Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1999-02-09
2002-04-09
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S075000
Reexamination Certificate
active
06369974
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to magnetic disk drives capable of constructing an indicated position signal from an integer track component and a fractional track component or fractional PES segment. The invention relates more particularly to a method of constructing an indicated position signal that is continuous from fractional PES segment to fractional PES segment and a related method of linearizing the fractional PES segments without destroying the continuity.
2. Description of the Related Art
A conventional disk drive has a head disk assembly (“HDA”) including at least one disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a transducer head for reading and writing data. The HSA forms part of a servo control system that positions the transducer head over a particular track on the disk to read or write information from that track.
The industry presently prefers a “rotary” or “swing-type” actuator assembly which conventionally comprises an actuator body that rotates on a pivot assembly between limited positions, a coil that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor, and an actuator arm that extends from the opposite side of the actuator body to support the HGA.
Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo track information is provided on this disk or another disk to determine the position of the head. A manufacturing fixture called a servo track writer (STW) is used to write the servo track information on the surfaces of the disks in an HDA. The STW mechanically moves the actuator to a given reference position precisely measured by a laser interferometer. The HDA is then driven to write servo track information for that position. The process of precisely measured displacement and servo track writing is repeated to write all required servo tracks across the disk.
The most prevalent servo system used in disk drives is called “sampled servo” or “embedded servo” wherein the servo track information is written in a plurality of servo sectors that are equally angularly spaced from one another and interspersed between data segments around the track. Each servo sector comprises a track identification (ID) field defining a gray code track number or servo track number and a pattern of servo burst fields. The transducer head reads the track ID field and the servo bursts to construct an indicated position signal formed from an integer component and a fractional component. The gray code track number (track ID) provides the integer component and the servo bursts provide the fractional component. The difference between the indicated position and a desired position forms a position error signal (overall PES) for use by the servo control system. Note that overall PES is different than the fractional component which is often simply called the “PES.” This application will refer to the latter as a “fractional component,” “fractional PES,” or “fractional PES segment” to avoid confusion.
The servo control system reads the track ID field and samples the servo bursts to position the transducer head at the desired radial position. The servo control system moves the transducer head toward a desired servo track during a coarse “seek” mode using the track ID field as a control input. Once the transducer head is generally over the desired servo track, the servo control system uses the servo bursts to keep the transducer head at a desired radial position relative to the servo track in a fine “track follow” mode.
One conventional servo burst pattern is a repeating pattern of four bursts, grouped as two burst pairs, where each pair is abutted along the “burst pair centerline” and wherein the pairs and associated burst pair centerlines are offset from one another by a fixed amount. The servo system constructs a fractional PES by computing a pair difference signal for each pair and choosing which pair difference signal to use based on which pair is closer.
The pair difference signal is zero if the head is positioned at a “seam” or burst pair centerline where a pair of bursts are abutted. The pair difference signal increases as the head moves away from the burst pair centerlines. In one conventional embodiment which makes efficient use of the STW, each burst is nominally one data track wide and the burst pair centerlines are spaced apart by one half of a data track. Because the read width of the head is less than a burst width, the servo system must switch or “commutate” between burst pairs when the head is at a “commutation position” near the mid-point between burst pair centerlines. Commonly, a fractional PES from a burst pair at track center is termed “P” or primary, while a fractional PES from the adjacent burst pair is termed “Q” or quadrature. In another embodiment used with narrower read heads such as magnetoresistive (MR) heads, the burst width may be two/thirds of a data track width and the burst pair centerlines are spaced apart by one/third of a data track width.
The indicated position signal should be continuous through the commutation position. An indicated position signal of ordinary construction, however, may have discontinuities owing to variations in the placement of the bursts during the servo writing process, non-linearity of the read head signals relative to true displacement from a burst pair centerline, or both. Discontinuities are especially prevalent with the current use of magnetoresistive (MR) heads, and are especially troublesome. First, MR heads have an inherently non-linear microtrack profile. Second, MR heads suffer from “head switch instability” in that when a head is selected, it may have undergone a state change which causes the head to exhibit a gain which is significantly shifted from its nominal value. Finally, to make matters worse, the servo system often operates with the head near the commutation position and in the vicinity of a potential discontinuity when “micro-jogging” (operating the head at an offset from a burst pair centerline) in one case to compensate for the offset between the read and write elements in an MR head.
The inventors are aware of MR head disk drives having servo control systems which provide some degree of hysteresis at the commutation position. Such disk drives tend to continue deriving the fractional PES from one burst pair centerline, or the other, even if the head moves a short distance to the other side of the commutation position. The hysteresis, therefore, reduces the likelihood of encountering a discontinuity while operating at the commutation position. Disk drives using hysteresis, however, continue to construct the fractional PES in a conventional manner such that a significant variance may still exist in the indicated position signal on either side of the commutation position. Accordingly, while hysteresis helps resolve the effect of discontinuities in the indicated position at the commutation position, it does nothing to prevent such variance in the first place and continues to allow for inherent instability while operating the MR head at or near the commutation position.
U.S. Pat. No. 5,825,579, assigned to IBM, is an example of others having tried to achieve a continuous PES at the commutation position. The IBM inventors, however, select one of four segments +P, +Q, −P, or −Q based on whether the head is over “track type” 0, 1, 2, or 3, and then “stitch” the segments together to form a position error signal (PES) portion of the overall position signal (fractional PES segment herein). Moreover, the IBM approach requires different algorithms for wide read heads (equation 5), narrow read heads (equations 6 and 7), and read heads with widths between these two extremes (function 11). The IBM approach is apparently head width dependent because it relies on linearizing the raw signal
Asgari Saeed
Hagen Mark D.
Hudspeth David
Shara Milad G
Western Digital Technologies Inc.
Wong K.
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