Tracking servo control method and device for preventing tremble of object lens during fast search

A tracking servo control method for preventing a tremble of an object lens installed in a pickup in an optical disk drive. A determination is made whether a distance between a target track and a current track where the pickup is presently placed is longer than a first distance. When the distance between the target track and the current track is longer than the first distance, both ends of a tracking coil through which a driving current for moving the object lens flows, are shorted. In the meantime, when the distance between the target track and the current track is shorter than a second distance as the pickup moves to the target track, both ends of the tracking coil are opened. The first distance is equivalent to a distance of 1000 tracks, and the second distance is equivalent to a distance of 100-200 tracks.

CROSS-REFERENCE TO RELATED APPLICATIONS
 This application claims the benefit of Korean Application No. 62132/1997,
 filed Nov. 22, 1997, in the Korean Patent Office, the disclosure of which
 is incorporated herein by reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an optical disk drive, and in particular,
 to a tracking servo control method and device for preventing a tremble of
 an object lens during a fast search mode.
 2. Description of the Related Art
 Compact disks (CD), a kind of an optical disk, are typically used for
 recording audio data such as music. Recently, however, the CDs are widely
 used as an auxiliary storage for text and computer data because of its
 high storage capacity.
 In general, the CDs have a large number of tracks arranged at fine
 intervals of 1.6 .mu.m, wherein the number of the tracks is about twenty
 thousand for recording music of one hour. Therefore, in a microscopic
 view, the CDs may rotate acentrically. From the standpoint of an optical
 pickup, each track with a width of about 0.5 .mu.m rotates (or proceeds)
 sinusoidally. A tracking servo allows the optical pickup to accurately
 search (or seek) the tracks and radiate a laser beam to them. A
 fundamental structure of the tracking servo is illustrated in FIG. 1.
 In FIG. 1, reference numeral 2 denotes an optical disk (i.e., CD),
 reference numeral 4 denotes a disk motor, and reference numeral 6 denotes
 a pickup. As illustrated, the pickup 6 includes an object lens 30, a
 tracking coil 32, a magnet 34, a 1/4-wavelength plate 36, a beam splitter
 38, an optical detector 40 and a laser diode 42. Further, reference
 numeral 8 denotes a feeding motor for feeding the pickup 6, reference
 numeral 10 denotes a tracking error detector 10, reference numeral 12
 denotes a phase compensator, reference numeral 14 denotes a system
 controller, and reference numerals 16 and 22 denote adders. Moreover,
 reference numerals 18 and 24 denote drivers, and reference numeral 20
 denotes a lowpass filter (LPF).
 In operation, the tracking error detector 10 generates a tracking error
 signal according to a beam trace status, and the phase compensator 12
 generates a phase compensation signal according to the tracking error
 signal. The tracking servo corrects the position of a laser beam spot by
 moving the object lens 30 and the pickup body 6 in any direction according
 to the phase compensation signal. After tracking, the pickup 6 follows a
 specific track. For moving the pickup 6, the adder 22 adds a search
 control signal output from the system controller 14 to an output signal of
 the lowpass filter 20 which lowpass-filters the phase compensation signal
 output from the phase compensator 12. The driver 24 then drives the
 feeding motor 8 according to an output signal of the adder 22, to move the
 pickup 6 in any direction. Further, for moving the object lens 30, the
 adder 16 adds the search control signal output from the system controller
 14 to the phase compensation signal output from the phase compensator 12,
 and the driver 18 drives an actuator (comprising the tracking coil 32 and
 the magnet 34) according to an output signal of the adder 16, to move the
 object lens 30.
 FIG. 2 illustrates a schematic block diagram of a conventional tracking
 servo control device for preventing a tremble of the object lens during a
 fast search (or course seek) mode. Referring to FIG. 2, a switch 54 has a
 common node connected to the input of the adder 16, a contact node A
 connected to an output of the phase compensator 12 and a contact node B
 connected to an output of an amplifier 52. In the conventional tracking
 servo control device, the pickup 6 outputs a central-point servo signal
 for positioning the object lens 30 at a stable central point, to prevent a
 tremble of the object lens 30 during the fast search. The central-point
 servo signal is amplified in the amplifier 52 and then applied to the
 contact node B of the switch 54. The phase compensator 12 outputs the
 phase compensation signal according to the tracking error signal from the
 tracking error detector 10 and provides the phase compensation signal to
 the contact node A of the switch 54.
 FIG. 3 illustrates a diagram for explaining the tracking error signal and
 the central-point servo signal, in a heterodyne tracking servo. In FIG. 3,
 reference numeral 60 denotes a photodiode divided into A, B, C and D
 areas, and reference numeral 62 denotes an optical spot. The tracking
 error signal output from the tracking error detector 10 has a value of
 (A+C)-(B+D), and the central-point servo signal output from the pickup 6
 has a value of (A+D)-(B+C). As can be appreciated, the central-point servo
 signal and the tracking error signal have opposite phases relative to each
 other.
 FIG. 4A illustrates a frequency and a servo-on time of the tracking error
 signal in the state where the object lens 30 does not tremble during slow
 search (or fine seek), and FIG. 4B illustrates a frequency and a servo-on
 time of the tracking error signal in the state where the object lens 30
 trembles during the fast search (or course seek). As illustrated, the
 servo-on time during the fast search of FIG. 4B is delayed by a time t as
 compared with the servo-on time during the slow search of FIG. 4A.
 Accordingly, track searching is delayed by the time t.
 To prevent the delay in searching the tracks, the system controller 14
 switches the common node of the switch 54 to the contact node B slightly
 before the servo-on time. The adder 16 then adds the search control signal
 to the central-point servo signal output from the amplifier 52 instead of
 the phase compensation signal output from the phase compensator 12. As a
 result, the driver 18 provides the tracking coil 32 with a driving current
 corresponding to the central-point servo signal. The central-point servo
 signal having the phase opposite to the phase of the tracking error
 signal, stabilizes the object lens 30 which was trembling by a driving
 current corresponding to a tracking signal.
 However, the conventional tracking servo control device is disadvantageous
 in that it cannot use a pickup which does not output the central-point
 servo signal. Further, although the pickup outputs the central-point servo
 signal, the central-point servo signal, the central-point servo signal
 itself may have a deviation, which makes it difficult to accurately
 stabilize the trembling object lens.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide a tracking
 servo control method and device for preventing a tremble of an object lens
 during a fast search mode without regard to a type of a pickup used to
 read information from a compact disk.
 It is another object of the present invention to provide a tracking servo
 control method and device for preventing a tremble of an object lens
 during a fast search mode without using a central-point servo signal.
 Additional objects and advantages of the invention will be set forth in
 part in the description which follows and, in part, will be obvious from
 the description, or may be learned by practice of the invention.
 To achieve the above and other objects of the present invention, there is
 provided a tracking servo control method of preventing a tremble of an
 object lens installed in a pickup in an optical disk drive. In the method,
 it is determined whether a distance between a target track and a current
 track where the pickup is presently placed is longer than a first
 distance. When the distance between the target track and the current track
 is longer than the first distance, both ends of a tracking coil through
 which a driving current for moving the object lens flows, are shorted. In
 the meantime, when the distance between the target track and the current
 track is shorter than a second distance as the pickup moves to the target
 track, both ends of the tracking coil are opened. Here, the first distance
 is equivalent to a distance of 1000 tracks, and the second distance is
 equivalent to a distance of 100-200 tracks.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 A preferred embodiment of the present invention will be described
 hereinbelow with reference to the accompanying drawings. In the following
 description, well known functions or constructions are not described in
 detail sine they would obscure the invention in unnecessary detail.
 FIG. 5 illustrates a tracking servo control device for preventing a tremble
 of the object lens during a fast search mode according to an embodiment of
 the present invention. Many of the elements shown in FIG. 5 are the same
 as the ones shown in FIG. 1. However, referring to FIG. 5, the tracking
 servo control device has a switch device 70 connected between the driver
 18 and the tracking coil 32 through which a driving current flows to move
 the object lens 30. The switch device 70 includes a first switch SW1
 connected between an end 72 of the tracking coil 32 and an output node 74
 of the driver 18, a second switch SW2 connected between another end 76 of
 the tracking coil 32 and another output node 78 of the driver 18, and a
 third switch SW3 connected between both ends 72 and 76 of the tracking
 coil 32.
 In operation, the system controller 14 performs the fast search when a
 target track is distanced by a predetermined number of tracks (e.g., 1000
 tracks) or more, from the current track where the pickup 6 is presently
 placed; otherwise, the system controller 14 performs the slow search.
 During the fast search, the system controller 14 closes (turns on) the
 third switch SW3 and opens (turns off) the first and second switches SW1
 and SW2, in response to a switching control signal SWC output from the
 system controller 14. Accordingly, both lens 72 and 76 of the tracking
 coil 32 for moving the object lens 30 are shorted together. In this
 circumstance, the actuator comprising the tracking coil 32 and the magnet
 34 is inactivated, so that the object lens 30 may not tremble even during
 the fast search. In the meantime, the first and second switches SW1 and
 SW2 are opened with respect to the tracking coil 32 during the fast search
 to protect the driver 18.
 In the meantime, when the current track gets near to the target track by a
 predetermined distance of 100-200 tracks as the pickup 6 moves toward the
 target track, the system controller 14 opens the third switch SW3 and
 closes the first and second switches SW1 and SW2. Then, the actuator
 (comprising the tracking coil 32 and the magnet 34) finely drives the
 object lens 30 to accurately position the object lens 30 at the target
 track. In effect, the slow search is performed at this time.
 As described above, the tracking coil constituting the actuator is shorted
 during the fast search to prevent a tremble of the object lens 30.
 While the invention has been shown and described with reference to a
 certain preferred embodiment thereof, it will be understood by those
 skilled in the art that various changes in form and details may be made
 therein without departing from the spirit and scope of the invention as
 defined by the appended claims.