Moisture-insulating strain gauges and how to insulate them against moisture ingress

Abstract

To provide a strain gauge insulated against moisture penetration and a method of manufacturing the strain gauge.SOLUTION: A strain gauge 31 against moisture penetration and a method of manufacturing the strain gauge 31 comprise the step of producing a coated base layer or coated cover layer 34 by forming a moisture barrier coating on the surface of the strain gauge 31.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to strain gauges that are insulated against the ingress of moisture, and to methods for manufacturing strain gauges. [Background technology]

[0002] Of particular interest are strain gauges used in load cells, but the method of the present invention is believed to be fully applicable to strain gauges used in other applications and is not intended to be limiting. Furthermore, the moisture insulation method of the present invention can be applied in one of three ways: a) during the strain gauge manufacturing process, b) to the new strain gauge itself, or c) to the strain gauge while attached to a load cell or any object where strain measurements are to be performed. The scope of the present invention extends to strain gauges and load cells that include moisture protective insulation produced using the method of the present invention.

[0003]

[0002] Conventional strain gauges currently available on the market typically have the form of a rectangular piece of polymer foil forming a support substrate, on which a metal resistor track is disposed in the form of a serpentine structure. The metal resistor track is disposed on the metal resistor foil material and combined with the support substrate to form a strain gauge by known lamination methods followed by known chemical etching methods. Connector electrodes or electrode pads for contacting the resistor track are also disposed on the metal resistor foil material. The connector electrodes are often fabricated together with the resistor track in a single operation and therefore often consist of the same material, often constantan due to its low temperature dependence. Depending on the application, the support substrate may be glass, a ceramic material, or, in many cases, a polymer, glass fiber-reinforced polymer, or composite. One or more strain gauges are adhesively attached to the surface of an unstressed object to measure mechanical deformation due to forces or stresses acting on the object. When an object is then subjected to a force or stress, the resulting deformation of the object produces a change in the electrical resistance of the strain gauge's metallic resistive conductive path, which can be measured. In the case of a strain gauge load cell, the strain gauge is attached to the surface of the load cell's elastically deformable body (also called the spring element), and measurements of the electrical resistance are used to determine the magnitude of gravitational forces acting on the load cell.

[0004]

[0003] The metallic resistor conductive path of a strain gauge can be covered by a cover layer, referred to herein as a mechanical protective cover. The purpose of this cover is to protect the sensitive resistor conductive path from direct mechanical contact and from the adhesion of dirt and dust. Therefore, the metallic resistor conductive path is sandwiched between the substrate foil and the mechanical protective cover, both of which are made of a polymer material approximately 10 to 20 μm (micrometers) thick. Polymer materials used in strain gauges include, for example, polyimide, phenolic aldehyde, polyether ether ketone (PEEK), and related types. However, despite their suitability for strain gauges, these materials have the disadvantage of being hygroscopic, retaining moisture at rates that vary depending on the humidity of the surrounding atmosphere. Upon absorbing moisture, the polymer material of the strain gauge expands in volume, resulting in slight deformation of the metallic resistor conductive path, which is tightly bonded to the polymer material. In addition to volume changes, moisture absorption also causes changes in the elastic modulus of the strain gauge's polymer material. In the case of strain gauge load cells, changes in the volume and elastic modulus of the strain gauge's polymer material can have a detrimental effect on the load cell's metrological performance, causing drift in the load cell's no-load signal (zero point) and / or drift in sensitivity (signal / load ratio) and / or creep (initial signal creep occurs when a load is applied to the load cell over an extended period of time). This causes the temperature to slowly deviate from the indicated value.

[0005]

[0004] Therefore, to ensure that strain gauge load cells maintain their metrological accuracy and reliability even during periods of varying atmospheric humidity, strain gauges must be protected against moisture ingress. Load cells used in so-called "legal for trade" applications are government regulated and internationally standardized in accordance with "OIML R60-Parts 1 and 2, Metrological Regulation for Load Cells," published by the OIML (Organisation Internationale de Métrologie Legale, Paris, France). Provisions in this regulation include standardized procedures for testing the metrological accuracy of load cells in environmental test chambers at various levels of atmospheric temperature and humidity. In the development and design of load cells, one of the mandatory target specifications is that the load cell must pass these required OIML tests at various levels of specific atmospheric temperature and humidity. In these standardized procedures, the atmospheric humidity during testing is necessarily varied in defined time cycles for a limited period of time.

[0006] To meet the aforementioned requirements, the strain gauges of the load cell must be insulated against the ingress of moisture from the surrounding atmosphere. For example, according to a first state-of-the-art solution described and illustrated in U.S. Pat. No. 4,957,177, this can be achieved in a cantilever beam load cell (also known as a bending beam load cell or moment-insensitive load cell) by enclosing the bending beam bearing the strain gauge within a corrugated metal bellows. Here, the edges of the bellows are welded to cylindrical terminal ends at each of the fixed and movable ends of the cantilever beam load cell. The bellows may be filled with a gas, such as dry nitrogen. While the bellows enclosure seals the strain gauge area of ​​the load cell and thus provides absolute protection for the strain gauge against atmospheric humidity, it also suffers from several significant drawbacks. Residual stresses introduced by the welding process can relax over time or upon application of a weighed load, reducing the load cell's measurement accuracy through drift and hysteresis in the indicated load value. The relative magnitude of these effects on weighing capacity is greatest for low-capacity load cells, e.g., 2 g for 6 kg and 1 g for 3 kg, which are typically used in retail scales, one of the most important applications for low-capacity strain gauge load cells. Additionally, the bellows, welding process, and subsequent leak testing of the hermetic seal substantially increase manufacturing costs.

[0007]

[0006] According to a second state-of-the-art solution, strain gauges for use in load cells are insulated against the ingress of moisture from the surrounding atmosphere by covering the region of the serpentine resistor trace with metal foil over an electrically insulating intermediate layer, leaving the connector tabs at the ends of the resistor trace exposed. As described, for example, in U.S. Pat. No. 4,557,150, the metal foil covering is applied to the strain gauge after it has been mounted in the load cell body. Alternatively, as described, for example, in U.S. Pat. No. 5,631,622, the strain gauge itself is manufactured and sold already including the metal foil covering. The metal foil protects the resistor trace against the ingress of moisture through the top surface. However, the relatively high elastic modulus of the metal foil material, combined with the viscosity of the strain gauge's polymer material and adhesive bonding layer, can significantly affect the load cell's measurement performance, causing drift in the load cell's no-load signal (zero point) and / or drift and / or creep (a slow shift from the initial reading as the load cell is loaded over time). As with the corrugated metal bellows described above, these effects are most pronounced in low-capacity load cells. Furthermore, holes or gaps in the intermediate layer between the resistor track and the foil cover pose a risk of shorting out portions of the serpentine resistor track.

[0008]

[0007] If a material other than metal is used for the foil to avoid the high modulus, the replacement low modulus material must be thicker to achieve the same protection. As with metal foils, increased thickness affects the measurement performance of the load cell in the same way as above.

[0009]

[0008] According to a third state-of-the-art solution, described, for example, in EP 1560011 A1, strain gauges for use in load cells are insulated against the ingress of moisture from the surrounding atmosphere by covering the strain gauge (except for the connector tabs at the ends of the resistor tracks) with a protective inorganic coating having multiple discrete layers of different materials or whose material composition varies continuously through the thickness of the coating. A surface-smoothing polymer layer is applied to the metallic resistor tracks, for example, by brushing, spraying, rolling, or tampon printing. A protective inorganic coating is then applied on top of the surface-smoothing polymer layer by plasma-enhanced chemical vapor deposition (PECVD). A multilayer inorganic coating having multiple discrete layers of different materials can consist of alternating discrete layers of silicon nitride and silicon oxide. Other possible materials include metals, carbides, and fluorides. Coatings whose material composition varies continuously through their thickness can be made of silicon oxynitride (SiO ). x N y where the ratio x / y varies through the thickness of the layer.

[0010]

[0009] The protective coating according to the third solution described above, as described in EP 1560011 A1, can be applied during the strain gauge manufacturing process. The resulting product is a strain gauge that is already protected against moisture before it is attached to the load cell. Alternatively, the protective coating can be applied to a conventional (i.e., uncoated) strain gauge after it has been attached to the load cell body. In the latter alternative, the deposition process for the protective coating significantly increases manufacturing costs, as the entire load cell must be placed in a deposition chamber.

[0011] According to U.S. Pat. No. 5,052,505, cantilever load cells can be protected against moisture by recessing the surface area where the strain gauges are to be mounted and then covering the recess with a moisture-proof cover sheet, such as a rubber sheet, so that there is no contact between the top surface of the strain gauge and the rubber sheet. In other words, each strain gauge is enclosed in its own recess. The main concern with this solution is that the narrow air gap under the moisture-proof rubber sheet of U.S. Pat. No. 5,052,505 makes leak testing unlikely, as opposed to the solution of the aforementioned U.S. Pat. No. 4,957,177, in which the strain gauges are enclosed in a metal bellows and can be leak tested.

[0012]

[0011] According to another state-of-the-art solution, described, for example, in EP 0 667 514 A1, a strain gauge for use with a flexure element includes a substrate made of a resin material, a resistor provided on the surface of the substrate, and a fusing layer provided on the surface opposite the resistor. The purpose of the fusing layer is to electrically insulate the strain gauge from the flexure element. The fusing layer is a thermoplastic polyimide layer that may contain, in addition to thermoplastic polyimide, a resin other than thermoplastic polyimide and / or a filler. Examples of fillers include inorganic particles of aluminum oxide, titanium oxide, boron nitride, and silicon oxide. The fusing layer is mixed with the filler to adjust the linear expansion coefficient of the thermoplastic resin to a value close to that of the flexure element, and therefore is applied only to the surface of the strain gauge facing the flexure element. Summary of the Invention [Problem to be solved by the invention]

[0013]

[0012] In view of the shortcomings of these prior art solutions, the object of the present invention is to provide a strain gauge that is insulated against the ingress of moisture, and a method for manufacturing said strain gauge. Yes, especially for strain gauges designed for use in load cells or already mounted on load cells, so that - Load cells equipped with moisture-insulated strain gauges according to this method not only pass the humidity test prescribed by the aforementioned standard OIML R-60, but also pass the same humidity test even if the test period is extended to approximately one year. - A measuring instrument incorporating one or more load cells with moisture-insulated strain gauges according to this method passes the humidity test prescribed by standard OIML R-76 for measuring instruments of accuracy classes I, II and III. - The metrological performance of the above humidity insulated load cells is not impaired as a result of the insulation when compared to an otherwise identical load cell that is not humidity insulated. - This insulation can be applied during the strain gauge manufacturing process, or it can be added to the finished strain gauge itself, or it can be applied to a strain gauge already attached to the body of the load cell. - Whether this insulation is applied to the strain gauge itself or to a strain gauge already attached to the body of the load cell, the method consistently delivers high quality results at lower manufacturing costs compared to existing state-of-the-art techniques. [Means for solving the problem]

[0014]

[0013] The above mentioned object is achieved by a strain gauge according to independent claim 1 and by a method according to independent claims 7, 12, 13 and 14. Detailed aspects, further developed versions and modifications of the strain gauge and method are set out in the dependent claims.

[0015]

[0014] All of the independent claims have in common that the surface of the polymer material of the strain gauge that is exposed to the environment is reduced, i.e., the polymer material used in the manufacturing or installation process of the strain gauge is coated with a moisture barrier coating on its surface.

[0016]

[0015] The strain gauge insulated against moisture ingress of the present invention comprises a base layer made of a polymer substrate foil material and a resistor conductive path layer on a metal resistor foil material, the resistor conductive path being in the form of a serpentine structure and having electrode pads for contacting the resistor conductive path. The resistor conductive path layer is laminated together with the base layer and produced on this laminated layer by a chemical etching method. According to the present invention, a moisture barrier coating is formed on all sides of the base layer by a deposition process so that the base layer is enveloped by the moisture barrier coating, and / or the strain gauge further comprises a cover layer made of a polymer film foil material. Additionally, a moisture barrier coating is formed on at least one side of the cover layer by a deposition process, and the overcoat cover layer is placed over the surface of the resistor conductive path layer to cover at least a portion of the strain gauge.

[0017]

[0016] The method of the present invention for manufacturing a strain gauge insulated against moisture ingress includes the steps of providing a base layer made from a polymer substrate foil material, providing a resistor conductive path layer on the metal resistor foil material, the resistor conductive path being in the form of a serpentine structure and having electrode pads for contacting the resistor conductive path, producing a coated base layer by forming a moisture barrier coating on all surfaces of the base layer by a deposition process so that the base layer is enveloped by the moisture barrier coating, laminating the resistor conductive path layer together with the coated base layer, and producing a strain gauge in the laminated layer by a chemical etching method.

[0018] Another method of the present invention for manufacturing a strain gauge that is insulated against moisture ingress includes the steps of providing a cover layer made from a polymer film foil material; producing the coated cover layer by forming a moisture barrier coating on the surface of the coated cover layer by a deposition process; and attaching the coated cover layer to the strain gauge to insulate at least part of the strain gauge. This alternative method for manufacturing a strain gauge can also be applied to strain gauges manufactured as described in the previous paragraph.

[0019]

[0018] After applying the covering layer, the surfaces of the strain gauge that remain uncovered are preferably covered with additional metal material from the resistor track layer, where the additional metal material is not conductively connected to the metal resistor track or the electrode pads. The additional metal material is made together with the resistor track and the electrode pads in one operation and therefore in most cases consists of the same material.

[0020]

[0019] When applying the overlay cover layer to the strain gauge, it is also advantageous to cover the entire strain gauge with a moisture insulating polymer film, in which case openings penetrate the overlay cover layer to the electrode pads of the strain gauge.

[0021] Suitable polymers for the base layer or cover layer include, but are not limited to, PET (polyethylene terephthalate), polyimide, and PEEK (polyether ether ketone). The thickness of the cover layer used in the method according to the present invention is typically in the range of several micrometers. Thicker thicknesses may be suitable for higher capacity load cells, such as shear beam load cells or cylindrical load cells. For example, a 500 μm (micrometer) thick overcoat cover layer may be suitable for such high capacity load cells and is considered within the scope of the present invention. In any event, it will be understood that the thickness of the cover layer is not a defining or limiting factor of the present invention.

[0022]

[0021] The moisture barrier coating is preferably a non-metallic inorganic coating having multiple separate layers of different materials or in which the material composition varies continuously in the thickness direction perpendicular to the coating surface, such as in the state-of-the-art solutions described in EP 1560011 A1, also owned by the assignee of the present invention. The advantages of such a layered structure of the moisture barrier coating are extensively explained in EP 1560011 A1, which is incorporated herein by reference. Non-metallic inorganic materials suitable for the moisture barrier coating used in the method according to the present invention include, for example, SiO2, Al2O3, TiO, and SiN. Inorganic-organic multilayer structures are also possible. Coating techniques that can be used to apply the moisture barrier coating include, for example, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), and atomic layer deposition (ALD).

[0023]

[0022] The thickness of the moisture barrier coating deposited on the base layer or cover layer is typically 200 nanometers or less. Furthermore, the moisture barrier coating can be deposited on one or both sides of the film foil, or the film foil can be wrapped with the coating on all sides, including a very thin surface area around the edges of the film foil.

[0024] A base layer or cover layer having a moisture barrier coating thereon is hereinafter referred to as a coated base layer or coated cover layer, respectively.

[0024] Conventional strain gauges are considered herein as strain gauges that are currently commercially available, ie, strain gauges that have a substrate without a moisture barrier coating.

[0025] According to the method of the present invention, the method step of applying a moisture insulating polymer film to a strain gauge can be carried out in three different ways A, B and C: A. The strain gauge is attached to the body of the load cell (or another object on which the strain measurement is being performed). The mounting and application of the coating cover layer onto the strain gauge are combined: the strain gauge is placed on the load cell body with the necessary adhesive, the coating cover layer is applied over at least a portion of the strain gauge with the necessary adhesive, the strain gauge and coating cover layer are secured to the load cell body with pressure clamps, and the entire assembly is heat cured in an oven to harden the adhesive bond. B. The attachment of the coating cover layer is performed on a strain gage that has already been attached and has had its adhesive bond heat cured. The coating cover layer, coated with the necessary adhesive, is placed over at least a portion of the strain gage, the attachment of the coating cover layer to the strain gage is secured by applying pressure with a clamp, and the entire assembly is heat cured in an oven to harden the adhesive bond. If a room temperature curing adhesive is used, oven curing after attachment of the coating cover layer is unnecessary. C. The application of the coating cover layer is performed during the strain gage manufacturing process, and the resulting product is a moisture-insulating strain gage. The application of the coating cover layer can be performed on individual strain gages, or on multi-unit sheets that are then cut into individual strain gages. Strain gages are typically produced in the form of multi-unit sheets that are then cut into their final form, the individual strain gages that make up the final product.

[0026] The method of the present invention has significant advantages over the assignee's previous solution in the above-mentioned EP 1560011 A1, particularly when the coating cover layer is applied to a strain gauge already disposed or permanently attached to the body of the load cell. The previous solution required the entire load cell to be placed in a deposition chamber to deposit a moisture-insulating coating on the surface-smoothing polymer layer. By comparison, in the method of the present invention, the moisture-insulating coating is applied to the cover layer, and the resulting coating cover layer then covers the strain gauge, which allows for more efficient use of the deposition chamber and results in significant cost savings.

[0027] Detailed embodiments of the present invention are shown in the accompanying drawings and are explained in the following description of the drawings and exemplary embodiments, in which like reference numerals in the several figures refer to the same or equivalent features. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a three-dimensional view of a double cantilever load cell. [Figure 2] 2 is a diagram of the load cell of FIG. 1 with a coating cover layer attached over the strain gauges. [Figure 3] FIG. 1 is a cross-sectional view of an overlay cover layer with a moisture barrier coating applied to one side of a polymer foil. [Figure 4] FIG. 1 is a cross-sectional view of an overlay cover layer with a moisture barrier coating applied to both sides of a polymer foil. [Figure 5] 1 is a cross-sectional view of a coated cover layer encased in a moisture barrier coating applied to both sides of the polymer foil and in a very thin surface area around the edges of the polymer foil. [Figure 6] FIG. 1 is a top view of the strain gauge without the mechanical protective cover. [Figure 7] Figure 7A is a cross-sectional view of a strain gauge without a mechanical protective cover, and Figure 7B is a cross-sectional view of a strain gauge without a mechanical protective cover, with a moisture barrier coating applied to both sides of the base layer and a very thin surface area around the edges of the base layer. [Figure 8] FIG. 1 is a top view of a strain gauge with a mechanical protective cover. [Figure 9] Figure 9A is a cross-sectional view of a strain gauge with a mechanical protective cover, and Figure 9B is a cross-sectional view of a strain gauge with a mechanical protective cover in which a moisture barrier coating is applied to both sides of the base layer and to a very thin surface area around the edges of the base layer. [Figure 10] 1 is a cross-sectional view of a strain gauge attached to the strain sensing area of ​​a load cell and having an overlay cover layer attached thereto, but without a mechanical protective cover. [Figure 11] 1 is a cross-sectional view of a strain gauge having a mechanical protective cover attached to a strain sensing area of ​​a load cell and having an overlay cover layer attached thereto. FIG. [Figure 12] Figure 12A is a top view of a strain gauge protected with an overcoat cover layer, Figure 12B is a top view of a strain gauge protected with an overcoat cover layer with the uncovered surface of the strain gauge capped with additional metal material that remains on the strain gauge after chemical etching, and Figure 12C is a top view of a strain gauge protected over its entire surface with the overcoat cover layer having openings for contacting electrode pads. [Figure 13] FIG. 1 is a cross-sectional view of a strain gauge protected by an overcoat cover layer but without a mechanical protective cover. [Figure 14] 1 is a cross-sectional view of a strain gauge having a mechanical protective cover protected by an overcoat layer. DETAILED DESCRIPTION OF THE INVENTION

[0029] FIG. 1 shows a double cantilever beam load cell 1 having top and bottom surfaces 2 and 3 (not visible), a load-receiving end 4 and a mounting end 5, and upper and lower bending beam sections 6 and 7. The load-receiving end 4 has two threaded holes 8 from the top surface 2 for attachment to a scale platform or other type of load receiver. Similarly, the mounting end 5 has two threaded holes from the bottom (not visible) for attaching the load cell 1 to a scale base (not shown) or any other type of support structure. The bending beam sections 6 and 7 are formed by machining an appropriately contoured opening 9 through the center of the load cell 1. The opening 9 is shaped to form thin bridge sections 10 in the bending beam sections 6 and 7. Four strain gauges 12 (two in the upper bending beam section 6 and two (not visible) in the lower bending beam section 7) are mounted in the centers of the thin bridge sections 10 with their resistor tracks precisely aligned. The principles of wiring four strain gauges 12 into a Wheatstone bridge circuit to obtain an electrical signal representative of the load being weighed on the load-receiving end 4 of the dual cantilever load cell 1 are well known in the art (see, for example, U.S. Pat. No. 5,052,505) and therefore will not be described in further detail here.

[0030] FIG. 2 shows that the overcoat cover layer 14 is made of a commercially available strain gauge adhesive, such as Vishay Precision Group, Micro-Measurements (951 1 shows the same load cell 1 after it has been mounted over strain gauges 12 with M-Bond 43B (Mr. Wendell Blvd., Wendell, NC 27591, USA). The area covered by the overlay cover layer 14 extends slightly beyond the edge of each strain gauge 12. The electrode pads 13 are left partially uncovered so that circuit wires can be soldered or welded to the electrode pads 13.

[0031]

[0030] The load cells of Figures 1 and 2 can also be replaced by rocker pin type load cells, or any object on which strain measurements are to be made, such as a pressure sensor or an airplane landing gear or a truck chassis frame, as well as machines for static and dynamic testing, and building structures.

[0032] The substrate of the overcoat cover layer 14 (see FIGS. 3-5) is a thin sheet of polymeric material 15. In accordance with the present invention, the polymeric sheet material 15 obtains its moisture insulating properties through a moisture barrier coating 17 of inorganic material applied to the polymeric sheet material 15 in a deposition process. The inorganic moisture barrier coating 17 can be applied to one side (FIG. 3), both sides (FIG. 4), or a very thin surface area around both sides and edges (FIG. 5) of the polymeric sheet material 15, so that the polymeric material 15 is enveloped by the moisture barrier coating 17. The thickness of the overcoat cover layer 14 for a low-capacity dual cantilever beam load cell 1 is in the range of a few micrometers. Thicker thicknesses are suitable for load cells with higher weighing capacities, such as shear beam load cells or cylindrical load cells. For example, a 500 μm (micrometer) thick overcoat cover layer 14 is believed to be suitable for such high-capacity load cells and is within the scope of the present invention. Suitable polymers for the basic polymer sheet material 15 include, for example, PET (polyethylene terephthalate), polyimide, PEEK (polyether ether ketone), and related materials such as KAPTON (a polyimide film developed by DuPont). Suitable non-metallic inorganic materials for the moisture barrier coating 17 used in the method according to the present invention include, for example, SiO2, Al2O3, TiO, and SiN. Inorganic-organic multilayer structures are also possible. Typical thicknesses of the moisture barrier coating 17 are less than 200 nanometers, but can be as thick as 2000 nanometers depending on the manufacturing process used.

[0033]

[0032] There are two types of commercially available strain gauges according to their layer structure. In the so-called open-type strain gauge 21A (shown in a top view in FIG. 6 and in a cross-section in FIG. 7A), the metal resistor conductive path 22 in the form of a meander structure terminated in an electrode pad 23 and backed by a base layer 20 of polymer substrate foil is exposed on its top surface, i.e., the open-type strain gauge 21A does not have a mechanical protective cover. In the so-called protected-type strain gauge 25A (shown in a top view in FIG. 8 and in a cross-section in FIG. 9A), the resistor conductive path 22 is protected by a mechanical protective cover 26, for example a polyimide layer. The method according to the invention is applicable to the open-type strain gauge 21A and the protected-type strain gauge 25A.

[0034]

[0033] Figures 7B and 9B show the same layer structure as strain gauge 21A and strain gauge 25A, except that a moisture barrier coating 17 is applied to the surface of base layer 20 before the resistor conductive path and electrode pads in the form of a serpentine structure are placed thereon.

[0035]

[0034] In the method according to the present invention, the step of applying a coating cover layer to the strain gauge can be carried out in a variety of ways, as exemplified by (but not limited to) the following detailed steps 1, 2, and 3.

[0036] In detailed procedure 1, the attachment of the strain gauges to the load cell body and the attachment of the covering layer to the surface of the strain gauge are combined with each other. After the strain gauges 31, 35 are placed in position on the load cell body 33 using strain gauge adhesive, as shown in FIG. 10 for the strain gauge 31 without a mechanical protective cover, and as shown in FIG. 11 for the strain gauge 35 with a mechanical protective cover 36, the covering layer 34 is attached using the aforementioned M-Bond A strain gauge adhesive 38, such as 43B, is applied to the strain gauges 31, 35 and the load cell body 33, covering the surfaces thereof, including the adjacent boundary region 37. The adhesive bond 38 can also function as a surface-smoothing layer. If the overcoat cover layer 34 has a one-sided moisture barrier coating (as in FIG. 3), the overcoat cover layer 34 is attached with the coated side facing outward. An inward-facing coated side is also possible and falls within the scope of the present invention, but is less effective. Here, the electrode pads 43 of the strain gauges are left at least partially uncovered by the overcoat cover layer so that circuit wires can be soldered or welded to the electrode pads 43. The strain gauges 31, 35 and overcoat cover layer 34 are secured to the load cell body 33 with clamps, and the load cell is placed in an oven, where the adhesive bond 38 of the strain gauges 31, 35 and overcoat cover layer 34 is heat-cured together.

[0037] In Detailed Step 2, an overlay cover layer 34 is attached to the strain gauges 31, 35, which have already been heat cured and permanently bonded to the load cell body 33. As in Detailed Step 1, the overlay cover layer 34 is applied over the strain gauges 31, 35 and the surface of the load cell body 33, including the adjacent boundary region 37, using, for example, the aforementioned M-Bond 43B. The overlay cover layer 14, having a one-sided moisture barrier coating 17, is attached with the coated side facing outward. The electrode pads 43 of the strain gauges 31, 35 are now secured to at least a small area of ​​the overlay cover layer 34 so that circuit wires can be soldered or welded to the electrode pads 43. The load cell is placed in an oven to heat cure the adhesive bond 38 of the overlay cover layer 34. Optionally, as a variation of Detailed Procedure 2, the overlay cover layer 34 can be attached using a room temperature cure adhesive, in which case the adhesive bond of the overlay cover layer 34 will harden within a specified time without oven curing.

[0038] The overlay cover layer 14, 34, 44 provides a high degree of protection because moisture cannot penetrate the moisture barrier coating 17 of the polymer film 15. Because the exposed edge of the adhesive layer is only 2-5 μm (micrometers) thick and the distance that water molecules must travel from the exposed edge to the resistor conductive path 22 of the strain gauge 31, 35 is relatively long, lateral penetration of moisture through the adhesive layer between the overlay cover layer 14, 34, 44 and the surface of the load cell body 33 is minimized.

[0039] In detailed step 3, the overcoat cover layer 14, 34, 44 is applied during the strain gauge manufacturing process, and the resulting product is a moisture-insulating strain gauge. Application of the overcoat cover layer 14, 34, 44 can be performed on individual strain gauges, or on multi-unit sheets that are then cut into individual strain gauges. Strain gauges are typically produced in the form of multi-unit sheets that are then cut into their final form, the individual strain gauges that become the final product. FIG. 12A shows a top view of strain gauges 41, 45 (FIGS. 13 and 14) to which the overcoat cover layer 44 has been applied by the method of the present invention. The strain gauge in FIG. 12A may be an open-type strain gauge 21A, 41 that does not have a mechanical protective cover over the resistor conductive path 42 (as shown in the cross-sectional views of FIG. 7A and FIG. 13), or may be a protected-type strain gauge 25A, 45 that has a mechanical protective cover 26, 46 between the resistor conductive path 42 and the overcoat cover layer 44 (as shown in the cross-sectional views of FIG. 9A and FIG. 14).

[0040] 2, 8 and 12A, when the electrode pads 13, 23, 43 are left at least partially uncovered by the overlying cover layer 14, 34, 44, the uncovered faces of the strain gauge can be covered with additional metal material 48 (see FIG. 12B) that remains on the strain gauge after chemical etching, such as the resistor conductive track and electrode pad 43. A slight gap ensures that there is no conductive connection between the additional metal material 48 and the resistor conductive track 42 or electrode pad 43.

[0041] As an alternative to leaving the electrode pads 13, 23, 43 at least partially uncovered by the overcoat layer 14, 34, 44 as shown in FIGS. 2, 8, and 12A, the overcoat layer 14, 34, 44 can be placed over the strain gauges 12, 21A, 21B, 25A, 25B, 31, 35, 41, 45. In this case, openings 47 extend through the overcoat layer 14, 34, 44 to the electrode pads 13, 23, 43 (see FIG. 12C) to ensure that circuit lines can be soldered or welded to the electrode pads 13, 23, 43. The strain gauges in FIG. 12C can be moisture-insulated open-type strain gauges 21B as shown in FIG. 7B, which lack a mechanical protective cover over the resistor conductive path 42, or moisture-insulated protected-type strain gauges 25B as shown in FIG. 9B.

[0042]

[0041] Although the invention has been described with particular reference to load cells, it is understood that other applications and other ways of carrying out the method of the invention are likewise within the teachings of the invention. In particular, the method of the invention for producing strain gauges insulated against the ingress of moisture is not limited to the moisture protection of strain gauges in load cells used in weighing, but can be used generally for strain gauges that require protection against moisture without the drawback of impairing measurement accuracy as a result of the protection measures. This applies, for example, to strain gauges used in pressure sensors or strain gauges used in airplane landing gear or strain gauges incorporated into truck chassis frames, as well as to static and dynamic testing of machines and building structures. Such applications and modifications of the concepts described and claimed herein are believed to be within the scope of protection sought for the invention hereby. [Explanation of symbols]

[0043] 1. Double cantilever load cell 2 Top side 3 Bottom 4 Load-bearing end 5 Mounting end 6 Upper curved beam 7 Lower curved beam 8 screw holes 9 Contoured opening 10 Thin bridge section 12 Strain gauge 13, 23, 43 Electrode pads 14, 34, 44 Cover layer 15 thin sheet of polymer material, 14 substrate, cover layer 17 Moisture Barrier Coating 20 Base layer 21A, 31, 41 Open type strain gauge 21B Moisture-insulated open strain gauge 22, 42 Resistor conductive path 25A, 35, 45 Protected strain gauge 25B Moisture Insulation Protection Type Strain Gauge 26, 36, 46 Mechanical protection cover 33 Load cell body 37 Boundary area adjacent to strain gauges 31 and 35 38 Adhesive Bonding Agents 47 Aperture 48 Additional Metal Materials

Claims

1. A method for attaching a strain gauge (12, 21B, 25B, 31, 35) to the body of a load cell (1), or to any object on which strain measurements are to be performed, comprising: The strain gauges (12, 21B, 25B, 31, 35) a base layer (20) made from a polymer substrate foil material; a resistor conductive path layer laminated on the base layer (20) and produced by a chemical etching method, the resistor conductive path layer having resistor conductive paths (22, 42) formed in a meandering structure and electrode pads (13, 23, 43) for contacting the resistor conductive paths (22, 42); a coating cover layer (14, 34, 44); The coated cover layer (14, 34, 44) comprises a single cover layer (15) made of a polymer film foil material, and a moisture barrier coating (17) of a non-metallic inorganic material formed on at least the top and bottom surfaces of the cover layer (15) by a deposition process; The covering layer (14, 34, 44) is placed on the surface of the resistor conductive path layer laminated on the base layer (20), The method comprises: Applying an adhesive (38) to a load cell body (1) or any object on which strain measurement is to be performed, and placing the base layer (20) on which the resistor conductive path layer is laminated; providing a cover layer (15) made of a polymer film foil material; forming a moisture barrier coating (17) on the surface of said cover layer (15) by a deposition process to produce a covering cover layer (14, 34); applying the adhesive bonding agent (38) to cover the overcoat cover layer (14, 34) to cover at least a portion of the base layer (20) on which the resistor conductive path layer is laminated; a step of attaching the base layer (20) on which the resistor conductive path layer is laminated and the covering cover layer (14, 34) to the load cell body (1) or any object on which strain measurement is performed, and fixing the combination of the base layer (20) and the covering cover layer (14, 34) under contact pressure; placing the load cell body (1) or the object on which strain measurement is performed in a furnace with the base layer (20) on which the resistor conductive path layer is laminated and the covering cover layer (14, 34) fixed in place to thermally harden and solidify the adhesive bonding agent between the base layer (20) on which the resistor conductive path layer is laminated and the covering cover layer (14, 34); A method comprising:

2. A method for manufacturing a load cell having strain gauges (12, 21B, 25B, 31, 35) attached to a load cell body (1) or an object on which strain measurements are to be performed, wherein the strain gauges (12, 21B, 25B, 31, 35) are strain gauges insulated against the ingress of moisture, and the strain gauges (12, 21B, 25B, 31, 35) are: a base layer (20) made from a polymer substrate foil material; a resistor conductive path layer laminated on the base layer (20) and produced by a chemical etching method, the resistor conductive path layer having resistor conductive paths (22, 42) formed in a meandering structure and electrode pads (13, 23, 43) for contacting the resistor conductive paths (22, 42); a coating cover layer (14, 34, 44); The coated cover layer (14, 34, 44) comprises a single cover layer (15) made of a polymer film foil material, and a moisture barrier coating (17) of a non-metallic inorganic material formed on at least the top and bottom surfaces of the cover layer (15) by a deposition process; The covering layer (14, 34, 44) is placed on the surface of the resistor conductive path layer laminated on the base layer (20), and the method includes: providing at least one base layer (20) on which the resistor conductive path layer is laminated, the base layer (20) being attached to the load cell body (1) or an object on which strain measurement is to be performed and having undergone thermal curing of an adhesive bonding agent; providing a cover layer (15) made of a polymer film foil material; forming a moisture barrier coating (17) on the surface of said cover layer (15) by a deposition process to produce a covering cover layer (14, 34); The coating cover layer (14, 34) Applying a heat-curing adhesive bond (38), or Applying a room temperature curing adhesive bond (38), a step of covering at least a portion of the base layer (20) on which the at least one resistor conductive path layer is laminated; a step of fixing the overlay cover layer (14, 34) to the base layer (20) on which the at least one resistor conductive path layer is laminated under contact pressure; With the covering cover layer (14, 34) fixed to the base layer (20) on which the at least one resistor conductive path layer is laminated, the load cell body (1) or the object on which strain measurement is performed is placing the coated cover layer (14, 34) in an oven to heat cure and harden the heat-curable adhesive bonding agent (38); or leaving the coated cover layer (14, 34) at room temperature for a predetermined time to cure and harden the room temperature curing adhesive bonding agent (38); A method comprising:

3. A method for manufacturing a strain gauge, the strain gauge being a strain gauge insulated against the ingress of moisture, the method comprising: a base layer (20) made from a polymer substrate foil material; a resistor conductive path layer laminated on the base layer (20) and produced by a chemical etching method, the resistor conductive path layer having resistor conductive paths (22, 42) formed in a meandering structure and electrode pads (13, 23, 43) for contacting the resistor conductive paths (22, 42); a coating cover layer (14, 34, 44); The coated cover layer (14, 34, 44) comprises a single cover layer (15) made of a polymer film foil material, and a moisture barrier coating (17) of a non-metallic inorganic material formed on at least the top and bottom surfaces of the cover layer (15) by a deposition process; The covering layer (14, 34, 44) is placed on the surface of the resistor conductive path layer laminated on the base layer (20), and the method includes: providing the base layer (20) on which at least two individual resistor conductive path layers are stacked, or providing at least one multi-unit sheet having the base layer (20) on which the resistor conductive path layers are stacked; providing a cover layer (15) made of a polymer film foil material; forming a moisture barrier coating (17) on the surface of said cover layer (15) by a deposition process to produce a covering cover layer (44); applying an adhesive bonding agent (38) to overlay the overlay cover layer (44) to cover at least a portion of the base layer (20) on which the at least two individual resistor track layers are laminated, or at least a portion of the base layer (20) on which the resistor track layers of the at least one multi-unit sheet are laminated; fixing the attachment of the overlay cover layer (44) to the base layer (20) on which the at least two individual resistor conductive path layers are laminated or to the base layer (20) on which the resistor conductive path layers of the at least one multi-unit sheet are laminated under contact pressure; curing the base layer (20) on which the resistor conductive path layer is laminated together with the coating cover layer (44) at a predetermined temperature for a predetermined time to harden the adhesive bond between the coating cover layer (44) and the base layer (20) on which the resistor conductive path layer is laminated; A method comprising:

4. 4. The method according to claim 1, wherein the surface of the base layer (20) on the side where the resistor conductive path layer is laminated, that is not covered by the covering cover layer (14, 34, 44), is covered with additional metal material (48) from the resistor conductive path, and no conductive connection exists between the additional metal material (48) and the resistor conductive path (22, 42) or the electrode pad (13, 23, 43).

5. 4. The method according to claim 1, wherein the covering layer (14, 34, 44) is applied to cover the entire surface of the base layer (20) on the side where the resistor conductive path layer is laminated; The method further comprises the step of creating an opening (47) through said overlying cover layer (14, 34, 44) to said electrode pad (13, 23, 43).

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