The present invention relates to an access method and apparatus for rapidly retrieving desired information from a high density record medium which stores information signals thereon, and more particularly to an access method and apparatus which rapidly and optically retrieves desired information from a rotating optical disk on which information signals are recorded concentrically or spirally at a high density.
The optical disk has a number of concentric or spiral tracks formed thereon at a predetermined pitch, and a number of sectors for delimiting data are formed for each track. In order to record external information at any position or reproduce or erase information recorded at any position, one of the tracks on the disk is sought and one sector on the track is then sought. That is, a macro-seek control (i.e., speed control and position control) in which a light spot is rapidly moved to a neighborhood of a target track, a tracking servo in which the light spot is maintained at a center of the track, and a micro-seek control (for example, jump control) in which a deviation from the target track is corrected are necessary. The access operation in such an optical disk storage is disclosed in Japanese Unexamined Patent Publication Nos. 58-91536 and 58-169370 (corresponding to U.S. patent application Ser. No. 06/443,399, continuation application Ser. No. 06/736,125, now U.S. Pat. No. 4,607,358 and EPC Patent Application No. 82110907.1). In the optical disk storage, the positioning of the light spot is controlled by cooperation of a coarse actuator, such as a linear motor and a fine actuator, such as a galvanomirror. The coarse positioning of the optical head is carried out by the course actuator and an external scale as a position detector, and the fine positioning to the track is carried out by the fine actuator by detecting an address signal recorded on the track. A scale pitch of the external pitch is a multiple of the track pitch.
The macro-seek operation for positioning the optical head to an approximate target position by using the external coarse scale usually effects speed control to reduce an access time. However, since the macro-positioning is carried out by using the external scale which is independent from the track eccentricity, the deviation from the target track varies significantly due to the eccentricity. As a result, the distance of movement of the light spot in the next micro-seek control (for example, jump control) is large and the access time increases. In order to control the speed, a means for detecting the speed of the head is necessary. However, when a separate detector (such as a tachometer) for detecting the speed is mounted, the weight of the head increases and an oscillation at a low frequency may be generated depending on the length of the tachometer. In addition, the cost increases. Accordingly, it is prederable to detect the speed by using the output of the external linear scale. A pulse frequency of a zero-crossing detection signal is proportional to the speed of the head. Accordingly, the zero-crossing detection signal is frequency-voltage converted so that the speed is detected based on a voltage. However, the frequency-voltage conversion does not provide an accurate conversion output when the frequency is low (head speed is low), because the pulse interval of the zero-crossing detection signal increases and the speed change in the interval cannot be detected. Accordingly, when the speed is controlled by using the output of the frequency-voltage conversion, the control is unstable when the speed is low and the system may overrun.