JP2004349973A - Ultrasonic sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an ultrasonic sensor having its responsiveness and reverberation improved. <P>SOLUTION: The ultrasonic sensor is provided with: an approximately cylindrical acoustic matching material 11 having a ceiling surface part 11a on one side and having an aperture part 11b on the other; and a piezoelectric element 12 of an expanding oscillation mode provided on the inside of the ceiling surface part 11a of the acoustic matching material 11. The ultrasonic sensor uses the thickness oscillation mode of the ceiling surface part 11a and a bending oscillation mode of the entire acoustic matching material 11. A damping material 15 having an acoustic impedance higher than that of the acoustic matching material 11 is provided in the aperture part 11b of the acoustic matching material 11 so as not to come into contact with the piezoelectric element 12. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an ultrasonic sensor that is used for an ultrasonic sensor, for example, a gas flow meter and transmits and receives ultrasonic waves.
[0002]
[Prior art]
[Patent Document 1]
JP-A-2002-188946
2. Description of the Related Art Conventionally, as an ultrasonic sensor requiring high accuracy like a gas flow meter, a sensor described in Patent Document 1 has been proposed by the present applicant. As shown in FIG. 6, the ultrasonic sensor includes a cylindrical acoustic matching layer 1 having a top surface 1a and an opening 1b, a piezoelectric element 2, and a backing layer 3. The backing layer 3 is adhered to the inside of the top surface 1a, and fills the entire opening 1b of the acoustic matching layer 1.
[0004]
In this ultrasonic sensor, the thickness vibration mode of the top surface 1a of the acoustic matching layer 1 and the bending vibration mode of the entire acoustic matching layer 1 are generated by vibrating the piezoelectric element 2 in the spread mode. The backing layer 3 is for suppressing unnecessary vibration of the piezoelectric element 2 and the acoustic matching layer 1.
[0005]
[Problems to be solved by the invention]
However, in the above-described conventional ultrasonic sensor, the backing layer 3 does not always function sufficiently, and there is a problem in response and reverberation characteristics. That is, if a hard material is used for the backing layer 3, the vibration is hindered because the material is in contact with the piezoelectric element 2, and the sensitivity is reduced. Therefore, a soft material is used. However, with this, the braking effect is small, sufficient response cannot be obtained, and reverberation remains.
[0006]
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 a break in a lead wire connected to a piezoelectric element or a soldered portion in addition to achieving the above object.
[0007]
Means and Action for Solving the Problems
In order to achieve the above object, an ultrasonic sensor according to the present invention includes a substantially cylindrical acoustic matching material having a top surface on one side and an opening on the other side, and is provided inside the top surface of the acoustic matching material. An ultrasonic sensor using the thickness vibration mode of the top surface portion and the flexural vibration mode of the entire acoustic matching material as resonance modes. It is characterized in that a braking material having a large impedance is provided.
[0008]
In the ultrasonic sensor according to the present invention, since the damping member having a larger acoustic impedance than the acoustic matching material is provided in the opening of the substantially cylindrical acoustic matching material, the piezoelectric member provided inside the top surface of the acoustic matching material is provided. Vibration of the acoustic matching material can be effectively damped without hindering vibration of the element.
[0009]
In the ultrasonic sensor according to the present invention, it is preferable that an acoustic impedance between the acoustic matching material and the damping material is set to a ratio of 1: 1.5 to 1:40.
[0010]
Acoustic impedance is represented by the product of the density of a material and the speed of sound propagating through it. Generally, the speed of sound is higher for a hard material or a material having a higher specific gravity. When the damping material has a larger acoustic impedance than the acoustic matching material, an effect of suppressing vibration of the acoustic matching material can be obtained. Therefore, the damping material is preferably made of a material having a higher hardness and / or a material having a higher specific gravity than the acoustic matching material.
[0011]
The piezoelectric element that vibrates in a spreading mode can be suitably used. Further, it is preferable that the lead wire is connected to the electrode of the piezoelectric element with a slack. Normally, the lead wire is led out through the braking material, but when the lead wire is pulled out in a hard material with a tension, the lead wire and its soldered portion are thermally deformed by the braking material. May be disconnected. By making the lead wires slack, such disconnection can be prevented.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of an ultrasonic sensor according to the present invention will be described with reference to the accompanying drawings.
[0013]
(1st Embodiment, FIGS. 1-3)
As shown in FIG. 1, the ultrasonic sensor 10A according to the first embodiment of the present invention includes a substantially cylindrical acoustic matching member 11, a piezoelectric element 12, a base substrate 13, an elastic member 14, and a damping member 15. It is composed of
[0014]
The acoustic matching material 11 has a top surface 11a on one side and an opening 11b on the other, and is used for matching acoustic impedance between the piezoelectric element 12 and an external medium. As the material, for example, a material obtained by mixing and curing an epoxy resin and a glass balloon is used, and the specific gravity is preferably 0.5 or less.
[0015]
The piezoelectric element 12 is formed by forming vibration electrodes on the front and back surfaces of a disk-shaped piezoelectric substrate, and vibrates in a spreading mode. The piezoelectric element 12 is attached to the inside of the top surface 11a of the acoustic matching material 11 with an epoxy adhesive or the like. One end of a lead wire 16a, 16b is soldered to the vibration electrode of the piezoelectric element 12, and the lead wire 16a, 16b is soldered to the back surface of the base substrate 13 through an elastic material 14 and a damping material 15 described below. Is attached. Further, the lead wires 17a and 17b of the coating type are drawn out to the outside through the damping material 15 by relaying the soldered portion.
[0016]
The base substrate 13 is provided in the opening 11 b of the acoustic matching material 11.
[0017]
The elastic material 14 is filled in a portion of the acoustic matching material 11 that contacts the piezoelectric element 12 on the top surface 11a side. The elastic member 14 absorbs the ultrasonic waves emitted from the piezoelectric element 12, but must not inhibit the vibration of the piezoelectric element 12. Therefore, as the elastic material 14, for example, a silicone resin having a hardness (JIS A) of 50 or less is used.
[0018]
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 not to contact the piezoelectric element 12. The damping material 15 suppresses unnecessary vibration of the acoustic matching material 11 and is made of a material having a larger acoustic impedance than the acoustic matching material 11. For example, an epoxy resin having a hardness (JIS D) of 80 or more is used.
[0019]
The ultrasonic sensor 10 </ b> A having the above configuration is arranged in a gas flow path with a pair of top surfaces 11 a facing each other, and a signal is input to the piezoelectric element 12 so that the ultrasonic sensor 10 </ b> A is based on the vibration of the piezoelectric element 12. Ultrasonic waves are emitted through the acoustic matching material 11. In addition, a reception signal is output from the piezoelectric element 12 by the ultrasonic wave received via the acoustic matching material 11.
[0020]
In this case, the piezoelectric element 12 vibrates in the spreading mode, the top surface 11a of the acoustic matching material 11 vibrates in the thickness mode, and the entire acoustic matching material 11, particularly the opening 11b vibrates in the bending mode. The resonance frequency of the disk-shaped piezoelectric element 12 in the expansion vibration mode is determined by the diameter of the piezoelectric substrate. Therefore, the diameter of the piezoelectric substrate is determined so that the resonance frequency exists near the center of the frequency band to be used.
[0021]
The resonance frequency of the thickness vibration mode of the top surface 11a can be adjusted by the thickness dimension of the top surface 11a. Further, it is easiest to adjust the resonance frequency of the entire flexural vibration mode of the acoustic matching member 11 by adjusting the size of the tapered portion 11c formed on the ridge of the top surface 11a.
[0022]
By the way, in the first embodiment, the damping material 15 having a larger acoustic impedance than the acoustic matching material 11 is provided in the opening 11b of the acoustic matching material 11, and the piezoelectric element 12 provided inside the top surface portion 11a is provided. Vibration of the acoustic matching member 11, in particular, the vibration of the bending mode of the opening 11 b can be effectively damped without obstructing the vibration of the acoustic matching member 11.
[0023]
FIG. 2 shows a 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 is apparent from a comparison between FIGS. 2A and 2B, in the example of the present invention, the vibration near the lower portion of the acoustic matching member 11 is limited, and the vibration leakage is eliminated.
[0024]
FIG. 3 is a histogram showing reverberation characteristics (reverberation level when a predetermined time has elapsed after receiving ultrasonic waves). 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 the comparison between FIGS. 3A and 3B, in the example of the present invention, the reverberation improving effect of 2.3 dB (about 30%) on average was obtained.
[0025]
In the ultrasonic sensor from which the data of FIGS. 2 and 3 was obtained, the acoustic impedance of the acoustic matching material 11 was 0.5 × 10 ^{6 to} 1.5 × 10 ⁶ kg / m ² · s, and the acoustic The impedance was 2.0 × 10 ^{6 to} 5.0 × 10 ⁶ kg / m ² · s.
[0026]
As described above, the acoustic impedance is represented by the product of the density of a material and the speed of sound propagating in the material, and the speed of sound is higher for a hard material or a material having a higher specific gravity. Therefore, a material having a higher hardness and / or a material having a higher specific gravity than the acoustic matching material 11 may be used as the damping material 15.
[0027]
However, if the acoustic impedance of the damping material 15 is too large (the acoustic impedance difference with the acoustic matching material 11 is too large), sound waves will be reflected at the interface, and the reverberation may be rather increased. The acoustic impedance of the acoustic matching material 11 is such that the acoustic impedance of the piezoelectric element 12 is about 1.5 × 10 ⁷ kg / m ² · s and the acoustic impedance of air as an external medium is about 4.4 × 10 ² kg / m ^2. S is considered, its geometric average of about 8 × 10 ⁴ kg / m ² s is ideal.
[0028]
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 preferred. In other words, the ratio of the acoustic impedance between the acoustic matching material 11 and the damping material 15 is preferably 1: 1.5 to 1:40. The polymer material satisfies such conditions and is a suitable material for the damping material 15.
[0029]
In the ultrasonic sensor 10A, the lead wires 16a and 16b are connected to the electrodes of the piezoelectric element 12 with slack. Although the lead wires 16a and 16b are embedded in the braking material 15, when the lead wires 16a and 16b are pulled out of the hard braking material 15 with tension, the leads are deformed due to thermal deformation of the braking material 15. There is a possibility that the wires 16a and 16b and their soldered portions may be broken. By making the lead wires 16a and 16b slack, such disconnection can be prevented beforehand.
[0030]
(Second embodiment, see FIG. 4)
As shown in FIG. 4, an ultrasonic sensor 10B according to a second embodiment of the present invention basically has the same configuration using the same components as the first embodiment, and is the same as FIG. The same reference numerals are given to the parts, and the duplicate description will be omitted.
[0031]
The difference from the first embodiment is that the base substrate is omitted, and the lead wires 16a and 16b are drawn directly to the outside through the elastic member 14 and the damping member 15. Also in the second embodiment, the damping material 15 provided in the opening 11b of the acoustic matching material 11 is made of a material having a larger acoustic impedance than the acoustic matching material 11. Therefore, the operation and effect are the same as those of the first embodiment.
[0032]
Further, also in the second embodiment, it is important that the leads 16a, 16b are connected to the electrodes of the piezoelectric element 12 with slack in order to prevent the leads 16a, 16b and their soldered portions from breaking. It is. Since the lead wire 16b goes down 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 a tension. If the lead wire 16a is drawn in this way, stress is generated in the lead wire 16a due to thermal deformation of the brake material 15, and there is a possibility of disconnection. Therefore, it is necessary that the lead wire 16a be artificially slackened. For this purpose, it is preferable to connect the piezoelectric element 12 so that the angle of the soldered portion to the electrode of the piezoelectric element 12 is 45 ° or less.
[0033]
(3rd Embodiment, see FIG. 5)
As shown in FIG. 5, an ultrasonic sensor 10C according to a third embodiment of the present invention has basically the same configuration using the same components as those in the second embodiment, and is the same as FIG. The same reference numerals are given to the parts, and the duplicate description will be omitted.
[0034]
The difference 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 braking member 15 does not contact the piezoelectric element 12. Also in the third embodiment, the damping material 15 provided in the opening 11b of the acoustic matching material 11 is made of a material having a larger acoustic impedance than the acoustic matching material 11. Therefore, the operation and effect are the same as those of the first and second embodiments.
[0035]
(Other embodiments)
The ultrasonic sensor according to the present invention is not limited to the above-described embodiment, but can be variously modified within the scope of the gist.
[0036]
For example, the structure of details such as an acoustic matching material, a piezoelectric element, and a damping material is arbitrary. The ultrasonic sensor according to the present invention is of course used for various purposes other than the gas flow meter.
[0037]
【The invention's effect】
As is clear from the above description, according to the ultrasonic sensor of the present invention, since the damping material having a larger acoustic impedance than the acoustic matching material is provided in the opening of the acoustic matching material, the vibration of the piezoelectric element is hindered. Thus, the vibration of the acoustic matching material is effectively damped, and an ultrasonic sensor excellent in responsiveness and reverberation removal can be obtained.
[Brief description of the drawings]
FIG. 1 is a sectional view showing an ultrasonic sensor according to a first embodiment of the present invention.
FIG. 2 is a graph showing a vibration velocity distribution on a side surface of an acoustic matching material, where (A) shows an example of the present invention (first embodiment) and (B) shows a comparative example.
3A and 3B are graphs showing reverberation characteristics of an ultrasonic sensor, wherein FIG. 3A shows an example of the present invention (first embodiment), and FIG. 3B shows a comparative example.
FIG. 4 is a sectional view showing an ultrasonic sensor according to a second embodiment of the present invention.
FIG. 5 is a sectional view showing an ultrasonic sensor according to a third embodiment of the present invention.
FIG. 6 is a sectional view showing an example of a conventional ultrasonic sensor.
[Explanation of symbols]
10A, 10B, 10C ... ultrasonic sensor 11 ... acoustic matching material 11a ... top surface portion 11b ... opening 12 ... piezoelectric element 14 ... elastic material 15 ... braking material 16a, 16b ... lead wire 18 ... space portion

Claims (6)

A substantially cylindrical acoustic matching member having a top surface on one side and an opening on the other side, and a piezoelectric element provided inside the top surface of the acoustic matching material, and a thickness vibration mode of the top surface as a resonance mode And the ultrasonic sensor using the flexural vibration mode of the entire acoustic matching material,
In the opening of the acoustic matching material, a damping material having a larger acoustic impedance than the acoustic matching material is provided,
An ultrasonic sensor characterized by the above-mentioned. The ultrasonic sensor according to claim 1, wherein the acoustic impedance between the acoustic matching material and the damping material is set to a ratio of 1: 1.5 to 1:40. The ultrasonic sensor according to claim 1, wherein the damping material is made of a material having a higher hardness than the acoustic matching material. 4. The ultrasonic sensor according to claim 1, wherein the damping material is made of a material having a specific gravity greater than that of the acoustic matching material. 5. 5. The ultrasonic sensor according to claim 1, wherein the piezoelectric element vibrates in a spreading mode. 6. The ultrasonic sensor according to claim 1, wherein a lead wire is connected to the electrode of the piezoelectric element with a slack.

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