BACKGROUND OF THE INVENTION  
       [0001]     This application claims the priority of Korean Patent Application No. 2002-70063, filed on Nov. 12, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a spindle motor and a micro-drive apparatus including the spindle motor, and more particularly, to a spindle motor having a slim stator improved for use in mobile micro-drive apparatuses and a slim micro-drive apparatus comprising the slim spindle motor.  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 1  is a side cross-section of a spindle motor  10  in a conventional IBM micro-drive apparatus. Referring to  FIG. 1 , the conventional spindle motor  10  includes a hub  11 , a magnet  13 , and yokes  15 . The hub  11  has ball bearings  17  in the space defined by the hub  11  and is rotatably supported by a base plate  19 . The magnet  13  is fastened to the hub  11  to provide a magnetic force. The yokes  15  are arranged around the outer boundary of the magnet  13  and wound with coils and generate turning forces to the spindle motor  10  by interaction between current flowing in the coils and the magnetic force of the magnet  13 . The yokes  15  are fixedly installed at the base plate  19 .  
         [0006]     Currently, most current mobile micro-drive apparatuses have a CompactFlash type I (CF-I) form, which is an existing flash memory standard and provides a thickness of 3.3 mm, and their application range is gradually extending. On the other hand, the conventional IBM micro-drive apparatus of  FIG. 1  is excellent in terms of capacity, unit cost per storage capacity, and future competitiveness but adopts a CF-II form which provides a thickness of 5.0 mm because of the difficulties of making slim. As shown in  FIG. 1 , the conventional CF-II IBM micro-drive apparatus has the spindle motor  10  in which the magnet  13  and the yokes  15  are arranged to have the same central axis, thereby maximizing the rotation efficiency of the spindle motor  10 . However, such an arrangement is not suitable for the CF-I form having a great demand. In particular, there is not enough space in which to install stator yokes because of the thickness of an actuator installed in the space where the stator yokes are to be located.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a slim spindle motor having an improved stator and a slim micro-drive apparatus using the slim spindle motor.  
         [0008]     According to an aspect of the present invention, there is provided a spindle motor including: a hub seating a disk thereon and rotatably mounted on a base plate by ball bearings; a ring-shaped magnet engaged to the outer boundary of the hub; and a stator formed by arranging a plurality of yokes in a round shape around the outer boundary of the ring-shaped magnet. The yokes include cores having ends curved to reduce the central axis offset between each of the yokes and the ring-shaped magnet and most of the yokes include coils that cover the cores.  
         [0009]     The curved ends are curved either upward or downward so as to be perpendicular to the central axis of the stator.  
         [0010]     According to an aspect of the present invention, there is also provided a spindle motor including: a hub seating a disk thereon and rotatably mounted on a base plate by ball bearings; a ring-shaped magnet engaged to the outer boundary of the hub and having an increased diameter; and a stator formed by arranging a plurality of yokes in a round shape around the outer boundary of the ring-shaped magnet. The yokes include cores with increased diameters arranged such as to reduce the central axis offset between each of the yokes and the ring-shaped magnet and most of the yokes include coils that cover the cores.  
         [0011]     According to an aspect of the present invention, there is also provided a spindle motor including: a hub seating a disk thereon and rotatably mounted on a base plate by ball bearings; a slim ring-shaped magnet engaged to the outer boundary of the hub; and a stator formed by arranging a plurality of slim yokes in a round shape around the outer boundary of the ring-shaped magnet. The slim yokes include cores arranged such as to reduce the central axis offset between each of the yokes and the ring-shaped magnet and most of the yokes include coils that cover the cores.  
         [0012]     According to another aspect of the present invention, there is provided a micro-drive apparatus including a base plate, a spindle motor, and an actuator. The spindle motor includes a hub seating a disk thereon and rotatably mounted on a base plate by ball bearings, a ring-shaped magnet engaged to the outer boundary of the hub, and a stator formed by arranging a plurality of yokes in a round shape around the outer boundary of the ring-shaped magnet. The yokes include cores having curved ends to reduce the central axis offset between the yokes and the ring-shaped magnet and most of the yokes include coils that cover the cores. The actuator includes a head, which moves over the disk to write/read data to/from the disk, and a suspension positioned at an end of the head.  
         [0013]     According to another aspect of the present invention, there is also provided another micro-drive apparatus including a base plate, a spindle motor, and an actuator. The spindle motor includes a hub seating a disk thereon and rotatably mounted on a base plate by ball bearings, a ring-shaped magnet engaged to the outer boundary of the hub and having an increased diameter, and a stator formed by arranging a plurality of yokes in a round shape around the outer boundary of the ring-shaped magnet. The yokes include cores with increased diameters arranged such as to reduce the central axis offset between the yokes and the ring-shaped magnet and most of the yokes include coils that cover the cores. The actuator includes a head, which moves over the disk to write/read data to/from the disk, and a suspension positioned at an end of the head.  
         [0014]     According to another aspect of the present invention, there is also provided still another micro-drive apparatus including a base plate, a spindle motor, and an actuator. The spindle motor includes a hub seating a disk thereon and rotatably mounted on a base plate by ball bearings, a slim ring-shaped magnet engaged to the outer boundary of the hub, and a stator formed by arranging a plurality of slim yokes in a round shape around the outer boundary of the ring-shaped magnet. The slim yokes include cores arranged such as to reduce the central axis offset between the yokes and the ring-shaped magnet and most of the yokes include coils that cover the cores. The actuator includes a head, which moves over the disk to write/read data to/from the disk, and a suspension positioned at an end of the head.  
         [0015]     The curved ends are curved either upward or downward so as to be perpendicular to the central axis of the stator.  
         [0016]     Preferably, the magnet has a thickness of 1 mm or less.  
         [0017]     Preferably, the cores have thicknesses of 0.5 mm or less.  
         [0018]     Preferably, the number of cores is less than the number of poles of the magnet.  
         [0019]     The micro-drive apparatus further includes a damper installed on the hub to firmly clamp the disk to the hub and a screw locked in the hub to fix the damper to the hub.  
         [0020]     Due to the use of the improved stator according to the present invention in which a central axis offset between the magnets and the yokes is reduced, a spindle motor is slimmed, and a micro-drive apparatus using the spindle motor are also be slimmed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0022]      FIG. 1  is a side cross-section of a spindle motor in a conventional IBM micro-drive apparatus;  
         [0023]      FIG. 2  is a side cross-section of a slim spindle motor according to an embodiment of the present invention;  
         [0024]      FIG. 3A  is a cross-section of a first embodiment of a stator that can be installed in the slim spindle motor of  FIG. 2 ;  
         [0025]      FIG. 3B  is a perspective view of the stator of  FIG. 3A ;  
         [0026]      FIG. 4  is a cross-section of a second embodiment of a stator that can be installed in the slim spindle motor of  FIG. 2 ;  
         [0027]      FIG. 5  is a graph showing variations in the torque of a slim spindle motor according to an embodiment of the present invention with respect to the angle of the torque;  
         [0028]      FIG. 6  is a graph showing variations in an axial electromagnetic force with respect to its angle; and  
         [0029]      FIG. 7  is an exploded perspective view of a disk drive apparatus in which the slim spindle motor of  FIG. 2  is installed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     Referring to  FIG. 2 , a slim spindle motor  30  according to an embodiment of the present invention includes a hub  31 , a ring-shaped magnet  33 , and yokes  35 . The hub  31  is rotatably fixed to a base plate  39  by ball bearings  37 . The circular magnet  33  is fastened to the outer boundary of the hub  31  to provide a magnetic force. The yokes  35  are arranged in a round shape along the outer boundary of the magnet  33 .  
         [0031]     The magnet  33  functions as a rotor, and the yokes  35  function as a stator. The yokes  35  are wound with coils and generate electromagnetic forces that interact with the magnetic force of the magnet  33  so as to rotate the spindle motor  30  in a predetermined direction, which depends on the direction of current received from a flexible printed circuit (FPC; not shown). A screw  38  is locked in the central axis of the hub  31  to fix a damper (not shown), which seat a disk and fix the seated disk.  
         [0032]     The spindle motor  30  according to an embodiment of the present invention includes the circular magnet  33  smaller than the magnet  13  of the conventional spindle motor  10  of  FIG. 1  and the yokes  35  installed along the outer boundary of the magnet  33 , thereby simply achieving slimming of a micro-drive apparatus. The yokes  35  are fixed onto the base plate  39  and rotate the spindle motor  30  by rotating the hub  31  to which the magnet  33  is fastened using an electromagnetic force. Making the central axes of the magnet  33  and each of the yokes  35  collinear can maximize a torque created by the electromagnetic force generated by interaction between the magnetic field of the magnet  33  and the electric field created by current flowing in coils  34  wound around the yokes  35  and can remove a force in an unnecessary Z-axis direction, that is, in the direction of the height of a drive apparatus.  
         [0033]     If an offset between the central axes of the magnet  33  and each of the yokes.  35  occurs, a force in the Z-axis direction is generated, which causes unstable operation (e.g., vibration) of the spindle motor  30 . If the spindle motor  30  operates unstably, a seated disk (not shown) does not rotate in a round shape but in a distorted round shape. Hence, a head may be positioned on a wrong track, which is referred to as runout. If such runout occurs, it is difficult to record data to an accurate area on a disk, and recorded data cannot be properly read.  
         [0034]     Hence, the spindle motor  30  of  FIG. 2  includes a first embodiment of a stator whose central axes offset is removed, though the magnet  33  and the yoke  35  become slimmer. However, since there is a limit in slimming the magnet  33  and the reduction of the thickness of the magnet  33  reduces the amount of generated magnetic force, the magnet  33  having a high magnetic flux must be employed. To overcome this limit, first and second embodiments of a stator that can have the magnet  33  whose thickness is not slimmed are shown in  FIGS. 3A and 4 .  
         [0035]      FIGS. 3A and 4  show first and second embodiments of a stator that can be installed in the slim spindle motor of  FIG. 2 .  FIG. 3B  is a perspective view of the stator of  FIG. 3A .  
         [0036]     In the first embodiment of the stator of  FIGS. 3A and 3B , yokes  35   a  have ends  34   a  curved upwardly such that the central axes of the yokes  35   a  are collinear with that of a magnet  33   a  and are arranged around the outer boundary of the magnet  33   a . The yokes  35   a  are each comprised of a core  34   a  and a coil  36   a  which covers the core  34   a . The magnet  33   a  is engaged to the hub  31 , and the yokes  35   a  are fixed onto the base plate  39 . If the magnet  33   a  is installed at a location lower than the yokes  35   a , the yokes  35   a  may have downwardly curved ends instead of the upwardly curved ends  34   a  so as to make the central axis of each of the cores  34   a  collinear with that of the magnet  33   a.    
         [0037]     Referring to  FIG. 3B , the ring-shaped magnet  33   a  is comprised of 12 poles, and 9 yokes  35   a  are arranged in a round shape around the outer boundary of the magnet  33   a . A stator  32  is comprised of the yokes  35   a  symmetrically arranged around the outer boundary of the magnet  33   a  and an outer ring which supports the yokes  35   a . As described above, the yokes  35   a  are each comprised of the core  34   a  and the coil  36   a  wound around the core  34   a . However, the cores  34   a  of some of the yokes  35   a  are not wound with a coil. In  FIG. 3B , two adjacent cores  34   a  are wound with coils  36   a , and the cores  34   a  on both sides of the pair of cores  34   a  wound with the coils  36   a  are not wound with coils. In this way, the yokes  35   a  are arranged.  
         [0038]     In the second embodiment of the stator of  FIG. 4 , the hub  31  is installed at a high location and the diameter of the magnet  33   b  is increased, thereby obtaining sufficient outer space. The yokes  35   b  are positioned in the obtained space at a higher location than the location where the yokes  35   a  in the first embodiment are positioned, such that the central axis of each of the yokes  35   b  is made collinear with that of the magnet  33   b . However, since the stator according to the second embodiment needs a space in which to install the yokes  35   b , the locations of other component elements must be adjusted.  
         [0039]     Hence, a spindle motor adopting the stator according to the first embodiment shown in  FIGS. 3A and 3B  is suitable for use in conventional CF-I disk drives.  
         [0040]      FIG. 5  is a graph showing variations in the torque of a slim spindle motor according to an embodiment of the present invention with respect to the angle of the torque. Reference numeral f 1  indicates an ideal case where an offset is 0, reference numeral f 2  indicates a case where an offset is 0.2 mm, reference numeral f 3  indicates a case where an offset has been removed by curving an end of a yoke upward by 0.2 mm, and reference numeral f 4  indicates a case where an offset is reduced from 0.12 mm to 0.08 mm by curving the end of the yoke upward by 0.2 mm.  
         [0041]     Referring to  FIG. 5 , cases f 1 , f 3 , and f 4  have similar torque variations, while the torque of case f 2  is reduced from those of the other three cases. Such a reduction in torque denotes a reduction in the turning force of a spindle motor, and then if the turning force is reduced, the spindle motor operates unstably. Unstable operation of the spindle motor causes a disk seated on the spindle motor to rotate irregularly, making it difficult to accurately write data to an area on the disk and accurately read data. Thus, a spindle motor according to the present invention can have a stable torque characteristic by adopting a structure in which an offset between the central axes of a magnet and a yoke is reduced or removed.  
         [0042]      FIG. 6  is a graph showing variations in an axial electromagnetic force with respect to its angle. Referring to  FIG. 6 , reference numeral g 1  indicates a force in a Z-axial direction, Fz, that represents 0 in an ideal case where an offset is zero. Reference numeral g 2  indicates Fz of about 0.2 N in the case f 2  of  FIG. 4 , and reference numeral g 4  indicates Fz of about 0.14 N in the case f 4  of  FIG. 4 . As shown in  FIG. 6 , a Z-axial offset generates the Z-axial force (Fz), and Fz increases as the Z-axial offset increases. As described above, as Fz increases, the rotation of the spindle motor becomes unstable.  
         [0043]      FIG. 7  is an exploded perspective view of a disk drive apparatus  40  in which the slim spindle motor of  FIG. 2  is installed. The disk drive apparatus  40  includes a disk  50  which stores information, a locking area  52  on which the disk  50  is seated, the spindle motor  30  which rotates the seated disk  50 , an actuator  43  having a head  47  capable of recording/reproducing data to/from the disk  50 , and a voice coil motor (not shown) comprised of a magnet assembly  46  and a voice coil  45  which drives the actuator  43 .  
         [0044]     The spindle motor  30  of  FIG. 2  is positioned under the locking area  52 . A stator installed in the spindle motor  30  can be any of the stators shown in  FIGS. 2 through 4 . Preferably, an offset is removed by making the central axis of the magnet  33 ,  33   a , or  33   b  collinear with that of the yoke  35 ,  35   a , or  35   b , respectively, so that a force in the Z-axial direction is removed.  
         [0045]     The actuator  43  receives an electrical signal from a printed circuit board (PCB; not shown) via a flexible printed circuit (FPC; not shown).  
         [0046]     The disk  50  is comprised of a parking zone minutely formed by laser at the inner area of the disk  50  and is fitted onto the spindle motor  30  so that the head  47  can be parked in the parking zone upon power-off. A data zone can be formed at the outside of the parking zone to record a magnetic signal on the data zone. The data zone has several tens of thousands of tracks on which a servo signal indicating locations where data is to be recorded has already been recorded along the circular shape of the disk  50 .  
         [0047]     The actuator  43  includes the voice coil motor (not shown) which drives the actuator  43 , a pivot bearing  48  around which the actuator  43  rotates, and the head  47  which has a write head to write data to the disk  50  and a read head to read data from the disk  50 . In particular, the actuator  43  is made very slim by attaching the voice coil  45  to a fantail molding portion, which is formed by extending a uni-mounting portion of a head gimbal assembly  41 .  
         [0048]     The PCB sends an electrical signal to the FPC, and the FPC transmits the received electrical signal to the actuator  43 . The electrical signal received by the actuator  43  is transmitted to the voice coil  45 . An electromagnetic force created by interaction between current flowing in the voice coil  45  and the magnetic force of the magnet assembly  46  rotates the actuator  43  around the pivot bearing  48  to move the actuator  43  from the parking zone to the data zone.  
         [0049]     The present invention provides a stator whose central axes offsets between the magnet and yokes are reduced or removed, thereby obtaining a slim spindle motor and a slim micro-drive apparatus.  
         [0050]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. For example, various stators in which the central axes of yokes are collinear with that of a magnet may be construed.  
         [0051]     As described above, the use of a stator in which the central axes of yokes are collinear with that of a magnet makes it possible to manufacture a slimmer and lighter spindle motor and a slimmer and lighter micro-drive apparatus that can maintain a stable driving performance and also be utilized as a mobile micro-drive apparatus.
A slim spindle motor and a micro-drive apparatus including the slim spindle motor are provided. In the spindle motor, a hub is rotatably mounted on a base plate by ball bearings. The hub is used to seat a disk thereon. A ring-shaped magnet is engaged to the outer boundary of the hub. A stator is formed by arranging a plurality of yokes in a round shape around the outer boundary of the ring-shaped magnet. The yokes include cores having curved ends to reduce the central axis offset between the yokes and the ring-shaped magnet. Most of the yokes include coils that cover the cores. Since the central axes of the yokes in the stator are made collinear with the central axis of the ring-shaped magnet, the slim spindle motor can operate stably.