Abstract:
In one embodiment, an arrangement of elements on a head includes a first group of data element spanning a first distance on the head and a second group of data elements spanning a second spanning distance on the head greater than the first spanning distance. The second group of elements overlaps the first group of elements such that some elements are common to both groups.

Description:
BACKGROUND 
   Tape drives are used to store very large amounts of digital information on rolls of magnetic tape and are often used to backup information stored in computer systems. In a typical linear tape open (LTO) drive, magnetic tape is stored on a supply reel contained in a removable cartridge. Information on the tape, including servo information, is arranged in a multitude of parallel tracks that extend along the length of the tape. During operation, the tape is passed along a series of rollers, defining the tape path, to a non-removable take up reel in the tape drive. The tape passes in close proximity to an array of magnetic head elements that read and record information on the tape. The head elements must be accurately positioned over the desired tracks so information can be read or recorded without loss and without corrupting adjacent tracks. An actuator positions the head elements by moving the head containing the elements across the width of the tape. During coarse positioning, the actuator moves the head so that a read element is close enough to a desired track to read servo information. Subsequently, during fine positioning, the servo information is read from the track and sent to servo control circuitry, which then sends a signal to the actuator to move the head so that the elements are directly over the desired tracks and to follow the small lateral motion of the track as it passes by the head. 
   The capacity of a linear recording tape is determined, in part, by the number of tracks that can be read and recorded across the width of the tape. To reliably read and record all tracks, the head, tape and servo positioning system must achieve accurate head to tape alignment within system tolerances, including the dimensional stability of the tape. Magnetic tapes tend to shrink over the useful life of the tape. In addition, magnetic tapes shrink and expand in response to changes in temperature and humidity. Hence, the width of the tape can and usually does vary over time. That is to say, the tape is not dimensionally stable. As the number of tracks on a tape increase, the adverse effect of tape dimensional instability on head to tape alignment also increases. 

   
     DRAWINGS 
       FIG. 1  shows computers networked to a tape drive. 
       FIG. 2  is a plan view illustrating an LTO drive that may be used to implement embodiments of the invention. 
       FIG. 3  shows a format typical of an LTO tape. 
       FIG. 4  is a schematic illustration of an array of head elements in a current/third generation LTO drive. 
       FIG. 5  illustrates one example of the layout of the elements in the array of  FIG. 4 . 
       FIG. 6  is a schematic illustration of an array of head elements arranged according to one embodiment of the invention. 
       FIG. 7  illustrates one example of the layout of the elements in the array of  FIG. 6 . 
       FIGS. 8-10  illustrate the head shown in  FIG. 6  positioned over a data band from the tape format of  FIG. 3 . In  FIG. 8 , the head is centered over the middle of the data band. In  FIG. 9 , the head is centered over the top of the data band. In  FIG. 10 , the head is centered over the bottom of the data band. 
       FIG. 11  is a schematic illustration of an array of head elements arranged according to one embodiment of the invention. 
       FIGS. 12-14  illustrate the head shown in  FIG. 11  positioned over a data band from the tape format of  FIG. 3 . In  FIG. 12 , the head is centered over the middle of the data band. In  FIG. 13 , the head is centered over the top of the data band. In  FIG. 14 , the head is centered over the bottom of the data band. 
       FIG. 15  shows a tape format that might be used, for example, in a next generation LTO tape. 
       FIG. 16  illustrates the head shown in  FIG. 6  positioned over a data band from the tape format of  FIG. 15 . 
       FIG. 17  illustrates a head similar to the head shown in  FIG. 11  positioned over a data band from the tape format of  FIG. 15 . 
   

   DESCRIPTION 
   Embodiments of the present invention were developed in an effort to reduce the adverse effect of tape dimensional instability on head to tape alignment. Embodiments of the invention will be described with reference to an LTO tape drive. The invention, however, is not limited to use in LTO drives but may be implemented in other tape drives or other recording devices. 
     FIG. 1  illustrates a tape drive  2  with a removable tape cartridge  4  networked to computers  6  through a wired or wireless link  8 .  FIG. 2  illustrates an LTO drive  10  such as might be used in the network of  FIG. 1 . In tape drive  10  in  FIG. 2 , magnetic tape  12  is wound on supply reel  14  inside removable cartridge  16 . When cartridge  16  is inserted into drive  10 , tape  12  passes around guide  18 , over head  20 , around guide  22 , to take up reel  24 . As described in detail below, head  20  contains an array of elements that read and record information on tape  12 . A “head element” or just “element” as used in this document means a transducer that converts an electrical signal to the form required to record the signal to a medium (a write element), or reads a signal from a medium and converts it to an electrical signal (a read element), or both. A servo element refers to a head element configured to read head positioning information. Head positioning information is often referred to as servo information. A data element refers to a head element configured to record, read or record and read information other than head positioning information, unless the data element is specially configured to also read head positioning information. Tape drives typically use magnetic head elements, where an electrical signal drives a time-varying magnetic field that magnetizes spots, or domains, on the surface of the magnetic tape. A CD-ROM drive typically uses an optical head, where an electrical signal drives a laser that varies the reflectivity of an optical medium. 
   Head  20  is mounted to an actuator  26  which moves head  20  across the width of tape  12 . An electronic controller  28  receives read and write instructions and data from a computer  6  ( FIG. 1 ) or other host device. Controller  28 , which may include more than one controller unit, includes the programming, processor(s) and associated memory and electronic circuitry necessary to control actuator  26 , head  20  and the other operative components of tape drive  10 . As actuator  26  carries head assembly  20  back and forth across the width of tape  12 , controller  28  selectively activates the head elements to read or record data on tape  12  according to instructions received from the host device. 
     FIG. 3  shows one format for an LTO tape  12 . Tape  12  is nominally 12.6 mm (½ inch) wide. Five servo bands  30 ,  32 ,  34 ,  36  and  38  border four data bands  40 ,  42 ,  44  and  46 . Edge guard bands  48  and  50  separate the top and bottom servo bands  30  and  38  from the edge of tape  12 . In a current generation LTO tape  12 , known to those skilled in the art as the second generation, each data band  40 ,  42 ,  44  and  46  includes  128  data tracks ( 512  tracks total). In an immediate next generation LTO tape  12  currently in development, known to those skilled in the art as the third generation, each data band  40 ,  42 ,  44  and  46  includes  176  data tracks ( 704  tracks total). It is expected that future generations of LTO tape  12  will include even more data tracks. 
     FIG. 4  is a schematic illustration of a head  52  that includes an array  53  of sixteen data elements  54 - 69  used in an immediate next generation LTO drive currently in development, known to those skilled in the art as the third generation. Head  52  also includes a servo element  70  above the data elements and a servo element  72  below the data elements. Servo elements  70  and  72  read servo information from the servo bands bordering each data band on tape  12  ( FIG. 3 ). For example, and referring also to  FIG. 3 , if array  53  on head  52  is positioned over data band  1   42 , then servo elements  70  and  72  read the servo positioning information recorded on servo band  1   32  and servo band  2   34 . Positioning head  52  occurs in two stages for a typical read or record operation. In a first “coarse” positioning stage, head  52  is brought close enough to the desired data band (data band  1   42  in this example) to read servo information on the bordering servo bands (servo bands  1   32  and  2   34  in this example). Then, in a second “fine” positioning stage, servo information read from servo bands  1   32  and  2   34  is used to position data elements  54 - 69  over the desired tracks within data band  1   42 . 
     FIG. 5  illustrates one example of the layout of the head elements in array  53  in a third generation LTO drive. Referring to  FIG. 5 , array  53  consists of two arrays  53 A and  53 B spaced apart from one another across head  52  in the direction the tape moves past head  52 . Each servo element  70 ,  72  consists of two read elements  70 A,  70 B and  72 A,  72 B. Servo elements  70 A and  72 A read servo information when the tape is moving in one direction past head  52  and servo elements  70 B and  72 B read servo information when the tape is moving in the opposite direction past head  52 . Each data element  54 - 69  consists of two element pairs  54 A- 69 A and  54 B- 69 B. Each element pair includes a read element, e.g., read elements  54 A(R) and  54 B(R), and a write element, e.g., write elements  54 A(W) and  54 B(W). Read elements in the A array and write elements in the B array (e.g.,  54 A(R) and  54 B(W)) read and record data on the tape when the tape is moving in one direction. Read elements in the B array and write elements in the A array (e.g.,  54 B(R) and  54 A(W)) read and record data on the tape when the tape is moving in the opposite direction. 
     FIG. 6  is a schematic illustration of a head  74  that includes an array  76  of data elements  78 - 103  and servo elements  104 ,  106 ,  108  and  110  arranged according to one embodiment of the invention. Referring to  FIG. 6 , array  76  is arranged into two groups of data elements  112  and  114 . There are sixteen elements in each group  112 ,  114 . First group  112  includes elements  78 - 83 ,  86 ,  89 ,  92 ,  95 , and  98 - 103 . Second group  114  includes more closely spaced elements  83 - 98 . Data elements  83 ,  86 ,  89 ,  92 ,  95 , and  98  are included on both groups  112  and  114 . The elements in first group  112  correspond to the sixteen data elements shown in  FIG. 4  that are used in third generation LTO drives. In the embodiment shown in  FIG. 6 , the span of the elements in second group  114  along head  74  is ⅓ the span of the elements in first group  112  and the second group elements are centered in the span of the first group elements. 
   To support one mode of use for head  74  described below with reference to  FIGS. 8-10 , elements  82  and  99  are configured to read and record data on data bands and to read servo information on servo bands. This dual “configuration” of elements  82  and  99  occurs in the control circuitry (not shown) that supports these elements by including both a data read channel and a servo read channel for each element  82  and  99 . The physical structure of elements  82  and  89  on head  74  is the same as the other data elements. As an alternative to using dual configuration data elements, discrete servo elements  116  and  118  may be added adjacent to data elements  82  and  99 . 
     FIG. 7  illustrates one example of the layout of the head elements in array  76  as they might appear in a fourth generation LTO drive. Referring to  FIG. 7 , array  76  consists of two arrays  76 A and  76 B spaced apart from one another across head  74  in the direction the tape moves past head  74 . Each servo element  104 ,  106 ,  108  and  110  consists of two read elements  104 A and  104 B,  106 A and  106 B,  108 A and  108 B, and  110 A and  110 B. Servo elements  104 A,  106 A,  108 A, and  110 A read servo information when the tape is moving in one direction past head  74  and servo elements  104 B,  106 B,  108 B, and  110 B read servo information when the tape is moving in the opposite direction past head  74 . Each data element  78 - 103  consists of two element pairs  78 A- 103 A and  78 B- 103 B. Each element pair includes a read element, e.g., read elements  78 A(R) and  78 B(R), and a write element, e.g., write elements  78 A(W) and  78 B(W). Read elements in the A array and write elements in the B array (e.g.,  78 A(R) and  78 B(W)) read and record data on the tape when the tape is moving in one direction and read elements in the B array and write elements in the A array (e.g.,  78 B(R) and  78 A(W)) read and record data on the tape when the tape is moving in the opposite direction. 
   The use of head  74  to read and record data on a tape formatted like tape  12  in  FIG. 3  will now be described with reference to  FIGS. 8-10 .  FIGS. 8-10  show head  74  positioned over, for example, data band  1   42  bordered by servo bands  1   32  and  2   34  along a portion of tape  12 . Data band  1   42  includes multiple tracks  120 ( 1 )- 120 ( n ). Head  74  may be used in two modes. In a first mode, when tape  12  is a third generation tape for example, then the elements in first group  112  (elements  78 - 83 ,  86 ,  89 ,  92 ,  95 , and  98 - 103  in  FIG. 6 ) are used to read and record data on tape  12  in connection with positioning information read by servo elements  106  and  108 . In the third generation LTO tape  12 , each data band includes  176  tracks. So, in this mode each of the sixteen first group  112  elements accesses eleven tracks during fine positioning in each data band. A set of sixteen tracks recorded simultaneously is called a wrap. The eleven wraps on each data band are recorded in a spiraling sequence. Positioning information on servo band  1   32  and servo band  2   34  read by servo elements  106  and  108  is used to control the movement of head  74  between and during each wrap. 
   In a second mode, if tape  12  is a fourth generation tape for example, then the elements in second group  114  (elements  83 - 98  in  FIG. 6 ) are used to read and record data on tape  12  in connection with positioning information read by servo elements  104 ,  106 ,  82 / 116 ,  99 / 118 ,  108  and  110 . In the embodiment of head  74  shown in  FIGS. 6-10 , the span of the elements in second group  114  along head  74  is ⅓ the span of the elements in first group  112 . In this second mode, therefore, second group elements  114  must be positioned at three different locations within data band  1   42  to read all tracks  120 ( 1 )- 120 ( n ).  FIG. 8  illustrates head  74  with second group  114  located during coarse positioning over the middle third of data band  1   42 . In this location, position information is read by servo elements  106  and  108 .  FIG. 9  illustrates head  74  with second group  114  located during coarse positioning over the top third of data band  1   42 . In this location, position information is read by servo elements  82 / 116  and  110 .  FIG. 10  illustrates head  74  with second group  114  located during coarse positioning over the bottom third of data band  1   42 . In this location, position information is read by servo elements  104  and  99 / 118 . In the fourth generation LTO tape  12  each data band may include as many as 288 tracks (1152 tracks total across the four data bands). So, each of the sixteen second group  114  elements would access eighteen tracks in each data band from three different locations (coarse positioning) covering six tracks in each location (fine positioning). 
   The effect of changes in the width of the tape on head to tape alignment is proportional to the total span of the array of head elements. Therefore, reducing the span of the head array will reduce the effect of changes in the width of the tape on head to tape alignment. For example, in a third generation LTO head such as head  52  shown in  FIGS. 4  hand  5 , the sixteen data elements in the array span approximately 2.5 mm. If this span is reduced by a factor of three in future generation heads, to approximately 0.83 mm, as in the group two elements of head  74  shown in  FIGS. 6 and 7 , then the contribution to misalignment from the dimensional instability of the tape can be reduced to approximately ⅓ of its current value. While any reduction in the span of the data elements can result in a corresponding reduction in misalignment due to tape dimensional instability, the degree of span reduction is effectively limited by current techniques for fabricating the data elements. A three factor reduction is presently preferred as the greatest reduction practicable within the constraints of current fabrication techniques. 
   The addition of the more widely spaced group one data elements on head  74  enables using the head with both third generation tapes and fourth generation tapes. This “backward compatibility” for the element array of head  74  shown in  FIG. 6  is evident in the mode one and mode two uses described above. 
     FIG. 11  is a schematic illustration of a head  122  that includes an array  124  of sixteen data elements  126 - 141  and six servo elements  142 - 147  arranged according to one embodiment of the invention.  FIGS. 12-14  show head  122  positioned over, for example, data band  1   42  bordered by servo bands  1   32  and  2   34  along a portion of tape  12 . Data band  1   42  includes multiple tracks  120 (1)- 120 ( n ). In this embodiment, data element array  124  spans approximately ⅓ of data band  1   42 . Array  124 , therefore, must be positioned at three different locations within data band  1   42  to read all tracks  120 (1)- 120 ( n ). Data band  1   42  typically will include many more than 48 tracks. Consequently, array  124  will be moved through multiple positions at each location to cover all tracks. For example, if there are 288 tracks across data band  1   42  (1152 tracks total across the four data bands), then each of the sixteen data elements in array  124  would access six tracks at each of the three locations. 
   In  FIG. 12 , array  124  is located over the middle third of data band  1   42 . At this location, position information is read from servo bands  1   32  and  2   34  by servo elements  143  and  146 . In  FIG. 13 , array  124  is located over the top third of data band  1   42 . At this location, position information is read from servo bands  1   32  and  2   34  by servo elements  144  and  147 . In  FIG. 14 , array  124  is located over the bottom third of data band  1   42 . At this location, position information is read from servo bands  1   32  and  2   34  by servo elements  142  and  145 . 
     FIG. 15  shows a new tape format that might be used, for example, in a next generation LTO tape  12 . Thirteen servo bands  150 - 162  border twelve data bands  164 - 175 . Edge guard bands  176  and  178  separate the top and bottom servo bands  150  and  162  from the edge of tape  12 . Again, tape  12  is nominally 12.6 mm (½ inch) wide. Hence, each data band  164 - 175  in this new format is approximately ⅓ the width of each data band on a tape formatted like the tape shown in  FIG. 3 . Correspondingly, each servo band  150 - 162  is proportionately more narrow than the servo bands on a tape formatted like the tape shown in  FIG. 3  because the head does not need to move as far to access all the tracks on each data band. 
     FIG. 16  shows head  74  (from  FIG. 6 ) positioned over, for example, data band  1   165  bordered by servo bands  1   151  and  2   152  along a portion of tape  12 . Data band  1   165  includes multiple tracks  180   a - 180   n . The second group  114  of elements on head  74  are located over data band  1   165  and position information is read by servo elements  82  and  99 . Using this format in a next/fourth generation LTO tape  12  with 1152 total tracks, each data band would only include 96 tracks. So, each of the sixteen second group  114  data elements would access eight tracks in each data band. 
     FIG. 17  shows head  182  positioned over, for example, data band  1   165  bordered by servo bands  1   151  and  2   152  along a portion of tape  12 . Head  182  is the same as head  122  shown in  FIG. 11  except that the top two servo elements and the bottom two servo elements of head  122  are omitted. Only two servo elements  144  and  145  adjacent to data elements  126 - 141  are needed to read servo information on servo bands  151 ,  152  because head  182  does not need to move far to access all tracks  180 (1)- 180 ( n ) on data band  1   165 . Using this format in a next/fourth generation LTO tape  12  with 1152 total tracks, for example, each data band would include 96 tracks. So, each of the sixteen data elements  126 - 141  would only have to access six tracks in each data band. 
   The exemplary embodiments shown in the figures and described above illustrate but do not limit the invention. Other forms, details, and embodiments may be made and implemented. Hence, the foregoing description should not be construed to limit the scope of the invention, which is defined in the following claims.