TAPPI PaperCon ’09 - Session 65 R. Naimimohasses, A. Hellstrom Next Generation Optical Caliper Sensor Technology St. Louis, May 31 – June 3, 2009
TAPPI PaperCon ’09 - Session 65
R. Naimimohasses, A. Hellstrom
Next Generation Optical Caliper Sensor TechnologySt. Louis, May 31 – June 3, 2009
2 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Caliper sensing technologies
Contacting Airbearing Laser / optical
3 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Basic laser triangulation device
4 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Cellulose fibers are semi-translucent
Linerboard
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Paper interaction with laser triangulatorAccuracy depends on how center of laser spot on the surface can be extracted from the intensity distribution and translated to distance
Measured light intensity reflected
Laser
CCD ArrayDetector
Inte
nsity
0 1024CCD element
Opaque solidmaterial
Inte
nsity
0 1024CCD element
Laser
Wood fiber cross section5 µm cell wall
6 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Fiber shape simulation resultsRound fibers Flat fibers
Center of gravity of the blue curve
-60 -40 -20 0 20 40 60-2
0
2
4
6
8
10
12
14
16x 10-6
Height [um]
Rel
ativ
e In
tens
ity
Surface ReflectanceSecondary RefectanceAll together
Center of gravity of the red curve
Center of gravity of the blue curve
-60 -40 -20 0 20 40 60-5
0
5
10
15
20x 10-6
Rel
ativ
e In
tens
ity
Height [um]
Surface ReflectanceSecondary RefectanceAll together
Center of gravity of the red curve
Sources:
K. Saarinen and K. Muinonen “Light scattering by wood fibers” Applied Optics 40(28), ( 2001) 5064-5077.
K. Muinonen and K. Saarinen “Ray optics approximation for Gaussian random cylinders” Journal of Quantitative Spectroscopy and Radiative Transfer 64 (2000), 201-218
7 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Comparison between simulated and measured distribution
-40 -30 -20 -10 0 10 20 30 400
0.5
1
1.5
2
2.5
3
3.5
4x 10
-5
-40 -30 -20 -10 0 10 20 30 400
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5x 10-5
Simulated: Round fiber Flat fiber
Measured by: Y.J. Sung, C.H. Ham, O. Kwon, H.L. Lee and D.S. Keller, “Applications of Thickness and Apparent Density Mapping by Laser Profilometry”, presented at 13th Fundamental Research Symposium, Cambridge, 2005.
Measured:
8 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Contacting Laser
C o m b in a tion 1
0
5 0
1 0 0
1 5 0
2 0 0
2 5 0
3 0 0
0 .5 1 1 .5 2 2 .5 3 3 .5 4 4 .5 5 5 .5 6 6 .5 7 7 .5 8 8 .5 9
tim e , s e c
µ
G TL a s e r
F ine 1
F ine 2
F ine 3B a g K ra ft
C a rd
Dynamic test – laser caliper vs. contacting caliper
9 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Smoothness / porosity problem – laser caliper
Contacting caliperSenses mainly surface peaksRight answer (Lab standard)
Fine paper
Laser caliperSenses mainly surface averageAdd light penetration in fibersDisagreement with lab standard
10 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Confocal optics solves light penetration issue
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Comparison of optical measurement methods
Penetration and scattering
Surface topography
Sources of errorTrue Surface
Paper
Confocal method
Paper
Laser Triangulation
“Glow-ball” False surfaceTrue surface
Laser
Paper
False surfaceTrue surface
Laser True Surface
Paper
12 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Confocal optics principle
Longitudinal chromatic aberration
White light source (LED)
Separated optically into its component colours
Each individual colour is in focus at a specific distance from the lens
Spectral analysis of the reflected light is done
Strongest peak in the reflected colour spectra gives distance of first surface seen
Fast DSP processing (4KHz)
13 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
A wide variety of grades on one linear calibration curvePaper grade Calibration
0
200
400
600
800
1000
1200
1400
1600
1800
0 200 400 600 800 1000 1200 1400 1600 1800
Lab caliper (µm)
Prec
itec
Opt
ical
Cal
iper
(µm
)
Caliper (ave)758 µm double Mylar1120 µm triple Mylarpaper samples (various)
Tissue grade
February 2007
14 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Image of paper surface by confocal probe
This x / y mapping probe has similar technology as the Optical Caliper Sensor
All details are in clear focus (12 µm spot ) independent of distance
No shadowing effects
15 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Sensor - simplified cross section
Air purge
Lens system(optic probe)
Fiber optic to remote processor
Temperaturecontrolled air
Ferrite targetSheet stabilizer
Optic targetOuter vacuumInner vacuum
CeramicSpacer Coaxial ferrite
sensing coil
Chromatic Light cone
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Optical Caliper lab performance
17 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Optical Caliper on high speed, wide LWC machine
1200 data box profiles (7.5 mm per box)Compare Optical and Contacting caliper profiles
Excellent profile agreementOptical detects small details better
January 2008
18 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Newsprint machine, 100% recycled Sensor gap, single profile, process up
Single gap profile ~350 microns variations + vibration.
Magnetic gap
Optic gap
January 2009
19 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Optical vs. contacting caliper - recycled newsprint
January 2009
20 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Optical Caliper vs. Lab Profiler, 100% recycled newsprint
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Sheet (mm)
Pro
file
devi
atio
n (µ
m)
Optical Caliper Lab profiler
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Sheet (mm)
Prof
ile d
evia
tion
(µm
)
Optical Caliper Lab profiler
60 µm, 31/03/2009
52 µm, 31/03/2009
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Machine economy improvement
Profile costTotal cost
22 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Long term stability
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StoraEnso Sachsen Optical Caliper installation
Customer feedback
Good measurement profiles
No marking or sheet damage
No holes picked
No waste paper from CD measurements
24 TAPPI PaperCon’09 - St. Louis, May 29 – June 3, 2009
Thank you for your participation
Questions, Please!