Abstract:
A method for monitoring a data storage medium is provided in which a virtual head fly profile is measured. In addition, a data storage device for implementing the monitoring method is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/383,051, filed May 23, 2002. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of data storage devices and, more particularly, to a method of screening for abnormal head fly profiles in a data storage device. 
     BACKGROUND 
     Typically, a data storage device such as a magnetic disc drive, an optical storage device, and a removable disc drive, among others, contains a medium or data storage surface on which data is stored by writing to the media using a transducer, or head. Similarly, data is retrieved from the media by using the head in a read mode. The transducer is normally positioned a specific distance from the media, commonly referred to as a head-disc gap. A spindle motor and a voice coil motor are able to change the relative position of the head and media by a positioning control means. A read/write channel allows communication between the head, positioning control means, and a controller. The controller&#39;s main functions are to receive and interpret commands from an outside source, usually an interface adapter on a computer system bus, and transmit control signals to the read/write channel and positioning control means. 
     A data storage surface, as used in a typical hard disc drive, is commonly divided into multiple data zones oriented in the radial direction. Each data zone typically comprises a set of tracks. Typically, data is stored on the tracks in sectors. Tracks in the outer data zones are longer than those in the inner data zones, thus the tracks in the outer data zones are able to store more data than the tracks in the inner data zones. In this configuration, the additional capacity in the outer data zones is utilized by having a larger number of sectors on each track in the outer data zones. This results in the number of data sectors per track varying from zone to zone. 
     With the use of hard disc drives, the concept of “flying” heads is generally adopted and can be similarly applied to other data storage devices having similar reading and writing mechanisms. The flying effect of a head is usually achieved by the special design of the air-bearing surface on the head structure that generates elevation whenever there is a difference in air pressure on the head caused by the spinning of the data storage surface. As the rotational speed of the data storage surface increases, so does the head-disc gap created by this phenomenon. At the point when the target rotational speed is reached, a controlled head-disc gap is created that enables the head to glide across the data storage surface effortlessly without actual contact with the data storage surface. 
     Any head that is flying too low to the data storage surface will incur at least two major risks. First, probability of flight disturbances will be high due to the presence of uneven micro-bumps on the data storage surface. This will often cause “skipped writes” and other read/write abnormalities. A skipped write is an abnormal write event where the writer/head experiences a sudden lift away from the disc surface. This is normally caused by a disturbance in the air flow or particle contact. This event is thought to be caused at times by the inherent lack of spacing between head and the disc where the probability of air disturbance or particle contact is higher. The final result of such an event is a badly written region which leads to user errors. Second, there is a higher risk of head to surface contact. This can result in smearing, scratching, or even a head crash. 
     All of these effects are often aggravated by changes in the device temperature. Any temperature increase will cause a corresponding change in air pressure, which may affect the head-disc gap and in turn will cause the head to fly lower. Any such effects are undesirable and may cause long-term reliability issues. 
     Historically, previous methods have measured head-disc gap by a number of different techniques. The previous methods have also tried to estimate the short-term effect of flight disturbances on the data storage device&#39;s operation. 
     However, there are problems with the previous methods. The previous methods do not predict long-term drive failure. Also, many of the previous methods require the addition of special hardware to the data storage device, thus increasing the cost and complexity of the device. Typically, the previous methods used servo data to perform calculations. This method can not be used to predict long term device reliability issues because servo data is usually recorded at a lower recording frequency than user data. This is due to the fact that the analysis of data recorded at a lower frequency results in less sensitive and accurate measurement of critical effects on a data storage device. Furthermore, the effect of flight disturbances on user data is ultimately what is important to the user, not the effect on servo data. 
     Even further, the previous methods do not effectively predict data storage device failure due to individual problem areas, collectively problematic regions of the data storage surface, problematic heads, or data storage surfaces in general. Also, the previous methods neglect the effect of varying data density on recording signal strength. Increased data density improves the storage capacity of the device, but can result in data interfering with neighboring data. This phenomenon is known in the art as Inter-Symbol Interference (ISI). Along with head-disc spacing, inter-symbol interference can also affect signal strength. Thus, fly-height abnormalities at an area of higher data density on the data storage surface will have a greater effect on signal strength than fly-height abnormalities at an area of lower data density. 
     The present invention provides a solution to these and other problems, and offers other advantages over the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention relates to data storage devices with concentric tracks of data which solve the above-mentioned problems. 
     In accordance with one embodiment of the invention, a method for monitoring a data storage medium having concentric data storage tracks is provided including writing test data at a constant frequency on a first diameter and at a second diameter of a data zone; reading the test data at the first diameter to produce a first read signal and at the second diameter to produce a second read signal; comparing the first read signal with the second read signal; and determining if there is a fly-height abnormality from the comparing. In addition, the invention also can be implemented as a data storage device itself. 
     These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a disc drive incorporating the preferred embodiment of the present invention. 
         FIG. 2  is a functional block diagram of the disc drive of  FIG. 1 . 
         FIG. 3  shows theoretical signal amplitude response to the increase in area density. 
         FIG. 4  shows theoretical adaptive gain response to increasing signal amplitude. 
         FIG. 5  shows a simplified example of a data surface with data zoning. 
         FIG. 6  shows adapted VGA gain response with and without zoning. 
         FIG. 7  shows adapted VGA gain response within a zone. 
         FIG. 8  shows a flowchart for determining a virtual head fly profile. 
     
    
    
     DETAILED DESCRIPTION 
     A disc drive  100  constructed in accordance with a preferred embodiment of the present invention is shown in  FIG. 1 . The disc drive  100  includes a base  102  to which various components of the disc drive  100  are mounted. A top cover  104 , shown partially cut away, cooperates with the base  102  to form an internal, sealed environment for the disc drive in a conventional manner. The components include a spindle motor  106  that rotates one or more discs  108  at a constant high speed. Information is written to and read from tracks on the discs  108  through the use of an actuator assembly  110 , which rotates during a seek operation about a bearing shaft assembly  112  positioned adjacent the discs  108 . The actuator assembly  110  includes a plurality of actuator arms  114  which extend towards the discs  108 , with one or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a read/write head  118  which includes an air bearing slider (not shown) enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 . 
     During a seek operation, the position of the read/write heads  118  over the discs  108  is controlled through the use of a voice coil motor (VCM)  124 , which typically includes a coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets  128  which establish a magnetic field in which the coil  126  is immersed. The controlled application of current to the coil  126  causes magnetic interaction between the permanent magnets  128  and the coil  126  so that the coil  126  moves in accordance with the well known Lorentz relationship. As the coil  126  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112 , and the heads  118  are caused to move across the surfaces of the discs  108 . 
     A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly includes a printed circuit board  132  to which head wires (not shown) are connected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and a preamplifier for amplifying read signals generated by the heads  118  during a read operation. The flex assembly terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive  100 . 
     As shown in  FIG. 1 , located on the surface of the discs  108  are a plurality of nominally circular, concentric tracks  109 . Each track  109  preferably includes a number of servo fields that are periodically interspersed with user data fields along the track  109 . The user data fields are used to store user data and the servo fields used to store servo information used by a disc drive servo system to control the position of the read/write heads. 
       FIG. 2  provides a functional block diagram of the disc drive  100  of  FIG. 1 , operably connected to a host computer  200 . As shown in  FIG. 2 , the disc drive  100  generally comprises or includes circuits or modules for spindle control  226 , servo control  228  and read/write channel control  212 , all operably connected to a system microprocessor  216 . Additionally, an interface  202  is shown connected to the read/write channel  212  and to the system microprocessor  216 , with the interface circuit  202  serving as a conventional data interface and buffer for the disc drive  100 . As will be recognized, the spindle control  228  controls the rotational speed of the spindle motor  106 . 
     In operation of the disc drive  100 , the servo control  228  receives servo position information from the tracks  109  via the read/write heads  118  and, in response thereto, provides a correction signal to the actuator coil  126  in order to position the heads  118  with respect to the discs  108 . The read/write channel  212  operates to write data to the tracks  109  in response to user data provided to the channel from the interface  202  by encoding and serializing the data and generating a write current utilized by the heads  118  to selectively magnetize portions of a selected track  109  on the discs  108 . Correspondingly, data previously stored on a track  109  are retrieved by the read/write channel  212  by reconstructing the data from the read signals generated by a head  118  as the head passes over the selected track  109  on the disc  108 . The operation of the read/write channel  212  in accordance with the preferred embodiment of the present invention will be discussed in greater detail below. 
     It will be noted that the various operations of the disc drive  100  may be controlled by the microprocessor  216 , in accordance with programming stored in system microprocessor memory  224 . Those skilled in the art will recognize that typical disc drives include additional circuitry and functions beyond those delineated above, but such are only of general interest to the present discussion and accordingly do not warrant further description. 
     As shown in  FIG. 3  when data is written at a constant frequency, theoretical signal amplitude  202  varies in a linear fashion to the radius measured from an outer diameter (OD) to an inner diameter (ID). Area density  204  (KBPI) in this case varies in a linear, inversely proportional manner to the radius measured from the OD to the ID. This is due to the effect of increasing Inter-Symbol Interference (ISI) as the area density increases. 
       FIG. 4  shows that the theoretical adaptive Variable Gain Amplifier (VGA) gain response  402  is inversely proportional to increasing the signal amplitude. 
       FIG. 5  is an illustrative example of data zoning on a disc. Zoning is done to help compensate for large differences in area density between the disc inner diameter (ID) track  504  and the disc outer diameter (OD) track  506 . This example contains four zones A, B, C, and D  502 , each zone preferably written at a constant frequency. Each of the zones also has an inner diameter track  508  and an outer diameter track  510 , as shown for Zone B  512  in  FIG. 4 . Because area density increases toward the ID, as shown in  FIG. 3 , the frequencies in the zones  502  follow this relationship: 
     Freq. Of Zone A&gt;Freq. Of Zone B.&gt;Freq. Of Zone C&gt;Freq. Of Zone D 
       FIG. 6  illustrates VGA values with zoning  610  and without zoning  620 . As can be seen in  FIG. 6 , when the head  118  goes into a new zone  506  (crosses over the boundary  602 ), there is an expected VGA gain drop due to the reduction of frequency. This can be seen when the head crosses over a zone boundary  602 . This change is due to the change in frequency from one zone  502  to the other, as the head moves from the OD to the ID. 
     Within a zone  502 , the effect as seen in  FIG. 3  can still be felt, though the difference in signal amplitude  202  is smaller. This is dependent on the size and number of zones  502  used.  FIG. 7  illustrates this difference, called ΔVGA  702 . Ideally, ΔVGA  702  is a negative value approximately around an optimum number (ΔVGA OPT ) . For example, if ΔVGA in a certain zone is well above ΔVGA OPT , it is highly likely the increase in signal amplitude  202  is due to a low head flight profile. This may indicate long-term reliability issues. Preferably, the method to measure this amount of amplitude change is to use an adaptive gain read-out of a channel amplifier (not shown) located in the read/write channel  212 . 
       FIG. 8  illustrates a process of screening for abnormal heads representative of the preferred embodiment. The process takes place either during manufacture or on-the-fly in a hard disc drive having a disc surface  108 . The disc surface contains M zones  502 , where M is a positive whole number. First, a constant frequency (single tone 2T) pattern is written  802  on the inner diameter  508  and the outer diameter  510  of each zone  502 . Starting at the first zone  804 , the ID track and the OD track of the zone is read  806  and ΔVGA is computed for the zone  506  by the following equation:
 ΔVGA=VGA OD −VGA ID   (1) 
This is preferably done for each zone  502 .
 
     Once ΔVGA is obtained for at least one zone, it can be used to screen for abnormal flying heads  808 . One advantage of screening for abnormal flying heads is to improve disc drive reliability. Three methods are proposed for screening abnormal flying heads. 
     The first screening method tests for whether the head is flying lower at the inner diameter (i.e.  508 ) of the zone than at the outer diameter (i.e.  510 ) of the zone. This can be accomplished by defining a threshold level Y 1  and then comparing Y 1  to ΔVGA. If ΔVGA is greater than or equal to Y 1  a warning signal is sent  810  to the microprocessor  216  to indicate a test failure because this should only happen when the head is flying lower at the inner diameter than at the outer diameter. This test condition may be represented as shown below:
 
ΔVGA(n)≧Y1  (2)
 
     This test condition usually indicates the head is flying lower at the inner diameter of the zone than at the outer diameter. If this condition is present, the drive may be able to read or write successfully in this zone, but this abnormal flight can compromise reliability for the medium or long term life of the drive. 
     A second screening method screens out heads that have a positive ΔVGA for more than one zone  502 . For the second method, ΔVGA is calculated from the zone containing the disc outer diameter  506  thru the current zone, n. This may be represented by the following equation: 
     
       
         
           
             
               
                 
                   
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                         Δ 
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                           VGA 
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     If this A(n) is greater than a predetermined value Y 2 , there may be a consistent problem over the majority of the disc surface and a warning signal  810  is sent to the microprocessor  216 . Typically, this screening method will screen out a low-flying head with many consecutive zones that are slightly problematic but are within the specifications tested by the first screening method. 
     The third proposed screening method screens head with overall low flying ability. The proposed screening method can be defined as:
 
A(m)≧Y3  (4)
 
where m represents the number of zones  502  and Y 3  is a predetermined value. This method will screen out heads with overall low flying ability and send a warning signal to the microprocessor  216  if the above equation 4 is true. Screening for overall low flying ability tries to detect if the head is flying generally low across the entire disc, or a substantial portion of the disc. In this situation, there is usually no sudden loss of fly-height at a specific radius. An overall low flying ability could be caused by an inherent error in the gram-loading of the suspension or an abnormally dimensioned head.
 
     In the preferred embodiment, all three conditions are iteratively checked until all of the zones  502  have been considered  812 . 
     It should be noted additional possibilities exist regarding where data is written and read from to perform calculations. Any portion of the disc surface that can be read or written can be utilized in the process of determining flight abnormalities. 
     It should also be noted the above conditions used to determine failure can be altered or substituted for any other method producing substantially the same information. 
     From the preferred embodiment above a number of advantages can be identified, including predicting problems related to short, medium, and long-term reliability. The above methods can be used to find flight abnormalities due to bad heads, discs, or both. Also, the above process can be applied in any combination to individual head testing, disc testing, drive testing, in manufacturing or can be used during the normal use of the drive to help warn or prevent failures over the lifespan of the drive. The above process can also find flight problems due to the head, disc, or both over a specific track on the disc, region of the disc surface, or the entire disc surface. Groups of tracks with individual, less severe problems can be considered problematic when looked at collectively. 
     The above methods provide excellent results because the above calculations can be done over the user data portions on the disc, rather than over the servo portions of the disc. Although it is possible to perform the above calculations using the servo portions on the disc; ultimately, the user data is what is important to the user of the disc drive. Further, calculations done using servo data may not be indicative of drive performance as far as the user is concerned. Furthermore, servo data is normally written at a lower frequency resulting in less sensitive measurements. Calculations over the data portions allow for more accurate measurement and better predictions of reliability and performance. 
     Additionally, the above calculations utilize knowledge of Inter-Symbol Interference by recognizing that flight abnormalities that occur further toward the inner diameter of a zone will have a greater effect on drive performance and are accounted for accordingly. 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.