RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Patent Application Serial No. 60/311,136 filed Aug. 9, 2001 and entitled “METHOD FOR IN-SITU DETECTION AND MEASUREMENT OF RECORDING TRANSDUCER MAGNETIC INSTABILITY.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This application relates to disc drives and more particularly to an apparatus and method for detecting, during the course of disc drive manufacture, assembly, test, and use, a transducer possessing the undesirable characteristic of instability.  
         BACKGROUND OF THE INVENTION  
         [0003]    Recorded data is detected by a read/write head in a disc drive when the field of a recorded signal is brought in close proximity to the head. An inductive head detects a change in magnetic flux and generates a current. Detection circuitry responds to the induced current, identifying it as indicative of stored data. Alternatively, when a magneto-resistive head reads a magnetic field, it alters its own resistance. The detection circuitry detects the change in head resistance by continually running a current through the head, and identifying changes in voltage across the head. Regardless of how detection is to be accomplished, it is essential that the head&#39;s response to encountering a localized magnetic field of a recorded signal be predictable and repeatable. Variance in head response would frustrate the detection circuitry&#39;s ability to recognize data and result in errors generated during read operations.  
           [0004]    Magneto-resistive heads (hereinafter, “MR head”) may possess a particular failure mechanism that is inconsistent with the goal of predictable response to magnetic field immersion. A magneto-resistive transducer is composed of layers of films in which resistance changes in the presence of a magnetic field. The films may contain a number of magnetic domains, which can change their orientation independently. Multiple magnetic domains acting independently within the films inside the transducer can result in an unpredictable, or nonlinear response in the MR head.  
           [0005]    This failure mechanism may be the result of manufacturing or assembly errors, or undesirable environmental events that occur during the lifetime of the MR head. For example, manufacturing errors that result from manufacturing defects include voids, contamination, or material defects in a given region within the MR head. Similarly, physical impacts, electrical discharges, temperature effects encountered by the magneto-resistive head, or various mishandling that occurs during the assembly process result in errors. Environmental events often include without limitation, degradation of materials and undesirable exposure to various levels of moisture, temperature, or debris.  
           [0006]    Failure mechanisms, or magnetic instability within the magnetic recording heads can be detected when the MR head reads a magnetic field transition. The MR head generates an output signal upon exposures to magnetic fields by changing its resistance. Repetitive and consistent reaction to the change of field is compromised when magnetic domains within the MR head reorient their magnetic moment, resulting in a change in readback signal.  
           [0007]    Head instability is likely to generate an increase in error rates in the disc drive. Since head stability can be a function of time or environmental variables, error rates due to an unstable head may get worse with age or changing environments. Therefore, the data within the read/write storage device may become unrecoverable. Moreover, if other conditions which contribute to error rates such as off track, or poor signal to noise ratios are present, head instability may become even more difficult to distinguish with present detection methods.  
           [0008]    It is the function of the transducer in the MR head to produce a signal of changing voltage or current as it travels over a recorded magnetic transition on the rotating disc. Where there are no recorded transitions, or when the disc is not spinning, there should be no readback signal generated by the readback transducer. Transducers with instabilities will frequently produce output signals independently, even without the presence of recorded transitions on a rotating disc. These signals can be of varying frequency and amplitude. They are typically the result of magnetic domain switching or reorientation of magnetic moments within the transducer or magneto-resistive head.  
           [0009]    The characteristics of detected instability signals may be indicative of an operable MR head, but may also be indicative of potential failing or progressively unreliable MR heads. Temperature in the disc drive environment may also accelerate failure of the MR head. Moreover, functional problems associated with defective MR heads, such as read/write errors or data recovery errors, may be later borne by the consumer.  
           [0010]    Present testing techniques and analysis of magnetic characteristics of an MR head are difficult to perform in-situ, i.e. once the transducer has been assembled and affixed to an actuator arm within the disc drive. Typical MR head instability detection techniques consist of tracking and analyzing poor error rates within data transfer of the recording heads. However, detection of poor error rates are translated without distinction of diverse, numerous problems. For example, poor error rates may be indicative of a head flying too high above a rotating disc, creating a low signal to noise ratio. Poor error rates can also be indicative of track misregistration, track encroachment, media defects or thermal decay. Known present art techniques do not separate and distinguish magnetic instability in MR heads from other error rate failure mechanisms.  
           [0011]    Against this backdrop embodiments of the present invention have been developed.  
         SUMMARY OF THE INVENTION  
         [0012]    The method and apparatus in accordance with embodiments of the present invention solves the aforementioned problem and other problems by isolating MR head instabilities associated with assembly/manufacturing errors and environmental events. The present invention uses available (in-situ) components to isolate MR instability failure mechanisms. One embodiment of the invention uses a detection method wherein the sensitivity of a thermal asperity (TA) detector is tuned to isolate and analyze noise or stability related events produced by the MR head. The functions of TA detector typically include detection of large amplitude events related to temperature changes in the MR head created by interference with debris or the disk surface. However, this embodiment method, by using increased thermal asperity sensitivity, is useful in characterizing the quality and predicted reliability of the MR head&#39;s function. Information provided through analysis of the thermal asperity detection system can be used in repair or maintenance of the MR head. Similarly, such MR head failure data may be used to reject the MR head&#39;s use and initiate data recovery schemes.  
           [0013]    The method and apparatus detects signals produced by the MR head, (typically in current or voltage), which may translate to magnetic domain activity, then adjusts control signals within the reading or writing head to compensate for MR head instability. MR head instability events are counted and analyzed in the method of the present invention. The in-situ method commences by selecting an MR head, which is being tested, then an initial TA detection threshold is set. A read bias based on MR head specific values is also set. The writer (inductive write element on the head) is energized and magnetic transitions are erased over a pre-determined track on the disc. Next, empirical data from signals that emanate from the MR head are collected and analyzed, typically through the use of a comparator unit. Erasure pointers, or flags, where the signals emanating from the biased MR head exceed the TA detection threshold, are detected and counted. The steps of biasing the MR head, energizing the writer and erasing magnetic transitions while detecting and counting erasure pointers are repeated within a subroutine of the method. The read bias is generally incremented or decremented during the method for a range of different biases. The range of biases is typically determined by the resistive properties specific to each individually selected MR head. Further steps of the present invention include without limitation, incrementing or decrementing the writer current, and TA detection threshold between each step of adjusting the read bias, while erasure pointers are detected and counted.  
           [0014]    The apparatus includes a read channel component or a pre-amplifier, containing a TA detector, which is used to adjust TA sensitivity or thresholds to assist in detecting and analyzing failure mechanisms associated with the target MR head. During the application of a read bias to the MR head, an erased track on the disc is used to distinguish signals indicative of failure mechanisms in the MR head. The read bias is incrementally adjusted during detection of the signals. Within the apparatus a comparator unit coupled to the TA detector is used to compare the signal from a biased MR head to the threshold set in the TA detector to further analyze and detect erasure pointers during a test routine. Additionally, the TA detector can also be adjusted while iteratively varying the read bias.  
           [0015]    This method and apparatus allows the monitoring and characterization of a MR head for purposes of adjusting both the read/write head, while placing criteria on it for possible rejection. Moreover, the method allows detection of error prone MR heads from the various stages of manufacturing and assembly throughout the lifetime use of the MR head within a disc drive. Furthermore, the method provides the ability for detection and possible correction of the MR head during these stages. Quantities of erasure pointers that exceed thresholds are further analyzed for adjusting the MR head to correct or prevent errors from occurring, while extending the life of the disc drive.  
           [0016]    These and various other features as well as advantages of the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a plan view of a disc drive incorporating an embodiment of the present invention.  
         [0018]    [0018]FIG. 2 provides a functional block diagram of the servo control system of the drive in FIG. 1, a representation of a portion of a selected track of the disc drive of FIG. 1.  
         [0019]    [0019]FIG. 3 is a flow diagram of a method of in-situ transducer magnetic instability detection in accordance with an embodiment of the invention.  
         [0020]    [0020]FIG. 4 is flow diagram of one another embodiment of a method of in-situ transducer magnetic instability detection in accordance with the present invention.  
         [0021]    [0021]FIG. 5 is a functional diagram as in FIG. 2 having a more detailed diagram of the read portion of the read/write channel of an embodiment of the apparatus.  
         [0022]    [0022]FIG. 6 is a set of signal waveforms depicting signal detection capabilities of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]    A disc drive  100  constructed in accordance with a preferred disc drive embodiment of the present invention is shown in FIG. 1. The disc drive  100  includes a base  102  to which various components of the disc drive  100  are mounted. A top cover  104 , shown partially cut away, cooperates with the base  102  to form an internal, sealed environment for the disc drive in a conventional manner. The components include a spindle motor  106  which rotates one or more discs  108  at a constant high speed. Information is written to and read from tracks (not shown) on the discs  108  through the use of an actuator assembly  110 , which rotates during a seek operation about a bearing shaft assembly  112  positioned adjacent the discs  108 . The actuator assembly  110  includes a plurality of actuator arms  114  which extend towards the discs  108 , with one or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a transducer head  118  which includes an air bearing slider enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 .  
         [0024]    During a seek operation, the track position of the heads  118  is controlled through the use of a voice coil motor (VCM)  124 , which typically includes a coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets  128  which establish a magnetic field in which the coil  126  is immersed. The controlled application of current to the coil  126  causes magnetic interaction between the permanent magnets  128  and the coil  126  so that the coil  126  moves in accordance with the well known Lorentz relationship. As the coil  126  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112 , and the heads  118  are caused to move across the surfaces of the discs  108 .  
         [0025]    The spindle motor  106  is typically de-energized when the disc drive  100  is not in use for extended periods of time. The heads  118  are typically moved over park zones  120  near the inner diameter of the discs  108  when the drive motor is de-energized. The heads  118  are secured over the park zones  120  through the use of an actuator latch arrangement, which prevents inadvertent rotation of the actuator assembly  110  when the heads are parked.  
         [0026]    A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly includes a preamplifier printed circuit board  132  to which head wires (not shown) are connected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and a preamplifier for amplifying read signals generated by the heads  118  during a read operation. The flex assembly terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive  100 .  
         [0027]    Referring now to FIG. 2, shown therein is a functional block diagram of the disc drive  100  of FIG. 1, generally showing the main functional circuits which are resident on the disc drive printed circuit board and used to control the operation of the disc drive  100 . The disc drive  100  is operably connected to a host computer  200  in a conventional manner. Control communication paths are provided between the host computer  200  and a disc drive microprocessor  216 , the microprocessor  216  generally providing top level communication and control for the disc drive  100  in conjunction with programming for the microprocessor  216  stored in microprocessor memory (MEM)  224 . The MEM  224  can include random access memory (RAM), read only memory (ROM) and other sources of resident memory for the microprocessor  216 .  
         [0028]    The discs  108  are rotated at a constant high speed by a spindle motor control circuit  226 , which typically electrically commutates the spindle motor  106  (FIG. 1) through the use of back electromotive force (BEMF) sensing. During a seek operation, wherein the actuator  110  moves the heads  118  between tracks, the position of the heads  118  is controlled through the application of current to the coil  126  of the voice coil motor  124 . A servo control circuit  228  provides such control. During a seek operation the microprocessor  216  receives information regarding the velocity of the head  118 , and uses that information in conjunction with a velocity profile stored in memory  224  to communicate with the servo control circuit  228 , which will apply a controlled amount of current to the voice coil motor coil  126 , thereby causing the actuator assembly  110  to be pivoted.  
         [0029]    Data is transferred between the host computer  200  or other device and the disc drive  100  by way of an interface  202 , which typically includes a buffer  210  to facilitate high-speed data transfer between the host computer  200  or other device and the disc drive  100 . Data to be written to the disc drive  100  is thus passed from the host computer  200  to the interface  202  and then to a read/write channel  212 , which encodes and serializes the data and provides the requisite write current signals to the heads  118 . To retrieve data that has been previously stored in the disc drive  100 , read signals are generated by the heads  118  and provided to the read/write channel  212 , which performs decoding and error detection and correction operations and outputs the retrieved data to the interface  202  for subsequent transfer to the host computer  200  or other device. Such operations of the disc drive  100  are well known in the art and are discussed, for example, in U.S. Pat. No. 5,276,662 issued Jan. 4, 1994 to Shaver et al.  
         [0030]    The transducer head  118  carries a magneto-resistive (MR) head read element and an inductive write element (writer) in the trailing edge of an air-bearing slider. The MR element will be referred to subsequently in this description as an MR head. The heads  118  in the disc drive  100  are positioned over an area of the rotating disc  108  where magnetic transitions have been erased in embodiments of the present invention. A thermal asperity (TA) detector circuit in the read channel  212  is used to detect and quantify the amount of output coming from the read transducer. Any output signal generated by the read transducer positioned over an area with no written transitions will be amplified, filtered, and compared to a known voltage within a comparator using the TA detector. The comparator threshold can be adjusted to make it possible to detect low or high amplitude signals coming from the read transducer. The drive electronics will detect and quantify the number of signal events that exceed the comparator threshold as erasure pointers on each MR head within a disc drive.  
         [0031]    Levels of head instability can also vary with the amount of bias current or voltage applied to the read transducer. Also, energizing the writing element of the recording head can produce changes in the level of stability within the head. The present invention energizes the write element of the head  118  and also applies a range of bias current or voltage to the read element during thermal asperity detection routines, in an attempt to expose these instabilities. The transducer magnetic instability detection method is used over a range of voltage/current bias settings, repeated over a number of write cycles, and further repeated while adjusting the TA threshold over a pre-determined range. Heads  118 , which show output or erasure pointers in the system, have been confirmed to be unstable, and are known to produce error rate related failures.  
         [0032]    [0032]FIG. 3 illustrates a simplified flow diagram of the operational environment of a transducer magnetic instability detection system  300  according to an illustrative embodiment. In this embodiment, and other embodiments described herein, the logical operations of the transducer magnetic instability detection system  300  may be implemented as a sequence of computer implemented steps or program modules running on a microprocessor, such as, a read channel within a disc drive coupled to a microprocessor, or a pre-amplifier operably connected to the MR head and microprocessor. It will be understood to those skilled in the art that the transducer magnetic instability detection system  300  may also be implemented as interconnected machine logic circuits or circuit modules within a computing system. Additionally, the transducer magnetic instability detection system may be implemented in a separate component of the disc drive, such as a dedicated servo controller. The implementation is a matter of choice dependent on the performance and design requirements of the disc drive. As such, it will be understood that the operations, structural devices, acts, and/or modules described herein may be implemented in software, in firmware, in special purpose digital logic, and/or any combination thereof without deviating from the spirit and scope of the present invention as recited within the claims attached hereto. Furthermore, the various software routines or software modules described herein may be implemented by any means as is known in the art. For example, any number of computer programming languages, such as “C”, “C++”, Pascal, FORTRAN, assembly language, Java, etc., may be used. Furthermore, various programming approaches such as procedural, object oriented or artificial intelligence techniques may be employed.  
         [0033]    Referring to FIG. 3, in one embodiment of the invention, steps of the transducer magnetic instability detection system  300  may be stored in some form of computer readable media. As used herein, the term computer-readable media may be any available media that can be accessed by a processor or component that is executing the functions or steps of the transducer magnetic instability detection system  300 . By way of example, and not limitation, computer-readable media might comprise computer storage media and/or communication media.  
         [0034]    Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EPROM, flash memory or other memory technology, CD-ROM, digital versatile discs (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disc storage or other magnetic storage devices, or any other medium that can be used to store the desired information and that can be accessed by the computer or processor which is executing the operating code.  
         [0035]    Communication media typically embodies computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared, and other wireless media. Combinations of any of the above should also be included within the scope of computer-readable media. Computer-readable media may also be referred to as computer program product.  
         [0036]    Within the present invention flow diagram in FIG. 3 represents a simplified embodiment of the transducer magnetic instability detection system. Within the instability detection system  300 , the process begins with threshold setting  310 , which includes the step of setting a threshold in a thermal asperity (TA) detector. Thresholds within the TA detector typically consist of pre-determined settings, but can be variable values within the present invention. By way of example, TA detector thresholds may consist of several settings equally divided among a pre-determined range of voltages, such as in one embodiment, where eight settings are spread over a range of 36 millivolts (mV) to 291 mV. Bias setting  320  includes the step of setting a bias over a predetermined range of values, where the range of values are usually dependent of a magnetic characteristic of the MR head, such as resistance. Such MR head metrics are used to determine a voltage target, or a predicted tolerance value specific to the MR head for determining the reliability and performance of the MR head. Reading an erased track  330  includes the steps of positioning the MR head over a pre-determined track to detect a signal emanating from the MR head. Counting occurrences  340  includes the steps of counting the number of signals that exceed the threshold, or events that occur when comparing the signal to the threshold.  
         [0037]    Instability detection and measurement process  400  in FIG. 4 represents another embodiment of the present invention. Within selection step  410  an MR head is selected for instability detection and measurement. Within a disc drive one or many MR heads are affixed to an actuator arm for reading/writing to one or many discs within the drive. However, one MR head is selected during process  400 , and the remaining heads are selected later for detection and measurement. Threshold setting step  420  includes the setting of a threshold in a TA detector. As previously discussed, TA thresholds may be selected from fixed, pre-determined thresholds or they may be adjustable thresholds. A read bias is set in step  430  to a value typically based on a magnetic characteristic specific to the selected MR head. However, the bias value can be adjusted to ranges independent of the magnetic characteristic of the MR head. Erasing step  440  includes energizing the writer of the MR head after positioning the head over a desired track to erase magnetic transitions on the track. Alternately, the MR head can be positioned over a known erased track to accomplish similar features of step  440 . Once an erased track has been identified or made available, signals are detected and counted in step  450 . Detecting and counting  450  includes without limitation reading a signal emanating from the MR head that exceed the threshold set in step  420 . Signals giving an indication where input exceeds threshold are flagged as erasure pointers. Erasure pointers may be also flagged through designated pins on a chip, detected by the present invention. These erasure pointers or events are counted within step  450 . Repetition decision step  460  repeats steps  420  through step  450  for a pre-determined number of repetitions until a condition is met. Generally, the number of repetitions selected is low, but sufficient to eliminate anomalies or to produce a statistically acceptable reading of signals or erasure pointers. For example, in one embodiment, the number of repetitions is set at five, and upon five iterations of step  460  the condition is satisfied and the process continues to step  470 . Upon satisfaction of the repetition decision, the read bias is changed to a new value in step  470 . Change bias step  470  includes incrementing or decrementing the read bias to a new value. The range of bias decision step  480 , includes without limitation repeating steps  440  through step  470  until the range of biases have been cycled through steps  440  through step  470 . Upon the completion of iteratively stepping through the range of biases, step  490  changes the threshold of the TA detector. Change threshold step  490  includes without limitation incrementing or decrementing the TA detector threshold. The range of TA threshold decision step  495 , includes repeating steps  420  through step  490  until desirable thresholds have been stepped through within the range of thresholds.  
         [0038]    In FIG. 5, apparatus  500  represents one embodiment of the present invention. Within an operating disc drive, one or more transducers  118  having MR heads  510  are attached to actuator arms positioned above the surfaces of one or more discs  108 . In operation, MR head  510  is arcuately positioned over spinning disc  108  to read/write data recorded on tracks within disc  108 . Read channel  212  is connected to the MR head  510  via read/write pre-amplifier  520 . Preamplifier  520  conditions signals from the read element of the head  510  for use within the read channel  212 , where read channel  212  encodes/decodes signal data for transfer via interface  202  and eventual use by host computer  200 . Components within the read channel  212  includes a filter and automatic gain control (AGC) unit  525 , a threshold voltage supply  530 , a comparator  535 , and a thermal asperity (TA) detector  540 . The filter and AGC unit  525  filters noise emanating from the MR head  510 . Threshold voltage supply  530  provides threshold value for signal comparison conducted within the comparator  535 , which is coupled to TA detector  540 . The comparator  535  compares signals to the threshold set within TA detector  540 , while counter  545  counts the number of occurrences in which a signal exceeds the threshold.  
         [0039]    In one embodiment of the present invention, the TA detector  540  (shown in FIG. 5) is a component within the read channel  212 . In an alternate embodiment the TA detector  540  may be a component connected to or subcomponent part of preamplifier  520 . In one embodiment of the present invention, TA detector  540  is a component within read channel  212 ; however, in alternate embodiments the TA detector  540  may be a component connected to or part of preamplifier  520 . For alternate embodiments of the present invention, read/write preamplifiers also have thermal asperity detection capabilities, which could be used in place of, or in addition to the read channel thermal asperity detector. The present invention could also be made to work with different numbers of write cycles, write currents, read bias current/voltage ranges, and will work on different locations of the rotating disc.  
         [0040]    In any of the embodiments of the present invention, the invention can be used in the disc drive test process, and can also be made active in the end user environment to provide notification that head instability conditions exist within the disc drive. Such detections at various stages of disc drive life cycles promote opportunity and adjustment of the MR head parameters such as current or voltage bias to realign or reorient magnetic domains within the MR head. These corrections improve transducer reliability and reduce error rates within the disc drive while extending the lifetime of the magneto-resistive head and the disc drive itself.  
         [0041]    [0041]FIG. 6 represents various signal outputs from the transducer using an oscilloscope within the present invention. By way of example, a selected track having erased magnetic transitions is sampled and analyzed after increasing the sensitivity of a TA detector. A TA comparator threshold voltage is chosen, and a read gate is established for detecting signals generated by the MR head while the read element is positioned over the area of the disk where magnetic transitions have been erased. Signals generated by the MR head due to magnetic instabilities are sensed while read gate is active. Where the signals produced by the head due to magnetic instabilities are of amplitude that exceeds the threshold set at the comparator, output pulses will be generated by the TA detector. The output pulses from the TA detector will be issued as erasure pointers that are to be counted. The number of erasure pointers that exceed the threshold are counted and analyzed for determining the reliability or predicted performance of the MR head. MR heads are then evaluated for possible maintenance schemes or rejection. The present invention provides an opportunity for this assessment while also providing the opportunity to recover data as needed prior to action taken related to the MR head.  
         [0042]    In summary, the present invention may be viewed as a method (such as  300 ) of detecting and measuring instability within the MR head (such as  510 ) within a disc drive (such as  100 ), in which the disc drive has a plurality of tracks and a magneto resistive (MR) head (such as  510 ) positioned above the tracks. The method includes setting a threshold in a thermal asperity detector (such as  310 ) connected to the MR head (such as  510 ) and applying a read bias to the MR head (such as  320 ). The method also includes reading a signal emanating from the MR head positioned over an erased track (such as  330 ), counting a number of occurrences of signals that exceed the threshold (such as  340 ) to determine transducer magnetic instability within the MR head based on the number of occurrences of signals that exceed the threshold. The further includes adjusting the read bias to a new value within a range of values (such as  470 ), while repeating the steps of reading, counting, and determining transducer instability (such as  450 ). The range of values is based on a characteristic of the MR head resistance. The method also includes re-setting the thermal asperity detector to a new threshold (such as  490 ) and repeating the steps of reading, counting, and determining transducer magnetic instability. The method further includes realigning the magnetic domains of the MR head if the number of signal occurrences exceeds a pre-determined number. Applying a pre-determined write or read current/voltage to the MR head may be used to perform the realignment.  
         [0043]    A computer readable medium having computer-executable instructions may be used for performing the steps within the above method. The method for detecting transducer magnetic instability in a MR head for an operating disc drive includes without limitation, setting a signal threshold in a thermal asperity detector in a disc drive read channel circuit (such as  420 ), setting a read bias in a read channel circuit (such as  430 ), reading an erased track on the drive to detect a signal emanating from the MR head (such as  440 ), and counting an occurrence of the signal if the signal exceeds the signal threshold (such as  450 ). The method further includes re-setting the read bias to a new bias (such as  470 ), while repeating the reading and counting steps of the method (such as  450 ). The method also includes performing the re-setting of the read bias (such as  470 ) and repeating the reading and counting steps (such as  450 ) for a pre-determined number of repetitions. Additionally, the method may include the re-setting (such as  470 ) and repeating the reading and counting steps (such as  450 ) of the method are repeated until there are no occurrences of signals that exceed the threshold. The method may also include performing such repetitions for five (5) cycles. In addition to the aforementioned steps of the method, the method may include re-setting the signal threshold to a new signal threshold (such as  490 ) and repeating the setting of a read bias (such as  470 ), the reading on a erased track (such as  450 ), and counting of the number signal occurrences exceeding the threshold (such as  450 ). The method also includes repeating the re-setting of the signal threshold (such as  490 ) and repeating the setting of a read bias (such as  470 ), the reading of an erased track (such as  450 ), and the counting of signal occurrences for a pre-determined number of repetitions.  
         [0044]    The method includes setting a first criterion based on a characteristic of the MR head, and comparing the counted number of occurrences of the signals that exceed the threshold to the first criterion to determine a reliability value to the MR head. The method also includes rejecting the MR head if the reliability value is outside a second criterion. The method may also include re-aligning magnetic domains within the MR head based on the reliability value by applying a pre-determined write or read current/voltage to the MR head. The current/voltage may be based on the reliability value. The method includes attenuating the signal emanating from the MR head to a level within a range of pre-determined signal thresholds, or amplifying the signal emanating from the MR head to a level within a range of pre-determined signal thresholds. These steps within the method may be performed by a computer readable medium having computer-executable instructions.  
         [0045]    The present invention may also viewed as an apparatus (such as  500 ) for detecting and measuring instability in a MR head (such as  510 ) in an operating disc drive (such as  100 ). The MR head (such as  510 ) has a magnetic orientation and is positioned over a predetermined track on a disc in the drive. The apparatus (such as  500 ) includes without limitation, a thermal asperity detector (such as  540 ) in the disc drive (such as  100 ) connected to the MR head (such as  510 ). The thermal asperity detector (such as  540 ) capable of having an adjustable threshold set to a pre-determined value. The apparatus (such as  500 ) also includes a read bias applied to the MR head (such as  510 ). The bias is selected from a range of values, in which the values are based on the MR head resistance to a magnetic field. The apparatus (such as  500 ) includes a signal generated by the MR head while the MR head is positioned over an erased track. The apparatus includes a software module connected to the thermal asperity detector (such as  540 ) for comparing the signal from emanating from the MR head to the pre-determined threshold. The software module also counts occurrences in which the signal exceeds the pre-determined threshold. The software within the apparatus may also include a comparator (such as  535 ) connected to the read channel (such as  212 ) for comparing the signal from the MR head (such as  510 ) to the pre-determined threshold, which also includes a counting unit (such as  545 ) counting occurrences in which the signal exceeds the pre-determined threshold. The apparatus may also include a thermal asperity detector (such as  540 ) connected to the MR head via a read channel (such as  212 ) and a software module connected to the thermal asperity detector (such as  540 ) via the read channel (such as  212 ). In addition, the TA detector (such as  540 ) of the apparatus may be connected to the MR head via a pre-amplifier (such as  520 ) with the software module connected to the TA detector (such as  540 ) via the pre-amplifier (such as  520 ). The apparatus includes a means for adjusting the write or read bias to re-orient the magnetic domains within the MR head based on the number of occurrences of signals exceeding the threshold. The apparatus also includes a means for adjusting the signal emanating from the magneto-resistive head. The means for adjustment may include attenuating or amplifying the signal to a level within the range of settings for the threshold.
The method and apparatus herein identifies instability events occurring within a magneto-resistive head. The method commences by positioning magneto resistive head over a selected area of a disk that has no transitions, and then iteratively setting a read bias, and a thermal asperity threshold for counting and analysis of magneto resistive head instability events. The sensitivity of the thermal asperity detector is tuned to detect events that indicate magneto resistive head instability generated by the MR head over regions where magnetic transitions have been erased. The analyzed events are then used to determine the action taken related to the reliability of the head, that is, whether to reject or attempt to reduce the amount of instability related output. The apparatus includes without limitation a preamplifier, a read channel with a thermal asperity detector, and a comparator for counting and analyzing the resultant signals tested via biasing the magneto resistive head in a disc drive.