Conventional disk drives can use a rotary voice coil motor (VCM) actuator for positioning the read/write heads. A servo control system receives servo positioning information read by the read/write heads from the data tracks, typically from equally-angularly-spaced servo sectors that extend generally radially across the tracks. The servo control system generates control signals to the VCM to maintain the read/write heads on track and move them to the desired tracks for reading and writing of data. The servo sectors also contain servo timing marks (STMs) that indicate the start of the servo sectors.
In some implementations, a set of spirals can be written on a blank disk. Timing and positioning information can be derived from the spirals for use in controlling the read/write heads to write servo sectors. The servo sectors can form a band of servo tracks. The servo tracks can be used to accelerate the read/write heads to a target velocity to write another set of spirals. The process of alternating between writing servo tracks and writing spirals can be repeated to write additional servo tracks on the blank disk. Additional details of this self-servo write (SSW) process can be found in U.S. Pat. No. 7,209,312, entitled “Self-servo-write Using Ramp-tracks”, which is incorporated by reference herein for all purposes.
When the read/write heads are positioned above a blank portion of a disk, servo sector timing marks are not available as a feedback source, for example, for regulating variations in target velocities of the VCM actuator. A back-electromotive-force (BEMF), from voice coils of a VCM, for example, may serve as a feedback source. However, the BEMF may include noise and be at a low resolution, so the BEMF may not be an ideal feedback source. Accordingly, while the servo tracks are being used to accelerate the VCM to the target velocity, an open loop control system may be used. For example, moving the read/write heads at a constant velocity and moving the read/write heads back to a starting track can be performed using the open loop control system.
As a VCM moves, torques on the VCM structure (e.g., cable bias, windage, head friction) may affect the momentum of the VCM, thereby creating variability in the velocity of the VCM.