1. Field of the Invention
The invention relates to a storage apparatus such as an optical disk drive or the like for recording and reproducing information to/from a medium such as an optical disk or the like and, more particularly, to a storage apparatus which can stably move a light spot to an adjacent track by a track jump even under a circumstance of an eccentricity of a disk, a frictional disturbance occurring by a positioner mechanism, or a disturbance from the outside of the apparatus.
2. Description of the Related Arts
In a conventional optical disk drive, to raise track-following performance of a light spot to a medium track, there is used a double stage (fine/coarse separation driving type) positioner comprising: a carriage actuator (VCM) for a seek control (also referred to as a coarse control) for moving a carriage supported by a ball bearing for a guide rail which is fixedly arranged; and a tracking actuator for a track-following control (also referred to as a precision control or tracking control) for moving a laser beam in the direction which transverses the tracks by the driving of an objective lens mounted on the carriage. In recent years, however, an apparatus using a single stage (fine/coarse integrated driving type) positioner in which a tracking actuator is omitted and only a carriage actuator is used is also widespread for the purpose of the reduction of costs of the apparatus. In the single stage positioner, a simple slide bearing is formed by removing a ball bearing from a bearing portion, thereby reducing the number of parts and costs.
Since the optical disk has a spiral track, a track jump for moving a light spot to an adjacent track is performed at a predetermined position for every rotation of the optical disk during a track-following control. As a conventional track jumping method, generally, a low band component of a control output of a feedback control system is held during the track jump, thereby stabilizing the track jump (JP-A-2-152020, JP-A-7-110950). In an apparatus such as a single stage apparatus in which the frictional disturbance of the positioner cannot be ignored, however, there is a case where a sufficient compensation is not performed.
FIG. 1A shows a positioner displacement 300 in the seeking direction of a carriage for a guide rail during a track-following control with respect to an apparatus of a conventional single stage positioner. FIG. 1B shows a positioner speed 302 in the seeking direction. FIG. 1C shows a positioner acceleration 304 in the seeking direction. Further, FIG. 1D shows a relation of a driving current 306 which is supplied to a VCM. For simplicity of explanation, it is assumed that an eccentricity of the disk comprises only a primary eccentricity, a rotational speed is set to 4500 rpm (75 Hz), and an eccentricity displacement amplitude is set to xc2x135 xcexcm. In the eccentricity tracking state, as shown in FIG. 1A, the positioner reciprocates on the rail synchronously with the eccentricity of the disk and causes the positioner speed 302 in FIG. 1B and the positioner acceleration 304 in FIG. 1C. If there is no friction or the like between the rail and the slide bearing portion of the carriage, the ideal track-following performance is realized by applying the VCM driving current 306 shown by a broken line in FIG. 1D. However, in the single stage positioner, a disturbance due to a Coulomb friction in the bearing portion between the rail and the carriage cannot be ignored. The disturbance due to the Coulomb friction usually becomes a disturbance which acts in the direction opposite to the moving direction, namely, in the direction so as to obstruct the movement. Due to the eccentricity tracking, the moving direction of the carriage which reciprocates on the rail is reversed twice a disk rotation. Actually, if the secondary eccentricity or the like cannot be ignored, there is also a case where the moving direction is reversed two or more times. A situation that the moving direction of the carriage is reversed twice a disk rotation and the Coulomb frictional disturbance occurs corresponds to a timing when the positioner speed 302 in FIG. 1B crosses the zero point and the sign is inverted as shown by circles 308-1 to 308-4. When such a Coulomb frictional disturbance is simply modeled, it can be regarded as a step-like disturbance in which the direction of the disturbance force is abruptly inverted in dependence on the sign of the positioner speed and becomes a frictional disturbance 310 as shown by a broken line in FIG. 1C. A case where a coefficient of friction (xcexc) is equal to xcexc=0.4 is presumed here. In such a storage apparatus, to realize an ideal track-following control, the drive current is not limited to the VCM driving current to generate the positioner acceleration but it is necessary to use a VCM driving current 312 as shown by a solid line in FIG. 1D by further multiplexing the driving current to cancel the frictional disturbance to the VCM driving current. When considering a case of the 1-track jump for moving the light spot to the adjacent track, it is considered that time that is required to move the light spot to the adjacent track is generally equal to about 500 xcexcsec or less. On the other hand, a disk rotating frequency is a relatively low frequency of, for example, 75 Hz (13.3 msec per period). In such an apparatus that the Coulomb frictional disturbance can be ignored, it is considered that a change in disturbance force that is presumed for a short time during the track jump is very small. A valid effect can be expected even if a low frequency component of an output of a feedback control system is multiplexed during the track jump as in the conventional method. In the apparatus of the single stage positioner in which the Coulomb frictional disturbance cannot be ignored, however, there exists a rotational phase area of the disk in which the VCM driving current needs to be steeply changed like a VCM driving current 312 shown by a solid line in FIG. 1D to compensate the abrupt change of the frictional disturbance 310 in FIG. 1C. Such an area exists at a position near a point shown by each of the circles 308-1 to 308-4 where the positioner speed 302 in FIG. 1B becomes zero. When considering a case where a 1-track jump command is received just before the positioner speed becomes zero, there is a large change between a disturbance amount at a start point of the track jump and that at an end point of the track jump. When the track jump command is received just before the positioner speed becomes zero, it is considered that the value of the low frequency component of the feedback control system is almost converged to the disturbance amount just before the track jump. In the conventional track jump, however, with respect to the low band component of the feedback control system, the value just before the track jump is held and a feed-forward outputted during the track jump. Therefore, at the end point of the track jump, a large error occurs between the actual disturbance amount and the value of the low band component which is feed-forward outputted, so that there is a case where the track jump becomes unstable or the track jump fails. Since the track jump command is instructed from an upper controller irrespective of the rotational phase of the disk, if the track jump command is instructed in such an area where the frictional disturbance changes, according to the conventional method of feed-forward outputting the low band component of the feedback control system during the track jump, there is a case where the output is incomplete. To solve such a problem, there is a method whereby the eccentricity speed is detected and the track jump command is outputted excluding an area where it is influenced by a static friction and the eccentricity speed becomes a value near zero (JP-A-9-35282). However, when the track jump command is issued in an area where the track jump is inhibited, a time lag occurs before the track jump command is executed and there is a fear of deterioration of random access performance.
According to the invention, a storage apparatus which can perform a stable track jump for an eccentricity of a disk, a frictional disturbance occurring by a positioner mechanism, or a disturbance from the outside of the apparatus is provided. According to the invention, there is also provided a storage apparatus for realizing a stable track jump even in an apparatus which uses a single stage positioner and in which a frictional disturbance cannot be ignored.
According to the invention, there is provided a storage apparatus comprising: a recording/reproducing unit for recording or reproducing information onto/from a track by a light spot; and a track-following control unit for driving a positioner to move a position of the light spot by a feedback control signal from a feedback control unit based on a tracking error signal TES showing a positional deviation of the light spot and the track and allowing the light spot to follow the track.
According to the invention, the storage apparatus is characterized by having a track jump control unit for adding a feed-forward signal for repetitive disturbance compensation which is used for the track-following control to a track jump signal for moving the light spot to the adjacent track, thereby driving the positioner. As mentioned above, by feed-forwarding the repetitive disturbance compensation signal of high precision captured by the learning control for the track-following control during the track jump, adding the feed-forward signal to the track jump signal, and driving the positioner, a repetitive disturbance such as eccentricity disturbance, Coulomb frictional disturbance, or the like can be accurately compensated.
The track jump control unit has a learning control unit for obtaining a function for one medium rotational period to set a positional deviation amount for the repetitive disturbance to zero by a learning algorithm as an approximated function which was approximated and presumed and storing such a function as a feed-forward signal for repetitive disturbance compensation. As for the learning by the learning control unit, even if it takes long time to settle the learning result due to a low learning gain, a compensation signal of a steep frictional disturbance of a high band in association with the inversion of the carriage moving direction can be also included in the learning result that is obtained finally. By adding the learning control signal to the track jump control system by the feed-forward control, the steep frictional disturbance can be removed from the track jump control system. By using the single stage positioner, a tracking error for the eccentricity of the medium can be fairly reduced and the precision of the track jump control can be improved even if there is a limitation due to the existence of a high-order resonance and a control band is low.
The track jump control unit has a function to hold a final value of a low band component just before the track jump in the feedback control signal for track-following control and has a sample holder for adding the held final value to the track jump signal and the feed-forward signal for repetitive disturbance compensation, thereby driving the positioner during the track jump. By feed-forwarding the value just before the track jump of the low band compensation component of the feedback control system for the track-following control during the track jump together with the repetitive disturbance compensation signal as mentioned above, in addition to the repetitive disturbance such as eccentricity disturbance, Coulomb frictional disturbance, or the like, a non-repetitive disturbance such as an oscillatory disturbance from the outside of the apparatus can be compensated with high precision.
During the track jump, the track jump control unit stops a low band compensating calculation in the feedback control unit for a time interval from a timing during the track jump to the elapse of a predetermined time during a lead-in control after the end of the track jump by holding internal variables just before the track jump and, thereafter, restarts the low band compensating calculation by using the held internal variables as initial values. As mentioned above, a stop of the calculation of a low band compensating filter performed during the track jump is continued during a transient response of the lead-in control after the end of the track jump. The calculation of the low band compensating filter is restarted at a timing near a timing when the transient response is settled and a tracking error signal TES is converged to a target track. Thus, a situation that the low band compensation component is vainly fluctuated due to the lead-in transient response is prevented and a deterioration of the low band compensation precision by the low band compensating filter after the lead-in is prevented as much as possible.
A low band emphasizing filter (low band compensating filter) is constructed by a PI compensating filter. In the feedback control unit, as a final value of the low band compensation component just before the track jump that is fed-forward during the track jump, a value obtained by multiplying the final value just before the track jump of the signal of either a xe2x80x9cPI type low band compensation componentxe2x80x9d or an xe2x80x9cI type low band compensation componentxe2x80x9d by a correction coefficient (corresponding to Kp1 or Kcomp in FIGS. 16, 17A, and 17B) is used. Any of those values is held by the sample holder and feed-forward added during the track jump. In case of using the feedback control unit, a calculation of an integration compensation calculating unit (low band emphasis calculating unit in the embodiment) is stopped simultaneously with the start of the track jump while holding and outputting an integration value just before the track jump. During the track jump and until the elapse of a predetermined time after the end of the track jump (end of the track jump signal), the integration compensation calculating unit continuously holds and outputs the integration value just before the track jump and, after that, restarts the integration compensating calculation by using the held integration value as an initial value. Since a selector is switched to the track jump signal side simultaneously with the start of the track jump, an output of a phase-lead calculation is not reflected to a drive command signal. However; a phase-lead compensating unit continues the calculation even during the track jump. That is, during the track-following control until a timing just before the track jump, a phase-lead calculation is performed by using a PI compensation output (PI type low band compensation component) as an input. After the start of the track jump, an addition value of the integration compensation output which was held to a constant value and outputted and a signal (this signal fluctuates in response to a TES change) obtained by multiplying the TES signal by Kp is used as an input, and the phase-lead calculation is continued. The selector is again switched to the phase-lead compensation output side simultaneously with the end of the track jump signal and the lead-in control is started. Since the phase-lead compensating calculation is continued as mentioned above, the valid driving signal for the lead-in operation is outputted from the phase-lead compensating unit instantaneously with the switching of the selector. Since the low band compensation signal held by the integration calculating unit is also included in such an output, the low band compensation signal which has been feed-forward outputted during the track jump by the selector is invalidated because the selector is switched to the 0 output side instantaneously with the start of the lead-in. After the elapse of a predetermined time from the start of the lead-in, the integration compensation calculating unit restarts the integrating calculation by using the held output as an initial value. From this time point, the feedback control unit is recovered to a feedback control system of a perfect form for track-following control also including the low band compensating calculation.
The low band emphasizing filter can be also implemented with phase-lag compensation. The phase-lag compensation can be equivalently realized by an additional synthesis of a direct transfer component (DC transfer component) of a proportional component and a result of a low pass filter calculation (the low band emphasis calculating unit in the embodiment).
In the feedback control unit, as a value that is fed-forward during the track jump, there is used a value obtained by multiplying the final value just before the track jump of a signal of either the xe2x80x9cI type low band compensation componentxe2x80x9d (actually, although it is an output of a low pass filter, it is called an I type low band compensation component for convenience in the invention) or the xe2x80x9cPI type low band compensation componentxe2x80x9d (also referred to a PI type for convenience because of the same reason as that mentioned above) by the correction coefficient (corresponding to Kp1 or Kcomp in FIGS. 16, 17A, and 17B). Any of those values is held by the sample holder and feed-forward added during the track jump. In case of using the feedback control unit, calculation of the low band emphasis calculating unit is stopped while holding internal variables in the filter just before the track jump and holding the output just before the track jump simultaneously with the start of the track jump. During the track jump and until a predetermined time elapses after the end of the track jump (end of the track jump signal), the low band emphasis calculating unit continuously holds and outputs the output value just before the track jump and, after that, restarts the low band emphasizing calculation by using the held filter internal variables and output value as initial values. Since the selector is switched to the track jump signal side simultaneously with the start of the track jump, the output of the phase-lead calculation is not reflected to a drive command signal. However, the phase-lead compensating unit continues the calculation even during the track jump. That is, during the track-following control until a timing just before the track jump, a phase-lead calculation is performed by using a phase-lag compensation calculation output (PI type low band compensation component) as an input. After the start of the track jump, an addition value of the output of the low band emphasis calculating unit which was held to a constant value and outputted and a signal (this signal fluctuates in response to the TES change) obtained by multiplying the TES signal by Kp is used as an input, and the phase-lead calculation is continued. The selector is again switched to the phase-lead compensation output side simultaneously with the end of the track jump signal and the lead-in control is started. Since the phase-lead compensating calculation is continued as mentioned above, the valid driving signal for the lead-in operation is outputted from the phase-lead compensating unit instantaneously with the switching of the selector. Since the low band compensation signal held by the low band emphasis calculating unit is also included in such an output, the low band compensation signal which has been feed-forward outputted during the track jump by the selector is invalidated because the selector is switched to the 0 output side instantaneously with the start of the lead-in. After the elapse of a predetermined time from the start of the lead-in, the low band emphasis calculating unit restarts the low band emphasizing calculation by using the held filter internal variables and output value as initial values. From this time point, the feedback control unit is recovered to the feedback control system of a perfect form for track-following control also including the low band compensating calculation. More generally, the phase-lag compensation is constructed by a filter to emphasize a low band to improve a low band gain for low band compensation. However, in case of a compensation in which filters are serially connected like
(phase-lag compensation)+(phase-lead compensation)
the low band emphasis filter unit usually includes the DC transfer component of the proportional component. That is, now assuming that a transfer function of the low band emphasizing filter is labelled as GL(s), it is expressed by                                           G            L                    ⁡                      (            s            )                          =                                                            b                m                            ⁢                              s                m                                      +            …            ⁢                          xe2x80x83                        +                                          b                1                            ⁢              s                        +                          b              0                                                                          a                n                            ⁢                              s                n                                      +            …            +                                          a                1                            ⁢              s                        +                          a              0                                                          (        1        )            
Generally, mxe2x89xa6n. If the low band emphasizing filter includes the DC transfer component of the proportional component, the degrees of a numerator and a denominator are equal and m=n. When m=n, the transfer function of the equation (1) can be generally dissolved as shown by the following equation.
GL(s)=GLxe2x80x2(s)+KGLxe2x80x83xe2x80x83(2)
GLxe2x80x2(s) is the transfer function that is strictly proper (namely, the degree of the numerator is smaller than the degree of the denominator). That is, when the numerator and the denominator of the transfer function GL(s) of the low band emphasizing filter are equal, as shown by the equation (2), it can be expressed by the sum of a direct transfer term (DC transfer term) (KGL) of the proportional component and the transfer function GLxe2x80x2(s) that is strictly proper. In the invention, GLxe2x80x2(s) is called a low band emphasis calculating unit.
In the feedback control unit which generally expresses the low band emphasizing filter, as a final value of the low band compensation component just before the track jump which is fed-forward during the track jump, a value obtained by multiplying the final value just before the track jump of the signal of either an xe2x80x9cI type low band compensation componentxe2x80x9d (although it is actually an output of a low pass filter, it is called an I type low band compensation component for convenience in the invention) or a xe2x80x9cPI type low band compensation componentxe2x80x9d (also called a PI type for convenience in the invention) by the correction coefficient (corresponding to Kp1 or Kcomp in FIGS. 16, 17A, and 17B) is used. Any of those values is held by the sample holder and feed-forward added during the track jump. In case of using the feedback control unit, a calculation of the low band emphasis calculating unit is stopped simultaneously with the start of the track jump while holding the filter internal variables just before the track jump and holding the output just before the track jump. During the track jump and until a predetermined time elapses after the end of the track jump (end of the track jump signal), the low band emphasis calculating unit continuously holds and outputs the output value just before the track jump and, after that, restarts the low band emphasizing calculation by using the held filter internal variables and output value as initial values. Since the selector is switched to the track jump signal side simultaneously with the start of the track jump, an output of a high band compensating calculation is not reflected to the drive command signal. However, a high band compensating unit continues the calculation even during the track jump. That is, during the track-following control until a timing just before the track jump, the high band compensating calculation is performed by using an output of the low band compensating calculation (PI type low band compensation component) as an input. After the start of the track jump, an addition value of the output of the low band emphasis calculating unit which was held to a constant value and outputted and a signal (this signal fluctuates in response to the TES change) obtained by multiplying the TES signal by (Kpxc3x97KGL) is used as an input, and the high band compensating calculation is continued. The selector is again switched to the high band compensation output side simultaneously with the end of the track jump signal and the lead-in control is started. Since the high band compensating calculation is continued as mentioned above, the valid driving signal for the lead-in operation is outputted instantaneously with the switching of the selector. Since the low band compensation signal held by the low band emphasis calculating unit is also included in such an output, the low band compensation signal which has been feed-forward outputted during the track jump by the selector is invalidated because the selector is switched to the 0 output side instantaneously with the start of the lead-in. After the elapse of a predetermined time from the start of the lead-in, the low band emphasis calculating unit restarts the low band emphasizing calculation by using the held filter internal variables and output value as initial values. From this time point, the feedback control unit is recovered to the feedback control system of a perfect form for track-following control also including the low band compensating calculation.
As a command value of the track jump signal to move the light spot to the adjacent target track by one track, the track jump control unit generates a command of a first kick pulse to output a predetermined first acceleration A1 for a predetermined time T1, subsequently generates a command of a second kick pulse to output a second acceleration A2 smaller than the first acceleration A1 until a passage of xc2xd track of the light spot is detected (zero-cross point of TES is detected), and then generates a command of a brake pulse to output a predetermined third acceleration (deceleration) A3 for a predetermined time T3 in such a manner that the track jump is finished at a position near a point that is xc2xc track before the target track or at a position near the peak of the tracking error signal TES near a position which is xc2xc track before the target track, and after that, shifts the control to the track-following control by the feedback control unit, and leads in the light spot to the adjacent target track. By the generation of such a track jump signal, for a case where the direction of the Coulomb frictional disturbance which causes a compensation error during the track jump is reversed from the direction during the track-following control and the acceleration becomes insufficient, the second kick pulse to compensate the insufficient acceleration is generated subsequently to the first kick pulse mainly for accelerating the positioner to thereby cope with such a case, thereby preventing the positioner from being moved backward for a period of time until the end of the track jump. The brake pulse of the track jump signal subsequent to the kick pulse is set to the deceleration A3 and continuation time T3 so that the track jump is finished at a position near a position that is xc2xc track before the target track, accurately, a position near a position where the tracking error signal TES becomes the peak, thereby setting a timing to shift the control to the lead-in control system (track-following control system) to an early timing. By this method, a compensation possibility due to the feedback control is raised. Thus, a stable track jump can be performed even in such an apparatus that the Coulomb frictional disturbance cannot be ignored like an apparatus using the single stage positioner.
A learning control unit obtains an unknown function (drive command signal function) for one medium rotational period to set a positional deviation amount for a repetitive disturbance such as a medium eccentricity or the like synchronized with the medium rotation to zero by a learning algorithm as an approximated function which was approximately by using a set of heights of rectangular functions for intervals obtained by dividing the time for one medium rotational period into N time segments and stores it. According to the learning control unit, even if it takes a slightly long time for settlement of the learning result due to a low learning gain, a compensation signal of a steep frictional disturbance of a high band in association with the reversal of the carriage moving direction can be also included in the learning result that is obtained finally. By adding the learning control signal to the feedback control system as a feed-forward compensation signal, the steep frictional disturbance can be removed from the feedback control system. By using the single stage carriage, even if there is a limitation due to the existence of the high-order resonance and the control band is low, the tracking error for the eccentricity of the medium is remarkably reduced and precision of the on-track control can be improved by the learning control.
The learning control unit of the invention is provided between the feedback control unit and the addition point. Assuming that the time for one medium rotational period is set to TL, an unknown driving current function Irepeat(t) (where, 0xe2x89xa6t less than TL;TL denotes one medium rotational period) which is repeated for a period of time from a start time t=0 for one medium rotational period to an end time t=TL is obtained by a learning algorithm as an approximated function I{circumflex over ( )}repeat(t) (where, 0xe2x89xa6t less than TL; TL denotes the one medium rotational period) which is approximately estimated by using a set of heights Ci of N rectangular functions indexed from i=0 to (Nxe2x88x921), obtained by dividing the time TL for one medium rotational period into N intervals and stored. Although the approximated function is expressed by
Îrepeat
it is expressed as xe2x80x9cI{circumflex over ( )}repeatxe2x80x9d in the specification.
The learning control unit comprises a memory, a sampling unit, an approximated function calculating unit, and a feed-forward output unit. The memory has a plurality of memory cells to store the height Ci of each rectangular function of the approximated function I{circumflex over ( )}repeat(t). The sampling unit samples the control signal IFB which is outputted from the feedback control unit. The approximated function calculating unit obtains the height Ci of each rectangular function of the approximated function I{circumflex over ( )}repeat(t) stored in each memory cell of the memory by the following learning law
xe2x80xa2xe2x80xa2i=Klearnxc3x97IFB
where, i denotes the index number of the rectangular function which is decided by time t and 0xe2x89xa6ixe2x89xa6(Nxe2x88x921);
for example, i=floor(t/T),
T denotes a time width per rectangular function, where T=TL/N
on the basis of the control signal IFG sampled by a sampling unit and a predetermined learning gain Klearn and updates the height Ci.
A feed-forward output unit (FF output unit) reads out the height Ci as a learning control signal, of each rectangular function of the approximated function I{circumflex over ( )}repeat(t) stored in the memory cells of the memory synchronously with the divisional period T (time width per rectangular function) of the medium rotation, adds it to the control signal IFB from the feedback control unit, and supplies a driving signal IVCM to the driving unit. Further, the feed-forward output unit reads out the height Ci of each rectangular function of the approximated function I{circumflex over ( )}repeat(t) stored in the memory cells of the memory synchronously with th e medium rotation, adds it to the control signal IFB from the feedback control unit, and supplies the driving signal IVCM to the driving unit.
The feed-forward output unit reads out the value of the approximated function I{circumflex over ( )}repeat(t) stored in each memory cell of the memory corresponding to the time that is advanced by a predetermined time xcex94tlead and outputs it. The learning control unit repeats the learning while feed-forward outputting the learning result at this time point. In this case, there is a time delay such as a phase delay or the like in the feedback control system. Unless it is compensated, the control becomes unstable. Therefore, with respect to the latest learning control result I{circumflex over ( )}repeat at that time point, the value corresponding to the time that is advanced from the present time by the predetermined time xcex94tlead is read out and outputted, so that the learning can be performed in a state where the control system is stable. The learning control unit outputs the approximated function I{circumflex over ( )}repeat(t) obtained by the learning algorithm after the learning synchronously with the medium rotation, thereby performing a feed-forward control. The learning control unit feed-forward controls in such a manner that an operation to obtain an approximated function by the learning algorithm is performed for a specific time at a timing just after the medium was inserted into the apparatus and, at the time of a track-following control after the learning, the obtained approximated function is outputted synchronously with the medium rotation and the repetitive disturbance is removed. In the storage apparatus, when the approximated function is obtained by learning at a specific position in the disk radial direction, for example, at a position near the center region on the disk, in the case where a pickup is sought and moved to another radial direction position and the track-following control is performed, there is such a situation that an error occurs so long as the obtained approximated function is used, so that the approximation is inadequate. For example, when circularity at the inner region of the track on the disk and that at the outer region on the disk are different, when a difference between the phases or amplitudes of the repetitive disturbance in association with the spindle rotation in the inner region and the outer region cannot be ignored, or when a pickup having a single stage structure is used, there is such a situation that magnitudes of friction in the inner region and the outer region differ. Therefore, at the learning control unit of the invention, the getting operation of the approximated function is performed at a plurality of positions in correspondence to the radial direction position of the disk. In the feed-forward mode, the approximated function is selected in accordance with the track address where the pickup is in the on-track state at that time (for example, the approximated function obtained by the learning in the nearest track address is selected) and the feed-forward is performed, so that the high precise track-following control can be realized irrespective of the track address to be in the on-track state. In the case where the getting operation of the approximated function is performed at a plurality of positions as mentioned above, there is hardly difference among the basic waveforms of the approximated functions and differences among the approximated functions are very small. Therefore, when there is approximated function data at another position, an initial value (initial value of the cell corresponding to the height of each rectangular function) of the approximated function data in the approximated function getting operation at the present position is not started from zero but is started by using the approximated function data at another position as an initial value, thereby enabling the learning time to be reduced. Further, the positioner of the storage apparatus has such a single stage structure that an objective lens is focus-controllably mounted on a carriage which is movable in the direction that transverses the tracks on the medium, and both a tracking control for allowing the light beam to trace the track and a seek control for allowing the light beam to be moved to an arbitrary track position are performed only by the movement of the carriage.