WO2015053253A1

WO2015053253A1 - Ultrasonic sensor and sensor device

Info

Publication number: WO2015053253A1
Application number: PCT/JP2014/076770
Authority: WO
Inventors: 秀嗣三神; 恒介渡辺; 純多保田
Original assignee: 株式会社村田製作所
Priority date: 2013-10-11
Filing date: 2014-10-07
Publication date: 2015-04-16

Abstract

An ultrasonic sensor is provided with a case (10) and a piezoelectric element (20) that is provided on the inner surface (12) of the bottom section (11) of the case and that is subjected to bending vibration together with the bottom section (11). The piezoelectric element (20) comprises a first piezoelectric layer (21) and a second piezoelectric layer (22) that are stacked in the direction leading away from the bottom section (11) with an electrode therebetween and that are electrically connected in parallel. The thickness of the second piezoelectric layer (22) is smaller than the thickness of the first piezoelectric layer (21). The present invention makes it possible to increase the acoustic pressure when transmitting an ultrasonic wave.

Description

Ultrasonic sensor and sensor device

The present invention relates to an ultrasonic sensor having a piezoelectric element, and a sensor device including such an ultrasonic sensor.

A general ultrasonic sensor forms a unimorph structure by bonding a piezoelectric element to the inner bottom surface of a metal case, and transmits and receives ultrasonic waves by bending vibration of the bottom of the metal case. The following Patent Documents 1 and 2 disclose an ultrasonic sensor including a stacked piezoelectric element. A stacked piezoelectric element is configured by stacking a plurality of piezoelectric layers and connecting the plurality of piezoelectric layers in parallel. The technology using the laminated piezoelectric element is disclosed in Patent Document 3 below, although the technical field is different from that of the ultrasonic sensor.

JP 2002-204497 A Japanese Patent Laid-Open No. 01-245799 JP 2003-337140 A

The present invention relates to an ultrasonic sensor capable of increasing the sound pressure when ultrasonic waves are transmitted using a laminated piezoelectric element as compared with the prior art, and a sensor device including such an ultrasonic sensor. The purpose is to provide.

An ultrasonic sensor according to a first aspect of the present invention includes a bottomed cylindrical case and a piezoelectric element that is provided on an inner surface of a bottom portion of the case and that bends and vibrates together with the bottom portion of the case. Includes a plurality of piezoelectric layers that are sequentially stacked in the direction away from the bottom with electrodes interposed therebetween and electrically connected in parallel, and the plurality of piezoelectric layers are provided farthest from the case side. The thickness of the piezoelectric layer is the smallest.

An ultrasonic sensor according to a second aspect of the present invention includes a bottomed cylindrical case and a piezoelectric element that is provided on an inner surface of a bottom portion of the case and that bends and vibrates together with the bottom portion of the case. Includes a first piezoelectric layer and a second piezoelectric layer, which are sequentially stacked in a direction away from the bottom portion with electrodes interposed therebetween and electrically connected in parallel, and the first piezoelectric layer has a thickness greater than the thickness of the first piezoelectric layer. 2 The thickness of the piezoelectric layer is smaller. Preferably, the ratio of the thickness of the first piezoelectric layer to the total thickness of the first piezoelectric layer and the second piezoelectric layer is from 58% to 80%.

An ultrasonic sensor according to a third aspect of the present invention includes: a bottomed cylindrical case; and a piezoelectric element that is provided on an inner surface of a bottom portion of the case and that bends and vibrates together with the bottom portion of the case. Includes a first piezoelectric layer, a second piezoelectric layer, and a third piezoelectric layer, which are sequentially stacked in a direction away from the bottom portion with an electrode interposed therebetween and electrically connected in parallel. The thicknesses of the second piezoelectric layer and the third piezoelectric layer become smaller in this order. Preferably, the ratio of the thickness of the first piezoelectric layer to the total thickness of the first piezoelectric layer, the second piezoelectric layer, and the third piezoelectric layer is 40% or more and 72% or less, The ratio of the total thickness of the first piezoelectric layer and the second piezoelectric layer is 76% or more and 90% or less.

An ultrasonic sensor according to a fourth aspect of the present invention includes: a bottomed cylindrical case; and a piezoelectric element that is provided on an inner surface of the bottom of the case and that bends and vibrates with the bottom of the case. Includes a first piezoelectric layer, a second piezoelectric layer, a third piezoelectric layer, and a fourth piezoelectric layer, which are sequentially stacked in a direction away from the bottom portion with electrodes interposed therebetween and electrically connected in parallel, The thicknesses of the first piezoelectric layer, the second piezoelectric layer, the third piezoelectric layer, and the fourth piezoelectric layer are reduced in this order. Preferably, the ratio of the thickness of the first piezoelectric layer to the total thickness of the first piezoelectric layer, the second piezoelectric layer, the third piezoelectric layer, and the fourth piezoelectric layer is 40% or more. 60% or less, and the ratio of the total thickness of the first piezoelectric layer and the second piezoelectric layer is 64% or more and 83% or less, and the first piezoelectric layer, the second piezoelectric layer, and the second piezoelectric layer. The ratio of the total thickness of the three piezoelectric layers is 84% or more and 95% or less.

An ultrasonic sensor according to a fifth aspect of the present invention includes a bottomed cylindrical case, and a piezoelectric element that is provided on an inner surface of the bottom of the case and bends and vibrates together with the bottom of the case. Includes a piezoelectric layer composed of n layers (n is an integer of 2 or more) stacked in a direction away from the bottom portion with an electrode interposed therebetween, and wherein the piezoelectric layer is composed of the n layers. When the total thickness of T is T, the thickness T1 of the piezoelectric layer positioned first from the bottom side is a value satisfying the following formula (1):

The thickness Tk of the piezoelectric layer located kth (k is an integer of 2 or more) from the bottom side is a value that satisfies the following formula (2).

Preferably, the electrode provided in the piezoelectric element is formed so as to be electrically connected to the external electrode, and is formed so as not to be electrically connected to the internal electrode connecting the piezoelectric layers in parallel. And a floating electrode. Preferably, the floating electrode is provided at a position symmetrical to the internal electrode with respect to a plane passing through the center in the stacking direction of the piezoelectric elements.

A sensor device according to the present invention includes the ultrasonic sensor according to the present invention and a signal generation circuit that outputs a signal for driving the ultrasonic sensor, and the ultrasonic sensor includes the signal generation circuit. The signal output from the circuit is input without being amplified.

According to the present invention, there is provided an ultrasonic sensor capable of increasing the sound pressure when transmitting ultrasonic waves using a multilayer piezoelectric element as compared with the conventional one, and such an ultrasonic sensor. A sensor device can be provided.

2 is a diagram illustrating an outline of a circuit configuration of a sensor device according to Embodiment 1. FIG. 3 is a diagram showing functional blocks of the sensor device in Embodiment 1. FIG. 1 is a cross-sectional view illustrating an ultrasonic sensor according to Embodiment 1. FIG. (A) is a top view which shows a part of ultrasonic sensor in Embodiment 1, (B) is sectional drawing which shows a part of ultrasonic sensor in Embodiment 1. FIG. FIG. 3 is a cross-sectional view showing a state when a part of the ultrasonic sensor according to Embodiment 1 is bending-vibrated. 3 is an enlarged cross-sectional view showing a metal case and a piezoelectric element of the ultrasonic sensor according to Embodiment 1. FIG. 3 is a diagram illustrating a circuit configuration of a first piezoelectric layer and a second piezoelectric layer that constitute a piezoelectric element of the ultrasonic sensor according to Embodiment 1. FIG. It is a figure for demonstrating the relationship between the thickness of a general piezoelectric element (piezoelectric body layer), and electric field strength. It is a figure for demonstrating the relationship between the piezoelectric material layer which has half thickness of the piezoelectric element shown in FIG. 8, and electric field strength. It is a figure for demonstrating the electric field strength at the time of laminating | stacking two of the piezoelectric elements shown in FIG. In the configuration of the first embodiment, the thickness ratio of the first layer is changed, and the voltage sensitivity and the charge sensitivity when receiving a reflected wave having a certain strength are numerically calculated by the finite element method. It is based on the voltage sensitivity and electric charge sensitivity which are shown in FIG. 11, and is a figure which shows the relationship between the thickness ratio of a 1st layer, and generated electric energy. FIG. 6 is an enlarged cross-sectional view showing a piezoelectric element in a second embodiment. 6 is a diagram illustrating a circuit configuration of each piezoelectric layer constituting a piezoelectric element of an ultrasonic sensor according to Embodiment 2. FIG. In the configuration of the second embodiment, the voltage sensitivity, the charge sensitivity, and the generated electric energy are numerically calculated by changing the thickness ratio of each piezoelectric layer. FIG. 6 is an enlarged cross-sectional view illustrating a piezoelectric element in a third embodiment. FIG. 6 is a diagram showing a circuit configuration of each piezoelectric layer constituting a piezoelectric element of an ultrasonic sensor in a third embodiment. In the configuration of the third embodiment, the voltage sensitivity, the charge sensitivity, and the generated electric energy are numerically calculated by changing the thickness ratio of each piezoelectric layer. FIG. 10 is an enlarged cross-sectional view illustrating a piezoelectric element according to a fifth embodiment. FIG. 9 is an equivalent circuit corresponding to the circuit configuration of Embodiments 1 to 5 in Embodiment 6. FIG. It is a figure which expands and shows the connection part of the amplifier circuit (transformer) and capacity | capacitance in FIG. It is a figure which shows the relationship between a frequency and a circuit gain. FIG. 10 is a diagram showing an outline of a circuit configuration of a sensor device in a sixth embodiment. FIG. 10 is a diagram illustrating functional blocks of a sensor device according to a sixth embodiment. FIG. 25 is a diagram showing functional blocks of a sensor device in a modification of the sixth embodiment.

Each embodiment will be described below with reference to the drawings. When referring to the number and quantity, the scope of the present invention is not necessarily limited to the number and quantity unless otherwise specified. The same parts and corresponding parts are denoted by the same reference numerals, and redundant description may not be repeated.

[Embodiment 1]
(Sensor device 1000)
The sensor device 1000 according to the first embodiment will be described with reference to FIGS. FIG. 1 is a diagram illustrating an outline of a circuit configuration of the sensor device 1000. The sensor device 1000 includes an ultrasonic sensor 100, a power source 200, a signal generation circuit 300, an amplification circuit 400, and a detection circuit 500.

The power source 200 outputs a DC voltage of 12 V, for example, and the DC voltage is input to the signal generation circuit 300 and converted into an AC voltage having a predetermined frequency. The AC voltage is supplied to the ultrasonic sensor 100 while being boosted by the amplifier circuit 400. The ultrasonic sensor 100 is driven, and ultrasonic waves are transmitted (transmitted) from the ultrasonic sensor 100 toward the air. Hereinafter, the sensor device 1000 will be described in more detail.

FIG. 2 is a diagram illustrating functional blocks of the sensor device 1000. The sensor device 1000 includes an IC, and the IC includes a microcomputer 600 and a memory 610. The microcomputer 600 reads data stored in the memory 610 and outputs a control signal suitable for driving the ultrasonic sensor 100.

The signal generation circuit 300 generates an AC voltage from the DC voltage based on the control signal output from the microcomputer 600, and outputs the AC voltage to the amplification circuit 400 (transformer). The AC voltage boosted by the amplifier circuit 400 is supplied to the ultrasonic sensor 100. The higher the voltage input to the ultrasonic sensor 100 is, the stronger the ultrasonic sensor 100 is driven, the stronger the sound pressure generated, and the longer the detectable distance.

The ultrasonic sensor 100 has a resistor 110 and a capacitor 120 connected in parallel. When the ultrasonic sensor 100 receives the reflected wave from the target, the received signal generated by the ultrasonic sensor 100 is sent to the reception amplifier 510 and input to the microcomputer 600 through the detection circuit 500. The microcomputer 600 makes it possible to grasp the presence / absence of the target and information related to movement.

(Ultrasonic sensor 100)
FIG. 3 is a cross-sectional view showing the ultrasonic sensor 100. 4A is a plan view showing only the metal case 10 and the piezoelectric element 20 in the ultrasonic sensor 100, and FIG. 4B is a plan view showing the metal case 10 and the piezoelectric element 20 in the ultrasonic sensor 100. It is sectional drawing which shows only. 3, 4A, and 4B, the ultrasonic sensor 100 includes a metal case 10, a piezoelectric element 20, a sound absorbing material 40, silicone 41 to 44, a relay substrate 51,

lead wires

52, 53,

Pin terminals

54 and 55, a connector 56, and a filling resin 60 are provided.

The metal case 10 includes a disk-shaped bottom part 11 and a cylindrical side wall part 14 provided along the periphery of the bottom part 11 and has a bottomed cylindrical shape as a whole. The bottom portion 11 has an inner surface 12 and an outer surface 13, and the piezoelectric element 20 is disposed on the inner surface 12 of the bottom portion 11, and is joined to the inner surface 12 using an adhesive or the like (not shown). The metal case 10 is made of aluminum having high elasticity and light weight, for example. The metal case 10 is manufactured by forging or cutting such aluminum, for example. The metal case 10 is an example of the “case” in the present invention. The material of the case is not necessarily limited to metal, and may be made of resin, for example.

The

silicones

41 and 42 have an annular shape, and are arranged so as to fix the piezoelectric element 20 to the inner surface 12 while surrounding the piezoelectric element 20. The sound absorbing material 40 (also referred to as a damper) is made of a molded body having high elasticity, and faces the piezoelectric element 20 with a space therebetween. The silicone 43 is provided so as to block the space on the side where the piezoelectric element 20 and the sound absorbing material 40 are arranged in the internal space of the metal case 10. The silicone 44 holds the relay substrate 51.

The piezoelectric element 20 is provided with electrodes (not shown) on the front and back surfaces. The electrode located on the inner surface 12 side of the bottom 11 when viewed from the piezoelectric element 20 is electrically connected to the outside (GND) through the metal case 10, the lead wire 52, the relay substrate 51, the pin terminal 54, and the connector 56. The electrode located on the side opposite to the bottom 11 when viewed from the piezoelectric element 20 is electrically connected to the outside through the lead wire 53, the relay substrate 51, the pin terminal 55, and the connector 56.

FIG. 5 is a cross-sectional view showing a state where the bottom 11 of the metal case 10 and the piezoelectric element 20 are bending-vibrated. A unimorph structure is formed by the bottom 11 of the metal case 10 and the piezoelectric element 20, and a stress neutral surface is formed during bending vibration. The stress neutral surface is a surface serving as a boundary between the portion where the tensile stress is generated and the portion where the compressive stress is generated, and is located at the position of the joint surface between the bottom portion 11 of the metal case 10 and the piezoelectric element 20. It is formed.

The amount of expansion / contraction on the stress neutral surface is almost zero, the amount of expansion / contraction is small near the stress neutral surface, and the amount of expansion / contraction increases as the distance from the stress neutral surface increases. When ultrasonic waves are transmitted and received by bending the bottom portion 11 of the metal case 10 and the piezoelectric element 20, the work near the stress-neutral plane formed by bending vibration is reduced during transmission and reception.

In such a situation, the piezoelectric element 20 provided in the ultrasonic sensor 100 adopts a laminated structure. Further, each piezoelectric layer has a configuration in which the piezoelectric layer farthest from the bottom 11 of the metal case 10 is thinner than the piezoelectric layer closest to the bottom 11 of the metal case 10. According to this configuration, the sound pressure at the time of transmitting an ultrasonic wave increases, and the maximum detection distance by the ultrasonic sensor increases. In addition, by setting the thickness of each piezoelectric layer to be within an optimal range, it is possible to efficiently extract energy at the time of reception. This will be specifically described below.

(Piezoelectric element 20)
FIG. 6 is an enlarged cross-sectional view showing the bottom 11 of the metal case 10 and the piezoelectric element 20. The piezoelectric element 20 has a two-layer structure including a first piezoelectric layer 21 (hereinafter referred to as a first layer 21) and a second piezoelectric layer 22 (hereinafter referred to as a second layer 22). The first layer 21 and the second layer 22 are produced by laminating a common electrode 32 between two piezoelectric layers made of thin piezoelectric ceramic having a strip shape, and firing them together. Is done.

The first layer 21 and the second layer 22 are laminated in this order in a direction away from the bottom 11, and an electrode 31 is provided on the opposite side of the electrode 32 of the first layer 21. An electrode 33 is provided on the opposite side. Two unit cells are constituted by the first layer 21 and the second layer 22 sandwiched between the electrodes 31 to 33 (internal electrodes). As shown in FIG. 7, these two unit cells are arranged on the side surface of the piezoelectric element. They are connected in parallel by provided external electrodes (electrode patterns or wiring members) (not shown). In addition, the white arrow in FIG. 6 shows the polarization direction of each piezoelectric material layer.

Referring to FIGS. 8 to 10, here, the piezoelectric element will be described as having a laminated structure. The white arrows in FIGS. 8 to 10 indicate the polarization direction of each piezoelectric layer. Referring first to FIG. 8, the driving force per unit thickness of the piezoelectric element is proportional to the electric field strength E applied to the piezoelectric element. A relationship of E = Vdd / t is established between the thickness t of the piezoelectric element having a single layer (one layer) structure and the electric field intensity E formed by the voltage Vdd applied to the piezoelectric element. The electric field strength E is proportional to the force F. That is, the force F increases as the thickness decreases.

On the other hand, since the power supply outputs a DC voltage of about 12V, for example, the AC voltage that can be generated by the signal generation circuit is 12V or less in principle. In order to realize a detection range on the order of several meters, 12 V or less is insufficient, and a mechanism for increasing the voltage is required. As a means for boosting the AC voltage, as shown in FIG. 2, a transformer (amplifying circuit 400) using magnetic coupling is simple and inexpensive.

As shown in FIG. 9, when the thickness of the piezoelectric element is halved (t / 2), the electric field strength is doubled. However, when the thickness of the piezoelectric element is halved, the volume that exerts the driving force per unit thickness of the piezoelectric element is also halved. Therefore, even if the electric field strength is doubled, the bending vibration of the piezoelectric element causes The resulting force is reduced.

Referring to FIG. 10, the piezoelectric element shown in FIG. 10 has a structure in which two piezoelectric elements (piezoelectric layers) shown in FIG. 9 are laminated. With this structure, even with a piezoelectric element having a thickness t, the electric field strength can be doubled. In other words, the same force as before lamination can be realized with half the voltage before lamination.

If the number of piezoelectric layers is increased to 3 layers and 4 layers, the required voltage can be reduced accordingly. For example, by dividing the piezoelectric element into layers that can be sufficiently driven by a voltage supplied from a power supply, the ultrasonic sensor can be driven without using a boosting means such as a transformer.

However, as shown in FIG. 10, since the direction of the electric field strength is reversed in the adjacent piezoelectric layers, adjacent layers are driven in order to drive all the piezoelectric layers in the same direction during bending vibration. Needs to reverse the polarization direction. Therefore, it is necessary to provide a common electrode inside the piezoelectric element, connect the electrodes in parallel, and connect each cell to the outside.

Referring to FIG. 6 again, in the present embodiment, a two-layer structure including the first layer 21 and the second layer 22 is employed. The thickness T2 of the second layer 22 is smaller than the thickness T1 of the first layer 21. That is, the second layer 22 farthest from the bottom 11 of the metal case 10 has a feature that it is thinner than the first layer 21 closest to the bottom 11 of the metal case 10. More specifically, the ratio of the thickness T1 of the first layer 21 to the total thickness (T1 + T2) of the thickness T1 of the first layer 21 and the thickness T2 of the second layer 22 is 58% or more and 80% or less. . That is, a relationship of 0.58 ≦ T1 / (T1 + T2) ≦ 0.80 is established between T1 and T2. T1 is, for example, 350 μm, and T2 is, for example, 150 μm.

The first layer 21 and the second layer 22 are polarized in the thickness direction as indicated by white arrows in FIG. The first layer 21 and the second layer 22 are polarized in opposite directions with the electrode 32 as a boundary.

When the ultrasonic wave is transmitted by bending the bottom portion 11 of the metal case 10 and the piezoelectric element 20, the work amount (F · m) is small in the portion near the stress neutral plane formed by the bending vibration. The further away from the elevation, the greater the work load. The sound pressure of the transmitted ultrasonic wave is larger in the portion far from the stress neutral surface than in the portion near the stress neutral surface.

The sound pressure when transmitting ultrasonic waves can be calculated by adding the sound pressure of each piezoelectric layer. In the case of the multilayer piezoelectric element, the influence of the work amount of the piezoelectric layer (the second layer 22 in the present embodiment) farthest from the bottom 11 of the metal case 10 is dominant. That is, compared with the case where the thickness of the 1st layer 21 and the 2nd layer 22 is the same, or the case where the 2nd layer 22 is thicker than the 1st layer 21, in the case of this Embodiment, the 2nd layer 22 is By being thinner than the first layer 21, it is possible to increase the sound pressure when transmitting ultrasonic waves. Furthermore, since the sound pressure at the time of transmitting an ultrasonic wave increases, the S / N at the time of receiving a reflected wave also increases. This is because the magnitude of N (noise) does not change, but the S / N increases relatively as S (signal) increases.

Next, the reason why the sensitivity when receiving the reflected wave by the above configuration is improved will be described. Here, there are two types of sensitivity when receiving reflected waves: voltage sensitivity and charge sensitivity, and the generated electrical energy expressed as ½ of the product of charge sensitivity and voltage sensitivity is large. It can be said that the sensor has a high S / N, that is, a high sensitivity during reception. The deformation stress generated in the piezoelectric layer due to bending vibration is reversed between positive and negative with respect to the stress neutral plane, and its magnitude increases in proportion to the distance from the stress neutral plane. Since the electric field generated in the piezoelectric layer is proportional to the deformation stress generated in the piezoelectric layer, the electric field generated in the piezoelectric layer increases in proportion to the distance from the stress neutral plane. As described above, the stress neutral surface is a joint surface between the metal case 10 and the piezoelectric element 20.

Since the voltage is obtained by integrating the electric field in the thickness direction of the piezoelectric layer, the voltage inside the piezoelectric layer increases in a quadratic function with respect to the thickness direction. Therefore, in the case of a laminated structure composed of a plurality of piezoelectric layers having the same thickness as in Patent Document 1 described at the beginning, a portion near the stress neutral plane, that is, a layer near the bottom of the metal case 10 is generated. An unbalanced state is formed in which the voltage is low and the generated voltage is high as the distance from the bottom of the metal case 10 increases.

In addition, since the layers of the piezoelectric layer are connected in parallel, the potential difference generated between the layers is consumed as heat by the internal resistance. As a result, it is impossible to take out all the charges generated in each layer. In other words, in a laminated structure composed of a plurality of piezoelectric layers having the same thickness, only a voltage obtained by averaging the voltages generated in each layer can be extracted outside. In order to avoid this, the loss is minimized by detecting the generated voltage only from the layer with the highest generated voltage, that is, the layer farthest from the bottom of the metal case 10 and taking it out as a received signal. Means can be considered (Patent Document 2).

However, a voltage is generated even in a portion where the generated voltage is low. In the configuration of Patent Document 2, the information extracted from the portion where the generated voltage is low is not reflected in the information extracted as the received signal, so that the voltage sensitivity is low. Furthermore, the configuration of Patent Document 2 has a demerit that requires a complicated circuit operation in which all layers are connected at the time of transmission and other than the outermost layer is disconnected at the time of reception.

On the other hand, in the configuration of the present embodiment, the ratio of the thickness T1 of the first layer 21 to the total thickness (T1 + T2) of the thickness T1 of the first layer 21 and the thickness T2 of the second layer 22 is 58%. More than 80%. As described above, since the voltage generated in the piezoelectric layer is a quadratic function, according to the configuration, the generated voltage of the first layer 21 and the second layer 22 is substantially the same, reducing the loss, and more Much of the generated electrical energy can be extracted. That is, the generated electrical energy can be greatly increased, and a highly sensitive ultrasonic sensor can be realized.

(Experimental example)
FIG. 11 shows the result of numerical calculation of the voltage sensitivity and the charge sensitivity when receiving a reflected wave having a certain intensity by changing the thickness ratio of the first layer in the configuration of the first embodiment by the finite element method. is there. FIG. 12 is based on the voltage sensitivity and the charge sensitivity shown in FIG. 11, and is a diagram showing the relationship between the thickness ratio of the first layer and the generated electric energy. As a sensor configuration under experimental conditions, the metal case 10 is made of aluminum with a Young's modulus of 70 GPa, and the bottom 11 has a disk shape with a thickness of 650 μm and a radius of 7 mm.

The piezoelectric element is made of, for example, lead zirconate titanate having a Young's modulus of 75 GPa, has a total thickness of 500 μm, and is a rectangular parallelepiped having a surface direction of 6.5 mm × 3.9 mm. The piezoelectric element is composed of two piezoelectric layers, and has a structure in which the polarization direction is reversed with a common electrode provided therein as a boundary. In the electrode of the piezoelectric element, the electrode provided on the adhesion surface with the metal case and the electrode on the opposite side are short-circuited, and are fixed to 0V. A common electrode provided inside the piezoelectric element is taken out as an electrode that doubles as drive and detection.

The charge sensitivity, voltage sensitivity, and generated electrical energy when the thickness ratio of the first layer was changed from 0% to 100% were determined. As shown in FIG. 11, the charge sensitivity increases the thickness ratio of the first layer. It increased monotonously. On the other hand, the voltage sensitivity was maximized when the thickness ratio of the first layer was approximately 62%, and thereafter decreased as the thickness ratio of the first layer increased.

As shown in FIG. 12, the generated electric energy becomes maximum when the thickness ratio of the first layer is about 70%, and the generated electric energy is about 20% compared to the case where the thickness ratio of the first layer is 50%. Increased. The generated electric energy is maximized when the thickness ratio of the first layer is approximately 70%, but a sufficient effect is exhibited even in the range of 58% to 80%.

[Embodiment 2]
(Piezoelectric element 20A)
The second embodiment will be described with reference to FIGS. Here, the difference from the piezoelectric element 20 in the first embodiment will be mainly described, and the overlapping description will not be repeated. FIG. 13 is an enlarged cross-sectional view of the piezoelectric element 20A according to the second embodiment. The piezoelectric element 20A includes a first piezoelectric layer 21 (first layer 21), a second piezoelectric layer 22 (second layer 22), and a third piezoelectric layer 23 (hereinafter referred to as a third layer 23). It has a layer structure.

The first layer 21, the second layer 22, and the third layer 23 are stacked in this order in the direction away from the bottom 11. An electrode 31 is provided between the first layer 21 and the bottom 11 of the metal case 10, an electrode 32 is provided between the first layer 21 and the second layer 22, and the second layer 22 and the third layer are provided. 23 is provided with an electrode 33, and an electrode 34 is provided on the surface of the third layer 23.

Three unit cells are constituted by the first layer 21, the second layer 22, and the third layer 23 sandwiched between the electrodes 31 to 34 (internal electrodes). As shown in FIG. They are connected in parallel by an external electrode (electrode pattern or wiring member) (not shown) provided on the side surface of the piezoelectric element.

Here, the ratio of the thickness T1 of the first layer 21 to the total thickness (T1 + T2 + T3) of the thickness T1 of the first layer 21, the thickness T2 of the second layer 22, and the thickness T3 of the third layer 23 is 40% or more and 72. % Or less. The ratio of the total thickness of the first layer 21 and the second layer 22 to the total thickness (T1 + T2 + T3) is 76% or more and 90% or less. As in the case of the first embodiment, in each piezoelectric layer, the piezoelectric layer farthest from the bottom 11 of the metal case 10 is thinner than the piezoelectric layer closest to the bottom 11 of the metal case 10. It has a configuration. Preferably, the first layer, the second layer, and the third layer may be configured such that each thickness decreases in this order.

The first layer 21, the second layer 22, and the third layer 23 are polarized in the thickness direction as indicated by white arrows in FIG. The first layer 21 and the second layer 22 are polarized in a direction away from each other with the electrode 32 therebetween. The second layer 22 and the third layer 23 are polarized in a direction approaching each other with the electrode 33 therebetween.

As described in the first embodiment, since the third layer 23 is thinner than the first layer 21, the sound pressure at the time of transmitting ultrasonic waves can be increased. Furthermore, since the voltage generated in the piezoelectric layer is a quadratic function, according to this configuration, the generated voltages of the first layer 21, the second layer 22, and the third layer 23 are substantially the same, reducing the loss. Therefore, it becomes possible to extract more generated electric energy. That is, the generated electrical energy can be greatly increased, and a highly sensitive ultrasonic sensor can be realized.

(Experimental example)
FIG. 15 shows the result of calculating the generated electric energy based on the numerical calculation of the voltage sensitivity and the charge sensitivity by the finite element method by changing the thickness ratio of each layer in the configuration of the second embodiment. As shown in Example A1 of FIG. 15, the generated electric energy becomes maximum when the thickness ratio of the first layer is about 58% and the total ratio of the first layer and the second layer is 82%. Compared to Example A, the generated electrical energy increased by about 25%. In the case of a three-layer structure, it is sufficient if the thickness ratio of the first layer is 40% or more and 72% or less and the ratio of the total thickness of the first layer and the second layer is 76% or more and 90% or less. (See Examples A2 to A4).

[Embodiment 3]
(Piezoelectric element 20B)
The third embodiment will be described with reference to FIGS. Here, the difference from the piezoelectric element 20 in the first embodiment will be mainly described, and the overlapping description will not be repeated. FIG. 16 is an enlarged cross-sectional view of the piezoelectric element 20B according to the third embodiment. The piezoelectric element 20B includes a first piezoelectric layer 21 (first layer 21), a second piezoelectric layer 22 (second layer 22), a third layer 23 (third layer 23), and a fourth piezoelectric layer (first layer). It has a four-layer structure consisting of four layers 24).

The first layer 21, the second layer 22, the third layer 23, and the fourth layer 24 are stacked in this order in a direction away from the bottom 11. An electrode 31 is provided between the first layer 21 and the bottom 11 of the metal case 10, an electrode 32 is provided between the first layer 21 and the second layer 22, and the second layer 22 and the third layer are provided. An electrode 33 is provided between the third layer 23 and the fourth layer 24, and an electrode 35 is provided on the surface of the fourth layer 24.

Four unit cells are constituted by the first layer 21, the second layer 22, the third layer 23, and the fourth layer 24 sandwiched between the electrodes 31 to 35 (internal electrodes). As shown in FIG. The two unit cells are connected in parallel by an external electrode (an electrode pattern or a wiring member) (not shown) provided on the side surface of the piezoelectric element.

Here, the ratio of the thickness T1 of the first layer 21 to the total thickness (T1 + T2 + T3 + T4) of the thickness T1 of the first layer 21, the thickness T2 of the second layer 22, and the thickness T3 of the third layer 23 is 40% or more 60 % Or less. The ratio of the total thickness of the first layer 21 and the second layer 22 to the total thickness (T1 + T2 + T3 + T4) is 64% or more and 83% or less. The ratio of the total thickness of the first layer 21, the second layer 22, and the third layer 23 to the total thickness (T1 + T2 + T3 + T4) is 84% or more and 95% or less. As in the case of the first and second embodiments, each piezoelectric layer has a piezoelectric layer farthest from the bottom 11 of the metal case 10 than a piezoelectric layer closest to the bottom 11 of the metal case 10. The structure is thin. Preferably, the first layer, the second layer, the third layer, and the fourth layer may be configured such that the thicknesses thereof are reduced in this order.

The first layer 21, the second layer 22, the third layer 23, and the fourth layer 24 are polarized in the thickness direction as indicated by white arrows in FIG. The first layer 21 and the second layer 22 are polarized in a direction away from each other with the electrode 32 therebetween. The second layer 22 and the third layer 23 are polarized in a direction approaching each other with the electrode 33 therebetween. The third layer 23 and the fourth layer 24 are polarized in a direction away from each other with the electrode 34 therebetween.

As described in the first embodiment, since the fourth layer 24 is thinner than the first layer 21, it is possible to increase the sound pressure when transmitting ultrasonic waves. Furthermore, since the voltage generated in the piezoelectric layer is a quadratic function, according to the configuration, the generated voltages of the first layer 21, the second layer 22, the third layer 23, and the fourth layer 24 are substantially the same. Thus, loss can be reduced and more generated electric energy can be extracted. That is, the generated electrical energy can be greatly increased, and a highly sensitive ultrasonic sensor can be realized.

(Experimental example)
FIG. 18 shows the results of calculating the generated electrical energy based on the numerical calculation of the voltage sensitivity and the charge sensitivity by the finite element method with the thickness ratio of each layer changed in the configuration of the third embodiment. As shown in Example B1 of FIG. 18, the generated electric energy is such that the thickness ratio of the first layer is 50%, the total ratio of the first layer and the second layer is 69%, and the first layer and the second layer When the total ratio of the third layer and the third layer is 87%, the maximum is obtained, and the generated electric energy is increased by about 25% as compared with the case of Comparative Example B. In the case of a four-layer configuration, the ratio of the thickness of the first layer is 40% or more and 60% or less, and the ratio of the total thickness of the first layer and the second layer is 64% or more and 83% or less, and When the ratio of the total thickness of the first layer, the second layer, and the third layer was 84% or more and 95% or less, a sufficient effect was exhibited (see Examples B2 to B5).

[Embodiment 4]
As described above, the voltage generated in the piezoelectric layer is a quadratic function. By optimizing the thickness of each layer so that the generated voltage of each layer is substantially the same, loss can be reduced and more generated electric energy can be extracted. This can be explained as follows, for example.

The piezoelectric element has a piezoelectric layer composed of n layers (n is an integer of 2 or more). The total thickness of the piezoelectric layer composed of n layers is T. Assuming that the joint surface between the piezoelectric element and the bottom portion 11 of the metal case 10 is zero potential, when the coordinate axis t is taken in the thickness direction of the piezoelectric element, the potential at an arbitrary position in the thickness direction is V (t). = At ² a is a constant including the piezoelectric constant, the material of the metal case 10, the received sound pressure, and the like.

Among the piezoelectric elements, the k-th coordinate (k is an integer between 1 and n) in the t-axis direction that divides the voltage V (t) at the coordinate t farthest from the bottom 11 of the metal case 10 into n equal parts. _{At k} , the voltage is ideally k * V (t) / n. When the equation of V (t) is used, a relationship of at _k ² = k * aT ² / n is established. When this is the formula of t _k, the following expression (A).

The thickness Tk of the piezoelectric layer located kth (k is an integer of 2 or more) from the bottom 11 side of the metal case 10 is a subtraction of coordinate values, so that Tk = t _k −t _k−1 . It can be calculated and is expressed by the following equation (B).

In order to make the voltage generated in each piezoelectric layer substantially the same (for example, ± 10%), the voltage V (t) at the coordinate t farthest from the bottom 11 of the metal case 10 among the piezoelectric elements. Is divided into n equal parts. If the value obtained at this time is the target voltage value, the generated voltage at each coordinate (t1, t2, t3...) When the coordinate t (thickness t) is divided into n is substantially the same as the target voltage value (for example, , ± 10%). Therefore, the thickness T1 of the piezoelectric layer located first from the bottom 11 side of the metal case 10 is a value that satisfies the following equation (1).

The thickness Tk of the piezoelectric layer located kth (k is an integer of 2 or more) from the bottom 11 side of the metal case 10 is a value that satisfies the following equation (2).

According to the above formulas (1) to (2), the generated voltage in each piezoelectric layer is substantially the same as the target voltage value (± 10%), reducing loss and extracting more generated electric energy. Can do.

[Embodiment 5]
(Piezoelectric element 20C)
Embodiment 5 will be described with reference to FIG. Here, differences from the piezoelectric element 20B in the above-described third embodiment will be mainly described, and overlapping description will not be repeated. FIG. 19 is an enlarged cross-sectional view of the piezoelectric element 20C according to the fifth embodiment. The piezoelectric element 20C includes a first piezoelectric layer 21 (first layer 21), a second piezoelectric layer 22 (second layer 22), a third layer 23 (third layer 23), and a fourth piezoelectric layer (first layer). It has a four-layer structure consisting of four layers 24).

The first layer 21, the second layer 22, the third layer 23, and the fourth layer 24 are stacked in this order in a direction away from the bottom 11. An electrode 31 is provided between the first layer 21 and the bottom 11 of the metal case 10, an electrode 32 is provided between the first layer 21 and the second layer 22, and the second layer 22 and the third layer are provided. An electrode 33 is provided between the third layer 23 and the fourth layer 24, and an electrode 35 is provided on the surface of the fourth layer 24. These electrodes 31 to 35 correspond to “internal electrodes” of the present invention.

A first external electrode 38 is provided on the first side surface of the piezoelectric element 20C. A second external electrode 39 is provided on the second side surface facing the first side surface. The electrode 31, the electrode 33, and the electrode 35 are electrically connected to the first external electrode 38, and the electrode 32 and the electrode 34 are electrically connected to the second external electrode 39.

Four unit cells are constituted by the first layer 21, the second layer 22, the third layer 23, and the fourth layer 24 sandwiched between the electrodes 31 to 35 (internal electrodes), and these four unit cells are piezoelectric elements. These are connected in parallel by

external electrodes

38 and 39 provided on the side surfaces of them and wiring members (not shown). In the present embodiment, when the plane passing through the center in the stacking direction of the piezoelectric element 20C is X, the floating electrode is provided so as to be plane-symmetric with the internal electrode with respect to the plane X.

Specifically, the first floating

electrodes

36a and 36b are provided so as to be plane-symmetric with the electrode 33 with respect to the plane X. Although the first floating

electrodes

36a and 36b are separated from each other, they may be integrally formed. In the stacking direction of the piezoelectric elements 20C, the dimension Ta from the position of the surface X to the position where the electrode 33 is formed is the dimension Ta from the position of the surface X to the position where the first floating

electrodes

36a and 36b are formed. Is equal to

Similarly, second floating

electrodes

37a and 37b are provided so as to be plane-symmetric with the electrode 34 with respect to the plane X. Although the second floating

electrodes

37a and 37b are separated from each other, they may be integrally formed. In the stacking direction of the piezoelectric elements 20C, the dimension Tb from the position of the surface X to the position where the electrode 34 is formed is the dimension Tb from the position of the surface X to the position where the second floating

electrodes

37a and 37b are formed. Is equal to

In the present embodiment, an electrode 35 is further provided so as to be plane-symmetric with the electrode 31 with respect to the plane X. In the stacking direction of the piezoelectric elements 20C, the dimension Tc from the position of the surface X to the position where the electrode 31 is formed is equal to the dimension Tc from the position of the surface X to the position where the electrode 35 is formed.

The floating

electrodes

36a, 36b, 37a, and 37b are electrodes that are not electrically connected to any of the internal electrodes (electrodes 31 to 35) and the

external electrodes

38 and 39. The floating

electrodes

36a, 36b, 37a, 37b are preferably made of the same material as the internal electrodes (electrodes 31 to 35), but may be made of another electrode material.

As described in the first embodiment, since the fourth layer 24 is thinner than the first layer 21, it is possible to increase the sound pressure when transmitting ultrasonic waves. Further, by providing the floating electrode, it is possible to prevent a warp that may occur due to a difference in shrinkage rate between the internal electrode and the ceramic during firing of the piezoelectric ceramic. The idea of providing a floating electrode to prevent warping can also be applied when the piezoelectric element is composed of a piezoelectric layer having a two-layer structure (Embodiment 1), and the piezoelectric element is a piezoelectric layer having a three-layer structure The present invention can be applied to the case where the piezoelectric element is composed of piezoelectric layers having a plurality of layer structures other than 2 to 4 (Embodiment 2). Also with the configuration of the present embodiment, the generated electrical energy can be significantly increased, and a highly sensitive ultrasonic sensor can be realized.

[Embodiment 6]
Referring to FIG. 20, an amplifier circuit 400 is used in the sensor device of each of the embodiments described above. In the fifth embodiment, the amplifier circuit 400 is not used. This configuration can be applied to any of Embodiments 1 to 5 described above. Hereinafter, the operation and effect when the amplifier circuit 400 is not used and the signal output from the signal generation circuit 300 is input to the ultrasonic sensor 100 without being amplified will be described.

FIG. 20 is an equivalent circuit corresponding to the circuit configuration of the first to fifth embodiments, and shows an equivalent circuit including the amplifier circuit 400 (transformer) and the ultrasonic sensor 100 (piezoelectric element). In addition to the function of transmitting ultrasonic waves, the ultrasonic sensor 100 also has a detection function of receiving ultrasonic waves, and the charge accumulated in the capacitor C2 is detected at the time of detection.

FIG. 21 is an enlarged view showing a connection portion between the amplifier circuit 400 (transformer) and the capacitor C2. An electromagnetic wave (external noise) that has entered from the connecting portion (wiring line or the like) of both flows as a current signal I in a circuit including L2 and C2. Here, the circuit including L2 and C2 constitutes an LC resonance circuit.

As shown in FIG. 22, since this LC resonance circuit has a very large amplification factor at the resonance frequency, a component close to the resonance frequency in the electromagnetic wave (external noise) is greatly amplified. Therefore, it can be said that the configuration having a transformer is more susceptible to external noise than the configuration having no transformer.

Therefore, as in the sensor device 1001 shown in FIGS. 23 and 24, if there is a structure without a transformer, it is possible to reduce the influence of external noise, and it can be said that the structure is strong against external noise. Further, since the number of parts can be reduced and the circuit can be simplified in the circuit configuration, the size and cost can be reduced. In the configurations shown in FIGS. 23 and 24, for example, the piezoelectric element may have a thickness of 340 μm and a stacked configuration including six piezoelectric layers.

In the first to fifth embodiments described above, since the ultrasonic sensor having a multilayered piezoelectric element is used, the step-up ratio of the transformer can be reduced. That is, since the size of the transformer can be reduced, an effect that the entire detection function can be made smaller than before can be obtained.

(Modification)
A sensor device 1002 shown in FIG. 25 is obtained by adding a booster circuit 210 to the configuration of FIG. The point that the transformer is not used is common to the case of FIG. 24, and the voltage can be raised from the power supply (12 V) to, for example, 40 V by the booster circuit 210. That is, the sound pressure of the ultrasonic sensor can be increased, the sensitivity of the ultrasonic sensor can be increased, and the detection distance can be increased.

As mentioned above, although each embodiment and each experimental example based on this invention were demonstrated, each disclosed embodiment and each experimental example are illustrations in all points, and are not restrictive. The technical scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

10 metal case, 11 bottom, 12 inner surface, 13 outer surface, 14 side wall, 20, 20A, 20B, 20C piezoelectric element, 21 first piezoelectric layer (first layer), 22 second piezoelectric layer (second layer) , 23 Third piezoelectric layer (third layer), 24 Fourth piezoelectric layer (fourth layer), 31 to 35 electrodes (internal electrodes), 36a, 36b Floating electrodes (first floating electrodes), 37a, 37b Floating electrode (second floating electrode), 38 External electrode (first external electrode), 39 External electrode (second external electrode), 40 Sound absorbing material, 41-44 Silicone, 51 Relay board, 52, 53 Lead wire , 54,55 pin terminal, 56 connector, 60 filling resin, 100 ultrasonic sensor, 110 resistor, 120 capacitor, 200 power supply, 210 booster circuit, 300 signal generation circuit, 400 increase Circuit 500 detecting circuit, 510 a receiving amplifier, 600 microcomputer, 610 memory, 1000,1001,1002 sensor device.

Claims

A bottomed cylindrical case,
A piezoelectric element provided on the inner surface of the bottom of the case and bending-vibrated with the bottom of the case;
The piezoelectric element includes a plurality of piezoelectric layers that are sequentially stacked in a direction away from the bottom with electrodes interposed therebetween and electrically connected in parallel.
The plurality of piezoelectric layers have the smallest thickness of the piezoelectric layer provided farthest from the case side,
Ultrasonic sensor.
A bottomed cylindrical case,
A piezoelectric element provided on the inner surface of the bottom of the case and bending-vibrated with the bottom of the case;
The piezoelectric element includes a first piezoelectric layer and a second piezoelectric layer that are sequentially stacked in a direction away from the bottom with electrodes interposed therebetween and electrically connected in parallel.
The thickness of the second piezoelectric layer is smaller than the thickness of the first piezoelectric layer;
Ultrasonic sensor.
The ratio of the thickness of the first piezoelectric layer to the total thickness of the first piezoelectric layer and the second piezoelectric layer is not less than 58% and not more than 80%.
The ultrasonic sensor according to claim 2.
A bottomed cylindrical case,
A piezoelectric element provided on the inner surface of the bottom of the case and bending-vibrated with the bottom of the case;
The piezoelectric element includes a first piezoelectric layer, a second piezoelectric layer, and a third piezoelectric layer that are sequentially stacked in a direction away from the bottom with electrodes interposed therebetween and electrically connected in parallel.
The thicknesses of the first piezoelectric layer, the second piezoelectric layer, and the third piezoelectric layer are reduced in this order.
Ultrasonic sensor.
The ratio of the thickness of the first piezoelectric layer to the total thickness of the first piezoelectric layer, the second piezoelectric layer, and the third piezoelectric layer is not less than 40% and not more than 72%. The ratio of the total thickness of the body layer and the second piezoelectric layer is not less than 76% and not more than 90%.
The ultrasonic sensor according to claim 4.
A bottomed cylindrical case,
A piezoelectric element provided on the inner surface of the bottom of the case and bending-vibrated with the bottom of the case;
The piezoelectric element includes a first piezoelectric layer, a second piezoelectric layer, a third piezoelectric layer, and a fourth piezoelectric layer, which are sequentially stacked in a direction away from the bottom portion with electrodes interposed therebetween and electrically connected in parallel. Including
The thicknesses of the first piezoelectric layer, the second piezoelectric layer, the third piezoelectric layer, and the fourth piezoelectric layer are reduced in this order.
Ultrasonic sensor.
The ratio of the thickness of the first piezoelectric layer to the total thickness of the first piezoelectric layer, the second piezoelectric layer, the third piezoelectric layer, and the fourth piezoelectric layer is 40% or more and 60% or less. The ratio of the total thickness of the first piezoelectric layer and the second piezoelectric layer is not less than 64% and not more than 83%, and the first piezoelectric layer, the second piezoelectric layer, and the third piezoelectric body The ratio of the total thickness of the layers is 84% or more and 95% or less,
The ultrasonic sensor according to claim 6.
A bottomed cylindrical case,
A piezoelectric element provided on the inner surface of the bottom of the case and bending-vibrated with the bottom of the case;
The piezoelectric element includes a piezoelectric layer composed of n layers (n is an integer of 2 or more) stacked in order in a direction away from the bottom with an electrode interposed therebetween and electrically connected in parallel.
When the total thickness of the piezoelectric layer composed of the n layer is T,
The thickness T1 of the piezoelectric layer positioned first from the bottom side is a value satisfying the following formula (1):

The thickness Tk of the piezoelectric layer located kth (k is an integer of 2 or more) from the bottom side is a value that satisfies the following expression (2).

Ultrasonic sensor.
The electrode provided in the piezoelectric element includes an internal electrode that is connected to the external electrode and connected in parallel to the piezoelectric layers, and a floating electrode that is formed not to be connected to the external electrode. And having
The ultrasonic sensor according to claim 1.
The floating electrode is provided at a position that is plane-symmetric with the internal electrode with respect to a plane passing through the center in the stacking direction of the piezoelectric elements.
The ultrasonic sensor according to claim 9.
The ultrasonic sensor according to any one of claims 1 to 10,
A signal generation circuit that outputs a signal for driving the ultrasonic sensor,
The ultrasonic sensor is input in a state where the signal output from the signal generation circuit is not amplified.
Sensor device.