A strain-gauge transducer comprises a strain gauge enclosed between pressure members which deform the strain gauge, the strain gauge comprising a plate-shaped substrate provided with strain-detection elements. At least one spring having a flexibility larger than that of the substrate is arranged between the strain gauge and at least one of the pressure members, said substrate and said spring being in contact with one another so as to be jointly subjected to deformations. The spring may be a blade spring, a helical spring, or a cylindrical buffer spring. The excursions of the pressure members can be limited by means of stops. In the case of a blade spring the contact between the substrate and the spring can be controlled by giving the substrate a toothed shape or by providing it with contact elements.

BACKGROUND OF THE INVENTION 
The invention relates to a strain-gauge transducer comprising: 
a strain gauge enclosed between pressure members, the strain gauge 
comprising a plate-shaped substrate provided with strain-detection 
elements, 
at least one spring arranged between said substrate and one of the pressure 
members so as to be deformed in conjunction with said substrate by 
pressures exerted by the pressure members. 
Such a transducer is used for the detection or measurement of forces, in 
particular in force transducers. By means of these transducers it is 
possible to construct, for example, scales, load detectors etc. 
In the analysis of the strength of materials it is known to place a sample 
on two supports and subsequently exert pressure on the other side of the 
sample in the middle of the two supports, use being made of its bending 
capacity. A similar device is utilised for measuring forces in transducers 
employing strain gauges formed by resistors arranged on a rigid substrate: 
two supports are fixedly connected to a first pressure member and the 
support arranged opposite thereto is fixedly connected to a second 
pressure member. This is described in, for example, "Strain sensitivity of 
thick-film resistors", J. S. SHAH, IEEE Trans. CHMT-3, no. 4, 1980, p. 
554. The supports may be knife-edge supports. The knife-edge supports 
should be accurately parallel to one another, which requires accurate 
machining of the various elements. 
A strain gauge generally comprises resistive elements whose resistance 
value varies with the deformation of the support on which they are 
arranged. Generally, several resistive elements are used which are 
connected in a Wheatstone bridge arrangement. In this way it is possible 
to obtain an electric signal which is directly proportional to the flexure 
of the support independently of temperature variations. 
Thus, the document DE-B-1,001,832 describes a transducer element 
constructed as a symmetrical spring having two blades, of which one blade 
is provided with resistive elements, the deformation of the blade enabling 
pressure forces applied to the transducer element to be determined. The 
two blades are connected to one another in such a way that their 
deformations are equal and oppositely directed. 
In order to ensure that the measurement by means of a transducer is linear 
and accurate the support which is used should have reproducible and linear 
mechanical properties. This is an advantage of gauges formed by a ceramic 
plate on which a resistive ink is deposited by screen-printing. Indeed, 
the ceramic plate has a very large elasticity range, which is limited only 
by the breaking point of the ceramic plate itself. There is neither 
plastic deformation nor hysteresis. For operation near the breaking point 
a means is required which prevents this point from being overstepped. 
On the other hand, a strain-gauge transducer is mainly intended for use in 
consumer applications, for example in scales or all kinds of force 
transducers. For such consumer applications perfectly machined elements 
cannot be used for reasons of cost. This leads to the use of elements of 
imperfect flatness, parallelism and structure. The technologies used 
should therefore be free from the restraints associated with large series, 
in particular spreads of mechanical tolerances of the parts. It is to be 
noted that for the ceramic plates used the deflection to be utilised is 
small. Moreover, the ceramic plates for mass-production uses are 
commercially available plates, which are generally non-polished and have a 
surface roughness or a curvature which may be even of the order of 
magnitude of the deflection amplitude. The mechanical tolerances are not 
constant from plate to plate. An adjustment may be correct for one plate 
and may be destructive or lead to inadequate deflection amplitudes for 
another plate. In practice, it is therefore difficult to limit or adjust 
the maximum deflection that can be handled by such a plate so as to obtain 
a correct operation without breaking. 
SUMMARY OF THE INVENTION 
The problem to be solved is therefore, using standard ceramic plates, to 
construct strain-gauge transducers having reproducible characteristics 
which are not dependent upon variable mechanical tolerances of the plates. 
These variable tolerances include the roughness, the curvature, the 
thickness, the flexibility. 
This is solved in that said substrate exclusively bears on said spring in a 
floating arrangement, said spring having a flexibility larger than that of 
the substrate. 
Preferably, the spring is a curved blade spring, because this is very 
suitable for large-scale production. Thus, the substrate which is 
subjected to the action of an applied force will deform, thereby 
compressing the curved blade spring supporting it, as a result of which 
this spring is deformed. Owing to the compression of this spring the 
substrate is situated closer to one of the pressure members. It is 
necessary that the flexibility of the substrate and that of the curved 
blade spring are such that they both deform, the spring being deformed to 
a larger extent that the substrate. The flexibility of the spring and of 
the substrate should be such that the substrate cannot reach its breaking 
point when the maximum excursion is reached. These excursions are limited 
by stops. 
A curved blade spring has a concave surface and a convex surface. The 
substrate can be in contact with either surface of the spring. 
When the substrate is arranged at the side of the concave surface of said 
spring the edges of this substrate are in contact with the spring. The 
edges may be provided with contact elements. Alternatively, the spring may 
be of the helical type or of the cylindrical buffer type. 
Suitably, according to the invention, the deformation of small amplitude of 
a deformed substrate is combined with a deformation of larger amplitude of 
a spring. In this way nearly the entire range of deformation of the 
substrate can be utilised without its breaking point being overstepped. 
In all these situations the excursions can be limited by means of stops, 
which may be arranged on the substrate or on the spring or on at least one 
pressure member. This enables the transducer to be loaded well beyond the 
breaking load of the substrate without the substrate being broken.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows diagrammatically a prior-art device in which a substrate 5 of 
a strain gauge 6 is arranged on two lower knife edges 4.sub.1, 4.sub.2, a 
pressure force being applied by an upper knife edge 3. Under the influence 
of the force the substrate is bent and exhibits a deflection f. In the 
case of fairly stiff substrates such as for example ceramic plates this 
deflection is comparatively small of the order of 5.10.sup.2 mm. In order 
to prevent the ceramic plate from breaking under the influence of 
excessive forces the value of said deflection should be limited. 
However, these ceramic plates, which are often made of sintered aluminium, 
have a surface roughness and a curvature of substantially the same order 
of magnitude as the deflection f. Therefore, very thin stops have to be 
used in order to limit the deflection of the plate. In view of the 
differences in curvature this is found to be very difficult in practice 
because the spreads in mechanical properties of the plates can be very 
large. Thus, it is evident that it is very difficult to control the 
deflection limits, for if the emphasis is placed on the breaking point of 
the plate not being overstepped it is not unlikely that the excursion 
becomes too small to carry out strain measurements. If the emphasis is 
placed on the range of the excursion the breaking point may be reached for 
certain plates and the plate may break. 
Since the transducer in accordance with the invention is intended for the 
use of ceramic plates without prior surface treatment these problems have 
to be solved. 
FIG. 2 shows a first type of transducer in accordance with the invention in 
which the spring is constituted by a curved blade. This transducer 
comprises two pressure members 10, 11 which are subjected to a pressure 
force F. This force is transmitted to a strain gauge which is supported on 
a curved blade spring 13. The strain gauge comprises a substrate 12 in the 
form of a plate (hereinafter referred to as "substrate") carrying the 
strain detection elements, for example the resistor 14. A plurality of 
such elements have been arranged on the surface of the substrate. For 
reasons of sensitivity to strain and immunity to temperature variations 
these detection elements are generally arranged as a Wheatstone bridge. 
In a first embodiment of this first type of transducer shown in FIG. 2 the 
substrate is supported on the concave surface of the blade spring. It is 
floating, i.e. the ends of the substrate 12 and of the spring 13 are not 
connected to one another. Moreover, the substrate 12 bears only on the 
spring 13. Consequently, the substrate bears on the spring 13 with edges 
15.sub.1, 15.sub.2. 
The pressure member 11 transmits the applied force F by means of an element 
16 which may: 
act substantially in a point, for example via a ball or a part having a 
small contact surface, or 
act over a certain length in a direction perpendicular to the longitudinal 
direction of the substrate, for example via a pin, which may be 
cylindrical in order to eliminate the effect of flatness deviations, or 
act in the above direction and in a direction perpendicular thereto over a 
certain length, for example by means of two cylindrical pins fixed to one 
another and forming a cross. This eliminates the effect of flatness 
deviations in two directions. 
This element 16 transmits the force F applied substantially to the centre 
of the substrate so as to act oppositely to the two edges 15.sub.1, 
15.sub.2 bearing on the spring 13. Under the influence of the force F the 
substrate 12 of the strain gauge will deflect but this also causes the 
spring 13 to be flattened on the pressure member 10. The flexibility of 
the spring 13 is selected in such a way that the substrate cannot break 
when flattening ceases, the substrate then bearing on a central point of 
support. 
FIG. 3 shows a second embodiment of this first type of transducer in which 
the substrate 12 bears on the convex surface of the spring 13. In this 
case the pressure member 11 applies the pressure force F to the substrate 
12 by means of two elements 16.sub.1, 16.sub.2, arranged near the ends of 
the substrate 12. Thus, the reaction force of the spring 13 on the 
substrate 12 opposes the forces acting on the substrate 12 via the 
elements 16.sub.1, 16.sub.2. In the same way as above the flexibility of 
the substrate and the spring are selected in such a way that the substrate 
cannot break. One of the ends of the spring 13 can be mounted in a recess 
17 formed in the pressure member 10. Any other equivalent mounting method 
is suitable. 
In operation the pressure members 10, 11 can move relative to one another. 
They should be movable in the direction of the excursion and be held in 
position in the other directions. This can be achieved, for example, by 
means of guides (not shown). 
In accordance with the invention the maximum deflection of the substrates 
12 without breaking is not controlled directly. It is controlled 
indirectly by controlling the maximum excursion of the spring 13 when it 
is flattened by the substrate. This is achieved by means of stops. 
FIG. 4A relates to the first embodiment shown in FIG. 2. Similar elements 
bear the same reference numerals. FIG. 4A by way of example shows three 
possibilities of arranging the stops. Preferably, these three 
possibilities should be used independently of one another. 
A first possibility is to arrange a stop 20 underneath the substrate 12 
substantially in its centre. Thus, when the force F is applied the maximum 
deflection of the substrate 12 and the maximum flattening of the spring 13 
are reached simultaneously when the stop 20 comes into contact with the 
concave surface of the spring 13. 
A second possibility is to arrange a stop 21 on the concave surface of the 
spring 13 substantially in its centre. The above maximum limits are now 
reached when the stop 21 comes into contact with the substrate 12. 
A third possibility is to arrange two stops 22.sub.1, 22.sub.2 on one of 
the pressure members, for example the pressure member 11. The advantage of 
this arrangement is that in the case of overloading no additional stress 
is transmitted to the substrate itself. 
For each of these possibilities the size of the stops depends on the 
flexibility of the substrate 12 and of the spring 13 and on the distance 
between the pressure members 10, 11. The size of the stops then remains 
the same for batches of substrates and springs having substantially 
homogeneous properties. 
FIG. 4B relates to the second embodiment shown in FIG. 3. In a manner 
similar to that described for FIG. 4A it is possible to use: 
two stops 32.sub.1, 32.sub.2 arranged on one of the pressure members, for 
example the pressure member 11, externally of the substrate 12, or 
two stops 33.sub.1, 33.sub.2 arranged at the ends of the substrate 12 which 
come into contact with the pressure member 10 when flattening is maximal. 
Preferably, the stops 33.sub.1, 33.sub.2 are arranged opposite the 
elements 16.sub.1, 16.sub.2 on the opposite surface of the substrate 12. 
All the stops may be separate parts secured by, for example, gluing or 
welding. They may also be integral with the element to which they belong. 
Thus, the stop 21 shown in FIG. 4A may be constructed for example as shown 
diagrammatically in FIG. 5A. In this case the stop is a stop 21 which is 
integral with the spring 13. Another construction may be, for example, as 
shown diagrammatically in FIG. 5B. In this case the stop is a stop 21 
secured to the pressure member 10, said stop 21 traversing a hole 30 
formed in the spring 13. Alternatively, the stop 21 may be constituted by 
a screw fitted in the pressure member 10 to adjust the length of the stop. 
A use of a strain-gauge transducer in accordance with the invention is, for 
example, to detect the weight of loads placed on the transducer. 
Determining this weight should be possible over a certain weight range. If 
the load placed on the transducer exceeds this range the transducer should 
not break down. An example of such a use is in cook-tops provided with 
strain gauges. The applied force F can be a weight or a pressure force. 
It has been found that the maximum permissible deflection in the case of a 
ceramic substrate is very small. A substrate of 31.times.16.times.1 mm 
arranged on two supports spaced 25 mm apart has a maximum deflection of 
5.10.sup.-2 mm for a weight of 10 kg in the centre. When the invention is 
not used and allowance is made for surface irregularities and warping it 
is found to be difficult to provide each substrate with the stops 
necessary for limiting the deflection to a maximum of 5.10.sup.-2 mm: 
either this leads to problems in manufacturing the stops, which is not 
permissible in mass-production, 
or the amounts of adhesive/solder cannot be measured out correctly. 
Owing to the invention these problems are overcome by means of a curved 
blade spring of, for example, spring steel, of CHRYSOCAL, or of any other 
material having suitable elastic properties. For example, in combination 
with the above ceramic substrate a spring-steel blade spring is used 
having a radius of curvature of 240 mm in its no-load condition and having 
such an elasticity that the deflection in the centre is approximately 0.5 
mm when the spring bearing on two supports spaced 31 mm apart is loaded by 
a weight of 10 kg. In the present example the ratio between the 
flexibility of the spring and of the substrate will be approximately 10. 
With such an arrangement special stops are not indispensable because for a 
load equal to or above 10 kg the blade spring is almost completely 
flattened and the substrate comes into contact with the spring. 
The curve I representing the sensitivity of such a transducer (FIG. 2) is 
shown in FIG. 6 in which the weight in kg of the applied load is plotted 
along the horizontal axis and the electric signal supplied by the strain 
gauge arranged on the substrate is plotted along the vertical axis. It 
appears that the curve I has two parts: a linearly rising part 
approximately between weights from 0 to 8 kg and a constant part 
approximately between weights of 10 to 20 kg. The transition zone between 
these two parts at approximately 9 kg relates to the load necessary to 
bring the substrate into contact with the spring. The electric signal 
supplied by the gauge between 0 and 8 kg is perfectly proportional to the 
weight. This signal becomes constant when the substrate comes into contact 
with the spring. Thus, the transducer can be overloaded without the 
ceramic substrate being broken. The position of the transition zone is 
reproducible from one substrate to the other and does not depend on 
surface irregularities of the substrate. This transition zone is dependent 
upon the reproducibility of the elastic deformation of the spring. In 
fact, the deflection of the spring is approximately 10 times as large as 
that of the substrate. Thus, it is the reproducibility of the spring 
characteristics which enables the load limit to be determined precisely. 
In FIG. 2 the substrate 12 and the spring 13 are in contact at the edges 
15.sub.1, 15.sub.2 of the substrate. In operation it is possible that one 
edge of the substrate is not in contact all along the edge but only along 
a part of the edge or even in one point. When the quality of the contact 
along parts of the two edges of the substrate is considered it is evident 
that these parts can move along the respective edges when the load is 
varied. This may lead to a linearity error for small loads. 
FIG. 7 shows how to correct this error. It is a plan view showing the 
spring 13 with which the substrate 11 is in contact at the edges 15.sub.1, 
15.sub.2. In order to ensure a good linearity the edges are toothed, the 
edge 15.sub.1 having two teeth 35.sub.1, 35.sub.2 separated by a gap 36 
and the edge 15.sub.2 having one tooth 35.sub.3 so that the substrate 12 
bears on the spring 13 at three points. Thus, the teeth 35.sub.1, 
35.sub.2, 35.sub.3 ensures a satisfactory reproducibility of the linearity 
curve from one transducer to the other. 
In order to provide a correct support and a good linearity the substrate 
can be supported on contact elements. This is illustrated in FIGS. 8A, 8B. 
The substrate 12 and the contact elements in FIG. 8A are shown in a 
sectional view taken on the line I--I in FIG. 8B. Three contact elements 
37.sub.1, 37.sub.2, 37.sub.3 are arranged at apposite sides of the 
substrate for a stable balance. Preferably, these contact elements 
correspond in shape to the edge of the substrate in order to engage with 
the substrate along the edge. Thus, in operation the contact points 
38.sub.1, 38.sub.2, 38.sub.3 are movable on the curved blade spring. 
Said contact elements may be secured to the substrate 12 by means of an 
adhesive or by any other means. It is possible to connect the elements to 
one another. Thus, they may form part of a mounting 50 shown in FIG. 9, 
which mounting comprises three contact elements 37.sub.1, 37.sub.2, 
37.sub.3 interconnected by a web or ribs or any other coupling element. 
This mounting 50 should be slightly curved in such a way that the 
substrate 12 and the mounting 50 are in contact via the contact elements. 
FIGS. 10A, 10B show a second type of transducer in accordance with the 
invention. Similar elements bear the same reference numerals. A part of 
FIG. 10A corresponds to a sectional view taken on the line II--II in FIG. 
10B. The curved blade spring is replaced by springs which are subjected to 
deformations along their axes. The springs may be, for example, helical 
springs 90.sub.1, 90.sub.2, 90.sub.3. The flexibility of the springs is 
also selected in such a way that the range of deformation of the springs 
is larger than the range of deformation of the substrate. Preferably, 
three springs are used, two springs being arranged at one end of the 
substrate and one spring being arranged at the other end in order to 
define a stable seating. It is possible to use an arrangement with two 
springs (one at each side) or four springs (two at each side). The helical 
springs may be replaced by cylindrical buffer-type springs.