Abstract:
An actuator for a magnetic head of a disk device has a rotating arm constructed of resin. The arm is supported rotatably in a housing on an electrically conducting shaft, and a magnetic head supported by a supporting member at one end of the arm is moved to various positions above the magnetic disk when the arm is rotated by a rotating mechanism. An electrically conducting mechanism conducts static electricity from the supporting member of the magnetic head to the shaft so that static electricity is prevented from accumulating on the head supporting member of the resin arm or in its vicinity.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a swing type actuator which drives a magnetic head in a magnetic disk device or the like. 
     Swing type actuators used in magnetic disk devices, for example, comprise an arm which pivots about a base, a magnetic head fixed to a tip of the arm, a coil attached to a base of the arm, and magnets facing both sides of the coil. 
     When the coil is energized, a magnetic force acts between the coil and magnets according to Fleming&#39;s left-hand rule, and this force displaces the magnetic head to a predetermined position on a magnetic disk. 
     The arm is, for example, made of diecast aluminum alloy. However, diecast arms are heavy, and it is difficult to make the coil-magnet drive combination more compact. 
     To resolve this problem, Tokkai Hei 4-229062, published by the Japanese Patent Office in 1992, proposes a one-piece construction wherein the arm and a coil holder provided on the arm are made of a thermoplastic resin. 
     However, in such a resin arm, static electricity is generated between the arm and the magnetic disk rotating at high speed. This static electricity tends to charge the arm&#39;s magnetic head support and surrounding parts, and therefore led to read/write errors of the magnetic head. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to prevent charging of a resin arm by static electricity in an actuator for driving a magnetic head. 
     It is a further object of the present invention to make an actuator for driving a magnetic head more compact. 
     In order to achieve the above objects, the invention provides an actuator which rotates a magnetic head of a disk device, comprising an electrically conducting shaft which supports an arm free to rotate about an axis in a housing, a supporting member for a magnetic head provided at a tip of the arm, a mechanism for rotating the arm, and a mechanism for electrically conducting static electricity from the supporting member to the conducting shaft. 
     It is preferable that the conducting mechanism comprises a metal sleeve provided in the arm so as to form a bearing at the axis, and an electrically conducting plate which electrically connects the supporting member with the sleeve. 
     It is also preferable that the conducting mechanism comprises a metal sleeve provided in the arm so as to form a bearing at the axis, and a metal film provided on the surface of the arm so as to connect the supporting member with the sleeve. 
     It is also preferable that the arm comprises a flexible printed circuit for relaying a detection signal from the magnetic head, and a screw mechanism for electrically connecting the flexible printed circuit with the conducting mechanism. In such an embodiment, it is further preferable that the screw mechanism has a further function of fixing the flexible printed circuit to the arm. 
     The details as well as other features and advantages of the invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a plan view of an arm according to the present invention. 
     FIG. 1B is a vertical sectional view of the arm. 
     FIG. 2 is a lateral view of a fixed part of a flexible printed plate according to the present invention. 
     FIG. 3 is a perspective view of component parts of the arm. 
     FIG. 4A is a plan view of an upper resin plate of the arm. 
     FIG. 4B is a vertical sectional view of the upper resin plate. 
     FIG. 5A is a plan view of a middle resin plate of the arm. 
     FIG. 5B is a sectional view of the middle resin plate. 
     FIG. 6A is a plan view of a lower resin plate of the arm. 
     FIG. 6B is a sectional view of the lower resin plate. 
     FIG. 7A is a plan view of an arm according to another embodiment of the invention. 
     FIG. 7B is a vertical sectional view of the arm of FIG. 7A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1A and 1B of the drawings, a resin arm 20 of an actuator which drives a magnetic head of a disk device comprises three superposed resin plates 20a-20c. Each of the plates 20a-20c includes an electrically conducting plate 21 inside it and is formed by injecting a thermoplastic resin into a mold so as to surround the plate 21. One end of each of the plates 21 projects from the resin plates 20a-20c, and fixing holes 22 for fixing a magnetic head are formed in the projecting ends. An axial hole 23 is formed at a predetermined position in the resin arm 20. The other ends of the plates 21 project some distance inside the axial hole 23. 
     When the plates 20a-20c are superposed, a sleeve 24, having a radius slightly less than the axial hole 23 and constructed of a metal such as stainless steel or the like, is disposed inside the hole 23 such that it comes in contact with the ends of the plates 21. Hence, by injecting thermoplastic resin between the sleeve 24 and the resin arm 20, the sleeve 24 and resin arm 20 are formed as a one-piece construction. The sleeve 24 comprises a bearing 25 for a conducting shaft, defining an axis, which is fixed to a housing of the disk device. 
     The resin arm 20 is supported free to pivot in the housing about the conducting shaft. The sleeve 24 protects the wall of the axial hole 23 in the thermoplastic resin, thereby assuring durability and reliability of the resin arm 20. The other ends of the plates 21 at the side of the hole 23 are wedge-shaped so that they can dig into the outer circumference of the sleeve 24. In this way, the plates 21 and sleeve 24 are firmly connected. 
     As shown in FIG. 3, in the three layer construction comprising the upper plate 20a, middle plate 20b and lower plate 20c of the resin arm 20, a positioning pin 26 and engaging holes 27 are formed in joint surfaces. The magnetic head, not shown, is attached to the fixing holes 22 in the plates 21 which project from the plates 20a-20c. A detection signal of the magnetic head is input to a control circuit of the disk device via a flexible printed circuit 42 shown in FIG. 1. The flexible printed circuit 42 is fixed by a plurality of screws, not shown, to a side of the resin arm 20. A projection 28, oriented downwards, is formed in the upper plate 20a of the resin arm 20 as shown in FIGS. 2 and 4B. A screw hole 43a is formed in the projection 28 extending from a surface of the printed circuit 42 to the plate 21 as shown in FIG. 4A. A projecting piece 44, that comes in contact with a screw inside this screw hole 43a, is first formed on the plate 21. The screw has the dual function of electrically connecting the printed circuit 42 and plate 21, and of fixing the printed circuit 42 to the resin arm 20. A recess 29, that accepts the projection 28, is formed in the middle resin plate 20b as shown in FIGS. 2, 5A and 5B. A pair of fixing members 31 for a moving coil 30 is also formed on the middle resin plate 20b. As in the case of the sleeve 24, the moving coil 30 is fixed by thermoplastic resin when positioned by the fixing members 31 in a mold. The parts shown by ANSI 31 type hatching in FIGS. 1A indicate thermoplastic resin formed after the resin plates 20a-20c are formed. 
     Desirably, the moving coil 30 comprises a coil 30b wound on a coil bobbin 30a since, by using the coil bobbin 30a, deformations arising in the coil 30b when the thermoplastic resin is injected are prevented. However a coil without a bobbin may also be used. As the coil bobbin 30a is lightweight, it may be formed of the same thermoplastic resin as the resin plates 20a-20c. 
     The upper plate 20a and middle plate 20b are superposed on the lower plate 20c shown in FIGS. 6A and 6B such that the pin 26 engages with the engaging holes 27 as shown in FIG. 3. The assembly is thereby formed as the resin arm 20 of one-piece construction by the thermoplastic resin which fixes the sleeve 24. 
     Permanent magnets are disposed in the housing of the disk device such that they enclose the moving coil 30. As a result, when the moving coil 30 is energized, a magnetic force acts on the coil 30 according to Fleming&#39;s left-hand rule, the resin arm 20 rotates around the axis of the bearing 25, and the magnetic head is moved to a predetermined position on the magnetic disk. 
     The magnetic head reads data from the magnetic disk, and its detection signal is input to a control circuit of the magnetic disk device via the flexible printed circuit 42. The printed circuit 42 is connected to the plates 21 via grounding screws, so that the plates 21 are connected to ground via the electrically conducting shaft in contact with the inner circumference of the sleeve 24. However, the grounding screws may be in direct contact with the sleeve 24 without passing through the plates 21. 
     Due to the aforesaid construction, even if static electricity is generated between the resin arm 20 and the disk rotating at high speed, the charge is leaked to the housing through the plates 21, sleeve 24 and electrically conducting shaft. This therefore prevents the arm 20 from becoming charged with static electricity. Data read/write errors of the magnetic head due to static electricity are thereby avoided, and the actuator is made more lightweight and compact by the use of thermoplastic resin without losing any reliability of data processing. 
     As the plates 21 increase the rigidity of the resin plates 20a-20c, the resin plates 20a-20c may be made thinner than in the case where they are constructed of thermoplastic resin alone. Further, grounding of the printed circuit 42 is simply achieved by the screw in the screw hole 43a. 
     FIGS. 7A-7B show an arm 20&#39; of another embodiment of the present invention. 
     According to this embodiment, a metal film 40 is formed on surfaces of plates 20a&#39;-20c&#39; by non-electrolytic plating or vapor deposition in place of the plates 21 of the first embodiment. Static electricity is then prevented from accumulating on the plates 20a&#39;-20c&#39; by the contact of the film 40 with a sleeve 24a. 
     Flanges are formed at both ends of the sleeve 24a, and a hole 41 is provided inside of an axial hole 23&#39; to assemble the sleeve 24a in the arm 20&#39;. The hole 41 is formed with a slightly greater diameter than that of the flanges. The sleeve 24a is disposed at a predetermined position from the hole 41 inside the axial hole 23&#39;, and is formed into a one-piece construction with the arm 20&#39; when thermoplastic resin is injected in the hole 41. 
     The fixing holes 22 of the magnetic head are formed by, for example, drilling the ends of the plates 20a&#39;-20c&#39; after finishing the metal film 40. The printed circuit 42 is attached to the side of the arm 20&#39; by a screw, and is grounded via the metal film 40, sleeve 24a and the electrically conducting shaft in contact with the sleeve 24a.