Abstract:
An optical disk drive having a laser source for irradiating an optical beam onto an optical disk, a lens unit to focus the optical beam from the laser source onto the optical disk, a drive unit for driving the lens unit, and a high-frequency signal output unit for producing a high-frequency signal. The drive unit drives the lens unit by applying the high-frequency signal before the optical disk is started to reproduce, and drives it without applying the high-frequency signal after the optical disk is started to reproduce. Thus, it is possible to effectively control the aberration correction mechanism using a linear actuator.

Description:
INCORPORATION BY REFERENCE  
       [0001]     The present application claims priority from Japanese application JP2005-181403 filed on Jun. 22, 2005, the content of which is hereby incorporated by reference into this application.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to an optical disk drive that reproduces optical disks  
         [0003]     Until now, a technique is proposed of a closed loop control using a movable lens and a position sensor for the way the aberrations in the optical disk drive are corrected as disclosed in, for example, JP-A-2002-352449.  
         [0004]     In addition, another technique for the aberration correction method has been offered in which the closed loop control is made by computing the amount of correction from the movable lens and reproduced signals with long and short periods as described in, for example, JP-A-2004-241102.  
       SUMMARY OF THE INVENTION  
       [0005]     As described in the above patent documents, an approach is employed to drive the aberration-correction movable lens in the optical-axis direction by using a linear actuator as one of the spherical aberration correction methods.  
         [0006]     However, the aberration-correcting movable lens is required to have an ability to firmly hold its position once fixed in the optical-axis direction, or retainability without being affected by the external vibration or the like.  
         [0007]     In addition, the movable lens has a problem that it is apt to faintly move in the direction perpendicular to the optical axis or to have a tilt to the optical axis due to the looseness between the movable portion and the drive shaft.  
         [0008]     Thus, it is an objective of the invention to provide an optical disk drive capable of solving the above problems and of highly reliable operation.  
         [0009]     According to the invention, in order to solve the above problems, there is provided an optical disk drive having a laser source for irradiating an optical beam onto an optical disk, a lens unit to focus the optical beam from the laser source onto the optical disk, a drive unit to drive the lens unit, and a high-frequency signal output unit for producing a high-frequency signal. Before the optical disk is started to reproduce, the drive unit impresses the high-frequency signal to drive the lens unit. After the optical disk is started to reproduce, it does not impress the high-frequency signal but drives the lens unit without that signal.  
         [0010]     According to this invention, it is possible to provide a highly reliable optical disk drive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a block diagram to which reference is made in explaining the elements of the construction of embodiments according to the invention.  
         [0012]      FIG. 2  is a block diagram to which reference is made in explaining the elements of the construction of the aberration correction control signal generator of embodiment 1.  
         [0013]      FIG. 3  is a block diagram to which reference is made in explaining the elements of the construction of the aberration correction control signal generator of embodiment 2.  
         [0014]      FIG. 4  is an explanatory diagram  1  for the adjustment of the operating point and sensitivity of the position signal generator.  
         [0015]      FIG. 5  is an explanatory diagram  2  for the adjustment of the operating point and sensitivity of the position signal generator.  
         [0016]      FIGS. 6A and 6B  are graphs to which reference is made in explaining the operation of the target range judging unit.  
         [0017]      FIG. 7  is graphs to which reference is made in explaining the operation for the addition of the dither signal.  
         [0018]      FIG. 8  is graphs to which reference is made in explaining the operation for the addition of the dither signal at the time of the start and stop of the dither signal.  
         [0019]      FIG. 9  is an explanatory diagram  1  for the operation of the gain control unit.  
         [0020]      FIG. 10  shows a table  1  for the setting of gains.  
         [0021]      FIG. 11  is an explanatory diagram  2  for the operation of the gain control unit.  
         [0022]      FIG. 12  shows a table  2  for the setting of gains.  
         [0023]      FIG. 13  is a flowchart for driving the aberration correcting element with the tracking control off.  
         [0024]      FIG. 14  is a flowchart for driving the aberration correcting element with the tracking control on. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Embodiments of the invention will be described in detail.  
       Embodiment 1  
       [0026]     The construction of an optical disk drive of the invention will be described first with reference to  FIGS. 1 and 2 .  
         [0027]     Referring to  FIG. 1 , there are shown a disk  1  for recording/reproducing data, an objective lens  2  for use in gathering beam flux on the disk  1 , a focusing actuator  3  to drive the objective lens  2  in the rotation-axis direction of the disk  1 , a tracking actuator  4  to drive the objective lens  2  in the radius direction of the disk  2 , an aberration correcting lens  5  for correcting the aberrations and an aberration correcting actuator  6  to drive the aberration correcting lens  5  in the optical-axis direction. In addition, there are shown an optical detector  7  for optically detecting the disk  1 , a position detector  8  for detecting the position of the aberration correcting lens, a position signal generator  9  for setting the operating point and sensitivity relative to the output from the position detector  8 , and an aberration correction control signal generator  10  to control the aberration correcting actuator  6  so that the aberration correcting lens  5  can be set in a predetermined position. Moreover, there are shown an aberration-correcting actuator drive unit  11  for driving the aberration correcting actuator, a focusing error signal generator  12  for generating a signal of the focusing-direction error of the objective lens relative to the disk, a focusing-control signal generator  13  to control the focusing actuator so that the beam spot can be located just on the recording surface or reproducing surface of the disk, a focusing-actuator drive unit  14  for driving the focusing actuator, and a tracking-error signal generator  15  for generating a signal of the tracking error of the objective lens relative to the track of the disk. Also, there are shown a tracking-control signal generator  16  to control the tracking actuator so that the beam spot can be located just on a predetermined track of the disk, a tracking-actuator drive unit  17  for driving the tracking actuator, a spindle motor  18  for rotating the disk, a frequency generator  19  for generating a signal proportional to the rotation speed of the spindle motor, a motor control unit  20  to control the spindle motor to rotate with a predetermined speed, and a temperature sensor  21 .  
         [0028]     In addition,  FIG. 2  is a block diagram of the aberration correction control signal generator  10 . In  FIG. 2 , there are shown a target position setting unit  101  for setting the target position of the aberration correcting lens, a low-pass compensation filter  102 , a phase compensation filter  103 , a target range judging unit  104  for judging whether the error between the aberration correcting lens position and the target position is within a predetermined range, and a dither signal generator  105 .  
         [0029]     The outline of the operation of each block and the relation between the blocks will be described next.  
         [0030]     Referring to  FIG. 1 , the focusing actuator  3  moves the objective lens  2  in the rotation axis direction of the disk, and the tracking actuator  4  moves the objective lens  2  in the radius direction of the disk. The optical detector  7  converts the reflected light into an electric signal, and supplies the electric signal to the focusing-error signal generator  12  and tracking-error signal generator  15 . The focusing-error signal generator  12  generates a focusing error signal based on the fed signal, and supplies it to the focusing-control signal generator  13  and aberration-correction control signal generator  10 . The focusing-control signal generator  13  generates a focusing control signal based on the fed signal, and supplies it to the focusing-actuator drive unit  14 . The focusing-actuator drive unit  14  drives the focusing actuator  3  in accordance with the fed signal. The tracking-error signal generator  15  generates a tracking error signal based on the fed signal, and supplies it to the tracking-control signal generator  16  and aberration-correction control signal generator  10 . The tracking-control signal generator  16  generates a tracking control signal based on the fed signal, and supplies it to the tracking-actuator drive unit  17 . The tracking-actuator drive unit  17  drives the tracking actuator  4  in response to the fed signal. In addition, the aberration correcting actuator  6  moves the aberration-correcting lens  5  in the optical-axis direction. The aberration-correcting lens position detector  8  converts the aberration correcting lens position into an electric signal, and supplies it to the position signal generator  9 .  
         [0031]     The position signal generator  9  corrects its operating point and sensitivity relative to the fed signal, and supplies the corrected signal to the aberration-correction control signal generator  10 . In the aberration correction control signal generator  10 , the target-position setting unit  101  compares the fed signal with a target value, and supplies the resulting signal to the low-pass compensation filter  102 , phase compensation filter  103  and target range judging unit  104 . The temperature sensor  21  converts the drive-inside temperature into an electric signal, and supplies it to the dither signal generator  105 . The target-range judging unit  104  uses the fed signal from the target-position setting unit  101  to judge whether the aberration-correcting lens is out of a predetermined range with respect to the target position, and supplies the judgment signal to the dither signal generator  105 . The dither signal generator  105  determines the frequency and amplitude of a dither signal based on the signal fed from the temperature sensor  21 , and turns the generation of this dither signal on or off in accordance with the signal fed from the target range judging unit  104 . The aberration correction control signal generator  10  produces a sum signal of the output signals produced from the low-pass compensation filter  102 , phase compensation filter  103  and dither signal generator  105 , and supplies it to the aberration-correcting actuator drive unit  11 . The aberration-correcting actuator drive unit  11  drives the aberration correcting actuator  6  in response to the fed signal. The spindle motor  18  drives the disk  1  to rotate. The frequency generator  19  converts the rotation speed information of the spindle motor  18  into an electric signal, and supplies it to the motor control unit  20 . The motor control unit  20  controls the spindle motor  18  for rotating the disk  1  to rotate with a predetermine speed based on the fed signal.  
         [0032]     The main blocks will be described in detail.  
         [0033]     The adjustment of the operating point and sensitivity of the position signal generator  9  to the input signal will be first described with reference to  FIG. 4 . Here, the adjustment of the operating point and sensitivity means that the offset and gain of the signal to the aberration correction control signal generator  10  that makes digital processing are adjusted before the supply of this signal to the generator  10  so that the maximum resolution can be surely obtained within the practical movement range of the aberration correcting lens. In other words, the operating point is adjusted by the offset control, and the sensitivity is adjusted by the gain control. Thus, this adjustment enables the aberration to be corrected with high precision.  
         [0034]     The aberration-correcting lens is required to move within a wide range and to control with high precision, and thus it needs a high resolution. In order for both wide dynamic range and high resolution to be achieved when the control system partially makes digital processing, it is considered to employ a method for increasing the bit number or bit-precision of the AD converter. However, since the specification of DSP (digital signal processor) is necessary to change, it is not easy to achieve. Therefore, first the operating point and sensitivity are adjusted according to the method shown in  FIG. 4 .  
         [0035]     As illustrated in  FIG. 4 , the movable range of the aberration correcting lens has a practical range and an unused range. The practical range is the region lying between the target positions of the aberration-correcting lens relative to the layers of the two-layer recording/reproducing disk. The unused range is the region corresponding to the distance by which the aberration-correcting lens can be additionally moved considering the optical pickup assembly tolerance, but it is not used in the actual operation. Although the location of the practical range in its movable range depends upon each optical disk drive, it suffices to detect the position signal within the practical range if it can be detected after the target position of the aberration-correcting lens is determined relative to the 0-layer or 1-layer of the disk on each drive. ( FIG. 4  shows the case in which the practical range lies substantially at the center of the movable range.)  
         [0036]     Accordingly, the operating point of the position signal generator  9  can be adjusted to lie at the center of the signal level by applying an offset to the input signal as indicated by (1). In addition, the practical range can be adjusted to enter in the whole dynamic range of DSP by controlling the gain as indicated by (2), thus assuring the resolution. When the target position of the aberration correcting lens is determined relative to the 0-layer or 1-layer, adjustment is performed so that the position signal within the movable range can be detected as shown in  FIG. 5 . In other words, the position signal generator  9  determines the target position under the conditions that an offset is applied to the input signal as indicated by (3) and that the gain is set as indicated by (4).  
         [0037]     Description will be made of the operation of the target position setting unit  101  at the focusing jump time when the beam spot moves between the layers of the two-layer disk. When the beam spot moves between the layers, it is necessary to also change the target position of the aberration-correcting lens. When the aberration-correcting lens moves slower than the focusing control in which the objective lens is moved in the focusing direction, the target position of the aberration-correcting lens is required to previously change to the destination layer. However, when the aberration-correcting lens is moved from the original-layer target position, the amplitudes of the focusing and tracking error signals are reduced, thus making the focusing and tracking control unstable. When the target position is abruptly changed in a single step as shown in  FIG. 6A , the operation of the aberration-correcting lens gives rise to an overshoot, incurring further instability. Thus, the target-position setting unit  101  changes from the original position to the destination target position in steps as shown in  FIG. 6B  to reduce the overshoot. Alternatively, the low-pass compensation filter  102  and phase compensation filter  103  may be changed in their characteristics to achieve the same effect. In addition, the tracking control in which the error signal amplitude could be remarkably reduced may be disabled before the aberration-correcting lens switches the target positions.  
         [0038]     The operation of the target-range judging unit  104  will be described in detail with reference to  FIG. 7 .  
         [0039]     The aberration-correcting lens position target range of the target-range judging unit  104  is set according to the suppression specification of the deviation and variation necessary for each layer as shown in  FIG. 7 .  FIG. 7  is graphs schematically showing the lens position for the movement of the beam spot between the layers, the on/off of the dither signal (high-frequency signal), and the aberration-correction control signal. Before the change of the target position, the target-range judging unit  104  turns on the output of the dither signal generator  105 . Then, the target-position setting unit  101  sets the target on the destination layer.  
         [0040]     The purpose of the application of the dither signal to the drive signal for driving the spherical aberration correcting element is to enable it to be smoothly driven. In other words, the linear actuator for use in driving the beam expander for correcting the spherical aberration enables the optical pickup to be small-sized as compared to the current stepping motor, and it has a merit of lower cost than the piezoactuator. However, the friction to the drive shaft increases for the necessity of looseness reduction and high retainability. Therefore, if the object to be driven is tried to control without application of dither signal, it suddenly moves, thus accurate control being difficult. In this case, if the dither signal is applied to the drive signal for the spherical aberration correcting element, or for the linear actuator, the actuator is continuously controlled to perate finely as indicated at the bottom graph in  FIG. 7 , thus less affected by the static friction so that it can be smoothly driven.  
         [0041]     The target range judging unit  104  maintains the dither signal generator  105  to operate until the output of the target position setting unit  101  enters in the target range. After the output of the target position setting unit  101  moves into the target range, the judging unit  104  controls the dither signal generator  105  to be made in the off-state. When the position of the actuator is being changed through the stepwise ranges toward the target range as described above, the dither signal generator  105  is kept in operation to output fine signals or to be ceased even if the output of the target position setting unit  101  comes into each of the stepwise ranges.  
         [0042]     In other words, the dither signal is applied only when the linear actuator is being driven. The dither signal is not impressed when it is not driven, or during the recording or reproduction. If the dither signal were always applied, the spherical aberration correcting element would continue to finely vibrate even after the arrival at the target position, thus adversely affecting the focusing control.  
         [0043]     The phrase “linear actuator is being driven” given above means that, when the target position is controlled to change, the high-frequency signal is continuously applied to the element to adjust its position until the element arrives in the target range.  
         [0044]     In addition, the driving of the linear actuator as described above is performed before the reproduction processing of the read-out signal from the optical disk, or before the demodulation of the read-out signal from the optical disk and production of video signal or audio signal.  
         [0045]     Moreover, the dither signal generator  105  generates such a signal as to start changing with a zero-amplitude phase and to stop at another zero-amplitude phase as shown in  FIG. 8 . Alternatively, it generates such a signal as to gradually increase the amplitude when starting to produce the signal and to gradually decrease the amplitude when stopping from producing the signal.  
         [0046]     The reason why the dither signal is started to apply at the zero-amplitude phase or stopped from applying at the zero-amplitude phase is that the sudden application or stop of the dither signal might adversely affect the control even if the dither signal is a very small oscillation as compared with the drive signal. In this connection, the start or stop of application of the dither signal at the zero-amplitude phase will result in smooth control, thus better results being acquired. The gradual increase or decrease of the amplitude of the dither signal at the start or stop of application will also result in smooth control.  
         [0047]     Description will be made of a method for determining the frequency and amplitude of the signal produced from the dither signal generator  105 . The signal from the dither signal generator  105  needs a predetermined frequency or below and a predetermined amplitude or above in order that the movable portion of the aberration correcting mechanism including the aberration correcting lens can be operated without influence of the static friction to the stationary part. In addition, in order to suppress the effect of the movement of the aberration correcting lens on the focusing control and tracking control, the frequency and amplitude of the signal must be increased above and decreased below predetermined values, respectively. The influence on the focusing control and tracking control is determined according to the amplitude variation of the focusing error signal and tracking error signal before the application of the dither signal. Alternatively, it is determined on the basis of the performance fluctuation of the reproduction of the data recorded on the disk. Thus, the dither signal generator  105  determines the amplitude and frequency for each temperature that meet these conditions, and generates the most appropriate dither signal based on the output from the temperature sensor  21 . In other words, the amplitude of the signal must be set so high as to reduce the effect of the static friction and so low as not to adversely affect the focusing control and tracking control. Similarly, the frequency needs to be determined so high as not to adversely affect the focusing control and tracking control and so low as to reduce the effect of the static friction.  
         [0048]     Here, the frequency will be specifically mentioned. The frequency f of the dither signal takes the following range. In other words, if the main resonance of the aberration correction driving actuator is represented by f0_s, the control bandwidth of the aberration correction driving actuator by fc_s, the control band of the focusing actuator by fc_f, and the control band of the tracking actuator by fc_t, then the following expressions can be obtained. 
 
f0_s&lt;f&lt;fc_s  (1) 
        or when fc_s&lt;fc_f, fc_t, 
 
fc_s&lt;f&lt;fc_f or fc_t (any smaller one)  (2)
 
fc_f or fc_t (any larger one)&lt;f  (3) 
    or when fc_f, fc_t&lt;fc_s, 
 
fc_f or fc_t (any larger one)&lt;f&lt;fc_s  (4) 
 
 In the present circumstances, the condition of fc_s=0.5 kHz&lt;fc_f, fc_t=5˜10 kHz is estimated. 
       
 
       Embodiment 2  
       [0051]     The construction of the optical disk drive of the invention will be described with reference to  FIGS. 1 and 3 .  
         [0052]     In the embodiment 2, the blocks  1  through  21  shown in  FIG. 1  are the same as in embodiment 1, and thus will not be described.  FIG. 3  is a block diagram of the aberration correction control signal generator  10  of the embodiment 2. The blocks  101  through  104  shown in  FIG. 3  are the same as in embodiment 1, and thus will not be described. In  FIG. 3 , there are shown a timer  106 , a gain control unit  107  for the aberration correction control, and a gain amplifier  108  of the aberration correction control loop.  
         [0053]     The outline of the operation of each block and the relation between the blocks will be described.  
         [0054]     The focusing control, tracking control and spindle control are the same as in embodiment 1, and thus will not be described. The aberration correcting actuator  6  moves the aberration correcting lens  5  in the optical-axis direction. The aberration-correcting lens position detector  8  converts the position of the aberration correcting lens into an electric signal, and supplies it to the position signal generator  9 . The position signal generator  9  corrects the operating point and sensitivity given for the fed signal, and supplies the corrected signal to the aberration correction control signal generator  10 . In the aberration correction control signal generator  10 , the target position setting unit  101  compares the fed signal and the target value, and supplies the compared result to the low-pass compensation filter  102 , phase compensation filter  103  and target range judging unit  104 . The temperature sensor  21  converts the drive-inside temperature into an electric signal, and supplies it to the gain control unit  107 . The timer  106  supplies time information to the gain control unit  107 . The target range judging unit  104  judges whether the aberration correcting lens is located out of a predetermined range of the target position on the basis of the fed signal, and supplies the judgment result signal to the gain control unit  107 . The gain control unit  107  determines a set value of gain on the basis of the signals from the temperature sensor  21  and target range judging unit  104 , and sets the gain of the gain amplifier  108  according to the set value. The aberration correction control signal generator  10  supplies the sum signal of the low-pass compensation filter  102  and phase compensation filter  103  to the aberration correcting actuator drive unit  11  through the gain amplifier  108 . The aberration correcting actuator drive unit  11  drives the aberration correcting actuator  6  according to the fed signal.  
         [0055]     The operation of the main blocks will be described in detail.  
         [0056]     The adjustment of the operating point and sensitivity of the position signal generator  9  is the same as in embodiment 1. The operation of the target position setting unit  101  at the time of focusing jump is the same as in embodiment 1.  
         [0057]     The operation of the target range judging unit  104  and gain control unit  107  will be described in detail with reference to  FIGS. 9 and 10 . The target range of the target range judging unit  104  about the position of the aberration correcting lens is set according to the deviation and variation suppression specification necessary for each layer as shown in  FIG. 9 . The target range judging unit  104  judges whether the signal fed from the target position setting unit  101  is within the target range, and supplies the judgment result signal to the gain control unit  107 . The gain control unit  107  judges the signal fed from the temperature sensor  21  by using thresholds T 1  and T 2  (T 1 &lt;T 2 ), and sets G 01 , G 02 , G 03 , G 11 , G 12  and G 13  in the gain amplifier  108  according to the judgment result signal fed from the target range judging unit  104 . The gains have the relations of G 01 &lt;G 11 , G 02 &lt;G 12 , G 03 &lt;G 13 . In addition, when the static friction between the movable portion and fixed portion of the aberration correcting mechanism decreases with the increase of temperature, the relations of gains are G 01 &lt;G 02 &lt;G 03 , and G 11 &lt;G 12 &lt;G 13 . When the static friction increases with the increase of temperature, the relations of gains are G 01 &gt;G 02 &gt;G 03 , G 11 &gt;G 12 &gt;G 13 .  
         [0058]     Here, the target range judging unit  104  may have one or more target ranges except the target range based on the suppression specification as shown in  FIG. 11 . In this case, the gains G 21 , G 22 , G 23 , G 31 , G 32  and G 33  are added on the table of  FIG. 10  as established gains. The gain control unit  107  has the table shown in  FIG. 11 . In this case, the gains respectively take the lowest values in the target range 0 that means that the lens has arrived at the target position, and take higher values in the other ranges as the lens approaches to the target range, that is, G 01 &lt;G 31 &lt;G 21 &lt;G 11 , G 02 &lt;G 32 &lt;G 22 &lt;G 12 , G 03 &lt;G 33 &lt;G 23 &lt;G 13 .  
         [0059]     In addition, the gain control unit  107  increases the gains to be set in the gain amplifier  108  to exceed the values shown in the above table according to the time information fed from the timer  106  when the signal fed from the target position setting unit  101  does not come into each target range in a predetermined time.  
       Embodiment 3  
       [0060]     The flowchart of a specific control in embodiment 1 will be described with reference to  FIGS. 13 and 14 .  
         [0061]     First, referring to  FIG. 13 , when the aberration correcting element needs to be driven with the focusing control on and with the tracking control off, condition setting is first performed in order to suppress the effect of the superposition of high-frequency signal and the drive signal for the aberration correcting element on the focusing control. At this time, the condition setting is made so that the focusing error signal can be observed when the focusing control and tracking control are both turned off. In addition, the high-frequency signal is not applied.  
         [0062]     Then, the focusing error signal amplitude is acquired under the condition that the high-frequency signal is not applied (S 11 ). This value is represented by Fe 0 .  
         [0063]     Next, the dither signal (high-frequency signal) is added to the drive signal for the aberration correcting element (S 12 ). At this time, the initial amplitude (Scd 0 ) of the high-frequency signal is assumed to be small enough such as zero.  
         [0064]     Then, the output amplitude of the position sensor is measured, and the amplitude of the high-frequency signal is increased ΔScd by ΔScd (S 15 ) until the measured amplitude (Se 1 ) becomes larger than a predetermined value (Seth) (S 14 ). The amplitude of the high-frequency signal satisfying the condition of Se 1 &gt;Seth is represented by Scd 1 .  
         [0065]     Then, the amplitude of the focusing error signal is measured (S 16 ), and the amplitude of the high-frequency signal is increased ΔScd by ΔScd (S 18 ) until the absolute value of the difference between the measured amplitude (Fe 1 ) and the previously given amplitude Fe 0  becomes larger than a predetermined value (Feth) (S 17 ). The amplitude of the high-frequency signal satisfying the condition of |Fe 1 −Fe 0 |&gt;Feth is represented by Scd 2 .  
         [0066]     The actually used high-frequency signal amplitude Scd is set to satisfy the condition of Scd 1 &lt;Scd&lt;Scd 2  by using the obtained values Scd 1  and Scd 2 . For example, the amplitude Scd may take an intermediate value between Scd 1  and Scd 2 , or as Scd=(Scd 1 +Scd 2 )/2.  
         [0067]     Referring to  FIG. 14 , when the aberration correcting element needs to be driven with the focusing control on and the tracking control on, it is necessary to suppress the effect of the superposition of the high-frequency signal and the drive signal for the aberration correcting element on the focusing control and tracking control. In this case, the processing shown in  FIG. 14  is necessary in addition to that shown in  FIG. 13 .  
         [0068]     The condition setting in the flowchart of  FIG. 14  is performed so that the tracking error signal can be observed with the focusing control on and tracking control off. In addition, the high-frequency signal is not applied.  
         [0069]     Then, the tracking error signal amplitude is acquired with the high-frequency signal not applied (S 101 ). This value is represented by Te 0 .  
         [0070]     Next, the high-frequency signal is added to the drive signal for the aberration correcting element (S 102 ). At this time, the initial amplitude (Scd 1 ) of the high-frequency signal is assumed to be the value detected in the flowchart shown in  FIG. 13 .  
         [0071]     Then, the tracking error signal amplitude is measured (S 103 ), and the amplitude of the high-frequency signal is increased ΔScd by ΔScd (S 105 ) until the absolute value of the difference between the measured amplitude (Te 1 ) and the above given Te 0  becomes larger than a predetermined value (Teth) (S 104 ). The amplitude of the high-frequency signal satisfying the condition of |Te 1 −Te 0 |&gt;Teth is represented by Scd 3 .  
         [0072]     The actual used high-frequency signal amplitude Scd is set to satisfy the condition of Scd 1 &lt;Scd&lt;Scd 2  or Scd 3  (any smaller one) by using the above Scd 1 , Scd 2  and Scd 3 . For example, the amplitude may take an intermediate value between Scd 1  and Scd 2  or Scd 3 , or as Scd=(Scd 1 +Scd 2  or Scd 3 )/2 in order to assure the margin to the environmental change such as temperature change.  
         [0073]     As described above, according to the above embodiments, when the aberration correcting lens is moved, the dither signal is superimposed on the control signal or the gain of the control loop is increased, thereby enabling the aberration correcting lens to be controlled with high precision. Therefore, the linear actuator can be used with less lens tilt and less looseness in the aberration correcting mechanism, and thus a small-sized and inexpensive optical disk drive can be provided.  
         [0074]     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.