Patent Publication Number: US-2022236442-A1

Title: Open Metal Detector

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of continuous wave detectors designed for the detection of unauthorized objects or materials in a protected access area. 
     STATE OF THE ART 
     It appears now necessary to monitor with great reliability the attempts to introduce certain products, for example but not exclusively weapons or explosive devices, into a sensitive area or the attempts to get them out of it. 
     The problem thus posed covers a very wide range of situations, which encompasses in particular and without limitation the attempt to introduce products into a protected area, such as a store, a school, a train station, a public or private organization, or the attempt to get products out of a defined perimeter, for example in case of theft in a company or on a protected site. 
     For many years, continuous wave walk-through detectors for detecting metallic objects have in particular been proposed, that is to say walk-through detectors using waves of constant amplitude and frequency in frequency ranges comprised between 70 Hz and 50 kHz typically, as opposed to the pulse detectors which work in the time domain and use “pulses” of magnetic field of approximately 100 μs to 500 μs and use the receiver to monitor the weakening of the magnetic field. 
     The general structure and the general operation of such continuous wave equipment are well known to those skilled in the art. Essentially, the walk-through detector comprises a transmitter assembly housed in a first column comprising transmitter coils which generate a magnetic field and a receiver assembly housed in a second column comprising receiver coils which detect disturbances of this field due to metallic objects carried by an individual passing through the walk-through detector. The first column and the second column are connected together by a cross member, which can carry complementary accessories such as a camera, a sound alarm and/or a visual alarm. Examples of such walk-through metal detectors will be in documents EP 1 750 147 and EP 1 892 542. 
     The transmitter and receiver coils are supplied and monitored by an electronic control unit. In order to allow the demodulation of the signal received by the receiver coils and to detect, with a reduced number of false alarms, metallic objects passing through the walk-through detector, the signals for driving the transmitter coils must have the same frequency and a phase coherent with the frequency and the phase of the demodulation signals of the receiver coils. This requirement is fulfilled by using a single clock to define the electrical signals transmitted to the transmitter coils and to demodulate the electrical signals transmitted by the receiver coils, and by connecting the control unit to both the transmitter assembly and the receiver assembly by wired means. 
     However, the current climate resulting from various attacks in public places has given rise to the need to be able to rapidly deploy checkpoints and security stations in order to allow detecting weapons at the entrance to public places, such as stadiums, concert halls, department stores, etc. Indeed, the surveillance of these public places requires a quick installation and withdrawal of the inspection equipment, insofar as the entrance to these public places also often serves as an emergency exit, so that all obstacles (including inspection equipment) must be able to be instantly withdrawn. 
     Yet, traditional walk-through detectors are not suitable for this type of situation insofar as their components must be assembled and disassembled systematically during their installation and their withdrawal. It is therefore necessary to bring and mount the two columns, the cross member as well as the control interface on the place of inspection, then to disassemble them, to put them in their storage box and then to remove them once the inspection is completed. 
     It has already been proposed to provide the pre-assembled walk-through detectors and to store them on adapted rolling trolleys in order to reduce the time for mounting the walk-through detectors. Such a solution is however not viable when the public place is large or when it does not have a suitable storage area for the trolleys and the pre-assembled walk-through detectors, which are very bulky. 
     DISCLOSURE OF THE INVENTION 
     One aim of the invention is to propose a continuous wave system for detecting metallic objects, which can be deployed and withdrawn quickly on a given area, while ensuring effective detection of the metallic objects with a reduced number of false alarms. 
     To this end, it is proposed, according to a first aspect of the invention, a continuous wave system for detecting metallic objects, comprising a transmitter assembly and a receiver assembly, in which:
     the transmitter assembly comprises at least one transmitter coil housed in a first column, a first clock configured to emit a first electrical signal at a first given frequency, at least one first frequency generator configured to transmit to a corresponding transmitter coil an electrical signal having a frequency which is synchronized to the first frequency such that the transmitter coil emits a magnetic field,   the receiver assembly comprises:   at least one receiver coil housed in a second column, distinct from the first column, said receiver coil being configured to produce an electrical signal as a function of the magnetic field emitted by the transmitter coil, a second clock configured to emit a second signal at a second given frequency, and at least one second frequency generator configured to determine an electrical signal having a frequency which is synchronized to the second frequency, and   a unit for comparing the electrical signal produced by the receiver coil with the electrical signal determined by the second frequency generator.   

     The system further comprises:
     a detector configured to detect an instant of zero crossing of the set of the electrical signals transmitted by the at least one first frequency generator or the at least one second frequency generator,   a signal generator configured to generate a phase realignment signal synchronized to the zero crossing instant detected by the detector and   transmission means comprising a wireless communication interface configured to transmit to the transmitter assembly or to the receiver assembly the phase realignment signal in order to realign the phase of the first clock and of the second clock.   

     Some preferred but non-limiting characteristics of the continuous wave detection system described above are as follows, taken individually or in combination:
     the second given frequency is substantially equal to the first given frequency.   the first clock is placed in the first column and the second clock is placed in the second column.   the first clock and the second clock are displaced outside the first column and the second column.   the detector is configured to detect an instant of zero crossing of the set of the electrical signals transmitted by the first frequency generator or the second frequency generator when said signals have a positive slope.   the first column is separated from the second column so that the detection system has no physical connection between the first column and the second column.   the transmitter assembly comprises at least two transmitter coils and as many associated first frequency generators, the receiver assembly comprises at least two receiver coils and as many associated second frequency generators.   the wireless communication interface comprises the at least one of the following elements: a radiofrequency interface, an optical interface and/or an inductive interface. And/or   the wireless communication interface comprises an inductive interface, said inductive interface comprising the at least one of the transmitter coils and of the receiver coils of the transmitter assembly and of the receiver assembly, respectively.   

     According to a second aspect, the invention proposes a method for detecting metallic objects with a continuous wave detection system as described above, said method comprising the following steps:
     S 1 : emitting a magnetic field at least at one transmitter coil from a corresponding electrical signal having a frequency which is a function of the first frequency of the first clock   S 2 : producing at least at one receiver coil at least one electrical signal as a function of the magnetic field emitted in step   S 3 : determining an electrical signal having a frequency which is a function of the second frequency of the second clock   S 4 : comparing the electrical signal produced in step S 2  and the electrical signal determined in step S 3     S 5 : detecting an instant of zero crossing of the set of the electrical signals whose frequency is a function of the first frequency or of the set of the electrical signals whose frequency is a function of the second frequency   S 6 : generating a phase realignment signal synchronized to the zero crossing instant detected in step S 5  and   S 7 : transmitting using a wireless communication interface the phase realignment signal of the first clock and of the second clock to the transmitter assembly or to the receiver assembly.   

     Some preferred but non-limiting characteristics of the method are as follows, taken individually or in combination:
     the steps S 2  and S 3  are simultaneous.   during step S 5 , the zero crossing instant is detected when the set of the electrical signals whose frequency is a function of the first frequency or the set of the electrical signals whose frequency is a function of the second frequency has a positive slope.   during step S 6 , the phase realignment signal is generated at the transmitter assembly and transmitted to the receiver assembly, so as to realign the phase of the second clock with that of the first clock. And/or   during step S 6 , the phase realignment signal is generated at the receiver assembly and transmitted to the transmitter assembly, so as to realign the phase of the first clock with that of the second clock.   

    
    
     
       DESCRIPTION OF THE FIGURES 
       Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and not limiting, and which should be read in relation to the appended drawings in which: 
         FIG. 1  schematically illustrates a detection system in accordance with a first embodiment of the invention; 
         FIG. 2  schematically illustrates a detection system in accordance with a second embodiment of the invention; 
         FIG. 3  illustrates the frequency of the electrical signal transmitted to three transmitter coils, the corresponding pulses received as input of the detector and the pulses generated by the corresponding signal generator of an exemplary embodiment of a detection system in accordance with the invention; 
         FIG. 4  illustrates the sinusoidal frequencies of the three electrical signals of  FIG. 3 , as well as the instant of zero crossing with a positive slope for these three electrical signals; 
         FIG. 5  illustrates one example of a detection assembly comprising three detection systems in accordance with the invention; and 
         FIG. 6  is a flowchart of steps of a detection method according to one embodiment of the invention. 
     
    
    
     In all of the figures, similar elements bear identical references. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The appended figures, in particular in  FIG. 1 , represent a continuous wave detection system in accordance with the present invention comprising two columns  2 ,  3  defining therebetween a channel  4  through which individuals  5  to be checked can pass. As a non-limiting example, the height of the columns  2 ,  3  can be comprised between 150 cm and 200 cm, advantageously between 150 cm and 180 cm and the deviation between the two columns  2 ,  3  is advantageously comprised between 70 cm and 100 cm. 
     By column  2 ,  3 , it is understood here any base, whatever its shape, capable of housing the detection means and of defining the passage channel for the individuals  5  to be checked. The column may thus have the shape of a substantially cylindrical or tubular post, of substantially planar panels or delimiting an ogive-shaped or elliptical space, etc. 
     The detection system  1  comprises a transmitter assembly  10 , a receiver assembly  20  and analysis means  30 . The transmitter assembly  10  comprises at least one transmitter coil Tx 1 , . . . , Txm housed in a first column  2  and configured to emit a magnetic field. The receiver assembly  20  comprises at least one receiver coil Rx 1 , . . . , Rxn housed in a second column  3 , distinct from the first column  2 , which is configured to detect disturbances of the magnetic field due to metallic objects. Finally, the analysis means  30  are suitable for analyzing the signals derived from the receiver coils to detect the presence of metallic objects carried by an individual  5  passing through said channel formed between the two columns  2 ,  3 . 
     The transmitter Tx 1 , . . . , Txm and receiver Rx 1 , . . . , Rxn coils preferably cover the entire height of the columns  2 ,  3 . They can be the object of numerous known embodiments, as used today in conventional walk-through metal detectors. Their operation in itself is also conventional. The structure and the operation of the transmitter Tx 1 , . . . , Txm and receiver Rx 1 , . . . , Rxn coils will therefore not be described in detail below. It will however be noted that preferably each transmitter Tx 1 , . . . , Txm or receiver Rx 1 , . . . , Rxn coil can be formed by several separate windings whose relative distribution over the height of the columns  2 ,  3  is adapted to optimize detection and is driven by the analysis means  30  to emit alternating inductive fields over a frequency range and receive all of these alternating inductive fields over said frequency range, respectively. 
     Preferably, the metal detector inductive fields generated by the transmitter Tx 1 , . . . , Txm and receiver Rx 1 , . . . , Rxn coils are in the frequency range comprised between 70 Hz and 50 kHz, preferably between 100 Hz and 50 kHz. 
     In order to quickly deploy and withdraw the system  1  in a given checkpoint, the first column  2  and the second column  3  of the system  1  are distinct and separate, i.e. they are no longer physically connected to each other by a cross member or by electric cables, and the signals of the transmitter assembly  10  and of the receiver assembly  20  are realigned in phase by wireless transmission means  40 . 
     For that, the transmitter assembly  10  further comprises a first clock  12  configured to transmit a first electrical signal at a first given frequency F 1  and at least one first frequency generator  14  configured to transmit to a corresponding transmitter coil Tx 1 , . . . , Txm an electrical signal having a frequency which is synchronized to the first frequency F 1  so that the transmitter coil Tx 1 , . . . , Txm emits a magnetic field. 
     Furthermore, the receiver assembly  20  comprises a second clock  22  configured to emit a second signal at a second given frequency F 2  substantially equal to the first given frequency F 1 , and at least one second frequency generator  24  configured to determine an electrical signal having a frequency which is synchronized to the second frequency F 2 , as well as a unit for comparing the electrical signal produced by the receiver coil Rx 1 , . . . , Rxn with the electrical signal determined by the second frequency generator  24 . 
     Finally, the system  1  also comprises:
     a detector  50  configured to detect an instant of zero crossing of the set of the electrical signals transmitted by the first frequency generator  14  or the second frequency generator  24 ,   a signal generator  60  configured to generate a phase realignment signal synchronized to the zero crossing instant detected by the detector  50  and   the wireless transmission means  40 , which are configured to transmit to the transmitter assembly  10  or to the receiver assembly  20  the phase realignment signal in order to realign the phase of the first clock  12  and of the second clock  22 .   

     It will be understood that the first clock  12  and the second clock  22  are not necessarily housed in the first column  2  and the second column  3  but can be fixed outside these columns  2 ,  3  or, as a variant, distant from said columns  2 ,  3  and connected, by wired or wireless means, to the corresponding frequency generators and to the wireless transmission means  40 . 
     In the following, the invention will be described for the sake of simplicity in the case where the first clock  12  and the second clock  22  are housed in the first column  2  and in the second column  3 , respectively. Furthermore, the invention will be described in the case where the first and the second frequency generator  14 ,  24  comprise frequency dividers. This is however not limiting, any programmable logic device capable of generating synchronized frequencies starting from the same clock  12 ,  22  can be used. 
     The first clock  12  and the second clock  22  emit electrical signals at a first and a second frequency F 1 , F 2 , respectively. In one embodiment, the first frequency F 1  is substantially identical to the second frequency F 2 . It will however be noted that the frequency of the clocks  14 ,  24  being generally based on the mechanical resonance of a quartz, the first and the second frequency F 1 , F 2  are necessarily slightly different due to the bias induced by the quartz oscillations. This also explains the need to realign the phase of the first clock  12  and of the second clock  22  in order to allow the demodulation of the signals received by the receiver coils Rx 1 , . . . , Rxn. 
     The Transmitter Assembly  10   
     The transmitter assembly  10 , which is housed in the first column  2  of the system  1 , comprises at least one transmitter coil Tx 1 , . . . , Txm. Preferably, the transmitter assembly  10  comprises several transmitter coils Tx 1 , . . . , Txm, for example between four and ten. 
     Each transmitter coil Tx 1 , . . . , Txm is configured to receive an electrical signal and emit a magnetic field which depends on the frequency of the received electrical signal. Preferably, the frequencies of the electrical signals transmitted to the transmitter coils Tx 1 , . . . , Txm are all different, while remaining a sub-multiple of the first frequency F 1  of the first clock  12 . For that, the first clock  12  is connected to a set of first frequency dividers  14  which are each connected to an associated transmitter coil Tx 1 , . . . , Txm and are each associated with a different division setpoint N 1 , . . . Nm. 
     By way of non-limiting example, the frequency Fi of the electrical signal received by a given transmitter coil Txi (i comprised between 1 and m) is equal to the quotient of the frequency of the first clock  12  and of a predefined division setpoint Ni, where the division setpoint Ni varies from one frequency divider  14  to another. Preferably, the frequencies Fi generated by the first frequency dividers  14  are different from each other (Ni≠Nj ∀ (i, j) ∈ [1; m]) 
     Furthermore, each transmitter coil Txi can be excited by at least one of these frequencies Fi (i ∈ [1; m])) 
     The Receiver Assembly  20   
     The receiver assembly  20 , which is housed in the second column  3  of the system  1 , comprises at least one receiver coil Rx 1 , . . . , Rxn. Preferably, the receiver assembly  20  comprises several receiver coils Rx 1 , . . . , Rxn, for example between four and ten. The receiver assembly  20  can comprise as many receiver coils Rx 1 , . . . , Rxn as the transmitter assembly  10  comprises transmitter coils Tx 1 , . . . , Txm, or a different number (m≠n). 
     Each receiver coil Rx 1 , . . . , Rxn is configured to detect disturbances of the magnetic field due to metallic objects and generate an electric signal whose frequency depends on the detected magnetic field. 
     The second clock  22  emits a second electrical signal having the second given frequency F 2 . This electrical signal is transmitted to the second frequency dividers  24 , which generate as output an electrical signal whose frequency is a sub-multiple of the second frequency F 2 . 
     The frequencies thus obtained by the second frequency dividers  24  are transmitted, for each receiver coil Rx 1 , . . . , Rxn and each second frequency divider  24 , to an associated comparison unit  26  which aims to demodulate the electrical signal generated by the receiver coil Rx 1 , . . . , Rxn. For this purpose, the comparison unit  26  compares, for each receiver coil Rx 1 , . . . , Rxn, the electrical signal produced by this receiver coil Rx 1 , . . . , Rxn with the electrical signal determined by the second frequency divider  24 . This information is then communicated to the analysis means  30  which deduce therefrom whether the magnetic field generated by the transmitter assembly  10  has been disturbed by metallic objects. 
     For example, the comparison unit  26  can comprise a subtraction unit which is configured to differentiate between the electrical signal produced by the receiver coil Rx 1 , . . . , Rxn and the electrical signal determined by the second frequency divider  24 . 
     In the exemplary embodiment illustrated in the figures, the comparison unit  26  comprises a mixer associated with each coil Rx 1 , . . . , Rxn configured to determine the in-phase (In-phase, I) and quadrature (Q) component of the electrical signal by mixing the electrical signal produced by the receiver coil Rx 1 , . . . , Rxn and the electrical signal determined by the second frequency divider  24 , this mixing being made by applying on the one hand a reference phase and on the other hand a phase shift of π/2 to the electrical signal generated by the associated second frequency divider  24 . These components are then sent to an analog-to-digital converter  32  which converts them into digital data and then communicates them to the analysis means  30 . 
     It will be noted that, analogously to the transmitter assembly  10 , the frequencies generated by the second frequency dividers  24  correspond to those generated by the first frequency dividers  14 , and that each coil Rx 1 , . . . , Rxn can be demodulated by one or several of these frequencies. 
     The Detector  50   
     The detector  50  is configured to detect an instant of zero crossing of the set of the electrical signals transmitted by the first frequency divider  14  or the second frequency divider  24  (see  FIGS. 3 and 4 ). 
     In the exemplary embodiment illustrated in  FIG. 1 , the phase of the second clock  22  is realigned with the phase of the first clock  12 . The detector  50  therefore receives of the set of the electrical signals transmitted by the first frequency divider(s)  14  and determines the instant of zero crossing of these electrical signals. As a variant, in  FIG. 2 , the phase of the first clock  12  is realigned with the phase of the second clock  22 . The detector  50  therefore receives the set of the electrical signals transmitted by the second frequency divider(s)  24  and determines the instant of zero crossing of these electrical signals. 
     Insofar as the electrical signals are sinusoidal ( FIG. 4 ), each electrical signal successively takes the zero value with a positive slope and a negative slope. In other words, the value of the signal can for example start from zero, then increase (positive slope) until reaching a local maxima, then decrease until reaching a local minima (negative slope) through zero. It follows that, on the set of the electrical signals, there are several instants of zero crossing of the set of the electrical signals, where some of these signals have a negative slope while others have a positive slope. In order to realign the phase of the electrical signal of the second clock  22  with the phase of the electrical signal of the first clock  12 , the detector  50  is configured to detect the instant of zero crossing of the set of the electrical signals transmitted by the first or the second frequency divider  14 ,  24  when these signals all have a positive slope. As a variant, it will of course be understood that the detector  50  can also be configured to detect the instant of zero crossing of these signals when they all have a negative slope. 
     The Signal Generator  60   
     Once this instant is detected, the signal generator  60  generates the phase realignment signal. This signal is synchronized to the zero crossing instant detected by the detector  50 . 
     The phase realignment signal can in particular comprise a pulse ( FIG. 3 ). 
     The signal generator  60  and the detector  50  are preferably housed in the column  2 ,  3  housing the frequency dividers  14 ,  24  whose detector  50  determines the zero crossing instant. 
     The Transmission Means  40   
     The transmission means  40  comprise communication interfaces  41 ,  42  configured to transmit to a communication interface of the transmitter assembly  10  or of the receiver assembly  20  the phase realignment signal in order to realign the phase of the first clock  12  and of the second clock  22 . The communication interfaces  41 ,  42  are connected either to the signal generator  60  (when the communication interface  41  is configured to transmit the phase realignment signal) or to the frequency dividers  14 ,  24  (when the communication interface  42  is configured to receive the phase realignment signal). The communication interfaces  41 ,  42  comprise a wireless interface in order to allow easily and rapidly placing the detection system  1 , for example an interface of the radiofrequency, Wi-Fi, Bluetooth type, by optical (typically infrared using photodiodes for example) or inductive communication, etc. It will be noted that, in the case of optical communication, the Applicant has noticed that the possible temporary masking of the infrared wireless interfaces would not be detrimental to the operation of the system insofar as the clocks  12 ,  22  remain very accurate and can realign with the next phase realignment signal. Where appropriate, when the communication interfaces comprise an inductive interface, said inductive interface can optionally comprise the at least one of the transmitter coils Tx 1 , . . . , Txm and of the receiver coils Rx 1 , . . . , Rxn of the transmitter assembly  10  and of the receiver assembly  20 , respectively. In other words, the inductive interface can use all or part of the transmitter Tx 1 , . . . , Txm and receiver Rx 1 , . . . , Rxn coils of the detection system  1  for transmitting and receiving the phase realignment signal in order to realign the phase of the first clock  12  and of the second clock  22 . 
     For example, the communication interfaces  41 ,  42  of the transmission means  40  comprise a modulator  41  configured to modulate a carrier signal with the phase realignment signal of the generator in order to communicate it, for example by radiofrequencies, to the transmitter assembly  10  or to the receiver assembly  20 , and a demodulator  42  configured to demodulate the carrier signal and extract the phase realignment signal and then transmit it to the receiver assembly  20  or to the transmitter assembly  10 , respectively. 
     This realignment signal is then communicated by the demodulator  42  to the set of the frequency dividers  14 ,  24  which is driven by the clock  12 ,  22  whose phase must be realigned. Typically, in the exemplary embodiment illustrated in  FIG. 1 , the realignment signal is transmitted to the set of the second frequency dividers  24 . More specifically, the realignment signal is transmitted to the reset input of the second frequency dividers  24  in order to realign them in phase and thus ensure the phase coherence in the detection system  1 . 
     The Analysis Means  30   
     As indicated previously, the comparison unit  26  associated with each receiver coil Rx 1 , . . . , Rxn sends to the analysis means  30  information on the electrical signal generated by the corresponding receiver coil Rx 1 , . . . , Rxn and demodulated using the electrical signal determined by the second frequency divider  24 . The analysis means  30  then deduce therefrom whether the magnetic field generated by the transmitter assembly  10  has been disturbed by metallic objects. 
     When the analysis means  30  determine that the magnetic field has been disturbed by one or several metallic object(s), the analysis means  30  send instructions for generating an alarm (sound and/or optical alarm) to one or several transmitters  70  (speaker, LED (Light-Emitting Diode), flashing lamp, etc.). The transmitters  70  can be housed in the second column  3  and/or in the first column  2 . When the alarm transmitters  70  are housed in whole or in part in the first column  2 , the alarm generation instructions are communicated to the transmitters  70  of the first column  2  via the transmission means  40  (see  FIG. 3 ). 
     The analysis means  30  can in particular comprise a processor, microprocessor, microcontroller, etc. type computer configured to execute instructions. 
     It will be noted that the communication interfaces  41 ,  42  of the transmission means  40  can, where appropriate, further be configured to transmit and receive diagnostic signals, or to allow the detection system  1  to communicate with another detection system  1  comprising, analogously, a transmitter assembly  10 , a receiver assembly  20 , a detector  50 , a signal generator  60  and transmission means  40 . Where appropriate, the settings of the detection systems  1  can then be synchronized, thanks to their communication interface  41 ,  42 , by wireless means. For example, the transmitter assembly  10  of a first detection system  1  can transmit the synchronization information to the transmitter assembly  10  of a second detection system  1 , via their respective communication interface  41 ,  42 . 
     Thus,  FIG. 5  illustrates a detection assembly comprising three detection systems  1  in accordance with the invention. Each detection system  1  comprises a transmitter assembly  10  and a receiver assembly  20  each comprising a communication interface  41 ,  42  in order to send and receive, respectively, the phase realignment signal from their respective clocks  12 ,  22 . Furthermore, the transmitter assembly  10  of each detection system  1  transmits, via its communication interface  41 ,  42 , synchronization information to the transmitter assembly of the adjacent detection system (communication channel  43 ). 
     The Detection Method S 
     The detection of metallic objects using a continuous wave detection system  1  can in particular be made in accordance with the following steps. In what follows, the invention will be described in the case where the transmitter assembly  10  comprises several transmitter coils Tx 1 , . . . , Txm and the receiver assembly  20  comprises several receiver coils Rx 1 , . . . , Rxn. As seen above, however, this is not limiting. 
     During a first step S 1 , the transmitter coils Tx 1 , . . . , Txm emit a magnetic field from an electrical signal having a frequency which is a function of the first frequency F 1  of the first clock  12 . 
     For that, the first clock  12  sends a first electrical signal having the first given frequency F 1  to the first frequency dividers  14 . Each first frequency divider  14  then divides the first frequency F 1  by its associated division setpoint N 1 , . . . , N m . For each transmitter coil Tx 1 , . . . , Txm, an oscillator then produces an electrical signal having the frequency thus determined by the first associated frequency divider  14  and transmits it to the transmitter coil Tx 1 , . . . , Txm in order to generate a magnetic field. 
     During a second step S 2 , the magnetic fields emitted in step S 1  induce an electrical signal in each receiver coil Rx 1 , . . . , Rxn. 
     During a third step S 3 , which is simultaneous with the second step S 2 , the second clock  22  sends a second electrical signal having the second given frequency F 2  to the second frequency dividers  24 . Each second frequency divider  24  then divides the second frequency F 2  by its associated division setpoint N 1 , . . . , N m . For each receiver coil Rx 1 , . . . , Rxn, an oscillator therefore produces an electrical signal having the frequency thus determined by the associated second frequency divider  24 . 
     During a fourth step S 4 , the electrical signal induced in each receiver coil Rx 1 , . . . , Rxn is compared with the electrical signal determined by the associated second frequency divider  24 . 
     For that, for each receiver coil Rx 1 , . . . , Rxn, the comparison unit  26  receives as input the electrical signal induced in the receiver coil Rx 1 , . . . , Rxn and the electrical signal determined by the second associated frequency divider  24 . Then, the comparison unit  26  determines the in-phase (I) and quadrature (Q) components of the electrical signal. 
     The in-phase (I) and quadrature (Q) components of each electrical signal thus determined are then transmitted to an associated analog-to-digital converter  32 , which converts them into digital data and then communicates them to the analysis means  30 . 
     In a manner known per se, the analysis means  30  then deduce whether the magnetic field generated by the emitter assembly  10  has been disturbed by metallic objects. 
     During a fifth step S 5 , the detector  50  detects an instant of zero crossing of the set of the electrical signals whose frequency is a function of the first frequency F 1  or of the set of the electrical signals whose frequency is a function of the second frequency F 2 . 
     When it is the phase of the second clock  22  that is realigned with the phase of the first clock  12  ( FIG. 1 ), during the fifth step, the detector  50  detects the instant of zero crossing of the set of the electrical signals transmitted by the first frequency divider(s)  14 . When it is the phase of the first clock  12  that is realigned with the phase of the second clock  22  ( FIG. 2 ), during the fifth step S 5 , the detector  50  detects the instant of crossing to zero of the set of the electrical signals transmitted by the second frequency divider(s)  24 . 
     In one embodiment, the zero crossing time is detected when the set of said electrical signals (transmitted by the first or second frequency dividers  14 ,  24 ) have a positive slope (i.e. when the value of the signal is negative immediately before the zero crossing and positive immediately after). As a variant, the zero crossing instant can be detected when the set of said electrical signals (transmitted by the first or second frequency dividers  10 ,  24 ) have a negative slope. 
     During a sixth step S 6 , a phase realignment signal which is synchronized to the zero crossing instant detected in step S 5  is generated. This phase realignment signal is generated by the signal generator  60 . 
     During a seventh step S 7 , the phase realignment signal is transmitted to the transmitter assembly  10  or to the transmitter assembly  10  to realign the phase of the first clock  12  and of the second clock  22 . 
     For example, when the phase of the second clock  22  is realigned with the phase of the first clock  12  ( FIG. 1 ), the phase realignment signal is generated at the transmitter assembly  10 . This signal is therefore transmitted to the communication interface  41  of the transmitter assembly  10 , which is then a modulator, so that it modulates the carrier signal with the phase realignment signal of the generator to communicate it to the communication interface  42  of the receiver assembly  20 . This interface then comprises a demodulator  42  configured to demodulate the carrier signal thus received and extract the phase realignment signal. The signal thus demodulated is then transmitted to the reset input  25  of the set of second frequency dividers  24  in order to realign them in phase with the first clock  12  and thus to ensure the phase coherence in the detection system  1 . 
     As a variant, when it is the phase of the first clock  12  that is realigned with the phase of the second clock  22  ( FIG. 2 ), the phase realignment signal is generated at the receiver assembly  20 . This signal is therefore transmitted to the communication interface  41  of the receiver assembly  20 , which is then a modulator, which communicates it to the communication interface  42  of the transmitter assembly  10 , analogously to what has been described previously. The signal thus demodulated is then transmitted to the reset input  15  of the set of the first frequency dividers  14  in order to realign them in phase with the second clock  22  and thus ensure the phase coherence in the detection system  1 .