Antenna structure for a motion metal detector with non-coplanar induction-balance coils

Abstract

The invention relates to an antenna structure for a motion metal detector comprising an antenna body, at least one receiver coil and a transmitter coil. One of the main aims of this invention is to provide an antenna structure capable of maintaining a stable induction balance while being less sensitive to impacts inherent in field use. For this purpose, the inventor proposes an innovative configuration of the coils, which are arranged with respect to a reference plane so as to receive or emit opposing signals in a central portion (113). In particular, the arrangement of the central portion of the antenna structure is designed such that, in response to an impact, signals received or emitted in the central portion (113) tend to cancel each other out while maintaining the induction balance.

Description

ANTENNA STRUCTURE FOR MOTION METAL DETECTOR WITH NON-COPLANAR COILS AND INDUCTION BALANCE

[0001] The invention relates to the field of antenna structures for motion metal detectors used for searching for metallic objects buried in the ground.

[0002] In particular, it concerns the antenna structures used in these detectors. Similar detectors are known from documents WO2017015730A1 and EP0654685.

[0003] Motion-activated metal detectors are widely used in various applications, including searching for buried metal objects, archaeology, security, and recreation.

[0004] These detectors rely on the principle of electromagnetic induction to detect the presence of metals in the soil or in other media.

[0005] It is known that motion and metal discrimination metal detectors generally use antenna structures that include transmit and receive coils to detect metallic objects in the ground.

[0006] These antenna structures are designed to be held and maneuvered by a user when searching for targets.

[0007] When the user moves the detector, shocks to the sensing disc can cause relative displacements between the transmitting and receiving coils.

[0008] These movements, even minimal ones, disrupt detection and generate spurious signals, thus reducing the accuracy and reliability of the detector.

[0009] Usually, these transmitting and receiving coils are positioned in induction balance, that is to say that the receiving coil overlaps the transmitting coil so that the electromagnetic signal received by the receiving coil from the transmitting coil is zero or substantially close to zero.

[0010] Traditionally, some of these transmitting and receiving coils are arranged non-coplanarly, with one coil inclined relative to the other. This arrangement helps compensate for the effects of shocks and vibrations on the detector, although this method has limitations in terms of efficiency.

[0011] An alternative configuration involves arranging the transmitting and receiving coils coplanarly or close to coplanarly in an antenna body, usually in a plane parallel to the ground, to compensate for shocks, but this also presents constraints in terms of efficiency.

[0012] This coplanar configuration helps to limit the sensitivity of the coils to deformations during shocks or movements of the coils, because the electromagnetic field generated by the transmitting coil varies little in its plane.

[0013] However, existing antenna structures are generally not designed to adequately compensate for shock effects, which can lead to false detections or reduced performance under harsh operating conditions.

[0014] Thus, there is a need for a motion metal detector antenna structure that can maintain a stable induction balance while being less sensitive to the shocks inherent in its use in the field.

[0015] The invention aims to solve, at least partially, this need.

[0016] In practice, the invention relates to an antenna structure for a motion metal detector.

[0017] In particular, the antenna structure includes: - an antenna body designed with respect to a user holding and maneuvering the motion metal detector to detect a target in the ground; - at least one receiving coil and at least one transmitting coil; and - a reference plane parallel to the ground that horizontally cuts the antenna body into an upper and a lower portion.

[0018] More specifically, the antenna body comprises: - lateral parts, - front-to-rear parts, and - a central part which is located between the lateral parts and the front-to-rear parts.

[0019] In addition, the transmitting coil and the receiving coil are arranged in the antenna body, each comprise multiple turns of conductors, partially overlap in the central part, and are positioned to create an induction balance.

[0020] Furthermore, in a section perpendicular to the reference plane that intersects the central portion where the transmitting and receiving coils partially overlap, called the overlap section, the central portion is arranged such that at least one of the coils, either the transmitting coil or the receiving coil, is configured non-coplanarly with respect to the reference plane, with positions at different heights relative to the reference plane, such that at least one coil is above or below the reference plane, creating at least one vertical offset, D, between the coils. The vertical offsets, D, are designed to allow the central portion to receive or transmit signals in opposite directions, so that, in response to a shock that causes a displacement substantially in the same direction of all or part of the receiving coil and / or the transmitting coil,The signals received or emitted in the central part tend to cancel each other out, while maintaining the induction balance.

[0021] In a first variant of the invention, the perimeter of the central part corresponds to at least 20% of the total perimeter of the antenna body.

[0022] In a second embodiment of the invention, in the central part, the transmitting coil and the receiving coil are both configured in a non-coplanar manner with respect to the reference plane, and the transmitting coil and the receiving coil are oriented in a complementary manner, so that when the receiving coil extends above the reference plane, the transmitting coil extends below the reference plane, and vice versa.

[0023] In a third variant of the invention, the vertical offsets, D, are distributed uniformly along the central part or variably distributed along the central part.

[0024] In a fourth variant of the invention, the antenna structure further includes mechanical stiffening elements arranged in the central part which are designed to maintain the vertical offsets, D, between the coils when shocks are applied.

[0025] In a fifth variant of the invention, the transmitting and receiving coils are arranged according to a three-dimensional configuration chosen from: - an angular offset between the principal planes of the coils, - a variation in the spacing between the coils along their perimeter, and - a relative inclination of the coils with respect to the reference plane.

[0026] Other features and advantages of the invention will be better understood from the description that follows and with reference to the attached drawings, given for illustrative purposes only and not for limitation.

[0027] Lare represents a first perspective view of the antenna structure according to the invention.

[0028] Lare represents a second perspective view of the antenna structure according to the invention.

[0029] Lare represents a third perspective view of the antenna structure according to the invention.

[0030] Figure 1 represents a top view of the antenna structure according to the invention.

[0031] Lare represents a first view of the overlap section according to the invention.

[0032] Lare represents a second view of the overlap section according to the invention.

[0033] The figures do not necessarily respect scales, particularly in thickness, for illustrative purposes.

[0034] Furthermore, some drawings are presented in transparency, as their representation in black and white is impossible. In particular, color is necessary in these drawings to discern details that would be lost if they were presented in black and white.

[0035] Preliminary remarks

[0036] In order not to obscure the description and distract the reader from understanding the teachings of the invention, our explanations will not go beyond what is considered necessary for understanding and appreciating the underlying concepts of the invention. Indeed, the embodiments illustrated in the description are, for the most part, composed of elements known to a person skilled in the art.

[0037] Objective of the invention

[0038] One of the main objectives of the invention is to provide an antenna structure for motion metal detectors that maintains a stable induction balance while being less sensitive to shocks inherent in its use in the field.

[0039] To achieve this, the inventors propose an innovative configuration of the transmitting and receiving coils, which are arranged relative to a reference plane in such a way as to capture or emit signals of opposite directions in a central part of the antenna structure.

[0040] In particular, the arrangement of the central part of the antenna structure aims to mutually cancel unwanted signals generated by shocks, while preserving the induction balance essential to the sensitivity of the detector.

[0041] This approach aims to significantly improve the accuracy and reliability of motion metal detectors, even under difficult operating conditions on rough terrain.

[0042] General structure of the invention

[0043] As illustrated on the, the, the and the antenna structure 100 for motion metal detector includes several components: an antenna body 110, at least one receiving coil 120, at least one transmitting coil 130 and a reference plane 140.

[0044] The term "antenna structure" refers to a set of elements that form the antenna of a handheld device used to detect metallic objects buried in the ground.

[0045] In the invention, the antenna body 110 is designed with respect to a user who holds and maneuvers the motion metal detector to detect a target in the ground, namely metallic objects such as ancient coins, buried jewelry, historical relics, or metallic debris.

[0046] The term "antenna body" refers to the main part of the antenna structure 100 that supports and contains the other components.

[0047] In the example of the, the antenna body 110 is arranged in a housing 10.

[0048] In the invention, as illustrated on the diagram, a reference plane 140 which is parallel to the ground cuts horizontally the antenna body 110 into an upper part and a lower part.

[0049] The term "reference plane" indicates an imaginary plane used as a reference point to define the position of the other elements of the antenna structure 100. It therefore serves as a reference point to describe the geometric configuration of the coils 120, 130 and their relative positions.

[0050] In practice, as illustrated in the figure, the antenna body 110 comprises several elements: side parts 111, front-rear parts 112 and a central part 113.

[0051] The term "lateral parts" refers to the sides of the antenna body 110.

[0052] The term "front-rear parts" refers to the front and rear sections of the antenna body 110.

[0053] The term "central part" corresponds to the area located in the middle of the antenna body 110 and which is situated between the lateral parts 111 and the front-rear parts 112.

[0054] In other words, the antenna body 110 has a three-dimensional structure with distinct parts, each playing a specific role in the overall configuration of the antenna structure 100.

[0055] The overall shape of the 110 antenna body can vary, but it is generally designed to optimize ground coverage while remaining ergonomic for the user. Common shapes include circular, elliptical, or DD (Double D) configurations.

[0056] In the invention, the transmitting coil 130 and the receiving coil 120 have several common characteristics.

[0057] The term "transmitting coil" refers to a winding of conductive wire that generates an electromagnetic field, while the term "receiving coil" refers to a winding of conductive wire that captures variations in the electromagnetic field.

[0058] In the invention, as illustrated in the figure, the transmitting coil 130 and the receiving coil 120 are arranged in the antenna body 110, that is to say inside the antenna body 110.

[0059] In addition, the transmitting coil 130 and the receiving coil 120 each comprise multiple turns of conductors, thus forming multiturn coils.

[0060] The term "conductor turns" indicates the turns of wire that form each coil 120, 130.

[0061] The exact number of turns may vary depending on the performance specifications required for the metal detector. These turns can be made of copper wire or any other suitable conductive material.

[0062] In the invention, as illustrated on the, the, the and the, the transmitting coil 130 and the receiving coil 120 partially overlap in the central part 113.

[0063] The term "partially overlap" refers to the spatial configuration in which the transmitting coil 130 and the receiving coil 120 occupy a common space in an incomplete or limited way within the central part 113 of the antenna structure.

[0064] This arrangement involves a partial overlap of the two coils, allowing a specific electromagnetic interaction within the central part 113.

[0065] In practice, this overlap can occur in both directions, i.e. the transmitting coil 130 can overlap the receiving coil 120 or conversely, the receiving coil 120 can overlap the transmitting coil 130.

[0066] Furthermore, in the invention, the transmitting coil 130 and the receiving coil 120 are designed and positioned so as to create an induction balance.

[0067] The term "induction balance" refers to the state in which the transmitting coil 130 and the receiving coil 120 are positioned so as to balance the electromagnetic forces received by the receiving coil 120, thus optimizing detection sensitivity.

[0068] This induction balance is achieved through various techniques, including the precise adjustment of the relative positions of coils 120 and 130, the optimization of the number of turns in each coil, and possibly the use of electronic compensation circuits. These methods aim to maintain a zero or at least minimal signal in the receiving coil 120, thus ensuring the sensitivity of the metal detector by facilitating signal amplification.

[0069] To establish this balance, a reference point is needed. For example, "0V" is often used as a reference for the equilibrium of electromagnetic forces. In this case, it corresponds to the state where, in the absence of a metallic target, the magnetic field induced in the receiving coil by the transmitting coil is perfectly compensated, resulting in a zero output voltage (0 Volts) at the receiving circuit.

[0070] In other words, the configuration of the transmitting coil 130 and receiving coil 120 is optimized to maximize the detection efficiency of the antenna structure 100.

[0071] In particular, the partial overlap of coils 120, 130 in the central part 113 leading to an induction balance makes it possible to obtain a more sensitive detection area and to improve detection accuracy.

[0072] In the invention, the central part 113 has a particular configuration in a specific section.

[0073] The term "section" refers to a specific cross-section of the central part 113.

[0074] In particular, this section is perpendicular to the reference plane 140 and intersects the central part 113. This section, illustrated on la and la, is called the overlap section 150.

[0075] In practice, in the overlap section 150, the central part 113 plays an important role in the operation of the antenna structure 100, as it optimizes the interaction between the emitted electromagnetic field and the received signals, thus improving the ability to detect metallic objects buried in the ground.

[0076] For this purpose, in the central part 113, the transmitting coil 130 and the receiving coil 120 together have a specific configuration.

[0077] In particular, as illustrated on laet la, in the central part 113, at least one of the coils 120, 130 is configured in a non-coplanar manner with respect to the reference plane 140.

[0078] The term "non-coplanar" for a 120, 130 coil indicates that it is not located on the same plane parallel to the reference plane 140.

[0079] Thus, the 120, 130 reels can have positions at different heights relative to the reference plane 140.

[0080] In other words, this non-coplanar configuration of at least one of the coils 120, 130 creates a complex three-dimensional geometry within the central part 113, in order to optimize detection sensitivity and reduce potential interference.

[0081] In practice, the configuration of the central part 113 is such that at least one coil 120, 130 is located above or below the reference plane 140.

[0082] As illustrated on the, the, the and the, this configuration creates at least one vertical offset D between coils 120, 130.

[0083] The term "vertical offsets" refers to; in the overlap section 150, the height differences between the coils 120, 130 which are measured relative, perpendicularly, to the reference plane 140, in the central part 113.

[0084] Thus, this arrangement of the coils 120, 130 relative to the reference plane 140, in the overlap section 150, allows for an optimal spatial distribution of electromagnetic fields within the central part 113. This configuration helps to improve the accuracy and sensitivity of the detection of metallic objects in the ground.

[0085] In the invention, the vertical offsets D are designed for a specific purpose.

[0086] In particular, in the overlap section 150, the vertical offsets D allow the central part 113 to capture or emit signals in opposite directions.

[0087] The term "opposite-sense signals" refers to electromagnetic signals that have polarities or phases opposite to each other, or to induced currents that flow in opposite directions.

[0088] In practice, this configuration with signals in opposite directions plays an important role in reducing interference related to shocks.

[0089] The term "shock" here refers to a sudden or repeated impact or vibration that affects the antenna structure 100.

[0090] The physical basis for this arrangement, which allows the reception or transmission of signals in opposite directions in different areas of the antenna, rests on the principle of electromagnetic induction and Lenz's law. Indeed, when a changing magnetic field passes through a conducting loop, it induces a current in that loop. And the direction of this induced current depends on the orientation of the loop relative to the magnetic field.

[0091] In the context of the overlap section 150, the coils 120 and 130 in the central part 113 are oriented differently with respect to the emitted or received magnetic field. Consequently, the currents induced in each of the coils 120 and 130 have opposite directions, resulting in signals of opposite sense.

[0092] In the invention, this configuration has a particular effect in the event of a shock.

[0093] Indeed, in response to a shock which causes a displacement substantially in the same direction of all or part of the receiving coil 120 and / or the transmitting coil 130, the signals captured or emitted in the central part 113 tend to cancel each other out.

[0094] This mutual cancellation of signals prevents, or at least limits, variations in the induction balance, even in the presence of shocks or vibrations. This represents a significant improvement over conventional antenna structures, which are often sensitive to mechanical disturbances.

[0095] The term "same direction" refers to the substantially identical spatial orientation in which all or part of the receiving coil 120 and / or the transmitting coil 130 move in response to an impact. This concept implies that the elements concerned follow parallel or overlapping trajectories, without necessarily having the same direction of movement.

[0096] In other words, this particular configuration of the 100 antenna structure offers a form of self-compensation in the event of mechanical disturbances. This helps reduce false signals that could be generated by sudden movements or shocks during detector use, thus improving the reliability and accuracy of detections under various operating conditions.

[0097] How the invention works

[0098] The antenna structure 100 operates on a principle of electromagnetic induction.

[0099] First, the transmitting coil 130 generates an electromagnetic field that penetrates the ground.

[0100] Then, when a metallic object is present, it disrupts this field.

[0101] In response, the receiving coil 120 detects these disturbances and variations in the field, thus allowing the object to be located.

[0102] In the invention, the configuration of the coils 120, 130 in the central part 113, combined with the vertical offsets D, optimizes detection while reducing interference.

[0103] Indeed, in the event of a shock or sudden movement, the opposing signals generated in the central part 113 tend to cancel each other out, thus minimizing false positives.

[0104] First embodiment: minimum proportion of the central part 113

[0105] In a first embodiment of the antenna structure 100, the perimeter of the central part 113 corresponds to at least 20% of the total perimeter of the antenna body 110.

[0106] The term "perimeter of central part 113" refers to the length of the line that delimits the outline of central part 113, while the term "total perimeter" refers to the total length of the line that delimits antenna body 110.

[0107] Second embodiment: complementary non-coplanar configuration of the transmitting and receiving coils

[0108] In a second embodiment of the antenna structure 100, as illustrated in the figure, the transmitting coil 130 and the receiving coil 120 are both configured in a non-coplanar manner with respect to the reference plane 140.

[0109] In addition, in the central part 113, the transmitting coil 130 and the receiving coil 120 are oriented in a complementary manner.

[0110] The term "oriented in a complementary manner" means that the coils 120, 130 are arranged so as to complement each other in their positioning relative to the reference plane 140.

[0111] This complementary orientation, in the central part 113, is manifested in the following way: when the receiving coil 120 extends above the reference plane 140, the transmitting coil 130 extends below the reference plane 140, and vice versa.

[0112] In other words, this particular configuration of coils 120, 130 in the central part 113 creates an alternating arrangement of coils 120, 130 with respect to the reference plane 140.

[0113] This alternating arrangement optimizes the interaction between emitted and received electromagnetic fields, helps to mitigate electromagnetic disturbances induced by sudden movements, thus significantly reducing spurious signals due to shocks.

[0114] Third embodiment: uniformity or variability of vertical offsets

[0115] In a third embodiment of the antenna structure 100, the vertical offsets D can have two types of distribution along the central part 113: a uniform distribution or a variable distribution.

[0116] In the case of a uniform distribution, the vertical offsets D between the coils 120, 130 maintain the same value over the entire length of the central part 113.

[0117] As an example, the term "uniform vertical offsets" may refer to constant height differences of 5 mm between coils 120, 130 along the central part 113.

[0118] In the case of a variable distribution, the vertical offsets D between the coils 120, 130 change in value along the central part 113.

[0119] For example, the term "variable vertical offsets D" may also include height deviations that gradually increase from 2 mm to 10 mm from the center of the central part 113 to the extremities of the central part 113, or an alternation of large and small deviations along the central part 113.

[0120] In other words, this configuration of vertical offsets D in the antenna structure 100 optimizes the spatial distribution of electromagnetic fields. The uniform or variable distribution of vertical offsets D provides flexibility in antenna design to adapt the compensation for shock effects, thus optimizing detector performance under various operating conditions.

[0121] Fourth embodiment: mechanical stiffening elements

[0122] In a fourth embodiment of the antenna structure 100, it includes mechanical stiffening elements, namely structural components designed to reinforce and stabilize the antenna structure 100.

[0123] In practice, the mechanical stiffening elements are arranged in the central part 113.

[0124] In particular, the mechanical stiffening elements fulfill the function of maintaining the vertical offsets D between the coils 120, 130, when shocks are applied.

[0125] As an example, mechanical stiffening elements may include composite material reinforcements, honeycomb structures, support ribs, or supports specially designed to fit the shape of the coils 120, 130 in the central part 113.

[0126] These components could be made from lightweight but strong materials, such as fiber-reinforced polymers, so as not to excessively burden the antenna structure 100 while ensuring its rigidity.

[0127] Fifth embodiment: advanced three-dimensional coil configurations

[0128] In a fifth embodiment of the antenna structure 100, the transmitting coils 130 and receiving coils 120 are arranged according to a three-dimensional configuration chosen from three options: an angular offset between the principal planes of the coils 120, 130, a variation in the spacing between the coils 120, 130 along their perimeter, and a relative inclination of the coils 120, 130 with respect to the reference plane 140.

[0129] The term "angular offset" refers to an arrangement where the planes containing the transmitting 130 and receiving 120 coils form an angle with each other.

[0130] As an example, the angular offset between the main planes of coils 120, 130 could be 15°, 30° or 45°.

[0131] This configuration optimizes the interaction between emitted and received electromagnetic fields, thus improving the performance of the antenna structure 100.

[0132] The "spacing variation" refers to a configuration where the distance between the transmitting coil 130 and receiving coil 120 is not constant around their circumference.

[0133] As an example, the variation in spacing between coils 120, 130 along their perimeter could follow a sinusoidal function or a linear progression.

[0134] This variation in spacing can help create a non-uniform distribution of the electromagnetic field, allowing the detection sensitivity to be adapted to the specific needs of the application.

[0135] The relative inclination of coils 120, 130 with respect to the reference plane 140 indicates that the transmitting coil 130 and receiving coil 120 are not necessarily parallel to the reference plane 140.

[0136] As an example, the relative inclination of coils 120, 130 with respect to the reference plane 140 could be 5°, 10° or 20°, depending on the specific requirements of the application.

[0137] This tilt can change the main direction of the electromagnetic field generated and captured by the antenna structure 100, thus providing additional flexibility in the design of the motion metal detector.

[0138] In other words, these three-dimensional configurations of the transmitting coil 130 and receiving coil 120 within the antenna structure 100 optimize the spatial distribution of electromagnetic fields. These arrangements can improve detection sensitivity, antenna directivity, or its ability to adapt to different operating conditions.

[0139] Conclusion

[0140] We have described and illustrated the invention. However, the invention is not limited to the embodiments we have presented. Indeed, numerous combinations of variants, alternatives, embodiments, and implementations can be envisaged without requiring substantial modifications to the invention. Thus, an expert in the field can deduce other variants, alternatives, embodiments, and implementations by reading the description and the accompanying figures, and taking into account the economic, ergonomic, and dimensional constraints to be respected.

[0141] In particular, when an expression uses the term "at least one", this means that the element or characteristic in question may be present in a single occurrence or in multiple occurrences, thus comprising one, two, three or more elements or characteristics, without any upper limit specified.

[0142] On the other hand, when an element is "designed" to fulfill a particular function, it means that this element is created specifically for the purpose of fulfilling that particular function.

[0143] However, depending on the needs and resources available, it may be possible to consider using an existing element, which will be modified or adapted to fulfill this particular function, without requiring substantial modifications to the invention.

[0144] The invention is capable of numerous variations and applications other than those described above. In particular, unless otherwise specified, the various structural and functional features of each particular embodiment described above should not be considered as combined and / or closely and / or inextricably linked to one another, but rather as mere juxtapositions. Furthermore, the structural and / or functional features of the various embodiments described above may be juxtaposed or combined, in whole or in part, in any different manner.

Claims

Antenna structure (100) for a motion metal detector, the antenna structure (100) comprising: - an antenna body (110) designed with respect to a user holding and maneuvering the motion metal detector to detect a target in the ground; - at least one receiving coil (120) and at least one transmitting coil (130); and - a reference plane (140) parallel to the ground that horizontally intersects the antenna body (110) into an upper and a lower portion, wherein: - the antenna body (110) has: - lateral portions (111); - front and rear portions (112); and - a central portion (113) located between the lateral portions (111) and the front and rear portions (112); - the transmitting coil (130) and the receiving coil (120) are arranged within the antenna body (110); each of multiple turns of conductors, -- partially overlap in the central part (113),and-- are positioned so as to create an induction balance, and wherein, in a section perpendicular to the reference plane (140) which intersects the central part (113) where the transmitting coils (130) and receiving coils (120) partially overlap, called the overlap section (150), the central part (113) is arranged such that at least one of the coils (120, 130) between the transmitting coil (130) and the receiving coil (120) is configured non-coplanarly with respect to the reference plane (140), with positionings at different heights with respect to the reference plane (140), such that at least one coil (120, 130) is above or below the reference plane (140), creating vertical offsets, D, between the coils (120, 130), the vertical offsets, D, being,- variably distributed along the central part (113), with at least two different values ​​of vertical offsets, D,and- designed to allow the central part (113) to receive or emit signals in opposite directions, so that, in response to a shock that causes a displacement substantially in the same direction of all or part of the receiving coil (120) and / or the transmitting coil (130), the signals received or emitted in the central part (113) tend to cancel each other out, while maintaining the induction balance. Antenna structure (100) according to claim 1, wherein the perimeter of the central part (113) corresponds to at least 20% of the total perimeter of the antenna body (110). Antenna structure (100) according to any one of claims 1 to 2, wherein, in the central part (113), - the transmitting coil (130) and the receiving coil (120) are both configured in a non-coplanar manner with respect to the reference plane (140), and - the transmitting coil (130) and the receiving coil (120) are oriented in a complementary manner, such that when the receiving coil (120) extends above the reference plane (140), the transmitting coil (130) extends below the reference plane (140), and vice versa. Antenna structure (100) according to any one of claims 1 to 3, further comprising mechanical stiffening elements arranged in the central part (113) which are designed to maintain the vertical offsets, D, between the coils (120, 130) when shocks are applied. Antenna structure (100) according to any one of claims 1 to 4, wherein the transmitting coils (130) and receiving coils (120) are arranged according to a three-dimensional configuration chosen from, - an angular offset between the principal planes of the coils (120, 130), - a variation of the spacing between the coils (120, 130) along their perimeter, and - a relative inclination of the coils (120, 130) with respect to the reference plane (140).

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