JPS6161238A - Focus pulling-in system - Google Patents

Abstract

PURPOSE:To make focus servo stably at a normally exact just focus point by providing a detector that detects the envelope waveform of a track error signal at a required threshold value or more and determining the timing of starting driving of a focus servo mechanism. CONSTITUTION:A signal generating circuit 28 makes initial driving of a condenser lens 5, and a switching circuit 26 changes a signal that drives the condenser lens from initial driving to a focus error signal. When the condenser lens is brought close to a recording medium from a distant position, a track error signal beings to appear rear the just focus point. A comparator 30 detects above threshold value Vr at a time T1 through a circuit 29 for detecting the envelope of the signal. At the same time, it outputs a switch changing signal (g) to change a switching circuit 26 to the focus error signal side. Thereafter, it is pulled in a focus servo system by the focus error signal around the normal just focus point.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a focus pull-in method for an optical disc device equipped with a focus servo mechanism.

2. Description of the Related Art In recent years, the amount of information processed by computers has continued to increase, and in order to record and reproduce this increased amount of information, optical disk devices that can record a large amount of data have attracted attention, and the technology has improved at a remarkable pace. Since the information recorded or to be recorded on the recording medium in an optical disk device has a minute focus of 1p or less, the laser beam is narrowed down to a minute spot of about 1 angle in diameter, and the laser beam is always focused regardless of the surface wobbling of the recording medium. A focus servo mechanism that illuminates the medium surface is essential, and the development of a highly accurate servo function is desired.

[Conventional technology]

FIG. 4 shows a configuration diagram of an optical disc device equipped with a conventional focus servo mechanism. In the figure, 1 is a semiconductor laser which is a light source, 2 is a collimating lens that converts the light beam emitted from the semiconductor laser 1 into parallel light, 3 and 4
5 is an optical element for separating the light beam, which is a polarizing beam splinter and a 74-wavelength plate; 5 is a condenser I equipped with a focus servo machine ζ for irradiating the light beam onto the recording medium 6;
7 is a reflection mirror for dividing the reflected light from the recording medium into two parts, 8 and 9 are condenser lenses for detecting a focus error signal (hereinafter abbreviated as focus error signal);
Divided photodiodes <10 and 11 are a condenser lens and a two-division photodiode for detecting a track error signal (hereinafter abbreviated as track error signal).

When the recording medium 6 rotates, the surface of the recording medium moves up and down due to surface deflection, and the shape change of the reflected light on the focus error signal detector of the two-split photodiode 9 at that time is shown in FIG. It becomes like this.

12, the distance X between the condenser lens 5 and the recording medium is the condenser lens 5.
13 shows the situation when the focal length matches the focal length, and 14 shows the situation when the focal length becomes shorter than the focal length. Therefore, a focus error signal is obtained from the differential signal AB of the two-split detector 9.

The focus error signal at this time is shown in FIG. In this figure, the vertical axis is the focus error signal V, and the horizontal axis is the distance X between the condenser lens 5 and the recording medium 6 in FIG.
It is. 16 is the just focus point, and the value of the focus error signal (2) crosses zero on the line of distance MX. It would be good if stable control could be performed within the range of the linearity changing portion 19 centered on this point 16, but since each condensing lens is formed extremely small, it is not an ideal lens, and the zero cross point or As shown in the figure, there are multiple points close to zero (
In this example, it appears at each point of 15, 17, and 18).

If the focus servo system is closed at the wrong time, it will become stable at another zero cross point of 15 or 17.18.
In the worst case, the condenser lens may come too close to the recording medium, causing damage to the recording medium or the condenser lens.

[Problem that the invention seeks to solve]

Conventionally, the timing for closing the focus servo system is 2.
A signal obtained by differentiating the sum signal or difference signal obtained from the split photodiode 11 was used, but both of them have poor stability, and the operation becomes unstable even with the slightest change in external conditions. There were drawbacks.

In view of the above-mentioned conventional drawbacks, the present invention aims to provide a focus pull-in method for stably performing focus servo at a normal just focus point at all times.

(Means for solving the problem) The purpose is to irradiate a light spot onto the surface of the recording medium through a condensing lens, and use the reflected light to determine the focus error necessary for driving the focus servo mechanism of the condensing lens. In an optical disc device comprising a detector for detecting a signal and a track error signal necessary for driving a track servo mechanism, the detector is provided for detecting an envelope waveform of the track error signal at a predetermined threshold or higher, The focus pull-in method of the present invention is characterized in that the timing for starting driving of the focus servo mechanism is determined by the output of the detector that appears during the initial forced movement of the condensing lens in the optical axis direction. This is achieved by providing

[Effect]

That is, the present invention provides means for reliably detecting the timing to close the focus servo system by utilizing the fact that the track error signal obtained from the reflected light of the recording medium is obtained only from the vicinity of the just focus point. .

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

Note that, in order to make the explanation of the configuration and operation easier to understand, the same parts are given the same reference numerals throughout all the figures, and repeated explanation thereof will be omitted.

FIG. 1 shows a diagram explaining the principle of the present invention. Waveform +a) is the 6th
The focus error signal and waveform (bl is the track error signal and its envelope waveform (hereinafter abbreviated as envelope), waveform TC) explained in the drawings indicate a timing signal for closing the focus servo system. In the figure, 2
1 is a track error signal, 22 is its envelope signal, 23 is a threshold value Vr for detecting a certain value or more, and 24 is a focus servo ON'' (this is just a saying. Focus servo oON' only at a normal just focus point) 4
You can see that issue g has been issued.

FIG. 2 shows a block diagram of a focus pull-in method according to the present invention. In the figure, 28 is a signal generation circuit for initially driving the condensing lens 5, 26 is a switching circuit for switching the signal for driving the condensing lens from the initial drive to a focus error signal, 25 is a focus servo drive circuit, and 27 is a focus error signal compensation circuit, 2
9 is a circuit for detecting the envelope of the track error signal, and 30 is a detector (comparator) for determining whether the envelope has exceeded the threshold value Vr.

FIG. 3 shows a time chart of the operation of each part in FIG. 2. In the figure, the waveform +d+ indicates the initial drive signal, and the waveform (el and track error signal, waveform (f) is the focus knee signal, and waveform 1g) indicates the switch switching signal.

First, the condenser lens 5 is recorded from a distance using an initial drive signal.
As the medium 6 approaches, a tracking error signal begins to appear near just focus. The comparator 3
0 detects that it is equal to or greater than the threshold νr at time T+, and at the same time outputs a switch change signal (gl) to switch the switching circuit 2.
6 is switched to the focus error signal side, and from then on, the focus servo system is driven by the focus error signal around the normal just focus point.

〔Effect of the invention〕

As explained in detail above, according to the focus pull-in method of the present invention, the timing to close the focus servo can be accurately detected, so the focus servo will not be activated at an incorrect stable point, which will damage the recording medium and the condensing lens. It has the effect of eliminating things.

[Brief explanation of the drawing]

Fig. 1 is a diagram explaining the principle of the present invention, Fig. 2 is a block diagram of the focus pull-in method according to the invention, Fig. 3 is a time chart of the operation of each part in Fig. 2, and Fig. 4 is a diagram showing the conventional focus servo mechanism. FIG. 5 is a diagram showing changes in the shape of reflected light on the focus error signal detector; FIG. 6 is a waveform diagram of the focus error signal near the just focus point. In the figure, 5 is a condenser lens, 6 is a recording medium, 9 is a two-split photodiode for detecting a focus error signal, 1
1 is a two-part photodiode for detecting a tracking error signal, 22 is an envelope waveform, 30 is a detector, and Vr is a threshold value.

Claims (1)

[Claims] A light spot is irradiated onto the recording medium surface through a condenser lens, and a focus error signal necessary for driving a focus servo mechanism of the condenser lens and a track error signal necessary for driving a track servo mechanism are generated from the reflected light. In an optical disk device comprising a detector for detecting each, a detector is provided for detecting an envelope waveform of the tracking error signal above a required threshold value, and the condenser lens is forcibly initially moved in the optical axis direction. A focus pull-in method characterized in that the timing for starting driving of the focus servo mechanism is determined based on the output of the detector that appears between the two.