Sensor and method for monitoring the evolution of the viscoelastic properties of a material.

Abstract

A sensor for measuring the viscoelastic properties of a material comprises a rigid support (10) and a membrane (11) fixed to the support (10), the material being intended to come into contact with an outer face (111) of the membrane (11). The sensor further comprises an active assembly (12) having at least one piezoelectric element bonded to an inner face of the membrane (11), the active assembly (12) being configured to generate the oscillation of the membrane (11) according to a resonance mode greater than or equal to the second resonance mode of the membrane (11). Method for measuring the viscoelastic properties of a material and method for monitoring milk coagulation using such a sensor. Figure for the abstract: Fig. 2

Description

Title of the invention: Sensor and method for monitoring the evolution of the viscoelastic properties of a material. Technical field

[0001] The invention relates to a sensor dedicated to monitoring the evolution of the viscoelastic characteristics of a material. It also relates to a method for monitoring the evolution of the viscoelastic properties of a material, in particular milk during the coagulation phase. • Previous technique#

[0002] Among the key steps in cheesemaking, milk coagulation is of particular interest to professionals. This step involves transforming milk into a gel, curd, through the action of a coagulating agent. This coagulation stage requires careful attention. Indeed, the cheesemaker must make numerous observations at short and regular intervals to detect the change in the milk's state and the evolution of the firmness of the resulting gel. This transition will impact all subsequent steps, such as cutting the gel, draining, acidification, and ripening, and ultimately the quality of the finished product. Therefore, during the milk coagulation stage, the cheesemaker must detect certain essential parameters, such as: • measuring the setting time during the transformation of milk into gel, • the measurement of an optimal gel firmness value corresponding to the time the cutting of this gel, this optimum being a function of the type of cheesemaking technology.

[0003] Characterizing milk during processing thus plays a key role. It is therefore necessary to offer instrumentation incorporating a sensor adapted to the production conditions.

[0004] Various methods exist for simultaneously measuring coagulation time and gel properties. A technique commonly used in the dairy industry employs a device called a "Formagraph." This device, designed in the early 1980s, sets in motion a container holding the product being coagulated and measures the transmission of this motion to a rod suspended in the liquid. The more viscous and gel-like the product becomes, the more motion is transmitted to the rod. Document FR 2 991 771 A1 adopts this technique, refining the method of measuring the motion on the rod.

[0005] This measurement method is not satisfactory insofar as it is necessary to take samples of the milk and coagulation occurs in a separate volume of The one where the cheese forms, therefore potentially subject to variations in ambient conditions. The coagulation conditions are thus not identical; in particular, the temperature may differ, such that the coagulation of the sample does not necessarily represent the coagulation of the milk in the vat.

[0006] The same defects are found with the device described in US document 9,494,475 Bl.

[0007] The document “PZN-PT based smart probe for high temperature fluid viscosity measurements” published by Chen Zhang et al. in 2016 by Elsevier, Measurement 94 (2016) 753-758, describes a measurement technique using a oscillating plunger. A piezoelectric cell is bonded to the plunger with epoxy resin. The plunger allows access to the viscosity properties of the liquid in which its tip is placed. The physical arrangement of the plunger does not allow the sensor to be positioned in a suitable location for real-time monitoring of milk coagulation. • Description of the invention#

[0008] The invention aims to provide a sensor and a method for monitoring the evolution of the viscoelastic properties of a material in situ.

[0009] With these objectives in mind, the invention relates to a sensor for measuring the viscoelastic properties of a material comprising a rigid support and a membrane fixed to the support, the material being intended to come into contact with an external face of the membrane. The sensor is characterized in that it further comprises an active assembly including at least one piezoelectric element bonded to an internal face of the membrane, the active assembly being configured to generate the oscillation of the membrane according to a resonance mode greater than or equal to the second resonance mode of the membrane. By providing a sensor having a membrane that can be in contact with the material to be measured, the sensor can be placed directly in contact with the material. The evolution of the material in its container can therefore be monitored in real time without having to take a sample.The first resonance mode of a membrane, or fundamental mode, is one in which, when a cross-section of the membrane is viewed, a single displacement maximum is observed between the two points where the membrane is anchored to the support. The second resonance mode is characterized by the simultaneous presence of two displacement maxima on the membrane. Using a resonance mode greater than or equal to the second resonance mode of the membrane allows for interaction between the membrane and the material with movements of lesser amplitude than with the first resonance mode. The inventors found that with such a resonance mode, the damping of the movement was less, whereas with the first resonance mode, the damping was so great that the measurements became... difficult, or even insignificant. The inventors also observed that the characteristics of the resonance mode changed depending on the properties of the material. Knowledge of the characteristics of the resonance mode thus provides access to the properties of the material.

[0010] According to one design, the membrane has the shape of a disc embedded around its periphery in the support. This shape is simple to produce and has shown good results. It also makes it possible to obtain a leak-proof sensor with good resistance.

[0011] According to one design, the active assembly only partially covers the inner face of the membrane. The addition of piezoelectric elements to the membrane surface stiffens it. By minimizing the surface area occupied by the piezoelectric elements, the increase in membrane stiffness is also minimized, thus allowing for a better match with the frequencies usable for characterizing the material.

[0012] According to one embodiment, the membrane has a center and the active assembly is off-center on said inner face. By having an active assembly off-center with respect to the center of the membrane, the emergence of an operating mode higher than the fundamental resonance mode is favored. This increases the sensitivity of the sensor.

[0013] According to one embodiment, the active element is arranged to generate differential oscillations of the membrane. Generating differential oscillations means moving the membrane in one direction at one location and in the opposite direction at another location. The generation of differential oscillations directly induces the membrane into a particular resonance mode.

[0014] According to an improvement, the outer surface has a wettability characterized by a contact angle of a 1 qL distilled water droplet of less than 20°, preferably less than 10°. It was observed that when the outer surface had low wettability, measurements could be erroneous under certain conditions. The inventors believe this may be due to poor coupling between the fluid and the membrane. By modifying the surface to improve wettability, they found that these effects no longer occurred.

[0015] According to a constructive arrangement, the external face of the membrane includes a coating or treatment to improve wettability.

[0016] According to one embodiment, the coating is selected from a polymer coating, a silicon oxide deposit, or an epoxy resin incorporating or not titanium oxide nanoparticles. A silicon oxide coating is, for example, obtained from a precursor, tetraethyl orthosilicate.

[0017] According to one embodiment, the external face is polished. This reduces the surface roughness, thereby improving wettability. For example, a roughness of less than 20 nanometers is obtained.

[0018] According to one embodiment, the sensor is intended for use in monitoring milk coagulation. The sensor can be of dimensions adapted to this measurement and to environmental constraints, particularly hygiene. It can be used in the manufacture of all types of dairy products.

[0019] According to one embodiment, the sensor is intended for use in controlling cheese production. The sensor can indeed be used to monitor production and to make decisions, particularly regarding the timing of certain operations.

[0020] The invention also relates to a method for measuring the viscoelastic properties of a material, characterized in that a sensor as described above is used, the material is placed in contact with the outer face of the membrane, and the electrical characteristics of the piezoelectric element are measured when it is powered at a frequency that induces a resonance mode in the membrane greater than or equal to the second mode. The measurements on the piezoelectric element are affected by the behavior of the membrane, which is itself affected by the presence and properties of the material in contact with the membrane. It is therefore possible to determine the properties of the material based on the electrical measurements across the terminals of the piezoelectric element.

[0021] The invention also relates to a method for monitoring milk coagulation, in which the impedance of the active assembly is measured at different frequencies, at least one of the following parameters is determined: the maximum phase q>max(t) of the impedance or admittance, the maximum magnitude Zmax(t) of the impedance or admittance, a shift in the anti-resonance frequency AFa or resonance frequency AFr, and the pseudo-resonance frequency for the phase Fq>max(t). The milk is determined to be sufficiently curdled when the monitored parameter(s) have changed by a predetermined value or in a predetermined ratio. When a voltage at a predetermined frequency is applied across the terminals of the active assembly, an electric current is observed having an intensity and phase related to the applied voltage. This ratio is called impedance, which is decomposed into a phase q and a magnitude Z. We can also work with admittance, which is the inverse ratio of impedance.By varying the frequency close to the frequency corresponding to a resonance mode, we observe that the phase and modulus also change. We can thus determine a maximum for both the phase and the modulus. The frequency at which the maximum for the phase or modulus is observed also varies according to the properties of the milk during the coagulation phase. This is called the frequency of . The resonance frequency Fr is the frequency at which the minimum modulus is reached. The anti-resonance frequency Fa is the frequency at which the maximum modulus is reached. The shift of the anti-resonance frequency AFa is the difference between the anti-resonance frequency at the initial time and the time of measurement. Similarly, the shift of the resonance frequency AFr is the difference between the resonance frequency at the initial time and the time of measurement. The pseudo-resonance frequency for the phase Fq>max(t) is the frequency at which the maximum for the phase is observed at a given instant. Various criteria can be used to determine whether the milk is sufficiently curdled, for example, based on the absolute pseudo-resonance frequency for the phase, or its shift relative to the start of the curdling phase, or its relative value, also relative to the start of the curdling phase.The criteria can also be a relative or absolute variation on the maximum modulus, or on the relative or absolute phase variation for this maximum. The criteria must be adapted to the type of cheese being made. • Brief description of the figures#

[0022] The invention will be better understood and other features and advantages will become apparent upon reading the following description, the description referring to the accompanying drawings, among which: • Fig. 1 is a view of a cheese-making vat in which a sensor according to the invention is mounted; • [Fig.2] is a cross-sectional view of the sensor in [Fig.1]; • [Fig.3] is a top view of a membrane of the sensor in [Fig.2] according to different versions; • [Fig.4] is a view of the membrane representing different modes of vibration; • [Fig.5] is a curve representing the evolution of the magnitude of the sensor impedance as a function of frequency; • [Fig.6] is a curve representing the evolution of the phase of the sensor impedance as a function of frequency; • [Fig.7] is a view showing the evolution of a parameter measured by the sensor as a function of time during milk coagulation. • Detailed description#

[0023] A viscoelastic property measurement sensor according to the invention can be implemented during cheesemaking, particularly during the milk coagulation stage and during the development of gel firmness. Figure 1 shows an example of a vat 2 into which milk is poured at a controlled temperature, and coagulant is added to trigger the coagulation step. Tank 2 is equipped with a sensor 1 according to the invention, being disposed in tank 2.

[0024] As shown in detail in [Fig. 2], the viscoelastic property measurement sensor 1 comprises a rigid, annular support 10 and a disc-shaped diaphragm 11 fixed to the support 10. The support 10 and the diaphragm 11 are made of stainless steel. The support 10 has a rebate 100 at the junction between an end face 101 and an internal bore 102. The diaphragm 11 fits into this rebate 100 such that its outer face 111 is substantially aligned with the end face 101. For manufacturing the sensor 1, a continuous weld is performed between the periphery of the diaphragm 11 and the support 10 such that the diaphragm 11 is tightly fitted onto the support 10. The outer face 111 of the diaphragm 11 receives a surface treatment to improve its wettability.

[0025] An active assembly 12 is also fixed to the inner face of the membrane 11 opposite the outer face 111. In the example shown, the active assembly comprises only a piezoelectric element 12 in the form of a pellet with a diameter smaller than the radius of the membrane 11. This pellet 12 has substantially the same thickness as the membrane 11 and is fixed to the membrane 11 by means of an epoxy-type adhesive in an off-center manner.

[0026] The cavity behind the membrane 11 can accommodate electronic components 13 necessary for connecting and powering the piezoelectric element. A cable, not shown, with electrical conductors passes radially through the support 10, the electrical conductors being connected to the electronic components. The cavity is closed by a bottom wall 14 opposite the membrane 11, which is also hermetically sealed around its periphery. The active assembly 12 is thus configured to generate the oscillation of the membrane 11 in a resonance mode greater than or equal to the second resonance mode of the membrane 11.

[0027] The material whose viscoelastic properties are to be evaluated is brought into contact in particular with the end face 101 and on the external face 111 of the membrane 11.

[0028] We will now describe a surface treatment which improves the wettability of the external face 111 of the sensor 1.

[0029] A coating is applied to the outer face 111 of the membrane 11 by a sol-gel process. The sol-gel is a chemical reaction in solution which, after gelation, gives a solid matrix. This process, here based on titanium dioxide (TiO2) and silicon dioxide (SiO2), comprises four steps.

[0030] First, the preparation of a SiO2 solution by mixing acidified water at pH 3 and tetraethyl orthosilicate (TEOS) in a molar ratio of 1:20. It is stirred for a minimum of 18 hours. The hydrolysis of the precursor (TEOS), by reaction between silica and water, takes place according to the reaction:

[0031] Si(OEt)4 + 4 H2O —> Si(OH)4 + 4 EtOH where Et is an ethyl group C2H5.

[0032] This reaction is followed by a condensation reaction, allowing the formation of SiO2:

[0033] 2 Si(OH)4 -> (OH)3-Si-O-Si(OH)3 + H2O

[0034] This condensation step leads to the formation of gels in which the Si-O-Si complexes are linked together.

[0035] (OH)3-Si-O-Si(OH)3 -> 2 SiO2+ 3 H2O

[0036] The second step is the preparation of the TiO2 solution. This is prepared from a mixture of distilled water / hydrochloric acid / glacial acetic acid / titanium tetraisopropoxide (TTIP), at 60°C for 2 hours, in a molar ratio of 88.6:1.4:5:5. The solution is then stirred for a minimum of 16 hours at room temperature before use. The reaction between TTIP, with the formula Ti[OCH(CH3)2]4, and acidified water allows the formation of TiO2 according to the following reactions:

[0037] Ti[OCH(CH3)2]4 + H2O -> Ti(OH)4 + 4 (CH3)2CHOH

[0038] 2 Ti(OH)4 -> (OH)3-Ti-O-Ti(OH)3 + H2O 2 TiO2 + 3 H2O

[0039] Then, a TiO2 / SiO2 mixture is then prepared and stirred for at least 1 hour according to molar ratios 30:70, 50:50 or 70:30.

[0040] The TiO2 / SiO2 mixture is deposited on the outer face 111 of the membrane 11 of the sensor 1 by spin-coating. During the spin-coating, the disk is positioned on a rotating support and a drop of solution is deposited in the center of the surface. The centrifugal force then distributes the solution uniformly over the entire surface.

[0041] The stainless steel membrane is pre-activated by O2 plasma before deposition. During centrifugal coating, the rotation speed is accelerated to 4000 rpm; a 1 to 2 ml drop of TiO2 / SiO2 mixture is deposited once the speed of 4000 rpm is reached. The disc rotates for a total time of 1 minute.

[0042] The final step consists of annealing. The solvents are evaporated by drying, and a heat treatment allows the film to densify. After deposition, the disc is placed in an oven at 80°C for 15 minutes to evaporate the solvent, then a heat treatment at 120°C for 25 minutes is carried out. A superhydrophilic coating with a thickness of a few tens of nanometers is obtained.

[0043] Different characterizations of the TiO2 / SiO2 coating on the disc are carried out to measure the wettability of the surface and its roughness.

[0044] The contact angle of a 1 µL drop of demineralized water deposited on the surface is measured with a goniometer. The roughness of the TiO2 / SiO2 film is determined by profilometry.

[0045] The angle of the drop is less than 20°.

[0046] Figure 4 illustrates different resonance modes of the membrane 11. Figure 4a shows the fundamental resonance mode, or first mode, in which the maximum vibration amplitude occurs at the center of the membrane 11. This resonance mode is not considered satisfactory for obtaining the measurements according to the invention. Figure 4b shows the second resonance mode, in which a bump 113 appears on one side of the membrane 11 while a symmetrical depression 114 appears on the other side. Figure 4c shows the third resonance mode in which two bumps 113' appear symmetrically with respect to the center while two depressions 114' also appear simultaneously, symmetrically with respect to the center but offset by a quarter turn with respect to the bumps 113'. These resonance modes occur at different frequencies to which the membrane 11 is subjected by the active assembly 12.When a piezoelectric element is placed over an area exhibiting high amplitudes in a specific resonance mode, this resonance mode is favored; that is to say, the resulting impedance is particularly variable depending on the frequency.

[0047] To implement the method according to the invention, the material is placed in contact with the outer face 111 of the membrane 11, and the electrical characteristics across the piezoelectric element are measured. For this purpose, an impedance measuring device is used, which supplies the sensor 1 with an alternating voltage of adjustable frequency. The active assembly 12 is powered at a frequency that induces a resonance mode in the membrane 11 greater than or equal to the second mode. The frequencies in the vicinity are explored by measuring, for example, the magnitude and phase shift of the impedance. Figures 5 and 6 show the evolution of the magnitude and phase, respectively, as a function of frequency near resonance. It can be seen that at the resonance frequency, Fr(t), the magnitude is minimal Zmin(t), then increases to a maximum value Zmax(t) at the antiresonance frequency Fa(t) before decreasing.The diagram shows a second curve with essentially the same shape but flatter and shifted towards higher frequencies. This second curve corresponds to a measurement with a more viscous material than the one obtained in the first curve. In [Fig. 6], we observe that the phase shift reaches a maximum. <e>max(t) for a frequency F <e>max(t), then decreases with increasing frequency. [Fig. 6] also shows a first and second curve corresponding to measurements with the same sensor 1 but a more viscous material in contact with the membrane 11.

[0048] These measures are applied to a method for monitoring milk coagulation. Tank 2 is filled with milk at a controlled temperature, and measurements are started upon the addition of the coagulant. [Fig. 7] shows the measurement of the evolution of the maximum phase <e>max(t) as a function of coagulation time, as well as a curve of variation of this value (derivative with respect to time).

[0049] It is observed that the value is initially stable, then begins a rapid change, corresponding to the setting time, followed by a slightly slower change. The milk is determined to be sufficiently curdled when the maximum phase <bmax(t) a évolué pour atteindre une valeur prédéterminée, par exemple 1,5°. cette prédéterminée est consigne que définit le fromager.

[0050] In various embodiments of the assembly, as shown in [Fig. 3], the active assembly 12 is arranged to generate differential oscillations of the membrane 11. Example (c) is the one previously used with a piezoelectric element in the form of a pellet with a diameter smaller than the radius of the membrane 11. In example (a), the surface of the membrane 11 is covered by an active assembly 12' comprising eight piezoelectric elements 120 in the form of sectors. The adjacent sectors are energized so that their action is in opposite phase. Example (b) comprises a piezoelectric element 12" in the form of a circular pellet centered on the membrane 11. In example (d), the active assembly 12" comprises four circular piezoelectric pellets 121 arranged at the apex of a square and inscribed within the diameter of the membrane 11.

[0051] The invention is not limited to the embodiment just described. The coating for improving wettability could be a polymer coating. The external face 111 could be polished and bare. The evolution of the maximum modulus could be monitored instead of the phase modulus, or the evolution of the resonance pseudofrequency. These parameters could also be combined. < / e> < / e> < / e>

Claims

Demands

1. Sensor for measuring the viscoelastic properties of a material comprising a rigid support (10) and a membrane (11) fixed to the support (10), the material being intended to come into contact on an external face (111) of the membrane (11), the sensor further comprising an active assembly (12) comprising at least one piezoelectric element bonded to an internal face of the membrane (11), the active assembly (12) being configured to generate the oscillation of the membrane (11) according to a resonance mode greater than or equal to the second resonance mode of the membrane (11), characterized in that the external face (111) has a wettability characterized by a contact angle of a 1 qL distilled water droplet of less than 20°, preferably less than 10°.

2. Sensor according to claim 1, wherein the membrane (11) has the shape of a disc embedded by its periphery on the support (10).

3. Sensor according to claim 1 or 2, wherein the active assembly (12) only partially covers the inner face of the membrane (11).

4. Sensor according to any one of claims 2 or 3, wherein the membrane (11) has a center and the active assembly (12) is off-center on said inner face.

5. Sensor according to any one of the preceding claims, wherein the active assembly (12) is arranged to generate differential oscillations of the membrane (11).

6. Sensor according to any one of claims 1 to 5, wherein the outer face (111) of the membrane (11) has a coating or treatment for improving wettability.

7. Sensor according to claim 6, wherein the coating is selected from a polymer coating and a deposit of titanium oxide and silicon oxide nanoparticles.

8. Sensor according to claim 6 or 7, wherein the outer face (111) is polished.

9. Sensor according to any one of claims 1 to 8, intended for use in monitoring milk coagulation.

10. Sensor according to any one of claims 1 to 8, intended for use in controlling cheese production.

11. A method for measuring the viscoelastic properties of a material, characterized in that a sensor is used according to one

12. In accordance with the preceding claims, the material is placed in contact with the external face (111) of the membrane (11) and the electrical characteristics of the piezoelectric element are measured when it is powered at a frequency that induces for the membrane (11) a resonance mode greater than or equal to the second mode. Method for monitoring milk coagulation, characterized in that a measurement method according to claim 11 is implemented and in which the impedance of the active assembly (12) is measured at different frequencies, at least one of the parameters is determined among the maximum phase q> max(t) of the impedance or admittance, the maximum magnitude Z max(t) of the impedance or admittance, a shift in the anti-resonance frequency AFa or resonance frequency AFr, the pseudo-resonance frequency for the phase Fq>max(t), and it is determined that the milk is sufficiently curdled when the monitored parameter(s) have changed by a predetermined value or in a predetermined ratio.

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