Background and Summary of the Invention

In current magnetic recording systems, such as hard disc drives, a large portion (as high as 30%) of the recorded medium is used to servo a head on track. Most of this servoing is performed in bursts in which the head is aligned with the track (typically at the beginning of the sector) and then the head travels unservoed for a portion of the disc rotation (e.g. for the rest of the sector). In a typical hard disc drive, the track width is typically 10-20 microns, guard bands of 2-5 microns each are placed between adjacent tracks, and a servo burst of up to 15-20% of the sector is utilized to align the head prior to data recording or retrieval. In floppy discs or diskettes, because of the anisotropic expansion and contraction of the medium, the actual portion of the floppy diskette which stores data or is available for storing recorded data is much less. As can be appre¬ ciated, present prior art servoing techniques require a significant portion of the magnetic recording medium to

be used for purposes other than data storage. This pro¬ vides obvious drawbacks but has been accepted by the prior art as necessary for ensuring the reliable reading and writing of data to and from the magnetic medium. The inventors herein have previously investigated this phenomenon and identified a technique which sought to maintain head alignment by sensing the track edge noise and, with a micro positioner, maintain the position of the head as it moved along a track and through a sec- tor. This technique was disclosed and described in the inventors' prior article entitled Noise Correlation of Magnetic Thin Film Media published in August of 1991 in the Japanese Journal of Applied Physics. Vol. 30, No. 8B, pages L1483-L1485. This investigation was conducted for a thin film medium, as explained more thoroughly in that article. Although strong correlative data were found, and a block diagram for a high performance track follow¬ ing servo was proposed, the technique and method de¬ scribed therein must be extremely sensitive in order to perform as power levels relating to head displacement with respect to track edges is in the micro watt range. At these levels, differences in power readings which must be sensed in order to determine if the head is on or off the track are difficult to discern and, consequently, difficult to implement in the laboratory, much less in commercial application.

To solve these and other problems in the prior art, and as an improvement and change in approach from their prior work, the inventors herein have succeeded in designing and developing a continuously monitored track following servo for maintaining alignment of a magnetic recording head over a track in a magnetic recording medi¬ um by measuring the differential power spectrum between a main recording head and a pair of secondary heads as the heads traverse a data encoded track. The primary head is aligned in the approximate center of the track, and each

of the secondary heads is aligned substantially adjacent one of the track edges. As the head assembly remains on track, there is a close correlation between the power- frequency distribution between the primary head and each of the secondary heads. However, as the head assembly drifts to the left, for example, the power frequency distribution remains closely correlated for the main head and the right side secondary head, but varies signifi¬ cantly, in the range of 200 milliwatts, between the pri- mary head and the left edge secondary head. As can be appreciated, this difference in power levels is several magnitudes greater than with the technique previously developed which relied on sensing track edge noise.

It is anticipated that a head assembly may be manufactured which would provide .continuous track head alignment in the submicron range. This is because once the track width is known, the head assembly may be manu¬ factured to extremely close tolerances using known manu¬ facturing techniques and methods. Once manufactured, the head assembly would be self-centering and adjustments and the like during operation would be anticipated to be minimally required.

The present invention has been demonstrated for use with pseudo-random data, with differential power fre- quency measurements made over an appropriate data length, with the window of sampled data moving as the head assem¬ bly moves through the track. For long sections of track, the inventors expect that the frequency spectra would look similar with respect to the various heads so care must be taken to use an appropriate data length in order to ensure the sensing of head alignment. While the in¬ ventors have not fully investigated to determine the limits of an appropriate data length, it is believed that data lengths between about 10 microns and 100 microns would be appropriate as the window of data to be used for calculating and comparing power frequency spectra. While

this range has been found to be effective, the inventors believe that the range of appropriate data lengths is dependent upon several factors including the magnetically recorded linear bit density and head speed. A run- length-limited code might be used to prevent long runs of contiguous DC tracks and data might be encoded different¬ ly in alternate tracks so that adjacent tracks containing identical data would still possess track edges readily sensed by this differential frequency power spectra tech- nique.

While the principal advantages and features of the present invention have been described above, a more com¬ plete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow. Brief Description of the Drawings

Figure 1 is a schematic diagram of a multiple head assembly with micro positioner and controller for imple¬ menting the track following servo of the present inven- tion; and

Figure 2 is a top view of a head assembly of the present invention aligned over a track between two adja¬ cent tracks. Detailed Description of the Preferred Embodiment As shown in Figure 1, the present invention 20 includes a primary head 22 for reading data encoded on the track of a magnetic medium 24. A secondary head assembly 26, which may be two separate heads (as shown in Figure 2) or two separate portions of a split head as- sembly for sensing the edges of a track on recording medium 24. Micro positioner 28 both mechanically fixes the relative positions of heads 22, 26 and also reposi¬ tions heads 22, 26 in response to the controller 30 which receives and processes the power spectrum measured by heads 22, 26, as is explained in greater detail below.

As shown in Figure 2, the primary head 22 is ideally centered over a track 32 which is defined by a left edge 34 and a right edge 36. Track 32 is surrounded by a track 38 to its left and a track 40 to its right, it being specifically noted that in the present invention there is no need for guard bands or dead spaces between adjacent tracks 32, 38, 40. The secondary head assembly 26 is comprised of a left track edge head 42 and a right track edge head 42. Each of track edge heads 42, 44 sense their respective associated edges 34, 36 and main¬ tain the alignment of primary head 22 as the head assem¬ bly traverses track 32 to read the magnetically encoded data 46.

The operation of the present invention may be readily understood by referring to Figure 2. When the head assembly is on track, both secondary heads 42, 44 produce similar spectral power differences with respect to the primary head 22. Thus, their differential power spectrum is well correlated for appropriate data lengths and the controller does not adjust the position of the head assembly. When the head assembly strays off track to the left by a distance such that left edge head 42 senses magnetically encoded data not only on track 32 but also on track 38, the power spectrum difference read by head 42 with respect to head 22 will change sharply while the power spectrum difference read by head 44 with re¬ spect to head 22 will change only slightly (due to the head sensitivity functions). This difference in power spectrum will be sensed by controller 30 and it will then cause micro positioner 28 to reposition the head back to the right and centered on track 32. Similarly, when the head assembly moves off track to the right, the power difference read by heads 22, 42 will be virtually un¬ changed while the power spectrum differences between heads 22, 44 will change sharply such that controller 30

will readjust through micro positioner 38 the position of the head assembly back onto track 32.

There are various changes and modifications which may be made to the invention as would be apparent to those skilled in the art. However, these changes or modifications are included in the teaching of the disclo¬ sure, and it is intended that the invention be limited only by the scope of the claims appended hereto.