High-frequency rheometer with dual actuators

Abstract

High-frequency rheometer with dual actuators. High-frequency rheometer (1) comprising a first element (2) and a second element (3), the first and second elements being movable relative to each other, the rheometer being intended to characterize the mechanical properties of a material (4) interposed between the first and second elements, the rheometer further comprising: - a first actuator (11) configured to generate a periodic high-frequency displacement of the first and / or second element, - a second actuator (12) configured to generate a low-frequency displacement of the first and / or second element, said low frequency being strictly lower than said high frequency, and - a first sensing means (6) configured to measure a response of said material to the displacement between the first and second elements. Figure for the abstract: Figure 1

Description

Title of the invention: High-frequency rheometer with double actuators Technical field of the invention

[0001] The invention relates to a rheometer. The invention relates in particular to a piezo-rheometer. The invention also relates to a method of using such a rheometer or piezo-rheometer. State of the prior art

[0002] Rheometry consists of determining the mechanical response of a material to mechanical stress. Two regimes of mechanical stress are usually distinguished: either the linear regime where the excitation is of sufficiently low amplitude so that it does not disturb the structure of said material, or the non-linear regime where the excitation is of greater amplitude and leads to a modification of the structure and the mechanical response of said material. In the linear regime, the excitation is generally carried out by the application of an oscillating deformation of variable frequency.

[0003] To determine the mechanical properties of materials, rheometers are known, among other things, configured to stress a material in shear between two plates, often circular, parallel to each other. A first plate is driven by a movement that can be oscillating in the plane of said plate. One of the two plates is connected to a sensor making it possible to determine the stress exerted by the sheared material. These rheometers make it possible to stress a material either in a linear regime or in a non-linear regime.

[0004] The rheometers known from the state of the art are limited in several areas. On the one hand, most commercial rheometers have an accessible frequency range limited to typically 100 Hz, while being capable of performing shear measurements in both linear and non-linear regimes. On the other hand, a few high-frequency rheometers are known from the state of the art, rheometers which have the following limitations: 1) A significant part of these instruments operate in compression (“squeeze flow”), the shear behavior being deduced by means of physical models whose relevance must be assessed on a case-by-case basis. 2) All known high-frequency rheometers in the state of the art measure mechanical properties exclusively in the linear regime, by applying nanometric deformations. 3) All high-frequency rheometers known from the state of the art estimate the applied deformation, without direct measurement. The coupling between the structure of the rheometer and the mechanical properties of the material can considerably affect the resonances of the system, the deformations actually exerted can vary by several orders of magnitude, and therefore generate major errors. Presentation of the invention

[0005] The aim of the invention is to provide a rheometer which overcomes the drawbacks stated above.

[0006] An object of the invention is a rheometer making it possible to characterize materials in areas not achievable by rheometers known from the state of the art. Summary of the invention

[0007] The invention relates to a high-frequency rheometer comprising a first element and a second element, the first element and the second element being movable relative to each other, the rheometer being intended to characterize the mechanical properties of a material interposed between the first element and the second element, the rheometer further comprising: - a first actuator configured to generate a high frequency periodic displacement of the first element and / or the second element, - a second actuator configured to generate a low frequency displacement of the first element and / or the second element, said low frequency being strictly lower than said high frequency, and - a first detection means configured to measure a response of said material to the displacement between the first element and the second element.

[0008] According to one embodiment, the first actuator is configured to generate a periodic displacement whose frequency is at least ten times greater than the frequency of the periodic displacement of the second actuator, in particular the first actuator being configured to generate a periodic displacement of frequency greater than or equal to 100 Hz.

[0009] According to one embodiment, the first actuator and / or the second actuator are piezoelectric actuators.

[0010] According to one embodiment, the first actuator is intended to generate a low amplitude periodic displacement of the first element and / or the second element, and the second actuator is intended to generate a higher amplitude periodic displacement than the first actuator of the first element and / or the second element.

[0011] According to one embodiment, the first actuator is configured to move one of the first element and the second element, and the second actuator is configured to move the other of the first element and the second element.

[0012] According to one embodiment, the second element is a reference element and the first element is a movable element relative to the reference element, the first actuator being configured to generate a displacement of the movable element, the second actuator being configured to generate a displacement of the first actuator.

[0013] The rheometer may comprise a second detection means configured to measure a displacement of a mobile element among the first element and the second element, and a servo means configured to regulate the displacement of the mobile element as a function of a signal provided by the second detection means.

[0014] According to one embodiment, the second detection means is also configured to determine an orientation of the mobile element.

[0015] The rheometer may comprise a means for adjusting the parallelism between the first element and the second element, in particular a ball joint connection means.

[0016] According to one embodiment, the first actuator is configured to generate a periodic displacement whose amplitude is at least one hundred times less than the amplitude of the periodic displacement of the second actuator, in particular the first actuator being configured to generate a periodic displacement of amplitude less than or equal to Ipm.

[0017] The rheometer may comprise a third sensing means configured to measure a normal force exerted on the material interposed between the first element and the second element.

[0018] According to one embodiment, the first element is a first plate, and the second element is a second plate extending parallel to the first plate, the first plate and the second plate being movable relative to each other parallel to the planes in which they extend, to stress said material in shear and / or perpendicular to the planes in which they extend to stress said material in compression.

[0019] The invention also relates to a method of using a rheometer as defined previously, the method comprising a relative displacement between the first element and the second element, the relative displacement being composed of a superposition of a first periodic displacement of high frequency generated by the first actuator, and of a second displacement of lower frequency than the first displacement generated by the second actuator. Presentation of figures

[0020] These objects, characteristics and advantages of the present invention will be explained in detail in the following description of two particular embodiments made without limitation in relation to the attached figures.

[0021] [Fig.l] is a schematic view of a rheometer according to a first embodiment of the invention.

[0022] [Fig.2] is a schematic view of a rheometer according to a second mode of rea lization of the invention. Detailed description

[0023] [Fig.l] schematically illustrates a rheometer 1 according to a first embodiment of the invention. Conventionally, the rheometer 1 comprises a reference element 2 and a movable element 3 relative to the reference element 2. A material 4 to be characterized is interposed between the movable element 3 and the reference element 2. The rheometer 1 may be intended to determine the mechanical properties of the material 4. The material 4 may be of any nature. Subsequently, the reference element 2 may also be designated by "first element" and the movable element may be designated by "second element".

[0024] The rheometer 1 further comprises an excitation means 5 configured to move the movable element 3 relative to the reference element 2, and a first detection means 6 configured to measure a response of said material 4 to the movement of the movable element 3.

[0025] It is assumed that the rheometer 1 rests on a horizontal surface. The Z axis designates the vertical axis. According to the first illustrated embodiment, the reference element 2 and the movable element 3 are respectively a first plate and a second plate. The first plate and the second plate both extend parallel to the horizontal plane. The first plate may extend below the second plate. An air gap may be defined as the distance along the vertical Z axis separating an upper surface of the lower plate and a lower surface of the upper plate. The material 4 to be characterized extends in this air gap.

[0026] According to the first embodiment presented, the first plate is movable relative to the second plate in the plane in which the first plate extends. Preferably, the first plate may be movable in translation parallel to a horizontal axis, so as to stress the material in shear. Alternatively, the first plate could be movable parallel to a vertical axis, so as to stress the material in compression, or even according to more complex kinematics. According to yet another embodiment, the first plate and the second plate could be movable in rotation relative to each other about a vertical axis. Generally, the material 4 is intended to be stressed in shear or compression, or a combination of the two, between the first plate and the second plate.

[0027] According to an alternative embodiment of the invention, the reference element 2 and the element mobile element 3 could have different shapes from a plate. The reference element 2 and the mobile element 3 could for example respectively comprise a first and a second surface of cylindrical or conical shape, these two surfaces being separated from each other by a given air gap. According to another variant embodiment, the first plate could be driven by a movement not parallel to the plane in which this plate extends.

[0028] Advantageously, the rheometer 1 comprises an adjustment means 7 configured to move the movable element 3 parallel to the vertical axis Z. The adjustment means 7 thus makes it possible to define a given air gap value. Alternatively or in addition, this adjustment means 7, associated with a detection means such as one or more force sensors, also makes it possible to define a force normal to the shear plane applied by the movable element 3 on the reference element 2 via the material 4 to be characterized. The adjustment means 7 can be configured so as to vertically move the excitation means 5 with the movable element 3. It can in particular comprise a high-precision motorized stage so as to carry out a very fine adjustment of the air gap and / or said normal force. The adjustment means 7 also makes it possible to apply given profiles of vertical movement speed of the movable element 3.This allows the operation of placing the material in the air gap to be controlled with great precision, which makes it possible to obtain reproducible characterization for materials with an internal structure sensitive to shear.

[0029] The rheometer may also comprise an opening means configured to easily position the material 4 to be characterized on the reference element 2. The opening means may in particular comprise a hinge 8 intended to pivot the assembly constituted by the mobile element 3, the excitation means 5 and the adjustment means 7. This assembly may in particular pivot around a horizontal axis. The Rheometer is thus practical to use and makes it possible to place a material to be characterized according to a well-reproducible process.

[0030] According to the invention, the excitation means 5 comprises on the one hand a first actuator 11 capable of generating a displacement of the movable element 3, and on the other hand a second actuator 12 capable of generating a displacement of the first actuator 11. The second actuator 12 is fixed to a base 10 of the rheometer, in particular by means of the adjustment means 7. The second actuator 12 comprises an arm 13 to which the first actuator 11 is fixed. The movable element 3 is therefore connected to the base 10 by means of the first actuator 11 and the second actuator 12. The first actuator 11 and the second actuator 12 are therefore arranged in series. The movable element 3 is thus moved relative to the reference element 2 according to a movement resulting from the superposition of the displacement generated by the first actuator 11 and the displacement generated by the second actuator 12. The base 10 supports the first actuator 11, the second actuator 12 but also the adjustment means 7.

[0031] The first actuator 11 and the second actuator 12 may be configured to generate a movement parallel to a first axis, and respectively parallel to a second axis. The first axis and the second axis are preferably horizontal axes. According to one embodiment, the first axis and the second axis may be parallel to each other. According to an alternative embodiment, the first axis and the second axis may form a non-zero angle. For example, the first axis and the second axis could be perpendicular to each other.

[0032] The first actuator 11 and the second actuator 12 may be configured to generate periodic movements. These periodic movements may be, for example, back-and-forth movements in translation parallel to a horizontal axis.

[0033] The first actuator 11 may be configured to generate high-frequency periodic movements while the second actuator 12 may be configured to generate lower-frequency movements. In particular, the frequency of the periodic movements generated by the first actuator 11 may be at least ten times, or even at least one hundred times, higher than the frequency of the periodic movements generated by the second actuator 12. The first actuator 11 may be configured to generate a periodic movement at a frequency greater than or equal to 1 Hz, in particular greater than or equal to 10 Hz, preferably greater than or equal to 100 Hz, preferably greater than or equal to 1 kHz, or even greater than or equal to 10 kHz. The second actuator 12 may be configured to generate a periodic movement at a frequency of between 0.1 Hz and 10 Hz, or even a frequency of between 0.1 Hz and 100 Hz.Alternatively, the second actuator 12 could be configured to generate a static displacement to pre-stress the material to be studied, for example to generate a static pre-stress in shear and / or compression. A static displacement can be considered as a displacement of zero frequency.

[0034] The first actuator 11 may be configured to generate periodic displacements of low amplitude while the second actuator 12 may be configured to generate displacements that may be of much greater amplitude. In particular, the amplitude of the periodic displacements generated by the first actuator 11 may be at least one hundred times, or even at least one thousand times, lower than the amplitude of the periodic displacements generated by the second actuator 12. The amplitude of the displacements generated by the first actuator 11 may be, for example, less than or equal to 1 pm, for example of the order of 100 nm. The amplitude of the displacements generated by the second actuator 12 may be of the order of one centimeter.

[0035] The second actuator 12 can, if necessary, be used to prepare the material in situ before measurements in a procedure called “rejuvenation” where the stress imposed by the actuator 12 makes it possible to erase the thermomechanical history of the material.

[0036] Preferably, the first actuator 11 and / or the second actuator 12 are piezoelectric actuators, that is to say that the displacement that they generate is obtained by means of PZ piezoelectric elements. The rheometer 1 can thus be described as a piezo-rheometer. In order to generate a shear stress on the material 4, each actuator can comprise a set of PZ piezoelectric elements, for example of the dl5 type. A piezoelectric actuator makes it possible to generate very well-controlled displacements without displacement hysteresis and whose frequency is relatively high.

[0037] As illustrated in [Fig. 1], the first actuator comprises an upper plate and a lower plate connected to each other via two piezoelectric elements PZ. The movable element 3 is fixed to the lower plate of the first actuator. The upper plate is connected to the arm 13.

[0038] The first detection means 6 comprises in particular one or more sensors arranged between a support 14 and the reference element 2. These sensors make it possible to measure the manner in which the material 4 transmits the stresses imparted by the mobile element 3 to the reference element 2. According to the first embodiment presented, the first detection means 6 comprises two sensors 15A, 15B. The sensors 15A, 15B can advantageously be piezoelectric sensors. They convert the force observed on the surface of the reference element 3 into an electric current which can then be acquired by electronic equipment.

[0039] The rheometer 1 further comprises a second detection means 16 configured to measure a displacement of the mobile element 3. It is thus possible to determine the excitation setpoint actually applied to the material 4. The second detection means 16 may for example comprise a plurality of accelerometers 17A, 17B, 17C connected to a printed circuit board 18 fixed to the mobile element 3, in particular via the lower plate of the first actuator 11. Each accelerometer may be intended to measure an acceleration of the mobile element 3 over a given acceleration range. The association of several accelerometers (for example three accelerometers as shown in [Fig.l]) thus makes it possible to measure an acceleration of the mobile element 3 over a large acceleration range. The displacement of the mobile element 3 can then be calculated on the basis of signals provided by the accelerometers.Alternatively, the second detection means 16 could comprise other types of sensors capable of measuring a displacement of the mobile element 3, for example optical sensors, ultrasonic sensors, eddy current sensors, capacitive sensors, etc.

[0040] The rheometer 1 further comprises a servo-control means 19 configured to regulate the movement of the mobile element 3 as a function of a signal provided by the second detection means 16. The servo-control means 19 may for example comprise a PID type regulator. The movement of the mobile element 3 is therefore a servo-controlled movement. Thus, it is possible to precisely control the stress which is applied to the material 4 by the mobile element 3.

[0041] According to the first embodiment presented, said servo-control means 19 is integrated in the form of a computer program recorded in a memory of an electronic equipment 20. The electronic equipment 20 is connected to the second detection means 16 and to the first actuator 11, in particular to the piezoelectric elements PZ of the first actuator 11. The electronic equipment 20 comprises a microprocessor configured to execute said computer program.

[0042] The rheometer may further comprise a ball joint connection means 21 interposed between the first actuator 11 and the second actuator 12. The ball joint connection means 21 comprises a sphere 22 and a structural element 23. The structural element 23 is secured to the arm 13. The sphere 22 is connected by an axis 24 to the first actuator 11. The ball joint connection means 21 also comprises a locking means 25 movable between a locking position and an unlocking position. When the locking means 25 is in the unlocking position, the sphere 22 is free to pivot about its center relative to the structural element 23, which makes it possible to orient the first actuator 11 relative to the arm 13. Conversely, when the locking means 25 is in the locking position, the sphere 22 is locked in position relative to the structural element 23.The first actuator is then rigidly connected to the arm 13 and the movements generated by the second actuator can be efficiently transmitted to the mobile element 3. Thus, the orientation of the mobile element 3 can be adjustable. The ball joint connection means 21 forms a means for adjusting the parallelism between the reference element 2 and the mobile element 3. Alternatively, other means for adjusting the parallelism could be envisaged, such as for example a three-screw adjustment means.

[0043] Advantageously, the signals provided by the accelerometers 17A, 17B and 17C and / or by a third detection means which will be described later can also be used to adjust the orientation of the mobile element 3. For this purpose, at least one accelerometer among the three accelerometers 17A, 17B, 17C can comprise two channels making it possible to measure the orientation of the mobile element 3 along two horizontal axes perpendicular to each other. It can thus be ensured that the mobile element extends parallel to the reference element 2.

[0044] The support 14 rests on the base 10 via a third detection means 26. The third detection means 26 is configured to measure a force normal exerted by the mobile element 3 on the reference element 2. The third detection means 26 may in particular comprise two force sensors 27A, 27B, or alternatively the entire number of force sensors. The third detection means 26 also makes it possible to detect the contact of the mobile element 3 with the material 4 during the descent of the assembly formed by the excitation means 5 and the mobile element 3 controlled by the adjustment means 7. The device also makes it possible to carry out compression measurements by means of the adjustment means 7 and the force sensor(s) 27A, 27B.

[0045] The electronic equipment 20 can be connected not only to the first actuator 11 and to the second detection means 16, but also to the second actuator 12 and / or to the first detection means 6 and / or to the adjustment means 7 and / or to the third detection means 26. Electrical connections between the electronic equipment 20 and these different components 6, 7, 11, 12, 16, 26 are represented by dotted lines in [Fig. 1].

[0046] The electronic equipment 20 thus makes it possible to control: - the amplitude and / or frequency of the movements generated by the first actuator 11, and / or - the amplitude and / or the frequency of the movements generated by the second actuator 12, and / or - the air gap separating the moving element 3 from the reference element 2 and / or the normal force applied by the moving element to the reference element. The electronic equipment 20 can also acquire the signals provided by each of the three detection means 6, 16 and 26, in particular in order to characterize the material 4.

[0047] To characterize a material 4 by means of the rheometer 1, the following procedure can be used. First of all, said material 4 can be positioned between the reference element 2 and the mobile element 3. This can be done simply by actuating the hinge 8. The correct orientation of the mobile element can also be checked by observing the signals supplied by the accelerometers 17A, 17B, 17C. If necessary, the orientation of the mobile element 3 can be adjusted with the ball joint connection means 21. For this purpose, the locking means 25 of the ball joint connection means 21 can be temporarily unlocked. Once the orientation of the mobile element has been correctly adjusted, the locking means is locked. Then, the air gap value and / or the normal force exerted by the mobile element 3 on the reference element 2 can be adjusted by actuating the motorized plate of the adjustment means 7.Once these operations have been carried out, the rheometer 1 is ready to characterize the material 4.

[0048] The first actuator 11 and the second actuator 12 can then be activated simultaneously and / or successively. The movable element 3 moves relative to the element of reference 2 according to a movement composed of a superposition of a first periodic displacement of low amplitude and high frequency generated by the first actuator 11 and a second displacement of high amplitude by the second actuator. It is thus possible to bring the material to a given sheared state with the second actuator 12 and to carry out a frequency sweep, for example from 1Hz to 10kHz with the first actuator 11, to know the shear modulus of the material at this imposed shear point.

[0049] Such a configuration makes it possible to obtain deformations of up to 10000% of the material, said deformation being defined as the ratio between the air gap and the amplitude of the displacement imposed by the second actuator 12. For example, a displacement of approximately 1 cm can be achieved with an air gap of 100 pm. This makes it possible, for example, to characterize the high-frequency linear behavior of the material around a non-linear state, to characterize the linear and non-linear anisotropic properties of the material by different combinations of stress axes.

[0050] The invention therefore makes it possible to improve the rheometers known from the state of the art since: 1) No high-frequency rheometer known from the state of the art measures the normal force exerted by the material on the plates. However, for a whole class of materials, this force conditions the mechanical properties of said materials. 2) No known high-frequency rheometer can pre-shear the material in order to erase its thermomechanical history. 3) No known high-frequency rheometer can control the displacement profile when placing the material in the air gap, an essential tool for controlling the mechanical history of the material before measurement. 4) While several conventional rheometers offer orthogonal measurements, no high-frequency instrument does.

[0051] [Fig. 2] illustrates a second embodiment of a rheometer according to the invention. The rheometer according to the second embodiment comprises the same components as the rheometer of the first embodiment but arranged differently. In particular, the second embodiment differs from the first embodiment mainly by the arrangement of the first actuator 11 and the second actuator 12. To facilitate and simplify the understanding of this second embodiment, the same reference signs are used to designate components having the same function. In addition, the differences compared to the first embodiment described above are simply described.

[0052] According to the second embodiment, the rheometer 1 therefore comprises two actuators 11, 12 having the same characteristics as the actuators 11, 12 of the first embodiment. In particular, the first actuator 11 may be configured to generate high-frequency periodic displacements while the second actuator 12 may be configured to generate displacements of lower frequency than the first actuator. The first actuator 11 may be configured to generate low-amplitude periodic displacements while the second actuator 12 may be configured to generate displacements that may be of greater amplitude than the first actuator.

[0053] According to the second embodiment, the two actuators 11, 12 are not arranged in series as in the first embodiment. On the contrary, the first actuator 11 and the second actuator 12 are independent of each other. The first actuator 11 is configured to move the second element 3 and the second actuator 12 is configured to move the first element 2. Thus, each of the two elements 2 and 3 are intended to move relative to the base 10 and can be described as mobile elements. The first element 2 may be intended to move at low frequency and high amplitude relative to the base 10. The second element 3 may be intended to move at high frequency and low amplitude relative to the base 10.

[0054] According to the second embodiment, the detection means 16 is configured to measure the displacement of the second element 3, which is only acted upon by the first actuator 11. The detection means 16 thus only measures the high-frequency component of the acti-vation of the material 4, which simplifies the regulation carried out by the servo-control means 19.

[0055] As a note, the terms "first" and "second" are simply intended to distinguish objects distinct from one another. These terms do not characterize any particular order relationship between the objects to which they relate. Thus, according to an alternative embodiment not shown, the first actuator 11 could be configured to move the first element 2 and the second actuator 12 could be configured to move the second element 3.

Claims

Claims

1. A high-frequency rheometer (1) comprising a first element (2) and a second element (3), the first element and the second element being movable relative to each other, the rheometer being intended to characterize the mechanical properties of a material (4) interposed between the first element and the second element, the rheometer further comprising: - a first actuator (11) configured to generate a high-frequency periodic displacement of the first element and / or the second element, - a second actuator (12) configured to generate a low-frequency displacement of the first element and / or the second element, said low frequency being strictly lower than said high frequency, and - a first detection means (6) configured to measure a response of said material to the displacement between the first element and the second element.

2. Rheometer (1) according to the preceding claim, characterized in that the first actuator (11) is configured to generate a periodic displacement whose frequency is at least ten times greater than the frequency of the periodic displacement of the second actuator (12), in particular the first actuator (11) being configured to generate a periodic displacement of frequency greater than or equal to 100 Hz.

3. Rheometer (1) according to one of the preceding claims, characterized in that the first actuator (11) and / or the second actuator (12) are piezoelectric actuators.

4. Rheometer (1) according to one of the preceding claims, characterized in that: - the first actuator (11) is intended to generate a periodic displacement of low amplitude of the first element and / or of the second element, and - the second actuator (12) is intended to generate a periodic displacement of higher amplitude than the first actuator of the first element and / or of the second element.

5. Rheometer (1) according to one of the preceding claims, characterized in that the first actuator (11) is configured to move one of the first element (2) and the second element (3), and in that the second actuator (12) is configured to move the other of the first member and the second member.

6. Rheometer (1) according to one of claims 1 to 4, characterized in that the second element (3) is a reference element and in that the first element is a movable element relative to the reference element, the first actuator (11) being configured to generate a displacement of the movable element, the second actuator (12) being configured to generate a displacement of the first actuator.

7. Rheometer (1) according to one of the preceding claims, characterized in that it comprises a second detection means (16) configured to measure a displacement of a mobile element among the first element and the second element, and a servo means (19) configured to regulate the displacement of the mobile element as a function of a signal supplied by the second detection means (16).

8. Rheometer (1) according to the preceding claim, characterized in that the second detection means (16) is also configured to determine an orientation of the movable element (3).

9. Rheometer (1) according to one of the preceding claims, characterized in that it comprises a means for adjusting the parallelism between the first element (2) and the second element (3), in particular a ball joint connection means (21).

10. Rheometer (1) according to one of the preceding claims, characterized in that the first actuator (11) is configured to generate a periodic displacement whose amplitude is at least one hundred times less than the amplitude of the periodic displacement of the second actuator (12), in particular the first actuator (11) being configured to generate a periodic displacement of amplitude less than or equal to Ipm.

11. Rheometer (1) according to one of the preceding claims, characterized in that it comprises a third detection means (26) configured to measure a normal force exerted on the material (4) interposed between the first element and the second element.

12. Rheometer (1) according to one of the preceding claims, characterized in that the first element (2) is a first plate, and in that the second element (3) is a second plate extending parallel to the first plate, the first plate and the second plate being movable relative to each other parallel to the planes in which they extend, to stress said material in shear and / or perpendicular to the planes in which they extend to stress

13. said material in compression. Method of using a rheometer (1) according to one of the preceding claims, characterized in that it comprises a relative displacement between the first element and the second element, the relative displacement being composed of a superposition of a first periodic displacement of high frequency generated by the first actuator (11), and of a second displacement of lower frequency than the first displacement generated by the second actuator (12).

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