Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2000-06-26
2001-10-30
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
Reexamination Certificate
active
06310742
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to rotating magnetic disk drives and, more particularly, to a method of learning and thereafter canceling repeatable runout written to a servo track during the servowriting process for accurately track following at a desired position relative to a servo track center
DESCRIPTION OF THE RELATED ART
A conventional disk drive has a head disk assembly (“HDA”) including at least one magnetic 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 information is provided on each disk or on another disk to determine the position of the head. The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo wedges that are angularly spaced from one another and interspersed between data sectors around each track of each disk. Each servo wedge generally comprises a track identification (ID) field and a group of servo bursts (an alternating pattern of magnetic transitions) which the servo control system samples to align the transducer head with or relative to a particular servo track or one or more corresponding data tracks. The servo control system moves the transducer toward a desired track during a coarse “seek” mode using the track ID field as a control input. Once the transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the transducer head over that track in a fine “track follow” mode. The transducer generally reads the servo bursts to produce a position error signal (PES) that is 0 when the transducer is at a particular radial position. The position where the PES=0 may or may not be at the servo track or data track center, however, depending on the magnetic characteristics of the transducer, the arrangement of the servo bursts, and the formula used to calculate the PES.
The general goal of the servo control system is to control the head position relative to a desired position—i.e. to get it there and to keep it there. There are numerous outside influences which make it difficult for the servo control system to achieve to control the head position, but a particularly troublesome influence is known as “runout.”
Runout generally refers to deviation from perfect circular motion and, more particularly, refers to variation in the distance between an external point of reference and a passing surface of a rotating object. “Repeatable runout” involves periodic deviations that occur with predictable regularity (hereafter “RRO”). “Nonrepeatable runout” involves random perturbations due, for example, to bearing slop, shock events, and so on (hereafter NRRO). Perturbations due to NRRO generally cannot be removed. The present invention is directed to RRO as R affects disk drives.
In the context of a disk drive, RRO is “repeatable” because it occurs in sync with the spinning disk. RRO comes from one or more of the following mechanical sources:
a) spindle motor runout;
b) disk slippage;
c) disk warping;
d) disturbances converted to RRO during the Servo Writing process due, for example, to NRRO, vibrations, resonances, media defects, or disk distortion due to clamping of the HDA.
RRO may also be caused by electromagnetic imperfections due to low quality servo bursts, even if they were mechanically recorded on the ideal circle. This is true because the low quality bursts will yield incorrect position information.
At least one other inventor has tried to reduce the effect of RRO in magnetic disk drives, as shown by U.S. Pat. No. 5,550,685, which issued to David M. Drouin on Aug. 27, 1996, was assigned to Syquest Technology, Inc., and is entitled “APPLYING ADAPTIVE FEED-FORWARD ALGORITHM AS A FREQUENCY SELECTIVE FILTER IN A CLOSED LOOP DISK DRIVE SERVO SYSTEM IN ORDER TO COMPENSATE FOR PERIODIC PERTURBATIONS WHICH OTHERWISE APPEAR IN THE SERVO SYSTEM POSITION ERROR SIGNAL.” The '685 patent primarily deals with tracking RRO at or above the “1F” rotational frequency of the disk (see 2:47-50 and 3:43-46). In other words, the '685 patent detrimentally tries to follow the RRO, rather than trying to cancel it altogether.
There remains a need, therefore, for a method of canceling RRO wherein the drive is independently capable of learning the RRO associated with each or with a selection portion of its servo wedges without need for attachment to a servowriter, for storing such RRO data, and for thereafter effectively canceling the effect of the RRO when the servo control system is track following on a servo track by correcting the position error signal (PES) from each such servo wedge based on the stored RRO data.
SUMMARY OF THE INVENTION
The proposed method learns about servo written RRO using the drive's own heads and the drive's own servo control system without need for extremely accurate positioning that must ordinarily be provided by a servo writer for this purpose. The method is an adaptive averaging method that converges on a best fit servo track according to conditions experienced by the drive's own head and servo control system during several revolutions of the drive's disk while the servo control system's open loop response is intentionally dulled with respect to higher frequencies in order to maintain stability.
In a first aspect, the present invention resides in a method for determining repeatable runout cancellation values in a disk drive having a magnetic disk with a plurality of tracks that each contain a plurality of servo wedges, a means for rotating the magnetic disk at a rotation frequency, a transducer head mounted on an actuator, a means for moving the actuator, and a sampled servo controller for reading signals from the transducer head and for providing servo compensation signals to the actuator moving means for positioning the transducer head.
The method more specifically comprises the steps of: (a) initializing a wedge runout value for each servo wedge in a current track; (b) initializing an average uncorrected runout value; (c) track following the current track with the sampled servo controller operating in a low bandwidth mode so that the sampled servo controller is less responsive to high frequency components of the repeatable runout; (d) waiting for a current servo wedge of the current track; (e) reading from the transducer head to produce a raw position error signal which may have a repeatable runout component for the current servo wedge; (f) computing an interim wedge runout estimate for the current servo wedge by adding a first multiple of the raw position error signal to a second multiple of the wedge runout value for the current wedge; (g) computing a corrected position error signal by subtracting the interim wedge runout estimate from the raw position error signal; (h) moving the transducer head based on the corrected position error signal; (i) computing a new wedge runout value for the current wedge by subtracting the average uncorrected runout value from the interim wedge runout estimate for the current wedge; (j) saving the new wedge runout value for the current wedge; (k) repeating steps (d) through (k) for each of the plurality of servo wedges in a complete revolution; (l) revising the average uncorrec
Nazarian Ara W.
Simmons Charles W.
Trieu Thao P.
Wong Richard K.
Davidson Dan I
Hudspeth David
Shara Milad G
Western Digital Technologies Inc.
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