Abstract:
A voice coil and actuator yoke assembly is fabricated by deforming the coil to exactly fit the inner perimeter and thickness of the yoke. The comb is placed in a tool that registers the comb with the yoke against the same flat surfaces that will squash the coil. The tooling is closed and placed in an appropriate loading device and the coils are squashed to the precise thickness of the yoke. This eliminates the in-plane tolerances of the current process for bonding a coil into the yoke. The coil perimeter expands as the coil is squashed to form an intimate press fit with the yoke. A low viscosity, high T g  adhesive is then applied to the coil. The low viscosity of the adhesive allows capillary action to fill the voids between the wires and bond the coil to the yoke.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates in general to an improved voice coil motor and, in particular, to an improved system, method, and apparatus for plastically deforming and potting the voice coil of a voice coil motor for a disk drive. 
   2. Description of the Related Art 
   Data access and storage systems generally comprise one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, one to five disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm). Hard disk drives have several different typical standard sizes or formats, including server, desktop, mobile (2.5 and 1.8 inches) and microdrive. 
   A typical HDD also uses an actuator assembly to move magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk. 
   A slider is typically formed with an aerodynamic pattern of protrusions on its air bearing surface (ABS) that enables the slider to fly at a constant height close to the disk during operation of the disk drive. A slider is associated with the functional side of each disk and flies just over the disk&#39;s surface. Each slider is mounted on a suspension to form a head gimbal assembly (HGA). The HGA is then attached to a semi-rigid actuator arm that supports the entire head flying unit. Several semi-rigid arms may be combined to form a single movable unit having either a linear bearing or a rotary pivotal bearing system. 
   The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop and settle directly over the desired track. 
   The motor used to rotate the disk is typically a brushless DC motor. The disk is mounted and clamped to a hub of the motor. The hub provides a disk mounting surface and a means to attach an additional part or parts to clamp the disk to the hub. In most typical motor configurations of HDDs, the rotating part of the motor (the rotor) is attached to or is an integral part of the hub. The rotor includes a ring-shaped magnet with alternating north/south poles arranged radially and a ferrous metal backing. The magnet interacts with the motor&#39;s stator by means of magnetic forces. Magnetic fields and resulting magnetic forces are induced via the electric current in the coiled wire of the motor stator. The ferrous metal backing of the rotor acts as a magnetic return path. 
   In prior art carriage and head stack assembly (HSA) performance, it has long been desired to stiffen the coil to improve its dynamic performance. Attempts at doing this have taken two directions. First, the coil has been cast in a plastic carrier or yoke and attached to the barrel of the comb. Drawbacks to this approach are that plastic is not the best structural material for drives requiring high dynamic performance, nor is it an effective conductor of heat for cooling the coil. This approach has found limited acceptance in the mobile drive market. Another approach has been to add a bobbin to the center of the coil. While increasing stiffness, it also increases mass, which detracts from power consumption and/or access time. 
   Another problem is that the coil generates heat and can reach 100° C. in server class drives and requires measures for cooling in the design of the HSA. The current method of mounting the coils for cooling is to bond the coil to the yoke with an adhesive that is filled with aluminum particulate to increase its thermal conductivity. There are a few problems with this approach. The width of the adhesive bond (about 0.5 mm) has relatively low modulus and stiffness as compared to the aluminum of the yoke. Variability in its width as well as variability in its properties, such as glass transition temperature (T g ), can cause variability in the dynamic performance of the HSA. In addition, variability in the homogeneity as well as the amount of aluminum particle filler will cause variability in the adhesives thermal conductivity and, consequently, variability in the power consumption or access speed of the HSA. 
   Moreover, the amount of adhesive required to bond the coil to the yoke is denser than an equivalent volume of coil windings. For a given carriage, this equates to about 0.2 g. Added mass either reduces the access time or increases power consumption. Furthermore, the aforementioned 0.5 mm bond line in effect positions the yoke legs outwards by this amount. This has an adverse effect on coil torsion modes. 
   Finally, the coil location in the z-axis of the carriage as well as planarity of the coil to the VCM magnets has associated tolerances. These tolerances exist with all coil attachment processes and can only be improved with better tooling and bonding fixtures. It is suspected that the coil not being centered or out of plane with the VCM creates coil torsion problems. Thus, an improved VCM assembly solution would be desirable. 
   SUMMARY OF THE INVENTION 
   One embodiment of a system, method, and apparatus for an improved voice coil motor is described. The principal idea behind the present invention is to fabricate the yoke and coil such that, as wound, the coil is a close fit to the inside perimeter of the yoke, and protrudes above the surface of the yoke by about 50 to 75 microns. With the coil in place in the yoke, it is squashed to fit exactly the inner perimeter and thickness of the yoke. A low viscosity, high T g  adhesive is then applied to the coil. The low viscosity of the adhesive allows capillary action to fill the voids between the wires and bond the coil to the yoke. By potting the coil with a high T g  adhesive, the coil becomes a structural part of the comb. By eliminating the need for a border of adhesive for attachment, the HSA is lighter with less inertia. Since the coil is pressed against the yoke, the heat transfer out of the coil is now metal-to-metal rather than through an adhesive. 
   The comb is placed in the tool that registers the comb with the yoke against the same flat surfaces that will squash the coil. The coil is set in place inside the yoke. The tooling is then closed and placed in an appropriate loading device such as a press or vise. The coils are squashed to the precise thickness of the yoke. This eliminates the in-plane tolerances of the current process for bonding a coil into the yoke. The coil perimeter expands as the coil is squashed to form an intimate press fit with the yoke. Experience with this procedure has shown that although the coil is free to expand inwards, the inner diameter or the coil actually expands. The back section of the coil that is unsupported by the yoke is constrained by the tooling and controls this area&#39;s final shape. 
   Once removed from the tool, and sitting in a free state (i.e., no fixtures), a capillary adhesive is applied to the outer surfaces of the coil. The adhesive should be chosen for low viscosity and ability to fill small voids by capillary action. The adhesive should have a high T g  and be compatible with the HDD environment. It is desirable for the adhesive to provide ease of mixing and a long shelf life. In one embodiment, approximately 50 micro liters of Epo-Tek 377 was applied with a micro-pipette to each side of the coil, avoiding the region of the barrel of the comb. The comb and coil assembly is set with the coil horizontal for 1 hour. Over this period of time most of the adhesive wicks into the coil. The adhesive is cured in an oven at about 130° C. for about 40 minutes. At elevated temperature the viscosity of the adhesive further decreases, and the remainder of the adhesive wicks into the coil leaving only a shiny brown luster to the coil. This temperature is excessive for the pivot cartridge, thus, the squash and pot procedure must be done prior to installation of the pivot. The volume of adhesive is dependent on the design of the coil which determines the volume of voids between the wires. A volume of adhesive that is greater than the volume of voids between the wires assures complete filling and compensates for part-to-part variations. Excess adhesive only makes for larger fillets between the outside wire windings. 
   The dynamic performance of the coil of the present invention is improved over prior art designs. Additional dynamic benefits of the coil are realized when the pivot is also made stiffer. The pivot may be stiffened by, e.g., bonding the pivot into the comb through the use of a capillary acting adhesive. The bore of the comb may be fabricated to be a slip fit with the sleeve of the pivot cartridge. A low viscosity adhesive is then wicked into the microscopic space between the sleeve and the comb bore, creating a stiff adhesive joint. 
   The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the features and advantages of the invention, as well as others which will become apparent are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only an embodiment of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
       FIG. 1  is an exploded isometric view of a system for constructing a disk drive voice coil and comb assembly and is constructed in accordance with the present invention. 
       FIG. 2  is a top view of the comb and coil assembly of  FIG. 1  prior to deformation. 
       FIG. 3  is a top view of the comb and coil assembly of  FIG. 1  after deformation. 
       FIG. 4  is an enlarged top view of a portion of the comb and coil assembly of  FIG. 2  prior to deformation. 
       FIG. 5  is an enlarged top view of a portion of the comb and coil assembly of  FIG. 3  after deformation. 
       FIG. 6  is a plot of an arm stack assembly low frequency response for a conventional comb and coil assembly. 
       FIG. 7  is a plot of an arm stack assembly low frequency response for a comb and coil assembly constructed in accordance with the present invention. 
       FIG. 8  is a plot of an arm stack assembly high frequency response for a conventional comb and coil assembly. 
       FIG. 9  is a plot of an arm stack assembly high frequency response for a comb and coil assembly constructed in accordance with the present invention. 
       FIG. 10  is a plan view of a disk drive constructed in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , one embodiment of a system, method, and apparatus for fabricating a voice coil motor for a data access storage device is disclosed. The present invention is ideally suited for stiffening an actuator comb and coil assembly. In one embodiment, an actuator comb  11  has a plurality of arms  125 , an axial bore  13  for a pivot  123  ( FIG. 10 ) and a yoke  15  opposite the arms  125 . The yoke  15  includes a pair of legs  17  that extend from the actuator comb  11  that define an inner perimeter  19  and an axial dimension  21 . 
   A coil  23  forms a portion of a voice coil motor  133  ( FIG. 10 ) and comprises a wound loop of conductive material, such as copper. The coil  23  has an axial dimension  25  and an outer perimeter  27 . The axial dimension  25  of the coil  23  is greater than the axial dimension  21  of the yoke  15 . The outer perimeter  27  is shaped complementary to the inner perimeter  19 , and the outer perimeter  27  is sized slightly smaller than the inner perimeter  19  ( FIGS. 2 and 4 ). 
   One embodiment of a block-like tool  31  used to form an assembly out of actuator comb  11  and coil  23  has a fixture  33  and a base  35  that is complementary to the fixture  33 . The fixture  33  has a pair of alignment pins  37 ,  39  protruding therefrom. The base  35  has a pair of apertures  41 ,  43  that are complementary to the alignment pins  37 ,  39 , respectively, and which receive the alignment pins  37 ,  39  when brought into engagement therewith. Alignment pin  37  is designed to extend through the axial bore  13  of the actuator comb  11 . 
   The base  35  also has a seat  45  for receiving the yoke  15  and the coil  23  located in the yoke  15  between legs  17 . When the fixture  33  and the base  35  are joined together, the coil  23  is plastically deformed ( FIGS. 3 and 5 ) as it is pressed between legs  17  so that the outer perimeter  27  expands outward to the inner perimeter  19 , and the axial dimension  25  of the coil  23  is decreased. In one version, the axial dimensions  25 ,  21  of the coil  23  and the yoke  15  are substantially equivalent after deformation. However, axial dimension  25  may still exceed axial dimension  21  after deformation (e.g., via mechanical stops to prevent further deformation.) The yoke  15  and coil  23  are loaded in the tool  31  and pressed by the fixture  33  and base  35  to deform the coil  23 . A back section  24  ( FIG. 3 ) of the coil  23  that is unsupported by the yoke  15  is constrained at wall  46  ( FIG. 1 ) with the tool  31  to control a final shape of the coil  23 . 
   In one embodiment, the yoke  15  and the coil  23  are fused together thereafter to form an actuator comb and coil assembly. The coil  23  may be fused by bonding to the yoke  15  with a low viscosity, high T g  adhesive  51  ( FIG. 5 ) that fills between the coil and the yoke by capillary action. The yoke  15  and coil  23  may be removed from the tool  31  before the coil  23  and the yoke  15  are fused. In one version, approximately 10 to 50 micro liters of adhesive  51  ( FIG. 5 ) are applied to each side of the coil  23  and yoke  15  (i.e., leg  17 ) while avoiding, for example, a region  14  ( FIG. 3 ) of a bore  13  of the comb to avoid contact therewith (e.g., the aluminum used to form the comb). The coil  23  and yoke  15  are set horizontal for 1 hour, and cured in an oven at about 130° C. for about 40 minutes. 
   In one embodiment, the coil and comb assembly ( FIGS. 3 and 5 ) are further stiffened by a pivot  123  ( FIG. 10 ) that is slip fit inside the bore  13  of the comb  11 . An adhesive  51  that is wicked between the pivot  123  and the bore  13  form a stiff adhesive bond joint therebetween. 
   The present invention also comprises a method of stiffening an actuator comb and coil assembly. One embodiment of the method comprises providing a comb  11  having a yoke  15  with an inner perimeter  19 , positioning and registering a coil  23  in the yoke  15 , pressing the coil  23  in place in the yoke  15  such that a perimeter  27  of the coil  23  is increased to the inner perimeter  19  and a thickness  25  of the coil  23  is decreased, and fusing the coil  23  with the yoke  15  to form a stiffened comb and coil assembly ( FIGS. 3 and 5 ). This may include loading the yoke  15  and coil  23  in a tool  31  and then pressing the tool  31  to deform the coil  23  while constraining a back section  24  of the coil  23  that is unsupported by the yoke  15  with the tool  31  to control a final shape of the coil  23 . 
   The method may comprise making the thickness  25  of the coil  23  substantially equivalent to the thickness  21  of the yoke  15 . The method may comprise bonding the coil  23  to the yoke  15  with a low viscosity, high T g  adhesive  51  that fills between the coil  23  and the yoke  15  by capillary action. This step may comprise applying approximately 50 to 100 micro liters of adhesive to each side of the coil  23  and yoke  15  while avoiding a region  14  of a bore  13  of the comb  11 , setting the coil  23  and yoke  15  horizontal for 1 hour, and then curing the coil  23  and yoke  15  in an oven at about 130° C. for about 40 minutes. The method may further comprise further stiffening the coil and comb assembly by providing a pivot  123  that slip fits inside the bore  13  of the comb  11 , and applying an adhesive  51  to wick between the pivot  123  and the bore  13  to form a stiff adhesive bond joint therebetween. In addition, the method may comprise providing the yoke  15  with a pair of legs  17  extending from the comb  11 , positioning the coil  23  between the legs  17  of the yoke  15 , and pressing the coil  23  in between the legs  17  of the yoke  15 . 
   Referring now to  FIG. 10 , a schematic drawing of one embodiment of an information storage system comprising a magnetic hard disk file or drive  111  for a computer system is shown and is constructed in accordance with the present invention. Drive  111  has an outer housing or base  113  containing at least one magnetic disk  115 . Disk  115  is rotated by a spindle motor assembly having a central drive hub  117 . An actuator  121  comprises a plurality of parallel actuator arms  125  (one shown) in the form of a comb that is pivotally mounted to base  113  about the pivot assembly  123 , mentioned above. A controller  119  is also mounted to base  113  for selectively moving the comb of arms  125  relative to disk  115 . 
   In the embodiment shown, each arm  125  has extending from it at least one cantilevered load beam and suspension  127 . A magnetic read/write transducer or head is mounted on a slider  129  and secured to a flexure that is flexibly mounted to each suspension  127 . The read/write heads magnetically read data from and/or magnetically write data to disk  115 . The level of integration called the head gimbal assembly is head and the slider  129 , which are mounted on suspension  127 . The slider  129  is usually bonded to the end of suspension  127 . The head is typically pico size (approximately 1250×1000×300 microns) and formed from ceramic or intermetallic materials. The head also may be femto size (approximately 850×700×230 microns) and is pre-loaded against the surface of disk  115  (in the range two to ten grams) by suspension  127 . 
   Suspensions  127  have a spring-like quality which biases or urges the air bearing surface of the slider  129  against the disk  115  to enable the creation of the air bearing film between the slider  129  and disk surface. A voice coil  133  housed within a conventional voice coil motor magnet assembly  134  (top pole not shown) is also mounted to arms  125  opposite the head gimbal assemblies. Movement of the actuator  121  (indicated by arrow  135 ) by controller  119  moves the head gimbal assemblies radially across tracks on the disk  115  until the heads settle on their respective target tracks. The head gimbal assemblies operate in a conventional manner and always move in unison with one another, unless drive  111  uses multiple independent actuators (not shown) wherein the arms can move independently of one another. 
   Referring now to  FIGS. 6 and 7 , at lower operational frequencies (e.g., 2.8–9.2 kHz), an improvement of about 1 kHz is possible when the entire system is made stiffer by potting the coil  23  (e.g., with adhesive  51 ) as disclosed herein and by stiffening the pivot  123 . Stiffening the coil  23  alone improves the frequency response by about 64 Hz, as shown by comparing plots  601  and  701 . But with a stiffened pivot  123 , the improvement is about 430 Hz. Further stiffening of the pivot  123  increases the dynamics by a total of about 1.1 kHz. The improvement in dynamics for stiffening the pivot alone is about 592 Hz. Even though stiffening the coil alone has a relatively small improvement in dynamics (64 kHz), when it is combined with a stiffer pivot a total improvement of about 1024 Hz is possible. It has been recognized before that a stiffer pivot is desirable for dynamics. 
   Referring now to plots  801  and  901  in  FIGS. 8 and 9 , respectively, a similar trend can be seen at the higher frequencies of about 8.8 to 21.6 kHz. The HSA level data is not as dramatic as the carriage level data. The trend for improvement still exists but is less due to several factors. The mass of an HSA is greater than the carriage with the added Head Gimbal Assemblies (HGA). The HGAs introduce their own modes of vibration that interact with the carriage. 
   The present invention has several advantages, including the ability to make the coil a structural part of the comb. By eliminating the need for a border of adhesive for attachment, the HSA is lighter with less inertia. Since the coil is pressed against the yoke, the heat transfer out of the coil is metal-to-metal rather than through an adhesive. The dynamic performance of the coil of the present invention is significantly improved over prior art designs. 
   While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention. For example, other types of adhesive may be used, such as anaerobic adhesives or those initiated with UV radiation. In addition, a higher viscosity adhesive may be used to provide the same function (e.g., filling voids between the coil and yoke). However, such an embodiment would not operate with capillary action. Rather it would require the squashed coil to be submerged in the adhesive, degassed in a vacuum and typically require a secondary cleaning operation for any excess coating in need of removal.