Disc device having disc in balance correcting arrangements

A disc device which includes a rotary drive mechanism for rotating a replaceable recording medium. An unbalance correcting mechanism adapted to correct unbalance is incorporated in a rotary body rotating unit in the rotary drive mechanism, and includes a holding system having a holding member for holding the rotary drive mechanism and a head having at least a reproducing function. Further included is a support member composed of resilient members, for supporting a casing and a unit holder to each other. The holding system has a natural frequency higher than 30 Hz but lower than 70 Hz, and exhibits a transmission characteristic having a lift-up degree of higher than 8 dB at a resonant point.

BACKGROUND OF THE PRESENT INVENTION
 The present invention relates to a disc device which rotates a disc-like
 replaceable recording medium at a high speed so as to carry out at least
 reproduction of data, such as, a disc removable type disc device for
 CD-ROM, DVD, MO, removable HDD or the like, and in particular to a disc
 device which is effective at a high rotational speed.
 A conventional CD-ROM device is composed of a rotary system for rotating a
 disc, and a pick-up system for reading data from the disc. On the disc,
 spiral or concentric recording pits are formed, onto which laser is
 projected from the pick-up system which is driven radially of the disc,
 and data are read through the reflection thereupon. The radial recording
 pitches of these recording pits are very fine, that is, 1.6 .mu.m, and
 highly precise positioning is required between the disc and the pick-up.
 The following two problems are main factors which cause hindrance to
 positioning accuracy.
 (1) Vibration caused by the spindle drive system and the pick-up drive
 system; and
 (2) External disturbance exerted from the outside.
 The typical those of the problems stated in (1) are unbalance vibration of
 a rotary system, electromagnetic vibration of a motor or drive reaction of
 a pick-up drive system. It is most important to ensure a required degree
 of positioning accuracy under the presence of these factors when designing
 the device. In particular, the unbalance vibration of the rotary system
 have been being materialized since the speed of rotation of a disc has
 been rapidly increased due to requirement of high speed data transfer.
 Accordingly, as disclosed in Japanese Laid-Open Patent No. H3-86968,
 balance correction using fluid has been proposed.
 It has been desired to increase the data transfer speed of the disc device
 due to the materialization of multi-media which concerns a large capacity
 of data of image or motion picture. The higher the rotational speed of a
 disc in a disc device, the higher the data transfer speed. Accordingly,
 these years, the rotational speed of an the disc has been being rapidly
 increased.
 The most serious problem which should be overcome for increasing the
 rotational speed of a disc, is an increase in unbalance vibration.
 Different from a hard disc device, an optical disc device for CD-ROM or
 the like is essential in view of the compatibility of a disc as a
 recording medium used therein. Since the disc is mass-producible by
 pressing, the manufacturing accuracy cannot be enhanced greatly, and
 accordingly, unevenness in thickness and eccentricity between the outer
 periphery and the inner periphery of a disc are large. Further, unbalance
 in weight caused by printing letters and a pattern on the disc is not
 negligible.
 If a system having such a large unbalance is rotated, unbalance vibration
 would occurs due to a cause such that the center of gravity of the rotary
 system does not precisely align with the rotating center thereof. The
 force causing unbalance vibration is exhibited by the following formula
 (1):
EQU F=m.times..epsilon..times..omega..sup.2 (1)
 Where .epsilon. is a distance between the center of gravity and the
 rotating center of the rotary system, m is a mass of a rotary body, and
 .omega. is a rotational speed (rotational frequency).
 As understood from formula (1), the unbalance vibration is proportional to
 the square of the rotational speed, and accordingly, it becomes serious
 rapidly if the rotational speed increases. The unbalance vibration not
 only vibrate the pick-up system so as to hinder the read/write of signals
 and to cause noise, but also vibrates the system itself to which the
 device is attached, thereby remarkably lowering the reliability of
 computer system itself.
 This is because of the higher speed rotation of a disc due to the higher
 speed data transfer. For example, in the case of an 8-X CD-ROM drive, the
 rotational speed is 4,200 r.p.m, and accordingly, the problem of unbalance
 vibration of a disc, as mentioned above, has been being materialized.
 OBJECT AND SUMMARY OF THE INVENTION
 One object of the present invention is to provide a highly reliable disc
 device which incorporates in its drive system composed of a disc and a
 drive system, a mechanism for automatically correcting unbalance of the
 rotary system so as to restrain generation of vibration at a high
 rotational speed in order to carry out high speed data transfer.
 In order to achieve the above-mentioned object, a rotary system having a
 natural frequency .omega.n and a rotational frequency .omega. is set so
 that .omega.n is lower than .omega., that is, .omega.n&lt;.omega., and an
 unbalance correcting mechanism including a mass system which can smoothly
 rotate about a rotary shaft, is provided, thereby it is possible to
 correct unbalance of a rotary body itself through self-aligning action
 inherent to the rotary system during rotation.
 Further, in addition to the above-mentioned unbalance correcting mechanism,
 an intermediate holding member which carries thereon a rotary drive system
 and an optical head is supported to a casing or a unit mechanism base
 through resilient elements (vibration proof legs). It is noted that the
 natural frequency of the support system composed of the intermediate
 member and the resilient elements is set to be higher than 30 Hz but lower
 than 70 Hz, and a lift value of the transmission characteristic thereof is
 set to be higher than 8 dB at a resonance point. Thus, the above-mentioned
 resilient elements are made of silicon group rubber or fluorine group
 rubber, and the intermediate holding member is supported by the resilient
 elements at three or four points around the outer periphery thereof.
 Further, in the unbalance correcting system, a ring-like groove around the
 rotary shaft as a center, is formed in a clamper for fixing a disc, a disk
 holding part (turntable) for fixing a disc, the rotor side surface of a
 motor or the interior of the motor, and more than two spherical correcting
 members are inserted in this groove.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 is an outside view which shows an optical disc (CD-ROM) device in
 which an embodiment of the present invention is incorporated. Explanation
 will be hereinbelow made of basic operation of the optical disc device.
 At first, a disc tray 31 for loading a disc 2 into the disc device (or
 unloading the disc 2 from the disc device) is extended from an entrance
 slot formed in a front panel 1. The disc tray 31 is extended and retracted
 by a disc loading mechanism which is not shown. Then, the disc is set on
 the extended disc tray 31. Thereafter, the disc tray 31 is retracted into
 the disc device by means of the loading mechanism, and then, the disc 2 is
 shifted onto a turntable 15 as a disc carrying part provided to the shaft
 of a motor (spindle motor). The disc 2 carried on the turntable 15 is held
 between a disc clamper 3 attached to a clamper holder 4 and the turntable
 under magnetic attraction effected by the disc clamper 3.
 It is noted that a rotary drive mechanism such as the spindle motor, an
 optical head for reading and writing data from and onto the disc, and a
 drive mechanism for the latter are provided on a unit mechanical chassis
 11 serving as an intermediate holding member.
 Next, the disc 2 is rotated at a predetermined rotational speed by the
 spindle motor 14, and data are written onto or read from the disc 2 by the
 optical head which is not shown, carried on the unit mechanical chassis 11
 located below the disc 2. The optical head incorporates an objective lens
 drive mechanism and a shift means by which the optical head can be moved
 radially of the disc 2. The unit mechanism chassis 11 is supported on a
 unit mechanical holder 12 secured to a unit mechanical base 18 through the
 intermediary of vibration proof legs 13a, 13b, 13c formed of resilient
 elements. Alternatively, it is supported directly to the base of a casing
 (unit mechanical housing) through the intermediary of the vibration proof
 legs without the unit mechanical holder 12 intervening therebetween.
 According to the present invention, an unbalance correcting mechanism for
 automatically correcting unbalance of the disc 2 is provided in the
 optical disc device in order to enhance the vibration-proof function of
 the optical disc device. Further, the natural frequency of a vibration
 system including the above-mentioned vibration proof legs 13 is set to a
 predetermined value.
 The disc 2 used in a CD-ROM device or a DVD device is a removable. Since
 the disc 2 is mass-produced by pressing or the like, the manufacturing
 accuracy thereof cannot be made to be so high, that is, the unevenness of
 the thickness of the disc 2, and the unevenness of concentricity between
 the outer and inner peripheries of the disc are relative large. Unbalance
 of the disc 2 caused by letters and a pattern printed on the disc 2 and
 caused by a sorting label or the like stuck to the disc 2 by the user is
 not negligible. That is, the disc 2 itself is unbalanced in weight by
 usually 1 gcm at maximum. Thus, if such a disc 2 is rotated at a high
 speed, unbalanced load which is greatly increased by the rotation of the
 disc 2 is exerted to the spindle motor 14 for carrying the disc 2.
 Vibration of rotational primary component of the disc 2 caused by the
 unbalance load is transmitted to the unit mechanical chassis through the
 intermediary of the spindle motor 14, resulting in the vibration of the
 device itself and interference contact between the components or the like,
 which are caused by the transmitted vibration, and accordingly, noise is
 produced.
 Next, the principle of the unbalance correcting mechanism will be
 explained, referring to FIGS. 6 and 7.
 FIGS. 6A to 6C are schematic views which show a motor and a disc attached
 to the motor, in which reference numeral O denotes the rotational center
 of a bearing, S denotes the rotational center of the disc at the surface
 thereof, and G denotes the center of gravity of the disc. In an ideal
 rotary body with no unbalance, O, S and G are completely coincident with
 one another as shown in FIG. 6A, but in an unbalanced rotary body, the
 shaft is drawn outward by the centrifugal force of the unbalance, causing
 deflection or deformation of the shaft.
 The phases of the points S, G, relative to the point O, vary, depending
 upon the relationship between the natural frequency .omega.n and the
 rotational frequency .omega. of the rotary system. This condition is shown
 in FIGS. 6B and 6C. If .omega.n is greater than .omega. (FIG. 6B), the
 point G is positioned outside of the point S. If .omega.n is equal to
 .omega. (critical), the points S, G are positioned on one and the same
 straight line. If .omega. is greater than .omega.n (FIG. 6C,
 overcritical), this relationship is reversed so that the point S is
 positioned outside of the point G. In order to correct the unbalancing,
 this overcritical condition is used.
 FIG. 7 is a schematic view which shows the relationship among unbalance
 correcting members 7a, 7b (spherical bodies) provided in the rotary body,
 and the rotational center, the center of gravity and the like.
 Two correcting members 7a, are provided so as to smoothly rotationally move
 in such a way that their centers move depict a locus S. It is estimated
 that the correcting members 7a, are secured in a condition as shown in
 FIG. 7A, until the overcritical condition occurs. The rotational speed
 increases to a constant rotational speed in the overcritical condition. In
 this condition, the positions of centrifugal forces F1, F2 exerted to the
 correcting members 7a, 7b and the points O, S, G are shown in FIG. 7A in
 which the center G of gravity is that of the entire rotary system
 including the correcting members 7a, 7b. In this condition, if the fixing
 of the correcting members 7a, 7b is released, the correcting members 7a,
 7b are shifted from the condition shown in FIG. 7A into a condition shown
 in FIG. 7B by the circumferential components of the centrifugal forces F1,
 F2. The shifting of the correcting members 7a, 7b causes the center G of
 gravity to be shifted in an upward direction as viewed in the figure, that
 is, the direction toward S, and accordingly, the difference between the
 points S, G which approach each other becomes less. That is, the points S,
 G, O are coincident with one another in a direction in which the
 unbalancing is decreased due to the shifting of the correcting members 7a,
 7b. That is, it continues until a condition in which the unbalancing is
 completely eliminated, occurs. Thus, the unbalance of the rotary system
 which exists in the initial stage can be automatically corrected by the
 correcting members 7a, 7b.
 Due to the provision of the above-mentioned automatic unbalance correcting
 mechanism for the rotary body, a disc 2 having a large unbalance can be
 rotated at a high speed without generation of vibration, thereby it is
 possible to transmit data at a high speed with no deterioration of
 reliability.
 FIG. 2 shows the external appearance of the unit mechanical part of the
 disc device (FIG. 1) incorporating the above-mentioned unbalance
 correcting mechanism.
 In this embodiment, the unbalance correcting mechanism has a such an
 arrangement that a plurality of correcting members (balance balls) 7a, 7b
 are provided in a groove 17 formed in the disc carrying part (turn table)
 of the spindle motor 14. The optical head 8 incorporating the objective
 lens can be moved radially of the disc along a guide rail 16. The unit
 mechanical chassis 11 having a substantially rectangular shape in ths
 embodiment is attached to the unit mechanical holder 12 by means of the
 vibration proof legs 13a. 13b, 13c and 13d.
 Next, a specific embodiment of the automatic balance correcting mechanism
 will be explained.
 FIG. 3 is a sectional view which shows a CD-ROM device provided with the
 unbalance correcting mechanism on the disc clamper 3 side.
 The disc 2 as a recording medium, is inserted into the device from the
 front panel 1 side by means of a tray of a loading device. At this time, a
 unit mechanical system composed of the spindle motor 14, the pick-up 9 and
 the drive system therefor, is retracted in order to hinder the loading of
 the disc 2. When the disc 2 is completely inserted into the device, the
 unit mechanical system initiates its movement, and when the height of the
 turntable 15 attached to the spindle motor 14 becomes equal to the height
 of the disc 2, it stops the movement. Simultaneously, the disc 2 is
 clamped by attracting forces of a magnet 27 on the turntable 15 and a
 magnetic member 6 provided to the clamper 3, and is secured by the clamp
 holder 4 and the clamp retainer 5. Thus, the disc 2 can be rotated by the
 spindle motor 14, and simultaneously, optical data on the disc 2 can be
 read by the optical head 8 attached to the unit mechanical system. It is
 noted that the unit mechanical system is incorporated in a unit mechanical
 housing 10, and is covered with a top cover 9.
 The unit mechanical base 18 is supported to the unit mechanical chassis 11
 by means of vibration proof legs 13 made of soft rubber or formed of
 springs. The purpose of the provision of the vibration proof legs 13 is to
 prevent unbalancing oscillation of the disc 2 from being transmitted
 outside of the device, to prevent vibration outside of the device from
 being transmitted to the unit mechanical base 18, and to effectively
 operate the unbalance correcting mechanism.
 FIG. 5 shows an example of the unbalance correcting mechanism. In this
 example, two rolling grooves 17a, 17b are formed, each incorporating
 therein with a correcting member 1. However, it is natural that a single
 groove can incorporate two correcting members. The explanation will be
 hereinbelow made with such an estimation that the single groove is used.
 The first essential feature of the present invention is the provision of an
 unbalance correcting mechanism integrally with the above-mentioned clamper
 3. This unbalance correcting mechanism is composed of a ball guide having
 an arcuate rolling groove 17, and two correcting members (balance balls)
 7a, 7b used in the groove 17.
 The rolling groove 17 which is formed in the ball guide, extending
 circumferentially of the latter, has a width which is slightly larger than
 the diameter of the correcting members 7a, 7b, and is inserted therein
 with the two correcting members 7a, 7b. The side surfaces and the bottom
 surface of the rolling groove 17, and as well the outer surface of the
 correcting members 7a, 7b are finished so as to be smooth in order to
 allow the correcting members 7a, 7b to freely move in the rolling groove
 17.
 The material of the correcting members 7 should be selected so that it is
 nonmagnetic, and has a high density, and it can hardly cause occurrence of
 aging effect such as occurrence of abrasion or corrosion at the outer
 surfaces of the correcting members. If it is magnetic, it is magnetized by
 a clamping magnet 27 provided in the clamper 3 so as to cause such a
 problem that the two correcting members 7a, 7b are attracted. If it has a
 low density, the degree of unbalance correction becomes small. The aging
 effect of the outer surface increases the frictional force which hinders
 the movement of the correcting members 7a, 7b. In view of these problems,
 the correcting members are suitably made of nonmagnetic stainless steel or
 glass.
 The maximum degree of unbalance which can be corrected is exhibited by the
 following formula in such a condition that the two balls assemble together
 in an unit body:
EQU W=2.times.mb.times.r
 Where mb is a weight of each of the correcting members 7a, 7b, r is the
 rolling radius of the same. The maximum degree w of unbalance of the disc
 2 can be considered by experience to be 1 gr.cm at maximum. If the rolling
 radius is r=1.75 cm which is substantially equal to the outer diameter of
 the clamper 3, a required weight md of each of the correcting members 7
 can be determined from the following formula;
EQU mb=w/(2.times.r)=1/(2.times.1.75)=0.28 gr
 The weight of a stainless balls having a diameter of 4 mm is 0.27 gr, and
 accordingly, the this ball can be used to cope with the above-mentioned
 maximum of unbalance of the disc 2.
 The second essential feature of this embodiment is the provision of such an
 arrangement that the natural frequency .omega.n which is exhibited by the
 following formula;
EQU .omega.n= (m/K)
 where K is a rigidity of the above-mentioned vibration proof legs 13 and M
 is a mass of the above-mentioned unit mechanical base 18, is set to be
 smaller than the rated rotational speed .omega. of the device, and the
 device is operated in an overcritical condition. Operation is made with
 the unbalance correcting mechanism having a configuration as mentioned
 above, and through the overcritical condition, the correcting members 7
 are shifted in a direction reverse to that of the unbalance of the disc 2
 since the phases of the center of gravity and the rotational center are
 reversed, and accordingly, the unbalance can be automatically corrected.
 In the case of a CD-ROM device, the rotational speed of the spindle motor
 14 is controlled so that the peripheral speed is constant. That is, when
 the pick-up 8 is located at the inner peripheral side, the rotational
 speed is high, but when it is located at the outer peripheral side, the
 rotational speed is low. The rotational speed at which an audio CD is
 reproduced, is called as a standard speed which is in a range between 3.8
 Hz at its outer periphery and 8.3 Hz at its inner periphery. Due to the
 demand of increasing the speed of data transmission speed, these days, the
 rotational speed of the CD-ROM device is planed to be 12 times as high as
 the standard speed (that is, 45 Hz at the outer periphery and 100 Hz at
 the inner periphery). The natural frequency .omega.n is set to be higher
 than the rotational frequency of the standard speed at the inner periphery
 but is lower than the rotational frequency of the used speed at the outer
 periphery. If the natural frequency .omega.n is set to a value lower than
 the standard rotational speed, the rigidity of the vibration proof legs 13
 has to be extremely small, and accordingly, the vibration proof legs 13
 would be greatly deformed when an external force is exerted thereto. Thus,
 it is anticipated that the disc 2 or the like cannot be stably supported,
 and accordingly, it is not practical. On the other hand, a rotational
 speed .omega. substantially equal to the standard speed does not cause a
 problem even though the unbalance vibration is not corrected since the
 unbalance vibration is small.
 Attention has to be taken for the damping rate of the vibration when the
 natural frequency .omega.n is set. In view of the vibration-proof, it is
 effective to use a material having a large damping rate in order to
 decrease the amplification factor of vibration peak. However, in this
 embodiment, the vibration proof legs 13 made of such a material having a
 high damping rate causes a problem. Since the material having a high
 damping rate delays the reversing of the phase. As mentioned above, since
 the reversing of the phases of the rotational center and the center of
 gravity during overcritical operation is used in the unbalance correcting
 mechanism according to the present invention, the phases cannot be
 completely reversed if a the material having a high damping rate is used,
 and accordingly, the effect of the correction is low. In the case of a
 damping rate which is 25%, even though .omega.n is set to one-third (1/3+L
 ) of .omega., the phase does not shift, exceeding an angle of 160 deg. in
 this condition, the effect of correction for unbalance is low. In this
 embodiment, the damping rate is set to 5% while .omega.n is set to 1/2+L
 of .omega., the reversing of the phases of about an angle of about 180
 deg. can be obtained, thereby sufficient unbalance correction can be
 expected.
 FIG. 4 is an embodiment in which the unbalance correcting mechanism is
 provided in the disc clamp 3 part.
 The essential feature of this embodiment is the provision of a first clamp
 19 on which the unbalance correcting mechanism composed of the correcting
 members 7 is provided, and a second clamp 20 incorporating a ball fixing
 mechanism which can be vertically moved, coaxial with the first clamp 19
 is provided. In the case of a CD-ROM device, since the rotation of the
 disc 2 is made in such a condition that the peripheral speed is constant,
 the spindle motor 14 is accelerated and decelerated in accordance with a
 position of the pick-up 8 in order to change the rotational speed. In the
 case of no fixing mechanism for the correcting members 7, since the
 correcting members 7 would turn at a constant rotational speed upon such
 acceleration and deceleration, relative rotation occurs between the
 correcting members 7 and the disc 2. With a design such that full
 attention is made to the ranges of both natural frequency on and
 rotational speed .omega., the correcting members 7 can be finally
 stabilized to their original correcting positions even thought the
 rotational speed .omega. varies. However, since the frictional force
 between the rolling surface of the rolling groove 17 and the correcting
 members 7 is set to be low in order to reduce the affection of the
 friction exerted to the correcting members 7, a time is required more or
 less until the correcting members are stabilized. If the period from the
 time when the rolling of the correcting members 7 is initiated due to
 acceleration and deceleration to the time when it is stabilized, exceeds a
 seek time by which the pick-up 8 moves, data has to be read in an unstable
 vibration condition, causing deterioration of the reliability of the
 device itself. Thus, the fixing mechanism for the correcting members 7 are
 provided so that the correction is once made, and in this condition, the
 correcting members 7 are fixed in this condition, thereby it is possible
 to solve the above-mentioned problems.
 The second clamp 20 is assembled in such a way that a protrusion 23 of the
 second clamp 20 is snugly fitted in a hollow shaft 22 of the first clamp
 19, which extends from the first clamp 19, which is hollow and which is
 formed on its outside with a first clamp groove 24 so that the second
 clamp 20 can be rotated simultaneously with the rotation of the first
 clamp 19 while it can be vertically moved. Further, a spring 26 is
 provided between the first clamp 19 and the second clamp 20 so that the
 first clamp 19 and the second clamp 20 are held to be opened if no
 vertical force is exerted thereto. The second clamp 20 is formed therein
 with several teeth 21 having a width which is narrower than the rolling
 groove 17, at a position just facing the rolling groove 17 for the
 correcting members 7, formed in the first camp 19, the several teeth 21
 being adapted to enter the ball rolling groove 17 of the first clamp 19
 over the entire periphery thereof when the second clamp 20 lowers. The
 pitches of these teeth 21 is slightly larger than the diameter of the
 correcting member 7, but are equally divided circumferentially. The height
 of the teeth 21 is set so that the gap between the bottoms of the rolling
 members 17 and the tip ends of the teeth 21 becomes smaller than the
 diameter of the correcting members 17 in such a condition that the first
 clamp 19 and the second clamp 20 are closed.
 In such a condition that the first clamp 19 and the second clamp 20 are
 opened, the correcting members 7 can freely be moved in the rolling groove
 17, but when they are closed, the correcting members 7 are clamped between
 the teeth 21 of the second clamp 20 so that they cannot be moved. In such
 a condition that the first clamp 19 and the second clamp 20 are opened,
 unbalance correction is carried out under overcritical condition, and
 after the correcting members 7 become stable, when the first and second
 clamps 19, 20 are closed, the corrected condition of the correcting
 members 7 can be held. The opening and closing of the first and second
 clamps 19, 20 are carried out through the vertical motion of the unit
 mechanical base 18 upon ejection of the disc 2. In a conventional device,
 upon insertion of the disc 2, the unit mechanical base 18 is retracted
 downward in order to prevent the spindle 14 from hindering the disc 2, and
 after the completion of insertion of the disc 2, the unit mechanical base
 18 is raised up to an operating height so as to clamp the disc 2 with the
 use of the attracting forces of the magnet 27 provided in the spindle
 motor 14 and the magnetic member 6 provided to the clamper. This operation
 is carried out by gear and cam mechanisms and a motor provided in the
 chassis 11 and the unit mechanical base 18. However, the essential feature
 of this embodiment in which the unit mechanical base 18 is raised upward
 from the retracted condition, and the first clamp 19 is attracted by the
 magnet 29 of the spindle motor 14 so as to secure the disc 2 which can be
 therefore rotated, is the provision of such an arrangement that there are
 established a condition in which the first clamp 19 and the second clamp
 20 are opened so that the correcting members 7 can be moved freely (which
 will be hereinbelow mentioned as correcting mode"), and a condition in
 which the unit mechanical base 18 is further moved so as to close the
 first clamp 19 and the second clamp 20 in order to secure the correcting
 members 7 (which will be mentioned as rated mode", FIG. 5B).
 Explanation will be hereinbelow made of the operation. When the disc 2 is
 inserted, the unit mechanical base 18 takes the lowest position. As the
 disc 2 is inserted, the unit mechanical base 18 is moved upward, and comes
 to a stop once in the correcting mode. In this condition, the first clamp
 19 is operated so as to clamp the disc 2. In this phase, the first clamp
 19 and the second clamp 20 are held to be opened, as shown in FIG. 4a, and
 accordingly, the correcting members 7 can freely move in the rolling
 groove 17. The positional relationship between the pick-up 8 and the disc
 2 has been established. After starting of the rotation of the disc 2 by
 the spindle motor 14, the disc 2 is rotated at a constant rotational speed
 in an overcritical condition, exceeding the natural frequency .omega.n of
 the rotary system. This rotational speed is determined in consideration
 with the damping characteristic of the above-mentioned vibration proof
 legs 13, but can be determined, irrespective of the rated rotational speed
 of the device itself. At the time when the correcting members 7 become
 stable at their correcting positions, the unit mechanical base 18 is
 further moved upward while the rotational speed being held, and comes to a
 stop in the rated mode. The first clamp 19 and the second clamp 20 are
 gradually closed through the movement of the unit mechanism 18, and
 accordingly, the correcting members 7 are secured by the teeth 21 formed
 in the second clamp 20 in the rated mode, at the correcting positions. By
 securing the correcting members 7 at the positions where the balance
 correction is made, the correcting members 7 are not moved even though the
 rotational speed is changed in association with a position of the pick-up
 8 in order to make the line speed constant, and accordingly, it is
 possible to prevent deterioration of the seek performance.
 In this embodiment, with the use of a vibration sensor such as an
 acceleration sensor, the manipulatability can be improved. For example, a
 vibration sensor is attached to the unit mechanical base 18. Explanation
 will be made of the operation of this embodiment in the case of the
 provision of the vibration sensor. When the disc 2 is inserted, the unit
 mechanical base 18 is moved to a rated position, and the correcting
 members 7 are secured at arbitrary positions. Thereafter, the rotation of
 the motor is started. In this condition, the measurement of vibration is
 carried out. If it is determined that the level of vibration is low, the
 data are read soon, but it is determined that the level of vibration is
 high, the unit mechanical base 18 is lowered down to the correcting mode
 position. Thereafter, the unbalance correction is carried outer in a
 procedure similar to that made in the case of no provision of the
 vibration sensor. In the case of the presence of the vibration sensor, no
 unbalance correction is required for every disc 2, and accordingly, it is
 possible to promote the operation steps from the insertion of the disc 2
 to the stating of the reading of data.
 Next, explanation will be hereinbelow made of the arrangement in which two
 rolling grooves 17a, 17b are formed, and the correcting members 7a, 7b are
 incorporated in these grooves, respectively.
 Should a plurality of correcting members 7 are incorporated in a single
 groove 17, the correcting members 7 impinge upon each other during
 unbalance correction, and cause occurrence of an unstable phenomenon.
 Further, in the case of using magnetic correcting members 7, since it is
 magnetized, the correcting members 7 are attracted to each other. Thus, in
 this embodiment, the plurality of rolling grooves 17 are formed, and the
 correcting members 7 are incorporated in these rolling grooves 17,
 respectively. Thereby it is possible to enhance the reliability of the
 balance correction.
 Next, explanation will be made of an arrangement in which the unbalance
 correcting mechanism is provided to the turntable of the spindle motor 14
 with reference to FIG. 8.
 A disc rotating system for an optical disc, is composed of a spindle motor
 14, a disc 2 serving as a recording medium, a disc clamper 3 for securing
 the disc 2 onto the turntable 15, a rubber element 39 for preventing the
 disc from slipping, and the like. The spindle motor is composed of the
 turntable 15 for carrying the disc 2, a shaft 37 press-fitted in the
 turntable 15, and a rotor 36 press-fitted onto the shaft 37. A magnetic
 circuit for generating a rotary drive power for the spindle motor 14 is
 formed in the rotor 36, and the rotor 36 serving as a rotary part is
 attached thereto with a cylindrical magnet multi-polarized. In the rotor
 36, a core serving as a stationary part, and a coil are attached to a
 stator substrate 38.
 The shaft 37 is rotatably supported to the stationary side by means of a
 bearing 43 such as a ball bearing or a slide bearing. Further, the
 stationary part is secured onto the stator substrate 38 to which a spindle
 motor control substrate is applied. Detailed explanation will be made of
 the turntable 15 incorporating the unbalance correcting mechanism as shown
 in the figure.
 The turntable 15 is formed therein with a ring-like groove 17 in which a
 disc securing magnet 41 is provided. A disc 2 carried on the turntable 15
 is clamped by the disc clamper 3 from above so that the disc 2 is secured.
 This is caused by the magnetic attracting force applied by an attracting
 iron plate 34 in the disc clamper 3, facing the disc securing magnet 41. A
 ring-like groove 17 (rolling groove) is also formed in the turntable 15
 outside of the former ring-like groove 17, and is incorporated therein
 with correcting members (balance balls) 7 for correcting unbalance. The
 groove 17 in which the correcting members 7 are incorporated, is
 completely covered with a dust protecting cover 35 for preventing dust
 from entering therein from the outside.
 As mentioned above, unbalance vibration caused by unbalance of the disc 2
 is exerted radially of the disc due to deflection and rotation of the
 shaft. If the unbalance correcting mechanism is incorporated in the
 turntable 15, the above-mentioned correcting members 7 also centrifugally
 act upon the rotary shaft. These two forces can be effected substantially
 in the same plane so as to correct unbalance without exerting an angular
 moment to the shaft 3. Further, the ring-like groove 17 formed in the
 turntable 15 is completely fixed to the shaft 37 as a rotary shaft, and
 accordingly, it can be easily formed, concentric with the rotary shaft,
 thereby it is possible to reduce the rotational eccentricity of the
 ring-like groove 17.
 Next, explanation will be made of the operation of the correcting members 7
 with reference to FIG. 10.
 FIG. 10 shows the configuration of the correcting members 7 set in the
 ring-like groove 17 formed in the turntable 15.
 The correcting members 7 are used by at least two in the unbalance
 correcting mechanism in the present invention. Further, each of the
 correcting members 7 is formed of a nonmagnetic true-circular rigid ball
 in order to prevent affection by a magnetic field. For example, in the
 case of the presence of disc unbalance in the lower part of the figure, a
 force caused by this disc unbalance is effected in a centrifugal direction
 (indicated by the arrow). On the contrary, in an ideal corrected
 condition, the correcting members 7 are located in the direction opposite
 to the direction of the disc unbalance. The unbalance correcting force at
 this time is effected in the direction of a component of a centrifugal
 force exerted to the correcting members 7, opposite to the direction of
 the unbalance. In the case of the presence of a plurality of the
 correcting members 7, the sum of the components of centrifugal forces
 exerted to the correcting members 7 gives an unbalance correcting force.
 Accordingly, the correcting members 7 located in the ring-like groove 17
 make contact with the pathway surface to which they cling under a
 centrifugal force while they are arranged, adjacent to one another, and in
 this condition, the positions of correcting members 7a, 7b at opposite
 ends of them, are located in directions which are directed from the
 rotational center, right angles to the disc unbalance so that the
 components of the centrifugal force becomes zero. That is, in the case of
 the arrangement of the plurality of correcting members in the correcting
 direction, the shape and the number of the correcting members should be
 determined in such a way that the correcting members at the opposite ends
 are set within an angular range of less than 180 deg. In this embodiment,
 estimating that the degree of unbalance is 1 gr.cm in the worst case in
 view of a degree of a disc unbalance in a CD-ROM device in general,
 stainless rigid balls having a diameter of about 2 to 3 mm are used by a
 number of 8 to 15 as the correcting members 7 which can be set in a groove
 17 having a shape that can be formed in the turntable 15 for a spindle
 motor available at present (the outer diameter of the balance ball pathway
 is about 25 mm). The effect of correction by these correcting members is
 0.5 to 0.8 gr.cm, and accordingly, even in the case of the disc unbalance
 of 1 gr.cm, vibration can be restrained below a degree corresponding to an
 unbalance load of 0.5 to 0.2 gr.cm. Further, by making the correcting
 members from a material having a heavy density and by using a correcting
 pathway having a large diameter, the correcting force can be further
 increased. Although the explanation has been made such that spherical
 bodies such as the balance balls are used as the correcting members 7 for
 correction of unbalance, it goes without saying that not only the
 spherical bodies but also those which can move through the correcting
 grooves 17 with a small frictional force, or fluid may be also used.
 Further, in such a case that the frequency of replacement of discs is less,
 the correcting members may be secured after correction of unbalance,
 similar to the arrangement in which the unbalance correcting mechanism is
 incorporated in the clamp part, as mentioned above.
 In the device in this embodiment, the unit mechanical chassis 11 shown in
 FIG. 2, is supported to the unit mechanical holder 12 by means of four
 vibration proof legs 13a, 13b, 13c, 13d. The correcting members 7 is
 shifted toward the disc unbalance side in a range of .omega.n&gt;.omega.
 where .omega.n is the natural frequency of this support system, and
 .omega. the rotational frequency of the spindle motor at a start of
 rotation, that is, before the rotational frequency exceeds the natural
 frequency. The support system is a primary vibration system, and
 accordingly, the phase is inverted by an angle of 180 deg. when it exceeds
 an eigenvalue. Thus, if the rotational frequency .omega. exceeds the
 natural frequency .omega.n, a force in a direction opposite to the
 direction of unbalance acts upon the correcting members 7. Accordingly,
 when the spindle motor is rotated at a rotational speed in a range of
 .omega.n&lt;.omega., the correcting members are 7 shifted in a direction of
 correction of unbalance. For the unbalance correction in this embodiment
 using this principle, the setting of a natural frequency .omega.n of the
 support system, in particular, the vibration proof legs 13, the way of the
 direction of the shift of the phase in a frequency range exceeding the
 natural frequency .omega.n, and so forth are important.
 FIG. 9 shows a vibration transmitting characteristic of the vibration proof
 leg 13 as mentioned above. The correcting members 7 located in the
 ring-like groove 17 shown in FIG. 8 , act in the direction of unbalance as
 shown in a range in which the rotational frequency .omega. of the spindle
 motor is less than the natural frequency .omega.n exhibited by the
 vibration proof legs 13, due to a centrifugal force given by a disc
 unbalance. If it exceeds the natural frequency .omega.n, the phase is
 inverted, exceeding an angle of 180 deg, the above-mentioned centrifugal
 force acts in a direction opposite to the direction of unbalance so that
 the correcting members 7 act in the direction of correction of unbalance.
 Next, explanation will be made of the setting of the natural frequency. As
 mentioned as to the action of the unbalance correcting mechanism, the
 natural frequency .omega.n of the support system should be set to be lower
 than the rotational frequency .omega.n of a disc. An objective lens
 actuator used in a CD-ROM device, a DVD device or the like, has two axial
 freedoms in order to allow its objective lens to follow a surface
 deflection (surface oscillation) or eccentric motion of the disc.
 Specifically, it is in general supported by a suspension having freedoms
 in the above-mentioned directions.
 The natural frequency .omega.n of the suspension system is set to about 30
 Hz for determining a d.c. sensibility of the actuator in order to follow
 rotational primary components of surface deflection of .+-.500 .mu.m and
 an eccentricity of .+-.70 .mu.m. Further, in order to carry out the
 correction of unbalance at the rotational speed higher than that of a 8x
 CD-ROM device which has remarkably exhibited unbalance frequency these
 days, the rotational speed of 8x is set to be lower than 70 Hz.
 Next, explanation will be made of the damping characteristic of the support
 system. In the case of making the vibration proof legs 13 of a material
 having a high degree of damping, the phase rotation is slow in a condition
 in which the rotational frequency exceeds the natural frequency .omega.n
 of the support system, due to the damping effect, and accordingly, a
 characteristic indicted by the chain line is exhibited. That is, the
 correcting members 7 are unstable around ideal positions by a degree
 corresponding to that the phase cannot be shifted toward the unbalance
 direction, that is, a sufficient effect of correction cannot be obtained.
 Accordingly, in the correction of unbalance using the correcting members
 7, the support by resilient members having a damping which is less than
 that of conventional support legs is preferable. With the resilient member
 having a small damping, when the rotational frequency exceeds the natural
 frequency .omega.n, the phase shift is fast in comparison with vibration
 proof legs 13 having a large damping, and accordingly, the correcting
 members 7 are shifted to positions which are substantially ideal.
 Accordingly, the explanation will be made of the characteristics of the
 vibration proof legs 13 which are required for the correction of unbalance
 according to the present invention, with reference to FIG. 11, as to a
 vibration ratio (rotational frequency .omega./natural frequency .omega.n),
 an amplitude amplification factor and a phase characteristic.
 For example, estimation is made such that the natural frequency of the
 support system given by the vibration proof legs 13, is 30 Hz, and the
 disc rotational speed of 5,500 revolution per minute (about 90 Hz)
 corresponding to that of a 10x CD-ROM. The vibration ratio should be three
 times, 90/30=3, and the damping ratio .zeta. should be less than 0.2 in
 view of a phase shift of an angle larger than 170 deg. The amplitude
 amplifcation factor is about 2.5 when the damping ratio .zeta. is 0.2.
 Accordingly, in the above-mentioned-condition, the degree of lift-up of
 the characteristic at the resonant point of the vibration proof legs 13
 should be set to be greater than 2.5 times (8 dB).
 FIG. 12 shows transmission ratios of materials of the resilient members
 used for the specific vibration proof legs at a resonant point. In
 general, butyl group rubber is used as the material of the resilient
 members for the vibration proof legs 13 in a CD-ROM drive or a DVD drive.
 As mentioned above, this is because of using the vibration legs having a
 high damping rate for preventing exciting force from being transmitted
 from the exterior to the interior. As to the vibration proof legs 13 made
 of butyl group rubber having a rubber hardness of 30 degrees, and
 incorporated in, for example, an actual device, the amplitude
 amplification factor (transmission rate) is about 2 times at the resonant
 point as indicated by the solid line in the figure. In this case,
 estimating that the damping ratio .zeta. is about 0.25 and the vibration
 ratio is 3 times, the phase is not shifted, exceeding an angle of about
 160 deg. and accordingly, ideal correction of balance cannot be made by
 the correcting members 7.
 On the contrary, in the case of using silicon group rubber having a
 hardness of about 20 degrees, the amplitude amplification factor is about
 7 times (when damping rate is obtained in the bandwise direction from the
 characteristics shown in FIG. 11, it becomes .zeta.=0.06). Further, in the
 case of using silicon group rubber having a hardness of 30 degrees, the
 amplification factor is about 20 times (when the damping rate is obtained
 from the characteristics shown in FIG. 11, it becomes .zeta.=0.03) as
 indicated by the one-dot chain line. As mentioned above, with the use of
 the vibration proof legs 13 made of silicon group rubber, the damping rate
 .zeta. can be easily reduced from 0.06 to 0.03, and accordingly, if the
 vibration proof legs 13 made of silicon group rubber is used, the phase is
 shifted by an angle of about 180 deg. under the same condition as
 mentioned above, the correcting members 7 are shifted to ideal correcting
 positions. With the vibration proof legs 13 having a usual shape, butyl
 group rubber can hardly increase the amplitude amplification factor up to
 about 2.5 times at a resonant point, and accordingly, it is found that
 silicon group rubber is preferable. Further, the silicon group rubber is
 excellent in its temperature characteristic, and accordingly, it is
 possible to provide a disc device having a high degree of reliability. It
 is noted that, in addition to the silicon group rubber, fluoro rubber
 having a small damping rate may be used in order to obtain similar
 effects.
 FIG. 13 shows a sectional shape of the vibration proof legs in an
 embodiment. The vibration proof legs 13 has a structure in which a
 shoulder is formed in the center part thereof. Thus, the shoulder of the
 vibration proof leg 13 is supported by the unit mechanical chassis 11, and
 then the vibration proof leg 13 is fastened onto a boss part 44 provided
 on the unit mechanical holder 12 by a retaining screw 40. A gap is defined
 between the retaining screw 40 and the inner surface side of the vibration
 proof leg 13. With this arrangement, the unit mechanical chassis 11 and
 the unit mechanical holder 12 are resiliently supported by the vibration
 proof leg 13. By using silicon group rubber or fluoro rubber as a material
 of the vibration proof leg 13, the unbalance correcting mechanism as
 mentioned above, is operated so as to effect the correction of unbalance
 with a high degree of accuracy.
 As mentioned above, with the use of the arrangement in which the unbalance
 correcting mechanism is incorporated in the turntable 15, the correction
 of unbalance can be made without great improvement in the drive mechanism.
 Further, since the groove 17 is formed in the turntable 15, the grooving
 process can be carried out so as to easily form a true-circular groove 17
 with no eccentricity with respect to the rotational center. Since
 nonmagnetic rigid balls are used for the correcting members 7, the degree
 of correction is given by the sum of mass systems of the correcting
 members 7, thus it is possible to obtain a large degree of correction.
 Further, since the correcting members 7 are nonmagnetic, the correcting
 members 7 can be prevented from being magnetically attracted, and
 accordingly, the reliability of rolling thereof is high. Further, since
 silicon group rubber or fluoro rubber having a low damping, is used, the
 calculation of an eigenvalue of the support system is easy, and the
 damping effect in a high frequency range is great. Further, by using
 silicon group rubber or fluoro rubber, the temperature characteristic is
 stable, and further, the aging effect is low, thereby it is possible to
 enhance the use life and the reliability.
 FIG. 14 shows another embodiment of the unbalance correcting mechanism. In
 this embodiment, it is provided in the disc clamper 3, similar to the
 embodiment shown in FIG. 3.
 This embodiment is the same as the embodiment shown in FIG. 3, except that
 teeth 21 for securing the correcting members (balance balls) are not
 formed on the calmper holder 4 side, that is, this arrangement corrects
 the unbalance by using such a principle that the correcting members) are
 shifted so that the phase thereof is reversed by an angle of 180 deg, with
 the use of vibration of the support system through the vibration proof
 legs 13, similar to the afore-mentioned embodiment.
 The effect and the like obtained thereby are similar to those as mentioned
 above. However, this embodiment exhibits such a disadvantage that a
 true-circular groove 17 in which the correcting members 7 are set cannot
 be easily formed with respect to the rotational center.
 FIG. 15 is an embodiment in which the unbalance correcting mechanism is
 provided at a side surface of the rotor 36 of the motor. That is, a
 ring-like member formed therein with a groove in which the correcting
 members are set, is fitted in the rotor 36. Specifically, the member 42
 having a ring-like groove 17 and the correcting members 7 set in the
 ring-like groove are incorporated in the rotor 36.
 In this case, since a bearing 43 for supporting the shaft 37 is present
 within the rotor 36, the moment exerted to the shaft 37 can be made to be
 small, and accordingly, the use life of the bearing 43 can be enhanced. In
 the case of using fluid as the correcting members 7, the vertical
 dimensions can be increased so as to increase the cubage of the correcting
 members 7, and accordingly, it is possible to enhance the effect of
 correction. Further, the above-mentioned unbalance correcting mechanism
 may be provided to the shaft 37 which is extended downward from the
 bearing 43, although it is not shown.
 FIG. 16 is a sectional view which shows another embodiment of the present
 invention. This embodiment is substantially the same as that shown in FIG.
 8, except that the unbalance correcting mechanism is incorporated in the
 turntable 15, including a holding member 45 for holding the correcting
 members 7. Explanation will be hereinbelow made of the operation and the
 effect of the holding member 45. The condition shown in this figure, the
 rotational speed of a disc 2 does not yet reach a resonant point of the
 rotational support system.
 In this embodiment, the holding member 45 is formed of a leaf spring which
 made of a resilient material, being bent at their outer peripheral part
 (forward end part). In the range of the rotational speed up to the
 resonant point, the correcting members 7 are pressed against the rolling
 groove 17 by means of the leaf spring so as to be held, being prevented
 from shifting. This pressing force can be about a value which is slightly
 greater than the rolling resistance between the correcting members 7 and
 the pathway surface, that is, the contact frictional force.
 When the rotational speed of the disc exceeds the resonant point, the
 centrifugal force caused by the rotation acts upon the forward end of the
 leaf spring, the bent forward end part thereof starts its deformation in a
 horizontal direction. Accordingly, the pressing force exerted to the
 correcting members 7 is eliminated, and accordingly, the correcting
 members 7 can be freely moved. When the rotational speed exceeds the
 resonant point, the correcting members 7 is shifted in a direction reverse
 to the direction of unbalance by an angle of 180 deg., thereby it is
 possible to correct the unbalance.
 Thus, the reason why the correcting members are held before the resonant
 point, is such that vibration caused by the rolling of the correcting
 members 7, which generates when passing the resonant point of the rotary
 support system in the case of a disc speed changing control system (For
 example, a CD-ROM drive or a DVD-ROM drive using a constant linear speed
 system) should be restrained. In the case of the constant disc speed
 control system (for example, an MO drive or a high speed CD-ROM system
 using an angularly constant system), the rotational speed rapidly rises up
 since the vibration which is caused by rolling of the correcting members
 when passing the resonant point of the rotary support system is
 restrained, similar to that mentioned above. Further, since the correcting
 members 7 can be released after completely passing the resonant point, and
 since a force can be prevented from being effected in an unbalance
 direction but can be effected in only a correcting direction, unbalance of
 the disc can be stably corrected.
 As mentioned above, the disc device according to the present invention, can
 automatically correct unbalance of the rotary system including unbalance
 of the disc, and can reduce unbalance vibration caused by the rotary
 system. With this arrangement, focus errors and tracking errors caused by
 the unbalance vibration can be restrained, and vibration and noise can
 also be decreased. Further, a disc device using the unbalance correcting
 mechanism according to the present invention can aim at increasing the
 rotational speed of a disc for high speed transmission of data, and at
 enhancing the degree of accuracy which is required when the density of
 data is increased to a high value.