Abstract:
A stator magnet according to a first embodiment of the present invention forms a voice coil motor with a closed coil. The closed coil is supported by an actuator arm in a rotatable manner and at a predetermined rotation angle. The closed coil has first and second side edges that extend along different lines in radial directions whose center is a center of rotation of the actuator arm. It also has an outer edge that connects edges of outer sides of the first and second side edges with viewed from the radial direction, and extends along an arc whose center is the center of rotation. A first magnetic pole region is located within a moving area of the first side edge to act on the first side edge. A second magnetic pole region is located within a moving area of the second side edge to act on the second side edge. The polarity of the second magnetic pole region is opposite to the polarity of the first magnetic pole region. A third magnetic pole region is located within a moving area of the outer edge to act on the outer edge. The polarity of the third magnetic pole region is the same as the polarity of the first magnetic pole region.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a disk drive, an actuator, and a stator magnet configuring a voice coil motor (hereinafter, a VCM) of the actuator, and in particular, to a configuration for improving a breathing phenomenon of a coil occurring during the operation of the VCM. 
     2. Description of the Related Art 
     FIG. 10 is a schematic of an actuator  100  used in a conventional hard disk drive. An actuator arm  101  is configured by a suspension  102  and a coil support part  103  in one piece, is rotatably supported by a rotary shaft  104  setting on a base (not shown), and is driven by a VCM, described later, in the direction shown by an arrow J or K. 
     A slider  109  is supported in an edge of the suspension  102 , and respective heads for reading and writing that are not shown are provided on this slider  109 . When the actuator arm  101  is positioned on a recording surface of a hard disk (not shown) rotating, the actuator  100  is configured so that the heads face the recording surface with keeping a predetermined gap between the recording surface and themselves by the slider  109  flying over the recording surface of the disk. 
     In the actuator arm  101 , the slider  109  is supported in the edge of the suspension  102  as described above. Nevertheless, a pair of coil supports  103   a  and  103   b  for sandwiching a flat coil  105  configuring the VCM is formed in the coil support part  103  positioned in the opposite side of the slider  109  against the rotation shaft  104 . A lower stator magnet retention plate  106  fixed on the base retains a stator magnet  107  below the flat coil  105 . The stator magnet  107  has a north pole  107   a  and a south pole  107   b , and these are formed with making a boundary  107   c  a borderline. The VCM is configured by these flat coil  105  and stator magnet  107 , and the actuator  100  is configured by this VCM and the actuator arm  101 . 
     In the configuration described above, the flat coil  105  obtains a force in a rotational direction shown by an arrow H in each of the side edges  105   a  and  105   b . This is because the flat coil  105  is located so that an electromagnetic action may occur between the flat coil  105  and stator magnet  107 . Therefore, the actuator arm  101  obtains a rotary force in a clockwise direction if current in the direction shown by an arrow m passes through the flat coil  105 . On the contrary, if the current passes through the flat coil  105  in the direction shown by an arrow n, the actuator arm  101  obtains a rotary force in the counterclockwise direction. This is because the flat coil  105  obtains a force in the rotary direction shown by an arrow I in each of the side edges  105   a  and  105   b.    
     On the other hand, an outer edge  105   c  of the flat coil  105  is not supported by the coil support part  103  because of lightening and miniaturizing the coil support part  103 , and further making a torque small. Nevertheless, the outer edge  105   c  receives a force in a radial direction shown by an arrow F or G according to the direction of the current passing and its rotary position. 
     FIGS. 11 and 12 are operational diagrams for explaining a force that the outer edge  105   c  of the flat coil  105  receives, but the suspension  102  of the actuator arm  101  (FIG. 10) is omitted. FIG. 11 shows such a state that the actuator arm  101  is present at a position (hereinafter, this is called an OD position) where the actuator arm  101  rotates at most in the direction, shown by an arrow H, within its rotatable range. At this position, the outer edge  105   c  of the flat coil  105  is present above the north pole  107   a  of the stator magnet  107 . Therefore, if current in the direction shown by an arrow m passes through the flat coil  105 , the outer edge  105   c  receives a force in the direction shown by an arrow F that heads from the shaft center of the rotary shaft  104  to the outside. On the contrary, if current in the direction shown by an arrow n, the outer edge  105   c  receives a force in the direction shown by an arrow G that heads toward the shaft center of the rotary shaft  104 . 
     FIG. 12 shows such a state that the actuator arm  101  is present at a position (hereinafter, this is called an ID position) where the actuator arm  101  rotates at most in the direction, shown by an arrow I, within its rotatable range. At this position, the outer edge  105   c  of the flat coil  105  is present above the south pole  107   b  of the stator magnet  107 . Therefore, if current in the direction shown by an arrow m passes through the flat coil  105 , the outer edge  105   c  receives a force in the direction shown by an arrow G. On the contrary, if current in the direction shown by an arrow n, the outer edge  105   c  receives a force in the direction shown by an arrow F. 
     FIGS. 13 and 14 are drawings of analyzing the deformation of the flat coil  105  and coil supports  103   a  and  103   b , sandwiching the flat coil  105 , when the flat coil  105  resonates at a predetermined frequency by alternately receiving forces in the directions shown by No arrows F and G, by numerical simulation using a finite-element method (FEM). As shown in FIG. 13, when the outer edge  105   c  of the flat coil  105  protrudes in the direction shown by an arrow F and hence the flat coil  105  is extended, an angle between the coil supports  103   a  and  103   b  sandwiching this decreases. On the other hand, as shown in FIG. 14, when the outer edge  105   c  of the flat coil  105  dents in the direction shown by an arrow G and hence the flat coil  105  is shrunk, an angle between the coil supports  103   a  and  103   b  sandwiching this increases. 
     Such a vibration mode wherein a coil is extended and shrunk is called a coil-breathing mode. A piezoelectric element  108  (FIG. 10) detects an amount of extension or shrinkage of the coil support  103   b  where the piezoelectric element  108  is fixed. In addition, as FIG. 13, the piezoelectric element  108  detects extension when the flat coil  105  is extended and hence the angle between the coil supports  103   a  and  103   b  decreases. Furthermore, the piezoelectric element  108  outputs, for example, plus voltage at a level according to the extension amount. On the contrary, as shown in FIG. 14, the piezoelectric element  108  detects shrinkage when the flat coil  105  is shrunk and hence the angle between the coil supports  103   a  and  103   b  increases. Furthermore, the piezoelectric element  108  outputs, for example, minus voltage at a level according to the shrinkage amount. In addition, a fixed position of the piezoelectric element  108  (FIG. 10) is determined so that it is possible to detect warpage occurring when the actuator arm  101  receives acceleration in a rotary direction. 
     FIGS. 15 a  and  15   b  show frequency characteristics of a transfer function from the drive current of the flat coil  105  to the output voltage of the piezoelectric element  108  in the actuator  100  (FIG. 10) configured as described above. In the frequency characteristic charts, the horizontal axis shows frequencies from 2 kHz to 16 kHz that are linearly graduated. In addition, the vertical axis in FIG.  15 ( a ) shows gains expressed in decibels, and the vertical axis in FIG.  15 ( b ) shows phases. Furthermore, dotted lines show frequency characteristics of a transfer function A 2 od(s) at the time when the actuator arm  101  is near the OD position shown in FIG.  11 . Moreover, continuous lines show frequency characteristics of a transfer function A 2 id(s) at the time when the actuator arm  101  is near the ID position shown in FIG.  12 . 
     As being apparent from FIG. 15, although the actuator  100  resonates at nearly 4 kHz, this is butterfly resonance caused by the warpage of the actuator arm  101 . In addition, although the phase largely changes near this frequency, two phases at different rotary positions of the actuator arm, that is, the OD position and ID position become the same. 
     On the other hand, resonance at nearly 10 kHz is coil-breathing resonance caused by the coil breathing described above. In this resonance, the phases at different rotary positions, that is, the OD position and ID position become opposite. This is because directions of the forces that the outer edge  105   c  receives become opposite against the current passing through the outer edge  105   c  since polarities of the stator magnets that the outer edge  105   c  of the flat coil  105  faces at the OD position and ID position are different. 
     Technology of actively damping the above-described butterfly resonance is disclosed in Japanese Patent Application No. 11-80723 filed by the present applicant. According to this, in a hard disk drive, by not only performing tracking control to drive a VCM of an actuator so as to make heads positioned above a desired track, but also driving the VCM of the actuator in the direction where warpage is removed through detecting a warpage component of the actuator with the above-described piezoelectric element, the stability of the tracking control is improved. 
     Owing to this, a control signal for the tracking control and a control signal for damping the butterfly resonance are superimposed, and the current passing through a flat coil configuring the VCM on the basis of this signal superimposed is controlled. Nevertheless, if such damping control technology is applied to an actuator having frequency characteristics shown in FIGS. 15 a  and  15   b , various problems arise. Thus, in a frequency band of 10 kHz or higher, coil breathing has large effect, and hence phases near rotary positions of the actuator arm  101 , that is, the OD position and ID position are largely different. In particular, in nearly 10 kHz, and 14 kHz and higher, respective phases become opposite, and hence it is impossible to make stable control near both rotary positions compatible. 
     In addition, there is a method for removing a high frequency range, where the coil breathing has effect, by a filter in a control loop. Nevertheless, in the actuator  100  (FIG. 10) that has a wide rotation angle and is used in a hard disk drive, it is not possible to narrow the width of the outer edge  105   c  of the flat coil  105 . Therefore, the resonance frequency of the coil breathing becomes low, and hence is present near a butterfly resonance frequency. Therefore, it is difficult to remove only this part by a filter. 
     Furthermore, so as to enlarge the torque of an actuator, it is common to extend magnetic poles of a stator magnet to a moving area of the outer edge  105   c  of the flat coil  105  as shown in FIG.  10 . Nevertheless, even if the magnetic poles are configured, for example, for damping the coil-breathing phenomenon so that the magnetic poles may be not extended to this moving area, the flat coil  105  is affected by leakage magnetic flux from adjacent north and south magnetic poles. Therefore, it is difficult to damp the coil-breathing phenomenon at a level where the phenomenon has no effect on the control. 
     Thus, it is an object of the present invention to provide an actuator that can provide stabilized control of an actuator arm regardless of a rotary position of the actuator arm if the butterfly resonance and further coil breathing resonance are actively damped. 
     SUMMARY OF THE INVENTION 
     A stator magnet according to a first embodiment of the present invention forms a voice coil motor with a closed coil. The closed coil is supported by an actuator arm in a rotatable manner and at a predetermined rotation angle. The closed coil has first and second side edges that extend along different lines in radial directions whose center is a center of rotation of the actuator arm. It also has an outer edge that connects edges of outer sides of the first and second side edges with viewed from the radial direction, and extends along an arc whose center is the center of rotation. A first magnetic pole region is located within a moving area of the first side edge to act on the first side edge. A second magnetic pole region is located within a moving area of the second side edge to act on the second side edge. The polarity of the second magnetic pole region is opposite to the polarity of the first magnetic pole region. A third magnetic pole region is located within a moving area of the outer edge to act on the outer edge. The polarity of the third magnetic pole region is the same as the polarity of the first magnetic pole region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a hard disk drive showing an embodiment of the present invention; 
     FIG. 2 is a schematic of a VCM of an actuator showing an embodiment of the present invention; 
     FIG. 3 is a drawing for explaining the operation of the present invention; 
     FIG. 4 is another drawing for explaining the operation of the present invention; 
     FIGS.  5 ( a ) and  5 ( b ) are frequency characteristic charts of a transfer function of the actuator; 
     FIG. 6 is a schematic showing an example of a to control system controlling the actuator; 
     FIG. 7 is a schematic showing another example of a stator magnet of the present invention; 
     FIG. 8 is a schematic showing still another example of a stator magnet of the present invention; 
     FIG. 9 is a schematic showing a further example of a stator magnet of the present invention; 
     FIG. 10 is a schematic showing the configuration of an actuator used in a conventional hard disk drive; 
     FIG. 11 is a drawing for explaining a force that a prior art flat coil  105  receives; 
     FIG. 12 is a drawing for explaining another force that prior art flat coil  105  receives; 
     FIG. 13 is a drawing showing a transformed state of the prior art coil in a resonant state; 
     FIG. 14 is a drawing showing another transformed state of the prior art coil in a resonant state; and 
     FIGS.  15 ( a ) and  15 ( b ) are frequency characteristic charts of a transfer function of the conventional actuator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a top view of a hard disk drive  1  showing an embodiment of the present invention. A disk  2  is supported in one piece by a hub  5  of a spindle motor  4  located on a base  3 , and is rotationally driven by the spindle motor  4 . An actuator arm  8  is configured by a suspension  6  and a coil support part  7  that are formed in one piece, and is rotatably supported by a rotary shaft  9  set on the base  3 . 
     Coil supports  7   a  and  7   b  supporting a flat coil  10  in the side opposite to the suspension  6  against the rotary shaft  9  are formed respectively in this coil support part  7 . The flat coil  10  configures a VCM with a stator magnet (not shown) fixed on the upper stator magnet retention plate  11  fixed on the base  3  above the flat coil  10 , and a stator magnet  16  (FIG. 2) fixed on a lower stator magnet retention plate  17  described later. 
     The VCM rotates the actuator arm  8  in the direction shown by an arrow A or B, and this VCM and the actuator arm  8  construct an actuator  12 . In addition, in FIG. 1, the upper stator magnet retention plate  11  is shown with an upper main part being cut for convenience, and its contour is shown by a dotted line. Furthermore, as shown in FIG. 2, the lower stator magnet retention plate  17  for supporting a stator magnet  16  is provided on the base  3  also below the flat coil  10 . 
     A slider  13  is supported in an end portion of the suspension  6 , and respective heads for reading and writing of signals that are not shown are provided in predetermined positions of this slider  13 . When the actuator arm  8  rotates in the direction shown by an arrow A and moves above a recording surface of the disk  2  rotating, the slider  13  flies above the recording surface of the disk  2 , and the heads face the recording surface with keeping a predetermined gap between the heads and recording surface. A tab  15  in the end portion of the suspension  6  is placed on a ramp  14  located on the base  3  when the actuator arm  8  is unloaded and is at a home position. 
     A piezoelectric element  18  detects the extension or shrinkage of a fixed portion of the coil support  7   b , where this piezoelectric element  18  is fixed, as described later, and transforms the extension or shrinkage into an electric signal to output the electric signal. 
     Although, in the above-described explanation, it is supposed that the disk  2  is a single one-sided disk for simple description, another suspension supporting heads scanning each recording surface is provided for double-sided recording. Furthermore, the suspension is fixed in the coil support part  7  at a position where the suspension overlaps the suspension  6 , shown in FIG. 1, in a predetermined gap. Moreover, in case of recording a plurality of double-sided hard disks, the plurality of hard disks are supported in one piece by the hub  5  in a predetermined gap in a direction of a rotary shaft of the spindle motor  4 . In addition, the number of suspensions each supporting heads scanning each recording surface corresponds to the number of recording surfaces. The suspensions are fixed in the coil support part  7  at positions where the suspensions overlap the suspension  6 , shown in FIG. 1, in predetermined gaps. Nevertheless, since these treatments themselves are well known, detailed description will be omitted. 
     FIG. 2 is a schematic of the VCM of the actuator  12  (FIG. 1) showing an embodiment of the present invention. For simple description, the suspension  6  (FIG. 1) of the actuator arm  8  is omitted. In addition, only the stator magnet  16  supported by the lower stator magnet retention plate  17  provided on the base  3  (FIG. 1) is shown. Nevertheless, actually as described above, the stator magnet having the same polarity is also located on the upper stator magnet retention plate  11  (FIG. 1) at a position facing the stator magnet  16  through the flat coil  10 . 
     The flat coil  10  has a substantially trapezoidal shape as shown in FIG. 2, and is wound in a substantially flat shape so that a closed loop may be formed. Furthermore, the flat coil  10  comprises: side edges  10   a  and  10   b  that extend along different lines in radial directions whose center is a center of rotation of the actuator arm  8 ; an outer edge  10   c  that connects edges of outer sides of both side edges with viewing in the radial directions and extends substantially along an arc whose center is the center of rotation of the actuator arm  8 ; and an inner edge  10   d  that connects edges of inner sides of both side edges with viewing in the radial directions and extends substantially along an arc whose center is the center of rotation of the actuator arm  8 . 
     The lower stator magnet retention plate  17  supports the stator magnet  16  below the flat coil  10 . In this stator magnet  16 , a north pole  16   a  and a south pole  16   b  are formed in one piece with being separated by a boundary  16   c  as shown in FIG. 2 so that the side edges  10   a  and  10   b  of the flat coil  10  may almost cover moving areas respectively. 
     Nevertheless, although the south pole  16   b  is formed with extending to a moving area of the outer edge  10   c  of the flat coil  10 , the north pole  16   a  does not include the moving area of the outer edge  10   c . In this area, an additional slim south pole  16   d  is located adjacently to the north pole  16   a . In addition, although the upper stator magnet is similarly configured, a north pole is formed with facing the south pole in the lower side and a south pole is formed with facing the north pole in the lower side. 
     In the configuration described above, if current in the direction shown by an arrow m is made to pass through the flat coil  10 , the side edges  10   a  and  10   b  of the flat coil  10  each receive a force in the direction shown by an arrow E showing the rotary direction since the flat coil  10  is located so that an electromagnetic action may occur between the flat coil  10  and stator magnet  16 . Therefore, the actuator arm  8  obtains a rotary force in a clockwise direction. On the contrary, if current in the direction shown by an arrow n is made to pass through the flat coil  10 , the side edges  10   a  and  10   b  of the flat coil each receive a force in the direction shown by an arrow F showing the rotary direction. Therefore, the actuator arm  8  obtains a rotary force in a counterclockwise direction. The outer edge  10   c  also receives a force in the direction shown by an arrow C or showing each radial direction according to the current passing through the outer edge  10   c.    
     FIGS. 3 and 4 are operational drawings for explaining the forces that the outer edge  10   c  of the flat coil  10  receives. Nevertheless, similarly to FIG. 2, the suspension  6  of the actuator arm  8  is omitted, and only the stator magnet  16  that is supported by the lower stator magnet retention plate  17  provided on the base  3  is shown. 
     FIG. 3 shows such a state that the actuator arm  8  is present at the ID position where the actuator arm  8  rotates at most in the direction shown by an arrow F within its rotation angle. This position is a rotary position at the time when the slider  13  shown in FIG. 1 flies at a most inner position of the disk  2 . Since the outer edge  10   c  of the flat coil  10  is present above the south pole  16   b  of the stator magnet  16  at this position, the outer edge  10   c  receives a force in the direction shown by an arrow D if the current in the direction shown by an arrow m passes through flat coil  10 . On the contrary, if current in the direction shown by an arrow n passes, the flat coil  10  receives a force in the direction shown by an arrow C. 
     On the other hand, FIG. 4 shows such a state that the actuator arm  8  is present at the OD position where the actuator arm  8  rotates at most in the direction shown by an arrow E within its rotation angle. This position corresponds to the home position of the actuator arm  8  described above. Since the outer edge  10   c  of the flat coil  10  is present above the additional south pole  16   d  of the stator magnet  16  at this OD position, the outer edge  10   c  receives a force in the same direction as that at the time when the actuator arm  8  is present at the ID position, which is shown in FIG.  3 . If the outer edge  10   c  of the flat coil  10  alternately receives forces in the directions shown by arrows C and D owing to the action described above, the coil-breathing phenomenon that is described in FIGS. 13 and 14 showing the analysis by the simulation arises. 
     Thus, as shown in FIG. 13, when the outer edge  10   c  of the flat coil  10  protrudes in the direction shown by an arrow C and hence the flat coil  10  is extended, an angle between the coil supports  7   a  and  7   b  sandwiching the flat coil  10  decreases. On the contrary, when the outer edge  10   c  of the flat coil  10  dents in the direction shown by an arrow D and hence the flat coil  10  is shrunk, an angle between the coil supports  7   a  and  7   b  sandwiching the flat coil  10  increases. 
     The piezoelectric element  18  (FIG. 1) detects extension when the angle between the coil supports  7   a  and  7   b  decrease, and outputs, for example, plus voltage at a level according to the extension amount. On the contrary, the piezoelectric element  18  detects shrinkage when the angle between the coil supports  7   a  and  7   b  increases, and outputs, for example, minus voltage at a level according to the shrinkage amount. In addition, a fixed position of the piezoelectric element  18  is determined so that it is possible to detect warpage occurred when the actuator arm  8  receives acceleration in the rotary direction. 
     FIGS. 5 a  and  5   b  show frequency characteristics of a transfer function from the drive current of the flat coil  10  to the output voltage of the piezoelectric element  18  in the actuator  12  (FIG. 1) configured as described above. In the frequency characteristic charts, the horizontal axis shows frequencies from 2 kHz to 16 kHz that are linearly graduated. In addition, the vertical axis in FIG.  5 ( a ) shows gains expressed in decibels, and the vertical axis in FIG.  5 ( b ) shows phases. Furthermore, dotted lines show frequency characteristics of a transfer function A 1 od(s) at the time when the actuator arm  8  is near the OD position shown in FIG.  4 . Moreover, continuous lines show frequency characteristics of a transfer function A 1 id(s) at the time when the actuator arm  8  is near the ID position shown in FIG.  3 . 
     As being apparent from FIGS. 5 a  and  5   b , although the actuator  12  resonates at nearly 6 kHz, this is butterfly resonance caused by the warpage of the actuator arm  8 . In addition, although the phase largely changes near this frequency, two phases at different rotary positions of the actuator arm  8 , that is, the OD position and ID position become the same. 
     On the other hand, resonance at nearly 11 kHz is coil-breathing resonance caused by the coil breathing described above. In this resonance, the phases at different rotary positions of the actuator arm  8 , that is, the OD position and ID position become the same by the actuator  12  of the present invention. As described above, this is because the actuator  10  is configured so that the polarities of the stator magnets that the outer edge  10   c  of the flat coil  10  faces may be the same at the OD position and ID position (a south pole in the lower stator magnet  16 ). 
     As described above, according to the present invention, it is possible to always keep a phase characteristic of the actuator  12  so as to be in the same phase regardless of a rotary position of an actuator arm. Therefore, it is possible to perform stable damping control of the butterfly phenomenon and further breathing phenomenon. 
     FIG. 6 is a schematic showing an example of a control system controlling the actuator  12 , configured as described above, in a hard disk drive according to the present invention. The actuator  12  outputs a regenerative signal s 2  from the head  20 , and an extensional signal s 1  outputted according to the extension and shrinkage of the piezoelectric element  18  described above. The tracking controller  22  takes out tracking error information from the regenerative signal s 2 , and outputs to an adder  23  an actuating signal s 4  for performing the tracking control on the basis of this error information. 
     The damping controller  21  receives the extensional signal si from the piezoelectric element  18 , and outputs to the adder  23  an actuating signal s 5  for controlling the actuator  12  in the direction where the extension and shrinkage is damped. The adder  23  adds the actuating signal s 4  to the actuating signal s 5  to generate an added signal s 6 . A driver  24  outputs drive current s 3 , passing through the flat coil  10 , to the actuator  12  so as to drive the VCM of the actuator  12  on the basis of this added signal s 6 . Owing to the control system of the actuator  12  that is configured as described above, the actuator  12  operates so as to damp the butterfly phenomenon and breathing phenomenon of the actuator  12 , described above, as well as the usual tracking control. 
     FIG. 7 is a schematic showing another example of a stator magnet of the present invention. The stator magnet  26  in this example is used instead of the stator magnet  16  shown in FIG.  2 . In this stator magnet  26 , a north pole  26   a  is formed with extending to a moving area of the outer edge  10   c  of the flat coil  10  (FIG.  2 ). Nevertheless, a south pole  26   b  does not include the moving area of the outer edge  10   c , but an additional north pole  26   d  is located in this area adjacently to the south pole  26   b.    
     FIG. 8 is a schematic showing still another example of a stator magnet of the present invention. The stator magnet  27  in this example is used instead of the stator magnet  16  shown in FIG.  2 . This stator magnet  27  is formed in one piece, and is magnetized into a north pole  27   a  and a south pole  27   b  with being separated by a boundary  27   c , as shown in FIG.  8 . The additional slim south pole portion  27   d  of the south pole  27   b  is a portion corresponding to the moving area of the outer edge  10   c  of the flat coil  10  (FIG.  2 ). 
     FIG. 9 is a schematic showing a further example of a stator magnet of the present invention. The stator magnet  28  in this example is used instead of the stator magnet  16  shown in FIG. 2. A main pole portion of this stator magnet  28  is formed in one piece, and is magnetized into a north pole  27   a  and a south pole  27   b  with being separated by a boundary  28   c . The additional south pole portion  28   d  separated from the main pole portion is adjacent to the main pole portion, and is located in a part corresponding to a moving area of the outer edge  10   c  of the flat coil  10 . Although a piezoelectric element is used as a sensing element detecting the deformation of the actuator arm  8  in the above-described embodiment, another element acting similarly can be used instead of it. 
     According to the present invention, it is possible to keep a phase characteristic of a transfer function of an actuator, and in particular, a phase characteristic around a coil breathing resonance frequency so as to be in the same phase regardless of the rotary position of the actuator arm. Therefore, it is possible to always perform stable control regardless the rotary position of the actuator arm when a butterfly phenomenon and further coil-breathing phenomenon of the actuator are actively damped. 
     In addition, according to the present invention, by considering the configuration of a stator magnet, an object is attained, and hence it is possible to avoid cost increase in connection with implementation of the present invention, and complexity of the configuration.