-CT systems

Mini-Probe

Calottecube

Ballplate

Stepgage

10 mm

Test procedures

10 mm

10 mm

Development of material standards

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Test procedures - Material standards

1 mm

Microtetrahedron

10 mm

Miniprobe

20 mm

Spherecalotte plate

10 mm

Spherecalotte cube

Length standards / test spheres for CT application

300 mm

Step cylinder

CT-specific standards

Multi-material ringsRing / Cylinder pair

25 mm

10 mm

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Advantages

Increased information content compared

to 2D standard

Traceable tactile calibration available

Standard can be used with

others sensors, too (multisensor)

Sphere calotte cube made from titanium

Calottes manufactured by eroding (EDM)

Cube features 3 facets (x, y, z)

with 5 x 5 calottes

Calottes: Ø 0.8 mm

form deviation: nominal < 2 µm

Optional:

Cavity at the bottom

Modified calotte grid

Test procedures and error correction

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Application of sphere calotte cube to reduce scale errors

Sphere distance deviation of a CT measurement of calotte cubebefore (red squares) and after (black squares) scale correction

0 2 4 6 8 10 12 14 m m

-8

-6

-4

-2

0

2

46

810

12

1416

µm

sp

here

dis

tan

ce

de

via

tion

m easured length

voxel size 15 µm

2775 lengths

red = uncorrected

black = corrected

Recent result

(other CT system):

Max. deviation

< 1 µm


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Application of sphere calotte cube

Spatial distribution of the residual position errors of a corrected CT system Cone beam CT 180 kV, (18.4 µm)3 voxel size

measurement position


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Probing error form PF after correction ≈ 2 µm (voxel size)Probing error form PF before correction < 5 µm

Micro tetrahedron(Ruby balls 0,5 mm)

Manufactured by PTBusing micro mechanics

CT measurement BAM 100 kV, (2 µm)3 voxel size

µm

1 mm

Correction of geometry errors of involved CT axesapplying micro tetrahedron


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No. of teeth 14

Normal modul mn 0.12 mm

Pressure angle α0 20°

Addendum modification coefficient x

0.12

Base circle radius rb 0.789 mm

Helix angle β0 0°

Tip diameter da 2.0 mm

Face width b 1.0 mm

Reference circle radius r0 0.840 mm

Example 1 – Micro spur-gear study

CAD of micro spur-gear

Parameters of micro spur-gear under study

Micro spur-gear

made from steel

2 mm

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Micro-CMM measurements (Carl Zeiss F25)using software Zeiss Calypso and Gear Pro

Tactile micro probeProbe diameter 120 µm

Analysis parameters:Tooth No. 1, 5, 8, 12

Flanks left and right

Radius / length of roll foot rSAP / lαSAP 0.845 mm / 0.301 mm

Radius / length of roll tip rEAP / lαEAP 0.985 mm / 0.589 mm

Reference for the height -0.05 mmof measurement (related to the datum face

of the gear)

Measurement range in height 0.7 mm

Measurements with F25 by Michael Neugebauer (PTB)

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used BAM micro-CT system

Voltage Filtering No. of angles Time Detector size

80 kV 0.25 mm Cu 1800 5 h 2048 x 2048

Data binning Reconstruction Voxel size Surface extraction No. of measurements

No Feldkamp (3.6 µm)3 adaptive 6

reconstructed volume reconstructed volumeconstant threshold surface with adaptive threshold

Parameters of micro CT measurement

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Registrationand analysisof data

Object

CMM-measurementwith Zeiss F25 CMM

Projection with µCT at BAM

Reconstruction

Voxel data

Adaptive threshold

Surface

Comparison

Best fit alignment

Report

Aligned flanks

Point Data

First:Define workpiececoordinate systemwith Calypso.

Second:Measure the flanks with gear specific software Gear Pro.Orientation of the x-axes is unknown!

Tooth flanks

Define coord. system

Define Zeiss F25 CMM workpiece

coordinate system

Best fit alignment with restricteddegrees of freedom

Probe radius correction

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Deviation of CT measurement (data-set 1 of 6)

Distribution of deviations without strong outliers

me

asure

me

nt err

or

in m

m

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Histogram of all 6 assessed measurements

Analysis of deviations

95% of all observed absolute deviations are less 0.0037 mm

Due to the statistics of 6 measurements (correction with t-distribution) a value of 0.0048 mm can be attributed as a first measureof the uncertainty of measurement

0

500

1000

1500

2000

2500

3000

3500

-0.006 -0.004 -0.002 0 0.002 0.004 0.006

measurement error in mm

freq

uen

cy

0

500

1000

1500

2000

2500

3000

0 0.001 0.002 0.003 0.004 0.005 0.006

absolute value of measurement error in mm

freq

uen

cy

0%

20%

40%

60%

80%

100%

cu

mu

lati

ve p

erc

en

tag

efrequency

cumulative %

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Example 2 – Internal micro gear study

No. of teeth 40

Normal module mn 0.12 mm

Pressure angle α0 20°

Addendum modification coefficient x

-0.79

Base circle radius rb 2.615 mm

Helix angle β0 0°

Tip circle diameter da 4.8 mm

Root circle diameter df 5.23 mm

Internal micro gear

made from steel

lengthca. 11 mm

� Reference measurement with micro CMM

Werth VideoCheck HA 400 with tactile-optical probe

Probe diameter: 61 µm

Modus of operation: single point probing

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Reproducibility test of CMM measurement (7 measurements)Result of gear analysis: Profile deviations (example: tooth 10, left flank, z = -0.4 mm)

Reproducibility better than 1 µm in analysis interval


-0.005

-0.0025

0

0.0025

0.005

0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3

length of roll in mm

pro

file

dev

iati

on

in

mm

tooth tip tooth rootanalysis interval

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Synchrotron CT measurement

at BESSY BAMLine

Report

Object

Threshold

Voxel data

Surface

Comparison

Reconstruction

Segmented slice data

Gear analysis

CMM meas.

Gear analysis


Energy: 60 keV

Voxel size: (2.2 µm)3

Detector: GdOS scintillator,

CCD-camera

(3209 x 801 pixel)

Exposure (each): 3 s

No. of angles: 2500 (0°-180°)

Projection

Point data

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Comparison of CT and CMM dataResult of gear analysis: Profile deviations (example: tooth 10, left flank)

Deviations < 2 µm in analysis interval


-0.005

0

0.005

0.01

0.015

0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3

length of roll in mm

pro

file

de

via

tio

n i

n m

m

CT, z = -0.4 mm

CT, z = -0.6 mm

CMM, z = -0.4 mm

CMM, z = -0.6 mm

tooth tip tooth rootanalysis interval

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� Complete dimensional measurements of microparts possible with micro-CT

� Smallest voxel size without optics approx 0.5 µm

� Successful application of material standards

� Standardization of dimensional CT:

- VDI/VDE 2617 Sheet 13 = VDI/VDE 2630 Sheet 1.3

available as Draft

- Work item CT in ISO TC 213

� Achieving traceability of CT measurements

is still a challenge

First results: measurement uncertainties of several

micrometer for small complex objects

� Vision in future:

CT is a fully accepted measurement technique

which is used coequal to conventional coordinate metrology

Conclusions

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Contact:

[email protected]

Thank you for your attention !

Activities are supported by the German Research Foundation (DFG)

project No. NE 757/2-2

and by the German Federal Ministry of Economics and Technology

project No. AZ: II D 5 – 07/06

dimensional measurements with micro- ct -test · pdf filewith micro- ct-test procedures and...

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