JPH07182660A - Method for retraction of focus servo, and focus servo apparatus - Google Patents

Abstract

PURPOSE:To obtain a retraction method making possible the normal start of focus servo by using and comparing different reference values with an error signal. CONSTITUTION:When an objective lens 1b is moved to increase or decrease a distance between the objective lens 1b and a recording medium 2 monotonously, a reference value-generating means 6 outputs two reference values V1, V2 sequentially approaching a predetermined value. The reference values V1, V2, are between an extreme value at a position closer to an initial position where the lens lb is started to move and tone predetermined value when the lens 1b is in focus. A comparison means 7 compares an error signal output from an error signal-generating means 3 with the reference values output from the means 6. A control means 8 forms a focus servo loop when the relationship of the error signal and reference value V2 i.e., that the error signal is larger (smaller) than the reference value V2 is reversed after the relationship of the error signal and reference value V1 is reversed. Focus servo is normally started.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus servo retracting method and a focus servo device. More specifically, the present invention relates to a focus servo pull-in method for guiding a focus servo loop required for an optical disk reproducing device or the like to an appropriate operating range, and a focus servo device suitable for the method.

The present invention is particularly applicable to, but not limited to, LD, CD and MD playback devices.

[0003]

2. Description of the Related Art In an optical disc reproducing apparatus, in order to accurately focus laser light emitted from a semiconductor laser or the like on an optical disc, the position of an objective lens for converging the laser light is always set on the optical disc. It is necessary to control the position of the objective lens so as to maintain the in-focus position.

Therefore, conventionally, for example, the condensing characteristic of a cylindrical lens in a single direction is used, and photodetectors are arranged in a line in the direction and a direction orthogonal to the direction, and the cylindrical direction is defined by these directions. An astigmatism method has been proposed in which the difference in the degree of dispersion of transmitted light of a lens is taken as a feedback element of a focus servo.

Such a focus error signal becomes 0 when the objective lens is at the in-focus position and + and-are reversed when the objective lens moves from the in-focus position to the optical disc in the perspective direction. It is known to have a characteristic that changes and asymptotically approaches 0 through one extreme value, that is, an S-shaped characteristic.

Further, in such a focus servo, since the proper operation range of the focus servo is limited, the objective lens is moved in the vicinity of the in-focus position in advance with the focus servo loop cut off, and then the focus servo is performed. It is known that a servo loop needs to be formed.

[0007]

Therefore, in the focus error signal having the above-mentioned characteristics, 0 is indicated only at the in-focus position. Therefore, the objective lens is moved in order to find out the in-focus position. It is expected that the location where the error signal indicates 0 is searched and the focus servo loop is formed at that location.

However, according to such a method, in practice, there is a possibility that noise may temporarily indicate 0 in a region where the above-mentioned focus error signal asymptotically approaches 0. If the focus servo loop is formed by misjudging that the position is the position, the focus servo may not be normally activated.

A technical problem of the present invention is to pay attention to such a problem and to normally activate the focus servo in the focus servo pull-in method and the focus servo apparatus.

[0010]

According to a first aspect of the present invention, there is provided a focus servo pulling-in method, wherein the light emitted from a light source is converged on a recording medium via a movable lens. Is a predetermined value when is in the in-focus position, and when the lens is moved from the in-focus position in the perspective direction with respect to the recording medium, the magnitude relationship with the predetermined value is changed so as to be reversed, respectively. The following is performed when forming a focus servo loop in which a detected value having a characteristic that asymptotically approaches the predetermined value via an extreme value is used as a feedback element.

The lens is moved so that the distance between the lens and the recording medium monotonically increases or decreases,
In parallel with the movement of the lens, a reference value set to a value between the extreme value closer to the initial position where the movement of the lens is started and the predetermined value is compared with the detected value.

Then, when an inversion of the magnitude relation is obtained in the comparison, the reference value is switched to a value close to the predetermined value to continue the comparison, and again in the comparison after switching the value of the reference value. If you get a reversal of the magnitude relationship,
The focus servo loop is formed.

A focus servo apparatus according to a second aspect of the present invention is shown in FIG.
As shown in FIG. 1, the recording medium 2 is irradiated with light 1a emitted from a light source through a lens 1b which is moved by an actuator 1c.
It is a focus servo device for converging upward.

Therefore, the focus servo apparatus according to a second aspect of the present invention has a predetermined value when the lens 1b is at the in-focus position, and when the lens 1b moves from the in-focus position to the recording medium 2 in the perspective direction. , And an error signal generating means 3 for outputting an error signal having a characteristic that the magnitude relationship with the predetermined value changes so as to be opposite to each other, and each asymptotically approaches the predetermined value via an extreme value.

Further, two reference values V ₁ and V ₂ which are between the extreme value closer to the initial position for starting the movement of the lens 1b and the predetermined value and which are closer to the predetermined value in order are set. It has a reference value generating means 6 for outputting.

Further, a comparison means 7 for comparing the error signal with the reference value, the lens 1b and the recording medium 2 are provided.
The lens so that the distance to and from it monotonically increases or decreases
Switching means for switching between forming a focus servo loop and sweep means 5 for outputting a signal for driving the actuator 1c for moving 1b, or applying the output signal of the sweep means 5 to the actuator 1c. 4 and.

In addition to such a configuration, the focus servo apparatus according to claim 2 applies the output signal of the sweeping means 5 to the actuator 1c, and in the comparison result of the comparing means 7, the error signal and the reference signal. After the inversion of the magnitude relationship with the value V ₁ is obtained, the control means 8 is provided for forming the focus servo loop when the inversion of the magnitude relationship between the error signal and the reference value V ₂ is obtained. There is.

Furthermore, the focus servo apparatus according to a third aspect of the present invention is configured such that the value of the reference value V ₂ is the same as the predetermined value.

Further, the focus servo apparatus according to claim 4, comprising: means for generating a reference voltages V _1, and means for dividing the reference voltages V ₁ to energization by the switching means is controlled, the switching state of the switching means The comparison means 7 controls the comparison result.

[0020]

In the focus servo pull-in method according to the present invention, the reference value used for the first comparison with the detected value can be set from the predetermined value while ensuring a proper noise margin. Even if noise is mixed in a region that asymptotically approaches the predetermined value, the region is skipped without making an erroneous determination (determined as not within the proper operating range).

Further, in the focus servo pull-in method according to the first aspect, when the inversion of the magnitude relation is obtained in the first comparison, the reference value used for the subsequent comparison is set close to a predetermined value to bring the focus into focus. The focus servo is normally started by finding a nearby position and forming a focus servo loop by reversing the magnitude relationship in the comparison at this time. The value of the reference value close to the predetermined value includes the value of the predetermined value.

According to another aspect of the focus servo apparatus of the present invention, in comparing the _two reference values V ₁ and V ₂ which are closer to a predetermined value in order with the error signal, the magnitude relationship between the error signal and the reference signal V ₁ is reversed. After being obtained, the error signal and the reference signal V
The focus servo pull-in method according to claim 1 is realized by the configuration in which the focus servo loop is formed when the inversion of the magnitude relation with ₂ is obtained.

Further, in the focus servo apparatus according to the third aspect of the present invention, by setting the reference value V ₂ to a predetermined value, it becomes possible to form a focus servo loop at the in-focus position itself, and the focus servo control start point. It is possible to suppress the discontinuity point (step response) in.

In the focus servo apparatus according to the present invention, the voltage dividing circuit for the reference voltage V ₁ is switched according to the comparison result, so that when the inversion of the magnitude relationship between the two signals to be compared is obtained. It is possible to use V ₁ to obtain a reference voltage V ₂ .

[0025]

EXAMPLES Next, examples of practical use of the focus servo pull-in method and the focus servo apparatus according to the present invention will be described.

[First Embodiment] First, referring to FIG.
The configuration of the first embodiment will be described based on the block diagram shown in (a). 10 is a laser diode. The laser light emitted from the laser diode 10 is the grating 1
1, collimator lens 12, half mirror 13, objective lens 1
After passing through 4, it is converged on the recording surface 15 of the optical disc.

A part of the laser light reflected by the recording surface 15 of the optical disk is reflected by the half mirror 13, passes through a cylindrical lens 16, and is converged on a photodetector 17.

The photodetection signal output from the photodetector 17 is connected to the focus error signal generator 18, and the focus error signal generator 18 generates the focus error signal 20.

FIG. 2B is a block diagram for explaining the details of the part 19, that is, the photodetector 17 and the focus error signal generator 18. The photodetector 17 is composed of photodetectors 17a, 17b, 17c and 17d.

The photodetectors 17b and 17d are arranged side by side in the condensing direction of the cylindrical lens 16 with the center of the laser light interposed therebetween. The photo detector 17a,
17c are arranged side by side in the direction perpendicular to the direction with the center of the laser beam interposed therebetween.

The photo detectors 17a, 17b, 17c, 17
The outputs of d are amplified by amplifiers 18a, 18b, 18c, and 18d, respectively, and the outputs of the photodetectors 17a and 17c are added by an adder circuit 18e, and the photodetector 17c
The outputs of b and 17d are added by the adder circuit 18f.

Further, a differential signal between the output of the adder circuit 18e and the output of the adder circuit 18f is generated in the differential amplifier 18g. The difference signal is the focus error signal 20
Is.

Returning to FIG. 3A again, the focus error signal 20 is transferred to the selection circuit SW22 via the phase compensation circuit 21.
Connected to the input a. In the selection circuit SW22, switching between the input a to the output c and the input b to the output c is performed according to the control signal 31.

The output c of the selection circuit SW22 is connected to the actuator 24 via the amplifier 23. In the actuator 24, depending on the input signal, the objective lens
The recording surface 15 is moved in the perspective direction with respect to the recording surface 15.

The output of the signal generator 33 is connected to the input b of the selection circuit SW22. The signal generator 33 has a port
The control signal 32a output from 32 is connected. In the signal generator 33, according to the control signal 32a,
The signal shown in (c) is generated.

[0036] When the control signal 32a instructs the reset at time t _50, the output of the signal generator 33, FIG.
As shown in (a), it shifts to V _MIN . Also, the control signal
When 32a gives an instruction to start at time t ₅₁ , the output of the signal generator 33 shifts to V _MAX as shown in FIG.

Here, the V _MIN is the signal generator 33.
Is input to the actuator 24, the objective lens 14 is set to a voltage that moves the farthest from the recording surface 15. Further, the V _MAX is set to a voltage at which the objective lens 14 moves closest to the recording surface 15.

At this time, in the same figure (a), the objective lens 14 moves so as to gradually approach the recording surface 15.

Further, the V _MIN and the V _MAX may be replaced with the settings. At this time, in FIG.
The objective lens 14 moves so as to gradually move away from the recording surface 15.

When the control signal 32a instructs the stop when the output voltage is changing as shown in FIG. 9 (a), the output of the signal generator 33 is maintained at the voltage at that time (Fig. (Not shown).

If the output of the signal generator 33 is V
When the control signal 32a to reach the _MAX does not instruct the stop, to the output voltage may be kept in the V _MAX (not shown), toward said V _MIN As shown in the diagram (c) You may make it change by turning back.

Returning to FIG. 4A again, CPU 36, ROM 3
7, the RAM 38 and the ports 27, 28, 30, 32 mutually transfer data via the bus 35 to which they are connected. The CPU 36 executes the control described later.
The ROM 37 stores the control procedure of the control. RAM
Data required for the control is stored in 38.

The control signal 31 instructing switching of the selection circuit SW22 is set by the CPU 36 via the port 30. The control signal 32a for controlling the signal generator 33 is set by the CPU 36 via the port 32.

The focus error signal 20 is also connected to the positive input of the voltage comparator 26. The output c of the selection circuit SW25 is connected to the negative input of the voltage comparator 26.

Reference voltages V ₁ and V ₂ are connected to the input a and the input b of the selection circuit SW25, respectively. The selection circuit SW
In 25, depending on the control signal 29, it is switched whether the input a is output to the output c or the input b is output to the output c, and the control signal 29 is transmitted via the port 28. It is set by the CPU 36.

The voltage comparator 26 outputs a positive value when the voltage at the positive input becomes equal to or higher than the voltage at the negative input. The output of the voltage comparator 26 is connected to the port 27 and read by the CPU 36 via the port 27.

The reference voltage V ₁ and the reference voltage V ₁
The desirable setting of ₂ will be described later.

[Regarding Focus Servo Pull-In Control] Next, an example of the control procedure of the focus servo pull-in control executed by the CPU 36 will be described with reference to the flowchart shown in FIG.

In step H70, the selection circuit SW
The control signal 31 is set so that the input b is output to the output c at 22. In step H70, the control signal 29 is set so that the selection circuit SW25 outputs the input a to the output c.

At this time, the focus servo loop is not formed, and the actuator 24 is controlled by the output of the signal generator 33. In the voltage comparator 26, the focus error signal 20 and the reference voltage V
A comparison with ₁ is made.

In the following step H71, the control signal 32a is set to instruct the reset. At this time, the V _MIN is applied to the actuator 24.

In the following step H72, the control signal is sent.
No. 32a is set to indicate the start. This
At this time, the actuator 24 is supplied with the V_MINFrom before
Note V _MAXA voltage that changes to is applied. And said
Objective lens 14 moves accordingly. And control
Shifts to step H73.

In step H73, the voltage comparator
26 outputs are read. If the output of the voltage comparator 26 indicates a positive value, control transfers to step H74. Otherwise, control returns again to step H73.

In step H74, the selection circuit SW
The control signal 29 is set so that the input b is output to the output c at 25. At this time, the voltage comparator 26 compares the focus error signal 20 with the reference voltage V ₂ . Then, the control is step H75.
Move to.

In step H75, the voltage comparator
26 outputs are read. If the output of the voltage comparator 26 indicates a positive value, control returns again to step H75. Otherwise, control transfers to step H76.

In step H76, the selection circuit SW
The control signal 31 is set so that the input a is output to the output c at 22. At this time, the focus error signal 20 is fed back to the actuator 24, and a focus servo loop is formed. Then, the control shifts to step H77.

In step H77, the selection circuit SW
The control signal 29 is set so that the input a is output to the output c at 25. Then, the control ends.

In the step H76, the control signal 32a may be set so as to instruct the stop. The step H77 may be omitted.

[Operation] Next, the operation of the configuration will be described based on the plan view, characteristic graph, and time chart shown in FIG.

FIG. 7A shows the photodetectors 17a and 17a.
The shapes of the laser beams applied to b, 17c, and 17d are shown. The figure shows the case where the objective lens 14 is at the in-focus position.

The drawing shows the case where the objective lens 14 is farther from the recording surface 15 than the in-focus position. This figure shows a case where the objective lens 14 approaches the recording surface 15 from the in-focus position.

At this time, the focus error signal 20
Is the singular value Vj in the above-mentioned state as shown in FIG.
Shows the objective lens 14 from the focus position to the recording surface.
As it moves in the perspective direction with respect to 15, it changes in the opposite direction according to the moving direction, and exhibits an S-shaped characteristic that gradually approaches the limit value through one extreme value.

The singular value Vj is generally 0,
The limit value is also generally equal to the singular value Vj.

The reference voltage V ₁ is the singular value V j
It is set between the extreme value of the objective lens 14 and the extremum closer to the initial position. Desirably, it is set as close to the singular value Vj as possible, but it is set so that a necessary noise margin is secured.

More specifically, assuming that the limit value on the initial position side of the objective lens 14 is V _LM , the extreme value closer to the initial position of the objective lens 14 is V _BB , and the noise margin is V _NS , Vj + V _NS < It is set to satisfy V ₁ <V _BB (when V _LM <V _BB ) Vj−V _NS > V ₁ > V _BB (when V _LM > V _BB ). However, as described above, generally, it is Vj = V _LM = 0.

The reference voltage V ₂ is set closer to the Vj than the reference voltage V ₁ . Desirably, it is set equal to Vj. In that case, since the Vj is generally 0, the reference voltage V ₂ is also generally 0.

When the steps H70, H71, H72 are executed in the focus servo pull-in control, the output of the signal generator 33 is applied to the actuator 24, and the objective lens 14 is moved to ((c) in the figure). Move like a) and (a). In FIG. 9C, t ₉₅ is the above step H71.
Is executed, and t ₉₆ is the time when step H72 is executed.

After the time t _{96, the} step H73 is repeatedly executed, but at the time t ₉₇ , the objective lens is
When the distance between 14 and the recording surface 15 reaches L ₉₀ , the same figure (b)
As shown in FIG. 7, the focus error signal 20 and the reference voltage V ₁ become equal, and the control is performed in the step H7.
4, the process proceeds to H75, and the step H75 is repeatedly executed.

Then, time t₉₈At the objective lens
When the distance between 14 and the recording surface 15 reaches Lj, the same figure (b)
As shown in FIG.
Pressure V ₂And become equal, and the step H76 is executed.
The focus servo loop is formed.

In the figure, the objective lens 14 is shown to gradually approach the recording surface 15 as shown in (a) of FIG. The objective lens 14 is moved to the recording surface 15 by reversing the moving direction of the objective lens 14 with respect to the input voltage.
May be gradually moved away from.

In this case, the vertical axis of FIG. 7C shows that the distance between the objective lens 14 and the recording surface 15 increases as it goes up. The setting conditions of the reference voltages V ₁ and V ₂ may be as described above.

Further, the cylindrical lens 16 may be rotated 90 degrees with the center of the incident laser light as the axis of rotation. In that case, in the figure (a), the above is replaced with the above, and accordingly, the polarity of the vertical axis is inverted in the figure (b). The setting conditions of the reference voltages V ₁ and V ₂ may be as described above.

Further, the plus input and the minus input of the differential amplifier 18g may be replaced with each other. In that case, the polarity of the vertical axis is reversed in FIG. The setting conditions of the reference voltages V ₁ and V ₂ may be as described above.

However, in the focus error signal 20, when the extremal value closer to the initial position of the objective lens 14 is smaller than the extremal value on the initial position side of the objective lens 14, the steps H73 and H75 are performed. Reverse the branching condition at.

[Regarding the Second Embodiment] Further, the reference voltage V ₂ and the selection circuit SW25 are replaced with a voltage dividing circuit for controlling energization by switching means, and the divided voltage of the voltage dividing circuit is changed to the reference voltage V2. _It may be ₂ . A specific example of the method will be described based on the block diagram shown in FIG.

The reference voltage V ₁ is connected to the negative input of the voltage comparator 26 via the resistor R ₁₀₀ . Resistance R
A series circuit of ₁₀₁ and the rectifying element 102 is connected to the output of the differential amplifier 103, and one end of the resistor R ₁₀₁ is connected to the negative input of the voltage comparator 26.

At this time, the rectifying element 102 is connected so as to form a forward direction from the resistor R ₁₀₁ to the output of the differential amplifier 103. The differential amplifier 103
The negative input of is connected to the voltage comparator through a resistor R ₁₀₅ .
Connected to 26 outputs. The resistor R ₁₀₄ is a feedback resistor.
The positive input of the differential amplifier 103 is grounded.

Here, the resistors R ₁₀₄ and R _{105 are}
Means that when the output Vo of the voltage comparator 26 is at the low potential V _OL and the high potential V _OH , respectively, the output of the differential amplifier 103 is taken as Va, and V ₁ <Va (Vo = V _OL ) V ₁ > Va (when Vo = V _OH ) is satisfied.

When this structure is used, step H74 in the focus servo pull-in control is unnecessary.

In the configuration, when the focus error signal 20 exceeds the reference voltage V ₁ ,
As shown in (a), the output Vo of the voltage comparator 26 is high potential V
_The voltage changes to _OH , the rectifying element 102 becomes conductive, and the negative input V _R of the voltage comparator 26 is (1 / (R ₁₀₀ + R ₁₀₁ )) × ((V _aL + V _DD ) R ₁₀₀
+ V ₁ · R ₁₀₁ ).

Here, V _aL is the potential of the output Va of the differential amplifier 103 when the output Vo of the voltage comparator 26 is at the high potential V _OH . V _DD is the rectifying element 102
It is the voltage drop that occurs in.

When the focus error signal 20 falls below the potential, as shown in (c) of FIG.
26 output Vo of the return to the low potential V _OL, said rectifying element 102 becomes non-conductive, the minus input V _R of the voltage comparator 26, the reference voltage as shown in (d) of FIG. (B) Return to V ₁ .

When the reference voltage V ₂ becomes larger than the reference voltage V ₁ due to the setting conditions of the reference voltages V ₁ and V ₂ , the rectifying element 102 is connected so that the polarities thereof are reversed. The resistors R ₁₀₄ and R ₁₀₅ may be set as described above. The divided voltage is (1 / (R ₁₀₀ + R ₁₀₁ )) × ((V _aH −V _DD ) R ₁₀₀
+ V ₁ · R ₁₀₁ ).

Here, V _aH is the potential of the output Va of the differential amplifier 103 when the output Vo of the voltage comparator 26 is at the low potential V _OL .

[0085]

According to the focus servo pull-in method of the first aspect, since the comparison with the error signal is performed by using the different reference values as described above, unlike the prior art, the noise margin is added to the first comparison. By applying the secured reference value and applying the reference value close to the error signal at the in-focus position to the subsequent comparison, even if noise is mixed in the area where the error signal asymptotically approaches the predetermined value. The area is 1
It is possible to skip in the second comparison, find the vicinity of the in-focus point in the subsequent comparison, and normally activate the focus servo.

In the focus servo apparatus according to the second aspect, as described above, in the comparison between the _two reference values V ₁ and V ₂ and the error signal, the inversion of the magnitude relation between the error signal and the reference value V ₁ is obtained. After that, the focus servo loop is formed when the inversion of the magnitude relationship between the error signal and the reference value V ₂ is obtained. Therefore, by ensuring the noise margin by the reference value V ₁ , the error signal Is asymptotically approaching a predetermined value, even if noise is mixed, the area is skipped by comparison with the reference value V ₁ and the reference value V ₁ is skipped.
_By comparing with ₂ , it becomes possible to find the vicinity of the in-focus point and activate the focus servo normally.

Further, in the focus servo apparatus according to the third aspect, as described above, since the value of the reference value V ₂ is set equal to the value of the error signal at the in-focus position, the reference value V ₂
By comparing with, it becomes possible to find the in-focus position itself.

Further, as described above, the focus servo apparatus according to the present invention divides the reference voltage V ₁ to generate the reference voltage V 1.
Since it is configured to generate ₂ , the selection circuit SW2
5. The reference voltage V _{2 and the} like are integrated, and the configuration is simplified.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic principle of the present invention.

FIG. 2 is a block diagram showing a first embodiment of the present invention and a time chart of sweep waveforms.

FIG. 3 is a flowchart showing an example of a control procedure of focus servo pull-in control.

FIG. 4 is a plan view and a time chart of the photo detector for explaining the operation.

FIG. 5 is a block diagram showing a second embodiment of the present invention.

[Explanation of symbols]

 1a Laser light 1b Objective lens 1c Actuator 2 Recording surface 3 Error signal generating means 4 Switching means 5 Sweeping means 6 Reference value generating means 7 Comparison means 8 Control means

Claims (4)

[Claims] 1. A method for retracting a focus servo for converging light emitted from a light source onto a recording medium through a movable lens, wherein a predetermined value is obtained when the lens is at a focus position.
When the lens moves from the in-focus position to the recording medium in the perspective direction, it changes so that the magnitude relationship with the predetermined value is reversed, and each asymptotically reaches the predetermined value via an extreme value. When forming a focus servo loop having a detection value having a characteristic of approaching as a feedback element, the lens is moved so that the distance between the lens and the recording medium monotonously increases or decreases, and movement of the lens In parallel, the reference value set to a value between the extreme value closer to the initial position for starting the movement of the lens and the predetermined value is compared with the detection value, and the magnitude relationship is compared in the comparison. If the reversal of the magnitude relation is obtained, the reference value is switched to a value close to the predetermined value to continue the comparison, and if the reversal of the magnitude relation is obtained again in the comparison after the value of the reference value is switched, A focus servo pull-in method characterized by forming a focus servo loop. 2. A focus servo device for converging light (1a) emitted from a light source onto a recording medium (2) via a lens (1b) moved by an actuator (1c), When the lens (1b) is at the in-focus position, the value becomes a predetermined value, and the lens (1b) moves from the in-focus position to the recording medium
When it moves in the perspective direction with respect to (2), it changes so that the magnitude relationship with the above-mentioned predetermined value becomes opposite, and outputs an error signal having a characteristic that asymptotically approaches the above-mentioned predetermined value via each extreme value. Error signal generating means (3), and two reference values between the extreme value closer to the initial position where the movement of the lens (1b) is started and the predetermined value, and closer to the predetermined value in order. Reference value generation means (6) for outputting the values V ₁ and V _2, and comparison means (7) for comparing the error signal with the reference value.
And a sweep means for outputting a signal for driving the actuator (1c) for moving the lens (1b) so that the distance between the lens (1b) and the recording medium (2) monotonously increases or decreases. (5), switching means (4) for switching between forming a focus servo loop or applying the output signal of the sweeping means (5) to the actuator (1c), and the sweeping means (5) Output signal to the actuator (1c)
And the magnitude relation between the error signal and the reference value V ₁ is obtained in the comparison result of the comparing means (7), and then the magnitude relation between the error signal and the reference value V ₂ is obtained. A focus servo device comprising: a control means (8) for forming the focus servo loop when inversion is obtained. 3. The focus servo apparatus according to claim 2, wherein the reference value V ₂ is the same as the predetermined value. 4. The focus servo apparatus according to claim 2, wherein the reference value generating means (6) divides the reference voltage V ₁ whose energization is controlled by a switching means and a means for generating a reference voltage V _1. Means for controlling the switching state of the switching means by the comparison result of the comparing means (7).