The role of an optical disk system as memory storage device is going to be increased as long as the multimedia has advanced. In the most recent study, high data transfer rate becomes inevitable necessities to improve the performance of an optical disk system. Therefore, there have studied about this technology because a high speed optical disk is required for many applications, such as picture storage, moving pictures storage, voice or a mixture of them in addition to conventional digitally code data file.
Because there are some mechanical limits in the development of new high-speed actuating system to enhance of the data transfer rate in the optical disk system, the multi-beam reading system based on new servo system of coarse actuator has been suggested as new method. The multi-beam reading system is accomplished by increasing the beam spot number as a reading source on a disk. The adaptive model following control as new coarse servo system help the multi-beam reading system have the effect of a multi-beam data transfer in whole disk area through the fast seek control algorithm.
The key point of a multi-beam reading optics system is simple in the view of the structure. The optical path is constructed more simple than the previous study on multi-beam optical system thanks to a focusing/tracking error signal detection method and a reading signal detection method on beam spot. That is, the astigmatism method for a focusing error detection and the differential phase tracking method for a tracking error detection on one beam spot. In addition, the characteristics of multi-beam optical system is analyzed such as the data transfer effect according to the multi-beam number and the cause of the track-off of the side beam in a multi-beam system. The overall multi-beam optics is established through the optical simulation and several experimental trials.
A new multi-beam reading system positioning is constructed on the grounds of tri-positioning systems and AMFC system of the coarse actuator. The tri-positioning systems are composed of a focusing and tracking actuating system basically and a newly developed beam rotating actuator. The focusing and tracking actuators is the same a lens actuator as that in the conventional one beam optical disk system. In order to trace individual beam spots of a multi-beam optical system on more than one track simultaneously, the conventional tracking method, which moves beam spots laterally across a track with only a tracking actuator, is insufficient. The reason is that the track pitch varies in appearance due to specially, eccentricity of a disk. In order to make up for this track variation apparently, it is necessary that beam array be rotated at the optical axis in addition to tracing beam spots on tracks in the radial direction. For this reason, the dove prism inserted in an optical path is used to rotate beam spots on disk for tracing multi-beam reading spots on tracks simultaneously. Therefore, the newly developed beam rotating actuator is proposed with no friction and the simple structure of a cantilever arm. The beam rotating actuator must have high resolution and high response enough to adapt to the environment of a multi-beam optical system. For this reason, a piezoelectric material is suggested as the actuating device. The dynamic model of the beam rotating actuator is derived. The beam rotating actuator is designed on the base of the dynamic model.
In the time optimal control problem of a coarse actuator, bang-bang control has been used because it is the theoretical time minimum solution. However, to improve tracking speed performance in the time optimal control, it is important to select a switching point accurately which makes the velocity zero near the target track. But it is not easy to select the switching point accurately because of the damping coefficient variation and uncertainties of modeling an actual system. The adaptive model-following control is implemented to relieve the difficulty and inconvenience of this task. The adaptive model-following control can make the controlled plant follow as closely as possible to a desired reference model whose switching point can be calculated easily and accurately, assuring the error between the states of the reference model and those of the controlled plant approaches zero. The performance of coarse actuator is improved more than 20% thanks to the adaptive model following control.
The beam rotating actuator servo is designed and constructed through the beam rotating mechanism analysis of side beam that is generated by the eccentricity of a disk. Simple PID control scheme based on a strain gage sensor is used to compensate for the track-off of side beam. It is shown experimentally that the proposed multi-beam reading system positioning has enough performance to adequate to optical disk system. The focus error of a focusing servo system is less or an allowable error limit, ±0.35um and the tracking servo system is satisfied with the condition of a tracking error limit, ±0.03um. The beam rotating actuator has the enough resolution of below ±0.0025°.
Through the effect analysis of multi-beam transfer and experiments, more or 2 times a data transfer for an ordinary single-beam optical disk, is realized in three multi-beam reading system. The data transfer performance of the overall multi-beam reading system is also more than 24% in the whole disk area due to 20% enhancement in the performance of a coarse actuator.
