Electricity: motive power systems – Positional servo systems – Adaptive or optimizing systems including 'bang-bang' servos
Utility Patent
1999-04-28
2001-01-02
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
Adaptive or optimizing systems including 'bang-bang' servos
C318S561000, C318S611000, 36, 36, C360S069000, C360S081000
Utility Patent
active
06169382
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to an apparatus and method for improving disc drive data storage capacity and performance by adaptively adjusting the velocity profile of a head during a seek in relation to the radial location of the destination track.
BACKGROUND
A disc drive is a data storage device used to store and retrieve computerized data in a fast and efficient manner. A typical disc drive comprises 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. The data are stored on the discs in a plurality of concentric circular tracks by an array of transducers (“heads”) mounted to a radial actuator for movement of the heads relative to the discs.
The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from an actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer diameter of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs so that the heads move in a plane parallel with the surfaces of the discs.
Typically, the actuator employs a voice coil motor to position the heads with respect to the disc surfaces. The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit with one or more permanent magnets. When controlled current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the permanent magnets to cause the heads to move across the disc surfaces.
The heads are supported over the discs by actuator slider assemblies which include air-bearing surfaces designed to interact with a thin layer of moving air generated by the rotation of the discs, so that the heads are said to “fly” over the disc surfaces. Generally, the heads write data to a selected data track on the disc surface by selectively magnetizing portions of the data track through the application of a time-varying write current to the head. In order to subsequently read back the data stored on the data track, the head detects flux transitions in the magnetic fields of the data track and converts these to a read signal which is decoded by read channel circuitry of the disc drive.
Control of the position of the heads is typically achieved with a closed loop, digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention and incorporated herein by reference. In such a system, servo (positional control) data are interspersed with user data fields used to store the user data, with the servo data being transduced by the heads and provided to the servo system to detect head position and velocity.
The servo system carries out two primary operations: seeking and track following. During track following, the selected head is maintained over a selected track on the corresponding disc. To this end, the servo system uses the servo data to generate a position error signal (PES) indicative of the relative distance between the head and the center of the selected track. In turn, the PES is used to generate correction signals which adjust the position of the head by adjusting the amount of current applied to the actuator coil.
During a seek operation, the selected head is moved from an initial track to a destination track on the associated disc surface. A velocity controlled approach is used wherein a velocity profile, indicative of a desired velocity trajectory for the head, is selected based on the distance between the initial and destination tracks. The head is first accelerated to a maximum velocity through the application of a large magnitude of current to the coil. The head then is maintained at this maximum velocity until it reaches a predetermined distance from the destination track, at which point a large amount of current of opposite polarity is applied to the coil to decelerate and settle the head onto the destination track. Adjustments in the velocity of the head are repetitively made during the seek in relation to the difference between the actual velocity and the velocity profile. The desired velocity at any given point during the seek is referred to as a demand velocity, with the velocity profile comprising a series of demand velocity values. If a fault occurs during the seek, the coil will be shorted, causing the actuator to brake.
To maximize data transfer performance, attempts are made to minimize the time required to perform each seek, as the disc drive can only transfer data during a track following mode of operation. Hence, disc drive designers attempt to provide a maximum velocity that is as high as practicable to allow the head to reach the destination track in a short amount of time.
There is an attendant danger of damage to the drive, however, should the servo system lose control of the actuator during a seek; such a loss of control can occur, for example, from a loss of power to the drive. Limit stops (sometimes referred to as “crash stops”) are provided to limit the maximum radial extension of the heads as they sweep across the disc surfaces. Such limit stops are typically arranged to contact a corresponding features of the actuator when the heads are at the innermost and outermost diameters of the discs. If the servo system fails to adequately reduce the velocity of the heads, the abrupt deceleration of the actuator upon contact with the associated limit stop can cause excessive movement of the heads, potentially damaging the heads and/or the disc surfaces.
One prior art approach to reducing damage to the disc drive from an over-velocity condition is disclosed by U.S. Pat. No. 4,967,291 issued Oct. 30, 1990 by Touchton et al. (“Touchton '291”), which limits the maximum velocity of the heads during a seek to a sufficient level so that, should a run-away condition occur, the heads are not damaged upon contact with the associated limit stop. Touchton '291 monitors the velocity of the head and interrupts the application of current to the actuator coil (such as by temporarily shorting the coil) when the velocity exceeds a safe threshold.
Another prior art approach is to make the limit stops sufficiently soft so as to safely decelerate the heads upon contact at full speed. A problem with this approach is that a relatively large deceleration zone is required proximate the stops to accommodate the deceleration of the actuator into the stops. Because the actuator is in contact with the limit stops while the heads are over the deceleration zones, the zones cannot be used to store data. Thus, a large deceleration zone requires a narrowing of the data recording band and limits the total data capacity of the drive.
While operable, there remains a continued need for improvements in the art to enable disc drive designers to achieve ever increasing data densities and levels of data transfer performance. It is to these ends that the present invention is directed.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for improving the operational performance of a disc drive using an adaptive velocity profile selected in relation to destination track location.
In accordance with preferred embodiments, a disc drive includes a rotatable disc with a recording surface on which a plurality of tracks are defined and an actuator which supports a head adjacent the tracks. The head is moved through application of current to a coil of an actuator motor, with limit stops being provided to limit the maximum radial excursion of the head. A servo circuit performs a velocity-controlled seek operation to move the head from an initial track to a destination track by accelerating and maintaining the head at a maximum velocity and then decelerating the head to the destinat
McKenzie Lealon R.
Wood Roy L.
Crowe & Dunlevy
Leykin Rita
Nappi Robert E.
Seagate Technology LLC
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