EP1721134A1

EP1721134A1 - Weight sensor

Info

Publication number: EP1721134A1
Application number: EP05732469A
Authority: EP
Inventors: Benoît LINGLIN; Didier Anthoine-Milhomme
Original assignee: SEB SA
Current assignee: SEB SA
Priority date: 2004-03-03
Filing date: 2005-02-28
Publication date: 2006-11-15
Also published as: CN1926410B; WO2005095903A1; US7441466B2; US20070186662A1; FR2867275B1; RU2369845C2; FR2867275A1; RU2006134732A; CN1926410A

Abstract

The invention relates to a weight sensor comprising strain gauges which are deposited in thick films on a support (2). The support is made from an electrically-insulating material which is intended to be applied to a metallic body (1) that is essentially subject to bending. According to the invention, the support (2) comprises a ceramic material which has a Young's modulus E2 that is equal to or less than that E1 of the biased metallic body (1) and which is applied to the latter by means of gluing.

Description

WEIGHT SENSOR

The present invention relates to a weight sensor, more particularly of the type using resistance strain gauges to detect deformations of a metal bar. Such a sensor can advantageously be used in a device of the personal scale, baby scale or household scale type.

A weighing device, such as a bathroom scale, comprises a plate whose upper surface is intended to receive the weight to be weighed and one or more sensors supporting on the one hand the plate and on the other hand bearing on a base or on the feet of the appliance. The sensor (s) include strain gauges connected to an electronic circuit capable of converting the deformations undergone by the gauges into electrical signals and transforming these into digital values corresponding to the measured weight which is then displayed by the device.

A strain gauge sensor is known from document FR 2 587 484 where the gauges and their connections are deposited on a support produced in the form of a thin plate of a ceramic material. The strain gauges are resistances applied by screen printing on one face of said support, its other face being fixed on the mechanical element whose tensions or deformations are to be detected locally. This type of sensor is called a thick film technology sensor. The support can be attached to the stressed element using screws or rivets, or even with a layer of adhesive, the deformations of the stressed element being transmitted to the strain gauge through its support. Such a support with strain gauges is easy to manufacture and to apply to the stressed part, but it has been found that the type of fixing and the type of material of the support greatly influence the measurement accuracy of the sensor.

Document FR 2 734 050 in the name of the applicant describes a weight sensor applied to a weighing device. The sensor is flat and has a bending bar on which a ceramic support is stuck. In the Applicant's applications, the strain gauges and their connections are arranged by screen printing on an alumina support. The support is then attached by bonding to the sensor bar, generally made of steel. Such an embodiment of the sensor is easy to implement, but it has the drawback of using a support which, while being a good electrical insulator, has mechanical properties which strongly attenuate the electrical signals supplied by the circuits of the gauges.

Another thick-film technology strain gauge sensor used to measure mechanical torque is described in document WO 99/22210. A resistive paste and conductive tracks are applied by means of an electrically insulating layer on a steel support constituting the mechanically stressed element. The electrically insulating layer is a paste based on glass frit which is first applied to the support requested by a printing technique, the strain gauges and their connections then being applied by screen printing on said insulating layer. The assembly thus prepared is baked at a temperature of about 750 ° to 900 ° C and the insulating layer is sintered with the upper surface of the support. This production technique has several disadvantages, the main one being that one must handle the mechanically stressed part during the operations of depositing the strain gauges and their connections, which imposes significant manufacturing and flow organization constraints. . Furthermore, in view of the very high sintering temperatures, the material of the metal support must be chosen so that it does not lose its mechanical properties with temperature.

The object of the present invention is to remedy these drawbacks at least in part and to propose a weight sensor comprising a metal bar comprising strain gauges deposited in a thick layer on an insulating support with improved mechanical properties, capable of providing a signal more important for the same stress applied to the sensor.

Another object of the invention is a weight sensor comprising a body metallic comprising strain gauges deposited in a thick layer on an insulating support, easy to handle, which can be applied to practically any type of metallic body, without limitation as to the type of material of the body and / or to the shape and dimensions of the latter , as well as increased sensitivity.

Another object of the invention is an easy to industrialize weight sensor, suitable for mass production for a lower manufacturing cost, while being reliable in operation.

These goals are achieved with a weight sensor with strain gauges deposited in a thick layer on a support made of an electrically insulating material intended to be applied to a metal body stressed essentially in bending, since said support is a ceramic material having a module of Young equal to or less than that of the metal body requested and that it is applied by gluing on the latter.

The expression “metal body essentially stressed in bending” includes the test body of a weight sensor, one end of which serves to fix the housing of the apparatus and the other receives the load applied to the plate. Such a body is subjected to a main stress in bending under the effect of the weight to be weighed applied to the platform, parasitic moments, such as torsional moments which can also occur due to the point of application of the weight on the platform located at a distance. of the sensor.

By support made of an electrically insulating material is understood a substantially flat plate or sheet, made of a ceramic material on which one can deposit, for example by screen printing, the different parts of the resistive circuit of the strain gauges, this support being sufficiently rigid so that it can be grasped and manipulated, with a view to its transfer onto the mechanically stressed body, without tearing and without undergoing permanent deformation. This makes it possible to carry out the delicate operation of depositing the resistive circuit and of baking at high temperature away from the part or the body which is stressed, which is, generally, of complex shape and of large dimensions related to said support, therefore difficult to integrate into an automated manufacturing flow. Several supports can thus be treated simultaneously during an automatic deposition for mass production, each support can then be separated from the others and attached by bonding to the metal body of which one wants to measure the tensions or deformations. Fixing by gluing is particularly advantageous in such an embodiment, the intermediate glue layer, well calibrated, playing the role of stress transmitter of the body biased towards the support of the gauges.

The metallic body that is essentially stressed in bending can be likened to a beam embedded at one of its ends, the other being subjected to a load whose value is to be determined by the sensor. The amplitude of deformation of such a beam depends on the value of the applied load and its section inertia. When a rigid support or plate, less deformable than the body of the beam, is made integral with one of the faces of the beam, the deformations of the assembly have a lesser amplitude. Thus, it was noted during tests carried out in the laboratory, that a steel bar (having a Young's modulus of 210 GPa) covered with an alumina plate (Young's modulus of 340 GPa) deforms much less than the bar alone, without plate, subjected to the same load. This has a direct influence on the reduction of the signal perceived by the sensor and, therefore, on the sensitivity of the latter.

However, with a sensor of the invention, it has been found that, for a support or plate of Young's module equal to or less than that of the metal body of the beam, the calculated slope of the sensor is very close to the actual measured slope during laboratory tests, as will be explained later.

Advantageously, said body has a rectangular section of thickness less than or equal to 15 mm.

It has been shown in laboratory tests that signal loss from sensor increases with the E ₂ / Ei ratio of the Young's moduli of the support and the body and decreases with the increase in the sectional inertia of the body. Thus, for a test body of square section of 15mm x 15mm, the loss of signal compared to a calculated ideal value is very small, but this loss of signal is amplified for test bodies of lesser thickness on which is applied a ceramic support having a high Young's modulus.

Usefully, said body is made of steel, a material chosen for its properties of mechanical strength and elasticity.

According to a first embodiment of the invention, said support is chosen from the group comprising a zirconia or yttria or cordierite or steatite ceramic.

A zirconia ceramic has a Young's modulus of 210 GPa, which is around 30% less than alumina, which limits the harmful effect on the sensitivity of the sensor. In addition, a zirconia ceramic is less friable than alumina, which can therefore be handled more easily. In addition, the coefficient of linear expansion is greater than that of alumina, which limits the stresses in the intermediate adhesive layer.

Other ceramic materials such as yttria and cordierite have a Young's modulus of around 140 GPa and soapstone has a Young's modulus of 120 GPa. Due to their low Young's modulus value compared to that of the steel body, these materials, when used as gauge supports, allow the latter to provide a real signal, not attenuated, to the electrical measurement circuit and even for test bodies of small cross-section.

Such supports made of ceramic materials can be obtained by sintering in the form of a plate of calibrated thickness, a plate which is then cut to the desired dimensions.

According to a second embodiment of the invention, said support is produced in a ceramic baked at low temperature.

Such a material may advantageously be a laminated type tape 951 Green Tape ^® from DuPont having a Young's modulus of 152 GPa. Such a ceramic generally comprises approximately 80% of alumina and 20% of glass frit with an organic binder. Such a ceramic is more particularly suitable for use with test bodies of small cross section, without degrading the sensitivity of the sensors.

When used as a gauge support, such a ceramic baked at low temperature can undergo a first firing step followed by a second operation during which it is cut or precut to the dimensions of the support on which the screen printing of the conductive and resistive tracks. This screen printing is then followed by baking, before the support is bonded to the test body. In a variant of the invention, the firing of such a cured ceramic strip can be carried out at the same time as that of the screen-printed paste deposited on said strip.

Usefully, the thickness of said support is between 0.05 and 0.5 mm.

The electrically insulating support supporting the strain gauges must have the thinnest possible thickness in order to better transmit the deformations of the test body, but while being easy to handle during the operations prior to its bonding on the test body and having effective electrical insulation with regard to the electrical voltages involved and with regard to the expected longevity of the sensors.

Preferably, the weight sensor of the invention comprises a test body in the form of a bar carrying strain gauges, one end of said bar being connected to a fixing element, the other end being connected to a load application element, where the test body flexes in an S shape with a symmetrical double overhang. By placing the strain gauges in the zones of the deformed test body mounted in double overhang where the radii of curvature due to the double bending of the beam are the smallest, we can therefore obtain amplified signals, more easy to process subsequently by the electrical circuit of the device.

Advantageously, the weight sensor of the invention is produced in the form of a metal plate comprising a fixing element in the form of a frame or a U, connected in the middle of its base to a first end of a test body extending inside the fixing element, the opposite end of the test body being connected to a U-shaped load receiving element, extending symmetrically with respect to the body, with the arms parallel to the body and oriented towards said first end of the body.

Such a sensor makes it possible to produce a weighing device with a thin profile, while being very precise and reliable in operation.

An electronic weighing device may include at least one weight sensor of the invention. Advantageously, such an apparatus can be provided with four sensors then with a test body of reduced section, while retaining good measurement accuracy.

The invention will be better understood from the study of the embodiments taken without any limitation being implied and illustrated in the appended figures in which:

- Figure 1a schematically shows a sensor of the state of the art in longitudinal section;

- Figure 1b is the cross section of the sensor of Figure 1a;

- Figure 1c is the transformed, theoretical section of that shown in Figure 1b;

- Figure 2 is a perspective view of an embodiment of a weight sensor using the features of the invention;

- Figure 3 is a cross-sectional view of the test body of the sensor of Figure 2;

- Figure 4 is a graph illustrating the sensitivity curves of a sensor weight as a function of the section inertia of the test body for different materials of the gauge support.

A force sensor essentially stressed in bending is represented in FIG. 1a by a symmetrical composite beam embedded at one of its ends, the load being able to be applied to the free end. This composite beam consists of two different materials: a body 1 made of steel and a support 2 made of alumina applied to the upper part of the body 1. Support 2 is applied by gluing and it can be assumed that there is no no sliding between the support 2 and the body 1, so that one can use the theory of simple beams according to which the elongations and contractions of the longitudinal fibers are proportional to the distance which separate them from the neutral axis . In FIG. 1 b, the rectangular section of the body 1 of width b and height a and that of the support 2 of width b and thickness β _j is noted. In Figure 1c is shown the transform of the section of Figure 1b, the elastic modulus E ₂ of alumina being greater than that E of steel, which is equivalent, from the point of view of bending to a much wider steel core, of width bi and thickness β _j . If we want the resistance moment of internal forces to remain unchanged for a given curvature, the thickness bi of the core must be such that

FIG. 2 is a perspective view of a weight sensor fitted to a scale as described in document FR 2 734 050 in the name of the applicant. The sensor comprises a fixing element 3 to the weight receiving plate of the device, more particularly in the form of a frame 3a. The frame 3a is connected by a test bar or body 1 to a U-shaped load application element 4. When the load is applied to the two opposite arms 4a, 4b parallel to the body 1 of the element 4, the test body 1, mounted in double overhang, deforms taking a symmetrical S shape. A support 2 carrying strain gauges 6 is applied to the body 1 over all or part of its length so that the deformations of the body 1 are transmitted to the stress gauges through the support 2. The stress gauges 6 are positioned in the deformation zones maximum of the body 1 in order to give more sensitivity to the sensor.

Figure 3 illustrates a cross section of the body 1 of the sensor of Figure 2 where the support 2 made of a ceramic material is applied via a layer of glue 5 on the body 1. Strain gauges 6 and conductive tracks 7 ensuring their connection to the electrical circuit of the device were previously applied by screen printing of resistive paste and respectively of conductive paste on the support 2.

In the context of such a sensor described by way of example, two gauges 6 connected in half-bridge are applied to the test body 1. The theoretical or calculated slope of such a sensor is given by the formula: Theoretical slope = 3 ^* k ^* d / 2 ^* E ^* b ^* a ² where k is the coefficient of resistance gauge which is a function of the type of resistive paste used (equal to 10 in this case); d is the distance between the gauges;

E is the Young's modulus of body 1 (equal to 210 GPa for a steel body); b is the width of the body 1 and a is the height of the body 1.

This theoretical slope corresponds to the ideal behavior of a sensor, it does not take into account the stiffness provided by the support 2.

FIG. 4 illustrates by a graphic representation the variation of the slope or of the sensitivity of a sensor as a function of the sectional inertia of its test body. Thus, curve A represents the theoretical slope of a sensor of the type described. Curves B, C and D are representations of the actual slopes measured with a sensor of the type described, but using different materials for its support 2. Curve B is the real curve of a sensor of the state of the art using an alumina support 2.

More particularly according to the invention, the support 2 is made of a material which has a Young's modulus equal to or less than that of the body 1, in the occurrence of a ceramic baked at low temperature (called LTCC) or a zirconia ceramic on a body 1 of steel.

Thus, curve D is the actual curve of a sensor according to the invention comprising a support 2 made of ceramic baked at low temperature or LTCC.

Curve C is the actual curve of a sensor according to the invention using a zirconia ceramic as the material of support 2.

It is clear from FIG. 4 that the sensitivity of the sensor is greatly improved by a judicious choice of the support material, in particular its elasticity, and this is all the more visible for test bodies having a low section inertia.

By comparing curves A and B in FIG. 4, it is noted that the maximum deviation is obtained for test bodies of small section, while for test bodies of larger section the deviation is very small. Thus, considering a weight sensor comprising an alumina support 2 fixed on a steel test body of square section 15mm x 15mm, used for example in a bathroom scale with a single sensor, the signal loss is about 0.13%. Measurements made with a household balance using the same alumina support and a rectangular section of the steel test body of 10mm x 3.5mm, the signal loss is 20.1%. While the same measurements made with a household balance with four sensors whose section of the test body also in steel and rectangular is 8mm x 1, 2mm, the gauge support also being in alumina, established a loss of signal 59.4%.

The invention in the following manner is believed to explain, or reconsidering Figure 1c where the section of inertia of the body 1 only are: _zz = b * a ^3/12, and _yy = a ^* b ^3/12. The section inertias of the new part and considering the distance from the center of gravity of the body 1 to the center of gravity of the support 2 approximately equal to half the width of the body 1, ie a / 2 are: \ _2Z = bXe _a , π ³ \ 2 + be _i * ^2/4, and

Therefore, the corrected section of inertia of the composite beam are Izz (totai) = b ^3/12 ej + b ^3/12 + biθj ^* a / 4, and otao ≈

From the previous formulas, it would seem that the lower the Young's modulus of the support material E ₂ compared to E ^, the less its influence on the section inertia of the composite beam. These theoretical recitals are at the origin of the realization of the sensor of the invention.

Thus, with the sensors of the invention using a test body 1 made of steel onto which a support 2 is applied by bonding 2 in a zirconia ceramic (curve C) or in a ceramic baked at low temperature or LTCC (curve D) on observe in FIG. 4 that the actual behavior of the sensor (actual slope) respects the theoretical curve linking the sensitivity of the sensor to the sectional inertia of the test body. The differences obtained with such supports are very small, and are established, for the curve D at maximum 18.2%, and for the curve C at maximum 26%, in the most unfavorable case of a test body of reduced section, the dimensions of the latter being 8mm x 1, 2mm.

The manufacture of such a weight sensor comprises the following steps. A first step consists in obtaining the metal body of the sensor, for example according to the contour shown in FIG. 2, for example by stamping or by cutting out a matrix from a flat metal sheet. In parallel, the sintered ceramic support (this support being a zirconia, yttria, cordierite or sintered soapstone ceramic or an already fired LTCC), in the form of a fairly large sheet, is precut to the dimensions of a individual sensor support. A first screen printing operation consists of apply the conductive tracks by applying a conductive paste, for example based on silver. This screen printing is followed by baking at around 850 ° C. A second screen printing step consists in applying a resistive paste, for example a glass frit with metallic particles, on the ceramic support followed by a second firing at 850 ° C. The precut blanks thus obtained are then cut and attached by bonding to the test body of the sensor. The adhesive is, for example, an epoxy adhesive which crosslinks at 200-250 ° C. The thickness of the adhesive layer is well calibrated in order to reduce its shearing when the test body drops to room temperature in order to be able to transmit the stresses coming from the test body towards the ceramic support and therefore the strain gauges . The calibrated thickness of the adhesive layer also makes it possible to obtain a good hysteresis and a good return to zero of the sensor.

Other variants and embodiments of the weight sensor of the invention can be envisaged without departing from the scope of these claims.

Thus, in a variant, it is possible to use as insulating support a strip laminated in a ceramic of the LTCC type on which a deposition is carried out by screen printing before the firing of the ceramic. Baking is then carried out at approximately 850 ° C. of the support assembly and screen-printed tracks deposited on said support. The assembly thus obtained may optionally undergo an additional screen printing step and it is then applied by gluing to the test body.

In another variant, it is already possible to glue the uncooked LTCC strip to the test body and then screen-print and bake the whole.

Claims

B.0661, Rm1eexxtι CLAIMS

1. Weight sensor with strain gauges deposited in a thick layer on a support (2) made of an electrically insulating material intended to be applied to a metal body (1) stressed essentially in bending, characterized in that said support (2) is a ceramic material having a Young's modulus E ₂ equal to or less than that of the stressed metal body (1) Ei and that it is applied by bonding to the latter.

2. Weight sensor according to claim 1, characterized in that said body (1) has a rectangular section of thickness less than or equal to 15 mm.

3. Weight sensor according to one of claims 1 or 2, characterized in that said body (1) is made of steel.

4. Weight sensor according to one of the preceding claims, characterized in that said support (2) is chosen from the group comprising a zirconia or yttria or cordierite or steatite ceramic.

5. Weight sensor according to one of claims 1 to 3, characterized in that said support (2) is made of a ceramic baked at low temperature.

6. Weight sensor according to one of the preceding claims, characterized in that the thickness of said support (2) is between 0.05 and 0.5 mm.

7. Weight sensor according to one of the preceding claims, characterized in that it comprises a bar-shaped test body (1) carrying strain gauges (6), one end of said bar being connected to a fastening element (3), the other end being connected to a load application element (4), where the test body (1) flexes in an S shape in a symmetrical double overhang .

8. Weight sensor according to claim 7, characterized in that it is produced in the form of a metal plate comprising a fixing element (3) in the form of a frame (3a) or of a U, connected in the middle of its base to a first end of a test body (1) extending inside the fixing element (3), the opposite end of the test body (1) being connected to a load receiving element (4) U-shaped, extending symmetrically relative to the body (1), with the arms (4a, 4b) parallel to the body (1) and oriented towards said first end of the body (1).

9. Electronic weighing apparatus comprising at least one sensor according to one of the preceding claims.