Abstract:
A data storage device is made by molding a portion of a motor into a first enclosure member, and then coupling the first enclosure member to a second enclosure member to define an interior environment for other data storage device components.

Description:
RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. 10/326,790, filed Dec. 20, 2002, now abandoned, and claims priority from U.S. provisional application Ser. No. 60/383,035, filed May 23, 2002. 
    
    
     FIELD OF THE INVENTION 
     This application relates generally to magnetic disc drives, and more particularly to a disc drive having a molded top cover where a top pole of a voice coil motor assembly is over-molded within the molded top cover of the disc drive. 
     BACKGROUND OF THE INVENTION 
     Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium, such as a disc. Modem disc drives include a head disc assembly comprising one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a drive motor for rotation at a constant high speed. Disc drive components within the head disc assembly, such as the hub of the drive motor, an actuator assembly, and a voice coil motor, are mounted to a base plate. A top cover attaches to the base plate to internally seal the head disc assembly. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers (“heads”) mounted to a radial actuator arm for movement of the heads relative to the discs. The read/write transducer, e.g. a magneto resistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. 
     The voice coil motor assembly is part of the actuator assembly and operates to rotate the actuator arms and the attached read/write heads in an arcuate path over the respective disc surfaces. The voice coil motor assembly includes a coil and a magnetic circuit comprising one or more permanent magnet sets and magnetically permeable pole pieces. The coil is mounted on a rear portion of the actuator body opposite the actuator arms so as to be immersed in the magnetic field of the magnetic circuit. When controlled direct current (DC) is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about a pivot shaft so that the actuator arms and the attached heads move in an arc across the disc surfaces. 
     Typically, a magnetically permeable bottom pole is mounted to the base plate and a magnetically permeable top pole is either mounted to the inner surface of the top cover or is mounted to the bottom pole (via spacers or “standoffs”) in spaced relation to both the bottom pole and the top cover. At least one permanent magnet set is positioned between and attached to one of the two poles. A gap between the magnet set and the opposite pole provides space for the coil to move in response to the application of varying DC signals to the coil. 
       FIG. 1  shows a head disc assembly of a conventional disc drive  100  that uses the spacer method to position the top pole. The disc drive  100  includes a base plate  102  to which various components of the disc drive  100  are mounted. A top cover  104 , shown partially cut away, cooperates with the base  102  to form an internal, sealed environment for the disc drive  100  in a conventional manner. The components include a drive motor  106  that rotates one or more discs  108  at a constant high speed. Information is written to and read from tracks on the discs  108  through the use of an actuator assembly  110 , which rotates during a seek operation about a bearing shaft assembly  112  positioned adjacent the discs  108 . The actuator assembly  110  includes a plurality of actuator arms  114  which extend towards the discs  108 , with one or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a head  118  which includes an air bearing slider enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 . 
     During a seek operation, the track position of the heads  118  is controlled through the use of a voice coil motor (VCM) assembly  120 , which typically includes a coil  122  attached to the actuator arm  114  on the opposite side of the bearing shaft assembly  112 , a top pole  124 , a magnet  126 , and a bottom pole  128 . The magnet  126  either defines a pair of magnets with opposite polarity lying in a common plane, or a single part with a transition zone between two faces of opposite polarity, so that the magnet establishes a magnetic field in which the coil  122  is immersed. The top pole  124  is attached in spaced relation to the bottom pole  128  with magnetically permeable standoffs or side posts  130 . The controlled application of current to the coil  122  causes magnetic interaction between the permanent magnet sets  126  and the coil  122  so that the coil  122  moves in accordance with the well known Lorentz relationship. The top pole  124  and the bottom pole  128  provide a return path for the magnetic field passing through the coil  122 . Furthermore, the standoffs or side posts  130  typically act together with the top and bottom poles  124  and  128  to form a closed magnetic field loop for the magnetic flux lines emanating from the magnet set  126 . As the coil  122  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112 , and the heads  118  are caused to move across the surfaces of the discs  108 . 
       FIG. 2  shows a generalized sectional view of the conventional voice coil motor  120  shown in  FIG. 1 . The bottom pole  128  is mounted to the base plate  102  by any conventional method, such as screws or an adhesive. The top pole  124  is mounted to the bottom pole  128 , and thus to the base plate  102  via standoffs  130  such that the top pole  124  is spaced apart from the bottom pole  128 . The top pole  124  and the top cover  104  typically form an air gap  123  therebetween. The permanent magnet set  126  is attached to the top pole  124  opposite the top cover  104 . The coil  122  is attached to the actuator assembly (not shown) and positioned within an air gap  125  between the magnet set  126  and the bottom pole  128 . 
     One of the problems with this conventional design is that the overall height of the disc drive  100  is fixed due to form factor limitations, and thus a portion of this limited space or “height” is wasted due to the inclusion of the air gap  123 . A second problem with this conventional design is that it requires extra parts, such as standoffs  130 , to both mount and properly position the top pole  124  within the head disc assembly. While the standoffs  130  help to maximize the magnetic field in the vicinity of the coil  122  by creating a closed magnetic path, it has been determined that the standoffs  130  tends to increase inductance within the windings of the coil  122 . Increased inductance results in an increased resistance to a change in the current within the windings of the coil  122  and thus slower “seek” times for the voice coil motor  120 . Specifically, the “seek” time of a disc drive  100  is the amount of time necessary to pivot the actuator assembly  110  so that the heads  118  move between different tracks on the discs  108 . Because a quick change in current through the coil  122  is necessary for fast seek times, it is axiomatic that an increase in inductance within the coil results in slower seek times. In addition to adding to the complexity and cost of manufacturing a disc drive, a further problem with the standoffs  130  is that engineering tolerances in the manufacture of the standoffs can lead to variations in the magnetic fields produced by different voice coil motors  120   
     As noted above, one solution to the problem of the air gap  123  and the manufacturing complexity and expense of the standoffs  130  is to mount the top pole  124  directly to an inner surface of the top cover  104 , such as by an adhesive or by welding the top pole  124  to the metallic cover  104 . However, adhesives are expensive and may cause outgassing that can corrupt normal disc drive operation, while welding creates the potential for gaps to form between the two parts that may allow contaminants to be trapped and possibly escape to the interior of the head disc assembly. In any event, the process of attaching the top pole  124  to the top cover  104  requires extra manufacturing steps that increases the build time and thus the cost of a disc drive  100 . A further difficulty with attaching the top pole  124  directly to the cover  104  is that modem top covers  104  are frequently formed with numerous recessed regions, cutouts and other features required to accommodate the different components within the drive  100 . As a result, it is difficult to provide a smooth mounting surface on the inner surface of the top cover  104  for attaching to the top pole  124 . Indeed, while a metal top cover  104  provides needed support for certain disc drive components such as the bearing shaft assembly  112 , it is expensive to mold or stamp aluminum top covers  104  with the intricate shapes required by current disc drive designs. 
     Accordingly there is a need for a voice coil motor assembly that minimizes wasted space within the disc drive while reducing manufacturing costs by eliminating unnecessary parts and simplifying manufacturing steps. The present invention provides a solution to these and other problems, and offers other advantages over the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a disc drive having a molded top cover that includes an embedded or “over-molded” top pole piece that cooperates with other components of a voice coil motor assembly attached to a base plate of the disc drive. The over-molded top pole piece minimizes wasted space within the interior of the drive and reduces the complexity and time required to assemble the voice coil motor assembly. 
     In accordance with one embodiment of the present invention, a disc drive has a base plate supporting a data storage disc and an actuator arm mounted adjacent the disc for positioning a transducer over the disc. A top cover attached to the base plate defines an internal environment of the disc drive, where the top cover is formed from a moldable material by an injection molding process and includes a metal top pole piece over-molded within the top cover. A voice coil motor assembly includes a voice coil attached to the actuator arm, a bottom pole mounted to the base plate, and a magnet attached to the bottom pole, wherein the coil is positioned adjacent to the permanent magnet between the bottom pole and the top pole piece over-molded within the top cover. In one embodiment, the moldable material is a polymer reinforced with carbon or steel fibers. 
     One embodiment of the metal top pole piece includes a pole body and a mounting surface extending from the pole body, wherein the mounting surface is secured to an actuator arm bearing shaft to provide a metallic anchor between the bearing shaft and the molded top cover. The metal top pole piece may also include an outer flange extending from the pole body, wherein the outer flange includes a hole aligned with an opening in the top cover to provide a reinforced contact point between the base plate and the molded top cover. 
     Connected in this manner, the over-molded metal top pole piece is fully supported by the top cover and is not directly connected to the bottom pole of the voice coil motor assembly. Additionally, the over-molded top pole piece does not extend within the internal environment of the disc drive. 
     The present invention can also be implemented as a top cover for a disc drive that includes a base plate supporting an actuator arm mounted adjacent a data storage disc and a voice coil motor assembly having a voice coil attached to the actuator arm, a bottom pole mounted to the base plate and a magnet attached to the bottom pole. The top cover includes a cover portion formed from a moldable material by an injection molding process, wherein the cover portion is adapted for attachment to the base plate to form an internal environment of the disc drive. A metal top pole piece is over-molded within the cover portion of the top cover during the injection molding process, and the metal top pole piece includes a pole body adapted to be aligned over the bottom pole of the voice coil motor assembly when the cover portion is attached to the base plate. In one embodiment, the moldable material is a polymer reinforced with fibers such as carbon or steel fibers. 
     One embodiment of the top cover includes a mounting surface extending from the pole body, wherein the mounting surface is over-molded within the cover portion and is adapted for attachment to a bearing shaft of the actuator arm to provide a metallic anchor between the bearing shaft and the cover portion. The top cover may also include an opening for attaching the cover portion to the base plate of the disc drive. An outer flange extending from the pole body of the metal top pole piece is also over-molded within the cover portion and includes a hole aligned with the opening in the cover portion to provide a reinforced contact point between the cover portion and the base plate. 
     The present invention can further be implemented as a disc drive having a base plate supporting an actuator arm mounted adjacent a data storage disc and a voice coil motor assembly having a voice coil attached to the actuator arm, a bottom pole mounted to the base plate and a magnet attached to the bottom pole. The disc drive includes a top cover attached to the base plate to define an internal environment of the disc drive, and means for supporting a top pole piece in alignment above the bottom pole of the voice coil motor assembly so that the top pole piece does not extend into the internal environment of the disc drive. In one embodiment, the means for supporting the top pole piece includes injection molding the top cover from a moldable material and over-molding the top pole piece within the top cover. In certain embodiments, the moldable material is a reinforced polymer. 
     These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially exploded, perspective view of a disc drive showing a prior art voice coil motor assembly with a top pole and a magnet set illustrated exploded from the remainder of the voice coil motor assembly. 
         FIG. 2  is a generalized section view of the prior art voice coil motor assembly shown in  FIG. 1 . 
         FIG. 3  is a perspective view of a disc drive illustrating a moldable top cover of the disc drive exploded from a base plate and a head disc assembly of the disc drive, and further illustrating a top pole of a voice coil motor assembly over-molded with the top cover in accordance with a preferred embodiment of the present invention. 
         FIG. 4  is an enlarged perspective view of a portion of the moldable top cover shown in  FIG. 3  illustrating the over-molded top pole of the present invention exploded away from the moldable top cover. 
         FIG. 5  is a is a generalized section view of the voice coil motor assembly shown in  FIG. 3  illustrating the over-molded top pole formed integrally with a moldable top cover in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment of the present invention is shown in  FIGS. 3-5 . A disc drive  300  ( FIG. 3 ) includes a base plate  302  and a top cover  304  formed from a moldable material such as a reinforced polymer plastic. The top cover  304 , described in greater detail below, connects to the base plate  302  to form an internal, sealed environment for the disc drive  300  similar to that shown in  FIG. 1  above. The internal components of the drive  300  include a drive spindle motor  306  that rotates one or more discs  308 , and an actuator assembly  310  that rotates about a bearing shaft  312  during a seek operation. The spindle motor  306  is preferably of a rotating shaft design that does not require a connection to the top cover  304  (as in the case of a stationary shaft design). The actuator assembly  310  includes a plurality of actuator arms  314  with one or more flexures  316  extending from each of the actuator arms  314  towards the discs  308 . Read/write heads  318  mounted at the distal end of each of the flexures  316  include air bearing sliders enabling the heads  318  to fly in close proximity above the corresponding surface of the associated disc  308 . 
     As noted above, the track position of the heads  318  is controlled during a seek operation through the use of a voice coil motor (VCM) assembly  320 . In the preferred embodiment shown in  FIG. 3 , the VCM assembly  320  includes a bottom pole  322  that is fixed to the base plate  302  in a conventional manner, such as by adhesive or a threaded fastener (not shown). A magnet set  324  is preferably attached to a top surface of the bottom pole  322 , where the magnet set  324  includes a pair of magnets with opposite polarity faces lying in a common plane. The magnet set  324  may be attached to the top surface of the bottom pole  322  by its own magnetic force or by a suitable adhesive. 
     The moldable top cover  304  is preferably formed integrally with a top pole  326  embedded or “over-molded” therein. In this manner, the top pole  326  is aligned over top of the magnet set  324  and the bottom pole  322  when the top cover  304  is attached to the base plate  302 . Specifically, the top pole  326  is suspended above the magnet set  324  so as to leave an air gap  328  ( FIG. 5 ) therebetween to allow for movement of a coil  330  within the magnetic field created by the magnet  324  and the top and bottom poles  326  and  322 , respectively. The coil  330  is attached to the rear of the actuator assembly  310  in a conventional manner so that movement of the coil  330  (upon the application of direct current to the coil windings) causes the actuator arms  314  and the heads  318  to move in an arcuate path over the surfaces of the discs  308 . 
       FIG. 4  is an exploded view illustrating the top pole  326  exploded away from the molded cover portion  332  of the top cover  304 . In a preferred embodiment, the top cover  304  is molded from a reinforced polymer material (e.g., plastic reinforced with carbon or stainless steel fibers), although any moldable material may be used. For the purposes of the present invention, the term “moldable material” encompasses those materials that may be injection molded and which harden to form a sufficiently stiff cover portion  332  capable of sealing the drive  300  and protecting the internal disc drive components. The term “moldable material” does not encompass materials such as stainless steel or aluminum which are commonly used to form the base plate  302  as well as prior art top covers. The use of a moldable materials such as reinforced polymers for the top cover  304  represents an improvement over prior metal top covers in terms of both cost and manufacturing ease. Specifically, plastic molding techniques provide the ability to form more complex shapes than is possible with metal molding or stamping techniques. Furthermore, with respect to the present invention, plastic molding techniques provide the ability to embed or “over-mold” a metal part within the molded plastic body. Thus, the metal top pole piece  326  (preferably formed from stainless steel) may be inserted within a plastic mold prior to injecting the molten reinforced polymer within the mold so that the metal piece is surrounded or “over-molded” by the molten polymer material during the formation of the top cover  304 . In a preferred embodiment, the cover portion  332  completely covers an upper portion of the top pole  326  so that a top surface of the over-molded top pole  326  does not protrude upward through a top surface of the top cover  304  (see the section view of the top cover  304  and the over-molded top pole  326  in  FIG. 5 ). 
       FIG. 4  illustrates the complex shape of the preferred pole piece  326 . Specifically, the pole piece  326  includes a pole body  340  defining the maximum thickness of the pole piece  326  (preferably 0.104 inches for a standard 3.5 inch form factor disc drive). The pole body  340  has a curved shape that matches the shape of the bottom pole  322  and the magnet set  324 . An outer flange  342  extends from an outer circumference of the curved pole body  340  and preferably has a reduced thickness relative to the thickness of the pole body  340 . The outer flange  342  preferably extends flush with a top surface of the pole body  340  to form a mounting hole  344  that aligns with a similar hole  346  formed in the molded top cover  304 . The holes  344  and  346  in turn align with a threaded hole  348  ( FIG. 3 ) formed in the base plate  302 . The inclusion of the metal flange  342  and the mounting hole  344  allows a reinforced metal-to-metal contact at one corner of the joined metal base plate  302  and the molded (e.g., plastic) top cover  304 . This metal-to-metal contact allows a threaded fastener (not shown) to make a more secure connection between the base plate  302  and the top cover  304 . 
     The pole piece  326  also preferably includes an inner feature  350  extending from an inner circumference of the curved pole body  340 . The inner feature  350  preferably has a reduced thickness relative to the thickness of the pole body  340  and includes a circular mounting surface  352  defining a mounting hole  354 . The mounting hole  354  is aligned with the bearing shaft  312  of the actuator assembly  310  when the top cover  304  is attached to the base plate  302  and provides increased stiffness (i.e., a metal-to-metal contact) at the contact point between the top cover  304  and a top end of the bearing shaft  312 . In this manner, the molded top cover  304  provides adequate support for the actuator assembly  310  similar to the level of support provided by prior art metal top covers (such as the aluminum or stainless steel top cover  104  shown in  FIG. 1 ). Specifically, the metal inner feature  350  helps to anchor the bearing shaft  312  of the actuator assembly  310  to prevent the reduced stiffness of the molded top cover  304  (relative to the prior art metal top covers  104 ) from introducing vibrations or amplifying existing vibratory modes during operation of the actuator assembly  310 . 
     While the outer flange  342  includes a mounting hole  344  for securing the top cover  304  to the base plate  302 , the top pole piece  326  may also include a separate through hole  360  at one end of the curved pole body  340 . As best shown in  FIG. 4 , the hole  360  aligns with a hole  362  formed in the molded top cover  304  and further aligns with a hole  364  formed in the bottom pole  322  beyond the end of the magnet  324 . The aligned holes  362 ,  360  and  364  may receive a threaded fastener to further secure and position the top pole  326  relative to the bottom pole  322 . However, in those cases where the reinforced polymer top cover  304  is sufficiently stiff, the holes  360 ,  362  and  364  may not be necessary to provide a sufficiently rigid connection between the elements of the voice coil motor assembly  320 . 
     Molding the top cover  304  from a polymer (or other moldable) material provides a number of benefits over prior art aluminum or steel top covers  104 . Initially, it has been found that forming the top cover  304  and the integral top pole piece  326  in a single molding step saves a significant amount of time (and thus money) over prior art manufacturing processes where the top cover  104  is formed separately and a top pole  124  is either attached to the top cover  104  (such as by an adhesive) or is connected directly to a bottom pole  128  by standoffs  130 . With respect to prior art drives  100  where the top pole  124  is attached by an adhesive to the top cover  104 , the present invention represents a major improvement by not incurring the costs or the potential outgassing hazards associated with the use of an adhesive. Indeed, it has been found that the use of a moldable top cover  304  with an over-molded top pole  326  provides a cost savings of approximately twenty percent over adhering a top pole to a metallic top cover. Furthermore, with respect to prior art drives that utilize standoffs  130  to support the top pole  124 , the present invention provides additional cost savings (in both material and manufacturing steps) by not requiring separate standoffs  130  to support the top pole. 
     Furthermore, with respect to all such prior art voice coil motor assemblies where the top pole is attached within the internal environment of the drive, the present invention provides for a more powerful voice coil motor assembly  320  by embedding the top pole  326  within the top cover  304 , thereby maximizing the remaining space within the interior of the disc drive  300 . That is, for a given form factor, each disc drive has a limited amount of vertical space or “height” within which to house the components of the voice coil motor. Thus, by moving the top pole piece  326  out of the drive interior and embedding or “over-molding” the top pole within the top cover  304  itself, the present invention provides additional space within the drive interior to accommodate a larger magnet  324  and a larger bottom pole  322 , as well as a larger coil  330  (compare the section view of  FIG. 5  with the prior art section view of  FIG. 2 ). The use of a larger magnet, coil and/or pole pieces in turn creates a stronger magnetic field that is capable of generating more torque and thus faster seek times for the disc drive  300 . 
     Additionally, forming the top cover  304  from a moldable material such as a fiber reinforced polymer allows for the inclusion of intricate features within the top cover, such as retaining features used to accurately position and firmly hold the top pole  326  in place. Such intricate features are possible as a result of the injection molding process that is preferably used to form the top cover  304 , and would not be possible with a stamped or poured aluminum top cover. For example,  FIG. 4  illustrates a depending border  370  that surrounds and retains a perimeter of the curved pole body  340  while providing an opening to allow the outer flange  342  to extend above and beyond the border  370 . Additionally, an expanded support surface  372  extends from an inner portion of the border  370  to support and retain the inner feature  350  of the top pole piece  326  so that the mounting hole  354  is properly aligned with the bearing shaft  312 . Other features, such as a servo writing opening  380 , are also easily formed within the moldable top cover  304  during the injection molding process. 
     As noted above,  FIG. 5  provides a generalized sectional view of the disc drive  300  and illustrates the use of the relatively larger components, such as the bottom pole  322 , the magnet  324  and the coil  330 , which is possible due to the fact that the top pole  326  no longer takes up valuable space within the drive interior.  FIG. 5  further illustrates the complete over-molding of the top pole  326  by the polymer top cover  304 . As noted above, while a fiber reinforced polymer is preferably used to form the moldable cover  304 , the present invention encompasses other moldable materials provided that the material is sufficiently rigid to protect the drive components while being sufficiently moldable (i.e., capable of injection molding) to form all of the required intricate features of the top cover  304 . 
     Connected in this manner, the over-molded metal top pole piece is fully supported by the top cover and is not directly connected to the bottom pole of the voice coil motor assembly. The present invention thus simplifies the manufacturing process and reduces manufacturing costs (both in material and in assembly time) by eliminating the requirement for adhesives or side posts to support the top pole piece. Any loss of efficiency within the voice coil motor resulting from the lack of side posts connecting the top and bottom poles is compensated for by the increased size (and thus power) of the magnet and the coil that is made possible by removing the top pole piece from the internal environment of the disc drive. 
     Numerous other changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the invention disclosed and as defined in the appended claims.