Abstract:
A method and apparatus are disclosed for characterizing asymmetries of a magneto-resistive type head ( 18 ) in proximity to a magnetic media ( 40 ) such as a magnetic disk of a hard disk drive ( 12 ). The method includes using the magneto-resistive type head ( 18 ) to read a continuous signal from the magnetic media ( 40 ) to provide a read back signal ( 110, 112 ). Energies contained in the read back signal which occur in excess of a predetermined threshold ( 111, 58 ) for the positive ( 110 ) and negative ( 112 ) portions of the read back signal are then determined. The energies may be determined by accumulating sampled signal values from the read back signal in registers ( 78 ) during times at which the read back signal exceeds the threshold ( 111, 58 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to improvements in manufacturing and operating techniques for mass data storage devices, and the like, and more particularly to improvements in methods and apparatuses for characterizing asymmetries of an MR read head, or the like, for use therein.  
           [0003]    2. Relevant Background  
           [0004]    Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected to further increase in the future.  
           [0005]    One class of mass data storage devices to which the present invention has particular applicability is hard disk drive systems. A hard disk drive system typically includes a rotating magnetic disk on which information is recorded. A read transducer is movably supported adjacent the magnetic disk for reading the prerecorded information from the disk. The read transducer typically flies above the surface of the disk, being supported by an “air bearing” that is created by the spinning disk, so that the transducer does not touch the surface of the disk in normal operation.  
           [0006]    Recently, magnetoresistive (MR) heads have been gaining wide popularity for use as the read transducer. The term “magnetoresistance” refers to the change in resistivity of the materials of the head in the presence of a magnetic field. The introduction of MR heads into disk drives has significantly increased the areal density. However, due to the characteristics of the head materials, changes in resistance in response to equal positive and negative magnetic fields are not necessarily symmetrical. This is referred to as head “asymmetry”.  
           [0007]    Typically, a circuit is included in the read channel of the mass data storage device in which the MR head is employed to compensate for such asymmetrical signal responses. However, the asymmetrical responses are in large part nonlinear, and are difficult to assess and compensate.  
           [0008]    What is needed, therefore, is a method for more accurately characterizing asymmetries of an MR head in a mass data storage device, or the like.  
         SUMMARY OF THE INVENTION  
         [0009]    In light of the above, therefore, it is an object of the invention to provide a method for characterizing asymmetries of an MR head in a mass data storage device, or the like.  
           [0010]    This and other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims.  
           [0011]    According to a broad aspect of the invention, a method is disclosed for characterizing asymmetries of a magnetoresistive type head in proximity to a magnetic media, such as a magnetic disk of a hard disk drive, or the like. The method includes using the magneto-resistive type head to read a continuous signal from the magnetic media to provide a read back signal. Energies contained in the read back signal which occur in excess of a predetermined threshold for the positive and negative portions of the read back signal are then determined. The energies may be determined by accumulating sampled signal values from the read back signal during times at which the read back signal exceeds the threshold.  
           [0012]    According to another broad aspect of the invention, a method is disclosed for characterizing asymmetries of a magneto-resistive type head in proximity to a magnetic media. The method includes writing a continuous signal onto the magnetic media, and using the magneto-resistive type head, reading back the continuous signal to provide a read back signal. The positive and negative portions of the read back signal are then compared to a threshold value, and energies contained in the read back signal occurring in excess of the threshold value for the positive and negative portions of the read back signal are determined, for example by accumulating sampled signal values from the read back signal during times at which the read back signal exceeds the threshold.  
           [0013]    According to another broad aspect of the invention, an apparatus for determining asymmetries of a magneto-resistive type head in proximity to a magnetic media is disclosed. The apparatus includes a comparator for determining when positive and negative signals produced by the head in reading a continuous signal from the disk exceed known thresholds, and an energy determining circuit for measuring energies contained in the positive and negative read back signals when the read back signals exceed the thresholds. The energy determining circuit may include a circuit for sampling the read back signals at least during times at which the read back signal exceeds the threshold to produce sampled signal values therefrom, and an accumulator for accumulating the sampled signal values. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The invention is illustrated in the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 is a block diagram of a generic disk drive system, illustrating one general environment in which the invention may be practiced.  
         [0016]    [0016]FIG. 2 is a block diagram showing a portion of a read channel of a typical hard disk drive showing an asymmetry compensation circuit that can be configured, in accordance with a preferred embodiment of the invention.  
         [0017]    [0017]FIG. 3 is a graph of amplitude vs. time to show the preamplified analog output of an MR head with an asymmetrical response in response to a prerecorded continuous signal.  
         [0018]    [0018]FIG. 4 is a block diagram showing a circuit for accumulating the energy values contained in positive and negative signal portions of the signal shown in FIG. 3, in accordance with a preferred embodiment of the invention.  
         [0019]    [0019]FIG. 5 is a graph of the output of a read channel analog-to-digital converter showing the sampling of the absolute value of the signal of FIG. 3.  
         [0020]    And FIG. 6 is a graph of a response of an MR head that is sampled in sections to enable a timing or slope asymmetry to be measured, in accordance with another preferred aspect of the invention. 
     
    
       [0021]    In the various figures of the drawing, like reference numerals are used to denote like or similar parts.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    [0022]FIG. 1 is a block diagram of a generic disk drive system  10 , which represents one general environment in which the invention may be practiced. It should be noted that although the invention is described in the context of a hard disk drive with a rotating magnetic media, the invention may be equally advantageously employed in other environments which use a magneto-resistive head that changes its electrical characteristics in the presence of magnetic flux from an associated magnetic media. The circuit  10  represents a method and apparatus for practicing the invention in which asymmetries may be detected during the operation of a mass data storage device, for example in its initial characterization during manufacture of the device.  
         [0023]    The system  10  includes a magnetic media disk stack  12  that is rotated by a spindle motor  14  and spindle driver circuit  16 . A data transducer or head  18  is locatable along selectable radial tracks (not shown) of the disk stack  12  by a voice coil motor  20 . The radial tracks may contain magnetic states that contain information, such as track identification data, location information, synchronization data, user data, and so forth. The head  18 , which may be a magneto-resistive (MR) head, is used to both record user data to and read user data back from the disk. The head  18  may also be used to detect signals that identify the tracks and sectors at which data is written, to detect servo bursts that enable the head to be properly laterally aligned with the tracks of the disk, and so on.  
         [0024]    Analog electrical signals that are generated by the head  18  in response to the magnetic signals recorded on the disk are preamplified by a preamplifier  22  for delivery to read channel circuitry  24 , which includes an asymmetry processing circuit  26 , as below described in detail. Servo signals are detected and demodulated by one or more servo demodulator circuits  28  and processed by a digital signal processor (DSP)  30  to control the position of the head  18  via a positioning driver circuit  32 .  
         [0025]    A microcontroller  34  is typically provided to control the DSP  30 , as well as an interface controller  36  to enable data to be passed to and from a host interface (not shown) in known manner. A data memory  38  may be provided, if desired, to buffer data being written to and read from the disk.  
         [0026]    It is well known that magneto-resistive heads of the type used in the environments described herein often have nonlinear responses to positive and negative signals. Moreover, the resistance of the head may vary more in response to exposure to magnetic flux in one direction than in the other direction. This is referred to herein as “asymmetry” of the head. In the past, asymmetry circuits have been provided between the preamplifier  22  and the read channel circuitry  24  to correct for such asymmetries. In many cases, asymmetry compensation circuitry  40  is included in the read channel itself before the analog signal developed by the MR head is digitized in an analog-to-digital converter  42 , as shown in FIG. 2. However, as mentioned above, because the asymmetries are nonlinear, it is difficult to accurately set up and adjust the asymmetry compensation circuitry.  
         [0027]    Thus, according to a preferred embodiment of the invention, a method and apparatus are provided to develop information that enables MR heads to be more accurately characterized on account of the asymmetries of the head responses. This information can be used to configure the asymmetry compensation circuitry in the hard disk drive read channel. According to a preferred embodiment of the invention, this information is developed by determining the energy contained in the positive and negative signal envelopes produced by the MR head in response to a continuous signal prerecorded on the disk. (A continuous signal means a single frequency pattern or sequence written to the disk, and may include, for example, an EPR4 signal that represents a sine wave.) Once the energy information is developed, the positive and negative envelope energies can be compared and the nature of the asymmetries assessed.  
         [0028]    For example, a waveform  100  that represents a typical MR head response to a symmetrical, non-overlapping consecutive positive and negative signals recorded on a disk drive is shown in FIG. 3, to which reference is now additionally made. The envelope of the waveform  100  has a positive portion  110  and negative portion  112 . However, the positive envelope portion  110  is not as large as the negative envelope portion  112 . Such an asymmetry in the head response may result when as a normal part of the operation of the head, a signal is produced by the head in response to a prerecorded continuous signal. As a result, an appropriate signal compensation should be made to the head output before the signal is digitized by the A/D converter in the read channel of the drive.  
         [0029]    According to a preferred embodiment of the invention, the energies contained in the respective positive and negative signal envelopes  110  and  112  of the read back signal in response to the prerecorded continuous signal of the disk are determined. This determination may be made, for example, by the integrating circuit  26  of FIG. 4, to which reference is now additionally made. A method and circuit for energy determination for positive and negative signal portions between zero crossings thereof is shown in U.S. Pat. No. 6,163,419, issued Dec. 10, 2000, entitled METHOD AND APPARATUS FOR DEMODULATING A SERVO BURST SIGNAL IN A HARD DISK DRIVE, assigned to the assignee hereof, and incorporated herein by reference.  
         [0030]    The integrating circuit  26  includes an absolute value circuit  62 , which has an input coupled to the output of the ADC  42  (FIG. 2) of the read channel  24 . The absolute value circuit  62  determines and outputs the absolute value of each digital sample generated by the ADC. If the digital samples are represented by signed binary numbers, this may involve simply removing the sign bit. The absolute value circuit  62  thus operates somewhat like a full-wave rectifier circuit.  
         [0031]    The operation of the absolute value circuit  62  is illustrated in FIGS.  5 , to which reference is now additionally made. FIG. 5 is a graph representing the output of the absolute value circuit  62  of the read channel, where the broken lines  110 ′ and  112 ′ represent the envelopes of the recovered and digitized signal  100 , and the solid lines  72  and  74  represent the samples taken by the ADC of the rectified positive and negative signal segments. Thus, the output of the absolute value circuit  62  converts the negative samples from the ADC to positive samples, in order to effectively achieve full-wave rectification. As evident from FIGS. 3 and 5, the disclosed embodiment preferably utilizes sampling at or above the Nyquist rate, which means taking samples at a rate equal to or greater than twice the highest frequency contained in the signal. The sampling in the disclosed embodiment involves about twelve samples per full cycle of the burst signal, or, in other words, about six samples per half-cycle, although it will be recognized that the specific sampling rate can be varied within the scope of the invention.  
         [0032]    The successive output values during a predetermined portion of the positive  72  or the negative  74  signal portions from the absolute value circuit  62  are supplied to an accumulator  74 , which adds up the successive digital values. More specifically, the accumulator  74  sums the sample values supplied to it during the time interval during which the read back signal exceeds a predetermined threshold voltage  58 , determined by a threshold detector  76 . It should be noted that although a specific threshold voltage is shown, in many applications, it may be desired to set the threshold to zero. Thus, the function of the threshold detector  76  may be served by a zero crossing detector. On the other hand, the use of a non-zero threshold may be desirable in some applications where, for example, the baseline  111  is mis-shaped or is otherwise difficult to detect due to noise, or the like.  
         [0033]    Since the signal is effectively rectified by the absolute value circuit  62 , in the embodiment illustrated, only a single threshold value  58  need be provided. Thus, the accumulator  74  essentially integrates the digital voltage values from the ADC of the read channel  24 , producing a value that represents the energy contained in the read back signal during the time that the read back signal exceeds the predetermined threshold value  58 .  
         [0034]    With continued reference to FIG. 4, at the end of a time interval during which the accumulator  74  has been adding up sample values, the accumulated values or sums are transferred to a register array  78 . Then, if desired, the accumulator  74  may be cleared to be ready to add up samples for another time interval, in a similar fashion to that above described. As the accumulator  74  acquires each accumulated value, it is inserted into an unused register of the register array  78 .  
         [0035]    On the other hand, the sample values for both the positive and negative portions of the MR head response  100  may be accumulated over the entire expanse of the disk with which the head is associated. In this embodiment, the total values may be stored in just two of the registers  78  for later use.  
         [0036]    As mentioned, one of the inherent advantages of the invention is that it can be used to characterize the MR read head response to enable asymmetry configuration circuit to be configured to correct for asymmetries in the head. Thus, as a part of the characterization of the drive on which the particular MR head having the characteristic curves  110  and  112  is used, the method of the invention can be used to measure the energy contained in the positive and negative signal envelopes.  
         [0037]    The basic method of the invention can be refined to measure head asymmetries by determining the slopes of the response curves, as illustrated in the graph  130  of FIG. 6, to which reference is now additionally made. The graph  130  shows positive  132  and negative  134  envelopes that are generated by an MR head in response to the continuous signals recorded on the disk  40 . By appropriately controlling the thresholds of the detector and timing for initiating the accumulation of samples, the energies in respective portions A, B, C, and D of the signals  132  and  134  can be separately collected.  
         [0038]    By comparing the energies of sections A and B, and C and D, the leading vs trailing or positive vs. negative pulse shape asymmetry can be precisely determined. Moreover, by applying multiple thresholds, the resolution of the measurement can be increased, as desired. In the past, this asymmetry has been compensated for in the continuous-time filter and/or the FIR of the read channel.  
         [0039]    Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.