A typical disc drive comprises a plurality of rigid magnetic storage discs which are axially aligned and arranged about a spindle motor for rotation at a constant high speed (such as around 10,000 revolutions per minute). An array of read/write heads are provided to transfer data between tracks of the discs and a host computer in which the disc drive is mounted. The heads are mounted to a rotary actuator assembly and are controllably positioned adjacent the tracks by a closed loop servo control system.
The servo control system primarily operates in one of two selectable modes: seeking and track following. A seek operation entails moving a selected head from an initial track to a destination track on the associated disc surface through the initial acceleration and subsequent deceleration of the head away from the initial track and toward the destination track. A velocity control approach is used whereby the velocity of the head is repeatedly measured and compared to a velocity profile defining a desired velocity trajectory for the seek. Once the head has settled on the destination track, the servo system enters a track following mode of operation wherein the head is caused to follow the destination track until the next seek operation is performed.
Both track seeking and track following operations typically require generation of a position error signal (PES) which gives an indication of the radial position of the head with respect to the tracks on the disc. In high performance disc drives, the PES is derived from either a prerecorded servo disc with a corresponding servo head (a dedicated servo system), or from servo information that is embedded on each recording surface among user data blocks at predetermined intervals (an embedded servo system).
The head provides the servo information to the servo system which generates the PES with a magnitude that is typically equal to zero when the head is positioned over the center of the track (“on track”), and is nominally linearly proportional to a relative off-track distance between the head and the center of the track.
The track center for reading and writing is generally defined by servo bursts patterns or fields that are read by the read head as a read back signal. The burst patterns typically comprise a Null Pattern that causes amplitude components of the PES to approach zero amplitude when the head is positioned between two tracks. At such low levels, noise in the system can dominate the PES components thereby reducing the quality of the PES components. As a result, position information that is obtained using such low quality PES components may be unreliable.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.