Patent Application
20220033254
The present disclosure relates to a wide-band MEMS (MicroElectroMechanical System) accelerometer for detecting accelerations having a high frequency, as, for example, in the case of high-frequency vibrations.
As is known, systems for monitoring vibrations are today very widespread and are widely used, for example, in the context of condition-based maintenance of machinery, and in particular maintenance based upon the state of usage. In fact, it is known how monitoring of the vibrations to which a mechanical component is subject enables, for example, detection in advance of the onset of problems and/or damage.
Typically, systems for monitoring vibrations use accelerometers in order to detect, for example, variations of amplitude or frequency of the forces that act upon the components. More in general, it is known to use accelerometric sensors with a sensitivity of up to frequencies in the region of 80 kHz.
In general, sensors with different bandwidths typically find different uses. Purely by way of example, in the context of monitoring of misalignment of the shafts of electric motors, accelerometric sensors are typically used that have small bandwidths (for example, up to 3 kHz), whereas, in the context of detection of failure of bearings of fans, accelerometric sensors are used having large bandwidths (for example, up to 80 kHz).
All this having been said, it may be desirable to find a good balance between the costs and the performance of the sensors. In this connection, MEMS sensors have been developed, as described, for example, in F. Gerfers et al., “Sub-μg Ultra-Low-Noise MEMS Accelerometers Based on CMOS-Compatible Piezoelectric AN Thin Films,” Transducers & Eurosensors 2007, which describes the use of a membrane structure, or else in T. Kobayashi et al., “A digital output piezoelectric accelerometer using CMOS-compatible AN thin film,” Transducers 2009, which describes is the use of a structure that includes a cantilever element.
In general, MEMS membrane structures are subject to residual mechanical stresses on account of the presence of distributed constraints and/or constraints in a relatively high number, which act upon the membrane; such mechanical residual stresses lead to a degradation in the performance. Instead, MEMS structures with cantilever elements prove less subject to residual mechanical stresses on account of the presence of a single constraint; however, these are substantially sensitive only to out-of-plane accelerations; i.e., they are basically uniaxial. Moreover, in the presence of a number of cantilever elements, the resonance frequencies of the cantilever elements inevitably differ from one another, with consequent reduction of sensitivity.
In various embodiments, the present disclosure provides an accelerometer that will overcome at least in part the drawbacks of the prior art.
In at least one embodiment, the present disclosure provides a MEMS accelerometer that includes a supporting structure. First and second deformable groups include, respectively, a first deformable cantilever element and a second deformable cantilever element, which are arranged in a direction parallel to a plane and each having a respective first end fixed to the supporting structure, and a respective second end. The first deformable group further includes a first piezoelectric detection structure mechanically coupled to the first deformable cantilever element, and the second deformable group further includes a second piezoelectric detection structure mechanically coupled to the second deformable cantilever element. A first mobile mass and a second mobile mass are fixed, respectively, to the second end of the first deformable cantilever element and to the second end of the second deformable cantilever element and are staggered with respect to the first and second deformable cantilever elements, respectively, in a vertical direction transverse with respect to the plane. A first elastic structure is configured to elastically couple the first and second mobile masses.
In at least one embodiment, the present disclosure provides a device that includes a supporting structure. A first cantilever has a deformable portion and a distal portion, and the deformable portion is fixed to the supporting structure. A first mobile mass is mechanically coupled to a first side of the deformable portion of the first cantilever. A second deformable cantilever has a deformable portion and a distal portion, and the deformable portion is fixed to the supporting structure. A second mobile mass is mechanically coupled to a first side of the deformable portion of the second cantilever. A first piezoelectric detection structure is mechanically coupled to a second side of the first deformable cantilever element that is opposite the first side of the first deformable cantilever element. A second piezoelectric detection structure is mechanically coupled to a second side of the second deformable cantilever element that is opposite the first side of the second deformable cantilever element. A first elastic structure is configured to elastically couple the first and second mobile masses.
For a better understanding of the present disclosure, preferred embodiments thereof will now be described, purely by way of non-limiting example, with reference to the attached drawings, wherein:
In greater detail, assuming an orthogonal reference system XYZ, the cavity 4 is delimited at the top and at the bottom, respectively, by a top surface Stop and a bottom surface Sbot, which are parallel to the plane XY; moreover, the cavity 4 is delimited laterally by a first side wall P11 and a second side wall P12, which are parallel to the plane YZ, and by a third side wall P13 and a fourth side wall P14 (visible in
The sensitive structure 6 is constrained to the first, second, third, and fourth side walls P11, P12 P13, P14 so as to be suspended over the bottom surface Sbot, at a distance therefrom.
In particular, the sensitive structure 6 comprises a first cantilever element 21, a second cantilever element 22, a third cantilever element 23, and a fourth cantilever element 24, as well as a first suspended group G1, a second suspended group G2, a third suspended group G3, and a fourth suspended group G4, and a first spring element 41, a second spring element 42, a third spring element 43, and a fourth spring element 44.
In detail, the first, second, third, and fourth cantilever elements 21, 22, 23, 24 are the same as one another and are arranged, in resting conditions, in a co-planar way, at a same height along the axis Z. In particular, the first, second, third, and fourth cantilever elements 21, 22, 23, 24 have a planar shape; purely by way of example, the first, second, third, and fourth cantilever elements 21, 22, 23, 24 may have a thickness (measured along the axis Z) comprised between 10 μm and 20 μm.
Furthermore, the first, second, third, and fourth cantilever elements 21, 22, 23, 24 are, for example, of silicon.
In greater detail, each one of the first, second, third, and fourth cantilever elements 21, 22, 23, 24 includes a respective deformable portion (designated, respectively, by 21*, 22*, 23* and 24*) and a respective distal portion (designated, respectively, by 21**, 22**, 23** and 24**).
The deformable portions 21*, 22*, 23* and 24* of the first, second, third, and fourth cantilever elements 21, 22, 23, 24 have the shape of parallelepipeds.
In particular, assuming a first plane of symmetry SP1 and a second plane of symmetry SP2, parallel, respectively, to the plane ZX and to the plane ZY, the deformable portions 21*, 22* of the first and second cantilever elements 21, 22 are elongated in a direction parallel to the axis X and are set in a symmetrical way with respect to the second plane of symmetry SP2, at a distance from one another. In greater detail, if H1 and H2 are the directions in which the centroidal axes of the deformable portions 21*, 22* of the first and second cantilever elements 21, 22, respectively, extend, the directions H1 and H2 to a first approximation coincide with and are parallel to the axis X. In addition, the deformable portion 21* of the first cantilever element 21 and the deformable portion 22* of the second cantilever element 22 have respective first ends, which are, respectively, fixed to the first and second side walls P11, P12, and respective second ends. In practice, the first and second cantilever elements 21, 22 are arranged so as to be opposite to one another.
The deformable portions 23*, 24* of the third and fourth cantilever elements 23, 24 are elongated in a direction parallel to the axis Y and are set in a symmetrical way with respect to the first plane of symmetry SP1, at a distance from one another. In greater detail, if H3 and H4 are the directions in which the centroidal axes of the deformable portions 23*, 24* of the third and fourth cantilever elements 23, 24 extend, respectively, to a first approximation, the directions H3 and H4 are found to coincide and to be parallel to the axis Y. In addition, the deformable portions 23*, 24* of the third and fourth cantilever elements 23, 24 have respective first ends, which are, respectively, fixed to the third and fourth side walls P13, P14, and respective second ends. In practice, the third and fourth cantilever elements 23, 24 are arranged so as to be opposite to one another.
The first and second planes of symmetry SP1, SP2 intersect along an axis HX parallel to the axis Z. Furthermore, the distal portions 21**, 22**, 23**, 24** of the first, second, third, and fourth cantilever elements 21, 22, 23, 24 are, respectively, fixed with respect to the second ends of the deformable portions 21*, 22*, 23*, 24* of the first, second, third, and fourth cantilever elements 21, 22, 23, 24, and overlie, respectively, the first, second, third, and fourth suspended groups G1, G2, G3, G4. As explained hereinafter, and without this implying any loss of generality, in top plan view the distal portions 21**, 22**, 23**, 24** of the first, second, third, and fourth cantilever elements 21, 22, 23, 24 have, respectively, the same shape as the first, second, third, and fourth suspended groups G1, G2, G3, G4.
In detail, the first, second, third, and fourth suspended groups G1, G2, G3, G4 are the same as one another, have, for example, the shape of prisms with polygonal bases with axes parallel (in resting conditions) to the axis Z; i.e., their sections in planes parallel to the plane XY do not vary as the co-ordinate along the axis Z varies. The details regarding the shape of the suspended groups are in any case irrelevant for the purposes of operation of the MEMS accelerometer 1. Purely by way of example, in
The first, second, third, and fourth suspended groups G1, G2, G3, G4 are, respectively, fixed underneath the distal portions 21**, 22**, 23**, 24** of the first, second, third, and fourth cantilever elements 21, 22, 23, 24.
Without this implying any loss of generality, in what follows it is assumed that each one of the first, second, third, and fourth suspended groups G1, G2, G3, G4 is formed by a corresponding main region (designated, respectively, by 31A, 32A, 33A, 34A), for example, of silicon, and by a corresponding top layer (designated, respectively, by 31B, 32B, 33B, 34B), for example, of silicon oxide.
In greater detail, with reference, for example, to the first suspended group G1 (but the same considerations also apply to the second, third, and fourth suspended groups G2, G3, G4), the corresponding top layer 31B is fixed at the top to the distal portion 21** of the first cantilever element 21 and overlies the underlying main region 31A. As mentioned previously and without this implying any loss of generality, the distal portion 21** of the first cantilever element 21, the underlying top layer 31B and the underlying main region 31A have a same shape in top plan view and moreover form a mobile mass 31, referred to in what follows as the first mobile mass 31. Purely by way of example, the top layer 31B and the underlying main region 31A have a thickness (measured along the axis Z) comprised, for example, respectively, in the ranges 1-2 μm and 250-500 μm. Likewise, the distal portions 22**, 23**, 24** of the second, third, and fourth cantilever elements 22, 23, 24 form, respectively, a second mobile mass 32, a third mobile mass 33, and a fourth mobile mass 34, respectively, with the underlying i) top layer 32B and main region 32A, ii) top layer 33B and main region 33A, and iii) top layer 34B and main region 34A. In practice, the first suspended mass 31 and the second suspended mass 32 are arranged in a symmetrical way with respect to the second plane of symmetry SP2. Likewise, the third suspended mass 33 and the fourth suspended mass 34 are arranged in a symmetrical way with respect to the first plane of symmetry SP1. From another standpoint, the first, second, third, and fourth suspended masses 31, 32, 33, 34 are arranged at a same radial distance from the axis HX and are set at equal angular distances apart from one another of 90° so as to be arranged according to the vertices of a hypothetical rhombus.
The first, second, third, and fourth spring elements 41, 42, 43, 44 each have a folded elongated shape, are, for example, of silicon, and may have, in resting conditions, the same thickness (along Z) as the first, second, third, and fourth cantilever elements 21, 22, 23, 24, with which they are co-planar.
In greater detail, each one of the first, second, third, and fourth spring elements 41, 42, 43, 44 has a respective first end and a respective second end. The first ends of the first, second, third, and fourth spring elements 41, 42, 43, 44 are fixed, respectively, to the distal portions 21**, 22**, 23**, 24** of the first, second, third, and fourth cantilever elements 21, 22, 23, 24, with which they are in direct contact and form a single piece. The first ends of the first, second, third, and fourth spring elements 41, 42, 43, 44 are therefore fixed, respectively, to the first, second, third, and fourth mobile masses 31, 32, 33, 34. The second ends of the first, second, third, and fourth spring elements 41, 42, 43, 44 are fixed to one another, around the axis HX, so as to form a single cross-shaped piece.
The first and second spring elements 41, 42 are rigid in a direction parallel to the axis Z, are compliant in a direction parallel to the axis X, and form a first spring structure M1. The third and fourth spring elements 43, 44 are rigid in a direction parallel to the axis Z, are compliant in a direction parallel to the axis Y, and form a second spring structure M2.
In practice, the distal portions 21**, 22** of the first and second cantilever elements 21, 22 are connected to one another by the first spring structure M1, which is compliant in a direction parallel to the axis X and is rigid in a direction parallel to the axis Z. Likewise, the distal portions 23**, 24** of the third and fourth cantilever elements 23, 24 are connected to one another by the second spring structure M2, which is compliant in a direction parallel to the axis Y and is rigid in a direction parallel to the axis Z. As mentioned previously, the first and second spring structures M1, M2 intersect one another and form, respectively, a first arm and a second arm of a cross-shaped structure, which are perpendicular to one another.
This having been said, the first spring structure M1 has a stiffness in the direction H1 (equivalently, H2) that is lower than the stiffness in the direction H1 of the deformable portions 21*, 22* of the first and second cantilever elements 21, 22. The second spring structure M2 has a stiffness in the direction H3 (equivalently, H4) that is lower than the stiffness in the direction H3 of the deformable portions 23*, 24* of the third and fourth cantilever elements 23, 24.
In addition, the centroids of the first and second mobile masses 31, 32 (represented qualitatively in
To a first approximation, and without this implying any loss of generality, the centroids B1, B2 of the first and second mobile masses 31, 32 are co-planar with respect to one another and with respect to the directions H1, H2; to a first approximation, the centroids B1, B2 and the directions H1, H2 lie in the first plane of symmetry SP1. Likewise, albeit not illustrated, the centroids of the third and fourth mobile masses 33, 34 are located at a smaller height than the directions H3, H4; to a first approximation they lie, together with the latter directions, in the second plane of symmetry SP2 and are laterally staggered, along the axis Y and in the direction of the axis HX, with respect to the second ends of the deformable portions 23*, 24* of the third and fourth cantilever elements 23, 24.
The MEMS accelerometer 1 further comprises a first piezoelectric detection structure 51, a second piezoelectric detection structure 52, a third piezoelectric detection structure 53, and a fourth piezoelectric detection structure 54, which are, for example, the same as one another and, albeit not illustrated, each comprise a respective pair of electrodes, interposed between which is a respective region of piezoelectric material (for example, of PZT).
Without this implying any loss of generality, the first, second, third, and fourth piezoelectric detection structures 51, 52, 53, 54 extend, respectively, on the deformable portions 21*, 22*, 23*, 24* of the first, second, third, and fourth cantilever elements 21, 22, 23, 24, and are therefore laterally staggered, respectively, with respect to the first, second, third, and fourth mobile masses 31, 32, 33, 34. Moreover, once again without this implying any loss of generality, the first, second, third, and fourth piezoelectric detection structures 51, 52, 53, 54 extend laterally as far as on the first ends of the deformable portions 21*, 22*, 23*, 24* of the first, second, third, and fourth cantilever elements 21, 22, 23, 24, which, as has been said, are fixed to the supporting structure 2 and represent areas subject to high deformations. Likewise, albeit not illustrated, variants are possible in which the first, second, third, and fourth piezoelectric detection structures 51, 52, 53, 54 extend partially also on part of the supporting structure 2.
In greater detail, considering, for example, the first piezoelectric detection structure 51 (but the same considerations also apply to the other piezoelectric detection structures), this is set, at a distance, above the centroidal axis of the ensemble formed by the deformable portion 21* of the first cantilever element 21 and by the same first piezoelectric detection structure 51, which to a first approximation can be assumed as coinciding with the centroidal axis of just the deformable portion 21* of the first cantilever element 21, and therefore with the aforementioned direction H1. Furthermore, the first piezoelectric detection structure 51 (but the same considerations also apply to the other piezoelectric detection structures and to the corresponding centroidal axes) has a planar shape symmetrical with respect to the underlying centroidal axis of the deformable portion 21* of the first cantilever element 21; i.e., in top plan view, it is divided into two equal parts by said centroidal axis.
All this having been said, considering the pair formed by the first cantilever element 21 and the first mobile mass 31, and neglecting for the moment the presence of the first spring structure M1, and therefore considering for simplicity the first mobile mass 31 as being decoupled from the second, third, and fourth mobile masses 32, 33, 34, we find what is described in what follows.
In the case where, as illustrated in
Furthermore, even in the case where, as shown in
Instead, in the case (not illustrated) where the first mobile mass 31 is subjected to an (inertial) acceleration parallel to and concordant with the axis Y (as in the case of a vibration such that the supporting structure 2 is subjected to an acceleration directed along the axis Y, but discordant with respect thereto), the deformable portion 21* of the first cantilever element 21 is subjected to a torsion about the direction H1, which entails that part of the first piezoelectric detection structure 51 is subjected to a tension and part is subjected to compression, with the consequence that the voltage V1 is approximately zero.
Considering, instead, the pair formed by the second cantilever element 22 and the second mobile mass 32, and again neglecting the presence of the first spring structure M1, we find what is described hereinafter.
In the conditions referred to in
In the conditions referred to in
By means of an electronic circuitry of a per se known type and here not illustrated, it is thus, for example, possible to generate a signal sVZ′=V1+V2 and a signal sVX=V1−V2, which, respectively, indicate the acceleration along Z and the acceleration along X to which the MEMS accelerometer 1 is subjected. In this connection, the bottom electrodes of the first and second piezoelectric detection structures 51, 52 can be set to ground, and the top electrodes of the first and second piezoelectric detection structures 51, 52 can be connected so as to generate the aforementioned signals sVZ′ and sVX. Similar considerations apply to the subsystem formed by the third cantilever element 23, the third suspended mass 33, and the third piezoelectric detection structure 53, and by the fourth cantilever element 24, the fourth suspended mass 34, and the fourth piezoelectric detection structure 54.
In particular, as illustrated in
In addition, with reference, for example, to the elastic coupling present, thanks to the first spring structure M1, between the first and second mobile masses 31, 32, we find what is described hereinafter.
As mentioned previously, in the direction H1, and therefore in a direction parallel to the axis X, the first spring structure M1 is compliant, unlike the deformable portions 21*, 22* of the first and second cantilever elements 21, 22. Consequently, any possible undesired mechanical stresses that act upon the supporting structure 2, for example, due to the package (not illustrated), are compensated by the first spring structure M1, without causing any deformation of the deformable portions 21*, 22* of the first and second deformable elements 21, 22, and therefore without altering the voltages generated by the first and second piezoelectric detection structures 51, 52.
For instance, the first spring structure M1 can absorb the stresses due to gluing or welding of the supporting structure 2 to a respective base of the package, which would otherwise induce tensile or compressive states of the first and second piezoelectric detection structures 51, 52. In addition, the deformable portions 21*, 22* of the first and second cantilever elements 21, 22, the first and second mobile masses 31, 32, and the first spring structure M1 form a mechanical system, which, if for the moment the presence of the second spring structure M2 and of the third and fourth cantilever elements 23, 24 are neglected, can be sized so that the mode in which the first and second mobile masses 31, 32 oscillate in phase has a desired frequency, lower than the frequency of the spurious mode, in which the first and second mobile masses 31, 32 oscillate in phase opposition. In this connection, in the absence of the first spring structure M1, the mechanical system formed by the deformable portion 21* of the first cantilever element 21 and by the first mobile mass 31 and the mechanical system formed by the deformable portion 22* of the second cantilever element 22 and by the second mobile mass 32 would be independent and would have independent modes of oscillation, with different resonance frequencies, which would require normalization of the electrical responses supplied by the first and second piezoelectric detection structures 51, 52 so as to neutralize the differences due to the different resonance frequencies of the modes of oscillation of the aforementioned two independent mechanical systems. In practice, there would in any case be a reduction of sensitivity.
The same considerations also apply to the second spring structure M2, to the direction H3, and to the deformable portions 23*, 24* of the third and fourth cantilever elements 23, 24, which absorb undesired stresses present on the first ends of the third and fourth cantilever elements 23, 24.
In practice, the deformable portions 23*, 24* of the third and fourth cantilever elements 23, 24, the third and fourth mobile masses 33, 34, and the second spring structure M2 form the aforementioned mechanical system, together with the deformable portions 21*, 22* of the first and second cantilever elements 21, 22, with the first and second mobile masses 31, 32, and with the first spring structure M1. This mechanical system can thus be sized so that the so-called operating mode, i.e., the mode excited by the accelerations to be detected, is the first mode, i.e., the mode in which the first, second, third, and fourth mobile masses 31, 32, 33, 34 oscillate in phase.
Albeit not illustrated, there may in any case be envisaged variants in which, for example, the third and fourth cantilever elements 23, 24, the third and fourth suspended groups G3, G4, and the second spring structure M2 are absent, in which case the MEMS accelerometer 1 is insensitive to accelerations directed in a direction parallel to the axis Y. Likewise, variants are possible in which the first and second cantilever elements 21, 22, the first and the second suspended groups G1, G2, and the first spring structure M1 are absent, in which case the MEMS accelerometer 1 is insensitive to accelerations directed in a direction parallel to the axis X.
In practice, the embodiment illustrated in
According to a different embodiment (illustrated in
In detail, the first, second, third, and fourth mobile masses 131, 132, 133, 134 have approximately the shape of parallelepipeds, are arranged at a same radial distance from the axis HX and are set at equal angular distances apart so as to be arranged at the vertices of a hypothetical square.
In particular, the first, second, third, and fourth mobile masses 131, 132, 133, 134 have approximately the shape of parallelepipeds with side walls parallel alternatively to the plane XZ or to the plane YZ, in resting conditions. Moreover, the first and third mobile masses 131, 133 are aligned in a direction parallel to the axis X; likewise, the second and fourth mobile masses 132, 134 are aligned in a direction parallel to the axis X. Furthermore, the first and fourth mobile masses 131, 134 are aligned in a direction parallel to the axis Y; likewise, the second and third mobile masses 132, 133 are aligned in a direction parallel to the axis Y. In resting conditions, the deformable portions 121*, 122* of the first and second cantilever elements 121, 122 have centroidal axes that lie, respectively, in a direction H1′ and a direction H2′, which are parallel to the plane XY, are parallel to one another, and to a first approximation coincide. In addition, the directions H1′, H2′ are inclined by −45° with respect to the axis X. To a first approximation, the directions H1′, H2′ coincide also with the centroidal axes of the two ensembles formed, respectively, i) by the deformable portion 121* of the first cantilever element 121 and the first piezoelectric detection structure 151, and ii) by the deformable portion 122* of the second cantilever element 122 and the second piezoelectric detection structure 152, said centroidal axes moreover in turn coinciding, to a first approximation, with the neutral axes of the corresponding ensembles.
The deformable portions 123*, 124* of the third and fourth cantilever elements 123, 124 have centroidal axes that lie, respectively, in a direction H3′ and a direction H4′, which are parallel to the plane XY, are parallel to one another, and to a first approximation coincide. In addition, the directions H3′, H4′ are inclined by +45° with respect to the axis X. The directions H3′, H4′ coincide, to a first approximation, also with the centroidal axes of the two ensembles formed, respectively, i) by the deformable portion 123* of the third cantilever element 123 and the third piezoelectric detection structure 153, and ii) by the deformable portion 124* of the fourth cantilever element 124 and the fourth piezoelectric detection structure 154, said centroidal axes moreover in turn coinciding, to a first approximation, with the neutral axes of the corresponding ensembles.
The deformable portions 121*, 122*, 123*, 124* of the first, second, third, and fourth cantilever elements 121, 122, 123, 124 are set, in top plan view, between the supporting structure 102 and, respectively, the corresponding distal portions 121**, 122**, 123**, 124**, which form, respectively, the first, second, third, and fourth mobile masses 131, 132, 133, 134. Furthermore, the deformable portions 121*, 122*, 123*, 124* of the first, second, third, and fourth cantilever elements 121, 122, 123, 124 are, respectively, overlaid by the first, second, third, and fourth piezoelectric detection structures 151, 152, 153, 154.
The first, second, third, and fourth spring elements 141, 142, 143, 144 each have a folded elongated shape, are, for example, of silicon and may have, in resting conditions, the same thickness (along Z) as the first, second, third, and fourth cantilever elements 121, 122, 123, 124, with which they are co-planar.
In greater detail, in what follows designated by F13 and F31 are the side surfaces parallel to and facing the second plane of symmetry SP2 of the first suspended mass 131 and, respectively, the third suspended mass 133; likewise, designated by F24 and F42 are the side surfaces parallel to and facing the second plane of symmetry SP2 of the second suspended mass 132 and, respectively, the fourth suspended mass 134. In addition, designated by F14 and F41 are the side surfaces parallel to and facing the first plane of symmetry SP1 of the first suspended mass 131 and, respectively, the fourth suspended mass 134; likewise, designated by F23 and F32 are the side walls parallel to and facing the first plane of symmetry SP1 of the second suspended mass 132 and, respectively, the third suspended mass 133.
All this having been said, the first spring element 141 has its ends fixed, respectively, to the side surfaces F13 and F31 of the first and third suspended masses 131, 133 so as to elastically couple the latter; moreover, the first spring element 141 is compliant in a direction parallel to the axis X.
The second spring element 142 has its ends fixed, respectively, to the side surfaces F24 and F42 of the second and fourth suspended masses 132, 134, so as to elastically couple the latter; moreover, the second spring element 142 is compliant in a direction parallel to the axis X.
The third spring element 143 has its ends fixed, respectively, to the side surfaces F23 and F32 of the second and third suspended masses 132, 133, so as to elastically couple the latter; moreover, the third spring element 143 is compliant in a direction parallel to the axis Y.
The fourth spring element 144 has its ends fixed, respectively, to the side surfaces F14 and F41 of the first and fourth suspended masses 131, 134 so as to elastically couple the latter; moreover, the fourth spring element 144 is compliant in a direction parallel to the axis Y.
With reference for brevity to the spring elements in contact with the third mobile mass 133, and therefore with reference to the first and third spring elements 141, 143 (but the same considerations apply also to the pairs of spring elements in contact with each one of the first, second, and fourth mobile masses 141, 142, 144), without this implying any loss of generality, we find what is described hereinafter. If VX denotes the side edge of the third mobile mass 133 parallel to the axis Z and facing the axis HX, the first and third spring elements 141, 143 contact, respectively, the side surfaces F31 and F32 in the proximity of the edge VX, albeit remaining at a distance from one another. Even though not illustrated, embodiments are possible in which the first and third spring elements 141, 143 are in contact with one another.
In practice, the first and third spring elements 141, 143 form, together with the third mobile mass 133, a first branch of the first spring structure, here designated by M1′, which elastically couples the first and second mobile masses 131, 132. In addition, the second and fourth spring elements 142, 144 form, together with the fourth mobile mass 134, a second branch of the first spring structure M1′, which also elastically couples the first and second mobile masses 131, 132 and is set, to a first approximation, parallel to the aforementioned first branch.
Likewise, the first and fourth spring elements 141, 144 form, together with the first mobile mass 131, a first branch of the second spring structure, here designated by M2′, which elastically couples the third and fourth mobile masses 133, 134. In addition, the second and third spring elements 142, 143 form, together with the second mobile mass 132, a second branch of the second spring structure M2′, which also elastically couples the third and fourth mobile masses 133, 134 and is set, to a first approximation, parallel to the corresponding first branch.
Operation of the MEMS accelerometer 101 is such that, in the case where it is subjected to an acceleration directed in a direction parallel to the axis Z, the first, second, third, and fourth mobile masses 131, 132, 133, 134 tend all together to rise or drop, according to the sense of the acceleration, as illustrated qualitatively in
In the case where the MEMS accelerometer 1 is, instead, subjected to an acceleration directed along the axis X, but with opposite sense, what is illustrated in
The aforementioned behavior is may be explained recalling what has been described regarding the MEMS accelerometer 1 referred to in
In the case where the MEMS accelerometer 1 is subjected to an acceleration directed along the axis Y, what is illustrated in
By means of an electronic circuitry of a per se known type and not illustrated herein, it is thus possible to generate a signal sVZ2, for example, equal to V1+V2+V3+V4, which indicates the acceleration along Z. In addition, it is possible to generate a signal sVX′=(V1+V4)−(V2+V3) and a signal sVY′=(V2+V4)−(V1+V3), which indicate the accelerations along X and along Y, respectively.
The embodiment illustrated in
In general, the advantages that the present accelerometer affords emerge clearly from the foregoing description. In particular, the present accelerometer enables, for example, detection with a high sensitivity of vibrations in mutually orthogonal directions of stimulus, these vibrations possibly having relatively high frequencies, and at the same time makes it possible to benefit from a high immunity to the spurious static effects induced by spurious voltages.
Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present disclosure.
For instance, the shapes, and therefore also the corresponding sections, of the cantilever elements and of the mobile masses may differ from what has been described, as likewise the shapes of the piezoelectric detection structures and the arrangements of the latter with respect to the cantilever elements. For instance, in principle, the piezoelectric detection structures may be arranged underneath the deformable portions of the cantilever elements.
A MEMS accelerometer may be summarized as including a supporting structure (2; 102) and at least one first deformable group (21*, 51; 121*, 151) and one second deformable group (22*, 52; 122*, 152), which include, respectively, a first deformable cantilever element (21*; 121*) and a second deformable cantilever element (22*; 122*), which are arranged in a direction parallel to a plane (XY) and each have a respective first end, which is fixed to the supporting structure (2; 102), and a respective second end; said first deformable group (21*, 51; 121*, 151) further including:
a first piezoelectric detection structure (51; 151), which is mechanically coupled to the first deformable cantilever element (21*; 121*);
said second deformable group (22*, 52; 122*, 152) further including:
a second piezoelectric detection structure (52; 152), which is mechanically coupled to the second deformable cantilever element (22*; 122*);
said MEMS accelerometer (1; 101) further including:
a first mobile mass (31, 131) and a second mobile mass (32, 132), which are fixed, respectively, to the second end of the first deformable cantilever element (21*, 121*) and to the second end of the second deformable cantilever element (22*, 122*) and are staggered with respect to the first and second deformable cantilever elements (21*, 121*; 22*, 122*), respectively, in a vertical direction (Z) transverse with respect to said plane
(XY); and
a first elastic structure (M1, M1′) configured to elastically couple the first and second mobile masses (31; 131; 32, 132).
The first and second deformable cantilever elements (21*, 22*; 121*, 122*) may extend in a first direction of detection (H1, H2; H1′, H2′) and may be are opposite to one another, so that, in the presence of accelerations directed in a direction parallel to said first direction of detection (H1, H2; H1′, H2′), the first and second deformable cantilever elements (21*, 22*; 121*, 122*) may be subjected to first parallel and concordant bending moments.
The first and second deformable groups (21*, 51, 22*, 52; 121*, 151, 122*, 152) may have, respectively, a first neutral axis (H1, H1′) and a second neutral axis (H2, H2′), which extend in said first direction of detection (H1, H2; H1′, H2′).
The first and second piezoelectric detection structures (51, 52; 151, 152) may be, respectively, staggered, in the vertical direction (Z), with respect to the first neutral axis (H1, H1′) and to the second neutral axis (H2, H2′) so that, in the presence of said first parallel and concordant bending moments, one of the first and second piezoelectric detection structures (51, 52; 151, 152) may be subjected to tension, whereas the other is subjected to compression.
The first and second piezoelectric detection structures (51, 52; 151, 152) may have planar shapes, arranged in a symmetrical way with respect to the first neutral axis (H1, H1′) and to the second neutral axis (H2, H2′), respectively.
The first and second piezoelectric detection structures (51, 52; 151, 152) may be arranged, respectively, on the first and second deformable cantilever elements (21*, 22*; 121*, 122*); and wherein the first and second mobile masses (31, 131; 32, 132) may be vertically staggered downwards, with respect to the first and second deformable cantilever elements (21*, 121*; 22*, 122*).
the first elastic structure (M1, M1′) may be compliant in a direction parallel to the first direction of detection (H1, H2; H1′, H2′).
The accelerometer may further include a third deformable group (23*, 53; 123*, 153) and a fourth deformable group (24*, 54; 124*, 154), which may include, respectively, a third deformable cantilever element (23*; 123*) and fourth deformable cantilever element (24*; 124*), which are arranged in a direction parallel to said plane (XY), extend in a second direction of detection (H3, H4; H3′, H4′), are opposite to one another and each have a respective first end, which is fixed to the supporting structure (2; 102), and a respective second end; said third deformable group (23*, 53; 123*, 153) may further include:
a third piezoelectric detection structure (53; 153), which is mechanically coupled to the third deformable cantilever element (23*; 123*);
said fourth deformable group (24*, 54; 124*, 154) may further:
a fourth piezoelectric detection structure (54; 154), which is mechanically coupled to the fourth deformable cantilever element (24*; 124*);
said MEMS accelerometer (1; 101) may further include:
a third mobile mass (33, 133) and a fourth mobile mass (34, 134), which are fixed, respectively, to the second end of the third deformable cantilever element (23*; 123*) and to the second end of the fourth deformable cantilever element (24*; 124*) and are, respectively, staggered, in the vertical direction (Z), with respect to the third and fourth deformable cantilever elements (23*, 123*; 24*, 124*) so that, in the presence of accelerations directed in a direction parallel to said second direction of detection (H3, H4; H3′, H4′), the third and fourth deformable cantilever elements (23*, 123*; 24*, 124*) are subjected to second parallel and concordant bending moments;
said accelerometer (7; 107) may further include a second elastic structure (M2, M2′) configured to elastically couple the third and fourth mobile masses (33, 133; 34, 134).
The third and fourth deformable groups (23*, 53, 24*, 54; 123*, 153, 124*, 154) may have, respectively, a third neutral axis (H3, H3′) and a fourth neutral axis (H4, H4′), which extend in the second direction of detection (H3, H4; H3′, H4′).
The third and fourth piezoelectric detection structures (53, 54; 153, 154) may be, respectively, staggered, in the vertical direction (Z), with respect to the third neutral axis (H3, H3′) and to the fourth neutral axis (H4, H4′) so that, in the presence of said second parallel and concordant bending moments, one of the third and fourth piezoelectric detection structures (53, 54; 153, 1524) is subjected to tension, whereas the other is subjected to compression.
The third and fourth piezoelectric detection structures (53, 54; 153, 154) may have planar shapes, arranged in a symmetrical way with respect, respectively, to the third neutral axis (H3, H3′) and to the fourth neutral axis (H4, H4′).
The second elastic structure (M2, M2′) may be compliant in a direction parallel to the second direction of detection (H3, H4; H3′, H4′).
The first direction of detection (H1, H2; H1′, H2′) and the second direction of detection (H3, H4; H3′, H4′) may be perpendicular to one another and to the vertical direction (Z).
The first and second mobile masses (31, 32; 131, 132) may have centroids, respectively, staggered, in the first direction of detection (H1, H2; H1′, H2′), with respect to the second ends of the first and second deformable cantilever elements (21*, 121*; 22*, 121*) so that, in the presence of accelerations directed in a direction parallel to the vertical direction (Z), the first and second deformable cantilever elements (21*, 121*; 22*, 121*) are subjected to parallel and opposite third bending moments; and wherein the third and fourth mobile masses (33, 34; 133, 134) have centroids, respectively, staggered, in the second direction of detection (H3, H4; H3′, H4′), with respect to the second ends of the third and fourth deformable cantilever elements (23*, 123*; 24*, 124*) so that, in the presence of accelerations directed in a direction parallel to the vertical direction (Z), the third and fourth deformable cantilever elements (23*, 123*; 24*, 124*) are subjected to parallel and opposite fourth bending moments.
The first, second, third, and fourth mobile masses (131, 132, 133, 134) may be arranged in a square so as to be set two by two parallel alternately to a first reference axis (X) or a second reference axis (Y), which are orthogonal to one another and are perpendicular to the vertical direction (Z); and wherein said first and second directions of detection (H1′, H2′; H3′, H4′) may be both transverse with respect to said first and second reference axes (X,Y); wherein said accelerometer (107) may further include:
a first spring (141), which is compliant in a direction parallel to the first reference axis (X) and elastically couples the first and third mobile masses (131, 133);
a second spring (142), which is compliant in a direction parallel to the first reference axis (X) and elastically couples the second and fourth mobile masses (132, 134);
a third spring (143), which is compliant in a direction parallel to the second reference axis (Y) and elastically couples the second and third mobile masses (132, 133); and
a fourth spring (144), which is compliant in a direction parallel to the second reference axis (Y) and elastically couples the first and fourth mobile masses (131, 134);
and wherein the first elastic structure (M1′) comprises a respective first branch, which includes the first and third springs (141, 143) and the third mobile mass (133), and a respective second branch, which includes the second and fourth springs (142, 144) and the fourth mobile mass (134); and wherein the second elastic structure (M2′) comprises a respective first branch, which includes the first and fourth springs (141, 144) and the first mobile mass (131), and a respective second branch, which includes the second and third springs (142, 143) and the second mobile mass (133).
The first, second, third, and fourth mobile masses (31, 32, 33, 34) may have a rhomboidal arrangement; and wherein the first and second spring structures (M1, M2) may include, respectively, a first arm (41, 42) and a second arm (43, 44), each having a folded elongated shape and being arranged to form a cross.
The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102020000018670 | Jul 2020 | IT | national |
