Abstract:
A metal detector ( 20 ) has a metallic enclosure ( 21 ) with entrance and exit apertures ( 30, 31 ). A coil system inside the enclosure has an energized transmitter coil. First and second receiver coils ( 24, 25 ) bound a detection zone ( 28 ) between the apertures, through which inspected objects ( 2 ) travel. The receiver coils are on opposite sides of the transmitter coil, with regard to the direction of travel ( 13 ). The receiver coils are connected in series, but their windings are wires oppositely in a rotational sense. Metal contamination in the object generates a detection signal in the receiver coils. A first and second flange ( 26 ), arranged at the entrance and exit, cancel the undesirable influence of metal contamination beyond a first distance upstream of and a second distance downstream of the coil system. The respective flanges differ from each other and, consequently, the first and second distances are different.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT/EP2012/063283, filed on 6 Jul. 2012, which in turn claims a right of priority under 35 USC §119 from European patent application 11173277.2, filed 8 Jul. 2011. The content of each application is incorporated by reference as if fully recited herein. 
    
    
     TECHNICAL FIELD 
     The invention relates to an industrial metal detector for the food-, beverage-, pharmaceuticals-, plastics-, chemicals-, packaging-, and other industries. 
     BACKGROUND 
     The main purpose of metal detectors of the kind described herein is to detect the presence of metal in an article, a bulk material, or generally any object being examined. Such metal detectors are widely used and integrated into production and packaging lines, for example to detect contamination of food by metal particles or components from broken processing machinery during the manufacturing process, which constitutes a major safety issue in the food industry. The generic type of metal detector that this invention relates to and which is known as balanced three-coil system with an encircling coil arrangement can be described as a portal through which the articles and materials under inspection are moving, for example individual packages riding on a horizontal conveyor belt through a vertical portal, or a stream of bulk material in free fall through a vertical duct or funnel passing through a horizontally arranged portal. 
     The portal is generally configured as a box-shaped metallic enclosure with an entrance aperture and an exit aperture. The operative part of the metal detector is a system of three electrical coils wound on a common hollow carrier or coil former made of a non-metallic material, which is arranged inside the metallic enclosure. The aperture cross-section of the coil former matches the size and shape of the entrance and exit apertures and lines up with them, so that the coil former and the entrance and exit apertures form a tunnel defining a detection zone through which the conveyor belt or other transport means moves the articles or materials under inspection. The cross-section of this detection zone tunnel is generally rectangular or circular, but could also have any other shape. 
     In state-of-the-art metal detectors of this type, the coils are exactly parallel to each other and, consequently, their parallel planes are orthogonal to their common central axis. The center coil, also variously called transmitter coil, emitter coil, or excitation coil, is connected to a high-frequency oscillator and thus generates a primary alternating electromagnetic field which, in turn, induces a first and a second alternating voltage, respectively, in the two coils on either side of the center coil, which are also called the first and the second receiver coil. The first and second receiver coils are connected in series with each other, but with their windings wired in opposition to each other. In other words, the coil wire runs continuously from a first output terminal through the windings of the first receiver coil, then with the opposite sense of rotary direction through the windings of the second receiver coil to a second output terminal. In addition the first and second receiver coils are located equidistant from the transmitter coil. Therefore, they are in all respects mirror images of each other in relation to the central plane of the transmitter coil, and thus the first and the second alternating voltage induced in them by the primary alternating electromagnetic field will cancel each other. In other words, the mirror symmetry of this state-of-the-art metal detector has the result that the voltage picked up between the first and second output terminals will be zero. 
     Symmetrical balance coil arrangements can also consist of multiple transmitter coils and/or multiple receiver coils that are arranged in such a way to achieve a so called null balance condition. Therefore the first receiver coil can form one or more entrance-side receiver coils, and the second receiver coil one or more exit-side receiver coils. Likewise the transmitter coil can be designed as one or more transmitter coils. 
     However, if a piece of metal passes through the coil arrangement, the electromagnetic field is disturbed, giving rise to a dynamic voltage signal across the output terminals of the serially connected receiver coils. 
     The foregoing concept, often referred to as “balanced-coil system”, “inductively balanced metal detector” and similar terms, is commonly known in the field of industrial metal detectors. The generic principle is described and illustrated for example in U.S. Pat. No. 4,563,645 (col. 1, lines 11-32, and FIG. 1) as well as U.S. Pat. No. 7,061,236 B2 (col. 1, lines 20-41, and FIGS. 1 and 2a). 
     The metallic enclosure surrounding the coil arrangement serves to prevent airborne electrical signals or nearby metallic items and machinery from interfering with the proper functioning of the metal detector. In addition, the metal enclosure adds strength and rigidity to the assembly, which is absolutely essential as even microscopic dislocations of the coils relative to each other and relative to the enclosure can disturb the detection system which is sensitive to signals in the nanovolt range. 
     An issue of concern in metal detectors of the foregoing description is their sensitivity to stationary and, even more so, to moving metal in areas outside the detection zone and, in particular, even far outside the enclosure of the metal detector. This is due to the fact that the electromagnetic field generated by the transmitter coil extends outside the entrance and exit apertures to a distance as far as two or three times the length of the detection zone. If there are stationary or moving metal parts within this range, for example the support frame or other components of a conveyor, the interaction of the electromagnetic field with the metallic parts in its reach will produce an unwanted output signal of the receiver coils which interferes with the actual detection signals originating from metallic contaminants in the material under inspection traveling through the metal detector. Therefore, unless special design measures are taken, a large space before the entrance aperture and after the exit aperture of the metal detector has to be kept free of all metal. The area that must be kept free of metal in order to ensure the proper operation of a balanced-coil metal detector is generally called the “metal-free zone” or MFZ. 
     The metal-free zone, in particular its length in the direction of the transport path, is normally specified as a multiple of the aperture height (or diameter) h for stationary metal and for moving metal. According to EP 0 536 288 B1 (col. 2, lines 6-8, and FIG. 1), the MFZ extends to about 1.5×h for stationary metal and to about 2×h for moving metal. In any given application, the MFZ will dictate the metal detector system design, specifically the insertion space, i.e. the amount of space that must be allowed in a packaging or process line to accommodate the metal detector and its MFZ. 
     In applications where the space available for the metal detector is limited and where the foregoing guideline can therefore not be met, the interference due to metallic objects in the ambient vicinity could be suppressed by lowering the sensitivity of the metal detector to the point where the spurious signals are no longer registered. Of course, this would simultaneously reduce the useful detection sensitivity for metal contaminants inside the detection zone, i.e. it would handicap the metal detector in a clearly undesirable way. 
     A solution whereby the metal-free zone in the type of metal detector described hereinabove is reduced or even eliminated is presented in EP 0 536 288 B1, which is hereby incorporated by reference in the present disclosure. One of the possible means for reducing or eliminating the MFZ described in EP 0 536 288 B1 has the form of metallic flanges or collars that may be integral with the rims of the entrance and exit apertures of the enclosure of the metal detector. These flanges or collars act as short-circuit coils in which a current is induced by the alternating electromagnetic field of the transmitter coil. The induced current, in turn, generates a secondary electromagnetic field which can, under certain conditions, nullify the primary field of the transmitter coil beyond a certain distance before the entrance coil and after the exit coil, even to the extent that the primary field outside the apertures of the enclosure is totally suppressed and the metal-free zones before the entrance aperture and after the exit aperture are effectively reduced to zero providing a so-called “zero metal-free zone” (ZMFZ). 
     Because a state-of-the-art metal detector using the ZMFZ concept according to EP 0 536 288 B1 can be operated at the full detection sensitivity that it was designed for, even with metallic structures or machinery adjacent to one or both of its apertures, it is advantageous for installations where there is not enough space available to allow for the metal-free zones that would be required with a metal detector of an earlier state of the art. Nevertheless, the full sensitivity of a metal detector equipped with aperture flanges according to the ZMFZ concept is lower than the full sensitivity of a conventional metal detector operating with the required metal-free zones. Thus, the present state of the art still represents a compromise: while a ZMFZ metal detector significantly reduces or even eliminates the need for metal-free zones upstream and/or downstream in the processing line, it comes at the expense of a somewhat lower detection sensitivity in comparison to a conventional metal detector installed in a longer insertion space that allows for the metal-free zones. 
     SUMMARY 
     It is therefore the object of the present invention to provide an improved metal detector which employs the ZMFZ concept while approaching or matching the detection sensitivity of a conventional metal detector with standard metal-free zones. 
     This requirement is met by a metal detector having the features named in the independent patent claims. Various embodiments and refinements of the invention are presented in the dependent claims. Typical production- or packaging lines incorporating a metal detector according to the invention are presented in further claims. 
     The metal detector according to the present invention has a metallic enclosure with an entrance aperture and an exit aperture and, arranged inside the metallic enclosure, a coil system with at least one transmitter coil and at least one first and at least one second receiver coil. The entrance and exit apertures and the first and second receiver coils form a tunnel-like detection zone through which objects under inspection move along a travel path that enters the metal detector through the entrance aperture and leaves the metal detector through the exit aperture. 
     Preferably, relative to this travel path, the first receiver coil or coils are arranged ahead of the transmitter coil, and the second receiver coil or coils are arranged after the transmitter coil or coils. The first and second receiver coils are wired in series with each other, they have an equal small number of winding turns (typically a single turn), and they are wound with the opposite sense of rotation relative to each other. 
     The metal detector of the present invention comprises means for cancelling the primary field beyond a certain distance from the coil system. The means for cancelling the primary field are preferably configured in the form of metallic flanges or collars that are connected to, or integral with, the rims of the entrance and exit apertures of the metallic enclosure of the metal detector. The flanges or collars perform the function of short-circuit coils in which an alternating current is induced by the primary electromagnetic field of the transmitter coil. This induced current, in turn, generates a secondary electromagnetic field which nullifies the primary field of the transmitter coil beyond a certain distance from the coil system. 
     The metal detector of the present invention is distinguished from the prior art by the fact that the means for cancelling the primary field on the side of the entrance aperture and the means for cancelling the primary field on the side of the exit aperture are no longer equal and symmetric to each other relative to a central plane of the metal detector. In other words the metal detector of the present invention has a first cancelling means arranged at the entrance aperture and a different, second cancelling means arranged at the exit aperture. The first and second cancelling means differ from each other not only in their configuration, but also in their ability to cancel the primary electromagnetic field. Consequently, the primary field is cancelled beyond a first distance from the entrance aperture and beyond a different, second distance from the exit aperture. Accordingly, the metal detector according to the invention has a first metal-free zone which, relative to a travel path of said objects under inspection, extends upstream to a first distance from the entrance aperture, and a different second metal-free zone which extends downstream to a second distance from the exit aperture. 
     Due to the inductive interaction that takes place between the cancelling means and the coil system, the asymmetric cancelling means would result in a large imbalance between the voltages induced in the receiver coils of a symmetrical coil system. Therefore, the concept of asymmetric cancelling means first of all requires a solution to compensate for this voltage imbalance between the receiver coils without thereby disturbing the electrical signal circuit in other ways. In the process of the present invention, it was found that this problem can be solved if the first and second receiver coils are placed in asymmetric positions relative to the transmitter coil or coils and relative to the metallic housing and to the first and second cancelling means, wherein the asymmetric positions are determined so that the first and second voltages cancel each other when there is no metal present in said objects under inspection. In other words, the coil system is still a balanced coil system, but lacks the geometric symmetry of the coil systems in prior-art metal detectors. 
     Interestingly, a metal detector according to the invention showed improved sensitivity compared to a state-of-the-art metal detector using the ZMFZ concept according to EP 0 536 288 B1. 
     As balance coil arrangements can also consist of multiple transmitter coils and/or multiple receiver coils that are arranged in such a way to achieve a so called null balance condition, in the context of the following description and claims of the inventive concept, the term “transmitter coil” and/or “receiver coil” may stand for “at least one transmitter coil” and/or “at least one receiver coil”. 
     In an exemplary embodiment the transmitter coil or coils are positioned in a central plane between the entrance aperture and the exit aperture and the receiver coils are arranged each at a different distance from the transmitter coil, i.e. asymmetrically with regard to their position from said central plane. Alternatively the transmitter coil is positioned out of center between the entrance aperture and the exit aperture whereas the receiver coils are arranged each at a different distance from the transmitter coil but not necessarily from said central plane. 
     Of particular interest is an embodiment of the foregoing concept where a cancelling means is present at only one of the apertures, so that the primary electromagnetic field is cancelled only on the side of one aperture of the metal detector while remaining essentially undiminished on the side of the other aperture. In a practical embodiment of what could be called a one-sided ZMFZ design, the transmitter coil is placed essentially halfway between the entrance and exit apertures, the receiver coil next to aperture that has no cancelling means is positioned so as to optimize the sensitivity of the coil system to metal contaminants in the objects under inspection, and the position of the receiver coil next to the aperture with the cancelling means is determined by computer modeling or experimentation, so that the coil system is electrically balanced. 
     The choice of a metal detector with an asymmetric configuration according to the foregoing description suggests itself specifically for installations where a packaging- or production apparatus unit containing metallic components is arranged in line with the metal detector and directly adjacent to either the entrance aperture or the exit aperture, and wherein few or no metallic components are present within a metal-free zone extending outside the other aperture. In such a situation, which is quite common, a metal detector according to the invention with either asymmetric or totally one-sided cancelling means can be used very advantageously in that the suppression of the primary field—and thus the reduction or elimination of the metal-free zone—can be concentrated on the side of the metal detector where it is needed, while the opposite side of the metal detector can be designed with the aim of optimizing the overall sensitivity of the metal detector. 
     In a metal detector according to the invention, the detection zone normally has the form of a tunnel with a constant cross-sectional profile over its length from the entrance aperture to the exit aperture. This cross-sectional profile of the detection zone is typically of rectangular, quadratic, circular, or elliptical shape, but can also have any other shape that may be required in a practical application. 
     Typically in a metal detector according to the invention, the first and second receiver coils and the transmitter coil are wound on a common coil former in the shape of a hollow tube which is made of an electrically insulating non-metallic material and whose inside conforms to the cross-sectional profile of the detection zone. 
     The space between the coil former and the enclosure may contain sensitive electronic circuit components and is typically filled with a potting compound. The latter has the functions of keeping out moisture and of fixating the coil system and electronic components in their positions relative to the housing. 
     Metal detectors according to the invention are typically used in packaging- or production lines. Consequently, the scope of the invention also encompasses any packaging- or production line in which the metal detector as claimed, described and/or, illustrated herein is incorporated. 
     Particularly advantageous is the installation of a metal detector with a one-sided cancelling means as described hereinabove in a processing line where a packaging- or production apparatus unit containing metallic components is arranged immediately upstream of the entrance aperture or downstream of the exit aperture and where no metallic components are present within a metal-free zone extending outside the other aperture. In such a case, the one-sided cancelling means is arranged at the aperture nearest to the apparatus unit containing the metallic components. 
     In a typical packaging- or production line incorporating a metal detector of the present invention, the latter is arranged so that the tunnel-like detection zone and the travel path of the objects under inspection are oriented in a horizontal direction. The travel path can be constituted by a state-of-the-art conveyor belt. 
     In another typical packaging- or production line incorporating a metal detector of the present invention, the latter is arranged so that the tunnel-like detection zone and the travel path of the objects under inspection are oriented in a vertical direction. The travel path can be constituted by a chute through which the objects under inspection move in free fall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The metal detector according to the invention will hereinafter be explained in more detail through examples and with references to the schematically simplified drawings, wherein: 
         FIG. 1A  is a side view of a processing line incorporating a metal detector according to the prior art and a reject punch arranged downstream of the metal detector; 
         FIG. 1B  is a top view of the same processing line; 
         FIG. 2  schematically illustrates a preferred embodiment of the metal detector according to the present invention in a sectional view; 
         FIG. 3A  illustrates a special case of the metal detector of  FIG. 2  with only one aperture flange; 
         FIG. 3B  shows the metal detector of  FIG. 3A  with field lines of the primary electromagnetic field; and 
         FIG. 4  shows a top view of a processing line incorporating a metal detector according to the invention and a reject punch arranged downstream of the metal detector. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a side view (A.) and a top view (B.) of a processing line incorporating a conventional metal detector  1  according to the state of the art before EP 0 536 288 B1, which serves to detect metal contaminations in articles  2  moving in a direction of travel  13  on a conveyor belt  3  through the detection zone  4  which extends from the entrance aperture  5  to the exit aperture  6  of the metallic enclosure  7 . The configuration of the metal detector  1  is symmetric relative to a symmetry plane SP. Substantially equal metal-free zones  8  and  9  extend upstream from the entrance aperture  5  and downstream from the exit aperture  6 . Metallic parts of the processing line are placed outside the periphery of the metal-free zones  8  and  9 . In the illustrated example this concerns specifically apparatus units such as the belt rollers  10  and the reject punch mechanism  11  which serves to push rejected (i.e. metal-containing) articles off the conveyor belt  3 , where they fall into a reject bin  12 . 
     The substantially equal lengths d and e of the metal-free zones  8  and  9  in  FIGS. 1A and 1B  are realistically proportioned in relation to the aperture height h and to the length z of the metal detector enclosure. This illustrates the substantial space allowance that has to be made for the metal-free zones  8  and  9 , unless one opts for a ZMFZ-solution according to EP 0 536 288 B1, where the metal-free zones on both sides of the metal detector are equally suppressed. However, as explained previously herein, this comes at the expense of a somewhat lower detection sensitivity for metal contaminants inside the detection zone  4 . 
       FIG. 2  represents a first embodiment of a metal detector  20  according to the present invention in a vertical sectional plane along a central axis  29 . For clarity and consistency, the metal detector  20  is shown with the same orientation as the metal detector  1  of  FIG. 1 , i.e. with its entrance aperture  30  facing to the left side and its exit aperture  31  facing to the right side of  FIG. 2 . The principal parts of the metal detector  20  are the enclosure  21 , the coil former  22  with the transmitter coil  23  and receiver coils  24 ,  25 , and aperture flanges  26 ,  27  at the entrance and exit apertures  30 ,  31 , respectively. The coils  23 ,  24 ,  25  are imbedded in the coil former  22 , and the rotary direction of the coil windings is reversed between the first receiver coil  24  and the second receiver coil  25 . With the exit aperture flange  27  being substantially larger than the entrance aperture flange  26 , the metal detector  20  in the embodiment of  FIG. 2  exemplifies an asymmetric ZMFZ concept as explained previously herein. The enclosure  21  and the aperture flanges  26 ,  27  must be made of metal in order to perform their function of confining the primary magnetic field generated by the transmitter coil  23 . The coil former  22 , on the other hand, must be made of a non-conductive but mechanically stable material such as, e.g., a fiber-reinforced plastic. The coil former  22 , the aperture flanges  26 ,  27 , and the entrance and exit apertures  30 ,  31  form a tunnel-like detection zone  28  through which a product under inspection (not shown in the drawing) moves for example on a conveyor belt, entering the metal detector  20  through the entrance aperture  30  and leaving the metal detector  20  through the exit aperture  31 . The inside space contained between the enclosure  21 , the coil former  22  and the aperture flanges  26 ,  27  is filled with a potting compound  33  which serves to keep out moisture and to hold the coil former  22 , the aperture flanges  26 ,  27  and the enclosure  21  rigidly in place relative to each other. For installations with a conveyor belt, where the central axis  29  of the metal detector  20  is oriented horizontally, the cross-sectional profile of the detection zone  28  is preferably rectangular, and the dimension h indicates in this case the aperture height. However, other orientations and other profile shapes of the detection zone are likewise possible, for example a round, vertically oriented detection zone through which the products under inspection move in free fall through a chute-like detection zone  28 . In this case, the dimension h would indicate the aperture diameter. 
     The aperture flanges  26 ,  27  act as short-circuit coils in which a current is induced by the alternating or pulsating primary electromagnetic field of the transmitter coil  23 . According to Lenz&#39;s rule, an induced current always flows in such a direction as to oppose the field change that causes it. Accordingly, the secondary electromagnetic field generated by the induced current in the aperture flanges  26 ,  27  opposes the primary field. This applies especially to those parts of the aperture flanges  26 ,  27  that are aligned horizontally in  FIG. 2 . Depending on the design and dimensions of the aperture flanges  26 ,  27 , the secondary electromagnetic field can reduce or even totally cancel the primary field of the transmitter coil to the outside of the entrance and exit apertures  30 ,  31 . 
     The transmitter coil  23  is preferably positioned in the center of the length z of the detection zone  28 . The first and second receiver coils  24 ,  25  are placed asymmetrically, i.e. at different respective distances a and b from the transmitter coil  23 . The distances a and b are determined, e.g., experimentally or by computer modeling, with the simultaneous objectives of optimizing the detection sensitivity and balancing the induced voltages in the receiver coils against each other. 
     Alternatively the transmitter coil  23  can be positioned out of center between the entrance aperture  30  and the exit aperture  31  whereas the first receiver coil  24  and the second receiver coil  25  are arranged each at a different distance a, b from the transmitter coil  23  but not necessarily from said center of the length z of the detection zone  28 . 
     However, in some applications, one or both of the aperture flanges  26 ,  27  can extend outside the metal enclosure  21  (not shown in the figures) and form a collar which is connected to or integral with the rim of one of the enclosure apertures  30 ,  31 . 
       FIGS. 3A and 3B  illustrate a special case of the metal detector of  FIG. 2 , where one of the aperture flanges, in this case the flange  26  at the entrance aperture  30 , has been completely left out. Rather than solving the problems of optimizing the detection sensitivity and balancing the induced voltages in the receiver coils simultaneously, one could in this case start by placing the first receiver coil  24  at a distance a′ which is selected so as to maximize the detection sensitivity. Next, the second receiver coil  25 , i.e. the coil on the side with the aperture flange  27 , is positioned relative to the transmitter coil  23  at a distance b′ where the respective voltages induced in the receiver coils  24 ,  25  by the primary field cancel each other when there is no metal being detected in the detection zone. The distance b′ will generally be smaller than a′ and will depend on the flange length f and aperture height h. 
     The effect of the secondary field of the aperture flange  27  on the primary electromagnetic field of the transmitter coil  23  is illustrated in  FIG. 3B  for the same metal detector  20  as shown in  FIG. 3A . To the outside of the exit aperture  31 , the primary field generated by the transmitter coil  23  is essentially cancelled by the secondary field of the aperture flange  27 . This is graphically illustrated by the field lines  35 . For the sake of clarity, field lines reaching into the potted section  33  of the enclosure  21  are not shown. 
     Field lines on the side of the exit aperture  31  are deflected back into the detection zone  28 . In contrast, the primary field is not opposed by a secondary field on the opposite side of the detection zone  28 . Therefore, the same field lines  35  that are deflected back on the exit side extend undeflected through the entrance aperture  30  to the outside of the metal detector  20 . As a result, the metal detector  20  requires essentially no metal-free zone at the exit aperture  31 , while a standard metal-free zone extending about as far as 1.5 to 2.5 times the aperture height h is required outside the entrance aperture  30 . 
       FIG. 4  shows a top view of a processing line incorporating the asymmetric metal detector  20  of  FIGS. 3A and 3B . To allow an easy comparison,  FIG. 4  is placed on the same page and drawn to the same scale as  FIG. 1 . While the metal-free zone  48  on the entrance side of the metal detector  20  extends to the same first distance d as the symmetric metal-free zones  8  and  9  of the metal detector  1  in  FIG. 1 , the metal-free zone  49  on the exit side is now substantially shorter, extending to a second distance e≈d/4 in the illustrated example. This makes it possible to arrange the reject punch mechanism  11  practically next to the exit aperture  31 , so that the metal detector  20  according to the invention can be installed in a significantly shorter insertion space than the conventional metal detector  1 . At the same time, as the metal detector  20  employs the ZMFZ concept only on one side, the useful detection sensitivity of the metal detector  20  is not appreciably diminished in comparison to the conventional metal detector  1 . 
     While the invention has been described through the presentation of specific examples, it is evident that, based on the knowledge provided by the present disclosure, the invention could be embodied in numerous other variations with asymmetric field-cancelling means, where the field-cancelling means on the entrance side of the metal detector is different from the field-cancelling means on the exit side in order to achieve reductions of the respective metal-free zones as required for a given installation, while retaining the highest possible degree of useful sensitivity of the metal detector. Furthermore, a means for cancelling the primary field could also be a coil that is actively energized by an electronic circuit, in contrast to the metallic flanges or collars which are passive carriers of induced currents. 
     Further embodiments of the invention are conceivable using other state of the art symmetrical balance coil arrangements that consist of multiple transmitter and/or multiple receiver coils that are arranged asymmetrically to achieve the null balance condition and optimized sensitivity within the asymmetric ZMFZ configuration. 
     It should be understood that all such variations and combinations are considered to be within the scope of the present invention.