JPH03156731A - Focus servo device - Google Patents

Abstract

PURPOSE:To attain accurate and stable detection by using two threshold levels so as to identify a focus error signal and detecting a correct focal point based on the sequence of occurrence of a change point of the signals. CONSTITUTION:When a controller 14 detects a trailing of an output S1 of a comparator 12 and a leading of an output S2 of comparator 13 succeedingly, it is discriminated that an objective lens is located near the focus and the controller 14 outputs a control signal S3 to close a switch 11. Then focus servo control is started and a focus error signal at a terminal 1 is fed to a phase compensation circuit 15 via the switch 11 to compensate a phase shift caused by a transfer function of the focus servo system. The signal is fed to an amplifier 17 via an adder circuit 16, a drive voltage is fed to an actuator coil 18 thereby driving the objective lens so that the focus error signal is always zero. Thus, the accurate and stable focal point is detected.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Industrial Application Field The present invention is a focus servo device for converging light irradiated from a light source through an objective lens onto a recording surface of a disk. The present invention relates to a focus servo device that performs focus servo control using only signals.

B1 Summary of the Invention The present invention is a focus servo device for converging light irradiated from a light source onto a recording surface of a disk via an objective lens. A focus that has the characteristic that signals with different polarities are generated when the objective lens is positioned before and after the in-focus position, and each signal with different polarity passes through one extreme value and approaches zero as it moves away from the in-focus position. A focus error signal generating means for generating an error signal; and a positive extreme value of the focus error signal that is smaller than the positive extreme value of the focus error signal generated by reflection from the recording surface of the disk and generated by reflection from other than the recording surface of the disk. a first detection means that compares a focus error signal with a second threshold that is larger than the first threshold and detects that the focus error signal is larger than the first threshold; and a focus error signal that is generated by reflection on the recording surface of the disk. The focus error signal is compared with a second threshold value which is larger than the negative extreme value of the focus error signal and smaller than the negative extreme value of the focus error signal generated due to reflections other than the recording surface of the disk, and the focus error signal is determined to be the second threshold value. The second detection means detects that the detection output is smaller than the threshold, and the fall of one detection output is detected for each detection output of the first detection means and the second detection means, and then the fall of the other detection output is detected. The control means generates a control signal when a rise is detected from the start, and the switch switches the focus servo control loop to a closed loop state in response to the output from the control means. The system identifies the signals, detects the correct in-focus position based on the order in which these change points occur, and closes the focus servo control loop.

C0 Conventional technology continues to emerge In optical disk devices such as dedicated optical disk devices, write-once optical disk devices, and rewritable magneto-optical disk devices, a disk-shaped recording medium (disk) is moved at a constant angular velocity or linear velocity using a spindle motor or the like. Drives constant rotation,
Data (information) is recorded and reproduced in a trunk H provided spirally or concentrically on the disk. When recording and reproducing data, the laser beam is guided to the target trunk or selector, and is focused and irradiated onto the recording surface of the disk. Data is reproduced by scanning the track with a spot and detecting changes in the intensity of reflected light from the recording surface of the disc.

A so-called optical system is known as a means of scanning tracks on a disk with a spot of laser light, and the focus of the objective lens of the optical head is always maintained at the recording surface (focus point position) of the disk. Focus servo control is known.

Here, a so-called focus error signal and focus servo control will be briefly explained. For example, in the so-called astigmatism method, a circular spot is formed when the objective lens is at the focused position, and a horizontally or vertically elongated spot is formed on the light receiving element when the objective lens deviates from the focused position. Therefore, the reflected light from the disk is made incident on a light-receiving element divided into four parts through an optical component that generates astigmatism, such as a cylindrical lens, and the sum signal of the light-receiving elements on each diagonal of the four-part light-receiving element is calculated. By comparing them, a focus error signal can be obtained. As shown in Figure 8, the characteristics of this focus error signal are that a circular spot is formed on the light-receiving element at the in-focus position, so the focus error signal becomes zero, and signals with different polarities before and after the in-focus position. is obtained, and the focus error signal increases as it moves away from the in-focus position, and as it moves further away, for example, the spot size becomes larger than the light-receiving surface, the output level of each light-receiving element decreases, and the focus error signal becomes zero again.

In other words, it has a so-called figure-eight curve characteristic.

Focus servo control is performed using the focus error signal having this figure-8 curve characteristic.

In other words, when the objective lens of the optical head is sweep-driven from a position away from the in-focus position and the focus error signal reaches zero after passing through the threshold, the objective lens is assumed to be located near the in-focus position and the focus servo control is performed. The loop is closed (or focus servo control is started), and then focus servo control is performed to drive the objective lens so that the focus error signal is always zero.

As a focus search method of sweepingly driving the objective lens to detect the focused position of the objective lens, for example, a playback signal (R
A method for detecting the level of the F-times signal, a method for detecting the zero-crossing point of the focus error signal as described above, a method for detecting the zero-crossing point of the focus error signal, and detecting that the so-called pull-in signal is above a certain level. Various methods are known, such as a method using so-called PLL lock information.

D0 Problem to be solved by the invention By the way, as an optical disk device used in automobiles, aircraft, etc., the optical disk and the drive device are separated for the purpose of dustproofing, moistureproofing, vibrationproofing, etc., and each is completely sealed. Structures have been proposed. Specifically, a magneto-optical disk device having the structure described below has been proposed.

FIG. 9 is a longitudinal cross-sectional view showing the structure of a sealed disk cartridge that accommodates a magneto-optical disk and the structure of a main part of a disk drive device for recording and reproducing data on the magneto-optical disk.

In FIG. 9, a disk cartridge 31 consists of an upper casing 32 and a lower casing 33 made of a metal having the required strength, such as aluminum, which are screwed together, and has a magneto-optical disk (hereinafter simply referred to as a disk) inside. ) 34 is rotatably housed. That is,
Circular disc storage recesses 32a and 33a are formed on the inner surfaces of the upper and lower housings 32 and 33, respectively, and the disc 34 is arranged in these disc storage recesses 32a and 33a.

Further, a groove 35 is formed in the outer periphery of the disk storage recess 32a in the upper housing 32, and an O-ring 36 is inserted into the groove 35 so as to be sandwiched between the upper housing 32 and the lower housing 33. The O-ring 36 maintains airtightness within the disc cartridge 31, and the disc 34 is completely isolated from the outside air.

In addition, a plurality of ribs 37.3 are formed radially from the center toward the outer periphery on the inner surfaces of both housings 32.33, that is, the disk storage recesses 32a and 33a, and these ribs 37.38 This ensures that both the casings 32 and 33 have the required rigidity.

The disk 34 is rotatably supported at its center. That is, a fixed shaft 39 is fixed to the center of the disk storage recess 33a of the lower housing 33 by a screw 40, and a ball bearing is connected to the fixed shaft 39. A hub 42 made of iron is rotatably supported via a hub 41, and a disk 34 is fixed to the hub 42 at its center with adhesive.

A rotor magnet 43 is mounted on the upper surface of the hub 42.
is fixed, and as will be described later, when the disk cartridge 31 is inserted into the disk drive device, this rotor magnet 43 is configured to correspond to the stator coil disposed on the disk drive device side. There is.

Further, a rotor cover 44 is attached to the upper housing 32 at a portion facing the rotor magnet 43. That is, a fixing hole 45 is formed in the center of the upper housing 32, and the rotor cover 44 is fitted into the fixing hole 45 so that its upper surface is flush with the upper plane of the upper housing 32. The center portion thereof is screwed onto the fixed shaft 39 with a screw 46, and the outer circumferential surface of the rotor cover 44 and the inner circumferential surface of the fixing hole 45 are bonded with adhesive.

As will be described later, the rotor cover 44 is made of a material, such as stainless steel, that has a small eddy current loss against the rotating magnetic field applied from the stator coil on the disk drive device side when the disk cartridge cartridge 31 is inserted into the disk drive device. is formed by. As the material with low eddy current loss, other non-magnetic materials having the required strength, such as hard plastics, can be used.

In addition, this rotor cover 44 protects the upper housing 32 from
It has a structure that allows it to be pressed down to prevent warping. That is, a flange 44a is formed protruding from the upper edge of the outer peripheral surface of the rotor cover 44 over the entire circumference, and correspondingly, a flange 44a is formed at the upper opening edge of the fixing hole 45 of the upper housing 32.
- A lower stepped portion 45a is formed, and with the flange 44a of the rotor cover 44 engaged in surface contact with the stepped portion 45a, the center portion of the rotor cover 44 is connected to the lower cover body 33 with a screw 46. By fixing to the fixed shaft 39 in the example, the upper casing 32 has its center portion attached to the rotor cover 4.
4 to suppress upward warping.

In addition, in the rotation support portion of the disk 34, the fixed shaft 3
The lower housing 33 and the fixed shaft 39 are connected to the groove formed on the lower end surface of the
An O-ring 47 is fitted so as to be sandwiched between the rotor cover 44 and the fixed shaft 39, and an O-ring 47 is fitted into a groove formed on the upper end surface of the fixed shaft 39 so as to be sandwiched between the rotor cover 44 and the fixed shaft 39. These O-rings 47 and 48 seal the inside of the disk cartridge to prevent outside air from entering through the holes for the screws 40 and 46, respectively.

In the disk cartridge 31 in which the disk 34 is rotatably and hermetically housed as described above, a window hole 49 is formed in the concave portion 33b formed in the front part of the lower housing 33 in the radial direction of the disk 34. The window hole portion 49 is entirely covered by a glass window 50 fixed to its inner surface, and laser light enters and exits through this glass window 50 during recording and reproduction.

Further, the lower housing 33 is provided with a shutter 51 that opens and closes the window hole portion 49. This shutter 51 is slidable in the front-rear direction, and is in the front sliding position covering the window hole 49 when not in use, that is, in the stored state, and when in use, that is, when the disc cartridge 3I is inserted into the disc drive device. The window hole portion 49 is exposed by sliding rearward.

Furthermore, a concave notch 52 is formed in the upper housing 32 corresponding to the window hole 49 from the front edge toward the center. This notch 52 has a predetermined depth, and the back side of the bottom thereof corresponds to the disk 34 with a small distance, and when the disk cartridge 31 is inserted into the disk drive device, this notch 52 An external magnetic field generating means for magneto-optical recording, which is provided on the disk drive side, is arranged in the disk drive device. Moreover, the bottom of this notch 52 is formed to be particularly thin compared to other parts.
That is, the thickness of the flat plate portions of both cylindrical bodies 32 and 33 is, for example, 2.
5 mm, whereas the thickness of the bottom of the notch 52 is approximately 1 mm, which allows the magnetic field of the external magnetic field generating means to be brought closer to the disk 34.

The disk cartridge 31 having the above structure is inserted and loaded into the disk drive device 60. This disk drive device 60 includes a stator yoke 61 and a stator coil 62 fixed to the lower surface of the stator yoke 61. When the disk cartridge 31 is inserted, this stake coil 62 is attached to the rotor magnet via the rotor cover 44. 43, and this stake coil 62 and rotor magnet 43 constitute a flat brushless morph. The rotor magnet 43 is rotated by energizing the stator coil 62 to generate a rotating magnetic field, and thus the disk 34 is driven to rotate together with the rotor magnet 43.

Furthermore, when the disk cartridge 31 is inserted, the optical handle 3 of the disk drive device 60 comes into a state corresponding to the window hole 49. The laser beam passes through the glass window 50 through the objective lens of the optical head 63 and forms a spot on the recording medium surface of the disk 34, thereby recording and reproducing data.

In addition, when the disk cartridge 3I is inserted, the magnet 64 that generates an external magnetic field for recording and reproducing data is placed in the notch 52, and the magnet 64 causes the recording medium of the disk 34 to be placed inside the notch 52. Apply an external magnetic field to the surface for magneto-optical recording.

Then, with this external magnetic field applied, the optical head 63
When the laser beam irradiated from the disk 34 is focused on the recording medium surface of the disk 34, magnetization is reversed by an external magnetic field, and data is thereby recorded.

In a magneto-optical disk drive having such a structure, reflection also occurs at the glass window 50, and as shown in FIG. Detecting a cross point does not result in detection of an effective in-focus position. In other words, the focus search method described above that detects the zero cross point of the focus error signal detects the true focus error signal caused by reflection from the recording surface of the disk. In addition to the (large figure 8 curve), an unnecessary focus error signal (small figure 8 curve) is generated due to reflection from surfaces existing in the optical path, such as the glass window 50, the surface of the disk, etc. Even if the cross point was detected, the in-focus position was not effectively detected, and the focus servo control could not be started normally.

Furthermore, - the method for detecting the level of the reproduced signal is such that focus servo control is activated based on the detection that the reproduced signal has reached a predetermined level. For this reason, the level of the playback signal may vary depending on the recording surface of the disc, for example.
If the conditions of the light source, light receiving element, etc. fluctuate, normal focus servo control cannot be activated.

Also, the method of detecting the zero cross point of the focus error signal and detecting that the pull-in signal is above a certain level is similar to the method of detecting the level of the playback signal described above. However, if the condition of the light source, light receiving element, etc. fluctuates, normal focus servo control cannot be started.

Furthermore, the method using the PLL lock information described above has a problem in that, for example, it takes a long time until the PLL is locked.

Furthermore, as described in J. above, if a signal other than the focus error signal, such as a pull-in signal or PLL lock information, is used, detection and signal processing of these signals are required, which poses a cost problem.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a focus servo device that can accurately and stably detect the in-focus position using only a focus error signal and that has a simple structure. With the goal.

89 Means for Solving the Problems The present invention provides a focus servo device for converging light irradiated from a light source onto a recording surface of a disk via an objective lens, the objective lens being at a focused position. Sometimes it becomes zero, and signals with different polarities are generated when the objective lens is located before and after the in-focus position, and each of the signals with different polarities passes through one extreme value and leaves the in-focus position. a focus error signal generating means for generating a focus error signal having a characteristic of approaching zero as the value approaches zero; A first detection step of comparing the focus error signal with a first threshold value larger than a positive extreme value of the focus error signal generated by reflection of the focus error signal, and detecting that the focus error signal is larger than the first threshold value. means, which is larger than the negative extreme value of the focus error signal generated by reflection from the recording surface of the disk, and smaller than the negative extreme value of the focus error signal generated by reflection from a surface other than the recording surface of the disk; a second detection means that compares a second threshold with the focus error signal and detects that the focus error signal is smaller than the second threshold; the first detection means and the second detection means; For each detection output of The above problem is solved by including a switch that closes the control loop.

F0 action In the focus servo device according to the present invention, the focus error signal is identified using two threshold values, and the focused point position is detected based on the order of occurrence of these change points, so that the focus is detected near the focused point position. Make the servo control loop closed.

G. Example Hereinafter, an example of a focus servo device according to the present invention will be described with reference to the drawings.

FIG. 1 is a block circuit diagram of main parts of a focus servo device according to the present invention.

Optical disk devices to which this focus servo device is applied include, for example, optical disk devices for continuous use, write-once optical disk devices, and rewritable magneto-optical disk devices, in which a disk-shaped recording medium (disk) is driven by a spindle motor. The disc is rotated at a constant angular velocity or linear velocity, and data (information) is recorded and reproduced on tracks provided in a spiral or concentric pattern on the disk. When recording and reproducing data, the laser beam spot is guided to the trunk and selector,
Always focus the objective lens on the recording surface of the disc.
In other words, the objective lens is held at the focal point position and the laser beam is irradiated.
Detects changes in the intensity of reflected light from the recording surface of the disc,
It is designed to play data.

An optical head is used as a means for moving and holding the laser beam spot in this way, and in order to always keep the objective lens of the optical head at the focused position, for example, the above-mentioned astigmatism method or Naifetsu method is used. Focus servo control is performed using the obtained focus error signal having a figure-8 curve characteristic. In the present invention, with the configuration shown in FIG. 1, focus servo control is activated using only the focus error signal having the above-mentioned figure 8 curve characteristic.

In the 111iJ, a focus error signal obtained by, for example, the astigmatism method described above is supplied to the terminal I. This focus error signal is sent to switch 11 and comparators 12 and 13. In the comparator 12, the focus error signal is compared with a first threshold value T H of a predetermined level, and the output S1 of the comparator 12 is supplied to the controller 14. Further, the focus error signal is compared with a second threshold value THz at a predetermined level in the comparator 13, and the output S2 of the comparator 13 is supplied to the controller 14. A control signal S3 from the controller 14 that controls the focus servo control loop controls the on/off of the switch 11. The switch 11 is connected to the phase compensation circuit 15, and the focus error signal inputted manually through the terminal I is sent to the phase compensation circuit 1 through this switch 11.
Sent to 5.

The output of the phase compensation circuit 15 is supplied to an adder circuit 16.

Further, the adder circuit 16 is supplied with an actuator sweep voltage via the terminal 2 for linearly driving the objective lens of the optical head in the optical axis direction. The output of adder circuit 16 is sent to amplifier 17. The output of the amplifier 17 is supplied to a coil 18 of a so-called focusing actuator, and an objective lens of νF is driven in the optical axis direction by the focusing actuator.

In the present invention, the threshold value T H used in the comparator 12 is determined by reflection from a surface other than the recording surface of the optical disc, for example, a surface existing in the optical path such as the glass window 50 shown in FIG. The level is set to be greater than the positive extreme value of the unnecessary focus error signal that occurs and smaller than the positive extreme value of the true focus error signal that occurs due to reflection on the recording surface of the optical disc. In addition, the threshold value TH used in Comparison 2; The level is set to be higher than the negative extreme value of the true focus error signal caused by reflection.

Next, the operation of the focus servo device will be explained.

When focus servo control is not being performed, such as when the power is turned on, a command (focus search command) to detect the focused position of the objective lens is input to the controller 14 via the terminal 3, and then the command is input to the controller 14 via the terminal 2. For example, a sawtooth wave actuator sweep voltage that sweeps and drives the objective lens of the optical head is supplied to the adder circuit 16 . Note that at this time, the switch 11 receives the control signal S3 from the controller 14.
is turned off by The actuator sweep voltage is amplified by an amplifier 17 and supplied to an actuator coil 18. As a result, the objective lens of the optical head is swept within its movement range, for example, in a direction away from the position closest to the optical disk. Alternatively, the optical disc is swept from the farthest position to the closest position. Focus error signals input to the switch 11 and the comparators 12 and 13 via the terminal 1 at this time are shown in FIGS. 2 and 3. FIG. 2 shows a focus error signal when the objective lens is swept from the farthest position, and FIG. 3 shows a focus error signal when the objective lens is swept from the closest position. In the focus error signals shown in FIGS. 2 and 3, the focus error signal with the small three-curve characteristic is, for example,
This is a focus error signal generated by reflection on a surface such as zero.

The operation of the focus servo device will be described below in the case where the objective lens is swept driven from the farthest position.

As shown in FIG. 2, as the objective lens approaches the recording surface of the optical disk, the focus error signal gradually increases, and at time 1. The threshold value TH becomes larger. As a result, the output S1 of the comparator 12 is inverted and becomes, for example, a high level (hereinafter referred to as H level). Further, as the objective lens approaches the recording surface of the optical disk, the focus error signal reaches an extreme value and then gradually decreases until time 1. is the threshold value TH,
The following is true. As a result, the output S1 of the comparator 12 is inverted and becomes L level. The falling edge of this output S1 is detected by the controller 14. When the objective lens further approaches the recording surface of the optical disk and is located at the focused position, the focus error signal becomes zero. When the objective lens passes the focused position and approaches the recording surface of the optical disk, the focus error signal gradually becomes smaller, and at time t,
becomes less than or equal to the threshold value TH. As a result, the output S2 of the comparator 13 is inverted and becomes, for example, H level. The rise of this output S2 is detected by the controller 14.

In this way, the controller 14 detects the fall of the output S1 of the comparator 12, and then detects the fall of the output S1 of the comparator 13.
2, the objective lens is assumed to be located near the in-focus position, and the controller 14 sends a control signal S3 to the switch 11 to turn on the switch ti. That is, the loop of focus servo control is brought into a closed loop state, and focus servo control is started. Specifically, a focus error signal input via the terminal l is supplied to the phase compensation circuit 15 via the switch 11. This phase compensation circuit 15 compensates for a phase shift caused by, for example, a transfer function of a focus servo system. This phase-compensated signal is sent to an amplifier 17 via an adder circuit 16 and amplified, and this amplified drive voltage is supplied to an actuator coil 18. As a result, the objective lens is driven so that the focus error signal is always zero. FIG. 4 shows a focus error signal when the focus servo control is started as described above and the objective lens is drawn into the in-focus position. Incidentally, when the objective lens described above is driven to sweep from the nearest position and the objective lens is drawn into the in-focus position, a focus error signal as shown in FIG. 5 is obtained. As shown in these figures, after the focus servo control is activated at time t3, the focus error signal quickly becomes zero.

By the way, when the objective lens is being swept in the vicinity of the in-focus position, for example, if the objective lens fluctuates due to vibration or the like and is not swept linearly as described above, the focus error signal will be as shown in FIG. 6, for example. In some cases, the three-figure curve characteristic may not be obtained. In this case, in the present invention, as described above, the switch 11 is turned on only when the fall of the output S1 of the comparator 12 and the rise of the output S2 of the comparator 13 are successively detected.
As shown in the figure, the second falling edge of the output St of the comparator 12 is detected at time t, and then the output S of the comparator 13 is detected.
When the rising edge of 2 is detected at time t, switch 1
1 turns on. As a result, it is possible to prevent the focus servo control loop from being erroneously closed when the objective lens is not near the in-focus position due to vibration or the like. That is, the in-focus position can be detected accurately and stably.

Next, the operation of the controller 14 will be explained with reference to the flowchart shown in FIG.

In FIG. 7, start is J: As mentioned above, it represents, for example, the state immediately after the power of the optical disk device is turned on, and the focus servo control is not yet performed.

In step STI, when a command (focus search command) to detect the focused position of the objective lens inputted via the terminal 3 is detected, the process proceeds to step ST2.

In step ST2, an actuator sweep voltage for sweeping driving the objective lens of the optical hand is supplied to the addition circuit 16 via terminal 2, and sweeping driving of the objective lens is started.
Proceed to step ST3.

In step ST3, when the falling edge of the output S1 of the comparator 12 is detected, the process proceeds to step ST4, and when the falling edge of the output S2 of the comparator 13 is detected, the process proceeds to step ST5.

In step ST4, a counter provided in the controller 14 is cleared, and the process proceeds to step ST6.

In step ST6, when the rising edge of the output S1 of the comparator 12 is detected again, the process returns to step ST4, and when the rising edge of the output S2 of the comparator 13 is detected, the process proceeds to step ST7.

On the other hand, in step ST5, a counter provided in the controller 14 is cleared, and the process proceeds to step STH.

In step ST8, when the falling edge of the output S2 of the comparator 13 is detected again, the process returns to step ST5, and when the rising edge of the output Sl of the comparator 12 is detected, the process proceeds to step ST7.

In step ST7, the count value of the counter in the controller 14 is determined, and if it is, for example, 4 or more, the process proceeds to step ST9. When it is less than 4, the process returns to step ST2. That is, the counter in the controller 14 is incremented (increased by l) at a predetermined period, and the time from detecting the fall of the output S1 of the comparator 12 to detecting the rise of the output S2 of the comparator 13 is , or when the time from detecting the falling edge of the output S2 of the comparator 13 to detecting the rising edge of the output S1 of the comparator 12 is longer than a predetermined interval, the process proceeds to step ST9.

In step ST9, a control signal S3 for turning on the switch 11 is sent to turn on focus servo control. In other words, the focus servo control loop is brought into a closed loop state.

As described above, when one of the falling edge of the output Sl of the comparator 12 or the falling edge of the output S2 of the comparator 13 is detected, and then the rising edge of the output of the other comparator is detected, the in-focus point position is determined for the first time. By assuming that we can detect
The focal point position can be detected accurately and stably. Also,
In particular, when an unnecessary focus error signal (small three-figure curve) is generated due to reflection from a surface existing in the optical path (for example, the glass window 50) as in the above-mentioned sealed magneto-optical disk device, the comparator 12 or 13 By setting the absolute value of the threshold value to be larger than the absolute value of the extreme value of the unnecessary focus error signal, the correct in-focus position can be detected. Furthermore, as mentioned above, when the output S1 of the comparator 12 falls or the output S2 of the comparator l3 falls and the comparator 1
When the interval between the rising edge of the output S2 of the comparator 12 or the rising edge of the output S1 of the comparator 12 is less than a predetermined interval, the switch 11 is not turned on and the in-focus position is detected again. For example, if the movement speed of the objective lens is high due to vibration or the like, and when the switch 11 is turned on, the objective lens may move outside the focus servo control pull-in range and normal focus servo control cannot be performed. It can be prevented. Furthermore, since the in-focus position is detected using only the focus error signal, the circuit configuration is simple and there are great economic advantages.

H1 Effects of the Invention As is clear from the above description, in the present invention, the in-focus position can be detected using only the focus error signal, so a focus servo device that can be easily configured at 8WI times can be obtained.

Further, in the figure-8 curve characteristic of the focus error signal, by detecting the true zero cross point of the focus error signal using two appropriate positive and negative thresholds, it is possible to accurately and stably detect the in-focus position. In particular, even when an unnecessary focus error signal is generated due to reflection from a surface existing in the optical path, the correct in-focus position can be detected.

In addition, as described in the embodiment, by setting a constraint on the time from detection using one threshold value to detection using the other threshold value, the movement speed of the objective lens becomes large due to vibration, for example, and focus sabot control This can prevent the device from being unable to operate properly.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram of main parts of a focus servo device according to the present invention, FIGS. 2 to 6 are diagrams showing time charts for explaining the operation of the focus servo device, and FIG. The figure is a flowchart for explaining the operation of the controller of the focus servo device, FIG. 8 is a waveform diagram of a focus error signal, and FIG. 9 is a diagram of a sealed disk cartridge for storing a magneto-optical disk. - It is a longitudinal cross-sectional view showing the structure of the nozzle and the structure of the main part of the disk drive device for recording and reproducing data on this magneto-optical disk. 1 l...l 2, 13 14...15...16...18...Switch...Comparator controller Phase compensation circuit Adder circuit Actuator coil

Claims (1)

[Claims] A focus servo device for converging light irradiated from a light source onto a recording surface of a disk via an objective lens, which becomes zero when the objective lens is at a focused position; A characteristic in which signals with different polarities are generated when the objective lens is located before and after the focal point position, and each signal with the different polarity passes through one extreme value and approaches zero as it moves away from the focal point position. a focus error signal generating means for generating a focus error signal having a focus error signal having a focus error signal that is smaller than a positive extreme value of the focus error signal generated by reflection from the recording surface of the disk and which is generated by reflection from a surface other than the recording surface of the disk; a first detection means for comparing the focus error signal with a first threshold value larger than the positive extreme value of the error signal, and detecting that the focus error signal is larger than the first threshold value; a second threshold that is larger than the negative extreme value of the focus error signal caused by reflection on the recording surface and smaller than the negative extreme value of the focus error signal caused by reflection from a surface other than the recording surface of the disk; and the focus error. a second detection means that compares the focus error signal with the second threshold and detects that the focus error signal is smaller than the second threshold; and one of the detection outputs of the first detection means and the second detection means. A control means that generates a control signal when a fall of a detection output is detected and then a rise of the other detection output is detected, and a focus servo control loop is brought into a closed loop state in accordance with the output from the control means. A focus servo device comprising a switch.