AU633536B2 - Metal detector - Google Patents

Description

COMMONWEALTH OF AUSTRALIA Form Patents Act 1952-1969 COMPLETE SPECiFI g T

(ORIGINAL)

FOR OFFICE USE: Class Int. Class Application No Lodged Complete Application No Specification Lodged Published Priority: Related art: TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: HALCRO NOMINEES PTY. LTD.

338 Greenhill Road, Glenside (formerly 205A Unley Road, Unley) State of South Australia, Commonwealth of Australia BRUCE HALCRO CANDY Care of COLLISON CO., 117 King William Street, Adelaide, South Australia, 5000 Complete Specification for the invention entitled: METAL DETECTOR The following statements is a full description of the best method of performing it known to us: this invention, including PAILNI, TRADE MARKS DESIGNS SUB-OFFICE 12 JAN 1990 SOUTH AUSTRALIA This invention relates to conducting metal discriminating detectors.

The problem to which this invention is directed relates to difficulties associated with discriminatory detection of target objects when within an environment that provides either substantive variable magnetic reactive and resistive components of re-transmitted magnetic signals such that it has been hitherto difficult to distinguish a target signal from a background signal.

Such an environment can be typically ironstone magnetic soils.

The object of this invention is to achieve a method and apparatus by which greater sensitivity can be achieved in such difficult environments.

Concept of the Invention: The invention in one form resides in a conducting metal discriminating detection apparatus comprising: transmission means for transmitting a discontinuous pulse voltage waveform to provide a magnetic field in a target region, the discontinuous pulse voltage waveform providing periods of non-transmission of the magnetic field; a detector coil for producing a detected signal by detecting changes in the magnetic field; measurement means for measuring the detected signal within a time interval which is separate from a period having a significant signal resulting from decay of ground eddy currents, wherein the measuring occurs during the periods of non-transmission of the magnetic field; and processing means for processing at least two of the measurements to provide an output signal substantially independent of an effect of nonelectrically conducting ferrite constituents in the target region, the processing means scale at least one of the measurements and subtract the scaled measurement from at least one of the other measurements, wherein the output signal is indicative of the existence of a metal object within the target region.

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In preference, the scaling of the measurements is selected so that a summation of the processed measurement after scaling is zero wherein the measurements before scaling correspond to a relaxation curve attributed to non-electrically conducting ferrite.

In preference, the discontinuous pulse voltage waveform comprises pulses of both positive and negative magnitudes for predetermined durations such that a magnetic field is produced in a target region and has a net time 1 0 average value of substantially zero.

Alternatively, in another form the invention resides in a conducting metal discriminating detection apparatus comprising: transmission means for transmitting a discontinuous pulse voltage waveform 1 5 to provide a magnetic field in a target region, the discontinuous pulse voltage waveform providing periods of non-transmission of the magnetic field; a detector coil for producing a detected signal by detecting changes in the magnetic field; measurement means for measuring the detected signal within a time interval which is separate from a period having a significant signal resulting from decay of ground eddy currents, wherein the measuring occurs during the periods of non-transmission of the magnetic field; means for synchronously demodulating the detected signals such that a first demodulated signal is derived from a first period following a transition in the transmission of the magnetic field and a second demodulated signal is derived from a second period following a transition in the transmission of the magnetic field, both first and second periods being subsequent to a time necessary for substantial decay of ground eddy currents, the demodulation occurring during the period of non-transmission; and processing means for processing the demodulated signals by comparing respective magnitudes of the demodulated signals to provide an output signal substantially independent of a background environment having a substantial quantity of material with a substantial magnetic effect and a reactive to resistive response ratio which is substantially independent of an interrogating frequency below 100kHz.

In preference, the transmission means provides a transmit signal which consists of a sequence of pulses repeated continuously for a selected time in which at 'ast one of the pulses in the sequence is of a different period than that of at least one other pulse in the sequence and the detected signals being synchronously demodulated with respect to the transmitted repetitively pulsed magnetic field.

In preference, the sequence of pulses has some pulses transmitted with a 1 0 positive magnitude and other pulses with a negative magnitude such that a net time averaged flux transmitted by the transmit coil is approximately zero.

In preference, the detected signals are synchronously demodulated such that the net time averaged linear combination of asynchronous background flux is substantially zero.

1 5 In preference, the means for synchronously demodulating comprises n synchronous demodulators and is operably configurable to satisfy the equation: n ISiPi O, i=1 where Si is a relative effective gain of an ith synchronous demodulator and the sign of Si is consistent with the polarity of the pulse being demodulated, and Fli is the relative period for which the ith synchronous demodulator is "on", In preference, there are filter means for filtering outputs of the demodulators, and means for inverting the detected signals, whereby the non-inverted detected signals are input to a first set of the demodulators and inverted detected signals are input to a second set of the demodulators.

In preference, in any of the above forms, the pulse voltage waveform has 30 substantially constant magnitude during at least two periods, the magnitude during a first period being positive and the magnitude during a second period being negative.

In preference, in any of the above forms, the duration of the pulses ranges from 0.5ms to L In preference, in any of the above forms, the transmission means comprises a transmit coil and a power supply means having a positive and a negative power rail, the power supply means being recharged by a back electromotive force resulting from the effect of the discontinuous voltage waveform being applied to the transmit coil.

In preference, in any of the above forms, the transmission means comprises first and second switching means, the first switching means having a first diode connected across its output and the second switching means having a second diode connected across its output such that the cathode of a first diode is connected to a positive voltage rail and the anode of second diode is connected to a negative voltage rail, the anode of the first diode is connected to the cathode of the second diode, and connected to the cathode of the second diode and in parallel with one of the diodes is a capacitor in series with a parallel combination of the transmit coil and damping resistor.

In preference, in any of the above forms, the transmission means comprises ground rail, a positive voltage rail, a negative voltage rail, first and second switching means, the first switching means having a first diode connected across its output and the second switching means having a second diode connected across its output such that the cathode of a first diode is connected to the positive voltage rail and the anode of a second diode is connected to the negative voltage rail, the anode of the first diode is connected to the cathode of the second diode, and connected to the cathode of the second diode is a parallel combination of the transmit coil and a damping resistor connected to the ground rail.

In preference, in any of the above forms, the measurement means measures the first magnitude of the detected signal at least once during a latter part of a transmission period of the pulse voltage waveform, and measures a second magnitude of the detected signal at least once during a nontransmission period following a delay period after commencement of the said non-transmission period, a proportion of the second magnitude being subtracted from the first magnitude.

In preference, in any of the above forms, the processing means subtracts an adjustable portion of the first magnitude measured by the measurement means during transmission from the second magnitude measured by the measurement means during non-transmission.

.r In preference, in any of the above forms, the transmit coil is substantially larger than the detector coil, the transmit coil is placed over the target region and the receive coil is moved within the transmit coil to effect searching for any metallic target object.

In preference, in any of the above forms, the measurement means comprises an amplifier means for amplifying the detected signal; a plurality of demodulating means for synchronously demodulating 1 0 the amplified detected signal; an inverting means for inverting the amplified detected signal, whereby the inverting means is connected to at least one of the demodulating means; and a low pass filter means for filtering the outputs of the plurality of 1 5 demodulating means.

In preference, in any of the above forms, the time constants of both the transmission means and detector coil are substantially short when compared to the time interval separate from the period having a significant signal resulting from decay of ground eddy currents.

Alternatively, in another form the invention resides in a method of conducting metal discriminating detection comprising the steps of: transmitting a discontinuous pulse voltage waveform to form a magnetic field in a target region, the discontinuous puise voltage waveform providing periods of non-transmission to the magnetic field; detecting changes in the magnetic field and providing a detected signal indicative of the changes; measuring the detected signal within a time interval which is separate from a period having a significant signal resulting from decay of ground eddy currents, the measuring occurring during the periods of non-transmission of the magnetic field; and processing a plurality of measurements to provide an output signal substantially independent of an effect of non-electrically conducting ferrite constituents in the target region, the processing including the steps of: scaling at least one of the measurements; and 'I wo subtracting the scaled measurement from at least one other measurement, the scaling being selected so that a summation of the processed measurements after scaling is zero when the measurements before scaling correspond to a relaxation curve attributed to non-electrically conducting ferrite, wherein the output signal is indicative of the existence of a metal object in the target region.

Alternatively, in another form the invention resides in a method of conducting metal discriminating detection comprising the steps of: transmitting a discontinuous pulse voltage waveform having pulses of both positive and negative magnitudes for predetermined durations such that a magnetic field is produced in a target region and has a net time average value of substantially zero; detecting changes in the magnetic field and providing a detected signal indicative of the changes; measuring average magnitudes of the detected signal during a selected period of time and within a time interval separate from a period having a significant signal resulting from decay of ground eddy currents; and processing a plurality of measurements by scaling the magnitude of at least one of the measurements and subtracting the scaled magnitude from the magnitude of at least one of the other measurements to provide an output signal substantially independent of an effect of non-electrically conducting ferrite constituents in the target region the output signal being indicative of the existence of a metal object in the target region.

In preference, the pulses of the pulse voltage waveform are separated by periods ot non-transmission and wherein some of the pulses are of different duration.

Alternatively, in another form the invention resides in a method of conducting metal discriminating detection comprising the steps of: generating and applying to a transmit coil a discontinuous pulse voltage waveform to form a magnetic field in a target region, wherein the discontinuous pulse voltage waveform provides periods of non-transmission to the magnetic field; T 1 6B detecting changes in the magnetic field and providing a detected signal indicative of the changes; synchronously demodulating the detected signals during nontransmission periods of the magnetic field to provide a first demodulated signal derived from a first period during one non-transmission period and a second demodulated signal derived from a second period during another non-transmission period such that both first and second periods begin a delayed time after the start of the respective non-transmission periods, the delayed time being of a duration necessary for substantial decay of ground eddy currents; and processing the demodulated signals by comparing the demodulated signals to provide an output signal substantially independent of a background environment which includes a substantial quantity of material with a substantial magnetic effect and a reactive to resistive response ratio which is substantially independent of an interrogating frequency below 100kHz.

In preference, the transmitted pulse magnetic field consists of a sequence of pulses repeated continuously for a selected time, a period between transitions within the sequence of pulses being different and the detected signals being synchronously demodulated with respect to the transmitted pulsed magnetic field.

In preference, the sequence of pulses has some pulses transmitted with a positive magnitude and some pulses transmitted with a negative magnitude such that a net time averaged flux is approximately zero.

In preference, the detected signals are synchronously demodulated such that the net time averaged linear combination of asynchronous background flux is substantially zero.

In preference, the demodulating step comprises the step of demodulating using n synchronous demodulated operably configurable such that the following equation is followed: n .SiPi 0, i=1 where Si is a relative effective gain of an ith synchronous demodulator and the sign of Si is consistent with the polarity of the pulse being demodulated, and Pi is the relative period for which the ith synchronous demodulator is on".

In preference, the pulse voltage waveform has a substantially constant magnitude during at least two periods, the magnitude during a first period being positive and the magnitude during a second period being negative.

In preference, the duration of the pulses ranges from 0.5ms to In preference, the method further comprises the steps of supplying power from a power source during a part of the pulse voltage waveform and recharging the power source during another part of the pulse voltage waveform.

In preference, the measuring step comprises the steps of measuring a first magnitude of the detected signal at least once during a latter part of the transmission period of the pulse voltage waveform, and measuring a second magnitude of the detected signal at least once during the non-transmission period following a delay period after commencement of the non-transmission period, a proportion of the second magnitude being subtracted from the first magnitude and compared to a magnitude of the detected signal measured at least once during non-transmission.

In preference, the processing step comprises the step of subtracting an adjustable portion of at least one of the measurements made during the transmission from at least one of the other measurements made during nontransmission.

PT

The most difficult ground for detecting highly conductive metal targets, such as coins, gold, underwater pipes etc., is that containing large concentrations of ironstone whose resistive to reactive ratio varies spatially, and worse still if the ground also contains moderately electrically conductive components. The best of the commercially available metal detectors transmit a roughly sinusoidal signal (distortion -20dB) at between a few kHz to a few 10's of kHz. The received signal is synchronously demodulated and passed through a low-pass filter to remove both noise and carrier related signals. Such art shall be called single frequency detectors.

The phase angle of the synchronous demodulator in S.F. detectors is set so that the detector is insensitive to components with a phase near to the reactive component, but is offset by usually less than several degrees 1 5 towards the resistive component. This phase angle can be varied, and is so in most detectors manually by means of the user varying a potentiometer.

The user is said to "ground balance" the detector to a local area of ground, so that the detector is relatively insensitive to the local area. This occurs in the detector when the demodulator "object" channel reference phase is at quadrature to the "resultant ground vector" of the local reactive and resistive vectors. The resultant ground vector varies spatially owing to both variations in the mildly conductive component as well as variations in ironstone resistive component relative to the reactive component of the soil. This adjustment need be made frequently for best results. Metal target objects are located by the detection of statistically significant irregularities in the "object" channel arising from the interrogation of different areas of ground. Such signals are amidst substantial background signals due to ground signals.

Another type of metal detector transmits a magnetic transient pulse rather than a sinusoidal signal. These are called pulse induction detectors. Usually a voltage of the order of 10 volts is switched on to the transmit coil though a series resistor for longer than several milliseconds, whereafter the switch is turned off and the resulting back e.m.f. from the 3 5 transmit coil is clamped by means of a voltage clamp such as a zenner diode. The voltage is clamped typically to a few hundred volts. Both the receive coil and transmit coil are critically damped with resistors.

8 In some detectors only one coil is used to both transmit and receive.

Decaying eddy currents following this pulse induced in conducting metal target objects induce signals in the receive coil and the amplified receive signal is gated by means of a S.D. to an averager and thence passed to indicator means.

No commercially available P.I. detectors are considered to be performance competitive with the sinusoidal transmitting detectors owing to their inability to ground-balance in magnetic soils which produce signals that 1 0 also persist after the cessation of transmission.

In Patent G.B. 2041532A, Poole describes a P.I. apparatus which subtracts a proportion of one sampled value of the received signal from another, each sample being taken at different periods following the 5 transmit pulse. In doing so, Poole claims to have solved the problem of signals arising from eddy current decay signals from the mildly electrically conducting ground components contaminating eddy current signals from metal objects. Poole claims that the invention is "particularly useful" for locating metal objects in salt water and sites the sea/sea floor interface as a specific problem area.

Poole states that the eddy current decay signal from the ground is more rapid than that from metal objects (other than very small metal objects) and thus if the said proportion is selected to minimise the local eddy current signals of the mildly conductive ground components, then metal objects will not be cancelled. However, the apparatus described by Poole will not universally cancel the mildly electrically conductive ground components owing to the fact that the decay rate depends on the relative orientation and position of the sensing coils relative to the interrogated S0 ground, as well as on the conductivity of the local ground. For example, sea water is considerably more conductive than moist sea side sand. Thus the decay signal from sea water is substantially slower than sea side sand, and hence the said proportion need be quite different to cancel each background medium. Poole has stated that the ground is "primarily 3 5 resistive in nature". It is however in most parts of the world primarily reactive in nature by typically a factor of 30 to 100 times at typical metal detector operating frequencies. This is reflected in the range of "ground balance" settings available from most well known commercial detectors.

Poole does not offer to cancel principal interfering ground signals arising from magnetic soils, as is the subject of this specification, which can be universally cancelled.

Commercial P.I. metal detectors are well known for their ability to be quite insensitive to mildly conductive ground components. All achieve this by simply sampling after the fast decaying ground eddy currents have substantially decayed away. This technique does however have the disadvantage that very small metal objects with eddy current decay 1 0 periods equally as fast as the mildly conductive ground components will not be detected for the same reason. In this specification, as stated later it is assumed that the apparatus is likewise adapted to allow eddy currents of mildly electrically conductive ground components to decay before commencement of accumulation of signals.

One method for substantially reducing the signals arising from the ground is described in Australian patent applications PH7889 and PJ0991. In these applications an apparatus adapted to transmit at least two substantially sinusoidal signals is described. The apparatus is adapted to select linear combinations of reactive and resistive signals of at least two transmitted signals such that the mildly conductive ground components are substantially cancelled, or the ironstone "resultant ground vector" is substantial: cancelled, or both, while maintaining sensitivity to interrogated target objects. Such art shall be called multiple frequency 2 5 detectors. However, even though in practice M.F. detectors are a considerable improvement on the S.F. detectors described above, nonlinearities in electronics place a limit on performance in soils containing substantial concentrations of ironstone owing to the large dynamic range required. M.F. detectors require a substantial number of stable and 3 0 accurate electronic components.

The subject of this patent is metal detector apparatus in which the signal applied to the transmit coil is essentially pulsed in voltage waveform where the transmit coil is for periods not driven by any (external) voltage 3 5 source and is substantially critically damped and where the said periods are at least several times longer than the critically damped transmit coil time constant. Thus for these said periods, there is effectively no transmission, hence the term "discontinuous transmission". Induced mod I signals in the receive coil are amplified and synchronous demodulated and thence passed by low-pass filters. These low-pass filtered signals are then processed further for interpretive means. It is easiest to consider such apparatus from the point of view of time domain analysis rather than frequency domain or phasors as is easiest for S.F. or M.F. art. Time domain analysis of pulsed transmit signal detectors can be adapted to be considered as counter-parts to the sinusoidal transmitting S.F. of M.F.

apparatus, although the final response to interrogated objects are itrinsically different. Means of this implementation is the subject of this invention, where the apparatus will be categorised as that is "time domain detectors". Novel P.I. apparatus and another type of T.D.D. are described.

One standard feature of many commercially available metal detectors (most is the discrimination against iron metal objects as these are usually of little value compared to non-iron objects, such as gold, silver, most coins etc.. The method for determining whether an metal object is made out of iron is in effect to compare the sign of the reactive component to that of the resistive component. The signs will be the same for iron objects but opposite for non-iron objects. One problem with the existing art is that for some iron objects the electrical conduction component masks out the effects of the magnetic properties of the iron, and hence such objects are determined to be non-iron objects by most detectors. The most common such iron objects are large links and rings. A means of substantially reducing this masking effect is described herein.

Receiving apparatus adapted to receive only when the transmitter has stopped transmitting such as P.I. has the advantage over the M.F. or S.F.

detectors owing to the considerably reduced requirement of dynamic range described above and hence accuracy of the electronics. P.I.

detectors however have a disadvantaged in that the power supply requirements are substantially greater than either S.F. or M.F.

counterparts. Part of this invention describes a novel pulse transmitting apparatus that requires less power than P.I. and yet achieves similar results.

To assist with an understanding of the present invention, definitions and back ground physics will be presented: L -a "First order objects" refers to target objects that can be represented magnetically as a single inductor L, to which the interrogation field is loosely coupled, loaded with a single resistor R. For these objects the characteristic frequency co is defined as equalling R/L.

Mildly electrically conducting soils, not containing "magnetic soils" such as ironstone can be represented by a continuum of first order objects where the distribution of co on scale sizes of the order of 1m (size of detection coil) is only significant at high frequencies and peaks at the order of lMHz, depending on the local ground conductivity. For larger scale sizes this said peak will be lower in value and for smaller scale sizes this peak will be higher in value.

1 5 "Ground channels" refer to low-pass filters and associated S.D.s whose outputs are substantially sensitive to magnetic soils. (In the case of S.F.

detectors these are called reactive channels.) "Object channels" refer to low-pass filters and associated S.D.s whose outputs are substantially insensitive to magnetic soils (but not necessarily completely) and are relatively sensitive to electrically conducting metal target objects. (In the case of S.F. detectors these are called ground balanced or resistive channels.) It is important to note that received purely magnetic component follows the applied field "instantly," that is, the value of the purely magnetic component is simply proportional to the instantaneous value of the applied field. Thus it is possible to cancel out the purely magnetic component by selecting S.D. gains and references with respect to the 3 0 transmitted signal such that the summed averaged contribution from lowpass filtered S.D. is substantially nulled (non-trivially) to interrogation of t the purely magnetic component. This is in fact achieved in S.F. detectors by the appropriate setting of the ground balance control. This said null is achieved by virtue of the fact that the received response is predictable, 3 5 except for the basic magnitude which is dependent on the amount, position and type of material within the influence of the interrogating field, which may be achieved by addition of one averaged portion of the received response from another averaged but different portion of the received response, such that the two said averages are substantially equal but opposite in sign.

In the case of half wave S.D.s in S.F. detectors two such said portions could be the low-passed synchronous demodulation taken between t=-7rn/(2co) and t=0 added to that taken between t=0 and t=nn/(2co) where k sin(wct) is the transmitted signal, or in other words, the low-passed synchronous demodulation taken between and t=tn/(2co) is nulled to the purely magnetic component.

In contrast to this, received loss components are dependent on the history of the applied field and are not proportional to the instantaneous applied field. However it is possible to substantially effect a null to any loss component which responds in a linear predictable manner to a specified transmitted signal. This can be achieved in a similar way to effecting a null to the purely magnetic component: namely; by selecting S.D. gains and S.D. references with respect to the transmitted signal such that the summed averaged contribution from low-pass filtered S.D. is substantially nulled (non-trivially) to interrogation of the said loss component. This said null is achieved by virtue of the fact that the received response is predictable, except for the basic magnitude which is dependent on the amount, position and type of material within the influence of the interrogating field, which may be achieved by subtraction of one averaged portion of the received response from another averaged but different portion of the received response, such that the two said averages are substantially equal.

We have noted that non-electrically conducting soil has reactive to resistive component ratios that are frequency independent to within a S0 fraction of a percentum below frequencies of the order of 100s of kHz.

This is typical property of all ferrites and indeed the major magnetic soil contributor is a ferrite, namely Fe304. It is assumed that H is sufficiently small to substantially avoid any magnetic field saturation. The relaxation temporal characteristics of all ferrites may be described as a multiplication constant, K, times a function of time, for the same change in the applied magnetic field. It is important to note that F(t) is the same for for all ferrites for the same applied field history except in the high frequency domain. K is different for different chemical ferrites, all else being equal, and indeed is dependent on the quantity and proximity of a particular ferrite to the transmit and receive coils. These "relaxation temporal characteristics" following (not instantly) a change in the applied field shall be called "ferrite relaxation decays," or F.R.D.

From a very simplistic quantum mechanical point of view, energy is required to change the (magnetic) quantum state of a part of a ferrite crystal is analogous to the well known photon absorption phenomena of a substance which changes it's quantum state. Also just as the said substance may then loose energy by the emission of photons upon returning to the original quantum state, so too may the said crystal in the ferrite return to it's original magnetic quantum state. Hence the phenomena of the delay in fully magnetising the ferrite and the magnetic decay of the ferrite following an applied magnetic field.

For all ferrites, the reactive (purely magnetic) component is much larger than the resistive component, and is substantially frequency independent below 100kHz. Thus materials for which the reactive to resistive component ratio is frequency independent also have the property that the loss per cycle about the loop per interrogation magnetic intensity (H) is substantially frequency independent at frequencies below 100kHz. K is proportional to the loss per cycle, all else being equal.

Thus it is possible to cancel the F.R.D. component of all ferrites as this component is predictable, except for the magnitude, so long as the transmitted signal has more than one (Fourier) frequency component.

In the analysis which follows, for the sake of simplicity, it is assumed that the electronic arrangement consists of a transmit signal generator S0 connected to a transmit coil and receiver coil which are substantially nulled to the transmit coil in "free space". A pre-amplifier is connected to the receive coil. S.D.s follow the pre-amplifier which are synchronized to the transmit pulses, and low-pass filters are connected to the outputs of the S.D.s. Also, the systems are linear, and the low-pass filters are temporally matched. Further the low-pass filters filter out transmit related Fourier components, and that the outputs of the low pass filters are connected to further processing means and thence the output indicator means.

S.D. "gain" refers to the relative gain of one S.D. compared to the others.

This is defined as the relative signal change at the output of the low-pass filter connected to the said S.D. resulting from a change in the mean input to the said S.D. during the "on" period of the said divided by the effective gain of the said low-pass filter. Here the effective impedance of the S.D.s connected to the said low-pass filter need be taken into account to calculate the said low-pass filter's gain. For this said definition, all other S.D.s connected to the said low-pass filter must have unchanging inputs during their respective "on" times.

T.D.D. apparatus adapted to transmit non-continuously, such as will be referred to as "non-continuous" transmission. There are essentially two different types of T.D.D apparatuses described herein: Both are noncontinuous transmission types. There are 5 possible voltage values, namely two positive values appearing for significant lengths of time across the transmit coil; +V1 and +V2 and two negative values -V3 and -V4 where V2>>V1 and V4>>V3, and zero volts. In the P.I. case V2 and V4 are applied to the output. The time period between a voltage transition between one and another said applied voltage value in all cases may consist of several different period values, but in all cases the resultant pulse sequence is of fixed repetitive pattern. The voltage transitions between one value and another may follow approximately exponential wave forms, but the transition time is small compared to the duration for which the said applied voltages are applied to the transmit coil. The S.D.

"on" and "off" times are synchronised to the transmit signal and also may consist of several different periods, where the "on" and "off" sequence is of fixed repetitive pattern. Table I contains a summary of different novel combinations utilising these possible T.D.D. concepts.

CASE FEATURE TRANSMISSION TYPE Snumber (c) I x x P.I.

II X I X P.I III X -I Novel, pulsed IV x I 4 P.I.

V X 4 4 Novel, pulsed ~OI Table I.

where: Feature donates "conventional" S.F. type ground balance facility.

Feature donates several S.D. reference periods in a repetitive sequence effecting insensitivity to F.R.D..

Feature donates several transmit and S.D. reference periods in a repetitive sequence effecting insensitivity to F.R.D..

In order to avoid sensitivity to magnetised ground or the earths magnetic field as the detector interrogation head is passed across the ground, and to accommodate the well known electronic drift problem resulting principally from noise, many commercial metal detectors have A.C. coupled preamplifiers. The problem with any filtering such as A.C. coupling is that received signals contain memory of preceding signals which then contaminate future signals. Part of this memory is derived from the (nonlinear) pre-amplifier slew rate limited periods following voltage transitions during transmission.

In all the T.D.D. concepts which follow, unless otherwise specified, it is assumed that the following novel concept is incorporated, namely: There will be at least two S.D.s where the sum of, the gain of each S.D.

times it's total "on" period, summed over all the contributing S.D.s is substantially zero and the preamplifiers are D.C. coupled. To effect this, the contribution from some S.D.s is of opposite sign to others, that is there is an effectively subtraction. This can be achieved by means of a subtractor, or by the pre-amplifier feeding the inputs of somrne S.D.s and an 0 unity gain invertor amplifier following the pre-amplifier feeding the inputs of other S.D.s, where the demodulated signals are then added. In summary, a linear sum of the signals passed by each of the said synchronous demodulators is added such that n ~,SiPi 0 i=l where Si is the relative effective gain of the ith synchronous demodulator which passes it's output to be added and low-pass filtered, where the sign of Si is consistent with the polarity of the pulse being demodulated, and Pi is the relative period for which the said synchronous demodulator is "on".

Such a system, if electrically balanced, cancels out any induced signals resulting from the movement of the interrogation coils relative to static magnetic fields, nor does it suffer from electronic drifts, while maintaining no loss in sensitivity to target objects. In other words, the net final lowpassed signal contains no component resulting from asynchronous received signals. If further, the said S.D.s are enabled only during periods of non-transmission and only after a sufficient delay period at least three times the sum of the critically damped receive and transmit coil time constants, then the said net final low-passed filter output will be independent of the purely magnetic soil component and transmit pulse received by the receive coil directly from the transmit coil (assuming fast electror.ics is implemented).

Another problem with the current art of pulsed detectors is their inability to effect ground balance owing to ferrite relaxation decays wnose magnitude varies according to the soil being interrogated. Effecting universal balance to soil is the principal subject of this patent.

There are several different concepts of T.D.D. described below which are listed case by case.

CASE I.

A novel means of effecting ground balance, which can be considered as a pulsed counterpart to S.F. art, is to select a linear combination of an object channel signal and a ground channel signal. The addition of these two signals may occur at the S.D.-low-pass filter interface, as described in patent application P17889.

To assist with the understanding of the present invention, reference will now be made to the accompanying illustrations.

Figure I shows a basic electronic block diagram of a means to effect a ground balance control according to a preferred embodiment for a P.I.

detector.

Figure II shows voltage wave forms at various stages in figure I for case I.

Figure III shows voltage wave forms at various stages in figure I for case

II.

Figure IV shows voltage wave forms at various stages in figure I for case

III.

Figure V shows an electronic circuit diagram of a means to implement a transmitter adapted to generate a voltage waveform such as that in figure IV according to a preferred embodiment for a T.D.D..

Figure VI shows voltage wave forms at various stages in figure I for case

IV.

Figure VII shows an alternative unipolar transmit T.D.D. to case IV.

Referring to the drawings in detail it is now noted as follows: A means for implementing a ground balanced P.I. concept is shown in figure I and voltage wave forms at various points are shown in figure II.

A receive coill is connected to a pre-amplifier 3 and loaded with a damping resistor 2. The output of 3 is connected to an inverting amplifier 4 and to the inputs of S.D.s 5 and 7 (solid state switches). The output of 4 is connected to the inputs of S.D.s 6 and 8. The references to 5, 6, 7, and 8 are synchronized to the transmit pulses. 7 and 8 are enabled by 11 and 12 during the transmit pulse (in figure II a high at 11 and 12 turns 7 and 8 "on" respectively), while 5 and 6 are enabled by 9 and 10 after a delay subsequent to the transmit pulse. 5 and 8 are turned "on" in association with one polarity while 6 and 7 are turned "on" in association for the other. The transmit coil 18 has a waveform 20 applied to it, which as shown, the back E.M.F. of which is voltage clamped following the application of a voltage V1 or -V3 to 18. The transmit coil is supplied by generator 19 and is critically damped when the low impedance source voltages V1 and -V3 are not applied to the transmit coil, nor when the back E.M.F. is voltage clamped. The receive signal at the output of 3 is 3 0 shown as waveform 21 and the waveform of 9 is shown by 22, and that of is shown by 23. The waveform of 11 is shown by 24, and that of 12 is shown by 25. The outputs of 5 and 6 are connected to each other and to resistor 13. The outputs of 7 and 8 are connected to each other and to variable resistor 14, which is used to control the ground balance. The wiper of 14 is connected to resistor 15 to which resistors 13 and capacitor 16 are connected, at which junction an output 17 may be adapted for further processing. Capacitor 16 is grounded, thereby 13, 14, 15 and 16 form a low-pass filter and adder whose relative proportions may be 1 varied by adjustment of 14.

14 may be adjusted to pass to 17 relatively more or less ground signal, which is substantially manifest at the pre-amplifier output during transmission. This is used to "ground balance" the T.D.D. by subtracting an averaged proportion of the soil's magnetic component from the averaged F.R.D. which appears during the non-transmit period. F.R.D.s are also manifest during transmission, but the predominant signal arising from magnetic soils during transmission is due to the magnetic component (by a factor of the order of 100). This arrangement of ground balance is a counterpart to that found in S.F. art.

Note that the waveform 21 shows an exponential droop in value during low impedance transmission. This results from the exponential decay in E.M.F. across the "pure" inductive component (Lt) of the transmit coil which has a finite "series" real resistance The exponential decay time constant is thus equal to Lt/Rt. The S.D. 5 "on" duration shown by 22 in figure II must be very similar to that of 10 shown by 23 to effectively cancel static magnetic fields.

Their are several advantages to this novel T.D.D. compared to high quality S.F. detectors: Firstly, it requires less expensive electronic components and secondly, it can be adapted to be relatively insensitive to conductive soil components such as salt water if the S.D. are "off" during a period following a transition in the transmit voltage waveform due to reasons given above. Thirdly, the "ground balanced T.D.D. has the ability to perform better than the S.F. counterpart owing to reduced dynamic range requirements in highly magnetic soils described above.

3 0 Despite these advantages, this concept does not universally cancel both the magnetic and ferrite loss ground components. The T.D.D.s can be adapted to achieve this property by several different arrangements, which may be considered as T.D.D. counterparts to the M.F. apparatus described in patent applications PH7889 and PJ0991. In essence, as the relaxation temporal characteristics are the same for all ferrites for the same change in the applied magnetic field (see above), then for a given transmit waveform, signals arising from interrogated ferrite produce a predictable function of time except for the scaling factor which is dependent on the amount of matter under the influence of the interrogating field. Also, as the decay function is dependent on the history of the applied field, it is possible by controlling the transmit signal to determine the subsequent ferrite response function, and hence by selecting a linear combination of contributions arising from different time periods with respect to the transmit pulse, or from different periods following different transmit pulses, or both, to cancel out the signals produced by ferrite. This can be achieved by a number of different ways. M.F. art being just one special case. Here, in the examples given, the voltage applied to the transmit coil is either constant in value for periods in time, or short transients. This is because these are easy and inexpensive to produce electronically. In principle though, the waveform may be any function of time, and signals from ferrite may be cancelled.

CASE II In this case a novel T.D.D. concept in which there is cancellation of both the magnetic, conductive soil and and ferrite loss soil components is described, which may be considered as T.D.D. counterparts to the M.F.

apparatus. This utilises P.I. art and one simple arrangement of such a system may be described by figure I where the components perform the same task as described in the above case I. In this case and all subsequent cases though, the one end of 14 shown connected to ground in figure I is left disconnected. The associated voltage wave forms are given in figure III.

19 produces a P.I. voltage waveform 26, similar to 20 in figure II. The receive waveform, 27 is likewise similar. The waveform of 9 is shown by 28, and that of 10 is shown by 30. The waveform of 11 is shown by 31, and that of 12 is shown by 29. As shown in figure III, the time variable T is zero at the termination of the transmit period, and T=T1 at the turn "on" transition of 5 (see 28). The turn "off" time of 5 occurs at T=T2 and the turn "on" time of 8 (see 29) occurs at T=T3 and then the turn "off" time of 8 occurs at T=T4.

For the sake of simplicity, assume that the short circuit transmit coil time constant is very much greater than the transmit pulse period, that is the degree of droop evident in the receive signal owing to the exponential decay time constant Lt/Rt is negligible. Also assume that the time constant of both the critically damped transmit coil and critically receive coil time constants are very short compared to the transmit period. Also for simplicity, assume that the wave forms associated with the positive transmit signal correspond in period and value (except for sign as the case may be) to the wave forms associated with the negative transmit period.

Let the value of 13 be R1, and that of 14 in series with 15 be R2.

So long as the time constant of the critically damped (undriven) transmit coil and receive coil are much shorter than T1, then the system is insensitive to the purely magnetic component as this decays away "instantly" with the transmit pulse. The F.R.D. and conducting soil components are manifest following the transmit transient pulse. This T.D.D. can be adapted to be relatively insensitive to conductive soil components such as salt water if the S.D. are "off" during a period following the back E.M.F. transients in the transmit voltage waveform for reasons given above.

We have calculated and confirmed by measurement that for a single "on" transmit applied constant reactive voltage followed by a short critically damped turn off transient, that in order to effect F.R.D. ground balance, fT2 1 T2+P\ T1 1l+P T4 r4+P T3 T3+P lo logr p O.log F Iog P+ T 2

P+

1 Ti P+1 T2 P+1 T3 R1 R2 0 (i must be satisfied, where P is the low impedance drive period.

We have also shown that this is also approximately true for a pulse train as shown in figure III and is relatively independent of the D.C. resistance of the transmit coil. The response to a first order object of characteristic ,3 0 frequency co is approximately proportional to fe-cT2_e-mT 1 e-mT4_e-omT 3 Obj(co) (1 -e-P-wP) R2 In figure III even though T2 shown to be similar to T3, this need not be the case; the S.D. gains must simply be adjusted to effect F.R.D. balance by means of selecting 13, 14 and 15 appropriately. 14 can be used manually

I

to achieve this.

If instead of there being two pairs of S.D. as shown in figure I with their associated resistors, there are n pairs, where j is the label referring to the jth S.D. pair in the following equations, where the jth pair has an associated resistor connecting the S.D. pair to the low-pass filter capacitor, and T=Tj is the turn "on" time of the said pair relative to the transmit voltage transition, and T=Tj' is the turn "off" time. For such a generalised T.D.D. F.R.D. ground balance occurs when n S Ilo R Tjg 0 j=1 (iii) The response to a first order object is approximately n Z sfe-TJ'-e-oTj Obj(co) (1-e-'P-coP) Se j (iv) j=1 Here S=+l or S=-l depending on the sign sense of the S.D. pair; see and Note that the any pair may turn "on" and "off" more than once during one transmit state in which case the contribution of this action to equation (iii) and (iv) appears as an extra term (or terms as the case may be) in the summation with the appropriate corresponding value of j and Tjs. Also note that it is possible to effect the same result by selecting a linear combination of two low-pass buffered outputs (which need be temporally matched). S is then taken to be the value of the effective gain of the buffered low-pass filter and added contribution of the jth S.D. pair (relative to that of others).

The principal advantages of this T.D.D. compared to M.F. detectors are the same as those of the T.D.D. in case I compared to S.F. art.

The function Obj(w) is zero for certain values of w for one or more values of w. Obj thus has a null response to certain target objects (but is not nulled to most). Different linear combinations of the constants in (iv) that satisfy (iii) yield different values of w to which Obj(w) is nulled. Thus in order to effect no nulls in the w domain, different Obj outputs need be combined in such a way that activity in an Obj output is in some way addressed. One way of achieving this is by full-wave rectification of each Obj signal, each of which may be high-passed by respective high-pass filters first, and then adding these full-wave rectified signals. Another way is to again full-wave rectification of each Obj signal, each of which may be high-passed by respective high-pass filters first, and then to pass each of these full-wave rectified signals to a selector which passes the 0 largest instantaneous rectified signal to the output. This final output then may be further low- and high-pass filtered. The final output can be used to control an audio output or trigger yet further electronics if a threshold is exceeded.

Thus it is possible to combine channel outputs in many ways to yield outputs relatively insensitive to soils and yet maintain sensitivity to a large range (in w) of metal target objects.

CASE III There is yet another novel T.D.D. concept in which there is cancellation of both the magnetic, conductive soil and and ferrite loss soil components which may be considered as T.D.D. counterparts to the M.F. apparatus.

This utilises a novel pulsed transmit waveform and such a system may be described by figure I where the components perform the same task as described above in case II. The associated voltage wave forms is given in figure IV.

19 produces a voltage waveform 32 and the receive and amplified 3 0 waveform at 3 is shown by 33. The waveform of 9 is shown by 34, and that of 10 is shown by 36. The waveform of 11 is shown by 37, and that of 12 is shown by 35. As shown in figure IV, the time variable T is zero at the transmit "off" transition, and T=T1 at the turn "on" transition of 5 (see 34). The turn "off" time of 5 occurs at T=T2 and the turn "on" time of 8 (see 37) occurs at T=T3 and then the turn "off" time of 8 occurs at T=T4.

The current flowing in the transmit coil is shown by 38. When this current reaches approximately zero following a transmit pulse "off" transition, T is defined as zero and the transmit driving impedance is switched from a low impedance to a (relatively high) resistance equal to that required for critical damping of the transmit coil. One way of achieving this is to measure the transmit current and as it approaches zero, turn off the transmit low impedance driving source and only low load the transmit coil with a damping resistor. A simple way of performing this function is illustrated in figure V. In this figure, a drive signal at 39 is fed to the drive input of a solid state switch 40 and a drive signal at 41 is fed to a complementary solid state switch 42. These said switches are connected in series between the supply rails 43 and 44. To the node 1 0 connecting the two said switches is connected one end of the transrait coil and to the other end is connected a blocking capacitor 46. 46 is also connected back to the power supply. In parallel across each solid state switch is connected a diode, 47 across 42 and 48 across 40, in the sense that the cathode of the diode connected to the positive voltage power 1 5 supply node is connected to it, and the anode of the diode connected to the negative voltage power supply node is connected to it. Damping resistor 49 is connected in parallel across the transmit coil. During the transmit cycle (P1 and P2 in figure IV), only one said switch is turned "on" and "off". The one turned "on" is so at the beginning of the transmit period for a period P1, at the end of which time it is turned "off". During P1 current increases from zero in the transmit coil and is supplied by the power supply. The back E.M.F. from the transmit coil at the end of P1 causes the diode connected across the inactive switch to conduct and pass the current stored in the inductor to the power supply, part of which may be considered as 46, in a "charging up" sense. In the complementary transmit cycle shown in figure V, the roles of the switches and diodes are reversed.

The similar action of this transmitter to a switch mode power supply should be noted. So long as the quality factor of the coil is high enough at the frequencies of the order of l/P1, and that the forward voltage drop 3 0 across the diodes is very much less than the supply voltage, P1-P2 0 and the average power supply current drawn from the power supply is low compared to the peak currents flowing in the transmit coil. This is obviously an advantage over detectors where the transmit power requirements are high such as P.I.

As shown in figure IV the S.D.s are only on during the non-transmit period. All comments which apply to the receive T.D.D. described in case II above apply except that equations through to (iv) are different.

is for the concept now being described [2(K2± 1) )log (K2± 1) )-K2lo g(K2) log 2(K1+1)log(K1±1)-Kllog(K1)-(K1+2)log(K1±2) }R1 If 2(K4+ 1)log(K4 1 )-K4log(K4)-(K4±2)log(K4+2) [2(K3±1)log(K3+1 )-K3log(K3)-(K3±2)log(K3+2) /R2 0 where Ki =Ti/Pi where i 1,2,3,4 and P2 =P1.

(ii) is now approximately e-coT4-e-coT 3 Obj(co) (0 -e-OP-coP){e RIo1 R2 f(i) 1 5 and (iii) is approximately for thi. case n I S (2(Kj±1l)log,(Kj'±1)-Kj'log(Kj')-(Kj'+2)log(Kj'+2) j=1 {2(Kj±1)log(Kj±1)-Kjlog(Kj)-(Kj±2)log(Kj±2)) 0 (iii)', where Kj' is equivalent to Tj'/Pi and Kj is equivalent to Tj/P1 above. (iv) in this case approximately is n Obj(w) (1-2ewPl~e-2wPl) I S f(e-wTj'-e-wTj)/Rj} j=1 Again, all comments which apply to the receive T.D.D. described in case II 3 0 above apply except that equations through to (iv) are different and the transmnit waveform is different.

The principal advantages of this T.D.D. compared to M.F. detectors are the same as those of the T.D.D. in case II compared to S.F. art. In addition, the 3 5 power supply requirements are lower than the P.1. T.D.D. counterpart to the M.F. detector described in case 11 above.

This novel T.D.D. concept can be adapted yet further to a substantial advantage in terms of sensitivity to depth over the existing art. This can be achieved by utilizing the concepts so far outlined in this case III, but by adapting the transmit coil to occupy a large area across the soil to be interrogated and then to move a smaller receive coil within the influence of the transmit coil. For example; the transmit coil could consist of a few turns in a roughly circular loop of 10m diameter laid on the ground, and the receive coil could have a diameter of 1/2m and be moved across the ground within and just outside of the loop. In the conventional art 1 0 interrogation coils are arranged in close proximity and the transmit and receive coils are substantially "nulled" in "free space" relative to each other. They are typically between 5 and 60cm in diameter and the transmit and receive coils are of similar size. The sensitivity to a small target object of such interrogation coils decreases as a sixth power law 1 5 with distance of the object from the sensing head for distances of the object from the head of the order of the dimension of the head or greater.

The sensitivity to a small target object of the (very) large transmit and "normal" sized receive interrogation coils decreases as a cube power law with distance of the object from the receive coil for distances of the object 2 0 from the receive coil of the order of the dimension of the receive coil but less than the dimensions of the transmit coil. Such a system has the advantage of being more sensitive to deeply buried objects for the same magnetic field interrogation strength in the region nearby the receive coil.

2 5 Similarly, the P.I. system describe in case II could be adapted to such advantage. However, in order to attain a reasonably high interrogation magnetic flux per unit area without sacrifice to performance, it is necessary to dissipate a large transmit power and the electronics should be able to cope with large back E.M.F.s. The system in case III does not 3 0 have these problems for the reasons outlined above.

The following two cases IV and V are similar to II and III respectively in that the block diagrams and basic transmit waveform are the same as each 3 5 other respectively, but in each case the transmitter transmits a fixed repetitive sequence of pulses in which the pulses constituting the sequence have different periods rather than all pulses being identical except for the sign sense.

26 CASE IV.

The block diagram for this case is given in it's simplest form by figure I where the blocks perform the same tasks as in case II except for the wave forms, which are given in in figure VI.

19 produces a voltage waveform 50 and the receive and amplified waveform at 3 has a similar waveform. The waveform of 9 is shown by 51, and that of 10 is shown by 52. The waveform of 11 is shown by 54, and that of 12 is shown by 53. In the example of this case IV shown in figure VI, the transmit sequence consists of a pair of complementary P.I.

wave forms of low impedance transmit period P1 (V1 and -V3 applied to the coil) each followed by a large critically damped back but clamped, voltage signal, followed by a train of four pairs of complementary P.I. wave forms of low impedance transmit period P2 (V1 and -V3 applied to the coil) each followed by a large critically damped back but clamped, voltage signal. The actual order of the pulses in this sequence may be different to that shown. This transmit waveform may consist of pulses of several different periods with some pulses having the same period as others, other than the example given, so long as the T.D.D. is adapted to cancel the three backgro d soil components, namely, the purely magnetic component, F.R.D.s and mildly electrically conducting soil components. Individual S.D.s may be enabled only following transmit pulses of a certain period as shown in figure VI, or they may be enabled following transmit pulses of various periods.

Again, as in the cases above, insensitivity to conductive soil components such as salt water are achieved if the S.D. are "off" during a period following a transition in the transmit voltage waveform.

Further, owing to S.D.s being "off" during transmission in this the passed S.D. signal is insensitive to the magnetic component, so long as the critically damped time constants of both the receive and transmit coil are short compared to the time delay between the cessation of transmission and the "on" signal transition of the S.D.s.

As in case II, the same equations apply under the same conditions as in

L

case II. That is, to effect F.R.D. balance, the added time averaged currents following through the said low-pass filter resistors must cancel when ferrite is being interrogated. As stated above in case II, equations (ii), (iii), and (iv) are approximately true for any Lt/Rt. In this case the scaling factor of any time averaged currents flowing through the low-pass filter resistor of the jth S.D. pair for a given demodulation waveform pattern following the termination of a transmit pulse, is a function of Lt/Rt and the duration of the transmit pulse period preceding the demodulation. That is, if the time averaged signal of demodulated signals following a transmitted pulse of one duration are subtracted from demodulated signals following a transmitted pulse of a different duration, then it is necessary to introduce a scaling factor in combination with equations (iii) and (iv) in order to determine the conditions for insensitivity to The longer the transmit pulse period, the larger the required scaling factor for the 1 5 contribution for the demodulated pulses following such a pulse. If Lt/Rt is effectively infinite compared to the pulse periods, no scaling factor is necessary. As shown in figure VI, in order to obtain the best signal-tonoise, the total time spent transmitting short pulses should be similar to the total time transmitting long pulses.

It is possible to apply only uni-polar low impedance drive signals to the transmit coil and still cancel asynchronous components as described above.

Such an arrangement is cheaper than the symmetric bi-polar implementation. The wave forms for such an implementation is given in figure VII. In this case all negative low-impedance drive periods and associated current discharge transients followed by the associated receive periods are absent from the voltage wave forms. Here the figure I performs the same sort of tasks, but the wave forms are different as shown in figure VII. 55 corresponds to 50, 56 to 51, 57 to 52, 58 to 53 and 59 to 54. As shown, the asynchronous components are cancelled by subtracting a component collected during a period substantially after termination of transmission from the components collected during periods following shortly after termination of transmission.

A T.D.D. utilizing this concept can be combined with a T.D.D. utilizing the concepts described in case II. All other comments which apply to case II also apply here.

M

CASE V In exactly the same way case IV is an extension of case II, so too is case V an extension of case III. In this case V, case III may be extended to include transmit pulse sequences consisting of pulses of different periods but fixed repetitive pattern. Just as between case II and IV a scaling factor need be introduced to accommodate the transmit coil's (short circuit) time constant (Lt/Rt), so too with the same conditions, should a 1 0 scaling factor be applied here to equations (iii)' and in case III to determine the conditions for balance to F.R.D.s All other comments which apply to case III also apply here. A T.D.D. may be adapted to combine both the concepts of case V and case III. In 1 5 addition, beside those comments specific to wave forms, all comments which apply to case IV apply here.

In all the above cases, iron object discrimination may be implemented.

This may consist of accumulating the received signal by synchronous demodulation towards the latter part of the transmit signal, such as waveform 24 in figure II, but with the "on" period starting at least half way through the "VI" and "V2" periods of the transmission shown in waveform 20. This is because the eddy current signal is most pronounced during the first part of the transmit period, while the reactive magnetic part is constant throughout the period (bar the effects of the transmit coil's time constant). Thus the signal from the magnetic iron part will be more pronounced. However, even though this assists with reducing the masking effect of the electrically conductive component, it is still not sufficient to overcome the masking effects satisfactorily. In addition, if a 3 0 proportion of I' t e received signal is synchronously demodulated during the non-transmit period, following a delay period of at least 1/4 or longer of that of the transmit period, which follows the commencernent of nontransmission, is subtracted from the said signal measured during the latter part of the transmit period, then the effects of this said masking will be 3 5 further reduced. This is because the conductive component manifest during the latter transmit period is similarly manifest at a similar delay following the commencement of non-transmission. Hence, the resultant will contain the true magnetic component less that contaminated by the electrically conductive component. This value is then effectively compared in sign to a received demodulated signal during non-transmission to determine whether a metal target is made of iron or not.

i

Claims (28)

1. A conducting metal discriminating detection apparatus comprising: transmission means for transmitting a discontinuous pulse voltage waveform to provide a magnetic field in a target region, the discontinuous pulse voltage waveform providing periods of non-transmission of the magnetic field; a detector coil for producing a detected signal by detecting changes in the magnetic field; measurement means for measuring the detected signal within a time interval which is separate from a period having a significant signal resulting from decay of ground eddy currents, wherein the measuring occurs during the periods of non-transmission of the magnetic field; and processing means for processing at least two of the measurements to provide an output signal substantially independent of an effect of non- electrically conducting ferrite constituents in the target region, the processing means being adapted to scale at least one of the measurements and subtract the scaled measurement from at least one of the other measurements, wherein the output signal is indicative of the existence of a metal object within the target region.

2. A conducting metal discriminating detection apparatus as in claim 1 in which the scaling of the measurements is selected so that a summation of the processed measurement after scaling is zero wherein the measurements before scaling correspond to a relaxation curve attributed to non-electrically conducting ferrite.

3. A conducting metal discriminating detection apparatus as in any one of the above claims in which the discontinuous pulse voltage waveform comprises pulses i4 both positive and negative magnitudes for predetermined durations such that a magnetic field is produced in a target region and has a net time average value of substantially zero. i L~t

4. A conducting metal discriminating detection apparatus comprising: transmission means for transmitting a discontinuous pulse voltage waveform to provide a magnetic field in a target region, the discontinuous pulse voltage waveform providing periods of non-transmission of the magnetic field; a detector coil for producing a detected signal by detecting changes in the magnetic field; measurement means for measuring the detected signal within a time interval 1 0 which is separate from a period having a significant signal resulting from decay of ground eddy currents, wherein the measuring occurs during the periods of non-transmission of the magnetic field; means for synchronously demodulating the detected signals such that a first demodulated signal is derived from a first period following a transition in the 1 5 transmission of the magnetic field and a second demodulated signal is derived from a second period following a transition in the transmission of the magnetic field, both first and second periods being subsequent to a time necessary for substantial decay of ground eddy currents, the demodulation occurring during the period of non-transmission; and processing means for processing the demodulated signals by comparing respective magnitudes of the demodulated signals to provide an output signal substantially independent of a background environment having a substantial quantity of material with a substantial magnetic effect and a reactive to resistive response ratio which is substantially independent of an interrogating frequency below 100kHz. A conducting metal discriminating detection apparatus as in claim 4 wherein the transmission means provides a transmit signal which consists of a sequence of pulses repeated continuously for a selected time in which at least one of the pulses in the sequence is of a different period than that of at least one other pulse in the sequence and the detected signals being synchronously demodulated with respect to the transmitted repetitively pulsed magnetic field. I_

6. A conducting metal discriminating detection apparatus as in claim 4 or wherein the sequence of puisees has some pulses transmitted with a positive magnitude and other pulses with a negative magnitude such that a net time averaged flux transmitted by the transmit coil is approximately zero.

7. A conducting metal discriminating detection apparatus as in any one of claims 4 to 6 wherein the detected signals are synchronously demodulated such that the net time averaged linear combination of asynchronous background flux is substantially zero.

8. A conducting metal discriminating detection apparatus as in any one of claims 4 to 7 in which the means for synchronously demodulating comprises n synchronous demodulators and is operably configurable to satisfy the equation: n _SiPi 0, i=1 where Si is a relative effective gain of an ith synchronous demodulator and the sign of Si is consistent with the polarity of the pulse being demodulated, and Pi is the relative period for which the ith synchronous demodulator is on",

9. A conducting metal discriminating detection apparatus as in claim 8 further comprising filter means for filtering outputs of the demodulators, and means for inverting the detected signals, whereby the non-inverted detected signals are input to a first set of the demodulators and inverted detected signals are input to a second set of the demodulators. A conducting metal discriminating detection apparatus as in any one of the above claims in which the pulse voltage waveform has substantially constant magnitude during at least two periods, the magnitude during a first period being positive and the magnitude during a second period being negative.

11. A conducting metal discriminating detection apparatus as in any one of the above claims in which the duration of the pulses ranges from 0.5ms to Ss ).h

12. A conducting metal discriminating detection apparatus as in any one of the above cla'ms in which the transmission means comprises a transmit coil and a power supply means having a positive and a negative power rail, the power supply means being recharged by a back electromotive force resulting from the effect of the discontinuous voltage waveform being applied to the transmit coil.

13. A conducting metal discriminating detection apparatus as in any one of the above claims in which the transmission means comprises first and second switching means, the first switching means having a first diode connected across its output and the second switching means having a second diode connected across its output such that the cathode ofe first diode is connected to a positive voltage rail and the anode of 4 second diode is connected to a negative voltage rail, the anode of the first diode is connected to the cathode of the second diode, and connected to the cathode of the second diode and in parallel with one of the diodes is a capacitor in series with a parallel combination of the transmit coil and damping resistor.

14. A conducting metal discriminating detection apparatus as in any one of th e=abeve claimsAin which the transmission means comprises ground rail, a positive voltage rail, a negative voltage rail, first and second switching means, the first switching means having a first diode connected across its output and the second switching means having a second diode connected &ke. across its output such that the cathode of first diode is connected to the positive voltage rail and the anode of a second diode is connected to the negative voltage rail, the anode of the first diode is connected to the cathode of the second diode, and connected to the cathode of the second diode is a parallel combination of the transmit coil and a damping resistor connected to the ground rail. A conducting metal discriminating detection apparatus as in any one of the above claims wherein the measurement means measures the first magnitude of the detected signal at least once during a latter part of a transmission period of the pulse voltage waveform, and measures a second magnitude of the detected signal at least once during a non-transmission period following a delay period after commencement of the said non- transmission period, a proportion of the second magnitude being subtracted from the first magnitude. rA7-.) ".L L

16. A conducting metal discriminating detection apparatus as in any one of the above claims wherein the processing means subtracts an adjustable portion of the first magnitude measured by the measurement means during transmission from the second magnitude measured by the measurement means during non-transmission.

17. A method of conducting metal discriminating detection comprising the steps of: 1 0 transmitting a discontinuous pulse voltage waveform to form a magnetic field in a target region, the discontinuous pulse voltage waveform providing periods of non-transmission to the magnetic field; detecting changes in the magnetic field and providing a detected signal indicative of the changes; 1 5 measuring the detected signal within a time interval which is separate from a period having a significant signal resulting from decay of ground eddy currents, the measuring occurring during the periods of non-transmission of the magnetic field; and processing a plurality of measurements to provide an output signal substantially independent of an effect of non-electrically conducting ferrite constituents in the target region, the processing including: scaling at least one of the measurements; and subtracting the scaled measurement from at least one other measurement, the scaling being selected so that a summation of the processed measurements after scaling is zero when the measurements before scaling correspond to a relaxation curve attributed to non-electrically conducting ferrite; wherein the output signal is indicative of the existence of a metal object in the target region.

18. A method of conducting metal discriminating detection comprising the steps of: transmitting a discontinuous pulse voltage waveform having pulses of both positive and negative magnitudes for predetermined durations such that a magnetic field is produced in a target region and has a net time average value of substantially zero; *1' .4 A detecting changes in the magnetic field and providing a detected signal indicative of the changes; measuring average magnitudes of the detected signal during a selected period of time and within a time interval separate from a period having a significant signal resulting from decay of ground eddy currents; and processing a plurality of measurements by scaling the magnitude of at least one of the measurements and subtracting the scaled magnitude from the magnitude of at least one of the other measurements to provide an 1 0 output signal substantially independent of an effect of non-electrically conducting ferrite constituents in the target region the output signal being indicative of the existence of a metal object in the target region.

19. A method of conr "-ting metal discriminating detection as in claim 18 1 5 wherein the pulses of the pulse voltage waveform are separated by periods of non-transmission and wherein some of the pulses are o, different duration. A method of conducting metal discriminating detection comprising the steps of: generating and applying to a transmit coil a discontinuous pulse voltage waveform to form a magnetic field in a target region, wherein the discontinuous pulse voltage waveform provides periods of non-transmission to the magnetic field; detecting changes in the magnetic field and providing a detected signal indicative of the changes synchronously demodulating the detected signals during non- transmission periods of the magnetic field to provide a first demodulated signal derived from a first period during one non-transmission period and a second demodulated signal derived from a second period during another 3 0 non-transmission period such that both first and second periods begin a delayed time after the start of the respective non-transmission periods, the delayed time being of a duration necessary for substantial decay of ground eddy currents; and -S "1 :I. 1 36 processing the demodulated signals by comparing the demodulated signals to provide an output signal substantially independent of a background environment which includes a substantial quantity of material with a substantial magrnetic effect and a reactive to resistive response ratio which is substantially independent of an interrogating frequency below 100kHz.

21. A method of conducting metal discriminating detection as in claim 1 0 wherein the transmitted pulse magnetic field consists of a sequence of pulses repeated continuously for a selected time, a period between transitions within the sequence of pulses being different and the detected signals being synchronously demodulated with respect to the transmitted pulsed magnetic field.

22. A method of conducting metal discriminating detection as in claims or 21 wherein the sequence of pulses has some pulses transmitted with a positive magnitude and some pulses transmitted with a negative magnitude such that a net time averaged flux is approximately zero.

23. A method of conducting metal discriminating detection as in any one of claims 20 to 22 wherein the detected signals are synchronously demodulated such that the net time averaged linear combination of asynchronous background flux is substantially zero.

24. A method of conducting metal discriminating detection as in any one of claims 20 to 23 wherein the demodulating step comprises the step of demodulating using n synchronous demodulators operably configurable such that the following equation is followed: n ISiPi 0, i=1 where Si is a relative effective gain of an ith synchronous demodulator and the sign of Si is consistent with the polarity of the pulse being demodulated, and Pi is the relative period for which the ith synchr-nous demodulator is "on". :'Z l ljib .4 C 37 A method of conducting metal discriminating detection as in any one of claims 20 to 24 wherein the pulse voltage waveform has a substantially constant magnitude during at least two periods, the magnitude during a first period being positive and the magnitude during a second period being negative.

26. A method of conducting metal discriminating detection as in any one of claims 20 to 25 wherein the duration of the pulses ranges from 0.5ms to

27. A method of conducting metal discriminating detection as in any one of claims 20 to 26 further comprising the steps of supplying power from a power source during a part of the pulse voltage waveform and recharging 1 5 the power source during another part of the pulse voltage waveform.

28. A method of conducting metal discriminating detection as in any one of claims 20 to 27 wherein the measuring step comprises the steps of measuring a first magnitude of the detected signal at least once during a latter part of the transmission period of the pulse voltage waveform, and measuring a second magnitude of the detected signal at least once during the non-transmission period following a delay period after commencement of the non-transmission period, a proportion of the second magnitude being subtracted from the first magnitude and compared to a magnitude of the detected signal measured at least once during non-transmission.

29. A method of conducting metal discriminating detection as in any one of claims 20 to 28 wherein the processing step comprises the step of subtracting an adjustable portion of at least one of the measurements made during the transmission from at least one of the other measurements made during non-transmission. A conducting metal discriminating dc'"ction apparatus as in any one of claims 1 to 16 wherein the transmit coil is substantially larger than the 3 5 detector coil, the transmit coil is placed over the target region and the receive coil is moved within the transmit coil to effect searching for any metallic target object. 7

31. A conducting metal discriminating detection apparatus as in any one of claims 1 to 16 or claim 30 wherein the measurement means comprises an amplifier means for amplifying the detected signal; a plurality of demodulating means for synchronously demodulating the amplified detected signal; an inverting means for inverting the amplified detected signal, whereby the inverting means is connected to at least one of the demodulating means; and 1 0 a low pass filter means for filtering the outputs of the plurality of demodulating means.

32. A conducting metal discriminating detection apparatus as in any one of claims 1 to 16, claim 30 or claim 31 in which the time constants of both the 1 5 transmission means and detector coil are substantially short when compared to the time interval separate from the period having a significant signal resulting from decay of ground eddy currents.

33. A conducting metal discriminating detection apparatus substantially as described with reference to and as illustrated by the accompany drawings.

34. A method of conducting metal discriminating detection substantially described with reference to and as illustrated by the accompanying drawings. Dated this 24th day of November 1992 HALCRO NOMINEES PTY. LTD. By their Patent Attorneys COLLISON CO M