JP2024098234A - Pulse wave sensor - Google Patents

Abstract

To provide a pulse wave sensor excellent both in sensitivity and in rigidity.SOLUTION: A pulse wave sensor includes a housing having a cylindrical side wall part, a strain element with no opening, whose outer peripheral part of a first face is fixed to a strain element fixing face provided at one end of the side wall part, and a plurality of strain gauges each provided with a sensing part, which are provided closer to a central side than the outer peripheral part in the first face. The strain element fixing face includes an annular part and two protrusions protruding toward the central side of the first face from the inner edge of the annular part, and opposed to each other with the center of the first face in between. The plurality of strain gauges are positioned on a first virtual straight line, which is the shortest straight line passing through the center of the first face and connecting the opposed sides of the two protrusions, and include a first strain gauge and a second strain gauge disposed opposed to each other with the center of the first face in between on a side closer to the protrusions than the center of the first face. The pulse wave sensor detects a pulse wave on the basis of a change in output of the sensing part of the plurality of strain gauges involved in the deformation of the strain element.SELECTED DRAWING: Figure 6

Description

The present invention relates to a pulse wave sensor.

Pulse wave sensors are known that detect pulse waves generated when the heart pumps blood. One example is a pulse wave sensor that is provided with a pressure-receiving plate that is a strain-generating body supported so that it can flex under the action of an external force, and a piezoelectric conversion means that converts the flexure of the pressure-receiving plate into an electrical signal. In this pulse wave sensor, the flexible area of the pressure-receiving plate is formed in a dome shape that is a convex curved surface facing outward, and a pressure detection element is provided on the inner surface of the apex of the pressure-receiving plate as the piezoelectric conversion means (see, for example, Patent Document 1).

JP 2002-78689 A

Pulse wave sensors need to detect minute signals, so a thin strain element is used to ensure the necessary sensitivity, but durability is also required, so it must be compatible with rigidity. However, with conventional pulse wave sensors, the compatibility of sensitivity and rigidity has not been fully considered.

The present invention was made in consideration of the above points, and aims to provide a pulse wave sensor that combines sensitivity and rigidity.

This pulse wave sensor has a housing with a cylindrical side wall, a flexure body without an opening, the outer periphery of which is fixed to a flexure body fixing surface provided at one end of the side wall, and a plurality of strain gauges each having a sensing part provided on the first surface closer to the center than the outer periphery. The flexure body fixing surface includes an annular part and two protrusions that protrude from the inner edge of the annular part toward the center of the first surface and face each other across the center of the first surface. The plurality of strain gauges include a first strain gauge and a second strain gauge that are located on a first virtual line that passes through the center of the first surface and is the shortest line connecting the opposing sides of the two protrusions, and are arranged opposite each other across the center of the first surface on the side closer to the protrusion than the center of the first surface. The pulse wave sensor detects a change in output of the sensing part of the plurality of strain gauges due to deformation of the flexure body.

The disclosed technology makes it possible to provide a pulse wave sensor that combines sensitivity and rigidity.

FIG. 1 is a perspective view (part 1) illustrating a pulse wave sensor according to a first embodiment. FIG. 2 is a second perspective view illustrating the pulse wave sensor according to the first embodiment; 1 is an exploded perspective view illustrating a pulse wave sensor according to a first embodiment; 1 is a cross-sectional view illustrating a pulse wave sensor according to a first embodiment. 1 is a view of a flexure body fixing surface of a side wall portion as viewed from a direction perpendicular to the flexure body fixing surface. FIG. 3 is a view of a first surface of a strain body viewed from a direction perpendicular to the first surface. FIG. This is the first simulation result regarding the distribution of strain occurring in a strain body. This is the second simulation result regarding the distribution of strain generated in a strain body. FIG. 2 is a plan view illustrating a first strain gauge according to the first embodiment. FIG. 2 is a cross-sectional view (part 1) illustrating the first strain gauge according to the first embodiment. FIG. 2 is a cross-sectional view (part 2) illustrating the first strain gauge according to the first embodiment.

Below, a description of the embodiment of the invention will be given with reference to the drawings. In each drawing, the same components are given the same reference numerals, and duplicated explanations may be omitted.

First Embodiment
FIG. 1 is a perspective view (part 1) illustrating a pulse wave sensor according to a first embodiment. FIG. 2 is a perspective view (part 2) illustrating a pulse wave sensor according to a first embodiment. FIG. 3 is an exploded perspective view illustrating a pulse wave sensor according to a first embodiment. FIG. 4 is a cross-sectional view illustrating a pulse wave sensor according to a first embodiment. Note that FIGS. 1 and 4 show a case where the pulse wave sensor is arranged so that the strain generating body is located on the upper side. In contrast to this, FIGS. 2 and 3 show a case where the pulse wave sensor is arranged so that the strain generating body is located on the lower side.

Referring to Figures 1 to 4, the pulse wave sensor 1 has a housing 10, a strain body 40, a first strain gauge 100A, a second strain gauge 100B, a third strain gauge 100C, and a fourth strain gauge 100D.

In the pulse wave sensor 1, the housing 10 is a portion that holds the strain generating body 40. The housing 10 includes a cylindrical side wall portion 20 and a lid portion 30 that closes an opening on one side of the side wall portion 20. The side wall portion 20 is, for example, a hollow cylinder, but is not limited to this. The side of the housing 10 opposite the lid portion 30 is open. The side wall portion 20 and the lid portion 30 can be made of, for example, metal or resin. The side wall portion 20 and the lid portion 30 may be made of the same material or different materials. The side wall portion 20 or the lid portion 30 may be provided with a hole for wiring.

In the illustrated example, the side wall portion 20 has four screw holes 20x provided at approximately equal intervals in the circumferential direction. In addition, the outer periphery of the lid portion 30 has four recesses 30x in which the heads of the screws 50 are disposed, which are provided at positions corresponding to the screw holes 20x. The lid portion 30 is fixed to the side wall portion 20 by the four screws 50. Note that this is just one example, and the lid portion 30 may be fixed to the side wall portion 20 by other methods such as adhesive or welding. Alternatively, the lid portion 30 may be formed integrally with the side wall portion 20.

The flexure body 40 is fixed, for example, by an adhesive, so as to cover the opening of the housing 10. In other words, the flexure body 40 is fixed to the side wall 20 of the housing 10 on the side opposite the lid 30. For example, the entire outer periphery of the flexure body 40 is bonded to the flexure body fixing surface 22 of the side wall 20 by an adhesive. The flexure body 40 is made of, for example, a metal. Examples of metals constituting the flexure body 40 include stainless steel, phosphor bronze, and aluminum. The flexure body 40 may be formed using inorganic materials such as alumina, zirconia, and glass. The flexure body 40 may also be formed using resin materials such as polycarbonate and polyether ether ketone.

The flexure body 40 is flat and has no openings. If an opening such as a slit is provided in the flexure body, the rigidity of the flexure body decreases and the flexure body becomes more susceptible to plastic deformation. In contrast, since the flexure body 40 does not have any openings, the rigidity can be increased.

In addition, when providing openings such as slits in the flexure body, methods such as press punching, chemical etching, and laser processing are used, which leads to increased costs for the flexure body. In contrast, flexure body 40 does not have openings, making it possible to reduce costs.

In addition, because the flexure body comes into direct contact with human skin, if an opening such as a slit is provided in the flexure body, there is a risk that human sweat, body hair, etc. may enter the inside of the pulse wave sensor through the opening as contaminants. In contrast, because the flexure body 40 does not have an opening, it is possible to prevent contaminants from entering the inside of the pulse wave sensor 1.

The flexure body 40 is, for example, circular, but is not limited to this. The flexure body 40 may be, for example, elliptical or rectangular. In the following, an example in which the flexure body 40 is circular will be described.

The diameter of the flexure body 40 can be, for example, about 14 mm or more and 20 mm or less. The thickness t of the flexure body 40 is preferably 40 μm or more and 200 μm or less. If the thickness t of the flexure body 40 is thinner, the sensitivity will be higher but the rigidity will be lower, and if the thickness t of the flexure body 40 is thicker, the rigidity will be higher but the sensitivity will be lower. By setting the thickness t of the flexure body 40 to be 40 μm or more and 200 μm or less, it is possible to achieve both rigidity and sensitivity.

The flexure body 40 has a first surface 40m and a second surface 40n. The flexure body 40 is used so that the second surface 40n side exposed from the housing 10 contacts the subject's radial artery. The flexure body 40 is flexible, and when a load is applied to the flexure body 40 in response to the subject's pulse wave, the flexure body 40 elastically deforms in response to the magnitude of the load. As the flexure body 40 elastically deforms, the output of the sensing parts of the first strain gauge 100A, the second strain gauge 100B, the third strain gauge 100C, and the fourth strain gauge 100D changes, and the pulse wave sensor 1 can detect the pulse wave based on this change in output.

The flexure body 40 may have a load section 45 that protrudes from the second surface 40n, which is the surface that contacts the subject, to the opposite side of the housing 10. The load section 45 is provided, for example, at a position including the center of the flexure body 40. The load section 45 may be, for example, a circle having a diameter of about 1/10 to 2/10 of the diameter of the flexure body 40. It is preferable that the center of the load section 45 coincides with the center of the flexure body 40. The amount of protrusion of the load section 45 based on the second surface 40n of the flexure body 40 may be, for example, about 0.1 mm. By providing the load section 45 that protrudes from the second surface 40n on the flexure body 40, it is possible to easily transmit a load corresponding to the subject's pulse wave to the flexure body 40.

Four strain gauges, a first strain gauge 100A, a second strain gauge 100B, a third strain gauge 100C, and a fourth strain gauge 100D, are provided on a first surface 40m of the strain body 40. Because the strain body 40 is flat, the strain gauges can be easily attached. By providing four strain gauges on the strain body 40, the strain of the strain body 40 can be detected by a full bridge.

Now, with reference to Figures 5 and 6, the positional relationship between the side wall portion 20 of the housing 10, the strain body 40, and the first strain gauge 100A, the second strain gauge 100B, the third strain gauge 100C, and the fourth strain gauge 100D will be described in detail.

Figure 5 is a view of the flexure body fixing surface of the side wall portion viewed from a direction perpendicular to the flexure body fixing surface. As shown in Figure 5, one end of the side wall portion 20 is provided with an outer peripheral protrusion 21 and a flexure body fixing surface 22 located inside the outer peripheral protrusion 21. A step is formed between the inner wall surface of the outer peripheral protrusion 21 and the flexure body fixing surface 22, and this step can be used for positioning when fixing the outer periphery of the first surface 40m of the flexure body 40 to the flexure body fixing surface 22. 40c indicates the position of the center of the first surface 40m when the flexure body 40 is fixed to the flexure body fixing surface 22.

The strain generating body fixing surface 22 includes an annular portion 23 of a substantially constant width, and protrusions 24a to 24d that protrude from the inner edge of the annular portion 23 toward the center 40c of the first surface 40m. The width of the annular portion 23 can be, for example, about 0.5 mm to 1.5 mm. Note that in FIG. 5, for convenience, the boundaries between the annular portion 23 and the protrusions 24a to 24d are shown by dashed lines, but the annular portion 23 and the protrusions 24a to 24d are formed integrally.

Protrusions 24a and 24c face each other across center 40c of first surface 40m. Protrusions 24b and 24d face each other across center 40c of first surface 40m. Protrusions 24a to 24d are preferably provided at approximately equal intervals along the inner edge of annular portion 23.

The width w of the protrusions 24a to 24d can be, for example, about 2 mm to 3 mm. The width w of the protrusions 24a to 24d is defined as the average value of the width of the protrusions measured perpendicularly to each imaginary line at each position on the extension of the first imaginary line V1 or the second imaginary line V2 shown in FIG. 6 and described below. It is preferable that the width w of each of the protrusions 24a to 24d is equal. Here, "equal" means that the width w of each of the protrusions 24a to 24d is within a range of ±10% of the average value of the width w of each of the protrusions 24a to 24d.

The projection amount p of the projections 24a to 24d based on the inner edge of the annular portion 23 can be, for example, about 0.5 mm to 1.5 mm. If the distance between the opposing sides of the opposing projections is too short, the radial strain of the strain body 40 will decrease, so the projection amount p is intended to be the minimum necessary. The projection amount p is defined as a position on an extension line of the first imaginary line V1 or the second imaginary line V2 shown in FIG. 6 and described below. It is preferable that the projection amounts p of the projections 24a to 24d are equal. Here, "equal" means that the projection amount p of each of the projections 24a to 24d is within a range of ±10% of the average value of the projection amounts p of each of the projections 24a to 24d.

Figure 6 is a view of the first surface of the flexure body viewed from a direction perpendicular to the first surface. In Figure 6, the outer periphery of the first surface 40m of the flexure body 40 that is fixed to the flexure body fixing surface 22 is shown by a dashed line. In other words, the outer periphery of the first surface 40m of the flexure body 40 is outside the dashed line in Figure 6. Also, in Figure 6, the code for the corresponding position on the flexure body fixing surface 22 is shown by a dashed line to show the positional relationship with the flexure body fixing surface 22 on the first surface 40m of the flexure body 40.

As shown in FIG. 6, on the first surface 40m of the strain body 40, four strain gauges, a first strain gauge 100A, a second strain gauge 100B, a third strain gauge 100C, and a fourth strain gauge 100D, are provided closer to the center 40c than the outer periphery.

In FIG. 6, the shortest line that passes through the center 40c of the first surface 40m and connects the opposing sides of the protrusions 24a and 24c is defined as a first virtual line V1. In addition, the line that passes through the center 40c of the first surface 40m and is perpendicular to the first virtual line V1 is defined as a second virtual line V2.

The first strain gauge 100A and the second strain gauge 100B are located on the first virtual line V1 and are arranged opposite each other across the center 40c of the first surface 40m, closer to the protrusions 24a and 24c than the center 40c of the first surface 40m.

In FIG. 6, 130R indicates the sensitive area where the resistor is formed in each strain gauge. The first virtual line V1 preferably passes through the sensitive area 130R of the first strain gauge 100A and the sensitive area 130R of the second strain gauge 100B. As will be seen from the simulation results described later, the side of the first virtual line V1 closer to the protrusions 24a and 24c than the center 40c of the first surface 40m is an area where large strain occurs. Therefore, by arranging the sensitive areas 130R of the first strain gauge 100A and the second strain gauge 100B in such an area, the strain detection sensitivity of the first strain gauge 100A and the second strain gauge 100B can be improved.

It is more preferable that the first virtual straight line V1 passes near the center of the sensitive area 130R of the first strain gauge 100A and near the center of the sensitive area 130R of the second strain gauge 100B. This can further improve the strain detection sensitivity of the first strain gauge 100A and the second strain gauge 100B. Note that the vicinity of the center of the sensitive area 130R is within a radius of 50 μm from the center of the sensitive area 130R.

The third strain gauge 100C and the fourth strain gauge 100D are preferably located on the second virtual straight line V2. The third strain gauge 100C and the fourth strain gauge 100D are preferably arranged opposite each other across the center 40c of the first surface 40m on the side closer to the center 40c of the first surface 40m than the first strain gauge 100A and the second strain gauge 100B. The protrusions 24b and 24d are preferably located on the second virtual straight line V2.

The second virtual line V2 preferably passes through the sensitive area 130R of the third strain gauge 100C and the sensitive area 130R of the fourth strain gauge 100D. As will be seen from the simulation results described below, the side of the second virtual line V2 closer to the center 40c of the first surface 40m is an area where large strain occurs. Therefore, by arranging the sensitive areas 130R of the third strain gauge 100C and the fourth strain gauge 100D in such an area, the strain detection sensitivity of the third strain gauge 100C and the fourth strain gauge 100D can be improved.

It is more preferable that the second virtual line V2 passes near the center of the sensitive area 130R of the third strain gauge 100C and near the center of the sensitive area 130R of the fourth strain gauge 100D. This can further improve the strain detection sensitivity of the third strain gauge 100C and the fourth strain gauge 100D.

Figure 7 shows the results of a simulation (part 1) regarding the distribution of strain occurring in the flexure body. Figure 7 shows the magnitude of strain in the flexure body 40 when a load is applied to the load portion 45 of the flexure body 40 in the direction of the thin arrow in the pulse wave sensor 1 having the shape shown in Figures 1 to 6. In this simulation, the material of the flexure body 40 is SUS304, the diameter of the flexure body 40 is 17 mm, and the thickness t of the flexure body 40 is 100 μm.

In FIG. 7, the four thick arrows indicate the positions of the four protrusions on the side wall portion's strain body fixing surface. Also, in FIG. 7, when displayed in black and white, the lighter the gray or white the area, the greater the strain. That is, in FIG. 7, large strain occurs near each protrusion and on the outer periphery of the load portion 45. From FIG. 7, it can be seen that when a load is applied to the load portion 45, the four protrusions indicated by the thick arrows become fulcrums, and large strain (tensile stress) occurs near the four protrusions. It can also be seen that large strain (compressive stress) occurs on the outer periphery of the load portion 45. In contrast, no large strain occurs other than near the four protrusions indicated by the thick arrows and on the outer periphery of the load portion 45.

Figure 8 shows the results of a simulation (part 2) regarding the distribution of strain generated in the flexure body. Figure 8 shows the results of a simulation under the same conditions as Figure 7 for a pulse wave sensor (comparative example) having the same shape as pulse wave sensor 1 shown in Figures 1 to 6, except that the four protrusions are not provided on the flexure body fixing surface of the side wall. Also, as with Figure 7, Figure 8 shows that when displayed in black and white, the areas closer to light gray and white have larger strains. Figure 8 shows that in the absence of protrusions that serve as fulcrums, strain is generated approximately evenly on the outer periphery of flexure body 40. The magnitude of this strain is approximately 20% smaller than the magnitude of strain generated near the four protrusions in Figure 7.

From the results of Figures 7 and 8, it was found that by providing four protrusions on the flexure body fixing surface of the side wall, a larger strain is generated near the protrusions of the flexure body 40 than when no protrusions are provided. In other words, it was found that the proportion of radial strain of the flexure body 40 can be increased and the proportion of circumferential strain can be reduced near the protrusions of the flexure body 40. From this, it can be said that the strain detection sensitivity can be improved by placing the first strain gauge 100A and the second strain gauge 100B at the positions shown in Figure 6. In addition, from the results of Figure 7, since the outer periphery of the load portion 45 of the flexure body 40 is an area where a large strain is generated, it can be said that the strain detection sensitivity can be improved by placing the third strain gauge 100C and the fourth strain gauge 100D at the positions shown in Figure 6.

In other words, with the arrangement shown in FIG. 6, tensile stress can be effectively detected by the first strain gauge 100A and the second strain gauge 100B, and compressive stress can be effectively detected by the third strain gauge 100C and the fourth strain gauge 100D. As a result, a large voltage output can be obtained by forming a full bridge circuit with the first strain gauge 100A, the second strain gauge 100B, the third strain gauge 100C, and the fourth strain gauge 100D.

In this way, by arranging four strain gauges at the positions shown in FIG. 6 on the first surface 40m of the strain body 40, which does not have an opening, a pulse wave sensor 1 that combines sensitivity and rigidity can be realized.

In FIG. 6, no strain gauges are placed near the protrusions 24b and 24d. Therefore, it is possible to provide only the protrusions 24a and 24c on the strain body fixing surface 22 of the side wall 20, without providing the protrusions 24b and 24d. In this case, too, a large strain is generated at the position where the strain gauge is placed in FIG. 6, so a highly sensitive pulse wave sensor can be realized.

However, distortion may occur when the flexure body 40 is bonded to the flexure body fixing surface 22 of the side wall portion 20. The distortion caused by bonding is not the distortion that should be measured, and becomes a noise component. If such noise components are a problem, the distortion caused by bonding can be reduced by evenly arranging the protrusions 24a to 24d. In this case, it is preferable to make the widths of the protrusions 24a to 24d all the same, and to make the protrusion amounts of the protrusions 24a to 24d all the same. This allows the flexure body 40 to be bonded to the flexure body fixing surface 22 of the side wall portion 20 in a balanced manner, further reducing the distortion caused by bonding.

Here, we will explain the first strain gauge 100A.

Figure 9 is a plan view illustrating the first strain gauge according to the first embodiment. Figure 10 is a cross-sectional view (part 1) illustrating the first strain gauge according to the first embodiment, showing a cross section along line A-A in Figure 9. Figure 11 is a cross-sectional view (part 2) illustrating the first strain gauge according to the first embodiment, showing a cross section corresponding to Figure 10.

Note that, although the following describes the first strain gauge 100A, the second strain gauge 100B, the third strain gauge 100C, and the fourth strain gauge 100D can also have a similar structure to the first strain gauge 100A. However, each strain gauge may have a partially different structure as needed. For example, the size of the substrate, the presence or absence of a cover layer, and other specifications may be changed as needed.

Referring to Figures 9 to 11, the first strain gauge 100A has a substrate 110, a resistor 130, wiring 140, electrodes 150, and a cover layer 160. For convenience, only the outer edge of the cover layer 160 is shown by a dashed line. The cover layer 160 may be provided as needed.

9 to 11, for the sake of convenience, the side of the first strain gauge 100A on which the resistor 130 of the substrate 110 is provided is referred to as the "upper side," and the side on which the resistor 130 is not provided is referred to as the "lower side." The surface located on the upper side of each part is referred to as the "upper surface," and the surface located on the lower side of each part is referred to as the "lower surface." However, the first strain gauge 100A can also be used upside down. The first strain gauge 100A can also be placed at any angle. The planar view refers to viewing the object in the normal direction from the top to the bottom relative to the upper surface 110a of the substrate 110. The planar shape refers to the shape of the object when viewed in the normal direction.

The substrate 110 is a member that serves as a base layer for forming the resistor 130 and the like. The substrate 110 is flexible. The thickness of the substrate 110 is not particularly limited and may be appropriately determined depending on the intended use of the first strain gauge 100A. For example, the thickness of the substrate 110 may be about 5 μm to 500 μm. The lower surface side of the first strain gauge 100A is joined to the strain body 40 via an adhesive layer or the like. From the viewpoint of the transferability of strain from the strain body 40 to the sensing part and dimensional stability against environmental changes, the thickness of the substrate 110 is preferably within the range of 5 μm to 200 μm. From the viewpoint of insulation, the thickness of the substrate 110 is preferably 10 μm or more.

The substrate 110 can be formed from an insulating resin film such as PI (polyimide) resin, epoxy resin, PEEK (polyether ether ketone) resin, PEN (polyethylene naphthalate) resin, PET (polyethylene terephthalate) resin, PPS (polyphenylene sulfide) resin, LCP (liquid crystal polymer) resin, polyolefin resin, etc. Note that a film refers to a flexible material with a thickness of about 500 μm or less.

When the substrate 110 is formed from an insulating resin film, the insulating resin film may contain fillers, impurities, etc. For example, the substrate 110 may be formed from an insulating resin film containing a filler such as silica or alumina.

Examples of materials other than resin for the base material 110 include crystalline materials such as SiO ₂ , ZrO ₂ (including YSZ), Si, Si ₂ N ₃ , Al ₂ O ₃ (including sapphire), ZnO, and perovskite ceramics (CaTiO ₃ , BaTiO ₃ ). In addition to the above-mentioned crystalline materials, amorphous glass or the like may be used as the material for the base material 110. Metals such as aluminum, aluminum alloys (duralumin), and titanium may also be used as the material for the base material 110. When a metal is used, an insulating film is provided on the metallic base material 110.

The resistor 130 is a thin film formed in a predetermined pattern on the upper side of the substrate 110. In the first strain gauge 100A, the resistor 130 is a sensing part that receives strain and produces a resistance change. The resistor 130 may be formed directly on the upper surface 110a of the substrate 110, or may be formed on the upper surface 110a of the substrate 110 via another layer.

The resistor 130 has a structure in which multiple elongated portions are arranged at regular intervals with their longitudinal direction in the same direction (the direction of line A-A in FIG. 9), and the ends of adjacent elongated portions are alternately connected, folding back in a zigzag pattern as a whole. That is, the resistor 130 includes multiple elongated portions arranged in parallel, and folded back portions that connect the ends of adjacent elongated portions. The longitudinal direction of the multiple elongated portions is the grid direction, and the direction perpendicular to the grid direction is the grid width direction (the direction perpendicular to line A-A in FIG. 9).

One end in the longitudinal direction of the two elongated portions located at the outermost sides in the grid width direction is bent in the grid width direction to form respective ends _130e1 and _130e2 in the grid width direction of the resistor 130. The respective ends _130e1 and _130e2 in the grid width direction of the resistor 130 are electrically connected to the electrodes 150 via the wiring 140. In other words, the wiring 140 electrically connects the respective ends _130e1 and _130e2 in the grid width direction of the resistor 130 to the respective electrodes 150.

The resistor 130 can be formed from, for example, a material containing Cr (chromium), a material containing Ni (nickel), or a material containing both Cr and Ni. That is, the resistor 130 can be formed from a material containing at least one of Cr and Ni. An example of a material containing Cr is a Cr mixed phase film. An example of a material containing Ni is Cu-Ni (copper-nickel). An example of a material containing both Cr and Ni is Ni-Cr (nickel-chromium).

Here, the Cr mixed phase film is a film in which Cr, CrN, Cr ₂ N, etc. are mixed together. The Cr mixed phase film may contain inevitable impurities such as chromium oxide.

The thickness of the resistor 130 is not particularly limited and may be appropriately determined depending on the intended use of the first strain gauge 100A. For example, the thickness of the resistor 130 may be about 0.05 μm to 2 μm. In particular, when the thickness of the resistor 130 is 0.1 μm or more, the crystallinity of the crystals constituting the resistor 130 (for example, the crystallinity of α-Cr) is improved. Furthermore, when the thickness of the resistor 130 is 1 μm or less, (i) film cracks and (ii) warping of the film from the substrate 110 caused by the internal stress of the film constituting the resistor 130 are reduced. The width of the elongated portion 31 can be optimized for the required specifications such as resistance value and lateral sensitivity, and can be set to, for example, about 10 μm to 100 μm, taking into consideration measures against breakage.

For example, when the resistor 130 is a Cr mixed-phase film, the stability of the gauge characteristics can be improved by making the resistor 130 mainly composed of α-Cr (alpha chromium), which is a stable crystal phase. Also, for example, when the resistor 130 is a Cr mixed-phase film, the resistor 130 mainly composed of α-Cr can make the gauge factor of the first strain gauge 100A 10 or more, and the gauge factor temperature coefficient TCS and the resistance temperature coefficient TCR within the range of -1000 ppm/°C to +1000 ppm/°C. Here, the "main component" means a component that occupies 50% by weight or more of the total material that constitutes the resistor 130. From the viewpoint of improving the gauge characteristics, it is preferable that the resistor 130 contains 80% by weight or more of α-Cr. Furthermore, from the same viewpoint, it is more preferable that the resistor 130 contains 90% by weight or more of α-Cr. Note that α-Cr is Cr with a bcc structure (body-centered cubic lattice structure).

Furthermore, when the resistor 130 is a Cr mixed-phase film, the Cr mixed-phase film preferably contains 20% by weight or less of CrN and Cr ₂ N. By containing the Cr mixed-phase film with 20% by weight or less of CrN and Cr ₂ N, a decrease in the gauge factor can be suppressed.

In addition, the ratio of CrN and Cr ₂ N in the Cr mixed phase film is preferably such that the ratio of Cr ₂ N is 80% by weight or more and less than 90% by weight with respect to the total weight of CrN and Cr _{2 N. More specifically, it is more preferable that the ratio of Cr 2 N} is 90% by weight or more and less than 95% by weight with respect to the total weight of CrN and Cr ₂ N. Cr ₂ N has a semiconductor property. Therefore, by setting the ratio of Cr ₂ N to 90% by weight or more and less than 95% by weight, the decrease in TCR (negative TCR) becomes more significant. Furthermore, by setting the ratio of Cr ₂ N to 90% by weight or more and less than 95% by weight, the ceramicization of the resistor 130 can be reduced, and the brittle fracture of the resistor 130 can be made less likely to occur.

On the other hand, CrN has the advantage of being chemically stable. By including more CrN in the Cr mixed-phase film, the possibility of unstable N being generated can be reduced, and a stable strain gauge can be obtained. Here, "unstable N" means a trace amount of _N2 or atomic N that may be present in the Cr mixed-phase film. These unstable N may escape to the outside of the film depending on the external environment (e.g., high temperature environment). When unstable N escapes to the outside of the film, the film stress of the Cr mixed-phase film may change.

In the first strain gauge 100A, when a Cr mixed-phase film is used as the material of the resistor 130, high sensitivity and miniaturization can be achieved. For example, while the output of a conventional strain gauge was about 0.04 mV/2 V, when a Cr mixed-phase film is used as the material of the resistor 130, an output of 0.3 mV/2 V or more can be obtained. In addition, while the size (gauge length x gauge width) of a conventional strain gauge was about 3 mm x 3 mm, when a Cr mixed-phase film is used as the material of the resistor 130, the size (gauge length Lg x gauge width Wg) of the strain gauge can be miniaturized to about 0.3 mm x 0.3 mm.

The area determined by the gauge length Lg × gauge width Wg shown in FIG. 9 is the sensitive area 130R shown in FIG. 6. In FIG. 6, the first strain gauge 100A and the second strain gauge 100B are preferably arranged with their grid directions parallel to the first virtual line V1. Also, the third strain gauge 100C and the fourth strain gauge 100D are preferably arranged with their grid directions parallel to the second virtual line V2.

By arranging the grid directions of the first strain gauge 100A and the second strain gauge 100B in a direction parallel to the first virtual line V1, and arranging the grid directions of the third strain gauge 100C and the fourth strain gauge 100D in a direction parallel to the second virtual line V2, the radial strain of the strain body 40 can be effectively detected.

The width of the protrusions 24a and 24c is preferably equal to the gauge width Wg of the first strain gauge 100A and the second strain gauge 100B. This effectively reduces the circumferential strain of the strain body 40 while increasing the radial strain. In other words, when the strain body 40 is deformed, a large strain can be generated in the sensitive area of the first strain gauge 100A and the second strain gauge 100B. Here, the width of the protrusions 24a and 24c being equal to the gauge width Wg of the first strain gauge 100A and the second strain gauge 100B includes a range of ±10% of the width of the protrusions 24a and 24c with respect to the gauge width Wg of the first strain gauge 100A and the second strain gauge 100B.

The wiring 140 is provided on the substrate 110. One end of the wiring 140 is electrically connected to both ends of the resistor 130, and the other end is electrically connected to the electrode 150. The wiring 140 is not limited to being linear, and can be in any pattern. In addition, the wiring 140 can be any width and any length.

The electrodes 150 are provided on the substrate 110. The electrodes 150 are electrically connected to the resistor 130 via the wiring 140. In a plan view, the electrodes 150 are formed in a generally rectangular shape, wider than the wiring 140. The electrodes 150 are a pair of electrodes for outputting to the outside a change in the resistance value of the resistor 130 caused by distortion. For example, a lead wire for external connection is joined to the electrodes 150.

The cover layer 160 is formed on the substrate 110, covers the resistor 130 and the wiring 140, and exposes the electrode 150. A part of the wiring 140 may be exposed from the cover layer 160. By providing the cover layer 160 that covers the resistor 130 and the wiring 140, mechanical damage to the resistor 130 and the wiring 140 can be prevented. Furthermore, by providing the cover layer 160, the resistor 130 and the wiring 140 can be protected from moisture and the like. The cover layer 160 may be provided so as to cover the entire portion except for the electrode 150.

The cover layer 160 is provided on the upper surface 110a of the substrate 110 as necessary so as to cover the resistor 130 and the wiring 140 and expose the electrode 150. Examples of materials for the cover layer 160 include insulating resins such as PI resin, epoxy resin, PEEK resin, PEN resin, PET resin, PPS resin, and composite resins (e.g., silicone resin, polyolefin resin). The cover layer 160 may contain a filler or a pigment. The thickness of the cover layer 160 is not particularly limited and can be appropriately selected depending on the purpose. For example, the thickness of the cover layer 160 can be about 2 μm to 30 μm. By providing the cover layer 160, it is possible to suppress mechanical damage, etc., to the resistor 130. In addition, by providing the cover layer 160, it is possible to protect the resistor 130 from moisture, etc.

In order to stabilize the gauge characteristics, the first strain gauge 100A may also include a functional layer 120 formed to a predetermined thickness, for example, by conventional sputtering, on the upper surface 110a of the substrate 110 as a base layer.

In this application, the functional layer 120 refers to a layer that has the function of promoting the crystal growth of at least the upper layer, the resistor 130. The functional layer 120 preferably also has the function of preventing oxidation of the resistor 130 due to oxygen and moisture contained in the substrate 110, and the function of improving the adhesion between the substrate 110 and the resistor 130. The functional layer 120 may also have other functions.

The insulating resin film that constitutes the substrate 110 contains oxygen and moisture, and since Cr forms a self-oxidizing film, particularly when the resistor 130 contains Cr, it is effective for the functional layer 120 to have the function of preventing oxidation of the resistor 130.

The material of the functional layer 120 is not particularly limited as long as it has the function of promoting the crystal growth of at least the upper layer, the resistor 130, and can be appropriately selected according to the purpose. For example, Cr (chromium), Ti (titanium), V (vanadium), Nb (niobium), Ta (tantalum), Ni (nickel), Y (yttrium), Zr (zirconium), Hf (hafnium), Si (silicon), C (carbon), Zn (zinc), Cu (copper), Bi (bismuth), Fe (iron), Mo (molybdenum), W (tungsten), Ru (ruthenium), Rh (rhodium), Re (rhenium), Os (osmium), Ir (iridium), Pt (platinum), Pd (palladium), Ag (silver), Au (gold), Co (cobalt), Mn (manganese), Al (aluminum) One or more metals selected from the group consisting of, an alloy of any of the metals in this group, or a compound of any of the metals in this group can be mentioned.

The functional layer 120 can be formed in a vacuum by conventional sputtering, for example, using a raw material capable of forming the functional layer 120 as a target and introducing Ar (argon) gas into a chamber. By using conventional sputtering, the functional layer 120 is formed while etching the upper surface 110a of the substrate 110 with Ar, so that the amount of the functional layer 120 formed can be minimized and the effect of improving adhesion can be obtained.

However, this is just one example of a method for forming the functional layer 120, and the functional layer 120 may be formed by other methods. For example, a method may be used in which the upper surface 110a of the substrate 110 is activated by a plasma treatment using Ar or the like before forming the functional layer 120, thereby improving adhesion, and then the functional layer 120 is vacuum-formed by magnetron sputtering.

When a functional layer 120 is provided on the upper surface 110a of the substrate 110 as an underlayer for the resistor 130, wiring 140, and electrode 150, the first strain gauge 100A has a cross-sectional shape as shown in FIG. 11. When the functional layer 120 is provided, the planar shape of the first strain gauge 100A is, for example, similar to the resistor 130, wiring 140, and electrode 150 in FIG. 9. However, as described above, the functional layer 120 may be formed solidly on part or all of the upper surface 110a of the substrate 110.

Although the preferred embodiments have been described above in detail, the present invention is not limited to the above-described embodiments, and various modifications and substitutions can be made to the above-described embodiments without departing from the scope of the claims.

For example, in the above embodiment, an example was shown in which the first strain gauge, the second strain gauge, the third strain gauge, and the fourth strain gauge were provided on the strain body. However, only the first strain gauge and the second strain gauge may be provided on the strain body. In this case, the strain of the strain body can be detected by the half bridge.

In addition, in the above embodiment, a resistor is used as the sensing part of each strain gauge, but this is not limited to this. For example, a magnetic material may be used as the sensing part instead of a resistor. In this case, each strain gauge detects and outputs magnetic changes caused by strain occurring in the strain body.

1 Pulse wave sensor, 10 Housing, 20 Side wall portion, 20x Screw hole, 21 Outer peripheral protrusion portion, 22 Strain body fixing surface, 23 Annular portion, 24a, 24b, 24c, 24d Protrusion portion, 30 Lid portion, 30x Recessed portion, 40 Strain body, 40m First surface, 40n Second surface, 45 Load portion, 50 Screw, 100A First strain gauge, 100B Second strain gauge, 100C Third strain gauge, 100D Fourth strain gauge, 110 Base material, 110a Top surface, 130 Resistor, 130R Sensing area, 140 Wiring, 150 Electrode, 160 Cover layer, 130e ₁ , 130e ₂ Termination

Claims (11)

A housing having a cylindrical side wall portion;
a strain body having no opening, the outer periphery of a first surface being fixed to a strain body fixing surface provided at one end of the side wall portion;
a plurality of strain gauges each having a sensing portion, the strain gauges being provided on the first surface closer to the center than the outer circumferential portion;
the strain-generating body fixing surface includes an annular portion and two protruding portions that protrude from an inner edge of the annular portion toward a center of the first surface and face each other across the center of the first surface,
the plurality of strain gauges include a first strain gauge and a second strain gauge that are located on a first virtual line that passes through a center of the first surface and is the shortest line connecting opposing sides of the two protrusions, and are disposed opposite each other across the center of the first surface on a side closer to the protrusions than the center of the first surface,
A pulse wave sensor that detects a pulse wave based on a change in output of the sensing parts of the plurality of strain gauges that accompanies deformation of the strain body. the plurality of strain gauges include a third strain gauge and a fourth strain gauge positioned on a second imaginary line that passes through a center of the first surface and is perpendicular to the first imaginary line;
2. The pulse wave sensor according to claim 1, wherein the third strain gauge and the fourth strain gauge are arranged opposite each other across the center of the first surface on a side closer to the center of the first surface than the first strain gauge and the second strain gauge. the strain-generating body fixing surface includes two second protruding portions that protrude from an inner periphery of the annular portion toward a center of the first surface and face each other across the center of the first surface,
The pulse wave sensor according to claim 2 , wherein the two second protrusions are located on the second imaginary line. The two second protrusions and the two protrusions all have the same width,
The pulse wave sensor according to claim 3 , wherein the two second protrusions and the two protrusions have an equal protrusion amount. The pulse wave sensor according to any one of claims 2 to 4, wherein the first virtual straight line passes through the sensitive area of the first strain gauge and the sensitive area of the second strain gauge. The pulse wave sensor according to claim 5, wherein the first virtual straight line passes through the vicinity of the center of the sensitive area of the first strain gauge and the vicinity of the center of the sensitive area of the second strain gauge. The pulse wave sensor according to claim 5 or 6, wherein the second virtual straight line passes through the sensitive area of the third strain gauge and the sensitive area of the fourth strain gauge. The pulse wave sensor according to claim 7, wherein the second virtual line passes through the vicinity of the center of the sensitive area of the third strain gauge and the vicinity of the center of the sensitive area of the fourth strain gauge. The pulse wave sensor according to any one of claims 1 to 8, wherein the sensing part is a resistor. The resistors of the first strain gauge and the second strain gauge are arranged such that a grid direction is parallel to the first virtual line,
The pulse wave sensor of claim 9 , wherein the width of the protrusion is equal to the gauge width. The pulse wave sensor according to claim 9 or 10, wherein the resistor is a Cr mixed phase film.

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