JP4191628B2 - Optical disk inspection device - Google Patents

Description

  The present invention relates to an optical disc inspection apparatus that inspects an optical disc by reproducing a signal recorded on the optical disc.

  Conventionally, after recording a signal by irradiating an optical disk such as a CD or DVD with a laser beam, the recorded signal is reproduced, or a signal recorded in advance on an optical disk is reproduced, and the jitter and waveform of the reproduced signal are reproduced. 2. Description of the Related Art An optical disk inspection apparatus that inspects an optical disk by measuring characteristics and the like is well known.

  In recent years, it has been demanded to reduce the time required for inspection by rotating the optical disk at a speed higher than the standard rotation speed defined in the optical disk standard. Even when this inspection time is shortened, it is required to obtain an evaluation result equivalent to the reference rotational speed, but the transfer characteristic of the actuator of the optical pickup device constituting the servo system changes depending on the frequency. The servo system response similar to the case of the reference rotational speed cannot be obtained only by changing the overall gain in accordance with the change of the rotational speed of the optical disk. Specifically, as shown in FIG. 8, the amount of displacement from the reference position of the optical disk (hereinafter simply referred to as the amount of displacement of the optical disk) is maintained at a large value in the low frequency region, and decreases as the frequency increases. Go. The amount of displacement in the low frequency region is based on the surface shake or eccentricity of the optical disk. The amount of displacement in the high frequency region is caused by acceleration such as surface wobbling or eccentricity of the optical disk. Then, when the rotational speed of the optical disk changes from a low state to a high state, the characteristic of the displacement amount of the optical disk changes so as to slide from the solid line to the high band side as shown by the broken line.

For this reason, conventionally, for example, as shown in the following patent document, a servo circuit has different characteristics of a lag-lead type (Lag-Lead type) or a lead-lag type (Lead-Lag type). A plurality of gain compensation circuits having a plurality of gain compensation circuits are arranged, and the plurality of gain compensation circuits are selectively used according to the rotation speed of the optical disk.
JP-A-8-30986

  However, the actuator has resonance determined by a loss factor caused by the mass of the movable part, the spring constant of the elastic body that supports the movable part, the friction generated when the movable part moves, and the like. Since the resonance Q is generally high, it is difficult to set the resonance frequency to a different frequency by electrical compensation. Therefore, even if the plurality of gain compensation circuits shown in the above prior art are selectively used according to the rotation speed of the optical disk, the rotation speed of the optical disk changes from a low state to a high state, and the characteristics of the displacement amount of the optical disk When the lens slides to the high frequency side (see FIG. 8), the optical pickup device, that is, the objective lens cannot be accurately servo-controlled by following the change in the displacement amount of the optical disk.

  The present invention has been made to address the above problems, and its purpose is to minimize the change in the response of the servo system even when the rotation speed of the optical disk is changed, and to be substantially equal to the reference rotation speed of the optical disk. It is an object of the present invention to provide an optical disk inspection apparatus capable of obtaining the above evaluation results.

In order to achieve the above object, the objective lens for condensing the laser light from the laser light source to form a light spot on the optical disk and the light receiving the reflected light from the optical disk by the light spot to indicate the light receiving state A photodetector that generates a signal, an actuator that drives the objective lens, and a servo control circuit that drives the actuator in accordance with a light reception signal from the photodetector and performs servo control so that a light spot is formed at a target position of the optical disk. In an optical disk inspection apparatus for inspecting an optical disk by reproducing a signal recorded on the optical disk based on a light reception signal by a photodetector while moving the light spot along the track of the optical disk, the displacement amount of the objective lens by the actuator a displacement amount detecting means for detecting, of the optical disk A rotation speed detecting means for detecting the rotation speed, generates a feedback control signal for variably controlling the feedback amount of displacement of the objective lens based on the displacement amount of the detected object lens by the displacement amount detecting means, the actuator And the feedback amount of the displacement amount of the objective lens by the generated feed control signal is detected by the rotation speed detecting means so that the resonance frequency of the servo control system including the servo control circuit changes according to the rotation speed of the optical disk. A feedback control signal generation circuit that controls according to the rotation speed of the optical disk, and compensates for the phase of the feedback control signal generated by the feedback control signal generation circuit, and also performs the servo control even if the resonance frequency of the servo control system changes. So that the resonance Q of the system is kept constant The a phase compensation circuit which is adapted to characteristics of the phase compensation is changed according to the rotational speed of the optical disc detected by the rotational speed detection means, to provide a feedback control signal the phase has been compensated for the servo control circuit And a phase compensation circuit added to the servo control.

  In this case, for example, the actuator is configured by a focus actuator that displaces the objective lens in the optical axis direction, the servo control circuit is configured by a focus servo control circuit that performs focus servo control on the objective lens, and the displacement amount detection is performed. The means may be constituted by an optical axis direction displacement detecting means for detecting an amount of displacement of the objective lens in the optical axis direction of the laser light.

  Further, the actuator is constituted by a tracking actuator for displacing the objective lens in the radial direction of the optical disc, the servo control circuit is constituted by a tracking servo control circuit for tracking servo controlling the objective lens, and the displacement amount detecting means Further, it may be constituted by a radial displacement amount detecting means for detecting the displacement amount of the objective lens in the radial direction of the optical disk.

In the present invention configured as described above, the feedback control signal generation circuit variably controls the feedback amount supplied to the servo control circuit according to the displacement amount of the objective lens via the phase compensation circuit. In this case, the feedback control signal generation circuit changes the feedback amount of the displacement amount of the objective lens by the feedback control signal so that the resonance frequency of the servo control system including the actuator and the servo control circuit changes according to the rotation speed of the optical disk. Control according to the rotation speed. Variation further than this feedback quantity, the spring constant of the elastic body supporting the movable part of the actuator is changed apparently (see Table 1 below). Therefore, the resonance frequency of the servo system including the servo control circuit and the actuator can be changed (see FIG. 7A). Further, the phase compensation circuit changes the phase compensation characteristic according to the rotational speed of the optical disc so that the Q of resonance of the servo control system is kept constant even if the resonance frequency of the servo control system changes. . Therefore, changing the resonant frequency, Q of the resonance of the servo control system is drip retaining substantially constant (see FIG. 7 (B)). Thus, even when the rotation speed of the optical disk is increased, a servo system response substantially equivalent to that obtained when the optical disk is rotated at the reference rotation speed can be obtained. Therefore, it is possible to optimize the gain characteristic of the servo system despite the high resonance Q, and the optical disk inspection based on the reproduction signal can be performed very well.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an overall schematic diagram of an inspection apparatus for inspecting an optical disk DK such as a CD or a DVD. The inspection apparatus includes a driving device 10 that rotates and drives an optical disc DK, and an optical pickup device 20 that irradiates the optical disc DK with laser light and receives reflected light from the optical disc DK.

  The drive device 10 includes a spindle motor 11 for rotating the optical disc DK and a feed motor 12 for moving the optical disc DK in the radial direction. A turntable 13 is fixed to a rotating shaft 11b of the spindle motor 11, and an optical disk DK is detachably assembled on the turntable 13.

The spindle motor 11 incorporates an encoder 11a that detects the rotation of the spindle motor 11, that is, the rotation of the turntable 13 (optical disk DK) and outputs a rotation detection signal indicating the rotation. This rotation detection signal repeats a high level and a low level by a predetermined minute rotation angle and a z-phase signal φ _Z generated every time the rotation position of the turntable 13 (optical disc DK) reaches one reference rotation position. It consists of a pulse train signal and an A phase signal φ _A and a B phase signal φ _B that are shifted in phase by π / 2. These rotation detection signals φ _Z , φ _A , and φ _B are supplied to a controller 50 described later, and are used by the controller 50 to detect the rotation speed of the spindle motor 11, that is, the optical disk DK.

  The rotation of the spindle motor 11 is controlled by a spindle motor control circuit 14. The spindle motor control circuit 14 uses the wobble signal supplied from the wobble signal take-out circuit 57 described later or the digital signal (recording / reproduction signal of the optical disk) supplied from the binarization circuit 47 described later. The rotational speed of the spindle motor 11 is controlled so that the spot moves relative to the optical disk DK with a constant linear velocity. Specifically, the pulse train signal generated from the wobble signal and the reference clock signal are synchronized, or the clock signal formed according to the digital signal using a PLL circuit or the like and the reference clock signal are synchronized. In addition, the rotation of the spindle motor 11 is controlled. The spindle motor control circuit 14 also receives a speed control signal from a controller 50 described later, and the linear speed of the light spot on the optical disk DK is switched by the speed control signal (for example, with respect to the reference speed). 2 times speed, 4 times speed, etc.). In this case, the frequency of the reference clock signal may be multiplied to synchronize the multiplied reference clock signal with the wobble signal or the clock signal.

  The feed motor 12 is connected via a screw rod 15 to a support member 16 that fixedly supports the spindle motor 11 and is allowed only to move in the radial direction of the optical disk DK. The screw rod 15 is connected to one end of the screw rod 15 so as to rotate integrally with the rotation shaft of the feed motor 12, and is screwed to a nut (not shown) fixed to the support member 16 at the other end. Therefore, when the feed motor 12 rotates, the spindle motor 11, the turntable 13, and the support member 16 are displaced in the radial direction of the optical disc DK by the screw mechanism including the screw rod 15 and the nut.

Also incorporated in the feed motor 12 is an encoder 12a that detects the rotation of the feed motor 12 and outputs rotation detection signals φ _Z , φ _A , and φ _B similar to those of the encoder 11a. The rotation detection signals φ _Z , φ _A and φ _B from the encoder 12 a are supplied to the thread servo circuit 17. In addition to the rotation detection signals φ _Z , φ _A , and φ _B , the sled servo circuit 17 controls the rotation of the feed motor 12 using a tracking servo signal, which will be described in detail later, to control the spindle motor 11, the turntable 13, The displacement of the support member 16 in the radial direction of the optical disk DK is controlled. Specifically, a signal indicating the radial position of the optical spot designated by the input device 66 on the optical disk DK is input from the controller 50, and the rotation detection signals φ _Z , φ _A , φ _B from the encoder 12a are used. The light spot is moved to the designated radial position. Further, using the tracking servo signal, control is performed so that the light spot moves following the track of the optical disk DK.

  The optical pickup device 20 includes a laser light source 21, a collimating lens 22, a polarizing beam splitter 23, a quarter wavelength plate 24, an objective lens 25, a convex lens 26, a cylindrical lens 27, and a photodetector 28. In the optical pickup device 20, the laser light from the laser light source 21 is condensed on the optical disk DK via the collimating lens 22, the polarization beam splitter 23, the quarter wavelength plate 24 and the objective lens 25, and the optical disk A light spot is formed on the DK. The reflected light from the light spot formed on the optical disk DK is guided to the photodetector 28 through the objective lens 25, the quarter wavelength plate 24, the polarization beam splitter 23, the convex lens 26, and the cylindrical lens 27, and received. Is done. The photodetector 28 is composed of four divided light receiving elements composed of four light receiving elements having the same square shape divided by dividing lines, and each light receiving element receives detection signals A, B, C, and D proportional to the amount of received light. Output as a received light signal. The detection signals A, B, C, and D represent the amount of light received by each light receiving element arranged clockwise from the upper left.

  The optical pickup device 20 also includes a focus actuator 31, a tracking actuator 32, and an objective lens position detector 33. The focus actuator 31 drives the objective lens 25 in the optical axis direction of laser light (perpendicular to the surface of the optical disk DK). The tracking actuator 32 drives the objective lens 25 in the radial direction of the optical disc DK. The objective lens position detector 33 detects each displacement amount from each reference position of the objective lens 25 in the optical axis direction of the laser light and the radial direction of the optical disk DK.

  Here, a specific assembly example of the focus actuator 31, the tracking actuator 32, and the objective lens position detector 33 will be described. FIG. 2 is a perspective view of an objective lens assembly for moving the objective lens 25 including the focus actuator 31, the tracking actuator 32, and the objective lens position detector 33. As shown in FIG. FIG. 3 is an exploded perspective view of the objective lens assembly, and FIG. 4 is a partially cutaway perspective view of the objective lens assembly shown with a part cut away.

  The objective lens assembly has a flat plate-like first base 71 made of a high magnetic permeability material fixed to a support of the optical pickup device. On the upper surface of the first base 71, a second base 72 made of a non-permeable material (low-permeability material) having a U-shaped cross section with an opening on the left side in the figure is erected and fixed. A pole piece portion 71a made of a high permeability material integrally formed with the base 71 is formed at the center of the upper surface of the first base 71, and a non-permeability material is formed on the upper surface of the pole piece portion 71a. A shaft 73 made of (low magnetic permeability material) is erected and fixed. Note that relay boards 74 for enabling connection to an external circuit are also assembled to the four corners of the second base 72, respectively.

  On the first base 71, a pair of fan-shaped focus magnets 75, 75 are fixed at symmetrical positions facing the pole piece portion 71 a, and their upper surfaces are flush with the focus magnets 75, 75. Focus yokes 76 and 76 made of a high magnetic permeability material are magnetically coupled and fixed. The focus magnets 75 and 75 are magnetized in the thickness direction, and a magnetic circuit having a radial magnetic field is formed by the focus magnets 75 and 75, the focus yokes 76 and 76, the first base 71, and the pole piece portion 71a. .

  The second base 72 has a pair of tracking yokes 77, 77 each having a circular cross section made of a high magnetic permeability material at symmetrical positions on both sides of the shaft 73 at the upper part inside the front and back surfaces. It is assembled and fixed. Inside the tracking yokes 77, 77, a pair of tracking magnets 78, 78 having a circular arc cross section are assembled and fixed. The tracking magnets 78 and 78 are polarized and magnetized along the circumferential direction of the shaft 73, and the magnetic flux emitted from one side of the inner surface of each tracking magnet 78 and 78 returns to the other side of the inner surface.

  In the first and second bases 71 and 72, a rectangular holder 80 formed by assembling the objective lens 25 is accommodated. A sleeve 81 penetrating vertically is fixed to the center portion of the holder 80. By assembling the sleeve 81 on the outer periphery of the shaft 73 so as to be displaceable in the axial direction and the circumferential direction, the holder 80 has first and second holders. It is displaceable in the vertical direction with respect to the bases 71 and 72 and is rotatable around the vertical axis. The holder 80 is elastically supported at the four corners of the second base 72 via the four wires 82a to 82d at the four corners. The laser light that enters and exits the objective lens 25 passes through a through hole 71 b provided in the first base 71.

  A focus coil 83 is assembled on the lower surface of the holder 80, and tracking coils 84 and 84 are assembled on the front and back surfaces thereof. Note that a counterweight 85 is also attached to the holder 80 in order to balance the moment with respect to the central axis of the shaft 73.

  The focus coil 83 has the axial direction thereof as the vertical direction of the holder 80. With the holder 80 assembled to the first and second bases 71 and 72, the focus magnets 75 and 75 and the focus yokes 76 and 76 It penetrates into a magnetic gap formed between the pole piece portion 71a. Thereby, by passing an electric current through the focus coil 83, the holder 80 including the objective lens 25 is displaced in the axial direction of the shaft 73, that is, the vertical direction by the electromagnetic force. In this case, the focus magnets 75 and 75, the focus coil 83, and the like constitute the focus actuator 31.

The tracking coils 84 and 84 have their axial directions directed from the front to the back of the holder 80, and are attached to the tracking magnets 78 and 78 in a state where the holder 80 is assembled to the first and second bases 71 and 72. Opposite. Each tracking coil 84, 84 is close to each tracking magnet 78, 78.
The magnetic flux interlinking with the tracking coils 84 and 84 substantially coincides with the normal direction of the tracking magnets 78 and 78. As a result, by passing a current through the tracking coils 84, 84, the holder 80 including the objective lens 25 rotates around the axis of the shaft 73 by electromagnetic force. The rotation of the holder 80 corresponds to the radial direction of the optical disc DK. In this case, the tracking magnets 78 and 78, the tracking coils 84 and 84, etc. constitute the tracking actuator 32.

  A prism 86 is incorporated on the opposite side of the objective lens 25 around the sleeve 81 of the holder 80. On the other hand, a light emitting element 87 is fixed on the upper surface of the first base 71 at a position below the prism 86, and the light emitting element 87 faces the reflecting surface of the prism 86. The prism 86 reflects light emitted from the light emitting element 87 in the direction opposite to that of the objective lens 25, and guides part of the light to the outside of the holder 80 through an aperture 80 a formed in the holder 80. On the left side surface of the second base 72, a photodetector 88 that receives light that has passed through the aperture 80a is assembled. The photodetector 88 is also composed of a four-divided light receiving element, like the photodetector 28, and each light receiving element outputs detection signals A, B, C, D proportional to the amount of received light as light receiving signals. In this case, adjustment is performed so that the light passing through the aperture 80a is irradiated to the dividing line intersection of the photodetector 88 in a state where the objective lens 25 (or the holder 80) is at a predetermined origin position.

  As will be described later, a displacement amount from the reference position of the objective lens 25 in the radial direction of the optical disk DK and a displacement amount from the reference position of the objective lens 25 in the optical axis direction are detected by the output signal from the photodetector 88. Therefore, the prism 86, the light emitting element 87, the photodetector 88, and the like constitute the objective lens position detector 33.

  Returning to the description of FIG. 1 again, the inspection apparatus includes an amplifier circuit 41 that is connected to the photodetector 28 and amplifies the detection signals A, B, C, and D. A reproduction signal generation circuit 42, a focus error signal generation circuit 43, and a tracking error signal generation circuit 44 are connected to the amplification circuit 41.

  The reproduction signal generation circuit 42 generates a reproduction signal (SUM signal composed of the sum signal A + B + C + D of the detection signals A to D from the photodetector 28) based on the signal from the amplifier circuit 41. The amplitude of the reproduced signal is corrected according to the frequency by a waveform equivalent circuit 45 configured by an equalize circuit, and is supplied to a waveform evaluation device 46 configured by a digital oscilloscope. The waveform evaluation device 46 evaluates the waveform of the reproduction signal, such as the symmetry of the reproduction signal and the amplitude ratio for each recording mark length. The output of the waveform equivalent circuit 45 is binarized by the binarization circuit 47, that is, converted into a digital signal, and is also supplied to the jitter measuring device 48. In the jitter measuring device 48, the jitter of the reproduction signal is measured.

  The waveform evaluation device 46 and the jitter measurement device 48 supply the controller 50 with evaluation results relating to the reproduced signal under the control of the controller 50 configured by a computer. Therefore, the controller 50 inspects the optical disc DK using these evaluation results. The digital signal from the binarization circuit 47 is also output to the spindle motor control circuit 14 for controlling the spindle motor 11 as described above.

  The focus error signal generation circuit 43 performs calculation using the detection signals A to D from the photodetector 28 via the amplifier circuit 41 (specifically, calculation of (A + C) − (B + D) by the astigmatism method). A focus error signal is generated and output to the focus servo circuit 51. The focus servo circuit 51 generates a focus servo signal based on the focus error signal and outputs the focus servo signal to the adder 52. The adder 52 adds a feedback signal to be described later to the focus servo signal and supplies it to the drive circuit 53. The drive circuit 53 drives and controls the focus actuator 31 according to the focus servo signal to which the feedback signal is added, and performs focus servo control by displacing the objective lens 25 in the optical axis direction. The focus error signal generation circuit 43, the focus servo circuit 51, and the drive circuit 53 constitute a focus servo control circuit of the present invention.

  The tracking error signal generation circuit 44 generates a tracking error signal by calculation using the detection signals A to D from the photodetector 28 (specifically, calculation of (A + B) − (C + D)) via the amplifier circuit 41. And output to the tracking servo circuit 54. The tracking servo circuit 54 generates a tracking servo signal based on the tracking error signal and outputs the tracking servo signal to the adder 55. The adder 55 adds a feedback signal to be described later to the tracking servo signal and supplies it to the drive circuit 56. The drive circuit 56 controls the tracking actuator 32 by displacing the objective lens 25 in the radial direction of the optical disk DK by driving and controlling the tracking actuator 32 according to the tracking servo signal to which the feedback signal is added. The tracking error signal generation circuit 44, the tracking servo circuit 54, and the drive circuit 56 constitute a tracking servo control circuit of the present invention.

  A control signal for controlling the gain of the servo system is supplied from the controller 50 to the focus servo circuit 51 and the tracking servo circuit 54. The focus servo circuit 51 and the tracking servo circuit 54 change and control the gains of the servo signals output to the adders 52 and 55 according to the control signal from the controller 50, respectively.

  The tracking error signal generated by the tracking error signal generation circuit 44 is also supplied to a wobble signal extraction circuit 57 composed of a band pass filter. The wobble signal extraction circuit 57 extracts the wobble signal from the tracking error signal and supplies it to the spindle motor control circuit 14. As described above, the supplied wobble signal is used in the spindle motor control circuit 14 to rotate the optical disk DK at a constant linear velocity. The tracking servo signal generated by the tracking servo circuit 54 is also supplied to the thread servo circuit 17. In the thread servo circuit 17, a DC component is extracted from the supplied tracking servo signal, and the extracted DC component is used for controlling the feed motor 12. As described above, the tracking servo signal is used by the sled servo circuit 17 to track the light spot.

  Detection signals A to D from the objective lens position detector 33 (photodetector 88) are supplied to an optical axis direction displacement detection circuit 61 and a track direction displacement detection circuit 62, respectively. The optical axis direction displacement detection circuit 61 performs an operation using the detection signals A to D from the objective lens position detector 33 (photodetector 88) (specifically, an operation of (A + B) − (C + D)) to perform an optical disc operation. An objective lens displacement amount signal representing a displacement amount from the reference position of the objective lens 25 in the optical axis direction of the laser beam to the DK (perpendicular to the surface of the optical disk DK) is generated and supplied to the feedback signal generation circuit 64. To do. The feedback signal generation circuit 64 generates a feedback signal according to the supplied objective lens displacement amount signal and outputs the feedback signal to the adder 52 via the phase compensation circuit 67.

  The track direction displacement detection circuit 62 performs the calculation using the detection signals A to D from the objective lens position detector 33 (photodetector 88) (specifically, the calculation of (A + D) − (B + C)). The objective lens displacement amount signal representing the displacement amount from the reference position of the objective lens 25 in the radial direction is generated and supplied to the feedback signal generation circuit 65. The feedback signal generation circuit 65 generates a feedback signal according to the supplied objective lens displacement amount signal, and outputs the feedback signal to the adder 55 via the phase compensation circuit 68.

  The feedback signal generation circuits 64 and 65 are supplied with a control signal from the controller 50 for changing the feedback amount including positive feedback and negative feedback of the feedback signal. The feedback signal generation circuits 64 and 65 change and control the feedback amounts in accordance with the control signal from the controller 50, respectively.

  The phase compensation circuits 67 and 68 are constituted by, for example, a lead-lag type (Lead-Lag type: phase advance compensation) circuit having phase compensation characteristics as shown in FIG. Are compensated for the phases of the feedback signals respectively output from the output signals to the adders 52 and 55, respectively. The phase compensation circuits 67 and 68 are supplied with a phase compensation characteristic, that is, a control signal for changing the phase advance amount at each frequency from the controller 50, and phase compensation is performed in accordance with the control signal from the controller 50. Each characteristic is controlled to change. The phase compensation circuits 67 and 68 are not limited to a lead-lag type (Lead-Lag type: phase advance compensation) circuit, and are, for example, a lag-lead type (Lag-Lead type: phase delay compensation). You may comprise with a circuit.

  An input device 66 such as a keyboard is also connected to the controller 50. The keyboard 66 is used to input an instruction from the user regarding the operation of the inspection apparatus. In the case of this embodiment, the keyboard 66 particularly instructs the magnitude of the linear velocity of the light spot on the optical disc DK (switching of the linear velocity). The controller 50 outputs a speed control signal representing the magnitude of the linear velocity of the light spot on the optical disc DK to the spindle motor control circuit 14 in accordance with the instruction of the input device 66, and the linear velocity (the rotation of the spindle motor 11). Speed) is controlled. The controller 50 also instructs the laser light source 21 to emit light.

Further, the controller 50 also receives the rotation detection signals φ _Z , φ _A and φ _{B of the} spindle motor 11, that is, the optical disk DK, from the encoder 11 a in the spindle motor 11. Then, the controller 50 calculates the rotation speed of the optical disc DK based on the rotation detection signals φ _Z , φ _A , φ _B , and sends the focus servo circuit 51 and the tracking servo circuit 54 based on the calculated rotation speed. A control signal to be supplied is generated. This control signal is for controlling the gain of the servo signal in the focus servo circuit 51 and the tracking servo circuit 54, and represents a gain that increases as the rotational speed of the optical disc DK increases.

  The controller 50 also generates a control signal to be supplied to the feedback signal generation circuits 64 and 65 based on the calculated rotation speed. This control signal is for controlling the feedback amount of the feedback signal generation circuits 64 and 65, and increases the feedback control amount that is negatively fed back as the rotational speed of the optical disc DK increases. Represents the feedback gain. Conversely, it represents a feedback gain of positive feedback that increases, that is, increases the feedback control amount that is positively fed back as the rotational speed of the optical disk DK decreases.

  The controller 50 also generates a control signal to be supplied to the phase compensation circuits 67 and 68 based on the calculated rotational speed. This control signal is for controlling the phase compensation characteristics of the phase compensation circuits 67 and 68, and is controlled to be changed to the phase compensation characteristics corresponding to the rotation speed of the optical disc DK.

  Next, the operation of the embodiment configured as described above will be described. The worker first places the optical disc DK to be inspected on the turntable 13 and fixes the optical disc DK on the turntable 13. Then, the input device 66 is operated to start the inspection. At this start, the optical disk DK is driven by the spindle motor 11 to rotate at a linear velocity instructed by the user, and is driven by the feed motor 12 to radially move from the position instructed by the user to the optical pickup device 20. Moving.

  The laser light source 21 also starts emitting laser light. The laser light emitted from the laser light source 21 is guided to the optical disk DK through the collimator lens 22, the polarization beam splitter 23, the quarter wavelength plate 24, and the objective lens 25, and is emitted to a predetermined radial position of the optical disk DK. A spot is formed. Then, the reflected light from the optical disk DK enters the photodetector 28 via the objective lens 25, the quarter wavelength plate 24, the polarizing beam splitter 23, the convex lens 26 and the cylindrical lens 27. The photodetector 28 supplies the detection signals A to D corresponding to the incident light to the reproduction signal generation circuit 42, the focus error signal generation circuit 43, and the tracking error signal generation circuit 44 via the amplification circuit 41.

  The detection signals A to D supplied to the reproduction signal generation circuit 42 are processed by the waveform equalization circuit 45 and the binarization circuit 47 and supplied to the waveform evaluation device 46 and the jitter measurement device 48. In the waveform evaluation device 46 and the jitter measurement device 48, the reproduction signal, that is, the optical disk DK is evaluated, and the evaluation result is supplied to the controller 50. In this way, the optical disc DK is inspected.

  The detection signals A to D supplied to the focus error signal generation circuit 43 are used for focus servo control of the objective lens 25 by the focus servo circuit 51 and the drive circuit 53. The detection signals A to D supplied to the tracking error signal generation circuit 44 are used for tracking servo control of the objective lens 25 by the tracking servo circuit 54 and the drive circuit 56 and also used for thread servo control by the thread servo 17. Is done. In these focus servo control and tracking servo control, their gains are controlled by a control signal corresponding to the rotation speed of the optical disk DK from the controller 50. The tracking error signal generated by the tracking error signal generation circuit 44 is also used for rotation control of the spindle motor 11 by the wobble signal extraction circuit 57 and the spindle motor control circuit 14.

  In the focus servo control of the objective lens 25, the reference position of the objective lens 25 in the optical axis direction of the laser beam detected by the objective lens position detector 33 by the feedback signal generation circuit 64, the phase compensation circuit 67 and the adder 52. The feedback control according to the amount of displacement of the objective lens representing the amount of displacement from is also taken into account. Further, in tracking servo control of the objective lens 25, the reference of the objective lens 25 in the radial direction of the optical disk DK is detected by the objective lens position detector 33 by the feedback signal generation circuit 65, the phase compensation circuit 68 and the adder 55. Feedback control according to the amount of displacement of the objective lens representing the amount of displacement from the position is also taken into account. Further, in these feedback controls, the encoder 11a, the controller 50, the feedback signal generation circuits 64 and 65, and the phase compensation circuits 67 and 68 change and control the feedback amount and its frequency characteristics according to the rotational speed of the optical disc DK. The

This feedback control (hereinafter referred to as feedback compensation) will be described in detail. Before describing feedback compensation, simple focus servo control and tracking servo control without feedback compensation will be described. Both servo controls are solid line parts excluding the broken line part (feedback part and phase compensation circuit) in FIG. It is represented by a block diagram. In this case, the transfer function G ₁ corresponds to the gain with respect to the deviation of the actual light spot position from the target light spot position, that is, the focus error signal generation circuit 43 or the tracking error signal generation circuit 44 of the above-described embodiment. The transfer function G ₂ (s) represents the transfer characteristics of the servo circuit and actuator, that is, the focus servo circuit 51, the drive circuit 53 and the focus actuator 31, or the tracking servo circuit 54, the drive circuit 56 and the tracking actuator 32 of the above-described embodiment. To express. As a result, the overall transfer function G (s) is expressed as the following equation (1).

Here, considering that the present embodiment is a discussion of a relatively low frequency band and that the transfer function G ₂ (s) is almost determined by the actuator, the transfer function G ₂ (s) is expressed by the following equation ( ₂ ). It can be approximated as a second order lag.

Further, if c / m = 2ζω _n and k / m = ω _n ² , the above formula 2 is transformed into the following formula 3.

This equation 3 is a quadratic equation _where the natural angular frequency ω _n = (k / m) ^1/2 , the loss factor ζ = c / 2 (mk) ^1/2 , and the gain | G ₂ (s) | = A / k. It is a general formula for delay. In this case, ω _n is the resonance angular frequency of the actuator, ζ is a loss resistance acting on the movable part of the actuator, and k is a spring constant acting on the movable part. A is a product of the gain of the servo circuit and the thrust constant by the actuator (the product of the effective magnetic flux density of the actuator magnetic circuit and the effective coil conductor length).

  On the other hand, the total transfer function when feedback compensation is performed (when the feedback control unit excluding the phase compensation circuit of FIG. 6 is included) is expressed by the following equation (4).

Here, when a transfer function of a portion G ₂ (s) / {1 + SG ₂ (s)} (a portion corresponding to G ₂ (s) in the above equation ₁ ) excluding the transfer function G ₁ is obtained, the following equation 5 is obtained. Become.

Here, as in the case of Equation 2, if c / m = 2ζ′ω _n ′ and (k + SA) / m = ω _n ² ′, Equation 5 is transformed into Equation 6 below.

This means that by performing feedback compensation, it is possible to obtain a transfer function of a second-order lag element having parameters different from those in the case of Equation 3. In other words, it means that by performing feedback compensation, the natural angular frequency ω _n and the loss coefficient ζ, which are the intrinsic parameters of the actuator, can be apparently changed. These are summarized in Table 1 below.

  Thus, it is understood that the spring constant k, which is a parameter unique to the actuator, can be apparently changed to k + SA by performing feedback compensation. As a result, the inherent breakpoint frequency, that is, the resonance frequency (refer to the prototype in FIG. 7A) determined by the movable part mass m of the actuator and the spring constant k that supports the movable part is changed. If the gain S of the displacement amount of the objective lens 25 is negative feedback with respect to the “+ side”, the resonance frequency of the actuator is apparently increased and the gain in the low band is slightly reduced (see the negative feedback in FIG. 7A). ). On the contrary, if the gain S of the displacement amount of the objective lens 25 is positive feedback with respect to the “− side”, the resonance frequency of the actuator is apparently lowered and the gain in the low band is slightly increased (in FIG. 7A). See positive feedback).

  Such a change of the gain S is controlled according to the rotational speed of the optical disc DK in the present embodiment. Specifically, when the rotational speed of the optical disc DK increases, the feedback amount by feedback compensation is increased to the negative side. Further, when the rotational speed of the optical disk DK decreases, the feedback amount by feedback compensation is increased to the positive side. Therefore, the apparent gain characteristics of the focus actuator 31 and the tracking actuator 32 change as shown in FIG. 7A due to the movement of the resonance frequency. Therefore, as the rotational speed of the optical disk DK changes from a low state to a high state, the displacement amount characteristic of the optical disk DK slides from the solid line to the high band side as shown in the broken line as shown in FIG. Even if it changes, the optical pickup device 20, that is, the objective lens 25 can be accurately servo-controlled by following the slide change of the same displacement amount characteristic. In addition, by controlling the output gains of the focus servo circuit 51 and the tracking servo circuit 54, it is possible to compensate for the gain change in the low frequency region shown in FIG. 7A due to the control of the feedback amount.

Further, by performing feedback compensation including the phase compensation circuit in FIG. 6, the resonance Q of the actuator can be kept substantially constant regardless of the resonance frequency (see FIG. 7B). That is, even if the rotation speed of the optical disk DK is changed from the reference rotation speed, a servo system response substantially equivalent to that obtained when the optical disk DK is rotated at the reference rotation speed can be obtained. The conditions for phase compensation set in the phase compensation circuit can be obtained as follows. Assuming that the transfer characteristic of the phase compensation circuit is G ₃ (S), when feedback compensation including the phase compensation circuit is performed, the total transfer characteristic is as shown in Equation 7 below.

Here, G ₃ (S) in Equation 7 is expressed by Equation 8 below.

In Equation 8, ω ₀ is the center frequency of phase compensation (the frequency at which the phase compensation amount is maximized in FIG. 5), and α is a coefficient representing the frequency range of phase compensation. Then, when the transfer function of the part corresponding to G ₂ (S) of the equation 1 is obtained using the equations 7 and 8, the following equation 9 is obtained.

  In Equation 9, since the portions corresponding to the natural angular frequency and loss factor in Table 1 depend on the frequency, it is difficult to solve the expression quantitatively, but feedback including the phase compensation circuit From the desired characteristics of the resonance frequency and resonance Q at the time of compensation and the values A, m, c, and k related to the actuator, the conditions for phase compensation to be set can be obtained approximately. In this case, since the obtained phase compensation condition is approximate, an optimum condition is further obtained by experiment.

  As a result, the displacement amount characteristic of the optical disk DK based on the rotational speed of the optical disk DK, which is the displacement amount of the optical disk DK due to the surface deviation or eccentricity of the optical disk DK, is the change in the apparent gain characteristics of the focus actuator 31 and the tracking actuator 32. The gain is corrected by the servo system gain control and compensated over the entire frequency range. Therefore, according to the above embodiment, even if the rotation speed of the optical disk DK is increased to shorten the inspection time of the optical disk DK, the change in the response of the servo system is minimized, and the reference rotation speed of the optical disk DK is reduced. And almost the same evaluation result can be obtained.

  Although one embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the object of the present invention.

  For example, in the above embodiment, the rotation speed of the spindle motor 11, that is, the optical disc DK is calculated using the encoder 11a and the controller 50, and the focus servo circuit 51 and the tracking servo circuit 54 are calculated based on the calculated rotation speed by the controller 50. The output gain of the feedback signal generation circuits 64 and 65 and the phase compensation characteristics of the phase compensation circuits 67 and 68 are controlled. However, the rotational speed of the optical disk DK can be given to the controller 50 from the outside by the input device 66. Therefore, the controller 50 controls the output gains of the focus servo circuit 51 and the tracking servo circuit 54 in accordance with the rotation speed given from the outside, and the feedback amounts and phase compensation circuits 67, 65 of the feedback signal generation circuits 64, 65. 68 phase compensation characteristics may be controlled.

  In this case, the output gains of the focus servo circuit 51 and the tracking servo circuit 54, the feedback amounts of the feedback signal generation circuits 64 and 65, and the phase compensation characteristics of the phase compensation circuits 67 and 68 are continuously set to the rotation speed of the optical disc DK. Since it cannot be made to follow, the effect of controlling the output gain, the feedback amount and the phase compensation characteristic is reduced as compared with the case of the above embodiment, but the focus actuator 31 and the tracking actuator 32 are controlled according to the rotational speed of the optical disk DK. It is possible to switch the apparent gain characteristic. For example, when the linear velocity is switched between two stages and three stages, the apparent gain characteristics of the focus actuator 31 and the tracking actuator 32 can be switched in stages. Inspection will be performed well.

  In the above embodiment, the gains of the focus servo system and the tracking servo system are changed by changing the output gains of the focus servo circuit 51 and the tracking servo circuit 54. However, instead of this, as shown by the broken lines in FIG. 1, the control signals supplied from the controller 50 to the focus servo circuit 51 and the tracking servo circuit 54 are guided to the drive circuits 53 and 56, and the drive circuits 53 and 56 The gains of the focus servo system and the tracking servo system may be changed by controlling each of the gains according to a control signal from the controller 50.

1 is a schematic view showing an entire optical disk inspection apparatus according to an embodiment of the present invention. It is a perspective view of the objective lens assembly for moving the objective lens of FIG. FIG. 3 is an exploded perspective view of the objective lens assembly. It is a partially broken perspective view which fractures | ruptures and shows a part of said objective lens assembly. It is a graph which shows the phase compensation characteristic of a phase compensation circuit. It is a block diagram which shows the transfer characteristic of a focus servo system and a tracking servo system. (A) is a graph showing the feedback compensation characteristic of the actuator before phase compensation, and (B) is a graph showing the feedback compensation characteristic of the actuator after phase compensation. FIG. 6 is a characteristic diagram showing frequency characteristics of an amount of displacement from a reference position of an optical disk due to surface blurring, eccentricity, and the like.

Explanation of symbols

DK: optical disk, 10: driving device, 11: spindle motor, 20: optical pickup device, 21: laser light source, 16: objective lens, 28: photo detector, 31: focus actuator, 32: tracking actuator, 33: detection of objective lens position , 43 ... Focus error signal generation circuit, 44 ... Tracking error signal generation circuit, 50 ... Controller, 51 ... Focus servo circuit, 52, 55 ... Adder, 54 ... Tracking servo circuit, 61 ... Optical axis direction displacement detection circuit 62, tracking direction displacement detection circuit, 64, 65 ... feedback signal generation circuit, 67, 68 ... phase compensation circuit.

Claims (3)

An objective lens for condensing the laser light from the laser light source to form a light spot on the optical disc;
A photodetector that receives light reflected from the optical disk by the light spot and generates a light reception signal indicating the light reception state;
An actuator for driving the objective lens;
A servo control circuit that drives the actuator according to a light reception signal from the photodetector and servo-controls the light spot to be formed at a target position of the optical disk, and moves the light spot along the track of the optical disk. However, in the optical disk inspection apparatus for inspecting the optical disk by reproducing the signal recorded on the optical disk based on the light reception signal by the photodetector,
A displacement amount detecting means for detecting a displacement amount of the objective lens by the actuator;
Rotation speed detection means for detecting the rotation speed of the optical disc;
A servo control system that generates a feedback control signal for variably controlling the feedback amount of the displacement amount of the objective lens based on the displacement amount of the objective lens detected by the displacement amount detection means, and includes the actuator and a servo control circuit The amount of feedback of the displacement amount of the objective lens by the generated feed control signal is controlled according to the rotational speed of the optical disk detected by the rotational speed detection means so that the resonance frequency of the optical disk changes according to the rotational speed of the optical disk. A feedback control signal generation circuit that
The phase of the feedback control signal generated by the feedback control signal generation circuit is compensated , and the resonance Q of the servo control system is kept constant even when the resonance frequency of the servo control system changes. A phase compensation circuit that changes a phase compensation characteristic in accordance with the rotational speed of the optical disk detected by the rotational speed detection means, and supplies the phase compensated feedback control signal to the servo control circuit. An optical disk inspection apparatus comprising a phase compensation circuit in addition to the servo control. In the optical disk inspection apparatus according to claim 1 ,
The actuator comprises a focus actuator that displaces the objective lens in the optical axis direction,
The servo control circuit comprises a focus servo control circuit that performs focus servo control of the objective lens, and the displacement amount detection means detects an amount of displacement of the objective lens in the optical axis direction of laser light. An optical disk inspection apparatus constituted by a quantity detection means. In the optical disk inspection apparatus according to claim 1 ,
The actuator comprises a tracking actuator that displaces the objective lens in the radial direction of the optical disc,
The servo control circuit includes a tracking servo control circuit that performs tracking servo control of the objective lens, and the displacement detection unit detects a displacement of the objective lens in a radial direction of the optical disc. An optical disk inspection device configured as described above.

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