WO2018219683A1

WO2018219683A1 - Integral deformation sensor and method for measuring the deformation of a surface of a solid

Info

Publication number: WO2018219683A1
Application number: PCT/EP2018/063015
Authority: WO
Inventors: Samuel BEHAR; Jean-Jacques BOIS
Original assignee: Nanolike
Priority date: 2017-05-29
Filing date: 2018-05-17
Publication date: 2018-12-06
Also published as: FR3066814A1; FR3066814B1

Abstract

The invention relates to a deformation sensor comprising at least one sensing element (12) of which one electrical characteristic varies as a function of the deformation thereof, applied to one face, referred to as support face (14), of a film part (11). The attachment face (20) of the sensor is entirely made up of a free face of the film part. A coating (17) is rigidly and permanently attached to said support face, extending entirely opposite the attachment face, over a thickness greater than that of said film part.

Description

MONOBLOC DEFORMATION SENSOR AND METHOD FOR MEASURING THE DEFORMATION OF A SURFACE OF A SOLID

The invention relates to a deformation sensor capable of measuring the deformation of a surface of a solid, and to a method for measuring the deformation of a surface of a solid using such a deformation sensor.

The measurement of the surface deformation of solids is useful in many applications for the control of the mechanical behavior of solids for example for prevention, design, maintenance, metrology, automation ... The measurement of the surface deformation of a solid makes it possible not only to determine the deformation undergone on the surface by the solid, but also to determine the forces and / or stresses experienced by the solid under the effect of stresses of all types: traction and / or compression and / or bending, twisting and / or shearing.

To measure the surface deformation of a solid, at least one sensitive element is applied, or at least one deformation sensor incorporating at least one such sensitive element on the surface of the solid. Such a sensitive element, generally improperly qualified as a "strain gauge" (the term "strain gauge" being more appropriate but less common) has at least one electrical characteristic (generally resistance or capacitance) which varies according to a state of deformation of the surface of the solid. The traditional strain gauges known for more than fifty years are formed of flexible films carrying a conductive material track (metal alloy) arranged in a coil. Piezoelectric strain gauges based on semiconductor materials are also known which have a better sensitivity but a less good linearity and a greater sensitivity to temperature variations. Strain gauges based on nanoparticles, for example metal nanoparticles, in particular gold or carbon, assembled by deposition on a flexible film, which are of interest for a very high gauge factor, are also known. wide deformation range and low power consumption (see in particular "High- According to Cosmin Farcau et al., ACS Nano, 2011, 5 (9), pp. 7137-7143, DOI: 10.1021 / nn201833y; or "Influence of the Humidity on Nanoparticle-Based Resistive Strain Gauges" Lucas Digianantonio et al, J. Phys. Chem. C, 2016, 120 (10), pp. 5848-5854, DOI: 10.1021 / acs.jpcc.6b00822; US7116209).

The use of such strain gauges poses many practical difficulties.

Firstly, the connection between the flexible film of the sensitive element and the surface of the solid must be perfect, that is to say in theory have an infinite rigidity, so that any deformation of the surface of the solid can be reflected on the sensing element. Mechanical assembly connections (screws, bolts, rivets, welds ...) are very long and expensive and are now abandoned in most applications. In most cases when the surface of the solid object is suitable, the strain gauges are glued by rigid adhesives such as cyanoacrylate or epoxy glues on the surface of the solid. Nevertheless, this bonding operation is in practice extremely delicate, relatively long (typically of several tens of minutes) and requires an important know-how that few operators master completely.

It has been proposed in the past to integrate one or more sensitive elements in a housing itself rigidly fixed to the surface of the solid which facilitates their retention in place during the curing of the adhesive and subsequently. It has also been proposed to coat a strain gauge in a coating applied after sticking the dipstick on the surface of the solid to form a moisture barrier and thermal insulation. All these solutions still require a particularly precise bonding of the flexible film of the sensitive element on the surface of the solid and actually increase the complexity of laying the sensor.

It has also been proposed to associate one or more strain gauges with a support, this support then being fixed on the surface of a solid part to test. These solutions, however, are not entirely satisfactory either, since the problem of fixing this support to the surface of the solid part to be tested remains. In addition, such intermediate assemblies necessarily induce transmission losses between the solid surface and each sensitive element, to the detriment of the overall gauge factor of the sensor thus formed. Since the strain gauges are already insensitive (requiring in most cases Wheatstone bridge arrangements), the use of such intermediate supports is only possible in applications in which the deformations of the surface of the solid or the values of the stresses or efforts are very important. More generally, such mechanical assemblies of the strain gauge to a support before it is bonded to the surface of the solid part to be tested necessarily imply an interference of the mechanical properties (mass, inertia, rigidity, etc.) of this support with respect to the strain gauge and with respect to the deformations of the surface of the solid part, and therefore the malfunctions or reliability of the latter.

WO 2016/030752 discloses a stretchable sensor patch comprising a stretchable elastic film layer carrying at least one elastic capacitive strip or at least one resilient resistive filament and an integrated circuit, and a flexible stretchable flexible protective layer for protecting said at least one strip and the integrated circuit. The elastic stretch film can also carry a wireless transmission circuit and a source of electrical power. The thickness of this patch is less than a few millimeters, and may for example be between 40 μηι and 20 μηι. In addition to being limited to measuring tensile deformities on human or animal organs, such a patch is fragile and difficult to handle, store, transport, pack, and install on a surface.

The invention aims to overcome these disadvantages.

It aims in particular to provide a deformation sensor whose attachment to a surface of a solid whose deformation must be measured is simplified, fast, reliable, and which also has a sufficiently large gauge factor and provides reliable results and specific. It also aims to provide such a deformation sensor whose handling, storage, transport, packaging and installation on a solid surface are simple, do not require special precautions, can be performed by operators non-specialized, and do not in any way affect the performance of the sensor.

It also aims to provide such a deformation sensor which is protected vis-à-vis the external environment, especially with respect to moisture or chemical or mechanical aggression, and temperature variations.

The object of the invention is more particularly to provide such a deformation sensor that can be used in a very large number of applications, including those in which the deformations to be measured are of low amplitude, and which is compatible with a large number of materials. different constituents of the surface of the solid whose deformation must be measured.

Moreover, a strain gauge is connected to an electronic circuit which makes it possible to measure the variation of the electrical characteristic of the strain gauge as a function of the deformation, and to deliver a corresponding measurement signal. Such an electronic circuit generally incorporates components for calibrating and thermally compensating the measurement signal delivered by the sensor.

The invention aims more particularly at providing such a deformation sensor which can integrate such an electronic circuit and which is therefore entirely prefabricated in the form of a sensor in one single autonomous, precalibrated.

The invention aims in particular to provide such a deformation sensor capable of delivering a measurement signal in a format compatible with most systems and automation capable of exploiting such a measurement signal.

The invention thus aims to propose a deformation sensor whose implementation is particularly simple and only requires its attachment to the surface a solid whose deformation must be measured, and the connection of the sensor to a system for exploiting the measurement signal delivered by the sensor.

The invention also aims at providing such a sensor whose manufacture is particularly simple, reliable and inexpensive.

The invention also aims at providing a method for measuring the deformation of the surface of a solid having the same advantages.

The invention therefore relates to a deformation sensor having a free face, called a fixing face, adapted to be applied to a surface of a solid whose deformation must be measured, and comprising:

at least one sensitive element whose electrical characteristic varies according to its deformation arranged in such a way that said electrical characteristic of the sensitive element varies as a function of a deformation state of at least a portion of said fixing face,

a coating extending around each sensitive element,

characterized in that

said fixing face of the sensor is entirely constituted by a free face of a piece of film, this piece of film having a face, said support face, opposite to said fixing face, and a constant thickness between said fixing face; and said support face,

at least one sensitive element is applied to said support face,

said coating:

o is rigidly fixed irremovably to said support face of said piece of film,

o extends from said support surface of said piece of film, entirely opposite said fixing face,

o over a thickness greater than the thickness of said piece of film.

The combination of the characteristics of a sensor according to the invention makes it possible to provide numerous novel advantages and to envisage in consequently a considerable development of the use of such deformation sensors.

The fixing face of a sensor according to the invention being a free face of a piece of film has a smooth surface, so smooth, continuous and free of asperities, discontinuities, slots, recesses, lights or d interfaces between several materials. (Throughout the text, the term "united" applied to a surface designates the fact that this surface is free of asperities and is continuous, that is to say can be entirely traversed by moving a pencil held in contact of this surface without having to break this contact.)

In addition, the attachment face is formed by a single piece of film, which is a solid material homogeneous by nature (that is to say a piece of film free of gaps, lights, or heterogeneities). Consequently, the sensor according to the invention can be fixed to the surface of a solid exclusively via this fixing face, in particular by bonding with an adhesive compatible with said piece of film and with the solid surface.

In a first possible embodiment, said piece of film is formed of a single layer of homogeneous material of constant thickness and has no adhesive layer. In this first variant, the fixing face is not itself adhesive and the bonding of the fixing face is achieved by means of a glue attached to this attachment face or on the surface of the solid on which the The sensor must be attached, or an adhesive that is an integral part of this surface, or even using a double-sided adhesive or a layer of transfer adhesive applied by transfer on the attachment face.

For example, in certain advantageous embodiments of a sensor according to the invention, said fixing face is adapted to be fixed by gluing to a surface of an object with an adhesive selected from the group consisting of cyanoacrylate adhesives, glues epoxy, polyurethane glues, silicone glues, nitrile glues and neoprene glues. In a second possible embodiment, said piece of film has an adhesive bonding layer forming said fixing face. In this second variant, said piece of film may advantageously be formed of a layer of flexible homogeneous dielectric material adapted to receive and support each sensitive element, and of a layer of adhesive composition superimposed on this layer of dielectric material. Such a layer of adhesive composition may be chosen in particular from pressure-sensitive adhesives at ambient temperature, heat-activatable adhesives, ultraviolet-activatable adhesives and chemically activatable adhesives (in particular by the humidity of the ambient air and or by addition of an activating agent).

This piece of film forming a support on which is applied at least one element sensitive to deformation is necessarily already inherently compatible with a collage. In addition, the coating extending around each sensitive element opposite the fixing face makes it possible to protect each sensitive element from various external aggressions, and, above all, facilitates the handling, the transport, the storage, the packaging and installation of a sensor according to the invention, and this by non-specialized operators, that is to say other than the operators specializing in the bonding operations of traditional strain gages.

For the installation of a deformation sensor according to the invention, it suffices to glue the said fixing face to the surface of the solid in a single step and to connect the sensor according to the invention to a system intended to exploit the signal of measurement delivered by the sensor.

The coating extending opposite the fixing face, no portion of the latter is formed by the coating. A sensor according to the invention is therefore capable of being fixed on a surface of a solid exclusively by said fixing face, and thus by means of said piece of film which constitutes the sole element of the sensor according to the invention. in contact with this solid surface. Thus, the coating does not come into contact with the surface of the solid on which the sensor is attached, does not interfere with this surface of the solid, and therefore has only a minimal influence on the mechanical behavior of each sensitive element and the sensor from the point of view of the detection of deformations. This influence is all the more negligible that the coating itself is sufficiently flexible and that the gauge factor of each sensitive element is in itself sufficiently important, the decrease of the gauge factor by the presence of the remaining coating acceptable.

Thus, although the invention is applicable with all kinds of sensitive element (s) whose at least one electrical characteristic varies as a function of its deformation (including strain gauges with tracks of metallic material, semiconductor or carbonaceous, solid or nanostructured, and piezoelectric strain gauges), a sensor according to some embodiments of the invention is advantageously at least one sensitive element formed of a nanoparticle-based strain gauge applied to said support face . More particularly, said nanoparticles are chosen from the group of metal nanoparticles, semiconductor nanoparticles and their aggregates, assembled via an insulating material or a semiconductor material. More particularly, these are nanoparticles chosen from the group of gold nanoparticles and carbon nanoparticles. In a particularly advantageous embodiment, the sensor according to the invention comprises a resistive strain gauge formed of at least one wire of colloidal gold nanoparticles extending between two electrodes as described by Cosmin Farcau et al. Lucas Digianantonio et al publication cited above or US7116209.

Such strain gauges based on nanoparticles have a very high gauge factor, especially greater than 20. Surprisingly, it has been found that a sensor according to the invention can thus have a gauge factor greater than 15, despite the presence of the coating. Thus, each sensitive element, said coating and said piece of film are chosen so that the sensor has (after bonding to a solid surface via a rigid glue such as a cyanoacrylate glue) a higher gauge factor. at 5, in particular greater than 10, in particular greater than 15. According to a first variant, the nanoparticles are directly deposited on said support face of said piece of film, the latter thus constituting the substrate supporting the nanoparticles forming the strain gauge. The film constituting said piece of film forming said fixing face and said support face is a flexible film supporting at least one deformation-sensitive element applied directly on said support face of this flexible film during manufacture. Consequently, the mechanical properties of this flexible film are, by their nature, perfectly adapted to the appropriate operation of each sensitive element for the detection of the deformations of the surface of the solid on which the sensor according to the invention is then fixed.

According to another possible variant, a sensitive element comprises a flexible film pellet on which the nanoparticles are deposited, and the strain gauge thus formed is itself applied to said support face of said piece of film, possibly via a layer of glue. In this variant, the flexible film forming the substrate supporting the nanoparticles is itself applied to said support face of said piece of film. Said piece of film may then be formed of a film of the same nature as the flexible film constituting the substrate supporting the nanoparticles. These two films can even be identical. On the contrary, nothing prevents the film constituting said piece of film from being different in its nature and from the dimensions of the film forming the substrate of a strain gauge of a sensor according to the invention.

In all cases, and in particular in the case of a strain gauge based on nanoparticles as mentioned above, said film constituting said piece of film is a film of polymeric material for example selected from the group consisting of polyimide polyolefins (especially polyethylene terephthalate (PET), polypropylene (PP), polyethylene naphthalate (PEN), and cycloolefin copolymer (COQ), polymethylmethacrylate (PMMA), polycarbonate (PC), polyetheretherketone (PEEK), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), and halocarbon polymers (especially fluorocarbons such as polytetrafluoroethene (PTFE) and polyvinylidene fluoride (PVDF)).

Advantageously, the constituent material of said piece of film is also chosen so as to have a high modulus of elasticity - in particular greater than 1 GPa, for example of the order of 2.5 Gpa- so as to transmit the deformations between the surface of the solid on which the sensor is applied and each sensitive element.

Nevertheless, in particular for the measurement of flexural deformations, said piece of film must be flexible, that is to say have a low flexural stiffness value, so that it can easily be deformed in flexion according to the flexural deformations of the solid surface. To do this, said piece of film advantageously has a thickness of less than 1 mm, in particular between 5 μηι and 50 μηι, for example of the order of 25 μηι-.

In a sensor according to the invention, in particular for the measurement of bending deformations, said piece of film has a flexural stiffness lower than that of the coating. Indeed, the coating extending opposite said fixing face may have a flexural stiffness a little larger than that of said piece of film.

Furthermore, advantageously, said coating of a sensor according to the invention is a solid-state coating which may be formed into a peripheral shape and of suitable dimensions which may be subject to numerous variations depending on the applications and embodiments of the sensor.

Advantageously, in certain possible embodiments of a sensor according to the invention said coating has a free peripheral outer surface forming a free peripheral outer surface of the sensor according to the invention. Thus, the peripheral outer surface of a sensor according to the invention is entirely formed by said coating and by said piece of film. In these embodiments, a sensor according to the invention may be free of rigid peripheral housing, said coating conferring on the sensor its mechanical strength and its general shape, allowing it to be handled in one piece.

However, nothing prevents alternatively to provide a sensor according to the invention is provided with a reinforcement or a protective cover. But this reinforcement or protective cover is arranged not to disturb the operation of each sensitive element of the sensor according to the invention, and in particular is in any case remote from said fixing face so as not to come into contact with the surface of the solid on which the sensor according to the invention is applied.

Said coating of a sensor according to the invention is sufficiently flexible to preserve a sufficiently large gauge factor value to the sensor according to the invention. In particular, advantageously and according to the invention, said coating is formed of at least one layer of at least one material, each material having a modulus of elasticity of less than 10 MPa. Thus, each constituent material of said coating has a modulus of elasticity lower than the modulus of elasticity of the material constituting said piece of film-in particular less than one-hundredth of this modulus of elasticity-.

However, said coating is also adapted, that is to say sufficiently rigid, to allow the handling, transport, storage, packaging, and fixing of the sensor on a solid part to be tested. Thus, a sensor according to the invention is a sensor in one piece (monoblock) mainly constituted, seen from the outside, of said piece of film and said coating. To do this, the modulus of elasticity of said coating - in particular of each material constituting said coating - is preferably greater than 100 kPa. Advantageously, said coating may be formed by molding directly on said support face.

In addition, said coating advantageously has a thickness greater than 2 mm, in particular between 5 mm and 5 cm, for example of the order of 0.5 cm to 1 cm -1. Thus, the thickness of the coating is greater than 5 times - especially between 100 times and 10,000 times, in particular is of the order of 1000 times - the thickness of said piece of film. It can be formed of a single block of the same solid material, a mixture of solid materials or a composite material; or on the contrary in several successive layers of the same material or of different materials, superimposed on said support face and parallel to the latter to each other, rigidly and irremovably connected to each other.

At least one first layer of the coating extending in contact with said support face is formed of a dielectric material covering at least each sensitive element applied to said support face. Preferably, the coating is entirely formed of dielectric material (s), that is to say constitutes a dielectric coating.

In certain advantageous embodiments of a sensor according to the invention said coating is formed of at least one polymeric material selected from the group of thermoplastic polymeric materials, viscoelastic polymeric materials, thermosetting polymeric materials, elastomers, their blends. and composite materials. More particularly, said polymeric material may advantageously be chosen from the group of flexible polyurethanes, flexible polyepoxides, and polysiloxanes (silicones).

According to a first possible embodiment of a sensor according to the invention, said coating comprises a first coating layer made of a first dielectric material - in particular flexible polyurethane - applied above each sensitive element and said support face and a block of a second polymeric material - in particular a polyepoxide - applied over said first layer, said block having a thickness greater than said first layer. According to a second possible embodiment of a sensor according to the invention, said coating is formed of a single block of a flexible polymeric material, in particular a polyepoxide or a polyurethane. Other embodiments are possible.

The coating of a sensor according to the invention is rigidly fixed irremovably to the support face of the piece of film, that is to say is not suitable to be removed after fixing the sensor on a surface of a solid. The coating may be applied to cover said support face by at least one deposition of a curable composition, each deposit forming a layer of the coating. Such deposition can be achieved by simple application or by molding of said curable composition. Each curable composition is cured in situ after deposition, and thus secured rigidly and immovably to the underlying layer, in particular to said piece of film. The constituent material of each layer of the coating can thus be chosen to promote this rigid connection between the layers, and in particular between the coating and said piece of film.

However, nothing prevents alternatively to provide that at least one layer of the coating is fixed rigidly by gluing with a glue on an underlying layer. In this embodiment, the glue, which is an integral part of the coating, must also be chosen to have mechanical properties which are those required for the coating.

The choice of the number, the nature, the orientation and the arrangement of the sensitive element (s) of a sensor according to the invention may be the subject of any variant, by application of the methods well known in this respect in the field of strain gages, depending on the applications and the deformation measurements to be made on the surface of the solid (which may be deformations in flexion and / or in compression and / or in tension and / or in shear and / or in torsion). A sensor according to the invention can thus comprise:

a single sensitive element applied on said support face;

a plurality of distinct sensitive elements, at least one of which is applied to said support face, in particular a plurality of sensitive elements applied to said support face;

a plurality of distinct sensitive elements all applied to said support face;

at least one sensitive element embedded in said coating, at a distance from said support face, one or more sensitive elements applied to said support face, and one or more sensitive elements embedded in said coating, at a distance from said support face.

In a deformation sensor according to the invention, said coating extends around each sensitive element and from the support face of the film piece. Consequently, each sensing element extends parallel to said support face in an area less than the total area of this support face. In addition, said support face has an area greater than that of each sensitive element parallel to said support face, and greater than the total area of said at least one sensitive element parallel to said support face, that is to say say greater than the total area parallel to said support surface of the surface occupied by the different sensitive elements parallel to said support face when the sensor comprises a plurality of such sensitive elements.

Each sensitive element of a sensor according to the invention has electrical connection pads for electrically connecting this sensitive element to an electronic circuit adapted to measure said electrical characteristic of the sensitive element. In some advantageous embodiments, a sensor according to the invention is also characterized in that it comprises an electronic circuit:

- electrically connected to the pads of each sensitive element,

adapted to measure said electrical characteristic of each sensitive element, and to deliver a signal, said measurement signal, representative of said electrical characteristic of each sensitive element,

and in that said electronic circuit is at least partly included in said coating.

In certain possible embodiments of a sensor according to the invention, said electronic circuit is integrally included inside the coating, and comprises for this purpose all the components necessary for the measurement of the electrical characteristic of each sensing element, and to the wireless transmission - particularly radio frequency - of the measurement signal outside the coating on sensor according to the invention for an external system such as a computer system. In these particular embodiments, the electronic circuit incorporates in particular a source of electrical power which can be formed for example of at least one battery and / or a photovoltaic component. Alternatively, the electronic circuit may be electrically powered from outside the coating, for example by induction or photoelectric effect.

However, in certain preferred advantageous embodiments a sensor according to the invention is also characterized in that it comprises a connection port which extends outside said coating and in that said electronic circuit is adapted to deliver said measurement signal on this connection port.

This connection port may be carried by a plate of said electronic circuit which protrudes outwardly from said coating, for example protruding outwardly from a peripheral face of said coating, in particular a side face of said coating. extending from a peripheral edge of said piece of film forming said attachment face.

It should be noted that the only source of rigidity incorporated in a sensor according to the invention is possibly the electronic circuit which can be formed from at least one plate (or printed circuit board) more or less rigid. However, nothing prevents forming the electronic circuit in flexible form, from a flexible substrate plate, at least in its part included inside the coating.

Furthermore, the electronic circuit of a sensor according to the invention may comprise a wafer applied in contact with said support face and coated by the coating, and / or a wafer embedded within the thickness of the coating, without coming into contact with said support face. In this second variant, the wafer of the electronic circuit interferes less mechanically with the deformation of said piece of film, and therefore with the operation of each sensitive element. The electronic circuit can be electrically connected to the pads of each sensing element by connecting wires included in said coating (by wire wiring, called "wire"). bonding "in English), or by direct connection (in particular by a flip-chip type assembly).

Thus, in certain possible embodiments, a sensor according to the invention consists of said piece of film forming said fixing face and a polymeric coating extending from said support face opposite to said fixing face, and wherein at least one of a single sensing element is embedded on said support face and said electronic circuit, said connection port extending outside the coating. According to one variant, the sensor further comprises a wireless communication module extending outside said coating and connected to said connection port. Other variants are possible as indicated above or below.

The connection port of a sensor according to the invention may be adapted to make a wired connection of the sensor according to the invention with an external system such as a computer system. Alternatively or in combination, the connection port of a sensor according to the invention may be adapted to receive a connector of a wireless communication module extending outside the coating and adapted to allow the establishing a communication link with an external system. Thus, in some embodiments a sensor according to the invention comprises a wireless communication module extending outside the encapsulation and electrically connected to said connection port. Such a wireless communication module can be chosen for example from a radio frequency communication module, an infrared communication module, an optical communication module, a magnetic communication module, an induction communication module. Advantageously, it may be a radiofrequency module for transmitting the measurement signal according to any appropriate radio frequency protocol (Wi-Fi®, Bluetooth®, ZigBee®, SigFox®, LoRaWan®, mobile telephony protocol (GPRS, UMTS , LTE, WiMax ...) ...).

In certain advantageous embodiments, such a wireless communication module may be formed of a rigid plate adapted to be able to extend along a flat face of the coating opposite to the fixing face. In certain embodiments, such a wafer forming a communication module extends away from this flat face, this wafer having a lateral connector adapted to be connected to said connection port extending projecting from a lateral face of the coating. In other possible variants, such a wafer forming a communication module is instead embedded in a recess conjugate coating. A sensor according to the invention thus formed is particularly compact and easy to handle and install.

The electronic circuit of a sensor according to the invention may comprise one or more electrical circuits extending between each sensitive element and said connection port. In certain advantageous embodiments, the electronic circuit is adapted to deliver a digital measurement signal, in particular in a format compatible with said wireless communication module and / or with a digital network, for example a format compatible with the Internet network. Thus, the electronic circuit is adapted to firstly perform an analog measurement of the electrical characteristic of each sensitive element, and secondly to transform this analog measurement into a digital signal delivered on said connection port, which can be a connector compatible with a computing device and / or with a wireless communication module. In these embodiments, a sensor according to the invention can be qualified as digital deformation sensor.

In certain possible embodiments of a sensor according to the invention comprising an electronic circuit, said piece of film and the attachment face extend opposite each sensitive element and at least a part of the electronic circuit. The area of the attachment face is greater than the area occupied by each sensitive element on said support face parallel to said attachment face.

A sensor according to the invention can have all shapes and sizes, depending on the applications, and in particular according to the number of sensitive elements and their nature. The general shape of a sensor according to the invention is defined by that of the coating. In particular, the coating may be generally prismatic or in the general parallelepiped shape (with the exception of the attachment face whose shape is adapted to that of the solid surface on which the sensor must be fixed as indicated below ). Advantageously, the coating has a planar face opposite to the attachment face as indicated above.

In certain possible embodiments, a sensor according to the invention has a generally parallelepipedal shape with a width greater than 0.5 cm - in particular less than 10 cm, for example of the order of 1 cm to 2 cm -1 and a length greater than 2 cm - especially less than 50 cm, for example of the order of 2 cm to 5 cm -1. All other forms (cylinder portion, cap, prismatic, non-remarkable shape ...) and dimensions are possible.

The shape of the fixing face may be arbitrary, and is adapted to that of the surface of the solid on which the sensor must be fixed. This form of the fixing face depends on that given to said piece of film, the latter depending on the coating superimposed on this piece of film, and which therefore determines its general shape at rest. In particular, the coating may be formed by molding as indicated above, a piece of the mold being applied against the fixing face during curing of the coating to give it the desired shape. In particular, the fixing face may be flat. Alternatively, the attachment face may be a left surface, for example cylindrical or a cap or a polyhedral surface or the like. Thus, the fixing face can be chosen for example from the group consisting of planar faces, cylindrical faces-notably cylindrical of concave-revolution, cylindrical faces-in particular cylindrical of convex-revolution, concave spherical caps, convex spherical caps. concave dish caps, convex dish caps, concave polyhedral faces and convex polyhedral faces. The attachment face may also have any unspecified left shape, neither convex nor concave.

As indicated above, the invention applies to a sensor comprising, as sensitive element, any sensitive element of which at least one electrical characteristic varies depending on its deformation. More particularly, in some preferred embodiments, at least one-in particular each-sensitive element is formed of a strain gauge whose impedance varies as a function of its deformation. The electronic circuit is then adapted to electrically power each sensitive element so as to allow the measurement of its impedance. Advantageously and according to the invention, at least one-in particular each-sensitive element is formed of a resistive strain gauge and the electronic circuit is adapted to electrically power each resistive strain gauge.

The invention particularly relates to a deformation sensor -in particular a flexural deformation sensor- having a free face, called a fixing face, adapted to be applied to a surface of a solid whose deformation-in particular the bending deformation - must be measured, and including:

a coating extending around each sensitive element,

said fixing face of the sensor is entirely constituted by a solid free face of a piece of film, this piece of film having a face, said support face, opposite to said fixing face, and a constant thickness between said face of said film face; fixing and said support face,

at least one sensitive element, in particular at least one sensitive element based on nanoparticles, is applied to said support face,

said coating:

o is rigidly fixed irremovably to said support face of said piece of film,

o extends from said support surface of said piece of film, entirely opposite said fixing face, characterized in that

said piece of film has a thickness of less than 1 mm, in particular between 5 μηι and 50 μηι, for example of the order of 25 μηι-,

said coating extends from said support surface of said piece of film to a thickness greater than-in particular greater than 5 times, in particular between

100 times and 10,000 times, more particularly of the order of 1000 times the thickness of said piece of film; in particular, said thickness of said coating is greater than 2 mm, in particular between 5 mm and 5 cm, more particularly of the order of 0.5 cm to 1 cm,

the modulus of elasticity of said coating - in particular of each material constituting said coating - is greater than 100 kPa, and less than 10 MPa,

said piece of film is formed of a material having a modulus of elasticity greater than 1 GPa,

said piece of film has a flexural stiffness lower than that of the coating,

- it includes an electronic circuit:

at least partly included in said coating,

o electrically connected to each sensitive element,

o adapted to measure said electrical characteristic of each sensitive element, and to deliver a signal, said measurement signal, representative of said electrical characteristic of each sensitive element.

Thus, the invention makes it possible in particular to obtain, for the first time, a digital deformation sensor in one piece (monoblock), autonomous in its operation, precalibrated, ready to be fixed on a solid surface in one step bonding and to be directly connected to a computer system by a simple connection or establishment of a wireless communication link.

The invention also extends to a method of using a sensor according to the invention. It therefore also extends to a method for measuring the deformation of a surface of a solid in which: a deformation sensor according to the invention is chosen,

the fixing face of the sensor is fixed on the surface of the solid.

Advantageously and according to the invention, the sensor is fixed on the surface of the solid by gluing -particularly exclusively by gluing- the fixing face of the sensor on the surface of the solid. In addition, advantageously and according to the invention, the sensor is connected to a computer system so as to transmit to this computer system the measurement signal delivered by the sensor. In particular, the sensor is connected to a computer system via the sensor connection port, and this by a wired connection or via a wireless communication module connected to said connection port.

The invention also extends to a method of manufacturing a sensor according to the invention. It therefore relates to a method of manufacturing a deformation sensor in which:

at least one sensitive element is applied to a face, called the support face, of a piece of film, this piece of film having a face, called the attachment face, opposite to the support face and a constant thickness between the face fixing and the support side,

a coating is fixed rigidly and irremovably to said support face so as to extend around each sensitive element, from said support face completely opposite to said attachment face, to a thickness greater than the thickness of said piece of film.

The invention also relates to a deformation sensor, a deformation measuring method and a method of manufacturing such a sensor characterized, in combination or not, by all or some of the characteristics mentioned above or below. Whatever the formal presentation given, unless expressly stated otherwise, the various characteristics mentioned above or below must not be considered as closely or inextricably linked together, the invention may relate to only one of these structural or functional features, or only some of these structural or functional characteristics, or only part of one of these structural or functional characteristics, or any grouping, combination or juxtaposition of all or part of these structural or functional characteristics.

Other objects, features and advantages of the invention will appear on reading the following non-limiting description which refers to the appended figures in which:

FIG. 1 is a schematic perspective view of a sensor according to a first embodiment of the invention,

FIG. 2 is a schematic view from above of the sensor of FIG. 1,

FIG. 3 is a schematic side view of the sensor of FIG. 1 shown fixed on a surface of a solid and connected to a computer system,

FIGS. 4 and 5 are schematic views in perspective and, respectively, of a sensor according to a second embodiment of the invention,

FIG. 6 is a schematic perspective view of a sensor according to a third embodiment of the invention

FIG. 7 is a schematic side view of the sensor of FIG. 6 shown fixed on a surface of a solid and connected to a computer system,

FIGS. 8 and 9 are schematic side views of a sensor according to a fourth embodiment of the invention, respectively according to a fifth embodiment of the invention,

FIG. 10 is a diagram illustrating test results for determining the gage factor of a sensor according to the invention,

FIGS. 11 and 12 are diagrams illustrating measurement signals delivered by a strain gauge according to the state of the art and, respectively, by a sensor according to the invention integrating the same strain gauge,

FIG. 13 is a block diagram of an exemplary embodiment of an electronic circuit of a sensor according to the invention. In the figures, the scales are not necessarily respected, especially in thickness, and this for the purposes of illustration. The one-piece sensor according to the first embodiment of the invention shown in FIGS. 1 to 3 comprises a rectangular chip 11 of a flexible film of polyimide having a thickness of the order of 25 μηι on one side, called the support face 14, of which is applied a strain gauge 12 formed of son of colloidal gold nanoparticles extending between two electrodes 13, which constitute electrical connection pads of the strain gauge 12, as described by the publication Cosmin Farcau et al. by Lucas Digianantonio et al, cited above.

An electronic circuit in the form of a printed circuit board also extends in contact with the support face 14, adjacent the strain gauge 12. Connecting wires 16 electrically connect the electrodes 13 of the strain gauge 12 to the electronic circuit.

A dielectric polymeric flexible coating 17, for example transparent, covers the entire support face 14 and extends in thickness over the latter by covering the strain gauge 12 and the electronic circuit, with the exception of a port 18 connection of the latter. The connecting wires 16 are embedded in the coating 17. The connection port 18 projects outwardly from a side face 19 of the coating 17. The coating 17 has a generally parallelepiped shape. In the figures, it is presumed to be transparent for illustrative purposes.

The pellet 11 has a rectangular face, called the fixing face 20, opposite the support face 14 and the coating 17. This fixing face 20 is a free face which allows the bonding of the sensor thus formed on the surface 21 d a solid whose deformations or stresses must be measured, as represented in FIG. 3, by means of a layer 24 of glue which is for example a layer of cyanoacrylate glue or a layer of epoxy glue.

The sensor according to the invention thus has a free peripheral outer surface 31 which is formed by said fixing face 20 by the edges peripherals of the pellet 11 and by the coating 17, except for the connection port 18 which protrudes outside the coating 17. This being so, this sensor is free of any rigid peripheral housing. Its mechanical properties making it possible on the one hand easy handling and, on the other hand, the proper operation of the strain gauge 12, are conferred exclusively by the flexible film pellet 11, by the electronic circuit and by the coating 17. .

The port 18 of connection is connected by wires 22 (preferably in the form of a network cable such as a USB cable or an Ethernet cable) to a computer system 23 which allows on the one hand the power supply of the circuit 15 electronics, on the other hand the transmission of the measurement signals delivered by the electronic circuit to the computer system 23.

The strain gauge 12 is a resistive strain gauge, that is to say that its resistance varies as a function of the deformation of the fastening face, which itself depends on the deformation of the surface 21 of the solid on which sensor is glued.

The second embodiment shown in FIGS. 4 and 5 differs from the first embodiment in that the sensor comprises a pressure sensitive adhesive layer at ambient temperature integral with the wafer 11, the fixing face 20 of the the pellet 11 being formed by the free face of this adhesive layer 25 which completely covers the face of the flexible film opposite to the support face 14. This layer 25 of adhesive is protected before use of the sensor by a removable sheet 26 formed for example of a silicone paper. To fix such a sensor on the surface of a solid, it is sufficient to remove the removable sheet 26 and apply the sensor with the adhesive layer 25 in contact with the surface 21 of the solid to achieve the bonding. This adhesive layer may be applied to the flexible film by printing or transfer, or be formed of a double-sided adhesive previously bonded to the flexible film.

The third embodiment shown in FIGS. 6 and 7 differs from the first embodiment on the one hand in that the coating 17 is formed of several layers, namely a first layer 28 placed overlapping the support face 14 and the strain gauge 12, of relatively small thickness, for example of the order of 1 mm to 2 mm, and a second layer 29 overlying the first layer 28. For example, the first layer 28 is formed of polyurethane, while the second layer 29 is formed of polyepoxide and has a greater thickness, for example of the order of 0.5 cm to 1.5 cm. In addition, the electronic circuit is not in contact with the wafer 11 of flexible film, but is embedded within the thickness of the second layer 29 of the coating 17, at a distance from the support face 14.

In addition, as shown in FIG. 7, the sensor may be connected in communication with a computer system 23 by a wireless communication module 27 connected to the connection port 18. This wireless communication module 27 allows remote transmission by radio frequency of the measurement signals to the computer system 23. Advantageously, the wireless communication module 27 may be a low-energy and / or long-distance transmission module. Any radio frequency transmission protocol can be envisaged: Wi-Fi®, Bluetooth®, ZigBee®, SigFox®, LoRaWan®, mobile phone protocol (GPRS, UMTS, LTE, WiMax ...) ... Nothing also prevents to consider an infrared communication module 27 or optically. The wireless communication module 27 is for example in the form of a rectangular plate and advantageously extends above the coating 17, that is to say parallel and along a free face 30 of this coating 17 opposite the fixing face 20. Preferably, this wireless communication module 27 is carried by the connection port 18 and extends away from the free face 30 of the coating 17, so as not to interfere with the deformation of the sensor. However, nothing prevents the wireless communication module 27 from being placed in another arrangement with respect to the coating 17.

In the embodiments described above, the attachment face 20 is a rectangular planar face. Nevertheless, any other form can be envisaged, and for example a concave cylindrical shape as shown in Figure 8, or a convex cylindrical shape as shown in Figure 9 or other.

The electronic circuit is formed of any suitable circuit for electrically supplying the strain gauge 12, and measuring its resistance. Such a circuit is well known in itself. It therefore comprises a voltage source supply circuit delivering a DC voltage VCC.

The strain gauge 12 is shown in FIG. 13 as a variable resistor connected in series between the ground and a series resistor forming a divider bridge fed by the voltage VCC. The output of the strain gauge 12 is connected to the non-inverting input of an operational amplifier 37 mounted with a resistor 36 for feedback, and two resistors 38, 39 parallel input.

The analog signal delivered by the operational amplifier 37 is converted into a digital signal by an analog / digital converter 38 powered by the voltage VCC. Preferably, the converter 38 provides a measurement signal in a format compatible with digital networks, in particular in a format compatible with the Internet. The measurement signal delivered on the connection port 18 by the converter 38 is therefore directly usable by any computer system 23. Such an electronic circuit can be miniaturized and extend over a printed circuit board of small dimensions, for example having a width of the order of 1 cm for a length of the order of 2 cm.

The following method can for example be used to manufacture a sensor according to the invention. In a first step, the strain gauge 12 is produced by deposition on the wafer 11 of flexible polyimide film. In a second step, the electronic circuit is placed on the support face 14 and the electrical connection is made between the strain gauge 12 and the electronic circuit with the connecting wires 16. In a third step, the pellet 11 of flexible film is placed on a mandrel having a shape corresponding to that of the solid surface 21 on which the sensor must be fixed, without sticking the pellet 11 on the mandrel, and so as to deform the flexible film according to the shape of the mandrel. In a fourth step, a mold is placed around the pellet 11 of flexible film, the connection port 18 passing through a recess of a side wall of the mold. In a fifth step, a moldable liquid composition is deposited in this mold above the support face 14 to form the coating 17 by embedding the strain gauge, the connecting wires 16 and the electronic circuit. This composition is then cured to obtain the coating 17. The deposition and hardening steps can be repeated if the coating 17 is formed of a plurality of successive layers.

In order to manufacture a sensor according to the third embodiment, it suffices to keep the electronic circuit in place at a distance from the pellet 1 during the hardening of the second layer 29 of the coating 17, for example by means of spacing studs formed of a material similar to, or compatible with, that of the coating.

Example 1

A sensor according to the third embodiment of the invention has been realized with the following characteristics:

- overall dimensions: 33 mm x 20 mm x 6 mm,

strain gauge of 5 mm x 3 mm based on gold nanoparticles deposited on a piece of polyimide film 33 mm x 20 mm x 6 mm, having a modulus of elasticity of 2, 5 Gpa,

the coating comprises a first layer of polyurethane resin 140 covering the strain gauge and the piece of polyimide film,

the electronic circuit is formed of a card of 28 mm x 14 mm x 2 mm placed next to the strain gauge and connected to the latter by connecting threads, but elevated with respect to the polyurethane layer by means of pads of PDMS (polydimethylsiloxane),

a parallelepipedal PDMS block having a modulus of elasticity of between 360 kPa and 870 kPa is molded above the polyurethane layer by drowning the electronic circuit, with the exception of the connection port which protrudes from a side face of this block.

FIG. 10 represents the variations of the relative electrical resistance AR / R ₀ of the strain gauge 12 as a function of the deformation ε. As can be seen, a gage factor of 14.34 is obtained.

Example 2

The sensor of Example 1 was tested on a MTS Criterion® traction bench marketed by MTS Systems Corporation, Eden Prairie, USA. A three-point bending test with a span of 90 mm was carried out using a 140 mm × 30 mm × 5 mm aluminum test tube carrying the sensor according to the invention and, for comparison, a dipstick. constraints identical to that of the sensor according to the invention but "bare", that is to say without any coating or coating, connected to an ohmmeter. The signals delivered are transmitted to an electronic interface card with a computer.

A prestress of 10 N was applied and five cycles were carried out successively, with a deformation of 0.1% of Γ test piece.

The diagram of FIG. 11 represents the signals obtained with the bare strain gauge of the state of the art. The diagram of FIG. 12 represents the signals obtained with the sensor according to the invention. As can be seen, the sensor according to the invention provides a measurement signal that is almost identical in its shape and amplitude to that of the strain gauge according to the state of the art.

Example 3

The adhesive strength of the sensor of Example 1 was evaluated by subjecting the specimen on which the sensor is bonded to a deformation ramp of 0 to 2%. No detachment of the sensor was found.

By exerting a manual bending on the coating 17 in order to seek to take off the sensor, it is found that the sensor finally detaches from the test piece, the coating 17 nevertheless remaining integral with the pellet 11. The invention can be the subject of many alternative embodiments and applications other than those described above. In particular, it goes without saying that unless otherwise indicated the various structural and functional characteristics of each of the embodiments described above should not be considered as combined and / or closely and / or inextricably linked to each other, but to contrary as mere juxtapositions. In addition, the structural and / or functional characteristics of the various embodiments described above may be wholly or partly the subject of any different juxtaposition or any different combination.

In particular, the strain gauge may be formed of nanoparticles deposited not directly on the support surface 14, but on the contrary on a substrate pellet of smaller dimensions than the rectangular pellet 11, this strain gauge thus formed being applied, in particular by gluing, on the support face 14. Nothing prevents to provide a cover placed above the coating, for example to protect the wireless communication module. Nothing also prevents embedding the wireless communication module and / or all or part of the electronic circuit in a recess formed from outside the coating.

Claims

1 / - deformation sensor having a free face, said face (20) of attachment, adapted to be applied on a surface of a solid whose deformation must be measured, and comprising:

at least one sensitive element whose electrical characteristic varies as a function of its deformation, arranged in such a way that said electrical characteristic of the sensitive element varies as a function of a state of deformation of at least a portion of said fixing face,

a coating (17) extending around each sensitive element (12), said sensor fixing face (20) consisting entirely of a free face of a piece (11) of film, this piece of film exhibiting a face, said face (14) of support, opposite to said fixing face and a constant thickness between said face (20) of attachment and said face (14) of support,

at least one sensitive element (12) applied to said support face (14), said coating (17):

o being rigidly fixed irremovably to said support face (14) of said piece (11) of film,

o extending from said support face (14) of said piece (11) of film, completely opposite said fixing face (20),

characterized in that said coating (17) extends from said support surface (14) of said film piece (11) to a thickness greater than the thickness of said film piece (11).

21 - Sensor according to claim 1 characterized in that said coating (17) is formed of at least one layer of at least one material, each material having a modulus of elasticity less than 10 MPa.

3 / - Sensor according to any one of claims 1 or 2 characterized in that said coating (17) has a free peripheral outer surface forming a free outer peripheral surface (31) of the sensor. 4 / - Sensor according to one of claims 1 to 3 characterized in that at least one element (12) sensitive is formed of a strain gauge based on nanoparticles applied to said face (14) of support.

5 / - Sensor according to one of claims 1 to 4 characterized in that said piece (11) of film has a bending stiffness lower than that of the coating.

6 / - Sensor according to one of claims 1 to 5 characterized in that each element (12) sensitive having pads (13) for electrical connection, it comprises an electronic circuit (15):

- electrically connected to the pads (13) of each sensitive element (12),

adapted to measure said electrical characteristic of each sensitive element (12), and to deliver a signal, said measurement signal, representative of said electrical characteristic of each sensitive element,

and in that said electronic circuit (15) is at least partly included in said coating (17).

11 - Sensor according to claim 6 characterized in that it comprises a connection port (18) which extends outside said coating (17) and in that said electronic circuit (15) is adapted to deliver said signal measurement on this port (18) of connection.

8 / - Sensor according to claim 7 characterized in that it comprises a wireless communication module (27) extending outside the coating (17) and electrically connected to said port (18) connection.

91 - Sensor according to one of claims 6 to 8 characterized in that said circuit (15) is adapted to deliver a digital measurement signal.

10 / - Sensor according to one of claims 6 to 9 characterized in that at least one sensitive element (12) is formed of a resistive strain gauge, and in that said electronic circuit (15) is adapted to electrically supplying each gauge (12) with resistive stresses. 11 / - Sensor according to one of claims 1 to 10 characterized in that said coating (17) is formed of at least one polymeric material selected from the group of thermoplastic polymeric materials, viscoelastic polymeric materials, thermosetting polymeric materials, elastomers, their blends and composite materials.

12 / - Sensor according to one of claims 1 to 11 characterized in that said piece (11) of film has a thickness of less than 1 mm and in that said coating (17) has a thickness greater than 2 mm.

13 / - Sensor according to one of claims 1 to 12 characterized in that:

the modulus of elasticity of each material constituting said coating is greater than 100 kPa,

said piece of film is formed of a material having a modulus of elasticity greater than 1 Gpa.

14 / - Method for measuring the deformation of a surface of a solid in which:

a deformation sensor according to one of claims 1 to 13 is chosen,

the fixing surface (20) of the sensor is fixed on the surface of the solid.

15 / - Method according to claim 14 characterized in that the sensor connects to a system (23) computer so as to transmit to the system (23) computer measurement signal delivered by the sensor.