1. Field of the Invention
The present invention relates to a track following control method and apparatus for a hard disk drive (HDD). More particularly, the invention relates to a track following control method in which estimators and controllers according to operation modes of an HDD separately exist and an apparatus suitable for performing the track following control method.
This application claims the benefit of Korean Patent Application No. 10-2006-0059244, filed on Jun. 29, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
2. Description of the Related Art
Information is stored on a hard disk drive (HDD) within a concentric arrangement of data storage tracks. The magnetic read/write head (hereafter “head”) of the HDD must be aligned over a specified target track in order to perform read/write operations. Thus, read/write operations require a certain degree of accuracy in the positioning of the head throughout read/write operations. A track following control apparatus is commonly used within the HDD to maintain proper head positioning.
Figure (FIG.) 1 is a general block diagram of a conventional track following control apparatus. The illustrated track following control apparatus includes an estimator 102 which estimates a position, velocity, and/or acceleration for the head based on a position error signal derived from a servo signal recorded at defined intervals on the disk. (The position velocity and/or acceleration are singularly or collectively referred to hereafter as “state information”. The track following control apparatus also includes a controller 104 generating a control signal that drives the head in accordance with the estimated state information obtained from estimator 102. The control signal provided by controller 104 is applied to a head disk assembly (HDA) 106 which mechanically positions the head.
FIG. 2 is a conceptual block diagram illustrating a mathematical model that describes the operation of the conventional track following control apparatus illustrated in FIG. 1.
Referring collectively to FIGS. 1 and 2, estimator 102 is assumed to output estimation values x1h, x2h, and x3h respectively indicating the estimated state information (e.g., position, velocity, and acceleration) for the head. The estimation values x1h, x2h, and x3h may be obtained using the following well known equations:x1h=x1b+L1*estimate,x2h=x2b+L2*estimate,x3h=x3b+L3*estimate.
Controller 104 may be implemented using amplifiers that respectively multiply the estimation values x1h and x2h by gain factors K1 and K2. Following gain factor multiplication, estimation values x1h and x2h are applied to a first adder. Estimation value x3h is applied to second adder with a unity gain factor along with the output of the first adder. The output of the second adder is applied to HAD 106 as the control signal.
However, this straight forward approach to implementing a track following control apparatus may be complicated by certain emerging HDD designs. For example, the greatly increased recording density of contemporary disks with HDDs often motivates the use of separate read and write heads. Within this configuration, write heads are commonly wider than read heads in order to provide improved write operation performance. This physical size difference between read and write heads necessitates the use of a different (e.g., more strict) positioning tolerance for the write head. In addition, read operation failures are typically considered less critical than write operation failures, because it is relatively easy to “retry” a failed read operation. In contrast, a significant positioning error for a write head during a write operation may result in an overwrite of data stored in adjacent track. Thus, a relatively strict positioning tolerance for a write head during a write operation is mandated.
As a result of the foregoing considerations, the tracking control implemented by an HDD controller is typically designed in relation to the write operation. The resulting tracking tolerance is also applied by the HDD controller to read operations,
Unfortunately, as track densities have continued to increase, the characteristics (e.g., the detected amplitude) of the position error signal has become a limiting factor in the design and implementation of track following control apparatuses within emerging HDDs. That is, increased track densities result in narrower track widths. Narrower track widths yield a narrower recording width for the servo signal from which the position error signal is derived. That is, a narrower recording width for the servo signal results in a position error signal having reduced (and therefore less easily detected) amplitude. As a result, the operating margin for the constituent track following control apparatus decreases with the decrease in the amplitude of the position error signal. This reduced operating margin affords less protection from disturbances within operation of the HDD.
However, since the tolerance for positioning errors during read operations is greater than that for write operations, read operations are generally less susceptible to the system design limitations cause by the reduced amplitude of the position error signal. However, conventional track following control apparatuses are designed with the same operating characteristics for both read and write operations. So, read operation tend to suffer under the same positioning error tolerances imposed by write operations.