AU2013206236A1 - Metal detector having constant reactive transmit voltage applied to a transmit coil - Google Patents

Abstract

An electronic metal detector having, a transmit coil arranged and adapted to transmit an alternating magnetic field associated with a reactive transmit coil voltage, the transmit coil being connected to transmit electronics arranged and adapted to generate a transmit signal, the transmit electronics having at least two power sources, a first power source and a second power source, wherein the first power source is adapted and arranged to connect to the transmit coil for at least a first period, and the second power source is adapted and arranged to connect to tie transmit coil for at least a second period, the said transmit electronics including at least one servo control negative feedback loop, a first servo control negative feedback loop, which is adapted and arranged to monitor a transmit coil current for at least part of the said first period, and to control at least part of the said transmit signal, the transmit electronics being adapted and arranged to control the transmit signal to produce the reactive transmit coil voltage to be approximately constant and approximately equal to zero whilst the transmit coil current is non-zero and approximately constant for at least part of the first period; and receive electronics which are adapted and arranged to receive and process a receive magnetic field during at least some of the first period to produce an indicator output. 6a 6b -- 23 24-; ... 21 8 29b 9a Sa 5b 14 15 10l Figure 1

Description

METAL DETECTOR HAVING CONSTANT REACTIVE TRANSMIT VOLTAGE APPLIED TO A TRANSMIT COIL This invention relates to a metal detector which transmits at least some periods of constant reactive transmit coil voltage equal to approximately zero whilst a transmit coil current is non-zero, and a 5 method of assisting accuracy of this constant reactive voltage generation. This transmission has particular application to metal detectors which are to operate in environments which may contain magnetic soils, are to be insensitive to transmit induced magnetic signals from such soils, including effects of such soils modulating the transmit coil's inductance and also including varying soil magnetic coupling of transmit coil with a receive coil, whilst the metal 10 detector is to produce an indicator output responsive to metal targets. TECHNICAL FIELD The general forms of most metal detectors which interrogate soils are either hand-held battery operated units, conveyor-mounted units, or vehicle-mounted units. Examples of hand-held products include detectors used to locate gold, explosive land-mines or ordnance, coins or treasure. Examples 15 of conveyor-mounted units include fine gold detectors in ore mining operations, and an example of a vehicle-mounted unit includes a unit to search for land-mines. These units usually consist of a transmit coil which may transmit an alternating magnetic field associated with a reactive transmit coil voltage, transmit electronics which may generate a transmit signal applied to the transmit coil, and receive electronics which may receive a magnetic field and process received signals to produce 20 an indicator output. The received magnetic field may be detected by a receive coil in most metal detectors, or the transmit coil in some metal detectors such as pulse induction units. The most numerous products of the above examples are the hand-held battery-operated products. It is desirable that these have good buried target detection range, especially in magnetic soils which may contain ferrimagnetic materials. Such metal detectors comprise receive electronics which processes a 25 received magnetic field such that the indicator output is responsive to metal targets buried in such soils but not responsive the magnetic soils.

2 US4942360, US5576624, US5537041, US6636044, US6653838, AU2006905485 describe various means of cancelling signals arising from magnetic soils. This invention is an improvement on these and WO 2005/047932. The below table lists these patents, the method disclosed for each, the relative problems of each 5 method and the relative advantage of this invention. Patent Disclosure Problems . New advantage 4942360 Simultaneous multi- Difficulty in linear Eliminates linear frequency with selected electronic demodulation problem receive frequency demodulation. during reception. Phase component linear Difficult to set reference problem combinations absolute phase effectively eliminated reference 5576624 Pulse induction with Overcomes above Higher sensitivity to slow selected receive time problems but time constant targets and domain component linear relative more power efficient combinations insensitivity to slow time constant targets. Poor power efficiency 5537041 Multi-period rectangular Difficulty in linear Eliminates linear pulses with selected electronic demodulation problem receive time domain demodulation. during reception. Phase component linear Difficult to set reference problem combinations absolute phase effectively eliminated. reference 6636044 Improved signal Applies to this invention processing signal-to noise 3 6653838 Multi-voltage pulse Improved signal- See 5576624 induction to-noise version of 5576624 with same problems 2005/047932 Constant transmit coil Improved transmission reactive voltage when method suitable for transmit coil current is efficient power non-zero consumption and accurate maintenance of constant reactive voltage 2006905485 Improved method for Applies to this invention magnetic soil signal suppression For state-of-the-art metal detectors metal target signals at the limit of the electronic noise produce voltage signals in a receive coil of the order of ten nanoVolts. In a metal detector with a "nulled" transmit and receive coil, the varying reactive voltage component from the highly magnetic soils, found in most Australian goldfields for 5 example, may be typically of the order of tens to hundreds of milliVolts across the receive coil. Reactive components are usually symbolised as 'X." Resistive soil components, usually symbolised by "R," are typically of the order of a hundred times less than the X component. The R soil component is predictable and thus the R soil component may be nulled as described in the patents in the table. As X is poorly 10 correlated with R in magnetic soils and thus unpredictable, the cancellation of the effects of X is essential in order to accurately null magnetic soil signals. X contamination in a resistive demodulated signal (and low pass filtered) is a problem for any metal detector system which transmits non-zero reactive voltages during resistive synchronous demodulation, such as for example multi-frequency sine-waves. One 15 problem of X contamination of resistive synchronous demodulation is receive electronics non-linear behaviour. In order that this component is less than the smallest detectable metal target signals, the receive electronics must be accurately linear to an order of 10. to 10" This is difficult and especially difficult at tens of kHz or higher. In 4 order that any absolute "phase" inaccuracy of a metal detector system which transmit non-zero reactive voltages during resistive synchronous demodulation does not significantly impede the cancellation of signals from magnetic soils, the absolute X contamination of the resistive components should be about two orders of magnitude less than the absolute resistive components. 5 Hence, the absolute accuracy of the "phase" needs to be of the order of 104. This too is difficult. The transmit coil may be approximated as an effective inductive component L, in series with an effective resistive component R, which may for mathematical convenience include resistance of cabling and connectors and some elements of the transmit electronics. Thus the transmit coil plus electronics effective resistance has the effective transmit coil time constant t = L/R. As v=Ld(i)/dt 10 where v is the transmit coil reactive voltage, if the transmit coil current i is constant, then v=0. Thus maintaining the transmit coil current constant for a period results in a zero transmit coil reactive voltage. The applied voltage across the transmit coil is u = v+iR. Hence when the transmit coil reactive voltage is zero, the signal applied across the coil including the transmit coil cable resistance is iR. These equations assume transmit coil, cable and electronics with no stay capacitance or 15 parallel resistance. For a given transmit alternating waveform repeating sequence, and for a constant coupling between the transmit coil and the receive coil, the receive coil voltage waveform will be of constant magnitude and form if the transmit coil alternating reactive voltage waveform v(t) is of constant magnitude and form. As i is proportional to I /L for a constant transmit coil reactive waveform v(t), the transmit coil current waveform i(t) is modulated by I /L and so too therefore is 20 Ri(t), and t too. Thus the applied voltage waveform need be modified as the transmit coil inductance is modulated. A method of maintaining a constant reactive transmit coil voltage waveform is described in WO 2005/047932 Al. Bi-polar transmitting cw systems such as sine-waves and rectangular-waves and examples given below of this invention have an advantage over similar fundamental frequency and power 25 consumption pulse induction systems in that they have intrinsically more gain to long time constant targets, such as large gold nuggets or unexploded ordnance. For example the resistive component of many cw systems asymptotically decreases as I /(target time constant) assuming the target has an 5 effective principal time constant, whereas a pulse induction system normally have a response proportional to 1 /(target time constant) 2 . SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a method for detecting a target 5 in a soil, the method including the steps of: generating a transmit signal, the transmit signal includes a repeating sequence, the repeating sequence includes a first high-voltage period and a first low-voltage period; transmitting a transmit magnetic field by applying the transmit signal to a transmit coil; sensing a current through the transmit coil during at least part of the first low-voltage period 10 of a first sequence of the repeating sequence to produce a first control signal; controlling at least part of the transmit signal during at least part of the first low-voltage period of the first sequence and/or during at least part of the first low-voltage period of one or more subsequent sequences using the first control signal, with the transmit signal during the first high voltage period unchanged, for maintaining the reactive transmit voltage across the transmit coil 15 during subsequent first low-voltage periods of the transmit signal at approximately constantly zero voltage, even when an inductance of the transmit coil is modulated by the soil, with a corresponding non-zero current through the transmit coil; receiving a receive magnetic field using a receive coil to produce a receive signal; processing the receive signal during at least a part of the first low-voltage period; and 20 producing an indicative output signal indicating the presence of the target based on the processing of the receive signal. In one form, the controlling at least a part of the transmit signal includes controlling a magnitude of the voltage of the transmit signal during the first low-voltage period. 25 In one form, each repeating sequence further includes a second high-voltage period and a second low-voltage period, the method further includes the steps of: sensing a current through the transmit coil during at least a part of the second low-voltage period of the first sequence to produce a second control signal; 30 controlling at least part of the transmit signal during at least part of the second low-voltage period of the first sequence and/or during at least part of the second low-voltage period of one or more subsequent sequences using the first control signal, with the transmit signal during the second high-voltage period unchanged, for maintaining the reactive transmit voltage across the transmit coil during subsequent second low-voltage periods of the transmit signal at approximately constantly 35 zero voltage even when an inductance of the transmit coil is modulated by the soil, with a corresponding non-zero current through the transmit coil; 5a wherein the processing includes processing the receive signal during at least part of the second low-voltage period. 5 In one form, the polarity of an average voltage during the first high-voltage period is opposite that of an average voltage during the second high-voltage period. In one forn, the method further includes the steps of: sensing a difference in an absolute value of a current through the transmit coil during the 10 first low-voltage period of a sequence and that of a current through the transmit coil during the second low-voltage period of the same or a different sequence; and controlling a ratio of energy transferred to the transmit coil during the first high-voltage period and the second high-voltage period of a subsequent sequence such that the difference in the absolute value of a current through the transmit coil during the first low-voltage period of the 15 subsequent sequence and that of a current through the transmit coil during the second low-voltage period of the subsequent sequence is approximately zero. In one form, a waveform of a current through the transmit coil is approximately a rectangular wave with slew rates approximately proportional to a voltage applied to the transmit coil during the first 20 high-voltage period and a voltage applied to the transmit coil during the second high-voltage period. In one form, an absolute value of an average voltage during the second high-voltage period is lower than that of an average voltage during the first high-voltage period. 25 In one fonn, the polarity of a current through the transmit coil changes from one sign to opposite sign during the second high-voltage period. In one form, the processing the receive signal further includes processing the receive signal during at least a part of a period with non-zero reactive transmit voltage across the transmit coil. 30 In one form, the processing of the receive signal during at least a part of the first low-voltage period includes demodulating or sampling the receive signal to produce a resistive receive signal, the processing of the receive signal during at least a part of the period with non-zero reactive transmit voltage includes demodulating or sampling the receive signal to produce a reactive signal, the 35 processing of the receive signal further including the steps of: differentiating the reactive receive signal to produce a differentiated receive signal; and combining linearly the differentiated receive signal and the resistive signal.

5b In one form, timings and gains for demodulating or sampling the receive signal is selected such that the resistive receive signal is approximately intrinsically statistically insensitive to signals from magnetic soils but is responsive to magnetically permeable material rate of change of coupling between the transmit magnetic field and the receive magnetic field, and the combining linearly is 5 arranged and adapted to approximately cancel a component of the resistive receive signal which is responsive to magnetically permeable material rate of change of coupling between the transmit magnetic field and the receive magnetic field. In one form, the controlling includes producing a constant voltage applied to the transmit coil during 10 the first low-voltage period using a sample-and-hold element. In one form, the transmit signal is controlled to include more than one receive periods during which the reactive transmit voltage across the transmit coil is maintained approximately constantly zero with a corresponding current through the transmit coil being non-zero, each of the receive periods is 15 controlled by separate control signals, and wherein the processing of the receive signal during at least a part of each of the receive periods includes demodulating or sampling the receive signal to produce resistive receive signals. According to a second aspect of the present invention, there is provided a metal detector 20 configurable to perform the method according to the first aspect and/or its various forms.

6 This invention thereby addresses these problems by, in the alternative, disclosing apparatus and methods capable of accurately setting the transmit coil reactive voltage v to near zero. The advantage of having v=0 or close to this during demodulation is that the signal received by the receive electronics is mostly free of X and thus also 5 avoids significantly the non-linearities and absolute phase problem described above. This invention discloses an apparatus and method conducive to high power efficiency, accurate transmit electronics which may generate a transmit signal applied to the transmit coil, which may transmit an alternating magnetic field associated with a reactive transmit coil voltage, such that the transmit signal of the transmit electronics is controlled 10 to produce the reactive transmit coil voltage to be approximately constant and approximately equal to zero whilst a transmit coil current is non-zero and approximately constant for at least a period during which a magnetic field may be received and processed by receive electronics to produce an indicator output which is relatively insensitive to receive signals from magnetic soils, and having relatively high 15 sensitivity to relatively long time constant metal targets compare to comparable pulse induction systems, possible reduced sensitivity to environmental magnetic interference and improvement over Aus2006 905485. DESCRIPTION OF THE DRAWINGS To assist with the understanding of this invention, a description of embodiments will 20 now be made with the assistance of drawings wherein Figure 1 is the circuit of a electronic system according to a first embodiment capable of producing an approximately constant reactive transmit coil voltage which is approximately equal to zero whilst a transmit coil current is non-zero and approximate constant for at least a first period, and independent of transmit coil inductance 25 modulation by magnetic soils. Figure 2 shows transmit and receive waveforms resulting from the embodiment; this example being an approximate transmit coil current square-wave. Figure 3 is an example of an electronics system with more detail than the embodiment of figure 1.

7 Figure 4 is a further embodiment of this invention. Figure 5 is an example of transmit and receive waveforms from the circuit shown in figure 4. DESCRIPTION OF EMBODIMENTS In Figure 1, transmit coil I approximately may be represented by an effective resistive component 3 5 in series with an effective inductive component 2. This is connected to transmit electronics 4, 5a, 5b, 6a, 6b, 9a, 9b, 16, 17, 19 and 20 which may generate a transmit signal applied to the transmit coil 1. A first power source 5a, connected to the system common ground 4, is connected to switch 21. A third power source 5b, connected to the system common ground 4, is connected to switch 22. Switch 21 and 22 are controlled to be "on" or "off' by timing control electronics 16 via control lines 17. 10 Switch 25 and switch 26 are connected to the system common ground 4 via resistor 8, to switch 21 and switch 22 respectively, and also to the transmit coil 1, and switch 23 and switch 24 respectively. Switch 23 is connected to a second power source 6a which is connected to the system common ground 4, and switch 24 is connected to a fourth power source 6b which is also connected to the system common ground 4. Switch 25, switch 26, switch 23, and switch 24 are controlled to be "on" 15 or "off" by timing control electronics 16 via control lines 17. Thus switch 21 connects or "switches" the transmit coil to the first power source 5a, switch 23 switches the transmit coil to the second power source 6a, switch 22 switches the transmit coil to the third power source 5b, switch 24 switches the transmit coil to the fourth power source 6b, switch 25 and switch 26 switch the transmit coil to the system common ground 4 via resistor 8. These switches 21, 22, 23, 24, 25, and 26 and 20 resistor 8 may be said to form switching electronics 20. Resistor 8 is of low resistive value and used for transmit coil current sensing by feeding a voltage across resistor 8 to an input of a first servo loop amplifier 9a, and a third servo loop amplifier 9b and a fourth servo loop amplifier 19. An output of the first servo loop amplifier 9a is connected to a control input of the first power source 5a, such that the first power source 5a, resistor 8, first servo loop amplifier 9a and the said switching 25 electronics 20 form a first servo control negative feedback loop which controls current flowing through resistor 8 to be approximately constant at least during a first period. The outputs of the second power source 6a and the fourth power source 6b are constant voltages, which may be 8 maintained by a shunt regulator or local servo control negative feedback loops. An output of the third servo loop amplifier 9b is connected to a control input of the third power source 5b, such that the third power source 5b, resistor 8, third servo loop amplifier 9b and the said switching electronics 20 form a third servo control negative 5 feedback loop which controls current flowing through resistor 8 to be approximately constant at least during a third period. Only one of switch 23, switch 25 and switch 21 are closed at any instant and only one of switch 24, switch 26 and switch 22 are closed at any instant. During the first period either switch 25 and switch 22 are closed, or switch 26 and switch 21 are closed. 10 During the second period either, switch 23 and switch 26 or 22 are closed, or, switch 24 and switch 25 or 21 are closed. Receive coil 29 receives a magnetic field via receive inductor 12 under the influence of an alternating magnetic field generated by the transmit coil 1. Receive coil 29 is connected to an amplifier 13 whose output is connected to synchronous demodulators 15 14 which are controlled by the timing electronics 16 via control lines 18. An output of synchronous demodulators 14 is connected to further receive electronics processing 15 which includes an indicator output 10. Amplifier 13, synchronous demodulator 14 and further receive electronics processing 15 may be said to form receive electronics 11. Transmit coil 1 and receive coil 12 may be said to form a metal detector sensing 20 coil 19. The synchronous demodulation may include similar principles of US6636044. The first power source 5a and third power source 5b may be combined into a single power source, a first combined power source, wherein its output is controlled to produce an output when switch 21 is closed during a first period at a voltage controlled by an output of the first servo loop amplifier 9a, and when switch 22 is closed during a 25 third period, the output voltage of the combined first power source is controlled by an output of the third servo loop amplifier 9b. The second power source 6a and fourth power source 6b may be combined into a single power source, a second combined power source, wherein its output may be controlled by an output of a single servo loop amplifier which monitors a voltage output of the second combined power source 30 to form a servo control negative feedback loop to maintain the output of the second combined power source at an approximate constant voltage. However, the second combined power source may consist of a simple power supply regulated with a shunt 9 constant voltage element, such as a zener diode for example, and have no need of any servo control negative feedback loop to maintain the output voltage. The transmit electronics in figure 1 may generate a transmit signal which when applied to the transmit coil 1 may transmit for example a bipolar alternating magnetic field containing at least a 5 first period with a reactive transmit coil voltage to be approximately constant and approximately equal to zero whilst a transmit coil current is non-zero. To assist with the understanding of the operation of the figure 1, figure 2 shows an example of possible signal waveforms. Figure 2 is an example of a transmit waveform symmetric both in time and polarity, which in this example, produce an approximate transmit coil current square-wave 5 1, 10 52, 53 and 54. Waveform 40, 41, 43 and 44 shows the reactive transmit coil voltage. A first period commencing at the transition between 43 and 40 and terminating at time 56, the transition between 40 and 44, is a period with zero reactive transmit coil voltage 40 whilst a non-zero constant transmit coil current 51 flows. Similarly in reversed transmit coil current flow sense compared to the first period, a third period commencing at the transition between 44 and 41 and terminating at time 55, 15 the transition between 41 and 43, is another period with zero reactive transmit coil voltage 41 whilst a non-zero constant transmit coil current 52 flows. During the first period, the first power source 5a is applied to the transmit coil, with for example switch 21 closed and switch 23 and switch 25 open. The opposite end of the transmit coil is connected to the system ground 4 via resistor 8 and via switch 26 being on, and switch 24 and switch 22 being off. The actual voltage applied across the 20 transmit coil 45 during the first period is approximately the reactive transmit coil voltage 40 plus a voltage resulting from the transmit coil current flowing through the transmit coil effective resistance 3. During a second period between time 56 and the transition between 44 and 41, the transmit coil is switched to the second power source 6a. This is shown as 44 in figure 2 and is reduced in scale for clarity. During the second period, the reactive voltage across the transmit coil is roughly the same as 25 the actual voltage across the transmit coil assuming this voltage is much higher than the voltage applied across the transmit coil 45 during the first period. During this second period, switch 23 is closed and switch 21 and switch 25 are open. The opposite end of the coil may be switched to either the system ground 4 via resistor 8, with switch 26 closed and switch 22 and switch 24 open, or to the third power source 5b with switch 30 10 22 closed and switch 26 and switch 24 open. Alternatively, during the second period, this end of the coil may be switched to the third power source for some of the third period and system ground for a different period within the second period. The transmit coil is switched to the third power source 5b in a reverse polarity sense 46 compared to 45 (when switched to the first power source) during a 5 third period. During the third period, the third power source 5b is applied to the transmit coil, with for example switch 22 closed and switch 24 and switch 26 open. The opposite end of the transmit coil is connected to the system ground 4 via resistor 8 and via switch 25 being on, and switch 23 and switch 21 being off. During a fourth period between time 55 and the transition between 43 and 40, the transmit coil is switched to the fourth power source 6b in a reverse polarity sense to the second 10 period. This is shown as 43 in figure 2 and is reduced in scale for clarity. During the fourth period, the reactive voltage across the transmit coil is roughly the same as the actual voltage across the transmit coil assuming this voltage is much higher than the voltage applied across the transmit coil 46 during the third period. During this fourth period, switch 24 is closed and switch 22 and switch 26 are open. The opposite end of the coil may be switched to either the system ground 4 via resistor 15 8, with switch 25 closed and switch 21 and switch 23 open, or to the first power source 5a with switch 21 closed and switch 25 and switch 23 open. Alternatively, during the fourth period, this end of the coil may be switched to the first power source for some of the fourth period and system ground for a different period within the fourth period. The transmit coil approximately square-wave has transitions 54 and 53 occurring during the second and fourth periods when the transmit coil is 20 connected to the second power source 6a and the fourth power source 6b. Fine details of the waveforms shown in figure 2 are not shown such as low level transmit coil current ringing associated with switching due to transmit coil and electronics capacitances and inductances, and such as switching "glitches." The slew rate during current changes 54 and 53 is approximately proportional to a voltage of the said second power source switched to the transmit coil during the 25 second period and a voltage of the said fourth power source switched to the transmit coil during the fourth period respectively. In figure 2 received signal decays 71 and 72 from magnetic soils are shown, as too is a soil signal 73 and 74 during the second and fourth periods. In figure 2, the currents of 51 and 52 are assumed to be the same absolute magnitude. However, unless they are controlled to be so, there is nothing to constrain an arbitrary offset which may drift. 30 If say the duration of the second period is fixed, and the duration of the fourth period may be controlled, and if a difference in value between the transmit coil current between the first period and the third period is monitored by a fourth servo-loop amplifier 19 which by controlling the ratio of the fourth period to the second period, duration controls the ratio of energy transferred to the transmit coil during the second 5 period and the fourth period. The fourth servo-loop amplifier 19, timing control electronics 16, control lines 17, switching electronics 20 and power sources form a fourth servo control negative feedback loop which controls the said difference in absolute values between the transmit coil current during the first period and the during the third period to be approximately zero. 1o The example waveforms shown in figure 2 is a useful waveform and may be thought of as "pulse induction like," but with finite transmit current flow during suitable periods of resistive synchronous demodulation, namely the first and third periods. For direct comparison of the waveforms in figure 2 with a similar pulse induction system, assume for the pulse induction system that the power consumed in the 15 transmit coil is the same for the waveforms in figure 2 and the pulse induction system. Assume that the pulse induction "on" time is during the first period with -V volts (e.g. -5V) applied to the transmit coil, the back emf occurs approximately during the second period (e.g. +200V), and the zero transmit coil current during the receive third and fourth period. Assume ideal component behaviour with the transmit coil current 20 approximately a linear ramp, with the reactive transmit coil voltage during the first period assumed to be approximately constant, and back-emf period assumed to be of "zero" duration. Assume the first, second, third and fourth period form a repeating sequence. As described in US5576624, US5537041 and US6636044, the total integrated 25 synchronous demodulation linear combination must equal zero in order to cancel out environmental static magnetic fields and 1/f electronic noise. As this must occur in just the third and fourth period of a pulse induction system compared to both the first and third period with the advantage of opposite polarity in the transmit coil current square wave system, less long time constant target attenuation occurs from the latter system 30 compared to the former system. In addition, as there are 2 periods of high voltage in the transmit coil current square-wave system compared to the pulse induction system, the resistive signal from fast time constant targets is roughly improved by 4/SQRT(6) 12 for the same power consumed in the transmit coil. Hence the target response is generally improved. In addition, if the synchronous demodulation gain profile is symmetric in the transmit coil current square-wave system, then even noise harmonics of the fundamental frequency are cancelled thus offering more immunity to environmental noise. 5 A bi-polar pulse induction system does not fair significantly better. Assuming that the losses in the switching electronics is small and the power sources efficient (e.g. switch mode power supplies with synchronous rectifiers) and the voltage drop across any series 10 linear regulator in the first power source is small (few 1/10'' of a volt), this transmit coil square wave current system must be near optimal in target signal-to-noise ratio for a large range of time constant targets for a given transmit power dissipation and a system suitable for cancelling out magnetic soil signals with resistive synchronous occurring during periods of zero transmit coil reactive voltage. 15 The signal from magnetic soil components during the third period from a single second period (44) followed by the said third period is proportional to i/t assuming the second period is effectively of "zero" duration, and assuming the soils have constant valued resistive components to very high frequencies. 20 For a continuous repeating sequence of figure 2, the signal is approximately proportional to (t/T) where $(t)={[(t+1)/2]- iV[t/2]}/2, W(t)= d ln(F(t))/dt and where T equals the duration of each of the first and third periods. 25 Figure 3 shows an expanded block diagram of figure 1. Many of the elements are labelled the same as in figure 1 as these perform the same function. Two extra switches 82 and 83 are included in the switching electronics for practical reasons, mostly because of the parallel diodes in FET switches 25 and 26, assuming these are FETs. Switch 82 and switch 83 are also controlled by the timing electronics 16 and control lines 17. Control lines 98 control the operations of servo-loop amplifiers 30 9a, 9b and 7. A power supply 80 (e.g. a switch mode power supply) in the first power source 5 may have an output which varies slightly as the transmit coil current changes. Hence for practical reasons, the first power source 5 should include electronic circuitry whose 35 13 output is of low impedance and whose output voltage switched to the transmit coil during the first period and third period may change at a substantially faster rate than any variation in the output of power supply 80. This may be implemented by the first power source 5 including a series linear regulator 92 (such as a series FET plus operational amplifier for example) whose output voltage is 5 controlled by an output of the servo-loop amplifier 9a during the first period when switch 21 , 26 and 82 are closed, and an output of the servo-loop amplifier 9b during the third period when switch 22, 25 and 83 are closed, where the output of the series linear regulator 92 connected to switch 21 and switch 22 may be tightly voltage controlled via a fast local negative feedback loop formed using the said operational amplifier for example. Hence the first power source 5 of low impedance has an 10 output connected to the switching electronics which may include a series linear regulator 92 including a local negative feedback loop. The voltage of the second power source 6 is shown as 43 and 44 (second and fourth periods) with a change in scale assuming that this voltage is relatively high (e.g. 200V). Not shown is fine waveform detail, in particular, assuming the value of storage capacitor 90 is relatively low value 15 (e.g. 2.2microFarad), the peaks of the waveforms 43 and 44 should show roughly small parabolic waveforms as the capacitor initially charges then discharges. Assuming that the second power source 6 contains a switch mode power supply 84, an output rectifying diode 85 is shown. The voltage across capacitor 90 is controlled by servo-loop amplifier 7 controlling power supply 84. The receive electronics 11 processors a receive signal from the said received magnetic field such that 20 the indicator output 10 is responsive to metal targets but not receive signals from magnetic soils which may include ferrimagnetic materials in accordance with similar principles disclosed in US5576624. Resistive receive signals may be generated by receive synchronously demodulation during the first and third periods as the transmit reactive voltage is approximately zero. As transmit electronics which generates a transmit signal repeating sequence applied to the transmit 25 coil as the electronics is not perfect, in order to reduce any inaccuracies in the waveform, each "independent" variable should be controlled with different servo control negative feedback loops.

14 In order to accurately ensure the reactive transmit coil voltage to be constant and approximately equal to zero whilst a transmit coil current is non-zero and constant for the receive resistive synchronous demodulation periods, whilst rapidly tracking changes in the inductance of the transmit coil, advantage may be gained by the use of sample-and-hold elements, for example, at the output of 5 servo loop amplifiers which may be controlled by the timing electronics 16 via control lines 98. Thus the transmit electronics may include at least one sample-and-hold element which feeds a first output signal to a control input of at least the first power source which controls the output of the first power source 5 during at least the first period, such that the said first output signal voltage is held constant during the first period. Similarly a sample-and- hold element in the output of the third servo 10 loop amplifier may control the first power source output constant during the third period. Suppose the transmit waveform consists of a repeating sequence of say a first period where a first constant low voltage (from the first power source, e.g. 45 in figure 2 of say u =+1 Volt) is applied to the transmit coil of a first polarity, followed by a second high voltage during a second period of short duration (say Ts) and of a second polarity (from the second power source, e.g. 44 in figure 2 of 15 say V = -200Volt), followed by third period where a third constant low voltage (from the first power source, e.g. 46 in figure 2 of say u = -1 Volt) is applied to the transmit coil of second polarity, followed by a fourth high voltage during a fourth period of short duration (say Ts) and of a first polarity (from the second power source, e.g. 43 in figure 3 of say +V = +200Volt), and then this sequence repeated. If the average transmit current for each complete sequence is zero, and the 20 transmit coil current is constant during the first period (e.g. say 51 in figure 2 of value +1) and third period (e.g. 52 in figure 2 of value -1) which results in a constant zero reactive transmit coil voltage (e.g. 40 and 41 in figure 2), then as the voltage applied across the coil during the second and fourth periods (-/+200V) is high compared to any voltage drop across effective resistance component impedance 3 (<=1 VI), the reactive transmit coil voltage approximately equals the voltage across the 25 transmit coil 1. Thus when this relatively high voltage is applied across the transmit coil during the second or fourth period, the instantaneous reactive transmit coil voltage is approximately defined and independent of the instantaneous effective inductive component impedance 2 which may be under the influence of magnetic soils and thus altering as the coil moves relative to the magnetic soils. Hence it is useful if the voltage of the second power source is more than a factor of 10 that of 15 the voltage of the first power source. The change in transmit current during the fourth period is DI=+VTs/L=+l--l = 21, where L is the transmit coil inductance which may be varied as the coil moves relative to magnetic soils. As L may vary, so too will DI assuming Ts and V are constant, and hence so too does I vary in proportion to I/L. 5 Thus u must also vary in proportion to 1 /L as the voltage across the transmit coil u = IR if the reactive voltage is zero, where R is the effective resistive component impedance 3. The same applies to the second period. The response time of a transmit electronics system combination including all the servo control negative feedback loops, must operate significantly faster, that is operate up to 10 an effective higher frequency than an information frequency bandwidth of the indicator output, that is in essence the effective frequency information bandwidth at the output of resistive signals including low pass filtering that is passed to an indicator output. This is because the response of the servo loops to correct say an inductance of the transmit coil altered by a varying proximity of the transmit coil to magnetic soils must rapidly 15 track such changes so that the effect of these changes does not significantly contribute to an indicator output signal. Unlike a pulse induction system, this transmit coil current square-wave system transmits a magnetic field whilst receiving resistive components, and these synchronous transmitted finite magnetic fields may induce a rate of change of flux signal as the 20 coupling between the transmit coil transmitted alternating magnetic field and a magnetic field received and processed by the receive electronics alters when for example, the coupling is modulated by the varying interrogation of magnetic soils. This component rate of change of receive resistive signal needs to be cancelled to stop it contaminating an indicator output signal. A reactive receive signal may be formed by synchronously 25 demodulating a receive signal during a period when the reactive transmit coil voltage is non-zero, for example during the second period and fourth period. To effect this said cancellation, the reactive receive signal is further processed by the receive electronics to be approximately differentiated to give a differentiated reactive receive signal, and a linear combination of the differentiated reactive receive signal and the resistive 30 receive signal is selected so that a component of the said resistive receive signal that is responsive to magnetically permeable material rate of change of coupling between the transmit coil transmitted alternating magnetic field and a magnetic field received and processed by the receive electronics is approximately cancelled, and the resultant 16 linear combination is further processed by the receive electronics to give an indicator output. Figure 5 shows another example of transmit waveforms which includes a third period and a first period of zero reactive transmit coil voltage, 58 and 61, whilst a non-zero transmit coil flows, 64 and 66, which may be implemented in figure 4. This includes the switching electronics 20 in figure 4 5 switching a fifth power source 93 to the transmit coil via switch 94 and via 21 or switch 22, and switch 82 during a fifth period, such that the transmit coil current 65 changes sign during the fifth period when an output voltage 63 of the fifth power source 93 (e.g. -5Volt) switched to the transmit coil during the fifth period is greater in absolute magnitude to a voltage of the first power source switched to the transmit coil during the first period (e.g. IVolt), and less that an absolute magnitude 10 of a voltage of the second power source switched to the transmit coil during the second period (e.g. 200Volt). The fifth power source 93 is connected to the system ground 4. Figure 5 shows the voltage 60 applied to the transmit coil for the first period, and the voltage 62 applied to the transmit coil for the third period, and the voltage 57 applied to the transmit coil for the second period. The fifth period is defined between times 76 and 77. The transmit coil reactive voltage 59 of the transmit coil 15 during the fifth period is proportional to exp(-t/t). The transmit coil current during the fifth period is Joexp(-t/r)+V/R(1-exp(t/r)) where V is the output voltage of power source 93 switched to the transmit coil and Jo is the initial current of the fifth period. The receive voltage signal 68 across receive coil 12 from magnetic soils from a single fifth period 20 pulse during the first period is approximately proportional to {Ei[t/T]-Ei[(t+1)/T]}exp[(-t+T)/] where Ei is the exponential integral across the receive inductor 12, where T is the duration of the fifth period. The actual receive signal from a repeated transmit waveform sequence also includes the decaying signals from previous transmissions. 25 The advantage of such a waveform wherein an output voltage of the fifth power supply switched to the transmit coil during the fifth period is greater in absolute magnitude to a voltage of the first power source switched to the transmit coil during the first period, and less that an absolute magnitude of a voltage of the second power source switched to the transmit coil during the second period, such as -5VoltS, is that the receive waveform form magnetic soils is very different between 30 the first 68 and 17 third 69 periods, thus potentially improving the capability of discriminating between metal targets and magnetic soils. Also, as described in US5576624 and US5537041, ferrous metal targets may be relatively well discriminated from non-ferrous targets if a reactive signal is derived from synchronously demodulating during the latter half of the fifth period. The signal 75 from magnetic 5 soils during period 5 is shown in figure 5. A servo control negative feedback loop is required for each "independent" variable of the transmit signal repeating sequence for best control of the transmit coil reactive voltage during the first and third periods. Hence for example, for the transmission of the waveform shown in figure 5, assuming that the voltage and duration of the second period, and the voltage at the output of the fifth power 10 source are fixed, then the following variables may be controlled by individual servo loops: the voltage 60 of the first power source 5 applied to the transmit coil during the first period, the voltage 62 of the first power source 5 applied to the transmit coil during the third period, and the duration of the fifth period when the fifth power source is switched to the transmit coil. A fifth servo loop amplifier 96 monitors a difference in transmit coil current between the first period 15 and the third period. An output of the fifth servo loop amplifier 96 is connected to a controlled duration timer 95 which includes an input selector. An output of the controlled duration timer 95 controls switch 94 to be on during the fifth period. The controlled duration timer 95, fifth servo loop amplifier 96, switch 94, switching electronics 20, and fifth power supply 93 form a fifth servo control negative feedback loop which maintains the difference in transmit coil absolute current 20 between the first period and the third period to be approximately zero. A second servo loop amplifier 99 monitors the output voltage of the second power source which comprises of a first capacitor 90 which is charged only by energy from the back emf of the transmit coil when the transmit coil is switched to the said first capacitor 90 during the second period. An output of the fifth servo loop amplifier 96 is connected to a controlled duration timer 95 which 25 includes an input selector. An output of the controlled duration timer 95 controls switch 94 to be on during sixth period between 32 and 75 in figure 5. During the sixth period, the fifth power supply is switched to the coil as shown by the applied transmit coil voltage 31, and the transmit coil current 33 increases absolutely. The controlled duration timer 95, second servo loop amplifier 99, switch 94, switching electronics, and fifth power supply 93 18 form a second servo control negative feedback loop. The increase in energy stored in the transmit coil resulting from the said increase in transmit coil current during the sixth period is discharged during part of at least the second period into the said first capacitor such that energy lost during the second period in the transmit coil and also in the switching electronics associated with switching the 5 transmit coil to the first capacitor 90, is compensated by the said increase in energy stored in the transmit coil resulting from the said increase in transmit coil current. The second servo control negative feedback acts to maintain the voltage across capacitor 90 constant. The power supply connected to the transmit coil during the sixth period may be a different power supply to the fifth. The transmit coil reactive voltage during the sixth period is shown as 30 in figure 4. 10 Hence the transmit electronics is arranged and adapted to comprises at least different servo control negative feedback loops to control the transmit waveform such that reactive transmit coil voltage is controlled to be constant and approximately equal to zero whilst a transmit coil current is non-zero during periods when the receive electronics is arranged and adapted to synchronous demodulate 15 receive resistive components, such as during the first and third periods for example. Varying the duration of the fifth period when the fifth power source is switched to the transmit coil is not ideal, but in practice power efficient, inexpensive and gives satisfactory performance. A better solution for this situation is disclosed in W02005/047932, but this is more expensive and less power 20 efficient. If the repeated transmit signal voltage waveform is that given in figure 5 and the voltages 57, 62, 60 are the same as 43, 45 and 46 respectively as above, the change in current Dl during the fifth period must equal the change in current during the second period and thus is also proportional to I/L. 25 In an alternative preferred embodiment power source 5 may be a controlled current source, but a system employing such a system exhibits ring transients from the transmit coil associated resonance. Note that the roles of various servo control negative feedback loop may be altered, which form combined servo-loop systems. 30 A detailed description of one or more preferred embodiments of the invention is provided above along with accompanying figures that illustrate by way of example the principles of the invention. While the invention is described in connection with such embodiments, it should be understood that the invention is not limited to any embodiment. On the contrary, the scope of the invention is limited 35 only by the appended claims and the invention encompasses numerous alternatives, modifications, and equivalents. For the purpose of example, numerous specific details are set forth in the description above in order to provide a thorough understanding of the present invention. The present 18a invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the present invention is not unnecessarily obscured. 5 Throughout this specification and the claims that follow unless the context requires otherwise, the words 'comprise' and 'include' and variations such as 'comprising' and 'including' will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. 10 The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge of the technical field.

Claims (16)

1. A method for detecting a target in a soil, the method including the steps of: generating a transmit signal, the transmit signal includes a repeating sequence, the repeating 5 sequence includes a first high-voltage period and a first low-voltage period; transmitting a transmit magnetic field by applying the transmit signal to a transmit coil; sensing a current through the transmit coil during at least part of the first low-voltage period of a first sequence of the repeating sequence to produce a first control signal; controlling at least part of the transmit signal during at least part of the first low-voltage 10 period of the first sequence and/or during at least part of the first low-voltage period of one or more subsequent sequences using the first control signal, with the transmit signal during the first high voltage period unchanged, for maintaining the reactive transmit voltage across the transmit coil during subsequent first low-voltage periods of the transmit signal at approximately constantly zero voltage, even when an inductance of the transmit coil is modulated by the soil, with a corresponding non-zero 15 current through the transmit coil; receiving a receive magnetic field using a receive coil to produce a receive signal; processing the receive signal during at least a part of the first low-voltage period; and producing an indicative output signal indicating the presence of the target based on the processing of the receive signal; 20 wherein a voltage of any said high-voltage period is greater in absolute value than a voltage of any said low-voltage period.

2. A method according to claim 1, wherein the controlling at least a part of the transmit signal includes controlling a magnitude of the voltage of the transmit signal during the first low-voltage 25 period.

3. A method according to claim 1, wherein each repeating sequence further includes a second high-voltage period and a second low-voltage period, the method further includes the steps of: sensing a current through the transmit coil during at least a part of the second low-voltage 30 period of the first sequence to produce a second control signal; controlling at least part of the transmit signal during at least part of the second low-voltage period of the first sequence and/or during at least part of the second low-voltage period of one or more subsequent sequences using the first control signal, with the transmit signal during the second high voltage period unchanged, for maintaining the reactive transmit voltage across the transmit coil during 35 subsequent second low-voltage periods of the transmit signal at approximately constantly zero voltage even when an inductance of the transmit coil is modulated by the soil, with a corresponding non-zero current through the transmit coil; 20 wherein the processing includes processing the receive signal during at least part of the second low-voltage period.

4. A method according to claim 3, wherein the polarity of an average voltage during the first 5 high-voltage period is opposite that of an average voltage during the second high-voltage period.

5. A method according to claim 3, wherein the method further includes the steps of: sensing a difference in an absolute value of a current through the transmit coil during the first low-voltage period of a sequence and that of a current through the transmit coil during the second 10 low-voltage period of the same or a different sequence; and controlling a ratio of energy transferred to the transmit coil during the first high-voltage period and the second high-voltage period of a subsequent sequence such that the difference in the absolute value of a current through the transmit coil during the first low-voltage period of the subsequent sequence and that of a current through the transmit coil during the second low-voltage 15 period of the subsequent sequence is approximately zero.

6. A method according to claim 3, wherein a waveforn of a current through the transmit coil is approximately a rectangular wave with slew rates approximately proportional to a voltage applied to the transmit coil during the first high-voltage period and a voltage applied to the transmit coil during 20 the second high-voltage period.

7. A method according to claim 3, wherein an absolute value of an average voltage during the second high-voltage period is lower than that of an average voltage during the first high-voltage period. 25

8. A method according to claim 3, wherein the polarity of a current through the transmit coil changes from one sign to opposite sign during the second high-voltage period.

9. A method according to claim 1, wherein the processing the receive signal further includes 30 processing the receive signal during at least a part of a period with non-zero reactive transmit voltage across the transmit coil.

10. A method according to claim 9, wherein the processing of the receive signal during at least a part of the first low-voltage period includes demodulating or sampling the receive signal to produce a 35 resistive receive signal, the processing of the receive signal during at least a part of the period with non-zero reactive transmit voltage includes demodulating or sampling the receive signal to produce a reactive signal, the processing of the receive signal further including the steps of: 21 differentiating the reactive receive signal to produce a differentiated receive signal; and combining linearly the differentiated receive signal and the resistive signal.

11. A method according to claim 10, wherein timings and gains for demodulating or sampling the 5 receive signal is selected such that the resistive receive signal is approximately intrinsically statistically insensitive to signals from magnetic soils but is responsive to magnetically permeable material rate of change of coupling between the transmit magnetic field and the receive magnetic field, and the combining linearly is arranged and adapted to approximately cancel a component of the resistive receive signal which is responsive to magnetically permeable material rate of change of 10 coupling between the transmit magnetic field and the receive magnetic field.

12. A method according to claim 1, wherein the controlling includes producing a constant voltage applied to the transmit coil during the first low-voltage period using a sample-and-hold element. 15

13. A method according to claim 1, wherein the transmit signal is controlled to include more than one receive periods during which the reactive transmit voltage across the transmit coil is maintained approximately constantly zero with a corresponding current through the transmit coil being non-zero, each of the receive periods is controlled by separate control signals, and wherein the processing of the receive signal during at least a part of each of the receive periods includes demodulating or sampling 20 the receive signal to produce resistive receive signals.

14. A metal detector configurable to perform the method according to claim 1.

15. A method substantially as herein described with reference to any one of the embodiments of 25 the invention illustrated in the accompanying drawings.

16. A metal detector substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings.