EP2038680A1 - Détecteur de métaux avec capacités de détection améliorées - Google Patents

Détecteur de métaux avec capacités de détection améliorées

Info

Publication number
EP2038680A1
EP2038680A1 EP07719081A EP07719081A EP2038680A1 EP 2038680 A1 EP2038680 A1 EP 2038680A1 EP 07719081 A EP07719081 A EP 07719081A EP 07719081 A EP07719081 A EP 07719081A EP 2038680 A1 EP2038680 A1 EP 2038680A1
Authority
EP
European Patent Office
Prior art keywords
coil
metal detector
coils
receive
receiving
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07719081A
Other languages
German (de)
English (en)
Inventor
Leon William Mitchell
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Minelab Electronics Pty Ltd
Original Assignee
Minelab Electronics Pty Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from AU2006903287A external-priority patent/AU2006903287A0/en
Application filed by Minelab Electronics Pty Ltd filed Critical Minelab Electronics Pty Ltd
Publication of EP2038680A1 publication Critical patent/EP2038680A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/15—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for use during transport, e.g. by a person, vehicle or boat

Definitions

  • the current invention relates to metal detectors having improved detection capabilities, in particular the metal detector includes at least two coils capable of simultaneously receiving a signal from a target excited by a transmit coil and preferably wherein the two coils capable of receiving the target signal are of a different effective configuration.
  • the invention has particular relevance to any field where knowledge of the depth, size and orientation of the target in the ground is important, such as is the case when searching for dangerous military hardware (such as unexploded ordnance or land mines) or treasure.
  • US patent 1812392 discloses a device for electromagnetically detecting and locating terrestrial conducting bodies having a plurality of receiving coils. This patent further discloses the step of measuring the ratio between the voltages induced in the receiving coils by the secondary field to determine the distance to the conducting body.
  • US patent 3471772 and similarly US patent 5721489 disclose a detector having a transmit coil and two receiving coils where the ratio of the two receive signals can be used to determine the approximate size and depth of the object to be detected.
  • US 4091322 discloses a detector for detecting pipe lines having three receive coils.
  • the metal detectors in the prior art that use two receive coils to process the received signals have at least three coils in the coil housing. (Two receive coils plus the transmit coil.)
  • a typical embodiment from the art is shown in Fig. 1. Having three coils in the coil housing adds complexity, weight and cost to the coil housing design.
  • depth and hence target size information is available as previously disclosed from the ratio of signals from the two receive coils, the two sets of information from said receive coils has very similar form (on account of the similar effective configuration of the coils) and hence contains little new information regarding the target.
  • Fig. 3 illustrates the calculated response of the first receive coil (RXl) and the second receive coil (RX2) as a coil arrangement, similar to that in Fig.
  • Target 1 is moved over two targets (in this case located 15 cm below the plane of the coil arrangement).
  • Target 1 is a target that can be excited to produce a dipole-like response only in the vertical (z) direction, such as a loop of wire with inductance L and resistance R which lies wholly in the horizontal (x, y) plane.
  • a flat conductive disk such as a metallic coin similarly orientated to a loop, produces a response with similar spatial variation.
  • Target 2 is a target that can be excited only to produce a dipole-like response only in the horizontal x direction (here defined to be in the direction of motion of the coil arrangement), such as a loop of wire or coin which lies wholly in the vertical (y, z) plane. From Fig.
  • the reason for the slow variation of the RX1/RX2 ratio with depth is that, far from the target, the secondary magnetic field produced by the target is approximately spatially uniform, so sampling by the two similarly-shaped, but differently sized (or similarly-shaped but different height), concentric receive coils produces a result which asymptotes to the ratio of the areas of RXl and RX2. (IfRXl and RX2 have a different number of turns, or different amplification gain, then the ratio is suitably modified.)
  • the mono/mono curve in Fig. 4 shows the calculated variation of the ratio RX1/RX2 as a function of the depth of the target below a coil arrangement similar to that depicted in Fig. 1.
  • the Dashed Curve in Fig. 4 represents the ratio (Area l)/(Area 2), to which the mono/mono curve asymptotes for large distances, as described above.
  • a further limitation to the existing art lies with the susceptibility of the detectors disclosed to external electro-magnetic noise.
  • detectors described in the art require a signal to be derived from a two receive coils RXl and RX2, wherein RX2 is, in some embodiments, significantly smaller than RXl.
  • RX2 is, in some embodiments, significantly smaller than RXl.
  • the system for receiving, amplifying and recording signals from RX2 must have both high gain (achievable by having a large number of turns on the RX2 coil, or high gain on the amplification circuit) and noise as low as achievable.
  • the RX2 coil is a simple circular coil, any external electromagnetic signals present in the local environment will induce voltage in the RX2 coil, which, because of the high gain required by the design, may cause noise problems in the detector which will further limit the depth resolution capabilities of the detector.
  • the current invention aims to improve on the ideas presented in these earlier disclosures.
  • a metal detector having a coil housing with at least two coils each capable of simultaneously receiving a respective signal from a target excited by a transmit coil and preferably wherein the two coils capable of receiving the target signal are of a substantially different effective configuration, and hence sensitive to different aspects of the secondary magnetic field from the target.
  • a different effective configuration includes coils that have a different physical configuration such as a circular coil versus an "8" shaped coil, and different effective configurations may also include coils having a similar physical configuration. Where the receive coils do have similar physical configurations, the receive signals from the respective coils may be formed either by addition or subtraction or some other mathematical manipulation to provide, in effect, different signal responses that would occur with coils of different physical configurations, thus having different effective configurations.
  • Flat, circular coils are examples of what are called “dipole” coils. If a source of electrical current is connected to a flat circular coil, effecting a current in a particular direction along the wire constituting the windings of the coil, a dipole-like magnetic field is generated.
  • Field lines can be used to describe the field, pictorially, in a generally accepted representation. Field lines are continuous. In the case of a flat, circular coil, the field lines on one side of the plane, near the plane and within the boundary of the coil, point toward the plane, while just on the other side of the plane the field lines within the boundary of the coil point away from the plane. This is the dipole field of the dipole coil.
  • a coil wound as a Figure ⁇ coil has two loops. Passing a current along the wire of the coil will produce a dipole field in one of the two loops, while the other loop also produces its own dipole field, but the sense of its field opposes that of the field of the first coil.
  • the arrangement produces a total field called a "quadrupole" field, it having four poles as opposed to the two poles of the dipole field.
  • Both types of effective configuration can be applied to the signals for the two receive coils in the one detector; thus, the one pair of coils can be effectively configured, simultaneously, as both an effective dipole coil and an effective quadrupole coil.
  • the one pair of coils can be effectively configured, simultaneously, as both an effective dipole coil and an effective quadrupole coil.
  • a metal detector including; a first coil for transmitting a signal and receiving a first response signal, a second coil for receiving a second response signal, where said first and second coils for receiving first and second response signals are of different effective configurations and, where said first and second response signals are processed to determine at least the depth, location, approximate size, shape, and/or other useful characteristic of an object that is detected by said metal detector.
  • a metal detector including; a transmit coil for transmitting a signal, at least two receive coils for receiving a first and second response signal, where said at least two receive coils are of different effective configurations and, where said first and second response signals are processed to determine at least the depth, location, approximate size, shape, and/or other useful characteristic of an object that is detected by said metal detector.
  • a metal detector where at least one coil for receiving a first response signal is configured substantially as a dipole coil and, at least one coil for receiving a second response signal is configured substantially as a quadrupole coil.
  • the metal detector has more than two coils for receiving response signals.
  • a metal detector including; a transmit coil for transmitting a signal, at least three receive coils for receiving a first, a second and a third response signals, where the said first receive coil is configured substantially as a dipole coil and the said second and third receive coils are configured substantially as an effective quadrupole receive coil through a superposition of emissions of a first and a second receive circuits, the second receive coil being connected to an input of the first receive coil and the third receive coil being connected to an input of the second receive circuit and, where the said first response signal and the said superposition of emissions of the first and second receive circuits are processed to determine at least the depth, location, approximate size, shape, and/or other useful characteristic of an object that is detected by said metal detector.
  • a metal detector including; a transmit coil for transmitting a signal, at least two receive coils for receiving two response signals, where said at least two receive coils being effectively configured, simultaneously, as an effective dipole coil and as an effective quadrupole coil, where a signal from the effective dipole coil and a signal from the effective quadrupole coil are processed to determine at least the depth, location, approximate size, shape, and/or other useful characteristic of an object that is detected by said metal detector.
  • At least one coil for receiving a response signal is of a circular configuration and at least another coil for receiving a second response signal is of an "8" shaped configuration. Examples of this embodiment are shown in Fig. 2 in the drawings.
  • At least one coil for receiving a response signal is of an effective circular configuration and at least another coil for receiving a second response signal is of an effective "8" shaped configuration. Examples of this embodiment are shown in Fig. 7, Fig.8, and Fig. 9 in the drawings.
  • the coils of the metal detector may be arranged as an array.
  • the metal detector includes pairs of coils of different effective configurations arranged as an array. An example of the coils arranged as an array is shown in Fig. 9.
  • the metal detector of the current invention may be a pulse induction (PI) type metal detector or a continuous wave (CW) metal detector, both types being well known in the metal detector industry.
  • the metal detector is a continuous wave metal detector
  • the same coil for transmitting and receiving the signals.
  • the Applicant has been able to achieve a single coil capable of both transmitting and receiving a continuous wave signal using novel signal processing techniques. This invention is disclosed in co-pending Australian provisional patent application, 2007901083, that is hereby incorporated by reference.
  • Fig. 1 illustrates a typical embodiment of the prior art having two receive coils and one transmit coil, all of the same circular physical configuration.
  • Fig. 2 illustrates one embodiment of the invention where one coil of circular configuration acts as the transmit coil and a receive coil, and a second coil of a fig. "8" configuration acts as a second receive coil.
  • Fig. 3 depicts a response curve using a coil configuration as depicted in Fig. 1 when moved over two targets.
  • Fig. 4 shows the calculated variation of the ratio of the two receive coils as a function of depth using a coil arrangement similar to that depicted in Fig. 1 and Fig 2.
  • Fig. 5 illustrates the calculated response of the two receive coils using a coil arrangement as shown in Fig. 2
  • Fig. 6 shows results obtained when passing coils having the arrangement shown in Fig. 2, over a coin target.
  • Fig. 7 illustrates a coil arrangement where the two receive coils have a similar physical configuration.
  • Fig. 8 illustrates a coil arrangement where the first coil is used to transmit and receive signals, and the second coil is used to receive a second receive signal.
  • Fig. 9 illustrates a coil arrangement of the current invention as an array.
  • Fig. 10 shows the experimental data using an array illustrated in Fig. 9.
  • Fig. 11 shows the experimental data using an array illustrated in Fig. 9 where the array has a forward and reverse pass of the target.
  • the transmit coil TX and the first receive coil RXl are one and the same coil which has, for example, a circular configuration, while the second receive coil RX2 is an "8' shaped coil located coplanar with the TX/RX1 coil.
  • This configuration which is only one embodiment of the present invention, is shown schematically in Fig. 2.
  • Fig. 5 illustrates the calculated response of the first receive coil (RXl) and the second receive coil (RX2) as the Fig. 2 coil arrangement is moved over two targets, in this case located 15 cm below the plane of the coil arrangement.
  • Target 1 is a target that can be excited to produce a dipole-like response only in the vertical (z) direction, such as a loop of wire with inductance L and resistance R which lies wholly in the horizontal (x, y) plane. It is well known among those skilled in the art that a flat conductive disk such as a metallic coin similarly orientated to a loop produces a response with similar spatial variation.
  • Target 2 is a target that can be excited only to produce a dipole-like response only in the horizontal x direction (here defined to be in the direction of motion of the coil arrangement), such as a loop of wire or coin which lies wholly in the vertical (y,z) plane.
  • Fig. 3 it is seen in Fig.
  • Fig. 6 shows experimental results obtained when passing a common coin over a detector of the type described in this invention, with the coil arrangement of Fig. 2, at a distance of 15 cm and with the orientation for each pass coinciding with Target 1 and Target 2 above.
  • the shapes and relative sizes of the RXl and RX2 response for each of the two coin orientations are in excellent agreement with the calculations of Fig. 5, and illustrate the rich target information that is inherent in the present invention. Processing of the signals in Fig. 5 using well-known techniques of inversion could be carried out in an on-board microprocessor to return both target depth and orientation for each of the signals depicted.
  • the mono/Fig 8 curve in Fig. 4 shows the calculated variation of the ratio RX1/RX2 as a function of the depth of the target below the coil arrangement depicted in Fig. 2.
  • the RX1/RX2 ratio varies almost linearly with depth across a wide range of depths.
  • the diameter of the outer RX coil in this case, but not always, the same as the TX coil.
  • the RX1/RX2 ratio will vary approximately linearly with depth, with smaller coils varying more quickly.
  • the linear variation of the ratio RX1/RX2 with depth means that there is no intrinsic loss in depth resolution for deeper targets, as is the case for prior art arrangements.
  • the interference-canceling properties of the "8" shaped receive coil configuration are well known among those familiar with the art, with most sources of external electromagnetic interference inducing approximately equal and opposite voltages in the opposite lobes of the "8" shaped coil, resulting in a net zero signal from the coil.
  • the signal from the "8" shaped RX2 coil in the present arrangement is most often significantly smaller than that from the mono-loop RXl coil, and as such the presence of external electro-magnetic noise has a potentially larger effect on the smaller RX2 signal compared with the larger RXl signal.
  • the interference-canceling properties of the "8" shaped RX2 coil provide a significant benefit over mono-loop RX2 configurations by providing an RX2 signal with much improved signal-to-noise ratio, allowing more accurate measurements of the spatial evolution of the signal and therefore providing better information on deep targets.
  • the pinpointing properties of the "8" shaped receive coil configuration are well known among those familiar with the art.
  • the signal output from the "8" shaped receive coil crosses through zero when the centre of the coil arrangement passes directly over the centre of the target, and this zero-crossing has been used to good advantage in signal processing to indicate the position of the target beneath the coil.
  • Fig. 7 the outer coil TX is used to transmit the primary magnetic field
  • two receive coils RXl and RX2 are arranged inside TX and connected to independent receive electronics.
  • Two receive signals required in the current invention can be generated as two effective receive coils RXl' and RX2' of different effective configuration by forming (in analogue electronics or by digital addition of digitized signals in the detector electronics) by a sum and a difference, respectively of the RXl and RX2 signals.
  • RXl ' RX1+RX2 gives an effective "mono-loop" response
  • RX2' RX1-RX2 gives an effective "8" shaped response.
  • the signals RXl ' and RX2' can then be analyzed to deliver all the outcomes described for the current invention.
  • the coil on the left is used for both TX and RXl, and the coil on the right is used for RX2.
  • Effective signals RXl ' and RX2' can be formed and analyzed as described above to give the desired benefits of the present invention, despite some additional complexity in analysis required to compensate for the lack of symmetry between the transmit and receive coils in this geometry.
  • a coil configuration such as that shown in Fig.
  • RX3 of similar geometry to RX2, orientated in the same plane but rotated 90 degrees relative to RX2. If connected to a third receive circuit, the information from RXl, RX2 and RX3 would provide a complementary set of information primarily sensitive to the three perpendicular excitation axes of a concealed target.
  • the configurations discussed above are well suited for use in a hand-held metal detector, since such a unit is typically swung from side to side to produce time- evolving signals such as those shown in Fig. 5.
  • the metal detecting array has found a number of applications, in areas as diverse as treasure hunting, landmine or UXO detection, and geophysical mapping. Such arrays are typically designed to cover a relatively wide area while being propelled in the forward direction only: either fixed to the front of a vehicle, or mounted on a pushcart, or carried by hand.
  • the present invention can be applied readily to metal detecting arrays, provided the coils are arranged so that the two receive coils of different effective configuration cross the target in the appropriate direction.
  • One possible embodiment is shown in Fig.
  • Fig. 9 depicts a nine-receive-coil array, this is for illustration only as any number of receive coils from two or more could be used.
  • the "forward" direction is indicated by the direction of the arrow.
  • the cross-track position of a target can be deduced from the relative amplitudes of the responses in the various receive coils as the array passes over the target.
  • FIG. 10 shows experimental data from a test array of this type, with the array passing the target first in the forward direction (RX3 leading), then in the reverse direction (RX3 trailing). Because the coils RX3 and RX7 have similar physical configuration and size, these data exhibit no information that readily relates to the depth of the target.
  • Fig. 11 shows the RXl ' and RX2' responses for the forward and reverse pass, and as described in detail above, the depth of the target can be inferred from comparison of the ratio RXl '/RX2' with calculation.
  • Experimental measurements with a test array of the type shown in Fig. 9, but with a smaller number of receive coils show good agreement between actual and inferred depth. For targets that produce a response across a number of array elements, depth measurements can be improved by forming linear combinations of the receive coil outputs to produce effective receive coils.
  • a means of receiving a signal from a CW metal detector using a single mono-loop coil for both transmitting and receiving such as that disclosed by the applicant in a co-pending provisional patent application 2005901108, could be employed to allow the RXl to be received and analysed from a coil which is not induction-balanced with respect to TX.
  • RXl could be inductively balanced with respect to a non-simple transmit coil by using techniques well known in the art, such as splitting the windings of the TX coil into a main coil and a counter-wound bucking coil configured to contribute zero net flux through either of RXl or RX2.
  • metal detectors - be they CW or PI - have the ability to provide the user with information regarding the characteristic frequency of a concealed target (or equivalently its time constant), and its ferrous content.
  • this information resides in response measured both in-phase and in-quadrature with the transmitted signal, while in a pulse induction detector time constant information comes from the shape of the characteristic decay curve of a target and ferrous information by measurements made during the transmit pulse.
  • the invention disclosed here can be implemented with either CW or PI technologies, and the additional target information supplied by the detector technology can be used to complement the invention.
  • two buried targets may have markedly different characteristic frequencies of ferrous content.
  • the breadth of the target response signal (quantified as the full width at half maximum (FWHM) of the receive signal) provides direct information about the depth of a target, since beyond a depth of approximately one coil diameter the FWHM increases approximately linearly with target depth. It is clear, then, that correlating target response with the position of the detector head has the potential to offer increased information about target depth.
  • FWHM full width at half maximum
  • the spatial FWHM of the RXl response is related to target depth, as is the curvature at the peak of the RXl response, and calculations and measurements show that so, too, is the separation between the positive and negative peaks of the RX2 response (see Target 1 in Fig. 6, for example), and the slope of the RX2 response as it passes through zero between the positive and negative peaks. Combinations of these quantities can be formed that can provide additional information on target depth, independently of the speed of the detector head over the target.
  • a response can be related with a high confidence level to a particular target depth, orientation, size and composition, it may be possible in certain circumstances to rule out targets of interest. For example in a minefield a small pieces of ferrous shrapnel may be readily distinguished from a landmine, and the user provided with a confidence level indication for the discrimination process.

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  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Environmental & Geological Engineering (AREA)
  • Geology (AREA)
  • Remote Sensing (AREA)
  • Physics & Mathematics (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Geophysics (AREA)
  • Geophysics And Detection Of Objects (AREA)

Abstract

L'invention concerne un détecteur de métaux doté d'un boîtier de bobines renfermant au moins deux bobines capables de recevoir simultanément un signal provenant d'une cible excitée par une bobine émettrice qui induit un champ magnétique et de manière préférentielle dans lequel les deux bobines capables de recevoir le signal de la cible présentent des configurations efficaces sensiblement différentes, qui sont par conséquent sensibles à différents aspects du champ magnétique secondaire émanant de la cible.
EP07719081A 2006-06-19 2007-06-18 Détecteur de métaux avec capacités de détection améliorées Withdrawn EP2038680A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
AU2006903287A AU2006903287A0 (en) 2006-06-19 Metal detector with improved detection capabilities
PCT/AU2007/000839 WO2007147199A1 (fr) 2006-06-19 2007-06-18 Détecteur de métaux avec capacités de détection améliorées

Publications (1)

Publication Number Publication Date
EP2038680A1 true EP2038680A1 (fr) 2009-03-25

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WO (1) WO2007147199A1 (fr)

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DE102012204580A1 (de) * 2012-03-22 2013-09-26 Robert Bosch Gmbh Handortungsgerät
DE102012218174A1 (de) * 2012-10-05 2014-04-10 Robert Bosch Gmbh Ortungsvorrichtung zur Bestimmung einer Objekttiefe
FR3009382B1 (fr) * 2013-08-05 2015-08-21 Elettroniche Ind Automatismi S P A C E I A S P A Costruzioni Detecteur portatif de metal pour utilisation dans un controle d'acces
WO2015025478A1 (fr) * 2013-08-21 2015-02-26 パナソニックＩｐマネジメント株式会社 Dispositif de chargement de terminal portable et automobile équipée de celui-ci
RU2569489C2 (ru) * 2014-03-25 2015-11-27 Виктор Олегович Арбузов Металлоискатель
RU2569488C2 (ru) * 2014-03-25 2015-11-27 Виктор Олегович Арбузов Датчик металлоискателя
US10141775B2 (en) * 2014-05-19 2018-11-27 Panasonic Intellectual Property Management Co., Ltd. Mobile-terminal-charging device and vehicle in which same is mounted
JP6461670B2 (ja) * 2015-03-26 2019-01-30 日本電産サンキョー株式会社 カードリーダおよびカードリーダの制御方法
WO2019245487A1 (fr) * 2018-06-21 2019-12-26 Nokta Muhendislik Ins. Elekt. Plas. Gida Ve Reklam San. Tic. Ltd. Sti. Procédé de fonctionnement d'un détecteur de métal qui peut mesurer une profondeur cible
CN110488358B (zh) * 2019-08-23 2020-10-16 清华大学 面向未爆弹的动定源结合式瞬变电磁探测仪及其探测方法
CN111273359B (zh) * 2020-02-17 2021-05-11 北京航空航天大学 高信噪比差分式金属收发探测器的线圈结构及其探测器
US20220291412A1 (en) * 2021-03-12 2022-09-15 Christopher Frank Eckman Metal detecting sensor array for discriminating between different objects
AU2021290359B2 (en) * 2021-03-25 2023-12-14 Minelab Electronics Pty. Limited An improved metal detector
CA3227826A1 (fr) * 2021-08-04 2023-02-09 Pierre-Alexandre RENINGER Systeme electromagnetique de prospection geophysique

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