JP2006228269A - Optical disk tilt compensation device and optical disk device using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical disk tilt compensation method and device for realizing the Rad and Tan AC tilt servo of small residual control errors for sufficiently suppressing disk tilts near the fundamental wave frequency and secondary harmonic components of a disk AC tilt, and an optical disk device. <P>SOLUTION: In an optical disk tilt compensation method for tilting an objective lens so as to minimize the coma aberration of an optical spot on a recording surface on the basis of the relative tilt of the objective lens for performing condensing irradiation on the recording surface of an optical disk and the optical disk, the AC components of the relative tilt of a specified band are made small by using signals synchronized with the rotation cycle of the optical disk such as a rotation frequency or the second harmonic components. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical disc apparatus such as an optical disc recording / reproducing apparatus and its tilt servo apparatus, and more particularly to an optical disc tilt control technique for correcting a tilt error.

  In an optical disk recording / reproducing apparatus such as a CD (Compact Disc) or a DVD (Digital Versatile Disc), in general, the optical axis of the light beam emitted from the optical pickup due to warpage of the optical disk and the normal of the optical disk at the irradiation position are shifted. Occurs. This misalignment angle is called a tilt angle, which occurs mainly in the radial direction of the optical disk (hereinafter referred to as “radial direction”), and causes factors such as coma in the optical system. Therefore, the occurrence of the tilt angle causes signal degradation such as crosstalk and jitter with adjacent tracks, which adversely affects the reproduction quality of the optical disc. In particular, when performing high-density recording such as a DVD, it is necessary to shorten the wavelength of the laser beam and increase the numerical aperture NA of the objective lens in order to reduce the spot diameter of the laser beam. Becomes smaller. That is, even if the optical disk is slightly inclined, the reproduction quality is greatly degraded. Therefore, in order to correct the aberration due to the tilt angle during reproduction of the optical disc, it is common to provide a tilt servo mechanism that corrects the tilt error based on the detected signal intensity of the reflected light beam.

  This type of tilt correction mechanism is disclosed in [Patent Document 1]. An optical pickup that records and reproduces information on an optical disc by irradiating a laser beam, a drive mechanism that moves the optical pickup in the radial direction of the optical disc, a detecting means that detects the tilt amount of the optical disc, and the optical pickup according to the tilt amount It is a tilt correction mechanism of an optical disc apparatus provided with a tilt adjusting means for tilting. A main guide structure on the driving side that determines the moving direction of the optical pickup and a sub guide on the dependent side that determines the attitude of the optical pickup are provided. The main guide structure and the sub guide are engaged with each other by a slidable connecting member, and the main chassis The main guide structure and the sub guide are inclined in synchronization with the optical disk surface in the radial direction by moving the connecting member relative to the optical axis. The inclination of the optical axis of the optical pickup can be mechanically and reliably corrected so that the disk recording surface is vertical.

  [Patent Document 2] also discloses a tilt servo apparatus for an optical disc recording / reproducing apparatus, in particular, a liquid crystal tilt servo apparatus having a liquid crystal panel element for correcting a tilt error. This is a tilt servo device capable of accurate and stable tilt correction even if there is surface deflection of the optical disk. In the apparatus of [Patent Document 2], sampling means for sampling the envelope signal intensity of the detection signal at every predetermined rotation angle based on the rotation signal synchronized with the rotation of the motor that rotates the optical disc, and the sampled envelope signal And calculating means for calculating an average value of the intensity. The correction amount is determined based on the average value, and tilt correction control is executed.

JP 2003-217155 A JP 2000-298862 A

  In recent years, in order to handle high-definition moving images and still images in a disk playback device such as an optical disk playback device, it is desired to increase the capacity of the disk and to reduce the size of the device, and further high density recording is required. Therefore, by shortening the wavelength of the laser light source of the optical pickup for recording and reproducing information signals and increasing the numerical aperture of the objective lens, the light spot diameter is narrowed down, and the track pitch and recording bits are reduced. .

  For example, there is a standard using a blue-violet laser having a wavelength of 405 nm, such as Blue-Ray Disc (hereinafter referred to as BD) or HD-DVD, as a next-generation large-capacity optical disk. In addition to shortening the wavelength to increase the capacity, there is an increase in the NA of the objective lens. However, if the NA of the objective lens is increased, the tilt margin of the optical disk decreases and control of the objective lens (hereinafter referred to as OL) tilt servo etc. Correction is necessary. In the case of BD, the substrate thickness is as thin as 0.1 mm to make it a tilt-resistant system, but there is a problem in compatibility with conventional discs such as DVD and CD. In the case of HDDVD, the same substrate thickness as DVD is set to 0.6 mm to make it compatible with conventional discs, but NA is also set to 0.65, which is the same as DVD, due to the limitation of tilt margin. BD has achieved 25GB, but HDDVD has difficulty in increasing density, and is 15GB. Therefore, a highly accurate tilt servo system is an essential technology to achieve a capacity of 25 GB and a substrate thickness of 0.6 mm, the same as DVD.

  As described above, in the invention of [Patent Document 1], the inclination of the optical axis of the optical pickup is mechanically corrected with high reliability so that the optical axis of the beam irradiated from the optical pickup is perpendicular to the disk recording surface. In the correction of the direct current component of the optical disc tilt, correction can be performed by moving the pickup mechanically, but the correction of the alternating current component of the optical disc tilt cannot be performed. For example, if the disc is rotating at a rotational speed of 2400 rpm, the fundamental wave component of the alternating current component of tilt is 40 Hz, so when trying to correct the fundamental wave component, the optical pickup is driven with sufficient acceleration at 40 Hz. There must be. Further, since the disc tilt includes not only the fundamental wave component but also many harmonic components thereof, it is necessary to drive on the order of several hundred Hz, which is difficult with the optical pickup in the current optical system.

  A tilt servo system that achieves a substrate thickness of 0.6mm and a capacity of 25GB is capable of correcting disk AC tilt with a disk rotation period and several times the frequency component in both the radial (Rad) and tangential (Tan) directions of the disk. is necessary. If the linear velocity is 6.0 m / s, the disc rotation frequency is about 40 Hz, and the disc tilt has components up to 10 times the bandwidth, so a control bandwidth of 500 Hz is necessary. With a tilt with a 500 Hz band, the PU tilt servo that tilts the entire pickup (hereinafter referred to as “PU”) is difficult because the mass of the moving part is large, and the OL tilt servo that tilts the small OL (objective lens) is optimal. is there.

  In the above-mentioned invention of [Patent Document 2], there is a calculating means for calculating the average value of the sampled envelope signal intensity, and the tilt correction control is executed by determining the correction amount based on the average value. Therefore, in the correction of the direct current component of the optical disc tilt, the liquid crystal correction element can be corrected by electrically moving, but the alternating current component of the optical disc tilt cannot be corrected. For example, assuming that the disk is rotating at a rotational speed of 2400 rpm, the fundamental wave component of the alternating current component of tilt is 40 Hz. Therefore, in order to correct the fundamental wave component, the liquid crystal correction element must be driven at 40 Hz. In addition, since the disc tilt includes not only the fundamental wave component but also many of its harmonic components, it is necessary to drive on the order of several hundred Hz, and it is difficult to correct the AC tilt in the current liquid crystal element.

  To tilt OL in both Rad and Tan directions, add 2-axis tilt drive for Rad tilt and Tan tilt to 2-axis OL actuator (hereinafter ACT) for focus (hereinafter Fo) and track (hereinafter Tr) servos. 4-axis drive ACT is required. The crossover frequency of the tilt servo (frequency at which the open loop gain becomes 0 dB) is about 500 Hz in the case of 4-axis drive ACT, which depends on the allowable power of ACT. Increasing the crossover frequency increases the high-frequency current, often resulting in poor thermal characteristics due to copper loss heat generation. From the viewpoint of control error, if the crossover frequency is lowered, it approaches the ACT primary resonance frequency, so that the open loop DC gain cannot be obtained and the control error cannot be sufficiently compressed. For this reason, there is a method of individually increasing the gain in the DC region to reduce the control error in the low region, but if the crossover frequency is low, the phase margin is reduced due to the phase delay caused by the DC gain increase, and the servo system becomes unstable. There is a problem of becoming.

  When the primary resonance frequency is f1 and the crossover frequency is f0, the relationship between the open loop gain and Hs is expressed by the following equation (1).

  Here, assuming that the primary resonance frequency f1 = 104 [Hz] and the crossover frequency f0 = 500 [Hz], the open loop gain is Hs = 27.2 [dB].

  When the 4-axis drive ACT is designed, the primary resonance frequency of the tilt axis is close to 100 Hz as shown in the above equation, and is usually lower than the primary resonance frequency of Fo and Tr, and the open loop gain of tilt is about 27 dB from equation (1). I ca n’t make it bigger. The control error ε can be obtained by the following equation (2).

  Where r is the tilt [rad] generated by the optical disc surface shake and Hs is the open loop gain of the control system. The control target value is the tilt r of the optical disc that is generated by this surface shake. If the maximum value rmax of the optical disc tilt is 0.2 [deg] and the open loop gain of the control system is 27.2 dB, the maximum value εmax of the control error is 8.7e-3 [deg] from the equation (2). For the purpose of stabilizing the servo system, a phase compensator is used in the control loop to advance the phase in the vicinity of the crossover frequency. The low frequency gain of this phase compensator is smaller than 1 and is about 1/3 of the conventional one. Therefore, the maximum value of the control error is further increased by 3 times, rmax = 0.026 [deg], and only an improvement effect of about one digit can be expected for the tilt of 0.2 [deg].

  On the other hand, the tilt of the disc to be controlled is localized at a frequency that is an integral multiple of the disc rotation frequency. In particular, the fundamental (hereinafter 1ω) component (about 40Hz) and the second harmonic (2ω) component (about 80Hz) is large. Since the open loop gain is as small as 27 dB and the 1ω and 2ω components of the tilt are not sufficiently suppressed, this component becomes a bottleneck and the residual control error cannot be reduced. In order to increase the gain in the vicinity of this frequency, DC gain correction is inserted. However, since this is close to the crossover frequency in the vicinity of this frequency, a sufficient gain cannot be obtained and the DC gain correction effect is small.

  Therefore, the present invention has been made in view of the above-described problems, and its problem is that the fundamental frequency of the disk AC tilt and the Rad with a small residual control error that sufficiently suppresses the disk tilt near the second harmonic component. Realizes Tan AC tilt servo and realizes optical disc tilt compensator and optical disc tilt compensator that minimizes coma generated by optical disc tilt, and solves the problems of the prior art described above using this optical disc tilt compensator It is an object of the present invention to provide an optical pickup apparatus and an optical information recording apparatus, an optical information reproducing apparatus and an optical disc apparatus such as an optical information recording / reproducing apparatus provided with the optical pickup.

  The optical disc tilt compensation method of the present invention is based on the relative tilt between the objective lens for condensing the light emitted from the light source onto the recording surface of the optical disc and the optical disc, and the light on the recording surface. In a tilt compensation method for an optical disc that tilts the objective lens so as to minimize the coma of a spot, the AC component of the relative tilt in a specific band is reduced using a signal synchronized with the rotation period of the optical disc. It is a feature.

  This makes it possible to increase the open-loop gain of the desired frequency without increasing the crossover frequency compared to the conventional tilt servo, so that a tilt servo system with a small control error can be constructed without flowing a large current to the actuator. Thus, an optical disc tilt compensation method that minimizes coma generated by tilting of the optical disc can be realized by high-precision tilt servo.

  By using a signal synchronized with the rotation period of the optical disc, the rotation frequency of the optical disc or the AC component of the relative tilt of the second harmonic component can be reduced. In this way, it is possible to suppress the disk AC tilt component such as the optical disk rotation speed component and the second harmonic component, which have a particularly large control error, so that the desired frequency can be increased without increasing the crossover frequency compared to the conventional tilt servo. The open-loop gain of the frequency can be increased, a tilt servo system with a small control error can be constructed without flowing a large amount of current through the actuator, and the coma caused by the tilt of the optical disk is minimized by the high-precision tilt servo. An optical disc tilt compensation method can be realized.

  The optical disc tilt compensation apparatus of the present invention includes an objective lens for condensing and irradiating light emitted from a light source onto a recording surface of the optical disc, and the objective lens based on a relative tilt between the objective lens and the optical disc. An objective lens tilt actuator for tilting the optical disc, a compensator for creating a relative tilt signal to be a drive signal for the objective lens tilt actuator, and an objective lens tilt actuator driver, and a signal synchronized with the rotation period of the optical disc. A correction signal of the objective lens tilt actuator drive signal is generated, and the correction signal is injected into a closed loop of the optical disk tilt control to compensate for the tilt of the optical disk.

  In such an optical disc tilt compensator, it is possible to suppress the disc AC tilt component such as the optical disc rotation speed component and the second harmonic component, which have a large control error, and therefore, the crossover frequency is increased as compared with the conventional tilt servo. Therefore, it is possible to increase the open-loop gain of a desired frequency without causing a large amount of current to flow to the actuator, so that a tilt servo system with a small control error can be constructed. An optical disc tilt compensation device that minimizes coma aberration can be realized.

  In the above configuration, a voltage control amplifier is used as a correction signal generator for the objective lens tilt actuator drive signal using a signal synchronized with the rotation period of the optical disk, and a relative tilt signal is applied to the gain control signal of the voltage control amplifier. It may be used.

  As a result, the disc AC tilt can be detected by using the signal from the tilt error signal spindle index signal as the control signal of the voltage control amplifier, so that the amplitude of the corresponding tilt frequency is detected as a stable DC signal. Therefore, the voltage control amplifier is controlled using the control signal, and a stable and accurate tilt correction signal can be generated. In particular, it can suppress disk AC tilt components, such as optical disk rotation speed components and second harmonic components, which have large control errors. Therefore, compared to conventional tilt servos, the open-loop gain of the desired frequency can be achieved without increasing the crossover frequency. Therefore, it is possible to construct a tilt servo system with a small control error without flowing a large amount of current through the actuator, and optical disc tilt compensation that minimizes coma caused by tilting of the optical disc by high-precision tilt servo. The device can be realized.

  A phase detector for detecting a phase between two signals of a relative tilt signal and a signal synchronized with a rotation period of the optical disc, and a low-pass filter (LPF) for extracting a low frequency component from the signal after the phase detection ), And the output signal of the low-pass filter may be used as an amplification factor control signal of the voltage control amplifier.

  In this way, it is possible to detect the disc AC tilt, which is smoothed by LPF from the tilt error signal spindle index signal to the control signal of the voltage control amplifier, and detected as a DC signal with stable amplitude of the corresponding tilt frequency. Therefore, the control signal becomes stable and accurate, and the voltage control amplifier is controlled using this control signal, so that a stable and accurate tilt correction signal can be generated.

  This is error correction that automatically follows the change in spindle rotation even if the spindle speed changes due to the combination of PSD and LPF. In particular, it can suppress disk AC tilt components, such as optical disk rotation speed components and second harmonic components, which have large control errors. Therefore, compared to conventional tilt servos, the open-loop gain of the desired frequency can be achieved without increasing the crossover frequency. Therefore, it is possible to construct a tilt servo system with a small control error without flowing a large amount of current through the actuator, and optical disc tilt compensation that minimizes coma caused by tilting of the optical disc by high-precision tilt servo. The device can be realized.

  The optical disc apparatus of the present invention records and erases information by condensing and irradiating the light emitted from the light source onto the recording surface of the optical disc, and receives transmitted or reflected light from the optical disc. In an optical disc apparatus such as an optical recording apparatus, an optical reproducing apparatus, an optical recording / reproducing apparatus, etc., for reproducing information by causing the element to detect or converging in a signal detection optical system and causing the light receiving element to detect the focused light. Any one of the optical disc tilt compensation apparatuses described above is provided.

  Since this optical recording / reproducing apparatus includes the optical disk tilt compensation apparatus of the present invention, the open-loop gain of a desired frequency can be increased without increasing the crossover frequency compared to the conventional tilt servo, so that the actuator Therefore, it is possible to construct a tilt servo system with a small control error without flowing a large amount of current, and to realize an optical disc tilt compensator that minimizes coma generated by tilting of the optical disc by high-precision tilt servo. As a result, it is possible to form a high-quality beam spot with the coma aberration suppressed to the limit on the optical disc board surface, to reduce the read error, to improve the writing quality, and to solve the problems of the prior art and the optical pickup. An information recording apparatus and an information reproducing apparatus provided can be realized.

  Compared to the conventional tilt servo, the open loop gain of the desired frequency can be increased without increasing the crossover frequency, so that a tilt servo system with a small control error can be constructed without flowing a large current to the actuator. Optical disk tilt compensation that minimizes coma generated by tilting of the optical disk can be realized by a highly accurate tilt servo.

  First, the principle of tilt compensation by a 4-axis actuator (hereinafter sometimes abbreviated as ACT) is shown. As shown in the explanatory diagram of FIG. 1, the objective lens 1 (OL) is usually installed in parallel with the optical disc (optical recording medium) 2 and supported by the wires of the actuators 3 and 3. When focusing is performed on the recording surface 2A of the optical disc in this state and the spot shape is observed, a circular spot 4 as shown in FIG. However, when the optical disk 2 is tilted as shown in FIG. 2, the spot shape becomes an ellipse, further coma occurs, and the irradiation light cannot be condensed. Therefore, as shown in FIG. 3, if the OL1 is tilted by the four axes ACT3 and ACT3 and the optical disc 2 and OL1 are made parallel, the spot becomes circular again, and the tilt of the optical disc 2 can be compensated. The 4-axis ACT3 can also control radial tilt and tangential tilt in addition to the focus and track control performed by the conventional ACT.

  The tilt compensation system will be described in more detail with reference to the schematic block diagram of FIG. An OL tilt sensor 5 for detecting the OL tilt is disposed below the OL 1. The OL tilt sensor senses OL tilt and converts it into an electrical signal. On the other hand, an optical disc tilt sensor (media tilt sensor) 6 is also arranged on the optical disc side to sense the tilt of the optical disc. The difference between the output signals of the two tilt sensors is taken and input to the OLACT driver 8 via the compensator 7. The OLACT driver 8 drives OLACT according to the difference signal of the sensor.

  Next, the operation of the system of FIG. 4 will be described. When the optical disk is inserted, SW1 is connected to the servo pull-in circuit 9, and SW2 is off. The servo pull-in operation is performed by the servo pull-in circuit 9, and the OL tilt servo is driven by connecting the SW1 to the compensator side at the timing when the OL tilt is zero. Next, SW2 is connected to make the OL tilt follow the optical disc tilt signal from the optical disc tilt sensor 6.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 5 shows a schematic block diagram of the tilt servo according to the present invention. As in this embodiment, if only the 1ω and 2ω components of the disk AC tilt, which is the bottleneck of the control error, can be reduced, the control error is reduced. Alternatively, if only the gain in this band can be increased, the control error is reduced.

  Waveform shaping of the signal synchronized with the rotation of the optical disk (hereinafter referred to as spindle index signal), adjust the phase and amplitude based on the tilt error signal, and cancel out the components that could not be suppressed by the open loop gain in addition to the ACT control signal . In order to detect each tilt component from the tilt error signal, lock-in detection is performed by a combination of PSD (Phase Sensitive Detector) 10 and LPF (Low Pass Filter) 11, and it reacts and responds quickly to changes in the rotational speed. The configuration is as follows.

  The difference between the disc tilt signal from the disc tilt sensor 6 and the OL tilt signal from the OL tilt sensor 5 is taken as a tilt error signal. Based on the tilt error signal, the compensator 7 generates a 4-axis ACT drive signal. The ACT is driven by this signal, and the OL is tilted so as to coincide with the disc tilt. The OL tilt sensor 5 detects the OL tilt and feeds it back to the tilt error calculation. The configuration so far is the conventional tilt servo. In the control method of the present embodiment, the tilt error signal is branched, and a signal synchronized with the disk rotation from the spindle index is multiplied by the tilt error signal using the PSD 10.

  When this signal is passed through the LPF 11 having a frequency lower than the spindle rotation frequency, only a component having a frequency matching the spindle index of the tilt error signal is converted into a DC voltage and output. When a spindle control signal is passed through a voltage controlled amplifier (hereinafter referred to as VCA: Voltage Controlled Amplifier) 12 using this DC signal as a gain control signal, a frequency component matching the spindle can be extracted from the tilt error signal. When this signal is added to the compensator output and the 4-axis ACT3 is driven, the gain of the frequency synchronized with the spindle rotation speed decreases in the closed loop, and the 1ω component, which is a large frequency component in the tilt error signal, can be removed.

  In this embodiment, the OL tilt sensor 5 and the disc tilt sensor 6 are used to obtain the relative tilt signal, the tilt is detected individually, and the difference is calculated to obtain the relative tilt signal. However, the present invention is not limited to this, and for example, a tilt method may be used in which a disk tilt sensor is provided in a portion that moves together with the OL such as an objective lens holder, and a relative tilt signal is directly detected. Alternatively, a relative tilt signal may be obtained from a signal such as an RF signal or a push-pull signal without providing a tilt sensor.

[Second Embodiment]
In the first embodiment described above, the signal extraction port is provided before the compensator 7, and the superimposition port is provided after the compensator 7. However, the reverse is also possible. Hereinafter, the case of the second embodiment in which the signal extraction port and the superimposition port are reversed will be described.

  FIG. 6 is a detailed view of the tilt servo block showing a circuit corresponding to the schematic block diagram of FIG. 5 in a more specific block. In FIG. 6, the input signal is a simulated signal that reproduces the disc tilt. FIG. 7 shows a block diagram of the simulated tilt signal generator 20. A tilt 1ω component (40 Hz, equivalent to 6.0 m / s) is generated by the oscillator 21, and random noise corresponding to sensor noise is superimposed. An LPF 23 with a cutoff of 200 Hz is inserted into the random noise 22 in accordance with the frequency characteristics of the sensor to remove high frequency components from the sensor output. Further, the tilt 1ω component is branched, binarized by the binarization unit 24, and simply output as a spindle index signal.

  A signal to be added to the disc tilt error is created from this spindle index signal. For this purpose, first, a quadrature two-phase signal generator 30 shown in a block diagram in FIG. 8 generates a two-phase signal, that is, two signals whose phases are shifted by 90 degrees from the input spindle index signal. The spindle index signal is a rectangular wave signal with a duty of 50%, and this signal is passed through an 8th order Butterworth LPF 31 with a cutoff of 60 Hz to extract a sine wave of a 1ω component. This signal is differentiated to obtain a cosine wave, and the parents are binarized again to generate two rectangular wave signals with a 90 deg phase shift. The quadrature two-phase signals are multiplied with error signals by separate PSD1 and PSD2 (Phase Sensitive Detector).

  The tilt error signal is extracted from the Act Cont signal after the lead lag filter 41. Since only the tilt error signal synchronized with the spindle index is output as a direct current voltage (actually pulsating), each PSD outputs only the DC component with LPF1 and LPF2. These LPFs are 8th order Butterworth LPFs with a cut-off of 10 Hz. The response characteristics of the controller are determined by the setting of the cut-off. The Cos DC and Sin DC are input as voltage control signals to VCA1 and VCA2 (voltage controlled amplifiers), and signals that can be subjected to voltage amplitude modulation are assumed to be quadrature two-phase signals Rect Cos and Rect Sin. However, since the signals Rect Cos and Rect Sin are rectangular waves, the signals are passed through LPF C1 and LPF C2, respectively, converted into sine waves, and then input to VCA1 and VCA2. Add the two VCA output signals, adjust the amplitude and phase, and return to the control loop. The place to add is immediately before the lead lag filter 41, and is added to the Tilt Error signal. Since the spindle index signal is output in a phase that is not related to the tilt AC component (frequency is the same), the phase adjuster 42 is inserted so that the error signal becomes the smallest between the phase of the disc tilt. It needs to be adjusted.

The simulation result of the tilt control error performed based on the block diagram of FIG. 6 is shown.
[Before correction] First, the waveform of each part was observed when the pseudo tilt signal was input with the correction signal injected into the Tilt Error in the block diagram of FIG. . FIG. 9 shows the observed waveform. The upper disc tilt waveform is applied to the input. In this case, random noise of ± 0.001 [deg] is superimposed on a signal of ± 0.1 [deg] and 40 Hz.

  In the figure, the horizontal axis is time t [s], and the upper waveform shows the output OL tilt signal [deg] (OL Tilt) and disc tilt signal [deg] (Disc Tilt), and the lower waveform. Indicates a tilt error signal [deg] (Tilt Error). The disc tilt signal and the OL tilt signal are almost the same, indicating that the tilt servo is correctly applied. If you look closely, an OL tilt signal is observed inside the disc tilt signal, and this difference is a tilt error. The lower row is a tilt error signal, which corresponds to a waveform obtained by subtracting the OL tilt signal from the upper disc tilt signal. An AC waveform of 40 Hz at ± 0.01 [deg] is observed, and the compression rate is about one digit = 20 dB at 40 Hz. FIG. 10 (Board diagram) shows the open loop characteristics and the closed loop characteristics of the tilt correction system before control. The gain of the open loop transfer function at a frequency of 40 Hz is 20.2 dB, which matches the ratio of the input waveform of the 40 Hz tilt signal and the amplitude of the tilt error signal in FIG.

  A graph (Frequency characteristic (amplitude characteristic) of the tilt signal waveform) obtained by Fourier transform of the tilt signal waveform shown in FIG. 9 is shown in FIG. In this figure, the horizontal axis is frequency (analysis order), and each waveform is as indicated in the figure: OL tilt signal [deg] (OL Tilt): Disc tilt signal [deg] (Disc Tilt ),: Tilt error signal [deg] (Tilt Error).

The frequency characteristics of the OL tilt signal and the disc tilt signal are in good agreement, indicating that the tilt servo is correctly applied in each frequency band. The horizontal axis represents the calculated order (linear display), and the frequency near 31 on the scale is 40 Hz. In the vicinity of the scale 100, the frequency is 78 Hz, and in the frequency region displayed on the horizontal axis, there is a tilt error signal about one digit below the OL tilt signal, that is, 20 dB below. This also coincides with the Bode diagram of FIG.
There is a sharp peak at a frequency of 40 Hz near the scale 31, which is the tilt of the 1ω component of the disk. In a normal tilt servo, the tilt error signal in the frequency characteristics is a fraction of the open-loop gain of the disc tilt signal (20 dB lower in this simulation). Therefore, if there is a sharp peak as shown in FIG. The error is dominant over the entire error.

  [After Correction] Next, in the control state in which the correction signal is injected into the Tilt Error in the block diagram of FIG. 6, the waveform of each part when the pseudo tilt signal is input was observed. FIG. 12 shows the observed waveform. The upper disc tilt waveform is applied to the input. Random noise of ± 0.001 [deg] is superimposed on the ± 0.1 [deg], 40 Hz signal. The output OL tilt signal is the upper waveform, which almost overlaps the disc tilt waveform.

  The disc tilt signal and the OL tilt signal are almost the same, indicating that the tilt servo is correctly applied. An OL tilt signal is observed inside the disc tilt signal within an elapsed time of 0.15 seconds, and this difference is a tilt error. It can be seen that the disc tilt signal and the OL tilt signal completely match after 0.15 seconds. The lower row is a tilt error signal, which is a waveform obtained by subtracting the OL tilt signal from the upper disc tilt signal. An AC waveform of 40Hz with ± 0.01 [deg] is observed within an elapsed time of 0.15 seconds. After 0.15 seconds, the 40Hz AC waveform of ± 0.01 [deg] becomes smaller, and the random noise component becomes dominant.

  The middle waveform in FIG. 12 is a VCA control signal, which is 0 within 0.1 seconds, then the signal falls, and is constant at -2 after 0.15 seconds, and within 0.15 seconds, the tilt error signal and spindle It can be seen that index synchronization detection is not stable and correct correction is not performed. After 0.15 seconds, synchronization detection is stable and the tilt error signal is small. The time until this signal becomes stable depends on the cutoffs of LPF1 and LPF2 in FIG. 6, and it can be seen that 10 Hz = 0.1 seconds in this simulation. When the cutoff frequency is increased, the time until stabilization is reduced, but on the contrary, the stability of the stable region is lowered, so that the cutoff frequency cannot be increased more than necessary.

  FIG. 13 shows a graph obtained by Fourier transforming the waveform when the tilt signal in FIG. 12 is in the stable region. The frequency characteristics of the OL tilt signal and the disc tilt signal are in good agreement, indicating that the tilt servo is correctly applied in each frequency band. The horizontal axis represents the calculated order (linear display), and the frequency near 31 on the scale is 40 Hz. In the vicinity of the scale 100, the frequency is 78 Hz, and in the frequency region displayed on the horizontal axis, there is a tilt error signal of about one digit or more with respect to the OL tilt signal, that is, below 20 dB. There is a sharp peak at a frequency of 40 Hz near the scale 31, which is the tilt of the 1ω component of the disk. In the tilt error signal before correction in FIG. 11, this peak was about 20 dB below the disc tilt signal.

  Thus, after the correction shown in FIG. 13, the disc tilt is the same 0.1 [deg], but the peak of the tilt error signal is not observed and the level is 0.0004 [deg]. The value is far below 0.01deg. It has been shown that the removal of the 40 Hz peak significantly reduces the control error and can reduce the control error without increasing the open loop gain of the main loop. In addition, since a two-phase rectangular wave is input to the PSD input, correction of the fundamental wave component also has an effect of suppressing odd harmonic components such as the third harmonic and the fifth harmonic.

It can be realized with an algorithm in the DSP, and high-precision control can be performed without increasing the cost.
In addition, the combination of PSD and LPF provides error correction that automatically follows changes in the spindle rotation speed.

[Third Embodiment]
In the second embodiment, suppression simulation of the fundamental wave component is performed, but tilt correction of the second harmonic component and the higher harmonic component is also possible.
FIG. 14 is a block diagram showing a configuration of a 2ω component generator 40 using a PLL according to the third embodiment. In a system similar to that shown in FIG. 6 (tilt servo block), a PLL (Phase Locked Loop) is inserted before the spindle index output enters the quadrature two-phase signal generator 20 to generate a 2ω component from the spindle 1ω component. . If a 2ω component is obtained directly, there is no need to insert a PLL. Also, a PLL is not required when a 2ω quadrature two-phase signal can be generated directly from a 1ω component by a quadrature two-phase signal generator. The other blocks in the system are the same as those in FIG. By inserting a block as shown in FIG. 14, the residual tilt due to the disk AC tilt localized in the 2ω component of the disk can be removed. In addition, since a two-phase rectangular wave is input to the PSD input, correction of 2ω component also has an effect of suppressing even harmonic components such as the fourth harmonic and the sixth harmonic.

  In addition, it is possible to remove both the 1ω component and the 2ω component by preparing sub-loops as shown in FIG. 6 for the 1ω component and the 2ω component and connecting them in parallel. Also, residual tilt of an arbitrary frequency component can be removed by creating and correcting a component of 3Ω or more using a PLL.

It is a typical block diagram for the tilt compensation principle explanation. It is a typical block diagram for the tilt compensation principle explanation. It is a typical block diagram for the tilt compensation principle explanation. It is a schematic block diagram for demonstrating a tilt compensation system. It is a schematic block diagram of a tilt servo. It is a detailed view of a tilt servo block according to the present invention. FIG. 3 is a block diagram of a simulated tilt signal generator according to the present invention. It is a block diagram of a quadrature two-phase signal generator according to the present invention. It is a figure which shows each part waveform in a known tilt servo system. It is a Bode diagram which shows the open loop characteristic and closed loop characteristic of the tilt correction system before control. 10 is a graph obtained by Fourier transforming the tilt signal waveform shown in FIG. 9. It is a figure which shows each part waveform in the tilt servo system at the time of control. 13 is a graph obtained by Fourier transforming the tilt signal waveform shown in FIG. It is a detailed view of a tilt servo block according to the present invention.

Explanation of symbols

1 ... OL (objective lens)
2 ... Optical disc (optical recording medium)
3 ... Actuator (4-axis ACT)
5 ... OL tilt sensor 6 ... Optical disc tilt sensor (media tilt sensor)
7 ... Compensator 8 ... OLACT driver 9 ... Servo pull-in circuit 10 ... PSD (Phase Sensitive Detector)
11 ... LPF (Low Pass Filter)
12 ... Voltage Controlled Amplifier
20 ... Simulated tilt signal generator 21 ... Oscillator 22 ... Random noise 23 ... LPF
24 ... Binarization unit 30 ... Quadrature two-phase signal generator 31 ... LPF
40... 2ω component generator 100... Tilt compensation system

Claims (6)

Based on the relative tilt between the objective lens for condensing the light emitted from the light source onto the recording surface of the optical disc and the optical disc, the coma aberration of the light spot on the recording surface is minimized. In the tilt compensation method of the optical disc for tilting the objective lens,
An optical disc tilt compensation method, wherein an AC component of the relative tilt in a specific band is reduced using a signal synchronized with a rotation period of the optical disc.   2. The optical disc tilt compensation according to claim 1, wherein an AC component of the relative tilt of the rotation frequency of the optical disc or a second harmonic component thereof is reduced using a signal synchronized with the rotation period of the optical disc. Method. An objective lens for condensing and irradiating the light emitted from the light source onto the recording surface of the optical disc, and an objective lens tilt actuator for tilting the objective lens based on the relative tilt of the objective lens and the optical disc; Comprising a compensator and an objective lens tilt actuator driver for generating a relative tilt signal to be a drive signal of the objective lens tilt actuator;
A correction signal for the objective lens tilt actuator drive signal is generated using a signal synchronized with the rotation period of the optical disc, and the tilt of the optical disc is compensated by injecting the correction signal into the optical disc tilt control closed loop. An optical disc tilt compensator.   A voltage control amplifier is used as a correction signal generator for the objective lens tilt actuator drive signal using a signal synchronized with the rotation period of the optical disc, and a relative tilt signal is used as an amplification factor control signal of the voltage control amplifier; The optical disc tilt compensation apparatus according to claim 3.   A phase detector for detecting a phase between two signals of the relative tilt signal and a signal synchronized with the rotation period of the optical disc, and a low-pass filter for extracting a low frequency component from the signal after the phase detection ( 5. The optical disc tilt compensator according to claim 4, further comprising: an LPF), wherein an output signal of the low-pass filter is used as an amplification factor control signal of the voltage control amplifier. Information is recorded or erased by condensing and irradiating the light emitted from the light source onto the recording surface of the optical disc, and transmitted light or reflected light from the optical disc is detected by the light receiving element, or signal detection is performed. In an optical disc apparatus such as an optical recording apparatus, an optical reproducing apparatus, an optical recording / reproducing apparatus, etc., that reproduces information by focusing in an optical system and causing the light receiving element to detect the focused light.
6. An optical disc apparatus comprising the optical disc tilt compensation apparatus according to claim 3.

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