JPS62134830A - Optical disk tracking device - Google Patents

Abstract

PURPOSE:To correctly detect a tracking error signal amount through the use of both a signal detected from elliptic bits and a signal from a groove in a 1/8 wavelength depth by disposing one or plural sets of elliptic bits so that they are previously arranged in the header area of a disk by shifting the right and left sides of the track. CONSTITUTION:In a low frequency area, the gain of a prewobbling tracking error signal 24 obtained from a set of elliptic bits 20 arrays by shifting the right and left sides of the track is set larger than a push-pull tracking error signal using the groove 15 in a 1/8 wavelength depth, while at the prescribed frequency it is added to a push-pull type error signal in an LPF after attenua tion. If the relationship between the frequency fe ensuring the same gain of the prewobbling error signal after passing through the LPF and the push-pull type error signal, a disk rotation speed fd and a number N in the header area during one rotation is selected as inequalities show, an offset component included in the push-pull tracking error signal can be suppressed, thereby carrying out an precise tracking.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical disc device, and particularly to a technique for performing suitable tracking in an optical disc file device that allows user recording.

[Conventional technology]

FIG. 2 is a diagram showing an example of the configuration of an optical system for recording and reproducing information on an optical disc. In Figure 2, 1,
2 is a semiconductor laser drive signal, 3 is a semiconductor laser drive circuit, and 4 is a semiconductor laser. Light emitted from the semiconductor laser 4 forms a light spot 11 on the surface of the disk 10 by a coupling lens 5, a beam splitter 6, a galver mirror 7, a quarter wavelength plate 8, and an objective lens 9. The reflected light from the disk returns to the above-mentioned optical system again, is reflected by the beam splitter 6, and is transmitted to the photodetector 13 (
131, 132) and converted into an electrical signal. FIG. 3 shows an example of the track structure of an optical disc conventionally used in such a recordable optical disc device.

Track 12 has a header area 121 and a data recording area 12
For example, 40% of each track is 64 sectors, with 2 as one set.

In the header area 121, a track address, a sector address, a synchronization signal, etc. are formed in advance with pits 1-14 having a depth of 1/4 wavelength.

A pre-groove 15 having a depth of 1/8 wavelength is formed in advance in the header area 121 and the data recording area 122, and data pits are recorded in the pre-groove 15'' of this data recording area.

FIG. 4 shows the light spot 11 on the photodetector 13 (13], , 132) when the 1/8 wavelength depth pregroove 15 is irradiated with the light spot 11, and the light spot 11 is not from the center of the group 15. This shows the diffraction light distribution.

If the optical spot IJ is shifted from the center of the track 15, the diffracted light will have an asymmetric distribution.

For this reason, in this type of optical disc device, as shown in FIG.

As shown in FIG. 5, two photodetectors 131 and 132 placed parallel to the track receive the diffracted light from the pregroove 15, and the difference in output between these two photodetectors 131 and 132 is taken. , the bush-pull tracking error signal 16 is detected.

Such a device is shown in Japanese Patent Laid-Open No. 49-60702.

FIG. 6 is a block diagram of a bush-pull tracking servo system using the pregroove shown in FIGS. 4 and 5, and FIG. 7 shows a Bode diagram of its cyclic transfer function. In FIG. 7, the servo system includes a track deviation detection element Kd17. Phase compensation element Gc 18° tracking actuator G
It is composed of a19.

[Problem that the invention seeks to solve]

However, in the so-called bush-pull tracking method in which the tracking error signal J6 is obtained from the difference signal of the diffracted light distribution from the pregroove 5 as described above, the light spot is set for tracking control as shown in FIG.
When the galver mirror 7 is moved to follow the eccentric component of the disk 10, the diffraction distribution on the photodetector 13 moves as shown in FIG.

Therefore, an offset occurs in the tracking error signal 16 due to the movement of the diffraction distribution. Further, even if the disk 1o is tilted, it moves in the same way, and even if the optical spot 11 is at the center of the track 15, the tracking error signal 16
A phenomenon in which the tracking offset does not become zero occurs, that is, a tracking offset occurs. As a result, the optical spot 11 cannot be accurately positioned at the center of the track 15, which is a drawback.

The present invention has been made in view of the above drawbacks, and provides optical tracking that eliminates the offset (deviation component) caused by the deflection of the diffracted light beam on the photodetector surface 13, thereby achieving more accurate tracking. The purpose is to provide a tracking device.

[Means for solving problems]

In order to achieve this object, the present invention arranges pre-wobble tracking pits consisting of a set of one or more oval pits, which are arranged in advance to the left and right of the track, in the header area of the disc. put. For data recording and reproduction, a pre-wobble tracking error signal detected from the pre-wobble pits placed in this header area and a conventional push-pull tracking error signal obtained from a pre-groove with a depth of 1/8 wavelength are used together. and perform tracking. Here, an example of a method for obtaining a pre-wobble tracking error signal from the pre-wobble pit 20 shown in the eighth article will be explained. FIG. 9 is a diagram for explaining the configuration and operation of a set of ellipses 2o arranged at left and right sides of the track for obtaining a pre-wobbling tracking error signal, and shows the relationship between the light spot position and the output waveform. FIG. In FIG. 9, when the laser spot 11 irradiates a set of pits 20 while crossing over time, the output signal 21 shifts to the left and right of the track depending on the left and right deviation of the center position of the light spot from the center of the track. Output optical signals from a set of oval pits arranged in a staggered manner have opposite polarities.

A difference signal 24 between the peak values 22.degree. 23 of the output signals of one set of pits becomes a track deviation error signal of the spot 11.

The above-mentioned pre-wobbling tracking error signal detection method shown in FIG. 9 does not use the diffraction distribution of light, so DC tracking offset is caused by the tilt of the disk, movement of the lens, and rotation of the galver mirror. Therefore, the amount of tracking error signal can be detected accurately.

Therefore, by using the push-pull tracking error signal and the pre-wobbling tracking error signal together, the offset component included in the push-pull tracking error signal can be suppressed, and accurate tracking becomes possible.

FIG. 10 shows a block diagram of a control system for performing tracking using both the push-pull tracking error signal and the pre-wobbling tracking error signal. In FIG. 10, the system includes a pre-wobble tracking error detection element 2 for converting the amount of track deviation ΔX into an electrical signal.
Two sets of tracking error detection elements, ie, a push-pull tracking error detection element 25 and a push-pull tracking error detection element 25, are used. The pre-wobble tracking error signal is an intermittent signal obtained only at the pre-wobble pits, which are provided around several dozen around the circumference of the track.

Therefore, the signal 24 is obtained by holding between the wobbling pits using the sample and hold circuit 27. The error signal 24 after holding contains unnecessary harmonic components, but only the low frequency components have meaning as a tracking error signal, so after attenuating frequencies above a predetermined level with the first-order low-pass filter 28, , and the push-pull tracking error signal 16 including the offset component in an adder circuit 32 to obtain a composite tracking error signal 33. The servo system then operates using the signal 33 as the final tracking error signal.

[Effect]

In the present invention, the relationship between the gain Kw of the pre-wobbling tracking error detection system, the gain Kd of the push-pull tracking error detection system, and the time constant of the low-pass filter 28 in the block diagram of FIG. 10 is important. Since the offset ε occurs only in the push-pull detection system, by configuring the system as shown in FIG.
) is reduced to Therefore, it can be seen that the larger Kw is, the greater the offset suppression effect is. However, the number of pre-wobble pits in one layer of the track is N.
, if the number of rotations of the disk is fcl, then from the sampling theorem, the frequency component for which signal 16 has meaning is at most 0.
．． 5.N-fd, it is necessary to make the error signal of the pre-wobbling system sufficiently smaller than that of the push-pull system in the high frequency region. Therefore, K
It can be seen that the value of w is limited by the number of prewobble pits N, the disk rotation fd, and the time constant of the first-order low-pass filter.

As a result of the applicant's analysis of this relationship through computer simulation, it has been found that if the servo system is configured under the following conditions, a stable servo system with an offset suppression effect can be achieved. If it is set to a large value, the system becomes unstable, and if it is set to a small value, the system becomes stable, but it is not very effective in suppressing the offset. That is, in FIG. 11, a is K
It is assumed that w represents the composite characteristic of the low-pass filter 28, and b represents the gain of Kd. Although a attenuates the high frequency side due to the characteristics of the filter, the frequency at which the gains of a and b are equal is set as fe.
Then, there is an upper limit for fe, and the system operates stably if . Also, from the point of view of offset suppression, it is meaningless if fe is too low - N -f cL < f e
It is practical within the range of (2). Therefore, by combining (1) and (2) and configuring the servo system so that fe is selected within the range, it is possible to realize a stable and high-performance servo system. The above is an overview of the present invention, and the present invention will be explained in more detail with reference to Examples.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. Here, the system explained in FIG. 10 will be specifically explained using numerical values.

If the transfer function of the sample and hold circuit 27 is Gh (S), then the transfer function of the first-order low-pass filter 28 is Gh (
s).

a r (s) = −(5) 1 +ST 1 Assuming that the transfer function of the phase compensation circuit 29 is G2(S) and the actuator 30 is a quadratic system, its transfer function Ga
(s) is. Next, give specific numerical values to each constant. 7 is the sampling period, and if the sampling frequency is T6, then ? =1/f s=], / (N-f d)
(8), where the disk rotation speed fd is 30Hz
, the number N of bliwobbles in the track-circumference is 16.

Shitagatte, f s = 480 (Hz), Z = 1/4
80 (sec). Assuming that the cutoff frequency of the low-pass filter is 10Hz here, T1 in equation (5)
is T1=1/2πX10=1.59X10-2(see)
(9). In the phase compensation circuit 29, α=
0.1. Let T2=1.8X10-'. When the damping coefficient ξ of the actuator 30 is 0.3 and the resonance frequency fO is 45Hz, ωo=2πf o=282.7 (rad/5ec)
(10). Further, the gain constant of the push-pull tracking system is assumed to be Kd=1000. In the above, numerical values have been given for all constants other than the gain constant Kw of the wobbling system in the system shown in FIG. Next, the gain constant of the wobbling system is determined based on equation (3). In this example, the sampling frequency is f8=480 (Hz), so
From formula (3), 9.6<fe<96 (Hz)
(11) is the range. Here f e=90 (
Hz), the maximum possible ratio of Kw and Kd is the first
It can also be determined as shown in Figure 2. That is, fe=
If the frequency is 90 (Hz), the first-order low-pass filter is 20
Since the gain decreases by dB/decade, 90Hz
Draw a straight line with a slope of 20 dB/decade passing through the O dB point. The gain shown by this straight line at the point where the cutoff frequency of the low-pass filter is 10 Hz is Kw/Kd, which in this case is 19 dB. Therefore, Kw=8.9Kd=8900 (12) is the maximum value that Kd can take in this case. In this case, the DC offset can be reduced to about 1/10. Although the influence of the sample-and-hold function (4) is omitted here, since the decrease in gain due to (4) is small in the sampling frequency region of 115 or less, it may be omitted.

Although Kw and Kd have been determined as described above, a computer simulation will show how the servo system will behave if, for example, KW becomes 8900 or more. Figure 13 shows a model constructed using the numerical values of this example, and the time response of the servo system calculated by numerical integration, where the horizontal axis is time;
The vertical axis indicates ΔX. The curve (2) is for Kw=8900, small vibrations are acceptable up to the first 10 m5ec, and the response becomes a triangular hunting waveform, and the interval between peaks is the sampling period, that is, τ. It can thus be seen that if Kw is increased beyond the condition of equation (3), such hunting appears and the servo system becomes unstable.

The above is the case where N=16, but N=10°16.24
．． By a similar computer simulation in the case of 32,
Figure 1 shows the results of finding the maximum possible gain curve. From this, it can be seen that Equation (1) holds true and that the larger N is, the larger Kw is =15-, and therefore the offset suppression effect is enhanced.

〔Effect of the invention〕

As described above, according to the present invention, push-pull tracking can be significantly suppressed and a stable servo system can be constructed, so that highly accurate tracking can be performed in an optical disc file device or the like.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the prewobble mark N in one track layer obtained by the present invention and a gain curve, FIG. 2 is a diagram showing an optical system for recording and reproducing information on an optical disc, and FIG. Figures 4 and 5 are diagrams for explaining the pregroove track structure of recording and reproducing optical discs. Figures 4 and 5 are diagrams for explaining the principle of bush-pull tracking error detection using diffraction light distribution. Figure 6 is a general diagram. Figure 7 for explaining the configuration of a continuous tracking servo system for an optical disc.
The figure is a Bode diagram for explaining the frequency characteristics of a continuous tracking servo system, FIG. 8 is a diagram for explaining the track structure of a pre-pit type recording/reproducing optical disc of the present invention, and FIG. A diagram illustrating a method for obtaining a tracking signal from a pre-wobble pit row,
Fig. 10 is a diagram for explaining the tracking servo system for tracking according to the present invention, Fig. 11 is a diagram for explaining the present invention in detail, and Fig. 12 is a diagram for explaining the application method of the conditional expression of the present invention. FIG. 13 is a diagram for explaining the response characteristics of the servo system when the conditional expression of the present invention is satisfied and when it is not satisfied. [Explanation of symbols] 1... Semiconductor laser drive signal (when reading), 2... Semiconductor laser drive signal (when writing), 3... Semiconductor laser drive circuit, 4... Semiconductor laser, 5... ...Coupling lens, 6...Beam splitter, 7...Galver mirror (deflector), 8...1/4 wavelength plate, 9...
Objective lens, 10... Disk, 11... Light spot, 13... Photodiode, 131... Photodiode, 132... Photodiode, 14...
・1/4 wavelength depth pit, 15...178 wavelength depth pregroove, 121...header area, 122...
Data recording area, 123...Header area, 16...
- Bush pull tracking error signal, 17... Track deviation detection element Kd, 18... Phase compensation element Gc, 19
. . . Tracking actuator Qa, 20 . . . Pre-wobble ring pit, 21 . . . Optical output signal, 22.
22'...Peak optical output signal, 23.23'...
Peak optical output signal, 24... Pre-wobbling tracking error signal, 25... Push-pull tracking deviation detection element Kd, 26... Pre-wobbling 1-racking deviation detection element Kw, 27... Sample hold circuit , 28...1st-order low-pass filter, 29...
Phase compensation element G2.30... actuator, 32.
...Addition circuit, 33...Synthetic tracking servo error signal.

Claims (1)

[Claims] A light spot is irradiated onto a rotating optical disk on which a pregroove with a depth of λ/8 wavelength and a header area arranged at equal temporal intervals are formed in advance along the rotation direction, The amount of deviation of the optical spot from the track center (push-pull tracking error signal) is detected using the diffraction caused by the 1/8 wavelength deep pregroove, and a part of the header area is pre-determined from the track center. Oval pit rows (pre-wobble pit rows) are arranged to be shifted left and right, and the pre-wobble tracking error signal detected when a light spot crosses the pre-wobble pit rows is detected and continuously obtained. In an optical disc device that performs tracking using both the push-pull tracking error signal and the pre-wobbling tracking error signal obtained intermittently in the header area, the gain of the pre-wobbling tracking error signal is calculated by the push-pull tracking error signal in the low frequency region. When the configuration is such that the pre-wobbling signal is made higher than the signal and is attenuated by a first-order low-pass filter at a frequency higher than a predetermined frequency, and then added to the push-pull tracking error signal to form a composite tracking error signal, the pre-wobbling after passing through the first-order low-pass filter The frequency at which the gain of the tracking error signal and the gain of the push-pull tracking error signal are equal is f_
e, an optical disk tracking device characterized by satisfying the following condition: 1/50・N・f_d<f_e<1/5・N・f_d, where f_d is the rotational speed of the disk, and N is the number of header areas during one rotation of the disk. .