JP2004273023A - Focus control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a focus control device which can perform a focus search operation with a focus point as acenter at the absolute minimum amplitude. <P>SOLUTION: This focus control device, in which an objective lens 6 is prevented from colliding on an optical information recording medium D based on a focus error signal, comprises: a region discriminating means 14 which discriminating that the focus error signal is in between a first threshold value and a second threshold value when the focus error signal value in which the objective lens exceeds a focus point and is in the direction of approaching to a recording plane side and the objective lens is set so as not to conflict to the optical recording medium is assumed to the first threshold value, and when the focus error signal value in which the objective lens exceeds a focus point and is in the direction of separating from the recording plane and the objective lens is set so as not to separate excessively from the optical recording medium is assumed to the second threshold value; and a lens control means 16 performing focus control so that the focus error signal is in between the first threshold value and the second threshold value based on a signal outputted from the region discriminating means. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention moves an objective lens of an optical pickup used in a device for recording or reproducing information on or from an optical information recording medium such as an optical disk device in a focus direction, thereby changing a focal position (focus) of the objective lens. The present invention relates to a focus control device that detects and controls a moving direction of an objective lens.
[0002]
[Prior art]
Generally, in an optical information recording medium, for example, in a device for recording or reproducing information on an optical disk, as a process before pulling in a focus servo, an objective lens of an optical pickup is moved in a direction perpendicular to the disk surface, That is, a focus search operation is performed in which the lens is moved in the focus direction to find the objective lens focal position (also referred to as a focal point).
In this case, the objective lens is fixed so that the focal position of the objective lens can be detected even if the distance from the neutral position of the objective lens to the focal point varies due to the warp or surface shake of the optical disk, the mounting accuracy of the lens actuator or the disk motor, and the like. The focus search operation is performed by largely moving in the focus direction with the amplitude (see, for example, Patent Document 1).
[0003]
The state at this time will be described with reference to FIG. FIG. 6 is a graph showing the positional relationship between the disk surface and the tip of the objective lens during the focus search operation. FIG. 6 shows an example of focusing on a high-density optical disc when there is a low-density optical disc and a high-density optical disc. When the focus search operation is started, the objective lens approaches the disk surface from a neutral position far away from the disk surface, passes through the focal point for the high-density optical disk, approaches the disk surface further, and then reverses the moving direction of the objective lens Then, it operates to move away from the disk surface. During this time, the disk surface is irradiated with laser light via the objective lens and reflected on the disk surface, and the reflected light is detected by an optical sensor (not shown) to continuously obtain a focus error signal. Therefore, it can be recognized that the tip of the objective lens has passed the focal point for the high-density optical disk. Thereafter, focus servo is performed by recognizing the position of the focal point, the movement of the objective lens is controlled based on the focus error signal, and the objective lens moves so as to straddle the focal point as a center. Will go.
[0004]
[Patent Document 1]
JP 10-55546 A [0005]
[Problems to be solved by the invention]
Incidentally, the light spot formed on the information recording surface of the optical disc is determined by the NA (Numerical Aperture) of the objective lens and the wavelength of the laser light. Since the size of the light spot is proportional to the wavelength / NA, the size of the optical spot is small. In order to increase the density, it is necessary to employ laser light having a short wavelength or to increase the NA in order to obtain a minute light spot.
Further, in order to reduce the influence of the inclination of the disk surface with respect to the optical axis of the objective lens, in a high-density optical disk, the thickness from the light incident surface to the information recording surface is reduced. Accordingly, the distance from the tip of the objective lens to the disk surface, that is, the so-called working distance (WD) has been shortened. For example, as shown in FIG. 6, in the case of the conventional optical disk, WD1, which is the distance from the focal point to the disk surface, is large, but in the case of the high-density optical disk, WD2 is considerably small.
[0006]
Further, since it is necessary to form a light spot on the information recording surface with high accuracy, the sensitivity of a focus error near the focal point has been increased. That is, as shown in FIG. 6, in the case of a high-density optical disk, the time width of the S-curve characteristic in the time axis direction of the focus error signal near the focal point is short.
Therefore, when the working distance is narrowed as described above, the focus search method conventionally performed to detect the focal position of the objective lens by moving the objective lens of the optical pickup largely at a constant amplitude in the direction perpendicular to the disk surface. In this case, as shown in FIG. 6, there is a possibility that the objective lens collides with the high-density optical disk.
[0007]
In addition, in order to stably pull in the focus servo, the temporal change rate of the focus error signal at the focal point needs to be equal to or lower than a predetermined level. The error change rate becomes large and the focus servo cannot be pulled in stably.
In order to solve this problem, it is conceivable to reduce the moving speed of the objective lens of the actuator in order to stably pull in the focus servo by the conventional method. However, this will increase the time required for the focus search, so it is adopted. I can't.
The present invention has been devised in view of the above problems and effectively solving the problems. An object of the present invention is to provide a focus control device capable of performing a focus search operation with a required minimum amplitude around a focal point.
[0008]
[Means for Solving the Problems]
The present invention focuses the irradiation light on a recording surface of an optical information recording medium via an objective lens, and the objective lens is configured to control the optical information recording medium based on a focus error signal obtained by returning light from the recording surface. A focus control device that prevents the object lens from colliding with the focusing error signal value at a focus position where the objective lens focuses the irradiation light on the recording surface; The focus error signal value set so that the objective lens does not collide with the optical information recording medium in a direction approaching the recording surface beyond A focus error set in such a direction as to move away from the recording surface beyond a position, so that the objective lens is not too far from the optical information recording medium. When the signal value is set to a second threshold value, the focus error signal is determined to be between the first threshold value and the second threshold value, and is output from the area determination means. When the focus error signal approaches the recording surface beyond the first threshold or returns from the recording surface beyond the second threshold, the focus error signal is returned to the in-focus position based on the focus error signal. Lens control means for controlling the focus error signal to be between the first threshold value and the second threshold value, and performing focus control without causing the objective lens to collide with the optical information recording medium. A focus control device characterized by the following.
According to the above configuration, it is possible to determine whether the objective lens is located closer to or farther from the disk surface than the focal point, so that the lens moving direction is reversed immediately after the objective lens passes through the focal point in the focus search operation. As a result, the focus search operation can always be performed with the minimum necessary amplitude centered on the focal point.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of a focus control device according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a block diagram showing a main part of an optical pickup including a focus control device according to the present invention, and FIG. 2 is a block diagram showing an area determination circuit of the focus control device.
In FIG. 1, an optical disc D, which is an example of an optical information recording medium, is driven to rotate by a spindle motor M. An optical pickup 2 is provided movably relative to the optical disc D. The optical pickup 2 has an objective lens 6 which is moved forward and backward in the direction of the surface of the optical disc D (in the direction of the optical axis) by an actuator 4, and a recording / reproducing laser beam emitted by a laser element (not shown) is used. A light spot is formed by irradiating the disk surface through an optical system (not shown) and the objective lens 6. Here, the description of the actuator in the tracking direction is omitted.
[0010]
The optical sensor 8 detects reflected light from the disk surface (recording surface). The focus error detection circuit 10 outputs a focus error signal FS based on a signal from the optical sensor 8. Note that the description of the tracking error detection circuit and the like is omitted.
This focus error signal is input to the focus control device 12 of the present invention. The focus control device 12 includes an area determination circuit 14 that determines whether the objective lens is located closer to or farther from the disk surface than the focal point based on the focus error signal FS. And a lens control circuit 16 for controlling the moving direction of the objective lens 6 according to the result of the determination.
[0011]
The lens control circuit 16 delays the output from the area determination circuit 14 by a predetermined time Td (see FIG. 4), and an actuator that forms a control signal for the actuator 4 based on the output of the delay circuit 18. It comprises a control circuit 20. Then, the output of the actuator control circuit 20 is input to the driver 22, where the actuator driver signal DS is output.
As shown in FIG. 2, the area determination circuit 14 includes two comparator circuits 22 and 24, two rising edge detection circuits 26 and 28 each including a logic circuit individually connected to these outputs, The output of the detection circuits 26 and 28 is constituted by an RS flip-flop 30 which is input to the S input and the R input, respectively. This RS flip-flop 30 outputs a region determination signal AS.
The threshold values + TH1 and -TH2 are input to the comparator circuits 22 and 24 as reference values. Each of the rising edge detection circuits 26 and 28 receives delay element circuits 26A and 28A for delaying a part of the outputs of the comparator circuits 22 and 24, and inputs the outputs of the comparator circuits 22 and 24 to one pin, and Are provided as AND pins 26B and 28B for receiving the outputs of the delay element circuits 26A and 28A.
[0012]
Next, the operation of the device configured as described above will be described.
First, the operation of the area determination circuit 14 will be described with reference to FIGS. FIG. 3 is a timing chart showing a relationship among a lens position, a focus error signal, an area determination signal, and a set (including reset) signal.
Normally, when the objective lens 6 (see FIG. 1) passes through the focal point in a direction approaching the disk surface from a distance, the focus error signal FS becomes zero (for example, period h1) → positive (for example, period h2) → negative (for example, period). h3) → zero (for example, period h4), and when passing through the focal point in a direction away from the disk surface, zero (for example, period h5) → negative (for example, period h6) → positive (for example, period h7) → zero (for example, period h7) Period h8) changes.
[0013]
First, when the focus search operation starts and the objective lens 6 approaches the focal point (periods h1 and h2), the level of the focus error signal FS input to each of the comparator circuits 22 and 24 of the area determination circuit 14 shown in FIG. Changes from zero in the positive direction and exceeds the positive threshold value + TH at time t1 in FIG. At this time, the output of the comparator circuit 22 changes from L to H, and a positive pulse having a width corresponding to the delay amount d of the delay element circuit 26A is output from the rising edge detection circuit 26 to the set input (S) of the RS flip-flop 30. However, since the RS flip-flop 30 is set at the H level at the start of the focus search, the area determination signal AS remains at the H level and does not change (time t1).
[0014]
Next, when the objective lens 6 passes the point P1 of the focal point, the focus error signal FS changes from zero in the negative direction (period h3) and then returns to the zero direction again at time t2 in FIG. Exceeded (focus error signal is negative). At this time, the output of the comparator circuit 24 changes from L to H, a positive pulse having a width of the delay amount d is output from the rising edge detection circuit 28 to the reset input (R) of the RS flip-flop 30, and the area determination signal AS is L, and it is determined that the objective lens 6 is located closer to the disk surface than the focal point (time t2).
When the objective lens 6 passes through the focal point and moves to a region near the disk surface (period h4), the actuator control circuit 20 (see FIG. 1) controls the objective lens 6 to return to the focal point. Reverses the moving direction at the point P2, and starts moving toward the focal point again (period h5).
[0015]
When the objective lens 6 approaches the focal point from a region near the disk surface, the level of the focus error signal FS changes from zero to a negative direction, then to a positive direction, and exceeds -TH2 at time t3 in FIG. 3 (period h6). At this time, the output of the comparator circuit 24 changes from L to H, and a positive pulse is output from the rising edge detection circuit 28 to the reset input (R) of the RS flip-flop 30, but the RS flip-flop 30 is already reset to L level. Therefore, the area determination signal AS remains at the L level and does not change (time t3).
[0016]
Next, when the objective lens 6 exceeds the focus point P3 again, the focus error signal FS changes from zero to the positive direction, returns to the zero direction again, and exceeds + TH1 at time t4 in FIG. 3 (period h7). At this time, the output of the comparator circuit 22 changes from L to H, a positive pulse is output from the rising edge detection circuit 26 to the set input (S) of the RS flip-flop 30, the area determination signal AS becomes H, and the objective lens 6 It is determined that it is in a region far from the focal point. Thereafter, similar steps will be repeated.
[0017]
Now, after understanding the basic operation of the area determination circuit 14 as described above, the focus search operation will be described with reference to FIGS. FIG. 4 is a timing chart showing a relationship among a lens position, an area determination signal, a delay circuit output, and an actuator drive signal.
First, the reflected light from the optical disc D is converted into an electric signal by the optical sensor 8 in the optical pickup 2 via the objective lens 6 and output to the focus error detection circuit 10. The focus error detection circuit 10 forms a focus error signal FS corresponding to the focal position of the light spot of the laser beam based on the electric signal converted by the optical sensor 8 and constitutes a part of the focus control device 12 of the present invention. It outputs to the area determination circuit 14.
[0018]
The area determination circuit 14 determines, as described above, whether the objective lens 6 is located closer to or away from the disk surface than the focused point based on the level change order of the focus error signal FS. The determination signal AS is output.
The area determination signal AS output from the area determination circuit 14 is output to the actuator control circuit 20 with a delay of a predetermined time Td by the delay circuit 18.
The actuator control circuit 20 separates the disk from the objective lens 6 when the objective lens 6 is located closer to the disk surface than the focal point, and determines when the objective lens 6 is located farther from the focal point based on the determination result of the area determination circuit 14. The actuator 4 is moved so as to approach the disk.
[0019]
First, at the start of the focus search operation, the state of the area determination circuit 14 is set to the H level (a state away from the focal point). Then, the output (region determination signal) of the region determination circuit 14 is output to the actuator control circuit 20 with a delay of a predetermined time Td (see FIG. 4) by the delay circuit 18. Further, the actuator control circuit 20 changes the voltage (actuator drive signal) applied to the actuator 4 in a ramp shape (inclined shape) so that the moving speed of the objective lens 6 becomes constant. This operation can be realized by integrating the delayed output AS of the area determination circuit 14 with the delay circuit 18.
Since the area determination signal AS, which is the output of the area determination circuit 14, is at the H level until the focal point is first reached, the voltage applied to the actuator 4 is ramped (inclined) in the positive direction in proportion to the passage of time. And the objective lens 6 approaches the focal point at a constant speed.
[0020]
Here, when the objective lens 6 passes through the focal point, the region determination signal AS output from the region determination circuit 14 changes to L level (a state closer to the disk surface than the focal point), and the input of the actuator control circuit 20 is also given. It changes to the L level with a delay of time Td. Therefore, the voltage applied to the actuator 4 changes in the form of a ramp in the negative direction in proportion to the elapse of time with a delay of Td from the time when the objective lens 6 passes through the focal point. Get closer to focus.
Thereafter, each time the objective lens 6 passes through the in-focus point, the area determination signal AS, which is the output of the area determination circuit 14, changes in a direction to bring the objective lens 6 closer to the in-focus point. Move to the center in the focus direction and repeat the focus search operation.
[0021]
The width of the focus search centered on the focal point can be controlled by the rate of change of the voltage applied to the actuator 4 with time (the constant of the integrator) and the delay time Td. FIG. 5 is a diagram for comparing the trajectories of the objective lens at the time of the focus search of the present invention and at the time of the focus search of the conventional device. As is apparent from this figure, in the focus search operation of the conventional apparatus, the objective lens might collide with the high-density optical disk. However, according to the apparatus of the present invention, the moving amplitude of the objective lens centering on the focal point is remarkably large. Therefore, it is possible to prevent the objective lens from colliding with the high-density optical disk.
[0022]
【The invention's effect】
As described above, according to the focus control device of the present invention, the following excellent operational effects can be exhibited.
Determines whether the objective lens is closer to or farther from the disk surface than the focal point based on the level change order of the focus error signal, and if it is closer to the disk surface, the direction to move away from the disk surface Since the objective lens is moved in the direction of approaching when it is far away, the objective lens can always perform the focus search operation with a small amplitude centering on the focal point.
Therefore, taking into account the variation of the focal point from the neutral position of the objective lens due to the warpage or surface deflection of the optical disk, the mounting accuracy of the lens actuator or the disk motor, etc., as in the conventional focus search method, the objective lens is increased in the focus direction. There is no need to move to find the lens focal position, and even when the working distance (WD) is narrow, collision between the objective lens and the optical information recording medium at the time of focusing servo pull-in can be prevented.
In addition, since the focus search can be performed with the minimum necessary amplitude, the lens speed when passing through the focal point can be set low, and the focus servo can be stably pulled in even in an optical disk device having high focus error sensitivity. it can.
Further, since the number of searches per time can be increased, it is possible to shorten the time until the focus servo is applied.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a main part of an optical pickup including a focus control device according to the present invention.
FIG. 2 is a block diagram illustrating an area determination circuit of the focus control device.
FIG. 3 is a timing chart showing a relationship among a lens position, a focus error signal, an area determination signal, and a set (including reset) signal.
FIG. 4 is a timing chart illustrating a relationship among a lens position, an area determination signal, a delay circuit output, and an actuator drive signal.
FIG. 5 is a diagram for comparing the trajectories of the objective lens at the time of the focus search of the present invention and at the time of the focus search of the conventional device.
FIG. 6 is a graph showing a positional relationship between a disk surface and a front end of an objective lens during a focus search operation.
[Explanation of symbols]
2 optical pickup, 4 actuator, 6 objective lens, 8 optical sensor, 10 focus error detection circuit, 12 focus control device, 14 area determination circuit (area determination means), 16 lens control circuit (lens) Control means), 18 delay circuits, 20 actuator control circuits, 22 and 24 comparator circuits, 26 and 28 rising edge detection circuits, 30 RS flip-flops.

Claims (1)

Focusing the irradiation light on the recording surface of the optical information recording medium via the objective lens, and the objective lens collides with the optical information recording medium based on a focus error signal obtained by returning light from the recording surface. Focus control device for preventing
With the focus error signal value at a focus position where the objective lens focuses the irradiation light on the recording surface as a reference value, in a direction in which the objective lens approaches the recording surface side beyond the focus position. The focus error signal value set so that the objective lens does not collide with the optical information recording medium is set as a first threshold value, and the objective lens moves beyond the focus position and away from the recording surface. When the focus error signal value set so that the objective lens is not too far from the optical information recording medium is set as a second threshold value, the focus error signal is set between the first threshold value and the second threshold value. Area determination means for determining and entering
Based on the focus error signal output from the area determination means, when the focus error signal exceeds the first threshold and approaches the recording surface, or when the focus error signal exceeds the second threshold and moves away from the recording surface The focus is controlled without causing the objective lens to collide with the optical information recording medium by controlling the focus error signal to be between the first threshold value and the second threshold value. Lens control means;
A focus control device comprising:

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