Abstract:
A pulse-induction type metal detector using a transmitter coil energizing pulse that selectively reduces the amplitude of background signals from conductive soils, ores and salt water. The detector can be operated with higher amplification of the received signals than conventional detectors, without driving the input amplifier into saturation. This makes it possible to detect land mines, tramp metal and gold in media whose characteristics make detection with conventional metal detectors difficult.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to metal detectors and specifically to detectors that are optimized for use with conductive media. The technology disclosed is applicable to pulse-induction type metal detectors. 
     BACKGROUND OF THE INVENTION 
     When metal detectors are used to detect targets in conductive media, the background signal amplitude may be so high that the input amplifier of the detector is driven into saturation. 
     When the gain of the input amplifier is lowered to make the detector operative, the sensitivity to targets is lowered correspondingly. 
     In prior art metal detectors, the background signals are dealt with by sampling the received signals at various times and by combining said samples algebraically, using multiplication factors that cause the background signals to essentially cancel. 
     US Application No. 20100148960 by Candy is an example of such technology. The above approach does not address the basic problem of the input amplifier being overloaded by high-amplitude background signals. 
     OBJECTS AND ADVANTAGES 
     The present invention provides a solution to the problem of input amplifier overload by minimizing the background signal amplitude at its very source, making it possible to maintain high amplification in the input amplifier, which results in higher sensitivity to targets. 
     THEORY OF THE INVENTION 
     The operating principle of the detector is based on the observation that when a very fast magnetic field step function is imposed on a target, one with a short time constant responds more than one with a long time constant. 
     Background signals usually have shorter time constants than the targets to be detected. 
     The background signals pose a problem mainly because the ground or a load on a conveyor belt has a very large volume compared to a target to be detected and despite the short time constant of the background signal, it will not have decayed enough to be insignificant at a time when a target signal is sampled. 
     Selectively minimizing the amplitude of the background signal without materially affecting the target signal is made possible by the use of a transmitter coil energizing wave form that combines a long charging interval for a target and a very short charging interval of the opposite polarity that essentially cancels eddy currents with a short time constant in the medium in which a target is located. 
     SUMMARY OF THE INVENTION 
     The salient distinction between the present invention and prior-art metal detectors is the inclusion of a short magnetic field step function at the end of a conventional charging interval of targets and the means required to regulate the magnitude of said step function, so that the background signal is essentially canceled. As the target signal amplitude is also lowered to some degree by said short magnetic step function, additional means are included in the invention to restore the sensitivity to targets with long time constants. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the Block Diagram of the invention. The functions of the various blocks are well known to those skilled in the art and detailed descriptions of the blocks are therefore omitted to eliminate obfuscation and prolixity. 
         FIG. 2  shows the transmitter coil pulse and the signals received by the receiver coil in a conventional pulse-induction metal detector. Trace  100  is the base line and trace  101  shows the shape of the transmitter coil current. Trace  111  shows the signal generated by the background and trace  113  shows the signal generated by the target. Intervals  120 ,  123  and  125  show the intervals of time where the signals from the receiver coil are sampled. 
         FIG. 3  shows the corresponding parameters in the present invention. Trace  301  shows the background signal canceling pulse. This is the salient departure from the conventional transmitter coil pulse wave form in pulse-induction metal detectors. 
         FIG. 4  shows the transmitter coil current waveform of  FIG. 3  in a bi-polar version, where the current wave form  401  is symmetrical with respect to the base line  400 . 
         FIG. 5  shows the programming steps executed by microprocessor  327  in  FIG. 1 . The functions can be carried out in any programming language. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Microprocessor  327 , shown in  FIG. 1 , delivers a digitally stored waveform to pulse generator  346 , which in turn activates coil driver  342  that converts the voltage waveform to a current with a corresponding shape, imposing it on transmitter coil  340 . 
     The magnetic field emanating from transmitter coil  340  engenders eddy currents in a target and its surrounding medium. The magnetic field generated by said eddy currents induces voltages in receiver coil  302 . After the termination of the coil pulse, the eddy currents in the target and the surrounding medium decay toward zero, each according to its own time constant. At any point in time, the voltages captured by receiver coil  302  are summed algebraically, and in the state-of-the-art metal detector they are inextricably intermingled. 
     In  FIG. 2 , trace  111  shows a voltage generated by the background medium and trace  113  shows the voltage generated by a target. The voltage with the shorter time constant is the background signal. As an example, a load of conductive ore on a conveyor belt may have a time constant of 10 uS and the smallest target of interest may have a time constant of 30 uS. 
       FIG. 3  shows the novel transmitter coil excitation wave form where the novel feature is indicated by trace  301 . This is a fast magnetic step function, termed the background signal canceling pulse. At that point, the coil current is returned to zero with the maximal speed that the electronic circuitry allows. The effect of this quick excursion of the magnetic field is to essentially cancel the voltage with the short time constant while having only a minor effect on the voltage with the long time constant. 
     In  FIG. 2 , traces  120 ,  123  and  125  show the sampling intervals for the sample-and-hold circuit  309  of  FIG. 1 . Interval  120  represents sampling interval one, interval  123  represents sampling interval two, and interval  125  represents sampling interval three. The analog samples are digitized by A/D converter  320  and sent to microprocessor  327  which derives a first time constant Tc 1 , from samples one and two, and a second time constant, Tc 2 , from samples two and three. The processor then subtracts Tc 1  from Tc 2  and adjusts the amplitude of background signal canceling pulse  301  according to the result of said subtraction. 
     An examination of the voltage waveforms shown in  FIG. 2 , shows that a time constant calculation using samples one and two yields a shorter time constant than a calculation using samples two and three, because the influence of the voltage with the shorter time constant is greater during the earlier sampling intervals than during the later sampling intervals. 
     The programmatic steps required to accomplish the proper regulation of the amplitude of pulse  301  are shown in  FIG. 5 . The steps can be implemented by many different processors, by code that varies from processor to processor. The code required to implement the various steps is known to those skilled in the art and it does not merit further elaboration. 
     In  FIG. 3 , voltage  111  with the shorter time constant has been essentially eliminated by background signal canceling pulse  301 . Hence, time constant calculations using samples one and two, and samples two and three yield essentially the same result. This is interpreted by the microprocessor as an indication that the amplitude of the background signal canceling pulse has been adjusted to the appropriate level. 
     The main object of the invention is to reduce the amplitude of the background signal so that the detector remains operative even when the background medium is highly conductive. The effectiveness of the background signal canceling procedure is evidenced by empirical data obtained by using a prototype of the invention: Referring to  FIG. 2 :
     The peak amplitude of coil pulse  101 =3 A   Background signal  111  time constant=10 uS   Target signal  113  time constant=30 uS   Background signal to target signal amplitude ratio=3   

     Referring to  FIG. 3 :
     The peak amplitude of the coil pulse=3 A   The peak amplitude of the background signal canceling pulse=−1 A   The ground signal amplitude=+/−0 V   Target signal amplitude is reduced by 3.2%   

     The measurements were taken with sampling interval one 10 uS from the trailing edge of the coil pulse, and sampling intervals one, two and three were separated by 20 uS. The time constants used by processor  327  to regulate the amplitude of pulse  301  were derived from the formula:
 
First time constant=( T 2− T 1)/ ln ( V 1/ V 2) and second time constant=( T 3− T 2)/ ln  ( V 2/ V 3)
 
Where T 1 , T 2  and T 3  refer to the points in time when samples one, two and three were taken. V 1 , V 2  and V 3  refer to the voltages sampled during said time intervals. Ln refers to the natural logarithm.
 
     Although the 3.2% reduction in sensitivity referred to above may not be significant, means to restore the sensitivity to what it would be without the conductive background medium is incorporated in the patent. 
     A lookup-table containing correction factors based on the correlation between the amplitude of the background signal canceling pulse and the time constant of the target is accessed by the processor and a corresponding correction signal is sent to input amplifier  307  to modify its gain as required. 
     The amplitude of the signal sampled during sampling interval one is compared with a preset threshold amplitude and when the threshold is exceeded, alarm circuit  349  is activated by the processor. 
     In  FIG. 1 , alarm circuit  349  activates sound generator  352 , but additional types of alarm could be used without deviating from the concept of the invention. 
       FIG. 3  shows a coil energizing pulse of only one polarity, but bipolar pulses are commonly used in industrial metal detectors. The advantage derived from this is that any DC offset present at the output of preamplifier  307  is cancelled during demodulation of the signals by processor  327 . Interference signals caused by extraneous magnetic fields are canceled in a similar fashion 
     The Preferred Embodiment of the Invention 
     The preferred embodiment of the invention is shown by  FIGS. 1 through 5  and described in the detailed description of the invention. The preferred transmitter coil current wave form is shown in  FIG. 4 .