This invention pertains to a servo control system for use in a magnetic disk storage device. More specifically, the invention concerns a servo control system that adaptively corrects dynamic and static alignment errors between a data head and a data track centerline on a data disk in the storage device.
Disk storage devices are used in data processing system for storing relatively large amounts of information which can generally be accessed within milliseconds. Structurally, a typical storage device comprises a rotating magnetizable disk medium having several surfaces, in the form of an asssembly of one or more stacked platters, on which data is magnetically sensed and/or recorded in addressable sectors located on circular data track centerlines. The disk assembly is mounted upon a drive spindle in the storage device that rotates it at a constant speed, about 3600 revolutions per minute. The storage device also includes one or more transducers, or read/write heads, associated with each surface of the disk. The transducers are mounted in spaced relation on an arm of a movable transducer carriage. A servo controller actuates the carriage in a controlled fashion to move all the data heads in unison radially over the disk surfaces thereby to position any one of the data heads over a selected track centerline. Since all of the data heads on the carriage move together, the device also includes control circuitry that selects one of the read/write heads to perform a data transfer operation.
The servo controller responds to commands from the data processing system. The controller does this by transforming these commands into an analog servo signal which ultimately drives, usually through a power amplifier, an electro-mechanical actuator that connects to the transducer carriage.
Typically, the disk device operates in one of two different modes. The first is a "seeking" mode in which the magnitude of the servo signal is used in a controlled fashion to drive the carriage, and thus the selected data head, to travel to the vicinity of a desired circular track centerline; once the data head reaches that vicinity, the system is then switched to a second or "track-following" mode. In the track-following mode, the position of the actuator is controlled to cause the center of the selected data head to align itself with the centerline of the data track. However, even in this mode there exists a finite alignment error between the center of the data head and the selected track centerline. The magnitude of this alignment error places an upper limit on the data track density and thus, on the data storage capacity of the storage device.
To minimize alignment error, servo systems typically employ formatting information prerecorded on the data disk to allow the controller to detect the displacement between the data head and the track centerline. A preferred format might include servo data that is continuously prerecorded along servo tracks on a dedicated surface of the disk assembly ("dedicated" servo data), together with servo data that is prerecorded in circumferentially spaced servo sectors interspersed, or embedded, between adjacent pairs of storage data sectors on a data surface of the disk assembly ("embedded" servo data). Dedicated servo data is read by a read-only servo head, while embedded servo is read along with the data by a read/write head and thereafter separated from the data by servo data processing circuitry.
The servo data from both the dedicated and data surfaces is decoded by the disk controller, thereby enabling it to modify a servo control signal, if necessary, and thus continuously maintain the position of the data head in alignment with a selected data track centerline. Several factors, however, limit the alignment accuracy, and thus the maximum attainable data track density, of a disk storage device. The most common of these factors stem from electrical and mechanical disturbances or noise. D.C. Bias forces and electrical offsets are examples of some disturbances. A most notable mechanical disturbance is spindle "runout", or "wobble", which is the difference between the actual centerline of a track and the effective centerline presented to a head positioned a fixed distance from the mounting center of the disk. It is typically caused by slight eccentricity in the mounting of the disk on its drive spindle. Runout is more prevalent in disk systems using exchangable disk cartridges and results from even the slightest off-center mounting (e.g., a fraction of a thousandth of an inch), as well as from slippage or tilt in seating of the disk cartridge after mounting. Carriage play between the transducer carriage and its guide rods, as well as misalignment due to uneven thermal expansion of the carriage, arms, disk, or transducers, further contribute to the mechanical disturbances. Generally, positioning tolerances should be within .+-.10% maximum of track pitch (e.g., spacing between adjacent track centerlines). Thus, for example, a 1000 track-per-inch servo system should maintain a data head within .+-.100 micro-inches of a data track centerline. With typical currently available exchangeable disk systems, such alignment accuracy is not readily attainable.
Control system lag is another factor that affects positioning accuracy. Lag is the time delay between the time that the controller detects an off-track condition and the time that the actuator begins to move the transducer into alignment with the data track centerline. Some of this delay is attributable to the electrical response characteristics of the servo control system, such as, for example, that resulting from a low sampling rate; the remaining delay is attributable to the mechanical response characteristics of the electromechanical actuator. These delays characterize the "bandwidth" of the servo control system. The greater the bandwidth, the faster the positioning system can respond to an off-track condition thereby providing tightly controlled positioning of the data head. A positioning system having high bandwidth provides increased data track density because centerlines can be followed within a smaller tolerance. There are other factors, as well, that contribute to misalignment during track following operations.
Conventional methods of increasing servo bandwidth include increasing the frequency of structural mechanical resonances, providing continuous position feedback from a dedicated servo surface, and providing a higher sample-rate position feedback emanating from the data surfaces, among others.