Abstract:
In a magnetic, disk apparatus, flows around disks are stabilized, flow-induced vibration generated in the disks and a head positioning actuator is reduced, and the positioning accuracy of the head is improved. 
     In a magnetic disk apparatus including plural magnetic disks which are attached to a rotating motor and stacked with a spacer in between and a static structure that surrounds outer circumferences of the magnetic disks, plural current plates supported by the static structure are inserted between a pair of the magnetic disks in a stacking direction of the magnetic disks.

Description:
TECHNICAL FIELD 
     The present invention relates to a magnetic disk apparatus and more particularly to a magnetic disk apparatus in which a flow-induced vibration generated in a disk is reduced. 
     BACKGROUND ART 
     The magnetic disk apparatus has a magnetic disk that is an information recording medium and a magnetic head to read and write magnetic information from and to the magnetic disk and includes a VCM actuator to support the magnetic head and move the magnetic head to a predetermined radial position over the magnetic disk. The magnetic information is written along a track circularly arranged on the magnetic disk. To correctly read and write the magnetic information, the magnetic head is required to be accurately positioned with respect to the track, so that, as the recording capacity and the recording density of the magnetic disk apparatus increase, a mechanism and a control method to achieve a higher positioning accuracy are required. One of the factors that degrade the positioning accuracy is the flow-induced vibration of the magnetic disk, which is called a disk flutter. 
     When the magnetic disk rotates, a complex air flow occurs in the magnetic disk apparatus. In particular, a high-speed and turbulent flow occurs around the magnetic disk, so that the disk flutter that is a vibration of the magnetic disk occurs due to pressure variation on the surface of the magnetic disk generated by the high-speed and turbulent flow. The disk flutter varies the position of the track to be followed by the magnetic head and degrades the positioning accuracy, so that reduction of the disk flutter is an important challenge to increase the recording density of the magnetic disk apparatus. 
     As one of the methods to reduce the disk flutter, a method for narrowing a gap between the magnetic disk and a shroud that covers the outer circumference of the magnetic disk, which is described in Patent Literature 1, and a method for inserting a partition member with a triangle cross section between disks, which is described in Patent Literature 2, are proposed. These methods suppress the turbulence of the air flow and reduce the disk flutter by stabilizing the flow around the magnetic disk by the shroud and the partition member. Further, as described in Patent Literature 3, a method for reducing the disk flutter by blowing air to the inner circumference of the disk is also known. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 11(1999)-232866 
     Patent Literature 2: Japanese Examined Patent Application Publication No. 4(1992)-47918 
     Patent Literature 3: Japanese Unexamined Patent Application Publication No. 11(1999)-297037 
     SUMMARY OF INVENTION 
     Technical Problem 
     The disk flutter is caused by turbulence of flow around the disk, and there is an essential instability described below in the flow between the disks because the disks rotate. The air between the disks makes a flow rotating around a central motor in accordance with the rotation of the disks. The air between the disks receives a centrifugal force due to the rotation and receives a force to the outer circumferences of the disks. In the magnetic disk apparatus, the outer circumferences of the disks are covered by the shroud, so that a pressure distribution as shown by a solid line in  FIG. 7 , in which the pressure is high near the shroud and the pressure is low on the side of the inner circumferences of the disks, is formed. As a result, as shown in  FIG. 3 , flows  24  directed to outside are generated near the disks by the centrifugal force and flows  25  directed from the outer circumferences to the inner circumferences are generated near the center between the disks by a pressure difference between the inner circumferences and the outer circumferences of the disks. At this time, the flow  24  directed to the outer circumference and the flow  25  to the inner circumference are adjacent to each other in a small gap between the disks, so that strong shearing resistance occurs between them and the flows become unstable and very turbulent. The same goes for the structure described in Patent Literature 2, in which a partition plate is inserted between the disks. The turbulence of the flows results in causing a vibration of the disks or an actuator on which the head is mounted and becomes a cause of degrading the positioning accuracy of the head. 
     A method of Patent Literature 3 is to reduce the pressure difference between the inner circumferences and the outer circumferences of the disks and eliminate the instability of the flows by directly introducing air from the motor side to the inner circumferences of the disks. However, in order to secure a sufficient amount of air flow, a motor hub or a spacer that supports the disks has to be processed largely, so that there is a problem that the strength and accuracy are degraded and a static deformation of the disks generated when the disks are assembled increases. 
     An object of the present invention is to stabilize the flows around disks in the magnetic disk apparatus, reduce the flow-induced vibration generated in the disks and a head positioning actuator, and improve the head positioning accuracy. 
     Solution to Problem 
     To address the above problem, in the present invention, plural plates are arranged between the disks with a constant gap between each of the plates. 
     A typical example of the magnetic disk apparatus of the present invention is a magnetic disk apparatus including plural magnetic disks which are attached to a rotating motor and stacked with a spacer in between and a static structure that surrounds outer circumferences of the magnetic disks. In the magnetic disk apparatus, plural current plates supported by the static structure are inserted between a pair of the magnetic disks in a stacking direction of the magnetic disks, 
     Advantageous Effects of Invention 
     According to the present invention, the instability of the flows around the disks are reduced by reducing the pressure difference between the inner circumference and the outer circumference of the disks, which is generated when the disks are rotated, and separating the flow near the disk, which is directed to the outside, and the flow at the center between the disks, which is directed to the inside, so that it is possible to reduce the flow-induced vibration of components inside the magnetic disk apparatus, such as the disks and the actuator, and improve the positioning accuracy of the head. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view showing a magnetic disk apparatus of a first embodiment of the present invention. 
         FIG. 2  is a perspective view showing the magnetic disk apparatus of the first embodiment of the present invention. 
         FIG. 3  is a schematic diagram showing flows between disks in a conventional technique. 
         FIG. 4  is a schematic diagram showing flows between disks in the first embodiment of the present invention. 
         FIG. 5  is a cross-sectional view showing a simulation result of flows between disks under a condition in which the conventional technique is simulated. 
         FIG. 6  is a cross-sectional view showing a simulation result of flows between disks under a condition in which the first embodiment of the present invention is simulated. 
         FIG. 7  is a graph showing pressure distribution obtained by simulations. 
         FIG. 8  is a graph showing a frequency spectrum of disk vibration obtained by the simulations. 
         FIG. 9  is a graph showing disk amplitude obtained by the simulations. 
         FIG. 10  is a cross-sectional view showing a magnetic disk apparatus of a second embodiment of the present invention. 
         FIG. 11  is a cross-sectional view showing a magnetic disk apparatus of a third embodiment of the present invention. 
         FIG. 12  is a plan view showing a plate of the third embodiment of the present invention. 
         FIG. 13  is an exploded perspective view showing a manufacturing method of the plates of the third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a cross-sectional view showing a first embodiment of the present invention. In a magnetic disk apparatus  1 , disks  2 , on both sides of which information can be recorded, and a spindle motor  3 , which drives and rotates the disks  2 , are mounted. The disks  2  are attached to the spindle motor  3  with a spacer  4  in between. Two plates  5  are inserted between the disks  2 . The spindle motor  3  and the plates  5  are attached to a base  6 . A cover  7  is attached to a base  6  and components and air in the apparatus are sealed against the outside. A portion of the base  6 , which faces the outer circumferences of the disks  2 , is called a shroud  8 , and a flow around the disks is arranged to be directed to the circumferential direction by the shroud  8 . An appropriate gap  9  is provided between the plates  5  and the shroud  8  so that air flows without resistance. The shroud  8  works as a static structure. A distance between a disk  2  and a plate  5  and a distance between the plates  5  need to be as large as not to disturb the flow of the air, and generally, it is assumed that these distances are desired to be substantially the same. 
       FIG. 2  is a perspective view showing the first embodiment of the present invention.  FIG. 2  shows a state in which the cover  7  and the uppermost disk  2  are removed. An actuator  10  on which a magnetic head is mounted is placed in a portion over the disk  2 . The plate  5  therefore has a fan shape formed by cutting off a part of a circular plate so that the plate  5  does not interfere with the actuator  10 . The plate  5  is fixed to the base  6  by screws or the like at plural fixing portions  11  provided at the outer circumference. An appropriate gap is provided between the outer edge of the plate and the shroud  8  except for the fixing portions  11 . 
       FIG. 3  is a schematic diagram showing flows between disks in a conventional technique. When the disks  21  are rotated, the air near disks  21  receives a force directed to the outside by a centrifugal force. Thereby, the pressure near a shroud  22  increases and the pressure near a spacer  23  decreases relatively. By this pressure difference, the air away from the disks  21  receives a force directed to the inside. Therefore, a flow  24  directed to the outer circumference occurs near a disk  21  and a flow  25  directed to the inside occurs at a portion away from the disk  21 . 
       FIG. 4  is a schematic diagram showing flows between disks in the first embodiment of the present invention. The air between the disks  2  are divided into three regions by the two plates  5 . In a region  31  in contact with the disk  2 , when the disks  2  are rotated, the air receives a centrifugal force directed to the outside. In a region  32  sandwiched by the plates  5 , the air is difficult to be influenced by the rotation of the disks  2 , so that the air in the region  32  receives a force directed to the inside due to the pressure difference between the outer circumference and the inner circumference. Therefore, a flow  33  directed to the outside occurs in the region  31  and a flow  34  directed to the inside occurs in the region  2 . As compared with the conventional technique in  FIG. 3 , in the present embodiment, the flow  33  directed to the outside and the flow  34  directed to the inside are separated from each other, so that the turbulence of the flow decreases. At the same time, the flow  34  directed to the inside alleviates the pressure difference between a portion near the shroud  8  and a portion near the spacer  4 , so that the pressure difference between them decreases. As a result, the instability generated by the centrifugal force and the pressure difference between the inner and the outer circumferences of the disks is alleviated. The flows between the disks are stabilized, so that it is possible to reduce the flow-induced vibration generated in the disks  2  and the actuator  10 . The plates  5  work as current plates. 
       FIG. 5  is a simulation result of flows between two rotating disks  42  surrounded by a shroud  41  of the conventional technique. The direction and the magnitude of the flow of the air between the disks  42  in a cross-section passing through the rotation center of the disks  42  are shown by arrows. In the simulation, the disks  42  and a rotor  43  are rotating and the shroud  41  is stationary. The simulation result shows that a flow directed to the outer circumference occurs near the disk and a flow directed to the inner circumference occurs near the center between the disks as shown in  FIG. 3 , and it is known that these flows opposite to each other are adjacent to each other. 
       FIG. 6  is a simulation result of flows in a case in which two stationary plates  53  are inserted between two rotating disks  52  surrounded by a shroud  51  of the first embodiment. In the same manner as in  FIG. 5 , the direction and the magnitude of the flow of the air between the disks  52  in a cross-section passing through the rotation center of the disks  52  are shown by arrows. The simulation result shows that a flow directed to the outer circumference occurs between the disk  52  and the plate  53  and a flow directed to the inner circumference occurs between the plates  53  as shown in  FIG. 4 , and these flows are separated from each other by the plate  53 . 
       FIG. 7  is a graph showing pressure distributions between the disks obtained by the simulations shown in  FIGS. 5 and 6 . The horizontal axis represents a radial position of a pressure measurement point. The left end represents the outside diameter of the spacer, the right end represents a wall surface of the shroud, and the outside diameter of the disk is located slightly left (inner) from the shroud. The vertical axis represents a time average value of the pressure at the height of the center between the disks. The solid line represents a result of a case  61  in which there is no plate and the dashed line represents a result of a case  62  in which two plates are inserted. Both simulation results are displayed by using the same origin so that the pressure near the shroud wall is 0 Pa. It is known that the pressure difference between a portion near the spacer and a portion near the shroud, which is the cause of the instability of the flows, significantly decreases when the two plates are inserted. 
       FIG. 8  is calculation values of a frequency spectrum of disk vibration obtained by the simulations. The gray line represents a spectrum  63  of the case in which there is no plate and the black line represents a spectrum  64  of the case in which two plates are inserted.  FIG. 9  shows amplitude values of the disks obtained by integrating the frequency spectra. It is known that the vibration in the case in which two plates are used is reduced compared with the case in which there is no plate. 
     Second Embodiment 
       FIG. 10  is a cross-sectional view showing a second embodiment of the present invention. Three plates  72  are inserted between two disks  71 . Although the number of the plates between the disks is two in the first embodiment, when the number of the plates is three or more, the function and effect to stabilize the flows and reduce the flow-induced vibration are the same. However, in an actual magnetic disk apparatus, the dimension in the height direction is limited. If the number of the plates is increased, the necessary height dimension increases according to the number of the plates. Therefore, a practical number of the plates is desired to be two that is the minimum number by which the effect of the present invention can be obtained. 
     Third Embodiment 
       FIG. 11  is a cross-sectional view showing a third embodiment of the present invention. Two plates  82  are inserted between two disks  81  and the plates  82  are connected by ribs  83 . The rib  83  has a rod shape long in the radial direction and does not block a flow  85  directed to the inner circumference in a region  84  between the plates  82 . The rigidity of the structure in which the plates  82  are connected by the ribs  83  is significantly increased compared with a case in which there is no rib  83 , so that deformation of the plates  82  due to impact acceleration or the like applied from the outside of the magnetic disk apparatus can be suppressed to be small. Although the rib  83  does not block the flow in the radial direction in the region  84  between the plates, the rib  83  blocks a flow in the circumferential direction. The flow velocity in the circumferential direction in the region  84  decreases, so that the centrifugal force applied to the air in the region  84  decreases. Therefore, the pressure difference between a portion near the shroud and a portion near the spacer further decreases compared with the case of the first embodiment in which there is no rib  83 . The ribs  83  may be provided by being inclined from the radial direction. The shape of the rib  83  is not limited to a long rod shape, but any ribs  83  which connect the two plates with a distance in between may be used, such as cylindrical ribs are separately provided between the plates. 
       FIG. 12  is a plan view showing the plate of the third embodiment of the present invention. The angle at which the ribs  83  are arranged is desired to be set so that the ribs  83  do not disturb the flow between the plates  82  in the radial direction and contribute to increase the out-of-plane bending rigidity of the plates  82 . Specifically, the angle θ between the ribs is desired to be in a range between 10 degrees and 90 degrees. 
       FIG. 13  is an exploded perspective view showing a manufacturing method of the plates shown in the third embodiment of the present invention. A plate  92  on which ribs  91  are integrally formed and another plate  93  are overlapped and the upper surfaces of the ribs  91  and the lower surface of the plate  93  that faces the upper surfaces of the ribs  91  are bonded together while positioning is performed by assembling holes  94 . The material of the plate  91  and the plate  93  is desired to be a metallic material, such as an aluminum alloy, with plating on the surface of the metallic material, or a resin material such as engineering plastics. 
     LIST OF REFERENCE SIGNS 
     
         
           1  Magnetic disk apparatus 
           2  Disk 
           3  Spindle motor 
           4  Spacer 
           5  Plate 
           6  Base 
           7  Cover 
           8  Shroud 
           9  Gap 
           10  Actuator 
           11  Fixing portion 
           21  Disk 
           22  Shroud 
           23  Spacer 
           24  Flow directed to the outside 
           25  Flow directed to the inside 
           31  Region in contact with the disk  2   
           32  Region sandwiched by the plates  5   
           33  Flow directed to the outside 
           34  Flow directed to the inside 
           41  Shroud 
           42  Disk 
           43  Rotor 
           51  Shroud 
           52  Disk 
           53  Plate 
           61  Case in which there is no plate 
           62  Case in which two plates are inserted 
           63  Spectrum of the case in which there is no plate 
           64  Spectrum of the case in which two plates are inserted 
           71  Disk 
           72  Plate 
           81  Disk 
           82  Plate 
           83  Rib 
           84  Region between the plates  82   
           85  Flow directed to the inner circumference 
           91  Rib 
           92  Plate 
           93  Plate 
           94  Assembling hole