Abstract:
A magnetic field generator ( 10 ) comprising an assembly ( 20 ) of permanent magnets ( 30 ) inside which the magnetic flux concentrates. The assembly comprises opposed first and second elements ( 21, 22 ) that include magnets ( 30 ). The elements ( 21 ) and ( 22 ) are arranged substantially in the same plane and surrounded by respective closing mechanisms ( 51, 52 ). The permanent magnets ( 30 ) are made up of parallelepipedal blocks, arranged substantially in an arc of a circle according to three areas: a central area ( 60 ), a first side area ( 70 ) on one side of the central area ( 60 ), and a second side area ( 80 ) on the other side of the central area ( 60 ). The permanent magnets ( 30 ) of the first and second ( 70, 80 ) side areas have opposite directions of magnetization. Two parts ( 91 ) and ( 92 ) of a ferromagnetic material, forming a magnetic flux concentrator ( 90 ), are arranged on either side of the air gap ( 40 ).

Description:
[0001]    This application is a National Stage Completion of PCT/CH2010/000143 filed Jun. 1, 2010 which claims priority from Swiss Application Serial No. 837/09 filed Jun. 2, 2009. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention concerns a magnetic field generator comprising at least one assembly of anisotropic permanent magnets for creating a magnetic field and defining an air gap within which the magnetic flux is concentrated, 
         [0003]    said assembly comprising a first element and a second element mounted across from each other and symmetrically relative to an axis perpendicular to the transverse axis of the air gap; 
         [0004]    each of said first and second elements comprising at least three permanent magnets, and 
         [0005]    said first and said second elements of said assembly of magnets being disposed generally in the same plane and at least partially surrounded, respectively, by mechanisms for closing the magnetic field. 
         [0006]    A further objective is a thermal magnetocaloric device comprising at least one magnetic field generator according to the invention and one magnetocaloric element traversed by a heat transporting fluid circulating alternately towards a first extremity of said thermal generator and towards its second extremity, as well as a means for magnetically activating and deactivating the displacement of the magnetocaloric element relative to said magnetic field generator. 
       BACKGROUND OF THE INVENTION 
       [0007]    In order to obtain a strong magnetic field in a defined area, the technique of forming an assembly of permanent magnets already exists. The literature describes such assemblies, in particular for magnetic resonance imaging applications in the medical domain. In this domain, rings of permanent magnets are formed and arranged side by side. The structure of the utilized permanent magnets, however, is difficult to achieve, which increases the cost of magnet assemblies. 
         [0008]    For this reason, it is not possible to transpose such magnetic structures, particularly in the domain of magnetocaloric thermal generators. Actually, with these generators, it is imperative to generate a uniform, intense and variable magnetic field in an air gap essentially corresponding to the volume of a material or a magnetocaloric element so that the magnetic field created can successively magnetically activate and deactivate one or more magnetocaloric materials alternately introduced and then withdrawn from the air gap. 
         [0009]    In order to create a strong magnetic field in a defined space, a method exists for forming an assembly of permanent magnets according to a Halbarch structure. The literature, particularly the following publications: J. Lee, J. M. Kenkel and D. C. Jiles, “Design of Permanent-Magnet Field Source for Rotary-Magnetic Refrigeration Systems,” IEEE Trans Magn 38 5 (2002), pp. 2991-2993; K. Halbach, “Nucl. Instr. Methods,” Vol. 169, p. 1 (1981); F. Bloch, O. Gugat, J.C. Toussaint and G. Meunier, IEEE Trans. Magn., Vol. 34, p. 2465 (1998); “CERN Courier”, Vol. 43, No. 3, p. 7 (2002); and S.J. Lee and D.C. Jiles, IEEE Trans. Magn., Vol. 36, No. 5, p. 3105 (2000) describes such assemblies, particularly for an application in the medical domain of magnetic resonance imaging. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention attempts to overcome the drawbacks of the prior art by proposing a device for generating an intense and uniform magnetic field that is easy to achieve and low cost. 
         [0011]    To achieve this, the invention concerns a magnetic field generator such as the one described in the preamble, characterized in that said permanent magnets on said first and second elements of said magnet assemblies consist of parallelepipedal shaped blocks, in that they are arranged in a generally circular arc in three zones, a central zone located opposite said air gap, a first lateral zone located beside said central zone, and a second lateral zone located on the other side of said central zone, with said permanent magnets of said first and second lateral zones magnetizing in the direction opposite to the perpendicular transverse axis of said air gap, and in that at least two pieces of ferromagnetic material, constituting a magnetic flux concentrator, are disposed on either side of said air gap, respectively, between the permanent magnets of said first lateral zones of the first and second elements of the magnetic assembly, located on one side of said central zones and between the permanent magnets of said second lateral zones of the first and second elements of the assembly of magnets, located on the other side of said central zones. 
         [0012]    Preferably the permanent magnets may consist of blocks with a parallelepipedal shape and with a rectangular and/or trapezoidal transverse cross-section. 
         [0013]    According to a preferred embodiment, the generator comprises several groups of permanent anisotropic magnet assemblies, with said magnet assemblies being identical and the group comprising a single air gap, each group of magnet assemblies creating a magnetic flux and comprising a means for concentrating the magnetic flux generated by said group of magnet assemblies inside said single air gap. 
         [0014]    Advantageously, the generator may comprise several groups of permanent anisotropic magnet assemblies, said magnet assemblies being different, juxtaposed and arranged to form a single air gap, each group of magnet assemblies creating a magnetic flux and comprising a means of concentrating the magnetic flux generated by said group of magnet assemblies inside said single air gap. 
         [0015]    According to a particularly advantageous embodiment, each of the mechanisms for closing the magnetic field on said first and second elements of said magnet assembly has an essentially circular arc interior profile corresponding to the circular arc arrangement of the three permanent magnet zones in said first and second elements of said magnet assembly. 
         [0016]    Preferably, said permanent magnets are arranged in said magnet assembly in such a way that 
         [0017]    in said central zone they are magnetized approximately tangentially to the adjacent surface of the mechanism for closing the corresponding magnetic field; and 
         [0018]    in said first and second lateral zones, they are magnetized is perpendicularly to the corresponding surface of the mechanism for closing the corresponding magnetic field. 
         [0019]    In a particularly advantageous manner, in said first and second lateral zones, the permanent magnets are magnetized perpendicularly to the adjacent surface of the two pieces of the corresponding magnetic flux concentrator. 
         [0020]    The permanent magnets in the two lateral zones may each be mounted on one of the pieces of the corresponding magnetic flux concentrator. 
         [0021]    The pieces of said magnetic flux concentrator may have oblique surfaces on one side corresponding in shape to the surface of the corresponding permanent magnets in the two lateral zones and on the other side, a projecting portion at the level of the air gap. 
         [0022]    According to one particular design, each first and each second element of said magnet assembly is respectively associated with one first and one second mechanism for closing the magnetic field. 
         [0023]    According to another particular design, each first and each second element of said magnet assembly is respectively associated with several first and several second mechanisms for closing the magnetic field. 
         [0024]    The invention also concerns a thermal magnetocaloric device such as the one defined in the preamble and characterized in that said magnetic field generator constitutes the means for magnetically activating and deactivating the magnetocaloric element, and in that said magnetocaloric element is located in the air gap of said magnetic field generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The present invention and its features will be more apparent from the following description of embodiments provided by way of non-limiting examples, with reference to the attached drawings, in which: 
           [0026]      FIG. 1  is a perspective view of a first embodiment of a magnetic field generator according to the invention; 
           [0027]      FIG. 2  is a perspective view of a second embodiment of a magnetic field generator according to the invention; 
           [0028]      FIG. 3  is an elevation illustrating the arrangement of the magnets in the embodiment of  FIG. 1 ; 
           [0029]      FIG. 4  is a schematic plane view representing the distribution of the field lines in the embodiment of  FIG. 1 ; 
           [0030]      FIG. 5  is a partial cross-section of the magnetic field generator shown in  FIG. 2 ; 
           [0031]      FIG. 6  is a partial cross-section of a variation of the embodiment of the magnetic field generator shown in  FIG. 2 ; and 
           [0032]      FIGS. 7A through 7C  are complete and partial perspectives, respectively, and an elevation, of a third embodiment of the magnetic field generator according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]      FIG. 1  represents an elementary embodiment of a magnetic field generator  10  according to the invention. This magnetic field generator  10 , in this case, is composed of an assembly  20  of permanent anisotropic magnets  30  which create a magnetic flux and define an air gap  40  inside which the magnetic flux is concentrated. The assembly  20  of magnets consists of two elements  21  and  22  mounted across from each other symmetrically in relation to an axis A-A, and perpendicular to a transverse axis B-B of the air gap  40 . Elements  21  and  22  are identical and arranged facing each other. In the shown example, they each comprise five permanent magnets  30 , a number that may vary depending upon the embodiment desired, arranged in two opposing circular arcs. In this case, these permanent magnets are essentially rectangular in cross-section, such that a generally triangular space exists between two adjacent magnets. These permanent magnets  30 , which form the two elements  21  and  22  of assembly  20 , are separated by a magnetic flux concentrator  90  consisting of two elements  91  and  92  made of ferromagnetic material. These elements  91  and  92  may be made of ferromagnetic steel, for example, and may have oblique surfaces  93  and  94  corresponding in shape to an adjacent surface of permanent magnets  30 , with these surfaces being in contact with said ferromagnetic elements  91  and  92 . A closed magnetic flux loop results, thanks to two mechanisms  51  and  52  for closing the magnetic field, made of ferromagnetic material, surrounding permanent magnets  30  and attached to the latter through magnetic attraction. The two closing mechanisms  51  and  52  are held in place by two transverse clamps screwed in place. Closing mechanisms  51  and  52  may also have a layered structure to increase their effectiveness. 
         [0034]    The configuration of the magnetic field generator  10 , shown in  FIG. 1 , permits the use of permanent magnets  30  with an easy to achieve shape. For this reason, the geometric shape of said permanent magnets  30  is a parallelopipedal block and they are anisotropically magnetized. The parallelopipedal blocks have a square cross-section, but could also be configured with a rectangular cross-section. Likewise, the parallelopipedal blocks could be cubical in shape. 
         [0035]    As  FIG. 3  shows in particular, permanent magnets  30  of assembly  20  are arranged at the level of each of the circular arcs they form in three zones, specifically, a central zone  60  located opposite said air gap  40  and which comprises, in the example shown, a single permanent magnet  30  that is magnetized tangentially to the closing mechanism  51  or  52  for the corresponding magnetic field, and two other zones called adjacent zones  70  and  80  that have magnets mounted on magnetic flux concentrator  90 , more specifically, on oblique surfaces  93 ,  94  of ferromagnetic pieces  91  and  92  constituting it. The permanent magnets  30  in the adjacent zones number two for each of said zones,  70  and  80 , respectively, for each element  21  and  22  of assembly  20 . Note that said zones adjacent to central zone  60 , called the first lateral zone  70  and the second lateral zone  80 , located on either side of central zones  60 , are magnetized in opposite directions. These directions are perpendicular to both said concentrator  90  and to ferromagnetic elements  91  and  92  constituting it, as well as to mechanisms  51  and  52  for closing the magnetic field. 
         [0036]    The space between the two pieces  91  and  92 , forming magnetic flux concentrator  90 , constitutes the air gap  40  of the magnetic field generator  10  shown. To further improve a magnetic flux density in this gap  40 , these two pieces  91  and  92  each comprise a projecting portion  55  extending into said gap  40 . 
         [0037]      FIG. 4  shows the magnetic flux lines in magnetic field generator  10  of  FIG. 1 . Note that the flux lines are concentrated at the level of gap  40 . Arched magnetic field closing mechanisms  51  and  52  and the two pieces  91  and  92  of magnetic flux concentrator  90  made of ferromagnetic material allow the formation of regular magnetic flux loops concentrated uniquely in the space of magnetic field generator  10 , which are dominant and uniformly distributed throughout gap  40 . This results in a generator  10  capable of generating an intense field in its gap  40 , despite using a reduced number of permanent magnets, giving it an advantageously simple, economical structure. 
         [0038]    Magnetic field generator  100  shown in  FIG. 2  is composed of a unit of assemblies  20  of identical juxtaposed magnets. In the example shown, the number of these magnet assemblies  20  is three, but it could either be limited to two or increased depending upon the parameters desired for a specific given application. Since the lateral dimension of each magnet assembly  20  is defined by the width of mechanisms  51  and  52  for closing the magnetic field, the magnet assemblies are placed against each other and held in position by a suitable mechanical attachment (not shown). 
         [0039]    In the embodiments shown in  FIGS. 1 and 2 , a closing mechanism  51 ,  52  is associated with each first and second element  21 ,  22 , respectively. This construction is more apparent in  FIG. 5 , which shows magnetic field generator  100  of  FIG. 2  in partial cross-section. 
         [0040]    It is also possible to associate several closing mechanisms  51 ,  52  with each first or second element  21 ,  22 . In such a configuration, shown by magnetic field generator  110  in  FIG. 6 , the length of permanent magnets  30  (along longitudinal axis C-C of air gap  40 ) is greater than the width of each closing mechanism  51 ,  52  taken independently. The advantage of this type of configuration resides in its ease of assembly. It could also be expected that for a given generator length, this configuration allows a stronger magnetic field to be obtained in air gap  40 . In magnetic field generator  110  shown in  FIG. 6 , the length of a permanent magnet  30  corresponds to the width of three closing mechanisms  51 ,  52  arranged side by side. However, this number is not limitative and it may vary depending on the design or on the applications at hand. 
         [0041]    A third embodiment of a magnetic field generator according to the invention is shown in  FIGS. 7A through 7C . As with magnetic field generator  10  in  FIG. 1 , magnetic field generator  120  comprises an assembly  20  of permanent magnets  30 , said magnet assembly being composed of two identical elements  21 ,  22  arranged opposite each other. It is noteworthy that in the designs described previously, permanent magnets  30  are shaped like blocks with a rectangular cross-section. Given that they are arranged in two opposing circular arcs, the geometry of their rectangular cross-section creates a generally triangular corner between two adjacent permanent magnets  30 . This space, filled with air, does not optimally conduct the field lines, causing loss of magnetism and, to some extent, reducing the strength of the magnetic field generated. This embodiment, illustrated by  FIGS. 7A through 7C , solves that problem by filling in the space between two blocks of adjacent permanent magnets. It is for this reason that at least one permanent magnet  31 , located between two adjacent permanent magnets  30  with rectangular cross-sections, has a trapezoidal cross-section that eliminates all the “openings” in the magnetic circuit and creates a sort of uninterrupted magnetic ring around elements  91  and  92  of magnetic flux concentrator  90 . Additionally, the geometry of mechanisms  51  and  52 , for closing the magnetic field in this embodiment, differs from the geometry of the embodiments shown in  FIGS. 1 through 6  in that it is semi-hexagonal in shape and not semi-cylindrical. 
         [0042]    Magnetic field generators  10 ,  100 ,  110  and  120  illustrated by all the drawings are particularly useful in a thermal magnetocaloric device comprising at least one magnetocaloric element. This magnetocaloric element may consist of one or more magnetocaloric materials and it is traversed by a heat-transporting fluid circulating alternately towards the first extremity of said thermal generator and then towards its second extremity, in a synchronized manner and with a means for magnetically activating and deactivating said magnetocaloric element. The purpose of this magnetic activation and deactivation means is to successively and alternately subject said magnetocaloric element to a magnetic field and then to a null field; this is achieved by the movement of magnetic field generator  10 ,  100 ,  110 ,  120  of the invention relative to said magnetocaloric element in order to achieve the variation in the magnetic field. Preferably, the magnetocaloric element that slides within the air gap of said magnetic field generator is driven in forward and backward translational movement. 
         [0043]    Possibilities for Industrial Application 
         [0044]    It is clear from this description that the invention achieves the stated goals, that is, providing a generator to create a magnetic field that is structurally simple, economical, and furnishes a strong magnetic field using relatively little magnetized material. Such a generator assuredly has industrial as well as domestic applications when integrated into a magnetocaloric thermal device designed for heating, air conditioning, temperature modulation, cooling or the like. It is competitively priced and compact in size.