Metal detector

Abstract

A metal detector for detecting at least one metallic target within a detection zone, comprising: At least one receiving coil arranged to detect a magnetic field emanating from the detection zone and to generate a signal corresponding to said magnetic field, referred to as the "detection signal"; A detection unit (24) comprising: A filtering module (28) arranged to receive the detection signal as input and to filter it according to at least: - first and second filtering configurations, the second filtering configuration having a shorter time support than the first filtering configuration; A control module arranged to control the filtering module (28) based on at least one piece of information contained in the detection signal and indicating the presence of a metallic object other than said target of interest. Figure for the abbreviation: Figure 2

Description

Title of the invention: Metal detector technical field

[0001] The invention relates to the general field of portable electromagnetic metal detectors. Technological background

[0002] Such detectors are used primarily for recreational purposes, for example, for searching for and discriminating between metallic objects buried in the ground, such as coins, jewelry, treasures, or gold nuggets, but also by professionals, for example, for demining, ballistics research, or locating pipes in the construction industry. These examples are not exhaustive. Modern handheld metal detectors allow the user not only to locate but also to identify a target buried in the ground.

[0003] A distinction is essentially made between metal detectors, known as "passive" and metal detectors, known as "active".

[0004] Passive metal detectors capture and evaluate deformations in the Earth's magnetic field. Active metal detectors, on the other hand, produce an electromagnetic field during their emission phase that induces eddy currents in metals.

[0005] The magnetometer is a passive detector that emits no signal. In particular, it detects anomalies in homogeneity, assuming that the Earth's magnetic field is homogeneous by nature. Thus, any metallic particle, magnetized by the Earth's field, disturbs this homogeneity to a greater or lesser degree, depending on its size, position, and metallurgical properties. The basic principle of a magnetometer relies on two coils that are sensitive to magnetic fields and are connected in opposite phase, so as to give the magnetic field a zero electrical measurement. If one of the coils encounters a disturbing field, the difference ceases to be zero, which generates a voltage, allowing a metallic object to be located and distinguished.

[0006] Typically, active metal detectors include an electromagnetic detection head, also called a detection disc, which constitutes the active part of the detector. This head is commonly mounted at the end of a handle. This detection head generally takes the form of a disc because it contains one or more inductive coils that can generate an incident electromagnetic field and receive a magnetic field modified by the immediate environment.

[0007] The detection head can also incorporate control electronics adapted to generate an alternating electromagnetic signal that creates the incident electromagnetic field, and to process a received electromagnetic detection signal corresponding to the resulting magnetic field. This processing enables the detection and discrimination of any metallic objects exposed to the incident electromagnetic field. Alternatively, the detection head incorporates only part of the electronics for processing the received electromagnetic detection signal, for example, a preamplifier, with the remainder of this electronics housed in a control unit to which the detection head is connected.

[0008] In portable electromagnetic detectors operating in continuous wave mode, also called continuous wave (CW) detectors, an alternating magnetic field is emitted continuously, and detection can be based on the variations in amplitude and phase between the frequency components of the emitted signal and those of the received signal. More specifically, CW detectors use the measurement of the amplitude and phase of the different components of the alternating electromagnetic signal emitted by at least one transmitting coil powered by an electrical voltage, and the components of the electromagnetic signal received by one or more receiving coils arranged close to the transmitting coil. At least one frequency of the signal emitted by the transmitting coil is generally less than 100 kHz. In some embodiments, the transmitting coil and the receiving coil(s) are a single coil.

[0009] Implementing the induction balance (IB) principle allows for the measurement of very small changes in the properties of the medium by induction. This is achieved by arranging the transmitting and receiving coils so that the mutual inductance of the receiving coil(s) and the transmitting coil(s) is as low as possible. This minimizes the electromagnetic field measured in the absence of a target. The coupling of the two coils is such that the signal received by the receiving coil is essentially zero when no metallic element is placed in the field of the transmitting coil. Since conditioning electronics often have a saturation amplitude, this optimizes the dynamic range of the measurement signal by measuring only the variations in the resulting electromagnetic field.

[0010] The most common configuration is that of the "double D" head, but other configurations are possible, for example with concentric coils, or by placing the coils at 90° to each other, or with auxiliary coils used to generate a compensation field, or with other geometries.

[0011] Furthermore, there are other types of metal detectors that, instead of operating in the "frequency" domain, operate in the "time" domain. In this case, it is not the frequency components of the emitted and received signals that are analyzed, but rather the shape of the received signal. This may seem similar because there is a correspondence (via Fourier transforms) between the time and frequency domains. However, this comparison may be misleading because the time analysis is generally performed during a relaxation time when induction phenomena do not manifest themselves in the same way. This often involves a different way of generating the emitted magnetic field.Indeed, while in the case of a CW detector the signal is emitted continuously, in the case of a pulsed induction (or "PI") detector, a pulsed signal is generated for a few tens of microseconds (ps) in the detection head. The transmitting coil is then released (or relaxed), and the signal received after this magnetic excitation of the surrounding medium is analyzed. The same coil can then be used alternately to generate a pulse and to analyze the received field. Alternatively, two coils can be used, one for transmission and the other for reception, in various configurations (for example, the aforementioned "double D" configuration, or others). In all cases, the received signal can be seen as the sum of different exponential decays that are representative of the medium in which it operates.Signals associated with soil or salt water generally exhibit rapid exponential decays, with time constants of less than a few ps, whereas targets of interest have longer time constants. Naturally, the excitation signal must be renewed periodically (usually with a recurrence of about 1 kHz) and the received signal is averaged for processing, but this processing occurs in the time domain, not the frequency domain.

[0012] We can also have detectors that work simultaneously in both modes, namely frequency analysis and time analysis.

[0013] During a detection session, the user walks around a designated area to be explored, sweeping the ground with the handheld detector using the carrying handle, thus avoiding the need to bend or stoop. More specifically, the user stands upright and moves the detection coil parallel to the ground using the handle, sweeping the detection head from left to right, then from right to left, and so on. This method of operation is suitable for dynamic detectors often used in recreational handheld detectors. Operation without this swinging motion is also possible for detectors by selecting a so-called static mode. The transmitting coil produces a magnetic field which, at the level of a possible fixed metallic target, varies over time. This is due to the relative movement of the detection head with respect to the target. This magnetic field generates eddy currents in the metallic target that one seeks to detect.

[0014] These currents in turn generate magnetic fields that are captured by the receiving coil (which is sometimes the same as the transmitting coil). The received electromagnetic signal, also called the target detection signal or simply the detection signal in what follows, has a response that varies as the detection head moves over the target, proportionally to the magnetic fields produced by the circulation of eddy currents generated in the target and received by the detection head. The received electromagnetic signal is then processed by the receiving electronics so as to isolate the component of the detection signal associated with the target being sought and to separate it from unwanted or parasitic components, such as industrial or meteorological interference, variations associated with the ground, the Earth's magnetic field, interfering objects, etc.

[0015] To achieve this, the receiving electronics conventionally include ground discrimination and suppression modules.

[0016] Discrimination refers to the processing applied to the detection signal that enables the detection of the presence of one or more types of materials, or even the identification of their type. This discrimination can be achieved, for example, by measuring and analyzing the ratio between the received reactive and resistive signals. Discrimination can be used to identify soil and / or unwanted metallic objects, also referred to as parasitic objects or debris in what follows. It should be noted that a parasitic object can be of the same nature as a target. In the context of the invention, a parasitic object is an object whose signal masks that of a target.

[0017] Ground suppression makes it possible to attenuate or even eliminate the ground-related component of the detection signal. It can be achieved using several techniques, as described below. Generally, ground suppression is achieved by combining at least two of these techniques, one based on the relative uniformity of the ground and the other on the electromagnetic properties of the ground. Each technique often only reduces the ground response while minimizing the target response to a lesser extent. Techniques based on the relative uniformity of the ground are suitable for dynamic detectors, that is, detectors whose operation requires continuous movement of the head through a scanning motion.

[0018] The suppression can be achieved using a specific antenna (traditionally figure-eight) or its electronic equivalent having two loops as described in particular in application WO2007147199. This antenna allows remove soils considered homogeneous using two loops arranged to subtract a uniform field.

[0019] Suppression can be achieved using a combined demodulator. This combination modifies the gain according to the characteristic frequency of the desired target. For a CW detector, the combination is usually performed between signals received at several frequencies.

[0020] The discrimination described above can also be used. In this case, the discrimination process classifies the target as ground. With such a technique, the ground might obscure targets mixed with it.

[0021] The suppression can also be achieved using a filtering module. Assuming the ground is substantially uniform, a high-pass or, more traditionally, band-pass filter is applied to suppress the slowest spectral components corresponding to the assumed slow variations in the ground.

[0022] US patents US4128803, US4514692, US4700139, US7148692, US9207315 and US 11067715 provide improvements to metal detectors. These improvements aim to enhance ground suppression, signal quality for discrimination, or sensitivity to a wider range of targets.

[0023] Generally, the user can choose the cutoff frequency to adapt the filtering to the scanning speed and / or the uniformity of the soil type. This cutoff frequency can be set independently or in conjunction with a set of parameters to simplify the detector's operation. This parameter is called reactivity or "recovery speed" depending on the manufacturer.

[0024] A low cutoff frequency will only slightly suppress the ground signal. However, it will allow for less reduction of the detection signals and maintain a wide detection range. Conversely, a high cutoff frequency will suppress the ground signal more, at the expense of the detector's range.

[0025] Drag is a common problem induced by filtering techniques. Filtering spreads the temporal response of a signal over time and consequently mixes two closely related signals in time.

[0026] Also, when two targets are located close to each other, they generate relatively close time signals during the scanning of their environment. Depending on the spacing and the filtering used, the data representing said targets may be mixed in the detection signal received by the detector.

[0027] During detection, it is possible to encounter targets of varying sizes in the vicinity. In some cases, the targets may be of the same type, for example, an anti-personnel mine near an anti-tank mine. This configuration risks obscuring the presence of the anti-personnel mine from the operator and consequently placing them in a dangerous situation. In other, more common cases, in addition to the target being sought, It is possible to encounter other stray metallic objects. Generally, these stray objects are not rejected by the filtering module but by the discrimination process. The filtering module may therefore mix the components related to the detected metallic objects and potentially mask the intended target.

[0028] This masking problem is recurring. Indeed, it is common for large extraneous objects to be shallowly buried while the targets of interest are both smaller and buried deeper.

[0029] There is therefore a need to further improve metal detectors, in particular to improve the detection of the target of interest in the presence of one or more extraneous metallic objects in its vicinity. Summary of the invention

[0030] The invention aims to achieve this objective and relates to a metal detector for detecting at least one metallic target in a detection zone, referred to as the "target of interest," said metal detector comprising - At least one receiving coil arranged to detect a magnetic field coming from the detection zone and to generate a signal corresponding to said magnetic field, called the "detection signal", - A detection unit configured to process said detection signal in such a way as to detect the presence of a target of interest and possibly its type, said detection unit comprising: • A filtering module arranged to receive the detection signal as input and to filter it according to at least two first and second filtering configurations, the second filtering configuration having a narrower time support than the first filtering configuration, • A control module arranged to control the filtering module based on at least one piece of information contained in the detection signal and indicating the presence of a metallic object different from said target of interest, called "parasitic object", said metallic object having a signal power greater than a predefined power threshold, said control module so as to apply the second filtering configuration to the detection signal so as to attenuate at least one component of the detection signal relating to said parasitic object.

[0031] By "target of interest" is meant a metallic target sought by the user and which the detector will discriminate from other types of metallic targets encountered in the detection zone. This discrimination can be carried out according to criteria The target of interest can be fixed or user-configurable. Generally, the target of interest can be a non-ferrous metal. Depending on the user, the target of interest may be an object made of gold, silver, or even a meteorite. These examples are not exhaustive.

[0032] The target of interest may be buried in the detection zone.

[0033] The term "detection signal" means a digital signal representative of the detected magnetic field. In the following, the detection signal may correspond to the detection signal as provided by the receiving coil(s) or to a signal resulting from pre-processing of said provided signal. The pre-processing may include, for example, amplification and / or demodulation of the signal. The detection signal may be single-frequency.

[0034] In what follows, "parasitic object" means a metallic object present in the vicinity of the target of interest generating a signal stronger than said target.

[0035] By "attenuate at least one component of the detection signal relating to said parasitic object", it is meant that the component relating to the parasitic object, present in the detection signal, is attenuated, or even eliminated, in the output signal of the filtering module.

[0036] The detection signal filtering can initially be performed according to a first configuration defined by the user. The invention then offers the possibility of adapting this filtering to the conditions encountered in the field in order to prevent the target of interest from being masked by interfering objects present in its vicinity. For example, in the event of detection of an interfering object generating a strong signal, the control unit adapts the detection signal filtering by applying the second filtering configuration to the detection signal. Applying the second filtering configuration makes it possible to attenuate, or even eliminate, the component of the detection signal related to this unwanted object. This consequently prevents the target of interest from being masked.Also, unlike prior art detectors, the detector according to the invention allows the use of filtering to both suppress the ground component of the signal and attenuate the signal components related to interfering objects, when necessary. This avoids the mixing of components related to the different metallic objects described above.

[0037] The claimed detector thus makes it possible to maintain a detection depth while avoiding the masking of the target of interest by larger extraneous objects present in the vicinity of said target. The detector allows the spatial separation of the target of interest while maintaining equivalent sensitivity.

[0038] The control module adapts the filtering when necessary without requiring user intervention. This improves performance. detection and / or to facilitate the use of the detector, even for inexperienced users.

[0039] Advantageously, the control module is configured to deliver a substantially continuous output signal from said filtering module during a configuration change.

[0040] Compared with prior art metal object detectors, the component relating to the parasitic object initially present in the detection signal is attenuated or even eliminated in the output signal of the filtering module.

[0041] Advantageously, the invention is independent of the type of detection. The detector according to the invention can be single-frequency.

[0042] The detector according to the invention can be multi-frequency.

[0043] The detector according to the invention can be temporal.

[0044] The detector according to the invention can be hybrid.

[0045] Preferably, said information data corresponds to information representative of the power, particularly Euclidean, of the detection signal.

[0046] Preferably, the control module is configured to control the filtering module in such a way as to: - apply the first filtering configuration to the detection signal by default, and - when the control module detects the presence of the parasitic object, apply the second filtering configuration, in particular in parallel or as a replacement, of the first filtering configuration.

[0047] In some embodiments, the detector may include at least one demodulator.

[0048] Said demodulator can be arranged to receive the detection signal as input and to apply demodulation to said detection signal.

[0049] Alternatively, the demodulator is arranged to apply demodulation to the signal at the output of the filtering module. Preferably, the input of the demodulator is connected to the output of the filtering module.

[0050] The term "connected" means a direct or indirect connection between the input of the demodulator and the output of the filtering module.

[0051] In embodiments, the filtering module comprises at least first and second distinct filters, the first filter being arranged to filter the detection signal according to the first filtering configuration so as to generate a first filtered signal and the second filter being faster than the first filter and being arranged to filter the detection signal according to the second filtering configuration so as to generate a second filtered signal.

[0052] Preferably, the time support of the second filter is narrower than the time support of the first filter. In other words, [0;AMax], 3 t0| V t 6 [ t0; + æ[? |^t)| < , hi and h2 being the impulse response of the first and second filters, respectively. For sufficiently small amplitudes, also called tolerance, the impulse response of the second filter is always shorter than that of the first filter, up to this tolerance.

[0053] Preferably, the filtering module includes a delay line connected to the second filter, said delay line being arranged to compensate for the latency differences that may exist between the two filters.

[0054] Preferably, the time support of the second filter is less wide than the time support of the component relating to the parasitic object.

[0055] Preferably, the time support of the second filter has a width of less than 500 ms, in particular less than 400 ms, better less than 300 ms.

[0056] Preferably, the first and / or second filter is a low-cut filter. By way of example, the first and / or second filter can be chosen from a derivatizer, a series of derivatizers, a high-pass filter, a band-pass filter, a low-pass filter, or a combination of at least two of these.

[0057] Preferably, the application of the second filtering configuration is maintained for a duration at least equal to the detection duration of the parasitic object, preferably for a duration greater than or equal to the detection duration of the parasitic object + 30 ms.

[0058] In some embodiments, the control module is arranged to control the filtering module in such a way as to: • Deliver, by default, the first filtered signal, and • When the presence of a parasitic object is detected, output the second filtered signal or a combination, in particular linear, of the first and second filtered signals.

[0059] Preferably, the filtering module further comprises a combiner arranged to receive the first and second filtered signals as input and to output a combination, in particular a linear combination, of the aforementioned filtered signals. The control module is advantageously configured to determine the weighting values ​​associated with said combination. These weighting values ​​preferably range from 0 to 1, with the sum of the aforementioned values ​​equal to 1.

[0060] Preferably, the maximum excursion of the control module is dependent on the maximum power of the detection signal.

[0061] Preferably, the maximum excursion is defined so that the signal representing the distance to the target is monotonic as a function of distance.

[0062] Preferably, the control module is arranged to determine the weights of the aforementioned combination as a function of information representative of the power of the signal component relating to the parasitic object.

[0063] Preferably, the control module is configured to determine the weighting values ​​of the two signals so as to obtain a substantially monotonous sound level.

[0064] Preferably, the control module is configured to control the combiner so as to apply by default a value of 1 to the weighting associated with the first filtering function.

[0065] Also, the signal delivered by the filtering module can correspond to a combination, in particular linear, of the first and second filtered signals, the weight associated with each filtered signal can go from 0 to 1. The default values ​​of the weights can correspond to 1 for the weight associated with the first filtered signal and 0 for the weight of the second filtered signal.

[0066] In embodiments, the detector may further include a discrimination module arranged to receive as input the signal at the output of the filtering module, said discrimination module being configured to process the input signal of the latter and to detect the presence of one or more metallic objects.

[0067] The discrimination module can be configured to determine information indicating the type of metallic object detected.

[0068] In this way, the information data can correspond to information indicating the type of metallic object detected.

[0069] In embodiments, the detector may include more than two filtered signals.

[0070] In embodiments, the signal delivered by the filtering module may correspond to a combination, in particular linear, of at least three filtered signals.

[0071] In some embodiments, the filtering module comprises a single configurable filter adapted to filter the detection signal according to at least the first and second configurations, said single filter having at least one adjustment parameter for its time support, the control module being arranged to adjust the value of the adjustment parameter according to said data contained in the detection signal. Preferably, the adjustment parameter for the time support comprises at least eight distinct values.

[0072] The single filter can be continuously configurable.

[0073] Alternatively, the single filter has a configuration granularity allowing for at least two steps, better six steps, even better eight steps, for example ten steps, between the first configuration and the second configuration.

[0074] The single filter may correspond to a low-cut filter. By way of example, the single filter may be chosen from a derivatizer, a series of derivatizers, a high-pass filter, a band-pass filter, a low-pass filter, or a combination of at least two of these.

[0075] The output of the single filter can be used by the discrimination module to calculate the information data.

[0076] The data contained in the discrimination signal is advantageously used by the control module to analyze the presence of a strong component related to a spurious object. Upon detection of a spurious object, the control module commands the single filter to reduce its time support and adopt the second filtering configuration.

[0077] The detector may include at least one transmitting coil arranged to generate an incident magnetic field.

[0078] To appreciate the similarity of the following descriptions with the preceding ones, it is possible to imagine that the combination of several coils moving relative to a target is similar to a filter. Obviously, the following descriptions are not limited to this similarity between filtering and combinations of moving coils.

[0079] The invention further relates, particularly in combination with the foregoing, to a metal detector for detecting at least one metallic target in a detection zone, referred to as the "target of interest," said metal detector comprising: - At least two receiving coils, each arranged to detect a magnetic field coming from the detection zone and to generate a signal corresponding to said magnetic field, called the "detection signal", - A detection unit configured to process detection signals in such a way as to detect the presence of a target of interest in the detection zone and possibly its type, said detection unit comprising: • a combination module configured to receive detection signals as input and to deliver as output a signal corresponding to a combination, in particular a linear combination, of said detection signals, • a control module configured to command the combination module based on at least one piece of information contained in the detection signals and indicating the presence of a metallic object different from the target of interest, referred to as the "object" parasitic”, said metallic object having a signal power greater than a predefined power threshold, said control module being configured to determine the weighting weights of the combination of said detection signals so as to attenuate at least one component of the detection signal relating to said parasitic object.

[0080] By "attenuate at least one component of the detection signal relating to said parasitic object", it is meant that the component relating to the parasitic object, present in the detection signal, is attenuated, or even suppressed in the output signal of the combination module.

[0081] The detector may further include a discrimination module.

[0082] The detector may include all or part of the elements described above.

[0083] The invention further relates, particularly in combination with the foregoing, to a metal detector for detecting at least one metallic target in a detection zone, referred to as the "target of interest," said metal detector comprising: - At least two transmitting coils, each arranged to generate a transmission signal to create a corresponding incident magnetic field, - At least one receiving coil arranged to detect magnetic fields emanating from the detection zone resulting from each of the incident magnetic fields and to generate a signal corresponding to each resulting magnetic field, called the "detection signal", - A detection unit configured to process detection signals in such a way as to detect the presence of a target of interest in the detection zone and possibly its type, said detection unit comprising: • a combination module configured to receive detection signals as input and to output a signal corresponding to a combination, in particular a linear combination, of said detection signals, • a control module configured to control the transmitting coils based on at least one piece of information contained in the detection signals and indicating the presence of a metallic object different from said target of interest, called a "parasitic object", said metallic object having a signal power greater than a predefined power threshold, said control module being configured to adjust the gains of the transmission signals of said coils so as to attenuate at least one component of the detection signal relating to said parasitic object.

[0084] By "attenuate at least one component of the detection signal relating to said parasitic object", it is meant that the component relating to the parasitic object, present in the detection signal, is attenuated, or even suppressed in the output signal of the combination module.

[0085] The detector may further include a discrimination module.

[0086] The detector may include all or part of the elements described above.

[0087] The control module can be configured to determine the weighting values ​​of the two signals so as to obtain a substantially monotonous sound level. Brief description of the figures

[0088] The following description, with reference to the accompanying drawings, given by way of non-limiting examples, will clearly explain what the invention consists of and how it can be implemented. In the accompanying figures:

[0089] [Fig.1] Fig.1 schematically represents a detector according to the invention during a detection session conducted by a user;

[0090] [Fig.2] The [Fig.2] is a functional diagram representing, in the form of a block diagram, the architecture of the detector of the [Fig.1];

[0091] [Fig.3] [Fig.3] represents a processing unit according to a first embodiment;

[0092] [Fig.4] [Fig.4] represents a processing unit according to a second embodiment;

[0093] [Fig. 5] [Fig. 5] represents a processing unit according to a third embodiment; and

[0094] [Fig.6] [Fig.6] is an example of curves representing the sound level likely to be generated by the detector according to the invention. Description of method(s) of implementation

[0095] In the figures, and unless otherwise specified, identical elements shall bear the same reference symbols.

[0096] Figure 1 shows an example of a metal detector 10 for detecting at least one metallic target within a detection zone, referred to as the "target of interest". As illustrated, the detector 10 comprises three elements communicating with each other via an analog or digital connection, wired or wireless, for example, a radio link. These three elements correspond to: - a detection disc 11 or detection head which incorporates the transmitting and / or receiving coil(s); - a control module 12, also called a remote control, which allows you to configure all the various functions of the detector using a adapted human-machine interface (or HMI, from the English "Human-Machine Interface"); - an audio playback device 13, which may be wired headphones or preferably wireless headphones, one or more loudspeakers, a haptic feedback system, or a bone conduction device, comprising electroacoustic transducers (e.g., one for each ear) capable of reproducing the detection signal in a form audible to the user. The audio playback device 13 is not required and may be replaced by the control module 12. Nevertheless, the device 13 is the preferred means of playback due to the sensitivity and dynamic range of the human ear compared to other human sensory organs.

[0097] In the illustrated example, all or part of the electronics for managing the transmission, reception, and processing of detection signals to achieve the detection and discrimination of targets may be integrated into the detection head, or into the remote control, or may be distributed between the detection head and the remote control.

[0098] In addition, the detector generally comprises a shaft 15 having a handle and an armrest. The end of the shaft 15 opposite the armrest and the handle is adapted to support the detection disc, which can be removably coupled to it. The remote control 12 is adapted to be fixed to the shaft 15, just in front of the handle as shown in [Fig. 1].

[0099] It is possible to decompose the architecture of detector 10 in the manner that will be described below with reference to the functional diagram of [Fig.2].

[0100] This functional breakdown allows us to distinguish the main functions performed by the detector, but it is not limiting to how these functions are implemented at the hardware level. Some functions may be grouped together or, conversely, separated into specific hardware components, which may also implement additional functions that are not considered here because they are not essential for describing embodiments of the invention. Digital components may be implemented in an analog manner unless otherwise specified.

[0101] The detector comprises an electromagnetic transmission / reception assembly 21, comprising the coil(s), namely at least one receiving coil and possibly at least one transmitting coil, knowing that these two coils may also be one. This assembly 21 is installed in the detection head 11 of [Fig. 1],

[0102] The detector also includes an analog front-end module and a digital front-end module. The assembly 22 of these two front-end modules ensures the emission (Tx) of the incident magnetic field.

[0103] In the receive direction (Rx), the front-end modules 22 perform the transposition of the analog signals adapted to the coils into signals, particularly at very low frequencies (generally below 50 Hz), representative of the speed at which the detection head passes over the target. This transposition can be carried out in different ways depending on the type of detector. Frequency transposition is generally applied for frequency-analyzing detectors, which can be achieved, for example, by demodulation and associated filtering. For a time-domain detector, the transposition is usually performed by integrating time windows. Most often, the analog front-end module integrates the power amplifiers of the transmit chain (Tx) and the low-noise amplifiers of the receive chains (Rx).Traditionally, frequency detectors are called VLF detectors, which stands for the frequency band from 3 kHz to 30 kHz. However, lower frequencies, and especially higher frequencies—that is, frequencies in a wider range than the VLF band, for example, between 1 kHz and 150 kHz—can also be used for metal detection in specific searches. This is why we refer to it as an "extended" VLF band. The analog front-end module can also integrate demodulators, but demodulators are most often implemented in the digital front-end module for narrowband detectors.

[0104] The person skilled in the art will also appreciate that the analog / digital separation depends on the design of the equipment, and is not limited, in practice, by the examples considered here.

[0105] The detector 1O further comprises a detection unit 24, which is adapted to apply detection treatments in order to determine the presence and possibly the type of the metallic target of interest.

[0106] The detection unit 24 can be implemented in the form of a miniaturized electronic circuit, which allows it, in certain metal detectors, to be integrated into the detection disc 11. Such a circuit is suitable for digitizing and analyzing the detection signals and for producing the detection data and the discrimination data, this data then being sent, in particular in real time, to the user interface 25, for example by digital radio link as shown in [Fig.1] or by wired connection, for sound playback by the audio headset 13 and possibly, in addition, for display at the remote control 12.

[0107] The performance of the detector 10 may be affected by the different levels of soil mineralization that may be encountered depending on the use case. These may include, for example, naturally occurring magnetic mineralizations such as iron oxide, ferrites, and magnetites. They may also include localized mineralizations linked to former sites of human occupation (which are also magnetic), for example, Hearths, terracotta, ferrites, slag, etc. It may also be seaside mineralizations which can range from magnetic grade (black sands) to electrically conductive grade (salt water), depending on the beaches and regions.

[0108] Detection is also affected by electromagnetic interference (or EMI, from the English "Electromagnetic interference") which is widespread, particularly in urban areas (high voltage line, electrical transformer, electric fence, power lines, radio relays, mobile phones, computers, televisions, other metal detectors operating nearby, etc.), as well as by metallic pollution.

[0109] In particular, detection may be disrupted by the presence of unwanted metallic objects which may obscure the target of interest.

[0110] The detection unit 24 includes a filtering module 28 which filters the ground component of the detection signal. The module attenuates the component of the detection signal related to one or more parasitic metallic object(s) significantly larger than the target of interest and present in its vicinity.

[0111] As will be described later, the filtering module 28 is arranged to receive the detection signal as input and to filter it according to at least first and second filtering configurations, the second filtering configuration having a shorter time support than the first filtering configuration. Advantageously, the filtering adaptation is maintained for a duration at least equal to the detection duration of the spurious object, preferably for a duration greater than or equal to the detection duration of the spurious object + 30 ms. The filtering adaptation, in particular the application of the second filtering function, occurs over a duration shorter than the detection duration of the target of interest + 30 ms.

[0112] The detection unit 24 further comprises a control module 30 arranged to control the filtering module 28 based on at least one piece of data indicating the presence of a parasitic object. This data may correspond to the power, for example Euclidean, of the detection signal so as to adapt the filtering applied to the detection signal in order to attenuate at least a power of the detection signal relative to at least one metallic object considered undesirable.

[0113] In the illustrated example, the detection unit 24 further includes an optional discrimination module 29 which receives as input the signal from the output of the filtering module 28. Such a discrimination module is optional. The discrimination module 29 is configured to process the input signal and to detect the presence of one or more metallic objects and, optionally, their type. The discrimination module is capable of determining information about the type of object. metallic detected and possibly whether said object corresponds to the target of interest or to a parasitic object.

[0114] When the detector includes the aforementioned discrimination module, the information data can be an output of the discrimination module. In this embodiment, the output of the discrimination module can correspond to information indicating the type of metallic object detected and, optionally, whether the metallic object corresponds to the target of interest or whether it is a spurious object.

[0115] The detector includes a human-machine interface, also called a user interface 25 (or HMI). The user interface 25 allows the equipment to be configured, on the one hand, and provides information indicating the possible presence of metallic targets and, possibly, information relating to their identification, as well as detection aids to assist the user in their search, on the other. As already mentioned in the introduction, audio output is the most commonly used interface for this purpose, and it is primarily this interface that is the subject of the present invention. The user interface 25 thus includes the headphones 13 of [Fig. 1] and / or any other electroacoustic transducer, for example, one or more loudspeakers.

[0116] However, all or part of the aforementioned information can also be displayed in another form, for example, visible on a screen. The user interface 25 includes the remote control 12 ([Fig. 1]) for this purpose. The remote control also allows the user to adjust the main detection settings such as sensitivity, discrimination, ground effects, tones, the frequency(ies) used for the signals emitted and processed by the detection head, volume, etc., as well as to select factory programs or those previously created by the user using the remote control.

[0117] Figures 3 to 5 illustrate embodiments of the detection unit 24 according to the invention.

[0118] In the embodiment illustrated in [Fig. 3], the filtering module 28 comprises a single configurable filter 40a capable of filtering the detection signal according to at least the first and second configurations. This single filter 40a comprises at least one adjustment parameter for setting its time support. The control module is arranged to adjust the value of this adjustment parameter according to the information data. Preferably, the adjustment parameter for the time support comprises at least eight distinct values.

[0119] The unique 40a filter can be continuously configured.

[0120] Alternatively, the single filter 40a has a configuration granularity allowing at least ten steps between the first configuration and the second configuration.

[0121] In the illustrated example, the single filter 40a corresponds to a low-cut filter.

[0122] In the embodiment illustrated in [Fig.4], the filtering module 28 comprises two separate filters 40b and 40c. The first filter allows the detection signal to be filtered according to the first filtering configuration so as to generate a first filtered signal.

[0123] The second filter 40c is faster than the first filter 40b and filters the detection signal according to the second filtering configuration to generate a second filtered signal. The time span of the second filter 40b is narrower than the time span of the component related to the parasitic object.

[0124] In the illustrated example, the first and / or second filter is a low-cut filter.

[0125] The filtering module 28 further comprises a delay line 42 connected to the second filter 40c, as illustrated, and allows to compensate for any differences in latency between the two filters 40b and 40c.

[0126] The possible discrimination module 29 is connected on one side to the filtering module 28, and on the other side to the control module 30. Also, depending on the output data of the discrimination module 28, the control module adapts the filtering in order to avoid the masking of the target of interest by a parasitic metallic object.

[0127] Also, the control module is arranged to control the filtering module in such a way as to: • Deliver at the output, particularly by default, the first filtered signal at the output of the first 40-bit filter, and • When the presence of a parasitic object is detected, output the second filtered signal resulting from the second filter 40c or a combination of the two filtered signals from the two filters 40b and 40c.

[0128] The filtering module 28 further comprises a combiner 44 arranged to receive as input the first and second filtered signals and to output a combination, in particular a linear combination, of the aforementioned filtered signals. The control module 30 is advantageously configured to determine the weighting values ​​associated with each filtered signal in said combination. These weighting values ​​range from 0 to 1, with the sum of the aforementioned values ​​equal to 1. The weights of the aforementioned combination are preferably determined based on at least one piece of information relating to the power of the signal component related to the parasitic object.

[0129] In the illustrated embodiment, the control module 30 is arranged to control the combiner 44 so as to apply a default value of 1 to weight associated with the first filtered signal. In other words, the filtering module 28 delivers the first filtered signal by default.

[0130] Also, the signal delivered by the filtering module 28 corresponds to a combination, in particular a linear one, of the first and second filtered signals. The default values ​​of the weights correspond to 1 for the weight associated with the first filtered signal and 0 for the weight of the second filtered signal.

[0131] Advantageously, the control module determines the values ​​of the weights associated with the filters 40a and 40b so as to obtain a substantially monotonous sound level, as illustrated in [Fig.6].

[0132] Filters 40b and 40c have different signal-to-noise ratios. In terms of signal-to-noise ratio, we hear AWGN (additive white Gaussian noise), which is representative of the thermal noise received by the receiving electronics. The faster 40c filter has a narrower bandwidth and therefore a lower signal-to-noise ratio than filter 40a. Generally, the sound level is a function of the signal strength. The operator's audio perception therefore differs depending on the filter, whether for noise, for a target, or for both.

[0133] Figure 6 shows representative curves of the sound level of the target of interest. depending on its depth. Curve A corresponds to the first filter, 40b. Curve B corresponds to the second filter, 40c. Switching at a fixed threshold without weighting corresponds to curve C. As illustrated, when switching from filter 40b to the second filter, 40c, without weighting, the sound level is not monotonic. This can influence the user's perception. Curve D, on the other hand, shows a transition weighted by the detection level of the component related to the unwanted object. As can be seen, such weighting makes it possible to obtain a monotonic sound level.

[0134] An example of a detection unit 24 according to a second embodiment has been illustrated in [Fig.5].

[0135] In the illustrated example, the information data used by the control module 30 is calculated from the second filtered signal at the output of the second filter 40c.

[0136] In this embodiment, the detection unit includes an additional discrimination module 43. This additional discrimination module 43 is connected to both the second filter 40c and the control module 30. Such a configuration has the advantage of allowing the filtering adaptation to be triggered earlier. This makes it possible to improve the aforementioned masking effects upstream (temporally), and more particularly the masking at the beginning of the filtering process.

[0137] Of course, the invention is not limited to the examples described.

[0138] The filtering module can be arranged to switch continuously from the first configuration to the second configuration. In such an embodiment, the filtering module may not include a combiner.

[0139] The detector may not include a discrimination module.

Claims

Demands

1. A metal detector for detecting at least one metallic target in a detection zone, referred to as the "target of interest," said metal detector comprising: - At least one receiving coil (21) arranged to detect a magnetic field emanating from the detection zone and to generate a signal corresponding to said magnetic field, referred to as the "detection signal," - A detection unit (24) configured to process said detection signal so as to detect the presence of the target of interest and optionally its type, said detection unit comprising: • A filtering module (28) arranged to receive the detection signal as input and to filter the latter according to at least first and second filtering configurations, the second filtering configuration having a smaller time support than the first filtering configuration,• A control module (30) arranged to control the filtering module (28) based on at least one piece of information contained in the detection signal and indicating the presence of a metallic object different from said target of interest, referred to as the "parasitic object", said metallic object having a signal power greater than a predefined power threshold, so as to apply the second filtering configuration to the detection signal so as to attenuate at least one component of the detection signal relating to said parasitic object.

2. Detector according to claim 1, said control module (30) being configured to deliver a substantially continuous output signal from said module during a configuration change.

3. Detector according to claim 1 or 2, the control module (30) being configured to control the filtering module (28) so as to: - apply the first filtering configuration to the detection signal by default, and - when the control module (30) detects the presence of the parasitic object, apply the second filtering configuration, in particular in combination, in parallel or in replacement of the first filtering configuration.

4. Detector according to any one of the preceding claims, said information data corresponding to information relating to the power, in particular Euclidean, of the detection signal.

5. Detector according to any one of the preceding claims, further comprising a discrimination module (29) arranged to receive as input the signal at the output of the filtering module (28), said discrimination module (29) being configured to process the input signal therefrom and to detect the presence of one or more metallic objects.

6. Detector according to the preceding claim, said information data corresponding to information indicating the type of object detected.

7. Detector according to any one of the preceding claims, the filtering module (28) comprising at least first and second separate filters (40b, 40c), the first filter (40b) being arranged to filter the detection signal according to the first filtering configuration so as to generate a first filtered signal and the second filter (40c) being faster than the first filter (40b) and being arranged to filter the detection signal according to the second filtering configuration so as to generate a second filtered signal.

8. Detector according to claim 7, the time support of the second filter (40c) being less wide than the time support of the component relating to the parasitic object.

9. Detector according to claim 7 or 8, the filtering module (28) comprising a delay line (42) connected to the second filter (40c), said delay line being arranged to compensate for the latency differences likely between the two filters (40b, 40c).

10. Detector according to any one of claims 7 to 9, the filtering module (28) further comprising a combiner (44) arranged to receive at least the first and second filtered signals as input and deliver at output a combination, in particular linear, of the aforementioned filtered signals, the control module (30) preferably being configured to determine the weighting values ​​associated with said combination, said weighting values ​​preferably ranging from 0 to 1 with the sum of the aforementioned values ​​equal to 1.

11. Detector according to any one of the preceding claims, the maximum excursion of the control module being dependent on the maximum power of the detection signal.

12. Detector according to the preceding claim, the maximum excursion being defined so that the signal representing the distance to the target is monotonic as a function of distance.

13. Detector according to any one of claims 1 to 6, the filtering module (28) comprising a single configurable filter (40a) adapted to filter the detection signal according to at least the first and second configurations, said single filter (40a) having at least one adjustment parameter for its time support, the control module (30) being arranged to adjust the value of the adjustment parameter according to said information data.

14. Detector according to the preceding claim, the time support adjustment parameter having at least eight distinct values.

15. Detector according to any one of claims 7 to 14, the first filter (40b), the second filter (40c) and / or the single filter (40a) being a low-cut filter.

16. Detector according to any one of claims 7 to 14, the first filter (40a), the second filter (40c) and / or the single filter (40a) being a low-pass filter.

17. Detector according to any one of the preceding claims, the application of the second filtering configuration being maintained for a duration at least equal to the detection duration of the spurious object, preferably for a duration greater than or equal to the detection duration of the spurious object + 30 ms.

18. Detector according to any one of the preceding claims, comprising at least one demodulator, said demodulator being arranged to receive the detection signal as input and to apply a demodulation to said detection signal.

19. A metal detector for detecting at least one metallic target in a detection zone, referred to as the "target of interest," said metal detector comprising: • At least two receiving coils (21), each arranged to detect a magnetic field emanating from the detection zone and to generate a signal corresponding to said magnetic field, referred to as the "detection signal," • A detection unit (24) configured to process the detection signals so as to detect the presence of a target of interest in the detection zone and optionally its type, said detection unit comprising: • a combining module (44) configured to receive the detection signals as input and to output a signal corresponding to a combination, in particular a linear combination, of said detection signals,• a control module (30) configured to control the combination module (44) based on at least one piece of information contained in the detection signals and indicating the presence of a metallic object different from said target of interest, referred to as a "parasitic object", said metallic object having a signal power greater than a predefined power threshold, said control module (30) being configured to determine the weighting factors of the combination of said detection signals so as to attenuate at least one component of the detection signal relating to said parasitic object.

20. Metal detector for detecting at least one metallic target within a detection zone, referred to as the "target of interest", said metal detector comprising: At least two transmitting coils (21), each arranged to generate a transmission signal to create a corresponding incident magnetic field, At least one receiving coil (21) arranged to detect the magnetic fields emanating from the detection zone resulting from each of the incident magnetic fields and to generate a signal corresponding to each resulting magnetic field, referred to as the "detection signal", A detection unit (24) configured to process the detection signals so as to detect the presence of a target of interest in the detection zone and optionally its type, said detection unit (24) comprising: • a combination module (44) configured to receive the detection signals as input and to deliver as output a signal corresponding to a combination, in particular a linear combination, of said detection signals, • a control module (30) configured to control the transmitting coils (21) according to at least one data contained in the detection signals and indicating the presence of a metallic object different from said target of interest, called "parasitic object", said metallic object having a signal power greater than a predefined power threshold, said control module (30) being configured to adjust the gains and phases of the transmission signals of said coils so as to attenuate at least one component of the detection signal relating to said parasitic object.

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