Abstract:
A brushless motor includes a rotor position detecting device disposed between stator teeth, rather than directly below a rotor magnet, so that the distance between a lower end of a rotor magnet and a stationary frame is minimized. This results in a thinner brushless motor than conventional designs. In the preferred embodiment of the present invention, the rotor position detecting device is a Hall device.

Description:
This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in Japanese Patent Application No. 2002-077860 filed on Mar. 20, 2000. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to brushless motors for driving storage disks, such as CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-RAM, and DVD-RW. More particularly, the present invention relates to brushless motors in which magnetic field variations, generated by a rotor magnet, are detected by a rotational position detecting means that is disposed in a particular position on the stator side, such that the stator current can be switched using the detected result. 
     2. Background Art 
     As shown in FIG. 6, by way of example, a brushless motor used to drive a disk comprises a stationary frame  100 , a shaft  104  rotatably supported by the stationary frame  100  through a bearing  102 , a rotor  106  mounted to the shaft  104  and rotating in union with the shaft  104 , a rotor magnet  108  attached to the rotor  106 , and a stator  110  supported by the stationary frame  100  in a position facing the rotor magnet  108 . 
     The stator  110  comprises a stator core  112  and stator windings  114  having, for example, 3 phases, which are wound over the stator core  112 . Also, the rotor magnet  108  is magnetized with multipolar magnetization, in which the rotor magnet is magnetized into different magnetic poles alternating in the circumferential direction. The direction of current supplied to the stator winding  114  of each phase is changed in correspondence with the rotation of the rotor magnet  108 . The torque of the motor is obtained through repeated attraction and repulsion between the magnetic poles of the stator  110  and the magnetic poles of the rotor magnet  108 . 
     In order to make the brushless motor rotate, the current supplied to the stator winding  114  of each phase must be switched in sync with the rotation of the rotor magnet  108 . The timing for switching the current is generated by detecting variations of a magnetic field generated by the rotation of the rotor magnet  108 . The field detecting means  116  is disposed in a particular position on the stationary side. A Hall device  116  is used as one known example of field detecting means for that purpose. 
     The Hall device  116  generates a voltage depending on the amount of magnetic flux penetrating the Hall device. Accordingly, the greater a change in the magnitude of the terminal voltage, more precisely a change in the amount of magnetic flux penetrating the Hall device can be measured with higher sensitivity. In the brushless motor, variations of magnetic flux caused by the rotation of the rotor magnet  108  are detected by the Hall device  116  that is disposed in a particular position on the stationary side. Detection sensitivity in the rotor rotation can be increased by arranging the Hall device  116  at a position where the magnetic flux penetrating the Hall device is maximally changed with the rotor rotation. 
     Further, the rotor magnet  108  is multipolar-magnetized such that different magnetic poles are alternating in the circumferential direction and in the radial direction. In a cross-section of the rotor magnet  108  shown in FIG. 6, the radially inner side of the rotor magnet is magnetized into an N (or S) pole and the radially outer side thereof is magnetized into an S (or N) pole. Then, lines of magnetic force generated by the rotor magnet  108  are radially extended from both poles and are deflected, to a large extent, depending on the arrangement of magnetic bodies disposed in the surroundings of the rotor magnet. It is usually thought that the sensitivity in detecting the rotor rotation is increased by arranging the Hall device in a position directly below, and closer to, the rotor magnet. In positions away from the position directly below the rotor magnet, the rotation detection sensitivity is reduced, while it is relatively increased by arranging the Hall device in a position where the lines of magnetic force are concentrated (and hence the density of magnetic flux is relatively high). 
     Recently, notebook personal computers capable of handling CD-ROMs or the like have been commercialized. The size and thickness of these disk drives for driving CD-ROMs or the likes have been reduced. Correspondingly, there is a demand for a reduction in the size and thickness of the brushless motors that are to be incorporated in these disk drives. 
     However, because the Hall device  116  is disposed directly below the rotor magnet  108 , as shown in FIG. 6, the presence of the Hall device  116  impedes an attempt at reducing the motor thickness in the axial direction. 
     On the other hand, when attempting to move the position of the Hall device  116  radially inward of the rotor magnet  108  to avoid such a drawback, there is not sufficient space to accommodate the Hall device, because the stator windings  114  are disposed radially inward of the rotor magnet as shown in FIG.  7 . Also, even if there is sufficient space, it would be difficult to precisely detect the rotor rotation because, in a position away from the rotor magnet, the density of magnetic flux is reduced and, therefore, the sensitivity in detecting the rotor rotation is reduced. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a brushless motor, in which a rotor position detecting device is able to detect changes in the density of magnetic flux that are caused by rotation of a rotor magnet, while reducing the size and thickness of the motor. 
     According to the present invention, a brushless motor includes a rotor position detecting device disposed between stator teeth, rather than directly below a rotor magnet, so that the distance between a lower end of a rotor magnet and a stationary frame is minimized. This results in a thinner brushless motor than conventional designs. In the preferred embodiment of the present invention, the rotor position detecting device is a Hall device. 
     Further, according to the present invention, the windings are wound over a stator in larger number on the inner peripheral side than they are on the outer peripheral side thereof. However, the total number of stator windings remains substantially equal to that in a conventional motor, so that sufficient space to accommodate the Hall device is defined between the adjacent teeth of a stator core. 
     In addition, the Hall device that is disposed in such a space is fixed in a position where the lines of magnetic force generated from the rotor magnet are concentrated (and hence the density of magnetic flux is relatively high), and the magnetically sensitive surface of the Hall device is inclined with respect to the axial direction of a shaft of the motor. This arrangement enables the Hall device to receive the most possible magnetic flux generated during the rotation of the rotor magnet. As a result, the Hall device can detect, with satisfactory accuracy, the timing of switching in a stator current supplied to the brushless motor. 
     With the arrangements described above, the present invention has succeeded in reducing the size and thickness of the brushless motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a brushless motor, according to the present invention; 
     FIG. 2 is a sectional view of a stator of the brushless motor of FIG. 1; 
     FIG. 3 is a perspective view of a Hall device of the brushless motor of FIG. 1; 
     FIG. 4 is a side view of the Hall device of FIG. 3; 
     FIG. 5 is a sectional view of a disk drive with the brushless motor of FIG. 1 disposed therein; 
     FIG. 6 is a sectional view of a portion of a conventional brushless motor; and 
     FIG. 7 is a plan view of a portion of the conventional brushless motor of FIG.  6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a sectional view of a brushless motor for driving a disk according to one embodiment of the present invention, and FIG. 2 is a sectional view of a stator used in the brushless motor. 
     It is to be noted that the “vertical direction” used in the following description of one embodiment of the present invention implies the vertical direction as defined on each of the drawings for the sake of convenience, but that the direction of the brushless motor as actually mounted is not limited to the illustrated direction. 
     The brushless motor for driving a disk according to the described embodiment illustrates a motor used in a disk drive for a CD-ROM or the like. The brushless motor comprises a frame  1 , a bushing  5  fixed to the frame  1  to stand in the vertical direction, a sleeve bearing  9  fitted to the bushing  5 , a shaft  7  rotatably supported by the sleeve bearing  9 , and a substantially cup-shaped rotor  17 . 
     The frame  1 , serving as a stationary member, has a central hole  3  formed therein, and the bushing  5  is fitted to the central hole  3 . The bushing  5  is fabricated from a magnetic material, such as iron or stainless steel, and has a substantially cylindrical shape. The bushing  5  is fixed to the frame  1  by plastically deforming a caulked portion  51   a , which is formed at a lower end of the bushing  5 , toward the outer peripheral side. The sleeve bearing  9  is fitted to an inner periphery of the bushing  5  on the upper side, and a closing plate  11  is attached to a lower end of the bushing  5  to enclose a bottom opening of the bushing  5 . A disk-shaped thrust bearing plate  13  is attached to an upper surface of the closing plate  11 , and the thrust bearing plate  13  and the closing plate  11  are both fixed to the bushing  5  by plastically deforming a caulked portion  51   b , which is formed at the lower end of the bushing  5 , toward the inner peripheral side. 
     A projection  52  is formed at the upper end of the bushing  5  and extends outwardly from its outer periphery. A hook  14  is attached to an inner portion of a rotor  17  so that it is capable of engaging the projection  52 , whereby the amount of axial movement of both the shaft  7  and the rotor  17  is restricted. 
     The rotor  17 , serving as a rotating member, is formed of a magnetic material, such as iron, by pressing. The rotor  17  comprises an upper wall portion  17   a , a peripheral wall portion  17   b  extending downward from an outer periphery of the upper wall portion  17   a , and a boss portion  17   c  erected at the center of the upper wall portion  17   a  and having a circular bore formed through the boss portion  17   c . Then, the boss portion  17   c  is fitted over an upper portion of the shaft  7  so that the shaft  7  and the rotor  17  are rotated in union with each other. 
     An upper surface of the upper wall portion  17   a  of the rotor  17  serves as a loading portion on which a disk, such as a CD-ROM, is loaded. A buffer member  21  is attached to an upper surface of an outer peripheral portion of the upper wall portion  17   a , and a disk (not shown) is placed on the upper wall portion  17   a  with the buffer member  21  interposed therebetween. 
     Further, a center boss  23  formed of a nonmagnetic material and fitted to a center hole of the disk is mounted to the boss portion  17   c  of the rotor  17 . The center boss  23  is provided with a plurality of chucks  25  which are movable in the radial direction and are arranged at equal angular intervals. Each of the chucks  25  is urged radially outward by a spring  26  disposed inside the chuck  25 . Accordingly, when the center hole of the disk is fitted to the center boss  23 , an inner peripheral edge of the disk pushes the chucks  25  radially inward against the biasing forces of the springs  26  acting radially outward. Then, when the disk is loaded in a position where it contacts the buffer member  21 , a distal end of each chuck  25  is positioned over an upper surface of an inner peripheral portion of the disk around the center hole, whereupon the chuck  25  now presses the disk against the upper wall portion  17   a  of the rotor  17  by the biasing force of the spring  26  acting radially outward. As a result, the disk is properly placed on the upper wall portion  17   a  of the rotor  17 . In addition, the center boss  23  is provided with a plurality of center aligning fingers  27  positioned between the chucks  25  in the circumferential direction. Upon loading of the disk, the center aligning fingers  27  contact the inner peripheral edge of the disk for center alignment of the disk. 
     The structure constituting the features of the present invention will now be described in detail with reference to FIGS. 1,  2 ,  3  and  4 . 
     As shown in FIG. 1, a cylindrical rotor magnet  19  is attached to an inner surface of the peripheral wall portion  17   b  of the rotor  17  and is positioned to face the stator  15  with a very small gap left between them in the radial direction. The stator  15  comprises a stator core  15   a  and windings  15   b  wound over teeth (not shown) projecting from a base portion of the stator core  15   a  in a radial pattern. The stator  15  is fitted to a stepped portion  53  formed in an upper outer peripheral portion of the bushing  5 . Further, an annular magnet  18  is attached to an upper surface of the base portion of the stator core  15   a . The annular magnet  18  is positioned to face the upper wall portion  17   a  of the rotor  17  in the axial direction for applying a magnetic bias to the rotor  17 . 
     As shown in FIG. 2, by way of example, the windings  15   b  are wound such that the number of windings is larger on the inner peripheral side than on the outer peripheral side. With this arrangement, a space sufficient to accommodate a Hall device  31  is ensured between the adjacent teeth of the stator core  15   a.    
     Corresponding to those spaces, a plurality of Hall devices  31  are attached to a circuit board  29  that is disposed on the frame  1 . In this embodiment, since the number of teeth of the stator core  15   a  is  12  and the windings  15   b  are wound in 3 phases, three Hall devices  31  are disposed between three pairs of the adjacent teeth of the stator core  15   a.    
     Further, as shown in FIGS. 3 and 4, the Hall devices  31  are each fixed in the above-mentioned space at a position where magnetic flux is maximally changed with the rotor rotation, and the magnetically sensitive surface  31   a  of each Hall device  31  is inclined at a predetermined angle with respect to the axial direction of the shaft  7 . The predetermined angle is selected to a value at which magnetic flux is maximally changed with the rotor rotation. With this arrangement, in spite of the Hall device being fixed to a location away from the position directly below the rotor magnet  19 , the Hall device can detect, with satisfactory accuracy, changes in the density of magnetic flux caused by the rotation of the rotor magnet  19 . Consequently, not only the stator current can be switched using the detected result to make the rotor rotate accurately, but also the rotor magnet  19  can be positioned closer to the upper surface of the frame  1 , with the circuit board  29  interposed between them. The resulting brushless motor has a smaller thickness than a conventional one. 
     As described above, by winding the windings  15   b  over the stator core  15   a  in a larger number on the inner peripheral side than on the outer peripheral side thereof, while keeping the total number of the stator windings  15   b  wound over each tooth of the stator core  15   a  substantially equal to that in the conventional motor, a space sufficient to accommodate the Hall device  31  is defined between the adjacent teeth of the stator core. Such unevenness in the number of windings can be realized by estimating a position where the sensitivity in detecting the rotor rotation is maximized by arranging the Hall device  31  in that position, and determining a manner of winding the windings, with which the space is created in that position. By thus ensuring the space, it is possible to adjust the position where the Hall device  31  is to be fixed. 
     The inner construction of a general disk drive  40  will now be described with reference to FIG.  5 . The disk drive  40  comprises a housing  42 , a brushless motor  44  fixedly disposed within the housing  42 , a removable disk  46  having the shape of a circular plate and held on the brushless motor  44 , and a pickup device  48  for writing and/or reading information in and/or from a predetermined position on the disk  46  during the motor rotation. 
     While one embodiment of the present invention has been described above, the present invention is not limited to the above-mentioned embodiment, but can be modified in various ways. 
     For example, the above-mentioned embodiment uses the Hall device  31  having the magnetically sensitive surface  31   a  inclined with respect to the axial direction of the shaft  7 . However, a Hall device having a magnetically sensitive surface parallel to the axial direction of the shaft may also be used. 
     Further, while a Hall device is used as a rotational position detecting means in the above description, the rotational position detecting means is not limited to the Hall device. 
     Moreover, the embodiment has been described in connection with the disk driving motor of the so-called outer rotor type in which the rotor magnet  19  is disposed on the side radially outward of the stator  15 . However, the present invention is also applicable to a disk driving motor of the so-called inner rotor type in which a rotor magnet is disposed on the radially inward side of a stator. In such a case, similar advantages in operation to those in the above-mentioned embodiment can also be obtained. 
     Additionally, while the embodiment of the present invention has been described in connection with the disk driving motor, the applicable range of the present invention is not limited to the field related to driving of disks. The present invention can also be employed in other various fields of applications, and similar advantages in operation to those in the above-mentioned embodiment can be obtained.