Abstract:
An optical disk apparatus includes a movable part having an objective lens for focusing a laser beam onto an optical disk. A lens-holding member holds the objective lens. A protecting member prevents contact of the objective lens with the optical disk. The protecting member is spaced apart from the optical disk. An elastic supporting member supports the movable part, and an actuator drives the movable part toward or away from the optical disk. The apparatus is constructed so as to maintain the protecting member spaced apart from the optical disk, even when the elastic supporting member is bent by gravity toward the optical disk.

Description:
This application claims priority from Japanese Patent Application No. 2004-183721, filed on Jun. 22, 2004, which is hereby incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a portable optical disk apparatus for recording or reproducing information signals on an optical disk, which employs an objective lens having an NA (numerical aperture) of 0.8 or higher. 
   2. Related Background Art 
   Optical disk apparatuses have been commercialized, which record information signals by focusing a laser beam in a fine spot by an objective lens onto an information signal recording layer of an optical disk or reproducing the recorded information signals by the reflected light. Lately, it has become possible to increase the NA of the objective lens of the optical disk apparatus to be 0.08 or higher, as the results of improvement in the design and production technique of the objective lens. Such a high-NA objective lens enables the formation of a finer light spot, to improve the resolution in information recording and reproduction, and to increase the capacity of the optical disk. 
   Conventionally, a typical optical disk has an information signal recording layer formed on a substrate plate having a thickness of 0.6-1.2 mm, and which is composed of a transparent resin material. The optical disk apparatus projects a laser beam through the substrate plate onto the information signal recording layer. However, an objective lens of a higher NA has a shorter focus length, requiring a shorter distance between the objective lens and the information signal recording layer. This makes it difficult to focus a laser beam through the substrate plate having a thickness of 0.6-1.2 mm on the information signal recording layer. To solve this problem, certain methods have been disclosed. In one method, to meet the increase of the NA of the objective lens, the information signal recording layer is covered on the face reverse to the substrate plate side with a transparent cover layer thinner than the substrate plate, and the laser beam is projected through the cover layer, and not through the substrate plate, as disclosed in Japanese Patent Application Laid-Open Nos. 2003-91833, 2004-30835, and so forth. 
     FIG. 4  illustrates schematically a constitution of such an optical disk apparatus of the prior art. In  FIG. 4 , reference numeral  1  denotes an optical disk, reference numeral  2  denotes an optical head, and reference numeral  3  denotes a spindle motor to rotate the optical disk. Optical head  2  is constituted of laser light source  6 , collimator lens  7 , beam splitter  8 , condenser lens  9 , photodetector  10 , objective lens  20 , and actuator  11  for controlling the focusing and tracking with the objective lens  20 . 
   Optical disk  1  is constituted of substrate plate  12 , information signal recording layer  13 , formed on substrate plate  12  from a phase changeable material capable of changing the reversible phase state, and cover layer  14  having a thickness of about 0.1 mm, which is composed of a transparent resin material. On information signal recording layer  13 , recording tracks are formed in spiral or concentric circles. The information signal recording layer  13  may be formed from a magnetooptical material, or a metallic reflecting film having pits (concave areas) formed thereon. Optical head  2  is placed to face cover layer  14  of optical disk  1 . 
     FIG. 5  shows a constitution of actuator  11 . Actuator  11  comprises immovable part  15  and movable part  16 . Immovable part  15  is constituted of permanent magnets  17   a ,  17   b , yoke  18 , and supporting stage  19 . Movable part  16  is constituted of objective lens  20 , focusing coil  21 , tracking coil  22 , and lens-holding member  23  for holding the above articles. Elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  are in a linear shape, and are elastic and highly electroconductive. The supporting members are fixed at the respective ends to supporting stage  19 , and hold, at other ends, movable part  16  to be movable freely in vertical and radial directions relative to optical disk  1 . Elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  are also connected electrically to focusing coil  21  and tracking coil  22  on lens-holding member  23 . 
   The optical disk apparatus, as shown in  FIG. 4 , has an error signal generating circuit  4  and a control circuit  5  for control of focusing and tracking by actuator  11 . Control circuit  5  applies electrical control current through elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  to focusing coil  21  and tracking coil  22 . 
   In recording the information signals, first, optical disk  1  is rotated by spindle motor  3 . With the optical disk kept rotated, a laser beam pulse-modulated in accordance with information signals and emitted from laser light source  6  is made parallel by collimator lens  7 , allowed to pass beam splitter  8 , and focused in a fine spot by objective lens  20  through cover layer  14  of optical disk  1  on information signal recording layer  13 . 
   The information signal recording layer of optical disk  1  is heated and cooled repeatedly by projection of a pulse-modulated laser beam to form recording marks by a phase change to an amorphous phase or a crystal phase by the heating-cooling process. 
   In reproduction of the recorded information signals, optical disk  1  is rotated similarly by spindle motor  3 . With the optical disk being kept rotated, a constant intensity of a laser beam emitted from laser light source  6  is focused in a fine spot through cover layer  14  on information signal recording layer  13 . The intensity of the light reflected from information signal recording layer  13  varies in accordance with the recorded marks, whereby the information signal is reproduced. 
   During the operation of recording and reproducing of the information signals, the light beam reflected by optical disk  1  is further reflected by beam splitter  8 , condensed by condenser lens  9 , and detected by photodetector  10 . Photodetector  10  has a light-receiving face divided into plural sections. Error signal generating circuit  4  generates focus error signals and tracking error signals according to the detected signals at the respective sections of the light-receiving face. 
   Control circuit  5  applies a control current, based on the focus error signal and the tracking error signal, through elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  to focusing coil  21  and tracking coil  22 . Actuator  11  drives movable part  16  to move perpendicularly toward or away from optical disk  1 , or in the radial directions crossing the recording track, by an electromagnetic force generated by an interaction of the control current and the magnetic fluxes of permanent magnets  17   a ,  17   b.    
   In such a manner, the focusing is controlled to focus precisely the light spot on the recording track to offset perpendicular displacement by axial deflection of a surface of optical disk  1 , or a like cause, and the tracking is controlled to scan the recording track by offsetting radial run-out of the track center in the radial direction. 
   In the above optical disk apparatus, since the gap (working distance) between objective lens  20  and the surface of cover layer  14  is as small as 0.2-0.6 mm, a stopper, or a like mechanism, cannot be provided at a position in the gap, to limit the displacement of movable part  16 , to prevent undesired contact of objective lens  20  with optical disk  1 . Naturally, the actuator normally working for the control will keep constant the gap between objective lens  20  and the surface of cover layer  14 . However, the actuator may fail in the control, owing to an abrupt impact, vibration, or a like cause, or an adverse effect of dirt or a scratch on optical disk  1  so that a normal error signal will not be generated. 
   In such an abnormal working state, movable part  16  is excessively displaced, inevitably to come into contact with optical disk  1 . Even when the contact occurs, at least objective lens  20  can be protected by providing a protrusion higher than objective lens  20  at a portion of lens-holding member  23  facing optical disk  1  and allowing the protrusion to touch optical disk  1 . 
   However, even in a non-working state, especially, with the power source turned off, when the optical disk apparatus is placed with optical disk  1  held horizontally, and with optical head  2  held above the disk, elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  may be bent by gravity acting on movable part  16 , to cause movable part  16  to contact with optical disk  1 . This contact state can sometimes be kept for a long time if the optical disk apparatus is left standing. 
   Usually, lens-holding member  23  and cover layer  14  of optical disk  1  are made from different kinds of resins. The different kinds of resins, which are kept in contact with each other under pressure, are liable to undergo not only usual environmental deterioration, but also, a physical change, such as local deformation and a chemical change, such as chemical deterioration. In particular, the protecting member should have essential properties for productivity (formability) and mechanical properties, such as rigidity, and the cover layer should have optical properties and physical properties, such as hardness. Therefore, physical and chemical durability cannot readily be given additional to the protecting member or the cover layer. The deterioration of optical properties of cover layer  14  caused by deformation or deterioration of the contact portion can presumably prevent normal recording and reproduction of the information signals disadvantageously. 
   In recent years, optical disk apparatuses are coming to be used as portable apparatuses for recording and reproducing voices, still-pictures, animated pictures, and so forth. The portable apparatuses are stored arbitrarily in various manners by users, and stored in many cases with the optical disk being kept inserted in the apparatus. Accordingly, the optical disk apparatus having the objective lens of a high NA will inevitably encounter the above-mentioned problems. 
   SUMMARY OF THE INVENTION 
   The present invention intends to provide an optical disk apparatus which does not cause undesired contact of an objective lens with the optical disk, regardless of the placement state of the optical disk apparatus. 
   According to an aspect of the present invention, an optical disk apparatus is constituted of a movable part having an objective lens for focusing a laser beam onto an optical disk, a lens-holding member for holding the objective lens, and a protecting member for preventing contact of the objective lens with the optical disk, an elastic supporting member for supporting the movable part, and an actuator for driving the movable part toward or apart from the optical disk, wherein the apparatus is constructed so as not to cause contact of the protecting member with the optical disk in a non-working state of the actuator, even when the elastic supporting member is bent by gravity toward the optical disk. 
   The optical disk preferably has a recording layer and a transparent cover layer formed on the recording layer, and the protecting member prevents contact of the objective lens with the cover layer. 
   The apparatus is preferably constituted to satisfy the condition below:
 
 S−S   ν   +S   0   −A&gt; 0,
 
where S denotes a distance between the optical disk and the protecting member with the actuator working, S ν  denotes the maximum decrease of the distance between the optical disk and the protecting member by variable factors, S 0  denotes a withdrawal distance with the actuator non-working, and A denotes displacement of the protecting member by gravity acting on the movable part.
 
   The optical disk apparatus is preferably portable. The objective lens preferably has an NA not less than 0.8. 
   According to the present invention, the moving part will not come into contact with the optical disk by bending by gravity (that is, the weight of the movable part) of the elastic supporting member for supporting the movable part toward the optical disk, even when the optical disk is in place during a non-working state, in particular, with the power source turned off, with the optical disk being kept horizontally, and with the optical head kept above the optical disk. Therefore, even if the apparatus is stored for a long time in the above-mentioned state, the contact of the movable part with the optical disk is prevented, so as not to impair the reliability of the apparatus. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a constitution of an actuator of an optical disk apparatus of the present invention. 
       FIG. 2  is a drawing for explaining the working of the optical disk of the present invention, placed in a vertical direction. 
       FIG. 3  is a drawing for explaining the working of the optical disk of the present invention, placed in a horizontal direction. 
       FIG. 4  schematically illustrates a constitution of an optical disk apparatus. 
       FIG. 5  illustrates a constitution of an actuator of a conventional optical disk apparatus. 
       FIG. 6  is a drawing for explaining the working of an optical disk of the present invention, placed in another horizontal direction. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The best mode for carrying out the invention is explained below in detail by reference to the drawings. The constitution and operation of the optical disk apparatus of the present invention is similar to the conventional one shown in  FIG. 4 , so that a detailed explanation thereof is omitted. The present invention relates to optical disk apparatuses, especially, to portable ones, for recording or reproduction of voices, still-pictures, animated pictures, or the like, and assumes the use and storage in an arbitrary placement direction of the apparatus. 
   Actuator  11  of the optical disk apparatus of the present invention is explained below in detail.  FIG. 1  shows a constitution of actuator  11 . In  FIG. 1 , the same symbols as in  FIG. 5  are used to denote corresponding members. Actuator  11  comprises immovable part  15  and movable part  16 . Immovable part  15  is constituted of permanent magnets  17   a ,  17   b , yoke  18 , and supporting stage  19 . Movable part  16  is constituted of objective lens  20  having an NA of 0.8 or higher, focusing coil  21 , tracking coil  22 , and lens-holding member  23  for holding the above articles. Lens-holding member  23  has protecting member  25  protruding a little higher than objective lens  20  toward the optical disk  1 . Protecting member  25  may be formed in integration with lens-holding member  23 , or may be formed separately and attached later thereto. 
   Elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  are in a linear shape, and are elastic and highly electroconductive. The supporting members are fixed at the respective ends to supporting stage  19 , and hold, at the other ends, movable part  16  to be movable freely in vertical and radial directions relative to optical disk  1 . Elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  are also connected electrically to focusing coil  21  and tracking coil  22  on lens-holding member  23 . 
   The optical disk apparatus, as shown in  FIG. 4 , has error-signal generating circuit  4  and control circuit  5  for performing focusing and tracking by actuator  11 . Control circuit  5  applies an electrical current for performing control through elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  to focusing coil  21  and tracking coil  22 . 
   In the above optical disk apparatus, since the gap (working distance) between objective lens  20  and the surface cover layer  14  is as small as 0.2-0.6 mm, a stopper, or a like mechanism, cannot be provided at a position in the gap to limit the displacement of movable part  16 , to prevent undesired contact of objective lens  20  with cover layer  14 . Naturally, the actuator normally working for the control will keep constant the gap between objective lens  20  and the surface of cover layer  14 . However, an actuator may fail in the control, owing to an abrupt impact, vibration, or a like cause, or owing to an adverse effect of dirt or a scratch on optical disk  1 , so that a normal error signal will not be produced. 
   When movable part  16  comes to be displaced excessively toward optical disk  1  in such an abnormal working state, protecting member  25  protruding higher than objective lens  20  is brought into contact with cover layer  14  of optical disk  1  to protect objective lens  20  and to prevent scratches from being formed on cover layer  14 . To prevent a serious adverse effect on the reliability of optical head  2  or optical disk  1 , protecting member  25  is formed from a slidable material and cover layer  14  is formed from a hard material, and immediately after detection of abnormal working, movable part  16  is withdrawn once away from optical disk  1  and later, the control is restarted. Thereby, the contact between protecting member  25  and optical disk  1  is limited to be instantaneous, not seriously affecting the reliability of optical head  2  and optical disk  1 . 
     FIG. 2  is a side view of actuator  11  and optical disk  1  of an optical disk apparatus of the present invention, with an optical disk set vertically. In this placement direction, gravity acts parallel to elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  as indicated by arrow mark G, so that the elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  are not bent by gravity acting on movable part  16 , namely, the weight thereof. This placement state, in which the elastic supporting members are not bent by gravity, is hereinafter referred to as a “non-gravitational placement state”. When actuator  11  is not working (hereinafter, referred to as a “neutral state”) in the non-gravitational placement state, for example, the power source of the apparatus is turned off, or elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  keep the position as shown by solid lines  51 , without elastic deformation. Here, the position of the top of protecting member  25  in the neutral state is denoted by a symbol P′. By operation of actuator  11  for performing focusing control, elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  are elastically deformed, as shown by one-dot chain lines  53 , to displace movable part  16  toward optical disk  1  by a distance S 0  from the neutral state. The focus-controlled state by actuator  11  is hereinafter referred to as a “controlled state”. The position of the top of protecting member  25  in the controlled state is denoted by a symbol P. The distance between position P′ of the protecting member in the neutral state and position P thereof in the controlled state is referred to as a “withdrawal distance” of protecting member  25 . In the controlled state, the distance between position P of the top of protecting member  25  and cover layer  14  (hereinafter, referred to as a “non-contact interval”) is kept invariable at a constant interval S. Thus, movable part  16  is installed such that position P′ of the top of protecting member  25  is apart from optical disk  1  more than a position P thereof, at the neutral state by withdrawal distance S 0 . Therefore, in the non-gravitational placement in the neutral state, the interval L between the top of protecting member  25  and cover layer  14  (hereinafter, referred to as a “neutral state interval”) is a sum of the non-contact interval S and withdrawal distance S 0 :
   L=S+S   0 . 
     FIGS. 3 and 6  are side views of actuator  11  and optical disk  1  of an optical disk apparatus of the present invention with optical disk  1  placed horizontally and optical head  2  placed above the disk, respectively. In this placement direction, gravity acts perpendicularly on elastic supporting members  24   a ,  24   b ,  24   c , and  24   d , as indicated by arrow mark G. In this placement direction, in a neutral state, elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  are bent by gravity acting on movable part  16 , namely, the weight thereof, toward optical disk  1  from the position in the non-gravitated non-gravitational placement in the neutral state, as shown by one-dot chain line  51  toward optical disk  1 , as shown by solid line  52 . The placement state of the optical disk apparatus in which the elastic supporting member is bent by gravity is referred to as a “gravitational placement state”. The position of the top of protecting member  25  in the neutral state in the gravitational placement state is indicated by a symbol P″. In this gravitational placement state, by performing focusing control by actuator  11 , movable part  16  is driven to cause elastic deformation of  24   a ,  24   b ,  24   c , and  24   d , as shown by one-dot chain lines and continuous line  51  to  53  in  FIGS. 3 and 6 , to keep constant, the non-contact interval S between position P of the top of protecting member  25  and cover layer  14 . The degree of bending (hereinafter, referred to as a “displacement distance”) of protecting member  25  by gravity from the position P′ in the neutral and non-gravitational placement state is indicated by a symbol A. Then, the neutral state distance D in the neutral state in the gravitational placement state is equal to non-contact interval S, plus withdrawal distance S 0 , minus displacement distance A:
   D=S+S   0   −A.    
   Actually, in any placement state, in the neutral state, the relative interval between the top of protecting member  25  and cover layer  14  is variable around the designed dimension owing to warpage and thickness error of optical disk  1 , variation of setting position of the optical head, or other causes. In consideration of a possible maximum deviation Sν toward the optical disk (or the cover layer) by variable factors, the minimum value L min  of the neutral state interval L in the non-gravitational placement state is represented by the equation below:
 
 L   min   =S−S   ν   +S   0 ,
 
and the minimum value D min  of the neutral state interval in the gravitational condition is represented by the equation below:
 
 D   min   =S−S   ν   +S   0   −A.  
 
In the case wherein the deviation A is larger than the minimum (L min =S−S ν +S 0 ) at the neutral state under a non-gravitational condition, the protecting member  25  is kept in contact with cover layer  14 . Therefore, to prevent the contact of protecting member  25  with cover layer  14 , L min  should be larger than A:
 
 D   min   =S−S   ν   +S   0   −A&gt; 0.  (1)
 
   The non-contact distance S in the controlled state is necessarily short, as the result of shortening of the focal distance resulting from an increase of NA of objective lens  20 . On the other hand, the decrease S ν  of the interval results from a variation in production of the optical disk and apparatus, and other variable factors, so that the reduction thereof is limited. Therefore, in the present invention, withdrawal distance S 0  and displacement distance A are suitably designed to satisfy the above equation (1). The above matter is explained more specifically by reference to the Examples discussed below. 
   The withdrawal of movable part  16  in the neutral state is conducted without employing a driving means requiring electricity. Therefore, the movable part  16  is withdrawn surely in a power turn-off state of the optical apparatus. 
   The above matter is explained below more specifically by reference to the following Examples. 
   Example 1 
   In the apparatus of this Example, the laser beam has a wavelength of 405 nm; objective lens  20  has an NA of 0.85; cover layer  14  of optical disk  1  has a thickness of 0.1 mm; the interval between the surface of cover layer  14  and objective lens  20  (working distance) is 0.3 mm at focusing of the laser beam on information signal recording layer  13 ; protecting member  25  protrudes higher than objective lens  20  by 0.05 mm; and the interval between position P of the top of protecting member  25  and cover layer  14 , namely, the non-contact interval S, is 0.25 mm during focusing control with the actuator, namely, in the controlled state. 
   Movable part  16  of optical head  2  has a mass of 270 mg. The total of the spring constants of elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  is 19 N/m. Displacement distance A is 0.14 mm with optical disk  1  placed horizontally. 
   At the neutral state, the maximum deviation S ν  to cause a decrease of the relative interval between the top of protecting member  25  and cover layer  14  by various variation factors is 0.2 mm. 
   In this state, if the position of movable part  16  in the neutral state is the same as the controlled position, namely, S 0 =0, the minimum D min  of the interval between protecting member  25  and cover layer  14  is calculated as below: 
                     D   min     =     S   -     S   v     +     S   0     -   A                 =     0.25   -   0.2   +   0   -   0.14                 =       -   0.09     ⁢           ⁢     (   mm   )               .         
Therefore, the protecting member actually comes into contact with the cover layer.
 
   In this Example, the protecting member is withdrawn by a withdrawal distance S 0  of 0.1 mm. Then, the minimum D min  of the interval between protecting member  25  and cover layer  14  is calculated as below: 
                     D   min     =     S   -     S   v     +     S   0     -   A                 =     0.25   -   0.2   +   0.1   -   0.14                 =     0.01   ⁢           ⁢     (   mm   )               .         
Thus, the relation of the above Equation (1) is satisfied. As the result, even when elastic supporting members  24   a ,  24   b ,  24   c ,  24   d  are kept bending toward optical disk  1  by gravity of movable part  16 , protecting member  25  does not come into contact with cover layer  14 .
 
   Example 2 
   In this Example also, cover layer  14  of optical disk  1  has a thickness of 0.1 mm; the laser beam has a wavelength of 405 nm; objective lens  20  has an NA of 0.85; the interval between the surface of cover layer  14  and objective lens  20  (working distance) is 0.3 mm at focusing of the laser beam on information signal recording layer  13 ; protecting member  25  protrudes more than objective lens  20  by 0.05 mm; and non-contact interval S is 0.25 mm in the controlled state. 
   Movable part  16  of optical head  2  has a mass of 230 mg, which is less than that in Example 1. The total of the spring constants of elastic supporting members  24   a ,  24   b ,  24   c , and  24   d  is 56 N/m, which is more than that in Example 1. Displacement distance A is decreased to 0.4 mm with the optical disk placed horizontally. 
   At the neutral state, the maximum deviation S ν  to cause a decrease of the relative interval between the top of protecting member  25  and cover layer  14  by various variation factors, is 0.2 mm. 
   The decrease of the mass of movable part  16  and the increase of the spring constant of elastic supporting member  24   a ,  24   b ,  24   c ,  24   d  enables a sufficient decrease of the displacement distance A. Thereby, without withdrawal of movable part  16  in the neutral state from the controlled position (S 0 =0), the minimum D min  of the interval between protecting member  24  and cover layer  14  is calculated as below: 
                     D   min     =     S   -     S   v     +     S   0     -   A                 =     0.25   -   0.2   +   0   -   0.04                 =     0.01   ⁢           ⁢     (   mm   )               .         
Thus, the relation of the above Equation (1) is satisfied. As the result, even when elastic supporting members  24   a ,  24   b ,  24   c , and  24   d  are bent toward optical disk  1  by gravity acting on movable part  16 , the protecting member  25  does not come into contact with cover layer  14 .
 
   In the above embodiments explained above, protecting member  25  is formed as a part of lens-holding member  23 , but it is not limited thereto. Protecting member  25  may be formed at any position of movable part  16  driven together with objective lens  20 , or on objective lens  20 , itself. For example, protecting member  25  may be formed from a transparent glass or a plastic material, to protrude from objective lens  20  in integration. In this case, protecting member  25  preferably has a surface film constituted of a material having high slidability, not to cause damage to objective lens  20  and cover layer  14  on contact with optical disk  1 .