Dynamic magnetic information storage or retrieval – Monitoring or testing the progress of recording
Reexamination Certificate
2001-06-01
2003-12-30
Faber, Alan T. (Department: 2651)
Dynamic magnetic information storage or retrieval
Monitoring or testing the progress of recording
C360S060000, C360S025000, C360S053000
Reexamination Certificate
active
06671111
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to data storage systems, and more particularly, to envelope detection in a magnetic data storage system.
2. Background of the Related Art
A typical magnetic data storage system 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. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator and passed over the surface of the rapidly rotating disks. The actuator typically includes one or more outwardly extending arms to which in-line suspensions are attached and onto which one or more air bearing sliders are mounted at a distal end of the suspensions. One or more transducers, in turn, are disposed on the air bearing slider. Airflow produced above the disk surface by the rapidly rotating disks results in the production of an air bearing upon which the aerodynamic slider is supported, thus causing the slider to fly a small distance above the rotating disk surface.
The actuator arms are interleaved into and out of the stack of rotating disks, typically by means of a rotary voice coil assembly mounted to the actuator. The rotary voice coil assembly generally interacts with a permanent magnet structure, and the application of current to the coil in one polarity causes the actuator arms, suspensions and sliders to shift in one radial direction, while current of the opposite polarity shifts the actuator arms and sliders in an opposite radial direction.
In a typical magnetic digital data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, closely spaced tracks comprising the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a plurality of sectors, with each sector comprising a number of information fields. One of the information fields is typically designated for storing data, while other fields contain sector identification, synchronization and radial position information, for example. Data is transferred to and retrieved from specified track and sector locations by the transducers being moved from track to track, typically under the control of a position controller.
The transducer, also referred to as a read/write head, is one of the most important components in a magnetic disk drive system. The transducer assembly typically includes a read element and a write element. A common type of read element is the magnetoresistive (MR) head. A conventional read head operates by sensing the rate of change of magnetic flux transitions stored on the surface of a magnetic disk. The MR head produces an electrical output signal in response to the sensed magnetic flux transitions. The MR head's output signal is velocity independent.
Writing data to a data storage disk generally involves passing a current through the write element of the transducer assembly to produce magnetic lines of flux which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a MR read element transducer sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the emanating field from the magnetized locations on the disk surface results in the production of electrical signals, commonly referred to as readback signals, in the read element.
MR heads represent an important improvement in magnetic disk drive systems. In particular, the output signal of a MR head is not dependent on the relative velocity between the head and the disk. MR heads may employ an inductive write element. In contrast to older head assemblies, a MR head uses a modified read element employing features such as a thin sensing element called an “MR stripe”. The MR stripe operates based upon the magnetoresistive effect. Namely, the resistance of the MR stripe changes in proportion to the magnetic field of the disk, passing by the MR stripe. If the MR stripe is driven with a constant bias current, the voltage across the MR stripe is proportional to its resistance. Thus, the MR stripe's voltage represents the magnetic signals encoded on the disk surface. In other arrangements, a constant voltage is applied to the MR stripe, and the resultant current is measured to detect magnetic signals stored on the disk surface.
Although highly beneficial, MR heads are especially susceptible to certain errors. Namely, the resistance of the MR stripe varies in response to heating and cooling of the MR stripe, in addition to the magnetic flux signals encoded on the disk surface. Normally, the MR stripe maintains a steady state temperature as the slider flies over the disk surface, separated by a thin cushion of air created by the rapidly spinning disk. In this state, the stored magnetic flux signals contribute most significantly to the MR stripe's output signals, as intended. An MR stripe, however, may experience heating under certain conditions, especially when the MR head inadvertently contacts another object on the disk.
Physical contact with the MR head may occur in a number of different ways. For instance, the MR head may contact a raised irregularity in the disk surface, such as a defect in the material of the disk surface or a contaminant such as a particle of dust, debris, etc. Also, the MR head may contact the disk surface during a high shock event, where G-forces momentarily bounce the MR head against the disk surface.
Such physical contact results in heating of the MR head, including the MR stripe. Heating of the MR stripe increases the stripe resistance, which distorts the MR stripe's output signal. This type of distortion is known in the art as a “thermal asperity.” A read channel in a magnetic disk drive, however, requires a reliable readback signal from the MR head, free from irregularities such as thermal asperities. Consequently, severe thermal asperities may prevent the read channel from correctly processing output signals of the MR head, causing a data error.
These data errors may be manifested in a number of different ways. For instance, severe distortions of the readback signal may cause the magnetic disk drive to shut down. Other data errors may simply prevent reading of data on the disk. Such data errors may also prevent writing of data, if the servo signal embedded in the disk cannot be read correctly, or it indicates that the head is too far off track to write data without overwriting data on an adjacent track. This condition is called a “write inhibit error”. If data errors of this type persist, the disk drive may deem the entire sector “bad”, causing a write inhibit “hard” error. Repeated thermal asperities may also cause a disk drive to fail a predictive failure analysis measure, falsely signaling an impending disk failure to the disk drive user. As shown by the foregoing, thermal asperities in magnetic disk drive systems may cause significant problems in disk drives that use MR heads.
It is now known that the thermal asperites and other heating/cooling events contribute a thermal signal component (baseline-wander) to the overall readback signal. As such, the readback signal may be understood as a composite signal comprising a magnetic component and the thermal component. A detailed discussion regarding these signal characteristics may be found in U.S. Pat. No. 6,088,176, entitled “Method and Apparatus for Separating Magnetic and Thermal Components from an MR Read Signal,” which is hereby incorporated by reference.
Despite its undesirability, the thermal signal component has been used to advantage in detecting any surface defects on disks. By monitoring the thermal signal component of a readback signal, the foregoing problems related to thermal asperties may be
Ottesen Hal Hjalmar
Smith Gordon James
Faber Alan T.
International Business Machines - Corporation
Moser Patterson & Sheridan LLP
Nock James R.
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