Abstract:
The invention provides an active element for an electromagnetic machine, the element comprising an alternating succession in a main direction of portions presenting a first magnetic property and of portions presenting a second magnetic property, wherein the element includes a non-magnetic covering that is thin relative to a thickness of the portions and that extends to cover a substantial fraction of an outside surface of the active element, the covering being secured to at least some of the portions and presenting sufficient strength to form a member for mechanically transmitting the magnetic forces to which the portions are subjected.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in International Application No. PCT/FR2007/001088 filed on Jun. 28, 2007 and French Patent Application No. 06 06335 filed on Jul. 12, 2006. 
     FIELD OF THE INVENTION 
     The invention relates to an active element for an electromagnetic machine, to a method of fabricating such an active element, and to a machine including such an active element. 
     BACKGROUND OF THE INVENTION 
     Linear electromagnetic machines are known that have two parts that move relative to each other in a sliding direction. At least one of the parts extending in the sliding direction includes active elements, each made up of a succession of portions having different magnetic properties. 
     In variable-reluctance machines, the portions of each active element alternate between ferromagnetic portions and non-magnetic portions. In permanent-magnet machines, the active elements of one of the parts comprise portions that alternate between ferromagnetic portion and non-magnetic portions, while the active elements of the other part comprise magnetized portions that alternate between portions magnetized in a first direction and portions magnetized in a second direction opposite to the first. 
     The two parts are placed to interact electromagnetically, one of the parts being associated with means for generating a magnetic field. Magnetic-field generation causes magnetic forces to appear that tend to move the parts relative to each other in the sliding direction. Alternatively, the machine may be operated as a generator by imparting a relative movement between the two parts. 
     In a first type of electromagnetic machine, the portions making up the active elements are in the form of plates or blades. The active elements of the two parts are parallel and form respective interleaved combs such that an active element of one of the parts extends between two active elements of the other part (naturally with the exception of the outermost active elements). 
     In an embodiment shown in FIG. 5 of document FR 2 588 133, the portions forming the active elements of the movable part are subdivided into two sub-portions extending on either side of a central support that holds the sub-portions at one end thereof and that takes up the magnetic forces to which the portions are subjected. The portions forming the active elements of the stationary part are likewise subdivided into two sub-portions, each being held via one of its ends by an external support. 
     Those supports are complex to fabricate and they receive each of the sub-portions via one end only, which means that the sub-portions are cantilevered out and that they are therefore subjected to stresses that tend to separate them from the supports that receives them. In addition, the end of each sub-portion that is engaged in a support is not in magnetic interaction with the sub-portions of the facing active elements, and those ends therefore do not participate in the operation of the machine. In addition, the presence of central supports and of external supports increases the overall size of the machine. The supports do not participate in the magnetic interaction between the active elements, so they take up precious space and tend to limit the volume power density of the machine. 
     In a second type of electromagnetic machine, as illustrated in the article “Actionneur linéaire synchrone machine à aimants permanents multi-tiges” [Synchronous linear actuators with multiple-rod permanent magnets], presented to the colloquium “Electrotechnique du futur, SUPELEC, Dec. 9-10, 2003”, the active elements of the movable part are in the form of rods and comprise cylindrical portions threaded onto a central support. The central support holds the portions and mechanically takes up the magnetic forces to which the portions are subjected, and it stiffens the rod. 
     As with plates, the central support does not contribute to the magnetic interactions and it occupies precious space, tending to limit the volume power density of the machine. 
     As suggested in document FR 2 588 133, it is possible to omit the support and to bond the portions of a given active element to one another by adhesive or by brazing. Nevertheless, that method assumes that the portions are mutually adjacent and that they are suitable for being bonded together by brazing or adhesive. The magnetic forces are then transmitted mechanically in traction through the bonds made in this way, which is not ideal from a mechanical point of view. 
     OBJECT OF THE INVENTION 
     An object of the invention is to provide a novel active element that provides greater efficiency. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to achieve this object, the invention provides an active element for an electromagnetic machine, the element comprising an alternating succession in a main direction of portions presenting a first magnetic property and of portions presenting a second magnetic property. According to the invention, the active element includes a non-magnetic covering that is thin relative to a thickness of the portions and that extends to cover a substantial fraction of an outside surface of the active element, the covering being secured to at least some of the portions and presenting sufficient strength to form a member for mechanically transmitting the magnetic forces to which the portions are subjected. 
     Thus, the covering is placed on the outside of the active element, i.e. in a zone of greatest second moment of area, such that the covering contributes greatly to stiffening the active element in spite of being thin. The electromagnetic interactions then take place through the covering. 
     The magnetic forces acting on the portions are transmitted mechanically in shear by the covering, and that is highly favorable from a mechanical point of view. The covering thus forms an effective force transmitter that is very compact. 
     Thus, in the context of an active element in the form of a plate or a blade, it is advantageous to provide a covering that has two thin walls substantially covering the two large faces of the active element through which the electromagnetic interactions take place. In the context of an active element in the form of a rod, it is advantageous to provide a covering that comprises a tubular thin wall that covers the cylindrical portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood in the light of the following description with reference to the figures of the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an electromagnetic machine of the invention including active elements in the form of plates; 
         FIG. 2  is a cutaway detail view of an active element of the machine shown in  FIG. 1 ; 
         FIG. 3  is a cutaway detail view of another active element of the machine shown in  FIG. 2 ; 
         FIG. 4  is a view of the  FIG. 3  active element shown during fabrication; 
         FIG. 5  is a view of the  FIG. 3  active element shown during fabrication in a variant embodiment; 
         FIG. 6  is a perspective view of an electromagnetic machine of the invention including active elements in the form of rods; and 
         FIG. 7  is a partially cutaway detail view of an active element segment of the machine shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is initially described with reference to a linear electromagnetic machine with multiple air gaps, such as the machine shown in  FIG. 1 . In known manner, such a machine comprises a cage  1  of ferromagnetic material, here receiving three coils  2  electrically powered so as to be offset successively by 120°. The magnetic field generated by the coils  2  is transmitted in an active zone of the cage  1  in which there extend in alternation:
         active elements in the form of stationary plates  10  that are mutually parallel and prevented from moving relative to the cage (only one stationary plate is referenced). Each of the stationary plates  10  comprises a succession of permanent magnets, as described in greater detail with reference to  FIG. 2 ;   active elements in the form of mutually parallel movable plates  20 , each extending between two stationary plates  10  so as to present a plurality of air gaps relative thereto (only one movable plate  20  is referenced). Each of the movable plates  20  comprises a succession of ferromagnetic portions and of non-magnetic portions, as described in greater detail below with reference to  FIGS. 3 to 5 . The movable plates  20  are secured to one another by means of pins  30  extending to pass through all of the movable plates  20 . The movable plates slide in a sliding direction X facing the stationary plates  10 .       

     As shown in  FIG. 2 , each stationary plate  10  comprises permanent magnets  11  oriented in a first direction disposed in alternation with permanent magnets  12  oriented in a second direction opposite to the first, as represented by the arrows drawn on the ends of the permanent magnets. The permanent magnets  11  and  12  are mutually touching bars and they form successive portions having alternating magnetic properties. According to an essential aspect of the invention, each stationary plate  10  has thin non-magnetic walls  15  that extend on either side of the stationary plate  10  over the large faces thereof and that are secured to the permanent magnets  11  and  12 , e.g. by adhesive. The large surfaces are made up of the adjacent side walls of the bars. 
     By way of example, the permanent magnets  11  and  12  present a thickness of about 1 millimeter (mm), while the thin walls  15  present a thickness of 0.1 mm. The thin wall  15  is made of bronze. 
     The thin walls  15  form support means for the permanent magnets  11  and  12  that are very compact. The full height of the permanent magnets  11  and  12  can thus interact electromagnetically with the movable plates  20  facing them through the thin walls  15  so that all of the magnets are used for producing mechanical forces on the movable plates  20 . 
     In addition, the thin walls  15  form the outer layers of a sandwich structure having its core constituted by the permanent magnets  11  and  12 , thereby giving the stationary plate  10  a high degree of stiffness in bending. 
     As shown in  FIG. 3 , each movable plate  20  comprises ferromagnetic portions  21  disposed in alternation with non-magnetic portions  22 . The portions  21  and  22  form touching bars. Each movable plate  20  also has non-magnetic ends  23  extending the portions  21  and  22  and serving to receive the pins  30  that secure the movable plates  21  to one another so as to form the movable element of the machine. According to an essential aspect of the invention, each movable plate  20  comprises thin walls  25  that extend on either side of the movable plate  20  over the large faces thereof, being secured to the portions  21  and  22  and also to the ends  23 , e.g. by adhesive. 
     In addition to their above-mentioned stiffening function, the thin walls also serve to transmit mechanically to ends  23  the magnetic forces to which the portions  21  and  22  are subjected. The mechanical transmission of these forces between the thin walls  25  and the portions  21  and  22  takes place in shear, which is an effective mode of transmission for adhesively-bonded assemblies. Here likewise, the thin walls  25  are made of bronze. 
     As shown in  FIG. 4 , and in a preferred embodiment, each movable plate  20  is obtained by using a sheet of non-magnetic material  26  having parallel windows  27  cut out therein for receiving the ferromagnetic portions  21 . To make a movable plate  20 , a thin wall  25  is adhesively bonded to one of the large faces of the cut-out sheet  26 , here on the bottom large face. Then the ferromagnetic portions  21  are placed in the windows  27 . One of the ferromagnetic portions  21  is shown while it is being put into place in one of the windows  27 . Thereafter, the other thin wall  25  is adhesively bonded to the top large face of the cut-out sheet  26 . The side margins of the cut-out sheet  26  are then cut away along the dashed lines so that the cut-out sheet  26  that remains between the ferromagnetic portions  21  constitutes the non-magnetic portions  22 . It then remains to drill the ends  23  in order to provide the orifices for receiving the pins  30 . 
     In the variant shown in  FIG. 5 , the cut-out sheet  26  is made of ferromagnetic material, while the windows are filled with liquid resin  28  in order to form the non-magnetic portions. 
     After the resin has set, the top thin wall  25  has been adhesively bonded, and the side margins have been cut away from the dashed lines, the cut-out sheet extending between the windows forms the ferromagnetic portions  21 , while the resin  28  forms the non-magnetic portions  22 . 
     The fabrication method shown in  FIGS. 4 and 5  can naturally be applied to a stationary plate, or indeed a plate without an end. Similarly, it is possible to leave the windows empty, with the air therein forming the non-magnetic portions. 
     In known machines, it is desired to keep empty spaces between the facing plates so as to ensure they do not touch, since the facing portions are not covered and therefore risk sliding one against another and catching one another because of the multiple edges at the boundaries between the portions. Keeping such spaces empty requires the movable plates to be externally guided relative to the stationary plates and gives rise to magnetic instabilities that tend to move the plates towards one another, and thus to stress them in bending, thereby leading to variable air gaps. These empty spaces enlarge the air gaps between the plates considerably, and thus reduce the efficiency of the machine. 
     In contrast, and in accordance with a particularly advantageous aspect of the invention, the machine of  FIG. 1  is designed so that in operation the stationary plates  10  and the movable plates  20  slide with very little clearance with contact between the plates being allowed. The accurate guidance provided in this way enables the air gaps between the magnetic portions of two facing plates to be kept substantially constant and equal at most to the thickness of the thin walls rubbing against one another plus the clearance for sliding. The plates come into contact via their thin walls  15  and  25  which are continuous and without edges, thereby facilitating sliding. 
     In addition, the accurate guidance provided in this way increases the stiffness, and thus the buckling strength, of the movable plates  20 , so they can be long. The movable plates  20  can thus be floatingly mounted on the pins  30 . 
     The material used for the thin walls  15  and  25  (bronze in this example) allows the plates to slide relative to one another with a coefficient of friction that is low. In a variant, the thin walls  15  and  25  could be made of some other non-magnetic material, and if the material does not possesses a favorable coefficient of friction, then the thin walls  15  and  25  can advantageously be coated in a surface layer having a low coefficient of friction, for example of polytetrafluoroethylene (PTFE), and compatible with the operating conditions of the machine, and in particular its internal temperature. 
     The invention is illustrated below with reference to a linear electromagnetic machine having rods as shown in  FIG. 6 . 
     This type of machine likewise comprises a cage  51  receiving three coils  52  that are phase-offset at 120° intervals. The magnetic field generated by the coils  52  is transmitted in an active zone of the cage  51  within which there extend:
         a stationary core  60  made up of permanent magnets in the form of disks with alternating magnetizations; and   mutually parallel movable rods  70  that extend through orifices in the stationary core  60  so as to present annular air gaps relative thereto (only one movable rod  70  is referenced). Each of the movable rods  70  comprises a succession of ferromagnetic portions and of non-magnetic portions, as described in greater detail below with reference to  FIG. 7 . The movable rods  70  are secured to one another by means of an end plate  80  to which the ends of the movable rods  70  are fastened. The rods slide in a sliding direction X.       

     The stationary core  60  performs the same function in this embodiment as the stationary plates  10 , and the movable rods  70  perform the same function as the movable plates  20 . 
     As can be seen in  FIG. 7 , each movable rod  70  has ferromagnetic portions  71  disposed in alternation with non-magnetic portions  72 . Each movable rod  70  also has threaded non-magnetic end portions  73  that extend in line with the portions  71  and  72  and that are designed to be received in one of the end plates  80  that secure the movable plates  70  to one another so as to form the moving element of the machine. According to an essential aspect of the invention, each movable rod  70  has a thin tubular wall  75  (shown partially cut away to clarify the figure) that extends around the movable rod  70  and that is secured to the portions  71  and  72  and also to the ends  73 , e.g. by adhesive. The electromagnetic interaction between the portions of the movable rods and the stationary core takes place through the tubular thin wall  75 . 
     As with the plates, the tubular thin wall  75  holding the portions  71  and  72  greatly stiffens the movable rod  70  and serves to transmit the magnetic forces to which the portions  71  and  72  are subjected mechanically to the ends  73  (transmission taking place in shear as above). 
     Furthermore, the absence of a central support makes it possible for the portions  71  and  72  to be made solid, having no central orifice, thereby improving the efficiency of the machine. 
     According to a particularly advantageous aspect of the invention, the movable rods  70  are slidably received in the orifices of the stationary core  60  with small clearance, thus allowing the movable rods  70  to come into contact with the stationary core  60 . As a result, the movable rods  70 , which may be long, are well guided, thereby increasing their stiffness and their ability to withstand buckling. Furthermore, the air gap is well controlled and substantially equal at most to the thickness of the tubular thin wall  75  plus the clearance for sliding. The tubular thin wall  75  is made of bronze, a material that presents a low coefficient of friction. The tubular thin wall  75  rubs directly against the orifice in the stationary core. Nevertheless, the thin wall  75  is continuous and without edges, thereby encouraging sliding of the associated movable rod. 
     The invention is not limited to the above description, but on the contrary covers any variant coming within the ambit defined by the claims. 
     In particular, although the description above relates to machines in which some of the active elements have magnetic portions, the invention can naturally be applied to other types of machine, for example to variable reluctance machines without permanent magnets, the portions making up the active elements then alternating between being ferromagnetic and non-magnetic. 
     Although the portions of an active element are shown herein as being adjacent, empty spaces could be left between these portions. Furthermore, one portion could be constituted entirely by an empty space. 
     Although the active elements shown herein have an outer covering in the form of one or more continuous thin walls covering all of an outside face of the active element, the covering could have other forms. In particular, the covering need not comprise continuous walls but could comprise strips extending in the sliding direction of the active element along the surface thereof. It is then preferable for the strips of one active element to be arranged to face strips of the facing active elements. Under all circumstances, it is important for the covering to be sufficiently strong to be capable of mechanically transmitting the magnetic forces to which the portions of the active element are subjected. 
     Although it is stated that the covering is bonded to the portions adhesively, other connection methods could be used. For example, the covering could be obtained by coating the portions with a hot material that forms the covering on cooling. The covering may also be sprayed onto the portions in a gas. 
     Although it is stated that all of the portions are secured to the covering, it is possible for some of them not to be secured thereto, for example for every other portion not to be secured. The magnetic forces are then transmitted to the thin wall(s) solely by those portions that are secured thereto, still in shear. 
     Finally, although the invention is described with reference to linear electromagnetic machines, the invention can be applied to rotary machines, e.g. having active elements in the form of disks. Under such circumstances, the portions extending along angular sectors alternate in a main direction around a circle. The covering then comprises two thin walls extending over the two large spaces of the disk formed by the adjacent side faces of the portions.