Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2001-10-19
2003-12-30
Kwok, Helen (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S602000
Reexamination Certificate
active
06668653
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for highly accurate measurements of propagation characteristics, in particular, phase velocity, of leaky surface acoustic waves (LSAWs) by an ultrasonic material characterization system using a focused ultrasonic beam.
The ultrasonic material characterization system has been developed as a new substance/material property analysis/evaluation technique, and it uses ultrasonic plane waves and ultrasonic focused waves for quantitative measurements. One of the quantitative measurement schemes using the ultrasonic focused waves is a V(z) curve analysis. This scheme is to measure propagation characteristics (phase velocity and propagation attenuation) of leaky surface acoustic waves (LSAWs) excited on a water-loaded specimen surface. The measurement can be done using an ultrasonic point-focus beam (PFB) and an ultrasonic line-focus beam (LFB). Now, a description will be given of an LFB ultrasonic material characterization system (see References (
1
) and (
2
)).
The LFB ultrasonic material characterization system determines LSAW propagation characteristics on the water/specimen boundary through analysis of a V(z) curve obtained by changing the relative distance z between an LFB ultrasonic device and the specimen. The principle of measurement by the LFB ultrasonic material characterization system will be described with reference to
FIG. 1
, which depicts in section the system wherein an ultrasonic device composed of an ultrasonic transducer
1
and an LFB acoustic lens
2
and a specimen
4
are disposed. The coordinate system is defined with its origin at the focal point of the acoustic lens
2
in water
3
as shown. Ultrasonic plane waves excited by the ultrasonic transducer
1
are converged by the LFB acoustic lens in wedge form onto the surface of the specimen
4
through the water
3
used as a coupler. When the specimen
4
is moved away from a focal plane
5
toward the ultrasonic device, only those components of the reflected wave from the specimen
4
which propagate through the acoustic lens
2
along paths #0 and #1 approximately shown in
FIG. 1
most contribute to the generation of the output from the ultrasonic transducer
1
due to the effect of the cylindrical surface of the acoustic lens
2
. The component #0 is a directly reflected component from the specimen
4
, and this component will hereinafter be referred to as a phasor V
0
(z). On the other hand, the component #1 is one that impinges on the specimen
4
at an LSAW excitation critical angle &thgr;
LSAW
and propagates as an LSAW on the surface of the specimen
4
—this component will hereinafter be referred to as a phasor V
1
(z). The phasors are given by the following equations (1) and (2) taking into account their phase and amplitude variations with the lengths z of their paths.
V
0
(
z
)=|
V
0
(
z
)|exp{
j
(−2
k
W
z+&phgr;
0
)} (1)
V
1
(
z
)=|
V
1
(
z
)|exp{
j
(−2
k
W
cos &thgr;
LSAW
z+&phgr;
1
)} (2)
k
W
=2&pgr;
f/V
W
(3)
where k
W
and V
W
are the wave number and velocity of a longitudinal wave in the water
3
, f is an ultrasonic frequency, and &phgr;
0
and &phgr;
1
are initial phases of the phasors. The transducer output V(z) is given by the following equation as the sum of the two phasors.
V
(
z
)=
V
0
(
z
)+
V
1
(
z
) (4)
Noting the amplitude of the transducer output signal, the amplitude V(z) of the phasor V(z) is given by the following equation (5).
V
(
z
)=|
V
(
z
)|=|V
0
(
z
)+
V
1
(
z
)| (5)
Therefore, the V(z) curve takes a waveform that is periodically maximized and minimized due to variations in the relative phase difference between the two phasors with the distance z, and the interference interval &Dgr;z of the V(z) curve is given by the following equation (6).
&Dgr;
z=
2
&pgr;/k
(&Dgr;
z
)=2&pgr;/2
k
W
(1−cos &thgr;
LSAW
) (6)
where k(&Dgr;z) is the wave number of the V(z) curve interference waveform on the V(z) curve. The LSAW velocity V
LSAW
is determined by the following equation (7) from the interference interval &Dgr;z of the V(z) curve.
V
LSAW
=
V
W
1
-
(
1
-
V
W
2
f
Δ
z
)
2
(
7
)
The LSAW propagation attenuation is also obtainable from the waveform attenuation factor of the V(z) curve.
FIG. 2
is a graph showing an example of the V(z) curve measured at an ultrasonic frequency f=225 MHz for a (111) GGG (Gadolinium Gallium Garnet) specimen with LSAWs propagating in the [{overscore (1)}{overscore (1)}2] direction.
A description will be given below of the procedure for the analysis of the V(z) curve to determine the LSAW velocity V
LSAW
.
FIG. 3
is a flowchart explaining the procedure for the V(z) curve analysis. Usually, the V(z) curve measured on a decibel scale (
FIG. 4A
) is converted to digital form and then read into a computer, wherein it is converted to a linear scale (step S
1
). A V
L
′ curve, which is an approximation of a V
L
(z) curve reflecting the characteristic of the ultrasonic device, is subtracted from the V(z) curve to obtain a V
I
′(z) curve (step S
2
). The V
L
′(z) curve used in this case is, for example, such a V(z) curve as depicted in
FIG. 4B
which was measured for a Teflon (trademark) specimen in which no leaky surface acoustic waves are excited.
The next step is to remove, by digital filtering, small interference components (on the V(z) curve in
FIG. 4A
) due to a spurious noise signal caused on the V
I
′(z) curve by a carrier leakage signal in an RF switching circuit for generating an RF tone burst signal used in the measurement system or multiple-reflected signals of ultrasonic waves in the acoustic lens (step S
3
). After this, interference components (interference interval &Dgr;z components) resulting from leaky surface acoustic waves are removed by digital filtering from the V
I
′(z) curve to synthesize a &Dgr;V
L
(z) curve representing low-frequency components including DC components (step S
4
). This is followed by subtracting &Dgr;V
L
(z) from V
I
′(z) calculated in step S
2
to obtain such a V
l
(z) curve as shown in
FIG. 5A
which is an interference output necessary for analysis (step S
5
).
The digital filtering is implemented, for example, by the moving average method. An FFT analysis of the V
I
(z) curve provides such a frequency spectrum distribution as depicted in
FIG. 5B
, and the interference interval &Dgr;z is obtained from the peak frequency (step S
6
). Since the longitudinal wave velocity V
W
in water is known as a function of temperature as set forth in Reference (
3
), it can be determined from the water temperature T
W
measured at the same time as the V(z) curve is measured (step S
7
). Therefore, the LSAW velocity V
LSAW
can be calculated by Eq. (7) from the interference interval &Dgr;z and the longitudinal wave velocity V
W
(step S
8
). While in the above the analysis scheme has been described on the assumption of only one leaky surface acoustic wave mode, it is a matter of course that when multiple modes are present, propagation characteristics can be measured for each mode. The V(z) curve in
FIG. 2
is shown to have removed therefrom by the moving average method the small interference components based on the above-mentioned spurious noise signal. Thus it can be seen that the V
LSAW
measurement accuracy depends mainly on the water temperature measurement accuracy, which determines the longitudinal wave velocity V
W
, and the translation accuracy of a Z (vertical translation) stage used in the system.
Now, a description will be given of the influence of a temperature measurement error on the V
LSAW
measurement accuracy.
FIG. 6
shows LSAW velocity measurement errors calculated by Eq. (7) with respect to water temperature measurement errors at LSAW velocities of 2000 m/s, 3000 m/s, 4000 m/s, 6000 m/s
Kushibiki Jun-ichi
Ono Yuu
Gallagher & Lathrop
Kushibiki Jun-ichi
Kwok Helen
Lathrop, Esq. David N.
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