Non-contact air-coupled velocity
measurements in planar samples
Qiang LIU1,2,Bogdan PIWAKOWSKI2,Zoubeir LAFHAJ1
1. Laboratoire de Mécanique de Lille (LML, UMR CNRS 8107), Ecole Centrale de Lille
2. Institut d’Electronique, Microélectronique et Nanotechnologies (IEMN UMR 8520), Ecole
Centrale de Lille
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The 13th International Symposium on Nondestructive Characterization of Materials
OUTLINE
I.
II.
III.
IV.
V.
Context and goal
Contact methods
Non-contact methods
Experimental results
Conclusions
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Context and goal
1. Necessity to mesure velocity.
2. Traditional methods are contact type.
3. We have developed a non-contact air-coupled system in order to:
a) Avoid coupling medium;
b) Avoid surface preparation;
c) Enable the automation of measurement.
Objective: To measure velocities by using non-contact
air-coupled technique with good accuracy.
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Contact methods
1. Pulse-echo:
Transmitter/Receiver
Need of coupling meduim
Need of planar contact surface
d
1st
Nth
Δt
2. Through-transmission:
Δt
Δt
1st
Transmitter
Nth
V
Receiver
2d
t
d
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Immersion method
Water
Receiver
Transmitter
Water
Receiver
Transmitter
d
VL 
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d
t p  tw 
d
Vw

d
t pw 
d
Vw
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Non-contact air-coupled method
Air
Receiver
Transmitter
Air
Receiver
Transmitter
0.01%
d
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Two problems:
The immersion solutions developed for water cannot be applied
directly for air because:
•The velocity in air is unstable, so it is better not to use velocity in air
for the computation of velocity in sample.
•The transmission coefficient is too small to allow the emitted energy
penetrate into the sample, so it is necessary to apply techniques which
improve the signal level.
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« Chirp» signal
A
A
Transmitter
Receiver
A
t
t
T
Correlation
s (t )  A sin( 2ft )  sin[ 2f (t )t ]
f (t )  f min  at
t  (0, T )
400
200
0
-200
0
0.002
0.004
0.006
time [s]
0.008
0.01
k (0)  S  A2 BT
NB
S
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N
 A2T
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Experimental setup
Power
Amplifier
PC
Motor control
Generator: Generation of chirp;
Memory card: Acquisition of signal;
LabView: Correlation.
Movements in x,
y ,z directions;
Pre
Amplifier
Receiver
Transmitter
z
Motor control
z y y
PC
x
x
Amplifier
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Transmitter
Receiver
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Calibration
Receiver
Before calibration
Transmitter
Beam axis
L
y
After calibration
Receiver
Transmitter
Beam axis
L
y
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Receiver
Transmitter
2
1
ΔL
L1
L2
y
Vair 
The moving
direction of receiver
First step: Calculation of Vair
L2  L1 L

ta 2  ta1 ta
(1)
ta1
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Receiver
d
Transmitter
The moving
direction of receiver
Second step: Calculation of VL
1
2
3
y
L
1
2
Δtp
Δtp
Δtp
VL _ Multiple 
VL _ Direct 
tp1
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2d
t p
(2)
d
t p1  ta 
d
Vair
d

t pa 
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d (3)
Vair
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Receiver
d
Transmitter
The moving
direction of receiver
Second step: Calculation of VL
1
2
3
y
L
1
2
Δtp
Δtp
Δtp
VL _ Multiple 
VL _ Direct 
tp1
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2d
t p
(2)
d
t p1  ta 
d
Vair
d

t pa 
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d (3)
Vair
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Third step : Calculation of VT
Receiver
Transmitter
β
α
tan  
The incident angle should greater than the
first critical angel and less than the second
critical angle.
Δy
y
2  d  cos 
αc1<α<αc2
y
1
2
Δy
VT _ Mutiple 
Δts
VT _ Direct 
2d
y  tan 
(ts 2  ts1 
)  cos 
Vair
d
d  cos(    ) / cos 
(ts1  ta 
)  cos 
Vair
VT _ Shift  Vair
ts1
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ts2
sin 
sin 
(4)
(5)
(6)
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Validation with Plexiglas plate
Comparing with the velocity obtained by contact method we find:
1. For longitudinal velocity, the ‘Multiple’ approach is more
precise than the ‘Direct’ approach, because the velocity in air
isn’t used here. As well the standard error of ‘Multiple’ approach
is smaller.
2. For shear velocity, the results of ‘Multiple’ and ‘Shift’
approaches are also more precise than the ‘Direct’ approach.
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Results obtained with porous material : mortar sample
Our conclusions are confirmed with a mortal sample.
Comparing with contact approach:
1.
For longitudinal velocity, our ‘Multiple’ approach
produces only 0.7% error while the ‘Direct’ approach
produces 43.8% error.
2.
For shear velocity, our ‘Multiple’ approach provides only
26.5% error, Shift’ approach provides 2.3% error white
the ‘Direct’ approach produces 38.3% error.
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Conclusions
1. ‘Direct’ approach, comes from immersion method, and commonly
used in non-contact air-coupled technique gives always a high error
when compares it with contact method.
2. ‘Multiple’ approach for longitudinal velocity and ‘Shift’ approach for
shear velocity provide higher accuracy which is comparable with the
contact method.
3. Non-contact air-coupled method has great potential for the rapid
measurements of velocity in ‘difficult’, porous, rough surface
materials, such as concrete.
4. The proposed approach can be easily automated and used in high speed
automatic NDT systems.
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Thank you for you attention!
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