Abstract:
A force measuring device has a rigid stem joined to an essentially flat deformable membrane. The membrane includes detectors for detecting a deformation of the membrane. A portion of the stem comes in contact with an element capable of being subjected to the action of a force. The stem has slots forming anchoring means for interacting with the element. The force measuring device is for use, in particular, in improving the transmission of loads to the deformable membrane.

Description:
PRIORITY CLAIM 
   This application is a U.S. nationalization of PCT Application No. PCT/FR2006/000996, filed May 3, 2006, and claims priority to French Patent Application No. 0504564, filed May 4, 2005. 
   TECHNICAL FIELD 
   The present invention concerns a force measuring device. 
   BACKGROUND 
   A sensor of any type of force (force, pressure, traction force, moment or angular or linear acceleration) can be integrated into any system in which a force is to be measured (pneumatic, video games joystick, and the like) or an acceleration is to be measured (triggering of an airbag in an automobile, pacemaker, and the like). 
   One such force sensor is described in U.S. Pat. No. 6,666,079, in particular. 
   SUMMARY 
   The present invention is generally concerned with a force measuring device comprising a rigid stem connected to a substantially plane deformable membrane including means for detecting a deformation of said membrane, the stem including at least a portion adapted to be in contact with an element that can be loaded by said force. 
   At least a portion or the whole of the rigid stem of the force measuring device is adapted to be in contact with an element able to be loaded by the force to be measured. 
   Thus the stem transmits the force applied to the element to the deformable membrane, the deformation whereof is proportional to the force to be measured. 
   An object of the present invention is to enable efficient transmission of the forces to be measured. 
   To this end, the present invention is directed to a force measuring device comprising a rigid stem connected to a substantially plane deformable membrane including means for detecting a deformation of the membrane, the stem including at least a portion adapted to be in contact with an element that can be loaded by the force. 
   According to the invention, the stem portion includes slots forming anchor means adapted to cooperate with said element. 
   Accordingly, in contrast to the state of the art in which the stem of the sensor generally has a smooth cylindrical shape, the slots present on at least a portion of the surface of the stem improve the adhesion of the stem to the element with which it is in contact. 
   This improved adhesion or anchorage achieves improved transmission of the forces, in particular when the force to which the element is subjected is a traction force that tends to separate the element from the force measuring device as well as in the case of repetitive loads. 
   This avoids the creation of incipient cracks in the element in contact with the sensor that can lead to errors in the measurement of the forces applied and possibly to the partial or total destruction of the element cooperating with the stem of the force measuring device. 
   In one embodiment of the invention, the slots are perpendicular to the membrane, thereby improving the transmission of a force tangential to the rigid stem. 
   Instead of this or in addition to this, the slots are parallel to the membrane, to improve the transmission of a normal force extending on the axis of the stem. 
   Alternatively, the anchor means are formed by an enlarged portion of the stem so that the area of contact between the stem and the element loaded by a force is increased. 
   The present invention is advantageously used if the stem portion is adapted to be embedded in said element loaded by the force or the stem portion is adapted to be nested in that element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will become more apparent in the course of the following description. 
     In the appended drawings, given by way of nonlimiting example: 
       FIG. 1  is a diagrammatic view in cross section of a force measuring device conforming to a first embodiment of the invention; 
       FIG. 2A  is a view of the force measuring device from  FIG. 1  seen from below; 
       FIG. 2B  is a view of an alternative embodiment of a force measuring device from  FIG. 1  seen from below; 
       FIG. 3  is a diagrammatic view in cross section of a force measuring device conforming to a second embodiment of the invention; 
       FIG. 4  is a view of the force measuring device from  FIG. 3  seen from below; 
       FIG. 5  is a diagrammatic view in cross section of a force measuring device conforming to a third embodiment of the invention; 
       FIG. 6  is a diagrammatic view in cross section of a force measuring device conforming to a fourth embodiment of the invention; 
       FIG. 7  is a diagrammatic view in cross section of a force measuring device conforming to a fifth embodiment of the invention; 
       FIG. 8  is a diagrammatic view in cross section of a force measuring device conforming to a sixth embodiment of the invention; 
       FIG. 9  is a diagrammatic view in cross section of a force measuring device conforming to a seventh embodiment of the invention; 
       FIG. 10  is a view of the force measuring device from  FIG. 9  from below; 
       FIGS. 11   a  to  11   k  are diagrams illustrating the succession of steps of a method of fabricating a measuring device conforming to the invention; and 
       FIGS. 12   a  to  12   k  are diagrams illustrating the steps of a method of fabricating a measuring device conforming to another embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   A first embodiment of a force measuring device conforming to the invention will be described first with reference to  FIGS. 1 and 2A . 
   The force sensor illustrated has a particular structure of the “nail” type. It includes a rigid stem  10  surmounted by a head  11 . 
   That head  11  includes a substantially plane deformable membrane  12  that can be deformed if the rigid stem  10 , here connected to the center of the membrane, is loaded by a force or a moment or if the complete structure of the measuring device is loaded by an acceleration, the stem then forming a seismic mass. 
   In this embodiment, the deformable membrane  12  is a solid circular membrane, connected to the stem  10  in a central area  13 . 
   This deformable membrane  12  could have a different structure, and could for example be produced from different arms extending from the central area  13  to a peripheral area  14  of the membrane. 
   This peripheral area  14  includes anchor points that have a stable position relative to the stem  10 , whether the membrane  12  is deformed or not. There is a multitude of continuous anchor points in this embodiment, extending over the peripheral area  14  of the membrane  12 . 
   A cap  15  is also provided for covering the membrane  12  at a distance, on the side of a face opposite that carrying the rigid stem  10 . 
   This deformable membrane  12  further includes means for detecting its deformation, for example consisting of piezo-resistive gauges  16  aligned in different directions in the plane of the membrane. 
   Thus these detection means can comprise eight piezo-resistive gauges disposed four by four in a double Wheatstone bridge, aligned in two perpendicular directions in the plane of the deformable membrane  12 . 
   The imbalance measured at the terminals of the Wheatstone bridges is directly proportional to the deformation of the membrane in the direction associated with the Wheatstone bridge. 
   The description of U.S. Pat. No. 6,666,079 can advantageously be referred to for information regarding the detection of the deformation of the membrane and the measurement of force associated with this device. 
   In this embodiment of the present invention, a stem portion, here corresponding to the free end  10   a  of the stem  10 , has a structure including anchor means  17 . 
   Here these anchor means  17  comprise slots  17  perpendicular to the plane of the membrane  12 . 
   As clearly shown in  FIG. 2A , these slots are rectilinear and extend parallel to each other in a plane corresponding to the terminal face of the free end  10   a  of the stem  10 . 
   These slots perpendicular to the membrane  12  therefore open onto the free end  10   a  of the stem  10 . Thus they form grooves of square or rectangular cross section that open onto the terminal face of the free end  10   a  of the stem. 
   In this embodiment, where at least the free end  10   a  of the stem is adapted to be embedded in an element  20 , these slots  17  improve the anchoring of the stem  10  in this element  20 . 
     FIG. 1  illustrates the force measuring device in which the entirety of the device, that is to say the head  11  and the stem  10  of the sensor are embedded in the material of the element  20 . In this  FIG. 1 , the element  20  is loaded by a force F x  tangential to the axis of the stem  10 . 
   The force to which the element  20  is subjected can therefore be transmitted perfectly to the stem  10  thanks to the adhesion and to the anchorage improved by the presence of the slots  17 . These slots  17  extend perpendicularly to the plane of the membrane  12 , and the anchorage is particularly improved when the force applied to the element  20  extends in the plane of this membrane  12 . 
   These slots could have a different shape. Accordingly, as illustrated in  FIG. 2B , the slots  17 ′ can be concentric circles opening onto a face, here the terminal face, of the free end  10   a  of the stem  10 . 
   The depth of the slots  17 ,  17 ′ must not weaken the stem which must remain rigid when acted on by the applied forces. Here a depth of the order of one quarter of the length of the stem  10  is a correct value. 
   A second embodiment of the invention is described next with reference to  FIGS. 3 and 4 . 
   The elements identical to the embodiment described hereinabove, and carrying the same reference numbers, are not described again in detail here. 
   In this embodiment, the stem  10  includes in its height anchor means  18  that are formed of slots parallel to the plane of the membrane at rest. 
   As shown clearly in  FIG. 4 , these slots are annular and open onto the perimeter of the cylindrical rigid stem  10 . In this embodiment, three circular slots  18  are superposed on the longitudinal axis of the stem  10 . 
   These slots  18  are formed of annular grooves of square or rectangular cross section one side whereof opens onto the longitudinal wall of the stem  10 . 
   Of course, a single slot could be produced in the body of the stem  10 , or a number equal to two or greater than three. 
   The slots  18  open onto the longitudinal wall of the stem  10 , and these anchor means are particularly suitable for transmitting a force to the membrane  12  when the force F z  loading the stem  10  is normal, i.e. on the axis of the stem  10 , perpendicularly to the membrane  12 . 
   Here also, the depth of the slots in the thickness of the stem must not weaken the latter. 
   In  FIG. 3  the sensor is illustrated buried in an element  20 , the latter being loaded by a traction force F z  tending to separate the element  20  from the stem  10 . 
   The anchorage of the stem  10  in the element  20  is improved by the presence of the slots  18 . 
   A third embodiment is described next with reference to  FIG. 5 , elements common to the preceding embodiments carrying the same reference numbers. 
   In this embodiment, the stem is structured so that the anchor means are formed by an enlarged portion of the stem  10 . 
   Here this enlarged portion has a frustoconical shape, the enlarged base of the frustum constituting a free end  10   a  of the stem  10 . 
   In this embodiment, in which the sensor is buried in an element  20 , when the loading force F z  is perpendicular to the membrane  12 , this enlarged portion of the free end  10   a  of the stem provides a mechanical anchorage favorable to the transmission of the force to the membrane  12 . 
   In the preceding embodiments, there is illustrated a sensor entirely buried in an element  20 , for example in a flexible rubber type material. 
   Of course, only the stem  10 , or the free end  10   a  of the stem  10  could be buried in the element  20 . 
   A fourth embodiment is described next with reference to  FIG. 6 , elements common to the preceding embodiments carrying the same reference numbers. 
   In this embodiment, the stem  10  comprises anchor means consisting both of slots  17  perpendicular to the membrane  12  and slots  18  parallel to the membrane  12 . 
   Here this force measuring device comprises slots  17  opening onto the free end wall  10   a  of the stem  10  and a slot  18  opening onto the longitudinal wall of the stem  10 . 
   Thus the two types of slots described hereinabove with reference to the first and second embodiments of the invention can be combined. 
   This embodiment is particularly suitable when the stem  10  cooperates with a rigid transmission element  21 , similar to a stem. Such a mounting of the force measuring device can be encountered in particular in a game control device of the joystick type. 
   Of course, the embodiments described hereinabove can be combined. 
   In particular, as clearly illustrated in  FIG. 7 , in a fourth embodiment, the stem  10  can have an enlarged portion  10   c  and slots  17  extending perpendicularly to the plane of the membrane  12 . 
   Thus in this embodiment the stem  10  has a first portion  10   b  of smaller diameter connected to the central area  13  of the membrane  12 . This smaller portion  10   b  is then extended by a portion  10   c  of increased diameter, achieving improved anchorage of the stem  10  in an element loaded by a force. 
   Here these two stem portions  10   b ,  10   c  are cylindrical and coaxial. 
   Alternatively, as illustrated in  FIG. 8 , in a sixth embodiment, the enlarged portion  10   c  of the stem can also have a frustoconical shape as described hereinabove with reference to  FIG. 5 . 
   It is connected by a portion  10   b  of smaller diameter to the central area  13  of the membrane  12 . 
   Such a structure improves both the transmission of forces tangential and perpendicular to the plane of the deformable membrane  12 . 
   A seventh embodiment of the invention is described next with reference to  FIGS. 9 and 10 . 
   As in the embodiments described with reference to  FIG. 7 , the stem  10  has an enlarged portion  10   c  attached by a portion  10   b  of smaller diameter to the central area  13  of the membrane  12 . 
   Also, the free end  10   a  comprises slots  19  clearly illustrated in  FIG. 10  that open both onto a longitudinal wall of the stem  10  and onto the terminal face of the free end  10   a  of the stem  10 . 
   The slots  19  are preferably distributed symmetrically with respect to the central longitudinal axis of the stem, and here are distributed regularly over the perimeter of the cylindrical stem  10 . 
   These anchor means  19  improve not only the transmission of tangential forces but also the transmission of a moment to which the stem  10  is subjected. 
   Of course, the embodiments described hereinabove are in no way limiting and can be combined with each other to improve the anchorage of the stem  10  in an element loaded by a force to be measured. 
   Moreover, the number and the shape of the slots  17 ,  17 ′,  18 ,  19  are in no way limiting. 
   A first fabrication method for producing slots perpendicular to the membrane  12  is described next with reference to  FIGS. 11   a  to  11   k.    
   The fabrication process described here uses microtechnology techniques. 
   Starting from an SOI (Silicon On Insulator) substrate, as illustrated in  FIG. 11   a , the first step is to etch the surface layers as illustrated in  FIG. 11   b.    
   This is followed by epitaxial growth as illustrated in  FIG. 11   c.    
   This step grows a layer of monocrystalline silicon from the surface monocrystalline silicon of the SOI substrate. 
   As illustrated in  FIG. 11   d , there are then formed on an upper face of the substrate resistive gauges forming detection means and conductors for connecting the gauges to form Wheatstone bridges. 
   As illustrated in  FIG. 11   e , a double mask  30 ,  31  is then produced on the lower face of the substrate. 
   A first deep etch as illustrated in  FIG. 11   f  begins the production of the stem  10 . 
   As illustrated in  FIG. 11   g , the first mask  30  is then eliminated, the second mask  31  remaining present to delimit a series of slots opening onto the free end of the stem  10  being formed. 
   Here, this second mask  31  forms a series of parallel strips spaced from each other at a regular pitch. 
   As illustrated in  FIG. 11   h , a second deep etch is performed and then the second mask  31  is also eliminated. This forms the slots  17 . 
   In  FIG. 11   i , a protection layer  32  is applied to the upper face carrying the detection means and the conductors. 
   Then, as illustrated in  FIG. 11   j , the sacrificed oxide layer of the SOI substrate is etched to obtain the smaller-diameter portion  10   b  of the stem  10 . 
   Finally, the protection  32  is removed from the upper face, as illustrated in  FIG. 11   k.    
   A second method of fabricating a force measuring device conforming to the invention in which the stem includes a slot extending parallel to the plane of the membrane  12  is described next with reference to  FIGS. 12   a  to  12   k.    
   As before, microtechnology techniques are used. 
   Starting from an SOI substrate as illustrated in  FIG. 12   a , the surface layers are etched as illustrated in  FIG. 12   b  followed by epitaxial growth as illustrated in  FIG. 12   c.    
   In this fabrication process, the substrate is then inverted, as illustrated in  FIG. 12   d , the upper face becoming the lower face and vice-versa. 
   Then, as illustrated in  FIG. 12   e , the detection means  16  and the electrical connection of these connection means  16  are produced on an upper face of the substrate in known manner. 
   There are then produced as illustrated in  FIG. 12   f  both a protection layer  33  on the upper face of the substrate, for example with the aid of a resin, then a double mask  34 ,  35  on the lower face of the substrate. This double masking can be effected with the aid of an oxide placed under a resin. 
   A first deep etch begins the structure of the stem  10  of the sensor as illustrated in  FIG. 12   g.    
   As illustrated in  FIG. 12   h , wet etching of the lower, face of the substrate etches the sacrificed oxide layer to produce a slot  18  extending parallel to the plane of the membrane. 
   The resin-based first mask  34  is then eliminated along with the protection layer  33  of the upper face of the substrate as illustrated in  FIG. 12   i.    
   A second deep etch is performed as illustrated in  FIG. 12   j  to produce both the plane of the membrane  12  and the body of the stem  10  in the region of its junction with the membrane  12 . 
   Finally, as illustrated in  FIG. 12   k , the oxide-based second mask is eliminated. 
   Of course, the fabrication methods described hereinabove are given by way of nonlimiting example only. 
   In particular, other more conventional techniques could be used to produce a force measuring device conforming to the invention. 
   In particular, the various parts of the structure could be produced by conventional machining and then assembled. 
   Moreover, the force measuring device can be mounted differently on the element loaded by a force and, in particular, a free space can exist between the loaded element and the deformable membrane of the device.