Abstract:
A method of detecting a change in fly-height comprising measuring a first baseline position error signal (PES) at a first diameter on a disk, measuring a second baseline PES at a second diameter, measuring a first transient PES at the first diameter subsequently to the first baseline PES, measuring a second transient PES at the second diameter subsequently to the second baseline PES, determining a first change in PES by comparing the first transient PES to the first baseline PES, determining a second change in PES by comparing the second transient PES to the second baseline PES, performing responsive action when one of the first change in PES exceeds a first threshold and the second change in PES exceeds a second threshold, and generating a general error condition when both the first change in PES and the second change in PES exceed respective thresholds.

Description:
RELATED APPLICATION  
       [0001]     The present application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/924,449, entitled DETECTION OF FLY HEIGHT CHANGE IN A DISK DRIVE, of concurrent ownership, filed on Aug. 24, 2004, the disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to computer storage products, and more particularly to detecting changes in fly height for disk drives.  
       BACKGROUND  
       [0003]     A disk drive is a data storage device that stores data in concentric tracks on a disk. Data is written to or read from the disk by spinning the disk about a central axis while positioning a transducer near a target track of the disk. During a read operation, data is transferred from the target track to an attached host through the transducer. During a write operation, data is transferred in the opposite direction.  
         [0004]     During typical disk drive operation, the transducer does not contact the surface of the disk. Instead, the transducer rides along a cushion of air generated by the motion of the disk. The transducer is normally mounted within a slider structure that provides the necessary lift in response to the air currents generated by the disk. The distance between the transducer/slider and the disk surface during disk drive operation is known as the “fly height” of the transducer.  
         [0005]     The fly height is controlled by the suspension attached to the slider and the airbearing of the slider. For magnetic purposes, the fly height is measured as a distance between the read/write elements and the magnetic surface. There are several conditions that create disturbances between the airbearing and the disk surface that can change the fly height. These conditions include altitude, temperature, and contamination. An extreme in any of these conditions will degrade the error rate performance of the drive. These conditions are taken into account during the development of the airbearing designs.  
         [0006]     Because the transducer is held aloft during disk drive operation, friction and wear problems associated with contact between the transducer and the disk surface are usually avoided. However, due to the extremely close spacing of the heads and disk surface, any contamination of the read-write heads or disk platters can lead to a head crash—a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film. For giant magnetoresistive head technologies (GMR heads) in particular, a minor head contact due to contamination (that does not remove the magnetic surface of the disk) could still result in the head temporarily overheating, due to friction with the disk surface, and renders the disk unreadable until the head temperature stabilizes.  
         [0007]     Therefore, what is needed is a disk drive that can monitor the fly-height and take corrective action upon the first indication of a change in the fly-height. Preferably, the monitoring would be accomplished without adding components that increase the cost of the drive.  
       SUMMARY  
       [0008]     Embodiments of the present invention as presented herein provide at least for a method of detecting a change in fly-height comprising measuring a first baseline position error signal (PES) representing an initial PES at a first diameter on a disk, measuring a second baseline PES representing an initial PES at a second diameter on the disk, measuring a first transient PES at the first diameter subsequently to the first baseline PES, measuring a second transient PES at the second diameter subsequently to the second baseline PES, determining a first change in PES by comparing the first transient PES to the first baseline PES, determining a second change in PES by comparing the second transient PES to the second baseline PES, performing at least one responsive action when one of the first change in PES exceeds a first threshold and the second change in PES exceeds a second threshold, and generating a general error condition when both the first change in PES exceeds the first threshold and the second change in PES exceeds the second threshold.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings.  
         [0010]      FIG. 1  is a diagrammatic view of an apparatus which is an information storage system that embodies aspects of the present invention.  
         [0011]      FIG. 2  is a flowchart illustrating a process for determining fly height decrease in a disk drive by monitoring the PES.  
         [0012]      FIG. 3  is a flowchart illustrating a process for determining fly height decrease in a disk drive by monitoring multiple PESs. 
     
    
     DETAILED DESCRIPTION  
       [0013]     In a disk drive, a position error signal (PES) is indicative of the position of the head with respect to the center of a particular track. Particularly, during track following, a servo system generates the PES from the received servo information and then uses the PES to generate a correction signal which is provided to a power amplifier to control the amount of current through the actuator coil, in order to adjust the position of the head accordingly. Typically, the PES is presented as a position dependent signal having a magnitude indicative of the relative distance between the head and the center of a track and a polarity indicative of the direction of the head with respect to the track center.  
         [0014]     A position error signal is determined by comparing the amplitude of the signals read from neighboring bursts. The PES is proportional to the difference between the signal amplitudes of the neighboring bursts, divided by the sum of their signal amplitudes. Thus, the PES represents the offset distance between the head and track centerline as defined by the servo information embedded in the disk. The PES is then used as part of a closed loop servo system to correct the position of the head with respect to the track.  
         [0015]      FIG. 1  is a diagrammatic view of an apparatus which is an information storage system  10 , and which embodies aspects of the present invention. The system  10  includes a receiving unit or drive  12  which has a recess  14 , and includes a cartridge  16  which can be removably inserted into the recess  14 .  
         [0016]     The cartridge  16  has a housing, and has within the housing a motor  21  with a rotatable shaft  22 . A disk  23  is fixedly mounted on the shaft  22  for rotation therewith. The side of the disk  23  which is visible in  FIG. 1  is coated with a magnetic material of a known type, and serves as an information storage medium. This disk surface is conceptually divided into a plurality of concentric data tracks. In the disclosed embodiment, there are about 50,000 data tracks, not all of which are available for use in storing user data.  
         [0017]     The disk surface is also conceptually configured to have a plurality of circumferentially spaced sectors, two of which are shown diagrammatically at  26  and  27 . These sectors are sometimes referred to as servo wedges. The portions of the data tracks which fall within these sectors or servo wedges are not used to store data. Data is stored in the portions of the data tracks which are located between the servo wedges. The servo wedges are used to store servo information of a type which is known in the art. The servo information in the servo wedges conceptually defines a plurality of concentric servo tracks, which have a smaller width or pitch than the data tracks. In the disclosed embodiment, each servo track has a pitch or width that is approximately two-thirds of the pitch or width of a data track. Consequently, the disclosed disk  23  has about 73,000 servo tracks. The servo tracks effectively define the positions of the data tracks, in a manner known in the art.  
         [0018]     Data tracks are arranged in a concentric manner ranging from the radially innermost tracks  36  to the radially outermost tracks  37 . User data is stored in the many data tracks that are disposed from the innermost tracks  36  to the outermost tracks  37  (except in the regions of the servo wedges).  
         [0019]     The drive  12  includes an actuator  51  of a known type, such as a voice coil motor (VCM). The actuator  51  can effect limited pivotal movement of a pivot  52 . An actuator arm  53  has one end fixedly secured to the pivot  52 , and extends radially outwardly from the pivot  52 . The housing of the cartridge  16  has an opening in one side thereof. When the cartridge  16  is removably disposed within the drive  12 , the arm  53  extends through the opening in the housing, and into the interior of the cartridge  16 . At the outer end of the arm  53  is a suspension  56  of a known type, which supports a read/write head  57 . In the disclosed embodiment, the head  57  is a component of a known type, which is commonly referred to as a giant magneto-resistive (GMR) head. However, it could alternatively be some other type of head, such as a magneto-resistive (MR) head.  
         [0020]     During normal operation, the head  57  is disposed adjacent the magnetic surface on the disk  23 , and pivotal movement of the arm  53  causes the head  57  to move approximately radially with respect to the disk  23 , within a range which includes the innermost tracks  36  and the outermost tracks  37 . When the disk  23  is rotating at a normal operational speed, the rotation of the disk induces the formation between the disk surface and the head  57  of an air cushion, which is commonly known as an air bearing. Consequently, the head  57  floats on the air bearing while reading and writing information to and from the disk, without direct physical contact with the disk. As stated above, the distance the head floats above the disk is known as the “fly-height.”  
         [0021]     The drive  12  includes a control circuit  71 , which is operationally coupled to the motor  21  in the cartridge  16 , as shown diagrammatically at  72 . The control circuit  71  selectively supplies power to the motor  21  and, when the motor  21  is receiving power, the motor  21  effects rotation of the disk  23 . The control circuit  71  also provides control signals at  73  to the actuator  51 , in order to control the pivotal position of the arm  53 . At  74 , the control circuit  71  receives an output signal from the head  57 , which is commonly known as a channel signal. When the disk  23  is rotating, segments of servo information and data will alternately move past the head  57 , and the channel signal at  74  will thus include alternating segments or bursts of servo information and data.  
         [0022]     The control circuit  71  includes a channel circuit of a known type, which processes the channel signal received at  74 . The channel circuit includes an automatic gain control (AGC) circuit, which is shown at  77 . The AGC circuit  77  effect variation, in a known manner, of a gain factor that influences the amplitude of the channel signal  74 . In particular, the AGC circuit uses a higher gain factor when the amplitude of the channel signal  74  is low, and uses a lower gain factor when the amplitude of the channel signal  74  is high. Consequently, the amplitude of the channel signal has less variation at the output of the AGC circuit  77  than at the input thereof.  
         [0023]     The control circuit  71  also includes a processor  81  of a known type, as well as a read only memory (ROM)  82  and a random access memory (RAM)  83 . The ROM  82  stores a program which is executed by the processor  81 , and also stores data that does not change. The processor  81  uses the RAM  83  to store data or other information that changes dynamically during program execution.  
         [0024]     The control circuit  71  of the drive  12  is coupled through a host interface  86  to a not-illustrated host computer. The host computer can send user data to the drive  12 , which the drive  12  then stores on the disk  23  of the cartridge  16 . The host computer can also request that the drive  12  read specified user data back from the disk  23 , and the drive  12  then reads the specified user data and sends it to the host computer. In the disclosed embodiment, the host interface  86  conforms to an industry standard protocol which is commonly known as the Universal Serial Bus (USB) protocol, but could alternatively conform to any other suitable protocol, including but not limited to the IEEE 1394 protocol.  
         [0025]     As the heads  57  get dirty, the fly height decreases. The decrease in the fly height increases the friction between the heads  57  and the disk  23 , which causes the slider to get off-track, thus increasing the PES. Therefore, monitoring the PES can be used to indicate a change in the fly height.  
         [0026]      FIG. 2  is a flowchart showing the process  200  for detecting the fly height change in the present invention. The process  200  begins at a START block  205 . Proceeding to block  210 , the process  200  establishes a baseline PES for the drive  12 . The baseline PES represents the PES at the initial life of the disk and may be stored in memory for comparison purposes.  
         [0027]     Proceeding to block  215 , the process  200  measures the current PES of the drive  12 . As stated above, over time the heads  57  of the drive  12  may get dirty and thereby affect the value of the PES. The PES at block  215  may be measured at a regular interval. Also, the PES may be measured on one or more heads  57  of the drive. After the current PES is measured, the process  200  proceeds to block  220 . In block  220 , the change in the average absolute PES is calculated by comparing the current measured PES (or an average of a predetermined number of measured PESs) to the baseline PES.  
         [0028]     Proceeding to block  225 , the change in the absolute PES is compared to a threshold value. The threshold value may be selected in a variety of manners, including being predetermined, measured, or calculated. If the change in the absolute PES is below the threshold value, the process  200  proceeds along the NO branch back to block  215  to measure the next current PES at an appropriate interval. If the change in the absolute PES is above the threshold value, the process  200  proceeds along the YES branch to block  230  where an error condition is generated by the drive  12 .  
         [0029]     Proceeding to block  235 , the process  200  determines whether the drive  12  has a head cleaner. If the drive  12  has a head cleaner, the process  200  proceeds along the YES branch to block  240  where a head cleaning procedure is initiated. As stated above, if the heads  57  of the drive  12  get dirty, then the PES may be changed. By cleaning the heads  57 , the fly height should return to normal and the PES should therefore return to a value close to the baseline PES. After the head cleaning is initiated, or if the drive  12  is determined not to be a removable hard drive in block  235 , the process terminates in END block  245 .  
         [0030]      FIG. 3  discloses a process  300  according to an alternative embodiment of the present invention. The process  300  begins at a START block  305 . Proceeding to block  310 , the process  300  detects the error condition generated in block  230  and then proceeds to block  315  where the absolute changes in the PES are compared at both the inner diameter  36  and the outer diameter  37  of the disk  23 . This is accomplished by having a first baseline PES measured at the inner diameter  36  and a second baseline PES measured at the outer diameter  37 . These first and second baseline PESs are then compared to corresponding first and second transient PESs.  
         [0031]     Proceeding to block  320 , the changes in both PES at both the inner diameter  36  and the outer diameter  37  are compared against corresponding thresholds. If the changes in the PES at both the inner diameter  36  and the outer diameter  37  exceed the threshold, the process  300  proceeds along the YES branch to block  325 . In block  325 , a general error condition is generated indicating a possible future failure of the drive. If the PES is changing due to contamination issues, the PES will often be affected at certain diameter first depending on the slider design. Thus, if the change in both the first and second PESs exceed the threshold, it is likely the result of another factor, such as a vibration.  
         [0032]     Returning to block  320 , if the change in one of the inner diameter  36  PES and the outer diameter  37  PES exceeds the threshold, but the change in the other does not exceed the threshold, the process proceeds along the NO branch to block  330 . At block  330 , one, or more, of a plurality of possible responsive actions is performed. In one embodiment, a head cleaning procedure is initiated, at block  330 . For example, a change in the inner diameter PES and not the outer diameter PES may be indicative of dirty heads  57 . By cleaning the heads  57 , the fly height should return to normal and the PES should therefore return to a value close to the baseline PES. In another embodiment, a decreasing fly height condition is generated, at block  330 .  
         [0033]     In another embodiment, where a performance of a responsive action at block  330  is already in progress, and the absolute changes in an inner diameter PES and an outer diameter PES both exceed the threshold the process proceeds along the YES branch to block  325 . A general error condition is generated, at block  325 , and the performance underway of the responsive action is canceled.  
         [0034]     After the one, or more, responsive action is performed, or after the general error condition is generated at block  325 , the process terminates at END block  335 . It will be appreciated that other types of responsive actions are possible, alternatively, or in addition to the responsive actions articulated in the foregoing.  
         [0035]     Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics.