Abstract:
An optical scanning device for reading data stored on a rotating recording medium having a scanning head; at least one guide device for guiding the scanning head along a predefined track; at least one bearing device connected to the scanning head for supporting the at least one guide device and having at least one elastic prestressing device to provide a static prestress on the at least one guide device to prestress the same and the at least one bearing device connected to the scanning head against each other to eliminate a relative displacement.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an optical scanning device for reading stored data on a rotating recording medium, in particular a scanning device having a scanning head, at least one guide device for guiding the scanning head along a predefined track and at least one bearing device connected to the scanning head for supporting the at least one guide device. 
   BACKGROUND INFORMATION 
   An optical scanning head of a scanning device is guided along a data track to read data stored on a rotating recording medium. 
   The scanning head directs a light beam onto the rotating recording medium through a lens attached to the scanning head to acquire the recorded information by scanning the data tracks applied to the recording medium. In order to avoid data reading errors, it is important in particular that the light beam is always guided exactly along the data tracks and is focused on the recording medium. 
   The housing of, e.g., a compact disc (CD) drive installed in a motor vehicle in particular is exposed to various shocks. If the playback device is subjected to vibrations, inertia forces act on the scanning device. At the same time, changes in the position of the laser beam relative to the data track occur since all components present tend to move with respect to each other due to the inertia forces. However, since the function of the device depends on a continuous exact positioning of the scanning head relative to the recording medium, there is in this case the danger of an interruption of function or data reading. 
   The tolerance for positional accuracy corresponds roughly to the geometric width of the data tracks. This is equivalent to approximately 1.6 μm in a compact disc and approximately 0.740 μm in a digital video disc (DVD). 
   The optical scanning head is moved on a radial track under the recording medium. Of critical significance for the vibration performance is the quality of the linear guidance of the optical scanning head on this radial track. In stationary operation, irregularities of guidance are compensated for by adjusting the optics. An electronic regulating device of the optical adjustment is adequately able to balance slowly changing deviations. 
   German Published Patent Application No. 196 42 343 describes a method used to regulate the focusing of a light beam of a scanning device directed onto a rotating recording medium to read data on the recording medium and the guidance of the light beam along the data tracks of the recording medium by at least one closed-loop control circuit. However, only translatory movements of the scanning device caused by vibrations in a lower frequency range up to approximately 100–200 Hz are correctable by this method. Vibrations in a frequency range above this limit cannot be compensated. 
   The manufacturing conditions cause fit tolerances among the individual components which cause them to undergo shocklike movements under vibration. However, the high-frequency components contained in the shocklike movements overtax the electronic regulating devices of the optics. This results in a malfunction of the read-write function. 
   These problems are illustrated in  FIGS. 5   a ,  5   b , and  6 . 
     FIGS. 5   a  and  5   b  illustrate the presence of a fit tolerance  4  between a guide device  11  which guides scanning head  10  along a predefined track and a bearing device  12  for the support of guide device  11 . 
   For cost reasons, scanning head  10  is guided on the other side only by a forked guide element. 
   For the sake of greater comprehensibility, pivot  20  and a section of data track  21  of recording medium  2  are shown in  FIG. 5 . 
     FIG. 6  shows a diagram of the vibration curves of the scanning head and of the entire recording device under acceleration events in the case of the presence of fit tolerances. A relative movement between the scanning head and the recording device due to inertia forces and the fit tolerance is clearly recognizable. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is generally to eliminate the fit tolerance between the scanning head or the bearing device attached to it and its guide device. 
   It is generally attempted to keep the fit tolerances of the sliding bearings as low as possible. In practice, requirements with regard to robustness (e.g., sensitivity to temperature and moisture variations, contaminations), service life, production capabilities, production tolerances and production costs place such narrow limits on these methods that it is impossible to prevent a relative movement of the scanning head at even low vibration. 
   The idea on which the present invention is based is that the optical scanning device has at least one elastic prestressing device to apply a static pre-stress of a specific magnitude and direction to the guide device in order to prestress it and the bearing device connected to the scanning head against each other. 
   In contrast to the known approaches, the optical scanning device of the present invention has the advantage that the scanning head is guided reliably and free from play even in the presence of vibrations and consequently the resistance to shock is increased. Using the invention makes it possible to reduce the vibration sensitivity of optical data scanning devices. This thus eliminates a weakness of many scanning devices available on the market and increases the selection of possible types for vibration-relevant uses. 
   The scanning device according to the present invention is the most efficient way to achieve the objective described since as a rule, it is only necessary to make slight design modifications on already existing components. 
   According to a preferred enhancement, the scanning head has a lens to focus a light beam onto the recording medium. 
   According to another preferred enhancement, the guide device is embodied as a shaft having helical grooves and may be driven by a motor, an electric motor in particular. The motor is used to induce a rotational movement in the shaft which is converted into a linear movement of the scanning head. 
   According to another preferred enhancement, the bearing device is made up of a bearing shell to accommodate the guide device and of an elastic matrix element coupled to the scanning head. The elasticity of the matrix element makes it possible to apply a force of a predefined magnitude and direction to the guide device. 
   According to another preferred enhancement, the matrix element has a plurality of teeth which engage the grooves of the guide device to convert the rotational movement of the guide device into a linear movement of the scanning head. The sizes of the grooves and the corresponding teeth are matched to ensure good guidance. This makes it possible to control the movement of the scanning head on the predefined radial track using the motor. 
   According to another preferred enhancement, the prestressing device is embodied as a spring element, as a leaf spring in particular, which may be attached to the elastic matrix element in such a manner that it is possible to press the plurality of teeth using a static prestress of a predefined magnitude and direction against the guide device. This eliminates the play between the guide device and the bearing device. 
   Thus the matrix element including the teeth applied to it assumes not only the function of a radial guidance but also the function of eliminating the fit tolerance of the relevant components. As a result, the scanning head is held rigid even under high-frequency vibrations in that the magnitude and direction of the prestress is determined in advance according to the strength and direction of the acceleration taking place. 
   According to another preferred enhancement, the prestressing device is embodied as an additional spring element which is attached in the at least one bearing device connected to the scanning head for applying a static prestress of a predefined magnitude and direction to the at least one guide device in order to prestress it and the bearing device against each other. This offers the advantage that it is only necessary to modify already existing parts slightly for improved resistance to shock in conventional drives. This type of tolerance fit elimination is thus a cost-effective means to achieve the objective. 
   According to another preferred enhancement, the recording medium is embodied, e.g., as a compact disc (CD), a digital video disc (DVD), a minidisc (MD) or as or as a magnetooptic disc (MOD). Since the mechanics of related storage media operate according to the same principle, namely a rotating medium and a linearly adjustable scanning head, the present invention is also applicable to all recording media functioning in this manner. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a perspective view of a matrix element and a spring element according to a first embodiment of the present invention. 
       FIG. 2  shows a top view of the matrix element including the spring element in installed condition according to the first exemplary embodiment of the present invention. 
       FIG. 3  shows a view in cross-section along line H—H from  FIG. 2 . 
       FIG. 4  shows a top view of a spring element in installed condition according to a second exemplary embodiment of the present invention. 
       FIG. 5   a  shows a top view of an optical scanning device. 
       FIG. 5   b  shows an enlarged view of the part of the optical scanning device labeled A in  FIG. 5   a.    
       FIG. 6  shows a diagram of the vibration curves of the scanning head and of the entire data recording device in an acceleration event when fit tolerances are present. 
   

   DETAILED DESCRIPTION 
   In the figures, identical reference symbols denote identical components or those having identical functions. 
     FIG. 1  shows a perspective view of an elastic matrix element  121  which is embodied as a part of a bearing device  12  to accommodate a guide device  11  shown in  FIG. 2  and a correspondingly embodied prestressing device  13  in the form of a leaf spring  13  according to a first exemplary embodiment of the present invention. 
   Matrix element  121  is made of a back section  123 , which may be affixed to scanning head  10 , and a front section  124 , which is elastically connected to back section  123 , both the top and bottom of connection point  125  between back section  123  and front section  124  being formed in the shape of a channel and thus being used as a hinge for an elastic movement. Due to the recess at connection point  125 , the stiffness is reduced there compared to the back and front section and connection point  125  acts as a hinge when force is applied to front section  124 . 
   In addition, a through hole  29  as well as pins  126  are placed both at the top and at the bottom of back section  123 . 
   Front section  124  has two arms  127 , each of which has a plurality of teeth  122  on its bottom and a contact point  128  on its top. 
   Prestressing device  13 , also shown in  FIG. 1  and embodied as a leaf spring  13 , has two symmetrical holes  131  which assume a shape appropriate to accommodate pins  126  on the top of back section  123  of matrix element  121 , and a hole  133  situated between the two holes  131 , this hole being formed to match through hole  129 . The use of additional fastening elements, which are not shown here, makes it possible to attach leaf spring  13  to the matrix element and it in turn to scanning head  10 . 
   Leaf spring  13  has a U shape, its two ends  132  being bent up somewhat at bending points in both legs. 
     FIG. 2  illustrates matrix element  121  according to the first exemplary embodiment of the present invention as installed on scanning head  10 . 
   Matrix element  121  together with leaf spring  13  is attached using a screw which may be inserted into overlapping holes  129  and  133  of the back section and of the leaf spring. 
   A bearing shell  120  is connected to scanning head  10 , the bearing shell together with matrix element  121  forming the for entire bearing device  12  to support guide device  11 , the guide device in this exemplary embodiment being embodied in the form of a spindle shaft  11  having helical grooves  110 . 
   Spindle shaft  11  is driven by an electric motor, which is not shown, to produce rotational movements and is located in bearing shell  120  of scanning head  10 . 
   A cross-section along line H—H from  FIG. 2  is shown in  FIG. 3 , a portion of recording medium  2  being also depicted above the system for improved comprehensibility. 
   The principle and the function of the first exemplary embodiment of the present invention is illustrated in this sectional view. 
   Spindle shaft  11  is located in bearing shell  120  of scanning head  10 . Teeth  122  are designed in such a manner that they are in contact with the tooth faces of grooves  110  of spindle shaft  11  and as a result convert the rotational movement of spindle shaft  11  into a linear movement of the scanning head along the radial track. 
   However, as may be seen in  FIG. 5   a , without prestressing device  13 , a gap  4  exists between teeth  122  and the spindle surface since both components are subject to tolerances. Bearing shell  120  also has a fit tolerance. 
   When such a device is installed in, e.g., a motor vehicle, it is exposed to specific accelerations, due to which the device experiences vibrations. Accelerations in the direction of the plane of recording medium  2 , which are applied parallel to the direction of travel of scanning head  10 , are in particular critical with respect to vibrations and accordingly for an interruption of data reading. 
   For that reason, the direction and magnitude of the prestress acting on spindle shaft  11  by means of prestressing device  13  must be determined appropriately in advance with respect to both a specific direction, namely in the plane of the acceleration occurring, and with respect to the force occurring as a result of the acceleration. The necessary elastic force of leaf springs  13  in the direction which is favorable in particular to prestressing the components involved with respect to the acceleration occurring is determined in advance with consideration of specific factors such as mass and center of gravity of the scanning head and the force of acceleration acting on the device in the special application. Spring element  13  is selected appropriately and is installed accordingly. 
   For matrix element  121  to also be able to exert prestressing force on spindle shaft  11 , in addition to the function of converting the rotational movement of spindle shaft  11  into a linear movement of scanning head  10 , leaf spring  13  is attached to matrix element  121  in such a manner that the elastic force acting on contact point  128  of matrix element  121  exerts a force on spindle shaft  11  and presses it against the bearing shell wall at least two places by means of the lever at connection point  125  of matrix element  121 . An appropriate design of front section  124  of matrix element  121  makes it possible to divert this prestress so that a sufficiently great force component is effective in the plane of the medium. 
   In motor vehicles, accelerations reaching a maximum of 5 g occur in the plane of the medium. From this, it is possible to calculate the force acting on spindle shaft  11  in acceleration events. Accordingly, spring element  13  must be selected in such a way that it exerts a prestress transferred to spindle shaft  11  via teeth  122  with a sufficiently great force component in the plane of the medium perpendicular to the spindle shaft and thus prestresses spindle shaft  11  against bearing shell  120  even if the maximum acceleration force occurs. 
   Similarly, it is possible to select the angle of the prestress relative to the plane of the medium by a special design of the system. Since, however, as mentioned above, accelerations in the direction of the plane of the medium parallel to the direction of travel of scanning head  10  are critical in particular, the angle must be selected to be as small as possible for as great as possible a prestress relative to this direction. 
   In order to prevent scanning head  10  from tipping relative to spindle shaft  11 , it is necessary to exert an appropriate prestress on spindle shaft  11  using the two arms  127  of front section  124  at least two points spaced apart by a predefined amount to prevent a torque. This ensures that the spindle is pressed onto the wall of bearing shell  120  opposite teeth  122  at least two different points. As a consequence, scanning head  10  does not tip or turn even in the presence of vibrations in an upper frequency range. 
     FIG. 4  illustrates a second exemplary embodiment of the present invention. 
   In this exemplary embodiment, leaf spring  13  from the first exemplary embodiment is replaced by an additional spring element  1300 . 
   Spring element  1300  is inserted, for example, in bearing device  12  or in bearing shell  120  connected to scanning head  10 , spring element  1300  being attached to a wall opposite spindle shaft  11  parallel to the plane of the medium and engaging two adjacent points on spindle shaft  11 . 
   Again, such a spring element  1300  must be selected to have adequate elastic force to counteract the acceleration forces occurring when driving and to create a continuous prestress of spindle shaft  11  and scanning head  10  or bearing shell  120  relative to each other. 
   As a result, the scanning head is guided reliably and free from play even in the event of vibration due to only a slight modification to the already existing parts. Thus the present invention eliminates a notorious weakness of the drives cited above and represents the most efficient way to implement damperless drives. 
   When used in conventional drives, a noticeable improvement, even by a factor of 2 to 3 in the most favorable cases, is attained in vibration performance, in vertical installation in particular. The expected advantage is so great that it is worthwhile to install it in existing products, even considering the expense of modification. 
   Although the present invention has been described above on the basis of a preferred embodiment, it is not limited to it but instead is modifiable in a variety of ways.