FR3068132B1 - DEVICE FOR DETERMINING A STRESS VALUE CORRESPONDING TO A PIC OF THE STRESS AND METHOD OF DETERMINING THE STRESS - Google Patents

Abstract

The device (1) comprises at least one stress sensor (2) comprising a piezoresistive element (3) and a piezoelectric element (4) capable of generating an electrical signal when a stress is applied to the stress sensor (2). The device (1) comprises a detector (5) configured to detect a peak of the stress applied to the stress sensor (2) by processing the electrical signal. The device (1) is configured to trigger, when the peak of the applied stress is detected by the detector (5), a measurement of an electrical quantity representative of an electrical resistance of the piezoresistive element (3) of the sensor. constraint (2). The device (1) is configured to determine a value of the stress applied to the stress sensor (2) using the measured electrical quantity.

Description

DEVICE FOR DETERMINING A CONSTRAINTS VALUE CONCEALED TO A PIC OF THE STRESS AND METHOD OF DEDETMINING THE STRESS

Field of the Invention [001] The field of the invention relates to the study of stress. More generally, the invention relates to a device comprising at least one strain sensor.

STATE OF THE ART [002] There are numerous solutions for characterizing stresses exerted on stress or pressure sensors.

[003] In the particular field of the shoe, it is possible to use pressure sensors judiciously positioned within a soleplate, for example, to determine the shocks suffered by the human body during the impact of the foot / or by example measure different pressures applied by the foot arch of the foot on the sole.

[004] A pressure sensor may be formed by a piezoresistive element that separates as a variable resistor whose value depends on the pressure applied to the pressure sensor. Such a pressure sensor has the advantage of enabling the detection of shocks or other brief phenomena, but has the disadvantage of requiring the use of a measurement electronics with high frequency of acquisition of electrical quantities each representative of an electrical resistance of the piezoresistive and stress-dependent element exerted to attempt to measure a representative value of the stress peak corresponding to one of these electrical magnitudes. A high acquisition frequency of the measurement electronics, especially greater than 200 Hz or 300 Hz, is energy consuming. Too high energy consumption is incompatible with embedded solutions that generally use battery to operate. Furthermore, another disadvantage is that, if the acquisition frequency is not high enough, the accuracy of the sensor is not satisfactory because it is possible that the measurement assimilated to a correspondence with the constraint is not made to good moment.

[005] In this sense, there is a need to find a solution for limiting the energy consumption required to measure the electrical magnitude representative of the electrical resistance corresponding to a peak pressure exerted on a pressure sensor using piezoresistive technology.

[006] The patent application CN104535229, or the utility modelCN204286650, seek to meet this need by using an additional pressure sensor piezoelectric type. This additional sensor serves as a wake-up module of piezoresistive sensors each forming a pressure sensor. The awakening of piezoresistive sensors is implemented as soon as a value is measured, starting from the piezoelectric sensor, which exceeds a predetermined threshold. This solution has the disadvantage of a high simultaneous consumption of the piezoresistive sensors as soon as the threshold is reached. Moreover, this solution has the disadvantage that the threshold may not be reached in the case of aging piezoelectric sensor, or in the case of application of a stress too low. As a function of the aging of the piezoelectric sensor, the voltage that it can generate will evolve, and it may be in a situation where it can not generate enough voltage for the threshold to be reached.

Object of the invention [007] The invention aims to remedy all or part of the aforementioned drawbacks.

[008] For this purpose, the invention relates to a device comprising at least one strain sensor comprising a piezoresistive element, the device ischaracterized in that: • the stress sensor comprises a piezoelectric element capable of generating an electrical signal when a constraint is applied to the stress sensor, • said device comprises a detector configured to detect a stress peak applied to the stress sensor by processing of the electrical signal, • the device is configured to trip, when the peak of the stress applied is detected by the detector, a measurement of an electrical magnitude indicative of an electrical resistance of the stressorresistive element of the stress sensor, and • the device is configured to determine a value of the stress applied to the stress sensor using the electromeasured magnitude.

[009] The device according to the invention may comprise one or more following features.

According to a characteristic of the device, the detector is configured to detect a local extremum of the electrical signal of the piezoelectric element corresponding to the peak of the applied stress.

According to a characteristic of the device, the detector is configured to use the derivative of the electrical signal to detect the local extremum of the electrical signal.

According to a characteristic of the device, the detector comprises: a diverter configured to determine the derivative of the electrical signal, a comparator configured to output an alternating logic signal between two levels, the transition to one of the levels indicating that the applied stress peak is detected, said comparator being configured tochange the level of the logic signal upon a zero crossing of the derivative of the electrical signal determined by the differentiator.

According to a feature of the device, the two levels of the logic signal correspond respectively to a first level and a second level, and the concomparator is configured to: • pass the logic signal from the first level to the second level during a zero crossing from the derivative of the electrical signal determined by the differentiator for a local maximum of the electrical signal, • passing the logic signal from the second level to the first level during a zero crossing of the derivative of the electrical signal determined by the differentiator for a local minimum of the signal electrical, the local extremum of the electrical signal being the local maximum of the electrical signal, the peak of stress is considered as detected during the passage of the first level to the second level, where the local extremum of the electrical signal being the local minimum of the electrical signal, the peak of stress is considered as detected during the passage of the second nive at the first level.

According to a feature of the device, the piezoresistive element and the piezoelectric element of the stress sensor are juxtaposed.

According to a feature of the device, the piezoelectric element and the piezoresistive element of the stress sensor are arranged such that: the piezoelectric element at least partially surrounds the piezoresistive element, or the piezoresistive element at least partially surrounds the élémentpiézoélectrique.

According to a feature of the device, the stress sensor is such that the piezoelectric element and the piezoresistive element are stacked.

According to a feature of the device, the device adopts at least partly the form of a shoe sole incorporating the stress sensor.

The invention also relates to a stress determination method from a device comprising at least one stress sensor comprising a piezoresistive element. This stress determination method also aims to overcome all or some of the aforementioned drawbacks. The stress determining method comprises a step of applying a stress to the stress sensor, and said stress sensor comprising a piezoelectric element, said stress determining method comprises: a step of processing an electrical signal generated by the stress sensor; a piezoelectric element in response to the stress applied to the stress sensor for detecting a peak of the stress applied to the stress sensor; and a step of measuring an electrical quantity representative of an electrical resistance of the piezoresistive element of the stress sensor. step of measuring the electrical magnitude of the electrical resistance of the piezoresistive element being triggered when the peak of the constraint applied is detected, • a step of determining a value of the stress applied to said strain sensor using the measured electrical quantity.

The stress determination method according to the invention may comprise one or more of the following characteristics.

According to a characteristic of the stress determination method, the processing step comprises a step of detecting a local extremum of electric signal corresponding to the peak of the applied stress.

According to a characteristic of the stress determination method, the step of detecting the local extremum of the electrical signal comprises a step of determining the derivative of the electrical signal, and a step of detecting a zero crossing of the derivative. of the electrical signal from which it results that leditextremum local electrical signal is detected.

According to a characteristic of the stress determination method, the step of detecting the zero crossing of the derivative of the electrical signal is implemented by a comparator of the device providing an output logic signalalternant between two levels, the peak of the applied stress being detected when the logic signal at the output of the comparator passes to one of the levels of the signallogic equated with a detected state of the local extremum of the electrical signal.

According to a characteristic of the stress determination method, the stress determination method comprises a step of determining, according to an acquisition frequency, a plurality of physical quantities each representative of the electrical resistance of the piezoresistive element, and detecting the peak of the applied stress causes the increase of the acquisition frequency to trigger the step of measuring the electrical magnitude representative of the electrical resistance of the piezoresistive element.

According to a characteristic of the stress determination method, the stress determination method comprises a plurality of stress sensors each comprising a piezoresistive element and a piezoelectric element, and the steps of applying a stress, a treatment, a measurement, and determining a value of the applied stress are implemented for each stress sensor.

BRIEF DESCRIPTION OF THE DRAWINGS [0025] Other advantages and features will become clear from the following description of particular embodiments of the invention given by way of nonlimiting example and represented in the accompanying drawings in which: FIG. curves, one of the curves corresponding to the evolution of the electrical resistance of a piezoresistive element over time, and the other of the curves corresponding to the evolution of the voltagegenerated over time by a piezoelectric element, - FIG. 2 illustrates a particular embodiment of a constraint sensor device according to the invention; FIG. 3 superimposedly illustrates the evolution of electrical signals given in volts as a function of time; FIG. 4 is an example of an implementation of FIG. of a device detector, - Figure 5 illustrates a view from above of a first embodiment of FIG. 6 is a sectional view along AA of FIG. 5, said section being perpendicular to the plane of FIG. 5; FIG. 7 illustrates a view from above of a second embodiment of the stress sensor of FIG. FIG. 8 is a sectional view along BB of FIG. 7, said section being perpendicular to the plane of FIG. 7; FIG. 9 illustrates a view from above of a third embodiment of the constraint sensor of the device; FIG. 10 is a sectional view along CC of FIG. 9, said section being perpendicular to the plane of FIG. 9, and FIG. 11 a sectional view along CC of FIG. 9 according to an alternative to FIG. 10; FIG. an example of interdigitated electrodes for a piezoresistive element, - FIG. 13 illustrates a particular implementation of the device according to the invention used in the field of footwear, - FIG. 14 is an example of implementation of FIG. a constraint ofdetermining method from the device comprising the sensor decontrainte.

In these figures, the same references are used to designate the same elements.

The elements shown in the figures are not necessarily scaled to make the figures more readable.

DESCRIPTION OF PARTICULAR EMBODIMENTS [0028] In the present description, "based on" corresponds to "comprising more or less".

A "piezoresistive element", also called piezoresistive sensor, corresponds to an element having an electrical resistance varying according to the stress to which it is subjected. In FIG. 1, a curve C1 shows the evolution of the electrical resistance of the piezoresistive element as a function of time during the application of successive stresses to the piezoresistive element each having a maximum value at the times indicated by the lines 11 and 12, or the same constraint whose intensity varies so as to locally present two maximum values at the times indicated by the lines 11 and I2.

A "piezoelectric element", also called a piezoelectric sensor, corresponds to an element capable of generating an electrical signal as a function of the stress to which it is subjected, for example this electrical signal represents the evolution of a voltage generated by the piezoelectric element as a function of time. In FIG. 1, a curve C2 shows the evolution of the voltage of the piezoelectric element as a function of time during the application of one or more stresses such as those described in combination with the curve C1. For each peak stress, the electrical signal has a local extremum. The local end may be a local maximum or a local minimum depending on the branch of the piezoelectric element. In FIG. 1, the local extremums are positive maximum voltage values referred to by the references P1 and P2.

By "local extremum of an electrical signal" is meant that for part of the electrical signal generated by the piezoelectric element there is a value of this signal part which can be, if appropriate, maximum or minimum, and nibble value, the derivative of the part of the electrical signal is zero (that is to say 0). By derivative, this is the first time derivative of the electrical signal. The constraint determining device and method described hereinafterprovides using a strain sensor comprising both a piezoelectric element and a piezoresistive element. The piezoelectric element comprises an active material capable of generating an electrical signal (for example an electrical voltage) which is a function of the stress applied without being powered: the consumption for generating the electrical signal is therefore zero. Thus, the use of the piezoelectric element and the use of the voltage it generates when it is under stress to detect a peak of stress consumes much less energy than the continuous breakdown of the high frequency piezoresistive element. especiallycomprise between 200Hz and 20kHz. However, the piezoelectric element alone does not make it possible to obtain a precise value of the stress because the latter is loaded when read by a low power electronics (that is to say a consumption strictly less than 10pW) because that the input impedance of this electronics is not infinite. Thus, for the same stress value, the voltage across the piezoelectric element may be different depending on how it was read. Therefore, it is preferred to use the piezoresistive element to that of the piezoelectric element to accurately determine a stress value. It is in this sense that the present invention proposes to use the piezoelectric element of the stress sensor to detect the peak of the stress applied to the stress sensor in order to trigger a measurement of an electrical quantity representative of an electrical resistance of the piezoresistive element at a convenient moment corresponding to the peak of the stress applied to the stress sensor. In comparison with the patent application CN104535229, the energy consumption related to the operation of the piezoresistive element can be limited to the closest to the peak of the constraint that one seeks to visualize. Moreover, the peak stress will always cause the presence of local unextremum of the electrical signal regardless of the aging state of the piezoelectric element.

In the present description, a peak of a stress applied to the stress sensor may correspond to a local maximum of the stress. In fact, undistinguished two cases. In a first case, the constraint is linked to a brief event where it is applied to the stress sensor, then completely relaxed: it then presents a unique maximum of stress. In a second case, the intensity of the stress applied to the stress sensor may vary, and it will then present local maxima that can adopt identical or different values. In the first case, the piezoelectric element will generate an electrical signal comprising a single local extremum corresponding to the peak, this local extremum being generated when the stress applied to the pressure sensor reaches its peak. In the second case, it will be possible to detect from the electrical signal generated by the piezoelectric element several local extrema each corresponding to a peak of the stress, that is to say to a local maximum value of the stress. In other words, a stress applied in the sense of the present description may have a single stress peak, or several stress peaks, each peak of the stress applied to the stress sensor then causes the presence of a corresponding local extremum of the electrical signal. A stress peak may be brief, i.e. the stress decreases immediately after the peak has been reached, or perhaps longer so that the stress is maintained for a period of time.

As illustrated in FIG. 2, there is now described a particular embodiment in which the device 1, also called a device for determining, or measuring, stress (in particular at least one stress peak), comprises less than a strain sensor 2. This device 1 may comprise one or more stress sensors 2, this point will be detailed later. The stress sensor 2 comprises a piezoresistive element 3 and a piezoelectric element 4. The device 1 is also called a device for determining at least one stress value from the piezoresistive element 3. The piezoelectric element 4 is able to generate a signal when the stress is applied to the pressure sensor 2. Thereafter, the reference to the electrical signal corresponds to that of the piezoelectric element 4, also called the electrical signal of the piezoelectric element 4. In particular, the electrical signal is an electrical signal detension representing the evolution of the electrical voltage generated by the piezoelectric element 4 as a function of time according to the stress applied to the stress sensor 2. When a stress is applied to the stress sensor 2, the piezoelectric element 4 and the piezoresistive element 3 are subject to this constraint e. Furthermore, the device 1 includes a detector 5 configured to detect a peak of the stress applied to the stress sensor 2 by processing the electrical signal of the piezoelectric element 4. In the following description, "peak of the applied stress" is understood to mean the peak of the stress applied to the stress sensor 2. "Electrical signal processing" means any operation applied to the electrical signal in order to detect the peak of the applied stress. If necessary, for the same constraint, the detector 5 can detect one or more peaks of the applied stress. Furthermore, the device 1 is configured to trigger, when said, and in particulareach peak, of the applied constraint is detected by the detector 5, a measurement of an electrical quantity representative of an electrical resistance of the piezoresistive element 3 of the constraint sensor. 2. Thus, the detection of the peak of the applied stress causes the triggering of the measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3. The device 1 thus gains in precision since it knows when to measure the electrical quantity representative of the resistance. of the piezoresistive element 3, which is then also representative of the peak of the applied stress. The measurement of the electrical magnitude representative of the electrical resistance of the piezoresistive element 3 can be achieved by electrically feeding the piezoresistive element 3, and measuring the voltage at its terminals by means of a resistive bridge for example, or by measuring the current circulating in a branch including the piezoresistive element 3.This measured electrical quantity is said to be representative of the stress applied to the stress sensor 2, and more particularly of the value of the peak of the applied stress, since it can be used to determine a value of the stress , that is to say, to develop a value of the constraint, when it reaches a peak. Typically, the measured electrical quantity is said to be representative of the electrical resistance of the piezoresistive element 3 because it depends on its electrical resistance. The electrical resistance of the piezoresistive element 3 in the context of this description results in the property of the piezoresistive element to be opposed to the passage of an electric current, although the unit of the electrical resistance is the ohm, the measured electrical magnitude may be a voltage, or a current, whose value depends on the electrical resistance of the piezoresistive element, or indirectly the electrical resistance. For example, the measured electrical quantity can be obtained from the measurement of the voltage at the terminals of the piezoresistive element 3 fed through a resistive bridge, which can then be traced back, if necessary, in a manner known per se by the a person skilled in the art, at the value of the stress, the latter being a function of the resistance, itself a function of tension. Furthermore, the device 1 is configured to determine a value of the stress applied to the stress sensor 2 by using, that is to say from, the measured electrical quantity. Thus, the value of the determined stress, corresponding to the value of the peak of the applied stress, can be determined reliably by limiting the power consumption of the device 1.

As illustrated in FIG. 2, the device 1 may comprise an electronic module 6 electrically connected to the stress sensor 2. The electronic module 6 may comprise the detector 5 and the means necessary for triggering and implementing the measurement of the representative electrical quantity. the electrical resistance of the piezoresistive element 3. The electronic module 6 may comprise a power supply 7, such as a battery, or be connected to an external power source for supplying the components of the electronic module 6. The module The electronics 6 may also have elements necessary for making measurements from the pressure sensor 2, such as, for example, measurements of the electrical magnitudes of the piezoresistive element 3 and measurements of the electrical quantities of the piezoelectric element 4, these electrical quantities varying according to the constraint. applied sensor c Thus, the device 1, and in particular the electronic module 6 of the device 1, can be configured to implement an electrical signal processing step, a step of measuring the electrical quantity, and a step of determining the value of the stress applied to said strain sensor using the measured electrical quantity as such will be described hereinafter in combination with the stress determination method. [0036] For example, in order to measure the electrical quantity representative of the electrical resistance of the piezoresistive element 3, the piezoresistive element 3 is electrically powered (arrow F1 of FIG. 2) by means of a resistive bridge (not shown) of the electronic module 6 and the measurement is made by reading (arrow F2 of FIG. 2) of the mid-point of the resistive bridge, for example using an analog-digital converter (not shown), also known by the acronym "ADC" for the English "Analog to Digital Converter ", of the electronic module 6. [0037] By "constraint" is meant, for example, pressure, or stretching, or force. More generally, a constraint may correspond to any external force exerted on the strain sensor 2 and which tends to distort it as long as the latter can provide the measured electrical quantity representative of the electrical resistance of the piezoresistive element 3, and dependent on the deformation of the piezoresistive element 3. The stress sensor may be a force sensor. According to a particular example, the stress applied to the stress sensor 2 can be a pressure applied to the, or more particularly on the stress sensor 2 by a body, for example a part of the human body or an organ of a machine. For example, the stress applied can be measured in kg / cm 2 and the stress applied can be between λg / cm 2 and 5 kg / cm 2, especially in the shoe field where the stress applied to the stress sensor is caused by a foot.

As mentioned above, the electrical signal generated by the piezoelectric element 4 has a local extremum when the stress applied to the stress sensor has a peak. In this sense, the detector 5 can be configured to detect this local extremum of the electrical signal of the piezoelectric element 4, said local extremum of the electrical signal corresponding to the peak of the constraintapplied to the stress sensor 2. The detection of the local extremum is easy to implement by processing the electrical signal, and allows effective detection of the peak of the applied stress. By "local extremum of the electric signal corresponding to the peak of the stress applied to the stress sensor 2", it is meant that the local extremum of the electrical signal is generated by the piezoelectric element 4 when, that is to say when, the Stress sensor 2 is subjected to the peak of the applied stress. The detector 5 can detect, for a stress applied to the stress sensor 2, at least one local extremum of electric signal, that is to say a local extremum or several local extremums. All that applies to the detection by the detector 5 of the local extremum of the electrical signal generated by the application of the stress to the stress sensor 2 canapply to the detection by the detector 5 of several local extremum of the electric signal generated by the application of the same constraint, each extremum localPosition of the electrical signal to be detected by the detector 5 corresponding to a piccorrespondant of the stress applied to the stress sensor 2. Thus, the device 1, and more particularly the electronic module 6 of the device, can beeconfigured to trigger, when the local extremum of electric signal corresponding to the peak of said stress applied to the stress sensor 2 is detected by the detector 5, the measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3 of the stress sensor 2.

The local extremum of the electrical signal to be detected may be a local maximum, for example a positive voltage, or a local minimum, for example a negative voltage, depending on the connection of the piezoelectric element 4. In this sense, knowing the type constraint that will be applied to the stress sensor 2, andthe characteristics of the piezoelectric element 4, the person skilled in the art is even to know whether a local maximum, or a local minimum, of the electrical signal is to be detected as a local extremum electrical signal.

It is understood from what has been said previously that there is a need for accurate detection of the local extremum of the electrical signal. The electrical signal has a derivative, in particular time derivative, and this derivative is zero, that is to say equal to zero, for the local extremum of the electrical signal. Thus, the detector 5 can be configured to use the derivative of the electrical signal to detect the local extremum of the electrical signal. In other words, the detection of the local end of the electrical signal by the detector 5 is achieved by the detection of a zero crossing of the derivative of the electrical signal. As illustrated in FIG. 2, to enable the implementation of this precise detection, the detector 5 preferably includes a diverter 8 (notably a voltage diverter 8when the electrical signal is a voltage signal) configured to determine the electrical signal's value. In fact, the diverter 8 can output the derivative in the form of a signal derived from the electrical signal. The electric signal derivative signal may be a voltage signal representing the evolution of the electric signal's time difference. Thus, the derivative of the electrical signal can be formed by voltage values. The diverter 8 may be connected to the piezoelectric element 4 which provides it with the electrical signal to be derived. Furthermore, the detector 5 also comprises a comparator 9 configured to output a logic signal alternating between two levels, for example between 0 or 1 (1représentant a level for example given by the supply voltage comcomparateur 9, and 0 representing for example 0 volts). The comparator 9 may comprise an operational amplifier mounted as a comparator or a low power comparator (for example less than 5 μΑ). The changeover to one of the levels indicates that the local extremum of the electrical signal, i.e., the applied peak stress, is detected. In this sense, said one of the levels indicates the detected state of the local extremum of the electrical signal. The comparator 9 is configured to change the level of the logic signal during a, especially during each zero crossing of the derivative of the electrical signal determined by the differentiator 8. In other words, the comparator 9 is a zero comparator. This bridge-based solution 8 and the comparator 9 is preferred because it helps to limit consumption to detect the peak of the applied stress. In a conventional manner, the comparator 9 may comprise means adapted to avoid false detections of the local extremum of the electrical signal, these means, known to those skilled in the art, may for example comprise logic components such as latches associated with the generation of time constant or delay cells whose use can be combined with that of thebules.

In general, the electronic module 6 may comprise an electronic system 10 whose function may be to trigger the measurement of the electrical magnitude representative of the electrical resistance of the piezoresistive element 3 in particular when the comparator 9 has detected the local extremum of the electrical signal, and therefore the peak of the applied stress. This electronic system 10, powered by the power supply source 7, may be a microcontroller with the advantage of limited power consumption. The electronic system 10 can be connected to the output of the comparator 9. More particularly, the output of the comparator 9 is connected to an interruption input of the electronic system 10 so that the electronic system 10 generates a cut-off interruption of said electrical variable representative of the electrical resistance of the piezoresistive element 3 when it detects the passage of the logic signal it receives on its interruption input at a level that it assimilates to a detected state of the local end of the electrical signal. The advantage of using a microcontroller as described is that the latter can occupy a state of waiting in which its power consumption is limited. This waiting state can be interrupted during the generation of the interrupt or when physical measurements / determinations are made at an acquisition frequency as will be described hereinafter.

The level of the logic signal indicating that the local extremum of the electrical signal has been detected may be fixed by design of the device, including taking into account whether the local extremum of the electrical signal to be detected is a local maximum of the generated electrical signal, or a local minimum of the generated electrical signal. The operation of the comparator 9 is also to be taken into account to consider what level of the logic signal corresponds to that indicating that the local extremum of the electrical signal has been detected.

According to a preferred example for detecting the peak of the applied constraint, the two levels of the logic signal correspond respectively to a first level and to a second level. The first and second levels are different, and are in particular each one chosen from a low level and a high level, for example each associated with a voltage of its own, for example OV or a supply voltage of the comparator 9. Moreover, The analyzer 9 is configured to: • pass the logic signal from the first level to the second level during a zero crossing of the derivative of the electrical signal determined by the detector 8 for a local maximum of the electrical signal, for example for a positive value of voltage, • to pass the logic signal from the second level to the first level during a zero crossing of the derivative of the electrical signal determined by the diverter 8 for a local minimum of the electrical signal, for example a negative voltage value.

It is then understood that if the local extremum of the electrical signal to be detected by the detector 5 is the local maximum of the electrical signal, then the peak of stress is considered as detected during the transition from the first level to the second level. Conversely, if the local extremum of the The electrical signal to be detected is the minimum local of the electrical signal, whereas the peak of stress is considered as detected during the transition from the second level to the first level. Such a method is advantageous in the sense that it can easily be implemented without requiring a significant energy consumption. Alternatively, although it requires greater power consumption, the microcontroller evoked previously can be configured to detect the peak of the stress, trigger the measurement of the electrical quantity, and determine the value of the stress. It is now described a concrete application of the preferred example. FIG. 3 shows, during the application of a constraint varying in the meantime to the stress sensor, the evolution of voltage signals S1, S1 ', SL depending on the time respectively representing the electrical signal S1, the derivative S1 of the electrical signal and the logic signal SL. The signal SL varies between 0 and a power supply of the comparator 9. According to this concrete application, the comparator 9 makes it possible to compare the derivative S1 'of the electrical signal S1 with zero, for example with a zero electrical potential, in particular ground. Here, the comparator 9 takes as input voltage values each corresponding to a value of the derivative S1 'of the electrical signal. Moreover, the comparator 9 is configured so that if the voltage corresponding to the derivative of the electrical signal at its input is a voltage different from zero and positive, then the level of the logic signal is high, and so that if the voltage corresponding to the derived from the electrical signal at its input is a negative voltage different from zero, the level of the logic signal is low. In this case, when the derivative ST of the electrical signal becomes zero, it then accompanies a change of sign of the derivative ST of the electrical signal. For example, if the local extremum of the electrical signal to be detected is a local maximum of the electrical signal S1, for example a positive voltage, the derivative ST of the electrical signal will be positive before reaching the local extremum of the electrical signal, then negative after reaching the local extremum of the electrical signal: it follows that the local extremum of the electrical signal can be detected following the regeneration at the output of the comparator 9 of a falling edge to go from the high level to the low level of the logic signal SL. On the contrary, if the local extremum of the electrical signal is a local minimum of the electrical signal S1, for example a negative voltage, the derivative ST of the electrical signal will be negative before reaching the local end of the electrical signal to be detected, then positive after reaching local extremum of the electrical signal to be detected: it follows that the local extremum of the electrical signal can be detected following the output generation of the comparator 9 of a rising edge to go from the low level of the logic signal SL to the high level of the logic signal.

More particularly, the output of the comparator 9 providing the signal may be connected to the interruption input of the electronic system 10. Thus, the electronic system 10 generates an interruption triggering the measurement of the electrical magnitude representative of the electrical resistance of the electronic system 10. the piezoresistive element 3 when said electronic system 10 detects the passage of signalallogic that it receives on its interruption input at a level that it assimilates to the detected state of the local extremum of the electrical signal. Of course, depending on the programming of the electronic system, a logic signal inverter may be placed between the output of the comparator 9 and the interrupt input of the electronic system 10: it will depend on how the electronic system 10 interprets the logic signal at its input. interruption as a function of the interpretation of the logic signal at the output of the comparator 9.

As illustrated in FIG. 4, the differentiator 8 may be a series RC cononfiguration circuit. In the field, an RC circuit is a circuit which comprises a resistive component 8a and a capacitor 8b. FIG. 4 illustrates a particular example of implementation of the differentiator 8 comprising an input En8 connected to the piezoelectric element and an output S8 connected to the comparator 9. The capacitor 8b of the differentiator 8 is connected to the output S8 and to the input En8 of the derivative 8. Furthermore, the resistive component 8a of the diverter 8 is connected, on the one hand, to ground, and on the other hand, to an electrical link connecting the capacitor 8b to the output S8 of the diverter 8. The advantage of such a derivative is that it consumes little energy and takes up very little space on the electronic board (two components). The capacitance of the capacitor 8b is between 1 μF and 100 μF, and the electrical resistance of the resistive component 8 a is between 1 μΩ and 100ΜΩ, and this does not affect the output voltage of the piezoelectric element whose capacity will typically be between 100pF and 10nF.

As illustrated in Figure 4, the comparator 9 may include a first input denoted "+" and a second input denoted "-". The first input "+" is connected to the output S8 of the diverter 8 and the second input "-" is connected to the line so as to allow the comparison to zero. The output S9 of the comparator 9 makes it possible to supply the logic signal mentioned above by comparison of the data supplied to the first and second inputs of the comparator 9. The level of the logic signal corresponding to the detected state of the peak of the stress applied depends in particular on the connection of the inputs of the comparator 9. to the differentiator8 and to the mass.

As illustrated in FIGS. 5 to 11, the device may comprise a substrate 11 on which the stress sensor 2 is arranged. The substrate 11 may be thin and coupled to allow its deformation when the stress is applied to the stress sensor 2. The substrate On the other hand, it can be rigid. The substrate 11 can be chosen so that it can be cut, for example in the format of a sole, to be inserted into a shoe. The substrate 11 may be of plastic type PEN (for polyethylene naphthalate), PET (for polyethylene terephthalate), PC (for Polycarbonate), Polymide (for example Kapton®) or other. Typically, the substrate 11 has a thickness that can be between 100pm and 200pm. The thickness of the substrate 11 given above and combined with a Young's modulus of the material of the substrate 11, for example between 0.5GPa and 10GPa, corresponds to the definition of a thin and flexible substrate 11. Alternatively, the substrate 11 may also be paper, textile, metal, or another typ adapted to the desired application. The range of thickness given in relation to the various examples of the substrate 11 makes it possible to obtain sufficient flexibility of the substrate for applications in the field of the shoe sole. [0048] As represented in FIGS. 5 to 11, the piezoelectric element 4 may comprise a stack comprising two electrodes 41, 42 (also called first electrode 41 and second electrode 42 of the piezoelectric element 4) separated by a layer of piezoelectric material 43, for example a dielectric material having piezoelectric properties. The electrodes 41, 42 of the piezoelectric element 4 can take the form of layers stacked with the layer of piezoelectric material 43 located between said two electrodes 41, 42. The layers forming the electrodes 41, 42 of the piezoelectric element 4 may each have a thickness of between 0.5 nm and 500 nm for electrodes produced by chemical vapor deposition and between 0.5 μm and 5 μm for electrodes produced by printing. The layer of piezoelectric material 43 may have a thickness of between 0.5 μm and 50 μm depending on the desired force of the voltage signal generated by the piezoelectric element 4. The thickness of the layer of piezoelectric material has an influence on the voltage generated by the piezoelectric element : the greater the estimated thickness plus the voltage of the electrical signal generated by the piezoelectric element 4 will be important. The electrodes 41, 42 of the piezoelectric element 4 may comprise metal (for example silver in the form of particles, nanoparticles or nanowires), or an electronically conductive polymer such as, for example, PEDOT-PSS or polyaniline. The electrodes 41, 42 of the piezoelectric element 4 can be obtained by printing from an ink containing the aforementioned metal, or the aforementioned electronic conductive polymer.Alternatively, the electrodes 41, 42 of the piezoelectric element 4 can be obtained by using a vacuum deposition method such as a CVD (chemical vapor deposition in French) or a PVD (physical vapor deposition) process or physical vapor deposition in English French). The two electrodes 41, 42 of the piezoelectric element 4 may be made of identical or different materials. PEDOT-PSS, also known as PEDOT: PSS, is a combination of two polymers, poly (3,4-ethylenedioxythiophene) (PEDOT) and sodium poly (styrene sulfonate) (PSS). PEDOT is a derivative conjugated polymer polythiophene of which a fraction of the sulfur atoms is protonated with a positive charge, whereas the PSS is a negatively charged sulfonated polystyrene of which a fraction of the sulfonate groups SC> 3 carries a sodium ion Na +. The layer of piezoelectric material 43 may comprise piezoelectric inorganic particles such as titanium lead zirconate particles (also known under the name PZT), or barium titanate (as for example BaTiOs), or still include an electro-active polymer such as poly (vinylidenefluoride-co-trifluoroethylene) (also known as P (VDF) -TrFE). The layer of piezoelectric material 43 may be made by depositing an ink comprising the inorganic particles or the electroactive polymer.

As represented in FIGS. 5 to 11, the piezoresistive element 3 may comprise two electrodes 31, 32 nested one inside the other in the form of an undefine: these are interdigital electrodes 31, 32 (also called first electrode 31 and second interdigitated electrode 32 of the piezoresistive element 3) as for example illustrated in FIG. 12 which represents an example of interdigitated electrodes 31, 32. Each of these interdigitated electrodes 31, 32 may have a thickness, given orthogonally with respect to the plane where they are interdigitated, between 1pm and 15pm (although higher values may also be possible), and preferably between 7pm and 8pm. The electrode 31 may comprise fingers 31a and the electrode 32 may comprise fingers 32a, the fingers are interdigitated so that each finger 31a of one of the electrodes 31 of the piezoresistive element 3 is separated from a finger 32a of the another electrode 32 of the piezoresistive element 3 with a distance of between 100 μm and 1000 μm. The width of the interdigitated fingers parallel to the plane where they are interdigitated can be between 100pm and 1000pm. According to a non-limiting example, the width of the fingers is 500 μm, and the spacing between two adjacent fingers is 500 μm. Each interdigital electrode may for example comprise eight fingers (Figure 12). The dimensions of the interdigital fingers, their spacing and their numbers are chosen according to the constraint that one wishes to measure and the desired sensitivity of the stress sensor. These electrodes 31, 32 of the piezoresistive element 3 may comprise metal (for example silver in the form of particles, nanoparticles or nanowires), or an electron conducting polymer such as, for example, PEDOT-PSS or polyaniline. The electrodes 31, 32 of the piezoresistive element 3 can be made by printing from inks comprising the materials described above, or be formed using a vacuum deposition method, for example by CVD or by PVD. A layer of piezoresistive material 33, in particular with a thickness of between 5 μm and 25 μm, is held facing the electrodes by a spacer element 34, which is called a spacer. More particularly, the piezoresistive material layer 33 is formed on a support 35 fixed to the spacer element 34. The piezoresistive material is FSR (for the English "Force Sensing Resistor"). The piezoresistive material may comprise at least one of: carbon particles, carbon nanotubes, graphene, and metal particles, mixed with an elastomeric binder. The piezoresistive material may alternatively be formed by an electrically conductive polymer (e.g. polyaniline, polypyrrole or polyacetylene). The bracing member 34 may be an acrylic or silicone adhesive. The support 35 may be made of polypropylene (PP), polyethylene (PE), or polyurethane (PUR) and in particular take the form of a layer of thickness between 25pm and 200pm. The spacer element 34 is arranged such that the piezoresistive element 33 remains at a distance, for example 50 μm, from the electrodes when no stress to be determined is applied to the stress sensor 2. Moreover, when a stress to be determined is applied to the stress sensor 2, it deforms the layer of piezoresistive material 33 which comes into contact with the interdigitated electrodes 31, 32: it follows that it is possible to supply the piezoresistive element 3 to measure a voltage ( allowing the electrical resistance of the piezoresistive element 3) representative of the deformation of the piezoresistive material layer 33, if applicable, to be able to be correlated with a stress value, such as a pressure. It is then understood that the bracing member 34 is sized according to the stresses that it is desired to determine.

When the stress sensor is not stressed, the layers of piezoelectric material and piezoresistive material are preferablyparallel between them. The device is arranged in such a way that the stress is applied parallel to the direction of the stack forming the piezoelectric sensor, this stacking direction being perpendicular to the plane of the substrate 11.

In order to improve the operation of the stress sensor 2, it is better that the piezoelectric element 4 and the piezoresistive element 3 are as close as possible to one another. This desired proximity allows the peak of the stress seen by the piezoelectric element 4 and the piezoresistive element 3 to be detected by the piezoelectric element 4 and then measured by the piezoresistive element at the same point or almost at the same point, thus improving the overall accuracy of the piezoelectric element. stress sensor 2.

According to a first embodiment of the stress sensor 2 illustrated in FIGS. 5 and 6, the piezoresistive element 3 and the piezoelectric element 4 of the pressure sensor 2 are juxtaposed, and in particular arranged side by side on the same substrate 11. The element piezoresistive 3 comprises the interdigital electrodes 31, 32, in particular in contact with the substrate 11, and the layer of piezoresistive material 33 formed on the support 35 separated from the substrate 11 by the spacer element 34 connecting the substrate 11 to said support 35. The piezoelectric element 4 comprises a layer of piezoelectric material 43 located between its two electrodes 41, 42, 41 of which is proximal to, and in particular in contact with, the substrate 11. This solution has the advantage of being easy to produce, but has the disadvantage of a lateral shift between the piezoresistive elements 3 and piezoelectric 4. Decalculation limits the accuracy of the stress sensor 2 when the stress is applied orthogonally to the plane of FIG. 5 because the detection of the stress peak from the piezoelectric element 4 and the measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3 will be split in two close but distinct points.

It is understood that there is a problem of improving the accuracy of the strain sensor 2. The second and third embodiments of the pressure sensor 2 described below allow to address this problem of improving the accuracy.

According to the second embodiment of the strain sensor 2 illustrated in FIGS. 7 and 8, the piezoelectric element 4 and the piezoresistive element 3 of the pressure sensor 2 are arranged so that the piezoelectric element 4 surrounds, in particular at least partially, the piezoresistive element 3 or vice versa. It is said that the piezoelectric element 4 and the piezoresistive element 3 can be concentric. For example, as illustrated in FIG. 7, the piezoelectric element 4 adopts an annular shape. The piezoelectric element 4 can form an open or closed ring according to the technological needs. In FIG. 7, the piezoelectric element 4 is arranged around the piezoresistive element 3, according to another formulation, the piezoelectric element 4 is disposed laterally to the piezoresistive element 3 so as to surround it. According to this second embodiment, the piezoelectric element 4 and the piezoresistive element are formed on the substrate 11 in the manner as described in connection with the first embodiment. This second realization makes it possible to improve the accuracy of the stress sensor 2 by improving the location of the stress relative to the piezoelectric elements 4 andpiezoresistive 3.

According to the third embodiment illustrated in FIGS. 9 to 11, the pressure sensor 2 is such that the piezoelectric element 4 and the piezoresistive element 3 are stacked, in particular this stack is made on the substrate 11. This embodiment is preferred, especially when the stress applied to the stress sensor 2 is parallel, or secant but not orthogonal, to the stacking direction of the stack comprising the piezoelectric element 4 and the piezoresistive element 3. According to this third embodiment, a stress sensor 2 is obtained compact and accurate since any force applied parallel to the stacking direction can be seen similarly by the piezoelectric elements 4 and piezoresistive 3.On FIGS. 10 and 11, the piezoresistive element 3 is formed on the piezoelectric element 4. The element piezoelectric 4 is formed as for him the substrate 11 which carries it. FIG. 11 shows successively from the substrate 11, a stack comprising one of the electrodes 41 of the piezoelectric element 4, the piezoelectric material layer 43, the other of the electrodes 42 of the piezoelectric element 4, a material element dielectric 12 (for example, in the form of a layer formed on the second electrode 42), and the interdigital electrodes 31, 32 of the piezoresistive element 3 formed on the element dielectric material 12. The element of dielectric material 12 can have a thickness between 1pm and 10pm, and be poly (fluoride divinylidene) (PVDF). The layer of piezoresistive material 33 is disposed opposite the interdigital electrodes 31, 32, and is kept at a distance from said interdigital electrodes 31, 32 via the spacer element 34 which can be fixed, on the one hand, on the element made of dielectric material 12 or else on the substrate 11et, on the other hand on the support 35 on which is formed the layer of piezoresistive material 33 oriented towards the interdigital electrodes 31, 32. According to an alternative illustrated in FIG. 10, the element in dielectric material 12 is not required, in this case the electrode 42 of the most resistant piezoelectric element 4 of the substrate 11 may be formed by one of the interdigitated electrodes 32 of the piezoresistive element 3 then arranged on the layer piezoelectric material43, this has the advantage of limiting the number of layers and the amount of material used: the stress sensor 2 can then to be more compact and therefore more precise.

Preferably, one of the electrodes of the piezoelectric element 4 and one of the electrodes of the piezoresistive element 3 are electrically connected to each other, this makes it possible to limit the number of electrical tracks to be made to connect the piezoelectric elements 4 and piezoresistive 3 to the electronic module, and limit the number of tracks to be formed on the same face of the substrate 11 to avoid overlapping tracks that would be electrically insulated. For the embodiments of the stress sensor 2 of FIGS. 6 and 8, it is possible to connect the electrode of the piezoelectric element 4 closest to the substrate 11 to one of the electrodes of the piezoresistive element 3. In FIG. the piezoelectric element 4 and the piezoresistive element 4 comprise a common electrode, which makes it possible to facilitate the manufacture of the device.

The different embodiments of the stress sensor 2 described above are particularly suitable for the device as described. However, such a stress sensor 2 can be used independently in any type of system that seeks to obtain information on a constraintapplied to a stress sensor 2 without being limited to the audipositive application as described. Indeed, the data from the piezoelectric element 4 and the piezoresistive element 3 can be used in a complementary manner to characterize a stress applied to the stress sensor.

The strain sensor 2 described may be wholly or partly obtained by printing suitable materials as mentioned above. The printing can be carried out by screen printing, by spraying also known by the English name "spray-coating", or by inkjet. The advantage of the use of printing in the production of a stress sensor 2 as described is that it is thus possible to deposit the materials in a suitable pattern that does not require etching and therefore the loss of material. The stress sensor 2 can thus be manufactured at low cost, over large areas and on flexible substrates.

It is now described a method for manufacturing the stress sensor 2 as illustrated in its various embodiments in Figures 5 to 11. The materials used in the present manufacturing process can be those described above in connection with the description of the piezoelectric element and the piezoresistive element. Such a manufacturing method comprises a step of forming the piezoelectric element 4 and a step of forming the piezoresistive element 3. As will be seen later, these two formation steps may be completely independent, or may include common technological techniques. The thicknesses described above are given orthogonally to the plane of the substrate for what is formed from, or above, the substrate, or possibly the support for the layer of piezoresistive material on the support.

The manufacturing method may comprise a substrate supply step 11 as previously described.

Before implementing the steps of forming the piezoelectric elements 4 and piezoresistive 3, the manufacturing process may comprise a cleaning step of the substrate 11 for example with ethanol or acetone, or by plasma treatment. The purpose of this cleaning is to obtain a surface conditionproductible substrate before forming the stress sensor.

It follows from what is shown in Figures 6, 8, 10 and 11 that the deformation step of the piezoelectric element 4 may comprise the following steps: a step of forming the first electrode 41, said lower electrode, del piezoelectric element 4, in particular on the substrate 11; a step of forming the layer of piezoelectric material 43 on the first electrode 41; a step of forming the second electrode 42, referred to as the upper electrode, of the piezoelectric element 4 on the layer of piezoelectric material 43.

The step of forming the first electrode 41 of the piezoelectric element 4 is in particular such that the first electrode 41 of the piezoelectric element 4 may be produced by printing (for example by screen printing) on the substrate 11, a metallic ink (for example based on silver or copper), or a polymeric ink (for example based on PEDOT-PSS or polyaniline), or can be produced by vacuum deposition, for example by PVD or CVD. When the first electrode 41 of the piezoelectric element 4 is made by printing an ink, the printed ink is dried and then annealed. The annealing can take place in an oven at a temperature between the ambient temperature (25 ° C °) and 200 ° C depending on the nature of the substrate 11 and the material used to form the first electrode 41 of the piezoelectric element 4.

In general, the drying of an ink allows a solvent evaporation it contains at a gentle temperature, typically between room temperature and 60 ° C, the annealing of the ink allows for it to evaporate the luster of solvent of the ink but to give the ink at the end of the annealing some desired properties of the element to be formed from the ink.

The step of forming the layer of piezoelectric material 43 is particularly such that the piezoelectric material layer 43 can be made by printing (for example by screen printing), on the first electrode 41 of the piezoelectric element 4, an ink The piezoelectric material is said to be dispersed and dissolved in the ink. Alternatively, the piezoelectric material chosen to form the piezoelectric material layer 43 may be deposited under vacuum, for example by PVDou CVD. When the layer of piezoelectric material is produced by printing, the printed ink is dried and then annealed. The annealing can take place in an oven at a temperature between room temperature (25 ° C) and200 ° C depending on the nature of the substrate 11 and the material used to form the piezoelectric material layer 43. The annealing is particularly carried out at a temperature allowing the piezoelectric material to crystallize in a form giving it its properties. The drying can be carried out here in an empty chamber at room temperature, or on a hot plate, or in an oven at a temperature below 60 ° C.

The step of forming the second electrode 42 of the piezoelectric element 4 is in particular such that said second electrode 42 may be made in the same manner as the first electrode 41 of the piezoelectric element 4, with the difference that its printing or deposit is made on the layer of piezoelectric material 43.

It follows from what is shown in Figures 6, 8, 10 and 11 that the deformation step of the piezoresistive element 3 may comprise the following steps: a step of forming the first and second electrodes 31, 32 interdigitated piezoresistive element 3 (for example as in FIG. 12); a step of associating the layer of piezoresistive material 33 with the first and second interdigitated electrodes 31, 32.

The step of forming the first and second interdigitated electrodes 31, 32 may comprise a deposit by printing (for example by screen printing) of an ink (as described in the context of the piezoresistive element) in a pattern adapted to the shapes The first and second interdigitated electrodes 31, 32 may alternatively be obtained from a deposition of suitable material, in particular by PVD or CVD technique. In particular, the first and second electrodes 31, 32 of the piezoresistive element 3 may be formed simultaneously with the second electrode 42 of the piezoelectric element 4 so that the second electrode 42 of the piezoelectric element 4 is electrically connected to one 32 of the first and second electrodes 31, 32 of the piezoresistive element 3, or, if appropriate, that the second electrode 42 of the piezoelectric element 4 is common to one of the first and second electrodes 31, 32 of the piezoresistive element 3 ( 10). When they are produced by printing, the first and second electrodes 31, 32 of the piezoresistive element 3 are dried, then annealed for example in an oven in the manner as described above in the context of the formation of the first electrode 41 of the piezoelectric element 4.

The step of combining the piezoresistive material layer 33 with the first and second interdigitated electrodes 31, 32 may comprise a step of forming the spacer element 34 around the first and second interdigitated electrodes 31, 32. formation of the spacer element can be achieved by printing a suitable material such as a resin or glue, or by laminating an adhesive film. The association step may then include a step of mounting the layer of piezoresistive material 33 to the spacer element 34 after its formation. The layer of piezoresistive material 33 may be formed on the support 35, for example by printing. This support 35 may be of the same type as the substrate 11 described above, and is bonded to the spacer element 34.

For example, to implement the first or second embodiment of the stress sensor 2, the step of forming the first and second interdigital electrodes 31, 32 of the piezoresistive element 3 may be such that these first and second interdigitated electrodes 31, 32 are formed on the substrate 11, and that this step of forming the first and second interdigitated electrodes 31, 32 of the piezoresistive element 3 is performed simultaneously with the step of forming the first electrode 41 of the element piezoelectric 4, or at the step of forming the second electrode 42 of the piezoelectric element4. The advantage of simultaneous training is to improve the production rate, for example by reducing the manufacturing time by using fewer levels to print, or by reducing the number of screen printing masks to be used.

For example, to implement the third embodiment of the strain sensor 2 (FIGS. 10 and 11), the step of forming the first electrode 41 of the piezoelectric element is such that said first electrode 41 is formed on the In addition, the first and second interdigitated electrodes 31, 32 of the piezoresistive element 3 can be formed on the dielectric material element 12 serving as separation (FIG. 11) formed on the second electrode 42 of the piezoelectric element before putting the step of forming the piezoresistive element on this element made of dielectric material 12.Alternatively (FIG. 10), again according to the third embodiment, the step of forming the first and second interdigital electrodes 31, 32 of the piezoresistive element 3 can be performed simultaneously with the step of forming the second electrode 42 of the piezoelectric element 4 from which it follows that the second electrode 42 of the piezoelectric element 4 and one of the first and second interdigital electrodes 31, 32 of the piezoresistive element 3 form a common electrode common to the piezoelectric element 4 and the piezoresistive element 3, this limits the amount of material used.

In particular, the method of manufacturing the stress sensor may be a step of a manufacturing method of the device as described.

The device 1 may comprise one or more stress sensors 2 as described. In FIG. 13, the device 1 adopts, at least in part, the shape of a shoe insole incorporating the or, where appropriate, the stress sensors 2. This has the advantage of being able to measure stresses, particularly peaks, related to the walk of an individual when wearing a shoe equipped with the device.

In particular, as illustrated in FIG. 13, the device 1 comprises a plurality of stress sensors 2, all that has been described in relation to a strain sensor 2 can be applied to each stress sensor 2 of the device 1. the example of Figure 13, eleven strain sensors 2 are distributed on the substrate 11 adopting the shape of a shoe sole. Thus, when the device 1 comprises a plurality of stress sensors 2, each stress sensor 2 comprises a piezoresistive element 3 and a piezoelectric element 4 capable of generating an electrical signal when a stress is applied to said strain sensor 2 (in FIG. the piezoelectric elements 4 and the piezoresistive elements 3 of the stress sensors 2 are, by way of example, represented stacked in the manner of FIG. 9). In FIG. 13, the device 1 comprises a plurality of detectors 5 schematized in line in the frame corresponding to the electronic module 6. Each stress sensor 2 is associated with one of the detectors 5, that is to say that it is connected to one of the detectors 5. In particular, each detector 5 and each piezoresistive element 3 is connected to the electronic system 10. Moreover, for each stress sensor 2: the detector 5 associated with said stress sensor 2 is configured to detect a peak of the stress applied to said sensor stress 2 byprocessing the electrical signal of the piezoelectric element 4 of said stress sensor 2, • the device is configured to trigger, when said stress peak is detected by said detector 5, a measurement of the electrical magnitude representative of the electrical resistance of the piezoresistive element 3 of said strain sensor, the device 1 is configured to terminating a value of the constraintapplied to said strain sensor 2 using the electrimetric magnitude.

Here, it is understood that the device has the advantage of allowing the detection of stress currents applied in several distinct locations of the device 1, these constraints then being preferentially associated with the same event as for example a support of a foot on the stress sensors 2 For example, in an application of a sports sole (FIG. 13), the device 1 can comprise the substrate 11 adopting the shape of a shoe sole, said substrate 11 having a face on which the strain sensors 2 of the plurality are arranged. detectors, this face being intended to be oriented towards the sole of the foot.

As illustrated in FIGS. 2 and 13, the device 1 may comprise a modulus of determination 13 of the value of the stress (in particular of the peak value of the applied stress), for example integrated in the electronic system 10, using the measured electrical quantity. from the piezoresistive element 3 of the strain sensor 2. This can be implemented in a manner known per se by the skilled person, for example by reading a correspondence table of the processing module 13 associating stress values with electrical quantities representative of the electrical resistance of the piezoresistive element 3.

The invention also relates to a method of determining stress (in particular of at least one stress value applied to a pressure sensor 2 of the device 1) from the, that is to say by using the , Device 1comportant said at least one stress sensor 2 comprising thepiezoresistive element 3 and the piezoelectric element 4, in particular the device may be as previously described. In particular, the advantages associated with this method of determining strain derive from those described above in the context of device 1. The method of determining stress comprises (FIG. 14): a step E1 of applying the stress to the stress sensor 2, a processing step E2 of the electrical signal generated by the piezoelectric element 4 in response to the stress applied to the load sensor 2 to detect the peak of the stress applied to the load sensor 2, a measuring step E3 of the electrical quantity representative of the electrical resistance of the piezoresistive element 3, the measurement step E3 of the electrical amplifier representative of the electrical resistance of the piezoresistive element 3 being triggered when the peak of the applied stress is detected, • a step of determining E4 of the value the stress applied to the strain sensor 2 using the a measured electrical quantity representative of the electrical resistance of the piezoresistive element 3.

Thus, it is the detection of the stress peak which causes the triggering of the measurement step E3. The sequence of these different steps has the advantage of triggering the measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3 at the appropriate time, which makes it possible, if necessary, to limit the overall consumption related to the operation of the piezoresistive element. and / or to perform the measurement of the electrical magnitude representative of the electrical resistance of the piezoresistive element 3 at the moment when the stress applied to the stress sensor 2presents a peak that one seeks to determine from said electrimetric magnitude. Thus, the measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3 is such that the measured electrical quantity is representative of the value of the peak of the applied stress. In particular, the processing step E2 can be implemented by the detector 5, the measurement step E3 can be triggered by the electronic system 10, and the determination step E4 can be implemented by the electronic system 10. The determining step E4 of the value of the applied stress, in particular the value of the peak of the applied stress, to said strain sensor 2 from the measured electrical magnifier, is known per se to those skilled in the art: as long as it has the measured electrical quantity, it can go back to the corresponding value of the stress, such as an applied pressure value.

In particular, the processing step E2 of the electrical signal comprises a detection step E2-1 of the local extremum of the electrical signal corresponding to thepick of the applied stress. Said local extremum of the electrical signal is generated by the piezoelectric element 4 at the moment when the stress sensor 2 is subjected to the peak of the stress applied to said strain sensor 2. Thus, it results from the detection of this local extremum of the electrical signal, a detection precise dupic of the applied constraint.

According to a particular implementation, the detection step E2-1 of the local end of the electrical signal comprises a step E2-1-1determination of the derivative of the electrical signal, and a detection step E2-1- 2a zero passage of the derivative of the electrical signal, that is to say the determined derivative, whereby it results that said local extremum of the electrical signal is detected. The detection step E2-1-2 of the zero crossing consists in detecting the presence of a point of the electrical signal for which the determined derivative of the electrical signal at this point is zero. The determination step E2-1-1 of the derivative can be implemented by the differentiator 8, and the detection step E2-1-2 of the zero crossing can be implemented by the comparator 9.

The detection step E2-1-2 of the zero crossing of the derivative of the electrical signal, that is to say of the determined derivative, can be implemented by the co-processor 9 of the device 1 supplying the output alternating logic signal between two levels, the peak of the applied stress being detected when the signal at the output of the comparator 9 goes to one of the levels of the logic signalassimilated to a detected state of the local extremum of the electrical signal. Here, the comparator 9 of the device 1 makes it possible to compare the derivative of the electrical signal with a zero reference value (equal to zero). In particular, the processing step E2comporte a step of changing the level of the logic signal of the comparator 9 each time the derivative becomes zero.

According to one embodiment of the stress determination method, the piezoresistive element 3 can be used to determine, preferably at regular intervals, physical quantities representative each of a corresponding electrical resistance of the piezoresistive element 3 from which it is possible to to know values of any constraints applied to the stress sensor 2 at different times. In this sense, the stress determination method may comprise a determination step E5, according to a frequency of acquisition, of a plurality of physical quantities each representative of an electrical resistance of the piezoresistive element 3. This acquisition frequency used during the determination step E5 is preferably between 10Hz and 20Hz to limit the power consumption required to determine these physical quantities. The detection of the peak of the stress applied provoquel'augmentation of the acquisition frequency to trigger the measurement step E3de the electrical quantity representative of the electrical resistance of thepiezoresistive element 3. In other words, the detection of the peak of the applied stress interrupts the determination step E5 of the physical quantities is temporarily measured to measure the electrical quantity representative of the electrical resistance of the piezoresistive element 3 then representative of the value of the applied stress peak. This increase is such that the acquisition frequency during the step E2 is then between 200Hz and 2kHz, but may be limited to one, five or even ten measurement points before returning between 10Hz and 20Hz to continue the implementation of the determination step E5 of the physical quantities. The obtaining of several measurement points of electrical quantities each representative of a corresponding electrical resistance of the piezoresistive element 3 at different times to characterize more endetails the peak of the applied stress, for example to know if it is about Brief or short support on the stress sensor 2. In other words, obtaining several measurement points at the moment when the stress peak is present makes it possible to obtain additional information as to the applied stress peak. The step E5 has the advantage of following the evolution of one or more constraints applied to the stress sensor 2: it is then possible to obtain information on one or more static or dynamic stresses out of one or more stress peaks.

Of course, if the device 1 comprises a plurality of pressure sensors 2 each comprising a piezoresistive element 3 and a piezoelectric element 4 capable of generating an electrical signal when a stress is applied to said stress sensor 2, the application steps of a constraint E1, E2 processing, E3 measurement and E4 determination of an applied stress value can be implemented for several, and especially each stress sensor 2. Of course, for each stress sensor, the constraint applied may be different. Here, each stress sensor 2 is independent in the sense that the measurement of the piezoresistive element 3 will take place when the peak of the applied stress has been detected from the electrical signal of the piezoelectric element 4 of the stress sensor 2: the electrical consumption connected the use of each stress sensor 2 is then limited independently to that of the other stress sensors 2. Therefore, the stress determination procedure may comprise a step of generating a mapping of the stress values undergone in different zones of the device 1 each zone being associated with one of the stress sensors 2 which is situated in said zone.

Furthermore, the piezoelectric element 4, although judiciously used to trigger the measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3 at the moment when the peak of the stress is applied to the stress sensor 2, may also be used to measure data related to the applied stress, for example shear stress data, or to measure additional information about the peak stress applied. It is in this sense that in FIG. 2, the piezoelectric element 4 is connected via the arrow F3 to the electronic system 10 which can, via an ADC, measure a value, for example of voltage, of the piezoelectric element 4 when the peak the stress applied to the stress sensor 2 is detected.

According to a variant (not shown), the stress sensor can also have several piezoresistive elements, and when the peak of the stress is detected from the use of the piezoelectric element of the stress sensor, this triggers, for each piezoresistive element of the stress sensor, a measurement of an electrical quantity representative of an electrical resistance. Thispresents the advantage of locally having the constraint sensor severalinformation characterizing the stress.

The industrial applications of the present invention are numerous. The device, and thus the stress determination method, can be used in any type of application for which it is desired to have a maximum value of a stress exerted while limiting the power consumption making it possible to obtain this maximum value. One application described relates to sport in the sense that the device may comprise a soleplate to recognize the pressures exerted by the foot of a runner for example to anticipate his injuries. In the medical field, the present invention can also be applied to detect the pressure zones of the body of a diabetic patient. The invention can also be applied to touch screens, vehicle tires, robotics, artificial limbs, gloves, safety shoes, or even the wings of airplanes which we seek to know the stresses inforeformation suffered during the flight of the aircraft.

Claims (7)

Device (1) comprising at least one strain sensor (2) comprising a piezoresistive element (3), characterized in that: • the stress sensor (2) comprises a piezoelectric element (4) capable of generating an electrical signal when a constraint is applied to the stress sensor (2), • said device (1) comprises a detector (5) configured to detect a peak of the stress applied to the stress sensor (2) by electric signal processing, • the device (1) is configured to trigger, when the peak of the constraint applied is detected by the detector (5), a measurement of an electrical quantity representative of an electrical resistance of the piezoresistive element (3) of the stress sensor (2), and • the device (1) is configured to determine a value of the constraintapplied to the stress sensor (2) using the electrimetric magnitude. 2. Device according to the preceding claim, characterized in that the detector (5) is configured to detect a local extremum of the electrical signal of the piezoelectric element (4) corresponding to the peak of the applied stress. 3. Device according to claim 2, characterized in that the detector (5) is configured to use the derivative of the electrical signal to detect the local extremum of the electrical signal. 4. Device according to any one of the preceding claims, characterized in that the detector (5) comprises: • a diverter (8) configured to determine the derivative of the electrical signal, • a comparator (9) configured to output a an alternating logic signal between two levels, changing to one of the levels indicating that the applied stress peak is detected, said comparator (9) being configured to change the level of the logic signal during a zero crossing of the derivative of the electrical signal determined by the differentiator (8). 5. Device according to the preceding claim, characterized in that the two levels of the logic signal respectively correspond to a first level and a second level, and in that the comparator (9) is configured to: • pass the logic signal of the first level at the second level during a zero crossing of the derivative of the electrical signal determined by the differentiator (8) for a local maximum of the electrical signal, • passing the logic signal from the second level to the first level during a zero crossing of the derived from the electrical signal determined by the differentiator (8) for a local minimum of the electrical signal, and in that: • the local extremum of the electrical signal being the local maximum of the electrical signal, the peak of stress is considered as detected during the switchover first level at the second level, or • the local extremum of the electrical signal being the local minimum of the electrical signal, the peak of cont. rainte is considered detected when passing from the second level to the first level. 6. Device according to any one of the preceding claims, characterized in that the piezoresistive element (3) and the piezoelectric element (4) strain sensor (2) are juxtaposed. 7. Device according to any one of claims 1 to 5, characterized in that the piezoelectric element (4) and the piezoresistive element (3) of the pressure sensor (2) are arranged so that: • the piezoelectric element ( 4) at least partially surrounds the piezoresistive element (3), or • the piezoresistive element (3) at least partially surrounds the piezoelectric element (4). 8. Device according to any one of claims 1 to 5, characterized in that the stress sensor (2) is such that the piezoelectric element (4) and the piezoresistive element (3) are stacked. 9. Device according to any one of the preceding claims, characterized in that it adopts at least partly in the form of a shoe sole incorporating the strain sensor (2). A method for determining stress from a device (1) comprising at least one strain sensor (2) having a piezoresistive element (3), said stress determining method comprising: an application step (E1) of a constraint to the stress sensor (2), characterized in that said stress sensor (2) having a piezoelectric element (4), said stress determination method comprises: a processing step (E2) of an electrical signal generated by the piezoelectric element (4) in response to the stress applied to the stress sensor (2) to detect a peak of the stress applied to the strain sensor (2), • a step of measuring (E3) a representative electrical quantity of an electrical resistance of the piezoresistive element (3) of the stress sensor (2), the step of measuring (E3) the electrical magnitude of the electrical resistance of the element piezoresistive ent (3) being triggered when the applied stress peak is detected, • a step of determining (E4) a value of the stress applied to said stress sensor (2) using the measured electrical quantity. 11. Stress determination method according to the preceding claim, characterized in that the processing step (E2) comprises a step of detecting (E2-1) a local extremum of the electrical signal corresponding to the applied stress peak. 12. Stress determination method according to the preceding claim, characterized in that the step of detecting (E2-1) the local extremum of the electrical signal comprises: a determination step (E2-1-1) of the derivative of the electrical signal, and • a step of detecting (E2-1-2) a zero crossing of the electrical signal derivative, whereby said local extremum of the electrical signal is detected. 13. A method of determining stress according to the preceding claim, characterized in that the step of detecting (E2-1-2) the deviation of zero crossing of the electrical signal is implemented by a comparator (9) of the device ( 1) outputting a logic signal alternating between two levels, the peak applied stress being detected when the logic signal outputcomcomparateur (9) goes to one of the levels of the logic signal assimilated to a statedétecté of the local extremum of electrical signal. 14. Stress determination method according to any one of claims 10 to 13, characterized in that it comprises a step ofdetermination (E5), according to an acquisition frequency, a plurality ofphysical quantities each representative of the electrical resistance of the piezoresistive element (3), and in that the detection of the peak of the applied stress causes the increase of the acquisition frequency to trigger the measurement step (E3) of the electrical quantity representative of the electrical resistance of the element piezoresistive (3). 15. Stress determination method according to any one of Claims 10 to 14, characterized in that it comprises a plurality of stress sensors (2) each comprising a piezoresistive element (3) and a piezoelectric element (4), and in that the steps of applying a constraint (E1), processing (E2), measuring (E3), and determining (E4) of an applied stress value are implemented for each stress sensor (2) .