FR3068132A1 - 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 STRESS VALUE CORRESPONDING TO A PIC OF THE STRESS AND METHOD OF DETERMINING STRESS

Field of the invention 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 There are numerous solutions making it possible to characterize the stresses exerted on stress or pressure sensors.

In the particular field of footwear, it is possible to use pressure sensors judiciously positioned within a sole to, for example, determine the shocks undergone by the human body during the impact of the foot and / or for example measuring different pressures applied by the arch of the foot on the sole.

A pressure sensor can be formed by a piezoresistive element which behaves like a variable resistance whose value depends on the pressure applied to the pressure sensor. Such a pressure sensor has the advantage of allowing the detection of shocks or other brief phenomena, but has the disadvantage of requiring the use of measurement electronics with a high acquisition frequency of electrical quantities each representative of a electrical resistance of the piezoresistive element and dependent on the stress exerted to try to measure a value representative of the peak of the stress corresponding to one of these electrical quantities. A high frequency of acquisition of the measurement electronics, in particular higher than 200Hz or 300Hz, consumes energy. Too high energy consumption is incompatible with on-board solutions which generally use a battery to operate. Furthermore, another drawback 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 to the peak of the stress is not made not at the right time.

In this sense, there is a need to find a solution to limit the energy consumption necessary for measuring the electrical quantity representative of the electrical resistance corresponding to a pressure peak exerted on a pressure sensor using technology. piezoresistive.

The patent application CN104535229, or even the utility model CN204286650, seek to meet this need by using an additional piezoelectric type pressure sensor. This additional sensor serves as a wake-up module for piezoresistive sensors, each forming a pressure sensor. The awakening of the piezoresistive sensors is implemented as soon as a value, from the piezoelectric sensor, which exceeds a predetermined threshold, is measured. This solution has the disadvantage of a high simultaneous consumption of piezoresistive sensors as soon as the threshold is reached. Furthermore, this solution also has the disadvantage that the threshold may not be reached in the event of aging of the piezoelectric sensor, or in the event of application of too weak a stress. Indeed, depending on the aging of the piezoelectric sensor, the voltage that it will be able to generate will change, and it may be that it finds itself in a situation where it will no longer be able to generate a sufficient voltage for the threshold to be reached.

OBJECT OF THE INVENTION The invention aims to remedy all or part of the aforementioned drawbacks.

To this end, the invention relates to a device comprising at least one strain sensor comprising a piezoresistive element, the device is characterized in that:

• the stress sensor comprises a piezoelectric element capable of generating an electrical signal when a stress is applied to the stress sensor, • said device comprises a detector configured to detect a peak of the stress applied to the stress sensor by processing the electric signal , • the device is configured to trigger, when the peak of the applied stress is detected by the detector, a measurement of an electrical quantity representative of an electrical resistance of the piezoresistive element of the stress sensor, and • the device is configured to determine a value of the stress applied to the stress sensor using the measured electrical quantity.

The device according to the invention may include one or more of the following characteristics.

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 feature 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 differentiator configured to determine the derivative of the electrical signal, • a comparator configured to provide an output of a logic signal alternating between two levels, the passage to one of the levels indicating that the peak of the applied stress is detected, said comparator 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.

According to a characteristic of the device, the two levels of the logic signal correspond respectively to a first level and to a second level, and the comparator is configured to:

• 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 differentiator for a local maximum of the electrical signal, • to pass the logic signal from the second level to the first level during a zero crossing of the derivative of the electric signal determined by the differentiator for a local minimum of the electric signal, the local extremum of the electric signal being the local maximum of the electric signal, the stress peak is considered to be detected during the passage from the first level to the second level, or the local extremum of the electrical signal being the local minimum of the electrical signal, the stress peak is considered to be detected when passing from the second level to the first level.

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

According to a characteristic of the device, the piezoelectric element and the piezoresistive element of the stress sensor are arranged so that:

• the piezoelectric element at least partially surrounds the piezoresistive element, or • the piezoresistive element at least partially surrounds the piezoelectric element.

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

According to a characteristic of the device, the device adopts at least partially the shape of a shoe sole incorporating the stress sensor. The subject of the invention is also a method for determining stress from a device comprising at least one stress sensor comprising a piezoresistive element. This stress determination method also aims to remedy all or part of the aforementioned drawbacks. The stress determination method comprises a step of applying a stress to the stress sensor, and said stress sensor comprising a piezoelectric element, said stress determination method comprises:

• a step of processing an electrical signal generated by the piezoelectric element in response to the stress applied to the stress sensor to detect a peak of the stress applied to the stress sensor, • a step of measuring a representative electrical quantity of an electrical resistance of the piezoresistive element of the stress sensor, the step of measuring the electrical quantity of the electrical resistance of the piezoresistive element being triggered when the peak of the applied stress is detected, • a step of determining a value of the stress applied to said stress sensor using the electrical quantity measured.

The stress determination method according to the invention may include 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 the electrical 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 follows that said local extremum of the electrical signal is detected.

According to a characteristic of the stress determination method, the step of detecting the passage through zero of the derivative of the electrical signal is implemented by a comparator of the device providing at output a logic signal alternating 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 logic signal assimilated to a detected state of the local extremum of the electric 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 the detection of the peak of the applied stress causes the increase in the acquisition frequency to trigger the step of measuring the electrical quantity 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, of treatment, measurement, and determination of a value of the applied stress are implemented for each stress sensor.

SUMMARY DESCRIPTION OF THE DRAWINGS Other advantages and characteristics will emerge clearly from the description which follows of particular embodiments of the invention given by way of nonlimiting example and represented in the appended drawings in which:

- Figure 1 represents two 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 voltage generated during the time by a piezoelectric element,

FIG. 2 illustrates a particular embodiment of a strain sensor device according to the invention,

- Figure 3 superimposes the evolution of electrical signals given in volts as a function of time,

- Figure 4 is an example of implementation of a detector of the device,

- Figure 5 illustrates a top view of a first embodiment of a strain sensor of the device,

FIG. 6 is a sectional view along A-A of FIG. 5, said section being perpendicular to the plane of FIG. 5,

- Figure 7 illustrates a top view of a second embodiment of the device's stress sensor,

FIG. 8 is a sectional view along B-B of FIG. 7, said section being perpendicular to the plane of FIG. 7,

- Figure 9 illustrates a top view of a third embodiment of the device's stress sensor,

FIG. 10 is a sectional view along C-C of FIG. 9, said section being perpendicular to the plane of FIG. 9, and FIG. 11 a sectional view along C-C of FIG. 9 according to an alternative to FIG. 10,

- Figure 12 illustrates an example of interdigitated electrodes for piezoresistive element,

FIG. 13 illustrates a particular implementation of the device according to the invention used in the field of footwear,

- Figure 14 is an example of implementation of a stress determination method from the device comprising the stress sensor.

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

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

Description of particular embodiments In the present description, "based on" corresponds to "comprising mainly".

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 when successive stresses are applied to the piezoresistive element each having a maximum value at the instants indicated by lines 11 and I2, or of the same constraint, the intensity of which varies so as to locally present two maximum values at the times indicated by 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 constraints such as those described in combination with the curve C1. For each stress peak, the electrical signal has a local extremum. The local extremum can be a local maximum or a local minimum depending on the connection of the piezoelectric element. In FIG. 1, the local extremes are maximum positive voltage values referred to by the references P1 and P2.

By “local extremum of an electric signal”, it is meant that for a part of the electric signal generated by the piezoelectric element there is a value of this part of the signal which can be, if necessary, maximum or minimum, and in this value, the derivative of the part of the electrical signal is zero (that is to say equal to 0). By derivative, we speak here of the first temporal derivative of the electrical signal. The device and method for determining stress described below propose to use a stress 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) as a function of the applied stress without being supplied: the consumption for generating the electrical signal is therefore zero. Thus, the use of the piezoelectric element and the use of the voltage which it generates when it is under stress to detect a stress peak consumes much less energy than the continuous measurement of the piezoresistive element at frequency high, especially 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 discharges when it is read by low-consumption electronics (that is to say a consumption strictly less than 10 pW ) because the input impedance of this electronics is not infinite. Thus, for the same stress value, the voltage across the terminals of the piezoelectric element can be different depending on how it was read. Therefore, the use of the piezoresistive element is preferred to that of the piezoelectric element to determine a stress value precisely. It is in this sense that the present invention proposes to use the piezoelectric element of the strain sensor to detect the peak of the stress applied to the strain sensor in order to trigger a measurement of an electrical quantity representative of an electrical resistance. of the piezoresistive element at a convenient time corresponding to the peak of the stress applied to the stress sensor. In comparison with patent application CN104535229, the energy consumption linked to the operation of the piezoresistive element can be limited to the closest to the peak of the stress that one seeks to visualize. Furthermore, the stress peak will always cause the presence of a local extremum of the electrical signal whatever the aging state of the piezoelectric element.

In the present description, a peak of a stress applied to the stress sensor can correspond to a local maximum of the stress. In fact, there are two cases. In a first case, the stress is linked to a brief event where it is applied to the stress sensor, then fully released: it then presents a single maximum stress. In a second case, the intensity of the stress applied to the stress sensor can vary, and it will then present local maximums which 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 stress 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 extremes each corresponding to a peak of the stress, that is to say to a maximum local value of the stress. In other words, a stress applied within the meaning 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 can be brief, that is, 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, a particular embodiment is now described in which the device 1, also called a device for determining, or measuring, stress (in particular at least one stress peak), includes at least one strain sensor 2. This device 1 can include one or more strain 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 suitable generating an electrical signal when a stress is applied to the strain sensor 2. Subsequently, the reference to the electrical signal corresponds to that generated by the piezoelectric element 4, also called the electrical signal from the piezoelectric element 4. In particular, the electrical signal is an electrical voltage signal 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 subjected to this constrain you. Furthermore, 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 of the piezoelectric element 4. In the following description by "peak of the applied stress" , the peak of the stress applied to the strain sensor 2 is understood. By “processing of the electrical signal”, is meant any operation applied to the electrical signal in order to detect the peak of the stress applied. If necessary, for the same stress, 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 particular each, 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 stress sensor 2. Thus, the detection of the peak of the applied stress triggers the measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3. The device 1 therefore gains in precision since it knows when to measure the electrical quantity representative of the electrical resistance of the piezoresistive element 3 which is then also representative of the peak of the applied stress. The measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3 can be carried out by electrically supplying the piezoresistive element 3, and by measuring the voltage at its terminals using a resistive bridge for example, or by measuring the current flowing in a branch including the piezoresistive element 3. This electrical quantity measured is said to be representative of the stress applied to the stress sensor 2, and more particularly to the value of the peak of the stress applied, since it can be used to determine a value of the stress, that is to say develop a value of the stress, when it reaches a peak. Typically, the electrical quantity measured 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 the present description results in the property of the piezoresistive element to oppose the passage of an electric current, although the unit of the electrical resistance is the ohm, the electrical quantity measured can be a voltage, or a current, the value of which depends on the electrical resistance of the piezoresistive element, or even directly the electrical resistance. For example, the electrical quantity measured can be obtained from the measurement of the voltage across the terminals of the piezoresistive element 3 supplied through a resistive bridge, which then makes it possible to go back, if necessary, in a manner known per se by a person skilled in the art, at the value of the stress, the latter being a function of the resistance, itself a function of the voltage. Furthermore, the device 1 is configured to determine a value of the stress applied to the stress sensor 2 using, that is to say from, the electrical quantity measured. 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 electrical consumption of the device 1.

As illustrated in Figure 2, the device 1 may include an electronic module 6 electrically connected to the strain sensor 2. The electronic module 6 may include the detector 5 and the means necessary to trigger and implement the measurement of the magnitude electrical representative of the electrical resistance of the piezoresistive element 3. The electronic module 6 may include an electrical power source 7, such as a battery, or be connected to an external power source for powering the components of the electronic module 6. The electronic module 6 can also include elements necessary to carry out measurements from the stress sensor 2, such as for example measurements of electrical quantities of the piezoresistive element 3 and measurements of electrical quantities of the element piezoelectric 4, these electrical quantities varying according to the stress applied to the co sensor stress 2. Thus, the device 1, and in particular the electronic module 6 of the device 1, can be configured to implement a step for processing the electrical signal, a step for measuring the electrical quantity, and a step for determining the value of the stress applied to said stress sensor using the measured electrical quantity as will be described below in combination with the stress determination method. For example, to measure the electrical quantity representative of the electrical resistance of the piezoresistive element 3, the piezoresistive element 3 is electrically powered (arrow F1 in Figure 2) using a resistive bridge (not shown) of the electronic module 6, and the measurement is carried out by reading (arrow F2 in FIG. 2) the midpoint of the resistive bridge, for example using an analog-digital converter (not shown), also known by the acronym "ADC For English "Analog to Digital Converter", of the electronic module 6. By "constraint" is meant for example a pressure, or a stretch, or a force. More generally, a stress can correspond to any external force exerted on the stress sensor 2 and which tends to deform it so that 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 can be a force sensor. According to a particular example, the stress applied to the stress sensor 2 can be a pressure applied to, 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 applied stress can be measured in kg / cm ² and the applied stress can be between Okg / cm ² and

5kg / cm ² , especially in the field of footwear 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 stress applied to the stress sensor 2. Detection of the extremum local 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 electrical signal corresponding to the peak of the stress applied to the strain 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 at the time where, 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 the electrical signal, that is to say a local extremum or several local extremes. 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 can apply to the detection by the detector 5 of several local extremums of the electrical signal generated by the application of the same stress, each local extremum of the electrical signal to be detected by the detector 5 corresponding to a corresponding peak of the stress applied to the stress sensor 2. Thus, the device 1, and more particularly the electronic module 6 of the device, can be configured to trigger, when the local extremum of the electrical 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 resistance of the piezoresistive element 3 of the strain sensor 2.

The local extremum of the electrical signal to be detected can 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 of stress which will be applied to the stress sensor 2, and the characteristics of the piezoelectric element 4, the person skilled in the art is able to know whether a local maximum, or a local minimum, of the electrical signal is to be detected by as the local extremum of the electrical signal.

We understand from what has been said previously that there is a need to allow precise detection of the local extremum of the electrical signal. The electrical signal has a derivative, in particular temporal, 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 extremum of the electrical signal by the detector 5 is carried out by the detection of a zero crossing of the derivative of the electrical signal. As illustrated in FIG. 2, to allow the implementation of this precise detection, the detector 5 preferably comprises a derivator 8 (in particular a voltage derivator 8 when the electrical signal is a voltage signal) configured to determine the derivative of the electrical signal. In fact, the derivator 8 can output the derivative in the form of a derivative signal of the electrical signal. The electrical signal derivative signal can be a voltage signal representing the evolution of the derivative of the electrical signal over time. Thus, the derivative of the electrical signal can be formed by voltage values. The diverter 8 can be connected to the piezoelectric element 4 which supplies it with the electrical signal to be derived. Furthermore, the detector 5 also includes a comparator 9 configured to output a logic signal alternating between two levels, for example between 0 or 1 (1 representing a level for example given by the supply voltage of the comparator 9, and 0 representing for example 0 volts). The comparator 9 may include an operational amplifier mounted as a comparator or a low-consumption comparator (for example less than 5 μΑ). Going to one of the levels indicates that the local extremum of the electrical signal, i.e. the peak of the applied 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, in particular during each, zero crossing of the derivative of the electrical signal determined by the differentiator 8. In other words, the comparator 9 is a comparator at zero. This solution based on the differentiator 8 and the comparator 9 is preferred because it helps to limit consumption to detect the peak of the applied stress. Conventionally, the comparator 9 may include means adapted to avoid false detections of the local extremum of the electrical signal, these means, known per se to those skilled in the art, may for example include logic components such as flip-flops associated with the generation of time constant or delay cells whose use can be combined with that of flip-flops.

In general, the electronic module 6 may include an electronic system 10, one function of which may be to trigger the measurement of the electrical quantity 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 electrical power source 7, can be a microcontroller having the advantage of limited electrical 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 interrupt input of the electronic system 10 so that the electronic system 10 generates an interrupt triggering the measurement of said quantity electrical representative of the electrical resistance of the piezoresistive element 3 when it detects the passage of the logic signal which it receives on its interrupt input at a level which it assimilates to a detected state of the local extremum of the electrical signal . The advantage of using a microcontroller as described is that the latter can occupy a waiting state in which its electrical consumption is limited. This waiting state can be interrupted during the generation of the interrupt or when readings / determinations of physical quantities are carried out at an acquisition frequency as will be described below.

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, in particular taking into account if the local extremum of the electrical signal to be detected is a local maximum of the signal generated electrical signal, or a local minimum of the generated electrical signal. The operation of comparator 9 is also to be taken into account to consider which 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 stress, 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 chosen from a low level and a high level, for example each associated with an electric voltage which is specific to it, for example 0V or a supply voltage of the comparator 9. Furthermore , comparator 9 is configured to:

• passing 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 differentiator 8 for a local maximum of the electrical signal, for example for a positive voltage value, • 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 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 stress peak is considered to be detected when passing from the first level to the second level. Conversely, if the local extremum of the electrical signal to be detected is the local minimum of the electrical signal, then the stress peak is considered to be detected when going from the second level to the first level. Such a method is advantageous in the sense that it can easily be implemented without requiring significant energy consumption. Alternatively, although this requires greater electrical consumption, the microcontroller mentioned 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. A concrete application of the preferred example is now described. In FIG. 3, the evolution of voltage signals S1, S1 ′, SL as a function of time respectively representing the electrical signal S1 is shown, when a time-varying stress is applied to the stress sensor. , the derivative S1 'of the electrical signal and the logic signal SL. The signal SL varies between 0 and a supply voltage 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 to zero, for example to a zero electrical potential, in particular the ground. Here, the comparator 9 takes as input voltage values each corresponding to a value of the derivative S1 ’of the electrical signal. Furthermore, the comparator 9 is configured so that if the voltage corresponding to the derivative of the electrical signal at its input is a non-zero and positive voltage, then the level of the logic signal is high, and so that if the corresponding voltage at the derivative of 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 S1 ’of the electrical signal becomes zero, this is then accompanied by a change of sign of the derivative S1’ 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 SI '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 generation at the output of comparator 9 of a falling edge to pass from the high level to the low level of the signal logic 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 S1 'of the electrical signal will be negative before reaching the local extremum of the electrical signal to be detected, then positive after reaching the local extremum of the electrical signal to be detected: it follows that the local extremum of the electrical signal can be detected following the generation at the output of comparator 9 of a rising edge to pass from the low level of the logic signal SL at the high level of the logic signal.

More particularly, the output of comparator 9 supplying the logic signal can 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 quantity representative of the resistance electrical of the piezoresistive element 3 when said electronic system 10 detects the passage of the logic signal which it receives on its interrupt input at a level which 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: everything will depend on how the electronic system 10 interprets the logic signal occurring at its interrupt input as a function of the interpretation of the logic signal at the output of comparator 9.

As illustrated in Figure 4, the differentiator 8 can be an RC circuit in serial configuration. In the field, an RC circuit is a circuit which includes a resistive component 8a and a capacitor 8b. FIG. 4 illustrates a particular example of implementation of the derivator 8 comprising an input En8 connected to the piezoelectric element and an output S8 connected to the comparator 9. The capacitor 8b of the derivator 8 is connected to the output S8 and to the input En8 of the differentiator 8. Furthermore, the resistive component 8a of the differentiator 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 differentiator 8. The advantage of such a diverter is that it consumes little energy and takes up very little space on the electronic card (two components). The capacitance of the capacitor 8b is between 1pF and 100pF, and the electrical resistance of the resistive component 8a is between 1ΜΩ and 100ΜΩ, so as not to influence the output voltage of the piezoelectric element whose capacitance will typically be between 100pF and 10nF.

As illustrated in Figure 4, the comparator 9 may include a first entry marked "+" and a second entry marked "-". The first input "+" is connected to the output S8 of the differentiator 8 and the second input "-" is connected to ground so as to allow comparison to zero. The output S9 of the comparator 9 makes it possible to supply the logic signal mentioned previously by comparing 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 applied stress will depend in particular on the connection of the comparator 9 inputs to derivative 8 and ground.

As illustrated in Figures 5 to 11, the device may include a substrate 11 on which is arranged the stress sensor 2. The substrate 11 can be thin and flexible to allow its deformation when the stress is applied to the stress sensor 2 The substrate 11 can also, on the other hand, 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 in a shoe. The substrate 11 can be made of plastic of PEN (for poly (ethylene naphthalate)), PET (for poly (ethylene terephthalate)), PC (for Polycarbonate), Polymide (for example Kapton®) or other types. Typically, the substrate 11 has a thickness which can be between 100 μm and 200 μm. 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 thin and flexible substrate 11. Alternatively, the substrate 11 can also be paper, textile, metal, or another type suitable for 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. As shown in Figures 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 layer of 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 can have each a thickness between 0.5nm and 500nm for electrodes produced by chemical vapor deposition and between 0.5pm and 5pm for electrodes produced by printing. The layer of piezoelectric material 43 can have a thickness of between 0.5 μm and 50 μm depending on the desired strength 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 thickness, the greater the voltage of the electrical signal generated by the piezoelectric element 4. The electrodes 41, 42 of the piezoelectric element 4 can 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 comprising 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 process such as, for example, a CVD process (for English “Chemical Vapor Deposition” or chemical vapor deposition in French). ) or a PVD process (for English “Physical vapor deposition” or physical vapor deposition in French). The two electrodes 41, 42 of the piezoelectric element 4 can be made of identical or different materials. PEDOT-PSS also noted PEDOT: PSS is an acronym for a mixture of two polymers, poly (3,4ethylenedioxythiophene) (PEDOT) and sodium poly (styrene sulfonate) (PSS). PEDOT is a conjugated polymer derived from polythiophene in which a fraction of the sulfur atoms is protonated with a positive charge, while PSS is a negatively charged sulfonated polystyrene in which a fraction of the sulfonate groups SC> 3 ^_ carries a sodium ion Na ⁺ . The layer of piezoelectric material 43 can comprise piezoelectric inorganic particles such as particles of lead titano-zirconate (also known under the name PZT), or barium titanate (such as for example BaTiOs), or also comprise an electro-active polymer such as poly (vinylidenefluoride-cotrifluoroethylene) (also known under the name P (VDF) -TrFE). The layer of piezoelectric material 43 can be produced by depositing an ink comprising the inorganic particles or the electro-active polymer.

As shown in Figures 5 to 11, the piezoresistive element 3 may include two electrodes 31, 32 nested one inside the other in the form of a comb: these are interdigitated 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 can 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 can comprise fingers 31a and the electrode 32 can comprise fingers 32a, the fingers are interdigitated so that each finger 31a of one of the electrode 31 of the piezoresistive element 3 is separated from a finger 32a of the other electrode 32 of the piezoresistive element 3 from a distance between 100pm and 1000pm. The width of the interdigitated fingers parallel to the plane where they are interdigitated can be between 100pm and 1000pm. According to a nonlimiting example, the width of the fingers is 500 μm, and the spacing between two adjacent fingers is 500 μm. Each of the interdigitated electrodes may for example have eight fingers (FIG. 12). The dimensions of the interdigitated fingers, their spacing and their number are chosen according to the stress which one wishes to measure and the desired sensitivity of the stress sensor. These electrodes 31, 32 of the piezoresistive element 3 can comprise metal (for example silver in the form of particles, nanoparticles or nanowires), or an electronic conductive polymer such as for example PEDOT-PSS or polyaniline . The electrodes 31, 32 of the piezoresistive element 3 can be produced by printing from inks comprising the materials described above, or can be formed using a vacuum deposition process, for example by CVD or by PVD. A layer of piezoresistive material 33, in particular of thickness between 5 pm and 25 pm is held facing the electrodes by a bracing element 34 also called a spacer. More particularly, the layer of piezoresistive material 33 is formed on a support 35 fixed to the bracing element 34. The piezoresistive material is FSR (for English "Force Sensing Resistor"). The piezoresistive material can comprise at least one of the materials chosen from: carbon particles, carbon nanotubes, graphene and metal particles, mixed with an elastomeric binder. The piezoresistive material can alternatively be formed by an electrically conductive polymer (for example polyaniline, polypyrrole or polyacetylene). The spacer 34 may be an acrylic or silicone adhesive. The support 35 can be made of polypropylene (PP), polyethylene (PE), or polyurethane (PUR) and in particular adopt the form of a layer with a thickness of between 25 μm and 200 μm. The bracing element 34 is arranged so 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. Furthermore, when a stress to be determined is applied to the stress sensor 2, this can deform the layer of piezoresistive material 33 which comes into contact with the interdigitated electrodes 31, 32: it follows that it is possible to power the piezoresistive element 3 to measuring a voltage (allowing, if necessary, to go back to the electrical resistance of the piezoresistive element 3) representative of the deformation of the layer of piezoresistive material 33, this voltage can then be correlated with a stress value, such as a pressure. It is then understood that the spacer element 34 is dimensioned as a function of the constraints that it is desired to determine.

When the stress sensor is not stressed, the layers of piezoelectric material and of piezoresistive material are preferably parallel to each other. The device is in particular arranged so that the stress is applied parallel to the direction of the stack forming the piezoelectric sensor, this direction of stack being perpendicular to the plane of the substrate 11.

In order to improve the operation of the strain sensor 2, it is preferable that the piezoelectric element 4 and the piezoresistive element 3 are as close as possible to each other. This sought-after proximity allows the peak of the stress seen by the piezoelectric element 4 and by 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 strain 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 stress sensor 2 are juxtaposed, and in particular arranged side by side on the same substrate 11. The piezoresistive element 3 comprises the interdigitated electrodes 31, 32, in particular in contact with the substrate 11, and the layer of piezoresistive material formed on the support 35 separated from the substrate 11 by the bracing element connecting the substrate 11 to said support 35 The piezoelectric element 4 comprises the layer of piezoelectric material 43 located between its two electrodes 41, 42 one of which 41 is proximal to, and in particular in contact with, the substrate 11. This solution has the advantage of being easy to be produced, but has the drawback of a lateral offset between the piezoresistive 3 and piezoelectric elements 4. This offset limits the accuracy of the stress sensor 2 when the stress is applied orthogonally to the plane of Figure 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 take place at two close but distinct points.

We understand that there is a problem of improving the precision of the stress sensor 2. The second and third embodiments of the stress sensor 2 described below make it possible to respond to this problem of improving the precision.

According to the second embodiment of the strain sensor 2 illustrated in Figures 7 and 8, the piezoelectric element 4 and the piezoresistive element 3 of the strain sensor 2 are arranged so that the piezoelectric element 4 surrounds, in particular at less 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 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 arranged 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 relation to the first embodiment. This second embodiment 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 and piezoresistive 3.

According to the third embodiment illustrated in Figures 9 to 11, the strain sensor 2 is such that the piezoelectric element 4 and the piezoresistive element 3 are stacked, in particular this stacking is carried out on the substrate 11. This embodiment is preferred , in particular 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, one obtains a compact and precise stress sensor 2 since any force applied parallel to the stacking direction can be seen in a similar manner by the piezoelectric elements 4 and piezoresistive 3. In FIGS. 10 and 11, the piezoresistive element 3 is formed on the piezoelectric element 4. The piezoelectric element 4 is formed in turn the substrate 11 which carries it. In FIG. 11, one finds successively from the substrate 11, a stack comprising one of the electrodes 41 of the piezoelectric element 4, the layer of piezoelectric material 43, the other of the electrodes 42 of the piezoelectric element 4, a element made of dielectric material 12 (for example in the form of a layer formed on the second electrode 42), then the interdigitated electrodes 31, 32 of the piezoresistive element 3 formed on the element made of dielectric material 12. The element made of dielectric material 12 may have a thickness of between 1 μm and 10 μm, and be of poly (vinylidene fluoride) (PVDF). The layer of piezoresistive material 33 is arranged opposite the interdigitated electrodes 31, 32, and is kept away from said interdigitated electrodes 31, 32 via the bracing element 34 which can be fixed, on the one hand, to the element in dielectric material 12 or on the substrate 11 and, on the other hand on the support 35 on which the layer of piezoresistive material 33 is formed oriented towards the interdigitated electrodes 31, 32. According to an alternative illustrated in FIG. 10, l element of dielectric material 12 is not necessary, in this case the electrode 42 of the piezoelectric element 4 furthest from the substrate 11 can be formed by one of the interdigitated electrodes 32 of the piezoresistive element 3 then placed on the layer of piezoelectric material 43, this has the advantage of limiting the number of layers and the quantity of materials used: the strain sensor 2 pe ut then 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 together, this makes it possible to limit the number of electrical tracks to be produced to connect the piezoelectric elements 4 and piezoresistive 3 to the electronic module, and to limit the number of tracks to be formed on the same face of the substrate 11 in order to avoid overlapping tracks which would have to be electrically insulated. For the embodiments of the strain sensor 2 in 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 the figure 10, the piezoelectric element 4 and the piezoresistive element 4 comprise a common electrode, which facilitates 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 which seeks to obtain information on a stress applied to a stress sensor 2 without being limited to the application to the device 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 stress sensor 2 described can be wholly or partly obtained by printing suitable materials as mentioned above. Printing can be carried out by screen printing, by spraying also known under the English name "spray-coating", or by inkjet. The advantage of using 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 which 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 of manufacturing the stress sensor 2 as illustrated according to 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 of 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 below, these two formation steps can be completely independent, or can include steps common technology. The thicknesses described above are given orthogonally to the plane of the substrate for what is formed from, or above, the substrate, or where appropriate the support for the layer of piezoresistive material formed on the support.

The manufacturing process may include a step of supplying the substrate 11 as described above.

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

It follows from what is shown in Figures 6, 8, 10 and 11 that the step of forming the piezoelectric element 4 may include the following steps: a step of forming the first electrode 41, called the lower electrode , of the 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, called 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 can 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 polymer 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 produced by printing an ink, the printed ink is dried and then annealed. Annealing can take place in an oven at a temperature between room 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 the solvents it contains to evaporate at a gentle temperature, typically between room temperature and 60 ° C, the annealing of the ink allows it to evaporate the rest of the ink solvent but to give the ink after annealing some of the desired properties of the element to be formed from the ink.

The step of forming the layer of piezoelectric material 43 is in particular such that the layer of piezoelectric material 43 can be produced by printing (for example by screen printing), on the first electrode 41 of the piezoelectric element 4, d an ink comprising said piezoelectric material. For this, it is said that the piezoelectric material is dispersed and dissolved in the ink. Alternatively, the piezoelectric material chosen to form the layer of piezoelectric material 43 can be deposited under vacuum, for example by PVD or CVD. When the layer of piezoelectric material is produced by printing, the printed ink is dried and then annealed. Annealing can take place in an oven at a temperature between room temperature (25 ° C) and 200 ° C depending on the nature of the substrate 11 and the material used to form the layer of piezoelectric material 43. The annealing is in particular put in works at a temperature allowing the piezoelectric material to crystallize in a form giving it its properties. Drying can be carried out here in a vacuum 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 can be produced in the same way as the first electrode 41 of the piezoelectric element 4 with the difference that its printing or depositing it on the layer of piezoelectric material 43.

It follows from what is shown in Figures 6, 8, 10 and 11 that the step of forming the piezoresistive element 3 can include the following steps: a step of forming the first and second electrodes 31, 32 interdigitated the 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) according to a suitable pattern. to the desired shapes of the first and second interdigitated electrodes 31, 32. Alternatively, the first and second interdigitated electrodes 31, 32 can be obtained from a suitable material deposit, in particular by PVD or CVD technique. In particular, the first and second electrodes 31, 32 of the piezoresistive element 3 can 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, where 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 (Figure 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 associating the layer of piezoresistive material 33 with the first and second interdigitated electrodes 31, 32 may include a step of forming the bracing element 34 around the first and second interdigitated electrodes 31, 32. L the step of forming the bracing element 34 can be carried out by printing a suitable material such as a resin or an adhesive, or by laminating an adhesive film. The association step can then include a step of mounting the layer of piezoresistive material 33 to the bracing element 34 after its formation. The layer of piezoresistive material 33 can be formed on the support 35, for example by printing. This support 35 can be of the same type as the substrate 11 described above, and is bonded to the bracing element 34.

For example, to implement the first or the second embodiment of the stress sensor 2, the step of forming the first and second interdigitated electrodes 31, 32 of the piezoresistive element 3 can 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 carried out simultaneously with the step of forming the first electrode 41 of the piezoelectric element 4, or in the step of forming the second electrode 42 of the piezoelectric element 4. The advantage of the simultaneous formation 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 use.

For example, to implement the third embodiment of the strain sensor 2 (Figures 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 substrate 11. Furthermore, the first and second interdigitated electrodes 31, 32 of the piezoresistive element 3 can be formed on the element of dielectric material 12 serving as separation (FIG. 11) formed on the second electrode 42 of l piezoelectric element before implementing the step of forming the piezoresistive element on this element of dielectric material 12. Alternatively (FIG. 10), still according to the third embodiment, the step of forming the first and second electrodes interdigitated 31, 32 of the piezoresistive element 3 can be carried out simultaneously with the step of forming the second electro of 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 interdigitated electrodes 31, 32 of the piezoresistive element 3 form a single electrode common to the 'piezoelectric element 4 and the piezoresistive element 3, this allows to limit the amount of material used.

In particular, the method for manufacturing the stress sensor can be a step in a method for manufacturing the device as described.

The device 1 may include one or more stress sensors 2 as described. In FIG. 13, the device 1 adopts at least partially the shape of a shoe sole integrating the, or if necessary the, stress sensors 2. This has the advantage of being able to measure stresses, in particular their peaks, related to the walking of an individual when wearing a shoe fitted with the device.

In particular, as illustrated in FIG. 13, the device 1 comprises a plurality of strain sensors 2, everything that has been described in relation to a strain sensor 2 can be applied to each strain sensor 2 of the device 1. In the example of FIG. 13, eleven stress 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 stress sensor 2 (on the FIG. 13, the piezoelectric elements 4 and the piezoresistive elements 3 of the stress sensors 2 are, for example, shown stacked in the manner of FIG. 9). In FIG. 13, the device 1 comprises a plurality of detectors 5 shown diagrammatically online 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. Furthermore, for each stress sensor 2:

The detector 5 associated with said stress sensor 2 is configured to detect a peak in the stress applied to said stress sensor 2 by processing 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 quantity representative of the electrical resistance of the piezoresistive element 3 of said stress sensor, • the device 1 is configured to determine a value of the stress applied to said sensor of stress 2 using the electrical quantity measured.

Here, it is understood that the device has the advantage of allowing the detection of peaks of stresses applied in several different places of the device 1, these stresses then being preferably associated with the same event such as for example a support of a foot on the sensors. 2. For example, in an application of a sports sole (FIG. 13), the device 1 may include the substrate 11 adopting the shape of a shoe sole, said substrate 11 having a face where the sensors for constraint 2 of the plurality of sensors, this face being intended to be oriented towards the sole of the foot.

As illustrated in Figures 2 and 13, the device 1 may include a module 13 for determining the value of the stress (in particular the peak value of the stress applied), for example integrated into the electronic system 10, using the electrical quantity measured from the piezoresistive element 3 of the stress sensor 2. This can be implemented in a manner known per se by those skilled in the art, 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 for determining stress (in particular at least one stress value applied to a stress sensor 2 of the device 1) from the, that is to say by using the device 1 comprising said at least one stress sensor 2 comprising the piezoresistive element 3 and the piezoelectric element 4, in particular the device can be such as that described above. In particular, the advantages linked to this stress determination method derive from those described above in the context of the device 1. The stress determination method comprises (FIG. 14):

• a step E1 of applying the stress to the stress sensor 2, • a step of processing E2 of the electrical signal generated by the piezoelectric element 4 in response to the stress applied to the stress sensor 2 to detect the peak of the stress applied to the strain sensor 2, • a step E3 of measuring the electrical quantity representative of the electrical resistance of the piezoresistive element 3, the step of measuring E3 of the electrical magnitude representative of the electrical resistance of the piezoresistive element 3 being triggered when the peak of the applied stress is detected, • a step E4 of determining the value of the stress applied to said stress sensor 2 using the 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 stages 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, this making it possible, if necessary, to limit the overall consumption linked to the operation of the piezoresistive element 3 and / or to carry out the measurement of the electrical quantity representative of the electrical resistance of the piezoresistive element 3 at the moment when the stress applied to the stress sensor 2 presents a peak which one seeks to determine from said electrical quantity measured. 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 step of determining E4 of the value of the applied stress, in particular the value of the peak of the applied stress, to said stress sensor 2 from the electrical quantity measured is known per se to those skilled in the art: as soon as it has the measured electrical quantity, it can go back to the corresponding value of the stress, such as for example an applied pressure value.

In particular, the step E2 of processing the electrical signal includes a step of detecting E2-1 of the local extremum of the electrical signal corresponding to the peak of the applied stress. Said local extremum of the electric 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 stress sensor 2. Thus, it results from the detection of this local extremum of the electric signal , precise detection of the peak of the applied stress.

According to a particular implementation, the step of detecting E2-1 of the local extremum of the electrical signal comprises a step of determining E2-1-1 of the derivative of the electrical signal, and a step of detecting E2- 1-2 of a zero crossing of the derivative of the electrical signal, that is to say of the determined derivative, from which it follows that said local extremum of the electrical signal is detected. The zero crossing detection step E2-1-2 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 step E2-1-1 of determining the derivative can be implemented by the differentiator 8, and the step E2-1-2 detecting 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 comparator 9 of the device 1 providing outputs the logic signal alternating between two levels, the peak of the applied stress being detected when the logic signal at the output of comparator 9 passes to one of the levels of the logic signal assimilated 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 E2 includes 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 each representative of a corresponding electrical resistance of the piezoresistive element 3 from which it is possible to know values of possible stresses applied to the stress sensor 2 at different times. In this sense, the stress determination method may include a step of determining E5, according to an acquisition frequency, of a plurality of physical quantities each representative of an electrical resistance of the piezoresistive element 3. This frequency of acquisition used during the determination step E5 is preferably between 10 Hz and 20 Hz to limit the electrical consumption necessary to determine these physical quantities. The detection of the peak of the applied stress causes the increase in the acquisition frequency to trigger the measurement step E3 of the electrical quantity representative of the electrical resistance of the piezoresistive element 3. In other words, the detection of the peak of the applied stress temporarily interrupts the step E5 of determining the physical quantities the time to measure the electrical quantity representative of the electrical resistance of the piezoresistive element 3 then representative of the value of the peak of the applied stress. This increase is in particular such that the acquisition frequency during step E2 is then between 200Hz and 2kHz, but can be limited to one, five or even ten measurement points before returning between 10Hz and 20Hz to continue the implementation of the step E5 of determining the physical quantities. Obtaining several measurement points of electrical quantities each representative of a corresponding electrical resistance of the piezoresistive element 3 at different times making it possible to characterize in more detail the peak of the applied stress, for example to know if it s '' acts of a short or short press on the stress sensor 2. In other words, obtaining several measurement points at the time when the stress peak is present makes it possible to obtain additional information as to the peak of the applied stress. 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 constraints outside of one or more peaks of constraint.

Of course, if the device 1 comprises a plurality of stress 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 steps d application of a constraint E1, of processing E2, of measurement E3, and of determination E4 of a value of the applied stress can be implemented for several, and in particular each, stress sensor 2. Of course, for each stress sensor, the applied stress 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 linked to the use of each stress sensor 2 is then limited independently to that of the other stress sensors 2. Consequently, the stress determination method may include a step of generating a map 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 located 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 when the stress peak is applied to the stress sensor 2 , can also be used to measure data related to the applied stress, for example shear force data, or to measure additional information regarding the peak of the applied stress. 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 of the stress applied to the stress sensor 2 is detected.

According to a variant (not shown), the stress sensor can also include several piezoresistive elements, and when the stress peak 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. This has the advantage of having local information to the stress sensor characterizing the stress.

The industrial applications of the present invention are numerous. The device, and therefore 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 electrical consumption allowing this maximum value to be obtained. One application described relates to that of sport in the sense that the device may include a sole allowing to know 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 pressure zones in the body of a diabetic patient. The invention can also be applied to touch screens, tires for vehicles, robotics, artificial limbs, gloves, safety shoes, or even aircraft wings for which the strain constraints are sought. suffered during the flight of the aircraft.

Claims (15)

1. 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 stress 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 strain sensor (2) by processing the electrical signal, • the device (1) is configured to trigger, when the peak of the stress 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 stress applied to the stress sensor (2) in using the electrical quantity measured. 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 derivator (8) configured to determine the derivative of the electrical signal, • a comparator (9) configured to supply as an output a logic signal alternating between two levels, the transition to one of the levels indicating that the peak of the applied stress 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 correspond respectively to a first level and to a second level, and in that the comparator (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 differentiator (8) for a local maximum of the electrical signal, • pass the logic signal from the second level to the first level during a zero crossing of the derivative of the electric signal determined by the differentiator (8) for a local minimum of the electric signal, and in that: • the local extremum of the electrical signal being the local maximum of the electrical signal, the stress peak is considered as detected when passing from the first level to the second level, or • the local extremum of the electrical signal being the local minimum of the electrical signal , the stress peak is considered to be detected when going 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) of the stress 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 stress 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 strain 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 partially the shape of a sole for a shoe incorporating the stress sensor (2). 10. Method for determining stress from a device (1) comprising at least one stress sensor (2) comprising a piezoresistive element (3), said method for determining stress comprising: • a step of applying (E1) a stress to the stress sensor (2), characterized in that said stress sensor (2) comprising 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 in the stress applied to the stress sensor (2) , • a measurement step (E3) of an electrical quantity representative of an electrical resistance of the piezoresistive element (3) of the stress sensor (2), the measurement step (E3) of the electrical quantity 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) a value of the stress applied to said stress sensor (2) using the electric quantity measured. 11. Stress determination method according to the preceding claim, characterized in that the processing step (E2) comprises a detection step (E2-1) of a local extremum of the electrical signal corresponding to the peak of the applied stress. 12. Stress determination method according to the preceding claim, characterized in that the detection step (E2-1) of the local extremum of the electrical signal comprises: • a step of determining (E2-1-1) the derivative of the electrical signal, and • a step of detecting (E2-1-2) a zero crossing of the derivative of the electrical signal from which it follows that said local extremum of the electrical signal is detected. 13. Stress determination method according to the preceding claim, characterized in that the step of detecting (E2-1-2) the passage through zero of the derivative of the electrical signal is implemented by a comparator (9) of the device (1) outputting a logic signal alternating between two levels, the peak of the applied stress being detected when the logic signal at the output of the comparator (9) passes to one of the levels of the logic signal assimilated to a detected state of l 'local extremum of the electrical signal. 14. Stress determination method according to any one of claims 10 to 13, characterized in that it comprises a step of determining (E5), according to an acquisition frequency, of a plurality of physical 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 in the acquisition frequency to trigger the measurement step (E3) of the electrical quantity representative of the electrical resistance of the piezoresistive element (3). 15. Stress determination method according to any one of claims 10 to 14, characterized in that it comprises a plurality of strain sensors (2) each comprising a piezoresistive element (3) and a piezoelectric element (4), and in that the steps of applying a stress (E1), processing (E2), measuring (E3), and determining (E4) a value of the applied stress are implemented for each sensor of constraint (2).