Abstract:
In a disc drive with a disc spun by a motor and a head positionable over the disc, a disc drive controller alternately leaves the head landed on successive landing spots in a landing zone on the disc for selected stop intervals to remove an increment of lubricant from the head with each stop and then automatically lifts the head by spinning the disc for selected run intervals between the stop intervals so the head does not stick to the disc.

Description:
The present invention claims priority to Provisional Application Ser. No. 60/086,482, filed May 22, 1998 and entitled DRIVE-LEVEL ALGORITHM TO REDUCE LUBE COLLECTION ON HEADS. 
    
    
     BACKGROUND OF THE INVENTION 
     Magnetic disc drives for the storage of digital information in computerized systems are known. In such disc drives, a read/write head flies above the surface of spinning disc having magnetic media at its surface. The head is positionable to read and write information on concentric tracks on the disc. Typically the disc and head will have multiple layers to provide a compact arrangement, and the disc drives are sealed and have lubricant distributed over the media surfaces over which the head flies. 
     As the disc drive is exercised in ordinary use, the lubricant accumulates undesirably on head surfaces over time. The lubricant can migrate from air bearing stagnation regions between the head and the disc to air bearing surface regions of the head by capillary action. The process of migration is complex and depends on many factors such as lubricant viscosity and surface tension, the peak roughness of the disc surface, the length of the slider, temperature, as well as the presence of humidity and other contaminants. 
     When the disc drive is not in use for some time, the heads are landed in a landing zone and the spinning of the disc is stopped. When there is a large enough accumulation of lubricant on the head, the lubricant can flow back into the narrowed interface between the head and the disc, resulting a high static friction, or stiction between the head and the disc. If the disc drive motor does not have sufficient torque to overcome the stiction, the discs will become stuck, making repair or replacement of the disc drive necessary. 
     There is a need for disc drive technology which reduces the stiction to a small enough level so that the disc does not become stuck due to lubricant accumulation after periods of normal exercise or use. 
     SUMMARY OF THE INVENTION 
     In a disc drive with a disc spun by a motor and a head positionable over the disc, a disc drive controller alternately lands the head on the disc for a selected stop interval to remove an increment of lubricant from the head then automatically lifts the head by spinning the disc a run interval after the stop interval so the head does not stick to the disc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a disc drive incorporating the present invention, with its upper casing removed. 
     FIG. 2 is a partial perspective view of a head for a disc drive. 
     FIG. 3 is a block diagram showing coupling of disc controller circuitry. 
     FIG. 4 is a flow chart of a process of removing lubricant from a head. 
     FIG. 5 is a flow chart of a process of leaving a head landed on a landing spot for a timed stop interval. 
     FIG. 6 is a flow chart of a process of lifting a head for a timed run interval. 
     FIG. 7 is a flow chart of a process of controlling a number of stop and run intervals. 
     FIG. 8 is a timing diagram of head position and spindle drive actuations as a function of time. 
     FIG. 9 is a graph of time in hours versus slider length in millimeters. 
     FIG. 10 is a graph of fly/stiction in grams versus dwell time in minutes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, disc drive  10 , shown with its upper casing removed, includes a multiple layer disc  12  coupled to disc drive motor  14  which spins disc  12 . A read/write head  16  which has multiple heads interleaved with the multiple disc layers is shown connected to head positioner  18 . Lubricant  20  is distributed over the surfaces of disc  12 . Disc  12  includes landing zone  13  with multiple spots or locations  15  for landing head  16  when the disc drive  10  is not in active use. Disc drive motor  14  and head positioner  18  are controlled by controller  22  which includes electronic circuitry  24  which executes an algorithm preventing excess accumulation of lubricant  20  on read/write head  16 . 
     In FIG. 2, read/write slider or head  16  is partially shown to includes shaped surfaces  32  on either side of magnetic transducer surface  34 . When disc  12  is spinning, shaped surfaces  32  fly over the lubricated surfaces of disc  12 . The relative motion between the disc surface and the shaped surfaces  32  creates an air bearing which permits the head to fly over the disc surface rather than remaining in contact with it. The air bearing, however, includes a stagnation region that can allow the transfer of excessive amounts of lubricant  20  to head  16 , where lubricant  20  accumulates undesirably. Without the use of the present invention, the head could become flooded with accumulated lubricant. When head  16  is landed one of the landing spots  15  and left there for a period of time, the accumulated lubricant could flow slowly back into the gap between the head and disc and eventually cause head  16  and disc  12  to stick together. 
     The static friction, also called stiction, between the surfaces can be so high that drive motor  14  does not have enough torque to overcome the stiction and the drive becomes stuck. Such sticking does not take place, however, because drive controller  22  includes circuit  24  which executes a lubrication algorithm described below. 
     In FIG. 3, the arrangement of disc drive  10  of FIG. 1 is shown schematically. 
     In FIG. 4, a method or process  40  (typically performed by controller  22 ) of incrementally removing lubricant accumulated on the head of a disc drive is shown as a flow chart. Program flow starts at  41 . First, a disc exercise timer is checked at  42  to detect whether disc drive  10  has been exercised enough to warrant servicing the heads by removing excess lubricant from the head. If disc drive  10  has not yet been exercised enough (NO), then disc drive  10  is returned at  43  to normal use or exercise operation. If the accumulated disc exercise time has been reached or exceed the limit set for the disc exercise time interval (YES), then the program flow proceeds to TIMED STOP step  44 . In other words, the process waits until a selected level of exercise time has accumulated before service is performed. In timed stop step  44 , an increment of lubricant is removed by automatically leaving head  10  landed on a landing spot in the landing zone on the disc for a selected stop interval. The timed stop interval is long enough to transfer only a portion of accumulated lubricant to the landing spot, but not long enough to allow the head to stick so much that the drive motor can&#39;t break it free. After completion of step  44 , TIMED RUN step  46  is performed. In TIMED RUN step  46 , the heads are lifted by spinning the disc for a selected timed run interval. The length of the timed run interval is selected to be long enough to ensure that the disc is spinning freely. After completion of TIMED RUN step  46 , COUNT step  44  is performed. In COUNT step  44 , the number of fly/stop steps or sequences( 44 , 46 ) that have been performed for the current service are counted by decrementing (or incrementing) a counter for each fly/stop sequence. When a desired, preset number of fly/stop sequences is completed, then the counter for fly/stops steps is reset and the disc drive is returned to normal operation at  50 . If the preset number of fly/stop steps is not yet completed, then the process is repeated by going back to  44  to perform another fly/stop sequence. The flow of the process is automatic and proceeds until a preset number of landings have been made to reduce the amount of lubricant on the head to ensure it won&#39;t stick. 
     If desired, at step  42 , a check can also be made to sense whether the disc drive has been put in a power saving mode. If the disc is not both in a power saving mode as well as having been exercised enough, then the disc drive can be returned to normal operation at  43  until both conditions have been met at  42 . 
     Examples of how steps  44 ,  46  and  48  of FIG. 4 can be performed are shown in more detail in FIGS. 5,  6  and  7 . The steps shown in FIGS. 4,  5 ,  6  and  7  are all controlled automatically by electronic circuitry  24  in controller  22 . Electronic circuitry  24  can comprise a microcontroller, programmed logic array or custom integrated circuit programmed to perform the desired logical steps, timing and control functions. 
     In FIG. 5, control sequence  60  is shown for performing a TIMED STOP process. At step  62 , the heads are moved to the landing zone, the spindle is stopped and a HEAD STOP timer is started. Next, at step  64 , the HEAD STOP timer is tested to find out if it is timed out or complete. If the HEAD STOP time interval is completed (YES), then program flow moves on at  66  to TIMED RUN (FIG.  6 ). If the HEAD STOP time is not complete (NO), however, a step  68  is then made to find out whether there is a user interrupt. If there is a user interrupt (YES), then the disc drive is returned ( 70 ) to normal operation without completing the service process. If there is no user interrupt (NO), however, the process continues back to step  64 . In other words, the process loops back until either the HEAD STOP time has elapsed or until there is a user interrupt. After the interrupt is handled, control can be returned (at  41  in FIG. 4) to complete the service process. 
     In FIG. 6, control sequence  80  is shown for performing a TIMED RUN process. At step  82 , the spindle is started and a SPINDLE RUN timer is started. Next, at step  84 , the SPINDLE RUN timer is tested to find out if it is timed out or complete. If the SPINDLE RUN time interval is completed (YES), then program flow moves on at  86  to a COUNT step (FIG.  7 ). If the SPINDLE RUN time is not complete (NO), however, a step  88  is then made to find out whether there is a user interrupt. If there is a user interrupt (YES), then the disc drive is returned ( 90 ) to normal operation without completing the service process. If there is no user interrupt (NO), however, the process continues back to step  84 . In other words, the process loops back until either the SPINDLE RUN time has elapsed or until there is a user interrupt. After the interrupt is handled, control can be returned (at  41  in FIG. 4) to complete the service process. 
     In FIG. 7, count control process  100  is shown. After completing each fly/stop sequence, the sequence is counted by decrementing a counter at step  102 . Next, the counter is tested to see if it has been counted down to zero at step  104 . If the counter has reached zero (YES), then the desired number of TIMED RUN and TIMED STOP sequences had been completed, the counter is set back to its starting setting at  106  and then the disc drive returns to normal operation at  108 . If the desired number of repetitions of TIMED RUN and TIMED STOP is not complete yet (NO), then the process continues on at  110  to loop back to an additional fly/stop sequences (FIG.  4 ). 
     In FIG. 8, a timing diagram of the head position  120  and the spindle drive actuation  122  is illustrated. With the head stopped in the park or landing zone, the spindle drive is alternated between run and stop (fly/stop sequences) as shown in FIG. 8 until the counter is counted down to N=0. 
     The lengths of various time interval settings and number of repetitions to be performed are dependent on factors associated with the lubricant selected and the design of the disc drive. The factors are set experimentally based on testing and then stored at the disc drive level. The factors can be stored in disc drive circuitry, or alternately, the disc drive circuitry can access such data stored on the disc itself. The arrangement allows for relatively transparent and automatic maintenance of a disc drive without having to rely on the user or service personnel to maintain the disc drive. 
     In one arrangement, the algorithm of the present invention may be executed after approximately three weeks of drive operation. Further, in one preferred embodiment, less than about 10 to about 20 percent of the air bearing surface of the slider is allowed to be flooded with lubricant before reinitiating flying. It has been found that if more than about 10 to about 20 percent of the air bearing surface is covered with lubricant, the motor may not be capable of breaking the initial stiction. In one arrangement, about 10 percent of the air bearing surface was found to be flooded with lubricant after between about 10 to 14 minutes. 
     A number of factors effect the distribution of lubricant across the air bearing surface. Lubrication collection on the head takes place due to the migration of lubricant from air bearing stagnation regions to the air bearing surface by a capillary action. A useful formula determines the time necessary for an air bearing with a fully flooded surface to engulf the total surface. The air bearing surface and the media surface form a capillary channel spaced apart by the combined head/medium roughness. If one assumes that the lubricant collects at the stagnation regions of the slider, as well as along the trailing edges of the slider, one can derive a formula to describe the time required for the lubricant to fill the entire length (L) of the slider in a one dimensional model: 
     
       
           t= 3η L   2 /( hγ )  EQUATION 1 
       
     
     Where η is the lubricant viscosity, γ is the surface tension of the lubricant, h is the capillary spacing determined by peak roughness, and L is the length of the slider. FIG. 9 is a graph showing the time for a droplet of lubricant collected at the trailing edge of a slider to wick into the interface versus slider length in millimeters. FIG. 9 was generated using Equation 1 in which η was 10 poise, γ was 20 dyne/cm, h was 20 nm and L ranged from zero to 2 mm. Thus, within the first two hours, over half of the slider surface has been flooded with liquid. FIG. 10 is a graph of fly/stiction in grams versus dwell time in minutes. These graphs illustrate that it takes a relatively large amount of time for liquid collected by the head to transfer into the interface. It is this effect that allows the present invention to release collected lubricant by “briefly” landing on the disc surface. 
     In considering the periods used in the present invention, there is a tradeoff which must be recognized between repeated takeoffs and landings versus the benefits of the reduced stiction. However, in typical embodiments, the number of additional contact start/stops required by the present invention will not exceed several hundreds of contact start/stops during the lifetime of the disc drive. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.