JPS61122934A - Focus position control device - Google Patents

Abstract

PURPOSE:To execute the stable pulling-in by detecting with the first comparator that the distance between a disk recording surface and an objective lens enters a surface oscillating detecting scope, detecting the point of time near the zero crossing with the second comparator and closing the servo loop. CONSTITUTION:When a photodetecting output signal A is processed through the focusing detector from a terminal 38 by the first comparator 33 and the distance between a disk recording surface and a objective lens is in the surface oscillating detecting scope, an output E of a comparator 33 goes to be H, and by the two detections, H is latched at an FF 37 at the rear step of two steps of D-type FF 36 and 37. Next, when an output F of the second comparator 34 is inverted to H and impressed to a clock terminal C near the zero crossing of the signal A, the contents H of the latch of the FF 37 is outputted, a close control signal I from a NOR gate 40 is outputted and the servo loop is closed. Consequently, when a reflecting ratio difference between a recording surface and a protecting surface is small, the stable servo pull-in can be executed near the zero crossing.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a focal position control device in an optical information recording/reproducing or reproduction-only device, typified by an optical video disc, a document file, or 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 disc 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. FIG. 2 shows an example of a conventional focus position control device. 1 is a concentric or spiral track with a track pitch of 1.5 to 2.5 μm and a length of 1 to 2.5 μm.
This is a disc in which information pits of about 4 μm are recorded as gradient information of shading or unevenness.

Reference numeral 2 denotes a motor whose rotation is controlled with high precision in order to extract information recorded from the disk 1 as time-series signals/signals, 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. Reference numeral 4 denotes a focusing actuator, which allows the beam spot narrowed down by the objective lens 3 to be irradiated at a constant constant level even when the recording surface is shaken due to the disc 1's sag, the inclination of the disc 1 when it is attached to the disc motor 2, etc. As if
A focusing actuator 4 constantly corrects the position of the objective lens 3. Reference numeral 5 is a λ/4 plate, and reference numeral 6 is a λ/4 plate through which the polarization plane of the light beam that has passed 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. T is a collimator lens for converting the laser light radially emitted from the semiconductor laser 8 into parallel light. 9 is a circuit for controlling the output power of the semiconductor laser 8.

10 is a collimator lens, 11i is a knife set at the center of the optical axis of the collimator lens 10, and 12 is a 2
Split focusing detector I.

■It is. 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 than the focused position by a knife lens 11, it is focused on one detector I, or , if the detector is far away, most of the reflected light will be incident on the other detector (2). Each of the above-mentioned components is housed in the pickup 13, and is adapted to be driven in the radial direction with respect to the disk 1.

In addition to the above-mentioned means, the pink-up 13 stores means for correcting track fluctuations due to track eccentricity, but is omitted here. The output signals of each of the above-mentioned detectors are converted into differential signals by a differential amplifier 14,
The focusing actuator is driven through a phase compensation circuit 16 for ensuring loop stability, a loop switch 17, a gain control circuit 18 for correcting gain fluctuations due to the reflectance of the disc, and a drive circuit 19. A focusing servo is configured so that the objective lens faithfully follows shake. 16 is detector I.

The H output is also a No. 1 summing amplifier, and is used as a control signal for the gain control circuit 18 for keeping the loop gain constant. If the differential signal of the differential amplifier 14 is of the so-called knife type as in this configuration example, the waveform example will be as shown in FIG. This is called a known S-shaped signal waveform, and Ds shown in the figure is approximately several tens of μm, although it varies depending on the optical element used. The distance between Ds is the detectable range for detecting ifn tracking of the disc, and the distance shown in the figure is . 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, and in order to retract the focusing servo from this state, the objective lens must be forced onto the disk surface. It is necessary to move closer and detect the above-mentioned Ds section. Therefore, as shown in FIG. 2, in response to the focusing servo ON signal 22, the retraction activation signal B
A start signal is applied to the focusing actuator via the drive circuit 19, and the objective lens 3 is brought close to the disk recording surface to a detectable range for detecting surface wobbling of the disk. The pull-in detection circuit 20 detects that the D8 period shown in FIG.
is "'ON" L, and the servo loop is closed. or,
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-D shown in the figure
correspond to M to D shown in FIG. 2, 1o indicates the start point of retraction activation, and tl indicates the point of time when the servo loop closes.

An example of the focus position control device has been described above.

In this example, the Knifezi method is adopted as a method for detecting surface runout, but as is known, various other methods such as the critical angle method and the astigmatism method can be used to detect runout. There is no major difference between them except for the detection method and some of the components inside the pickup.

Problems to be Solved by the Invention The configuration shown in item iii has the following problems. FIG. 5 & is a figure 8 waveform when the recording surface has a high reflectance such as an aluminum vapor-deposited film. In the figure, in addition to the 8 characters indicating the recording surface, there is a very small 8 character waveform, which is
The figure 8 waveform represents the surface of PlaMA, which is a protective film for the recording surface. It can be seen from the amplitudes M8 and M in the figure that the reflectance of the surface of the PMM person is sufficiently lower than the reflectance of the aluminum vapor deposited film.

Therefore, by comparing the output signals with a comparator set to a voltage slightly higher than A, the time point D1 at which the servo loop is closed can be detected. In the figure, Do reaches the operating point of the servo loop and reaches the zero cross point as described above. The difference between D and Do can be made smaller. When the servo is closed at point 04, the uni7) function of amplitude is applied to the servo loop, but as is clear from the figure, the amplitude AT is sufficiently small compared to As, and it is pulled 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 (0<
In the case of a write-once optical disc using <2), an example of the waveform is shown in FIG. In the figure, the figure 8 waveform with amplitude As' is the PMM human blood surface width As' is the recording surface. Incidentally, in order to obtain As2As/, the magnification of the vertical axis in FIG. 5A is considerably larger than that in FIG. 6A. Fifth
In the figure, as in Figure 5a, when the output signal M is compared with a comparator set to the potential A, it is found that PMMA
When D3' on the film surface and the original D1' are detected, and the objective lens is gradually brought closer to the disk from a distance, D3' will be detected first.
If detected, the beam will be focused on the surface of the PMM membrane. Therefore, it is necessary to perform pull-in detection using a comparator set to a sufficiently higher voltage input voltage than the voltage input voltage. However, in this case, when the servo loop is closed by detecting the pull-in at D2', a unit function with an amplitude ~' is 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 loose 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 focal position control device of the present invention uses a first comparator to detect when the distance between the objective lens and the disk recording surface is within the shake detectable range. The second saw/palator detects that the point is near the first zero crossing point from the time of detection, and closes the servo loop at that point.

Embodiment Hereinafter, an embodiment of the focal position control device of the present invention will be described 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-J in FIG. 1S correspond to M-J in FIG. 1A. The operation of this embodiment will be explained using FIGS. 1a and 1b. It should be noted that the constituent elements other than the four pull-in detection paths according to the present invention are the same as those of the prior art (1), and their explanation will be omitted. The comparator levels of the first comparator 33 and the second comparator 34 in FIG.
．． 26, the potential v1. When the voltage becomes V2, the output of each comparator is pulled up to the power supply voltage by resistors 27 and 28. Also, diodes 31, 32 and resistor 2
9.30 is for not applying a negative input to the logic element in the next stage.

Figure 1. Focusing servo roof'O at the time
The N signal 22 becomes "ON" and becomes "L", the reset of the first D launch 36 and the second D latch 37 is released, and the output signals H and D of both remain "L" until the next input signal is input. Become. Also, the pull-in start signal 41 is “H”
Then, an activation signal is output from the retraction activation circuit 21 shown in FIG. 2, and the objective lens 3 is brought closer to the disk 1. When the objective lens Z approaches the disk, first PliLM
An S-shaped signal due to the surface of the A film is output from the differential amplifier 14.

Since the S-curve amplitude is larger than v2, F becomes L during the period (from 1 to t2) that is larger than v2.

At the time t2 when F changes from low to high, the second D bottom 3
7 latches all the data, but since Hid ``L''f is turned off, the output also becomes L''.As the disk approaches further, the S-shaped signal on the recording surface is output from the differential amplifier.The S-curve amplitude on the recording surface is larger than vl, so the period (t4
to t5), E becomes "H". The first latch 36 latches the data at time t4 when E changes from "L" to "H", but at this time G is t)lm, so the output H becomes IIH11 from that point on.Furthermore, F becomes " At time t6 when the signal changes from "L" to "H", the second latch latches the data. At this time, since H is "H", the output is also 'H'.
This D signal becomes the pull-in detection signal, and this signal turns the loop switch on and off, so the recording surface S
Since the servo loop is closed near the zero crossing of the signal, it is no longer necessary to apply a large step input to the servo loop as in the conventional case. Since D becomes "H" at time t6, the pull-in start signal J becomes 'L'
The activation face will be canceled.

By simply adding a simple circuit like this, stable servo 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 detects when the distance between the disk recording surface and the objective lens falls within the surface vibration detection range by the first comparator. After detecting this, the second comparator detects the point near zero cross and closes the servo loop to provide stable retraction.

[Brief explanation of the drawing]

FIG. 1a is an electric circuit diagram showing an example of the retraction detection circuit in the focus position control device according to the present invention, FIG. 1 is a waveform diagram of each signal in FIG. Block diagram, Fig. 3 is a waveform diagram of the focal position error signal of the Knifetsu focusing servo, Fig. 4 is a waveform diagram of each signal in the conventional example, Fig. 5 &b is the reflectance of the recording surface and the protective film surface. FIG. 4 is a diagram showing an example of an S-shaped signal waveform when the difference between the two is large and the difference is small. 1...disc, 3...objective lens,
4... Focusing actuator, 8...
... Semiconductor laser, 12 ... Focusing detector, 16 ... Phase compensation circuit, 1A...
...Loop switch, 18...Gain control circuit, 19...Drive circuit, 20...
...Retraction detection circuit, 21...Retraction starting circuit, 33.34...Comparator. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 3
Figure 4

Claims (1)

[Claims] an optical means for guiding a light beam from a light source to a desired reflecting surface via an objective lens; and driving the objective lens in an optical axis direction in accordance with a control signal to focus the light beam on the reflecting surface. a driving means, 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, and an optical means for directing the output of the photodetector to the driving means. 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, a retraction starting circuit that drives the drive means to search for a focused position, and a focus position control loop that and a comparator that respectively 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; means for turning on the switch and closing the control loop when the comparator detects that the output level of the device exceeds the second reference level after exceeding the first reference level twice in succession; A focal position control device comprising: