Piezoelectric sensor

Abstract

This piezoelectric sensor comprises an insulating base material having a first upper main surface and a first lower main surface, a plurality of electrodes including a first electrode formed on the first upper main surface and formed at least on the first upper main surface, and a piezoelectric film having a second upper main surface and a second lower main surface and disposed so that the second lower main surface faces the first electrode. In a region where the insulating base material and the piezoelectric film overlap in a plan view, the plurality of electrodes do not overlap each other in a plan view.

Description

Piezoelectric Sensor

[0001] The present invention relates to a piezoelectric sensor for detecting deformation.

[0002] A known example of a conventional invention related to a piezoelectric sensor is the piezoelectric sensor described in Patent Document 1. The piezoelectric sensor described in Patent Document 1 is disposed on the inner wall surface of a cylindrical housing, is pressed from the outer peripheral side surface (pressing surface) of the housing, and detects deformation due to the pressure. The piezoelectric sensor described in Patent Document 1 includes an insulating substrate having two opposing main surfaces, a first electrode, a second electrode, a piezoelectric film having an upper surface and a lower surface, and a conductive thin film member. The first electrode and the second electrode are formed on both main surfaces of the insulating substrate, respectively. The second electrode is attached to the inner wall surface of the housing with an adhesive or the like. The lower surface of the piezoelectric film is attached to the upper surface of the first electrode. A conductive thin film member is attached to the upper surface of the piezoelectric film.

[0003] International Publication No. 2016 / 194690

[0004] In the piezoelectric sensor described in Patent Document 1, if the thickness of the insulating substrate varies, the sensitivity of the piezoelectric sensor to deformation changes, and there is a risk that the detection accuracy of the piezoelectric sensor to deformation will decrease.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a piezoelectric sensor that can suppress changes in sensitivity to deformation due to variations in the thickness of the insulating substrate.

[0006] A piezoelectric sensor according to one embodiment of the present invention comprises: an insulating substrate having a first upper main surface and a first lower main surface; a plurality of electrodes including a first electrode formed on the first upper main surface, the plurality of electrodes being formed on at least the first upper main surface; and a piezoelectric film having a second upper main surface and a second lower main surface, the second lower main surface being arranged so that the second lower main surface faces the first electrode; and in a region where the insulating substrate and the piezoelectric film overlap in a planar view, the plurality of electrodes do not overlap each other in a planar view.

[0007] When a piezoelectric sensor is deformed by, for example, pressure in the normal direction of the insulating substrate, the upper principal surface of the piezoelectric sensor contracts and the lower principal surface expands. In the structure of the piezoelectric sensor described in Patent Document 1, the position of the insulating substrate in a cross-sectional view is close to the center position of the thickness of the piezoelectric sensor. Therefore, in the structure of the piezoelectric sensor described in Patent Document 1, the neutral plane of stress (the portion that does not expand or contract) of the piezoelectric sensor is located in the insulating substrate. Therefore, in the structure of the piezoelectric sensor described in Patent Document 1, the sensitivity of the piezoelectric sensor varies significantly depending on the thickness of the insulating substrate. In contrast, in a piezoelectric sensor according to one embodiment of the present invention, in the region where the insulating substrate and the piezoelectric film overlap in a planar view, multiple electrodes do not overlap each other in a planar view. This prevents the neutral plane of stress from being located in the insulating substrate when the piezoelectric sensor is pressed. Furthermore, when the piezoelectric sensor is pressed, the insulating substrate can be prevented from being located in the range from the piezoelectric film to the neutral plane of stress. As a result, a piezoelectric sensor according to one embodiment of the present invention can suppress changes in sensitivity to deformation due to variations in the thickness of the insulating substrate.

[0008] According to the present invention, it is possible to suppress changes in sensitivity to deformation due to variations in the thickness of the insulating base material.

[0009] FIG. 1 is a perspective view of a piezoelectric sensor 1. FIG. 2 is an exploded perspective view of the piezoelectric sensor 1. FIG. 3 is a plan view of the first electrode 3, the second electrode 4, and the third electrode 5 viewed from below. FIG. 4 is a plan view of the shielding tape 11 viewed from below. FIG. 5 is a plan view of the shielding film 7 viewed from above. FIG. 6 is a perspective view of a piezoelectric sensor 50 according to a comparative example. FIG. 7 is a cross-sectional view of the piezoelectric sensor 50. FIG. 8 is a cross-sectional view of the piezoelectric sensor 1. FIG. 9 is a schematic cross-sectional view of the piezoelectric sensor 50 and the piezoelectric sensor 1 in a first simulation. FIG. 10 is a diagram showing the amount of strain of each member in the first simulation of the piezoelectric sensor 50. FIG. 11 is a diagram showing the amount of strain of each member in the first simulation of the piezoelectric sensor 1. FIG. 12 is a diagram showing changes in the sensitivity of the piezoelectric sensors 50 and 1 relative to changes in the thickness of each member of the piezoelectric sensors 50 and 1 in the first simulation. FIG. 13 is an example diagram showing the amount of strain of each member in a second simulation of the piezoelectric sensor 1. FIG. 14 is an example of a diagram showing the amount of strain of each component in a second simulation of the piezoelectric sensor 1. FIG. 15 is an exploded perspective view of the piezoelectric sensor 1a. FIG. 16 is a plan view of the piezoelectric sensor 1a. FIG. 17 is a cross-sectional view taken along line A-A in FIG. 16. FIG. 18 is a cross-sectional view taken along line B-B in FIG. 16. FIG. 19 is a diagram showing the amount of strain of each component in a third simulation of the piezoelectric sensor 1a. FIG. 20 is a diagram showing the amount of strain of each component in a simulation of the piezoelectric sensor 1 when the pressed point overlaps with the second electrode 4, as viewed from above. FIG. 21 is a plan view showing the area where the second electrode 4 and the piezoelectric film 10 overlap, as viewed from above.

[0010] [First Embodiment] The configuration of a piezoelectric sensor 1 according to a first embodiment of the present invention will be described below with reference to the drawings. Fig. 1 is a perspective view of the piezoelectric sensor 1. Fig. 2 is an exploded perspective view of the piezoelectric sensor 1. Fig. 3 is a plan view of the first electrode 3, the second electrode 4, and the third electrode 5 as viewed from below. Fig. 4 is a plan view of the shielding tape 11 as viewed from below. Fig. 5 is a plan view of the shielding film 7 as viewed from above.

[0011] In this specification, directions are defined as follows. As shown in FIG. 1 , the direction in which the long sides of the upper main surface US2 of the FPC 2 extend is defined as the left-right direction. The direction in which the short sides of the upper main surface US2 of the FPC 2 extend is defined as the front-rear direction. The direction in which the FPC 2 and the piezoelectric film 10 are aligned is defined as the up-down direction. The left-right direction, front-rear direction, and up-down direction are mutually orthogonal. Viewing from above or below corresponds to a "planar view" according to the present invention. However, the left-right direction, front-rear direction, and up-down direction in this specification are defined for the convenience of explanation and may not coincide with the left-right direction, front-rear direction, and up-down direction when the piezoelectric sensor 1 is in use. Furthermore, in each drawing, the left and right directions may be interchanged, the front and rear directions may be interchanged, and the up and down directions may be interchanged.

[0012] The piezoelectric sensor 1 detects deformation of a measurement object. Furthermore, the piezoelectric sensor 1 is highly flexible and can deform flexibly. This allows the piezoelectric sensor 1 to be closely attached to the measurement object even if the measurement object has irregularities. As shown in FIG. 2 , the piezoelectric sensor 1 includes an FPC 2, a first electrode 3, a second electrode 4, a third electrode 5, a coverlay 6, a shielding film 7, double-sided tape 8, hot melt 9, a piezoelectric film 10, and a shielding tape 11. The piezoelectric sensor 1 is attached to the measurement object so that the piezoelectric film 10 is positioned directly below the pressing point of the measurement object. When viewed from above, the FPC 2 overlaps the first electrode 3, the second electrode 4, the third electrode 5, the coverlay 6, the shielding film 7, the double-sided tape 8, the hot melt 9, the piezoelectric film 10, and the shielding tape 11. In the piezoelectric sensor according to the present invention, the third electrode 5, coverlay 6, shielding film 7, double-sided tape 8, hot melt 9, and shielding tape 11 are not essential components. The FPC 2, shielding film 7, double-sided tape 8, hot melt 9, and shielding tape 11 correspond to the "insulating substrate," "second conductive thin-film member," "third adhesive member," "first adhesive member," and "first conductive thin-film member" according to the present invention, respectively. The first electrode 3, second electrode 4, and third electrode 5 correspond to the "plurality of electrodes" according to the present invention.

[0013] The FPC2 is a flexible insulating substrate. Examples of materials for the FPC2 include polyimide, PET (Poly Ethylene Terephthalate), and liquid crystal polymer. The FPC2 has an upper main surface US2 and a lower main surface DS2 arranged in this order along a downward direction. The upper main surface US2 and the lower main surface DS2 each have a rectangular shape with two long sides extending in the left-right direction and two short sides extending in the front-to-back direction. The upper main surface US2 and the lower main surface DS2 correspond to the "first upper main surface" and the "first lower main surface" of the present invention, respectively. The shape and arrangement of the insulating substrate of the present invention are not limited to those of the FPC2.

[0014] The first electrode 3 is formed on the upper main surface US2 of the FPC 2. The upper and lower surfaces of the first electrode 3 each have a rectangular shape with two long sides extending in the left-right direction and two short sides extending in the front-to-back direction. The first electrode 3 functions as a signal electrode for outputting the potential difference generated by the piezoelectric film 10 as an electric charge. Note that the shape and arrangement of the first electrode according to the present invention are not limited to those of the first electrode 3.

[0015] The second electrode 4 is formed on the lower main surface DS2 of the FPC 2. The upper and lower surfaces of the second electrode 4 each have a rectangular shape with two short sides extending in the left-right direction and two long sides extending in the front-rear direction. The second electrode 4 is connected to the ground potential. This allows the second electrode 4 to function as a shield electrode. Note that the shape and arrangement of the second electrode according to the present invention are not limited to the shape and arrangement of the second electrode 4.

[0016] In the piezoelectric sensor according to the present invention, the second electrode 4 is not an essential component. That is, the plurality of electrodes according to the present invention may be formed only on the upper main surface US2 of the FPC 2, not on the lower main surface DS2 of the FPC 2.

[0017] The third electrode 5 is formed on the upper main surface US2 of the FPC 2. That is, in this embodiment, the first electrode 3, the second electrode 4, and the third electrode 5 are formed on the upper main surface US2 and the lower main surface DS2 of the FPC 2. The upper and lower surfaces of the third electrode 5 each have a rectangular shape with two short sides extending in the left-right direction and two long sides extending in the front-rear direction. In this embodiment, the third electrode 5 is disposed to the left of the first electrode 3. The third electrode 5 is connected to the ground potential. As a result, the third electrode 5 functions as a shield electrode. Note that the shape and arrangement of the third electrode according to the present invention are not limited to the shape and arrangement of the third electrode 5. Note that the third electrode 5 is not an essential component of the piezoelectric sensor according to the present invention.

[0018] The coverlay 6 is flexible and insulating. The coverlay 6 is formed on the lower main surface DS2 of the FPC 2 so as to cover the second electrodes 4. In this way, the coverlay 6 protects the second electrodes 4. The material of the coverlay 6 is, for example, polyimide. Note that the shape and arrangement of the coverlay are not limited to those of the coverlay 6.

[0019] The shielding film 7 is flexible and conductive. The shielding film 7 is flat. The shielding film 7 has an upper principal surface US7 and a lower principal surface DS7 arranged in this order along a downward direction. The upper principal surface US7 is disposed so as to face the lower principal surface DS2 of the FPC 2. The shielding film 7 is partially bent to cover the lower principal surface DS2 of the FPC 2 and the coverlay 6. The shielding film 7 is connected to a ground potential. The connection of the shielding film 7 to a ground potential can be achieved, for example, by providing an opening in the coverlay 6 and connecting the shielding film 7 to the second electrode 4. This allows the shielding film 7 to function as a shield conductor. The upper principal surface US7 and the lower principal surface DS7 correspond to the "fourth upper principal surface" and "fourth lower principal surface," respectively, according to the present invention. Note that the shape and arrangement of the second conductive thin-film member according to the present invention are not limited to the shape and arrangement of the shielding film 7.

[0020] The double-sided tape 8 is flexible. The double-sided tape 8 is in the form of a flat film. The upper and lower surfaces of the double-sided tape 8 are adhesive. The double-sided tape 8 is provided on the lower main surface DS7 of the shielding film 7. The upper surface of the double-sided tape 8 is bent in part to be in close contact with the lower main surface DS7 of the shielding film 7. When using the piezoelectric sensor 1, the lower surface of the double-sided tape 8 is brought into close contact with the object to be measured, thereby attaching the piezoelectric sensor 1 to the object to be measured. Note that the shape and arrangement of the third adhesive member according to the present invention are not limited to the shape and arrangement of the double-sided tape 8.

[0021] The piezoelectric film 10 is flexible. The piezoelectric film 10 is flat. The piezoelectric film 10 covers the upper surface of the first electrode 3. The piezoelectric film 10 has an upper principal surface US10 and a lower principal surface DS10 arranged in this order along a downward direction. The lower principal surface DS10 is disposed to face the first electrode 3. When viewed in the vertical direction, the upper principal surface US10 and the lower principal surface DS10 each have a rectangular shape having two long sides extending in the left-right direction and two short sides extending in the front-to-back direction. The upper principal surface US10 and the lower principal surface DS10 correspond to the "second upper principal surface" and the "second lower principal surface" according to the present invention, respectively. Note that the shape and arrangement of the piezoelectric film according to the present invention are not limited to the shape and arrangement of the piezoelectric film 10.

[0022] The piezoelectric film 10 is polarized by deformation, generating a potential difference between the upper principal surface US 10 and the lower principal surface DS 10. The potential difference generated between the upper principal surface US 10 and the lower principal surface DS 10 depends on the amount of deformation of the piezoelectric film 10.

[0023] The piezoelectric film 10 is, for example, a film formed from a chiral polymer. The chiral polymer is, for example, polylactic acid (PLA), such as poly-L-lactic acid (PLLA) or poly-D-lactic acid (PDLA). PLA has a helical structure in its main chain. PLA has piezoelectric properties due to the orientation of its molecules through uniaxial stretching. The piezoelectric film 10 has a piezoelectric constant of d14.

[0024] The PLA is stretched in a uniaxial stretching direction OD. The uniaxial stretching direction OD of the PLA forms a 45-degree angle with respect to both the left-right direction and the front-back direction. The 45-degree angle may be within a range of approximately 45 degrees ±10 degrees. When the piezoelectric film 10 is stretched or compressed along the left-right direction, a potential difference is generated between the upper principal surface US10 and the lower principal surface DS10. Similarly, when the piezoelectric film 10 is stretched or compressed along the front-back direction, a potential difference is generated between the upper principal surface US10 and the lower principal surface DS10. In this embodiment, the magnitude of the potential difference generated between the upper principal surface US10 and the lower principal surface DS10 is proportional to the differential value of the deformation amount of the piezoelectric film 10.

[0025] The hot melt 9 is flexible. The hot melt 9 is disposed between the lower main surface DS10 of the piezoelectric film 10 and the first electrode 3. The hot melt 9 is in the form of a flat film. The hot melt 9 is adhesive. The hot melt 9 is solid at room temperature. When heated, the hot melt 9 melts and becomes liquid. The hot melt 9 fixes the piezoelectric film 10 to the upper surface of the first electrode 3. Note that the shape and arrangement of the first adhesive member according to the present invention are not limited to the shape and arrangement of the hot melt 9.

[0026] The shielding tape 11 is flexible and conductive. The shielding tape 11 is flat. The shielding tape 11 has an upper principal surface US11 and a lower principal surface DS11 arranged in this order along a downward direction. The lower principal surface DS11 of the shielding tape 11 is adhesive. The shielding tape 11 is disposed on the upper principal surface US10 of the piezoelectric film 10. The lower principal surface DS11 of the shielding tape 11 is in close contact with the upper principal surface US10 of the piezoelectric film 10. The shielding tape 11 also covers the upper principal surface US10 of the piezoelectric film 10. Furthermore, by bending the left end of the shielding tape 11, the left end of the shielding tape 11 contacts the upper surface of the third electrode 5. This electrically connects the shielding tape 11 to the third electrode 5. As a result, the shielding tape 11 is connected to ground potential. Therefore, the shielding tape 11 functions as a shielding conductor. In summary, in this embodiment, the first electrode 3 functions as a signal electrode, and the second electrode 4, the third electrode 5, the shielding film 7, and the shielding tape 11 function as shielding conductors. The upper principal surface US11 and the lower principal surface DS11 correspond to the "third upper principal surface" and the "third lower principal surface" according to the present invention, respectively. Note that the shape and arrangement of the first conductive thin-film member according to the present invention are not limited to the shape and arrangement of the shielding tape 11.

[0027] 3 , in this embodiment, the position and shape of an area A1 where the FPC 2 and the piezoelectric film 10 overlap when viewed in the vertical direction coincide with the position and shape of the first electrode 3 when viewed in the vertical direction. In the area A1 where the FPC 2 and the piezoelectric film 10 overlap when viewed in the vertical direction, the first electrode 3, the second electrode 4, and the third electrode 5 do not overlap one another when viewed in the vertical direction. That is, in the area A1 where the FPC 2 and the piezoelectric film 10 overlap when viewed in the vertical direction, the third electrode 5 does not overlap the first electrode 3 and the second electrode 4 when viewed in the vertical direction. In this embodiment, the second electrode 4, the third electrode 5, and the first electrode 3 are aligned in this order along the rightward direction when viewed in the vertical direction. Note that the second electrode 4, the third electrode 5, and the first electrode 3 do not necessarily have to be arranged in this order along the rightward direction when viewed in the vertical direction, and it is sufficient that the first electrode 3, the second electrode 4, and the third electrode 5 do not overlap one another when viewed in the vertical direction in the region A1 where the FPC 2 and the piezoelectric film 10 overlap one another when viewed in the vertical direction. In other words, the first electrode 3, the second electrode 4, and the third electrode 5 may overlap one another when viewed in the vertical direction in regions other than the region A1 where the FPC 2 and the piezoelectric film 10 overlap one another when viewed in the vertical direction. Furthermore, the position and shape of the region A1 where the FPC 2 and the piezoelectric film 10 overlap one another when viewed in the vertical direction do not necessarily have to match the position and shape of the first electrode 3 when viewed in the vertical direction.

[0028] 4, in this embodiment, the shielding tape 11 overlaps the entire piezoelectric film 10 when viewed in the vertical direction. Note that the shielding tape 11 may overlap only a portion of the piezoelectric film 10 when viewed in the vertical direction.

[0029] 5, in this embodiment, the shielding film 7 overlaps the entire piezoelectric film 10 when viewed in the vertical direction. Note that the shielding film 7 may overlap only a portion of the piezoelectric film 10 when viewed in the vertical direction.

[0030] The inventors of the present application performed two types of simulations (hereinafter referred to as the first simulation and the second simulation) for the piezoelectric sensor 1 and the piezoelectric sensor 50 according to the comparative example to confirm changes in sensitivity of the piezoelectric sensor to deformation due to variations in the thickness of the FPC 2. First, the piezoelectric sensor 50 according to the comparative example will be described with reference to the drawings. Fig. 6 is a perspective view of the piezoelectric sensor 50 according to the comparative example.

[0031] 6 , the piezoelectric sensor 50 includes an FPC 2, a first electrode 3, a second electrode 4, a third electrode 5, a coverlay 6, double-sided tape 8, hot melt 9, a piezoelectric film 10, and a shielding tape 11. The configuration of the piezoelectric sensor 50 conforms to the configuration of the piezoelectric sensor 20 described in Patent Document 1 (WO 2016 / 194690). The configuration above the FPC 2 in the piezoelectric sensor 50 is the same as the configuration above the FPC 2 in the piezoelectric sensor 1, and therefore a description thereof will be omitted.

[0032] The piezoelectric sensor 50 does not include a shielding film 7. In the piezoelectric sensor 50, the second electrode 4 covers substantially the entire lower main surface DS2 of the FPC 2. The coverlay 6 covers substantially the entire lower surface of the second electrode 4. The double-sided tape 8 covers substantially the entire lower surface of the coverlay 6.

[0033] FIG. 7 is a cross-sectional view of the piezoelectric sensor 50. As shown in FIGS. 6 and 7 , in the piezoelectric sensor 50, in an area A1 where the FPC 2 and the piezoelectric film 10 overlap when viewed in the vertical direction, the first electrode 3 and the second electrode 4 overlap each other when viewed in the vertical direction. Also, in the piezoelectric sensor 50, the second electrode 4 and the third electrode 5 overlap each other when viewed in the vertical direction. FIG. 8 is a cross-sectional view of the piezoelectric sensor 1. On the other hand, as shown in FIGS. 3 and 8 , in the piezoelectric sensor 1, in an area A1 where the FPC 2 and the piezoelectric film 10 overlap each other when viewed in the vertical direction, the first electrode 3, the second electrode 4, and the third electrode 5 do not overlap each other when viewed in the vertical direction.

[0034] Next, a first simulation and a second simulation will be described with reference to the drawings. The first simulation is a simulation to confirm the change in sensitivity to deformation of the piezoelectric sensor 50 and the piezoelectric sensor 1 due to variations in the thickness of the FPC 2. The second simulation is a simulation to confirm the effect of the thickness of the double-sided tape 8 on the piezoelectric sensor 1.

[0035] First, the first simulation will be described with reference to the drawings. Fig. 9 is a schematic cross-sectional view of the piezoelectric sensor 50 and the piezoelectric sensor 1 in the first simulation. Fig. 10 is a diagram showing the amount of strain of each member in the first simulation of the piezoelectric sensor 50. Fig. 11 is a diagram showing the amount of strain of each member in the first simulation of the piezoelectric sensor 1. The horizontal axes in Figs. 10 and 11 each represent the distance x [mm] from the starting point O. The vertical axes in Figs. 10 and 11 each represent the amount of strain of each member.

[0036] As shown in FIG. 9 , in the first simulation, a plate 60 having a first main surface S1 and a second main surface S2 was prepared. The plate 60 was the measurement object. The plate 60 was made of SUS (Steel Use Stainless Steel). Next, either the piezoelectric sensor 1 or the piezoelectric sensor 50 was attached to the center of the second main surface S2 of the plate 60 using double-sided tape 8. Next, the left and right ends of the plate 60 were supported by support members 70. Then, the center (starting point O) of the first main surface S1 was pressed, and the change in sensitivity to deformation due to variations in the thickness of each component was confirmed. The pressed point (starting point O) and the second electrode 4 did not overlap each other when viewed vertically. In the first simulation, the thickness of the plate 60 was 1 mm. The thickness of the double-sided tape 8 was 100 μm.

[0037] 10 and 11 , in regions where the strain amount is negative, each member is compressed. Furthermore, in regions where the strain amount is positive, each member is stretched. When the strain amount is 0, each member is neither compressed nor stretched. As shown in FIG. 10 , for example, in the case of the plate 60 in the first simulation of the piezoelectric sensor 50, the plate 60 was compressed in the range where the distance x from the starting point O was longer than 0.0 mm and shorter than approximately 0.46 mm. Furthermore, the plate 60 was stretched in the range where the distance x from the starting point O was longer than approximately 0.46 mm and shorter than 1.0 mm. Furthermore, the plate 60 was neither compressed nor stretched at a point where the distance x from the starting point O was approximately 0.46 mm. In other words, when the center (starting point O) of the first main surface S1 is pressed, the upper part of the plate 60 (in the range where the distance x from the starting point O is longer than 0.0 mm and shorter than approximately 0.46 mm) is compressed, the lower part of the plate 60 (in the range where the distance x from the starting point O is longer than 0.0 mm and shorter than approximately 0.46 mm) is stretched, and the interface between the upper and lower parts of the plate 60 (at the point where the distance x from the starting point O is approximately 0.46 mm) is neither compressed nor stretched. In this case, the interface between the upper and lower parts of the plate 60 (at the point where the distance x from the starting point O is approximately 0.46 mm) is a neutral plane of stress.

[0038] In the first simulation of the piezoelectric sensor 50, the neutral plane of stress was located at the plate 60, the double-sided tape 8, and the FPC 2. The coverlay 6 and the second electrode 4 were each entirely compressed. The first electrode 3, the hot melt 9, the piezoelectric film 10, and the shielding tape 11 were each entirely stretched.

[0039] 11 , in the first simulation of the piezoelectric sensor 1, the neutral plane of stress was located at the plate 60, the double-sided tape 8, and the first electrode 3. The shielding film 7 and the FPC 2 were each entirely compressed. The hot melt 9, the piezoelectric film 10, and the shielding tape 11 were each entirely stretched. Therefore, the FPC 2 in the piezoelectric sensor 1 was not located in the range from the piezoelectric film 10 to the neutral plane of stress immediately adjacent to the piezoelectric film 10 (the neutral plane of stress located at the first electrode 3).

[0040] 12 is a diagram showing the change in sensitivity of the piezoelectric sensor 50 and the piezoelectric sensor 1 with respect to a change in the thickness of each component of the piezoelectric sensor 50 and the piezoelectric sensor 1 in the first simulation. The vertical axis in FIG. 12 represents the change in sensitivity of the piezoelectric sensor 50 and the piezoelectric sensor 1 with respect to a change in the thickness of each component of the piezoelectric sensor 50 and the piezoelectric sensor 1. For example, if the sensitivity of the piezoelectric sensor 50 changes by 1% when the thickness of the double-sided tape 8 of the piezoelectric sensor 50 changes by 1%, the change in sensitivity of the piezoelectric sensor 50 with respect to the change in thickness of the double-sided tape 8 of the piezoelectric sensor 50 is 1. Furthermore, if the sensitivity of the piezoelectric sensor 50 changes by 0.5% when the thickness of the double-sided tape 8 of the piezoelectric sensor 50 changes by 1%, the change in sensitivity of the piezoelectric sensor 50 with respect to the change in thickness of the double-sided tape 8 of the piezoelectric sensor 50 is 0.5.

[0041] 12 , in the case of piezoelectric sensor 50, the change in sensitivity of piezoelectric sensor 50 in response to a change in the thickness of FPC 2 was approximately 0.2, whereas in the case of piezoelectric sensor 1, the change in sensitivity of piezoelectric sensor 1 in response to a change in the thickness of FPC 2 was approximately 0.07. In other words, the first simulation confirmed that, compared to piezoelectric sensor 50, piezoelectric sensor 1 is able to suppress the change in sensitivity in response to a change in the thickness of FPC 2.

[0042] In the piezoelectric sensor 1, in an area A1 where the FPC 2 and the piezoelectric film 10 overlap when viewed in the vertical direction, the first electrode 3, the second electrode 4, and the third electrode 5 do not overlap one another when viewed in the vertical direction. As a result, when the piezoelectric sensor 1 is pressed, the neutral plane of stress is not located on the FPC 2. Furthermore, when the piezoelectric sensor 1 is pressed, it is possible to prevent the FPC 2 from being located in the range from the piezoelectric film 10 to the neutral plane of stress immediately adjacent to the piezoelectric film 10 (the neutral plane of stress located on the first electrode 3). As a result, the piezoelectric sensor 1 can suppress changes in sensitivity to deformation due to variations in the thickness of the FPC 2.

[0043] Next, the second simulation will be described with reference to the drawings. FIG. 13 is an example of a diagram showing the amount of strain of each member in the second simulation of the piezoelectric sensor 1. Note that FIG. 13 shows the case where the thickness of the double-sided tape 8 is 20 μm. FIG. 14 is an example of a diagram showing the amount of strain of each member in the second simulation of the piezoelectric sensor 1. Note that FIG. 14 shows the case where the thickness of the double-sided tape 8 is 10 μm. The horizontal axes in FIGS. 13 and 14 each represent the distance x [mm] from the starting point O. The vertical axes in FIGS. 13 and 14 each represent the amount of strain of each member.

[0044] In the second simulation, the thickness of the plate 60 was 1 mm, as in the first simulation. In the second simulation, the thickness of the double-sided tape 8 was set to either 20 μm or 10 μm.

[0045] 13 , when the thickness of the double-sided tape 8 is set to 20 μm, the neutral plane of stress is located at the plate 60, the double-sided tape 8, and the first electrode 3. The shielding film 7 and the FPC 2 are each compressed as a whole. The hot melt 9, the piezoelectric film 10, and the shielding tape 11 are each stretched as a whole. Therefore, the FPC 2 in the piezoelectric sensor 1 is not located in the range from the piezoelectric film 10 to the neutral plane of stress immediately adjacent to the piezoelectric film 10 (the neutral plane of stress located at the first electrode 3).

[0046] 14 , even when the thickness of the double-sided tape 8 is set to 10 μm, the neutral plane of stress is located at each of the plate 60, the double-sided tape 8, and the first electrode 3. The shielding film 7 and the FPC 2 are each compressed as a whole. The hot melt 9, the piezoelectric film 10, and the shielding tape 11 are each stretched as a whole. Therefore, the FPC 2 in the piezoelectric sensor 1 is not located in the range from the piezoelectric film 10 to the neutral plane of stress immediately adjacent to the piezoelectric film 10 (the neutral plane of stress located at the first electrode 3).

[0047] However, for the first electrode 3, the neutral plane of stress is located near the bottom surface. If the thickness of the double-sided tape 8 is thinner than 10 μm, the neutral plane of stress will be located at the FPC 2, as in the piezoelectric sensor 50. Therefore, it is desirable that the thickness of the double-sided tape 8 be 10 μm or more. Even if the double-sided tape 8 is made thicker, the neutral plane of stress will be located at the first electrode 3, not the FPC 2. Therefore, even if the double-sided tape 8 is made thicker, the FPC 2 in the piezoelectric sensor 1 will not be located in the range from the piezoelectric film 10 to the neutral plane of stress immediately adjacent to the piezoelectric film 10 (the neutral plane of stress located at the first electrode 3). However, if the double-sided tape 8 is made thicker, the piezoelectric sensor 1 will also become thicker. Therefore, when the piezoelectric sensor 1 is incorporated into a device, it is desirable that the thickness of the double-sided tape 8 be 200 μm or less so as not to hinder the device from being made thinner.

[0048] In the piezoelectric sensor 1, the shielding tape 11 is electrically connected to the third electrode 5. This connects the shielding tape 11 to the ground potential. As a result, the piezoelectric sensor 1 can prevent noise caused by the potential difference generated by the piezoelectric film 10 from being transmitted to external devices, and prevent noise generated by external devices from being transmitted to the piezoelectric film 10. Furthermore, when viewed in the top-bottom direction, the shielding tape 11 overlaps the entire piezoelectric film 10. Therefore, the piezoelectric sensor 1 can further prevent noise caused by the potential difference generated by the piezoelectric film 10 from being transmitted to external devices, and prevent noise generated by external devices from being transmitted to the piezoelectric film 10.

[0049] In the piezoelectric sensor 1, when viewed from the top to bottom, the shielding film 7 overlaps the entire piezoelectric film 10. Therefore, the piezoelectric sensor 1 can further suppress the transmission of noise caused by the potential difference generated by the piezoelectric film 10 to external devices, and the transmission of noise generated by external devices to the piezoelectric film 10.

[0050] [First Modification] The configuration of a piezoelectric sensor 1a according to a first modification of the present invention will be described below with reference to the drawings. FIG. 15 is an exploded perspective view of the piezoelectric sensor 1a. FIG. 16 is a plan view of the piezoelectric sensor 1a. FIG. 17 is a cross-sectional view taken along the line A-A in FIG. 16. FIG. 18 is a cross-sectional view taken along the line B-B in FIG. 16. Note that, for the piezoelectric sensor 1a according to the first modification, only the differences from the piezoelectric sensor 1 according to the first embodiment will be described, and the rest will be omitted. Note that in FIGS. 17 and 18, the reference numerals for the FPC 2, first electrode 3, second electrode 4, third electrode 5, coverlay 6, shielding film 7, piezoelectric film 10, and shielding tape 11 have been omitted.

[0051] As shown in FIG. 15 , the piezoelectric sensor 1 a according to the first modification differs from the piezoelectric sensor 1 according to the first embodiment in that it includes a second adhesive member 12 instead of the double-sided tape 8 .

[0052] The second adhesive member 12 is flexible. The second adhesive member 12 is flat-film-shaped. The lower surface of the second adhesive member 12 is adhesive. The second adhesive member 12 is disposed on the upper main surface US11 of the shielding tape 11. When viewed in the vertical direction, the second adhesive member 12 has an area A2 that does not overlap with the FPC 2 and the piezoelectric film 10. In this modification, when viewed in the vertical direction, the area A2 that does not overlap with the FPC 2 and the piezoelectric film 10 has an annular shape. As shown in FIGS. 16 to 18 , the second adhesive member 12 is partially bent to cover the FPC 2, the first electrode 3, the second electrode 4, the third electrode 5, the coverlay 6, the shielding film 7, the piezoelectric film 10, and the shielding tape 11. As a result, in the piezoelectric sensor 1a, the second adhesive member 12 fixes the FPC 2, the first electrode 3, the second electrode 4, the third electrode 5, the coverlay 6, the shielding film 7, the piezoelectric film 10, and the shielding tape 11 to the object to be measured. When using the piezoelectric sensor 1a, the piezoelectric sensor 1a is attached to the object to be measured by bringing the lower surface of the second adhesive member 12 into close contact with the object to be measured. Note that the shape and arrangement of the second adhesive member according to the present invention are not limited to the shape and arrangement of the second adhesive member 12. In addition, the upper surface of the second adhesive member 12 may be adhesive.

[0053] The inventors of the present application conducted a simulation (hereinafter referred to as the third simulation) for the piezoelectric sensor 1a to confirm changes in sensitivity to deformation due to variations in the thickness of the FPC 2. The third simulation will be described below with reference to the drawings. FIG. 19 is a diagram showing the amount of strain of each component in the third simulation of the piezoelectric sensor 1a. The horizontal axis in FIG. 19 represents the distance x [mm] from the starting point O. The vertical axis in FIG. 19 represents the amount of strain of each component. The method for the third simulation was the same as that for the first simulation. However, in the third simulation, the piezoelectric sensor 1a was attached to the center of the second main surface S2 of the plate 60 with the second adhesive member 12.

[0054] 19 , in the piezoelectric sensor 1a, the neutral plane of stress is located at the plate 60 and the hot melt 9. The shielding film 7, the FPC 2, and the first electrode 3 are all compressed. The piezoelectric film 10 and the shielding tape 11 are all stretched. Therefore, in the piezoelectric sensor 1a, the FPC 2 is not located in the range from the piezoelectric film 10 to the neutral plane of stress immediately adjacent to the piezoelectric film 10 (the neutral plane of stress located at the hot melt 9).

[0055] In the piezoelectric sensor 1a, when the piezoelectric sensor 1a is pressed, the neutral plane of stress is not located at the FPC 2. Furthermore, when the piezoelectric sensor 1a is pressed, it is possible to prevent the FPC 2 from being located in the range from the piezoelectric film 10 to the neutral plane of stress immediately adjacent to the piezoelectric film 10 (the neutral plane of stress located at the hot melt 9). As a result, the piezoelectric sensor 1a can suppress changes in sensitivity to deformation due to variations in the thickness of the FPC 2.

[0056] Other Embodiments The piezoelectric sensor according to the present invention is not limited to the piezoelectric sensors 1 and 1a, and can be modified within the scope of the present invention. In addition, the structures of the piezoelectric sensors 1 and 1a may be combined arbitrarily.

[0057] When the pressed portion and the second electrode 4 overlap each other in a vertical view, the member on which the neutral plane of stress is located changes. Fig. 20 is a diagram showing the amount of strain of each member in a simulation of the piezoelectric sensor 1 when the pressed portion overlaps the second electrode 4 in a vertical view. Fig. 21 is a plan view showing the area where the second electrode 4 and the piezoelectric film 10 overlap each other in a top view. Fig. 21 shows a case where the area US10S of the upper principal surface US10 and the area DS10S of the lower principal surface DS10 are equal.

[0058] When viewed from the top-bottom direction, if the pressed area overlaps with the second electrode 4, the neutral plane of stress is located on the plate 60, the double-sided tape 8, and the FPC 2, as shown in Fig. 20. The shielding film 7, the coverlay 6, and the second electrode 4 are all compressed. The first electrode 3, the hot melt 9, the piezoelectric film 10, and the shielding tape 11 are all stretched. When viewed from the top-bottom direction, if the pressed area overlaps with the second electrode 4, the neutral plane of stress is located on the FPC 2, and therefore, it becomes difficult to suppress changes in sensitivity to deformation due to variations in the thickness of the FPC 2.

[0059] Here, the piezoelectric film 10 is positioned directly below the pressed area. In the piezoelectric sensors 1 and 1a, the second electrode 4 does not overlap with the pressed area because it does not overlap with the piezoelectric film 10 when viewed in the vertical direction. On the other hand, in the piezoelectric sensor 50 according to the comparative example, the second electrode 4 overlaps with the piezoelectric film 10 when viewed in the vertical direction, and therefore overlaps with the pressed area. Therefore, it is considered that the probability of suppressing changes in sensitivity in response to changes in the thickness of the FPC 2 changes depending on whether the area S of the region where the piezoelectric film 10 and the second electrode 4 overlap when viewed in the vertical direction is larger or smaller than the area US10S of the upper principal surface US10 or the area DS10S of the lower principal surface DS10 of the piezoelectric film 10. 21 , like the piezoelectric sensors 1 and 1a, when the area S of the overlapping region between the piezoelectric film 10 and the second electrode 4 when viewed in the vertical direction is 0 (when the area S of the overlapping region between the piezoelectric film 10 and the second electrode 4 when viewed in the vertical direction is 0% of the area US10S of the upper main surface US10 of the piezoelectric film 10 or the area DS10S of the lower main surface DS10 of the piezoelectric film 10), the probability of suppressing the change in sensitivity to a change in the thickness of the FPC 2 is highest. On the other hand, when the second electrode 4 overlaps with the entire piezoelectric film 10 when viewed in the vertical direction (when the area S of the overlapping region between the piezoelectric film 10 and the second electrode 4 when viewed in the vertical direction is 100% of the area US10S of the upper main surface US10 of the piezoelectric film 10 or the area DS10S of the lower main surface DS10 of the piezoelectric film 10), the probability of suppressing the change in sensitivity to a change in the thickness of the FPC 2 is lowest. The area S of the region where the piezoelectric film 10 and the second electrode 4 overlap when viewed in the vertical direction is preferably 0% to 50% of the area US10S of the upper main surface US10 or the area DS10S of the lower main surface DS10 of the piezoelectric film 10. Furthermore, the area S of the region where the piezoelectric film 10 and the second electrode 4 overlap when viewed in the vertical direction is preferably 0% to 10% of the area US10S of the upper main surface US10 or the area DS10S of the lower main surface DS10 of the piezoelectric film 10.

[0060] The present invention has the following configuration.

[0061] (1) A piezoelectric sensor comprising: an insulating substrate having a first upper main surface and a first lower main surface; a plurality of electrodes including a first electrode formed on at least the first upper main surface; and a piezoelectric film having a second upper main surface and a second lower main surface, the second lower main surface being arranged so that the second lower main surface faces the first electrode; wherein, in a planar view, in a region where the insulating substrate and the piezoelectric film overlap, the plurality of electrodes do not overlap each other in a planar view.

[0062] (2) The piezoelectric sensor according to (1), further comprising: a first adhesive member disposed between the second lower principal surface and the first electrode.

[0063] (3) The piezoelectric sensor according to (1) or (2), wherein the plurality of electrodes includes a second electrode formed on the first lower principal surface.

[0064] (4) The piezoelectric sensor according to (3), wherein an area of ​​a region where the piezoelectric film and the second electrode overlap in a plan view is 0% to 50% of an area of ​​the second upper principal surface or an area of ​​the second lower principal surface.

[0065] (5) The piezoelectric sensor according to (3), wherein an area of ​​a region where the piezoelectric film and the second electrode overlap in a plan view is 0% or more and 10% or less of an area of ​​the second upper principal surface or an area of ​​the second lower principal surface.

[0066] (6) The piezoelectric sensor according to any one of (3) to (5), wherein the plurality of electrodes includes a third electrode formed on the first upper main surface, and in a planar view, the third electrode does not overlap with the first electrode and the second electrode in a region where the insulating substrate and the piezoelectric film overlap.

[0067] (7) The piezoelectric sensor according to (6), further comprising a first conductive thin film member disposed on the second upper main surface, the first conductive thin film member being electrically connected to the third electrode.

[0068] (8) The piezoelectric sensor according to (7), wherein the first conductive thin film member overlaps the entire piezoelectric film in plan view.

[0069] (9) The piezoelectric sensor according to (7) or (8), further comprising a second adhesive member, wherein the first conductive thin film member has a third upper main surface and a third lower main surface, the second adhesive member is disposed on the third upper main surface, and in plan view, the second adhesive member has an area that does not overlap with the insulating substrate and the piezoelectric film.

[0070] (10) A piezoelectric sensor described in any of (1) to (9), further comprising a second conductive thin film member having a fourth upper main surface and a fourth lower main surface, the second conductive thin film member being arranged so that the fourth upper main surface faces the first lower main surface, and in a planar view, the second conductive thin film member overlaps the entire piezoelectric film.

[0071] (11) A piezoelectric sensor described in any one of (1) to (8), further comprising: a second conductive thin film member having a fourth upper main surface and a fourth lower main surface, the second conductive thin film member being arranged so that the fourth upper main surface faces the first lower main surface; and a third adhesive member provided on the fourth lower main surface, wherein the thickness of the third adhesive member is 10 μm or more and 200 μm or less.

[0072] (12) The piezoelectric sensor according to any one of (1) to (11), wherein the piezoelectric film contains polylactic acid stretched in at least one axial direction.

[0073] 1, 1a: Piezoelectric sensor 2: FPC 3: First electrode 4: Second electrode 5: Third electrode 6: Coverlay 7: Shielding film 8: Double-sided tape 9: Hot melt 10: Piezoelectric film 11: Shielding tape 12: Second adhesive member 60: Plate 70: Support member A1, A2: Area DS2, DS7, DS10, DS11: Lower main surface DS10S, S, US10S: Area O: Starting point OD: Uniaxial stretching direction S1: First main surface S2: Second main surface US2, US7, US10, US11: Upper main surface x: Distance

Claims

1. A piezoelectric sensor comprising: an insulating substrate having a first upper principal surface and a first lower principal surface; a plurality of electrodes including a first electrode formed on at least the first upper principal surface; and a piezoelectric film having a second upper principal surface and a second lower principal surface and arranged so that the second lower principal surface faces the first electrode; wherein in a planar view, in a region where the insulating substrate and the piezoelectric film overlap, the plurality of electrodes do not overlap each other in a planar view.

2. The piezoelectric sensor according to claim 1, further comprising a first adhesive member disposed between said second lower principal surface and said first electrode.

3. The piezoelectric sensor according to claim 1 or 2, wherein the plurality of electrodes includes a second electrode formed on the first lower main surface.

4. The piezoelectric sensor according to claim 3, wherein the area of ​​the region where the piezoelectric film and the second electrode overlap when viewed in a plane is greater than or equal to 0% and less than 50% of the area of ​​the second upper principal surface or the area of ​​the second lower principal surface.

5. The piezoelectric sensor according to claim 3, wherein the area of ​​the region where the piezoelectric film and the second electrode overlap when viewed in a plane is between 0% and 10% of the area of ​​the second upper principal surface or the area of ​​the second lower principal surface.

6. A piezoelectric sensor as described in any one of claims 3 to 5, wherein the plurality of electrodes includes a third electrode formed on the first upper main surface, and in a planar view, the third electrode does not overlap with the first electrode and the second electrode in a region where the insulating substrate and the piezoelectric film overlap.

7. The piezoelectric sensor according to claim 6, further comprising a first conductive thin film member disposed on said second upper main surface, said first conductive thin film member being electrically connected to said third electrode.

8. The piezoelectric sensor according to claim 7, wherein, in a plan view, the first conductive thin film member overlaps the entirety of the piezoelectric film.

9. A piezoelectric sensor as described in claim 7 or claim 8, further comprising a second adhesive member, the first conductive thin film member having a third upper main surface and a third lower main surface, the second adhesive member being disposed on the third upper main surface, and in a planar view, the second adhesive member having an area that does not overlap with the insulating substrate and the piezoelectric film.

10. A piezoelectric sensor as described in any one of claims 1 to 9, further comprising a second conductive thin film member having a fourth upper principal surface and a fourth lower principal surface, the second conductive thin film member being arranged so that the fourth upper principal surface faces the first lower principal surface, and in a planar view, the second conductive thin film member overlaps the entirety of the piezoelectric film.

11. A piezoelectric sensor as described in any one of claims 1 to 8, further comprising: a second conductive thin film member having a fourth upper principal surface and a fourth lower principal surface, the second conductive thin film member being arranged so that the fourth upper principal surface faces the first lower principal surface; and a third adhesive member provided on the fourth lower principal surface, wherein the thickness of the third adhesive member is 10 μm or more and 200 μm or less.

12. The piezoelectric sensor according to any one of claims 1 to 11, wherein the piezoelectric film contains polylactic acid stretched in at least one axial direction.

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