Abstract:
An apparatus and process are provided for reducing the electromagnetic field intensity in selected regions for a field produced when ac current flows through one or more induction coils through which a workpiece moves for induction heating of the workpiece. The electromagnetic shield has transverse screen elements through which the workpiece moves at opposing ends of the one or more induction coils. One or two longitudinal screen elements connect the transverse screen elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/482,825 filed Jun. 26, 2003, hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electromagnetic shields or screens used with induction heating coils that inductively heat a workpiece moving through the induction coils. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  illustrates a single turn induction coil  102  surrounding a metal strip  104  that continuously moves through the coil in the direction indicated by the arrow. Terminals  106  and  108  of coil  102  are connected to a suitable ac power source with instantaneous current flow through the coil as indicated by the arrows. The current flowing through the coil establishes a magnetic field that inductively heats the strip as it passes through the coil. However unless some type of electromagnetic shielding is used, the generated magnetic field intensities in regions beyond the region in which the coil and strip are magnetically coupled may exceed permissible levels. 
     One known method of reducing field intensities in the desired regions is to place electrically conductive screens  110  and  111 , such as a rectangular copper form, around the strip as shown in FIG.  1 . As understood from Faraday&#39;s law, instantaneous current flow induced by the magnetic field in each screen will be opposite in direction to the instantaneous current flow in the induction coil. In the arrangement in  FIG. 1 , current will flow in the same direction in the upstream (element  111 ) and downstream (element  110 ) screens. In  FIG. 2 , two single turn coils  102   a  and  102   b  are used with instantaneous current flow in opposite directions established by connecting terminals  106   a  and  108   a , and terminals  106   b  and  108   b , to one or more suitable ac power supplies. With the arrangement of  FIG. 2 , current flow will be in opposite directions in the upstream and downstream screens. One disadvantage of the arrangements in FIG.  1  and  FIG. 2  is that each screen is closed around the strip, which makes lateral movement of the strip in and out of the screens impossible. Such lateral movement is desirable for changing induction coils without cutting a continuous workpiece. 
     U.S. Pat. No. 5,034,586 discloses a method of using decoupling rings (elements  20  and  22 ) with a switch (element  24 ) in the rings. The switch opens to allow a workpiece to be moved laterally out of, or into, the decoupling rings. The decoupling rings reduce inter-coil magnetic coupling between adjacent induction coils. One disadvantage of this method is that an electrically conductive component, namely the decoupling rings, must include a mechanical switch that reduces reliability and increases operational complexity and maintenance. 
     Therefore there is the need for a means of electromagnetic shielding of an induction coil, through which a moving workpiece passes to be inductively heated, that would allow the workpiece to laterally move in and out of the electromagnetic shielding without the use of mechanical switches. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the present invention is an apparatus for, and method of, electromagnetically shielding the magnetic field produced around one or more induction coils through which a workpiece continuously moves. The electromagnetic shield comprises first and second transverse screen elements between which the one or more induction coils are disposed. In one example of the invention the first and second transverse screen elements are connected together by a single longitudinal screen element longitudinally aligned along one side of the workpiece so that a continuous single current path is provided through the two transverse screen elements and the longitudinal screen element. In another example of the invention the first and second transverse screen elements are connected together by two longitudinal screen elements, longitudinally aligned on opposing sides of the workpiece to provide opposing current paths in the screen halves formed between the workpiece. In another example of the invention one of the screen halves is hinged to allow further longitudinal opening between the screen halves. In another example of the invention, the one or two longitudinal screen elements are extended around the perimeter of the first and second transverse screen elements to form, respectively one substantially closed conducting box, or a substantially closed conducting box formed from two half-sections, around the one or more induction coils, while allowing the workpiece to laterally move in and out of the electromagnetic shielding formed by the box structure in place without the use of mechanical switches. 
     Other aspects of the invention are set forth in this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
         FIG. 1  is a diagrammatic view of a prior art induction coil used to inductively heat a workpiece continuously moving through it and electromagnetic screens used to reduce field intensities produced by ac current flow in the coil in regions beyond the region in which the coil and strip are magnetically coupled. 
         FIG. 2  is a diagrammatic view of a prior art arrangement of two induction coils with opposing instantaneous ac current flow used to inductively heat a workpiece continuously moving through it and electromagnetic screens used to reduce field intensities produced by current flow in the coil in regions beyond the region in which the coil and strip are magnetically coupled. 
         FIG. 3  is a diagrammatic view of one example of the electromagnetic screen of the present invention. 
         FIG. 4  is a diagrammatic view of another example of the electromagnetic screen of the present invention. 
         FIG. 5  is a diagrammatic view of another example of the electromagnetic screen of the present invention wherein the longitudinal screen element shown in  FIG. 3  is extended around the induction coils to form a box-shaped electromagnetic screen. 
         FIG. 6  is a diagrammatic view of another example of the electromagnetic screen of the present invention wherein the longitudinal screen elements shown in  FIG. 4  are extended around the induction coils to form a box-shaped electromagnetic screen. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like numerals indicate like elements, there is shown in the  FIG. 3 , one example of the electromagnetic shield or screen  10  of the present invention that forms an electromagnetic shield. The screen comprises first transverse screen element  10   a , second transverse screen element  10   b , and longitudinal screen element  10   c , which connects the first and second transverse elements together as shown in FIG.  3 . Longitudinal screen element  10   c  consists of first and second longitudinal screen sections  10   c ′ and  10   c ″ that are substantially parallel to each other and separated by gap h 1 . In this non-limiting example of the invention, two single turn coils  12   a  and  12   b  are suitably connected to one or more ac power sources so that instantaneous ac current flows in opposing directions in the two coils as indicated by the arrows. The coils are disposed between the first and second transverse screen elements of screen  10 . A continuous workpiece  104 , such as a metal strip, moves through the induction coils and screen, and is inductively heated by current flow in the coils. In general, but not by way of limitation, the planes of the first and second transverse screen elements are substantially perpendicular to the direction in which the workpiece moves. All drawings of the present invention are diagrammatic; positioning of the screen elements relative to the workpiece can be adjusted to suit a particular application. In this non-limiting arrangement ac current flow in screen  10  is as indicated by the arrows. The gap between the first and second longitudinal screen sections of longitudinal screen element  10   c , namely h 2 , provides a means for lateral movement of the strip from in and out of the shield. The opening h 2  is sufficiently small so that opposing electromagnetic fields established by opposing instantaneous currents in the two longitudinal screen sections that comprise longitudinal screen element  10   c  substantially cancel each other out. If gap h 2  is too small to allow lateral movement of the strip in and out of the screen, then screen  10  must include flexing means for moving the two longitudinal screen sections apart so that the workpiece can be moved laterally away from shield. For example the section of the first and second transverse screen elements located adjacent to the edge of the workpiece opposite the edge of the workpiece adjacent to gap h 2  may be formed from a material that allows non-deforming flexing apart of the two longitudinal screen sections to increase the size of gap h 2 . 
     Gap h 2  establishes a discontinuity in the first and second transverse screen elements. First longitudinal screen section  10   c ′ is connected between the first end of gap h 2  in the first transverse screen element and the first end of gap h 2  in the second transverse screen element; second longitudinal screen section  10   c ″ is connected between the second end of gap h 2  in the first transverse screen element and the second end of gap h 2  in the second transverse screen element. In this arrangement a closed screen circuit path is established by the first transverse screen element, the first longitudinal screen section, the second transverse screen element, and the second longitudinal screen section. As previously stated the first and second longitudinal screen sections  10   c ′ and  10   c ″ that make up the longitudinal screen element  10   c  are disposed relative to each other so that opposing electromagnetic fields established by opposing instantaneous currents in the first and second longitudinal screen sections substantially cancel each other out. 
     In other examples of the invention the first and second longitudinal screen sections that comprise longitudinal screen element  10   c  can be extended around induction coils  12   a  and  12   b  to form a substantially closed conducting box around the coils. For example as shown in  FIG. 5 , wherein coils  12   a  and  12   b  are not shown for clarity, the first and second longitudinal screen sections  10   c ′ and  10   c ″ are extended (partially shown as cross hatched regions in  FIG. 5 ) around the outer perimeters of first and second transverse elements  10   a  and  10   b  to form a substantially closed box-shaped electromagnetic shield around the coils and the workpiece. The box is open along gap h 2 , and sufficiently open at each end at least to allow the workpiece to pass through. 
       FIG. 4  illustrates another example of the present invention. In this arrangement first and second transverse screen elements  11   a  and  11   b , respectively, are connected together by first and second longitudinal screen elements  11   c  and  11   d  to form shield or screen  11 . Longitudinal screen element  11   c  consists of first and second longitudinal screen sections  11   c ′ and  11   c ″, respectively, that are substantially parallel to each other and separated by gap h 3 . Longitudinal screen element  11   d  consists of first and second longitudinal screen sections  11   d ′ and  11   d ″, respectively, that are substantially parallel to each other and separated by gap h 4 . Generally, but not by way of limitation, gaps h 3  and h 4  are equal to each other. In this arrangement an integral half screen is formed on either side of the workpiece. In this non-limiting example of the invention, two single turn coils  12   a  and  12   b  are suitably connected to one or more ac power sources so that instantaneous ac current flows in opposing directions in the two coils as indicated by the arrows. The coils are disposed between the first and second transverse screen elements of screen  11 . A continuous workpiece  104 , such as a metal strip, moves through the induction coils and screen, and is inductively heated by current flow in the coils. In general, but not by way of limitation, the planes of the first and second transverse screen elements are substantially perpendicular to the direction in which the workpiece moves. In this non-limiting arrangement ac current flows in each of the two half screens as indicated by the arrows. The openings h 3  and h 4  are sufficiently small so that opposing electromagnetic fields established by opposing instantaneous currents in the two longitudinal screen sections that comprise screen longitudinal elements  11   c  and  11   d  substantially cancel each other out. 
     For the example of the invention shown in  FIG. 4  one, or both, screen halves may be retracted from around the induction coils without impacting the inductive coupling between the induction coils and the workpiece. Unlike the previous example of the invention illustrated in  FIG. 3 , if the gaps between longitudinal screen sections comprising longitudinal screen elements  11   c  and  11   d  are not large enough to laterally remove the workpiece from within the there is no need for flexible parts in the screen to enlarge the gap to allow the lateral movement of the workpiece in and out of the screen. Alternatively, hinge means can be provided to rotate one or both of the screen halves about a longitudinal axis, such as longitudinal axis Z 1  in FIG.  4 . 
     In the example of the invention in  FIG. 4 , gap h 3  establishes a first discontinuity in the first and second transverse screen elements, and gap h 4  establishes a second discontinuity in the first and second transverse screen elements. First longitudinal screen section  11   c ′ is connected between the first end of gap h 3  in the first transverse screen element and the first end of gap h 3  in the second transverse screen element; second longitudinal screen section  11   c ″ is connected between the second end of gap h 3  in the first transverse screen element and the second end of gap h 3  in the second transverse screen element. Third longitudinal screen section  11   d ′ is connected between the first end of gap h 4  in the first transverse screen element and the first end of gap h 4  in the second transverse screen element; fourth longitudinal screen section  11   d ″ is connected between the second end of gap h 4  in the first transverse screen element and the second end of gap h 4  in the second transverse screen element. In this arrangement a first closed screen circuit path is established by the first half of the first transverse screen element, the first longitudinal screen section, the first half of the second transverse screen element, and the third longitudinal screen section, all of which comprise the first screen half. Similarly a second closed screen circuit path is established by the second half of the first transverse screen element, the second longitudinal screen section, the second half of the second transverse screen element, and the fourth longitudinal screen section, all of which comprise the second screen half. As previously stated the first and second longitudinal screen sections  11   c ′ and  11   c ″ that make up the first longitudinal screen element  11   c , are disposed relative to each other so that opposing electromagnetic fields established by opposing instantaneous currents in the first and second longitudinal screen sections substantially cancel each other out, as are the third and fourth longitudinal screen sections  11   d ′ and  11   d ″ that make up the second longitudinal screen element  11   d.    
     In other examples of the invention the first and second longitudinal screen sections that comprise longitudinal screen elements  11   c  and  11   d  can be extended around induction coils  12   a  and  12   b  to form a substantially closed conducting box around the coils. For example as shown in  FIG. 6  wherein coils  12   a  and  12   b  are not shown for clarity, the first and third longitudinal screen sections  11   c ′ and  11   d ′, respectively, are extended around the outer perimeters of the first halves of the first and second transverse elements  11   a  and  11   b , and the second and fourth longitudinal screen sections  11   c ″ and  11   d ″, respectively, are extended around the outer perimeters of the second halves of the first and second transverse elements  11   a  and  11   b  to form a substantially closed box-shaped electromagnetic shield around the coils and the workpiece. The box is open along gaps h 3  and h 4 , and sufficiently open at each end at least to allow the workpiece to pass through. 
     The term “screen” is used in all examples of the invention to describe a substantially electrically conductive material in any form such as, but not limited to, a solid or mesh material formed from copper or an aluminum based alloy. 
     Two individual single turn coils are used in the examples of the invention only to illustrate and not limit the scope of the invention. Generally the arrangement of induction coils, both quantities and types of coils, between the two transverse screen elements, and directions of instantaneous current in the induction coils can be any arrangement that results in the screen currents as disclosed in the above examples of the invention. Further the coil arrangements disclosed in U.S. Pat. No. 5,495,094 and No. 5,837,976 may be used in the examples of the invention, and both of these patents are incorporated herein by reference in their entirety. 
     The foregoing examples do not limit the scope of the disclosed invention. The scope of the disclosed invention is further set forth in the appended claims.