Abstract:
A disc drive with improved servo-mechanical frequency response includes a rotary actuator assembly incorporating stiffening members disposed along the sides of its actuator arms. In one embodiment the stiffening members comprises edge rails. The edge rails may rise from only the upper surface of the actuator arm or from both the upper and lower surfaces. In one embodiment the stiffening members comprise wings which project upward from the sides of the actuator arm.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This patent application claims priority from U.S. Provisional Application No. 60/322,419 filed on Sep. 14, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to data storage devices. More specifically, but not by way of limitation, the present invention concerns actuators for accessing data in hard disc drives.  
         BACKGROUND OF THE INVENTION  
         [0003]    Until recently, the width (radial direction) to length (circumferential) direction aspect ratio of a group of magnetic particles storing one bit of data in hard disc drives has typically been about 15-to-1. In recent years, in order to increase data storage capacities, designers have decreased bit width so that more tracks can be squeezed onto the disc surface. However, an increase in track density requires a corresponding increase in overall servo-mechanical performance. Furthermore, as track width decreases the effect of external disturbances such as spindle vibrations and disc flutter and wobble becomes more significant. Off-track head displacement induced by such disturbances is significant relative to the narrow inter-track spacing.  
           [0004]    A disc drive servo system&#39;s susceptibility to spindle vibration and associated low frequency disturbances may be reduced by increasing the servo-system&#39;s bandwidth. However, in general the servo system bandwidth frequency is limited to about 20% of the lowest mechanical resonance frequency of the actuator assembly.  
           [0005]    An actuator system has four primary bending modes, each having a resonant frequency a designer must be concerned with. One such bending mode, conventionally known as a “first bending mode,” involves bending of the actuator arm out of the rotational plane of the actuator, where the bending takes place near the pivot cartridge. Another bending mode, conventionally known as a “second bending mode,” similarly involves bending out of the rotational plane of the actuator, but where the bending takes place further away from the pivot axis, near the flexure support end of the actuator arm. A third bending mode is the “first torsion mode,” in which the actuator arm twists about a longitudinal axis of the actuator arm, such that the plane of the actuator intersects but is no longer parallel to the rotational plane of the actuator. A fourth primary bending mode is the “first sway mode,” in which the actuator arm bends within the rotational plane of the actuator. A further limiting mechanical resonance frequency is due to vibration in the so-called “butterfly” mode. In the butterfly mode the read/write head end of the actuator arms and the fantails swing simultaneously to left and right relative to pivot assembly in the manner of a butterfly&#39;s wings flapping about its body. The butterfly mode resonant frequency is determined by the sway modes of the coil structure and arm structure and limited by the lower of these frequencies. As the servo system directs the actuator to move the head from track to track, the actuator will vibrate in these various modes. As long as the frequencies generated by the servo system remain below the various resonant frequencies of the actuator, the drive will continue to function properly. It should be clear that the speed at which the drive may operate is limited by the resonant frequencies of the actuator system. It is generally a goal of actuator design, therefore, to raise the natural resonant frequencies of the actuator system to allow for faster drive operation.  
           [0006]    One approach to increasing the resonance frequency of actuator arms has been to increase the mechanical resonant frequency by making the actuator arms out of stiffer and lighter materials. However that approach entails using materials which may be expensive and/or difficult to work with.  
           [0007]    Another approach to providing a servo system with improved frequency response characteristics has been the use of a secondary microactuator. Here, a first coarse servo system is used to move the actuator head across large distances and a second lighter servo system effects movement in a smaller microactuator to handle fine seek requests. However, microactuators at his point in time are expensive and much more research and development will be necessary before they can be effectively implemented in hard disc drives.  
           [0008]    What the prior art has been lacking is a low-cost method for improving servo system frequency response characteristics without significantly increasing costs associated with materials and/or manufacturing, and is easily fabricated using standard technologies.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    According to one embodiment of the present invention there is provided a stamped actuator arm for a disc drive of the type incorporating a rotary actuator assembly, the actuator arm including one or more stiffening members arranged to increase said arm&#39;s resistance to lateral bending. These stiffening members may take the form of two raised stiffening rails located along opposing long edges of the arm.  
           [0010]    Additional features and benefits will become apparent upon a review of the attached figures and the accompanying description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a stylized top view of a hard disc drive incorporating an actuator of the present invention.  
         [0012]    [0012]FIG. 2 is an isometric view of a typical stamped actuator arm.  
         [0013]    [0013]FIG. 3 is an isometric view of an actuator arm according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 4 is an isometric view of an actuator arm according to a further embodiment of the present invention.  
         [0015]    [0015]FIG. 5 is an isometric view of an intermediate stage in the production of the actuator arm of FIG. 6.  
         [0016]    [0016]FIG. 6 is an isometric view of an actuator arm according to a further embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIG. 1 is a stylized top view of a hard disc drive  2  with cover removed to reveal its inner workings. The disc drive includes a stack of magnetic platters  4  of which only the uppermost is visible. Each platter takes the form of a rotatable storage disc operatively rotated at a constant speed of several thousand RPM by a spindle motor (not shown). Each platter  4  typically comprises a disc substrate having a surface on which a magnetic material is deposited. Digital data is stored on the disc as a series of variations in magnetic orientation of the disc&#39;s magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data.  
         [0018]    Data is written to and read from concentric tracks on each magnetic platter  4  by each of a number of read/write head assemblies or sliders  6  of which the uppermost one is visible. Each read/write head assembly  6  includes a magnetoresistive (MR) head unit supported by a corresponding suspension assembly  8 . Each slider  6  glides over the surface of a corresponding one of the platters  4 . Each slider  6  is coupled to a corresponding actuator arm  10  via a suspension  8  which rotates about pivot assembly  12 . The actuator arms  10  are stacked one above the other to form a rotary actuator assembly. Actuator arms  10  may be formed by stamping them out of a flat plate of aluminum.  
         [0019]    Each actuator arm  10  has a rear “fan-tail” portion  15  into which a voice coil  16  is mounted. Attached to the actuator arm assembly is a printed circuit cable (PCC)  14  which serves to transmit electrical signals to and from read/write and servo system circuitry mounted on printed circuit board (PCB)  18 .  
         [0020]    [0020]FIG. 2 shows an actuator arm  10  as typically used in a hard disc drive  2 . The arm  10  may be formed by stamping it out from a metallic plate. Aluminum has been found to be satisfactory material of manufacture, primarily because of its low inertia, low cost and ease of manufacture. However, it is contemplated that any number of materials could be used without departing from the spirit of the invention. This arm  10  may be used alone, as a single-stage actuator carrying a single suspension  8  and head  6  for accessing a single side of a disc  4 . Alternatively, an arm similar to the one shown in FIG. 2 may be produced without the coil support portion  15 . In this instance, a number of arms may be vertically aligned and “stacked” atop one another. A single coil support portion  15  is then provided, and an overmold is provided uniting the arms  10  and coil support  15  together in a single unit. Commonly known as a stacked actuator, this type of actuator may be provided with a large number of suspensions  8  and heads  6  (typically two heads  6  to each arm  10 ) for accessing a larger number of discs  4 .  
         [0021]    Where a stamped, monolithic actuator arm  10  such as the one illustrated in FIG. 2 is used, the bending resistance of the actuator arm in the lateral Y-axis direction (i.e. the off-track direction) is often found to be undesirably low. This is due at least in part to a lack of material, especially about the circular hole  22  formed to receive a disc drive&#39;s pivot assembly. It is believed that the low bending resistance in the Y-axis direction is also a contributing factor to the actuator having a low butterfly mode resonance frequency. Bending resistance in the Z direction, transverse to the surface of the disc  4 , is also a problem with actuator arms, particularly with a stamped arm such as the one illustrated in FIG. 2. Again, pivot hole  22  is one cause of this. Bending in this way is commonly called the “first bending mode.” 
         [0022]    An actuator arm  10  according to a first embodiment of the present invention is shown in FIG. 3. It will be noted that the actuator has been formed with raised stiffening members in the form of rails  24  and  25  located along opposite edges of the upper surface  26  of the actuator arm and adjacent pivot assembly hole  22 . The inventors have found that addition of the rails increases the elastic section modulus, S, of the actuator since:  
         
       S=I/c  
     
         [0023]    where I is the second moment of inertia of the area, and c is the centroid of the cross section.  
         [0024]    Provision of the rails thus confers a resistance to lateral and transverse, vertical bending without substantially adding to the weight of the actuator which would be undesirable.  
         [0025]    [0025]FIG. 4 depicts a further embodiment of the present invention wherein rails  24 ,  25  and  27  (a fourth rail protruding from the underside  28  beneath rail  25  is not visible) are located both on the upper surface  26  and lower surface  28  of the actuator. The addition of the extra rails on the underside of the actuator arm further increases the arm&#39;s resistance to lateral bending and so increases the mechanical resonance frequency of the arm.  
         [0026]    Yet another embodiment of the present invention is illustrated in FIG. 6. The embodiment of FIG. 6 is formed by firstly stamping out an actuator of the shape shown in FIG. 5 with lateral wings  30  and  32  in a single piece and then folding or bending up the wings to form stiffening members in the form of rails  34  and  36  of FIG. 6. One of ordinary skill in the art would recognize that the stamping and bending steps may be performed in a single step by appropriately configuring the die used to perform the stamping operation.  
         [0027]    In each of the embodiments shown in FIGS. 3, 4 and  6  the presence of the rails increases the arms&#39; stiffness and thereby significantly increases the butterfly mode resonance frequency. The inventors have tested the embodiment of FIG. 3 with a rail height of 1.5 mm (60 mils) above the upper surface of the arm and found that the resonant frequency is increased by 200 Hz over that of a prior art actuator arm of the type shown in FIG. 1.  
         [0028]    It should be understood that the configuration of the rails  24 , 25 , 26 , 27 , 34 ,  36  may be “tuned” so as to vary the resonance characteristics of an actuator arm  10 . For example, lengthening rails along the longitudinal extent of the arm  10  would increase the stiffness of the arm with respect to lateral bending, and would increase resistance to transverse bending along the extent of the rails. Similarly, changing the height of the rails significantly increases the resistance to transverse bending while also contributing to an increase in resistance to lateral bending. The precise shape of the rails may also be modified so as to tune the stiffness of the actuator arm with respect to its various bending modes and resonant frequencies.  
         [0029]    Accordingly, a hard disc drive incorporating at least one actuator arm according to the present invention may be operated with a larger servo system bandwidth than would be the case if prior art actuator arms were used. Consequently finer control of read/write head position may be achieved and narrower inter-track spacings used.  
         [0030]    It is to be understood that ever though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustratively only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the fill extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example in the embodiments thus far described the rails have been contiguous with the actuator arm along their length, however, the rails might instead be fixed only at each end to the actuator arm. Furthermore, they may be of different cross section to those described. In addition, although the present invention has been described with reference to preferred embodiments, it will be appreciated by those skilled in the art that the teaching of the present invention can be applied to other systems without departing from the scope and spirit of the present invention.