JP2020148658A - Strain gauge and load cell - Google Patents

Abstract

To provide a load cell with a stable output by reducing variations in the output of a strain gauge due to moisture absorption of a base.SOLUTION: A strain gauge 17 is attached to a strain part 12, which is deformed by an applied load. The strain gauge 17 includes: an insulating base 18 attached to the strain part 12; and a strain resistor 19 attached to the base 18. The strain resistor 19 has an electric resistance value varied by the degree of deformation of the strain part 12. At least a part of the base 18 is made of a liquid crystal polymer.SELECTED DRAWING: Figure 4

Description

The present invention relates to a strain gauge for measuring strain generated in a strain generating portion and a load cell having a strain generating portion to which a strain gauge is attached.

Conventionally, a strain gauge having a strain resistor whose electric resistance value changes according to the amount of deformation of the surface of the strain-causing portion and an insulating base to which the strain resistor is attached is known. For example, Patent Document 1 (Japanese Unexamined Patent Publication No. 2015-64296) describes a load cell having a strain gauge having a thin-film base made of a polyimide resin, an epoxy resin, or the like, and a strain generating portion to which a strain gauge is attached. It is disclosed. In this load cell, in order to reduce unintentional fluctuations in the electrical resistance value of the strain resistor due to the base absorbing moisture and deforming, a strain gauge is provided in the recess formed on the surface of the strain generating portion. It has a structure in which the dent is covered with a cover.

However, even if the dent provided inside the strain gauge is covered with a cover or the like, moisture gradually infiltrates into the dent due to long-term use of the load cell, and the base is deformed by moisture absorption to output the strain gauge. May fluctuate.

One object of the present invention is to provide a load cell with a stable output by reducing fluctuations in the output of the strain gauge due to moisture absorption of the base.

The strain gauge according to the first aspect of the present invention is attached to a strain-causing portion that deforms when a load is applied. The strain gauge includes an insulating base attached to the strain generating portion and a strain resistor attached to the base. The electric resistance value of the strain resistor changes according to the degree of deformation of the strain generating portion. At least part of the base is made of liquid crystal polymer.

At least a part of the base of this strain gauge is made of a liquid crystal polymer having a small water absorption rate and a small amount of deformation due to absorption and release of water as compared with conventional materials such as polyimide resin. As a result, fluctuations in the electrical resistance value of the strain resistor due to deformation due to moisture absorption of the base are reduced, so fluctuations in the output of the strain gauge can be suppressed.

The strain gauge according to the second aspect of the present invention is the strain gauge according to the first aspect, and the base is a layer formed of a liquid crystal polymer, which is a layer parallel to a plane including a predetermined sensitivity direction. Including. The sensitivity direction is the direction in which the length of the strain resistor changes due to the deformation of the strain generating portion.

The strain gauge according to the third aspect of the present invention is the strain gauge according to the second aspect, and the base has one liquid crystal polymer layer formed of a liquid crystal polymer. The sensitivity direction is along the direction in which the linear expansion coefficient of the liquid crystal polymer layer is the lowest.

The strain gauge according to the fourth aspect of the present invention is the strain gauge according to the third aspect, and the sensitivity direction is along the longitudinal direction of the liquid crystal polymer molecules forming the liquid crystal polymer layer.

The strain gauge according to the fifth aspect of the present invention is the strain gauge according to the second aspect, and the base has a first liquid crystal polymer layer and a second liquid crystal polymer layer formed of a liquid crystal polymer. The first liquid crystal polymer layer and the second liquid crystal polymer layer each have an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption. The expansion direction of the first liquid crystal polymer layer and the expansion direction of the second liquid crystal polymer layer intersect.

の関係式を満たす場合、
角度θ_１は、θ_ＴＨ１−２０°からθ_ＴＨ１＋２０°までの範囲内の角度であり、
角度θ_２は、θ_ＴＨ２−２０°からθ_ＴＨ２＋２０°までの範囲内の角度であり、
角度θ_０を

の関係式から求められる角度とした場合、第１液晶ポリマー層の膨張方向と、第２液晶ポリマー層の膨張方向とが成す角度は、θ_０−２０°からθ_０＋２０°までの範囲内の角度である。上式において、第１液晶ポリマー層の膨張方向の線膨張率がＥ_１ａであり、第１液晶ポリマー層の収縮方向の線膨張率がＥ_１ｂであり、第１液晶ポリマー層の収縮方向と感度方向とが成す角度がθ_１であり、第１液晶ポリマー層の厚みがｔ_１であり、第２液晶ポリマー層の膨張方向の線膨張率がＥ_２ａであり、第２液晶ポリマー層の収縮方向の線膨張率がＥ_２ｂであり、第２液晶ポリマー層の収縮方向と感度方向とが成す角度がθ_２であり、第２液晶ポリマー層の厚みがｔ_２である。 The strain gauge according to the sixth aspect of the present invention is a strain gauge according to the fifth aspect.
Angle θ _TH1 and angle θ _TH2

If the relational expression of
The angle θ ₁ is an angle within the range from θ _TH1 −20 ° to θ _TH1 + 20 °.
The angle θ ₂ is an angle within the range from θ _TH2 −20 ° to θ _TH2 + 20 °.
Angle θ ₀

If the angle obtained from the equation, the expansion direction of the first liquid crystal polymer layer, the angle of the direction of expansion forms the second liquid crystal polymer layer, in the range of theta ₀ -20 ° to θ _{0 +} 20 ° The angle. In the above equation, the linear expansion coefficient in the expansion direction of the first liquid crystal polymer layer is E _1a , the linear expansion coefficient in the contraction direction of the first liquid crystal polymer layer is E _1b , and the contraction direction and sensitivity of the first liquid crystal polymer layer. The angle formed by the directions is θ ₁ , the thickness of the first liquid crystal polymer layer is t ₁ , the linear expansion coefficient in the expansion direction of the second liquid crystal polymer layer is E _2a , and the contraction direction of the second liquid crystal polymer layer. The coefficient of linear expansion of the second liquid crystal polymer layer is E _2b , the angle formed by the contraction direction and the sensitivity direction of the second liquid crystal polymer layer is θ ₂ , and the thickness of the second liquid crystal polymer layer is t ₂ .

The strain gauge according to the seventh aspect of the present invention is the strain gauge according to the second aspect, and the base has at least three liquid crystal polymer layers formed of the liquid crystal polymer. Each liquid crystal polymer layer has an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption. The expansion directions of the two liquid crystal polymer layers adjacent to each other intersect each other.

の関係式を満たす角度がθである場合、
液晶ポリマー層の膨張方向と、感度方向とが成す角度は、θ−２０°からθ＋２０°までの範囲内の角度である。上式において、液晶ポリマー層の膨張方向の線膨張率がＥ_１ａであり、液晶ポリマー層の収縮方向の線膨張率がＥ_１ｂであり、液晶ポリマー層の厚みがｔ_１であり、非液晶ポリマー層の線膨張率がＥ_２であり、非液晶ポリマー層の厚みがｔ_２である。 The strain gauge according to the eighth aspect of the present invention is a strain gauge according to the second aspect, and the base is a liquid crystal polymer layer formed of a liquid crystal polymer and a crystalline polymer or a non-crystalline polymer other than the liquid crystal polymer. It has a non-liquid crystal polymer layer formed of. The liquid crystal polymer layer has an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption. Also,

When the angle that satisfies the relational expression of
The angle formed by the expansion direction and the sensitivity direction of the liquid crystal polymer layer is an angle within the range of θ-20 ° to θ + 20 °. In the above equation, the linear expansion coefficient in the expansion direction of the liquid crystal polymer layer is E _1a , the linear expansion coefficient in the contraction direction of the liquid crystal polymer layer is E _1b , the thickness of the liquid crystal polymer layer is t ₁ , and the non-liquid crystal polymer. The coefficient of linear expansion of the layer is E ₂ , and the thickness of the non-liquid crystal polymer layer is t ₂ .

The strain gauge according to the ninth aspect of the present invention is a strain gauge according to any one of the first to eighth aspects, and further includes a cover member for covering the strain resistor.

The strain gauge according to the tenth aspect of the present invention is the strain gauge according to the ninth aspect, and at least a part of the cover member is made of a liquid crystal polymer.

The load cell according to the eleventh aspect of the present invention includes a strain gauge according to any one of the first to tenth aspects and a strain generating portion. The strain-causing portion is deformed when a strain gauge is attached and a load is applied. The strain gauge is attached to the surface of the strain-causing portion, which intersects the direction of deformation when a load is applied.

The strain gauge according to the present invention can reduce fluctuations in output due to moisture absorption of the base, and can provide a load cell with stable output.

It is a perspective view of the load cell 10 including the strain gauge which concerns on 1st Embodiment. It is a side view of the load cell 10. It is the schematic of an example of the scale 100 using the load cell 10. It is an external view of the strain gauge 17. It is a side view of the strain gauge 17. It is a side view of the strain gauge 17 attached to the strain measurement part of the upper beam part 15 of a strain raising part 12. It is a figure which showed typically the liquid crystal polymer molecule which forms the liquid crystal polymer layer. It is a side view of the strain gauge 17 in the 2nd Embodiment. It is a figure which showed typically the layered structure of the base 18. It is a figure for demonstrating the calculation method of the angle formed by the 1st expansion direction D1a and the 2nd expansion direction D2a. It is a side view of the strain gauge 17 in 3rd Embodiment. It is a figure which showed typically the layered structure of the base 18. It is a side view of the strain gauge 17 in 4th Embodiment.

An embodiment of the present invention will be described with reference to the drawings. The embodiments described below are one of the specific examples of the present invention and do not limit the technical scope of the present invention.

― First Embodiment ―
(1) Configuration of Load Cell FIG. 1 is a perspective view of a load cell 10 including a strain gauge 17 according to the first embodiment of the present invention. FIG. 2 is a side view of the load cell 10. The load cell 10 is mainly composed of a strain generating portion 12 and four strain gauges 17.

The strain generating portion 12 uses an aluminum alloy as a raw material. The aluminum alloy is an aluminum alloy (Al—Cu based alloy) in the 2000 series defined by the Japanese Industrial Standards (JIS).

As shown in FIG. 2, the strain generating portion 12 has a reverbal shape. The strain generating portion 12 has a fixed rigid body portion 13 and a movable rigid body portion 14. The fixed rigid body portion 13 and the movable rigid body portion 14 are arranged at both ends in the longitudinal direction of the strain generating portion 12.

The fixed rigid body portion 13 and the movable rigid body portion 14 are connected by two beam portions 15 and 16. The two beam portions 15 and 16 are composed of an upper beam portion 15 and a lower beam portion 16. As shown in FIG. 2, the upper beam portion 15 and the lower beam portion 16 are arranged side by side in the height direction orthogonal to the longitudinal direction of the strain generating portion 12 in the side view. The upper beam portion 15 connects the upper portion of the fixed rigid body portion 13 in the height direction and the upper portion of the movable rigid body portion 14 in the height direction. The lower beam portion 16 connects the lower portion of the fixed rigid body portion 13 in the height direction and the lower portion of the movable rigid body portion 14 in the height direction.

The upper surface of the fixed rigid body portion 13 and the upper surface of the movable rigid body portion 14 are at the same position in the height direction. The lower surface of the fixed rigid body portion 13 and the lower surface of the movable rigid body portion 14 are at the same position in the height direction. The upper surface of the upper beam portion 15 is located below the upper surfaces of the fixed rigid body portion 13 and the movable rigid body portion 14 in the height direction. The lower surface of the lower beam portion 16 is located above the lower surfaces of the fixed rigid body portion 13 and the movable rigid body portion 14 in the height direction.

The load cell 10 has holes penetrating both side surfaces in the width direction of the strain generating portion 12 at the central portion in the height direction and the central portion in the longitudinal direction of the strain generating portion 12. The width direction is a direction orthogonal to the longitudinal direction and the height direction. The hole of the load cell 10 is a space surrounded by a fixed rigid body portion 13, a movable rigid body portion 14, and two beam portions 15 and 16. The hole of the load cell 10 is composed of two notch portions 30 and a communication portion 31 connecting the two notch portions 30. The height dimension of the notch portion 30 is longer than the height dimension of the communication portion 31.

The upper beam portion 15 has two thin-walled portions 15a and one uniform-thickness portion 15b. The lower beam portion 16 has two thin-walled portions 16a and one uniform-thickness portion 16b.

The thin-walled portions 15a and 16a are portions of the beam portions 15 and 16 whose dimensions (thickness) in the height direction are smaller than those of the other portions. The thin-walled portions 15a and 16a are the portions having the lowest rigidity in the strain-causing portion 12. In the longitudinal direction of the strain-causing portion 12, the thin-walled portions 15a and 16a are at the same positions as the notch portion 30. The thicknesses of the thin portions 15a and 16a are the smallest at the center of the notch portion 30 in the longitudinal direction of the strain generating portion 12, and increase as the distance from the center of the notch portion 30 along the longitudinal direction of the straining portion 12.

The uniform thickness portions 15b and 16b are portions sandwiched between the two thin wall portions 15a and 16a and having a uniform thickness. The uniform thickness portions 15b and 16b are the thickest portions in the beam portions 15 and 16. In the longitudinal direction of the strain generating portion 12, the thickening portions 15b and 16b are at the same positions as the communicating portion 31.

Each of the two beam units 15 and 16 is provided with two strain measuring units. Specifically, two strain measuring units are provided on the upper surface of the upper beam unit 15, and two strain measuring units are provided on the lower surface of the lower beam unit 16. The strain measuring unit is provided at the same position as the thin-walled portions 15a and 16a or at a position near the thin-walled portions 15a and 16a in the longitudinal direction of the strain-causing portion 12. One strain gauge 17 is attached to each of the four strain measuring portions provided on the two beam portions 15 and 16.

(2) Example of Scale Using Load Cell The load cell 10 is used, for example, as one element constituting the scale. FIG. 3 is a schematic view of an example of a scale 100 using the load cell 10. The scale 100 measures the weight of the object to be measured G.

In the example shown in FIG. 3, the load cell 10 is attached with the base unit 21 and the saucer 22. The fixed rigid body portion 13 of the strain generating portion 12 is fixed to the base unit 21 by a screw or the like. A saucer 22 is attached to the movable rigid body portion 14 of the strain generating portion 12 by a screw or the like.

When the object to be measured G is placed on the saucer 22, the load P of the object to be measured G acts on the movable rigid body portion 14 of the strain generating portion 12, and the movable rigid body portion 14 is displaced downward. On the other hand, the fixed rigid body portion 13 fixed to the base unit 21 is not displaced. As a result, the strain generating portion 12 is deformed, the upper beam portion 15 extends in the longitudinal direction of the strain generating portion 12, and the lower beam portion 16 contracts. As a result, tensile strain is generated in the two strain measuring portions of the upper beam portion 15, and compression strain is generated in the two strain measuring portions of the lower beam portion 16.

The four strain gauges 17 of the load cell 10 are connected to a Wheatstone bridge. The strain gauge 17 can detect the strain due to the load P in each of the four strain measuring portions of the beam portions 15 and 16 by the change of its own electric resistance value. The load P is calculated from the strain detection results of the four strain gauges 17, and the weight of the object to be measured G is obtained.

(3) Configuration of Strain Gauge FIG. 4 is an external view of the strain gauge 17. FIG. 5 is a side view of the strain gauge 17. FIG. 6 is a side view of the strain gauge 17 attached to the strain measuring portion of the upper beam portion 15 of the strain generating portion 12.

The strain gauge 17 is mainly composed of a base 18 and a strain resistor 19. The base 18 is a thin film-shaped electrical insulator. At least part of the base 18 is made of liquid crystal polymer. The base 18 is attached to the strain measuring portions of the beam portions 15 and 16 with an adhesive or the like. The base 18 electrically insulates the strain resistor 19 and the strain generating portion 12.

The strain resistor 19 is an electrical resistor attached to the base 18. As shown in FIG. 4, for example, the strain resistor 19 is formed by forming a photo-etched metal foil or the like on the base 18 in a meandering shape. In FIGS. 4 to 6, the longitudinal direction of the strain resistor 19 formed in a meandering shape is indicated by an arrow L. The strain gauge 17 is attached to the strain generating portion 12 so that the longitudinal direction of the strain resistor 19 is along the longitudinal direction of the strain generating portion 12. When the strain-causing portion 12 is deformed by the load P, the beam portions 15 and 16 expand and contract in the longitudinal direction of the strain-causing portion 12. As a result, the strain resistor 19 expands and contracts in the longitudinal direction thereof. Hereinafter, the direction in which the strain resistor 19 expands and contracts due to the deformation of the strain generating portion 12 (longitudinal direction of the strain resistor 19) is referred to as a sensitivity direction.

As shown in FIG. 4, the strain resistor 19 occupies most of the portion extending along the sensitivity direction. When the strain resistor 19 extends along the sensitivity direction, its cross-sectional area becomes smaller and the electric resistance value increases. When the strain resistor 19 shrinks along the sensitivity direction, its cross-sectional area increases and the electrical resistance value decreases. Therefore, the electric resistance value of the strain resistor 19 changes according to the degree of deformation of the strain generating portion 12.

As shown in FIG. 6, the strain gauge 17 attached to the strain generating portion 12 is covered with the cover member 20. For example, after the strain gauge 17 is attached to the strain generating portion 12, the cover member 20 is attached to the strain gauge 17 and the strain generating portion 12 so as to cover the strain resistor 19. The cover member 20 may be attached to the strain gauge 17 in advance so as to cover the strain resistor 19 before the strain gauge 17 is attached to the strain generating portion 12. The cover member 20 is formed of a material having excellent adhesion to the base 18 and having sufficient strength. For example, a composite film of Ta ₂ O ₅ and SiN is used as the cover member 20.

(4) Structure of base of strain gauge The thin film-shaped base 18 has a layered structure. Specifically, the base 18 is composed of one or more layers having insulating properties. When the base 18 is composed of a plurality of layers, each layer is laminated in the height direction of the strain generating portion 12. The sensitivity direction of the strain resistor 19 attached to the base 18 is along the plane formed by the layer of the base 18.

In this embodiment, the base 18 is composed of only one liquid crystal polymer layer made of liquid crystal polymer, as shown in FIG. The liquid crystal polymer is, for example, a polymer classified as an aromatic polyester from the viewpoint of molecular structure. The liquid crystal polymer has a property that the molecules of the liquid crystal polymer are oriented along the longitudinal direction thereof when melted or in a solution state, and the oriented structure is almost maintained even when solidified. Therefore, the liquid crystal polymer layer has anisotropy in terms of linear expansion coefficient due to the higher-order structure of the liquid crystal polymer molecules. Specifically, the liquid crystal polymer layer has an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption.

The sensitivity direction of the strain resistor 19 attached to the base 18 is along the direction in which the linear expansion coefficient of the liquid crystal polymer layer is the lowest. The direction in which the linear expansion coefficient of the liquid crystal polymer layer is the lowest is the direction along the longitudinal direction of the liquid crystal polymer molecules forming the liquid crystal polymer layer. FIG. 7 is a diagram schematically showing the liquid crystal polymer molecules forming the liquid crystal polymer layer when the base 18 is viewed from above. In FIG. 7, the liquid crystal polymer molecules are indicated by hatched rectangles, and the longitudinal direction of the liquid crystal polymer molecules is along the long side of the rectangle. The longitudinal direction (sensitivity direction) of the strain resistor 19 is along the longitudinal direction of the liquid crystal polymer molecule.

As the liquid crystal polymer used as the material of the base 18, for example, the E5000 series manufactured by Sumitomo Chemical Co., Ltd. is used.

(5) Features Conventionally, polyimide resin, epoxy resin and the like have been used as the base material of the strain gauge. While these resins have high heat resistance and excellent soldering resistance, they easily absorb moisture in the outside air and expand easily. When the base expands due to moisture absorption, the strain resistor attached to the base also expands, and the electrical resistance value of the strain resistor increases. On the contrary, when water is released from the base and the base contracts, the strain resistor also contracts, and the electric resistance value of the strain resistor decreases. As described above, the output of the conventional strain gauge tends to change according to the amount of water absorbed in the base. Further, since the output of the strain gauge and its changing direction change depending on the relative relationship between the amount of water absorbed in the base and the humidity of the outside air, for example, it is difficult to correct the output of the strain gauge according to the humidity. is there.

In this embodiment, a liquid crystal polymer is used as the material of the base 18 of the strain gauge 17. Since the liquid crystal polymer molecule does not contain a hydrophilic functional group such as a hydroxyl group, it is difficult to bind to a water molecule. In addition, since the liquid crystal polymer molecules have rigidity and are difficult to bend, the molecules tend to be regularly arranged next to each other. As a result, there are few gaps between the liquid crystal polymer molecules through which water molecules can permeate, and the gaps are less likely to be expanded by the water molecules. Therefore, the liquid crystal polymer has low water absorption, and its dimensions are unlikely to change due to moisture absorption.

Therefore, the liquid crystal polymer has a smaller water absorption rate and a smaller amount of deformation due to absorption and release of water as compared with conventional materials such as polyimide resin. For example, the water absorption rate of the liquid crystal polymer is about 1/10 of the water absorption rate of polyimide. Therefore, by using the liquid crystal polymer as the material of the base 18, the fluctuation of the electric resistance value of the strain resistor 19 due to the deformation of the base 18 due to moisture absorption is reduced, and the fluctuation of the output of the strain gauge 17 can be suppressed.

Therefore, by using the liquid crystal polymer as the material of the base 18 of the strain gauge 17, it is possible to suppress the deformation of the base 18 due to moisture absorption and reduce the fluctuation of the output of the strain gauge 17. As a result, fluctuations in the output of the scale 100 using the load cell 10 having the strain gauge 17 are also reduced, so that a highly accurate scale 100 that is not easily affected by the external environment can be realized.

Further, even if the liquid crystal polymer is solidified from the molten state, it is easy to maintain a higher-order structure in which the liquid crystal polymer molecules are regularly oriented along the longitudinal direction thereof. Therefore, the base 18 made of a liquid crystal polymer has anisotropy in the coefficient of linear expansion when deformed due to moisture absorption or the like. For example, it is described in "Table 2 (page 39)" of the paper "Development of LCP film" published on pages 37-42 of "Sumitomo Chemical Technical Journal 2003-I (issued on May 30, 2003)". As described above, the liquid crystal polymer film has anisotropy in terms of coefficient of thermal expansion. By attaching the strain resistor 19 to the base 18 so that the sensitivity direction of the strain resistor 19 follows the direction in which the linear expansion coefficient of the base 18 is the lowest, the output of the strain gauge 17 due to the deformation of the base 18 due to moisture absorption Fluctuations can be minimized. As a result, the dispersion of the fluctuation of the output of the strain gauge 17 is suppressed as compared with the case where the strain resistor 19 is attached to the base 18 without considering the anisotropy of the linear expansion coefficient of the base 18, and the strain gauge 17 The output stabilizes.

-Second Embodiment-
FIG. 8 is a side view of the strain gauge 17 according to the second embodiment of the present invention. In this embodiment, only the configuration of the base 18 of the strain gauge 17 is different from that in the first embodiment. Hereinafter, the configuration of the base 18 will be mainly described.

In the present embodiment, the base 18 has a two-layer structure composed of a first liquid crystal polymer layer 18a formed of a liquid crystal polymer and a second liquid crystal polymer layer 18b formed of a liquid crystal polymer. The first liquid crystal polymer layer 18a and the second liquid crystal polymer layer 18b may be formed of the same material or may be formed of different materials. A strain resistor 19 is attached to the first liquid crystal polymer layer 18a. The second liquid crystal polymer layer 18b is attached to the strain measuring portions of the beam portions 15 and 16 with an adhesive or the like.

FIG. 9 is a diagram schematically showing the layered structure of the base 18. The first liquid crystal polymer layer 18a has a first expansion direction D1a that expands due to moisture absorption and a first contraction direction D1b that contracts due to moisture absorption. The second liquid crystal polymer layer 18b has a second expansion direction D2a that expands due to moisture absorption and a second contraction direction D2b that contracts due to moisture absorption. In FIG. 9, the directions of the first liquid crystal polymer layer 18a and the second liquid crystal polymer layer 18b along the hatching lines represent the first expansion direction D1a and the second expansion direction D2a, respectively. When the base 18 is viewed in a plan view, the first expansion direction D1a and the second expansion direction D2a intersect.

FIG. 10 is a diagram for explaining a method of calculating the angle formed by the first expansion direction D1a and the second expansion direction D2a. FIG. 10 shows the first expansion direction D1a, the first contraction direction D1b, the second expansion direction D2a, the second contraction direction D2b, and the sensitivity direction S of the strain resistor 19.

First, the angle θ _TH1 and the angle θ _TH2 that satisfy the relational expression of the following equation (1) are obtained.

In the formula (1), E _1a is the linear expansion coefficient of the first liquid crystal polymer layer 18a in the first expansion direction D1a. E _1b is the linear expansion coefficient of the first liquid crystal polymer layer 18a in the first contraction direction D1b. t ₁ is the thickness of the first liquid crystal polymer layer 18a. E _2a is the coefficient of linear expansion of the second liquid crystal polymer layer 18b in the second expansion direction D2a. E _2b is the coefficient of linear expansion of the second liquid crystal polymer layer 18b in the second contraction direction D2b. t ₂ is the thickness of the second liquid crystal polymer layer 18b. In the equation (1), the angle θ _TH1 and the angle θ _TH2 may be obtained so that the lvalue is close to 1.

Next, the angle θ ₀ is obtained from the relational expression of the following equation (2).

In the formula (2), θ ₁ is an angle formed by the first shrinkage direction D1b of the first liquid crystal polymer layer 18a and the sensitivity direction S. θ ₂ is an angle formed by the second shrinkage direction D2b of the second liquid crystal polymer layer 18b and the sensitivity direction S. The angle θ ₁ is an angle within the range from θ _TH1 −20 ° to θ _TH1 + 20 °. Angle theta ₁ is preferably an angle in the range of up to θ _TH1 + 10 ° from theta _TH1 -10 °, more preferably an angle within the range of θ _TH1 + 5 ° from theta _TH1 -5 °, even more The angle is preferably in the range of θ _TH1-1 ° to θ _TH1 + 1 °. The angle θ ₂ is an angle within the range from θ _TH2 −20 ° to θ _TH2 + 20 °. The angle θ ₂ is preferably an angle in the range of θ _TH2 −10 ° to θ _TH2 + 10 °, more preferably an angle in the range of θ _TH2 −5 ° to θ _TH2 + 5 °, and even more. The angle is preferably in the range of θ _TH2 -1 ° to θ _TH2 + 1 °.

In the case where the base 18 viewed from the top, the angle formed between the first expansion direction D1a and the second expansion direction D2a is is the angle in the range of theta ₀ -20 ° to θ ₀ + 20 °, preferably theta ₀ - An angle in the range of 10 ° to θ ₀ + 10 °, more preferably an angle in the range of θ _0-5 ° to θ ₀ + 5 °, and even more preferably an angle in the range of θ ₀ -1 ° to θ _0. The angle is within the range up to + 1 °.

In this embodiment, the base 18 has a multilayer structure composed of two liquid crystal polymer layers. The two liquid crystal polymer layers are laminated so that the expansion directions of the layers intersect in a plan view. As a result, the deformation of each liquid crystal polymer layer due to moisture absorption cancels each other out, so that the amount of deformation due to moisture absorption of the entire base 18 can be reduced.

Further, in the present embodiment, as shown in FIG. 6 regarding the first embodiment, the strain gauge 17 attached to the strain generating portion 12 may be covered with the cover member 20.

―Third Embodiment―
FIG. 11 is a side view of the strain gauge 17 according to the third embodiment of the present invention. In this embodiment, only the configuration of the base 18 of the strain gauge 17 is different from that in the first embodiment. Hereinafter, the configuration of the base 18 will be mainly described.

In the present embodiment, the base 18 has a first liquid crystal polymer layer 18c formed of a liquid crystal polymer, a second liquid crystal polymer layer 18d formed of a liquid crystal polymer, and a third liquid crystal polymer formed of a liquid crystal polymer. It has a three-layer structure composed of layers 18e. The first liquid crystal polymer layer 18c, the second liquid crystal polymer layer 18d, and the third liquid crystal polymer layer 18e may be formed of the same material or may be formed of different materials.

FIG. 12 is a diagram schematically showing the layered structure of the base 18. In the height direction, the first liquid crystal polymer layer 18c, the second liquid crystal polymer layer 18d, and the third liquid crystal polymer layer 18e are laminated from top to bottom in this order. A strain resistor 19 is attached to the first liquid crystal polymer layer 18c. The third liquid crystal polymer layer 18e is attached to the strain measuring portions of the beam portions 15 and 16 with an adhesive or the like. In FIG. 12, the direction along the hatching line of each liquid crystal polymer layer represents the expansion direction.

Each of the liquid crystal polymer layers 18c, 18d, 18e has an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption. When the base 18 is viewed in a plan view, the expansion directions of the two liquid crystal polymer layers 18c, 18d, 18e adjacent to each other intersect each other. As a result, the deformation of each liquid crystal polymer layer due to moisture absorption cancels each other out, so that the amount of deformation due to moisture absorption of the entire base 18 can be reduced.

Further, in the present embodiment, as shown in FIG. 6 regarding the first embodiment, the strain gauge 17 attached to the strain generating portion 12 may be covered with the cover member 20.

― Fourth Embodiment ―
FIG. 13 is a side view of the strain gauge 17 according to the fourth embodiment of the present invention. In this embodiment, only the configuration of the base 18 of the strain gauge 17 is different from that in the first embodiment. Hereinafter, the configuration of the base 18 will be mainly described.

In the present embodiment, the base 18 has a two-layer structure including one liquid crystal polymer layer 18f formed of a liquid crystal polymer and one non-liquid crystal polymer layer 18 g. The non-liquid crystal polymer layer 18 g is a layer formed of a crystalline polymer other than the liquid crystal polymer or a non-crystalline polymer. The non-liquid crystal polymer layer 18 g is a layer made of a polyimide resin, an epoxy resin, or the like. A strain resistor 19 is attached to the liquid crystal polymer layer 18f. The non-liquid crystal polymer layer 18 g is attached to the strain measuring portions of the beam portions 15 and 16 with an adhesive or the like.

The liquid crystal polymer layer 18f has an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption. Here, let θ be an angle that satisfies the relational expression of the following equation (3).

In the formula (3), E _1a is the linear expansion coefficient in the expansion direction of the liquid crystal polymer layer 18f. E _1b is the linear expansion coefficient in the shrinkage direction of the liquid crystal polymer layer 18f. t ₁ is the thickness of the liquid crystal polymer layer 18f. E ₂ is the coefficient of linear expansion of 18 g of the non-liquid crystal polymer layer. t ₂ is the thickness of the non-liquid crystal polymer layer 18 g.

When the base 18 is viewed from above, the angle formed by the expansion direction of the liquid crystal polymer layer 18f and the sensitivity direction of the strain resistor 19 is an angle within the range of θ-20 ° to θ + 20 °, preferably θ. An angle in the range of −10 ° to θ + 10 °, more preferably an angle in the range of θ-5 ° to θ + 5 °, and even more preferably in the range of θ-1 ° to θ + 1 °. The angle. The angle θ may be determined so that the lvalue of the equation (3) is close to 1.

In the present embodiment, the base 18 has a multilayer structure including one liquid crystal polymer layer and one non-liquid crystal polymer layer. As a result, the amount of deformation of the entire base 18 due to moisture absorption can be reduced.

Further, in the present embodiment, as shown in FIG. 6 regarding the first embodiment, the strain gauge 17 attached to the strain generating portion 12 may be covered with the cover member 20.

-Modification example-
(1) Modification A
In the third embodiment, the base 18 has a multilayer structure composed of three liquid crystal polymer layers 18c, 18d, 18e formed of the liquid crystal polymer. However, the number of liquid crystal polymer layers contained in the base 18 may be greater than three. In this case, the plurality of liquid crystal polymer layers contained in the base 18 are laminated so that the expansion directions of the two liquid crystal polymer layers adjacent to each other intersect with each other.

When the base 18 has a plurality of liquid crystal polymer layers, it is preferable that the plurality of liquid crystal polymer layers are laminated so that the expansion direction of the liquid crystal polymer layers changes by a predetermined angle along the stacking direction. For example, when the base 18 is composed of four liquid crystal polymer layers, it is preferable that the four liquid crystal polymer layers are laminated so that the expansion direction of the liquid crystal polymer layers changes by 90 ° along the stacking direction.

(2) Modification B
In the fourth embodiment, the base 18 has a multilayer structure including one liquid crystal polymer layer 18f formed of a liquid crystal polymer and one non-liquid crystal polymer layer 18 g. However, the number of liquid crystal polymer layers contained in the base 18, the number of non-liquid crystal polymer layers, and the stacking order of the layers are arbitrary. For example, the base 18 may have two or more liquid crystal polymer layers and one non-liquid crystal polymer layer, or may have one liquid crystal polymer layer and two or more non-liquid crystal polymer layers. It may have two or more liquid crystal polymer layers and two or more non-liquid crystal polymer layers. In the base 18 having a multilayer structure, the position of the non-liquid crystal polymer layer may be between two liquid crystal polymer layers, between two non-liquid crystal polymer layers, or between a liquid crystal polymer layer and a non-liquid crystal polymer layer. .. The liquid crystal polymer layer is preferably adjacent to another liquid crystal polymer layer from the viewpoint of reducing the amount of deformation due to moisture absorption of the entire base 18 by canceling the deformation of each liquid crystal polymer layer due to moisture absorption.

(3) Modification C
FIG. 6 shows a cover member 20 that covers the strain gauge 17. As the material of the cover member 20, a liquid crystal polymer may be used. Specifically, at least a part of the cover member 20 may be made of a liquid crystal polymer. For example, a thin film made of a liquid crystal polymer may be used as the cover member 20 previously attached to the strain gauge 17 so as to cover the strain resistor 19. The liquid crystal polymer used as the material of the cover member 20 may be the same as the liquid crystal polymer used as the material of the base 18 of the strain gauge 17.

Further, as a material of the cover member 20 previously attached to the strain gauge 17 so as to cover the strain resistor 19, a polyimide resin, an epoxy resin or the like may be used. For example, the cover member 20 may be a thin film made of a polyimide resin, an epoxy resin, or the like.

Further, after the strain gauge 17 is attached to the strain generating portion 12, rubber such as butyl rubber is used as the material of the cover member 20 attached to the strain gauge 17 and the strain generating portion 12 so as to cover the strain resistor 19. May be good. For example, the cover member 20 may be a thin film made of rubber such as butyl rubber.

10 Load cell 12 Distortion part 17 Strain gauge 18 Base 19 Strain resistor 20 Cover member

Japanese Unexamined Patent Publication No. 2015-64296

Claims (11)

A strain gauge attached to a strain-causing part that deforms when a load is applied.
With an insulating base attached to the strain generating part,
A strain resistor attached to the base and whose electrical resistance value changes according to the degree of deformation of the strain-causing portion.
With
At least part of the base is made of liquid crystal polymer.
Strain gauge. The base is a layer made of a liquid crystal polymer, including a layer parallel to a plane containing a predetermined sensitivity direction.
The sensitivity direction is a direction in which the length of the strain resistor changes due to the deformation of the strain generating portion.
The strain gauge according to claim 1. The base has one liquid crystal polymer layer made of liquid crystal polymer.
The sensitivity direction is along the direction in which the linear expansion coefficient of the liquid crystal polymer layer is the lowest.
The strain gauge according to claim 2. The sensitivity direction is along the longitudinal direction of the liquid crystal polymer molecules forming the liquid crystal polymer layer.
The strain gauge according to claim 3. The base has a first liquid crystal polymer layer and a second liquid crystal polymer layer formed of a liquid crystal polymer.
The first liquid crystal polymer layer and the second liquid crystal polymer layer each have an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption.
The expansion direction of the first liquid crystal polymer layer and the expansion direction of the second liquid crystal polymer layer intersect with each other.
The strain gauge according to claim 2. The linear expansion rate in the expansion direction of the first liquid crystal polymer layer is E _1a , the linear expansion rate in the contraction direction of the first liquid crystal polymer layer is E _1b , and the contraction direction of the first liquid crystal polymer layer. The angle formed by the sensitivity direction and the sensitivity direction is θ ₁ , and the thickness of the first liquid crystal polymer layer is t ₁ .
The linear expansion rate in the expansion direction of the second liquid crystal polymer layer is E _2a , the linear expansion rate in the contraction direction of the second liquid crystal polymer layer is E _2b , and the contraction direction of the second liquid crystal polymer layer. The angle formed by the sensitivity direction and the sensitivity direction is θ ₂ , and the thickness of the second liquid crystal polymer layer is t ₂ .
Angle θ _TH1 and angle θ _TH2

If the relational expression of
The angle θ ₁ is an angle within the range from θ _TH1 −20 ° to θ _TH1 + 20 °.
The angle θ ₂ is an angle within the range from θ _TH2 −20 ° to θ _TH2 + 20 °.
Angle θ ₀

When the angle is calculated from the relational expression of
Said expansion direction of the first liquid crystal polymer layer, wherein the angle between the direction of expansion forms the second liquid crystal polymer layer is an angle in the range of theta ₀ -20 ° to θ _{0 +} 20 °,
The strain gauge according to claim 5. The base has at least three liquid crystal polymer layers made of liquid crystal polymer.
Each of the liquid crystal polymer layers has an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption.
The expansion directions of the two liquid crystal polymer layers adjacent to each other intersect each other.
The strain gauge according to claim 2. The base has a liquid crystal polymer layer formed of a liquid crystal polymer and a non-liquid crystal polymer layer formed of a crystalline polymer other than the liquid crystal polymer or a non-crystalline polymer.
The liquid crystal polymer layer has an expansion direction that expands due to moisture absorption and a contraction direction that contracts due to moisture absorption.
The linear expansion coefficient of the liquid crystal polymer layer in the expansion direction is E _1a , the linear expansion coefficient of the liquid crystal polymer layer in the contraction direction is E _1b , and the thickness of the liquid crystal polymer layer is t ₁ .
The coefficient of linear expansion of the non-liquid crystal polymer layer is E ₂ , and the thickness of the non-liquid crystal polymer layer is t ₂ .

When the angle that satisfies the relational expression of
The angle formed by the expansion direction of the liquid crystal polymer layer and the sensitivity direction is an angle within the range of θ-20 ° to θ + 20 °.
The strain gauge according to claim 2. A cover member that covers the strain resistor is further provided.
The strain gauge according to any one of claims 1 to 8. At least a part of the cover member is made of a liquid crystal polymer.
The strain gauge according to claim 9. The strain gauge according to any one of claims 1 to 10,
A strain-causing part to which the strain gauge is attached and deforms when a load is applied,
With
The strain gauge is attached to the surface of the strain-causing portion, which intersects the direction of deformation when a load is applied.
Load cell.

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