r. naimimohasses, a. hellstrom - tappi · contacting airbearing laser / optical. 3 tappi...

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


Basic laser triangulation device


Cellulose fibers are semi-translucent

Linerboard


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


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


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:


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


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


Confocal optics solves light penetration issue


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


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)


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


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


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


Optical Caliper lab performance


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


Newsprint machine, 100% recycled Sensor gap, single profile, process up

Single gap profile ~350 microns variations + vibration.

Magnetic gap

Optic gap

January 2009


Optical vs. contacting caliper - recycled newsprint

January 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


Machine economy improvement

Profile costTotal cost


Long term stability


StoraEnso Sachsen Optical Caliper installation

Customer feedback

Good measurement profiles

No marking or sheet damage

No holes picked

No waste paper from CD measurements


Thank you for your participation

Questions, Please!

r. naimimohasses, a. hellstrom - tappi · contacting airbearing laser / optical. 3 tappi...

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