Abstract:
A method of manufacturing a disk drive where the width of the read element is measured at servo-writing time in order to establish a write unsafe (WUS) limit corresponding to a maximum distance during writing that the write element is permitted to move radially offtrack from the centerline of a data track before writing is disabled. The method includes the steps of measuring a width of the read element with a servo track writer and determining a write unsafe (WUS) limit based on the data track pitch and the measured width of the read element. Varying the WUS limit on a drive-by-drive basis enhances the performance of some drives that would otherwise have capability that goes unused.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to magnetic disk drives (disk drives), and more particularly to a method of manufacturing a disk drive by using a servo track writer (STW) for measuring the width of the read element to set the write unsafe (WUS) limit. 
     2. Description of the Related Art 
     This application is directed to varying an operating parameter known as the write-unsafe limit, or “WUS limit”, based on the width of a read element and, in some embodiment, on the width of a write element. As explained below, the WUS limit has historically been fixed for large groups of disk drives without regard to the actual widths of the read and write elements in a given disk drive. 
     1) An Exemplary Disk Drive and its Read/Write Elements 
     Referring to FIG. 1, a conventional disk drive  10  has a head disk assembly (HDA)  20  including at least one disk  23 , a spindle motor  22  for rapidly rotating the disk  23 , and a head stack assembly (HSA)  40  that includes an actuator assembly  50  and a head gimbal assembly (HGA) (not numbered) with a transducer head  80  for reading and writing data. The HSA  40  is part of a servo control system that positions the transducer head  80  over a particular track on the disk to read or write information from that track. The HSA  40  earns its name from the fact that it generally includes a plurality of HGAs that collectively provide a vertical arrangement of heads called a “head stack.” 
     The transducer heads  80  of several years ago were “merged” devices where reading and writing were accomplished with a single inductive element. The transducer head  80  commonly used today, however, is a composite (MR and inductive) transducer head  80  that has separate read and write elements. FIG. 2 is a highly simplified representation of a composite transducer head  80  having it&#39;s a write element  81  of width W and it&#39;s a read element  82  of width R. The transducer head  80  shown is a “write wide, read narrow” device in that the read element&#39;s width R is typically about 50-65% of the write element&#39;s width W. 
     Composite transducer heads  80  are very small devices that are manufactured in large batches using photolithographic wafer process techniques. As a result, operating characteristics such as the widths of the read and write elements  81 ,  82  tend to vary over a normal distribution curve for a given number of heads, wafers or an manufacturers. As explained further below, the wide variability of read width R and write width W is problematic when combined with a fixed WUS limit. 
     FIG. 3 is an exploded perspective view of a fully-assembled HDA  20  having servo-writing access ports  25 ,  26  (discussed below) and the controller circuit board  30  that is usually installed after servo-writing. The controller circuit board  30  suitably positions the actuator assembly  50  and then reads or writes user data in accordance with commands from a host system (not shown). 
     Returning to FIG. 1, the industry presently prefers a “rotary” or “swing-type” actuator assembly  50  that conventionally comprises an actuator body  51  which rotates on a pivot assembly between limited positions, a coil  52  that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor (VCM), and an actuator arm  54  that extends from the opposite side of the actuator body to support the HGA. 
     2. An Exemplary Servo Pattern 
     A disk drive is ultimately used to store user data in one or more “data tracks” that are most commonly arranged as a plurality of concentric data tracks on the surface of its disk or disks. Special servo information is factory-recorded on at least one disk surface so that the disk drive&#39;s servo control system may control the actuator assembly  50 , via the VCM, to accurately position the transducer head to read or write user data to or from the data tracks. In colloquial terms, the servo information provides the servo control system with the position of the head relative to the written track. In operation, the disk drive&#39;s servo control system intermittently processes (read only) the pre-recorded servo information just before the disk drive processes (reads or writes) user data in the data tracks. 
     3. The Write Unsafe Limit 
     FIGS. 4A,  4 B and  4 C are data path diagrams that explain why a WUS limit has been used to date and why it is generally set to a small, “narrow” or “tight” value when a single WUS limit is used for a family of drives. 
     FIG. 4A shows a hypothetical data path  501  of a nominally wide write element  81  that is 70% as wide as the track pitch. As shown, the write element  81  settles in along a damped oscillatory path  501  after the servo control system has moved the write element  81  to the desired track in a track seek mode and then entered a track following mode. The WUS limit relates to when writing will be terminated as a function of the oscillatory deviations of the write element&#39;s path  501  relative to track center (T/C). The WUS limit, to put it another way, corresponds to the maximum off-track distance of the write element  81  before writing is disabled. The tighter the WUS limit, the more frequently that writing will be disabled. A higher frequency of disabling writing will reduce the performance of the drive. 
     The WUS limit is usually specified in terms of a percentage track pitch from track center T/C (e.g. ±16%). In FIG. 4A, the write element&#39;s excursions from track center T/C are signified by vertical arrows, varying from +5%, to −10%, to +18%, to −14%, to +5%, to −3%. The disk drive&#39;s servo control system stops writing the moment that the write element moves beyond the WUS limit due to resonant vibrations, a shock event, or the like. In FIG. 4A, assuming the WUS limit is set to 16%, and writing is disabled just prior to the 18% excursion. What may not be so apparent from FIG. 4A is that the WUS limit is chosen to minimize or eliminate the detrimental effect of reading erroneous data with a narrow read element. The WUS limit, in more detail, reduces so-called “sliver” errors, i.e. errors that arise from reading a sliver of old data that remains when new data is written to the same track. 
     FIG. 4B shows a “new” data path  502 . As shown, most of the old data path  501  has been overwritten beneath the new data path  502 . Exposed adjacent to the new data path  502 , however, are some slivers of old data  501 - 1 ,  501 - 2 ,  501 - 3  and  501 - 4 . 
     FIG. 4C shows a relatively narrow read element  82  attempting to read the data in the new data path  502 . This particular read element  82  is represented as being 32% as wide as the data track pitch. As shown, if the data had been written from position “A” onward, i.e., with an extremely liberal WUS limit of 33%, the read element  82  may read the old data track slivers  501 - 1 ,  501 - 2  and  501 - 3  while trying to read the data on the new data path  502 . This is completely unacceptable, of course, because it constitutes a data integrity error. There is no resulting ECC error to alert the disk drive&#39;s firmware to the problem. The problem simply goes undetected and the disk drive provides the host with garbled data masquerading as good data. 
     A WUS limit is useful for preventing sliver errors. The problem, however, is that a single WUS limit is usually applied to an entire family of disk drives even though the width of the read element varies from drive to drive. Under this one size fits all approach, the WUS limit is set to 50% of: (1) the narrowest width of the read elements used in the drive family in order to guarantee that there are no sliver errors; (2) a compromise between (i) an overly-narrow WUS limit that causes too many disk drives to fail during Initial Burn-In (IBI) for repeatedly trying to satisfy the WUS limit and (ii) an overly-wide WUS limit that permits disk drives to pass through IBI with one or more narrow read elements that make the drive susceptible to sliver errors. 
     The designers choose a narrow WUS limit to eliminate sliver errors from virtually all drives that pass through IBI. Unfortunately, wide read element drives are limited by an unnecessarily narrow WUS limit even though a wider WUS limit could be used for increased performance. 
     There remains a need for a method of manufacturing a disk drive that allows for variability of the WUS limit in order to enhance the performance of some drives that would otherwise have capability that goes unused. 
     SUMMARY OF INVENTION 
     The invention may be regarded as method of manufacturing a disk drive formed from a head disk assembly (HDA) containing at least one magnetic disk with a magnetic surface and a head stack assembly (HSA) that includes a transducer head with a write element for writing data to the magnetic disk and a read element for reading data from the magnetic disk, the method comprising the steps of: mounting the HDA in a servo track writer and moving the HSA to desired positions over the magnetic disk while writing servo tracks onto the magnetic disk to define a data track pitch; measuring a width of the read element with the servo track writer; and determining a write unsafe (WUS) limit based on the data track pitch and the measured width of the read element, the WUS limit corresponding to a maximum distance during writing that the write element is permitted to move radially offtrack from the centerline of a data track before writing is disabled. 
     In a more specific context, the step of determining a WUS limit is based on the data track pitch and the measured width of the read element being within a discrete number of predefined width ranges. 
     In a preferred embodiment of addition, the determined WUS limit is communicated forward for subsequent use by suitable firmware contained in a controller card that is attached to the HDA. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The just summarized invention may best be understood with reference to the Figures of which: 
     FIG. 1 an exploded perspective view of a magnetic disk drive  10  having a head disk assembly  20  (“HDA”) including a head stack assembly  40  (“HSA”) which carries a transducer  80  over concentric data tracks and associated servo bursts on the surface of a disk  23 ; 
     FIG. 2 is a simplified representation of a magneto-resistive transducer head  80  that has two elements that vary in width over a normal distribution curve, namely a write element  81  and a read element  82 . 
     FIG. 3 is an exploded perspective view of a fully-assembled HDA  20  having servo-writing ports  25 ,  26  and the controller circuit board  30  that is usually installed after servo-writing; 
     FIGS. 4A,  4 B, and  4 C illustrate the relationship between a write unsafe limit (WUS limit) and a potential “sliver” error including (A) a first data path  501 , (B) a second data path  502  written only partially over the first data path  501 , and (C) a narrow read element  82  that will suffer a sliver error if the WUS limit is set too broad; 
     FIGS. 5 and 6 illustrate how a narrow WUS limit (e.g. 16%) inhibits writing while a broader WUS limit (e.g., 20%) permits writing to continue; 
     FIG. 7 shows how a broader WUS limit corresponds to better write performance while a narrower WUS limit permits narrower heads to be used without possibility of sliver error; 
     FIG. 8A shows how the prior art approach of using the WUS limit needed for a narrow read head for all read head widths constitutes a lost opportunity to have better write performance with nominal and wide heads; 
     FIG. 8B shows how varying the WUS limit based on the width of the read element recaptures some of the lost opportunity illustrated by FIG. 8A; 
     FIG. 9A is a flow-chart of a method of manufacturing according to this invention; 
     FIG. 9B is an extension of the flow-chart of FIG. 9A; 
     FIG. 10 shows how read elements are characterized into three discrete ranges identified as narrow, nominal or wide for use in certain embodiments of the invention; 
     FIG. 11 shows how write elements are characterized into three discrete ranges identified as narrow, nominal or wide for use in certain embodiments of the invention; 
     FIG. 12 illustrates a decision table for use with an embodiment of the invention where the WUS limit is set only on the basis of the write element&#39;s width; and 
     FIG. 13 illustrates a presently preferred decision table for determining the WUS limit based on the read element&#39;s width and, where the read element is classified as “wide”, based on the write element&#39;s width in order to avoid encroachment issues. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This patent application is directed to a new, innovative method of manufacturing a disk drive and to a disk drive made in accordance with the method. 
     FIGS. 5 and 6 illustrate how a narrow WUS limit (e.g. 16%) inhibits writing while a broader WUS limit (e.g., 20%) permits writing to continue. In Case # 1  of FIG. 5, the disk drive is writing with a narrow WUS limit of 16%. In FIG. 5, if the read element that is guiding the write element deviates from track center T/C by more than 16%, then the write gate closes, as shown, and writing stops. In Case # 2  of FIG. 6, by contrast, a broader WUS limit of 20% is used. In this case, writing does not stop in the face of the same deviation from track center T/C that stopped writing in Case # 1 , and writing continues. 
     FIG. 7 shows how a broader WUS limit corresponds to better write performance while a narrower WUS limit permits narrower heads to be used without possibility of sliver error. The WUS limit is traditionally set on the basis of the narrowest expected read element. Preferably, a rule of thumb which may be applied is that a WUS limit that is ½ of the narrowest expected read element will prevent sliver errors for heads of that width or wider. If the narrowest expected read element is 32% of a track pitch, then the WUS limit is 16%. 
     FIG. 8A shows how the prior art approach of using the WUS limit needed for a narrow read head, for all read head widths, constitutes a lost opportunity to have better data transfer performance during write operations with nominal and wide heads. The nominal and wide read elements, in other words, could operate with a broader WUS limit that provides better write performance while still being less than ½ of the read element&#39;s width in order to preclude sliver errors. 
     FIG. 8B shows how varying the WUS limit based on the width of the read element recaptures some of the lost opportunity illustrated by FIG.  8 A. Here, the WUS limit is varied as a function of the read element&#39;s width in order to take advantage of the presence of additional read element width. 
     FIG. 9A is a flowchart of a preferred method of manufacturing a disk drive according to this invention. The illustrated method operates on an HDA  20  like that shown in FIGS. 1 and 3, i.e. an HDA  20  containing at least one magnetic disk  23  with a magnetic surface and a head stack assembly  40  that includes a two-element transducer head  80 . The HDA  20  is usually placed into an STW in “bare” form (i.e. without the controller circuit board  30 ), but as discussed below, it may be desirable to marry the HDA  20  with the controller circuit board  30  before mounting the disk drive  10  in the STW and recording servo tracks. 
     A presently preferred embodiment of the method proceeds as follows: 
     At step  210 , the method begins by mounting the HDA  20  in an STW for moving the HSA  40  to desired positions over the magnetic disk. 
     At step  220 -A, the method proceeds by measuring a width “R” of the read element  82  with the STW. (Step  220 -B is optional). 
     At step  230 , the method proceeds by determining a WUS limit based on the measured width “R” of the read element  82 . 
     Measuring Methods 
     With reference to measuring steps  220 -A and  220 -B (optional), any suitable measuring algorithm may be used for measuring the width of the read element R and write element W, as described for example, in application Ser. No. 09/920,665, filed on Jul. 31, 2001, and commonly owned by the assignee of this application. The entire content of this prior application is hereby incorporated by reference as if fully set forth herein. 
     The STW ultimately records servo tracks corresponding to a data track pitch of some specified number of tracks per inch (TPI). A single TPI value may be used for an entire family of drives or, as taught in application Ser. No. 09/920,665, the TPI may be varied from drive to drive based on the widths of the read and write elements as measure by the STW prior to performing the actual servo-writing process. The present invention may be practiced in connection with a fixed TPI or with a variable TPI such as may be established in accordance with application Ser. No. 09/920,665. If the TPI is varied according to the teachings of application Ser. No. 09/920,665, then the width of a track would be increased or decreased relative to some nominal track and the measurement of the read element as a percentage ratio of track width would change accordingly. After that point, however, the methodology of determining a WUS limit according to this invention remains the same. 
     Determining the WUS Limit 
     With reference to determining step  230 , a variety of approaches may be taken to determine a WUS limit based on the measured width “R” of the read element  82  and, optionally, also based on the measured width “W” of the write element  81 . The presently preferred approach involves the classification suitably measured elements  81 ,  82  into a discrete number of predefined width ranges, e.g. three discrete size ranges that are aptly named “narrow,” “nominal,” and “wide.” 
     FIG. 10 is an exemplary table of narrow, nominal and wide width ranges for a read element  81 . Here, the widths are expressed as percentages of a data track pitch, but it is equally valid to specify the widths as absolute measurements of suitable units. 
     FIG. 11 is similar to the table of FIG. 10 except that its width ranges are for use with a write element  82  rather than a read element  81 . The inductive write element  82 , as described above in the background section, tends to be wider than the magnetoresistive read element  81 . As a point of reference, the data path diagrams of FIGS. 4A,  4 B and  4 C assumed a 70% write element and a 32% read element, i.e. a nominal width write element and a narrow read element. The elements in any given drive, of course, may be different. 
     FIG. 12 illustrates a first decision table for determining the WUS limit based on the width of the read element  82  alone. Before, all disk drives were manufactured to use a WUS limit of 16%. Here, by contrast, only a narrow read element results in a WUS limit of 16% whereas nominal and wide read elements result in broader WUS limits of 20% and 24% respectively. These percentages are exemplary only. The actual WUS limits used may vary somewhat from these values. 
     FIG. 13 illustrates a presently preferred decision table for determining the WUS limit based on the width of the read element  82  and, where the read element is regarded as wide, on the width of the write element  81  as well. The concept here is that a narrow read element is always paired with a tight WUS limit of 16% and a nominal read element is always paired with a somewhat wider WUS limit of 20%. A wide read element is paired with an even larger WUS limit of 24% provided that the write element  81  is regarded as narrow or nominal. If the write element  81  is regarded as wide, however, then the WUS limit is dialed back from 24% to 20% so that the wide write element is less likely to encroach into an adjacent data track owing to such wide excursion from track center. 
     A plurality of WUS limits may be established on a head-by-head basis or on a drive-based WUS limit may be established for each drive based on all heads using, for example, worst case or average measurements. 
     Communication Forward 
     As suggested by FIG. 9B, which may be regarded as a continuation of FIG. 9A, the preferred embodiment further comprises the step  240  of communicating the determined WUS limit forward for subsequent use by suitable firmware contained in a controller card that is attached to the HDA. 
     There are a number of ways of accomplishing the communicating step. The preferred approach involves associating the determined WUS limit with drive identification data, electronically transmitting the determined WUS limit and HDA identification data over a communications network, and combining the WUS limit with the HDA identified by the HDA identification data during a manufacturing process that is subsequent to the servo track writer. Another approach involves-encoding the WUS limit into a label that is applied to the HDA and reading that label to apply the WUS limit to the controller card that will be combined with the HDA. In such case, the label is likely to comprise a bar code label.