Dynamic magnetic information storage or retrieval – Monitoring or testing the progress of recording
Reexamination Certificate
1999-05-26
2001-09-11
Faber, Alan T. (Department: 2651)
Dynamic magnetic information storage or retrieval
Monitoring or testing the progress of recording
C360S075000, C360S078140, C360S065000
Reexamination Certificate
active
06288856
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to data storage systems and, more particularly, to a system and method for estimating head-to-disk clearance in real-time during data storage system operation.
BACKGROUND OF THE INVENTION
A typical data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to write and read magnetic data respectively to and from the medium. A typical disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute (RPM).
Digital information is typically stored in the form of magnetic transitions on a series of concentric, spaced tracks formatted on the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a number of sectors, with each sector comprising a number of information fields, including fields for storing data, and sector identification and synchronization information, for example.
An actuator assembly typically includes a plurality of outwardly extending arms with one or more transducers and slider bodies being mounted on flexible suspensions. The slider body lifts the transducer head off the surface of the disk as the rate of spindle motor rotation increases, and causes the head to hover above the disk on an air bearing produced by high speed disk rotation. The distance between the head and the disk surface, which is typically on the order of 40-100 nanometers (nm), is commonly referred to as head-to-disk clearance or spacing.
Within the data storage system manufacturing industry, much attention is presently being focused on reducing head-to-disk clearance as part of the effort to increase the storage capacity of data storage disks. It is generally desirable to reduce the head-to-disk clearance in order to increase the readback signal sensitivity of the transducer to typically weaker magnetic transitions associated with the higher recording density written on disks. When decreasing the transducer-to-disk clearance, however, the probability of detrimental contact between the sensitive transducer and an obstruction on the disk surface significantly increases.
In the continuing effort to minimize head-to-disk clearance, manufacturers of disk drive systems recognize the importance of establishing a nominal head flyheight that is sufficient to avoid disk surface defects, such as protruding asperities. As head-to-disk clearances are reduced to achieve additional improvements in disk drive performance, detecting changes in head-to-disk clearance becomes increasingly important. Unexpected changes in head-to-disk clearance of a particular head, which may or may not result in deleterious head-to-disk contact, are generally indicative of a problem with the particular head or head assembly. By way of example, an appreciable decrease in head-to-disk clearance may be indicative of a suspect head.
A number of screening approaches have been developed for use during disk drive manufacturing to identify heads that are flying with insufficient head-to-disk clearance. One such method for determining head-to-disk clearance is referred to as a Harmonic Ratio Flyheight (HRF) clearance test. Although the HRF clearance test provides accurate head-to-disk spacing measurements, the HRF testing approach typically requires employment of a dedicated tester which may take several minutes to complete HRF testing of a disk drive. Moreover, a HRF testing procedure, even if implemented in-situ a disk drive system, must be performed during idle periods or periods during which user data is neither transferred to nor obtained from the disk.
Other head flyheight evaluation techniques require dedicated tracks of information over which each of the heads must pass in order to obtain head flyheight measurements. Accordingly, such evaluation approaches, like the HRF testing approach, must be performed during idle periods in which information is not being transferred between the head and disk. Using dedicated tracks of information to evaluate head flyheight thus reduces the number of tracks available for storing data, and requires an allotment of time, such as 50 milliseconds (ms) to 100 ms, to perform a flyheight determination procedure for each head.
There exists a need in the data storage system manufacturing community for an apparatus and method for detecting low flying heads during disk drive manufacturing and, importantly, during subsequent use in the field. There exists a further need for an apparatus and method for detecting head-to-disk clearance changes in real time, and without the need for dedicated test tracks. There exists yet a further need to provide such an apparatus and method which is suitable for incorporation into existing data storage systems, as well as into new system designs, and one that operates fully autonomously in-situ a data storage system. The present invention is directed to these and other needs.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for detecting changes in clearance between a read/write transducer and a data storage medium. A readback signal is acquired from the data storage medium using the transducer. A change in spacing between the transducer and the data storage medium is detected using the readback signal. The spacing and changes in spacing may be detected and quantified in real time.
Detecting the spacing change involves using a readback signal obtained from a location on the data storage medium at which magnetic information is stored, such as a data sector or a servo sector of the data storage medium. The magnetic information from which the spacing and/or spacing change is detected may be obtained from a Gray code portion of a servo sector, such as from an isolated di-bit. Alternatively, the magnetic information may be a pre-written isolated pulse situated at one or more selected locations on the data storage medium. An isolated pulse represents a pulse that is substantially free of inter-symbol interference and filter dynamics.
In one embodiment, detecting the spacing change further involves using a slope of an edge of an isolated pulse, such as the leading edge. A readback signal comprising one or more isolated pulses, such as one or more di-bit responses, is digitized and differentiated. Maximum differentiated leading edge values of the respective isolated pulses are determined and then averaged from which transducer-to-medium spacing or spacing change is computed.
Generally, an average of several maximum differentiated leading edge values obtained from a number of isolated pulses are compared to preestablished slope/head-to-medium spacing data typically established at manufacturing time in order to determine transducer flyheight. For example, the maximums of several differentiated leading edges of isolated pulses within one or more Gray code zones may be averaged and used in the slope/head-to-medium spacing computation.
Spacing data obtained for a particular transducer may be compared to spacing data of other transducers within a common system in order to identify poor performing heads. A large deviation in spacing for a particular head relative to the spacing of other heads is generally indicative of a suspect head. Spacing deviations of approximately two to three sigma or more of the spacing deviation distribution are generally of concern. Such deviations may warrant further evaluation of a suspect head, and notification of the identified suspect head to a host system.
In accordance with another embodiment, detecting the spacing change for particular transducer involves using a pulse width of one or more isolated pulses obtained from a magnetic data storage medium. The width of the leading edge of each isolated pulse is estimated when the respective isolated pulses reach a preestablished amplitude, such as 50% of peak amplitude. The preestablished amplitude may range between approximately 10% and 70% of isolated signal peak
Ottesen Hal Hjalmar
Smith Gordon James
Faber Alan T.
Hollingsworth Mark A.
International Business Machines - Corporation
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