Correction of dynamic track spacing errors in storage devices

Details Correction of dynamic track spacing errors in storage devices Correction of dynamic track spacing errors in storage devices

2001-06-29

2004-08-31

Hudspeth, David (Department: 2651)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

C360S077080

Reexamination Certificate

active

06785084

ABSTRACT:

FIELD OF THE INVENTION
This application relates generally to disc drive data storage devices and more particularly to an apparatus and method of compensating for track spacing errors.
BACKGROUND OF THE INVENTION
Disc drives are the most common means of storing electronic information in use today. Typical disc drives have one or more magnetic media discs attached to a spindle; the spindle and discs are rotated at a constant velocity by a spindle motor. An actuator assembly, attached to a bearing shaft assembly next to the discs, radially traverses over the surface of the discs. The actuator assembly has a plurality of actuator arms, each with one or more flexures extending from the end of each actuator arm. A read/write head is attached to the distal end of each flexure. The actuator assembly is rotated about the bearing shaft assembly by a servo positioner. The servo positioner receives signals from a controller, rotates the actuator assembly, and positions the read/write head relative to the disc surface.
Information is transferred to and from the discs by the read/write heads attached to the flexures at the end of the actuator arms. Each head includes an air bearing slider that enables the head to fly on a cushion of air in close proximity to the corresponding surface of the associated disc. Most heads have a write element and a read element. The write element is used to store information to the disc, whereas the read element is used to retrieve information from the disc.
Discs, to facilitate information storage and retrieval, are radially divided into concentric circles known as “servo tracks” or “tracks”. Tracks are given a track number, among other identifying information, so that the servo positioner can align the read/write head over desired track. Information is stored or retrieved from the disc after the read/write head is aligned over the desired track. The process of switching between different tracks is called “seeking”, whereas remaining over a single track while information is stored or retrieved is called “following”.
Each track is linearly subdivided into “segments” or “sectors”. The two most common types of sectors are informational data sectors and servo data sectors. In a typical disc drive, the informational data sectors usually contain information generated or stored by the user such as programs files, application files, or database files. There may be ten to a hundred, or even more, informational data sectors dispersed around a single track.
The servo sectors, on the other hand, contain information that is used by the servo positioner to determine the radial, and linear, position of the head relative to the disc surface and relative to a track center. Servo sectors typically consist of a Gray code field, which provides coarse position information to the servo positioner such as the track and cylinder number, and a servo burst field, which provides fine position information to the servo positioner such as the relative position of the head to the track center. Generally speaking, the burst field creates a specific magnitude signal on one side of the track centerline and a different specific magnitude signal on the other side of the track centerline. The read head can be aligned directly over a track centerline by positioning the read head at the “null” position, or the position in which the sum of the burst field magnitudes cancel each other and equal zero.
Servo sectors are usually embedded between adjacent informational data sectors located on a single track. The servo sector provides positional information to the servo positioner so that the read/write head can be properly aligned over the subsequent informational sector. A clock signal mechanism is used to insure that data intended to be stored in a servo sector does not overwrite data in an information sector (and vice versa).
The number of tracks located within a specific area of the disc is called the “track density”. The greater the number of tracks per area, the greater the track density. The track density may vary as the disc is radially traversed. Disc manufacturers attempt to increase track density in order to place more information on a constant size disc. Track density may be increased by either decreasing the track width or by decreasing the space between adjacent tracks.
An increase in track density necessitates increased positioning accuracy of the read/write elements in order to prevent data from being read from or written to the wrong track. Manufacturers attempt to fly the read/write head elements directly over the center of the desired track when the read/write operation occurs to insure that the information is being read from and written to the correct track. Hitting the track center target at high track densities requires that the tracks be as close to perfectly circular as possible when written to the disc surface.
Tracks are usually written on the disc during disc drive manufacturing using one of two means: 1) a servowriting machine, or 2) self-propagated servo writing. A servowriting machine is a large piece of external equipment that writes servo tracks on a disc drive. A typical servowriting machine uses a large actuator with laser interferometer position feedback and a pushpin to position the arm of the disk drive. The write element, which is attached to the arm, is aligned to where the desired track is to be written on the disc surface. A track is written on the disc once the write element is correctly aligned. The head/arm positioner then moves the write element a predetermined distance to the next desired track location. The head/arm positioner, therefore, controls both the track placement and track-to-track spacing.
Although accurate, a servowriter has several drawbacks. First, a typical disc may contain more than 60,000 servo tracks. The process of aligning and writing each track on the disc is very time consuming and expensive. Next, although very accurate at lower track densities, the servowriter cannot meet the accuracy requirements dictated by higher track densities. Finally, track spacing and track shape errors, caused by spindle wobble, vibrations, disc slip, and thermal expansion among others, are introduced during the servowriting process.
The second means of writing tracks on a disc is called self-propagating servo writing. Oliver et al first described this method of servo track writing in U.S. Pat. No. 4,414,589. Several other patents have disclosed slight variations in the Oliver patent, but the same basic approach is used. Under the basic method, the drive's actuator assembly is positioned at one of its travel-range-limit stops. A first reference track is written with the write head element. The first reference track is then read with the read element as the head is radially displaced from the first reference track. When a distance is reached such that the read element senses a predetermined percentage of the first reference track's amplitude, a second reference track is written. The predetermined percentage is called the “reduction number”.
For example, the read element senses 100% of the first reference track's amplitude when the read element is directly over the first reference track. If the reduction number is 40%, the head is radially displaced from the first reference track until the read element senses only 40% of the first reference track's amplitude. A second reference pattern is written to the disc once the 40% is sensed by the read element. The head is then displaced in the same direction until the read head senses 40% of the second reference track's amplitude. A third reference track is then written and the process continues. The process ends when the actuator arm's second travel-range-limit stop is reached and the entire disc surface is filled with reference tracks. The average track density is then calculated using the number of tracks written and the length of travel of the head.
If the average track density is too high, the disc is erased, the reduction number is lowered so that a larger displacement occurs between

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