Abstract:
Method and apparatus for characterizing the flight characteristics of a read/write head. A disc is accelerated to a selected rotational velocity sufficient to aerodynamically support the head. The head is positioned over a portion of the disc non-accessible to customer data and a magnetoresistive element of the read/write is biased using a suitable voltage or current. A media noise landing signature obtained as the element transduces white noise from the surface identifies the head as a low-flying head.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Application No. 60/368,360 filed Mar. 28, 2002 entitled Detecting Head Fly Height Using Media Noise. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to identifying a low-flying read/write head of a disc drive based on a media noise landing signature.  
         BACKGROUND  
         [0003]    Disc drives are used for data storage in modern electronic products ranging from digital cameras to computers and network systems. Typically a disc drive includes a mechanical portion and an electronics portion in the form of a printed circuit board assembly that controls functions of the mechanical portion while providing a communication interface to a host being serviced by the disc drive.  
           [0004]    Typically, the mechanical portion, or head-disc assembly, has a disc with a recording surface rotated at a constant speed by a spindle motor assembly and an actuator assembly positionably controlled by a closed loop servo system for use in accessing the stored data. The actuator assembly commonly supports a magnetoresistive read/write head that writes data to and reads data from the recording surface. Normally, the magnetoresistive read/write head uses an inductive element, or writer, to write data to and a magnetoresistive element, or reader, to read data from the recording surface.  
           [0005]    The disc drive market continues to place pressure on the industry for disc drives with increased capacities, higher data rates and lower costs. A key aspect of achieving lower costs is an identification of marginal components as early as practical in the manufacturing process to preclude needless accrual of additional manufacturing costs and costly rework operations in subsequent processes. Additionally, an ability to identify, remove and replace marginal components from a disc drive prior to shipment is an aid in reduction of field failure and warranty expense.  
           [0006]    A critical component of a disc drive is the magnetoresistive read/write head. As each read/write head passes through manufacturing processes in preparation for use in a disc drive, costs associated with those processes accrue and contribute to the overall cost of the disc drive. By measuring characteristics of the read/write head throughout the manufacturing process, defective and marginal read/write heads can be culled from the process before additional costs are needlessly applied.  
           [0007]    Fly height of a read/write head is an important operating characteristic of the read/write head for proper operation of the disc drive. A read/write head with a fly height greater than a specified nominal fly height will typically display poor data transfer characteristics and is generally replaced. However, a read/write head with a fly height less than the specified nominal fly height will typically display good data transfer characteristics and, unless detected, is generally not replaced. An undetected low-fly head within a disc drive poses an increased risk to subsequent failure of the disc drive over the useful life of the disc drive.  
           [0008]    As such, challenges remain and a need persists for effective techniques for determining a low-flying read/write head within a disc drive throughout the disc drive manufacturing process. It is to this and other features and advantages set forth herein that embodiments of the present invention are directed.  
         SUMMARY OF THE INVENTION  
         [0009]    As exemplified herein, embodiments of the present invention are directed to categorization of a fly height status of a read/write head of a disc drive as either a low-flying read/write head or as a non low-flying read/write head.  
           [0010]    Categorization of the fly height status of the read/write head is based on detection of an occurrence of a media noise landing signature prior to encountering a predetermined rotational velocity threshold. The media noise landing signature is preferably determined from a readback response of the head to white noise (i.e., a readback signal obtained from a nonrecorded region of the disc).  
           [0011]    During a landing procedure, the media noise landing signature is provided by a readback signal generated from white noise present in a magnetic recording surface of a disc of the disc drive. The readback of the white noise present in the recording surface occurs as a disc decelerates from a nominal operating rotational velocity to a stationary state during the landing procedure. The media noise landing signature is formed just prior to the read/write head landing on the disc. 
       
    
    
       [0012]    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  
       [0013]    [0013]FIG. 1 is a top plan view of a disc drive that incorporates a read/write head screened for low-fly height.  
         [0014]    [0014]FIG. 2 is a functional block diagram of a circuit for controlling operation of the disc drive of FIG. 1 and in determining a fly height status of the read/write head of FIG. 1.  
         [0015]    [0015]FIG. 3 is a graphical representation of a typical media noise landing signature of the white noise present in the recording surface read by the read/write head prior to landing on a disc.  
         [0016]    [0016]FIG. 4 is a flow chart of a characterization process for characterizing the fly height status of a read/write head of the disc drive of FIG. 1.  
         [0017]    [0017]FIG. 5 provides a functional block diagram of a system configured to carry out the routine of FIG. 4 in accordance with preferred embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring to the drawings in general, and more particularly to FIG. 1, shown therein is a top view of a disc drive  100 , also referred to herein as a data storage device, constructed in accordance with preferred embodiments of the present invention.  
         [0019]    The disc drive  100  includes a basedeck  102  supporting various data storage device components including a spindle motor assembly  104  that supports one or more axially aligned rotatable discs  106  forming a disc stack  108 , each disc  106  having at least one, and usually two, recording surfaces  109 .  
         [0020]    Adjacent the disc stack  108  is a head stack assembly  110  (also referred to as an actuator assembly) that pivots about a bearing assembly  112  in a rotary fashion. The actuator assembly  110  includes an actuator arm  114  that supports a load arm  116 , which in turn supports a read/write head  118  corresponding to the rotatable recording surface  109 . The recording surface  109  is divided into concentric information tracks  120  (only one depicted) over which the read/write head  118  is positionably located. The information tracks  120  accommodate head position control information written to embedded servo sectors (not separately depicted).  
         [0021]    Between the embedded servo sectors are data sectors used for storing data in the form of bit patterns. The read/write head  118  includes a reader element (not separately shown) offset radially and laterally from a writer element (not separately shown). The writer element writes data to the concentric information tracks  120  while the reader element controls the positioning of the read/write head  118  relative to the concentric information tracks  120  during write operations. During read operations the reader element reads data from the concentric information tracks  120  for passage to a host (not shown) serviced by the disc drive  100  and for use by a servo control system.  
         [0022]    The term “servoing,” or “position-controlling,” as used herein, means maintaining control of the read/write head  118  relative to the rotating recording surface  109  during operation of the disc drive  100 . When servoing to or servoing on a selected information track  120 , the actuator assembly  110  is controllably positioned by a voice coil motor assembly  122 . The voice coil motor assembly  122  includes an actuator coil  124  immersed in a magnetic field generated by a magnet assembly  126 . A pair of steel plates  128  (pole pieces) mounted above and below the actuator coil  124  provides a magnetically permeable flux path for a magnetic circuit of the voice coil motor  122 .  
         [0023]    During operation of the disc drive  100 , current passes through the actuator coil  124  forming an electromagnetic field, which interacts with the magnetic circuit of the voice coil motor  122 , causing the actuator coil  124  to move relative to the magnet assembly  126 . As the actuator coil  124  moves, the actuator assembly  110  pivots about the bearing assembly  112 , causing the read/write head  118  to move over the rotatable recording surface  109 , thereby allowing the read/write head  118  to interact with the information tracks  120  of the recording surface  109 .  
         [0024]    To provide the requisite electrical conduction paths between the read/write head  118  and read/write circuitry of the disc drive (not shown), read/write head wires (not shown) affixed to the read/write head  118  are attached to a read/write flex circuit  130 . The read/write flex circuit  130  is routed from the load arm  116  along the actuator arm  114  and into a flex circuit containment channel  132  and secured to a flex connector body  134 .  
         [0025]    The flex connector body  134  supports the flex circuit  130  during passage through the basedeck  102  and into electrical communication with a printed circuit board assembly (PCBA), (not shown) typically mounted to the underside of the basedeck  102 .  
         [0026]    The flex circuit containment channel  132  also supports read/write signal circuitry including a preamplifier/driver (preamp)  136  that conditions read/write signals passed between the read/write circuitry and the read/write head  118 . The printed circuit board assembly mounted to the underside of basedeck  102  provides the data storage device read/write circuitry that controls the operation of the read/write head  118 , as well as other interface and control circuitry for the disc drive  100 .  
         [0027]    Turning to FIG. 2, position-controlling of the read/write head  118  is provided by the positioning mechanism (not separately shown) operating under the control of a servo control circuit  142  programmed with servo control code, which forms a servo control loop.  
         [0028]    The servo control circuit  142  includes a micro-processor controller  144  (also referred to herein as controller  144 ), a volatile memory or random access memory (VM)  145 , a demodulator (DEMOD)  146 , an application specific integrated circuit (ASIC) hardware-based servo controller (“servo engine”)  148 , a digital to analog converter (DAC)  150  and a motor driver circuit  152 . Optionally, the controller  144 , the random access memory  145 , and the servo engine  148  are portions of an application specific integrated circuit  154 .  
         [0029]    Typically, a portion of the random access memory  145  is used as a cache for data read from the information track  120  awaiting transfer to a host connected to the disc drive  100  and for data transferred from the host to the disc drive  100  to be written to the information track  120 . The components of the servo control circuit  142  are utilized to facilitate track following algorithms for the actuator assembly  110  (of FIG. 1) and more specifically for controlling the voice coil motor  122  in position-controlling the read/write head  118  relative to the selected information track  120  (of FIG. 1).  
         [0030]    The demodulator  146  conditions head position control information transduced from the information track  120  of the rotatable recording surface  109  to provide position information of the read/write head  118  relative to the information track  120 . The servo engine  148  generates servo control loop values used by the controller  144  in generating command signals such as seek signals used by voice coil motor  122  in executing seek commands. Control loop values are also used to maintain a predetermined position of the actuator assembly  110  during data transfer operations.  
         [0031]    The command signals generated by the controller  144  and passed by the servo engine  148  are converted by the digital to analog converter  150  to analog control signals. The analog control signals are used by the motor driver circuit  152  in position-controlling the read/write head  118  relative to the selected information track  120 , during track following, and relative to the rotatable recording surface  109  during seek functions.  
         [0032]    In addition to the servo control code program of the application specific integrated circuit  154 , control code is also programmed into the application specific integrated circuit  154  for use in executing and controlling data transfer functions between a host  156  and the disc drive  100 . Read/write channel electronics  158 , operating under control of the controller  144  executing the control code, passes data received from the host  156  to the read/write head  118  for storage on the disc  106  and passes data read by the read/write head  118  from the disc  106  back to the host  156 .  
         [0033]    The read/write channel electronics  158  includes a servo variable gain amplifier (SVGA)  160 , which amplifies an amplitude of a head position control signal read from the information track  120 . The amplified amplitude of the head position control signal, provided by the servo variable gain amplifier  160 , is stored in a servo variable gain amplifier register  162  for subsequent release to, and processing by, the servo engine  148 .  
         [0034]    It will be recognized that the height (distance) that a given read/write head  118  flies above an associated disc surface will generally depend upon the rotational speed of the discs  106  and the particular characteristics of the head. While all of the heads  118  in a particular disc drive are designed to nominally fly at the same height for a selected rotational speed of the discs  106  (i.e., a nominal operational fly height), some amount of manufacturing variations will tend to be present in a given population of heads.  
         [0035]    Thus, in a given disc drive  100 , some heads will tend to fly at a slightly higher than nominal operational fly height while other heads will tend to fly at a slightly lower than nominal operational fly height. The lowest flying head  118  in a disc drive  100  will typically land (i.e., contact the associated disc surface) before the other heads  118  in the drive as the discs  106  are decelerated to rest.  
         [0036]    A particularly low flying head can accordingly pose a long term reliability risk for the drive. Drive manufacturers have employed a number of different methodologies in an attempt to screen for low flying heads during disc drive manufacturing operations.  
         [0037]    The present invention (as embodied herein and as claimed below) provides a novel approach to characterizing the fly height characteristics of a selected head  118  through evaluation of the readback response of the head  118  to white noise (i.e., a readback signal obtained from a nonrecorded region of the associated disc  106 ).  
         [0038]    As will be recognized, an amplitude of a readback signal obtained from a selected head  118  will generally increase as the head  118  comes into closer proximity to the disc  106 . It has been found that a peak amplitude of a readback signal obtained from white noise on the disc  106  will reach a maximum value just prior to the head  118  landing on the disc surface. Thus, monitoring the readback response of a head to white noise provides a distinct media landing noise signature that can be used to accurately determine the fly height characteristics of the head.  
         [0039]    [0039]FIG. 3 provides a graphical representation of a readback signal  170  obtained from a selected head  118  of the disc drive  100  in accordance with preferred embodiments of the present invention. The signal  170  is plotted against a disc speed x-axis (in decreasing revolutions per minute, rpm) and a signal amplitude y-axis.  
         [0040]    The signal  170  was obtained as the head  118  was flown over the landing zone ( 120 , FIG. 1). The head  118  had not been previously used to write data to this portion of the disc  106 ; rather, the white noise exhibited in the signal  170  arose from the existing, substantially randomly directed magnetization of the particles in the magnetic recording layer of the disc  106 . The signal  170  was obtained while the head  118  flew in an ambient environment (for example, air) while the disc speed was gradually reduced.  
         [0041]    It will be noted from FIG. 3 that portion  172  of the readback signal  170  represents the response of the head  118  while the head was supported adjacent the disc surface. As the head  118  came into close proximity with the disc  106 , an increase in signal amplitude was encountered, culminating in a peak amplitude at point  174 . The head then subsequently came into substantially constant contact with the disc  106  at point  176  (and, due to friction forces, the disc  106  decelerated quickly to a full stop). It has been determined that this characteristic media landing noise signature (as shown in FIG. 3) can be used as a reliable and repeatable indicator of the landing characteristics of the head  118 .  
         [0042]    Generally, it has been found that higher flying heads tend to land at a lower rpm and lower flying heads tend to land at a higher rpm. By applying a threshold value suitable for the environment in which the discs  106  are rotated, heads that exhibit the peak amplitude above said threshold value can be characterized as insufficiently low flying heads and removed from the manufacturing operation.  
         [0043]    [0043]FIG. 4 provides a flow chart for a head flight characterization routine  200 , generally illustrative of steps carried out in accordance with preferred embodiments of the present invention. The routine is preferably carried out during manufacturing using a test stand or other suitable test equipment for a population of heads (such as  118 ). The routine can also be carried out within the confines of a disc drive (such as  100 ), as desired. Also, the routine can be carried out using an ambient environment or a reduced density environment. The reduced density environment may be obtained by replacing a portion of the ambient atmosphere with an inert gas such as helium. The first head to be tested is selected at step  202 , and the associated disc (such as  106 ) is accelerated at step  204  to a nominal rotational velocity. This results in the generation of an air bearing sufficient to fly the head  118  adjacent the disc surface, as indicated by step  206 . The head  118  is moved at step  208  to a position adjacent a region of the disc surface inaccessible for storage of customer data (such as the landing zone  120 ).  
         [0044]    An appropriate biasing of the head  118  (such as through application of a low level read bias current) is applied at step  210  to enable the head  118  to output a readback signal in response to the white noise of the random magnetization of the disc surface. Monitoring of the readback response of the head is initiated at step  212 . At this point, the head  118  will generally provide a baseline response such as shown at portion  172  of signal  170  in FIG. 3.  
         [0045]    The disc surface is next decelerated beginning at step  214  and continuous speed measurements are obtained as the disc surface decelerates to rest. As the head  118  comes closer to the disc surface, a peak amplitude of the media landing noise signature (such as point  174 , FIG. 3) will be determined, step  216 . This can be carried out by monitoring the output of a digital oscilloscope configured to display the readback response. The peak amplitude can also be determined through sample averaging of adjacent values in the readback signature.  
         [0046]    The associated disc speed S 1  corresponding to the peak amplitude is next identified at step  218 , and this disc speed is compared to a preselected threshold value ST at step  220 . As shown by decision step  222 , when the disc speed S 1  is found to be greater than the threshold value ST, the head is determined to have unsuitably low flying characteristics and the flow passes to step  224  where the head is replaced or otherwise rejected from the manufacturing operation.  
         [0047]    On the other hand, when the disc speed S 1  is less than the threshold value ST, the head is accepted for further operations. The routine then passes to decision step  226  which inquires whether additional heads remain to be tested. If so, the next head is selected at step  228  and the routine is repeated for the next selected head. Finally, when all heads have been tested in turn, the routine ends at step  230 .  
         [0048]    In an alternate embodiment, each of the plurality of disc drives (such as  100 ) are placed in an altitude chamber, the rotational velocity of the disc (such as  106 ) is maintained at the nominal operating rotational velocity and a white noise present in a recording surface (such as  106 ) of the disc is read while increasing the effective altitude experienced by the disc drive.  
         [0049]    Upon encountering the media noise landing signature, the effective altitude experienced by the disc drive concurrent with the occurrence of the media noise landing signature is logged for each of the plurality of disc drives of the particular configuration being evaluated. The data are reviewed and a determination is made, taking into consideration any additional margin of safety thought appropriate, regarding an effective altitude threshold, and an effective altitude threshold is set.  
         [0050]    In an additional alternate embodiment, an ambient atmosphere within each of the plurality of disc drives (such as  100 ) is displaced with a lower density atmosphere, such as helium, at a predetermined rate; rotational velocity of the disc (such as  106 ) is maintained at substantially the nominal operating rotational velocity; and a white noise present in the recording surface (such as  109 ) of the disc is read while the ambient atmosphere is displaced. Displacement of the ambient atmosphere with a lower density atmosphere decreases the fly height of the read/write head (such as  118 ).  
         [0051]    Upon encountering the media noise landing signature, the rate dependent elapse time of the atmosphere displacement procedure is logged for each of the plurality of disc drives of the particular configuration being evaluated. The data are reviewed and a determination is made, taking into consideration any additional margin of safety thought appropriate, and an effective rate dependent elapse time threshold is set.  
         [0052]    [0052]FIG. 5 provides a system  300  configured to carry out the routine of FIG. 4 in accordance with preferred embodiments of the present invention. The system  300  includes several components discussed above including the disc  106 , head  118 , preamp  136  and read/write channel  158  shown in FIG. 2. The system further preferably includes a housing  302  in which at least the head  118  and the disc  106  are disposed, a motor  304  used to rotate the disc  106  at a desired rotational speed, a control circuit  306  which provides overall control of the system  300 , and an analysis and display module  308 .  
         [0053]    In a preferred embodiment, the system  300  is incorporated into a spin-stand in which multiple discs  106  and heads  118  are supported. In such case the heads  118  are preferably evaluated as part of a servo track writing operation in which the aforementioned servo information is written to the disc surfaces  109 . The control circuit  306  in this configuration can comprise a host computer alone or in conjunction with selected circuitry from FIG. 2 configured to carry out the routine of FIG. 4. The module  308  can comprise a monitor of the computer or a separate data acquisition device (such as a digital oscilloscope).  
         [0054]    In an alternative embodiment, the system  300  is embodied within the disc drive  100  so that the housing  302  corresponds to the housing formed by the base deck  102  and top cover  103 , the motor  304  corresponds to the spindle motor  104  (FIG. 1) and the control circuit  306  corresponds to the controller  144  (FIG. 2).  
         [0055]    The module  308  can comprise a separate data acquisition device (such as a digital oscilloscope) with test probes placed in electrical communication with appropriate test points on the disc drive printed circuit board assembly to obtain data as shown in FIG. 3. The display module  306  can also be incorporated into the circuitry of FIG. 2 such as by using suitable programming of the controller  144  so that the signature is detected in relation to baseline and peak values from the readback signals obtained from the head  118 .  
         [0056]    Accordingly, embodiments of the present invention are generally directed to categorization of a read/write head (such as  118 ) of a disc drive (such as  100 ) as a low-flying read/write head or a non low-flying read/write head. The categorization is based on an occurrence of a media noise landing signature (such as  170 ) occurring prior to an encountering of a predetermined threshold. The media noise landing signature is based on a readback of a white noise present in a recording surface (such as  109 ) located within a region of the disc inaccessible for storage of customer data. The readback of the white noise present in the recording surface preferably occurs while a disc (such as  106 ) decelerates from a nominal operating rotational velocity to a stationary state during a landing procedure.  
         [0057]    For purposes of the appended claims, it will be understood that the disclosed structure corresponding to the recited first means comprises the circuitry shown in FIG. 5.  
         [0058]    It will be clear that the present invention is well adapted to attain the ends and advantages mentioned as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the appended claims.