WO2005124394A1

WO2005124394A1 - Method and apparatus for detecting and classifying metallic objects

Info

Publication number: WO2005124394A1
Application number: PCT/AU2005/000883
Authority: WO
Inventors: Kenneth Brian Roberts; Raymond Leslie Seidel
Original assignee: Kenneth Brian Roberts; Raymond Leslie Seidel
Priority date: 2004-06-21
Filing date: 2005-06-20
Publication date: 2005-12-29

Abstract

A method and apparatus for detecting and classifying metallic objects, the apparatus (100) including a transmitter having a transmit coil (102) for transmitting a magnetic field in response to a driving electrical signal, and a receiver having a detector coil (104) for generating a detected electrical signal in response to changes in magnetic fields. The method detects metallic objects within a target volume of the apparatus, and includes driving the transmitter coil with a piecewise linear current waveform (400) having at least two linear segments (402, 404). The method further includes measuring a detected signal waveform (708) at the detector coil, and analysing the detected waveform to identify features characteristic of the presence of metallic objects within said target volume. The apparatus includes a driver (120, 122, 124, 126) configured to drive the transmit coil (102) with a piecewise linear waveform (400), and an analyser configured to analyse the corresponding detected signal waveform. The method and apparatus substantially mitigates problems associated with the transmission of non linear magnetic fields commonly associated with known metal detection apparatus and methods.

Description

METHOD AND APPARATUS FOR DETECTING AND CLASSIFYING METALLIC OBJECTS FIELD OF THE INVENTION The present invention relates to metal detectors, and more particularly to an improved method and apparatus for detecting conductive 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, a metal detector may be used to detect buried metallic objects of value, such as nuggets of gold, or other precious metal, or man made artefacts.

Metal detection apparatus is also used in applications such as security screening, wherein a metal detector may be used to identify metallic objects concealed about a person's body. As will be appreciated, many metallic objects that may be encountered in the environment are of little value or interest to most users of metal detectors.

For example, discarded aluminium cans and other items of metallic refuse are of little interest to a gold prospector. Similarly, items such as metallic keys or coins are generally of no interest in security screening. Accordingly, it is highly desirable for a metal detector to be able to perform at least some level of discrimination between different types of metallic objects that may be encountered in use. A further problem that arises, particularly in relation to metal detectors used for identifying buried metallic objects, is that the magnetic properties of the environment may interfere with the ability of the detector to reliably identify and discriminate amongst metallic objects of interest. For example, common soils contain minerals which, through their own magnetic and electrical properties, interact with the fields transmitted by metal detectors. Since the metallic objects of interest may be of greatly varying size, distance from the detector, and electrical and magnetic properties, there may be significant difficulties in reliably detecting and discriminating amongst the objects of interest in such an environment. In the most general terms, a metal detector may be defined as 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 coils through which an electrical current is passed in order to generate a magnetic field. The magnetic field propagates into the surrounding environment, and the target volume may be defined as that volume of the surrounding environment in which the transmitted magnetic field induces eddy currents in metallic target objects, such that the induced currents generate fields that are detectable at a receiver of the metal detector. The receiver generally also includes at least one detector coil in which electrical currents and voltages will be induced in response to an external magnetic field. Depending upon the design of the metal detector, a single coil may be used for both transmission and reception of magnetic fields. Accordingly, all known metal detectors operate by generating magnetic fields which induce currents in metallic target objects within the target volume, such that a target object generates its own magnetic field while the induced currents persist, and it is these return fields generated by the target object that are used by the metal detector to detect the presence of the target. The two major known categories of metal detectors are frequency domain detectors, and time domain detectors. Frequency domain detectors operate generally by transmitting, receiving and processing signals, such as continuous sinusoidal waveforms, that are most readily compared in terms of frequency and relative phase. Time domain detectors, on the other hand, operate by generating and processing signals as they evolve over time. The metal detector subject of the present invention is a time domain detector, and may more specifically be categorised as a pulse induction detector. Pulse induction detectors are characterized in that they transmit transient magnetic pulses, and target objects are detected by analysis in the time domain of return signals received in response to the transmitted pulses. Known pulse induction detectors generally operate by applying a transient voltage pulse across the transmitting coil of the detector for a specified period, generating a corresponding current in the coil. The response of the coil current to a voltage step is typically exponential in form, with a time constant that depends upon the inductance and resistance of the coil. Furthermore, since the inductance of the coil depends upon the permeability of the surrounding environment of the target volume, this time constant is dependent upon the environment in which the detector is used. The instantaneous magnetomotive force (mmf) is proportional to the coil current, and accordingly also has an exponential form. Similarly, the currents induced in a target object within the target volume are exponential, as are the return fields generated by these currents. The presence of a target will therefore ultimately result in the detection of a corresponding voltage at the terminals of the receiver coil that is also exponential in form. The exact form of the voltage detected at the terminals of the receiver coil depends upon the characteristics of the target object. The form of the eddy currents induced in a metallic object depends upon a number of factors, including the size, composition, and proximity of the target. For some objects the functional form of the induced eddy currents may be well-categorised by a single time constant. In other objects, it is more appropriate to model the response of the eddy currents to an externally applied field using two or more time constants. In particular, ferrous objects, which have high magnetic permeability, exhibit substantial surface currents characterized by a relatively short time constant, as well as internal bulk material currents characterised by a longer time constant. The generation of short lived distinct surface currents is also known as the "skin effect". Accordingly, the time constants associated with the eddy currents induced in response to external magnetic fields may provide useful information in relation to the nature, composition and/or size of a detected target object. However, a number of problems inherent in prior art metal detectors have caused significant difficulties in reliably extracting this information in order to discriminate amongst detected target objects. One significant problem relates to the fact that the inductance of the transmit and receive coils is unknown and environmentally dependent. In a detector having separate transmit and receive coils, the coils are coupled by a similarly unknown mutual inductance, and accordingly, the transmitted magnetic field makes an indeterminate contribution to the received magnetic field. In detectors in which a single coil is used for both transmission and reception, it is generally the case that the received signal is detected and analysed once the driving voltage pulse has been removed, however the length of time required for the corresponding transmitted magnetic field to decay is again dependent upon the unknown inductances and corresponding time constants. It has therefore been difficult in known metal detectors to distinguish between the time constants associated with the metal detector coils and circuitry, and those of target objects. As a result, information that may be useful in detecting target objects, and discriminating amongst different types of target object, has not previously been fully utilized. It is accordingly an object of the present invention to provide a method and apparatus for detecting and classifying metallic objects that is able to mitigate these problems of the prior art by enabling the undesired contributions, to the received signal made by the detector itself, and the surrounding environment, to be substantially reduced. 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 the appended claims. SUMMARY OF THE INVENTION In one aspect the present invention provides, in a metal detection apparatus that includes a transmitter having a transmit coil for transmitting a magnetic field in response to a driving electrical signal, and a receiver having a detector coil for generating a detected electrical signal in response to changes in magnetic fields, a method of detecting metallic objects within a target volume of the apparatus including the steps of: driving the transmitter coil with a piecewise linear current waveform having at least two linear segments; measuring the detected signal waveform at the detector coil; and analysing the detected signal waveform to identify features characteristic of the presence of metallic objects within said target volume. In accordance with the present invention, therefore, the instantaneous mmf generated by the transmit coil has a piecewise linear form. The corresponding direct-coupled detector coil voltage is proportional to the time derivative of the instantaneous mmf, and therefore consists of a plurality of segments of substantially constant voltage having step changes therebetween. Accordingly, the method of the present invention avoids the generation of an instantaneous mmf, and corresponding direct-coupled detector coil voltage, including exponential and/or other non linear components, thereby enabling the detected signal waveform to be more easily and accurately analysed than is the case with prior art detection methods based upon analysing a received response to a transmitted impulse. Furthermore, in the presence of a metallic object within the target volume, following a transition between linear segments of differing gradient in the transmit coil current there will be a corresponding change in the detected signal waveform from an initial value just prior to said transition towards a final value that would be reached after a sufficiently long settling time, assuming no further such transitions in transmit coil current occur. The presence of the metallic object within the target volume determines the rate and manner in which the detected signal waveform approaches said final value. Accordingly, the method of the present invention enables the detected signal waveform in response to a transition between linear segments of the transmit coil current to be analysed, thereby enabling the effect of the presence of a metallic object within the range of the metal detection apparatus upon the response of the received signal waveform to a corresponding transition in instantaneous mmf to be determined. It is preferred that the step of measuring includes measuring the detected signal waveform simultaneously with the transmission of the magnetic field. Preferably the piecewise linear current waveform includes at least one segment during which the magnitude of the current is linearly increasing, and at least one segment during which the magnitude of the current is linearly decreasing. It is also preferred that the net change in current over said piecewise linear current waveform is zero. In particular, it is preferred that the initial and final current values be zero. In a particularly preferred embodiment, the piecewise linear current waveform is periodic. The piecewise linear current waveform may include a continuous-current section having a first segment of increasing current magnitude adjacent to a second segment of decreasing current magnitude. Preferably, there is a known ratio N between the magnitude of the rate of decrease in the second segment and the magnitude of the rate of increase in the first segment. It will be appreciated that in this case the known ratio N will also be the ratio between the magnitude of the direct-coupled detector coil voltage during the second segment and its magnitude during the first segment. In preferred embodiments, the magnitude of the rate of increase of the current during the first segment is less than the magnitude of the rate of decrease of the current during the second segment, and the duration of the first segment is greater than the duration of the second segment. In a particularly preferred embodiment the ratio between the duration of the first segment and the duration of the second segment is equal to the ratio N of the magnitudes of the rate of change of current in the second segment and the rate of change of current in the first segment, such that the net change in current between the start of the first segment and the end of the second segment is zero. The step of analysing the detected signal waveform preferably includes analysing a portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil current. The step of analysing said portion of the detected signal waveform may include determining, at a fixed time following a transition between adjacent linear segments of the transmit coil current, whether the detected signal waveform has reached or exceeded a predetermined threshold between the initial value of the detected signal prior to the step and a predicted final value of the detected signal waveform. It will be appreciated that the failure of the detected signal waveform to reach an appropriately determined threshold within the fixed time interval indicates a relatively slow response in the detected waveform which is characteristic of the possible presence of a metallic object. In preferred embodiments of the invention, the method includes carrying out a number of successive measurements over a number of periods of the transmitted waveform, and if the detected signal waveform fails to reach said predetermined threshold on a predetermined number of successive occasions, for example five successive occasions, then forming a determination that a metallic object may be present within the target volume. The method may also include determining a time derivative of said portion of the detected signal waveform, and if a peak of said time derivative exceeds a predetermined threshold then forming a preliminary determination that a metallic object may be entering the target volume of the metal detection apparatus. The step of analysing the detected signal waveform may also include estimating at least one time constant associated with a change in detected signal waveform following a transition between adjacent linear segments of the transmit coil current. In a preferred embodiment, this includes measuring a first time taken for the detected signal waveform to change from a first value measured at a first time instant to a value that is determined as a known fraction of the difference between said first value and a predicted final value of the detected signal range form that would be reached after a sufficiently long settling time. The step of estimating at least one time constant may include estimating a second time constant. In a preferred embodiment, this includes measuring a second time taken for the detected signal waveform to change from a second value measured at a second time instant subsequent to the first time instant of the first time constant measurement, to a value that is determined as a known fraction of the difference between said second value and a predicted final value of the detected signal waveform that would be reached after a sufficiently long settling time. In a particularly preferred embodiment, if the two time constant measurements are substantially different, the method includes forming a determination that a ferrous metallic object may be present within the target volume. The method of analysing the detected signal waveform may alternatively or additionally include analysing the frequency content of the portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil current. In a preferred embodiment, this includes measuring at least a high frequency component of said portion of the detected signal waveform, and a low frequency component of said portion of the detected signal waveform. It is particularly preferred that the time evolution of said high frequency and low frequency components then be compared to determine a likelihood that a ferrous metallic object is present within the target volume. It is further preferred that the step of analysing the detected signal waveform includes measuring a substantially steady state value of the detected signal waveform prior to a transition between adjacent linear segments of the transmit coil current. In a particularly preferred embodiment, said steady state value is used to predict a final value of the detected signal waveform that would be reached after a sufficiently long settling time following said step in transmitted field intensities. In particular, predicting said final value may include multiplying the magnitude of said steady state value by the known ratio N. The steady state value is preferably measured over successive periods of the transmitter field waveform, to determine whether an increase in magnitude of said steady state value occurs. The method then preferably includes, if an increase is detected, forming a determination that a ferrous object may be entering the target volume. It is preferred that this determination is only carried out if it has previously been determined that a metallic object is present. The method of detecting metallic objects may also include the further step of classifying detected metallic objects. The step of classifying may include comparing the at least one estimated time constant with values characteristic of known metallic materials and/or objects. The step of classifying may further include classifying a detected metallic object as ferrous or nonferrous using any or all of the aforementioned preferred methods for identifying ferrous metallic objects, either independently or in combination. Accordingly, a preferred method in accordance with the invention enables metallic objects to be detected and additionally classified according to their character and/or composition, and also as to whether they are ferrous or nonferrous metallic objects. In preferred embodiments the step of analysing the detected signal waveform includes substantially cancelling or reducing the component of the detected signal waveform caused by direct coupling of the field generated by the transmitter. This most preferably involves subtracting a suitably scaled replica of the time derivative of the transmitted waveform from the detected signal waveform. The appropriate scale factor may be determined, for example, by prior calibration of the metal detection apparatus. The step of analysing the detected signal waveform also preferably includes substantially cancelling or reducing the effect upon the detected signal waveform of mains frequency fields, such as those generated by power transmission lines and the like. This may include detecting ambient mains frequency fields and substantially cancelling corresponding frequency components present in the detected signal waveform. Preferably, the effect of mains frequency fields is further mitigated by using a periodic signal for the transmitted waveform having a frequency that is not a fundamental or harmonic frequency of the mains frequency. It is particularly preferred that the transmitted waveform has a frequency that is set midway between two consecutive harmonics of the mains frequency. Detected ambient mains frequency fields may be used to synchronise the transmitted waveform to the desired frequency. An exemplary preferred nominal frequency is 325 Hz. It will be appreciated that 325 Hz lies approximately midway between successive harmonies of 50 Hz and 60 Hz mains frequencies. Preferably, the period of the transmitted waveform, and the duration of the piecewise linear current waveform segments, may be adapted in response to detected characteristics of a target object. In particular, it is preferred that the period and duration may be increased if a target object is detected that exhibits a time constant of the detected signal waveform that is comparable to, or longer than, the initial duration of one or more of the piecewise linear segments. The step of analysing the detected signal waveform also preferably includes substantially cancelling or reducing the effect upon the detected signal waveform of static magnetic fields, such as those generated by magnetised objects. It will be appreciated that the effect of static fields is to generate a relatively slowly-varying offset in the amplitude in the detected signal waveform, and accordingly in preferred embodiments cancelling or reducing the effect of such fields includes measuring a substantially steady state value of the detected signal waveform during a period in which no magnetic field is transmitted, and subtracting said measured value from the detected signal waveform throughout at least the subsequent transmission period. According to another aspect, the present invention provides a metal detection apparatus including: a transmitter having a transmit coil for transmitting a magnetic field in response to a driving electrical signal; a receiver having a detector coil for generating a detected electrical signal in response to changes in magnetic fields; a driver configured to drive the transmit coil with a piecewise linear waveform having at least two linear segments; and an analyser for analysing the detected electrical signal waveform to identify features characteristic of the presence of metallic objects within a target volume of the metal detection apparatus. The driver preferably includes at least one controlled current source connected to the transmit coil and modulated with a piecewise linear control waveform. The driver may include multiple controlled current sources connected in parallel which may be modulated by the same control waveform. In a particularly preferred embodiment, the piecewise linear control waveform is generated by integrating a digitally composed step wise constant waveform. The receiver preferably includes an amplifier for amplifying the detected electrical signal. In a preferred embodiment, the receiver gain is adaptable, such that when the detected signal is of relatively high amplitude, the gain may be reduced to improve the dynamic range of the metal detection apparatus. Alternatively, the transmitted field amplitude may be reduced in such circumstances, by reducing the current output of the driver. The analyser preferably includes a substantive object detector configured to analyse a portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil current waveform. The substantive object detector may be further configured to determine, at a fixed time following said transition, whether the detected signal waveform has reached or passed a predetermined threshold between its initial value prior to the step and a predicted final value. The substantive object detector also preferably includes a validator configured to perform successive measurements over a number of periods of the transmitted waveform, and to generate an indication that a metallic object may be present within the target volume if the detected signal waveform fails to reach said predetermined threshold on a predetermined number of successive occasions, for example, five successive occasions. The analyser may also include a liminal object detector configured for determining a time derivative of said portion of the detected signal waveform and for generating an indication that a metallic object may be entering the detected range of the metal detection apparatus if a peak of said time derivative exceeds a specified threshold. The analyser preferably also includes a time constant estimator for estimating at least one time constant associated with a change in detected signal waveform following a transition between adjacent linear segments of the transmit coil current waveform. The time constant estimator may be configured to measure a first time taken for the waveform to change from a first value measured at a first time instance to a value that is determined as a known fraction of the difference between said first value and predicted final value of the detected signal waveform that would be reached after a sufficiently long settling time. The time constant estimator may also be configured to estimate a second time constant. In a preferred embodiment, the time constant estimator is configured to measure a second time taken for the detected waveform to change from a second value measured at a second time instant subsequent to the first time instant of the first time constant measurement, to a value that is determined as a known fraction of the difference between said second value and a predicted final value of the detected signal waveform that would be reached after a sufficiently long settling time. It is particularly preferred, that if the two time constants are substantially different, the time constant estimator generates an indication that a ferrous metallic object may be present. The analyser may also include a frequency analyser for analysing the frequency content of the portion of the detected signal waveform received subsequently to a transition between adjacent linear segments of the transmit coil current waveform. In a particularly preferred embodiment, the frequency analyser includes a low pass filter and a high pass filter. Preferably, the frequency analyser is configured to compare the filter outputs and to generate an indication when it is likely that a ferrous metallic object is present. In preferred embodiments of the metal detection apparatus, the analyser further includes a sampler for capturing a value of the detected signal waveform prior to a transition between adjacent linear segments of the transmit coil current waveform. The captured value may be used by the time constant estimator to predict a final value of the detected signal waveform that would be reached after a sufficiently long settling time following said transition. It is further preferred that the analyser includes a permeability analyser that is configured to compare captured values of the detected signal waveform prior to a transition between adjacent linear segments of the transmit coil current over successive periods of the transmitter waveform. The permeability analyser is preferably configured to determine whether an increase in magnitude of the captured values occurs. It is particularly preferred that the permeability analyser is further configured to generate an indication that a ferrous object may be entering the detection range of the metal detection apparatus if an increase in magnitude of the captured value is detected. The metal detection apparatus may also include a classifier for classifying detected metallic objects. The classifier preferably includes a resistivity classifier configured to compare the at least one estimated time constant with values characteristic of known metallic materials and/or objects. It is particularly preferred that the classifier includes a discriminator for further classifying a detected metallic object as either ferrous or nonferrous using any or all of the ferrous object indications generated by the time constant estimator, frequency analyser and/or permeability analyser, either independently or in combination. The discriminator is preferably configured to generate an indication as to whether a detected metallic object is likely to be either ferrous or nonferrous. The discriminator may further be configured to generate an indication of the probable metal type of a detected object. In preferred embodiments, the analyser includes a direct coupling canceller for substantially cancelling or reducing the component of the detected signal waveform caused by direct coupling of the field generated by the transmitter. The analyser preferably further includes a mains harmonic canceller for substantially cancelling or reducing the effect upon the detected signal waveform of mains frequency fields generated by power transmission lines and the like. Also preferably, the analyser includes a static magnetic field canceller for substantially cancelling or reducing the effect upon the detected signal waveform of static magnetic fields, such as those generated by magnetised objects. In the following detailed description one preferred embodiment of a metallic detection apparatus according to the invention is described, and the further features and advantages of the invention will be apparent therefrom to those skilled in the art. In the described embodiment, the various components of the metal detection apparatus are implemented using a combination of analogue and digital electronic circuitry, however it will be appreciated that this implementation should not be considered limiting of the invention as defined in any of the proceeding statements or the claims appended hereto. It will be appreciated by persons skilled in the art that many alternative arrangements are possible to implement a metal detection apparatus in accordance with the invention. For example, the detected signal waveform could be digitally sampled at the output of the receiver, and all of the processing conducted by the analyser and classifier of the preferred embodiment could be carried out by software executing on a microprocessor system incorporated into the metal detection apparatus. Similarly, the generation of the transmitted signal waveform, which in the preferred embodiment is carried out using digital and analogue electronics, could also be performed under software control. Accordingly, it will be understood that many alternative embodiments of the invention including differing combinations of analogue electronics, digital electronics, and software processing would be readily apparent to a skilled practitioner. BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the invention will now be described, without limitation of the invention as defined in the proceeding statements and the claims appended hereto, with 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; Figure 2 illustrates an electrical model of the transmitter target and receiver in accordance with the preferred embodiment of the invention; Figure 3 shows a composed digital waveform in accordance with the preferred embodiment of the invention; Figure 4 shows a transmit coil drive current waveform in accordance with the preferred embodiment of the invention; Figure 5 shows a transmit coil terminal voltage waveform in accordance with the preferred embodiment of the invention; Figure 6 shows a direct-coupled detector coil voltage waveform corresponding to a transmit coil drive current waveform in accordance with the preferred embodiment of the invention; Figure 7 shows comparative waveforms of transmitter coil current, transmitter coil voltage, receiver coil voltage, and induced target eddy currents in accordance with the preferred embodiment of the invention; Figure 8 shows comparative receiver coil voltage waveforms corresponding to no target present, a nonferrous target present, and a ferrous target present, in accordance with the preferred embodiment of the invention; Figure 9 shows comparative receiver coil voltage waveforms, when no target is present corresponding to a target volume consisting of free space, dry sand, and wet conductive sand, in accordance with the preferred embodiment of the invention; Figure 10 shows a simplified schematic diagram of receiver signal processing circuitry in accordance with the preferred embodiment of the invention; Figure 11 shows a close up of a portion of the received signal waveform, offset to eliminate the affect of soil permeability, in accordance with the preferred embodiment of the invention; Figure 12 shows a gated received waveform in accordance with the preferred embodiment of the invention; Figure 13 shows a block diagram of a target validator in accordance with the preferred embodiment of the invention; Figure 14 shows a simplified block diagram of a processor for making a ferrous decision and classifying resistivity of a target in accordance with the preferred embodiment of the invention; Figure 15 shows a simplified schematic diagram of circuitry for measuring a time constant in accordance with the preferred embodiment of the invention; Figure 16 shows a simplified block diagram of a processor for making a ferrous decision based on target frequency response in accordance with the preferred embodiment of the invention; Figure 17 is a flow chart illustrating the general methods of detection and analysis employed by the preferred embodiment of the invention; Figure 18 is a flow chart illustrating a method of liminal metallic object detection according to the preferred embodiment of the invention; Figure 19 is a flow chart illustrating a method of substantive metallic object detection and validation in accordance with the preferred embodiment of the invention; Figure 20 is a flow chart illustrating the method of analysis, consensus, discrimination and classification employed by the preferred embodiment of the invention; Figure 21 is a flow chart illustrating a method of time constant estimation according to the preferred embodiment of the invention; Figure 22 is a flow chart illustrating a method of soil permeability estimation according to the preferred embodiment of the invention; and Figure 23 is a flow chart illustrating a method of frequency analysis according to the preferred embodiment of the invention. DETAILED DESCRIPTION OF THE 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 transmitter having a transmit coil 102 for transmitting a magnetic field in response to a driving electrical signal, and a receiver having a detector coil 104 for generating a detected electric signal in response to changes in magnetic fields. All major functions of the metal detector 100 are controlled and coordinated by the digital sequencer and logic control block 110, which in the preferred embodiment includes digital electronic circuitry. It will be appreciated by those of skill in the art that control of the metal detector 100 may equally be effected through use of a microprocessor executing suitable software code, by a combination of digital logic circuitry and microprocessor control, or by any other means commonly known in the electronic arts. Overall timing for the digital sequencer and logic control block may be derived from crystal clock oscillator 111. However, the metal detector 100 may be operated in an environment where there is significant electromagnetic interference from mains frequency signals generated, for example, by overhead or underground power lines. In order to mitigate the impact of such interference, in the preferred embodiment 100 circuitry is provided to ensure that the transmitted fields avoid any undesirable synchronisation with the mains frequency fields over either short or long time periods. This circuitry includes antenna 112, which receives any mains frequency fields, and mains harmonic amplifier 113 that selects and amplifies such fields. Mains fundamental filter 114 selects the fundamental frequency components of the interfering fields, which would typically be either 50 Hz or 60 Hz depending upon the country which the detector 100 is operated. The phase locked loop frequency multiplier 115 is used to generate a derived clock signal that is not a fundamental or harmonic of the mains frequency, but is maintained at a frequency midway between two consecutive harmonics of the mains frequency in order to avoid synchronization with the mains frequency. In the preferred embodiment, the preferred derived frequency is 325 Hz. The level sensitive clock selector 116 selects between the crystal clock isolator 111 and the mains derived clock 115 depending upon the level of the output of the mains fundamental filter 114. That is, when significant mains frequency interference is detected, the derived clock is used. Otherwise, the clock generated using the crystal oscillator 111 is used. Furthermore, in the presence of significant mains frequency interference, lamp 117 is lit to alert the user of the detector 100 to the presence of mains interference. Figure 2 depicts electrical circuit models for the transmitter coil 102, the detector coil 104, and a target object 200. A simple model of the target 200 consists of a circuit loop containing an equivalent inductor 202 and equivalent resistor 204, representing the inductance and the resistivity of the target object respectively. The transmitter coil model similarly includes the coil inductance 210 and parasitic resistance 212. The detector coil is also modelled as an inductor 220 having parasitic resistance 222. According to the present invention, it is preferred to use separate transmitter and detector coils, in order to facilitate the simultaneous transmission of magnetic fields, and detection of any return fields generated by target objects. The transmitter and detector coils are coupled, that is fields generated in the transmitter coil will induce a corresponding voltage across the detector coil. The coupling is modelled by mutual inductance 230. Similarly, fields generated by transmitter coil 102 may induce currents in target object 200, and corresponding fields generated by target object 200 will induce a voltage across detector coil 104. This coupling between transmitter and target is modelled by mutual inductance 232, and the coupling between target and detector is modelled by mutual inductance 234. It will be appreciated that, in normal use, the magnitude of inductances 202, 210, 220 and mutual inductances 230, 232 and 234 are unknown. However, as will become apparent from the following description of the operation of the preferred embodiment of the invention, the detector 100 is self calibrating in operation such that knowledge of these inductances is not required in order to determine the properties of target 200. In the preferred embodiment, the digital sequencer and logic control block 110 signals digital differential composer 120 to generate a stepped digital waveform 300, as shown in Figure 3. This composed waveform is periodic and each period consists of first constant voltage segment 302, and second constant voltage segment 304. As can be seen in Figure 3, first segment 302 is of positive voltage while second segment 304 is of negative voltage. The amplitudes and duration of the two segments 302, 304 are in proportion, such that the ratio of the amplitude of second segment 304 to the amplitude of first segment 302 is equal to the ratio of the duration of first segment 302 to the duration of second segment 304. This ratio may generally be any convenient value N, however for reasons that will become apparent it is particularly preferred that the duration of first segment 302 be substantially longer than duration of second segment 304. In the preferred embodiment, as shown in Figure 3, the ratio N equals 9. The composed digital waveform 300 is input to integrator 122, which in the preferred embodiment is an integrated circuit configured as a Miller integrator. The output of integrator 122 is waveform 400 shown in Figure 4. As will be appreciated by those skilled in the art, waveform 400 includes first linear segment 402 and second linear segment 404, the gradient of each segment being proportional to the amplitude of the corresponding segment in the composed digital waveforms. As a result of the balanced ratios between amplitude and duration of the two segments 302 and 304 of the composed digital waveform, the net change in the output of the integrator over one period of the waveform is zero. Waveform 400 is used to drive a controlled current source 124, which in the preferred embodiment is a current amplifier with constant input and variable gain controlled by the output of integrator 122. Feedback loop 126 between the outputs of amplifier 124 and integrator 122 is provided to maintain the linearity of the current drive to transmit coil 102 in the presence of various parasitic circuit and environmental effects. As will be appreciated, it may be desirable to implement current source 124 as a number of current sources in parallel, in order to distribute generated heat and improve the linearity of current output. Waveform 400 at the output of integrator 122 will therefore be understood to be equally representative of the drive current to the transmit coil 102. While the drive current 400 in the preferred embodiment of the detector 100 consists of two linear segments 402, 404, it should be understood that a wide variety of piecewise linear current waveforms may be employed in accordance with the invention. As will become apparent from the further description of operation of the detector 100, the key requirement for operation of the invention is the provision of a piecewise linear current waveform driving the transmitter coil. As a result, the transmitter coil generates an instantaneous mmf that also has a piecewise linear form, such that the corresponding direct-coupled detector coil voltage consists of a plurality of segments of substantially constant amplitude and having step changes therebetween. Accordingly, waveform 400 represents only one preferred possibility. Although waveform 400 includes first ascending segment 402 and second descending segment 404, a waveform consisting of a descending segment followed by an ascending segment would be equally applicable. Furthermore, it is not necessary that the first segment be of longer duration than the second segment, and while this arrangement is employed in the preferred embodiment, in alternative embodiments the relative durations of the two segments may be reversed. Furthermore, the drive current waveform may include more than two piecewise linear segments, and various current segments, including ascending and descending segments of different rise and fall rates may be composed in order to provide a suitable drive current to the transmit coil 102. According to the preferred embodiment 100, the transmitter coil 102 and receiver coil 104 are separately electrostatically shielded, to stabilise its transitional behaviour due to variations of coil winding capacitance and coil to ground capacitance variations. Figure 5 illustrates the transmit coil terminal voltage waveform 500 that is generated in response to the drive current waveform 400. The voltage waveform 500 includes linear segments 502, 504 with step changes at the respective start and end of these segments, and a large step change therebetween. The step changes in voltage result from the changes in drive current gradient, since the voltage across the inductive coil is proportional to the first derivative of the driving current. The linear ramping observed in segments 502, 504 is due to the additional voltage generated across the coil resistance in response to the change in coil current. The amplitude of the corresponding direct-coupled detector coil voltage, the waveform 600 of which is shown in Figure 6, consists of two substantially constant amplitude segments 602, 604. No ramping of the field waveform is evident, because the direct-coupling detector coil voltage amplitude is proportional only to the derivative of the transmit coil current, and is unaffected by the additional terminal voltage generated as a result of the coil resistance. The transmitted magnetic field induces corresponding eddy currents in a metallic target within the target volume of the detector 100. These induced eddy currents will themselves generate a magnetic field, that will induce a corresponding voltage across the detector coil 104 due to the mutual inductance therebetween, as illustrated and previously described in relation to Figure 2. This target-induced voltage will be superimposed upon the direct-coupled detector coil voltage 600. Figure 7 shows a series of comparative waveforms illustrating the transmitter coil current 702, the corresponding transmitter coil voltage 704, the current 706 induced in a target object as a result of the transmitted magnetic field, and the overall detector coil voltage 708. According to the simple target model 200, the target current 706 responds to the transitions in the piecewise linear transmitter coil current in an exponential manner. Accordingly, during the period of linear increase of transmitter coil current 702 the eddy currents induced in the target have sufficient time to reach a steady state value. During the period of linear decrease of transmitter coil current 702, the target current again responds to the corresponding step in transmitted magnetic field with an exponential response, and depending upon the time constant of the target current response, may or may not reach a steady state value prior to the time at which the transmitter coil current is switched off, causing a corresponding deactivation of the transmitted magnetic field. As can be seen from target current waveform 706, the induced eddy currents in the target then decay away in an exponential manner. As can be seen from detector coil voltage 708, the received waveform will consist of a combination of the stepped direct-coupled detector coil voltage 600, which is directly coupled between transmitter and detector by mutual inductance 230, and the exponentially varying component contributed by the received magnetic field generated by the induced target current 706. Analysis of the received waveform is carried out in the preferred embodiment during the time period 710 commencing just prior to the transition in transmitter current from a linear increase to linear decrease, and extending until the end of the interval of linear decrease. Figure 8 illustrates the detector coil voltage over this interval of time in greater detail. Three sample waveforms are illustrated, each divided into time segment 802 prior to the transition in transmitter current, and time segment 804 subsequent to the transition in transmitter current. Waveform 802a, 804a represents the typical response when no target is present, the shape of the waveform being primarily influenced by circuit effects and the permeability of the environment, such as the type of soil over which the detector is passed. Waveform 802b, 804b represents the typical response in the presence of a nonferrous target, ie a metallic target of low permeability exhibiting minimal skin effect. Waveform 802c, 804c represents the typical response in the presence of a ferrous target, being a target of high magnetic permeability exhibiting significant skin effect resulting in an initial response with a relatively short time constant, followed by a bulk response with a longer time constant. In the example shown in Figure 8, the steady state detector coil voltage prior to the transition in transmitter current is approximately the same in all three cases, and accordingly the waveforms 802a, 802b and 802c are apparently coincident. Following the transition, the three waveforms 804a, 804b and 804c are distinguishable due to the differing magnetic response of the environment alone, the presence of a nonferrous target, and the presence of a ferrous target respectively. Accordingly, by analysing the detector coil voltage waveform over this time interval it is possible to identify the presence of a target, and also to classify any detected target as being either ferrous or nonferrous, and by further analysis of the detector voltage waveform possibly to further classify a detected target according to its probable metal type. The way in which this analysis is carried out according to the preferred embodiment of the invention will now be described with further reference to Figures 8 to 16. As shown in Figure 8, detection of the probable presence of a target may be carried out a time instant t_s 808 shortly after the transition in transmitter current. An appropriate threshold voltage V_th 806 may be established and the magnitude of the detector coil voltage compared with threshold voltage 806 at time instant 808. As can be seen in Figure 8, in the absence of a substantial target presence the detector coil voltage will have passed threshold 806 prior to time instant 808, and accordingly will be greater than the established threshold 806. On the other hand, in the presence of a substantial metallic target, either ferrous or nonferrous, the contribution of the target current results in a delayed exponential response in the detector coil voltage, for example 804b, 804c, which prevents the detected voltage from reaching threshold 806 until after time instant 808. Accordingly, in the presence of such a target, the detector coil voltage will be less than threshold 806 at time instant 808. It will therefore be appreciated that the failure of the detector coil voltage waveform to reach such an appropriately determined threshold within a fixed time interval following the transition in transmitter current indicates a relatively slow response in the detected waveform which is characteristic of the possible presence of a metallic object. In order to set the threshold voltage V_th 806 to an appropriate value, it is necessary to estimate the expected final steady state voltage that will be established at the detector coil after the transition in transmitter current. This steady state voltage will depend upon the values of mutual inductances 230, 232 and 234, which in turn depend upon the permeability of the surrounding environment. Accordingly these inductances vary over time, and are dependent upon the environment in which the metal detector is used. In order to account for such environmental variation, in a preferred embodiment the invention samples the detector coil voltage waveform at a time instant just prior to the transition in transmitter current. This time instant 908 is illustrated in Figure 9, which shows possible detector coil voltage waveforms in the absence of a target, and in four different operating environments. Segments 902a, 902b, 902c and 902d represent the four waveforms at their steady state values just prior to the transition in transmitter current, whereas time segments 904a, 904b, 904c and 904d represent the response to the transition. Waveform 902a, 904a and waveform 902b, 904b represent the response when the surrounding environment is free space or dry sandy soil respectively, and as these environments have similar magnetic properties they are indistinguishable in Figure 9. Waveform 902c, 904c represents the response in the presence of a wet, conductive quartz sand, which has slightly lower magnetic permeability, resulting in a small shift in the amplitude of the response. Waveform 902d, 904d represents the response in the presence of a high permeability soil, such as a soil containing considerable quantities of iron oxide. This soil has a significantly higher permeability, resulting in a corresponding increase in the amplitude of the detector coil voltage waveform. Since the time derivative of the generated mmf following the transition in transmitter current is a known multiple of its value prior to the transition, the value of detector coil voltage sampled at time instant 908 can be used to predict the ultimate steady state value of the detector value voltage following the transition. In the described embodiment, the ratio N of transmitted field amplitudes is 9 to 1 , and accordingly the steady state value of detector coil voltage after the transition is predicted to be nine times greater than the value sampled at time instance 908 just prior to the transition. The threshold voltage V_tri 806 may then be set to an appropriate fixed value below this anticipated steady state voltage. Figure 10 shows a simplified schematic of the signal processing circuitry in the receiver of the preferred embodiment of the detector 100. The effect of the soil permeability is cancelled by permeability canceller 1002, which captures a sample of the detector coil voltage just prior to the transition in transmitter current, at time instant 908, multiplies it by the appropriate factor (being, in the exemplary embodiment, a factor of 9), and then subtracts the scaled sample value from the detected voltage waveform. The resulting waveform at point 1004 in the circuit is shown in Figure 11. In Figure 11 , waveform 1104a is the response in the absence of a metallic target, while waveform 1104b is the response in the presence of a metallic target. As is apparent in the figure, both waveforms have now been referenced to zero volts through the operation of soil permeability canceller 1002. A translated threshold voltage V_th' 1106 is set at a fixed negative value such that at time instant 808 the detector coil voltage is above threshold voltage 1106 in the absence of the target, and detector coil voltage 1104b is below threshold 1106 in the presence of a target. In the preferred embodiment 100 a transmission gate 140 is used to select the portion of the waveform following time instance 808 for further analysis. Under the control of digital sequencer and logic control 110 the transmission gate passes the segment of the detector coil voltage that is of interest, and the resulting waveform at point 1006 in the circuit is shown in Figure 12. Prior to the gated period, the voltage at point 1006 is zero volts, whereas following time instant 808 the voltage at point 1006 follows the offset detector coil voltage 1004. In the absence of any metallic target object, the waveform 1204a remains at approximately zero volts, whereas in the presence of a metallic target, the waveform 1204b has a slowed response due to the magnetic field generated by the eddy currents in the target. Comparator 1008 compares the detected voltage with fixed threshold voltage 1012, such that the signal at point 1010 indicates at time instant 808 whether or not it is probable that a metallic target is present. This is the substantive detection signal of the preferred embodiment of the detector 100. In addition to the substantive detection, the gated waveform 1204 is also differentiated using capacitor 1016 and resistor 1018 to produce a derivative signal at point 1020 in the circuit. This derivative signal is compared with a further threshold in comparator 1014 to produce liminal detection signal at point 1022 in the circuit. The liminal detection signal is more sensitive to the possible presence of a target, and therefore may be used as an early indication signal. However, it is also more easily affected by noise and other environmental factors, and is therefore not a reliable indication of the presence of a target. While it may therefore be useful in providing some feedback to a user of the detector, it is not used in the subsequent processors of substantive detection, validation, and classification of the target. In order to guard against false detection of a target, any substantive detection signals are validated by the preferred embodiment of the detector 100. The process of validation consists of correlating substantive detection events with transmitted magnetic field pulses. A block diagram of the target validator of the preferred embodiment 100 is illustrated in Figure 13. The target validator 1300 includes received pulse counter 1302 and transmitted pulse counter 1304. The received pulse counter increments each time a substantive detection event occurs, while the transmitted pulse counter increments upon each transmitted pulse. The values of the counters may be compared by comparer 1306, and if over a number of periods of the transmitted waveform the number of transmitted pulses coincides with the number of substantive detection events, a decision may be made that a target has been validly detected. For example, if on five consecutive occasions a transmitted pulse produces a corresponding substantive detection event, it may be reliably concluded that a target is present. Once a target has been detected and validated, in the preferred embodiment detector 100 uses a number of distinct processes to assess whether the detected target is ferrous or nonferrous, and to further classify the target according to its probable metal type. The first technique used to effect a decision in relation to whether the target is ferrous or nonferrous is based upon the effect that a ferrous target has upon the apparent permeability of the surrounding environment. Since, as illustrated in Figure 9, an increase in the permeability of the target volume results in an increase in amplitude of the detector coil voltage waveform, this may be used to identify the intrusion of a ferrous object of significant size into the target volume. Since a substantial target object will usually enter the target volume progressively over a number of periods of the transmitted magnetic field waveform, this results in an apparent increase in permeability of the environment over the time during which the target is entering the target volume. This will be reflected in an increase in amplitude of the detector coil voltage at time instant 908 over each successive period of the transmitted waveform. This effect is utilized by iron decision circuit 1030 illustrated in Figure 10. A further sample of the detected waveform at time instant 908 is captured and held, and is maintained at point 1032 in the circuit when a substantive detection event occurs. This value is then compared with the corresponding value held at circuit point 1003 over the subsequent periods of the transmitted waveform. If a ferrous object enters the target volume over these periods of time, a significant difference will arise between the voltages at point 1032 and 1003 which will be reflected in the output of comparator 1034. This output therefore represents the first indication as to the likelihood that the target object is either ferrous or nonferrous. The second method employed by the preferred embodiment of the detector 100 to classify the target object is based upon the presence of multiple time constants, resulting from skin and bulk effects, that is a distinctive characteristic of objects of high magnetic permeability. This method is illustrated with reference to Figures 12 and 14. The technique involves measuring time intervals over which the detected waveform decays by a predetermined fractional amount. Figure 12 illustrates two such time intervals 1208 and 1212. The time period 1208 is the time taken for waveform 1204b to decay from voltage level 1106 to voltage level 1206, which is a fixed fraction of voltage 1106. The time period 1212 is the subsequent time taken for voltage waveform 1204b to decay from voltage 1206 to voltage 1210, which is the same fixed fraction of voltage 1206. Since the fractional voltages 1206 and 1210 represent the same fractional decay, for a pure exponential decay with a single time constant the two time intervals 1208 and 1212 will be identical. However, in the presence of significant skin effect causing a response having multiple time constants, these two time periods will be different. Furthermore, the duration of time periods 1208 and 1212 may also provide further information characteristic of the type and composition of the detected object that may be useful in classifying the object more precisely. The process may be continued to measure further time intervals in the same manner. Figure 14 shows a block diagram of a processor for carrying out the method just described with reference to Figure 12. The processor 1400 includes, in the preferred embodiment, three time constant measurement steps 1402, 1404, and 1406. Each of these measures a successive time period, eg 1208, 1212, which are input to a decision process 1408 which, by comparing the time intervals, is able to make a decision 1410 as to whether the detected target object is likely to be ferrous or nonferrous. Furthermore, the later detected time periods are input to resistivity classifier 1412 which, by comparing the measured time periods with time constants characteristic of known metallic materials, is able to broadly classify the detected target object as being of low, medium, or higher resistivity. Figure 15 illustrates a simplified schematic of circuitry suitable for measuring time intervals, eg 1208, 1212, in accordance with the preferred embodiment of detector 100. The gated signal is input at point 1502 and buffered by buffer 1504, as well as being fed forward to comparator 1518. A sample is taken at the beginning of the detection interval using transmission gate 1506, and held at point 1508 on the corresponding sample hold capacitor. This voltage is buffered by buffer amplifier 1510, which applies the held voltage across the voltage divider formed using resistors 1514 and 1516. Accordingly, point 1512 maintains a voltage that is a fixed fraction of the initial sampled input voltage. Comparator 1518 continuously compares the fractional voltage with the evolving input voltage, during which interval counter 1522 maintains a count of the elapsed time. Once the input voltage crosses the fractional threshold, the counter is stopped, and its value represents a measure of the time taken for the input signal to decay from the initial value to the fixed fraction of the initial value. Circuit 1500 therefore is able to perform the time interval measurement required by processor 1400 for performing a time constant based classification of a target object. The third method used by the preferred embodiment of detector 100 for classifying a detected target object is based on a frequency domain analysis of the detected signal waveform. Figure 16 shows a block diagram of a processor 1600 for making a ferrous decision based on target frequency response. The gated received signal is input into high pass filter 1604 and low pass filter 1602. The high pass filter will pass high frequency components of the detected signal, which will correspond to a target response having a short time constant, such as the skin effect response of a ferrous object. The low pass filter, on the other hand, will pass components of the detected signal having a longer time constant, such as the bulk material response of a ferrous or nonferrous metallic object. The output of the two filters are rectified and compared by DC rectifier and level comparator 1606. The rectified filter outputs will be impulses, the magnitude and duration of which represent the low and high frequency content of the detected signal as it evolves in response to corresponding currents in the target object. The time evolution of the relative high frequency and low frequency content of the signal may therefore be used to make a decision 1608 as to whether the detected target object is likely to be ferrous or nonferrous. In the preferred embodiment 100, the low frequency cut off of high pass filter 1604, and high frequency cut off of low pass filter 1602 are set to approximately 75kHz, which is the preferred frequency for discriminating between short and long time constant components due to skin and bulk material effects. All three iron decision signals, as well as the resistance decision signal, are fed to the consensus and discrimination block 150 of the preferred embodiment of the detector 100. The logic circuitry in consensus and discrimination block 150 compares and combines the three iron decision signals and resistance decision signals in order to generate user feedback in relation to a detected target object. According to the preferred embodiment, the user feedback includes a visual indicator 152 as to whether or not the detected target object is considered more likely than not to be composed of iron. Additionally, the consensus and discrimination block 150 generates signals driving audio composer 160 which producers audible signals in headphones 162 which are worn by the user while operating the detector 100. These audible signals alert the user to the presence and likely composition of a target object, and may use any properties of the audio signal desired to provide this information to the user. For example, inferred characteristics of a detected object, such as composition, size, resistivity and so on, may be converted into audible signals of differing frequency, volume, or spatial position within a stereo sound space. It will be appreciated, however, that the particular means of user feedback provided may take an alternative form, such as a visual display, and that the manner in which the information derived from the processing conducted in accordance with the present invention is presented does not form a part of the invention as disclosed herein. A number of the methods employed by the preferred embodiment of the present invention are further illustrated by the flow charts shown in Figures 17 to 23. Figure 17 is a flow chart showing in general terms the overall method 1700 employed by the metal detector 100. At step 1702 the transmit coil is driven with a piecewise linear current waveform, and the resulting detected signal, as received by the detector coil, is measured at step 1704. The measured waveform is then analysed in various ways, as represented by the step 1706. The analysis includes: liminal detection (ie detection of possible metallic objects entering the target volume of the detector), described below with reference to Figure 18; substantive detection (ie detection of metallic objects within the target volume), described below with reference to Figure 19; and discrimination/classification of detected metallic objects (eg assessment of likelihood that objects are ferrous or non-ferrous, and/or assessment of metal type or nature of object), described below with reference to Figures 20 to 23. Figure 18 is a flow chart illustrating the preferred method 1800 of liminal detection of metallic objects. At step 1802 the liminal detector receives the, detected waveform segment used in the analysis, being specifically the portion of the received waveform during the time period 710 commencing just prior to the transition in transmitter current from a linear increase to linear decrease, and extending until the end of the interval of linear decrease. At step 1804 the time derivative of the received waveform segment is obtained, and in step 1806 a comparison is carried out to determine whether the time derivative exhibits a large peak value, being one that exceeds the predetermined threshold (in the preferred embodiment this operation is carried out by comparator 1014, as previously described). If a significant peak is not identified, then there is no liminal object detection, and the process may be repeated commencing again at step 1802. If a significant peak in the time derivative is identified, then at step 1808 the possible liminal detection of a metallic object is indicated, and again the process may be repeated. The preferred method 1900 of substantive object detection is illustrated by the flow chart shown in Figure 19. At step 1902, the relevant detected waveform segment is received, and at step 1904 this waveform segment is analysed to assess whether the detected response to the signal generated by the transmit coil is relatively slow. As previously described, with reference to Figure 8 in particular, a relatively slow response is one in which the detected rise in detector coil voltage over the relevant interval of time is slowed relative that which is observed in the absence of any metallic target within the target volume. The slowing of the response is due to the influence of magnetic fields generated by metallic objects, in which currents have been induced by the transmitted field. If a relatively slow response is not identified, then there is no substantive object detection, and the method is restarted at step 1902. However, in the event of a relatively slow response being detected, it is possible that a substantive detection of a metallic target has occurred, and accordingly in step 1906 a further waveform segment is received, corresponding to a subsequent period of the transmitted signal waveform. Again, at step 1908, the waveform segment is analysed to assess whether a relatively slow response has been detected. If not, then the initial detection of a relatively slow response has not been validated, and the method resets to step 1902. However, the repeated detection of a relatively slow response indicates the probable validation of the initial substantive object detection, and at step 1910 a test is carried out to confirm whether or not a predetermined number (which is five in the preferred embodiment) of successive object detections have occurred. If not, then the method returns to step 1906 to perform a further test for a relatively slow response on a subsequent received waveform segment corresponding with a subsequent period of the transmitted waveform. Following five consecutive substantive detection events, a validated substantive target object detection is indicated at step 1912. Figure 20 shows a flowchart 2000 illustrating the overall method of target discrimination and classification used in the preferred embodiment. The method 2000 employs three parallel analysis techniques, each of which generates outputs regarding the properties of a detected metallic object, including at least an indication of whether the detected object is likely to be ferrous or non ferrous. The three methods employed are time constant analysis 2100, soil permeability analysis 2200, and frequency domain analysis 2300, each of which is described in greater detail with reference to Figures 21 to 23. The outputs of these methods are processed at step 2002 in order to establish a consensus as to whether a detected metallic object is either ferrous or non ferrous, and to perform further discrimination and classification as previously described. Figure 21 illustrates the preferred method 2100 for time constant estimation. At step 2102 the relevant detected waveform segment is received. At steps 2104 and 2106 first and second time constant measurements, in the form of rise or decay time measurements, are performed over first and second time intervals commencing at corresponding first and second time instants. The first time instant is selected to be after the transition in transmitter current from a linear increase to linear decrease, and the second time instant is selected subsequent to the first time instant. It is to be noted that in the preferred embodiment a third time constant measurement is also included (not shown in Figure 21), commencing at a still later third time instant. At step 2108 the decay or rise time intervals are compared, with a difference in these intervals over the relevant segment of the detected waveform being indicative, in the manner previously described, of the presence of a ferrous metallic object. Accordingly, at step 2110 a decision is made wherein a significant difference in the time constants results in an indication, at step 2112, of a possible ferrous object detection, while no significant difference results, at step 2114, in an indication of a possible non ferrous object detection. Figure 22 illustrates the preferred method 2200 of soil permeability estimation. At step 2202 the relevant segment of the detected waveform is received, and at step 2204 a sample of the initial value of the waveform segment, ie at a time instant just prior to the transition in transmitter current from a linear increase to linear decrease, is sampled and held. A subsequent detected waveform segment, corresponding with a subsequent period of the transmitted waveform, is received at step 2206. At step 2208 the initial value of this subsequent waveform segment is compared with the previous initial value that has been sampled and held at step 2204. An increase in amplitude of successive samples indicates a permeability increase within the target volume, and decision 2210 results accordingly in either an indication of a possible ferrous object detection (step 2212) or a possible non ferrous object detection (step 2214). In either case, the method may be continuously repeated commencing again at step 2206 in order to detect any increase or otherwise in the permeability of the target volume over a number of successive periods of the transmitted waveform. Figure 23 illustrates a preferred method 2300 of frequency analysis. At step 2302 the relevant detected waveform segment is received, and at step 2304 the frequency content over time of the portion of the waveform segment following the transition in transmitter current from linear increase to linear decrease is analysed as previously discussed. The low and high frequency components of the detected signal waveform segment correspond respectively with bulk and skin effects observed in ferrous metallic objects, and accordingly these frequency components are compared in step 2306. The decision based upon the comparison as to whether a substantial skin effect has been detected is made at step 2308. If so, then at step 2310 a possible ferrous object detection is indicated, otherwise a possible non ferrous object detection is indicated at step 2312. In the preferred embodiment 100, the metal detector also includes some additional processing of the detector coil voltage prior to carrying out the substantive detection and classification steps previously described with reference to Figures 10 to 16. The purpose of this additional processing is to condition the signal so that the detection and classification may be carried out with greater accuracy and reliability. The detector coil voltage is first filtered and amplified using radio frequency filter and amplifier 130. The function of these elements is simply to eliminate frequencies that are of no interest in the detector processing, and which therefore contribute only noise to the signal. The further function is to amplify the detector coil voltage to an appropriate level for further processing. Direct coupling canceller 132 has as one input the output of the digital differential composer 120, which is a stepped waveform having the same functional shape as the anticipated detector coil voltage in the absence of any target. By scaling this signal appropriately, the direct coupling canceller is able to substantially remove the effect of direct coupling between transmitter coil 102 and detector coil 104 through mutual inductance 230. The appropriate scale factor can be determined in a precalibration process that takes place in a controlled environment prior to use of the metal detector 100. It will be appreciated that the direct coupling cancellation does not need to be perfect, however any reduction in this component will improve the dynamic range available for analysis of the target response. Mains harmonic canceller 134 has as one input the output of mains harmonic amplifier 113, which represents the detected waveform of any mains frequency oscillating frequencies that are present in the external environment, and which may therefore be picked up by detector coil 104, and contribute to the detector coil voltage. By inverting, and again appropriately scaling, the detected mains frequency signals, the mains harmonic canceller 134 is able to substantially remove these oscillating signals from the detected voltage waveform. It will be appreciated that the appropriate scale factor can again be precalibrated under controlled conditions. In the preferred embodiment, the detector 100 also includes static magnetic field canceller 136. In the presence of a static magnetic field, such as a field generated by a magnetised object, or by the earth's own magnetic field, a constant offset will be introduced in the detector coil voltage, which may influence the zero voltage referencing procedure employed in the soil permeability canceller. Accordingly, the detector in its preferred embodiment 100 samples the value of the detector coil voltage during the period prior to transmission of the magnetic field pulse, and any voltage offset detected during this period resulting from the presence of a static magnetic field is subsequently subtracted from the detected voltage waveform. Also, if such a static field offset is detected, a magnetic field visual indicator 137 is activated to alert the user to the presence of a detectable static magnetic field, which may result from the presence within the target volume of a magnetised object. In addition, the preferred embodiment of detector 100 may include adaptive functionality to further improve its ability to detect and classify metallic objects. Two forms of adaptive behaviour are readily incorporated into a preferred embodiment of the invention, in order to account for the effect of target objects of high magnetic permeability and/or long time responses respectively. These effects are most likely to be observed in the presence of very substantial ferrous objects. As previously described in relation to Figure 9, if the surrounding environment is of high magnetic permeability, as may be the case where the detector is used near soils with very high ferro-magnetic content, but which may also occur simply because there is a large ferrous object within the target volume, the resulting amplitude of the detector coil voltage may increase significantly. In order to prevent such large increases in detector coil voltage from saturating the components in the receiver circuitry, and causing a corresponding loss in accuracy and dynamic range of the detector, the gain of the receiver circuitry and in particular the amplifier in filter and amplifier block 130, may be adapted accordingly. That is, if a relatively high voltage amplitude is detected at sample time 908, the gain of the receiver may be reduced to compensate for the anticipated corresponding increase in detector coil voltage following the step in transmitted magnetic field. Alternatively, in such circumstances the output of current amplifier 124 may be reduced. It will also be appreciated that in the presence of an object within the target volume having substantial size, and relatively long time response, the detected signal may not reach a steady state value prior to sampling time instant 908. Again, such an event may be detected where there are significant fluctuations in the amplitude of the detected signal at time instant 908 as the substantial object enters the target volume. In this event, the period of the transmitted waveform may be successively increased until a consistent and stable value is sampled at time instance 908. The increase in the overall period of the waveform provides additional time for the detected signal to settle to a steady state value prior to the step in transmitted magnetic field. Once the object that caused the adaptive behaviour to be initiated is no longer present, the period of the transmitted waveform may be reduced back to its initial preferred value. It should be noted that in the preferred embodiment of the detector 100, the preferred frequency of the periodic transmitted waveform is approximately 325 Hz, as previously stated, and the preferred duration of the transmitted magnetic pulse waveform corresponding to the linearly increasing and linearly decreasing transmitter coil current segments is approximately 400 microseconds, including 360 microseconds increasing current and 40 microseconds decreasing current such that the ratio N therebetween is 9. 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 that such variations fall within the scope of the invention. For example, different methods of processing the detector coil voltage waveform are possible, using analogue or digital techniques or a combination thereof. Different combinations of piecewise linear transmitter coil current waveforms are also possible, and would result in corresponding detector coil voltage waveforms that could be analysed using the techniques employed in at least the preferred embodiment of the invention. The time periods, specific voltage levels, scale factors, relative amplitudes and particular frequencies employed in the preferred embodiment are exemplary only, and it will be readily understood that many variations on these specifically chosen values would be possible within the scope of the invention.

Claims

CLAIMS:

1. A method of detecting metallic objects within a target volume of a metal detection apparatus that includes a transmitter having a transmit coil for transmitting a magnetic field in response to a driving electrical signal, and a receiver having a detector coil for generating a detected electrical signal in response to changes in magnetic fields, said method including the steps of: driving the transmitter coil with a piecewise linear current waveform having at least two linear segments; measuring the detected signal waveform at the detector coil; and analysing the detected signal waveform to identify features characteristic of the presence of metallic objects within said target volume.

2. The method of claim 1 wherein the measuring step includes measuring the detected signal waveform simultaneously with the step of driving the transmitter coil.

3. The method of claim 1 or claim 2 wherein the piecewise linear current waveform includes at least one segment during which the magnitude of the current is linearly increasing, and at least one segment during which the magnitude of the current is linearly decreasing.

4. The method of any one of the preceding claims wherein the net change in current over said piecewise linear current waveform is zero.

5. The method of claim 4 wherein the initial and final current values are zero.

6. The method of any one of the preceding claims wherein said piecewise linear current waveform is periodic.

7. The method of any one of the preceding claims wherein said piecewise linear current waveform includes a continuous current section having a first segment of increasing current magnitude adjacent to a second segment of decreasing current magnitude.

8. The method of claims 7 wherein there is a predetermined ratio (N) between the magnitude of the rate of decrease in said second segment and the magnitude of the rate of increase in said first segment.

9. The method of claim 8 wherein the ratio between a duration of said first segment and a duration of said second segment is equal to said ratio (N) of the magnitudes of the rate of change of current in the second segment and the rate of change of current in the first segment.

10. The method of claim 9 wherein said duration of said first segment is greater than said duration of said second segment.

11. The method of any one of the preceding claims wherein the step of analysing the detected signal waveform includes analysing a portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil current.

12. The method of claim 11 wherein the step of analysing includes determining whether a relatively slow response of the detected signal is received at the detector coil following said transition between adjacent linear segments of the transmit coil current, which is characteristic of the possible presence of a metallic object within the target volume.

13. The method of claim 12 wherein determining whether a relatively slow response is received includes determining, at a fixed time following said transition between adjacent linear segments of the transmit coil current, whether the detected signal waveform has reached or exceeded a predetermined threshold between an initial value of the detected signal prior to said transition, and a predicted final value of the detected signal waveform, whereby a failure to reach or exceed said threshold is indicative of the presence of a metallic object within the target volume.

14. The method of either claim 12 or claim 13 including carrying out a number of successive measurements of response speed over a number of periods of a periodic transmitted waveform wherein if a relatively slow response is received on a predetermined number of successive occasions then determining that a metallic object may be present within the target volume.

15. The method of any one of claims 11 to 14 wherein the step of analysing further includes determining a time derivative of said portion of the detected signal waveform and if a peak of said time derivative exceeds a predetermined threshold then forming a preliminary determination that a metallic object may be entering the target volume of the metal detection apparatus.

16. The method of any one of the preceding claims wherein the step of analysing includes estimating at least one time constant associated with a change in the detected signal waveform following a transition between adjacent linear segments of the transmit coil current.

17. The method of claim 16 including measuring first and second time constant wherein the first time constant is measured over a first time interval commencing at a first time instant following said transition, and the second time constant is measured over a second time interval commencing at a second time instant subsequent to said first time instant.

18. The method of claim 17 including, if the two time constant measurements are substantially different, then determining that a ferrous metallic object may be present within the target volume.

19. The method of any one of the preceding claims wherein the step of analysing includes analysing the frequency content of a portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil current.

20. The method of claim 19 including measuring at least a high frequency component of said portion of the detected signal waveform, and a low frequency component of said portion of the detected signal waveform, and comparing said high frequency and low frequency components to determine a likelihood that a ferrous metallic object is present within the target volume.

21. The method of any one of the preceding claims wherein the step of analysing the detected signal waveform includes: measuring a substantially steady state value of the detected signal waveform prior to a transition between adjacent linear segments of the transmit coil current over at least two periods of a periodic driving current waveform; determining whether there is an increase in magnitude of said measured steady state values between said measurements; and if an increase has occurred, then forming a determination that a ferrous object may be entering the target volume.

22. The method of any one of the preceding claims including the further step of classifying detected metallic objects.

23. The method of claim 22 including the steps of: estimating at least one time constant associated with a change in the detected signal waveform following a transition between adjacent linear segments of the transmit coil current; and comparing the at least one estimated time constant with values characteristic of known metallic materials and objects.

24. The method of either one of claims 22 or 23 wherein the step of classifying further includes classifying a detected metallic object as ferrous or non ferrous.

25. The method of claim 24 including classifying a detected metallic object as ferrous or non ferrous by comparing time constants measured over differing time intervals of a portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil current, and if said measured time constants are substantially different then classifying the detected metallic object as ferrous.

26. The method of either one of claims 24 or 25 including classifying a detected metallic object as ferrous or non ferrous by comparing time evolutions of measured high frequency and low frequency components of a portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil current.

27. The method of any one of the preceding claims wherein the step of analysing the detected signal waveform includes reducing a component of the detected signal waveform caused by direct coupling of the field generated by the transmitter.

28. The method of claim 27 including substantially cancelling the component of the detected signal waveform caused by direct coupling of the field generated by the transmitter to reduce or substantially cancel said component of the detected signal waveform caused by direct coupling of the field generated by the transmitter.

29. The method of either one of claims 27 or claim 28 including subtracting a scaled replica of the time derivative of the driving current waveform from the detected signal waveform.

30. The method of any one of the preceding claims wherein the step of analysing includes reducing an effect upon the detected signal waveform of mains frequency fields.

31. The method of claim 30 including detecting ambient mains frequency fields and cancelling corresponding frequency components present in the detected signal waveform.

32. The method of either one of claims 30 or 31 wherein the effect of mains frequency fields is reduced by using a periodic drive current waveform having a frequency that is not a fundamental or harmonic frequency of the mains frequency.

33. The method of claim 32 wherein detected ambient mains frequency fields are used to synchronize said periodic waveform to the desired frequency.

34. The method of any one of the preceding claims wherein a duration of each of said piecewise linear current waveform segments is adapted in response to detected characteristics of a target object.

35. The method of claim 34 wherein a duration of at least one of said piecewise linear current waveform segments is increased if a target object is detected that exhibits a time constant of the detected signal waveform that is comparable to, or longer than, the initial duration of said at least one piecewise linear segment.

36. The method of any one of the preceding claims wherein the step of analysing includes reducing an effect upon the detected signal waveform of static magnetic fields, such as those generated by magnetised objects.

37. The method of claim 36 wherein reducing the effect of static magnetic fields includes measuring a substantially steady state value of the detected signal waveform during a period in which no magnetic field is transmitted, and subtracting said measured value from the detected signal waveform throughout at least a subsequent transmission period.

38. A metal detection apparatus including: a transmitter having a transmit coil for transmitting a magnetic field in response to a driving electrical signal; a receiver having a detector coil for generating a detected electrical signal in response to changes in magnetic fields; a driver configured to drive the transmit coil with a piecewise linear waveform having at least two linear segments; and an analyser configured to analyse the detected electrical signal waveform to identify features characteristic of the presence of metallic objects within a target volume of the metal detection apparatus.

39. The apparatus of claim 38 wherein the driver includes at least one controlled current source connected to the transmit coil and modulated with a piecewise linear control waveform.

40. The apparatus of claim 39 wherein the piecewise linear control waveform is generated by integrating a digitally composed step-wise constant waveform.

41. The apparatus of any one of claims 38 to 40 wherein a current output of the driver is reduced in response to a high amplitude of the detected electrical signal.

42. The apparatus of any one of claims 38 to 40 wherein the receiver includes an amplifier having a gain for amplifying the detected electrical signal, said gain being adaptable to reduce the amplification in response to a high amplitude of the detected electrical signal.

43. The apparatus of any one of claims 38 to 42 wherein the analyser includes a substantive object detector configured to analyse a portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil drive waveform in order to detect the presence of a metallic object within the target volume.

44. The apparatus of claim 43 wherein the substantive object detector is configured to determine, at a fixed time following said transition, whether the detected signal waveform has reached or exceeded a predetermined threshold between an initial value prior to said transition and a predicted final value, whereby a failure to reach or exceed said threshold is indicative of the presence of a metallic object within the target volume.

45. The apparatus of claim 44 wherein the substantive object detector is configured to receive successive said determinations of the detected signal reaching or exceeding said threshold over a number of periods of a periodic transmitted waveform, and to generate an indication that a metallic object may be present within the target volume if the detected signal waveform fails to reach said predetermined threshold on a predetermined number of successive occasions.

46. The apparatus of claim 43 wherein the analyser includes a liminal object detector configured to determine a time derivative of said portion of the detected signal waveform, and to generate an indication that a metallic object may be entering the target volume of the metal detection apparatus if a peak of said time derivative exceeds a predetermined threshold.

47. The apparatus of any of claims 38 to 46 wherein the analyser includes a time constant estimator for estimating at least one time constant associated with a change in detected signal waveform following a transition between adjacent linear segments of the transmit coil drive waveform.

48. The apparatus of claim 47 wherein the time constant estimator is configured to measure at least first and second time constants, wherein the first time constant is measured over a first time interval commencing at a first time instant following said transition, and the second time constant is measured over a second time interval commencing at a second time instant subsequent to said first time instant.

49. The apparatus of claim 48 wherein the time constant estimator is configured to generate an indication that a ferrous metallic object may be present if said measured time constants are substantially different.

50. The apparatus of any of claims 38 to 49 wherein the analyser includes a frequency analyser for analysing the frequency content of a portion of the detected signal waveform received subsequent to a transition between adjacent linear segments of the transmit coil drive waveform.

51. The apparatus of claim 49 wherein the frequency analyser includes a low pass filter and a high pass filter, and is configured to compare outputs of said filters and to generate an indication that a ferrous metallic object is present based upon said comparison.

52. The apparatus of any one of claims 38 to 51 wherein the analyser includes a sampler configured to capture a value of the detected signal waveform prior to a transition between adjacent linear segments of the transmit coil drive waveform, and a permeability analyser configured to compare successive values captured by said sampler, the permeability analyser being further configured to determine if an increase in magnitude of said sampled values occurs, and if so then to generate an indication that a ferrous object may be entering the target volume.

53. The apparatus of any one of claims 38 to 51 further including a resistivity classifier configured to compare at least one time constant associated with a change in detected signal waveform following a transition between adjacent linear segments of the transmit coil drive waveform with values characteristic of known metallic materials and objects.

54. The apparatus of claim 49 including a discriminator configured to classify a detected metallic object as being either ferrous or non ferrous on the basis of said indication generated by the time constant estimator.

55. The apparatus of claim 51 including a discriminator configured to classify a detected metallic object as either ferrous or non ferrous based on the indicating generated by said frequency analyser.

56. The apparatus of claim 52 including a discriminator for classifying a detected metallic object as either ferrous or non ferrous on the basis of said indication generated by the permeability analyser.

57. The apparatus of any one of claims 54 to 56 wherein the discriminator is configured to generate an indication of whether a detected metallic object is likely to be either ferrous or non ferrous.

58. The apparatus of claim 57 wherein the discriminator is further configured to generate an indication of the probable metal type of a detected metallic object.

59. The apparatus of any one of claims 38 to 58 wherein the analyser includes a direct coupling canceller for reducing a component of the detected signal waveform caused by direct coupling of the magnetic field transmitted by the transmitter.

60. The apparatus of any one of claims 38 to 59 wherein the analyser includes a mains harmonic canceller for reducing an effect upon the detected signal waveform of mains frequency fields generated by power transmission lines.

61. The apparatus of any one of claims 38 to 60 wherein the analyser includes a static magnetic field canceller for reducing an effect upon the detected signal waveform of static magnetic fields, such as those generated by magnetised objects.