Abstract:
An electric motor comprises a stator ( 1, 5, 8 ) having an air-gap which extends around an axis and in which a rotating magnetic field is generated, and a rotor ( 11, 12, 13 ) comprising a shaft ( 11 ) which is rotatably held by a bearing ( 10 ) and a permanent magnet ( 13 ) which is held in said air-gap and is driven to rotate around said axis by said rotating magnetic field. The stator ( 1, 5, 8 ) is adapted to generate, in addition to said rotating magnetic field, a stationary magnetic field which is rotationally asymmetric with respect to said axis.

Description:
FILED OF INVENTION  
       [0001]     The present invention relates to an electric motor, in particular but not exclusively to a brushless DC motor.  
       BACKGROUND  
       [0002]     In recent years, demand has grown for miniature electric motors of extremely flat design, which can be used to build flat drives for disk-shaped data carriers such as floppy disks, CD-ROMs, DVDs etc.  
         [0003]     The shaft of such a motor has a certain bearing clearance which allows the shaft to sway. Obviously, the sway angle of the shaft is the larger, the smaller the axial dimension of the bearing is (if the shaft is held by a single bearing) or the smaller the distance between bearings at opposite ends of the shaft is. If a disk is rotated by the motor, the axis of rotation of the disk may move in space, and its orientation may vary. This causes a problem when data are read from the disk: Conventionally, the data are arranged on concentric tracks on the surface of the disk, and if the axis of rotation of the disk moves, a servosystem of an actuated reading head of the disk drive may have more problems to follow the track during a turn of the disk, so that data cannot be read consecutively. The distance between the head and the surface of the disk may vary in addition and may sometimes leave the range in which reading is possible.  
         [0004]     Part of these variations are periodic, with a period equalling the rotation period of the disk. These variations can be predicted quite accurately based on observation, and they can be compensated by moving the reading head periodically, so that it faces the same track of the disk and does not leave the proper reading distance range during a complete turn. These periodic variations are also referred to as repeatable runout.  
         [0005]     However, there is also a non-repeatable runout which is not easily compensated and which may cause serious tracking problems. Reducing this non-repeatable runout by reducing the bearing clearance is a rather expensive solution to the problem, because the components of the motor have to be manufactured with very strict tolerances.  
         [0006]     US 2004/0007929 A1 discloses a flat electric motor in which a bearing extends over most of the axial dimension of the motor. There is nothing in this motor that might prevent the shaft from swaying over the entire angle allowed by the bearing clearance.  
       SUMMARY OF THE INVENTION  
       [0007]     The object of the present invention is to provide an electric motor in which a rotor shaft is prevented from swaying over the entire angle allowed by the clearance of a bearing, which holds the shaft.  
         [0008]     This object is achieved by an electric motor comprising a stator having an air-gap which extends around an axis and in which a rotating magnetic field is generated, and a rotor comprising a shaft which is rotatably held by a bearing and a permanent magnet which is held in said air-gap and is driven to rotate around said axis by said rotating magnetic field, the electric motor being characterized in that the stator is adapted to generate, in addition to said rotating magnetic field, a stationary magnetic field which is rotationally asymmetric with respect to said axis.  
         [0009]     This asymmetric stationary magnetic field will impose an excentric force on the rotor which is in particular perpendicular to the rotational torque on the rotor around its rotor axis and which will therefore cause the rotor to tilt. Such an eccentric force causes a change of the angular momentum vector of the rotor, which is perpendicular to the torque vector. The rotor can fit its axis to this change of angular momentum as far as the bearing clearance allows. The axis of the rotor will therefore assume a constant and well-defined orientation, regardless of how large the bearing clearance is. The non-repeatable runout is reduced considerably.  
         [0010]     Preferably, the stator comprises a number of radially oriented spokes made of soft-magnetic material. Using radial spokes instead of a solid disk reduces formation of eddy currents, which would counteract the rotor movement.  
         [0011]     According to a first preferred embodiment, as a very simple means to generate the asymmetric stationary magnetic field, at least on of the cross section and the length of the spokes in a first sector of the stator is made different, e.g. in thickness or length, from that of the spokes in a second sector.  
         [0012]     Alternatively, the number of spokes per unit angle in the first sector may be made different from that in the second sector. The first and second sectors may extend over about half of the circumference of the stator, respectively.  
         [0013]     In order to facilitate the assembly of the motor, the spokes preferably have their inward ends or their outward ends unitarily connected to a ring. Alternatively, they may be embedded in or mounted on a non-conductive material. Preferably, this non-conductive material is a circuit board.  
         [0014]     According to a second preferred embodiment, the stator comprises a plurality of coils distributed uniformly around the axis, and in a first sector of the stator, the permeability of a material within the coils is different from that of the coils in a second sector of the stator.  
         [0015]     According to a third preferred embodiment, the stator comprises a plurality of coils distributed uniformly around the axis, for generating the magnetic field, and in a first sector of the stator, the number of windings per coil is different from that in a second sector, in order to generate the asymmetric field component.  
         [0016]     The motor may be made particularly flat if the coils have their axes oriented parallel to the axis of rotation. The coils may be formed further very economically by printing on a circuit board. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Further features and advantages of the present invention will become apparent from the subsequent description of embodiments thereof, referring to the appended drawings.  
         [0018]      FIG. 1  is an exploded view of a motor according to the first embodiment of the invention;  
         [0019]      FIG. 2  is an axial cross section of the motor of  FIG. 1 ;  
         [0020]      FIG. 3  is a perspective view showing the bottom side of the motor of  FIG. 1 ;  
         [0021]      FIG. 4  is a perspective view showing the top side of the motor of  FIG. 1 ;  
         [0022]      FIG. 5  is an exploded view, analogous to  FIG. 1 , of a motor according to the second embodiment of the invention;  
         [0023]      FIG. 6  is an axial cross section of the motor according to the second embodiment;  
         [0024]      FIG. 7  is a perspective view showing the top side of the motor of  FIG. 5 ;  
         [0025]      FIG. 8  is a perspective view showing the bottom side of the motor of  FIG. 5 ;  
         [0026]      FIG. 9  is a bottom view of a circuit board according to a third embodiment of the motor;  
         [0027]      FIG. 10  is a bottom view of a circuit board according to a fourth embodiment of the motor;  
         [0028]      FIG. 11  is a plane view of a circuit board according to a third embodiment of the motor; and  
         [0029]      FIG. 12  is an electrical connection and placement scheme of coils  9   a  and  9   b  according to a fifth embodiment of the motor. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]     A first embodiment of the motor will be described referring to FIGS.  1  to  4 . The motor has a base member  1  made of non-magnetic material, i.e. aluminum, and shaped as a short hollow cylinder with two shoulders  2 ,  3  extending around the outer periphery of the cylinder and a third shoulder  4  in its inner cavity. The shoulder  2  supports a yoke  5  cut from sheet iron. The yoke  5  is formed of a central ring  6  and a large number of spokes  7   a ,  7   b , which extend radially from the ring  6 . In a first half of the yoke  5 , facing the viewer in  FIG. 1 , the spokes  7   a  are shorter than the spokes  7   b  of the other half. The long spokes  7   b  extend up to the outer edge of an annular circuit board  8 , which is supported by the shoulder  3  of the base member  1 . Six coils  9  are printed on the upper surface of the circuit board  8 , opposite the yoke  5 . Base member  1 , yoke  5  and circuit board  8  form a stator of the motor.  
         [0031]     On the inner shoulder  4  of base member  1 , an outer ring of a ball bearing  10  is supported. A shaft  11  is fitted snugly into an inner ring of the ball bearing  10 . The ball bearing  10  has a clearance.  
         [0032]     A radial flange of the shaft  11  carries a rotor disk  12  and a ring magnet  13 , which has magnetic poles facing the coils  9 . The field axes of the coils  9 , being perpendicular to the surface of the circuit board, are parallel to the axis of shaft  11 .  
         [0033]     As is known in the art, the rotor formed of shaft  11 , disk  12  and ring magnet  13  is driven by a rotating magnetic field which is generated by supplying alternating currents of different phases to the coils  9 , causing a magnetic flux in a magnetic circuit formed of ring magnet  13 , rotor disk  12 , shaft  11 , ball bearing  10 , base member  1  and yoke  5 .  
         [0034]     Due to the different lengths of spokes  7   a  and  7   b  of yoke  5 , the air gap width is bigger and the yoke area is smaller in the sector having the shorter spokes  7   a  than in the sector having the long spokes  7   b , and accordingly, the magnetic attraction force experienced by the sector of ring magnet  13  facing the short spokes  7   a  is less than that of the sector facing the long spokes  7   b . Therefore, whenever the motor is in operation, a torque is applied to the rotor, which tends to tilt the rotor around an axis which is perpendicular to the angular momentum vector causing the rotation of the rotor.  
         [0035]     It is known that in a freely spinning top, such a torque would cause an axis variation of the angular momentum vector and, hence, of the axis of the top. This is because in a spinning top, the torque vector rotates in synchronism with the direction to which the top is inclined, so that the torque vector is always orthogonal to the angular momentum vector. In the present case, in contrast, the torque vector does not rotate, so that the rotor axis will only tilt in a predetermined direction as far as the clearance of ball bearing  10  will allow. Thus, in spite of the clearance which would allow the shaft  11  to sway, the shaft  11 , when driven to rotate by the magnetic fields of the coils  9 , will assume a well-defined position and orientation, and a non-repeatable runout is reduced to a minimum.  
         [0036]     In the above-described embodiment, the yoke  5  has the inner ring  6  which is supported on shoulder  2  of base member  1  and from which the spokes  7   a ,  7   b  extend radially to the outside. The spokes  7   a ,  7   b  differ only in length, but not in cross section.  
         [0037]     Alternatively, the field asymmetry required for defining the position and orientation of the shaft might be caused by spokes  7   a ,  7   b , which differ not in length but in cross section area.  
         [0038]     As another alternative, the angular spacing of the spokes might be made different in first and second sectors of yoke  5 .  
         [0039]     As a further alternative, the spokes might be connected in one piece by an outer ring, the free ends extending radially to the inside and being supported on shoulder  2  of base member  1 . In this case an electrically isolating layer should be provided between the free ends of the spokes and the base member in order to prevent the flow of eddy currents from the outer ring through a first spoke and the base member and back to the outer ring through a second spoke. Another alternative is, that the spokes  7   a  and  7   b , connected in one piece by an outer ring, don&#39;t extend to the base member  1  but are only supported by a connection to the circuit board  8 , for example by glueing.  
         [0040]     A second embodiment of the motor is described referring to FIGS.  5  to  8 . Components of this motor that have identical counterparts in the motor of the first embodiment have the same reference numerals as these and are not described again.  
         [0041]     In the motor of this second embodiment, the yoke  5  has spokes  7  of identical shape which extend radially to the outside from an inner ring at regular angular intervals. An asymmetric magnetic field component is generated in this embodiment by the fact that among the coils  9  on the circuit board  8 , there is one group formed of three consecutive coils  9   b , which have a metal core  14 , whereas the remaining three coils  9   a  that form the other group do not. These metal cores  14  may simply be fixed to the surface of circuit board  8 , e.g. by glueing; preferably as shown in the cross section of  FIG. 6  and the bottom view of  FIG. 8 , they are fitted into holes that are formed in the circuit board  8 . The metal cores  14  are also effective to decrease the air gap width in the half of the circuit board  8  occupied by the coils  9   b , so that in this half, the ring magnet  13  is subject to a stronger magnetic attraction force than in the half bearing the empty coils  9   a . Just like in the first embodiment, the asymmetry of the magnetic field causes the shaft  11  to assume a well-defined, slightly tilted orientation.  
         [0042]      FIG. 9  is a bottom view of a circuit board according to a third embodiment of the invention. In this embodiment, instead of a one-part yoke, there is only a number of radial spokes  7  which are not directly interconnected but fixed, e.g. by glueing, to the bottom side of the circuit board  8 . Similar to the embodiment of FIGS.  5  to  8 , the part of the coils formed on the top side of circuit board  8 , not shown, have metal cores  14  which extend through holes of the circuit board  8 . For mounting the spokes  7  on the circuit board  8 , conventional technology for assembling electronic circuits from discrete components may be employed. Another alternative could be a single plastic yoke part  8   a , with integrated spokes  7 , the plastic yoke part made by e.g. an injection moulding process, in accordance with  FIG. 11 .  
         [0043]     It should be noted, according to a fourth embodiment, that by placing identical spokes  7  individually on the circuit board  8 , it is particularly easy to form a circuit board as shown in  FIG. 10 , in which a magnetic field asymmetry is obtained by placing the spokes at smaller angular intervals in one half of the circuit board  8  than in the other.  
         [0044]     Another approach for obtaining a magnetic field asymmetry is illustrated in  FIG. 12 , showing an electrical connection scheme and placement scheme of coils  9   a , coils  1 ,  3 ,  5  in  FIG. 12  having n windings, and coils  9   b , coils  2 ,  4 ,  6  in  FIG. 12  having n+x windings, with different amounts of windings n, according to a fifth embodiment of the invention. The coils  5 ,  1 ,  3  in one half of the circuit board  8  have a smaller number of windings than the coils  4 ,  2 ,  6  of the other half, so that when the two groups of coils are supplied with identical currents, as shown in the connection scheme of  FIG. 12 , the magnetic attraction between the ring magnet and the coils  9   a  will be less than between the ring magnet and coils  9   b.