1. Field of the Invention
The present invention relates to a sector servo system access controller.
2. Related Art of the Invention
As an access controller for, e.g., a magnetic disk apparatus, a sector servo system access controller is well known. In the case of this system, a magnetic head is positioned on a destination track on the basis of servo data recorded in advance in a servo area on a data surface of a magnetic disk serving as a recording medium. In the servo area, servo data consisting of servo patterns shown in FIG. 1A is recorded for each track block of, for example, 16 tracks.
In FIG. 1A, each dotted line represents the center of a corresponding one of tracks 0 to 15 in each block. As shown in FIG. 1B, for example, servo data corresponding to servo patterns is 5-bit data having bits P, Q, R, L, and P. The bit data "1", "0", and "X" of bits P, Q and R represent "presence", "absence", and "indeterminate state", respectively. The bits LP are bits of a 4-track period determined on the basis of servo patterns A, B, C, and D. Therefore, as shown in FIGS. 1A and 1B, when a head 20 is present on the track 0, servo data "X0100" is obtained.
In a conventional access controller, a peak value of each of the servo patterns A to D is held in correspondence with a track center wherein the magnetic head is positioned, as shown in FIG. 1C. Thereafter, peak value differences "X=A-B" and "Y=C-D" are calculated, the bits LP of the 4-track period being determined in accordance therewith. More specifically, if "X.gtoreq.0, and Y&gt;0", the bits LP are "00"; if "X&gt;0, and Y.ltoreq.0", they are "01"; if "X.ltoreq.0", and Y&lt;0", they are "10"; and if "X&lt;0", and Y.gtoreq.0", they are "11". As a result, the whole servo data is obtained from the bits P, Q, R, and LP.
In the conventional access controller, the track position of a head in a track block is determined in accordance with servo data of 5 bits, i.e., the bits P, Q, R, and LP. Therefore, the head position on a recording medium can be determined in accordance with data representing a block-in track position and data representing a track block position. For example, when the servo data is "01100", it is determined that the head is positioned on the track 8 of the track block. However, due to an influence of disturbance on the controller, a defect of the recording medium, or the like, there is a case that incorrect servo data such as "00011" is generated. Conventionally, in such a case, the current track position of the head cannot be determined, and thus the head position cannot be controlled.