Abstract:
A method and apparatus are provided for implementing an in-situ digital harmonic computation facility for direct access storage device (DASD). A digital sequence output of a DASD channel from a constant write frequency field on a disk surface is applied to a plurality of adders for modulo-n summing the digital sequence. A harmonics computation block provides discrete Fourier transform computing based upon the summed modulo-n digital sequence to produce a plurality of harmonic coefficients. A plurality of estimate functions utilize predetermined ones of the plurality of harmonic coefficients for estimation of predefined conditions in the DASD. The plurality of estimate functions include a flyheight estimate function, a readback signal asymmetry estimate function and a thermal activity estimate function.

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 implementing an in-situ digital harmonic computation facility for direct access storage device (DASD). 
     DESCRIPTION OF THE RELATED ART 
     Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. 
     Data located on a particular track on a disk surface is read or written by properly positioning a data transducer head directly over the track. In order to maintain the head in proper position over the data track, track-following servo systems often are incorporated into disk drives. Servo position control is used to position the data heads in registration with the data information tracks. A sector servo system uses the data heads as servo transducers in a time-multiplexed fashion. As a head is following a particular track on a rotating disk, the head will pick up data information and servo information alternately. Servo information, interlaced with data information, is prewritten on the disk surfaces at manufacturing time within narrow radial sectors as spokes on a wagon wheel. A servo system that interlaces data and servo information is commonly referred to as sector servo or embedded servo. Another servo system uses a dedicated servo transducer head to read position signals recorded in servo information tracks on a dedicated disk surface. The data heads are ganged with the servo head for simultaneous movement relative to the data information tracks and the servo information tracks. To access the disk drive unit, a feedback sector servo controlled drive system locates the head in a desired position, where data is to be written or read. 
     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) typically are used to estimate flying height and Clearance Modulation Detection (CMD) typically used to detect the presence of raised disk asperities in direct access storage device (DASD). Currently many direct access storage devices (DASDs) include a General Error Measurement (GEM) facility for error detection. 
     It is desirable to provide an improved method and apparatus for detection of error and critical conditions in a DASD. It is desirable to provide a simpler and more computationally efficient method to perform functions than presently executed in the GEM facility. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide a method and apparatus for implementing an in-situ digital harmonic computation facility for direct access storage device (DASD). Other important objects of the present invention are to provide such method and apparatus for implementing an in-situ digital harmonic computation facility for direct access storage device (DASD) substantially without negative effect; and that overcome many of the disadvantages of prior art arrangements. 
     In brief, a method and apparatus are provided for implementing an in-situ digital harmonic computation facility for direct access storage device (DASD). A digital sequence output of a DASD channel from a constant write frequency field on a disk surface is applied to a plurality of adders for modulo-n summing the digital sequence. A harmonics computation block provides discrete Fourier transform computing based upon the modulo-n summed digital sequence to produce a plurality of harmonic coefficients. A plurality of estimate functions utilize predetermined ones of the plurality of harmonic coefficients for estimation of predefined conditions in the DASD. 
     In accordance with features of the invention, the plurality of estimate functions include a flyheight estimate function, a readback signal asymmetry estimate function and a thermal activity estimate function. 
    
    
     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 an in-situ digital harmonic computation facility in accordance with the preferred embodiment; and 
     FIG. 2 is a block diagram representation illustrating the in-situ digital harmonic computation facility 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 to a digital harmonic computation (DHC) facility  120  of the preferred embodiment and onto a data recording channel  122 . A disk drive supervisor  124  monitors the results of the DHC facility  120  and may as a result alter features within the data recording channel  122  or may notify the system operator of drive status and impending problems. Data is output to the host system as indicated at a line labeled DATA OUTPUT. 
     In accordance with features of the preferred embodiment, the digital harmonic computation (DHC) facility  120  is used to monitor various conditions of the DASD  100 , eliminating the need for the conventional General Error Measurement (GEM) facility. Digital harmonic computation (DHC) facility  120  enables a digital implementation of all current GEM methods using a simpler harmonic analysis. 
     Referring to FIG. 2, a detailed diagram of the digital harmonic computation (DHC) facility  120  of the preferred embodiment is shown. The digital sequence x(n) is obtained from a write recovery field (WRF) which is the preamble to each servo sector on the disk surface or other constant write frequency field on a disk surface. For example, in one known DASD spinning at 10,000 RPM, the write recovery field has a written, frequency of fw=33.125 MHz and is sampled at fs=265 MHz, which is exactly eight times the written frequency. In this example, fs=8*fw. In general, the sampling frequency fs is a multiple integer of the write frequency fw. In FIG. 2, the digital sequency x(n) on line  210  is indexed on a modulo-8 basis, such that every eight sample of x(n) goes to a respective adder S i  including S 0    220 , S 1    222 , S 2    224 , S 3    226 , S 4    228 , S 5    230 , S 6    232 , and S 7    234 . As shown in the following Equation 1, a sequence x(n) of length N is summed modulo-8 by the adder S i . Note that the sequence length N is divisible by eight. Examples are N=16, 24, 32 or the like. It should be understood that the principles of the present invention are not limited to modulo-8 summing, various modulo-n can be used, where n represents an integer number.                  S   i     =       ∑     m   =   0       N   /   8            x        (     i   +     8      m       )           ,                i   =   0     ,   1   ,   2   ,   …              ,   7.           Equation                 1                                
     The sequence x(n) modulo-8 summing may be obtained from part of a write recovery field (WRF) within a servo sector or it could be the whole WRF, or it could be a modulo-8 sum of all WRFs around a given track. An alternative to using the WRF is to write similar patterns at other places on a disk surface. The only requirement is that each adder has the same number of samples. Note that the computational requirements for the modulo-8 summing by the eight adders S 0    220 , S 1    222 , S 2    224 , S 3    226 , S 4    228 , S 5    230 , S 6    232 , and S 7    234  are very low. After the desired number of samples N (divisible by eight) have been taken, then a discrete Fourier transform (DFT) is computed at a harmonic computations block  236 , based upon the sums Si, i=0, 1, 2, . . . , 7, contained in the eight adders S 0    220 , S 1    222 , S 2    224 , S 3    226 , S 4    228 , S 5    230 , S 6    232 , and S 7    234 . This is done for the first and third harmonic according to the algorithms set forth in the following Equations 2 and 3, respectively. 
     
       
           X ( N/ 8)= R   1 + q*R   2 − j ( I   1 + q*I   2 ), where  q= 0.707  Equation 2 
       
     
     
       
           X (3 N/ 8)= R   1 + q*R   2 − j ( I   1 + q*I   2 ), where  q= 0.707  Equation 3 
       
     
     Where j=sqrt(−1), and the coefficients in terms of the adder sums are defined as follows: 
     
       
           R   1 = S   0 − S   4 ,  R   2 = S   1 − S   3 − S   5 + S   7   Equation 4 
       
     
     
       
           I   1 = S   2 − S   6 ,  I   2 = S   1 + S   3 − S   5 − S   7   Equation 5 
       
     
     In similar fashion, the even harmonics have the zeroth and second harmonic defined by Equations 6 and 7, respectively. 
     
       
           X ( 0 )= S   0 + S   1 + S   2 + S   3 + S   4 + S   5 + S   6 + S   7   Equation 6 
       
     
     
       
           X (2 N/ 8)=( S   0 − S   2 + S   4 − S   6 )− j ( S   1 − S   3 + S   5 − S   7 )  Equation 7 
       
     
     Note that the total number of multiplications required in harmonic computations block  236  to produce the four harmonic components in Equations 2, 3, 6 and 7 are equal to four no matter the length N of the sequence X(n). 
     Estimation of special conditions in the DASD  100  are performed by an estimate flyheight block  238 , an estimate asymmetry block  240 , an estimate thermal block  242  and an estimate block  244 . Estimate flyheight block  238 , estimate asymmetry block  240 , estimate thermal block  242  and estimate block  244  use the harmonic coefficients X 0 , X 1 , X 2 , and X 3  for estimation of special conditions in the DASD  100 . For example, if the magnitudes of the four harmonic coefficients X 0 , X 1 , X 2 , X 3  are M 0 , M 1 , M 2 , M 3 , respectively, then an estimate of the flyheight d using a harmonic ratio frequency (HRF) technique in estimate flyheight block  238  can be found using an inverse of Wallace&#39;s equation as set forth in the following Equation 8. 
     
       
           d= (λ 1 /4π)ln( M   1   /M   3 )  Equation 8 
       
     
     where λ 1  is the spatial wavelength of the written pattern. 
     In accordance with features of the preferred embodiment, the asymmetry in the readback signal can be interpreted as a damaged read element or head instability. Asymmetry shows up as distortion in the readback signal and appears in the second harmonic component. A normalized second harmonic distortion Q 2  estimated in the estimate asymmetry block  240  can be defined as set forth in the following Equation 9. 
     
       
           Q   2   =M   2   /M   1   Equation 9 
       
     
     The larger the value of Q 2 , the more asymmetry is present or a larger value of Q 2  means that more asymmetry is present. 
     In the estimate thermal block  242 , the severity of any short term, thermal activity, such as resulting from baseline wander and thermal asperities, will manifest itself proportionally in the normalized zeroth harmonic component Q 0  is defined as set forth in the following Equation 10. 
     
       
           Q   0   =M   0   /M   1   Equation 10 
       
     
     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.