1. Field of the Invention
The present invention is directed to an improved control system for controlling the head of a disk drive in track-following and, in particular, to a high sample frequency track-following compensator control system for improving the accuracy of track-following while reducing the manufacturing cost of such system.
2. Description of the Related Art
The reliability of a disk drive in storing and retrieving information is largely dependent on the accuracy by which a data head may be aligned with a data track while reading or writing data. One technique for maintaining alignment between the head and track is to embed servo information fields in the data track. The servo information fields are provided with servo bursts that allow the position of head to be located relative to the center line of the data track. As the disk spins, the servo information fields can be read resulting in a periodic sampling of servo information. An adjustment to the position of the head can then be determined and applied. The correction is typically applied as a current to an actuator motor. Each head, of potentially many heads, is coupled to a respective load beam by a flexure that is in turn connected to the actuator body and motor, thus allowing control over the position of the head. The intended consequence of each position adjustment is to move the head toward alignment with a predetermined data track's centerline. These adjustments are, however, effectively applied as a fixed force for the duration of the period of the sample frequency.
The position of the head may thus oscillate about the center line of a track as a consequence of drifting past the center line between position corrections. The effective frequency of such head oscillations is dependent on the positional amplitude of the head about the track center line and the rate of position correction, which is in turn dependent on the sample rate of the servo information fields.
Consequently, a problem has been found to exist when a head assembly has a resonant frequency that is within or is a multiple of a frequency within the sample frequency of the servo information fields. The resonance of concern arises from the fact that the head, load beam and flexure are to a certain degree flexible in a plane parallel to the surface of the disk. Naturally, the resonant frequency is primarily dependent on the stiffness of the load beam and flexure in that plane, the mass of the load beam and head, and the length of the load beam.
Given the goal of fast position correction and that position corrections are periodic at the servo information field sample rate, overshoot of the track center line will inevitably occur. That is, as the head settles into following along a single track, it may begin to repeatedly and increasingly overshoot the track centerline. As this oscillation approaches the resonant frequency of the load beam and head, the head may actually be thrown completely off track, or at least sufficiently off track to compromise the reading and writing of data to the disk until the position i corrected.
A natural approach to increasing the resonant frequency of the head assembly is to increase the mechanical stiffness of the load beam. The flexure is not easily subject to stiffening given its primary goal of readily allowing the head to float on the air bearing layer that forms on a spinning disk surface. The head itself is quite stiff given its normally ceramic composition.
A conventional method of stiffening the load beam involves adhering a metal strip along the length of the load beam. Unfortunately, this solution necessitates additional precision construction steps in the manufacture of the head assembly. Typically, this results in an approximately one dollar per head assembly additional cost. Consequently, the cost, complexity and difficulty of precision manufacture of the head assembly is significantly increased. Further, the likelihood of failure over time is increased. In addition, the actuator's maximum position correction acceleration rate may be reduced by some degree due to the cumulative increased mass of the load beams.