Patent Publication Number: US-7900885-B2

Title: Electromagnetic actuator with permanent magnets which are disposed in a V-shaped arrangement

Description:
The invention relates to an electromagnetic actuator having permanent magnets arranged in the form of a V. 
     BACKGROUND OF THE INVENTION 
     Document FR 2 865 238 discloses an electromagnetic actuator having an actuating member associated with an armature that can move under the action of an electromagnet, comprising a coil and a core suitable for channeling the flux of the coil so as to form a return path in the armature, the core having a base from which branches extend, including a central branch around which the coil extends. The electromagnet comprises two permanent magnets which are incorporated into the core in such a way that the latter channels the flux of the permanent magnets so as to form a return path in the armature, the flux of the coil passing through the magnets. In one of the embodiments illustrated in that document, the permanent magnets are placed obliquely in the lateral branches of the core, thereby making it possible to house, in the core, magnets having a length substantially equal to the height of the coil without correspondingly increasing the height of the electromagnet. 
     However, such an arrangement means that the laminations of the core have to be cut so as to allow the magnets to be inserted, thereby mechanically weakening the laminations and posing assembly problems. Furthermore, the gap between the core laminations and the permanent magnets depends on the precision with which the laminations are cut, something that is therefore difficult to control. 
     SUBJECT OF THE INVENTION 
     The subject of the invention is an electromagnetic actuator having oblique magnets that is easier to assemble. 
     BRIEF DESCRIPTION OF THE INVENTION 
     To achieve this objective, the invention provides an electromagnetic actuator, having an actuating member associated with an armature and capable of moving under the action of at least one electromagnet, which comprises: a coil; a core designed to channel the flux of the coil so as to form a return path in the armature, the core having a base from which branches extend, including a central branch around which the coil extends; and two permanent magnets which are associated with the core so that the latter channels the flux of the permanent magnets so as to form a return path in the armature, the flux of the coil passing through the magnets. According to the invention, the two magnets are placed in the central branch of the core so as to form a V, which separates the central branch into a support part, which supports the magnets and is integral with the base, and an end part lying above the magnets. 
     Thus, the core is separated into a main part, incorporating the part for supporting the magnets, the access to which, for positioning the permanent magnets, is completely free, and an end part, which is attached to the magnets placed on the support part so as to lie above them, the end part being centered by itself on the V formed by the permanent magnets. The assembly of the actuator is thereby made easier. 
     It is therefore sufficient to exert a compressive force on the end part in order to reduce, or even eliminate, the gap between the permanent magnets and the laminations constituting the core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more clearly understood in the light of the following description with reference to the figures of the appended drawings in which: 
         FIG. 1  is a partial schematic sectional view of an actuator according to the invention; 
         FIG. 2  is a partial schematic view of the actuator of  FIG. 1 , illustrated in the course of being mounted; and 
         FIG. 3  is a view in partially exploded, cast-away perspective of a double electromagnetic actuator according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1 , the electromagnetic actuator of the invention comprises an electromagnet  1  with a core  2  and a coil  3 . The electromagnet  1  exerts an electromagnetic force in a controlled manner on an armature  4  integral with a pushrod  5  that can move along the X axis. 
     Such an actuator is, for example, used to actuate an internal combustion engine valve, the actuator being placed in such a way that the pushrod  5  extends along the sliding axis of the valve. As is known, the actuator includes another electromagnet (not shown) that extends opposite the electromagnet  1  so as to selectively attract the armature  4  in the opposite direction. The end of the pushrod  5  and the end of the valve are returned to each other by opposing springs (not shown) that define an equilibrium position of the pushrod/valve assembly in which the armature extends substantially at mid-path between the two electromagnets. 
     The core  2  of the electromagnet  1  has a base  10  from which two lateral branches  11  and a central branch extend, the coil  3  extending around said central branch. The central branch comprises two portions  12  with facing inclined faces integral with the base  10 . The portions  12  form a support part, for supporting the core  2 , said part being designed to accommodate permanent magnets  13  so that the latter extend obliquely to the X axis and form a V, the point of which here is turned toward the base  10 . A wedge  14  forming an end part of the central branch is thus formed in the V. 
     The path of the flux lines generated by the permanent magnets  13 , which pass through the core  2  so as to form a return path in the armature  4 , is depicted as the bold dashed lines in  FIG. 1 . The wedge  14  has an end face  15  in which a groove  17  lies parallel to the permanent magnets  13 . The groove  17  ensures that there is a sharp separation between the respective flux lines of the two permanent magnets  13  that pass on either side of the groove  17 . 
     As may be seen in  FIG. 3  (in which the core is illustrated upside-down with respect to  FIG. 1 ), the actuator is mounted as follows. After having formed the core  2  by assembling the laminations that form the base  10 , the lateral branches  11  and the support portions  12 , the permanent magnets  13  are put into position on the support portions  12 . In this regard, the support portions  12  include steps  50  making it easier to position the magnets  13 . After having formed the wedge  14 , by assembling the corresponding laminations, the wedge  14  is then attached to the permanent magnets  13  as indicated by the arrow. The wedge  14  then lies above the permanent magnets  13  and is self-centered by the V formed by the permanent magnets  13 . 
       FIG. 3  shows a double actuator intended for actuating two pushrods (not shown). In this regard, the double actuator comprises two coils  3  and a common core  2  obtained by juxtaposing two cores identical to that illustrated in  FIG. 1  and by forming a single branch of the two juxtaposed lateral branches. As previously, the core  2  has central branches around which the coils  3  extend, the central branches having support portions  12  that support the permanent magnets  13 , and wedges  14 . 
     To keep the whole assembly in place, nonmagnetic clamps  18  are used, each of these having, on the one hand, an elongate part  19  that is housed in the groove  17  of the active face  15  of the wedge  14 , and on the other hand, braces  20  that extend into holes passing through the wedge  14 , then between the permanent magnets  13  and finally in holes in the core  2  (these not being visible) so as to be fastened to the latter, for example by screwing or by riveting (as a variant, the braces could pass through the core  2  so as to be fixed directly to the body  100 ). 
     The nonmagnetic clamps make it possible to exert a compressive force so as to take up, or even eliminate, the residual gap that may remain owing to the manufacturing tolerances between, on the one hand, the support portions  12  and the permanent magnets  13  and on the other hand, the permanent magnets  13  and the wedge  14 . This gap take-up allows the magnetic efficiency of the actuator to increase. 
     According to one particular aspect of the invention, more particularly visible in  FIG. 1 , the end face  15  of the wedge  14  lies set back by an amount h relative to the end faces  16  of the lateral branches  11 . 
     Thus, when the armature  4  butts against the core  2 , said armature butts only on the end faces  16  of the lateral branches  11  and not on the central branch. In general, and more particularly when the permanent magnets are produced by sintering powder materials, the permanent magnets are very sensitive to shocks. The set-back h makes it possible to protect the permanent magnets  13  from the shocks when the armature  2  strikes the core  4 , thereby increasing the lifetime of the actuator. 
     Furthermore, in the absence of such a set-back, the manufacturing tolerances on the core would give rise to residual gaps between the armature and the branches of the actuator, causing magnetic hysteresis that would disturb the repeatability of the separation of the armature  4  from the core  2 . The set-back makes it possible for this hysteresis to be reduced, or even eliminated. For this purpose, a set-back h of the order of a few tenths of a millimeter is preferably chosen, and therefore substantially greater than the gaps, which are of the order of a few tens of microns, so that the set-back h forms, between the armature and the central branch, a large gap, the influence of which predominates over that of the residual gaps when the armature is close to the core. This makes it possible to reduce, or even eliminate, effects of the magnetic hysteresis caused by the residual gaps. 
     In practice, a set-back h of greater than 0.1 millimeters will preferably be chosen, while still remaining less than 0.35 millimeters, so as not to prejudice the performance of the actuator. 
     According to one particular aspect of the invention more particularly visible in  FIGS. 1 and 2 , the core  2  includes a space  30  that extends between the permanent magnets  13  near the tip of the V, in which space the tip  31  of the wedge  14  is engaged. The clamping of the wedge  14  by means of the clamp  18  may result, because of the relatively acute angle of the wedge  14 , in a large dispersion in the position of the tip  31  along the X axis. The space  30  allows this dispersion to be absorbed, while preventing any contact between the tip  31  and the rest of the core  2 . 
     Furthermore, the space  30  will be chosen to be deep enough to form a sufficiently large gap between the wedge  14  and the base  10 , preventing, in service, magnetic flux from passing between the wedge  14  and the base  10 , which would short-circuit the permanent magnets  13 . 
     Finally, the space  30  forms a nonmagnetic region at the base of the permanent magnets  13  at the place where the latter form the tip of the V, thereby making it possible for there to be a sharp separation between the flux lines of the permanent magnets  13  in this region of the core. 
     The invention is not limited to what has just been described, rather quite to the contrary it encompasses any variant falling within the scope defined by the claims. 
     In particular, although actuators have been illustrated here in which the permanent magnets form a V, the tip of which is turned toward the base of the core, it will also be possible to place the magnets in such a way that they form a V with the tip directed toward the armature. The magnet support parts of the base will have inclined faces no longer facing each other but being turned toward the lateral branches, whereas the end part of the central branch will no longer have a wedge shape but a hat shape.