WO1987007019A1

WO1987007019A1 - Force pick-up with deformation gauges for the direct measurement of vertical stress

Info

Publication number: WO1987007019A1
Application number: PCT/FR1987/000156
Authority: WO
Inventors: Serge Lafond
Original assignee: Societe Sebalares
Priority date: 1986-05-12
Filing date: 1987-05-12
Publication date: 1987-11-19
Also published as: FR2598504A1; EP0267214A1

Abstract

Force pick-up with double nesting provided with deformation gauges for the direct measurement of vertical stress, characterized in that its body (1)is comprised of a sandwich structure formed of assembled parts comprising a lower flexion body (2), an upper flexion body (3) which are separated at their ends by means of symmetrical intercalated parts which are in bearing contact with the non recessed zones of said intercalated parts acting as struts and in that the ends of at least one flexion body are integral with the intercalated parts at the recesses (16) by an interlocking means or material. Said invention is of particular interest for manufacturers of force pick-ups and users employing such force pick-ups in the fields of weighting, pressure measurements, etc.

Description

FORCE SENSOR WITH DEFORMATION GAUGES FOR DIRECT MEASUREMENT OF VERTICAL EFFORTS.

The present invention relates to a force sensor with strain gauges for the direct measurement of vertical forces.

Force sensors are widely used in weighing instruments and apparatus in association with a mechanical demuItipIication assembly.

This is the case, in particular, for the majority of bathroom scales in which the bending sensor is of the simple embedding type.

However, the dimensions of this type of sensor grow rapidly with the value of the applied force. Thus, for the range of forces used, the direct PTO on the sensor would lead to completely disproportionate dimensions with the possible dimensions of the devices.

This major drawback led researchers to choose double-embedded sensors. In fact, in these sensors, the blade working in bending can withstand a greater force with equal cross-section, thus making it possible to increase the load.

Immobilized by conventional means, the bending blade mounted directly on a support, frame or chassis of an apparatus, has the following two major drawbacks:

. tendency to slip due to immobilization which cannot be perfect: source of hysteresis. transmission of deformations from the support to the bending blade: source of measurement errors. However, the following various embodiments are not satisfactory.

The bending blade blocked directly at each of its ends on the support of the device is too sensitive to mechanical deformations of said support to provide usable information in weight measurements.

The method of locking or fixing by screws or by clamping brings in addition a cycle of hysteresis and secondary drawbacks not very compatible with the requirements of industrial exploitation.

More elaborate methods of direct attachment to the support would not make it possible to completely overcome deformations of said support.

The object of the invention is precisely to remedy the drawbacks of the previous sensors and to realize a force sensor with strain gauges whose structure, method of assembly and connection to the support, base, frame or chassis of the device, make it almost insensitive to deformations of said support.

It further aims to obtain sufficient qualities of stability and precision in the measurements, necessary for the applications envisaged.

To this end, the invention relates to a strain gauge force sensor characterized in that its body consists of a sandwich structure formed of assembled parts comprising a lower bending body, an upper bending body separated at the end by symmetrical and hollowed-out intermediate pieces, said flexure bodies being fixed at each of their ends to the faces of the intermediate pieces by fastening elements through the recesses of the intermediate pieces in contact with support on the non-hollowed areas of said intermediate pieces playing the role of spacer and in that the ends of at least one bending body are secured to the intermediate pieces at the level of the recesses by a means or a material of solidification.

The force sensor according to the present invention offers many advantages, the main of which will be cited below by way of nonlimiting example. . almost total independence of body movements from bending with respect to its support; . precision of the measurements by dimensional stability of the supports relating to the bending bodies; . economical, moderate cost sensor compatible with industrial requirements;

. reduced size compared to performance; . wide range of capacity;

. no expensive machining operation is involved in its manufacture; . relative insensitivity to shock;

. the mobile part transmitting the force moves along a line and therefore does not provide lateral forces on the fulcrum.

The invention will be clearly understood on reading the description which follows, given by way of nonlimiting example on an embodiment of the invention with reference to the accompanying drawings in which:. Figure 1 is a perspective view of the strain gauge sensor according to the invention; . Figure 2 is a. so-called exploded perspective view of the strain gauge sensor according to the invention; . Figure 3 is a longitudinal sectional view with figuration of the base or chassis and the lower support on the base; . Figures 4 to 6 are perspective views of different forms of cutouts relating to the intermediate pieces; . Figure 7 is a schematic view of a continuous series of spacers for example according to the second variant before their individual isation;

. Figures 8 to 10 are perspective views of several types of spacers with obviously receptacIe.

It is specified here that the dimensions of the sensor can be adapted to the load in a very wide range of capacities, in particular in the range of loads difficult to cover with the sensors used commonly namely the range of 10 to 100 kg.

The general inventive idea consists in using a force sensor with double embedding comprising two bending bodies in symmetrical sandwich structure whose upper body is mounted by each of its ends on the lower body through symmetrical end pieces on which the flexure bodies are applied in floating support, making the entire sensor relatively independent of movement with respect to the carrier support.

One will be described below. sensor whose bending bodies are formed of blades. It is understood that the invention extends to bending bodies of different shapes, in particular to profiles with curved face (s), grooved (s) or any other shape in sufficient plane contact with the intermediate parts. .

The sensor according to the invention has a rectilinear body 1 of general paraIleIepiped shape according to a sandwich structure with three levels assembled according to two superimposed metal plates, lower 2 and upper 3, forming bending blades, separated at the end by intermediate pieces 4 and 5 also metallic, playing the role of spacers to delimit between the two bending blades a clearance space 6.

The preferred material is aluminum, which has sufficient anticorrosion guarantees.

The lower plate or lower bending blade 2 communicates with the support 7 of the device, chassis or frame, by a support element 8 in the middle lower part. It carries, for example, above the support element 8 a stop piece 9 intended to delimit the movement of movement of the two bending blades 2 and 3 in approach towards one another. The upper plate 3 is a bending blade capable of deformation following a support transverse to its plane given by a pressure piece 10 in straight line movements, represented schematically by a needle.

The end pieces 4 and 5 are short in relation to the total length of the bending blades, lower and upper. As indicated, they play the role of spacer between the blades 2 and 3, forming in the central part between them a sufficient gap to allow the clearance following the bending deformation of the blades.

A perfectly symmetrical sandwich structure has thus been formed, making it possible to virtually eliminate the movements of the support on which it is in contact.

The upper bending blade 3 receives force or pressure on its upper face. It has for this purpose a slight hollow 11, forming a bowl, for example conical, located in the central part, which marks the position of the fulcrum given by the pressure piece 10, needle or other, for mechanical transmission of the rectilinear force. support from the surface subjected to pressure which does not provide lateral forces on the support point. One or more strain gauges 12 suitably arranged on one or the other of the bending blades 2 or 3, are electrically connected to a measurement circuit.

As a variant, the flexion blade can be cut at its support on the intermediate pieces according to symmetrical zones such as 13 and 14, so as to constitute an isoflexion or stress concentration surface 15 avoiding having to place the gauge (s) 12 precisely. In order to ensure the stability and precision of the measurements, the spacers have the following characteristics. The faces are flat and parallel, so as to guarantee good surface contact, metal on metal, with the bending volumes.

Furthermore, the intermediate parts have a recess 16 of suitable shape, open at least at one of the ends of the intermediate parts, called to contain the solidification material, for example a mastic, an adhesive and in particular a neck with low shrinkage. and strong attachment power based on epoxy resins or polyesters loaded with fibers or a metal powder, injected through the openings after mechanical assembly of the sensor.

As shown in the figures, the recesses 16 are of two types: the openings or passages 17 constituted by real cuts in the thickness of the intermediate parts and the cavities 18 forming the receptacle volume for the glue on each of the faces of the intermediate parts.

The general shapes have an interior area for receiving the adhesive delimited by an open perimeter along a transverse end side 19. The opposite end is offset from the adjacent transverse edge so as to form a transverse metal support area. on metal which continues along the longitudinal sides to form a true support border 21.

The securing elements, screws or the like, such as 22 and 23 are embedded in the volume of glue allowing them to be sealed perfectly to avoid any micro-displacement at the level of the screws.

One of the elements, for example 22, is arranged at the end near the transverse support zone 20.

The opening of the perimeter at the end provides all the ease necessary for the insertion of the glue which is injected after assembly of the bending blades on the intermediate end pieces. Various forms of cutouts and volumetric receptacle provided in the intermediate pieces are possible. Without limitation, certain characteristic forms are shown by way of example in the figures from 4 to 10.

We can thus consider trapezoid shapes

24 with a small base located on the open outside side 19, the large base of which continues in the middle part towards the other end by a rectangular recess 25 for the passage of the screw 22 (f i gure 4).

It is also possible to envisage divergent shapes 26 with an open base on the outer side 19, the apex of which is extended by a circular space 27 around the screw 22 near the closed end, making a cut in the form of a keyhole 28 (FIG.

5).

It is also possible to envisage rectangular shapes 29 slightly rounded at the end 30 or in a warhead on the closed transverse side (FIG. 6). These cuts could advantageously be made by punching in a continuous strip of metal thus forming a continuous linear series of juxtaposed pieces to then be individualized by fractionation along transverse section lines 31 (FIG. 7).

This technique of making the intermediate pieces leads to part shapes having lateral projections 32 and 33 at the end delimiting with the longitudinal edges and the blades of the additional cavities 34 and 35 for the injection of the glue (case shown in FIG. 6) .

Similar shapes are provided by double recesses or cavities 18 formed on each face by chemical erosion or electroerosion or any other similar technique.

The bonding or sealing material called solidification material may in certain applications be replaced by a metal contribution and be constituted by the metal of the upper and lower blades, and thus form the equivalent of a one-piece sensor of the same technical forms. It is thus possible to melt the intermediate parts which will serve as a connection for the production of a one-piece sensor. This type of sensor is then manufactured in one piece by molding.

The present invention is susceptible to variations, adaptations and variants by modifications, substitutions of equivalent means, addition of inoperative elements and other non-inventive changes without departing from the scope of this protection.

Claims

R EV END I CAT I ONS

1. Doub I e built-in force sensor provided with deformation gauges for the direct measurement of vertical forces, characterized in that its body (1) consists of a sandwich structure formed from assembled parts comprising a lower bending body ( 2), an upper bending body (3) separated at the end by symmetrical intermediate pieces (4) and (5) having recesses (16), said bending bodies carrying directly and being fixed at each of their ends on the faces intermediate pieces by fastening elements through the recesses (16) of the intermediate pieces in direct bearing contact on the recessed areas of said intermediate pieces playing the role of spacers and in that the ends of at least one body of bending are secured to the intermediate pieces only at the recesses (16) by a means or a fastening material.

2. Sensor according to claim 1 characterized in that the bending volumes are metal blades of which one of the faces of each blade or a part of these is secured to the intermediate pieces (4) and (5) by a solidification material with strong catching power and low shrinkage.

3. Sensor according to claims 1 and 2 characterized in that the soIidarization material is an adhesive based on epoxy resins or charged polyesters.

4. Sensor according to claims 1 and 2 characterized in that the soIidarisation material is a metal supply.

5. Sensor according to claim 1 characterized in that the intermediate parts are melted and provide a one-piece connection between the Iames.

6. Sensor according to claims 1 and 5 characterized in that it is manufactured by molding.

7. Sensor according to claims 1 and 2 characterized in that the recesses (16) of the end intermediate pieces (4) and (5) are openings or passages (17) through each piece.

8. Sensor according to claims 1 and 2 characterized in that the recesses (16) of the end intermediate pieces (4) and (5) are cavities (18) present on and in the upper and lower faces forming receptacle volume for glue.

9. Sensor according to claims 1, 2, 7 and 8 taken as a whole, characterized in that the general shapes of the contours of the recesses have an open side (19) located at the end of the corresponding bending blade and an opposite offset end relative to the outer transverse edge so as to form a transverse support zone (20), metal on metal, which continues with a border (21) along the longitudinal sides.

10. Sensor according to claim 7 characterized in that the shape of the opening (17) is a trapezoid (24) with small base located on the outside, the large base of which continues in the middle part towards the other end by a rectangular recess (25) for the passage of the fixing element.

11. Sensor according to claim 7 characterized in that the shape of the opening (17) is of the keyhole type (28).

12. Sensor according to claim 7 characterized in that the shape of the opening (17) is substantially rectangular rounded at the end (27) on the closed transverse side.

13. Sensor according to claims 7 and 8 characterized in that the shapes of the openings (17) are those of the contours of the cavities (18).