EP2409122A1

EP2409122A1 - Sealed sensor with strain gauges

Info

Publication number: EP2409122A1
Application number: EP09824254A
Authority: EP
Inventors: Jean-Pierre Aumard; Vincent Bourdarel
Original assignee: S C A I M E SA
Current assignee: S C A I M E SA
Priority date: 2009-03-19
Filing date: 2009-03-19
Publication date: 2012-01-25
Also published as: US20110232393A1; CA2747851A1; WO2010106388A1; US8561482B2

Abstract

The invention relates to a sealed sensor comprising a test body in the form of a deformable simple parallelogram, characterised in that it includes a one-piece test body (1) having a generally H- or X-shaped central opening (2) that extends through the body (1), the shape of which determines the position of the four hinges (a, b, c, d) of the deformable parallelogram, between the end of each arm of the opening (2) and one face of the test body (1), and the thickness thereof. At least one of the hinges is a measuring hinge located on one side of the test body, the outer face of said body being provided with a recess (5) and a housing (8) in which the strain gauges and the associated electric circuit are disposed respectively. At least one pair of side cavities (12) define therebetween the width of the measuring hinge(s) and the thickness of the sheets of material (13) connecting the side edges of this measuring hinge to the side edges of the test body corresponding to said hinge, such as to prevent strains generated in the measuring hinges from being affected by strains originating from the side edges of the test body (1). At least one metal cover is welded to the test body in order to seal the recess (5) and the housing (8), the latter including a passage to the exterior of the test body intended to receive a sealed raceway.

Description

Watertight strain gauge sensor

The present invention relates to a measuring sensor of the type comprising a monolithic test body provided with strain gauges fixed on this test body in at least one of its zones of elastic deformation under the effect of an applied force. audit body.

Generally the test body is formed of a monolithic deformable bar which can have the form of a deformable parallelogram whose one end is fixed on a fixed frame and the other is arranged to receive a plate on which is placed the load to measure. Such sensors are described for example in US 4,150,729, US 4,546,838 or US 4,107,985.

The present invention aims at the realization of such a sensor that can be used in a hostile or corrosive environment, salt water, chlorine or acid, etc., which requires to protect the strain gauges to prevent their deterioration.

Generally the strain gauges are protected by a coating or film of a material resistant to corrosive agents. However, experience shows that it is always this protection that degrades in the first place, requiring the replacement of the sensor although its mechanical part, the test body, is still intact.

Sensors have been proposed in which the strain gauges are enclosed in sealed cavities as described, for example, in US Pat. No. 4,488,611. This embodiment, however, requires the welding of covers or metal protective elements on the test body in the immediate vicinity of the deformation zones thereof, to encapsulate sealingly the strain gauges. This way of doing things allows long-term protection of the strain gauges but deteriorates the accuracy and reliability of the measurement. Indeed, the fact of welding these covers or protections in the immediate vicinity of the deformation zones of the test body induces modifications structural elements of the material constituting the test body in these areas including changing its elastic characteristics which are the source of inaccuracies or measurement errors that can not be easily corrected electrically.

In addition these sealed sensors require a measurement beam separate from the two arms of the deformable parallelogram of the test body, generally placed in the central recess of this test body, which complicates its machining and thus greatly increases the sealed sensor.

The licensee has developed a leakproof sensor described in EP-1319931 in which the strain gauges and their associated electrical circuit are placed in a cavity of the test body whose edges to be closed by a welded plate are located in a zone of the test body not participating in the measurement, these edges being decoupled in particular from the measuring beam. However here also the test body comprises a main beam, connected to the measuring beam, distinct from the sides of the deformable parallelogram and through the central cavity of the test body whose machining remains complex and expensive.

The object of the present invention is the production of a strain-gage-proof sensor in which, on the one hand, the cavities receiving the strain gauges and their associated electrical circuit are arranged in such a way that their edges intended to be connected by welding to a metal cover are remote and mechanically decoupled from the deformation zones used for the measurement and, secondly, whose test body in the form of deformable parallelogram has a simple machining and in particular does not include a beam passing through the cavity central of this test body, so that we obtain a precise and cheaper sensor.

Thus, the purpose of the present invention is to provide a sensor comprising a monolithic proof body and encapsulated strain gauges, thus protected from the environment, which obviates the aforementioned drawbacks, that is to say which is resistant, the protection of gauges does not influence the measurement characteristics and is a lower cost of manufacture.

The present invention relates to a sealed measuring sensor comprising a monolithic proof body provided with strain gauges which is distinguished by the characteristics listed in the independent claim 1. The particular embodiments of the invention are defined in the claims dependent.

The accompanying drawing illustrates schematically and by way of example two embodiments of the sensor according to the invention. Figure 1 is a perspective view of the sensor without its closure cap.

Figure 2 is a top plan view of the sensor shown in Figure 1.

Figure 3 is a bottom plan view of the sensor shown in Figure 1.

FIG. 4 is a side view of the sensor illustrated in FIG. 1. FIG. 5 is a section along the line A-A of FIG. 4.

Figure 6 is an enlarged detail of Figure 5.

Figure 7 is a perspective view of the sensor shown in Figure 1, its closure cap being welded in place on the test body.

Figure 8 is a top view of a second embodiment of the sensor.

Figure 9 is a side view of the sensor shown in Figure 8.

Figure 10 is a bottom view of the sensor shown in Figure 8.

Figure 11 is a perspective representation of the sensor according to the second embodiment. The subject of the present invention is a strain-gage-type weighing sensor of the single parallelogram type. To achieve the proposed purpose, a deformable parallelogram proof body is used to accept the load eccentricities. This sensor does not have, as is generally the case, a third complementary beam passing through the opening of the parallelogram, in order to simplify as much as possible the machining of the test body and thus reduce the cost price thereof. This test body is formed of a parallelogram consisting of four hinges of which at least one but preferably two have a particular shape characterized by a narrow central portion connected on both sides by a very thin veil (0.1 to 0.4mm ) to the two outer parts sufficiently thick to support the welding of the metal closure sheet and thus form a sealed cavity accommodating the gauges and thus providing protection vis-à-vis the aggressive outdoor environment. The narrow central parts (4 to 8mm) called hinges consist of a section of material between the flat part where the strain gauges are bonded and the concave face (radius of 6mm) so that the deformations are limited to these only thinned zones thus constituting a parallelogram having a fairly high stiffness.

In contrast, the connecting webs of the hinges to the outer parts are designed so as to present, thanks to their shape and their small thickness, the maximum possible flexibility in order to perfectly decouple the parallelogram of measurement of the external parts which ensure in association with the closing plate sealing the assembly.

With reference to FIGS. 1 to 7 of the drawing, it can be seen that the measuring sensor according to the present invention comprises a monolithic proof body 1 constituting a deformable parallelogram made of steel, stainless steel or alloy thereof, aluminum or other elastic material. , one end 1a is intended to be fixed on a frame while the other end 1b is equipped to receive a tray on which is placed a load to be weighed or applied a force to be measured. This test body (see FIG. 4) is traversed transversely by a central recess 2 having the general shape of an H defining the four hinges a, b, c, d of the deformable parallelogram of which at least one (a) is a hinge measurement (see Figure 6). Each end of the vertical branches of the H that forms the central recess 2 has a cylindrical shape whose generatrices are perpendicular to the longitudinal axis of the test body 1.

The two lower hinges c, d serving to deform the test body but not to the measurement are thin and can be further weakened by openings 3, 4 made in the lower wall of the test body 1. This is more particularly useful in the case of test bodies for low nominal loads.

The lower hinges c and d have a thickness of the order of 0.5mm to 1mm for example, depending on the nominal load of the sensor. The upper hinges a, b are those parts of the proof body which not only will ensure the deformability of the test body but are used to measure the bending stresses that will be measured using strain gauges. These measurement hinges a, b, in some variants only one of these hinges is used for measurement, have a greater thickness, of the order of 1 mm to 2 mm for example, also depending on the nominal load of the sensor. The figures given above are valid for sensors rated for a nominal load of 5 to 10 kilograms and as indicated above may vary depending on the rated load of the intended sensor. The higher the rated load, up to a few hundred kilograms, the heavier the hinges.

The upper face of the test body 1 comprises on the one hand a shallow recess 5 intended to receive strain gauges 6.7 fixed on the bottom of this recess 5 to the right of the measurement hinges a, b and, d on the other hand, a deeper housing 8 intended to house the electrical circuits 9 associated with the strain gauges 6, 7. This recess 5 and this housing 8 have a flange 10 located in the same plane, parallel to the upper face of the test body 1. This flange 10 allows to position a closure plate or cover 11 covering and closing the recess 5 and the housing 8 which will be welded for example laser on said flange 10 to ensure perfect sealing of the recess 5 and the housing 8.

To be certain that the zone of the test body 1 situated around the hollow 5 does not participate in the measurement, these zones being able to be affected as regards the characteristics of the material by the welding of the cover 11, pairs of cavities 12 are machined on both sides of the test body delimiting a width of the hinges a, b used for the measurement, of the order of 4 to 8mm for test bodies of nominal capacity of 5 to 10 kilograms . These pairs of lateral cavities also define the thickness of the webs 13 connecting the hinges a, b to the corresponding edges of the test body 1.

Thus, the material of the test body 1 which is used for the measurement of the stresses is limited to the measuring hinges a and b having a specific thickness and width. The sails 13 of material connecting the hinge has, respectively b, to the upper lateral edges of the test body 1 are thin, of the order of 0.1mm to 0.4mm, and ensure a total decoupling between the hinges a , b and the upper edge 10 of the test body 1 to be welded to the lid 11. Thus it can be ensured that the stresses measured in the material hinges a and b are not influenced by the deformations of these sails 13 during the loading of the test body 1.

On the other hand, the machining of the test body 1 is made much simpler because of the shape of the central opening 2 and the lateral cavities 12 and the absence of a central beam passing through this central opening 2.

In the case where only one of the upper hinges b is used for the measurement, the lateral cavities 12 made in the test body 1 can encompass only the area surrounding this single hinge measurement.

A bore 14 connects the recess 5 to the housing 8 to pass the son connecting the strain gauges 6, 8 to the electrical circuit 9. Such drilling 14 may be omitted in the case of the embodiment described so far since she does not comprises a cover 11 and the housing 8 communicates with the recess 5. This drilling is essential only in the embodiments where the recess 5 and the housing 8 do not communicate and are each closed by a separate cover as 8. Finally, a hole 15 connects the housing 8 to the outside of the test body 1 which can be equipped with a sealed cable passage for connecting the electrical circuit 9 to the apparatus located outside the body. test 1.

The principle of this sensor is to produce a central recess 2 in the test body 1 whose shape defines four hinges or deformation zones of the parallelogram thus produced. This central recess 2 also determines the thickness of the measurement hinge (s) a, b. The width of these measuring hinges a, b is then determined by the lateral cavities 12 which also determine the thickness of the webs 13 connecting the measuring hinges a, b to the edge of the test body 1 intended to receive a cover 11 welded. With a minimum of machining of the test body 1, a perfect decoupling is thus achieved between the measurement hinge (s) a, b and the zones of the test body intended to be welded to the cover, which makes it possible to produce a sensor. waterproof precise and inexpensive.

The second embodiment of the sensor according to the invention illustrated in Figures 9 to 11 comprises a monolithic test body 1 having a central recess 2 having the general shape of an X whose end of each branch has a cylindrical shape whose generatrices are perpendicular to the longitudinal axis of the test body 1.

The two upper ends of the central recess 2 define the position and the thickness of the measuring hinges a, b while the two lower ends of this central recess define the position and the thickness of the measuring hinges c and d.

Pairs of cavities 12 are machined on either side of the test body to the right of each hinge a, b, c, d determining the width of these hinges, all of which are measuring hinges, as well as the thickness of the sails 13 connecting these hinges to the adjacent edge of the test body 1, as is the case in the first embodiment for the measuring hinges a, b . In this embodiment the four hinges a, b, c and d are measuring hinges and each of them is surrounded by a pair of lateral cavities 12.

In this second embodiment of the test body 1 it comprises on a first face, the upper face here, a recess 5a for accommodating the strain gauges as in the first embodiment and the drilling 14a allows connecting these strain gauges to their associated circuits housed in the housing 8a also opening on the upper face of the test body and having a shoulder 10b. The housing 8a and the recess 5a are closed by separate covers welded to the shoulders 10a, 10b respectively.

In this embodiment, the underside of the test body comprises a recess 5b housing the strain gauges associated with measurement hinges c and d and a second hole 14b for connecting them to their associated circuits housed in the housing 8a. This housing 8a is connected to the outside of the test body by a passage 15a for receiving a sealed cable passage. In this embodiment, and in all those having separate covers for the recess 5 housing the strain gauges and the housing 8 enclosing the electrical circuits, the test body can be tested once the recess (s) 5 are closed by their welded lids, then make the necessary adjustments to the corresponding electrical circuits if necessary before closing the housing 8 by its welded cover. The welding of the housing cover 8 no longer modifies the characteristics of the test body 1 since this housing is located in a non-deformable portion of the test body. It is thus possible to further improve the accuracy and linearity of the electrical response of the sensor to the loads it measures. As has been seen from the foregoing, the present sensor always comprises a test body in the form of a single deformable parallelogram comprising a central recess 2 traversing transversely the test body and having the general shape of an H or a X. The ends of the branches of the H-shaped or X-shaped central recess 2 have a cylindrical shape whose generatrices are perpendicular to the longitudinal axis of the test body 1 and parallel to the upper and lower faces of this test body. This form of the central recess 2 makes it possible to form the four hinges of the test body 1, the flexural deformations of at least some of them will be used to act on the strain gauges.

The sensor may have one, two, three or four measuring hinges. When it has one or two, these measurement hinges are preferably located on the same side of the test body 1.

The central recess 2 determines the position and the thickness of the measuring hinges.

The test body also has pairs of lateral cavities 12, the cavities 12 of each pair being located on either side of each measurement hinge and determining the width of said measurement hinges and the thickness of the sails 13 connecting each measurement hinge to one side of the test body. These thin sails 13 allow a decoupling of the edges of the test body 1 with respect to hinges of measurement, so that these edges of the test body do not participate in the deformations of the hinges during bending of the test body. Thus, even if the elastic characteristics of the material of the upper and lower edges of the test body are modified by the welding of the cover (s) sealingly closing the recesses 5, 5a, this has no influence on the stresses measured at the measuring hinges.

Finally, the housing 8, 8a is located in an area of the test body not undergoing deformation, so that the welding of the cover closing this housing does not influence the measurement either. It is also noted that the machining of the test body is simple, consisting for the most part of milling, which reduces the manufacturing cost of this sensor.

Claims

claims

1. Strain-proof type strain-proof sensor in the form of a simple deformable parallelogram for receiving a load plate, characterized in that it comprises a one-piece proof body (1) comprising a central recess (2) having the general shape of an H or an X traversing transversely this test body (1) and whose shape defines the position of the four hinges (a, b, c, d) of the deformable parallelogram lying between the end of each branch of the recess (2) and a face of the test body (1) and their thickness; at least one of these hinges being a measurement hinge located on one side of the deformable parallelogram of the test body comprising on its outer face a recess (5) and a housing (8) in which the strain gauges are respectively arranged and their associated electrical circuit; at least one pair of lateral cavities (12) defining between them the width of the measurement hinge (s) and the thickness of the material webs (13) connecting the lateral edges of this measuring hinge to the lateral edges of the test body corresponding to this hinge, so as to avoid that the stresses caused in the measuring hinges are affected by stresses coming from said lateral edges of the test body (1); and in that at least one metal cover is welded to the test body to sealingly seal the recess (5) and the housing (8), the latter having a passage connecting it to the outside of the body test piece for receiving a sealed cable passage.

2. Sealed sensor according to claim 1, characterized in that the ends of the branches of the recess (2) central H-shaped or X have a cylindrical shape whose generatrices are perpendicular to the longitudinal axis of the body of and parallel to the upper and lower faces bottom of the test body.

3. Sealed sensor according to claim 1 or claim 2, characterized in that the cavities of a pair of lateral cavities (12) are arranged on either side of at least one measuring hinge.

4. Sealed sensor according to claim 3, characterized in that the cavities of a pair of lateral cavities (12) are arranged on either side of two measuring hinges located on the same face of the test body.

5. Sealed sensor according to one of claims 1 to 3, characterized in that it comprises two measuring hinges (a, b) close to the upper surface of the test body (1).

6. Sealed sensor according to one of claims 1 to 4, characterized in that it comprises two measuring hinges (c, d) close to the lower surface of the test body (1).

7. Sensor according to claim 6, characterized in that it comprises a second recess (5a) in the lower face of the test body (1).

8. Sealed sensor according to one of the preceding claims, characterized in that it comprises at least one strain gauge (6, 7) fixed on the bottom of the recess (5) facing a measuring hinge.

9. Sealed sensor according to claim 8, characterized in that it comprises a strain gauge fixed on the bottom of the recesses (5, 5a) facing each measurement hinge (a, b, c, d). _

13

10. Sensor according to one of the preceding claims, characterized in that it comprises two separate covers closing one the housing (8) and the other the recess (5).

11. The sensor of claim 10, characterized in that it comprises a third cover closing the recess (5a) of the underside of the test body (1).