JP2004349973

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DESCRIPTION JP2004349973
An ultrasonic sensor with improved response and reverberation is obtained. A substantially
cylindrical acoustic matching material 11 having a top surface 11a on one side and an opening
11b on the other, and a piezoelectric element of a spreading vibration mode provided inside the
top surface 11a of the acoustic matching material 11. And an ultrasonic sensor using a thickness
vibration mode of the top surface portion 11a and a bending vibration mode of the entire
acoustic matching material 11. A damping member 15 having a larger acoustic impedance than
the acoustic matching member 11 is provided in the opening 11 b of the acoustic matching
member 11 so as not to contact the piezoelectric element 12. [Selected figure] Figure 1
Ultrasonic sensor
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic sensor, for example, an ultrasonic sensor used in a gas flow meter or the like to
transmit and receive ultrasonic waves. [0002] Conventionally, as an ultrasonic sensor that
requires high accuracy as in a gas flow meter, the one described in Patent Document 1 is
disclosed. It has been proposed by the applicant. As shown in FIG. 6, this ultrasonic sensor
comprises a cylindrical acoustic matching layer 1 having a top surface 1 a and an opening 1 b, a
piezoelectric element 2 and a backing layer 3, and the piezoelectric element 2 is an acoustic
matching layer 1. The backing layer 3 is bonded to the inside of the top surface portion 1 a, and
the backing layer 3 is filled in the entire opening 1 b of the acoustic matching layer 1. In this
ultrasonic sensor, by vibrating the piezoelectric element 2 in the expansion mode, a thickness
vibration mode of the top surface portion 1 a of the acoustic matching layer 1 and a bending
vibration mode of the entire acoustic matching layer 1 are generated. The backing layer 3 is for
suppressing unnecessary vibration of the piezoelectric element 2 and the acoustic matching layer
1. However, in the above-described conventional ultrasonic sensor, the backing layer 3 does not
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always function sufficiently, and has problems in responsiveness and reverberation
characteristics. That is, when a hard material is used for the backing layer 3, the vibration is
inhibited because the backing layer 3 is in contact with the piezoelectric element 2, and the
sensitivity is lowered. Therefore, although a soft material is used, the damping effect is small and
sufficient responsiveness can not be obtained, and reverberation remains. Accordingly, an object
of the present invention is to provide an ultrasonic sensor with improved responsiveness and
reverberation. Another object of the present invention is to provide an ultrasonic sensor capable
of preventing disconnection in a lead wire connected to a piezoelectric element or a soldered
portion in addition to achieving the above object. SUMMARY OF THE INVENTION In order to
achieve the above object, an ultrasonic sensor according to the present invention is a
substantially cylindrical acoustic matching device having a top surface on one side and an
opening on the other side. An acoustic wave sensor comprising a material and a piezoelectric
element provided inside the top surface of the acoustic matching material, wherein the ultrasonic
sensor uses a thickness vibration mode of the top surface and a flexural vibration mode of the
entire acoustic matching material as a resonance mode; A damping material having an acoustic
impedance larger than that of the acoustic matching material is provided at the opening of the
matching material. In the ultrasonic sensor according to the present invention, the damping
member having an acoustic impedance larger than that of the acoustic matching material is
provided at the opening of the substantially cylindrical acoustic matching material, so that the
ultrasonic sensor is provided inside the top surface of the acoustic matching material. The
vibration of the acoustic matching material can be effectively damped without inhibiting the
vibration of the provided piezoelectric element.
In the ultrasonic sensor according to the present invention, the acoustic impedance between the
acoustic matching material and the damping material is preferably set to a ratio of 1: 1.5 to 1:40.
The acoustic impedance is represented by the product of the density of the material and the
speed of sound propagating inside it. In general, the speed of sound is faster for hard materials
and materials with higher specific gravity. When the damping material has an acoustic
impedance larger than that of the acoustic matching material, an effect of suppressing the
vibration of the acoustic matching material can be obtained. Therefore, the damping material is
preferably made of a material having a hardness higher than that of the acoustic matching
material and / or a material having a large specific gravity. As the piezoelectric element, one that
vibrates in the spread mode can be suitably used. Furthermore, it is preferable that the lead wire
is connected to the electrode of the piezoelectric element with slack. Normally, the lead wire is
led to the outside through the damping material, but if the lead wire is pulled out with the
material in the damping material with a firm material, the lead wire and its soldered portion are
thermally deformed by the damping material. May break. By providing the lead wire with slack,
such disconnection can be prevented in advance. BEST MODE FOR CARRYING OUT THE
INVENTION Embodiments of an ultrasonic sensor according to the present invention will be
described below with reference to the attached drawings. First Embodiment, See FIGS. 1 to 3 As
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shown in FIG. 1, an ultrasonic sensor 10A according to a first embodiment of the present
invention has a substantially cylindrical acoustic matching material 11 and a piezoelectric
element. 12, a base substrate 13, an elastic member 14, and a damping member 15. The acoustic
matching material 11 has a top surface portion 11a on one side and an opening portion 11b on
the other side, and is used to match acoustic impedance between the piezoelectric element 12
and an external medium. As the material, for example, one obtained by mixing and curing an
epoxy resin and a glass balloon is used, and the specific gravity is preferably 0.5 or less. The
piezoelectric element 12 has vibrating electrodes formed on the front and back surfaces of a
disk-shaped piezoelectric substrate, and vibrates in a spread mode. The piezoelectric element 12
is attached to the inside of the top surface portion 11 a of the acoustic matching material 11 with
an epoxy adhesive or the like. Further, one end of lead wires 16a and 16b is soldered to the
vibrating electrode of the piezoelectric element 12, and the lead wires 16a and 16b are soldered
to the back surface of the base substrate 13 through the elastic material 14 and damping
material 15 described below. It is attached. Furthermore, the lead wires 17 a and 17 b of the
coating type are drawn out through the damping member 15 by relaying this soldered portion.
The base substrate 13 is provided in the opening 11 b of the acoustic matching material 11. The
elastic member 14 is filled in a portion in contact with the piezoelectric element 12 on the top
surface portion 11 a side of the acoustic matching member 11. The elastic material 14 absorbs
the ultrasonic wave emitted from the piezoelectric element 12, but it should not inhibit the
vibration of the piezoelectric element 12. Therefore, as the elastic member 14, for example, a
silicon resin having a hardness (JIS A) of 50 or less is used. The damping material 15 is filled so
as to include the base substrate 13 in the entire opening 11 b of the acoustic matching material
11 and so as not to be in contact with the piezoelectric element 12. The damping member 15
suppresses unnecessary vibration of the acoustic matching member 11 and is made of a material
having a larger acoustic impedance than the acoustic matching member 11. For example, an
epoxy resin having a hardness (JIS D) of 80 or more is used. The ultrasonic sensor 10 A having
the above configuration is arranged in the gas flow path, with the pair of top surfaces 11 a facing
each other, and inputting a signal to the piezoelectric element 12 causes vibration of the
piezoelectric element 12. The ultrasonic waves are emitted through the acoustic matching
material 11 based on Further, the ultrasonic wave received through the acoustic matching
material 11 causes the piezoelectric element 12 to output a reception signal. In this case, the
piezoelectric element 12 vibrates in the spread mode, the top surface portion 11a of the acoustic
matching material 11 vibrates in the thickness mode, and the entire acoustic matching material
11, in particular, the opening 11b vibrates in the bending mode. The resonance frequency of the
spreading vibration mode of the disk-like piezoelectric element 12 is determined by the diameter
of the piezoelectric substrate. Therefore, the diameter of the piezoelectric substrate is determined
so that the resonant frequency exists near the center of the frequency band to be used. The
resonant frequency of the thickness vibration mode of the top surface portion 11a can be
adjusted by the thickness dimension of the top surface portion 11a. In addition, the resonance
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frequency of the entire bending vibration mode of the acoustic matching material 11 is most
easily adjusted by the size of the tapered portion 11 c formed in the ridge line portion of the top
surface portion 11 a. In the first embodiment, the damping member 15 having an acoustic
impedance larger than that of the acoustic matching member 11 is provided in the opening 11 b
of the acoustic matching member 11 and provided inside the top surface 11 a. The vibration of
the bending mode of the acoustic matching material 11, in particular, the vibration of the
bending mode of the opening 11b can be effectively damped without inhibiting the vibration of
the piezoelectric element 12. FIG. 2 shows the vibration velocity distribution on the side surface
of the acoustic matching material 11.
FIG. 2A shows measurement data of the ultrasonic sensor 10A according to the first embodiment,
and FIG. 2B shows measurement data in a state where the elastic material 15 is removed as a
comparative example. As apparent from comparison between FIGS. 2A and 2B, in the example of
the present invention, the vibration near the lower portion of the acoustic matching material 11
is limited, and the vibration leakage is removed. Further, FIG. 3 shows a reverberation
characteristic (a reverberation level when a predetermined time has elapsed after ultrasonic wave
reception) by a histogram. FIG. 3A shows measurement data of the ultrasonic sensor 10A
according to the first embodiment, and FIG. 3B shows measurement data in a state where the
elastic material 15 is removed as a comparative example. As is clear from a comparison of FIGS.
3A and 3B, in the example of the present invention, a reverberation improving effect of 2.3 dB
(about 30%) in average value was obtained. In the ultrasonic sensor from which the data in FIG. 2
and FIG. 3 were obtained, the acoustic impedance of the acoustic matching material 11 was 0.5
× 10 <6> to 1.5 × 10 <6> kg / m <2 The acoustic impedance of the damping material 15 was
2.0 × 10 <6> to 5.0 × 10 <6> kg / m <2> · s. As described above, the acoustic impedance is
represented by the product of the density of the material and the speed of sound propagating
inside the material, and the speed of sound is faster for a hard material or a material having a
large specific gravity. Therefore, the damping material 15 may use a material having a hardness
higher than that of the acoustic matching material 11 and / or a material having a large specific
gravity. However, if the acoustic impedance of the damping material 15 is too large (if the
difference between the acoustic matching material 11 and the acoustic impedance is too large),
the sound wave may be reflected at the interface, and the reverberation may be rather large. The
acoustic impedance of the acoustic matching material 11 is about 1.5 × 10 <7> kg / m <2> · s
for the piezoelectric element 12 and about 4.4 × 10 <7 for the air as the external medium.
Considering that 2> kg / m <2> · s, about 8 × 10 <4> kg / m <2> · s which is its geometric mean
is ideal. Assuming that the acoustic impedance of the acoustic matching material 11 is about 8 ×
10 <4> kg / m <2> · s, the acoustic impedance of the damping material 15 is about 1.2 × 10 <5>
to 3 2 × 10 <6> kg / m <2> · s is preferable.
In other words, the ratio of the acoustic impedance of the acoustic matching material 11 to the
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damping material 15 is preferably 1: 1.5 to 1:40. The polymeric material satisfies such
conditions, and is a material suitable as the damping material 15. In the ultrasonic sensor 10A,
the lead wires 16a and 16b are connected to the electrodes of the piezoelectric element 12 with
slack. The lead wires 16a and 16b are embedded in the damping member 15. However, when the
lead wires 16a and 16b are pulled out in the damping member 15 made of a hard material, the
leads are deformed by the thermal deformation of the damping member 15. There is a possibility
that the wires 16a, 16b or their soldered portions may break. By providing the lead wires 16a
and 16b with slack, such disconnection can be prevented in advance. Second Embodiment, See
FIG. 4 As shown in FIG. 4, an ultrasonic sensor 10 B according to a second embodiment of the
present invention basically uses the same parts as those of the first embodiment. The same
components as those in FIG. 1 are denoted by the same reference numerals and redundant
description will be omitted. The difference from the first embodiment is that the base substrate is
omitted, and the lead wires 16 a and 16 b are directly pulled out through the elastic member 14
and the damping member 15. Also in the second embodiment, the damping material 15 provided
at the opening 11 b of the acoustic matching material 11 is made of a material having a larger
acoustic impedance than the acoustic matching material 11. Therefore, the effect is the same as
that of the first embodiment. Furthermore, also in the second embodiment, the lead wires 16a
and 16b are connected to the electrodes of the piezoelectric element 12 with slack in order to
prevent disconnection of the lead wires 16a and 16b and the soldered portions thereof. This is
very important. Since the lead wire 16b turns downward from the upper surface side of the
piezoelectric element 12, a slack is naturally formed. However, the lead wire 16a can be pulled
straight downward with tension, and if it is pulled out in this way, stress is generated in the lead
wire 16a due to thermal deformation of the damping material 15, which may cause
disconnection. Therefore, the lead wire 16a needs to be artificially slackened, and for this
purpose, it is preferable to connect so that the angle of the soldered portion to the electrode of
the piezoelectric element 12 is 45 ° or less. Third Embodiment, See FIG. 5 As shown in FIG. 5, an
ultrasonic sensor 10 C according to a third embodiment of the present invention basically uses
the same components as those of the second embodiment. The same components as those in FIG.
2 are denoted by the same reference numerals and redundant description will be omitted.
What differs from the second embodiment is that the elastic member 14 is omitted, and a space
18 is formed inside the acoustic matching member 11 so that the damping member 15 does not
contact the piezoelectric element 12. . Also in the third embodiment, the damping material 15
provided in the opening 11 b of the acoustic matching material 11 is made of a material having a
larger acoustic impedance than the acoustic matching material 11. Therefore, the effect is the
same as that of the said, 1st and 2nd embodiment. Other Embodiments The ultrasonic sensor
according to the present invention is not limited to the above embodiment, and can be variously
modified within the scope of the invention. For example, the structure of details such as the
acoustic matching material, the piezoelectric element, and the damping material is arbitrary.
Further, it goes without saying that the ultrasonic sensor according to the present invention is
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used for various applications other than the gas flow meter. As is apparent from the above
description, according to the ultrasonic sensor according to the present invention, a damping
material having an acoustic impedance larger than that of the acoustic matching material is
provided at the opening of the acoustic matching material. Therefore, without inhibiting the
vibration of the piezoelectric element, the vibration of the acoustic matching material can be
effectively damped, and an ultrasonic sensor excellent in responsiveness and dereverberation can
be obtained. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view showing an
ultrasonic sensor according to a first embodiment of the present invention. FIG. 2 is a graph
showing vibration velocity distribution on the side surface of the acoustic matching material, in
which (A) shows an example of the present invention (first embodiment) and (B) shows a
comparative example. FIG. 3 is a graph showing the reverberation characteristics of the
ultrasonic sensor, where (A) shows an example of the present invention (first embodiment) and
(B) shows a comparative example. FIG. 4 is a cross-sectional view showing an ultrasonic sensor
according to a second embodiment of the present invention. FIG. 5 is a cross-sectional view
showing an ultrasonic sensor according to a third embodiment of the present invention. FIG. 6 is
a cross-sectional view showing an example of a conventional ultrasonic sensor. Explanation of
the code 10A, 10B, 10C ... ultrasonic sensor 11 ... acoustic matching material 11a ... top surface
11b ... opening 12 ... piezoelectric element 14 ... elastic material 15 ... damping material 16a, 16b
... lead wire 18 ... space
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