OPTICAL PICK-UP DEVICES

Abstract: An optical pick-up device (1) comprising an objective lens (23) having an optical axis for focusing a light beam emitted from a light source onto an optical recording medium. The ob­ jective lens (23) is carried on a support member (19) which is coupled with a base (2) through a coupling unit (10). The coupling unit (10) has a first rotation axis parallel to the optical axis for permitting rotation of the support member (19) about the first rotation axis in a tracking direction perpendicular to the optical axis and a second rotation axis perpendicular to the op­ tical axis for permitting rotation of the support member (19) about the second rotation axis in a focusing direction parallel to the optical axis. The support member (19) has a centre of gravity positioned substantially on the first rotation axis. A light beam reflected from the optical recording medium is sensed for deriving a force to be exerted on the support mem­ ber (19) to move the objective lens (23) in the focusing and/or tracking directions so as to adjust the optical axis to a desired position.

Description: This invention relates to optical pick-up devices suitable for use in recording on and reproducing from an optical recording medium. Upon recording of data from a music or image source on an optical disc recording medium, the data are converted into a digital signal having logic "0" and "1" levels. According to the logic level, pits are formed to provide a continuous spiral recording track in the optical disc recording medium. An optical pick-up device is used to reproduce the recorded data by radiating a light beam from a light source such as a semiconductor laser through an objective lens which focuses the light beam on the optical disc recording medium, and sensing the light beam reflected from the optical recording medium to determine the presence of pits. For this purpose, the objective lens is supported in such a manner that its optical axis can follow the spiral recording track while the optical disc is rotating during the data reproducing operation. The objective lens is supported at a predetermined constant distance from the optical disc recording medium to focus the light beam so as to produce a light spot of a predetermined constant diameter. To achieve high accuracy detection of the pits formed in the optical disc recording medium, the error in the distance of the objective lens from the optical disc recording medium should be less than the pit depth, that is, about 0.1 nm. However, it is very difficult to maintain the distance with such a high accuracy due to the poor finishing accuracy of the optical disc recording medium, and inaccuracy in the spindle mechanism on which the optical disc is placed. It is, therefore, current practice to avoid this difficulty by moving the objective lens in the focusing direction parallel to the optical axis so as to adjust the distance between the objective lens and the recording medium when the distance deviates from the predetermined value. As described above, optical pick-up devices have normally been designed to move the objective lens in a tracking direction perpendicular to the optical axis and also in a focusing direction parallel to the optical axis for the purpose of adjustment of the optical axis to a desired position. However, such an optical pick-up device tends to deviate from a desired position in the tracking direction when it is subject to an impact or sudden movement particularly in the tracking direction. Our US patent specification US-A-4 449 213, referred to in more detail below, discloses an optical pick-up device having features corresponding to the pre-characterizing part of claim 1. According to the present invention there is provided an optical pick-up device comprising a base, an objective lens having an optical axis for focusing a light beam emitted from a light source onto an optical recording medium, a support member carrying said objective lens thereon, a coupling unit coupling said support member to said base for permitting movement of said support member, and means responsive to a light beam reflected from said optical recording medium for exerting a force on said support member to move said objective lens in the focusing and/or tracking direction so as to adjust said optical axis to a desired position; said coupling unit having a first rotation axis parallel to said optical axis for permitting rotation of said support member about said first rotation axis in a tracking direction perpendicular to said optical axis, and said coupling unit having a second rotation axis perpendicular to said optical axis for permitting rotation of said support member about said second rotation axis in a focusing direction parallel to said optical axis; the optical pick-up device being characterized in that: said support member has a centre of gravity positioned substantially on said first rotation axis. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a perspective view showing a previously proposed optical pick-up device; Figure 2 is a plan view showing another type of previously proposed optical pick-up device; Figure 3 is a plan view showing one embodiment of optical pick-up device in accordance with the invention; Figure 4 is a section taken on line II-II in Figure 3; Figure 5 is an enlarged perspective view of the optical pick-up device of Figure 3; Figure 6 is an enlarged exploded perspective view of the optical pick-up device of Figure 3; Figure 7 is a plan view showing a second embodiment of optical pick-up device in accordance with the invention; Figure 8 is section taken on line VI-VI in Figure 7; Figure 9 is an enlarged perspective view of the optical pick-up device of Figure 7; Figure 10 is an enlarged exploded perspective view of a coupling member in Figure 7; Figure 11 is a plan view of a third embodiment of optical pick-up device in accordance with the invention; Figure 12 is a section taken on line X-X of Figure 11; Figure 13 is an enlarged perspective view of the optical pick-up device of Figure 11; and Figure 14 is an enlarged exploded perspective view of the optical pick-up device of Figure 11. Prior to the description of the preferred embodiments of the invention, the previously proposed optical pick-up devices of Figures 1 and 2 will be briefly described in order to provide a basis for a better understanding of the difficulties attendant thereon. Figure 1 shows a optical pick-up device disclosed, for example, in our US patent US-A-4 449 213. The optical pick-up device a includes a support member b which is coupled to a sliding member d through a pair of parallel resilient arms e such as leaf springs. An objective lens c is mounted on the support member b with its optical axis directed in a focusing direction. In this structure, the sliding member d can slide on a suitable member (not shown) in a direction parallel to the optical axis, permitting the support member b to move in the focusing direction, whereas the parallel resilient arms e can move in a direction perpendicular to the optical axis, permitting the support member b to move in the tracking direction. When an impact is applied to the optical pick-up device a, however, a moment moves the support member b a distance X in the tracking direction. Figure 2 shows another type of optical pick-up device as disclosed, for example, in our US patent US-A-4 482 988. The optical pick-up device f includes a support cylinder g placed around a support shaft i extending through a through-hole h formed centrally in the support cylinder g. An objective lens j is mounted on the support cylinder g with its optical axis directed in a focusing direction. A balancer weight k is provided on the support cylinder at a position opposite to the objective lens j with respect to the support shaft i to minimize the moment which would tend to rotate the support cylinder g when it is subject to an impact. The support shaft i is placed with a clearance in the through-hole h for permitting the support cylinder g to rotate about and move along the support shaft i. When an impact occurs, the support cylinder g will move in the tracking direction within the clearance. Referring to Figures 3 to 6, there is shown one embodiment of optical pick-up device in accordance with this invention. The optical pick-up device 1 includes an objective lens 23 for use in focusing a light beam emitted by a light source (not shown) onto an optical disc recording medium (not shown). The objective lens 23 is attached to a support member 19 with its optical axis directed to the optical disc recording medium. The support member 19 is mounted on a base substrate 2 through a coupling unit 10 which has a first rotation axis parallel to the optical axis for permitting rotation of the support member 19 about the first rotation axis in a tracking direction perpendicular to the optical axis, and also a second rotation axis perpendicular to the optical axis for permitting rotation of the support member 19 about the second rotation axis in a focusing direction parallel to the optical axis. In greater detail, the objective lens 23 is fixed within an annular lens holder 24. The support member 19 has an aperture 20 having its inner surface conforming generally to the circumferential surface of the lens holder 24. An annular ring 20a is formed integrally on the inner surface of the aperture 20 substantially intermediate between the opposite ends of the aperture 20. The lens holder 24 is press fitted in the aperture 20 with its bottom in abutment with the annular ring 20a. A first balancer weight 25 is fixedly fitted in the lower half portion of the aperture 20 with its top in abutment with the annular ring 20a. The support member 19 has a pair of rearwardly extending parallel wings 19b each having a hole 22 formed therein near its rear end. The parallel wings 19b carry a second balancer weight 26 which is of a cylindrical shape having small diameter portions 26a on the opposite ends thereof. The second balancer weight 26 is mounted between the parallel wings 19b with their small diameter portions 26a fitted respectively in the holes 22. The parallel wings 19b also have inwardly extending projections formed near their forward ends to provide grooves 21 facing each other. A pair of drive coil assemblies 27 are attached on the opposite side surfaces of the support member 19, each including a rectangular coil frame 28 around which a focusing coil 30 is wound to produce magnetic lines of force directed in the focusing direction when it is energized. The focusing coils 30 are attached on the opposite side surfaces of the support member 19. A pair of square coil plates 29 are attached side by side on the outer side surface of each of the focusing coils 30 with each carrying a tracking coil 31 which produces magnetic lines of force directed in the tracking direction when it is energized. The coupling unit 10 includes a mounting base 11 secured on the base substrate 2, a parallel link 13, and a hinge member 15, these parts being made integrally of a synthetic resin. The mounting base 11 is provided near its opposite ends with bolt holes 12 whereby it may be bolted to the base substrate 2. The hinge member 15 is of a cubic shape having cut-outs formed in the opposite side surfaces thereof to provide a flexible thin hinge connection 15c between front and rear hinge members 15a and 15b, the hinge connection 15c having a hinge or first rotation axis parallel to the optical axis about which the front hinge member 15a can rotate with respect to the rear hinge member 15b. The parallel link 13 comprises upper and lower hinge members 13a and 13b extending in parallel-spaced relation between the mounting base 11 and the hinge member 15. The upper hinge member 13a is of a trapezoidal shape having at its one end a flexible thinned portion connected with the upper edge of the mounting base 11 and at the other end thereof a flexible thinned portion connected with the upper edge of the rear hinge member 15b. Similarly, the lower hinge member 13b is of an inverted-trapezoidal shape having at its one end a flexible thinned portion connected with the lower edge of the mounting base 11 and at the other end thereof a flexible thinned portion connected with the lower edge of the rear hinge member 15b. The coupling unit 10 is coupled to the support member 19 with its front hinge member 15a being fitted and stuck in the grooves 21 and with the second balancer weight 26 being placed between the upper and lower hinge members 13a and 13b. The parallel link 13 has a second rotation axis perpendicular to the optical axis about which the hinge member 15 can rotate in the focusing direction with respect to the mounting base 11. It is, therefore, apparent from the foregoing that the coupling unit 10 has a first rotation axis parallel to the optical axis for permitting rotation of the support member 19 about the first rotation axis in the tracking direction perpendicular to the optical axis, and a second rotation axis perpendicular to the optical axis for permitting rotation of the support member 19 about the second rotation axis in the focusing direction parallel to the optical axis. In other words, the coupling unit 10 permits the objective lens 23 to rotate in the focusing and/or tracking direction with respect to the base substrate 2. The base substrate 2 has a mounting platform 5 formed on the rear end portion thereof at a position substantially intermediate between the opposite side edges. The mounting platform 5 is provided near its opposite ends with threaded holes 6 in which bolts 17 are threaded to secure the mounting base 11 on the base substrate 2. Washers 18 are interposed between the mounting base 11 and the bolt heads. The base substrate 2 is also provided with a centre opening 4 formed in a shape which corresponds to and is slightly greater than that of the support member 19. A pair of inner yokes 7 is provided on the base substrate 2 adjacent to the side edges of the centre opening 4. The rectangular coil frames 28 are placed loosely around the respective inner yokes 7 to permit the support member 19 to move in the focusing and tracking directions. The base substrate 2 is also provided thereon with a pair of outer yokes 8 facing in parallel-spaced relation to the respective inner yokes 7. Magnets 9 and 9' formed in the same shape as the inner and outer yokes 7 and 8, are attached on the inner surface of each of the outer yokes 8 in such a manner that the inner and outer yokes 7 and 8 and the magnets 9 and 9' establish a magnetic circuit producing magnetic lines of force perpendicular to the optical axis in which the drive coil assemblies 27 are placed. The base substrate 2 is provided at its corners with bolt holes 3 whereby it may be bolted on an optical block (not shown) which carries a semiconductor laser for producing a laser beam, a quarter-wave plate and a collimation lens through which the laser beam is guided to the objective lens 23, a polarizing beam splitter which separates the incident beam into the laser beam emitted from the semiconductor laser and the beam reflected on the optical disc, a photo sensor for sensing the reflected beam, and a control circuit responsive to the photo sensor for producing focusing and tracking error signals which are used to produce correction currents for the focusing and tracking coils 30 and 31. The laser beam emitted from the semiconductor laser passes upward through the centre opening 4 in the base substrate 2, as indicated by the two-dotted arrows in Figure 4. It is to be noted that the first and second balancer weights 25 and 26 are provided for the purpose of making up the balance of the weight of the support member 19 in such a manner as to position the centre of gravity of the support member 19 on the first rotation or hinge axis of the hinge member 15. This is effective to minimize the possibility of movement of the support member 19 causing the objective lens 23 to deviate in the tracking direction from a correct position when the optical pick-up device 1 is subject to an impact or a sudden movement particularly in the tracking direction. It is preferable to adjust the weight balance of the support member 19 in such a manner as to position the centre of gravity on the first and second rotation axes to minimize the tendency of the objective lens 23 to deviate in the tracking and focusing directions from a correct position when the optical pick-up device 1 is subject to an impact or a sudden movement. The operation is as follows. The laser beam from the semiconductor laser device passes through the objective lens 23 and is reflected on the optical disc. A part of the reflected beam from the optical disc returns through the objective lens 23 to the optical block which contains a control unit. The control unit receives a signal from the photo sensor and reads deviations of the position of the objective lens 23 in the focusing and tracking directions. The control unit provides correction currents, which correspond to the sensed deviations, for the focusing and tracking coils 30 and 31. The current flows through the respective focusing coils 30 disposed in the magnetic flux produced in the magnetic circuit cause forces on the respective focusing coils 30 to move the support member 19 in the focusing direction so as to correct the distance between the objective lens 23 and the optical disc recording surface. The direction of movement of the support member 19 is dependent both on the direction of the current flow through the focusing coils 30 and on the direction of the magnetic flux produced in the magnetic circuit. In a similar manner, the current flows through the respective tracking coils 31 disposed in the magnetic flux produced in the magnetic circuit cause forces on the respective tracking coils 31 to rotate the support member 19 in the tracking direction about the first rotation or hinge axis of the hinge member 15 so as to direct the objective lens optical axis to the centre of the optical disc recording track. The direction of rotation of the tracking coils 31 is dependent on the directions of the current flows through the tracking coils 31 and also on the direction of the magnetic flux produced in the magnetic circuit. Referring to Figures 7 to 10, there is shown a second embodiment of the invention which differs from the first embodiment only in the support arm arrangement. Accordingly, like parts are designated by like reference numerals. In this embodiment, a coupling unit 32 includes a parallel link comprising two resilient plates such, for example, as leaf springs 33, which are placed in parallel-spaced relation to couple the hinge member 15 with the mounting base 11. The upper leaf spring 33 has a front end portion stuck on the upper surface of the rear hinge member 15b and a rear end portion stuck on the upper surface of the mounting base 11. The lower leaf spring 33 has a rear end portion stuck on the lower surface of the rear hinge member 15b and a rear end portion attached on the lower surface of the mounting base 11. A washer 34 is secured by the bolts 17 on the upper surface of the mounting base 11 to press the upper leaf spring rear end portion against the upper surface of the mounting base 11. The lower leaf spring rear end portion is sandwiched between the mounting base 11 and the mounting platform 5. The parallel link coupling unit 32, comprising the upper and lower leaf springs 33, has a resilient property permitting rotation of the support member 19 about a second rotation axis in the focusing direction with respect to the mounting base 11. It is, therefore, apparent that, also in this embodiment, the coupling unit 32 has a first rotation axis parallel to the optical axis for permitting rotation of the support member 19 about a second rotation axis in the focusing direction parallel to the optical axis. Referring to Figures 11 to 14, there is shown a third embodiment of the invention comprising a support member 37 which carries the objective lens 23 thereon. The support member 37 differs from that of the first embodiment in that the parallel wings are connected integrally by a rear frame 38. The rear frame 38 also serves as a second balancer weight as described in connection with the first embodiment. The coupling unit includes a parallel link comprising two resilient plates such, for example, as leaf springs 39 placed in parallel-spaced relation to connect the hinge member 15 with the support member rear frame 38. The upper leaf spring 39 has a front end portion stuck on the upper surface of the rear hinge member 15b and a rear end portion stuck on the lower surface of the rear frame 38. The front hinge member 15a is stuck on the rear surface of a mounting member 36 provided on the base substrate 2. The base substrate 2 is formed with a circular opening 35 which corresponds to the circular aperture 20 provided in the support member 37. The parallel link, comprising the upper and lower leaf springs 39, has a resilient property permitting rotation of the support member 19 about a second rotation axis in the focusing direction with respect to the mounting member 36. It is, therefore, apparent that, also in this embodiment, the coupling unit has a first rotation axis parallel to the optical axis for permitting rotation of the support member 37 about the first rotation axis in the tracking direction perpendicular to the optical axis and a second rotation axis perpendicular to the optical axis for permitting rotation of the support member 37 about the second rotation axis in the focusing direction parallel to the optical axis. A balancer weight 25 is secured in the circular opening 20 in the same manner as described in connection with the first embodiment to position the centre of gravity of the support member 37 on the first rotation or hinge axis of the hinge member 15. In the optical pick-up devices described, the support member carrying an objective lens thereon is coupled to a base substrate through a coupling unit which has a first rotation axis for permitting rotation of the support member about the first rotation axis in a tracking direction and a second rotation axis for permitting rotation of the support member about the second rotation axis in a focusing direction. The support member is designed to have a centre of gravity positioned substantially on the first rotation axis so as to minimize the possibility of a deviation of the optical axis of the objective lens in the tracking direction from a desired position when the optical pick-up device is subject to an impact or sudden movement particularly in the tracking direction.

Claims: An optical pick-up device (1) comprising a base (2), an objective lens (23) having an optical axis for focusing a light beam emitted from a light source onto an optical recording medium, a support member (19) carrying said objective lens (23) thereon, a coupling unit (10) coupling said support member (19) to said base (2) for permitting movement of said support member (19), and means (30, 31) responsive to a light beam reflected from said optical recording medium for exerting a force on said support member (19) to move said objective lens (23) in the focusing and/or tracking direction so as to adjust said optical axis to a desired position; said coupling unit (10) having a first rotation axis parallel to said optical axis for permitting rotation of said support member (19) about said first rotation axis in a tracking direction perpendicular to said optical axis, and said coupling unit (10) having a second rotation axis perpendicular to said optical axis for permitting rotation of said support member (19) about said second rotation axis in a focusing direction parallel to said optical axis; the optical pick-up device being characterized in that: said support member (19) has a centre of gravity positioned substantially on said first rotation axis. An optical pick-up device (1) according to claim 1 wherein said coupling unit (10) includes a mounting base (11) secured on said base (2), a hinge member (15) secured on said support member (19) for permitting rotation of said support member (19) about said first rotation axis, and a parallel link (13) coupled between said mounting base (11) and said hinge member (15) for permitting rotation of said support member (19) about said second rotation axis. An optical pick-up device (1) according to claim 2 wherein said mounting base (11), said hinge member (15) and said parallel link (13) are made integrally of a synthetic resin. An optical pick-up device (1) according to claim 2 wherein said parallel link (13) comprises a pair of resilient plates placed in parallel-spaced relation between said mounting base (11) and said hinge member (15). An optical pick-up device (1) according to claim 1 wherein said coupling unit (10) includes a mounting base (36) secured on said base (2), a hinge member (15) secured on said mounting base (36), and a parallel link (39) coupled between said hinge member (15) and said support member (19) for permitting rotation of said support member (19) about said second rotation axis, said hinge member (15) permitting rotation of said support member (19) about said first rotation axis. An optical pick-up device according to claim 2 or claim 5 wherein at least one weight (25,26,38) is mounted on said support member (19,37) for positioning said centre of gravity substantially on said first rotation axis. An optical pick-up device according to claim 5 wherein said parallel link (39) comprises a pair of resilient plates placed in parallel-spaced relation between said hinge member (15) and said support member (37).

Inventors: GODA, YOSHIMASA PATENTS DIVISION, OINOUE, HIROSHI PATENTS DIVISION
Applicants: SONY CORPORATION
Application year: 1986
Main classification: G11B7
IPC codes: G11B7, G02B7, G02B26