Abstract:
A linear-motor stator is configured by connecting a plurality of magnetic circuits along the axis of travel of the forcer (along the x-axis). The magnetic circuits are furnished with yokes and pluralities of field magnets fixed to the yokes. High-magnetic permeability members of magnetic permeability higher than that of the yokes are provided where adjoining magnetic circuits are connected, straddling the connections.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    Certain embodiments of the invention relate to linear motors. 
         [0003]    2. Description of Related Art 
         [0004]    Linear motors are used to convert electrical energy into linear motion.  FIG. 1  is a perspective view of a conventional linear motor. As illustrated in  FIG. 1 , the linear motor  2   r  is furnished with a forcer  10  and a stator  20   r . The stator  20   r  comprises a yoke  22  including a pair of yoke backs  22   a  and  22   b  disposed in opposition, between which the forcer  10  is sandwiched, and a plurality of field magnets  24  provided on inner-side lateral faces S 1   a  and S 1   b  of the yoke backs  22   a  and  22   b , paralleling the axis along which the forcer  10  can move (x-axis direction). The plurality of field magnets  24  are glued on according to a predetermined magnetic-pole pitch, and such that the N poles and S poles appear in alternation. The yoke  22  and the field magnets  24  form a magnetic circuit. 
         [0005]      FIGS. 2A and 2B  are perspective views illustrating the stator  20   r . In a linear motor  2   r  in which the moving range of the forcer is large, it is necessary to make the yoke  22  long, but constituting a longer yoke  22  in such instances is problematic, owing to cost as well as difficulties with machining precision, and consequently longer yokes  22  are often constructed by manufacturing several shorter yokes  23  as indicated in  FIG. 2A , and connecting them together as indicated in  FIG. 2B . 
         [0006]      FIGS. 3A and 3B  are plan views of the yoke  22 . As indicated in  FIG. 3A , when shorter yokes  23  are connected to construct a longer yoke  22 , gaps (air strata)  26  originating in discrepancies in manufacturing the yokes  23  arise in the connecting area  25  between them. These gaps  26  turn out to be areas of high magnetic resistance (accordingly, of low magnetic permeability). 
         [0007]    The magnetic flux Φ that a field magnet  24  generates passes through the yoke  22  and flows into a neighboring field magnet  24 . However, in a case in which the yoke  22  is not an integrated component but has an articulated structure, magnetic flux Φ cannot negotiate the high-magnetic-resistance sections of the connecting area  25 , such that a portion Φ EXT  of the magnetic flux leaks to the exterior of the yoke  22 . Alternatively, if the surface of the yokes  23  is plated as indicated in  FIG. 3B , the plating  27  turns out to be a section of high magnetic resistance, becoming the cause of magnetic field leakage. 
         [0008]    A technique for reducing magnetic field leakage by especially devising the form of the connecting area between the yokes is disclosed in Japanese Unexamined Utility Model App. Pub. No. H05-8793. 
         [0009]    Although the leakage flux density of conventional linear motors, being on the order of several tens of mT, does not lead to problems in the majority of applications, in devices employing electron beams, and in like applications in which magnetic fields have an impact on the target object, reducing magnetic field leakage to still lower levels has been desired. 
       SUMMARY 
       [0010]    The invention provides a linear motor that reduces a stray magnetic field. 
         [0011]    According to an aspect of the invention, there is provided a linear motor. The linear motor includes a movable element and a stator that includes a plurality of magnetic circuits connected in a movable direction of the movable element. Each of the magnetic circuits includes a yoke and a plurality of field magnets fixed to the yoke. A high-magnetic permeability member of which magnetic permeability is higher than magnetic permeability of the yoke is provided at a connecting portion between adjacent magnetic circuits so as to extend across the connecting portion. 
         [0012]    According to this aspect, since the high-magnetic permeability member is provided at the connecting portion, a path having low magnetic resistance is formed so as to bypass portions that have high magnetic resistance and are formed at the connecting portion. Accordingly, it is possible to reduce magnetic flux that leaks out of the yoke. 
         [0013]    Both ends of the high-magnetic permeability member may overlap at least a part of the field magnets of the magnetic circuit. 
         [0014]    Accordingly, since it is possible to guide the magnetic flux, which is generated by the field magnets, to the inside of the high-magnetic permeability member, it is possible to reduce a stray magnetic field. 
         [0015]    The high-magnetic permeability member may be embedded in the yoke. 
         [0016]    Accordingly, it is possible to increase a contact area between the high-magnetic permeability member and the yoke. Therefore, it is possible to guide more magnetic flux to the inside of the high-magnetic permeability member. 
         [0017]    The high-magnetic permeability member may be provided on a surface of the yoke. In this case, the assembling of the magnetic circuits becomes easy. 
         [0018]    The yoke may include a pair of back yokes that are provided so as to face each other with the movable element interposed therebetween in a direction perpendicular to the movable direction. The plurality of field magnets may be provided on inner surfaces of the back yokes. A recess to which the high-magnetic permeability member is fitted may be formed in an end face of the back yoke. 
         [0019]    The yoke may include a pair of back yokes that are provided so as to face each other with the movable element interposed therebetween in a direction perpendicular to the movable direction. The plurality of field magnets may be provided on inner surfaces of the back yokes. The high-magnetic permeability member may be provided on an outer surface of adjacent back yoke. 
         [0020]    In this case, the assembling of the magnetic circuits becomes easy. 
         [0021]    A groove may be provided on an outer surface of the back yoke and the high-magnetic permeability member may be embedded in the groove. 
         [0022]    Accordingly, since it is possible to improve assembling accuracy during the assembling of the high-magnetic permeability member with the back yoke, and the back yoke and the high-magnetic permeability member can be positioned so as to be flush with each other. Therefore, it is possible to reduce a stray magnetic field. 
         [0023]    The high-magnetic permeability member may have the shape of a plate. The high-magnetic permeability member may have the shape of a rod. 
         [0024]    According to another aspect of the invention, there is provided a stage device. The stage device may include any one of the above-mentioned linear motors. 
         [0025]    It is to be noted that any arbitrary combination or rearrangement of the above-described structural components and so forth is effective as and encompassed by the present embodiments. 
         [0026]    Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
           [0028]      FIG. 1  is a perspective view of a linear motor in the related art. 
           [0029]      FIGS. 2A and 2B  are perspective views of a stator. 
           [0030]      FIGS. 3A and 3B  are plan views of a yoke. 
           [0031]      FIGS. 4A and 4B  are views showing a stator of a linear motor according to an embodiment. 
           [0032]      FIG. 5  is an assembly diagram of the stator. 
           [0033]      FIG. 6  is a plan view of the stator. 
           [0034]      FIGS. 7A and 7B  are perspective views of a stator according to a first modification. 
           [0035]      FIG. 8  is a perspective view of a stator according to a second modification. 
           [0036]      FIGS. 9A and 9B  are views of a stator of a linear motor according to another embodiment. 
           [0037]      FIGS. 10A and 10B  are perspective views of stators according to third and fourth modifications. 
           [0038]      FIG. 11  is a perspective view of a stator according to a fifth modification. 
           [0039]      FIGS. 12A and 12B  are cross-sectional views of a stator according to a sixth modification. 
           [0040]      FIG. 13  is a plan view of a stage device using the linear motor according to the embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    The invention will now be described based on preferred embodiments which do not intend to limit the scope of the present invention but exemplify the invention. All of the features and the combinations thereof described in the embodiment are not necessarily essential to the invention. 
       First Embodiment 
       [0042]      FIGS. 4A and 4B  are views showing a stator  20  of a linear motor according to a first embodiment. The stator  20  includes a plurality of magnetic circuits  30  that are connected in a movable direction of a movable element (the x-axis direction). Each of the magnetic circuits  30  includes a yoke  22  and a plurality of field magnets  24  fixed to the yoke  22 . High-magnetic permeability members  40  of which the magnetic permeability is higher than the magnetic permeability of the yoke  22  are provided at a connecting portion  32  between adjacent magnetic circuits  30  so as to extend across the connecting portion  32 . 
         [0043]    In this embodiment, the plate-like high-magnetic permeability members  40  are embedded in the yoke  22 .  FIG. 5  is an assembly diagram of the stator  20 . In  FIG. 5 , only one back yoke  22   a  of a pair of back yokes  22   a  and  22   b , which are provided so as to face each other with the movable element interposed therebetween in a direction (Y direction) perpendicular to the movable direction (the x-axis direction), is shown and the other back yoke  22   b  is omitted. The field magnets  24  are provided on an inner surface S 1   a  of the back yoke  22   a . End faces (joint surfaces) S 2   a  of the back yokes  22   a  of the respective adjacent magnetic circuits  30  come into contact with each other. A recess  42  to which the high-magnetic permeability member  40  is fitted is formed in the end face S 2   a  of the back yoke  22   a . Adjacent magnetic circuits  30  are connected to each other in a state in which the high-magnetic permeability member  40  is fitted to the respective recesses  42 . The back yoke  22   b  also has the same structure. 
         [0044]    The material of the high-magnetic permeability member  40  is not particularly limited, and a material of which the magnetic permeability is higher than that of the yoke  22  may be selected according to the material of the yoke  22 . For example, an iron material (SS400) or low-carbon steel having a relative permeability μ/μ 0  of about 1000 is used for the yoke  22 . In this case, a material of which the relative permeability (magnetic permeability) is higher than that of the iron material (SS400) or the low-carbon steel, such as pure iron having higher purity, a permalloy (μ/μ 0 =8000, μ=1.0×10 −2 H/m), or an iron-cobalt alloy, may be selected for the high-magnetic permeability member  40 . In a case in which a permalloy is used for the yoke  22 , a material of which the magnetic permeability is higher than that of the permalloy, such as an iron-cobalt alloy or pure iron having high purity, can be used for the high-magnetic permeability member  40 . 
         [0045]    The structure of the stator  20  according to a first embodiment has been described above. Subsequently, the advantage of the stator  20  will be described. 
         [0046]      FIG. 6  is a plan view of the stator  20 . Magnetic flux Φ is shown in  FIG. 6  by a dashed-dotted line. Since the magnetic permeability of the high-magnetic permeability member  40  is higher than that of the yoke  22 , the magnetic flux density B 1  of magnetic flux passing through the high-magnetic permeability member  40  is higher than the magnetic flux density B 2  of magnetic flux passing through the yoke  22 . In other words, magnetic flux Φ, which is generated by the field magnets  24 , is concentrated on the high-magnetic permeability member  40 . Accordingly, magnetic flux Φ, which leaks out of yoke  23 , can be reduced in the structure in the related art shown in  FIG. 3 . 
         [0047]    The advantage and effect of the stator  20  according to a first embodiment has been described above. 
         [0048]    In order to improve the advantage and effect, it is important to effectively guide the magnetic flux Φ, which is generated by the field magnets  24 , to the inside of the high-magnetic permeability member  40 . Accordingly, it is preferable that both ends of the high-magnetic permeability member  40  overlap at least a part of the field magnets  24  of the magnetic circuit  30 . The high-magnetic permeability member  40  overlaps the field magnets by preferably ¼ or more and more preferably ½ or more of the width W of the field magnet  24  in the movable direction (the x-axis direction). That is, when the overlap width is denoted by W OL , it is preferable that W OL ≧W/2 is satisfied. 
         [0049]    Further, it is preferable that the high-magnetic permeability member  40  also overlaps at least apart of the field magnets  24  in a height direction (Z direction). In this embodiment, the height h of the high-magnetic permeability member  40  is larger than the height H of the field magnet  24 . Accordingly, all the magnetic flux generated from the back of the field magnet  24  (the surface of the field magnet  24  coming into contact with the yoke  22 ) passes through the high-magneticpermeabilitymember  40  in the height direction. 
         [0050]    It is possible to suitably reduce stray magnetic fields by setting the size of the high-magnetic permeability member  40  and the arrangement relationship between the high-magnetic permeability member  40  and the field magnets  24  as described above. 
       First Modification 
       [0051]    Subsequently, a modification relating to the first embodiment will be described.  FIGS. 7A and 7B  are perspective views of a stator  20  according to a first modification. In this modification, the height h of a high-magneticpermeabilitymember  40  is substantially equal to the height H of a field magnet  24 . The same effect as the first embodiment can be obtained with this modification as well. 
       Second Modification 
       [0052]      FIG. 8  is a perspective view of a stator  20  according to a second modification. In this modification, high-magnetic permeability members  40  have the shape of a rod. A plurality of holes (recesses)  46  are formed in an end face S 2  of each of back yokes  22   a  and  22   b . High-magnetic permeability members  40  are inserted into corresponding holes  46 . The same effect as the first embodiment can be obtained with this modification as well. 
       Other Modifications 
       [0053]    In addition, the shape of the high-magnetic permeability member  40  may be an arbitrary shape without being limited to the shape of a plate and the shape of a rod. Further, the number of high-magnetic permeability members  40 , which are provided at the connecting portion, is also not particularly limited. 
       Second Embodiment 
       [0054]      FIGS. 9A and 9B  are views of a stator  20  of a linear motor according to a second embodiment. In the second embodiment, the high-magnetic permeability members  40  have been embedded in the yoke  22 . However, high-magnetic permeability members  50  are attached to the surface of a yoke  22  in the second embodiment. 
         [0055]    Specifically, the high-magnetic permeability members  50  are provided on outer surfaces S 3  of the respective back yokes  22   a  and  22   b . A groove  44  is provided at an end portion of the outer surface S 3  of each of the back yokes  22   a  and  22   b . The high-magnetic permeability members  50  are embedded in the grooves  44 . It is preferable that the surface of the high-magnetic permeability member  50  is flush with the surface of the back yoke  22   a  ( 22   b ) without a step as shown in  FIG. 9B . It is possible to reduce a stray magnetic field, which is generated from a step, that is, a discontinuous portion by removing the step between the high-magnetic permeability member  50  and the back yoke  22   a  ( 22   b ). 
         [0056]    According to the second embodiment, the high-magnetic permeability members  50  having high magnetic permeability are provided on the surfaces of the yoke  22  at the connecting portion between the magnetic circuits  30 . Accordingly, magnetic flux density inside the high-magnetic permeability member  50  is increased and magnetic flux density outside the yoke  22  is relatively reduced. That is, since magnetic flux, which is to leak out of the surface of the yoke  22 , can be made to enter the high-magnetic permeability member  50 , stray magnetic fields can be reduced. 
         [0057]    Further, the second embodiment has an advantage of easily assembling the stator  20  in comparison with the first embodiment. 
       Third and Fourth Modifications 
       [0058]    Subsequently, modifications relating to the second embodiment will be described.  FIGS. 10A and 10B  are perspective views of stators  20  according to third and fourth modifications. In the third modification of  FIG. 10A , the height h of a high-magnetic permeability member  50  is substantially equal to the height H of a field magnet  24 . The same effect as the first embodiment can be obtained with this modification as well. 
         [0059]    In the fourth modification of  FIG. 10B , a high-magnetic permeability member  50  having a U shape is provided at a connecting portion between adjacent magnetic circuits  30  so as to cover outer surfaces S 3   a  and S 3   b  of back yokes  22   a  and  22   b  and a bottom S 4  of a yoke  22 . According to this modification, since it is sufficient for one high-magnetic permeability member  50  to be used at one connecting portion, assembling is easier. Further, stray magnetic fields, which are generated from the bottom of the yoke  22 , can also be reduced. 
       Fifth Modification 
       [0060]      FIG. 11  is a perspective view of a stator  20  according to a fifth modification. The stator  20  includes first high-magnetic permeability members  40  and second high-magnetic permeability members  50 . The first high-magnetic permeability members  40  are embedded in a yoke  22  as described in the first embodiment. The second high-magnetic permeability members  50  are provided on the surfaces of the yoke  22  as described in the second embodiment. Since the high-magnetic permeability members  40  and  50  are used together with each other, stray magnetic fields can be further reduced. 
       Sixth Modification 
       [0061]      FIGS. 12A and 12B  are cross-sectional views of a stator  20   c  according to a sixth modification. In the above-mentioned embodiments or modifications, the U-shaped yoke  22  of one magnetic circuit  30  has been formed of one component. However, in the sixth modification, a yoke  22   c  includes a plurality of parts  70  and  72  that are connected to each other. Mechanical coupling means, such as screws  74 , may be used for the connection of the parts  70  and  72 , or an adhesive may be used for the connection of the parts  70  and  72 . 
         [0062]    For example, one  70  of the plurality of parts may have an L-shaped cross-sectional shape, and the other  72  may have an I-shaped cross-sectional shape. A high-magnetic permeability member  78  of which the magnetic permeability is higher than that of the yoke  22   c  is provided on joint surfaces  76  of the plurality of parts  70  and  72  so as to extend across the joint surfaces  76  in a direction orthogonal to the joint surfaces  76 . The high-magnetic permeability member  78  may have the shape of a plate. 
         [0063]    In the modification of  FIG. 12A , the high-magnetic permeability member  78  is attached to the bottom of the U-shaped yoke  22   c . The length L of the high-magnetic permeability member  78  is longer than the length  11  of the bottom. A recess  80  to which the high-magnetic permeability member  78  is fitted is formed in the I-shaped part  72 . The parts  70  and  72  are connected to each other in a state in which the high-magnetic permeability member  78  is fitted to the recess  80 . The length  12  of the recess  80  may be determined so that l 1 +l 2 ≅L is satisfied. The length  12  may be determined in consideration of the thickness d 1  and strength of the I-shaped part  72 , the thickness d 2  and strength of the high-magnetic permeability member  78 , and the like. 
         [0064]    According to this modification, since it is possible to reduce leakage flux at the connecting portion even though the U-shaped yoke  22   c  is designed so as to be divided into a plurality of parts, it is possible to achieve performance that is not inferior to the performance of the integrated U-shaped yoke  22 . 
         [0065]    In  FIG. 12B , the division form of the L-shaped part  70  and the I-shaped part  72  is different from that of  FIG. 12A  and others are the same as those of  FIG. 12A . 
         [0066]    The U-shaped yoke  22   c  has been divided into two parts  70  and  72  in this modification, but the shapes of the parts are not particularly limited. For example, the U-shaped yoke  22   c  may be divided into two L-shaped parts at the middle of the bottom thereof. Alternatively, the U-shaped yoke  22   c  may be divided into three or more parts. 
         [0067]    Further, the high-magnetic permeability member  78  has been attached to the bottom of the part  70  of the yoke  22   c  in the sixth modification. However, the invention is not limited thereto, and a groove  44  may be formed as shown in  FIG. 9 or 10  and the high-magnetic permeability member  78  may be embedded in the groove  44 . 
         [0068]    Furthermore, the shape of the high-magnetic permeability member  78  may be an arbitrary shape without being limited to the shape of a plate and the shape of a rod. Moreover, the number of high-magnetic permeability members  78 , which are provided at each joint surface  76 , is also not particularly limited. 
         [0069]    In the first embodiment or second embodiment and the first to fifth modifications, it is possible to grasp that one long yoke is divided into a plurality of parts for the respective magnetic circuits in the movable direction of the movable element. Accordingly, the following technical idea is derived from the entire specification. 
         [0070]    Certain aspects of the invention relate to a linear motor that includes a movable element and a stator. A yoke may be divided into a plurality of parts. The yoke may be provided with a high-magnetic permeability member which is provided so as to extend across joint surfaces of the plurality of parts in a direction orthogonal to the joint surfaces and of which the magnetic permeability is higher than the magnetic permeability of the plurality of parts of the yoke. 
         [0071]    Finally, the use of a linear motor  2  will be described.  FIG. 13  is a plan view of a stage device  100  using the linear motor  2  according to the first embodiment. The stage device  100  is called an XY stage, and positions an object in the X direction and the Y direction. 
         [0072]    The stage device  100  mainly includes a Y stage  120 , an X stage  130 , and a surface plate  140 . The Y stage  120  includes a pair of sliders  124  and a horizontal member  122  that is horizontally provided between the pair of sliders  124 . An X linear motor  2 X, which moves the X stage  130  in the X direction, is provided on the horizontal member  122 . The X linear motor  2 X includes a stator  20  that is fixed to the horizontal member  122  and extends in the X direction, and a movable element (coil)  10  that is joined to the lower surface of the X stage  130 . Accordingly, the X stage  130  is positioned in the X direction by the control of the movable element  10  of the X linear motor  2 X. 
         [0073]    A pair of Y linear motors  2 Y are provided on both ends of the surface plate  140 . Each of the Y linear motors  2 Y includes a movable element  10  and a stator  20 . The above-mentioned sliders  124  are fixed to the stators  20  of the Y linear motors  2 Y. The Y stage  120  is positioned in the Y direction by the control of the movable elements  10  of the Y linear motors  2 Y. 
         [0074]    The structure of the stage device  100  has been described above. The linear motor  2  according to the first embodiment can be suitably used for the X linear motor  2 X or the Y linear motor  2 Y of the stage device  100 . The stage device  100  can be used to position a wafer or a glass substrate of an exposure device, or can also be used for an actuator or the like used for a scanning electron microscope (SEM). 
         [0075]    While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.