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Terahertz: An Emerging Technology for Process Control of Printing, Coating and Various
Other Aspects of Paper Manufacturing
Mark Kemper, Picometrix, LLC
Jeffrey White, Picometrix, LLC
Overview
• Introduction to Time‐Domain Terahertz Technology (THz)
• Measurements of Interest–Simultaneous Coat Weight–Substrate Basis Weight–Moisture Measurement
• Summary
What is Terahertz•Portion of EM spectrum between Microwaves and Far‐IR
•Used by Astronomers for yearsGas Phase spectroscopy of interstellar objects
• Typically used liquid He cooled Bolometer detectors and very long acquisition times (min)
What is Terahertz?•Terahertz is…
–An emerging technology for Non‐destructive Testing (NDT) combining imaging and chemical analysis
–Non‐contact, non‐destructive, non‐invasive –Safe, not harmful to operators or inspected objects
–Electromagnetic properties: 0.02 – 4 THz, 0.7 – 133 cm‐1,15 – 0.075 mm
•Terahertz can…–See through most materials
–Determine chemical composition
–Measure multiple properties simultaneously
•Moisture, density, weight, thickness…
Advantages• FAST (measurements can be done 100 times per second and soon to be 1000 times per second…eventually 4000 times per second)
•Non‐destructive, non‐contact, non‐invasive• Transparent to most materials
• Safe to user and material (µW energy at the sample, Class I laser)
•Physical and spectral properties can be measured
• Fiber optic flexibility and versatility•Allows on‐line and off‐line measurements that are difficult to achieve with any other technique
Advantages• FAST (measurements can be done 100 times per second and soon to be 1000 times per second…eventually 4000 times per second)
•Non‐destructive, non‐contact, non‐invasive• Transparent to most materials
• Safe to user and material (µW energy at the sample, Class I laser)
•Physical and spectral properties can be measured
• Fiber optic flexibility and versatility•Allows on‐line and off‐line measurements that are difficult to achieve with any other technique
General Areas of Application
•Imaging
•Spectroscopy•Time domain (physical parameters)
•Can be used for non‐destructive testing on‐ and off‐line
splitter
Typical TD‐THz System
GDC splittervariable delay
power meter
splitter
power meter
100fs Laser
Time Domain Terahertz (TD‐THz)
Time Domain Terahertz (TD‐THz)
Reason for Interest
•In Fall 2007, the Domestic Nuclear Detection Office (DNDO) of the Homeland Security (HLS) Department awarded a $1,000,000 grant to develop a functional economic replacement for nuclear gauges (Alpha, Beta, Gamma)
•The goal of the HLS office was to eventually remove all “orphaned” nuclear material from the private sector
Applications and Benefits for Paper
•Coating Thickness•Basis Weight
•Caliper Thickness•Moisture Measurement
•Ink Mileage
•Simultaneous Measurements (simplicity)
•Remote Measurements
•In Process Measurements
Sensor Installed on CLC
Samples
• S1C1W1• S1C1W2• S1C1W3• S1C1W4• S1C1W5• S2C1W1• S2C1W2• S2C1W3• S3C1W1• S3C1W2
S1 = RecycledS2 = Free SheetS3 = LWC
C1 = High SolidsC2 = Low Solids
• S1C2W1• S1C2W2• S2C2W1• S2C2W2• S2C2W3• A2C2W4
Results – B ‐ Scan
Coating Start
Coating Start
Velocity = 2500 ft/min
Velocity = 350 ft/min
Substrate Basis Weight
Caliper Thickness
Sample Caliper Thickness (mm ± 0.002) THz Thickness (mm) Deviation
HCC80 0.226 0.226 < 1 µm
HCC100 0.285 0.285 < 1 µm
K184 0.249 0.252 3 µm
K219 0.188 0.187 1 µm
K247 0.297 0.297 < 1 µm
K247 0.297 0.296 1 µm
Wet Coating Measurement
Cured Coating Measurement
Cured Coating Measurement
Coat Weight Measurement
Detection of Coating
-2 -1 0 1 2 3 4-0.2
-0.1
0.0
0.1
0.2
0.3
Light Weight Coating Up - Deconvolution / Fitting
Dec
onvo
lved
TD
-TH
z Si
gnal
(Arb
)
Waveform Time (ps)
Decon Sum2PkFit Sum3PkFit
-2 -1 0 1 2 3 4
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Light Weight Coating Up - CPT Fits
Tim
e-D
omai
n Te
rahe
rtz S
igna
l (V
olts
)
Waveform Time (ps)
Fit1 Fit2 Fit3
-2 -1 0 1 2 3 4
-0.2
-0.1
0.0
0.1
0.2
0.3Light Weight Coating Dn - Deconvolution / Fitting
Tim
e-D
omai
n Te
rahe
rtz S
igna
l (Vo
lts)
Waveform Time (ps)
Decon Sum2PkFit Sum3PkFit
-2 -1 0 1 2 3 4
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
Light Weight Coating Dn - CPT Fits
Tim
e-D
omai
n Te
rahe
rtz S
igna
l (V
olts
)
Waveform Time (ps)
Fit1 Fit2 Fit3
Basis Weight VariationFormation
Sample D Δt
5 10 15 20 25 30 35 40 45
5
10
15
20
25
30
35
40
45
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
Basis Weight Measurement
4 6 8 10 12 14 16 180.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
6 Paper/Card Stock Types - 4 Paper Weights, 4 Card Stock Weights
R^2=0.999
Paper Basis Weigth Measurement
Mea
sure
d 4
Pass
TH
z Ti
me
Del
ay (p
s)
Paper Weight (g)
Measured Values Linear Fit
Summary
•THz has potential to make measurements of coating thickness, basis weight, caliper thickness and moisture simultaneously. Ink Mileage may also be possible.
•Freely positionable (single sided measurement, fiber optic connected) sensor easy to operate for laboratory measurements or for use in process.
•THz measurements can provide accurate process control to allow improved quality and consistency.