Abstract:
Head stack fixtures for securing head stack assemblies to spin stand testing systems, and spin stand testing systems incorporating such fixtures, are disclosed. Exemplary head stack fixtures comprise a base supporting a piezoelectric actuator. The base includes an attachment mechanism for securing the HSA in such a way that the HSA will pivot relative to the base. When the HSA is secured to the base, the piezoelectric actuator engages the HSA. The piezoelectric actuator is therefore able to pivot the HSA relative to the base for fine positioning of a head of the HSA.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The disclosure relates generally to the field of magnetic disk drives, and more particularly to apparatus and methods related to testing disk drive components. 
   2. Description of the Prior Art 
   Magnetic disk drives are used to store and retrieve data for digital electronic apparatuses such as computers. A typical magnetic disk drive comprises a head, including a slider and a transducer, in very close proximity to a surface of a rotatable magnetic disk. The transducer, in turn, includes a write element and/or a read element. As the magnetic disk rotates beneath the head, a very thin air bearing is formed between the surface of the magnetic disk and an air bearing surface of the slider. The air bearing causes the head to “fly” above the surface of the magnetic disk with a separation (“fly height”) that is typically less than 40 nanometers in contemporary disk drives. As the head flies over the magnetic disk, the write element and the read element can be alternately employed to write and read data bits along a magnetic “track” on the magnetic disk. 
   It will be appreciated that the head is a highly complex component and, accordingly, heads are preferably qualified before being assembled into disk drives. Commonly, heads are tested after being assembled into either a Head Gimbal Assembly (HGA) or after one or more HGAs are assembled into a Head Stack Assembly (HSA). The HGA typically comprises the head, a load beam, a gimbal that attaches the head to the load beam, a swage mount, and electrical traces to facilitate electrical connection of the transducer of the head to a pre-amplifier circuit. The HSA comprises one or more HGAs in a stacked arrangement for use with one or both sides of a magnetic disk and/or multiple magnetic disks. The HSA also typically comprises an actuator arm structure that can pivot in response to torques applied by a voice coil motor. 
   A spin stand is a common testing apparatus for testing writing and reading characteristics of heads. Generally, the spin-stand includes a rotatable disk and a positioning device that secures a HGA or HSA. The positioning device allows the head to be moved to a desired position over the disk. Accordingly, the positioning device typically includes two coarse motors for moving the head in two orthogonal directions relative to the disk. 
   One issue with spin stand testing relates to the problem of keeping the head aligned with a track on the disk. In a disk drive, a feedback loop known as a closed-loop servo is employed to keep the head properly aligned. The closed-loop servo relies on regularly spaced patterns on the disk, known as servo bursts, to determine any misalignment, and then corrects for the misalignment by adjusting the position of the head. However, spin stands historically have been “open-loop” systems, meaning that there is no feedback mechanism for keeping the head aligned with a track. Open-loop systems designed to have very high mechanical precision worked adequately when tracks were relatively wide. However, the demand for increased information storage density and associated improvements in write and read elements have lead to narrower track widths and narrower read elements, and therefore, what were once considered to be very small sources of mechanical imprecision now can cause the head to move significantly off-track during testing. 
   Accordingly, what is needed is a spin stand that is able to dynamically adjust the position of the head to keep the head aligned with a track during testing. 
   SUMMARY 
   According to an embodiment of the invention, a head stack fixture for securing a head stack assembly during spin stand testing comprises a base and a fine positioner including a piezoelectric actuator. The base includes an attachment mechanism for pivotably attaching the head stack assembly to the base. The fine positioner is attached to the base and to the head stack assembly and is configured to pivot the head stack assembly relative to the base. 
   In some embodiments, the fine positioner further includes an actuator housing having a shape of a parallelogram. In some of these embodiments, the piezoelectric actuator is disposed within the parallelogram. The parallelogram can also include notches that reduce the thickness of the actuator housing near the corners of the parallelogram to deform more easily in response to the piezoelectric actuator. The piezoelectric actuator can deform the parallelogram, in some embodiments, in order to pivot the head stack assembly. In some of these embodiments the fine positioner further includes a set screw disposed against a first end of the piezoelectric actuator, and the piezoelectric actuator works against the set screw to deform the parallelogram. The set screw can also provide a pre-load to the piezoelectric actuator. 
   According to an embodiment of the invention, a spin stand testing system comprises a rotatable magnetic recording disk including a track, a head stack fixture for securing a head stack assembly as described above, and a controller configured to actuate a piezoelectric actuator of the head stack fixture to optimize a read signal from a head of the head stack assembly. In some embodiments, the controller actuates the piezoelectric actuator by applying a voltage thereto. The read signal can be generated by reading alignment information on the track with the head, and optimizing the read signal can include using a closed-loop servo. Optimizing the read signal can include maximizing an intensity of the read signal or converging on a target value. In some instances the track includes two parallel sub-tracks each encoding a different frequency, and in some of these embodiments optimizing the read signal includes minimizing a difference between intensities of sub-signals read from the sub-tracks. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a schematic illustration of an exemplary spin stand testing system according to an embodiment of the invention. 
       FIG. 2  shows a schematic illustration of a head stack fixture according to an embodiment of the invention. 
       FIGS. 3 and 4  show, respectively, a cross-section and a top view of a fine positioner according to an embodiment of the invention. 
       FIG. 5  shows exemplary components of a spin stand testing system according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic illustration of an exemplary spin stand testing system  100  according to an embodiment of the invention. The spin stand testing system  100  comprises a disk  102  and a positioning device  104 . The disk  102  is representative of a magnetic recording disk that would be used in a disk drive and is configured to rotate around an axis at a variable rotation rate. The positioning device  104  secures a head stack assembly  106 , including a head  108 , and is configured to position the head  108  to a desired position over the disk  102 . The positioning device  104  includes both apparatus for course adjustment and a head stack fixture including a fine positioner for use in a closed-loop servo system as described in more detail with respect to  FIGS. 2–5 . 
   As shown in  FIG. 1 , the exemplary positioning device  104  comprises an apparatus for course adjustment, which in this example includes two platforms on orthogonal rail systems. More specifically, the head stack assembly  106  is fixed to a base  110  on a first platform  112 . The first platform  112  includes a set of rails  114  upon which the base  110  can move back and forth in a first direction relative to the first platform  112 . A motor (not shown) is one example of a mechanism for driving the base  110  relative to the first platform  112 . Similarly, the positioning device  104  also comprises a second platform  116  including a set of rails  118 . In this embodiment the first platform  112  is configured to move upon the rails  118  relative to the second platform  116  in a second direction. 
   By moving the base  110  relative to the first platform  112 , and by moving the first platform  112  relative to the second platform  116 , the head  108  can be positioned at a desired disk radius, r, and skew angle, α (an angle formed between a longitudinal axis of the HSA  106  and a tangent to a radial line through the head  108 ). It will be appreciated, however, that neither the head stack fixtures or the spin stand testing systems disclosed herein are limited by the particular course positioning apparatus employed by the positioning device  104 . The platform and rails system is described herein merely for illustrative purposes; other course positioning mechanisms can just as readily be employed. 
     FIG. 2  shows a schematic illustration of a head stack fixture  200  for securing a head stack assembly  106  during spin stand testing. As shown in  FIG. 2 , the head stack fixture  200  comprises a base  210 , including an attachment mechanism  220 , and a fine positioner  230 . In general terms, the head stack fixture  200  can be a module for use with any course positioning mechanism of any spin stand testing system. In those embodiments in which the course positioning mechanism is the positioning device  104  ( FIG. 1 ), the base  210  can be the platform  110  ( FIG. 1 ). 
   The attachment mechanism  220  allows the head stack assembly  106  to be pivotably attached to the base  210 , preferably in the same manner as if the head stack assembly  106  were installed in a disk drive. The fine positioner  230  is attached to the base  210  and engages the head stack assembly  106 . When actuated, the fine positioner  230  is configured to pivot the head stack assembly  106  relative to the base  210 . In some embodiments, as shown in  FIG. 2 , the head stack assembly  106  includes a voice coil  240  with a tang  250 , and the fine positioner  230  engages the tang  250 . 
   The fine positioner  230 , in some embodiments, comprises a piezoelectric actuator that includes a piezoelectric material such as lead zirconium titanate (PZT). By applying a voltage across the piezoelectric material, the piezoelectric material can be made to controllably expand or contract, depending on the polarity of the applied voltage. It can be seen that as the fine positioner  230  activates laterally, the head stack assembly  106  pivots around the attachment mechanism  220  causing the head  108  to also move laterally. 
     FIGS. 3 and 4  show, respectively, a cross-section and a top view of an exemplary fine positioner  300 . Fine positioner  300  is an embodiment of the fine positioner  210  ( FIG. 2 ). The cross-section of  FIG. 3  is taken along the line  3 — 3  of  FIG. 4 . The fine positioner  300  includes a clamp  310  fixedly attached to a lower portion  320 . The clamp  310  is configured to secure the tang  250 , for example, by tightening a bolt or set screw  330 . 
   The lower portion  320  includes an actuator housing  340  and a piezoelectric actuator  350 . Although the actuator housing  340  and the clamp  310  are shown in  FIG. 3  as separate components fixedly attached, in some embodiments these components are integrally formed. In the embodiment shown by  FIG. 3 , the actuator housing  340  is formed in the shape of a hollow parallelogram  355  including a top cross-member  360 , and the piezoelectric actuator  350  is attached inside of the parallelogram  355  and to the top cross-member  360 . It will be appreciated that other arrangements of the parallelogram  355  and piezoelectric actuator  350  are also possible; another possible arrangement is described further herein. The hollow parallelogram  355  is preferably open at both ends to allow the parallelogram  355  to deform, as described in more detail herein. It will be appreciated that although the shape of the actuator housing  340  is described herein as a parallelogram  355 , the parallelogram  355  is preferably a rectangle as illustrated in  FIG. 3 . 
   The piezoelectric actuator  350  moves the clamp  310  in a lateral direction  370  in order to translate the tang  250  laterally so that the head stack assembly  106  pivots around the attachment mechanism  220 , thus causing the head  108  to move laterally. The piezoelectric actuator  350  is able to move the clamp  310  laterally, in some embodiments, because it is configured to work against a fixed object such as a set screw  380  disposed through apertures in the actuator housing  340  in order to press against a first end  385  of the piezoelectric actuator  350 . As noted above, a voltage applied to a piezoelectric material can make the piezoelectric material controllably expand or contract. Thus, application of a voltage to the piezoelectric actuator  350  can cause the piezoelectric actuator  350  to expand or contract in a lateral direction with respect to the first end  385 . Since the first end  385  is constrained by the set screw  380 , this expansion or contraction deforms the parallelogram  355  of the actuator housing  340  thereby moving the attached clamp  310 . 
   The parallelogram  355  is able to more easily deform in response to the piezoelectric actuator  350 , in some embodiments, because the parallelogram  355  includes notches  390  that reduce the thickness of the actuator housing  340  near the corners of the parallelogram  355 , as shown in  FIG. 3 . Where the parallelogram  355  is thinned the parallelogram  355  flexes more easily. The remaining material of the parallelogram  355  at each notch  390  behaves as a spring that acts to restore the parallelogram  355  to its original shape when the force from the piezoelectric actuator is removed. As the notches  390  are made bigger and the remaining material is made thinner, the parallelogram  355  deforms more easily but the restorative force is reduced. Accordingly, these two considerations should be balanced for particular applications. 
   The set screw  380  is also able to apply a pre-load to the piezoelectric actuator  350 . The pre-load is an external force applied to the piezoelectric actuator  350  to compress the piezoelectric actuator  350 . To appreciate the benefit of the pre-load it is helpful to understand the behavior of piezoelectric materials in further detail. As is well known, not only will an applied voltage cause a dimensional change in a piezoelectric material, but causing a dimensional change by mechanically straining or compressing the piezoelectric material will produce a voltage across the piezoelectric material. This reciprocal relationship between dimensional change and voltage is commonly referred to as the piezoelectric effect. 
   The piezoelectric effect, however, is not symmetric with respect to compression and tension. That is, most piezoelectric materials can be made to expand much more than they can be made to contract. More particularly, each piezoelectric material has a linear response range in both compression and tension in which the relationship between applied voltage and dimensional change (or between dimensional change and induced voltage) is linear. Most piezoelectric materials exhibit a greater linear response range in tension than in compression. 
   It is desirable to be able to combine the linear ranges for tension and compression in order to obtain the greatest actuation range from the piezoelectric material of the piezoelectric actuator  350 . In some embodiments this is achieved by applying either positive or negative voltages as needed to make use of both linear ranges. However, in other embodiments, the piezoelectric material of the piezoelectric actuator  350  is initially compressed by the pre-load. The pre-load compresses the piezoelectric material to near the end of the compressive linear range and causes a negative voltage to be induced across the piezoelectric material. Thereafter, the full linear range of actuation can be accessed with applied positive voltages rather than having to switch polarity between the tension linear response range and the compression linear response range. 
   It will be appreciated that the relationship of the piezoelectric actuator  350  to the actuator housing  340  in  FIG. 3  and  FIG. 4  is merely illustrative and other arrangements are also possible. Thus, in some embodiments the piezoelectric actuator  350  is located outside of the actuator housing  340 . As an example, and with reference to  FIG. 3 , the piezoelectric actuator  350  can be placed to the left of the actuator housing  340  such that the piezoelectric actuator  350  is between the set screw  380  and the actuator housing  340  and abuts a sidewall  395  of the actuator housing  340  (which does not house the piezoelectric actuator  350  in this embodiment). In this embodiment, as above, the piezoelectric actuator  350  works against the set screw  380  to deform the parallelogram  355  of the actuator housing  340  to laterally translate the clamp  310 . 
   In addition to the head stack fixtures described above, spin stand testing systems comprising such head stack fixtures are also disclosed.  FIG. 5  shows exemplary components of an embodiment of a spin stand testing system  500 . As noted previously, since head stack fixtures of the invention can be employed with any course positioning apparatus, a course positioning apparatus has been omitted from spin stand testing system  500 . 
   The spin stand testing system  500  includes a head stack fixture  200 , a disk  102  having a track  510 , and a controller  520 . In operation, a positioning device (not shown) such as positioning device  104  ( FIG. 1 ) is used to bring the head  108  to a desired location over track  510 . As a read element (not shown) of the head  108  reads the track  510 , a read signal is produced. The read signal is transmitted to the controller  520  along an electrical path  530  (common steps such as amplification and noise filtering are omitted here for simplicity). Although the controller  520  is shown in  FIG. 5  as a discrete component, it will be understood that the controller  520  can also be integrated into the software or firmware of the spin stand testing system  500 . 
   The controller  520  maintains a closed-loop servo feedback to lock the head  108  to the track  510  by optimizing the read signal through actuation of the fine positioner  230 . To enable the closed-loop servo, the track  510  includes alignment information. The alignment information can be in the form of servo bursts at regularly spaced intervals around the track  510 , as is commonly used on tracks in disk drives. Such servo bursts include information that, when read by the head  108 , can be used to determine a misregistry of the head  108  relative to the track  510 . The misregistry typically represents the displacement between a centerline of the track  510  and a center of a read element of the head  108 . Alternately, because the track  510  is used for testing purposes and not particularly for data storage, the alignment information can be continuous along the length of the track  510  rather than in short servo bursts. 
   Accordingly, the controller  520  receives the read signal from a servo burst (or continuously, as the case may be) and seeks to reposition the head  108  to optimize the read signal. The head  108  is repositioned by adjusting an actuation signal applied to the fine positioner  230  by the controller  520  along an electrical path  540 . The actuation signal is preferably a voltage to be applied to the piezoelectric actuator  350 . Though typically the read signal is optimized by aligning the center of the read element to the centerline of the track  510 , this is not always the case. For example, in some testing circumstances it is desirable to offset the head  108  from the track  510  by some finite amount. 
   Where the goal of optimization is to center the head  108  over the track  510 , one alternative is to maximize an intensity of the read signal. In these instances, the controller  520  initially increases or decreases the voltage to the fine positioner  230  and based on the resulting change in the read signal intensity either continues to move the voltage in the same direction, or moves the voltage in the opposite direction, in order to find the position of the maximum read signal intensity. In some embodiments, however, a two-frequency servo burst (or continuous track) is employed. In these embodiments the alignment information is provided as two parallel sub-tracks divided by the centerline of the track  510 , where each sub-track encodes a different frequency. Centering the head  108  over the track  510 , in these embodiments, requires separating from the overall read signal the individual sub-signals from the two sub-tracks and minimizing a difference between the intensities of the two sub-signals. 
   Where the goal of optimization is to align the head  108  over the track  510  so that it is offset by a specific amount from the centerline, similar strategies can be employed. However, rather than simply maximizing a signal or minimizing a difference between sub-signals, in these embodiments the feedback loop converges on an intermediate target value. Accordingly, in some embodiments a calibration file of the radial position of the head  108  as a function of the read signal is first established. When a particular offset is desired when aligning the head  108 , a target value for the read signal, or the difference between sub-signals, is determined from the calibration file. The calibration file can be established, for instance, by stepping the head  108  across the track  510  in known radial increments while measuring the read signal at each step. 
   In the foregoing specification, the invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the invention is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, the invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art.