FLEXIBLE PIEZOELECTRIC DEFORMATION MEASUREMENT DEVICE, UNIDIRECTIONAL, BIDIRECTIONAL OR OMNIDIRECTIONAL

Abstract

A single-crystal piezoelectric thin element (PIEZO) in the form of a plate extending in an extension plane (xy) defined by a first direction (x) and a second direction (y) normal to the first direction, with dimensions (LX, LY) in the first and second directions each greater than 100 µm and a thickness (LZ) less than 50 µm, a ratio of the thickness to dimension (LX) in the first direction or dimension (Ly) in the second direction being less than 0.1, the piezoelectric element exhibiting a first sensitivity (Sx) to deformation along the first direction and a second sensitivity (Sy) to deformation along the second direction, a crystalline orientation of the element (PIEZO) being such that abs(Sy / Sx) < 0.1, abs((Sy+Sx) / Sx) < 0.1, or for at least two first directions (x) of the extension plane (xy) making an angle between them between 30° and 60°, abs((Sx-Sy) / Sx) < 0.1.Figure to be published with the abbreviation: Fig. 3.

Description

Title of the invention: FLEXIBLE PIEZOELECTRIC DEFORMATION MEASUREMENT DEVICE, UNIDIRECTIONAL, BIDIRECTIONAL OR OMNIDIRECTIONAL TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to strain sensors based on a piezoelectric measuring element, i.e. taking advantage of the piezoelectric behavior of a material to transform into an electrical signal a deformation which it undergoes under the effect of external forces, for example when it is glued to a surface and the latter deforms. TECHNOLOGICAL BACKGROUND

[0002] Strain sensors make it possible to estimate the surface deformation which an element of any mechanical system undergoes when it is subjected to external forces (force and moments of force which are applied to it by external elements), in a wide range of applications: aerospace, machine tool, transport, industry in hydrocarbons, food or even chemistry.

[0003] Two technologies are used to construct strain sensors.

[0004] The first technology is based on resistive strain gauges. This type of gauge comprises an electrically conductive element with a geometry that gives it different sensitivities depending on the orientation of the strain applied to it, which is referred to as its directivity. Figure 1 represents such a conductive element CE. The variation of its electrical resistance under the effect of the strain, combined with its known directivity, makes it possible to deduce the amplitude of the strain and, if applicable, the applied force that generated this strain. The arrows SA for "sensitive axis" and TA for "transverse axis" indicate, respectively, an axis of maximum sensitivity and an axis of minimum sensitivity, the latter being perpendicular to the former. In practice, the axis of maximum sensitivity is aligned with the expected direction of the strain to be characterized.Resistive gauges have good directivity, but suffer from low sensitivity and require an electrical power supply to operate.

[0005] The second technology is based on piezoelectric sensors, which are generally more sensitive but also less directional than resistive gauges. This type of sensor includes a so-called "piezoelectric" element, which generates, through the piezoelectric effect, the appearance of electrical charges on its faces under the effect of an applied stress. Several types of such sensors exist.

[0006] A first type corresponds to a deformation sensor comprising a housing A rigid housing containing a crystal Q, such as a piezoelectric quartz crystal, as illustrated by the DynSens sensor in [Fig. 2], which shows in A) a cross-sectional view and in B) a top view of the sensor. The rigid housing serves to limit the amplitude of deformation of the crystal, which, being relatively thick, cannot withstand excessive deformation without breaking. The crystal is attached to a wall of the housing designed to be fixed to a flat, smooth surface by bonding with a high-strength adhesive. The housing can also contain an amplifier AMP connected to the crystal and to the outside via electrical connection wires CAB. The TA and SA axes are defined in the same way as for the resistive gauge in [Fig. 1]. An application example is given by Fabio LM dos Santos et al., dans « THE USE OF DYNAMIC STRAIN SENSORS AND MEASUREMENTS ON THE GROUND VIBRATION TESTING OF AN F-16 AIRCRAF », International Forum on Aeroelasticity and Stuctural Dynamics, IFASD 2015-122. .

[0007] A second type is that of piezoelectric surface deformation sensors. The first advantage of these sensors is that they are passive and therefore do not require an electrical power supply. Such sensors comprise a piezoelectric element, generally a quartz crystal, and a rigid and relatively bulky mechanical structure that restricts the deformation of the piezoelectric element to a single direction. These sensors are generally fixed to a flat surface by screwing, as described, for example, in US patent document 4,314,481 A. The use of a rigid and relatively bulky mechanical structure, which requires a specific geometry for implementation and intrinsically influences the measurement performed, significantly limits the application range of this type of sensor.

[0008] The rigid housing and mechanical structure of these first two types of sensor strongly limit the ranges of deformations to which they are likely to be applied.

[0009] A third type consists of composite structures comprising PZT (or lead zirconate titanate) bars situated between sheets of polymer materials and interdigitated electrodes, structures known as "Micro-Fiber Composites" in English terminology. Their advantages include directivity achieved through the geometry of the PZT bars and greater flexibility than the piezoelectric sensors mentioned above, but they have a complex structure and require polarization achieved by applying an electric field. See the conference paper by Wilkie, WK et al.: "Low-cost piezocomposite actuator for structural control applications," Proceedings of the SPIE, Volume 3991, pp. 323-334 (2000). A disadvantage of such sensors is their temperature dependence due to the presence of polymer material components and insufficient flexibility for certain applications.

[0010] A fourth type is that of strain sensors based on a piezoelectric polymer film, called "PVDF" because of the material used to form the piezoelectric film: piezoelectric polyvinylidene fluoride, known as PVDF in English terminology. These are flexible and lightweight sensors, but they must be polarized, are imprecise, non-directional, and subject to drift over time, particularly due to the lack of polarization stability, or with temperature. Reference can be made to US patent application 2021 / 0102850 A1.

[0011] A fifth type is that of a vibration sensor described in patent document FR 3 122 985 and based on a thin layer of piezoelectric ceramic carried on a flexible support. This sensor is flexible, but non-directional, and is not intended for quantitative measurement of deformation or force.

[0012] Despite a wide variety of devices intended to assume the function of passive strain sensors, a device combining directivity, sensitivity, conformability, lightness and allowing precise quantitative measurements over large strain ranges does not appear to be available. Description of the invention

[0013] The applicant's objective is to propose a strain sensor device combining advantages that can be found individually in the various existing strain sensors, based on the use of a thin piezoelectric element.

[0014] To achieve this goal, a first aspect of the invention is a thin, single-crystal piezoelectric element in the form of a plate extending in a plane of extension defined by a first direction and a second direction normal to the first direction, with dimensions in the first and second directions each greater than 100 pm and a thickness less than 50 pm, a ratio of the thickness to the dimension in the first direction or the dimension in the second direction being less than 0.1, the piezoelectric element having a first sensitivity Sx to deformation along the first direction and a second sensitivity Sy to deformation along the second direction, a crystalline orientation of the element being such that abs(Sy / Sx) <0.1, corresponding to a so-called "unidirectional" sensitivity, abs((Sy+Sx) / Sx) <0.1, corresponding to a so-called "bidirectional" sensitivity,or for at least two initial directions of the extension plane forming an angle between them between 30° and 60°, abs((Sx-Sy) / Sx) <0.1, corresponding to a so-called "omnidirectional" sensitivity.

[0015] Such a thin piezoelectric element is suitable for forming the basis of a passive strain sensor, which can combine precision, sensitivity, conformability, flexibility, lightness, stability, linearity, directivity and applicability to wide ranges of deformations.

[0016] Thus, this piezoelectric element makes it possible to measure deformations greater than 5000 micrometers per meter with a resolution on the order of 1 nanometer of deformation per meter. These figures can be compared with those of conventional resistive gauges, which can measure deformations up to 12000 micrometers per meter but with a much lower resolution, on the order of 1 micrometer of deformation per meter, or with those of piezoelectric gauges in a housing, which are only capable of measuring deformations limited to approximately 300 micrometers per meter with a resolution of 1 nanometer per meter.

[0017] This piezoelectric thin element can be provided with a pair of electrically conductive layers located respectively on two opposite faces of the piezoelectric thin element.

[0018] The invention extends to a strain sensor comprising at least one thin piezoelectric element according to the invention, located on a flexible sheet.

[0019] According to additional non-limiting features of the sensor according to the invention, considered individually or in any technically feasible combination:

[0020] - at least one thin piezoelectric element can be encapsulated between the sheet flexible and another flexible sheet;

[0021] - the sensor may include at least one charge amplifier connected to the minus a thin piezoelectric element;

[0022] - at least one charge amplifier can be integrated on the flexible sheet;

[0023] - the sensor may comprise a plurality of thin piezoelectric elements according to the invention, oriented in different directions with at least 30° of difference between them;

[0024] - the sensor may comprise a first, a second and a third thin element piezoelectric, each exhibiting the characteristic abs(Sy / Sx) < 0.1, the first direction of the second piezoelectric thin element being able to make an angle of 90° with the first direction of the first piezoelectric thin element, the first direction of the third piezoelectric thin element being able to make an angle of 45° with the first direction of the first piezoelectric thin element;

[0025] - the sensor may comprise a first, a second and a third thin element piezoelectric, each of which can exhibit the characteristic abs(Sy / Sx) < 0.1, the first direction of the second piezoelectric thin element being able to make an angle of 120° with the first direction of the first piezoelectric thin element, the first direction of the third piezoelectric thin element being able to make an angle of 240° with the first direction of the first piezoelectric thin element;

[0026] - the sensor may comprise a plurality of charge amplifiers each connected to one of the respective thin piezoelectric elements; and

[0027] - a sensor can combine at least two sensors according to the invention connected electrically tricement en parallel.

[0028] The invention also extends to a device equipped with at least one of the deformation sensors according to the invention. The device may be a ring equipped with at least two unidirectional or omnidirectional deformation sensors, the ring being configured to serve as a human-machine interface, or a smartphone equipped with at least one deformation sensor and configured to warn a user when a deformation measured on the basis of signals from at least one sensor exceeds a predetermined threshold or to serve as a human-machine interface. BRIEF DESCRIPTION OF THE FIGURES

[0029] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0030] [Fig. 1] The [Fig. 1] represents a known resistive strain gauge;

[0031] [Fig.2] The [Fig.2] represents a known piezoelectric strain sensor;

[0032] [Fig.3] The [Fig.3] represents a thin piezoelectric element;

[0033] [Fig.4] Fig.4 illustrates the rotations used in the sensitivity calculations in a plane for a given crystalline orientation in a single crystal;

[0034] [Fig.5] The [Fig.5] represents a polar diagram of strain sensitivities of a thin piezoelectric element according to its crystallographic orientation;

[0035] [Fig.6] The [Fig.6] represents a linear diagram of the sensitivities of the diagram of the [Fig.5];

[0036] [Fig.7] The [Fig.7] illustrates a perfectly unidirectional deformation sensitivity of a thin piezoelectric element;

[0037] [Fig.8] The [Fig.8] illustrates the sensitivities to deformation and stress of the same thin piezoelectric element;

[0038] [Fig.9] The [Fig.9] illustrates a strain sensor incorporating a thin piezoelectric element;

[0039] [Fig. 10] The [Fig. 10] illustrates a charge cell equipped with thin piezoelectric elements exhibiting a first behavior;

[0040] [Fig. 11] The [Fig. 11] illustrates an application of a sensor based on a thin piezoelectric element exhibiting a second behavior;

[0041] [Fig. 12] The [Fig. 12] illustrates an application of a sensor based on a thin piezoelectric element exhibiting a third behavior;

[0042] [Fig. 13] The [Fig. 13] represents a first piezoelectric sensor;

[0043] [Fig. 14] The [Fig. 14] represents a second piezoelectric sensor;

[0044] [Fig. 15] The [Fig. 15] represents a third piezoelectric sensor;

[0045]

[0046]

[0047]

[0048]

[0049]

[0050]

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[0053] [Fig. 16] The [Fig. 16] represents a fourth piezoelectric sensor; [Fig. 17] Figure 17 represents a first device equipped with strain sensors; and [Fig. 18] The [Fig. 18] represents a second device equipped with strain sensors. DETAILED DESCRIPTION OF THE INVENTION The objective of the present invention is to provide piezoelectric elements suitable for use in sensors that combine the advantages of the various types of sensors mentioned above, particularly with regard to sensitivity and directivity, but also accuracy, conformability, flexibility, lightness, stability, linearity, directivity, and applicability to wide deformation ranges. The sections below explain how the inventors determined the characteristics necessary for piezoelectric elements to produce such advantages when integrated into sensors. Starting point of the invention The behavior of piezoelectric materials can be described using three tensors: the dielectric permittivity tensor sT at constant stress, the compliance tensor sE at constant electric field, and the piezoelectric tensor d. In this document, Voigt notation is used. The strain tensor 5 and the electrical displacement D in the material, for example a piezoelectric single crystal, are related to the electric field E and the stress tensor T by the following equations 1 and 2, known as the "piezoelectric equations": s E T + d l E Eq. 1 D-dT+e T E Eq.2 The approach of studying the directivity of the piezoelectric behavior of piezoelectric crystals by analyzing the directivity of the piezoelectric tensor d is well known in the literature, as can be seen for example in the article by W. Yue and J. Yi-jian, "Crystal orientation dependence of piezoelectric properties in LiNbO3 and LiTaO3", Opt. Mater., vol.23, no.1, p.403-408, July 2003. The tensor d represents the electric charge generated by the piezoelectric material when a stress, represented by the tensor T, is applied to it. Typically, analyzing the coefficient d33 of the piezoelectric tensor d gives the charge generated at the faces of a piezoelectric element in response to a stress component that is normal to those faces. The coefficient dM gives the charge generated on these same faces in response to a stress component parallel to them.

[0054] The behavior of the material for different crystal orientations, or orientations of the piezoelectric element with respect to the crystallographic axes of the crystal from which it originates, is obtained by applying the relevant transformations to the tensors defining the behavior of the material (d, se). Reference may be made, for example, to US patent application 4,314,481 A.

[0055] However, the inventors of the present invention have opted for a different approach from those of the analyses found in the literature. More specifically, instead of focusing on the response of a piezoelectric element to a given stress, the inventors focused on the electric charge generated by a piezoelectric element in response to its deformation. As we will see below, this approach leads to the investigation of particular crystal orientations, exhibiting interesting directivity characteristics for the unidirectional, bidirectional, or omnidirectional detection of deformations.

[0056] Such an approach finds practical application, for example, in the case of piezoelectric elements thin enough to form conformable strain sensors, that is, capable of conforming to the shape of a surface on which they are applied. Elements of single-crystal materials that are too thick are too rigid and brittle for such an application. Thin piezoelectric element - Modeling

[0057] We consider a thin piezoelectric element formed of a single crystal. Considering a thin piezoelectric element allows us to simplify equations 1 and 2 by making the following two assumptions.

[0058] First, the dimensions of the piezoelectric thin element PIEZO, considered in its plane of extension, are much larger than its thickness. The thin element can thus be described as having the shape of a plate. A piezoelectric thin element extending in an xy plane, hereafter referred to as the "plane of extension," can be considered, defined by two directions x and y normal to each other and normal to a third direction z, which are directions specific to the piezoelectric thin element. A parallelepiped-shaped thin element with dimensions Lx, LY, and Lz, respectively, in the x, x, and z directions, which define an orthogonal coordinate system, can be considered, as illustrated in [Fig. 3] (for clarity, the scales are not to scale).We consider thin elements satisfying the conditions under which Lx and Ly are at least ten times, preferably at least one hundred times, greater than Lz, the latter dimension corresponding to the thickness of the thin element.

[0059] Secondly, we will consider a unidirectional deformation, along a direction of the extension plane.

[0060] A thin piezoelectric element whose dimensions in the plane of extension are

[0061]

[0062]

[0063]

[0064]

[0065] A thin piezoelectric element, much larger than its thickness (Lx and Ly > lOLz, preferably Lx and Ly > lOOLz), and equipped with two electrically conductive layers on the two faces parallel to the extension plane, behaves electrically like a parallel-plate capacitor whose two electrically conductive plates correspond to the surfaces normal to the z-direction, denoted Top and Bot, respectively. In this capacitor, the electric field is normal to the plates: the electric field has no component in the xy-plane. When the thin piezoelectric element is used in a load-mode sensor, the electrodes, which cover the two planes parallel to the extension plane, are connected to a charge amplifier that creates a low-impedance path between the two electrodes. Consequently, the voltage between the two electrodes is zero, and the electric field in the normal z-direction is also zero.In conclusion, there is no electric field in the piezoelectric thin element when it is connected to a charge amplifier. Equations 1 and 2 can then be simplified, becoming the following equations 3 and 4, which are sufficient to describe the piezoelectric behavior of the thin element within the framework of the assumptions specified above: S = s^T Eq. 3 D-dT EqA Study of the sensitivity of the thin element In thin layers of a material, the stress in the normal direction is often small compared to the stresses in the plane of the material. It is therefore common to use the plane stress approximation to model the material's behavior, assuming that the stresses normal to the material are zero. This approximation greatly simplifies the constitutive and mechanical equilibrium equations. In this case, we are working within the framework of this approximation, with a thin piezoelectric element Piezo (Lx and Ly > lOLz), considered, as in a real-world practical application, as having one of its faces fixed to a structure and the opposite face free, and which will be in a state of stress in its xy extension plane, with the stresses in the normal z direction being comparatively negligible. In this situation, we can further simplify the piezoelectric equations: the T3, T4, and T5 components of the stress tensor T are zero, and the piezoelectric tensor d can be reduced to a 3x3 matrix, denoted d, with 9 components. Thus, it can be demonstrated that the electrical displacement generated by the thin element piezoelectric in charging mode when a deformation is applied to him / her.

[0066]

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[0070]

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[0074] to be expressed by equation Eq. 5 [If] Eq. 5 6 in which cE represents the reduced compliance matrix, obtained by removing rows and columns 3, 4 and 5 from the compliance matrix cE, and T represents the reduced stress tensor, obtained by removing components 3, 4 and 5 from the stress tensor T. In the case of uniaxial deformation along the x-direction, the electrical displacement D is expressed by equation Eq. 6 T = dT = dcE o Si Eq. 6 D. Equation Eq. 6 shows that the electrical displacement caused by uniaxial strain in a thin piezoelectric element depends not only on the piezoelectric tensor d after rotation, but also on the reduced compliance tensor cE after rotation. Analyzing only the piezoelectric tensor d after rotation, as is commonly done in the literature, is insufficient to characterize the sensitivity of a thin piezoelectric element to a strain applied in a given direction. We are only interested in the third component D3 of the electric displacement tensor: this is the quantity that will actually be measured during a practical use of the piezoelectric thin element, by means of a pair of electrodes placed on the faces of the element parallel to the xy extension plane. The sensitivity Sx to deformation is obtained by integrating the D3 component of the electrical displacement over the entire surface of the thin piezoelectric element covered on both sides by electrodes, a sensitivity calculated for a deformation uniaxial We thus obtain Sx = AxD3 / Si, where A is the surface area of The thin piezoelectric element is covered on both sides by electrodes. The sensitivity Sx is expressed in pC / (micro_m / m), and represents the electric charge generated by the piezoelectric element when a deformation of 1 micro_m / m is applied to it along the x direction. Sensitivity of single-crystal thin elements - Directivity The objective here is to evaluate the directivity of the sensitivity of a single-crystal thin element for a given crystal orientation of a bulk single crystal. Crystal orientations will be considered as defined in the IEEE standards. piezoelectricity in “IEEE Standard on Piezoelectricity”, ANS1 / 1EEE Std 176-1987, 1988.

[0075] The directivity of the single-crystal piezoelectric thin element can be calculated analytically by applying an Rt sequence of rotations to the thin element with respect to the crystallographic axes, corresponding to the definition of IEEE (YXwl) 0i / 02 crystal orientations and illustrated by [Fig.4].

[0076] The Rt sequence of rotations is composed of 4 successive rotations, Rx(90), Ry(0i), Rx(02) and Rz(03) in that order.

[0077] The first rotation Rx(90) is done at an angle of 90° around the X axis, the xyz frame specific to the thin element being transformed into an x'y'z' frame.

[0078] The second rotation Ry(0i) is done by rotation through an angle 0i around the y' axis, the x'y'z' frame being transformed into an x”y”z” frame”.

[0079] The third rotation Rx(02) is done by rotation around the x axis”, the x”y”z” frame being transformed into an x”'y”'z”’ frame.

[0080] The fourth rotation Rz(03) is done by rotation through an angle 03 around the axis z'”, the frame x'”y”'z”' being transformed into a frame x””y””z””.

[0081] Each of the frames xyz, x'y'z', x”y”z” and x'”y'”z'” is respectively defined by the triplets of direction x, y and z, x', y' and z', x”, y” and z”, and x'”, y'” and z'”, and x””, y”” and z””, by successive application of the 4 rotations Rx(90), Ry(0i), Rx(02) and Rz(03) to the piezoelectric element whose eigendirection x, y and z initially correspond to the eigendirections X, Y and Z of the bulk single crystal considered.

[0082] The first three rotations Rx(90), Ry(01) and Rx(02) are used to define the crystalline orientation of the thin element in the bulk crystal. The fourth rotation Rz(03) is used to study the directivity of the sensitivity for a plane of crystalline orientation defined by the first three rotations.

[0083] Thus, the electrical charges generated on the opposite faces Top and Bot are calculated according to the orientation of the crystallographic axes defined by the angles 0H 02 and 03 for a uniaxial deformation along the x'” direction of the bulk single crystal. From this charge calculation, the sensitivity Sx = AxD3 / Si to deformation is deduced. It is therefore possible to test the sensitivity of all possible crystal orientations for a bulk single crystal.

[0084] Calculations performed by scanning the values ​​of θi and, for each value of θb, the values ​​of θ2, show that, for a given value of θi and depending on the value of θ2, the generated charges vary very differently with the angle θ3. For some pairs of values ​​(θ3 θ2), approximately the same amount of charge is generated regardless of the value of θ3. For other pairs of values ​​(θi θ2), the generated charge can be negative, positive, or zero depending on the value of θ3. For still other pairs of values ​​of (θi θ2), the generated charge is zero regardless of the value of 03.

[0085] Fig. 5 thus illustrates a polar diagram representing the quantity of electric charges generated on the Top and Bot faces of a thin lithium tantalate element in response to a deformation, and therefore the sensitivity to this deformation, calculated for an angle 0i fixed here at 7° and for angles 02 of 10°, 90°, 118° and 159° each corresponding to one of the curves represented on the diagram, angle 03 being the polar angle of the diagram.

[0086] The polar diagram of [Fig.5] allows us to deduce, for various crystal orientations defined by their values ​​of 01 and 02, the sensitivity ratios between a so-called "transverse" sensitivity, considered according to a direction perpendicular to a direction corresponding to a maximum sensitivity, and the so-called "longitudinal" sensitivity considered according to the direction of maximum sensitivity, with ratios of 55%, 100%, 0% and -99% for values ​​of 02 of 10°, 90°, 118° and 159°, respectively.

[0087] For 02=159°, four lobes are formed on the polar diagram and the ratio of the transverse sensitivity (03=95°) to the longitudinal sensitivity (03=5°) is close to 1 in absolute value with a ratio of -99%. This indicates that a piezoelectric sensor incorporating a thin piezoelectric element having a crystalline orientation defined by the angles 0i=7° and 02=159° will be a bidirectional sensor, particularly sensitive to deformations occurring along two directions normal to each other.

[0088] For 02=118°, only two lobes are formed on the polar diagram and the ratio of the transverse sensitivity (03=8°) to the longitudinal sensitivity (03=98°) is zero. This means that a piezoelectric sensor incorporating a thin piezoelectric element having a crystalline orientation defined by the angles 0i=7° and 02=118° will be very directional, even perfectly unidirectional, particularly sensitive to deformation occurring along a single direction.

[0089] For 02=10°, no lobe is formed, the curve representing the sensitivities according to the value of 03 forming essentially a circle, the sensitivity ratio is 1, with a sensitivity ratio of 100%. This indicates that a piezoelectric sensor incorporating a thin piezoelectric element having a crystalline orientation defined by the angles 0i=7° and 02=90° will be omnidirectional, detecting with the same sensitivity a deformation regardless of its orientation in the plane of extension of the thin piezoelectric element.

[0090] For 02=90°, no lobe is formed; the curve representing the sensitivities according to the value of 03 forms a figure without any particularly remarkable characteristics. Such a crystal orientation has no obvious application for forming a strain sensor.

[0091] Figure 6 reproduces the four sensitivities illustrated by Figure 5, but this time on a linear graph, the sensitivities are expressed as a function of the angle 03 expressing the orientation of a deformation imposed in the extension plane with respect to the proper x axis of the piezoelectric thin element, for angles 03 between 0° and 180°.

[0092] For the angle 02 of 159°, there are two directions normal to each other, each exhibiting a high sensitivity, these sensitivities being substantially equal in absolute values ​​but of opposite signs (bidirectional sensitivity).

[0093] For the angle 02 of 118°, there is a direction of high sensitivity, the sensitivity of a normal direction being zero (unidirectional sensitivity).

[0094] For the angle 02 of 90°, the sensitivity is substantially constant over all orientations, and here of negative sign (omnidirectional sensitivity).

[0095] For the angle 02 of 10°, the sensitivity is constantly negative and fluctuating, without any notable a priori point.

[0096] Each of the above behaviors, but essentially the first three, can be used in the design of sensors for particular applications, as explained below.

[0097] By evaluating the directivity of thin piezoelectric elements as described above by sweeping the different values ​​of 0i and 02, it is possible to determine the crystal orientations allowing the formation of thin piezoelectric elements having strain sensitivity ratios of particular interest.

[0098] In particular, the sensitivities Sx and SY in two directions of the extension plane can be considered normal to each other. In [Fig. 5], for example, Sx can be considered as the sensitivity in the direction of the extension plane defined by θ3 = 7° and SY as the sensitivity in the direction of the extension plane defined by θ3 = 97°, called "transverse sensitivity". Particular attention may be paid to (i) so-called "unidirectional" sensitivity planes exhibiting a transverse sensitivity SY close to 0 and a non-zero sensitivity Sx, preferably of high value, (ii) so-called "omnidirectional" sensitivity planes exhibiting sensitivities Sx and SY close to each other for all orientations in the plane, and (iii) so-called "bidirectional" sensitivity planes exhibiting an orientation with sensitivities Sx and SY close to each other in absolute values ​​and of opposite signs.

[0099] A criterion for qualifying the behavior of a piezoelectric thin element as unidirectional is that the absolute value of a ratio of sensitivities Sx and Sy to deformation in its plane of extension (which corresponds to its crystalline orientation in the bulk single crystal from which it originates) in a first direction and a second direction normal to each other (i.e. abs(SY / Sx) or |Svf SJ in mathematical notation) is less than 10%, preferably 5%, more preferably 2%, even more preferably 1%, and most preferably 0.1%.

[0100] Similarly, a criterion for classifying the behavior of a thin piezoelectric element as bidirectional is that the absolute value of the ratio of the sum of the piezoelectric sensitivities to strain in a first direction and a second direction normal to each other to the piezoelectric sensitivity to strain in the first direction (i.e., abs((Sx+SY) / Sx) or |($^ + 5^J in mathematical notation) is less than 10%, preferably 5%, more preferably 2%, even more preferably 1%, and most preferably 0.1%.

[0101] The behavior of a piezoelectric thin element can be described as omnidirectional when, for all orientations in its extension plane (which corresponds to its crystalline orientation in the bulk single crystal from which it originates), the ratios of the difference in strain sensitivities Sx and Sy respectively in a first direction and in a second direction normal to the first direction, both in the extension plane, to the sensitivity Sx (i.e., abs((Sx-SY) / Sx) or |(5V - 5Q / 5| in mathematical notation)) are less than 10%, preferably 5%, more preferably 2%, even more preferably 1%, and most preferably 0.1%. From a practical point of view, the thin element will be considered to exhibit truly omnidirectional behavior when the criterion on the value of abs((Sx -SY) / Sx) is satisfied for two first directions forming an angle between them of between 30° and 60°, preferably between 40° and 50°.

[0102] The method developed above for calculating strain sensitivity makes it possible to determine the crystal orientations defined by the angles θi and θ2 that have perfectly unidirectional sensitivity: there exists a direction θ3 for which the ratio SY / SX is perfectly zero, up to the accuracy of the calculation. Figure 7 illustrates such a case, representing the strain-sensitivity to strain in a polar diagram as a function of θ3 for a lithium tantalate crystal orientation defined by the angles θi = 7° and θ2 = 118°. The same approach can be used for omnidirectional or bidirectional sensitivities.

[0103] It should be noted that the fact that, for a given crystal orientation, the stress sensitivity exhibits perfectly unidirectional behavior does not imply that the strain sensitivity will also be perfectly unidirectional. Figure 8 illustrates the result of calculations for the stress and strain sensitivities, Stress-Sens and Strain-Sens, respectively, for a lithium tantalate crystal orientation defined by the angles θ1 = 7° and θ2 = 110°. It can be seen that even though there is an angle θ3 for which the stress sensitivity ratio (SY / SX) is zero, indicating perfectly unidirectional behavior in response to an applied stress, the strain sensitivity ratio (SY / SX) is close to 18%, meaning that this strain sensitivity cannot be considered... formation as unidirectional.

[0104] Unlike conventional approaches focused on applied stress, the method explained above, centered on the deformation applied to a thin piezoelectric element, makes it possible to choose the crystal orientations adapted to a desired behavior in response to unidirectional deformation. This can be unidirectional, omnidirectional, or bidirectional behavior, depending on the practical applications envisaged.

[0105] The examples given above were obtained for lithium tantalate LiTaO3 in single-crystal form, which belongs to space group 3m. However, the method can be applied to all space groups of piezoelectric materials. Without limiting the method to these, other materials of interest include: lithium niobate LiNbO3 (group 3m), lead magnesium niobate MgNb2(PbO3)3 (group PI), aluminum nitride AIN (group P63mc), barium titanate BaTiO3, potassium niobate KNbO3 and lead titanate TiPbO3 (all three of group P4mm). Applications - Sensors

[0106] Figure 9 illustrates in (A) a cross-sectional view of a SENS piezoelectric sensor based on a PIEZO piezoelectric thin element with an extension plane chosen to exhibit a particular behavior—unidirectional, omnidirectional, or bidirectional—in response to a unidirectional deformation applied to it in its extension plane. The sensitivity behavior of the piezoelectric thin element (unidirectional, omnidirectional, or bidirectional) is transferred to the sensor incorporating this piezoelectric thin element. Such sensors can be particularly suited to specific situations, as illustrated by Figures 10 to 12 discussed below, but they can also be used in more general applications, as will become apparent later.

[0107] To take advantage of the thinness, and therefore the flexibility and conformability, of the PIEZO thin element, the SENS sensor comprises a flexible SH1 sheet to which the PIEZO thin element is attached. The sheets are preferably made of flexible materials chosen according to the intended application and may, for example, be made of metal, polyvinyl chloride (PVC), polyimide (PI), polyethylene terephthalate (PET), biaxially oriented polyethylene terephthalate (Mylar®), or a composite material of epoxy resin and glass fibers. The PIEZO thin element can be attached to the SH1 sheet by means of a flexible adhesive such as an anisotropic conductive film (ACF), which also allows for electrical contact as described in patent document FR 3 122 985. In use, the SENS sensor can be attached to a surface to be characterized by means of an adhesive, for example, cyanoacrylate glue or epoxy resin.

[0108] In addition to the thin PIEZO element, in the example of [Fig. 9], a charge amplifier C.AMP is also fixed to the SH1 sheet and functionally connected to two conductive layers ELI and EL2 acting as electrodes, formed respectively on two opposite faces of the thin PIEZO element. The charge amplifier's function is to produce a voltage corresponding to the input load, which corresponds to the charge generated by the PIEZO element during its deformation, for the purpose of electronically processing the generated electrical potential and effectively measuring the deformation of the PIEZO element. Although not shown, a wired connection element, such as a ribbon cable, is connected to the charge amplifier to connect the sensor to an external measuring device.An alternative would be to connect the external measuring device directly to the ELI and EL2 electrodes, or to use an amplifier located between the SENS sensor and the external measuring device. Reference can also be made to patent application published under number FR 3 122 985, the principles of which are applicable to the subject matter of this document.

[0109] Fig. 9 illustrates in (B) a sensor similar to that of the configuration illustrated in (A), but further comprising a second sheet SH2, which may be of the same nature as the sheet SH1, which allows the thin PIEZO element and C.AMP charge amplifier to be encapsulated by sandwiching between the sheets SH1 and SH2.

[0110] The thin piezoelectric element PIEZO preferably has a thickness of less than 50 pm, more preferably less than 25 pm, and even more preferably less than 10 pm. If we consider a piezoelectric element defined as illustrated in [Fig. 3], a ratio of the thickness of the PIEZO element to its dimension Lx in a first direction of its extension plane, and / or to a dimension Ly in a second dimension of its extension plane normal to the direction Lx, is less than 0.1, preferably less than 0.05, and more preferably less than 0.01.

[0111] The SH1 sheet and, where applicable, the SH2 sheet, may have a thickness between 5 and 300 pm.

[0112] However, it is preferable that the SENS piezoelectric sensor, considered as a whole, be sufficiently flexible to follow the deformations of the object to be characterized on which it is fixed. The practitioner can decide, for each application, on the characteristics of the PIEZO piezoelectric thin element, its support, and other elements such as the electrode layers or the means for making electrical contacts.

[0113] As an alternative to using the flexible SH1 and SH2 sheets which serve as support and protection for the thin piezoelectric element, more rigid supports could also be used, such as a rigid metal casing, which would, however, not allow the exploitation of the flexibility and conformability aspects of a piezoelectric element. thin electrical as described in this document.

[0114] A sensor based on a thin piezoelectric element with unidirectional behavior, and therefore with unidirectional behavior itself, can be used to characterize the deformation of a mechanical part subjected to deformations in multiple directions, but where the component of these deformations in a particular direction is sought. It is also possible to know the expected direction of deformation of the mechanical part and seek to characterize this expected deformation in particular. An advantage of a sensor based on an element with unidirectional behavior is that it does not measure the disturbances resulting from parasitic external forces to which the system might be exposed in its environment. Thus, the [Fig.Figure 10] represents in (A) a load cell CELL at rest, equipped with four sensors SENS, each based on a thin piezoelectric element with unidirectional behavior, oriented according to the expected deformation of the cell, which is specifically designed to deform in a preferred direction. Under load from the application of a force F, as illustrated in (B), the load cell deforms and two of the four sensors are in a state of compression Comp while the other two are in a state of extension Ext.

[0115] A thin piezoelectric element with omnidirectional behavior can be used when the mechanical part under consideration is subjected to radial deformation and / or if the deformation is the same in both the axial and transverse directions, for example, a membrane subjected to a variation in acoustic pressure and whose deformation one wishes to characterize. Figure 11 illustrates such a situation, with a rigid circular armature (ARM) fixing the periphery of a membrane (MEM) that can be set into vibration, for example, by an acoustic wave. Here, the membrane can consist of a thin piezoelectric element deposited on a flexible support.

[0116] A thin piezoelectric element with bidirectional behavior can be used, for example, when a cylindrical beam (BEAM) is subjected at its ends to a couple of opposing forces FT+ and FT about the longitudinal axis of the beam (torsion). In this situation, each point on the beam surface simultaneously undergoes compression Comp along a first direction and extension Ext along a second direction normal to the first direction, as illustrated in [Fig. 12]. In such a situation, placing a sensor equipped with a thin piezoelectric element with bidirectional behavior, with its x and y axes aligned respectively with the direction of compression and the direction of extension, is suitable for efficient deformation detection, making it possible to determine the couple of forces experienced by the beam using a single sensor.Indeed, given the characteristics of the sensor, with sensitivities to deformation of opposite signs in the normal directions. Given the characteristics of the deformation, with extension in one direction and compression in a second direction normal to the first, the sensor can be sensitive to both directions of deformation provided it is properly positioned on the cylindrical beam. For similar characterization by conventional means, two resistive strain gauges would be necessary due to their largely unidirectional sensitivity.

[0117] In the examples in Figures 10, 11, and 12, the flexibility of the thin piezoelectric element is advantageously exploited to fix one of its faces in close contact with the curved surface of the mechanical part under study. In the example of [Fig. 11], the deformability provided by the flexibility of the thin piezoelectric element is particularly utilized.

[0118] The geometry described above represents a basic configuration. It is also possible to combine several PIEZO elements to form a sensor. Figures 13, 14, and 15 illustrate several possible configurations for sensors combining thin piezoelectric elements with unidirectional behavior along a Sens_Dir direction specific to each element. Each of these sensors has a structure analogous to that of the SENS sensor illustrated in [Fig. 9], the thin piezoelectric elements of these sensors sharing the same support Supp on which they are placed, a support possibly formed from the flexible SH1 sheet of the SENS sensor. Furthermore, each of the thin elements can be connected to a dedicated amplifier, as in the SENS sensor described above, possibly integrated onto the Supp support. The amplifier can be a charge amplifier.

[0119] Figure 13 illustrates a SENS_1 sensor comprising two piezoelectric thin elements PIEZO_0° and PIEZO_90° fixed to the same support Supp and having their respective Sens_Dir directions oriented at 90° to each other. This example illustrates a configuration with two piezoelectric thin elements having a specific orientation relative to each other, but the invention is not limited to this particular configuration, and the number and respective orientations of the piezoelectric thin elements are not limited to this particular configuration and can be adapted to the requirements of any application, as decided by the user.

[0120] Figure 14 illustrates a SENS_2 sensor comprising three thin piezoelectric elements PIEZO_0°, PIEZO_90°, and PIEZO_45° fixed to the same support Supp. The Sens_Dir directions of PIEZO_90° and PIEZO_45° are inclined at 90° and 45° respectively with respect to the Sens_Dir direction of PIEZO_0°. Furthermore, each of the three thin piezoelectric elements satisfies the characteristic abs(Sy / Sx)<0,l, i.e., they exhibit unidirectional behavior.

[0121] Figure 15 illustrates a SENS_3 sensor comprising three thin piezoelectric elements PIEZO_0°, PIEZO_120° and PIEZO_240° fixed on the same Supp support, The directions Sens_Dir of PIEZO_120° and PIEZO_240° are inclined at 120° and 240° respectively with respect to the direction Sens_Dir of PIEZO_0°. Furthermore, the three thin piezoelectric elements each satisfy the characteristic abs(Sy / Sx) <0.1, that is, they exhibit unidirectional behavior.

[0122] The SENS_2 and SENS_3 sensor configurations implement the known principle of combining resistive strain gauges (see [Fig. 1]) to form what are called strain gauge rosettes, which make it possible to characterize the surface deformation of a mechanical part even when this deformation is not unidirectional. While the principle of placing thin elements to efficiently derive the deformation components from measurements is known, replacing resistive gauges with excellent directivity with piezoelectric thin elements exhibiting unidirectional behavior according to the invention makes it possible to obtain a strain sensitivity that is ordinarily unattainable for comparable applications. This replacement is made possible by determining specific crystal orientations that give the piezoelectric thin elements according to the invention unidirectional behavior.

[0123] It is convenient to use certain specific orientations (0°, 45°, 90°, 120°, 240°) for the thin piezoelectric elements constituting the rosettes, but there is no fundamental reason to limit oneself to the specific configurations of the SENS_2 and SENS_3 sensors, and any relative orientations can be used as long as two of the sensors are not oriented parallel to each other. When referring to these specific orientations, it is understood that the values ​​given are the optimal values ​​and that the values ​​on an actual device may differ slightly from these optimal values. For example, the effective angles between the orientations can be given with an inaccuracy of up to ±10°, preferably ±5°, and even more preferably ±1°.

[0124] More generally, determining crystal orientations using the approach detailed above, based on studying the deformation of a thin piezoelectric element, makes it possible to define crystal orientations leading to precisely chosen and controlled behaviors in response to deformation. Implemented in strain sensors, such thin piezoelectric elements offer practitioners great flexibility and allow the development of sensors perfectly suited to the intended applications.

[0125] Sensor coupling

[0126] One possibility of using the thin piezoelectric elements according to the invention is their coupling, that is to say, configuring them in such a way that the electrical charges generated by at least two piezoelectric elements during their use add up, for example by electrically connecting their respective electrodes. One application could be correcting their directionality errors.

[0127] Consider, for example, a crystal orientation of omnidirectional sensitivity, for which abs(Sx-Sy) / Sx) <10%. Two piezoelectric sensors PIEZO'_0° and PIEZO'_90° exhibiting this crystal orientation, called "elementary sensors," can be integrated onto the same support, oriented at 90° to each other, and electrically connected in parallel. In such a configuration, the two electrodes ELI are connected to each other and to a connection surface PAD1 by connection elements Cl, and the two electrodes EL2 (not shown in [Fig. 16], refer to [Fig. 12]) are connected to each other and to a connection surface PAD2 by connection elements C2.

[0128] The output of the resulting sensor, called the "combined sensor," will be the sum of the electrical charges generated by the two sensors. The advantage is that the coupling thus achieved in the combined sensor compensates for the maximum 10% directionality error of each of the two elementary sensors taken individually: the combined sensor will be perfectly omnidirectional.

[0129] The same principle can be applied to other systems, for example to several imperfectly unidirectional elementary sensors to obtain a perfectly directional combined sensor, the elementary sensors being able to be of different sizes and of relative orientations determined so as to correct the imperfections in directionality of these sensors.

[0130] Application examples

[0131] Figure 17 illustrates in (A) a perspective view of a first application example with a ring R instrumented by three SENS sensors according to the invention. These sensors are preferably of the unidirectional or omnidirectional type.

[0132] This ring instrumentation allows it to be transformed into a human-machine interface in order to control, for example, an electronic device such as a computer, a smartphone, a television, or a multimedia player. Three SENS piezoelectric strain sensors are bonded to the ring, which integrates the EL electronic equipment and possibly the power source necessary to measure the electrical charges generated by the sensors, analyze them, and transmit the information wirelessly to an external device such as those mentioned above.

[0133] As illustrated in (B) of [Fig. 17], when the ring R is held between two fingers Fl and F2, a compressive force F is applied on either side of the ring by the fingers, creating a localized deformation that can be measured using the three SENS sensors. When the ring is rolled between the fingers in a rotational movement Rot as illustrated in (C), the position at which this force is applied moves, and this movement can be measured by comparing the signals from the three sensors. The following actions can thus be detected and associated with predetermined actions, such as (i) a single press by pressing between the fingers associated with a confirmation action, (ii) a double press associated with a back action in a menu of a ring-controlled application, and (iii) a rotation in one direction or the other associated with scrolling up or down in a list or with volume control of a music player.

[0134] Fig. 18 illustrates a second application example with a SMART smartphone equipped with four thin piezoelectric SENS sensors according to the invention.

[0135] A first application of such a system addresses the problem that a smartphone can be damaged when subjected to excessive deformation, which typically occurs when it is placed in a back trouser pocket and the user sits down. Integrating one or more SENS sensors according to the invention into a smartphone makes it possible to continuously measure the deformation of the phone and to warn the user with a visual or audible message when the deformation exceeds a predefined threshold beyond which the smartphone could be damaged. In this example, the SENS sensors are attached to the RF back cover intended to protect the main body MB of the SMART smartphone and are connected to the smartphone's data processing system.The four sensors are arranged to measure deformations along the two principal axes (defined by the directions of the long and short sides of the smartphone), as well as the torques applied to the phone in all directions. The sensors are preferably unidirectional, with two sensors oriented parallel to a long side of the smartphone and two others oriented parallel to a short side of the smartphone.

[0136] For a second application of the system in [Fig. 18], preferably omnidirectional SENS sensors are used to detect and locate a press on the back of the phone, with a view to using the sensors as a human-machine interface. The signals measured by the four sensors are compared: the closer the press is to a sensor, the greater the measured deformation. The press can thus be detected and located using the four sensors. Based on this detection and localization, the following user interactions with their phone can be identified and associated with control actions: (i) single press, (ii) double press, (iii) pressing a key on a virtual touch keypad located on the back of the smartphone, each key corresponding to a respective press zone on the back of the smartphone.

[0137] Fabrication of a piezoelectric thin film

[0138] A piezoelectric thin film according to the invention, equipped with its electrodes, can be manufactured as described below. A substrate called a "piezoelectric on insulator" (or POI for Piezoelectric On Insulator in English terminology), a structure comprising a piezoelectric layer fixed to a substrate, an oxide layer of Commercially available silicon (interposed between these two layers) is used as the source of the piezoelectric thin film. This type of substrate can be obtained for any crystal orientation and for any type of piezoelectric material.

[0139] In a first step, a first layer of platinum electrode 400 nm thick is deposited on the free surface of the piezoelectric thin layer by chemical vapor deposition (for example by PECVD for Plasma Enhanced Chemical Vapor Deposition).

[0140] In a second step, a glass support with a separation layer (for example a so-called "LTHC" layer for "Light to Heat Conversion release Coating") is temporarily bonded to the platinum electrode deposited in the first step, using a liquid polymerized by exposure to ultraviolet radiation, so as to form a glass-POI wafer.

[0141] In a third step, the glass-POI stack is thinned by mechanical grinding of the free face of the POI substrate until approximately the buried silicon oxide layer of the POI is reached. Dry etching removes the remaining silicon and the oxide layer, selectively stopping at the piezoelectric layer.

[0142] All that remains of the glass-POI wafer as a result of the third step is the thin piezoelectric layer under which lies the first layer of platinum electrode supported by the glass substrate.

[0143] In a fourth step, a second 400 nm platinum electrode layer is then deposited on the exposed face of the piezoelectric thin layer by chemical vapor deposition.

[0144] Finally, the stack consisting of the thin piezoelectric film between its two electrode layers is transferred and bonded to a final substrate. The glass support is then removed by peeling at the level of the separation layer (by laser irradiation in the case of an LTHC separation layer) used to peel off the polymerized liquid, leaving the thin piezoelectric cell and its electrode layers on its final substrate.

[0145] This process enables the fabrication and transfer of large piezoelectric thin films with electrodes onto various types of substrates. The piezoelectric cell and the substrate then need to be cut to the correct dimensions according to the intended applications. In this description, passages relating to an element associated with a given identifier refer to another element with the same identifier in a different figure.

[0146] In this description, describing a strain sensor or the behavior of this sensor as unidirectional, bidirectional, or omnidirectional is equivalent to saying that this sensor incorporates a thin piezoelectric element whose sensitivity is unidirectional. tional, bidirectional or omnidirectional as defined in the description, respectively.

[0147] Of course the invention is not limited to the embodiments described above and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Demands

1. A single-crystal piezoelectric thin element (PIEZO) in the form of a plate extending in an extension plane (xy) defined by a first direction (x) and a second direction (y) normal to the first direction, of dimensions (Lx, LY) in the first and second directions each greater than 100 pm and of thickness (Lz) less than 50 pm, a ratio of the thickness to the dimension (Lx) in the first direction or the dimension (Ly) in the second direction being less than 0.1, the piezoelectric element having a first sensitivity (Sx) to deformation along the first direction and a second sensitivity (Sy) to deformation along the second direction, a crystalline orientation of the element (PIEZO) being such that: - abs(Sy / Sx) <0.1, corresponding to a so-called "unidirectional" sensitivity, - abs((Sy+Sx) / Sx) <0.1, corresponding to a so-called "bidirectional" sensitivity,or - for at least two initial directions (x) of the extension plane (xy) forming an angle between them between 30° and 60°, abs((Sx-Sy) / Sx ) < 0.1, corresponding to a so-called "omnidirectional" sensitivity.

2. Piezoelectric thin element (PIEZO) according to claim 1, provided with a pair of electrically conductive layers (EL1, EL2) located respectively on two opposite faces of the piezoelectric thin element.

3. Strain sensor (SENS, SENS_1, SENS-2, SENS_3) comprising at least one thin piezoelectric element (PIEZO) according to claim 1 or 2, located on a flexible sheet (SH1).

4. Strain sensor (SENS, SENS_1, SENS-2, SENS_3) according to claim 3, at least one thin piezoelectric element being encapsulated between the flexible sheet (SH1) and another flexible sheet (SH2)

5. Strain sensor (SENS, SENS_1, SENS-2, SENS_3) according to claim 2 and claim 3 or 4, comprising at least one charge amplifier (C.AMP) connected to at least one piezoelectric thin element (PIEZO).

6. Strain sensor (SENS, SENS_1, SENS-2, SENS_3) according to claim 5, at least one charge amplifier (C.AMP) being integrated on the flexible sheet (SH1).

7. Strain sensor (SENS_1, SENS_2, SENS_3) according to one any of claims 3 or 4, comprising a plurality of thin piezoelectric elements (PIEZO_0°, PIEZO_90°, PIEZO_45°, PIEZO_120°, PIEZO_240°) according to claim 1 or 2, oriented in different directions having at least 30° of difference between them.

8. Strain sensor (SENS-2) according to claim 7, comprising a first (PIEZO_0°), a second (PIEZO_90°) and a third (PIEZO_45°) piezoelectric thin elements, each having the characteristic abs(Sy / Sx) < 0.1, the first direction (x) of the second piezoelectric thin element making an angle of 90° with the first direction (x) of the first piezoelectric thin element, the first direction (x) of the third piezoelectric thin element making an angle of 45° with the first direction (x) of the first piezoelectric thin element.

9. Strain sensor (SENS_3) according to claim 7, comprising a first (PIEZO_0°), a second (PIEZO_120°) and a third (PIEZO_240°) piezoelectric thin elements, each having the characteristic abs(Sy / Sx) < 0.1, the first direction (x) of the second piezoelectric thin element making an angle of 120° with the first direction (x) of the first piezoelectric thin element, the first direction (x) of the third piezoelectric thin element making an angle of 240° with the first direction (x) of the first piezoelectric thin element.

10. Strain sensor (SENS_1, SENS_2, SENS_3) according to claim 2 and any one of claims 7 to 9, comprising a plurality of charge amplifiers (C.AMP) each connected to one of the respective thin piezoelectric elements.

11. Sensor combining at least two sensors according to claim 2 and any one of claims 3 to 6, electrically connected in parallel.

12. A device equipped with at least one of the strain sensors according to any one of claims 3 to 10 or a combination of sensors according to claim 11.

13. An apparatus according to claim 12, the apparatus being a ring equipped with at least two unidirectional or omnidirectional deformation sensors, the ring being configured to serve as a human / machine interface.

14. A device according to claim 12, the device being a smartphone equipped with at least one strain sensor and configured to warn a user when a strain measured on the basis of signals from at least one sensor exceeds a predetermined threshold or to serve as a human / machine interface.

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