Abstract:
An actuator assembly for use in a data storage device is described that has features that reduce off-track writes due to coil popping. The fantail portion of the actuator includes thermal restraint features that engage with the over-mold, and the thermal restraint feature are positioned prevent the over-mold from separating from the actuator as a result of the over-mold and the actuator having different coefficients of thermal expansion. The thermal restraint features reduce the length of effective interface between the actuator and the over-mold acted upon by shear forces due to coefficient of thermal expansion mismatch. In one example, the thermal restraint features are holes that extend through the actuator, to allow the over-mold to flow through the holes and connect on both sides of the fantail during molding, effectively interlocking the fantail portion to the over-mold.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority of U.S. provisional application Serial No. 60/419,752, filed Oct. 18, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This application relates generally to data storage devices, and more particularly to a method and apparatus to reduce off-track writes in a disc drive due to coil popping.  
         BACKGROUND OF THE INVENTION  
         [0003]    The storage medium for a disc drive is a flat, circular disc capable of retaining localized magnetic fields. The data is arranged on the disc in concentric, circular paths known as tracks. A disc drive uses a magnetically sensitive head (transducer) to detect the data. The transducer is mounted upon an actuator, which is attached to a voice coil. The voice coil is immersed in a magnetic field generated by permanent magnets. This causes the actuator to move when a current is applied to the actuator.  
           [0004]    A cost-effective and dynamically robust way to attach the voice coil to the actuator is to use an over-mold of structural plastic. The voice coil is held in a fantail section of the actuator, and the over-mold surrounds the voice coil and the fantail section of the actuator. However, when an over-mold is used to attach the voice coil to the actuator, a phenomenon known as “coil popping” can occur.  
           [0005]    Coil popping occurs when the over-mold separates from the actuator as a result of the over-mold and the actuator each having a different coefficient of thermal expansion. During normal seek operations, current though the coil causes the temperature of the actuator to increase. This temperature increase causes the over-mold and the actuator to both expand at different rates. The actuator quickly decreases in temperature when the actuator stops at a desired location in order to perform a read or write operation, causing the actuator and the over-mold to both quickly shrink at different rates, which creates stress at the interfaces between the actuator and the over-mold. Coil popping is a physical shift that occurs in response to the stress.  
           [0006]    When coil popping occurs, the actuator shudders, driving the transducer off track. If coil popping occurs during a write process, then data stored on adjacent tracks can be corrupted.  
           [0007]    Accordingly there is a need for a method and apparatus to reduce off-track writes due to the phenomenon of coil popping.  
         SUMMARY OF THE INVENTION  
         [0008]    Against this backdrop, embodiments of the present invention has been developed. According to one exemplary embodiment, thermal restraint features are included in the fantail section of the actuator. The thermal restraint features are located on the fantail section to reduce the length of an interface between the over-mold and the actuator acted upon by shear forces generated by the mismatch of the coefficients of thermal expansion between the over-mold and the actuator. The thermal restraint features operate to reduce the effect of the thermal stress in order to prevent popping. According to one exemplary embodiment, the thermal restraint features are one or more spaced holes along each leg of the fantail section of the actuator that extend through the fantail section of the actuator. Over-mold material fills these holes and effectively minimizes coil popping from occurring thus minimizing off-track writes due to coil popping phenomenon.  
           [0009]    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  
       [0010]    [0010]FIG. 1 is a plan view of a disc drive incorporating an embodiment of the present invention, with portions broken away to show the primary internal components.  
         [0011]    [0011]FIG. 2 is an enlarged plan view of a fantail portion of an actuator incorporating an embodiment of the present invention, with the voice coil and over-mold depicted in dashed lines.  
         [0012]    [0012]FIG. 3 is a perspective view of the fantail portion of the actuator shown in FIG. 2.  
         [0013]    [0013]FIG. 4 is a perspective view of an alternative embodiment of the fantail portion of the actuator shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0014]    A disc drive  100  constructed in accordance with a preferred embodiment of the present invention is shown in FIG. 1. The disc drive  100  includes a base  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 in a conventional manner. The components include a spindle motor  106  which 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 a fluid bearing slider enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 .  
         [0015]    During a seek operation, the track position of the heads  118  is controlled through the use of a voice coil motor (VCM)  124 , which typically includes a voice coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets  128  which establish a magnetic field in which the coil  126  is immersed. The controlled application of current to the voice coil  126  causes magnetic interaction between the permanent magnets  128  and the voice coil  126  so that the coil voice  126  moves in accordance with the well known Lorentz relationship. As the voice coil  126  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 . The voice coil  126  is covered by an over-mold  140 .  
         [0016]    The spindle motor  116  is typically de-energized when the disc drive  100  is not in use for extended periods of time. The heads  118  are moved over park zones  120  near the inner diameter of the discs  108  in the embodiment shown when the drive motor is de-energized. The heads  118  are secured over the park zones  120  through the use of an actuator latch arrangement, which prevents inadvertent rotation of the actuator assembly  110  when the heads are parked.  
         [0017]    A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly includes a printed circuit board  132  to which head wires (not shown) are connected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and a preamplifier for amplifying read signals generated by the heads  118  during a read operation. The flex assembly terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive  100 .  
         [0018]    [0018]FIG. 2 is an enlarged, partial plan view of an actuator assembly  110  incorporating an embodiment of the present invention, with the voice coil and over-mold depicted in dashed lines. The actuator assembly  110  includes an actuator body  201 , the voice coil  126 , and the over-mold  140 . The actuator body  201  includes a fantail portion  204 , extending opposite the actuator arms  114 . The fantail portion  204  has two spaced apart legs  206  and  208 , forming a yoke for holding the voice coil  126 . Leg  206  includes a bias bar  210  that is press fit through the fantail portion  204 . The leg  206  also includes thermal restraint features  220  and  222 , spaced along the length of the leg  206 . The leg  208  includes insulated connector pins  212  and  214 . The leg  208  also includes thermal restraint features  224  and  226 , spaced along the leg  208 . The voice coil  126 , positioned between the legs  206  and  208 , and the over-mold  140 , formed around and over the legs  206  and  208 , are shown with dotted lines so that the legs  206  and  208  within the over-mold  140  can be seen.  
         [0019]    The voice coil  126  is configured to move (rotate) the actuator body  201  when a current is applied to the voice coil  126  via pins  212  and  214 . The over-mold  140  surrounds the legs  206  and  208 , and the voice coil  126  so that the voice coil  126  is coupled to the actuator body  201 .  
         [0020]    The bias bar  210  is a piece of steel that, under the influence of the magnetic field produced by magnets  128 , provides a bias force on the actuator body  201  in a clockwise direction. Pins  212  and  214  are coil termination pins that are covered with a plastic insulator and are thus insulated from the fantail portion  204 . Beginning and end wires of the voice coil  126  are coupled to pins  212  and  214 . The voice coil  126  receives current from the disc drive servo-control system (not shown) via pins  212  and  214  for moving the actuator. The bias bar  210  and pins  212  and  214  do not limit coil popping, because they are positioned too close to the proximal end of the legs  206  and  208  to effectively limit the length of the legs  206  and  208 .  
         [0021]    [0021]FIG. 3 is an enlarged separate perspective view of the fantail section  204  of the actuator body  201  shown in FIG. 2. FIG. 3 shows the actuator body  201  before the over-mold  140  and certain other features are added. The leg  206  has a hole  310  for receiving the bias bar  210  (shown in FIG. 2). The leg  206  also has holes  220  and  222 . The leg  208  has holes  312  and  314  for receiving pins  212  and  214  (shown in FIG. 2) respectively. The leg  208  also has holes  224  and  226 . The bias bar  212  and pins  214  and  216  are installed before over-molding. When the over-mold  140  is molded onto the actuator body  201 , the over-mold  140  surrounds the bias bar  212 , and pins  214  and  216 . According to an example in which the thermal restraint features ( 220 ,  222 ,  224 , and  226 ) are holes, the over-mold  140  extends through holes  220 ,  222 ,  224 , and  226 , interlocking the over-mold to the legs  206  and  208 . This interlock effectively divides the legs  206  and  208  into short lengths in which relative movement over-mold with respect to the fantail  204  is substantially precluded. This minimizes occurrences of coil popping.  
         [0022]    [0022]FIG. 4 is an enlarged separate perspective view of an alternative embodiment of the fantail section  204  of the actuator body  201  shown in FIG. 2. In FIG. 4, thermal restraint features  420 ,  422 ,  424 , and  426  are pins that extend through the fantail portion  204  and extend vertically from the fantail portion  204 . Pins  420 ,  422 ,  424 , and  426  may be press fit in place, glued, or otherwise fastened to the fantail  204 . Alternatively, pins  420 ,  422 ,  424 , and  426 , may be integrally formed in the fantail portion  204 .  
         [0023]    In the embodiment shown in FIG. 3, thermal restraint features  220 ,  222 ,  224 , and  226  are holes in the actuator that extend through the fantail portion  204 . The over-mold  140  extends through holes  220 ,  222 ,  224 , and  226 . Alternatively, as shown in FIG. 4, thermal restraint features  420 ,  422 ,  424 , and  426  may be pins that extend vertically from the actuator body  201 . According to yet another alternative, thermal restraint features  220  and  222  may be bridges or walls positioned across the width of the leg  206 , and thermal restraint features  224  and  226  may be bridges or walls positioned across the width of the leg  208 .  
         [0024]    The over-mold  140  engages the thermal restraint features ( 220 ,  222 ,  224 , and  226  or  420 ,  422 ,  424 , and  426 ) to prevent the over-mold  140  from separating from the actuator body  201  as a result of the over-mold  140  and the actuator body  201  each having a different coefficient of thermal expansion. The thermal restraint features are located such that the length of the effective interface between the over-mold  140  and the actuator body  201  that is acted upon by shear forces caused by the mismatch of thermal coefficients of expansion between the actuator body  201  and the over-mold  140  is reduced. Thermal loads produced during a temperature change are transferred to the structure of the fantail portion  204  without substantial slippage. This reduces the effects of the thermal stress in order to limit coil popping.  
         [0025]    The presence of a thermal restraint feature on the legs of the fantail portion prevents thermal expansion of the components beyond the location of the thermal restraint feature. Since the amount of thermal expansion is proportional to length, the presence of a thermal restraint feature reduces the effective length of the leg. For example, if a leg had one thermal restraint feature approximately halfway between the proximal and distal ends of the leg, the effective length of the leg would be reduced by half, and the thermal expansion of each section would be reduced by approximately half. If a leg has two thermal restraint features that divide the leg into approximately three equal parts, then thermal expansion of each section is reduced to approximately one third. (This is the configuration depicted in FIG. 2, FIG. 3, and FIG. 4). This configuration significantly decreases effects of thermal stresses caused by the mismatch of thermal coefficients of expansion between the actuator body  201  and the over-mold  140 , and therefore significantly limits thermal popping. In contrast, a hole or a pin that is near the distal or proximal end of a leg of a fantail portion of an actuator does not act as a thermal restraint feature, because it has little or no effect on thermal expansion.  
         [0026]    Although round thermal restraint features are shown in FIG. 2, FIG. 3, and FIG. 4, any shape may be used for the thermal restraint features, as the shape of the thermal restraint feature makes no appreciable difference with regard to coil popping. However, it may be more cost effective to use a round shape.  
         [0027]    The actuator body  201  may be composed of aluminum. Alternatively, the actuator body  201  may be composed of another metal or non-metallic material.  
         [0028]    The over-mold  140  may be composed of structural plastic. Coil popping may be reduced by utilizing an over-mold material having a coefficient of thermal expansion that closely matches the coefficient of thermal expansion of the material that the actuator is composed of. For example, the vendor RTP Company of Winona, Minn. supplies a material called 1399x94017J, and the supplier LNP ENGINEERING PLASTICS, INC. of Exton, Pa. supplies a material called PDX01787CCSGN4A421. PDX-01787CCSGN4A421 and 1399×94017J have coefficients of thermal expansion that more closely match the coefficient of thermal expansion of aluminum.  
         [0029]    It will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While a presently preferred embodiment has been described for purposes of this disclosure, various changes and modifications may be made which are well within the scope of the present invention. For example, a variety of features shapes may be used for the thermal restraint features. Also, other configurations of the thermal restraint features are possible. For example, each of the legs may have more or less thermal restraint feature than shown. As another example, the positioning of the thermal restraint features may be different, as long as the thermal restraint features are positioned to reduce the effective length of the leg that it is on. 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.