JP2012033989A - Ultrasonic sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high shock resistant ultrasonic sensor with which a piezoelectric element is hardly separated and mass production can be performed easily.SOLUTION: An ultrasonic sensor 201 is provided with a piezoelectric element 21 and a bumper 22 which is an ultrasonic transmission/reception wave member to which the piezoelectric element 21 is attached, and a flexible member 23 and an ultrasonic transmission member 24 are arranged between the piezoelectric element 21 and the bumper 22. The piezoelectric element 21 is provided with an electrode formed in the upper and lower faces, and it spreads and vibrates in the radial direction by applying an alternation voltage. The ultrasonic transmission member 24 has a smaller Young's modulus than the bumper 22 and the piezoelectric element 21, and transmits the elastic wave which is generated in the piezoelectric element 21 to the bumper 22. Since the bumper 22 flexural-vibrates in a comparatively wide range, the narrow directivity and the high sensitivity characteristics are obtained.

Description

  The present invention relates to an ultrasonic sensor attached to, for example, a vehicle bumper or a resin portion of a vehicle.

  Patent Document 1 discloses an ultrasonic sensor attached to a vehicle bumper or a resin portion of a vehicle. FIG. 1 is a cross-sectional view of an ultrasonic sensor disclosed in Patent Document 1. An ultrasonic transmission portion 4 is formed of a material different from the material of the housing 2 on a part of the bottom surface portion 3 of the housing 2 interposed between the piezoelectric element 1 and the bumper 11. Inside the housing 2, a vibration absorber 6, a stopper 7, a spacer 8, a circuit board 9, a connector 10, and the like are provided. When the ultrasonic sensor 100 is attached, the cover 12 is covered so that the protrusion 11a and the protrusion 12a are fitted.

  The acoustic impedance of the ultrasonic transmission unit 4 is closer to the intermediate value of the acoustic impedances of the piezoelectric element 1 and the bumper 11 than the acoustic impedance of the housing 2. Thus, at the time of transmission / reception of ultrasonic waves by the piezoelectric element 1, transmission of ultrasonic waves on the bottom surface portion 3 of the housing 2 is mainly performed via the ultrasonic transmission portion 4. In Patent Document 1, it is described that the vibration part in the bumper 11 is restricted by the above action, and the directivity can be prevented from becoming excessively narrow or irregular.

JP 2007-142967 A

The ultrasonic sensor disclosed in Patent Document 1 has the following problems to be solved.
(1) Since the acoustic impedance of the ultrasonic transmission part 4 is intermediate between the acoustic impedances of the piezoelectric element 1 and the bumper 11, at least a rubber-like elastic member cannot be used. When a hard material is used, it may be peeled off by a thermal shock caused by a difference in thermal expansion coefficient between the piezoelectric element 1 and the bumper 11.

(2) Since the ultrasonic transmission portion has no cushioning property, when a foreign object hits the bumper, an impact is directly applied to the piezoelectric element, and the function of the sensor is likely to be damaged.

(3) In the ultrasonic sensor disclosed in Patent Document 1, thickness longitudinal vibration is used. However, in thickness longitudinal vibration, a piezoelectric element having a thickness of about 20 mm at 60 kHz is required if it is a piezoelectric element. The mass productivity is low.

  Therefore, an object of the present invention is to provide an ultrasonic sensor that solves these problems, that is, an ultrasonic sensor in which a piezoelectric element is hardly peeled off, has high impact resistance, and is easily mass-produced.

An ultrasonic sensor of the present invention includes a piezoelectric element that vibrates by application of an alternating voltage, and an ultrasonic wave transmitting / receiving member to which the piezoelectric element is attached,
The piezoelectric element spreads and vibrates by application of the alternating voltage,
An ultrasonic wave having a Young's modulus smaller than that of the ultrasonic wave transmitting / receiving member that transmits an elastic wave (longitudinal wave) generated in the piezoelectric element to the ultrasonic wave transmitting / receiving member between the piezoelectric element and the ultrasonic wave transmitting / receiving member. A transmission member is provided.

  With this configuration, the ultrasonic transmission member has a small thermal shock due to a difference in thermal expansion coefficient between the piezoelectric element and the ultrasonic transmission / reception member (a bumper or a resin part of the vehicle), and the problem of peeling is solved. Further, since the ultrasonic transmission portion acts as a cushioning material, the impact when a foreign object hits the ultrasonic transmission / reception member is mitigated, and damage to the piezoelectric element can be avoided. Furthermore, since the thickness dimension required for the piezoelectric element can be made relatively thin, mass productivity is high.

  In addition, for example, a flexible member surrounding the ultrasonic transmission member is disposed between the ultrasonic transmission / reception member and the piezoelectric element together with the ultrasonic transmission member. According to this structure, the ultrasonic transmission member can be provided by filling the space formed by the piezoelectric element, the bumper, and the flexible member.

  According to the present invention, the problem of peeling of the piezoelectric element due to thermal shock is solved. Moreover, damage to the piezoelectric element when a foreign object hits the ultrasonic wave transmitting / receiving member can be avoided. Furthermore, there is an effect that the mass productivity of the piezoelectric element is high.

FIG. 1 is a cross-sectional view of an ultrasonic sensor disclosed in Patent Document 1. 2A is a plan view of the ultrasonic sensor 201 according to the first embodiment, and FIG. 2B is a cross-sectional view of a BB portion in FIG. 2A. FIG. 3 is a diagram schematically illustrating the displacement of each part of the ultrasonic sensor 201 due to the vibration of the piezoelectric element 21. 4A is a reflected waveform of the ultrasonic sensor 201 according to the first embodiment, and FIG. 4B is a comparative example. A piezoelectric element is bonded to the metal case, and the bottom surface of the metal case is bent and vibrated. It is the reflected waveform of the ultrasonic sensor of conventional structure. FIG. 5 shows the result of measuring the displacement of the vibration surface of the ultrasonic sensor with a laser Doppler measuring device. FIG. 6A is a diagram showing the frequency characteristics of the impedance of the piezoelectric element provided in the ultrasonic sensor according to the embodiment, and FIG. 6B is a diagram showing the frequency characteristics of the impedance of the piezoelectric element in the state of the ultrasonic sensor. It is. FIG. 7 is a diagram showing the directivity characteristics of the ultrasonic sensor 201. FIG. 8 is a cross-sectional view of the ultrasonic sensor 202 according to the second embodiment. FIG. 9 is a cross-sectional view of an ultrasonic sensor 203 according to the third embodiment.

<< First Embodiment >>
The configuration and characteristics of the ultrasonic sensor according to the first embodiment of the present invention will be described with reference to the drawings.
2A is a plan view of the ultrasonic sensor 201 according to the first embodiment, and FIG. 2B is a cross-sectional view of a BB portion in FIG. 2A. The ultrasonic sensor 201 includes a piezoelectric element 21 and a bumper 22 to which the piezoelectric element 21 is attached. A flexible member 23 and an ultrasonic transmission member 24 are disposed between the piezoelectric element 21 and the bumper 22. Yes. The piezoelectric element 21 has electrodes formed on the upper and lower surfaces thereof, and vibrates by expanding in the radial direction when an alternating voltage is applied. The ultrasonic transmission member 24 has a Young's modulus smaller than that of the bumper 22 and the piezoelectric element 21, and transmits an elastic wave (longitudinal wave) generated in the piezoelectric element 21 to the bumper 22. This bumper corresponds to an “ultrasonic wave transmitting / receiving member” according to the present invention.

The dimensions of each part of the ultrasonic sensor 201 shown in FIG. 2 are as follows.
D1 = 30.0mm
D2 = 70.0mm
D3 = 40.0mm
d3 = 25.0mm
t1 = 6.6mm
t2 = 2.5mm
t3 = 2.0mm
Here, for the purpose of quantitative evaluation, the bumper 22 is a disk-shaped resin made by adding rubber to polypropylene, which is a resin material used as a pan bar. The size of the disk is considered to be sufficiently larger than the range of vibration due to the vibration of the piezoelectric element.

  The ultrasonic transmission member 24 is a disc-like layer made of an elastic material such as silicone rubber. The ultrasonic transmission member 24 is filled in a ring-shaped flexible member 23. Here, it is a two-component room temperature curing type self-adhesive liquid silicone rubber (silicone rubber for potting), and its hardness is 40. The flexible member 23 is a molded body in which sponge or rubber is molded into a ring shape.

  The ultrasonic sensor 201 shown in FIG. 2 uses this flexible member 23 as an adhesive material, sticks the piezoelectric element 21 to the bumper 22, and transmits ultrasonic waves to the space between the bumper 22 and the piezoelectric element 21. It is configured by filling the member 24.

  FIG. 3 is a diagram schematically illustrating the displacement of each part of the ultrasonic sensor 201 due to the vibration of the piezoelectric element 21. However, the flexible member 23 shown in FIG. 2 is not shown because it is irrelevant to the operation. The piezoelectric element 21 expands in the radial direction and vibrates as shown by a broken line in FIG. 3 when an alternating voltage is applied between the electrodes. Along with this spreading vibration, it also vibrates in the thickness direction. Since the ultrasonic transmission member 24 has a Young's modulus smaller than that of the piezoelectric element 21, the ultrasonic transmission member 24 does not hinder the spreading vibration of the piezoelectric element 21. However, since the thickness dimension is much smaller (thin) than the radial dimension, the displacement in the thickness direction of the piezoelectric element 21 is efficiently transmitted to the bumper 22. Therefore, the bumper 22 bends and vibrates as indicated by a broken line in FIG.

  4A is a reflected waveform of the ultrasonic sensor 201 according to the first embodiment, and FIG. 4B is a comparative example. A piezoelectric element is bonded to the metal case, and the bottom surface of the metal case is bent and vibrated. It is the reflected waveform of the ultrasonic sensor of conventional structure. The piezoelectric element included in the ultrasonic sensor of this comparative example has a diameter of 7 mm and a thickness of 0.15 mm, and the metal case has an outer diameter of 14 mm and a height of 9 mm. The reflective target used for the measurement was a pole having a diameter of 60 mm, and was placed at a distance of 60 cm.

  The total sensitivity of the ultrasonic sensor having a conventional structure was 0.97 Vpp, and the 1 V cross reverberation time was 0.9 ms. The total sensitivity of the ultrasonic sensor according to the embodiment was 2.19 Vpp, and the 1 V cross reverberation time was 1.35 ms. Thus, although the reverberation has a long attitude in the ultrasonic sensor according to the embodiment, the intensity of the reflected signal is about twice as large.

  Incidentally, when the thickness dimension of the piezoelectric element is 3.0 mm, the total sensitivity is 1/10. In addition, when the thickness dimension of the piezoelectric element was set to 15 mm, the reverberation time was increased and the total sensitivity could not be measured. Therefore, it is expected that the thickness dimension of the piezoelectric element has the best point.

  When the thickness of the ultrasonic transmission member was 1 mm and 5 mm, the total sensitivity was 1/10. The thickness dimension of the ultrasonic transmission member has a best point at or near 2 mm.

  When the hardness of the ultrasonic transmission member was 25, the total sensitivity was about 1/4, and when the hardness of the ultrasonic transmission member was 80, the total sensitivity was about 1/10. Therefore, the hardness of the ultrasonic transmission member is preferably about 40 in the middle.

  When the inner diameter of the flexible member 23 was 15 mm, that is, when the diameter of the ultrasonic transmission member 24 was 15 mm, the total sensitivity was 1/10. In order to transmit the vibration energy of the piezoelectric element 21 to the bumper 22, a size corresponding to the diameter of the piezoelectric element 21 is required, and in this example, 25 mm or the vicinity thereof is the best.

  FIG. 5 shows the result of measuring the displacement of the vibration surface of the ultrasonic sensor with a laser Doppler measuring device. FIG. 5A shows the displacement of the vibration surface (bumper) of the ultrasonic sensor 201 according to the embodiment. The right side of FIG. 5A is a diagram showing the circle of the measurement range and the amount of displacement within the circle in terms of concentration. The measurement range is about 25 mm in diameter. The left side of FIG. 5A is a diagram showing the displacement at the central transverse line portion of the measurement range shown on the right side, where the horizontal axis is the position and the vertical axis is the amount of displacement. FIG. 5B shows a displacement of the vibration surface of an ultrasonic sensor having a conventional structure provided with the metal case as a comparative example. The right side of FIG. 5 (B) is a diagram showing the circle of the measurement range and the amount of displacement within the circle in terms of concentration. This measuring range is about 13 mm in diameter. The left side of FIG. 5B is a diagram showing the displacement at the central transverse line portion of the measurement range shown on the right side, where the horizontal axis is the position and the vertical axis is the displacement amount.

  In both cases, measurement was performed by applying a continuous sine wave having an effective voltage of 10V. According to the ultrasonic sensor of the conventional structure, the displacement on the vibration surface is ± 1.6 μm, whereas the displacement of the vibration surface of the ultrasonic sensor 201 according to the embodiment is ± 0.15 μm. Thus, although the displacement amount is about 1/10, the total sensitivity is more than doubled as shown in FIG. As shown in FIG. 3, it is expected that the ultrasonic sensor of the present invention operates by a mechanism different from that of the conventional ultrasonic sensor. This suggests that sound waves are important not only as the amplitude of the vibration surface but also as a flow of vibration energy.

  FIG. 6A is a diagram showing the frequency characteristics of the impedance of the piezoelectric element provided in the ultrasonic sensor according to the embodiment, and FIG. 6B is a diagram showing the frequency characteristics of the impedance of the piezoelectric element in the state of the ultrasonic sensor. It is. A curve Z represents the absolute value of the impedance, and a curve P represents the phase.

  The resonance frequency of the spreading vibration in the radial direction of the piezoelectric element alone is 66 kHz, but it is slightly peaked at 58 kHz by being attached to the bumper as the ultrasonic wave transmitting / receiving member via the silicone rubber as the ultrasonic transmitting member. Has occurred. This frequency is the resonance frequency of the ultrasonic sensor, and the vibration shown in FIG. 3 is generated by driving at this frequency of 58 kHz.

FIG. 7 is a diagram showing the directivity characteristics of the ultrasonic sensor 201. The horizontal axis represents the azimuth angle with the direction perpendicular to the vibration plane (front) as 0 °, and the vertical axis represents the total sensitivity with the peak represented as 0 dB. The azimuth angle at which the total sensitivity decreases to −6 dB is ± 8 °, that is, the half-value angle is 16 °. Since the half-value angle of the ultrasonic sensor having the conventional structure as the comparative reference is 40 °, it can be seen that a good narrow directivity can be obtained. This is presumably due to the fact that the vibration surface that vibrates in the same phase of the bumper, which is an ultrasonic wave transmitting / receiving member, is larger than the ultrasonic sensor of the conventional structure.
Further, as is clear from FIG. 7, not only the directivity is sharp, but also the side lobe is small.

  According to the ultrasonic sensor shown in the first embodiment, the ultrasonic transmission member has a small thermal shock due to the difference in thermal expansion coefficient between the piezoelectric element and the bumper, and the problem of peeling is solved. In addition, since the ultrasonic transmission portion acts as a cushioning material, the impact when a foreign object hits the bumper is mitigated, and damage to the piezoelectric element can be avoided. Furthermore, since the thickness dimension required for the piezoelectric element can be made relatively thin, mass productivity is high.

<< Second Embodiment >>
FIG. 8 is a cross-sectional view of the ultrasonic sensor 202 according to the second embodiment. The ultrasonic sensor 202 includes a piezoelectric element 21 and a bumper 22 to which the piezoelectric element 21 is attached. A flexible member 23 and an ultrasonic transmission member 24 are disposed between the piezoelectric element 21 and the bumper 22. Yes. Further, the bumper 22 is curved and a gap is formed between the bumper 22 and the flexible member 23, and this gap is filled with a silicone rubber filler 25. This filler is a dealcohol-free one-component liquid silicone rubber that reacts with moisture (water) in the air and cures into a rubbery elastic body.

The piezoelectric element 21 has electrodes formed on the upper and lower surfaces thereof, and vibrates by expanding in the radial direction when an alternating voltage is applied. The ultrasonic transmission member 24 has a Young's modulus (acoustic impedance) smaller than that of the bumper 22 and the piezoelectric element 21, and transmits an elastic wave (longitudinal wave) generated in the piezoelectric element 21 to the bumper 22. At this time, since the filler 25 hardly inhibits the vibration of the bumper 22, it acts in the same manner as the ultrasonic sensor shown in the first embodiment.
Thus, an ultrasonic sensor can be configured by attaching a piezoelectric element to the curved surface of a bumper that is an ultrasonic wave transmitting / receiving member.

<< Third Embodiment >>
FIG. 9 is a cross-sectional view of an ultrasonic sensor 203 according to the third embodiment. The ultrasonic sensor 203 includes a piezoelectric element 21 and a bumper 22 to which the piezoelectric element 21 is attached. An ultrasonic transmission member 24 is disposed between the piezoelectric element 21 and the bumper 22. Further, the piezoelectric element 21 and the ultrasonic transmission member 24 are attached to the bumper 22 with a hook 22F via a buffer material 26. That is, the laminated body of the ultrasonic transmission member 24, the piezoelectric element 21, and the buffer material 26 is sandwiched between the inner surface of the bumper 22 and the hook 22F. In this example, the hook 22 </ b> F protrudes toward the inner surface of the bumper 22 and is integrally formed with the bumper 22.

  The buffer material 26 has elasticity, and presses the piezoelectric element 21 and the ultrasonic transmission member 24 against the inner surface of the bumper 22 with a necessary pressing force. In this way, the ultrasonic sensor can be configured by arranging the ultrasonic transmission member 24 that is pre-molded instead of filling.

  Note that oil or grease may be interposed between the bumper 22 and the ultrasonic transmission member 24 or between the ultrasonic transmission member 24 and the piezoelectric element 21.

<< Other embodiments >>
In the embodiment described above, the piezoelectric sensor and the ultrasonic transmission member are provided on the bumper of the vehicle to configure the ultrasonic sensor. However, the piezoelectric element and the ultrasonic transmission member are provided on the decorative board provided in the vehicle and other devices. An ultrasonic sensor can be configured similarly.

  Alternatively, a plurality of piezoelectric elements may be attached to the ultrasonic transmission / reception member via the ultrasonic transmission member to form an array. As a result, the synthetic aperture surface is widened, and the width of the directional beam can be further reduced.

21 ... piezoelectric element 22 ... bumper (ultrasonic wave transmitting / receiving member)
22F ... Hook 23 ... Flexible member 24 ... Ultrasonic transmission member 25 ... Filler 26 ... Buffer material 201-203 ... Ultrasonic sensor

Claims (2)

In an ultrasonic sensor comprising a piezoelectric element that vibrates by application of an alternating voltage, and an ultrasonic wave transmitting / receiving member to which the piezoelectric element is attached,
The piezoelectric element spreads and vibrates by application of the alternating voltage,
An ultrasonic wave having a Young's modulus smaller than that of the ultrasonic wave transmitting / receiving member and the piezoelectric element that transmits an elastic wave generated in the piezoelectric element to the ultrasonic wave transmitting / receiving member between the piezoelectric element and the ultrasonic wave transmitting / receiving member. An ultrasonic sensor comprising a transmission member.   The ultrasonic sensor according to claim 1, wherein a flexible member surrounding the ultrasonic transmission member is disposed between the ultrasonic transmission / reception member and the piezoelectric element.