Abstract:
A method and apparatus are provided for detection of the presence of slider airbearing resonance using surface analysis test (SAT) primary defect list (P-list) data. The P-list in the disk drive is selected. Checking for a cluster threshold of adjacent tracks within a sector to identify a SAT cluster is performed. Responsive to identifying the SAT cluster, checking for multiple defects on some tracks within the SAT cluster is performed. Responsive to identifying multiple defects on some tracks within the SAT cluster, the SAT cluster is converted to a binary matrix map. A histogram is generated for the binary matrix map. The harmonic magnitude content of the histogram from harmonics centered about a predetermined slider airbearing resonance frequency is identified. A harmonic power ratio (HPR) for the SAT cluster is computed and compared with a harmonic power ratio threshold Responsive to the computed HPR being greater than the harmonic power ratio threshold, a wave-front frequency of the SAT cluster is computed.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the data processing field, and more particularly, relates to a method and apparatus for detection of the presence of slider airbearing resonance using surface analysis test (SAT) P-list data. 
     DESCRIPTION OF THE RELATED ART 
     Techniques for detecting disk surface defects are known. Most commercially available disk drives store a standard primary defect list (P-list) and a grown defect list (G-list) in a protected area of the disk drive, often referred to as disk defect logs. The P-list is generated for each disk file at manufacturing time and stores information of specific locations of magnetic surface defect sites and the alternate site for storing data. The G-list is generated and periodically updated while the disk drive is in use including stored information of grown defects that occurred after manufacturing. 
     Special manufacturing slider-glide-test procedures, such as Harmonic Ratio Flyheight (HRF) and Clearance Modulation Detection (CMD) typically are used to detect the presence of airbearing resonances in direct access storage device (DASD) sliders. 
     It is desirable to provide a method and apparatus for detection of slider airbearing resonance that eliminates the need for Special manufacturing slider-glide-test procedures, such as Harmonic Ratio Flyheight (HRF) and Clearance Modulation Detection (CMD). A need exists for an improved method and apparatus for detection of slider airbearing resonance. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide an improved method and apparatus for detection of the presence of slider airbearing resonance. Other important objects of the present invention are to provide such method and apparatus for detection of the presence of slider airbearing resonance substantially without negative effect and that overcome many of the disadvantages of prior art arrangements. 
     In brief, a method and apparatus are provided for detection of the presence of slider airbearing resonance using surface analysis test (SAT) P-list data. The P-list in the disk drive is selected. Checking for a cluster threshold of adjacent tracks within a sector to identify a SAT cluster is performed. Responsive to identifying the SAT cluster, checking for multiple defects on some tracks within the SAT cluster is performed. Responsive to identifying multiple defects on some tracks within the SAT cluster, the SAT cluster is converted to a binary matrix map. A histogram is generated for the binary matrix map. The harmonic magnitude content of the histogram from harmonics centered about a slider airbearing resonance frequency fa is identified. 
     In accordance with features of the invention, a harmonic power ratio (HPR) for the SAT cluster is computed and compared with a harmonic power ratio threshold. Responsive to the computed HPT being greater than the harmonic power ratio threshold, a wave-front frequency of the SAT cluster is computed. The wave-front frequency fw of said SAT cluster is represented by:                fa   ~   fw     =         ∑     k   =     k   1         k   2                         A        (   k   )            f        (   k   )               ∑     k   =     k   1         k   2                       A        (   k   )                   Equation                 1                                
     where A(k) is the harmonic magnitude of the DFT at harmonic frequency f(k) and k is the harmonic frequency index and k 1  and k 2  are computed harmonic indices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein: 
     FIG. 1 is a block diagram representation illustrating a direct access storage device (DASD) for implementing methods for detection of slider airbearing resonance using surface analysis test (SAT) P-list data in accordance with the preferred embodiment; 
     FIG. 2 is a chart illustrating P-list data with track number shown along the vertical axis and sector identification (SID) shown along the horizontal axis; 
     FIG. 3 is a flow chart illustrating exemplary sequential steps for detection of slider airbearing resonance using surface analysis test (SAT) P-list data in accordance with the preferred embodiment; and 
     FIGS. 4A,  4 B,  4 C,  5 A,  5 B,  5 C,  6 A,  6 B, and  6 C are charts illustrating example P-list data in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Having reference now to the drawings, in FIG. 1, there is shown a direct access storage device (DASD) of the preferred embodiment generally designated by the reference character  100 . As shown in FIG. 1, direct access storage device (DASD)  100  includes a recorded disk  110  that is spun at constant speed and a recording head  112  carried by a slider  113  that is positioned on a given track for reading information stored on that track. The readback signal r(t) is highpass-filtered by an arm electronic (AE) module  114 , and its filtered output is bandpass-filtered through a channel equalizer  116 . An equalized channel equalizer output X(t) is sampled by an analog-to-digital converter (A/D)  118  to provide a discrete-time digital sequence X(n). The digital sequence X(n) is then sent onto a data recording channel  120  and to a surface analysis test (SAT) facility  122 . A slider airbearing resonance detector  124  can access P-list data and G-list data from the SAT facility  122 . 
     Slider airbearing resonance detector  124  is suitably programmed to execute the flow charts of FIG. 3 of the preferred embodiment. 
     In accordance with features of the invention, the detection of airbearing resonance in hard disk drive sliders is achieved by special processing of the P-list or G-list data. The presence of slider airbearing resonance is mainly synonymous to physical or near-physical head-to-disk contact by a protruding defect or asperity. The invention enables early identification of a protruding defect which is especially important, since it may avoid a potential disk crash and extend the useful life of the disk drive immensely. The commonly used glide tests, such as HRF and CMD are no longer needed. Thus, the cost of testing in manufacturing is reduced substantially. Disk drives that show airbearing resonance will not be shipped, but will be reworked. This will improve disk drive quality and reliability. 
     All hard disk drives undergo a surface analysis test (SAT), which essentially maps all error sites on the disk surface by location including head, track and sector. These SAT sites are minute sites where data cannot be read back correctly. The location of these SAT sites are located or stored in a file called a P-list. During normal operation, new or grown SAT sites occur. These new SAT sites are stored in a G-list. All SAT sites are relocated as a complete track-sector-site to another track-sector-site where data can be written and read reliably. A typical section of a long P-list with multiple SAT sites on the same tracks is shown in Table 1 below. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 section of a P-list 
               
             
          
           
               
                   
                 Head 
                 track 
                 sect1 
                 SID1 
                 sect2 
                 SID2 
               
               
                   
                   
               
               
                   
                 3 
                 7415 
                 79 
                 527 
                 79 
                 538 
               
               
                   
                 3 
                 7417 
                 79 
                 239 
                 79 
                 249 
               
               
                   
                 3 
                 7418 
                 79 
                 200 
                 79 
                 261 
               
               
                   
                 3 
                 7419 
                 79 
                 218 
                 79 
                 279 
               
               
                   
                 3 
                 7419 
                 79 
                 513 
                 79 
                 553 
               
               
                   
                 3 
                 7420 
                 79 
                 266 
                 79 
                 282 
               
               
                   
                 3 
                 7420 
                 79 
                 519 
                 79 
                 537 
               
               
                   
                 3 
                 7420 
                 79 
                 794 
                 79 
                 809 
               
               
                   
                 3 
                 7421 
                 79 
                 244 
                 79 
                 276 
               
               
                   
                 3 
                 7421 
                 79 
                 483 
                 79 
                 546 
               
               
                   
                 3 
                 7421 
                 79 
                 747 
                 79 
                 808 
               
               
                   
                 3 
                 7422 
                 79 
                 197 
                 79 
                 264 
               
               
                   
                   
               
             
          
         
       
     
     As shown in Table 1, the P-list has data organized in six columns. The first column, head is the head number, the second column, track is the track number, the third column, sect 1  is start sector, the fourth column, SID 1  is the beginning sector identification (SID) count, the firth column, sect 2  is the end sector, and the sixth, SID 2  is the end SID count. Each sector contains a fixed number of SID counts, for example, a sector may contain 3070 SID counts. Note in Table 1 that all SAT sites are located on surface head  3  and in sector  79 . The first entry shows that track  7415  has a single SAT site starting at SID count  527  and ending at SID count  538 . Track  7416  has no SAT site in sector  79 . Track  7417  has a single SAT site starting at SID count  239  and ending at SID count  249 . Track  7419  has a double SAT site, starting at SID count  218  and ending at SID count  279 , and starting at SID count  513  and ending at SID count  553 . If there are many SAT sites on adjacent tracks within the same sector on the same surface, then the SAT sites are combined into a SAT cluster. Various cluster algorithms are known for locating clusters within P-lists. A surface defect can either be a magnetic void, a pit, a bump, or a combination of these. A large bump is very often associated with an airbearing resonance, caused by a bouncing action of the slider  113  in the wake of the protruding bump. The larger the bump, the more bounce of the slider  113 . Large surface bumps are notorious for causing physical head-to-disk contact, which is the precursor for a fatal disk crash. All users of hard disk drives dread even the thought of a fatal disk crash, since all data stored on that unfortunate disk drive may be lost forever. 
     FIG. 2 illustrates an unexpected discovery of the nature of multiple SAT sites within SAT clusters as a result of an initial P-list analysis. Note that the within-the-cluster SAT sites look like wave-fronts. Similar illustrations of FIG. 2 are shown in FIGS. 3A,  4 A, and  5 A. The graph represented in FIG. 2 has a corresponding binary (only zeros and ones) matrix M mapping. This matrix has dimensions (N×3070), where N is the number of tracks in the cluster and 3070 is the maximum SID count within a sector. Note that the second dimension of the binary matrix M corresponds to the circumferential direction on the disk surface. A “1” in the matrix represents part of a SAT site. By radial projection or a summation along the individual columns in mapping matrix M, a cumulative defect histogram can be obtained. Such SAT histograms are shown in FIGS. 3B,  4 B, and  5 B. 
     Discrete Fourier transform (DFT) analysis is then applied to the circumferential, cumulative defect histogram to reveal the frequency content of the SAT site wave-fronts. Theoretical calculations verified by laboratory experiments can determine the airbearing resonance frequency f a  of a particular slider to fall within a small percentage, such as 5-10%. A subset of the harmonic wave-front frequencies f(k) in the vicinity of f a  is then extracted. A centroidal method of Equation 1 below, applied to the harmonic frequency subset can determine the main frequency f w  in the wave-front. Comparison of the main wave-front frequency f w  with the precomputed airbearing resonance frequency of the slider f a  can determine when they are almost identical. The estimated airbearing resonance frequency of the slider f a  can be approximately equal to the wave-front frequency f w . The formula for computing the wave-front frequency f w  is set forth in Equation 1 below.                fa   ~   fw     =         ∑     k   =     k   1         k   2                         A        (   k   )            f        (   k   )               ∑     k   =     k   1         k   2                       A        (   k   )                   Equation                 1                                
     where A(k) is the harmonic magnitude of the DFT at harmonic frequency f(k) and k is the harmonic frequency index. The harmonic indices k 1  and k 2  are computed as follows. Let the time between SID counts be Δt and let the total number of SID counts within the data sector be Q, then the DFT frequency resolution Δf is equal to: 
     
       
           Δf =1/( QΔt )  Equation 2 
       
     
     For a disk drive where Q=3070 and Δt=10.78×10 −9  seconds, this corresponds to a frequency resolution of Δf=16.47 kHz. The harmonic indices k 1  and k 2  must be computed such that the inequalities in the following Equation 3 are satisfied. 
     
       
           k   1   Δf&lt;f   a&lt;   k   2   Δf   Equation 3 
       
     
     FIG. 3 illustrates exemplary sequential steps for detection of slider airbearing resonance using surface analysis test (SAT) P-list data in accordance with the preferred embodiment. As indicated in a block  300 , after initialization of variables, head and sector numbers, a P-list is selected from the disk drive. The head number is first set to zero and a cluster algorithm as indicated in a decision block  302  checks for clusters, checking for more than NN adjacent tracks within a given sector. The cluster threshold NN may typically be set to NN˜0.5*Wpad/Wtrack, where Wpad is the width of the trailing slider-pad and Wtrack is the track width. For example, the cluster threshold NN˜60 for a disk drive where Wpad=220 microns and Wtrack=1.86 micron. If no clusters are found, then the head number is indexed by one as indicated in a block  314  and the next surface is analyzed. If one or more clusters are found at block  302 , then another algorithm is performed as indicated in a decision block  304  checking successively for multiple SAT sites on some tracks within the identified clusters. If none is found, the head number is indexed by one again at block  314 . If multiple SAT sites have been identified within one or more clusters, then the corresponding P-lists are used to generate one or more binary matrices M of a dimension (N×P) as indicated in a block  306 . Here N is the number of tracks in the identified cluster and P is the maximum SID count within the data sector, for example P=3070 in one known disk drive. In the binary mapping matrix M, a “1” implies a SAT defect and a “0” does not. A histogram is then obtained by summing all P columns of mapping matrix M at block  306 . Next as indicated in a block  308 , the histograms are successively transformed using a discrete Fourier transform (DFT) which computes only one harmonic DFT bin at a time. It is only necessary to compute a few harmonic magnitudes A(k) for corresponding frequencies straddling the airbearing resonance frequency. A typical number may be eight harmonics. This speeds up the process and makes in-situ operations possible. To validate the suitability of the P-list data in the cluster, a harmonic power ratio (HPR) is computed for the selected cluster as indicated in a decision block  310 . This is achieved using the following Equation 4.              HPR   =           ∑     k   =     k   1         k   2                         A   2          (   k   )             ∑     k   =     m   1         m   2                         A   2          (   k   )           &gt;   R             Equation                 4                                
     where the sum in the denominator has twice as many harmonic terms as the numerator and the denominator must contain the harmonics in the numerator. Consider the following example, k 1 =9, k 2 =12, m 1 =7, and m 2 =14. Cluster examples are illustrated in FIGS. 4A,  4 B,  4 C,  5 A,  5 B,  5 C,  6 A,  6 B and  6 C. A typical value of the harmonic power ratio threshold R is R=0.6. If the HPR falls below the threshold R, then the head number is indexed by one at block  314 . Otherwise, when the HPR is greater than the threshold R, the wave-front frequency f w  is computed using Equation 1 as indicated in a block  312 . If this frequency value f w  is close to the precomputed airbearing resonance frequency f a , then the likelihood of airbearing resonance is high. Thus, the findings show that the SAT defect wave-fronts are synonymous with airbearing resonance action in the slider, and the airbearing resonance action reveals a protruding defect. The estimated airbearing resonance frequency f a  is then f a ˜f w . 
     The readback signal amplitude is inverse-exponentially related to the head-to-disk separation. If the head flies high, then the readback signal is small. The readback signal is large when the head-to-disk separation is small. The SAT site wave-fronts are most likely being set up by the vertical airbearing oscillation of the slider. At the higher fly height points the readback signal amplitude is too small to be read reliably, thus a SAT site is generated. For many laser-bumps in test bump drives described below, the recording channel lost synchronization during the bump. In the case, the SAT algorithm will default the whole sector as bad by placing a zero in column 4 and 3070 in column 6 of the P-list. However, analysis of the generalized error measurement (GEM) flyheight facility reveals airbearing resonance action across the laser bumps. Using an external synchronization method or fake sync when a loss-of-sync is detected in the P-list will again reveal the wave-front action in the binary mapping matrix M. 
     In accordance with features of the preferred embodiment, the method allows for the detection of airbearing resonance by simple analysis of the P-list during SAT test. Minimal computational resources are required. Disk drives that show sites of significant airbearing resonance will not be shipped, but instead reworked. This improves disk drive reliability and quality. 
     Referring to FIGS. 4A,  4 B,  4 C,  5 A,  5 B,  5 C,  6 A,  6 B, and  6 C, there are shown charts illustrating analysis of P-list data in accordance with the preferred embodiment. An analysis of P-lists from special disk drives having laser bumps along the same sector on head surface  2  and  3  has been made. In FIGS. 4A,  5 A, and  6 A, three multiple SAT sites from one of the special disk drives are shown. In these FIGS. 4A,  5 A, and  6 A, the SAT sites have been plotted on a graph of SID count versus track number. The SAT sites look like wave-fronts. FIGS. 4B,  5 B, and  6 B illustrate the corresponding histograms and FIGS. 4C,  5 C, and  6 C depict the associated harmonic magnitude content. The computed airbearing frequencies for the three projections were 175.0 kHz, 171.9 kHz, and 171.3 kHz. The airbearing resonance for the special disk drive slider is 180 kHz+/−10%. Thus, the actual findings using only the P-lists show that the SAT defect wave-fronts are synonymous with airbearing resonance action in the slider. Analysis of other surfaces that did not include the purposely made laser bumps did not reveal this wave-front phenomenon. When new SAT sites occur and are stored on the G-list, the G-list data is immediately analyzed by the algorithm of the preferred embodiment. 
     While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.