Patent Publication Number: US-2010128380-A1

Title: Ses assisted write fly height monitor and control

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for determining the flying height of a head of a hard disk drive. 
     2. Background Information 
     Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA&#39;s are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks. 
     HGA transducers include three primary elements: a reader sensor, a writer structure and a head protrusion control element, also known as fly-on-demand (“FOD”). The reader sensor is commonly made of an MR structure. The writer structure includes a coil and a magnetic flux path structure made with high permeability and high magnetization material. The head protrusion control element (FOD device) is typically includes a heater coil. When a current is applied, the coil generates heat and causes the writer and reader elements to move closer to the media. The FOD device is used to dynamically set writer spacing and reader spacing to the disk surface during the operation of the disk drive. 
     During operation, each head is separated from a corresponding disk surface by an air bearing. The air bearing eliminates mechanical interference between the head and the disks. The FOD device is used to further set reader and writer positions above the disk surface, based on a pre-calibrated target. The strength of the magnetic field from the disk is inversely proportional to the height of the reader head spacing to the disk. Reduced spacing results in a stronger magnetic field on the disk, and vice versa. 
     The flying height of a head may vary during the operation of the drive. For example, a shock load on the drive may create a vibration that causes the heads to mechanically resonate. The vibration causes the heads to move toward and then away from the disk surfaces in an oscillating manner. Particles or scratch ridges in the disk may also cause oscillating movement of the heads. The oscillating movement may occur in either a vertical or in-plane direction relative to the flexure arm. Environment changes, such as temperature and altitude can also cause a change in the head flying height. 
     If oscillation of the heads occurs during a write routine of the drive, the resultant magnetic field from the writer on the disk will vary inversely relative to the flying height of the writer. The varying magnetic field strength may result in poor writing of data. Errors may occur when the signal is read back by the drive. 
     Knowing and controlling the flying heights of the heads is critical for both disk drive reliability and data integrity. With the introduction of FOD technology, the disk drive can dynamically control head flying height. To accurately operate the FOD device and achieve the desirable writer and reader spacings to the disk, flying height measurement techniques have been developed. The most common technique is to use playback signal components in frequency domain. 
     The FOD device can be used to adjust head flying height in real time. The relative flying change for a given FOD device condition can be accurately characterized. If the head flying height relative to a desirable target can be measured, the offset can then be compensated by proper fine tuning of the FOD device setting (adjust either current or voltage). A spacing error signal (SES) of a head is defined as an indicator of a spacing offset between an actual head position and a desirable head position. The concept of SES is very similar to a position error signal (“PES”) of a disk drive servo system. One can view SES as the PES of head in the direction perpendicular to the disk surface. 
     There are various methods for creating spacing error signals (“SES”) that are used to control the flying height through feedback schemes. Practical construction of spacing error signals (“SES”) is limited by available electrical/mechanical signals and disk drive hardware capability. One type of SES is to use a servo automatic gain control (“AGC”) signal where a signal (AGC) embedded into a dedicated field of a servo sector is read and used to calculate SES in accordance with an AGC process. There are also schemes to utilize an AGC that reads data from a data field of the track sector. Finally, SESs can be generated by analyzing the 1st and 3rd harmonics, or ratio of harmonics, from an embedded signal(s) in a dedicated track. 
     Prior art schemes used to determine flying height are performed during the read operation of a drive. Errors due to excessive flying height may occur during the write process. Such errors are not identified until the written data is read back by the drive. It would be desirable to determine the flying height during a write operation of a hard disk drive. 
     BRIEF SUMMARY OF THE INVENTION 
     A hard disk drive that includes a disk, and a head that is separated from the disk by a flying height. The disk drive also includes a circuit that determines the flying height from a signal read from the disk. The circuit performs a calibration routine to determine a temperature dependent variable of the signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of an embodiment of a hard disk drive; 
         FIG. 2  is a top enlarged view of a head of the hard disk drive; 
         FIG. 3  is a schematic of an electrical circuit for the hard disk drive; 
         FIG. 4  is a schematic showing function blocks of a read channel of the drive; 
         FIG. 5  is an illustration showing a track sector of a disk; 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed is a hard disk drive that includes a disk, and a head that is separated from the disk by a flying height. The disk drive also includes a circuit that determines the flying height from a signal read during a write operation of the drive. The circuit performs a calibration routine to determine a temperature dependent variable of the signal to offset any temperature effects on the signal used to determine the flying height. The calibration routine can be performed using a spacing error signal (“SES”) generated by the drive. 
     Referring to the drawings more particularly by reference numbers,  FIG. 1  shows an embodiment of a hard disk drive  10  of the present invention. The disk drive  10  may include one or more magnetic disks  12  that are rotated by a spindle motor  14 . The spindle motor  14  may be mounted to a base plate  16 . The disk drive  10  may further have a cover  18  that encloses the disks  12 . 
     The disk drive  10  may include a plurality of heads  20  located adjacent to the disks  12 . As shown in  FIG. 2  the heads  20  may have separate write  24  and read elements  22 . The write element  24  magnetizes the disk  12  to write data. The read element  22  senses the magnetic fields of the disks  12  to read data. By way of example, the read element  22  may be constructed from a magneto-resistive material that has a resistance which varies linearly with changes in magnetic flux. The heads also contain a heater coil  25 . Current can be provided to the heater coil  25  to generate heat within the head  20 . The heat thermally expands the head  20  and moves the read and write elements closer to the disk. 
     Referring to  FIG. 1 , each head  20  may be gimbal mounted to a flexure arm  26  as part of a head gimbal assembly (HGA). The flexure arms  26  are attached to an actuator arm  28  that is pivotally mounted to the base plate  16  by a bearing assembly  30 . A voice coil  32  is attached to the actuator arm  28 . The voice coil  32  is coupled to a magnet assembly  34  to create a voice coil motor (VCM)  36 . Providing a current to the voice coil  32  will create a torque that swings the actuator arm  28  and moves the heads  20  across the disks  12 . 
     The hard disk drive  10  may include a printed circuit board assembly  38  that includes one or more integrated circuits  40  coupled to a printed circuit board  42 . The printed circuit board  40  is coupled to the voice coil  32 , heads  20  and spindle motor  14  by wires (not shown). 
       FIG. 3  shows an electrical circuit  50  for reading and writing data onto the disks  12 . The circuit  50  may include a pre-amplifier circuit  52  that is coupled to the heads  20 . The pre-amplifier circuit  52  has a read data channel  54  and a write data channel  56  that are connected to a read/write channel circuit  58 . The pre-amplifier  52  also has a read/write enable gate  60  connected to a controller  64 . Data can be written onto the disks  12 , or read from the disks  12  by enabling the read/write enable gate  60 . 
     The read/write channel circuit  58  is connected to a controller  64  through read and write channels  66  and  68 , respectively, and read and write gates  70  and  72 , respectively. The read gate  70  is enabled when data is to be read from the disks  12 . The write gate  72  is to be enabled when writing data to the disks  12 . The controller  64  may be a digital signal processor that operates in accordance with a software routine, including a routine(s) to write and read data from the disks  12 . The read/write channel circuit  58  and controller  64  may also be connected to a motor control circuit  74  which controls the voice coil motor  36  and spindle motor  14  of the disk drive  10 . The controller  64  may be connected to a non-volatile memory device  76 . By way of example, the device  76  may be a read only memory (“ROM”). The non-volatile memory  76  may contain the instructions to operate the controller and disk drive. Alternatively, the controller may have embedded firmware to operate the drive. 
       FIG. 4  is a schematic showing functional blocks of a read channel and pre-amp of the disk drive for servo signal processing. The read channel includes an amplifier  80  coupled to a head(s) (not shown). The amplifier  80  adjusts the amplitude of a signal read by the head. The amplified signal is filtered by filter  82  and converted to a digital bit string by an analog to digital (“ADC”) converter  84 . 
     The gain of the amplifier  80  is adjusted by an automatic gain control circuit  86 . The automatic gain control circuit  86  receives as input the digital output of the ADC  84  and provides an analog control signal to the amplifier  80 . 
     The automatic gain control signal is inversely proportional to the amplitude of the read signal. A weak signal will result in a larger control signal. A larger control signal will increase the gain of the automatic gain control circuit and boost the amplitude of the read signal. The signal read by the head is inversely proportional to the head fly height. Consequently, the control signal is proportional to the flying height. 
       FIG. 5  is an illustration of a track sector of a disk. The sector typically includes a sync field  102  and a servo field  104  as is known in the art. The sector also has a data field  106 . 
     A read signal generated by the sync field can be used to determine the flying height of a head. A flying height F s  calculated from the sync signal can be expressed as: 
         F   s   =F   ref   +F   sp   +F   t    (1) 
     Where; 
     
         
         F ref =a reference spacing under specific read conditions. 
         F sp =the change in flying height. 
         F t =is an error due to temperature change. 
       
    
     For short term changes in flying height the temperature error is non-existent because the drive temperature will not vary rapidly. Knowing the reference spacing F ref  and measuring the sync signal amplitude F s  the change in flying height F sp  can be calculated from equation (1) (i.e., F t =0). The reference spacing F ref  may change per sector. A look up table for the various sectors may be generated and called to determine the change in flying height for a specific sector. F ref  is a function of the magnetic properties of the read signal. Any variations due to magnetic properties can be nulled out of the F ref  before using it in equation (1). 
     The flying height can be determined during a write operation. During a write operation, the system reads the sync field and servo fields to insure that the heads are properly aligned with the disk tracks. Therefore, utilizing the sync field allows the flying height to be determined during a write operation. 
     Over time, the temperature error F t  may be introduced into the measured signal F s . If the measured signal F s  exceeds a threshold the system may perform a calibration routine to determine the temperature error. The calibration routine may also be performed during regular time intervals, or before each write operation. 
     A SES signal F ses  can be expressed as a function of F s  and F t  by the equation: 
         F   SES   =F   ref   +F   sp   =F   s   −F   t    (2) 
     The F SES  signal is obtained from a read signal during the reading of data during the calibration process. The F SES  can be generated in accordance with the method described in application Ser. No. ______, filed on ______, entitled Harmonic Measurement For Head-Disk Spacing Control Using User Data, which is hereby incorporated by reference. The temperature error can therefore be calculated by subtracting the SES signal F ses  from the sync signal F s . The calculated temperature error F t  is then used in equation (1) to determine the change in flying height F sp . 
     SES calibration values from other tracks can be used by utilizing a spacing profile. This may allow for a spacing profile equation that is a function of disk radius described as follows: 
         F ( r )= F   p ( r )+ F   c ( a )− F   p ( a )   (3) 
     Where; 
     
         
         F p (r)=the spacing profile as a function of radius. 
         F c (a)=the SES calibration results at radius a. 
         F p (a)=the spacing profile at radius a. 
         F(r)=the spacing at any radius. 
       
    
     Alternatively, the temperature dependent spacing change can be calculated by using the 1 st  and 3 rd  harmonics of a read signal and determining a temperature dependent gain G(T) with the following equation: 
         G ( T )=√{square root over ( V ( f ) 3   /V (3 f ))}{square root over ( V ( f ) 3   /V (3 f ))}  (4) 
     The gain G(T) can be calculated during a read operation and during the calibration process. Once the gain G(T) is calculated the dependent spacing d can be computed from either the following 1 st  or 3 rd  harmonic equations: 
         V ( f )= G ( T ) e   −2λdf    (5) 
         V (3 f )= G ( T ) e   −6πdf    (6) 
     The spacing d can be determined during a write operation. 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.