Bi-directional servo track writing to minimize sidewall writing at high skew angles

#The present invention relates generally to disc drives. In particular, the present invention relates to servo track writing.

#The typical disc drive in a computer has at least one disc that stores information. Referring to FIG. 1, a disc 100 is shown that has an associated actuator 110. At the distal end of actuator 110 is a head 120. The information is written and read in concentric tracks—one is designated 130—on the disc by head 120. The disc drive then must be able to follow each data track to read and write the information. To assist in such data track follow, servo information is written on each data track at intervals. The servo information is used by the disc drive to, among other things, keep the head aligned with the desired data track. The servo information is typically written prior to writing information to the disc.

#Due to the structure of the disc drive, the head azimuth usually has a non-zero azimuth angle with respect to the data track where the servo information is written. This is known as skew. Also due to that structure, the skew changes as the head moves between the inner diameter (ID) and outer diameter (OD). At some point between the ID and OD, the head skew transitions between positive and negative.

#FIG. 1 shows a disc and associated actuator track following a data track on the disc.

#FIG. 2 shows undesired magnetic transition overwriting caused by head skew.

#FIGS. 3A-3C generally shows stitching of servo information.

#While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not to be limited to the specific embodiments described.

#An issue with head skew is that it causes unwanted overwriting of previously written servo information. In general, the skew causes not only the magnetic flux from the leading edge of the writer to create magnetic transitions on the disc, but also the magnetic flux of the sidewall edge of the writer to create magnetic transitions on the disc. In particular, for longitudinal recording, a writer tip of the head is rectangular. Referring to FIG. 2, a writer tip 200 has a leading edge W, a sidewall edge L and a longitudinal axis A. The leading edge W of the trailing pole of the writer 200 writes magnetic transitions 210 on the disc. A line T represents the center of the servo track portion that is being written. Line T is tangent to the radial position at that point. The angle between line T and axis A defines the skew angle. The effect of the sidewall edge L—shown by 220—is proportional to the skew angle and the dimension of the sidewall edge L. In perpendicular recording, the writer tip of the head has a square footprint, i.e. the sidewall edge is shorter than that of the longitudinal writer sidewall edge L. This writer dimension change combined with the properties of perpendicular recording makes the undesired sidewall writing more severe in perpendicular recording.

#The sidewall writing described above is detrimental to the servo tracks. When servo tracks are written, at least two writing passes are typically used to write one data track. The stitching between two writing passes is very vulnerable to the sidewall writing. FIGS. 3A-C demonstrate the servo track writing (STW) process and the stitching. In FIG. 3A, a first pass for servo track writing on data track n is performed. In FIG. 3B, a second pass for servo track writing on data track n is performed. FIG. 3C shows a third pass, which is the start of servo track writing for data track n+1. The stitching shown in FIG. 3C shows how the first two passes (for data track n) abut.