Method for attaching a strain gauge to a component, and component producible by such a method

Abstract

The invention relates to a method for attaching a strain gauge (1) to a component (7). The strain gauge (1) is placed on the component (7), aligned, and fixed. According to the invention, it is proposed that the strain gauge (1) located on the component (7) is subsequently overmolded with a plastic (9a). Furthermore, a component that can be manufactured using such a process will be placed under protection.

Description

[0001] The invention relates to a method for attaching a strain gauge to a component. The invention also relates to a component that can be manufactured by such a method.

[0002] Strain gauges are commonly used to measure strain or compression on the surface of components. This allows for the detection of mechanical stresses and thus the loads on a component. A common design is the foil strain gauge. In a foil strain gauge, a measuring grid film made of meandering resistance wire is laminated onto a thin plastic substrate, photolithographically etched, and provided with electrical connections. After the strain gauge is attached to a component, its electrical connections can be connected to electrical leads, which in turn can be connected to a suitable power supply and to appropriate evaluation electronics for transmitting and processing the generated signals.

[0003] Attaching a strain gauge to a component, also known as strain gauge application, is typically done manually. This requires additional adhesive or resin to bond the strain gauge to the component. The strain gauge must then be covered with a suitable cover to protect the measuring point from environmental influences. This common procedure has the disadvantage that the adhesive or resin material differs from the material of the actual structural component to which the strain gauge is attached. Beyond the additional labor and higher costs, this can also negatively impact the component's strength and service life.

[0004] For example, EP 0 469 323 A2 discloses a method for manufacturing and applying a strain gauge. Specifically, an electrically conductive layer is first applied to a substrate, from which several measuring grids are then produced using a photolithographic process. Next, the back side of the substrate opposite the measuring grids is coated with a phenolic resin layer, which is pre-dried but not fully cured. After cutting out individual strain gauges, the strain gauge is applied to a component to be tested, such as a force transducer. For this purpose, the strain gauge is first aligned and fixed to the component using adhesive tape. Then, the strain gauge is pressed onto the component using a clamping device, and the entire assembly is baked in an oven to cure. The phenolic resin layer cures under the influence of the heat and bonds firmly to the surface of the component.

[0005] US Patent 10,119,869 B1 describes a method for manufacturing a strain gauge device, in which a first substrate with a first, malleable substrate film for receiving electronic components is present. A strain gauge is then printed onto the first substrate using a printed circuit board process, thus bonding it firmly to the substrate. The first substrate containing the printed strain gauge is then formed, which can be done by thermoforming, cold forming, vacuum forming, pressure forming, or high-pressure forming. Finally, a layer of material is applied over the strain gauge by injection molding, thereby embedding the strain gauge in the cast material layer.

[0006] The present invention aims to propose a method for attaching a strain gauge to a component that is simple and cost-effective. Furthermore, the method should enable reliable and permanent attachment of the strain gauge to the component. A further objective of the invention is to propose a component that can be manufactured using the method according to the invention.

[0007] This problem is solved by a method having the features of claim 1 and by a component having the features of claim 7. Advantageous embodiments or further developments of the invention can be found in the dependent claims.

[0008] The invention relates to a method for attaching a strain gauge to a component. The strain gauge is placed on the component, aligned, and fixed in place. This fixing action initially prevents the strain gauge from slipping on the component.

[0009] According to the invention, the DMS located on the component is subsequently overmolded with a plastic. In this way, the DMS is permanently attached to the component and also well protected against environmental influences. Furthermore, the inventive method also makes it possible to simultaneously injection-mold one or more functional parts onto the component during the injection molding process while the DMS is being overmolded. This eliminates the need for a separate step in the process. After the desired injection-molded part is completed, a separate step for applying the DMS is not required.

[0010] Following further training, it is proposed that the strain gauge be aligned and fixed using a recess incorporated into the component. This has the advantage that the strain gauge can be inserted into the recess quickly and easily. This ensures that the strain gauge is already aligned and secured against slippage along its surface. The recess is thus shaped to accommodate the strain gauge. The outline of the recess preferably corresponds to the outline of the strain gauge. To facilitate insertion of the strain gauge into the recess, the outline of the recess is preferably chosen to be slightly larger than the outline of the strain gauge. The recess can preferably be embossed into the component, which represents a simple method for creating the recess.

[0011] According to an alternative training method, the alignment and fixation of the strain gauge can also be achieved using several pin- or needle-like elements connected to the component. These pin- or needle-like elements are positioned relative to each other so that the strain gauge can be inserted between them. Once inserted, the strain gauge is aligned and simultaneously fixed, thus preventing it from slipping along its surface. This method also allows for simple alignment and fixation of the strain gauge.

[0012] Another method proposes connecting the strain gauge to electrical supply lines using its electrical connection cables, thereby simultaneously using these supply lines for aligning and fixing the strain gauge to the component. This has the advantage that components already required for aligning and fixing the strain gauge can be used, eliminating the need for a separate fixing device. The electrical supply lines can also be used to power the strain gauge and connect it to evaluation electronics.

[0013] According to another embodiment of the invention, the strain gauge can also be fixed to the component by creating a vacuum on the side of the strain gauge that rests on the component after the strain gauge has been placed and aligned. This offers the advantage that the strain gauge can also be securely fixed after manual alignment and will not slip during the injection molding process.

[0014] Preferably, the vacuum can be generated in a channel that opens into a designated area on the component where the strain gauge is placed. The channel is subjected to vacuum at least during the injection molding process. The channel can, for example, be pre-drilled into the component as a bore. After the injection molding process, in which the strain gauge is overmolded with plastic, the channel can be sealed again in a subsequent injection molding process if necessary.

[0015] As mentioned at the beginning, the present invention is also intended to protect a component which can be manufactured using the inventive method.

[0016] A strain gauge is attached to such a component, the strain gauge being covered by a plastic overmolding. The component according to the invention is designed such that the strain gauge is at least partially received in a recess of the component or arranged between pin- or needle-like elements attached to the component. With such a component, stress measurements can be reliably carried out due to the secure retention and protection of the strain gauge.

[0017] After the component has been formed, the depth of the recess is dimensioned such that the strain gauge, when positioned within it, does not protrude beyond the recess. In other words, the recess is chosen to be deep enough that the strain gauge inserted into it is, at most, flush with the surface of the component surrounding the recess. Such a design can, for example, have a beneficial effect on the injection molding process when overmolding the strain gauge, as it reduces the resistance encountered by the injection molding material.

[0018] In an alternative design of the component, where pin- or needle-like elements are present, these elements are rounded at the ends that protrude from the component. This design facilitates the manufacturing of the component, specifically the insertion of the strain gauge between the pin- or needle-like elements.

[0019] The component itself can be made of different materials. For example, it could be made of metal, plastic, or even wood. A component made of fiber-reinforced plastic is also considered a plastic component.

[0020] The process can be easily automated and thus accelerated. For example, it is conceivable that the component could first be positioned in an injection mold using a robot. The robot could then place the strain gauge into the recess that serves as a fixing and alignment aid, or between the pin- or needle-like elements. Finally, the injection mold can be closed and the injection molding process carried out to overmold the strain gauge.

[0021] This results in a component that features a captive strain gauge and is well protected against environmental influences. Simultaneously with the overmolding of the strain gauge, an additional functional component can also be injection-molded if required. This could be, for example, a connecting flange, a threaded fitting, etc.

[0022] Preferred embodiments of the invention are illustrated in the figures and are explained in more detail in the following description with reference to the figures. This also clarifies further features and advantages of the invention. Identical reference numerals, even in different figures, refer to identical, comparable, or functionally equivalent components. Corresponding or comparable properties and advantages are achieved even if no repeated description or reference is made to them. The figures are not, or at least not always, to scale. In some figures, proportions or distances may be exaggerated to more clearly emphasize features of an embodiment.

[0023] They show, schematically, each one Fig. 1. A representation of a foil DMS according to the state of the art, Fig. 2 A partial representation of a component equipped with a strain gauge in an injection mold and in a first embodiment, during an injection molding process, Fig. 3 a representation of the DMS according to view III from Fig. 2, after the injection molding process, Fig. 4 a partial representation of a component equipped with a DMS, comparable to the view in Fig. 3, however, in a second embodiment, before the injection molding process, Fig. 5 a representation of the DMS according to view V from Fig. 4 and Fig. 6 a partial representation of a component equipped with a strain gauge in an injection mold comparable to the view according to Fig. 2, however in a third embodiment and during the injection molding process.

[0024] In Fig. Figure 1 shows an example of a strain gauge 1 (SG). The SG 1 has a rectangular outline, but can also have any other outline, e.g., round or oval. The SG 1 is designed as a so-called foil strain gauge. It has a carrier layer 2. This is preferably foil-like. A measuring grid 3 is applied to the carrier layer 2, for example, by a photolithographic process. The measuring grid 3 preferably consists of an electrically conductive material, for example, constantan.

[0025] Furthermore, it can be seen that extensions of the measuring grid 3 are equipped with electrical contacts 4. Electrical connecting leads 5 are attached to the electrical contacts 4. These serve to connect the strain gauge to electrical supply lines for a power supply and evaluation electronics (not shown).

[0026] Based on the Fig. 2 and Fig. Section 3 now describes a first embodiment of how the DMS 1 can be attached, i.e., applied, to a component 7: In a step not shown, component 7 was first placed into a first part 6a of an injection mold 6. In a subsequent process step, the strain gauge 1 was placed into a recess 7a formed in component 7. The recess 7a was preferably pre-pressed into component 7. This allows the strain gauge 1 to take the pressure in the Fig. 2 and Fig. 3. The position is visible. In this position, the DMS 1 is thus aligned to a desired orientation and fixed in this orientation, i.e., secured against slipping.

[0027] It is evident that the recess 7a is dimensioned in height such that the strain gauge 1 does not protrude beyond the recess 7a. Preferably, the height of the recess 7a is dimensioned such that the strain gauge 1 lies approximately flush with a surface of the component 7 that surrounds the recess 7a.

[0028] In a further step, a second part 6b of the injection mold 6 is placed onto the first part 6a of the injection mold 6, thus closing the injection mold 6. This creates a cavity 6c between the strain gauge 1 and the second part 6b of the injection mold 6.

[0029] This is followed by a further work step in which molten plastic 9a is injected into the cavity 6c via a feed channel 6d in the second part 6b of the injection mold 6. For this purpose, plastic granules 9 contained in a storage container 8 are heated in a heat and pressure generator 10 and fed into the cavity 6c under pressure. Due to the recess 7a, the strain gauge 1 does not slip even under the forces acting on it during the injection molding process.

[0030] After the subsequent opening of the injection mold 6, the DMS 1 is provided with a plastic overmolding 9b (indicated by dashed lines) which covers the entire DMS 1 (see figure). Fig. 3) This ensures that it is permanently attached to component 7 and well protected from environmental influences.

[0031] As an alternative to recess 7a, it is also conceivable that the DMS 1 is aligned and fixed to a component 7b using pin-like elements 11. This is shown in the Fig. 4 and Fig. Figure 5 illustrates a further embodiment, showing only component 7b with the strain gauge 1 aligned and fixed to it. An injection mold is not shown.

[0032] In the exemplary embodiment, four pin-like elements 11 are present, each pin-like element 11 being connected to component 7b. This can be achieved by each pin-like element 11 having a free end 11a and an end 11b embedded in component 7b. Alternatively, it is also conceivable that the pin-like elements 11 are integrally connected to component 7b, which can be accomplished, for example, in a prior injection molding process if component 7b and the pin-like elements 11 are made of plastic. In any case, the pin-like elements 11 are positioned relative to each other such that the strain gauge 1 can be easily inserted between them. Specifically, the pin-like elements 11 are positioned such that the strain gauge 1 is fixed at diagonally opposite corners 1a and 1c. Alternatively or additionally, the strain gauge 1 can also be fixed at other corners 1b and 1d by pin-like elements 11.Furthermore, the pin-like elements 11 can be dimensioned in length such that they project beyond the strain gauge 1. It is also conceivable to dimension the pin-like elements 11 in length such that they remain below an upper edge of the strain gauge or are flush with such an edge. To facilitate insertion of the strain gauge 1 between the pin-like elements 11, these are preferably rounded at their free ends 11a.

[0033] Furthermore, in a deviation from the exemplary embodiment, it is also conceivable that pin-like elements are connected to a part of an injection mold and positioned relative to each other in such a way that, when the injection mold is closed, they hold down the strain gauge 1 aligned with component 7b (not shown). This has the particular advantage that it reliably prevents plastic from getting under the strain gauge 1 during the injection molding process.

[0034] Finally, in the Fig. Figure 6 shows another embodiment in which a component 7c, together with a previously placed and aligned strain gauge 1, was inserted into an injection mold 6 and the mold was closed. This again formed a cavity 6c.

[0035] Beneath the strain gauge 1, i.e., beneath the side where the strain gauge 1 rests on component 7c, there is a channel 7d in component 7c. The channel 7d opens into a surface 7e of component 7c, on which the strain gauge 1 rests. The other end of the channel 7d is fluidically connected to a through-opening or to another channel 6e located in a second part 6b of the injection mold 6. A vacuum pump 12 is connected to this channel 6e by means of a connecting part 13. The vacuum pump 12 creates a vacuum in the channel 7d, thus fixing the strain gauge 1, which is aligned with component 7c. Subsequently, analogous to the Fig.2. An injection molding process was started and molten plastic 9a was injected under pressure into cavity 6c.

[0036] After the injection mold 6 is subsequently opened, the DMS 1 is again covered with a plastic overmolding that completely encapsulates the DMS 1 (not shown). This ensures that it is permanently attached to component 7c and well protected from environmental influences.

[0037] The previously mentioned work steps, namely positioning a component in an injection mold, placing and, if necessary, aligning a strain gauge on the component, and removing the finished component with the overmolded strain gauge from the injection mold, can all be automated and carried out using a suitable handling robot. Reference symbol list 1 strain gauge (SG) 1a-1d corners Page 1 2 Carrier layer 3 measuring grids 4 electrical contacts 5 electrical connection cables 6 injection molds 6a first part of the injection mold 6b second part of the injection mold 6c cavity 6d Feed channel 6th channel 7, 7b, 7c Component 7a Advanced 7d Channel 7e Surface 8 storage containers 9 plastic granules 9a molten plastic 9b Plastic overmolding 10 heat and pressure generators 11 pen-like elements 11a free ends 11b recessed ends 12 Vacuum pump 13 Connection part QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 0 469 323 A2

[0004] US 10 119 869 B1

[0005]

Claims

[1] Method for attaching a strain gauge (1) to a component (7, 7b, 7c), wherein the strain gauge (1) is placed on the component (7, 7b, 7c), aligned and fixed, characterized by , that the strain gauge (1) located on the component (7, 7b, 7c) is subsequently overmolded with a plastic (9a). [2] Method according to the preceding claim, characterized by , that the strain gauge (1) is aligned and fixed by means of a recess (7a) which is provided in the component (7). [3] Method according to claim 1, characterized by , that the strain gauge (1) is aligned and fixed by means of several pin- or needle-like elements (11) which are connected to the component (7b), wherein the pin- or needle-like elements (11) are positioned relative to each other in such a way that the strain gauge (1) can be inserted with its outline between the pin- or needle-like elements (11). [4] Method according to claim 1, characterized by , that the strain gauge (1) is connected to electrical supply lines by means of electrical connecting leads (5) of the strain gauge (1) and is thereby aligned and fixed. [5] Method according to claim 1, characterized by , that after placing and aligning the DMS (1) on the DMS (1) a vacuum is applied to such a side (1e) of the DMS (1) with which the DMS (1) rests on the component (7c). [6] Method according to the preceding claim, characterized by , that the negative pressure is generated at a channel (7d) which opens into such an area on the component (7c) on which the strain gauge (1) is placed. [7] Component (7, 7b, 7c), producible by a method according to one of the preceding claims, wherein a strain gauge (1) is attached to the component (7, 7b, 7c) and the strain gauge (1) is covered by a plastic overmolding (9b), characterized by, that the strain gauge (1) is at least partially received in a recess (7a) of the component (7) or arranged between pin- or needle-like elements (11) attached to the component (7b). [8] Component (7) according to the preceding claim, characterized by , that the recess (7a) is dimensioned in height such that the strain gauge (1) does not protrude beyond its position in the recess (7a). [9] Component (7b) according to claim 7, characterized by , that the pin- or needle-like elements (11) are rounded at the ends (11a) with which they protrude from the component.

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