METAL DETECTOR USING AMPLITUDE CROSS-CORRELATION

Abstract: A detector for conducting metals within mineralised ground where the detector uses an integrated cross correlator to control a ground balancing element. The detector contains a coil for transmitting sinusoidal magnetic signals and coil for receiving retransmitted signals from the ground and objects. The received signal is demodulated to extract signals predominant in the ground signal, X, the reactive channel, and predominant in object signals, R, the loss channel. Changes in time of these signals, Xa and Ra, when suitably modified to remove signals intrinsically arising from functions of the detector, are cross correlated by a multiplier (15), the output of which is integrated (17). The integrator controls the ground balancing control, the reference phase to the R channel demodulator by means of the phase shifter (18).

Description: This invention relates to metal detection apparatus and in particular to maintaining substantial discriminatory sensitivity in mineralized envlroments. In this specification the term "metal detector" is used to refer to detectors of a type used for the purpose of discriminating metal within the 5. ground but it can also be used to refer to a single detector or an array of detectors where used to detect metal objects in soil on a moving conveyor system. Such a system may be used, for example, to locate gold nuggets in soil moving on the conveyor system. The invention is directed to some of the difficulties encountered when 10. using a detector in the presence of ground containing varying proportions of minerals magnetic characteristics of which may vary from one location to another. These minerals, such as ferrous oxides may produce strong signals in the detector which make identification of a target signal (for example, a signal from is . a gold nugget) difficult to identify; that is these stong variable ground signals can "mask out" the signal from a target object. The extent of this difficulty 1s very dependent on ground conditions. There are a number of types of metal detectors each having a different method of operation. There is however a common principle of operation common to all 20. these types of detectors. This involves the production of a magnetic field by a coil. _ A conducting material within an effective range will interact with thi field and change characteristics of the field which characteristics can then b analyse . 25. This invention is directed to a type of metal detector known as a alternating magnetic field metal detector where the the field variations ar substantially sinusoidal. This will include detection electronic circuitry which compares the phas and magnitude of an emf signal induced in a receiver coil with that of 30. transmitted signal. The object of this invention is to provide means by which there can be more effective detecting in a discriminatory way in such conditions ground containing mineralisation, particularly where the magnetic characteristics are variable spatially . 5. It is well known that an Induced signal in any material may have a phase relationship that is useful in identifying it's magnetic characteristics; this phase relationship can be referred to in terms of quadrature components of the signal. These can be characterised by a purely reactive component and at 90 degrees to this, a purely resistive component which Is some times refered to as the loss 10. . component . In general, mineralised ground will provide a large reactive component and a small loss component whereas conducting objects, because of eddy currents generated within them have large loss components by comparison. It is this difference which allows for the possibility of locating conducting is i objects even when mineralisation in the ground produces a large return signal. The best available conducting metal target detectors use two synchronous demodulators known as the "X" and "R" channel, such that the "X" channel has it's sensitive component axis aligned within a small angle of the purely reactive retransmitted components, and the "R" channel has it's sensitive component axis 20.. aligned within a smal «l. angle of the purely resistive retransmitted components. The sensitive axis of the "R" channel can be manually varied in order to align it so that it is at quadrature with the ground vector. The detector is said to be gound balanced for the areas of ground whose vector is aligned at quadrature with the sensitive axis of the "R" channel. For these detectors the "R" channel is known as 25 the "object channel". Ground balancing can also be achieved by adding a proportion of the "X" channel to the "R" channel by control of the proportion added. For detectors using this latter ground balancing method the composite signal is called the "object channel". Many detectors do not provide for either of these controls. U.S. Pat. No. 4,128,803 uses an "X" and "R" signals channel and a third 30. demodulator between these two channels to detect and identify the target. U.S. Pat. No. 4,024,468 an attempt is made to discriminate between different types of metal objects by amplitude discrimination. U.S. Pat. No. 4,099, 1 16 and 4,300,097 uses a phase angle setting to discriminate between various types of ferrous and non ferrous objects. Various feedback circuits from the "X" channel are used to compensate for the mineralised component signal in the discriminator circuit. 5. U.S. Pat. No. 4,303,879 discloses an automatic tuning circuit between mode selection, with manual ground cancellation. U.S. Pat. No. 4,344,034 uses a double detector circuit in which the second detector circuit attempts to eliminate ground signals from the first discriminating circuit. 10. In all these patents together with U.S. Pat. No.4,507,612, U.S. Pat. No. 4,325,027, U.S. Pa No. 4,470,015 and U.S. Pat. No. 4,514692 the emphasis is in target identification in mineralised soil conditions. However the common difficulty encountered by all these techniques concerns the variability of the ground. The setting of the electronic controls which will, to a large extent, is . eliminate the interference from the ground have to be changed as the detector is moved from one region to another and the ground mineralisation composition changes. Sometimes this ground variation is minimal , slowly varying, and only substantially involves variations in the concentration of mineralisation, whereas other ground is also characterised by rapid spacial changes In the angle of the 20. ground vector relative to the "pure" axes described above. One well known ' technique to try and overcome this difficulty is to run the detector in the "Auto" or "A.C." mode. In this mode of operation low frequency components of the signal are filtered out and in this way slow variations due to ground mineralisation are 25. removed and only the high frequency components of the signal (which may be due to the required object) remain. Thus the compensating feedback circuits described in some of the above patents and other techniques employing high pass filtering ( U.S. Pat. No.4,470,015 and U.S. pat. No. 4,507,612) and differentiating circuits ( U.S. Pat. No.4,514,692 ) 30. are employed. U.S. Pat. No. 4,507,612 employs high Q high pass filters to remove ground signals and then uses a feedback circuit to determine the phase angle of the target, for target Identification purposes. In all mineralised ground it is necessary to periodically manually adjust the ground; balance control to minimise the signal due to the changes in the mineralisation concentration for optimum performance. This procedure, although inconvenient, Is satisfactory provided the character of the mineralisation Is 5. constant over regions extending over many metres. In areas where the phase angle of the ground spatially varies rapidly, manual ground balance adjustment is impractical but, If not carried out, will result in large signals in the object channel due to changing mineralisation concentration, thereby substantially reducing the ability to detect signals from 10.. target objects. These high frequency components of the ground signal can be much greater than remote or small target signals, and this is particularly the case near ironstone reefs where the adjustment may have to be made at intervals as close as one- metre or even less. Detectors with no means for ground balancing are particularly inadequate in these conditions. is. Prospecting under these conditions is extremely difficult and tedious and, despite their potential for producing gold, these regions are accordingly avoided by prospectors. An object to which this invention is directed then is to assist in reducing the above difficulties. 20. . In accordance with this invention there is apparatus for detecting remote metal objects in soil by interrogation using substantially sinusoidal magnetic transmitted signals for such interrogation the apparatus including a correlator, t e-apparatus being characterised In that there are means to effect at least two signals derived from incoming interrogation signal or signals by synchronous 25. . demodulation referenced to the said transmitted signal, the apparatus being adapted such that one of the said signals contains Information distinctive of changes during different soil interrogation over time in soil background magnetic response and any target metal object at the transmitted frequency, and the other signal will contain substantially that information arising from changes over time 3Q. . in the said retransmitted magnetic field from interrogation from a moving source of transmitted signal relative to a target of any target conducting metal object, but relatively little information arising from differences in the ground background magnetic response from Interrogated location to location, means to provide for amplitude cross correlation between the two signals by means of the 35. correlator, the said correlator being adapted to alter the said second signal in such a way that the said correlation between the said signals is minimised within a selected response period as the apparatus interrogates different areas of soil locations. In accordance with one form of this invention an alternating current is 5. produced In a transmitting coll producing a magnetic field which interacts with the immediate environment. The Induced emf signal in a receive coil Is demodulated with reference to the phase of the alternating current in the transmitting coil to produce an "R" channel and an "X" channel described above. The induced signal in the receive coil results from two sources, namely, directly iQ. from the currents flowing in the transmitting coil, and varying magnetic sources in the local environment under the influence of the transmitted magnetic field. The transmitter coil and the receiver coil are normally set up so that when the coils are remote from the ground or other objects the signal induced in the • receiver coil is substantially a minimum. is. In this situation, 1t Is said that the sensing coils are "nulled". Consider for the sake of clarity, an ideal situation where the capacitance between windings of both the transmitting coll and the receiving coil Is negligible. Also consider that the load presented to the receiving coil by the detection electronics is effectively infinite. Furthermore consider that Induced 20. eddy currents in the tra « nsmitter or receiver coil are negligible. Thus for the description which follows, the receive signal may be considered as the induced emf resulting from the alternating flux due to the transmitting coll so that for each Fourier component transmitted the corresponding voltage component in the receive coll has a phase angle of 90 * 25. relative to the current in the transmitter coil. This Induced component will be called the purely reactive component, where the measured so called "X" component has a component vector within 10 degrees of the purely reactive component. Any Induced received Fourier components with the same ( or opposite ) phase as the transmitted current will be called purely resistive components, where the 30. measured so called "R" component has a component vector within 10 degrees of the purely resistive component. Received signals resulting from local environmental sources in general induce both resistive and reactive components in the receiving coil. Two sources dominate in most ground. One is from ferrous oxides which have a reactive component much larger 5. than the resistive component, and the second from mildly electrically conductive sources such as moisture containing salts, clays and carbon deposits, all of which have small reactive components and large resistive components at audio frequencies. In ground containing very heavy ferrous oxide deposits (heavy ironstone deposits) the reactive component can dominate the resistive component ιo . by as much as 100: 1. Usually the resistive component variations of the second source above in the ground is not correlated with the reactive component variations. However, the resistive component of the ferrous oxides is correlated well with its reactive component is . The invention described and claimed herein comprises metal detector apparatus incorporating a system for correlating changes in the "X" and object channel signal with time, as the sensing coil passes from one area of soil to another, on a continuous or repeating basis to produce a correction signal which can be used to minimise those component variations in the "X" signal correlated 20. with those simultaneous variations in the object channel in the object channel. Changes over time in the "X" and "R" or object channel can be determined by the use of high pass filters. _ The method for effecting ground balancing can consist of either a system for automatically rotating the reference phase angle for the "R" component, or Z-r. shifting the phase of the signal from the receive coil, or multiplication of the "X" component by a factor determined by the correlated signal and subtracted from the "R" component signal. By this means the detector Is continuously adjusted to substantially reject signals in the "R" channel arising from changes in the magnitude of the 30.- mineralised component in the ground as the character of the mineralisation changes. The strength of the interrogated signal in hand held detectors is modulated by the swinging action of users as the head Is swept across the ground from side to side, thereby enhancing these changes in the interrogated magnitude of the ground signal, but this is action is by no means a necessary requirement for- his continuous ground balancing system using the method of correlation, as the-ground inhomogeneitles produce variations the recleved signal's magnitude. The advantages of using a time averaged correlated signal between changes in the reactive and object channels are that the detector measures the amount of 5. predominant ground signal present in the object channel and rapidly and continually adjusts this signal to zero. Furthermore the correlator technique will adjust out this ground component with an accuracy which is comparable with the most careful manual adjustment and with a speed which is many times faster. Difficulties can arise in providing a continuing ground balancing effect 10. using the principle of correlation, namely getting a feedback loop to respond quickly but maintain its stability for a wide range of ground conditions. This problem can arise out of the fact that the strength of the integrated correlation signal will be proportional to the strength of the reactive signal. A small error in the ground balance control in the presence of a large reactive is . signal will produce a large component in the object channel giving rise to a large correcting signal. This will lead to instability if the loop gain is too high; indeed, if the loop gaif. is too high, the system displays many typical characteristics of non-linear systems, such as bification. When ground containing low concentrations of ferrous oxides is encountered the reactive signal is 20. proportionally small and large errors in the degree of ground balance produce only a small component in « the object channel giving rise to small correcting signals. This produces a very long time constant in the loop If the loop gain is too low, and although the loop is stable, an unacceptable delay is produced In correcting the ground balance. 25. The problem can be overcome by providing means to detect the extent of total reactive signal and means to adjust loop gain so that with higher magnetic concentration in the target ground location the loop gain will be accordingly reduced. This can be achelved as follows: the signal from the reactive channel X or 30. high passed reactive singal Xa is rectified and passed to a peak detector to determine the strength of the reactive signal. The peak detector can have a droop time constant of the order of several seconds. A peak detector is more satisfactory than a low pass filter with a similar time constant because, unlike th filter, transient responses in "X" do not cause momentary Instabilities since the gain is immediately reduced. The output of the peak detector is fed to a divider as the divisor which attenuates the loop gain. The operation of this divider is such that the divisor has a lower limit, thereby not enabling division by zero. With reference to Fig. 1 it is possible to set the "constant of 5 . proportionality" of the divider such that the "gain" of the high passed object signal, "Rm", fed through the divider can be considered constant for values of the divisor above its preset lower limit. In this instance the gain of "Rm" is defined as the pulse response magnitude of "Rm" to a specific step response in the reactive component for a specific misalignment of the "R" demodulator to the 10 .. ground vector. Thus the gain of the feedback loop responsible for the "automatic ground balance" is independent of ironstone concentration if the divisor is greater than- Its preset lower limit, and the "free-air" value of 'much less than the operating value of "X". The invention may be better understood by reference to a preferred is. embodiment which shall now be described with reference to the accompanying drawings wherein _ " FIG. 1 shows- a block drawing illustrating the arrangement for a first embodiment FIG. 2 is a block drawing showing the arrangement for a second embodiment 20 . and FIG. 3 is a graphical display illustrating some typical outputs. FIG. 4, FIG.5 and FIG. 6 are each circuit details of the preferred embodiment the:circuits shown being in each case a circuit part interrelated to the other parts by having common letters identifying circuit lines intended to be connected 2 5 . together. The first embodiment as shown in functional blocks in Fig 1 involves the use of a correlator for correcting the ground signal component in the object channel, "Rm". "R" and "X" signals are produced by demodulators 1 and 2 . The circuit is thus arranged that each signal passes through a pair of low pass filters 30. substantially matched in temporally response 4 and 5 with a (D.C.) output signal for sensing the presence of an object being taken from output 1, 5. From the low pass filters 4 and 5 the signals are passed through a pair of high pass filters 7 and 8 substantially matched in temporal response, giving Xa and Ra, with the reactive component Ra passed through either a high gain amplifier; or a comparitor 14 to produce a signal of fixed amplitude but whose 5. sign depends on the sign of Xa. The resistive component Ra passes to a subtractor 1 1 where a multiple 12 of the reactive component Xa Is subtracted from It The purpose for the subtractor will be explained later. The resistive component is then passed as the dividend to a divider 13 where the divisor is a processed signal 10 from the reactive channel. The quotient from ιu . the divider is then multiplied by multiplier! 5, the cross correlator, by the comparitor/amplifier output and passed through a gain control 16 to an integrator 17 with the output of the Integrator controlling the phase set relative to the transmitted phase, to the demodulators 1,2, and 3. The 90 * phase shift required for the reactive channel 1 and for the input signal to the demodulator 3 comes 15. from the fixed phase shif ter 19. The sections of the circuit comprising 3,6,9,10, 1 1, 12, 13, are for the purpose of stability and * time constant. control in the feedback loop. An additional problem arises out of signal components In "Ra" which result intrinsically from the effects of changing the phase angle of the "R" demodulator 20. for a non-zero value of "X". Consider for example the situation where the magnetic environment of the sensing coil is steady and the value of "X" is non-zero. If the phase reference Rd referenced to the transmitted signal Is shifted, the value of "R" changes owing to the change in the projection of the unbalanced signal at the demodulator inputs on the sensitive vector axis of the "R" demodulator. These 25. components detract from the intrinsic correlation process as they are unrelated intrinsically to the correlated ground components and can be eliminated by subtracting a signal S" from "Ra" to give "Rm" which is free of these components. The method for removing these components Is as follows. Through the action of the demodulator 3, S is proportional to the sine of the phase difference 30. between Xd and the transmitted phase ( here the "d" indicated the digital nature of these signals), such that S=0 if Xd and the transmitted current phase are 90 * out of phase, and S Is a maximum for the in-phase condition. S' Is the signal after passing through low 6 and high 9 pass filters matched in temporal response to the filters in the "X" and "R" channels, so that S' represents the change in the phase angle. Thus the output of the multiplier 12 is proportional to S'*X = S" as Indicated, which in turn Is proportional to the intrinsic signal to be removed from 5. "Ra". Subtraction of S" from "Ra" ( scaled appropriately) therefore removes this unwanted component of Ra. Note: "Ra" and S" can be correctly scaled by making sure Rm is a minimum for changes in the phase of Rd, for a fixed input signal that results in a non-zero value of X. 10.. Note that the part of the system intended to remove signals in "Ra" intrinsic to.-variations of phase in Rd (viz; the"S" channel) may be omitted, but with a corresponding reduction in performance. There is yet a further problem in providing continuously ground balancing detectors using the principle of correlation, namely as the sensing coils pass over is . a target object in the ground, the presence of the "X" and "R" signal from the target modifies the ' measured "X" and "R" signal from that of the local soil. Indeed it is precisely this modification which Is used to detect the presence of an object To overcome the difficulty of the correlator system attempting to correct the phase angie of the "R" channe.1 owing to the measured resultant of the ground plus object, 20.. thereby unbalancing the detector from the ground balanced phase position, the integrator is inhibited by a switch 46. The switch is opened if the averaged absolute value of Rm is ' exceeded by a fixed multiple of the absolute value of Rm . This relatve level sensing of Rm is processed in the object sensor 20. The fixed 90 * phase shifter in Fig_ 1 may be some other angle than 90 * , for 25. example, 45 β . This may be advantageous in situations where the sought after objects have a specific loss to inductive ratio, for example 45 degrees, in which case the correlator does not attempt to correct for the object, but rather, to a first order approximation ignores it and is only sensitive to the ground. 30. It should be noted however that the elimination of the unwanted components intrinsic to the process of changing the phase angle, Is most accurately achieved when the phase Is set approximately 90' relative to the transmitted phase plus the phase shift of the preamplifier. The above control, which adjusts the variation of the gain of the feedback loop in Fig. 1 can be performed manually for "manual" detectors. In this instance, the operator perceives the tuning-in time or Instability of the detector and adjusts the feedback gain accordingly. 5. This does however require that the operator perform adjustments regularly in relatively inhomogeneous ground for optimum results. In practice this procedure Is not desirable because if the reactive component suddenly increases, the feedback loop can become unstable which may result in a substantial rapid phase angle change. Upon reducing the feedback gain ιo. to regain stability, a lengthy correction period Is required. Using the control described above, can be made possible, and the tuning-in time can be set by the operator to obtain the best compromise between the tuning-in time and stability. This adjustment can be achieved by varying the "fine gain controls" In * Fig.1. is . The AC. output of the detector is shown as output 2 in Fig.1 where the phase correction "noise" has been removed from the "R" signal. Note: the RC decoupling combinations simply remove d.c. offsets from the divider, and depending on the technology, may not be necessary. In addition some of the functional blocks may be placed in different positions, for example the multiplier 20. 15 may be placed between the subtractor 1 1 and the divider 13, without changing the principle of operation. The system can be realised in many different types of electronic technologies, for example Fig. 2 shows the correlation principle employed to determine the amount of high passed "X" signal present in the high passed "R" 25. channel and then subtracting this component directly from the high passed "R" channel without changing the phase angle. This procedure requires very high precision with a large dynamic range to cover all possible ground conditions and can be implemented (though not necessarily ) with digital techniques using a microprosessor. The signal from the 30. x ι and "R" 2 channel pass through low pass filters 4,5 to prevent aliasing and are then converted to digital form using high precision analog to digital converters 3,6. They are then ( digitally ) filtered 7,8 and a fraction of the "X" channel ( determined by the correlation ) is then ( digitally) subtracted 1 1 from the "R" channel which Is then ( digitally ) low pass filtered 14 to reduce noise to produce the output channel. The correlator operates in the same way as previously described with the 5. sign of the "X" channel 12 multiplying the processed "R" channel 15 and integrated 17 to determine the extent of the "X" channel remaining in the "R" channel. The usual sensing procedures is used to determine and correct 13 for the gain in the feedback loop. Again the integrator is inhibited by the same procedure described above, that Is when the output signal deviates from zero by a ID . statistically significant amount 20, the integrator is Inhibited by the opening of the.- switch 46. The functional blocks can again be shifted without changing the basic principle of operation. For example, the division process can preceed the correlation process before the integration process. Here the microprocessor can is. be used to advantage by the implementation of various "adaptive" processes such as filters which continually adjust according to the various ground conditions. Fig.3 shows a typical time trace for the "X" and "R" channels for the detector as the correlator corrects for the "X" component present in the "R" signal. Note the strong correlation between the resistive channel and the reactive channel 20. at the start of the trace. As the phase control adjusts the phase angle, the component of "X" in "R". is reduced to almost zero with only the "true" resistive component remaining. * Some dry sandy beaches and other soils may produce very little interfering signal, while other ground conditions containing perhaps heavy Ironstone (ferrous 25 : . oxide) concentrations, and/ or mildly electrically conducting components such as some clay deposits, moist salty conditions, produce severe interference, making detection of valuable objects in such soils difficult and In some cases almost Impossible. One reason for the importance of obtaining a detection system which can 30. operate under these conditions is that such conditions are often encountered when prospecting for gold. They result from the weathering of the ironstone-quartz reefs which contain the gold. The ironstone mineralisation is distributed in wide areas around the reef and Is bound in the clays and soils which often contain the gold nuggets originating from the reef. There are well known techniques for minimising this interference from the ground which will become apparent as the content of this invention Is explained. The input signal from the receiver coil goes to the circuit 21 consisting of two amplifiers 22 and 23. The demodulators 1 and 2 switch the signal using 5. MOSFET switches 24 and 25. Switch 24 produces the reactive channel and switch 25 produces the resistive channel. After demodulation the signal passes through low pass filters 4, 5 which remove high frequency components of the switching. From filter amplifiers 27 and 28 the signal then passes to high pass IΌ .. filters 7 and 8. Filter 7 consists of a single pole RC filter followed by a high gain amplifier 30. The D.C. output signal is taken from the output of amplifier 28 (output 1 ). From the high pass filter 8 (line C) the signal goes to subtracting circuit 1 1 which uses an amplifier 54 operating as a summing amplifier. As the phase of the signal is reversed in the S circuit the summing is. amplifier subtracts unwanted phase shift signal from the "R" channel. The signal from high pass filter 7 and amplifier 30 is further amplified at 53 and 52 to produce a clipped wave form to operate switches 49 and 50. From the output of subtractor 1 1 an output A.C. signal (output 2) can be taken from the amplifier 54. A signal is also taken from the output of the "X" channel low pass filter 4 (line A) 20. and goes to the peak de * tector and droop circuit 10. This signal is first full wave rectified by amplifier 33 and then the peak is determined by the action of the amplifier 34 charging the capacitor 35 through a diode. The capacitor is discharged with a time constant t determined by resistive chain 36. In this way the circuit "memorises" the magnitude of the reactive signal 25. * which is present and this signal becomes the divisor of a dividing circuit 13. To obtain the required dynamic range, precision, and temperature stability necessary for the division a digital/analog hybrid circuit dividing circuit 13 is used. Two 4 bit up/down counters 41, 42 drive two 8 bit digital to analog 30. converters (D/A) 39, 40 which use the same data bits. These two D/A converters track each other and one 40 uses a feedback circuit which includes the comparitor 38. The sign of the comparator determines whether the counters count up or down and in this way the output from 40 tracks the output from 37. The clock rate for the counters 41, 42 and the data latch 47 are taken from a divided frequency from the crystal oscillator circuit 71. 5 « The multiplying D/A 39 and associated operational amplifier are connected as a dividing circuit with one Input (dividend) coming from subtractor 54 and the other (divisor) from peak droop circuit 37 through the 8 input bits. The output of the divider circuit passes through a high pass filter 43, 44 to a voltage follower driver 48, the output of which is switched by the switches 49, 50. i Q . The purpose of the high pass RC filter is to remove any D.C. component from the divider. The multiplying circuit 15 multiplies the signal from the divider by the sign of the high passed signal from the "X" channel through the action of the inverting/noninverting switches 49, 50 and amplifier 51. is . This signal then passes to the integrator circuit 17 composed of integrating amplifier 45 together with an integrating gain controller 16. The integration process is inhibited if the FET switch 46 is opened. This is enabled by the action of. the object detecting circuit 20 which has its input T, as the "AC" ouput It comprises a full-wave rectifier 84 and 83, and an averaging 20. circuit, namely, the low pass filter 81. If a multiple of the absolute value of the input signal exceeds the averaged value, the comparing amplifier 82 opens the switch and thus inhibits the Integrator. Line G from the integrator alters the phase angle of the demodulating circuit by controlling the voltage controlled phase shifting circuit 19. This 25. circuit is driven by the crystal oscillator 70 and dividing circuit 72 and consists of a phase locked loop whose phase follows the phase of the current signal in the transmitting Tx coil through the action of the resistance 59 and amplifier 62. Driver amplifiers 55,56,57, 58, drive the resonant tank circuit composed of the inductance of the transmitting coil and capacitor 60. The value of capacitor 60 is adjusted to make the natural resonant frequency of the tank circuit equal the driving frequency. Filter 73 filters out the harmonics from the crystal oscillator frequency dividing circuit 72 and drives the coil drivers (line R). Output from 62 passes through the demodulator 63 which 5. . is Integrated 65 to control the switching level point on a quasi triangular wave generator from the oscillator 70 and by this means lock the phase of the output of 68 to the phase at the 62 (phase shift 90 * ). The additional 90 * phase shift from 69 drives the "R" demodulator. Phase shift is controlled by the voltage on line G which causes a current to flow to the 10. virtual earth of the inverting input of the integrator causing a corresponding shift in the D.C. component at the demodulator 63 output to maintain locked phase. Thus the phase of the phase locked loop Is shifted with respect to the output of 62. The phase range can be centred with the potentiometer 66. The demodulator 3 Is also driven by the current sensing amplifier 62 with is . the output of the demodulator passing through the low pass filter 6 and high pass filter 9. Line J from the high pass filter goes to multiplier 12 which uses an A/D tracking converter which operates on the same principle as the divider. However in this case line J goes to the reference of the multiplying D/A 76 where the signal is multiplied by the- 10 bits to produce an output signal 20.. attenuated by 80 and . going to line H. Tracking is accomplished using the comparitor 74. Potentiometer 80 is used to change the gain from the multiplier and this is adjusted to null out the inherent phase shift signal present in the "R" channel.

Claims: 1. Apparatus for detecting remote metal objects in soil by interrogation using substantially sinusoidal magnetic transmitted signals for such Interrogation the apparatus including a correlator, the apparatus being characterised In that there are means to e fect at least two signals derived 5. from incoming interrogation signal or signals by synchronous demodulation referenced to the said transmitted signal, the apparatus being adapted such that one of the said signals contains information distinctive of changes during different soil interrogation over time In soil background magnetic response and any target metal object at the transmitted frequency, and the 10. . other signal will contain substantially that Information arising from changes over time in the said retransmitted magnetic field from interrogation from a moving source of transmitted signal relative to a target of any target conducting metal object, but relatively little information- arising from differences in the ground background magnetic is. . response from Interrogated location to location, means to provide for amplitude cross correlation between the two signals by means of the correlator, the said correlator being adapted to alter the said second signal in such a way that the said correlation between the said signals is minimised within a selected response period as the apparatus interrogates 20. different areas of soil locations. A metal detection system for detecting the presence of target conducting metal targets in soil, where the interrogated soil in the absence of conducting metal targets is predominantly characterised by materials exhibiting significant magnetic reponse and having predominant enhansing 5. - and relatively non-disipative properties of magnetic fields at frequencies below one hundred klloHertz, Including detector means, Including a coil means, and oscillator means for generating an alternating magnetic signal and means for detecting any retransmitted magnetic signal from the ground and any target, the retransmitted signal processing means including a first lo. and second synchronous demodulator referenced to the said generated magnetic field to interrogate any said received signal and provide a lowpassed output by means of a first and second low pass filter connected to the first and second synchronous demodulator respectively, each low pass filter providing a first and second signal respectively, the said first is: . and second signal being passed through a first and second high pass filter respectively to produce a third and fourth signal respectively, In the third , whereby the signal contains substantially Information distinctive of changing background magnetic response of the soil from location to location 2Q__: . over time and changing magnetic reponse of conducting metal objects as the said generated magnetic field is moved relative to the target object, in the fourth case with a second synchronous demodulator reference phase selected whereby the signal contains information arising substantially from any changes in the retransmitted magnetic signal of a target metal 25. conducting object as the said generated magnetic field is moved relative to the target object, but relatively little information arising from changes distinctive of background magnetic response of the soil from location to location over time, then adapted to effect a change on a continuous or regular repeating basis in the said second phase position by means of a 30. controlled phase shifter, the said second phase position being effected by control of the integrated cross correlation between the said third and fourth signal in such a way that the said cross correlation Is minimised within selected response time as the said coil interrogates different locations of soil, the said cross correlation being implemented by means of a correlator 35. and the integration by means of an Integrator, whereby the integration process is inhibited if a presence of a metal object is substantially manifest In the said fourth signal. . 3. A metal detection system for detecting the presence of target conducting metal targets in soil, where the interrogated soil In the absence of conducting metal targets is predominantly characterised by materials having a magnetic reponse having predominant enhansing and relatively non- 5. dlsipative properties of magnetic fields at frequencies below one hundred kiloHertz, Including detector means, Including a coil means and oscillator means for generating an alternating magnetic signal and means for detecting any retransmitted magnetic signal from the ground and any target, the retransmitted signal processing means including a first and second lo . synchronous demodulator referenced to the said generated magnetic field to interrogate any said received signal and provide a lowpassed output by means of a first and second low pass filter connected to the first and second synchronous demodulator respectively, each low pass filter providing a first and second signal respectively, the said first and second is. signal being passed through a first and second high pass filter respectively to produce a third and fourth signal respectively, in the third case, with the said first synchronous demodulator reference phase selected whereby the signal contains substantially information distinctive of changing background magnetic response of the soil from location to location over 20. time and changing magnetic reponse of conducting metal objects as the said generated magnetic field Is moved relative to the target object, in the fourth case with a second synchronous demodulator reference phase selected whereby the ' signal contains Information arising substantially from any changes in the retransmitted magnetic signal of a target metal 25. conducting object as the said generated magnetic field is moved relative to the target object, but relatively little information arising from changes distinctive of background magnetic response of the soil, then adapted to effect a change on a continuous or regular repeating basis in the said fourth signal by substracting a selected proportion of the said third signal from 30. the said fourth signal to give a fifth signal, where the said proportion is controlled by an integrated cross correlation between the said third and fifth- signal in such a way that the said cross correlation is minimised within selected response time as the said coil interrogates different soil locations, the said cross correlation being Implemented by means of a 35. correlator and the integration by means of an integrator, whereby the integration process is inhibited if a presence of a metal object is substantially manifest in the said fifth signal. 4. Apparatus as in claim 2 further Including means for controlling the gain of a feedback control containing the said correlator, the said integrator and the said controlled phase shifter. 5. r Apparatus as ' in claim 3 further Including means for controlling the gain of a feedback control containing the said correlator, the said integrator and the said proportionality constant. 6. Apparatus as In claim 4 wherein the means controlling the said gain of the said feedback is selected to be inversely proportioned to a selected mean absolute peak value of either the said first or third signal. 7. Apparatus as in claim 5 wherein the means controlling the said gain of the said feedback is selected to be inversely proportion to a selected mean absolute peak value of either the said first or third signal. 8. Apparatus as in claim 6 further characterised In that the said mean peak detector is adapted to select its value, by memorising peaks of the absolute value of the input signal, whereby the said memory decays In magnitude with a selected response time until the said absolute value of the . said memorised value Is updated by the currently existing peak value. 9. " Apparatus as in claim 7 further characterised that the said mean peak detector is adapted to select Its value, by memorising peaks of the . absolute value of it's input signal, whereby the said memory decays in magnitude with a selected response time until the said absolute value of the 5. input signal exceeds the said decaying memorised value at which stage the said memorised value is updated by the currently existing peak value. 10. Apparatus as In claim 8 wherein the value of the said peak memorised signal is constrained to be greater than a selected lower limit 1 1. Apparatus as in claim 9 wherein the value of the said peak memorised signal is constrained to be greater than a selected lower limit 12. Apparatus as in claim 10 wherein the said first low pass filter has substantially the same temporal response as the said second low pass filter, the said first high pass filter has substantially the same temporal response as the said second high pass filter, Including means to generate a 5. fifth signal which is proportional to changes in the phase of the said phase reference of the said second synchronous detector relatve to the said generated magnetic signal, the said fifth signal being produced either by synchronously demodulating the generated magnetic signal using the a phase reference fixed relative to the said reference phase of the second 10. demodulator, or by determining the control level to the said controlled phase shifter if the relationship between this level and the resulting phase shift Is linear, for * either method of determining the phase, the said determined phase angle amplitude is passed through a third low pass filter of the same temporal characteristics as the said second low pass filter, is . this said low passed phase signal Is passed through a third high pass filter of the same temporal characteristics as the said second high pass filter, this said high passed phase signal being the said fifth signal which Is multiplied by the said first signal by means of a first multiplier; a selected fixed proportion of this said product being subtracted from the said fourth 20.. signal to give a sixth signal, whereby the said proportion of the said product Is selected so that components in the said fourth signal intrinsic to changes in the phase of the said phase shifter are substantially cancelled; the said sixth signal being divided by the output of the said mean peak detector by a divider, the quotient of the said divider being either passed directly to a 25. secon mu p er, e sa corre a r, passed by a fourth high pass filter; the said correlator adapted to multiply this- said quotient or it's said high passed signal by the sign of the said third signal; the product of the correlator being applied to the said integrator which Is inhibited if the absolute value of the sixth signal 30. exceeds some selected value, thereby indicating the possible presence of a conducting metal object within the changing Influence of the generated magnetic field; the output of the integrator adapted to control the said phase shifter; the said sixth signal being useful for Indicating information about potential presence of conducting objects within the influence of the 35. generated magnetic field as it contains no information related intrinsically to phase shifts and furthermore substantially has most of the dominant background soil Interrogated signal removed owing to the correlation feedback process. IX Apparatus as in claim 1 1 wherein the said first low pass filter has substantially the same temporal response as the said second low pass filter, the said first high pass filter has substantially the same temporal response as the said second high pass filter, the said fifth signal being 5. multiplied by the sign of the said third signal by means of a first multiplier, the said correlator; the product of the said correlator being divided by the output of the said mean peak detector; the quotient of the said division being integrated by the said integrator; the said integrated signal adapted to control the said controlled attenuator; the said integrator being inhibited if lo . the absolute value of .the said fifth signal exceeds a selected value, thereby indicating the possible presence of a conducting metal object within the changing influence of the generated magnetic field; the said fifth signal is useful for indicating Information about potential presence of conducting objects within the influence of the generated magnetic field as it is _ . substantially has most of the dominant background soil interrogated signal removed owing to the correlation feedback process. 14. Apparatus as in claim 10 wherein the said first low pass filter has substantially the same temporal response as the said second low pass filter, the said first high pass filter has substantially the same temporal response as the said second high pass filter, including means to generate a 5. . fifth signal which is proportional to changes in the phase of the said phase reference of the said second synchronous detector relatve to the said generated magnetic signal, the said fifth signal being produced either by synchronously demodulating the generated magnetic signal using a phase re 1 0. demodulator, or by determining the control level to the said controlled phase shifter if the relationship between this level and the resulting phase shift is linear, for either method of determining the phase, the said determined phase angle amplitude being passed through a third low pass filter of the same temporal characteristics as the said second low pass is. filter, this said low passed phase signal being passed through a third high pass filter of the same temporal characteristics as the said second high pass filter, this said high passed phase signal being the said fifth signal which is multiplied by the said first signal by means of a first multiplier; a selected fixed proportion of this said product being subtracted from the 20.. said fourth signal to give a sixth signal, whereby the said proportion of the said product is selected so that components in the said fourth signal intrinsic to changes in the phase of the said phase shifter are substantially cancelled, the said sixth signal being multiplied by the sign of the said third signal by means of a second multiplier, the said correlator; the 25.. product of this said correlator being divided by the output of the said mean peak detector by a divider; the quotient of the said divider being applied to the said integrator which is inhibited if the absolute value of the said sixth signal exceeds some selected value, thereby indicating the possible presence of a conducting metal object within the changing influence of the 30. generated magnetic field; the output of the integrator controlling the said phase shifter; the said sixth signal being useful for indicating information about potential presence of conducting objects within the influence of the generated magnetic field as it contains no information related intrinsically to phase shifts and furthermore substantially has most of the dominant 35. background soil interrogated signal removed owing to the correlation feedback process. 15. Apparatus as in claim 1 1 wherein the said first low pass filter has substantially the same temporal response as the said second low pass filter, the said first high pass filter has substantially the same temporal response as the said second high pass filter, the said fifth signal being 5. adapted to be divided by the output of the said mean peak detector; the quotient of the said division being multiplied by the sign of the said third signal by means of a first multiplier, the said correlator; the product of the said correlator being integrated by the said Integrator; the said integrated signal adapted to control the said controlled attenuator; the said integrator lo. being inhibited if the absolute value of the said fifth signal exceeds a selected value, thereby indicating the possible presence of a conducting metal object within the changing influence of the generated magnetic field; the said fifth signal being useful for Indicating Information about potential presence of conducting objects within the Influence of the generated is.. magnetic field as it substantially has most of the dominant background soil interrogated signal removed owing to the correlation feedback process. 16. Apparatus for detection of conducting object substantially as described in the specification with reference to and as illustrated by the accompanying Illustrations.

Inventors: CANDY BRUCE HALCRO, CANDY, BRUCE, HALCRO
Applicants: MINELAB ELECTRONIC LTD, MINELAB ELECTRONIC INDUSTRIES LTD.
Application year: 1986
Main classification: G01V3
IPC codes: G01V3