Abstract:
A metal detector using a linear current ramp followed by an abrupt current transition to energize the transmitter coil. The constant emf imposed on the target during the current ramp permits separation of transient voltages generated in response to eddy currents in the target and its environment from the voltages arising as a result of an inductive imbalance of the coil system The temporal separation of the various voltages makes reliable differentiation between ferrous and non-ferrous targets possible.

Description:
CROSS-REFERENCE TO RELATED APPLICATONS 
       [0001]    This application claims the benefit of U.S. provisional application No. 60/611,070, filed on Sep. 15, 2005, which is incorporated herein, by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to metal detectors, and specifically to detectors that discriminate between ferrous and non-ferrous metals. 
         [0003]    Discriminating metal detectors are well known. More than 80 patents claim to have solved the problems inherent in such detectors, but the improvements in the art have been introduced in such small increments that a level of performance that satisfies the demands of the market place has not yet been attained. 
         [0004]    Discrimination is said to be functional “if ground permits” or “if the signal is not too weak”. Graphical representations of the state-of-the-art discrimination capability show that it is operative to a target depth that amounts to less than a quarter of the maximal detection depth. 
         [0005]    The difficulties encountered by the state-of-the-art detectors stem from the fact that the resistive and reactive components of the signal intercepted by the receiver coil are intermingled and the various methods used to separate them are very complex and the results obtained are not good enough to achieve reliable identification of ferrous metals. 
         [0006]    Instead of attempting to analyze the complex target signals, the present invention separates them into their components at their very source, by using a unique coil-current wave form. 
         [0007]    A partial separation of the received signals into in-phase and quadrature signals, has been achieved in prior art. 
         [0008]    Karbowski, in U.S. Pat. No. 4,906,973, describes such a system. However, his system fails to differentiate between signals of different origins, the effect of which will be discussed below in further detail. 
         [0009]    The above-cited patent senses the reactive component only when a target is present, as determined by the presence of a resistive signal--a feature in common with the present patent. The purpose for doing that is materially different in the present patent, however. Karbowski&#39;s purpose is to differentiate between a human body and a metallic object; in the present patent, the object is to differentiate between ferrous and nonferrous targets. 
         [0010]    The purpose for initiating the reactive-component sensing when the resistive component is present, is that the reactive components originating in the background must be nulled out immediately prior to the sensing of reactive signals caused by the target. This must be accomplished dynamically, as the environment and the coil assembly alignment change. Such a dynamic alignment requires an electronic feedback loop. Karbowski describes only mechanical means for nulling the detector. 
         [0011]    Other means for nulling or “balancing” a detector are also used in prior-art hand-held detectors. For example, a synchronous demodulator, with the signal gating pulse straddling the zero crossing point, as shown in  FIG. 1 , yields one output signal polarity for ferrous targets and the opposite polarity for non-ferrous targets. This is true, when the coil system has been balanced for a signal that contains both resistive and reactive components. In the balanced condition, discrimination between ferrous and non-ferrous targets is possible, however, presence of magnetic minerals in the ground often causes misidentification of a target. A change in the reactive component of the signal causes a phase shift, as shown in  FIG. 2 . The detector can not determine whether this is caused by a target, or a change in the soil. 
         [0012]    Attempts to separate the target signal from the background signal have resulted in the design of detectors which use multiple operating frequencies and process the resulting signals with extremely complicated circuits. The drawbacks of such methods are obvious when one considers the problems of alignment, power consumption and drift of complex systems, under the temperature and humidity conditions that metal detectors are expected to endure. 
         [0013]    The object in the state-of-the-art detectors is to make use of the fact that the ratios between the resistive and reactive components of the target signal vary with the nature of the target and the frequency used. Thus, it is theoretically possible to identify a target by comparing the received signal to memorized “signatures” of desirable and undesirable targets. 
         [0014]    Owing to the fact that some undesirable objects like “hot rocks” present a infinite variety of signatures, this type of system represents only a partial solution to the problem. The fact that many signatures of desirable and undesirable targets overlap is another shortcoming of such a system. 
         [0015]    To overcome the limitations of such sine-wave systems, pulse-induction systems have been developed. In those systems, the target signal is sampled at a time when the primary field is absent, thus eliminating signals owing to mutual inductance between the receiver and transmitter coils. This expedient greatly reduces a detector&#39;s sensitivity to magnetic minerals in the soil, but it also eliminates the detector&#39;s ability to discriminate between ferrous and non-ferrous targets. Such “all-metal” detectors are used mostly in industrial applications, where any metal contamination must be detected and removed from a product. 
         [0016]    A certain level of in-phase signal is present at nearly all times, owing to soil conductivity or the presence of minerals that exhibit magnetic viscosity or energy absorption and release effects. 
         [0017]    This background signal must be eliminated so that it does not affect the threshold at which a target is detected. 
         [0018]    The simplest method of prior art uses capacitive coupling between the demodulator and the level sensor to eliminate the DC component of the signal. 
         [0019]    Such detectors are known as “motion detectors”. Capacitive coupling of the signal eliminates one problem but introduces another. 
         [0020]    A transient, negative-going excursion of the signal, caused by a void or an inert rock in the ground produces a rebound, which results in a positive signal. A human ear may be able to learn to recognize the characteristic sound produced by this phenomenon, but a level sensor can not. An industrially usable detector must issue a non-ambiguous signal such as a logic pulse, to stop a conveyor belt or to operate automatic target processing machinery. 
         [0021]    More sophisticated detectors use what is known as a “self-adjusting threshold”, which periodically restores the background signal to zero. In some instruments, the rate at which this occurs is variable, to make it possible to strike a reasonable balance between instability and loss of sensitivity. 
         [0022]    Owing to the possibility of cancelling out a target, this method is not usable in detectors monitoring a conveyor belt, or in other sensitive applications. 
         [0023]    In the present invention, the background signal is eliminated by a method which is not subject to the above limitations. 
         [0024]    The degree of mineralization of the ground may change during the very time the target signal is being acquired, thereby affecting the target identification process. This problem is not addressed in prior-art-detectors. 
         [0025]    In the present invention, the background change is assessed by evaluating the background immediately before and after the target signal is acquired. By this means, the effect of the changing background is eliminated. 
         [0026]    A similar problem arises when several targets are located in close proximity and one of them exhibits undesirable characteristics. In prior-art detectors, signals from the undesirable target cause all the targets to be misidentified. 
         [0027]    The present invention comprises a means to detect the presence of multiple targets, lessening the probability of a good target being masked by an undesirable one. 
         [0028]    Owing to the above-described problems with state-of-the-art metal detectors, there is a need for an improved detector, particularly for use in gold mines, recycling facilities, security applications, food processing and for land mine detection. 
         [0029]    Even detectors used by hobbyists benefit from the ability to discriminate between worthless and valuable targets, without the need to interpret ambiguous visual and auditory cues. 
         [0030]    The failure of prior-art technology to solve the problems associated with target identification can be largely attributed to the use of techniques which do not identify and counteract the various signals that are elicited when a magnetic flux pulse penetrates the ground and the signals generated owing to mutual inductance between the transmitter and receiver coils. 
         [0031]    In contrast, the present invention detects and effectively separates the signals engendered by the coil pulse and provides the means to nullify the ones that impair detector performance. THEORY 
         [0032]    To facilitate the understanding of the operation of the present invention, the underlying physical principles are outlined below. 
         [0033]      FIG. 3-A  shows the shape of the coil current. In accordance with Faraday&#39;s Law of Electromagnetic Induction, a changing magnetic field produces a voltage which is proportional to the time-derivative of the magnetic field. 
         [0034]    In the case of a linearly changing field, the induced voltage is a steady DC level, as shown by trace  312 , in  FIG. 3-B . 
         [0035]    When such a voltage is generated in a conductive object, a current results. The magnitude of the current is initially zero, and it gradually attains a maximal value which is determined by the induced voltage and the resistance in the current path. This current is generally know as a Foucault current, or more popularly, as an eddy current. 
         [0036]    The speed with which the current attains its maximal value depends on the ratio between the inductance and the resistance of the current path. This quantity has been given the name “time constant”, according to the formula: 
         [0000]    
       
      
       T=L/R  
      
     
         [0000]    where T is the time constant, L is the inductance and R is the resistance. 
         [0037]    During the build-up phase, the eddy current follows an exponential path, which is defined by the equation: 
         [0000]        I=I .sub. max×(1 −e  .sup. (− t/T )) 
         [0000]    where I is the current at time t, I .sub. max is the steady-state current after the build-up period, e is the base of the natural logarithm and T is the time constant of the target. 
         [0038]    Such a changing current generates a magnetic field, referred to as a secondary magnetic field, in metal detector terminology. 
         [0039]    The secondary magnetic field induces a voltage in the receiver coil of a metal detector, and owing to the fact that the time derivative of an exponential function is an exponential funtion, the resultant voltage has the same time constant as the eddy current, as shown by trace  324 , in  FIG. 3-C . Owing to its origin, this voltage is referred to as the “eddy-current voltage”. 
         [0040]    The eddy-current voltage changes according to the formula: 
         [0000]        E=E  .sub. 0× e  .sup. (− t/T ) 
         [0000]    where E is the voltage at time t and E .sub. 0 is the initial voltage. 
         [0041]    It is important to note here, that the eddy-current voltage approaches zero, asymptotically. Thus, if the current ramp in  FIG. 3-A  is long enough, the eddy-current voltage is substantially zero at the end of the ramp. 
         [0042]    While the eddy current does not induce a voltage in the receiver coil after it has achieved steady state, it continues to have an effect on the mutual inducatnce of the coil system. 
         [0043]    In prior-art detectors, a gross reduction of the inductive coupling between the transmitter and receiver coils is usually accomplished by mechanical means, and any residual signal is removed by adding a compensating signal of the appropriate magnitude and polarity to the preamplifier input. 
         [0044]    When an object having a higher magnetic permeability than air is brought into the vicinity of such a balanced coil system, the balance is upset, and a voltage is generated in the receiver coil. Such a voltage is termed “mutual-inductance voltage”. 
         [0045]    A steady current flowing in a conductive object near the coil system produces an analogous mutual-inductance voltage, but of the opposite polarity. 
         [0046]    When a target having both magnetic and conductive properties is brought into the vicinity of the coil system, the signals produced are antagonistic, and the difference between them will be manifested. 
         [0047]    During the build-up phase of the eddy current, the eddy-current signals and the mutual-inductance signals are subtracted algebraically. As the eddy-current signal decays, the mutual-inductance signal becomes predominant, as show by traces  408  or  410 , in  FIG. 4-C . 
         [0048]    When a steady state is attained near the end of the current ramp, the residual signal represents the difference between the magnetic imbalance and the effect caused by a steady-state eddy current in the target. 
         [0049]    It has been observed that in most targets that are both magnetic and conductive, the magnetic effect predominates, making it possible to determine the magnetic property of the target simply by noting the polarity of the mutual-inductance signal. 
         [0050]    This condition is illustrated by trace  410 , in  FIG. 4-C . 
         [0051]    In contrast, a non ferrous target generates signals that are additive during the eddy-current build- up period, as shown by trace  408 , in  FIG. 4-C   
         [0052]    After the build-up effects have subsided, the residual mutual-inductance signal has the opposite polarity of that caused by a ferrous target. 
         [0053]    In ferrous targets that sustain large eddy currents, owing to a shape that represensts a large area perpendicular to the magnetic flux from the transmitter coil, the sum of the mutual inductance signals may be of indeterminate polarity. 
         [0054]    It is therefore advantageous to remove the signal component that represents the steady-state eddy current. 
         [0055]    The signal present during the constant-current interval  304 , in  FIG. 3-A , is a measure of the steady-state current, and it can be sampled there, while the mutual-inductance signal is absent. Sampling this signal at interval  315 , shown in  FIG. 3-C , and subtracting the value from the sample at interval  328 , will make the magnetic imbalance signal more prominent. 
         [0056]    This technique will also eliminate another artifact which is termed the “dynamic imbalance signal”. 
         [0057]    While the coil system is in motion, relative to magnetic material in the vicinity, the changing mutual inductance between the transmitter and receiver coils will generate a transitory magnetic imbalance signal. Additionally, the realignment of the domains will generate a transitory resistive signal, owing to absorption or release of energy. These signals have essentially the same magnitude at the end of the current ramp and during the constant-current interval, and they can thus be substantially eliminated by the above-mentioned sampling and subtraction. 
         [0058]    The signals caused by energy absorption and release can be significantly reduced in amplitude if the coil current is made unipolar, in contrast to the current practice of using bipolar coil excitation. Using a biphasic coil current, as shown in  FIG. 4-D , has the advantage of using less energy for a given length of the current ramp, but at the cost of more interference from ground signals. 
         [0059]    The choice between the unipolar and biphasic options depends on the application of the detector. 
         [0060]    When energy consumption of the detector is of importance, as in battery-powered detectors, extending the current ramp long enough to allow transitory signals to decay to substantially zero, may be impractical. 
         [0061]    There is an alternate method of assessing the magnitude of the mutual-inductance signal, however. 
         [0062]    In  FIG. 4-C , the eddy-current signals  408  and  340  should have the same time constant when calculated from the samples taken at intervals  326 ,  328  and  342  and  344 , respectively. When the signal amplitudes are assigned the designations V 1 , V 2 , V 3  and V 4 , in sequence, the time constants can be calculated according to the equation: 
         [0000]        T 1= t /log ( V 1/ V 2) and  T 2= t /(log ( V 3 /V 4) 
         [0000]    where T 1  is the time constant during the ramp and T 2  is the time constant after the coil pulse. The time difference between the sampling pulses=t, and log is the natural logarithm. 
         [0063]    Assuming that there is no significant eddy-current carry-over from the ramp interval to the after-pulse interval, by making the constant-current interval  304  appropriately long, the two time constants should be essentiall the same. 
         [0064]    It will be found however, that the two time constants may differ significantly, and this can be attributed to the influence of the mutual-inductance signal. 
         [0065]    When DAC  812  in  FIG. 8  is programmed to inject a signal into preamp  806  of such a magnitude and polarity during the current ramp that the two time constants become substantially equal, then, the polarity of the injected signal is an indication of the nature of the imbalance caused by the the target. 
         [0066]    Magnetite, or “black sand”, in the parlance of prospectors, is often found in areas where gold is found. Having a higher specific gravity than the country rock, magnetite is concentrated in the same gravity traps that catch gold nuggets. This circumstance constitutes a major problem for state-of-the-art detectors, since it affects the balance of the coil system. 
         [0067]    When only one target is present in the vicinity of the coil system, the demodulated envelopes of the signals of eddy-current origin and those of mutual-inductance origin are essentialy synchronous, as shown by traces  508 ,  604  and  606 , in  FIGS. 6-A  and  6 -D. Trace  606  is the response to a ferrous target and trace  604 , to a non-ferrous target. 
         [0068]    Occasionally, more than one target is detected at the same time, causing state-of-the-art detectors to misidentify a non-ferrous target, when a ferrous target is also present. 
         [0069]    This problem is eliminated when the signal envelopes of the eddy-current and mutual-inductance signals are compared with respect to shape and the point in time at which the signals peak. 
         [0070]    As shown in  FIG. 5-F , the zero-crossing points of the differentiated signals will differ, when two targets with differing magnetic characteristics are present. The rising edges of traces  514  and  520  define the points in time when the two signals reach their peak amplitudes. The difference in the times is used to define the length of “misalignment pulse”  522 . When a preset limit of the length of pulse  522  is exceeded, that is taken as an indication of the presence of more than one target, and a ferrous indication, which might normally be issued, is disabled. 
         [0071]    The above criterion for multiple-target detection is shown by way of example only. Other criteria may also be used to measure the tracking between the eddy-current and mutual-inductance signals by programming the appropriate algorithm into the microcontroller. 
         [0072]    In light of what has been discussed hereinabove, and from what can be seen in  FIGS. 3-A , B and C, it can be concluded that by using the appropriate coil energizing current waveform, the various signals intercepted by the receiver coil can be temporally separated. The amplitudes of the various signal components can be ascertained by sampling the signals at appropriate times. 
         [0073]    Thus, the use of complicated and ineffective signal-processing means are obviated. 
       DEFINITIONS OF TERMS 
       [0074]    To further facilitate the understanding of the present invention, terms whose definitions may be distinct from usage in prior art and terms that denote phenomena not described in prior art, are defined below: 
         [0075]    The term “dynamic magnetic mutual inductance voltage” as used herein refers to a transient signal caused by a change in inductive balance, owing to the introduction of magnetic material into the vicinity of the coil system. The voltage is present only while the magnetic material is in motion relative to the coil system. 
         [0076]    The term “dynamic resistive mutual inductance voltage” as used herein refers to a transient signal caused by a change in inductive balance, owing to the introduction of conductive material into the vicinity of the coil system. The voltage is present only while the conductive material is in motion relative to the coil system. 
         [0077]    The term “eddy-current voltage” as used herein refers to the voltage induced in the receiver coil by the “seconday field”, caused by eddy currents in a target. 
         [0078]    The term “external interference” as used herein refers to voltages induced in the receiver coil by permanent magnets in motion relative to the coil or by external fields, caused by power lines, e.g. 
         [0079]    The term “magnetic mutual-inductance voltage” as used herein refers to the voltage caused by a change in inductive coupling owing to introduction of magnetic material into the vicinity of a coil system. 
         [0080]    The term “magnetic viscosity” as used herein refers to the delayed reaction of magnetic domains to an imposed field, causing a transient magnetic field that is shifted in phase, relative to the imposed field. 
         [0081]    The term “primary field” as used herein refers to the magnetic field imposed on the target and its environment by the transmitter coil of the detector. 
         [0082]    The term “ramp voltage” as used herein refers to the voltage induced in the receiver coil by the linearly changing ramp current in the transmitter coil. 
         [0083]    The term “resistive mutual inductance voltage” as used herein refers to the voltage caused by a change in inductive coupling owing to the introduction of conductive material into the vicinity of a coil system. 
         [0084]    The term “secondary field” as used herein refers to the field caused by eddy-currents in the target. 
         [0085]    The term “signal envelope misalignment” as used herein refers to the lack of tracking between the envelopes of the demodulated eddy-current and mutual-inductance signals. 
         [0086]    The term “target voltage” as used herein refers to the voltage generated in the target owing to the primary field. 
         [0087]    The term “voltage-controlled current source” as used herein refers to an electronic circuit that converts an input voltage into an output current, with the identical wave-shape. 
       SUMMARY 
       [0088]    The metal detector of the present invention uses a linear coil-current ramp to enable separation of the resistive and reactive components of the target signal and the ground. 
         [0089]    The salient departure from the prior art resides in the method of resolving the signals intercepted by the receiver coil into components that are not comingled. In the prior art, various signal-processing methods are used to analyse the target signal. In the present invention, the signal components are separated temporally, by virtue of the coil-current waveform, obviating the need for complex means of signal processing. 
         [0090]    This approach results in a detector that is simpler in design and better able to distinguish between ferrous and non-ferrous targets. 
       OBJECTS AND ADVANTAGES 
       [0091]    It is an object of the present invention to provide a metal detector that reliably identifies ferrous and non-ferrous targets. 
         [0092]    It is another object of the invention to provide a metal detector whose capability of distinguishing between ferrous and non-ferrous targets is unaffected by the medium surrounding the target or the distance at which the target is found 
         [0093]    It is another objective of the present invention to provide a detector whose activation threshold is not subject to variation owing to the influence of magnetic minerals surrounding the target. 
         [0094]    It is yet another object of the present invention to provide a detector that does not misidentify a non-ferrous target as being ferrous, when it is located in close proximity to a ferrous target. 
         [0095]    It is a further object of the present invention to provide a metal detector yielding a non-ambiguous output that can be interfaced with automatic target-processing machinery without human intervention. 
         [0096]    It is another object of the invention to provide a metal detector having a single-turn transmitting coil which presents the shortest possible recovery time from a current transient, when critically damped. 
         [0097]    It is another object of the present invention to provide a metal detector that will maintain its optimal performance characteristics without periodic readjustments of its circuitry. 
         [0098]    The advantages of the present invention become obvious when comparing its simplicity with the complexity of prior-art detectors that purport to accomplish the same task. In the present invention, the distinction between ferrous and non-ferrous targets is made by a simple determination of the polarity of a signal and not by a complicated comparison of reactive and resistive components of signals at multiple frequencies. 
         [0099]    Further objects and advantages will become apparent from a consideration of the drawings and the description below. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0100]      FIG. 1  shows a prior-art method of “ground balancing” a metal detector. 
           [0101]    When the detector is ground-balanced, the signals bounded by base line  100  and the segments  104  and  106 , respectively, are equal in magnitude and opposite in polarity, resulting in zero output from the demodulator. 
           [0102]    The above condition is reached by varying the signal amplitude by raising and lowering the coil assembly above the ground, while adjusting the position of sampling gate  108 , until no change in output results. 
           [0103]      FIG. 2  shows that an output results in the above prior-art circuit, when the phase of the received signal is shifted. A phase shift signifies that the correlation between the reactive and resistive components has changed. Ferrous targets shift the phase in one direction and non-ferrous targets in the opposite direction, provided that the gate was initially nulled with a signal containing both resistive and reactive components. A shift in the non-ferrous direction can also be caused by a ferrous target that exhibits magnetic viscosity. 
           [0104]      FIG. 3-A  shows the coil-current waveform in the present invention. Since the magnetic field generated by a coil is directly proportional to the current through it, this is also representative of the waveform of the magnetic flux imposed on the target and its environment. 
           [0105]      FIG. 3-B  shows the voltages imposed on the target and the resulting eddy currents. The target voltages are proportial to the derivative of the magnetic flux imposed on the target. 
           [0106]      FIG. 3-C  shows the voltage induced in the receiver coil owing to the eddy current in the target. 
           [0107]      FIG. 4-A  shows the coil current, as a reference for mutual inductance signals and eddy-current signals, shown in  FIG. 4-B  and  FIG. 4-C , respectively. 
           [0108]      FIG. 4-B  shows the voltages that arise when the mutual inductance between the transmitter and receiver coils is changed. Targets having a predominantly reactive characteristic generate voltages of one polarity and targets having a predominantly resistive characteristic generate voltages of the opposite polarity. 
           [0109]      FIG. 4-C  shows the simultaneous presence of voltages from inductive imbalance and from eddy currents in the target. It should be noted that when the transitory eddy-current signals, shown by traces  408  and  410 , have decayed to substantially zero, the remaining signal represents an inductive imbalance of the coil system. 
           [0110]      FIG. 4-D  shows an alternate coil-current wave shape of the present invention. This wave form doubles the length of the linear current ramp for a given power dissipation, but the bi-phasic nature of the current increases the magnitude of the background signal owing to energy absorption by causing the magnetic dipoles to traverse more of their hysteresis curve. 
           [0111]      FIG. 4-E  shows the sampling of the preamplifier output between the coil pulses to eliminate DC offsets and signals induced in the receiver coil by permanently magnetized minerals. This refers to unipolar coil energization pulses. 
           [0112]      FIG. 4-F  shows a second alternative coil current waveform of the present invention. 
           [0113]      FIG. 5-A  shows the demodulated target signal, combined with the background signal, in reference to the base line. As most soils contain minerals that generate a resistive signal that is electrically indistiguishable from a target signal, the relatively constant background signal must be separated from the transitory target signal. 
           [0114]      FIG. 5-B  shows the result of differentiating the combined background and target signals, and 
           [0115]      FIG. 5-C  shows the result obtained when the differentiated signal is re-integrated. It should be noted that the DC component of the combined signal has been removed, bringing the signal below the detection threshold  510 , until the target signal exceeds it at point  509 . 
           [0116]      FIG. 5-D  shows the pulse generated by the level sensor, when the signal first exceeds and then falls below the detection threshold. 
           [0117]      FIG. 5-E  shows the pulse generated when the differentiated signal, as shown in  FIG. 5-B , crosses zero, while the target signal is above the detection threshold. This pulse, termed “center pulse”, indicates that the center of the target signal envelope has been passed. Such an indication is useful in hand-held detectors for “pin-pointing” purposes and for marking the load of a conveyor belt to indicate the position of a target buried in the load. 
           [0118]      FIG. 5-F  shows the differentiated waveforms of the eddy-current and mutual inductance signals respectively, as traces  504  and  516 . The zero crossing points of the above signals coincide with the rising edges of the center pulses  514  and  520 . The misalignment pulse  522  is initiated and terminated by the center pulses, and the length of pulse  522  is a measure of the misalignment between the eddy-current and mutual inductance signals. 
           [0119]      FIG. 6  shows the relationship between the signals sampled during the current ramp and after the abrupt return of the coil current to zero. When target signal  508  exceeds the detection threshold  510 , pulse  512  in  FIG. 6-B , is generated, to indicate the time interval during which the threshold is exceeded. During that interval, the negative feedback, which normally nulls the mutual inductance of the coil assembly, is disabled. As a consequence, the signal sampled at the end of the current ramp will indicate in which direction the coil system is influenced by the target. Signal  604  in  FIG. 6-D  represents a non-ferrous target and signal  606  represents a ferrous target. The actual polarities of the signals are arbitrary, since they depend on how the coils are wired. The significant fact is that they are of opposite polarities for ferrous and non-ferrous targets. 
           [0120]      FIG. 6-C  shows the sampling pulse generated after the target has passed the coil system. 
           [0121]      FIG. 7  shows the signal relationships when the background signal varies significantly while the target moves relative to the coil system. Sampling pulse  602  in  FIG. 7-C  is generated immediately after the target signal falls below the detection threshold. If the background signal  702  in  FIG. 7-D  has changed while the target was being detected, the average change is subtracted from the mutual inductance signal  606 , so that the influence of the background is removed. Trace  704  in  FIG. 7-E  shows the combined background and mutual inductance signal after integration. Trace  708  shows the the mutual inductance signal, after the changing background signal has been subtracted. 
           [0122]      FIG. 7-F  shows a target signal and a changing resistive background signal, as sampled after the coil pulse. It can be seen that owing to the increasing background signal, the combined signal does not fall below the detection threshold after the target is passed. The output of the level sensor is thus not able to determine when the target has been passed, as shown in  FIG. 7-G   
           [0123]      FIG. 7-H  shows the center pulse, triggered at the peak of the target signal, as shown in  FIG. 5-E . The length of the center pulse is determined by the distance between the triggering point  509  and the peak of the target voltage. The falling edge  516  of the center pulse is used to terminate pulse  512  in  FIG. 7-I  and to trigger the post-target sampling pulse in  FIG. 7-J . The premise is that the relative speed between the coil and the target remains essentially constant during the sampling interval. Thus, the target signal envelope is symmetrical around the peak signal. 
           [0124]      FIG. 8  shows a block diagram of the present invention. The blocks contain functional units which are well known to those skilled in the art and require no further description. There is one possible exception to the above statement, and to remove the need for any undue experimentation, a detailed circuit diagram is given of the voltage-controlled current source, below. 
           [0125]      FIG. 9  shows a circuit diagram of the voltage-controlled current source. An input voltage generated by a digital-to-analog converter is transformed by this circuit into a current with the same waveshape as the voltage. 
           [0126]      FIG. 10  shows a flow chart of the operation of the preferred embodiment of the invention. 
           [0127]      FIG. 11  shows a flow chart of the operation of an alternate embodiment of the invention. 
           [0128]      FIG. 12  shows the flow chart of an additional alternate embodiment of the invention. 
       
    
    
     REFERENCE NUMERALS USED IN THE DRAWINGS 
       [0129]      100  Zero baseline of amplifier. Prior art 
         [0130]      102  Background signal. Prior art 
         [0131]      104  Positive segment of background signal. Prior art. 
         [0132]      106  Negative segment of background signal. Priot art. 
         [0133]      108  Signal sampling pulse. Prior art. 
         [0134]      300  Coil current waveform. 
         [0135]      302  Linear current ramp. 
         [0136]      304  Constant current interval. 
         [0137]      306  Abruptly decreasing current interval. 
         [0138]      308  Zero current interval. 
         [0139]      310  Voltages imposed on the target and the resulting eddy currents. 
         [0140]      312  Voltage generated in target during linear current ramp. 
         [0141]      314  Eddy current generated in target during current ramp. 
         [0142]      316  Decay of eddy current during constant current interval. 
         [0143]      318  Eddy current generated during rapid decrease of coil current. 
         [0144]      320  Decaying eddy current in target after the end of coil pulse. 
         [0145]      322  Voltages induced in the receiver coil by eddy currents in target. 
         [0146]      324  Receiver coil voltage during current ramp. 
         [0147]      326  First gating interval. 
         [0148]      328  Second gating interval. 
         [0149]      340  Eddy-current voltage, generated by eddy currents in target. 
         [0150]      342  Third gating interval. 
         [0151]      344  Fourth gating interval. 
         [0152]      402  Voltages generated in receiver coil owing to inductive coupling to transmitter coil. 
         [0153]      404  Resistive mutual inductance voltage. 
         [0154]      406  Reactive mutual inductance voltage. 
         [0155]      408  Eddy current voltage combined with positive mutual inductance voltage. 
         [0156]      410  Eddy current voltage combined with negative mutual inductance voltage. 
         [0157]      412  Base line of biphasic coil current pulse. 
         [0158]      414  Linear current ramp of biphasic coil current pulse. 
         [0159]      420  Signal sampling interval during constant current segment of coil pulse. 
         [0160]      422  Signal sampling interval between the coil pulses 
         [0161]      424  Alternative coil current waveform. 
         [0162]      426  First target signal gating interval. 
         [0163]      428  Second target signal gating interval. 
         [0164]      430  Alternate mutual inductance signal gating interval. 
         [0165]      432  Eddy current imbalance compensation signal gating interval. 
         [0166]      440  Mutual inductance and eddy-current-induced voltages with alternate coil current waveform. 
         [0167]      500  Zero reference for demodulated target signal. 
         [0168]      502  Combined background and target signals, sampled after the coil pulse. 
         [0169]      504  Combined target and background signals, after differentiation. 
         [0170]      506  Zero-crossing point of differentiated target and background signals. 
         [0171]      508  Re-integrated target signal. 
         [0172]      509  Triggering point of level sensor. 
         [0173]      510  Detection threshold of level sensor. 
         [0174]      512  Output pulse of level sensor. 
         [0175]      514  Output from pulser circuit, indicating the center of the signal sampled after the coil pulse. 
         [0176]      516  Differentiated mutual inductance signal. 
         [0177]      518  Zero crossing point of differentiated mutual inductance signal. 
         [0178]      520  Pulse indicating the center of the mutual inductance signal. 
         [0179]      522  Misalignment indication pulse. 
         [0180]      602  Signal sampling pulse after passage of target. 
         [0181]      604  Demodulated mutual inductance signal of first polarity, owing to presence of a target. 
         [0182]      606  Demodulated mutual inductance signal of second polarity, owing to presence of a target. 
         [0183]      702  Mutual inductance signal during ramp, owing to changing ground mineralization. 
         [0184]      704  Target and background signals owing to mutual inductance change, after integration. 
         [0185]      706  Zero base line. 
         [0186]      708  Mutual inductance signal of target, after correction for changing background. 
         [0187]      710  Changing resistive background signal. 
         [0188]      802  The receiver coil. 
         [0189]      804  Critical damping network for receiver coil. 
         [0190]      806  Preamplifier. 
         [0191]      808  Signal gating circuit. 
         [0192]      809  Gating pulse connection. 
         [0193]      810  Analog-to-digital converter. 
         [0194]      811  Microcontroller. 
         [0195]      812  Digital-to-analog converter. 
         [0196]      814  Transmitter coil. 
         [0197]      816  Critical damping network for transmitter coil. 
         [0198]      818  Voltage-controlled current source. 
         [0199]      820  Digital-to-analog converter for coil pulse. 
         [0200]      822  Ouput circuit. 
         [0201]      824  Alarm circuit. 
         [0202]      826  Level sensor for target signal. 
         [0203]      828  Digital-to-analog converter for processed target signal. 
         [0204]      830  Center signal indicator. 
         [0205]      832  Pulser for generating target center pulse. 
         [0206]      834  Zero crossing detector. 
         [0207]      836  Oscillator for microcontroller. 
         [0208]      838  Power supply. 
       PARTS LIST FOR FIG.  9   
       [0209]    R 1 =0.1 OHMS; R 2 =100 OHMS; R 3 =1 KILOOHM; R 4 =100 OHMS; R 5 =1 KILOOHM R 6 =100 OHMS 
         [0210]    C 1 =0.001 uF; C 2 =470 uF. 
         [0211]    U 1 =LM6364; Q 1 =IRF520 
       DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0212]      FIG. 8  shows a block diagram of the preferred embodiment of the invention. Receiver coil  802  is connected to the input of preamplifier  806  which feeds an amplified signal to gating circuit  808 . The output of the gating circuit is connected to analog-to-digital converter  810 , which interfaces with microcontroller  811 . The gating pulses for circuit  808  are generated by the processor  811  and delivered via connection  809 . 
         [0213]    R-C network  804  provides critical damping of the receiver coil 
         [0214]    Processor  811  feeds a digitally stored waveform to digital-to-analog converter  820  and the corresponding analog signal is fed to voltage-controlled current source  818 , which energizes transmitter coil  814 . The shape of the current wave form is shown in  FIG. 3-A   
         [0215]    The transmitter coil is terminated by R-C network  816 , to minimize oscillations following an abrupt change in the coil current. In conjunction with a low transmitter coil inductance, the component parameters in the voltage-controlled current source are chosen to allow a flux slew rate exceeding one million Ampere-turns per second. The circuit elements involved in energizing the coil are collectively called the “coil-excitation means”. 
         [0216]    Processor  811  is also connected to preamplifier  806  via digital-to-analog converter  812  in a negative feeedback loop, implemented by a program in the processor. This feedback loop is used to neutralize the mutual inductance signals  404  and  406 , shown in  FIG. 4-B . 
         [0217]    After demodulation, differentiation and integration in processor  811 , the signal is fed to DAC  828  and further to the level sensor  826 , which is activated when predetermined signal levels have been exceeded. Alarm circuit  824  receives an additional input from processor  811  via feedback line  825  to suppress or allow an alarm when the nature of the target is either ferrous or non-ferrous, as predetermined by a setting in the program of the processor. 
         [0218]    When an alarm is issued, output circuit  822  converts it to a usable form, which may be an auditory or visual signal or a machine-readable pulse. The circuit elements that sample, process and convert the signals to usable form are collectively termed “read-out means”. 
         [0219]    Processor  811  also feeds a differentiated signal waveform to zero crossing detector  834  which triggers pulser circuit  832 . Pulse  514  is issued, when the peak of the target signal is passed at the zero-crossing point  506 . This relationship is shown in  FIGS. 5-B  and  5 -E 
         [0220]    Pulser circuit  832  activates center signal circuit  830  which issues an indicium which can be used for “pin-pointing” the location of the target in portable applications, or marking the conveyor belt in fixed installations. 
         [0221]    Some of the functional blocks shown as discrete analog functions may also be implemented by digital logic in the program running in processor  811  and alternatively, the whole circuitry may be implemented using discrete components, without using a processor. Thus, the diagram is to be interpreted as an example only and not as a limitation of the scope of the invention. 
       DESCRIPTION OF THE OPERATION OF THE PREFERRED EMBODIMENT 
       [0222]    Transmitter coil  802  in  FIG. 8  is preferably a one-turn coil, wound with heavy-gauge wire. (One Ampere flowing through a 10-turn coil and 10 Amperes flowing through a one-turn coil yield the same magnetic flux, but the latter has one-hundredth the inductance of the former.) The inductance of the coil resonates with its self-capacitance and the resulting circuit is caused to oscillate by any abrupt transition of the coil current waveform. To avoid sampling this interfering signal, the signal gating pulse must be delayed until the oscillations have ceased. 
         [0223]    The advantage of a one-turn coil over a reduced-eddy-current coil, such as described by Candy in U.S. Pat. No. 4,980,064 becomes clear when one considers that gold veins and nuggets with irregular shapes have very short time constants and thus, a transmitter coil with low inductance and no interwinding capacitance is better adapted for use in detectors for prospecting applications. 
         [0224]    A one-turn transmitter coil requires the use of a high-current pulse, and generating such pulses has become practical since the introduction of FET switches with milliohm “on” resistances. Transistor Q 1  in  FIG. 9  is preferably of this type. 
         [0225]    Receiver coil  814  in  FIG. 8  is also preferably implemented with a minimum number of turns, just enough to raise the target signal amplitude to a level where the noise figure of the preamplifier  806  ceases to be a significant factor. As a general observation, the performance of a metal detector is not limited by the noise in the system but rather the fact that the target signals are mingled with unrelated signals from the environment in which the target is found. 
         [0226]    To further shorten delay  341  between the end of the coil pulse and signal gating pulse  342 , in  FIG. 3-C , preamplifier  806 , in  FIG. 8 , should be of a type that has a high slew rate and a short recovery time from an overload condition. 
         [0227]    To prevent the amplifier from being over-driven to a point where the recovery time is invoked, a compensating pulse is injected into the amplifier input during the rapidly changing current phase. 
         [0228]    The correct amplitude and polarity for this pulse is derived from the balancing signal required for nulling in the negative feedback loop during the current ramp. Thus, the compensating pulse is dynamically adjusted, as the ground characteristics change. 
         [0229]    Using all of the described techniques, gating pulse delay  317 , in  FIG. 3-C , can be reduced to a few tens of nanoseconds. This is in sharp contrast to the delays of tens of microseconds described in prior art. The ability to sample the target signal close to the trailing edge of the coil pulse makes it possible to detect what have been termed “invisible gold nuggets”, in prior art. 
         [0230]    The linear current ramp  302  in  FIG. 3-A  induces a constant-voltage pulse  312 , shown in  FIG. 3-B . It can be seen that the eddy current  314  in  FIG. 3-B  will assume a steady state if the ramp is significantly longer that the time constant of the target. 
         [0231]      FIG. 3-C  shows the induced voltages as a result of the eddy currents circulating in the target. It can be seen that when the eddy currents reach a steady state, the induced voltage is essentially zero. 
         [0232]    The signal sampling intervals  326 ,  328 ,  342  and  344  of  FIG. 3-C , used by gating circuit  808  in  FIG. 8  are generated in the software of processor  811  and sent to the gating circuit via connection  809 . 
         [0233]    As pointed out above, toward the end of current ramp  302 , in  FIG. 3-A , the eddy current signal has decayed to substantially zero and thus, any signal present during gating interval  328 , in  FIG. 3-C , is caused by inductive imbalance of the coil system. 
         [0234]    For maximal detection efficiency, the transmitter and receiver coils must be mounted in a concentric orientation. Inevitably, this results in inductive coupling between the coils. Voltage waveform  312  in  FIG. 3-B  is also representative of the the voltage induced in the receiver coil owing to mutual inducance between the coils. However, since this voltage is a simple DC level, it can be conveniently nulled out by a signal of the opposite polarity injected into the summing junction of preamplifier  806  via the the digital-to-analog converter  812 . 
         [0235]    A negative feedback loop implemented in the software of processor  811  can thus keep the mutual inductance signal nulled out indefinitely, even when the mutual inductance between the coils changes, owing to mechanical movement between the coils or owing to introduction of magnetic materials into the vicinity of the coils. This can be done because the reactive signal is not used for detection purposes. The reactive signal is only used after a target has been detected, and then only to determine the nature of the target. 
         [0236]    The abrupt transition  306  of the coil current, shown if  FIG. 3-A , generates voltage pulse  317  in the target, as a result of which, eddy current  318  is generated in it. As soon as the amplitude of the corresponding eddy-current signal  340 , as shown in  FIG. 3-C , exceeds a predetemined amplitude, when sampled during gating interval  342  and processed by level sensor  826  in  FIG. 8 , the negative feedback loop implemented by processor  811  is suspended. Thus, when the target continues to pass by the coil system, it will influence the mutual inductance of the coil system and generate a signal, as shown in  FIG. 4-B . The signal will have polarity  404  or  406 , depending on whether the target is ferrous or non-ferrous. 
         [0237]    This mutual inductance signal will combine with the eddy current signals from the target as shown by traces  408  and  410  in  FIG. 4-C . 
         [0238]    It can be seen, that to assess the magnitude and polarity of the mutual-inductance signal, one must wait until the eddy current signal has decayed to substantially zero. Before sampling interval  328 , in  FIG. 4-C , the mutual-inductance and eddy-current signals either add or subtract and can not be effectively separated. Such separation is essential for accurate target identification. 
         [0239]    In  FIG. 4-C , points closer to the beginning of the waveform correspond to higher frequencies in sine-wave systems and points further along on the time axis correspond to lower frequencies. Owing to the fact that the eddy-current signals which originate in the target decay exponentially, whereas the mutual-inductance signals remain constant, signal separation becomes greater at lower frequencies. The theoretical limit is 0 Hz, which is what is represented by the linear current ramp, producing DC pulses  404  and  406 , as shown in  FIG. 4-B . 
         [0240]    The permeability of magnetic targets increases the mutual inductance between the transmitter and receiver coils whereas steady-state eddy currents decrease it. It is the difference between the two effects that remains when the build-up of eddy currents in the target and the environment has been completed. 
         [0241]    It has been found that in most ferrous objects, the magnetic effect predominates, but in some mildly magnetic alloys or ferrous targets that present a large surface area to the coil flux, the eddy-current signal may override the magnetic effect. To eliminate the possibility of misidentification of such targets, the eddy-current signal component of the mutual-inductance signal is cancelled by sampling the eddy-current signal immediately after the end of the ramp and by subtracting this signal from the composite mutual-inductance signal. 
         [0242]    Hot rocks are identified as ferrous objects by the present invention, since the resistive signal they generate during the energy absorption phase is short and disappears altogether, once the magnetic domains are maximally aligned wth the external field. 
         [0243]    The signal that is sampled after the cessation of the coil pulse is essentially resistive in nature. After demodulation in the processor, the signal assumes a shape that is illustrated by trace  502  in  FIG. 5-A . It can be seen that there is a DC component present, shown as the distance between the flat portions of trace  502  and the baseline  500 . 
         [0244]    Since the DC component would affect the detection threshold of the detector, it must be eliminated. This is accomplished by a program loop in the processor. Trace  504  in  FIG. 5-B  shows the signal after differentiation and trace  508  in  FIG. 5-C  shows the signal reconstituted by integration. As in mathematical integration, an integration constant must be added to recreate the original signal. In this application, the constant is not added, resulting in the elimination of the DC component. Thus, the signal from a target is brought below the level sensor threshold  510 , causing the level sensor to fire at point  509 , when the target signal increases sufficiently and to disengage when the signal falls below the threshold. 
         [0245]    Although not illustrated, it is obvious to those skilled in the art, that the above-described method does not suffer from the rebound-effect, inherent in the method of capacitive coupling, when subjected to a negative excursion of the signal. 
         [0246]    Trace  512 , in  FIG. 5-D , represents the output of level sensor  826  in  FIG. 8 . 
         [0247]    Zero crossing detector  834 , in the same figure, is triggered at point  506  and its output fires pulser  832 , whose output is shown as trace  514 , in  FIG. 5-E . 
         [0248]    The signal sampled at the end of the coil-current ramp is also demodulated in a software loop in controller  811 , however, owing to the negative feedback loop described above, the signal remains substantially at zero, until the feedback loop is disabled, during the time interval that the eddy-current signal exceeds the detection threshold, as shown in  FIGS. 6-B  and  6 -D. 
         [0249]    If the background signal remains constant while the target passes the coil system, detecting the beginning and end of the target signal presents no problem. If, on the other hand, the background signal changes rapidly, as shown by trace  710  in  FIG. 7-F , the level sensor may not be able to detect the end of the target signal, reliably. 
         [0250]    Therefore, the preferred embodiment of the invention uses a different method of defining the end of the target signal. 
         [0251]    Processor  811  measures the time interval between trigger point  509  and center point  506 , as shown in  FIGS. 5-B  and  5 -C, and extends center pulse  514 , in  FIG. 5-E , to the measured length. 
         [0252]    Thus, as shown in  FIGS. 7-H  and  7 -I, pulse  512 , the output of the level sensor, is terminated by the trailing end  516  of the center pulse. Trace  512 , in  FIG. 7-G , shows that the level sensor would not be able to terminate the pulse, since the background signal has increased to a level above the triggering threshold. 
         [0253]    The degree of mineralization of the ground may also change during the time interval that the target signal is acquired. 
         [0254]    Trace  702  in  FIG. 7-D  shows an increase in the resistive ground signal during the detection of a target. The ground signal is integrated, along with the mutual-inductance signal generated by the target. Trace  704  in  FIG. 7-E  shows the integral of the combined signals. In the shown instance, the resistive ground signal has outweighed the magnetic inductive-imbalance signal. 
         [0255]    To correct for this error, the background signal is sampled immediately after the termination of the target signal, by pulse  602 , in  FIG. 7-C . Thus, the contribution of the background signal is assessed and subtracted from the composite integral, yielding trace  708 , in  FIG. 7-E , which correctly identifies the target as ferrous. 
         [0256]    In a similar fashion, the reactive ground signal may change while a target is being detected. A correction as described above, will ensure that the target is correctly identified. 
         [0257]    When eddy-current signals and mutual inductance signals are generated by the same target, the demodulated signal envelopes are temporally aligned, as shown in  FIGS. 6-A  and  6 -D. 
         [0258]    When several targets are in close proximity, the differentiated signals from the eddy-current and mutual-inductance channels are likely to cross zero at different times, as shown by traces  504  and  516 , in  FIG. 5-F . 
         [0259]    Both zero crossings generate pulses, with rising edges  514  and  520 , respectively. The misalignment between the pulses is used to generate “misalignment pulse”  522 , whose length is a measure of the degree of misalignment. 
         [0260]    When the length of pulse  522  exceeds a predetermined value, it is assumed that multiple targets are present and that the output of the mutual inductance channel is not valid for all targets. The check for “envelope alignment” in the flow chart of  FIG. 10  refers to this test. 
         [0261]    Thus, when identification of the target is unreliable, owing to the presence of multiple targets, it is assumed that one of the targets may be valuable, and a non-ferrous indication is issued. 
         [0262]    Other criteria for misalignment may also be used. For example, if two targets are vertically aligned but have different sizes, the signal envelopes peak at the same time, but the beginning and ending points occur at different times. This is another criterion that can be used to detect the presence of multiple targets. 
         [0263]    In a stationary installation of the metal detector, such as might be applicable to tramp metal detection on a conveyor belt, the current consumption of the detector is not critical. 
         [0264]    Thus, the length of the current ramp  302 , as shown if  FIG. 3-A , can be extended to a point where it is certain that the eddy currents in likely targets have achieved a steady state, at the end of the ramp. 
         [0265]    In battery powered detectors however, this approach represents wasted energy, leading to shortened battery life. 
         [0266]    Since data elicited from signals present during the ramp are not useful until a target has in fact been detected, the ramp can be very short, until a signal exceeding the detection threshold has been detected after the coil pulse. 
         [0267]    Furthermore, a suffiencient length of the ramp can be determined by sampling the target signal at least twice, as shown in  FIG. 3-C , at intervals  326  and  328 . 
         [0268]    When the signals at the two sampling intervals are essentially the same, the ramp is long enough, and the signal sampled at interval  328  represents substantially a mutual inductance signal. 
         [0269]    The flow chart for the preferred embodiment is shown in  FIG. 10 . 
       DESCRIPTION OF THE OPERATION OF AN ALTERNATE EMBODIMENT 
       [0270]    It also possible to elicit the magnitude of the mutual inductance signal without waiting for the eddy-current signal to decay to essentially zero. 
         [0271]    When the time constant of the target signal is calculated from samples taken during sampling intervals  342  and  344 , in  FIG. 4-C , it will be found that the computation yields a value that is different from the one obtained when using data from sampling intervals  326  and  328 . This is caused by the addition of the mutual-inductance signal to the eddy-current signal from the target. 
         [0272]    A ferrous target causes a mutual-inductance signal which creates the appearance of a shorter time constant. 
         [0273]    Thus, a simple comparison of time constants yields information about the nature of the target. 
         [0274]    With a shorter ramp, less current is expended, but more time is required to process the information. In a portable detector, where the search coil moves relatively slowly over the ground, this is not seen as a limitation. 
         [0275]    A flow chart corresponding to the above embodiment is shown in  FIG. 11   
       DESCRIPTION OF THE OPERATION OF AN ADDITIONAL ALTERNATE EMBODIMENT 
       [0276]    In this procedure, a correction signal is added to the samples taken during the current ramp. When the magnitude and polarity of the correction signal is such that the computed time constants are the same for the signals during and after the current ramp, the correction signal represents the inductive imbalance during the current ramp. 
         [0277]    The polarity of the correction signal is then an indication of the nature of the target. 
         [0278]    A flow chart corresponding to the above embodiment is shown in  FIG. 12 . 
       RAMIFICATIONS AND SCOPE OF THE INVENTION 
       [0279]    The above embodiments are not to be construed as limitations as to the manner in which the invention can be implemented, but rather as examples of many possibilities. Likewise, the applications of the invention should not be considered to be limited to any one field. The ability of a detector to reliably distingush between ferrous and non-ferrous metals may find use in treasure hunting, mining, recycling, detection of land mines, security and in other fields. 
         [0280]    Consequently, the scope of the invention should not be determined by the specifications but rather by the claims that follow.