Abstract:
Detection of head disk interference is provided by monitoring a hard disk drive characteristic related to disk rotation rate. A number of characteristics can indicate HDI. These include the magnitude of decreases in rotation rate, changes in the time-derivative (time-rates of change) of rotation rate or other changes in time-profiles of rotation rate, changes in spin motor current, differences between maximum and minimum values of spin motor current, changes in the time-derivative (time-rates of change) of spin motor current or other changes in time-profiles of spin motor current or combinations or indicators thereof. These approaches to detecting HDI provide several potential advantages. Detection of HDI can be achieved relatively early in a test or other procedure, can be performed relatively rapidly, can distinguish HDI from at least some other anomalies and can indicate the location and/or magnitude of HDI occurrences.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed from U.S. Provisional Application Ser. No. 60/722,391 filed Sep. 30, 2005, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to detection of head-disk interference in a disk drive. In particular, the present invention is directed to detection of head-disk interference using a characteristic of disk location, such as the profile (or other feature) of the spin motor current. 
     BACKGROUND INFORMATION 
     Data storage devices including, e.g., those normally provided as part of, or in connection with, a computer or other electronic device, can be of various types. In one general category, data is stored on a rotating (or otherwise movable) data storage medium. A read head, a write head and/or a read/write head is positioned adjacent desired locations of the medium for writing data thereto or reading data therefrom. The head may include separate or integrated read and write elements. One common example of a data storage device of this type is a disk drive (often called a hard disk drive, “HDD,” or “fixed” disk drive). 
     Typically, the information is stored on each disk in nominally concentric tracks, which are divided into sectors. The read/write head (or transducer) is mounted on an actuator arm capable of moving the head to access various radial positions of the disk. Accordingly, the movement of the actuator arm allows the head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the head to access different sectors on the disk. 
     Although many concepts and aspects pertaining to the present invention will be described herein in the context of a disk drive, those skilled in the art, after understanding the present disclosure, will appreciate that advantages provided by the present invention are not necessarily limited to disk drives. 
     In an idealized drive configured with nominally concentric data tracks, if a read/write head is kept a constant radial distance from the (nominal) axis of rotation, there will be no change in the radial distance (if any) from the read/write head to the desired data track, as the disk rotates. In actuality, however, many factors can contribute to deviations from this ideal condition such that small tracking correction forces must be applied to the read/write head to maintain the head sufficiently aligned with a desired data track, as the disk rotates, although some amount of tracking error can be tolerated. Most modern disk drives provide a servo tracking system for seeking a target track and/or making tracking corrections to assist in maintaining tracking within acceptable ranges. 
     Typically, as part of a manufacturing or setup procedure (prior to normal use for data read/write), a hard disk drive is provided with a plurality of servo “bursts,” markers or sectors. The purpose of these bursts is to provide location information to components of the head-positioning and/or tracking system. Generally, a plurality of servo bursts are positioned around a given track. Typically, over various portions (“zones”) of the radial extent of the disk, the bursts are circumferentially aligned, from one track to the next, defining a plurality of servo “wedges.” 
     Proper operation of a disk drive typically involves maintaining the read write head at a preferred location with respect to the adjacent disk surface. Many hard disks are configured to provide their best performance when the read write head is maintained at a distance (or “fly height”) from the disk surface of a few nanometers. If the read/write head is located more than a tolerance amount from the preferred nominal fly height, there could be loss of data and/or effective loss of data storage capacity of the disk drive. Further, if the read write head is sufficiently close to the disk surface, a condition known as “head-disk interference” (HDI) occurs. Head-disk interference can (but need not always) involve contact of the head with the disk surface and has the potential to cause temporary or permanent physical damage to the HDD. 
     In many HDD manufacturing processes, attempts are made to detect whether HDDs that are being manufactured have occurrences of HDI or other anomalies. The detection of an anomaly may, depending on severity or other conditions, result in “failing” the drive. A drive which is “failed” (tagged as defective) may be subjected to various treatments, including removal from the product stream, repair and/or analysis. While it is useful to identify, during manufacturing, those HDDs which have occurrences of HDI, the amount of time involved in such detection can adversely affect the throughput and/or effective cost per unit of HDD manufacturing. Furthermore, previous HDI detection typically occurred after (or in conjunction with) a substantial amount of other configuration or testing procedures. Meaning that, by the time a drive was “failed” for HDI occurrences, an undesirably large amount of effort and funds had already been expended on the drive, again adversely affecting HDD manufacturing throughput and effective per-unit cost. Accordingly, it would be useful to provide a method, system and apparatus for detecting HDI which is of relatively short duration and/or occurs relatively earlier in the manufacturing process, as compared to previous approaches. 
     In some previous approaches, HDDs which had occurrences of HDI might be “failed” in a manner that may not specifically indicate that HDI was the cause of the failure (such as tests which detect an anomaly that can arise from any of a number of causes). Such failure data has limited utility in identifying possible problems in equipment or procedures of manufacturing. Further, some previous approaches provided little, if any, information pertinent to the location on the disk where HDI occurs, and/or the number of HDI occurrences and/or the severity of HDI. Thus, these approaches provided little information usable in deciding the disposition of the failed drive (repair, disassemble and use selected parts, scrap, etc.). Accordingly it would be useful to provide a method, system and apparatus which can distinguish HDI occurrences from other sources of failure or potential failure, and/or can provide information regarding the location of, number of, and/or severity of HDI occurrences. 
     SUMMARY OF THE INVENTION 
     The present invention includes a recognition and/or appreciation of the existence, source and/or nature of problems in previous approaches, including those described herein. 
     One embodiment of the invention involves detecting HDI using information generally indicative of disk rotation rate. It has been found that HDIs are typically associated with a change in, or a tendency for change in, disk rotation rate. Without wishing to be bound by any theory, it is believed that when the read/write head is undesirably close to the disk surface, there is an increase in the (aerodynamic and/or frictional) drag force which tends to resist rotation. Such increase in drag will, unless compensated, slow the rotation rate of the disk. Accordingly, in one embodiment, disk rotation rate (e.g., revolutions per minute (RPM)) is directly or indirectly monitored and any decreases (e.g., of a magnitude more than a threshold magnitude) and/or changes (e.g., differences between minimum and maximum amounts) in RPM and/or rates of change of RPM are used as at least partial indicators of HDI. 
     Many HDDs rotate the disk using a spin motor. Typically, a servo loop is provided, using any of a number of servo loop designs, as will be understood by those of skill in the art. Typically, a servo loop responds to a detected change in disk rotation rate (or an indication thereof) by adjusting the magnitude of the current (or other electrical parameter) supplied to the spin motor. For example, in some situations, a normal spin current value might be in range of about 300 milliamps to 500 milliamps. Many servo systems have a very rapid response and will compensate for any decrease in RPM (such as may be induced by HDI) almost immediately. According to one embodiment of the invention, the magnitude of the supplied spin motor current is monitored while the actuator arm is controlled to perform one or, preferably, more sweeps, at a substantially constant sweep rate, across the radial extent of the disk. Increases, e.g., above a threshold, and/or differences (e.g., between maximum and minimum values during the sweep) and/or rates of change, in spin motor current, especially if substantially localized over a portion of the radial sweep, are taken as indicative of HDI. Generally, any single sweep will be used for detecting HDI only over a (generally spirally-shaped) portion of the disk surface. Thus, multiple sweeps can be used to detect HDI over substantially the entire disk surface. Preferably, the circumferential and/or radial locations of HDI occurrences are recorded. 
     It is possible to use spin current or other characteristics related to disk rotation rate, without necessarily performing data read/write prior to, or as part of the test. Thus, HDI detection procedures can be of relatively short duration and can be performed relatively early (e.g., prior to one or more procedures which involve reading or writing data to the disk). 
     In one embodiment of the invention, detection of head disk interference is provided by monitoring a hard disk drive characteristic related to disk rotation rate. A number of characteristics can indicate HDI. These include the magnitude of decreases in rotation rate, changes in the time-derivative (time-rates of change) of rotation rate or other changes in time-profiles of rotation rate, changes in spin motor current, differences between maximum and minimum values of spin motor current, changes in the time-derivative (time-rates of change) of spin motor current or other changes in time-profiles of spin motor current or combinations or indicators thereof. These approaches to detecting HDI provide several potential advantages. Detection of HDI can be achieved relatively early in a test or other procedure, can be performed relatively rapidly, can distinguish HDI from at least some other anomalies and can indicate the location and/or magnitude of HDI occurrences. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective, partially broken-away view of a hard disk drive of a type which can be used in connection with embodiments of the present invention; 
         FIG. 2  is a flowchart of a process according to previous approaches; and, 
         FIG. 3  is a flowchart of a process according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A disk drive  10  is illustrated in  FIG. 1 . The disk drive comprises a disk  12  that is rotated by a spin motor  14 . The spin motor  14  is mounted to a base plate  16 . The disk drive  10  also includes an actuator arm assembly  18  having a head  20  (or transducer) mounted to a flexure arm  22 , which is attached to an actuator arm  24  that can rotate about a bearing assembly  26  that is attached to the base plate  16 . The actuator arm  24  cooperates with a voice coil motor  28  in order to move the head  20  along an arcuate path relative to the disk  12 . The spin motor  14 , voice coil motor  28  and head  20  are coupled to a number of electronic circuits  30  mounted to a printed circuit board  32 . In some configurations, the coupling includes a ribbon-like flexure connection  34  having a plurality of conductive traces. The electronic circuits  30  typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device. Instead of a single actuator arm, there may be more than one, such as one actuator arm for each side of the disk. Instead of a one-disk configuration (shown in  FIG. 1 ), the disk drive  10  may include a plurality of disks  12  and, therefore, a plurality of corresponding actuator arm assemblies  18 . 
     In many HDD manufacturing processes, certain of the HDD testing procedures are performed in conjunction with servo track writing, such as while the HDD is coupled to a servo track writer (STW). Some of the testing which is performed according to previous approaches, typically in conjunction with servo track writing processes, includes a process generally as depicted in  FIG. 2 . 
     The first step is a spin up of the drive  212 , i.e., activating the spin motor until it achieves a stable target rotation rate. This is followed by servo positioning the actuator  214 . The next test  216 , which is sometimes called the “best head test” is performed. Running the test  218  involves writing and reading back data. This is performed at various locations of the disk, preferably over substantially the majority of the disk surface. During the best head test  216 , the system obtains performance data (such as error rates, signal strengths and the like). The “best head test”  216  is typically looped  222  until such time as sufficient data is obtained  224  to reliably pass or fail the drive  226 . If the drive is not failed  228 , additional procedures are typically performed, such as writing clock bits  232  and/or writing certain servo data  234 . An exemplary best head test can be found in U.S. patent application Ser. No. 10/860,603 filed Jun. 2, 2004, which is incorporated by reference. 
     Although occurrences of head disk interference can result in the “best head test” failing the drive  228 , there are also other items which can cause or contribute to failing the drive, such as improperly functioning heads, or malfunction in the servo track writer apparatus. Accordingly, the fact of failing the drive as a result of the “best head test” did not, in previous approaches, necessarily indicate the occurrence of HDI. 
     As seen from  FIG. 2 , in previous approaches, when the drive is failed  228  whether because of the occurrence of HDIs (or other causes), such failing of the drive occurred only relatively late in the process flow, and specifically only after performance of the “best head test”  216 . The fact that failing the drive  228  occurs after the “best head test”  216  is even more problematic because the “best head test” can be of a relatively long duration, e.g., because of the fact that it involves time-consumptive read and write operations and/or involves looping  222  as needed to achieve sufficient data. 
       FIG. 3  illustrates a procedure, according to an embodiment of the present invention, which can provide at least some HDI testing prior to and/or without the need for performing a “best head test,” and which can at least partly distinguish between failures arising from HDI and those arising from other causes. The procedure of  FIG. 3  typically has a shorter duration than a “best head test,” and can provide information about the location and/or severity of HDI. 
     According to the embodiment depicted in  FIG. 3 , HDI detection  314  is performed immediately following spin up  312 . Thus, in the configuration of  FIG. 3 , it is not necessary to perform the “best head test” in order to detect HDI. Indeed, in the procedure of  FIG. 3 , HDI detection  314  is performed prior to the time a “best head test”  334  would be performed. Preferably, some or all steps of the procedure of  FIG. 3  are performed using a programmed microprocessor. 
     As part of the HDI detect  314 , optionally, the disk is burnished  316  (e.g., to remove or accommodate for thermal asperities, as understood by those skilled in the art). The procedure  314  then provides for sweeping the disk (i.e., substantially continuously moving the head along its arcuate path) substantially between the outer diameter and inner diameter, while monitoring the spin current  318 . The difference (A) is calculated between the maximum spin current and the minimum spin current during the sweeps  322 . 
     In the procedure of  FIG. 3 , the occurrence of HDI is indicated if the magnitude of Δ exceeds a threshold  324 . For example, a threshold value of about 10% above or below normal values of Δ can be used. In this case, an “HDI bit” is set and an indication of the magnitude (such as the value of Δ) and the location of the HDI(s) (such as the location or locations of the head at the time the greatest excursion of spin current occurs) is logged or stored in a memory. In the embodiment of  FIG. 3 , the process is halted and a shut-down sequence is performed. In cases where the HDI detection is performed during a servo track writing process, it is the STW process that is halted  328  and an STW shut down sequence  332  is performed. 
     In one embodiment, different procedures can be followed depending on the magnitude of the HDI. For example, the procedure can be configured to fail the drive if Δ exceeds the threshold by 50% or more, but to merely set the flag and, thereafter, to monitor the spin current, if Δ exceeds the threshold by more than 10% but less than 50%. 
     If the value of Δ does not exceed the threshold, then the procedure can go forward, e.g., by performing a “best head test”  334 . In this way, if a drive failure is declared, it will be known whether the failure occurred as a result of detection of a HDI  326  or from some other cause such as a bad head or the like, e.g., which occurred during the “best head test.” 
     Features of the procedure of  FIG. 3  are preferably configurable. For example, user-definable aspects preferably include features such as the number of sweeps  318 , the value of the threshold  324  and the like. 
     Preferably, the number of sweeps is selected to provide coverage of substantially the entire disk surface, while avoiding consuming more time than necessary. Typically, the number of sweeps will be dependent on the features of the particular model of disk drive being processed. 
     Although  FIG. 3  depicts using a particular electrical characteristic (namely, the magnitude of Δ) as an indicator of HDI, it is possible to use other indicators without the need to (first) perform or initiate a “best head test.” For example, HDI can be indicated by the value of the spin current (rather than Δ) exceeding (or falling below) a threshold value. HDI can be indicated by the presence of a particular time profile of spin current (or values derived from or related thereto), such as the time rate of change of spin current (di/dt). HDI can be indicated by changes in, or values of, RPM of the disk and the like. 
     A number of variations and modifications of the invention can also be used. Although the invention has been described in the context of detecting HDI during servo tests and servo writing procedures, it is possible to use some or all aspects of the present invention in other situations, including during normal use of the disk drive. Although procedures have been depicted and described in connection with embodiments of the present invention, it is possible to use other procedures in connection with the present invention including procedures having more or fewer steps and/or procedures in which steps are performed in an order different from those depicted. 
     In light of the above description, a number of advantages of the present invention can be seen. The present invention can make it possible to achieve early detection of HDI, e.g., without the need to first wait until the initiation of a “best head test.” The present invention can achieve HDI detection more rapidly than at least some previous approaches, such as without the need to perform time-consumptive procedures such as the read and write procedures involved in the “best head test.” For example, in one configuration, HDI detection procedures can be performed in less than about 4 seconds. 
     The present invention can provide an indication of the location of HDI occurrences, e.g., allowing problem areas of the disk to be quarantined or remapped. 
     The present invention makes it possible to distinguish, particularly to distinguish early in a testing procedure, HDI from other anomalies. Early detection of HDI and/or detection of the severity of HDI increases the ability to select appropriate action upon detection of HDI (e.g., such as deciding whether to remap portions of the drive, downgrade the drive, fail the drive and/or reuse some or all portions of the drive, or rebuild the drive). The present invention can shorten the time a servo track writer or other device or procedure takes to detect a bad drive due to HDI. The present invention can be used (e.g., as part of a statistical process control) to monitor the drive build process (e.g., to halt the build process whenever the number of failures of this test exceeds a pre-defined control limit). The present invention can be used as an indicator of the status of a particular STW or other apparatus. For example, an auto shut down trigger can be activated if a particular STW on a production line has more than a pre-defined number of HDI failures within a given time interval. 
     The present invention, in various embodiments, includes components, methods, processes, systems and/or apparatus substantially similar to those depicted and described herein, including various embodiments, sub-combinations, and subsets thereof. Those skilled in the art will understand how to make and use the present invention after understanding the present disclosure. The present invention, and various embodiments, includes providing the devices and processes in the absence of items not depicted and/or described herein or in various embodiments hereof, including in the absence of such items as may have been used in previous devices or processes, e.g., for improving performance, achieving ease of implementation and/or reducing cost of implementation. The present invention includes items which are novel, and terminology adapted from previous and/or analogous technologies, for convenience in describing novel items or processes, do not necessarily retain all aspects of conventional usage of such terminology. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the forms or form disclosed herein. Although the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope of the invention, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.