Hybrid strain sensor combining a piezoelectric gauge and a resistive gauge

Abstract

Disclosed is a 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 in such a way that the piezoelectric strain gauge and the resistive strain gauge generate a piezoelectric signal and a resistive signal, respectively, in response to a strain on an object instrumented with the hybrid sensor (HYB), and the electronic module being configured to perform a linear combination of a first signal (sRep.RES) obtained from the resistive signal (SRES) and of a second signal (sRep.PIEZO) obtained from the piezoelectric signal (SPIEZO), so as to generate a first output signal (SOut1) that is representative of the strain (Def) on the instrumented object (CELL).

Description

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] Deformation measurements 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 aircraft wing, or even biological measurements on 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 these, we can mention resistive and piezoresistive strain gauges, piezoelectric strain gauges, capacitive strain gauges, and magnetostrictive strain gauges.

[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 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 integrated on an object to be characterized, remains unmet.

[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] To achieve this aim, 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 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 perform a linear combination of a first signal (s Rep.RES ) from the resistive signal (S RES ) and a second signal (s Rep.PIEZO ) from the piezoelectric signal (S PIEZO ), so as to generate a first output signal (S Out1) which is representative of the deformation (Def) of the instrumented object (CELL).

[0008] Such a hybrid strain 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 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 characteristics of the sensor according to the invention, considered individually or in any technically feasible combination:

[0013] - a direction of greater sensitivity of the piezoelectric strain gauge can be aligned with a direction of greater sensitivity of the resistive strain gauge;

[0014] - the hybrid sensor can be configured in such a way that the first signal from the resistive signal and the second signal from the piezoelectric signal are representative of an amplitude of the deformation given at the same time;

[0015] - 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;

[0016] - the first signal from the resistive signal can be generated by a deformation measurement chain 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 can be equal to within at least 10%;

[0017] - the measurement chain for generating the first signal from the resistive signal may comprise the application of a first gain, the measurement chain for generating the second signal from the piezoelectric signal may comprise 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%;

[0018] - the first gain may have a value equal to at least 10% of 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;

[0019] - the electronic module can be configured so as to generate a second output signal 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;

[0020] - the second low-pass filter and the second high-pass filter may 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 and the cut-off frequency of the second high-pass filter are each at least 5 times higher than the cut-off frequency of the first low-pass filter and the cut-off frequency of the first high-pass filter;

[0021] - 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 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 reduce 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;

[0022] - the electronic module being able to 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 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;

[0023] - the piezoelectric gauge and the resistive gauge being able to be (i) integrated into a support substrate and next to each other or (ii) integrated into a support substrate and superimposed on each other;

[0024] - the electronic module can be integrated into the support substrate;

[0025] - the resistive gauge may comprise an electrically conductive element formed in an electrically conductive layer deposited on an electrode of the piezoelectric gauge;

[0026] - the piezoelectric gauge and the resistive gauge can each be part of a rosette; and

[0027] - the strain gauge may comprise 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 with 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.

[0028] The invention extends to a force sensor comprising a test body on which the hybrid sensor according to the invention is fixed.

[0029] Such a force sensor benefits from the advantages of the strain sensor described above, and more particularly from its frequency sensitivity range, and is therefore capable of carrying out static, quasi-static or dynamic force measurements.

[0030] The invention extends to a scale comprising a mechanical load receiving element; at least one force sensor according to the invention, configured to generate an electrical signal representative of a mechanical load applied to the mechanical load receiving element; and a measuring device configured to receive and process the electrical signal, and optionally display a weight resulting from the measurement. BRIEF DESCRIPTION OF THE FIGURES

[0031] 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:

[0032] It represents a hybrid strain measurement sensor, combining a resistive gauge and a piezoelectric gauge;

[0033] This represents a first mode of operation of the hybrid sensor of the;

[0034] It represents a simulation of the first mode of operation;

[0035] Larepresents a piezoelectric element that can be used in a piezoelectric gauge of the hybrid sensor of the;

[0036] Larepresents particular implementations of a hybrid sensor integrating a piezoresistive gauge and a piezoelectric gauge;

[0037] It represents an experimental device for validating the operation of the hybrid sensor of the;

[0038] Represents validation measurements obtained using the device;

[0039] Illustrates different configurations of a hybrid sensor combining one or more resistive gauges and one or more piezoelectric gauges;

[0040] Larepresents a force sensor employing hybrid sensors such as those of Figures 1 and 8;

[0041] It represents a second mode of operation of the hybrid sensor of the;

[0042] It represents a third mode of operation of the hybrid sensor of the;

[0043] La represents the characteristic transfer function modules of a pair of complementary filters;

[0044] Illustrates a generic mode of operation of the hybrid sensor of the;

[0045] Illustrates a preferential relative positioning between a piezoelectric gauge and a resistive gauge in the hybrid sensor;

[0046] Illustrates examples of force cells; and

[0047] Illustrates a scale equipped with force sensors using hybrid sensors of the. DETAILED DESCRIPTION OF THE INVENTION

[0048] First embodiment

[0049] An embodiment of the present invention is described by means of Figures 1 to 9 and the associated passages below.

[0050] 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.

[0051] The electronic module has a first pair of terminals T1 RES and T2 RES dedicated to the measurement of the electrical resistance of the resistive gauge, a second pair of terminals T1 PIEZO and T2 PIEZO dedicated to the measurement of electrical charges generated by the piezoelectric gauge, and a pair of outputs S1 and S2 dedicated to the transmission to the outside of a signal representative of the deformation of the support film.

[0052] The electronic module may comprise electronic elements conventionally associated with gauges of these types for the detection and transmission of the signals they generate.

[0053] The resistivity variations of the resistive gauge can be detected by a Wheatstone bridge, which converts the variation in the resistance of the gauge into a voltage measurable by the acquisition system.

[0054] The variations in the electrical charge appearing on 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 consider using an IEPE (Integrated Electronics Piezo-Electric) type assembly, i.e. with a charge amplifier integrated into the sensor, which preconditions the signal so that it is easier to transport on cables without adding noise.

[0055] 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 filtering operations, which can be more easily implemented digitally.

[0056] Principle of measurement by hybrid sensor

[0057] The principle is to form a linear combination of at least one signal generated by a processing chain of a signal generated by a resistive strain gauge and at least one signal generated by a processing chain of a signal generated by a piezoelectric strain gauge on the other hand. These signals generated by their respective processing chains are combined linearly with each other 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. It is understood that this is

[0058] Generally speaking, a linear combination of a number N of signals S means a weighted sum Sum LC of these signals S, for example according to the following equation:

[0059]

[0060] where i represents a natural number, the amplitude of an i èmesignal and a weighting coefficient of i ème signal. The coefficients can be positive or negative. The sensor can be designed to use identical coefficients, for example all equal to 1.

[0061] The illustrates the operation of a hybrid sensor represented by the, 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 is also applied to the support film SPRT and therefore simultaneously 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 the, 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.

[0062] The blocks 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 implementation of which is not detailed here because 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.

[0063] The combination 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.

[0064] The deformation Def induces the generation of a signal d inof mechanical deformation 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 the), the mechanical deformation signal is directly generated by the deformation of the gauges, integral with the surface of the instrumented object.

[0065] 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 Sens RES(which can be expressed in mV / µdef) allowing the generation of a first electrical signal (which can be expressed in mV) from the mechanical deformation signal (which can be expressed in µdef, 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 Add1. The gauge RES generates a resistive signal S RES representative of the signal d in of the mechanical deformation Def of the instrumented object.

[0066] 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 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.

[0067] 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 Sens PIEZO (which can be expressed in pC / µdef) in the case of a measurement in electrical charges or in mV / µdef in the case of a measurement in voltage, allowing the generation of a second electrical signal (which can be expressed in pC or in mV) from the mechanical deformation signal (which can be expressed in µdef) and (ii) a high-pass unity gain filter Filt HP1 . The PIEZO gauge generates an S signal PIEZO representative of the signal d in of the mechanical deformation Def of the instrumented object.

[0068] The high-pass filter Filt HP1reflects 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.

[0069] 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 related 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.

[0070] 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.

[0071] 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 H HP (ω) of a high-pass filter with infinite impulse response of order 1 can be written:

[0072] Eq. 1

[0073] Or is the pulsation, the frequency, the cut-off pulse, the cutoff frequency, and j the complex number defined such that j²=-1.

[0074] The S signals RES and S PIEZO are provided as inputs to the terminals of the EL.MOD electronic module. A gain G RES is applied and signal S RES and a gain G PIEZO is applied to signal S PIEZO .

[0075] In addition, the signal S RES is passed through a unity gain low-pass filter Filt LP1 after the gain G RES has been applied to it. The low-pass filter Filt LP1 preferably has the same cutoff frequency that the Filt filter HP1 , 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 Filt LP1 is a first-order infinite impulse response filter, then its transfer function H HL (ω) can be written:

[0076] Eq. 2

[0077] Filt filters HP1 and Filt LP1 represent two examples of unity gain filters: the modulus of their transfer function in their bandwidth (f>>fc for a high pass, f< <fc pour un passe bas) est égal à 1. Filt HP1 and Filt LP1 are also two so-called "complementary" filters, in that, whatever the frequency, the module of the sum of their respective transfer functions, And check equality this is due to the fact that their cutoff frequencies have the same value.

[0078] Figure 12 illustrates this situation with curves representing respectively Sum, And for complementary unitary filters with a cutoff frequency of 1 Hz, the abscissa axis representing the frequency f and the ordinate axis the gain g.

[0079] Finally, (i) the signal S RES filtered (indicated as the signal S Rep.RESon the) and (ii) the signal S PIEZO to which the gain G PIEZO was applied (indicated as signal S Rep.PIEZO on the) pass through an adder Add2 which sums these two signals to give an output signal S O ut 1.

[0080] In this situation, the equation Eq. 3 below expresses, in the frequency domain, the constitution of the output signal :

[0081] Eq. 3

[0082] where S O ut1 , represents the output signal of the hybrid sensor, the signal of the deformation of the instrumented object and therefore of the support film, the noise signal Nois, s r the value of sensitivity Sense RES from the resistive gauge to the signal , s p the value of sensitivity Sense PIEZO from the piezoelectric gauge to the signal , the transfer function of the high-pass filter Filt HP1 , the transfer function of the low-pass filter Filt LP1 , g r the value of the gain G RES , and g p the value of the gain G PIEZO .

[0083] Eq. 3 can be rewritten as Eq. 4:

[0084] Eq. 4

[0085] For the signal is 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 ​​g r and g p of the two gain blocks G RES and G PIEZO .

[0086] When we introduce the equations Eq. 1 and Eq. 2 ( ) in equation Eq. 4 and that the values ​​of the gains are chosen in such a way that And , then we obtain the equation Eq. 5:

[0087] Eq. 5

[0088] We note that the output signal is equal to the signal of the deformation Def applied to the hybrid sensor, plus a noise term introduced by the resistive gauge. In practice, we will choose for g r and g p values ​​respectively equal to 1 / s r and 1 / g p to at least 10%, preferably 5%, more preferably 1%, and even more preferably to 0.1%.

[0089] The noise term present on the output signal 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 Filt LP1 . The higher the cutoff frequency will be small, the less noise there will be in the output signal .

[0090] A resistive gauge used alone only provides a very noisy measurement signal which can be expressed in the form , 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 , but only for dynamic deformations due to the very operation of a piezoelectric gauge.

[0091] On the other hand, by processing and performing a linear combination of the signals from the two gauges of the hybrid sensor, the resistive gauge and the piezoelectric gauge, as described above, 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 the 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.

[0092] 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 allowing to characterize a deformation of the support film and therefore of the instrumented object. The Meas.Ch measurement chain REScorresponding to the resistive gauge RES is modeled as being formed from the sensitivity Sens RES , a source of the noise signal Nois, the adder Add1, the gain G RES and the Filt filter LP1 . The Meas.Ch measuring chain PIEZO dedicated to the PIZEO piezoelectric gauge is modeled as including the sensitivity Sens PIEZO , the Filt filter HP1 and the gain G PIEZO .

[0093] These measurement chains are defined by the set of operations applied to the input signal d in until 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.

[0094] Illustrates the 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 µdef, and modeled as a square wave function.

[0095] In (A), it shows the RA response of the resistive gauge (without filtering), which perfectly follows the deformation window but is very noisy.

[0096] In (B), Figure 3 shows the RB response of the resistive gauge filtered by means of a low-pass filter with cut-off frequency fixed at 0.1 Hz. The resulting response is less noisy than the RA response, but does not follow the deformation window, due to a response time corresponding to 1 / .

[0097] In (C), the RC response of the piezoelectric gauge is shown. This 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 gate is not maintained.

[0098] 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 to , , and the high-pass and low-pass filters have the same cutoff frequency .

[0099] Filt filters HP1 and Filt LP1may be filters other than those exemplified above, such as second-order filters or sets of first-order filters in series. For accurate quantitative measurements, the main selection criterion for filters Filt HP1 and Filt LP1 is that the modulus of the sum of their transfer functions is constant. We then consider that these are so-called "complementary" filters.

[0100] For example, for Filt we will have HP1 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 Filt filter LP1 to ensure complementarity between the two filters. Other cases are possible, for example with a load measurement of the piezoelectric gauge.

[0101] It is the choice of these filters that determines the fidelity of the electrical response of the hybrid sensor with respect to the mechanical deformation that it characterizes. If we take the example of the, we understand that the "complementary" aspect of the two filters makes it possible to obtain an output signal S Out1 representative of the deformation Def, with a continuous and quantitatively exact junction between (i) a signal S Rep.RES consisting mainly of low-frequency components obtained by means of the resistive gauge and (ii) a signal S Rep.PIEZO consisting mainly 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 S Out1 comes mainly from the resistive signal S RES while the information content of the high-frequency components of the first output signal S Out1 comes mainly from the piezoelectric signal S PIEZOWe will speak of low frequencies and high frequencies for frequencies respectively lower and higher than the common cutoff frequency of the two filters.

[0102] Figures 2 and 3 illustrate a particularly advantageous example of implementation of a hybrid sensor combining a resistive gauge and a piezoelectric gauge. Other implementation modes can be considered, modulating the respective contributions of the signals from the resistive gauge and the piezoelectric sensor in order to form an output signal S Sout1 resulting from a linear combination of at least a first signal from a first measuring chain integrating a resistive strain gauge and at least a second signal from a second measuring chain integrating a piezoelectric strain gauge.

[0103] This principle is illustrated with the SPRT support film undergoing a deformation Def which induces the generation of a signal din mechanical deformation of the support film. This signal in is mechanically transmitted to the resistive strain gauge RES and the piezoelectric strain gauge PIEZO, which are integrated into a Meas.Ch measuring chain RES of the resistive gauge and a Meas.Ch measuring chain PIEZO of the resistive gauge, respectively. The RES gauge and the PIEZO gauge generate simultaneously and in response to the same deformation Def detection signals SRES and SPIEZO, respectively, representative of this deformation at a given instant.

[0104] The two Meas.CH measuring chains RES and Meas.Ch PIEZO include Trt signal processing stages RES and Trt PIEZO , to the inputs of which the signals S are supplied RES and S PIEZO , respectively. These processing stages may include filters, amplifiers, signal adders configured to process the signals SRES and S PIEZO , for example in order to apply to these signals the functions described in the electronic module EL.MOD illustrated by the.

[0105] Thus, the signal processing stages Trt RES and Trt PIEZO generate the S signals Rep.RES and S Rep.PIEZO , representative of the S signals RES and S PIEZO and therefore of the deformation Def at the given instant. Indeed, one can either consider that the variation in time of the events detected by the gauges is negligible in view of a potential difference in duration between the processing of the signals by the signal processing stages, or, if this is not the case, arrange a delay within the fastest stage, so as to compensate for its higher speed to ensure that the signals S Rep.RES and S Rep.PIEZO are well synchronized, so that their linear combination corresponds well to a representative combination of a deformation at the same time.

[0106] Finally, an output signal S Out1 can be generated by linear LC combination of the S signals Rep.RES and S Rep.PIEZO , according to any method known to those skilled in the art. The two measuring chains and the device implementing the linear combination can be applied by the EL.MOD electronic module for data acquisition and processing.

[0107] A hybrid sensor configured to implement the principle described above makes it possible to combine the advantages of both types of gauge, the resistive gauge and the piezoelectric gauge.

[0108] A hybrid sensor such as that illustrated by the and whose operating principle is illustrated by figures 2 and 3 or a hybrid sensor of principle illustrated by the can be implemented by means of resistive gauges and piezoelectric gauges known to those skilled in the art.

[0109] The SPRT carrier 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.

[0110] The support film could be directly made of a flexible printed circuit (or FPCB for Flexible Printed Circuit Board in English terminology), with the electrical contacts soldered directly onto this element.

[0111] 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.

[0112] The piezoelectric strain gauge operates 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 used in piezoelectric strain gauges to detect the deformation of objects to which they are attached. There are different types of piezoelectric strain gauges, each of which can be used to form the piezoelectric strain gauge of the hybrid sensor illustrated in the figure.

[0113] A first type of piezoelectric strain gauge is based on the use of a crystal of piezoelectric material with relatively good sensitivity to strain and stability over time, but thick and rigid, often housed in a metal case 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 in a hybrid sensor combining a resistive gauge and a piezoelectric gauge (resistive gauges are very thin and flexible).

[0114] 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 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.

[0115] 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 S x to the deformation in the first direction and a second sensitivity S y to the deformation in the second direction perpendicular to the first, a crystalline orientation of the element being able to be such that abs(S y / S x) < 0.1, corresponding to a so-called “unidirectional” sensitivity, abs((S y +S x ) / S x ) < 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((S x -S y ) / S x ) < 0.1, corresponding to a so-called “omnidirectional” sensitivity.

[0116] 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 strain ranges, as described in detail in French patent application FR2303635.

[0117] 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 from such a thin element to cover a range of applications as wide as that of resistive gauges, without presenting any additional integration constraints.

[0118] This piezoelectric element can measure strains greater than 5000 micrometers per meter with a resolution of around 1 nanometer per meter of strain. These figures compare with those of conventional resistive gauges which can measure strains of up to 12000 micrometers per meter (for only a small number of cycles) but with a much lower resolution, of around 1 micrometer of strain per meter.

[0119] In comparison, the first type of packaged piezoelectric gauges are only capable of measuring deformations limited to around 300 micrometers per meter, which would further limit the range of a hybrid sensor using this technology.

[0120] 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.

[0121] A piezoelectric strain gauge may comprise at least one thin piezoelectric element as described above, attached to a flexible sheet.

[0122] According to additional characteristics, considered individually or in any technically feasible combination:

[0123] - the thin piezoelectric element can be encapsulated between the flexible sheet and another flexible sheet;

[0124] - the piezoelectric gauge may comprise a charge amplifier connected to the thin piezoelectric element;

[0125] - the charge amplifier can be integrated on the flexible sheet;

[0126] On the other hand, the thin monocrystalline piezoelectric 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 of a shape that is not necessarily flat) is essential here, and its sensitivity makes it possible to detect deformations of low amplitudes.

[0127] The piezoelectric element can be formed from lithium tantalate LiTaO3 in monocrystalline form, which belongs to the 3m space group, but also from lithium niobate LiNbO3 (group 3m), lead magnesium niobate MgNb2(PbO3)3 (group P1), aluminum nitride AlN (group P63mc), barium titanate BaTiO3, potassium niobate KNbO3 or lead titanate TiPbO3 (all three from the P4mm group).

[0128] Thus, the HYB hybrid sensor can use a PIEZO piezoelectric gauge 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.

[0129] In order to benefit from the thinness and therefore the flexibility and conformability of the EL.PIEZO thin element, the SPRT substrate on 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 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. 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.

[0130] In addition to the thin EL.PIEZO element, in the example of the, a charge amplifier C.AMP is also fixed on the SPRT substrate and operatively 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.

[0131] 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 strain or acceleration measurement using piezoelectricity. An electronic circuit transforms the high impedance signal of the piezoelectric gauge into a low impedance voltage signal, 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.

[0132] Although not shown, a wire connection element, such as a ribbon cable, is connected to the charge amplifier to connect the gauge to external measuring electronics.

[0133] The thin piezoelectric element EL.PIEZO preferably has a thickness of less than 50 µm, more preferably less than 25 µm, even more preferably less than 10 µm. Considering a piezoelectric element defined as illustrated in, a ratio of the thickness L z of the EL.PIEZO element in a third direction z on its dimension L X in a first direction x of its extension plane, and / or of a dimension L y in a second direction y of its plane of extension normal to the direction L x, is less than 0.1, preferably less than 0.05, 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.

[0134] The SPRT support can have a thickness between 5 and 300 µm.

[0135] However, it is preferable that the PIEZO piezoelectric gauge considered as a whole be sufficiently flexible to fit the surface of a curved object to which it is to 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.

[0136] 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.

[0137] The SPRT substrate may be made of or replaced by a flexible support such as a flexible printed circuit called "flex PCB", composed of layers of electrically insulating polymer and layers of copper, allowing the 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.

[0138] 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.

[0139] The experimental setup used is illustrated by the, and comprises a horizontal steel plate H.Pl 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.

[0140] 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.

[0141] The answers are expressed in µdef. The mass is applied at t=29.7s and removed at t=31.1s. The experimental parameters ensured that , , and that the high-pass and low-pass filters had the same cutoff frequency , fixed here at 0.8 Hz. These experimental results confirm the simulation results illustrated by the: 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).

[0142] More specifically, the RMS (Root Mean Square) noise floor is 0.53 µdef for the resistive gauge, while it is only 0.01 µdef 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.

[0143] In fact, the output signal of the hybrid sensor is therefore both low-noise and, with a zero or almost zero response time, has an immediate response. There is no need for stabilization time after the mass has paused to be able to accurately measure the deformation.

[0144] L illustrates only one particular configuration for a hybrid sensor combining a resistive gauge and a piezoelectric gauge. L illustrates other possible configurations for the hybrid sensor, possibly combinable with each other.

[0145] In (A), illustrates a configuration that differs from that of in that the EL.MOD electronic module is not attached to the SPRT support on which the resistive gauge and the piezoelectric gauge are attached. A wired connection can functionally connect the electronic module to the two strain gauges.

[0146] In (B), a different configuration is illustrated 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 attached to the piezoelectric gauge, attached to the SPRT holder via the PIEZO gauge. Alternatively, the piezoelectric gauge could be attached to the holder via the resistive gauge.

[0147] In (C), illustrates a configuration combining those of and in (B): the electronic module is fixed on the SPRT support and the two gauges are superimposed on each other.

[0148] In (D), a rosette is illustrated, 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 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 the.

[0149] 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 S Out1 .

[0150] 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 metal layers deposited on this electrode. This guarantees that the resistive gauge and the piezoelectric gauge measure exactly the same strain, in particular thanks 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 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.

[0151] 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 EL1 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 the, which represents a sectional view of the sensor HYB along a plane passing through points A and B of the(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 elements EL.PIEZO and EL.RES 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 shown 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.

[0152] 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, and in particular combined according to a linear combination.

[0153] Illustrated in (B) is 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, 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 charge amplification itself. In addition, the electronic module is preferably fixed to a surface of the SPRT substrate opposite to that hosting 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.

[0154] Illustrated in (D) is 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 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.

[0155] Illustrated in (E) is another variation of the HYB hybrid sensor shown 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.

[0156] Illustrated in (G) is another variation of the HYB hybrid sensor shown in (E). In this configuration, the EL.RES element is also embedded within the SPRT substrate, with 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.

[0157] Illustrated in (F) is another variation of the HYB hybrid sensor shown in (A) and (C). In this configuration, a substrate supporting the EL.RES and EL.PIEZO elements is not used: the EL.PIEZO element is self-supporting and 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 EL1 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.

[0158] Illustrated in (H) is a variation of the HYB hybrid sensor 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 Cond layer and the EL.RES element and between the Cond layer and the EL.PIEZO element and its electrodes. The Cond layer may have the function of functionally connecting 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 may also have the function of preventing capacitive coupling between the EL.RES element and the EL.PIEZO element. The COND layer is then typically connected to the sensor ground.

[0159] Illustrated in (H) is a variation of the HYB hybrid sensor shown 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.

[0160] Force sensor

[0161] A hybrid sensor as illustrated in Figures 1 and 8 is primarily intended for characterizing the deformation of an instrumented object, on which the hybrid sensor is attached. 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.

[0162] Thus, the represents an F.SENS force sensor consisting of a test body consisting of a CELL force cell equipped with four HYB hybrid strain sensors. 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. 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), the measurement of the deformation of the latter by means of the hybrid strain 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.

[0163] A load cell can have many other geometries than that illustrated by the, and be equipped with one or more hybrid sensors. The illustrates in (A), (B) and (C) examples of load cells CELL equipped with hybrid strain sensors HYB which can have applications in the field of weighing, forming load cells known under the designations of bending load cell (A), central fulcrum load cell (B) and tension load cell (C).

[0164] It is thus possible to replace the strain gauges conventionally used in weighing systems with hybrid strain sensors described in this document, with the advantages of improved measurement accuracy and dynamic use. While conventional systems are usually used statically, a weighing system, and force measurement system in general, employing one or more hybrid strain sensors as defined in this document, can also operate dynamically, which makes it possible, for example, to detect the stabilization of the system, accelerate the measurement, and detect dynamic events such as mechanical shocks.

[0165] Illustrates in (B) a BAL scale comprising a load receiving platform PL mounted on a plurality of F.SENS force sensors (often four force sensors are used in this type of configuration) operatively connected to a measuring MES system. The force sensors of this configuration can for example be compression weighing sensors, as illustrated in (A) of the, with here a cylindrical CELL force cell equipped with hybrid strain sensors HYB instead of the resistive gauges of conventional systems. The measuring MES system can be an electronic system processing the electrical signals generated by the HYB sensors to carry out a weight measurement in a conventional manner, or so as to benefit from the advantages of HYB sensors and carry out the detection and / or characterization of the stabilization of the system, shocks, or other transient events.

[0166] Illustrates a preferential relative positioning between a piezoelectric gauge and a resistive gauge, and reproduces the device described using (A).

[0167] When a PIEZO electric piezoelectric strain gauge and a RES resistive strain gauge are integrated on the same support or on the same mechanical part to be instrumented, it may, depending on the intended application, be particularly advantageous to align their respective directions of greater sensitivity.

[0168] In fact, a resistive gauge RES, such as the one illustrated in, is designed to detect an elongation of the electrically resistive element RES.Elmt which constitutes it. This resistive element is for the most part parallel to a particular direction, which defines a sensitive axis SA, or direction of greater sensitivity. The resistive gauge will be essentially sensitive to a deformation tending to elongate the resistive element in the direction SA. We can speak of an element with uniaxial sensitivity.

[0169] Furthermore, a piezoelectric gauge can be formed from a piezoelectric element having a particularly high sensitivity in a particular direction and lower or even negligible sensitivity in the other directions, to the point that we can also speak here of a piezoelectric element having a uniaxial sensitivity: its sensitivity has an orientation defined by a given axis, defined relative to the crystalline axes of the crystal forming the piezoelectric element. We can for example refer to patent document WO 2024 / 213465 A1. Such an element can be formed from a thin layer of piezoelectric material which will have a direction of greater sensitivity Sens-Dir.

[0170] When the two gauges of a hybrid sensor, the resistive gauge and the piezoelectric gauge, are formed from an element with so-called uniaxial sensitivity as described above, it is advantageous to align their respective directions of greatest sensitivity with each other. As illustrated by the, the resistive gauge and the piezoelectric gauge can be arranged in such a way that the direction SA can be parallel to the direction Sens_Dir, in order to characterize a deformation in the same direction for both gauges.

[0171] This geometric characteristic of the hybrid sensor is particularly advantageous for quantitative measurements of the deformations undergone by an object equipped with the hybrid sensor, since these deformations can then be quantitatively characterized in a homogeneous manner over a spectral band covering the entire spectrum covered by the two gauges in combination. It can also be said that this geometric configuration aligns the contributions of the two gauges in terms of spectrum and amplitude, allowing a quantitative characterization of the deformation.

[0172] Of course, this feature can optionally be applied to any combination of a resistive gauge and a piezoelectric gauge, such as the combinations illustrated in Figures 1, 5, 8 and 9.

[0173] Second embodiment

[0174] The first embodiment is based on the identity of the cutoff frequencies of the high-pass filter Filt HP1of the piezoelectric gauge and the low-pass filter Filt LP1 applied to the signal generated by the resistive gauge. These two filters are considered to form a first filtering stage.

[0175] However, in practice, the cutoff frequency of the high-pass filter Filt HP1 , which corresponds to the physical characteristics of the piezoelectric gauge, cannot always be estimated precisely. Especially when the piezoelectric gauge is used in voltage mode, the cut-off 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 cut-off frequency of the low-pass filter applied to the signal from the resistive gauge must be equal to the cut-off frequency of the high-pass filter of the piezoelectric gauge so that the output signal S O ut1be quantitatively speaking faithful to the distortion. A difference between these cutoff frequencies will generate distortion of the output signal, especially at frequencies close to these cutoff frequencies.

[0176] This second proposed embodiment aims to minimize this effect. It consists of placing a second filtering stage using a second low-pass filter Filt LP2 and a second high-pass filter Filt HP2 of the same cutoff frequency c2 , higher than the cutoff frequency aim for first stage filters Filt LP1 and Filt HP1 .

[0177] We then use the signal from the resistive gauge up to the frequency f c2 above the cutoff frequency of the first stage, and the signal from the piezoelectric gauge is only used above this second cutoff frequency c2The filters in 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. c2 , which guarantees an output from the second filter stage without signal distortion.

[0178] We will typically choose the cutoff frequency of the second stage 5 times greater than , preferably 10 times larger than so as to verify the inequality . More is high, the less the difference between the cutoff frequencies of the first stage filters Filt LP1 and Filt HP1 will have influence on the output signal, but the higher the residual noise on the output signal will be.

[0179] 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 Filt LP2 , Filt HP2 , and an adder Add3. For the other elements of the, we can refer to the first embodiment and to the.

[0180] The adder Add3 is configured to sum between (i) the output signal S O ut1 to which a filtering by the Filt filter HP2 was applied and (ii) the signal from the resistive gauge RES to which the gain G RES then filtering by the low-pass filter Filt LP2 were applied. At the output of the adder Add3, we obtain an output signal S Out2 , corresponding to the signal S Out1 largely freed from the influence of the difference between the cutoff frequencies of the Filt filtersLP1 et Filt HP1 from the first floor.

[0181] Third embodiment

[0182] Another problem that can 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, 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 depending on the 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.

[0183] 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. This first problem exists for all sensors, and many solutions have been developed to address it.

[0184] 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 G RES and G PIEZO , whose gain values ​​g r and g p are considered fixed in the hybrid sensor modeling used in this paper.

[0185] For a reference temperature T0, we can thus choose gains g r and g p so that the S signalsRep.RES and S REP.PIEZO have the same amplitude when measuring a given deformation. However, if the two gauges have sensitivity drifts (s r and s p ) at different temperatures, then at a temperature different from T0 the signals S Rep.RES and S R ep .PIEZO will have different amplitudes for the same deformation, and the output signal will be deformed relative to the measured deformation. In other words, in the first embodiment, illustrated by the, the equality between the respective overall gains s r ×g r and s p ×g p of the two Meas.Ch measuring chains RES and Meas.Ch PIEZO may be lost due to differential drift between the characteristics of the two gauges.

[0186] The third embodiment addresses this problem by ensuring that, at each instant, (i) the respective gains of the two measurement chains Meas.Ch RESand Meas.Ch PIEZO remain equal to each other and, (ii) the respective cut-off frequencies of the two measuring chains Meas.Ch RES and Meas.Ch PIEZO , which are those of the two Filt filters LP1 and Filt HP1 , are equal to each other. This identity between the overall gains and cut-off frequencies of the two measurement chains Meas.Ch RES and Meas.Ch PIEZO is in this embodiment ensured by means of an adaptive filtering technique described below by means of the functional block diagram of the.

[0187] Adaptive filtering according to this embodiment has the principle of adjusting in real time (i) the cut-off frequency of the low-pass filter Filt LP1 applied to the resistive gauge signal and (ii) the value g p_Corr of a gain G PIEZO_Corr placed in series between the gain G PIEZO and the adder Add2.

[0188] The cutoff frequency of the low-pass filter Filt LP1is constantly adjusted so as to make it tend towards that of the high-pass filter Filt HP1 of the acquisition chain of the piezoelectric gauge, the characteristics of which are likely to vary depending on the temperature or other physical parameters.

[0189] Similarly, the value g p_ Corr of gain G PIEZO_Corr is constantly adjusted so as to make the overall gain tend to p ×g p ×g p_Corr of the Meas.Ch measuring chain PIEZO towards the overall gain s r ×g r of the Meas.Ch measuring chain RES .

[0190] The diagram represents a functional block diagram explaining the algorithm implemented for adjusting the cutoff frequency of the filter Filt LP1 and the value g p_Corr correction of the gain of the Meas.Ch measuring chain PIEZO. The principle of the first embodiment illustrated by the is taken up, to which is added adaptive filtering represented by the Filt elements HP_Est , LMS Alg , Sub and G PIEZO_Corr . For the other elements of the, one can refer to the first embodiment and to the.

[0191] 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 from the resistive gauge. There is thus a redundancy of information that can be used to continuously calibrate the cut-off frequency of the filter Filt LP1 , and the value g p_Corr of gain G PIEZO_Corr . For this, we use a classic adaptive filtering strategy employing for example an LMS algorithm. Algleast squares (LMS) to adapt the characteristics of the transfer function of the filter Filt LP1 and the overall gain of the Mes.Ch measuring chain PIEZO .

[0192] Low signal frequencies, typically below 100 Hz, can be used to ensure that the resistive gauge signal is not too noisy. To do this, for example, the signals entering the LMS algorithm block can be filtered using a low-pass filter with a cutoff frequency of 100 Hz. Alg .

[0193] The resistive signal S RES of the resistive gauge to which the gain G RES was applied is (i) filtered by a high-pass filter Filt HP _Est of estimated transfer function H HP EST(ω), whose gain and cutoff frequency parameters are variable, and (ii) sent as input to the LMS algorithm Alg . A second input to the LMS algorithm Algis fed by an error signal e defined as (i) the signal S RES of the resistive gauge adjusted by means of the gain G RES and filtered by the estimated high-pass filter Filt HP_Est , from which we subtract (ii) the signal S Rep.PIEZO representative of the piezoelectric signal S PIEZO using the Sub block. The result of this subtraction is an error signal e which is representative of a difference between (i) the signal S Rep.RES from the Meas.Ch measuring chain RES before it is filtered by Filt LP1 but which is filtered by Filt HP_est and (ii) the signal S Rep.PIEZOissu de la chaîne de mesure Meas.ChPIEZOavant son passage par le gain GPIEZO_Corr. Les signaux SRep.PIEZOet SRep.PIEZOsont respectivement issus du signal piézoélectrique SPIEZOet du signal résistif SRES.

[0194] By default, the FiltHP_Est filter is initialized with a gain of 1 and a cutoff frequency equal to that of the Filt filters LP1 and Filt HP1 before drift and before modification by the LMS block Alg .

[0195] At least for low frequencies, this error signal must be minimized, which is done here in a classical manner by means of adaptive filtering.

[0196] The LMS algorithm Alg is conventionally configured to iteratively modify the gain and cutoff frequency of the estimated high-pass filter Filt HP _ EST so as to minimize the error e. At each iteration, or when deemed necessary, the cutoff frequency of the estimated filter Filt HP_ESTest transférée au filtre FiltLP1, et le gain gp_correst mis à jour avec l’inverse du gain du filtre estimé FiltHP_EST. Autrement dit, sur la base d’une comparaison entre (i) le premier signal représentatif sRep.RESdu signal résistif SRESavant qu’il ne passe par le premier filtre passe-bas FiltLP1et (ii) le second signal sRep.PIEZOreprésentatif du signal piézoélectrique SPIEZO, on obtient une erreur e à minimiser par le filtrage adaptatif, ce dernier garantissant (i) que les filtres FiltHP1et FiltLP1ont la même fréquence de coupure, et (ii) que les signaux générés respectivement par la jauge résistive et la jauge piézoélectrique ont la même amplitude avant leur sommation, à tout instant, même si les paramètres des filtres évoluent au cours du temps. En procédant spécifiquement comme illustré par la, la dérive de la sensibilité de la sortie du capteur hybride est égale à celle de la jauge résistive, qui en général dérive moins vite que la sensibilité du de la jauge piézoélectrique.

[0197] The parameter transfer is conventionally represented by a diagonal arrow in the background of the Filt filters LP1 and Filt HP _ Est and variable gain G PIEZO_Corr . Especially for copying the parameters of gain and cutoff frequency, it is convenient to implement this third embodiment digitally.

[0198] In the example shown above, the cutoff frequency of the filter Filt LP1 , which here can be a digital filter with a variable cutoff frequency defined on command, is adjusted so as to follow the drift of the cutoff frequency of the filter Filt HP1 . Thus the cutoff frequencies of the Filt filters LP1 and Filt HP1 are kept equal.

[0199] Still in this example, the overall gain of the Mes.Ch measurement chain PIEZO is corrected, by means of the gain G PIEZO_Corr , so as to ensure its equality with the overall gain of the Ch.Mes measurement chain RES An alternative based on the same principle would be to correct the overall gain of the Mes.Ch measuring chain RES so as to ensure its equality with the overall gain of the Ch.Mes measurement chain PIEZO . : it would be enough to place a variable gain on the Meas.Ch measurement chain RESand to control its gain value using a suitable algorithm.

[0200] 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, see the article by Shynk JJ "Adaptative IIR filtering", 1989, EEE ASSP Magazine, 6, 4-21.

[0201] Other methods for estimating the gain and cutoff frequency are possible, for example by using a Fast Fourier Transform (FFT) or adaptive least squares filtering (RLS). The problem is mathematically posed as a problem of identifying a first-order system from a noisy signal, which is a very classic and widely documented problem.

[0202] In the present embodiment, only two filters are involved, and the filter Filt is adjusted LP1 in response to the changing characteristics of the Filt filter HP1 . More generally, it is a question of making the cut-off frequency and the gain of the entire Meas.Ch measurement chain equal. PIEZO . and those of the entire Meas.Ch measuring chain PIEZOby reducing the gap between them. The cut-off frequency of the Meas.Ch measuring chain can be made to converge RES to that of the Meas.Ch measuring chain PIEZO as illustrated by the, or, conversely, converge the value of the cut-off frequency of the measuring chain Meas.Ch PIEZO to that of the Meas.Ch measuring chain RES Symmetrically, we can converge the overall gain of the Meas.Ch measurement chain PIEZO to that of the Meas.Ch measuring chain RES , 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 by the. "Converging" means an operation which consists of reducing a difference between two values.

[0203] 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.

[0204] Of course, the variants and embodiments detailed above can be combined with each other in any technologically feasible combination.

[0205] 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%.

[0206] The above description focuses in particular on the linear combination of signals from two types (resistive and piezoelectric) of strain sensors. Other ways of exploiting the signals generated in parallel by the two strain gauges of a hybrid sensor can be considered. For example, we can cite the use of Kalman filters using these signals as inputs, which would make it possible to estimate the deformation of the instrumented part from noisy or incomplete observations, here consisting of the sensor signals.

[0207] 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

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 (T1 RES , T2 RES , T1 PIEZO , T2 PIEZO ) of the electronic module, the hybrid sensor being configured so that the piezoelectric strain gauge and the resistive strain gauge generate a piezoelectric signal (S PIEZO ) and a resistive signal (S RES ), respectively, in response to a deformation (Def) of an object (CELL) instrumented by the hybrid sensor (HYB), and the electronic module being configured to perform a linear combination of a first signal (s Rep.RES ) from the resistive signal (S RES ) and a second signal (s Rep.PIEZO ) from the piezoelectric signal (S PIEZO), so as to generate a first output signal (S Out1 ) which is representative of the deformation (Def) of the instrumented object (CELL). The hybrid sensor of claim 1, wherein a direction of greater sensitivity (Sens_Dir) of the piezoelectric strain gauge is aligned with a direction of greater sensitivity (SA) of the resistive strain gauge. The hybrid sensor according to claim 1 or 2, configured such that the first signal (s Rep.RES ) from the resistive signal (S RES ) and the second signal (s Rep.PIEZO ) from the piezoelectric signal (S PIEZO ) are representative of a given deformation amplitude (Def) at the same time. The hybrid sensor according to any one of claims 1 to 3, wherein:- the first output signal (S Out1) includes low-frequency components and high-frequency components, which correspond respectively to a frequency content for frequencies below a given threshold (f c ) and 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 (S RES ); and- an informative content of the high-frequency components of the first output signal comes mainly from the piezoelectric signal (S PIEZO ). The hybrid sensor according to any one of claims 1 to 4, wherein:- the first signal (s Rep.RES ) from the resistive signal (S RES ) is generated by a deformation measurement chain (Def) comprising a first low-pass filter (Filt LP1 ) ;- the second signal (s Rep.PIEZO ) from the piezoelectric signal (S PIEZO) is generated by a deformation measurement chain (Def) comprising a first high-pass filter (Filt HP1 ), in which a cutoff frequency of the first low-pass filter (Filt LP1 ) and a cutoff frequency of the first high-pass filter (Filt HP1 ) are equal to at least 10%. The hybrid sensor according to claim 5, in which:- the measurement chain for generating the first signal (s Rep.RES ) from the resistive signal (S RES ) includes the application of a first gain (G RES ),- the measurement chain for generating the second signal (s Rep.PIEZO ) from the piezoelectric signal (S PIEZO ) includes the application of a second gain (G PIEZO ), the first gain (G RES ) and the second gain (G PIEZO ) are configured in such a way that a signal (S Rep.RES ) from the resistive gauge (RES) and a signal (S Rep.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%. The hybrid sensor according to claim 6, configured such that:- the first gain (G RES ) has a value (g r ) equal to at least 10% near an inverse of the sensitivity (s r ) of the resistive gauge (RES), and- the second gain (G PIEZO ) has a value (g p ) equal to at least 10% near an inverse of the sensitivity (s p ) of the piezoelectric gauge (PIEZO). The hybrid sensor according to any one of claims 5 to 7, wherein the electronic module (EL.MOD) is configured to generate a second output signal (S Out2 ) representative of the deformation of the instrumented object by summation (i) of the first signal (S Out1) representative of the deformation of the instrumented object to which a second high-pass filter (Filt HP2 ) is applied and (ii) the resistive signal (S RES ) to which a second low-pass filter (Filt LP2 ) is applied. The hybrid sensor according to claim 8, wherein:- the second low-pass filter (Filt LP2 ) and the second high-pass filter (Filt HP2 ) 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 (Filt LP2 ) and the cutoff frequency of the second high-pass filter (Filt LH2 ) are each at least 5 times higher than the cutoff frequency of the first low-pass filter (Filt LP1 ) and that the cutoff frequency of the first high-pass filter (Filt LH1 ). 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. The hybrid sensor according to claim 10, the electronic module (EL.MOD) being integrated into the support substrate (SPRT). 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 electrically conductive layer deposited on an electrode (EL2) of the piezoelectric gauge (PIEZO). The hybrid sensor according to any one of the preceding claims 1 to 12, 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, of dimensions (L X , L Y ) in the first direction and the second direction each greater than 100 µm and of thickness (L Z ) less than 50 µm, a ratio of the thickness to the dimension (L X ) in the first direction or dimension (L y ) in the second direction being less than 0.

1. 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 13 is fixed. Scale (BAL) comprising:- a mechanical load receiving element (PL, CELL);- at least one force sensor (F.SENS) according to claim 14, configured to generate an electrical signal representative of a mechanical load applied to the mechanical load receiving element; and- a measuring device (MES) configured to receive and process the electrical signal, and optionally display a weight resulting from the measurement.

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