Abstract:
A device in which the receiver coil of a pulse inductive metal detector is capable of switching between a differential configuration (i.e., gradiometer configuration) to a non-differential or summing configuration under control of the operator or computer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application No. 60/507,816, filed on Oct. 1, 2003, which is incorporated herein by reference in its entirety. 
     
    
     STATEMENT OF GOVERNMENTAL INTEREST  
       [0002]     This invention was made with U.S. Government support under Army contract no. N00024-98-D-8124. The U.S. Government has certain rights in this invention. 
     
    
     BACKGROUND  
       [0003]     There are an estimated 100 million mines and countless millions of acres of land contaminated with unexploded ordnance (UXO) worldwide. Thus, there is a need for sensor systems and methods that can detect and identify large and small metal objects buried in soil. Such objects often are mines and UXOs.  
         [0004]     A commonly used sensor for mine and UXO detection is the electromagnetic induction (EMI) metal detector. Conventional EMI metal detectors use either frequency-domain (FD) or time-domain (TD or pulse) eddy current methods and can detect small metal targets (such as plastic-cased low-metal content mines) at shallow depths and large metal targets (such as metal-cased high-metal content mines and UXOs) at both shallow and deep depths under a wide range of environmental and soil conditions. However, metal non-mine (i.e., clutter) objects commonly found in the environment pose a major problem in identifying mines. That is because these clutter objects create false alarms when detected by a metal detector. For time-efficient and cost-effective land clearing, the detected metal targets must be classified as to their threat potential: mine, UXO or clutter. Preferably, these metal targets need to be classified in real-time or near real-time.  
         [0005]      FIG. 1  is a block diagram of a conventional pulsed EMI metal detector and method of operation. A current loop transmitter  10  is placed in the vicinity of the buried metal target  12 , and a steady current flows in the transmitter  10  for a sufficiently long time to allow turn-on transients in the soil (soil eddy currents) to dissipate. The transmitter loop current is then turned off. The transmitter current is typically a pulsed waveform. For example, a square-wave, triangle or saw-tooth pulsed waveform, or a combination of different positive and negative current ramps.  
         [0006]     According to Faraday&#39;s Law, the collapsing magnetic field induces an electromotive force (EMF) in nearby conductors, such as the metal target  12 . This EMF induces eddy currents to flow in the conductor. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field that is detected by a magnetic field receiver  14  located above the ground and coupled to the transmitter  10  via a data acquisition and control system  16 .  
         [0007]     Pulse induction metal detector (PIMD) antennas (transmitter and receiver coil) come in two basic types as shown in  FIGS. 2   a  and  2   b . The first type of PIMD shown in  FIG. 2   a  illustrates a single combined transmitter and receiver coil  23  and damping resistor  22  with multiple loops of wire forming the coil  23 . A current pulse is sent through the multiple turn coil  23  and the received metal detection signal is sensed by the same coil  23 . The small voltage generated by the metal target is typically amplified by a high gain electronic amplifier  25  (typical gain factor of 100 to 1000). A protection circuit  24  is provided to protect the sensitive amplifier from the high kick-back voltage pulse generated by switching the inductive coil off abruptly (V=L di/dt, where L is the inductance of the transmitter coil and di/dt is the slope of the current decay in the coil).  
         [0008]     The second type of PIMD illustrated in  FIG. 2   b  uses a separate coil  27  and damping resistor  26  for the transmitter and a coil  29  and damping resistor  28  for the receiver. This configuration provides isolation between the transmitter circuit and the receiver circuit and allows for more flexibility in the receiver coil  29  (e.g., different number of turns, size or differential coil configuration) and amplifier circuit design (e.g., single ended operation of electronics). The high gain amplifier  25  also sees the high kick-back voltage pulse generated by switching the transmitter coil  27  off abruptly and protection circuitry  24  is needed to protect it from damage.  
         [0009]     Typically, for low-metal mine detection the transmitter and receiver coils (commonly called the antenna) are concentric and are about 8″ to 12″ in diameter. This antenna size is to facilitate the portability of the metal detection sensor (e.g., light weight) and to aid in localizing the detected metal (small detection footprint). A smaller antenna size limits the detection depth and a larger antenna size hinders the portability and localization capability of the metal detection sensor for small metal objects.  
         [0010]     Metal target classification places additional constraints on a metal detector antenna design. For small metal classification, the above antenna size is appropriate since the low-metal mine metal targets are much smaller than the antenna size (typically, less than 0.25″). For example, the metal object is fully illuminated (with a relatively uniform magnetic field) by the transmitting magnetic field coil and the re-radiated magnetic field is fully received by the receiver coil. The exact placement of the antenna over the metal object is not critical since the transmitter and receiver magnetic field uniformity is relatively large. However, when metal objects are encountered that are on the order of or larger than the metal detector antenna, the metal signature from the detected magnetic field is not unique and can confound a metal classification process. The random placement of the metal detector antenna over a large metal object excites the metal object with different magnetic field vector angles and strengths. The resulting magnetic field signatures are then dependent on the exact placement and orientation of the antenna and the unknown metal object. Since the relative positional information is not known for a hidden or buried object, classification of the metal object is difficult.  
         [0011]      FIG. 3  shows the basic configuration of a differential receiver coil for a pulse inductive metal detector (PIMD). These antennas are generally larger than the above single transmitter/receiver coil antennas to allow for a relatively large receiver coil to be placed inside the single transmitter coil. The configuration of the receiver coil is known by different names like a  FIG. 8 , a gradiometer, or a balanced coil. There are many advantages to a differential receiver coil arrangement. In low-metal content mine detection the major advantage of the differential receiver coil configuration is passive ground return cancellation which allows the metal detector to sense small metal objects in the presence of mineralized soil.  
         [0012]     Another advantage of the differential receiver coil is the decoupling of the transmitter pulse from the receiver signal which allows for a faster responding metal detection signal (high bandwidth). A disadvantage of the differential coil arrangement is the need for placing two receiver coils within a given transmitter coil dimension. This naturally limits the receiver coil size and hence the detection depth (metal detection sensitivity and detection depth are functions of the transmitter and receiver coil size). This size constraint does not adversely effect a typical metal detector for landmines since small metal objects characterized by low-metal mines are generally placed within the top few inches of the ground. However, when a large metal object such as a UXO or high metal content mine is encountered with a differential configured receiver coil, the size of the object is sometimes as large or larger than the receiver coil. The receiver coil does not measure the time decay signature of the object correctly for classification purposes. Large objects do not necessarily need the ground balancing of the differential coil arrangement (e.g., large metal object signature is much larger than ground response signature) nor the other advantages convened by the differential coil arrangement (large metal objects have a lower bandwidth compared to small metal objects). Large metal objects are sometimes buried deep in the ground and a differential receiver coil arrangement (for a given transmitter coil size) has relatively poor depth sensitivity.  
         [0013]     Thus, there is a need to combine the advantages of a differential receiver coil antenna design (for small metal objects) with a single receiver coil design (for large metal objects) into one metal detection device. By combining the features of both receiver coil configurations into one device we overcome the limitations of each device separately.  
       SUMMARY  
       [0014]     The present invention is a pulse inductive metal detector that can selectively change the receiver coil configuration from a differential configuration good for detecting and classifying smaller metal objects to a summing configuration good for detecting and classifying larger metal objects. A transmitter coil induces eddy currents in a nearby metal target. The operator can then select to configure the receiver coil in a differential configuration for small metal target detection and classification or a summing configuration for larger metal target detection and classification. The conversion from one mode to the other is achieved by a switch coupled with the receiver coil that switches couplings in the receiver coil to create either a differential or summing configuration. A computer can be coupled with the switch for assisting the operator in controlling the switch setting.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  illustrates a block diagram of a conventional pulsed EMI metal detector and method of operation.  
         [0016]      FIG. 2   a  illustrates a PIMD having a single transmit and receiver coil with multiple loops of wire forming the coil.  
         [0017]      FIG. 2   b  illustrates a PIMD having a separate transmit and receiver coils with multiple loops of wire forming each coil.  
         [0018]      FIG. 3  is a circuit diagram of an example PIMD differential coil configuration.  
         [0019]      FIG. 4  is a circuit diagram of an example PIMD summing coil configuration.  
         [0020]      FIG. 5   a  is a an example physical circuit layout for the differential circuit of  FIG. 3 .  
         [0021]      FIG. 5   b  is a an example physical circuit layout for the summing circuit of  FIG. 4 .  
         [0022]      FIG. 6  illustrates an embodiment of a switched receiver coil including a switch mechanism for selecting between a differential or summing configuration of the receiver coil. 
     
    
     DETAILED DESCRIPTION  
       [0023]     The present invention applies to pulse inductive metal detectors (PIMDs) and allows for the receiver coil to be re-configurable as either two coils connected in a differential coil configuration or a single coil, the single coil being a summing of the two coils comprising the differential receiver coil configuration.  
         [0024]      FIG. 3  illustrates a transmitter coil  30  and a receiver coil  31  configured as a differential coil. Receiver coil  31  includes coil components  32   a ,  32   b  (such as coil windings  32   a ,  32   b ) having respective outputs  33   a ,  33   b  feeding respective inputs of a summing amplifier  34 . Outputs  33   a ,  33   b  are signals (e.g., current) indicative of changing magnetic flux detected by or coupled into respective coils  32   a ,  32   b . The position of black dots  35   a ,  35   b  in  FIG. 3  indicates that the windings  32   a  and  32   b  are connected together such that they are wound in opposite directions or senses relative to each other. As a result, outputs  33   a ,  33   b  have opposite polarities, that is, outputs  33   a ,  33   b  form a differential pair of signals as applied to the inputs of summing amplifier  34 .  
         [0025]      FIG. 4  illustrates a transmitter coil  30  and a receiver coil  36  configured as a summing coil. The position of black dots  37   a ,  37   b  in  FIG. 4  indicates the windings  32   a  and  32   b  are connected together such that they are wound in the same direction or sense relative to each other. As a result, outputs  33   a ,  33   b  have the same polarities, that is, outputs  33   a ,  33   b  form a summing or additive pair of signals as applied to the inputs of summing amplifier  34 .  
         [0026]      FIG. 5   a  is a an example physical circuit layout for differential coil  31  of  FIG. 3 . Differentially configured receiver coil  31  is in the pattern of a figure-eight, as described in more detail below. In the embodiment of  FIG. 5   a , each of coil components or windings  32   a ,  32   b  includes a substantially planar single winding or loop of wire, however, each of windings  32   a ,  32   b  typically includes multiple windings or loops of wire. The wire may be wound around an air core (i.e., an air-wound coil), or alternatively, a ferromagnetic core.  
         [0027]     Each loop in winding  32   a  includes opposing ends  38   a  and  40   a , and each loop in winding  32   b  includes opposing ends  38   b  and  40   b . Opposing ends  38   a ,  40   a  are connected to opposing ends  38   b ,  40   b  in a coupling or connecting region  44 , in a manner described below in connection with  FIG. 6 . In the embodiment of  FIG. 5   a , opposing ends  38   a ,  40   a  are connected to opposing ends  38   b ,  40   b  in a criss-cross (“X”) manner so as to configure receiver coil  31  overall as a “figure-eight.” In this configuration, windings  32   a ,  32   b  are wound in opposite directions, as indicated by clockwise and counter-clockwise arrows A and B. Arrows A and B also respectively indicate the directions of currents induced in windings  32   a ,  32   b  when a changing magnetic field intersects the windings. Thus, outputs  33   a ,  33   b  have opposite polarities or senses, that is, are differential.  
         [0028]      FIG. 5   b  is a an example physical circuit layout for summing coil  36  of  FIG. 4 . In the embodiment depicted in  FIG. 5   b , opposing ends  38   a ,  40   a  are connected to opposing ends  38   b ,  40   b  in connecting region  44  so as to not criss-cross (“X”). In this non-criss-cross configuration, windings  32   a ,  32   b  are wound in the same directions, as indicated by clockwise arrows A and B. Currents flow in each of coils  32   a ,  32   b  in the same direction. Thus, outputs  33   a ,  33   b  have the same polarities or senses, that is, are summing outputs.  
         [0029]     In both of the embodiments of  FIGS. 5   a  and  5   b , coil windings  32   a ,  32   b  are substantially planar, and are adjacent to but spaced-apart from each other so that they do not overlap one another. That is, the coils are arranged side-by-side so as to not overlap each other.  
         [0030]      FIG. 6  illustrates an example switched receiver coil  50  including a switch  60  connected between windings  32   a ,  32   b  for selectively configuring coil  50  between differential configuration  31  and summing configuration  36 , responsive to a control signal  61  from a computer controller  62 . An operator can control positions (depicted at (a) and (b) in  FIG. 6 ) of switch  60 , and thus, the configuration of coil  50 , via an interface with computer  62 .  
         [0031]     Switch  60  includes terminals  62   b  connected to ends  38   b ,  40   b  of windings  32   b , and terminals  62   a  connected to ends  38   a ,  40   a . Switch  60   a  has a selectable first position (depicted at (b) in  FIG. 6 ) that connects end  38   a  to end  38   b , and end  40   a  to end  40   b , thereby configuring receiver coil  50  as summing coil  36 , as depicted in  FIG. 5   b.    
         [0032]     Switch  60   a  has a selectable second position (as depicted at (a) in  FIG. 6 ) that connects end  38   a  to end  40   b , and end  40   a  to end  38   b , thereby configuring receiver coil  50  as differential coil  31 , as depicted in  FIG. 5   a.    
         [0033]     In the summing configuration (position (b) in  FIG. 6 ), the two receive coils form a physically larger coil that can detect the eddy current decay signature of large targets more effectively and with greater sensitivity (better depth detection). Typically, a mine detection differential antenna is about 10″ by 20″ with a receiver coil about 10″ by 10″. If a buried metal object that is on the order of these dimensions is encountered at a depth of many inches, the excitation magnetic field and received magnetic field by the differential receiver is complex making target classification difficult. If the receiver coils were connected in a differential configuration (position (a) in  FIG. 6 ), the signal from a large object beneath the coils would be close to zero after performing the differential algorithm unless the metal object was off to one side of the sensor head (transmitter/receiver coil). Either way, classifying such a relatively large object would be much more difficult in a differential receiver coil configuration.  
         [0034]     If the receiver coil encountered the same metal object and the operator switched the receiver coil to a summing configuration, the larger receiver coil configuration can more easily process the eddy current decay signature of the metal object.  
         [0035]     Thus, the operator can toggle between small and large metal target modes using the same metal detector by simply altering the configuration of the receiver coil.  
         [0036]     It is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.