Track pitch control using head offset measurement for...

Details Track pitch control using head offset measurement for... Track pitch control using head offset measurement for...

2001-12-19

2004-07-20

Hudspeth, David (Department: 2651)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

Reexamination Certificate

active

06765744

ABSTRACT:

FIELD OF THE INVENTION
This application relates generally to self-servowriting tracks in a disc drive and more particularly to controlling track pitch by using the offset measured between the read and write elements in a disc drive.
BACKGROUND OF THE INVENTION
A disc drive is a data storage device that stores digital data in magnetic form on a rotating storage medium called a disc. Modern disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Each surface of a disc is divided into several thousand tracks that are tightly packed concentric circles similar in layout to the annual growth rings of a tree. The tracks are typically numbered starting from zero at the track located outermost the disc and increasing for tracks located closer to the center of the disc. Each track is further broken down into sectors and servo bursts. A sector is normally the smallest individually addressable unit of information stored in a disc drive and typically holds 512 bytes of information plus additional bytes for internal drive control and error detection and correction. This organization of data allows for easy access to any part of the disc. A servo burst is a particular magnetic signature on a track, which facilitates positioning of heads over tracks.
Generally, each of the multiple discs in a disc drive has associated with it two heads (one adjacent the top surface of the disc and another adjacent the bottom) for writing and reading data to or from a sector. A typical disc drive has two or three discs. This usually means that there are four or six heads in a disc drive carried by a set of actuator arms. Data is accessed by moving the heads from the inner to outer part of the disc (and vice-versa) driven by an actuator assembly. The heads that access sectors on discs are locked together on the actuator assembly. For this reason, all the heads move in and out together and are always physically located at the same track number (e.g., it is impossible to have one head at track
0
and another at track
500
). Because all the heads move together, each of the tracks on all discs is known as a cylinder for reasons that the tracks are equal-sized circles stacked one on top of the other in space forming a cylinder. So, for example, if a disc drive has four discs, it would normally have eight heads (numbered
0
-
7
), and a cylinder number
680
would be made up of a set of eight tracks, one per disc surface, at track number
680
. Thus, for most purposes, there is not much difference between tracks and cylinders since a cylinder is basically a set of all tracks where all the heads are currently radially located.
Servo fields are embedded among the sectors on each track to enable the disc drive to control the position of the heads over the center of the track. Generally, the servo fields are written to the discs during the manufacture of a disc drive using a highly precise external servowriter. The servowriter typically utilizes the heads of the disc drive to write the servo fields. As the servo fields are subsequently used to define the tracks, it is important to precisely control the position of the heads as the servo fields are written to the disc surfaces. Thus, a typical servo track writer comprises an actuator positioning system which advances the position of the heads, a laser based position detector which senses the position of the heads, and control circuitry which provides the servo information to be written to the servo fields on the discs. The positioning system in an external servowriter includes a pusher pin assembly that engages the actuator assembly through an opening in the disc drive base deck. The position detector detects the position of the heads by detecting the radial position of the pusher pin assembly.
Alternatively, tracks may be written by a self-servowriter. The self-servowriter controls the position of the heads directly by applying current to the coil of the disc drive voice coil motor. Self-propagated servo track writing was first described in U.S. Pat. No. 4,414,589 (Oliver et al.). 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 arm is positioned at one of its travel range limit stops. A first reference track is written with the write 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 self-servowriting process ends when the actuator arm's second limit stop is reached and the entire disc surface is filled with reference tracks. The conventional servowriters then checks to see whether a target number of tracks are written on the disc. If the total number of written tracks is unacceptably higher than the target number, the disc is erased, the reduction number is lowered so that a larger displacement occurs between tracks, and the process is repeated. Likewise, if the total number of written tracks is unacceptably lower than the target number, the disc is erased, the reduction number is increased so that a smaller displacement occurs between tracks, and the self-servowriting process is repeated.
The conventional self-servowriting technique such as that shown in Oliver et al. indicates that position information of the new tracks is derived based on the signal generated in the previously written track measured by the read element. However, the total number of tracks that are to be written on a disc is often not predictable or otherwise very difficult to determine until all tracks are written on the disc. This is because the pitch of a track written on a disc cannot be determined unless there is a way to compare the pitch of the track being written with the pitch of a prewritten reference tracks. The conventional self-servowriters such as that shown in Oliver et al. typically cannot predict or determine the total number of tracks that are going to be written on a disc until all tracks are written on the disc. For this reason, the conventional self-servowriting technique such as that shown in Oliver et al. typically erases the entire disc if the average track density of the written tracks is too high or too low. A second pass of servo track writing is required by rewriting of the whole disc.
The use of an external servowriter to create a prewritten reference track increases the cost of servo track writing and further creates a possibility of disc drive contamination since the clock head and the pusher pin of the external servowriter have to be inserted inside the disc drive through an opening throughout the entire servowriting process.
The cost associated with using an external servowriter and the possibility of the disc drive contamination associated with using an external servowriter can be substantially eliminated if the pitch of a track and a total number of tracks to be written on the disc can be computed on-the-fly. Accordingly there is a need for ways to control the pitch of the track as the track is being written.
SUMMARY OF THE INVENTION
Against this backdrop the present invention has been developed. The prese

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