1. Field of the Invention
The present invention relates to a hard disk drive, and more particularly, to a write control method of a hard disk drive, a hard disk drive adapting the method, and a recording medium storing a program for executing the method.
2. Description of the Related Art
A hard disk drive (HDD) is a magnetic writing device used to store information. Information is written in concentric tracks formed on a surface of a disk. Disks are mounted on a spindle motor to be rotated, and information is accessed by a read/write head mounted on an actuator arm rotated by a voice coil motor (VCM). The VCM is excited by current to rotate an actuator and move heads mounted on an actuator arm across the disk. When the HDD operates, the read/write head should be precisely aligned on tracks of the disk so as to ensure reading and writing information.
Traditionally, the position of the head is controlled by a servo control circuit. The servo control circuit detects and controls the position of the head using a servo bursts written on the disk.
In order to let the head follow the track correctly, servo information including servo bursts should be written on the track. STW (servo track write) is a process for writing such servo information on the disk.
To precisely control the position of the head, the quality of the servo bursts written in the STW process, that is, both the intensity and the phase of the servo bursts are important.
As a storage density of the HDD increases, a track density increases together. As the track density increases, in contrast, a track width decreases, and, accordingly, the precision necessary to write the servo information on the disk comes up against a limit.
As a result, servo bursts written on the disk are hardly uniform by track and even by servo sector in one track.
The HDD determines the position of the head using servo bursts. Thus, when the quality of the servo bursts is poor, it becomes difficult to calculate the position of the head correctly, and in worst case particular, the possibility of damaging data on adjacent tracks increases during a write operation.
FIG. 1 shows the written state of servo bursts according to a 4-servo burst technique. In FIG. 1, “n” denotes a track number, “ID” denotes an inner diameter of the disk, and “OD” denotes an outer diameter of the disk. The 4-servo burst technique uses four kinds of servo bursts, that is, A, B, C, and D servo bursts, and every servo burst is separated from each other by a predetermined distance in a direction of a track and in a direction of the diameter of a disk. Generally speaking, servo bursts are disposed radially on the disk and alternatively in every track or in every half of a track.
FIG. 2 shows profiles of a servo burst signals read by the head versus an amount of an offtrack. The longitudinal axis represents the position of the head in the diameter direction of the disk, and the perpendicular axis represents the magnitudes (or intensities) of servo bursts read by the head. It can be understood that the position of the head is indicated by a degree (offtrack) as much as deviation of the head from a center of a target track, marked with 0 in the longitudinal axis, as shown in FIG. 1.
Assuming that servo bursts are written uniformly in sectors and tracks, maximum values of each profile read from servo bursts should be identical, and peaks of servo burst signals read from the A, B, C, and D servo bursts should be disposed in positions off-tracked by −50%, 0%, 50%, or 100%, respectively.
The servo control circuit of the HDD calculates the position of the head by comparing magnitudes of four servo burst signals read by the head. Thus, in order to precisely calculate the position of the head, servo bursts should be written uniformly in all sectors and all tracks.
The HDD with the 4-servo burst technique generates a position error signal (PES) using Equation 1 as follows.
If (−7%<=target offtrack<=7%)PES=PES_(A−B)=A−B
Else if (−43%<target offtrack<−7%) or (7%<target offtrack<43%)PES=PES—N=(A−B)−(C−D)PES=PES—P=(A−B)+(C−D)
Else if (−50%<=target offtrack<=43%) or (43%<=target offtrack<=50%)PES=PES_(C−D)=C−D,  (1)
Where, the target offtrack indicates an offtrack from a center of the target track to the target position. This notation is useful in the case when there are several data tracks between adjacent servo tracks because the position of the target data track is expressed by an offtrack from the center of target servo track. In addition, the symbols including A, B, C, and D are used to represent servo burst signals or servo bursts, respectively. Values of PESs obtained by a difference (A−B) or (C−D) between two adjacent servo burst signals are referred to as primary PESs, and values of PESs obtained by the sum PES_P and a difference PES_N of the primary PESs are referred to as secondary PESs hereinafter.
FIG. 3 shows the relationship between the target offtrack and the PES. Referring to FIG. 3, it can be understood that an area A−B uses PES (PES_(A−B)) calculated by A−B (A minus B) within a range of −7 to 7% from a track center, an area C−D uses PES (PES_(C−D)) calculated by C−D within ranges of −43 to −50% and 43 to 50%, and an area NP uses PES (PES_NP) calculated by N (=(A−B)−(C−D)) or P (=(A−B)+(C−D)) within ranges of −43 to −7% or 7 to 43%.
With respect to PES_(A−B) or PES_(C−D), the linearity of PES_(A−B) or PES_(C−D) may be distorted due to saturation of (A−B) or (C−D). On the other hand, with respect to PES_NP, saturation of PES_NP does not occur even if saturation of (A−B) or (C−D) occurs. In addition, with respect to PES_NP, discontinuity of PES_NP is generated by offset in the place where PES_N and PES_P are changed, e.g. at the origin in FIG. 3. Thus, the PES should be calculated separately according to areas divided by the magnitude of the target offtrack, as shown in FIG. 3.
Circles shown in FIG. 3 represent portions where the saturation of (A−B) and (C−D) occurs.
To calculate the value of a PES using Equation 1 is based on the assumption that normal servo bursts, as illustrated in FIGS. 1 and 2, were written.
FIG. 4 shows a flowchart of a conventional write control method of the HDD. Referring to FIG. 4, it can be understood that a write operation is started depending on the determination as to whether the value of PES, which is calculated by A−B in the A−B area, exists within a predetermined range, or as to whether the value of PES, which is calculated by C−D in the C−D area, exists within a predetermined range.
In operation S402, it is determined whether the target offtrack exists in the area A−B, or exists in an area C−D.
If it is determined that the target offtrack exists in the area A−B or the area C−D, based on the determination result in operation S402, a PES is calculated using A−B or C−D in operation S404, respectively. Referring to FIG. 3, a PES is calculated using A−B in the area A−B, and a PES is calculated using C−D in the area C−D.
If it is determined that the target offtrack does not exist in the area A−B or C−D in operation S402, a PES is calculated using N or P in operation S406. Referring to FIG. 3, a PES is calculated using N or P in an area NP.
In operation S408, it is determined whether the PES calculated in operation S404 or S406 is within a predetermined write bump limit. The calculated PES to be considered in operation S404 is PES_A−B in the area A−B, PES_C−D in the area C−D, or PES_NP in the area NP.
If it is determined that the calculated PES is within the predetermined write bump limit in operation S408, the write operation is performed in operation S412, otherwise the write operation is prohibited in operation S410.
FIG. 5 shows another profile set of servo burst signals read by the head. Referring to FIG. 5, it can be seen that the peak of a servo burst signal is distorted in a portion marked by a circle, and the peaks of the servo burst signals are not precisely disposed in positions off-tracked by −50%, 0%, 50%, or 100%, respectively. Here, the term “distortion” means that a PES does not have linearity.
FIG. 6 shows the distortion of a PES caused by a defective servo burst as illustrated in FIG. 5. Referring to FIG. 6, it can be understood that PES (PES_(A−B)) cannot be linear and is distorted due to a servo burst signal B smaller in magnitude than a servo burst signal A as shown in a portion marked by a circle.
If some of servo bursts are defective, as shown in FIG. 5 or if the width of the read head is narrow, distortion may occur in PES (PES_(A−B), PES_(C−D)) calculated by A−B or C−D. Thus, the correct position of the head is not reflected into the PES. In other words, indeed it is determined that a PES is within the write bump limit, but, the head may be not within a range corresponding to the write bump limit. If servo bursts are not written uniformly in all sectors and all tracks, the profile of the servo burst is distorted as shown in FIG. 5, so that the position of the head cannot be calculated correctly.
If the position of the head cannot be calculated during a write operation, data may be written in a place away from the center of a data track so that written data cannot be precisely reproduced and data on an adjacent track can be damaged.
For this reason, a defect processing process which detects defective servo sectors or tracks and processing them not to be written or read of data is needed in a manufacturing process of an HDD.
In the defect processing process, for example, servo bursts are determined as defective when a PES has a value more than a threshold value at the position where data is read or written, and then servo sectors or tracks having those servo bursts are defect-processed.
However, although a PES is satisfactory at the position where the write operation is started, data can be written in incorrect position or data in adjacent tracks may be erased in the case when a defective servo burst exists in some distance from the position.