Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-11-16
2003-06-24
Psitos, Aristotelis M. (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S030150
Reexamination Certificate
active
06584048
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a focus jump device for jumping between recording layers in a storage medium, such as an optical disc, having at least two recording layers.
2. Description of Related Art
In a conventional optical disc reproduction apparatus such as a Compact Disc player, a focus servo control is performed as follows. First, a predetermined drive signal of a sawtooth waveform, for example, is supplied to a drive circuit of an actuator, and then the drive circuit applies an output corresponding to the drive signal to a focus coil provided in the actuator. Thus, the actuator moves toward or away from the disc. Then, a laser beam is irradiated on the optical disc by the objective lens of the actuator, and the light reflected by the optical disc is converted into an electric signal by the photodetector to produce a focus error signal. The focus error signal has a zero-level output when the relative distance between the objective lens and the disc is equal to a reference value, and has an S-shaped characteristic in which the output level continuously varies dependently upon the displacement from the reference distance. The focus error signal thus produced is compared with the reference value, and the comparison result is inputted to the servo loop control unit as a positional error. The servo loop control unit recognizes the zero-crossing point of the focus error signal from the inputted positional error, and inputs the focus error signal to the drive circuit of the actuator when the focus sum signal, indicating the total quantity of the reflected light, is larger than a given level and the focus error signal is at the zero-crossing point. Thus, the focus servo loop is established by the actuator, the photodetector and the drive circuit of the actuator. When the focus servo loop is established, the focus error signal is subjected to the phase compensation by the equalizer amplifier, and then inputted to the drive circuit of the actuator. The drive circuit of the actuator outputs the drive signal to the actuator, based on the inputted focus error signal, to drive the objective lens such that the relative distance between the objective lens and the optical disc is always equal to the reference value. In this way, in a conventional CD player, the pickup is driven by the focus coil to constantly maintain the relative distance between the objective lens and the optical disc to be constant.
Recently, a disc having a multi-layer structure including a first reflective layer and a second reflective layer to achieve high density recording has been put into practice along with the increase of the recording information on an optical disc. In reproducing such an optical disc having multi-layer structure, the focus servo control should be reliably performed for the reproduction layer from which recorded information is to be read out, when the reproduction layer is changed from one to another. For example, in order to change the reproduction layer from the first reproduction layer to the second reproduction layer during the reproduction, the actuator should be jumped to move the focused beam spot from the first reproduction layer to the second reproduction layer. The interlayer jump of the beam spot like this is generally called as “focus jump”. Specifically, the focus jump is performed as follows. In response to the jump start command, an acceleration pulse of a constant amount is applied to the drive circuit of the actuator, and a deceleration pulse of a constant amount is then applied to the drive circuit at a desired timing. By this, an open control of feed-forward type is carried out, and then the above described focus servo control is resumed after the objective lens reaches the target position. The target value of the focus servo is zero, and the detection system shows a linear characteristic when the focus error stays around zero at the focus servo closing timing. However, the output of the detector shows non-linear characteristic such as a sinusoidal waveform when the interlayer jump (i.e., focus jump) is performed. Therefore, conventionally, the feedback control for stabilization is not performed during the, focus jump period.
However, the open control of the feed-forward type is susceptible to external disturbance. For example, the control may be unstable due to the vibration of the disc surface at the time of the focus jump, or variation of the interlayer spacing. Further, the servo may not be quickly set at the time of closing the focus servo after the focus jump, or the focus servo may lose control, thus leading to the deterioration of performance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a focus jump device which performs stable focus jump even if external disturbance such as the vibration of the disc surface or variation of the interlayer spacing exists.
According to one aspect of the present invention, there is provided a focus jump device, adapted to be used in an information reproduction apparatus for reproducing information from a storage medium having at least two recording layers or used in an information recording apparatus for recording information onto the storage medium, which moves a focus position of a light beam in a direction perpendicular to the storage medium to the focus position corresponding to a target recording layer so that the light beam is irradiated on the target recording layer, the device including: a focus position moving unit for moving the focus position of the light beam; a focus error signal detector for detecting a focus error signal based on a variation of a returning light from the storage medium, the variation of the returning light being based on a variation of the focus position of the light beam with respect to the recording layer; a feedforward controller for supplying a drive signal to the focus position moving unit to move the focus position of the light beam in an acceleration state in an acceleration control and to move the focus position of the light beam in a deceleration state in a deceleration control; and a position controller for comparing the focus error signal, which is detected by the focus error signal detector at the time of movement of the focus position of the light beam to the target recording layer triggered by the acceleration control, with a predetermined target value to generate an error, and for feeding back the drive signal to the focus position moving unit to reduce the error thereby to control the focus position of the light beam.
In accordance with the focus jump device thus configured, when the jump instruction is outputted by an external controller or else, the feedforward controller conducts on the focus position moving unit. By this, the focus position of the light beam gradually starts moving, and the relative distance between the focus position of the light beam and the recording layer varies. In addition, by this variation, the returning light from the storage medium also varies, and this variation is detected by the focus error signal detector as the focus error signal. The focus error signal thus detected is compared with a predetermined target value to obtain an error therebetween, and a drive signal is fed back to the focus position moving unit such that the position controller reduces the error, thereby to perform the focus position control of the light beam. On the other hand, after the acceleration control, the feedforward controller performs the deceleration control at the given timing so that the focus position of the light beam reaches the target position of the recording layer. Thus, according to the present invention, with the aid of the feedforward control, the focus position of the light beam is securely moved according to the movement distance between the recording layers, which is longer than the movement distance in the focus servo control. In addition, the focus error signal, serving as a position information corresponding to the movement, is controlled to follow the given target value
Takahashi Kazuo
Tateishi Kiyoshi
Ladas & Parry
Pioneer Corporation
Psitos Aristotelis M.
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