Flexible piezoelectric device for unidirectional, bidirectional, or omnidirectional deformation measurement

Abstract

The present invention relates to a single-crystal thin-film piezoelectric element (PIEZO) in the form of a plate extending in an extending plane (xy) defined by a first direction (x) and a second direction (y) perpendicular to the first direction, wherein the dimensions (L x , L y ) in the first and second directions are each greater than 100 μm, the thickness (L z ) is less than 50 μm, and the ratio of the thickness to the dimension (L x ) in the first direction or the dimension (L y ) in the second direction is less than 0.1. The piezoelectric element has a first sensitivity (S x ) to deformation in the first direction and a second sensitivity (S y ) to deformation in the second direction. The crystal orientation of the element (PIEZO) is such that abs(S y / S x ) < 0.1, abs((S y +S x ) / S x ) < 0.1, or for at least two first directions (x) of the extending plane (xy) forming an angle of 30° to 60°, abs((S x -S y ) / S x ) < 0.1. The present invention relates to a single-crystal thin-film piezoelectric element (PIEZO).

Description

Technical Field

[0001] The present invention relates to a deformation sensor based on a piezoelectric measurement element, that is, for example, a deformation sensor that utilizes the piezoelectric behavior of a material to convert the deformation received under the influence of an external force into an electrical signal when attached to a surface and the surface is deformed.

Background Art

[0002] A deformation sensor can be used to estimate the surface deformation received by an element of any mechanical system when subjected to an external force (force and moment applied by an external element) in a wide range of applications such as the aerospace industry, machine tool industry, transportation industry, oil and gas industry, food industry, and chemical industry.

[0003] Two technologies are used to create a deformation sensor.

[0004] The first technology is based on a resistive strain gauge. This type of gauge includes a conductive element having a geometry that gives different sensitivities depending on the orientation of the applied deformation, called its orientation. FIG. 1 shows such a conductive element CE. The variation of its electrical resistance under the influence of deformation, combined with its known orientation, makes it possible to infer the amplitude of the deformation and, in some cases, the applied force that generated this deformation. The arrow SA of the "sensitivity axis" and the arrow TA of the "horizontal axis" indicate the maximum sensitivity axis and the minimum sensitivity axis, respectively, with the latter being perpendicular to the former. In practice, the maximum sensitivity axis is aligned with the expected direction of the deformation to be characterized. Resistive strain gauges have good orientation but low sensitivity and require a power supply to operate.

[0005] The second technology is based on a piezoelectric sensor, which is generally more sensitive than a resistive gauge but has low orientation. This type of sensor includes a so-called "piezoelectric" element that generates the appearance of electric charges on its surface under the influence of the applied stress due to the piezoelectric effect. There are several types of such sensors.

[0006] The first type corresponds to a deformation sensor with a rigid housing HOUS that houses a crystal Q, such as piezoelectric quartz, as shown by the sensor DynSens in Figure 2, where A) is a cross-sectional view of the sensor and B) is a top view of the sensor. The function of the rigid housing is to limit the amplitude of deformation of the crystal, which is relatively thick and cannot withstand excessive deformation without breaking. The crystal is integral with the walls of the housing, designed to be mounted on a flat, smooth surface using a high-rigidity adhesive. The housing can also house an amplifier AMP connected to the crystal and the outside via electrical connection wires CAB. Axes TA and SA are defined in the same manner as the resistance gauges in Figure 1. An example of application is given by Fabio LMdos Santos et al. "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] The second type is the piezoelectric surface deformation sensor. The primary advantage of these sensors is that they are passive and therefore do not require a power source. Such sensors comprise a piezoelectric element, typically a quartz crystal oscillator, and a rigid, relatively bulky mechanical structure that restricts the deformation of the piezoelectric element in one direction. These sensors are generally screwed onto a flat surface, as disclosed, for example, in U.S. Patent No. 4,314,481(A). The use of a rigid, relatively bulky mechanical structure requires a geometry adapted to its mounting configuration, which inherently affects the measurements performed and severely limits the application area of ​​this type of sensor.

[0008] The rigid housing and mechanical structure of these first two types of transducers greatly limit the range of deformations to which they can be applied.

[0009] A third type of composite structure, known as a "microfiber composite," is a structure in which a bar of PZT (i.e., lead zirconate titanate) is sandwiched between a sheet of polymer material and a comb-shaped electrode. These have the advantage of directionality achieved by the geometry of the PZT bar and greater flexibility than the piezoelectric sensors described above, but they have a complex structure and require polarization obtained by applying an electric field. See conference paper Wilkie, WKet al. "Low-cost piezocomposite actuator for structural control applications," Proceedings of the SPIE, Volume 3991, pp.323-334 (2000). Disadvantages of such sensors include temperature dependence due to the presence of the polymer material portion and insufficient flexibility for specific applications.

[0010] The fourth type is a deformation sensor based on a piezoelectric polymer film, named "PVDF" after the material used to form the piezoelectric film, namely piezoelectric polyvinylidene fluoride (PVDF). These sensors are flexible and lightweight, but must be polarized, inaccurate, non-directional, and subject to drift over time, particularly due to a lack of polarization stability or with temperature. See U.S. Patent Application Publication 2021 / 0102850(A1).

[0011] The fifth type is a vibration sensor based on a thin layer of piezoelectric ceramic supported by a flexible support, as described in French Patent No. 3,122,985. This sensor is flexible but non-directional and is not intended for quantitative measurement of strain or stress.

[0012] Despite the wide variety of devices designed to perform the functions of passive deformation sensors, a device that combines directionality, sensitivity, adaptability, and lightness to enable accurate quantitative measurements over a large deformation range does not appear to be available.

[0013] [Disclosure of the Invention] The object of the present applicant is to provide a deformation sensor device that combines the advantages individually found in various existing deformation sensors based on the use of thin piezoelectric elements.

[0014] To achieve this object, a first aspect of the present invention is a single-crystal thin piezoelectric element in the form of a plate extending in an extending plane defined by a first direction and a second direction perpendicular to the first direction, wherein the dimensions in the first direction and the second direction are each greater than 100 μm, the thickness is less than 50 μm, the ratio of the thickness to the dimension in the first direction or the dimension in the second direction is less than 0.1, and the piezoelectric element has a first sensitivity S to deformation in the first direction and a second sensitivity S to deformation in the second direction y and the crystal orientation of the element corresponds to a so-called "one-direction" sensitivity, abs(S y / S x ) < 0.1, corresponds to a so-called "two-direction" sensitivity, abs((S y +S x ) / S x ) < 0.1, or corresponds to a so-called "omnidirectional" sensitivity for at least two first directions of the extending plane forming an angle of 30° to 60°, abs((S x -S y ) / S x ) < 0.1, and is such a single-crystal thin piezoelectric element.

[0015] Such thin piezoelectric elements are suitable for forming the basis of passive deformation sensors that can combine accuracy, sensitivity, compatibility, flexibility, light weight, stability, linearity, directivity, and applicability for a wide range of deformations.

[0016] This piezoelectric element can measure deformations exceeding 5,000 micrometers per meter with a deformation resolution on the order of 1 nanometer per meter. These figures are comparable to those of conventional resistance deformation gauges, which can measure deformations up to 12,000 micrometers per meter but have a much lower resolution of 1 micrometer per meter, or to piezoelectric built-in deformation gauges, which can only measure deformations limited to approximately 300 micrometers per meter with a resolution of 1 nanometer per meter.

[0017] This thin piezoelectric element can provide a pair of conductive layers located on two opposing surfaces of the thin piezoelectric element.

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

[0019] According to additional non-limiting features of the sensor according to the present invention, individually or in any technically feasible combination: - At least one thin piezoelectric element can be enclosed between one flexible sheet and another flexible sheet; - The sensor may include at least one charge amplifier connected to at least one thin piezoelectric element; - At least one charge amplifier can be integrated into the flexible foil; - The sensor may comprise multiple thin piezoelectric elements according to the present invention, oriented in different directions at least 30° apart; - The sensor may comprise first, second, and third thin piezoelectric elements, each having characteristic abs(S y / S x The first direction of the second thin piezoelectric element may be at an angle of 90° with the first direction of the first thin piezoelectric element, and the first direction of the third thin piezoelectric element may be at an angle of 45° with the first direction of the first thin piezoelectric element; - The sensor may comprise first, second, and third thin piezoelectric elements, each having characteristic abs(S y / S x The first direction of the second thin piezoelectric element may have a value of <0.1, the first direction of the second thin piezoelectric element may be at an angle of 120° with the first direction of the first thin piezoelectric element, and the first direction of the third thin piezoelectric element may be at an angle of 240° with the first direction of the first thin piezoelectric element; - The sensor may comprise multiple charge amplifiers, each connected to a thin piezoelectric element; and, - The sensors can be a combination of at least two sensors according to the present invention that are electrically connected in parallel.

[0020] The present invention also extends to a device compatible with at least one of the deformation sensors according to the present invention. The device may be a ring compatible with at least two unidirectional or omnidirectional deformation sensors and configured to act as a man / machine interface, or a smartphone equipped with at least one deformation sensor and configured to alert the user when the deformation measured based on the signal from at least one sensor exceeds a predetermined threshold, or to act as a man / machine interface. [Brief explanation of the drawing]

[0021] Other features and advantages of the present invention will become apparent from the following detailed description of the invention with reference to the accompanying drawings. [Figure 1] A known resistance-deformation gauge is shown. [Figure 2] A known piezoelectric deformation sensor is shown. [Figure 3] This shows a thin piezoelectric element. [Figure 4] This shows the rotation used in in-plane sensitivity calculations for a given crystal orientation in a single crystal. [Figure 5] This shows a polar coordinate diagram of the deformation sensitivity of a thin piezoelectric element due to crystallographic orientation. [Figure 6] Figure 5 shows the sensitivity diagram. [Figure 7] This demonstrates the sensitivity of a thin piezoelectric element to complete unidirectional deformation. [Figure 8] This shows the deformation sensitivity and stress sensitivity of the same thin piezoelectric element. [Figure 9] This shows a deformation sensor incorporating a thin piezoelectric element. [Figure 10] This shows a load cell equipped with a thin piezoelectric element exhibiting the first behavior. [Figure 11] This demonstrates the application of a sensor based on a thin piezoelectric element that exhibits a second type of behavior. [Figure 12] This demonstrates the application of a sensor based on a thin piezoelectric element that exhibits a third type of behavior. [Figure 13] The first piezoelectric sensor is shown. [Figure 14] The second piezoelectric sensor is shown. [Figure 15] This shows the third piezoelectric sensor. [Figure 16] The fourth piezoelectric sensor is shown. [Figure 17] The first device, adapted to the deformation sensor, is shown. [Figure 18] This shows a second device adapted to the deformation sensor. [Modes for carrying out the invention]

[0022] The object of the present invention is to provide a piezoelectric element suitable for use in sensors that combine the advantages of the various types of sensors described above, particularly with respect to sensitivity and directionality, and with respect to accuracy, adaptability, flexibility, lightness, stability, linearity, directionality, and applicability over a wide range of deformations. The following sections describe how the inventors succeeded in determining the properties required for the piezoelectric element so that it can provide such benefits when integrated into a sensor.

[0023] Disclosure of the invention The behavior of piezoelectric materials is governed by three tensors, namely the dielectric constant tensor ε at a constant stress. T , the compression tensor s in a constant electric field EIt can be described using the piezoelectric tensor d. Voigt notation is used in this book.

[0024] In a material, such as a piezoelectric single crystal, the deformation tensor S and electrical displacement D are related to the electric field E and stress tensor T by the following equations 1 and 2, known as the "piezoelectric equations".

[0025]

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[0026] The approach of studying the directionality of piezoelectric behavior in piezoelectric crystals by analyzing the directionality of the piezoelectric tensor d is well known in the literature, as can be seen, for example, in the paper W. Yue and J. Yi-jian, "Crystal orientation dependence of piezoelectric properties in LiNbO3 and LiTaO3", Opt. Mater, vol. 23, No. 1, pp. 403-408, July 2003.

[0027] Tensor d represents the charge generated by a piezoelectric material when a stress represented by tensor T is applied to the piezoelectric material. Typically, the coefficient d of the piezoelectric tensor d is... 33 The analysis gives the charge generated on these surfaces in response to stress components perpendicular to the surfaces of the piezoelectric element. Coefficient d 31 This provides loads generated on these same surfaces in response to stress components parallel to these same surfaces.

[0028] The material behavior with respect to different crystal orientations, or the orientation of the piezoelectric element relative to the crystal axis from which it originates, is determined by the relevant transformations, using the tensor (d,ε) that defines the material behavior. T ,s E This can be obtained by applying it to ( ). For example, see U.S. Patent No. 4,314,481(A).

[0029] However, the inventors of this invention have chosen a different approach from the analyses found in the literature. More specifically, instead of focusing on the response of a piezoelectric element to a given stress, the inventors are interested in the charge generated by the piezoelectric element in response to deformation. As shown below, this approach leads to the investigation of specific crystal orientations and presents interesting directional properties for unidirectional, bidirectional, or omnidirectional deformation detection.

[0030] Such an approach has practical applications, for example, in the case of piezoelectric elements that are thin enough to form adaptable deformation sensors, i.e., piezoelectric elements that can conform to the shape of the surface on which they are applied. Single-crystal material elements that are too thick are too hard and brittle for such applications.

[0031] Thin piezoelectric element - Modeling We consider thin piezoelectric elements made of single crystals. By considering thin piezoelectric elements, equations 1 and 2 can be simplified by making the following two assumptions.

[0032] Firstly, the dimensions of the thin piezoelectric element considered in its extending plane are much larger than its thickness. Therefore, the thin element can be described as having a plate shape. We consider a thin piezoelectric element extending in the xy plane, hereafter referred to as the "extending plane," defined by two directions x and y, which are perpendicular to each other and perpendicular to a third direction z. As shown in Figure 3, the dimensions L in the x, x, and z directions define an orthogonal reference frame. X , L Y , and L Z We consider a thin parallelepiped element having L (scale is not considered for clarity of expression). x and L y is L Z Thin elements are considered that satisfy the condition that they are at least 10 times, preferably at least 100 times, larger than (the dimension corresponding to the thickness of the thin element).

[0033] Secondly, unidirectional deformation along one direction of the extending surface is considered.

[0034] A thin piezoelectric element, where the dimensions of the extending surface are much larger than its thickness (Lx and Ly > 10 Lz, preferably Lx and Ly > 100 Lz), and where two conductive layers are provided on both sides parallel to the extending surface, electrically behaves like a planar capacitor, with its two conductive armatures corresponding to surfaces perpendicular to the z-direction, labeled Top and Bot, respectively. In this capacitor, the electric field is perpendicular to the armature, and the electric field has no component in the xy-plane. When a thin piezoelectric element is used in a charge-mode sensor, electrodes spanning two surfaces parallel to the extending surface are connected to a charge amplifier that forms a low-impedance path between the two electrodes. As a result, the voltage between the two electrodes is zero, and the electric field in the z-normal direction is also zero. In conclusion, when connected to a charge amplifier, there is no electric field in the thin piezoelectric element.

[0035] Equations 1 and 2 can then be simplified to equations 3 and 4, which are sufficient to describe the piezoelectric behavior of the thin element under the assumptions specified above.

[0036]

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[0037] Sensitivity research of thin-film devices In thin layers of material, stress in the normal direction is often lower than stress in the plane of the material. Therefore, it is common to use the plane stress approximation to model material behavior by assuming that stress perpendicular to the material is zero. This approximation greatly simplifies the equations of behavior and mechanical equilibrium.

[0038] In this case, this approximation is used with a thin piezoelectric element (Lx and Ly > 10Lz) where, as in actual practical applications, one of its faces is attached to a structure, the opposing face is free, and it is in a stressed state on its extending surface xy, with the stress in the normal direction z being considered relatively negligible. In this situation, the piezoelectric equation can be further simplified, with components T3, T4, and T5 of the stress tensor T being zero, and the piezoelectric tensor d being a 3x3 matrix of 9 components.

[0039]

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[0040] It can be reduced to this size.

[0041] Therefore, deformation

[0042]

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[0043] When applied, it can be shown that the electrical displacement generated by a thin piezoelectric element in charge mode can be expressed by equation Eq. 5.

[0044]

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[0045] During the ceremony,

[0046]

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[0047] This is the complexity matrix c E This represents the reduced complexity matrix obtained by deleting rows and columns 3, 4, and 5 of the matrix.

[0048]

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[0049] This represents the reduced stress tensor obtained by removing components 3, 4, and 5 from the stress tensor T.

[0050] For uniaxial deformation in direction s, the electrical displacement D is expressed by equation Eq. 6.

[0051]

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[0052] Equation 6 shows that the electrical displacement caused by uniaxial deformation in a thin piezoelectric material is not only due to the piezoelectric tensor d after rotation, but also to the reduced compression tensor after rotation.

[0053]

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[0054] This also indicates that it depends on [another factor]. Analyzing only the post-rotation piezoelectric tensor d, as is commonly done in the literature, is insufficient to characterize the sensitivity of a thin piezoelectric element to deformation applied in a given direction.

[0055] Only the third component D3 of the electrical displacement tensor is of interest, as this is a quantity that is actually measured during the practical use of thin piezoelectric elements by a pair of electrodes located on the surface of the element parallel to the extending plane xy.

[0056] Sensitivity S x This is obtained by integrating the component D3 of electrical displacement across the entire surface of a thin piezoelectric element covered on both sides by electrodes, and is the sensitivity calculated for uniaxial deformation.

[0057]

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[0058] This is S x The value given is =A × D3 / S1, where A is the surface area of ​​the thin piezoelectric element covered on both sides by electrodes. Therefore, the sensitivity S is x This is expressed as pC / (μm / m), and represents the charge generated by a piezoelectric element when a deformation of 1 μm / m is applied in direction x.

[0059] Sensitivity-Directivity of Thin Single Crystal Devices The objective here is to evaluate the directional sensitivity of a thin single-crystal device to a given crystal orientation of a bulk single crystal. The crystal orientation is considered to be that defined in the IEEE standard on piezoelectricity, "IEEE Standard on Piezoelectricity," ANSI / IEEE Std 176-1987, 1988.

[0060] The directivity of a single-crystal piezoelectric thin-film device corresponds to the IEEE(YXwl)q1 / q2 definition, and is shown in Figure 4, with a rotational sequence R of the thin-film device relative to the crystal axis. t By applying this, it can be calculated analytically.

[0061] Rotation sequence R t is, R x (90), R y (q1), R x (q2), and R z It consists of four consecutive rotations in the order of (q3).

[0062] First rotation R x (90) is performed at a 90° angle around the X-axis and converts the xyz reference frame specific to the thin element to an x'y'z' reference frame.

[0063] Second rotation R y (q1) is performed by rotating the x'y'z' reference frame by an angle q1 around the y' axis, thereby transforming it into an x''y''z'' reference frame.

[0064] Third rotation R x(q2) is performed by rotating the x''y''z reference frame by an angle q2 around the x'' axis, thereby transforming it into an x'''y'''z''' reference frame.

[0065] Fourth rotation R z (q3) is performed by rotating by an angle q3 around the z''' axis to transform the x'''y'''z''' reference frame into an x''''y''''z'''' reference frame.

[0066] Each of the xyz, x'y'z', x''y''z'', and x'''y'''z'''' reference frames has four rotations R x (90), R y (q1), R x (q2), and R z (q3) is defined by three directions x, y, and z, x', y', and z', x', 'y'', and z'', and x''', y''', and z''', respectively, by applying (q3) sequentially to a piezoelectric element whose intrinsic directions x, y, and z initially correspond to the intrinsic directions X, Y, and Z of the solid single crystal under consideration.

[0067] The first three rotations R x (90), R y (q1), and R x (q2) is used to define the crystal orientation of the thin element in the bulk crystal. Fourth rotation R z (q3) is used to study the sensitivity of the crystal orientation to the plane defined by the first three rotations.

[0068] In this way, the charges generated on opposing surfaces Top and Bot are calculated according to the orientation of the crystal axis, defined by the angles q1, q2, and q3 with respect to the uniaxial deformation in the direction x''' of the solid single crystal. From this load calculation, the sensitivity S to deformation is calculated. x The equation = A × D3 / S1 is inferred. In this way, it is possible to test the sensitivity of all possible crystal orientations of a massive single crystal.

[0069] Calculations performed by scanning the values ​​of q1 and q2 for each value of q1 show that, for a given value of q1 and depending on the value of q2, the generated load varies considerably with respect to the angle q3. For a particular pair of values ​​(q1, q2), approximately the same amount of charge is generated, regardless of the value of q3. For other pairs of values ​​(q1, q2), the generated charge can be negative, positive, or zero, depending on the value of q3. For yet another pair of values ​​(q1, q2), the generated charge is zero, regardless of the value of q3.

[0070] Figure 5 shows a polar coordinate diagram representing the amount of charge generated on the top and bottom surfaces of a thin lithium tantalate element in response to deformation, and therefore the sensitivity to this deformation. The sensitivity is calculated here for an angle q1 set to 7°, and for angles q2 of 10°, 90°, 118°, and 159°, corresponding to one of the curves shown in the figure, respectively, where angle q3 is the polar angle in the figure.

[0071] The polar coordinate diagram in Figure 5 allows for the inference of the sensitivity ratio between the "transverse" sensitivity, considered along the direction perpendicular to the direction corresponding to the maximum sensitivity, and the "longitudinal" sensitivity, considered along the direction of maximum sensitivity, for various crystal orientations defined by q1 and q2, in ratios of 55%, 100%, 0%, and -99% for q2 values ​​of 10°, 90°, 118°, and 159°, respectively.

[0072] When q2 = 159°, four lobes are formed on the polar coordinate diagram, and the ratio of the lateral sensitivity (q3 = 95°) to the longitudinal sensitivity (q3 = 5°) is close to 1 in absolute value, with a ratio of -99%. This indicates that a piezoelectric sensor incorporating a thin piezoelectric element with a crystal orientation defined by angles q1 = 7° and q2 = 159° becomes a bidirectional sensor that is particularly sensitive to deformations occurring in two directions perpendicular to each other.

[0073] When q2 = 118°, only two lobes are formed on the polar coordinate diagram, and the ratio of lateral sensitivity (q3 = 8°) to longitudinal sensitivity (q3 = 98°) is zero. This means that a piezoelectric sensor incorporating a thin piezoelectric element with a crystal orientation defined by angles q1 = 7° and q2 = 118° will be highly directional, or even completely unidirectional, and particularly sensitive to deformation occurring in a single direction.

[0074] When q2 = 10°, no lobe is formed, and the curve representing the sensitivity depending on the value of q3 forms a substantially circular shape, with a sensitivity ratio of 1 and a sensitivity ratio of 100%. This means that a piezoelectric sensor incorporating a thin piezoelectric element with a crystal orientation defined by angles q1 = 7° and q2 = 90° is omnidirectional and can detect deformation with the same sensitivity regardless of the orientation of the deformation on the extending surface of the thin piezoelectric element.

[0075] When q2 = 90°, no lobe is formed, and the curve representing sensitivity depending on the value of q3 forms a pattern without any particularly remarkable features. Such a crystal orientation does not have any obvious applications for forming strain sensors.

[0076] Figure 6 shows the four sensitivities shown in Figure 5, but this time they are on line graphs and represent sensitivities based on an angle q3 between 0° and 180°, which represents the orientation of the deformation imposed on the extending surface of the thin piezoelectric element with respect to the x-axis.

[0077] For an angle q2 of 159°, there are two directions perpendicular to each other, each with high sensitivity, and these sensitivities are approximately equal in absolute value but opposite in sign (bidirectional sensitivity).

[0078] For an angle q2 of 118°, there is a direction of high sensitivity, while the sensitivity in the normal direction is zero (unidirectional sensitivity).

[0079] When the angle q2 is 90°, the sensitivity is approximately constant in all orientations and in this case has a negative sign (omnidirectional sensitivity).

[0080] For an angle q2 of 10°, the sensitivity is always negative, variable, and there are no a priori significant points.

[0081] Each of the above behaviors, but primarily the first three, can be used to design sensors for specific applications, as described below.

[0082] By scanning different values ​​of q1 and q2 and evaluating the directivity of the thin piezoelectric element as described above, it is possible to determine the crystal orientation for forming a thin piezoelectric element with a particularly interesting deformation sensitivity ratio.

[0083] In particular, sensitivity S X and S Y This can be considered in two directions of the extending plane that are perpendicular to each other. In Figure 5, for example, S X This is defined as the sensitivity in the direction of the extending surface defined by q3 = 7°, and S Y This can be considered as the sensitivity in the direction of the extending surface, defined by q3 = 97°, which is known as "lateral sensitivity". Of particular interest is (i) the lateral sensitivity S close to 0. Y and non-zero sensitivity S X (ii) A so-called "unidirectional" sensitive surface having (ii) all orientations within the surface, with respect to each other's sensitivities S X and S Y (iii) Sensitivity surfaces S having "omnidirectional" sensitivity, and (iii) absolute values ​​close to each other and opposite signs X and S Y It is a "bidirectional" sensitive surface having an orientation that includes [a specific characteristic].

[0084] One criterion for qualitatively defining the behavior of a thin piezoelectric element as unidirectional is its sensitivity S to deformation in its extension plane (corresponding to its crystal orientation in the bulk single crystal from which it originates) in two mutually perpendicular directions, first and second directions. x and S y The absolute value of the ratio (i.e., in mathematical notation, abs(S) Y / S X ) or |S y / S xThe percentage of |) is less than 10%, preferably less than 5%, more preferably less than 2%, even more preferably less than 1%, and most preferably less than 0.1%.

[0085] Similarly, a criterion for qualitatively defining the behavior of a thin piezoelectric element as bidirectional is the absolute value of the ratio of the sum of the piezoelectric sensitivities to deformation in a first and second direction perpendicular to each other to the piezoelectric sensitivities to deformation in the first direction (i.e., mathematically expressed as abs((S) X +S Y ) / S X ) or |(S x +S y ) / S x The percentage of |) is less than 10%, preferably less than 5%, more preferably less than 2%, even more preferably less than 1%, and most preferably less than 0.1%.

[0086] The behavior of a thin piezoelectric element is determined by the deformation sensitivity S in both a first direction and a second direction perpendicular to the first direction, for all orientations on its extending surface (corresponding to the crystal orientation in the bulk single crystal from which it originates). x and S y The difference and sensitivity S x The ratio of (or, in mathematical notation, abs((S x -S Y ) / S X ) or |(S x +S y ) / S x If the percentage of |) is less than 10%, preferably less than 5%, more preferably less than 2%, even more preferably less than 1%, and most preferably less than 0.1%, it can be described as omnidirectional. From a practical standpoint, thin elements are abs((S x -S Y ) / S X Omnidirectional behavior is considered to occur when the criteria for the value of ) are met for the first two directions that form an angle between 30° and 60°, preferably 40° and 50°.

[0087] The method developed above for calculating sensitivity to deformation is used to determine the crystal orientation defined by the angles q1 and q2 having completely unidirectional sensitivity, and within the accuracy of the calculation, the ratio S Y / S X there is a direction q3 for which is exactly zero. Figure 7 shows such a case, showing the strain-sensitivity to deformation in a q3-dependent polar plot for a lithium tantalate crystal orientation defined by the angles q1 = 7° and q2 = 118°. The same approach can be employed for omnidirectional or bidirectional sensitivity.

[0088] Note that for a given crystal orientation, the fact that the stress sensitivity behaves completely unidirectionally does not mean that the deformation sensitivity is also completely unidirectional. Figure 8 shows the calculation results of the stress-sensitivity Stress-Sens and the deformation-sensitivity Strain-Sens for lithium tantalate crystal orientations defined by the angles q1 = 7° and q2 = 110° respectively. The ratio of the stress sensitivity (S Y / S X ) stress is zero, and even when there is an angle q3 that exhibits completely unidirectional behavior in response to the applied stress, the ratio of the deformation sensitivity (S Y / S X ) deformation is close to 18%, meaning that the deformation sensitivity cannot be regarded as unidirectional.

[0089] Unlike the conventional approach that focuses on the applied stress, the above method that focuses on the deformation applied to the thin piezoelectric element enables accurate selection of a crystal orientation suitable for the desired behavior in response to unidirectional deformation. This can be made unidirectional, omnidirectional, or bidirectional according to practical applications.

[0090] The above example was obtained for lithium tantalate LiTaO3 in single-crystal form, belonging to space group 3m. However, this method can be applied to all space groups of piezoelectric materials. Other materials of interest include, but are not limited to, lithium niobate LiNbO3 (group 3m), lead magnesium niobate MgNb2(PbO3)3 (group P1), aluminum nitride AlN (group P63mc), barium titanate BaTiO3, potassium niobate KNbO3, and lead titanate TiPbO3 (all three belonging to group P4mm).

[0091] Applications - Sensors Figure 9 shows a cross-sectional view of a piezoelectric sensor SENS based on a thin piezoelectric element PIEZO, which has an extending surface selected to exhibit a specific unidirectional, omnidirectional, or bidirectional behavior in response to a unidirectional deformation applied to its extending surface. The sensitivity behavior (unidirectional, omnidirectional, or bidirectional) of the thin piezoelectric element is transmitted to the sensor incorporating this thin piezoelectric element. Such sensors can be particularly well-suited to specific situations, as shown in Figures 10 to 12, which will be discussed below, but can also be used for more general applications, as will become clear later.

[0092] To benefit from the thinness, and therefore flexibility and adaptability, of the thin element PIEZO, the sensor SENS comprises a flexible sheet SH1 to which the thin element PIEZO is mounted. The sheet is preferably made of a flexible material selected to suit the intended application, and can be made of, for example, metal, polyvinyl chloride (PVC), polyimide (PI), polyethylene terephthalate (PET), biaxially oriented polyethylene terephthalate (Mylar®), or a composite material of epoxy resin and glass fiber. The thin element PIEZO can be mounted to the sheet SH1 by a flexible adhesive such as an anisotropic conductive film (ACF), which further enables electrical contact as described in French Patent No. 3,122,985. When in use, the sensor SENS can be mounted to a characteristic surface using an adhesive such as a cyanoacrylate adhesive or epoxy resin.

[0093] In addition to the thin PIEZO element, in the example shown in Figure 9, a charge amplifier C.AMP is also mounted on sheet SH1 and functionally connected to two conductive layers EL1 and EL2, which act as electrodes and are formed on the two opposing surfaces of the thin PIEZO element, respectively. The function of the charge amplifier is to generate a voltage corresponding to the charge applied to the input, which corresponds to the charge generated by the PIEZO element when it is deformed, for the purpose of electronically processing the generated potential and performing effective measurement of 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. Alternatively, the external measuring device can be directly connected to electrodes EL1 and EL2, or the amplifier can be connected between the sensor SENS and the external measuring device. See also French Patent No. 3,122,985, the teachings of which are applicable to the purposes of this specification.

[0094] Figure 9 shows a sensor (B) that is similar in configuration to (A), but further includes a second sheet SH2 which may have the same properties as sheet SH1, and encloses a thin PIEZO element and a charge amplifier C.AMP by sandwiching them between sheets SH1 and SH2.

[0095] The PIEZO thin piezoelectric element is preferably less than 50 μm thick, more preferably less than 25 μm thick, and even more preferably less than 10 μm thick. Considering a piezoelectric element as defined in Figure 3, the thickness of the PIEZO element and its dimension L in a first direction of its extending surface are given. X The ratio of dimension Ly in the second dimension of its extending surface perpendicular to direction Lx is less than 0.1, preferably less than 0.05, and more preferably less than 0.01.

[0096] The SH1 sheet, and the SH2 sheet where applicable, can have a thickness of 5 to 300 μm.

[0097] However, it is preferable that the piezoelectric sensor SENS as a whole be sufficiently flexible to follow the deformation of the object to be characterized on which it is attached. Those skilled in the art can determine the characteristics of the thin piezoelectric element PIEZO, its support, and other elements such as means for making electrode layers or electrical contacts for each application.

[0098] As an alternative to using flexible sheets SH1 and SH2 to support and protect the thin piezoelectric elements, a more rigid support such as a rigid metal housing could be used; however, this would not allow for the utilization of the flexible and conformable aspects of the thin piezoelectric elements as described herein.

[0099] Sensors based on thin piezoelectric elements that exhibit unidirectional behavior, and therefore unidirectional behavior itself, can be used to characterize the deformation of mechanical parts that are subjected to deformation in multiple directions, but are looking for components of these deformations in a particular direction. It is also possible to determine the expected deformation direction of a mechanical part and, in particular, to characterize this expected deformation. One advantage of sensors based on elements with unidirectional behavior is that the system does not measure disturbances arising from parasitic external forces to which it may be exposed in its environment. For example, Figure 10 shows a stationary load cell CELL equipped with four sensors SENS in (A), each sensor based on a thin piezoelectric element with unidirectional behavior, oriented according to the expected deformation of the cell and precisely designed to be subjected to deformation in a preferred direction. Under a load due to the application of force F, the load cell deforms as shown in (B), with two of the four transducers in a compressed state Comp and the other two in an extended state Ext.

[0100] When the mechanical component in question is subjected to radial deformation, and / or the deformation is the same in the axial and transverse directions, for example, in the case of a film subjected to fluctuations in sound pressure whose deformation is to be characterized, a thin piezoelectric element with omnidirectional behavior can be used. Figure 11 shows such a situation, for example, with a rigid circular frame ARM mounted around a film MEM that can be vibrated by sound waves. Here, the film can consist of a thin piezoelectric element placed on a flexible support.

[0101] A thin piezoelectric element exhibiting bidirectional behavior is, for example, a cylindrical beam at its end, which exhibits a pair of opposing forces FT around the longitudinal axis of the beam. + and FT -It can be used when subjected to (torsion). In this situation, each point on the beam surface is simultaneously subjected to compression in a first direction (Comp) and extension in a second direction perpendicular to the first direction (Ext), as shown in Figure 12. In such a situation, a sensor fitted with a thin piezoelectric element having bidirectional behavior, positioned with its x and y axes aligned with the compression and extension directions, respectively, is suitable for efficient deformation detection, enabling the force torque acting on the beam to be traced by a single sensor. In fact, in light of the characteristics of a sensor having sensitivity to deformations of opposite signs in directions perpendicular to each other, and the characteristics of deformation that extends in the first direction and compresses in the second direction perpendicular to the first direction, the sensor can be sensitive to both directions of deformation when suitably positioned on a cylindrical beam. In the case of similar characterization by conventional means, two resistance strain gauges are required because they are mainly unidirectionally sensitive.

[0102] In the examples shown in Figures 10, 11, and 12, the flexibility of the thin piezoelectric element is advantageously utilized to mount one entire surface of it in close contact with the curved surface of the mechanical part under study. In the example in Figure 11, the deformability provided by the flexibility of the thin piezoelectric element is particularly well utilized.

[0103] The geometric shapes described above represent the basic configuration. Multiple PIEZO elements can also be combined to form a sensor. Figures 13, 14, and 15 show several possible configurations for a sensor that combines thin piezoelectric elements, each having unidirectional behavior along a direction specific to Sens_Dir. Each of these sensors has a structure similar to that of the sensor SENS shown in Figure 9, and the thin piezoelectric elements of these sensors share the same support Supp on which they are placed, which is sometimes formed by the flexible sheet SH1 of the sensor SENS. Thus, each of the thin elements can be connected to a dedicated amplifier, which may be integrated on the support Supp, as with the sensor SENS described above. The amplifier can be a charge amplifier.

[0104] Figure 13 shows sensor SENS_1 comprising two thin piezoelectric elements PIEZO_0° and PIEZO_90° mounted on the same support Supp and having respective directions Sens_Dir oriented at 90° relative to each other. While this example shows a configuration with two thin piezoelectric elements having a specific orientation relative to each other, the present invention is not limited to this particular configuration, and the number and orientation of the thin piezoelectric elements can be adapted to the requirements of any application, as can be determined by those skilled in the art, and is not limited to this particular configuration.

[0105] Figure 14 shows sensor SENS_2, which comprises three thin piezoelectric elements PIEZO_0°, PIEZO_90°, and PIEZO_45° mounted on the same support Supp, with the directions Sens_Dir of PIEZO_90° and PIEZO_45° tilted by 90° and 45°, respectively, relative to the direction Sens_Dir of PIEZO_0°. Additionally, each of the three thin piezoelectric elements confirms a characteristic abs(Sy / Sx) < 0.1, i.e., exhibits unidirectional behavior.

[0106] Figure 15 shows sensor SENS_3, which comprises three thin piezoelectric elements PIEZO_0°, PIEZO_120°, and PIEZO_240° mounted on the same support Supp, with the direction Sens_Dir of PIEZO_120° and PIEZO_240° tilted by 120° and 240°, respectively, with respect to the direction Sens_Dir of PIEZO_0°. Additionally, each of the three piezoelectric thin-film elements confirms a characteristic abs(Sy / Sx) < 0.1, i.e., exhibits unidirectional behavior.

[0107] Sensor configurations SENS_2 and SENS_3 realize the well-known principle of combining resistance strain gauges (see Figure 1) to form a so-called strain gauge rosette, which can be used to characterize deformation even when the deformation of the surface of a mechanical part is not unidirectional. While the principle of arranging thin elements to efficiently trace back measurements to deformation components is well known, by replacing the resistance gauges, which have excellent directionality, with the thin piezoelectric elements having unidirectional behavior according to this specification, it becomes possible to achieve a sensitivity to deformation that would not normally be obtainable in equivalent applications. This replacement is made possible by determining a specific crystal orientation that gives the thin piezoelectric element according to the present invention unidirectional behavior.

[0108] While it is convenient to use specific orientations (0°, 45°, 90°, 120°, 240°) for the thin piezoelectric elements forming the rosette, there is no reason in principle to be limited to specific configurations of sensors SENS_2 and SENS_3; any relative orientation can be chosen as long as two of the sensors are not oriented parallel to each other. When describing these specific orientations, it should be understood that the values ​​given are optimal values, and the values ​​on the actual device may differ slightly from these optimal values. For example, the effective angle between orientations can be given with an accuracy of up to ±10°, preferably ±5°, and even more preferably ±1°.

[0109] More generally, based on studies of deformation of thin piezoelectric elements, it becomes possible to define crystal orientations that result in perfectly selected and controlled deformation-responsive behavior by determining the crystal orientation through the approaches detailed above. When implemented in deformation sensors, such thin piezoelectric elements offer great flexibility to those skilled in the art, enabling the development of sensors that are perfectly suited to the targeted application.

[0110] Sensor integration One possibility for using the thin piezoelectric elements according to the present invention is their coupling, that is, configuring them such that the charges generated by at least two piezoelectric elements during their use are summed, for example, by electrically connecting their respective electrodes. One application could be the correction of their directional errors.

[0111] For example, consider an omnidirectional sensitive crystal orientation where abs(Sx-Sy) / Sx) < 10%. Two piezoelectric sensors with this crystal orientation, PIEZO'_0° and PIEZO'_90°, known as "basic sensors," can be employed and integrated onto the same support by orienting them at 90° to each other, and they can be electrically connected in parallel. In such a configuration, the two electrodes EL1 are connected to each other and to the connection surface PAD1 by a connecting element C1, and the two electrodes EL2 (not shown in Figure 16, see Figure 12) are connected to each other and to the connection surface PAD2 by a connecting element C2.

[0112] The resulting output of the "composite sensor" is the sum of the charges generated by the two sensors. The advantage is that the coupling thus achieved in the composite sensor compensates for the directional error of up to 10% of each of the two individual basic sensors, making the composite sensor completely omnidirectional.

[0113] The same principle can be applied to other systems; for example, by applying it to several imperfect unidirectional basic sensors, a fully directional composite sensor can be obtained, and the basic sensors can be of different sizes and relative orientations, determined to compensate for the directional imperfections of these sensors.

[0114] Exemplary Uses Figure 17 shows a perspective view of a first application example having a ring R equipped with three sensors SENS according to the present invention (A). These sensors are preferably unidirectional or omnidirectional.

[0115] This instrumentation of the ring transforms it into a man / machine interface for controlling electronic devices such as computers, smartphones, televisions, or multimedia players. Three piezoelectric deformation sensors SENS are coupled to the ring, and the ring incorporates the electronic device EL and, if necessary, the power supply required to measure and analyze the charges generated by the sensors and wirelessly transmit the information to external devices such as those mentioned above.

[0116] As shown in (B) of [Figure 17], when the ring R is grasped between two fingers F1 and F2, a compressive force F is applied by the fingers to both sides of the ring, creating a local deformation that can be measured using three sensors SENS. When the ring is rotated between the fingers in a rotational movement as shown in (C), the position where this force is applied shifts, and this shift can be measured by comparing the signals from the three sensors. In this way, actions such as (i) a single press due to pressure between the fingers associated with a verification action, (ii) a double press associated with a backward action in a menu of an application controlled by the ring, and (iii) a unidirectional or bidirectional rotation associated with controlling scrolling up or scrolling down in a list or controlling the volume of a music player can be detected and associated with a predetermined action.

[0117] Figure 18 shows a second application example using a smartphone SMART equipped with four thin piezoelectric sensors SENS according to the present invention.

[0118] A first application of such a system addresses the problem that smartphones can be damaged when subjected to excessive deformation, which typically occurs when a smartphone is placed in a back pocket and the user sits down. By incorporating one or more sensors SENS according to the present invention into a smartphone, it becomes possible to continuously measure the deformation of the phone and to warn the user by a visual or auditory message if the deformation exceeds a predetermined threshold that could damage the smartphone. In this example, the sensor SENS is mounted on the rear RF designed to cover the body MB of the smartphone SMART and is connected to the smartphone's data processing system. The four sensors are arranged to measure deformation along two principal axes (defined by the directions of the long and short sides of the smartphone), as well as torque applied to the phone in all directions. The sensors are preferably unidirectional, with two sensors oriented parallel to the long sides of the smartphone and the other two oriented parallel to the short sides of the smartphone.

[0119] In a second application example of the system shown in Figure 18, an omnidirectional sensor SENS is preferably used to detect and locate pressure on the back of the phone, with the aim of using the sensor as a man / machine interface. To do this, the signals measured by the four sensors are compared, but since the deformation measured is greater the closer the pressure is to the sensor, it is possible to use all four sensors to detect and locate the pressure. Based on this detection and locatering, the following user interactions with the phone can be identified and associated with control actions: (i) a single press, (ii) two presses, and (iii) presses on keys on a virtual touchpad located on the back of the smartphone, where each key corresponds to the respective press area on the back of the smartphone.

[0120] Manufacturing of piezoelectric thin films The piezoelectric thin film according to the present invention, in which the electrodes of the present invention are provided, can be manufactured as described below. A commercially available piezoelectric on-insulator (POI, a structure comprising a piezoelectric layer mounted on a substrate, with a silicon oxide layer sandwiched between the two layers) substrate is used as the source for the piezoelectric thin film. This type of substrate can be obtained for any crystal orientation and any type of piezoelectric material.

[0121] In the first step, a 400 nm thick platinum electrode layer is deposited on the free surface of the piezoelectric thin film by chemical vapor deposition (e.g., PECVD (Plasma Enhanced Chemical Vapor Deposition)).

[0122] In the second step, a glass support having a release layer (e.g., LTHC ("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 light to form a glass-POI wafer.

[0123] In the third step, the glass-POI stack is ground from the free surface of the POI substrate until it almost reaches the embedded silicon oxide layer of the POI. Dry etching removes the remaining silicon and oxide layers, selectively stopping at the piezoelectric layer.

[0124] After the third step, only a thin piezoelectric layer remains on the glass-POI wafer, beneath which is the first platinum electrode layer supported by the glass substrate.

[0125] In the fourth step, a second 400 nm platinum electrode layer is then deposited onto the exposed surface of the piezoelectric thin film by chemical vapor deposition.

[0126] Finally, the stack of thin piezoelectric films between the two electrode layers is transferred and bonded to the final substrate. The glass support is then removed by peeling off the separation layer (by laser irradiation in the case of the LTHC separation layer) that was used to lift off the polymerized liquid, leaving the thin piezoelectric cell and its electrode layers on the final substrate.

[0127] This method enables the fabrication of large piezoelectric thin films with electrodes and their transfer onto various substrates. The piezoelectric cells and substrates then need to be cut to the correct dimensions for their intended applications. In this disclosure, sections from this disclosure relate to elements associated with identifiers, given to other elements having the same identifier in other figures.

[0128] In this disclosure, when a deformation sensor or the behavior of such sensor is referred to as unidirectional, bidirectional, or omnidirectional, it means that the sensor incorporates a thin piezoelectric element whose sensitivity is unidirectional, bidirectional, or omnidirectional, as defined in this disclosure.

[0129] Naturally, the present invention is not limited to the implementation forms described above, and modified embodiments can be added thereto without departing from the scope of the invention as defined by the claims.

Claims

1. A single-crystal thin piezoelectric element (PIEZO) in the form of a plate extending within an extending plane (xy) defined by a first direction (x) and a second direction (y) perpendicular to the first direction, wherein the dimension (L) in the first direction and the second direction x , L y ) are each greater than 100 μm, and the thickness (L z ) is less than 50 μm, and the thickness and the dimension (L) in the first direction x ) or the dimension (L) in the second direction y The ratio to ) is less than 0.1, and the piezoelectric element has a first sensitivity (S) to deformation in the first direction. x ) and the second sensitivity (S) to deformation in the second direction y ) and the crystal orientation of the element (PIEZO) is - Corresponding to the so-called "unidirectional" sensitivity, abs(S y / S x ) < 0.1, - ABS ((S) that supports so-called "bidirectional" sensitivity. y +S x ) / S x ) < 0.1, or For at least two first directions (x) of the extending surface (xy) that form an angle between -30° and 60°, abs((S) corresponds to so-called "omnidirectional" sensitivity. x -S y ) / S x A single-crystal thin piezoelectric element (PIEZO) such that ) < 0.

1.

2. The thin piezoelectric element (PIEZO) according to claim 1, wherein a pair of conductive layers (EL1, EL2) are provided, each located on two opposing surfaces of the thin piezoelectric element.

3. A deformation 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. The deformation sensor (SENS, SENS_1, SENS_2, SENS_3) according to claim 3, wherein at least one thin piezoelectric element is sealed between the flexible sheet (SH1) and another flexible sheet (SH2).

5. The deformation sensor (SENS, SENS_1, SENS_2, SENS_3) according to claim 3 or 4, comprising at least one charge amplifier (C.AMP) connected to the at least one thin piezoelectric element (PIEZO).

6. The deformation sensor (SENS, SENS_1, SENS_2, SENS_3) according to claim 5, wherein at least one charge amplifier (C.AMP) is integrated into the flexible sheet (SH1).

7. A deformation sensor (SENS_1, SENS_2, SENS_3) according to claim 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 separated by at least 30°.

8. The device comprises a first thin piezoelectric element (PIEZO_0°), a second thin piezoelectric element (PIEZO_90°), and a third thin piezoelectric element (PIEZO_45°), each having characteristic abs(S y / S x The deformation sensor (SENS-2) according to claim 7, wherein the first direction (x) of the second thin piezoelectric element forms a 90° angle with the first direction (x) of the first thin piezoelectric element, and the first direction (x) of the third thin piezoelectric element forms a 45° angle with the first direction (x) of the first thin piezoelectric element.

9. The system comprises a first thin piezoelectric element (PIEZO_0°), a second thin piezoelectric element (PIEZO_120°), and a third thin piezoelectric element (PIEZO_240°), each having characteristic abs(S y / S x The deformation sensor (SENS_3) according to claim 7, wherein the first direction (x) of the second thin piezoelectric element has a value of < 0.1, the first direction (x) of the second thin piezoelectric element forms an angle of 120° with the first direction (x) of the first thin piezoelectric element, and the first direction (x) of the third thin piezoelectric element forms an angle of 240° with the first direction (x) of the first thin piezoelectric element.

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

11. A sensor comprising at least two sensors according to any one of claims 3 to 6, electrically connected in parallel.

12. A device comprising at least one of the deformation sensors described in any one of claims 3 to 10, or a combination of sensors described in claim 11.

13. The device according to claim 12, wherein the device is a ring on which at least two deformation sensors of the unidirectional or omnidirectional type are provided, and the ring is configured to act as a man / machine interface.

14. The device according to claim 12, wherein the device is a smartphone equipped with at least one deformation sensor and configured to warn the user when a deformation measured based on a signal from the at least one sensor exceeds a predetermined threshold, or to function as a man / machine interface.

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