Method and apparatus for improved PES demodulation in the presence of thermal asperities

#The present invention relates generally to the data processing field, and more particularly, relates to a method and apparatus for improved position error signal (PES) demodulation in the presence of baseline wander offset in disk drives.

#Modern disk drives are becoming more sophisticated with lower flyheight, higher areal recording densities, and faster rotational spindle velocities. Many new high capacity hard disk drives (HDDs) are storing an increasingly large amount of data on the magnetically coated disk surfaces. The areal recording densities are exceeding 10 Megabits per square inch. This large areal recording density requires the recording heads to fly even closer to the disk surface. The reason for the lower flyheight is to obtain the necessary signal strength in the readback signal from the transducer MR-head to discern the recorded data-signal from noise. In other words, the signal-to-noise (S/N) ratio in the readback signal has to exceed a certain level for reliable reading and writing of the recorded signal.

#Average flyheights of less than one microinch or 25 nanometers are common. The dynamic flyheight separation between the head transducer and the disk surface can be considered a random process with a mean value equal to the average flyheight and an erratic variance. Factors involved in the flyheight variation are topological disk surface variations, turbulence in the airflow making up the airbearing cushion underneath the slider, mechanical resonances in the actuator arm/suspension structure, and the like.

#The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein:

#FIG. 1 is a block diagram representation illustrating a data channel for implementing methods for improved position error signal (PES) demodulation in the presence of thermal asperities in accordance with the preferred embodiment;

#FIG. 2 is a diagram illustrating a prior art servo sector format with three tracks;

#Having reference now to the drawings, in FIG. 1, there is shown a direct access storage device (DASD) or disk drive generally designated by 100 for implementing methods for identifying a position error signal (PES) in accordance with the preferred embodiment. As shown in FIG. 1, DASD 100 includes a recorded disk 110 that is spun at constant speed and a recording head 112 that is positioned on a given track for reading information stored on that track. The readback signal is highpass-filtered by an arm electronic (AE) module 114 and applied to a variable gain amplifier (VGA) 116. A recording channel 118 coupled to the VGA 116 provides user data at it output. The recording channel 118 is coupled to a nonlinear time-windowed algorithm position error signal (PES) demodulator 120 for PES generation in accordance with the preferred embodiment. The PES demodulator 120 performs a nonlinear time-windowed algorithm for improved PES generation of the preferred embodiment as illustrated and described with respect to FIGS. 4A and 4B.

#FIG. 2 illustrates a very simplified prior art diagrammatic formatting of one radial servo sector with three tracks, N−1, N, N+1, generally designated by the reference character 200. Referring to FIG. 2, the footprint of the recording MR-head 112 is shown to the left on track (N). The footprint shows that the MR-head 112 is perfectly positioned on the track N as indicated by an arrow labeled H. The MR-head 112 is stationary, while the disk surface 110 with the prewritten information is moving to the left, as indicated by an arrow labeled D. The vertically striped region in FIG. 2 consists of two fields, a write-recovery-field (WRF) and a Gray code field (GCF). In FIG. 2, only two servo pattern fields, the A-field and the B-field, are shown. Typically the vertically striped WRF and GCF region is roughly twice as wide as the crosshatched AB region that consists of the servo pattern fields. Note that both the A-field and the B-field are offset one-half track-pitch and writing in the A-field and B-field is alternately skipped in the radial (vertical) direction. Writing is only done in the crosshatched areas. Each of the crosshatched areas consists usually of several cycles P of a single high-frequency pattern. The readback signal from the MR-head 112 as the crosshatched pattern moves underneath the head will appear as a sinusoidal-like burst signal. Today, most high performance disk drives use four servo pattern fields (A, B, C, D). The (A, B)-field is used for purposes of simplicity in the understanding of position error signal (PES) generation. Typically, the MR head 112 is slightly narrower than the track width.

#Assume that the recording head has a uniform read-sensitivity along the MR-element and that it is positioned as shown in FIG. 2. Then as the A-field followed by B-field pass underneath the head 112, two distinct sinusoidal-type signal-bursts, A(t) and B(t), are present in the MR-head readback signal x(t). Here continuous time is designated by the variable t. The bursts are separated in time, but have the same amplitude.