1. Field of the Invention
This invention relates to tape drive data storage systems. More particularly, the invention is directed to thin film tape heads for reading and writing data on magnetic recording tape.
2. Description of the Prior Art
Thin film tape heads for magnetic information storage systems (e.g., tape drives) have been constructed using thin film fabrication techniques that are similar to those used in the manufacture of disk drive transducers. In a typical tape head constructed for linear recording (i.e., with data tracks oriented in the direction of tape movement) there are two or more adjacently mounted transducer modules. Each module comprises a linear array of reader and/or writer transducer elements arranged in a cross-track direction that is perpendicular to the direction of tape movement. Each transducer element in a given transducer array is positioned to write or read a separate longitudinal track on the tape. This arrangement is shown in FIG. 1, which depicts a transducer module “M” having an array of thin film transducer elements “E” whose gaps “G” engage a tape “T” in alignment with tracks “TR” that extend in the direction of tape movement “D. In a “piggy back” design (see FIG. 2A), the transducer array “E” would comprise a write transducer “W” and a closely spaced read transducer “R” at each track position. In an interleaved design (see FIG. 2B), the transducer array “E” would comprise alternating read and write elements “R” and “W.” In each design, the transducer array “B” may also include a pair of servo read transducers “SR” that align with servo tracks “ST” used for head positioning.
As shown in FIG. 3, the module “M” of FIG. 1 can be secured to a mounting block “MB” in association with a complimentary tape head “M′” comprising either piggy back or interleaved read and write elements. The resultant assembly, which may be referred to as a “tape head,” will have read/write element pairs that are aligned in the trackwise direction of the tape “T.” In the piggy back design, there will be two read/write element pairs per track (see FIG. 4A). In the interleaved design, the read and write elements of each module “M” and “M′” will be arranged so that there is one read element and one write element for each track (see FIG. 4B). The dual module arrangement allows data recording (and playback) to be performed in both tape directions and provides conventional read-while write capability in which data written to the tape “T” is immediately read back and checked for errors. Other conventional tape head designs include heads in which all of the data transducer elements are read elements or write elements. Read-while write capability may then be achieved by combining a read-only module and a write-only module in a single tape head to provide trackwise-aligned read and write element pairs. As shown in FIG. 5, bi-directional recording with read-while write capability can be provided by placing a read-only module “M′” between a pair of write-only modules “M.”
A characteristic of tape head constructions as described above is that the gap pitch within the transducer array “E” is usually much larger than the gap width, such that for every track being read or written by the array, there will be space between the tracks where no transducing occurs. Thus, for every pair of tracks aligned with adjacent read and write elements “R” and “W,” there is inter-track white space on the tape “T” that is not transduced concurrently with the selected pair. The white space regions can be recorded with data by stepping the tape head in a cross-track direction during multiple transducing passes. Tape tracks can also be written at less than the gap width of the write transducers using a process known as “shingling.” According to this technique, the tape head is stepped by less than the write element gap width for each successive transducing pass, such that the edge of a previously written track is overwritten during the next pass, much like shingles on a roof.
Although the foregoing track writing techniques allow data to be densely packed on a tape, a continuing unresolved problem is track misregistration caused by tape dimensional changes between transducing (either reading or writing) operations. For example, the tape “T” may be written with data under one set of temperature and humidity conditions, and then later read following exposure to different environmental conditions. For conventional tape material, the dimensions can change by as much as 0.12%. These tape dimensional changes will widen or narrow the tape track spacing geometry, resulting in track misregistration with the tape head (whose gap spacing geometry is substantially unchanged). Providing a head that is statically rotated to a nominal predetermined angle addresses the misregistration problem because small changes in rotation change the effective track pitch of the transducer array “E.” However, this solution requires sophisticated mechanics and skew compensation circuitry.
The track misregistration problem is exacerbated in conventional tape heads due to the relatively large gap spacing of the transducer array “E.” which is mandated to a large extent by the size of the transducers themselves. This is due to the fact that for any percentage change in tape dimension, the actual misregistration between written tracks and outermost transducers depends on the span between the transducers. To illustrate, if the transducer array “E” has a transducer element gap pitch of x μm, and the percentage change in tape dimension is 0.12%, the resultant change in the spacing of the tape tracks under the outermost transducer elements of a sixteen transducer array will be 15×0.0012x=0.018x μm. If x is a typical value of 167 μm (for current generation tape heads), then 0.018x=3 μm. This is a large part of the TMR (Track MisRegistration) budget. On the other hand, if the transducer array “E” has a transducer element gap pitch of 0.5x μm, then a 0.12% change in tape dimension will only change the tape track spacing under the outermost transducer elements by 15×0.0006x μm=0.009x. Again assuming x is a typical value of 167 μm, then 0.009x=1.5 μm. The 0.5x gap pitch transducer array will thus experience only half of the tape dimensional change that is experienced by the x gap pitch array, such that track misregistration is less likely. Unfortunately, reducing track pitch using current thin film transducer fabrication techniques is not a trivial challenge due particularly to the size requirements of the write element structures. Absent the use of alternative transducer designs that permit reductions in track pitch (as previously proposed by one of the applicants herein in commonly-owned patent application filings), or the use of complicated head rotation techniques as referred above, there is no conventional technique for dealing with the thermally induced track misregistration.