JPH02206025A - Focus servo device - Google Patents

Abstract

PURPOSE:To prevent contact between an objective lens and an optical disk by detecting the reduction of the quantity of reflected light from the optical disk and suppressing driving of an actuator in response to the detection output. CONSTITUTION:The reflected laser light from an optical disk is received by two photodetectors 1 and 2 provided on a head, and light reception outputs of these photodetectors are supplied to a differential amplifier 3, and a focus error signal corrected in accordance with the reflectivity change of the disk is outputted from an AGC 9 which has the gain switched and controlled in accordance with the quantity of reception light, and the servo control of the position of the objective lens is performed through a driving circuit 11, etc., by the actuator. Simultaneously, the signal corresponding to the quantity of reception light by a summing amplifier 6 is monitored by a light quantity monitor circuit 13 to which a reference voltage is applied from a reference voltage circuit 14, and a switch 15 is switched by the inverted output of the circuit 13 at the time of the reduction of the quantity of reception light which is caused by light reception from an area other than the recording/reproducing area of the optical disk of low reflectivity and cannot be corrected by the AGC 9. Thus, driving of the actuator is suppressed to prevent contact between the optical disk and the objective lens.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical disc device for recording or reproducing information on an optical disc. The present invention relates to a focus servo device that controls an objective lens of an optical head and an optical disk at a constant interval so as to achieve (just focus).

2. Description of the Related Art The structure of a conventional focus servo device for an optical disc device will be described with reference to FIGS. 2 to 4.

In FIG. 2, numeral 21 is an optical disk, and 21 is an optical head. The output laser light of the semiconductor laser 22 passes through an optical system such as a collimator lens for beam shaping, an optical system U for optical isolation such as a polarizing beam splitter, and is further narrowed down by an objective lens 5 to be used for recording. The laser beam is irradiated onto the optical disc as a reproduction laser beam. The reflected light of this laser beam passes through the objective lens 5 and the optical system, and further passes through the reflected light detection optical system and is sent to the photodetector 27.
The light is received by the

The laser beam for recording or reproduction needs to be focused on the information recording layer of the optical disc, but this focus adjustment is performed by moving the objective lens 5 toward the optical axis using a 7-unit unit 293 (voice coil motor, etc.). This is done by displacing it. The focus servo device is
The actuator 2 is moved so that the distance between the optical disc and the objective lens section is maintained at the standard distance (the distance required to bring it into focus).
This is a device that drives 9.

For such control, the distance error (focus error) between the objective lens 5 and the optical disk is detected from the detection output of the photodetector 27. There are various methods for detecting this focus error, and the configuration of the reflected light detection optical system 26 differs depending on the method. Further, the photodetector 27 generally has a divided light receiving surface that functions as two or more photodetecting elements. The number of light-receiving surface divisions is determined by the focus error detection method.
For example, in the case of the double knife method, the light-receiving surface of the photodetector 27 is divided into four parts as shown in FIG. 3, and spots 30 and 31 of reflected light are imaged, for example, as shown in FIG.

A focus error signal is obtained by taking the difference or the sum of the outputs of two or more dividing surfaces (or two or more photodetecting elements) of the photodetector 27, and in order to set this signal to zero, The focus servo device is actuator 29
to drive.

FIG. 4 is a block diagram of such a conventional focusing device. To simplify the explanation, here, we will assume that the photodetector 27 in FIG. It is a detection element. In the focused state, the amounts of light received by the photodetecting elements 1 and 2 are the same, but when the focus of the laser beam is shifted in front of or behind the information recording layer, the amounts of light received by each of them increase or decrease symmetrically.

The detection outputs of the photodetecting elements 1 and 2 are amplified by a preamplifier 3.4 and input to a differential amplifier 5 and a sum amplifier 6, and a difference signal (focus error signal) FE between the respective signals is generated.
I and a sum signal (light reception level signal) FSI are generated.

During the recording operation and the reproduction operation, the power of the laser beam irradiated onto the optical disk is switched, and the light detecting elements 1 and 2 are switched.
Naturally, the received light intensity also changes. To compensate for this effect,
The focus error signal FE2 and the received light level signal FS2 that pass through the gain switching circuit 5 and 7 and 8, in which the amplification gain is switched between the recording operation and the playback operation, are sent to the AGC (automatic gain control) circuit. Enter 9. Incidentally, the gain switching circuits 7 and 8 are not necessary for a read-only optical disc device.

The AGC circuit 9 compensates for the difference in reflectance between the information-recorded part and the non-recorded part of the optical disc by changing the amplification gain of the focus error signal FE2 according to the received light level signal F'S2. A focus error signal FE3 is output. This focus error signal FE
3 is passed through a phase compensation circuit 10 for increasing the phase margin at the gain intersection of the focus servo to stabilize the servo operation and increase the servo gain at low frequencies;
This becomes a focus error signal FE4. Reference numeral 11 denotes a drive circuit that drives the drive coil 12 of the focus adjustment actuator 4 according to the focus error, and supplies the drive coil 12 with an excitation current having a magnitude and polarity corresponding to the voltage and polarity of the focus error signal FE4.

When the laser beam is focused on the information recording layer of the 6-page optical disc (just focus state), the level of the focus error signal FE (FEZ, FE2.FE3°FE4) is zero, but the focus of the laser beam is When the information recording layer shifts from front to back (defocus state), the polarity and level of the focus error signal FE change depending on the direction and amount of the shift (focus error). The drive coil 1
By automatically controlling the magnitude and polarity of the excitation current No. 2, the distance between the objective lens and the optical disk is adjusted to a specified distance even if there is surface wobbling of the optical disk, and a just-focus state is maintained.

Problems to be Solved by the Invention However, according to the configuration of such a focus servo device,
When the irradiation position of the laser beam is outside the recording/reproducing area of the optical disc, the focus servo becomes abnormal, which can cause problems such as damage to the optical disc due to contact between the objective lens and the optical disc.

71\-7 That is, when the irradiation position of the laser beam deviates from the recording/reproducing area of the optical disc as shown by the chain line 32 in FIG.
Since the reflectance of the optical disc is extremely low compared to the recording and reproducing area where there is an information recording layer path with high reflectance, the photodetecting element 1
, 2, the detection output level is significantly reduced. For example, in the case of the four-split photodetector shown in FIG. 3, the reflected light spots 30, 31 become smaller outside the recording/reproducing area as shown by the broken line.

This level drop cannot be sufficiently compensated for by the AGC circuit 9, which aims to compensate for changes in reflectance within the recording/reproducing area, so the focus servo may not work properly.1.
An excessive excitation current may flow through the drive coil 12 of the actuator, causing the objective lens to collide with the optical disk.

Note that, conventionally, a heater is generally provided inside the drive circuit 11 to prevent overcurrent. The function of this heater prevents the drive coil 12 from burning out due to excessive excitation current. However, this method only works after the excitation current actually exceeds its maximum value, and there is a long time delay until the fuse blows out, so it is not always possible to reliably prevent accidental contact between the objective lens and the optical disk. do not have. Another problem is that if a fuse blows out, a new fuse must be replaced, which takes time and effort to restart the device.

The present invention has been made in view of the above-mentioned problems, and prevents contact accidents between the objective lens and the optical disk when the laser beam irradiation position is out of the recording/reproducing area, without using overcurrent prevention means such as a fuse. An object of the present invention is to provide a focus servo device that reliably prevents this.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides means for detecting deviation of the laser beam irradiation position from the recording/reproducing area of the optical disk by monitoring the amount of reflected light from the optical disk, and The present invention includes a means for suppressing driving of an actuator of an objective lens in response to a focus error in response to a detection output.

Effect 9 In the above-described configuration, the laser beam irradiation position deviates from the recording/reproducing area of the optical disk, the amount of reflected light is extremely reduced compared to the amount of reflected light within the recording/reproducing area, and normal focus servo is not possible. Even if this becomes impossible, a deviation from the recording/playback area is detected due to an abnormal drop in light intensity, and the drive of the objective lens actuator according to the focus error is inhibited, thereby preventing accidents between the objective lens and the optical disk. .

Embodiment FIG. 1 is a block diagram of a focus servo device according to an embodiment of the present invention. In FIG. 1, the same reference numerals as those in FIG. 4 are equivalent parts, so the previous explanation will be used for these parts.

In this focus servo device, a light amount monitoring circuit 13 and a reference voltage circuit 14 are added for monitoring the amount of reflected light and detecting deviation from the recording/reproducing area.
A switch 15 is inserted between the path 9 to the AGC circuit 10 and the phase compensation circuit 10 to open the servo loop when the recording/reproduction area is deviated and to suppress actuator drive according to the focus error.

In this embodiment, the light amount monitoring circuit 13 is a circuit that pulls up the output of an analog comparator 16 using a resistor R2, and the reference voltage circuit 14 divides the negative power supply voltage using a resistor R1 and a variable resistor VRI to generate a reference voltage. This is a circuit that obtains the following. If the output FSI (proportional to the amount of reflected light) of the sum amplifier 6 is, for example, about -3V when the laser beam irradiation position is inside the recording/playback area of the optical disc, the reference voltage is, for example, half of that, -1, The variable resistor VRI is set so that the voltage is about 5V.

In the above configuration, when the laser beam irradiation position is within the recording/reproducing area, the output FS3 of the analog comparator 16 is at the L level (OV), so the switch 1
5 is turned on and the servo loop is closed. Therefore, an excitation current according to the focus error signal flows through the drive coil 12 of the actuator, and the focus servo functions.

On the other hand, when the laser beam irradiation position moves out of the recording/reproducing area, the amount of reflected light becomes extremely small, the output FSI of the sum amplifier 6 becomes almost 0, and the output of the analog comparator 16 becomes H. level (+5 V). That is, since the light amount monitoring circuit 13 outputs a detection output of the deviation from the recording/reproducing area, the switch 15 is turned off and the servo loop is opened. As a result, the drive of the actuator according to the focus error is suppressed (the drive coil 12
Either no current flows at all, or only a predetermined current flows without fear of the objective lens coming into contact with the optical disk).

In this way, the drive coil 12
Unlike the method of cutting off the excitation current with a heater when the current of the drive coil 12 becomes excessive, actuator drive is inhibited before the excitation current of the drive coil 12 becomes excessive when it deviates from the recording/reproduction area. There is no need to worry about accidental contact between the disc and the optical disc.

In addition, when the laser beam irradiation position returns to within the recording/reproducing area and the output FSI of the sum amplifier 6 exceeds the reference voltage, the switch 1
5 is turned on again, and the suppression of actuator drive according to the focus error is released. Unlike the method here where the excitation current is cut off using a fuse, there is no need to take time to restart the device after it leaves the recording/reproducing area.

Effects of the Invention As is clear from the above description, the present invention detects deviation of the laser beam irradiation position from the recording/reproducing area of the optical disk by a decrease in the amount of reflected light, and adjusts the objective lens actuator according to the focus error. Immediately inhibiting the drive of the optical disk makes it possible to reliably prevent an accidental contact between the objective lens and the optical disk, and also facilitates restarting the optical disc after the optical disk deviates from the recording/reproducing area.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a focus servo device according to an embodiment of the present invention, FIG. 2 is a schematic configuration diagram of an optical head of an optical disk device, FIG. 3 is an explanatory diagram of a photodetector for detecting reflected light, and FIG. The figure is a block diagram of a conventional focus servo device. 1.2... Photodetection element, 5... Differential amplifier, 6...
・Sum amplifier, 9...AGC circuit, 11...drive circuit,
12... Drive] 3.\-7 Moving coil, 13... Light amount monitoring circuit, 14... Reference voltage circuit, Adding... Optical disk, 21... Optical head,
5... Objective lens, F... Photodetector, 4... Actuator. Name of agent: Patent attorney Shigetaka Awano and one other person

Claims (1)

[Claims] means for detecting an error in the distance between the optical disk and the objective lens of the optical head from a reference distance based on reflected light of a laser beam irradiated from the optical head to the optical disk; and according to the error detected by the means, means for driving an actuator for displacing the objective lens relative to the optical disk; and means for detecting deviation of the irradiation position of the laser beam from the recording/reproducing area of the optical disk by monitoring the amount of the reflected light. A focus servo device for an optical disk device, comprising: and means for suppressing driving of an actuator according to the error in response to the detection output of the means.