Commercial magnetic disk drive areal densities now exceed 300 Gbits/sq. in., and track densities can be greater than 250,000 tracks per inch. Advanced servo techniques are required to further improve disk drive performance. At the needed track densities, the heads must stay centered on the narrow tracks to within a tolerance on the order of a few nanometers as the disk rotates under the heads at thousands of rpm. The servo fields, which encode positional information, are permanently written onto the disk during the manufacturing process. The servo information is processed by an electronics control system that adjusts the position of the actuator on which the heads are mounted.
The process by which the position information is written onto the disks is referred to as servowriting. Conventionally servowriting is performed by a dedicated device called servowriter that is distinct from the disk drive itself. This method of servowriting is performed in a clean room and uses sensors that are inserted into the head disk assembly to provide the precise angular and radial position information to write the servo patterns. Another approach is have the disk drive heads write the servo information, i.e., to perform self-servowriting.
One technique for improving areal densities is to alter the physical organization of the magnetic thin films on the disks by forming a pattern in the films. Conventional magnetic disks have continuous thin films in which the magnetic transitions are recorded. Discrete track media have been proposed in which the tracks are formed from continuous strips or rings of magnetic material separated by small gaps where the magnetic material has been removed. More recently bit-patterned media (BPM) have been proposed that use nanometer scale magnetic islands or dots arranged in tracks on the recording surface. Discrete track media and bit-patterned media place constraints on servo pattern design, in that any pre-patterned features need to be compatible with appropriate planarization methods. In general, this planarization constraint requires that all pre-patterned features are constructed of grooves and lands with common dimensions (i.e., servo patterns have the same land and groove dimensions as data tracks). The planarization constraint allows grooves to vary in orientation and absolute position, provided land and groove dimensions are generally fixed.
The servo data on the disk provides several fundamental functions and is conventionally arranged in four distinct fields that are disposed in sequence in each servo sector along the direction of the track. First, it supplies a timing mark (known as the Servo Track Mark (STM) or Servo Address Mark (SAM)) which is used to synchronize data within the servo fields, and also provides timing information for write and read operations in the data portions of the disk. Second, the servo area supplies a 10-30 bit digital field, which provides a integer track-ID (TID) number and additional information to identify the physical servo sector number. The TID is typically written in Gray code as the presence or absence of recorded dibits. During seek operations, when the head is moving across tracks, the head can typically only read a portion of the Gray-code in each TID. The Gray-code is constructed so that pieces of the TID, in effect, can be combined from adjacent tracks to give an approximate track location during a seek. The servo field also includes a position error field, which provides the fractional-track Position Error Signal (PES). Auxiliary functions, such as amplitude measurement or repeatable run-out (RRO) fields are sometimes also used. During read or write operations the drive's servo control system uses the PES servo information recorded on the disk surface as feedback to maintain the head in a generally centered position over the target data track. The typical PES pattern is called a quad-burst pattern in which the bursts are identical sets of high frequency magnetic flux transitions. Unlike the track-ID (TID) field number, the PES bursts do not encode numerical information. In contrast to the TID, it is the position of the bursts that provide information on where the head is relative to the centerline of a tracks. The quad-burst pattern is repeated for each set of two tracks, so only local information is provided. Each servo wedge has four (A,B,C,D) sequential slots reserved for PES bursts. Each track has a centered PES burst in only one of the four slots. Thus, when the head is centered over a selected track, it will detect the strongest signal from a burst centered on the selected track, but it will also detect a weaker signal from bursts on the adjacent tracks. For example, when the head is centered over a track with a burst in the A-position, it might also detect a subsequent weak B-burst on the adjacent track on the right and then a weak D-burst from the adjacent track on the left. When the head passes over the PES pattern, the bursts that are within range generate an analog signal (waveform) that indicates the position of the head over the disk and is used as feedback to adjust the position of the head.
The overhead on the disk to support these functions is a large factor in the drive's format efficiency. Typically, the servo fields can consume between 5% and 10% of the recording surface of the disk. As areal density gains in the magnetic and data signal processing components become harder and harder to achieve, the servo overhead becomes a more and more attractive target for reduction, and relief of necessary areal density targets to achieve particular HDD capacity points.
Published US patent application 20090097160 by Yamamoto (Apr. 16, 2009) describes a magnetic recording disk with pre-patterned servo sectors, wherein each data bit is stored in a magnetically isolated data island on the disk. The servo sectors include a synchronization pattern of generally radially directed discrete magnetized marks, and first and second position error signal (PES) fields of generally radially directed discrete magnetized stripes. Each stripe in each of the two fields comprises a plurality of radially spaced discrete servo islands, each island having a radial width of approximately Tp, where Tp is the radial spacing of the track centerlines. In each field, the servo islands in alternating stripes in the along-the-track direction are shifted radially by approximately Tp. In the first PES field, the islands are centered at the midline between two adjacent track centerlines, and in the second PES field the islands are centered at a track centerline. All of the servo islands in the two PES fields have the same magnetization direction.
U.S. Pat. No. 7,656,605 to Sutardja (Feb. 2, 2010) describes a method for repeatable run-out (RRO) compensation in a disk drive using self-servo writing. RRO is described as an error signal that is repeated with each rotation of the disk, but Sutardja does not specify how the error signal is obtained for disk prior to servo-writing. Sutardja's RRO compensation system processes residual error metrics through a plant model of a control system to generate updates to an RRO compensation table. Sutardja states that when the plant model and the repeatable component are known exactly, the lookup table stores the correct value of the RRO error after a single update step. When the plant model is not known exactly, the repeatable component is nonetheless reduced after each iteration and the algorithm is convergent. The RRO lookup table is created before self-servo write operations and is used during self-servo write operations to compensate for the RRO so that the final written servo tracks include minimal RRO effects.
Published US patent application 20080239906 by Akagi et al. (Oct. 2, 2008) describes self-servo writing (SSW) on the servo region formed from a flat section of a discrete track medium. Pre-patterned radial grooves on the disk are used as SSW timing detection patterns. The radial grooves are formed simultaneously with the grooves that separate the discrete tracks. Repeatable run-out (RRO) error signals used for positioning control along the radius are written in the servo information on a flat section of the disk. A servo writing method for correcting eccentricity between the pattern forming section (data track) and the flat section (servo track) is implemented by the following sequence: (1) Self-servo writing is first performed according to the timing pattern. (2) Data is recorded on the data region at a single frequency while following the servo pattern written by SSW. (3) A read-back waveform is acquired from the single frequency recording pattern just recorded. The read waveform is modulated by the read head cutting across the groove on the discrete track. This modulation envelope is made up of a 1st order synchronization oscillation (RRO). (4) Analysis of the read waveform envelope gives the RRO value, and the eccentricity quantity calculated in order to cancel out the modulation component in the read waveform. (5) SSW is again performed based on the eccentricity quantity in (4), and a servo track is obtained along the patterned data track. (6) The above steps (1) through (5) are repeated several times as needed to achieve servo write with reduced eccentricity. Akagi describes storing the eccentricity compensation value as an 8 bit RRO field in the servo pattern written on the disk.
Published PCT patent application WO2008105334 by Ono describes a method of detecting eccentricity in a track on a discrete track disk without referencing a servo pattern. The method includes writing data in a predetermined region while moving the head stepwise radially and then reading the data with the head “at rest.”
Published US patent application 20090168229 by Albrecht; et al. describes a method of fabricating servo sectors of a patterned storage media with two arrays of discrete islands in a servo sector. The first array defines at least two burst fields. The second array also defines at least two burst fields. The second array is formed with a track-wise offset (i.e., an offset in the radial direction) from the first array. The offset between the first array and the second array may be about a half track offset, but the amount of offset between the arrays may vary depending on design preferences. Another step of the method comprises performing a servo writing process to define the polarity of the islands in the arrays. The servo writing process is performed by circumferentially writing one or more rows of islands to define the servo pattern in the servo sector. The servo pattern may be burst fields, sync fields, etc. The servo pattern generated by the servo writing process allows a quadrature signal to be generated when a read/write head passes over a track of the patterned storage media.
One way to form servo patterns on a BPM disk is to create readable servo patterns from unipolar magnetized bit lands formed by direct current (DC) magnetization of the entire disk. One characteristic of patterned media is that the circular tracks formed on the disk by thin film processes are generally not centered with the center of rotation of the disk once it is mounted on the spindle. Therefore, the servo system must detect the deviations of the track geometry and adjust the position of slider based on this knowledge in order to accurately follow the track.
One problem with available planarization methods is that relatively large depressions, where magnetic material has been removed, that are included in the servo patterns cannot be filled. For planarization using liquids, for example, the grooves must be small enough for to capillary forces to planarize the liquid before curing. For other planarization techniques, such as deposition and etch-back planarization, the land to groove width ratio should be constant. But prior art servo patterns inherently have varying shapes and sizes that do not fit into this constraint.
Published US patent application 20100033868 by Hattori; et al. describes a method for writing servo patterns that includes using different drive currents for an actuator pushed against a crash stop to define a swing range of the actuator. A write operation is performed using at least one of the plurality of different drive currents to write servo pattern tracks including a plurality of separate servo pattern sectors. The drive current is also gradually changed while pushing the actuator into the crash stop to gradually move a read element in searching for a servo pattern track already written by the write element.