Abstract:
In a rigid magnetic disk drive data storage device, the protective lubricant coating on the magnetic disk migrates toward the outer diameter during operation with the result that the radially inward portions of the data surface become depleted. Further, the depletion is nonlinear across the band of data tracks. Since most of the operating time in a typical application is consumed by idle periods between read and write commands, it is necessary that the slider be positioned during such periods where device life shortening head-disk contacts are least likely. The present invention adaptively allocates the idle position to the outermost data tracks where the most robust lubricant film remains. However, to avoid a contaminant buildup that could lead to a catastrophic head crash, it is necessary to periodically sweep the disk surface by accessing the innermost diameter of the surface and progressively moving the transducer carrying slider to the outermost diameter track to sweep any debris radially outward.

Description:
FIELD OF THE INVENTION 
     The invention pertains to rigid magnetic disk drives and more particularly to a method of controlling the idling location of the transducer supporting sliders over the disk surface in response to the migration disk surface lubricant over time to extend the useful life of the drive. 
     BACKGROUND OF THE INVENTION 
     When rigid magnetic disks are manufactured, a typical lubricant is applied to protect the data surface from damage arising from occasional, accidental contact with the transducer head that flies over the surface during disk drive operation. The lubricant is normally a type that forms a chemical bond with the underlying disk surface coating. However such bonding is limited to a thin layer at the interface surface, with the result that a thin film of lubricant is bonded and stationary while the remainder is mobile and subject to migration when subjected to centrifugal forces as the disk rotates at high rotational velocities or head pressure as the head flies over and is supported above the surface by a film of air. 
     Centrifugal forces cause the lube to migrate toward the outer edges of the disks leaving the inner radii with less lube and therefore more susceptible to wear during intermittent slider-to-disk contact. If sliders are allowed to idle near the inner tracks after the lube has migrated from that vicinity, the result will be more frequent slider-disk contact that will accelerate failure. 
     SUMMARY OF THE INVENTION 
     This invention recognizes the occurrence of lubricant migration toward the perimeter of the disk as a result of high rotational speed disk rotation. If sliders are allowed to idle near the inner diameter tracks after lube has migrated from that location, failure will be accelerated. After a power on operational time period, during which experience has shown that significant lube migration has taken place, the sliders are moved during idle time toward the outer (larger radius) tracks to reduce the amount of time the sliders spend over the low lube regions at smaller radii on the disk surfaces. Initially, the lube thickness is substantially identical over the entire disk data surface. However, it has been found that after a period of about three months of continuous operation (spinning of the disks) much of the mobile portion of the lube has migrated toward the outer radius of the disk surface, leaving the inner radius tracks with little but the bonded lubricant remaining. 
     One response to the problem is to change the track distribution for idle to follow the lube migration over time. At low power on hours, the track distribution is broad and covers the entire disk surface. This is important because it prevents any area of the disk from becoming a collection area for debris. If, for example, an inner radii region of the disks were completely neglected during idle, a ridge of debris may develop at the edge of the idle zone. An analogy is the debris that accumulates at the edge of highways where vehicles seldom pass over. If a car, for example, moves onto the shoulder of a highway, it is likely that some debris will be hit. In the same way, if debris collects at the edge of an idle zone, it can cause a head crash when struck for the first time. Therefore, it is necessary to periodically sweep the entire disk data surface from inner diameter to outer diameter even after much of the lube has migrated to the outer disk radii and the idle function has been modified to position the sliders only over disk surface portions that are most adequately lubricated. 
     Thus, although the sliders are controlled to idle over a band of tracks at the outer diameter of the disk data surfaces to match the lube migration, the sliders are periodically moved to the innermost track and then moved to the outer diameter track in order to sweep the entire disk surface and eliminate debris. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a typical disk drive with the cover and portions of the voice coil motor removed to illustrate the principal assemblies of such a device. 
     FIG. 2 is a flow diagram illustrating a portion of a disk drive control sequence including the idle routine of the present invention. 
     FIG. 3 graphically illustrates the radial distribution of lubricant across the disk data band at operating times POH=0, POH=3 months, and POH=6 months. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a typical rigid disk drive  11  for storing data including one or more disks  10  mounted for rotation about a spindle  12  which is supported on a rigid base or frame  14 . An actuator assembly  16  carries the sliders  18  that carry the transducers that record data on and read data from surfaces  20 . Actuator assembly  16  includes a stationary portion  21  secured to base  14  and a pivotable portion  24  which pivots about a stationary post  26 . Actuator arms  28  carry flexures  30  which in turn support the sliders  18  that confront the disk surfaces. The flexure  30  allows the slider  18  to move toward and away from the disk surface  20  and a gimble connection (not shown) between flexure and slider allows the slider to pitch and roll during flight. An arm  32 , at the side of the pivotable actuator assembly opposite the transducer carrying arms  28 , supports a voice coil  34  which is part of the voice coil motor (VCM) that drives and controls the pivotal motion of the actuator assembly. The VCM includes permanent magnets  36  supported on the bottom plate  38  formed of magnetically permeable material. An upper plate member (not shown) of magnetically permeable material is attached to the portion  21  to form a flux path which is interrupted by a gap across which magnetic flux flows. The coil  34 , supported on an arm  32 , is positioned in the air gap and drives the actuator pivotable portion one direction when current flows through the coil in one direction and drives the actuator pivotable portion in the opposite direction when the current through the coil is reversed. When the drive is shut down, the actuator is pivoted toward the outer diameter of the disk where the flexures  30  engage ramps  40  and cause the sliders to be lifted away from the disk surface  20  before disk rotation is stopped, to prevent sliders from landing on the disk surface. A cover (not shown) is secured to base or frame member  14  to form a hermetic seal and thereby exclude particulate or contaminant materials from the enclosure. 
     One the disk surface  20 , the entire band of data tracks extends from an innermost diameter data track  42  to an outermost diameter data track  44 . The one third of the tracks adjoining the outer diameter extends from track  43  to track  44 . The remainder of the data tracks from track  42  to track  43  are the two thirds of the tracks adjoining the inner diameter of the data recording surface. 
     Rigid magnetic data storage disks have a lubricant applied to the surface during manufacture which is of uniform thickness and can be characterized as having a fixed or bonded portion, that forms a chemical bond with the material of the disk surface to which the lube is applied and a remainder that is mobile. When the disk is rotated in a disk drive, the mobile lubricant is dispersed over time and migrates toward the outer diameter of the disk. After three to six months of continuous operation there is probably no more than the bonded lube resident at the inner diameter of the disk storage track band. This leaves the inner tracks of the data storage surface with less lubricant causing that portion of the data surface to be more vulnerable to the occasional disk-slider contact that can damage the disk coatings and is cumulatively the most common cause of ultimate drive failure. To overcome this redistribution of effective disk lubrication, it is necessary to adaptively adjust drive operation to achieve extended drive life. 
     Since in most disk drive applications the majority of time during operation is spent idling between read or write data commands, the idle routine offers the greatest opportunity to adaptively control slider position to assure that the slider flies over an adequately lubricated surface of the disk. In the present invention this is achieved by causing the slider to idle variously over the outer one third of the data track band where lube migration has least reduced the mobile portion of the lubricant coating. 
     Positioning over an adequately lubricated surface can be accomplished by randomly positioning the slider during the idle routine nearer the outermost track  44  (FIG. 1) than the innermost track  42 . Typically the outer one third of the data tracks, from track  43  to track  44  is the preferred site for allowing the sliders to idle. The slider should be allowed to dwell in one location for only a limited time and thus after a prescribed time, the dwell at this location is terminated and the idle routine recycled to position the slider at another random location within the track band including the radially outer one third of the data tracks. 
     Another mode of slider positioning is to randomly position the slider within the radially outward third of the data tracks and thereafter advance or index the slider to the outermost track. When the outermost track is reached, the advance and dwell over the disk surface is terminated and the routine recycled to again randomly position the slider. This mode of operation causes the period of slider idle dwell to increase to a maximum at the outermost track location. This would tend to parallel the distribution of lube caused by migration which after extensive disk drive operation tends to diminish lube over much of the disk to only the fixed with the mobile lube being continuously more concentrated at the outer diameter of the data surface. 
     Although restriction of the head idle location to the outer tracks of the data band assures that the slider idles over the surface with optimum lubrication, further action must be taken to assure that such restriction of the slider location does not cause an adjacent buildup of a ridge of debris that could lead to catastrophic failure. To prevent a localized buildup of debris, it is important to periodically sweep across the data surface during the idle routine. Periodically, the sliders are moved to the innermost data track location and progressively moved to the outermost diameter to sweep the surface and urge any particulates or other contaminant material toward the outer edge of the disk. This action tends to move contaminants away from the data surface before any accumulation can occur that is capable of slider or transducer damage or the destruction of recorded data. 
     FIG. 3 illustrates why the disk radii used during idle should mimic or adapt to the lube thickness radial distribution. As shown in the upper graph, at POH=0 the lube has a uniform thickness over the entire disk surface with the result that lube thickness is not a factor in selecting head position since an adequate lube thickness exists at every location. As time progresses (to POH=3 months in the middle graph and POH=6 months in the lower graph of FIG. 3) the lubricant thickness increases toward the outer disk diameter (larger radii) and decreases at the inner diameter (smaller radii). The idle time radial position of the sliders should track the radial distribution of lubricant so that as the time (POH) increases, more of the idle time is spent at the outer diameter. For the reasons discussed above, the adaptive idle routine should be interrupted by a periodic sweep across the disk surfaces. 
     FIG. 2 is a flow diagram illustrating an embodiment of the present invention for the control of the slider position during the operational idle periods between disk drive commands. After start, a decision is made at  51  which determines whether the drive has been in operation for a minimum number of power on hours (POH) indicative of the occurrence of significant lube migration resulting in material lube depletion in the region of the inner tracks of the track band used to store data. Prior to significant lube migration, the lubrication over the entire surface is sufficiently uniform to permit the head to idle at any radial location above the disk in accordance with normal drive control sequences. 
     When the predetermined POH value indicative of disk lubricant migration has been exceeded, the idle control method of the present invention is invoked. Initially it is determined at block  52  whether a time period delta T has been exceeded since the last sweep. If so, the actuator moves the sliders to the innermost track and advances the sliders from track to track to the outermost track to urge any particulates or other contaminants radially outward on the disk surface. Any contaminants encountered during the sweep may be propelled from the disk to be captured by the drive recirculating filter or at least urged beyond the outer diameter of the data surface of the disk over which the slider must fly during drive operation. 
     After the sweep, the idle routine does not initiate another sweep until the time interval delta T has again elapsed. The idle routine at  54  controls the actuator to place the sliders over tracks within the outer one third of the band of data tracks (from tracks  43  to track  44  of FIG. 1) to assure that the sliders confront an adequately lubricated disk surface when idling between disk drive commands. After a prescribed time, the idle routine returns to block  52  and until the next sweep, again positions the slider over a location within the outer one third of the band of data tracks. 
     The idling location of the slider over the outer one third of the data track band may be randomly altered during each successive prescribed time period or may be advanced outwardly during successive prescribed time periods to distribute the period of dwell over the entire band of tracks where more adequate surface lubrication is known to be present. Knowing that lube migration is progressive toward the outer diameter, the idle routine may be controlled to have a greater number of idle time periods occur adjacent the outermost track  44  than adjacent track  43  (FIG.  1 ). 
     Whenever a drive command  58  is received, the idle routine is interrupted and the command processed at  59 . If the command is to read or write (R/W) data, the read or write operation is processed to completion and actuator control is returned to the idle routine. A stop command causes the drive to stop by following shut down procedures and exiting the overall routine. Similarly, a standby command puts the drive system in a power down standby mode until operation is resumed by a start command. 
     While the invention has been shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.