verifying the measurement accuracy for multiple-scan x-ray ... · pdf fileperfect spheres...

Verifying the measurement accuracy for multiple-scan X-ray cone-beam CT measurements

iCT Conference 2018, 2018-02-08

Daniel Weiß, Carl Zeiss IMT GmbH, Business Unit X-ray

2Carl Zeiss IMT GmbH, Daniel Weiß, Business Unit X-ray 2018-01-23

Agenda

123

Design of the reference standard

Results

Motivation


Agenda

123


Results

Motivation


Standardization: guidelines VDI/VDE 2617 + 2630 for DIN EN ISO 10360

Created to establish characteristics describing the accuracy of coordinatemeasurement devices with CT sensors

Scanning errors PF and PS3d error behavior in asmall measurementvolume, measured onperfect spheres

Length measurement error E3d error behavior of the overall system

Form errorPF = Rmax - Rmin

Size errorPS = Da - Dr

Measured either with a gauge block (using 2 single measurement points):E = La - Lr

Or using 2 spheres (center to center):E = Lka – Lkr + PS ± PF


Guideline requirements

- Measure 5 lengths, at 7 orientations, 3 times each, at least two magnifications- Smallest test length must be ≤ 30 mm- Largest test length must be ≥ 66% of measurement volume diagonal

- “METROTOM Check” (has been used forZEISS metrology CTs for more than 10 years):

- Arrangement of 22 or 27 spheres- Suitable for tactile calibration, stable with

regard to temperature and time- Maximum test lengths of 56 and 113 mm- Does not scale to large test lengths…

56 mm max


METROTOM specifications

Required test length: 305 655 ∗ 0.66 477 mm

Composite scan uses „stitching“ along z-axis


Agenda

123


Results

Motivation


Design of the reference standard:Current overview of designs

Source: Industrial X-Ray Computed Tomography, Springer 2017,Ed. Simone Carmignato, Wim Dewulf, Richard Leach



- The purpose is to apply the acceptance test MPESD (SD: sphere center distance) according to guideline VDI/VDE 2617/2630, to volume composite measurements

- Design according to guideline: "7 spatial directions, measuring 5 test lengths each 3x in each direction, smallest test length ≤ 30 mm, largest test length ≥ 66% of the room diagonal of the measuring volume“

- Selected maximum lengths per direction: 165, 168, 203, 246, 310, 457, 500 mm

- The tilt angles of the test lengths to the horizontal were evenly distributed between 0° and 90°

- 4 spheres per rail result in 6 sphere pairs or distances- Ruby spheres (diameter 8 mm, grade 5) glued onto quartz

rods


Design of the reference standard:Sphere rails are used as test lengths

- The shortest test length on each sphere rail is 30 mm- The position of the third sphere is chosen so that the resulting distances between

sphere pairs are equally spaced- Outer cylinders protect the spheres


Design of the reference standard:DAkkS calibration of the test lengths

- A special calibration device for linear reference standards such as gauge blocks, using a laser interferometer distance sensor, was used for the tactile measurement


Design of the reference standard:DAkkS calibration of the test lengths

(at 20°C)(All sphere centers are projected onto the line through the first and last sphere center.)


Design of the reference standard:Measurement uncertainty of the DAkkS calibration

Nominal (mm)

U1 (µm) U2 U at nominal (µm)

165 1,1 2,0 1,43168 1,1 2,0 1,44203 1,1 2,0 1,51246 1,1 2,0 1,59310 1,1 2,0 1,72457 1,1 3,0 2,47500 1,1 3,0 2,6

Extended (k = 2, 95% probability) measurement uncertainty U = U1 + U2 * 10-6 * l, with l the distance of the sphere centers (in μm)(Thermal expansion of the measuring length 500 mm: 0.26 μm/K)


Design of the reference standard:Final prototype

Height of large cylinder: 640 mmOuter diameter: 230 mm


Design of the reference standard:Alternative design (not built)

Problems:- Lack of torsional rigidity- Metal rings and the

titanium parts in the ThermoFit elements create CT artifacts


Design of the reference standard:Composite of two scans

Bottom half (z = 290) Top half (z = 30)

#1 (165 mm)

#6 (457 mm)

#7 (500 mm)

#5 (310 mm)

#2 (168 mm)

#3 (203 mm)

#4 (246 mm)


Design of the reference standard:Z movement is critical

- Because the subvolumes are “stitched” in z, any kind of unintentional z movement by source, object, or detector is critical, and will be converted 100% into measurement error!


Agenda

123


Results

Motivation


Results on METROTOM 1500:Scan parameters

Detector settings

2048x2048, 200 µm pixel pitch,800 ms exposure time

Voxel pitch 148 µm

Sourcesettings

180 kV @ 360 W, reflection target, 0.25 mm Cu prefilter

Source trajectories

2x circular “step-and-go”,2000 projections per scan


Results on METROTOM 1500

4,5 + L / 50

sphe

re distance error (m

m)

Lengths going from 1st to 2nd subvolume


Thank you

Thank you to the team at ZEISS:

Wolfgang KimmigRonald LonardoniMarc IlzhöferTimo SchneiderChristoph Kuhn

verifying the measurement accuracy for multiple-scan x-ray ... · pdf fileperfect spheres...

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