Abstract:
A disc drive including a rotatable data track, a dual-stage actuator with a primary actuator motor supporting an actuator arm, a read/write head supported by the actuator arm and communicating with a secondary actuator motor, and steps for controlling range of motion of the secondary actuator motor. The controlling steps include supplying and sustaining a bias signal to a single-sided unipolar device driver that then apply a bias voltage the secondary actuator motor to induce the secondary actuator motor to expand substantially one half of its expansion capabilities. And, confining correction signals provided by a control circuit of the disc drive, used in correcting mechanical position of the secondary actuator motor, to a voltage ranging substantially between a positive “+” and negative “−” voltage substantially equal to the applied bias voltage.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 60/202,885 filed May 10, 2000, entitled Single-Sided PZT Driver in Disc Drive. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to incorporation of a single-sided unipolar piezoelectric transducer driver for a disc drive. 
     BACKGROUND 
     Disc drives are used for data storage in modem electronic products ranging from digital cameras to computer systems and networks. Typically, a disc drive includes a mechanical portion, or head disc assembly (HDA), and electronics in the form of a printed circuit board assembly (PCB), mounted to an outer surface of the HDA. The PCB controls HDA functions and provides an interface between the disc drive and its host. 
     Typically, a HDA comprises a magnetic disc surface affixed to a spindle motor assembly for rotation at a constant speed and an actuator assembly position-controlled by a closed loop servo system. The actuator assembly supports a read/write head that traverses generally concentric magnetic tracks radially spaced across the disc surfaces. Disc drives using magneto resistive heads typically use an inductive element to write data to the tracks in the form of magnetic flux transitions and a magneto resistive element to read data, such as servo data, from the track during drive operations. Servo data are typically written to the track during the manufacturing process by a servo track writer and are used by the closed loop servo system for controlling read/write head position during drive operations. 
     Continued demand for disc drives with ever-increasing levels of data storage Continued demand for disc drives with ever-increasing levels of data storage capacity, faster data throughput and decreasing price per megabyte have led disc drive manufacturers to seek ways to increase the storage capacity and improve overall operating efficiencies of the disc drive. Present generation disc drives typically achieve areal densities of several gigabits per square centimeter, Gbits/cm 2 . Increasing recording densities can be achieved by increasing the number of bits stored along each track or bits per inch (BPI), generally requiring improvements in the read/write channel electronics, and/or by increasing the number of tracks per unit width or tracks per inch (TPI), generally requiring improvements in servo control systems. 
     One approach taken by disc drive manufacturers to improve servo control systems has been through the introduction of dual-stage actuator systems. One such system utilizes a suspension based bipolar piezoelectric transducer (PZT) operating in parallel with the VCM and driven by a bipolar driver. To date, attempts at utilizing more cost-effective single-sided unipolar drivers in a dual-stage actuator application have been unsuccessful since the D.C. component of the position signal and the D.C. component of the PZT driver affects both the VCM and the PZT transducer control signals. 
     As such, challenges remain and a need persists for advancing dual-stage actuator art with economical and effective solutions that overcome the constraints present in disc drives with dual-stage actuator systems. 
     SUMMARY OF THE INVENTION 
     The present invention provides an economical method for position-controlling a mechanical position of a micro-actuator of a disc drive, through use of a single-sided unipolar device driver. By supplying a bias voltage to the micro-actuator, rather than as an offset to a reference signal, to preset the mechanical position of the micro-actuator relative to a selected data track of the disc drive, the single-sided unipolar device driver can be used to adjust the position of the micro-actuator, in either a positive or negative position, relative to the preset position, through use of a single polarity input voltage. 
     In a preferred embodiment the micro-actuator is a bipolar piezoelectric transducer that responds to positive voltage input by expanding in a predetermined direction, while contracting in response to the application of a negative voltage. The bias voltage supplied to the piezoelectric transducer is a positive voltage that expands the piezoelectric transducer by substantially one half of the expansion capabilities of the piezoelectric transducer. Correction signals generated by the control circuit of the disc drive are effective in changing the mechanical position of the micro-actuator relative to a selected data track when the correction signal has a voltage in the range of between a positive “+” or a negative “−” voltage substantially equal to the applied bias voltage. Correction signals of negative voltage reduce the voltage supplied to the micro-actuator, thereby causing the piezo electric transducer to contract. Whereas correction signals of positive voltage increase the voltage supplied to the micro-actuator, thereby causing the piezoelectric transducer to expand. 
     By confining the correction signals to a voltage range between a positive “+” or negative “−” voltage substantially equal to the applied bias voltage, a single-sided unipolar driver is effective in controlling the bipolar piezoelectric transducer. And, as the piezoelectric transducer is affixed to the load arm of the head stack assembly, changes in mechanical position of the micro-actuator relative to the selected data track results in changes in mechanical position of the read/write head relative to the selected data track, thereby facilitating position-control of the read/write head relative to the selected data track. 
     These and various other features and advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a disc drive incorporating a single-sided unipolar driver for driving a micro-actuator of the disc drive in accordance with a method of the present invention. 
     FIG. 2 is a functional block diagram of control circuitry of the disc drive of FIG.  1 . 
     FIG. 3 provides a simplified block diagram of a closed-loop suspension-based dual-stage actuator system of the disc drive of FIG.  1 . 
     FIG. 4 is a flow chart of a method for controlling the micro-actuator of the disc drive of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings in general, and more particularly to FIG. 1, shown therein is a top view of a disc drive  100  constructed in accordance with the present invention. Numerous details of and variations for the construction of the disc drive  100  are not included in the following description as such are well-known to those skilled in the art and are believed to be unnecessary for the purpose of describing the present invention. 
     The disc drive  100  includes a basedeck  102  supporting various disc drive components, including a spindle motor assembly  104 . The spindle motor assembly  104  supports at least one axially aligned rotatable disc surface  106  forming a disc stack  108  (also referred to as a “disc pack”). Adjacent the disc stack  108  is a dual-stage actuator assembly  110  (also referred to as an “E-block” or a head stack assembly (HSA)), which pivots about a primary actuator motor support  112  (also referred to as a “bearing assembly”) in a rotary fashion. The HSA  110  includes at least one actuator arm  114  that supports a load arm  116 . Each load arm  116  in turn supports at least one read/write head  118  (also referred as heads  118 ) that correspond to each disc surface  106 . Each disc surface  106  is divided into concentric circular data tracks  120  (only one shown) over which the read/write heads  118  are positionably located, and on which head position control information are written to embedded servo sectors (not separately shown). The embedded servo sectors separate a plurality of data sectors (not separately shown) for use by customers to store data. 
     The HSA  110  is controllably positioned by a primary actuator motor  122  (also referred to as a “voice coil motor assembly” (VCM)), comprising an actuator coil  124  immersed in the magnetic field generated by a magnet assembly  126 . A magnetically permeable flux path is provided by a steel plate  128  (also called a top pole piece) mounted above the actuator coil  124  to complete the magnetic circuit of the VCM  122 . During operation of the disc drive  100 , current is passed through the actuator coil  124  and an electromagnetic field is setup which interacts with the magnetic circuit of the VCM  122  to cause the actuator coil  124  to move relative to the magnet assembly  126  in accordance with the well-known Lorentz relationship. As the actuator coil  124  moves, the HSA  110  pivots about the bearing assembly  112  (also referred to as a primary actuator motor support), causing the heads  118  to move over the surfaces of the discs  106 , thereby achieving a coarse positioning of the heads  118  adjacent a selected data track  120  of the disc surfaces  106 . 
     To attain fine position control of the heads  118  relative to the selected data track  120 , the HSA  110  further includes a micro-actuator  130  (also referred to as a secondary actuator motor) supported by the load arm  116 . In a preferred embodiment the micro-actuator  130  includes a bipolar piezoelectric transducer (not separately shown) that responds to positive voltage inputs by expanding in a predetermined direction, while contracting in the predetermined direction to application of a negative voltage. As the micro-actuator  130  is affixed to the load arm  116  of the HSA  110 , changes in mechanical position of the micro-actuator  130  relative to the selected data track  120  results in changes in mechanical position of the read/write head  118  relative to the selected data track  120 , thereby facilitating fine position control of the read/write head  118  relative to the selected data track  120 . 
     To provide the requisite electrical conduction paths between the read/write heads  118  and disc drive read/write circuitry (not shown), read/write head conductors (not separately shown) are affixed to a read/write flex circuit  132 . Next, the read/write flex circuit  132  is routed from the load arms  116  along the actuator arms  114  and into a flex circuit containment channel  134 , then on to a flex connector body  136 . The flex connector body  136  supports the flex circuit  132  during passage of the read/write flex circuit  132  through the basedeck  102  and into electrical communication a disc drive printed circuit board assembly (PCBA) (not shown) mounted to the underside of the basedeck  102 . The flex circuit containment channel  134  also supports read/write signal circuitry, including preamplifier/driver (preamp)  138  used to condition read/write signals passed between the read/write circuitry (not shown) and the read/write heads  118 . The PCBA of the disc drive supports read/write circuitry, which controls the operation of the heads  118 , as well as other interface and control circuitry for the disc drive  100 . It will be understood, data drivers can be alternatively configured to output analog control signals to the VCM  122  and the micro-actuator  130  in response to digital input values. 
     The disc drive  100  has two primary assemblies, the PCBA (not shown) and a head disc assembly (HDA)  140  attached to the PCBA. Typically, included within the HDA  140  are the HSA  110 , the VCM  122  and the disc pack  108 . 
     Turning to FIG. 2, position-control of the heads  118  is provided by a control circuit  142  that includes the control processor  144 , a demodulator (demod)  146 , an application specific integrated circuit (ASIC) hardware-based servo controller (“servo engine”)  148 , a set of digital to analog converters (DACs)  150  and a motor driver circuit  152 . The components of the control circuit  142  discussed to this point are utilized to facilitate track following algorithms for the HSA  110  (not shown) and more specifically for controlling the VCM  122  in attaining a coarse positioning of the heads  118  relative to the selected data track  120  (not shown). 
     The demodulator  146  conditions head position control information transduced from the disc surface  106  to provide position information of the read/write head  118  relative to the data track  120 . The servo engine  148  generates servo control loop values used by control processor  144  in generating command signals such as velocity-based seek signals used by VCM  122  in executing seek commands, and to maintain position of the HSA  110  during data transfer operations. The command signals are converted by the DACs  150  to analog control signals for use by the motor driver circuit  152  in directing coarse positioning of the heads  118  relative to the selected data track  120  and seek functions of the HSA  110 . 
     In a preferred embodiment dual-stage actuator  110  has a secondary actuator in the form of a piezoelectric transducer-based micro-actuator  130  attached to the load arm  116  (not shown) to provide fine position control of a selected read/write head  118  relative to the corresponding selected data track  120 . For the micro-actuator  130  embodiment, the DACs  150  convert and forward positioning and correction signals received from the servo engine  148  to a zero-order hold device  154  (ZOH  154 ) that continually maintains the positioning signal as a voltage level provided to a summing junction  156  until updated by a subsequent positioning signal issued by the servo engine  148 . The summing junction  156  combines the positioning signal received from the ZOH  154  with a bias signal  158  used for setting and maintaining a range of motion of the micro-actuator  130  during operation of the disc drive  100 . Incorporation of the bias signal  158  enables the use of a single-sided unipolar driver  160  for driving the micro-actuator  130  during operation of the disc drive  100 . The term position-controlling and/or position-control as used herein means, maintaining control of the read/write head  118  relative to the rotating disc surface  106  of disc drive  100  (of FIG. 1) throughout all operations of disc drive  100 . In other words, whether positioning the read/write head  118  relative to a selected data track  120  of the rotatable disc surface  106  during track seek operations or maintaining a position of the read/write head relative to the data track  120  during track following operations, the position of the read/write head  118  relative to the rotatable disc surface  106  is under the control of the control circuit  142  through effecting mechanical positions of the HSA  110 . 
     In a preferred embodiment the bias signal  158  is representative of a bias voltage signal, the single-sided unipolar driver  160  is a single-sided unipolar PZT driver  160  and the micro-actuator  130  is a bipolar piezo electric transducer  130  (hereafter PZT  130 ). The PZT  130  is used for fine position-control of the read/write head  118  relative to the data track  120  and to maintain the mechanical position of the PZT  130  relative to the selected data track  120 , based on the voltage level received from the single-sided unipolar PZT driver  160 . The single-sided unipolar PZT driver  160  maintains a voltage level used to drive the PZT  130  in the form of a position voltage, until the positioning voltage is updated. Once the position voltage is updated, the single-sided unipolar piezo driver  160  induces a change in mechanical position of the PZT  130 , relative to the selected data track  120 , which changes the alignment of the selected read/write head  118  relative to the selected data track  120 . 
     FIG. 3 provides a simplified functional block diagram of the servo control loop of FIG.  2 . For a preferred embodiment, the single-sided unipolar driver  160 , of FIG. 2, is a single-sided unipolar PZT driver  160 , the micro-actuator  130 , of FIG. 2, is a PZT  130 , and the bias signal  158  is a constant output bias signal representative of a constant bias voltage level. Block VCM  162  represents the dynamics of the motor driver circuit  152 , of FIG. 2, with the primary actuator motor  122  of the HSA  110 , of FIG. 1 acting on the read/write head  118  relative to the disc surface  106 . And, block PZT  164  represents the dynamics of the interaction of the single-sided unipolar piezo driver  160 (not shown separately) and the PZT  130  (not shown separately) acting on the read/write head  118  relative to the disc surface  106 . 
     The primary actuator, the HSA  110 , and secondary actuator, the PZT  130 , act in parallel so that the displacements XvCM  166  and XPZT  168  produced by the block VCM  162  and the block PZT  164 , respectively, sum to form the total displacement of the selected read/write head  118  relative to the selected data track  120  of the disc surface  106 . 
     The desired position, represented by reference signal  170  (ref.), and the actual position of the selected read/write head  118  relative to the selected data track  120 , represented by position signal  172 , are fed to the control circuit  142  to produce control signals U VCM    174  and U PZT    176 . Control signal U VCM    174  is passed to block VCM  162 , while U PZT    176  is passed to the summing junction  156 , combined with the constant output bias signal, then passed to the block PZT  164 . 
     Application of the constant output bias signal, represented by bias signal  158 , in the form of a bias voltage to the PZT  130  results in the ability to use the single-sided unipolar piezo driver (represented by  160  of FIG. 2) for controlling the PZT  130 . 
     The ability to operate the single-sided unipolar device driver  160  between any two supply voltages, such as minus five volts and plus twelve volts (−5 v and +12 v) or zero and plus twelve volts (0 v and +12 v) or zero and plus five volts (0 v and +5 v) or zero and plus twenty volts (0 v and +20 v) and so on, is contemplated by this disclosure. Additionally, voltage levels expressed in descriptions of preferred embodiments are used for disclosure clarity and are non-limiting. In a preferred embodiment, the single-sided unipolar piezo driver has an output operating range of between zero voltage and a positive forty-five volts (0 v to +45 v), and the PZT  130  has an operating range of between a minus forty volts and a positive forty volts (40 v to +40 v). And, for purposes of brevity and clarity of disclosure, it is to be assumed that a mechanical response of the PZT  130  responding to changes in voltage applied to the PZT  130  is a symmetrically repeatable linear response. In other words, if applying a positive voltage to the PZT  130 , such as +20 v, results in an expansion of the PZT  130  in a dimension of the PZT  130  equaling one half (½) of its overall expansion capability along that dimension, then application of a −20 v will cause the PZT  130  to contract along the same dimension, but in the opposite direction of its expansion, an amount equaling one-half (½) of its overall contraction capabilities. And, if the PZT  130  has a maximum capability of expanding to 120% of its dimension along one of its dimensions, it also has the maximum capability of contracting to 80% of its dimension along the same dimension. 
     Continuing with the example of a preferred embodiment, by supplying the summing junction  156  with the bias signal, representative of a continuous output bias voltage of a positive twenty volts (+20 v), prior to receipt of the position signal  172  by control electronics  142 , the single-sided unipolar piezo driver  160  drives the PZT  130  to expand to one-half (½) of its expansion capability. Having the PZT  130  preset at ½ of its expansion capability gives the control electronics  142 , the ability to operate with reference signal  170  and position signal  172  to output the control signal UPZT  176  representing a voltage of between negative twenty volts and positive twenty volts (−20 v to +20 v). By combining the control signal UPZT  176 , representing a voltage of between (−20 v to +20 v), with the continuous bias signal  158 , representing a voltage of +20 v, the summing junction  156  provides, to the single-sided unipolar piezo driver  160 , an output signal that is representative of a voltage of between zero volts and plus forty volts (0 v to +40 v). The single-sided unipolar piezo driver  160  in turn drives the PZT  130  with a voltage of between zero volts and plus forty volts (0 v to +40 v). And the PZT  130  responds by reducing its expansion from ½ of its expansion capability to zero expansion, for the case of zero voltage output from the single-sided unipolar piezo driver, or by expanding to 100% of its expansion capabilities for the case of +40 v output from the single-sided unipolar piezo driver. This ability to either expand or contract the mechanical position of the PZT  130  from the preset {fraction ( 1 / 2 )} full expansion capability, through use of a single polarity input voltage, results in the ability to reposition the read/write head  118  either toward or away from the inner diameter of the rotatable disc surface  106 . In other words, by biasing the secondary actuator motor (such as  130 ) of a dual-stage actuator (such as  110 ), a single-sided unipolar driver (such as  160 ) can be used to reposition heads (such as  118 ) of a disc drive (such as  100 ) relative to a selected track (such as  120 ), while retaining the current design and functionality of a control circuit (such as  142 ) of the disc drive. 
     Also shown by FIG. 3 is a disturbance input represented by force vector d  178 , which exemplifies external disturbances such as a vibration or windage from the HSA  110 , or disc slippage or disc pack imbalance from the disc pack  108 , that act on the system and result in an output disturbance signal (not separately shown) included in the position signal  172 . The control system is designed to reject the output disturbance signal by producing motions XvCM  166  and XPZT  168  that counteract the effects of the disturbance force vector “d”  178 . 
     FIG. 4 shows a micro-actuator control process  200  beginning at start process step  202 . The micro-actuator control process  200  continues with process step  204  that sets an initial mechanical position of a secondary actuator motor (such as  130 ) of a dual-stage actuator (such as  110 ) of a disc drive (such as  100 ) utilizing a bias signal (such as  158 ) to preset an initial mechanical position of the secondary actuator motor, and to maintain the initial position of the secondary actuator motor absent a determination, based on a position signal (such as  172 ), to adjust the position of the secondary actuator motor. By presetting the mechanical position of the secondary actuator motor, with a bias signal, a single-sided unipolar driver (such as  160 ) can be utilized to facilitate positional adjustments, in both a positive and negative direction relative to the initial preset mechanical position of the secondary actuator motor, while doing so with the single polarity signal. 
     In a preferred embodiment, the bias signal is a bias-voltage signal. The voltage level of the bias-voltage signal is predetermined empirically by calculating a mean of the displacement responses experienced by each of a representative sample of micro-actuators  130  of a selected PZT material, responding to an application of a known voltage level to each of the representative samples. 
     In step  206  of the micro-actuator control process  200 , the desirability of adjustments of the micro-actuator relative to the selected data track are calculated based on a position signal (such as  172 ) that includes a displacement signals emanating from disc drive environment disturbances, represented by a force vector (such as  178 ). If an adjustment in alignment of the selected head relative to the selected data track is determined by the calculations made in step  206  by a control circuit (such as  142 ), an adjustment signal is generated. In generating an adjustment signal the control circuit compares the position signal to a reference signal (such as  170 ) to form the adjustment signal capable of adjusting the mechanical position of the secondary actuator motor relative to the selected data track. And, in step  208  the adjustment signal is provided to a summing junction (such as  156 ) to be combined with the bias signal in step  210  and applied to the secondary actuator motor in step  212  by the single-sided unipolar driver, thereby controlling the mechanical position of the secondary actuator motor. 
     With the initial mechanical position of the secondary actuator motor set by the bias signal, the changes in mechanical position of the micro-actuator relative to the selected data track calculated, an appropriate adjustment signal generated and combined with the bias signal and then applied to the micro-actuator, the micro-actuator control process  200  ends at step  214 . 
     It is noted, the term micro-actuator or secondary actuator motor, as used herewithin, refers to a device (such as  130 ) capable of altering mechanical position of a selected head (such as  118 ) relative to a selected data track (such as  120 ) independent from or in conjunction with a primary actuator motor (such as  122 ) to alter mechanical position of the selected head relative to the selected data track. It is also noted that: the driver used to implement the invention need not be a single-sided unipolar driver (such as  160 ), but may be a dual-sided bipolar driver; the initial mechanical position of the secondary actuator motor need not be half its the expansion capability, but may set anywhere within the range of its capabilities; and the bias signal (such as  158 ) need not be held at a continuous output level, but may be set to a particular level to accommodate a desired function, such as an ability to expand from full contraction to full expansion to facilitate data collection, manufacturing processing steps or drive operations related to head to disc positioning. 
     Accordingly, the present invention is directed to an apparatus and method for incorporation of a single-sided unipolar device driver for a piezoelectric transducer in a disc drive. In accordance with one aspect, steps are performed of supplying a bias-voltage signal to the micro-actuator to set an initial mechanical position of the micro-actuator relative to a data track of the disc drive, step  204 ; generating a position signal indicative of an alignment between the read/write head of the disc drive and data track for use in determining change in mechanical position of the micro-actuator relative to the data track to align the read/write head to the data track, step  206 ; provided an adjustment signal based on the position signal for use in adjusting mechanical position of the micro-actuator relative to the data track, step  208 ; combining the adjustment signal with the bias-voltage signal forming a correction signal to correct the mechanical position of the micro-actuator relative to the data track, step  210 ; and applying a correction signal to the micro-actuator and inducing a predetermined change in mechanical position of the micro-actuator relative to the data track in response to the application of the correction signal, step  212 . 
     The many features and advantages of the present invention are apparent from the written description. It is intended by the appended claims to cover all such features and advantages of the invention. As numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention.