Head assembly with small distance between read/write heads

Abstract: A magnetic head assembly, for use in a tape drive wherein a tape is moved past the head assembly in a longitudinal direction and the tape exhibits transverse tape movement correlated with the position of the tape in the longitudinal direction, and wherein a minimum longitudinal distance exists for which the correlation is maintained, has a carrier with first and second gap and coil arrangements (15,16) disposed on the carrier. The respective gaps (10,12) in the first and second gap and coil arrangements (15,16) are disposed on the carrier separated by a spacing (Drw) which is less than the aforementioned minimum longitudinal distance. If one gap and coil arrangement is used as a write arrangement, and the other gap and coil arrangement is used as a read arrangement for verifying the writing of data by the write arrangement, tracking errors are minimized, or substantially avoided, because the write and read arrangements are so close to each other as to be effectively at the same position in the context of the transverse tape movements.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention is directed to a magnetic head assembly of the type used in a high-speed tape drive to transfer data to and from a magnetic tape moving past the magnetic head assembly in the drive. Description of the Prior Art and Related Applications In high-speed tape drives wherein data is transferred to and from a magnetic tape moving at high speed in the drive, using a magnetic head assembly, the data are written in a large number of closely adjacent, or overlapping, data tracks. The tracks extend in a longitudinal direction parallel to the direction of tape transport, and are disposed side-by-side in a direction perpendicular to the transport direction. In such systems, it is known to employ the read head of the magnetic head assembly to verify the data which has been written in a pass by the write head, so that any errors in tracking or data transfer can be immediately identified, and corrective steps can be taken. A system wherein the verification takes place in the same pass as the writing of the data is known as a read-while-write system. It has been observed and experimentally confirmed by the present inventor that if a track is recorded on a magnetic tape in a belt-driven cartridge using a write head, and if the tape in the cartridge is then rewound to exactly the same position as where the recording began and is then driven, the tape will exhibit the same transverse tape movements (TTM) as the tape exhibited when the first track was recorded. Such transverse tape movements occur in a direction substantially perpendicular to the direction of tape transport. By means of a servo track or tape edge monitoring, the write head can thus be made to precisely follow these transverse movements so that the "guard bands" between tracks can be made small to prevent overwriting from track-to-track (or, if there is intentional track-to-track overtap, this overlap can be precisely controlled). The details of this phenomenon, and supporting experimental data, are set forth in the inventor's pending application entitled "Method and Apparatus for Improving the Accuracy of a Tape Servo Track Seek Algorithm," Serial Number 08/034,573, filed in the United States Patent and Trademark Office on March 19, 1993. The aforementioned phenomenon can be summarized as a recognition that the tape transversal movements are strongly correlated with the longitudinal tape position. A graphical representation of such TTM as described in the aforementioned pending application is shown in Figure 1. The correlation is very strong, except for a slow shift or drift in the transverse direction over a relatively long time. Even when sudden, unpredictable changes take place, for example, due to external mechanical vibrations or shocks to which the drive is subjected, the correlation is maintained, and is superimposed on the total tape transversal movements. In conventional read-while-write head assemblies, a relatively long spacing exists between the write and read gaps, primarily for the purpose of avoiding crosstalk or other interference between the gaps. This relatively long distance results in a phase shift of the TTM waveform between the write and read gaps. In such conventional head assemblies, therefore, the inherent tracking repeatability of the write gap, upon playback of a previously recorded track, cannot be utilized to its fullest extent, since the read gap is not located at the same position as the write gap was located at the time the data was written, due to the aforementioned phase shift. Moreover, the correlation between longitudinal position of the tape and the transverse tape movement ceases to exist, or is at least not reliably predictable, for very short TTM wavelengths. This is caused by random TTM stemming from physical phenomena other than "weaving" caused by the rotating parts of the cartridge, which gives rise to the TTM. Such random TTM may result, for example, from transverse-mode "string" vibrations of the tape, in the same way as the well-known longitudinal mode Instantaneous Speed Variations (ISV) of the tape are caused by random frictional forces and the dynamics of the running tape. The minimum longitudinal distance at which longitudinal correlation in TTM can be seen will be referred to herein as the correlation distance, designated D corr . In the past several read-while-write systems have been developed: The document US-A-5 218 498 discloses a composite read-while-write magnetic head assembly, including a substrate having first and second opposing planar surfaces. On its opposite side, the substrate carries a read head and a write head, respectively. In order to avoid tracking errors and cross talk between the magnetic heads, the assembly has been designed in a specific way. Firstly, the substrate which carries the magnetic heads, simmultaneously serves as an isolating layer; secondly, it is preferred to use a read head of the magnetoresistive type, thereby allowing to minimize the distance between the magnetic heads. In the document US-A-4 972 336 a read-while-write magnetic head assembly is described in which the magnetic heads are provided at a very small distance. The problem of cross talk is solved by a feed-through shield, shielding the magnetoresistive thin film transducer, used as the read head, from the very closely positioned inductive thin film tranducer, used as the write head. The document US-A-5 189 580 discloses a magnetic transducer having a plurality of aligned transducing gaps. The individual tranducers are joined together by any bonding technique, and substrates provide non-magnetic spacers between the adjacent transducers. Cross talk between the adjacent transducers is reduced by the provision of shielding layers. Examples of materials for realizing such shielding layers are given in the documents JP-A-58 108 024 and JP-A-62 022 205, e.g. copper or copper alloys. SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetic head assembly for undertaking track verification in a high-speed tape drive wherein the respective gaps of the write and read heads are disposed at a distance that, in the context of transverse tape movement, they can be considered to be at the same position. Further, the assembly should not be limited to a specific kind of magnetic head design. The above object is achieved by the magnetic head assembly of claim 1. Typically, the aforementioned correlation distance, below which the predictability or repeatability of transverse tape movements no longer holds, is within a range of about 2 millimeters to about 3 millimeters. In conventional read-while-write head assemblies, the gaps of the respective heads have intentionally not been placed so close together as to be at or less than this correlation distance, because of the concern over crosstalk or other interference arising between the gap and coil arrangement which is used for reading and the gap and coil arrangement which is used for writing. In accordance with the principles of the present invention, a magnetic head assembly wherein the gaps are spaced a distance from each other which is equal to the correlation distance is achieved by a back-to-back arrangement of two thin-film heads, separated by a layer of shielding to prevent crosstalk or other interference between the two thin-film arrangements. The shielding is formed by a copper deposit or copper alloy disposed between the two thin-film arrangements, and connected to ground. DESCRIPTION OF THE DRAWINGS Figure 1, as noted above, is a graphical representation of TTM correlation in a tape drive. Figure 2 shows a time-domain waveform for a typical TTM, for use in explaining the invention. Figure 3 is a fast Fourier transform (FFT) of the TTM waveform of Figure 2. Figure 4 is a plan view of a first embodiment of a magnetic head assembly constructed in accordance with the principles of the present invention. Figure 5 is a sectional view taken along line 13 in Figure 4. Figure 6 is a plan view of a second embodiment of a magnetic head assembly constructed in accordance with the principles of the present invention. Figure 7 is a sectional view taken along line 13 of Figure 6. Figure 8 is a plan view of a third embodiment of a magnetic head assembly constructed in accordance with the principles of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a magnetic head assembly constructed in accordance with the principles of the present invention is shown in plan and sectional views in Figures 4 and 5. The magnetic head assembly shown therein, as well as in the other embodiments, is for use as a write/read head of the read-while-write (R-w-R) type. The head assembly can be used for track verification as explained in co-pending Tandberg Data Application Serial No. 08/037,903, filed in the United States Patent and Trademark Office on March 26, 1993 ("Head System Suitable for Magnetic or Optical High-Density Recording Devices," Solhjell et al.). Before describing the structure of the various embodiments of the magnetic head assembly, a description of the physical mechanism which causes the loss of correlation generally discussed above will be described. The most important mechanism resulting in the loss of correlation is a correlation loss which results because the read gap in known drive systems, such as the Quarter-Inch Cartridge (QIC) system which has heretofore been standard, is located 0,85 cm (1/3 of an inch) downstream of the write gap. If the "wavelength" of the written track movements with respect to the tape edge is 1,7 cm (2/3 of an inch), a maximum error will occur. This wavelength can be designated λ 0 with k 0 = 2π/λ 0 being the corresponding spatial frequency, and A being the amplitude of the movement. The write gap will record a waveform in the track which is Asin(kx). When this undulating track is attempted to be read, the read head will lag by half a wavelength, and the path described by the read gap will follow the curve Asin(kx-π) = -Asin(kx). Therefore, the mistracking will be Asin(kx) minus -Asin(kx) = 2Asin(kx). With a tape speed of 305 cm/sec (120 IPS), the critical frequency is 200 Hz, where loss of correlation causes a doubling of peak deviations. The write gap tracks perfectly in this model; the read gap is "off" by twice the tape movement amplitudes This model can be generalized for all wavelengths, both below and above the lowest critical wavelength described above. The phase lag is 2πD rw /λ, wherein D rw is the distance between the read and write gaps. Even if the phase lag corresponds to 1/16 of a period, for 305 cm/sec (120 IPS) and 12.5 Hz, the error will be noticeable. If at the same time it so happens that the actual TTM has its maximum peak at this frequency at 305 cm/sec (120 IPS), this will account for most of the dynamic tracking error. The mathematical model for the error amplitude A err can be represented as follows: A err (k,x,Φ) = A TTM sin(kx) - A TTM sin(kx-Φ) A err (k,x,Φ) = A TTM sin (kx) (1-cosΦ) + A TTM cos(kx)(sinΦ). The maximum error occurs when the derivative of the error is 0, i.e., d/dx(A err (k,x,Φ)) = 0, which means d/dx(A err (k,x,Φ)) = kA TTM (1-cosΦ)cos(kx)-kA TTM sin(kx)sin(Φ) = 0. The product kx max = arctan((1-cosΦ)/sinΦ)), and the maximum error can be found by inserting this value for kx max into the equation for A err (k,x,Φ). As an example for applying the above model, Figure 2 shows a time-domain waveform for a typical TTM, with the vertical scale being in micrometers and the horizontal scale in seconds. Figure 3 shows the corresponding fast Fourier transform F x of this waveform, designated f x in Figure 3. As can be seen from Figure 3, the 26 Hz component has a peak amplitude of about 11.5 micrometers. For 26 Hz and a tape speed of 305 cm/sec (120 IPS), a written wavelength is 4.615 inches and 1/3 inch read head lag corresponds to a phase lag of 26°. Inserting these values into the formula for kx max yields 13° as the phase position at which the maximum error occurs. This is as expected, because the maximum error always occurs at half the phase error angle. Inserting 13° and 26° into the formula for A err (k,x,Φ) shows: A err (k,x,Φ) = (11.5µm)(sin13°)(1-cos26°) + (11.5µm)cos13°sin26° = 5.17µm. One can thus see that even for a small amplitude of 11.5µm and a phase lag of 26°, tracking error can arise which may be large enough to cause mistracking. Each write assembly and read assembly is formed by a gap and coil arrangement, are shown in the exemplary embodiments as being constructed using thin-film magnetic technology. In the embodiment of Figures 4 and 5, the write assembly is generally referenced 15, and has a center line 13 extending substantially parallel to a direction of tape transport, and a width 8 extending substantially perpendicular to the center line 13. The read arrangement is generally referenced 16, and has a center line 14, substantially parallel to the center line 13, and a width 9. The width 8 of the write arrangement 15 is shown slightly larger than the width 9 of the read arrangement, as is typical for conventional head assemblies. The center lines 13 and 14 are spaced from each other by a distance 11 which is equal to the specified spacing between the respective center lines of two adjacent data tracks on a tape to be used with the head assembly. Data is transferred to and from the tape using the head assembly in successively recorded tracks. In the arrangement shown in Figure 4, data will be written on one track by the write arrangement 15 while the data previously written on the immediately preceding track is being verified by the read arrangement 16. Since the write arrangement 15 is assumed to be writing on the n th track, this type of arrangement is known in conventional assemblies as an "n-1" verification system. The write arrangement 15 and the read arrangement 16 each consist of a gap and coil arrangement constructed using thin-film technology with the two arrangements being disposed back-to-back separated by a shielding or screen layer 17, which is connected to ground to minimize or substantially prevent crosstalk and other interference between the arrangements 15 and 16. Each of the arrangements 16, therefore, consists of the same sequence of layered elements, arranged mirrored relative to the shield 9. For this reason, the identical layers in each arrangement have been provided with the same reference numerals. The arrangements 15 and 16 are built on separate substrates referenced 1. The substrates 1, as noted above, are separated by a shielding layer 17, which consist of a copper deposit or copper alloy. The substrates 1 may consist of alumina (Al 2 O 3 ), crystallized glass, or ferrite materials. It is also possible for the read head arrangement 15 to be of a magnetoresistive design, although this is not shown in the drawings. A layer 2 of spinned-on glass or silicon dioxide is disposed on the substrate 1 in each arrangement. Respective portions of the layer 2, forming the read head arrangement 16 and the write head arrangement 15, are covered by a magnetic film 3 of any known type. A layer 4 of silicon dioxide also is disposed on the layer 2, and also covers a portion of the magnetic film 3. The part of the layer 4 which covers the magnetic film 3 in each arrangement forms the gap of the respective arrangement, the write gap being referenced 12 and the read gap being referenced 10. Each gap is covered by an overlying extension of the magnetic film 3, spaced therefrom. In the spacing between the overlying magnetic film 3 and the portion of the layers 4 forming the respective gaps 10 and 12, one or more thin-film coils 5 are present. In the embodiment of Figures 4 and 5, only one such thin-film coil 5 is shown, however, a plurality of such coils may be used. The volume surrounding the thin-film coil 5 between the portion of the layer 4 and the overlying magnetic film 3 is filled with a filler material 6, such as alumina. Since the overlying portion of the magnetic film 3 is raised slightly higher from the remainder of the magnetic film 3, this height difference must be filled by filler material 6. Top and bottom substrates 7 are then applied. Although the same filler material 6 is shown both between the magnetic film 3 and the top and bottom substrates 7 and surrounding the thin film coil 5 within the magnetic film 3, it is possible that a different type of filler material may be used to surround the thin-film coil 5 to enhance or adjust the magnetic properties, depending on particular design specifications. The left-most edge of the gap 12 of the write arrangement 15 and the right-most edge of the gap 10 of the read arrangement 16 (in the orientation shown in Figure 1) are separated by a spacing D RW . This spacing D RW is less than or equal to the correlation distance D corr . As also noted above, the correlation distance D corr is typically in the range of about 2 millimeters to about 3 millimeters, and therefore the spacing D RW will less than or equal to the distances in that range. The embodiment shown in Figures 6 and 7 is for conducting verification on the same data track on which data is currently being written. Therefore, the read arrangement 16 follows the write arrangement 15 on the same center line 13. The only other departure from the embodiment shown in Figures 4 and 5 is that in the embodiment of Figures 6 and 7 the read arrangement 16 has a width 9a which is equal to the width 8 of the write arrangement 15, rather than being less than the width 8. Again, the respective facing edges of the gaps 10 and 12 are spaced a distance D RW from each other, which is less than or equal to D corr . In both of the embodiments of Figures 4 and 5, and 6 and 7, verification should take place in the same direction as the direction the track was moving when the data thereon was previously written. In most systems in use today, writing and reading of data can take place in both forward and reverse directions of tape movement, and therefore in such systems two arrangements of the type shown in Figures 4 and 5, or two arrangements of the type shown in Figures 6 and 7, will be used, with one arrangement for the lower half of the tape (forward) and one for the upper half of the tape (reverse), for example.

Claims: A magnetic head assembly for use in a tape drive wherein a tape is moved past said assembly in a longitudinal direction and the tape exhibits transverse tape movements correlated with the position of said tape in said longitudinal direction, with a minimal longitudinal distance for which the correlation exists, said minimal longitudinal distance corresponding to the distance between a reference position on said tape and the position along the tape corresponding to the next maximum of the correlation of said transverse tape movement, said magnetic head assembly comprising: a carrier; first means disposed on said carrier for transferring data relative to said tape including a first gap (10) and a first coil (5); second means disposed on said carrier for transferring data relative to said tape including a second gap (12) and a second coil (5); characterized in that said first and second gaps (10, 12) being disposed at a spacing from each other on said carrier in said longitudinal direction which is substantially equal to said minimal longitudinal distance. A magnetic head assembly as claimed in claim 1 wherein said spacing is about 3 mm. A magnetic head assembly as claimed in claim 1 wherein said spacing is about 2 mm. A magnetic head assembly as claimed in claim 1 for use with a tape on which a plurality of data tracks are successively disposed side-by-side in a direction perpendicular to said longitudinal direction, each data track having a center line, and wherein said first means for transferring data has a center line (13) substantially coinciding with a center line of a first data track and wherein said second means for transferring data has a center line (14) substantially coinciding with a center line of a data track immediately adjacent said first data track. A magnetic head assembly as claimed in claim 1 for use with a tape on which a plurality of data tracks are successively disposed side-by-side in a direction perpendicular to said longitudinal direction, each data track having a center line, and wherein said first means for transferring data has a center line (13) substantially coinciding with a center line of a first data track and wherein said second means for transferring data has a center line substantially coinciding with a center line of a data track with said center line of said first data track. A magnetic head assembly as claimed in claim 1 for use with a tape on which a plurality of data tracks are successively disposed side-by-side in a direction perpendicular to said longitudinal direction, each data track having a center line, and wherein said first means for transferring data has a center line (13) substantially coinciding with a center line of a first data track and wherein said second means for transferring data has a center line (14) substantially coinciding with a center line of a data track spaced from said first data track with one data track therebetween. A magnetic head assembly as claimed in one of the preceding claims wherein said first and second means are thin-film means, and said first and second coils are thin-film coils (5); and shielding means (17) are provided for preventing cross talk between said first and second means for transferring data disposed between and connecting said first and second means for transferring data, said shielding means extending in a direction perpendicular to said longitudinal direction. A magnetic head assembly as claimed in claim 7 wherein said shielding means (17) consists of a copper layer. A magnetic head assembly as claimed in claim 7 wherein said shielding means (17) consists of a copper alloy layer. A magnetic head assembly as claimed in one of the preceding claims wherein said first and second means are thin-film means, and said first and second coils are thin-film coils (5); and shielding means (18) are provided for preventing cross talk between said first and second means for transferring data disposed between and connecting said first and second means for transferring data, said shielding means extending in a direction parallel to said longitudinal direction.

Inventors: PAHR OLAF, PAHR, OLAF
Applicants: TANDBERG DATA, TANDBERG DATA ASA
Application year: 1993
Main classification: G11B5
IPC codes: G11B5