Ultrasonic sensor

Abstract

A case (110) has a bottom plate (111) and a peripheral wall (112), and has a cylindrical shape. A piezoelectric element (120) is mounted on the bottom plate (111) in a recess (113) surrounded by the bottom plate (111) and the peripheral wall (112). When viewed from a direction perpendicular to the bottom plate (111), the recess (113) extends in a longitudinal direction and a lateral direction orthogonal to the longitudinal direction, the longitudinal and lateral directions being parallel to the bottom plate (111), and the maximum dimension of the recess (113) in the longitudinal direction is greater than the maximum dimension of the recess (113) in the lateral direction. Openings (114) communicating with the recess (113) are formed in the peripheral wall (112) at positions above the bottom plate (111) on both sides in the longitudinal direction. When viewed from the direction perpendicular to the bottom plate (111), the piezoelectric element (120) extends along the longitudinal direction, and the maximum dimension of the piezoelectric element (120) in the longitudinal direction is greater than the maximum dimension of the piezoelectric element (120) in the lateral direction.

Description

Ultrasonic sensor

[0001] The present invention relates to an ultrasonic sensor.

[0002] As a prior art document disclosed for an ultrasonic transceiver, there is Japanese Patent Application Laid-Open No. 2006-345271 (Patent Document 1). In the ultrasonic transceiver described in Patent Document 1, a piezoelectric element is bonded inside the bottom surface of a bottomed cylindrical case provided with a depression that is relatively long in one direction and relatively short in another direction to form a unimorph vibrator. An opening is provided on the side surface of the bottomed cylindrical case on the relatively short side so as not to exceed the width of the shorter side of the depression.

[0003] Japanese Patent Application Laid-Open No. 2006-345271

[0004] An ultrasonic sensor is required to have a desired directivity and high efficiency. The present invention has been made in view of the above problems, and an object thereof is to provide an ultrasonic sensor that has a desired directivity and is highly efficient.

[0005] The ultrasonic sensor according to the present invention includes a case and a piezoelectric element. The case has a bottom plate and a peripheral wall and is cylindrical. The piezoelectric element is mounted on the bottom plate within a recess surrounded by the bottom plate and the peripheral wall. When viewed from a direction perpendicular to the bottom plate, the recess extends in a longitudinal direction parallel to the bottom plate and in a short direction orthogonal to the longitudinal direction, and the maximum dimension of the recess in the longitudinal direction is larger than the maximum dimension of the recess in the short direction. Openings communicating with the recess are formed in the peripheral wall at positions above the bottom plate on both sides in the longitudinal direction. When viewed from a direction perpendicular to the bottom plate, the piezoelectric element extends along the longitudinal direction, and the maximum dimension of the piezoelectric element in the longitudinal direction is larger than the maximum dimension of the piezoelectric element in the short direction.

[0006] According to the present invention, it is possible to make the ultrasonic sensor have a desired directivity and be highly efficient.

[0007] This is a perspective view of the ultrasonic sensor according to Embodiment 1 of the present invention, viewed from the bottom plate side. This is a perspective view of the ultrasonic sensor according to Embodiment 1 of the present invention, viewed from the side opposite to the bottom plate side. This is a cross-sectional view of the ultrasonic sensor in Figure 2, viewed from the direction of the arrow III-III. This is a plan view showing the dimensional relationship between the case and the piezoelectric element of the ultrasonic sensor according to Embodiment 1 of the present invention. This is an equivalent circuit diagram of the ultrasonic sensor according to Embodiment 1 of the present invention. This is a graph showing the change in vibration velocity with respect to the position of the bottom plate in the X-axis direction when the piezoelectric element is driven and the bottom plate vibrates, in the ultrasonic sensor according to the comparative example and the ultrasonic sensor according to the embodiment. This is a graph showing the change in the force coefficient when the length of the piezoelectric element in the X-axis direction is changed, in the ultrasonic sensor according to the comparative example and the ultrasonic sensor according to the embodiment. This is a graph showing the change in the electromechanical coupling coefficient when the length of the piezoelectric element in the X-axis direction is changed, in the ultrasonic sensor according to the comparative example and the ultrasonic sensor according to the embodiment. This is a graph showing the relationship between the frequency characteristics of the impedance in a series circuit of piezoelectric elements, the force coefficient and the electromechanical coupling coefficient. This is a plan view showing the shape of the piezoelectric element of the ultrasonic sensor according to the first modified example of Embodiment 1 of the present invention. This is a plan view showing the shape of the piezoelectric element of the ultrasonic sensor according to the second modified example of Embodiment 1 of the present invention. This is a plan view showing the shape of the piezoelectric element of an ultrasonic sensor according to a third modification of Embodiment 1 of the present invention. This is a plan view showing the shape of the piezoelectric element of an ultrasonic sensor according to a fourth modification of Embodiment 1 of the present invention. This is a cross-sectional view of an ultrasonic sensor according to a fifth modification of Embodiment 1 of the present invention. This is a plan view showing the shape of the piezoelectric element of an ultrasonic sensor according to a sixth modification of Embodiment 1 of the present invention. This is an equivalent circuit diagram of an ultrasonic sensor according to Embodiment 2 of the present invention. This is a graph showing the impedance frequency characteristics in a parallel circuit and a series circuit of piezoelectric elements of an ultrasonic sensor according to Embodiment 2 of the present invention. This is a graph showing the overall sensitivity frequency characteristics of ultrasonic sensors according to Embodiments 1 and 2. This is a graph showing the impedance frequency characteristics in a parallel circuit and a series circuit of piezoelectric elements of an ultrasonic sensor according to Embodiment 3 of the present invention. This is a graph showing the overall sensitivity frequency characteristics of ultrasonic sensors according to Embodiments 1 and 3. This is a diagram for explaining the detection frequency range.This graph shows the change in the detection frequency range when the length of the piezoelectric element in the X-axis direction is changed in the ultrasonic sensor according to Embodiment 3. This graph shows the frequency characteristics of the impedance in a parallel circuit and a series circuit of piezoelectric elements when the inductance of the secondary winding Lp is increased while the total capacitance of the capacitance Cp and the equivalent parallel capacitance Cd is decreased in the ultrasonic sensor according to Embodiment 3 of the present invention.

[0008] The ultrasonic sensors according to each embodiment of the present invention will be described below with reference to the figures. In the following description of the embodiments, the same or corresponding parts in the figures will be denoted by the same reference numerals, and their descriptions will not be repeated.

[0009] (Embodiment 1) Figure 1 is a perspective view of the ultrasonic sensor according to Embodiment 1 of the present invention, viewed from the bottom plate side. Figure 2 is a perspective view of the ultrasonic sensor according to Embodiment 1 of the present invention, viewed from the side opposite to the bottom plate side. Figure 3 is a cross-sectional view of the ultrasonic sensor of Figure 2, viewed from the direction of the arrow III-III. Figure 4 is a plan view showing the dimensional relationship between the case and the piezoelectric element of the ultrasonic sensor according to Embodiment 1 of the present invention.

[0010] As shown in Figures 1 to 4, the ultrasonic sensor 100 according to Embodiment 1 of the present invention comprises a case 110 and a piezoelectric element 120. The ultrasonic sensor 100 further comprises a first terminal 131, a second terminal 132, a first wiring 141, a second wiring 142, a filler material 150, and a lid 160.

[0011] The case 110 is cylindrical and has a bottom plate 111 and a peripheral wall 112. The bottom plate 111 has a disc shape with a diameter of D1. The peripheral wall 112 extends upward from the periphery of the bottom plate 111. The case 110 has a recess 113 surrounded by the bottom plate 111 and the peripheral wall 112.

[0012] As shown in Figure 4, when viewed from a direction perpendicular to the base plate 111 (Z-axis direction), the recess 113 extends in the longitudinal direction (X-axis direction) parallel to the base plate 111 and in the short direction (Y-axis direction) perpendicular to the longitudinal direction (X-axis direction). The maximum dimension L1 of the recess 113 in the longitudinal direction (X-axis direction) is greater than the maximum dimension W1 of the recess 113 in the short direction (Y-axis direction). When viewed from a direction perpendicular to the base plate 111 (Z-axis direction), the recess 113 has a roughly rectangular shape with rounded corners.

[0013] As shown in Figures 1 to 4, the peripheral wall 112 has openings 114 that communicate with recesses 113 on both sides in the longitudinal direction (X-axis direction) above the bottom plate 111. In this embodiment, the openings 114 are spaced apart from the bottom plate 111, but the openings 114 may be adjacent to the bottom plate 111. The height dimension of the openings 114 in the direction perpendicular to the bottom plate 111 (Z-axis direction) is not particularly limited.

[0014] As shown in Figure 4, the maximum opening width W3 of the opening 114 in the shorter direction (Y-axis direction) is, for example, 50% to 100% of the maximum dimension W1 of the recess 113 in the shorter direction (Y-axis direction). The maximum opening width W3 of the opening 114 in the shorter direction (Y-axis direction) may be 80% to 100% of the maximum dimension W1 of the recess 113 in the shorter direction (Y-axis direction), or 90% to 100% of the maximum dimension W1 of the recess 113 in the shorter direction (Y-axis direction).

[0015] The case 110 is made of a conductive material. In this embodiment, the case 110 is made of, for example, an aluminum alloy. The case 110 is formed, for example, by forging or cutting.

[0016] As shown in Figures 3 and 4, the piezoelectric element 120 is mounted on the bottom plate 111 within the recess 113. When viewed from a direction perpendicular to the bottom plate 111 (Z-axis direction), the piezoelectric element 120 is positioned such that its center lies on the center of the bottom plate 111. When viewed from a direction perpendicular to the bottom plate 111 (Z-axis direction), the piezoelectric element 120 extends along the longitudinal direction (X-axis direction). The maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) is greater than the maximum dimension W2 of the piezoelectric element 120 in the short direction (Y-axis direction). In this embodiment, when viewed from a direction perpendicular to the bottom plate 111 (Z-axis direction), the piezoelectric element 120 is a rectangle with a longer side extending in the longitudinal direction (X-axis direction).

[0017] As shown in Figure 3, the piezoelectric element 120 includes a piezoelectric body 121 and a first electrode 122 and a second electrode 123 that sandwich the piezoelectric body 121 between them. The piezoelectric body 121 is made of piezoelectric ceramics. In this embodiment, the piezoelectric body 121 is made of PZT (lead zirconate titanate) ceramics. However, the piezoelectric body 121 is not limited to PZT ceramics and may be made of other piezoelectric materials.

[0018] A unimorph type piezoelectric vibrator is formed by attaching a piezoelectric element 120 to the base plate 111. Alternatively, a bimorph type piezoelectric vibrator or a multimorph type piezoelectric vibrator may be used instead of the unimorph type piezoelectric vibrator.

[0019] The first electrode 122 is connected to the base plate 111 by adhesive. The peripheral wall 112 is electrically connected to the first terminal 131 through the first wiring 141. As a result, the first terminal 131 and the first electrode 122 are electrically connected. The second electrode 123 is electrically connected to the second terminal 132 through the second wiring 142.

[0020] When a voltage is applied between the first electrode 122 and the second electrode 123, the piezoelectric element 120 is driven to vibrate. As the piezoelectric element 120 vibrates, the bottom plate 111 vibrates.

[0021] Furthermore, when the bottom plate 111 of the case 110 vibrates due to external ultrasonic waves, the piezoelectric element 120 also vibrates in conjunction with this vibration. The vibration of the piezoelectric element 120 generates an electric charge, which converts the ultrasonic waves into an electrical signal in the piezoelectric element 120. This electrical signal is output from a pair of electrodes consisting of a first electrode 122 and a second electrode 123.

[0022] As shown in Figure 3, the recess 113 and the opening 114 are filled with a filler material 150. The filler material 150 is, for example, silicone resin. The filler material 150 may be composed of two or more types of resin. The recess 113 is closed by a lid 160. The lid 160 is made of, for example, an insulator. The first terminal 131 and the second terminal 132 pass through the lid 160. The filler material 150 and the lid 160 prevent water and foreign matter from entering the case 110. The lid 160 is not necessarily required.

[0023] Figure 5 is an equivalent circuit diagram of an ultrasonic sensor according to Embodiment 1 of the present invention. As shown in Figure 5, the equivalent circuit 12 of the piezoelectric element 120 of the ultrasonic sensor 100 has an equivalent series inductance Ls, an equivalent series capacitance Cs, an equivalent series resistance Rs, an equivalent parallel capacitance Cd, and an equivalent parallel resistance Rd. The equivalent series inductance Ls, equivalent series capacitance Cs, and equivalent series resistance Rs are connected in series with each other to form a series circuit 13. The series circuit 13, the equivalent parallel capacitance Cd, and the equivalent parallel resistance Rd are connected in parallel between the first terminal T1 and the second terminal T2.

[0024] Here, we will explain the experimental results regarding the change in vibration velocity with respect to the position of the bottom plate 111 in the longitudinal direction (X-axis direction) when the piezoelectric element 120 is driven and the bottom plate 111 vibrates, for an ultrasonic sensor according to a comparative example in which no opening is formed in the peripheral wall and an ultrasonic sensor according to the embodiment. The ultrasonic sensor according to the comparative example differs from the ultrasonic sensor according to the embodiment only in that no opening is formed in the peripheral wall. In the ultrasonic sensor according to the embodiment, the maximum dimension W3 of the opening width of the opening 114 in the short direction (Y-axis direction) is set to 88% of the maximum dimension W1 of the recess 113 in the short direction (Y-axis direction). Note that if the maximum dimension W3 of the opening width of the opening 114 in the short direction (Y-axis direction) is 50% or more and 100% or less of the maximum dimension W1 of the recess 113 in the short direction (Y-axis direction), the effect of improving the efficiency of the ultrasonic sensor 100 described below can be stably obtained. However, the maximum dimension W3 of the opening width of the opening 114 in the shorter direction (Y-axis direction) is not limited to 50% or more and 100% or less of the maximum dimension W1 of the recess 113 in the shorter direction (Y-axis direction), but may be 20% or more and 100%, 30% or more and 100%, or 40% or more and 100%.

[0025] Figure 6 is a graph showing the change in vibration velocity with respect to the position of the bottom plate in the longitudinal direction (X-axis direction) when the piezoelectric element is driven and the bottom plate vibrates, for the ultrasonic sensor according to the comparative example and the ultrasonic sensor according to the embodiment. In Figure 6, the vertical axis shows the vibration velocity (m / s) of the bottom plate 111 in the Z-axis direction, and the horizontal axis shows the position of the bottom plate 111 in the longitudinal direction (X-axis direction). The data for the ultrasonic sensor according to the embodiment is shown by the solid line E, and the data for the ultrasonic sensor according to the comparative example is shown by the solid line F.

[0026] As shown in Figure 6, in both the ultrasonic sensor according to the embodiment and the ultrasonic sensor according to the comparative example, the vibration velocity of the base plate 111 in the Z-axis direction was highest at the center of the base plate 111 in the longitudinal direction (X-axis direction) and decreased as it approached both ends of the base plate 111 in the longitudinal direction (X-axis direction). In the ultrasonic sensor according to the comparative example, there was almost no vibration at both ends of the base plate 111 in the longitudinal direction (X-axis direction), but it was confirmed that the ultrasonic sensor 100 according to the embodiment vibrated relatively large up to both ends of the base plate 111 in the longitudinal direction (X-axis direction).

[0027] From these experimental results, it was confirmed that the opening 114 is formed, the piezoelectric element 120 extends along the longitudinal direction (X-axis direction), and the maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) is greater than the maximum dimension W2 of the piezoelectric element 120 in the short direction (Y-axis direction). In other words, the piezoelectric element 120 extends in a direction approaching the opening 114, allowing the vibration region to extend to both ends of the bottom plate 111 in the longitudinal direction (X-axis direction), thereby enabling the ultrasonic sensor 100 to operate with high efficiency.

[0028] By extending the vibration region of the base plate 111 in the longitudinal direction (X-axis direction), it is possible to narrow the directivity of the ultrasonic sensor 100 in the longitudinal direction (X-axis direction) while ensuring a wide directivity in the short direction (Y-axis direction). In other words, the ultrasonic sensor according to this embodiment is highly efficient while having the desired directivity.

[0029] Next, we will explain the experimental results regarding the changes in the force coefficient and electromechanical coupling coefficient when the length of the piezoelectric element in the longitudinal direction (X-axis direction) is changed in the ultrasonic sensor according to the comparative example and the ultrasonic sensor according to the example.

[0030] The force coefficient A (N / V) is the value obtained by dividing the force applied to the bottom plate 111 by the voltage applied to the ultrasonic sensor 100. If the mode attenuation is D (N・s / m), the mode stiffness is K (N / m), and the mode mass is M (kg), then the equivalent series resistance Rs = D / A 2 The relationship satisfies, and the equivalent series capacitance Cs = A 2The relationship / K is satisfied, and the equivalent series inductance Ls = M / A 2 It satisfies the relationship.

[0031] Figure 7 is a graph showing the change in the force coefficient when the length of the piezoelectric element in the longitudinal direction (X-axis direction) is changed for the ultrasonic sensor according to the comparative example and the ultrasonic sensor according to the embodiment. In Figure 7, the vertical axis shows the force coefficient A, and the horizontal axis shows the ratio (L2 / D1) (%) of the maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) to the diameter dimension D1 of the base plate 111. The force coefficient A is shown normalized to 1.0 when the above ratio (L2 / D1) is 32%. When the above ratio (L2 / D1) is 32%, the outer shape of the piezoelectric element 120 is square when viewed from a direction perpendicular to the base plate 111. The maximum dimension W2 of the piezoelectric element 120 in the short direction (Y-axis direction) is kept constant. The data for the ultrasonic sensor according to the embodiment is shown by the solid line E, and the data for the ultrasonic sensor according to the comparative example is shown by the solid line F.

[0032] As shown in Figure 7, in the ultrasonic sensor 100 according to the embodiment, the force coefficient A increased in proportion to the ratio of the maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) to the diameter dimension D1 of the bottom plate 111. In the ultrasonic sensor according to the comparative example, the rate of increase of the force coefficient A due to the increase in the length of the piezoelectric element 120 in the longitudinal direction (X-axis direction) was lower compared to the ultrasonic sensor according to the embodiment, and the rate of increase of the force coefficient A decreased as the ratio (L2 / D1) increased from 45%.

[0033] Figure 8 is a graph showing the change in the electromechanical coupling coefficient when the length of the piezoelectric element in the longitudinal direction (X-axis direction) is changed for the ultrasonic sensor according to the comparative example and the ultrasonic sensor according to the embodiment. In Figure 8, the vertical axis shows the electromechanical coupling coefficient kp, and the horizontal axis shows the ratio (L2 / D1) (%) of the maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) to the diameter dimension D1 of the base plate 111. The electromechanical coupling coefficient kp is shown normalized to 1.0 when the above ratio (L2 / D1) is 32%. The data for the ultrasonic sensor according to the embodiment is shown by the solid line E, and the data for the ultrasonic sensor according to the comparative example is shown by the solid line F.

[0034] As shown in Figure 8, the ultrasonic sensor according to the comparative example had a lower rate of increase in the electromechanical coupling coefficient kp with increasing length of the piezoelectric element 120 in the longitudinal direction (X-axis direction) compared to the ultrasonic sensor according to the example. The rate of increase in the electromechanical coupling coefficient kp decreased as the ratio (L2 / D1) increased from 39%, and when the ratio (L2 / D1) exceeded 45%, the electromechanical coupling coefficient kp began to decrease, and the rate of decrease in the electromechanical coupling coefficient kp increased as the ratio (L2 / D1) increased from 45%. In the ultrasonic sensor 100 according to the example, the electromechanical coupling coefficient kp increased with increasing length of the piezoelectric element 120 in the longitudinal direction (X-axis direction) until the ratio (L2 / D1) reached 71%, and when the ratio (L2 / D1) exceeded 71%, the electromechanical coupling coefficient kp decreased gradually with increasing length of the piezoelectric element 120 in the longitudinal direction (X-axis direction).

[0035] Figure 9 is a graph showing the relationship between the frequency characteristics of impedance in a series circuit of piezoelectric elements, the force coefficient, and the electromechanical coupling coefficient. In Figure 9, the vertical axis represents the impedance in the series circuit 13 of piezoelectric elements 120, and the horizontal axis represents frequency. The data when the above ratio (L2 / D1) is 32% is shown by a dotted line, and the data when the above ratio (L2 / D1) is 45% is shown by a solid line.

[0036] As shown in Figure 9, when the ratio (L2 / D1) is 45%, the impedance Zs in the series circuit 13 of the piezoelectric element 120 decreases compared to when the ratio (L2 / D1) is 32%. In other words, the impedance Zs in the series circuit 13 of the piezoelectric element 120 decreases as the force coefficient and electromechanical coupling coefficient increase. As the impedance Zs in the series circuit 13 of the piezoelectric element 120 decreases, the ultrasonic sensor 100 becomes more efficient.

[0037] In this embodiment, the maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) is 39% to 77% of the diameter D1 of the base plate 111. This allows the ultrasonic sensor 100 to be made more efficient. The maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) may be 45% to 71% of the diameter D1 of the base plate 111. This allows the ultrasonic sensor 100 to be made even more efficient. The maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) may be 60% to 70% of the diameter D1 of the base plate 111. This allows the ultrasonic sensor 100 to be made even more efficient.

[0038] The following describes the first to fourth modifications of Embodiment 1 of the present invention. In the following first to fourth modifications, only the shape of the piezoelectric element differs from that of the ultrasonic sensor 100 according to Embodiment 1; therefore, the same configuration as that of the ultrasonic sensor 100 according to Embodiment 1 will not be repeated in the description.

[0039] Figure 10 is a plan view showing the shape of the piezoelectric element of an ultrasonic sensor according to a first modification of Embodiment 1 of the present invention. As shown in Figure 10, in the ultrasonic sensor 100a according to the first modification of Embodiment 1 of the present invention, the second electrode 123a is an ellipse with a major axis extending in the longitudinal direction (X axis direction) when viewed from a direction perpendicular to the bottom plate 111 (Z axis direction). Specifically, the second electrode 123a is an ellipse located inside the virtual ellipse n where the vibration nodes of the bottom plate are located. When viewed from a direction perpendicular to the bottom plate 111 (Z axis direction), the piezoelectric body 121 is rectangular.

[0040] When the base plate vibrates, the area outside the virtual ellipse n is in opposite phase to the area inside the virtual ellipse n. Therefore, the smaller the vibration outside the virtual ellipse n, the higher the efficiency of the ultrasonic sensor. In the ultrasonic sensor 100a according to the first modified example, the second electrode 123a is an ellipse with a major axis extending in the longitudinal direction (X-axis direction), which reduces the vibration outside the virtual ellipse n and increases the efficiency of the ultrasonic sensor 100a. Furthermore, the second electrode 123a is an ellipse with a major axis extending in the longitudinal direction (X-axis direction), which allows for an increase in the electromechanical coupling coefficient kp while decreasing the equivalent parallel capacitance Cd, thus increasing the efficiency of the ultrasonic sensor 100a.

[0041] Furthermore, in order to obtain the above effect, at least one of the first electrode 122 and the second electrode 123a may be an ellipse having a major axis extending in the longitudinal direction (X-axis direction). That is, both the first electrode 122 and the second electrode 123a may be ellipses having a major axis extending in the longitudinal direction (X-axis direction), or the first electrode 122 may be an ellipse having a major axis extending in the longitudinal direction (X-axis direction) instead of the second electrode 123a.

[0042] Figure 11 is a plan view showing the shape of the piezoelectric element of an ultrasonic sensor according to a second modification of Embodiment 1 of the present invention. As shown in Figure 11, in the ultrasonic sensor 100b according to the second modification of Embodiment 1 of the present invention, the piezoelectric body 121b is also an ellipse with a major axis extending in the longitudinal direction (X-axis direction) when viewed from a direction perpendicular to the bottom plate 111 (Z-axis direction). That is, the entire piezoelectric element is an ellipse with a major axis extending in the longitudinal direction (X-axis direction). In the ultrasonic sensor 100b according to the second modification, the efficiency of the ultrasonic sensor 100b can be made higher because the entire piezoelectric element is located inside the virtual ellipse n where the vibration nodes of the bottom plate are located.

[0043] FIG. 12 is a plan view showing the shape of a piezoelectric element of an ultrasonic sensor according to a third modification of Embodiment 1 of the present invention. As shown in FIG. 12, in an ultrasonic sensor 100c according to the third modification of Embodiment 1 of the present invention, when viewed from the direction perpendicular to the bottom plate 111 (Z-axis direction), the second electrode 123c has a shape in which the four corners of a rectangle having a long side extending in the longitudinal direction (X-axis direction) are chamfered. Specifically, the second electrode 123c is an octagon that is substantially entirely located inside a virtual ellipse n where a node of the vibration of the bottom plate is located. When viewed from the direction perpendicular to the bottom plate 111 (Z-axis direction), the piezoelectric body 121 is rectangular.

[0044] Also in the ultrasonic sensor 100c according to the third modification, since the second electrode 123c has a shape in which the four corners of a rectangle having a long side extending in the longitudinal direction (X-axis direction) are chamfered, the vibration outside the virtual ellipse n can be reduced and the efficiency of the ultrasonic sensor 100c can be increased. Further, since the second electrode 123c has a shape in which the four corners of a rectangle having a long side extending in the longitudinal direction (X-axis direction) are chamfered, the equivalent parallel capacitance Cd can be decreased while increasing the electromechanical coupling coefficient kp, so that the efficiency of the ultrasonic sensor 100c can be increased.

[0045] In order to obtain the above effects, at least one of the first electrode 122 and the second electrode 123c may have a shape in which at least one of the four corners of a rectangle having a long side extending in the longitudinal direction (X-axis direction) is chamfered. That is, it is not limited to the case where all four corners of the rectangle of the second electrode 123c are chamfered, and any one of the four corners may be chamfered. Also, both the first electrode 122 and the second electrode 123c may have a shape in which at least one of the four corners of a rectangle having a long side extending in the longitudinal direction (X-axis direction) is chamfered, or instead of the second electrode 123c, the first electrode 122 may have a shape in which at least one of the four corners of a rectangle having a long side extending in the longitudinal direction (X-axis direction) is chamfered.

[0046] FIG. 13 is a plan view showing the shape of a piezoelectric element of an ultrasonic sensor according to a fourth modification of Embodiment 1 of the present invention. As shown in FIG. 13, in an ultrasonic sensor 100d according to the fourth modification of Embodiment 1 of the present invention, when viewed from a direction perpendicular to the bottom plate 111 (Z-axis direction), the piezoelectric body 121d also has at least one of the corners of a rectangle having a long side extending in the longitudinal direction (X-axis direction) chamfered. That is, the entire piezoelectric element has a shape in which at least one of the corners of a rectangle having a long side extending in the longitudinal direction (X-axis direction) is chamfered. In the ultrasonic sensor 100d according to the fourth modification, since at least one corner of the piezoelectric element is chamfered, the portion of the piezoelectric element located outside the virtual ellipse n where the node of the vibration of the bottom plate is located can be reduced, so that the efficiency of the ultrasonic sensor 100d can be increased. When all four corners of the piezoelectric element are chamfered, the efficiency of the ultrasonic sensor 100d can be further increased by positioning substantially the entire piezoelectric element inside the virtual ellipse n where the node of the vibration of the bottom plate is located.

[0047] Hereinafter, a fifth modification of Embodiment 1 of the present invention will be described. In the fifth modification, since only the arrangement of the electrodes and wirings of the piezoelectric element is different from the ultrasonic sensor 100 according to Embodiment 1, the description of the configuration similar to the ultrasonic sensor 100 according to Embodiment 1 will not be repeated.

[0048] FIG. 14 is a cross-sectional view of an ultrasonic sensor according to a fifth modification of Embodiment 1 of the present invention. In FIG. 14, it is shown in the same cross-sectional view as FIG. 3. As shown in FIG. 14, in an ultrasonic sensor 100e according to the fifth modification of Embodiment 1 of the present invention, the first electrode 122 is folded back so as to cover the end face of the piezoelectric body 121 and is arranged to be spaced apart from the second electrode 123 on the upper surface of the piezoelectric body 121, whereby the first terminal 131 and the first electrode 122 are directly connected through the first wiring 141. That is, the first terminal 131 and the first electrode 122 are electrically connected to each other without passing through the case 110. As shown in FIG. 14, the piezoelectric body 121 has a first surface P1 located on the bottom plate 111 side and a second surface P2 located on the side opposite to the first surface P1.

[0049] The following describes a sixth modification of Embodiment 1 of the present invention. The sixth modification limits the arrangement of electrodes of the piezoelectric element in the ultrasonic sensor 100e according to the fifth modification of Embodiment 1, and the description of configurations that are the same as those of the ultrasonic sensor 100e according to the fifth modification of Embodiment 1 will not be repeated.

[0050] Figure 15 is a plan view showing the shape of the piezoelectric element of an ultrasonic sensor according to a sixth modification of Embodiment 1 of the present invention. As shown in Figure 15, in the ultrasonic sensor 100f according to the sixth modification of Embodiment 1 of the present invention, the second electrode 123f is provided on the second surface P2 and has a substantially rectangular shape with a notch N formed at the center of the longitudinal direction (X-axis direction) and at one end of the short direction (Y-axis direction) when viewed from a direction perpendicular to the bottom plate 111 (Z-axis direction). Specifically, the second electrode 123f has a shape in which a rectangular notch N is formed at the center of the longitudinal direction (X-axis direction) and at one end of the short direction (Y-axis direction) of a rectangle having a longer side extending in the longitudinal direction (X-axis direction) when viewed from a direction perpendicular to the bottom plate 111 (Z-axis direction).

[0051] The first electrode 122f is provided on the first surface P1 and is folded back to cover the end face of the piezoelectric element 121, extending into the notch N on the second surface P2 at a distance from the second electrode 123f. Specifically, the first electrode 122f includes a portion formed on the first surface P1 at a position opposite the rectangle of the second electrode 123f, and a portion that is folded back from the central part of the longitudinal direction (X-axis direction) of the said portion to cover the end face of the piezoelectric element 121, extending into the notch N on the second surface P2 at a distance from the second electrode 123f.

[0052] In the ultrasonic sensor 100f according to the sixth modification of Embodiment 1 of the present invention, as shown in Figure 14, the first wiring 141 and the second wiring 142 can be connected to the first electrode 122f and the second electrode 123f, which are located on the same plane (second plane P2), respectively. This facilitates wiring connections, and by positioning the portion of the first electrode 122f located on the second plane P2 in the center of the longitudinal direction (X-axis direction), the symmetry of the piezoelectric element's vibration can be maintained.

[0053] (Embodiment 2) Hereinafter, an ultrasonic sensor according to Embodiment 2 of the present invention will be described with reference to the figures. The ultrasonic sensor according to Embodiment 2 of the present invention differs from the ultrasonic sensor according to Embodiment 1 of the present invention in that it includes a circuit connected in parallel to the piezoelectric element, so the same configuration as the ultrasonic sensor according to Embodiment 1 of the present invention will not be described again.

[0054] Figure 16 is an equivalent circuit diagram of an ultrasonic sensor according to Embodiment 2 of the present invention. As shown in Figure 16, the ultrasonic sensor according to Embodiment 2 of the present invention includes a piezoelectric element 120, a secondary winding Lp of a transformer 20 for supplying a driving voltage to the piezoelectric element 120, a resistor Rp, and a capacitance Cp, all connected in parallel. The secondary winding Lp, resistor Rp, capacitance Cp, equivalent parallel capacitance Cd, and equivalent parallel resistor Rd are connected in parallel between a first terminal T1 and a second terminal T2, forming a parallel circuit 40. The electrical resonant frequency of the parallel circuit 40 and the electrical resonant frequency of the series circuit 13 of the piezoelectric element 120 are set to be the same as each other.

[0055] The transformer 20 consists of a primary winding Lo and a secondary winding Lp. The primary winding Lo is connected in series between the third terminal T3 and the fourth terminal T4. The voltage applied between the third terminal T3 and the fourth terminal T4 is boosted to a voltage obtained by multiplying the turns ratio of the primary winding Lo and the secondary winding Lp, and this voltage is applied from the secondary winding Lp between the first terminal T1 and the second terminal T2, and supplied to the piezoelectric element 120. The transformer 20, resistor Rp, and capacitance Cp are each composed of elements arranged on the circuit board 30 connected to the first terminal T1 (131) and the second terminal T2 (132).

[0056] In the ultrasonic sensor according to Embodiment 2, the resistance value of resistor Rp is less than or equal to the resistance value of the equivalent series resistance Rs. Figure 17 is a graph showing the frequency characteristics of impedance in a parallel circuit of the ultrasonic sensor and a series circuit of piezoelectric elements according to Embodiment 2 of the present invention. In Figure 17, impedance is shown on the vertical axis and frequency on the horizontal axis.

[0057] As shown in Figure 17, in the ultrasonic sensor according to Embodiment 2, where the resistance value of resistor Rp is less than or equal to the resistance value of the equivalent series resistance Rs, the curve showing the frequency characteristics of the impedance Zs in the series circuit 13 of the piezoelectric element 120 and the curve showing the frequency characteristics of the impedance Zp in the parallel circuit 40 do not intersect. Therefore, the parallel circuit 40 functions to reduce the overall impedance of the ultrasonic sensor circuit.

[0058] Figure 18 is a graph showing the frequency characteristics of the overall sensitivity of the ultrasonic sensors according to Embodiment 1 and Embodiment 2. In Figure 18, the vertical axis represents the overall sensitivity of the ultrasonic sensor, and the horizontal axis represents frequency. The overall sensitivity of the ultrasonic sensor is the output value from the first terminal T1 and the second terminal T2. The data for the ultrasonic sensor according to Embodiment 1 is shown by a dotted line, and the data for the ultrasonic sensor according to Embodiment 2 is shown by a solid line.

[0059] As shown in Figure 18, the ultrasonic sensor according to Embodiment 2 has a lower overall impedance due to the parallel circuit 40, resulting in higher overall sensitivity compared to the ultrasonic sensor 100 according to Embodiment 1. Thus, in the ultrasonic sensor according to Embodiment 2, high sensitivity can be achieved by making the resistance value of resistor Rp less than or equal to the resistance value of the equivalent series resistance Rs.

[0060] (Embodiment 3) Hereinafter, an ultrasonic sensor according to Embodiment 3 of the present invention will be described with reference to the figures. The ultrasonic sensor according to Embodiment 3 of the present invention differs from the ultrasonic sensor according to Embodiment 2 of the present invention in that the resistance value of the resistor Rp is greater than the resistance value of the equivalent series resistor Rs. Therefore, the same configuration as that of the ultrasonic sensor according to Embodiment 2 of the present invention will not be repeated in the description.

[0061] Figure 19 is a graph showing the frequency characteristics of impedance in a parallel circuit of an ultrasonic sensor and a series circuit of a piezoelectric element according to Embodiment 3 of the present invention. In Figure 19, the vertical axis represents impedance and the horizontal axis represents frequency.

[0062] As shown in Figure 19, in the ultrasonic sensor according to Embodiment 3, where the resistance value of resistor Rp is greater than the resistance value of the equivalent series resistance Rs, the curve showing the frequency characteristics of the impedance Zs in the series circuit 13 of the piezoelectric element 120 intersects with the curve showing the frequency characteristics of the impedance Zp in the parallel circuit 40. Therefore, the parallel circuit 40 functions to broaden the frequency range of the impedance peak in the entire ultrasonic sensor circuit.

[0063] Figure 20 is a graph showing the frequency characteristics of the overall sensitivity of the ultrasonic sensors according to Embodiment 1 and Embodiment 3. In Figure 20, the vertical axis represents the overall sensitivity of the ultrasonic sensor, and the horizontal axis represents frequency. The data for the ultrasonic sensor according to Embodiment 1 is shown by a dotted line, and the data for the ultrasonic sensor according to Embodiment 3 is shown by a solid line.

[0064] As shown in Figure 20, the ultrasonic sensor according to Embodiment 3 has a wider frequency range of the peak impedance of the entire ultrasonic sensor circuit due to the parallel circuit 40. Therefore, compared to the ultrasonic sensor 100 according to Embodiment 1, the frequency band with high overall sensitivity is expanded. Thus, in the ultrasonic sensor according to Embodiment 3, a wider bandwidth can be achieved because the resistance value of resistor Rp is greater than the resistance value of the equivalent series resistance Rs.

[0065] Here, we define the detection frequency bandwidth as the frequency band having a sensitivity of 6 dB or more lower than the maximum overall sensitivity of the ultrasonic sensor, and we will explain the experimental results of how the detection frequency bandwidth changes when the length of the piezoelectric element in the longitudinal direction (X-axis direction) is changed.

[0066] Figure 21 is a diagram illustrating the detection frequency bandwidth. As shown in Figure 21, the detection frequency bandwidth Wd is a frequency band that has a sensitivity of 6 dB lower (Max-6) or more than the maximum overall sensitivity (Max) of the ultrasonic sensor.

[0067] Figure 22 is a graph showing the change in the detection frequency width when the length of the piezoelectric element in the longitudinal direction (X-axis direction) is changed in the ultrasonic sensor according to Embodiment 3. In Figure 22, the vertical axis shows the detection frequency width, and the horizontal axis shows the ratio (L2 / D1) (%) of the maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) to the diameter dimension D1 of the base plate 111. The detection frequency width Wd is shown normalized to 1.00 when the above ratio (L2 / D1) is 32%. The data for the ultrasonic sensor according to Embodiment 3 is shown by the solid line E, and the data for the ultrasonic sensor according to the comparative example is shown by the solid line F.

[0068] As shown in Figure 22, the ultrasonic sensor according to the comparative example had a lower rate of increase in the detection frequency width Wd due to an increase in the length of the piezoelectric element 120 in the longitudinal direction (X-axis direction) compared to the ultrasonic sensor according to Embodiment 3. The rate of increase in the detection frequency width Wd decreased as the ratio (L2 / D1) increased from 39%, and when the ratio (L2 / D1) exceeded 45%, the detection frequency width Wd began to decrease, and the rate of decrease in the detection frequency width Wd increased as the ratio (L2 / D1) increased from 45%. In the ultrasonic sensor according to Embodiment 3, the detection frequency width Wd increased with increasing length of the piezoelectric element 120 in the longitudinal direction (X-axis direction) until the ratio (L2 / D1) reached 71%, and when the ratio (L2 / D1) exceeded 71%, the detection frequency width Wd gradually decreased with increasing length of the piezoelectric element 120 in the longitudinal direction (X-axis direction).

[0069] In this embodiment, the maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) is 39% to 77% of the diameter D1 of the base plate 111. This allows for effective broadbanding of the ultrasonic sensor. The maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) may be 45% to 71% of the diameter D1 of the base plate 111. This allows for even more effective broadbanding of the ultrasonic sensor. The maximum dimension L2 of the piezoelectric element 120 in the longitudinal direction (X-axis direction) may be 60% to 70% of the diameter D1 of the base plate 111. This allows for even more effective broadbanding of the ultrasonic sensor.

[0070] The above method of broadening bandwidth uses the real part of the impedance, but it is also possible to broaden bandwidth using the imaginary part of the impedance. Specifically, by increasing the inductance of the secondary winding Lp while decreasing the total capacitance of the capacitance Cp and the equivalent parallel capacitance Cd, the ultrasonic sensor can be broadened. In this case, if the inductance of the secondary winding after increasing it is Lp2, it is set so that the relationship Lp2 = (Cp + Cd)Lp / Cd is satisfied. In order to decrease the total capacitance (Cp + Cd) of the capacitance Cp and the equivalent parallel capacitance Cd, the capacitance Cp is set to 300 pF or less.

[0071] Figure 23 is a graph showing the frequency characteristics of impedance in a parallel circuit and a series circuit of piezoelectric elements in an ultrasonic sensor according to Embodiment 3 of the present invention, when the inductance of the secondary winding Lp is increased while the total capacitance of the capacitance Cp and the equivalent parallel capacitance Cd is decreased. In Figure 23, the vertical axis represents impedance and the horizontal axis represents frequency. The solid line shows the impedance Zp when the inductance of the secondary winding Lp is increased while the total capacitance of the capacitance Cp and the equivalent parallel capacitance Cd is decreased, and the dotted line shows the impedance Zp before the total capacitance of the capacitance Cp and the equivalent parallel capacitance Cd and the inductance of the secondary winding Lp are changed.

[0072] As shown in Figure 23, by increasing the inductance of the secondary winding Lp, the slope of the portion of the curve showing the frequency characteristics of the impedance Zp of the parallel circuit 40 at lower frequencies below the peak can be made gentler. By decreasing the total capacitance (Cp + Cd) of capacitance Cp and equivalent parallel capacitance Cd, the slope of the portion of the curve showing the frequency characteristics of the impedance Zp of the parallel circuit 40 at higher frequencies above the peak can be made gentler. As a result, the frequency range Wc at the intersection of the curve showing the frequency characteristics of the impedance Zs in the series circuit 13 of the piezoelectric element 120 and the curve showing the frequency characteristics of the impedance Zp of the parallel circuit 40 is widened. This makes it possible to widen the bandwidth of the ultrasonic sensor.

[0073] As described above, from the perspective of broadening the bandwidth by using the imaginary part of the impedance, the capacitance Cp may be 50pF or more and 300pF or less. The capacitance Cp may be 100pF or more and 250pF or less. The capacitance Cp may be 150pF or more and 200pF or less. However, the capacitance Cp is not limited to 300pF or less.

[0074] (Note) The exemplary embodiments described above will be understood by those skilled in the art to be specific examples of the following embodiments.

[0075] <1> An ultrasonic sensor comprising a cylindrical case having a bottom plate and a peripheral wall, and a piezoelectric element mounted on the bottom plate within a recess surrounded by the bottom plate and the peripheral wall, wherein, when viewed from a direction perpendicular to the bottom plate, the recess extends in a longitudinal direction parallel to the bottom plate and in a short direction perpendicular to the longitudinal direction, and the maximum dimension of the recess in the longitudinal direction is greater than the maximum dimension of the recess in the short direction, and openings communicating with the recess are formed in the peripheral wall on both sides in the longitudinal direction at positions above the bottom plate, when viewed from a direction perpendicular to the bottom plate, the piezoelectric element extends along the longitudinal direction, and the maximum dimension of the piezoelectric element in the longitudinal direction is greater than the maximum dimension of the piezoelectric element in the short direction.

[0076] <2> The ultrasonic sensor according to <1>, wherein the maximum dimension of the opening width of the opening in the short direction is 50% or more and 100% or less of the maximum dimension of the recess in the short direction.

[0077] <3> The ultrasonic sensor according to <1> or <2>, wherein the maximum dimension of the piezoelectric element in the longitudinal direction is 39% or more and 77% or less of the diameter of the base plate.

[0078] <4> The ultrasonic sensor according to <3>, wherein the maximum dimension of the piezoelectric element in the longitudinal direction is 45% or more and 71% or less of the diameter of the base plate.

[0079] <5> The ultrasonic sensor according to <4>, wherein the maximum dimension of the piezoelectric element in the longitudinal direction is 60% or more and 70% or less of the diameter of the base plate.

[0080] <6> The ultrasonic sensor according to any one of <1> to <5>, wherein the piezoelectric element includes a piezoelectric body and a pair of electrodes sandwiching the piezoelectric body between them, and when viewed from a direction perpendicular to the bottom plate, at least one of the pair of electrodes is elliptical with a major axis extending in the longitudinal direction.

[0081] <7> The ultrasonic sensor according to any one of <1> to <5>, wherein the piezoelectric element includes a piezoelectric body and a pair of electrodes sandwiching the piezoelectric body between them, and when viewed from a direction perpendicular to the bottom plate, at least one of the pair of electrodes has a shape in which at least one corner of a rectangle having a long side extending in the longitudinal direction is beveled.

[0082] <8> The ultrasonic sensor according to any one of <1> to <5>, wherein the piezoelectric element includes a piezoelectric body and a first electrode and a second electrode sandwiching the piezoelectric body between them, the piezoelectric body has a first surface located on the bottom plate side and a second surface located on the opposite side of the first surface, the second electrode is provided on the second surface and has a substantially rectangular shape with a notch formed at the center of the longitudinal direction and at one end of the short direction when viewed from a direction perpendicular to the bottom plate, and the first electrode is provided on the first surface and is folded back so as to cover the end face of the piezoelectric body and extends into the notch on the second surface at a distance from the second electrode.

[0083] <9> An ultrasonic sensor according to any one of <1> to <8>, comprising a circuit in which the piezoelectric element, the secondary winding of a transformer for supplying a driving voltage to the piezoelectric element, a resistor, and a capacitance are connected in parallel to each other, wherein the piezoelectric element has an equivalent series inductance, an equivalent series capacitance, an equivalent series resistance, an equivalent parallel capacitance, and an equivalent parallel resistance, and the resistance value of the resistor is less than or equal to the resistance value of the equivalent series resistance.

[0084] <10> An ultrasonic sensor according to any one of <1> to <8>, comprising a circuit in which the piezoelectric element, the secondary winding of a transformer for supplying a driving voltage to the piezoelectric element, a resistor, and a capacitance are connected in parallel to each other, wherein the piezoelectric element has an equivalent series inductance, an equivalent series capacitance, an equivalent series resistance, an equivalent parallel capacitance, and an equivalent parallel resistance, and the resistance value of the resistor is greater than the resistance value of the equivalent series resistance.

[0085] <11> The ultrasonic sensor according to <10>, wherein the capacitance is 300 pF or less.

[0086] In the description of the embodiments described above, the combinable configurations may be combined with each other.

[0087] The embodiments and examples disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the claims rather than the foregoing description, and all modifications within the meaning and scope of the claims are intended to be included.

[0088] 12 Equivalent circuit of piezoelectric element, 13 Series circuit of piezoelectric element, 20 Transformer, 30 Circuit board, 40 Parallel circuit, 100, 100a, 100b, 100c, 100d, 100e, 100f Ultrasonic sensor, 110 Case, 111 Bottom plate, 112 Peripheral wall, 113 Recess, 114 Opening, 120 Piezoelectric element, 121, 121b, 121d Piezoelectric body, 122, 122f First electrode, 123, 123a, 123c, 123f Second electrode, 131, T1 First terminal, 132, T2 Second terminal, 141 First wiring, 142 Second wiring, 150 Filling material, 160 Lid, Cd Equivalent parallel capacitance, Cp Capacitance, Cs Equivalent series capacitance, Lo Primary winding, Lp Secondary winding, Ls Equivalent series inductance, N Notch, P1 First surface, P2 Second surface, Rd Equivalent parallel resistance, Rp Resistance, Rs Equivalent series resistance, T3 Third terminal, T4 Fourth terminal, Wd Detection frequency width, Zp, Zs Impedance, kp Electromechanical coupling coefficient, n Virtual ellipse.

Claims

1. An ultrasonic sensor comprising: a cylindrical case having a bottom plate and a peripheral wall; and a piezoelectric element mounted on the bottom plate within a recess surrounded by the bottom plate and the peripheral wall, wherein, viewed from a direction perpendicular to the bottom plate, the recess extends in a longitudinal direction parallel to the bottom plate and in a short direction perpendicular to the longitudinal direction, and the maximum dimension of the recess in the longitudinal direction is greater than the maximum dimension of the recess in the short direction, and openings communicating with the recess are formed in the peripheral wall on both sides in the longitudinal direction at positions above the bottom plate, and, viewed from a direction perpendicular to the bottom plate, the piezoelectric element extends along the longitudinal direction, and the maximum dimension of the piezoelectric element in the longitudinal direction is greater than the maximum dimension of the piezoelectric element in the short direction.

2. The ultrasonic sensor according to claim 1, wherein the maximum dimension of the opening width of the opening in the short direction is 50% or more and 100% or less of the maximum dimension of the recess in the short direction.

3. The ultrasonic sensor according to claim 1 or 2, wherein the maximum dimension of the piezoelectric element in the longitudinal direction is 39% or more and 77% or less of the diameter of the base plate.

4. The ultrasonic sensor according to claim 3, wherein the maximum dimension of the piezoelectric element in the longitudinal direction is 45% or more and 71% or less of the diameter of the base plate.

5. The ultrasonic sensor according to claim 4, wherein the maximum dimension of the piezoelectric element in the longitudinal direction is 60% or more and 70% or less of the diameter of the base plate.

6. The ultrasonic sensor according to any one of claims 1 to 5, wherein the piezoelectric element includes a piezoelectric body and a pair of electrodes sandwiching the piezoelectric body between them, and when viewed from a direction perpendicular to the bottom plate, at least one of the pair of electrodes is elliptical with a major axis extending in the longitudinal direction.

7. The ultrasonic sensor according to any one of claims 1 to 5, wherein the piezoelectric element includes a piezoelectric body and a pair of electrodes sandwiching the piezoelectric body between them, and when viewed from a direction perpendicular to the bottom plate, at least one of the pair of electrodes has a shape in which at least one corner of a rectangle having a long side extending in the longitudinal direction is beveled.

8. The ultrasonic sensor according to any one of claims 1 to 5, wherein the piezoelectric element includes a piezoelectric body and a first electrode and a second electrode sandwiching the piezoelectric body between them, the piezoelectric body has a first surface located on the bottom plate side and a second surface located on the opposite side of the first surface, the second electrode is provided on the second surface and has a substantially rectangular shape with a notch formed at the center of the longitudinal direction and at one end of the short direction when viewed from a direction perpendicular to the bottom plate, and the first electrode is provided on the first surface and is folded back so as to cover the end face of the piezoelectric body and extends into the notch on the second surface at a distance from the second electrode.

9. An ultrasonic sensor according to any one of claims 1 to 8, comprising a circuit in which the piezoelectric element, the secondary winding of a transformer for supplying a driving voltage to the piezoelectric element, a resistor, and a capacitance are connected in parallel to each other, wherein the piezoelectric element has an equivalent series inductance, an equivalent series capacitance, an equivalent series resistance, an equivalent parallel capacitance, and an equivalent parallel resistance, and the resistance value of the resistor is less than or equal to the resistance value of the equivalent series resistance.

10. An ultrasonic sensor according to any one of claims 1 to 8, comprising a circuit in which the piezoelectric element, the secondary winding of a transformer for supplying a driving voltage to the piezoelectric element, a resistor, and a capacitance are connected in parallel to each other, wherein the piezoelectric element has an equivalent series inductance, an equivalent series capacitance, an equivalent series resistance, an equivalent parallel capacitance, and an equivalent parallel resistance, and the resistance value of the resistor is greater than the resistance value of the equivalent series resistance.

11. The ultrasonic sensor according to claim 10, wherein the capacitance is 300 pF or less.

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