JP5713615B2 - Load cell - Google Patents

Description

  The present invention relates to a load cell to which environmental measures are taken.

  Generally, a load cell is provided with a metal elastic body, and a strain generating portion that generates a stretching strain by the action of a load of an object to be weighed is formed on the metal elastic body. Four strain gauges are basically attached to detect the amount of strain. These four strain gauges are connected to a bridge circuit having two sheets for detecting the amount of elongation strain and two sheets for detecting the amount of contraction strain on opposite sides, and a predetermined voltage is applied to the input terminal of the bridge circuit.

  When a load is applied to the metal elastic body, the amount of expansion / contraction strain of the strain generating portion changes in proportion to the magnitude of the load, and the strain gauge resistance changes correspondingly to the output terminal of the bridge circuit. An output voltage signal corresponding to the magnitude of the load is generated. In the weighing device, the output voltage signal is used as a load signal for calculating the weight measurement value of the object to be weighed.

FIG. 5A shows an overall perspective view of a conventional parallelogram type (roberval type) load cell.
A load cell 100 shown in FIG. 5A includes a metal elastic body 102 made of an aluminum alloy formed in a parallelogram shape having a through hole 101. This elastic metal body 102 has a total of four thin strain-generating portions 103 sandwiched between the inner wall surface and the outer peripheral surface of the through-hole 101 in the vertical and horizontal directions, and each strain-generating portion 103 has a strain gauge 104. It is pasted with an adhesive.

FIG. 5B shows a plan view of a strain gauge used in a conventional load cell.
The strain gauge 104 shown in FIG. 5B is formed by forming resistance wires made of a Cu—Ni alloy called a grid 105 on a thin plate made of a material such as an imidoamide resin called a base 106. The structure is also protected by a thin plate made of a material such as polyester resin 107.

  In the load cell 100 shown in FIG. 5A, (A) conventional environmental resistance measures applied to the metal elastic body and (B) conventional environmental resistance measures applied to the strain gauge will be described below.

<Conventional environmental resistance measures for metal elastic bodies>
Since the metal elastic body 102 is an aluminum alloy, it is very easily corroded. When the load cell 100 is used by being exposed to the external environment, the following measures (1) to (3) are performed on the metal elastic body 102, for example.
(1) After anodized, it is further coated with silicon resin.
(2) The allodin process which is a chemical film process is performed.
(3) After alumite processing, further taffram processing for infiltrating the fluororesin is performed.

The countermeasure (1) by the alumite processing has a problem that, when used in an environment cleaned with hypochlorous acid, corrosion occurs due to residual adhered chlorine.
The countermeasure by the allodynic treatment (2) has a problem that a pinhole is present in the chemical film, and the aluminum alloy is directly exposed to the atmosphere through the pinhole, and corrosion occurs from that portion.
Note that even if silicon resin is applied on all anodized or allodynic elastic bodies, the silicon resin is porous and has a low gas barrier property. Therefore, hypochlorous acid water is used as a cleaning liquid for the measuring section. And free residual chlorine in hypochlorous acid water adheres to other substances such as sulfur oxides that adhere to the hypochlorous acid water during the drying process. When the pH of hypochlorous acid water is lowered, it gasifies and enters into the metal surface of the metal elastic body together with gas such as water vapor, so that corrosion occurs.

  In addition, even with the above-mentioned countermeasures by means of the taphram processing of (3), fluorine does not have a complete gas barrier property. Therefore, if water enters the porous anodized through the fluorine-impregnated portion and becomes moisture, it is contained in the alloy. There is a problem that the aluminum flows as an anode and Cu acts as a cathode due to the Cu, and the current flows and the aluminum is ionized and corroded.

  Even if silicon, resin, etc. are applied directly on the all-genus elastic body, these polymer materials are porous, so that they have gas barrier properties as well as problems in strain gauge covers and base materials described later. Since the invasion of water vapor and gas cannot be prevented, the environmental resistance is not significantly improved.

  By the way, there is a load cell in which a SUS alloy having a much higher corrosion resistance than an aluminum alloy is used as the all-genus elastic body, but the SUS alloy is difficult to process. In addition, the material cost is high, and hysteresis characteristics and creep characteristics in the stress-strain characteristic as a metal elastic body material appear more than aluminum alloy, and the measurement accuracy is lowered, so that there is a limit to applications. Further, even a SUS alloy causes corrosion when a liquid containing a chlorine component, for example, an aqueous solution of hypochlorous acid used for cleaning adheres.

<Conventional environmental measures for strain gauges>
In the strain gauge 104, since the material of the base 106 and the cover 107 is a resin, when the gas including water vapor, chlorine, sulfur component, etc. enters and dehydrates due to a decrease in temperature, the thickness and hardness of the base 106 are reduced. Since the change or the corrosion of the resistance wire of the grid 105 occurs, a zero point change or a span change occurs in the load signal. Therefore, measures are taken such as forming a layer having no gas permeability on the strain gauge 104.

  As a moisture-proof measure for the strain gauge 104, for example, a technique for attaching a non-gas permeable cover on the strain gauge 104 has been proposed (see, for example, Patent Document 1). As specific measures, techniques of vacuum deposition of metal on a resin layer, sputtering, and adhesion of a metal layer with a metal foil having a thickness of 2 to 3 μm have been proposed (see, for example, Patent Documents 2 and 3).

Japanese Utility Model Publication No. 57-201909 JP-A-60-227129 JP-A-60-227140

In the techniques according to Patent Documents 2 and 3 described above, there is a problem that even if the metal is vacuum-deposited or sputtered on the resin surface, there is a gap in the metal particles attached to the resin surface and gas barrier properties cannot be sufficiently obtained. There is a point. Further, if the metal foil is too thin and stretched, there is a problem that the rigidity of the foil plate is basically too weak to be handled at the time of sticking.
Further, if the metal foil is made too thin, there is a minute gap and the thickness is limited to 2 to 3 μm, and cannot be made thinner than this. As a result, the metal foil mechanically affects the metal elastic body. For example, in a small load cell having a capacity of about 5 kg, there is a problem that the measurement accuracy is lowered due to hysteresis and creep characteristics of the metal foil.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a load cell excellent in environmental resistance that can withstand use in a state exposed to an external environment.

In order to achieve the above object, a load cell according to the present invention comprises:
In a load cell comprising a metal elastic body having a strain generating portion, and used in a state where the strain generating portion is exposed to the outside air,
A strain gauge unit product in which a resistance wire portion made of metal foil and a lead wire connection portion connected to the resistance wire portion are integrally formed on the upper surface of an insulating substrate made of resin, and solder bonding in the lead wire connection portion A strain gauge unit product formed by forming a DLC film on the entire surface of the outer shape is pasted on the strain generating portion , leaving the portion .
Here, DLC is an abbreviation for diamond-like carbon.

In the present invention, a strain gauge single-piece formed by depositing a DLC film on the entire outer surface is attached to the strain generating portion, leaving a solder joint.
Here, the DLC film is a general term for carbon and an amorphous carbon film having a carbon bond, and is formed by a physical vapor deposition method such as an ion plating method or a sputtering method, a plasma CVD method, or a chemical vapor deposition method. Since it has high properties and no crystal grain boundaries, it has a feature of high gas barrier properties, and is suitably used as a coating for preventing wear of metals, resins and rubber materials used in sliding parts.
Therefore, according to the present invention, there is an effect that it is possible to obtain a load cell excellent in environmental resistance that can withstand use in a state exposed to the external environment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. Explanatory drawing of the environmental resistance improvement means with a metal elastic body alone or a set with a strain gauge Explanatory diagram of measures to improve environmental resistance of strain gauge alone Explanatory drawing of the environmental resistance improvement means of the strain gauge affixed to the strain generating part Overall perspective view of conventional load cell (a) and plan view of strain gauge (b)

  Next, specific embodiments of the load cell according to the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic structural explanatory diagram of a measuring instrument incorporating a load cell according to an embodiment of the present invention.

<Description of schematic configuration of weighing device>
A weighing device 1 shown in FIG. 1 includes a housing 2, a load cell 3, and a weighing pan 4.

<Description of schematic configuration of load cell>
The load cell 3 mainly includes a metal elastic body 5 and a plurality of strain gauges 6a, 6b, 6c, 6d. These strain gauges 6a, 6b, 6c, and 6d are collectively referred to as “strain gauge 6” as necessary.

The metal elastic body 5 is made of, for example, an aluminum alloy and has a parallelogram type (Roberval type) provided with a through hole 7. The metal elastic body 5 has a total of four strain-generating portions 8a, 8b, 8c, and 8d that are sandwiched between the inner wall surface and the outer peripheral surface of the through hole 7 in the vertical and horizontal directions. These strain generating portions 8a, 8b, 8c and 8d are collectively referred to as “strain generating portion 8” as necessary.
One end side of the metal elastic body 5 is fixed to the housing 2 via an elastic body support fitting 9.
A weighing pan 4 is attached to the other end of the metal elastic body 5 via a weighing pan support fitting 10. When the object to be weighed 11 is placed on the weighing pan 4 and the load of the object to be weighed 11 acts on the metal elastic body 5 via the weighing pan support fitting 10, the strain generating portions 8a, Elongation distortion occurs in 8c, and compression distortion occurs in the strain generating portions 8b and 8d.

  The strain gauges 6a and 6b are attached to the upper surface of the metal elastic body 5 by using an adhesive means so as to correspond to the strain generating portions 8a and 8b, and the strain gauges 6c and 6d correspond to the strain generating portions 8c and 8d. It is affixed on the lower surface of the metal elastic body 5 using an adhesive means.

<Overview of environmental resistance improving means>
In the present embodiment, the properties of a DLC film, that is, a diamond-like carbon film, are used for the metal elastic body 5 of the load cell 3 in which the strain-generating portion 8 is exposed to the outside air, and the load cell 3. The following environmental resistance improving means is applied to the strain gauge 6 and the strain gauge attaching part.

<Description of DLC film>
Here, the DLC film is a generic term for carbon and an amorphous carbon film having a carbon bond, and is formed by a physical vapor deposition method such as an ion plating method or a sputtering method, a plasma CVD method, or a chemical vapor deposition method. Since it has high uniformity and no crystal grain boundaries, it has the characteristic of high gas barrier properties, and is suitable for use as a coating for preventing wear of metals, resins and rubber materials used in sliding parts. .
According to the coating technique using a DLC film, the gas barrier property of a PET (made of polyethylene terephthalate resin) bottle can be improved by 20 times or more with a thin film of 200 mm. In addition, since the temperature at the time of film formation is as low as 60 ° C. or less, even if the DLC film is directly formed on the metal elastic body 5 or the strain gauge 6, they are not altered.

<Description of means for improving environmental resistance of metal elastic body>
As shown in FIGS. 2 (a) and 2 (a) ', the entire metal elastic body 5 is directly or, if the metal elastic body 5 is made of an aluminum alloy, anodized, and then the entire trum A DLC film 12A is formed on the entire metal elastic body 5 made of aluminum alloy that has been processed. In short, the DLC film 12A is formed on the metal elastic body 5 before the strain gauge 6 is attached. The cost may be reduced by forming the DLC film 12A only on the strain generating portion 8 and its periphery.

<Explanation of means for improving environmental resistance with metal elastic body and strain gauge set>
As shown in FIGS. 2B and 2B ′, a strain gauge 6 is first attached to the metal elastic body 5, and then a DLC film 12 B is formed on the entire surface of the metal elastic body 5 and the strain gauge 6.
Note that electronic components such as a temperature compensation resistor (not shown) may be mounted in the vicinity of the attached portion of the strain gauge 6, and the DLC film 12B is formed including these electronic components. Further, the strain gauge 6 is affixed to the metal elastic body 5 with or without a protective cover covering the surface of the strain gauge 6, and whether or not any resin is applied to the entire metal elastic body 5 and the strain gauge 6. Then, a DLC film 12B is formed.

<Explanation of the environmental resistance improvement means of the strain gauge alone>
A strain gauge 6A shown in FIG. 3A is a resistance wire portion (made of a metal foil such as a Cu-Ni alloy) formed on a thin rectangular plate-like insulating substrate (base) 13 made of a material such as imidoamide resin. (Grid) 14 and a lead wire connecting portion 15 connected to the grid 14 are integrally formed, and a lead wire 16 is connected to the lead wire connecting portion 15 by solder 17.
The strain gauge 6B shown in FIG. 3B has a simple structure in which the grid 14 and the lead wire connecting portion 15 are integrally formed on the upper surface of the base 13.
The strain gauge 6C shown in FIG. 3C has a structure in which a thin plate-like protective cover 18 made of a material such as polyester resin is mounted on the surface of the strain gauge 6A shown in FIG. Is.
The strain gauge 6D shown in FIG. 3 (d) has a structure in which a protective cover 19 is mounted on the surface of the strain gauge 6B shown in FIG. 3 (b) except for the lead wire connecting portion 15. is there.

As shown in FIGS. 3A and 3C, if the strain gauges 6A and 6C have a structure in which the lead wires 16 are wired, the entire surface as shown in FIGS. A DLC film 20A is formed including the periphery and back surface.
On the other hand, as shown in FIGS. 3B and 3D, if the lead wires 16 are strain gauges 6B and 6D having an unwired structure, as shown in FIGS. A DLC film 20B is formed on the entire surface, including the periphery and the back surface, leaving the solder joints 21 required for soldering in the lead wire connection portions 15.

<Explanation of the environmental resistance improvement means of the strain gauge affixed to the strain generating part>
As the environmental resistance improving means when any of the strain gauges 6A to 6B shown in FIGS. 3A to 3D is attached to the strain generating portion 8 of the metal elastic body 5, the following is provided. Can be mentioned.
As a representative example, as shown in FIG. 4A, the environment resistance improving means when the strain gauge 6A shown in FIG. 3A is attached to the strain-generating portion 8 of the metal elastic body 5 is shown. A protective cover 22 is produced by forming a DLC film on a resin sheet that can cover the entire strain gauge 6A attached to the strain-generating portion 8 of the metal elastic body 5 and its peripheral portion. The cover 22 may be affixed to the strain generating portion 8 so as to cover the strain gauge 6A.
In FIG. 4B, the strain gauge 6A of FIG. 3A according to the above representative example is pasted on the DLC film 23 already formed on the strain-generating portion 8 of the metal elastic body 5. The example of an aspect of the environment-proof improvement means in the case of being present is shown, and the protective cover 22 can be similarly applied.
Whereas conventional nickel foil has a thickness of several microns, the DLC film is formed to a thickness of several hundreds of millimeters, and the gas is blocked while preventing the characteristics of the metal elastic body 5 from deteriorating in the strained portion 8. To do.

<Description of the effects of this embodiment>
In the present embodiment, the DLC film 12A is formed on the entire surface of the metal elastic body 5 or on the peripheral portion including at least the strain-generating portion 8. Further, the DLC film 12B is formed directly on the metal elastic body 5 on which the electronic component including the strain gauge 6 attached to the strain generating portion 8 is mounted or on the resin layer protecting the electronic component. Further, strain gauges 6A ′, 6C ′; 6B ′, 6D ′ formed by forming DLC films 20A; 20B on all or part of the outer surface of the strain generating portion 8 are attached. Further, the strain gauge 6A attached to the strain generating portion 8 and the peripheral portion of the strain gauge 6A are covered with a protective cover 22 made of a resin formed with a DLC film. Therefore, there is an effect that it is possible to obtain the load cell 3 excellent in environmental resistance that can withstand use in a state exposed to the external environment.

  As mentioned above, although the load cell of the present invention has been described based on one embodiment, the present invention is not limited to the configuration described in the above embodiment, and the configuration is appropriately changed without departing from the spirit of the present invention. It is something that can be done.

  Since the load cell of the present invention has the property of being excellent in environmental resistance, it can be suitably used as a load sensor of a weighing device used in a state exposed to the external environment.

DESCRIPTION OF SYMBOLS 1 Weighing device 3 Load cell 5 Metal elastic body 6 Strain gauge 8 Strain part 12A, 12B DLC film 20A, 20B DLC film 22 Protective cover

Claims (1)

In a load cell comprising a metal elastic body having a strain generating portion, and used in a state where the strain generating portion is exposed to the outside air,
A strain gauge unit product in which a resistance wire portion made of metal foil and a lead wire connection portion connected to the resistance wire portion are integrally formed on the upper surface of an insulating substrate made of resin, and solder bonding in the lead wire connection portion A load cell in which a strain gauge unit product formed by forming a DLC film on the entire surface of the outer shape is pasted on the strain generating portion , leaving a portion .

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