JP7079704B2 - Piezoelectric sensor - Google Patents

Description

The present invention relates to a piezoelectric sensor that is flexible and applicable to MEMS.

MEMS (Micro Electro Mechanical Systems) is a technology that is indispensable for miniaturization, high functionality, and low cost of electronic devices, and its practical application is being promoted in a wide range of fields. Piezoelectric materials are expected to be applied to MEMS such as microsensors and microactuators because they can convert mechanical energy and electrical energy. Known examples of the piezoelectric material include ceramics such as lead zirconate titanate (PZT), polymers such as polyvinylidene fluoride (PVDF) and polylactic acid, and composites in which piezoelectric particles are filled in a polymer matrix. .. Among them, PZT, PVDF and the like have a large piezoelectric strain constant. Therefore, the electric charge generated by the applied load becomes large, and a highly sensitive sensor can be configured.

For example, for the purpose of health management, detection and treatment of illness, or for estimating the health condition of a driver in a driver monitoring system, a technique for measuring a respiratory condition and a heart rate by a piezoelectric sensor is being developed.

Patent Documents 1 and 2 describe a biological information measurement panel in which a strain gauge is arranged on a metal or plastic floor plate. The biological information measurement panel is laid under the body of the subject and used. For this reason, elastic sheet materials are arranged on both the upper and lower surfaces of the floor board in order to give the subject a soft feeling. Patent Document 3 includes biometric information having a plurality of strain generating plates in which detection units such as strain gauges and piezoelectric elements are arranged and connected in the plane direction, and a rubber base sheet that supports the strain generating plates. The measurement sensor is described. Patent Document 4 describes a panel for measuring biological information in which a piezoelectric film sensor is sandwiched between two sheets of silicone rubber.

Japanese Unexamined Patent Publication No. 2008-06779 Japanese Unexamined Patent Publication No. 2009-11246 Japanese Unexamined Patent Publication No. 2012-205803 Japanese Unexamined Patent Publication No. 2014-124310

Although the biometric information measurement sensors described in Patent Documents 1 to 3 have an elastic rubber sheet material or the like, they have a hard strain gauge and a hard strain plate, and therefore have poor flexibility as a whole. For this reason, when the subject lies on top of it, it is easy to feel a sense of discomfort such as hardness and stiffness. On the other hand, in the biometric information measurement sensor described in Patent Document 4, a silicon rubber floor plate is used instead of the metal bending plate. Then, a piezoelectric sensor is arranged between the two floor plates so as not to overlap the subject, and the distortion of the floor plate caused by the biological activity of the subject is detected by the piezoelectric sensor. In this case, the vibration generated by the biological activity of the subject is transmitted through the floor plate and reaches the piezoelectric sensor. However, since the silicone rubber floor plate has viscoelasticity, the vibration generated by the biological activity of the subject is greatly attenuated until it is transmitted to the piezoelectric sensor. Therefore, it is difficult to accurately detect weak vibrations such as respiration and heartbeat.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a piezoelectric sensor having a desired sensitivity and less likely to cause a sense of discomfort even when a person comes into contact with it.

In order to solve the above problems, the piezoelectric sensor of the present invention has a piezoelectric layer and a pair of electrode layers arranged on both the front and back surfaces of the piezoelectric layer, and has a plurality of piezoelectric elements arranged apart from each other in the plane direction. It is made of an elastic body having an elastic coefficient of 10 MPa or less, and is characterized by comprising an elastic body layer arranged on at least one side in the front and back directions of the plurality of piezoelectric elements.

In the piezoelectric sensor of the present invention, a plurality of piezoelectric elements are arranged apart from each other in the plane direction, and an elastic layer having an elastic modulus of 10 MPa or less is arranged on at least one side thereof. Since the piezoelectric elements are not continuous in the plane direction and are covered with a relatively flexible elastic layer, even if hard ceramics or resin is used as the piezoelectric layer, these hardnesses are unlikely to appear, and even if they come into contact with people, they are hard. It is hard to feel discomfort such as pods and foreign matter. Further, since the number of piezoelectric materials used can be reduced as compared with the case where one piezoelectric element continuous in the plane direction is arranged, the cost can be reduced. Further, since it can be folded in the gap between the piezoelectric elements, it is easy to make it compact.

According to the piezoelectric sensor of the present invention, by selecting a piezoelectric material suitable for the usage and usage environment such as frequency dependence and heat resistance and using it for the piezoelectric layer, the performance of the sensor according to the usage and usage environment can be obtained. be able to. For example, even a weak vibration due to breathing and heartbeat can be detected with high accuracy even in a harsh environment such as high temperature and high humidity indoors or in a car.

It is a top view of the piezoelectric sensor of 1st Embodiment. FIG. 2 is a cross-sectional view taken along the line II-II of FIG. It is a schematic diagram which shows the setting example of a pressure sensitive part. It is a top view of the piezoelectric sensor of Example 1. FIG. It is explanatory drawing of the measuring method of the piezoelectric strain constant.

First, one embodiment of the piezoelectric sensor of the present invention will be described, and then another embodiment will be described.

<First Embodiment>
[Piezoelectric sensor configuration]
First, the configuration of the piezoelectric sensor of this embodiment will be described. FIG. 1 shows a top view of the piezoelectric sensor of the present embodiment. FIG. 2 shows a cross-sectional view taken along the line II-II of FIG. In FIG. 1, the piezoelectric element is transmitted and shown. In FIGS. 1 and 2, the vertical direction corresponds to the front-back direction, the thickness direction, and the stacking direction of the member, and the front-back and left-right directions correspond to the surface direction of the member.

As shown in FIGS. 1 and 2, the piezoelectric sensor 1 includes two piezoelectric elements 10, a front elastic body layer 20, a back elastic body layer 30, and a wiring 40.

The two piezoelectric elements 10 are interposed between the front elastic body layer 20 and the back elastic body layer 30. The two piezoelectric elements 10 are juxtaposed with a gap d in the left-right direction. The two piezoelectric elements 10 have the same configuration, shape, and size. The two piezoelectric elements 10 each have a piezoelectric layer 11 and a pair of electrode layers 12a and 12b.

The piezoelectric layer 11 is made of lead zirconate titanate (PZT). The piezoelectric layer 11 has a square thin plate shape having a length of 10 mm, a width of 10 mm, and a thickness of 1 mm. The thickness of the piezoelectric layer is about 20% when the thickness of the entire piezoelectric sensor is 100%. The electrode layer 12a is arranged on the entire upper surface of the piezoelectric layer 11, and the electrode layer 12b is arranged on the entire lower surface of the piezoelectric layer 11. Both the electrode layers 12a and 12b are silver thin films having a thickness of 7 μm formed by the baking method. Wiring 40 is connected to both left and right ends of the electrode layer 12a. Similarly, wiring 40 is connected to both left and right ends of the electrode layer 12b. The electrode layers (12a-12a, 12b-12b) of the adjacent piezoelectric elements 10 are electrically connected by the wiring 40.

The shortest distance (interval d) between the two piezoelectric elements 10 is 2 mm. The two piezoelectric elements 10 are arranged so as to be spaced apart from each other by a minimum length (1 mm) or more of the piezoelectric layer 11. As shown in FIG. 1 with dotted line hatching, when the piezoelectric sensor 1 is viewed from above, a rectangular region (including the two piezoelectric elements 10 and their gaps) partitioned by connecting the outermost edges of the two piezoelectric elements 10 is included. Region) becomes the pressure sensitive portion S. The total area ratio of the two piezoelectric elements 10 to the area of the pressure sensitive portion S is about 91%.

The front elastic body layer 20 is arranged on the upper surface side of the piezoelectric element 10. The front elastic body layer 20 covers the two piezoelectric elements 10 and their gaps from above. The front elastic body layer 20 is composed of a first elastic body layer 21 and a second elastic body layer 22 laminated in the thickness direction. The first elastic body layer 21 is arranged on the piezoelectric element 10 side, that is, inside, and the second elastic body layer 22 is arranged on the outside. The first elastic body layer 21 and the second elastic body layer 22 have different elastic moduli. The first elastic body layer 21 is made of silicone rubber having a thickness of 1 mm. The elastic modulus of the silicone rubber (first elastic body layer 21) is 0.1 MPa. The second elastic body layer 22 is made of silicone rubber having a thickness of 1 mm. The elastic modulus of the silicone rubber (second elastic body layer 22) is 1.25 MPa. The inner first elastic layer 21 has a smaller elastic modulus than the outer second elastic layer 22. The thickness of the front elastic body layer 20 is twice the thickness of the piezoelectric layer 11.

The back side elastic body layer 30 is arranged on the lower surface side of the piezoelectric element 10. The backside elastic body layer 30 covers the two piezoelectric elements 10 and their gaps from below. The back side elastic body layer 30 is composed of a first elastic body layer 31 and a second elastic body layer 32 laminated in the thickness direction. The first elastic body layer 31 is arranged on the piezoelectric element 10 side, that is, inside, and the second elastic body layer 32 is arranged on the outside. The first elastic body layer 31 and the second elastic body layer 32 have different elastic moduli. The first elastic body layer 31 is made of silicone rubber having a thickness of 1 mm. The elastic modulus of the silicone rubber (first elastic body layer 31) is 0.1 MPa. The second elastic body layer 32 is made of silicone rubber having a thickness of 1 mm. The elastic modulus of the silicone rubber (second elastic body layer 32) is 1.25 MPa. The inner first elastic layer 31 has a smaller elastic modulus than the outer second elastic layer 32. The thickness of the backside elastic layer 30 is twice the thickness of the piezoelectric layer 11. The total thickness of the front elastic body layer 20 and the back elastic body layer 30 is four times the thickness of the piezoelectric layer 11.

[Piezoelectric sensor movement]
Next, the operation of the piezoelectric sensor of this embodiment will be described. The electrode layers 12a and 12b are electrically connected to a control circuit unit (not shown) by wiring 40. When a load is applied to the piezoelectric sensor 1, an electric charge is generated in the piezoelectric layer 11. The generated charge (output signal) is detected by the control circuit unit as a change in voltage or current.

[Action effect]
Next, the operation and effect of the piezoelectric sensor of the present embodiment will be described. According to the piezoelectric sensor 1, two piezoelectric elements 10 are arranged apart from each other in the left-right direction, and on both sides thereof, a flexible front elastic body layer 20 and a back side that continuously cover the gap between the two piezoelectric elements 10 and their gaps. The elastic body layer 30 (collectively referred to as the elastic body layers 20 and 30 in this embodiment) is arranged. As a result, even if the piezoelectric layer 11 is formed of hard PZT and the electrode layers 12a and 12b are made of hard silver, the hardness is unlikely to appear, and even if a person comes into contact with it, it is difficult to feel a sense of discomfort such as hardness or foreign matter. Further, as compared with the case where one piezoelectric element is arranged in the entire pressure-sensitive portion S, the number of piezoelectric materials used can be reduced, so that the cost can be reduced. Further, since it can be folded in the gap between the two piezoelectric elements 10, it is easy to make it compact.

Since the piezoelectric sensor 1 uses PZT having a relatively large piezoelectric strain constant, the sensitivity of the sensor is high. Therefore, even a weak vibration due to respiration and heartbeat can be detected with high accuracy.

The distance d between the piezoelectric elements 10 is a length equal to or larger than the thickness (minimum length) of the piezoelectric layer 11. The total thickness of the front elastic body layer 20 and the back elastic body layer 30 is more than twice the thickness of the piezoelectric layer 11. The thickness of the piezoelectric layer 11 is 33% or less of the thickness of the entire piezoelectric sensor 1. These are effective in reducing discomfort. Further, the total area ratio of the two piezoelectric elements 10 to the area of the pressure sensitive portion S is 80% or more. As a result, both flexibility and sensitivity are achieved.

The elastic layers 20 and 30 are each composed of two elastic layers having different elastic moduli. Explaining with respect to the front side electrode layer 20, the front side electrode layer 20 is composed of a first elastic body layer 21 and a second elastic body layer 22, and the first elastic body layer 21 having a smaller elastic modulus is arranged on the piezoelectric element 10 side. Has been done. When the second elastic body layer 22 which is relatively hard is arranged on the outside in this way, the stimulus can be received in a wide range, and the stimulus is easily transmitted from the flexible first elastic body layer 21 to the piezoelectric layer 11, so that the flexibility is achieved. It is possible to improve the sensitivity while imparting.

<Other embodiments>
The embodiment of the piezoelectric sensor of the present invention is not limited to the above embodiment, and is carried out in various embodiments with modifications and improvements that can be made by those skilled in the art within the scope of the gist of the present invention. be able to. The piezoelectric sensor of the present invention includes a plurality of piezoelectric elements and an elastic body layer. Hereinafter, each component will be described in detail.

[Piezoelectric element]
The number of piezoelectric elements may be two or more. The shape of the piezoelectric element may be a polygon such as a square, a rectangle, or a rhombus, a circle, an ellipse, or the like, and the size is not particularly limited. The shape and size of the piezoelectric elements do not have to be the same. The plurality of piezoelectric elements are arranged apart from each other in the plane direction (direction substantially perpendicular to the stacking direction of the piezoelectric layer and the electrode layer). The arrangement form may be regular or irregular.

The distance between the piezoelectric elements (distance between the piezoelectric elements) is not particularly limited as long as the piezoelectric sensor can be made flexible enough to reduce discomfort even when a person comes into contact with it. As the distance between the piezoelectric elements, the shortest distance between adjacent piezoelectric elements may be adopted. For example, it is desirable that the spacing between the piezoelectric elements is equal to or greater than the minimum length of the piezoelectric layer. When piezoelectric layers of different sizes are included, the minimum length of all the piezoelectric layers may be used as a reference. The spacing between the piezoelectric elements does not have to be the same.

The ratio of the piezoelectric element to the gap in the pressure sensitive portion is not particularly limited. In order to achieve both flexibility and sensitivity, for example, it is desirable that the total area ratio of the piezoelectric element to the area of the pressure sensitive portion is 80% or more and 98% or less. It is more preferable to make it 85% or more and 98% or less. Here, the pressure-sensitive portion is a region defined by connecting the outermost edges of a plurality of piezoelectric elements when viewed from the front and back directions of the piezoelectric sensor (the laminating direction of the constituent members). FIG. 3 schematically shows a setting example of the pressure sensitive portion. FIG. 3 shows a form in which a plurality of piezoelectric elements 100a to 100d having different shapes and sizes are arranged on the upper surface of the elastic body layer 200. In this embodiment, as shown by the thick line in FIG. 3, the pressure-sensitive portion S (the region where the dotted line hatching is applied) may be set by connecting the outermost edges of the piezoelectric elements 100a to 100d.

(1) Piezoelectric layer The material of the piezoelectric layer is not particularly limited. Lead zirconate titanate (PZT), barium strontium titanate (BST), bismas lanthanum titanate (BLT), bismuth strontium tantalate (SBT) potassium niobate (KN), barium titanate (BT), sodium potassium niobate If piezoelectric ceramics such as (KNN), other ceramics having a perovskite structure and having piezoelectricity, polymers such as polyvinylidene fluoride (PVDF), polylactic acid, and polyurea, and composites containing piezoelectric particles in a polymer are used. good. Further, resin electlets such as porous polypropylene, fluororesin, acrylic resin, polyamide and polyester, and inorganic electlets such as quartz, crystal, silicon oxide (SiO ₂ ) and silicon nitride (SiN) can also be used. ..

According to the finding that in a thin-film piezoelectric sensor, the smaller the cross-sectional area perpendicular to the tensile direction of the piezoelectric layer, the greater the sensitivity to the applied load, the thickness of the piezoelectric layer in one piezoelectric element is 2 mm. It is desirable that it is as follows. On the other hand, in consideration of durability, it is desirable that the thickness of the piezoelectric layer is 10 μm or more, more preferably 20 μm or more. Further, from the viewpoint of increasing flexibility, the thickness of the piezoelectric layer is preferably 33% or less, more preferably 25% or less of the thickness of the entire piezoelectric sensor.

(2) Electrode layer The material of the electrode layer is not particularly limited. Depending on the piezoelectric layer, metallic conductive materials such as silver, gold, copper, and nickel, carbon-based conductive materials such as carbon black, carbon nanotubes, and thin-layered graphite, conductive polymers, conductive cloths, and conductive materials for polymers. A complex or the like in which the above is dispersed may be used. Considering flexibility, ease of manufacture, and cost, a composite in which a conductive material is dispersed in an elastomer such as acrylic rubber, silicone rubber, and urethane rubber is suitable. When the electrode layer is formed from a composite, a conductive paint containing a polymer, a conductive material, or the like may be applied to a base material or a piezoelectric layer and dried to form the electrode layer. When the electrode layer is formed from a metal, the metal may be baked or vapor-filmed on the piezoelectric layer to form the electrode layer.

[Elastic body layer]
The elastic body layer is made of an elastic body having an elastic modulus of 10 MPa or less. When the elastic modulus of the elastic body is 5 MPa or less, further 1 MPa or less, the flexibility is further improved. On the other hand, the elastic modulus of the elastic body may be 0.03 MPa or more, more preferably 0.1 MPa or more, for the reason of suppressing stress attenuation. In the present specification, the elastic modulus is calculated from the slope of the linear region of the stress-elongation curve obtained by the tensile test specified in JIS K6251: 2014. The tensile test shall be performed using a dumbbell-shaped No. 3 test piece (parallel portion thickness 5 mm, initial distance between marked lines 20 mm) and a tensile speed of 50 mm / min.

For example, when the piezoelectric layer is made of a relatively flexible material such as a mixture of piezoelectric particles in an elastomer, when a force is applied from the front and back directions of the piezoelectric sensor (when the piezoelectric sensor is compressed), the elastic body layer is oriented in the plane direction. It stretches and a shearing force acts on the piezoelectric layer. As a result, a tensile force in the surface direction is applied to the piezoelectric layer in addition to the pressing force in the front and back directions, and the strain of the piezoelectric layer increases. As a result, the amount of charge generated in the piezoelectric layer increases, and the sensitivity of the sensor is improved.

The type of elastic body is not particularly limited. For example, as an elastomer having a relatively small elastic coefficient, silicone rubber, natural rubber, isoprene rubber, butyl rubber, acrylic rubber, urethane rubber, urea rubber, fluororubber, nitrile rubber and the like are suitable. In particular, when used for measuring human biological information, silicone rubber and urethane rubber having good affinity with the living body are desirable, and it is desirable that they do not contain substances extracted over time such as plasticizers. When an elastomer is used as the elastic body, the Poisson's ratio of the elastomer is about 0.5, so that the force applied in the thickness direction acts as a force in the surface direction as it is. Therefore, when the piezoelectric layer is made of a relatively flexible material such as an elastomer mixed with piezoelectric particles, the effect of increasing the strain of the piezoelectric layer by the elastic layer becomes large, and the sensitivity of the sensor can be improved. ..

The elastic layer is arranged on one side or both sides in the front-back direction of the plurality of piezoelectric elements. The elastic layer may be arranged in any way as long as it covers a plurality of piezoelectric elements. For example, it is effective to improve the flexibility of the piezoelectric sensor by continuously arranging the plurality of piezoelectric elements and the gap between the piezoelectric elements so as to cover them. On the other hand, in the piezoelectric sensor, the elastic layer does not necessarily have to be continuous. For example, the elastic layer may be separately laminated for each piezoelectric element or for each of several piezoelectric elements. That is, the laminated body of the elastic body layer and the piezoelectric element may be arranged in the plane direction at a predetermined interval. The material of the elastic layer may be the same or different in the plane direction. When the elastic layer is composed of a plurality of layers, the material of each layer may be the same or different.

The thickness of the elastic layer may be appropriately determined in consideration of the balance between flexibility and sensitivity. For example, the thickness of the elastic layer is preferably 2 times or more and 10 times or less the thickness of the piezoelectric layer. When the elastic layer is arranged on both sides of the piezoelectric element, the total thickness thereof may be compared with the thickness of the piezoelectric layer. If the thickness of the elastic layer is too small, the desired flexibility cannot be obtained. Conversely, if the elastic layer is too thick, the sensitivity of the sensor will decrease. The thickness of the elastic layer in the present specification is the thickness of the portion laminated on the piezoelectric element.

The elastic layer may be one layer or two or more layers. When the elastic layer is arranged on both sides of the piezoelectric element, the respective configurations may be the same or different. When the elastic layer is composed of a plurality of layers, the elastic modulus of each is set to 10 MPa or less. It is desirable that the elastic body layer has a first elastic body layer and a second elastic body layer which are laminated in the thickness direction and have different elastic moduli. By stacking a plurality of elastic layers having different elastic moduli, it becomes easy to balance flexibility and sensitivity. The difference in elastic modulus should be at least twice. For example, when the one arranged on the piezoelectric element side (inside) is the first elastic body layer, it is desirable that the elastic modulus of the first elastic body layer is smaller than that of the outer second elastic body layer. By doing so, it is possible to improve the sensitivity while imparting flexibility as in the first embodiment. On the contrary, when the elastic modulus of the inner first elastic layer is made larger than that of the outer second elastic layer, that is, when the more flexible elastic layer is arranged on the outside, the flexibility is further improved. It is effective in reducing discomfort.

[Piezoelectric sensor]
In the piezoelectric sensor of the present invention, the wiring connected to the electrode layer may be formed linearly or meanderingly. For example, by using meandering wiring for connecting adjacent piezoelectric elements, it is possible to suppress disconnection and deterioration of conductivity when the wiring is bent in a gap between the piezoelectric elements.

When the piezoelectric sensor of the present invention is used as a biological information sensor for detecting weak vibrations such as respiration and heartbeat, it is desirable that the sensor has high sensitivity. For example, the piezoelectric strain constant of the piezoelectric sensor is preferably 50 pC / N or more.

The piezoelectric sensor of the present invention can be manufactured by overlapping and crimping a piezoelectric element and an elastic body layer. At this time, the space between the piezoelectric element and the elastic layer, or the layers of the elastic body composed of a plurality of layers may be adhered with an adhesive or the like. When the elastic layer consists of two layers, the material for the inner elastic layer is injected and cross-linked, and the material for the outer elastic layer is further injected and cross-linked with the piezoelectric element floating to form the elastic body. Layers can be formed. Furthermore, these may be crimped.

Next, the present invention will be described in more detail with reference to examples.

<Manufacturing of piezoelectric sensors>
[Example 1]
(1) Manufacture of piezoelectric element Electrode layers are arranged on both sides (both front and back sides) of the square thin film-shaped piezoelectric layer in the thickness direction, and crimped using a laminator (“LPD3223” manufactured by Fujipura Co., Ltd.) to obtain the piezoelectric element. I manufactured two. As the piezoelectric layer, a polyvinylidene fluoride (PVDF) resin film “KF Piezo 40 μm” manufactured by Kureha Corporation was used. The elastic modulus of the PVDF resin film is 3000 MPa. The electrode layer was manufactured as follows. First, 100 parts by mass of an epoxy group-containing acrylic rubber polymer (“Nipol® AR51” manufactured by Nippon Zeon Corporation) as an elastomer was dissolved in methyl ethyl ketone to prepare a polymer solution. Next, 10 parts by mass of conductive carbon black (“Ketchen Black EC600JD” manufactured by Lion Corporation) was added to the prepared polymer solution and dispersed by a bead mill to prepare a conductive coating material. Subsequently, the conductive paint was applied on a film made of polyethylene terephthalate (PET) which had been mold-released by a bar coating method. This was heated at 150 ° C. for 1 hour to produce an electrode layer having a thickness of 0.015 mm (15 μm). The dimensions of the piezoelectric layer and the electrode layer are shown in Table 1 below (the same applies to the following samples).

(2) Manufacture of piezoelectric sensor The two manufactured piezoelectric elements were arranged in series on the upper surface of the elastic body layer with an interval of 0.5 mm. As the elastic layer, silicone rubber (“LIM Silicone KE1950-5” manufactured by Shin-Etsu Chemical Co., Ltd., elastic modulus 0.1 MPa) was used. The elastic layer has a rectangular thin plate shape having a length (short side) of 30 mm, a width (long side) of 60 mm, and a thickness of 3 mm. FIG. 4 shows a top view of the piezoelectric sensor of the first embodiment. As shown in FIG. 4, the piezoelectric sensor 50 includes an elastic body layer 51 and two piezoelectric elements 52 juxtaposed on the upper surface of the elastic body layer 51. Each of the two piezoelectric elements 52 has a square shape with a side of 20 mm. The distance d between the two piezoelectric elements 52 is 0.5 mm. Assuming that the rectangular region (the region including the two piezoelectric elements 52 and their gaps) defined by connecting the outermost edges of the two piezoelectric elements 52 is the pressure sensitive portion S (the same applies hereinafter), it occupies the area of the pressure sensitive portion S. The total area ratio of the two piezoelectric elements 52 is 97.6%.

[Example 2]
(1) Manufacture of Piezoelectric Elements Two piezoelectric elements were manufactured by forming silver electrode layers on both sides (both front and back sides) of the rectangular thin plate-shaped piezoelectric layer in the thickness direction by a baking method. As the piezoelectric layer, PZT (“C6” manufactured by Fuji Ceramics Corporation) was used. The elastic modulus of the PZT is 50,000 MPa.

(2) Manufacture of Piezoelectric Sensor The two manufactured piezoelectric elements were arranged in series on the upper surface of the same elastic body layer as in Example 1 with an interval of 2 mm in the long side direction. The total area ratio of the two piezoelectric elements to the area of the pressure-sensitive portion is 90.9%.

[Example 3]
A piezoelectric sensor was manufactured in the same manner as in Example 2 except that the shape of the piezoelectric layer was changed to a square thin plate (the long side of the rectangle was shortened to 10 mm) and the elastic layer was changed to two layers. Among the elastic layer, as the inner elastic layer to be arranged on the piezoelectric element side, the same silicone rubber as in Example 2 was used after changing the thickness to 1 mm. As the outer elastic body layer arranged outside the inner elastic body layer, silicone rubber (“LIM Silicone KE1950-40” manufactured by Shin-Etsu Chemical Co., Ltd., elastic modulus 1.25 MPa, thickness 1 mm) was used. The outer elastic body layer has a rectangular thin plate shape having the same size as the inner elastic body layer.

[Example 4]
A piezoelectric sensor was manufactured in the same manner as in Example 3 except that the arrangement inside and outside the two elastic layers was changed.

[Example 5]
A piezoelectric sensor was manufactured in the same manner as in Example 2 except that the distance between the two piezoelectric elements was changed to 0.5 mm. The total area ratio of the two piezoelectric elements to the area of the pressure-sensitive portion is 97.6%.

[Example 6]
A piezoelectric sensor was manufactured in the same manner as in Example 2 except that the thickness of the elastic layer was changed to 9 mm.

[Comparative Example 1]
Example 1 except that the shape of the piezoelectric layer was changed to a rectangular thin film (the two sides of the square were lengthened to 40 mm), the number of piezoelectric elements was changed to one, and the material of the elastic layer was changed. A piezoelectric sensor was manufactured in the same manner. As the elastic layer, silicone rubber (“LIM Silicone KE1950-20” manufactured by Shin-Etsu Chemical Co., Ltd., elastic modulus 0.7 MPa, thickness 1 mm) was used. Since only one piezoelectric element is arranged, the area of the pressure-sensitive portion is equal to the area of the piezoelectric element in the manufactured piezoelectric sensor. That is, the total area ratio of the piezoelectric element to the area of the pressure sensitive portion is 100%.

[Comparative Example 2]
Only the piezoelectric element having no elastic layer was used as the piezoelectric sensor of Comparative Example 2. As the piezoelectric element, the piezoelectric element of Example 2 in which the shape of the piezoelectric layer was changed to a square thin plate shape (the short side of the rectangle was lengthened to 20 mm) was used. Also in the case of this piezoelectric sensor, the total area ratio of the piezoelectric element to the area of the pressure sensitive portion is 100%.

<Evaluation of piezoelectric sensor>
The sensitivity of each piezoelectric sensor and the presence or absence of discomfort were evaluated. The evaluation method is as follows.

[sensitivity]
In order to evaluate the sensitivity of the piezoelectric sensor, the piezoelectric strain constant (d33) of the piezoelectric sensor was measured with a d33 meter (“Model ZJ-4B Piezo d33 meter”, importer: Alpha Co., Ltd.). FIG. 5 shows an explanatory diagram of a method for measuring the piezoelectric strain constant. The reference numerals in FIG. 5 correspond to those in FIG. 4 above. As shown in FIG. 5, first, the piezoelectric sensor 50 was placed on a silicone rubber plate 53 having a thickness of 12 mm with the piezoelectric element 52 facing down. In the two piezoelectric elements 52, the positive electrode layer and the negative electrode layer are each connected to the d33 meter by wiring (not shown). Next, a disk-shaped bakelite plate 54 (elastic modulus 5 GPa) having a diameter of 20 mm and a thickness of 3 mm was placed on the upper surface of the elastic body layer 51 at the center of the two piezoelectric elements 52. Then, as shown by the white arrows in FIG. 5, a load of 4N was applied from above the bakelite plate 54, and the amount of electric charge generated in the piezoelectric element 52 was measured. The piezoelectric strain constant (pC / N) is calculated by dividing the measured amount of generated charge (pC / m ² ) by the added load (N / m ² ).

When measuring the respiratory state, heart rate, etc., the piezoelectric strain constant needs to be 20 pC / N or more, and it is desirable that the piezoelectric strain constant is 50 pC / N or more. Therefore, the case where the piezoelectric strain constant is 50 pC / N or more is suitable for respiration and heart rate measurement (indicated by a circle in Table 1 below), and the case where it is 20 pC / N or more and less than 50 pC / N is applied. It was possible, but the sensitivity was slightly inferior (indicated by a triangle in the table).

[Presence / absence of discomfort]
Similar to the method for measuring the piezoelectric strain constant, first, the piezoelectric sensor was placed on a 12 mm thick silicone rubber plate (elastic modulus 0.1 MPa) with the piezoelectric element facing down (see Fig. 5 above). .. Next, the hardness of the piezoelectric sensor (elastic body layer + piezoelectric element) was measured using a hardness meter (“Ascar rubber hardness meter CS type” manufactured by Polymer Meter Co., Ltd.). Separately, only the piezoelectric layer constituting the piezoelectric sensor was placed on the silicone rubber plate, and the hardness of the elastic layer was measured using the same hardness tester. Then, the hardness of the elastic body layer was subtracted from the hardness of the piezoelectric sensor to obtain the difference in hardness. When the difference in hardness was less than 8, there was no discomfort (indicated by ◯ in Table 1 below), and when it was 8 or more, there was discomfort (indicated by x in the same table).

Table 1 shows the evaluation results together with the configuration of the manufactured piezoelectric sensor and the dimensions of each member. In Table 1, for the piezoelectric sensor in which the elastic body layer is composed of one layer, the one layer is described as the inner elastic body layer.

As shown in Table 1, it was confirmed that there was little discomfort according to the piezoelectric sensor of the example in which two piezoelectric elements were arranged. In particular, when the piezoelectric elements are arranged at intervals equal to or larger than the minimum length of the piezoelectric layer as in the piezoelectric sensors of Examples 1 to 4 and 6, the effect of reducing discomfort is high. When the thickness of the elastic layer is increased as in the piezoelectric sensor of the sixth embodiment, the elasticity is increased and the effect of reducing the discomfort is large, but the sensitivity is lowered. The decrease in sensitivity of the piezoelectric sensor of Example 6 is largely due to the measurement position of the sensitivity. In this embodiment, as shown in FIG. 5 above, the sensitivity is measured at the center of two piezoelectric elements separated from each other. For example, if the sensitivity is measured on one of the piezoelectric elements, a higher sensitivity can be obtained. Be done.

In this respect, when the elastic layer is composed of two layers having different elastic moduli as in Examples 3 and 4, the elastic layer improves the sensitivity as compared with Example 2 while maintaining the flexibility. I was able to. Comparing Example 3 and Example 4, Example 3 in which the elastic modulus with a small elastic modulus is arranged inside has higher sensitivity, and the example in which the elastic modulus with a small elastic modulus is arranged on the outside is higher. In No. 4, the difference in hardness was smaller and the effect of reducing discomfort was greater.

On the other hand, in the piezoelectric sensor of Comparative Example 1 in which only one piezoelectric element is arranged, the difference in hardness is 8 even though the elastic body layer having an elastic modulus of 10 MPa or less is provided, which gives a feeling of strangeness. occured.

The piezoelectric sensor of the present invention is useful as a biological information sensor for measuring a respiratory state, a heart rate, etc. in the fields of medical care, rehabilitation, nursing care, health management, training, a driver monitoring system for performing vital sensing of an automobile, and the like. It is also suitably used for applications that require flexibility, such as warning applications for automatic doors of trains, elevators, stores, etc., and applications that are laid on the floor to detect the movement of people, objects, and animals. It is also suitably used for switches and panel sensors that are touched by humans in IOT (Internet of Things) home appliances and smart houses.

1: Piezoelectric sensor, 10: Piezoelectric element, 11: Piezoelectric layer, 12a, 12b: Electrode layer, 20: Front side elastic body layer, 21: First elastic body layer, 22: Second elastic body layer, 30: Back side elastic body Layer, 31: First elastic body layer, 32: Second elastic body layer, 40: Wiring, 50: Piezoelectric sensor, 51: Elastic body layer, 52: Piezoelectric element, 53: Silicone rubber plate, 54: Bakelite plate, 100a ~ 100d: Piezoelectric element, 200: Elastic body layer, d: Spacing, S: Pressure sensitive part.

Claims (8)

A plurality of piezoelectric elements having a piezoelectric layer and a pair of electrode layers arranged on both the front and back surfaces of the piezoelectric layer and arranged apart from each other in the plane direction.
It is composed of an elastic body having an elastic modulus of 10 MPa or less, and includes an elastic body layer arranged on at least one side in the front-back direction of the plurality of piezoelectric elements .
The piezoelectric sensor is characterized by having a first elastic body layer and a second elastic body layer that are laminated in the thickness direction and have elastic moduli that differ by two times or more . The piezoelectric sensor according to claim 1, wherein the plurality of the piezoelectric elements are arranged at intervals equal to or larger than the minimum length of the piezoelectric layer. The piezoelectric sensor according to claim 1 or 2, wherein the elastic body layer is continuously arranged so as to cover a plurality of the piezoelectric elements and a gap between the piezoelectric elements. The piezoelectric sensor according to any one of claims 1 to 3, wherein the elastic layer has an elastomer. The piezoelectric sensor according to any one of claims 1 to 4, wherein the thickness of the elastic layer is 2 times or more and 10 times or less the thickness of the piezoelectric layer. The piezoelectric sensor according to any one of claims 1 to 5, wherein the first elastic body layer is arranged on the piezoelectric element side and has a smaller elastic modulus than the second elastic body layer. The piezoelectric sensor according to any one of claims 1 to 6 , wherein the thickness of the piezoelectric layer in one piezoelectric element is 33% or less of the thickness of the entire piezoelectric sensor. When the region defined by connecting the outermost edges of the plurality of piezoelectric elements is defined as the pressure-sensitive portion when viewed from the front and back, the total area ratio of the piezoelectric element to the area of the pressure-sensitive portion is 80% or more and 98%. The piezoelectric sensor according to any one of claims 1 to 7 below.

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