Patent Publication Number: US-2012044593-A1

Title: Temperature induced head skew

Description:
SUMMARY 
     Implementations described and claimed herein address the foregoing problems by providing a method including calculating a DC head-skew compensation factor by relating a first DC head-skew at a first temperature to a second DC head-skew at a second temperature. 
     Other implementations are also described and recited herein. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1A  illustrates a plan view of an example disc drive assembly including a first actuator aim positioned over a storage disc, a second actuator arm positioned under the storage disc, and a temperature sensor located in the vicinity of the storage disc. 
         FIG. 1B  illustrates an elevation view of the disc drive assembly of  FIG. 1A  with two skewed transducer heads above and below a storage disc. 
         FIG. 2  illustrates an elevation view of an example disc drive assembly with two storage discs and four skewed transducer heads. 
         FIG. 3  illustrates example operations for calculating a temperature-induced DC head-skew correction factor for a multi-head disc drive assembly. 
         FIG. 4  illustrates example operations for applying a temperature-induced DC head-skew correction factor to a multi-head disc drive assembly. 
     
    
    
     DETAILED DESCRIPTIONS 
     Multi-disk writing (MDW) technology combines two or more transducer heads capable of reading and writing bits of data to/from two or more stacked storage discs sharing a common axis of rotation within a storage media drive. The transducer heads are each attached to an actuator arm in a stack of actuator aims also with a common axis of rotation and are intended to be vertically aligned so that the transducer heads align with the same relative position on each corresponding storage disc. 
     Exact vertical alignment of the transducer heads is very difficult or entirely unfeasible. As a result, during assembly and certification of the storage media drive, a head-skew calibration process is performed to compensate for any head-skew of the transducer heads. The head-skew calibration process yields parameters that allow the storage media drive to adjust head positioning to accommodate for differences in alignment among multiple heads. After the head-skew calibration process, the storage media drive is able to have high servo performance during head switching seek operations. 
     The head-skew may have both AC and DC components. The AC component is not significantly affected by temperature change and is not addressed by the presently disclosed technology. The DC component is typically compensated for during certification of the storage media drive. However, the DC skew may change over time or following an environmental event that disrupts the transducer head alignment (e.g., a shock to the storage media drive, a head stack tilt event, and/or and changing environmental factors such as temperature). 
     After a shock or head stack tilt event, the DC head-skew component may have permanent changes. A power-on full DC head-skew re-calibration can compensate for the permanently changed DC skew. However, the full DC head-skew re-calibration is time consuming and affects drive performance (i.e., results in longer time to ready (TTR)). As a result, the storage media drive may also use quick head-skew checks to quickly determine if DC head-skew exceeds a predetermined triggering threshold, thus indicating that a full DC head-skew re-calibration is necessary. The quick head-skew checks may be performed periodically (e.g., upon start-up of the storage media drive) or only after an event that would likely cause a change in DC head-skew (e.g., a detected shock or head stack tilt event). The quick head-skew checks minimize the number of full DC head-skew re-calibrations, which minimizes time to read (TTR) for the storage media drive. 
     A storage media drive is typically certified at a known average operating temperature (e.g., 40-50 degrees Celsius). However, when the operating temperature of the storage media drive varies significantly from the temperature during drive certification, the DC head-skew change may be significantly. This is especially likely on small, compact storage media drives with limited cooling capability (e.g., notebook drives). However, the presently disclosed technology is applicable to all types of storage media drives. 
     If the DC head-skew caused by temperature change is not compensated for, head switching seek operations for the storage media drive will have an acoustic issue and data access time will be impacted. More specifically, the DC head-skew is an error in head position. Head position errors cause sharps changes in voice coil motor current during head switching operations under feedback control. The sharp changes in voice coil motor current create the acoustic issue and data access time suffers. Conversely, if a full DC head-skew re-calibration is performed every time the storage media drive operating temperature significantly changes, TTR will suffer. 
     One way to minimize full DC head-skew re-calibrations rendered necessary by changes in storage media drive operating temperature is to adjust the triggering threshold used by the quick head-skew checks as a function of operating temperature. However, merely adjusting the triggering threshold used by the quick head-skew checks is insufficient to yield high performance head switching seek operations while maintaining low TTR in storage media drives operating at large temperature variations from the certification temperature. Alternate systems and methods of compensating for temperature-induced DC head-skew are disclosed herein. 
       FIG. 1A  illustrates a plan view of an example disc drive assembly  100  including a first actuator arm  106  positioned over a storage disc  108 , a second actuator arm  110  positioned under the storage disc  108 , and a temperature sensor  116  located in the vicinity of the storage disc  108 . In various implementations, the storage disc (or media platter)  108  is an optical or magnetic storage medium. Further, the storage disc  108  may be a bit-patterned media. The storage disc  108  includes an outer diameter  102  and an inner diameter  104  between which are a number of data tracks (e.g., data track  132 ), illustrated by circular lines. The data tracks may be on one or both sides of the storage disc  108 . Further, the data tracks are substantially circular and generally concentric with one another and the storage disc  108 . In one implementation, the storage disc  108  rotates at a constant high speed about disc axis of rotation  112  as information is written to and read from the data tracks on the storage disc  108 . In another implementation, the storage disc  108  rotation speed is variable. 
     The actuator arms  106 ,  110  extend over and under the storage disc  108 , respectively, to write information to and read information from the storage disc  108 . Further, the actuator arms  106 ,  110  rotate about an actuator axis of rotation  114  during a seek operation to locate desired data track(s) on each side of the storage disc  108 . At the distal end of each of the actuator anus  106 ,  110  facing the storage disc  108  is a transducer head (e.g., transducer head  120 ), each of which flies in close proximity above/below the storage disc  108  while reading and writing data from/to the storage disc  108 . In other implementations, there are more than two transducer heads and actuator arms and more than one storage disc in the disc drive assembly  100  (see e.g.,  FIG. 2 ). 
     One or more flex cables  130  provides the requisite electrical connection paths for the transducer heads while allowing pivotal movement of the actuator arms  106 ,  110  during operation of the disc drive assembly  100 . The flex cable  130  routes along the actuator arm  110  and connects a printed circuit board (PCB) (not shown) to the transducer heads. The PCB typically includes circuitry for controlling the write currents applied to the transducer heads during a write operation and a preamplifier for amplifying read signals generated by the transducer heads during a read operation. Further, the PCB may contain circuitry used to implement the presently disclosed technology described in detail with regard to  FIGS. 1B and 2 . 
     The temperature sensor  116  is located within the disc drive assembly  100 , preferably near the transducer heads. However, the temperature sensor  116  may be located anywhere within the disc drive assembly  100 , so long as the temperature sensor  116  can accurately measure the ambient temperature at the transducer heads. The temperature sensor  116  may be of any type including but not limited to carbon resistors, film thermometers, wire-would thermometers, and coil elements. Further, if the temperature sensor  116  is a thermistor, any materials with a generally linear temperature-resistance relationship may be used for its construction. In other implementations, a thermocouple may be used in place of the thermistor for temperature sensor  116 . 
       FIG. 1B  illustrates an elevation view of the disc drive assembly  100  of  FIG. 1A  with two skewed transducer heads  118 ,  120  above and below a storage disc  108 . The disc drive assembly  100  is exemplary of a 1-disc drive with two transducer heads. As described with respect to  FIG. 1A , the storage disc  108  rotates about disc axis of rotation  112  as information is written to and read from the data tracks on the storage disc  108 . Both a top surface  122  and a bottom surface  124  of the storage disc  108  are divided into a number of data tracks as described above with respect to  FIG. 1A . 
     Transducer head  120  is configured to read/write data to/from data tracks on the top surface  122  and transducer head  118  is configured to read/write data to/from data tracks on the bottom surface  124 . Ideally, transducer head  120  is centered over the same data track on the top surface  122  as transducer head  118  is on the bottom surface  124 . However, rather than vertically aligned, transducer head  118  is skewed from transducer head  120  as illustrated by head-skew  126 . Head-skew  126  is exaggerated for visual effect and is not drawn to scale. When the disc drive assembly  100  switches from transducer head  120  to transducer head  118 , or vice versa, the track difference between the transducer head  120  location on the top surface  122  to transducer head  118  location on the bottom surface  124  is the head-skew  126 . The presently disclosed technology is directed at providing formulae for compensating for head-skew  126  rather than adjusting the head-skew  126  itself. 
       FIG. 2  illustrates an elevation view of an example disc drive assembly  200  with two storage discs  208 ,  228  and four skewed transducer heads  218 ,  220 ,  230 ,  232 . The disc drive assembly  200  is exemplary of a 2-disc drive with four transducer heads. Similar to the 1-disc drive assembly  100  of  FIGS. 1A &amp; 1B , the storage discs  208 ,  228  rotate about disc axis of rotation  212  as information is written to and read from the data tracks on the storage discs  208 ,  228 . Both top surface  222  and bottom surface  224  of the storage disc  208  and top surface  234  and bottom surface  236  of the storage disc  228  are divided into a number of data tracks as described above with respect to  FIGS. 1A &amp; 1B . 
     Transducer head  220  is configured to read/write data to/from data tracks on the top surface  222  of storage disc  208 . Transducer head  218  is configured to read/write data to/from data tracks on the bottom surface  224  of storage disc  208 . Transducer head  232  is configured to read/write data to/from data tracks on the top surface  234  of storage disc  228 . Transducer head  230  is configured to read/write data to/from data tracks on the bottom surface  236  of storage disc  228 . Ideally, transducer heads  218 ,  220 ,  230 , &amp;  232  are centered over the same data track on the bottom surface  224 , top surface  222 , bottom surface  236 , and top surface  234 , respectively. 
     However, rather than vertically aligned, the transducer heads  218 ,  220 ,  230 , &amp;  232  are skewed from one another as illustrated by head-skews  222 ,  238 ,  240 ,  242 . Head-skews  222 ,  238 ,  240 ,  242  are exaggerated for visual effect and not drawn to scale. Head-skew  222  illustrates head-skew between transducer heads  220 ,  218 . Head-skew  238  illustrates head-skew between transducer heads  218 ,  232 . Head-skew  240  illustrates head-skew between transducer heads  232 ,  230 . In this implementation, each head-skew  222 ,  238 ,  240  corresponds to head-skew between adjacent transducer heads and are approximately equal and varying in the same direction. In other implementations, head-skew between adjacent transducer heads may not be equal or skewed in the same direction. The total head-skew  242  illustrates head-skew between transducer heads  220 ,  230 , the uppermost and bottommost transducer heads in the disc drive assembly  200 . In still other implementations, the disc drive assembly may contain more than two storage disks with data tracks on one or both sides of the storage discs. Further, there may be more than four transducer heads in the disc drive assembly. 
     In an example implementation, a 2-disc drive test assembly with four transducer heads, similar to that of  FIG. 2 , was aligned at 5 degrees Celsius. The temperature of the test assembly was increased to 55 degrees Celsius and the head-skew of each pair of adjacent transducer heads was measured as they were moved from the inner diameter to the outer diameter of the test assembly. The measured DC head-skew between each pair of adjacent heads was generally linear as the transducer heads were moved from the inner diameter to the outer diameter of the test assembly. Further, even in implementations where the DC head-skew does not change linearly as the transducer heads are moved from the inner diameter to the outer diameter of a storage media, the temperature compensation factor contemplated herein will still apply to tracks at or near the inner diameter as well as tracks at or near the outer diameter of the storage media. There was also some measured AC component head-skew and the linearity of the DC component head-skew decreased close to the outer diameter of the test assembly. However, the linear DC head-skew was dominant. 
     A measured head-skew between each pair of adjacent heads ranged from approximately 12 to 28 tracks, with each pair of adjacent heads varying by no more than 10 tracks from inner diameter to the outer diameter of the test assembly. In one implementation, the threshold criteria for executing a full power-on DC head-skew re-calibration is set around 30 tracks. While, the skew of individual pair of adjacent heads at 55 degrees Celsius is not sufficient to trigger the full power-on DC head-skew re-calibration, the skew between each pair of adjacent heads is combined to find the head-skew between the top-most transducer head and the bottom-most transducer head in the test assembly. As a result, the skew between the top-most transducer head and the bottom-most transducer head in the test assembly ranged from approximately 52 to 65 tracks, which is significantly above the threshold criteria for executing a full power-on DC head-skew re-calibration. 
     If the temperature-induced DC component head-skew is compensated for at each transducer head in a linear manner from the inner diameter to the outer diameter of the test assembly, any residual DC error is small and can be compensated for using a servo control loop, gain scheduling, and/or other known compensation techniques during head switching operations. 
     In another example implementation, head-skew of a 2-disc drive test assembly with four transducer heads was tracked from 0 to 60 degrees Celsius. A generally linear slope of DC head-skew versus temperature was observed for both adjacent heads in the 2-disc drive test assembly and for head-skew between the top-most transducer head and the bottom-most transducer head in the test assembly. For example, the head-skew between the top-most transducer head and the bottom-most transducer head ranged from approximately +49 tracks at 0 degrees Celsius to −7 tracks at 60 degrees Celsius, with an approximately linear slope between the two temperature extremes. As a result, temperature-induced DC head-skew may be primarily compensated for using a linear function. Many power-on DC head-skew re-calibrations may be avoided if temperature-induced DC head-skew is compensated for using a linear function based on operating temperature of a disc drive. 
       FIG. 3  illustrates example operations  300  for calculating a temperature-induced DC head-skew correction factor for a multi-head disc drive assembly. In measuring operation  310 , DC head-skew (DCskew 1 ) is measured at a first arbitrary temperature (T 1 ) of the disc drive. In one implementation, DCskew 1  equals approximately 0 tracks at 55 degrees Celsius (T 1 ). In setting operation  320 , the disc drive temperature is reset to a second arbitrary temperature (T 2 ). In measuring operation  330 , DC head-skew (DCskew 2 ) is measured at T 2 . In one implementation, DCskew 2  equals approximately 28 tracks at 25 degrees Celsius (T 2 ). 
     In an optional calculating operation  340 , critical temperature(s) (T c ) for DC head-skew compensation are calculated. A DC head-skew correction factor is only necessary when the drive operating temperature falls outside of a range defined by T c . For example, when the code space and/or processor bandwidth required for implementing calculating operation  350  for the entire operating temperature range of the disc drive is too great, optional calculating operation  340  may be used. Since optional calculating operation  340  is a simplified way to compensate for temperature-induced DC head-skew, less code space and/or processor bandwidth are required. After calculating operation  340 , calculating operation  350  yields DC head-skew correction factor(s) only for temperature ranges outside the range(s) defined by T c . 
     In implementations that do not utilize calculating operation  340 , the operations  300  proceed directly from measuring operation  330  to calculating operation  350 . Other individual operations of operations  300  may also be optional in various implementations of the presently disclosed technology. In calculating operation  350 , the DC head-skew correction factor (e.g., DC head-skew slope between T 1  and T 2 ) is calculated by relating DCskew 1  at T 1  to DCskew 2  at T 2 . The following formula is one implementation of this relation. 
     
       
         
           
             correctionfactor 
             = 
             
               
                 
                   DCskew 
                   2 
                 
                 - 
                 
                   DCskew 
                   1 
                 
               
               
                 
                   T 
                   2 
                 
                 - 
                 
                   T 
                   1 
                 
               
             
           
         
       
     
     Since in the example implementation referenced above, DCskew 1  equals approximately 0, the slope of DC head-skew becomes: 
     
       
         
           
             correctionfactor 
             = 
             
               
                 DCskew 
                 2 
               
               
                 
                   T 
                   2 
                 
                 - 
                 
                   T 
                   1 
                 
               
             
           
         
       
     
     The DC head-skew correction factor may be stored in memory (e.g., servo adaptive parameters (SAP) or global memory) or it may be computed on-the-fly using DC head-skew and corresponding temperature values stored in memory. Further, more than two DC head-skew and temperature values may be used to calculate the DC head-skew correction factor. In one implementation, multiple correction factors between multiple pairs of measured DC head-skew (DCskew x ) at temperature points (T x ) may be averaged to find an overall DC head-skew correction factor. In another implementation, two different correction factors between two pairs of measured DC head-skew may be used to calculate two DC head-skew correction factors (e.g., one for higher temperature operation and one for lower temperature operation). More than two DC head-skew measurements may also be used to calculate separate DC head-skew correction factors for more than two temperature bands. 
     The servo control loop for the multi-head disc drive assembly is able to compensate for some DC skew without a DC head-skew correction factor (e.g., 25 tracks of DC head-skew). Using a line plotted between DCskew 1  at T 1  and DCskew 2  at T 2 , a critical temperature (T c ) may be found at the maximum DC head-skew that the servo control loop can compensate for (e.g., DCskew c ). For example, when DCskew 1  equals approximately 0 tracks at 55 degrees Celsius and DCskew 2  equals approximately 28 tracks at 25 degrees Celsius, DCskew c  may equal approximately 25 tracks at 30 degrees Celsius (T c ). A low-temperature DC head-skew correction factor may be applied when the disc drive temperature is below T c , while no DC head-skew correction is applied when the disc drive temperature is above T c . 
     Further, if the disc drive temperature is expected to rise above a second critical temperature (T c′ ) corresponding to the maximum number of tracks that may be compensated for using the servo control loop (e.g., −25 tracks of DC head-skew), a high-temperature DC head-skew correction factor may also be calculated using a similar procedure as the low-temperature DC head-skew correction factor. In some implementations, the high-temperature DC head-skew correction factor and the low-temperature DC head-skew correction factor are the same. 
     For example, when DCskew 2  equals approximately 28 tracks at 25 degrees Celsius and DCskew 1  equals approximately 0 tracks at 55 degrees Celsius, high-temperature DCskew c′  may equal approximately −25 tracks at 80 degrees Celsius (T c′ ). When there is a high critical temperature (e.g., 80 degrees Celsius) and a low critical temperature (e.g., 30 degrees Celsius), so long as the disc drive temperature remains between 30 degrees and 80 degrees Celsius, no head-skew correction factor is needed. However, a low-temperature DC head-skew correction factor is applied if the disc drive temperature drops below the low critical temperature (e.g., 30 degrees Celsius) and a high-temperature DC head-skew correction factor is applied if the disc drive temperature rises above the high critical temperature (e.g., 80 degrees Celsius). Further, if the slope of DC head-skew over temperature is non-linear, multiple DC head-skew correction factors may be calculated for temperature ranges below the low critical temperature and/or above the high critical temperature. 
       FIG. 4  illustrates example operations  400  for applying a temperature-induced DC head-skew correction factor to a multi-head disc drive assembly. In a power-up operation  410 , the multi-head disc drive assembly is powered-up for operation. In measuring operation  420 , a temperature sensor located somewhere in the disc drive assembly (preferably near the transducer heads) measures the current disc drive temperature (T x ). 
     In one implementation, a correctionfactor parameter (see e.g.,  FIG. 3 ) is only available for certain temperature ranges. In optional decision operation  430 , it is determined whether the disc drive operating temperature is within a range for DC head-skew correction. If the disc drive operating temperature is not within a range for DC head-skew correction, the operations  400  proceed to performing operation  450  (described below). 
     If the disc drive operating temperature is within a range for DC head-skew correction, than in applying operation  440 , a head-skew correction factor corresponding to the relevant range for DC head-skew correction is applied to correct the DC head-skew. For example, a low-temperature DC head-skew correction factor may be applied when the disc drive temperature is below a critical temperature, while no DC head-skew correction is applied when the disc drive temperature is above the critical temperature. The critical temperature refers to the number of tracks of DC head-skew that the servo control loop is able to compensate for without a DC head-skew correction factor. 
     In implementations that do not utilize decision operation  430 , the operations  400  proceed directly from measuring operation  420  to applying operation  440 . Other individual operations of operations  400  may also be optional in various implementations of the presently disclosed technology. In applying operation  440 , a head-skew correction factor is applied based on the measured temperature to correct the DC head-skew. For example, if the correctionfactor parameter was calculated as described above with regard to  FIG. 3 , then a DC head-skew correction value at T x  (DCskew x ) is calculated as follows. 
       DCskew x =DCskew 2 +correctionfactor*( T   x   −T   2 ) 
     In performing operation  450 , a quick head-skew check is performed to determine if a power-on full DC head-skew re-calibration is necessary. Assuming that the power-on full DC head-skew re-calibration is unnecessary due perhaps to the application of the correction factor applied in operation  440 , the disc drive is ready for operation. After application of the correction factor, any residual DC head-skew should be small enough that the servo control loop can compensate for it without a power-on full DC head-skew re-calibration, even at extreme temperature conditions of the disc drive assembly. As a result, the power-on full DC head-skew re-calibration will be skipped unless a shock to the storage media drive or a head stack tilt event has occurred. This will keep adequate drive servo performance while minimizing TTR. 
     Operations  420 - 450  may be periodically repeated over time, during certain drive functions, or during drive idle time. Further, operations  420 - 450  may be repeated only when a significant temperature change has been detected. Still further, operations  410 - 450  may be repeated every time the disc drive assembly is powered up or only the first time the disc drive assembly is powered up. 
     The embodiments of the invention described herein are implemented as logical steps in one or more computer systems. The logical operations of the present invention are implemented (1) as a sequence of processor-implemented steps executing in one or more computer systems and (2) as interconnected machine or circuit modules within one or more computer systems. The implementation is a matter of choice, dependent on the performance requirements of the computer system implementing the invention. Accordingly, the logical operations making up the embodiments of the invention described herein are referred to variously as operations, steps, objects, or modules. Furthermore, it should be understood that logical operations may be performed in any order, unless explicitly claimed otherwise or a specific order is inherently necessitated by the claim language. 
     The above specification, examples, and data provide a complete description of the structure and use of exemplary embodiments of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. Furthermore, structural features of the different embodiments may be combined in yet another embodiment without departing from the recited claims.