Transducer instability detection in a digital channel

Details Transducer instability detection in a digital channel Transducer instability detection in a digital channel

2000-10-27

2003-04-29

Hoff, Marc S. (Department: 2857)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Electrical signal parameter measurement system

C324S076390

Reexamination Certificate

active

06556933

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to component monitoring systems, and more particular to systems for monitoring the stability of high performance transducers such as those employed in information handling systems.
BACKGROUND OF THE INVENTION
Information handling systems have undergone explosive growth over the past several years. New technologies are allowing today's common components to outperform state-of-the-art components of five years ago. In disc drives, for example, areal density has doubled each year for the past several years. This has been made possible, in part, by the development of better magnetic transducers than previously existed. Not surprisingly, most modern transducers are highly sophisticated and temperamental.
Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers mounted to a radial actuator for movement of the transducers relative to the discs. Typically, such radial actuators employ a voice coil motor to position the transducers with respect to the disc surfaces. Sliders carrying the transducers are mounted via flexures at the ends of a plurality of arms which project radially outward from a substantially cylindrical actuator body. The actuator body pivots about a shaft mounted to the housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the transducers each move in a respective plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the transducer arms so as to be immersed in the magnetic field of an array of permanent magnets. When controlled DC current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnets and causes the coil to move in accordance with the well-known Lorenz relationship. As the coil moves relative to the magnets, the actuator body pivots about the pivot shaft and the transducers are moved across the disc surfaces.
Typically, the transducers are supported over the discs by actuator slider assemblies which include air-bearing surfaces designed to interact with a thin layer of moving air generated by the rotation of the discs, so that the transducers are said to “fly” over the disc surfaces. Generally, the transducers write data to a selected data track on the disc surface by selectively magnetizing portions of the data track through the application of a time-varying write current to the transducer. In order to subsequently read back the data stored on the data track, the transducer detects flux transitions in the magnetic fields of data track and converts these to a signal which is decoded by read channel circuitry of the disc drive.
Most high-performance disc drives use magneto-resistive (MR) transducers, which typically comprise one or two thin strips of ferromagnetic material such as NiFe magnetized along an easy axis of low coercivity. The strips are mounted in the transducer such that the easy axis is transverse to the direction of disc rotation and parallel to the plane of the disc. Magnetic flux from the disc surface causes rotation of the magnetization vector of the strip, which in turn causes a change in electrical resistivity. A sense current is passed through the thin strip and the magnetic flux can be detected by measuring the change in voltage across the element as a result of the changing resistivity.
As with any mass-produced component, a nominal amount of variation in the width of transducers is typically present as a result of tolerances inherent in large-scale manufacturing. As a result of these variances and irregularities in materials used, subtle and often intermittent performance problems can occur. In a disc drive, moreover, variations in baseline level may also be caused by gross fly-height issues, media problems, or the use of topography on the disc surface (servo patterned or bit patterned media).
Current magnetic read sensors often exhibit “instability,” in the sense that the “readback” transducer signal can exhibit random fluctuating behavior that can make the detection of both servo and user data extremely difficult. For example, either pulse “shouldering” (asymmetry) or baseline pop/shift can cause significant problems in demodulating the readback signal.
Existing methods of detecting an unstable behavior may not permit cost-effective diagnosis of such instability. This can cause a slow or inappropriate response, wasting an opportunity in cases where an effective response is available. Thus, there is a need for a more effective approach to diagnosing and responding to an unstable behavior.
SUMMARY OF THE INVENTION
The present invention is used in a data handling system that effectively detects and diagnoses manifestations of excessive transducer instability. For example, some embodiments of this invention identify shouldering and baseline pop instabilities in a digital servo channel. This is important since integration of the servo and read/write channels into a disc drive typically means the use of all digital channels.
A preferred method of the present invention involves detecting a precise time at which a first feature in the received signal is encountered. Then, after a preset delay, a first level of the received signal is sampled. Then, after waiting until after a second feature in the received signal occurs, a second level of the received signal is sampled. Finally, at least one digital signal is generated so as to indicate whether an arithmetic combination of the first and second levels falls outside a stability-indicative range.
A preferred device of the present invention includes a transducer, a peak detector configured to receive a signal from the transducer, two delay elements, two sampling circuits, and an output. One delay element is configured to generate a first sampling signal a predetermined time after a first peak is detected. The other delay element is configured to generate a second sampling signal a predetermined time after a second peak. One of the sampling circuits responds to each of these delay elements to extract a respective sample level from the transducer signal. The output generates a first digital signal indicative of whether a first arithmetic combination of the first and second levels falls outside a stability-indicative range.
Other features and advantages of the present invention will become apparent upon a review of the following figures and their accompanying description.


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