Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1999-09-13
2002-05-21
Hudspeth, David (Department: 2753)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S077030
Reexamination Certificate
active
06392834
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to disc drive data storage systems. More particularly, the present invention relates to compensation for errors in servo systems.
BACKGROUND OF THE INVENTION
Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position. A multi-rate servo system samples the position of the head relative to a particular track at a particular sampling rate and adjusts the position of the head at a rate that is a multiple of the sampling rate by estimating the position of the head between the measured position samples.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of error that prevents circular paths is known as a track following error. Track following errors arise as a head attempts to follow the path defined by the servo fields. The track following errors can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system is unable to respond fast enough to high frequency changes in the path defined by the servo fields.
Written-in errors are often referred to as repeatable run-out errors because they cause the same errors each time the head passes along a track. As track densities increase, these repeatable run-out errors begin to limit the track pitch. Specifically, variations between the ideal track path and the actual track path created by the servo fields can result in an inner track path that interferes with an outer track path. This is especially acute when a first written-in error causes a head to be outside of an inner track's ideal circular path and a second written-in error causes the head to be inside of an outer track's ideal circular path. To avoid limitations on the track pitch, a system is needed to compensate for repeatable run-out errors.
The present invention provides a solution to this and other problems and offers other advantages over the prior art.
SUMMARY OF THE INVENTION
The present invention relates to compensating for written-in repeatable run-out in disc drives.
One embodiment of the present invention is directed to a method of compensating for written-in repeatable run-out in a disc drive. A written-in repeatable run-out compensation value is stored in a servo field residing on a track. The compensation value is subtracted from a corresponding servo value to obtain a compensated servo value. Then the head is positioned relative to the track based upon the compensated servo value.
In one embodiment of the present invention, the servo value from which the written-in repeatable run-out compensation value is subtracted is the position error value.
Further, in one embodiment, the written-in repeatable run-out compensation values are determined by first rotating the disc about its spindle axis. The head is then positioned over the intended center of the track so that the head follows an essentially circular path relative to the disc surface. For each revolution of the disc, a servo value indicating the position of the head relative to the track at the servo field is determined. This step of determining a servo value is repeated over a plurality of revolutions and the servo values obtained are averaged in order to extract a non-repeatable portion of the run-out, resulting in a written-in repeatable run-out compensation value.
Another embodiment of the present invention is directed to a disc drive having a servo loop, a disc, a head, a servo controller, an actuator and a compensation circuit. The disc is capable of storing data on a track. The track includes servo fields capable of storing servo information used to indicate the position of a head relative to the track. The servo fields are also capable of storing a written-in repeatable run-out compensation value indicative of an amount of written-in repeatable run-out of the track at a given portion of the track. The head is capable of sensing the servo information located on the disc and of producing a servo position signal therefrom. The servo position signal is combined with a reference signal to produce a position error signal indicative of the difference between the actual and desired positions of the head relative to the track. The servo controller is capable of generating a servo control signal in response to a received position error signal. The actuator is coupled to the servo controller and is capable of moving the head in response to the servo control signal. The compensation circuit is capable of retrieving a written-in repeatable run-out compensation value stored in a servo field of the track and providing the compensation value to a subtractor. The subtractor is capable of subtracting the written-in repeatable run-out compensation value from a servo signal in the servo loop.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.
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Hudspeth David
Kelly Joseph R.
Seagate Technology LLC
Westman Champlin & Kelly P.A.
Wong K.
LandOfFree
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