Device and method for inspecting the leg of a person to determine whether the person is carrying unauthorized objects

Description

DOMAINE DE L'INVENTION

[0001] The present invention relates to the field of detectors designed for the detection of unauthorized objects or materials in a protected access area. ARRIERE-PLAN TECHNOLOGIQUE

[0002] Today it appears necessary to control with great reliability the attempts to introduce or remove certain products, for example but not exclusively explosive materials, into or out of a sensitive area.

[0003] The problem thus posed covers a very wide range of situations, which includes in particular and not limited to the attempt to introduce products into a protected area, such as a store, a school, a station, a public or even private body, or the attempt to remove products from a defined perimeter, for example in the event of theft in a company or on a protected site.

[0004] For several years, body scanners (commonly referred to by their English terminology "body scanner") have been developed to detect weapons, explosives, etc. hidden under the clothing of individuals entering a protected area. All these systems use technologies based on the detection of radiation energies modulated or emitted by the body of the individuals being inspected. The radiation energies thus used include X-rays, microwaves, millimeter waves, infrared light, terahertz waves and ultrasound.

[0005] Despite using several types of radiation energies and imaging geometries, these body scanners all work by creating an electronic image of the individual on which the individual's clothing is transparent. This image is then displayed on a screen and viewed by an operator so that the operator can determine whether the individual is carrying a target object. To do this, the operator, who is trained in target object detection, must be able to determine whether the objects identified by the body scanner correspond to human anatomy, an authorized object such as a lighter, a handkerchief or coins, or a target object such as a weapon or explosive.

[0006] It appears that individuals attempting to smuggle a product out of a protected area or to introduce such a product now often use shoes, boots or equivalent, or socks to conceal the product in question. This phenomenon appears to be mainly due to the fact that this area is difficult to easily control visually or by manual palpation.

[0007] However, it appears that conventional body scanners are not capable of detecting such products. On the one hand, detection by body scanners is interfered with by noise generated by ground infrastructure. Thus, generally, body scanners can only analyze the part of an individual located at a certain distance above the ground. On the other hand, conventional body scanners are limited in their detection due to the thickness of the upper of the shoe or boot, which forms a shield and does not allow, based on current technologies, to determine the shape of the foot and therefore to identify target objects.

[0008] This is why operators sometimes systematically require individuals wishing to enter or exit a sensitive area to remove their shoes, in an attempt to improve inspection. Such a request generates significant constraints and discomfort.

[0009] The applicant therefore proposed devices which comprise a frame which includes a support base formed from a rectangular step-shaped platform whose flat upper surface includes a design or imprint and a stop intended to accommodate and position a single foot of an individual wearing a shoe, two symmetrical side panels which house detection means, and an information module.

[0010] Examples of this device can be found in documents FR 2860631, EP 1574879, FR 2889338 (which corresponds to US 2007 / 073492) and FR 2911212.

[0011] Document US 2015 / 253422 discloses a portal comprising transmitter / receiver means adapted to inspect a complete individual.

[0012] The detection means described in the documents mentioned may be formed of coils for detecting metals, sampling means, for example in the form of suction nozzles, for sampling vapors or traces of particles, for example narcotics or explosives, analysis means based on nuclear magnetic resonance comprising for example Helmholtz coils, or even complex impedance analysis means or radioactive radiation detectors. RESUME DE L'INVENTION

[0013] One objective of the invention is to propose new detection means making it possible to improve the detection of target objects likely to be camouflaged in a shoe or on the leg of an individual.

[0014] One objective of the invention is in particular to propose detection means which are effective regardless of the location of attachment of a fraudulent object on the leg of an individual, more precisely on the calf (whether on the front, the back, an external lateral part or an internal lateral part of the leg, or even a combination of these arrangements).

[0015] These objectives are achieved according to the invention thanks to a device adapted to inspect the leg of an individual, according to claim 1.

[0016] The invention also relates to a method for detecting fraudulent objects carried on the leg of an individual using the aforementioned device, according to claim 13. BREVE DESCRIPTION DES DESSINS

[0017] Other characteristics, aims and advantages of the present invention will appear more clearly on reading the detailed description which follows, and with regard to the appended drawings given as non-limiting examples and in which: there figure 1 represents a perspective view of a device according to the present invention, the figure 2 represents the same device by schematically illustrating the positioning of the lower part of an individual's leg in the device, the figure 3 represents a schematic view in vertical longitudinal section of the device according to the present invention, the figure 4a schematically represents in side view the microwave emissions from a panel towards the opposite panel with an object not permeable to the microwave inserted between the two panels and the figures 4b, 4c, 4d, 4e, 4f , schematically represent the signals received on the receivers on the second panel according to their positioning relative to said object, the figure 5a schematically represents in top view the microwave emissions / receptions in the case of a leg inserted between the two panels, and the figures 5b et 5c schematically represent respectively a signal transmitted outside the leg area without delay and without attenuation and a signal reflected by the leg, the figure 6a schematically represents in top view the microwave emissions / receptions in the case of a leg inserted between the two panels and laterally carrying a fraudulent object and the figure 6b schematically represents the signal reflected by this object, and the figure 7a schematically represents in top view the microwave emissions / receptions in the case of a leg inserted between the two panels and carrying on the front a fraudulent object and the figure 7b schematically represents the signal transmitted through this object characterized by a specific time delay and attenuation. DESCRIPTION DETAILLEE D'UN MODE DE REALISATION

[0018] The attached figures show a device in accordance with the present invention comprising a frame 100 which includes: a support base 110 adapted to receive a foot of an individual and formed of a rectangular step-shaped plate whose flat upper surface preferably includes a design or marking 112 and a stop 114 intended to receive and position a single foot wearing a shoe, two symmetrical side panels 120 adapted to be placed on either side of the leg of an individual and which house detection means, and an information module 130.

[0019] The device shown in the attached figures may conform in terms of its geometry, its dimensions, the layout of the positioning mark 112, of the stop 114, of the nature of the messages displayed on the module 130, to the provisions described in the aforementioned documents FR 2860631, EP 1574879, FR 2889338 and FR 2911212.

[0020] More specifically according to the invention, as indicated previously, further comprises: means 140 for positioning the knee of an individual, relative to the support base 110 and to the panels 120, microwave transmitter / receiver means 200 arranged on each side panel 120 opposite the side panel facing it, and means 300 for analyzing the signals detected on the microwave receivers corresponding to the signals transmitted from a panel 120 to the opposite panel and to the signals reflected from a panel 120 towards this same panel.

[0021] The knee positioning means 140 are preferably formed from a crosspiece 142 connecting the internal faces of the two side panels 120 at a height of the order of 40 cm.

[0022] The microwave transmitter / receiver means 200 preferably comprise a plurality of microwave transducers arranged in a matrix of horizontal rows and vertical columns on each panel 120. As shown in the figures 1, 2 And 3 , five horizontal rows are thus preferably provided, each comprising three microwave transducers.

[0023] By way of non-limiting examples, the horizontal distance between two transducers 200 of a row is of the order of 75 mm, the vertical distance between two rows of transducers 200 is of the order of 55 mm, the highest row of transducers 200 is located at a height of the order of 350 mm above the surface of the support base 110.

[0024] Preferably, each microwave transducer 200 is associated with a focusing horn 202. The output of each focusing horn 202 is preferably located approximately 130 mm from the vertical median plane of the device.

[0025] The transducers 200 preferably operate in a frequency range between 5 and 30 GHz and very preferably between 12 and 20 GHz.

[0026] The different transducers 200 located on a panel 120, for example the fifteen transducers distributed in five vertical rows of three transducers, are located respectively coaxially with a transducer located opposite on the opposite panel 120.

[0027] Each transducer 200 can, under the control of the means 300, work alternately as a transmitter or receiver or as a transmitter and receiver.

[0028] Each transmitter 200 is preferably adapted to transmit toward a coaxial receiver 200 on the opposing panel 120, as well as toward transducers adjacent to that opposing coaxial transducer 200.

[0029] Likewise, each receiver 200 is preferably adapted to receive from a coaxial transmitter 200 located on the opposing panel 120, as well as from transducers adjacent to that opposing coaxial transmitter transducer 200.

[0030] As seen on the figures 4 , when an object X composed of a substance not permeable to microwaves is worn by the calf of an individual located opposite the transducer matrix 200, the rays transmitted between an emitter 200 located on a first panel 120 in the direction of one or more receivers 200 located on the second opposite panel 120 which are not obscured by the object X reach the receivers with a slight delay due to transmission in the air, without notable attenuation in amplitude ( figures 4b, 4e et 4f ).

[0031] On the other hand, the rays emitted by an emitter 200 in the direction of the object X are stopped by the latter and do not reach the receiver 200 opposite ( figures 4c et 4d ).

[0032] The control of the transmitter / receiver pairs 200 and the analysis of the signals from a receiver 200 therefore makes it possible to detect the presence on the leg of an individual of an object X not permeable to microwaves.

[0033] As seen on the figures 5 , the leg of an individual, due to its water density, reflects almost all microwaves.

[0034] Thus, the rays transmitted between an emitter 200 located on a first panel 120 in the direction of one or more receivers 200 located on the second opposite panel 120 which are not obscured by the leg again reach the opposite receivers, practically without delay and without notable attenuation in amplitude ( figure 5b ).

[0035] On the other hand, the rays emitted by a transmitter 200 in the direction of the leg are reflected by the leg in the direction of the transmitter operating as a receiver with a delay equal to 2*d / c, d representing the distance between the transmitter and the leg and c representing the speed of the microwaves in the air ( figure 5c ).

[0036] THE figures 6 represent the operation of the device for detecting a fraudulent object X1 worn laterally on the calf of an individual.

[0037] Here again, the rays transmitted between an emitter 200 located on a first panel 120 towards one or more receivers 200 located on the second opposite panel 120 which are not obscured by the leg reach the opposite receivers, practically without delay and without significant attenuation in amplitude, as shown in the figure 5b .

[0038] On the other hand, the rays emitted by an emitter 200 in the direction of the object X1 are reflected on the one hand by the external face of the object X1 and on the other hand by the leg in the direction of the emitter operating as a receiver, as shown in the figure 6b .

[0039] As shown in the figure 6b , the receiver 200 receives in this case two successive echoes, with different delays: a first echo due to the reflection on the substance X1 and a second echo due to the reflection on the leg.

[0040] Generally, according to tests carried out by the inventors, the second echo on the leg has a greater amplitude than the first echo on the substance X1 which is only partially reflective.

[0041] Comparative analysis of signals of the type illustrated in the figure 5b, 5c And 6b , allows by comparison to detect a fraudulent object worn laterally on the calf of an individual.

[0042] THE figures 7 represent the operation of the device for detecting a fraudulent object X2 worn on the front of an individual's calf.

[0043] Here again, the rays transmitted between an emitter 200 located on a first panel 120 towards one or more receivers 200 located on the second opposite panel 120 which are not obscured by the leg reach the opposite receivers, practically without delay and without significant attenuation in amplitude, as shown in the figure 5b .

[0044] On the other hand, the rays emitted by an emitter 200 in the direction of the object X2 without being obscured by the leg are transmitted with a delay and attenuation due to the substance X2 to the opposite receiver located on the opposite panel.

[0045] Comparative analysis of signals of the type illustrated in the figure 5b, 5c And 7b , allows by comparison to detect a fraudulent object worn on the front (or the back) of an individual's calf.

[0046] Those skilled in the art will understand from reading the foregoing description that the invention allows the detection of fraudulent non-metallic substances, for example explosives or drugs, carried on the lower leg of an individual by detecting the delay and amplitude of microwave signals transmitted through the substance and / or reflected by the substance, as well as by the leg.

[0047] Preferably, the transducer matrix 200 is adapted to cover the entire lower part of the leg between the ankle and the knee and the analysis means 300 proceed by comparing the transmitted and / or reflected signals with transmission values ​​in the air and / or signals reflected on a leg free of fraudulent object.

[0048] The means 300 are adapted to generate an alarm when a significant deviation exceeding a threshold is detected compared to the reference values.

[0049] The positioning of the leg on the marking 112 and the correct positioning of the knee resting on the crosspiece 142 can be checked by suitable optical cells placed on the panels 120.

[0050] Preferably, within the framework of the invention, the means 300 are adapted to carry out at least one of the following steps and preferably the combination of all of the following steps: measuring the delay and amplitude of a direct microwave transmission between each pair of coaxial 200Tx transmitters and 200Rx receivers located on the two opposite panels 120, measuring the delay and amplitude of an oblique microwave transmission between each 200Tx transmitter located on a panel 120 and the 200Rx receivers surrounding the receiver located on the opposite panel 120 coaxial with the transmitter, measuring the delay and amplitude of the microwave wave emitted by each 200Tx transmitter transducer and reflected towards the same 200Rx receiver transducer or towards the 200Rx receivers surrounding the latter, by a leg or by a fraudulent (non-metallic) substance placed on or around the leg, detecting the presence of a double echo of microwave waves reflected by a fraudulent substance and by the leg,comparison of the directly transmitted microwave waves with a reference value of transmission in a vacuum and transmission of an alarm in case of detection of a delay greater than a threshold and with an amplitude corresponding to a range of substances which delay and attenuate the microwaves, comparison of the directly transmitted microwave waves with microwaves transmitted on adjacent transducers and generation of an alarm in case of a difference greater than a threshold detected between the different signals (similar to the presence of a non-metallic substance which is not permeable to microwaves), comparison of the oblique microwave waves with a reference value in a vacuum and generation of an alarm in case of detection of a delay greater than a threshold and with an amplitude corresponding to a predetermined range representative of substances which delay and attenuate the microwaves,comparison of obliquely transmitted microwave waves with directly adjacent transmitted microwave waves and generation of an alarm in the event of a difference greater than a threshold detected between the different signals (similar to the presence of a non-metallic substance not permeable to microwaves), comparison of a detected double echo with transmission values ​​in a vacuum and generation of an alarm in the event of detection of a delay greater than a threshold between the peaks of the two echoes with an amplitude greater than a threshold (representative of a substance which reflects a part of the wave (first echo) and which delays and attenuates the non-reflected signal (second echo)).

[0051] The device according to the present invention which has just been described can be supplemented by auxiliary equipment, for example by means forming a metal detector based on windings integrated into the side panels 120 and / or by means for sampling and analyzing substances, vapor or traces of particles and / or by means of nuclear magnetic resonance type analysis, means of complex impedance analysis and / or means for detecting radioactive radiation.

[0052] Such means are known per se in their general structure and will therefore not be described in more detail below.

[0053] The device in accordance with the present invention described above can also be supplemented by the means which will now be described which correspond to the subject of the patent applications filed in France under the numbers FR 1653385, FR 1655726 and FR 1655729 and which can be integrated into the base 110 and / or the side panels 120.

[0054] Thus, within the framework of the invention, the device 100 may comprise microwave transmitter / receiver means 400 (for example in the 5GHz-30GHz range, advantageously in the 12GHz-20GHz range) preferably integrated into the base of each side panel 120 and distributed along the length of the support base 110, (these microwave transmitter means and associated microwave receiver means placed respectively on either side of the sole of the shoe are advantageously adapted for detection on a receiver means of the signal coming from a transmitter means placed directly opposite and of the signal coming from a transmitter means placed obliquely relative to the receiver means, at the same vertical level or at a different height), means for measuring the width of an element, typically a sole, inserted between these microwave transmitter / receiver means 400,means 300 for analyzing at least one parameter of the transmission delay between the microwave transmitter / receiver means 400 and / or of the amplitude of the signal transmitted between the microwave transmitter / receiver means 400, and means for normalizing the aforementioned analysis with respect to a standard width size unit obtained on the basis of the width measuring means.

[0055] The means 300 can also be adapted to analyze the amplitude of the signals from the receivers to deduce information on the thickness of the sole.

[0056] These means 400 form at least one microwave transmitter means placed on one side of the base 110 and at least one microwave receiver means placed on the opposite side of the base 110 so that the microwaves emitted by the microwave transmitter means pass through the sole of the shoe placed on the support base 110 before reaching the associated microwave receiver means placed on the opposite side of the base 110.

[0057] The means for measuring the width of the element interposed between the microwave transmitter / receiver means 400 comprise, for example, a plurality of infrared transmitter / receiver means adapted to measure the round-trip propagation time between an infrared transmitter and the associated infrared receiver.

[0058] Several pairs of infrared emitting means and associated infrared receiving means may thus be provided, placed on each side of the base 110, so that the infrared rays emitted by each infrared emitting means are reflected on the sole of the shoe placed on the support base 110 before reaching the associated infrared receiving means placed on the same side of the base 110.

[0059] The device may also comprise means, preferably based on microwaves and preferably a plurality of such means distributed along the length of the device, adapted to detect stratification by vertical stacking, in the sole, by detecting successive echoes following an emission of waves towards the sole.

[0060] Means may be provided for normalizing the signal from the means for detecting vertical stratification, on the basis of a signal representative of the estimated height of the sole,

[0061] The device may further comprise means for measuring the electrical capacitance formed by the sole of a shoe placed on the support base 110. Such means may comprise electrodes on handles 122 placed in the upper part of the panels 120. These electrodes formed on the handles 122 are for example made of an electrically conductive material embedded in the mass of the panels 120. An electrical generator (generating for example a voltage between 0.1V and 10V, preferably of the order of 1 volt) is connected in series with the handles 122 provided on side panels 120 and electrodes placed on the support base (110), for example electrodes placed on the support base 110 and preferably concentric with horns associated with microwave transducers 450 adapted to generate microwaves vertically in a sole.

[0062] Of course, the present invention is not limited to the embodiments previously described, but extends to all variants in accordance with its spirit.

[0063] In particular, the implementation of the present invention has been previously described in the context of a device whose bases have been previously described in documents FR 2860631, EP 1574879, FR 2889338 and FR 2911212, comprising a support base 10 formed of a step whose upper surface comprises an imprint 12 and a stop 14 intended to accommodate and position a single foot of an individual wearing a shoe.

[0064] However, the invention is not limited to this particular embodiment. As shown in the attached figure 23, the present invention can also be applied to devices in which the support base is adapted to simultaneously receive the two feet P of an individual.

[0065] In this case, however, it is preferable, to allow detection respectively and individually on each of the two feet P of an individual under test, to provide projecting from the upper surface of the base three blocks 320, 330 and 340 between which the two feet P must be positioned so that the central block 330 is placed between the two feet P, while the two lateral blocks 320 and 340 are arranged respectively on the outside of the feet. The central block 330 houses detection means associated respectively with the blocks 320 and 340 to allow the various aforementioned measurements respectively on each of the two feet P. Thus, the central block 330 in this case preferably houses transmitter means 142 / receiver means 144 for measuring the absorption amplitude of the microwaves and the propagation time, as well as infrared transducers 152 / 154 for measuring the width of each of the two soles S, separately.

Claims

1. A device adapted to inspect the leg of an individual, which comprises: - a support base (110) adapted to receive a foot of an individual, - two lateral panels (120) adapted to be placed on either side of the leg of an individual, - positioning means (112, 114, 142) of the foot and of the leg relative to the support base (110) and to the panels (120), - microwave sender / receiver transducer means (200) arranged on each lateral panel (120) opposite the lateral panel which is facing it, and the device being characterized in that: each emitter (200) is adapter to emit towards a coaxial receiver (200) on the opposite panel (120), as well as towards transducers (200) adjacent to this coaxial opposite receiver (200), each receiver (200) is adapter to receive from a coaxial emitter (200) on the opposite panel (120), as well as from transducers (200) adjacent to this coaxial opposite emitter (200), and it comprises analysis means (300) of signals detected on the microwave receivers (200) corresponding to the signals transmitted from a panel (120) to the opposite panel and corresponding to the signals reflected from a panel (120) towards this same panel.

2. The device according to claim 1, characterized in that the analysis means (300) perform analysis of the delay and amplitude of the signals received relative to a reference.

3. The device according to any one of claims 1 or 2, wherein the analysis means (300) undertake comparison of signals received on a receiver with reference signals representative of the transmission in a void and / or reflection on a leg.

4. The device according to any one of claims 1 to 3, wherein the analysis means (300) generate an alarm in case of detection of a gap greater than a threshold between the received signals and a reference.

5. The device according to any one of claims 1 to 4, characterized in that it comprises positioning means (140) of the knee formed by a cross-beam (142) connecting the internal faces of the two lateral panels (120).

6. The device according to any one of claims 1 to 5, characterized in that the microwave sender / receiver means (200) comprise several microwave transducers arranged according to a matrix of horizontal lines and vertical columns on each panel (120).

7. The device according to any one of claims 1 to 6, wherein the microwave receiver / sender means (200) comprise a row of microwave transducers, the highest located at a height of the order of 350mm above the surface of the support base (110).

8. The device according to any one of claims 1 to 7, wherein the microwave receiver / sender means (200) comprise five horizontal rows of microwave transducers each comprising three microwave transducers, the horizontal gap between two transducers (200) of a row being of the order of 75mm, whereas the vertical gap between two rows of transducers is of the order of 55mm.

9. The device according to any one of claims 1 to 8, wherein each microwave transducer (200) is associated with a focalisation cone (202).

10. The device according to any one of claims 1 to 9, wherein the transducers (200) operate in a range of frequencies between 5 and 30GHz and very preferably between 12 and 20GHz.

11. The device according to any one of claims 1 to 10, wherein each transducer (200) is adapted to work alternatively as sender or receiver or as sender and receiver.

12. The device according to any one of claims 1 to 11, wherein the analysis means (300) are adapted to execute at least the one of the following functions: - measuring the delay and amplitude of direct transmission of microwaves between each pair of coaxial senders (200Tx) and receiver (200Rx) located on the two opposite panels (120), - measuring the delay and amplitude of oblique transmission of microwaves between each sender (200Tx) located on a panel (120) and the receivers (200Rx) which enclose the receiver located on the opposite coaxial panel (120) of the sender, - measuring the delay and amplitude of the microwave wave emitted by each transducer sender (200Tx) and reflected towards the same transducer forming a receiver (200Rx) or towards the receivers (200Rx) enclosing the latter, by a leg or by a fraudulent substance placed on or around the leg, - detection of the presence of a double echo of microwave waves reflected by a fraudulent substance and by the leg, - comparison of microwave waves transmitted directly with a transmission reference value in a void and transmission of an alarm in case of detection of delay greater than a threshold and with an amplitude corresponding to a range of substances which delays and attenuates the microwave, - comparison of microwave waves transmitted directly with microwaves transmitted on adjacent transducers and generation of an alarm in case of a gap greater than a threshold detected between the different signals, - comparison of oblique microwave waves with a reference value in a void and generation of an alarm in case of detection of delay greater than a threshold and with an amplitude corresponding to a range predetermined representative of substances which delay and attenuate the microwave, - comparison of microwave waves transmitted obliquely with the adjacent microwave waves transmitted directly and generation of an alarm in case of a gap greater than a threshold detected between the different signals, - comparison of a double echo detected with transmission values in a void and generation of an alarm in case of detection of a delay greater than a threshold between the peaks of the two echoes with an amplitude greater than a threshold.

13. A method for inspection of the leg of an individual, by means of a device according to any one of claims 1 to 15, comprising the following steps : - positioning a foot and a leg of an individual relative to the support base (110) and to the panels (120), - emitting microwaves from an emitter on a panel towards a coaxial receiver (200) on the opposite panel (120), as well as towards transducers (200) adjacent to this coaxial opposite receiver (200), - receiving microwaves from a coaxial emitter (200) on the opposite panel (120), as well as from transducers (200) adjacent to this coaxial opposite emitter (200), - analysing the signals detected by the microwave receivers (200) corresponding to the signals transmitted from a panel (120) to the opposite panel and corresponding to the signals reflected from a panel (120) towards this same panel and - generating an alarm when analysis reveals a gap in the transmission time or in the amplitude of the signals, relative to a reference, greater than a threshold.

14. The method according to claim 13, characterized in that it performs at least one of the following steps and preferably a combination of the following steps: - measuring the delay and amplitude of direct transmission of microwaves between each pair of coaxial senders (200Tx) and receivers (200Rx) located on the two opposite panels (120), - measuring the delay and amplitude of oblique transmission of microwaves between each sender (200Tx) located on a panel (120) and the receivers (200Rx) which enclose the receiver located on the opposite coaxial panel (120) of the sender, - measuring the delay and amplitude of the microwave wave emitted by each transducer sender (200Tx) and reflected towards the same transducer forming a receiver (200Rx) or towards the receivers (200Rx) enclosing the latter, by a leg or by a fraudulent substance placed on or around the leg, - detection of the presence of a double echo of microwave waves reflected by a fraudulent substance and by the leg, - comparison of microwave waves transmitted directly with a transmission reference value in a void and transmission of an alarm in case of detection of a delay greater than a threshold and with an amplitude corresponding to a range of substances which delays and attenuates the microwave, - comparison of microwave waves transmitted directly with microwaves transmitted on adjacent transducers and generation of an alarm in case of a gap greater than a threshold detected between the different signals, - comparison of oblique microwave waves with a reference value in a void and generation of an alarm in case of detection of delay greater than a threshold and with an amplitude corresponding to a predetermined range representative of substances which delay and attenuate the microwave, - comparison of microwave waves transmitted obliquely with the adjacent microwave waves transmitted directly and generation of an alarm in case of a gap greater than a threshold detected between the different signals, - comparison of a double echo detected with transmission values in a void and generation of an alarm in case of detection of a delay greater than a threshold between the peaks of the two echoes with an amplitude greater than a threshold.