WO2006037176A1

WO2006037176A1 - Multi coil metal detector

Info

Publication number: WO2006037176A1
Application number: PCT/AU2005/001531
Authority: WO
Inventors: Kenneth Brian Roberts; Raymond Leslie Seidel
Original assignee: Kenneth Brian Roberts; Raymond Leslie Seidel
Priority date: 2004-10-06
Filing date: 2005-10-05
Publication date: 2006-04-13

Abstract

A metal detection apparatus (100) which is operable to provide a selectable target volume. The detection apparatus (100) includes a plurality of inductive coils (102,104,106,108). A driver (109, 110, 112, 114) is configured to drive a selected one or more of said inductive coils (102, 104) with corresponding driving electrical signals. The selected coils (102, 104) transmit a magnetic field in response to the driving signals. A receiver (116, 118, 120, 122, 124) is configured to receive electrical signals generated in one or more of the inductive coils (106, 108) in response to changes in magnetic field. A controller (126) controls the selection of inductive coils (102, 104) driven by the driver (109, 110, 112, 114), and/or the selection of the inductive coils (106, 108) from which the receiver (116, 118, 120, 122, 124) receives electrical signals. The apparatus (100) is thereby operable to provide a selectable target volume by controlled selection of driven and/or receiving coils. The provision of a selectable target volume enables many advantages to be realised, including the measurement of magnetic properties of the environment adjacent to a detected target object, the estimation of the size and proximity of a target object, and the provision of enhanced audio feedback regarding a detected target object to a user of the apparatus (100).

Description

MULTI COIL METAL DETECTOR FIELD OF THE INVENTION

The present invention relates to metal detectors, and more particularly to an improved method and apparatus for identifying and localising metallic target objects within the natural environment.

BACKGROUND OF THE INVENTION

Metal detectors are devices used to detect the presence of conductive metal objects, and most usually those objects that are concealed from plain view.

For example, handheld metal detectors may be used to detect buried metallic objects of value, such as nuggets of gold or other precious metals. Handheld metal detectors are also used for locating man made artefacts, including valuable articles and dangerous concealed objects such as landmines.

In general terms, a metal detector is an apparatus used to detect the presence of a conductive metal target object within a specified target volume. Known metal detectors include means for transmitting magnetic fields, and typically the transmitter includes one or more inductive coils through which an electrical current is passed in order to generate a magnetic field. The target volume is determined by the characteristics of the metal detector, and is defined herein as that three dimensional region of the surrounding environment in which the transmitted magnetic field is able to induce eddy currents in metallic target objects, such that the induced currents generate fields that are detectable at a receiver of the metal detector.

The target volume therefore depends generally upon the placement and geometry of transmit and receive coils of the metal detector. It will be appreciated that the target volume therefore effectively determines a "field of view" of the detector.

In the case of a handheld metal detector, such as is used for the detection of buried objects, localisation of a concealed metallic object and estimation of its size may be achieved by a user operating the detector in a sweeping motion. Accordingly, in the vicinity of a concealed metallic object, the detected signal strength increases as the detector passes over the object, providing the user with an indication of the location and extent of the object. However, while use of a handheld detector in a sweeping motion may provide some indication of the location and size of a concealed metallic object in a plane parallel to the plane of motion of the detector, the technique is limited in that it does not provide any information in relation to the depth of the concealed object. For example, a small object located close to the surface may be indistinguishable from a larger object located further beneath the surface. Accordingly, operating a metal detector in a sweeping motion provides, at best, an approximate indication of the size and location of a concealed metallic object, and provides little information in relation to depth. Furthermore, the sweeping technique is only effective once a target object is substantially within the target volume of the detector. It is therefore also desirable to be able to detect, at least provisionally, potential target objects that are located at the periphery of the field of view of the detector. However, the target volume is generally localised around the transmit and receive coils of conventional metal detectors, and therefore any field generated by induced currents in more distant objects tends to be obscured by the field generated by currents induced in closer objects, and by fields resulting from the magnetic properties of the surrounding environment, which may include soils containing minerals which, through their own magnetic and electrical properties, interact with the fields transmitted by metal detectors.

It is also desirable for a metal detector to have some ability to discriminate between different types of metallic target object. In particular, it may be desirable to distinguish between objects having a high iron content, which are generally of low value, and objects composed of more valuable metals, such as gold. One of the characteristics of ferrous objects is their high magnetic permeability, which increases the mutual inductance, and thereby the coupling between transmit and receive coils of a metal detector. However, soil permeability may vary significantly, depending upon the composition of the soil, and accordingly it may be difficult to distinguish between the effect on permeability of a ferrous object within the target volume, and that of the surrounding soil.

Indeed, as is well known in the art, certain types of high permeability mineralised soils interact with magnetic fields generated by metal detectors to produce ground component signals at the receive coils that bear some similarity to the signals produced by a ferrous target object. Such ground component signals may therefore cause problems in reliably detecting target objects, and in discriminating between ferrous and nonferrous target objects.

In principal, the influence of the soil permeability may be taken into account, if known. For example, an assumption may be made that the permeability of the soil surrounding a detected target object is similar to a measured permeability of the soil prior to the detection of the object. However, this may be a poor assumption in environments in which the soil composition is inhomogeneous. Furthermore, accurate estimation of soil permeability requires that the detector be operated at a constant distance from the surface of the soil, such that the volume of soil contained within the target volume of the detector remains substantially constant.

Alternatively, a number of signal processing techniques are known in the art that attempt to distinguish between target signals and ground component signals on the basis of differences in frequency and/or impulse response of mineralized soils as compared with typical target objects. However, such techniques are imperfect, and at best provide only an indirect and approximate measure of the magnetic properties of the soil surrounding a target object.

Accordingly, it is also desirable to provide an improved method and apparatus that enables soil permeability measurements to be made in the region immediately surrounding an identified target object.

Additionally, communicating information to the user of a metal detector regarding the size, likely composition, and relative spatial location of a target object is a closely related problem in the art. In the case of a handheld metal detector, it is generally beneficial to provide feedback to the user in an audible form, to avoid distracting the user from maintaining visual surveillance of the surrounding terrain while using the metal detector.

Accordingly, it is common for such metal detectors to provide an audio interface to the user, often in the form of an earphone or headphones that are worn while using the detector. The use of headphones enables information to be conveyed through the use of readily distinguished properties of the audio signal, such as its volume and/or pitch. However, it is believed that further improvements in the audio feedback provided by metal detectors are possible, and it is desirable in particular that improved feedback be provided to the user in relation to the probable size, composition, and/or relative spatial location of target objects within the target volume.

Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art on or before the priority date of any of the statements herein.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a metal detection apparatus operable to provide a selectable target volume, the apparatus including: a plurality of inductive coils; a driver configured to drive a selected one or more of said inductive coils with corresponding driving electrical signals, whereby the selected one or more coils transmits a magnetic field in response to said driving signals; a receiver configured to receive electrical signals generated in one or more of said inductive coils in response to changes in magnetic field; and a controller for controlling the selection of inductive coils driven by the driver, and/or the selection of the inductive coils from which the receiver receives electrical signals such that the metal detection apparatus is operable to provide a selectable target volume by controlled selection of driven and/or receiving coils.

Accordingly, by suitable placement and control of the inductive coils used for transmission and reception of magnetic fields, it is possible to control the target volume, and hence the effective field of view of the metal detector. The invention enables the target volume to be tailored for particular purposes in the course of detecting, classifying and/or localising a target object, and discriminating between different types of target objects. For example, a metal detection apparatus operable to provide a selectable target volume, in accordance with an embodiment of the invention in this aspect, may be used to perform measurements over different target volumes in order to assess the relative size, location, and/or depth of a target object located beneath the ground. Alternatively, measurements may be performed over target volumes located adjacent to or surrounding a detected target object for probing the magnetic properties of the surrounding environment, such as the soil surrounding a buried object. In a preferred embodiment, the signals received from multiple coils are combined by the receiver. Such combination of signals may be in phase, for example by adding received signals, or in antiphase, for example by subtracting received signals. Furthermore, arbitrary linear combinations of received signals may be formed by adding and/or subtracting signals in desired proportions. Preferably, the driving electrical signals are derived by the driver from a common driving waveform. Driving signals applied by the driver to each one of a plurality of selected coils may be scaled and/or inverted versions of the common driving waveform, so as to transmit a resultant total magnetic field that may be an arbitrary linear combination of the corresponding magnetic fields transmitted by each of the selected coils.

Accordingly, magnetic fields may be generated within a selectable target volume having any desired time evolution, such as impulses, sinusoidal waveforms, or any other arbitrary waveform in accordance with the common driving waveform. While each of the plurality of inductive coils may be operable as either a transmit or detector coil, in preferred embodiments the metal detection apparatus includes dedicated transmit coils, used only for transmitting magnetic fields when selected, and dedicated detector coils used only for detecting changes in magnetic fields when selected. In a particularly preferred embodiment, dedicated transmit and detector coils are arranged in a plurality of coil assemblies, wherein each assembly includes a transmit coil and a detector coil having substantially the same geometrical arrangement.

A particular advantage provided by this preferred arrangement is that it is possible to achieve reciprocity between the transmit and detector coils. Such reciprocity ensures that the region of space within which currents are induced in a target object by magnetic fields transmitted by the transmit coil of an assembly is matched to the region of space within which fields generated by such induced currents in the target object will result in detectable signals in the corresponding detector coil of the assembly. Such reciprocity thereby provides a high level of control over the target volumes.

Preferably, each coil assembly is electrostatically shielded, for example using concentric layers of high conductance shielding material. The electrostatic shield termination points are preferably located such that the induced magnetic fields resulting from electrostatically induced discharge currents are cancelled.

It is further preferred that the coil windings are bifilar wound in order to minimise the coil winding capacitance. Preferably, low resistance wire is used for the transmit coils in order to minimise resistive power loss.

In a particularly preferred embodiment of a handheld metal detector the driver and associated circuitry are physically located within a search head of the detector, along with the coil assemblies, in order to stabilise the net coil inductance variations due to physical vibration, and to minimise power loss. The receiver coil is preferably centre tapped, and may be formed from thinner wire than the transmit coil, since its winding resistance is not a significant factor in the operation of the detector.

It is further preferred that each coil assembly additionally includes a single turn coil with a high loop resistance, said single turn coil being coaxial and separate from the transmit and detector coils. Advantageously, the inclusion of such a coil defines a minimum fast time constant target that will be detected by the detector coil, in order to stabilise the detection threshold over low conductivity soils by swamping out high resistance variations.

The coils may be substantially circular, such that the target volume defined by each coil or coil assembly when used in isolation has a substantially circular cross section about a central axis through the coils.

Alternatively, the coils may be of a different shape, such as, for example, an elliptical shape such that the corresponding target volume is of greater extent along a major axis of the ellipse than along a minor axis. Accordingly, by aligning the major axis of the ellipse with the directions of a typical sweeping motion employed when using a handheld metal detector, an enhancement in the lateral scanning capability will be provided to the operator of the metal detector. In a preferred embodiment, a plurality of coils and/or coil assemblies are arranged in an array having a common axis. The coils/assemblies may be arranged along said common axis and/or maybe concentrically arranged about said axis. In a particularly preferred arrangement, coil assemblies of successively decreasing size are arranged along the central axis. Advantageously, when transmit coils arranged in this manner are driven in phase by the driver using a common driving signal, a magnetic focussing effect may be obtained, whereby a magnetic field is generated having increased intensity and extent along the direction of the central axis and extending from the end of the coil array at which the smallest coil is located. Further, by combining the corresponding received signals in phase, the view of the receiver is similarly focused along the common axis of the coil array. There is thereby produced a substantially focused target volume, that may be useful for the detection of distant objects located on the central axis, such as target objects buried relatively deeply below the surface of the ground.

Alternatively, by driving one or more selected transmit coils in antiphase with other of the coils, the corresponding transmitted magnetic fields may substantially cancel along the central axis, and a magnetic field may be generated having a generally annular form about the central axis. Again, by combining the received signals in like manner, the view of the receiver will have a similar annular form. Accordingly, a target volume that is substantially annular in form with a central "blind zone" may be generated. Such a target volume may be useful in the detection of the properties of the environment, such as mineralised soils, that surrounds a target object located within the central zone.

Each coil may be wound substantially within a single plane with all windings being of approximately the same radial dimension. In this case, for example, a magnetic focusing effect may be produced by the arrangement of such coils of successively decreasing size along a central axis. Alternatively or additionally, one or more of the coils may be wound in a helical manner, with each winding being of successively decreasing radial dimension in order to produce a coil having a flared shape. A degree of magnetic focusing may thereby be produced at the narrower end of the flare by virtue of this specialised coil geometry.

The controller may be implemented using a combination of analogue electronic circuitry, digital electronic circuitry and/or software executing on a microprocessor system incorporated into the metal detection apparatus.

The controller may be configured to enable a user to select a target volume of the metal detection apparatus by operating input controls, such as switches or buttons, to control the selection of coils driven by the driver, and from which the receiver receives electrical signals. Preferably, however, the controller is configured to control the selection of coils so as to select a predetermined sequence of target volumes corresponding to a desired function of the metal detection apparatus. For example, the controller may be configured to sequentially select target volumes corresponding to individual coil assemblies used alone and in combination in order to estimate the relative size and/or depth of a detected target object.

In a particularly preferred embodiment two coil assemblies are provided, each of which includes a transmit coil and a receive coil, each assembly being aligned about a common central axis, being of differing diameter, and being aligned parallel to one another but located on different lateral planes along the common axis. The larger coil assembly may be, for example, approximately 38cm in diameter, while the smaller coil assembly may be, for example, approximately 12.5cm in diameter. It is preferred that the coil assemblies are arranged within a search head of a handheld metal detection apparatus, and that the smaller coil assembly is arranged in a lateral plane that is closer to the ground surface when the detector is in use.

Accordingly, each of the coil assemblies may be employed separately, in combination in phase with one another in order to provide a focused target volume, or in combination in antiphase in order to provide an annular target volume having a substantially "blind zone" along the central axis. Accordingly, in this particularly preferred embodiment, a metal detection apparatus is provided having four distinct selectable target volumes.

In particular, by operating only the smaller coil assembly of the preferred embodiment, the metal detection apparatus is most sensitive to the presence of target objects located relatively close to the search head, and more particularly to smaller target objects located relatively close to the search head. Further, by operating only the larger coil assembly, the metal detection apparatus is most sensitive to the presence of target objects located in a middle range distance from the search head, and more particularly to medium sized objects located in a middle range distance from the search head. By operating both coil assemblies in phase with one another, the metal detection apparatus is most sensitive to the presence of objects located at a relatively greater distance from the search head, and more particularly to large objects located at a greater distance from the search head.

Advantageously, therefore, by using the information available from operation of the coil assemblies individually and in combination, it is possible to provide a user of the metal detection apparatus with additional feedback in relation to the relative distance, and also preferably the relative size, of a detected target object.

In another aspect, the present invention provides a method of using a metal detection apparatus that is operable to provide a selectable target volume, to measure the magnetic properties of the environment adjacent to a metallic target object, the method including the steps of: selecting a first target volume of the metal detection apparatus having a first region of sensitivity to the presence of a target object; and when a target object is detected within said region of sensitivity of the first target volume, executing the further steps of: selecting a second target volume of the metal detection apparatus having a second region of sensitivity located adjacent to said first region of sensitivity; and measuring the magnetic properties of the material located substantially within said second region of sensitivity.

Accordingly, once a target object has been detected within the first selected target volume, the metal detection apparatus is reconfigured to employ a second target volume occupying a volume of space adjacent to the first target volume and therefore also adjacent to the target object. In this way, the method enables the magnetic properties of the environment, such as mineralised soil, immediately adjacent to a target object to be measured, such that it is not necessary to rely on estimates of properties such as soil permeability that have been made using earlier measurements, or derived using imperfect signal processing techniques. By operating a metal detection apparatus having a selectable target volume in accordance with this method, it is thereby possible to obtain measurements of environmental magnetic properties that are most directly influencing measurements upon the target object itself.

It is preferred that the second target volume has a region of low sensitivity that coincides with the first region of sensitivity of the first target volume. Accordingly, the measurement of the magnetic properties of the material located substantially within the second region of sensitivity will not be significantly affected by the presence of the target object itself, since the target object is located within a region of low sensitivity of the second target volume.

In preferred embodiments the first region of sensitivity is a volume of space having a substantially circular or elliptical cross section about a central axis, the peak sensitivity within said region being located substantially along said axis. Accordingly, in preferred embodiments when the first target volume is selected, the metal detection apparatus is most sensitive to the presence of target objects located on the central axis. In the case of a typical handheld metal detector used to detect buried objects, the central axis may be directed substantially perpendicular to the surface of the ground, such that when the first target volume is selected, the metal detector is most sensitive to metallic objects located directly below the detector head.

It is particularly preferred that the second region is a volume of space having a substantially annular cross section about said central axis, such that the peak sensitivity is located substantially around an annulus surrounding the central axis, and a local sensitivity minimum is located along the central axis. Accordingly, when the second target volume is selected, the metal detection apparatus is most sensitive to the magnetic properties of material surrounding, but not located on, the central axis. Therefore, a target object detected within the first target volume, and located substantially on the central axis, will have minimal influence upon measurements performed following a selection of the second target volume. Furthermore, by using an annular region surrounding the detected target object, all of the surrounding environmental material, such as mineralised soil, is included in the subsequent measurements. This may result in an optimum estimate of the influence of the magnetic properties of the material surrounding the target object upon the measurements of the target object itself. In preferred embodiments, the method further includes measuring the magnetic properties of material located substantially within the first region of sensitivity of the first target volume, and processing the results of said measurements of the magnetic properties of material located within the first and second regions, to substantially isolate the magnetic properties of the target object from those of the surrounding environment.

In at least preferred embodiments, therefore, the method enables improved detection and discrimination of metallic objects within environments, such as mineralised soils, in which the magnetic properties of the environment may influence measurements of the properties of the detected target object. In still another aspect, the present invention provides a method of using a metal detection apparatus having a selectable target volume, to estimate the size and proximity of a metallic target object, the method including the steps of: selecting a first target volume having a first region of sensitivity to the presence of a target object; measuring a first response of a target object to magnetic fields generated within said first target volume; selecting a second target volume having a second region of sensitivity to the presence of a target object, said second region being more extensive than said first region such that the first region is substantially contained within the second region; measuring a second response of the target object to magnetic fields generated within said second target volume; and comparing said first and second response measurements in order to estimate the size and proximity of the target object. According to this method, therefore, at least two measurements are performed in order to estimate the possible size and proximity of a target object. A first measurement determines the response of the target object to fields generated within a first, and smaller, target volume, while the second measurement determines the response of the object to fields generated within a second, and larger, target volume. By comparing the two measurements, facilitated by the provision of a metal detection apparatus operable to provide a selectable target volume, an estimate of the relative size and proximity of a target object may be established.

For example, an object that produces a relatively strong response to a measurement over the first target volume, and a weaker response to a measurement over the second target volume, is likely to be a small object located close to the source of the magnetic fields. This is because such an object may substantially occupy the first target volume, but occupy a relatively smaller proportion of the second target volume.

As a further example, a target object producing a small response to a measurement over the first target volume, and a significantly larger response to a measurement over a second target volume is likely to be a relatively large object located at a greater distance from the source of the magnetic fields. This is because such an object may extend only to the periphery of the first target volume, whereas it may occupy a more substantial portion of the second, larger, target volume.

The method is particularly advantageous in the case of handheld metal detectors used to detect buried objects, since it is able to provide the user with an estimate of the relative size and depth of an object concealed beneath the ground. This estimate assists the user in determining, for example, how large a hole will need to be dug in order to uncover the buried object.

In preferred embodiments, said first and second regions are volumes of space having substantially circular or elliptical cross sections about a central axis, the peak sensitivity within said regions being located substantially along said axis. Accordingly, the method will be most sensitive to objects that are located along the central axis, which in the case of a handheld metal detector used for detecting buried objects would typically correspond to objects located directly below the detector head.

In a particularly preferred embodiment, the method further includes the additional steps of: selecting a third target volume having a third region of sensitivity to the presence of a target object; and measuring a third response of the target object to magnetic fields generated within said third target volume, wherein at least one of said first, second and third target volumes is a focused target volume in which the region of sensitivity extends substantially beyond those of the other two volumes along said central axis.

Such particularly preferred embodiments of the invention enable improved estimation of the size and proximity of objects located along the central axis of the regions of sensitivity. Again, this is particularly beneficial in the case of a handheld metal detector used for detecting buried objects, since the central axis may be arranged to extend substantially perpendicular to the surface of the ground, so that the method is optimized for the estimation of the size and depth of objects buried in the ground directly below the metal detector. In such a detector, it may be possible, for example, to detect objects located further beneath the surface than is possible using conventional methods, and further to estimate the relative size of such objects. For example, a large object located a relatively great distance beneath the surface of the ground may not be detected at all within the smallest target volume, may produce only a small response to measurements within a larger target volume, but when located directly below the metal detector may produce a substantial response to measurements within the focused target volume. On the other hand, a medium sized object located at the same depth may be not be detected at all in either the smaller or larger unfocused target volumes, but may produce a detectable response within the focused target volume.

In a further aspect, the present invention provides a method of providing audio feedback in relation to a detected target object to a user of a metal detection apparatus, the method including the steps of: generating a first detected target signal indicative of the presence of a metallic target object located in relatively close proximity to the metal detection apparatus; generating a second detected target signal indicative of the presence of a metallic target object located at a relatively greater distance from the metal detection apparatus; generating from each of said first and second detected target signals respective first and second audio frequency signals, wherein each audio frequency signal varies in volume in accordance with the corresponding detected target signal; and providing said first and second audio frequency signals aurally to the user of the metal detection apparatus via a binaural audio interface. Preferably, the binaural audio interface includes first and second distinct audio channels for providing audio signals to first and second ears of the user. In a preferred embodiment, the first and second audio signals are provided to the user via said first and second audio channels respectively. For example, the first audio channel may be directed to the left ear of the user, while the second audio channel may be directed to the right ear of the user. Of course, the first and second audio channels may alternatively be directed to the right and left ears of the user respectively.

In a particularly preferred embodiment, the method of providing audio feedback further includes the steps of: generating a third detected target signal indicative of the presence of a metallic object located at an intermediate distance from the metal detection apparatus; generating from the third detected target signal a third audio frequency signal which varies in volume in accordance with the corresponding detected target signal; and providing the third audio frequency signal aurally to the user of the metal detection apparatus via the binaural audio interface.

Preferably the fundamental frequencies of the audio frequency signals are harmonically related. For example, in one embodiment of the invention the audio frequency signals are harmonically related such that the ratio of the frequencies of the first, second and third audio signals is 3:1 :2. For example, the frequency of the first audio signal may be 900Hz, the frequency of the second audio signal may be 300Hz and the frequency of the third audio signal may be 600Hz. In a particularly preferred embodiment, the first audio signal is provided to the user via the first audio channel only, the second audio signal is provided to the user via the second audio channel only, and the third audio signal is provided to the user via both the first and second audio channels. Accordingly, a perception is created for the user that the third audio signal is provided via a third channel, located centrally between the first and second audio channels.

In a preferred embodiment, the audio frequency signals are pulse width modulated audio signals wherein the duration of each pulse in a periodic series of pulses generated at each respective fundamental frequency of the first, second and third audio frequency signals is proportional to the amplitude of the corresponding detected target signal. According to this preferred arrangement, the apparent volume of each audio frequency signal increases with an increase in amplitude of the corresponding detected target signal. Further, the spectral purity of the respective audio frequency signals may also increase with increasing amplitude of the corresponding detected target signals.

In a particularly preferred embodiment, the first, second and third audio frequency signals are combined with a noise-background signal having a volume such that as the amplitude of the detected target signals increases, causing corresponding increases in volume of the respective audio frequency signals, the audio frequency signals will be perceived by the user to emerge progressively from the noise background. The noise-background signal may be, for example, a white noise signal.

The metal detection apparatus may further be arranged to provide a ground survey signal representing the permeability of the environment, such as soil, within the target volume of the detector, and surrounding a detected target object. It is then preferred that the method of providing audio feedback includes generating a ground survey signal, and modifying the noise-background signal in accordance with said ground survey signal. For example, in embodiments of the invention the noise-background signal may be a white noise signal that is progressively modified towards a brown noise signal as the permeability of the environment is detected to increase.

Advantageously, preferred embodiments of the invention are therefore able to provide a user with audio feedback in relation to the surrounding environment. Such additional feedback may be useful, for example in geographical areas where gold deposits are associated with particular soil types having distinctive magnetic properties, such as magnetic black sand.

It will be appreciated from the foregoing that, in at least preferred embodiments, the method of providing audio feedback is able to provide a user of a metal detection apparatus, through standard stereo headphones, with an audio presentation that blends target presence and target intensity, and which may be spread across harmonically related frequencies and corresponding spatial locations, according to the relative proximity and size of a metallic target object. This combination of target information may convey to the operator the probable size and depth of a target object with greater clarity than has previously been possible, in a centre weighted, spatio-temporal binaural sound format combined with a low level noise background, which may further have the ability to convey the presence of black sand deposits. In a particularly preferred embodiment, the metal detection apparatus is arranged to generate the first, second and third detected target signals using two inductive coil assemblies each of which includes a transmit coil and a detector coil, each assembly being aligned about a common central axis, being of differing diameter, and being aligned parallel to one another but located on different lateral planes along said common axis.

The coil assemblies may be arranged within a search head of a handheld metal detection apparatus, and the smaller coil assembly is preferably arranged in a lateral plane that is closer to the ground surface when the detector is in use. Accordingly, when the first coil assembly is operated alone, it may be used to generate the first detected target signal. By operating both coil assemblies in combination and in phase with one another, the second detected target signal may be generated. Further, by operating the larger coil assembly alone, the third detected target signal may be generated. BRIEF DESCRIPTION OF THE DRAWINGS Further preferred features and advantages of the present invention will be apparent to those skilled in the art from the following description of preferred embodiments of the invention. It will be understood, however, that the preferred embodiments are not limiting of the scope of the invention as defined in any of the preceding statements. The description makes reference to the accompanying drawings in which:

Figure 1 shows a simplified block diagram of a preferred embodiment of a metal detection apparatus in accordance with the present invention; Figures 2(A) and 2(B) illustrate a coil arrangement of an embodiment of a metal detection apparatus according to the present invention, in section and plan views respectively;

Figure 3 shows a magnetic field profile generated by the larger diameter transmit coil of the coil arrangement of Figure 2; Figure 4 shows a magnetic field profile generated by the smaller diameter transmit coil of the coil arrangement of Figure 2;

Figure 5 shows a magnetic field profile generated by the larger and smaller diameter transmit coils of the coil arrangement of Figure 2, when operating in antiphase; Figure 6 shows a magnetic field profile generated by the larger and smaller diameter transmit coils of the coil arrangement of Figure 2, when operating in phase;

Figure 7A illustrates schematically a series of target volumes selectable using the coil arrangement of Figure 2; Figure 7B is a flowchart illustrating a method of using a metal detector with a selectable target volume to measure the magnetic properties of the environment adjacent to a target object according to an embodiment of the invention;

Figure 8 illustrates a flared coil arrangement of an alternative embodiment of a metal detection apparatus according to the present invention;

Figure 9 shows comparative magnetic field profiles generated by the transmit coils of the coil arrangement shown in Figures 2 and 8 respectively;

Figure 10A illustrates schematically relative target responses corresponding with different selectable target volumes according to an embodiment of the invention;

Figure 10B is a flowchart illustrating a method of using a metal detection apparatus with a selectable target volume to estimate the size and proximity of a target object according to a preferred embodiment of the invention; Figure 11 illustrates schematically target depth estimation using selectable target volumes according to an embodiment of the invention;

Figure 12A illustrates schematically subjective audio response to targets of varying size and at varying depth according to an embodiment of the invention; and

Figure 12B is a flowchart illustrating a method of using a metal detection apparatus with a selectable target volume to provide audio feedback to a user according to a preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT A simplified block diagram of a preferred embodiment of a metal detection apparatus 100 in accordance with the invention is illustrated in Figure 1. The metal detector 100 includes a plurality of inductive coils, being in the preferred embodiment the four coils 102, 104, 106, 108. The metal detector 100 further includes a driver configured to drive selected coils with corresponding driving electrical signals. According to the preferred embodiment, the driver includes waveform generator 109, circuitry 110, for controlling the phase and level of the driving signal and selecting the driven coils, and current amplifiers 112, 114. These components together form a driver for driving one or both of the transmit coils 102, 104 with a predefined current waveform determined by the waveform generator 108, whereby the selected driven coils transmit a magnetic field in response to the driving currents.

The metal detector 100 also includes receiver circuitry associated with inductive coils 106, 108. Electrical currents and voltages are induced in the detector coils 106, 108 in response to changes in magnetic field. The receiver circuitry includes radio frequency filters 116, 118 and preamplifiers 120, 122 associated with each of the two detector coils 106, 108. Further receiver circuitry 124 performs conditioning and pre-processing of the received electrical signals. This further processing includes additional amplification of the received signals, the cancellation of signals directly coupled between transmit coils 102, 104 and detector coils 106, 108, as well as the cancellation or mitigation of the effects of static magnetic fields and other signal offsets that may be introduced into the received signals. It will be appreciated that the various functions of the driving and receiving circuitry may be provided in accordance with methods and apparatus known in the art, and utilised in a variety of different types of metal detectors. In the preferred embodiment, the operation of the driving and receiving circuitry is substantially in accordance with the methods disclosed in the present inventors' prior International patent application no. PCT/AU2005/000883, which is incorporated herein in its entirety by reference.

The metal detection apparatus 100 further includes digital sequencer and logic control circuitry 126, which provides a controller for controlling the selection of inductive coils 102, 104 driven by the driver, and the selection of the inductive coils 106, 108 from which the receiver receives electrical signals.

The digital sequencer and logic control circuitry 126 controls the transmit sequencer 128, which in turn provides signals to the selection, phase and level control circuitry 110 to selectively drive transmit coils 102, 104. Output signals 130, 132, 134, 136 from transmit sequencer 128 respectively instruct control circuitry 110 to drive both transmit coils in phase, both transmit coils out of phase, only transmit coil 104, or only transmit coil 102. Each of these four driving configurations results in a different configuration of the generated magnetic field, resulting in a different target volume of the metal detector 100. The digital sequencer and logic control circuit 126 also controls receiver combining circuit 138. In a manner analogous to the operation of transmit sequencer 128 and control circuitry 110, the receiver combiner 138 provides output signals 140, 142, 144, 146 that are respectively the in phase sum of both detector coils, the antiphase sum (i.e. the difference) of both detector coils, detector coil 108 only, and detector coil 106 only.

As will be described in the following paragraphs, the ability of the metal detection apparatus 100 to control the selection of inductive coils driven by the driver, and/or the selection of the inductive coils from which the receiver receives electrical signals enables the apparatus to be operated to provide a selectable target volume. As will be apparent from the foregoing description, in the preferred embodiment separate coils 102, 104 are provided for transmission, and further distinct coils 106, 108 are provided for reception of magnetic fields. However, in alternative embodiments individual inductive coils may be operable as both transmit and detector coils, for example by switching between transmit and detection functions at different times in a detection cycle. It is also notable that it is preferred to derive the driving signals for all transmit coils from a single waveform generator 109, which provides a common driving waveform. However, distinct signal generators may alternatively be provided for each transmit coil. It will further be understood that the combination of transmit and receive signals in phase, out of phase, and in independent operation, do not represent all possible ways of combining signals either at the transmitter or receiver. Rather, arbitrary and linear combinations of transmitted or received signals may be formed by controlling the relative levels of transmitted and received signals that are combined additively, i.e. in phase, and/or subtractively, i.e. out of phase.

Figure 2(A) and 2(B) illustrate a physical coil arrangement 200 in a preferred embodiment of the present invention.

Figure 2(A) shows a cross section through the coil assemblies, whereas Figure 2(B) shows the coil assemblies from above in a plan view. A core 202, which may be manufactured, for example, from a suitable lightweight dielectric material such as a plastic foam, is provided to support the coil. The four coils 102, 104, 106, 108 are provided in two separate coil assemblies 204, 206. The upper coil assembly 204 is a larger diameter assembly including transmit coil 102 and detector coil 106. The lower coil assembly 206 is a smaller diameter coil assembly including transmit coil 104 and detector coil 108. In the preferred embodiment the larger coil assembly 204 has a diameter of approximately 38 centimetres while the smaller coil assembly 206 has a diameter of approximately 12.5 centimetres. As shown, in the preferred embodiment the two coil assemblies are substantially circular, and are arranged about a common central axis 208. However, it will be appreciated that the coils may be of an alternative shape, such as, for example, an elliptical shape.

The complete coil arrangement, including the coil assemblies 204, 206 and the foam core 202 may advantageously be located within the search head of a hand held metal detector. Furthermore, it is advantageous also to include the transmit coil driver and associated circuitry within the search head, along with the coil assemblies, in order to stabilise the net coil inductance variations due to physical vibration, and to minimise power loss.

Each of the coil assemblies 204, 206 are electrostatically shielded using concentric layers of conductive shield material. The electrostatic shield termination points are located in the preferred embodiment such that the induced magnetic fields resulting from electrostatically induced discharge currents are cancelled. The coil windings 102, 104, 106, 108 are bifilar wound in order to minimise the coil winding capacitance. For the transmit coils 102, 104 low resistance wire is used in order to minimise resistive power loss. The detector coils 104, 108 are centre tapped, and formed from thinner wire than the transmit coil, since the winding resistance of the receiver coils is not a significant factor in the operation of the preferred embodiment of the metal detector. Each of the two coil assemblies 204, 206 additionally includes a single turn coil with a high loop resistance (not shown), the single turn coils being separate from the transmit and receive coils, and being centred around the same common axis 208. The purpose of the high resistance single turn coil is to provide an effective minimum fast time constant target that will be detected by the detector coils, and which will thereby stabilise the detection threshold when the metal detector is operated over low conductivity soils. By driving the transmit coils 102, 104, and monitoring the detector coils

106, 108 either independently or in combination, the target volume of the metal detector 100 is controlled. Figures 3 to 6 illustrate the magnitudes of the electromagnetic field generated by operating the transmit coils 102, 104 in different selectable configurations. Each of the graphs 300, 400, 500, 600 shown in Figures 3 to 6 represents the amplitude of the vertical field component as would be measured along a line through the central axis 208 located below the coil assemblies 204, 206 and running parallel to the plane of the coil windings.

Figure 3 illustrates the field amplitude when only the larger diameter transmit coil 102 is operated. The X axis 302 represents a distance from the central axis 208, while the Y axis 304 represents the field amplitude. The field amplitude has a maximum 306 located along the central axis of the coils 208, and accordingly the greatest depth of penetration of the magnetic field is located aiong this axis. The field changes phase at points 308, 310 located substantially below the coil windings 102, and the magnitude of the field decays away 312, 314 outside the diameter of the transmit coil windings 102.

Accordingly, when only transmit winding 102 is driven, a magnetic field is generated that has substantially circular symmetry, with diameter comparable to the diameter of the winding 102, and with a maximum intensity and extent along the central axis 208.

Figure 4 illustrates the corresponding field profile graph 400 when only the smaller transmit coil winding 104 is driven. Once again, X axis 402 represents distance from the central axis 208, and Y axis 404 represents field amplitude. The field profile 400 is a proportionally reduced replica of the field profile 300 generated by the larger transmit coil 102. Again, a local maximum field amplitude 406 is obtained along the central axis 208. A change in phase in the field amplitude occurs at points 408, 410 located substantially beneath the smaller coil windings 104. The field magnitude decays away 412, 414 with increasing distance from the central axis 208.

Figure 5 shows a graph of the field profile 500 that results when the two transmit coil windings 102, 104 are operated in antiphase, that is with a current circulating in opposing directions in each of the respective coils. Again, X axis 502 represents a distance from the central axis 208, and Y axis 504 represents total field amplitude. By appropriately adjusting the magnitude of the driving currents, the opposing magnetic flux lines generated along the central axis 208 may be arranged to substantially cancel one another, resulting in a field null 506, or central "blind zone" along the central axis 208. Moving away from the central axis, the superposed fields produce a resultant maximum amplitude at points 508, 510 located between the smaller and larger coil diameters. Accordingly, it will be appreciated that the resulting field has an annular form, in which the maximum amplitude, and corresponding depth penetration, occurs about a ring centred on central axis 208. Once again, the field decays away 512, 514 with increasing distance from the central axis 208. Figure 6 illustrates a graph 600 of the field profile when the coil windings

102, 104 are driven in phase, that is with a current circulating in the respective coils in the same direction. Again, X axis 602 represents a distance from central axis 208, and Y axis 604 represents the total resultant field amplitude from operation of both coils 102, 104 together. Constructive superposition of the magnetic flux along the central axis 208 results in a significantly increased peak field amplitude 606 at the centre of the two sets of coils. This constructive superposition may be understood as a magnetic focussing effect, whereby the generated magnetic field has increased intensity and depth penetration along the central axis 208, particularly below the coil arrangement, that is extending from the side of the arrangement closest to the smaller coil windings 104. As shown in Figure 6, the combined field also exhibits "shoulders" 608, 610 between the diameters of the smaller and larger transmit coils 104, 102. Once again, the field decays away 612, 614 with increasing distance from the central axis 208.

While Figures 3-6 illustrate the field profiles corresponding to the two coil assembly arrangement 200 of the preferred embodiments, it will be appreciated that by arranging different numbers of coils in different geometrical configurations, a larger variety of different resultant field profiles are possible by operating such coils either singly or in various combinations, and either in phase, or in antiphase. Accordingly, the present invention is not limited to the two coil assembly arrangement 200 of the preferred embodiment.

As has previously been described, with reference to Figure 2, the detector coil windings 106, 108 are located in common assemblies 204, 206 with the corresponding transmit coil windings 102, 104. The particular advantage provided by the arrangement is that reciprocity is achieved between the transmit and detector coils. As a result, the region of space within which currents are induced in a target object by a magnetic field transmitted by the transmit coil of each assembly is matched to the region of space within which fields generated by such induced currents in the target object will result in detectable signals in the corresponding detector coils. To put this another way, the volume within which each detector coil is able to detect the presence of a target object is matched to the volume within which each transmit coil generates magnetic fields. In this way, a high level of control is achieved over the corresponding target volumes, which it will be understood are determined by the combination of both transmit and detector coil geometry.

Figure 7A illustrates schematically a set of spatial target volumes corresponding to the operation of the transmit and receive coils in various combinations. Configuration 702 illustrates the target volume 704 obtained by operating only the smaller diameter coil assembly 206. Configuration 706 illustrates the target volume 708 obtained by operating only the larger diameter coil assembly 204. Configuration 710 illustrates the target volume 712 obtained by operating the two coil assemblies 204, 206 in phase. Configuration 714 illustrates the target volume 716 obtained by operating the two coil assemblies 204, 206 in antiphase. Configuration 718 illustrates a virtual target volume 720 that is obtained from the differential of the target volumes 712, 716.

As may be seen from the illustrations in Figure 7A, operating the smaller diameter coils 206 alone results in a target volume 704 that is generally localised beneath the smaller diameter coils 206. Operating the larger diameter coils 204 alone results in a target volume 708 that is generally located beneath the larger diameter coils 204, and which is therefore somewhat larger in both radial extent, and depth penetration, than the target volume 704 obtained from the smaller diameter coils 206. Generally speaking, therefore, operation of the smaller diameter coils 206 alone produces a target volume 704 that is better adapted for detection of smaller target objects located at relatively shallow depth beneath the search head. Operation of the larger diameter coils 204 results in a target volume 708 that is better adapted for the detection of larger sized target objects located at relatively greater depth.

By operating both coil assemblies 204, 206 in phase, a target volume 712 is obtained that is generally broader than target volumes 704, 708 in proximity to the coil arrangements 200 within the detector search head, and that has substantially greater depth penetration due to the constructive superposition and magnetic focussing effect. Accordingly, target volume 712 is better adapted for the detection of still larger target objects and/or target objects located at greater depth beneath the metal detector search head.

Operation of the two coil assemblies 204, 206 in antiphase results in substantially annular target volume 716. As can be seen in configuration 714, this target volume has a substantially "blind zone" along the central axis 208. A target object located within the blind zone may be detected in one of the previous configurations 702, 706, 710, that is within one or more of the target volumes 704, 708, 712. However, such a target object may not be detected in configuration 714, if it is not of sufficient extent to substantially overlap with annular target volume 716.

A particularly preferred application of the annular target volume 716 is to measure the magnetic properties of the environment adjacent to a metallic target object, which has been detected within one of the target volumes 704, 708, 712. According to a preferred method, the metal detector 100 is operated to first select a first target volume, being one of the target volumes 704, 708, 712. When a metallic target object is detected within such first target volume, the configuration of the apparatus is switched to that of a second target volume, being the annular target volume 716. By measuring the magnetic properties of the material located substantially within the annular target volume 716, the apparatus 100 is in effect measuring the magnetic properties of the environment adjacent to the detected metallic target object. It will be appreciated from the foregoing description that the purpose of this method is firstly to detect and localise the target object along the central axis 208 beneath the detection head of the metal detector 100, and then to measure the magnetic properties of the adjacent environment. The measurement of the magnetic properties of the environment enables the effects of such properties to be compensated for in the measurements of the target object carried out using the first configuration having one of the target volumes 704, 708, 712.

Figure 7B is a flowchart 750 illustrating this preferred method of measuring the magnetic properties of the environment adjacent to a metallic target object. At step 752 a first target volume, preferably being one of the central target volumes 704, 708, 712, is selected. When an object is detected within the first target volume, in accordance with decision 754, operation of the metal detector 100 is adapted to select a second target volume in step 756, the second target volume preferably being the annular volume 716.

The magnetic properties within the second target volume, being the properties of the environment surrounding the detected target object, are measured at step 758. Having measured the surrounding environment, it is possible to compensate in step 760 for the effect of the environment on the detected properties of the metallic object. An alternative application of the annular target volume 716 in configuration 714 is specifically to detect target objects that may be located near the periphery of the field of view of the metal detector, while minimising interfering influences from objects located closer to the central axis 208 of the search head. In the embodiment of the invention described with reference to Figures 2 to

7, each of the coils 102, 104, 106, 108 is wound substantially within a single plane with all windings being of approximately the same radial dimension. An alternative winding arrangement is illustrated in Figure 8, which may provide for an improved magnetic focussing effect beneath the metal detector search head. In the arrangement 800 shown in Figure 8 a flared core 802 is provided, and each of two exemplary windings 804, 806 are wound about the flared core 802. Accordingly, each of the coils 804, 806 is wound in a helical manner, with each winding being of successively decreasing radial dimension in order to produce a coil having a flared shape. The flared shape preferably follows an exponential function. The flux lines, e.g. 808, 810, shown in Figure 8 illustrate the resulting magnetic focussing action. The more densely packed flux lines emerging from the narrow end of the flare 802 represent a magnetic field having greater intensity. Accordingly, greater depth penetration of the magnetic field beneath the search head of a metal detector may be obtained. The more widely dispersed flux lines entering the larger end of the flare represent lower intensity magnetic fields advantageously reducing the sensitivity of the metal detector to metal objects located above, or to the sides of, the search head.

Figure 9 shows a comparison of the magnetic field profiles generated by the planar and helical windings, in the form of the field amplitude as a function of distance from the central axis of the coils beneath the search head. The X axis 902 represents distance from the central axis, while the Y axis 904 represents the normalised field amplitude. The dashed curve at 908 represents the field profile for the flared geometry, while the solid line at 906 represents the field profile for the planar wound geometry. As can be seen, the flared geometry results in a more focussed magnetic field about the central axis. Furthermore, the secondary peak in field amplitude, located outside the diameter of the coil, is smaller in amplitude 912 for the flared geometry, than for the corresponding peak 910 for the planar wound geometry. Accordingly, the flared geometry is less sensitive to the presence of objects located outside the diameter of the coil windings.

Turning now to Figure 1OA, there is shown a table 1000 that illustrates schematically relative target responses corresponding with different selectable target volumes according to the preferred embodiment of the invention having two sets of coil assemblies. The columns of the table represent the relative depth of target objects, ranging from objects located near the surface in column 1002 to objects located at significant depth in column 1004. The rows of the table 1000 are grouped according to the operation of the transmit and receive coils, there being a group 1006 corresponding to the small coil operated alone, a group 1008 corresponding to the large coil operated alone, and a group 1010 corresponding to the two coils operated in phase. Within each group, separate rows are provided representing small medium and large target objects. Thus, for example, in group 1006 corresponding to operation of the small coil, row 1012 represents relatively small objects, row 1014 represents medium sized objects, and row 1016 represents relatively large objects. Each box in the table 1000 includes shaded and/or unshaded regions, the shaded regions representing the relative strength of the received signal under the corresponding conditions and in the presence of the correspondingly sized and located target object. Thus, for example, a box that is entirely unshaded represents a situation in which a negligible signal would be received. Conversely, a box that is entirely shaded represents a situation in which a relatively large received signal would be obtained.

By reference to the table 1000 shown in Figure 10A, it is possible to see how the metal detector 100 may be operated, by controlling the selectable target volumes, to estimate the relative size and depth of a detected target object. For example, a medium sized target object located at a mid depth beneath the search head will result in a small received signal 1018 when the small coil assembly 206 is operated alone. The same object results in a larger received signal 1020 when the larger coil assembly 204 is operated alone. A slightly larger received signal 1022 is generated when the two coil assemblies 204, 206 are operated together in phase.

If a similar medium sized object is located more deeply beneath the search head, a negligible signal 1024 may be obtained by operating the small coil assembly 206 alone. When the larger coil assembly 204 is operated alone, a small but readily detectable signal 1026 may be obtained. A slightly larger signal 1028 may be obtained by operating both the small and larger coil assemblies at 206, 204 in phase together. Accordingly, it will be appreciated that a metal detection apparatus having a selectable target volume in accordance with preferred embodiments of the present invention may be characterised and calibrated to provide estimates of the relative size and depth of detected target objects.

Figure 10B shows a flowchart 1050 illustrating a preferred method of operating the metal detector 100 to estimate the size and proximity of a metallic target object. At step 1052 a first target volume is selected, for example by operating only the small transmit coil of the metal detection apparatus 100. Alternatively, at step 1052 the large coil may be operated, or the coils may be operated in combination, and indeed it may be desirable to switch periodically between target volumes during the process of initial object detection, to maximise the probability of detecting objects of differing sizes located at differing depths.

Once a potential target object has been detected, in accordance with decision 1054, the first target response corresponding with the first selected target volume is recorded at step 1056. Subsequently, a second target volume is selected and a corresponding second target response recorded at step 1058. Additionally, according to the preferred embodiment of the invention a third target volume is selected and a third target response recorded at step 1060. It should be understood, however, that useful information regarding target size and proximity may be obtained from two measurements using two different target volumes, and also that in alternative embodiments more than three different target volumes may be employed. Accordingly, the invention is not limited to the use of three target volume measurements, and usefully encompasses a variety of arrangements employing at least two target volume measurements.

Once the three target responses have been measured, the target size and depth may be estimated 1062 by a comparison of the first, second and third target responses. For example, comparison of the three measured target responses with typical target responses, as illustrated in Figure 10A, may be used to estimate target depth and size. As an example, if the first target volume corresponds with operation of the large coil of the metal detector 100, the second target volume corresponds with operation of the small coil, and the third target volume corresponds with operation of the coils in combination, and a relatively small target response is measured in association with the first target volume, no target response is measured in association with the second target volume, and a mid-level target response is recorded in association with the third target volume, then it may be inferred that the detected target is of medium size and located relatively deeply beneath the surface. That is, the exemplary measurements correspond with typical target responses 1026, 1024 and 1028 illustrated in Figure 1OA.

Figure 11 illustrates an alternative arrangement for providing quantitative estimates of target size and depth through the use of the annular target volume 716 and the virtual differential target volume 720 that were described previously with reference to Figure 7. It should be understood that the virtual target volume 720 does not represent an actual transmitted magnetic field distribution, but rather is computed at the receiver by subtracting the signals obtained from operation of both sets of coil assemblies in phase and out of phase respectively. In this regard, it should also be noted that while it is necessary to switch the driver configuration in order to change the transmitted magnetic field volume, all four possible combinations of detector coil signals are available simultaneously, as represented by signals 140, 142, 144, 146 in Figure 1.

In the configuration 1100 shown in Figure 11 , the user of a hand held metal detector is operating the search head 1102 in the conventional manner, by sweeping it parallel to the ground surface 1104. Accordingly, any target object located beneath the surface of the ground 1104 will pass through volume 716, and then into volume 720, as the search head 1102 is swept above the target object. As a result, assuming that the sweeping motion is performed at approximately a constant speed, the time during which the target object is detected within each of the respective volumes 716, 720 depends upon its size and depth. Accordingly, an object located at shallow depth 1106 will spend a greater proportion of time within volume 716, since at that depth the width of volume 716 is greater than that of volume 720 in approximately the ratio 12:4. By comparison, at greater depth 1108 the situation is reversed, and the object spends a greater time within the wider volume 720, the ratio of the width of volume 716 to volume 720 at depth 1108 being approximately 4:13.

At still greater depth 1110 the object is detected only within volume 720. Accordingly, by timing the passage of a detected object through the selectable volumes 716, 720 a quantitative measure of the depth of a target object may be estimated based on the time ratios.

It will be appreciated from the foregoing discussion that a metal detection apparatus having a selectable target volume in accordance with embodiments of the present invention enables relative target size and depth to be estimated, as well as enabling the magnetic properties of the environment surrounding or adjacent to a detected target to be measured, and taken into account in the process of discriminating the nature of the target object. The signals generated by the receiver combiner 138 may be processed in any conventional manner, and accordingly the selectable target volume provided by embodiments of the present invention may be utilised in combination with various methods and apparatus of conventional metal detectors.

However, it is particularly preferred that the present invention be utilised in combination with the processing methods and apparatus disclosed in International patent application no. PCT/AU2005/000883, also filed in the names of the present inventors. Accordingly, various processing circuits are provided in accordance with the preferred detection apparatus, the most significant of which are illustrated in Figure 1. Circuit blocks 148, 150, 152 and 154 are concerned with the detection and validation of metallic target objects, to produce signals indicating the presence, or possible presence, of target objects. Circuits 156, 162, 164 and 166 are concerned with discrimination of the target objects, including estimating the likelihood that a target is either ferrous or non ferrous, and classifying the target resistance. Circuit block 168 processes the various output signals produced by other circuit blocks, in order to perform detected target discrimination. The detailed operation of these various circuit blocks does not form part of the present invention and will therefore not be discussed in greater detail herein. While the information provided by the output signals from the selectable target volumes in relation to relative size and depth of target objects may be presented in a variety of forms, including various types of visual display, it is preferred to provide an audio feedback to a user of the metal detection apparatus 100. Audible feedback is preferred to enable the user to maintain visual surveillance of the surrounding terrain while using the metal detector 100, without the need to maintain observation of an output display panel.

According to the preferred embodiment of the present invention, audio feedback may be provided in a relatively direct manner from the various signals received using the different selectable target volumes.

According to the preferred embodiment, the metal detection apparatus 100, under the control of controller 126, cycles through operation using the small coil assembly 206, the larger coil assembly 204, and the combined coil assemblies 204, 206 to provide target volumes 704, 708, 712 respectively. At each stage of this cycle, a signal is obtained, the relative strength of which depends upon the size and depth of a target object, in the manner illustrated schematically in Figure 10. In response to each corresponding received signal, an audible tone of a respective frequency is generated, with amplitude that is dependent upon the strength of the received signal. By choosing three frequencies that are harmonically related, a particularly effective form of audio feedback is obtained.

It will therefore be appreciated that the preferred method of providing audio feedback generally consists of generating first, second and third detected target signals indicative of the presence of metallic target objects respectively located close to, at a relatively greater distance from, and at an intermediate distance from the metal detection apparatus. In the preferred embodiment the signals are provided by using the corresponding selectable target volumes. From each of the first, second and third detected target signals there are then generated respective first, second and third audio frequency signals, wherein each audio frequency signal varies in volume in accordance with the magnitude of the corresponding detected target signal and wherein preferably the fundamental frequencies of the audio frequency signals are harmonically related. These audio frequency signals are then provided aurally to the user of the metal detection apparatus via a suitable audio interface.

The preferred process is illustrated specifically in Figure 12A. The figure shows three sets of generated first, second and third audio tone waveforms, corresponding to shallow targets 1202, relatively deeper targets 1206, and targets at intermediate depth 1204 respectively. The columns in the figure represent smaller targets 1208, medium size targets 1210 and large targets 1212 at the respective depths. As will be seen, the figure illustrates the amplitude of signals generated in response to operation of the small coil assembly 206, the combined coil assemblies 204, 206 and the large coil assembly 204. In the preferred embodiment, a 900 hertz tone is generated in response to the signal received from the small coil assembly 206, a 600 hertz tone is generated in response to the signal received from the large coil assembly 204, and a 300 hertz tone is generated in response to the signal received from the combined coil assemblies. Accordingly, the preferred ratio in which the first, second and third harmonically related frequencies are provided is 3:1 :2.

The three tones thus generated are then blended by sound blender 1220 to produce a binaural output. In the preferred embodiment, the 900 hertz signal generated in response to the operation of the small coil 206 is provided at 1214 to the left channel of sound blender 1220. The tone generated in response to operation of the large coil 204 is provided to a centre channel 1216 of the sound blender 1220. The tone generated in response to the combined coils 204, 206 is provided to the right channel 1218 of sound blender 1220.

Figure 12B is a flowchart 1250 illustrating a preferred method of providing audio feedback to the user, corresponding with the process described above with reference to Figure 12A. According to the preferred embodiment, in steps 1252, 1254 and 1256 respectively the metal detector 100 is operated to select a "shallow", a "mid-depth" and a "deep" target volume. In each case, the response is measured. At step 1258 audio frequency signals are generated which correspond with the measured responses. Finally, at step 1260 the audio frequency signals are combined and provided to the user, for example via a stereo headset. As may be concluded from the table 1000 shown in Figure 1OA, and the corresponding audio tone amplitudes illustrated in Figure 12A, small target objects generally result in smaller amplitude received signals, and therefore in audible tones having a lower volume. Medium or large sized targets will result in correspondingly higher volume levels. Furthermore, targets located at shallow depths result in detected signals, and corresponding audio tones, of relatively high amplitude when detected using the small coil assembly 206. However, target objects located at greater depth result in only small amplitude signals, or negligible signals, using the small coil assembly 206 alone. However, such deeper target objects result in more substantial signals, and therefore in audible tones of greater volume, when detected using the larger coil 204, and/or the combined coils 204, 206.

The overall effect of these different detected signal amplitudes, as illustrated in the schematic head set diagrams 1222, 1224, 1226 in Figure 12A, is that small target objects, and deeper target objects, result in audible tones of lower volume. Furthermore, deeper target objects, that are more strongly detected using the large coil 204, or combined coils 204, 206, are located more towards the right hand side of the binaural soundscape. Accordingly, the combination of volume and/or spatial location alone provides an indication to a user of the metal detection apparatus with regard to the size and/or depth of a detected target object. This perception is further enriched by the use of harmonically related audio frequencies to represent differing target depth.

In addition, the preferred method for generating the audio tones is through the use of pulse width modulation. According to this preferred method, triangular waves of the respective 300 hertz, 600 hertz and 900 hertz signals are generated, and input to corresponding comparators whose threshold levels are set according to the current detected signal strength of the combined coil assemblies, large coil assembly and small coil assembly respectively. The output of these comparators is therefore a square wave of the corresponding audio frequency that has a duty cycle proportional to the corresponding received signal strength. Therefore, the perceived volume of these pulse width modulated signals increases with increasing received signal strength, and also the spectral purity of the tones increases. The variation in spectral purity obtained using this preferred method therefore also provides a further perceptible indication of target size and depth.

While the foregoing discussion of the generation of audible feedback describes a preferred arrangement in which three audio channels are generated having harmonically related fundamental frequencies, it will be understood that a two-channel implementation is also within the scope of the invention. For example, a two-channel audio interface may include the generation of first and second detected target signals indicative of the presence of metallic target objects respectively located close to and at a relatively greater distance from, the metal detection apparatus, from which there are then generated respective first and second audio signals. The first and second audio signals may then be provided to the user via the left and right channels of a binaural audio interface. Furthermore, while it is preferred that the audio frequencies employed are harmonically related, it will be understood that there need not be a harmonic relationship between the audio frequencies, and that instead, for example, identical or unrelated frequencies may be employed.

The preferred embodiment of the metal detection apparatus 100 also provides a white noise signal in the centre channel 1216 provided to sound blender 1220, which is represented by the diamonds, e.g. 1228, shown in Figure 12. As a consequence, when a detectible target object comes into the target volume of the detection apparatus 100, the corresponding tones will be perceived by the user as emerging from this white noise background.

Returning to Figure 1 , it will be noted that circuit block 156, in addition to assisting in the classification of target objects, also produces an output 158 indicative of the soil permeability. Note that soil permeability measurements in the presence of a target object may be enhanced, as previously described, through the use of the annular target volume. The soil permeability signal 158 is used in the preferred embodiment 100 to drive a white noise modulator 160. The white noise modulator 160 may be used to modify the white noise 1228 incorporated in the centre channel 1216 provided to sound blender 1220. For example, in the preferred embodiment the noise-background signal may be progressively modified towards a brown noise signal as the permeability of the soil is detected to increase. The user of the metal detection apparatus 100 is thereby enabled to perceive changes in the surrounding environment through audio feedback in the form of changes in the noise-background signal. This feedback may be particularly useful, for example, in geographical areas where gold deposits are associated with particular soil types having distinctive magnetic properties, such as magnetic black sand. In this situation, the user will actually be enabled to hear changes in soil properties that may be indicative of desirable metal deposits contained therein.

As can be seen from the block diagram of the preferred embodiment of the metal detection apparatus 100, all of the signals required to generate the audio feedback in accordance with the process illustrated in Figures 12a and 12B are readily available within the apparatus 100. Input signals 172, 174 indicate the presence of a target object, and the corresponding detected signal strength respectively. Complex modulation signal 178 includes output from the white noise modulator 160. Audio circuitry 170 is therefore able to apply the signals to generate the waveforms as illustrated in Figure 12. The output from audio circuitry 170 is the left channel 180 and right channel 182 signals input to headphones 184 that are worn by the user of the metal detection apparatus 100.

It will be appreciated that the foregoing description is of one preferred embodiment of the invention only, and that many variations that would be apparent to a person of skill in the art are possible, and such variations fall within the scope of the invention. For example, any or all of the described functions of the metal detection apparatus 100 may be performed using analogue or digital circuits and techniques or a combination thereof. Digital techniques, in this context, includes the use of a microprocessor and/or digital signal processor in combination with analogue to digital, and digital to analogue conversion as required. Different geometric arrangements of transmit and receive coils are also possible, that would also result in the provision of a metal detection apparatus having a selectable target volume.

Claims

CLAIMS:

1. A metal detection apparatus operable to provide a selectable target volume, the apparatus including: a plurality of inductive coils; a driver configured to drive a selected one or more of said inductive coils with corresponding driving electrical signals, whereby the selected one or more coils transmits a magnetic field in response to said driving signals; a receiver configured to receive electrical signals generated in one or more of said inductive coils in response to changes in magnetic field; and a controller for controlling the selection of inductive coils driven by the driver, and/or the selection of the inductive coils from which the receiver receives electrical signals such that the metal detection apparatus is operable to provide a selectable target volume by controlled selection of driven and/or receiving coils.

2. A metal detection apparatus according to claim 1 wherein the receiver is configured to form a linear combination of signals generated in two or more inductive coils by adding and/or subtracting said signals in predetermined proportions.

3. A metal detection apparatus according to claim 2 wherein the receiver is configured to combine said signals in phase, by adding the signals.

4. A metal detection apparatus according to claim 2 wherein the receiver is configured to combine said signals in antiphase, by subtracting the signals.

5. A metal detection apparatus according to any one of the preceding claims wherein the driver is configured to derive the driving electrical signals from a common driving waveform, and to drive each one of a plurality of selected inductive coils with a scaled and/or inverted replica of the common driving waveform, so that a resultant total magnetic field is transmitted that is a linear combination of the corresponding magnetic fields transmitted by each of the selected coils.

6. A metal detection apparatus according to any one of the preceding claims including dedicated transmit coils used only for transmitting magnetic fields when selected, and dedicated detector coils, used only for detecting changes in magnetic fields when selected.

7. A metal detection apparatus according to claim 6 wherein the dedicated transmit and detector coils are arranged in a plurality of coil assemblies, each assembly including a transmit coil and a detector coil having substantially the same geometrical arrangement.

8. A metal detection apparatus according to claim 7 wherein each said coil assembly is electrostatically shielded.

9. A metal detection apparatus according to claim 8 including electrostatic shield termination points located such that the induced magnetic fields resulting from electrostatically-induced discharge currents are substantially cancelled.

10. A metal detection apparatus according to any one of claims 6 to 9 in which low-resistance wire is used for the transmit coils.

11. A metal detection apparatus according to any one of claims 6 to 9 wherein each receiver coil is centre tapped.

12. A metal detection apparatus according to claim 7 wherein each coil assembly additionally includes a single turn coil having a high loop resistance, and which is coaxial with, and separate from, the transmit and detect coils.

13. A metal detection apparatus according to any one of the preceding claims wherein the coil windings are bifilar wound.

14. A metal detection apparatus according to any one of the preceding claims which is a handheld metal detector having a search head including the inductive coils, and wherein the driver and associated circuitry are physically located within the search head.

15. A metal detection apparatus according to any one of the preceding claims wherein the inductive coils are substantially circular, such that a corresponding target volume defined by each coil or coil assembly when operated alone has a substantially circular cross-section about a central axis through the coils.

16. A metal detection apparatus according to any one of claims 1 to 14 wherein the inductive coils are substantially elliptical such that a corresponding target volume defined by each coil or coil assembly when operated alone has a substantially elliptical cross-section of greater extent along a major axis of the ellipse than along a minor axis.

17. A metal detection apparatus according to any one of the preceding claims wherein the plurality of coils or coil assemblies are arranged in an array having a common central axis.

18. A metal detection apparatus according to claim 17 wherein coil assemblies of successively decreasing size are arranged along the central axis.

19. A metal detection apparatus according to claim 18 wherein the controller is configured to select transmit and receive coils to be respectively driven and combined in phase to produce a substantially focussed target volume for detection of metallic objects located proximate to the central axis.

20. A metal detection apparatus according to claim 18 wherein the controller is configured to select transmit and receive coils respectively driven and combined in antiphase to produce a substantially annular target volume about the central axis.

21. A metal detection apparatus according to any one of the preceding claims wherein windings of each coil are arranged within a single corresponding plane and wherein all windings of the coil are of approximately the same radial dimension.

22. A metal detection apparatus according to any one of claims 1 to 20 wherein one or more of the inductive coils is wound in a helical manner, with each winding of said coil being of successively decreasing radial dimension such that the coil has a substantially flared shape.

23. A metal detection apparatus according to any one of the preceding claims wherein the controller is configured to control the selection of coils so as to select a predetermined sequence of target volumes corresponding with a desired function of the metal detection apparatus.

24. A metal detection apparatus according to claim 23 wherein the controller is configured to sequentially select target volumes corresponding to individual coil assemblies used alone and in combination in order to estimate the relative size and/or depth of a detected target object.

25. A metal detection apparatus according to claim 1 wherein two coil assemblies are provided, each of which includes a dedicated transmit coil and a dedicated receive coil, each assembly being aligned about a common central axis, being of differing diameter, and being aligned parallel to one another and located on different lateral planes along said common axis.

26. A metal detection apparatus according to claim 25 including a larger coil assembly of about 38 centimetres in diameter, and a smaller coil assembly of about 12.5 centimetres in diameter.

27. A metal detection apparatus according to either one of claims 25 or 26 which is a handheld metal detector having search head, and wherein the coil assemblies are arranged within the search head such that a smaller coil assembly is arranged in a lateral plane that is closer to the ground surface than a larger coil assembly when the detector is in use.

28. A method of using a metal detection apparatus that is operable to provide a selectable target volume, to measure the magnetic properties of the environment adjacent to a metallic target object, the method including the steps of: selecting a first target volume of the metal detection apparatus having a first region of sensitivity to the presence of a target object; and when a target object is detected within said region of sensitivity of the first target volume, executing the further steps of: selecting a second target volume of the metal detection apparatus having a second region of sensitivity located adjacent to said first region of sensitivity; and measuring the magnetic properties of the material located substantially within said second region of sensitivity.

29. A method of using a metal detection apparatus according to claim 28 wherein the second target volume has a region of low sensitivity that substantially coincides with the first region of sensitivity of the first target volume.

30. A method of using a metal detection apparatus according to either one of claims 28 or 29 wherein the first region of sensitivity is a volume of space having a substantially circular or elliptical cross-section about a central axis, the peak sensitivity within said region being located substantially along said axis.

31. A method of using a metal detection apparatus according to claim 30 wherein, in use, the central axis is directed substantially perpendicular to a ground surface beneath the detector, such that when the first target volume is selected the detector is most sensitive to metallic objects located directly below the detector.

32. A method of using a metal detection apparatus according to either one of claims 30 or 31 wherein the second region is a volume of space having a substantially annular cross-section about the central axis, such that a corresponding peak sensitivity is located substantially around an annulus surrounding the central axis, and a local sensitivity minimum is located along the central axis, whereby when the second target volume is selected, the metal detection apparatus is most sensitive to the magnetic properties of material surrounding, but not located on, the central axis.

33. A method of using a metal detection apparatus according to any one of claims 28 to 32 further including measuring the magnetic properties of material located substantially within the first region of sensitivity of the first target volume, and processing the results of said measurements of the magnetic properties of material located within the first and second regions to substantially isolate the magnetic properties of a target object located in the first region from those of the surrounding environment.

34. A method of using a metal detection apparatus having a selectable target volume, to estimate the size and proximity of a metallic target object, the method including the steps of: selecting a first target volume having a first region of sensitivity to the presence of a target object; measuring a first response of a target object to magnetic fields generated within said first target volume; selecting a second target volume having a second region of sensitivity to the presence of a target object, said second region being more extensive than said first region such that the first region is substantially contained within the second region; measuring a second response of the target object to magnetic fields generated within said second target volume; and comparing said first and second response measurements in order to estimate the size and proximity of the target object.

35. A method of using a metal detection apparatus according to claim 34 wherein the first and second regions are volumes of space having substantially circular or elliptical cross-sections about a central axis.

36. A method of using a metal detection apparatus according to claim 35 further including the steps of: selecting a third target volume having a third region of sensitivity to the presence of a target object; and measuring a third response of the target object to magnetic fields generated within the third target volume, wherein at least one of said first, second and third target volumes is a focussed target volume in which the region of sensitivity extends substantially beyond those of the other two volumes along the central axis.

37. A method of providing audio feedback in relation to a detected target object to a user of a metal detection apparatus, the method including the steps of: generating a first detected target signal indicative of the presence of a metallic target object located in relatively close proximity to the metal detection apparatus; generating a second detected target signal indicative of the presence of a metallic target object located at a relatively greater distance from the metal detection apparatus; generating from each of said first and second detected target signals respective first and second audio frequency signals, wherein each audio frequency signal varies in volume in accordance with the corresponding detected target signal; and providing said first and second audio frequency signals aurally to the user of the metal detection apparatus via a binaural audio interface.

38. A method of providing audio feedback according to claim 37 wherein the binaural audio interface includes first and second distinct audio channels for providing audio signals to first and second ears of the user.

39. A method of providing audio feedback according to either one of claims 37 or 38 wherein the audio frequency signals include harmonically related fundamental frequencies.

40. A method of providing audio feedback according to any one of claims 37 to 39 further including the steps of: generating a third detected target signal indicative of the presence of a metallic object located at an intermediate distance from the metal detection apparatus; generating from the third detected target signal a third audio frequency signal which varies in volume in accordance with the corresponding detected target signal; and providing the third audio frequency signal aurally to the user of the metal detection apparatus via the binaural audio interface.

41. A method of providing audio feedback according to claim 40 wherein the audio frequency signals are harmonically related such that the ratio of the frequencies of the first, second and third audio signals is 3:1 :2.

42. A method of providing audio feedback according to either one of claims 40 or 41 wherein the binaural audio interface includes first and second distinct audio channels for providing audio signals to first and second ears of the user, and the first audio signal is provided to the user via the first audio channel only, the second audio signal is provided to the user via the second audio channel only, and the third audio signal is provided to the user via both the first and second audio channels.

43. A method of providing audio feedback according to any one of claims 40 to 42 wherein the audio frequency signals are pulse width modulated audio signals wherein the duration of each pulse in a periodic series of pulses generated at each respective fundamental frequency of the first, second and third audio frequency signals is proportional to the amplitude of the corresponding detected target signals.

44. A method of providing audio feedback according to any one of claims 37 to 43 wherein the audio frequency signals are combined with a noise-background signal, and wherein volumes of the respective audio frequency signals increase in accordance with increasing amplitude of the corresponding detected target signals, and wherein the volume of the noise-background signal is selected such that said audio frequency signals are perceived by the user to emerge progressively from the noise background.

45. A method of providing audio feedback according to claim 44 wherein the noise-background signal is a white noise signal.

46. A method of providing audio feedback according to claim 44 wherein the metal detection apparatus provides a ground survey signal representing the permeability of the environment within the target volume of the detector, and wherein the method includes modifying the noise-background signal in accordance with said ground survey signal.

47. A method of providing audio feedback according to claim 46 wherein the noise-background signal is a white noise signal which is progressively modified to produce a brown noise signal in response to a detected increase in the permeability of the environment.