Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2000-05-09
2004-09-07
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
Reexamination Certificate
active
06788489
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to direct access storage drives having a rotating media. More specifically, the present invention relates to manners of improving data access in a direct access storage drive using knowledge of the particular values of off-track read capability of the heads within the drive.
2. The Relevant Technology
Computer systems generally utilize auxiliary storage devices on which data can be written and from which data can be read for later use. A direct access storage device (DASD) is a common auxiliary storage device which incorporates rotating magnetic disks for storing data in magnetic form on concentric, radially spaced tracks on the disk surfaces. Transducer heads driven in a path generally perpendicular to the drive axis are used to write data to the disks and read data from the disks.
Current DASDs typically utilize a head for reading or writing data onto the various tracks and an actuator connected by a support arm assembly to the head for moving the head to the desired track and maintaining it over the track centerline during read or write operations. The movement of the head to a desired track is referred to as data seeking, or merely “seeking.” Maintaining the head over the center line of the desired track during a read or write operation is referred to as track following or “tracking.”
DASD systems are required to position each data head over the proper radial location for writing a track and return the data head very close to the same location when reading the track. Current DASD systems, such as hard disk drives, use a voice coil type of actuator and a control device with a feedback response to locate and consistently maintain the head over a given track. Typically, seeking and tracking are enabled by magnetically written patterns on the disk surface. The patterns generally take the form of prerecorded tracking servo marks. The servo marks are read by the transducer head and transmitted to a control unit, which utilizes the servo marks to set and correct the trajectory of the transducer head.
The actuator is typically a voice coil motor (VCM) which comprises a coil movable throughout the magnetic field of a permanent magnetic stator. The application of current to the VCM causes the coil and thus the attached head to move radially. In the absence of bias forces, the acceleration of the coil is proportional to the applied current. This current is applied by a power amplifier in response to a control input. If the input control is small enough, then the applied current is proportional to the control input and the power amplifier is nonsaturated; if the control input is too large, the applied current reaches a maximum possible value, and the power amplifier is saturated.
In disk files which have a relatively high density of data tracks on the disk, it is necessary to incorporate a servo control system to efficiently move the head between tracks and to maintain the head precisely over the centerline of the desired track during read or write operations. This is accomplished by utilizing prerecorded servo information either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information sensed by the read/write head (or the dedicated servo head if a dedicated servo disk is used) is demodulated to generate a position error signal (PES) which is an indication of the position error of the head away from the nearest track centerline and to detect the track number or position sample.
In a DASD digital servo control system, a microprocessor utilizes a control signal algorithm to calculate a digital control signal based upon the digital values of certain state variables such as head position, VCM current, and head velocity. The digital control signal is converted to an analog signal and amplified to provide input current to the VCM. Such a digital servo control system is described in assignee's U.S. Pat. No. 4,679,103, incorporated by reference herein. The described system, as a part of the computation of the control signal to the actuator, makes use of the state estimator algorithm to estimate the state (i.e., position, velocity, and acceleration or current input to the VCM) of the head. In this type of system, a microprocessor receives, at discrete sample times, digital values corresponding to the PES, position sample, and the VCM input current, and computes through the use of the state estimator algorithm, a digital control signal. The digital control signal is then converted to an analog signal to provide a power amplifier control signal. A power amplifier then generates a new VCM input current.
One area of system performance in which there is room for improvement is in dealing with the off-track read capability (OTRC) of a head. As the track pitch of a DASD becomes smaller, manufacturing variations of the head manifests itself as a larger relative distribution of read and write performance. Coupled with other factors, such as servo positioning or channel robustness, this distribution can be an important component of the overall system performance.
Variations in head sizes that result from the manufacturing of a DASD can yield OTRC values that range from 15 to 30 percent of the track pitch. Head width is the primary variable in the OTRC of a head. Other variables, including head composition and sensitivity, spindle rotational speed, track pitch, and fly height of the head. While these latter variables must be considered, they generally do not vary within a DASD design.
Also, in order to improve the overall performance of the drive, seek order scheduling is conducted to improve throughput. That is, the seeks are not performed in the order received, but the order is rearranged so that more seeks can be conducted in a given time. To best rearrange the order, the microprocessor is generally provided with a table of read and write times versus seek length to predict how long a seek will take.
The prior art currently selects a single OTRC and incorporates that OTRC into its table of seek time versus seek length estimates. Only a single preselected OTRC is used for each head in a DASD and for each DASD of a given design. Generally, the OTRC value used must be a worst case or near worst case OTRC for all heads of all DASDs of the given design. Such methods have been found by the inventors to leave much room for improving seek latency.
Accordingly, a need exists for an apparatus and method for applying the head-specific OTRC to seeking and tracking operations within a DASD, such that seek latency may be improved and tracking may be optimized.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available direct access storage devices. Thus, it is an overall objective of the present invention to provide a system and method for employing off-track read capability in data access operations in a direct access storage device.
To achieve the foregoing object, and in accordance with the invention as embodied and broadly described herein in the preferred embodiments, a DASD system and method for employing customized off-track read capability (OTRC) in accessing data on a direct access storage device (DASD) is provided.
In one embodiment, the DASD comprises a rotating data storage medium; a read head positionable for interaction with the data storage medium; and a control unit for positioning the read head with respect to the data storage medium. Preferably, the control unit comprises an off-track read capability (OTRC) storage module configured to store an indicator of an OTRC particular to the read head and an access module configured to apply the OTRC in accessing data from the data storage medium. The data access module is, in one embodiment, configured to apply the OTRC to generate an improved read time estimate for optimal scheduling
Chang Thomas Young
Serrano Louis Joseph
Hitachi Global Storage Technologies - Netherlands B.V.
Hudspeth David
Kunzler & Associates
Wong K.
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