Force sensor with extended operating range

Description

TECHNICAL FIELD AND STATE OF THE PRIOR ART

[0001] The present invention relates to a force sensor having an increased operating range, and in particular to a resonant microelectromechanical accelerometer with increased operating range.

[0002] A force sensor converts the displacement of a test body into a force that is transmitted. In the case of a resonant force sensor, the force is transmitted to a resonant beam, the test body being set in motion, for example, by acceleration, a magnetic field, or pressure.

[0003] A resonant accelerometer is an example of a resonant force sensor comprising a mass suspended from a support and movable in the plane or in an out-of-plane direction under the effect of an acceleration and at least one resonator suspended between the support and the mass. The resonator is for example formed by a vibrating beam, i.e. a beam vibrated at its resonant frequency by electrodes. When the mass moves under the effect of an acceleration, the beam which is mechanically integral with the mass is compressed or stretched, which modifies its resonant frequency. The variation of its resonant frequency is detected, which makes it possible to trace the value of the acceleration. The resonant beam is used as a force sensor.

[0004] The article "Microfabricated Neuroaccelerometer: Integrating Sensing and Reservoir Computing in MEMS" by Bruno Barazani et al. describes an accelerometer with intrinsic processing capabilities, which combines sensing and computing functions in the same MEMS. In particular, the article describes a test body electrically (but not mechanically) coupled to a strain gauge in the form of a vibrating sensing beam.

[0005] Accelerometers manufactured using MEMS&NEMS (Microelectromechanical systems & Nanoelectromechanical systems) technology comprise a mass of micrometric dimensions, i.e. having a footprint of a few 100 µm on each side to a few mm on each side, for example formed by a 1000 µm square. The thickness of the mass can reach, for example, up to 700 µm due to the thickness of the substrate, and one or more resonant beams or resonators of nanometric dimensions, called nanoresonators. Nanoresonators are beams whose dimensions can be as follows: a length of a few µm to a few tens of µm and a nanometric section of a few tens of nm to 1 µm on each side.

[0006] By miniaturizing the beams relative to the mass, it is possible to achieve a much greater sensitivity to acceleration than that achieved by accelerometers comprising a seismic mass and micrometric beams. Furthermore, the dynamic range of a resonator used as a force sensor is limited by the maximum stresses admissible by the vibrating beam. In compression, this stress is limited by the buckling phenomenon. In tension, the rupture of the beam material is the limiting phenomenon. In the case of nanometric resonators, these limits dimension the maximum operating range of the sensor.

[0007] For example, in the case of a silicon beam, buckling of a nanometric resonator, having a cross-section of 250 nm 2< and 10 µm length, is achieved at a compressive stress of 300 MPa, while tensile failure occurs for a tensile force greater than 1 GPa.Therefore it is the buckling phenomenon which limits the operating range.

[0008] However, the nanometric layer in which the beam is made is generally subjected to an internal compressive stress which can be of the order of 150 MPa, this internal stress appears during manufacturing. The operating range is then reduced to ± 150 MPa, the measurement range being symmetrical, or even less to maintain a “safety” margin with respect to buckling.

[0009] The same phenomenon can occur in the case of a force sensor using one or more piezoresistive gauges.

[0010] Therefore the operating range of MEMS&NEMS accelerometers is significantly reduced. STATEMENT OF THE INVENTION

[0011] It is therefore an object of the present invention to provide a force sensor having an extended operating range compared to prior art force sensors.

[0012] The aim stated above is achieved by a resonant force sensor comprising a micrometric-sized test body and a micrometric-sized strain gauge at least mechanically separated, and electrostatic coupling means between the mass and the strain gauge which, on the one hand, apply in the rest state a stretching stress induced by an initial electrostatic force, which makes it possible at least to cancel all or part of the internal stress in the strain gauge. On the other hand, these means make it possible, when the test body is moved, to apply a compressive or stretching stress to the strain gauge while having a mechanically separated test body and strain gauge.

[0013] Mechanical separation allows a release of part of the internal stresses of the strain gauge. The application of a stretching stress, due to electrostatic coupling means, cancels another part of the internal stresses. Thus the operating range is then extended, preferably at least by the value of the released internal stresses.

[0014] The strain gauge can be a resonator formed by a nanometer-sized vibrating beam or a nanometer-sized piezoresistive gauge.

[0015] Advantageously, the stretching stress can be such that the beam is put under tension, which further increases the operating range.

[0016] In an exemplary embodiment, the force sensor comprises a carriage of low mass relative to that of the test body, which transmits the stress to the strain gauge via a lever arm. The electrostatic coupling means may comprise interdigital combs.

[0017] It is also an object of the present invention to provide a force sensor offering increased sensitivity. For this, the force sensor comprises two differentially mounted strain gauges, the electrostatic forces are applied in a balanced manner to the test body, canceling the electrostatic forces at rest and thus repelling the "pull-in" or risk of "collapse". It is then possible to apply a higher electrical voltage and / or to reduce the air gaps, and thus increase the sensitivity of the sensor.

[0018] In other words, a force sensor is produced in which the displacement of the test body is transmitted to a strain gauge by an electrostatic coupling, which makes it possible to mechanically decouple the test body from the beam and to release part of the internal stresses of the strain gauge, and which also makes it possible to apply a permanent tensile stress to the strain gauge, advantageously at least of the value of the part of the internal stresses not released by the mechanical decoupling. The electrostatic coupling makes it possible to transmit a variable tensile / compressive stress to the strain gauge depending on the displacement of the test body.

[0019] Mechanical decoupling means that there is no direct mechanical connection between the test body and the strain gauge, the test body and the beam being suspended on the sensor support.

[0020] The present invention is defined by the independent claim, and advantageous embodiments are described in the dependent claims.

[0021] Preferably, the force sensor comprises a second nanometric cross-section strain gauge mounted in differential relation to the first strain gauge, and second electrostatic coupling means between the test body and said second strain gauge configured to ensure mechanical decoupling between said second strain gauge and the test body, so that at rest, said second electrostatic coupling means generate traction on the second strain gauge, and so that under the effect of an acceleration they modify the stress state of the second strain gauge.

[0022] The first nanoscale strain gauge is equipped with a vibrating beam. This vibrating beam is suspended from a suspension beam.

[0023] The suspension beam has no detection role and is not vibrated around its resonant frequency.

[0024] The vibrating beam has a smaller section than the suspension beam.

[0025] In particular, the suspension beam has a section at least 10 times, preferably 100 times greater, and more advantageously 1000 times greater than that of the vibrating beam.

[0026] The suspension beam has a micrometric section ranging from 1µm to several hundred micrometers.

[0027] The suspension beam helps to release the internal stresses resulting from the manufacturing of the layer in which the vibrating beam is etched. It thus helps to extend the operating range.

[0028] The first and / or second electrostatic coupling means may be configured to generate, at rest, an electrostatic force so as to at least cancel the residual stresses in the first strain gauge and / or the second strain gauge.

[0029] In an exemplary embodiment, the first and / or second electrostatic coupling means each comprise at least one first and one second facing electrode, the first electrode being integral in movement with the seismic mass and the second electrode being fixed to a longitudinal end of the first and / or second strain gauge.

[0030] The first strain gauge and / or the second strain gauge may each be connected to a second electrode by a lever arm rotatably articulated in the plane of the sensor.

[0031] The first and / or second electrostatic coupling means each comprise, for example, at least one pair of interdigitated combs provided with fingers, one comb being integral in movement with the test body and the other being fixed to the lever arm.

[0032] The test body and the means for suspending the test body are configured so that the test body moves in translation in the plane along a given direction.

[0033] According to an additional characteristic, the first and / or second electrostatic coupling means comprise at least one carriage carrying at least one comb and mechanically connected to the lever arm. Preferably, the carriage has a reduced mass compared to that of the test body, the mass of the carriage being at least 10 times less than the mass of the test body.

[0034] For example, the carriage comprises an outer frame delimited by uprights from which fingers of the comb protrude, and the at least one comb integral in movement with the test body is arranged inside the frame and its fingers are placed between the fingers of the comb of the carriage.

[0035] Advantageously, the force sensor comprises several pairs of interdigitated combs so as to provide a large electrostatic coupling surface.

[0036] For example, the first and / or second coupling means comprise two carriages each connected to the first strain gauge by a lever arm and a transmission connected to the two lever arms.

[0037] According to another example, the first and / or the second strain gauge is and / or are suspended by a longitudinal end from a first lateral face of a suspension beam deformable in bending in the given direction (X), and a second face of the suspension beam opposite the first face is opposite an edge of the test body forming therewith the first and / or the second electrostatic coupling means.

[0038] According to another example, the force sensor comprises a pivot joint by which the mass is suspended from the support and by means of which the mass is rotatable in the plane of the force sensor, and the first and / or second coupling means are formed by a lateral face of the lever arm and a facing edge of the test body.

[0039] Very advantageously, said at least one strain gauge is a vibrating beam resonator.

[0040] Another subject of the present application is an accelerometer comprising a force sensor according to the invention, in which the test body is a seismic mass. Another subject of the present application is a gravimeter comprising at least one accelerometer according to the invention and the means for suspending the seismic mass are configured to buckle when the seismic mass is subjected to 1g of the Earth's acceleration. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The present invention will be better understood on the basis of the following description and the attached drawings in which: Figure 1A is a schematic representation of an example of an accelerometer according to the invention according to a first embodiment. Figure 1B is a representation of the accelerometer of the Figure 1A when an electrostatic force is generated with a magnified view. Figure 2is a schematic representation of another example of an accelerometer according to the invention according to a first embodiment. Figure 3A is a schematic representation of an example of an accelerometer according to the invention according to a second embodiment. Figure 3B is an enlarged view of the Figure 3A . Figure 4A is a schematic representation of another example of an accelerometer according to the invention according to the second embodiment. Figure 4B Figure 4C Figure 4D Figure 4E are detailed views of the accelerometer of the Figure 4A . Figure 5 is a schematic representation of an accelerometer according to the invention according to a third embodiment. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0042] The force sensor according to the invention will be described more particularly in its application to a microelectromechanical accelerometer, but it can be implemented in other applications as will be described below.

[0043] On the Figures 1A and 1B , we can see an example of an embodiment of a microelectromechanical accelerometer ACC1 according to a first embodiment.

[0044] In the present application, the term "microelectromechanical accelerometer" means a microelectromechanical and nanoelectromechanical accelerometer comprising a part, in particular the seismic mass of micrometric dimensions, and another part, in particular the measuring means, of nanometric dimensions.

[0045] The accelerometer ACC1 comprises a support 2, which is generally referred to as a substrate in microelectronics, a seismic mass 4 suspended from the support 2 by suspension means 6. The suspension means 6 are such that the seismic mass 4 is able to move in the XY plane of the accelerometer along the X direction under the effect of an acceleration.

[0046] The plane of the accelerometer is a plane parallel to the support.

[0047] In the example shown, the suspension means 6 comprise beams extending perpendicular to the X direction in the plane of the accelerometer and configured to deform in bending in the X direction. In this example, the seismic mass 4 is square in shape and there are four beams, parallel to each other and connecting a top of the mass 4 to the support 2. Other suspension means and other arrangements are conceivable. For example, the beams can be folded so as to reduce the stiffness in the X direction while limiting the buckling of the suspension elements. More than or at least four suspension elements are conceivable, for example six suspension elements can be implemented. Preferably, the suspension means are symmetrical to reject as much as possible parasitic movements, in particular the rotation of the mass in the plane.

[0048] Furthermore, the shape of the mass is not limiting.

[0049] In this example, the accelerometer further comprises means 8 for measuring the displacement of the seismic mass comprising a vibrating beam 10. A vibrating beam or resonant beam is vibrated at its resonant frequency by one or more actuating electrodes, any tensile or compressive stress experienced by the beam results in a variation in its resonant frequency, this variation makes it possible to deduce the stress experienced. The suspension of the vibrating beam is such that the beam is capable of vibrating. The vibrating beam 10 is parallel to the X direction and is suspended by one longitudinal end from the support 2 and by another longitudinal end from a suspension beam 12 which is perpendicular to the X direction, the suspension beam 12 being anchored to the support by its ends 12.1, 12.2. The suspension beam 12 is capable of deforming in bending in the X direction. The vibrating beam 10 can be anchored directly to a pad.The measuring means then comprise capacitive detection means. Preferably, the detection electrode is distinct from the actuating electrode. In another example, the detection means are of the piezoresistive type and the end of the vibrating beam 10 is connected to a pad via a piezoresistive gauge.

[0050] The accelerometer is a MEMS&NEMS system (Microelectromechanical systems & Nanoelectromechanical systems in English terminology) or microelectromechanical and nanoelectromechanical systems. The accelerometer then comprises a mass of micrometric size, i.e. having a footprint of a few 100 µm on each side to a few mm on each side, for example formed by a 1000 µm square. The thickness of the mass can reach for example up to 700 µm due to the thickness of the substrate, and nanometric size measuring means, i.e. the vibrating beam can have the following dimensions: a length of a few µm to a few tens of µm and a nanometric section of a few tens of nm to 1 µm on each side, for example 250 nm. The ACC1 accelerometer is manufactured using microelectronic technologies by layer deposition and etching, the vibrating beam 10 is etched in a layer whose thickness is a few tens to a few hundred nanometers.This layer is subjected to an internal compressive stress, for example of the order of 150 MPa.

[0051] In the ACC1 accelerometer of the Figures 1A and 1B the suspension beam 12, from which the vibrating beam 10 is suspended, is free to deform in the direction X which is the longitudinal direction of the vibrating beam 10, the internal stresses resulting from the manufacture of the layer in which the vibrating beam 10 is etched are partly released.

[0052] The accelerometer ACC1 also includes first electrostatic means C1 ensuring coupling between the seismic mass 4 and the vibrating beam 10.

[0053] The first coupling means C1 comprise a variable capacitance capacitor formed by a side 14 of the seismic mass 2 and by a lateral face 12.3 of the suspension beam 12. The side 14 and the lateral face 12.3 are opposite and separated by an air gap g, they form the plates of the variable capacitance capacitor.

[0054] By applying a potential difference between the seismic mass 2 and the suspension beam 12, an electrostatic force appears between the side 14 and the side face 12.3, which ensures a coupling between the seismic mass 4 and the vibrating beam.

[0055] This coupling translates into: at rest; on the one hand, by the fact that there is an attraction between the mass and the suspension beam 12, which deforms the suspension beam 12 in bending towards the seismic mass 4 and reduces the air gap between the mass and the suspension beam 12, exerting a tensile force on the vibrating beam 10. This tensile force has the effect of canceling at least part of the remaining internal stresses in the vibrating beam 10. The potential difference is preferably chosen to at least completely cancel the internal stresses. in a state under acceleration on the other hand, by the fact that a displacement of the seismic mass in the X direction, under the effect of an acceleration along this axis, deforms the suspension beam 12 in bending and applies a tensile or stretching stress to the vibrating beam 10. It is possible to measure the acceleration via the stress applied to the vibrating beam 10.Indeed, the electrostatic force being inversely proportional to the square of the value of the air gap, when the mass under the effect of an acceleration is moved towards the suspension beam 12, the value of the air gap is reduced and the traction force applied to the vibrating beam increases. On the . Figure 1B , the displacement of the seismic mass 4 is symbolized by the arrow D. We can see the beams of the suspension means which are deformed. On the Figure 1B , the arrow T symbolizes the tensile force exerted on the vibrating beam 10 at its end connected to the suspension beam 12, by the electrostatic force.

[0056] By cancelling the internal stresses in the vibrating beam 10, the operating limit in compression is shifted by the value of the internal stresses, for example 150 MPa. For a beam with a cross-section of 250 nm 2< and 10 µm long, the buckling limit is 300 MPa. Thus the operating range, instead of being reduced to + / -50 MPa due to the internal stresses, extends to + / - 200 MPa if all the internal stresses are released.

[0057] In one operating mode, it is possible to choose to apply a potential difference between the mass and the suspension beam 12 such that the tensile force seen by the vibrating beam 10 is greater than the internal stresses to be released. The operating range is further increased. It should be noted that the theoretical tensile failure for silicon is greater than 1 GPa, the level of tensile force applicable to the beam is therefore high.

[0058] The operation of the ACC1 accelerometer is as follows.

[0059] At rest, a potential difference between mass 4 and suspension beam 12 makes it possible to release all or part of the internal constraints in the vibrating beam.

[0060] When the seismic mass 4 moves in the X direction under the effect of an acceleration, due to the electrostatic coupling between the mass and the suspension beam 12, the displacement of the seismic mass applies a traction on the vibrating beam 10. The vibrating beam 10 is furthermore vibrated at its resonant frequency. This traction has the effect of varying the resonant frequency of the beam. A processing unit to which the accelerometer is connected measures the variation in resonant frequency, determines the displacement of the mass and calculates the acceleration it has undergone.

[0061] On the figure 2, we can see another example of the embodiment of an ACC2 accelerometer according to the first embodiment.

[0062] The ACC2 accelerometer differs from the ACC1 accelerometer in that it has a second 10' vibrating beam mounted differentially with the 10 vibrating beam.

[0063] The vibrating beam 10' is arranged opposite the vibrating beam 10 with respect to the seismic mass. The vibrating beam 10' extends along the X direction and is suspended by one longitudinal end from the support 2 and is suspended by another longitudinal end from a suspension beam 12' extending perpendicular to the X direction and capable of deforming in bending. The suspension beam 12' is anchored to the support by its two longitudinal ends.

[0064] The suspension beam 12' extends parallel to an edge 16 of the mass 4 parallel to the edge 14. The edge 16 forms with the lateral edge of the suspension beam 12' a variable capacitance capacitor with an air gap g', forming second electrostatic coupling means C1' ensuring a coupling between the seismic mass 2 and the vibrating beam 10'. As for the vibrating beam 10, the connection of one of the ends of the vibrating beam 10' to the suspension beam 12' makes it possible to release part of the internal stresses in the beam 10'.

[0065] Furthermore, the application of a potential difference between the mass and the vibrating beam 10' generates electrostatic forces exerting a tensile force on the vibrating beam 10' and therefore the at least partial, and preferably total, cancellation of the internal stresses.

[0066] The implementation of a differential assembly and two coupling means C1, C1' very advantageously makes it possible to increase the sensitivity of the accelerometer, which is inversely proportional to the cube of the width of the air gaps and proportional to the square of the polarization voltage. Indeed, since the electrostatic forces are applied to the opposite faces 14 and 16 of the mass 4, the mass 4 is maintained in equilibrium between the two suspension beams 12, 12'. It is then possible to increase the electrostatic forces between the mass 4 and the beams 12 and 12' by increasing the applied potential difference, and thus to reduce the width of the air gaps while limiting the risks of sticking between the mass and the beams. Reduced air gap widths and a higher voltage make it possible to achieve high sensitivity.

[0067] The operation of the ACC2 accelerometer is as follows: The movement of the seismic mass under the effect of acceleration causes the air gaps g and g' to vary differentially. For example, when the mass 4 moves towards the vibrating beam 10, the width of the air gap g decreases, the electrostatic force applied increases, which amplifies the tensile stress on the vibrating beam 10; the width of the air gap g' increases, reducing the electrostatic force and reducing the tensile stress on the vibrating beam 10'.

[0068] These variations in tension stress cause the resonance frequencies of each of the vibrating beams 10, 10' to vary, which makes it possible to trace the acceleration undergone by the mass.

[0069] On the Figures 3A and 3B , we can see an example of an embodiment of an ACC3 accelerometer according to a second embodiment.

[0070] The ACC3 accelerometer differs from the ACC1 and ACC2 accelerometers, notably in that it uses lever arms.

[0071] The accelerometer ACC3 comprises a support 102, a seismic mass 104 suspended from the support 102 by suspension means 112. In the example shown, these are beams that are deformable in flexion and provide translational guidance of the seismic mass along the Y direction, similar to those of the accelerometers ACC1 and ACC2. In this example, the seismic mass 104 is rectangular in shape and two beams 112 extend on each side of the mass parallel to the X direction.

[0072] The ACC3 accelerometer comprises means for measuring the displacement of the mass comprising two vibrating beams 110, 110' mounted in differential mode and electrostatic coupling means C3, C3' between the measuring means and the mass 104.

[0073] Each vibrating beam 110, 110' is suspended by one end from the support 102 and is suspended by another end from a rigid beam 114, 114' of the coupling means C3, C3', said rigid beam 114, 114' being articulated in rotation relative to the support 102 around an axis Z orthogonal to the plane of the accelerometer. The rigid beams form lever arms.

[0074] Since the coupling means are similar or identical, only the C3 coupling means will be described in detail.

[0075] A pivot joint 116 is provided between a longitudinal end 114.1 of the rigid beam 114 and the support 102. As can be seen in detail in the Figure 3B , in this example the pivot connection 116 is formed by a beam deformable in flexion connecting the end 114.1 of the beam to an anchoring pad 118.

[0076] The vibrating beam 110 is suspended from the rigid beam 114 at a distance I from the pivot connection 116 along the beam 114. The lever arm 114 has a length L. The vibrating beam 110 is located so that, at rest under the action of the coupling means C3, the vibrating beam 110 undergoes a tensile force. In this example, the vibrating beam 110 is located opposite the pivot connection relative to the beam 114.

[0077] The electrostatic coupling means C3 also comprise a carriage 120 suspended from the support 102 and guided in translation along the direction X. In the example shown, the suspension means formed by beams 122 deformable in flexion ensure the translational guidance of the carriage 120 along the direction X. In the example shown, the carriage 120 has a rectangular external shape and two beams 122 extend from two opposite sides of the carriage parallel to the direction X. The carriage is secured to the end 114.2 of the lever arm 114, which is opposite that 114.1 articulated on the support.

[0078] The electrostatic coupling means comprise two pairs of interdigitated combs 124, 124', 126, 126'; one pair of combs 124, 124' is secured to the seismic mass 104 and the other pair of combs 126, 126' is secured to the carriage 120. The combs are oriented so that, when electrostatic forces are generated between the combs, a displacement in the X direction is generated between the mass 104 and the carriage 120.

[0079] In the example shown, an arm 128 extends along the Y direction from an edge of the mass perpendicular to the X direction, and carries fingers 130 extending along the X direction on either side of the arm 128. These fingers define the combs 124, 124' on either side of the arm 128.

[0080] The combs 126, 126' are integrated with the carriage and are located inside the carriage. The carriage has an outer frame and fingers 132, 132' extend from the edges of the frame parallel to the Y direction defining a comb 126, 126' on each edge of the frame.

[0081] The fingers 130, 130' of the combs 124, 124' are housed between the fingers 132, 132'. Variable air gaps in the Y direction are delimited between a face of a finger 130 and a facing face of a finger 132. The carriage is connected to the other end 114.2 of the rigid beam 114 by a beam 133 having rigidity in the Y direction while allowing coexistence between a translation of the carriage and a rotation of the lever arm.

[0082] In this example, the distances between fingers 130, 132 and between fingers 130', 132' are such that they are minimal so that the electrostatic forces cause a positive displacement of the carriage 120 in the Y direction and a negative displacement of the carriage 120' in the Y direction, the mass 104 being balanced by the two electrostatic steps.

[0083] The position of the vibrating beam 110 relative to the pivot 116 determines whether the movement of the carriage induces an elongation or a compression of the beam 110. In the example, the force (in the positive Y direction) generated by the carriage 120 induces an elongation of the beam 110.

[0084] The coupling means C3' comprise a second carriage 120' forming, with a second comb secured to the seismic mass, second electrostatic coupling means between the mass and the vibrating beam 110'.

[0085] The rigid carriages preferably have a very low mass compared to that of the seismic mass, for example the ratio between the mass of a carriage and that of the seismic mass is of the order of 1 / 10, preferably 1 / 20. The integration of the combs inside the contour of the carriages makes it possible to achieve great compactness, but also to reduce the mass of the carriages.

[0086] The implementation of interdigitated combs makes it possible to obtain a large coupling surface between the seismic mass and each carriage, which makes it possible to increase the electrostatic sensitivity, i.e. the sensitivity between the movement of the mass and the generated electrostatic force. Indeed, this sensitivity is proportional to the coupling capacity, therefore to its surface.

[0087] The electrostatic force attracting the two electrodes of a capacitor is: Force = 1 2 C g V pol 2

[0088] With C the capacity and g the distance between the facing surfaces.

[0089] The capacity is given by the facing surfaces S, the permittivity ε 0 and the air gap g: C = ε 0 S g

[0090] The arrangement of the interdigitated combs is not limiting, for example the arm 128 carrying the combs could be carried by the carriage and the mass would comprise combs arranged on either side of the shaft. Furthermore, the number of comb fingers is also not limiting.

[0091] The vibrating beams have some of their internal stresses released thanks to the mobility of the lever arm assembly 114, 114' and carriage 120, 120' relative to the seismic mass. In addition, by applying a potential difference between the seismic mass and the carriages, an electrostatic force is generated between the fingers of the interdigitated combs, the carriages are attracted towards the seismic mass, which causes the lever arms 114 to pivot, which apply a tensile force to the vibrating beams, which makes it possible to cancel all or part of the unreleased internal stresses, or even to place the vibrating beams so that they are always in a state of stretching during the measurements.

[0092] Alternatively, the air gaps between the fingers of the mass and the carriages 120 and 120' are arranged so that the carriage 120 moves negatively in the Y direction and the carriage 120' moves positively in the Y direction. The location of the vibrating beams is chosen relative to the lever so that they still undergo a tensile force.

[0093] The operation of the ACC3 accelerometer will now be described.

[0094] The seismic mass moves along the Y direction under acceleration, which causes the air gaps between the combs carried by the mass 104 and those carried by the carriages 120, 120' to vary differentially. For example, reducing the width of the air gaps with the carriage 120 increases the electrostatic force acting on and amplifies the tensile stress on the vibrating beam 110, and increasing the width of the air gaps with the carriage 120' reduces the electrostatic force and reduces the tensile stress on the other vibrating beam 110'.

[0095] These variations in tension stress cause the resonant frequencies of each of the resonant beams 110, 110' to vary, which makes it possible to trace the displacement of the mass and the acceleration at the origin of this displacement. Depending on the tension stresses applied to the vibrating beams in the rest state, one of the beams goes into a state of compression or remains in tension, but under reduced tension.

[0096] The buckling stress is no longer limiting due to the application of the electrostatic force at rest. It is the maximum displacement of the mass before instability that limits the dynamic range. However, the stress in the vibrating beam equivalent to the mass displacement is greater than the buckling stress in a state-of-the-art resonant accelerometer, thus resulting in an increase in the dynamic range.

[0097] An accelerometer similar to the ACC3 accelerometer comprising only a vibrating beam and a trolley does not depart from the scope of the present invention.

[0098] On the Figures 4A to 4E , we can see an example of an ACC4 accelerometer according to the second embodiment.

[0099] In a similar manner to the ACC3 accelerometer, the ACC4 accelerometer uses interdigitated carriages and combs forming the electrostatic coupling means. In this example, the mass 204 is suspended by suspension means 206 formed by a blade deformable by bending in the XY plane and folded back on itself, also called a "folding" spring. These suspension means also ensure the guidance of the mass along the X direction.

[0100] The measuring means comprise two vibrating beams 210, 210' mounted in differential mode.

[0101] The electrostatic coupling means C4, C4' are similar for both vibrating beams, only the coupling means C4 coupling the vibrating beam 210 to the mass 204 will be described in detail.

[0102] The coupling means C4 comprise two carriages 220, 220', each carriage is suspended from the support 202 and guided in translation along the direction X by suspension means 212 similar to the mass suspension means. In the example shown, four springs are implemented.

[0103] Only the 220 trolley will be described in detail; this is more particularly visible on the Figure 4C The carriage 220 comprises two pairs of combs 226, cooperating with the combs 224 secured to the mass 204. The combs 224, 226 are oriented so as to exert a force in the X direction. The fingers of all the combs are parallel and the combs are interdigitated two by two.

[0104] The carriage 220 has a very low mass relative to that of the seismic mass 204, this can be advantageously obtained thanks to the integration of the combs in the carriages and the relatively hollow structure of the carriages.

[0105] Each carriage 220 is connected to the vibrating beam 210 by a lever arm 214 formed by a beam articulated in rotation on the support 202 by a pivot connection 216 ( figure 4D). Each carriage 220 is connected to one longitudinal end of the lever arm 214 and the vibrating beam 210 is connected to the other end of the lever arm 214 and the pivot connection 216 is located between the two ends. In this example the vibrating beam 210 is connected to the longitudinal ends opposite the lever arm connected to the carriage 220 and to the lever arm connected to the carriage 220'. The connection is made in the example shown by a U-shaped transmission 222, each end of the branches of the U being fixed on a longitudinal end of a lever arm, and the vibrating beam 210 is connected to the bottom of the U ( Figure 4E ).

[0106] Since the lever arms 214, 214' and the carriages are structurally decoupled from the mass, part of the internal stresses of the vibrating beams can be released. Furthermore, by applying a potential difference between the seismic mass 204 and the carriages 220, 220', the carriages 220, 220' move closer to the seismic mass, causing the lever arms to rotate and applying a tensile force to the vibrating beam 210 via the transmission. This force is preferably such that it cancels out the other part of the internal stresses, or even puts the vibrating beam under tension.

[0107] The same phenomenon occurs on the 210' vibrating beam thanks to the C4' coupling means.

[0108] An accelerometer similar to the ACC4 accelerometer comprising only a vibrating beam and a trolley does not depart from the scope of the present invention.

[0109] The operation of the ACC4 accelerometer is similar to that of the ACC3 accelerometer. On the Figure 5 , we can see an example of an embodiment of an ACC5 accelerometer according to a third embodiment.

[0110] The ACC5 accelerometer differs from other embodiments in that the seismic mass 304 is mounted to rotate about a Z axis normal to the plane of the accelerometer.

[0111] The mass 304 is suspended relative to the support 302 by a pivot joint 306. In this example, the pivot joint comprises two beams 307 each anchored by one end to an anchoring pad of the support and fixed by another end to the seismic mass 304. The beams 307 connect to the seismic mass 304 at the same point through which the rotation axis Z passes. In this example the shape of the seismic mass 304 is defined by an assembly of a square and an isosceles triangle, the base of the isosceles triangle being merged with a side of the square. The beams 307 of the pivot joint connect to the seismic mass 304 at the apex of the isosceles triangle.

[0112] The accelerometer also includes measuring means; in the example, it includes two vibrating beams 310, 310' mounted in differential mode and electrostatic coupling means C5, C5'.

[0113] The electrostatic coupling means C5, C5' are similar to those of the accelerometers ACC3 and ACC4, they implement two lever arms 314, 314' articulated in rotation on the support and in electrostatic coupling with the seismic mass 304

[0114] In this example, the coupling is made directly between a lateral face of the lever arm 314, 314' and a side of the seismic mass 304. In the example shown, this is a side of the square.

[0115] The mass has a Y axis of symmetry intersecting the Z axis of rotation.

[0116] The lever arms 314, 314' are arranged symmetrically on either side of the seismic mass with respect to the Y axis. By arranging the axis of rotation of the mass and the axes of rotation of the lever arms on the same side, the opposite faces of the air gaps remain substantially parallel.

[0117] As for the ACC4 accelerometer, the articulation of each lever arm 314, 314' is located between the vibrating beam 310, 310' and the electrostatic coupling zone and the vibrating beam 310, 310' is arranged opposite the pivot articulation with respect to the lever arm. Alternatively, the locations of the articulation of each lever arm and that of the resonator can be reversed.

[0118] When a potential difference is applied between the seismic mass and the lever arms, the lever arms move closer to the seismic mass in the X direction, applying a tensile force to the vibrating beams 310, 310', which makes it possible to reduce or even cancel out the internal stresses, which are partly released elsewhere due to the structure of the accelerometer.

[0119] The ACC5 accelerometer works similarly to the accelerometers described above.

[0120] The implementation of a rotating mobile seismic mass advantageously makes it possible to avoid losses linked to the translational guidance of the carriages of the 1 / D electrostatic coupling means.

[0121] In the examples described above, the interdigitated electrostatic combs are implemented with a lever arm. It will be understood that a configuration in which the carriage provided with electrostatic combs interdigitates with the mass combs and in which the carriage is directly connected to the resonant beam does not depart from the scope of the present invention.

[0122] The accelerometer according to the present invention is particularly suitable for application as a gravimeter.

[0123] Indeed, in gravimetry an accelerometer is subjected to the acceleration of gravity (1g) and measures the small displacements of the seismic mass around this equilibrium point, representing among other things the fluctuations of the Earth's gravity. To measure such displacements, the accelerometer requires high sensitivity. There is a gravimeter implementing capacitive detection and means of suspending the seismic mass of infinitely low stiffness, the hinges are designed to buckle when the seismic mass is subjected to 1g of the Earth's acceleration. Such a gravimeter is described in the document W. Wu et al., "Measurement of Tidal Tilt by a Micromechanical Inertial Sensor Employing Quasi-Zero-Stiffness Mechanism," J. MICROELECTROMECHANICAL Syst., vol. 29, no. 5, p. 10, 2020 .

[0124] Thanks to the present invention, it is possible to produce a gravimeter implementing a nanometric section vibrating beam detection having an increased sensitivity compared to capacitive detection, while not altering the infinitely low stiffness of the suspension means, and this thanks to the implementation of electrostatic coupling means which make it possible to mechanically decouple the mass of the vibrating beams. The implementation of electrostatic coupling means is all the more interesting since the electrostatic force depends on the square of the air gap variation, therefore on the displacement of the seismic mass.

[0125] The gravimeter comprises an accelerometer according to the invention, having differential measuring means and suspension means whose dimensions have been calculated to buckle when the seismic mass is subjected to 1g of the Earth's acceleration. The buckling takes place in a direction contained in the plane of the accelerometer, i.e. in the direction of movement of the mass. The suspension means described in the document W. Wu et al., "Measurement of Tidal Tilt by a Micromechanical Inertial Sensor Employing Quasi-Zero-Stiffness Mechanism," J. MICROELECTROMECHANICAL Syst., vol. 29, no. 5, p. 10, 2020 can be implemented.

[0126] As already explained above, the smallest possible air gap width is sought. However, manufacturing processes do not allow this width to be reduced below a ratio of 20 to the thickness of the micrometric layer. Thus, an air gap of 1 µm width can be produced for a micrometric layer of 20 µm. Thanks to the initial deflection of the mass and the generation of electrostatic force, the width of the air gap is reduced at rest.

[0127] It should be noted that in the use of an accelerometer according to the invention as a gravimeter, a sensitive sensor is primarily sought and not a sensor with a wide operating range.

[0128] In another embodiment, the measuring means comprise at least one piezoresistive gauge, advantageously two mounted in differential. Thanks to the electrostatic coupling means, all or part of the internal stresses are cancelled in the gauge(s), or even the gauge(s) are energized. All the examples described above can implement one or more piezoresistive gauges in place of the vibrating beam(s).

[0129] The force sensor according to the invention can be used in other applications, for example such as a magnetometer using a magnetic material or current lines, or a pressure sensor in which the test body is a membrane. All the examples described above in their application to an accelerometer apply to any other force sensor.

[0130] It will be understood that the examples of embodiment are not limiting. Other suspension means can be implemented to guide the masses in translation and other pivot joint configurations can also be provided.

[0131] It will also be understood that the electrostatic coupling means comprising interdigitated combs can be applied to a rotating mobile seismic mass.

[0132] Furthermore, in the exemplary embodiments, the vibrating beam(s) or the piezoresistive gauge(s) is(are) aligned with the direction of movement of the seismic mass moving in translation; any other orientation is possible, for example by using return means. In the examples using lever arms, the vibrating beams or the piezoresistive gauges may be oriented orthogonally to the hinge-forming beam.

[0133] Furthermore, the vibrating beams or piezoresistive gauges and / or the carriages can be connected to the lever arms in locations other than the ends thereof. The force sensors according to the invention can be produced by microelectronics methods, in particular MEMS&NEMS technology well known to those skilled in the art, implementing deposition and etching steps.

[0134] The manufacturing processes are such that the vibrating beams or piezoresistive gauges are manufactured in a nanometric layer and the electrostatic coupling means and the seismic mass are made in a stack of nanometric and micrometric layers. The surfaces facing the electrostatic coupling means are defined in part by the thickness of the coupling means which is of micrometric dimension.

Claims

1. Force sensor comprising a support (2), at least one test body (4) which is suspended by suspension means from the support, and which is capable of moving in a plane parallel to the support, means (8) for measuring the displacement of the test body (4), which means comprise at least a first strain gauge provided with a vibrating beam (10), the sensor further comprising first electrostatic coupling means (C1, C3') for electrostatically coupling the test body (4) and said at least one first strain gauge (10), which coupling means are configured to mechanically uncouple said first strain gauge (10) and the test body (4), so that at rest, said first electrostatic coupling means (C1) generate traction on the first strain gauge (10), and so that, under the effect of an external force, said first electrostatic coupling means modify the state of strain of the first strain gauge (10), the vibrating beam (10) having a nanometric cross-section between several tens of nanometres and 1 micrometre, the vibrating beam (10) being suspended, at one end, from said support (2), and at another end, from a suspension beam (12, 214, 114', 114, 314), the suspension beam (12, 214, 114', 114) having a cross-section that is at least ten times greater than the cross-section of the vibrating beam (10), the suspension beam having a micrometric cross-section between 1µm and several hundred micrometres.

2. Force sensor according to claim 1, comprising a second strain gauge of nanometric cross-section (10') with increased differential relative to the first strain gauge (10), and second electrostatic coupling means (C1') for electrostatically coupling the test body (4) and said second strain gauge (10'), which coupling means are configured to mechanically uncouple said second strain gauge (10') and the test body (4), so that at rest, said second electrostatic coupling means (C1') generate traction on the second strain gauge (10'), and so that under the effect of acceleration, the state of strain of the second strain gauge (10') is modified.

3. Force sensor according to claim 1 or 2, wherein the first (C1) and / or second (C1') electrostatic coupling means are configured to generate, at rest, an electrostatic force so as to at least cancel the unrecovered strain in the first strain gauge (10) and / or the second strain gauge (10').

4. Force sensor according to claims 1 to 3, wherein the first electrostatic coupling means (C1) comprise said suspension beam (12, 214, 114', 114, 314), such that an electrostatic coupling is produced between the suspension beam and the test body (4).

5. Force sensor according to claims 1 to 4, wherein the first electrostatic coupling means (C1) comprise at least a first and a second electrode facing one another, the first electrode being formed by a side of the test body (4) and the second electrode being formed by a side face of said suspension beam (12, 214, 114', 114, 314) attached to a longitudinal end of the vibrating beam (10).

6. Force sensor according to one of claims 1 to 4, wherein said suspension beam forms a lever arm and is rotatably articulated in the plane of the sensor, the lever arm being electrostatically coupled with the test body.

7. Force sensor according to one of claims 1 to 3, wherein the first electrostatic coupling means each comprise at least one pair of interdigitated combs with fingers, one comb being rigidly connected to the test body to move as one therewith, and the other comb being attached to a lever arm formed by said suspension beam.

8. Force sensor according to one of claims 1 to 7, wherein the test body (4) and the suspension means (6) of the test body (4) are configured so that the test body (4) translates in the plane in a given direction (X).

9. Force sensor according to claim 1, wherein the first electrostatic coupling means comprise a carriage carrying at least one comb and mechanically connected to a lever arm formed by said suspension beam.

10. Force sensor according to claim 9, wherein the carriage has a lower mass than the test body, the mass of the carriage being at least 10 times less than the mass of the test body.

11. Force sensor according to claim 9 or 10, wherein the carriage comprises an outer frame defined by uprights from which fingers of the comb project, and wherein the at least one comb rigidly connected to the test body to move as one therewith is arranged inside the frame and its fingers are placed between the fingers of the comb of the carriage.

12. Force sensor according to claim 9, 10 or 11, comprising a plurality of pairs of interdigitated combs so as to provide a large electrostatic coupling surface.

13. Force sensor according to claim 6, comprising a pivot joint via which the test body is suspended from the support and by means of which the test body is rotatable in the plane of the force sensor, and wherein the first and / or the second coupling means are formed by a side face of the suspension beam forming the lever arm and an edge facing the test body.

14. Accelerometer comprising a force sensor according to one of the preceding claims, wherein the test body is a seismic mass.

15. Gravimeter comprising at least one accelerometer according to claim 14 and wherein the suspension means of the seismic mass are configured to buckle when the seismic mass is subjected to 1g of the Earth's acceleration.