Patent Publication Number: US-2015085631-A1

Title: Tracking control method, tracking control device, and optical disc device

Description:
TECHNICAL FIELD 
     The present invention relates to an optical disc device, and particularly relates to a tracking control method and a tracking control device. 
     BACKGROUND ART 
     Recently, optical disc devices compatible with optical discs such as a BD (Blu-ray Disc), a DVD (Digital Versatile Disc), and a CD (Compact Disc) have been broadly used. In the optical disc device, a surface deflection (i.e., a vibration in a focusing direction) and an eccentricity (i.e., a vibration in a tracking direction) may occur during a rotation of the optical disc. The surface deflection and eccentricity are in the form of sine waves having a period corresponding to a time required for one rotation of the optical disc, and having amplitudes respectively corresponding to a surface deflection amount and an amount of eccentricity. A tracking control is performed so as to make an object lens follow the eccentricity of the optical disc as follows. 
     Generally, a tracking pull-in (i.e., a transfer to the tracking control) is performed after a speed at which the object lens crosses a track (i.e., a track crossing speed) becomes smaller than a predetermined value. A limit value of the track crossing speed varies depending on a configuration of the optical disc device and a kind of the optical disc (i.e., BD/DVD/CD or read-only/write-once/rewritable type), but is usually close to zero. Therefore, the tracking pull-in is generally performed after the track crossing speed becomes almost zero. 
     For example, in a technology disclosed in Patent Document 1, a rotation angle at which a displacement amount of the optical disc due to eccentricity becomes the maximum is learned, and the amount of eccentricity of the optical disc is detected. The tracking pull-in is performed at the rotation angle at which the displacement amount of the optical disc due to eccentricity becomes the maximum, and the object lens is moved by an amount corresponding to the detected amount of eccentricity. 
     In a technology disclosed in Patent Document 2, a position at which the eccentricity becomes the minimum is detected during a rotation of the optical disc. The tracking pull-in is performed at the position at which the eccentricity becomes the minimum. 
     In this regard, the tracking pull-in need be performed in a state where an influence of vibration of the object lens is eliminated. Therefore, before the tracking pull-in is performed, it is desirable to detect a lens error signal corresponding to displacement of the object lens, and to perform a control for maintaining the object lens at a predetermined neutral position (hereinafter referred to as a lens middle point control) by feeding back the lens error signal. 
     For example, in a technology disclosed in Patent Document 3, the maximum or minimum value of the lens displacement is detected based on the lens error signal for each rotation phase of the optical disc, and is stored. After the lens middle point control is performed, the tracking pull-in is performed at the rotation angle at which the displacement of the optical disc due to eccentricity becomes the maximum. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         [Patent Document 1] Japanese Laid-Open Patent Publication 2008-299963 (see, Abstract) 
         [Patent Document 2] Japanese Laid-Open Patent Publication 2004-062992 (see, Abstract) 
         [Patent Document 3] Japanese Laid-Open Patent Publication 2008-269662 (see, Abstract) 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, the optical disc may not only have a recording region, but may also have a non-recording region (a region where information is not recorded). The recording region and the non-recording region are different from each other in light reflection condition, and therefore the lens error signal becomes discontinuous at a boundary between both regions. Therefore, if the lens middle point control is performed straddling the recording region and the non-recording region of the optical disc (i.e., if the object lens, passes through the boundary between the recording region and the non-recording region), the object lens is overcontrolled at a point where the lens error signal becomes discontinuous. Therefore, the vibration of the object lens increases, and there is a possibility that the tracking pull-in may not be stably performed. In this case, the tracking pull-in may not only take time, but may also be unsuccessful. Therefore, recording and reproduction of information may not be possible. 
     For example, the technology disclosed in Patent Document 3 is based on a premise that a rotation phase of the optical disc where the displacement of the optical disc due to eccentricity becomes the maximum or minimum is the same as a rotation phase of the optical disc where the track crossing speed is almost zero. In order to establish the premise, it is necessary to suppress the vibration of the optical disc. In other words, it is necessary to perform the lens middle point control before the tracking pull-in. 
     However, if the lens middle point control is performed straddling the recording region and the non-recording region of the optical disc, the object lens is overcontrolled as described above, and the vibration of the object lens increases. As a result, the above-described premise may not be established, and the tracking pull-in may be performed at the rotation phase where the track crossing speed is not zero. Therefore, the tracking pull-in may be unsuccessful. 
     The present invention is intended to solve the above described problems, and an object of the present invention is to enable performing a tracking pull-in more stably. 
     Means for Solving the Problems 
     A tracking control method according to the present invention is a tracking control method for an optical pickup including an object lens for focusing laser light on an information recording surface of an optical disc, and a light receiving element that receives reflected light from the information recording surface of the optical disc and converts the light to an electric signal. The tracking control method includes a lens middle point control step of controlling a position of the object lens so as to suppress vibration of the object lens based on a signal obtained by feedback of a lens error signal generated from the electric signal, a tracking pull-in step of performing tracking pull-in processing to control a position of the object lens to follow a track of the optical disc, and a step of lowering a loop gain of the feedback of a lens middle point control in the lens middle point control step before the tracking pull-in step is started. 
     A tracking control device according to the present invention is a tracking control device performing a tracking control for an optical pickup including an object lens for focusing laser light on an information recording surface of an optical disc, and a light receiving element that receives reflected light from the information recording surface of the optical disc and converts the light to an electric signal. The tracking control device includes a lens middle point control unit that controls a position of the object lens so as to suppress vibration of the object lens based on a signal obtained by feedback of a lens error signal generated from the electric signal, a tracking pull-in unit that performs tracking pull-in processing to control a position of the object lens to follow a track of the optical disc, and a loop gain control unit that lowers a loop gain of the feedback of a lens middle point control by the lens middle point control unit before the tracking pull-in unit starts the tracking pull-in processing. 
     The optical disc device according to the present invention includes the above described tracking control device. 
     Effect of the Invention 
     According to the present invention, the loop gain of the feedback of the lens middle point control is lowered before the tracking pull-in is started, and therefore a stable tracking pull-in is achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a basic configuration of an optical disc device including a tracking control device according to Embodiment 1 of the present invention. 
         FIG. 2  is a diagram showing respective signal waveforms when a lens middle point control and a tracking pull-in are performed using technology disclosed in Patent Document 3. 
         FIG. 3  is a diagram showing respective signal waveforms when a lens middle point control and a tracking pull-in are performed according to Embodiment 1 of the present invention. 
         FIG. 4  is a flowchart showing a tracking control method performed by the tracking control device according to Embodiment 1 of the present invention. 
         FIG. 5  is a block diagram showing a basic configuration of an optical disc device including a tracking control device according to Embodiment 2 of the present invention. 
         FIG. 6  is a flowchart showing a tracking control method by the tracking control device according to Embodiment 2 of the present invention. 
     
    
    
     EMBODIMENT FOR CARRYING OUT THE INVENTION 
     Embodiment 1 
       FIG. 1  is a block diagram showing a basic configuration of an optical disc device  10  including a tracking control device according to Embodiment 1 of the present invention (i.e., a device capable of performing a tracking control method according to Embodiment 1 of the present invention). The optical disc device  10  is a device that performs recording and/or reproducing using an optical disc  40 . The optical disc  40  is, for example, a BD, DVD or CD. The BD, DVD and CD may respectively include discs classified into a read-only type, a write-once type and a rewritable type. 
     As shown in  FIG. 1 , the optical disc device  10  includes an optical pickup  11 , a spindle motor  12 , a laser control unit  13 , a spindle control unit  14 , a tracking error signal generation unit  15 , a lens error signal generation unit  16 , an object lens drive control unit  17 , and a central control unit  30 . 
     The central control unit  30  performs at least a tracking control in the optical disc device  10 . The central control unit  30  may be constituted by, for example, a computer having a CPU (Central Processing Unit). Further, the central control unit  30  includes a storage unit  31  such as a memory that stores various kinds of data and programs required for the tracking control. In this regard, the central control unit  30  may be configured to entirely control the optical disc device  10 . 
     The optical pickup  11  includes main optical components for recording information on or reproducing information from the optical disc  40 . To be more specific, the optical pickup  11  includes an object lens  11   a  and a light receiving element  11   b.    
     The object lens  11   a  focuses laser emitted by a laser emission unit of the laser control unit  13  described later on an information recording surface of the optical disc  40 . The light receiving element  11   b  receives reflected light from the information recording surface of the optical disc  40  irradiated with the laser light. The light receiving element  11   b  converts a received light signal into an electric signal, and outputs the electric signal. 
     The optical pickup  11  further includes an actuator  11   c  that drives the object lens  11   a  in a radial direction of the optical disc (i.e., a tracking direction), and in a direction perpendicular to the information recording surface of the optical disc (i.e., a focusing direction). The actuator  11   c  includes, for example, electromagnetic coils fixed to a lens holder that holds the object lens  11   a , magnets provided so as to face the electromagnetic coils, and the like. 
     The spindle motor  12  is controlled by the spindle control unit  14 , and rotates the optical disc  40 . Further, the spindle motor  12  outputs a rotation angle of the optical disc  40  to the central control unit  30 . 
     The spindle control unit (i.e., a rotation control unit)  14  controls a rotation of the spindle motor  12 . Information on the number of rotations and a rotation method according to a kind of the optical disc stored in the storage unit  31  of the central control unit  30  is inputted into the spindle control unit  14 . The spindle control unit  14  controls the rotation of the spindle motor  12  based on the inputted information so as to rotate the optical disc  40 . 
     In this regard, the number of rotations and the rotation method of the spindle motor  12  differ depending on the kind of optical disc  40 . For example, the rotation method is mainly divided into CAV (Constant Angular Velocity) in which an angular velocity is constant, and CLV (Constant Linear Velocity) in which a linear velocity is constant. 
     The laser control unit  13  has a laser emission unit that emits laser light to irradiate the information recording surface of the optical disc  40  via the object lens  11   a . A laser power value according to the kind of the optical disc stored in the storage unit  31  of the central control unit  30  is inputted into the laser control unit  13 . The laser control unit  13  emits laser light based on the inputted laser power value. 
     The tracking error signal generation unit  15  generates a tracking error signal based on the electric signal converted by the light receiving element  11   b  of the optical pickup  11 . The “tracking error signal” is a signal detected when the object lens  11   a  of the optical pickup  11  crosses a track of the optical disc  40 . As a method for generating the tracking error signal by the tracking error signal generation unit  15 , conventional methods may be used. The conventional methods are, for example, a push-pull method, a DPP (Differential Push-Pull) method, and a DPD (Differential Phase Detection) method. 
     The lens error signal generation unit  16  generates a lens error signal corresponding to a position of the object lens  11   a  of the optical pickup  11  based on the electric signal converted by the light receiving element  11   b  of the optical pickup  11 . For example, as a method for generating the lens error signal, a method disclosed in Patent Document 3 (Japanese Laid-Open Patent Publication No. 2008-269662) may be used. However, the method for generating the lens error signal is not limited thereto. 
     The object lens drive control unit  17  controls the position of the object lens  11   a  of the optical pickup  11 . To be more specific, the object lens drive control unit  17  controls driving of the object lens actuator  11   c  of the optical pickup  11  to move the object lens  11   a  in the tracking direction (i.e., the radial direction of the optical disc  40 ) so that the object lens  11   a  follows the track of the optical disc  40 . The object lens drive control unit  17  also performs a control to move the object lens  11   a  in the focusing direction so that the object lens  11   a  follows a surface deflection of the information recording surface of the optical disc  40 . 
     The storage unit  31  of the central control unit  30  stores the laser power values of the optical pickup  11 , the numbers of rotations and the rotation methods of the spindle motor  12  for respective kinds of optical discs  40 . Based on the kind of the optical disc to be used, the central control unit  30  selects the laser power value of the optical pickup  11 , the number of rotations and the rotation method of the spindle motor  12  from the storage unit  31 , and outputs the selected values to the laser control unit  13  and the spindle control unit  14 . 
     The central control unit  30  further includes a tracking pull-in unit  32 , a rotation angle reading unit  34 , a lens middle point control unit  35 , and a loop gain control unit  36 . These components may be realized by, for example, hardware such as electronic circuits, or software such as programs mounted to the computer. 
     The tracking pull-in unit  32  outputs a tracking control signal to the object lens drive control unit  17 . The “tracking control signal” is a feedback signal generated based on the tracking error signal generated by the tracking error signal generation unit  15 . The object lens drive control unit  17  performs a control to move the object lens  11   a  in the tracking direction based on the inputted tracking control signal so that the object lens  11   a  follows eccentricity of the optical disc  40 . 
     The rotation angle reading unit  34  continuously reads a rotation angle of the optical disc  40  based on information on the rotation angle of the optical disc  40  outputted from the spindle motor  12 . 
     The lens middle point control unit  35  adds a feedback signal, which is generated based on the lens error signal generated by the lens error signal generation unit  16 , to the tracking control signal. The lens middle point control unit  35  outputs a signal obtained by the addition to the object lens drive control unit  17 . The object lens drive control unit  17  drives the object lens  11   a  in the tracking direction based on the inputted signal so as to suppress the vibration of the object lens  11   a . This lens middle point control is performed prior to the tracking pull-in. 
     The loop gain control unit  36  performs processing to lower (i.e., decrease) a loop gain during the lens middle point control. The tracking pull-in is preferably performed in a state where the vibration of the object lens  11   a  is suppressed by the lens middle point control. A vibration suppression ability for the object lens  11   a  is determined by the loop gain of feedback of the lens middle point control. That is, it becomes easy to suppress the vibration of the object lens  11   a , as the loop gain increases (i.e., as a responsiveness becomes higher). In contrast, a lens error becomes discontinuous at a boundary between a recording region and a non-recording region of the optical disc as described later, and therefore oscillation of feedback loop is of particular concern. Therefore, in this embodiment, the loop gain is lowered (that is, the responsiveness is lowered) during the lens middle point control to thereby certainly eliminate the influence of vibration of the object lens  11   a , and the tracking pull-in is performed in this state. 
     In this regard, the central control unit  30  shown in  FIG. 1  corresponds to a tracking control device in this embodiment. However, the tracking control device is not limited to the configuration shown in  FIG. 1 . 
       FIG. 2  shows respective signal waveforms when the lens middle point control and the tracking pull-in are performed using technology disclosed in Patent Document 3.  FIG. 2  shows waveforms of predetermined periods before and after a start (ON) of the lens middle point control.  FIG. 2  does not belong to this embodiment, but will be described using the same marks as the components of this embodiment for convenience. 
       FIG. 2(A)  shows a tracking error signal waveform.  FIG. 2(B)  shows a lens error signal waveform.  FIG. 2(C)  shows a top envelope waveform of a reproduction signal (hereinafter, referred to as TOPENV).  FIG. 2(D)  shows a tracking control signal waveform. 
     The tracking error signal waveform shown in  FIG. 2(A)  is generated by the tracking error signal generation unit  15  using the push-pull method, the DPP method or the like. 
     The lens error signal waveform shown in  FIG. 2(B)  is generated by the lens error signal generation unit  16 . An undulation of the lens error signal waveform prior to the lens middle point control represents a vibrational component of the object lens  11   a.    
     The TOPENV waveform shown in  FIG. 2(C)  is a top envelope waveform of the reproduction signal generated based on the electric signal outputted from the light receiving element  11   b  of the optical pickup  11 . A level of the TOPENV waveform changes between the recording region and the non-recording region of the optical disc. 
     The tracking control signal waveform shown in  FIG. 2(D)  is a waveform of a drive control signal outputted by the object lens drive control unit  17  to the object lens  11   a  of the optical pickup  11 . During the tracking pull-in control, the tracking control signal waveform is generated by feedback of the tracking error signal waveform of  FIG. 2(A) . During the lens middle point control, the tracking control signal waveform is generated by feedback of the lens error signal waveform of  FIG. 2(B) . 
     As shown in  FIG. 2(B) , at the boundary between the recording region and the non-recording region of the optical disc  40 , the lens error signal becomes discontinuous (as shown by mark E in  FIG. 2(B) ). If the lens middle point control is performed based on such discontinuous lens error signal, an overcontrol of tracking occurs as shown by a circle in  FIG. 2 , and the lens error signal waveform is disturbed. This means that vibration of the object lens  11   a  increases when the lens middle point control is performed straddling the recording region and the non-recording region. For this reason, the tracking error signal waveform ( FIG. 2(A) ) becomes irregular. 
     In a state where the tracking is OFF, the tracking error signal waveform is a regular waveform in which sparse portions (i.e., portions with a long waveform period) and dense portions (i.e., portions with a short waveform period) are repeated in a direction of time. However, the tracking error signal waveform shown in  FIG. 2(A)  contains a lot of sparse portions. In the technology disclosed in Patent Document 3, the tracking pull-in is performed when displacement of the optical disc  40  due to eccentricity becomes the maximum. However, if the lens middle point control is performed straddling the recording region and the non-recording region of the optical disc  40  as shown in  FIG. 2 , the tracking pull-in may not be performed at a desired timing, and there is a possibility that the tracking pull-in may fail. 
       FIG. 3  shows respective signal waveforms when the lens middle point control and the tracking pull-in according to this embodiment are performed.  FIG. 3(A)  shows the tracking error signal waveform.  FIG. 3(B)  shows the lens error signal waveform.  FIG. 3(C)  shows the TOPENV waveform of the reproduction signal.  FIG. 3(D)  shows the tracking control signal waveform. In this regard, in  FIG. 3 , the lens middle point control has already been started at time  0  (i.e., left end). 
     As shown in  FIG. 3 , in this embodiment, the loop gain of the feedback of the lens middle point control is lowered during the lens middle point control (i.e., before performing the tracking pull-in). When the loop gain is lowered (i.e., when the responsiveness is lowered), the overcontrol of tracking becomes less likely to occur even if the lens middle point control is performed straddling the recording region and the non-recording region of the optical disc  40 . Therefore, regularity of the tracking error signal waveform (i.e., relationship between sparse and dense portions) can be maintained. 
     In this regard, if the loop gain of the feedback of the lens middle point control is lowered from the start, the amount of the vibration of the object lens  11   a  cannot be suppressed within a predetermined range. Therefore, it becomes difficult to eliminate the influence of the vibration of the object lens  11   a  when performing the tracking pull-in. 
     Therefore, in this embodiment, the loop gain of the feedback of the lens middle point control is lowered from halfway. With such an arrangement, it becomes possible to perform the tracking pull-in before the object lens starts vibrating, and to prevent the overcontrol of tracking to thereby maintain the regularity of the tracking error signal waveform (i.e., the relationship between sparse and dense portions). 
     Here, description will be made of a timing when the loop gain of the feedback of the lens middle point control is lowered, and a lowered value of the loop gain. 
     The timing when the loop gain of the feedback of the lens middle point control is lowered is a predetermined time prior to a timing when the tracking pull-in is started. The predetermined time can be expressed using arbitrary indexes. For example, if it is assumed that the tracking pull-in is performed when the optical disc  40  reaches a rotation angle at which displacement of the optical disc  40  due to eccentricity becomes the maximum, the loop gain may be lowered when the optical disc  40  reaches a predetermined rotation angle prior to the rotation angle at which the displacement of the optical disc  40  becomes the maximum. It is also possible to preliminarily determine a rotation angle after the loop gain of the feedback of the lens middle point control is lowered and before the object lens  11   a  starts vibrating. In this case, the loop gain is lowered when the optical disc  40  reaches the preliminarily determined rotation angle prior to the rotation angle of the optical disc  40  at which the tracking pull-in is started. 
     Moreover, the lowered value of the loop gain of the feedback of the lens middle point control need only be in a range in which the regularity of the tracking error signal (i.e., the relationship between sparse and dense portions) is maintained. For example, it is possible to determine the value of the loop gain with which the regularity of the tracking error signal is maintained, by performing experiments while varying the value of the loop gain and observing what kind of tracking signal is generated. It is also possible that the gain value is zero. 
     Next, a tracking control method according to this embodiment, i.e., a tracking control performed by the tracking control device will be described with reference to  FIG. 4 . 
       FIG. 4  is a flowchart showing an example of the tracking control performed by the tracking control device according to this embodiment. Here, this flowchart is performed by the central control unit  30  according to the program stored in the storage unit  31 . 
     When the tracking control is started, first, whether the tracking pull-in processing is to be started or not is judged (step S 1 ). If the tracking pull-in processing is to be started (YES in step S 1 ), the control proceeds to next step S 2 . If the tracking pull-in processing is not to be started (NO in step S 1 ), judgment in step S 1  is repeated. The judgment in step S 1  is performed by, for example, the central control unit  30 . The central control unit  30  stores a result of the judgment in the storage unit  31 . 
     Next, judgment whether the focusing control is ON and the tracking control is OFF is performed (step S 2 ). To be more specific, for example, if an amplitude value of the tracking error signal generated by the tracking error signal generation unit  15  exceeds a certain threshold value, it is judged that the tracking control is OFF. In contrast, if the amplitude value of the tracking error signal does not exceed the threshold value, it is judged that the tracking control is ON. The judgment in step S 2  is performed by, for example, the central control unit  30 . The central control unit  30  stores a result of the judgment in the storage unit  31 . In this regard, a judgment method and judgment means in step S 2  are not limited to the above described example. 
     In step S 2 , if it is judged that the focusing is ON and the tracking is OFF (YES in step S 2 ), the control proceeds to step S 4 . Otherwise (NO in step S 2 ), the control proceeds to step S 3 . 
     In step S 3 , the focusing control is turned ON and the tracking control is turned OFF. That is, if the focusing control is OFF in the above described step S 2 , the focusing control is turned ON. If the tracking control is ON in the above described step S 2 , the tracking control is turned OFF. Processing in step S 3  is performed by, for example, the central control unit  30 . In this case, the object lens drive control unit  17  moves the object lens  11   a  in the focusing direction based on a focusing control signal (obtained by feedback of a focusing error signal) outputted by the central control unit  30 . As to the tracking direction, the object lens drive control unit  17  moves the object lens  11   a  based on the addition signal outputted by the lens middle point control unit  35  in the lens middle point control (step S 4 ) described later. 
     In this regard, the reason why the focusing control is turned ON in step S 3  is because neither the lens error signal nor the tracking error signal is generated in a state where the laser light is not focused on the information recording surface of the optical disc  40 . Therefore, the lens middle point control and the tracking pull-in control are performed in a state where the focusing control is ON and in a state where the laser light is focused on the information recording surface of the optical disc  40 . 
     After step S 3  is completed, the control returns to step S 2  again. The judgment whether the focusing control is ON and the tracking control is OFF is performed. Processing of step S 2  and processing of step S 3  are repeated until the judgment in step S 2  becomes YES. 
     Next, the lens middle point control is started (step S 4 ). The lens middle point control is a control for suppressing the vibration of the object lens  11   a  and performed by the object lens drive control unit  17 . The loop gain of the feedback of this lens middle point control is preferably determined so that a vibration amount of the object lens  11   a  is within a predetermined range. 
     Step S 4  is performed by, for example, the lens middle point control unit  35  and the loop gain control unit  36  of the central control unit  30 . The lens middle point control unit  35  sends instruction to the object lens drive control unit  17  so as to perform the lens middle point control. That is, the lens middle point control unit  35  adds the feedback signal generated based on the lens error signal generated by the lens error signal generation unit  16  to the tracking control signal, and outputs the addition signal to the object lens drive control unit  17 . The object lens drive control unit  17  drives the object lens  11   a  in the tracking direction based on the inputted signal. 
     Next, reading of the rotation angle of the optical disc  40  is performed (step S 5 ). The reading of the rotation angle of the optical disc  40  is performed by, for example, the rotation angle reading unit  34  of the central control unit  30 . Information on the rotation angle of the optical disc  40  is outputted by the spindle motor  12 , and is inputted into the rotation angle reading unit  34 . 
     Further, after the reading of the rotation angle of the optical disc  40  is started in step S 5 , the rotation angle reading unit  34  continuously reads the rotation angle of the optical disc  40 , and stores the rotation angle in the storage unit  31  of the central control unit  30 . 
     The rotation angle of the optical disc  40  closely relates to an eccentricity phase angle of the optical disc  40 , and relationship therebetween depends on a chucking state of the optical disc  40 . The chucking state is a state where the optical disc  40  is held between a turntable mounted to the spindle motor  12  and a clamper facing the turntable. As long as the chucking state is the same, the relationship between the rotation angle and the eccentricity phase angle of the optical disc  40  is the same. Once the optical disc  40  is ejected, the relationship ends. The relationship between the rotation angle and the eccentricity phase angle of the optical disc  40  may be determined at any timing and using any method after the optical disc  40  is inserted. 
     In this regard, the relationship between the rotation angle and the eccentricity phase angle of the optical disc  40  is preliminarily determined before the tracking control of  FIG. 4  is started. This is for determining the timing to perform the tracking pull-in in step S 9  described later. The tracking pull-in is performed at the timing when the displacement of the optical disc  40  due to eccentricity becomes the maximum. In the case where an eccentricity component is approximated by a sine wave, the timing when the displacement of the optical disc  40  due to eccentricity becomes the maximum is a timing when the eccentricity phase angle is 90 degrees or 270 degrees. Therefore, the relationship between the rotation angle and the eccentricity phase angle of the optical disc  40  is preliminarily determined, and the tracking pull-in is performed when the rotation angle of the optical disc  40  becomes a rotation angle corresponding to the eccentricity phase angle (i.e., 90 degrees or 270 degrees) at which the displacement of the optical disc due to eccentricity becomes the maximum. 
     Next, judgment whether the rotation angle of the optical disc  40  read in the above described step S 5  reaches the predetermined rotation angle at which processing to lower the loop gain is to be performed (step S 6 ). Step S 6  is performed by, for example, the central control unit  30 . Here, the predetermined rotation angle is set to, for example, a predetermined rotation angle prior to the rotation angle at which the tracking pull-in (step S 9 ) is performed. 
     If the rotation angle of the optical disc  40  reaches the predetermined rotation angle (YES in step S 6 ), the control proceeds to next step S 7 . If the rotation angle of the optical disc  40  does not reach the predetermined rotation angle (No in step S 6 ), the processing of step S 6  and the processing of S 7  are repeated until the predetermined rotation angle is reached. In this regard, after the rotation angle reading unit  34  of the central control unit  30  starts reading the rotation angle of the optical disc  40  in the above described step S 5 , the rotation angle reading unit  34  continuously reads the rotation angle of the optical disc  40 , and stores the rotation angle in the storage unit  31  of the central control unit  30 . 
     If the judgment in the above described step S 6  is YES, the processing to lower the loop gain of the feedback of the lens middle point control is performed (step S 7 ). The gain value when the loop gain is lowered need only be in a range where the sparse and dense portions of the tracking error signal are regularly outputted. For example, the gain value may be determined by preliminarily observing the tracking error signal waveform, or may be set to zero. Step S 7  is performed by, for example, the loop gain control unit  36  of the central control unit  30 . The loop gain control unit  36  sends instruction to the object lens drive control unit  17  so as to lower the loop gain. Therefore, the object lens drive control unit  17  lowers the loop gain, and controls driving of the object lens  11   a.    
     Next, judgment whether the rotation angle of the optical disc  40  reaches the rotation angle at which the tracking pull-in is to be started is performed (step S 8 ). As described in relation to step S 9 , the tracking pull-in is performed when the rotation angle of the optical disc  40  becomes the rotation angle corresponding to the eccentricity phase angle (i.e., 90 degrees or 270 degrees) at which the displacement of the optical disc becomes the maximum. 
     If the rotation angle of the optical disc  40  reaches the predetermined rotation angle (YES in step S 8 ), the control proceeds to next step S 9 . If the rotation angle of the optical disc  40  does not reach the predetermined rotation angle (NO in step S 8 ), steps S 8  and S 9  are repeated until the predetermined rotation angle is reached. Step S 8  is performed by, for example, the central control unit  30 . 
     Then, the lens middle point control is terminated, and the tracking pull-in is performed (step S 9 ). That is, the object lens  11   a  is controlled to follow the eccentricity of the optical disc  40 . Step S 9  is performed by, for example, the tracking pull-in unit  32  of the central control unit  30 . That is, the tracking pull-in unit  32  outputs the tracking control signal obtained by feedback of the tracking error signal generated by the tracking error signal generation unit  15  to the object lens drive control unit  17 . The object lens drive control unit  17  moves the object lens  11   a  in the tracking direction based on the tracking control signal so that the object lens  11   a  follows the track of the optical disc  40 . In this regard, it is desirable that the tracking pull-in is performed at the same as the termination of the lens middle point control. 
     When the processing of step  9  is completed, the tracking control shown in  FIG. 4  is terminated. 
     As described above, according to the tracking control method, the tracking control device and the disc device of Embodiment 1 of the present invention, the lens middle point control is performed, and the loop gain of the lens middle point control is lowered before the tracking pull-in is performed. 
     Therefore, it becomes possible to prevent the overcontrol of tracking caused by straddling the recording region and the non-recording region. Therefore, the vibration of the object lens  11   a  can be certainly suppressed, and the stable tracking pull-in can be achieved. 
     Embodiment 2 
       FIG. 5  is a block diagram showing a basic configuration of an optical disc device  10  including a tracking control device according to Embodiment 2 of the present invention. In the tracking control device according to Embodiment 2, a recording/non-recording region judgment unit  33  for judging a recording region and a non-recording region is added to the tracking control device ( FIG. 1 ) described in Embodiment 1. Other components are the same as those of the tracking control device of Embodiment 1. 
     In Embodiment 2, signals for judging the recording region and the non-recording region of the optical disc  40  are detected. The loop gain of the feedback of the lens middle point control is lowered only when the recording region and the non-recording region are straddled (i.e., when the object lens  11   a  passes through the boundary between the recording region and the non-recording region of the optical disc  40 ) during the lens middle point control. In this case, there is an advantage that, if the recording region and the non-recording region are not straddled during the lens middle point control, the lens middle point control can be performed without lowering the loop gain. 
       FIG. 6  is a flowchart showing an example of a tracking control performed by the tracking control device according to Embodiment 2. In the flowchart of  FIG. 6 , steps S 10  through S 12  are added to the flowchart of  FIG. 4 . Steps S 1  through S 9  of  FIG. 6  are the same as steps S 1  through S 9  of  FIG. 4 , and explanations thereof will be omitted. 
     In the tracking control shown in  FIG. 6 , after the lens middle point control is started in step S 4 , detection of a signal representing the recording region or the non-recording region of the optical disc  40  is started (step S 10 ). This is for judgment whether the recording region and the non-recording region are straddled or not during the lens middle point control in subsequent step S 12 . 
     For example, the TOPENV signal may be used as the signal representing the recording region or the non-recording region of the optical disc  40 . The judgment in step S 10  is performed by, for example, the recording/non-recording region judgment unit  33  of the central control unit  30 . The recording/non-recording region judgment unit  33  continuously monitors a level value of the TOPENV signal generated by a not shown reproduction signal generation unit, and stores the level value in the storage unit  31 . 
     Next, waiting for one rotation of the optical disc  40  is performed (step S 11 ). Step S 11  is performed by, for example, monitoring the rotation angle of the spindle motor  12  by the central control unit  30 . 
     A reason for waiting for one rotation of the optical disc  40  is as follows. That is, when the tracking control is OFF, the object lens  11   a  does not follow the eccentricity of the optical disc  40 . Therefore, the object lens  11   a  relatively moves on a trajectory corresponding to the eccentricity of the optical disc  40 . This trajectory is a sine wave whose period corresponds to one rotation of the optical disc  40 . Therefore, in order to judge whether the boundary between the recording region and the non-recording region is straddled, it is only necessary to wait for one rotation of the optical disc  40 . That is, if the boundary is not straddled during at least one rotation of the optical disc  40 , it means that the boundary need not be taken into consideration when performing the tracking pull-in. 
     In following step S 6 , judgment whether the rotation angle of the optical disc  40  reaches the predetermined rotation angle is performed as described in Embodiment 1, and then judgment whether the recording region and the non-recording region are straddled during the lens middle point control is performed (step S 12 ). 
     This judgment is based on a change in a level of the above described signal (for example, the TOPENV) representing the recording region and the non-recording region during one rotation of the optical disc  40 . This judgment is performed by, for example, the recording/non-recording region judgment unit  33  of the central control unit  30 . 
     As a result of the judgment in step S 12 , if it is judged that the recording region and the non-recording region are straddled during the lens middle point control (YES in step S 12 ), the loop gain of the lens middle point control is lowered in step S 7  as in Embodiment 1, and then the control proceeds to step S 8 . 
     In contrast, as a result of the judgment in step S 12 , if it is judged that the boundary between the recording region and the non-recording region is not straddled during the lens middle point control (NO in step S 12 ), the control proceeds to step S 8  without performing the processing of step S 7 . The processing after the step S 8  is the same as that described in Embodiment 1. 
     As described above, according to the tracking control device and the tracking control method of Embodiment 2 of the present invention, the loop gain is not lowered if the recording region and the non-recording region are not straddled during the lens middle point control. Therefore, in addition to the advantage described in Embodiment 1, the vibration of the object lens  11   a  can be suppressed more certainly. 
     The tracking control device and the tracking control method in the respective embodiments may be realized only by hardware resources such as electronic circuits, or may be realized by cooperation of hardware resources and software resources. In the case of the cooperation of hardware resources and software resources, the tracking control device and the tracking control method are realized in such a manner that, for example, a computer program is executed by a computer. To be more specific, the tracking control device and the tracking control method are realized in such a manner that the computer program stored in a recording medium such as a ROM (Read Only Memory) is read out by a main memory unit, and is executed by a CPU. The computer program may be stored in the recording medium such as an optical disc which is readable by the computer, or may be provided via a network such as internet. 
     The present invention is not limited to the above described respective embodiments, and various modifications may be made without departing from the scope of the present invention. 
     EXPLANATION OF MARKS 
       10  . . . optical disc device,  11  . . . optical pickup,  11   a  object lens,  11   b  . . . light receiving unit,  11   c  . . . actuator,  12  . . . spindle motor,  13  . . . laser control unit,  14  . . . spindle control unit,  15  . . . tracking error signal generation unit, . . . lens error signal generation unit,  17  . . . object lens drive control unit,  30  . . . central control unit,  31  . . . storage unit,  32  . . . tracking pull-in unit,  33  . . . recording/non-recording region judgment unit,  34  . . . rotation angle reading unit,  35  . . . lens middle point control unit,  36  . . . loop gain control unit,  40  . . . optical disc.