Abstract:
A micro-actuator is comprised of a piezoelectric motor mounted on a flexure tongue with offsetting hinges, to perform a fine positioning of the magnetic read/write head. The substantial gain in the frequency response greatly improves the performance and accuracy of the track-follow control for fine positioning. The simplicity of the enhanced micro-actuator design results in a manufacturing efficiency that enables a high-volume, low-cost production. The micro-actuator is interposed between a flexure tongue and a slider to perform an active control of the fly height of the magnetic read/write head. The induced slider crown and camber are used to compensate for thermal expansion of the magnetic read/write head, which causes the slider to be displaced at an unintended fly height position relative to the surface of the magnetic recording disk. The enhanced micro-actuator design results in reduced altitude sensitivity, ABS tolerances, and reduced stiction. The controlled fly height of the magnetic read/write head prevents a possibility of a head crash, while improving the performance and data integrity.

Description:
PRIORITY CLAIM 
     The present application claims the priority of U.S. provisional patent application Ser. No. 60/421,727, filed on Oct. 28, 2002, titled “Active Fly Height Control Crown Actuator,” which is assigned to the same assignee as the present application, and which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to data storage systems such as disk drives, and it particularly relates to a read/write head, such as a thin film head, a MR head, or a GMR head for use in such data storage systems. More specifically, the present invention provides a novel design of a micro-actuator, such as a piezoelectric micro-actuator, that is interposed between a flexure tongue and a slider to perform an active control of the fly height of the magnetic read/write head. 
     BACKGROUND OF THE INVENTION 
     In a conventional magnetic storage system, a magnetic head includes an inductive read/write transducer fabricated on a slider. The magnetic head is coupled to a rotary voice coil actuator assembly by a suspension over a surface of a spinning magnetic disk. 
     In operation, a lift force is generated by the aerodynamic interaction between the magnetic 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 fly height is maintained over a full radial stroke of the rotary actuator assembly above the surface of the spinning magnetic disk. The fly height is the distance between the read/write elements of the head and the magnetic layer of the media. 
     One objective of the design of magnetic read/write heads is to obtain a very small fly height between the read/write element and the disk surface. By maintaining a fly height closer to the disk, it is possible to record high frequency signals to replace (high frequency signals), thereby achieving high density and high storage data recording capacity. 
     The slider design incorporates an air-bearing surface to control the aerodynamic interaction between the magnetic 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 read/write elements are located. 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 fly 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 magnetization bits are deposited for data recording. Each of these tracks is further divided into sectors where the digital data are registered. 
     As the demand for large capacity magnetic storage continues to grow, 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 areal or track density without significantly altering the geometry of the storage media. 
     Accompanied with the increase in the areal density of the magnetic media, the current trend in the magnetic storage technology has also been pushing the slider design toward a near zero fly height in order to reduce the magnetic flux spacing, thereby increasing the data recording capacity. Furthermore, to attain high linear or areal density, such a slider design may include a giant magnetoresistive (GMR) read/write sensor. 
     In principles, by reducing the fly height, the performance of the magnetic read/write head can be greatly enhanced, thereby enabling a higher signal to noise ratio (SNR) and lower read/write error rates. 
     However, in the conventional slider design with a near zero fly height, these advantages may not be fully realized due to a number of technical problems imposed by the operation of the magnetic read/write head with near zero fly height. 
     One such problem is the possibility of the read/write transducer coming into contact with the magnetic recording disk, which may consequently result in a catastrophic failure of the entire magnetic disk drive or head crash. The possibility of physical contact of the read/write transducer with the magnetic recording disk may be brought about by a number of causes, such as a thermal expansion process or low ambient air pressure associated with high elevation. 
     During a typical operation, the magnetic read/write head is subjected to various thermal sources that can adversely affect the magnetic read/write head. Both ambient and localized adverse heating effects of the magnetic read/write head will be described later in more detail. 
     Ambient heating sources are: 1) the heat dissipated by the motor that drives the magnetic recording disk; 2) a heat source results from the electrical power supplied to the VCM and drive electronics; 3) a small thermal source is attributed to a heat transfer process to the slider from the air friction generated by the rapidly spinning magnetic recording disk. 
     Localized heating effects arise from the operation of the read/write heads themselves. The write head has Joule heating input from the write current passing through its coils, as well as eddy current heating input from the eddy currents generated in its poles. The read head has Joule heating input from the read sense current passing through the GMR read/write sensor and a smaller amount of Joule heating from that current in the sensor leads. In general, the net ambient air temperature that the slider can experience may range from a room temperature of about (5° C.) to as high as 85° C. 
     The temperature increase consequently causes a thermal expansion of the pole tip region of the magnetic read/write head in all directions, but most adversely in the direction toward the magnetic recording disk. These thermal expansions, in effect, reduce the fly height, and in the worst-case results in a physical contact of the read/write transducer that causes a catastrophic failure of the magnetic disk drive. 
     Yet another problem also related to the near zero fly height is the altitude sensitivity of magnetic disk drives. As a magnetic disk drive operates at a higher altitude, the lower atmospheric pressure generates accordingly a reduced aerodynamic lift force. Consequently, the magnetic read/write head operates at a less than optimal fly height since the slider is not sufficiently lifted above the surface of the magnetic recording disk. In the presence of environmental temperature fluctuation, the risk of a magnetic read/write head contact therefore may become more pronounced. 
     On the other hand, the magnetic read/write head may operate at a fly height sufficiently distant from the surface of the magnetic recording disk. Even in the presence of the thermal expansion process, the fly height may deviate from its intended specification, but not low enough to present a head crash problem. 
     While the possibility of a head crash may be substantially alleviated, the performance of the magnetic read/write head may significantly suffer from the varying fly height. Since the magnetic permeability is proportional to the fly height, which affects the magnetic flux density, the deviation of the fly height may degrade the ability for the magnetic read/write head to register binary data onto the magnetic recording head. Furthermore, the variation in the fly height causes a varying performance of the magnetic read/write head, thus posing as a data integrity issue and a potential quality assurance problem. 
     To address this deficiency, a number of designs have been proposed. One such design utilizes electrostatic and piezoelectric actuators, this design would also require a complete redesign of the read/write transducer so it can be placed onto the movable part of a microactuator attached to the slider. Such a solution impedes the ability to optimize the design and current ease of fabrication process of the read/write transducer and therefore is less practical to implement. 
     Another design utilizing an active control method for head gimbal assembly was proposed in U.S. Pat. No. 5,991,114. The active fly height control is achieved through operating a gram load reducer between the support arm and the load beam to adjust the net force acting on the slider, which causes the slider to move closer to or farther from the surface of the magnetic storage disk as desired. 
     It is thus realized that the current attempts to address the control of the fly height of a magnetic read/write head still remains unsatisfied. It is therefore recognized that a further enhancement in the slider design for controlling the fly height of the magnetic read/write head is beneficial to the reliability and performance of a hard drive. Preferably, new slider design would afford all the advantages resulting from the near zero fly height, and at the same time would overcome the shortcomings with a conventional slider design. 
     Furthermore, the new slider design would achieve a controlled near zero fly height under most operational constraints. This in turn would result in performance advantages over the convention slider design. 
     SUMMARY OF THE INVENTION 
     It is a feature of the present invention to provide a novel enhanced micro-actuator slider design for actively controlling the fly height of a magnetic read/write head. The enhanced micro-actuator slider according to the present invention is designed to maintain a near zero fly height by controlling the crown or camber of the slider to compensate for the thermal expansion effect that causes an uncontrolled fly height that is either too small or too large, which could otherwise result in a performance degradation due to improper signal registration, or in the worst case a physical contact of the magnetic read/write transducer with the magnetic recording disk, resulting in a head crash or a catastrophic failure of the magnetic read/write head. 
     According to a preferred embodiment, the present invention features a novel application of a piezoelectric motor (also referred to as microactuator) in the form of a monolithic block that is suitable for low cost and manufacturing efficiency. The piezoelectric monolithic motor can be either a bulk or multi-layer type that is sandwiched between the flexure tongue and the slider. The piezoelectric motor is bonded to two hinged islands on the flexure tongue on one side, while the other side is bonded to the slider top surface. The piezoelectric motor is of a dual use for controlling both the fly height and track position, or in another embodiment, is utilized for fly height control alone. 
     A novel adhesive pad is used to bond the piezoelectric motor to the slider. An optimal pattern has been derived to provide a suitable means for controlling the fly height. 
     According to a preferred embodiment, a dual stage tracking fly height active control system commands a voltage to the piezoelectric motor, thus causing it to either expand or contract uni-directionally as desired in accordance with the voltage polarity. The elongation or contraction of the piezoelectric motor is restrained by the bond joint to the slider, thus inducing a combined extensional deflection and bending deflection. The contractional deflection of the piezoelectric motor-slider assembly is used for controlling the track position, while the bending deflection results in a curvature in the slider along either the longitudinal or transverse axis of the flexure tongue, also known as the slider crown or camber, respectively. 
     The adjustment of the slider crown and/or camber enables the fly height to be actively controlled, whereby a positive crown and/or negative camber would cause the magnetic read/write transducer to move farther from the surface of the magnetic recording disk and vice versa. This motion thus compensates for any pole tip protrusion and thermally induced slider crown caused by the thermal expansion effect. The novel design can also permit a gross adjustment of the fly height to a suitable value as needed. 
     The advantages of this novel slider design lie in the effectiveness and simplicity of the method for controlling the fly height, thus resulting in an improved manufacturing efficiency. 
     In addition to the fly height compensation for thermal expansion effect, other advantages afforded by the novel slider design may include a reduced altitude sensitivity, ABS process tolerances whereby a less than optimal pressure distribution can be compensated by adjusting the slider curvature, a reduced stiction which results in a faster spin-up of the magnetic recording disk, and minimal heat addition to the magnetic read/write transducer. 
    
    
     
       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, adhesive pads, and a piezoelectric motor, made according to the preferred embodiment of the present invention; 
         FIG. 3  is an exploded view of the head gimbal assembly of  FIG. 2 , illustrating a load beam, a flexure, a dielectric layer, a copper trace, a piezoelectric motor, and a slider; 
         FIG. 4  is a side view of the head gimbal assembly of  FIG. 2 , made according to a preferred embodiment of the present invention; 
         FIG. 5  is an enlarged, perspective view of the flexure shown secured to the piezoelectric motor and slider of  FIG. 4 ; 
         FIG. 6  is an enlarged, schematic illustration of a bottom view of a flexure tongue of the head gimbal assembly of  FIGS. 2 ,  4 , and  5 , illustrating two hinged islands of the flexure tongue that are made according to a preferred embodiment of the present invention; 
         FIG. 7  is a bottom view of the piezoelectric motor shown bonded to the flexure tongue and an adhesive pad, for bonding the slider of the head gimbal assembly of  FIG. 2  to the piezoelectric motor; 
         FIG. 8  is an ABS (bottom) view of the slider shown bonded to the piezoelectric motor of  FIG. 3 ; 
         FIG. 9  is a perspective view of the slider-flexure assembly, illustrating the slider crown and camber; 
         FIG. 10  is a mathematical description of the slider crown of  FIG. 9 ; 
         FIG. 11  is comprised of  FIGS. 11A and 11B , and is an illustration of the adhesive pad patterns; 
         FIG. 12  is a plot of the fly height for various adhesive pad patterns; 
         FIG. 13  is a plot of the slider curvature and tracking stroke sensitivity as a function of the adhesive pad bond length; 
         FIG. 14  is a plot of the fly height sensitivity as a function of the adhesive pad bond length; 
         FIG. 15  is a plot of the fly height sensitivity as a function of the adhesive pad side bond length; 
         FIG. 16  is a plot of the crown build dispense pattern of the preferred embodiment; 
         FIG. 17  is a cross sectional view of the slider undergoing thermal expansion process resulting in pole tip protrusion and thermal crown; 
         FIG. 18  is a top view of a magnetic storage disk encoded with servo bits and data bits; 
         FIG. 19  illustrates track position signals and track-follow control signals; 
         FIG. 20  illustrates read signals and fly height control signals for a static operation; 
         FIG. 21  illustrates read signals and fly height control signals for a dynamic operation; 
         FIG. 22  is a block diagram for a single stage tracking fly height control system; 
         FIG. 23  is a block diagram for a dual stage tracking fly height control system employed in the preferred embodiment; 
         FIG. 24  is a frequency response plot for various modes of control actuation for the dual stage tracking fly height control system of  FIG. 23 ; 
         FIG. 25  illustrates the physical principle of the piezoelectric motor actuation; and 
         FIG. 26  is comprised of  FIGS. 26A and 26B , and represents two perspective views showing the slider-piezoelectric motor assembly in a deflected (or crowned) position. 
     
    
    
     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 voice coil  20  attached to the block  19  in a position diametrically opposite to the actuator arms  18 A,  18 B,  18 C. The voice coil  20  cooperates with a magnetic circuit (not shown), comprising in total a voice coil motor (VCM) for rotating in an arc about the actuator axis  16 . Energizing the voice coil  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 generally similar 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  FIGS. 2 through 4 , the HGA  28  includes a suspension  33 , a piezoelectric motor  60  of the present invention, and a read/write head  35 . The suspension  33  includes a load beam  36  and a flexure  40 . The top surface of the piezoelectric motor  60  is bonded to the flexure  40  by means of a plurality of adhesive pads  62  ( FIG. 4 ), and to a read/write head  35  on its underside via an adhesive pad  70 . 
     The read/write head  35  is formed of a slider  47  and a read/write transducer  50  that is supported within the slider  47 , and is secured to the piezoelectric motor  60 . 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)  58  of the slider  47 . 
     With more specific reference to  FIG. 3 , the load beam  36  is generally flat and has an elongated shape with a taper width. The load beam  36  can assume a conventional design, with various features provided therein in the form of protrusions and cutouts that are positioned through the load beam  36  to provide connections to the flexure  40  and the actuator arm  18 A. These features include, for example, a lift tab  32  and an elliptical alignment slot  34 . The load beam  36  is connected to the actuator arm  18 A by swaging the base plate to it. 
     With reference to  FIG. 3 , the flexure  40  is made of stainless steel and is generally flat with an elongated shape. A number of protrusions and cutouts are made throughout the flexure  40 , such as a flexure tongue  48 , a T-shaped forward tab  42  and an elliptical alignment slot  44 . A serpentine strip  46  extends the main body of the flexure  40  to provide a surface onto which a dielectric material is deposited, conductive traces are routed, and termination pads are supported. 
     The flexure  40  is affixed to the underside of the load beam  36  by means of spot welding. The flexure  40  is positioned relative to the load beam  36  in a manner such that the alignment slots  34  and  44  of the load beam  36  and the flexure  40 , respectively, are coincident. 
     The flexure  40  includes the flexure tongue  48 , which, according to a preferred embodiment, has a generally rectangular shape, and is located in the forwardmost region of the flexure  40  adjacent to the T-shaped forward tab  42 . The flexure tongue  48  incorporates two substantially rectangular hinged islands  80  and  82  designed to provide means for pivotally securing the piezoelectric motor  60  to the flexure tongue  48 . The details of the flexure tongue  48  will be further described in connection with  FIGS. 5 to 10 . 
     In connection with  FIG. 3 , a dielectric layer  90  is attached to the underside of the flexure  40 . The dielectric layer  90  is composed of a conventional dielectric material such as polyimide, to provide electrical insulation between the stainless steel flexure  40  and conductive traces  110 . The dielectric layer  90  is formed on the underside of the flexure  40  by a CIS deposition or TSA subtractive method. 
     The dielectric layer  90  provides a layout for the electrical path to the read/write transducer  50  and piezoelectric motor  60  to be secured thereto. Two rectangular dielectric pads  92  and  94  of the dielectric layer  90  are formed onto, or secured to the two hinged islands  80  and  82  of the flexure tongue  48 , respectively. 
     The dielectric inner paths  96  and  98  are routed away from the forwardmost region of the dielectric layer  90  and merged with a narrow outer path loop  100  into two larger main paths  102  and  104 , respectively. The two main paths  102  and  104 , in turn, merge into a serpentine path  106 , which conforms to the serpentine strip  46  of the flexure  40 . 
     As further illustrated in  FIG. 3 , a conductive trace, such as a copper trace  110 , is deposited onto the underside of the dielectric layer  90 . The copper trace  110  provides the electrical connection to the read/write transducer  50  and piezoelectric motor  60 , and generally conforms to the layout of the dielectric layer  90 . The copper trace  110  is comprised of six separate electrical wiring paths  120 ,  122 ,  124 ,  126 ,  128 , and  130 . These respective wiring paths terminate on one distal end at six corresponding termination pads  132 ,  133 ,  134 ,  135 ,  136 , and  137 . 
     The two inner electrical wiring paths  120  and  122  connect at their other distal ends to two pair of rectangular electrical wiring loops  112 ,  113 , and  114 ,  115 , respectively. The wiring loops  113  and  114 , whose corresponding bond pads  62  and  64  ( FIG. 4 ) respectively, are preferably a electrically conductive epoxy, supply the electrical signal to the piezoelectric motor  60 . The corresponding bond pads  62  and  64  for loops  112  and  115  are preferably electrically non-conductive. The four outer electrical wiring paths  124 ,  126 ,  128 , and  130  connect at their other distal ends to four termination pads  116 ,  117 ,  118 , and  119  for reading and writing information to and from the storage media. 
     Referring now to  FIGS. 5 and 6 , the tongue  48  of the flexure  40  has a substantially rectangular shape, and is located in the forwardmost region of the flexure  40 . The flexure tongue  48  includes two hinged islands  80  and  82  that are formed by, and separated from the main body of the flexure tongue  48  by two narrow gaps  84  and  86 , respectively. 
     The gaps  84  and  86  are generally similar in design, and have the shape of the letter G, to enclose the hinged islands  80  and  82  in part. The dimensions of the gaps  84 ,  86  are such that they allow free motion (mainly rotation) of the hinged islands  80  and  82  therewithin. 
     As more clearly illustrated in  FIG. 6 , the hinged islands  80  and  82  are generally disposed opposite to each other relative to a center of symmetry C, at which a transverse axis  200  and a longitudinal axis  206  intersect. In  FIG. 6 , the flexure tongue is schematically represented by a rectangular borderline, to simplify the description of the hinged islands  80  and  82 . The hinged islands  80  and  82  are defined by two tabs (or paddles)  140  and  142 , and two elongated hinges  144  and  146 , respectively. Though the tabs  140  and  142  are shown to be generally rectangularly shaped, it should be clear that they can assume any other suitable shape. 
     The tabs  140  and  142  are similar in shape and construction, and provide bonding surfaces for attaching the piezoelectric motor  60  ( FIG. 5 ) by means of the adhesive pads  62  and  64  ( FIG. 4 ), respectively. In the embodiment illustrated herein, the tabs  140  and  142  are generally oriented along the transverse axis (or direction)  200 , and have the following approximate dimensions: 1 mm in length and 0.3 mm in width. 
     The tabs  140  and  142  are further separated by a distance of approximately 0.7 mm, from the inner edge  202  of the tab  140  to the inner edge  204  of the tab  142 . The two hinges  144  and  146  are formed of thin, short, substantially shouldered (stepped) rectangular sections that protrude from the inner edges  202  and  204  of the tabs  140  and  142 , respectively, and generally extend along the longitudinal axis  206  of the flexure  40 . Non shouldered (straight) hinges are also suitable and are consistent with the present invention. Furthermore, the two hinges  144  and  146  are offset by a distance  148  along the transverse axis  200 . The offset  148  is designed to enable the hinged islands  80  and  82  to freely rotate within the gaps  84  and  86  during a track-follow control actuation. 
     With reference to  FIGS. 7 ,  8 , and  9 , the piezoelectric motor  60  has a generally rectangular shape that is preferably, but not necessarily, similar in dimensions to those of the slider  47 . According to a preferred embodiment of the present invention, the dimensions of the piezoelectric motor  60  may be defined by a length of approximately 1.25 mm, a width of approximately 1 mm, and a thickness of approximately 0.2 mm. Thinner construction of the piezoelectric motor  60  may enable a lower profile head gimbal assembly (HGA)  28  and would be obvious within the context of this invention. 
     The piezoelectric motor  60  is positioned relative to the flexure tongue  48 , such as its length extends along the longitudinal axis  206  of the flexure  40 . The piezoelectric motor  60  is preferably, but not necessarily made of PZT material or any other similar material, and can be of either a bulk type or a multi-layer type. 
     A bulk-typed piezoelectric motor  60  is formed by firing the molded PZT powders followed by polarization, while a multi-layer typed piezoelectric motor  60  is comprised of a number of stratified sections of piezoelectric material that are superimposed to form a desired thickness of the piezoelectric motor  60 . Reference is made for example, to U.S. Pat. No. 6,246,552 for further composition details. 
     In certain applications, the multi-layer typed piezoelectric motor  60  is preferred over the bulk-typed piezoelectric motor  60  due to its high stroke sensitivity, because a larger electric field can be generated if voltages are applied to thinner layers, with the stroke being proportional to the electric field. The electrical contacts to the piezoelectric motor  60  are provided by the rectangular pads  113  and  114  ( FIG. 3 ), for supplying a controlled voltage as defined by the control system(s). 
     According to the preferred embodiment, the piezoelectric motor  60  may be of a dual use for controlling the track position as well as the fly height position. 
     The piezoelectric motor  60  is attached to the flexure tongue  48  by means of the adhesive pad sets  62  and  64 , which are positioned against the rectangular tabs  140  and  142  of the respective hinged islands  80  and  82 . For the purpose of illustration, two pairs of adhesive pads  62  &amp;  64  are shown in  FIG. 9  affixed to the hinged islands  80  and  82  respectively. The backside of the slider  47  is then affixed against the exposed surface of the piezoelectric motor  60 , by means of an adhesive (or adhesives)  70 . 
     With reference to  FIG. 6  and  FIG. 9 , by definition, a slider crown  240  is defined as a curvature of the slider  47  along the longitudinal axis  206 . A positive slider crown  240  results in a convexity of the slider  47  with respect to the longitudinal axis  206 , meaning the slider ABS  58  is convex facing the magnetic disk  14  as illustrated in  FIG. 9  and  FIG. 26 . Conversely, a negative slider crown  240  causes the slider  47  to concave facing the magnetic disk  14 . 
     Similarly, a slider camber  242  is defined as a curvature of the slider  47  along the transverse axis  200 . As with the slider crown  240  definition, a positive or negative slider camber  242  ( FIG. 9 ) corresponds to either a convexity or concavity, respectively, of the slider  47  curvature with respect to the transverse axis  200 . Positive slider camber  242  corresponds to the slider ABS  58  facing the magnetic disk  14  as convex. 
     With reference to  FIG. 10 , the curvature of the slider crown  240  may assume a parabolic shaped profile  245  as defined by the equation  244 . The slider crown  240  may be computed by the equation  246  using a half-length of the slider  47  and the coefficient of the squared term in the equation  244 . 
     According to the preferred embodiment, the adhesive bonding area  70  may be formed in various patterns.  FIG. 1A  illustrates a rectangular shape  70  that may be similar in dimensions to the piezoelectric motor  60 . It should be understood that the adhesive bonding area  70  may also be defined by other rectangular dimensions as suited to a particular application. 
     With reference to  FIG. 11B , the adhesive bonding area  70  is preferably formed of a cruciform shape. The cruciform-shaped adhesive pad  70  generally is comprised of a main bond pad  220  along the longitudinal axis  206  and a side bond pad  222  along the transverse axis  200 . The main bond pad is defined by a bond length  224  and bond width  226 . The side bond pad  222  is defined by a side bond length  228  and side bond width  230 . 
     By adjusting these dimensions, various crown and camber effects can be obtained to achieve a suitable fly height control, that is slider ABS  58  design specific. An ABS design is assumed to facilitate the following illustrations. As an example,  FIG. 12  illustrates a number of patterns. Pattern  250  is defined by a cruciform shape having a dominant bond length  224  and equal bond width  226  and side bond length  228 . Pattern  252  is defined by a cruciform shape having equal bond length  224  and side bond width  230 , as well as equal bond width  226  and side bond length  228 . Pattern  254  is defined by a cruciform shape having a dominant bond length  224  and a bond width  226  greater than a side bond length  228 . Pattern  256  is simply a rectangle, which can be considered as a limiting case of a cruciform shape when the bond width  226  and the side bond width  230  are equal. 
     With reference to  FIG. 13 , generally, increasing the bond length  224  would increase the slider crown  240  and slider camber  242  without significantly affecting the micro-actuator tracking stroke sensitivity. Likewise, decreasing the side bond length  228  would decrease the fly height adjust sensitivity. Depending on the ABS  58  design, crown and camber effects tend to cancel, so those effects are taken into account within the calculations of the delta fly height that can be induced via the piezoelectric motor  60  control voltage. 
     Thus, for the assumed ABS design pattern  256  may be used for the adhesive pad  70  to maximize the fly height controllability range. Furthermore, if pattern  250  is used in combination with a thinner slider construction, a Femco that has a thickness for example of 0.2 mm instead of 0.3 mm typical of a pico slider, a substantial boost in the sensitivity of the fly height could be realized. 
     With reference to  FIG. 14 , where the bond pattern is purely rectangular as in  FIG. 11A , the bond length  224  can be adjusted to achieve a certain fly height sensitivity. As an example, at the mid-diameter of the magnetic storage disk  14 , a 0.9 mm bond length would achieve a fly height delta of 0.076 μin, as compared to a fly height delta of 0.034 μin corresponding to a 0.3 mm bond length. Generally, the fly height sensitivity decreases with an increase in the bond length  224  up to about 0.9 mm, beyond which camber contributions begin to become more significant and fly height sensitivity reduces. 
     With reference to  FIG. 15 , the fly height sensitivity can also be obtained by tailoring the side bond length  228 . As an example, a 0.5 mm side bond length would achieve a fly height reduction of 0.13 μin. Increasing or decreasing the side bond length  228  from 0.5 mm reduces the fly height sensitivity. These illustrations are ABS  58  design specific, so other bond dimensions optimized for another ABS  58  design should be understood as being within the scope of this invention. 
     Using  FIGS. 14 and 15 , the pattern for the adhesive pad  70  may be optimized. Referring now to  FIG. 16  illustrating a slider crown  240  build dispense pattern for the adhesive pad  70 , the bond length  224  preferably, but not necessarily, is of a dimension of 1.2 mm. Moreover, the bond width  226  and the side bond length  228  preferably are of a dimension of 0.5 mm. 
     To gain further appreciation for the novelty of the present invention, the problem with a conventional slider design may now be described in connection with  FIG. 17B  illustrating the thermal expansion effect on the slider  47 . 
     During a typical operation, the slider  47  on which the read/write transducer  50  is mounted, is flying over the spinning magnetic storage disk  14  thereunder. The rapid rotation of the magnetic storage disk  14  generates a sufficient differential pressure between the top and bottom of the slider  47 , which is also the ABS  58 , to create a lift force  250 , which causes the slider  47  to tend to be airborne. A suspension gram load  252  equal to the lift force  250  is exerted downward onto the slider  47  to maintain the slider  47  in a static equilibrium. 
     Generally, the suspension gram load  252  and the lift force  250  are not co-linear such that the suspension gram load  252  is typically closer to the trailing edge  55  of the slider  47  than the lift force  250 . This force offset results in a torque or moment acting on the slider  47  to cause it to pitch in the counter clockwise direction. As a result, the read/write transducer  50  mounted at the trailing edge  55  is displaced closer to the surface of the magnetic storage disk  14 . The vertical gap between the bottom of the slider  47  at the trailing edge  55  and the surface of the magnetic storage disk is called as the fly height  254 . 
     In theory, the fly height  254  is precisely controlled at a very close proximity to the surface of the magnetic storage disk  14 . This near zero fly height  254  is necessary for an optimal magnetic flux induction during recording data onto the magnetic storage disk  14 . 
     In practice, however, during operation, the read/write head  35  is subjected to heating by various thermal sources such as the writer coil when writing data, reader sense current when reading data, air friction, spindle motor, drive electronics and VCM heating via power dissipation and external elevated ambient temperature. This thermal heating causes the air temperature in the vicinity of the pole tip region of the read/write transducer  50  to rise. 
     Accordingly, a heat conduction process takes place to redistribute the temperature within read/write head  35 . As various components of the conventional read/write head  35  register a temperature increase, they undergo an elongation of varying degrees in accordance with their specific coefficients of thermal expansion (CTE). Thus, in general the localized pole tip region of the read/write transducer  50  is protruded outwardly in a closer proximity to the surface of the magnetic disk  14 , resulting in an undesired reduction in the fly height  254 . 
     Furthermore, with regard to conventional slider and HGA designs, because the CTE of the flexure  40  generally is not equal to that of the slider  47 , their respective dimensions, therefore, do not necessarily elongate at the same rate. As a result of bonding the slider  47  to the flexure  40 , a shear strain is developed within their interface bonding to induce a curvature in the slider  47 , resulting in unwanted slider crown  240  and a slider camber  242 . 
     As exemplified by  FIG. 17A , because of the coefficients of thermal expansion (CTE) for the piezoelectric motor  60  nearly match that of the slider  47 , further appreciation for the novelty of the present invention is evident. 
     The combination of the pole tip protrusion and the thermally induced slider crown  240  causes the fly height  254  to deviate from its specification. In the worst-case scenario, a physical contact of the magnetic read/write transducer  50  with the surface of the magnetic storage disk  14  would develop, thereby resulting in a catastrophic head crash. 
     An almost equally adverse scenario is the possibility that the fly height  254  substantially deviates from its specification due to manufacturing and/or thermal variations. This would cause the read/write transducer  50  to be either too close or too far from the magnetic storage disk  14 , thereby resulting in a significantly degraded performance of the magnetic disk drive  10  as the magnetic flux lines may under-saturate or over-saturate the magnetic data bits  264  on the storage disk  14  for a proper data recording. 
     With reference to  FIG. 18 , in accordance with the industry standard, the magnetic storage disk  14  is encoded with a plurality of servo bits  260  circumferentially along a data track  262 . The servo bits  260  are placed on both sides, and adjacent to, the data track  262 . Data bits  264  are generally located on the data track  262  between the servo bits  260 . 
     With reference to  FIG. 19A , during a typical operation, the magnetic read/write head  35  generally travels along the data track  262 . As the magnetic read/write head  35  crosses the servo bits  260 , a raw track position signal  270 , composed of both low and high frequency components, is detected by the read/write transducer  50 . For the purpose of illustration, the raw track position is made up of a single low frequency sine wave, for example due to warpage of the storage disk  14 , with a superimposed high frequency sine wave, for example due spindle motor bearing vibration. This track position signal  270  is used as feed back to the control system(s) the information on the position of the read/write transducer  50  relative to the data track  262 . 
     In a dual stage tracking control system, and with reference to  FIG. 19B , the voice coil  20  (sometimes referred to as the primary actuator) receives a low frequency command signal  272  to correct for the raw position error  270  and drives the HGA  28 , which includes the read/write transducer  50 , to the center of data track  262 . 
     With reference to  FIG. 19C , the micro-actuator (sometimes referred to as the secondary actuator) or piezoelectric motor  60 , receives a command signal  276  comprised only of the high frequency portion of the raw track position  270 , these disturbances encountered by the magnetic read/write head  35 , are not correctable by the VCM (due to inherent resonances) in a single stage control system with high track density, thus the need and benefit of dual stage control. Track-follow command signal  276 , acting to fine position the read/write transducer  50 , is typically small in amplitude, high in frequency and accommodates following requirements driven heavily by areal density growth in disk drives. 
     To gain further understanding the novelty of active fly height control of the present invention, two types of operation will be described in details: static operation whereby the fly height  254  is time invariant and dynamic operation whereby the fly height  254  is time variant. 
     With reference to  FIG. 20A , under an assumption that the fly height  254  is optimal and at its correct specification at all times, the read/write transducer  50  would register a reference constant amplitude, very high frequency read signature  280  from the data bits  264 . The reference read signature  280  is generally considered as a nearly ideal read signal for an optimal performance of the magnetic read/write head  35 . 
     In a static operating environment, however, a thermally induced slider crown  240 , perhaps pole tip protrusion and an elevation change in combination would cause the fly height  254  to be either too small or too large, thus resulting in a constant read signature  282  of either too low or too high in amplitude, as compared to the amplitude of the reference read signature  280  under an ideal situation. Thus, in a static operating environment, this affects a static offset in the fly height  254 , whose amplitude is time invariant. 
     When the amplitude of the read signal  282  is too low as illustrated in  FIG. 20C , the read/write transducer  50  is positioned further away from the surface of the magnetic disk  14 , resulting in a larger fly height  254  than intended. Similarly, when the amplitude of the read signal  282  is too high as illustrated in  FIG. 20C , the magnetic read/write head  35  operates at a smaller fly height  254  than intended. 
     With reference to  FIGS. 22 and 23 , according to a preferred embodiment, a single stage tracking active fly height control system  290  or preferably a dual stage tracking active fly height control system  292  is deployed to compensate for the offset in the fly height  254  as well as the dynamic error in the track position signal  270 . 
     With more specific reference to FIGS.  19 A–C and  20 A–E, the dual stage tracking control system  292  uses the raw track position error signal  270  to compute and distribute the track-follow control command signals  272  and  274  to the VCM and micro-actuator respectively. A read amplitude error signal  288  (RAES) is needed for the fly height control systems  320  and  360 , for single and dual stage implementation respectively. The magnitude of the RAES  288  represents the difference between optimal and actual real-time read signature amplitudes. In proportion to, and with an appropriate polarity, the RAES is used to compute a fly height command signal  284 , which is superimposed onto an appropriate nominal DC voltage established initially for the piezoelectric motor  60 . In one example application, this nominal DC bias voltage  274  may be set at 20V. The fly height control system  320  or  360  then operates to maintain a zero RAES continuously by applying differential corrective command voltages  284  on top of the reference bias DC voltage  274 , resulting in a DC operating range, for example, of 10V to 30V on the piezoelectric motor  60  doing fly height control. 
     Generally, for a static operation, the fly height control signal  284  is a DC voltage. With reference to  FIG. 20D , to compensate for too large of read amplitude  282 ,  FIG. 20B  (low fly height  254 ), the fly height control signal  284  is the sum of the reference DC bias voltage and a positive differential voltage. Similarly, to compensate for too small of read amplitude  282 ,  FIG. 20C  (high fly height  254 ), the fly height control signal  284  is the sum of the reference DC bias voltage and a negative differential voltage as illustrated in  FIG. 20E . It should be understood that the sign convention for the differential voltage is simply for the purpose of exemplification; hence the converse may be equally applicable. 
     The overall control signal or the micro-actuator command signal  286  corresponding to  FIGS. 20F–G , which is sent to the piezoelectric motor  60 , is the sum of the track-follow control signal  276  as illustrated in  FIG. 19C  and the fly height control signal  284  as shown in  FIG. 20D  if read signal amplitude  282  is too large, and  20 E if read signal amplitude  282  is too low. Upon actuation of the piezoelectric motor  60 , the feedback implementation of the dual stage tracking control system  292  will restore the position of the read/write transducer  50  to the center of the desired data track  262  as well as the fly height  254  to its intended value. In this manner, the read signal  282  is brought into agreement with the reference read signal  280 . The DC bias voltage  284  adjusts according to the needs of the fly height control, and the AC voltage component  274  performs the track following duties. 
     In practice, the magnetic read/write head  35  frequently operates in a dynamic environment, wherein the fly height  254  is generally time variant. During a typical operation, the temperature rise usually varies with time. Lack of flatnesss and warpage, for example, of the disk  14  can cause unwanted fly height variation. Thermally induced pole tip protrusion is not constant with time, this also implies that the fly height  254  is generally time variant. In addition, mechanical disturbances such as shock or resonance may also contribute to the time variant nature of the fly height  254 . 
     Referring now to  FIG. 21A , a typical read signature  282  in a dynamic operating environment is as illustrated. The envelope is no longer bound by a constant amplitude. Rather, the amplitude is time varying, resulting in a sinusoidal wave envelope (for purposes of this illustration) of the read signature  282 . The width of the read signature envelope, at any point in time, indicates the corresponding fly height  254 . For example, if the envelope width is small (low signal amplitude), the fly height  254  is correspondingly too high, and vice versa. 
     The fly height control signal  284  is computed by the fly height control system  320  or  360  in the usual manner as the sum of the bias DC voltage and a differential voltage. However, since the fly height  254  is time variant, the differential voltage must accordingly be time varying as well. The resulting fly height control signal  284  is a sinusoidal wave with a DC offset as illustrated in  FIG. 21B . 
     With reference to  FIG. 21C , the micro-actuator command signal  286  is now characterized by a high frequency sine wave for track-follow control modulating on top of a low frequency sine wave with DC offset for fly height control. 
     Using a feedback implementation, the dual stage tracking control system  292  actively controls the fly height  254  during a typical operation in a dynamic environment to achieve its objective of maintaining the read signal  282  as close to the reference read signal  280  as possible so that the performance of the magnetic read/write head  35  is at an optimum. 
     With reference to  FIG. 22 , the single stage tracking control system  290  which may be used according to the preferred embodiment, is generally comprised of two independent feedback control loops: single stage tracking control loop  300  and fly height control loop  320 . 
     An existing or conventional single stage tracking control loop  300  uses the track position signal  270  as an input. The VCM  20  drives the magnetic read/write head  35  to its intended track center position. The loop bandwidth is usually limited to about 2 kHz and the sample rate is typically about 20 kHz. 
     The fly height control loop  320  uses the read signal  282 , and more specifically the read amplitude error signal (RAES)  288  as an input. The RAES  288  is then gained and filtered, for example, by a 500 Hz low pass compensator  322 . The conditioned signal is then sent to a bias voltage driver  324  having low bandwidth, whereupon it is converted into a fly height control signal  284 . The fly height control signal  284  is then used to actuate the piezoelectric motor  60  to achieve a desired slider crown  240  for controlling the fly height  254 . The loop bandwidth would be on the order of 100–300 Hz and uses sampling at a rate of about 3 kHz. 
     Generally, the single stage tracking control system  290  is less effective than the dual stage tracking control system  292  because of the lack of the track-follow control feature for fine track adjustment, which would render the magnetic read/write head susceptible to track alignment error. 
     Referring now to  FIG. 23 , the tri-stage tracking control system  292  employed in the preferred embodiment is generally comprised of two coupled feedback control loops: a dual stage tracking control loop  340  and a fly height control loop  360 . 
     For the purposes of this invention &amp; illustrations herein, the description of the dual stage servo is simplified. The dual stage tracking control loop  340  uses the track position signal  270  as an input. The track position signal  270  is then split into two identical signals: one passing through a compensator  342  tailored (typically for low and mid-frequencies) for driving the voice coil  20 , and the other passing through a compensator  344  tailored (typically for mid and high-frequencies) for driving the micro-actuator. The low pass conditioned signal from compensator  342  is then sent to a current driver  346 , which converts the signal into the command current signal  272  for the VCM  20 . 
     The high pass conditioned signal from compensator  344  is sent to an AC voltage driver  348  to modulate and convert the signal into the track-follow control signal  276 . The track-follow control signal  276  then combines with the fly height control signal  284  from the fly height control loop  360  to form the microactuator command signal  286  to actuate the dual-purpose piezoelectric motor  60  in conjunction with the VCM  20  actuation using the command current signal  272  to achieve a desired track position. The dual stage loop bandwidth is typically greater than 3 kHz with sampling at a rate typically about 30 kHz. The voice coil actuator  20  (primary) and micro-actuator  60  (secondary) typically have equal gain somewhere in the region 500–1000 Hz, with VCM dominating below that frequency, micro-actuator dominating above that frequency. 
     The fly height control loop  360  uses the read signal  282 , and more specifically the read amplitude error signal (RAES)  288  as an input. The RAES  288  is then gained and filtered, for example, by a 500-Hz low pass compensator  362 . The conditioned signal is then sent to a bias DC voltage driver  364 , whereupon it is converted into a fly height control signal  284 . The fly height control signal  284  then combines with the track-follow control signal  276  from the track-follow control loop to form the micro-actuator command signal  286  to actuate the piezoelectric motor  60  to achieve a desired slider crown  240  for controlling the fly height  254 . The fly height loop bandwidth would be on the order of 100–300 Hz and could use sampling at a rate of about 3 kHz. 
     The frequency responses of the various modes of actuation associated with the tri-stage tracking control system  292  are illustrated in  FIG. 24 . Generally, the VCM  20  is designed to perform well under 500 Hz. Above 500 Hz, the performance of the VCM is limited by resonances starting at about 5 kHz. To compensate for this performance deficit, the micro-actuator or the piezoelectric motor  60  generally has a complimentary frequency response, which allows it to provide the track position control in high frequency region wherein the VCM is ineffective. 
     In addition to providing a high frequency response for track position control, the piezoelectric motor  60  is also used for controlling the fly height  254  by inducing the slider crown  240 . Since the objective of controlling the fly height  254  is to reduce the read amplitude error signal  288  due to many known undesirable effects, the frequency response of the slider crown  240  actuation generally matches the typically low frequency demands of thermal expansion &amp; other environmental fly height altering effects. 
     Thus, the piezoelectric motor  60  provides both a low frequency actuation for the fly height control via the slider crown  240 , and a high frequency actuation for the track position control. 
     The dual purpose of the piezoelectric motor  60  becomes more apparent in connection with  FIGS. 25A–C , which describes in details the working principle of the slider crown  240 . Upon receiving a voltage from the micro-actuator command signal  284 , the piezoelectric motor  60  undergoes a displacement that is proportional to the voltage. For a pure slider crown  240 , the displacement as either an extension or contraction is along the longitudinal axis  206 . For example,  FIG. 25A  illustrates a contraction of the piezoelectric motor  60 . 
     Since the bottom surface  62  of the piezoelectric motor  60  is affixed to the slider  47  via the adhesive pad  70 , the displacement at the bottom surface  62  is therefore greater than that at the top surface  64  of the piezoelectric motor  60  due to the restraint by the slider  47 . The resulting displacement strain field  65  is shaped as a trapezoid as illustrated in  FIG. 25A . 
     With reference to  FIGS. 25B–25C , applying a principle of statics, the displacement strain field  65  is considered as a sum of a uniform displacement strain field  66  and a triangular displacement strain field  67 . It is a well-known fact that the uniform displacement strain field  66  is of a characteristic of a pure compression, while the triangular displacement strain field  67  corresponds to a flexure or bending. In effect, the piezoelectric motor  60  displays both an compressional deflection, causing it to compress, and a flexural deflection, causing a curvature on its surfaces. 
     The AC compressional deflection of the piezoelectric motor  60  affects primarily the track-follow control as it causes the two hinged islands  80  and  82  on the flexure tongue  48  to pivot as a means for controlling a track position. 
     On the other hand, the DC flexural deflection of the piezoelectric motor  60  causes the slider  47  to conform to the curvature of the piezoelectric motor  60 , thereby inducing the slider crown  240 . Thus, the flexural deflection of the piezoelectric motor  60  is used as a means for controlling the fly height  254 . 
     With reference to  FIG. 26  ( FIGS. 26A ,  26 B), the deflected shape of the slider  47  and the piezoelectric motor  60  results in a positive slider crown  240 , thus compensating for the thermally induced slider crown  240 .  FIGS. 26A and 26B  represent two perspective views showing a first position ( FIG. 26A ) of the slider-piezoelectric motor assembly and a subsequent sposition ( FIG. 26B ) of the slider-piezoelectric motor assembly shown deflected (or crowned). 
     The present invention offers several advantages over the conventional slider design. Using the tri-stage tracking fly height control system  292 , it can be seen that both the track position and the fly height  254  can be controlled simultaneously. This is beneficial and highly effective as track position and the fly height  254  in general are interdependent. 
     The slider crown  240  offers a novel means for controlling the static offset as well as the dynamics of the fly height  254  by compensating for the thermal expansion effect of the slider  47 . 
     In addition, a considerable improvement in the altitude sensitivity of the magnetic read/write head  35  may be realized with the present invention, since the fly height  254  can be statically offset accordingly to achieve a desirable ABS pressure distribution. 
     Similarly, the novel design of the present invention permits a greater ABS tolerances, hence lower production cost, since any variation in the fly height  254  can be eliminated by proper adjustment of the fly height  254  using the dual stage tracking fly height control system  292 . 
     Another advantage may be a decrease in the spin-up time due to a reduction in the stiction, since the slider crown  240  may be used to decrease the contact area of the slider  47  with the surface of the magnetic storage disk  14  when the magnetic read/write head  35  comes to a rest. 
     Some other advantages may be realized, including a controlled fly height  254  at various radial positions on the magnetic storage disk  14 , and thus minimal heat addition in the fly height adjust mechanism. 
     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. As an example, while the various motors have been described herein to be comprised of piezoelectric materials, it should be clear that other active materials, such as, electro-strictive material, memory alloy, smart material, electroactive polymers and so forth, could alternatively be employed.