Ultrasonic Sensors

Abstract

In an ultrasonic sensor (100), a piezoelectric body (130) has a first surface (131) and a second surface (132). A first electrode (140) is provided on the first surface (131). A second electrode (150) is provided on the second surface (132). The second electrode (150) is opposite to the first electrode (140) via the piezoelectric body (130). A third electrode (160) is provided on the second surface (132). The third electrode (160) is separated from the second electrode (150). The third electrode (160) is electrically connected to the first electrode (140). When viewed from the thickness direction (Z) which is the direction in which the first surface (131) and the second surface (132) are arranged, the second electrode (150) extends to the two end edges (132A) of the second surface (132) in the first direction (D1) and is separated from the two end edges (132B) of the second surface (132) in the second direction (D2) which is orthogonal to the first direction (D1).

Description

Technical Field

[0001] The present invention relates to ultrasonic sensors. Background Art

[0002] As documents disclosing the structure of an ultrasonic sensor, there are Japanese Unexamined Patent Publication No. 7-154898 (Patent Document 1) and International Publication No. 2013 / 047544 (Patent Document 2). Also, as a document disclosing the structure of a piezoelectric element, there is Japanese Utility Model Application Laid-Open No. 63-59499 (Patent Document 3).

[0003] The ultrasonic sensor disclosed in Patent Document 1 has a disc-shaped acoustic matching plate and a disc-shaped piezoelectric element plate. The disc-shaped piezoelectric element plate is bonded to the upper surface of the disc-shaped acoustic matching plate. Two electrode layers are formed on the surface and back of the disc-shaped piezoelectric element plate. The electrode layer located on the acoustic matching plate side has an electrical lead-out terminal. The electrical lead-out terminal extends to the surface side.

[0004] The ultrasonic sensor disclosed in patent document 2 includes a shell and a piezoelectric element. The shell is a bottomed cylindrical shape and has a bottom portion. The bottom portion is a vibration area. The piezoelectric element has a piezoelectric substrate, a first electrode, a second electrode, and a third electrode. The piezoelectric substrate has a first surface and a second surface. The second surface is opposite to the first surface. The first electrode is arranged on the first surface. The first electrode is bonded to the bottom portion. The second electrode is arranged on a portion of the second surface. The third electrode is arranged on a portion of the second surface separately from the second electrode. The third electrode is connected to the first electrode. When the bottom portion is viewed from above, the piezoelectric element is arranged at a position where its center is different from the center of the vibration area.

[0005] The piezoelectric element disclosed in Patent Document 3 comprises a piezoelectric body, a first electrode portion, a second electrode portion, and a third electrode portion. The piezoelectric body vibrates longitudinally in the thickness direction. The piezoelectric body has a first surface and a second surface which are perpendicular to the vibration direction and opposite to each other. The first electrode portion is arranged on the first surface. The second electrode portion is arranged on the second surface and connected to the first electrode portion. The second electrode portion has a lead connection portion. The third electrode portion is arranged on the second surface separately from the second electrode portion. The third electrode portion has a lead connection portion. The lead connection portion of the second electrode portion and the lead connection portion of the third electrode portion are arranged on the outer edge portion of the second surface. The shape of the outer edge portion of either the second electrode portion or the third electrode portion is set to be substantially the same as the shape of the outer edge portion of the first electrode portion.

[0006] Prior Art Literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 7-154898

[0009] Patent Document 2: International Publication No. 2013 / 047544

[0010] Patent Document 3: Japanese Utility Model Publication No. 63-59499 Summary of the invention

[0011] Problem that the invention aims to solve

[0012] The conventional ultrasonic sensors disclosed in Patent Documents 1 and 2 are configured so that wiring can be directly connected to electrodes of a piezoelectric element without passing through a member other than the piezoelectric element, such as a housing.

[0013] However, in the ultrasonic sensor disclosed in Patent Document 1, the shape of the portion where the two electrodes are opposed to each other, that is, the portion of the electrode constituting the vibration region, is low in symmetry when viewed from the thickness direction of the piezoelectric body with respect to the center of the entire piezoelectric element. Therefore, due to the vibration of the entire piezoelectric element accompanying the vibration of the vibration region of the piezoelectric element, stray vibration is generated, and the stray vibration is a vibration based on a mode of a stray frequency different from the desired resonance frequency. Even in the case where the ultrasonic sensor is designed so that the reverberation time of the vibration mode based on the resonance frequency becomes shorter, it is not designed so that the reverberation time of the stray vibration becomes shorter. As a result, due to the reverberation of the stray vibration, the reverberation time of the ultrasonic sensor becomes longer, and the reverberation characteristics of the ultrasonic sensor are degraded.

[0014] In the ultrasonic sensor disclosed in Patent Document 2, when viewed from the thickness direction of the piezoelectric body, the shape symmetry of the portion of the electrode of the piezoelectric element constituting the vibration region becomes high with respect to the center of the housing. However, similarly to the piezoelectric element disclosed in Patent Document 1, the shape symmetry of the electrode constituting the vibration region of the piezoelectric element is low with respect to the center of the entire piezoelectric element. Therefore, the reduction in the reverberation characteristics of the ultrasonic sensor is not sufficiently suppressed.

[0015] In addition, Patent Document 3 discloses a piezoelectric element in which the shape of the portion of the electrode constituting the vibration region is relatively symmetrical with respect to the center of the entire piezoelectric element when viewed from the thickness direction of the piezoelectric body. However, the area of ​​the portion of the electrode constituting the vibration region is relatively small relative to the entire piezoelectric element, and the vibration intensity of the piezoelectric element is low.

[0016] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an ultrasonic sensor capable of directly connecting wiring to electrodes of a piezoelectric element and suppressing a decrease in reverberation characteristics while increasing the strength of vibration caused by the piezoelectric element.

[0017] Technical solutions to solve problems

[0018] The ultrasonic sensor according to the present invention comprises a shell and a piezoelectric element. The shell has a bottom and a peripheral wall. The piezoelectric element is arranged on the bottom in the shell. The piezoelectric element includes a piezoelectric body, a first electrode, a second electrode, and a third electrode. The piezoelectric body has a first surface and a second surface. The first surface is located on the bottom side. The second surface is located on the opposite side to the first surface. The first electrode is arranged on the first surface. The second electrode is arranged on the second surface. The second electrode is opposite to the first electrode via the piezoelectric body. The third electrode is arranged on the second surface. The third electrode is separated from the second electrode. The third electrode is electrically connected to the first electrode. When viewed from the thickness direction which is the direction in which the first surface and the second surface are arranged, the second electrode extends to the two end edges of the second surface in the first direction and is separated from the two end edges of the second surface in the second direction orthogonal to the first direction.

[0019] Effects of the Invention

[0020] According to the present invention, the electrodes of the piezoelectric element can be directly connected to the wiring, and the intensity of the vibration caused by the piezoelectric element can be improved while suppressing the reduction of the reverberation characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a cross-sectional view showing the structure of the ultrasonic sensor according to the first embodiment of the present invention.

[0022] Figure 2 From the direction of the arrow on line II-II Figure 1 A cross-sectional view of an ultrasonic sensor for observation.

[0023] Figure 3 This is a cross-sectional view showing the structure of an ultrasonic sensor according to a modified example of Embodiment 1 of the present invention.

[0024] Figure 4 : is a cross-sectional view showing the structure of the ultrasonic sensor according to Comparative Example 1.

[0025] Figure 5 : is a cross-sectional view showing the structure of an ultrasonic sensor according to Comparative Example 2.

[0026] Figure 6 This is a graph showing an impedance curve obtained by measuring the impedance of the ultrasonic sensor according to Example 1.

[0027] Figure 7 This is a graph showing an impedance curve obtained by measuring the impedance of the ultrasonic sensor according to the second embodiment.

[0028] Figure 8 This is a graph showing an impedance curve obtained by measuring the impedance of the ultrasonic sensor according to Comparative Example 1.

[0029] Fig. 9This is a graph showing an impedance curve obtained by measuring the impedance of the ultrasonic sensor according to Comparative Example 2.

[0030] Fig.10 It is a cross-sectional view showing the structure of an ultrasonic sensor according to Embodiment 2 of the present invention.

[0031] Fig.11 From the direction of the arrow on the XI-XI line Fig.10 A cross-sectional view of an ultrasonic sensor for observation.

[0032] Fig.12 It is a plan view showing the structure of a piezoelectric element in an ultrasonic sensor according to Embodiment 3 of the present invention.

[0033] Fig.13 It is a plan view showing the structure of a piezoelectric element in an ultrasonic sensor according to a fourth embodiment of the present invention. DETAILED DESCRIPTION

[0034] Hereinafter, ultrasonic sensors according to various embodiments of the present invention will be described with reference to the drawings. In the following description of the embodiments, the same or corresponding parts in the drawings are denoted by the same reference numerals, and their description will not be repeated.

[0035] (Implementation Method 1)

[0036] Figure 1 It is a cross-sectional view showing the structure of the ultrasonic sensor according to the first embodiment of the present invention. Figure 2 From the direction of the arrow on line II-II Figure 1 A cross-sectional view of an ultrasonic sensor. Figure 1 as well as Figure 2 As shown, the ultrasonic sensor 100 according to the first embodiment of the present invention includes a housing 110 and a piezoelectric element 120 .

[0037] The housing 110 has a bottom portion 111 and a peripheral wall portion 112. Figure 2 As shown, when the ultrasonic sensor 100 is viewed from the opening end of the housing 110 on the opposite side to the bottom 111, the bottom 111 has a circular outer shape as a whole. The diameter of the bottom 111 as a whole when viewed from the opening end is, for example, 15.5 mm.

[0038] In the first embodiment of the present invention, when viewed from the opening end of the housing 110 located on the opposite side of the bottom 111, the portion 111A of the bottom 111 facing the inside of the housing has a line-symmetric outer shape with an imaginary line passing through the center point C1 of the portion 111A and parallel to the first direction D1 as the axis of symmetry. In addition, when viewed from the above-mentioned opening end, the above-mentioned portion 111A has a line-symmetric outer shape with an imaginary line passing through the above-mentioned center point C1 and parallel to the second direction D2 as the axis of symmetry, wherein the second direction D2 is orthogonal to the first direction D1.

[0039] In the first embodiment of the present invention, specifically, when viewed from the open end side of the housing 110, the portion 111A of the bottom 111 facing the inside of the housing 110 has a rectangular shape. When viewed from the open end side of the housing 110, the shape of the portion 111A of the bottom 111 has two long sides parallel to the first direction D1 and two short sides parallel to the second direction D2. In addition, the shape of the portion 111A of the bottom 111 may also have two long sides parallel to the second direction D2 and two short sides parallel to the first direction D1.

[0040] When viewed from the opening end of the housing 110 , the length of the portion 111A of the bottom 111 in the first direction D1 is, for example, 13 mm, and the length in the second direction D2 is, for example, 7 mm.

[0041] In the ultrasonic sensor 100 according to the first embodiment of the present invention, the portion 111A of the bottom portion 111 facing the inside of the housing 110 serves as a main vibration region of the housing 110 .

[0042] like Figure 1 as well as Figure 2 As shown, the area of ​​the opening end of the peripheral wall portion 112 when viewed from the opening direction is larger than the area of ​​the above-mentioned portion 111A.

[0043] In the first embodiment of the present invention, the housing 110 is made of a conductive material such as aluminum or an aluminum alloy. Alternatively, the housing 110 may be made of an insulating material.

[0044] like Figure 2 As shown, the piezoelectric element 120 is disposed on the bottom 111 in the housing 110. The piezoelectric element 120 is bonded to the bottom 111 by an adhesive such as epoxy resin.

[0045] like Figure 1 as well as Figure 2 As shown, the piezoelectric element 120 includes a piezoelectric body 130 , a first electrode 140 , a second electrode 150 , and a third electrode 160 .

[0046] The piezoelectric body 130 has a first surface 131 and a second surface 132. The first surface 131 is located on the bottom 111 side. The second surface 132 is located on the opposite side to the first surface 131. In the first embodiment of the present invention, the first surface 131 and the second surface 132 are located parallel to each other.

[0047] like Figure 2 As shown, when viewed from the thickness direction Z, which is the direction in which the first surface 131 and the second surface 132 are arranged, the piezoelectric body 130 has a line-symmetrical shape with an imaginary line L passing through the center point C3 of the piezoelectric body 130 and parallel to the first direction D1 as the symmetry axis. In addition, the piezoelectric body 130 has a line-symmetrical shape with an imaginary line passing through the center point C3 of the piezoelectric body 130 and parallel to the second direction D2 as the symmetry axis. In addition, the above-mentioned thickness direction Z is the same as the direction when the bottom 111 is observed from the opening end side.

[0048] In the first embodiment of the present invention, specifically, the piezoelectric body 130 has a rectangular shape when viewed from the thickness direction Z. When viewed from the thickness direction Z, the piezoelectric body 130 has two long sides parallel to the first direction D1, and has two short sides parallel to the second direction D2. The length of the piezoelectric body 130 in the first direction D1 when viewed from the thickness direction Z is, for example, 6.5 mm, and the length in the second direction D2 is, for example, 5.2 mm or more and 5.8 mm or less. In addition, the piezoelectric body 130 may also have long sides parallel to the second direction D2, and short sides parallel to the first direction D1.

[0049] In the first embodiment of the present invention, when viewed in the thickness direction Z, the center point C3 of the piezoelectric body 130 and the center point C1 of the portion 111A of the bottom 111 facing the inside of the housing 110 coincide with each other.

[0050] In the first embodiment of the present invention, when viewed in the thickness direction Z, the center point C3 of the piezoelectric body 130 and the center point C1 of the portion 111A of the bottom 111 facing the inside of the case do not necessarily have to coincide with each other. Figure 3 FIG. 1 is a cross-sectional view showing the structure of an ultrasonic sensor according to a modified example of the first embodiment of the present invention. Figure 3 In Figure 2 The ultrasonic sensor 100 according to the first embodiment of the present invention is shown in the same cross-sectional view.

[0051] like Figure 3As shown, in the ultrasonic sensor 100a according to the modification of the first embodiment of the present invention, the center point C3 of the piezoelectric body 130 is located at a position separated in the first direction D1 from the center point C1 of the portion 111A of the bottom 111 facing the inside of the housing 110 when viewed in the thickness direction Z. The other structures of the ultrasonic sensor 100a according to the modification of the first embodiment of the present invention are the same as those of the ultrasonic sensor 100 according to the first embodiment of the present invention.

[0052] In the ultrasonic sensor 100 according to the first embodiment of the present invention, the piezoelectric body 130 includes, for example, lead zirconate titanate (PZT).

[0053] like Figure 1 As shown, the first electrode 140 is provided on the first surface 131. In the first embodiment of the present invention, the first electrode 140 is provided on the entire surface of the first surface 131. That is, when viewed from the thickness direction Z, the first electrode 140 has the same outer shape as the piezoelectric body 130. In addition, the first electrode 140 may be provided to be separated from at least one of the two end edges of the first surface 131 in the second direction D2.

[0054] In the first embodiment of the present invention, the first electrode 140 is electrically connected to the bottom portion 111 via an adhesive for bonding the piezoelectric element 120 to the bottom portion 111. Alternatively, the first electrode 140 may not be electrically connected to the bottom portion 111.

[0055] like Figure 1 As shown, the second electrode 150 is provided on the second surface 132. The second electrode 150 faces the first electrode 140 with the piezoelectric body 130 interposed therebetween.

[0056] like Figure 2 As shown in FIG. 1 , when viewed from the thickness direction Z, the second electrode 150 extends to the two end edges 132A of the second surface 132 in the first direction D1. Thus, the area of ​​the region of the second electrode 150 that faces the first electrode 140 can be increased, thereby improving the vibration strength of the piezoelectric element 120. When viewed from the thickness direction Z, the second electrode 150 is separated from the two end edges 132B of the second surface 132 in the second direction D2 that is orthogonal to the first direction D1. The separation distance between the second electrode 150 and the two end edges 132B is, for example, 1.3 mm.

[0057] In the first embodiment of the present invention, the second electrode 150 has a line-symmetric shape with the imaginary line L parallel to the first direction D1 and passing through the center point C3 of the piezoelectric body 130 as the axis of symmetry when viewed from the thickness direction Z. In addition, the second electrode 150 has a line-symmetric shape with the imaginary line parallel to the second direction D2 and passing through the center point C3 of the piezoelectric body 130 as the axis of symmetry when viewed from the thickness direction Z.

[0058] In the first embodiment of the present invention, the second electrode 150 specifically has a rectangular outer shape when viewed in the thickness direction Z. When viewed in the thickness direction Z, the second electrode 150 has two sides parallel to the first direction D1 and two sides parallel to the second direction D2.

[0059] like Figure 1 as well as Figure 2 As shown, the third electrode 160 is provided on the second surface 132. The third electrode 160 is separated from the second electrode 150. The third electrode 160 is located on one side of the second electrode 150 in the second direction D2. When viewed from the thickness direction Z, the third electrode 160 extends to reach the end edges 132B located near the third electrode 160 of the two end edges 132B of the second surface 132 in the second direction D2. When viewed from the thickness direction Z, the third electrode 160 extends to reach the two end edges 132A of the second surface 132 in the first direction D1.

[0060] In the first embodiment of the present invention, the third electrode 160 has a rectangular outer shape when viewed from the thickness direction. When viewed from the thickness direction Z, the third electrode 160 has two sides parallel to the first direction D1 and two sides parallel to the second direction D2.

[0061] The third electrode 160 is electrically connected to the first electrode 140 . Specifically, the piezoelectric element 120 further includes a connection electrode 170 , and the third electrode 160 is connected to the first electrode 140 via the connection electrode 170 .

[0062] The position of the connection electrode 170 is not particularly limited as long as it is provided on the piezoelectric body 130 away from the second electrode 150. In the first embodiment of the present invention, the connection electrode 170 is provided on the side surface of the piezoelectric body 130 on the side where the third electrode 160 is located, as viewed from the center point C3 of the piezoelectric body 130.

[0063] like Figure 1 As shown, in the first embodiment of the present invention, the ultrasonic sensor includes a conductive member 191 and a filling member 192. Figure 2 In the figure, the conductive member 191 and the filling member 192 are not shown.

[0064] The conductive member 191 specifically includes an FPC (flexible printed circuit board) having a resin sheet and wiring and two wiring parts connected to the FPC. The conductive member 191 is configured to be led out from the inside of the housing 110 through the open end of the housing 110 to the outside of the housing 110. Inside the housing 110, the conductive member 191 connects one of the two wiring parts to the second electrode 150 via the FPC, and connects the other of the two wiring parts to the third electrode 160.

[0065] The filling member 192 fills all or part of the gaps between other members inside the housing 110. In the first embodiment of the present invention, the filling member 192 includes a first filling member 193 having different foaming ratios and a second filling member 194 located on the opening end side of the housing 110 relative to the first filling member 193. Alternatively, the filling member 192 may have the same foaming ratio as a whole. The filling member 192 includes, for example, silicone foam.

[0066] The mechanism of transmitting and receiving ultrasonic waves of the ultrasonic sensor 100 according to the first embodiment of the present invention is briefly described. First, when the ultrasonic sensor 100 is used to transmit ultrasonic waves, a pulse voltage is applied between the second electrode 150 and the third electrode 160 via the conductive member 191, thereby applying a pulse voltage between the second electrode 150 and the first electrode 140 connected to the third electrode 160. Moreover, by applying a pulse voltage between the first electrode 140 and the second electrode 150, the charges in the piezoelectric body 130 between the first electrode 140 and the second electrode 150 and the external charges attract or repel each other. As a result, the piezoelectric body 130 repeatedly expands and contracts. As the piezoelectric body 130 repeatedly expands and contracts, the piezoelectric element 120 vibrates. As the piezoelectric element 120 vibrates, the above-mentioned portion 111A of the bottom 111 connected to the piezoelectric element 120 mainly vibrates. As a result, ultrasonic waves are transmitted from the bottom 111.

[0067] When the ultrasonic sensor 100 is used to receive ultrasonic waves, the ultrasonic waves hitting the above-mentioned portion 111A of the bottom 111 vibrate the above-mentioned portion 111A of the bottom 111. The above-mentioned portion 111A vibrates, so that the piezoelectric element 120 vibrates, and the piezoelectric body 130 repeatedly expands and contracts. The piezoelectric body 130 repeatedly expands and contracts, so that charges opposite to each other are generated on the first surface 131 and the second surface 132. As a result, a voltage is applied between the first electrode 140 and the second electrode 150. The voltage between the second electrode 150 and the third electrode 160 connected to the first electrode 140 is sensed via the conductive member 191. In this way, the ultrasonic sensor 100 can be used to receive ultrasonic waves.

[0068] In the ultrasonic sensor 100, the time from when the voltage application to each electrode stops until the vibration of the piezoelectric element 120 and the housing 110 stops is called the reverberation time. When the ultrasonic wave transmitted by the ultrasonic sensor 100 is reflected by an object and the reflected ultrasonic wave is received by the same ultrasonic sensor 100, the reverberation time after the ultrasonic wave is transmitted becomes important. This is because when the object is located near the ultrasonic sensor 100, the time from the transmission of the ultrasonic wave to the reception is relatively short, so the shorter the reverberation time becomes, the higher the receiving sensitivity of the ultrasonic sensor 100 is.

[0069] Hereinafter, a first test example of the ultrasonic sensor 100 according to the first embodiment of the present invention will be described. In the first test example, first, ultrasonic sensors according to Example 1, Example 2, Comparative Example 1, and Comparative Example 2 were fabricated.

[0070] The ultrasonic sensor according to Example 1 is configured to be the same as the ultrasonic sensor 100 according to Embodiment 1 of the present invention, and further, the length of the piezoelectric body 130 in the first direction D1 when viewed from the thickness direction Z is set to 6.5 mm, and the length of the piezoelectric body 130 in the second direction D2 is set to 5.8 mm. In addition, the separation distance between the end edge 132B on the third electrode 160 side and the second electrode 150 in the second direction D2 when viewed from the thickness direction Z is set to 1.3 mm, the diameter of the bottom 111 when viewed from the thickness direction Z is set to 15.5 mm, the length of the portion 111A of the bottom 111 facing the inside of the housing 110 in the first direction D1 is set to 13 mm, and the length of the portion 111A in the second direction D2 is set to 7 mm. Specifically, in the ultrasonic sensor according to Example 1, the housing 110 is made of aluminum, and the piezoelectric body is made of PZT. The piezoelectric element 120 is bonded to the bottom 111 of the housing 110 using epoxy resin.

[0071] The ultrasonic sensor according to the second embodiment is configured to have the same structure as the ultrasonic sensor 100a according to the first variant of the first embodiment of the present invention, and further, the piezoelectric element 120 is arranged so that the center point C3 of the piezoelectric body 130 is 0.3 mm away from the center point C1 of the portion 111A of the bottom 111 facing the inside of the housing 110 toward the third electrode in the second direction D2. The material constituting the housing 110, the material of the piezoelectric body 130, and the method of joining the piezoelectric element 120 to the bottom 111 of the housing 110 are the same as those of the ultrasonic sensor according to the first embodiment.

[0072] The ultrasonic sensor according to Comparative Example 1 is different from the ultrasonic sensor according to Example 1 only in the structure of the second electrode and the arrangement position of the piezoelectric element. Figure 4is a cross-sectional view showing the structure of the ultrasonic sensor according to Comparative Example 1. Figure 4 In Figure 2 The ultrasonic sensor 100 according to the first embodiment of the present invention is shown in the same cross-sectional view.

[0073] like Figure 4 As shown, in the ultrasonic sensor 800 according to the comparative example 1, the second electrode 150 extends to reach the end edge 132B of the second surface 132 on the opposite side to the third electrode 160 side, of the two end edges 132B of the second surface 132 in the second direction D2. In addition, in the ultrasonic sensor 800 according to the comparative example 1, the piezoelectric element 120 is arranged at a position where the center of the second electrode 150 and the center point C1 of the portion 111A of the bottom 111 facing the inside of the housing 110 are substantially coincident with each other, as viewed in the thickness direction Z. Therefore, the center point C3 of the piezoelectric body 130 is located at a position 0.3 mm away from the center point C1 of the portion 111A of the bottom 111 facing the inside of the housing 110 in the second direction D2 toward the third electrode 160.

[0074] The ultrasonic sensor according to Comparative Example 2 is different from the ultrasonic sensor 800 according to Comparative Example 1 only in the arrangement position of the piezoelectric element. Figure 5 is a cross-sectional view showing the structure of an ultrasonic sensor according to Comparative Example 2. Figure 5 In Figure 4 The ultrasonic sensor 800 according to the comparative example 1 shown is shown in the same cross-sectional view.

[0075] like Figure 5 As shown, in the ultrasonic sensor 900 involved in Comparative Example 2, the center point C3 of the piezoelectric body 130 is located 0.3 mm away from the center point C1 of the portion 111A of the bottom 111 facing the inside of the shell 110 toward the third electrode 160 in the second direction D2, and is located 0.2 mm away from one side in the first direction D1.

[0076] Then, for each ultrasonic sensor involved in Example 1, Example 2, Comparative Example 1 and Comparative Example 2, a voltage was applied between the second electrode and the third electrode connected to the first electrode, and the impedance was measured. An impedance analyzer (Keysight 4194A, manufactured by Keysight Technologies) was used for the impedance measurement. In the impedance measurement, a constant voltage was applied while the power supply frequency was continuously changed, thereby measuring the impedance. For each example and each comparative example, an impedance curve as a measurement result of the impedance relative to the power supply frequency is shown in the following Figures 6 to 9 .

[0077] Figure 6This is a graph showing an impedance curve obtained by measuring the impedance of the ultrasonic sensor according to Example 1. Figure 7 This is a graph showing an impedance curve obtained by measuring the impedance of the ultrasonic sensor according to the second embodiment. Figure 8 This is a graph showing an impedance curve obtained by measuring the impedance of the ultrasonic sensor according to Comparative Example 1. Fig. 9 This is a graph showing an impedance curve obtained by measuring the impedance of the ultrasonic sensor according to Comparative Example 2.

[0078] like Figures 6 to 9 As shown in the figure, in the ultrasonic sensors involved in the embodiments and the comparative examples, when the power supply frequency is around 65kHz, a high impedance of 2000Ω or more is shown. That is, in the embodiments and the comparative examples, the structure of the ultrasonic sensor is designed so that the resonant frequency is around 65kHz. Therefore, in these ultrasonic sensors, the reverberation time is shortened when transmitting ultrasonic waves with a frequency around 65kHz.

[0079] like Figure 8 as well as Fig. 9 As shown, in the ultrasonic sensor 800 involved in Comparative Example 1 and the ultrasonic sensor 900 involved in Comparative Example 2, when the power supply frequency is around 75kHz, the impedance value becomes about 200Ω. Therefore, in Comparative Examples 1 and 2, when the ultrasonic sensor vibrates at a frequency around 65kHz, a stray vibration that vibrates relatively strongly at a frequency around 75kHz is generated. Therefore, when the ultrasonic sensor transmits an ultrasonic wave with a frequency around 65kHz, a beat is generated by mixing the stray vibration with the reverberation vibration of the ultrasonic sensor, and the reverberation time is long.

[0080] On the other hand, Figure 6 as well as Figure 7 As shown, in the ultrasonic sensors according to the first and second embodiments, there is no power supply frequency where the impedance value is about 200Ω in the frequency band different from the power supply frequency near 65kHz. That is, in the first and second embodiments, when the ultrasonic sensor vibrates at a frequency near 65kHz, no large stray vibration is generated. Therefore, the stray vibration mixed in the reverberation vibration of the ultrasonic sensor is also reduced, and the reverberation time is shortened.

[0081] In addition, if Figure 6 as well as Figure 8 As shown, when Example 1 and Comparative Example 1, in which the center positions of the second electrode 150 are substantially identical, are compared, it is found that in Example 1, the generation of stray vibrations at a frequency of around 75 kHz is reduced compared to Comparative Example 1.

[0082] In addition, if Figure 7 as well as Fig. 9 As shown, when comparing Example 2 and Comparative Example 2, in which the positions of the centers of the second electrodes 150 are substantially consistent with each other, it can be seen that in Example 2, the generation of stray vibrations at a frequency of about 75 kHz is reduced relative to Comparative Example 2. As described above, although in Example 2, the position of the center point C3 of the piezoelectric body 130 in the first direction D1 is inconsistent with the above-mentioned center point C1, since the symmetry of the second electrode 150 with respect to the entire piezoelectric element 120 is improved, the generation of stray frequencies can be reduced in the ultrasonic sensor involved in Example 2.

[0083] Furthermore, if Figure 6 as well as Figure 7 As shown, when comparing Example 1 and Example 2, the impedance at a frequency near 75 kHz is further reduced in Example 1. In this way, by aligning the position of the center point C3 of the piezoelectric body 130 with the center point C1, the stray vibration at a frequency near 75 kHz can be further reduced, and the reverberation time can be further reduced.

[0084] Next, a second test example of the ultrasonic sensor 100 according to the first embodiment of the present invention is described. In the second test example, for each ultrasonic sensor according to Examples 3 to 8 and Comparative Examples 3 and 4, a change in the intensity of stray vibration when the separation distance between the end edge 132B on the opposite side of the third electrode 160 and the second electrode 150 in the second direction D2 is changed is analyzed by simulation. Specifically, the intensity of stray vibration is measured by piezoelectric resonance analysis based on finite element analysis simulation.

[0085] Each ultrasonic sensor according to Examples 3 to 5 has the same structure as the ultrasonic sensor 100 according to Embodiment 1 of the present invention, and further, the length of the piezoelectric body 130 in the first direction D1 when viewed from the thickness direction Z is set to 6.5 mm, and the length of the piezoelectric body 130 in the second direction D2 is set to 5.8 mm. In addition, the separation distance between the end edge 132B on the third electrode 160 side and the second electrode 150 in the second direction D2 when viewed from the thickness direction Z is set to 1.3 mm, the diameter of the bottom 111 is set to 15.5 mm, the length of the portion of the bottom 111 facing the inside of the housing 110 in the first direction is set to 13 mm, and the length of the portion in the second direction is set to 7 mm. In addition, the housing 110 is made of aluminum, and the piezoelectric body is made of PZT. The piezoelectric element 120 is bonded to the bottom 111 of the housing 110 using epoxy resin.

[0086] Moreover, when viewed from the thickness direction Z, the separation distance between the end edge 132B on the opposite side of the third electrode 160 of the two end edges 132B in the second direction D2 and the second electrode 150 is set to 1 mm in Example 3, 1.3 mm in Example 4, and 1.8 mm in Example 5.

[0087] In each of the ultrasonic sensors according to Examples 6 to 8, the length of the piezoelectric body 130 in the second direction D2 when viewed from the thickness direction Z is set to 5.2 mm. Regarding Examples 6 to 8, other structures are the same as those of Examples 3 to 5, respectively.

[0088] In the ultrasonic sensor according to Comparative Example 3, the second electrode extends to the end edge of the piezoelectric body on the opposite side to the third electrode side, of both end edges in the second direction when viewed in the thickness direction Z. The other structures are the same as those of Examples 3 to 5.

[0089] In the ultrasonic sensor according to Comparative Example 4, the second electrode extends to the end edges of the piezoelectric body in the second direction opposite to the third electrode when viewed in the thickness direction Z, and the other structures are the same as those of Examples 6 to 8.

[0090] Table 1 below shows the ratio of the vibration intensity (amplitude) of the stray vibration at a frequency of 72 kHz of each ultrasonic sensor involved in Examples 3 to 5 measured by the above simulation to the vibration intensity (amplitude) of the stray vibration at a frequency of 72 kHz of the ultrasonic sensor involved in Comparative Example 3. In addition, Table 2 below shows the ratio of the stray vibration intensity at a frequency of 72 kHz of each ultrasonic sensor involved in Examples 6 to 8 measured by the above simulation to the vibration intensity of the stray vibration at a frequency of 72 kHz of the ultrasonic sensor involved in Comparative Example 4.

[0091] [Table 1]

[0092] Comparative Example 3 Example 3 Example 4 Example 5 Separation distance between the second electrode and the edge [mm] 0 1 1.3 1.8 Vibration intensity ratio 1 0.6 0.5 0.6

[0093] [Table 2]

[0094] Comparative Example 4 Example 6 Example 7 Example 8 Separation distance between the second electrode and the edge [mm] 0 1 1.3 1.8 Vibration intensity ratio 1 0.7 0.1 0.6

[0095] As shown in Table 1, in Examples 3 to 5, it is known that when viewed from the thickness direction Z, the closer the separation distance between the end edge 132B on the side opposite to the third electrode 160 and the second electrode 150 in the second direction D2 is to 1.3 mm, the smaller the vibration intensity ratio becomes, that is, the closer the separation distance is to 1.3 mm, the smaller the stray vibration becomes. Here, in Examples 3 to 5, the separation distance between the end edge 132B on the side of the third electrode 160 and the second electrode 150 in the second direction D2 when viewed from the thickness direction Z is 1.3 mm. Therefore, it is known that the higher the symmetry of the second electrode 150 in the second direction D2 on the piezoelectric body 130 becomes, the smaller the stray vibration becomes. Furthermore, it is known that when the separation distances between the second electrode 150 and the end edges 132B in the second direction D2 are equal to each other as in Example 4, the stray vibration becomes the smallest. In Examples 6 to 8 in Table 2 above, the same tendency as in Examples 3 to 5 is shown.

[0096] As described above, the ultrasonic sensor 100 according to the first embodiment of the present invention includes the housing 110 and the piezoelectric element 120. The housing 110 includes the bottom 111 and the peripheral wall 112. The piezoelectric element 120 is arranged on the bottom 111 in the housing 110. The piezoelectric element 120 includes a piezoelectric body 130, a first electrode 140, a second electrode 150, and a third electrode 160. The piezoelectric body 130 includes a first surface 131 and a second surface 132. The first surface 131 is located on the bottom 111 side. The second surface 132 is located on the opposite side to the first surface 131. The first electrode 140 is provided on the first surface 131. The second electrode 150 is provided on the second surface 132. The second electrode 150 is opposite to the first electrode 140 via the piezoelectric body 130. The third electrode 160 is provided on the second surface 132. The third electrode 160 is separated from the second electrode 150. The third electrode 160 is electrically connected to the first electrode 140. When viewed from the thickness direction Z, which is the direction in which the first surface 131 and the second surface 132 are arranged, the second electrode 150 extends to the two end edges 132A of the second surface 132 in the first direction D1, and is separated from the two end edges 132B of the second surface 132 in the second direction D2 orthogonal to the first direction D1.

[0097] Thus, in the ultrasonic sensor 100 according to the first embodiment of the present invention, the first electrode 140 and the second electrode 150 can be electrically connected and wired without passing through a member other than the piezoelectric element 120 such as the housing 110, and the intensity of the vibration caused by the piezoelectric element 120 can be increased. Furthermore, when viewed in the thickness direction Z of the piezoelectric body 130, the symmetry of the outer shape of the second electrode 150 relative to the outer shape of the piezoelectric body 130 is improved, so that stray vibration can be suppressed, and the reduction of the reverberation characteristics caused by the reverberation of the vibration can be suppressed.

[0098] In the first embodiment of the present invention, when viewed in the thickness direction Z, the second electrode 150 has a line-symmetric outer shape with the imaginary line L that is parallel to the first direction D1 and passes through the center point C3 of the piezoelectric body 130 as the axis of symmetry.

[0099] As a result, the symmetry of the outer shape of the second electrode 150 with respect to the outer shape of the piezoelectric body 130 when viewed in the thickness direction Z is further improved, so that the degradation of the reverberation characteristics of the ultrasonic sensor 100 can be further suppressed.

[0100] In the first embodiment of the present invention, when viewed in the thickness direction Z, the center point C3 of the piezoelectric body 130 and the center point C1 of the portion 111A of the bottom 111 facing the inside of the housing 110 coincide with each other.

[0101] As a result, the symmetry of the outer shape of the second electrode 150 with respect to the outer shape of the portion 111A of the bottom 111 facing the inside of the housing 110 is improved when viewed in the thickness direction Z, so that the degradation of the reverberation characteristics of the ultrasonic sensor 100 can be further suppressed.

[0102] In the first embodiment of the present invention, the housing 110 includes a conductive material. The first electrode 140 is electrically connected to the bottom 111 .

[0103] Thus, by connecting the housing 110 to the ground potential, the first electrode 140 can be used as a ground electrode in the piezoelectric element 120 .

[0104] (Implementation Method 2)

[0105] Hereinafter, an ultrasonic sensor according to Embodiment 2 of the present invention will be described. The ultrasonic sensor according to Embodiment 2 of the present invention is different from the ultrasonic sensor 100 according to Embodiment 1 of the present invention in that the piezoelectric element further includes a fourth electrode. Therefore, the same structure as that of the ultrasonic sensor 100 according to Embodiment 1 of the present invention will not be described again.

[0106] Fig.10 It is a cross-sectional view showing the structure of an ultrasonic sensor according to Embodiment 2 of the present invention. Fig.11 From the direction of the arrow on the XI-XI line Fig.10 A cross-sectional view of an ultrasonic sensor. Fig.10 as well as Fig.11 As shown, in the ultrasonic sensor 200 according to the second embodiment of the present invention, the piezoelectric element 120 further includes a fourth electrode 280. The fourth electrode 280 is separated from the second electrode 150 on the second surface 132 when viewed in the thickness direction Z. The fourth electrode 280 is located on the opposite side of the third electrode 160 with respect to the second electrode 150 in the second direction D2.

[0107] Through the above structure, on the second surface 132, a pair of electrodes are arranged adjacent to each other with the second electrode 150 in a roughly symmetrical manner, so that the internal stress of the piezoelectric body 130 on the second surface 132 side is reduced, the occurrence of cracks in the piezoelectric body 130 can be suppressed, and the mechanical reliability of the ultrasonic sensor 200 can be improved.

[0108] Furthermore, in the ultrasonic sensor 200 according to the second embodiment of the present invention, specifically, when viewed from the thickness direction Z, the fourth electrode 280 has an outer shape that is line-symmetrical with the third electrode 160 about an imaginary line L that passes through the center point C3 of the piezoelectric body 130 and is parallel to the first direction D1 as an axis of symmetry. Thus, the symmetry of the electrodes on the second surface 132 is further improved, and the internal stress of the piezoelectric body 130 can be further reduced.

[0109] (Implementation method 3)

[0110] The ultrasonic sensor according to the third embodiment of the present invention is described below. The ultrasonic sensor according to the third embodiment of the present invention is different from the ultrasonic sensor 100 according to the first embodiment of the present invention mainly in the outer shape of the piezoelectric element when viewed from the thickness direction. Therefore, the same structure as that of the ultrasonic sensor 100 according to the first embodiment of the present invention will not be described again.

[0111] Fig.12 FIG. 1 is a plan view showing the structure of a piezoelectric element in an ultrasonic sensor according to Embodiment 3 of the present invention. Fig.12 In, from Figure 2 Only the piezoelectric element 320 of the ultrasonic sensor according to the third embodiment of the present invention is shown in the same direction as the piezoelectric element 120 in the first embodiment of the present invention.

[0112] like Fig.12 As shown, in the third embodiment of the present invention, the piezoelectric element 320 has a substantially circular outer shape when viewed from the thickness direction Z of the piezoelectric body 130. When viewed from the thickness direction Z, the piezoelectric body 130 has a circular outer shape.

[0113] In the ultrasonic sensor according to the third embodiment of the present invention, similarly to the ultrasonic sensor 100 according to the first embodiment of the present invention, the third electrode 160 is provided on the second surface 132. The third electrode 160 is separated from the second electrode 150. The second electrode 150 extends to both end edges 132A of the second surface 132 in the first direction D1 when viewed from the thickness direction Z, and is separated from both end edges 132B of the second surface 132 in the second direction D2 orthogonal to the first direction D1. Thus, the electrodes can be connected to wiring without passing through other members such as a housing, and the intensity of the vibration caused by the piezoelectric element 320 can be increased while suppressing the reduction of the reverberation characteristics.

[0114] (Implementation 4)

[0115] Hereinafter, an ultrasonic sensor according to a fourth embodiment of the present invention will be described. The ultrasonic sensor according to the fourth embodiment of the present invention is different from the ultrasonic sensor 200 according to the second embodiment of the present invention mainly in the outer shape of the piezoelectric element when viewed in the thickness direction. Therefore, the same structure as that of the ultrasonic sensor 200 according to the second embodiment of the present invention will not be described again.

[0116] Fig.13 FIG. 4 is a plan view showing the structure of a piezoelectric element in an ultrasonic sensor according to Embodiment 4 of the present invention. Fig.13 In, from Fig.11 The piezoelectric element 120 in the second embodiment of the present invention is shown in the same direction as the piezoelectric element.

[0117] like Fig.13 As shown, in the fourth embodiment of the present invention, the piezoelectric element 420 has a circular outer shape when viewed from the thickness direction Z of the piezoelectric body 130. When viewed from the thickness direction Z, the piezoelectric body 130 has a circular outer shape.

[0118] In the ultrasonic sensor according to the fourth embodiment of the present invention, similarly to the ultrasonic sensor 100 according to the second embodiment of the present invention, the third electrode 160 is provided on the second surface 132. The third electrode 160 is separated from the second electrode 150. When viewed from the thickness direction Z, the second electrode 150 extends to both end edges 132A of the second surface 132 in the first direction D1, and is separated from both end edges 132B of the second surface 132 in the second direction D2 orthogonal to the first direction D1. Thus, the wiring can be connected to the electrodes without passing through other members such as a housing, and the intensity of the vibration caused by the piezoelectric element 420 can be increased while suppressing the reduction of the reverberation characteristics.

[0119] The embodiments disclosed this time should be considered as illustrative in all aspects and not restrictive. The scope of the present invention is indicated by the claims rather than the above description, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0120] Description of Reference Numerals

[0121] 100, 100a, 200, 800, 900: ultrasonic sensor, 110: shell, 111: bottom, 111A: part, 112: peripheral wall, 120, 320, 420: piezoelectric element, 130: piezoelectric body, 131: first surface, 132: second surface, 132A, 132B: end edge, 140: first electrode, 150 second electrode, 160: third electrode, 170: connecting electrode, 191: conductive member, 192: filling member, 193: first filling member, 194: second filling member, 280: fourth electrode.

Claims

1. An ultrasonic sensor comprising: A housing having a bottom and a peripheral wall; and a piezoelectric element, arranged on the bottom in the housing, The piezoelectric element comprises: A piezoelectric body having a first surface located on the bottom side and a second surface located on the opposite side to the first surface; A first electrode, disposed on the first surface; a second electrode disposed on the second surface; and a third electrode, which is provided on the second surface separately from the second electrode and is electrically connected to the first electrode; The second electrode is opposed to the first electrode via the piezoelectric body. When viewed from the thickness direction which is a direction in which the first surface and the second surface are arranged, the second electrode extends to both end edges of the second surface in a first direction and is separated from both end edges of the second surface in a second direction orthogonal to the first direction.

2. The ultrasonic sensor according to claim 1, in, When viewed in the thickness direction, the second electrode has a line-symmetric outer shape with an imaginary line that is parallel to the first direction and passes through a center point of the piezoelectric body as a symmetry axis.

3. The ultrasonic sensor according to claim 2, in, The center point of the piezoelectric body and the center point of a portion of the bottom facing the inside of the case coincide with each other when viewed in the thickness direction.

4. The ultrasonic sensor according to any one of claims 1 to 3, in, The housing comprises a conductive material, The first electrode is electrically connected to the bottom portion.

5. The ultrasonic sensor according to any one of claims 1 to 3, in, The piezoelectric element further includes a fourth electrode separated from the second electrode on the second surface when viewed in the thickness direction. The fourth electrode is located on the opposite side of the third electrode with respect to the second electrode in the second direction.

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