1. Field of the Invention
This invention relates generally to reading and writing data on a magnetic storage medium and in particular to reading and writing data at a predetermined location on a magnetic storage medium by using servo position information embedded within the data.
2. Prior Art
Typically, a disk drive contains one or more circular planar disks that are coated on each side with a magnetic medium. The disk or disks are mounted on a spindle that extends through the center of each disk so that the disks may be rotated at a predetermined speed, usually about 3600 rpm. Usually, one read/write head is associated with each side of the disk that is coated with a magnetic medium. The read/write head flies a small distance above the disk surface as the disk rotates. The read/write head, in response to signals from electronics associated with the disk drive, writes data at a predetermined location in the magnetic medium. Similarly, the read/write head, in response to other signals from the electronics associated with the disk drive, reads the stored data at a predetermined location.
The configuration of the data on the magnetic surface is instrumental in the operation of the disk drive. Data are recorded by the read/write head in concentric circular tracks on the disk. Corresponding tracks on different disk surfaces are cylindrically aligned. Typically, each track is segmented into one or more parts that are referred to as sectors. Thus, the disk drive must move the read/write head across the disk surface to locate the track for reading or writing data and then must follow that track until the desired sector passes under the read/write head. Hence, the read/write head is positioned at a predetermined position over the disk surface.
In a disk drive, each read/write head is usually affixed by an arm to a carriage and the carriage is moved so that the read/write head is positioned over the specified track. This operation is referred to as a track seek, or sometimes just a seek. In an open-loop disk drive, a stepper motor is used to move the carriage while in a closed-loop disk drive a servo system is used to move the carriage.
Many different servo systems have been developed for use in hard disk drives. One type of servo system is an embedded servo system where a servo field identifying the data location is placed in front of each data sector in a track. For example, in U.S. Pat. No. 4,823,212 issued to Knowles et al. on Apr. 18, 1989 each track is divided into an equal number of sectors. Each sector includes a section of servo code, referred to as a servo field, at the beginning of the sector. Each servo field is the same length and includes, starting at its leading edge, a write splice area, an automatic gain control section, a sector mark section, an index sector identifier, a defect bit, a Gray code track number section, and a track position section followed by another write splice area.
The write splice areas are used to compensate for disk rotational speed variations so that the servo code is not overwritten by data. The automatic gain control section is used to normalize the signals from the read/write head so that subsequent servo sections are properly detected and processed. The sector mark section is used to establish a timing reference for the servo signals that follow. The index sector identifier identifies the first sector on each track, i.e., provides an index pulse. The defect bit is used to indicate that the data sector associated with the servo code is defective. The Gray code track number section is a set of magnetic dibits that contain the track address. As is known to those skilled in the art, the track addresses are addresses that are encoded using a Gray code sequence so that any decoding uncertainty is limited to plus or minus one half track. With the Gray code, only one bit in the track address changes from track to track. Finally, the track position section is used to generate signals that are used for track following.
An embedded servo system where each servo field includes a full Gray code track address permits rapid seeks and rapid recovery following a seek error. However, the relatively large servo field limits the amount of data that can be stored in a track.
An alternative to the full Gray code track address in each servo field is to use a different system that has only a modulo track address. In the modulo track address system, the tracks are divided into bands. For a modulo n track address system, each band has tracks numbered from zero to (n-1).
The key to a modulo track address system is (i) having a known reference point from which to start a seek and (ii) keeping an accurate account of the movement of the read/write head from that reference point. The servo system samples at predetermined time intervals as the read/write heads move radially across the disk. During the sampling period, the modulo track address is read by the disk drive electronics. Thus, a track address is available only during the sampling period, but the read/write heads are moving between sampling periods. The disk drive electronics must maintain an accurate count of the number of bands that the read/write heads must traverse to reach the target track.
The disk drive electronics has no means to determine that more than one band has been traversed between consecutive samples. Thus, if the read/write heads move over more than one band during consecutive samples, a seek error occurs. When a seek error occurs, the reference point is lost. The maximum velocity of the read/write head is limited by the number of bits in the modulo track address, because the number of bits determines the number of tracks in a band. The maximum actuator velocity and the number of tracks in a band is selected so that the read/write heads cannot fly over an entire band of tracks during the sample period. Hence, the seek performance of the modulo track address embedded servo system is typically slower than the full track address embedded servo system.
Since the modulo track address is smaller in length than the full track address, the servo field length of the modulo embedded servo system is smaller than the full address embedded servo system. Hence, the modulo embedded servo system provides a reduction in servo overhead in comparison to the full address servo system. However, for both the modulo embedded servo system and the full address embedded servo system, each servo field on the disk has the same length.
Several different approaches have been used in the track position section of the servo field to encode information that results in accurate track following. For examples of track positioning techniques, see U.S. Pat. No. 4,823,212 issued to Knowles et al. on Apr. 18, 1989; U.S. Pat. No. 4,530,019 issued to Penniman on Jul. 16, 1985; U.S. Pat. No. 4,424,543 issued to Lewis et al. on Jan. 3, 1984; and U.S. Pat. No. 4,669,004 issued to Moon et al. on May 26, 1987, which are incorporated herein by reference in their entirety.
As is known to those skilled in the art, some servo systems do not incorporate the automatic gain section described above. However, most disk drives for portable computers having an embedded servo system use the automatic gain section in each servo field on the disk to assure reliable performance of the system. The automatic gain section contributes significantly to the servo overhead.