JP3880047B2 - Ultrasonic sensor - Google Patents

Description

【００１９】
実施の形態は、従来構造と電極構成が異なるために、同じ値の静電容量と感度は得られないが、静電容量と感度のそれぞれの平均値に対するばらつきの大きさの比を比較すると、実施の形態は比較例の半分以下で、ばらつきが小さいことが分かる。
【００２０】
また、前述の超音波センサについて、マイナス４０℃とプラス１００℃の温度を繰り返す熱衝撃試験を実施し、超音波センサの静電容量の変化を観測した。前記静電容量は、圧電セラミックスの誘電率と電極間距離、電極面積よって決まる値であるが、熱衝撃試験にて接着層に劣化があると振動子と振動板の間の拘束に変化が生じるため、前記静電容量が下がる傾向がある。比較のため、図７に示した従来構造の超音波センサについても、同様の試験を実施した。熱衝撃試験経過に伴う静電容量の変化率を図６に示す。
【００２１】
図６から、従来構造では、５００サイクル経過時点で５％程度の静電容量の変化があるのに対し、本発明による構造では、ほとんど劣化は見られなかった。この実験結果から、本構造では温度差による特性の劣化は従来構造に比し少ないことが確認された。
【００２２】
更に、前述の超音波センサを１０００個試作するときの１個当りの製造時間とコストを従来構造品と比較し、比較結果を表２に示した。
【００２３】
【表２】

【００２４】
表２から、本発明の超音波センサは比較例に比べ、約６０％の低コストで製造することが可能であることが分かる。
【００２５】
【発明の効果】
上記のように、本発明によれば、ケースの底面と円筒部分を圧電セラミックスで一体成型することで、初期特性のばらつきが少ない超音波センサを安価に提供すると同時に、底面に交差指電極や渦巻き状の電極を形成して、電極間の圧電セラミックスを分極して駆動源とすることによって、温度差の影響を受けにくく、環境による劣化を起こすことの少ない超音波センサを得ることが可能となった。
【図面の簡単な説明】
【図１】本発明による超音波センサの一実施の形態を示す断面図。
【図２】吸音材や弾性材料を充填する前の図１に示した有底筒状ケースの平面図。
【図３】交差指電極に電圧を印加した時の振動板の分極状態を示す図。
【図４】２本以上の同心円状の交差指電極を示す図。
【図５】一定間隔で２本の渦巻き状の電極を示す図。
【図６】熱衝撃試験経過に伴う静電容量の変化率を示す図。
【図７】公知文献に記載されている超音波センサの構造を示す図。
【符号の説明】
１，２５ 有底筒状ケース
２ 振動板
２ａ 振動板の内面
３，４ 交差指電極
５，６ ランド電極
７ 吸音材
８ 弾性材料
９，１０ リード線
１１，１２ 信号線
２１ 分極方向を示す矢印
２６ 振動板
２７ 圧電振動素子
２８ａ，２８ｂ 素子電極
２９ 吸音材
３０ 弾性体
３１ はんだ点
３２，３３ リード線
３４，３５ 信号線[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic sensor that transmits an ultrasonic signal and receives a reflected wave from an obstacle to detect the presence of the obstacle, and is particularly suitable for a drip-proof type suitable for back sonar and corner sonar of an automobile. The present invention relates to an ultrasonic sensor.
[0002]
[Prior art]
An ultrasonic sensor performs sensing using ultrasonic waves. By vibrating a diaphragm using a piezoelectric vibration element as a drive source, an ultrasonic pulse signal is intermittently transmitted, and a detected object existing ahead is detected. The reflected wave from the object is received by the piezoelectric vibration element through the diaphragm, and the object is detected.
[0003]
Conventionally, there is JP-A-2001-169392 as a known document regarding this type of ultrasonic sensor. FIG. 7 shows the structure of the ultrasonic sensor described in the known document. The ultrasonic sensor has a structure in which a bottom surface of a bottomed cylindrical case 25 made of a metal such as aluminum is a diaphragm 26, and a piezoelectric vibration element 27 is joined to the inner surface of the diaphragm 26. ing. Electrical connection from the piezoelectric vibration element 27 is performed by lead wires 32 and 33. One lead wire 32 is bonded to the element electrode 28a facing the bonding surface of the piezoelectric vibration element 27 by soldering. The other lead wire 33 is soldered to the metal case 25, and is electrically connected to the element electrode 28 b on the joint surface of the piezoelectric vibration element 27 via the metal case 25. The bottomed cylindrical case 25 is filled with a sound absorbing material 29 made of sponge or the like so as not to restrain the piezoelectric vibration element 27, and in order to prevent raindrops or the like from entering the case from above the sound absorbing material 29. Are filled with an elastic body 30 such as a silicone resin. The lead wires 32 and 33 are electrically connected to the signal wires 34 and 35 by soldering inside the resin.
[0004]
The ultrasonic sensor operates as follows. When a drive voltage is intermittently applied from the signal lines 34 and 35 to the piezoelectric vibration element 27 to vibrate the piezoelectric vibration element 27, the diaphragm 26 vibrates, and ultrasonic waves are emitted forward from the diaphragm. Since the drive is performed intermittently, the ultrasonic wave reflected from the detected object reaches the piezoelectric vibration element 27 via the vibration plate 26 after a predetermined time has elapsed within the time during which transmission is stopped. Then, the received ultrasonic wave is converted into a voltage signal by the piezoelectric vibration element 27 and output from the signal lines 34 and 35. Here, since the time until the reflected wave returns and the distance to the object to be detected are in a proportional relationship, the distance from the object to be detected can be measured by observing the elapsed time from transmission to reception. .
[0005]
[Problems to be solved by the invention]
As a matter required for an ultrasonic sensor used to detect obstacles around the automobile, it is required that the sensitivity and directivity angle of the ultrasonic sensor do not change due to environmental fluctuations such as a temperature difference and humidity. In addition, there is a demand for the above-mentioned characteristics not to be changed and deteriorated by the entry of raindrops, i.
[0006]
However, in the structure in which the piezoelectric vibration element is bonded to a metal case such as aluminum as disclosed in Japanese Patent Application Laid-Open No. 2001-169392, an actual thermal expansion coefficient of about 10 times is bonded to aluminum and piezoelectric ceramics. In a use environment, stress concentrates on the adhesive layer between the aluminum and the piezoelectric ceramic due to repeated temperature changes, and the adhesive force gradually decreases. In this type of ultrasonic sensor, the piezoelectric ceramic expansion and contraction due to AC voltage is used as a drive source to emit film by vibrating the diaphragm on the bottom of the case. Therefore, if the adhesive force between the piezoelectric ceramic and diaphragm changes, There is a problem that the transmission changes, and as a result, the performance of the ultrasonic sensor, such as sound pressure, sensitivity, and directivity, changes from the initial value due to environmental fluctuations and time and does not return.
[0007]
In general, such a metal case is manufactured by machining, but the thickness of the bottom of the bottomed cylindrical case that becomes the diaphragm, the undulation is the resonance frequency, sensitivity, reverberation characteristics, etc. of the ultrasonic sensor. It has a great influence on the various characteristics. Therefore, in order to reduce the variation in the initial characteristics, it is necessary to process the metal case with high accuracy, resulting in an increase in cost.
[0008]
Accordingly, the present invention has been made to solve the above-described problems, and provides an inexpensive ultrasonic sensor that has little initial characteristic variation, is less susceptible to temperature differences, and is less susceptible to environmental degradation. The purpose is to do.
[0009]
[Means for Solving the Problems]
According to the present invention, there is provided an ultrasonic sensor that transmits and receives ultrasonic waves using the bottom surface of a bottomed cylindrical case as a diaphragm, and the bottom surface and the cylindrical portion of the bottomed cylindrical case are integrally formed of a piezoelectric ceramic material. Thus, an ultrasonic sensor can be obtained, which has a structure in which a counter electrode is formed on a part of the bottom surface to impart piezoelectricity, and a part of the bottom surface is used as a vibration driving source and a reflected wave receiving source.
[0010]
Further, according to the present invention, the bottom surface of the bottomed cylindrical case has three or more parallel-line or concentric cross-finger electrodes or a pair of spiral electrodes having a land electrode at an end. An ultrasonic sensor is obtained.
[0011]
In the present invention, the bottom surface and the cylindrical portion of the bottomed cylindrical case are integrally formed of piezoelectric ceramics, a counter electrode is formed on the bottom surface of the bottomed cylindrical case made of this piezoelectric ceramic, and the counter electrodes are formed using the counter electrodes. The piezoelectric ceramic is polarized to impart piezoelectricity to form a diaphragm, and an AC voltage is applied between the counter electrodes to vibrate the diaphragm. Since the outer surface and the periphery of the diaphragm have no piezoelectricity, the diaphragm vibrates unevenly due to the expansion and contraction of the piezoelectric ceramics on the inner surface of the outer peripheral fixed diaphragm, and emits ultrasonic waves in front of the outer surface. The reflected wave can be detected.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below in detail with reference to the drawings.
[0013]
FIG. 1 is a cross-sectional view showing an embodiment of an ultrasonic sensor according to the present invention. In FIG. 1, a bottomed cylindrical case 1 having an outer diameter of 20 mm and a height of 10 mm is obtained by firing a molded body of a lead zirconate titanate piezoelectric ceramic material. The bottom of the bottomed cylindrical case 1 has a thickness. A 1.5 mm diaphragm 2 is integrally formed, cross finger electrodes 3 and 4 are formed on the inner surface 2a of the diaphragm 2 by sputtering, and land electrodes 5 formed at ends of the cross finger electrodes 3 and 4; 6 is soldered with lead wires 9 and 10 and is further filled with a sound absorbing material 7 made of sponge, felt or the like provided with a hole for passing the lead wires 9 and 10 inside the bottomed cylindrical case 1. The sound absorbing material 7 has a structure in which an insulating elastic material 8 made of silicone resin or urethane resin is filled in order to prevent moisture and raindrops from entering.
[0014]
FIG. 2 is a plan view of the bottomed cylindrical case shown in FIG. 1 before filling the sound absorbing material 7 and the elastic material 8. The lead wires 9 and 10 are soldered to the signal wires 11 and 12 in the elastic material 8.
[0015]
FIG. 3 shows the polarization state of the diaphragm when a voltage is applied to the interdigitated electrodes 3 and 4 in FIG. When an alternating voltage is applied to the crossed finger electrodes 3 and 4 by polarizing the piezoelectric ceramic on the inner surface side of the diaphragm 2 as shown by an arrow 21 in FIG. 3, the piezoelectric ceramic vibrates due to expansion and contraction of the piezoelectric ceramic on the inner surface side of the diaphragm. The plate can vibrate unevenly and excite vibration.
[0016]
In the above-described embodiment, the electrode formed on the inner surface of the diaphragm has been described as a cross finger electrode. However, two or more concentric cross finger electrodes or two spiral electrodes arranged at a fixed interval may be used. Similarly, it is possible to excite the vibration by exciting the vibration of the diaphragm by the expansion and contraction of the piezoelectric ceramics on the inner surface side of the diaphragm by application of the AC voltage, and an ultrasonic sensor can be configured. An example of the two or more concentric cross finger electrodes is shown in FIG. FIG. 5 shows an example of the two spiral electrodes arranged at regular intervals.
[0017]
The initial characteristics of capacitance and sensitivity were measured for the ultrasonic sensor prototyped in the above embodiment. For comparison, the same measurement was performed on the ultrasonic sensor having the same shape and the conventional structure shown in FIG. The number of measurement samples is 20 for each. The results are shown in Table 1. The capacitance is a value determined by the dielectric constant of the piezoelectric ceramic, the distance between the electrodes, and the electrode area, but stress is generated as the adhesive hardens in the bonding process, and the stress affects the dielectric constant of the piezoelectric ceramic. As a result, the capacitance tends to change.
[0018]
[Table 1]

[0019]
In the embodiment, since the electrode structure is different from the conventional structure, the same value of capacitance and sensitivity cannot be obtained, but when comparing the ratio of the magnitude of variation to the average value of capacitance and sensitivity, It can be seen that the embodiment is less than half of the comparative example and the variation is small.
[0020]
In addition, the above-described ultrasonic sensor was subjected to a thermal shock test in which temperatures of minus 40 ° C. and plus 100 ° C. were repeated, and changes in the capacitance of the ultrasonic sensor were observed. The capacitance is a value determined by the dielectric constant of the piezoelectric ceramic, the distance between the electrodes, and the electrode area, but if the adhesive layer is deteriorated in the thermal shock test, the constraint between the vibrator and the diaphragm changes. There exists a tendency for the said electrostatic capacitance to fall. For comparison, the same test was performed on the ultrasonic sensor having the conventional structure shown in FIG. FIG. 6 shows the rate of change in capacitance with the course of the thermal shock test.
[0021]
FIG. 6 shows that the conventional structure has a capacitance change of about 5% after 500 cycles, whereas the structure according to the present invention hardly shows any deterioration. From this experimental result, it was confirmed that the deterioration of the characteristics due to the temperature difference is less in this structure than in the conventional structure.
[0022]
Furthermore, the manufacturing time and cost per 1000 prototypes of the above-described ultrasonic sensors were compared with those of the conventional structure, and the comparison results are shown in Table 2.
[0023]
[Table 2]

[0024]
From Table 2, it can be seen that the ultrasonic sensor of the present invention can be manufactured at a low cost of about 60% as compared with the comparative example.
[0025]
【The invention's effect】
As described above, according to the present invention, by integrally molding the bottom surface and the cylindrical portion of the case with piezoelectric ceramics, an ultrasonic sensor with little variation in initial characteristics can be provided at a low cost, and at the same time, cross-finger electrodes and spirals are formed on the bottom surface. By forming a cylindrical electrode and polarizing the piezoelectric ceramic between the electrodes as a drive source, it is possible to obtain an ultrasonic sensor that is less susceptible to temperature differences and less susceptible to environmental degradation. It was.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an embodiment of an ultrasonic sensor according to the present invention.
FIG. 2 is a plan view of the bottomed cylindrical case shown in FIG. 1 before filling with a sound absorbing material or an elastic material.
FIG. 3 is a diagram illustrating a polarization state of a diaphragm when a voltage is applied to a crossed finger electrode.
FIG. 4 is a view showing two or more concentric cross finger electrodes.
FIG. 5 is a diagram showing two spiral electrodes at regular intervals.
FIG. 6 is a diagram showing the rate of change in capacitance with the course of a thermal shock test.
FIG. 7 is a diagram showing the structure of an ultrasonic sensor described in a known document.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1,25 Cylindrical case 2 with a bottom 2 Diaphragm 2a Inner surface 3,4 of crossing electrode 5,6 Land electrode 7, Sound absorbing material 8, Elastic material 9,10 Lead wire 11,12 Signal line 21 Arrow 26 which shows polarization direction Diaphragm 27 Piezoelectric vibration elements 28a and 28b Element electrode 29 Sound absorbing material 30 Elastic body 31 Solder points 32 and 33 Lead wires 34 and 35 Signal line

Claims (5)

An ultrasonic sensor for transmitting and receiving ultrasonic waves using a bottom surface of a bottomed cylindrical case as a diaphragm, wherein the bottom surface and the cylindrical portion of the bottomed cylindrical case are integrally formed of a piezoelectric ceramic material, and the bottom surface An ultrasonic sensor having a structure in which a counter electrode is formed on a part of the substrate to impart piezoelectricity, and a part of the bottom surface is used as a driving source for vibration and a receiving source for reflected waves. 2. The ultrasonic sensor according to claim 1, wherein three or more parallel-fingered interdigitated electrodes are provided on a bottom surface of the bottomed cylindrical case. 2. The ultrasonic sensor according to claim 1, wherein two or more concentric intersecting finger electrodes are provided on a bottom surface of the bottomed cylindrical case. 2. The ultrasonic sensor according to claim 1, wherein two spiral electrodes are provided at regular intervals on a bottom surface of the bottomed cylindrical case. 5. The ultrasonic sensor according to claim 2, further comprising a land electrode at an end of an electrode formed on a bottom surface of the bottomed cylindrical case.

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