Abstract:
A microelectromechanical (MEM) actuator incorporates a dual-stator design operated electrostatically in conjunction with a rotor to affect a fine positioning of a thin film magnetic read/write head. The substantial gain in the frequency response bandwidth greatly improves the performance and accuracy of the track-follow control for fine positioning of the thin film read/write head. The stators are comprised of a plurality of electrodes arranged perpendicularly along a plurality of stationary radial spokes, which are interleaving and oppositely disposed to the same plurality of moveable radial spokes formed on the rotor. A track-follow control commands a voltage to be supplied to the rotor and stators, thereby inducing an electrical potential field to generate an electrostatically attractive force for each pair of radial spokes between each of the stator and the rotor. These circumferentially acting forces result in a torque that causes the rotor on which the slider is mounted to displace in a pure clockwise or counterclockwise rotation about the center of the flexure.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to data storage systems such as disk drives, and it particularly relates to a thin film read/write head for use in such data storage systems. More specifically, the present invention provides a new microelectromechanical (MEM) actuator incorporating a dual-stator design operated electrostatically in conjunction with a rotor to affect a fine positioning of the thin film magnetic read/write head. The substantial gain in the frequency response bandwidth greatly improves the performance and accuracy of the track-follow control for fine positioning of the thin film read/write head. 
     BACKGROUND OF THE INVENTION 
     In a conventional magnetic storage system, a thin film magnetic read/write head includes an inductive read/write transducer mounted on a slider. The read/write head is coupled to a rotary actuator magnet and a voice coil assembly by a suspension and an actuator arm positioned over a surface of a spinning magnetic disk. In operation, a lift force is generated by the aerodynamic interaction between the read/write head and the spinning magnetic disk. The lift force is opposed by equal and opposite spring forces applied by the suspension such that a predetermined flying height is maintained over a full radial stroke of the rotary actuator assembly above the surface of the spinning magnetic disk. The flying height is defined as the spacing between the surface of the spinning magnetic disk and the lowest point of the slider assembly. One objective of the design of magnetic read/write heads is to obtain a very small flying height between the read/write element and the disk surface. By maintaining a flying height close to the magnetic disk, it is possible to record short wavelength or high frequency signals, thereby achieving high density and high storage data recording capacity. 
     The slider of the read/write head incorporates an air bearing surface to control the aerodynamic interaction between the read/write head and the spinning magnetic disk thereunder. Air bearing surface (ABS) sliders used in disk drives typically have a leading edge and a trailing edge at which thin film read/write transducers are deposited. Generally, the ABS surface of a slider incorporates a patterned topology by design to achieve a desired pressure distribution during flying. In effect, the pressure distribution on the ABS contributes to the flying characteristics of the slider that include flying height, pitch, and roll of the read/write head relative to the rotating magnetic disk. 
     In a conventional magnetic media application, a magnetic recording disk is comprised of several concentric tracks onto which magnetized bits are deposited for data recording. Each of these tracks are further divided into sectors wherein the digital data are registered. As the demand for large capacity magnetic storage continues to grow at an ever increasing pace, the current trend in the magnetic storage technology has been proceeding toward a high track density design of magnetic storage media. In order to maintain the industry standard interface, magnetic storage devices increasingly rely on reducing track width as a means to increase the track density without significantly altering the geometry of the storage media. 
     As the track width becomes smaller, this poses several mechanical and electrical problems to the operation of magnetic disk drives. One such problem lies in its actuation and control feature, which is critical to the operation of a magnetic disk drive. In order to appreciate the magnitude of this problem, it is necessary to further describe the control aspect of a typical magnetic read/write head. In a conventional magnetic disk drive, a read/write head includes a transducer mounted on a slider. The slider in turn is attached to a stainless steel flexure. The flexure and the load beam to which the flexure is attached together form a suspension arm. The suspension arm is then connected to an actuator arm, which is driven by a voice coil motor (VCM) to cause it to rotate at its mid-length about a pivot bearing. The suspension arm exerts an elastic force to counteract the aerodynamic lift force generated by the pressure distribution on the ABS of the slider. The elastic force together with the stiffness of the suspension arm controls the stability of the actuator arm with respect to the pitch, roll, and yaw orientations. With respect to the control feature of the magnetic disk drive, during each read or write operation, there are usually two types of positioning controls: a track-seek control and a track-follow control. 
     A track-seek control is typically commanded when data are to be retrieved from or new data are to be written to a particular sector of a data track. Electronic circuitry incorporating an embedded feedback control logic supplies a necessary voltage to the VCM to actuate it to drive the actuator arm, onto which the read/write head is attached, to a target track. Thus, a track-seek control performs a low-resolution or coarse positioning of the read/write head from one data track to another data track. 
     Upon the completion of a track-seek control, subsequent data operation is typically confined to within the target track. In the earlier stage of the magnetic storage technology, a typical data track is sufficiently wide so that small variations in the position of the read/write head resulting from external disturbances to the track-seek control plant do not cause the position of the read/write head to exceed the prescribed control error allowance. Therefore, no further control implementation in addition to the track-seek control is necessary. This type of control implementation is usually referred to as a single-stage actuation, which incorporates the VCM in the feedback loop to effect a total positioning of the read/write head. 
     As the track width reduces as a means to increase the track density and hence the storage capacity of magnetic disk drives, the foregoing single-stage actuation encounters a significant degree of difficulty, chiefly due to the excessive control error of the track-seek control using the VCM in the loop. In particular, a single-stage actuation using the VCM for low-resolution positioning of the read/write head is found to be inadequate because the resulting control error due to external disturbances such as inertial shock loading or noise sometimes could cause the read/write head to unintentionally position over the adjacent tracks, thus possibly causing a magnetic field disturbance of the existing data thereon. In the worst case, the data disturbances can result in a total erasure of data in the adjacent tracks after several repetitive write operations, or data corruption upon reading. 
     Moreover, the VCM employed in a single-stage actuation is typically subjected to a mechanical resonance at a low natural frequency in the range of 2000-3000 Hz due to the flexibility of the suspension arm assembly. The response of the servo-system further limits the frequency bandwidth to about 1500 Hz. As a result, this low frequency bandwidth imposes a severe penalty on the frequency response of the single-stage actuation system in such a manner that the track-seek control is unable to rapidly respond to a change in the position of the read/write head, thus causing a significant degradation in the performance of the magnetic disk drive. 
     To address this technical difficulty, it is recognized that in order to maintain the position of the read/write head in a manner that it follows a concentric path within a narrow track width of a target data track, necessary corrections to the motion of the actuator arm are required. This provision is made possible by a track-follow control, which uses a feedback on the track error signal to make an appropriate correction to the motion of the actuator arm so as to maintain the position of the read/write head to follow a concentric path of the target data track within a prescribed control error allowance. Thus, in the presence of external disturbances, variations in the position of the read/write head would not cause the position of the read/write head to significantly deviate from the target position in excess of the control error allowance. 
     To implement this track-follow control plant, a microactuator is frequently incorporated in the control feedback loop. Various types of microactuator have been proposed, including piezoelectric (PZT) actuators, electrostatic microelectromechanical systems (MEMS), and electromagnetic MEMS. By adjusting the voltage supplied to the microactuator, the track-follow control makes necessary corrections to the position of the actuator arm in the presence of external disturbances so that the read/write head is maintained to follow the target data track with a reasonable degree of precision. The implementation of a new, separate track-follow control for high-resolution positioning in addition to the usual track-seek control for low-resolution positioning as in the single-stage actuation is typically referred to as a dual-stage actuation, which constitutes the predominant control system employed in high capacity magnetic disk drives. 
     There currently exist a number of different microactuator designs in use for high-resolution positioning of read/write heads in high capacity magnetic disk drives. One such widely used conventional microactuator design is based on an electrostatically coupled rotor-stator concept. In principle, the rotor of the example conventional microactuator is physically connected to the outer stator by means of a plurality of elastic springs. 
     Moreover, the rotor is also electrostatically coupled to the outer stator by means of a plurality of radial spokes extending outwardly from the rotor and interleaving with the same plurality of similarly featured radial spokes extending inwardly from the stator. When a track-follow control plant commands a voltage to be sent to the example conventional microactuator in order to make a necessary correction to the motion of the actuator arm, an electrostatic potential field is induced within the radial spokes of the rotor and stator. By controlling the voltage polarity, an electrostatically attractive force can be generated between each pair of radial spokes between the rotor and stator. These circumferentially acting forces effectively result in a torque that causes a rotational motion of the rotor on which the slider containing the read/write head is mounted. 
     Notwithstanding the ability to achieve the track-follow control objective, the conventional electrostatically coupled rotor-stator microactuator design suffers a number of shortcomings that may offset the advantages it offers. These shortcomings may manifest into a number of problems as follows: 
     Because of the elastic spring connection of rotor to the stator and the mass of the rotor itself, the conventional microactuator of electrostatically coupled rotor-stator design possesses some natural frequencies of vibration. In particular, these natural frequencies are of low values because of the relatively substantial length of the elastic springs connecting the rotor to the stator. These low natural frequencies of vibration can easily be excited by an inertial force due to a sudden motion as commanded by the track-follow control, thereby rendering the microactuator susceptible to shock and vibration. Thus, in order to minimize this susceptibility to shock and vibration, the track-follow control may have to command a more gradual motion to reduce the inertial force loading. In so doing, the track-follow control performance may be degraded because of the reduced speed of actuation. 
     Furthermore, as the conventional microactuator of electrostatically coupled rotor-stator design is subjected to a typical excitation force during operation, the ensuing vibration of the rotor connected to the elastic springs manifests into an uneven oscillating forces at the spring supports on the outer stator. These forces may act transversely to the axis of the suspension arm assembly to which the stator is attached and present themselves as potential excitation forces to the suspension arm assembly. If these forces are sufficiently large and possess the excitation frequency near the natural frequency of the suspension arm, a resonant vibration of the suspension arm assembly would ensue, thereby causing an undesirable disturbance problem for the track-follow control system. 
     Yet another problem associated with the conventional electrostatically coupled rotor-stator microactuator design lies in its inherent non-linearity. The non-linearity is a manifestation of the force dependence on the voltage and the inverse dependence of the electrostatic force on the gap/engagement between the fingers. As in most physical systems, linearity is a highly desired virtue since it greatly simplifies the electromechanical conversion process. Furthermore, most modern control logics are built upon the premise of linearity in the system to be controlled. Hence, the non-linearity in the conventional electrostatically coupled rotor-stator microactuator design adds a significant degree of complexity in the operation and control of the conventional microactuator. 
     In light of the foregoing shortcomings with the conventional microactuator of electrostatically coupled rotor-stator design, it is recognized that a further enhancement in the microactuator design for a fine positioning of the read/write head is needed. Preferably, the enhanced microactuator would have a greater frequency response than the convention microactuator without adversely affecting the load beam vibration mode of the suspension arm assembly. Moreover, the enhanced microactuator design should have an enhanced linearity in order to reduce the complexity of the mechanical actuation. 
     SUMMARY OF THE INVENTION 
     In order to solve the foregoing difficulties, it is a feature of the present invention to provide a new enhanced microelectromechanical (MEM) actuator design for fine positioning of the read/write head during a track-follow control operation. The enhanced microactuator according to the present invention is designed to be used in a collocated dual-stage actuation servo system that substantially boosts the servo frequency bandwidth to enhance the track-seek and track-follow controls for an extremely high capacity magnetic storage devices. The present invention features an electrostatic MEM microactuator that is designed to rotate in response to a track-follow control voltage command. 
     According to a preferred embodiment, the electrostatic MEM microactuator of the present invention is comprised of a rotor electrostatically coupled to two stators arranged in an alternating manner. The stators are further comprised of a plurality of electrodes arranged perpendicularly along a plurality of stationary radial spokes, which are interleaving and oppositely disposed to the same plurality of moveable radial spokes formed on the rotor. 
     A track-follow control commands a voltage to be supplied to the rotor and stators, thereby inducing an electrical potential field to generate an electrostatically attractive force for each pair of radial spokes between each of the stator and the rotor. These circumferentially acting forces result in a torque that causes the rotor on which the slider is mounted to displace in a pure clockwise or counterclockwise rotation about the center of the flexure. The enhanced microactuator design of the present invention demonstrates the following improvements: 
     1. The use of two stators alternating with a rotor results in a greater elastic stiffness of the microactuator of the present invention due to the shorter spring length, thus substantially raising the natural frequencies of rotor. The increased natural frequencies translates into an improved frequency response of the microactuator of the present invention, thus enabling it to rapidly achieve a precise positioning of the read/write head; 
     2. The increased stiffness of the elastic connection of the rotor to the stators greatly diminishes the susceptibility of the microactuator of the present invention to shock and vibration, and further reduces excitation forces to the suspension arm assembly in the load beam modes; 
     3. Dividing the motor into quadrants to act in a pull hard—pull easy mechanism could enhance the motor linearity. In the current design, the doubling of the number of quadrants as a result of the circular symmetry of the microactuator of the preferred embodiment as compared to the conventional microactuator effectively enhances the linearity of the motor, thus reducing the complexity of the operation and control thereof. 
     4. The enhanced microactuator design of the preferred embodiment preserves the industry standard interface with a conventional suspension arm assembly without the necessity for a modification thereof, thus, rendering the performance of the enhance microactuator of the present invention virtually unaffected by various different form factors of the slider/suspension assembly such as Pico and Femto form factors. 
     A number of alternative embodiments are derived from the aforementioned novelties of the present invention. One such alternative embodiment includes a different physical arrangement of electrodes on non-equidistant radial spokes extending from the two stators with the outer stator closer to the rotor than the inner stator, thus resulting in combined electrostatically attractive forces between the radial spokes of the rotor and stators, which lead to a pull hard—pull easy arrangement instead on a pull only mechanism. As a result, the linearity of this microactuator could be further enhanced. 
     Another alternative embodiment of the present invention pertains to a microactuator employing a plurality of rotors and stators positioned relative to one another in an alternating arrangement. Thus, the resultant torque that causes a rotation of the slider is created by a combination of a series of forces acting between the rotors and stators. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention and the manner of attaining them, will become apparent, and the invention itself will be understood by reference to the following description and the accompanying drawings, wherein: 
     FIG. 1 is a fragmentary perspective view of a data storage system including the head gimbal assembly, made according to a preferred embodiment of the present invention; 
     FIG. 2 is a perspective top view of the head gimbal assembly of FIG. 1 comprised of a suspension, a slider, and a microactuator, made according to the present invention; 
     FIG. 3 is a cross-sectional view of the suspension/slider assembly of FIG. 2 showing a suspension, a slider mounted on a plurality of mounting pads, which are attached to the microactuator, made according to a preferred embodiment of the present invention; 
     FIG. 4 is a bottom view of a conventional microactuator comprised of a stator and a rotor; 
     FIG. 5 is a bottom view of the microactuator of FIG. 2 comprised of two stators and a rotor positioned midway between the two stators and electrostatically coupled by radial spokes of equal lengths having a plurality of eclectrostatically charged comb-like fingers, made according to a preferred embodiment of the present invention; 
     FIG. 6 is a bottom view of the head gimbal assembly of FIG. 2 before and after a track-follow control actuation; 
     FIG. 7 illustrates an electrical diagram of the radial spokes of the microactuator of FIG. 5, made according to a preferred embodiment of the present invention; 
     FIG. 8 is a diagram illustrating electrostatic transverse forces engaged between the comb-like fingers attached to the radial spokes of the microactuator of FIG. 5, made according to a preferred embodiment of the present invention; 
     FIG. 9 illustrates an electrostatic force concept of the microactuator of FIG. 5, made according to a preferred embodiment of the present invention; 
     FIG. 10 is a bottom view of the microactuator of FIG. 5, shown with force diagram during a track-follow control actuator; 
     FIG. 11 is a bottom view of the slider of FIG. 2 after a track-follow control actuation; 
     FIG. 12 is a schematic diagram of a typical manufacturing sequence of the microactuator of FIG. 5, made according to a preferred embodiment of the present invention; 
     FIG. 13 is a bottom view of a microactuator of FIG. 2 comprised of two stators and a rotor positioned in a close proximity to the outer stator and electrostatically coupled of radial spokes of unequal lengths having a plurality of electrostatically charged comb-like fingers, made according to an alternative embodiment of the present invention; 
     FIG. 14 illustrates an electrical diagram of the radial spokes of the microactuator of FIG. 13, made according to an alternative embodiment of the present invention; 
     FIG. 15 is a diagram illustrating electrostatic transverse forces engaged between the comb-like fingers attached to the radial spokes of the microactuator of FIG. 13, made according to an alternative embodiment of the present invention; 
     FIG. 16 is a schematic diagram of a typical manufacturing sequence of the microactuator of FIG. 13, made according to an alternative embodiment of the present invention; and 
     FIG. 17 is a bottom view of a microactuator of FIG. 2 comprised of a plurality of stators alternating with a plurality of rotors electrostatically coupled of radial spokes of equal lengths having a plurality of electrostatically charged comb-like fingers, made according to another alternative embodiment of the present invention. 
    
    
     Similar numerals in the drawings refer to similar elements. It should be understood that the sizes of the different components in the figures might not be in exact proportion, and are shown for visual clarity and for the purpose of explanation. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a disk drive  10  comprised of a head stack assembly  12  and a stack of spaced apart smooth media magnetic data storage disks or smooth media  14  that are rotatable about a common shaft  15 . The head stack assembly  12  is rotatable about an actuator axis  16  in the direction of the arrow C. The head stack assembly  12  includes a number of actuator arms, only three of which  18 A,  18 B,  18 C are illustrated, which extend into spacings between the disks  14 . 
     The head stack assembly  12  further includes an E-shaped block  19  and a magnetic rotor or voice coil motor (VCM)  20  attached to the block  19  in a position diametrically opposite to the actuator arms  18 A,  18 B,  18 C. The VCM  20  cooperates with a stator (not shown) for rotating in an arc about the actuator axis  16 . Energizing a coil of the rotor  20  with a direct current in one polarity or the reverse polarity causes the head stack assembly  12 , including the actuator arms  18 A,  18 B,  18 C, to rotate about the actuator axis  16  in a direction substantially radial to the disks  14 . The actuator arms  18 A,  18 B,  18 C are identical in design and geometry. Therefore, only one of these actuator arms,  18 A, is further referenced herein, with the understanding that this reference also applies to the plurality of the actuator arms  18 A,  18 B,  18 C. 
     According to a preferred embodiment of the present invention, a head gimbal assembly (HGA)  28  is secured to each of the actuator arms, for instance  18 A. With reference to FIG. 2, the HGA  28  is comprised of a suspension  33 , a microactuator  60  of the present invention, and a read/write head  35 . The suspension  33  includes a resilient load beam  36  and a flexure  40 . With further reference to FIG. 3, the microactuator  60  of the present invention is bonded to the flexure  40  on its base  68 , and supports a read/write head  35  via a protective cap  104  attached on its underside. The read/write head  35  is formed of a slider  47  secured to the microactuator  60  of the present invention, and a read/write magnetic sensor/transducer (or element)  50  supported by the slider  47 . The read/write element  50  is mounted at the trailing edge  55  of the slider  47  so that its forwardmost tip is generally flush with the air bearing surface (ABS)  65  of the slider  47 . 
     In order to appreciate the novelty and advantages of the present invention, it is necessary to describe a conventional microactuator of a prior art so as to provide a clear, distinct contrast with the microactuator  60  of the present invention. In connection with FIG. 4, a conventional microactuator  360  of a prior art is comprised of an outer stator  362  and an inner rotor  364 . 
     The outer stator  362  is generally formed of a circular ring and further is positioned stationary with respect to the head gimbal assembly  28 . Extending radially inward from the inner wall of the stator  362  are a plurality of stator radial spokes  366 . Each of the stator radial spokes  366  is further comprised of a plurality of electrostatically charged stator electrodes  368  mounted transversely at equidistant on the stator radial spokes  366 . These electrostatic charge carrying stator electrodes  368  form an array that resembles a comb, henceforth will be referred herein to as comb-like stator fingers  368 . 
     The inner rotor  364  is formed of a circular mass onto which the slider  47  is mounted and is rotatable with respect to the head gimbal assembly  28 . Extending radially outward from the rotor  364  is a plurality of rotor radial spokes  370  interleaving with the stator radial spokes  366 . Each of the rotor radial spokes  370  is further comprised of a plurality of electrostatically charged rotor comb-like fingers  372  mounted transversely at equidistant on the rotor radial spokes  370  and alternating with the comb-like stator fingers  368 . The inner rotor  364  is elastically attached to the inner wall of the outer stator  362  via a plurality of elastic springs  374 . 
     The problems and concerns associated with the conventional microactuator  360  of the prior art are now described. Among the disadvantages of the conventional microactuator  360  of the prior art is due to the elastic springs  374  that connect the inner rotor  364  to the inner wall of the outer stator  362 . Because of the stiffness of the elastic springs  374  and the mass of the inner rotor  364 , there exist a plurality of natural frequencies of vibration associated with the inner rotor  364 . 
     Furthermore, it is a well-known fact that the extensional stiffness of an elastic spring is inversely proportional to the length of the elastic spring, while the flexural stiffness varies inversely with the cubic power of the length. Since the length of the elastic springs  374  is of a substantial measure, the stiffness associated thereof is expected to be low. Hence, the natural frequencies of the inner rotor  364  are also low accordingly. The low natural frequencies of the rotor  364  pose several problems with the operation and control of the conventional microactuator  360  of the prior art. 
     One such problem is the susceptibility of the conventional microactuator  360  of the prior art to shock and vibration which in turn may result in a performance degradation. For instance, if the disk drive  10  or the head gimbal assembly  28  is subjected to a shock or harmonic load, this shock or harmonic load would excite one or more natural modes of vibration of the inner rotor  364 , causing it to oscillate together with the slider  47  mounted to it. This oscillation consequently may cause the track-follow control plant to improperly execute its error correction to maintain the desired position of the read/write element  50  within a target track. 
     In the worst case, this oscillation may subsequently cause the read/write element  50  to deviate from its intended position on a target data track of the magnetic disk  14  to an adjacent data track, thereby causing an unintended disturbance of the magnetic material therein. This phenomenon is also referred to as side-writing. The side-writing action may, in the worst case, result in an accidental complete erasure of data in the adjacent track. Thus, without data verification and correction in between each write operation, the data quality of a magnetic disk would be significantly compromised. Moreover, the oscillation of the inner rotor  364  may also impart a lateral dynamic side force to the flexure  40  of the suspension  33 , thereby causing a vibration of the suspension  33  in a flexural load beam mode. This flexure load beam mode vibration may also lead to the same consequence of the foregoing side-writing over a larger range of adjacent data tracks. 
     Furthermore, because of the relatively low natural frequencies of the inner rotor  364 , the track-follow control system would have to be designed accordingly so as not to impart an undesirable shock load to the inner rotor  364 . In so doing, the track-follow control system would have to command a gradual voltage change to effect the position of the slider  47 , hence also inner rotor  364 . This, in effect, slows down the frequency response and the ability of the track-follow control system to seek a target track, thereby significantly degrading the performance of the disk drive  10  which is typically measured by its seek time. 
     Yet another issue with the conventional microactuator  360  of the prior art is its inherent non-linearity in the actuation forces that cause the rotation of the inner rotor  364 . This non-linearity is directly proportional to the applied voltage and inversely proportional to the separation gap between the outer stator  362  and the inner rotor  364 , which is spanned by the elastic springs  374 . The non-linearity is usually not a desirable feature of any microactuator device, because the associated forces and motions are often described by a complex relationship with the applied voltage, thus introducing an uncertainty in the track-follow control of the conventional microactuator  360  of the prior art. 
     In light of the foregoing problems and issues associated with the conventional microactuator  360  of the prior art, it is recognized that in order to increase the natural frequencies of the inner rotor  364 , the length of the elastic springs  374  would have to decrease. However, in so doing, the size and hence the mass of the inner rotor  364  would also have to increase, thus negating the attempt to increase the natural frequencies by decreasing the length of the elastic springs  374 . A similar argument could be made with respect to the attempt to reduce the non-linearity of the conventional microactuator  360  of the prior art. 
     Thus, in an effort to attain an enhanced microactuator design based on electrostatically coupled stator-rotor concept, it is a novelty and feature of the present invention to introduce the enhanced microactuator  60  of the preferred embodiment that utilizes a novel two-stator design concept which effectively resolves all the foregoing problems and concerns with the conventional microactuator  360  of the prior art by reducing the length of the elastic springs  374  and the separation between the outer stator  362  and the inner rotor  364  without affecting the mass of the inner stator  364 . 
     Referring now to FIG. 5, in accordance with a preferred embodiment of the present invention, the microactuator  60  is comprised of an outer stator  62 , an inner stator  64 , and a rotor  66  positioned midway and concentrically between the outer stator  62  and the inner stator  64 . With further reference to FIG. 3, the outer stator  62  is generally made of a U-shaped plate, with its vertical “legs” forming a circular rim  70  circumscribing the horizontal circular base  68 . The base of the outer stator  68  is bonded to the flexure  40  of the suspension  33 , hence is stationary with respect to the head gimbal assembly  28 . 
     Extending radially inwardly from the inner wall of the circular rim  70  of the outer stator  62  are a plurality of outer stator radial spokes  72  equally spaced circumferentially except at the 12, 3, 6, and 9 o&#39;clock positions. Further, the outer radial spokes  72  terminate in a close proximity to the outer wall of the rotor  66 . Each of the outer stator radial spokes  72  is further comprised of a plurality of electrostatically charged outer stator fingers  74  mounted transversely and equally spaced on one side of the outer stator radial spokes  72 . With reference to FIG. 5, the electrostatic charge carrying outer stator fingers  74  form an array that resembles a comb pointing in a clockwise direction. 
     With reference to FIGS. 3 and 5, the inner stator  64  is generally formed of a circular disk located at the innermost and fixed to the circular base  68  of the outer stator  62  of the microactuator  60  of the preferred embodiment. A plurality of inner stator radial spokes  76  extend radially outward therefrom and are equally spaced circumferentially at the periphery thereof. Further, the inner stator radial spokes  76  terminate in a close proximity to the inner wall of the rotor  66 . 
     The number of inner stator radial spokes  76  is generally the same as the number of outer stator radial spokes  72 , with the understanding that these numbers could also be made different without deviating from the intent of the present invention. Each of the inner stator radial spokes  76  is further comprised of a plurality of electrostatically charged inner stator fingers  78  mounted transversely and equally spaced on one side of the inner stator radial spokes  76 . The electrostatic charge carrying inner stator fingers  78  form an array that resembles a comb pointing in a clockwise direction. 
     With reference to FIGS. 3 and 5, the rotor  66  is generally formed of a circular ring positioned midway and concentrically between the outer stator  62  and inner stator  64 . The rotor  66  is suspended above the circular base  68  and elastically connected to the outer stator  62  by four elastic springs  80  that are attached at one end to the outer wall of the rotor  66  and at the other end to the circular rim  70  of the outer stator  62  at the 12, 6, 3, and 9 o&#39;clock positions. It should be understood that the number of elastic springs  80  can be different from four without departing from the teaching of the present invention. In this manner, the rotor  66  can be made to rotate with respect to the head gimbal assembly  28  by deflecting the elastic springs  80 . Attached to the outer wall of the rotor  66  are a plurality of outward rotor radial spokes  82  that extend radially outward and alternate with the outer stator radial spokes  72  in a close proximity. 
     Further, the outward rotor radial spokes  82  terminate in a close proximity to the inner wall of the circular rim  70  of the outer stator  62 . In general, the number of the outward rotor radial spokes  82  is the same as the number of the outer stator radial spokes  72 . Each of the outward rotor radial spokes  82  is further comprised of a plurality of electrostatically charged outward rotor fingers  84  mounted transversely and equally spaced on one side of the outward rotor radial spokes  82 . The electrostatic charge carrying outward rotor fingers  84  form an array that resembles a comb pointing in a counterclockwise direction and alternate in between the outer stator fingers  74 . 
     Similarly, attached to the inner wall of the rotor  66  are a plurality of inward rotor radial spokes  86  that extend radially inward and alternate with the inner stator radial spokes  76  in a close proximity. Further, the inward rotor radial spokes  86  terminate in a close proximity to the inner stator  64 . In general, the number of the inward rotor radial spokes  86  is the same as the number of the inner stator radial spokes  76 . Each of the inward rotor radial spokes  86  is further comprised of a plurality of electrostatically charged inward rotor fingers  88  mounted transversely and equally spaced on one side of the inward rotor radial spokes  86 . The electrostatic charge carrying inward rotor fingers  88  form an array that resembles a comb pointing in a counterclockwise direction and alternate in between the inner stator fingers  78 . 
     Furthermore, with reference to FIG. 3, a protective cap  104  is attached to the rotor  66  on one of its sides, while the other side supports the slider  47 . The protective cap  104  generally is formed of a circular washer having an outer diameter approximately equal to the diameter of the outer stator  62  and an inner diameter inscribed the width of the slider  47  in the transverse direction of the suspension arm  33 . The protective cap  104  is rested upon the rotor  66  and raised above the outer stator rim  70  to enable it and the slider  47  to freely rotate in unison with the rotor  66  during a track-follow control actuation. When viewed from the underside of the slider  47 , the protective cap  104  provides a complete areal coverage of the rotor  66 . This areal coverage provided by the protective cap  104  is designed to prevent micro-contamination. This micro-contamination is due to a large electrostatic potential field existing among the various fingers  74 ,  78 ,  84 , and  88  that tends to promote dust particles to collect therein, thereby potentially causing short-circuit failures and/or mechanical binding of the rotor  66 . 
     In order to appreciate the advantages afforded by the novel features of the preferred embodiment of the present invention, it is necessary to further describe the functionality of the preferred embodiment in connection to FIGS. 6 through 11. 
     During a track-seek control, the actuator arm  18 A is driven by the VCM  20  to provide a coarse positioning of the read/write head  35  by pivoting about a pivot bearing  21  about the axis  16 . Upon arriving at a desired target data track, the track-follow control takes over the primary function of the magnetic disk drive  10  by maintaining the read/write head  35  to follow the target data track in a high resolution mode, which is also known as fine positioning. 
     An active control system is deployed by means of an embedded logic to enable the track-follow control function. The track-follow control system senses the deviation in the position of the read/write head  35  relative to the track position  26 . A correction is made to reduce this deviation by the track-follow control, which commands a necessary voltage to the microactuator  60  to cause it to rotate along with the slider  47 , thus restoring the desired position of the read/write head  35 . 
     This operation is illustrated in FIG.  6 . In particular, FIG. 6 shows that the error correction requires a clockwise rotation of the slider  47  as viewed from the ABS. It should be understood that a counter clockwise rotation of the slider  47  may be also required as needed to bring about a reduction in the error of the position of the read/write head  35 . 
     With reference to FIG. 7, generally, the rotor  66  is maintained at a positive polarity, while the outer stator  62  and inner stator  64  are maintained at ground. When a track-follow control commands a voltage to be sent to the microactuator  60  of the preferred embodiment, the voltage generates a current that flows through both the outward rotor radial spokes  82  and the inward rotor radial spokes  86  and in turn through the outward rotor fingers  84  and inward rotor fingers  88 . This current flow generates an electrostatic potential field that permeates in the air gap among the various fingers  74 ,  78 ,  84 , and  88 , as a result of the voltage potential difference between the rotor  66  and the outer stator  62  and inner stator  64 . 
     With reference to FIG. 8, the electrostatic potential field existing between the various fingers  74 ,  78 ,  84 , and  88  induces an electrostatic force  90  between the each pair of the outer stator radial spokes  72  and the outward rotor radial spokes  82 , and an electrostatic force  92  between each pair of the inner stator radial spokes  76  and the inward rotor radial spokes  86 . The electrostatic forces  90  and  92  acting in the circumferential direction of the rotor  66  are attractive in nature to work on adjusting the track-follow control voltage command accordingly to set up different electrostatic charges among the various fingers  74 ,  78 ,  84 , and  88 . 
     With reference to FIG. 9, the physical principle of the creation of the electrostatic forces  90  and  92  is described in further details. Upon a voltage command from the track-follow control, a voltage potential difference exists between the rotor  66  and the outer stator  62  and inner stator  64 . This potential difference sets up an electrostatic potential field in the permeable air gap among the various fingers  74 ,  78 ,  84 , and  88 , which induces a plurality of the electrostatic forces  90  and  92  between the outer stator  62  and the rotor  66 , and between the inner stator  64  and the rotor  66 , respectively. Each of the electrostatic forces  90  and  92  is further comprised of two force mechanisms: one due to the electrostatic potential field in the fingers  74 ,  78 ,  84 , and  88 ; and the other due to the electrostatic potential field in the outer stator radial spokes  72 , the inner stator radial spokes  76 , the outward rotor radial spokes  82 , and the inward rotor radial spokes  86 . These force mechanisms may be referred to as finger actuation force  94  and parallel-plate actuation force  96 , respectively. The finger actuation force  94  is defined according to the formula: 
     
       
           F =[0.5* — 0 *th*v {circumflex over ( )}2]/g  (1) 
       
     
     where F is the finger actuation force  94 ,_0 is a physical constant that defines air permissibility (permittivity), th is the thickness of the fingers  74 ,  78 ,  84 ,  88  in a direction normal to the paper, v is the applied voltage, and g is the separation gap between the outer stator radial spokes  72  or inner stator radial spokes  76  and the top of the outward rotor fingers  84  or inward rotor fingers  88 . 
     Similarly, the parallel-plate actuation force  96  is defined according to the formula: 
     
       
           P =(0.5* — 0 *w*th*v {circumflex over ( )}2)/( g−x ){circumflex over ( )}2  (2) 
       
     
     where P is the parallel-plate actuation force  96 , w is the width of the outward rotor fingers  84  or inward rotor fingers  88 , x is the engagement distance between the outward rotor fingers  84  or the inward rotor fingers  88  and the outer stator fingers  74  or inner stator fingers  78 , and the remaining parameters are the same as the aforementioned definition. 
     It is evident from the formulas (1) and (2) that both the finger actuation force  94  and the parallel-plate actuation force  96  comprising the electrostatic force  90  or  92  are non-linear with respect to the voltage command from the track-follow control. Furthermore, the finger actuation force  94  and the parallel-plate actuation force  96  are inversely proportional to second order power of the separation gap between the outer stator radial spokes  72  or inner stator radial spokes  76  and the top of the outward rotor fingers  84  or inward rotor fingers  88 . Thus, to generate an electrostatic force  90  and  92 , the various fingers  74 ,  78 ,  84 , and  88  are brought closer together during a track-follow control actuation. Conversely, when track-follow control is not needed, the various fingers  74 ,  78 ,  84 , and  88  are sufficiently separated. 
     With reference to FIG. 10, the electrostatic forces  90  and  92  that are formed between each pair of the outer stator radial spokes  72  and the outward rotor radial spokes  82 , and between each pair of the inner stator radial spokes  76  and the inward rotor radial spokes  86 , respectively, act in the clockwise circumferential direction of the microactuator  60  of the preferred embodiment. In this manner, the electrostatic forces  90  and  92  collectively form two force couples that result in a clockwise acting torque  98  between the outer stator  62  and the rotor  66  and a clockwise acting torque  100  between the inner stator  64  and the rotor  66 . In turn, these clockwise acting resultant torques  98  and  100  reinforce each other to form a clockwise acting driving torque  102  that actuates the rotor  66  in a clockwise rotation. 
     Referring now to FIG. 11, the clockwise acting driving torque  102  causes the elastic springs  80  to stretch and deflect correspondingly forming counter-clockwise restoring forces. When the driving voltage is removed, the spring forces dominate and act to return the fingers to their un-actuated positions. The clockwise rotation of the rotor  66  brings about the same rotation of the slider  47 , which is attached to the rotor  66  via the protective cap  104 . The resulting clockwise rotation of the slider  47  allows the read/write element  50  to be positioned at a desired location within a target data track. 
     The advantages of the microactuator  60  of the present invention over the conventional microactuator  360  of the prior art now become apparent as described subsequently. The incorporation of the inner stator  64  in the microactuator  60  of the present invention effectively reduces the length of the elastic springs  80  by a factor of two as compared to the elastic springs  374  of the conventional microactuator  360  of the prior art without a necessity of increasing the size of the rotor  66 . In fact, the rotor  66  can be made lighter than the inner rotor  364  of the conventional microactuator  360  of the prior art by varying the wall thickness of the rotor  66 . 
     The reduction in length of the elastic springs  80  brings about several beneficial consequences with respect to the performance of the microactuator  60  of the present invention. The halving of the length of the elastic springs  80  effectively doubles the extensional stiffness of the elastic springs  80  and increases the flexural stiffness of the elastic springs  80  by a factor of 8. Since the rotor  66  is actuated in a rotational direction, the flexural stiffness of the elastic springs  80  is of a greater significance than the extensional stiffness of the elastic springs  80 , which governs the lateral motion of the rotor  66 . As a result, the natural frequencies of the rotor  66  in the rotational direction nearly triples the natural frequencies of the rotor  364  of the conventional microactuator  360  of the prior art. The substantial gain in the natural frequencies of the rotor  66  translates into many advantages. 
     Among these advantages is the accompanied gain in the frequency response of the track-follow control, which enables the track-follow control to command a more rapid actuation to effect the high resolution positioning of the read/write head  35 , thus resulting in a substantially shorter seek time as needed for a competitive advantage. The increases in the extensional and flexural stiffnesses of the elastic springs  80  greatly enhance the protection of the microactuator  60  of the present invention from external shock and vibration, which could adversely affect the performance of the read/write head  35  and result in unwanted side-writing effect. Furthermore, the same increases in stiffnesses of the elastic springs  80  also result in a substantial reduction in the potential excitation of the suspension arm assembly  33  in the load beam mode since the potential oscillation of the rotor  66  is greatly minimized. 
     Referring now to FIGS. 12A and 12B which illustrates a typical manufacturing sequence of the microactuator  60  of the preferred embodiment, the outer stator  62  and inner stator  64  are formed on a wafer  106  using a lithographic etching process as shown in FIG.  12 A. Subsequently, the rotor  66  is created on another wafer  108  using the same lithographic etching process as shown in FIG.  12 B. Thereafter, the rotor  66  is transferred from the wafer  108  to assemble with the outer stator  62  and inner stator  64  on the wafer  106  to form the microactuator  60  of the preferred embodiment. 
     In general, the microactuator  60  of the preferred embodiment is referred to as a coupled stator design. This reference can be exhibited by FIG. 7, which illustrates an electrical diagram of the microactuator  60  of the preferred embodiment. Both the outer stator  62  and inner stator  64  are connected to a ground potential, whereas the rotor  66  is at a positive potential. Thus, the electrostatic forces  90  and  92  induced by the electrostatic potential field always act in the same sense. That is, both the electrostatic forces  90  and  92  are attractive. This sameness in the force direction developed in the microactuator  60  of the preferred embodiment results in a coupling between the outer stator  62  and inner stator  64 , which limits the ability to customize the actuation forces in ways that would further enhance the microactuator  60  of the preferred embodiment. 
     With reference to FIG. 13, an improved microactuator  160  of an alternative embodiment is introduced. The microactuator  160  is also comprised of an outer stator  162 , an inner stator  164 , and a rotor  166 . In contrast to microactuator  60  of the preferred embodiment, the rotor  166  of the microactuator  160  of the alternative embodiment is positioned closer to the outer stator  162  than the inner stator  164 . The outer stator  162  is further comprised of a plurality outer stator radial spokes  172  equally spaced circumferentially except at the 3 and 9 o&#39;clock positions. Each of the outer stator radial spokes  172  is comprised of a plurality outer stator fingers  174  that point in a counterclockwise direction. 
     Similarly, the inner stator  164  is comprised of a plurality of inner stator radial spokes  176  equally spaced circumferentially except at 12 and 6 o&#39;clock positions. Each of the inner stator radial spokes  176  is further comprised of a plurality inner stator fingers  178  the point in a clockwise direction. Thus, orientations of the outer stator fingers  172  and the inner stator fingers  178  are in the opposing sense, in contrast to the outer stator fingers  72  and inner stator fingers  78  in the microactuator  60  of the preferred embodiment. Further, the outer stator radial spokes  172  are shorter than the inner stator radial spokes  176  as a result of the rotor  166  being closer to the outer stator  162  than the inner stator  164 . 
     The rotor  166  is comprised of a plurality of outward rotor radial spokes  182  that alternate with the outer stator radial spokes  172  and a plurality of inward rotor radial spokes  186  that alternate with the inner stator radial spokes  176 . Each of the outward rotor radial spokes  182  is further comprised of a plurality of outward rotor fingers  184  that point in a clockwise direction opposing the outer stator fingers  174 . Similarly, each of the inward rotor radial spokes  186  is comprised of a plurality of inward rotor fingers  188  that point in a counterclockwise direction opposing the inner stator fingers  178 . Further, the outward rotor radial spokes  182  are shorter than the inward rotor radial spokes  186 . 
     The rotor  166  is elastically attached to two outer elastic springs  190  that anchor to the outer stator  162  at the 3 and 9 o&#39;clock positions and two inner elastic springs  192  that anchor to the inner stator  164  at the 12 and 6 o&#39;clock positions. 
     Referring now to FIG. 14 which illustrates an electrical diagram of the microactuator  160  of the alternative embodiment, both the outer stator  162  and the inner stator  164  are now connected to a positive potential, whereas the rotor  166  is at a ground potential. This electrical arrangement effectively decouples the outer stator  162  and the inner stator  164  from each other. This decoupling results in a beneficial effect in that the induced electrostatic forces between the outer stator  162  and the rotor  166  and between the inner stator  164  and the rotor  166  can de independently controlled. 
     With reference to FIG. 15, during a track-follow control actuation, the voltage command is split into two different voltages to be sent to the outer stator  162  and the inner stator  164 . The electrostatic potential field induces an electrostatic force  194  between each pair of the outer stator radial spokes  172  and the outward rotor radial spokes  182 , and an electrostatic force  196  between the inner stator radial spokes  176  and the inward rotor radial spokes  186 . Further, these electrostatic forces  194  and  196  can be independently controlled in amplitude by adjusting the amplitude of the two split voltages accordingly. This independent force control enables a wider range of latitude in the performance of the microactuator  160  of the alternative embodiment by customizing the electrostatic forces  194  and  196 . The electrostatic forces  194  and  196  act in opposite directions to offset the driving torque without the need to reduce the voltage command. This mode of force actuation, in fact, can significantly improve the stability of the track-follow control. 
     Referring now to FIG. 16 which illustrates a manufacturing sequence of the microactuator  160  of the alternative embodiment, the outer stator  162  is created on a wafer  198  using a lithographic etching process. The rotor  166  and the inner stator  164  are then created on wafers  200  and  202 , respectively, using the same lithographic etching process Thereupon, the inner stator  164  is transferred from the wafer  202  to assemble with the rotor  166  on the wafer  200 . Thereafter, the rotor  166 /inner stator  164  assembly is transferred from the wafer  200  to assemble with the outer stator  162  on the wafer  198  to form the microactuator  160  of the alternative embodiment. 
     The alternative embodiment may be modified substantially to provide a new embodiment. With reference to FIG. 17, a microactuator  260  of another alternative embodiment of the present invention is now introduced. The microactuator  260  is comprised of a plurality of stators alternating with a plurality of rotors. For example, the microactuator  260  is shown in FIG. 17 as being comprised of three stators  262 ,  264 , and  266 ; and two rotors  268  and  270  alternating with the stators  262 ,  264 , and  266 . The rotors  268  and  270  may be positioned midway between two adjacent stators as shown, or closer to one stator than the other. The various fingers of the stators  262 ,  264 , and  266 , and the rotors  268  and  270  may be oriented suitably in various manners such as those in the microactuator  60  of the preferred embodiment or the microactuator  160  of the earlier alternative embodiment of the present invention. 
     It should be understood that the geometry, compositions, and dimensions of the elements described herein can be modified within the scope of the invention and are not intended to be the exclusive; rather, they can be modified within the scope of the invention. Other modifications can be made when implementing the invention for a particular environment. For example, though the stators and the rotors have been described as having a generally circular shape, it should be appreciated that other shapes can alternatively be used.