Abstract:
A metal detector ( 1 ) used for identifying contaminants ( 35 ) in products ( 35 ). The detector ( 1 ) includes an oscillator coil assembly ( 10 ) that may be formed as a combination of pairs of series wound coils ( 15, 18 ) and pairs of parallel wound coils ( 16, 17 ). A pair of input coils ( 13, 14 ) defines the boundaries of a region ( 39 ) within which the oscillator coil assembly ( 10 ) resides. A first signal ( 8 ) is generated by the first input coil ( 13 ) in response to the presence of a metallic object ( 35 ) while a second signal ( 24 ) is generated by the second input coil ( 14 ) in response to the presence of the metallic object ( 35 ). By measuring the ratio of the first signal ( 8 ) to the second signal ( 24 ) the physical location of a metal object within the metal detector cavity ( 7 ) can be determined.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    This invention pertains generally to the field of radio frequency metal detection, and more particularly to improving the flux density of the magnetic field used in such a device. 
         [0003]    2. Description of Prior Art 
         [0004]    Metal detectors are used in the food processing industry, for example, to detect contaminants within a product. The unwanted material may include very small metallic particles having differing compositions. The typical metal detector is housed in an enclosure containing a longitudinal aperture through which the product under test is transported, usually by means of a conveyor belt. The metal detector includes a radio frequency transducer or oscillator that radiates a magnetic field by means of some arrangement of coils that serve as a radio frequency antenna. 
         [0005]    The typical metal detector includes a search head which contains both radiating and receiving coils, the search head being formed to surround the aperture or passageway through which the product travels. The oscillator coil is a continuous wire loop formed within the search head. The oscillator coil surrounds the aperture and receives radio frequency excitation from an oscillator circuit. The search head also includes an input coil connected to produce a zero input signal when no metal is present. A disturbance in the radiated magnetic field is sensed by the input coil and processed in order to detect a metal contaminant within the product passing through the detector aperture. A nonzero input coil signal is due to either mechanical imbalances in the construction of the search head, inherent electrical changes in the circuitry such as frequency drift, metal being introduced into the aperture, or the effect of the product itself. 
         [0006]    Modem digital signal processing techniques resolve the input signal into two signal components, one component being resistive and the other signal component being reactive. The “product effect” caused by the product passing through the aperture is due primarily to electrical conduction via salt water within the product, the electrical conduction causing large magnitude resistive signals and relatively smaller reactive signals. 
         [0007]    When a metal detector is used in the food processing industry, the detector is typically placed at a location which is a part of the existing food processing line. Due to constraints in processing the food items, there is often little discretion in choosing where the detector will reside. The detector is often placed in close proximity to other metal objects, such as conduits, casings, cabinets and other metal fixtures. Equipment, such as pumps and conveyors can vibrate or move with respect to the fixed position of the metal detector. The magnitude of the effect of such equipment on the metal detector is dependent on the size of the detector aperture, the detector operating frequency, the magnitude of the operating current, the type of material being tested and the size and location of any surrounding equipment which may include other metal detectors. 
         [0008]    In many situations, the magnitude of external interference is sufficient to cause the metal detector to falsely indicate the presence of metal in the product under test. In order to provide some measure of electromagnetic shielding, the detector enclosure is usually constructed of metal, but this typically requires that the coils be separated some distance from the walls of the metal enclosure in order to minimize the effects caused by enclosure vibration, heating and aging. Vibration caused by relative movement of the enclosure with respect to the coils causes a disturbance in the radiated electromagnetic field that may easily be mistaken as the sensing of contaminant metal. The magnitude of a vibration related disturbance increases as the distance between the coils and the metallic enclosure walls is reduced. In general, any metal residing within or near the detection aperture is likely to be sensed as a contaminant particle or object even when the metal is part of the detector structure or enclosure. A method of canceling or accounting for such residual or nearby metal is a major challenge affecting the design of metal detection equipment. 
         [0009]    Within the metal detector aperture is an area which may be properly termed as the detection zone. The detection zone is a region in which the product is subjected to the peak magnetic radiation of the oscillator coil and any disturbance in the magnetic field is assumed to be attributable to the presence of unwanted metal contaminants. Unfortunately, since the magnetic field extends beyond the detection zone, there is an additional region in which no metal should reside, typically referred to as the “metal free zone”. In real world metal detector installations, the desired metal free zone is often several times larger than the volume of the detector aperture. 
         [0010]    Achieving a substantial metal free zone often creates problems in a food processing or other production environment. One method of reducing the volume of the metal free zone is to reduce the physical boundaries or extent of the magnetic field produced by the oscillator coil. An example of a device which employs this technique is disclosed in U.S. Pat. No. 5,572,121, entitled METAL DETECTOR INCLUDING A METAL SCREENING FOR PRODUCING A SECONDARY MAGNETIC FIELD TO REDUCE THE METAL FREE ZONE, issued on Nov. 5, 1996 to Beswick. The Beswick device places a metal screen or grid adjacent to the oscillator coil. The metal screen induces a secondary magnetic field which is in opposition to the primary magnetic field, thereby constricting the size of the primary magnetic field and the volume of the metal free zone. The Beswick forms a continuous shield around the inside of the aperture such that the starting and ending edges of the screen are electrically connected. 
         [0011]    Ideally, the magnetic flux density created by the oscillator coil should be as large as possible but without enlarging the size of the metal free zone. A high density magnetic field must be created in a physically small volume. A larger flux density increases the magnitude of current induced in the receive coil and thus increases sensitivity to small contaminants. In an effort to address the problems presented by prior devices, the novel approach disclosed in the present invention utilizes multiple oscillator coils. 
         [0012]    Prior efforts to utilize multiple coils have been attempted. For example, U.S. Pat. No. 5,504,428, entitled MAGNETIC METAL DETECTOR MOUNTED IN A FEED ROLL OF A HARRISTING MACHINE, issued to Johnson, discloses a single oscillator coil with multiple input coils. Johnson does not disclose multiple interconnected oscillator coils. U.S. Pat. No. 5,199,545, entitled METAL BODY DISCRIMINATING APPARATUS, issued to Takamisawa et al., discloses multiple adjacent oscillator coils that are physically parallel but which are not electrically interconnected in either a parallel or series relationship. Rather, the Takamisawa et al. coils reside individually in discrete electrical circuits. U.S. Pat. No. 6,342,835, entitled SENSOR PANEL AND A DETECTION APPARATUS INCORPORATING THE SAME, issued to Nelson-White, discloses adjacent coils in a detector that having an aperture that is large enough to accommodate a human being and which is inherently unsuitable for detecting minute metallic particles passing rapidly through a small aperture. None of the foregoing patents address the problem of specific coil geometries that may be used to enhance flux densities in a small physical space while performing a high speed signal analysis of the materials passing near the coils. 
       SUMMARY OF THE INVENTION 
       [0013]    The current invention relates to improvements in the function of a metal detector and includes techniques for improving the magnetic flux associated with the use of search heads which must be dimensioned primarily based on the aperture needed for a particular metal detection application. One embodiment of the present invention uses two oscillator coils connected in a parallel relationship to generate up to twice the magnetic flux density for a given excitation voltage whenever a relatively larger search head is required. 
         [0014]    In the case of a small search head, the inductance is necessarily smaller due to the smaller size. In this circumstance, a given voltage will result in a relatively high current. The current is typically so high, in fact, that the voltage must be reduced to prevent the coil current from exceeding a value that would cause signal distortion, thereby raising the background noise level and hence reducing detector performance. An alternate embodiment of the present invention uses two oscillator coils interconnected in a series relationship so as to produce approximately twice the magnetic flux density for a given excitation voltage whenever a relatively smaller search head is required. 
         [0015]    The present invention includes an input coil formed as two coils that are coaxial with the oscillator coil. The resultant three-coil arrangement is connected such that a zero net input voltage is produced under quiescent conditions. 
         [0016]    The present invention also includes two additional types of coil arrangements that tend to increase the magnetic flux in the search head and thereby increase the signal strength in the receiving coils for any given size of metal contaminant. Search heads are presented that combine a single oscillator coil with coils that are connected in series and with coils that are connected in a parallel electrical relationship. 
         [0017]    For example, a search head of intermediate dimensions is formed with a single oscillator coil supplemented by two series interconnected coils spaced relatively close to the input coil windings. Both the single coil and the two series coils are excited by the same voltage source or, in an alternate embodiment, by two separate sources having different voltage levels. 
         [0018]    In other embodiments of the present invention, relatively larger search heads a combination of both series interconnected and parallel interconnected can be used together to increase received signal levels. Relatively smaller search heads can be enhanced by the use. of multiple sets of series interconnected coils. 
         [0019]    All of the techniques of the present invention together produce cumulative improvements in the resultant metal detecting apparatus. For example, the use of a series coil arrangement in conjunction with a relatively smaller search headcan reduce the size of the smallest detectable contaminant. Typically this means that a detector previously capable of detecting a particle having a diameter of 0.8 mm will be able to detect a particle having a diameter of 0.6 mm. The ability to ignore the effects of case vibration permits greater sensitivity. This further reduces the diameter of detectable contaminants from 0.8 mm to approximately 0.4 mm. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a perspective view of the aperture structure of a metal detector constructed according to the principles of the present invention; 
           [0021]      FIG. 2  is a perspective view of a metal detector incorporating the aperture structure depicted in  FIG. 1 ; 
           [0022]      FIG. 3  is a perspective view of a coil arrangement utilized in a first embodiment of the metal detector depicted in  FIG. 2 ; 
           [0023]      FIG. 4  is a perspective view of a coil arrangement utilized in a second embodiment of the present invention; 
           [0024]      FIG. 5  is a simplified perspective view of the oscillator coil arrangement utilized in a third embodiment of the present invention; 
           [0025]      FIG. 6  is a sectional view taken along line  6 - 6  of  FIG. 3 ; 
           [0026]      FIG. 7  is a sectional view taken along line  7 - 7  of  FIG. 4 ; 
           [0027]      FIG. 8  is a sectional view taken along line  8 - 8  of  FIG. 5 ; 
           [0028]      FIG. 9  is a graph depicting the relationship of the signals produced by the metal detector illustrated in  FIG. 2 ; 
           [0029]      FIG. 10  is a simplified perspective view of an oscillator coil arrangement utilized in a fourth embodiment of the metal detector depicted in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Referring to  FIG. 2 , a metal detector constructed according to the principles of the present invention is shown generally at 1. The metal detector  1  includes a metal or conductive cabinet  2 , typically stainless steel or aluminum that is supported by shock absorbing feet  3  and  4 . The cabinet is formed to include a generally rectangular first sidewall  5 . The sidewall  5  includes an opening or first aperture  6  which permits access to the interior volume of cavity  7 . The cavity  7  is bounded by a coaxially aligned second aperture  9 , such that an article  54  may enter the cavity  7  through first aperture  6  and exit cavity  7  through the second aperture  9 . The article may enter the cavity  7  by traveling generally in the direction of arrow  36  via a conveyor which may be a belt or chain as well as gravity feed mechanism or a pump forwarding the article through a conduit. In an alternate embodiment the aperture may have a circular shape, and the entire cabinet may also be generally circular or toroidal in configuration. In yet another embodiment the metal detector can be formed to have an open aperture, such as is commonly used in the carpet industry; in which the metal detector has a single sensitive face in which the active components are embedded. 
         [0031]    An oscillator coil assembly  10  resides within the cabinet  2 . The oscillator coil assembly  10  surrounds and substantially bisects the cavity  7 . As seen in  FIG. 3 , for example, the oscillator coil assembly  10  is excited or driven by a radio frequency oscillator or transmitter  11 , which is housed within the region  12  of cabinet  2 . Symmetrically spaced on opposite sides of oscillator coil assembly  10  is a front input coil  13  and a rear input coil  14 . Coils  13  and  14  are connected to each other in series opposition. 
         [0032]    The oscillator  11  produces a low frequency magnetic signal typically in the range of thirty kilohertz to two megahertz. The oscillator  11  is coupled to the oscillator coil  10  through leads  20  and  21 . The oscillator coil  10  acts as an antenna, radiating the signal produced by oscillator  11  and producing a magnetic field within and somewhat beyond cavity  7 . The input coils  13  and  14  reside within the magnetic field produced by the oscillator coil assembly  10 . 
         [0033]    In the absence of a metal contaminant  35 , and due to their series opposition interconnection, the signal induced in the front coil  13  from oscillator coil assembly  10  is of the same magnitude but of opposite polarity as the signal induced in the rear coil  14 , thereby producing a resultant signal of zero volts. When metal is present, or due to small irregularities in the position of coils  13  and  14 , or the location of the case  2 , or the presence of contaminant metal  35 , the symmetry of the magnetic field produced by oscillator coil assembly  10  is distorted, thereby causing signals of different magnitude to appear on coils  13  and  14 . This imbalance produces a signal having a magnitude that is greater than zero volts. 
         [0034]    The strength of the signal produced by the input coils  13  and  14  is a function of the size, shape and composition of the coils  13  and  14 , the absolute strength of the magnetic field produced by oscillator coil assembly  10 , the size of the metal object  35 , the composition of the metal object  35 , the distance between the metal object  35  and the input coils  13  and  14 , and the distance of the metal object  35  from the oscillator coil assembly  10 . The closer the metal object is to the oscillator coil assembly  10 , the greater will be the distortion of the magnetic field created by oscillator coil assembly  10 . A relatively greater field distortion appearing on one input coil produces a relatively greater unbalance in the signals induced by the combination of the input coils  13  and  14 . Similarly, the closer the metal object  35  is to either receiver coil  13  or  14 , the greater the imbalance in the amount of signal induced in either coil. Thus, a greater distance between the metal object  35  and any of the coils  10 ,  13  or  14  reduces the magnitude of the unbalanced signal. Typically, the input coils  13  and  14  are arranged and spaced in such a manner so as to maximize the magnitude of the unbalanced signal when the metal object is passed through the center of the cavity  7 . 
         [0035]    The oscillator  11  operates at a discrete, continuous wave radio frequency typically in the range of 0.030 to 2.00 megahertz. The magnetic field emitted by oscillator coil assembly  10  is coupled to the adjacent input coils  13  and  14  by magnetic induction creating signals in both coils  13  and  14 . Since the coils  13  and  14  are wired in series opposition, there is no resultant output signal when metal  35  or other electromagnetic field distorting medium is absent from the vicinity of the emitted magnetic field. 
         [0036]    Referring also to  FIG. 9 , when a moving metal article  35  is brought into proximity with the radiated magnetic field of the oscillator coil assembly  10 , the magnetic field effectively undergoes amplitude and phase modulation, that is, the magnetic field density varies with respect to time. The modulation frequency is typically in the range of 0.10 to 100 hertz. The amplitude of the modulation is dependent on the size and speed of the metal  35  or product  54  passing through the aperture  6  as well as the coil separation. 
         [0037]    The signal  8  received by input coil  13 , for example, will have an amplitude and an amplitude modulation frequency that is dependent on the metal or product characteristics as well as the velocity of the metal as it passes through the detector cavity  7 . The signal waveform  8  depicts the change in voltage across the input coil  13  when metal passes by the coil. The change in voltage is of the order of a few tens of nanovolts for a small contaminant  35  superimposed on a standing voltage of up to ten volts. The standing voltage is the same on each coil  13  and  14 . By wiring the coils  13  and  14  in an anti-phase relationship, the relatively large standing voltage is removed. The axis  46  represents the magnitude or amplitude of the signal  8  in units such as volts, while axis  47  represents the distance traveled or horizontal displacement of the metal contaminant  35 . 
         [0038]    The point  48  of signal  8  represents a point in time when no metal or product is in the detector cavity  7 . The signal  8  is representative of the signal appearing on one input coil  13 , which remains the same regardless of the signal on the other input coil  14 . As metal  35  enters the location in the detector cavity  7  and approaches the first input coil  13 , the magnitude of signal  8  changes and has an amplitude modulation envelope peak  50  which corresponds to the metal object passing between oscillator coil and the input coil  13 . Signal  24  represents a second amplitude modulation envelope peak  49  which corresponds to the metal object passing between oscillator coil and the input coil  14 . The time difference between the envelope peaks  50  and  49  along axis  47  indicates the absolute frequency of the amplitude modulation, which is dependent on the parameters such as the speed and size of the contaminant  35 . The closer the input coils  13  and  14  are to the center of the cavity  7 , and hence to the oscillator coil assembly  10  which resides at or near the center of the cavity  7 , the higher the frequency of the amplitude modulation due to the shorter period of time needed for passage of the object over the input coils  13  and  14 . 
         [0039]    If metal  35  resides at the aperture center  51 , the magnitude of the voltage induced in each input coil  13  and  14  will be substantially equal, corresponding to the magnitude of, for example, point  52 . Similarly, if the metal object  35  is nearer to input coil  13 , the magnitude  53  of the voltage induced in coil  13  is greater than the magnitude  23  of the voltage induced in coil  14  at the same moment. In other words, the magnitude of the voltage induced in the nearer coil is measurably greater than the voltage induced in the more distant coil. Only metal residing in the center  20  of the cavity  7  will produce a ratio of voltages in input coils  13  and  14  that is approximately 1:1. 
         [0040]    By monitoring the ratio of the voltages  8  and  24  induced in each coil, respectively, at the same instant, the position of the metal within the cavity can be calculated. A recognition that the signal produced on the input coils  13 ,  14  is the result of metal located within the cavity permits signals not corresponding to an in cavity location to be excluded as either the result of vibration or attributable to metal external to the metal detector  1 . By monitoring the position of the each contaminant using the ratio method just described, the position of a second contaminant can be determined and its effect on the signal attributable to the first contaminant can be recognized. 
         [0041]    The oscillator  11  is capable of delivering a current of approximately 11 amperes root mean square (11 A RMS). When the oscillator coil assembly  10  surrounds an aperture  6  having dimensions of, for example, 350 mm by 150 mm, the coil inductance limits the actual coil current to 7.5 A RMS at 300 kHz because the peak to peak voltage from the oscillator is also limited, in this case to 40 v. An aperture size of 600 mm by 200 mm will create a current of only 3 A, which represents a significant reduction in radiated magnetic flux from that which is theoretically available from the oscillator coil assembly  10 . 
         [0042]    Referring also to  FIG. 1 , a first improved oscillator coil arrangement can be understood. The oscillator coil assembly  10  is composed of several separate coils  15 ,  16 ,  17  and  18 . Thus, the coil assembly  10  spans the entire region  41 . While four coils are shown residing within coil assembly  10 , more than four coils may be used in alternate embodiments of the present invention. In one version of the present invention, the input coils  13  and  14  define the boundaries of a region  39  within which the region  41 , occupied by individual coils  15 - 18  of coil assembly  10 , resides. In other configurations, the input coils  13  and  14  reside inboard of the outermost oscillator coils  15  and  18 . 
         [0043]    Referring also to  FIGS. 3 and 6 , two separate sets of oscillator coils are depicted. The first set of oscillator coils is composed of coils  16  and  17  which are seen in  FIG. 3  to be interconnected in an electrically parallel relationship. Spaced apart from the coil  16  is the coil  15 , while coil  18  is separated from coil  17  by distance  55 . The coils  15  and  18  form a second set of oscillator coils that are interconnected in an electrically series relationship. The series coils  15  and  18  may be wound in either an in phase or antiphase relationship. In either case both sets of oscillator coils reside between the input coils  13  and  14 . 
         [0044]    The inner set of oscillator coils  16 ,  17  are in parallel and the outer set of coils  15 ,  18  are in series. The inner set of coils  16  and  17  may be wound in either an in phase or antiphase relationship. Such an arrangement is particularly advantageous for relatively larger coil assemblies  10  and results in the creation of increased signal levels. All of the coils  15 - 18  are interconnected to the oscillator  11  by leads  20  and  21 . In an alternate embodiment of the present invention the parallel coils  16 ,  17  are excited by a first oscillator and the series coil  15 ,  18  are excited by a second oscillator. The two oscillators can operate at different frequencies and at differing power levels. Each of the input coils  13  and  14  are interconnected to receiver  27  by leads  25  and  26 . In another embodiment of the present invention, the coils  15  and  18  are omitted, leaving a pair of parallel interconnected oscillator coil  16  and  17 . The coils  16  and  17  can be interconnected in either an in phase or antiphase relationship. 
         [0045]    The two series oscillator coils  15  and  18  are spaced apart a sufficient distance  41  to permit the generation of largely separate magnetic fields. The input coils  13  and  14  reside outside of the volume defined by the series oscillator coils  15  and  18  as well as the parallel oscillator coils  16  and  17 , the input coils being connected to input signal processing circuitry  27 . The optimum spacing  40  between coils  16  and  17  is approximately half the distance  55  between the oscillator coils  17  and  18 . The two oscillator coils  16  and  17  are wound in parallel to create a twin coil which is fed by oscillator leads  20  and  21 . The coils  16  and  17  may be arranged so that their fields are either additive or in phase opposition. The total current drawn by the combined twin coils  16  and  17  is substantially greater than a single coil having the same dimensions. For an aperture size of 600 mm by 200 mm, the current increases from approximately 3 A for a single coil to approximately 5 A for the twin coil  31 . 
         [0046]    The radiated magnetic flux is directly proportional to the current drain, the increased flux thereby causing the absolute magnitude of the peak signals  49  and  50  to increase, thereby simplifying their detection and measurement. Further, because each input coil  13  and  14  is physically closer to its adjacent oscillator coil  15  and  18 , respectively, the signal to noise ratio is further improved. In theory, the overall net gain achieved with the combination of the series coils  16  and  17  with the parallel coils  16  and  17  is substantially greater than a single oscillator coil configuration. This gain improvement translates into detection of metal particles that are approximately 25% smaller. Table I compares the expected performance of the present arrangement of combined series and parallel coils with the previous single oscillator coil method for an actual metal detector. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 300 kHz Peak Signal Test 
               
             
          
           
               
                   
                   
                   
                 Combined 
                   
               
               
                   
                   
                   
                 Series and 
                 Combined Series 
               
               
                   
                   
                   
                 Parallel 
                 and Parallel Coils 
               
               
                   
                   
                 Single Coil 
                 Coils Peak 
                 Peak Signal 
               
               
                   
                 Test sample 
                 Peak Signal 
                 Signal 
                 Improvement 
               
               
                   
                   
               
               
                   
                 0.8 mm Fe 
                 140 
                 290 
                 2.07 
               
               
                   
                 0.8 mm NFe 
                 165 
                 320 
                 1.94 
               
               
                   
                 1.2 mm SS 
                 143 
                 275 
                 1.92 
               
               
                   
                 1.5 mm SS 
                 435 
                 805 
                 1.85 
               
             
          
           
               
                   
                 Average Improvement 
                 1.95 (+95%) 
               
               
                   
                   
               
             
          
         
       
     
         [0047]    Table I shows that the peak signal reading produced by the combination of dual parallel and series interconnected oscillator coils produces significantly higher peak detector readings. In particular, the improvement afforded by the multiple coil system of the present invention shows an average improvement factor of 1.95 for a variety of potential metal contaminants  35 . 
         [0048]    Referring also to  FIGS. 4 and 7 , a second improved oscillator coil configuration can be understood. An oscillator coil assembly  10  of intermediate size may have dimensions of approximately 350 by 150 mm such as would be associated with an intermediate sized aperture. A single coil will have a relatively intermediate value of inductance which will benefit from an inductance enhancement. In such a case, the single oscillator coil  57  is placed between two additional coils  15  and  18  which are interconnected in series. The series coils  15  and  18  are symmetrically spaced so as to be relatively close to the nearest input coil  14  and  13 , respectively. In other words the distances  42  and  56  are approximately equal and greater than the spacing between oscillator coil  15  and input coil  14 . Both the single coil  57  and the two series coils  15  and  18  are excited from the same voltage source  11  although they may be driven at different voltage levels. The coils  57 ,  15  and  18  may be wound in either an in phase or antiphase relationship. 
         [0049]    For relatively smaller aperture sizes, the small inductance value produced by a single oscillator coil necessarily means that the oscillator coil will draw a relatively greater current. Since the oscillator  11  typically cannot deliver more than 11 A RMS, the driving voltage delivered by the oscillator  11  to the oscillator coil must be limited in order to prevent excessive current demand. A typical standard sized oscillator coil may have a potential difference of 40 volts peak to peak (40 Vp-p), whereas a smaller dimensioned oscillator coil may permit an oscillator voltage of only 10 Vp-p. 
         [0050]    Referring also to  FIGS. 5 and 8 , the improved oscillator coil assembly  10  is constructed to address the inherent shortcomings of a relatively smaller search head and includes two sets of series wound oscillator coils  16 ,  17  and  15 ,  18 . The adjacent oscillator coils  16  and  17  are interconnected at region  19  to create an electrically series relationship. The coils  16  and  17  may be arranged so that their fields are either additive or in phase opposition. The typical theoretical improvement in the radiated magnetic flux is approximately 4:1, which increases to approximately 5:1 if the coils  16  and  17  are placed such that the ratio of distance  38  to distances  37  or  39  is relatively small. Altematively, for a given voltage, the current can be reduced by a factor of 5:1 if the original level of magnetic flux is to be maintained. However, since the current drain has been reduced by a factor of five, the voltage produced by oscillator  11  can be increase by a factor of five without exceeding the maximum permissible current drain, thereby permitting an increase of 5:1 in the radiated magnetic flux. In practice, the actual improvement in magnetic flux is often limited to about 3:1 due to the proximity of the case  2 . 
         [0051]    The two oscillator coils  18  and  15  are spaced so as to be relatively closer to their adjacent input coil  13  and  14 , respectively. This increased spacing between the oscillator coils  15  and  18  causes each oscillator coil  18  and  15  to act substantially independently and behave as two inductors in a series circuit. The net inductance increase is therefore 2:1 which corresponds to a doubling of the radiated magnetic flux when the voltage of oscillator  11  is doubled. However, a metal object passing close to input coil  13 , for example and its associated oscillator coil  18  will cause a minute change in the oscillator voltage across coil  18 . The change in oscillator voltage causes a corresponding change in the magnitude of the current drawn from the oscillator  11 , the change being perceived or forwarded to the other oscillator coil  15  since the two coils  18  and  15  are wound in series. The result of the voltage and current change is to produce an additive signal which creates a potential flux improvement of approximately 2.5:1. A radiated flux improvement of 2.5:1 corresponds to a decrease in size of the minimum detectable contaminant of approximately 35%. 
         [0052]    The foregoing improvements embodied in the present invention are by way of example only. Those skilled in the metal detecting field will appreciate that the foregoing features may be modified as appropriate for various specific applications without departing from the scope of the claims. For example, as illustrated in  FIG. 10 , the input coils  13  and  14  reside between the outermost series oscillator coils  15  and  18 , wound in an antiphase relationship, and the innermost series oscillator coils  16  and  17 .