MAGNETIC FIELD DETECTION DEVICE AND MAGNETIC FIELD MEASURING SYSTEM WITH SUCH A DEVICE

Description

[0001] This patent application claims priority from French patent application FR22 / 10915. Domaine technique

[0002] This description generally concerns the measurement of static or time-varying magnetic fields. Technique antérieure

[0003] For some applications, it would be desirable to be able to measure a static or time-varying magnetic field with a spatial resolution of less than 0.1 mm. In addition, for some applications, it would be desirable to be able to measure a time-varying magnetic field, particularly a pulsed or high-frequency magnetic field. In addition, for some applications, it would be desirable to be able to measure a low-intensity or high-intensity magnetic field with the same device.

[0004] An example of a commercially available magnetic field detection device is a Hall effect sensor. However, such a sensor typically has a spatial accuracy better than 0.1 mm and is therefore not suitable for measuring a highly localized magnetic field. In addition, existing Hall effect sensors have a limited bandwidth that does not exceed 100 kHz and are therefore not suitable for measuring a high-frequency magnetic field. Furthermore, existing Hall effect sensors cannot be driven synchronously with a magnetic pulse source and are therefore not suitable for measuring a pulsed magnetic field.

[0005] Document US 5,966,008 A discloses a device for detecting a magnetic field comprising, in particular, an acoustic waveguide and an electroacoustic transducer. Résumé de l'invention

[0006] One embodiment overcomes all or part of the drawbacks of known magnetic field detection devices and magnetic field measurement systems comprising such devices.

[0007] An object of an embodiment is that the magnetic field detection device allows the measurement of a static or time-varying magnetic field with a spatial resolution of less than 0.2 mm.

[0008] An object of an embodiment is that the magnetic field detection device allows the measurement of a magnetic pulse.

[0009] An object of an embodiment is that the magnetic field detection device can be manufactured at reduced cost.

[0010] One embodiment provides a device for detecting a magnetic field comprising: a first tapered acoustic waveguide having a first base and a first tapered end; a first electrically conductive wire rigidly coupled to the first tapered end; and an electroacoustic transducer rigidly coupled to the first base.

[0011] According to one embodiment, the first tapered acoustic waveguide extends along an axis from the first base to the first tapered end, the cross-section of the first tapered acoustic waveguide decreasing from the first base to the first tapered end, and the first electrically conductive wire comprises a portion orthogonal to said axis at the first tapered end.

[0012] According to one embodiment, the electroacoustic transducer is an electroacoustic transducer with transverse waves polarized perpendicular to the plane defined by the axis of the first tapered acoustic waveguide and the tangent to the first electrically conductive wire when it is orthogonal to the axis at the first tapered end and which propagate in the first base-first tapered end direction.

[0013] According to one embodiment, the first tapered acoustic waveguide has the shape of a cone or a truncated cone.

[0014] According to one embodiment, the first tapered acoustic waveguide has an apex angle of less than 15°.

[0015] According to one embodiment, the first tapered acoustic waveguide has the shape of a truncated cone, the first tapered end comprising a planar surface.

[0016] According to one embodiment, the planar surface has a diameter less than or equal to half a wavelength of the phase velocity of the bending waves in the first tapered end.

[0017] According to one embodiment, the flat surface has a radius less than 1 mm.

[0018] According to one embodiment, the first tapered acoustic waveguide includes a notch in the planar surface receiving the first electrically conductive wire.

[0019] According to one embodiment, the first tapered acoustic waveguide has the shape of a triangular-based prism.

[0020] According to one embodiment, the first electrically conductive wire comprises an electrically conductive track deposited on the first tapered acoustic waveguide.

[0021] According to one embodiment, the first electrically conductive wire comprises an electrically conductive blade extending along a flank of the first tapered acoustic waveguide spaced from said flank and connected to the first tapered end.

[0022] According to one embodiment, the electroacoustic transducer is a piezoelectric shear wave transducer or an electromagnetic shear wave transducer.

[0023] According to one embodiment, the first tapered acoustic waveguide is made of a non-magnetic material.

[0024] According to one embodiment, the first tapered acoustic waveguide is made of a material selected from the group comprising glass, silicon, ceramic, non-magnetic metals, austenitic steel and non-magnetic metal alloys.

[0025] According to one embodiment, the melting temperature of the first tapered acoustic waveguide and the melting temperature of the first electrically conductive wire are greater than 1000°C.

[0026] According to one embodiment, the device comprises a support made of an electrically insulating material surrounding the first tapered acoustic waveguide midway between the first base and the first tapered end.

[0027] According to one embodiment, the device comprises an electrically conductive skirt surrounding the first tapered acoustic waveguide, spaced from the tapered acoustic waveguide, and comprising an opening for the passage of the first tapered end.

[0028] According to one embodiment, the electroacoustic transducer comprises a second tapered acoustic waveguide comprising a second base and a second tapered end, a second conductive wire electrically rigidly coupled to the second tapered end, and a first permanent magnet opposite the second tapered end.

[0029] According to one embodiment, the first tapered acoustic waveguide and the second tapered acoustic waveguide form a single piece.

[0030] According to one embodiment, the device further comprises a third tapered acoustic waveguide rigidly coupled to the first tapered acoustic waveguide, the third tapered acoustic waveguide comprising a third base and a third tapered end, a third electrically conductive wire rigidly coupled to the third tapered end, and a second permanent magnet opposite the third tapered end, the electroacoustic transducer being located between the first base and the third base.

[0031] According to one embodiment, the electroacoustic transducer comprises two piezoelectric plates located opposite each other and operating in phase opposition.

[0032] An embodiment also provides a system for measuring a magnetic field comprising a device for detecting a magnetic field as defined previously, and a control and acquisition device connected to the device for detecting a magnetic field comprising a generator configured to provide at least one current pulse in the first electrically conductive wire and an acquisition chain for detecting an electrical signal provided by the electroacoustic transducer or a generator configured to provide at least one voltage pulse controlling the electroacoustic transducer and an acquisition chain for detecting an electrical signal provided by the first electrically conductive wire.

[0033] According to one embodiment, the magnetic field corresponds to a magnetic pulse and the control and acquisition device is configured so that the generator provides a current pulse in the first conductive wire electrically synchronous with the magnetic pulse.

[0034] According to one embodiment, the control and acquisition device comprises a generator control module configured to receive a binary synchronization signal and a signal representative of a delay and to control the generator to provide the current pulse or the voltage pulse at the end of the delay after receiving the synchronization signal.

[0035] According to one embodiment, the generator is configured to provide the current pulse having a duration twice as short as the period corresponding to the maximum variation frequency of the magnetic field.

[0036] According to one embodiment, the control and acquisition device comprises an amplifier receiving the measurement signal whose gain is programmable in increasing steps, each step corresponding to a possible variation range of the amplitude of the magnetic field to be measured. Brève description des dessins

[0037] These and other features and advantages will be set forth in detail in the following description of particular embodiments given without limitation in relation to the attached figures, among which: there figure 1 is a partial, schematic, sectional view of one embodiment of a magnetic field detection system; figure 2 , there figure 3 , there figure 4 , there figure 5 , there figure 6 , and the figure 7 are partial and schematic sectional views of other embodiments of the probe of the detection system; figure 8 and the figure 9 are block diagrams of the detection system of the figure 1 illustrating embodiments of the control and acquisition device of the detection system; figure 10 is a partial, schematic sectional view of an embodiment of a transducer of the probe of the magnetic field detection system; figure 11 is a partial and schematic sectional view of an alternative embodiment of the probe 20 shown in figure 10 ; there figure 12 is a partial, schematic top view of another embodiment of a magnetic field detection system; figure 13 is a partial, schematic side view of the probe of the magnetic field detection system of the figure 12 ; there figure 14 is a partial and schematic top view of a variant of the probe shown in figure 12 ; there figure 15 is a partial and schematic top view of an alternative embodiment of the probe and the electroacoustic transducer of the magnetic field detection system shown in figure 12 ; there figure 16 is a partial and schematic top view of an alternative embodiment of the probe and the electroacoustic transducer of the magnetic field detection system shown in figure 12 ; there figure 17 is a partial, schematic top view of another embodiment of a magnetic field detection system; figure 18 is a partial, schematic side view of the probe of the magnetic field detection system of the figure 17 ; there figure 19 is a partial, schematic top view of another embodiment of a magnetic field detection system; figure 20 is a partial, schematic side view of the probe of the magnetic field detection system of the figure 19 ; and the figure 21 , there figure 22 , there figure 23 , there figure 24 , there figure 25 , and the figure 26 represent curves of the evolution over time of the measurement signal provided by the magnetic field detection system obtained during tests. Description des modes de réalisation

[0038] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0039] For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been represented and are detailed.

[0040] Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") together, this means that these two elements can be connected or be connected by means of one or more other elements.

[0041] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the figures or to a probe in a normal position of use.

[0042] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%. In the case of angle, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10°, preferably to within 5°. Furthermore, the terms "insulator" and "conductor" are considered here to mean "electrically insulating" and "electrically conducting", respectively.

[0043] There figure 1 is a partial and schematic sectional view of an embodiment of a system 10 for detecting a component B z of a magnetic field B.

[0044] The system 10 comprises a magnetic field detection device 20, hereinafter called a probe, and a control and acquisition device 30 connected to the probe 20. The probe 20 comprises an acoustic waveguide 40, an electrically conductive wire 50, and an electroacoustic transducer 60.

[0045] The acoustic waveguide 40 has a tapered shape along an axis D with a cross-section whose surface area decreases from a base 41 to a tapered end 42, hereinafter called a tip, opposite the base 41. The acoustic waveguide 40 is hereinafter called a tapered guide 40. The conductive wire 50 extends along the side wall 43 of the tapered guide 40 to the tip 42. According to one embodiment, the conductive wire 50 is folded over the tip 42 and fixed to the tip 42 by a bonding material 70.

[0046] According to one embodiment, the tapered guide 40 has a general shape of a cone or truncated cone. Preferably, the tapered guide 40 has a symmetry of revolution around the axis D. According to another embodiment, the tapered guide 40 has a general prismatic shape, in particular with a triangular base. At least in a plane containing the axis D, the tapered guide 40 has a triangular cross-section with an apex angle α of less than 15°, preferably less than 10°, more preferably less than 5°. When the tapered guide 40 has a general shape of a cone or truncated cone, the apex angle α corresponds to the apex angle of the cone. When the tapered guide 40 has a general shape of a prism with a triangular base, the apex angle α corresponds to the apex angle of the triangular base on the side of the tip 42 of the tapered guide 40.The conductive wire 50 has a portion 52 covering the tapered guide tip 40 in which the current flows substantially perpendicular to the axis D of the tapered guide 40.

[0047] In operation, the tip 42 of the tapered guide 40 is placed at the location where the magnetic field B is located. The system 10 makes it possible to measure the component B z , along the axis D of the tapered guide 40, of the magnetic field B.

[0048] According to one embodiment, hereinafter called direct operating mode, the control and acquisition device 30 is configured to provide a current pulse in the conductive wire 50. The current pulse has an intensity I and a duration Δt. In the presence of the magnetic field B at the tip 42 of the tapered guide 40, a Lorentz force F appears in the part of the conductive wire 50 located at the tip 42 of the tapered guide 40, the amplitude F of which is defined by the following relationship: F = IΔ tVB where V is the speed of electrons in the conductive wire 50.

[0049] In direct operating mode, the tapered guide 40 makes it possible to transform the Lorentz force F into a pulsed ultrasonic bending wave which remains broadband during its propagation in the tapered guide 40. The amplitude of the ultrasonic bending wave is in particular proportional to the component B z of the magnetic field B to be measured, to the intensity I of the current pulse, and to the duration of Δt of the current pulse. The bending wave is polarized in the direction of the vector product I^B perpendicular to the plane of the figure 1 defined by the axis D and the tangent to the conductive wire 50 defining the current vector I where the conductive wire 50 is rigidly fixed to the end of the tip 42. Due to its generally tapered shape with a small angle at the apex, the tapered guide 40 promotes the propagation of an acoustic bending mode generated at the end of the tip 42 to the base 41. The intrinsic impedance of the material constituting the tapered guide 40 is equal to the product of the fundamental velocity of a transverse wave in the material by the density of the material. The mechanical radiation impedance of the tip 42 of the tapered guide 40 is defined as the product of the phase velocity of an acoustic wave at the coupling location by the density of the material constituting the tapered guide 40. The mechanical radiation impedance is particularly low, typically 3 to 4 times lower than the intrinsic impedance of the material.This characteristic is advantageously suited to an efficient transfer of the Lorentz force appearing in the conductive wire 50 rigidly coupled to the tip 42 by bonding. The acoustic wave reaches the electroacoustic transducer 60 which converts it into an electrical signal S, for example a voltage. The control and acquisition device 30 is configured to measure the electrical signal S provided by the electroacoustic transducer 60 and to deduce therefrom the amplitude of the component B z of the magnetic field B and the polarization of the magnetic field B present at the tip 42 of the tapered guide 40.

[0050] According to one embodiment, the intensity I of the current pulse is preferably as high as possible, in practice between 1 A and 100 A, preferably approximately 50 A. According to one embodiment, the duration Δt of the current pulse is as short as possible, for example between 1 ns and 500 ns, preferably approximately 10 ns. In the case where the magnetic field to be measured corresponds to a magnetic pulse, the duration Δt of the current pulse is less than the duration of the magnetic pulse to be measured. In the case where the magnetic field to be measured corresponds to a sinusoidal magnetic field, the duration Δt of the current pulse is less than at least twice the period of the sinusoidal magnetic field to be measured. Generally, in the case where the magnetic field to be measured varies over time, the duration Δt of the current pulse is less than at least twice the period corresponding to the maximum variation frequency of the magnetic field.

[0051] In direct operating mode, the tapered guide 40 advantageously introduces an acoustic propagation delay of 1 µs to 100 µs between the electrical pulse present at the tip 42 of the tapered guide 40 and the supply of the electrical signal by the electroacoustic transducer 60. This makes it possible to avoid interference with the reception amplifier of the control and detection device 30 by direct aerial coupling between the electrical pulse and the reception electronics.

[0052] According to one embodiment, hereinafter called reciprocal operating mode, the control and acquisition device 30 is configured to control the electroacoustic transducer 60 for the generation of an acoustic wave in the tapered guide 40. In reciprocal operating mode, the tapered guide 40 makes it possible to propagate the ultrasonic wave from the base 41 to the tip 42. The ultrasonic wave causes the tip 42 to bend. The movement of the tip 42, and therefore of the conductive wire 50 in the magnetic field B causes the appearance of an electromotive force FEM in the conductive wire 50. The electromotive force FEM is proportional to the component B z of the magnetic field B to be measured. The control and acquisition device 30 is configured to measure the electromotive force FEM at the ends of the conductive wire 50 and to deduce therefrom the component B z of the magnetic field B and the polarization of the magnetic field B present at the tip 42 of the tapered guide 40.

[0053] According to one embodiment, the conductive wire 50 has a cylindrical cross section. According to another embodiment, the conductive wire 50 has a cross section that is not cylindrical. In this case, the diameter of the conductive wire 50 is equal to the diameter of the disc with the same surface area as the surface area of ​​the cross section of the conductive wire 50. According to one embodiment, the conductive wire 50 has a diameter varying from 10 µm to 200 µm, for example equal to approximately 40 µm. According to one embodiment, the conductive wire 50 comprises a conductive core surrounded by an insulating sheath, for example an enameled wire. According to another embodiment, the conductive wire 50 corresponds to a conductive track deposited on the tapered guide 40.

[0054] According to one embodiment, the tip 42 of the tapered guide 40 comprises a surface 44, called the bearing surface 44 hereinafter, on which the portion 52 of the conductive wire 50 rests.

[0055] According to one embodiment, the bearing surface 44 is flat and perpendicular to the axis D of the tapered guide 40. According to one embodiment, the bearing surface 44 is inscribed in a disc. The diameter of the bearing surface 44 is then called the diameter of the disc in which the bearing surface 44 is inscribed. According to one embodiment, the bearing surface 44 is not flat. In this case, the diameter of the bearing surface 44 is called the diameter of the disc in which the bearing surface 44 is inscribed when viewed along the axis D.

[0056] According to one embodiment, the bearing surface 44 has a diameter between the diameter of the conductive wire 50 and five times the diameter of the conductive wire 50, for example between twice and five times the diameter of the conductive wire 50. According to one embodiment, the bearing surface 44 has a diameter between twice and five times the diameter of the conductive wire 50. According to one embodiment, the bearing surface 44 has a diameter equal to the diameter of the conductive wire 50. The periphery of the bearing surface 44 then constitutes a bearing surface protecting the conductive wire 50. The closer the diameter of the conductive wire 50 is to the diameter of the bearing surface 44, the less propagation effects in the vicinity of the tip 42 are felt. For example, for a conductive wire 50 with a diameter equal to 40 µm, the diameter of the bearing surface 44 may be close to 100 µm.

[0057] Furthermore, to avoid crushing the conductive wire 50 during measurements, a notch of depth equal to the diameter of the conductive wire 50 can be made in the plane of the bearing surface 44 so as to bury the conductive wire 50 in the tip 42.

[0058] There figure 2 , there figure 3 , there figure 4 , and the figure 5 are partial and schematic sectional views of other embodiments of the probe 20. The sectional plane of the figures 2 à 5 contains the axis D of the tapered guide 40 and is perpendicular to the cutting plane of the figure 1 .

[0059] In the embodiment illustrated in figure 2 , the diameter of the bearing surface 44 is equal to the diameter of the conductive wire 50, and the bearing surface 44 comprises a notch 45 in which the conductive wire 50 is buried. This embodiment is particularly suitable for low amplitude magnetic fields. The bonding material 70 may be distributed in the notch 45. The portion 52 of the conductive wire 50 resting on the bearing face 44 extends substantially rectilinearly along an axis perpendicular to the axis D of the tapered guide 40. Preferably, the length of the rectilinear portion 52 of the conductive wire 50 does not exceed half a wavelength of the phase velocity of the transverse waves, i.e. typically less than 0.5 mm at 1 MHz and 0.1 mm at 5 MHz. The embodiment illustrated in figure 2 allows the detection of the magnetic field B with the best spatial resolution compared to the embodiments illustrated in the figures 3 à 5 .

[0060] In the embodiment illustrated in figure 3 , the bearing surface 44 is flat and the diameter of the bearing surface 44 is equal to approximately 3 times the diameter of the conductive wire 50. The conductive wire 50 is laid flat on the bearing surface 44 and is protected by the bonding material 70.

[0061] In the embodiment illustrated in figure 4 , the diameter of the bearing surface 44 is equal to approximately 3 times the diameter of the conductive wire 50 and the bearing surface 44 comprises a notch 45 in which the conductive wire 50 is buried.

[0062] In the embodiment illustrated in figure 5 , the tapered guide 40 includes a through opening 46 in the tip 42 near the end of the tip 42, for example at a distance from the end of the tip 42 varying from one to two times the diameter of the conductive wire 50. The diameter of the through opening 46 is slightly greater than the diameter of the conductive wire 50, and the conductive wire 50 passes through the opening 46. The conductive wire 50 is rigidly coupled to the tapered guide 40 by the bonding material 70 or by compression and deformation of the end of the tip 42 which slightly pinches the conductive wire 50.

[0063] The embodiments illustrated on the figures 3, 4 , And 5 are advantageously more robust than the embodiment illustrated in figure 2 . The embodiments illustrated on the figures 3, 4 , And 5are particularly suitable for applications for which it is envisaged to be able to put the probe 20 in contact with a magnetic surface. In addition, the risks of crushing the conductive wire 50 in the embodiments illustrated in the figures 4 And 5 are reduced compared to the embodiments illustrated on the figures 2 et 3 .

[0064] The larger the diameter of the bearing surface 44, the less disturbance there is in the measurement when the tip 42 comes into contact with a hard surface. Indeed, the radiation impedance of the tip 42 drops considerably when approaching the end of the tip 42. Its blocking by a hard surface could limit the amplitude of the mechanical pulse generated by the Lorentz force which is applied to a small volume of material, laterally free. This could distort the measurement by reducing the amplitude of the signal. In addition, the contact of the tip 42 with a solid medium can generate ultrasonic waves in the medium which can return to the tip 42 and distort the measurement.

[0065] Advantageously, the lateral size of the tip 42 of the probe 20 is at least ten times smaller than the lateral size of the Hall effect probes currently available commercially.

[0066] According to one embodiment, the conductive wire 50 comprises at least one core made of an electrically conductive material, possibly surrounded by an electrically insulating sheath. According to one embodiment, the conductive material is chosen from copper, a material whose electron mobility is greater than 30 cm 2 < / V / s (in particular gold, silver, or a semiconductor material such as graphene), or a mixture of at least two of these compounds. The nature of the conductor making up the conductive wire 50 is important whether for direct or reciprocal operation when it is desired to increase the sensitivity of the probe 20 to low-intensity magnetic fields. By default, an enameled copper conductive wire 50 can be used for magnetic fields ranging from a few milliteslas to several tens of Tesla.But for applications requiring high sensitivity to low intensity magnetic fields, typically when they are comparable to or lower than the millitesla or the Earth's magnetic field, materials with higher electron mobility than copper (which is equal to 30 cm 2 < / V / s) can be used, for example gold (43 cm 2 < / V / s), silver (68 cm 2 < / V / s) or semiconductor materials such as graphene. It is also possible to choose a copper wire covered with a layer of graphene with an electron mobility of up to 200,000 cm 2 < / V / s. The electrical voltage that appears across the conductive wire 50 in reciprocal mode is thus increased by the higher electron mobility of the material constituting the conductive wire 50 and the probe 20 produces a usable signal and an improved signal-to-noise ratio at lower magnetic fields compared to the case where an enameled copper conductive wire 50 would simply have been used.

[0067] According to one embodiment, the tapered guide 40 is made of a non-magnetic solid material, in particular a material chosen from the group comprising glass, silicon, ceramic, non-magnetic metals (in particular aluminum, copper, or titanium), austenitic steel and non-magnetic metal alloys, in particular alloys based on aluminum, copper, and / or titanium.

[0068] The tapered guide 40 makes it possible, due to its tapered geometric shape, to advantageously create a thermal buffer between the measurement zone at the tip 42 and a reception zone at the electroacoustic transducer 60. The measurement zone can then be brought to a high temperature of several hundred degrees Celsius while the reception zone can be subjected to a lower temperature compatible with the temperature range tolerated by the electroacoustic transducer 60.

[0069] The bonding material 70 may be a cyanoacrylate resin, or an epoxy resin, or a polyimide resin, or a ceramic resin, or a molten glass, or an enamel coating the conductive wire 50. According to one embodiment, the bonding material 70 is suitable for withstanding high temperatures, for example up to 1000°C, or up to the lowest of the melting temperatures of the tip 42 of the tapered guide 40, and the melting temperature of the conductive wire 50. The bonding material 70 corresponds for example to the high-temperature glue marketed under the name Ceramabond by the company Aremco.

[0070] In the direct operating mode, the acoustic transducer 60 is configured to receive an acoustic wave and provide an analog electrical signal, for example a voltage or a current, hereinafter called a measurement signal. The amplitude of the measurement signal depends on the amplitude of the acoustic wave, preferably is proportional to the amplitude of the acoustic wave. In the direct operating mode, the acoustic transducer 60 receives a packet and provides an electrical measurement signal comprising at least one peak, and generally several positive and negative peaks. In the reciprocal operating mode, the acoustic transducer 60 is configured to receive an analog electrical signal, for example a voltage or a current, hereinafter called a control signal, and provide an acoustic wave. The amplitude of the acoustic wave depends on the amplitude of the control signal, preferably is proportional to the amplitude of the control signal.The acoustic transducer 60 may be a transverse wave acoustic transducer. According to one embodiment, the acoustic transducer 60 is a piezoelectric transducer or an electromagnetic acoustic transducer. The transducer 60 corresponds, for example, to the piezoelectric transducer marketed by the company Olympus under the name V153, which may have a central frequency around 1 MHz.

[0071] There figure 6 is a partial and schematic sectional view of another embodiment of the probe 20. The probe 20 shown in the figure 6 includes all the elements of the probe 20 represented in figure 1 and further comprises a straight acoustic waveguide 80 interposed between the tapered guide 40 and the electroacoustic transducer 60. The straight acoustic waveguide 80 has a constant cross-section and extends in a rectilinear manner and is hereinafter called the straight guide 80. According to one embodiment, the straight guide 80 extends along the axis D and comprises two opposite faces 81 and 82. The electroacoustic transducer 60 is fixed to the face 82 of the straight guide 80. According to one embodiment, the tapered guide 40 and the straight guide 80 are separate parts. The base 41 of the tapered guide 40 is then fixed to the face 81 of the straight guide 80. According to another embodiment, the tapered guide 40 and the straight guide 80 are manufactured from a single piece. The base 41 of the tapered guide 40 is then included in the face 81 of the straight guide 80. In the embodiment illustrated in figure 6 , the straight guide 80 has a cylindrical shape with a circular base and the face 81 of the straight guide 80 has the same diameter as the base 41 of the tapered guide 40.

[0072] The probe 20 further comprises a support 83 for the tapered guide 40 fixed to the straight guide 80. The support 83 is made of an electrically insulating material, for example plastic, in particular a polymer. The support 83 allows in particular the manipulation of the tapered guide 40. The conductive wire 50 can be fixed to the support 83. The probe 20 may further comprise an additional support 84 fixed to the tapered guide 40, for example halfway up the tapered guide 40 between the tip 42 and the base 41. The conductive wire 50 can be fixed to the additional support 84. The additional support 84 is made of an electrically insulating material, for example plastic, in particular a polymer.

[0073] There figure 7 is a partial and schematic sectional view of another embodiment of the probe 20. The probe 20 shown in the figure 7 includes all the elements of the probe 20 represented in figure 6 with the difference that the straight guide 80 comprises two successive cylindrical portions 85 and 86 with a circular base of different diameters. The face 81 of the straight guide 80 has the same diameter as the base 41 of the tapered guide 40 and the face 82 of the straight guide 80 has substantially the same diameter as the electroacoustic transducer 60. In the embodiment illustrated in figure 7 , the probe 20 further comprises a skirt 87 completely surrounding the side wall 43 of the tapered guide 40. In the case where the tapered guide 40 has a conical shape, the skirt 87 may correspond to a hollow truncated cone. A gap 88 is provided between the skirt 87 and the tapered guide 40 so that the skirt 87 is not in direct contact with the tapered guide 40. The skirt 87 may be held by the support 83. The skirt 87 may be made of a metallic material. The skirt 87 comprises an opening 89 through which the tip 42 of the tapered guide 40 and the portion 52 of the conductive wire 50 folded over the tip 42 emerge. The skirt 87 protects the tapered guide 40 and constitutes a shielding screen for the portions of the conductive wire 50 extending along the side wall 43 of the tapered guide 40.

[0074] There figure 8 represents a block diagram of the detection system 10 illustrating an embodiment of the control and acquisition device 30 for implementing the direct operating mode. According to one embodiment, the electroacoustic transducer 60 is a piezoelectric transducer.

[0075] The control and acquisition device 30 comprises in particular a control chain 31 and an acquisition chain 32. The control chain 31 comprises: a current pulse generator 33 in the conductive wire 50 connected to both ends of the conductive wire 50; and a control module 34 of the current pulse generator 33 receiving a synchronization signal Sync.

[0076] Acquisition chain 32 includes: a programmable amplifier 35 receiving the analog measurement signal S supplied by the electroacoustic transducer 60 and providing an amplified analog measurement signal S amp substantially equal to the measurement signal S multiplied by an amplification gain G; and a processing module 36 receiving the amplified measurement signal supplied by the amplifier 35.

[0077] The control and acquisition device 30 further comprises: a microcontroller 37 connected to the amplifier 35, to the processing module 36, to the control module 34, and to the generator 33; a computer 38 connected to the microcontroller 37; and a human-machine interface 39 connected to the microcontroller 37 and / or to the computer 38 and comprising in particular a display screen.

[0078] Alternatively, the control module 34 and / or the processing module 36 may be integrated into the microcontroller 37.

[0079] The control and acquisition device 30 illustrated in figure 8 is configured for the generation of a current pulse in the conductive wire 50 of intensity I and duration Δt and possibly occurring after the lapse of a delay T following a pulse of the synchronization signal Sync in the case of a synchronous measurement.

[0080] The computer 38 is configured to exchange signals with the microcontroller 37, for example via a UART port (English acronym for Universal asynchronous receiver-transmitter), in particular the values ​​of the delay T, the duration Δt, the intensity I, and the value of the gain G of the programmable amplifier 35.

[0081] According to one embodiment, the value of the gain G of the programmable amplifier 35 is determined from the amplitude B z of the magnetic field determined during the previous measurement. The lower the amplitude B z determined during the previous measurement, the greater the value of the gain G, for example according to steps corresponding to measurement ranges of the amplitude B z .

[0082] According to one embodiment, the triggering of the measurement process occurs from the synchronization signal Sync if the measurement is synchronous. According to another embodiment, the triggering of the measurement process occurs automatically and periodically with a measurement period defined by the user if the measurement is asynchronous. In this case, the measurement period is preferably greater than the damping time of the acoustic pulse propagating in the tapered guide 40 due to the previous current pulse.

[0083] According to one embodiment, the generator 33 applies a current pulse of intensity I in the conductive wire 50 coupled to the tip 42 of the tapered guide 40 with a delay T and a duration Δt defined in the control module 34. In the presence of the magnetic field B, an ultrasonic acoustic wave, the peak amplitude of which is proportional to the component B z , rises from the tip 42 towards the base 41 of the tapered guide 40. The acoustic wave is converted into an electrical measurement signal S by the acoustic transducer 60 coupled to the base 41, and the electrical measurement signal S is amplified by the programmable amplifier 35 to provide the amplified measurement signal S amp .

[0084] According to one embodiment, the processing module 36 is configured to detect the peak amplitude of the amplified measurement signal S amp and provide an analog value of the detected peak amplitude to the microcontroller 37. According to one embodiment, the processing module 36 is further configured to determine a binary value Polar I / O representative of the positive or negative sign of the first pulse of the amplified measurement signal S amp . Indeed, the phase of the acoustic wave changes by 180° depending on whether the polarization of the component B z is oriented in one direction or the opposite direction. The microcontroller 37 may further comprise an analog / digital converter adapted to receive the analog value of the detected peak amplitude and provide a digital signal of the peak amplitude.

[0085] According to one embodiment, the microcontroller 37 is configured to directly receive the amplified measurement signal S amp and perform sampling of the amplified measurement signal S amp , for example over a depth of 10 to 16 bits and at a rate of 5 to 12 mega samples per second over a time window of 1 µs to 100 µs. According to one embodiment, the microcontroller 37 is configured to perform interpolation of the measurement points so as to finely reconstruct the amplified measurement signal and obtain a precise value of the peak amplitude and the phase of the amplified measurement signal.

[0086] According to another embodiment, the microcontroller 37 is configured to determine the Fourier transform of the amplified measurement signal S amp . Preferably, the Fourier transform is determined from the time trace of the amplified measurement signal S amp including the peaks of the amplified measurement signal S amp and excluding the parasitic coupling of the current or voltage pulse so that only the peaks due to the reception of the acoustic wave packet by the transducer 60 with a zero measurement signal before its arrival and after its arrival remain in the time trace.

[0087] According to one embodiment, the amplified measurement signal S amp is processed so that its values ​​are zero after the fourth or fifth zero crossing of the amplified measurement signal S amp , which corresponds to the moment when the bulk of the acoustic wave packet has been received by the transducer 60 and the tail of the acoustic wave packet is entered. The spectral line of maximum amplitude of the spectrum obtained corresponds to the central frequency of the acoustic wave packet. The amplitude of the spectral line is representative of the maximum amplitude value of the amplified measurement signal S amp .

[0088] Determining the peak amplitude of the amplified measurement signal S amp from the Fourier transform of the amplified measurement signal S amp rather than directly from the amplified measurement signal S amp is more accurate and more independent of the analog noise existing in the amplified measurement signal S amp . This allows access to weak magnetic fields close to the Earth's magnetic field. In these extreme cases, the sensitivity is increased by replacing the current pulse with a pulse train, comprising for example 2 to 10 pulses and preferably 4 equidistant current pulses, with a carrier centered on the center frequency of the electromechanical transducer 60. Furthermore, advantageously, the Fourier transform of the amplified measurement signal S amp is independent of the propagation time of the acoustic waves in the tapered guide 40.The effect of a change in temperature of the tapered guide 40 is then reduced to an advance or delay in the arrival of the acoustic wave packet at the transducer 60, or simply a change in phase in Fourier space.

[0089] The microcontroller 37 is configured to determine the value of the field B z by multiplying the peak value by a calibration coefficient. The value of the magnetic field B z and its polarization (north, south) are displayed on the display 39 in a chosen unit of measurement or transmitted to the computer 38 for further processing.

[0090] According to one embodiment, the control and acquisition device 30 can drive one, two, or more than two probes 20 from the same synchronization signal Sync, each of the probes 20 being able to be excited with an identical or different delay T relative to the other probes 20 so that the spatial and temporal sampling can be multiplied by the number of probes 20 used. If several probes 20 are excited with slightly different delays T and arranged practically at the same location of a relatively spatially extended but very rapidly oscillating magnetic source, the number of magnetic field sampling points can be multiplied by the number of probes 20 used.

[0091] There figure 9 represents a block diagram of the detection system 10 illustrating an embodiment of the control and acquisition device 30 for implementing the reciprocal operating mode. The control and acquisition device 30 represented in figure 9 includes the same elements as the control and acquisition device 30 shown in figure 8 with the difference that the measurement signal S amp received by the amplifier 35 corresponds to the voltage at the terminals of the conductive wire 50 and that the current pulse generator 33 is replaced by a voltage pulse generator 33 supplied to the transducer 60.

[0092] There figure 10 is a partial and schematic sectional view of an embodiment of the electroacoustic transducer 60 of the probe 20. In figure 10 , the electroacoustic transducer 60 is a shear wave electroacoustic transducer comprising an acoustic waveguide 61, an electrically conductive wire 62, and a permanent magnet 63.

[0093] The acoustic waveguide 61 may have the same shape as the tapered guide 40, i.e. a tapered shape along an axis D' from a base 64 to an end 65, hereinafter called the tip, opposite the base 64. The acoustic waveguide 61 is hereinafter called the tapered guide 61. According to one embodiment, the tapered guide 61 has a general shape of a cone of revolution with axis D', possibly truncated, with a small angle at the apex. According to one embodiment, the conductive wire 62 is folded over the tip 65 of the tapered guide 61 and is fixed to the tip 65 by a bonding material 66. The conductive wire 62 forms a half-loop substantially coplanar with the half-loop formed by the conductive wire 50. The permanent magnet 63 is located close to the tip 65 of the tapered guide 61, preferably almost in contact with the tapered guide 61 without however touching it, for example at a distance of 100 µm for a magnet with a diameter equal to 2 mm or 3 mm.The permanent magnet 63 generates a reference magnetic field B r at the tip 65 of the tapered guide 61. In . figure 10 , the axis of revolution D of the tapered guide 40 and the axis of revolution D' of the tapered guide 61 are parallel, preferably coincident.

[0094] In figure 10 , the probe 20 further comprises the straight guide 80 comprising two opposite faces 81 and 82 and a support element 83 of the straight guide 80. In the embodiment illustrated in figure 10 , the straight guide 80 has a cylindrical shape with a circular base. According to one embodiment, the tapered guide 61, the straight guide 80 and the tapered guide 40 are separate pieces. The base 41 of the tapered guide 40 is then fixed to the face 81 of the straight guide 80 and the base 64 of the tapered guide 61 is fixed to the opposite face 82 of the straight guide 80. According to another embodiment, the tapered guide 61, the straight guide 80 and the tapered guide 40 are manufactured from a single piece. The base 41 of the tapered guide 40 is then included in the face 81 of the straight guide 80 and the base 64 of the tapered guide 61 is then included in the face 82 of the straight guide 80.

[0095] The operation of the electroacoustic transducer 60 shown in figure 10 is as follows: when the ultrasonic wave from the tapered guide 40, whether in the direct or reciprocal operating mode, arrives at the base 64 of the tapered guide 61, it propagates to the tip 65 of the tapered guide 61, amplifying as it approaches the tip 65. The bending mode imposed on the tip 65 of the tapered guide 61 generates, in the presence of the reference magnetic field B r , an electromotive force FEM in the conductive wire 62. This provides a low-cost method for detecting a quasi-punctual broadband ultrasonic wave according to the same principle as the method for generating the broadband ultrasonic wave used for measuring the field B z .

[0096] There figure 11 is a partial and schematic sectional view of an alternative embodiment of the probe 20 of the figure 10 . Probe 20 shown in figure 11 includes all the elements of the probe 20 represented in figure 10 and further comprises a curved acoustic waveguide 90 interposed between the tapered guide 40 and the straight guide 80. The curved acoustic waveguide 90 has a constant cross-section and extends in a curve. The curved acoustic waveguide 90 is referred to as the curved guide 90 hereinafter. Alternatively, the curved guide 90 is interposed between the straight guide 80 and the tapered guide 61. The tapered guide 40, the curved guide 90, the straight guide 80, and the tapered guide 61 may be separate pieces or may be manufactured from a single piece. The curved guide 90 may have a circular cross-section.

[0097] The curved guide 90 allows the axis of revolution D of the tapered guide 40 and the axis of revolution D' of the tapered guide 61 not to be parallel, which can facilitate the mounting of the probe 20 on a positioning arm. The condition for curvature of the curved guide 90 is that the radius of curvature is large compared to the diameter of the curved guide 90, typically at least 3 times larger, so that the propagation of the ultrasonic waves in the curved guide 90 is slightly disturbed.

[0098] There figure 12 is a partial and schematic top view of another embodiment of the system 10 for detecting a magnetic field and the figure 13 is a side view of the probe 20 of the system 10 for detecting a magnetic field of the figure 12 .

[0099] The system 10 for detecting a magnetic field shown in the figure 12 includes all the elements of the system 10 for detecting a magnetic field represented in figure 6 in which the tapered guide 40 has the shape of a right triangular prism and the straight guide 80 has the shape of a rectangular parallelepiped. The side wall 43 of the tapered guide 40 comprises two main faces 47 each corresponding for example to an isosceles triangle and sides 48 corresponding to rectangles. The tapered guide 40 can be in the materials described previously for the embodiment illustrated in figure 1 . In particular, in the case where the tapered guide 40 is made of a conductive material, an insulating layer, not shown, can be interposed between the tapered guide 40 and the conductive track 50. The tapered guide 40 can be produced on a glass wafer or a semiconductor material, for example silicon, with a thickness of the order of 100 µm and preferably less than 1000 µm using photoetching techniques. In the present embodiment, the conductive wire 50 corresponds to a conductive track deposited on a triangular main face 47 of the tapered guide 40, for example by cathode sputtering or electrochemical growth. The electrical connections are made by means of connection wires fixed to conductive pads 54 located for example on the straight guide 80. The conductive track 50 can have a width of the order of a micrometer at the tip 42 of the tapered guide 40. As shown in figure 12 , the conductive track 50 has a substantially constant width.

[0100] When the tapered guide 40 corresponds to a prism, the bending mode in the tapered waveguide 40 that appears in operation corresponds more precisely to the first antisymmetric Lamb mode A 0 .

[0101] There figure 14 is a partial and schematic top view of a variant of the probe 20 in which the conductive track 50 has a width which increases from the tip 42 to the base 41 of the tapered guide 40. This makes it possible to reduce the ohmic resistance of the conductive track 50.

[0102] According to one embodiment, the triangular face 47 of the tapered guide 40 is partially covered with an electrically conductive layer providing electrical shielding.

[0103] There figure 15 is a partial and schematic top view of an alternative embodiment of the probe 20 and the electroacoustic transducer 60 of the system 10 for detecting a magnetic field illustrated in figure 12 . The probe 20 and the electroacoustic transducer 60 shown in figure 15 include all the elements of the probe 20 and the electroacoustic transducer 60 shown in figure 10 except that the tapered guide 40, the straight guide 80, and the tapered guide 61 of the electroacoustic transducer 60 each have the shape of a prism. More specifically, the tapered guide 40 has the structure described previously in relation to the figures 12 et 13 . The electroacoustic transducer 60 is a shear wave electroacoustic transducer comprising a prismatic guide 61 having the same shape as the tapered guide 40. In addition, the conductive wire 62 corresponds to a conductive track deposited on a triangular face 67 of the tapered guide 61, for example by cathode sputtering or electrochemical growth. The tapered guide 40, the straight guide 80, and the tapered guide 61 can be manufactured from a single piece by cutting a wafer.

[0104] There figure 16 is a partial and schematic sectional view of an alternative embodiment of the probe 20 and the electroacoustic transducer 60 of the system 10 for detecting a magnetic field illustrated in figure 15 . The probe 20 and the electroacoustic transducer 60 shown in figure 16 include all the elements of the probe 20 and the electroacoustic transducer 60 shown in figure 11 except that the tapered guide 40, the curved guide 90, the straight guide 80, and the tapered guide 61 of the electroacoustic transducer 60 have prismatic structures. The tapered guide 40, the curved guide 90, the straight guide 80, and the tapered guide 61 can be manufactured from a single piece by cutting a wafer.

[0105] There figure 17 is a partial and schematic top view of another embodiment of the probe 20 and the figure 18 is a side view of probe 20 of the figure 17 .

[0106] The probe 20 comprises all of the elements of the probe 20 shown in figure 12 with the difference that the tapered guide 40 is made of an electrically conductive material, for example metal, and that the conductive wire 50 corresponds to an electrically conductive blade extending along a side 48 of the tapered guide 40 from the tip 42 of the tapered guide 40, at a distance from the side 48 of the tapered guide 40, and which is connected to the tapered guide 40 at the tip 42 of the tapered guide 40. According to one embodiment, the tapered guide 40, and the conductive wire 50 are manufactured from a single piece by cutting a metal plate. This makes it easier to finely mount / position the conductive wire 50 at the tip 42 of the tapered guide 40. In particular, the radius and shape of the conductive wire 50 at the tip 42 of the tapered guide 40 can be adapted so as to optimize the Lorentz force on a segment of material oriented perpendicular to the axial component B z .According to one embodiment, the conductive wire 50 is arranged in a "U" shape at the tip 42 of the tapered guide 40.

[0107] According to one embodiment, the electroacoustic transducer 60 comprises first and second piezoelectric plates 68, 69 fixed to the center of the straight guide 80 of the probe 20, for example by gluing or by soldering, the first piezoelectric plate 68 being fixed on a face 87 of the straight guide 80, coplanar with the triangular face 47 of the tapered guide 40 and the second piezoelectric plate 69 being fixed on the face 88 of the straight guide 80 opposite the face 87.

[0108] There figure 19 is a partial and schematic top view of another embodiment of the probe 20 and the figure 20 is a side view of probe 20 of the figure 19 .

[0109] Probe 20 shown in figure 19 includes all the elements of the probe 20 represented in figure 17 and further comprises a calibration device 100 comprising an acoustic waveguide 101, a conductive wire 102, and a permanent magnet 103 emitting a reference magnetic field B r . The acoustic waveguide 101 comprises a base 104 which is connected to the tapered guide 40 via the straight guide 80. The acoustic waveguide 101 may have the same structure as the tapered guide 40 and is referred to as the tapered guide 101 hereinafter. The conductive wire 102 may have the same structure as the conductive wire 50. This means that the tapered guide 101 is made of an electrically conductive material, for example metal, and that the conductive wire 102 corresponds to an electrically conductive blade extending along a flank 105 of the tapered guide 101 from the tip 106 of the tapered guide 101, at a distance from the flank 105 of the tapered guide 101, and which is connected to the tapered guide 101 at the tip 106 of the tapered guide 101.

[0110] The tapered guide 40, the conductive wire 50, the straight guide 80, the tapered guide 101, and the conductive wire 102 are made of an electrically conductive material, for example metal. According to one embodiment, the tapered guide 40, the conductive wire 50, the straight guide 80, the tapered guide 101, and the conductive wire 102 are manufactured from a single piece by cutting or chemically etching a metal wafer.

[0111] According to one embodiment, the probe 20 shown in the figures 19 et 20 forms a self-calibrated biconical probe. According to one embodiment, the probe 20 operates in the reciprocal operating mode described above. Ultrasonic signals are provided at the same time by the piezoelectric plates 68 and 69. According to one embodiment, the piezoelectric plates 68 and 69 are ferroelectric plates of polarization P with an axis perpendicular to the face 47. They are mounted opposite each other and vibrate in phase opposition when they are subjected to an electrical pulse, for example between 50 V and 500 V and preferably around 100 V. This generates an antisymmetric Lamb mode A0 starting from the center of the probe 20 both in the direction of the tip 42 of the tapered guide 40 and in the direction of the tip 104 of the tapered guide 101.

[0112] The tapered guide 101 associated with the known reference field B r makes it possible to automatically determine the field to be measured B z which is proportional to it with a proportionality component directly dependent on the ratio between the peak amplitude A cm of the ultrasonic measurement signal and the peak amplitude A cr of the reference ultrasonic signal. The tip 42 of the tapered guide 40 vibrates with a peak amplitude perpendicular to the face 47 of the tapered guide 40. This generates in the presence of the magnetic field B z the measurement electromotive force FEM m . The tip 106 of the tapered guide 101 vibrates with a peak amplitude perpendicular to the main face 107 of the tapered guide 101. This generates in the presence of the field Br the reference electromotive force FEM r .

[0113] The magnetic field B z to be measured is obtained by the following relation: B z = K A cm A cr B r where K is a correction factor.

[0114] The determination of the ratio of the peak amplitudes A cm and A cr can be carried out directly from the determination of each peak amplitude A cm and A cr . Alternatively, the Fourier transforms of the sampled measurement and reference signals can be determined and the ratio of the peak amplitudes A cm and A cr can be obtained by determining the ratio of the amplitudes of the spectral components of the spectra of the measurement and reference signals at a frequency corresponding to the case where the width of the piezoelectric plates 68 and 69 is equal to half the wavelength of the Lamb mode A0.

[0115] The intensity of the reference magnetic field B r , emitted by the permanent magnet 63 can vary from 10 mT to 500 mT. According to one embodiment, each tapered guide 40 and 101 and the straight guide 80 has a thickness of 0.05 mm to 0.5 mm, preferably of 0.05 mm to 0.3 mm, for example approximately 0.2 mm. According to one embodiment, each tapered guide 40 and 101 has an apex angle of less than 5° and the base of the triangular face 47, 67 has a width of less than 1 mm. For example, the biconical probe 20 may have a thickness equal to 0.1 mm, a total length, measured from the tip 42 of the tapered guide 40 to the tip 104 of the tapered guide 101, equal to 20 mm, a width equal to 1 mm, each tapered triangular guide 40 and 101 having a height of approximately 9.5 mm and each conductive wire 50 and 102 having a width equal to 100 µm and a thickness equal to 100 µm.

[0116] According to one embodiment, the working frequency is between 500 kHz and 10 MHz and preferably at a frequency corresponding to the case where the product of the frequency by the thickness of each tapered guide 40 and 101 is close to 1 MHz.mm. For example, for tapered guides 40 and 101 each having a thickness equal to 0.1 mm, the working frequency may be close to 10 MHz.

[0117] According to one embodiment, it can be determined that the tapered guide 40 is not damaged from monitoring the tip echo traveling from the tapered guide 40 to the tapered guide 101. If the tapered guide 40 is damaged, the signal will be modified. It is sufficient to monitor the amplitude of the echo from the tapered guide 40 with a delay equal to the total transit time from one end of the probe 20 to the other.

[0118] According to another embodiment, each tapered guide 40 and 101 and the straight guide 80 may be made of an insulating material, and, for example, manufactured by etching in an insulating plate. In this case, the conductive wires 50 and 102 may be produced by depositing a conductive ink or paste on the sides of the tapered guide 40 and 101.

[0119] According to one embodiment, two probes 20 can be arranged in a biaxial arrangement oriented at 90° to each other, or three probes 20 can be arranged in a triaxial arrangement oriented at 90° to each other, the probes being joined at the tips. This makes it possible to locally determine the 2D or 3D vector components of the magnetic field to be measured.

[0120] According to one embodiment, the tapered guide 40 and the tapered guide 101 have the same dimensions so that the measurement signals relating to the wave packets received from the conductive wire 50 and the conductive wire 102 are identical when the reference magnetic field Br is equal to the component B z to be measured. In the event of a difference greater than a certain tolerance, for example 3%, a correction factor K, close to unity and reflecting a difference in performance between the tapered guide 40 and the tapered guide 101, can be adjusted.

[0121] According to one embodiment, the system 10 for detecting a magnetic field is used as a reading head for magnetized surfaces. The permanent magnet 63 providing the reference magnetic field Br can become very small and encapsulated in a containment enclosure for the field lines Br. The value of Br can be lower or higher than the maximum conceivable amplitude of the field to be measured B z . It nevertheless has a sufficient value so that the signal / noise ratio allows a measurement of the reference field Br reproducible to better than 1% from one measurement to another or another predefined tolerance and between 0.1% and 5% for the system 10.

[0122] Examples of applications of the embodiments of the magnetic field measurement system described above relate in particular to the study of the electromagnetic compatibility of components, the characterization of inductive components or high-intensity inductive probes, in particular the spreading of field lines at a distance or electromagnetic leaks through slots or openings in a shield, or the characterization of the magnetic susceptibility of materials. Another example of application is the production of a reading head for producing magnetic field maps or reading data stored in magnetic form by detecting a binary local polarization or a field of given amplitude.

[0123] The embodiments of the magnetic field measurement system described above make it possible to sample a transient or oscillating magnetic field, synchronously, and very localized in space and also very localized in time via a very short sampling period. In particular, the spatial measurement resolution can be less than 0.1 mm, the sampling duration can be less than 10 ns. It is thus in particular possible to measure pulsed magnetic fields with a minimum duration of approximately 10 ns or variable magnetic fields whose maximum frequency is of the order of 100 MHz.

[0124] Tests were carried out. For the first, second, third, fourth, and fifth tests, the probe 20 was used to measure the static magnetic field emitted by a permanent magnet composed of an alloy of neodymium, iron, and boron. The permanent magnet is cylindrical in shape with a circular base of diameter equal to 3 mm placed almost in contact with the tip 42 of the tapered guide 40, the axis of the permanent magnet being aligned with the axis D of the tapered guide 40. For all tests, the electroacoustic transducer 60 is the piezoelectric transducer marketed by the company Olympus under the name V153 centered on 1 MHz. For all tests, the tapered guide 40 is made of aluminum and the conductive wire 50 is an enameled copper wire bonded with a cyanoacrylate resin to the bearing surface 44 of the tapered guide 40. For all tests, the amplification gain of the programmable amplifier is equal to 48 dB.

[0125] For the first and second tests, the probe 20 is used in the direct operating mode and the current pulse passing through the conductive wire 50 has a duration Δt equal to 270 ns and has an intensity I equal to 50 A.

[0126] In the first test, probe 20 shown on the figure 3 is used. The height of the tapered guide 40, measured along its axis of revolution D, is 82 mm and the diameter at the base 41 is 7 mm. The diameter of the bearing surface 44 is 0.6 mm. The angle at the top of the tapered guide 40 is 4.6 °. The conductive wire 50 has a diameter equal to 150 µm. The diameter of the bearing surface 44 is therefore equal to four times the diameter of the conductive wire 50. The magnetic field of the permanent magnet has an amplitude of 445 mT.

[0127] There figure 21 represents a curve of evolution C1 as a function of time of the amplified measurement signal S amp supplied by the amplifier 35 of the detection system 10 for the first test. The instant t0, not visible in figure 21 , corresponds to the sending of the synchronization signal Sync for the control of the current pulse generator 33, knowing that the duration between the rising edge of the synchronization signal Sync and the instant of sending the current pulse in the conductive wire 50 lasts from 1 ns to 100 ns and typically 6 ns. The peak amplitude is representative of the magnetic field B z along the axis D of the tapered guide 40.

[0128] In the second test, probe 20 shown on the figure 1 is used. The height of the tapered guide 40, measured along its axis of revolution, is 40 mm and the diameter at the base 41 is 5.5 mm. The angle at the apex of the tapered guide 40 is 7.8°. The lateral extent of the tip 42 is approximately 150 µm. The conductive wire 50 has a diameter equal to 40 µm. The magnetic field of the permanent magnet has an amplitude of 480 mT. Two measurements are made. The orientation of the magnet for the first measurement is the inverse of the orientation of the magnet for the second measurement.

[0129] There figure 22 represents evolution curves C2_1 and C2_2 as a function of time of the amplified measurement signal S amp supplied by the amplifier 35 of the detection system 10 for the second test. The instant t0 corresponds to the sending of the synchronization signal Sync for the control of the current pulse generator 33. The orientation of the magnet for curve C2_1 is the inverse of the orientation of the magnet for curve C2_2. It appears that depending on whether the north pole (curve C2_1) or the south pole (curve C2_1) of the permanent magnet is presented, the Lorentz magnetic force is reversed and the phase of the signal supplied by the electroacoustic transducer 60, with a central frequency equal to 769 kHz in the second test, flips 180°.The tapered guide 40 introduces in the second test a delay of approximately 13 µs sufficient to properly separate the parasitic reception signal corresponding to the instant of the current pulse from the reception signal of the ultrasonic wave whose amplitude is proportional to the magnetic field B z . In the second test, the end 42 of the tapered guide 40 being quasi-punctual, the Lorentz force is exerted on a volume which is also quasi-punctual and there is no acoustic end resonance which could interfere with the waveform. The transmission of the Lorentz force in the form of stress carried by a source of low mechanical radiation impedance is then more efficient and the output electrical signal is increased compared to the truncated tip of the tapered guide 40 of the first test, by a factor of 2.4 (400 mV peak compared to 170 mV peak), but, generally speaking, this factor can be greater than 10.

[0130] In the third and fourth tests, probe 20 and the permanent magnet from the second test are used. For the third and fourth tests, probe 20 is used in the reciprocal mode of operation. Electroacoustic transducer 60 is excited by a 60 V voltage pulse of 270 ns duration.

[0131] There figure 23 represents evolution curves C3_1 and C3_2 as a function of time of the amplified measurement signal S amp supplied by the amplifier 35 of the detection system 10 for the third test. The instant t0 corresponds to the sending of the synchronization signal Sync for the control of the current pulse generator 33. The orientation of the magnet for curve C3_1 is the inverse of the orientation of the magnet for curve C3_2. It appears that depending on whether the north pole (curve C3_1) or the south pole (curve C3_1) of the permanent magnet is presented, the Lorentz magnetic force is reversed and the phase of the electromotive force in the conductive wire 50 flips 180°.

[0132] There figure 24 represents evolution curves C4_1 and C4_2 as a function of time of the amplified measurement signal S amp supplied by the amplifier 35 of the detection system 10 for the fourth test. Curve C4_1 is obtained in the presence of the permanent magnet and curve C4_2 is obtained in the absence of the permanent magnet. In the presence of the permanent magnet, the output signal reaches 100 mV peak, i.e. between 3 and 4 times less than in the direct configuration. Curve C4_2 illustrates that no thermoacoustic effect is observed in the absence of a magnetic field B z in the reciprocal operating mode.

[0133] In the fifth test, probe 20 and the permanent magnet from the second test are used. For the fifth test, probe 20 is used in the direct operating mode.

[0134] There figure 25 represents evolution curves C5_1 and C5_2 as a function of time of the amplified measurement signal S amp supplied by the amplifier 35 of the detection system 10 for the fourth test. Curve C5_1 is obtained in the presence of the permanent magnet and curve C5_2 is obtained in the absence of the permanent magnet. As shown in figure C4_2, in the direct operating mode, a thermoacoustic signal is observed which is therefore characterized by the presence of an ultrasonic signal even when the magnetic field B z to be measured is zero. However, the thermoacoustic effect reaches 80 mVc while the magnetoacoustic signal in the presence of the magnetic field reaches 1.2 V, i.e. 15 times more than the thermoacoustic signal. The thermoacoustic signal therefore represents a noise of approximately 32 mT. By recording the thermoacoustic signal and subtracting it from the measurement signal, the thermoacoustic noise can be reduced to less than 1 mT.This improves sensitivity to weak fields. Thermoacoustic noise decreases as the diameter of the conductive wire 50 increases.

[0135] In the sixth test, a first measurement is carried out using the probe 20 and the permanent magnet of the second test and a second measurement is carried out using the probe 20 of the second test and an electromagnet as the source of the magnetic field B z to be measured. The electromagnet comprises nine turns of a conductive wire of diameter equal to 2 mm surrounding a ferrite core. For the sixth test, the probe 20 is used in the direct operating mode and the current pulse passing through the conductive wire 50 has a duration Δt equal to 400 ns and has an intensity I equal to 50 A. The magnetic field provided by the permanent magnet has an amplitude of 450 mT.

[0136] There figure 26 represents evolution curves C6_1 and C6_2 of the electrical signal supplied by the amplifier 35 of the detection system 10 for the sixth test. Curve C6_1 is obtained during a measurement of the magnetic field emitted by the permanent magnet. Curve C6_2 is obtained during a measurement of the magnetic field emitted by the electromagnet. The electromagnet is controlled for a pulse of duration equal to 400 ns. The current pulse in the conductive wire 50 is simultaneous with the current pulse in the electromagnet.

[0137] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art. For example, the embodiment of the probe 20 described in connection with the figure 6 comprising supports 83 and 84 can be implemented with the embodiments of the tapered guides 40 described in relation to the figures 2 à 5 . Similarly, the embodiment of the probe 20 described in relation to the figure 7 comprising a skirt 87 can be implemented with the embodiments of the tapered guides 40 described in relation to the figures 2 à 5 .

[0138] Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art from the functional indications given above.

Claims

1. A device (20) for detecting a magnetic field (B2) comprising: - a first tapered acoustic waveguide (40) having a first base (41) and a first tapered end (42); - a first electrically conductive wire (50) rigidly coupled to the first tapered end (42); and - an electroacoustic transducer (60) rigidly coupled to the first base (41).

2. The device according to claim 1, wherein the first tapered acoustic waveguide (40) extends along an axis (D) from the first base (41) to the first tapered end (42), the cross-section of the first tapered acoustic waveguide (40) decreasing from the first base (41) to the first tapered end (42), and wherein the first electrically conductive wire (50) comprises a portion (52) orthogonal to said axis (D) at the first tapered end (42).

3. The device according to claim 2, wherein the electroacoustic transducer (60) is a transverse-wave electroacoustic transducer oriented in the direction of said portion (52).

4. The device according to any one of claims 1 to 3, wherein the first tapered acoustic waveguide (40) is shaped like a cone or truncated cone.

5. The device according to claim 4, wherein the first tapered acoustic waveguide (40) has an apex angle of less than 15°.

6. The device according to claim 4 or 5, wherein the first tapered acoustic waveguide (40) is shaped like a truncated cone, the first tapered end (42) comprising a flat surface (44).

7. The device according to any one of claims 1 to 3, wherein the first tapered acoustic waveguide (40) is shaped like a prism with a triangular base.

8. The device according to any one of claims 1 to 7, wherein the electroacoustic transducer (60) is a piezoelectric transverse wave transducer or an electromagnetic transverse wave transducer.

9. The device according to any one of claims 1 to 8, comprising a support (84) of electrically insulating material surrounding the first tapered acoustic waveguide (40) midway between the first base (41) and the first tapered end (42).

10. The device according to any one of claims 1 to 9, comprising an electrically conductive shell (87) surrounding the first tapered acoustic waveguide (40), at a distance from the tapered acoustic waveguide (40), and comprising an opening (89) for the passage of the first tapered end (42).

11. The device according to any one of claims 1 to 10, wherein the electroacoustic transducer (60) comprises a second tapered acoustic waveguide (61) comprising a second base (61) and a second tapered end (65), a second electrically conductive wire (62) rigidly coupled to the second tapered end (65), and a first permanent magnet (63) facing the second tapered end (65).

12. The device according to claim 11, wherein the first tapered acoustic waveguide (40) and the second tapered acoustic waveguide (61) form a single-piece part.

13. The device according to any one of claims 1 to 10, further comprising a third tapered acoustic waveguide (101) rigidly coupled to the first tapered acoustic waveguide (40), the third tapered acoustic waveguide (101) comprising a third base (104) and a third tapered end (106), a third electrically conductive wire (102) rigidly coupled to the third tapered end (106), and a second permanent magnet (103) opposite the third tapered end (65), the electroacoustic transducer (60) being located between the first base (41) and the third base (104).

14. A magnetic field measurement system (10) comprising a device (20) for detecting a magnetic field according to any one of claims 1 to 13, and a control and acquisition device (30) connected to the device (20) for detecting a magnetic field comprising a generator (33) configured to supply at least one current pulse in the first electrically conductive wire (50) and an acquisition chain for detecting an electrical signal supplied by the electroacoustic transducer (60) or a generator (33) configured to supply at least one voltage pulse controlling the electroacoustic transducer (60), and an acquisition chain for detecting an electrical signal supplied by the first electrically conductive wire (50).

15. The system according to claim 14, wherein the magnetic field corresponds to a magnetic pulse, and wherein the control and acquisition device (30) is configured so that the generator (33) provides a current pulse in the first electrically conductive wire (50) synchronous with the magnetic pulse.