AU593139B2 - Method of discrimination detection using two frequencies - Google Patents

Description

4k COMMONWEALTH OF AUSTRALIA 5 9 3 1 3 9 Patents Act 1952-1969 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int. Class Application Number Lodged Complete Application No. Specification Lodged Published 'riority: elated art: elated art: This document contains thc amenstnents made unde Secdtk 49.

Ind M mwio ex~ pebel".

I 1 me of Applicant: dress of Applicant: ual Inventor: ress for Service: TO BE COMPLETED BY APPLICANT BRUCE HALCRO CANDY Hunters Road, Basket Range, State of South Australia, Commonwealth of Australia BRUCE HALCRO CANDY Care of COLLISON CO., 117 King William Street, Adelaide, South Australia, 5000.

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t Complete Specification for the invention entitled: "METHOD OF DISCRIMINATION DETECTION USING TWO

FREQUENCIES"

The following statement is a full description of this invention, including the best method of performing it known to me: T~r~-PIp~~3-T1~7I~ 2.

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 reactive or resistive components of re-transmittced signals such that it has been hitherto difficult to distinguish a target signal from a background signal.

Such an environment can be typically ironstone or salt water or indeed both.

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: According to this invention there is provided a conducting metal discriminating detection apparatus comprising means to transmit at least two alternating magnetic fields of different frequency ea.ch below 100 kHz, means to receive respective retransmitted signals arising from each of the transmitted frequencies from a target,, ,herein there are means whereby the respective received signals will be demodulated for assessment of reactive and/or resistive component magnitude which respective levels will then be compared such that an output signal can be available, an interpretable ,2O. characteristic of which is substantially independent of a background environment where this includes substantial material which has a substantial magnetic effect and has a reactive to resistive response ratio which is substantially independent of any interrogating frequency below 100kHz.

In preference, the detected signals are compared such that a predominantly reactive component from each received signal is subtracted one from the =d other.

In preference, three different frequencies are used and such three frequencies are simultaneously transmitted and the detected re-transmission from the target environment is separately detected and treated so that an at least predominantly reactive component of such signals in the one case, or I A

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at least a predominantly resistive component of the signals in the other, are separately handled so that by use of comparison of the responses, greater sensitivity can be achieved.

In preference, if ironstone is the background environment, it is found that if the reactive synchronously demodulated received signals of different retransmitted signals are subtracted one from the other, then the result is such that the influence of ironstone is substantially reduced and in many cases effectively nulled entirely.

According to a further form of this invention, it can reside in a method of effecting discriminating detection of a conducting metal target which comprises the steps of interrogating the target with at least two discrete ct-cI dC o.ul> .a •frequencies, detecting/ any reultaM generated magnetic field in respect of .:each of said discrete frequencies, distinguishing reactive and resistive S":components of such detected received signals in respect of each of the said .:discrete frequencies and then combining the results in predetermined .".manner whereby the said output is substantially independent of selected background materials in the target environment.

In preference, the method is extended to the use of three discrete frequencies.

A background environment of concern comprises ferrites, and particularly I iron ferrites.

A further background environment of concern relates to salt water.

In preference, the method when effecting reduction of effect from background ferrite, includes the steps of subtracting the respective incoming signal which comprises dominantly the reactive component from each of the discrete frequencies.

In preference, when the background to be avoided is salt water, the output is based upon predominantly the resistive components of at least three Q I I !4.

discrete frequencies, and when the cum of the detected signals in the resistive component are equated to zero.

It is understood throughout this specification that such an output can then be used in combination with other detection techniques and discriminating techniques to further assist in detection of materials.

In preference, the discrete frequencies used lie within the range of 500 Hz to 100 kilohertz.

An alternating magretic field metal detector consists of electronic circuitry in which an alternating current signal is produced which is ted to a transmitting coil, and detection electronics which compares an emf signal induced in a ,receiver coil to the transmitted signal. The induced signal in the receiver coil ,cresults from two sources, namely, from alternating currents flowing in the transrnitting coil, and from retransmitting magnetic sources in the local environment under the influence of the transmitted magnetic field.

Consider for the sake of clarity, an ideal situation where the capacitance between windings of both the transmitting coil and receiving coil can be considered negligible.

Also consider that the load presented to the receiving coil by the detection electronics is effectively infinite. Furthermore consider that irduced eddy 2Q. currents in the transmitter or receiver coil may be considered for the following analysis to be negligible. Thus for the analysis which follows, the received signal may be considered as the induced emf resulting from Salternating magnetic flux with an amplitude directly proportional to the rate of change of flux passing through the receiver coil.

Under these circumstances, the romponent of the received signal resulting from currents flowing in the transmitting coil is such that for each Fourier component transmitted, the induced emf in the receiver coil has a phase angle of 90 degrees relative to the component's current flowing in the transmitter coil. This induced component will be called the "magnetic* or "reactive" component.

r. -1 1 Any induced received Fourier component with the same phase as the transmitted current, will be called the "loss" or "resistive" component.

Received signals resulting from local retransmitting environmental sources induce both resistive and reactive components in the receiving coil. Two sources dominate in most ground. One results from ironstone in which the reactive component is usually much greater than the resistive component (usually by more than 100 times), and the second results from mildly electrically conductive components such as moist ground containing salts.

The most difficult ground for detecting highly conductive metal targets, such as coins, gold, underwater p'oes etc., is that containing large concentrations of ironstone whose resistive to reactive ratio varies spatially, and worse still if C4the ground also contains moderately electrically conductive components.

o Usually the reactive to resistive component ratios in most ground is of the S° order of 10. The best of the existing 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.

The phase angle of the demodulator is set so that the detector is insensitive to components with a phase near to the reactive component, but offset by usually less than several degrees 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 insensitve to the local area. This occurs when the detector's 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 its reactive component. This adjustment need be made frequently for best results.

This invention describes means of reducing these interfering signals intrinsic to the ground from both the sources described above; viz: the mildly conducting material in the ground and varying reactive to resistive component ratios in ironstone by using two or more simultaneous .j interrogation frequencies while still maintaining sensitivity to highly electrically conducting metal targets.

Furthermore, a detector using three or more simultaneous interrogation frequencies, using the technology described herein to reduce ground signals, can be employed to advantage over existing art to furnish information about the onductive nature of the interrogated metal target object.

To assist with an understanding of the present invention, definitions and back ground mathematics will be presented; "Resistive channel outputs" refer to the outputs of low-passed synchronous demodulator outputs which have reference phases set to substantially pass :-*resistive component information (but not reactive component information).

99 999 9 ":'"Reactive channel outputs" refer to the outputs of low-passed synchronous demodulator outputs which have reference phases set to substantially pass 15. reactive component information (but not resistive component information).

"Channel gain" refers to the magnitude of the response from the output of the low-passed synchronous demodulator signal arising from one of it's associated interrogation frequencies, resulting purely from the interrogation tC of an object which is purely reactive at frequencies of the order of the ,0c, interrogation frequencies and has a response independent of frequencies at these frequencies, with the synchronous demodulator reference phase adjusted so that the low-pass filtered output is substantially sensitive to reactive components (and substantially insensitive to resistive components), all else being equal.

"First order objects" refers to objects that can be represented magnetically as a single inductor L loaded with a single resistor R. For these objects the characteristic frequency Wo is defined as R/L.

In the analysis which follows, for the sake of simplicity, it is assumed that the transmitter and receiver coils are substantially nulled, the systems are linear, and that the low-pass filters are temporally matched. Further, it is 7.

assumed that each low-pass demodulator signal is sensitive only to the selected interrogation frequency and not others simultaneously transmitted.

ii 1The interrogation of a first order object results in resistive channel outputs LCi being pr'oportional to LCio(GiLWoWi/(Wo 2 +Wi 2 and the reactive channel outputs MCi being proportional to MCio( GiMWi 2 /(Wo 2 +Wi 2 .where the Wi are the interrogation frequencies and Gi are the associated S channel gains. The subscript refers to the resistive or loss(L) and ":'*:magnetic(M) responses, and i is a label refering to the ith transmitted frequency.

S* S Most ferrites have frequency independent resistive components at Sfrequencies of the order of typical interrogation frequencies, that is the loss per cycle per interrogation field strength is substantially frequency independent below a certain frequency, usually approximately 100kHz.

For most such materials, the reactive component is much larger than the resistive component, and is substantially frequency independent below the said frequency. Thus materials for which the loss per cycle per interrogation field strength is substantially frequency independent, also have the property c, that the reactive to resistive component ratio is frequency independent. For materials with this property, we have calculated and confirmed by measurement that the difference between to reactive channel outputs of equal channel gain, called the reactive difference channel RDi is proportional to RDi o( log(Wh/WI) where Wh is the higher frequency and WI the lower. Furthermore for materials with this property, we have calculated and confirmed by 8.

measurement that for channels of equal gains the ratio of the resistive channel output LCi to reactive difference RDi is proportional to LCi/RDi a I l/(2og(Wh/W1) (4) where the logarithm is natural.

We have noted that non-electrically conducting ground has reactive to resistive component ratios that are frequency independent to within several percentum. That is the predominant ground signals behave like most ferrites, and indeed the major magnetic soil contributer is a ferrite, namely Fe304.

Thus it is possible to combine channel outputs in many ways to yield outputs relatively insensitive to non-electrically conducting ground. This can be t t cachieved by directly subtracting resistive channels of equal gain such that tthe output LDi equals S LD=LC2-LC1 S or by subtracting reactive difference channels RD1 and RD2, such that the output RDDi is proportional to RDDo RD1*log(W4/W3)-RD2*log(W2/W1), (6) where the channel RD1 is sensitive to MC2-MC1 and RD2 is sensitive to MC4-MC3. (Note W3 may equal W2.) Furthermore this can also be achieved by subtracting a reactive channel from an reactive difference channel, such that the output LRDi is proportional to LDi Ilog(W2/W1)*LC-RD1*rT/2 (7) Note that most electrically conductive metal objects yield non-zero responses in equations and Nulls in the Wo domain occur in all the responses defined in equations and However it should be noted that the nulls do not occur at the same frequencies.

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

wherein Figures I and II show an electronic block diagram of a "reactive difference channel" according to a preferred embodiment,.

Figure III shows a block diagram of a transmitting coil reactive voltage stabilizer according to the preferred embodiment and Figure IV is a functional block diagram according to the same preferred embodiment showing the receiver layout.

Figure shows an example in greater detail of an embodiment of a quasischematic diagram of a detector which transmitts simultaneously three substantially sinusoidal signals of frequency W1, W2 and W3 respectively Referring to the drawings in detail it is now noted as follows.

9 t 'Essentially the "reactive" synchronously demodulated received signals of different transmitted sinusoidal signals are subtracted at the demodulator/low-pass filter interface as shown in figures and (11).

The received signal induced in the receive coil 1 is amplified by preamplifier 2 and then passed to the analogue inputs of two low distortion doublebalanced mixers with current source outputs 4 and 5. To these demodulators 2o. is applied reference signals 6 and 7 respectively each phase-locked to a different transmitted signal, such that the phase of the references are each in-phase (or out of phase) with the reactive components at the preamplifier output.

The mixers' current outputs 8 and 9 respectively are added in the sense that the reactive components from the two frequencies are subtracted. The current from the outputs are fed to the supply 12 via two equal valued resistors 10 and 11. The difference between the potential at the outpul of the mixers is measured by a difference amplifier 13 to produce an output 14.

Low-pass filtering is achieved by capacitors 15 and 16 connected between the mixer outputs and ground. In figure II, the received signal induced in the receive coil 17 is amplified by preamplifier 18 and then passed to the analogue inputs of two solid-state switches "20 and 21.

J 4 Digital reference signals 22 and 23 each phase-locked to a different transmitted signal are applied to the switch controls such that the phase of the references are in-phase (or out of phase) with the reactive components at the preamplifier output.

The outputs of the switches are each fed via a resistor, 24 and respectively to a capacitor 26 connected to ground. A standard "multiple feed back" low-pass filter, comprising of the operational amplifier 28, resistors 27, 31 and the effective input load of 24 and 25 in series with the switches, and capacitors 26 and 30, passes a demodulated and low-passed signal to the output 29.

The digital reference phases are selected in the sense that the reactive S components from the two frequencies are subtracted. "Four quadrant" dual Ssynchronous demodulators using solid state switches can be similarly implemented by summing the outputs of two pairs of switches, each pair having one switch with both the reference and analogue inputs out-of-phase ,relative to the other.

St "Subtraction at the demodulator stage is substantially more satisfactory than subtracting the low-passed demodulated components as this latter technology requires extremely well temDorally matched low-pass filters.

r t Similarly, the resistive components can be subtracted at the demodulator Or, stage before low-passing this difference. Yet further, the subtractions of equation can be made at the demodulator stage with the appropriate gains selected by the choice of channel gains and resistors.

The simplest means of realising equation 7 with two frequencies is to subtract the outputs of two synchronous demodulators, the reference signal of one is phase locked to one of the transmitted frequencies with the phase selected such that the demodulator substantially passes reactive components, while the reference signal to the other is phase locked to the other transmitted frequency with the phase selected such that the demodulator passes a linear combination of reactive and resistive components, such that the said linear combination and channel gains are selected so that equation is satisfied. The principles of subtraction at. the demodulatorlow-pass filter ioterface can be applied to the selection of linear 1 combinations at this said 27 _i ~IDI of many synchronous demodulator outputs combined together interface.

I The outputs have two be combined in a way such that the output has no nulls in the Wo domain. This can be achieved by full-wave rectification of RDD, LC and/or LDR outputs followed by an adder which adds each rectified output, or a selector which passes the largest instantaneous rectified output. This final output then may be further low- and high-pass filtered. The final output can be used two control an audio output or trigger yet further electronics if a threshold is exceeded.

io. The above principles cope well with most soils not containing significant electrically conductive components. Most soils, even if substantially wet fall ,in this category. Wet brackish soils do not, and means to cope with these are S now given:

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t ~3 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 Wo on scale sizes of the order of 10cm is only significant at high frequencies, typically much higher than typical metal detector frequencies (2 to To a first order approximation targets with Wo at these frequencies, yields an approximate response in equation of Ci GiLWVWo and equation is approximately zero. Thus any reactive channel essentially contains little information arising from mildly electrically conductive components (on the scale size of the order of 10cm), and resistive channels respond to the salt component in proportion to the channels associated interrogation frequency to a first order approximation.

Thus in order for any resistive component associated output to be insensitive to mildly conducting ground components, a further constraint must be made so that for interrogation frequencies Wi, the mildly conductive insensitive output LD is proportional to, LDD o( i1 GiLLCi 12.

where i GiLWi=0 For soils containing ironstone and salt water, signal differences in reactive difference channels satisfying will suffice, as do signal differences in resistiva difference channels satisfying and i GiL=0 (11).

For example a system with 3 frequencies realising these equations such that no nulls occur in the frequency response could consist of a resistive or loss difference differencer (LDD output) and a reactive difference differencer o. (RDD output). For this system, LDD is proportional to r 1 LDDo( G1L-G2L+G3L (12), z c where G2=G1(W3-W1)/(W3-W2) (13) and o G3=G2-G1 (14).

The above equations indicate that the equal channel gain reactive difference outputs (RD) have advantages over the current art single frequency resistive channel output (LC) detectors with interrogation frequency near to the geometric mean of the frequencies of the two reactively compared components, for the detection of objects with predominant characteristic frequencies near to and within the frequency span defined by the two interrogation frequencies: For less than 16:1 frequency ratio, the ratio of object signals to signals arising from magnetic soils in RD are greater than those in LC. More importantly, RD is relatively insensitive to mildly electrically conducting ground components unlike LC. In addition, the measurement of RD rather than LC has advantage for metal detectors in which the design aim is to detect objects with predominant Wo 0r"\ 13.

within this said frequency span and not outside it (such as "coin detectors"), owing to the fact that RD has this very property unlike LC, as stated in the equations.

As above approximations are not perfect; that is the reactive to resistive ratio of ironstone is not entirely frequency independent (at audio frequencies), for best results small adjustments need to be made to parameters (gains or phase angles), to "ground balance" the detector for interrogation of different soils. These adjustments can be performed manually.

It is important that the reactive transmitting coil voltage does not change significantly owing to changes in the coil's inductance as the coil interrogates different magnetic soils. Such reactive voltage changes will o induce spuricus signals owing to varying retransmitting background Sresistive components. Further more in general different frequencies' reactive voltages will be effected differently.

I i5. This will cause spurious signals especially in RD signals. In order to Sovercome this problem the transmitting coils reactive signal voltages at each frequency should be measured and the level should be stabilised by a servo loop.

41 jAs an example a means of realising this for two simultaneously transmitted signals of frequency W1 and W2, is given in figure (III). The transmitter coil 32 is connected in series with an inductor 33 across which the reference transmitted current is measured. (This may in fact be any load, example resistor or capacitor.) The voltage across 33 is fed to two phase-locked loops 34 and 35. 34 is locked to W1 and 35 to W2. The digital outputs of these are fed to two synchronous demodulators 36 and 37 respectively. To both ths analogue inputs of 36 and 37 are fed a voltage 38 t ~ich appears across 32 and 33.

The output phases of 34 and 35 are selected so that the synchronous j" demodulators are sensitive to the transmitter coil's reactive voltage and substantially not it's resistive voltage at the respective frequencies. The outputs of 36 and 37 are passed to the servo-loop filters 39 and respectively. The outputs of 39 and 40 are used to control the level of the input signals frequencies Wt and W2 respectively) by gain controlled y~~LC-~ r1 i 14.

stages 41 and 42 respectively. The outputs of 41 and 42 are feed to a summing amplifiers 43 which is connected to the transmitter coil, at which node the combined signal 38 appears.

As an example of all of the above concepts, bar the reactive transmitted signal level controls, figure (IV) shows a block diagram of a detector in which 3 simultaneous magnetic interrogation signals are transmitted of different frequencies W1, W2 and W3, where W3>W2>W1.

The received emf in the receiver coil 44 is amplified by an preamplifier the output 46 of which is feed to the analogue inputs of synchronous lo. demodulators 48 via a gain controlled stage 47, and synchronous demodulators 49, 50, 51, 52, and 53. The amplification factor of 47 is Sdetermined by a control level 54.

The reference digital inputs to the demodulators 48, 49 and 50 are the three different frequencies substantially reactive references, where the reference 5 phase of W2 is in the opposite sense the other two, as per equation with W3=W2 in this said equation. Demodulators 51, 52 and 53 have S" substantially resistive digital references at the three different frequencies.

The sense of the reference of W2 is opposite to that of the other two. The digital reference of 51 is phase shifted by small angles for ground balancing purposes.

The demodulator outputs are combined by means described above, by means of the combiners and low-pass fillers 56 and 57. The outputs of 56 and 57 are passed to high-pass filters 58 and 59 respectively. The outputs of 58 and 59 are passed to fui'-wave rectifiers 60 and 61 respectively. The highest instantaneous output of the outputs of 60 and 61 is selected by a selector 62 and passed to the output 64 which may be first low- and highpass filtered by a filter 63.

The channel gains are selected to substantially obey equations (12), (13) and It must be noted that many aditional outputs with the same concepts shown in figure IV and manifest in equations to (13) can be obtained using 3 frequencies that are relatively independent of ironstone, or mildly electrically conducting ground components or both.

As an example in greater detail of an embodimient figure shows a quasischematic diagram of a detector which transmitts simultaneously three substantially sinusoidal signals of frequency W1, W2 and W3 respectively, by means of a si.mming amplifier 65 applying a composite signal to a transmitting coil 66. The current flowing through this coil flows through a small valued inductor 67 for sensing the said current. The output of a lowpass filter 68 is feed to one of the inputs of 65. The input to 68 is a squarewave of frequency WI, the amplitude of which is controlled by a voltage controlled amplifier 69 to which input is feed the source of W1 from a frequency divider 70. The gain of 69 is controlled by a voltage 71. Signal 71 is controlled by the level of the reactive component across 66 at the frequency Wl by means of a servo loop. The loop filter consists of the operational amplifier 73, an input resistor 74, damping resistor 75, and Sintegrating capacitor 76. A two quadrant synchronous demodulator solidstate switch 77 has it's reference phase 78 locked to the phase of the A transmitting coil's current at frequency W1 and selected so that the servo :r"loop substantially keeps the reactive voltage across 66 constant but not the resistive voltage. The input to the solid state switch is the composite voltage 79 from the output of 65. The other two inputs to 65 are from the outputs and 81 of 82 and 83 respectively, which are servo-loop level stablisers and correspond to the building block of the same form as 72 for producing signals at frequencies W2 and W3 respectively. 72 consists of 68,69,71,73,74,75,76 and 77. As in the block 72, 82 and 83 are each fed 79, Sand to each is fed two digital signals. One of 'the the said digital signals is the ,2,f source of frequency W3 84 feeding 82 and the source W2 85 feeding 83, and the other is the synchronous demodulator reactive reference phase, 86 feeding 82 and 87 feeding 83. The operation of 82 and 83 is the same as that of 72 (except for the operating frequencies). A phase-locked loop 88 is locked to Wl, the input signal 89 being the voltage across 67 amplified by amplifier 90. Also to this phase-locked loop is feed the digital signal input to 72, namely 91. Another digital signal 92 at four times the frequency of W1 and !ocked to it, is fed to 88 from 70. The composite signal 89 is fed to the synchronous demodulator of 88, the solid-state switch 93. The output 93 is feed to the loop filter consisting of the operational amplifier 94, an input resistors 95, damping resistor 96, and integrating capacitor 97. The output of 94 Is attenuated by resistors 98 and 99 and fed to comparitor 100. The other input of 100 is 92. The output of 100, namely 101 is fed to the clock inputs of two D-type flip-flops, 102 and 103. The D input of 102 is connected f~l~L'~ lot 16.

to 91 and the D-input of 103 is connected to the 0 output 104 of 102. The 0 output of 103, namely 105, feeds the reference phase of 93. The phase of the loop can be off-set by a voltage 107 which feeds a current to the inverting input of 94 via a resistor 106. The transfered phase of 68 and said comparitor input sense is selected so that when zero volts is applied to 107, the phase-locked loop locks so that 105 is substantially at quadrature with the Wl component in 89. Phase locked loops 108 and 109 are locked to the components of W2 and W3 respectively in 89. 108 and 109 are the same type of building blocks as 88, but with different frequency signal digital lo. inputs. The square-wave source of W2 feeds 108 via 110, while the source of W3 feeds 109 via 111. A phase locked signal of four times W2 feeds 108 via 112 and the corresponding input for 109 is via 113. The flip-flop outputs of 108 and 109 corresponding to 104 and 105 for 88, appear at 114 and 116 respectively for 108, and 115 and 117 respectively for 109. The phase Vcan be off-set in 108 and 109 by the application of a voltage at 1!8 and 119 P respectively, corresponding to the input 107 of 88. The received magnetic o signal induces an emf in receiver coil 120, which is amplified by amplifier c 121. The output of 121 feeds synchronous demodulators, combiners and low-pass filters 122 and 123, described above. The possible reference inputs to these are numerous, and the channel gains and phases must satisfy the equations described above, depending on application. For Sexample, a general purpose hand-held metal detector that is to be used to interrogate ground containing ironstone and brackish water could have three synchronous demodulators in both 122 and 123, where the outputs of the three in 122 are combined with reactive reference phases selected, and likewise in 123 but witi; resistive phase references selected. The channel gains in 122 need satisfy equation and in 123 the gains should satisfy equation (13) and In equation W2=W3 and corresponds to W2 in this example, and W4 in equation corresponds to W3 in this example.

The outputs of 122 and 123 are fed to high-pass filters 124 and 125 respectively. The outputs of 124 and 125 are fed to full-wave rectifiers 126 and 127 respectively. The outputs of 126 and 127 are compared by comparitor 128, the output of which controls a solid-state switch 129 such 4, that the larger instantaneous output of 126 and 127 is selected to be passed to the low-pass filter 130, the output of which is fed to a high-pass filter 131.

The output of 131, namely 132 is the system output which may be fed to a meter or audio controller. For best results Wl, W2 and W3 are phase locked.

If the transfered phase response of 121 is substantially zero (or one hundred

WAM"'WWWO

Bat -1I*I1 i-i 7 and eighty degrees) at frequencies W1, W2 and W3, then 104 and it's complement are the reactive phase references for W1, and 114 and it's complement, and 115 and it's complement are the reactive reference phases of W2 and W3 respectively. Also 105 and it's complement are the resistive phase references for W1, and 116 and it's complement, and 117 and it's complement are the resistive reference phases of W2 and W3 respectively for this said phase condition of 121.

There is yet another advantage of the processing described herein, namely that discrimination of characteristic frequencies of electrically conducting targets in soil can be determined at a relatively remote distances using the processed sigtnals described herein compared to the current art techniques which simply measure the reactive to resistive ratio. The very large background reactive signal due to magnetic soils found in most soils, .substantially places a limit on the ability of these sort of single frequency 1 "*reactive/resistive discriminators. Measurement and analysis of at least three S'",different reduced ground sensitive outputs yield unique information about ":Wo without large contaminating ground signals.

c t s SThe frequencies selected for this purpose are 1kHz, 4kHz and 16 kHz. In trials conducted these frequencies used in the manner described provided the advantages of the inveniion.

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Claims (5)

18. THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A conducting metal discriminating detection apparatus comprising means to transmit at least two alternating magnetic fields of different frequency each below 100 kHz, means to receive respective retransmitted signals arising from each of the transmitted frequencies from a target lwherein there are means whereby the respective received signals will be demodulated for assessment of reactive and/or resistive component magnitude which respective levels will then be compared such that an output signal can be available, an interpretable characteristic of which is substantially independent of a background tt environment where this includes material which has a magnetic effect and has a reactive to resistive response ratio which is substantially independent of any interrogating frequency. 4io_ soid C. 2. An apparatus as in claim 1 further characterised in thatl.- I oretransmitted signal from the ground and/or electrically conducting target metal object will be received such that the received signal is synchronously demodulated by at least one synchronous demodulator for each transmitted signal component, synchronous demodulator references beinc: derived from the said transmitted signal components, t and wherein a low-passed output of each said demodulator is sensitive to one only of the transmitted frequencies, and each said demodulator having its reference selected whereby each said low-passed synchronously demodulated output will contain information proportional to either predominantly reactive components of the magnetically ,",interrogated environment at a frequency Wi, where i is the mathematical label of the i-th transmitted frequency to which the demodulator is sensitive, the output of such a demodulator's low-pass filter being called RCi the output of the reactive channel of Wi or, predominantly resistive components of the magnetically interrogated environment at the frequency, Wi, to which the demodulator is sensitive, the output of such a demodulator's low-pass filter being called LCi, the output of the resistive channel of Wi, and further including an interpretive means including means adapted to compare by subtraction at least a selected ratio of one of the reactive channel's outputs with r. WIN qj~ i 7' C r t Si S* S S.r S S 411 .5.5.5 5 -19- another to result in a difference signal called a reactive difference signal RDk, where k is the mathematical label of the kth reactive difference signal such that RDk yields little or no change when material that is or nearly is purely reactive and non-resistive below 100kHz is moved within the influence of the transmitted fields and this occurs when the reactive channel gains are selected to be nearly equal. 3. An apparatus as in either of claims 1 or 2 further characterised in thlat the means to transmit an alternating magnetic field are adapted to provide three different frequencies, each less than 100 kHz and above 500 Hz, and further including in addition to, an interpretive means including means adapted to compared by subtraction at least a selected ratio of one of the reactive difference signals with another to result in a difference signal RDDm where m is the mathematical label of the mth difference of reactive difference signals such that RDDm yields no change when magnetic material with frequency independent reactive to resistive component ratios at frequencies below 100kHz is moved within the influence of the transmitted fields, that is if RDDma (the said selected ratio)*RDk-RDn, means multiply), then d/dt(RDDm)=0 for the interrogation of such said material only and this results when the channel gains are selected such that RDDmaRDk*log(W4/W3) -RDn*Log(W2/W1) where the log is natural, RDk equals the difference in the resistive channels of W1 and W2, and RDn equals the difference in the resistive channels of W3 and W4, where the channel gains are equal, and W2>W1 and W4>W3, and in the case of three simultaneously transmitted signals, W2 may equal W3 or W4, or W3 may equal W1. 4. A conducting metal discriminating detection apparatus including means to transmit an alternating magnetic field which will at least have two different frequencies, being each less than 100 KHz and above 500 Hz, whereby a retransmitted signal from the ground and/or electrically conducting target metal object will be received by receiving means such that the received signal is synchronously demodulated by at least one synchronous demodulator for each transmitted signal component, the output of the said synchronous demodulators is low I i, 1 1 19a pass filtered, the synchronous demodulator references being derived from the said transmitted signal components, and wherein the low- 4~ 0 *000 0* 0* 0 0w 4 0 eq 0 *0*q S *0 4 .q S I .4 .45444 4 *544 t 4. 4 .1 I .4 r r-A It 9.9.* S a ar ar 94 *r 9 I passed output of each said demodulator is sensitive to one only of the transmitted frequencies, and each said demodulator having its reference selected whereby each said low-passed synchronously demodulated output will contain information proportional to either predominantly reactive components of the magnetically interrogated environment at the frequency Wi where i is the mathematical label of the ith transmitted frequency to which the demodulator is sensitive, the output of such a demodulator's low-pass filter being called RCi the output of the reactive channel of Wi, or, predominantly resistive components of the magnetically interrogated environment at the frequency Wi, to which the demodulator is sensitive, the output of such a demodulator's low pass filter being called LCi the output of the resistive channel of Wi, and further including an interpretive means including means adapted to compare by subtraction at least a selected ratio of one of the resistive channel's outputs with another result in a difference signal called a resistive difference signal LDk where k is the mathematical label of the kth resistive difference signal such that LDk yields little or no change when magnetic material which has a magnetic effect and has a reactive to resistive component response ratio which is substantially independent of any interrogating frequency below 100kHz is moved within the influence of the transmitted fields and this occurs when the resistive channel gains are selected to be nearly equal, 25 5. A conducting metal discriminating detection apparatus comprising means to transmit and means to receive an alternating magnetic field which will at least have three different frequencies, being each less than 100 kHz, and further including an interpretive means including means adapted to compare by subtraction at least a selected ratio of one of the resistive difference signals with another to result in a difference signal LDDm where m is the mathematical label of the mth difference of resistive difference signals such that LDDm yields substantially no change when either magnetic material which has a substantial magnetic effect and has a reactive to resistive component response ratio which is substantially independent of any interrogating frequency at frequencies below 100kHz or non-magnetic material that is mildly electrically conductive or both is moved within the influence of the transmitted magnetic fields, that is if LDDmO(the said selected 1

21. ratio)*LDk-LDn, then d/dt(RDDm)=0 for the interrogation of such said material types only; this results when the resistive channel gains are selected such that the sum of the channel gains resulting in LDDm equals zero and the sum of the channel gains, each multiplied by the value of its associated transmitted frequency, equals zero. 6. A conducting metal discriminating detection apparatus including means to transmit and means to receive an alternating magnetic field which will have at least two different frequencies, being each less than 100 kHz, and being such whereby a retransmitted signal from the ground and/or electrically conducting target metal object will be received such that the received signal is synchronously demodulated by at least one synchronous demodulator for each transmitted signal component, the output of the said synchronous demodulator is low-pass filtered, the synchronous demodulator ,t -references being derived from the said transmitted signal components, f t and wherein the low-passed output of each said demodulator is sensitive to one only of the transmitted frequencies, and each said demodulator having its reference selected whereby each said low- passed synchronously demodulated output will contain information proportional to either predominantly reactive components of the magnetically interrogated environment at the frequency Wi where i is the mathematical label of the ith transmitted frequency to which the demodulator is sensitive, the output of such a demodulators low-pass filter being called RCi the output of the reactive channel of Wi, or, predominantly resistive components of the magnetically interrogated environment at the frequency Wi, to which the demodulator is sensitive, the output of such a demodulator's low-pass filter being called LCi the output of the resistive channel of Wi, or, a linear combination of resistive and reactive components of the magnetically interrogated environment soto at the frequency Wi, to which the demodulator is sensitive, the output of such a demodulator's low-pass filter being called RLCI the output of the resistive/reactive channel of Wi, and further including an interpretive means including means adapted to produce a signal that is a linear combination of, either two different reactive channel outputs and a resistive channel output or a resistive/reactive channel output RLCi and reactive channel output RCj where i is different to j, and indice similar to 0 0 4ic r 21 a. idesignating a variable associated to the jth transmit frequency, to result in a reactive difference resistive difference signal LRDk, where k is the a ~t I *9I14I eei I I e It I II 11 t I IC II I I I 14-4 1, 1 r-

22. mathematical label of the kth such signal such that LRDk yields little or no change when magnetic material which has a magnetic effect and has a reactive to resistive response ratio which is substantially independent of any interrogating frequency below 100kHz is moved within the influence of the transmitted fields; adapted whereby this results when the channel gains are selected so that LRDk <LCj- "^T /(2log(Wh/WI)*RDk means multiply) where RDk equals the difference in the reactive components of Wh and WI, WI being the higher transmitted frequency and Wh being the lower transmitted frequency, i.e. Wh>Wl, where the channel gains are equal, which for two transmitted signals is equivalent to subtracting a reactive channel output RCi from RLCj, such that when RLCj is mathematically split into a predominantly reactive component, RCj and a predominantly resistive e component LCj, where j=h and i=l, or j=l and i=h, then for the effective gain of RCj being equal to RCi, the effective gain of LCi is selected whereby LRDk<LCj- 7/(2log(Wh/WI)*RDk)where RDk equals the difference in the reactive components of RCi and RCj,Wh>WI. 7. An apparatus as in any one of the preceding apparatus claims further adapted whereby the apparatus includes processing of the said interpretive signals to give interpretive signals relating to the nar.'re of the interrogated conducting metal target objects and thereby enabling discrimination. 8. An apparatus as in any one of the preceding apparatus claims further characterised in that the said apparatus is adapted whereby comparisons are carried out at the synchronous demodulator stages before the respective signal components are low-passed. 9. An apparatus as in any one of the preceding apparatus claims wherein the said difference signals are combined by further interpretive means being adapted whereby the said difference signals are each passed through respective high-pass filters, the outputs of these said high-pass filters being each full-wave rectified through respective full- wave rectifiers, the outputs of these said full-wave rectifiers being each added by an adder to produce a final interpretive output. J11A

23. An apparatus as in any one of the preceding apparatus claims wherein the said difference signals are combined by further additional interpretive means whereby the said difference signals are each passed through respective high-pass filters, the outputs of these said high-pass filters being each full-wave rectified through respective full- wave rectifiers, the outputs of these said full-wave rectifiers being fed to a selector which passes the largest instantaneous said full-wave rectified output to a final interpretive output. 11. A method of effecting discriminating detection of a conducting metal target which comprises the steps of interrogating the taraet with at least two discrete transmit frequencies below 100kHz, detectingany resultant generated magnetic field due to the ground and/or target "o object in respect of each of said discrete frequencies distinguishing reactive and resistive components of such detected received signals in respect of each of the said discrete frequencies and then combining the results in predetermined manner whereby the said output is or nearly is independent of selected background materials in the target environment. 12. A method of effecting discriminating detection of a conducting metal target as in the last preceding c'aim further characterised in that the interrogating frequencies are within the range of 500 Hz to 100kHz. 13. A method of effecting discriminating detection of a conducting rE metal target as in either of the two last preceding claims further tl characterised in that the selected background material is that which has a magnetic effect and has a reactive to resistive response ratio which is substantially independent of the interrogating frequencies. 14. A method of effecting discriminating detection of a conducting metal target as in any one of the three last preceding claims further characterised in that the selected background material includes salt water and may include other materials. A method of effecting discriminating detection of a conducting metal target as in any one of the four last preceding c!aims further characterised in that at least three discrete transmit frequencies used rh IrV- Li4 4is D

24. simultaneously to interrogate the target and each of the at least three frequencies are detected and combined in predetermined manner whereby the said output is, or nearly is independent of selected background materials in the target environment. 16. A method of effecting discriminating detection of a conducting metal target as in any one of the last five preceding claims further characterised in that the frequencies are transmitted simultaneously. 17. A method of effecting discriminating detection of a conducting metal target as in any one of the six last preceding claims further characterised in that the manner of combining the signals comprises, in the case of two discrete transmit frequencies, subtracting the magnitude of the one signal from the magnitude of the other. S18. A method of effecting discriminating detection of a conducting metal target substantially as described in the specification with reference to and as illustrated by the accompanying drawings. 19. A conducting metal discriminating detection apparatus substantially as described in the specificatior with reference to and as illustrated by he accompanying drawings. Dated this 6th day of October 1989 BRUCE HALCRO CANDY By his Patent Attorneys COLLISON CO. I f-