Position Error Signal Generation From User Data Track With a Multiple Reader Array in Shingled Magnetic Recording

Hard disk drive (HDD) systems today have embedded servo wedges spanning the revolution of the disk that periodically provide the head with crosstrack position information as it flies over the servo wedges. The head is subjected to forces such as from turbulence, disk-waviness, shock, and vibration that cause its position to deviate from its desired on-track position. It is the job of the servo controller to use the information in the position error signal (PES), calculated from the readback signal as the head flies over the servo wedges, to reposition the head back on-track. There are a couple of disadvantages of the embedded servo system in today’s HDDs. The first is that the overhead of the servo wedges consumes some of the areal density reducing the amount of storage capacity for the user’s data. Today, that overhead is in the range of 3%–5% of the total disk surface. The second disadvantage is that between servo wedges, the servo control loop has no position information and the head is flying blind. In this paper, we propose an alternative method for the servo controller to obtain the PES it needs to position the head on-track without the need for servo wedges. Our method assumes the usage of a multiple-reader head that, as of today, seems to be a likely upgrade to HDD systems in view of the benefits promised by shingled magnetic recording and 2-D magnetic recording.