Abstract:
A sensor for measuring a physical parameter is described having a frame, at least one support which is resiliently flexible under the influence of a physical parameter and which is integral with the frame, and at least one resonator integral with the support for supplying a signal representative of the physical parameter in response to flection of the support, the sensor also having an abutment and counter abutment limiting the flextion of the support, in particular before it is fixed to the frame. This sensor has applications in the measurement of a physical parameter such as, for example, acceleration.

Description:
FIELD OF THE INVENTION 
     The invention relates to sensors for measuring physical parameters. 
     More specifically it relates to a sensor of the type having a support which is resiliently flexible under the influence of external stress such as acceleration and which is in particular adapted to receive detection means responsive to this stress. 
     DESCRIPTION OF THE PRIOR ART 
     Sensors provided with flexible supports of this type are already known. Sensors of this type have been used for a long time in instruments measuring forces, such as balances, or in instruments for measuring other physical parameters such as pressure or acceleration. These latter applications have, moreover, proved very valuable in the automotive field where these sensors are located in suitable parts of a vehicle to measure the acceleration undergone by the latter and to consequently modify certain parameters or trigger various safety devices. 
     In their simplest form, sensors of this type comprise a flexible support substantially having the shape of a beam of rectangular section fixed in overhanging manner onto a frame. This flexible support which is designed to deflect along at least one sensitive axis in response to external stress has a transducer. This transducer is fixed to a large face of the flexible support equidistant from one or other of its extremities. When the beam is flexed the transducer, which is, for example composed of a quartz resonator in the shape of a double tuning fork, sends signals in the form of frequency variations towards a signal treatment circuit. One or more other transducers are often used to reduce the effects of interference (temperature). 
     In this type of sensor, before being fitted to the frame the support is already equipped with its transducers. The latter are firmly fixed to the support to give maximum sensitivity to detect the slightest movement of the support with the result that, in the case of quartz transducers, the transducers often break during handling before and during installation since they are extremely fragile. 
     This risk of damage consequently makes it necessary to set up additional inspections in order to avoid installing defective sensors in their respective frames. Moreover, damage of this kind is not easily detected with the naked eye, thus making it necessary to use inspection instruments. 
     It should be noted that inspection steps of this kind are all the more necessary since these sensors are generally intended for automobiles or other vehicles for passenger transport in which they give information which could have a direct effect on the safety of these passengers. 
     OBJECTS OF THE INVENTION 
     It is thus an object of the invention to overcome this disadvantage by providing a sensor for measuring a physical parameter capable of being manipulated during its installation without the risk of damage to the transducers mounted thereon. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention thus relates to a sensor for measuring a physical parameter of the type comprising: 
     at least one support, said support having a base adapted to be mounted on said frame and from which extends an elongated member capable of flexing in relation to the base under the action of said physical parameter, 
     a mass integral with the elongated member of said frame comprising a cavity closely surrounding the mass, 
     detection means integral with said elongated member capable of supplying a signal representative of said physical parameter, and 
     means for limiting the flexion of said elongated member, in particular before its mounting on the frame, wherein the limiting means comprise at least one arm integral with the base and extending parallel to and adjacent to the elongated member, the extremity of said arm also extending into a space provided in said mass. 
     Due to these features, flexion of the support is advantageously controlled by the elongated member which limit flexion of the elongated member and consequently limit the stress on the detection means thereof up to a predetermined rupture-resistance threshold without extensively increasing the cumbersoneness of the sensor. 
     According to one embodiment, the elongated member is formed by an overhanging beam extending from the base and where said mass is formed by two assembled parts between which a space is provided into which said arm extends, this space defining the amplitude of maximum flexion of said elongated member. 
     The invention also relates to a sensor for measuring a physical parameter of the type comprising: 
     a frame, 
     at least one support, said support having a base adapted to be mounted on said frame and from which extends an elongated member capable of flexing in relation to the base under the action of said physical parameter, 
     detection means integral with said elongated member adapted to provide a signal representative of said physical parameter, and 
     means form limiting the flexion of said elongated member wherein said limitation means comprise at least one arm integral with the base, said arm extending parallel to and adjacent to the elongated member and comprising at least one wing which extends therefrom, said wing being integral with said arm or located thereon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will appear from study of the following detailed description of two embodiments of the invention, given by way of non limiting example, with reference to the accompanying drawings in which: 
     FIG. 1 is an exploded perspective view of a measurement sensor of the invention, 
     FIG. 2 is a view from above of the support of the sensor of FIG. 1 provided with various functional members and ready to be mounted on the frame, and 
     FIG. 3 is a sectional view along the line III--III of FIG. 2. 
     FIG. 4 shows a second embodiment of the stop means and counter-stop means of the sensor of the invention. 
     FIG. 5 is a sectional view along the line V--V of FIG. 4, 
     FIG. 6 is a similar view to FIG. 4, but representing a third embodiment of the stop means and counter-stop means of the sensor of the invention, and 
     FIG. 7 is a sectional view along the line VII--VII of FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring firstly to FIG. 1, this shows one preferred embodiment of the measurement sensor of the invention designated by the general reference numeral 1. 
     This sensor 1 has a frame 2 which is composed of two shells, 2a and 2b respectively, of similar shape and designed to be assembled using fixing means, such as screws 4, only one of which has been shown in FIG. 1. 
     The frame 2 is adapted to receive a functional unit 6. 
     This functional unit comprises a support 8 one part at least of which is resiliently flexible under the influence of a physical parameter to be recorded or measured. 
     In this specific example, two half masses 10 connected together by an axle 12 driven therein, are fixed to the support 8. The axle 12 passes completely through the support 8 so that this support is sandwiched between the two half masses 10. 
     In addition, the support 8 also has two detection means 14. In the example shown the detection means are composed of two quartz resonators shaped like a double tuning fork mounted on both sides of the support 8. Each resonator 14 is electrically connected to an associated printed circuit board 16 having electronic components 18 mounted on the surface thereof, only one of which has been shown here. The elements 16 and 18 constitute an electronic circuit 20. 
     The resonators 14 are connected to oscillating circuits, not provided with reference numerals, in a conventional manner, the resonance frequencies of which vary as a function of the flexion of the resonators and consequently as a function of the flexion of the support 8. The resonating circuits deliver signals which are treated and analysed in the electronic circuit 20. This delivers a variable signal as a function of the flexion of the support 8 and thus, in this example, of the acceleration undergone by the two half masses 10. 
     This embodiment therefore consists of two printed circuit boards 16 arranged one on each side of the support 8 and fixed thereto by means of press studs 19 or other similar means such as screws, not shown. 
     It should also be noted that at the area where the resonators 14 are mounted, the support 8 has under each of its detection or resonating means 14 a corresponding aperture 22 permitting the resonator 14 to vibrate freely. 
     In this specific example, the support 8 has a base 24 which is adapted to be mechanically fixed to the frame 2. Extending from this base 24 is a member 26 of substantially elongated shape which is particularly capable of resiliently flexing in relation to the base 24. This elongated member has the general shape of a beam of rectangular section, mounted in the example shown overhanging the frame 2. This beam is adapted to deflect in sensitive manner in one direction perpendicular to its large faces, under the influence of external stress, and this along a sensitive axis Xs to trigger the detection means 14. 
     In this embodiment the measurement sensor 1 and more specifically the support 8 has two arms 28 which are connected together to form a closed frame about the elongated member or beam 26. 
     The two arms 28 extend in a direction substantially parallel to that of the elongated member 26 from the base 24 and beyond the free extremity of the elongated member 26. 
     The two arms 28 are thus connected together beyond the free extremity of the elongated member 26 by means of a junction part 30 provided with a positioning opening 32. 
     It should be noted that the base 24, the elongated member 26, the two arms 28 as well as the junction part 30 are all provided in the same plan to form a single member and constitute the support 8 in such a way that the latter can advantageously be made of a stamped metal sheet. The support 8 can thus be mass produced using simple operations and at low cost. 
     It will be noted that the stamping operation needed for this manufacturing process does not require any additional stage to prepare the support 8 provided with the elongated member and the frame formed by the base 24, the junction part 30 and the two arms 28. 
     The base 24 also comprises a positioning opening 34 which is located, as is the positioning opening 32, on a longitudinal axis X1 of the beam 26, termed the positioning axis. The two positioning openings, 32 and 34 respectively, are located as far as possible from each other to ensure the correct positioning of the frame formed by the arms 28 and hence of the support 8 in the frame 2. The base 24 also has a passage opening 36 adapted to receive a screw 38 to fix the support to the lower shell 2b of the frame 2. Interposed between the screw 38 and the base 24 is a collar 23 which bears against the base 24 to coincide with the edge which overhangs the elongated member or beam 26. 
     The two positioning openings 32 and 34 are so arranged as to cooperate respectively with the studs 40 and 42 designed to fit precisely into the upper and lower shells 2a and 2b. 
     The positioning means 32, 34, 40 and 42 make it possible to adjust with a predetermined play the two half masses 10 inside the two cavities 44 and 46 provided in shells 2b and 2a respectively. 
     It will thus be noted that the two half masses 10 constitute a single mass having two parts assembled by an axle 12 between which is provided a space E into which the two arms 28 partially extend. This space E defines on both sides of the arms 28, and between each of these and the two half masses 10, a play determining the maximum flexion amplitude of the elongated member 26. 
     It will thus be understood that, before it is mounted in the frame 2, the functional unit 6 is protected against unwanted flexions which the elongated member 26 could undergo and which could lead to the damage or breakage of the resonators 14 because the amplitude of flexion of the elongated member 26 is limited to a predetermined threshold by the mechanical contact between the opposing faces of, respectively, the half masses 10 and the arms 28 forming the stop and counter-stop respectively. 
     The stop means are thus formed by the two half masses 10 which are integral with the elongated member 26. 
     Referring now to FIGS. 4 to 7, these show two embodiments of a support 8 of the invention in which those elements identical to those previously described have been designated with the same reference numerals. 
     Referring in particular to FIGS. 4 and 5, the support 8 has a single arm 28 which has two wings 50 extending from this latter and which are mounted on this arm by means of a press stud 52, these parts thus being mechanically fixed to the free extremity of the arm 28. The two wings 50 extend along planes substantially parallel to that of the elongated member 26, in a direction perpendicular thereto and alongside this elongated member to limit its displacement in both directions during flexion thereof. The elongated member 26 and the two wings 50 thus form the stop and counter-stop respectively. 
     It follows that in an even more simplified embodiment it would be sufficient to have only one wing which would limit movement of the elongated member 26 in a single direction. In another embodiment, the wing(s) are integral with the arm 28 and are provided therein by stamping and then bending in the appropriate manner. 
     Referring now to FIGS. 6 and 7, the elongated member 26 comprises two extensions 54 which are mounted by means of a press stud 56 close to the free extremity of the elongated member 26 and the two arms 28 which are integral with the base 24 extending partially between the wings 54. 
     A simplified embodiment has one extension 54 limiting the movement of the beam 10 in a single direction. 
     In another embodiment the extension(s) is/are integral with the beam 26 and is/are bent in appropriate manner to extend alongside the arm(s) 28. 
     It will also be noted that the extensions 54 constitute stop means whereas the arms 28 constitute associated counter-stop means. 
     It emerges from all these embodiments that the stop means and counter-stop means are integral with the support 8 and are mechanically independent of the frame 2 with the result that the stop means and the counter-stop means limit the flexion of the support 8 before it is fitted to the frame 2. The detection means 14 can consequently be effectively protected during the handling that precedes the operations to fit the functional unit 6 in its frame 2.