Abstract:
An apparatus of the present invention is an optical disk apparatus for irradiating a track on an information carrier on which information is recorded with a light beam so as to reproduce information from the track. The optical disk apparatus includes: a tracking error detector for detecting a tracking error signal representing a displacement between the light beam and the track; a fine movement section for moving the light beam in a substantially radial direction of the information carrier; a coarse movement section for moving the fine movement section in the substantially radial direction of the information carrier; a tracking controller for controlling the fine movement section and the coarse movement section based on the tracking error signal detected by the tracking error detector so that the light beam is positioned along the track; and an eccentricity detector for detecting an amount of eccentricity of the track on the information carrier. The tracking controller controls the coarse movement section based on the amount of eccentricity detected by the eccentricity detector.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for focusing a light beam from a light source such as a semiconductor laser onto a rotating disk-shaped recording medium (hereinafter, referred to as an “optical disk”) so as to record/reproduce signals on/from the optical disk. More particularly, the present invention relates to a tracking control for positioning the light beam along a track on the optical disk. 
     2. Description of the Related Art 
     With a conventional optical disk apparatus, a signal is reproduced from an optical disk by irradiating the optical disk with a light beam of a relatively small but constant light amount so as to detect the reflected light from the optical disk whose intensity has been modulated by the optical disk. A signal is recorded on the optical disk by writing information on a recording material film of the optical disk with a light beam while modulating the intensity thereof according to the signal to be recorded (e.g., Japanese Laid-Open Publication No. 52-80802). 
     A read-only optical disk is produced by recording a plurality of information pits on the disk in a spiral pattern. A recordable/reproducible optical disk is produced by providing an optically recordable/reproducible material film through a process such as vapor deposition on a surface of a substrate which includes spiral concave/convex tracks thereon. 
     To properly record information on an optical disk or reproduce the recorded information from the optical disk requires a focusing control and a tracking control. The focusing control is for controlling the optical disk in a direction normal to the optical disk surface (hereinafter, referred to as the “focusing direction”) so that the light beam is always in a predetermined focused state at the recording material film. The tracking control is for controlling the optical disk in a radial direction of the optical disk (hereinafter, referred to as the “tracking direction”) so that the light beam is always positioned along a predetermined track. 
     A conventional tracking control for an optical disk will be described with reference to FIG. 8. A disk-shaped optical disk  1  is rotated by a disk motor  50 . An optical head  10  includes a semiconductor laser  11 , a coupling lens  12 , a polarization beam splitter  13 , a ¼ wave plate  14 , a focusing actuator  16 , a tracking actuator  17 , a detection lens  18 , a cylindrical lens  19  and a 4-divided photodetector  20 . 
     The optical head  10  can be traversed by a traverse motor  43  in the tracking direction. A light beam generated from the semiconductor laser  11  is collimated by the coupling lens  12 , passes through the polarization beam splitter  13  and the ¼ wave plate  14 , and is then focused by a focusing lens  15  on the optical disk  1 . 
     The light beam is reflected by the optical disk  1 , passes through the focusing lens  15  and the ¼ wave plate  14 , and is then reflected by the polarization beam splitter  13 . Thereafter, the reflected light passes through the detection lens  18  and the cylindrical lens  19  so as to be incident upon the 4-divided photodetector  20 . 
     The focusing lens  15  is supported by an elastic body (not shown). The focusing lens  15  is moved in the focusing direction by applying a current to the focusing actuator  16  and in the tracking direction by applying a current to the tracking actuator  17 . 
     The photodetector  20  detects a light amount signal and sends the detected light amount signal to a focusing error detector (hereinafter, referred to as the “FE generator”)  30  and to a tracking error detector (hereinafter, referred to as the “TE generator”)  40 . 
     Using the light amount signal from the photodetector  20 , the FE generator  30  calculates an error signal (hereinafter, referred to as an “FE signal”) which indicates the focused state of the light beam at the information surface of the optical disk  1 , and sends the FE signal to the focusing actuator  16  via a focusing linear filter (hereinafter, referred to as the “Fc linear filter”)  31 . The focusing actuator  16  controls the focusing lens  15  in the focusing direction so that the light beam is focused on the recording surface of the optical disk  1  in a predetermined state. Thus, the focusing control is performed. 
     Using the light amount signal from the photodetector  20 , the TE generator  40  also calculates an error signal (hereinafter, referred to as a “TE signal”) which indicates the positional relationship between the light beam and an intended track on the optical disk  1 , and sends the TE signal to the tracking actuator  17  via a tracking linear filter (hereinafter, referred to as the “Tk linear filter”)  41 . 
     The tracking actuator  17  controls the focusing lens  15  in the tracking direction so that the light beam properly follows a track. The tracks of the optical disk  1  exist over a large area of the optical disk  1 , extending from the inner periphery to the outer periphery of the optical disk  1 . Therefore, the focusing lens  15  needs to be movable over a large extent in order to irradiate the intended track with the light beam. 
     Since the motion range of the tracking actuator  17  is limited, the optical head  10  needs to be driven in the tracking direction. Therefore, a drive signal output from the Tk linear filter  41  to the tracking actuator  17  is sent to the traverse motor  43  via a traverse linear filter  42 , an average calculator  45  and a pulse generator  44  so as to move the optical head  10  in the tracking direction through the rotation of the traverse motor  43 . 
     Thus, the optical head  10  moves in the tracking direction so that the drive signal to the tracking actuator  17  approaches zero or, in other words, so that the focusing lens  15  takes a normal position with respect to the optical head  10 . By the two devices, i.e., the tracking actuator  17  and the traverse motor  43 , operating as described above, the light beam follows a track on the optical disk  1 . Thus, the tracking control is performed. 
     Typically, as compared with the tracking actuator  17 , the traverse motor  43  is only responsive to an input signal having a relatively low frequency. The traverse linear filter  42  extracts a low-band component of the signal from the Tk linear filter  41 , for which the traverse motor  43  can sufficiently follow the track, through a low-pass filter having a low-pass characteristic as illustrated in FIG.  9 . Thus, the traverse motor  43  is driven by the extracted low-band component. 
     To move the optical head  10  by the traverse motor  43  requires a driving force which overcomes the frictional force of the traverse motor  43  itself or the frictional force of a mechanism for traversing the optical head  10 . 
     Moreover, when the optical disk  1  has some eccentricity, the drive signal from the Tk linear filter  41  includes eccentricity components so that the light beam can properly follow the track. When the optical disk  1  rotates at a high speed, it is difficult for the traverse motor  43  to follow the eccentricity components of the drive signal. Therefore, it is necessary to drive the traverse motor  43  to eliminate an influence of the eccentricity. 
     In an optical disk apparatus  800  illustrated in FIG. 8, a signal from the traverse linear filter  42  is sent to the traverse motor  43  via the average calculator  45  and the pulse generator  44 . 
     An operation of the optical disk apparatus  800  will be described with reference to FIGS. 10A to  10 D. FIG. 10A illustrates a signal from the disk motor  50 , FIG. 10B illustrates a signal from the traverse linear filter  42 , FIG. 10C illustrates a signal from the average calculator  45 , an FIG. 10D illustrates a signal from the pulse generator  44 . 
     Referring to FIG. 10A, the disk motor  50  outputs one cycle of a square wave signal (hereinafter, referred to as the “FG signal”) for each revolution thereof. Referring to FIG. 10C, the average calculator  45  calculates the average value of the signal output from the traverse linear filter  42  during a time period from the rising edge t 1  to the rising edge t 2  of the FG signal, and outputs the average value for the next time period from the rising edge t 2  to the rising edge t 3 . 
     Since the average value is calculated for each revolution of the optical disk  1 , the signal from the average calculator  45  is not influenced by the eccentricity of the optical disk  1 . Referring to FIG. 10D, the pulse generator  44  outputs a pulse signal having a wave height and a pulse width which are respectively predetermined to be sufficient for driving the traverse motor  43 , when the signal from the average calculator  45  exceeds a predetermined level SL (see FIG. 10C) (e.g., Japanese Laid-Open Publication No. 7-98877). 
     In the above-described conventional tracking control, the pulse width and the wave height of the drive signal to the traverse motor  43  are both fixed values. Therefore, as the frictional force of the traverse motor  43  itself or the frictional force of the mechanism for traversing the optical head  10  increase over time, the driving force of the traverse motor  43  may not overcome such frictional forces. 
     Moreover, since the average value is calculated for each revolution of the disk motor  50 , the response speed of the traverse motor  43  is determined by the number of revolutions of the disk motor  50 . Thus, the response speed of the traverse motor  43  decreases as the number of revolutions of the disk motor  50  decreases. 
     SUMMARY OF THE INVENTION 
     According to one aspect of this invention, there is provided an optical disk apparatus for irradiating a track on an information carrier on which information is recorded with a light beam so as to reproduce information from the track. The optical disk apparatus includes: a tracking error detector for detecting a tracking error signal representing a displacement between the light beam and the track; a fine movement section for moving the light beam in a substantially radial direction of the information carrier; a coarse movement section for moving the fine movement section in the substantially radial direction of the information carrier; a tracking controller for controlling the fine movement section and the coarse movement section based on the tracking error signal detected by the tracking error detector so that the light beam is positioned along the track; and an eccentricity detector for detecting an amount of eccentricity of the track on the information carrier. The tracking controller controls the coarse movement section based on the amount of eccentricity detected by the eccentricity detector. 
     In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an output of the tracking controller. 
     In one embodiment of the invention, the tracking controller includes: a fine tracking controller for controlling the fine movement section based on the tracking error signal so that the light beam is positioned along the track; and a coarse tracking controller for controlling the coarse movement section so that an amount of movement of the fine movement section by the fine tracking controller is zero on average. 
     In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an output of the coarse tracking controller. 
     In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an amplitude of an alternating current component of a control signal which is used by the coarse tracking controller for controlling the coarse movement section. 
     In one embodiment of the invention, the coarse tracking controller includes: a traverse linear filter for outputting a first drive signal for driving the coarse movement section based on an output of the fine tracking controller; and a traverse drive generator for outputting a second drive signal for driving the coarse movement section based on the first drive signal output from the traverse linear filter and based on the amount of eccentricity detected by the eccentricity detector. The eccentricity detector detects the amount of eccentricity based on the first drive signal output from the traverse linear filter. 
     In one embodiment of the invention, the tracking controller includes a switch for inactivating a control of the coarse movement section. The eccentricity detector detects the amount of eccentricity while inactivating the control of the coarse movement section by the switch during an operation of the tracking controller. 
     In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an output from the fine tracking controller. 
     In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an amplitude of an alternating current component of a control signal which is used by the fine tracking controller for controlling the fine movement section. 
     In one embodiment of the invention, the fine tracking controller includes a tracking linear filter for outputting a control signal for controlling the fine movement section. The eccentricity detector detects the amount of eccentricity based on an amplitude of an alternating current component of the control signal which is output from the tracking linear filter. 
     In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an output of the tracking error detector. 
     In one embodiment of the invention, the eccentricity detector detects the amount of eccentricity based on an amplitude of an alternating current component of the output of the tracking error detector. 
     In one embodiment of the invention, the coarse tracking controller includes: a traverse linear filter for outputting a first drive signal for driving the coarse movement section based on an output of the fine tracking controller: and a traverse drive generator for outputting a second drive signal for driving the coarse movement section based on the first drive signal output from the traverse linear filter and based on the amount of eccentricity detected by the eccentricity detector. 
     In one embodiment of the invention, the fine tracking controller includes a tracking linear filter for outputting a control signal for controlling the fine movement section. 
     In one embodiment of the invention, the eccentricity detector includes a dead zone width calculator for calculating a dead zone width representing a range in which a value of a drive signal for driving the coarse movement section is substantially zero, based on the amount of eccentricity. The tracking controller controls the coarse movement section based on the dead zone width calculated by the dead zone width calculator. 
     In one embodiment of the invention, the eccentricity detector includes an offset calculator for calculating a drive offset to be added to a drive signal for driving the coarse movement section based on the amount of eccentricity. The tracking controller controls the coarse movement section based on the drive offset calculated by the offset calculator. 
     Thus, the invention described herein makes possible the advantages of: (1) providing an optical disk apparatus capable of driving a traverse motor even when the frictional force of the traverse motor itself or the frictional force of the mechanism for traversing the optical head increases over time; and (2) providing an optical disk apparatus where the traverse motor has a desirable response speed even when the number of revolutions of the disk motor is small. 
     These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating an optical disk apparatus according to Example  1  of the present invention: 
     FIG. 2 is a graph illustrating the relationship between an input and an output of a traverse drive generator; 
     FIG. 3A illustrates an FG signal used when adjusting the traverse drive generator; 
     FIG. 3B illustrates an adjustment command signal; 
     FIG. 3C illustrates an input signal to the traverse drive generator; 
     FIG. 3D illustrates an output signal from a Max detector: 
     FIG. 3E illustrates an output signal from a Min detector; 
     FIG. 3F illustrates an output signal from a dead zone width calculator; 
     FIG. 3G illustrates an output signal from an offset calculator; 
     FIG. 3H illustrates a signal representing a set value prm 1  from a setter; 
     FIG. 3I illustrates a signal representing a set value prm 2  from another setter; 
     FIG. 4 is a block diagram illustrating an optical disk apparatus according to Example 2 of the present invention; 
     FIG. 5A illustrates an FG signal used when adjusting the traverse drive generator; 
     FIG. 5B illustrates an adjustment command signal; 
     FIG. 5C illustrates an input signal to the traverse drive generator; 
     FIG. 5D illustrates an output signal from a Max detector; 
     FIG. 5E illustrates an output signal from a Min detector; 
     FIG. 5F illustrates an output signal from a dead zone width calculator; 
     FIG. 5G illustrates an output signal from an offset calculator; 
     FIG. 5H illustrates a signal representing a set value prm 1  from a setter: 
     FIG. 5I illustrates a signal representing a set value prm 2  from another setter: 
     FIG. 6 is a block diagram illustrating an optical disk apparatus according to Example 3 of the present invention; 
     FIG. 7A illustrates an FG signal used when adjusting the traverse drive generator; 
     FIG. 7B illustrates an adjustment command signal; 
     FIG. 7C illustrates an input signal to the traverse drive generator; 
     FIG. 7D illustrates an output signal from a Max detector; 
     FIG. 7B illustrates an output signal from a Min detector: 
     FIG. 7F illustrates an output signal from a dead zone width calculator; 
     FIG. 7G illustrates an output signal from an offset calculator; 
     FIG. 7H illustrates a signal representing a set value prm 1  from a setter; 
     FIG. 7I illustrates a signal representing a set value prm 2  from another setter; 
     FIG. 8 is a block diagram illustrating a conventional optical disk apparatus; 
     FIG. 9 illustrates a frequency characteristic of a traverse linear filter; 
     FIG. 10A illustrates an FG signal used when performing a tracking control by the optical disk apparatus of FIG. 8; 
     FIG. 10B illustrates an output signal from a traverse linear filter; 
     FIG. 10C illustrates an output signal from an average calculator; and 
     FIG. 10D illustrates a drive signal from a pulse generator to a traverse motor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Various examples of the present invention will now be described. 
     Example 1 
     FIG. 1 is a block diagram illustrating an optical disk apparatus  100  according to Example 1 of the present invention. Elements in FIG. 1 having like reference numerals to those shown in FIG. 8 will not further be described. 
     The optical disk apparatus  100  includes the disk motor  50  for driving the optical disk  1 , the optical head  10 , the traverse motor  43 , the FE generator  30 , the Fc linear filter  31 , the TE generator  40 , a tracking controller TC, an eccentricity detector ED, a cycle counter  51  and a management processor  52 . 
     The optical head  10  includes the semiconductor laser  11 , the coupling lens  12 , the polarization beam splitter  13 , the ¼ wave plate  14 , the focusing actuator  16 , the tracking actuator  17 , the detection lens  18 , the cylindrical lens  19  and the photodetector  20 . 
     The tracking controller TC includes a fine tracking controller TF, a coarse tracking controller TL and a switch  46 . The fine tracking controller TF includes the Tk linear filter  41 . The coarse tracking controller TL includes the traverse linear filter  42  and a traverse drive generator  47 . 
     The eccentricity detector ED includes a band-pass filter  60 , a Max detector  61 , a Min detector  62 , a dead zone width calculator  63 , an offset calculator  64 , a setter  65  and another setter  66 . 
     The output of the traverse linear filter  42  is connected to the traverse drive generator  47 , and a drive signal from the traverse drive generator  47  is sent to the traverse motor  43  via the switch  46 . The switch  46  is closed when an adjustment command signal from the management processor  52  is at a low level. 
     FIG. 2 shows the relationship between the input and the output of the traverse drive generator  47 , wherein the horizontal axis represents the input value of the traverse drive generator  47 , and the vertical axis represents the output value thereof. 
     If the absolute value of the input value of the traverse drive generator  47  is less than or equal to a set value prm 1 , the output value is zero. Where the absolute value is greater than the set value prm 1 , if the input value is positive, the output value is the input value plus a set value prm 2 , and if the input value is negative, the output value is the input value minus the set value prm 2 . The values prm 1  and prm 2  are set values which influence the input/output relationship of the traverse drive generator  47 . The values prm 1  and prm 2  can be externally set. 
     The signal from the traverse linear filter  42  is input to the Max detector  61  and the Min detector  62  via the band-pass filter  60 . A signal from the cycle counter  51  is input to the band-pass filter  60 . The band-pass filter  60  suppresses frequency components other than the revolution cycle of the disk motor  50  obtained from the cycle counter  51  so as to extract the disk motor  50  revolution component from the signal from the traverse linear filter  42 . 
     When the rising edge of the adjustment command signal (see FIG. 3B, for example, to be described later) from the management processor  52  is detected, the Max detector  61  initializes its output with the output from the band-pass filter  60  at that time. Thereafter, based on the initial value, the Max detector  61  outputs to the dead zone width calculator  63  the maximum value of the signal from the band-pass filter  60 . 
     Similarly, when the rising edge of the adjustment command signal from the management processor  52  is detected, the Min detector  62  initializes its output with the output from the band-pass filter  60  at that time. Thereafter, based on the initial value, the Max detector  61  outputs to the dead zone width calculator  63  the minimum value of the signal from the band-pass filter  60 . 
     The dead zone width calculator  63  calculates one half of the value obtained by subtracting the output value of the Min detector  62  from the output value of the Max detector  61 , and outputs the obtained value to the offset calculator  64  and to the setter  65 . The offset calculator  64  subtracts the value output from the dead zone width calculator  63  from a predetermined value, and outputs the obtained value to the setter  66 . 
     The setter  65  latches the value output from the dead zone width calculator  63  when the falling edge of the adjustment command signal from the management processor  52  is detected. The setter  66  latches the value output from the offset calculator  64  when the falling edge of the adjustment command signal from the management processor  52  is detected. The traverse drive generator  47  uses the value output from the setter  65  as the set value prm 1 , and the value output from the setter  66  as the set value prm 2 . 
     The FG signal from the disk motor  50  is input to the cycle counter  51 . The cycle counter  51  measures the time period from one rising edge of the FG signal to the next rising edge thereof, and outputs the measured time period to the management processor  52  and to the band-pass filter  60 . 
     The drive waveform for driving the traverse motor  43  is first generated by the traverse linear filter  42 . As described above, the output signal from the traverse linear filter  42  has a fluctuation due to the eccentricity of the optical disk  1  as illustrated in FIG.  10 B. 
     In order to prevent the traverse motor  43  from being influenced by the fluctuation, the drive signal to the traverse motor  43  is set to zero if the output waveform from the traverse linear filter  42  is less than or equal to a predetermined level. This is realized by the dead zone determined by the set value prm 1  from the traverse drive generator  47 . 
     The drive offset is determined by the set value prm 2  of the traverse drive generator  47  so that the magnitude of the drive signal to the traverse motor  43  is equal to or greater than a predetermined level so as to obtain a driving force which overcomes the frictional force when traversing the optical head  10  by the traverse motor  43 . 
     In the optical disk apparatus  100  illustrated in FIG. 1, the magnitude of the fluctuation of the output signal from the traverse linear filter  42  caused by the eccentricity of the optical disk  1  is measured, so as to set the dead zone (the set value prm 1 ) and the drive offset (the set value prm 2 ) of the traverse drive generator  47  according to the measurement. 
     A sequence for determining the dead zone (the set value prm 1 ) and the drive offset (the set value prm 2 ) will be described with reference to the waveform diagrams of FIGS. 3A to  3 I. FIG. 3A illustrates the FG signal from the disk motor  50 , FIG. 3B illustrates the adjustment command signal from the management processor  52 , FIG. 3C illustrates a signal from the traverse linear filter  42 , FIG. 3D illustrates a signal from the Max detector  61 , FIG. 3E illustrates a signal from the Min detector  62 , FIG. 3F illustrates a signal from the dead zone width calculator  63 , FIG. 3G illustrates a signal from the offset calculator  64 , FIG. 3H illustrates the set value prm 1  of the traverse drive generator  47 , and FIG. 3I illustrates the set value prm 2  of the traverse drive generator  47 . 
     The management processor  52  constantly obtains the revolution cycle of the disk motor  50  from the signal from the cycle counter  51 . In a normal control state, the management processor  52  sends a low level signal to the switch  46 , the Max detector  61 , the Min detector  62 , the setter  65  and the setter  66 . 
     To initiate the sequence, the time period for one revolution of the disk motor  50  is measured by the cycle counter  51 . As illustrated in FIG. 3B, the management processor  52  outputs the adjustment command signal which is at a high level during one revolution of the disk motor  50  during a time period from time tA to time tB. 
     While the adjustment command signal is at a high level, the traverse motor  43  is inactivated so as to eliminate the influence of the TE generator  40  on the TE signal which may be caused by the activation of the traverse motor  43 . To do this, while the adjustment command signal is at a high level, the switch  46  is controlled so that the drive signal from the traverse drive generator  47  to the traverse motor  43  is zero. 
     The magnitude of the fluctuation of the output signal from the traverse linear filter  42  caused by the eccentricity of the optical disk  1  is measured by calculating the difference between the maximum value and the minimum value of the output signal from the traverse linear filter  42  during one revolution of the disk motor  50 . 
     When a rising edge of the adjustment command signal is detected, the Max detector  61  and the Min detector  62  initialize their respective output values. Then, until the adjustment command signal from the management processor  52  goes low, the Max detector  61  detects the maximum value of the signal from the band-pass filter  60  and the Min detector  62  detects the minimum value thereof. By the use of the band-pass filter  60 , the detection can be done while eliminating the signal noise component and the disturbance component. 
     As illustrated in FIGS. 3D and 3E, the Max detector  61  and the Min detector  62  initialize their respective output values with the value from the band-pass filter  60  at time tA, and then respectively measure the maximum value and the minimum value of the signal from the band-pass filter  60  until time tB. 
     As illustrated in FIG. 3F, the dead zone width calculator  63  constantly outputs one half of the value obtained by subtracting the output value of the Min detector  62  from the output value of the Max detector  61 . The output value of the dead zone width calculator  63  corresponds to the magnitude of the disk motor  50  revolution cycle component of the fluctuation of the output signal from the traverse linear filter  42  caused by the eccentricity of the optical disk  1 . The output value of the dead zone width calculator  63  is the set value prm 1  of the traverse drive generator  47  used for realizing the dead zone. 
     As illustrated in FIG. 3G, the offset calculator  64  subtracts the signal from the dead zone width calculator  63  from a predetermined level. This value is the drive offset (the set value prm 2  of the traverse drive generator  47 ) which is used so that the output from the traverse drive generator  47  is equal to or greater than a predetermined level when the signal from the traverse linear filter  42  exceeds the dead zone width. 
     By using the initial drive signal level of the traverse motor  43 , which is determined by the system frictional force, as the above-described predetermined level, the drive signal from the traverse drive generator  47  efficiently drives the traverse motor  43 . 
     Thereafter, when a falling edge of the adjustment command signal from the management processor  52  is detected at time tB, the setter  65  latches the output value of the dead zone width calculator  63  at that time, as illustrated in FIG.  3 H. The latched value is used by the traverse drive generator  47  as the set value prm 1 . At this time, the setter  66  latches the output value of the offset calculator  64 , as illustrated in FIG.  3 I. The latched value is used by the traverse drive generator  47  as the set value prm 2 . 
     When the adjustment command signal from the management processor  52  goes low, the switch  46  allows the drive signal from the traverse drive generator  47  to be input to the traverse motor  43 , so that the traverse motor  43  is driven based on the set values prm 1  and prm 2  of the traverse drive generator  47 . 
     Thus, when the signal from the traverse linear filter  42  is only influenced by the eccentricity of the optical disk  1 , the traverse motor  43  is not activated, and the light beam is only tracking-controlled by the tracking actuator  17 . 
     As the DC drive component of the tracking actuator  17  increases, and the signal from the traverse linear filter  42  becomes greater than that which is only influenced by the eccentricity of the optical disk  1 , the traverse drive generator  47  then outputs a drive signal which is sufficient to activate the traverse motor  43 . 
     This operation continues until the DC drive component of the tracking actuator  17  is reduced below the above-described dead zone by the activation of the traverse motor  43 . During such an operation, the activation of the traverse motor  43  is ensured because the above-described drive signal increases as the DC drive component of the tracking actuator  17  increases. 
     Where the mass eccentricity of the optical disk  1  is relatively large, as the number of revolutions of the optical disk  1  changes, the actual amount of eccentricity varies depending upon the mass eccentricity and the number of revolutions of the optical disk  1 . Therefore, the set values prm 1  and prm 2  of the traverse drive generator  47  are readjusted when the number of revolutions of the optical disk  1  has changed by a predetermined number from that when the set values prm 1  and prm 2  were previously adjusted. 
     The management processor  52  constantly obtains the number of revolutions of the optical disk  1  from the signal from the cycle counter  51 . The management processor  52  stores the number of revolutions of the optical disk  1  when the set values prm 1  and prm 2  of the traverse drive generator  47  are adjusted. Thus, when the difference between the stored number of revolutions and the current number of revolutions exceeds a predetermined value, the management processor  52  outputs the adjustment command signal so as to initiate the above-described adjustment operation. 
     As described above, Example 1 of the present invention realizes a tracking control which efficiently uses the supplied power and is not influenced by the eccentricity of the optical disk  1 . Moreover, the wave height of the drive signal to be applied to the traverse motor  43  is not fixed. Thus, even when the frictional force substantially in creases over time, the wave height can be increased so as to reliably drive the traverse motor  43 . 
     Furthermore, the output timing of the drive signal to the traverse motor  43  is not dependent upon the rotational position of the disk motor  50 . Moreover, even when the actual eccentricity varies depending upon the number of revolutions of the disk motor  50 , a readjustment operation can be performed to account for such a variation. 
     Example 2 
     FIG. 4 is a block diagram illustrating an optical disk apparatus  200  according to Example 2 of the present invention. Elements in FIG. 4 having like reference numerals to those shown in FIG. 1 will not further be described. 
     The signal from the Tk linear filter  41  is input to the Max detector  61  and to the Min detector  62  via a band-pass filter  67 . The signal from the cycle counter  51  is input to the band-pass filter  67 , and the band-pass filter  67  amplifies the frequency component of the disk motor  50  revolution cycle obtained from the cycle counter  51  with the same gain as that of the traverse linear filter  42 , while suppressing the other frequency components, thereby extracting the disk motor  50  revolution component from the signal from the Tk linear filter  41 . 
     In the optical disk apparatus  200  illustrated in FIG. 4, the magnitude of the fluctuation of the output signal from the Tk linear filter  41  caused by the eccentricity of the optical disk  1  is measured, so as to set the dead zone and the drive offset of the traverse drive generator  47  according to the measurement. 
     A sequence for determining the dead zone and the drive offset will be described with reference to the waveform diagrams of FIGS. 5A to  5 I. FIG. 5A illustrates the FG signal from the disk motor  50 , FIG. 5B illustrates the adjustment command signal from the management processor  52 , FIG. 5C illustrates a signal from the Tk linear filter  41 , FIG. 5D illustrates a signal from the Max detector  61 , FIG. 5E illustrates a signal from the Min detector  62 , FIG. 5F illustrates a signal from the dead zone width calculator  63 , FIG. 5G illustrates a signal from the offset calculator  64 , FIG. 5H illustrates the set value prm 1  of the traverse drive generator  47 , and FIG. 5I illustrates the set value prm 2  of the traverse drive generator  47 . 
     The management processor  52  constantly obtains the revolution cycle of the disk motor  50  from the signal from the cycle counter  51 . In a normal control state, the management processor  52  sends a low level signal to the switch  46 , the Max detector  61 , the Min detector  62 , the setter  65  and the setter  66 . 
     To initiate the sequence, the time period for one revolution of the disk motor  50  is measured by the cycle counter  51 . As illustrated in FIG. 5B, the management processor  52  outputs the adjustment command signal which is at a high level during one revolution of the disk motor  50  during a time period from time tC to time tD. 
     While the adjustment command signal is at a high level, the traverse motor  43  is inactivated so as to eliminate the influence of the TE generator  40  on the TE signal which may be caused by the activation of the traverse motor  43 . To do this, while the adjustment command signal is at a high level, the switch  46  is controlled so that the drive signal from the traverse drive generator  47  to the traverse motor  43  is zero. 
     The magnitude of the fluctuation of the output signal from the Tk linear filter  41  caused by the eccentricity of the optical disk  1  is measured by calculating the difference between the maximum value and the minimum value of the output signal from the Tk linear filter  41  during one revolution of the disk motor  50 . 
     When a rising edge of the adjustment command signal is detected, the Max detector  61  and the Min detector  62  initialize their respective output values. Then, until the adjustment command signal from the management processor  52  goes low, the Max detector  61  detects the maximum value of the signal from the band-pass filter  67  and the Min detector  62  detects the minimum value thereof. By the use of the band-pass filter  67 , the detection can be done while eliminating the signal noise component and the disturbance component. 
     As illustrated in FIGS. 5D and 5E, the Max detector  61  and the Min detector  62  initialize their respective output values with the value from the band-pass filter  67  at time tC, and then respectively measure the maximum value and the minimum value of the signal from the band-pass filter  67  until time tD. 
     As illustrated in FIG. 5F, the dead zone width calculator  63  constantly outputs one half of the value obtained by subtracting the output value of the Min detector  62  from the output value of the Max detector  61 . This value is obtained by multiplying the magnitude of the disk motor  50  revolution cycle component of the fluctuation of the output signal from the Tk linear filter  41  caused by the eccentricity of the optical disk  1 , by the gain of the traverse linear filter  42 . The output value of the dead zone width calculator  63  is the set value prm 1  of the traverse drive generator  47  used for realizing the dead zone. 
     As illustrated in FIG. 5G, the offset calculator  64  subtracts the signal from the dead zone width calculator  63  from a predetermined level. This value is the drive offset (the set value prm 2  of the traverse drive generator  47 ) which is used so that the output from the traverse drive generator  47  is equal to or greater than a predetermined level when the signal from the traverse linear filter  42  exceeds the dead zone width. 
     By using the initial drive signal level of the traverse motor  43 , which is determined by the system frictional force, as the above-described predetermined level, the drive signal from the traverse drive generator  47  efficiently drives the traverse motor  43 . 
     Thereafter, when a falling edge of the adjustment command signal from the management processor  52  is detected at time tD, the setter  65  latches the output value of the dead zone width calculator  63  at that time, as illustrated in FIG.  5 H. The latched value is used by the traverse drive generator  47  as the set value prm 1 . At this time, the setter  66  latches the output value of the offset calculator  64 , as illustrated in FIG.  5 I. The latched value is used by the traverse drive generator  47  as the set value prm 2 . 
     When the adjustment command signal from the management processor  52  goes low, the switch  46  allows the drive signal from the traverse drive generator  47  to be input to the traverse motor  43 , so that the traverse motor  43  is driven based on the set values prm 1  and prm 2  of the traverse drive generator  47 . 
     Thus, when the signal from the traverse linear filter  42  is only influenced by the eccentricity of the optical disk  1 , the traverse motor  43  is not activated, and the light beam is only tracking-controlled by the tracking actuator  17 . 
     As the DC drive component of the tracking actuator  17  increases, and the signal from the traverse linear filter  42  becomes greater than that which is only influenced by the eccentricity of the optical disk  1 , the traverse drive generator  47  then outputs a drive signal which is sufficient to activate the traverse motor  43 . 
     This operation continues until the DC drive component of the tracking actuator  17  is reduced below the above-described dead zone by the activation of the traverse motor  43 . During such an operation, the activation of the traverse motor  43  is ensured because the above-described drive signal increases as the DC drive component of the tracking actuator  17  increases. 
     Where the mass eccentricity of the optical disk  1  is relatively large, as the number of revolutions of the optical disk  1  changes, the actual amount of eccentricity varies depending upon the mass eccentricity and the number of revolutions of the optical disk  1 . Therefore, the set values prm 1  and prm 2  of the traverse drive generator  47  are readjusted when the number of revolutions of the optical disk  1  has changed by a predetermined number from that when the set values prm 1  and prm 2  were previously adjusted. 
     The management processor  52  constantly obtains the number of revolutions of the optical disk  1  from the signal from the cycle counter  51 . The management processor  52  stores the number of revolutions of the optical disk  1  when the set values prm 1  and prm 2  of the traverse drive generator  47  are adjusted. Thus, when the difference between the stored number of revolutions and the current number of revolutions exceeds a predetermined value, the management processor  52  outputs the adjustment command signal so as to initiate the above-described adjustment operation. 
     As described above, Example 2 of the present invention also realizes a tracking control which efficiently uses the supplied power and is not influenced by the eccentricity of the optical disk  1 . Moreover, the wave height of the drive signal to be applied to the traverse motor  43  is not fixed. Thus, even when the frictional force substantially increases over time, the wave height can be increased so as to reliably drive the traverse motor  43 . 
     Furthermore, the output timing of the drive signal to the traverse motor  43  is not dependent upon the rotational position of the disk motor  50 . Moreover, even when the actual eccentricity varies depending upon the number of revolutions of the disk motor  50 , a readjustment operation can be performed to account for such a variation. 
     Example 3 
     FIG. 6 is a block diagram illustrating an optical disk apparatus  300  according to Example 3 of the present invention. Elements in FIG. 6 having like reference numerals to those shown in FIG. 1 will not further be described. 
     The signal from the TE generator  40  is input to the Max detector . 61  and to the Min detector  62  via a band-pass filter  68 . The signal from the cycle counter  51  is input to the band-pass filter  68 , and the band-pass filter  68  amplifies the frequency component of the disk motor  50  revolution cycle obtained from the cycle counter  51  with the same gain as that of the Tk linear filter  41 , while suppressing the other frequency components, thereby extracting the disk motor  50  revolution component from the signal from the TE generator  40 . 
     In the optical disk apparatus  300  illustrated in FIG. 6, the magnitude of the fluctuation of the output signal from the TE generator  40  caused by the eccentricity of the optical disk  1  is measured, so as to set the dead zone and the drive offset of the traverse drive generator  47  according to the measurement. 
     A sequence for determining the dead zone and the drive offset will be described with reference to the waveform diagrams of FIGS. 7A to  7 I. FIG. 7A illustrates the FG signal from the disk motor  50 , FIG. 7B illustrates the adjustment command signal from the management processor  52 , FIG. 7C illustrates a signal from the TE generator  40 , FIG. 7D illustrates a signal from the Max detector  61 , FIG. 7E illustrates a signal from the Min detector  62 , FIG. 7F illustrates a signal from the dead zone width calculator  63 , FIG. 7G illustrates a signal from the offset calculator  64 , FIG. 7H illustrates the set value prm 1  of the traverse drive generator  47 , and FIG. 7I illustrates the set value prm 2  of the traverse drive generator  47 . 
     The management processor  52  constantly obtains the revolution cycle of the disk motor  50  from the signal from the cycle counter  51 . In a normal control state, the management processor  52  sends a low level signal to the switch  46 , the Max detector  61 , the Min detector  62 , the setter  65  and the setter  66 . 
     To initiate the sequence, the time period for one revolution of the disk motor  50  is measured by the cycle counter  51 . As illustrated in FIG. 7B, the management processor  52  outputs the adjustment command signal which is at a high level during one revolution of the disk motor  50  during a time period from time tE to time tF. 
     While the adjustment command signal is at a high level, the traverse motor  43  is inactivated so as to eliminate the influence of the TE generator  40  on the TE signal which may be caused by the activation of the traverse motor  43 . To do this, while the adjustment command signal is at a high level, the switch  46  is controlled so that the drive signal from the traverse drive generator  47  to the traverse motor  43  is zero. 
     The magnitude of the fluctuation of the output signal from the TE generator  40  caused by the eccentricity of the optical disk  1  is measured by calculating the difference between the maximum value and the minimum value of the output signal from the TE generator  40  during one revolution of the disk motor  50 . 
     When a rising edge of the adjustment command signal (FIG. 7B) is detected, the Max detector  61  and the Min detector  62  initialize their respective output values. Then, until the adjustment command signal from the management processor  52  goes low, the Max detector  61  detects the maximum value of the signal from the band-pass filter  68  and the Min detector  62  detects the minimum value thereof. By the use of the band-pass filter  68 , the detection can be done while eliminating the signal noise component and the disturbance component. 
     As illustrated in FIGS. 7D and 7E, the Max detector  61  and the Min detector  62  initialize their respective output values with the value from the band-pass filter  68  at time tE, and then respectively measure the maximum value and the minimum value of the signal from the band-pass filter  68  until time tF. 
     As illustrated in FIG. 7F, the dead zone width calculator  63  constantly outputs one half of the value obtained by subtracting the output value of the Min detector  62  from the output value of the Max detector  61 . This value is obtained by multiplying the magnitude of the disk motor  50  revolution cycle component of the fluctuation of the output signal from the TE generator  40  caused by the eccentricity of the optical disk  1 , by the gain of the Tk linear filter  41  and the gain of the traverse linear filter  42 . The output value of the dead zone width calculator  63  is the set value prm 1  of the traverse drive generator  47  used for realizing the dead zone. 
     As illustrated in FIG. 7G, the offset calculator  64  subtracts the signal from the dead zone width calculator  63  from a predetermined level. This value is the drive offset (the set value prm 2  of the traverse drive generator  47 ) which is used so that the output from the traverse drive generator  47  is equal to or greater than a predetermined level when the signal from the traverse linear filter  42  exceeds the dead zone width. 
     By using the initial drive signal level of the traverse motor  43 , which is determined by the system frictional force, as the above-described predetermined level, the drive signal from the traverse drive generator  47  efficiently drives the traverse motor  43 . 
     Thereafter, when a falling edge of the adjustment command signal from the management processor  52  is detected at time tF, the setter  65  latches the output value of the dead zone width calculator  63  at that time, as illustrated in FIG.  7 H. The latched value is used by the traverse drive generator  47  as the set value prm 1 . At this time, the setter  66  latches the output value of the offset calculator  64 , as illustrated in FIG.  7 I. The latched value is used by the traverse drive generator  47  as the set value prm 2 . 
     When the adjustment command signal from the management processor  52  goes low, the switch  46  allows the drive signal from the traverse drive generator  47  to be input to the traverse motor  43 , so that the traverse motor  43  is driven based on the set values prm 1  and prm 2  of the traverse drive generator  47 . 
     Thus, when the signal from the traverse linear filter  42  is only influenced by the eccentricity of the optical disk  1 , the traverse motor  43  is not activated, and the light beam is only tracking-controlled by the tracking actuator  17 . 
     As the DC drive component of the tracking actuator  17  increases, and the signal from the traverse linear filter  42  becomes greater than that which is only influenced by the eccentricity of the optical disk  1 , the traverse drive generator  47  then outputs a drive signal which is sufficient to activate the traverse motor  43 . 
     This operation continues until the DC drive component of the tracking actuator  17  is reduced below the above-described dead zone by the activation of the traverse motor  43 . During such an operation, the activation of the traverse motor  43  is ensured because the above-described drive signal increases as the DC drive component of the tracking actuator  17  increases. 
     Where the mass eccentricity of the optical disk  1  is relatively large, as the number of revolutions of the optical disk  1  changes, the actual amount of eccentricity varies depending upon the mass eccentricity and the number of revolutions of the optical disk  1 . Therefore, the set values prm 1  and prm 2  of the traverse drive generator  47  are readjusted when the number of revolutions of the optical disk  1  has changed by a predetermined number from that when the set values prm 1  and prm 2  were previously adjusted. 
     The management processor  52  constantly obtains the number of revolutions of the optical disk  1  from the signal from the cycle counter  51 . The management processor  52  stores the number of revolutions of the optical disk  1  when the set values prm 1  and prm 2  of the traverse drive generator  47  are adjusted. Thus, when the difference between the stored number of revolutions and the current number of revolutions exceeds a predetermined value, the management processor  52  outputs the adjustment command signal so as to initiate the above-described adjustment operation. 
     As described above, Example 3 of the present invention also realizes a tracking control which efficiently uses the supplied power and is not influenced by the eccentricity of the optical disk  1 . Moreover, the wave height of the drive signal to be applied to the traverse motor  43  is not fixed. Thus, even when the frictional force substantially increases over time, the wave height can be increased so as to reliably drive the traverse motor  43 . 
     Furthermore, the output timing of the drive signal to the traverse motor  43  is not dependent upon the rotational position of the disk motor  50 . Moreover, even when the actual eccentricity varies depending upon the number of revolutions of the disk motor  50 , a readjustment operation can be performed to account for such a variation. 
     The present invention has been specifically described above with respect to Examples 1 to 3, though the present invention is not in any way limited to those specific examples set forth above. While a rotational motor is used as the traverse motor in the above-described examples, the traverse motor may alternatively be a linear motor. 
     As described above, the present invention provides an optical disk apparatus capable of driving a traverse motor even when the frictional force of the traverse motor itself or the frictional force of the mechanism for traversing the optical head increases over time. 
     The present invention also provides an optical disk apparatus where the traverse motor has a desirable response speed even when the number of revolutions of the disk motor is small. 
     With the optical disk apparatus of the present invention, the eccentricity of the optical disk is measured, and the traverse motor is controlled based on the measurement. Thus, it is possible to optimize the accuracy of the movement of the traverse motor according to the optical disk being used. 
     Moreover, the traverse motor is not activated by the eccentricity. When there occurs a DC drive component of the tracking actuator, the traverse motor can be driven with a high accuracy with a delay less than or equal to a time period corresponding to one revolution of the disk. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.