Dynamic magnetic information storage or retrieval – Monitoring or testing the progress of recording
Reexamination Certificate
1999-05-18
2001-07-24
Neal, Regina Y. (Department: 2651)
Dynamic magnetic information storage or retrieval
Monitoring or testing the progress of recording
C360S046000, C360S053000
Reexamination Certificate
active
06266199
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to disk drives, and in particular to a method to characterize and limit the effect of disk damage due to head/disk contact in a hard disk drive.
2. Description of Related Art
Moving magnetic storage devices, especially magnetic disk drives, are the storage devices of choice. This is due to their expanded non-volatile memory storage capability combined with a relatively low cost.
Magnetic disk drives are information storage devices which utilize at least one rotatable magnetic media disk having concentric data tracks defined for storing data, a magnetic recording head or transducer for reading data from and/or writing data to the various data tracks, a slider for supporting the transducer in proximity to the data tracks typically in a flying mode above the storage media, a suspension assembly for resiliently supporting the slider and the transducer over the data tracks, and a positioning actuator coupled to the transducer/slider/suspension combination for moving the across the media to the desired data track and maintaining the transducer over the data track center line during a read or a write operation. The transducer is attached to or is formed integrally with the slider which supports the transducer above the data surface of the storage disk by a cushion of air, referred to as an air-bearing, generated by the rotating disk.
The need for higher data density on magnetic disks has imposed a requirement to read and write more data on narrower tracks located on the disk. The achievement of higher data density requires increasingly narrower transducer gaps and increasingly less spacing or clearance, commonly called flying height, between the magnetic transducer and the disk recording surface. In normal operation of the systems designed to reliably record and read data at the higher data density, a magnetic head slider flies on an air bearing about 30 nanometers away from the disk surface. Should the head slider develop abnormal flying characteristics, it can contact the disk surface causing damage to both the head and disk which may lead to damage which results in the loss of stored data.
Various methods are found in the prior art for detecting the high probability of failure as described above. For example, the prior art describes methods and apparatus to measure the flying height of the magnetic heads and providing a warning for taking corrective action when the measurement of flying height indicates an imminent failure condition. IBM's U.S. Pat. No. 4,777,544 granted to Brown et al. describes a harmonic ratio flyheight technique for calculating head/disk spacing. IBM's U.S. Pat. No. 5,410,439 granted to Egbert et al. describes a method and apparatus for measuring head/disk clearances in a disk drive and providing warning of impending failure caused by a head/disk “crash”.
Other methods for detecting impending failure include methods of monitoring the readback signal amplitude of heads in a disk drive and comparing with a preset amplitude to monitor disk magnetic coating degradation. These methods assume that loss of readback signal amplitude is associated with disk surface physical damage (for example, abrasive wear damage) caused by the slider contacting the disk and data is either rewritten at a different location or the disk drive is shutdown before a crash occurs.
However, in some situations readback signal degradation may be due to thermal effects caused by head/disk contact or to accumulated effects of small stresses from stray magnetic fields, etc. In these situations, the prior art approaches are not applicable because readback signal degradation is not associated with physical damage of the head/disk interface and the integrity of the disk drive is not at risk.
It therefore can be seen that there is a need for a method and apparatus for monitoring the readback signal quality and for determining whether signal degradation is due to disk magnetic layer damage or whether signal quality can be restored without compromising data integrity.
SUMMARY OF THE INVENTION
In order to address the problems found in the prior art as described above, it is an object of the present invention to disclose a method and apparatus to monitor a readback signal amplitude of a magnetic read transducer in a magnetic disk drive.
It is another object of the present invention to disclose a method and apparatus to monitor a readback signal amplitude by monitoring the amplification level of an automatic gain control (AGC) circuit in a read channel.
It is a further object of the present invention to disclose a method of analyzing readback signal amplitude degradation to determine whether or not permanent surface damage to the disk has occurred.
In accordance with the principles of the present invention, there is disclosed a method of monitoring the quality of the readback signal amplitude from a magnetic read transducer and of analyzing data tracks on disk surfaces showing signal degradation by refreshing the data and reexamining the quality of the rewritten data. In the preferred embodiment of the invention, the amplitude of a readback signal is monitored by continuously monitoring the level of an automatic gain control (AGC) signal during readback operations. The AGC is a circuit in the signal processor that provides control of the signal amplification in order to maintain a nearly constant signal amplitude to the data channel electronic circuitry. A decrease of the readback signal amplitude results in an increase of the level of the AGC signal and increased amplifier gain in the signal processor circuit.
With commonly used Al—Mg alloy disk substrates, degradation of the readback signal amplitude usually results from physical damage induced by slider contact with the magnetic coating on the disk surface. With the harder glass substrates being used in many disk files, slider contact with the magnetic coating on the disk surface does not always result in the severe physical damage observed with Al—Mg substrates. However, because of the poor thermal conductivity of the glass substrates, head/disk contact may cause severe local heating of the magnetic coating on the disk resulting in readback signal degradation as temperatures approach the Curie temperature of the magnetic coating. Signal degradation by this thermal mechanism is very different from signal degradation by a wear or other physical damage mechanism since the thermal mechanism does not result in permanent damage to the magnetic coating. Therefore, when dealing with glass substrates or other substrates having similar thermal properties, it is critical to determine the probable mechanism of signal degradation before taking remedial action to relocate or to rewrite the affected data.
In the preferred embodiment of the present invention, when the level of the AGC signal in the signal processing channel exceeds a first predetermined level representing a signal amplitude for a critically degraded readback signal, the location on the data track of the degraded signal amplitude data is flagged. The segment of flagged data is then rewritten by the write head on the same location on the flagged segment of the data track. A second read operation is then performed on the newly rewritten data in the flagged location. The level of the AGC signal is again monitored during the second read operation and is compared with a second predetermined level representing a signal amplitude for an acceptable readback signal amplitude. If the level of the AGC signal is smaller than the second predetermined level, the flagged location of the disk surface is judged to be undamaged (readback signal amplitude fully recovered by the rewrite operation) and the rewritten data is left at this location. If, on the other hand, the level of the AGC signal is greater than the second predetermined value, the flagged location of the disk surface is judged to be damaged (readback signal amplitude not fully recovered by the rewrite operation) and the data is moved to a new location and t
Gillis Donald Ray
Kroeker Richard Mark
Suk Mike
Wolter Reinhard Ferdinand
International Business Machines - Corporation
Krall Noreen A.
Martin Robert B.
Neal Regina Y.
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