GB2183051A - Strain gage sensor - Google Patents

Abstract

The invention relates to a strain gage sensor which includes a flexible element (3) formed of two beams (1 and 2 or 27 and 28,29) of the same flexibility interconnected at their ends by rigid cross-pieces (4,5). A force (P) to be measured is applied at the center of one of the beams whilst a mounting reaction (R) is applied to the center of the other. Electrical resistance strain gauges are formed on one surface of the beam (1,27). <IMAGE>

Description

SPECIFICATION Strain gage sensor The invention relates to a strain gage sensor used for measuring forces or displacements.

A strain gage sensor generally includes a flexible element on which resistive gages are stuck or printed and are fed from an electrical voltage source. The printing of the gages is generally carried out by a photo-lithographic process. The flexible element is subjected on the one hand to a force to be measured and on the other hand to a reaction force imposed by a frame in which this element is mounted.

The flexible elements are generally subjected to S-shaped distortions. The application of the forces to be measured has the effect of stretching or compressing the flexible element in flexion and the gages that are applied to it.

Consequently the electrical resistances of these gages vary and their variations are measured by means by a measuring bridge. The measurement of these variations represents the physical phenomenon to he analyzed.

When they are installed on a site, these sensors are subject to heating. Under this heating the flexible element expands. This has two consequent results. Firstly, the gages also expand, which changes their characteristic value. Secondly, the points of application of forces are relatively displaced with respect to each other. If the points of appliction of forces are maintained fixed, however, this results in parasitic traction-compression stresses in the flexible element. These parasitic stresses disturb the flexion effect to which this element is subjected and which we are seeking to measure in isolation. The measurement is therefore falsified.

It is known in the state of the art that the disadvantage of the shift in the characteristic value of the gages can be palliated by the addition of resistive elements not subjected to the mechanical strain and whose coefficient of resistivity varies with temperature inversely compared with that of the gages. Or the measuring bridge is simply calibrated on site. It remains however that the parasitic traction compression stresses that are superimposed on the flexion stresses to be measured can only be either ignored or made negligible by a certain mobility of the point of application of forces.

Furthermore, the production of flexible elements raises manufacturing problems. On the one hand, their sensitivity depends on the mesuring range in which they are made to work.

Their dimensions, in particular their thickness, are directly dependent on this range. Similarly, the arrangement of the gages on the flexible element varies according to the ranges. On the other hand the printing of the various gages on each of the flexible elements is carried out as many times as there are locations of these elements that can only be accessed in different ways. Each time a gage printing operation is undertaken on the one flexible element there is a risk of shifting the location of that gage with respect to the theoretical position that it should have with respect to the locations of the other gages. The direct result of these disadvantages is a scatter in the characteristics of the manufactured sensors corresponding with a given range.This scatter of characteristics gives rise to incovenient calibration operations on each sensor that must be carried out part by part.

The present invention relates to a strain gage sensor provided with a flexible element whose geometric configuration makes it possible to palliate the mentioned disadvantages.

On the one hand all the gages of a same element can be printed at the same time. On the other hand, whatever the range of the sensor may be, the locations of the various gages can be standardized.

The invention relates to a strain gage sensor of the type having a flexible element provided with gages on one of its surfaces and acted upon on the one hand by a force to be measured and on the other hand by a mounting reaction, the flexible element being formed from at least two beams interconnected at their ends by rigid cross-pieces, characterized in that it includes means for the mounting reaction and the force to be measured to be applied to the centers of the beams being located in a plane which is perpendicular to the axis of the beams.

The invention willbe better understood on reading the following description and on examining the figures that accompany it. This description is given by way of indication and in no way limits the invention. The same elements are given the same references in the figures. They show: Figures la and ib, perspective views of each of the beams of the invention; Figure 2, a perspective view of the flexible element of the invention; Figures 3a to 3e, the setting of the flexible element of the invention in the body of a sensor; Figure 4, an alternative embodiment of the flexible element according to the invention; Figure 5, a particular characteristic of embodiment of the beams of the invention; Figure 6, a table of manufacturing dimension as a function of the desired measuring range; Figure 7, a circuit diagram of the connection of the gages.

Figs. la and 1b respectively show two beams 1 and 2 of a same flexible element 3 (Fig. 2). These beams have the same flexibility. This is obtained in particular by producing them from the same material and with the same thickness e. In one example, this material is a steel alloy with a Young's modulus of 19600 kg/mm2. The strain gages refer enced J1 to J4 are all mounted on only one of the beams: beam 1. In order to form the flexible element 3, the beams 1 and 2 are interconnected at their ends by two rigid cross-pieces 4 and 5. A force to be measured P is applied to the center of one of the beams (beam 2) while the mounting reaction R is applied symmetrically on either side of the center of the other (beam 1). Under these conditions these two forces R and P are located in the same plane perpendicular to the axis-X'X of the beams.

In order to apply these forces, each beam 1 or 2 is provided with a central strengthening piece 6 and 7 oriented parallel to the crosspieces. Due to -the crosspieces 4 and 5 and the strengthening pieces 6 and 7, the beams cannot suffer distortion and the flexible parts of the element 3 are confined to the-sections of the beams which separate these central strengthening pieces from the cross-pieces.

The two central strengthening pieces are located, with respect to the surfaces of the beams, on the same side as the cross-pieces.

This gives, by means of a residual play 8 between these two strengthening pieces, the possibility of stopping the force P when it exceeds a nominal value for which the flexible element has been calculated.

The reaction force R is applied symmetrically to the central strengthening piece 6 by means of two lugs, 9 and 10, which extend perpendicularly to the central strengthening piece 6 and orthogonally to the axis X'X of the beam 1. This arrangement is such that the resultant of the mounting reaction is colinear with the force P to be measured. As the structure is symmetrical, this colinearity is maintained whatever the value of the force P may be. The direction of application of the force to be measured can be invariable no matter what the value of that force may be. It is noted that, in the example, the mounting reactions and the force to be measured are always located in a same plane which is perpendicular to the axis of the beams.The lugs thus make it possible to clear the upper surface 11 of beam 1 so that it can be polished and prined each time in a single operation.

In a practical embodiment, the two beams 1 and 2 are each provided with half crosspieces, 41, 51 and 42, 52 respectively. These two half cross-pieces which can be of equal height are connected to each other by welding to form the flexible element 3. In order to prevent this welding operation from altering the respective dimensions of beam 1 with respect to beam 2, these welds 43 and 53 are preferably spot welds. They are produced for example by means of a laser or by electronic bombardment. By spot welding, causing a general fusion of the surfaces of the half cross-pieces to be joined is avoided. This would have the effect, by flow or fusion, of modifying the anticipated heights of these cross-pieces as well as their alignment. In addition, spot welding does on cause as great a heating up of the half cross-pieces as a general weld. Hence it does not alter their mechanical strength.

The advantages provided by the structure and the geometrical shape of the flexible element of the invention are as follows. Firstly, there is no parasitic traction stress over the sensitive sections of the beams. On the one hand, under the effect of force P, the Sshaped distortion of each of these sections causes a mutual bringing together of the cross-pieces. This is not disadvantageous precisely because the cross-pieces are free, given that they are also connected to another beam which has the same flexibility characteristics.

Consequently, the mean lengths of the useful sections of the beams, and more particularly of beam 1 on which the gages are printed, remain constant no matter what the force P may be. On the other hand, the thermal expansion of beam 1 is balanced by an equivalent thermal expansion of beam 2 which is in the same ambient conditions as itself.

Secondly, the industrial manufacture of the beams is greatly facilitated. In fact, apart from the lugs 9 and 10, beam 1 is totally identical to beam 2. More accurately, this identity applies to the essential dimensions of the beams, namely their thickness e, the length L of the useful sections and the symmetrical positions of each of these useful sections with respect to the center of the beam. It is therefore possible to arrange for all the beams to be machined in one batch: it suffices to align the parallelepipedic blocks to be machined, parallel with each other, and start to machine in parallel with the alignment of these blocks.

In addition, before the printing of the gages on beam 1, it is necessary to polish the surface 11 of this beam on which the gages will be arranged. This polishing can be carried out while checking the thickness of beam 1 taking the upper surface 12 of the lug 9 as reference. It is also easy to measure the thickness of the beam as both of its surfaces are accessible. As nothing interferes in the middle, the sections 13 and 14 of the beam 1 can be polished at the same time.

A final advantage is in the fact that all the gages are on only one surface (surface 11) of one of the beams. Consequently, all the gages can be produced at the same time. The result of this is that a possible fault in the positioning one of the gages (J1) with respect to a given abscissa on the X'X axis can be compensated by an equal positioning fault of opposite sign in a corresponding gage (J4). In fact the distances separating the gages are fixed once and for all as they are imposed by the masks used to produce the photolithographic printing of these gages. The printing itself is also facilitated by the fact that the beams, including the half cross-pieces, are not very thick, the machines used for the printing being generally provided for small thicknesses, those of semiconductor chips.During the ionic printing operations of the resistive layer (for example made of nickel-chrome) the homogeneousness of the applied electrical fields is more easily obtained. This results in a greater ease and a greater accuracy in the implementation of the photolithographic process.

Finally, if through an installation fault the force to be measured P is not applied exactly perpendicular to the surface 11 of beam 1, the distortions suffered by the two useful sections of this beam, sections 13 and 14, will not be identical. It is possible to obtain a compensation for this difference in distortion by connnecting the corresponding gages in parallel and therefore measuring an unbalance in the measuring bridge corresponding with the mean of these distortions.

Figs. 3a to 3e show how a sensor according to the ivention appears when installed in a body 15. The body 15 is of circular cylindrical shape. It is symmetrical with respect to a median plane 16 containing the axis X'X of the flexible element 3. The body 15 is provided with a slot 17 slightly wider than the flexible element 3 so that this element can move within it easily. Two slots 18 and 19 are created, on either side of the plane 16, on the top of the body 15 in order to receive the lugs 9 and 10 respectively. The slots are provided with pis 20 and 21 respectively for insertion in the holes 22 and 23 made in the lugs. The body 15 is also provided, in its base, with a hole 24 (shown here in dotted line as it is not visible) through which the force P to be measured can be applied.So that the flexible element 3 is not ejected out of the body 15 when the force P is a thrust force, the pins 20 and 21 are welded onto the lugs 9 and 10 respectively after they have been inserted. This weld is preferably also a spot weld produced by means of a laser or electronic bombardment.

Two ceramic substrates 25 and 26 (Fig. 3a) are provided to receive circuits in thin layers having the effect of compensating for the variations in the characteristic resistances of the gages under the effect of temperature. These two substrates 25 and 26 are fixed to the body 15, at its top, on either side of the surface 11 of beam 1 of element 3. Fig. 3e shows an overall view of the sensor. The substrates 25 and 26 are cut as sections of a circle so that they can be fitted into the extension of the generatrices of the cylinder 15.

Fig. 4 shows an alternative embodiment of the flexible element 3. It is possible to imagine that this alternative is derived from the previous one by cutting the beam not fitted with gages into two parts and attaching each of these two half-beams 28 and 29 on either side of and parallel to the measuring beam 27 so as to form flanges. The total width of the two half-beams 28 amd 29 is equal to the width of the beam 27. In order to avoid torsion couples, it is preferable that the width of each of these two half-beams is equal to half that of the central beam. The flexible element functions in exactly the same way as before: the reaction force R is applied to the central strengthening pieces such as 30 of the halfbeams, the force P to be measured being itself applied to the central strengthening piece 31 of beam 27.The additional advantage is in an even simpler machining of the various beams: the cuttinhg of the metal block from which they are produced including a sawing stage to provide the separations 32 and 33 between the various beams. Consequently, the welds between the half cross-pieces are no longer required since the sawing is stopped before complete separation. Also, during the polishing operation, the polishing machine, which can be applied over a wider surfece, does not risk producing uneven surfaces.

Fig. 5 shows, in an exaggerated way here, the distortions that the sensitive element of Fig. 3 is subjected to when it is submitted to a force P to be measured and to a reaction force R. Fig. 5 shows that for a same applied force P the displacement of the central strengthening piece 7 is equal to twice that which would be obtained if beam 2 were a rigid beam. Under the left hand section of Fig.

5, a diagram of the strains at each point in section 13 of beam 1 as a function of its abscissa is shown. The diagram of this strain, or superficial extension dl of the surface 11, theoretically appears as a straight line passing through zero at the point of inflexion of section 13.

In order to obtain a certain tolerance in the placing of the gages, it is convenient to "flatten" this strain curve in the area of its maxima. This is obtained by connecting the beams to the cross-pieces and to the central strengthening pieces by means of fillets of given radius r. Because of symmetry, the radii of all the fillets are equal. These fillets are ensured during the machining of the beams.

For this purpose it is noted that the unobstructed nature of the areas to bge machined facilitates the accurate machining of these fillets. In fact, the flexible element of the invention, even in the case of the alternative, is freely accessible on all of its surfaces. There are no parts to be machined that are enclosed.

The thickness e of the beams is modified according to the range of forces P to be measured. For the industrial manufacture of the beams, the applicant has determined that a fillet radius r exists, which depends on the abscissa X of the gages but in particular is independent of the thcikness e of the beams, and for which the flattening of the strain curves is located at an invariable position. It is of course at this position that the gages are installed. In a manuacturing batch which is used by the applicant, the abscissa X of the start of the gages, from the inner edges of the cross-pieces or from the edges of the central strengthening pieces is 1000 micrometes; the spread of the gages is 800 micrometers and the fillet radius is 700 micrometers: which is therefore between 20 and 40% less than the abscissa of the start of the gages.In these conditions, it is possible to obtain a same nominal unbalance signal from the measuring bridge when the thickness of the beams and the range of corresponding applied forces are those given in the table in Fig.

6. This choice of a single fillet radius is therefore very important. It enables the manufacture of sensors giving a standardized response for ranges distributed in a ratio of 1 to 20.

In fact, the installation and the length of the gages are thus the same no matter what range the sensor is intended for. Similarly, the machining of the fillets will be the same no mater what the range may be; all that is necessary from one range to another is to change the thickness and therefore, in fact, the dimension with respect to the machining devices on which the unfinished parts to be cut stand. Rationalization of the manufacture of sensors is therefore greatly increased. In short, it can be said that if the abscissa X of the start of the gages is given, the fillet radius is significantly less than 30% of the abscissa of the start of the gages. The limits within which this choice of dimensions is possible, are determined by the thickness of the beams; the thickness of the thickest beam must be less (for example by 15%) than the value of the fillet radius.For example, in the table in Fig. 6, the maximum range corresponding to 80 newtons imposes a thickness of 580 micrometers for a material with a Young's modulus quoted earlier. This thickness is less, by about 15%, than the fillet radius used: 700 micrometers. The fillet radius must be modified for higher range assemblies. The spread D of the gages comes between the fillet radius and the abscissa of the start of the gages.

Fig. 7 is a diagrammatic representation of an example of a measuring bridge which can be installed with the gages of the invention.

The gages J1, J2, J3 and J4 are connected in series. The circuit is fed from a voltage source 34. Voltage unbalance is measured by an indicator 35 between the two center points of this circuit. In order to obtain compensation for a fault in the positioning of the gages when they are printed, this bridge can be modified as follows: gages J1 and J4 are connected in parallel with each other, as are the gages J2 and J3. These two pairs of gages are placed in series with each other and are fed from the voltage source 34. Another branch of the bridge is then formed by a pair of equal standard resistors, and the indicator 35 is connected to the center point of the pairs of gages and to the center point of the standard resistors. In these conditions, the differences in flexion that can be suffered by gages J1 and J4 or J3 and J2 are taken into acount for their mean value, whether these differences are due to a fault in the application of the forces to be measured, or due to a shift in the printing location of the gages.

Claims (12)

1. Strain gage sensor of the type having a flexible element (2) provided with gages (J1-J4) on one of its surfaces (11) and acted upon by a force (P) to be measured and by a mounting reaction (R), the flexible element being formed fromat least two beams (2,3) interconnected at their ends by rigid cross-pieces (4,5), characterized in that it includes means (9,10 or 28,29) for the mounting reaction and the force to be measured to be applied to the centers of the beams being located in a plane which is perpendicular to the axis (X'X) of the beams.

2. Sensor according to Claim 1, characterized in that the gages are arranged on only one surface of only one of the beams.

3. Sensor according to Claim 1 or Claim 2, characterized in that one of the beams (1) includes lugs (9, 10) placed at the center of that beam in order that the mounting reaction or the force to be measured is applied to it symmetrically on either side of the said beam.

4. Sensor according to Claim 3, characterized in that it includes only two beams and in that each of the beams is provided with a half cross-piece (41, 42, 51, 52) at each of its ends which is connected to a half cross-piece of the other beam.

5. Sensor according to Claim 4, characterized in that the connection between the half cross-pieces is obtained by means of spot welding (43, 53).

6. Sensor according to any of Claims 1 to 5, characterized in that each beam is provided with a central strengthening piece (6,7) extending parallel to the cross-pieces.

7. Sensor according to Claim 1 or Claim 2, characterized in that it includes three beams (27, 29) placed side by side in a plane perpendicular to the direction of application of forces, and each provided with a central strengthening piece (30, 31) through which the force to be measured or the mounting reaction acts.

8. Sensor according to Claim 7, characterized in that the three beams are obtained by machining from a single block and by partial separation of that block.

9. Sensor according to Claim 6 or Claim 7, characterized in that the cross-pieces and the central strengthening piece are connected to each beam with a fillet (r).

10. Sensor according to Claim 9, characterized in that the fillet radius is independent of the dynamic range of the forces to be measured and therefore from the thickness of the beams, but its value is between 60 and 80% of the abscissa (X) of the start of the gages with respect to the cross-pieces or to the central strengthening pieces concerned.

11. Sensor according to Claim 9 or Claim 10, characterized in that the spread (D) of each gage is between the fillet radius and the abscissa of the start of the gages.

12. A strain gage sensor substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.