Method for generating reference data for classifying conductive objects using a metal detector, method for classifying conductive objects, metal detector, device, and measurement device

Abstract

The invention relates to a method for generating reference data (R) for classifying a conductive object (G) in a state (Z) by means of a metal detector (MD) having a primary coil (PS), a measurement and operating circuit (MB), an input (E), and a plurality of coil pairs (SP), wherein the repeated execution of a procedure at various relative positions (x) between the metal detector (MD) and the object (G) comprises at least the following steps: generating a primary magnetic field (PF) using the primary coil (PS) of the metal detector (MD) in order to generate a secondary magnetic field (SF) via eddy currents induced in the electrically conductive object (G); detecting a plurality of amplitudes of voltages which are applied to the input (E) of the measurement and operating circuit (MB) of the metal detector (MD) and are induced in the plurality of coil pairs (SP) by the secondary magnetic field (SF); storing the detected amplitudes for each relative position (x); providing reference data (R) which includes the stored amplitudes from the various procedures for the electrically conductive object (G) in the state (Z).

Description

[0001] Method for generating reference data for classifying conductive objects with a metal detector, method for classifying conductive objects, metal detector, device and measuring device

[0002] The invention relates to a method for generating reference data for classifying and determining the position of concealed electrically conductive objects using a metal detector, and to a method for classifying and determining the position of concealed electrically conductive objects. The invention further relates to a metal detector that can apply the inventive method, a device that can apply the inventive method to a medium in a container, and a measuring device that measures a measurand of a medium in a container and can also apply the method.

[0003] A metal detector is a device for locating hidden metallic objects. It is used, for example, to locate pipelines, conductive objects in containers, especially foreign bodies in media, electrical conductors, electrically conductive objects, or objects concealed on people. In light of current events, the use of metal detectors for locating munitions such as mines, munitions, and unexploded ordnance is of particular interest. Common metal detectors comprise a transmitting coil, or primary coil, which is fed with an alternating current by an electronic circuit to generate the widest possible (primary) magnetic field, as well as one or more receiving coils, or secondary coils.

[0004] US 10056186B2 describes a metal detector as a device for detecting electrically conductive materials, comprising multiple electromagnetic coils and circuit boards. The circuit boards are positioned so that their metallic surfaces and layers reduce or eliminate the influence of the coils on the detection of electrically conductive materials.

[0005] Document WO0225318A1 teaches a device and method for detecting electrically conductive materials, comprising a primary coil, referred to as the excitation coil, which is operated with alternating voltage, a secondary coil, referred to as the detection coil, which detects output signals, and a subtraction unit that subtracts one detected signal from the other. The invention makes it possible to eliminate a background signal and enables the detection of metal particles.

[0006] The document US2022107439A1 teaches a device for detecting objects hidden on people, consisting of a metal detector with at least three transducers or coils arranged around a passage, so that a combination of an induced voltage allows an inaccurate localization of the object.

[0007] Document BG3298U1 teaches a production line for filling and dosing a product in the confectionery industry, including, among other things, a metal detector. This document describes a mechanism connected to the metal detector, which is partly responsible for removing part of the product from the production line, for example, to prevent contamination of the product with metallic objects.

[0008] Document CN213800188U teaches a landmine detection device mounted on an unmanned aerial vehicle. The device includes, among other components, a microcontroller, a metal detector, and a GPS positioner. The device can help ensure the safety of deminers, reduce detection time, and improve the efficiency of mine clearance.

[0009] A hidden electrically conductive object is detected with a metal detector by inducing a voltage signal suitable for evaluation in one or more secondary coils. Each voltage signal induced in a secondary coil contributes to the overall information about the detected electrically conductive object and allows, at least in principle, to assign properties to the detected object, such as a spatial size, a quantity of conductive material, or a distance from the metal detector. However, properties cannot be uniquely assigned: different objects with different properties can induce a comparable voltage signal in the same secondary coil, so voltage signals from a plurality of secondary coils are combined to determine the properties of an object.

[0010] Document US5721489A teaches a metal detector with two receiving antennas of different sizes, whose received signals are digitally processed using Fourier transformation to determine a frequency band containing information about the target signal. The information about the target signal is used to identify the target, with the distance of the target to the metal detector being determined by comparing the signals from the two receiving antennas.

[0011] Document W02006037176A1 teaches a metal detector with a plurality of transmit and receive coils, allowing the volume to be examined with the metal detector to be adjusted. This makes it possible to obtain a wide range of information about an electrically conductive object within the volume, allowing conclusions to be drawn about, among other things, the properties of the volume, the size, and the metal content of the object.

[0012] State-of-the-art metal detectors often provide no, or at best only inaccurate, information about the type and size of a detected electrically conductive object, and they also provide, at best, inaccurate information about the object's position relative to the metal detector. A metal detector is often adapted to the search conditions, for example, by varying the properties and arrangements of the secondary coils. In particular, the sizes of the receiving coils are adapted to the distance between the object and the metal detector in order to maximize the voltage induced in the secondary coils by the secondary magnetic field. Consequently, to detect concealed electrically conductive objects at an unknown distance, it is sometimes necessary to replace the receiving coils. Furthermore, conventional metal detectors rarely have more than two receiving coils and can therefore only provide limited information about an object.

[0013] The invention is based on the object of providing a method by means of a metal detector, by which a detected electrically conductive object is classified according to its type, nature and size, and by which the relative position between the electrically conductive object and the metal detector can be determined.

[0014] The invention solves the problem by the method according to independent claim 1.

[0015] The inventive method for generating reference data for classifying an electrically conductive object in a state, by means of a metal detector, wherein the metal detector comprises a primary coil, a measuring and operating circuit with an input, and a plurality of coil pairs connected to the measuring and operating circuit, wherein the method comprises repeatedly performing a procedure at different relative positions of the metal detector and the object, wherein the procedure comprises at least the following steps: generating a primary magnetic field with the primary coil of the metal detector for generating a secondary magnetic field by eddy currents induced in the electrically conductive object; detecting a plurality of amplitudes of voltages applied to the input of the measuring and operating circuit of the metal detector and induced in the plurality of coil pairs by the secondary magnetic field;Storing the detected amplitudes for the respective relative position; Providing reference data containing the stored amplitudes of the various procedures for the electrically conductive object in the state;

[0016] In a further development of the method according to the invention, the coil pairs are each formed by two secondary coils; wherein the magnetic coupling of the two secondary coils is identical to the primary coil except for tolerances; wherein several coil pairs have a symmetrical arrangement with respect to an axis of symmetry or plane of symmetry of the primary magnetic field; wherein one or more coil pairs have secondary coils with an asymmetrical arrangement with respect to the axis of symmetry or plane of symmetry of the primary magnetic field; wherein the secondary coils are connected to an input of the measuring and operating circuit in such a way that voltages of the two secondary coils induced by the primary magnetic field and present at the input have opposite signs; wherein the amplitude of a sum of voltages induced in a coil pair is detected and stored.

[0017] In a further development of the method according to the invention, several coil pairs have secondary coils with a symmetrical arrangement with respect to an axis of symmetry or plane of symmetry of the primary magnetic field.

[0018] In a further development of the method according to the invention, one or more coil pairs have secondary coils with an asymmetric arrangement with respect to the axis of symmetry or plane of symmetry of the primary magnetic field.

[0019] In a further development of the method according to the invention, the state of the material includes, for example, the strength of the reflection of the primary magnetic field, magnetic fields present in the material, the conductivity which may depend on temperature and humidity, and other magnetic properties of the material covering the electrically conductive object.

[0020] In a further development of the method according to the invention, the state comprises a quantity of conductive material contained in the electrically conductive object; the conductive material comprises, for example, copper, silver, gold, and metallic alloys.

[0021] In a further development of the method according to the invention, the state comprises a spatial size of the electrically conductive object.

[0022] In a further development of the method according to the invention, an algorithm, in particular artificial intelligence, generates the reference data.

[0023] A method according to the invention for classifying and determining the position of an electrically conductive object with a state, by means of a metal detector with a primary coil, a measuring and operating circuit with an input, and a plurality of coil pairs connected to the measuring and operating circuit, wherein a relative movement takes place in the direction between the electrically conductive object and the metal detector, comprising at least the following steps: generating a primary magnetic field with the primary coil of the metal detector, for generating a secondary magnetic field by eddy currents induced in the electrically conductive object; detecting a plurality of amplitudes of a plurality of coil pairs by the measuring and operating circuit, and storing the sequences as ordered time series; comparing the ordered time series with reference data generated using a method according to the invention;Classifying the object with respect to a result of the comparison, with respect to at least one of the following properties: an amount of conductive material, a spatial size, a property of the material covering the object; Determining the position of the object by determining at least one orthogonal projection and / or the minimum distance of the position of the object to a trajectory resulting from a reference point within the metal detector and the relative movement.

[0024] In a further development of the method according to the invention for classifying and determining a position of an electrically conductive object, a comparison of ordered time series and reference data is carried out by an algorithm; wherein the algorithm determines the position of the object, in particular by cross-correlation; wherein an object and a state are determined by the algorithm, in particular by a convolutional neural network.

[0025] A metal detector according to the invention, configured for classifying and determining the three-dimensional position of an electrically conductive object, comprising: a measuring and operating circuit with an amplifier having an input, a gain factor, and a saturation voltage value; a primary coil configured to be supplied with an alternating current by the measuring and operating circuit and configured to generate a primary magnetic field with an axis of symmetry or a plane of symmetry; several coil pairs, each comprising two secondary coils with magnetic couplings with the primary coil that are identical, up to tolerances, and connected to an input of the measuring and operating circuit in such a way that voltages of the two secondary coils induced by the primary magnetic field and present at the input have opposite signs;wherein one or more coil pairs have secondary coils with a symmetrical and / or asymmetrical arrangement with respect to an axis of symmetry or plane of symmetry of the primary magnetic field; wherein a total amplitude of a coil pair is formed by an amplitude of a sum of voltages induced in the coil pair; wherein the measuring and operating circuit is configured to record sequences of measured values ​​of total amplitudes of several coil pairs and to store them as ordered time series; wherein the measuring and operating circuit is configured to store reference data generated using a method according to the invention and to process the ordered time series independently according to a method according to the invention for classification and for determining a position; wherein the object is classified during processing, preferably a state and a position of the object are determined.

[0026] In a further development of the metal detector according to the invention, a first secondary coil of a coil pair has a main loop and a secondary loop, wherein the secondary loop is coupled to the main loop; wherein the main loop has a magnetic coupling with the primary coil M 1 H and the second secondary coil of the coil pair has a magnetic coupling with the primary coil M2, so that < 0.05; where the magnetic coupling of the first

[0027] Secondary coil with the primary coil and magnetic coupling of the second secondary coil with the primary coil is tuned such that an amplitude of the voltage induced by the primary magnetic field and applied to the input is smaller than - preferably smaller than half of - the saturation voltage of the amplifier divided by the gain factor.

[0028] In a further development of the metal detector according to the invention, the secondary loop is part of a plurality of secondary loops; wherein each secondary loop from the plurality of secondary loops has a magnetic coupling with the primary coil that is smaller than the magnetic coupling with the primary coil of the main loop; wherein any number of secondary loops from the plurality of secondary loops can be coupled to the main loop by means of one or more short-circuit bridges.

[0029] In a further development of the metal detector according to the invention, the first secondary coil of a coil pair comprises conductor tracks on a circuit board, in particular is formed by conductor tracks on a circuit board; wherein the second secondary coil of a coil pair comprises conductor tracks on a circuit board, and in particular is formed by conductor tracks on a circuit board; and wherein the primary coil comprises conductor tracks on a circuit board, and in particular is formed by conductor tracks on a circuit board.

[0030] In a further development of the metal detector according to the invention, the amplification factor with which the amplifier amplifies a voltage signal from the secondary coils is at least 1000 - preferably at least 10000.

[0031] In a further development of the metal detector according to the invention, the metal detector is attached to an aircraft, in particular a drone, or to a vehicle, in particular a rover, or to a swimming vehicle.

[0032] A device according to the invention for classifying an electrically conductive object comprises: a storage unit configured to store reference data generated according to a method according to the invention; a receiving unit configured to receive data series; a computing unit configured to be programmed with an algorithm; wherein the algorithm is configured, in particular, to process the data series and to classify them according to a method according to the invention using the stored reference data.

[0033] A further development of the device according to the invention, designed to detect electrically conductive objects in a medium and to classify them according to a method according to the invention and to determine their position, further comprises: a container designed to guide a medium; a metal detector according to the invention.

[0034] A measuring device according to the invention, configured to measure a measurand of a medium, comprises: a device according to the invention; a measuring sensor to which a medium can be applied; wherein the measuring sensor is configured to generate at least one measurement signal dependent on the measurand; wherein the receiving unit of the device is configured to receive the measurement signal and transmit it to the computing unit for evaluation. The invention has the advantage that a multitude of information about a hidden, electrically conductive object can be provided by an algorithm more quickly and precisely than is possible with the prior art. The invention further has the advantage that it can be applied to a multitude of possible objects to be detected, in particular for searching for munitions such as mines, munitions, and unexploded ordnance, whereby these can be differentiated from other objects.The invention further has the advantage of enabling automated detection and classification of concealed electrically conductive objects when the classification is performed by an algorithm, particularly when a metal detector according to the invention is mounted on autonomous vehicles, such as a rover, and autonomous aircraft, such as a drone. The invention further has the advantage that suitable secondary coils with a symmetrical shape and arrangement with respect to an axis or plane of symmetry of the primary magnetic field can provide a clear indication that an object to be detected is being crossed.The invention further has the advantage that two secondary coils of a secondary coil pair can be arranged asymmetrically with respect to a plane of symmetry of the primary magnetic field in order to enable the distance of a conductive object to the trajectory of the metal detector in a horizontal plane, which enables a precise determination of the three-dimensional position of the object.

[0035] The invention is explained using the following figures.

[0036] Fig. 1 shows a flowchart of an embodiment of the method according to the invention for creating reference data.

[0037] Fig. 2 shows a circuit diagram of an embodiment of a metal detector according to the invention.

[0038] Fig. 3 shows schematically an embodiment of the secondary coils of a coil pair of the metal detector used for the method according to the invention.

[0039] Fig. 4 shows a side view of an embodiment of the method according to the invention for classifying and determining the position of an object.

[0040] Fig. 5 shows a flowchart of an embodiment of the inventive method for classifying and determining the position of an object. Fig. 6 shows a circuit diagram of an embodiment of the inventive device.

[0041] Fig. 7 shows a cross section of an embodiment of the measuring device according to the invention.

[0042] The flowchart of an embodiment of the method according to the invention shown in Fig. 1 describes the creation of reference data of an electrically conductive object G in a known state Z using a metal detector comprising a primary coil, a measuring and operating circuit with an input, and several coil pairs connected to the measuring and operating circuit. The state Z of the object G includes, for example, the spatial size and / or the nature of the object G, in particular the type and quantity of the conductive material, as well as properties of the material U surrounding and covering it, for example, its nature and / or conductivity.The following procedure is repeatedly performed for a plurality of positions x: the primary magnetic field is generated by the primary coil; a secondary magnetic field is generated by eddy currents induced in the object by the primary magnetic field; the secondary magnetic field generates voltages in the secondary coils of the coil pairs of the metal detector by induction; the amplitudes of the voltages induced in the secondary coils are detected by the measuring and operating circuit; the measuring and operating circuit stores the amplitudes together with the position x and the state Z for the object G. The sum of the stored data forms the reference data R. It is advantageous to store amplitudes for a plurality of objects G as reference data. For example, G can be a coin or a mine.

[0043] The circuit diagram of an embodiment of the metal detector according to the invention shown in Fig. 2 comprises a measuring and operating circuit MB, which supplies a primary coil PS with alternating current and is connected to an amplifier VS having an input E, wherein the plurality of coil pairs SP are connected to the input E, to which the voltage amplitudes induced by a magnetic field are applied. A coil pair SP consists of a first secondary coil S1 and a second secondary coil S2 and comprises at least one main loop HS and a plurality of secondary loops VNS, wherein a secondary loop NS from the plurality of secondary loops VNS is coupled to the main loop HS by one or more short-circuit bridges KB. In this embodiment, the coupling of a secondary loop NS to a main loop HS can increase or decrease its magnetic coupling to the primary coil PS.

[0044] The design of the secondary coils of a coil pair of the metal detector shown schematically in Fig. 3a comprises a lower secondary coil S1 and an upper secondary coil S2, both of which have identical magnetic couplings with the primary coil PS within tolerances and are arranged symmetrically to an axis of symmetry and / or plane of symmetry S of the primary coil PS, and thus also to the primary magnetic field.

[0045] The design of the secondary coils of a coil pair of the metal detector, shown schematically in Fig. 3b, comprises a lower secondary coil S3 and an upper secondary coil S4, both of which have identical magnetic couplings with the primary coil PS within tolerances and are arranged asymmetrically to an axis of symmetry and / or plane of symmetry S of the primary coil PS, and thus also to the primary magnetic field. In this example, the secondary coils have a triangular shape.

[0046] The side view shown in Fig. 4 of an embodiment of the method according to the invention for classifying and determining the position of an object comprises a metal detector MD with a reference point RP, which moves in a direction RV with a relative speed v relative to the object G with position y. The object G is surrounded and / or covered by a material U, which in this example can be earth. The metal detector MD radiates a primary magnetic field PF, inter alia, in the direction of the object G, wherein eddy currents are induced in the object G, generating a secondary magnetic field SF, which is radiated inter alia, in the direction of the metal detector MD.A minimum distance of the object G from the metal detector MD is given by an orthogonal projection OP of the position y of the object G onto a trajectory, for example given by a line through the reference point RP within the metal detector MD and a direction RV. In this example, the metal detector can be attached to a rover or a drone. The flow chart shown in Fig. 5 represents an embodiment of the method according to the invention for classifying and determining the position of an object with an unknown state Z using a metal detector MD with relative movement v. The relative movement gives rise to relative positions x between the metal detector MD and the object G, wherein suitable coil pairs consisting of symmetrical secondary coils S1, S2 arranged symmetrically to the primary magnetic field PF can provide information about when the metal detector MD is at a minimum distance from the object G.For each relative position x, the following procedure is performed: a primary magnetic field PF is generated by the primary coil PS; a secondary magnetic field SF is generated by eddy currents induced in the object G by the primary magnetic field PF; the secondary magnetic field SF induces voltages in the secondary coils of the coil pairs of the metal detector MD, the amplitudes of which are detected by the measuring and operating circuit of the metal detector MD; the measuring and operating circuit stores the amplitudes together with the position x as an ordered time series. The ordered time series are compared with reference data R, whereby the state Z of the object G is classified, and the position y is determined.

[0047] The circuit diagram of an embodiment of the device according to the invention shown in Fig. 6 comprises a storage unit SE configured to store reference data, and a receiving unit EE configured to receive data series D, for example, from a metal detector. A computing unit is connected to the storage unit SE and the receiving unit EE and is configured to compare the received data series with the stored reference data, thus implementing a method according to the invention for classifying and determining the position of an electrically conductive object.

[0048] The cross-section of an embodiment of the measuring device according to the invention shown in Fig. 7 comprises a container B, which carries a medium ME and can be a pipe. Connected to the container are a measuring sensor MA, which is configured to be exposed to the medium ME and to generate at least one measurement signal dependent on the measured variable, and a metal detector MD according to the invention, which is connected to a device VO according to the invention for classifying electrically conductive objects and supplies it with data series. List of Reference Symbols

[0049] R Reference data

[0050] G electrically conductive object

[0051] Z condition

[0052] MD metal detector

[0053] PS primary coil

[0054] MB measuring and operating circuit

[0055] VS amplifier

[0056] E Entrance

[0057] KB short-circuit bridge

[0058] SP coil pairs

[0059] S axis of symmetry or plane of symmetry x relative position of object and metal detector

[0060] PF Primary magnetic field

[0061] SF Secondary magnetic field

[0062] S1,S2 Symmetrically arranged secondary coil of a coil pair

[0063] S3,S4 Asymmetrically arranged secondary coil of a coil pair

[0064] U Material covering an object v Relative motion

[0065] RV Direction of relative movement

[0066] GZ Ordered time series

[0067] OP Orthogonal projection

[0068] RP Reference point y Three-dimensional position

[0069] SE storage unit

[0070] EE receiving unit

[0071] RE computing unit

[0072] D data series

[0073] VO device

[0074] B Container

[0075] MA sensor

[0076] ME Medium

Claims

Patent claims 1. A method for generating reference data (R) for classifying an electrically conductive object (G) in a state (Z) by means of a metal detector (MD), wherein the metal detector (MD) comprises a primary coil (PS), a measuring and operating circuit (MB) with an input (E), and a plurality of coil pairs (SP) connected to the measuring and operating circuit, wherein the method comprises the repeated execution of a procedure at different relative positions (x) of the metal detector (MD) and the object (G), wherein the procedure comprises at least the following steps: • Generating a primary magnetic field (PF) with the primary coil (PS) of the metal detector (MD) for generating a secondary magnetic field (SF) by eddy currents induced in the electrically conductive object (G); • Detecting several amplitudes of voltages present at the input (E) of the measuring and operating circuit (MB) of the metal detector (MD) and induced by the secondary magnetic field (SF) in the several pairs of coils (SP); • Saving the recorded amplitudes for the respective relative position (x); • Providing reference data (R) containing the stored amplitudes of the various procedures for the electrically conductive object (G) in the state (Z).

2. Method according to claim 1, • wherein the coil pairs (SP) are each formed by two secondary coils (S1, S2; S3, S4); • wherein the magnetic couplings of the two secondary coils (S1, S2; S3, S4) are identical to the primary coil except for tolerances; • wherein the secondary coils are connected to an input (E) of the measuring and operating circuit (MB) in such a way that voltages of the two secondary coils (S1, S2) induced by the primary magnetic field (PF) and present at the input (E) have opposite signs; • where the amplitude of a sum of voltages induced in a pair of coils is detected and stored.

3. Method according to one of claims 1 or 2, • wherein a plurality of coil pairs (SP) have secondary coils (S1, S2) with a symmetrical arrangement with respect to an axis of symmetry or plane of symmetry (S) of the primary magnetic field.

4. Method according to one of claims 1 to 3, • wherein one or more coil pairs comprise secondary coils (S3, S4) with an asymmetric arrangement with respect to the axis of symmetry or plane of symmetry (S) of the primary magnetic field.

5. Method according to one of claims 1 to 4, • wherein the state (Z) includes, for example, the strength of the reflection of the primary magnetic field (PF), magnetic fields present in the material, the conductivity, which may be dependent on temperature and humidity, and other magnetic properties of the material (U) covering the electrically conductive object (G).

6. Method according to one of claims 1 to 5, • wherein the state (Z) comprises an amount of conductive material contained in the electrically conductive article (G); • wherein the conductive material includes, for example, copper, silver, gold, and metallic alloys.

7. Method according to one of claims 1 to 6, • wherein the state (Z) comprises a spatial size of the electrically conductive object (G).

8. Method according to one of claims 1 to 7, • where an algorithm, in particular an artificial intelligence, generates the reference data.

9. Method for classifying and determining a position (y) of an electrically conductive object (G) with a state (Z), by means of a metal detector (MD) with a primary coil (PS), a measuring and operating circuit (MB) with an input (E), and a plurality of coil pairs (SP) connected to the measuring and operating circuit (MB), wherein a relative movement (v) in the direction (RV) takes place between the electrically conductive object and the metal detector (MD), comprising at least the following steps: • Generating a primary magnetic field (PF) with the primary coil (PS) of the metal detector (MD), for generating a secondary magnetic field (SF) by eddy currents induced in the electrically conductive object (G); • Recording of several amplitudes of several coil pairs (SP) by the measuring and operating circuit (MB), and storing the sequences as ordered time series (GZ); • comparing the ordered time series (GZ) with reference data (R) which are generated using a method according to one of claims 1 to 8; • Classifying the object (G) with regard to a result of the comparison, with regard to at least one of the following properties: o an amount of conductive material, o a spatial size, o a property of the material (U) covering the object (G); • Determining the position (y) of the object by determining at least one orthogonal projection (OP) and / or the minimum distance of the position of the object (y) to a trajectory resulting from a reference point (RP) within the metal detector (MD) and the relative movement (v).

10. Method according to claim 9, • where a comparison of ordered time series (ZG) and reference data (R) is carried out by an algorithm; • wherein the algorithm determines the position (y) of the object (G) in particular by cross-correlation; • wherein an object (G) in a state (Z) is determined by the algorithm, in particular by a convolutional neural network.

11. A metal detector configured to classify and determine the three-dimensional position (y) of an electrically conductive object (G), comprising: • a measuring and operating circuit (MB), with an amplifier (VS) with an input (E), a gain factor, and a value of the saturation voltage; • a primary coil (PS) arranged to be supplied with an alternating current by the measuring and operating circuit (MB) and arranged to generate a primary magnetic field with an axis of symmetry or a plane of symmetry (S); • several coil pairs (SP), each comprising two secondary coils (S1, S2; S3, S4) with magnetic couplings with the primary coil (PS) that are identical up to tolerances, which are connected to an input (E) of the measuring and operating circuit (MB) in such a way that voltages of the two secondary coils (S1, S2; S3, S4) induced by the primary magnetic field (PF) and present at the input (E) have opposite signs; • wherein one or more coil pairs (SP) have secondary coils (S1, S2) with a symmetrical and / or asymmetrical arrangement with respect to an axis of symmetry or plane of symmetry (S) of the primary magnetic field; • wherein a sum amplitude of a coil pair (SP) is formed by an amplitude of a sum of voltages induced in the coil pair (SP); • wherein the measuring and operating circuit (MB) is designed to record sequences of measured values ​​of sum amplitudes of several coil pairs (SP) and to store them as ordered time series (GZ); • wherein the measuring and operating circuit (MB) is configured to store reference data (R) according to one of claims 1 to 8 and to process the ordered time series (GZ) independently according to a method according to claim 9; • wherein during processing the object (G) is classified, preferably a state (Z) and a position (y) of the object (G) are determined.

12. Metal detector according to claim 11, • wherein a first secondary coil (S1; S3) of a coil pair (SP) has a main loop (HS) and a secondary loop (NS), the secondary loop (NS) being coupled to the main loop (HS); • where the main loop (HS) has a magnetic coupling with the primary coil M 1 Hand the second secondary coil (S2; S4) of the coil pair (SP) has a magnetic coupling with the primary coil M2, so that < 0.05; • wherein the magnetic coupling of the first secondary coil (S1; S3) with the primary coil and the magnetic coupling of the second secondary coil (S2; S4) with the primary coil is tuned such that an amplitude of the voltage induced by the primary magnetic field and applied to the input (E) is smaller than - preferably smaller than half of - the saturation voltage of the amplifier (VS) divided by the gain factor.

13. Metal detector according to one of claims 11 or 12, • wherein the secondary loop (NS) is part of a plurality of secondary loops (VNS); • wherein each sub-loop of the plurality of sub-loops (VNS) has a magnetic coupling with the primary coil (PS) that is smaller than the magnetic coupling with the primary coil (PS) of the main loop (HS); • wherein any number of secondary loops from the plurality of secondary loops (VNS) can be coupled to the main loop (HS) by means of one or more short-circuit bridges (KB).

14. Metal detector according to one of claims 11 to 13, • wherein the first secondary coil (S1; S3) of a coil pair (SP) comprises conductor tracks on a printed circuit board, in particular is formed by conductor tracks on a printed circuit board; • wherein the second secondary coil (S2; S4) of a coil pair comprises conductor tracks on a printed circuit board, in particular is formed by conductor tracks on a printed circuit board; and • wherein the primary coil (PS) comprises conductor tracks on a printed circuit board, in particular is formed by conductor tracks on a printed circuit board.

15. Metal detector according to one of claims 11 to 14, • wherein the amplification factor with which the amplifier (VS) amplifies a voltage signal from the secondary coils (S1, S2; S3, S4) is at least 1000 - preferably at least 10000.

16. Metal detector according to one of claims 11 to 15, • wherein the metal detector (MD) is attached to an aircraft, in particular a drone, or to a vehicle, in particular a rover, or to a floating vehicle.

17. An apparatus for classifying an electrically conductive object, comprising: • a storage unit (SE) configured to store reference data (R) generated by a method according to one of claims 1 to 8; • a receiving unit (EE) configured to receive data series (D); • a computing unit (RE) adapted to be programmed with an algorithm; • wherein the algorithm is configured to process the data series (D) in particular and to classify them according to a method according to one of claims 9 or 10 by means of the stored reference data (R).

18. Device according to claim 17, adapted to detect electrically conductive objects (G) in a medium and to classify them according to a method according to one of claims 7 or 8 and to determine their position (y), further comprising: • a container (B) designed to carry a medium (ME); • a metal detector according to one of claims 11 to 16.

19. Measuring device, designed to measure a measurand of a medium (ME), comprising: • a device (VO) according to claim 18; • a measuring sensor (MA) that can be supplied with a medium (ME); • wherein the measuring sensor (MA) is designed to generate at least one measuring signal dependent on the measured variable; • wherein the receiving unit (EE) of the device (VO) is configured to receive the measurement signal and to transmit it to the computing unit (RE) for evaluation.

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