Abstract:
A disc drive system includes a recording disc rotatable about an axis, a slider supporting a transducing head for transducing data with the disc, and a dual-stage actuation assembly supporting the slider to position the transducing head adjacent a selected radial track of the disc. The dual-stage actuation assembly includes a support structure supporting the slider in proximity to a surface of the disc, a microactuator and a capacitive position sensor. The support structure is coarsely positionable by a main actuator. The microactuator includes a stator attached to the support structure and a movable rotor operatively attached to the slider, the rotor being connected to the stator by at least one flexible beam. The capacitive position sensor connects the stator to the rotor, and provides a relative position signal representing a state of displacement of the microactuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Provisional Application No. 60/146,244 filed Jul. 28, 1999 for “Magnetic Microactuator With Capacitive Position Sensing” by W. Bonin, P. Crane and Z. Boutaghou. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a disc drive microactuator, and more particularly to a microactuator employing a capacitive position sensor to indicate the relative position of the microactuator rotor with respect to the stator during operation of the microactuator. 
     The density of concentric data tracks on magnetic discs continues to increase (that is, the size of data tracks and radial spacing between data tracks are decreasing), requiring more precise radial positioning of the head. Conventionally, head positioning is accomplished by operating an actuator arm with a large-scale actuation motor, such as a voice coil motor, to radially position a head on a flexure at the end of the actuator arm. The large-scale motor lacks sufficient resolution to effectively accommodate high track-density discs. Thus, a high resolution head positioning mechanism, or microactuator, is necessary to accommodate the more densely spaced tracks. 
     One promising approach for high resolution head positioning involves employing a high resolution microactuator in addition to the conventional lower resolution actuator motor, thereby effecting head positioning through dual-stage actuation. Various microactuator designs have been considered to accomplish high resolution head positioning. One successful microactuator design is disclosed in copending U.S. application Ser. No. 09/315,006, entitled “Magnetic Microactuator,” filed May 19, 1999 by P. Crane, W. Bonin and B. Zhang, which is hereby incorporated by reference. 
     In addition to the advances in the movement mechanisms of microactuators, it is also desirable to provide an apparatus to indicate a relative position of the microactuator rotor with respect to the stator. Such an apparatus is provided by the present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a disc drive system employing a microactuator and a capacitive position sensor to provide information relating to the relative position of the microactuator. The disc drive system includes a recording disc rotatable about an axis, a slider supporting a transducing head for transducing data with the disc, and a dual-stage actuation assembly supporting the slider to position the transducing head adjacent a selected radial track of the disc. The dual stage actuation assembly includes a support structure supporting the slider in proximity to a surface of the disc, the support structure being coarsely positionable by a main actuator. A microactuator is also provided as part of the dual-stage actuation assembly, including a stator attached to the support structure and a movable rotor operatively attached to the slider, the rotor being connected to the stator by at least one flexible beam. A capacitive position sensor is employed connecting the stator to the rotor, the capacitive position sensor providing a relative position signal representing a state of displacement of the microactuator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a disc drive actuation system for positioning a slider over tracks of a disc. 
     FIG. 2 is an exploded perspective view of a portion of a disc drive including a microactuator employing a capacitive position sensor according to the present invention. 
     FIG. 3 is a perspective view of a microactuator system employing a capacitive position sensor according to the present invention for positioning and sensing the relative position of a slider over the tracks of a disc. 
     FIG. 4 is an enlarged perspective view of a portion of the microactuator system employing the capacitive position sensor according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a plan view of a disc drive actuation system,  10  for positioning slider  24  over a track  29  of disc  27 . Actuation system  10  includes voice coil motor (VCM)  12  arranged to rotate actuator arm  16  on a spindle around axis  14 . Head suspension  18  is connected to actuator arm  16  at head mounting block  20 . Flexure  22  is connected to an end of head suspension  18 , and carries slider  24 . Slider  24  carries a transducing head (not shown in FIG. 1) for reading and/or writing data on concentric tracks of disc  27 . Disc  27  rotates around axis  28 , so that windage is encountered by slider  24  to keep it aloft a small distance above the surface of disc  27 . 
     VCM  12  is selectively operated to move actuator arm  16  around axis  14 , thereby moving slider  24  between tracks  29  of disc  27 . However, for disc drive systems with high track density, VCM  12  lacks sufficient resolution and frequency response to position a transducing head on slider  24  precisely over a selected track  29  of disc  27 . Therefore, a higher resolution actuation device is necessary. 
     FIG. 2 is an exploded perspective view of a portion of a disc drive including microactuator  30  according to the present invention. Flexure  22  is attached to load beam  18 , and microactuator  30  is attached to flexure  22  to carry slider  24  above a surface of disc  27  (FIG.  1 ). A transducing head (not shown) is carried by slider  24  to transduce data with the disc. Capacitive position sensor  32  is provided on microactuator  30  to enable a relative position status of microactuator  30  to be determined. 
     In operation of the disc drive, load beam  18 , flexure  22  and microactuator  30  carrying slider  24  are all moved together as coarse positioning is performed by VCM  12  (FIG. 1) moving actuator arm  16  (FIG.  1 ). To achieve fine positioning of the transducing head, microactuator  30  generates a force which causes bending of the beam springs of the microactuator. As a result, the portion of microactuator  30  carrying slider  24  moves slightly with respect to flexure  22  in the direction of arrows  31 , displacing the transducing head carried by slider  24  with high resolution for precise positioning over a selected track of the disc. A relative position signal is provided by capacitive sensor  32  to indicate the state of displacement of microactuator  30  during operation thereof. 
     FIG. 3 is a perspective view of microactuator  30  according to the present invention. Microactuator  30  includes outer frame  38  serving as the microactuator stator, and inner frame  40  serving as the microactuator rotor. Inner frame  40  is connected to outer frame  38  by beams  33  and  35 , which are deformable in response to lateral force applied by lateral movement of magnets  62 ,  64 ,  66  and  68  to alter the position of inner frame  40  (and thus slider  24 ) with respect to fixed outer frame  38 . A tub  69  having a bottom surface lined with a ferromagnetic keeper material, such as cobalt-iron (CoFe) in one embodiment, is formed in the substrate of microactuator  30  opposite the transducing head carried at the trailing edge of slider  24 . Magnets  62 ,  64 ,  66  and  68  are located in tub  69 , attached to the ferromagnetic lining on the bottom surface of tub  69 . In an exemplary embodiment, magnets  62 ,  64 ,  66  and  68  are composed of samarium-cobalt (SmCo) or a similar magnetic material. An embodiment employing only two microactuator magnets may also be used. Standoffs  54 ,  56 ,  58  and  60  are formed on respective standoff bases  44 ,  46 ,  48  and  50  on outer microactuator frame  38 , to be borne upon so as to apply pre-load force to microactuator  30  as it supports slider  24  over the surface of the disc. The configuration of magnets  62 ,  64 ,  66  and  68 , the ferromagnetic lining of tub  69  and a coil and top keeper provided on the overhanging flexure (such as flexure  22 , FIG. 2) creates a vertical magnetic circuit that is operable to cause lateral movement of magnets  62 ,  64 ,  66  and  68  and therefore move microactuator inner frame  40  with respect to outer frame  38 . The details of the movement generated by microactuator  30  are discussed in the aforementioned U.S. application Ser. No. 09/315,006, which has been incorporated herein by reference. 
     Capacitive position sensor  32  is provided between magnets  62 ,  64 ,  66  and  68  and the leading edge of microactuator  30  to generate a signal representative of the relative movement of magnets  62 ,  64 ,  66  and  68  and inner frame  40  with respect to outer frame  38  of microactuator  30 . Bond pads  80  and  82  are provided on outer frame  38 , and conductive traces  84  and  86  are arranged to electrically connect alternate electrode panels of capacitive position sensor  32  that are connected to outer frame  38  to respective bond pads  80  and  82 . Bond pad  90  is provided on outer frame  38 , and conductive trace  92  is arranged to traverse flexible beams  33  to electrically contact the electrode panels of capacitive position sensor  32  that are connected to inner frame  40 . Bond pads  80 ,  82  and  90  are operatively connected to sense/control circuitry  94  in a manner known in the art, such as by an overhanging flex circuit in one embodiment. Sense/control circuitry  94  is operable to correlate the changing capacitance of capacitive position sensor  32  to a relative position of microactuator  30 . The details of the construction and operation of capacitive position sensor  32  are discussed below with respect to FIG.  4 . 
     FIG. 4 is an enlarged perspective view of a portion of microactuator  30  employing capacitive position sensor  32  according to the present invention. In the drawing of FIG. 4, some of the microactuator magnets are removed and a portion of inner microactuator frame  40  is cut away to more clearly depict the configuration of capacitive position sensor  32 . Sensor  32  includes electrode panels  72  attached to outer microactuator frame  38  interdigitated with electrode panels  74  attached to inner microactuator frame  40 . When microactuator  30  is operated to laterally move inner frame  40  with respect to outer frame  38 , the configuration of the gaps between electrode panels  72  and electrode panels  74  is altered. Patterned insulator  76  is provided in electrode panels  72  and  74  in such a manner that divides each of electrode panels  72  into two portions. The portions of electrode panels  72  on opposite sides of each of the interdigitated electrode panels  74  are connected to respective bond pads  80  and  82 . A differential voltage is applied between electrode panels  74  and the portions of electrode panels  72 . As the dimension of the gap between electrode panels  72  and electrode panels  74  changes due to movement of microactuator inner frame  40  with respect to outer frame  38 , the capacitance associated with the electrode panels also changes, according to the following formula:        C   =       ɛ                 A     d                            
     where C is the capacitance between the electrode panels, ε is the permittivity of the gap region between the electrode panels, A is the area of the electrode panels, and d is the gap distance between the electrode panels. The capacitance between electrode panels  72  and  74  can be measured by conventional sense/control circuitry  94  known in the art, and capacitance values can be correlated with states of movement of microactuator  30  in such a manner that capacitive position sensor  32  provides a signal representative of the relative position of microactuator inner frame  40  with respect to outer frame  38 . 
     Since the capacitance between electrode panels  72  and  74  is directly proportional to the area of the electrode panels, a larger electrode panel area will provide larger values of capacitance. Large electrode panel areas also provide a lower source impedance to the sense amplifier connected to capacitive position sensor  32 . Capacitive position sensor  32  therefore must be designed so that electrode panels  72  and  74  have a sufficient area to provide a signal to noise ratio (SNR) greater than a predetermined threshold for the sense amplifier and capacitance measuring circuit connected thereto. In an exemplary embodiment, electrode panels  72  and  74  have a length of about 500 micro-meters (μm) and a height (equal to the microactuator wafer thickness) of about 200 μm, giving an area of about 100,000 μm 2 . With a nominal gap of 20 μm between electrode panels  72  and  74 , the sense capacitance is about 1.0 pico-Farads (pF). This value of capacitance, as well as the changes in capacitance due to gap changes on the order of a few micrometers or less, are large enough that the capacitance sensing circuitry may be located at least several inches from electrode panels  72  and  74 , while still achieving a sufficient SNR for effective operation. With such a configuration, a single remotely placed chip could be used to drive all of the capacitive position sensors on a multiple head stack disc drive. In another embodiment, multiple drive circuit chips may be located directly at each capacitive position sensor  32  in the head stack, which would improve the SNR and enable a reduction in the length of electrode panels  72  and  74  to about 50 μm or less. 
     In addition to providing a signal representative of the relative movement of microactuator  30 , the interdigitated electrode configuration of capacitive position sensor  32  also acts as a viscous damper to reduce the amplitude of resonant vibrations in the microactuator structure. The damping effect of electrode panels  72  and  74  can be particularly beneficial for high frequency resonant vibrations beyond the servo bandwidth of the positioning system. For resonant vibration at a frequency lower than the servo bandwidth, active damping may be performed by initiating microactuator movement to cancel out the resonant vibrations sensed by capacitive position sensor  32 . 
     The present invention has been described above with capacitive position sensor  32  located toward the leading edge of microactuator  30 . However, it should be understood that capacitive position sensor  32  may also be located between microactuator inner frame  40  and outer frame  38  at the trailing edge of microactuator  30  as well. In some arrangements, locating electrode panels  72  and  74  of capacitive position sensor  32  at the trailing edge of microactuator  30  serves to improve the vibration damping effects of the device. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.