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JPH09307998

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DESCRIPTION JPH09307998
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
oblique angle sensor using a polymeric piezoelectric material. More specifically, the present
invention is an oblique angle that can be used for high performance flaw detection suitable for
detecting a defect with electrical low noise, short dead band and high resolution (wide band)
using polymer piezoelectric material. It relates to a sensor.
[0002]
2. Description of the Related Art Conventionally, ferroelectric ceramics are used as piezoelectric
materials used for transmission and reception of ultrasonic waves, and oblique angle sensors
using such ceramic vibrators are known as oblique angle sensors. There is. However, in flaw
detection by an oblique angle sensor using a generally used ceramic vibrator, when adjacent
defects and cracks are detected, it is difficult to separate reflected echoes as noise due to poor
resolution. Further, the oblique angle sensor using ceramics has a problem that the S / N ratio is
small since there are many piezoelectric (acoustic) noises, and moreover, since the dead zone is
long, it is difficult to detect a short distance defect.
[0003]
In particular, in oblique angle flaw detection, for example, when a sensor with a frequency of 5
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MHz and a transducer size of 10 mm × 10 mm or more is used, the near-field can not be
ignored. This is particularly due to the complexity of the sound pressure distribution in the width
direction of the sensor, which affects the detection and evaluation of flaws near the surface of the
object. In order to reduce the influence of such a near-field, it may be considered to reduce the
size of the vibrator, but in this case, the drop in echo height due to distance becomes remarkable,
which makes it possible to detect flaws at long distances. Affect.
[0004]
Also, in recent years, vibrators using a polymeric piezoelectric material instead of ceramics have
appeared. A vibrator using this polymeric piezoelectric material is characterized by being able to
be made thinner and more flexible than a ceramic vibrator, so that it can also cope with
complicated shapes. However, although there is freedom in the shape of use in the sensor using
this polymer piezoelectric material, there remains a problem that sound leaks on the back side
and echo echoes overlap in ultrasonic irradiation, and application to ultrasonic characteristics
and ultrasonic measurement There is still a problem in obtaining a good echo with high electrical
energy efficiency and little noise as a sensor or with high precision.
[0005]
SUMMARY OF THE INVENTION From the viewpoint of taking advantage of the characteristics of
a polymeric piezoelectric material, the present inventors have specifically conceived the idea of
adding a resonant reflector having a thickness of λ / 4 to a polymeric piezoelectric material as a
vibrator. As a result of earnest examination, we succeeded in solving the above problems.
[0006]
That is, an object of the present invention is to provide an oblique angle sensor using a polymer
piezoelectric material, which has high resolution (wide band) and low electrical noise, has a short
dead zone and can detect a short distance defect. .
[0007]
Another object of the present invention is an oblique angle sensor which enables detection of
relatively shallow flaws and high accuracy flaw detection, in particular, in the near field, with less
wave interference and less complicated sound field change. To provide.
[0008]
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Still another object of the present invention is a practical oblique angle sensor that can eliminate
noise leaking to the back side and can be widely used for detecting defects of structures and
devices in a nondestructive area with high accuracy, material evaluation and physical property
measurement, etc. To provide.
[0009]
The oblique angle sensor of the present invention achieves the above object, and comprises an
electrode A for extracting an electrical signal obtained from the input of electrical energy and
stress strain on a wedge slope. An oscillator made of a polymeric piezoelectric material, and a
resonant reflection plate having a function as a printed circuit board on which energy from the
oscillator is reflected in the wedge and on which the electrode B and the electric matching circuit
can be mounted The basic structure is to arrange them, and in this case, the electrode A can also
serve as a protective member that can also be a matching layer.
[0010]
Further, the oblique angle sensor according to the present invention comprises a protective
member which can also be a matching layer, an electrode A for extracting an electrical signal
obtained from an input of electrical energy and stress distortion, and a polymeric piezoelectric
material on a wedge slope. And an electric reflection plate having a function as a printed circuit
board on which the electric energy from the vibrator is reflected in the wedge and on which the
electrode B and the electric matching circuit can be mounted. It is a thing.
[0011]
The oblique angle sensor of the present invention includes the following preferred embodiments.
a.
The polymer piezoelectric material is a polymer piezoelectric material capable of λ / 2 and / or
λ / 4 modes.
b.
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The polymer piezoelectric body is made of one selected from polyvinylidene fluoride, a
copolymer of vinylidene fluoride and ethylene trifluoride or ethylene tetrafluoride, and a
copolymer of cyanovinylidene and vinyl acetate Being
c.
The piezoelectric polymer body is made of a fluorine-based polymer. d. The polymer
piezoelectric body is a composite piezoelectric body made of a piezoelectric polymer material and
a ceramic or the like. e. A resonance coil and a matching coil are further disposed on the
wedge. f. The resonant reflection plate is made of a metal having an acoustic impedance
larger than that of a polymeric piezoelectric material.
[0012]
DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention will be
specifically described below with reference to the drawings. The basic structure as a preferred
embodiment of the oblique angle sensor of the present invention is an oscillation comprising an
electrode A for extracting an electrical signal obtained from the input of electrical energy and
stress distortion on the inclined surface of the wedge, and a piezoelectric polymer body In this
structure, a resonator and a reflection plate for reflecting energy from the vibrator are disposed
in the wedge, and the reflection plate for resonance further includes a print on which an
electrode B for the electrode A and an electric matching circuit are mounted. It also has a
function as a substrate.
[0013]
FIG. 1 is a model diagram for explaining an oblique angle sensor of the present invention, and is
a side sectional view of the oblique angle sensor. Moreover, FIG. 2 is a front view seen from the
arrow direction (arrow direction) of FIG.
[0014]
In FIG. 1, in the oblique angle sensor, a protective member 2, an electrode A 3, a vibrator 4 made
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of a polymer piezoelectric material, and a reflection plate 5 for resonance are sequentially
stacked on the inclined surface of the wedge 1. The protective member 2 and the electrode A3
are separated into two layers in FIG. 1, but in the embodiment of the present invention, they may
have a single layer structure having both integrated functions. As shown in FIGS. 1 and 2, a
resonance coil 6 and a matching coil 7 are further fixed to the inclined surface of the wedge 1
and wired as shown in FIG. 2 to form a circuit. Each of these layers is usually fixed with an
adhesive.
[0015]
Another feature of the present invention resides in the use of a polymer piezoelectric material for
the vibrator 4, and the piezoelectric polymer material constituting this polymer piezoelectric
material can transmit and receive frequencies in the MHz band. In the present invention, a
polymer compound having a polar crystal structure is particularly used. The piezoelectric
polymer material is easy to process into a film-like material having a large area, is flexible and
can be formed into a complex shape, has flexibility, and is shock resistant. In addition to the
characteristics inherent to the polymer, it has characteristics such as low acoustic impedance and
low dielectric constant, which are not found in piezoelectric inorganic materials such as ceramics.
[0016]
Piezoelectric polymer materials used in the practice of the present invention include compounds
having a CF2 group or CN group having a large dipole moment, such as polyvinylidene fluoride
(PVDF), vinylidene fluoride (VDF) and trifluoride Copolymer P (VDF-TrFE) with ethylene (TrFE),
copolymer P (VDF-TeFE) with vinylidene fluoride (VDF) and tetrafluoroethylene (TeFE) and copolymerization of cyanovinylidene with vinyl acetate Although a polymer P (VDCN-VAc) etc. are
mentioned, a fluorine-type high molecular compound is a ferroelectric substance, and copolymer
P (VDF-TrFE) of a vinylidene fluoride and a trifluoro ethylene is especially preferable.
[0017]
The proportion of the vinylidene fluoride (VDF) component in this copolymer P (VDF-TrFE) is
preferably in the range of 65 mol% to 90 mol%, and more preferably in the range of 70 mol% to
85 mol% .
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Incidentally, the density ρ of the polymer piezoelectric material P (VDF-TrFE) at 20 ° C. is 1.88
× 10 3 kg / m 3, the speed of sound v is 2,400 m / s, and the acoustic impedance Z is 4.51 ×
10 6 kg / m 3. It is m2 s.
[0018]
The polymeric piezoelectric material constituting the vibrator 4 of the present invention is
basically composed of such a piezoelectric polymeric material, but other substances may be
further blended within the range not to impair the effects of the present invention Also included
are composite piezoelectrics which are composited by sticking or arranging. For example, a
composite piezoelectric material formed by mixing and mixing other materials such as a
ferroelectric ceramic powder with such a piezoelectric polymer material having a large dielectric
constant as long as the effects of the present invention are not impaired can also be used. In the
case of the ceramic powder, the mixing ratio can be up to 95% by weight according to the
purpose.
[0019]
In the present invention, such a composite piezoelectric body can be produced by arranging rodlike ceramic piezoelectric bodies in a matrix and connecting the peripheries thereof with a
polymeric piezoelectric material, or by mixing ceramic powder in the polymeric piezoelectric
material, Can be obtained by a method of forming a film or the like.
[0020]
The polymeric piezoelectric material used in the present invention is capable of λ / 2 mode and
λ / 4 mode.
Since the speed of sound in the polymer P (VDF-TrFE) is 2400 m / sec, for example, in the case of
λ / 4 mode at 5 MHz, the thickness (t) of the piezoelectric element is t = v / 4 f (f is the
frequency), From this, t = 2400 × 10 6/4 × 5 × 10 6 = 120 μm.
[0021]
In the case of a polymeric piezoelectric material used in the present invention, in-plane vibration
is constrained at frequencies in the MHz band, and only thickness vibration occurs.
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[0022]
One of the other features of the present invention is the use of the reflector 5 for resonance.
The reflection plate 5 for resonance of the present invention reflects energy from the vibrator 4
of the polymeric piezoelectric material into the wedge 1 and also has a function as a printed
circuit board on which the electrode B and the electric matching circuit are mounted.
[0023]
Basically, it is preferable to use an extremely different substance of the piezoelectric polymer and
the acoustic impedance from the viewpoint of the resonance reflection plate 5 of the present
invention, and a metal is mainly used by also serving as the electrode B.
[0024]
The acoustic impedance Z of the above-mentioned polymeric piezoelectric material is as small as
4 to 5 (× 10 6 kg / m 2 s).
For example, the acoustic impedance Z at 20 ° C. of P (VDF-TrFE) is 4.51 × 10 6 kg / m 2 s,
and as the reflector 5 for resonance of the present invention, a metal having a larger acoustic
impedance Z is desirable. For example, copper (acoustic impedance Z = 45.8 to 46.4 × 10 6 kg /
m 2 s), brass (Z = 31 × 10 6 kg / m 2 s), ring copper (Z = 37.6 to 38.4 ×) 106 kg / m 2 s) and
the like, but not limited thereto. The reflection plate 5 for resonance according to the present
invention is selected from materials whose acoustic impedance Z is preferably 10 × 10 6 kg / m
2 s or more larger than that of the polymer piezoelectric material, and precision is particularly
required for oblique angle sensor applications In those cases, those with excellent machinability
are preferably used.
[0025]
The reflector for resonance 5 according to the present invention has the best acoustic energy
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efficiency as a reflector, and the ultrasonic energy generated by the polymer piezoelectric
material is most efficiently obtained as a signal with a small amount of noise to obtain a good
echo waveform. Therefore, the thickness is determined to be λ / 4 of the frequency f determined
by the thickness vibration. Incidentally, in the embodiment of the present invention, the
reflection plate 5 for resonance may be, for example, 12 to 240 μm, but is not limited thereto.
[0026]
In the present invention, the reflector plate 5 for resonance is considered to be substantially
equivalent to a polymer piezoelectric such as a plastic such as a fluorine resin having
characteristics equal to the acoustic impedance of the polymer piezoelectric instead of the above
metal. Although it is possible to use a material, in this case, a thin electrode layer can be formed
on the front surface of the polymeric piezoelectric material, and a resonant reflector can be
provided on the back surface of the polymeric piezoelectric material. The reflection plate for
resonance at this time is determined to have a thickness of λ / 2.
[0027]
It is particularly preferable that the resonance reflecting plate 5 of the present invention also has
a function as an electrode (backing electrode) capable of inputting and outputting electric signals
and a function as a printed board on which an electric matching circuit can be mounted. is there.
[0028]
In addition, the electrode A3 (backing electrode) of the present invention sends electric energy to
the vibrator 4 (polymer piezoelectric material) to apply stress and strain, and at the same time,
cuts off electric energy and receives stress and generates electric energy. When it does, it
becomes the terminal port which transmits and receives electric energy.
Thus, the electrode A for driving the polymeric piezoelectric material can be formed by means of
metal deposition, sputtering or the like using various metals such as gold, copper, aluminum and
the like. The thickness of the electrode A is at most about 100 angstroms to 1 μm.
[0029]
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The surface of the electrode A3 of the present invention is preferably covered with the protective
member 2. The protective member 2 has a role of a protective layer for protecting the electrode
A3 from water or other media, and a role of a matching layer for smooth acoustic coupling
between the contact medium and the polymeric piezoelectric material.
[0030]
The protective member 2 is preferably made of plastic such as polyethylene, polyimide, epoxy
resin and the like, and the thickness thereof is optional, for example, angstrom level to about 50
μm, more preferably angstrom level to 30 μm.
[0031]
As one of the preferred embodiments of the present invention, such an electrode A3 may double
as a protective member.
[0032]
In the present invention, as shown in FIG. 1 and FIG. 2, the resonance coil 6 and the matching
coil 7 are further fixed on the inclined surface of the wedge 1, and as in the electric circuit of FIG.
Each is connected to the reflection plate 5 for resonance and wired.
[0033]
The wedge 1 used in the present invention is used for transversal wave conversion to a steel
material or the like by utilizing the refraction angle according to Snell's law for longitudinal
ultrasonic waves in the wedge 1, and acrylic resin as a material of the wedge 1 And polyimide
resins and ultem resins, but acrylic resins are preferably used.
The wedge is a size for bonding the sensor to the inclined surface, and the wedge inclination
angle I is determined by Snell's law, and in the case of an acrylic resin wedge, the inclination
angle I is usually 27.6 ≦ I ≦ 57. .7 degrees.
[0034]
The oblique angle sensor of the present invention is effectively used for flaw detection of the
joint portion of steel materials such as gas pipes.
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[0035]
(Example 1) A polyethylene-based matching layer protective film, a copper electrode, and a highmolecular polymer having an acoustic impedance of 4.51 × 10 6 kg / m 2 s on an inclined
surface of a wedge with a refraction angle of 53.3 degrees made of acrylic resin. A 10 × 10 mm
vibrator made of P (VDF-TrFE) and a copper reflective plate (printed board) having an acoustic
impedance of 46.4 × 10 6 kg / m 2 s are fixed with an adhesive, respectively. Thus, an oblique
angle sensor was prepared in which a coil for resonance and a coil for matching were wired.
[0036]
The sound field was measured using an analog flaw detector using this oblique angle sensor.
As a result of measurement of the sound field, according to the flaw detector using the oblique
angle sensor of the present invention, the maximum echo was obtained at the depth of 12 mm to
15 mm at the central axis of the beam.
Incidentally, in the case of a normal oblique angle sensor using a ceramic vibrator, the maximum
echo was obtained for the first time at a depth of 20 mm to 22 mm at the central axis of the
beam.
[0037]
Moreover, in the measurement result of the sound field by the digital flaw detector using the
same angle of refraction sensor of 53.3 degrees, the peak of the echo was obtained at the beam
center around the depth of 14 mm. In the case of the angle sensor, the echo height peaks were
obtained at two points apart from the central axis even at a depth of 15 mm or more.
[0038]
Therefore, in the vibrator of the present invention, since a simple sound pressure distribution is
shown from a relatively short distance, it is particularly suitable for evaluation of a flaw or the
like at a short distance.
[0039]
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It is to be noted that, in place of the reflection plate for resonance in the oblique angle sensor of
the present invention, using a baking pedestal of 5 mm in thickness and using an electrode
having a backing copper plate attached thereto determined by λ / 4, It carried out like.
As a result, the echo passes through the λ / 4 backing copper plate and enters the bake and is
reflected at the back edge of the bake thickness of 5 mm, resulting in an overlapping signal and
low resolution.
Example 2 An oblique angle sensor similar to that of Example 1 was prepared.
However, by suppressing the reflection plate for resonance to λ / 4 or less, back noise from the
reflection plate for resonance is eliminated, and in the state in which the electric resonance
circuit is also inserted, the volume (size) is practically the same as the commercially available
ceramic transducer. Sensitivity was
[0040]
The refraction angle of ultrasonic waves and the equivalent dimensions of the vibrator were
measured using this oblique angle sensor, and the near-field of the oblique angle sensor was
obtained by calculation. In the case of a normal ceramic vibrator oblique angle sensor, The
maximum echo was obtained at about 0.7 times the calculated value of the distance sound field
limit distance l0 in the center axis of the ultrasonic beam, whereas in the oblique angle sensor of
the present invention, the sound field is the largest in the ceramic vibrator. The maximum echo
was obtained at about 0.5 times the complex l0 around the ultrasonic beam central axis, and the
distance resolution was excellent.
[0041]
According to the sensor of the present invention, since the dead zone is short and the distance
resolution is excellent, the complex sound field peculiar to the near sound field is not remarkable.
It is effectively used for evaluation.
[0042]
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Further, according to the present invention, a polymer piezoelectric transducer having a high
damping property and a low wave number is used, and a resonator reflection plate or the like is
laminated as described above to form a polymer transducer. It compensates for the low
sensitivity part, which is a drawback of the above, with an electric matching circuit, (1) can be
used for high-performance flaw detection particularly suitable for short distance defect detection,
and (2) less interference of waves in near field. Compared with the conventional sensor, the
complex near-field does not appear noticeably, and (3) as described above, the echo height at the
beam center axis if it is about 0.5 times or more of the near-field limit distance l0 The maximum
of is obtained.
Furthermore, the sound leaking to the back side can be completely eliminated, and it can be
widely used with high accuracy for detection of defects in structures and devices, material
evaluation, physical property measurement and the like.
[0043]
Brief description of the drawings
[0044]
FIG. 1 is a model diagram for explaining an oblique angle sensor of the present invention, and is
a side sectional view of the oblique angle sensor.
[0045]
FIG. 2 is a front view seen from the arrow direction (arrow direction) of FIG.
[0046]
Explanation of sign
[0047]
DESCRIPTION OF SYMBOLS 1 ... Wedge 2 ... Protection member 3 ... Electrode A 4 ... Vibrator
(polymer piezoelectric material) 5 ... Reflection board for resonance (electrode B, printed circuit
board) 6 ... Coil for resonance 7 ・ ・ ・ Correction coil
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