Magnetic-field sensor for measuring a current differential

Abstract

The invention relates to a magnetic-field sensor (15) for measuring a current differential, intended to be subjected to a magnetic field that is supposed to be measured and is induced by a current flowing through at least two primary conductors, at least one segment of the primary conductors extending along an axis (Z), said current flowing through at least one of the primary conductors (4a) in an outward direction along the axis (Z), and through at least one of the primary conductors (4b) in a return direction opposite the outward direction. This sensor is special in that it comprises: • - an inner magnetic circuit (16) configured to extend fully around the primary conductors, • - an outer magnetic circuit (17) enclosing the inner magnetic circuit (16), • - at least one coil (18) of superparamagnetic material SPM, which is arranged between the inner magnetic circuit (16) and the outer magnetic circuit (17).

Description

[0001] DESCRIPTION

[0002] Title: Magnetic field sensor for measuring a differential current

[0003] Technical field

[0004] The present invention relates to the field of contactless measurement of a differential electric current in a set of primary conductors whose sum of currents is zero in normal use, by means of the measurement of the circulation of the magnetic field induced by this differential current. The invention relates more particularly to a magnetic field circulation sensor integrating at least one coil based on a super-paramagnetic material, suitable for measuring direct current.

[0005] Prior art

[0006] In an electrical installation, a leakage current is a current that flows through an unintended path, with the exception of short circuits. Leakage current results from an anomaly in the installation and must be detected so that the problem can be resolved. In the following description, the terms "leakage current" and "differential current" are interchangeable and refer to the same current.

[0007] In order to produce a leakage current detector, a detector as illustrated in Figs. 1 and 2 is commonly used. On an electrical installation, a primary conductor 4 can be defined, in which a current from a primary current source 6 flows. The primary conductor 4 comprises, for example, a forward conductor 4a, located upstream, in the direction of the current, of an electrical load in which it is desired to be able to detect the leakage current, and a return conductor 4b, located downstream of said load. The current flowing through the forward conductor 4a is denoted I+, and the current flowing through the return conductor 4b is denoted !.. The current in the return conductor 4b flows in a direction opposite to the current in the forward conductor 4a. In the absence of leakage current, we therefore have 1+ = - !.. The leakage current IA is equal to 1+ + !..

[0008] The detector comprises a first assembly 1 composed of a first magnetic core 1a, around which a first coil 1b is wound, and a second assembly 2, composed of a second magnetic core 2a, around which a second coil 2b is wound. Around the first coil 1b and the second coil 2b, a feedback coil 3 is wound.

[0009] If we consider Ci a the closed contour formed by the first magnetic core 1a around the first coil 1b, the feedback coil 3 and the forward conductors 4a and return conductors 4b, according to Ampère's theorem, we can write:

[0010] With 1st Ampere turns the current flowing through the feedback winding 3, and Iib Ampere turns the current flowing through the first coil 1b.

[0011] We then seek to obtain a zero magnetic field on the contour Ci a , in order to obtain the following relationship: llb

[0012] To obtain this state, the detector comprises a current generator 7, configured to generate in the first coil 1b an excitation bringing the first magnetic core 1a into alternately positive and negative saturation, according to an excitation frequency E e . Due to the high magnetic permeability of the first magnetic core 1a, made for example of an Eer / Nickel alloy, at least of the order of a few thousand times the magnetic permeability of vacuum, a certain time is necessary to bring it into saturation. For this reason, the excitation frequency is limited to a few tens of Hertz. The aim is for the magnetic field on the contour Cia to be zero on average over an observation period equal to one or an integer number of excitation periods 1 / F e. In this situation, the positive and negative alternations of the signal Iib are symmetrical, so that Iib is zero on average over the observation period. We then obtain the following relationship: 1A 1st

[0013] To obtain this zero field state, at the output of the first coil 1b, the detector comprises a module 8. The module 8 is configured to analyze the current Iib flowing through the first coil 1b, and adjust the current I cr running through the feedback coil 3 in order to obtain this symmetry of the positive and negative alternatives of the current Iib, corresponding to a zero field in the first magnetic core la. The detector includes an error amplifier U1 and possibly a PI (proportional / integral) or PID (proportional / integral / derivative) type corrector in order to more precisely control the current I cr .

[0014] Once the zero field state is reached, the feedback current I is measured crby means of a voltage measurement using a first measuring resistor 9. A low-pass filter 11 is used, the cut-off frequency Fc of which is significantly lower than Fe, in order to suppress variations in the excitation signal.

[0015] It is thus possible to measure the DC component of the leakage current: , DC = — lcr,DC

[0016] The second set 2 is then used to measure the alternating component of the leakage current If, AC. If we consider C2a the closed contour formed by the second magnetic core 2a around the second coil 2b, the feedback coil 3 and the forward 4a and return 4b conductors, according to Ampère's theorem, we can write:

[0017] In a situation of zero field on the contour Cib, we have the following relation:

[0018] IA 1st ^2b

[0019] Due to the fact that the continuous components of IA and I crcompensate each other, and in the absence of excitation of the second coil 2b, the DC component of the current hb is zero and the signal of the AC component of hb, not interfered by a DC component which usually has a large amplitude, is usable and can be measured. Indeed, without this compensation of the DC component, the magnetic measuring circuit would be saturated, and the AC component unmeasurable.

[0020] Once the zero field state is reached, the alternating component of the current flowing through the second coil hb is measured by means of a voltage measurement using a second measuring resistor 10. A high-pass filter 12 is used, the cut-off frequency Fc of which is equal to the cut-off frequency of the low-pass filter 11.

[0021] The high-pass filter 12 allows the AC component of the current I to be removed from the signal. cr, which is at a frequency lower than the frequency Fc. It is thus possible to measure the alternating component of the leakage current: , AC = — 12b, AC

[0022] Finally the leakage current can be calculated in a sum module 13, and sent to an output 14:

[0023] 1A = 1A, DC + 1A, AC

[0024] Such a detector is relatively complex, bulky and heavy. If one only wishes to measure the DC component of the leakage current, one can make an identical sensor without the second assembly 2, but the sensor remains heavy and bulky and the bandwidth is then limited to a few tens of Hertz.

[0025] Document US 2012 / 038360 A1 discloses a current sensor comprising a superparamagnetic core that forms a closed circuit comprising a "U"-shaped core and a hoop, these two elements being able to be separated in order to be able to insert the conductor inside the magnetic circuit. This sensor is not intended for differential current measurement. In addition, this sensor, in order to operate, requires a core of large cross-section having a high concentration of SPM material, i.e. a large quantity of SPM material, which makes it a very expensive solution.

[0026] Documents US2013 / 234722 Al and US2023 / 135229 Al propose sensors for measuring differential current between two primary conductors, going and returning, in which a magnetic core surrounds a section of the primary conductors.

[0027] CN111157777 A proposes a similar sensor, in which two magnetic cores surround the primary conductors. These sensors do not solve the problems mentioned above.

[0028] Statement of the invention

[0029] The present invention aims to overcome these drawbacks by proposing a magnetic field sensor comprising a coil of super-paramagnetic material SPM surrounded by two high-permeability magnetic circuits, the magnetic field induced by the differential current being particularly visible by the SPM coil, the signal of the current flowing through the SPM coil making it possible to deduce the DC and AC components of the differential current. For this purpose, the invention relates to a magnetic field sensor for measuring a differential current intended to be subjected to a magnetic field to be measured induced by a current flowing through at least two primary conductors, at least a portion of said primary conductors extending along an axis, said current flowing through said at least one of said primary conductors in a forward direction along said axis, and at least one of said primary conductors in a return direction opposite to the forward direction.

[0030] This sensor is special in that it includes at least:

[0031] - an internal magnetic circuit configured to form a closed contour around said primary conductors,

[0032] - an external magnetic circuit configured to form a closed contour surrounding said internal magnetic circuit,

[0033] - at least one set of one or more coils of super-paramagnetic material SPM, arranged between said inner and outer magnetic circuits and forming a closed contour surrounding said inner magnetic circuit.

[0034] Thanks to these arrangements, the differential current can be detected by means of a sensor of small dimensions and weight, and of simple construction, the alternating component of the differential current being able to be detected up to several kHz. The present invention takes advantage of the interesting characteristics of the SPM coils with zero field circulation, mainly their absence of magnetic offset, while having a reduced size.

[0035] Said primary conductor may comprise a primary forward conductor and a primary return conductor, said current flowing through said primary forward conductor in a forward direction along said axis, and said primary return conductor in a return direction opposite to the forward direction, which is an embodiment particularly suitable for the sensor according to the invention. Said at least one assembly may comprise at least two substantially identical SPM coils connected in series, arranged between said inner and outer magnetic circuits, the assembly forming a closed contour surrounding said inner magnetic circuit, which has the advantage of increasing the signal / noise ratio by reducing the level of the parasitic excitation signal in the SPM signal.

[0036] Said sensor may comprise at least two substantially identical sets of at least one SPM coil, each set forming a closed contour surrounding said internal magnetic circuit, which makes it possible to increase the signal / noise ratio by reducing the influence of the leakage current components at the analysis frequency.

[0037] The dimension of the inner and upper magnetic circuits along the Z axis may be greater than the spacing between said magnetic circuits, which makes it possible to satisfactorily reduce interference from external magnetic fields on the detection of the magnetic field induced by the differential current.

[0038] Said sensor may comprise a transducer comprising said set of one or more SPM coils, said transducer being electrically coupled to an excitation module and to an analysis module, and:

[0039] - said transducer may comprise at least one pair of SPM coils, the SPM coils being substantially identical and being connected in series between two end terminals of the coils, the common connection point of the SPM coils being connected to a reference potential;

[0040] - the excitation module is configured to generate and inject into the transducer an excitation current le at a predefined excitation frequency Fe, and comprises at least:

[0041] . a center-tapped coil mounted in parallel across the transducer terminals;

[0042] . an excitation voltage generator mounted between the reference potential and said midpoint;

[0043] . an excitation impedance configured to form with the SPM coils a first series RLC type circuit with a resonant frequency Fres_e substantially equal to the excitation frequency Fe;

[0044] - the analysis module may include at least:

[0045] . an analysis impedance connected to the outer terminals of the mid-point coil, the excitation impedance being configured to form with the SPM coils a second series REC type circuit with an analysis resonance frequency Fres_a substantially equal to an analysis frequency Fa;

[0046] . a means for analyzing the current passing through the analysis impedance at the analysis frequency to extract a component at said analysis frequency Fa equal to an even multiple of the excitation frequency Fe.

[0047] Advantageously, the analysis frequency Fa is equal to 2.Fe.

[0048] According to one embodiment, the excitation impedance is connected between the excitation generator and the midpoint of the midpoint coil.

[0049] According to another embodiment, the excitation impedance is connected between the reference potential and the common connection point of the SPM coils.

[0050] According to one variant, the excitation impedance comprises at least one excitation capacitor.

[0051] According to another variant, the excitation impedance comprises at least one excitation capacitor and one excitation inductor, the excitation inductor being connected between the excitation capacitor and the excitation voltage generator.

[0052] In practice, the center-point coil can be made up of two substantially identical windings wound on the same magnetic core.

[0053] Advantageously, the analysis impedance can be constituted by an analysis capacitor and an analysis resistor connected in series between the two external terminals of the mid-tapped coil.

[0054] The analysis impedance may also include an analysis inductor connected in series with the analysis capacitor and the analysis resistor between the outer terminals of the center-tapped coil.

[0055] According to another embodiment, the magnetic field sensor may further comprise a transformer mounted between the mid-tap coil and the analysis impedance, said transformer being constituted by a primary winding and a secondary winding, the primary winding being connected to the outer terminals of the mid-tap coil, and the secondary winding being connected to the terminals of the analysis impedance.

[0056] According to another embodiment, the magnetic field sensor may further comprise a transformer comprising a primary winding formed from the mid-tapped coil and a secondary winding connected across the analysis impedance.

[0057] According to another embodiment, the SPM coils of the transducer form a first pair of SPM coils, and the sensor may further comprise a second pair of SPM coils substantially identical to said first pair of SPM coils. In this other embodiment, the SPM coils of the second pair are connected in series, the extreme terminal of the first pair of SPM coils is connected to one of the coils of the second pair, and the extreme terminal of the first pair of SPM coils is connected to the other coil of the second pair. Advantageously, the sensor may further comprise a feedback module formed of at least one feedback voltage generator configured to generate a feedback current.In the case of a two-coil SPM sensor, the feedback voltage generator is preferably mounted across the transducer's end terminals, and in the case of a four-coil SPM sensor, the feedback voltage generator is preferably mounted between the SPM coils of the second pair and configured to generate a feedback current.

[0058] Alternatively, the feedback voltage generator may be formed from two voltage sources referenced to the reference potential.

[0059] According to another embodiment, the magnetic field sensor may further comprise a calibration module configured to search for an optimal value Fopt of the excitation frequency Fe to be injected into the transducer.

[0060] Alternatively, the calibration module may be configured to:

[0061] - varying the excitation frequency Fe of the excitation generator in a frequency range [Fe_min; Fe_max] and measuring the corresponding excitation currents Ic; and

[0062] - identify the optimal excitation frequency Fopt_e corresponding to the excitation frequency Fe for which the excitation current level le is maximum.

[0063] For example, the calibration module can be configured to measure the excitation current across a resistor mounted between the reference potential and the common connection point of the transducer's SPM coils.

[0064] The calibration module can also be configured to measure the excitation current across the excitation capacitor when the excitation impedance is mounted between the reference potential and the common connection point of the two SPM coils of the transducer. In another variant, the calibration module can be configured to:

[0065] - vary the excitation frequency Fe of the excitation generator in a frequency range [Fe_min; Fe_max] and measure the sensitivity S of the excitation module;

[0066] - identify the optimal excitation frequency Fopt_e corresponding to the excitation frequency Fe for which said sensitivity S is maximum.

[0067] The magnetic field sensor may comprise at least one superparamagnetic material transducer SPM intended to be subjected to a magnetic field to be measured induced by a current passing through at least one of said primary conductors, said SPM transducer having a longitudinal central axis and two opposite free ends along the longitudinal central axis, the SPM transducer being formed of a feedback winding coupled to at least one SPM coil extending between the two free ends, said internal magnetic circuit having at least two plane surfaces parallel to each other and perpendicular to said longitudinal central axis, the two plane surfaces being positioned opposite the respective free ends of the SPM transducer; and in that said sensor is positioned relative to said primary conductors so that said axis is perpendicular to the longitudinal axis and parallel to said plane surfaces.

[0068] Thanks to these provisions, the sensor according to the invention makes it possible to measure both the primary current on at least one of the primary conductors, and the differential current between two primary conductors, making it possible to detect a leakage current, by a sensor of small dimensions and weight, and of relatively simple manufacture.

[0069] Said super-paramagnetic material SPM transducer may comprise:

[0070] - at least one additional SPM coil formed from a core of longitudinal axis based on SPM material around which at least one electrical conductor is wound along the longitudinal axis; and - at least one feedback winding; characterized in that the SPM transducer further comprises:

[0071] - a rigid body with a longitudinal central axis (X), and two flat faces at each of the opposite ends of the body in the direction of the longitudinal central axis, these two flat faces being substantially perpendicular to the longitudinal central axis;

[0072] - at least one support channel formed in the body and in which the SPM coil is housed, the support channel extending parallel to the longitudinal central axis (X) and opening onto the two flat faces; and in that the feedback winding is formed of an electrical conductor wound on the external surface of the body and along the longitudinal central axis.

[0073] These arrangements make it possible to take advantage of the interesting characteristics of zero-field circulation SPM transducers, mainly their absence of magnetic offset, while presenting a reduced footprint.

[0074] Brief description of the drawings

[0075] The present invention and its advantages will appear better in the following description of several embodiments given as non-limiting examples, with reference to the appended drawings, in which:

[0076] [Fig 1] Fig. 1 is a perspective view of a state-of-the-art leakage current sensor.

[0077] [Fig 2] Fig. 2 is a simplified electronic diagram of the sensor of Fig. 1.

[0078] [Fig 3] Fig. 3 is a curve showing the relationship between primary field H(A / m) and measured field Hmes(A / m) with an open-loop sensor based on a known SPM material,

[0079] [Fig 4] Fig. 4 is a perspective view of a sensor according to a first embodiment of the invention, [Fig 5] Fig. 5 is a perspective view of the sensor of Fig. 4, in which the external magnetic circuit is partially cut to make the SPM coils visible,

[0080] [Fig 6] Fig. 6 is a cross-sectional view of the sensor of Fig. 4,

[0081] [Fig 7] Fig. 7 is a longitudinal sectional view of the sensor of Fig. 4, [Fig 8] Fig. 8 is a cross-sectional view of the sensor of Fig. 4, with the magnetic field lines induced by the current flowing through the primary conductors, as they would be without the action of the inner and outer magnetic circuits,

[0082] [Fig. 9] Fig. 9 is a graph showing the variation of magnetic field illustrated in Fig. 8 along the contour defined by the SPM coil,

[0083] [Fig 10] Fig. 10 is a cross-sectional view of the sensor of Fig. 4, with the magnetic field lines induced by the current flowing through the primary conductors, as they are with the action of the inner and outer magnetic circuits,

[0084] [Fig. 11] Fig. 11 is a graph showing the variation of magnetic field illustrated in Fig. 10 along the contour defined by the SPM coil,

[0085] [Fig. 12] Fig. 12 is a simplified electronic diagram of the sensor according to one embodiment, in which the impedance Ze is formed by a capacitor Ce,

[0086] [Fig. 13] Fig. 13 is a simplified electronic diagram of the sensor according to another embodiment, in which an inductance Le is added to the excitation impedance Ze,

[0087] [Fig. 14] Fig. 14 is a simplified electronic diagram of the sensor according to another embodiment, in which an inductance La is added to the analysis impedance Za,

[0088] [Fig. 15] Fig. 15 is a simplified electronic diagram of the sensor according to another embodiment, in which a matching transformer Tl is added, [Fig. 16] Fig. 16 is a simplified electronic diagram of the sensor according to another embodiment, in which the functions of the transformer Tl and the mid-tapped coil of Fig. 15 are grouped in a single component T2, [Fig. 17] Fig. 17 is a simplified electronic diagram of the sensor according to another embodiment, in which the excitation impedance Ze is mounted between the common terminal of the transducer and the common potential,

[0089] [Fig. 18] Fig. 18 is a simplified electronic diagram of the sensor according to another embodiment, comprising two additional SPM coils,

[0090] [Fig. 19] Fig. 19 is a simplified electronic diagram of the sensor of Fig. 18 comprising a feedback circuit, according to another embodiment,

[0091] [Fig. 20] Fig. 20 is a simplified electronic diagram of the sensor of Fig. 18 comprising a feedback circuit, according to another embodiment,

[0092] [Fig 21] Fig. 21 is a perspective view of a sensor according to a second embodiment of the invention,

[0093] [Fig 22] Fig. 22 is a perspective view of the sensor of Fig. 21, in which the external magnetic circuit is shown in transparency to make the SPM coil visible,

[0094] [Fig 23] Fig. 23 is a cross-sectional view of the sensor of Fig. 21,

[0095] [Fig 24] Fig. 24 is a perspective view of an additional SPM transducer according to a particular embodiment of the invention,

[0096] [Fig 25] Fig. 25 is a sectional view of the additional SPM transducer of Fig. 24, along plane AA.

[0097] Description of the embodiments

[0098] In the illustrated embodiments, identical elements or parts bear the same reference numbers. Furthermore, terms that have a relative meaning, such as vertical, horizontal, right, left, front, rear, above, below, etc., must be interpreted under normal conditions of use of the invention, and as shown in the figures. The X, Y and Z axes are defined by an orthonormal reference frame illustrated in figures 4, 21 and 22. Furthermore, the geometric positions indicated in the description and the claims, such as “perpendicular”, “parallel”, “symmetrical” are not limited to the strict sense defined in geometry, but extend to geometric positions that are close, that is to say which accept a certain tolerance in the technical field considered, without influence on the result obtained.This tolerance is notably introduced by the adverb “sensiblement”, without this term necessarily being repeated before each adjective.

[0099] With reference to the figures, the magnetic field sensor 15 according to the invention, an exemplary embodiment of which is illustrated in FIGS. 4 to 7, is configured to measure a differential current via the magnetic field induced by a current flowing through at least two primary conductors 4a, 4b along an axis Z. The sum of the currents flowing through the primary conductors is zero during normal operation of these conductors.

[0100] The sum of the currents flowing through said primary conductors at said sensor is zero in normal use. The differential current is a leakage current, resulting from an anomaly, and it is for example of an intensity at least 100 times, and often 1000 or even 10000 times lower than the maximum intensity of the currents flowing through the primary conductors.

[0101] In the remainder of the description, embodiments are described in which the primary conductor comprises a forward conductor 4a, located upstream, in the direction of the current, of an electrical load in which it is desired to be able to detect the leakage current, and a return conductor 4b, located downstream of said electrical load. However, other configurations are possible within the scope of the present invention; for example, the differential current can be the algebraic sum of three, four, five or even six primary conductors without this affecting the operating principle of the sensor according to the invention.

[0102] The current flowing through the forward conductor 4a is denoted I+, and the current flowing through the return conductor 4b is denoted !.. At sensor 15, the current in the return conductor 4b flows in a direction opposite to the current in the forward conductor 4a. In the absence of leakage current, we therefore have 1+ = - !.. The leakage current IA is equal to 1+ + I.

[0103] The sensor according to the invention comprises an inner magnetic circuit 16, forming a closed contour along the X and Y axes around the primary conductors going 4a and returning 4b. The sensor 15 also comprises an outer magnetic circuit 17 surrounding the inner magnetic circuit 16. The inner 16 and outer 17 magnetic circuits preferably have a similar shape along the X and Y axes, so that the distance between the inner 16 and outer 17 magnetic circuits is substantially identical throughout the closed contours that they form around the primary conductor 4a, 4b. The dimensions of the inner 16 and outer 17 magnetic circuits along the Z axis are preferably substantially identical.

[0104] The sensor 15 according to the invention comprises at least one set of at least one super-paramagnetic coil SPM 18, arranged between the inner 16 and outer 17 magnetic circuits, so as to form a closed contour around the closed contour defined by the inner magnetic circuit 16, and inside the closed contour defined by the outer magnetic circuit 17.

[0105] Each set of SPM coils 18 therefore forms a closed contour around the contour formed by the internal magnetic circuit 16, each of these contours being formed from a single SPM coil 18 or from several SPM coils 18.

[0106] The closed contours formed by the inner 16 and outer 17 magnetic circuits, as well as by the assembly of one or more SPM coils 18, are preferably perpendicular to the Z axis, in order to limit the size of the sensor 15 as much as possible.

[0107] Advantageously, the set of SPM coils 18 may comprise two SPM coils 18, substantially identical, arranged between the inner 16 and outer 17 magnetic circuits, as illustrated in FIG. 7. The set of these SPM coils 18 then forms a closed contour around the inner magnetic circuit 16.

[0108] The sensor 15 preferably comprises at least two sets of SPM coils 18, the sets being substantially identical, that is to say that they each comprise the same number of substantially identical SPM coils 18.

[0109] In particular, current sensors are known that use SPM 18 coils, known as Néel® Effect, described for example in document FR2891917. The particularity of this type of sensor is based on the use of a transducer made up of coils whose cores are based on a composite loaded with nanoparticles exhibiting super-paramagnetic (SPM) properties.

[0110] Traditionally, such a superparamagnetic (SPM) transducer consists of a wire conductor wound around and along a flexible, elongated magnetic core. The winding along the magnetic core serves a dual function as an excitation and measurement coil. However, it is customary to implement suitable feedback means to maintain the magnetic field flow in the core at a substantially zero value. The excitation coil can be used to provide the feedback function, but it is also possible to provide a specific winding superimposed on the excitation winding to act as a feedback coil.

[0111] The use of this type of transducer has the advantage of not having any magnetic offset, since an SPM material has the particularity of being free of hysteresis.

[0112] However, the characteristics of SPM materials are such that it is necessary to make a compromise between measurement dynamics and sensitivity. Indeed, the magnetization M(H) of an SPM material follows a Langevin function. Figure 3 shows the relationship between primary field H(A / m) and measured field H m es(A / m) with an open-loop sensor based on a known SPM material. In the case illustrated in Figure 3, we see that the linearity range is very small, and that the relationship is not bijective, each measured field value being able to correspond to two primary field values. In the example illustrated in Figure 3, the linearity range, and therefore the measurement range, is limited to H max = 1100 A / m.

[0113] Thus, the use of SPM material with an Hma value Xlow, for example 1100 A / m, allows good sensitivity but the measurement range remains restricted, and is therefore not suitable for measuring field circulation with a large field variation along the measurement contour.

[0114] The use of materials with an Hma value X higher, for example 10 kA / m, to have a larger linearity range, will however decrease the sensitivity of the transducer, and is therefore not suitable for measuring small currents.

[0115] The inner magnetic circuit 16 makes it possible to greatly limit the fluctuations of the magnetic field near its outer face. Figs. 8 to 11 show this effect. Figure 8 illustrates the fluctuations of the magnetic field induced by the current flowing through the primary conductors going 4a and returning 4b in the absence of leakage current, and in the absence of the inner 16 and outer 17 magnetic circuits. Fig. 9 shows the variation in amplitude of the field on the closed contour defined by the assembly of one or more SPM coils 18. This variation is significant, between substantially -2000 and 2000 A / m. The magnetic field is substantially zero on average along this closed contour, but due to its very significant variation, it makes undetectable a possible magnetic field induced by a leakage current, the amplitude of which would be much lower.

[0116] Figures 10 and 11 correspond to figures 8 and 9, taking into account the action of the inner 16 and outer 17 magnetic circuits on the magnetic field induced by the current flowing through the primary conductors going 4a and returning 4b. As illustrated in fig. 10, the field lines, upon entering the inner magnetic circuit 16, tend to close by the shortest path, i.e. in the contour defined by the inner magnetic circuit 16. Figure 11 shows that the variation in amplitude of the field on the closed contour defined by the assembly of one or more SPM coils 18 is then very small, of the order of -5 to 5 A / m. This phenomenon is all the more important as the thickness of the inner magnetic circuit is high. A compromise must then be found with the size of the sensor as well as its weight, and the use of the materials which compose it.It should be noted that the external magnetic circuit 17 also participates in this phenomenon of homogenization of the magnetic field induced by the current flowing through the primary conductors 4a, 4b, at the level of the assembly of one or more SPM coils 18.

[0117] Advantageously, the action of the internal magnetic circuit 16 on the magnetic field induced by the primary currents 1+ and I. does not extend to the magnetic field induced by the leakage current IA. Indeed, the magnetic field induced by 1+ and I. in the absence of leakage current results from two currents of the same amplitude and circulating in opposite directions. Only the lines of this primary field, corresponding to back and forth movements of the same current, are captured to a large extent by the internal magnetic circuit 16, as illustrated in fig. 10. The magnetic field induced by the leakage current IA passes through the internal magnetic circuit 16 without being significantly modified, and is therefore clearly visible by the assembly of one or more SPM coils 18.

[0118] The external magnetic circuit 17 makes it possible to limit the interference resulting from external magnetic fields on the measurement of the differential current. These external fields also include the magnetic field induced by the primary currents 1+ and I. which circulate along the Z axis at levels other than the sensor 15. The internal magnetic circuit 16 also participates in this limitation of interference. This technical effect is all the more effective as the dimension of the external 17 and internal 16 magnetic circuits is large along the Z axis. Once again, a compromise must then be found with the size of the sensor as well as its weight, and the use of the materials which compose it.

[0119] In order to obtain as accurate a measurement as possible, it is advantageous to limit the spacing between the inner 16 and outer 17 magnetic circuits. This arrangement makes it possible to further reduce the influence of external magnetic fields on the assembly of one or more SPM coils 18.

[0120] The spacing between the inner 16 and outer 17 magnetic circuits is for example less than 8 mm, preferably between 3 and 6 mm. In a preferred embodiment of the invention, the spacing between the inner 16 and outer 17 magnetic circuits is less than their dimension along the Z axis.

[0121] This technical effect is also all the more effective as the spacing between the outer 17 and inner 16 magnetic circuits is small.

[0122] The inner 16 and outer 17 magnetic circuits have, for example, a thickness along the plane formed by the X and Y axes of between 1 and 3 mm, a depth along the Z axis of between 15 and 25 mm, and the spacing between the two magnetic circuits 16, 17 is, for example, between 2 and 4 mm. These examples are not limiting, these dimensions must of course be adapted to the geometry of the primary conductors going 4a and return 4b, and to the level of current flowing through them.

[0123] The material of the inner 16 and outer 17 magnetic circuits is preferably based on a material with high permeability and low coercive field (for example based on 80% EeNi). Indeed, the higher the permeability, the more the field induced by the leakage current is visible by the assembly of one or more SPM coils 18, and the less the field induced by the primary current (without leakage current) is visible by the assembly of one or more SPM coils 18.

[0124] The sensor 15 preferably comprises an excitation module, configured to send an ISPM current into the set of one or more SPM coils 18 in order to obtain a zero field on the closed contour formed by the set of one or more SPM coils 18. When the field is zero, the differential current IA can be deduced from the ISPM current:

[0125] In the remainder of the description, several particular architectures are described in order to produce the sensor 15 according to the invention, with the support of figures 12 to 20. However, these are only particular embodiments of the invention, and other architectures can be used without departing from the scope of the present invention.

[0126] In these embodiments, the sensor 15 comprises a transducer preferably formed of at least one pair of SPM coils 18 coupled to the excitation module and to a conditioner module or analysis module. The transducer is intended to be subjected to the magnetic field to be measured. In practice, the magnetic field to be measured includes frequencies ranging from DC to a frequency substantially lower than the excitation frequency, preferably at least ten times lower. The excitation module is configured to generate and inject into the transducer an excitation signal, for example in the form of a so-called “excitation” current of frequency corresponding to a predefined excitation frequency Fe. The conditioner module is configured to recover and analyze a raw SPM measurement signal, for example the electromotive force at the terminals of the transducer, representative of the temporal variation of the magnetic induction in the SPM coils 18.The analysis of this raw SPM signal consists in particular of the generation of an easily exploitable useful signal containing the useful information representative of the current to be measured. The analysis includes for example the elimination of the unwanted frequency components of the SPM signal to keep only the useful frequency component of the SPM signal.

[0127] A single excitation frequency Fe, in particular a high excitation frequency, for example of the order of several hundred kHz, can be used in the sensor 15, and the useful information relating to the current to be measured is therefore found in the even harmonics of the SPM measurement signal. Therefore, the analysis of the SPM signal can consist of isolating or extracting the frequency component located at an even multiple of the excitation frequency Fe. In other words, the analysis of the SPM signal must be carried out at an analysis frequency Fa equal to an even multiple of the excitation frequency Fe, preferably at the frequency 2.Fe for which the level of the SPM signal is maximum.

[0128] In practice, the injection of a single excitation frequency Fe into the SPM coils requires the use of a quality excitation signal in order to limit as much as possible the parasitic signals likely to be found in the raw SPM measurement signal and likely to complicate its processing or the extraction of useful information.

[0129] In particular, a parasitic signal at the excitation frequency Fe may appear at the reception chain of the conditioner module 101. It is however possible to reduce its level in a relatively simple manner without excessively affecting the response time of the system, because the difference between the analysis frequency Fa and the parasitic frequency is equal to Fe. A low to medium selectivity filter that does not excessively affect the bandwidth of the system could, for example, be used.

[0130] Furthermore, since the SPM coils 18 act as a frequency mixer, the presence of harmonic components in the excitation current can induce, in the SPM measurement signal, parasitic components of a non-negligible level likely to drown out the useful component. For example, with a single-frequency excitation field of 500A / m and a field to be measured of 1A / m, the useful component for measuring the super-paramagnetic effect (namely the second harmonic of the excitation frequency, i.e. at frequency 2.Fe), has an amplitude of the order of 20mA / m. If the excitation field has a second harmonic level of 0.01%, i.e. 50mA / m, the signal measured in the absence of the field to be measured corresponds to approximately 2.5A / m, therefore much higher than the useful component. However, obtaining a harmonic rate for rank 2 of 0.01% is particularly difficult and directly measuring a component representing 0.004% of the total signal is particularly delicate.

[0131] In practice, this second harmonic in the excitation current is very difficult to avoid.

[0132] For example, when generating an excitation current with pulse width modulation techniques, the second harmonic is necessarily present because of the differences in rise and fall times, but also because of the differences in output resistances between the high state and the low state.

[0133] With excitation current generation techniques based on signal synthesis by digital / analog converter and power amplifier, the sources of the second harmonic are found in the nonlinearities of the digital / analog converter and the amplifier. Active filtering of the signal from the digital / analog converter makes it possible to obtain a signal with a low second harmonic content if the output current is low. However, the required excitation current can be between a few tens and a few hundred mA. Commercially available amplifiers offering low distortion rates for these current levels and frequencies are rare and expensive, and the realization of a power stage with discrete components compatible with the required performance is complex and bulky.

[0134] In order to be able to use cheap commercial amplifiers or a pulse width modulation system, while guaranteeing a low harmonic 2 rate, a solution to improve the quality of the excitation signal injected into the SPM coils can consist of inserting an excitation impedance Ze in the excitation circuit or module.

[0135] The sensor according to the invention preferably comprises a transducer 103 comprising at least one pair of SPM coils 18a, 18b, the SPM coils 18 being substantially identical and being connected in series between two end terminals 132, 132 of the transducer. The SPM coils 18a, 18b of this pair are identified by the references LN1 and LN2 in FIGS. 12 to 20. The common connection point 130 of the SPM coils 18a, 18b is connected to a reference potential.

[0136] The excitation module 102 can be configured to generate and inject into the transducer 103 an excitation current Ie at a predefined excitation frequency Fe, and comprise at least:

[0137] - a coil Pl 1 / P12 with midpoint 120 mounted in parallel to the terminals of the transducer 103,

[0138] - an excitation voltage generator VEXC mounted between the reference potential and said midpoint 120, and

[0139] - an excitation impedance Ze configured to form with the SPM coils 18a, 18b a first series RLC type circuit of resonant frequency

[0140] Fres_e substantially equal to the excitation frequency Fe.

[0141] The analysis module 101 may comprise at least:

[0142] - an analysis impedance Za connected to the external terminals of the mid-point coil Pl 1 / P12, the excitation impedance being configured to form with the SPM coils 18a, 18b a second series RLC type circuit with an analysis resonance frequency Fres_a substantially equal to an analysis frequency Fa, and

[0143] - a means for analyzing the current passing through the analysis impedance Za at the analysis frequency Fa to extract a component at said analysis frequency Fa equal to an even multiple of the excitation frequency Fe.

[0144] Two-coil SPM embodiment, Ze = Ce

[0145] An electronic assembly of the sensor according to a first embodiment is illustrated in Figure 12, in which an excitation impedance Ze comprising a capacitor Ce, is added for the attenuation of the harmonic component at rank 2, i.e. at 2.Fe, in the excitation signal.

[0146] The sensor comprises a transducer 103 formed by a pair of SPM coils 18, identified respectively by LN1 and LN2 in FIG. 12, of inductance LN, connected in series between the two extreme terminals 131, 132 of the transducer. The common connection point 130 of the two coils LN1 and LN2 is connected to a reference potential, for example a ground. The coils LN1 and LN2 are intended to be subjected to an external magnetic field to be measured induced for example by a primary current source IP.

[0147] The sensor further comprises an excitation module 102 electrically coupled to the transducer 103 and configured to generate and inject into the transducer 103 an excitation current at a predefined excitation frequency Fe. The excitation module 102 comprises:

[0148] - a mid-point coil Pl 1-P12 consisting of two substantially identical windings wound on the same magnetic core;

[0149] - an excitation voltage generator VEXC at a frequency equal to the excitation frequency Fe;

[0150] - an excitation impedance Ze comprising a capacitor Ce.

[0151] The two outer terminals 121, 122 of the coil Pl 1-P12 are connected respectively to the two extreme terminals 131, 132, of the transducer 103, the capacitor Ce of the excitation impedance Ze is connected between the excitation generator VEXC and the midpoint 120 of the coil Pl 1-P12.

[0152] The coils LN1 and LN2 of the transducer 103 constitute a series resistance / inductance (RL) type load. Inserting the capacitor Ce between the mid-tapped coil Pl 1-P12 and the excitation voltage generator VEXC amounts to loading the latter with a series RLC type circuit characterized by its resonant frequency Eres_e, hereinafter referred to as the "excitation resonant frequency". The excitation resonant frequency Eres_e is defined by:

[0153] Thus, if the value of the capacitor Ce is such that the resonance frequency Eres_e is substantially equal to the excitation frequency Le (Eres_e = Le), this circuit will act as a bandpass filter on the excitation current and will attenuate the harmonic component of the excitation signal at frequency 2. Le. This attenuation depends on the quality factor Q of the circuit, given by the following expression:

[0154] Furthermore, the gain at 2.Eres_e is given by:

[0155] G ( 2 F -.) = 7?fe s HAS

[0156] The sensor further comprises an analysis module or conditioner 101 configured to recover and analyze a raw SPM measurement signal in order to eliminate unwanted frequency components of the SPM signal and keep only the useful frequency component of the SPM signal. The analysis module 101 comprises:

[0157] - an analysis impedance Za constituted by an analysis capacitor Ca and an analysis resistor Ra connected in series between the two external terminals 121, 122 of the mid-point coil Pl 1-P12;

[0158] - an analysis chain A configured to provide an SPM signal containing useful information to 2. The which can be used by an external control unit or integrated into the sensor, this analysis chain comprising for example amplification and filtering circuits.

[0159] The current flowing through the analysis impedance Za thus corresponds to the raw SPM measurement signal and the analysis of this current flowing through the analysis impedance Za at an analysis frequency Fa equal to an even multiple of the excitation frequency Fe, for example at an analysis frequency Fa equal to 2.Fe, makes it possible to isolate or extract a useful signal containing the information relating to the field or current to be measured.

[0160] As for the excitation circuit, the combination of coils LN1 and LN2 and impedance Za forms a series RLC type circuit with resonance frequency Fres_a, hereinafter called the “analysis resonance frequency”. This analysis resonance frequency is:

[0161] In practice, the analysis frequency Fa is advantageously substantially equal to the analysis resonance frequency Fres_a.

[0162] Two-coil SPM embodiment, Ze = Ce, Le

[0163] An electronic assembly of the sensor according to another embodiment is illustrated in Figure 13, in which the excitation impedance Ze comprises a capacitor Ce and an inductance Le.

[0164] It is shown above that the attenuation of the level of the second harmonic depends on the quality factor Q of the circuit. However, by construction, SPM coils generally have a relatively poor quality factor at the frequencies considered, of the order of 3 to 5, which would make it possible to achieve an attenuation (the inverse of the gain) of the order of 4 to 8. To improve this attenuation, it is possible to add to the assembly of figure 12, an excitation inductance Le (for example a coil) with a high quality factor, with an inductance value significantly higher than that of the SPM coils and with negligible resistance compared to that of the SPM coils.

[0165] The assembly of figure 13 thus corresponds to the assembly of figure 12 in which the excitation impedance Ze further comprises an excitation inductance Le connected between the excitation capacitor Ce and the excitation voltage generator VEXC, in order to improve the quality factor of the excitation to reduce the level of distortion at the frequency 2.fe.

[0166] In this configuration, the resonant frequency of the excitation circuit Fres_e is now: , the overall quality of the circuit is now close to that of the Le coil thus added. It is thus possible to achieve a quality factor of the order of 30 to 50, which leads to attenuations of the order of 45 to 75, ten times more than with the SPM coils alone.

[0167] Optionally, when the measurement of the excitation current is necessary or envisaged, an excitation current measurement resistor Rsh can be added between the reference potential and the common terminal 130 of the transducer 103. The measurement of the excitation current is particularly useful when it is desired to carry out a preliminary calibration step in order to search for the optimal value of the excitation frequency to be injected into the SPM coils for which the excitation current level is maximum. Such a calibration for the search for the optimal excitation frequency will be described in more detail below.

[0168] Two-coil SPM embodiment, Ze=Ce,Le and Za=Ca,La

[0169] The assembly of Figure 13 assumes a near-perfect matching of the SPM coils so that only the useful frequency 2.Fe appears in the signal detected at the receiving circuit, namely the current flowing through the impedance Za. However, this perfect matching can be difficult to obtain in practice, and the imbalance between the SPM coils induces at the output signal (namely the current flowing in the impedance Za) a component at the excitation frequency Fe which can be of amplitude much higher than the frequency of the useful signal, thus degrading the signal / noise ratio of the system.

[0170] The SPM effect in an SPM coil can be modeled by a voltage source whose level is proportional to the magnetic field in which the SPM coil is immersed and whose source impedance is made up of the SPM coil itself. Thus, if the receiving circuit (the impedance Za) consists of a resistor Ra and a capacitor Ca, the charging circuit of the SPM voltage source is a series RLC circuit. We are therefore again in the presence of a bandpass circuit whose resonance frequency will be positioned at the analysis frequency 2.Fe. The parasitic component at the frequency Fe will therefore be attenuated, the attenuation level being all the higher as the quality factor of this circuit is high. If we only use the capacitor, the quality factor is very low, since it is necessarily lower than that of the SPM coils due to the addition of the measurement resistor Ra which is generally higher than the resistance of the circuit made up of the SPM coils.We can therefore count on a quality factor of around 2 to 3.

[0171] Thus, in the same way as for the excitation circuit, it is possible to add a series inductance in the analysis impedance Za of the conditioning module to significantly increase the quality factor of the conditioning circuit.

[0172] The assembly of Figure 14 thus corresponds to the assembly of Figure 13 in which the analysis impedance Za also comprises an analysis inductance La. The inductance La, the capacitor Ca and the resistor Ra are thus connected in series between the external terminals 121, 122 of the mid-point coil Pl 1 / P12. In this configuration, the analysis resonant frequency Fres_a of the analysis circuit is:

[0173] Furthermore, if the analysis frequency Fa is set to 2.Fe and if the excitation frequency Fe has been predefined to maximize the excitation current level, via for example a calibration step mentioned above, the quality factor of the conditioning circuit (or receiving circuit) should preferably be limited so that, taking into account the tolerances on the components La and Ca, the attenuation remains reasonable for all the frequency values ​​conceivable for the excitation frequency Fe.

[0174] For example, to guarantee an attenuation lower than 3dB with a dispersion on the excitation frequency Fe of 3% and a dispersion on the resonance frequency of the analysis circuit Fres_a also of 3%, a limitation to a quality coefficient of order 8 can be considered.

[0175] The resonant frequency of the excitation can vary due to tolerances on the components (here + / -5% on the value of the series excitation coil). It is then necessary that the analysis frequency which is twice the excitation frequency is in the bandwidth of the reception filter.

[0176] Two-coil embodiment SPM, Ze, Za and transformer Tl

[0177] In the arrangement shown in Figure 15, a transformer Tl is inserted between the mid-tapped coil Pl 1-P12 and the analysis impedance Za, in order to adapt the impedance of the analysis circuit and increase the output level of the signal received by the impedance Za. The transformer Tl is constituted by a primary winding PI and a secondary winding SI. The primary winding PI is connected to the outer terminals 121, 122 of the mid-tapped coil Pl 1-P12, and the secondary winding SI is connected to the terminals of the analysis impedance Za.

[0178] The operation of this circuit is similar to that of Figure 14.

[0179] Two-coil embodiment SPM, Ze, Za and transformer T2

[0180] In the arrangement shown in Figure 16, a single transformer T2 is used to perform the functions of the mid-tapped coil Pl 1-P12 and the transformer T1 of the circuit of Figure 15. Thus, the primary winding of the transformer T2 which is connected to the terminals 131, 132 of the transducer is configured to perform the function of the mid-tapped coil of Figure 15 and the secondary winding which is connected to the terminals of the analysis impedance Za is configured to perform the matching function of the transformer T1 of the circuit of Figure 15.

[0181] Two-coil SPM embodiment, impedance position Ze

[0182] In the arrangement shown in Figure 17, unlike the arrangement in Figure 16, the excitation impedance Ze is connected between the common terminal 130 of the transducer and the reference potential.

[0183] The advantage of this configuration lies in particular in the possibility of measuring the excitation current at the terminals of the capacitor Ce for example, which makes it possible to eliminate the measurement resistance Rsh which tends to degrade the quality factor of the circuit, the measurements being generally referenced to the reference potential. The quality factor of the excitation is therefore improved and the common mode voltage at the midpoint of the coil Pl 1-P12 and the external terminals 131, 132 of the coils LN1, LN2 is reduced. The configurations illustrated in figures 12 to 17 implementing two SPM coils excited by a single excitation frequency, without a feedback circuit, are particularly suitable for measurements where the linearity of the response is not a very important parameter, in particular when the measurement is made in a little disturbed electromagnetic environment. This may be the case in the present context of the measurement of a leakage current.However, the sensitivity of such an architecture is roughly proportional to the temperature in kelvins. For example, an increase of 10K around 300K (27°C) produces a sensitivity variation of around 3%, which is very high. However, the operating range of such sensors often goes from -10°C to +55°C, or even from -25°C to +85°C. In the latter case, we can expect a sensitivity variation of around 20% compared to operation at room temperature. This temperature sensitivity limits the possible applications of these open-loop architectures.

[0184] Two-coil SPM embodiment, with VCR feedback

[0185] The configuration of Figure 17 above can be improved by the conventional addition of a VCR feedback circuit or module. Conventionally, the principle of feedback consists of adjusting the feedback so as to obtain a zero SPM measurement signal, and the value of the feedback is then directly proportional to the field to be measured. This configuration thus offers good linearity, and is particularly suitable for disturbed electromagnetic environments while maintaining a certain simplicity in its implementation.

[0186] Similarly to the assembly of Figure 19, the feedback can thus be achieved by adding to the assembly of Figure 17, a feedback voltage generator VCR at the extreme terminals 131, 132 of the transducer, a capacitor Ce between terminals 121, 131 and another capacitor Cd between terminals 122, 132. The feedback voltage generator VCR is configured to generate a feedback current, and the capacitors Ce and Cd, of the same value CCR, are configured to prevent the feedback current from flowing in the mid-tapped coil Pl 1-P12, so that the feedback current only flows through the SPM coils in the useful frequency range. Furthermore, it may also be advisable to isolate the feedback voltage generator VCR from the reference potential by adding, for example, a power supply transformer and an isolated amplifier.

[0187] The excitation resonance frequency Fres_e and the analysis resonance frequency Fres_a are of course modified by the presence of these capacitors Ce and Cd forming a resonant circuit.

[0188] So :

[0189] In a variant similar to the assembly of Figure 20, the feedback voltage generator VCR can be formed by two voltage sources VCR1 and VCR2 referenced to the reference potential. The two voltage sources VCR1 and VCR2 are configured to generate voltages of the same value VCR / 2 but of opposite polarity, and are connected to the SPM coils LN1 and LN2 through respective amplifiers A1 and A2. In addition, to suppress the common mode noise induced by these two voltage sources VCR1 and VCR2, a common mode inductance LMC with a value much higher than the inductance value of the SPM coils and the value of the excitation inductance Le can be added. This configuration makes it possible to avoid the use of an isolated power supply and an isolated amplifier, which reduces costs.

[0190] 4-coil SPM embodiment, without feedback

[0191] To improve the elimination of a signal at the analysis frequency Fa which could be present in the SPM signal of the 2-coil SPM transducer in the setups of the previous figures, and thus disturb the measurement, a second set of additional SPM coils LN3 and LN4 can be added.

[0192] For example, as illustrated in Figure 18, the assembly of Figure 16 can thus be modified by coupling the first pair, or first set, of coils LN 1 and LN2, to a second pair, or second set, of super-paramagnetic coils 18 LN3 and LN4 substantially identical to the coils of the first pair.

[0193] In particular, the coils LN3 and LN4 of the second pair are connected in series and the two pairs of coils are connected in parallel, so that the extreme terminal 131 of the first pair of coils is connected to the coil LN3 and the extreme terminal 132 of the first pair of coils is connected to the coil LN4.

[0194] In practice, the SPM coils LN3 and LN4 are subjected to substantially the same magnetic fields as the SPM coils LN1 and LN2.

[0195] Thus, at the analysis frequency Fa, the voltage induced by the field variation, excluding the SPM effect, in the first pair of coils LN1 and LN2 is substantially identical to the induced voltage 634 in the second pair of coils LN3 and LN4. In the assembly of Figure 18, the polarity points of each coil in the assembly of Figure 20 with respect to the primary field are such that the voltage between the extreme terminals 131 and 132 developed by the first pair of coils LN1 and LN2 is equal to the voltage ei2, and the voltage between the extreme terminals 131 and 132 developed by the pair LN3 and LN4 is equal to -634. The SPM coils LN1 to LN4 being of substantially identical impedance, the voltage between the extreme terminals 131 and 132 is then equal to 612 / 2-634 / 2, or approximately 0.Furthermore, since the coils LN3 and LN4 are not traversed by the excitation current Ic, they do not produce any SPM effect, and the useful SPM voltage (VSPM) produced in the first pair of coils LN1 and LN2 is therefore found between the extreme terminals 131 and 132 but divided by 2 compared to the configuration with a pair of SPM coils in figure 17.

[0196] Thus, although the sensitivity of the circuit is divided by 2, this configuration with four identical SPM coils immersed in the same magnetic field nevertheless makes it possible to eliminate the primary component present at frequency Fa.

[0197] Implementation: four SPM coils, with VCR feedback

[0198] As with the two-coil embodiment, it is also possible to add a VCR feedback circuit to the assembly of Figure 18, to obtain a sensor more suited to disturbed electromagnetic environments.

[0199] As illustrated in Figure 19, the feedback can thus be achieved by adding to the assembly of Figure 18, a feedback voltage generator VCR between the coils LN3 and LN4 of the second pair of SPM coils. The VCR generator is thus mounted between the extreme terminals 133 and 134, and is configured to generate a feedback current.

[0200] Capacitors Ce and Cd, of the same CCR value, positioned respectively on the output lines, namely between terminals 121 and 131, and between terminals 122 and 132, are configured to prevent the flow of the feedback current in the mid-point coil Pl 1-P12, so that the feedback current only flows through the SPM coils in the useful frequency range.

[0201] In practice, the VCR feedback generator has a low impedance at the analysis frequency Fa. Furthermore, to ensure that the excitation current only flows through the first pair of coils LN 1 and LN2, it may also be advisable to isolate the VCR feedback voltage generator from the reference potential by adding, for example, a power supply transformer and an isolated amplifier.

[0202] The resonant frequency of the excitation circuit Fres_e and the resonant frequency of the analysis circuit Fres_a are of course modified by the presence of these capacitors Ce and Cd forming a resonant circuit.

[0203] So :

[0204] Implementation: four SPM coils, with VCR1 and VCR2 feedback

[0205] The feedback function can also be obtained with two voltage sources VCR1 and VCR2 referenced to the reference potential, as shown in Figure 20. The two voltage sources VCR1 and VCR2 are configured to generate voltages of the same value VCR / 2 but of opposite polarity, and are connected to the SPM coils through respective amplifiers A1 and A2.

[0206] As in Figure 19, to avoid the use of expensive components related to the realization of a feedback voltage source isolated from the reference potential, two voltage generators VCR1 and VCR2 connected to the reference potential and a common mode inductance LMC inserted between the generators and the coils LN3 and LN4 are used. The common mode inductance LMC is of a value much higher than the inductance value of the coils SPM and the value of the excitation inductance Le (preferably ten times higher). The common mode impedance of the branch formed by the coils LN3 and LN4 is then significantly increased, thus forcing the excitation current to flow through the coils LN1 and LN2.

[0207] However, for coils LN3 and LN4 to perform their role, there must be a near short circuit between the external terminals of the transducer at the analysis frequency Fa. To achieve this short circuit, a CMD capacitor can be placed between the external terminals of the transducer, which will allow differential mode currents, i.e. those flowing in the same direction in coils LN1 to LN4, to flow correctly at the analysis frequency Fa.

[0208] Thus, in the assembly of figure 20:

[0209] - the common mode coil LMC prevents the flow of excitation current in the pair of coils LN3 and LN4, while ensuring a high bandwidth for the feedback loop; and

[0210] - the CMD capacitor ensures a short circuit of the pair of coils LN3 and LN4 at the analysis frequency Fa.

[0211] In the embodiments described above the feedback function is integrated into the SPM coils 18. It is also possible, in other embodiments, for the sensor 15 to comprise at least one feedback coil separate from the SPM coils 18. The feedback coil is then wound around the set of SPM coils 18, or if the sensor 15 comprises at least two sets of SPM coils 18 around the sets of SPM coils 18. An example of such an embodiment can be obtained by modifying the architecture illustrated in fig. 20 as follows:

[0212] - replacement of the CMD capacitor with a short circuit,

[0213] - removal of the common mode coil LMC,

[0214] - connection of the outputs of amplifiers A1 and A2 to the terminals of the feedback coil, the feedback coil being arranged around coils LN1, LN2, LN3 and LN4.

[0215] Thanks to the present invention, and in particular to the use of a high-sensitivity superparamagnetic material, the sensor 15 makes it possible to capture the DC and AC components of the differential current with a single SPM coil 18. Indeed, due to its very low magnetic permeability, the SPM coil 18 can be excited at a very high excitation frequency, and thus restore the AC component of the differential current up to several kHz.

[0216] As illustrated in Figures 21 to 23, the sensor 15 may further comprise an additional SPM transducer 19, intended to be subjected to a magnetic field to be measured induced by a current passing through at least one of said primary conductors 4a, 4b. The additional SPM transducer 19 makes it possible to measure the current passing through at least one of the primary conductors 4a, 4b.

[0217] These examples are not exhaustive, these dimensions must of course be adapted to the geometry of the primary conductors going 4a and returning 4b, and to the level of current flowing through them.

[0218] An exemplary embodiment of the additional SPM transducer 19 is illustrated in Figures 24 to 25. This embodiment is described below as an example, other types of SPM transducers being able to be used as additional transducer 19. This additional SPM transducer 19 is formed of a body with a longitudinal central axis X. The body can thus be in the form of a solid cylinder, for example made of rigid plastic-based material.

[0219] The body 210 thus has an external surface 200, an internal volume and two planar faces 201, 202 at each of its opposite ends in the direction of the longitudinal central axis X. The two planar faces 201, 202 extend substantially perpendicular to the longitudinal central axis X.

[0220] Separate support channels 203 are formed in the internal volume of the body 210, in practice a pair, for example four support channels. Each support channel 203 is configured to house an additional SPM coil 302. The support channels 203 extend parallel to the longitudinal central axis X and are arranged in the body 210, around the axis X, symmetrically to each other or not, each channel 203 opening at the two flat faces 201, 202 of the body 210.

[0221] Thus, the additional SPM coils 302 arranged in these channels 203 extend parallel to the longitudinal central axis X and also open at the two flat faces 201, 202 of the body. Each additional SPM coil 302 is notably formed of a flexible core 220 with a longitudinal axis based on SPM material around which at least one electrical conductor 221 is wound along the longitudinal axis of the core 220.

[0222] A feedback coil 303 formed of an electrical conductor is wound on the outer surface 200 of the body 210 along the longitudinal central axis X.

[0223] The flat faces 201, 202 may be in the form of a coil flange or disc with a diameter greater than that of the body 210. These two flat faces 201, 202 thus delimit with the external surface 200 of the body 210 an external volume in which the feedback winding 303 is arranged. In other words, the transducer is in the form of a can or tube for a cable reel, the feedback winding being held axially via the rims or flanges, and the SPM coils being housed in the tube.

[0224] The internal magnetic circuit 16 comprises two flat portions or surfaces 251, 252 positioned opposite the respective free ends 201, 202 of the additional SPM transducer 19, these free ends 201, 202 advantageously being flat faces. The height (along the Z axis) and the length (along the Y axis) of a flat surface 251, 252 of the internal magnetic circuit 16 are such that the flat surface 251, 252 covers at least the flat face of the free end of the additional SPM transducer 19. The size of the flat surface 251, 252 may result from a compromise between performance and volume of magnetic material impacting on the price, size and weight of the sensor 15. In practice, an electrical insulator is interposed between the different elements of the sensor 15, so that the internal magnetic circuit 15 is preferably bonded to the electrical insulator, to be as close as possible to the primary conductor and additional SPM transducer assembly 19.

[0225] The additional SPM transducer 19 consisting of additional rectilinear SPM coils 302 is inserted between the two primary forward and reverse conductors 4a, 4b so as to create an open measurement contour C connecting the two flat surfaces 251, 252 of the magnetic circuit 5. The open measurement contour C is here similar to a path passing at both ends of the SPM transducer and extending in the X axis.

[0226] In practice, the primary conductor 4a carries a current 1+ = Ip which can vary in normal use between values ​​of -I ma x and +I ma x, and the primary return conductor 4b carries a current I. = -I p. The feedback winding 303 carries an ICR current, and is formed of a winding of N turns wound around a hollow support containing additional conventional SPM coils 302 for measuring the circulation cir of a magnetic field H on the open contour C. The circulation cir is given by the equation: cir = ® H dl

[0227] I

[0228] The feedback current ICR is preferably continuously adjusted so as to obtain a circulation cir = 0, so that N. ICR = -Ip.

[0229] The SPM material is preferably a high sensitivity SPM material characterized by a maximum operating field Hmax significantly lower than the maximum field H pmax generated by the primary conductors.

[0230] The shape and dimensions of the magnetic circuit and the feedback coil are preferably adjusted so as to obtain fluctuations of the magnetic field along the open contour C lower than the maximum operating field H max , when cir = 0 and l p =lp max-

[0231] For example, the open contour C is preferably of reduced length, for example of the order of 50 mm. The two flat surfaces 251, 252 of the internal magnetic circuit 16 can thus be considered as being approximately 50 mm apart along the X axis.

[0232] If we consider a closed contour C consisting of a part of the open contour C and a portion of contour C m has g connecting the two flat surfaces 251, 252 and surrounding the primary conductor going 4a or returning 4b, then Ampère's theorem leads to: with H the magnetic field, and l p the current flowing in the primary conductor considered.

[0233] If the relative permeability p r and / or the thickness of the magnetic material is sufficiently high, we can neglect the second term, so as to only consider:

[0234] Furthermore, if the feedback winding surrounding the SPM coils carries a current generating ICR Ampere turns, then:

[0235] When the circulation sensor, i.e. the SPM transducer, provides a zero value, we therefore have: ICR = -Ip.

[0236] The feedback winding must therefore be capable of generating 500A.t for a primary current of 500A, and the average filling rate of a winding is around K r = 0.6. If we limit ourselves to a current density of J ma x = 5 A / mm 2 , it will therefore be preferable to have a minimum Ssobine winding section of:

[0237] 500 ç Jpmax

[0238] •-'Coil = 167 mm 2rJmax 5 x 0.6

[0239] This means a winding thickness of between 3 and 4 mm.

[0240] In the embodiment illustrated in Figures 21 to 23, the inner 16 and outer 7 magnetic circuits take a particular shape whose function is to form closed contours comprising flat surfaces, with respect to the flat surfaces of the additional SPM transducer 19, and taking up as little space as possible in order to surround the primary conductors 4a, 4b. Other geometries are of course possible, the shapes and dimensions of course having to be adapted to the geometry of the primary conductors going 4a and return 4b, and to the level of current flowing through them.

[0241] In some applications, it may also be of interest, particularly for protection purposes, to be able to measure overload currents greater than the maximum current that can be compensated by the feedback current. For this purpose, it is possible to place a Hall effect sensor between the two primary conductors 4a, 4b. This Hall effect sensor will advantageously be supported by a printed circuit configured to connect the additional SPM transducer 19.

[0242] Finally, short-circuit currents can be measured by Rogowski coil or winding type devices. Thus, advantageously, the sensor 15 can integrate one or more Rogowski coils, each placed around one of the primary conductors 4a, 4b. These Rogowski coils can advantageously be integrated into the printed circuit. By way of example, the Rogowski coil in the form of a printed circuit described in document EP 3 268 754 and / or the Hall effect sensor described in document FR 2 947 060 can be implemented in the sensor of the invention, without these examples being limiting since any type of commercial sensor can be suitable.

[0243] The present invention is of course not limited to the embodiments described but extends to any modification and variant obvious to a person skilled in the art within the limits of the appended claims. Furthermore, the technical characteristics of the different embodiments and variants mentioned above may be, in whole or in some cases, combined with each other.

Claims

Claims

1. Magnetic field sensor (15) for measuring a differential current intended to be subjected to a magnetic field to be measured induced by a current flowing through at least two primary conductors, at least a portion of said primary conductors extending along an axis (Z), said current flowing through at least one of said primary conductors (4a) in a forward direction along said axis (Z), and at least one of said primary conductors (4b) in a return direction opposite to the forward direction, said magnetic field sensor (15) comprising at least: - an internal magnetic circuit (16) configured to form a closed contour around said primary conductors, - an outer magnetic circuit (17) configured to form a closed contour surrounding said inner magnetic circuit (16), characterized in that at least one set of one or more coils of super-paramagnetic material SPM (18), arranged between said inner (16) and outer (17) magnetic circuits and forming a closed contour surrounding said inner magnetic circuit (16).

2. Magnetic field sensor according to claim 1, characterized in that said primary conductor comprises a primary forward conductor (4a) and a primary return conductor (4b), said current flowing through said primary forward conductor (4a) in a forward direction along said axis (Z), and said primary return conductor (4b) in a return direction opposite to the forward direction.

3. Magnetic field sensor according to any one of claims 1 to 2, characterized in that said at least one assembly comprises at least two substantially identical SPM coils (18) connected in series, arranged between said inner (16) and outer (17) magnetic circuits, the assembly forming a closed contour surrounding said inner magnetic circuit (16).

4. A magnetic field sensor according to any one of claims 1 to 3, characterized in that said sensor comprises at least two substantially identical sets of at least one SPM coil (18), each set forming a closed contour surrounding said inner magnetic circuit (16).

5. Magnetic field sensor according to any one of claims 1 to 4, characterized in that the dimensions of the inner (16) and upper (17) magnetic circuits along the axis (Z) are greater than the spacing between said inner (16) and upper (17) magnetic circuits.

6. Magnetic field sensor according to any one of claims 1 to 5, characterized in that W comprises a transducer (101) comprising said set of one or more SPM coils (18), said transducer being electrically coupled to an excitation module (102) and to an analysis module (101), and in that: - said transducer (101) comprises at least one pair of SPM coils (LN1, LN2), the SPM coils being substantially identical and being connected in series between two end terminals of coils (131, 132), the common connection point of the SPM coils (LN1, LN2) (130) being connected to a reference potential; - the excitation module (102) is configured to generate and inject into the transducer (101) an excitation current Ie at a predefined excitation frequency Fe, and comprises at least: . a coil (Pl 1 / P12) with a midpoint (120) mounted in parallel to the terminals of the transducer (102); . an excitation voltage generator (VEXC) mounted between the reference potential and said midpoint (120); . an excitation impedance (Ze) configured to form with the SPM coils (LN1, LN2) a first series RLC type circuit with resonance frequency Fres_e substantially equal to the excitation frequency Fe; - the analysis module (101) comprising at least: . an analysis impedance (Za) connected to the outer terminals of the mid-point coil (Pl 1 / P12), the excitation impedance (Ze) being configured to form with the SPM coils (LN1, LN2) a second series RLC type circuit with an analysis resonance frequency Fres_a substantially equal to an analysis frequency Fa; . a means for analyzing the current (A) passing through the analysis impedance (Za) at the analysis frequency (Fa) to extract a component at said analysis frequency Fa equal to an even multiple of the excitation frequency Fe.

7. Magnetic field sensor according to claim 6, characterized in that the SPM coils (LN1, LN2) of the transducer form a first pair of SPM coils, and in that the sensor further comprises a second pair of SPM coils (LN3, LN4) substantially identical to said first pair of SPM coils (LN1, LN2), the SPM coils (LN3, LN4) of the second pair being connected in series, the extreme terminal (131) of the first pair of SPM coils (LN1, LN2) being connected to one of the coils (LN3) of the second pair and the extreme terminal (132) of the first pair of SPM coils (LN1, LN2) being connected to the other coil (LN4) of the second pair.

8. Magnetic field sensor according to any one of claims 1 to 7, characterized in that it comprises at least one additional SPM superparamagnetic material transducer (19) intended to be subjected to a magnetic field to be measured induced by a current passing through at least one of said primary conductors (4a, 4b), said additional SPM transducer (19) having a longitudinal central axis (X) and two free ends (201, 202) opposite the longitudinal central axis (X), the additional SPM transducer (19) being formed by a feedback winding (303) coupled to at least one additional SPM coil (302) extending between the two free ends (201, 202), said internal magnetic circuit (16) having at least two flat surfaces (251, 252) parallel to each other and perpendicular to said longitudinal central axis (X), the two flat surfaces (251, 252) being positioned opposite the respective free ends (201, 202) of the additional SPM transducer (19); and in that said sensor (15) is positioned relative to said primary conductors (4a, 4b) so that said axis (Z) is perpendicular to the longitudinal axis (X) and parallel to said flat surfaces (251, 252).

9. Magnetic field sensor according to claim 8, characterized in that said additional superparamagnetic material SPM transducer (19) comprises: - at least one additional SPM coil (302) formed of a core (220) of longitudinal axis based on SPM material around which at least one electrical conductor (221) is wound along the longitudinal axis; and - at least one feedback winding (303); characterized in that the additional SPM transducer (19) further comprises: - a rigid body (210) with a longitudinal central axis (X), and two flat faces (201, 202) at each of the opposite ends of the body (210) in the direction of the longitudinal central axis (X), these two flat faces (201, 202) being substantially perpendicular to the longitudinal central axis (X); - at least one support channel (203) formed in the body (210) and in which the additional SPM coil (302) is housed, the support channel (203) extending parallel to the longitudinal central axis (X) and opening onto the two flat faces (201, 202); and in that the feedback winding (303) is formed of an electrical conductor wound on the external surface (200) of the body (210) and along the longitudinal central axis (X).

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