Patent Publication Number: US-6982849-B2

Title: Method and apparatus for providing positional information on a disk

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is a non-provisional application of a provisional application, assigned Provisional Application Ser. No. 60/232,649, and filed Sep. 14, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to disk storage systems and more particularly, methods and apparatus for providing positional information on a disk in a hard drive assembly. 
   2. Description of the Related Art 
   Disk drives are magnetic recording devices used for the storage of information. The information is typically recorded on concentric tracks on either surface of one or more magnetic recording disks. To facilitate the storage and retrieval of data in an orderly manner, disks are typically organized in blocks called sectors. These sectors are located on the disk by a set of unique specifiers called cylinder (or track), head (or side) and sector number. The disks are rotatably mounted to a spin motor and information is accessed by means of read/write heads that are mounted to actuator arms which are rotated by a voice coil motor. The voice coil motor is excited with a current to rotate the actuator and move the heads. The read/write heads must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information. 
   To accurately write and read data, it is desirable to maintain the head on the center of the track. To assist in controlling the position of the head, each sector of the disk typically contains a number of servo bits accurately located relative to the centerline of the track. The raw signals produced by the servo bits are typically demodulated into a position signal which is utilized by a servo system to determine the position of the head relative to the track, and to move the actuator arm if the head is not located on the track centerline. 
   Due to defects in the servo patterns, the read head does not return to its original position after one revolution, as shown in FIG.  1 A. This results in a gap between the original (starting) position and the position of the read/write head after one revolution. The resulting position signal is an anomaly, and takes the form of a spike, as shown in FIG.  1 B. 
   Accordingly, there is a need in the technology for a method and apparatus for providing servo information on a disk in a hard drive assembly while overcoming the aforementioned problems. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is a method and apparatus for positioning a read/write head in a hard disk drive. The method comprises providing a disk having a at least one side with a plurality of tracks, where each of the tracks has a servo field with servo bits. The servo bits are read to provide a position signal for positioning a read/write head. The method determines a difference in position between an initial and a subsequent position of the read/write head on a track, where the subsequent location occurs after the read/write head has moved one revolution from the initial position on the track. The initial and subsequent positions are offset laterally. The method generates a compensation signal based on the initial position, the subsequent position and the difference. The position signal and the compensation signal are combined to provide a compensated position signal for positioning the read/write head. Various embodiments are described. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  illustrates the starting position of a typical read head and the subsequent position of the read head after one revolution. 
       FIG. 1B  illustrates a spiked signal resulting from the error as shown in FIG.  1 A. 
       FIG. 2A  illustrates two embodiments of a process for providing correction of non-centered position signal, in accordance with the principles of the invention. 
       FIG. 2B  illustrates one embodiment of a process for providing a corrected position signal. 
       FIG. 2C  illustrates the result of applying the position signal correction process to the error in FIG.  1 B. 
       FIG. 3  illustrates a hard disk drive which utilizes the methods of the invention. 
       FIG. 4  illustrates the general layout of the servo field region of a track. 
       FIG. 5  is a block diagram of portions of an integrated circuit read channel in accordance with the present invention. 
       FIG. 6A  illustrates one embodiment of a typical position signal PES used to center a read head along the centerline of a track. 
       FIG. 6B  illustrates one embodiment of a correction signal PES COR  used to correct the position signal PES in providing a centered position signal. 
       FIG. 6C  illustrates one embodiment of the resulting signal obtained when the position signal PES is combined with the correction signal PES COR . 
       FIG. 7A  illustrates one embodiment of a typical position signal PES used to center a read head along the centerline of a track. 
       FIG. 7B  illustrates a second embodiment of a correction signal PES COR  used to correct the position signal PES in providing a centered position signal. 
       FIG. 7C  illustrates one embodiment of the resulting signal obtained when the position signal PES is combined with the correction signal PES COR . 
       FIG. 8  is a flow chart illustrating one embodiment of an initialization process that may be implemented prior to the position signal correction process. 
       FIGS. 9A and 9B  are flow charts illustrating one embodiment of the position signal correction process provided in accordance with the principles of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention maybe used in conjunction with a defect management system, as described in U.S. patent application Ser. No. 09/952,683, entitled “Servo Defect Management Scheme in Hard Disk Drives” filed on Sep. 13, 2001, which has been assigned to the assignee hereof, and which is hereby fully incorporated by reference. 
   As discussed earlier, due to defects in the servo patterns, the read head does not return to its original position after one revolution, as shown in FIG.  1 A. This results in a gap G between the original (starting) position P 1  and the position P 2  of the read/write head after one revolution. The resulting position signal is an anomaly, and takes the form of a spike, as shown in FIG.  1 B. The present invention provides and apparatus and methods for eliminating the anomalous signal, by providing a correction term to the position signal used to direct the read head. 
     FIG. 2A  illustrates two embodiments of a process for providing correction of non-centered position signal, in accordance with the principles of the invention. In one embodiment as illustrated by the line A in  FIG. 2A , the corrected position signal PES directs the read head at a position N 1  sectors before the gap G, to move substantially linearly to the original position P 1 . In an alternate embodiment, as illustrated by the line B in  FIG. 2A , the corrected position signal PES directs the read head from a position N 2  sectors before the gap G, passes through the mid point of the gap G, to move substantially linearly to a position P 3  that is located after the original position P 1 . In one embodiment, N 2  is N 1 /2, and P 3  is located at a position N 2  after the gap G. 
     FIG. 2B  illustrates one embodiment of a process for providing a corrected PES signal. The servo reference signal r is typically combined with an original PES signal X 0  and the resulting signal is provided to the voice coil motor VCM, which controls movement of the read head. In accordance with the principles of the invention, a signal X, representing the value of the PES correction term, is added to the servo reference signal r and the original PES signal X 0 , and the resulting signal, X 1  is provided to the VCM. By adding X to the servo reference signal and the original PES signal X 0 , the VCM will control the read head to travel along one of the two paths described in FIG.  2 A and the corresponding text. 
   Referring to the drawings more particularly by reference numbers,  FIG. 3  shows a hard disk drive  100 . The disk drive  100  includes a disk  102  that is rotated by a spin motor  104 . The spin motor  104  is mounted to a base plate  106 . Also mounted to the base plate  106  is an actuator arm assembly  108 . The actuator arm assembly  108  includes a number of heads  110  mounted to corresponding flexure arms  112 . The flexure arms  112  are attached to an actuator arm  114  that can rotate about a bearing assembly  116 . The assembly  108  also contains a voice coil  118  that is coupled to the magnets  119  that are mounted to the base plate  106 . Energizing the voice coil  118  moves the heads  110  relative to the disk  102 . There is typically a single head for each disk surface. The spin motor  104 , voice coil  118  and the heads  110  are coupled to a number of electronic circuits  120  mounted to a printed circuit board  122 . In the following discussion, only one head  110  is referenced. The electronic circuits  120  typically include a read channel circuit, a microprocessor-based controller and a random access memory (RAM) device. 
   As shown in  FIG. 4 , data is typically stored within sectors of radially concentric tracks located across the disk  102 . A typical sector will have an automatic gain control (AGC) field  150 , a synchronization (sync) field  152 , a gray code field  154  that identifies the track, an identification (ID) field  156  that defines the sector, a servo field  158  which includes a number of servo bits A, B, C, D, a data field  160  which contains the data and an error correction code field  162 . In operation, the head  110  is moved to a track and the servo information provided in servo field  158  is read and provided to the electronic circuits  120 . The electronic circuits  120  utilize the variation in the servo bits (A-B) or (C-D) to generate Q, a positioning signal for aligning the head  110 . 
     FIG. 5  is a block diagram of an electronic circuit  120  of the drive. The electronic circuit  120  includes a preamplifier  172  which is coupled to a read/write (R/W) channel circuit  174 . The R/W channel circuit  174  includes a R/W Automatic Gain Control (AGC), a filter circuit  176 , a fullwave rectifier  178  and a peak detector  180 . The electronic circuit  120  further comprises a microprocessor-based servo controller  182  which includes an analog-to-digital converter (ADC)  184 , a digital signal processor (DSP)  186 , a burst sequencer and timing circuit  188  and a memory  190 , such as a random access memory (RAM) device. The DSP  186  includes a logic circuit  192 , a summing circuit  194  and a control logic circuit  198 . 
   The electronic circuit  120  is coupled to one of the magnetic heads  110  which senses the magnetic field of a magnetic disk  102 . When reading the servo information located in the servo field region  10  on the disk  102 , the head  110  generates a read signal that corresponds to the magnetic field of the disk  102 . The read signal is first amplified by the preamplifier  172 , and then provided to the R/W channel circuit  174 . The AGC data included in the read signal is provided to the R/W AGC and filter circuit  176 . The R/W AGC circuit in circuit  176  monitors the AGC data provided by the read signal and the read signal is then filtered by the filter circuit located in the R/W AGC and filter circuit  176 . The fullwave rectifier  178  rectifies the read signal and provides the rectified read signal to the peak detector  180 . The peak detector  180  detects the amplitude of the read signal. The read signal is then provided to the ADC  184  which provides digitized samples of the analog read signal. The digitized signal is then provided to a logic circuit  192  located within the DSP  186 . The logic circuit  192  generates a position signal X O , based on the servo bits A, B, C and D that are read by the head  110 . The position signal X O  is provided to the summing circuit  194 . The logic circuit  192  also generates a PES correction signal X, based on the servo bits A, B, C, and D. 
   The PES correction signal X is added to the position signal XO. A servo reference signal X is also added to XO. Based on the sum of r, XO and X, a corrected PES signal, X, is generated and provided to the control logic circuit  198 . The control logic circuit  198  calculates a compensated signal as control signal Q. The resulting control signal Q is stored in memory  190 . The control signal Q is subsequently provided to the actuator arm assembly  108  to move the heads  110 . Alternatively, the control signal Q can be provided directly to the actuator arm assembly  108  to move the heads  110 . 
     FIG. 6A  illustrates one embodiment of a typical position signal PES used to center a read head along the centerline of a track. As discussed earlier, the read head typically does not return to its original position after one revolution, as shown in FIG.  1 A. As a result, a correction signal is added to the original position signal PES to correct this anomaly.  FIG. 6A  illustrates one embodiment of a PES signal modeled as a sawtooth waveform. As shown, the period T 1  of the waveform corresponds to the time it takes for the disk to complete one revolution. The magnitude of the waveform corresponds to an off-track position of +/−5%.  FIG. 6B  illustrates one embodiment of a correction signal PESCOR used to correct the position signal PES in providing a centered position signal. The PES gap C in  FIG. 2A  corresponds to the peak-to-peak value S of the waveform. By implementing the techniques of the invention, the corrected PES signal will result in the form as shown by line A′ corresponding to the paths A as shown in FIG.  2 A. In one embodiment, the magnitude SCOR of the correction signal PESCOR is the same as the magnitude S of the original position signal PES. However, the period T 1  of the signal S is equal to the period T 3  of the correction signal PESCOR and T 2  the interval between each correction signal.  FIG. 6C  illustrates one embodiment of the resulting position signal obtained using the correction signal of FIG.  6 B. In one embodiment, the magnitude SR of the resulting correction signal is equal to S/2 if the original position signal S is symmetric. 
     FIG. 7A  illustrates a second embodiment of an uncorrected position signal used in providing a centered position signal.  FIG. 7A  illustrates one embodiment of a PES signal modeled as a sawtooth waveform. As shown, the period T 1  of the waveform corresponds to the time it takes for the disk to complete one revolution. The magnitude of the waveform corresponds to an off-track position of +/−5%.  FIG. 7B  illustrates one embodiment of a correction signal PESCOR used to correct the position signal PES in providing a centered position signal. In this embodiment, the correction signal is a dipulse signal having a period of T 5 , where T 5 &lt;T 1 . The PES gap G in  FIG. 2A  corresponds to the peak-to-peak value S of the waveform. By implementing the techniques of the invention, the corrected PES signal will result in the form as shown by line B′ corresponding to the paths B as shown in FIG.  2 A. In one embodiment, the magnitude SCOR of the correction signal PESCOR is the same as the magnitude S of the original position signal PES. However, the period T 1  of the signal S is equal to the period T 5  of the correction signal PESCOR and T 2  the interval between each correction signal.  FIG. 7C  illustrates one embodiment of the resulting position signal obtained using the correction signal of FIG.  7 B. In one embodiment, the magnitude SR of the resulting correction signal is the same as that of the original PES signal. 
     FIG. 8  is a flow chart illustrating one embodiment of the initialization process used prior to the position signal correction process of the invention. The process of the invention utilizes variables stored in a file. Before arriving at a target cylinder, various variables are initialized. The process proceeds as follows. Beginning from a START state, the process  800  proceeds to process block  810 , where the gap closure location CL and the gap closure magnitude CM are read back from a closure defect list or file. The process  800  then determines CS, the sector number at which the closure compensation process is to begin, as shown in process block  820  in the following manner:
 If ( CL−CN/ 2)≧0, then  CS =( CN/ 2) Otherwise  CS=CL− ( CN/ 2)+ NS   
   Where:
         CL is the gap closure location (measured by sector number);   CN is the gap compensation value (measured by sector number);   NS is the number of sectors per revolution on the disk.       

   Thus, if the gap closure location is more than half of the gap compensation value, then CS is initialized as half of the gap compensation value. Otherwise, it is initialized as the difference between the sum of the gap closure location and the number of sectors per revolution, and half the gap compensation value. The process  800  then returns to the main process flow. 
     FIGS. 9A and 9B  are flow charts illustrating one embodiment of the position signal correction process provided in accordance with the principles of the invention. The process  900  determines PC, the PES correction value to use based on various criteria as described below. The process  900  proceeds from a START state to decision block  905 , where it queries if the current sector SN is greater than CS, the sector number at which the closure compensation process is to begin. CS had previously been determined as shown in FIG.  8  and the corresponding text. If SN is greater than CS, the process  900  proceeds to process block  910 , where it determines if the difference between SN and CS is greater or equal to CN, the length of the gap compensation (in sectors). If so, PC, the PES compensation value is set to zero (process block  915 ). The process  900  then returns to the main process flow. During the main process flow, the original position signal is combined with the PES compensation value to provide the resulting compensated PES value. The compensated PES value is then used to position the read/write head. 
   If at decision block  910 , (SN−CS) is determined to be less than CN, the process  900  proceeds to process block  920 , where it queries if (SN−CS) is less than (CN/2). If so, the process  900  proceeds to process block  925 , where PC is determined as follows:
 
 PC=CM *( SN−CS +1)/( CN +1)
 
   where CM is the magnitude of the gap. The process  900  then proceeds to return to the main process flow. 
   If, at decision block  920 , the process  900  determines that (SN−CS) is not less than (CN/2), the process proceeds to process block  931 , where PC is determined as follows:
 
 PC=CM *( SN−CS−CN )/( CN +1)
 
   The process  900  then returns to the main process flow. 
   If, at decision block  905 , the process determines that the current sector SN is not greater than CS, the process proceeds to decision block  935 , where it determines if (SN−CS+NS) is greater than or equal to CN. If so, the process  900  proceeds to process block  940 , where PC is set to zero. The process  900  then returns to the main process flow. 
   Otherwise, the process  900  proceeds to decision block  945  where it determines if (SN−CS+NS) is less than (CN/2). If so, PC is determined as follows (process block  950 ): 
   PC=CM*(SN−CS+NS+1)/(CN+1). The process  900  then returns to the main process flow. 
   Otherwise, the process  900  determines PC as follows (process block  955 ):
 
 PC=CM *( SN−CS+NS−CN )/( CN +1).
 
   The process  900  then returns to the main process flow.