Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-07-05
2004-04-13
To, Doris H. (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044280, C360S077040
Reexamination Certificate
active
06721247
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a track-following control method and a disk apparatus thereof which calculates the control signal for suppressing lead dislocation due to periodic disturbances, such as medium eccentricity by learning control, and executes feed-forward control in a control system to perform feedback control on the moving position of the carriage so that the dislocation amount of the head, with respect to the track center, becomes small (almost zero).
2. Description of the Related Art
A disk apparatus using such a disk as a magnetic disk, optical disk and magneto optical disk as a storage medium is widely used as a memory for images, data and music. In such a disk apparatus, the head must follow the track of the disk to read or write data on the track of the disk. Therefore, track-following control to detect the dislocation of the head from the track and to position the head to the track center is necessary. Recently since the track pitch of a disk is reaching high densities as the storage capacity of the disk increases, periodic disturbance along with the rotation of the disk must be compensated, and for this reason track-following control to compensate for the periodic disturbance is required.
Eccentricity of the disk is generally known as a periodic disturbance, but a periodic disturbance with a frequency higher than an eccentric frequency also exists. In the case of an optical disk unit, for example, in order to improve the track-following performance of the optical beam to the medium track, a double drive type head mechanism is used, which is comprised of a carriage actuator for seek control (also called coarse control) which moves the carriage supported by a sliding bearing section on the stationary-installed guide rail, and a track actuator for tracking control (also called fine control) which moves the optical beam in the direction to cross the track by driving the objective lens mounted on the carriage.
Recently, however, a single drive type head mechanism that has only the carriage actuator, without using the track actuator, is widely used to decrease the cost of the unit. In a single drive type head mechanism, the ball bearing is eliminated from the sliding bearing section so as to decrease the number of parts and to decrease cost.
When such a disk unit is used, that is, when the head mechanism is a single drive type comprised of only the carriage actuator where the ball bearing is eliminated from the sliding bearing section of the carriage, the positioning control of the optical beam on the tracking center based on the tracking error signal is strongly influenced by the solid friction of the carriage bearing section.
FIG. 30
shows the characteristics of solid friction in the single drive type carriage. Here, the moving speed ‘V’ and the friction force ‘F’ have plus or minus values depending on the moving direction of the carriage. Now consider the case when the moving speed ‘V’ of the carriage changes from minus to plus. While the carriage is moving at a moving speed ‘V’ with a minus value, the roughly constant kinetic friction force ‘F
1
’ is being generated regardless the moving speed. When the moving speed ‘V’ of the carriage with respect to the guide rail becomes zero and the carriage begins to move in the opposite direction, a drive force exceeding the static friction force ‘−F
2
’ is required, then a roughly constant kinetic friction force ‘−F
1
’ is generated.
When the moving speed of the carriage inverts like this, the abrupt change of the friction force acts on the control system as a disturbance, and in order to sufficiently compensate for this disturbance, a feedback control system with a high band is generally required.
The moving speed of the carriage is inverted by the track-following control which periodically compensates the dislocation of the track due to the eccentricity of the disk medium. In other words, when the carriage is controlled so as to follow the eccentricity of the medium, the movement of the carriage with respect to the guide rail is a reciprocating motion synchronizing with the eccentricity period. Therefore the moving speed of the carriage inverts at least twice during a rotation of the medium, and a disturbance due to an abrupt change of the friction force in steps is received each time inversion occurs.
FIG. 29
shows waveform diagrams during track-following under the influence of Coulomb's friction disturbance, which is an acceleration of the positioner (carriage) for track-following, a change of the friction disturbance, and a waveform of the ideal drive current ‘I ideal’ of the positioner required for track-following, sequentially from the above. The rotation period of the disk is 13.33 ms, and waveforms for three rotation periods of the disk are shown. To simplify description, the static friction factor and the dynamic friction factor are equally &mgr;=0.4.
As
FIG. 29
shows, in order to implement high precision track following, the ideal drive signal for the positioner should include not only the drive signal for generating the acceleration of the positioner, but also the drive signal that changes abruptly to cancel the friction changes. By a feedback controller, it is difficult to generate such drive signals which include sudden changes because of the limitation of the control band thereof. In other words, a sufficient control band cannot be achieved because of the restriction of the mechanical resonance of the positioner, and as a result, large tracking errors occur.
As
FIG. 30
shows, Coulomb friction disturbance, which is a function of the speed of the positioner, can be compensated for using the speed information of the positioner. However, in the case of a disk unit, such as a magneto-optical disk unit, as the velocity information is not obtained with respect to the guide rail due to not have a sensor to measure the location of the positioner with respect to the guide rail, such a compensation cannot be performed. Therefore by considering periodic disturbance synchronizing with disk rotation, feed-forward compensation by learning and obtaining the repetitive disturbance compensation signal is proposed.
As a typical method to compensate such a periodic disturbance, repetitive control is known. In repetitive control, the basic period of a periodic disturbance is divided, for example, by a sampling period of the feedback control system, memory corresponding to each divided period is prepared, and the periodic disturbance is compensated. In a disk unit, such as a magneto-optical disk unit, however, the sampling period of the feedback control system is relatively short. So if the rotation period of the disk is divided by the sampling period, memory length is very long. For example, when the disk rotation frequency is 75 Hz (4500 rpm), the sampling rate is 55 kHz, and memory length is 733. If the resolution of one memory length is 256 bits, a 187 kbit capacity is required.
In order to decrease the memory length, it is possible to set the dividing period of the repetitive control system to a multiple integer of the sampling period of the feedback control system, and skip the feedback information obtained at the sampling period, but learning an accurate feed-forward compensation signal is difficult since feedback information is not effectively utilized.
Therefore the present inventor and others proposed a learning control system where information obtained at the sampling points of the feedback control system can be effectively used for the convergence of learning, even if the sampling frequency of the feedback control system and the dividing period of the learning waveforms (memory length of learning result) are independently set (an example of this is stated in the ASPE (American Society for Precision Engineering) 1999 Meeting paper “A Precise Track following Control using a Single-stage Tracking Mechanism for Magneto-optical Disk Drive).
This conventional proposal will be described with reference to
FIG. 31
to FIG.
33
.
FIG. 3
Greer Burns & Crain Ltd.
Ortiz-Criado Jorge L
To Doris H.
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