Electrical computers and digital processing systems: memory – Address formation – Address mapping
Reexamination Certificate
2001-10-04
2003-12-30
Sparks, Donald (Department: 2187)
Electrical computers and digital processing systems: memory
Address formation
Address mapping
C360S075000, C360S076000, C360S077020
Reexamination Certificate
active
06671790
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to a method of optimizing reader to writer offset compensation for a disc drive.
BACKGROUND
Disc drives are used for data storage in modem electronic products ranging from digital cameras to computer systems and networks. A typical disc drive includes a head-disc assembly (HDA), housing the mechanical portion of the drive, and a printed circuit board assembly (PCB) attached to the HDA, for controlling operations of the HDA and providing a communication link between the disc drive and its host.
Typically, an HDA comprises a recording surface affixed to a spindle motor assembly for rotation at a constant speed and an actuator assembly positionably controlled by a closed loop servo system. The actuator assembly supports a read/write head that traverse a plurality of radially spaced, generally concentric magnetic tracks on the recording surface. Disc drives using magneto resistive read/write heads typically use an inductive element, or writer, of the read/write head to write data to the magnetic tracks and a magneto resistive element, or reader, to read data from the magnetic tracks during drive operations. One type of data recorded to and read from the magnetic tracks is servo data. Servo data, including a physical track identification portion (also referred to as a servo track number or physical track number), written to the recording surface define each specific physical track of a number of physical tracks written on the recording surface. A servo track writer typically writes a predetermined number of physical tracks, also referred to as servo tracks, to each recording surface during the manufacturing process. The physical tracks are used by the closed loop servo system for controlling the position of the read/write head relative to the recording surface during disc drive operations.
High performance disc drives of the present generation typically achieve aerial bit densities measured in several gigabits per square centimeter, Gbits/cm
2
. Higher recording densities can be achieved by increasing the number of bits stored along each magnetic track or bits per inch (BPI), and/or by increasing the number of magnetic tracks provided per unit width or tracks per inch (TPI) across each recording surface. Increased BPI generally requires improvements in the read/write channel electronics to enable the data to be written (and subsequently read) at a correspondingly higher frequency.
Higher TPI generally requires improvements in servo control systems to enable the read/write heads to be more precisely positioned relative to the magnetic tracks. With increasing TPIs, separation between the reader and the writer of a fixed geometry MR read/write head, expressed as a number of magnetic tracks separating the two, increases at the same rate as the growth in TPI. As widths of the magnetic track decrease, the need to more precisely position the reader of the read/write head at track center of the magnetic track being read escalates. The heightened need to more precisely position the reader arises because the amplitude of the read signal drops off more sharply with each increment of lateral separation between the read element tracking at track center and data written off track center. The term track center is defined as the center of the servo data recorded on the magnetic track being read.
Because the reader is both laterally and longitudinally offset from the writer, and because the read/write head scribes an arch across the plurality of generally concentric magnetic tracks of the recording surface, the relative position of the reader in relation to track center of the magnetic tracks changes with the physical location of the read/write head relative to the recording surface. Near an outer diameter of the recording surface, the offset of the writer relative to track center of a magnetic track near the outer diameter of the recording surface and selected for writing data to, is at a first maximum. While near the inner diameter of the recording surface, the offset of the writer relative to track center of a magnetic track selected for writing data to, is at a second maximum. Near the center of the recording surface, the offset of the writer relative to track center of a magnetic track selected for writing data to, is substantially zero. The direction of offset of the second maximum offset from its track center is opposite from the direction of offset of the first maximum offset from its track center. Depending on the direction of offset, the compensation applied to the position signal during track following will be added as either a positive offset or a negative offset.
Typically, a plurality of logical tracks, each including a logical track identification portion (also referred to as a data track number or logical track number) is assigned and recorded onto the recording surface during a drive testing portion of the manufacturing process. Each logical track is associated with one of the physical tracks. During the servo write process, the physical location of each of the plurality of physical tracks, i.e., track spacing for the magnetic tracks and the physical track identification portions are defined and assigned to each magnetic track of the recording surface. Generally, a data track number does not correspond in value to a servo track number assigned the magnetic track. For example, data track “0” may be assigned, positioned and logical block addresses written to a magnetic track that coincides with servo track “1450” written to the recording surface during the servo write process.
Once each logical track has been assigned to one of the plurality of physical tracks, either a look-up table linking the relationship between a value of the logical track identification portion and a value of its associated physical track identification portion is provided, or a value to be used in translating between the value of the logical track identification portion and the value of its associated physical track identification portion is provided. During disc drive operations either the look-up table, the value or a conversion algorithm is used by a controller of the disc drive to convert from a requested logical track number to a corresponding servo track number, which the servo system uses to position the read/write head relative to the requested logical track.
During the process of assigning logical track numbers to each magnetic track, a calibration process is executed on selected logical tracks to determine an amount of compensation to be added by the servo system to position the writer to track center of the servo track associated with the selected logical track. Aligning maximum amplitude of the data being written to the selected logical track with track center of the servo track associated with selected logical track facilitates maximum recovery of the data during readback operation of the disc drive. The value of the compensation, measured in steps of the actuator, varies across the recording surface as a function of the diameter of the recording surface and an angle of presentation of the reader to the recording surface, which changes from the OD to the ID, across the recording surface.
The calibration process entails servoing on a physical track associated with a selected logical track; writing data to the selected logical track; stepping the reader over a half track to one side of the physical track and recording an amplitude of the data written to the logical track at that position; stepping the reader over a half track to the other side of the physical track and recording an amplitude of the data written logical track at that position; then collecting and analyzing amplitude measurements between those two positions to determine a maximum amplitude for the data written to the logical track; and then recording the number of steps and direction from track center needed to compensate the offset into a compensation table, also referred to as a
Gay Sam Alfredo
Young John Edward
Fellers , Snider, et al.
Seagate Technology LLC
Sparks Donald
Takeguchi Kathy
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