Abstract:
In order to construct a brushless electric motor comprising a housing, comprising a rotor and comprising a stator, each stator unit including pole shoes, formed as claw poles, as well as a coil positioned following the rotor in the direction of the rotor axis, by means of which the pole shoes can be magnetized, at the lowest possible cost and as simply as possible, it is proposed that the stator unit has two pole shoe elements of which a first pole shoe element has a first pole shoe carrier as well as first pole shoes formed integrally onto this carrier and of which a second pole shoe element has a second pole shoe carrier as well as the second pole shoes formed integrally onto this carrier and that a bearing support made of plastics is molded onto the second pole shoe carrier of the stator unit and is thereby secured to it.

Description:
The present disclosure relates to the subject matter disclosed in European application No. 02 025 775.4 of Nov. 16, 2002, which is incorporated herein by reference in its entirety and for all purposes. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a brushless electric motor, in particular a miniature motor, comprising a housing, comprising at least one rotor provided with magnetized regions and mounted rotatably about a rotor axis on bearing supports of the housing, and comprising a stator having at least one stator unit, each stator unit including a set of first pole shoes, formed as claw poles, and a set of second pole shoes, formed as claw poles, which are disposed around the rotor axis, as well as a coil positioned following the rotor in the direction of the rotor axis and with its windings arranged to encircle the rotor axis, by means of which the first and second pole shoes can be magnetized. 
   Such electric motors are known in the prior art, for which the problem is to construct such motors at the lowest possible cost and as simply as possible. 
   SUMMARY OF THE INVENTION 
   This object has been achieved for an electric motor of the type described in the opening paragraph in accordance with the invention in that the stator unit has two pole shoe elements of which a first pole shoe element has a first pole shoe carrier which extends transversely with respect to the rotor axis and is disposed on a side of the coil facing the rotor, as well as the first pole shoes formed integrally onto this carrier, which first pole shoes extend away from the first pole shoe carrier in a first direction approximately parallel to the rotor axis, and of which a second pole shoe element has a second pole shoe carrier which extends transversely with respect to the rotor axis and is disposed on a side of the coil facing away from the rotor, as well as the second pole shoes formed integrally onto this carrier, which second pole shoes also extend in the first direction away from the second pole shoe carrier approximately parallel to the rotor axis beyond the rotor, and that a bearing support made of plastics is molded onto the second pole shoe carrier of the stator unit and is thereby secured to it. 
   The advantage of the solution according to the invention is to be seen in its enabling a simple and low-cost construction of the stator unit. 
   A particularly favourable solution provides that a connecting piece which establishes a magnetic circuit between the pole shoe carriers is connected to the second pole shoe carrier to form a unit and that the bearing support is molded onto this unit. 
   This makes it possible to connect the bearing support fixedly and reliably to both the second pole shoe carrier and to the connecting element. 
   It is particularly favourable when the connecting element is formed as a sleeve. 
   A further advantage exists if the bearing support is formed to engage into the connecting element, since this provides a particularly stable and reliable connection between the respective stator unit and the bearing support. 
   So far, no precise details have been given concerning the molding-on of the bearing support. Basically, all conceivable plastics molding techniques can be used. A particularly suitable method has proved to be that of using injection molding to mold the bearing support onto the second pole shoe carrier. 
   So far, no precise details have been given concerning the connection of the connecting element to the second pole shoe carrier. For example, if the bearing support is molded onto the connecting element and onto the second pole shoe carrier, the connection between the second pole shoe carrier and the connecting element can also be established via the bearing support. 
   However, to make it easier to handle the unit consisting of the second pole shoe carrier and the connecting element when molding the respective bearing support on to it, it is preferable if the second connecting element is connected to the second pole shoe carrier by means of joining. 
   Such joining, for example, could take the form of bonding or soldering. A particularly favourable method in terms of manufacturing is when the connecting element is welded to the second pole shoe carrier. 
   Joining the material by welding is preferably achieved using a laser, either spot welding or the formation of a closed, continuous weld seam being possible. 
   The subsequent connection between the connecting element and the first pole shoe carrier can also be effected using various methods. Here it is advantageous if the first pole shoe carrier is directly connected to the connecting element. 
   Such a connection is preferably achieved by joining the first pole shoe carrier and the connecting element, this joining preferably suitably being effected by welding, particularly by laser welding, in the same way as described in relation to the connection between the connecting element and the second pole shoe carrier. 
   So far, no precise details have been given concerning the fixing of the coil with respect to the stator units. A particularly favourable solution provides for the connecting element to form a carrier for the coil so that on mounting the stator unit according to the invention onto the connecting element connected to the second pole shoe carrier, the coil can be put on before the first pole shoe carrier is placed onto the connecting element and these are connected together. 
   To achieve sufficient electrical insulation between the coil and the connecting element as well as the second pole shoe carrier, it is particularly favourable if the connecting element and the second pole shoe carrier on the side facing the coil are provided with an electrically insulating coating. 
   Moreover, it is even more preferable if the first pole shoe carrier is also provided with an electrically insulating coating on the side facing the coil. 
   The coating is preferably selected in such a way that it has a thickness of less than 10 μm, even more preferably less than 5 μm, so as not to impair the precise alignment of the individual elements of the stator unit relative to each other. 
   Concerning the type of coating, a large variety of possibilities are conceivable. For example, paint-like or similar coatings are conceivable. A particularly suitable coating provides for a glass-like consistency. 
   To additionally ensure that the pole shoe elements are protected against corrosion, it is preferable that all the pole shoe elements are provided with a corrosion resistant coating. 
   This coating need not necessarily be identical to the electrically insulating coating. 
   A particularly favourable embodiment of the solution according to the invention, however, provides that the electrically insulating coating acts at the same time as a corrosion resistant coating. 
   Both the first as well as the second pole shoe carriers are preferably coated with this coating. 
   So far, no precise details have been given concerning the arrangement of the pole shoes with respect to the other elements of the stator unit. It is particularly suitable if the second pole shoes overlap the coil and the coil is thus disposed in the direction of the rotor axis axially offset in relation to the respective rotor unit. 
   Moreover, it is basically conceivable to arrange the first pole shoes at a different radial distance from the rotor axis than the second pole shoes. To achieve optimum torque for the electric motor, it is preferably provided that the first and second pole shoes lie on the same cylindrical surface about the rotor axis and that the one pole shoes are disposed in the gaps between the other pole shoes. 
   Moreover, it is preferable if the pole shoes disposed successively in the azimuthal direction around the rotor axis have identical angular spacings from each other. 
   Moreover, it is particularly favourable within the solution according to the invention if the first and second pole shoes extend so far in the first direction that their ends lie in a common plane running perpendicular to the rotor axis. 
   In principle, it is possible for an electric motor as according to the invention to have a single stator unit. 
   However, to be able to clearly define the rotational direction of the rotor, it is preferably provided that the electric motor has a stator with two stator units and a rotor having a rotor unit associated with each stator unit in question, the rotor units being seated on a common shaft. 
   The pole shoes can then be so arranged that they point in the same direction. 
   A particularly favourable solution provides that the stator units are arranged in such a way that their pole shoes face each other. This makes it possible for the rotor units to be disposed alongside each other in a particularly expedient way. 
   To make the best possible use of the available space while achieving the best possible performance, it is preferable if, for both stator units, all pole shoes are disposed on the same cylindrical surface about the rotor axis. 
   A solution which can be produced at particularly low cost further provides that both stator units are of identical construction so that they can be constructed from identical parts. 
   Moreover, to clearly establish the rotational direction of the electric motor according to the invention, it is a particular advantage if holding positions of the rotor units determined by the magnetic effect, i.e. attraction, relative to the respective stator units are rotationally displaced in relation to each other by half a pole space. 
   Further characteristics and advantages of the invention form the subject matter of the description below as well as the illustration in drawings of a number of embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  a longitudinal view through an embodiment of the electric motor according to the invention; 
       FIG. 2  an enlarged section similar to  FIG. 1  in the region of a stator unit of the electric motor according to the invention; 
       FIG. 3  a second pole shoe element with connecting element of the electric motor according to the invention; 
       FIG. 4  a view from above in the direction of the arrow A in  FIG. 3 ; 
       FIG. 5  an exploded view of the stator unit shown in section in  FIG. 2  and 
       FIG. 6  a perspective assembled view of the stator unit illustrated in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of an electric motor according to the invention as illustrated in  FIG. 1 , preferably a stepper motor, includes a housing  10  with a housing sleeve  12  which extends between a first bearing support  14  and a second bearing support  16 , both of which are fixedly connected to the housing sleeve  12 . 
   In each of the bearing supports  14  and  16 , a respective rotary bearing  20  and  22  is mounted, both preferably formed as ball bearings, by means of which a shaft  24  is mounted about an axis  26  to be rotatable with respect to the housing  10 . 
   Moreover, the first bearing support  14  carries a connection board  28 , on which electric terminals  30 ,  32  are disposed for connection of a power supply of the electric motor. 
   The connection board  28  is preferably then located on an outer surface  34  of the bearing support  14  and encloses a retaining ring  36  of the bearing support  14 , which in turn forms a receptacle  38  for the rotary bearing  20 . 
   Both a stator  40 , formed by two identically constructed stator units  42  and  44  arranged in mirror image to each other, as well as a rotor  50 , formed by two rotor units  52  and  54 , are disposed in the housing  10 , the rotor  50  with both of its rotor units  52  and  54  being seated fixedly on the shaft  24  and thus, in the same way as the shaft  24 , being mounted about the axis  26 , representing the rotor axis, by means of the rotary bearings  20  and  22 , to be rotatable with respect to the housing  10 . In addition, each of the rotor units  52 ,  54  has magnetized regions  56  and  58 . 
   As shown in an enlarged view in  FIGS. 2 and 3 , the stator unit  44  includes a first pole shoe element  60 , which includes a first set of pole shoes  62  formed as claw poles which extend approximately parallel to the axis  26 . 
   Here, the first set of pole shoes  62  is formed integrally on a first pole shoe carrier  64  which preferably extends in a plane  66  running perpendicular to the axis  26 . 
   The first pole shoe element  60  is suitably a punched bent part and can thus be manufactured at low cost. 
   A second pole shoe element  70  includes a set of second pole shoes  72  which, in the same way as the first pole shoes  62 , extend approximately parallel to the axis  26  and are then formed integrally on a second pole shoe carrier  74  which preferably extends in a plane  76  perpendicular to the axis  26 . 
   The first and second pole shoes  62 ,  72  are preferably located on an enveloping surface  82  running circular-cylindrically to the axis  26 . 
   The second pole shoe element  70  is also suitably a punched bent part. 
   A magnetic circuit between the first pole shoe carrier  64  and the second pole shoe carrier  74  is produced by a connecting element depicted as a whole by  78  which, as shown in  FIGS. 3 and 4 , is initially connected to the second pole shoe element  74  in the region of a first end  77  and by the second pole shoe carrier  74  having a central recess  84  into which the first end  77  of the connecting element  78  engages, an outer surface  86  of the connecting element fitting snugly in the recess  84 . 
   Precise alignment of the connecting element  78  in relation to the second pole shoe carrier  74  is preferably effected by means of projections  88  provided on the second pole shoe carrier  74  which engage in cutouts  90  of the connecting element  78 , the projections  88  extending out beyond the contour of the recess  84  in the direction of the rotor axis  26  in order to engage into the cutouts  90 . 
   Moreover, connection of the first pole shoe carriers  64  to the connecting element  78  in the region of a second end  79  is effected by the second end  79  of the connecting element  78  engaging into a central recess  92  of the first pole shoe carrier  64 . 
   The invention provides that first of all the second pole shoe element  70  is manufactured as a punched bent part and after the connecting element  78  has been inserted into the recess  84 , welding then takes place by production of a weld seam  94  which preferably runs on a side of the second pole shoe carrier  74  facing away from the first pole shoe carrier  64  and which produces a connection between the recess  84  of the second pole shoe carrier  74  and the outer surface  86  of the connecting element  78 . 
   The unit illustrated in  FIG. 3 ,  4  consisting of the second pole shoe element  70  and the connecting element  78  is then connected to the bearing support  16  using plastics injection molding by molding-on of the bearing support  16 , as shown in  FIG. 2 , the bearing support  16  having a supporting plate  96  on one side, noses  98  of which engage into cutouts  75  in the second pole shoe carrier  74  which are provided radially between the pole shoes  72  and run radially inwards in relation to the rotor axis  26 , in order to receive the second pole shoe carrier  74  centered with respect to the axis  26 , so that when the bearing support  16  is molded-on using plastics injection moulding, the second pole shoe carrier  74  is at least partly embedded in the supporting plate  96 . 
   In addition, when the bearing support  16  is molded onto the supporting plate  96 , a sleeve-shaped receiving portion  100  is formed, the receiving portion  100  extending into the connecting element  78  formed as a sleeve, thus resulting in the unit consisting of the second pole shoe element  70  and the connecting element  78  being better secured to the bearing support  16 . 
   After the bearing support  16  has been injection molded onto the unit consisting of the second pole shoe element  70  and the connecting element  78 , a coil  104 , pre-wound onto a coil former  102 , is mounted, as shown in  FIG. 4 , so that the coil former  102  engages against the connecting element  78 , in particular the outer surface  86  of the same. Finally, the first pole shoe element  60  together with the first pole shoe carrier  64  is mounted, projections  108  on the first pole shoe carrier  64  engaging into the cutouts  110  for fixedly securing the first pole shoe carrier  64 , to the connecting element  78 , while the outer surface  86  of the connecting element  78  engages into a cutout  92  in the first pole shoe carrier  64 , thus making it possible to position the first pole shoe carrier  64  accurately in a positive-fit with respect to the connecting element  78  ( FIG. 3 ,  5 ). 
   Welding then preferably takes place between the connecting element  78  and the first pole shoe carrier  64 , preferably on a side  112  of the first pole shoe carrier  64  facing away from the second pole shoe carrier  74 , by application of a weld seam  114  to bridge a gap between the connecting element  78  and the first pole shoe carrier  64  in the region of the cutout  92  and the outer surface  86 . 
   In this way, the coil  104  is mounted fixedly between the pole shoe carriers  64  and  74 , the second pole shoes  72  projecting beyond the coil  104 . 
   Moreover, the second pole shoes  72  extend in the first direction  80  to the end  73  and in addition the first pole shoes  62  also extend to their end  63 , the ends  73  and  63  of the pole shoes  72  and  62  lying in a plane  118  which runs perpendicular to the axis  26 . 
   To achieve a sufficiently large spacing between the first pole shoe carrier  64  and the second pole shoes  72 , the first pole shoe carrier  64 , as illustrated particularly in  FIG. 4 , is provided with recesses  65  between the pole shoes  62 , cut radially inwards in the direction of the axis  26  so that the first pole shoe carrier  64  has a sufficiently great spacing from the second pole shoes  72 . 
   As illustrated in  FIG. 6 , all the pole shoes  62 ,  72  have a width in an azimuthal direction with respect to the axis  26  which represents an angular spacing WB, and respective pole shoes  62 ,  72  which follow one another at a pole spacing of PA are disposed at an angular spacing of WA from each other in relation to the axis  26 , which is identical between all pole shoes  62 ,  72 . 
   To electrically insulate the coil  104  in relation to the pole shoe elements  60 ,  70 , at least the connecting element  78  in the region of the outer surface  86  as well as the first pole shoe carrier  64  on the side  120  thereof facing the coil  104  and the second pole shoe carrier  74  on the side  122  thereof facing the coil  104  are provided with a coating  124  which has a layer thickness of less than 10 μm, still better less than 5 μm, and is electrically insulating, with a dielectric strength of up to 0.5 kV. 
   The coating  124  is preferably a glass-like layer, in particular a CVD layer, and is preferably also used to protect the pole shoe elements  60 ,  70  against corrosion, so that the entire pole shoe elements  60 ,  70  as well as the connecting element  78  are covered with this coating  124 . 
   In the embodiment of an electric motor according to the invention, the stator units  42 ,  44 , are formed identical to each other, as can be seen in  FIG. 1 , with each bearing carrier  14 ,  16  being molded onto at least the second pole shoe carrier  74 , so that the pole shoes  62 ,  72  of both stator units suitably lie on the same cylindrical enveloping surface  82  about the axis  26  and are also disposed with their ends  63 ,  73  facing each other. This allows the respective rotor unit  52 ,  54  to be located in each space  126 ,  128  enclosed by the pole shoes  62 ,  72 . 
   For example, the two rotor units  52 ,  54  can be disposed separately on the motor shaft  24  and connected fixedly to the shaft so that, for example, it is possible to arrange the rotor units  52 ,  54  on the shaft  24  with a rotational offset in relation to each other, preferably rotationally offset in relation to each other by approximately half a pole distance PA, while the pole shoes  62 ,  72  are located exactly opposite each other.