Error detection/correction and fault detection/recovery – Pulse or data error handling – Memory testing
Reexamination Certificate
1998-11-30
2001-06-19
Chung, Phung M. (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Memory testing
C360S053000, C714S718000
Reexamination Certificate
active
06249890
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of disc drive data storage devices and more particularly, but without limitation, to a method and apparatus for predicting a future failure of a disc drive based upon degradation of head readback response characteristics.
BACKGROUND
Hard disc drives are used in modern computer systems and computer networks to enable users to quickly access vast amounts of electronically stored data. A typical disc drive houses five to ten magnetic discs which are axially aligned and mounted to a spindle motor for rotation at a constant, high speed. An array of read/write heads are controllably positioned adjacent magnetic recording surfaces of the discs in order to store and retrieve the data from tracks defined on the disc surfaces. The heads fly adjacent the recording surfaces on air bearings established by air currents set up by the rotation of the discs.
Of particular interest are heads of the so-called “magneto-resistive” variety, which utilize magneto-resistive (MR) elements to sense the selective magnetization of the tracks during disc drive data transfer operations. A typical MR element is formed from an alloy of materials so as to have a baseline electrical resistance which varies in the presence of a magnetic field of a selected orientation. By passing a bias current through the MR element, the selective magnetization of a corresponding track can be determined in relation to variations in voltage detected across the MR element.
It is common in present generation disc drive manufacturing processes to individually select read bias current magnitudes for each of the MR heads of a disc drive in order to optimize disc drive performance. For example, test data are typically written and then read in turn a number of times using a range of different read bias current magnitudes. Those read bias current magnitudes providing optimum performance are then stored in memory utilized by the drive so that, when a particular head is selected during subsequent operation, the disc drive applies the appropriate read bias current to the particular head.
MR heads are known to be delicate and must be handled and operated with a certain degree of care so as to prevent inadvertent damage which can degrade the reliability of the heads. Although the application of a relatively larger bias current will generally enhance the sensitivity of an MR head during a read operation by providing a higher signal to noise ratio in a recovered readback signal, it is important to ensure that the maximum power dissipation capability of the head is not exceeded. As will be recognized, because the MR element operates as a (highly sensitive) resistance, the power P dissipated by the MR element will be generally proportional to the resistance R of the MR element multiplied by the square of the bias current I (i.e., P=I
2
R). Accordingly, there is an upper limit on the magnitude of the bias current that can be applied to any given MR head, and the application of too large a bias current, even momentarily, can stress the MR head and adversely affect its operational reliability over time.
Even if an MR head is not overstressed, the readback response characteristics of the MR head can still vary significantly over time due to a number of environmental and operational factors. Such readback response characteristics include nominal electrical resistance, as well as signal amplitude, asymmetry and linearity. Electrical resistance is the baseline direct current (dc) resistance of the read element, which will typically be from about 20 to 90 ohms (&OHgr;). Signal amplitude is a measure of the voltage range between maximum positive and negative peaks in the readback signal. Asymmetry is a measure of a difference between the absolute magnitudes of maximum positive and negative peaks in the readback signal, and linearity is a measure of higher-order response characteristics of the read element at positive or negative transitions in the readback signal. Each of these readback response characteristics affect the ability of an MR head to transduce data from a track.
Accordingly, there remains a continual need for improvements in the art whereby the reliability of a disc drive can be maintained by ensuring that head performance does not degrade to a level where previously stored data cannot be retrieved from a disc.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for predicting disc drive failure due to head readback performance degradation.
As exemplified by presently preferred embodiments, a disc drive includes a controllably positionable head adjacent a rotatable storage disc for reading data from the disc, the head comprising a magneto-resistive element having a changed electrical resistance in the presence of a magnetic field of selected orientation.
The disc drive further comprises a read/write channel which transfers data between the disc and a host device associated with the disc drive. A servo circuit controls the position of the head relative to the disc, and a control processor provides top level control of the disc drive.
In predicting failure of the disc drive the drive determines and stores a baseline level for a readback response characteristic of the head indicative of head performance as data are read back from the disc. The drive next periodically determines a subsequent level for the readback response characteristic of the head. The drive predicts a possibility of a future failure of the disc drive in relation to a difference between the baseline level and the subsequent level or the readback response characteristic of the head. The drive then provides an indication of the possibility of the future failure to allow a host device with which the disc drive is associated to reallocate data stored on the disc before the failure of the disc drive.
In one preferred embodiment, the readback response characteristic comprises an electrical resistance of the head. Preferred approaches to predicting disc drive failure based on head resistance degradation include selecting a read bias current value and positioning the head sufficiently away from data stored on the disc so that the data do not affect the electrical resistance of the magneto-resistive element; measuring a voltage across the magneto-resistive element while applying the read bias current to the magneto-resistive element; determining the electrical resistance of the magneto-resistive element in relation to the voltage measured across the magneto-resistive element and the read bias current applied to the magneto-resistive element; and storing the electrical resistance and the applied read bias current.
In another preferred embodiment, the readback response characteristic comprises amplitude response of the head, determined as a function of a range between maximum and minimum voltage peaks in a readback signal generated by the head as data are read from the disc. One preferred approach to detecting amplitude response degradation involves monitoring gain used by a variable gain amplifier. Steps include positioning the head over a track on the disc having stored data arranged in a constant frequency data pattern; reading the constant frequency data pattern; selecting a gain for a variable gain amplifier that provides optimum response by the read channel in decoding data from the disc, the gain indicative of the amplitude response of the head; and storing the gain. Another preferred approach to detecting amplitude response degradation evaluates track address detection thresholds of the servo circuit, the detection thresholds used to decode track address information. Steps include positioning the head over a selected track; instructing the servo circuit to temporarily ignore indications that the head is no longer positioned over the selected track in response to errors obtained as the track address fields are read; reading the track address fields while sequentially varying amplitude detection thresholds used to detect peaks in a track address signal; and identif
Enarson Karl L.
Ukani Anish A.
Chung Phung M.
Crowe & Dunlevy
Seagate Technology LLC
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