HYBRID DEFORMATION 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 so 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 that is representative of the deformation of the instrumented object. Figure to be published with the abbreviation: Fig. 1

Description

Title of the invention: HYBRID DEFORMATION 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 strain gauges. TECHNOLOGICAL BACKGROUND

[0002] Deformation measurements of an object, whether integrated or not into a complex system, have numerous applications in various fields such as integrated health monitoring (IHM), 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 aircraft wing, or even biological measurements on posture, respiration, and swelling of a part of a subject's anatomy.

[0003] Various measurement methods exist for these purposes. 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, and 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, integration into a structure, etc. For example, resistive strain gauges are capable of static measurements but produce a rather noisy signal, which complicates their use for high-frequency dynamic measurements. Conversely, piezoelectric strain gauges are capable of providing low-noise signals, but are not suitable 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. Description of the invention

[0006] The applicant's objective is to provide a simple-to-use strain sensor that can be easily integrated into an object to be characterized, at low cost, and capable of performing both static and low-noise dynamic measurements.

[0007] For the purpose of achieving this goal, a first aspect of the invention is a 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 so 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 strain sensor can be simply attached to an object to be characterized by gluing, requires 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 for the characterization of both 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 parameterized to generate measurement signals that faithfully reproduce, 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 features of the sensor according to the invention, considered individually or in any technically feasible combination:

[0013] - the first output signal may include 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 the deformation including a first low-pass filter; the second signal from the piezoelectric signal can be generated by a deformation measurement chain including a first high-pass filter, in which a cutoff frequency of the first low-pass filter and a cutoff frequency of the first high-pass filter can be equal to within at least 10%;

[0015] - the measurement chain for generating the first signal from the resistive signal can including 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 can have a value equal to, within at least 10%, an inverse of the sensitivity of the resistive gauge, and the second gain can have a value equal to at least 10% to 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 summing the first signal representative of the deformation of the instrumented object to which a second high-pass filter is applied and the resistive signal to which a second low-pass filter is applied;

[0018] - the second low-pass filter and the second high-pass filter can 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: reduce a difference between (i) a cutoff frequency of a measurement chain integrating the resistive strain gauge and a cutoff frequency of a measurement chain integrating the piezoelectric strain gauge, and / or reduce a difference between (i) a gain of a measurement chain integrating the resistive strain gauge and (ii) a gain of a measurement chain integrating the piezoelectric strain gauge;

[0020] - the electronic module can be configured so as to, on the basis of the 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 cutoff frequency of the first low-pass filter to the cutoff frequency of the first high-pass filter using adaptive filtering, and / or adjust the value of a correction gain placed in series on one (i) of the measurement chain incorporating the resistive strain gauge and (ii) the measurement chain incorporating the piezoelectric strain gauge, so as to reduce the gap between (i) the gain of the measurement chain incorporating the resistive strain gauge and (ii) the gain of the measurement chain incorporating the piezoelectric strain gauge;

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

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

[0023] - the resistive gauge may include 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 include a thin piezoelectric element single crystal occurring in the form of a plate extending in a plane of extension defined by a first direction and a second direction normal to the first direction, of dimensions in the first direction and the second direction each greater than 100 pm and of 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 strain sensor described above, and more particularly from its sensitivity frequency range, and is therefore capable of performing static, quasi-static, or dynamic force measurements. BRIEF DESCRIPTION OF THE FIGURES

[0028] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

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

[0030] [Fig.2] The [Fig.2] represents a first mode of operation of the hybrid sensor of the [Fig.1];

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

[0032] [Fig.4] Fig.4 represents a piezoelectric element that can be used in a piezoelectric gauge of the hybrid sensor of the [Fig.l];

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

[0034] [Fig.6] The [Fig.6] represents an experimental device for validating the operation of the hybrid sensor of the [Fig.1];

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

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

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

[0038] [Fig. 10] The [Fig. 10] represents a second operating mode of the hybrid sensor of the [Fig.1];

[0039] [Fig. 11] Fig. 11 represents a third operating mode of the hybrid sensor of Fig. 1; and

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

[0041] First embodiment

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

[0043] Fig. 1 illustrates a first implementation of the invention, in the form of a hybrid sensor HYB comprising a substrate formed of a SPRT support film integrating a piezoelectric strain gauge PIEZO, a resistive strain gauge RES, and an electronic module EL.MOD for data acquisition and processing, the latter being made up 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 include electronic elements conventionally associated with gauges of these types for the detection and transmission of the signals they generate.

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

[0047] The variations in electric charge appearing at the electrodes of the piezoelectric gauges can be detected by means of a charge amplifier or directly to a voltage input of a data acquisition system, with or without a voltage preamplifier. Alternatively, an IEPE (Integrated Electronics Piezo-Electric) type circuit can be used, i.e., with a load amplifier integrated into the sensor, which preconditions the signal to make it easier to transmit over 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. These signals are combined in such a way as to obtain an output signal in which the low-frequency and high-frequency information comes predominantly from the resistive gauge and the piezoelectric gauge, respectively.

[0051] Figure 2 illustrates the operation of the hybrid sensor of Figure 1 by means of a functional diagram. When the hybrid sensor HYB is in operation, a deformation Def of the object to whose surface it is fixed is also applied to the support film SPRT and therefore to the resistive strain gauge RES and the piezoelectric strain gauge PIEZO. The object in question will be referred to below as the "instrumented object." The instrumented object is a test specimen in the case where the sensor is used to form a force sensor as illustrated by Figure 9, but any object whose deformations are to be analyzed can constitute the instrumented object: aircraft wing, pipeline, operating machine, live cable, bridge component, etc.

[0052] The blocks in [Fig. 2] inscribed within the boxes marked with dashed lines and labeled "Tr.RES" and "Tr.PIEZO" indicate transducers based on the RES gauge and the PIEZO gauge, respectively. These are functional blocks, the implementation of which is not detailed here as it is 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 forms transducers that transform the mechanical input signal into signals of an electrical nature, in a manner well known in the field.

[0054] The deformation Def induces the generation of a mechanical deformation signal din of the SPRT support film, which constitutes the input signal of the HYB hybrid sensor. In the case where the sensor does not include the SPRT support film (see variants (F), (H) and (I) illustrated by [Fig. 5]), the mechanical deformation signal is directly generated by the deformation of the gauges, which are fixed to 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) allowing the generation of 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 continuum 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 S res s 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 sensitivity SensP1EZo (which can be expressed in pC / pdef) in the case of an electrical charge measurement or in mV / pdef in the case of a voltage measurement, allowing the generation of a second electrical signal (which can be expressed in pC or mV) from the mechanical deformation signal (which can be expressed in pdef), and (ii) a unity-gain high-pass filter FiltHPi. The PIEZO gauge generates a signal SP1Ezo representative of the mechanical deformation signal Def of the instrumented object.

[0058] The FiltHPi high-pass filter reflects the fact that piezoelectric gauges are not capable of measuring the static component of a deformation and that it is common to eliminate the low frequencies of the piezoelectric gauge's response 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 due to leakage of these charges through the piezoelectric material used and through the charge amplifier (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 with 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 is the capacitance of the piezoelectric gauge.

[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 changing the leakage resistance of the gauge, its capacitance, or the charge amplifier, if applicable. The transfer function Hhp(co) of a first-order high-pass filter with infinite impulse response can be written as: [00®1 Hflr(w)=-g.Eq.l

[0063] where œ = 2 jtI is the angular frequency, f the frequency, = 2tt / ' the cutoff angular frequency, fcl the cutoff frequency, and j the complex number defined such that j2=-l.

[0064] The Sres and SPiEZ0 signals are provided as inputs to the terminals of the EL.MOD electronic module. A gain of GP1EZ0 is applied to the SRES signal and a gain of GP1EZ0 is applied to the SPiEZ0 signal.

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

[0066] hlp (œ) = j” Eq. 2

[0067] The FiltHPi and FiltLPi filters represent two examples of unity-gain filters: the magnitude 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 also two so-called "complementary" filters, in that, regardless of the frequency, the magnitude Sum of the sum of their respective transfer functions, Hhp(<ω) and HLP(ω), satisfy the equality Sum - \Hhp(uj) + Hlp ( œ ) | = Iceci being due to the fact that their cutoff frequencies have the same value.

[0068] Figure 12 illustrates this situation with curves representing respectively Sum, \H HP(^ | and \HLP^) | for complementary unitary filters having a cutoff frequency of 1 Hz, the x-axis representing the frequency f and the y-axis the gain g.

[0069] Finally, (i) the filtered SRES signal (indicated as the SRep.REs signal in [Fig. 2]) and (ii) the Spiezo signal to which the Gpiezo gain has been applied (indicated as the SRep.piEzo signal in [Fig. 2]) pass through an Add2 adder which combines these two signals to give an output signal So ut i-

[0070] In this situation, equation Eq. 3 below expresses, in the frequency domain, the constitution of the output signal so«fi:

[0071] + «)8rHLP (œ) + ditfpgpHHp(œ) Eq. 3

[0072] where So utn represents the output signal of the hybrid sensor, din the deformation signal 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 d^ sp the value of the sensitivity SensP1EZo of the piezoelectric gauge to the signal d^ Hhp{ œ) the transfer function of the high-pass filter FiltHPi, Hlp((ü) the transfer function of the low-pass filter FiltLPi, gr the value of the gain GREs, and gp the value of the piezo- gain

[0073] Equation Eq. 3 can be rewritten in the form of the following equation Eq. 4:

[0074] ^out i = ngHLP (œ) + (srgHLP (œ) + spHhp (œ) g^din Eq' 4

[0075] For the signal sOiai 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 GRes and Gpiezo-

[0076] When equations Eq. 1 and Eq. 2 are introduced into equation Eq. 4 and the gain values ​​are chosen such that g = ] j sr and g? = 11sp, then equation Eq. 5 is obtained:

[0077] Eq.5 ~ ain+ n

[0078] It can be seen that the output signal is equal to the signal din of the deformation Def applied to the hybrid sensor, plus a noise term hl^n introduced by the resistive gauge. In practice, values ​​for gr and gp will be chosen respectively equal to l / sr and l / gp to within at least 10%, preferably 5%, more preferably 1%, and even more preferably 0.1%.

[0079] The noise term present in the output signal sOutt 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 FiltLP1. The smaller the cutoff frequency f, the less noise there will be in the output signal sout.

[0080] A resistive gauge used alone provides only a very noisy measurement signal that can be expressed in the form {din + n / Sr), even though 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 j ( to ) )' ma^s only for dynamic deformations due to the very operation of a piezoelectric gauge.

[0081] Conversely, by processing and combining the signals from the two gauges of the hybrid sensor—the resistive gauge and the piezoelectric gauge—in this way, we benefit from their respective, highly complementary advantages. The portion of the output signal from the resistive gauge allows us to characterize static or low-frequency deformation, while limiting noise thanks to the low-pass filter. The portion of the output signal from the piezoelectric gauge, which is naturally low-noise, allows us to precisely track 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 and the other to the piezoelectric measurement of a deformation, allowing the characterization of a deformation of the support film and thus of the instrumented object. The Meas.ChRES measurement chain corresponding to the resistive gauge RES is modeled as consisting of the Sensés sensitivity, a noise signal source Nois, the Addl adder, the GRES gain, and the FiltLPi filter. The Meas.ChP1EZ0 measurement chain dedicated to the piezoelectric gauge PIZEO is modeled as comprising the SensP1EZo sensitivity, the FiltHPi filter, and the GP1EZ0 gain.

[0083] These measurement chains are defined by the set of 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 side of the resistive gauge and on the other hand on the side of the piezoelectric gauge.

[0084] Figure 3 illustrates simulation results of the operation of a hybrid sensor as modeled by means of 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 niche 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 cutoff frequency f fixed at 0.1 Hz. The resulting response is less noisy than the RA response, but does not follow the distortion waveform, due to a response time corresponding to Vf Y

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

[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 closely follows (very low noise) the strain pulse, 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 to g - ïj sr^ gp= s?, the high-pass and low-pass filters have the same cutoff frequency fv

[0089] The FiltHPi and FiltLPi filters can be different from those given as examples above, such as second-order filters or sets of first-order filters in series. For precise quantitative measurements, the main selection criterion for FiltHPi and FiltLPi filters is that the magnitude of the sum of their transfer functions be constant. They are then considered to be so-called "complementary" filters.

[0090] For example, FiltHPi will be a first-order filter when the piezoelectric gauge's charge is measured as a voltage; in this case, a first-order filter will also be chosen 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] The choice of these filters determines the accuracy of the hybrid sensor's electrical response to the mechanical deformation it characterizes. Taking 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 accurate junction between (i) a signal Srep.res essentially consisting of low-frequency components obtained using the resistive gauge and (ii) a signal SRep.PiEzo essentially consisting of high-frequency components obtained using the piezoelectric gauge.Thus, the informational content of the low-frequency components of the output signal Somi comes mainly from the resistive signal SRES, while the informational content of the high-frequency components of the first output signal SOuti comes mainly from the piezoelectric signal SP1EZo. 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 using 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, 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.

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

[0095] The resistive strain gauge comprises an electrical resistance formed of a conductive material such as a nickel-copper alloy, integrated on a flexible support, as described for example in patent application FR2693795 or US patent 2386879. 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 can 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 made of 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. 1].

[0097] A first type of piezoelectric strain gauge is based on the use of a piezoelectric crystal material with relatively good sensitivity to deformation and stability over time, but which is thick and rigid, often housed in a metal casing to which it is mechanically attached. 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 gauge is based on the use of composite structures comprising PZT (or lead zirconate titanate) bars located between sheets of polymer materials, or on polymer piezoelectric films known as "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 some of the applications targeted for the hybrid sensor described above.

[0099] A third type of piezoelectric strain gauge, particularly suited to the hybrid sensor described above, employs as a sensing element a thin single-crystal 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 and second directions each greater than 100 pm and a thickness less than 50 pm, a ratio of the thickness to the dimension in the first direction or the dimension in the second direction being less than 0.1.The piezoelectric element can exhibit 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 crystalline orientation of the element being such that abs(Sy / Sx ) < 0.1, corresponding to a so-called "unidirectional" sensitivity, abs((Sy+Sx) / Sx) < 0.1, corresponding to a so-called "bidirectional" sensitivity, or for at least two first directions of the extension plane making an angle between them 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 accuracy, sensitivity, conformability, flexibility, lightness, stability, linearity, directivity and applicability to wide ranges of deformations, as described in detail in French patent application FR2303635.

[0101] In particular, this thin piezoelectric element can have characteristics 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 of 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 makes it possible to measure deformations greater than 5000 micrometers per meter with a resolution on the order of 1 nanometer per meter of deformation. These figures can be compared with those of conventional resistive gauges which make it possible to measure deformations up to 12000 micrometers per meter (for only a small number of cycles) but with a much lower resolution, on the order of 1 micrometer of deformation per meter.

[0103] In comparison, the enclosed piezoelectric gauges of the first type of 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 can 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 include at least one thin piezoelectric element as described above, fixed on a flexible sheet.

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

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

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

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

[0110] In contrast, the single-crystal piezoelectric thin element described above meets 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 whose shape is not necessarily planar) is paramount here, and its sensitivity allows for the detection of small-amplitude deformations.

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

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

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

[0114] In addition to the thin element EL.PIEZO, in the example of [Fig. 1], a charge amplifier C.AMP is also fixed to the SPRT substrate and functionally connected to two electrically conductive layers acting as electrodes, formed respectively on two opposite faces of the thin element EL.PIEZO. The charge amplifier's function is to produce a voltage corresponding to the input load, which corresponds to the charge generated by the EL.PIEZO element during its deformation, for the purpose of electronically processing the generated electrical potential and effectively measuring the deformation of the EL.PIEZO element.

[0115] The charge amplifier can be of the IEPE type, i.e., powered by a direct current, for example between 4 and 20 mA, generated by a data acquisition system, and returning the measurement signal to the power supply cable, 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 ensures 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 substrate 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, and 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 to 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, and more preferably less than 0.01. The xy extension plane 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 coordinate system.

[0118] The SPRT support can have a thickness between 5 and 300 pm.

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

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

[0121] The SPRT substrate may consist of, or be replaced by, a flexible support such as a flexible printed circuit board, known as a "flex PCB," composed of layers of electrically insulating polymer and copper layers, 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 with 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 the electronic control module for these elements. Alternatively still, 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 using 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 setup used is illustrated in [Fig. 6] and comprises a horizontal steel plate H.P1 rigidly fixed to a vertically extending support V.Sprt. A resistive gauge RES and a piezoelectric gauge PIEZO are fixed to the steel plate. The experiment consists of placing a mass M on the plate and 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 of the piezoelectric gauge.

[0125] The responses are expressed in pdef. The mass is placed at t=29.7s and removed at t=31.1s. The experimental parameters ensured that g ≈ 1 / Sr, g ≈ [ / Sp], and that the high-pass and low-pass filters had the same cutoff frequency f, fixed here at 0.8 Hz. These experimental results confirm the simulation results illustrated in [Fig. 3]: the measurement performed using the hybrid sensor corrects 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, whereas it is only 0.01 pdef for the hybrid sensor. The noise level is therefore reduced by a factor of 50, representing a 34 dB noise reduction, which is a very significant reduction.

[0127] Indeed, the output signal of the hybrid sensor is therefore both low-noise and, moreover, exhibits an immediate response with a zero or near-zero response time. There is no need for a stabilization time after the mass has paused in order 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. Figure 8 illustrates other possible configurations for the hybrid sensor, which can potentially be combined with each other.

[0129] In (A), [Fig. 8] illustrates a configuration that differs from that of [Fig. 1] in that the EL.MOD electronic module is not fixed to the SPRT support on which the resistive and piezoelectric strain gauges are mounted. A wire connection can functionally link the electronic module to the two strain gauges.

[0130] In (B), [Fig. 8] illustrates a different configuration from that shown in (A), in that the resistive gauge RES and the piezoelectric gauge PIEZO are superimposed on one another. Here, the resistive gauge RES is shown as fixed to the piezoelectric gauge, which is 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.1] 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 gauge pairs and their orientations Different relative positions are possible, such as 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 in the configuration illustrated by [Fig. 1].

[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 SOuti- signal

[0134] It is also possible for a resistive strain gauge to be formed from a variable electrical resistance element etched into an electrode of a piezoelectric gauge or formed from one or more metallic layers deposited on this electrode. This ensures that the resistive gauge and the piezoelectric gauge measure exactly the same strain, particularly due to the thinness of the piezoelectric element. This cointegration appears more complicated with other technologies, such as PZT bars. The hybrid sensor could naturally include such a combination of a resistive gauge and a piezoelectric gauge. The resistive gauge could then be considered attached to the support film via the piezoelectric gauge.

[0135] Figure 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 sensing 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 Figure 5, which represents a cross-sectional view of the HYB sensor along a plane passing through points A and B of Figure 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 may be interposed between the electrically conductive element and the electrode EL2. The EL.PIEZO and EL. elementsThe RES elements are integrated into a substrate formed from a flexible SPRT support film as described above, attached to one of the surfaces of this support as illustrated here. A C.AMP charge amplifier 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 considered for the hybrid sensor, as long as these gauges are configured to characterize the same deformation, preferably simultaneously, and the signals they generate can be combined.

[0137] Figure 5 illustrates in (B) a variation of the HYB hybrid sensor shown 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. However, in this configuration, a charge amplifier separate from the rest of the acquisition electronics is not required, as its function is integrated into an EL.MOD electronic module that directly manages the charge amplification. Furthermore, the electronic module is preferably attached to a surface of the SPRT substrate opposite to that on which the EL.PIEZO and EL.RES elements are mounted. Indeed, when attaching the hybrid sensor to 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 being instrumented.

[0138] Figure 5 illustrates in (D) another variation of the HYB hybrid sensor shown 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 contained between two meanders of the EL.RES element so that the signals generated therein are as representative as possible of the same deformation of the instrumented object.

[0139] Figure 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 multi-layered SPRT substrate, 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] Figure 5 illustrates in (F) another variation of the HYB hybrid sensor shown 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 attached to the surface of an object to be instrumented, in the same way as if it had an SPRT support substrate.

[0142] Figure 5 illustrates in (H) a variation of the HYB hybrid sensor shown in (F). In this configuration, an electrically conductive layer, Cond, is present, positioned here between the EL.RES element and the EL.PIEZO element and its electrodes. It is electrically isolated from these elements by two electrically insulating layers, Ins, respectively interposed between the Cond layer and the EL.RES element, and between the Cond layer and the EL.PIEZO element and its electrodes. The Cond layer can serve to functionally connect the EL.RES element and / or the EL.PIEZO element to an external electronic control device such as the MOD.EL module. The Cond layer can also serve to prevent capacitive coupling between the EL.RES element and the EL.PIEZO element. In this case, the COND layer is typically connected to the sensor ground.

[0143] Fig. 5 illustrates in (H) a variation of the hybrid sensor HYB 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 in Figures 1 and 8 is primarily intended for characterizing the deformation of an instrumented object to which the hybrid sensor is attached. However, it can also 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 illustrated example), to a test specimen. The function of the test specimen 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 comprising a force cell CELL equipped with four hybrid strain sensors HYB. In (A), with no force applied, the force cell is at rest. Under load from the application of a force F, as 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. Following 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), measuring its deformation using the hybrid strain sensors allows the amplitude of the force F applied to the cell to be deduced. Of course, other types of test bodies can be used.

[0147] Second embodiment

[0148] The first embodiment is based on the identity of the cutoff frequencies of the high-pass filter FiltHPi of the piezoelectric gauge and the applied low-pass filter FiltLPi 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 precisely estimated. 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 sensing 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 for the output signal So uti to be 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 employing a second low-pass filter FiltLP2 and a second high-pass filter FiltHP2 with the same cutoff frequency f c2, higher than the cutoff frequency f targeted for the filters of the first stage FiltLPi and FiltHPi.

[0151] The resistive gauge signal is then used up to the frequency / c2 above the cutoff frequency f of the first stage, and the piezoelectric gauge signal is used only above this second cutoff frequency / c2. The filters of this second filtering stage are implemented in the same way (either both analogically or both digitally), so it is easy to ensure that they have exactly the same cutoff frequency / c2, which guarantees an output of the second filtering stage without signal distortion.

[0152] Typically, the cutoff frequency of the second stage, f, will be chosen to be 5 times greater than fr, preferably 10 times greater than f, in order to verify the inequality f2 > fc. The higher f2 is, the less the difference between the cutoff frequencies of the first-stage filters FiltLPi and FiltHPi will influence the output signal, but the higher the residual noise on the output signal will be.

[0153] Figure 10 illustrates, by means of a functional diagram, the operation of the hybrid sensor according to this second embodiment. The second embodiment differs 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 Figure 10, reference can be made to the first embodiment and to Figure 2.

[0154] The Add3 adder is configured to sum (i) the output signal So uti, to which filtering by the FiltHP2 filter has been applied, and (ii) the signal from the gauge The resistive RES signal, to which the gain Grès and then filtering by the low-pass filter FiltLP2 were applied, is obtained at the output of the adder Add3. This output signal, SOut2, corresponds to the SOuti signal, largely freed from the influence of the difference between the cutoff frequencies of the FiltLPi and FiltHPi filters of the first stage.

[0155] Third embodiment

[0156] Another problem that can be encountered in practice when using the hybrid sensor is the effects of temperature. Every sensor is subject to a drift in its sensitivity with temperature; that is, 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 with temperature. Since the physical principles on which resistive and piezoelectric gauges are based are completely different, the sensitivities of two gauges of these two types are expected to drift differently with temperature.

[0157] The first problem is that temperature drift introduces bias into measurements taken with the hybrid sensor if it is not compensated for. The less the sensitivity drifts with temperature, the better the sensor is considered to be of good quality. This first problem exists for all sensors, and numerous solutions have been developed to address it.

[0158] The second problem arises from the differential drift between the two gauges, the resistive gauge and the piezoelectric gauge. Indeed, for the sensor output signal, which results from the combination of 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 deformation amplitude. This identity between the amplitudes is ensured, in the preceding embodiments, by the two gain blocks Grès and GP1Ezo, whose gain values ​​gr and gp are considered fixed in the hybrid sensor modeling used in this document.

[0159] For a reference temperature To, gains gr and gp can be chosen so that the SRep.REs and SreP.Piezo signals have the same amplitude when measuring a given strain. However, if the two gauges have different temperature-dependent sensitivity drifts (sr and sp), then at a temperature other than 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 Meas.Ch measurement chains RES and Meas.ChP1EZ0 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 every instant, (i) the respective gains of the two measurement chains Meas.ChREs and Meas.ChP1EZ0 remain equal to each other and, (ii) the respective cutoff frequencies of the two measurement chains Meas-Ch^s and Meas.ChP1EZO, which are those of the two filters FiltLPi and FiltHPi, are equal to each other. This identity between the overall gains and cutoff frequencies of the two measurement chains Mcas.ChR|S and Meas.ChP1EZ0 is ensured in this embodiment by means of an adaptive filtering technique described below using the functional block diagram in [Fig. 11].

[0161] An adaptive filtering according to this embodiment has the principle of adjusting in real time (i) the cutoff frequency of the low-pass filter FiltLPi applied to the signal of the resistive gauge and (ii) the value gp_CoiT of a gain GP1EZO_cOrr placed in series between the gain GP1EZ0 and the adder Add2.

[0162] The cutoff 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, whose characteristics are likely to vary depending on the temperature or other physical parameters.

[0163] Similarly, the gp_ Coir value of the GP1EZO_cOrr gain is continuously adjusted so as to make the overall gain spxgpxgp Coir of the Meas.ChP1EZ0 measurement chain tend towards the overall gain srxgr of the Meas-Ch^s measurement chain.

[0164] Figure 11 shows a functional block diagram explaining the algorithm implemented for adjusting the cutoff frequency of the FiltLPi filter and the gain correction value GP1EZO_cOrr of the Meas.ChP1EZ0 measurement chain. The principle of the first embodiment illustrated in Figure 2 is repeated, to which is added adaptive filtering represented by the elements FiltHP Est, LMSAig, Sub and GP1EZO_cOrr. For the other elements of Figure 11, refer to the first embodiment and Figure 2.

[0165] The principle employed 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 exploited to continuously calibrate the cutoff frequency of the FiltLPi filter, and the gp_CoiT value of the GP1EZO_cOn gain. For this, a classic adaptive filtering strategy is used, employing, for example, a Least Mean Squares (LMSAig) algorithm to adapt the characteristics of the transfer function of the FiltLPi filter and the overall gain of the Mes.ChP1EZ0 measurement chain.

[0166] The low frequencies of the signal, typically below 100 Hz, can be used to prevent the resistive gauge signal from becoming too noisy. This can be achieved, for example, by filtering the signals entering the LMSAig algorithm block using a low-pass filter with a cutoff frequency of 100 Hz.

[0167] The resistive signal Sres of the resistive gauge to which the gain Grès has been applied is (i) filtered by a high-pass filter FiltHP_Est with estimated transfer function HhpEST(co), whose gain and cutoff 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 SRES signal of the resistive gauge adjusted by means of the gain Grès and filtered by the estimated high-pass filter FiltHPEst, from which is subtracted (ii) the SRep.PiEzo signal representing the piezoelectric signal SPiezo by means of the Sub block. The result of this subtraction is an error signal e which is representative of a difference between (i) the SRep.REs signal from the Meas.ChREs measurement chain before it is filtered by FiltLPi but which is filtered by FiltHP_estet (ii) the SRep.PiEzo signal from the Meas.ChP1EZo measurement chain before it passes through the GplEzo_corr* gain. The SRep signals.PiEzo and SRep.PiEzo are respectively derived from the piezoelectric signal SPiezo and the resistive signal SREs-.

[0168] The FiltHP_Est filter is initialized by default 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 LMSai8 block.

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

[0170] The LMSAig algorithm is conventionally configured to iteratively modify the gain and cutoff frequency of the estimated high-pass filter FiltHP_Esrde so as to minimize the error e. At each iteration, or when deemed necessary, the cutoff frequency of the estimated filter FiltHP_EsT is transferred to the filter FiltLPi, and the gain gp_COIT is updated with the inverse of the gain of the estimated filter FiltHP_Esrde. 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.Using PiEzo as a representative of the piezoelectric signal SPiEzo, we obtain an error e to be minimized by adaptive filtering. This filtering ensures (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 given time, even if the filter parameters change over time. By proceeding specifically as illustrated in [Fig. 11], the drift of the sensitivity of the hybrid sensor output is equal to that of the resistive gauge, which generally drifts less rapidly than the sensitivity of the piezoelectric gauge.

[0171] The transfer of parameters is conventionally represented by a diagonal arrow in the background of the FiltLPi and FiltHP Est filters and the variable gain GP1EZo_cOrr. In particular, for copying parameters such as gain and cutoff frequency, it is convenient to implement this third embodiment numerically.

[0172] In the example shown above, the cutoff frequency of the FiltLPi filter, which can be a digital filter with a variable and controllable cutoff frequency, is adjusted to follow the drift of the cutoff frequency of the FiltHPi filter. Thus, the cutoff 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, via the gain GPIEZo_co™, to ensure its equality with the overall gain of the measurement chain Ch.MesRES. An alternative based on the same principle would be to correct the overall gain of the measurement chain Mes.ChRES to ensure its equality with the overall gain of the measurement chain Ch.MesP1EZ0: this would simply involve placing a variable gain on the measurement chain Meas.ChRES and controlling its gain value using 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 needed (a gain and a cutoff frequency for a first-order high-pass filter), adaptation of known methods can be implemented by those skilled in the art. For example, see the article by Shynk JJ, "Adaptive IIR filtering," 1989, EEE ASSP Magazine, 6, 4-21.

[0175] Other methods for estimating the gain and cutoff frequency are possible, for example by using a Fast Fourier Transform (FFT) or Recursive Least Squares (RLS). Mathematically, the problem is a matter of identifying a first-order system from a noisy signal, which is a very classic and well-documented problem.

[0176] In the present embodiment, only two filters are involved, and the FiltLPi filter is adjusted in response to the changing characteristics of the FiltHPi filter. More generally, the aim is to make the cutoff frequency and gain of the entire Meas.ChP1EZ0 measurement chain equal to those of the entire Meas.ChP1EZ0 measurement chain by reducing the difference between them. The convergence can be achieved by The cutoff frequency of the Meas-Ch^s measurement chain can be adjusted to match that of the Meas.ChP1EZ0 measurement chain, as illustrated in [Fig. 11], or conversely, the cutoff frequency of the Meas.ChP1EZ0 measurement chain can be adjusted to match that of the Meas.ChREs measurement chain. Symmetrically, the overall gain of the Meas.ChP1EZ0 measurement chain can be adjusted to match that of the Meas.ChRES measurement chain, or vice versa. This can be achieved by adjusting the gain and cutoff frequency parameters of a single filter, or by adjusting the gain and cutoff frequency parameters of filters combined together, for example, in series, or by using a corrective gain as illustrated in [Fig. 11]. "Convergence" refers to an operation that reduces the difference between two values.

[0177] Furthermore, depending on the conditions (drift of the cutoff frequency but not of the gain or vice versa, drift occurring at the level of the piezoelectric gauge or at the level of the resistive gauge or vice versa), the electronic module can be configured to adjust only one or the other of the gain and the cutoff 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, to say of two values ​​that they 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 alternative embodiments may be made without departing from the scope of the invention as defined by the claims.

Claims

1. Demands 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, T1P1EZO, T2P1EZ0) 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.piEzo) from the piezoelectric signal (SP1EZ0), so as to generate a first output signal (SOuti) which is representative of the deformation (Def) of the instrumented object (CELL); in which: - the first signal (sRep RES) from the resistive signal (SRES) is generated by a deformation measurement chain (Def) including a first low-pass filter (FiltLPi); - the second signal (sRep.pjEzo) from the piezoelectric signal (SPiEZ0) is generated by a deformation measurement chain (Def) including a first high-pass filter (FiltHPi); - the measurement chain for generating the first signal (sRep REs) from the resistive signal (Sres) includes the application of a first gain (GRES); - the measurement chain for generating the second signal (sRepPiEZo) from the piezoelectric signal (SP1EZ0) includes the application of a second gain (GPjEZo), - a cutoff frequency of the first low-pass filter (FiltLPi) and a cutoff frequency of the first high-pass filter (FiltHPi) are equal to within at least 10%, and the first gain (GRES) and the second gain (GPiEZ0) are configured such that a signal (SRep REs) from the resistive gauge (RES) and a signal (SRep.piEzo) from the piezoelectric gauge (PIEZO) have the same amplitude for a given deformation amplitude (Def) of the instrumented object, to within at least 10%.

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 (fj) and a frequency content for frequencies above the given threshold; - an informational content of the low-frequency components of the first output signal originates mainly from the resistive signal (Sres); and - an informational content of the high-frequency components of the first output signal originates mainly from the piezoelectric signal (Spiezo).

3. The hybrid sensor according to claim 1, configured such that: - the first gain (GRES) has a value (gr) equal to at least 10% to an inverse of the sensitivity (sr) of the resistive gauge (RES), and - the second gain (GP1EZo) has a value (gp) equal to at least 10% to an inverse of the sensitivity (sp) of the piezoelectric gauge (PIEZO).

4. The hybrid sensor according to any one of claims 1 to 3, 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 (Souti) 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.

5. The hybrid sensor according to claim 4, wherein: - the second low-pass filter (FiltLP2) and the second high-pass filter (FiltHP2) 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 (FiltLP2) and the cutoff frequency of the second high-pass filter (FiltLH2) are each at least 5 times higher than the cutoff frequency of the first low-pass filter (FiltLPi) and that the cutoff frequency of the first high-pass filter (FiltLHi).

6. The hybrid sensor according to any one of the preceding claims 1 to 5, the electronic module being further configured so as to, based on a comparison (e) between (i) the first signal (sRepRES) from the resistive signal (SRES) before it passes through the first low-pass filter (FiltLPi) and (ii) the second signal (sRep.PiEzo) from the piezoelectric signal (SP1EZo): - reduce a difference between (i) a cutoff frequency of a measurement chain (Ch.MeasnEs) incorporating the resistive strain gauge (RES) and a cutoff frequency of a measurement chain (Ch.MeasP1EZo) incorporating the piezoelectric strain gauge (PIEZO), and / or - reduce a difference between (i) a gain (sr, gr) of a measurement chain (Ch.MeasRES) incorporating the resistive strain gauge (RES) and (ii) a gain (sp, gp, gpCorr) of a measurement chain (Ch.MeasP1EZ0) integrating the piezoelectric strain gauge (PIEZO).

7. The hybrid sensor according to claim 6, the electronic module being configured so as to, based on the comparison (e) between (i) the first signal (sRepRES) from the resistive signal (Sres) before it passes through the first low-pass filter (FiltLPi) and (ii) the second signal (sReP.piEzo) from the piezoelectric signal (SPiezo): - converge the cutoff frequency of the first low-pass filter (FiltLPi) to the cutoff frequency of the first high-pass filter (FiltHPi) by means of adaptive filtering (FiltLPi, FiltHP Est, GP1EZo_corr LMSAig), and / or - adjust the value (gp_con-) of a correction gain (GPiEZo_cot) placed in series on one of (i) the measurement chain (Ch.Meas^s) incorporating the resistive strain gauge (RES) and (ii) the measurement chain (Ch.MeasP1EZ0) integrating the piezoelectric strain gauge (PIEZO), so as to reduce the gap between (i) the gain (sr, gr) of the measurement chain (Ch.(Meas^s) integrating the resistive strain gauge (RES) and (ii) the gain (sp, gp, gp_cOrr) of the measurement chain (Ch.MeasP1EZ0) integrating the piezoelectric strain gauge (PIEZO).

8. The hybrid sensor according to any one of the preceding claims 1 to 7, the piezoelectric gauge and the resistive gauge being (i) integrated into a support substrate (SPRT) and side by side or (ii) integrated into a support substrate (SPRT) and superimposed on one another.

9. The hybrid sensor according to claim 8, the electronic module (EL.MOD) being integrated into the support substrate (SPRT).

10. The hybrid sensor according to any one of the preceding claims 1 to 7 and 9, the resistive gauge (RES) comprising an electrically conductive element (EL.RES) formed in an electrically conductive layer deposited on an electrode (EL2) of the piezoelectric gauge (PIEZO).

11. The hybrid sensor according to any one of the preceding claims 1 to 10, the piezoelectric gauge and the resistive gauge each being part of a rosette.

12. The hybrid sensor according to any one of the preceding claims 1 to 11, wherein the strain gauge (PIEZO) comprises a single-crystal piezoelectric thin 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, of dimensions (Lx, LY) in the first and second directions each greater than 100 pm and of thickness (Lz) less than 50 pm, a ratio of the thickness to the dimension (Lx) in the first direction or the dimension (Ly) in the second direction being less than 0.

1.

13. Force sensor (F.SENS) comprising a test body (CELL) on which is fixed the hybrid sensor (HYB) according to any one of the preceding claims 1 to 2.

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