STRAIN GAUGE MODULE

A present strain gauge module includes a film-like strain detection device that is to be attached to a measurement target and includes a terminal on an upper surface thereof, and is configured to detect strain generated in the measurement target; and a thin-plate metal substrate including an upper surface and a lower surface. The film-like strain detection device is attached to the upper surface of the thin-plate metal substrate via an adhesive. The lower surface of the thin-plate metal substrate serves as an attachment surface that is to be attached to the measurement target.

TECHNICAL FIELD The present invention relates to strain gauge modules.

BACKGROUND ART

As film-like devices, for example, strain gauges including a resistor formed on a flexible base of a polyimide or the like are known (see, for example, Patent Literature 1). Such film-like devices are not readily attached to measurement targets because the base is flexible. In addition, a polyimide is a material that is poorly adhered, and thus special adhesion methods (heating and pressurizing) are required for attachment to the measurement target.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Application Publication No. 2018-185346

SUMMARY OF THE INVENTION

Technical Problem

The present invention has been made in view of the above, and it is an object of the present invention to provide a strain gauge module that is readily attached to a measurement target.

Solution to Problem

A present strain gauge module (1) includes: a film-like strain detection device (30) that is to be attached to a measurement target and includes a terminal (34,35) on an upper surface thereof, and is configured to detect strain generated in the measurement target; and a thin-plate metal substrate (10) including an upper surface (10a) and a lower surface (10b). The film-like strain detection device (30) is attached to the upper surface (10a) of the thin-plate metal substrate (10) via an adhesive (20). The lower surface (10b) of the thin-plate metal substrate (10) serves as an attachment surface that is to be attached to the measurement target.

The above reference numerals in the parentheses are given for ease of understanding, and are merely illustrative. The present invention is not limited to the embodiments as illustrated in the drawings.

Advantageous Effects of the Invention

According to the disclosed technique, it is possible to provide a strain gauge module that is readily attached to a measurement target.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the embodiments of the present invention will be described with reference to the drawings. In the drawings, the same components are denoted by the same symbols, and duplicate description thereof may be omitted.

Strain Gauge Module

FIG.1is a plan view illustrating an example of the strain gauge module according to the present embodiment.FIG.2is a cross-sectional view illustrating the example of the strain gauge module according to the present embodiment, and illustrates a cross section along line A-A inFIG.1.FIG.3is a diagram for describing the surface roughness Ra and the thickness of the adhesive.

As illustrated inFIG.1andFIG.2, the strain gauge module1includes a thin-plate metal substrate10, an adhesive20, and a film-like strain detection device30.

The thin-plate metal substrate10is a member in which the film-like strain detection device30is disposed. Here, the thin-plate metal substrate refers to a metal substrate having a thickness of 200 μm or smaller. The thickness of the thin-plate metal substrate10is preferably 20 μm or larger and 120 μm or smaller. When the thickness of the thin-plate metal substrate10is set to be 20 μm or larger, strain can be stably detected. When the thickness of the thin-plate metal substrate10is set to be 120 μm or smaller, strain can be detected with high sensitivity.

The thickness of the thin-plate metal substrate10is more preferably 20 μm or larger and 80 μm or smaller, and further preferably 20 μm or larger and50um or smaller. When the thickness of the thin-plate metal substrate10is set to be smaller, strain can be detected with higher sensitivity. Further, when the thickness of the thin-plate metal substrate10is set to be 20 μm or larger and 80 μm or smaller, the thin-plate metal substrate10can be readily fixed to even a curved measurement target. When the thickness of the thin-plate metal substrate10is set to be 20 μm or larger and 50 μm or smaller, the thin-plate metal substrate10can be further readily fixed to even a curved measurement target.

The thin-plate metal substrate10is preferably rectangular in a plan view in order to detect strain in a specific direction with high sensitivity. The direction in which strain is detected is, for example, a direction parallel to the longitudinal direction of the rectangular shape of the thin-plate metal substrate10.

The surface roughness Ra of the upper surface10aof the thin-plate metal substrate10is preferably 382m or higher and 20 μm or lower. When the surface roughness Ra of the upper surface10aof the thin-plate metal substrate10is set to be 3 μm or higher and 20 μm or lower, it is possible to reduce an impact of the surface roughness Ra of the upper surface10aof the thin-plate metal substrate10on the film-like strain detection device30, and measure strain readily and accurately.

The surface roughness Ra of the upper surface10aof the thin-plate metal substrate10is more preferably 3 μm or higher and 10 μm or lower, and further preferably 3 μm or higher and 5 μm or lower. When the surface roughness Ra of the upper surface10aof the thin-plate metal substrate10is set to be smaller, it is possible to further reduce an impact of the surface roughness Ra of the upper surface10aof the thin-plate metal substrate10on the film-like strain detection device30, and measure strain further readily and further accurately.

Here, the surface roughness Ra is one of the numerical values indicating surface roughness, and is called an arithmetic average roughness. Specifically, as illustrated inFIG.3, the surface roughness Ra is obtained by measuring absolute values of the height varying within a reference length l from the surface that is an average line, followed by arithmetically averaging.

As a material of the thin-plate metal substrate10, SUS (stainless steel) having high hardness (readily propagating strain) is suitable for transmitting strain. However, this is by no means a limitation. The material of the thin-plate metal substrate10may be aluminum, a copper alloy, or the like. SUS is also suitable because of easy availability. No particular limitation is imposed on the size of the thin-plate metal substrate10, which is, however, larger than the size of the film-like strain detection device30in a plan view.

The film-like strain detection device30is attached to the upper surface10aof the thin-plate metal substrate10via the adhesive20. As the adhesive20, for example, an epoxy-based resin or the like may be used. A flexural modulus of the adhesive20may be, for example, 3 GPa or higher and 20 GPa or lower. The adhesive20may include a filler, if necessary. When the adhesive20includes a filler, the filler included may be an inorganic filler or may be an organic filler. When the adhesive20includes an inorganic filler, the filler diameter is preferably 5 μm or less. When the adhesive includes an organic filler, the filler diameter is preferably 10 μm or less.

A thickness T of the adhesive20is preferably 30 μm or smaller. When the thickness T of the adhesive20is 30 μm or smaller, strain of the thin-plate metal substrate10can be efficiently transmitted to the film-like strain detection device30. As illustrated inFIG.3, the thickness T of the adhesive20is a distance from the tip of the projecting portions denoted by a solid line, excluding the abnormal projection (spike), to the lower surface of a base31in the upper surface10aof the thin-plate metal substrate10. The projections and recesses denoted by a dashed line inFIG.3are virtual images. The gaps between the projections and the recesses in a portion U ofFIG.3, i.e., the gaps between the projections and the recesses of the upper surface10aof the thin-plate metal substrate10, are filled with the adhesive20.

The film-like strain detection device30includes a base31, a resistor32, an interconnect33, and terminals34and35. No particular limitation is imposed on the size of the film-like strain detection device30. However, from the viewpoint of downsizing the strain gauge module1, the film-like strain detection device30is preferably compact. For example, the base31may be formed into a square or rectangular shape in which the length of one side is from about 1.5 mm through about 2 mm.

The base31is a member serving as a base layer on which the resistor32is to be formed, and has flexibility. No particular limitation is imposed on the thickness of the base31, which may be appropriately selected in accordance with the intended purpose. The base31may be formed of an insulating resin film, such as a film of a PI (polyimide) resin or the like.

The resistor32is a thin film formed on the upper surface of the base31, for example, in a zigzag pattern, and is a sensitive part that causes resistivity change in response to receiving strain. The resistor32may be formed directly on the upper surface of the base31or may be formed via other layers on the upper surface of the base31. No particular limitation is imposed on the thickness of the resistor32, which may be appropriately selected in accordance with the intended purpose. However, the thickness of the resistor32is, for example, 15 μm or larger and 50 μm or smaller.

The resistor32may be formed of, for example, a material including Cr (chromium), a material including Ni (nickel), or a material including both of Cr and Ni. That is, the resistor32may be formed of a material including Cr, Ni, or both. Examples of the material including Ni include Cu—Ni (copper nickel) and the like. Examples of the material including both of Cr and Ni include Ni—Cr (nickel chromium) and the like.

As the resistor32, a Cr mixed phase film may be used. Here, the Cr mixed phase film is a film of mixed phases of Cr, CrN, Cr2N, and the like. The Cr mixed phase film may include unavoidable impurities, such as chromium oxide and the like. When the Cr mixed phase film is used as the resistor32, the film-like strain detection device30can be made highly sensitive and can be downsized.

The terminals34and35are formed on the upper surface near the ends of the interconnect33. The terminals34and35are connected to both ends of the resistor32via the interconnect33formed of copper or the like, and are formed into, for example, a rectangular shape in a plan view. The terminals34and35are a pair of electrodes configured to output strain-derived change in the resistivity of the resistor32. The terminals34and35are formed of, for example, copper or the like. A gold film or the like may be stacked on the surface of copper or the like.

The strain gauge module1may include a resin over the thin-plate metal substrate10, the resin covering the film-like strain detection device30. The resin may be formed, for example, to expose a part of or the entirety of the terminals34and35of the film-like strain detection device30. When the resin covering the film-like strain detection device30is provided over the thin-plate metal substrate10, it is possible to increase the mechanical strength of the terminals34and35of the film-like strain detection device30and the resistance of the film-like strain detection device30to environmental factors (humidity, gas).

As the resin covering the film-like strain detection device30, in order to suppress the output (offset) of the film-like strain detection device30with no strain being applied, it is preferable to use a material including no filler or a material including an inorganic or organic filler of 3 μm or less. Alternatively, as the resin covering the film-like strain detection device30, it is preferable to use a material having a hardness suitable for strain propagation, i.e., a hardness of from D90 through A15, and a tensile strength of from 0.3 MPa through 10 MPa. Examples of such a material include thermosetting or photocurable silicone-based resins and epoxy-based resins. When such a low-stress resin is used as the resin covering the film-like strain detection device30, it is possible to reduce an impact of the covered resin on the characteristics (sensitivity) of the film-like strain detection device30.

Because the lowermost layer of the strain gauge module1is the thin-plate metal substrate10, the lower surface10bof the thin-plate metal substrate10serving as an attachment surface can be readily fixed to the measurement target with an adhesive or a tacky agent in the form of liquid or film (tape). The thickness of the adhesive or tacky agent may be, for example, about 25 μm.

That is, the lowermost layer of the strain gauge module1is not formed of a flexible resin (e.g., a polyimide) unlike in the existing strain gauge, and thus the strain gauge module1is readily attached to the measurement target. In addition, a polyimide is a material that is poorly adhered, and thus special adhesion methods (heating and pressurizing) are required for attachment to the measurement target. However, such a special adhesion method is unnecessary for attaching the thin-plate metal substrate10to the measurement target.

Percentage of Resistivity Change of Resistor32

The film-like strain detection device30that does not include the thin-plate metal substrate10and the adhesive20(hereinafter referred to as the film-like strain detection device30alone) was studied for change in the resistivity of the resistor32when the film-like strain detection device30alone was attached to surfaces with different surface roughness.

FIG.4is a graph indicating the percentage of resistivity change of the film-like strain detection device alone (part 1) and indicates the percentage of resistivity change when the film-like strain detection device30alone was attached to a surface having a surface roughness Ra of from 3.0 μm through 5.0 μm. There is no spike on the surface to which the film-like strain detection device30alone is attached.

InFIG.4, Probe is a resistivity obtained through measurement performed by a measuring instrument connected to the terminals34and35of the film-like strain detection device30alone. Also, Contact is a resistivity obtained through measurement performed by a measuring instrument connected to the terminals34and35of the film-like strain detection device30alone after the film-like strain detection device30alone was attached to a surface having a surface roughness Ra of from 3.0 μm through 4.0 μm. InFIG.4, the resistivity of Contact is shown as the percentage of resistivity change with the resistivity of Probe being treated as an initial value. As shown inFIG.4, the percentage of resistivity change when the film-like strain detection device30alone was attached to the surface having the surface roughness Ra of from 3.0 μm through 5.0 μm is 2% or lower, and the variation therein is very small.

FIG.5is a graph indicating the percentage of resistivity change of the film-like strain detection device alone (part 2) and indicates the percentage of resistivity change when the film-like strain detection device30alone was attached to a surface having a surface roughness Ra of 30 μm. There is no spike on the surface to which the film-like strain detection device30alone is attached. As shown inFIG.5, the percentage of resistivity change when the film-like strain detection device30alone was attached to the surface having the surface roughness Ra of 30 μm is from about 1% through about 3%, and the variation therein is larger than in FIG.4.

FIG.6is a graph indicating the percentage of resistivity change of the film-like strain detection device alone (part 3) and indicates the percentage of resistivity change when the film-like strain detection device30alone was attached to a surface having a surface roughness Ra of 30 μm and including spikes of from 50 μm through 80 μm. As shown inFIG.6, the absolute value and the variation of the percentage of resistivity change when there are spikes are greatly increased compared to when there is no spike as shown inFIG.5.

In this way, when the film-like strain detection device30is directly attached to the measurement target, because the base31is a flexible resin, the characteristic variation due to the surface roughness Ra of the measurement target occurs, and the strain cannot be accurately measured. As in the case ofFIG.4, if the film-like strain detection device30is attached to the measurement target having a low surface roughness Ra, the strain can be accurately measured. In practice, however, the film-like strain detection device30is attached to various measurement targets, and thus it is challenging to always accurately measure the strain of different measurement targets.

Meanwhile, the film-like strain detection device30of the strain gauge module1is attached via the adhesive20to the thin-plate metal substrate10in which the upper surface10ahas a desired surface roughness Ra. With this structure, the impact of the surface roughness of the measurement target is eliminated, and strain can be measured readily and accurately. In addition, because spikes of the upper surface10aof the thin-plate metal substrate10can be managed, measurement with little variation is possible. Especially, the strain can be more accurately measured by setting the surface roughness Ra of the upper surface10aof the thin-plate metal substrate10to be 3 μm or higher and 20 μm or lower.

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 claims recited.

The present international application claims priority under Japanese Patent Application No. 2021-178003 filed on Oct. 29, 2021, and the entire contents of Japanese Patent Application No. 2021-178003 are incorporated in the present international application by reference.

REFERENCE SIGNS LIST