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

Abstract

To provide a strain gauge insulated from 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) are provided, the method of manufacturing the strain gauge comprises the step of producing a coating base layer or a coating cover layer (34) by forming a moisture barrier coating on a surface the strain gauge (31).SELECTED DRAWING: Figure 10

Description

[Technical field]

[0001] The present invention relates to a strain gauge that is insulated against the ingress of moisture, and to a method for manufacturing the strain gauge. [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 manufacturing process of the strain gauge, b) to the new strain gauge itself, or c) to the strain gauge as it is 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] 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 metal resistor tracks are arranged in the form of a serpentine structure. The metal resistor tracks are arranged on a metal resistor foil material, which is combined with the support substrate to become a strain gauge by known lamination methods followed by known chemical etching methods. The metal resistor foil material is also arranged with connector electrodes or electrode pads for contacting the resistor tracks. The connector electrodes are often made together with the resistor tracks in one working operation and therefore often consist of the same material, which is often constantan due to its low temperature dependency. Depending on the field of application, the support substrate may be glass, a ceramic material, often a polymer, a glass fiber reinforced polymer, or a composite. To measure mechanical deformations due to forces or stresses acting on the object, one or more strain gauges are adhesively attached to the surface of the object that is not subjected to stress. 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 metal resistor conductive path of the strain gauge, which can be measured. In the case of a strain gauge load cell, the strain gauge is attached to the surface of the elastically deformable body (also called the spring element) of the load cell, and measurements of the electrical resistance are used to determine the magnitude of gravitational forces acting on the load cell.

[0004] The metallic resistor tracks of the strain gauge can be covered by a cover layer, referred to herein as a mechanical protective cover, the purpose of which is to protect the sensitive resistor tracks from direct mechanical contact and to protect the resistor tracks from dirt and dust adhesion. The metallic resistor tracks are therefore sandwiched between a substrate foil and a mechanical protective cover, both of which are made of a polymeric material with a thickness of about 10-20 μm (micrometers). Polymeric materials used in strain gauges include, for example, polyimide, phenolic aldehyde, polyether ether ketone (PEEK), and related types of these materials. On the other hand, apart from the properties that make them suitable for strain gauges, these materials have the disadvantage of being hygroscopic, which means that they retain moisture in a ratio that varies depending on the humidity of the surrounding atmosphere. When absorbing moisture, the volume of the polymeric material of the strain gauge swells, which causes a slight deformation of the metallic resistor tracks that are tightly bonded to the polymeric material. In addition to volume change, moisture absorption also causes a change in the elastic modulus of the strain gauge's polymeric material. In the case of strain gauge load cells, the changes in volume and elastic modulus of the strain gauge's polymeric material can have a detrimental effect on the metrological performance of the load cell, causing a drift in the load cell's no-load signal (zero point) and / or a drift in sensitivity (signal / load ratio) and / or creep (the initial signal strength decreases as the load cell is continued to be loaded for an extended period of time). This causes the temperature to slowly deviate from the indicated value.

[0005]

[0004] Therefore, to ensure that the metrological accuracy and reliability of strain gauge load cells are maintained over time with varying atmospheric humidity, the strain gauges must be protected against the ingress of moisture. Load cells used in so-called "legal for trade" applications are government regulated and internationally standardized according to "OIML R60-Parts 1 and 2, Metrological Regulation for Load Cells" issued by OIML (Organisation Internationale de Metrologie Legale, Paris, France). Provisions of 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 cells must pass these required OIML tests at various levels of specific atmospheric temperature and humidity. In these standardized procedures, the atmospheric humidity during the test is necessarily changed in defined time cycles for a limited period of time.

[0006]

[0005] 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 US Pat. No. 4,957,177, in a cantilever load cell (also known as a curved beam load cell, or a moment-insensitive load cell), this can be achieved by enclosing the curved beam with the strain gauge in a corrugated metal bellows. Here, the edges of the bellows are welded to cylindrical end pieces at the fixed and movable ends of the cantilever load cell, respectively. The bellows may be filled with a gas, for example dry nitrogen. Although the bellows enclosure seals the strain gauge area of ​​the load cell and thus provides absolute protection of the strain gauge against atmospheric humidity, it also has some significant drawbacks. Residual stresses are introduced by the welding process, which may relax over time or when the weighed load is applied, reducing the measurement accuracy of the load cell by drift and hysteresis of 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 weighing scales, one of the most important applications of low capacity strain gauge load cells. Additionally, the bellows, welding process, and subsequent leak testing of the hermetic seals 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 area of ​​the serpentine resistor track with a metal foil over an electrically insulating intermediate layer, leaving the connector tabs at the ends of the resistor track 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, the strain gauge itself is manufactured and sold already including a metal foil covering, as described, for example, in U.S. Pat. No. 5,631,622. The metal foil protects the resistor track against the ingress of moisture through the top surface. However, due to the relatively high elastic modulus of the metal foil material, combined with the viscosity of the strain gauge's polymeric material and adhesive bonding layer, metal foils have a significant effect on the measurement performance of the load cell, causing the load cell's no-load signal (zero point) to drift and / or its sensitivity (signal / load ratio) to drift and / or creep (slowly deviating from the initial indication when the load is continued on the load cell for a long period of time). As with the corrugated metal bellows mentioned above, these effects are most pronounced in load cells with low metering capacity. Furthermore, there is a risk that holes or gaps in the intermediate layer between the resistor track and the foil cover could cause a short circuit that bypasses a portion 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 manner as described above.

[0009]

[0008] According to a third state of the art solution, described for example in EP 1560011 A1, a strain gauge for use in a load cell is 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 a number of separate 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 multi-layer inorganic coating having a number of separate layers of different materials can consist of alternating separate layers of silicon nitride and silicon oxide. Other possible materials include metals, carbides and fluorides. Coatings whose material composition varies continuously through the thickness are often called 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 aforementioned third solution as described in EP1560011A1 can be applied during the manufacturing process of the strain gauge. The resulting product is a strain gauge that is already protected against moisture before it is mounted on the load cell. Alternatively, the protective coating can be applied to a conventional (i.e. uncoated) strain gauge after it has been mounted on the load cell body. In the latter alternative, the deposition process for the protective coating significantly increases the manufacturing costs, since the entire load cell has to be placed in a deposition chamber.

[0011] According to US Patent No. 5,052,505, cantilever load cells can be protected against moisture by recessing the surface area on which the strain gauges are to be mounted and then covering the recess with a moisture-proof cover sheet, e.g., 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 US Patent No. 5,052,505 makes leak testing unlikely, in contrast to the solution of the aforementioned US Patent No. 4,957,177, where 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 curved element comprises a substrate made of a resin material, a resistor provided on a surface of the substrate, and a fusing layer provided on a surface opposite to the surface provided with the resistor. The purpose of the fusing layer is to electrically insulate the strain gauge with respect to the curved element. The fusing layer is a thermoplastic polyimide layer, which may contain, in addition to the 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 so as to adjust the linear expansion coefficient of the thermoplastic resin to a value close to that of the curved element, and is therefore applied only to the surface of the strain gauge facing the curved element. Summary of the Invention [Problem to be solved by the invention]

[0013] Considering 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 laid down by standard OIML R-76 for measuring instruments of accuracy classes I, II and III. - the metrological performance of said humidity insulated load cells is not impaired as a result of the insulation when compared with an otherwise identical load cell which is not humidity insulated. - This insulation can be applied during the manufacturing process of the strain gauge, 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 technology. [Means for solving the problem]

[0014]

[0013] The above mentioned object is achieved by a strain gauge according to independent claim 1 and by the methods according to independent claims 7, 12, 13 and 14. Detailed aspects, further developed versions and variants of the strain gauge and the 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 provided with a moisture barrier coating on its surface.

[0016]

[0015] The strain gauge insulated against the ingress of moisture 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 meandering 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 overlying cover layer is placed over the surface of the resistor conductive track 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 such 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 producing a strain gauge that is insulated against moisture ingress includes the steps of providing a cover layer made from a polymeric film foil material, producing the coated cover layer by forming a moisture barrier coating on a surface of the coated cover layer by a deposition process, and attaching the coated cover layer to the strain gauge to insulate at least one 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 application of the coating cover layer, the surfaces of the strain gauges 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 tracks or to the electrode pads, and is made in one work operation together with the resistor tracks and the electrode pads and therefore in most cases consists of the same material.

[0020]

[0019] It is also advantageous when applying the overlay cover layer to the strain gauge that the entire strain gauge can be covered with a moisture insulating polymer film, in which case openings pass through 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 invention is typically in the range of a few micrometers. A thicker thickness may be suitable for higher capacity load cells, such as shear beam load cells and 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 to be within the scope of the invention. In any event, it will be understood that the thickness of the cover layer is not a defining or limiting factor of the invention.

[0022]

[0021] The moisture barrier coating is preferably a non-metallic inorganic coating having a plurality of separate layers of different materials or in which the material composition varies continuously in the thickness direction perpendicular to the coating surface, such as for example the state of the art solutions described in EP1560011A1, 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 EP1560011A1, which is incorporated herein by reference in the present description. Suitable non-metallic inorganic materials for the moisture barrier coating used in the method according to the present invention include, for example, SiO 2 , Al 2 O 3 , 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]

[0023] The base layer or cover layer having the 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 currently available on the market, ie, strain gauges having a substrate without a moisture barrier coating.

[0025] According to the method of the present invention, the method step of applying the moisture insulating polymer film to the strain gauge can be carried out in three different ways A, B and C: A. Attachment of the strain gauge to the body of the load cell (or another object on which the strain measurement is being performed) The mounting and attachment of the coating cover layer onto the strain gauge are combined: the strain gauge is placed on the load cell body with the application of the necessary adhesive, the coating cover layer is draped over at least a portion of the strain gauge with the application of the necessary adhesive, the mounting of the strain gauge and coating cover layer to the load cell body is secured under pressure with clamps, and the entire assembly is heat cured in an oven to set the adhesive bond. B. The attachment of the coating cover layer is performed on a strain gage that has already been attached and subjected to heat curing of its adhesive bond. 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 under pressure with clamps, 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 manufacturing process of the strain gage 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 it can be performed 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 of individual strain gages which are the final product.

[0026]

[0026] The method of the present invention has important advantages over the assignee's previous solution in the above-mentioned EP1560011A1, particularly when the coating cover layer is applied to a strain gauge already placed or permanently attached to the body of the load cell. In the previous solution, it was necessary to place the entire load cell in a deposition chamber in order 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 a cover layer, and the resulting coating cover layer then covers the strain gauge, which makes more efficient use of the deposition chamber and therefore reduces costs significantly.

[0027]

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

[0028] [Figure 1] FIG. 2 is a three-dimensional view of a double cantilever load cell. [Diagram 2] FIG. 2 is a diagram of the load cell of FIG. 1 with a coating cover layer attached over the strain gauges. [Diagram 3] FIG. 2 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. 2 is a cross-sectional view of an overlay cover layer with a moisture barrier coating applied to both sides of a polymer foil. [Diagram 5] FIG. 1 is a cross-sectional view of an overlay 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. 2 is a top view of the strain gauge without the mechanical protective cover. [Figure 7] 7A and 7B are cross-sectional views of a strain gauge without a mechanical protective cover and with a moisture barrier coating on both sides of the base layer and on a very thin surface area around the edges of the base layer. [Figure 8] FIG. 2 is a top view of a strain gauge with a mechanical protective cover. [Figure 9] 9A and 9B are cross-sectional views 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] FIG. 2 is a cross-sectional view of a strain gauge mounted on the strain sensing area of ​​a load cell and having an overlying cover layer attached thereto, without a mechanical protective cover. [Figure 11] FIG. 2 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 overlying cover layer attached thereto. [Figure 12] Figure 12A shows a top view of a strain gauge protected with an overcoat cover layer, Figure 12B shows 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 shows a top view of a strain gauge protected over its entire surface with the overcoat cover layer having openings for contacting the electrode pads. [Figure 13] FIG. 2 is a cross-sectional view of a strain gauge protected by an overcoat cover layer but without a mechanical protective cover. [Figure 14] FIG. 2 is a cross-sectional view of a strain gauge having a mechanical protective cover protected by an overlying cover layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] FIG. 1 shows a double cantilever beam load cell 1 having top and bottom surfaces 2 and 3 (not visible), load receiving end 4 and mounting end 5, and upper and lower bent beam sections 6 and 7. Load receiving end 4 has two threaded holes 8 from the top surface 2 for mounting to a scale platform or other type of load receiving part. Similarly, mounting end 5 has two threaded holes from the bottom (not visible in the figure) for mounting load cell 1 to a scale base plate (not shown) or any other type of support substructure. The bent beam sections 6, 7 are formed by machining a suitably contoured aperture 9 through the center of the load cell 1. The aperture 9 is shaped to form a thin bridge section 10 in the bent beam sections 6, 7. Four strain gauges 12 (two in the upper bent beam section 6 and two (not visible) in the lower bent beam section 7) are mounted with their resistor conductive paths precisely aligned and positioned in the center of the thin bridge section 10. 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 the strain gauges 12 with M-Bond 43B (Mr. Wendell Blvd., Wendell, NC 27591, USA). The area covered by the overcoat cover layer 14 extends a little beyond the end 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 may also be replaced by rocker pin type load cells, or any object on which strain measurements are to be made, such as pressure sensors or airplane landing gear or truck chassis frames, as well as machines for static and dynamic testing, and building structures.

[0032]

[0031] The substrate of the overcoat cover layer 14 (see Figures 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 by 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 (Figure 3), both sides (Figure 4), or to a very thin surface area around both sides and edges (Figure 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 low capacity double cantilever beam load cells 1 is in the range of a few micrometers. For load cells of higher weighing capacity, such as shear beam load cells or cylindrical load cells, a thicker thickness is suitable. For example, a thickness of 500 μm (micrometer) 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 invention include, for example, SiO 2 , Al 2 O 3 , 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 up to 2000 nanometers depending on the manufacturing process used.

[0033] According to their layer structure, there are two types of commercially available strain gauges. In the so-called open strain gauge 21A (shown in top view in FIG. 6 and in cross section in FIG. 7A), the metal resistor conductive path 22 in the form of a meander structure terminating in an electrode pad 23, backed by a base layer 20 of a polymer substrate foil, is exposed on the top, i.e. the open strain gauge 21A has no mechanical protective cover. In the so-called protected strain gauge 25A (shown in top view in FIG. 8 and in 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 strain gauge 21A and the protected 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 placing the resistor conductive path and electrode pads in the form of a serpentine structure.

[0035]

[0034] In the method according to the present invention, the step of applying a coating cover layer to the strain gauge can be performed in various 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 gauges are combined with each other. After the strain gauges 31, 35 are placed in position on the load cell body 33 using the 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 by bonding with the aforementioned M-Bond. A strain gauge adhesive 38 such as 43B is used to cover the strain gauges 31, 35 and the surface of the load cell body 33 in areas including the adjacent border area 37. The adhesive bond 38 can also act 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. A coated side facing inward is also possible and falls within the scope of the 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]

[0036] In detailed step 2, a coating 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 coating cover layer 34 is applied over the strain gauges 31, 35 and the surface of the load cell body 33, including the adjacent border area 37, using, for example, M-Bond 43B as previously described. The coating cover layer 14 having a one-sided moisture barrier coating 17 is attached with the coated side facing outwards. The electrode pads 43 of the strain gauges 31, 35 are now exposed to at least a small portion of the coating 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 overcoat cover layer 34. Optionally, as a variation of Detailed Procedure 2, the overcoat cover layer 34 can be attached using a room temperature cure adhesive, in which case the adhesive bond of the overcoat cover layer 34 will set within a specified time without oven curing.

[0038]

[0037] The overcoat cover layer 14, 34, 44 provides a high degree of protection since moisture cannot penetrate the moisture barrier coating 17 of the polymer film 15. Lateral ingress of moisture through the adhesive layer between the overcoat cover layer 14, 34, 44 and the surface of the load cell body 33 is minimized because the exposed ends of the adhesive layer are only 2-5 μm (micrometers) thick and the distance that water molecules have to travel from the exposed ends to the resistor conductive tracks 22 of the strain gauges 31, 35 is relatively long.

[0039]

[0038] In detailed step 3, the coating cover layer 14, 34, 44 is applied during the manufacturing process of the strain gauge, and the resulting product is a moisture insulating strain gauge. The application of the coating 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 are the final product. Figure 12A shows a top view of strain gauges 41, 45 (Figures 13 and 14) to which a coating 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 without a mechanical protective cover over the resistor conductive path 42 (as shown in the cross-sectional views of FIG. 7A and FIG. 13), or it may be a protected-type strain gauge 25A, 45 having 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 tracks and electrode pads 43. A small gap ensures that there is no conductive connection between the additional metal material 48 and the resistor tracks 42 or electrode pads 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 Figures 2, 8 and 12A, the strain gauges 12, 21A, 21B, 25A, 25B, 31, 35, 41, 45 can be covered with the overcoat layer 14, 34, 44. In this case, an opening 47 penetrates the overcoat layer 14, 34, 44 to the electrode pads 13, 23, 43 (see Figure 12C) so that the circuit lines can be soldered or welded to the electrode pads 13, 23, 43. The strain gauge in Figure 12C can be a moisture-insulated open-type strain gauge 21B as shown in Figure 7B without a mechanical protective cover over the resistor conductive path 42, or a moisture-insulated protected-type strain gauge 25B as shown in Figure 9B.

[0042]

[0041] Although the invention has been described with a particular example relating to load cells, it is considered self-evident that other applications and other ways of carrying out the method of the invention are likewise covered by 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-proofing of strain gauges of load cells used in weighing, but can be used generally for strain gauges which require protection against moisture without the drawback of impairing the measurement accuracy as a result of the protection measures. This can be applied, for example, to strain gauges used in pressure sensors or strain gauges used in airplane landing gear or strain gauges integrated into the chassis frames of trucks, 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. [Item 1] A strain gauge insulated against the ingress of moisture, the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) comprising: a base layer (20) made from a polymeric substrate foil material; a resistor conductive path layer on a metal resistor foil material, the resistor conductive path (22, 42) being in the form of a serpentine structure, the resistor conductive path layer having electrode pads (13, 23, 43) for contacting the resistor conductive path (22, 42); The resistor conductive path layer is laminated together with the base layer (20), and the strain gauges (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) are formed in the laminated layers by a chemical etching method, The moisture barrier coating (17) is formed on all sides of the base layer (20) by a deposition process such that the base layer (20) is enveloped by the moisture barrier coating (17); and / or The strain gauges (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) Further comprising a cover layer (15) made of a polymeric film foil material; a moisture barrier coating (17) is formed on at least one side of the cover layer (15) by a deposition process; A strain gauge, characterized in that the covering layer (14, 34, 44) is placed on a surface of the resistor conductive path layer to cover at least a portion of the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45). [Item 2] Item 1. The strain gauge according to item 1, an additional metal material (48) from the resistor conductive path layer is applied to the surface of the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) that is not covered by the coating cover layer (14, 34, 44); A strain gauge, characterized in that there is no conductive connection between said additional metal material (48) and said resistor conductive path (22, 42) or said electrode pad (13, 23, 43). [Item 3] Item 1. The strain gauge according to item 1, the covering layer (14, 34, 44) is placed so as to entirely cover the strain gauges (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45); A strain gauge, characterized in that an opening (47) is made through the overcoat cover layer (14, 34, 44) to the electrode pad (13, 23) of the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45). [Item 4] The strain gauge according to any one of items 1 to 3, The moisture barrier coating (17) On the top and bottom surfaces of the cover layer (15), or On all sides of the cover layer (15) such that the cover layer (15) is enveloped by the moisture barrier coating (17). A strain gauge, characterized in that it is deposited. [Item 5] 5. The strain gauge according to any one of the preceding claims, characterized in that the moisture barrier coating (17) has a thickness of 200 nanometers or less. [Item 6] Strain gauges (12, 21A, 21B, 25A, 25B, 31A, 31B, 32A, 32B, 32C, 32D, 32E, 32F) attached using an adhesive bond (38) to the load cell body (1) or to any object on which strain measurements are to be performed. , 35, 41, 45) in a load cell having A load cell, characterized in that the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) is one type of strain gauge according to any one of items 1 to 5. [Item 7] 1. A method for manufacturing a strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) insulated against the ingress of moisture, said method comprising the steps of: Providing a base layer (20) made from a polymeric substrate foil material; providing a resistor track layer on a metal resistor foil material, the resistor track (22, 42) being in the form of a serpentine structure and having electrode pads (13, 23, 43) for contacting the resistor track (22, 42); laminating the resistor track layer together with the base layer (20); producing strain gauges (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) in said laminated layers by chemical etching, the method further comprises the step of producing a coated substrate by forming the moisture barrier coating (17) on all surfaces of the substrate (20) by a deposition process, prior to the lamination of the resistor conductive track and the substrate (20), such that the substrate (20) is enveloped by a moisture barrier coating (17); and / or The method further comprising: Providing a cover layer (15) made from a polymeric film foil material; forming a moisture barrier coating (17) on a surface of said cover layer (15) by a deposition process to produce a covering cover layer (14, 34, 44); and applying the overcoat cover layer (14, 34, 44) to the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) to cover at least a portion of the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45). [Item 8] 8. The method according to item 7, characterized in that the surface of the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) that is not covered by the covering layer (14, 34, 44) is covered with additional metal material (48) from the resistor conductive path layer, 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). [Item 9] The method according to item 7, characterized in that the covering layer (14, 34, 44) is applied so as to entirely cover the strain gauges (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45), The method further includes creating an opening (47) through the overcoat cover layer (14, 34, 44) to the electrode pad (13, 23) of the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45). [Item 10] 10. The method according to any one of items 7 to 9, characterized in that the moisture barrier coating (17) has a thickness of less than or equal to 200 nanometers. [Item 11] The method according to any one of items 7 to 10, wherein the moisture barrier coating (17) is one side of the cover layer (15), or On both sides of the cover layer (15), or All surfaces of the cover layer (15) are covered by the moisture barrier coating (17). The method of claim 1, wherein the [Item 12] A method for mounting a strain gauge (12, 21A, 21B, 25A, 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 steps of: Applying an adhesive bond (38) to the load cell body (1) or any object on which strain measurements are to be performed and placing a conventional strain gauge (12, 21A, 25A, 31, 35) or a strain gauge (21B, 25B) according to item 1; Providing a cover layer (15) made from a polymeric film foil material; forming a moisture barrier coating (17) on a surface of said cover layer (15) by a deposition process to produce a covering cover layer (14, 34); applying the adhesive bond (38) over the overlay cover layer (14, 34) to cover at least a portion of the strain gauges (12, 21, 25, 31, 35); - fixing the combined attachment of the strain gauges (12, 21, 25, 31, 35) and the overcoat cover layer (14, 34) to the load cell body (1) or to any object on which strain measurements are to be performed under contact pressure; placing the load cell body (1) or the object on which strain measurements are to be performed with the strain gauges (12, 21, 25, 31, 35) and the coating cover layer (14, 34) fixed in place in an oven in order to thermally cure and harden the adhesive bond between the strain gauges (12, 21, 25, 31, 35) and the coating cover layer (14, 34); A method comprising: [Item 13] A method for attaching an overlay cover layer (14, 34) to an already attached conventional strain gauge (12, 21A, 25A, 31, 35) or a strain gauge (21B, 25B) according to item 1, comprising: Providing a load cell body (1) or an object on which strain measurements are to be performed, having at least one strain gauge (12, 21A, 21B, 25A, 25B, 31, 35) attached thereto and subjected to thermal curing of an adhesive bond; Providing a cover layer (15) made from a polymeric film foil material; forming a moisture barrier coating (17) on a surface of said cover layer (15) by a deposition process to produce a covering cover layer (14, 34); The covering layer (14, 34), Applying a heat-curing adhesive bond (38) or Apply room temperature curing adhesive (38) and covering at least a portion of the at least one attached strain gauge (12, 21A, 21B, 25A, 25B, 31, 35); Fixing the attachment of the overlay cover layer (14, 34) to the at least one strain gauge (12, 21A, 21B, 25A, 25B, 31, 35) under contact pressure; With the coating cover layer (14, 34) fixed to the at least one attached strain gauge (12, 21A, 21B, 25A, 25B, 31, 35), the load cell body (1) or the object on which strain measurements are performed is subjected to placing in an oven to heat cure and harden the heat curable adhesive bond (38) of the overcoat cover layer (14, 34); or leaving the laminate at room temperature for a predetermined period of time to cure and harden the room temperature curing adhesive bonding agent (38) of the overcoat cover layer (14, 34); A method comprising: [Item 14] A method for attaching a coating cover layer (44) during the manufacturing process of a conventional strain gauge (41, 45) or a strain gauge (21B, 25B) according to item 1, so that the resulting product is a moisture-insulated strain gauge (21B, 25B, 41, 45), comprising: At least two individual strain gauges (21B, 25B, 41, 45) or strain gauges providing at least one multi-unit sheet of the page (21B, 25B, 41, 45); Providing a cover layer (15) made from a polymeric film foil material; forming a moisture barrier coating (17) on a surface of the cover layer (15) by a deposition process to produce a cover layer (44); applying an adhesive bonding agent (38) over the overlay cover layer (44) to cover at least a portion of the individual strain gauges (21B, 25B, 41, 45) or at least a portion of the at least one multi-unit sheet of strain gauges (21B, 25B, 41, 45); fixing the attachment of the overlay cover layer (44) to the at least one multi-unit sheet of individual strain gauges (41, 45) or strain gauges (21B, 25B, 41, 45) under contact pressure; curing the strain gauges (21B, 25B, 41, 45) together with the coating cover layer (44) at a defined temperature for a defined time to harden the adhesive bond between the coating cover layer (44) and the strain gauges (21B, 25B, 41, 45); A method comprising: [Item 15] 15. The method according to any one of items 12 to 14, characterized in that the surfaces of the strain gauges (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45) that are not covered by the coating cover layer (14, 34, 44) are 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). [Item 16] The method according to any one of items 12 to 14, characterized in that the covering layer (14, 34, 44) is applied so as to entirely cover the strain gauges (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45); The method further includes creating an opening (47) through the overcoat cover layer (14, 34, 44) to an electrode pad (13, 23, 43) of the strain gauge (12, 21A, 21B, 25A, 25B, 31, 35, 41, 45). [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 polymeric material, 14 substrate, cover layer 17 Moisture Barrier Coating 20 Base layer 21A, 31, 41 Open type strain gauge 21B Moisture-insulated open-type 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, 35 38 Adhesive Bonding Agents 47 Aperture 48 Additional Metal Materials

Claims

1. A strain gauge insulated against the ingress of moisture, the strain gauge (12, 21B, 25B, 31, 35, 41, 45) comprising: a base layer (20) made of 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 polymeric 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 by a deposition process; A strain gauge, wherein the covering layer (14, 34, 44) is placed on a surface of the resistor conductive path layer laminated on the base layer (20).

2. 2. The strain gauge according to claim 1, a portion of the surface of the base layer (20) on the side on which the resistor conductive path layer is laminated that is not covered by the covering cover layer (14, 34, 44) is covered with an additional metal material (48) from the resistor conductive path layer; A strain gauge, characterized in that there is no conductive connection between said additional metal material (48) and said resistor conductive path (22, 42) or said electrode pad (13, 23, 43).

3. 2. The strain gauge according to claim 1, The covering layer (14, 34, 44) is placed so as to cover the entire surface of the base layer (20) on the side on which the resistor conductive path layer is laminated, A strain gauge, characterized in that an opening (47) is made through said overcoat cover layer (14, 34, 44) to said electrode pad (13, 23).

4. The strain gauge according to any one of claims 1 to 3, The moisture barrier coating (17) On all sides of the cover layer (15) such that the cover layer (15) is enveloped by the moisture barrier coating (17). A strain gauge, characterized in that it is deposited.

5. 5. A strain gauge according to any one of claims 1 to 4, characterized in that the moisture barrier coating (17) has a thickness of less than or equal to 200 nanometers.

6. A load cell having a strain gauge (12, 21B, 25B, 31, 35, 41, 45) attached to a load cell body (1) or to any object on which strain measurements are to be performed using an adhesive bond (38), A load cell, characterized in that said strain gauge (12, 21B, 25B, 31, 35, 41, 45) is one type of strain gauge according to any one of claims 1 to 5.

7. A method for manufacturing a strain gauge (12, 21B, 25B, 31, 35, 41, 45) insulated against the ingress of moisture, said method comprising the steps of: Providing a base layer (20) made from a polymeric substrate foil material; laminating a resistor conductive path layer onto the base layer (20); and generating, by a chemical etching method, resistor conductive paths (22, 42) in the form of meandering structures in the resistor conductive path layer and electrode pads (13, 23, 43) for contacting the resistor conductive paths (22, 42). The method further comprising: Providing a single cover layer (15) made from a polymeric film foil material; Producing a coating cover layer (14, 34, 44) by forming a moisture barrier coating (17) of a non-metallic inorganic material on the surface of said cover layer (15) by a deposition process; and attaching the covering layer (14, 34, 44) to a surface of the resistor conductive path layer laminated on the base layer (20). On both sides of the cover layer (15), or The method of claim 1, wherein the cover layer (15) is deposited on all sides of the cover layer (15) such that the cover layer (15) is enveloped by the moisture barrier coating (17).

8. 8. The method according to claim 7, characterized in that an additional metal material (48) from the resistor conductive path layer is applied to a portion of the surface of the base layer (20) on the side on which the resistor conductive path layer is laminated, the portion not covered by the covering cover layer (14, 34, 44), 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).

9. 8. The method according to claim 7, characterized in that the covering layer (14, 34, 44) is applied so as to cover the entire surface of the base layer (20) on the side on which the resistor conductive path layer is laminated, The method further comprises creating an opening (47) through said overlying cover layer (14, 34, 44) to said electrode pad (13, 23).

10. 10. The method according to any one of claims 7 to 9, characterized in that the moisture barrier coating (17) has a thickness of less than or equal to 200 nanometers.

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