Abstract:
A method for reducing a delay, between a transition of a transmission of a metal detector and a process of a receive signal received by a receive coil of the metal detector, due to a critically damped time constant of the receive coil, including: introducing a negative capacitance into the receive coil to reduce the critically damped time constant.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to a method for detecting fast time constant targets using a metal detector. 
       INCORPORATION BY REFERENCE 
       [0002]    This patent application claims priority from:
       Australian Provisional Patent Application No 2012900870 titled “A Circuit of a Metal Detector” filed 6 Mar. 2012.       
 
         [0004]    The entire content of this application is hereby incorporated by reference. 
       BACKGROUND ART 
       [0005]    Time domain metal detectors usually synchronously demodulate (or sample) a receive signal from a receive coil commencing after a short delay following a voltage transition of a transmit signal, for example, after the back emf of a pulse induction (PI) transmit pulse, or after a switch from a high voltage (e.g. 200V) to a low-voltage (e.g. 5V) or to zero voltage. The short delay is to avoid the processing electronics from demodulating potential spurious interfering signals (unwanted signals) from mildly conductive soils. Typically, these signals from mildly conductive soil have signal components of particularly short duration; they will decay very rapidly. Detection of these signals is undesirable in a metal detector. Given that they were the only difficulty to be overcome, a short delay of the onset of demodulation after the transition of transmit signal would be sufficient to reduce their effect upon detection. In most detectors, however, the demodulation must be delayed by more than is required to remove the effect of the signals from mildly conductive ground. In most detectors, the minimum of the practical delay is usually limited by the time constant of the critically damped receive coil, either because the decaying transient signal components in the receive coil, due to a voltage transition of the transmit signal, contain signals related to the commonly major soil component known in the field of metal detection, namely the signal components due to soil reactive magnetic permeability, which is normally required to be nulled out, or to prevent said transients from causing receive signal electronic to overload, or both. 
         [0006]    The required delay before demodulation, (or sampling), is undesirable for the detection of fast constant metal targets, because much of the decaying signal from the fast time constant targets following a transition of the transmit signal occurs during the delay, and thus most of this target signal is not added into the receive demodulation. Many sought buried metal targets have short time constants: for example; some minimum metal land mines, small gold nuggets, and fine jewellery. Improving the sensitivity to very short time constant targets requires reduction of the delay before demodulation, in turn requiring reduction of the time constant of the critically damped receive coil. Hitherto, this was usually achieved by choosing a low inductance for the receive coil, or eliminating the capacitive loading of the receive coils by the capacitance of the connecting cable, through installation of the receive preamplifier within the coil housing. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    According to a first aspect of the present invention, there is provided a method for reducing a delay, between a transition of a transmission of a metal detector and a process of a receive signal received by a receive coil of the metal detector, due to a critically damped time constant of the receive coil, including: introducing a negative capacitance into the receive coil to reduce the critically damped time constant. 
         [0008]    In one form, the step of introducing the negative capacitance includes: connecting a negative capacitance generator to the receive coil. 
         [0009]    In one form, the negative capacitance generator and the receive coil are connected to receive electronics for further processing. 
         [0010]    In one form, the negative capacitance generator includes an amplifier with its non-inverting input connected to the receive coil. 
         [0011]    In one form, the negative capacitance generator reduces an effective parallel capacitance of the receive coil. 
         [0012]    In one form, the negative capacitance generator further amplifies the receive signal. 
         [0013]    In one form, a feedback loop of the amplifier includes a small capacitance for improving the stability of the negative capacitance generator. 
         [0014]    In one form, the negative capacitance generator includes at least a circuit element that compensates for distributed inductances and capacitances of the receive coil. 
         [0015]    According to a second aspect of the present invention, there is provided a metal detector including a circuit to perform the method of the first aspect. 
         [0016]    To assist with the understanding of this invention, reference will now be made to the drawings: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  depicts a simple circuit of a metal detector to describe the problem addressed by the present invention; and 
           [0018]      FIGS. 2 to 9  depict various embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present invention offers an alternative method over the prior art for detecting fast time constant targets using a metal detector. The method is to cancel part of the capacitance associated with the receive coil, both from its self-capacitance and the capacitance of the connecting cable, using a negative capacitance impedance generator. In practice, the critically damped time constant can be improved by a factor of about 0.5 to 0.7, e.g. a time constant of say 0.3 μs reduced to one of say 0.2 or 0.15 μs, with no signs of instability or any sort of undesirable artefacts at the time of writing. 
         [0020]      FIG. 1  depicts a simple circuit of a metal detector, including a transmit winding  1  and a receive winding  4 , to describe the problem addressed by the present invention. 
         [0021]    The transmit winding  1  is connected  2  to transmit electronics (not shown) for receiving a transmit signal. A sudden transition in the transmit winding  1  can be capacitively coupled to the receive winding  4  through the virtual capacitance  3 . If the magnetic null between the windings is imperfect, there can be magnetic coupling (not shown) of the sudden transitions as well. The receive winding  4 , in parallel with its self-capacitance  5 , forms a self-resonant element  9 . The sudden transitions coupled to the receive winding  4  cause this self-resonant element  9  to oscillate at its natural frequency. 
         [0022]    In a receive circuit of a metal detector, a resistor  6  is included to damp the oscillation of the self-resonant element. In order to quench the oscillations in minimum time, the value of the resistor  6  is selected that the resulting damping is at, or close to, critical damping. The damped signal is fed to the Rx pre-amplifier  7 , and the output signal  8  is further processed. 
         [0023]    Signals from some targets have very short time constants. Thus, their eddy currents decay to very low levels very quickly. In order for the detector to be sensitive to such targets, demodulation of the receive signal must commence very soon after sudden transitions of the transmission that excites the eddy currents. 
         [0024]    In prior art, the commencement of demodulation must be designed to not incorporate significant contribution to its input from the self-oscillation of the self-resonant element  9 , incorporating the receive winding  4 . Thus, signals from targets with short time constants are almost completely decayed before demodulation begins. Such signals are lost to the detector after demodulation. There will be no indication, by the detector, of the presence of such targets. 
         [0025]    The present invention is to introduce a circuit that presents an apparently negative capacitance to the receive winding  4 , making it possible to adjust the value of the damping resistor  6  such that the decay of the oscillation of the self-resonant element  9  occurs more quickly than it would otherwise. This allows the earlier commencement of demodulation after sudden transition of transmission, facilitating the detection of targets with eddy currents of shorter time constants. 
         [0026]      FIG. 2  shows a general form of a circuit in accordance with an aspect of the present invention designed to reduce the time taken for critical damping of oscillations in the receiver  12  ,which can be a receive coil; this figure includes amplification of the signals induced in the receive winding  13 . The receiver  12  is modelled as a receive inductor  13  in parallel with a capacitor  14 ; resistance of the receive inductor  13  is not shown in this model. The capacitor  14  is a combination of the self-capacitance of the receive inductor  13 , input capacitance of the amplifier  16 , capacitance of any cables connected to the receive inductor  13  and the capacitance associated with associated connecting tracks and other physical features of the electronics. 
         [0027]    The circuit  26  presents what appears to be a negative capacitance at the non-inverting input  20  of the amplifier  11 . The amplifiers  11  and  16  can be operational amplifiers. The value of that negative capacitance is negative the value of the capacitor  24  multiplied by the value of resistance  21  and divided by the value of resistance  22 . Resistance  22  is connected between the system reference ground  17  and the inverting input  25  of the amplifier  11 . The amplifier  11  has a very high gain and very high input impedances at its inverting input  25  and its non-inverting input  20 . The feedback resistor  21  is connected between the output  15  and the inverting input  25  of the amplifier  11 . Capacitor  24  and resistor  23  are connected together in series; collectively they are connected between the output  15  and the non-inverting input  20  of the amplifier  11 , providing a positive feedback path. The resistor  23  reduces the amount of positive feedback at high frequencies, but has a small value compared to the value of the reactance of the capacitor  24  near the resonant frequency of the receive inductor  13  in parallel with the capacitor  14 . 
         [0028]    The damping resistor  19  is connected across the receive inductor  13  and is also connected between the non-inverting input  20  of the amplifier  16  and the system reference ground  17 . The value of the resistor  19  is selected so as to produce critical, or near-critical, damping of the circuit consisting of the receiver  12  connected to the circuit  26  via the connection to the non-inverting input  20 . The output  18  of the amplifier  16  provides an amplified signal for further amplification and demodulation. 
         [0029]      FIG. 3  shows an example of a basic negative capacitance generator. The output  31  of the generator is the non-inverting input of op-amp  30 , and is connected to capacitance  32 , which is connected between the op-amp output  33  and non-inverting input (which also acts as output  31 ) as a positive feedback path. Resistor  37  is connected between the op-amp output  33  and inverting input  34 , to which resistor  36  is also connected. The other end of resistor  36  is connected to the system reference ground  35 . If the value of resistor  37  is x times that of resistor  36 , and the capacitance of capacitor  32  is c, then the impedance Z generated at  31  relative to ground  35  is 
         [0000]    
       
         
           
             Z 
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                 1 
                 
                   j 
                    
                   
                       
                   
                    
                   ω 
                    
                   
                       
                   
                    
                   xc 
                 
               
             
           
         
       
     
         [0000]    when ω is the frequency in rad/s. 
         [0030]    Similarly,  FIG. 4  shows a negative resistance generator where resistors  47  and  46  play the same role as resistors  37  and  36  in  FIG. 3 , and op-amp  40  with inverting input at  44 , non-inverting at  41 , output at  43 , corresponds to op-amp  30 . However, in place of a capacitor being connected between the op-amp output  43  and non-inverting input  41 , is resistor  42  of resistance R. The impedance Z presented at  41  equals 
         [0000]    
       
         
           
             - 
             
               R 
               x 
             
           
         
       
     
         [0031]    Similarly,  FIG. 5  shows a negative resistance and negative capacitance generator where resistors  57  and  56  play the same role as resistors  37  and  36  in  FIG. 3 , and op-amp  50  with inverting input at  54 , non-inverting at  51 , output at  53 , corresponds to op-amp  30 . However, in place of only a capacitor being connected between the op-amp output  53  and non-inverting input  51 , is a resistor  52 B of resistance R in parallel with capacitor  52 A of capacitance c. The impedance Z presented at  51  is −R/x in parallel with −1/(jωxc). 
         [0032]    An alternative negative capacitance generator is shown in  FIG. 6 . The output of the generator  61  is the non-inverting input of op-amp  60 , and is connected to resistor  66 , which is connected between the op-amp output  63  and non-inverting input as a positive feedback path. Resistor  67  is connected between the op-amp output  63  and inverting input  64 , to which capacitor  62  is also connected. The other end of capacitor  62  is connected to the system reference ground  65 . If the value of resistor  66  is x times that of resistor  67 , and the capacitance of capacitor  62  is c, then the impedance Z generated at  51  relative to ground  65  is 
         [0000]    
       
         
           
             Z 
             = 
             
               - 
               
                 1 
                 
                   j 
                    
                   
                       
                   
                    
                   ω 
                    
                   
                       
                   
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                   xc 
                 
               
             
           
         
       
     
         [0033]      FIG. 7  shows another circuit capable of improving the detection of targets having short time constants. The output of the negative capacitance generator presented at  71  is the non-inverting input of op-amp  70 , and is connected to series resistor  83  and capacitor  84 , which is connected between the op-amp output  75  and non-inverting input  71  as a positive feedback path. Resistor  81  is connected between the op-amp output  75  and inverting input  85 , to which resistor  82  is also connected. The other end of resistor  82  is connected to the system reference ground  77 . 
         [0034]    This generator is connected to the receive coil  72 , effectively consisting of inductance  73  and effective receive coil capacitance  74 , comprising a combination of the self-capacitance of the receive coil, the capacitance of any connecting cable, the input capacitance of the pre-amplifier  76 , and any other stray capacitance associated with the receive coil. The series resistance of the receive coil is not shown. 
         [0035]    A small-valued capacitor  86  (e.g. ˜10 pF etc.) is connected in parallel with resistor  81 . Both capacitor  86  and resistor  83  are included for stability. The small-valued capacitor  86  adds more negative feedback at higher frequencies than the resonant frequency of the receive inductor  73  in parallel with capacitor  74 . 
         [0036]    For simplicity, assume that resistor  82  and resistor  81  have equal value, capacitor  84  has capacitance c, resistor  83  has resistance r, and that capacitance of capacitor  86  is negligibly small. The impedance presented to the receive coil at  71  equals 
         [0000]    
       
         
           
             
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         [0037]    Suppose at a frequency ω=1/(critically damped time constant), 1/(ωc)&gt;&gt;r. For example, suppose c=150 pF, r=150Ω, then at say ω=1/0.25μ rads, 1/(ωc)=1.71 kΩ&gt;&gt;150Ω. 
         [0038]    Thus the impedance presented at  71  of the negative impedance generator approximately equals a negative capacitance of 150 pF (with a relatively small effect of the 150Ω series resistance). 
         [0039]    Further, supposing that the value of inductance  73  is 400 μH and the value of the capacitance  74  is 220 pF, the net effective capacitance in parallel with the inductor  73  is 70 pF owing to the negative capacitance, producing a critically damped time constant of approximately sqrt(70/220)=0.56 of that of just the critically damped coil without the active negative impedance generator. If the effect of resistor  83  is taken into account, the time constant is, now, effectively slightly more than 0.53 of the original time constant. 
         [0040]    Resistor  79  is selected for critical damping with the active negative impedance generator connected, and the output  78  of preamplifier  76  is fed to the synchronous demodulators, or samplers or an Analogue-to-Digital Convertor. It is also possible to include a ferrite bead  87  in parallel with resistor  83 . Such an arrangement (indicated by dotted lines) may bring extra benefits in reducing the amount of positive feedback at high frequencies, but resistor  83  has a small impedance value compared with that of the capacitor  84  near the resonant frequency of the inductor  73  in parallel with capacitor  74 . The role of the, ferrite bead  87  in effect is as a damped inductor to compensate to some extent a more realistic model of a real physical receive coil with its distributed inductances and capacitances at frequencies higher than the said resonant frequency of the inductor  73  in parallel with capacitor  74 . 
         [0041]    In  FIG. 8 , the receive coil  96  and its effective parallel capacitance  97  combine to produce a self-resonant element  99 . A wideband preamplifier  90  is employed as both the receive preamplifier and for actively reducing the receive coil  96  effective parallel capacitance  97 . This is achieved via a positive feedback path consisting of capacitor  92  in series with resistance  94  connected from the output  93  of preamplifier  90 , to its non-inverting input  91  to which the receive coil  96  is also connected. Resistor  98  is connected across the receive coil  96  and selected for critical damping. 
         [0042]    If the gain of the wideband preamplifier  90  is g, capacitor  92  of capacitance c, and resistor  94  of resistance r, then the impedance presented at node  91  by the positive feedback path is 
         [0000]    
       
         
           
             Z 
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         [0043]    Again, as long as 1/(ωc)&gt;&gt;r, then this positive feedback path approximately acts to lower the effective input capacitance  97  and thus allows for a shorter receive coil critically damped time constant, thus allowing for demodulation to commence earlier compared to having no active feedback, and thus allowing for better detection of very short time constant metal targets. 
         [0044]      FIG. 9  shows another example of the one op-amp  100  being employed as the active element in a circuit to both reduce the effective parallel capacitance of the receive coil  105  and to amplify the receive signal from the receive coil  105 . The effective parallel capacitance of the receive coil  105  is not shown in this figure. 
         [0045]    Feedback resistor  101  and feedback capacitor  102 , connected in parallel with each other, are connected between the output  110  and the inverting input  111  of op-amp  100 . A resistor  103  is connected between the inverting input  111  of op-amp  100  and the system reference ground. Together, resistors  101  and  103  largely determine the low-frequency gain of the pre-amplifier. The resistor  104  is connected between the output  110  of op-amp  100  and the system for further processing the receive signal, and can shield the output  110  from any effects of the input  109  to the processing system. 
         [0046]    The receive coil  105  and the damping resistor  106  are connected, in parallel, between the non-inverting input  112  and the system reference ground  108 . The positive feedback capacitor  107  is connected between the non-inverting input  112  and the output  110  of the op-amp  100 . The apparent impedance presented by the output at the non-inverting input  112  is −1/j(g−1)ωC, where C is the value of the positive feedback capacitor  107  and g is the gain of the preamplifier at low frequencies ˜1+R101/R103. The value of the damping resistor  106  is selected so as to produce critical, or near-critical, damping at the frequency of the receive coil  105  with its associated virtual capacitances (not shown) in combination with the negative capacitance presented at the non-inverting input  112 . 
         [0047]    In all the above examples, the receive coil may also act as a transmit coil, and by altering the forward phase of the op-amp/preamp slightly, the effects of the resistors  83  (of  FIG. 7 ) or  94  in the active feedback paths may be reduced. 
         [0048]    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 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 invention may be practised 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. 
         [0049]    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. 
         [0050]    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.