JP2001291255A - Pickup servo system having revolution number synchronizing type narrow-band passing filter for optical disk - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase servo gain of an optical pickup, with the system stabilized, for the purpose of dealing with the high speed of an optical disk. SOLUTION: In a recording/reproducing device for an optical disk, a narrow- band passing filter 1 is inserted to an arbitrary point in the servo circuit of the optical pickup, with a bypass of the gain one simultaneously provided which goes around the filter and added to the output of the filter. In this case, the center frequency of the narrow-band passing filter 1 is synchronized with the disk motor rotation frequency 4 taken out of the frequency generator of a disk motor 7 or the Hall element output of magnetic pole detection so that both frequencies are constantly coincided. Thus, the gain can be increased only in the frequency component of surface wobbling, eccentricity, and the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention Music compact disk (CD), MD, CD-ROM, MO, CD-R, CD
-RW, DVD-Video, DVD-ROM, DVD
In the recording and reproduction of an optical disc such as -R, DVD-RAM, DVD-RW, the distance between the objective lens and the disc signal surface must always follow the set working distance with an error of ± 1 μm or less. Number 100 on disk
Since there is usually a deviation of μm, a feedback system called a focus servo is adopted to avoid the influence. In addition, the tracking servo is applied so that the spot narrowed down by the objective lens does not deviate from the track not only in the direction perpendicular to the disk but also in the horizontal direction, so that the influence of the eccentric component of the disk is reduced. Have been. The present invention
It relates to a servo circuit used for these optical disks.

[0002]

2. Description of the Related Art A conventional optical pickup servo is constructed as shown in FIG. In this figure, the focus servo is taken as an example, but the configuration is the same for the tracking servo. FIG. 2 shows a block diagram thereof. In FIG. 2, the surface runout 19 of the disk is set to a maximum of ± 400 μm according to the specification of the disk, but the focus error 18 must be ± 1 μm or less due to the requirement of the optical system. In order to solve the problem, a displacement servo is applied so that the vertical displacement 17 of the objective lens follows the surface deviation 19 of the disk. FIG. 3 shows a pickup 1
The basic frequency of the actuator 10 in the focus direction is 35 Hz, and the time constant T ₁ of the phase lead circuit 14 is 15 kHz.
z, _{T 2} is 600 Hz, the DC gain 20 shows a Bode diagram when the 65 dB.

[0003]

Since the optical disk has been used as a peripheral device of a computer, high-speed data recording and high-speed reproduction have been required, and the rotation speed of the disk has been increased to eight times the normal speed. 16 times faster. As a result, the frequency of the disk runout and the eccentric component also increase, and the conventional optical pickup servo, which suddenly decreases when the servo gain is around 50 Hz or more, cannot be used. For this reason, at present, the user is requested to use a blank disk of which quality is specially controlled for high-speed recording, or is recommended to use the disk at a reduced rotation speed. In addition, since the disturbance is not limited to surface runout and eccentricity, and also adds vibration due to high-speed rotation of the disk motor, it is necessary to further increase the servo gain at the rotation frequency of the motor from the current level. In the servo system represented by the Bode diagram shown in FIG. 3, if the focus error 18 is ± 1 μm or less, the permissible surface deviation range 25 is represented as a frequency characteristic as shown in FIG. This figure is derived from the principle that the degree of disturbance suppression is proportional to the servo gain. The servo gain referred to here is d of the open loop gain 21 and the closed loop gain 24.
It is B difference. The normal rotation speed of the disk is 200 rpm at the outermost circumference and 500 rpm at the innermost circumference.
The innermost circumference of the double speed is 4000 rpm, and the frequency is 67 Hz. The runout at this time is ±
When represented by a line spectrum assuming that it is 400 μm, it becomes as shown in FIG. Although this surface wobble includes an integer multiple of harmonics, if the second harmonic has a distortion of 20%, an 8 × speed surface wobble harmonic component 2
The result is a line spectrum shown as 7. These two spectra are completely suppressed because they are within the allowable deviation range 25, and do not cause abnormal operation. Next, looking at the state at 16 × speed, it is as shown in FIG. The two line spectra of the 16-times velocity fluctuation component 28 and the 16-times velocity harmonic component 29 both exceed the permissible range. That is, the servo breaks down and skips. In this case, about 10d
Since the servo gain of B is insufficient, a normal operation cannot be expected unless the disk runout is ± 130 μm or less. Considering that vibration due to the rotation of the eccentric disk is added as a disturbance, it is inevitable to use a disk with a surface runout of about ± 50 μm or less.
In order to solve this problem, it is sufficient if the gain can be increased as a whole, but the system becomes unstable because the higher-order resonance of the actuator 10 is around 10 kHz. Further, even with a method using a conventional automatic control technique, for example, a method of combining a phase lead circuit and a phase delay circuit, the entire system becomes unstable. This means that if you try to increase the gain at a certain frequency, it will be 20-30
This is because a phase change occurs up to twice as high frequency, and the phase margin decreases.

[0004]

SUMMARY OF THE INVENTION The present invention is directed to the fact that the disk runout and the eccentricity of the disk include only a frequency component that is an integral multiple of the rotational frequency of the disk, and the vibration component due to the eccentricity of the disk is similar Focus on the fact that it contains only the rotational frequency of the disk and a frequency component that is an integral multiple of the rotational frequency of the disk, and attempts to increase the gain of only those frequencies with a sharp narrow-bandpass filter. The sharper the gain characteristic of this filter, the more the phase rotation has no effect except in the vicinity of the frequency, so that the servo gain can be greatly increased. FIG. 6 shows a configuration example in which the present invention is applied only to the basic rotation frequency of the disk. A narrow band pass filter 1 having a single resonance of Q of about 10 to 50 and a peak gain of about 5 to 30 dB is inserted at an arbitrary point of the servo circuit, and a bypass of gain 1 is provided to bypass the filter at the same time. Add to the output of the filter. Here, the center frequency of the narrow band pass filter 1 is synchronized with the frequency generator of the disk motor 7 or the disk motor rotation frequency 4 extracted from the Hall element output of the magnetic pole detection so as to always have the same value.

[0005]

Although these means cannot secure the stability and accuracy of the frequency and gain of a narrow band filter in an analog circuit, a circuit having a desired performance can be easily obtained by a digital filter. realizable. In general, the servo circuit of the pickup is often digitally configured. Therefore, if this frequency synchronization type filter is incorporated in a part of the LSI, the present invention can be implemented without increasing the cost. FIG. 7 shows a block diagram of the present invention. The disk motor rotation frequency 4 does not necessarily have to match the desired frequency, and may be an integer multiple of the desired frequency or a fraction of an integer. The point is that the center frequency of the filter and the rotation frequency of the disk should be the same. FIG.
In this example, a symmetrical second-order filter is given as an example of the transfer function of the narrow band-pass filter 1, but the slope characteristic is not limited as long as it has a sharp peak. Also, the gain of the bypass need not necessarily be 1;
It may be determined by the distribution of the loop gain. When a symmetrical second-order filter is used as the narrow band-pass filter 1, a calculation including the bypass results in a transfer function shown as an inverted T-bridge circuit 2 in FIG. This function is an inverse function of a notch filter called a so-called T-bridge circuit. In addition, all of the solutions, drawings, and calculation examples have been described with focus servo. However, the same applies to tracking servo, and it goes without saying that the invention can be applied to both.

[0006]

The center frequency of the narrow band-pass filter 1 is set to 133H which is the inner rotation frequency at 16 times speed.
z, Q is 50, and peak gain is 20 dB.
Calculate the servo characteristics in advance. The coefficients a to d are as follows. FIG. 9 shows a Bode diagram. Compared with the open loop gain characteristic 21 and the open loop phase characteristic 22 of the conventional circuit, the gain characteristic changes within one octave and the phase characteristic changes within two octaves. The closed-loop gain characteristic 32 and the closed-loop phase characteristic 33 are not different from the conventional characteristics, and it can be seen that the inserted filter has no adverse effect on the stability of the system. However, 133Hz
If the gain of any of the active elements in the loop is reduced by 40 to 60 dB due to some trouble, the system becomes unstable because the phase between the frequency of 1 Hz and 160 Hz exceeds -180 °. When the gain decreases by 60 dB or more, the operation becomes stable again.
It is a so-called conditionally stable servo. ± 40 in FIG.
This figure shows a state in which a disk having a run-out of the standard limit of 0 μm is recorded and reproduced at 16 × speed. The line spectrum of the 16-times velocity fluctuation component 28 is 1 within the permissible fluctuation range 34.
It fits with a margin of 0 dB. In this state, if a large amount of harmonics is included in the run-out, a sound skip occurs at 266 Hz. In this case, as shown in FIG. A synchronized narrow bandpass secondary filter 2 is added in parallel. The block diagram at that time is shown in FIG.
FIG. 13 shows a block diagram when a type bridge circuit is used. 14 and 15 show calculation examples. It can be seen that the line spectrum of the 16 × speed surface vibration harmonic component 29 is completely within the surface fluctuation allowable range 39. If necessary, increase the number of third and fourth order filters.

[Brief description of the drawings]

FIG. 1 shows a configuration of a conventional optical pickup servo.

FIG. 2 is a block diagram of a conventional servo.

FIG. 3 is a Bode diagram of a conventional servo.

FIG. 4 shows the relationship between the permissible range of the conventional servo runout and the 8x speed runout.

FIG. 5 is a relationship between a permissible range of a conventional servo runout and a 16x speed runout.

FIG. 6 shows the configuration of an optical pickup servo according to the present invention.

FIG. 7 is a block diagram of a servo of the present invention.

FIG. 8 is a block diagram of the servo of the present invention represented by an inverted T-type bridge circuit.

FIG. 9 is a Bode diagram of the servo of the present invention.

FIG. 10 shows the permissible run-out range and 16 of the servo of the present invention.
Double speed runout relationship

FIG. 11 shows a configuration of an optical pickup servo of the present invention reinforced in two stages.

FIG. 12 is a block diagram of a servo of the present invention reinforced in two stages.

FIG. 13 shows the second embodiment of the present invention represented by an inverted T-bridge circuit.
Block diagram of step servo

FIG. 14 is a Bode diagram of the two-stage servo of the present invention.

FIG. 15 shows the relationship between the permissible range of the runout of the two-stage servo of the present invention and the runout at 16 × speed.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Narrow band pass filter 2 Narrow band pass secondary filter 3 Frequency multiplier 4 Disk motor rotation frequency 5 Inverted T type bridge circuit 6 Inverted T type bridge secondary circuit 7 Disk motor 8 Objective lens 9 Disk 10 Actuator 11 Optical pickup 12 Focus Error detector 13 Head amplifier 14 Phase lead circuit 15 Power amplifier 16 Actuator transfer function 17 Vertical displacement of objective lens 18 Focus error 19 Disc surface shake 20 DC gain 21 Open loop gain characteristic 22 Open loop phase characteristic 23 Closed loop gain characteristic 24 Closed Loop Phase Characteristics 25 Plane Shake Tolerance Range 26 8X Speed Plane Shake Component 27 8X Speed Plane Shake Harmonic Component 28 16X Speed Plane Shake Component 29 16x Speed Plane Shake Harmonic Component 30 Open Loop Geometry Emission characteristic 31 open loop phase characteristic 32 closed-loop gain characteristic 33 closed-loop phase characteristic 34 face shake tolerance

Claims (4)

[Claims] 1. A narrow-band digital filter whose center frequency is synchronized with the rotation frequency of an optical disk is inserted into a servo system of an optical pickup. (B) The input signal of the filter is branched and added to the output signal of the filter. A recording / reproducing apparatus for an optical disk having a servo system configured as described above. 2. A recording system for an optical disc, wherein a digital inverted T-type bridge circuit whose center frequency is synchronized with the rotation frequency of the optical disc is inserted into a servo system of the optical pickup.
Regenerator. 3. The servo system according to claim 1, wherein a second narrow-band-pass digital filter whose center frequency is synchronized with twice the rotation frequency of the optical disk is added in parallel. An optical disk recording and playback device. 4. An optical disk recording / reproducing apparatus according to claim 2, further comprising a second digital inverted T-type bridge circuit whose center frequency is synchronized with twice the rotation frequency of the optical disk.