HYBRID STRAIN SENSOR COMBINING A PIEZOELECTRIC GAUGE AND A RESISTIVE GAUGE

Abstract

Hybrid sensor (HYB) comprising: a piezoelectric strain gauge (PIEZO); a resistive strain gauge (RES); and an electronic module (EL.MOD), the piezoelectric strain gauge and the resistive strain gauge being connected in parallel to terminals (T1RES, T2RES, T1PIEZO, T2PIEZO) of the electronic module, the hybrid sensor being configured such that the piezoelectric strain gauge and the resistive strain gauge generate a piezoelectric signal and a resistive signal, respectively, in response to a deformation of an object instrumented by the hybrid sensor (HYB), and the electronic module being configured to combine a first signal from the resistive signal and a second signal from the piezoelectric signal, so as to generate a first output signal which is representative of the deformation of the instrumented object. Figure to be published with the abstract: Fig. 1

Description

Title of the invention: HYBRID STRAIN SENSOR COMBINING A PIEZOELECTRIC GAUGE AND A RESISTIVE GAUGE TECHNICAL FIELD OF THE INVENTION

[0001] The context of the invention lies in the field of characterizing the deformation of an object, and more particularly in the instrumentation of an object by means of deformation gauges. TECHNOLOGICAL BACKGROUND

[0002] Measurements of the deformation of an object, whether integrated or not in a complex system, have numerous applications in various fields such as integrated health monitoring (CSI) which consists of detecting and characterizing damage in a mechanical structure, predictive maintenance, the measurement of a structural load for example at the level of an airplane wing, or even biological measurements on the posture, breathing, swelling of a part of the anatomy of a subject.

[0003] For these purposes, various measurement methods exist. Some are based on the deformation of resonant structures, others on optical principles, and still others on electrical phenomena. Among the latter, we can mention strain gauges with resistive and piezoresistive effects, strain gauges with piezoelectric effects, strain gauges with capacitive effects, or strain gauges with magnetostrictive effects.

[0004] Each of these methods has its strengths and weaknesses in terms of cost, ease of installation, practicality of use, integrability into a structure, etc. For example, resistive gauges are capable of static measurements but produce a rather noisy signal, which makes their use complicated for high-frequency dynamic measurements. Conversely, piezoelectric gauges are capable of providing low-noise signals, but cannot be used for static measurements.

[0005] Despite the existence of a wide variety of methods for measuring deformations, a need for instruments capable of measurements covering several domains, static and dynamic for example, with low measurement noise, while being easily integrable on an object to be characterized, remains unmet. Statement of the invention

[0006] The applicant's objective is to provide a simple-to-use deformation sensor, easily integrated into an object to be characterized, at low cost, and capable of carrying out static measurements as well as dynamic measurements with low noise.

[0007] With a view to achieving this aim, a first aspect of the invention is a sensor hybrid sensor comprising: a piezoelectric strain gauge; a resistive strain gauge; and an electronic module, the piezoelectric strain gauge and the resistive strain gauge being connected in parallel to terminals of the electronic module, the hybrid sensor being configured such that the piezoelectric strain gauge and the resistive strain gauge generate a piezoelectric signal and a resistive signal, respectively, in response to a deformation of an object instrumented by the hybrid sensor, and the electronic module being configured to combine a first signal from the resistive signal and a second signal from the piezoelectric signal, so as to generate a first output signal which is representative of the deformation of the instrumented object.

[0008] Such a hybrid deformation sensor can be simply attached to an object to be characterized by gluing, requiring only simple instrumentation.

[0009] Crucially, by combining the capabilities of a resistive gauge and those of a piezoelectric gauge, this hybrid sensor allows low-noise measurements both for the characterization of a static or quasi-static phenomenon and a dynamic phenomenon, the entire frequency range from the static situation to the dynamic situation being covered continuously and with low noise.

[0010] The hybrid sensor can also be configured to generate measurement signals faithfully reproducing, quantitatively speaking, a deformation of the object to be characterized, for all frequencies, low or high.

[0011] Furthermore, this sensor, by combining the two signals from the resistive gauge and the piezoelectric gauge, allows the user to enjoy the advantages of two separate sensors while benefiting from the ease of use of a single sensor.

[0012] According to additional non-limiting characteristics of the sensor according to the invention, considered individually or according to any technically feasible combination:

[0013] - the first output signal may comprise low frequency components and high-frequency components, which correspond respectively to a frequency content for frequencies below a given threshold and to a frequency content for frequencies above the given threshold; an informative content of the low-frequency components of the first output signal may come mainly from the resistive signal; and an informative content of the high-frequency components of the first output signal may come mainly from the piezoelectric signal;

[0014] - the first signal from the resistive signal can be generated by a measurement chain of the deformation comprising a first low-pass filter; the second signal from the piezoelectric signal can be generated by a deformation measurement chain comprising a first high-pass filter, in which a cut-off frequency of the first low-pass filter and a cut-off frequency of the first high-pass filter may be equal to at least 10%;

[0015] - the measurement chain for generating the first signal from the resistive signal can include the application of a first gain, the measurement chain for generating the second signal from the piezoelectric signal may include the application of a second gain, the first gain and the second gain may be configured in such a way that a signal from the resistive gauge and a signal from the piezoelectric gauge have the same amplitude for a given deformation amplitude of the instrumented object, to within at least 10%;

[0016] - the first gain may have a value equal to at least 10% near an inverse of the sensitivity of the resistive gauge, and the second gain may have a value equal to at least 10% of an inverse of the sensitivity of the piezoelectric gauge;

[0017] - the electronic module can be configured to generate a second signal of output representative of the deformation of the instrumented object by summation of the first signal representative of the deformation of the instrumented object to which a second high-pass filter is applied and of the resistive signal to which a second low-pass filter is applied;

[0018] - the second low-pass filter and the second high-pass filter may each have a cutoff frequency, these cutoff frequencies being equal to within 10%, preferably to within 1%; and the cutoff frequency of the second low-pass filter and the cutoff frequency of the second high-pass filter are each at least 5 times higher than the cutoff frequency of the first low-pass filter and the cutoff frequency of the first high-pass filter;

[0019] - the electronic module can further be configured so as to, on the basis of a comparison between (i) the first signal from the resistive signal before it passes through the first low-pass filter and (ii) the second signal from the piezoelectric signal: reducing a difference between (i) a cut-off frequency of a measuring chain integrating the resistive strain gauge and a cut-off frequency of a measuring chain integrating the piezoelectric strain gauge, and / or reducing a difference between (i) a gain of a measuring chain integrating the resistive strain gauge and (ii) a gain of a measuring chain integrating the piezoelectric strain gauge;

[0020] - the electronic module being able to be configured so as to, on the basis of the com comparison between (i) the first signal from the resistive signal before it passes through the first low-pass filter and (ii) the second signal from the piezoelectric signal: converge the cut-off frequency of the first low-pass filter towards the cut-off frequency of the first high-pass filter by means of adaptive filtering, and / or adjust the value of a correction gain placed in series on one of (i) the measuring chain integrating the resistive strain gauge and (ii) the measuring chain integrating the piezoelectric strain gauge, so as to reduce the difference between (i) the gain of the measuring chain integrating the resistive strain gauge and (ii) the gain of the measuring chain integrating the piezoelectric strain gauge;

[0021] - the piezoelectric gauge and the resistive gauge being able to be (i) integrated into a substrate support substrate and one next to the other or (ii) integrated into a support substrate and superimposed on each other;

[0022] - the electronic module can be integrated into the support substrate;

[0023] - the resistive gauge may comprise an electrically conductive element formed in an electrically conductive layer deposited on an electrode of the piezoelectric gauge;

[0024] - the piezoelectric gauge and the resistive gauge can each be part of a rosette; and

[0025] - the strain gauge may comprise a thin mono piezoelectric element crystalline material in the form of a plate extending in an extension plane defined by a first direction and a second direction normal to the first direction, with dimensions in the first direction and the second direction each greater than 100 pm and with 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.

[0026] The invention extends to a force sensor comprising a test body on which the hybrid sensor according to the invention is fixed.

[0027] Such a force sensor benefits from the advantages of the deformation sensor described above, and more particularly from its frequency range of sensitivity, and is therefore capable of carrying out measurements of static, quasi-static or dynamic forces. BRIEF DESCRIPTION OF THE FIGURES

[0028] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

[0029] [Fig. 1] [Fig. 1] represents a hybrid strain measurement sensor, combining a resistive gauge and a piezoelectric gauge;

[0030] [Fig.2] [Fig.2] represents a first mode of operation of the hybrid sensor of [Fig.l];

[0031] [Fig.3] [Fig.3] represents a simulation of the first mode of operation;

[0032] [Fig.4] [Fig.4] represents a piezoelectric element suitable for use in a piezoelectric gauge of the hybrid sensor of [Fig.l];

[0033] [Fig.5] [Fig.5] represents particular implementations of a hybrid sensor integrating a piezoresistive gauge and a piezoelectric gauge;

[0034] [Fig.6] [Fig.6] represents an experimental device for validating the operation of the hybrid sensor of [Fig.l];

[0035] [Fig.7] [Fig.7] represents validation measurements obtained using the device of [Fig.6];

[0036] [Fig.8] [Fig.8] illustrates different configurations of a hybrid sensor combining one or more resistive gauges and one or more piezoelectric gauges;

[0037] [Fig.9] [Fig.9] represents a force sensor employing hybrid sensors such as those in Figures 1 and 8;

[0038] [Fig. 10] [Fig. 10] represents a second mode of operation of the hybrid sensor of [Fig.l];

[0039] [Fig. 11] [Fig. 11] represents a third mode of operation of the hybrid sensor of [Fig.l]; and

[0040] [Fig. 12] [Fig. 12] represents the characteristic transfer function modules of a pair of complementary filters. DETAILED DESCRIPTION OF THE INVENTION

[0041] First embodiment

[0042] One embodiment of the present invention is described by means of Figures 1 to 9 and the associated passages below.

[0043] [Fig.l] illustrates a first implementation of the invention, in the form of a hybrid sensor HYB comprising a substrate formed from a support film SPRT integrating a piezoelectric strain gauge PIEZO, a resistive strain gauge RES, and an electronic module EL.MOD for acquiring and processing data, the latter consisting of signals generated by the resistive gauge and the piezoelectric gauge.

[0044] The electronic module has a first pair of terminals TIres and T2reS dedicated to measuring the electrical resistance of the resistive gauge, a second pair of terminals T1P1EZO and T2P1EZ0 dedicated to measuring electrical charges generated by the piezoelectric gauge, and a pair of outputs SI and S2 dedicated to transmitting to the outside a signal representative of the deformation of the support film.

[0045] The electronic module may comprise electronic elements conventionally associated with gauges of these types for the detection and transmission of the signals they generate.

[0046] The resistivity variations of the resistive gauge can be detected by a bridge of Wheatstone, which converts the variation in the gauge resistance into a voltage measurable by the acquisition system.

[0047] The variations in the electrical charge appearing at the electrodes of the piezoelectric gauges can be detected by means of a charge amplifier or directly at the voltage input of an acquisition system, with or without a voltage pre-amplifier. It is also possible to envisage using a type assembly IEPE (Integrated Electronics Piezo-Electric in English terminology), that is, with a charge amplifier integrated into the sensor, which preconditions the signal so that it is easier to transport on cables without adding noise.

[0048] When the electronic module is configured to process digital signals, it is equipped with one or more analog-to-digital converters to digitize the signal before carrying out the filtering operations, which can be more easily implemented digitally.

[0049] Principle of measurement by the hybrid sensor

[0050] The principle is to combine the signals from a resistive strain gauge on the one hand, and from a piezoelectric strain gauge on the other hand. These signals are combined so as to obtain an output signal whose low-frequency and high-frequency information comes predominantly from the resistive gauge and the piezoelectric gauge, respectively.

[0051] [Fig.2] illustrates the operation of the hybrid sensor of [Fig.l] by means of a functional diagram. When the hybrid sensor HYB is in operation, a deformation Def of the object to the surface of which it is fixed also applies to the support film SPRT and therefore to the resistive strain gauge RES and to the piezoelectric strain gauge PIEZO. The object in question will be referred to below as the “instrumented object”. The instrumented object is a test body in the case where the sensor is used to form a force sensor as illustrated by [Fig.9], but any object whose deformations are to be analyzed can constitute the instrumented object: aircraft wing, pipeline, machine in operation, live cable, element of a bridge etc.

[0052] The blocks in [Fig.2] inscribed in the dotted boxes named "Tr.RES" and "Tr.PIEZO" indicate transducers based on the RES gauge and the PIEZO gauge, respectively. These are functional blocks, the implementations of which are not detailed here because they are well known to those skilled in the art. Only the parts necessary for understanding the measurement principle of the hybrid sensor are mentioned in the remainder of this document.

[0053] The association of the sensitive elements of the resistive gauge and the piezoelectric gauge with the data acquisition and processing electronics form transducers transforming the mechanical input signal into electrical signals, in a manner well known in the field.

[0054] The deformation Def induces the generation of a signal din of mechanical deformation of the support film SPRT, which constitutes the input signal of the hybrid sensor HYB. In the case where the sensor does not include the support film SPRT (see variants (F), (H) and (I) illustrated by [Fig.5]), the mechanical deformation signal is directly generated by the deformation of the gauges, integral with the surface of the instrumented object.

[0055] The RES gauge forms the sensitive element of the Tr.RES transducer whose behavior during a measurement can be modeled by means of (i) a sensitivity Sensés (which can be expressed in mV / pdef) making it possible to generate a first electrical signal (which can be expressed in mV) from the mechanical deformation signal (which can be expressed in pdef, a dimensionless unit used in continuous media mechanics to express deformations) and (ii) the addition of a noise Nois (which can be expressed in mV) to said first electrical signal by an adder Addb The RES gauge generates a resistive signal SRES representative of the signal din of the mechanical deformation Def of the instrumented object.

[0056] The noise Nois reflects the fact that a resistive gauge is, in practice, noisy: the variations in the measured voltages are small, which makes low-noise measurements difficult, for reasons related to the gauge itself or to the measurement system. Thus, the signal generated in response to the detection of a deformation by a system based on a resistive gauge is considered noisy.

[0057] The PIEZO gauge forms the sensitive element of the Tr.PIEZO transducer whose behavior during a measurement can be modeled by means of (i) a piezo sensitivity Sens (which can be expressed in pC / pdef) in the case of a measurement in electrical charges or in mV / pdef in the case of a voltage measurement, making it possible to generate a second electrical signal (which can be expressed in pC or in mV) from the mechanical deformation signal (which can be expressed in pdef) and (ii) a high-pass unity gain filter FiltHPi. The PIEZO gauge generates a signal SP1EZo representative of the signal din of the mechanical deformation Def of the instrumented object.

[0058] The high-pass filter FiltHPi reflects the fact that piezoelectric gauges are not capable of measuring the static component of a deformation and that it is common to eliminate low frequencies from the response of the piezoelectric gauge to a deformation. It is possible to acquire the signals they generate using two different methods.

[0059] A first method is to use a charge amplifier, which integrates the charges produced by the piezoelectric gauge. In this case, in order to avoid a constant drift of the signal linked to the leakage of these charges through the piezoelectric material used and through the charge amplifier (one can refer to figures 4 and 5 and the associated text passages for the typical configuration of a piezoelectric gauge, which integrates a piezoelectric element between two electrodes), it is common to filter the output signal by a first-order high-pass filter.

[0060] A second method is to use a voltage amplifier, which presents a finite impedance to the piezoelectric gauge. This forms a high-pass RC filter, where R is the resistance resulting from the leakage resistance of the piezoelectric material and the impedance of the voltage amplifier, and C the capacitance of the piezoelectric gauge. electric.

[0061] In both methods, the signal generated by the piezoelectric strain gauge is passed through a first-order high-pass filter, the cutoff frequency of which can be adjusted by varying the gauge's leakage resistance, its capacitance, or the charge amplifier, if applicable. The transfer function Hhp(co) of a first-order infinite impulse response high-pass filter can be written as:

[0062]

[0063] where w = 2 nf is the pulsation, / the frequency, the cut-off pulsation, / H the cutoff frequency, and j the complex number defined such that j2=-l.

[0064] The signals Sres and SP1EZ0 are provided as inputs to the terminals of the electronic module EL.MOD. A GRES gain is applied to the signal Srbs and a GPiEZo gain is applied to the signal SPiEZ0.

[0065] Furthermore, the SRES signal is passed through a low-pass unity gain filter FiltLPi after the GRES gain has been applied to it. The low-pass filter FiltLPi preferably has the same cutoff frequency f { as the filter FiltHPi, at least to within 10%, preferably to within 5%, more preferably to within 1%, and even more preferably to within 0.1%. If the filter FiltLPi is a first-order infinite impulse response filter, then its transfer function Hhl(w) can be written:

[0066] H <o) = Eq. 2

[0067] The filters FiltHPi and FiltLPi represent two examples of unity gain filters: the modulus of their transfer function in their passband (f»fc for a high pass, f«fc for a low pass) is equal to 1. FiltHPi and FiltLPi are furthermore two so-called “complementary” filters, in that, whatever the frequency, the modulus Sltm of the sum of their respective transfer functions, Hw) and HT P( w) verify the equality Sum = \Hhp( w) + HIP(tc) | = This is due to the fact that their cutoff frequencies have the same value.

[0068] Figure 12 illustrates this situation with curves representing respectively Sum, \Hhp( <û) j and \Hlp((ù) j for complementary unitary filters having a cut-off frequency of 1 Hz, the abscissa axis representing the frequency f and the ordinate axis the gain g.

[0069] Finally, (i) the filtered SRES signal (indicated as the SRep REs signal in [Fig.2]) and (ii) the SP1EZO signal to which the GP1EZ0 gain has been applied (indicated as the SRep PiEZo signal in [Fig.2]) pass through an adder Add2 which sums these two signals to give an output signal SOuti-

[0070] In this situation, the equation Eq. 3 below expresses, in the fre- domain quential, the constitution of the output signal sOut\; [°071l = {srdm + n)gHLP(^) + dinspgpHHP(^) Eq. 3

[0072] where South represents the output signal of the hybrid sensor, din the signal of the deformation of the instrumented object and therefore of the support film, n the noise signal Nois, sr the value of the sensitivity Sensés of the resistive gauge to the signal din, sp the value of the sensitivity SensP1EZ0 of the piezoelectric gauge to the signal din, Hhp{iü) the transfer function of the high-pass filter FiltHPi, HLP((jd) the transfer function of the low-pass filter FiltLPi, gr the value of the gain GRES, and gp the value of the gain GP1EZ0.

[0073] Eq. 3 can be rewritten as Eq. 4:

[0074] + (SrgtHLpM + 4

[0075] For the signal sOuti to be quantitatively representative of the measured deformation, it is necessary that the signals from the resistive gauge and the piezoelectric gauge have the same amplitude for a given deformation amplitude. This identity between the amplitudes is ensured by the gain values ​​gr and gp of the two gain blocks Grès and GPiEZ0.

[0076] When we introduce the equations Eq. 1 and Eq. 2 w) + Hlp( w) = 1) into the equation Eq. 4 and we choose the values ​​of the gains in such a way that g = 1 / and g = 1 / sP, then we obtain the equation Eq. 5:

[0077] . Eq. 5

[0078] We note that the output signal soun is equal to the signal din of the deformation Def applied to the hybrid sensor, plus a noise term HLP[d) introduced by the gauge " resistive. In practice, values ​​for gr and gp will be chosen respectively equal to l / sr and l / gp to at least 10%, preferably 5%, more preferably 1%, and even more preferably 0.1%.

[0079] The noise term present on the output signal somi generated by the hybrid sensor is much lower than the noise term in the signal generated by the resistive gauge because it has been filtered by the low-pass filter FiltLPi. The smaller the cut-off frequency f, the less noise there will be in the output signal s™t.

[0080] A resistive gauge used alone only provides a very noisy measurement signal that can be expressed in the form (din + njsr), even if static and dynamic measurements are possible. Conversely, a piezoelectric gauge used alone makes it possible to obtain very low-noise measurement signals that can be expressed in the form (dinHHP( œ)), but only for dynamic deformations due to the very operation of a piezoelectric gauge.

[0081] On the other hand, by processing and combining in this way the signals from the two hybrid sensor gauges, the resistive gauge and the piezoelectric gauge, we benefit from their respective advantages, which are very complementary to each other. The part of the output signal from the resistive gauge makes it possible to characterize a static or low-frequency deformation, while limiting noise thanks to the low-pass filter. The part of the output signal from the piezoelectric gauge, naturally low in noise, makes it possible to precisely monitor a dynamic deformation of the instrumented object.

[0082] As described above, the hybrid sensor can be modeled as two parallel measurement chains, each dedicated to one of the resistive measurement and the piezoelectric measurement of a deformation and making it possible to characterize a deformation of the support film and therefore of the instrumented object. The measurement chain Meas.ChRES corresponding to the resistive gauge RES is modeled as being formed of the sensitivity Sensés, a source of the noise signal Nois, the adder Addl, the gain Grès and the filter FiltLPi. The measurement chain Meas.ChP1EZ0 dedicated to the piezoelectric gauge PIZEO is modeled as comprising the sensitivity SensP1EZo, the filter Filt HPi and the gain GPjEZo.

[0083] These measurement chains are defined by all the operations applied to the input signal din up to the summation of the resulting signals at the level of the adder Add2, on the one hand on the resistive gauge side and on the other hand on the piezoelectric gauge side.

[0084] [Fig. 3] illustrates simulation results of the operation of a hybrid sensor as modeled using Figures 1 and 2. This is the response of the different elements forming the hybrid sensor to a deformation applied for 1s of amplitude 10 pdef, and modeled as a square wave function.

[0085] In (A), [Fig.3] shows the RA response of the resistive gauge (without filtering), which perfectly follows the deformation slot but is very noisy.

[0086] In (B), Figure 3 shows the RB response of the resistive gauge filtered by means of a low-pass filter with a cut-off frequency / set at 0.1 Hz. The resulting response is less noisy than the RA response, but does not follow the deformation slot, due to a response time corresponding to 1 / / r

[0087] In (C), [Fig.3] shows the RC response of the piezoelectric gauge. This is not noisy, but due to the high-pass filter which is an integral part of the gauge it tends to return towards 0. The response time is short, but the slot is not maintained.

[0088] In (D), Figure 3 shows the RD response of the hybrid sensor, consisting of the sum of the filtered RB response of the resistive gauge and the RC response of the piezoelectric gauge. This RD response precisely follows (very low noise) the deformation window, without drift or latency. The remaining noise corresponds to the residual noise of the resistive gauge filtered by the low-pass filter. The simulation conditions correspond at g = 1 / sr, gp = 1 / sp, and the high-pass and low-pass filters have the same cutoff frequency fv

[0089] The filters FiltHPi and FiltLPi may be filters other than those exemplified above, such as second-order filters or sets of first-order filters in series. For precise quantitative measurements, the main selection criterion for the filters FiltHPi and FiltLPi is that the modulus of the sum of their transfer functions is constant. These are then considered to be so-called "complementary" filters.

[0090] For example, for FiltHPi, we will have a first-order filter when the charge of the piezoelectric gauge is measured in voltage, in which case, we will also choose a first-order filter for the FiltLPi filter to ensure complementarity between the two filters. Other cases are possible, for example with a charge measurement of the piezoelectric gauge.

[0091] It is the choice of these filters which conditions the fidelity of the electrical response of the hybrid sensor with respect to the mechanical deformation which it characterizes. If we take the example of [Fig.12], we understand that the “complementary” aspect of the two filters makes it possible to obtain an output signal SOuti representative of the deformation Def, with a continuous and quantitatively exact junction between (i) a signal SRep.REs essentially consisting of low-frequency components obtained by means of the resistive gauge and (ii) a signal SRep.PiEzo essentially consisting of the high-frequency components obtained by means of the piezoelectric gauge.Thus, the information content of the low-frequency components of the output signal SOuti comes mainly from the resistive signal Sees while the information content of the high-frequency components of the first output signal SOuti comes mainly from the piezoelectric signal SPiezo* We will speak of low frequencies and high frequencies for frequencies respectively lower and higher than the common cutoff frequency of the two filters.

[0092] The hybrid sensor illustrated by [Fig. 1] and whose operating principle is illustrated by Figures 2 and 3 can be implemented by means of resistive gauges and piezoelectric gauges known to those skilled in the art.

[0093] The SPRT support film is preferably made of one or more flexible materials chosen according to the intended application, and may be, for example, 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.

[0094] The support film could directly consist of a flexible printed circuit (or FPCB for Flexible Printed Circuit Board in English terminology), with soldering of the electrical contacts directly onto this element.

[0095] The resistive strain gauge comprises an electrical resistor formed from a conductive material such as a nickel and copper alloy, integrated on a flexible support, as described for example in patent application FR2693795 or patent US2386879. More generally, any type of resistive gauge whose operation is based on the variation of an electrical resistance under the effect of the deformation of its support may be suitable for the formation of the present hybrid sensor.

[0096] The piezoelectric strain gauge operates based on the piezoelectric effect, which can be described as the appearance of electrical charges on the surface of an element formed from a so-called "piezoelectric" material under the effect of deformation. This effect is exploited in piezoelectric gauges to detect the deformation of objects to which they are attached. There are different types of piezoelectric gauges, each of which can be used to form the piezoelectric gauge of the hybrid sensor illustrated in [Fig.l].

[0097] A first type of piezoelectric strain gauges is based on the use of a crystal of piezoelectric material with relatively good sensitivity to strain and stable over time, but thick and rigid, often housed in a metal case to which it is mechanically secured. These latter characteristics make it difficult to integrate and lead to a loss of some of the advantages that resistive gauges have for integration into a hybrid sensor combining a resistive gauge and a piezoelectric gauge (resistive gauges are very thin and flexible).

[0098] A second type of piezoelectric strain gauges is based on the use of composite structures comprising PZT (or lead zirconate titano) bars located between sheets of polymer materials, or on polymer piezoelectric films called "PVDF" for poly(vinylidene fluoride) in English terminology. These structures are relatively flexible and can be integrated alongside a resistive gauge, but too unstable over time and too sensitive to temperature for certain applications targeted for the hybrid sensor described above.

[0099] A third type of piezoelectric strain gauge, particularly suitable for the hybrid sensor described above, uses as a sensitive element a thin monocrystalline piezoelectric element in the form of a plate extending in an extension plane defined by a first direction and a second direction normal to the first direction, with dimensions in the first direction and the second direction each greater than 100 μm and a thickness less than 50 μm, 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 may have a first sensitivity Sx to deformation along the first direction and a second sensitivity Sy to deformation along the second direction perpendicular to the first, a crystal orientation of the element may be such that abs(Sy / Sx) <0.1, cor 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 first directions of the extension plane making an angle between them of between 30° and 60°, abs((Sx-Sy) / Sx) <0.1, corresponding to a so-called “omnidirectional” sensitivity.

[0100] Such a thin piezoelectric element is suitable for forming the basis of a piezoelectric strain gauge, which can combine precision, sensitivity, conformability, flexibility, lightness, stability, linearity, directivity and applicability to wide ranges of strains, as described in detail in French patent application FR2303635.

[0101] In particular, this thin piezoelectric element can have characteristics that are completely compatible with those of conventional resistive gauges in terms of flexibility and form factor: flexibility, thickness and lateral dimensions can be the same as those of conventional resistive gauges, which would allow a hybrid sensor equipped with a piezoelectric gauge formed from such a thin element to cover a range of applications as wide as that of resistive gauges, and this without presenting any additional integration constraints.

[0102] This piezoelectric element can measure deformations greater than 5000 micrometers per meter with a resolution of the order of 1 nanometer per meter of deformation. These figures compare with those of conventional resistive gauges which can measure deformations of up to 12000 micrometers per meter (for only a small number of cycles) but with a much lower resolution, of the order of 1 micrometer of deformation per meter.

[0103] In comparison, the first type of gauges in a box piezoelectric gauges are only capable of measuring deformations limited to approximately 300 micrometers per meter, which would limit the range of a hybrid sensor using this technology.

[0104] The piezoelectric thin element may be provided with a pair of electrically conductive layers located respectively on two opposite faces of the piezoelectric thin element.

[0105] A piezoelectric strain gauge may comprise at least one thin piezoelectric element as described above, fixed on a flexible sheet.

[0106] According to additional characteristics, considered individually or according to any technically feasible combination:

[0107] - the thin piezoelectric element can be encapsulated between the flexible sheet and a other flexible sheet;

[0108] - the piezoelectric gauge may comprise a charge amplifier connected to the thin piezoelectric element;

[0109] - the charge amplifier can be integrated on the flexible sheet;

[0110] On the other hand, the thin monocrystalline piezoelectric element described above meets to all the criteria necessary for good integration in combination with a resistive gauge: dimensions, flexibility and sensitivity. The integrability of the piezoelectric element (ease of placing it in intimate contact with a structure of a shape that is not necessarily flat) is essential here, and its sensitivity makes it possible to detect deformations of low amplitudes.

[0111] The piezoelectric element can be formed from lithium tantalate LiTaO3 in monocrystalline form, which belongs to the space group 3m, but also from lithium niobate LiNbO3 (group 3m), lead magnesium niobate MgNb2(PbO3)3 (group PI), aluminum nitride AIN (group P63mc), barium titanate BaTiO3, potassium niobate KNbO3 or lead titanate TiPbO3 (all three from the group P4mm).

[0112] Thus, the hybrid sensor HYB of [Fig. 1] can use a piezoelectric gauge PIEZO based on a thin piezoelectric element EL.PIEZO of extension plane chosen to present a particular behavior, unidirectional, omnidirectional or bidirectional to a unidirectional deformation which is applied to it in its extension plane. The sensitivity behavior of the thin piezoelectric element (unidirectional, omnidirectional or bidirectional) is transferred to the strain gauge integrating this thin piezoelectric element.

[0113] In order to benefit from the thinness and therefore the flexibility and conformability of the thin EL.PIEZO element, the SPRT substrate on which the thin EL.PIEZO element is fixed is preferably flexible. Thus, the substrate is preferably made of one or more 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 thin EL.PIEZO element can be attached to the SPRT substrate by means of a flexible adhesive such as an anisotropic conductive film (ACF) which further allows electrical contacting as described in patent document FR 3 122 985. In use, the PIEZO gauge can be attached to the surface of an object to be instrumented by means of an adhesive, for example a cyanoacrylate glue or an epoxy resin.

[0114] In addition to the thin EL.PIEZO element, in the example of [Fig.l], a charge amplifier C.AMP is also fixed on the SPRT substrate and functionally connected to two electrically conductive layers acting as electrodes, respectively formed on two opposite faces of the thin EL.PIEZO element. The function of the charge amplifier is to produce a voltage corresponding to the charge applied at the input and which corresponds to the charge generated by the EL.PIEZO element during its deformation, for the purpose of electronic processing of the generated electrical potential and to carry out an effective measurement of the deformation of the EL.PIEZO element.

[0115] The charge amplifier can be of the IEPE type, i.e. powered by a direct current, with an intensity of, for example, between 4 and 20 mA, generated by an acquisition system, and returning the measurement signal to the cable used for power supply, by modulating its voltage. Piezoelectric sensors with an integrated charge amplifier are well known in the field of stress or acceleration measurement using piezoelectricity. An electronic circuit transforms the high impedance signal from the piezoelectric gauge into a low impedance voltage signal, which is easier to transmit to the measurement electronics. Such a charge amplifier guarantees the compatibility of the piezoelectric gauge with a wide range of electronic measurement systems. In this case, the EL.MOD electronic module is not integrated into the same support as the sensor.

[0116] Although not shown, a wired connection element, such as a ribbon cable, is connected to the charge amplifier to connect the gauge to external measuring electronics.

[0117] The thin piezoelectric element EL.PIEZO preferably has a thickness of less than 50 pm, more preferably less than 25 pm, even more preferably less than 10 pm. Considering a piezoelectric element defined as illustrated in [Fig.4], a ratio of the thickness Lz of the EL.PIEZO element in a third direction z to its dimension Lx in a first direction x of its extension plane, and / or of a dimension Ly in a second direction y of its extension plane normal to the direction Lx, is less than 0.1, preferably less than 0.05, more preferably less than 0.01. The extension plane xy of the EL.PIEZO element is here defined by the two directions x and y. The first, second and third directions x, y and z form an orthogonal reference frame.

[0118] The SPRT support may have a thickness of between 5 and 300 μm.

[0119] It is however preferable that the piezoelectric gauge PIEZO considered as a whole is sufficiently flexible to fit the surface of a curved object to which it must be fixed and capable of following its deformations. The practitioner will be able to decide for each application the characteristics of the thin piezoelectric element EL.PIEZO, its support, and other elements such as the electrode layers or the means of making electrical contacts.

[0120] The flexibility of the thin piezoelectric element is advantageously exploited to fix the whole of one of its faces in intimate contact with the possibly curved surface of an object to be instrumented (through an electrode and a possible adhesive film). In this way, this piezoelectric element is integral with the object to be instrumented, undergoes the same deformations as those undergone by this object where the piezoelectric element is fixed, and its deformation is therefore representative of that of the object.

[0121] The SPRT substrate may be made of or replaced by a flexible support such as a flexible printed circuit board called "flex PCB", composed of layers of electrically insulating polymer and layers of copper, allowing signals to be routed between the different components of an electronic circuit. Each sensor assembly may comprise a plurality of EL.PIEZO piezoelectric elements, each having its own support, just as the electronic module controlling them may have its own. Alternatively, a single flex PCB support may accommodate all the EL.PIEZO elements and electronics for controlling these elements. Alternatively, a first flex PCB support may be common to all the EL.PIEZO elements and a second flex PCB support may be dedicated to the electronic control module.

[0122] [Fig.7] illustrates the results of measurements carried out by means of a hybrid sensor comprising a conventional resistive gauge and a piezoelectric strain gauge of the third type described above, equipped with a thin piezoelectric element EL.PIEZO.

[0123] The experimental device used is illustrated by [Fig.6], and comprises a horizontal steel plate H.P1 rigidly fixed to a support V.Sprt extending vertically. A resistive gauge RES and a piezoelectric gauge PIEZO are fixed on the steel plate. The experiment consists of placing a mass M on the plate, then removing it.

[0124] [Fig.7] illustrates in (A) the response of the resistive gauge without filtering, in (B) the response of the resistive gauge with low-pass filtering, in (C) the response of the piezoelectric gauge, and in (D) the response of the hybrid sensor which combines the responses of the resistive gauge with low-pass filtering and the piezoelectric gauge.

[0125] The answers are expressed in pdef. The mass is placed at t=29.7s and removed at 1=31.1s. The experimental parameters ensured that g = I / sr, gp — 1 / sp, and that the high-pass and low-pass filters had the same cut-off frequency / p fixed here at 0.8 Hz. These experimental results confirm the simulation results illustrated by [Fig.3]: the measurement carried out using the hybrid sensor makes it possible to correct the defects of the resistive gauge used alone (noise) and those of the piezoelectric gauge used alone (drift).

[0126] More specifically, the RMS (Root Mean Square) background noise is 0.53 pdef for the resistive gauge, while it is only 0.01 pdef for the hybrid sensor. The noise level is therefore reduced by a factor of 50, which represents a 34 dB reduction in noise, which is a very significant reduction.

[0127] Indeed, the output signal of the hybrid sensor is therefore both low-noise, and also presents, with a zero or almost zero response time, an immediate response. There is no need for stabilization time after the mass has paused to be able to accurately measure the deformation.

[0128] [Fig. 1] illustrates only one particular configuration for a hybrid sensor combining a resistive gauge and a piezoelectric gauge. [Fig.8] illustrates other possible configurations for the hybrid sensor, which can possibly be combined with each other.

[0129] In (A), [Fig.8] illustrates a configuration which differs from that of [Fig.l] in that the electronic module EL.MOD is not fixed to the SPRT support on which the resistive gauge and the piezoelectric gauge are fixed. A wired connection can functionally connect the electronic module to the two strain gauges.

[0130] In (B), [Fig.8] illustrates a different configuration than that shown in (A), in that the resistive gauge RES and the piezoelectric gauge PIEZO are superimposed on each other. Here, the resistive gauge RES is shown as fixed on the piezoelectric gauge, fixed to the SPRT support via the PIEZO gauge. Alternatively, the piezoelectric gauge could be fixed to the support via the resistive gauge.

[0131] In (C), [Fig.8] illustrates a configuration combining those of [Fig.l] and [Fig.8] in (B): the electronic module is fixed on the SPRT support and the two gauges are superimposed on each other.

[0132] In (D), [Fig.8] illustrates a rosette, consisting of three pairs of resistive and piezoelectric gauges, arranged to have principal axes oriented at 120° to each other, as is well known in the field of strain gauges. Other configurations in which the number of pairs of gauges and their relative orientations are different are possible, such as for example three pairs of gauges whose principal axes form angles of 45° or two pairs of gauges with principal axes oriented at 90° to each other. Here, the electronic module is fixed to the support, it could be located outside the support, as for the configuration illustrated by [Fig.l],

[0133] In the case of a combination involving several pairs of gauges, one resistive and the other resistive for each pair, the electronic signal acquisition and processing module is configured to process the signals from each pair so as to provide an output signal for each pair of gauges, each signal having been generated as the signal OUti-

[0134] It is also possible for a resistive strain gauge to be formed at from a variable electrical resistance element etched into an electrode of a piezoelectric gauge or formed from one or more metal layers deposited on this electrode. We then have the guarantee that the resistive gauge and the piezoelectric gauge measure exactly the same deformation, in particular thanks to the fineness of the thin piezoelectric element. This cointegration appears more complicated with other technologies, such as PZT bars. The hybrid sensor could naturally include such an association of a resistive gauge and a piezoelectric gauge. One could then consider the resistive gauge attached to the support film via the piezoelectric gauge.

[0135] [Fig.5] illustrates in (A) a top view of a hybrid sensor HYB based on the configuration of a conventional piezoelectric gauge with a piezoelectric element EL.PIEZO equipped with its two electrodes ELI and EL2, combined with a resistive gauge RES. An electrically conductive element EL.RES of variable electrical resistivity forms the sensitive element of the resistive gauge RES. This conductive element is formed in a layer of an electrically conductive material formed on the electrode EL2, as illustrated in (C) of [Fig.5], which represents a sectional view of the sensor HYB along a plane passing through points A and B of [Fig.5](A). The conductive element EL.RES is formed of meanders etched in an electrically conductive layer deposited on the electrode SH2. An electrically insulating layer Ins can be interposed between the electrically conductive element and the electrode EL2. The EL.PIEZO and EL. elementsRES are integrated into a substrate formed from a flexible support film SPRT as described above, attached to one of the surfaces of this support as illustrated here. A charge amplifier C.AMP is also integrated into the SPRT substrate, here attached to the same surface of the substrate as the EL.PIEZO and EL.RES elements. The EL.PIEZO and EL.RES elements could also be integrated inside the support.

[0136] More generally, any type of association between a resistive gauge and a piezoelectric gauge can be envisaged for the hybrid sensor, as long as these gauges are configured to characterize the same deformation, preferably simultaneously, and the signals that they generate can be combined.

[0137] [Fig.5] illustrates in (B) a variation of the HYB hybrid sensor illustrated in (A) and (C). In this configuration, the piezoelectric gauge, the resistive gauge and an EL.MOD electronic module are integrated on the same SPRT substrate, but in this configuration, a charge amplifier separate from the rest of the acquisition electronics is not necessary, its function being integrated into an EL.MOD electronic module directly managing the amplification of the charge itself. In addition, the electronic module is preferably fixed to a surface of the SPRT substrate opposite that receiving the EL.PIEZO and EL.RES elements. Indeed, when fixing the hybrid sensor on an object to be instrumented, an electronic module located on the same side as these elements would hinder their application as close as possible to the surface of the object to be instrumented.

[0138] [Fig.5] illustrates in (D) another variation of the HYB hybrid sensor illustrated in (A) and (C). In this configuration, the EL.RES element is not formed on the EL.PIEZO element, but next to it, on the SPRT substrate. The EL.PIEZO element could be included between two meanders of the EL.RES element so that the signals generated there are as representative as possible of the same deformation of the instrumented object.

[0139] [Fig.5] illustrates in (E) another variation of the HYB hybrid sensor illustrated in (A) and (C). In this configuration, obtained for example by using a SPRT substrate formed of several layers, the EL.RES element is embedded within the SPRT substrate, while being superimposed on the EL.PIEZO element.

[0140] [Fig.5] illustrates in (G) another variation of the HYB hybrid sensor illustrated in (E). In this configuration, the EL.RES element is also embedded within the SPRT substrate, the EL.RES and EL.PIEZO elements being superimposed on each other, separated and electrically isolated from each other by a layer forming the SPRT substrate, i.e. by a part of the SPRTR substrate itself.

[0141] [Fig.5] illustrates in (F) another variation of the HYB hybrid sensor illustrated in (A) and (C). In this configuration, a substrate for supporting the EL.RES and EL.PIEZO elements is not used: the EL.PIEZO element is self-supporting and itself serves as a support for the EL.RES element. This is a minimalist configuration limited to the following elements: the EL.PIEZO element and its pair of electrodes ELI and EL2, the EL.RES element formed superimposed on the EL.PIEZO element, and an electrically insulating layer Ins separating the EL.RES element from the EL.PIEZO element and its electrodes. In such a case, the sensor can be directly fixed to the surface of an object to be instrumented, in the same way as if it included a SPRT support substrate.

[0142] [Fig.5] illustrates in (H) a variation of the hybrid sensor HYB illustrated in (F). In this configuration, an electrically conductive layer Cond is present, here arranged between the EL.RES element and the EL.PIEZO element and its electrodes, and electrically insulated from these elements by two electrically insulating layers Ins, respectively interposed between the layer Cond and the EL.RES element and between the layer Cond and the EL.PIEZO element and its electrodes. The layer Cond may have the function of functionally connecting the element EL.RES and / or the element EL.PIEZO to an external electronic control device such as the module MOD.EL. The layer Cond may also have the function of avoiding capacitive couplings between the element EL.RES and the element EL.PIEZO. The layer COND is then typically connected to the ground of the sensor.

[0143] [Fig.5] illustrates in (H) a variation of the HYB hybrid sensor illustrated in (F). In this configuration, the EL.RES element is formed directly on the EL.PIEZO element, next to the EL2 electrode and not superimposed on it.

[0144] Force sensor

[0145] A hybrid sensor as illustrated by figures 1 and 8 is primarily dedicated to the characterization of the deformation of an instrumented object, on which is fixed the hybrid sensor. However, it can be used to form, for example, a force sensor. The principle is to attach a hybrid sensor or a plurality of hybrid sensors, four in the example illustrated, to a test body. The function of the test body is to produce a physical quantity that can be measured when it is subjected to a force that one seeks to characterize.

[0146] Thus, [Fig.9] represents a force sensor F.SENS consisting of a test body consisting of a force cell CELL equipped with four hybrid deformation sensors HYB. In (A), without force applied, the force cell is at rest. Under load by application of a force F, situation illustrated in (B), the force cell deforms and two of the four hybrid sensors are in a compression state Comp while the other two are in an extension state Ext. At the end of a calibration step of the force cell (application of a known force to the force cell and measurement of the response, to calculate the sensitivity), the measurement of the deformation of the latter by means of the hybrid deformation sensors makes it possible to deduce the amplitude of the force F applied to the cell. Of course, other types of test body can be used.

[0147] Second embodiment

[0148] The first embodiment is based on the identity of the cut-off frequencies of the high-pass filter FiltHPi of the piezoelectric gauge and the low-pass filter FiltLPi applied to the signal generated by the resistive gauge. These two filters are considered to form a first filtering stage.

[0149] However, in practice, the cutoff frequency of the high-pass filter FiltHPi, which corresponds to the physical characteristics of the piezoelectric gauge, cannot always be estimated precisely. In particular when the piezoelectric gauge is used in voltage mode, the cutoff frequency depends on the leakage resistance of the piezoelectric element forming the sensitive element of the piezoelectric gauge, which can vary depending on the temperature and the deformation applied to the sensor. This can be problematic since the cutoff frequency of the low-pass filter applied to the signal from the resistive gauge must be equal to the cutoff frequency of the high-pass filter of the piezoelectric gauge so that the output signal SOuti is quantitatively faithful to the deformation. A difference between these cutoff frequencies will generate distortion of the output signal, especially at frequencies close to these cutoff frequencies.

[0150] This second proposed embodiment aims to minimize this effect. It consists of placing a second filtering stage using a second low-pass filter FiltLP2 and a second high-pass filter FiltHP2 with the same cut-off frequency fc2, higher than the cut-off frequency f targeted for the filters of the first stage FiltLPi and FiltHPi.

[0151] The signal from the resistive gauge is then used up to the frequency fc2 above the cutoff frequency f { of the first stage, and the piezoelectric gauge signal is only used above this second cutoff frequency fc2. The filters of this second filter stage are implemented in the same way (either both analog or both digital), so it is easy to ensure that they have exactly the same cutoff frequency fc2, which guarantees an output from the second filter stage without signal distortion.

[0152] Typically / 2 will be chosen as the cut-off frequency of the second stage 5 times greater than jv, preferably 10 times greater than f H so as to verify the inequality f > f c. The higher f is, the less influence the difference between the cut-off frequencies of the filters of the first stage FiltLPi and FiltHPi will have on the output signal, but the higher the residual noise on the output signal will be.

[0153] [Fig. 10] illustrates by means of a functional diagram the operation of the hybrid sensor according to this second embodiment. The second embodiment is distinguished from the first embodiment by the presence of a second filtering stage comprising the filters FiltLP2, FiltHP2, and an adder Add3. For the other elements of [Fig.10], reference may be made to the first embodiment and to [Fig.2].

[0154] The adder Add3 is configured to make the sum between (i) the output signal So utl to which filtering by the filter FiltHP2 has been applied and (ii) the signal from the resistive gauge RES to which the gain Grès then filtering by the low-pass filter FiltLP2 have been applied. At the output of the adder Add3, an output signal SOut2 is obtained, corresponding to the signal SOuti largely freed from the influence of the difference between the cut-off frequencies of the filters FiltLPi and FiltHPi of the first stage.

[0155] Third embodiment

[0156] Another problem that may be encountered in practice when using the hybrid sensor is the effects of temperature. Any sensor is subject to a drift in its sensitivity with temperature, that is to say that the conversion factor between the measured quantity (here the deformation) and the sensor output (a voltage in mV or a charge in pC for example) varies as a function of temperature. Since the physical principles on which resistive gauges and piezoelectric gauges are based are completely different, it is expected that the sensitivities of two gauges of these two types will drift in different ways with temperature.

[0157] The first problem is that temperature drift generates a bias in the measurements made with the hybrid sensor, if it is not compensated. The less the sensitivity drifts with temperature, the better the sensor is considered to be. This first problem exists for all sensors, and many solutions have been developed to address it.

[0158] The second problem originates from the differential drift between the two gauges, the resistive gauge and the piezoelectric gauge. Indeed, for the output signal of the sensor, which results from the combination between the signals from the two gauges, the resistive gauge and the piezoelectric gauge, to be quantitatively representative of the measured deformation, it is necessary that the signals from the resistive gauge and those from the piezoelectric gauge have the same amplitude for a given amplitude deformation. This identity between the amplitudes is ensured, in the previous embodiments, by the two gain blocks Grès and GP1EZ0, whose gain values ​​gr and gp are considered fixed in the modeling of the hybrid sensor used in this document.

[0159] For a reference temperature To, gains gr and gp can thus be chosen so that the SRep.REs and Sreepiezo signals have the same amplitude when measuring a given strain. However, if the two gauges have sensitivity drifts (sr and sp) at different temperatures, then at a temperature different from To the SRep.REs and SRep.PiEzo signals will have different amplitudes for the same strain, and the output signal will be distorted relative to the measured strain. In other words, in the first embodiment, illustrated by [Fig.2], the equality between the respective overall gains srxgr and spxgp of the two measurement chains Meas-Ch^s and Meas.ChP1EZo may be lost due to a differential drift between the characteristics of the two gauges.

[0160] The third embodiment addresses this problem by ensuring that, at each instant, (i) the respective gains of the two measurement chains Meas.Ch^s and Meas.ChP1EZ0 remain equal to each other and, (ii) the respective cut-off frequencies of the two measurement chains Meas.Chj^s and Meas.ChP1EZ0, which are those of the two filters FiltLPi and FiltHPi, are equal to each other. This identity between the overall gains and cut-off frequencies of the two measurement chains Meas.Chj^s and Meas.ChP1EZ0 is in this embodiment ensured by means of an adaptive filtering technique described below by means of the functional block diagram of [Fig. 11].

[0161] Adaptive filtering according to this embodiment has the principle of adjusting in real time (i) the cut-off frequency of the low-pass filter FiltLPi applied to the signal of the resistive gauge and (ii) the value gp_CoiT of a gain GPIEZO_coir placed in series between the gain GP1EZ0 and the adder Add2.

[0162] The cut-off frequency of the low-pass filter FiltLPi is continuously adjusted so as to make it tend towards that of the high-pass filter FiltHPi of the acquisition chain of the piezoelectric gauge, the characteristics of which are likely to vary depending on the temperature or other physical parameters.

[0163] Similarly, the value gp_CoiT of the gain GPiEZo_corr is continuously adjusted so as to make the overall gain spxgpxgp Corr of the measurement chain Meas.ChP1EZ0 tend towards the overall gain srxgr of the measurement chain Meas-Ch^s.

[0164] [Fig. 11] represents a functional block diagram explaining the algorithm implemented for adjusting the cutoff frequency of the FiltLPi filter and the gp_ CoiT value for correcting the gain of the Meas.ChP1EZ0 measurement chain. The principle of the first embodiment illustrated by [Fig.2] is repeated, to which is added adaptive filtering represented by the elements FiltHP Est, LMSAig, Sub and GP1EZo_con-- For the other elements of [Fig.l 1], one can refer to the first embodiment and to [Fig.2],

[0165] The principle used is based on the hypothesis, verified in practice, that, at low frequencies, the information contained in the signal generated by the piezoelectric gauge is also contained in the signal of the resistive gauge. There is thus a redundancy of information that can be used to continuously calibrate the cut-off frequency of the FiltLPi filter, and the value gp_Coir of the gain GP1EZO_cOrr- For this, a classic adaptive filtering strategy is used, for example using a least squares LMS Aig algorithm (designated by the acronym LMS for Least Mean Square) to adapt the characteristics of the transfer function of the FiltLPi filter and the overall gain of the Mes.ChP1EZ0 measurement chain.

[0166] Low frequencies of the signal can be used, typically below 100 Hz so that the signal from the resistive gauge is not too noisy. To do this, for example, the signals entering the LMSai8 algorithm block can be filtered using a low-pass filter with a cutoff frequency of 100 Hz.

[0167] The resistive signal Srbs of the resistive gauge to which the gain Grbs has been applied is (i) filtered by a high-pass filter FiltHP_Est with an estimated transfer function HhpEST(co), whose gain and cut-off frequency parameters are variable, and (ii) sent as input to the LMSAig algorithm. A second input to the LMSAig algorithm is fed by an error signal e defined as (i) the signal SRES of the resistive gauge adjusted by means of the gain G^s and filtered by the estimated high-pass filter FiltHP Est, from which (ii) the signal SRep.P1Ezo representative of the piezoelectric signal SP1EZ0 is subtracted using the block Sub. The result of this subtraction is an error signal e which is representative of a difference between (i) the signal SRep.REs from the measurement chain Mcas.ChR| S before it is filtered by FiltLPi but which is filtered by FiltHP estet (ii) the SRep.PiEzo signal from the Meas measurement chain.ChP1EZ0 before passing through the gain GP1EZO_cOn-- The signals SRep PiEZo and are respectively derived from the piezoelectric signal SP1EZ0 and the resistive signal SRES.

[0168] By default, the FiltHP_Est filter is initialized with a gain of 1 and a cutoff frequency equal to that of the FiltLPi and FiltHPi filters before drift and before modification by the LMSAig block.

[0169] At least for low frequencies, this error signal must be minimized, which is done here in a conventional manner by means of adaptive filtering.

[0170] The LMSAig algorithm is conventionally configured to iteratively modify the gain and the cutoff frequency of the estimated high-pass filter FiltHPEST so as to minimize the error e. At each iteration, or when deemed necessary, the cutoff frequency of the estimated filter FiltHPEST is transferred to the filter FiltLPi, and the gain gP_corr is updated with the inverse of the gain of the estimated filter FiltHP EST.In other words, based on a comparison between (i) the first representative signal sRepRES of the resistive signal SRES before it passes through the first low-pass filter FiltLPi and (ii) the second signal sRep P1EZ0 representative of the piezoelectric signal SP1EZ0, we obtain an error e to be minimized by adaptive filtering, the latter guaranteeing (i) that the filters FiltHPi and FiltLPi have the same cutoff frequency, and (ii) that the signals generated respectively by the resistive gauge and the piezoelectric gauge have the same amplitude before their summation, at any time, even if the parameters of the filters evolve over time. By proceeding specifically as illustrated by [Fig. 11], the drift of the sensitivity of the output of the hybrid sensor is equal to that of the resistive gauge, which in general drifts less quickly than the sensitivity of the piezoelectric gauge.

[0171] The transfer of the parameters is conventionally represented by a diagonal arrow in the background of the filters FiltLPi and FiltHP Est and the variable gain GPn7o_cm- In particular for the copying of the parameters that are the gain and the cut-off frequency, it is convenient to implement this third embodiment digitally.

[0172] In the example presented above, the cut-off frequency of the FiltLPi filter, which here can be a digital filter with a variable cut-off frequency defined on command, is adjusted so as to follow the drift of the cut-off frequency of the FiltHPi filter. Thus the cut-off frequencies of the FiltLPi and FiltHPi filters are kept equal.

[0173] Still in this example, the overall gain of the measurement chain Mes.ChP1EZ0 is corrected, by means of the gain GP1EZo_coir, so as to ensure its equality with the overall gain of the measurement chain Ch.McsR|S. An alternative based on the same principle would be to correct the overall gain of the measurement chain Mes.ChRES so as to ensure its equality with the overall gain of the measurement chain Ch.MesP1EZ0.: it would be sufficient to place a variable gain on the measurement chain Meas.ChRES and to control its gain value by means of a suitable algorithm.

[0174] LMS type algorithms are generally used for finite impulse response (FIR) filters. Their implementation for infinite impulse response (IIR) filters is more complicated, but in the present case, where only two parameters are sought (a gain and a cutoff frequency of a first-order high-pass filter), the adaptation of known methods can be implemented by those skilled in the art. For example, reference may be made to Shynk JJ's article "Adaptative IIR filtering", 1989, EEE ASSP Magazine, 6, 4-21.

[0175] Other methods for estimating the gain and the cut-off frequency are conceivable, for example by using a fast Fourier transform (or FFT in English for Fast Fourier Transform), or adaptive least squares filtering (or RLS for Recursive Least Square in English terminology). The problem is posed mathematically as a problem of identifying a first-order system from a noisy signal, which is a very classic and widely documented problem.

[0176] In the present embodiment, only two filters are used, and the FiltLPi filter is adjusted in response to the changing characteristics of the FiltHPi filter. More generally, this involves making the cutoff frequency and the gain of the entire measurement chain Meas.ChP1EZ0. and those of the entire measurement chain Meas.ChP1EZo equal by reducing the gap between them. The cutoff frequency of the measurement chain Meas-Ch^s can be made to converge towards that of the measurement chain Meas.ChP1EZ0 as illustrated in [Fig. 11], or, conversely, the value of the cutoff frequency of the measurement chain Meas.ChP1EZ0 can be made to converge towards that of the measurement chain Meas-Ch^s. Symmetrically, the overall gain of the Meas.ChP1EZ0 measurement chain can be made to converge towards that of the Meas.Ch REs measurement chain, or vice versa.This may involve adjusting the gain and cutoff frequency parameters of a single filter, or adjusting the gain and cutoff frequency parameters of filters combined together, for example in series, or any other method, such as using a corrective gain as illustrated in [Fig. 11]. "Converging" means an operation which consists of reducing a difference between two values.

[0177] Furthermore, depending on the conditions (drift of the cut-off frequency but not of the gain or vice versa, drift occurring at the piezoelectric gauge or at the resistive gauge or vice versa), the electronic module can be configured to adjust only one or the other of the gain and the cut-off frequency.

[0178] Of course, the variants and embodiments detailed above can be combined with each other in any technologically feasible combination.

[0179] In this description, saying that two values ​​are "equal to within at least n%" means that a relative difference expressed as a percentage between these two values ​​is less than n%.

[0180] The invention is not limited to the embodiments described above and variant embodiments may be made without departing from the scope of the invention as defined by the claims.

Claims

Claims

1. Hybrid sensor (HYB) comprising: - a piezoelectric strain gauge (PIEZO); - a resistive strain gauge (RES); and - an electronic module (EL.MOD), the piezoelectric strain gauge and the resistive strain gauge being connected in parallel to terminals (T1RES, T2rbs, T1p1EZo, T2p1EZo) of the electronic module, the hybrid sensor being configured so that the piezoelectric strain gauge and the resistive strain gauge generate a piezoelectric signal (SP1EZ0) and a resistive signal (Sres), respectively, in response to a deformation (Def) of an object (CELL) instrumented by the hybrid sensor (HYB), and the electronic module being configured to combine a first signal (sRep.REs) from the resistive signal (SreS) and a second signal (sRep.PIEZ0) from the piezoelectric signal (SPIEZO), so as to generate a first output signal (SOuti) which is representative of the deformation (Def) of the instrumented object (CELL).

2. The hybrid sensor according to claim 1, wherein: - the first output signal (SOuti) comprises low-frequency components and high-frequency components, which correspond respectively to a frequency content for frequencies below a given threshold (fc) and to a frequency content for frequencies above the given threshold; - an informative content of the low-frequency components of the first output signal comes mainly from the resistive signal (Srbs); and - an informative content of the high-frequency components of the first output signal comes mainly from the piezoelectric signal (S PIEZo)-

3. The hybrid sensor according to claim 1 or 2, wherein: - the first signal (sRepRES) from the resistive signal (Srbs) is generated by a deformation measurement chain (Def) comprising a first low-pass filter (FiltLPi); - the second signal (sRepP1EZ0) from the piezoelectric signal (SP1EZo) is generated by a deformation measurement chain (Def) comprising a first high-pass filter (FiltHPi), in which a cut-off frequency of the first low-pass filter (FiltLPi ) and a cutoff frequency of the first high-pass filter (FiltHPi) are equal to at least 10%.

4. The hybrid sensor according to claim 3, wherein: - the measurement chain for generating the first signal (sRepRES) from the resistive signal (SRES) comprises the application of a first gain (G®), - the measurement chain for generating the second signal (sRepP1EZ0) from the piezoelectric signal (SP1EZo) comprises the application of a second gain (GpiEZo), the first gain (GRES) and the second gain (GP1EZ0) are configured in such a way that a signal (SRep REs) from the resistive gauge (RES) and a signal (SRep.P1EZo) from the piezoelectric gauge (PIEZO) have the same amplitude for a given deformation amplitude (Def) of the instrumented object, to within at least 10%.

5. The hybrid sensor according to claim 4, configured in such a way that: - the first gain (Grès) has a value (gr) equal to at least 10% near an inverse of the sensitivity (sr) of the resistive gauge (RES), and - the second gain (GPiEZ0) has a value (gp) equal to at least 10% near an inverse of the sensitivity (sp) of the piezoelectric gauge (PIEZO).

6. The hybrid sensor according to any one of claims 3 to 5, wherein the electronic module (EL.MOD) is configured to generate a second output signal (SOut2) representative of the deformation of the instrumented object by summing (i) the first signal (S omi) representative of the deformation of the instrumented object to which a second high-pass filter (FiltHP2) is applied and (ii) the resistive signal (SRES) to which a second low-pass filter (FiltLP2) is applied.

7. The hybrid sensor according to claim 6, wherein: - the second low-pass filter (FiltLP2) and the second high-pass filter (FiltHP2) each have a cut-off frequency, these cut-off frequencies being equal to within 10%, preferably to within 1%; and - the cut-off frequency of the second low-pass filter (FiltLP2) and the cut-off frequency of the second high-pass filter (FiltLH2) are each at least 5 times higher than the cut-off frequency of the first low-pass filter (FiltLPi) and the cut-off frequency of the first high-pass filter (FiltLHi).

8. The hybrid sensor according to any one of the preceding claims 3 to 7, the electronic module being further configured so as to, on the basis of a comparison (e) between (i) the first signal (sRep.REs) from the resistive signal (Sres) before it passes through the first low-pass filter (FiltLPi) and (ii) the second signal (sRep.P1EZo) from the piezoelectric signal (SP1EZo): - reduce a difference between (i) a cut-off frequency of a measuring chain (Ch.MeasRES) integrating the resistive strain gauge (RES) and a cut-off frequency of a measuring chain (Ch.MeasP1EZ0) integrating the piezoelectric strain gauge (PIEZO), and / or - reduce a difference between (i) a gain (sr, gr) of a measuring chain (Ch.MeasRBs) integrating the resistive strain gauge (RES) and (ii) a gain (sp, gp, gp con) of a measuring chain (Ch.MeasP1EZ0) integrating the piezoelectric strain gauge (PIEZO).

9. The hybrid sensor according to claim 8, the electronic module being configured so as to, on the basis of the comparison (e) between (i) the first signal (sRep.REs) from the resistive signal (Srbs) before it passes through the first low-pass filter (FiltLPi) and (ii) the second signal (s RepPiEzo) from the piezoelectric signal (SPiEZ0): - converge the cut-off frequency of the first low-pass filter (FiltLPi) towards the cut-off frequency of the first high-pass filter (FiltHPi) by means of adaptive filtering (FiltLPi, FiltHP Est, GPiEZO_corr LMSAig), and / or - adjust the value (gp_cOrr) of a correction gain (GP1EZO_cor) placed in series on one (i) of the measurement chain (Ch.MeasRBs) integrating the resistive strain gauge (RES) and (ii) of the measuring chain (Ch.Meas piezo) integrating the piezoelectric strain gauge (PIEZO), so as to reduce the difference between (i) the gain (sr, gr) of the measuring chain (Ch.MeasRBs) integrating the resistive strain gauge (RES) and (ii) the gain (sp, gp, gp con) of the measuring chain (Ch.MeasP1EZ0) integrating the piezoelectric strain gauge (PIEZO).

10. The hybrid sensor according to any one of the preceding claims 1 to 9, the piezoelectric gauge and the resistive gauge being (i) integrated into a support substrate (SPRT) and next to each other or (ii) integrated into a support substrate (SPRT) and superimposed on each other.

11. The hybrid sensor according to claim 10, the electronic module (EL.MOD) being integrated into the support substrate (SPRT).

12. The hybrid sensor according to any one of the preceding claims 1 to 9 and 11, the resistive gauge (RES) comprising an electrically conductive element (EL.RES) formed in an elec- electrically conductive deposited on an electrode (EL2) of the piezoelectric gauge (PIEZO).

13. The hybrid sensor of any one of preceding claims 1 to 12, the piezoelectric gauge and the resistive gauge each forming part of a rosette.

14. The hybrid sensor according to any one of the preceding claims 1 to 13, in which the strain gauge (PIEZO) comprises a thin monocrystalline piezoelectric element (EL.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 direction and the second direction each greater than 100 pm and with a 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.

15. Force sensor (F.SENS) comprising a test body (CELL) on which the hybrid sensor (HYB) according to any one of the preceding claims 1 to 14 is fixed.

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