JPH0470698B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a focus position control device in optical information recording/reproducing or reproduction-only devices, typified by optical video disks, document files, and the like.

BACKGROUND OF THE INVENTION In recent years, various methods have been devised and developed for such focal position control devices, and these devices are actually used in optical video disk players, document files, and the like. An example of the above-mentioned conventional focus position control device will be described below with reference to the drawings. Second
The figure shows an example of a conventional focus position control device. Reference numeral 1 denotes a disk in which information pits of about 1 to 4 .mu.m in length are recorded as shading or uneven information on concentric circles or spiral tracks with a track pitch of 1.5 to 2.5 .mu.m. Reference numeral 2 denotes a motor whose rotation is controlled with high precision in order to extract the information recorded from the disk 1 as a time-series signal, and is called a disk motor. Reference numeral 3 denotes an objective lens, which focuses a laser beam onto the recording surface of the disk 1 and reads out information signals with high precision. 4 is a focusing actuator, which controls the warping of disk 1 and the disk motor 2.
The focusing actuator 4 adjusts the focus of the objective lens 3 so that the beam spot focused by the objective lens 3 is always irradiated at a constant level against the surface fluctuation of the recording surface caused by the tilting of the recording surface when the recorder is attached to the recording surface. The position is constantly being corrected. 5 is a λ/4 plate, and the polarization plane of the light beam that passes through it changes. Reference numeral 6 denotes a polarizing prism, which separates the light incident on the disk and the reflected light by utilizing the change in the plane of polarization of the two. Reference numeral 7 denotes a collimator lens for collimating the laser light emitted radially from the semiconductor laser 8. 9 is a semiconductor laser 8
This is a circuit that controls the output power of the 10 is a collimator lens, 11 is a collimator lens 1
A knife edge 0 is set at the center of the optical axis, and 12 is a two-part focusing detector. The reflected light from the disk surface is configured to be focused on a focusing detector 12 by a collimator lens 10, and when the disk 1 and objective lens 3 are closer to each other than the in-focus position by a knife edge 11, the light reflected from the disk surface is focused on one of the disks. , or if it is far,
Most of the reflected light is made incident on the other disk. Each of the above components is stored in the pickup 13 and is adapted to be driven in the radial direction relative to the disk 1. In addition to the above-mentioned means, the pickup 13 stores means for correcting track fluctuations due to track eccentricity, but these are omitted here. The output signals of the respective detectors are converted into differential signals by a differential amplifier 14, which includes a phase compensation circuit 16 for ensuring loop stability, a loop switch 17, and a circuit for correcting gain fluctuations due to disk reflectance. A focusing servo loop is configured to drive a focusing actuator via a gain control circuit 18 and a drive circuit 19 so that the objective lens faithfully follows the surface wobbling of the disk. Reference numeral 15 denotes a summing amplifier for the output signal of the detector, which is used as a control signal for the gain control circuit 18 for keeping the loop gain constant. If the differential signal A of the differential ramp 14 is of the so-called knife-edge type as in this configuration example, it will have a waveform example as shown in FIG. This is called a well-known S-shaped signal waveform, and D _s shown in the figure varies depending on the optical element used, but is a divisor.
It is about 10 μm. The distance D _s is the detectable range for detecting surface runout of the disk, and D _p in the figure is the operating point of the focusing servo. Normally, before the focusing servo is retracted, the objective lens is placed at a considerable distance from the disk recording surface. It is necessary to detect the above-mentioned D _s interval by forcibly approaching . Therefore, as shown in FIG. 2, the focusing servo loop ON signal 22 causes the retraction activation circuit 21 to generate the retraction activation signal B.
A starting signal is applied to the focusing actuator via the drive circuit 19, and the objective lens 3 is brought close to the recording surface of the disk to a detectable range for detecting surface wobbling of the disk. The pull-in detection circuit 20 detects that the _Ds period shown in FIG. 3 has entered and turns the loop switch 17 "ON" to close the servo loop. At the same time, the retraction start signal is turned "OFF". FIG. 4 shows an example of the waveform of each signal from the start of the pull-in activation until the servo loop is closed. A to D shown in the figure correspond to A to D shown in FIG. 2, where t ₀ indicates the start point of retraction activation, and t ₁ indicates the point in time when the servo loop closes.

An example of the focus position control device has been described above. In this example, the knife edge method is adopted as a method for detecting surface runout, but as is known, there are various other methods for detecting surface runout, such as the critical angle method and the astigmatism method. There is no major difference between them except for the detection method and some of the components in the pickup.

Problems to be Solved by the Invention The above configuration has the following problems. FIG. 5a shows an S-shaped waveform when the recording surface has a high reflectance such as an aluminum vapor-deposited film. In the figure, in addition to the S-shape that indicates the recording surface, there is a very small S-shaped waveform, but this is a protective film for the recording surface.
This is an S-shaped waveform showing the surface of PMMA. It can be seen from the amplitudes A _S and A _P in the figure that the reflectance of the PMMA surface is sufficiently lower than that of the aluminum vapor-deposited film. Therefore, by comparing the output signal A with a comparator set to a potential _AT slightly higher than _AP , the time point _D1 at which the servo loop is closed can be detected. In the figure,
D ₀ is the operating point of the servo loop as mentioned above,
This is the zero crossing point. Since A _P is considerably smaller than A _S , the difference between D ₁ and D ₀ can be made small. When the servo is closed at time _D1 , a unit function of amplitude A _T is applied to the servo loop, but as is clear from the figure, the amplitude A _T
is sufficiently smaller than A _S and is drawn in stably.

On the other hand, if the reflectance of the recording surface is not very high compared to the reflectance of the protective film surface, for example, TeOx (O
In the case of write-once optical discs using <x<2),
An example waveform is shown in FIG. 5b. In the figure, amplitude
The S-shaped waveform of A _P ' is the surface of the PMMA film, and the S-shaped waveform of amplitude A _S ' is the recording surface. In order to make A _S =A _S ', the magnification of the vertical axis in FIG. 5b is considerably larger than that in FIG. 5a. In Figure 5b, Figure 5a
Similarly, if the output signal A is compared with a comparator set to the potential of A _T , the PMMA film surface
When D ₃ ′ and the original D ₁ ′ are detected, and the objective lens is gradually brought closer to the disk from a distance, D ₃ ′ will be detected first and will be focused on the PMMA film surface. Therefore, it is necessary to perform pull-in detection using a comparator set to a voltage A _T that is sufficiently higher than A _P '. However, in this case, if pull-in is detected at D ₂ ' and the servo loop is closed, a unit function with amplitude A _T ' will be applied to the servo loop, and stable pull-in may not be possible.

In view of the above problems, the present invention provides stable pull-in by closing the servo loop near the zero cross point even when the difference in reflectance between the recording surface and the protective film surface is small.

Means for Solving the Problems In order to solve the above problems, the first step is to compare the output level of the photodetector for detecting the deviation from the focal point of the present invention with a first reference level. and a second reference level whose absolute value is smaller than the first reference level, and when the focus position control is pulled in, the output level of the photodetector and the first reference level are After detecting that the output level of the photodetector exceeds the first reference level and falls within the first reference level again by detecting that the magnitude relationship between the photodetector and The switch is turned on when the second comparator detects a change in the magnitude relationship between the output level of the instrument and the second reference level. It is detected that the distance between the recording surfaces has entered the range where surface runout can be detected, and the second comparator detects that the distance is near the first zero crossing point from the time of detection, and at that point the servo loop starts. It works to close.

Embodiment An embodiment of the focal position control device of the present invention will be described below with reference to the drawings.

FIG. 1a is a circuit diagram of an embodiment, which corresponds to the pull-in detection circuit 20 in the block diagram of FIG. Signal waveform examples A to J in Figure 1b are shown in Figure 1a.
It corresponds to A to J in . Figure 1 a and b
The operation of this embodiment will be explained using the following. It should be noted that the components other than the pull-in detection circuit according to the present invention are the same as those of the prior art, and a description thereof will be omitted. Figure 1a, 1st
The comparator levels of the comparator 33 and the second comparator 34 are set by connecting the power supply voltage Vcc to the resistor 2.
The potentials V ₁ and V ₂ are divided by voltages 4, 25, and 26, and the output of each comparator is connected to resistors 27 and 28.
It is pulled up to the power supply voltage. Furthermore, the diodes 31 and 32 and the resistors 29 and 30 are used to prevent a negative input from being applied to the next stage logic element.

When the focusing servo loop ON signal 22 becomes "ON" at time _t0 in FIG. H and D remain "L" until the next input signal is input. Further, the retraction starting signal 41 becomes "H", a starting signal is output from the retraction starting circuit 21 shown in FIG. 2, and the objective lens 3 is brought closer to the disk 1. When the objective lens approaches the disk, an S-shaped signal from the PMMA film surface is first output from the differential amplifier 14.

Since the S-curve amplitude is larger than V ₂ , F becomes "L" during the period (from _{t 1 to} t ₂ ) that is larger than V 2 .
At time _t2 when F changes from "L" to "H", the second D latch 37 latches the data, but since H is "L", the output D also becomes "L". As the disk approaches the disk further, an S-shaped signal from the recording surface is output from the differential amplifier. Since the S-curve amplitude of the recording surface is larger than V ₁ ,
During the period greater than V ₁ (from t ₄ to t ₅ ), E is “H”
becomes. At time _t4 when E changes from "L" to "H", the first latch 36 latches the data, but since G is "H" at this time, the output H becomes "H" from that point on. Furthermore, the point in time when F changes from “L” to “H” t ₆
Then, the second latch 37 latches the data. At this time, since H is "H", the output D also becomes "H". This D signal becomes the pull-in detection signal, and since this signal turns the loop switch on and off, the servo loop is closed near the zero cross of the S-curve signal on the recording surface, making it difficult to input large step inputs to the servo. No need to feed it to the loop. Since D becomes "H" at time _t6 , the pull-in starting signal J becomes "L" and the starting state is canceled.

By simply adding a simple circuit like this, stable serve pull-in is possible and the reliability of the device is improved.

Effects of the Invention As described above, the present invention uses two comparators each having a different voltage setting in the pull-in detection circuit, and the first comparator detects when the distance between the disk recording surface and the objective lens falls within the surface vibration detection range. After detecting this, the second comparator detects the point near zero cross and closes the servo loop to provide stable pull-in.

[Brief explanation of drawings]

FIG. 1a is an electric circuit diagram showing an example of a retraction detection circuit in a focus position control device according to the present invention, and FIG. 1b
1A is a waveform diagram of each signal in FIG. 1A, FIG. 2 is a block diagram of a conventional focus position control device, FIG. Waveform diagrams of each signal in the example, FIGS. 5a and 5b are diagrams showing examples of S-shaped signal waveforms when the difference in reflectance between the recording surface and the surface of the protective film is large and small. DESCRIPTION OF SYMBOLS 1...Disk, 3...Objective lens, 4...Focusing actuator, 8...Semiconductor laser, 12
... Focusing detector, 16... Phase compensation circuit, 17... Loop switch, 18... Gain control circuit, 19... Drive circuit, 20... Pull-in detection circuit, 21... Pull-in starting circuit, 33, 34... Comparator.

Claims (1)

[Claims] 1. Optical means for guiding a light beam from a light source to a desired reflective surface via an objective lens; and an optical means for directing the objective lens in an optical axis direction in accordance with a control signal in order to focus the light beam on the reflective surface. a photodetector for detecting deviation from the focused point; an optical means for guiding the reflected light flux from the reflecting surface to the photodetector; a focus position control loop that applies voltage to the drive means via a phase compensation circuit that compensates for delays in the mechanical system of the drive means; a retraction starting circuit that drives the drive means to search for a focused position; and a focus position control loop. a first comparator that compares the output level of the photodetector with a first reference level and a second reference level that is smaller in absolute value than the reference level; A second comparator is provided, and the output level of the photodetector is adjusted by detecting that the magnitude relationship between the output level of the photodetector and the first reference level changes continuously during focus position control pull-in. After detecting that the output level of the photodetector has exceeded the first reference level and then entered the first reference level again, the second and means for turning on the switch and closing the control loop when detected by a comparator.