Dynamic magnetic information storage or retrieval – General recording or reproducing – Signal switching
Reexamination Certificate
1998-03-23
2001-09-18
Sniezek, Andrew L. (Department: 2651)
Dynamic magnetic information storage or retrieval
General recording or reproducing
Signal switching
C360S076000, C360S077040, C360S078050
Reexamination Certificate
active
06292320
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method in a disk drive to adjust the radial offset between heads of a dual stage actuator, where the dual stage actuator uses either a milli-actuator or a micro-actuator, to position a read/write head during a head switch.
BACKGROUND OF THE INVENTION
A huge competitive market exists for disk drives that store massive amounts of data in computers. This competition requires disk drive manufacturers to provide ever-increasing data storage capacity and higher performance in their products. One way to increase capacity is to provide more data tracks on a given disk surface, which generally requires that the tracks be more densely packed since the form factor of the disk is limited by compatibility standards. In current disk drives, track density or pitch of 10,000 tracks per inch is becoming available.
As tracks are placed closer together, the problem of maintaining a read/write transducer in position over the track becomes more difficult. The industry presently prefers a rotary type actuator, which employs a voice coil motor to pivotally swing an array of vertically stacked arms carrying read/write transducers over tracks disposed on surfaces of a corresponding stack of disks. A sampled servo system reads servo wedges interspersed at equal intervals around the data tracks to maintain the position of a transducer over a track. At some point in the march toward higher track densities, these conventional actuators and servo systems may be unable to provide the bandwidth and precision control required to keep the head positioned over a target track without encroaching adjacent tracks.
Dual stage actuators have been proposed as a solution to the bandwidth and precision control problem. The dual stage actuator generally provides two pivot points and two motors with a second stage motor and pivot point being smaller and positioned closer to the transducer, therefore providing for higher control bandwidth and precision. Typically the span or range of motion of the second stage actuator is limited to a few tracks or even to the width of a single track or less.
Dual stage actuators may be broadly characterized as milli-actuators or micro-actuators. In a milli-actuator design, the second stage actuator moves a suspension arm that suspends the read/write head. The milli-actuator can adjust the radial offset of the read/write head by moving the suspension arm, and thus the read/write head, in relation to the first stage pivot point.
In the micro-actuator design, the second stage, mounted on the suspension arm, moves the read/write head directly. The micro-actuator can adjust the position of the read/write head by moving the read/write head in relation to the suspension.
The performance of disk drives employing dual stage actuators, as in conventional disk drives, is and will be impacted by the time required to position a transducer over a target data track to perform a read or write operation, characterized as access time. One major component of access time is the time required to move or swing the actuator from a present track to a target track, known as seek time. Another significant component of access time is the time required to perform a head switch, which entails selecting a target transducer head on a different one of the vertically stacked arms of the actuator and establishing the precise position of that head over the present track. The head switch occurs frequently during read or write operations of sequential data where data is conventionally recorded on a stack of respective tracks at the same radial location on each disk surface, known as a cylinder. This recording technique has been historically employed to minimize the delay in continuing sequential data transfer because a head switch can be accomplished in less time than a seek.
Although the stacked arms and transducer heads in a disk drive are closely aligned, there are sufficient variations between the disks, arms and heads to require that the target head be re-positioned when a head switch is performed, even though the previous head was perfectly aligned over a corresponding track. This is especially true with more densely spaced tracks and the problem may even be exacerbated with dual stage actuators. The instantaneous difference in position between heads on the stacked arms owing to these variations may be termed radial offset. As a consequence, there is a continuing need to improve head switch times and provide higher performance disk drives.
SUMMARY OF THE INVENTION
An object of the invention is to minimize the time required to perform a head switch from a current head to a target head. The invention takes advantage of the fact that a dual stage actuator provides an individual actuator for each head and therefore advantageously aligns a target head with a target track with the target head second stage actuator before a head switch occurs. The radial offset required to align the target head with the target track may be either calibrated at startup during a calibration period or may preferably be measured dynamically during normal disk operations. The invention can be applied to dual stage actuators using a milli-actuator structure or a micro-actuator structure.
In one aspect, the invention provides a method to measure the radial offset of a read/write head for head switching operations in a disk drive. The disk drive has a plurality of disk surfaces, a head actuator having a first stage actuator means and a plurality of head arms rotatable in unison about a pivot by the first stage actuator means. Each head arm has a head for reading and writing on a respective disk surface. Each head is independently movable by a respective second stage actuator means. The disk drive has a servo controller and a servo read channel for reading servo signals from a head and positioning the first and second stage actuator means. The method comprises the steps of: track following at a selected cylinder location with a first head with the servo controller to establish a first position error signal; switching to a target head; reading a second position error signal with the servo controller at the selected cylinder location with the target head; computing a measure of the radial offset between the first head and the target head based on a difference between the first position error signal and the second position error signal; and writing the radial offset into a radial offset table.
Preferably the radial offset measurement is performed over a predetermined number of head switches wherein the radial offset is averaged over the predetermined number of head switches. The radial offset averaging may be performed during a calibration period or may be performed during normal disk operations.
The invention also provides a method to adjust the radial offset of a head for performing a head switch from a first head to a target head over a target track. The method is employed in a disk drive having a plurality of disk surfaces, a head actuator having a first stage actuator means and a plurality of head arms rotatable in unison about a pivot by the first stage actuator means. Each head arm has a head for reading and writing on a respective disk surface and each head is independently movable by a respective second stage actuator means. The disk drive further comprises a radial offset table for storing mutual radial offsets between heads, and a servo controller for reading servo signals from a head for positioning the first and second stage actuator means. The method of the invention reads a radial offset for a target head from the radial offset table and calculates an offset compensation signal for the target head second stage actuator for positioning the target head based on the radial offset. The method of the invention then applies the offset compensation signal to the target head second stage actuator before the head switch occurs from the first head to the target head.
In an alternate embodiment, the disk drive has a plurality of servo read channels and the method includes the steps of track followin
Mason Jeffrey E.
Stich Michael Charles
Shara Milad G
Sniezek Andrew L.
Western Digital Technologies Inc.
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