Controlling low frequency track shape errors in hard disc drives employing self-propagated servo track writing

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

#In hard disc drives the head positioning servo system requires accurate

#measurement of the location of the recording head relative to the disc surface. This is usually accomplished by writing special servo marks on the surface of the discs during production. Typically, these servo marks are written on a high accuracy servo track writing machine. However, the conventional servo track writing technique has serious limitations. As track density of hard disc drives increases, the servo track writing machine is not able to write the servo marks on the disc surface with the desired accuracy. Therefore, new techniques are being sought to overcome this problem.

#FIG. 1 illustrates a disc and actuator portion of a disc drive.

#FIGS. 2A-2B illustrate servo wedges on a disc and certain information contained therein.

#FIG. 3 illustrates track shape errors that are propagated during self-servo writing.

#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.

#FIG. 1 shows certain portions of a hard disc drive 10. Included in hard disc drive 10 are a disc 100 and actuator 110. At the distal end of actuator 110 is a read/write head 120. Read/write head 120 is used to read and write data to disc 100 in a track format, such as a track 130. In a typical hard disc drive, servo sectors are used to determine the position of the head relative to the track center. The servo sectors are interspersed with data sectors along each track. FIG. 2A shows a disc 200 that is configured with servo track sectors 210.

#Referring to FIG. 2B, servo sectors typically consist of a Grey code field 220 to provide coarse position information (track number), and a servo burst field 230 to provide fine position information. Servo bursts consist of one or more transitions that generate signals in the read transducer (head). This burst pattern is usually referred to as quadrature (or ABCD) burst pattern. The position of the head can be determined accurately by measuring the magnitude of the signal generated by the bursts as the head travels through the pattern.