Patent Publication Number: US-2007104047-A1

Title: Optical disc drive and focus control method thereof

Description:
This application claims the benefit of Taiwan application Serial No. 94139501, filed Nov. 10, 2005, the subject matter of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates in general to an optical disc drive and a focus control method therefor, and more particularly to an optical disc drive that can prevent its object lens from crashing the optical disc and to a focus control method therefor.  
      2. Description of the Related Art  
      In an optical disc drive, an optical pickup head focuses a light beam via an object lens on a rotating optical disc, and receives a reflected light from the optical disc by using a light detector. However, in addition to normal disc rotation, the optical disc in rotation has abnormal movement such as disc vibration and eccentric rotation, where the disc vibration may lead to focusing errors. Thus, a prerequisite for performing read and write operations on the optical disc is to obtain a signal indicating focus error according to which the position of the optical disc can be adjusted.  
      It is desirable to develop new optical data storage media for the next generation to store much more data in order to achieve high-density storage in a single medium. This goal can be achieved by making the light spot smaller through a laser beam of a reduced wavelength for accessing data or an object lens with a larger numerical aperture (NA). However, a larger NA will reduce the working distance (WD) measured between the object lens and the optical disc.  
      When the working distance WD is reduced, the object lens might crash the optical disc, resulting in permanent damage to the object lens or the optical disc. Manufacturing error of the actuator or inappropriate servo control may cause this undesired situation when the object lens moves upwards or downwards to perform focus servo control. It is appreciated that a more accurate method for focus control should be provided as high-density optical storage media is developing.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the invention to provide an optical disc drive and a focus control method such that the object lens focuses on the optical disc correctly and crash of the object lens and optical disc will be prevented.  
      According to an embodiment of the invention, a focus control method is provided for use in an optical disc drive, wherein the optical disc drive includes an optical pickup head to access an optical disc, and the optical pickup head includes an object lens. The method includes the following steps. First, a light beam is emitted to the optical disc and a corresponding reflected light beam is then received to generate a summation signal and a focus error signal, wherein the object lens is located at an original position. Next, it is determined whether the summation signal is greater than a threshold value. If the summation signal is greater than a threshold value, the object lens is moved towards the optical disc for a first distance from the original position and away from the optical disc for the first distance from the original position. In addition, the object lens is focused on the optical disc correctly according to an S-curve of the focus error signal. If the summation signal is not greater than the threshold value, the object lens is moved away from the optical disc for a second distance, and then it is determined whether an S-curve is generated according to the focus error signal. If so, the object lens is focused on the optical disc correctly. Otherwise, the object lens is moved to the original position and then towards the optical disc for a third distance, and the object lens is focused on the optical disc correctly according to an S-curve of the focus error signal.  
      According to another embodiment of the invention, an optical disc drive is provided for accessing an optical disc. The optical disc drive includes an optical pickup head and a controller. The optical pickup head includes an object lens. The object lens is located at an original position. The optical pickup head is used for emitting a light beam to the optical disc and receiving a corresponding reflected light beam to generate a summation signal and a focus error signal. The controller is used for determining whether the summation signal is greater than a threshold value, wherein the controller controls the object lens to move towards and away from the optical for a first distance from the original position, and to focus the object lens on the optical disc correctly according to an S-curve of the focus error signal if the summation signal is greater than a threshold value. If the summation signal is not greater than the threshold value, the controller controls the object lens to move away from the optical disc for a second distance. If an S-curve is generated according to the focus error signal, the controller controls the object lens to focus on the optical disc correctly. If no S-curve is generated according to the focus error signal, the controller controls the object lens to move to the original position and then towards the optical disc for a third distance, and controls the object lens to focus on the optical disc correctly according to an S-curve of the focus error signal.  
      Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A-1C  illustrate detection of light.  
       FIG. 1D  shows the relationship between an error focus signal FE and a working distance.  
       FIG. 2  illustrates the working distance.  
       FIG. 3  illustrates the relative position of the object lens and the optical disc according to an embodiment of the invention.  
       FIG. 4  shows a flowchart illustrating a method for focus control according to a preferred embodiment of the invention.  
       FIG. 5A  illustrates the situation when the distance x is in a first state.  
       FIGS. 5B and 5C  illustrate the situation when distance x is in a third state.  
       FIG. 5D  illustrates the situation when the distance x is in a fourth state.  
       FIG. 6  shows an optical disc drive according to a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIGS. 1A, 1B , and  1 C, a photo-detector module of an optical pickup head is illustrated. The photo-detector module includes four split photo-detectors A, B, C, and D. When the optical disc is located at different positions before or after the focus, the light spot correspondingly forms different shapes on the four split photo-detectors and the split photo-detectors receives the light beam and generate signals VA, VB, VC, and VD, respectively. A focus error signal FE can then be obtained by signal processing of these signals generated by the split photo-detectors. The value of the focus error signal can be determined by: 
 
 FE =( VA+VB )−( VC+VD ). 
 
      As illustrated in  FIG. 1A , the light spot forms a circle on the photo-detector module when the object lens focuses on the optical disc correctly. In this case, the value of the focus error signal is zero. As shown in  FIG. 1B , the light spot forms an ellipse on the photo-detector module when the object lens locates too close to the optical disc. The value of the focus error signal in this case is less than zero. Referring to  FIG. 1C , the light beam also forms an ellipse on the photo-detector module when the object lens is located too far from the optical disc. The value of the focus error signal in this case is greater than zero.  
      Referring to  FIG. 1D , a graph illustrates the relationship of the values of the focus error signal and the working distance, wherein a curve similar to the letter “S” is shown, called an S-curve. In  FIG. 1D , it is assumed that the distance is defined as zero when a state as indicated in  FIG. 1A  occurs; that is, the focus is correct. For a state as indicated in  FIG. 1B , the distance ranges between −0.1 mm to 0 mm. For a state as indicated in  FIG. 1C , the distance ranges between +0.1 mm to 0 mm. In a state where the distance is greater than +0.1 mm or less than −0.1 mm, the value of the focus error signal FE tends to about zero.  
      Referring to  FIG. 2 , the working distance is illustrated in an optical disc drive. The optical disc drive  200  includes a turntable  205 , a spindle motor  201 , an optical pickup head  206 , a guided rod  203 , and a board  204 . The optical pickup head  206  includes an object lens  202 . The turntable  205  supports the optical disc while the spindle motor  201  rotates the optical disc. The optical pickup head  206  emits a light beam to the optical disc and receives the reflected beam, through the object lens  202 . The optical pickup head  206  accesses the optical disc for data by way of the movement of the guided rod  203 . All of the parts described above are configured on the board  204 .  
      When the optical disc is accessed, the distance between the object lens  202  and the optical disc, that is, the distance at which the object lens  202  focuses on the optical disc correctly, is defined as the working distance (WD). In  FIG. 2 , the perpendicular distance (WD 1 ) between the object lens  202  and the upper surface of the turntable  205 , as the optical disc is actually accessed, is required to reach a suitable state to enable the object lens  202  to focus on the optical disc correctly.  
      Referring to  FIG. 3 , the relative positions between the object lens and the optical disc are illustrated according to the invention. Since the optical disc  310  may vibrate while the optical disc  310  is accessed, the distance R is defined to indicate a reasonable range of vibration. When the object lens  202  focuses on the optical disc  310  correctly, the distance between the object lens  202  and optical disc  310  is the WD, that is, the distance measured along with the horizontal axis from 0 to −WD. If the optical disc  310  is located at the coordinate of −WD, measured along with the horizontal axis from zero, the optical disc  310  vibrates in a range of coordinates between −(WD+R) to −(WD−R).  
      The movement of the object lens  202 , measured along with the horizontal axis, as indicated in  FIG. 3 , can be divided into ranges I, II, III, and IV. The coordinates of the range I on the horizontal axis are between −b to −d. The coordinates of the range II on the horizontal axis are between +d to −d. The coordinates of the range III on the horizontal axis are between +d to +c. The coordinates of the range IV on the horizontal axis are between +c to +a.  
      The perpendicular distance between the object lens  202  and optical disc  310  is designated as x. As above discussed, when the object lens  202  is in the range I, the range of the distance x is expressed by: WD−(R+b)≦x≦WD+(R−d).  
      When the object lens  202  is in the range II, the range of the distance x is expressed by: WD−(R+b)≦x≦WD+(R+d).  
      When the object lens  202  is in the range Ill, the range of the distance x is expressed by: WD−(R−d)≦x≦WD+(R+c).  
      When the object lens  202  is in the range IV, the range of the distance x is expressed by: WD−(R−c)≦x≦WD+(R+a).  
      According to the above four ranges of x and the possible range of movement of the optical disc  310 , the value of the distance x between the object lens  202  and optical disc  310  may fall into four ranges indicating four different states of the object lens  202  as follows.  
      In a first state where x&lt;WD−d, the distance x between the object lens  202  and optical disc  310  is not enough for focusing and the object lens  202  needs to move far from the optical disc  310 .  
      In a second state where WD−d&lt;x&lt;WD+d, the distance x between the object lens  202  and optical disc  310  is close to an ideal working distance WD, and the object lens  202  will focus on the optical disc  310  correctly if the object lens  202  moves within a specific range.  
      In a third state where WD+d&lt;x&lt;WD+(R+c), the distance x between the object lens  202  and optical disc  310  is far away from the ideal working distance WD. Making the object lens  202  close to the optical disc  310  will smooth the way for correct focusing.  
      In a fourth state where WD+(R+c)&lt;x&lt;WD+(R+a), the distance x between the object lens  202  and optical disc  310  is larger than that in the third state. As compared to the third state, the object lens  202  needs to move closer to the optical disc  310  in order to smooth the way for correct focusing.  
      Referring to  FIG. 4 , a focus control method in accordance with a preferred embodiment of the invention is shown in block diagram form. First, as indicated in step  401 , the optical pickup head  206  is enabled to emit a light beam to the optical disc  310  via the object lens  202  and receive a reflected light beam from the optical disc  310  so as to generate a summation signal, denoted by “SUM”, and a focus error signal, denoted by “FE”. The object lens  202  is located at an original position. The summation signal, i.e. SUM, is defined by: 
 
 SUM =( VA+VB+VC+VD ). 
 
      Next, in step  402 , a determination is made as to whether the summation signal SUM is greater than a threshold value Th. If so, it is determined that the distance x indicates the second state, where WD−d&lt;x&lt;WD+d, and then step  403  is performed to enable the object lens  202  to move, from the original position, towards and away from the optical disc  310  for a first distance, for example, WD/2 or 2 d. In step  404 , the focus error signal FE is detected and the S-curve is generated accordingly. As in step  405 , after the object lens  202  focuses on the optical disc  310  correctly by determination with the S-curve, the method ends.  
      In step  402 , if the summation signal SUM is not greater than the threshold Th, step  406  is performed to move the object lens  202  away from the optical disc  310  for a second distance, where it is supposed that the distance x indicates the first state, i.e. x&lt;WD−d, and the second distance is, for example, equal to k 1 (WD+d). Next, as indicated in step  407 , a determination is made as to whether an S-curve is generated according to the focus error signal FE. If so, step  408  is performed to make the object lens  202  focus on the optical disc  310  correctly according to the S-curve.  
      In step  407 , if it is determined that no S-curve is generated according to the focus error signal FE in step  406 , it shows that the distance x neither indicates the first state nor the second state. Step  409  is then performed where the object lens  202  is moved to a third distance close to the optical disc  310  after returning to its original position. In this case, it is supposed that the distance x indicates the third state where WD+d&lt;x&lt;WD+(R+c), and the third distance is, for example, equal to k 2 (R+c+d).  
      Next, as shown in step  410 , it is determined whether an S-curve is generated according to the focus error signal FE in step  409 . If so, according to the S-curve, the object lens  202  is enabled to focus on the optical disc  310  correctly. The method then ends.  
      In step  410 , if it is determined that no S-curve is generated according to the focus error signal FE in step  409 , the method proceeds to step  412 . In step  412 , the object lens  202  is moved to be close to the optical disc  310  for a fourth distance after returning to its original position. In this case, it is supposed that the distance x indicates the fourth state where WD+(R+c)&lt;x&lt;WD+(R+a), and the fourth distance is, for example, equal to k3(R+a+d). After that, an S-curve is generated according to the focus error signal FE, as indicated by step  413 . After the object lens  202  correctly focuses on the optical disc  310  according to the S-curve, as shown by step  414 , the method ends.  
      In step  403 , since the distance x indicates the second state, the object lens  202  needs to move close to or far away from the optical disc  310  for at least a distance d in order to make the distance x be the working distance WD. In this way, the object lens  202  can correctly focuses on the optical disc  310 . Thus, in addition to WD/2 or 2 d, the first distance can be a value leading to the generation of an S-curve and prevention of crash involving the object lens  202  and optical disc  310 .  
      Referring to  FIG. 5A , the distance x indicates the first state, wherein the distance x in this example indicates the worst case that would occur in the first state, that is, the object lens  202  probably crashing the optical disc  310 . In step  406 , the object lens  202  needs to move away from the optical disc  310  for at most a distance of WD+d in order to generate an S-curve. However, since the actuator that moves the object lens  202  may cause errors or has a specification error, it requires that the actuator drives the object lens  202  according to a distance of (WD+d) times a parameter k 1  in order to move the object lens  202  far away from the optical disc  310  for a distance of WD+d. As an example, the parameter k 1  is usually taken as about 1.3, which can also be adjusted depending on the error.  
      Referring to  FIGS. 5B and 5C , the distance x indicates the third state, wherein the distance x in this example is the maximum value in the third state. When the distance x equals to WD+(R+c), the optical disc  310  is located at the coordinate of −(WD+R) and the object lens  202  is located at the coordinate of +c. In this case, the coordinate to which the object lens  202  is moved can lead the object lens  202  to focus correctly on the optical disc  310  is −R. In step  409 , in order to obtain an S-curve, the object lens  202  needs to be moved close to the optical disc  310  for the third distance k 2 (R+c+d). The parameter k 2  is required for the similar reason to that for the parameter k 1 . This reason will no longer repeated here for the sake of brevity.  
       FIG. 5C  illustrates an example where the distance x is the minimum value in the third state, wherein the distance x is equal to WD+d. In order to prevent the object lens  202  from crashing the optical disc  310  as approaching the position for the third distance, the following condition should be fulfilled: 
   k 2( R+c+d )&lt; WD+d.    It follows that:    c &lt;( WD+d )/ k 2−( R+d ).  
 According to this inequality, a range of the distance c can be determined for use in setting the value of distance c. 
 
      Referring to  FIG. 5D , the distance x indicates the maximum value in the fourth state, as an example. In  FIG. 5D , the optical disc  310  is located at the coordinate of −(WD+R) and the object lens  202  is located at the coordinate of +a. In this case, the object lens  202  can focus on the optical disc  310  correctly when being moved to the coordinate of −R. In step  412 , the object lens  202  is moved close to the optical disc  310  for the fourth distance so as to generate an S-curve. The fourth distance has to be equal to k 3 (R+a+d). The parameter k 3  is required for the similar reason to that for the parameter k 1 . This reason will no longer repeated here for the sake of brevity.  
      In the fourth state, the fourth distance has to be less than the possible minimum value of the distance x in the fourth state in order to prevent the object lens  202  from crashing the optical disc  310 , so that: 
 
 k 3( R+a+d )&lt; WD+c+R.  
 
It follows that: 
 
 a &lt;( WD+c+R )/ k 3−( R+d ). 
 
 According to this inequality, a range of the distance a can be determined for use in setting the value of distance a. 
 
      Referring to  FIG. 6 , an optical disc drive is illustrated according to a preferred embodiment of the invention. The optical disc drive  200  is used for accessing the optical disc  310 . The optical disc drive  200  includes an optical pickup head  206  and a controller  220 . The optical pickup head  206  includes an object lens  202 . The optical disc  310  is a blu-ray disc (BD), for example. The controller  220  drives the optical pickup head  206  according to the summation signal SUM 1  and the focus error signal FE, in order to change the perpendicular distance between the object lens  202  and optical disc  310 . The optical disc drive  200  applies a focus control method, as described in  FIG. 4 .  
      The above description discloses an optical disc drive and a focus control method thereof according to the embodiments of the invention. Accordingly, in a high-density optical storage medium where a reduced working distance is required, the object lens not only can focus correctly and accurately on the storage medium, but also prevent the object lens from crashing the storage medium. Permanent damage to the object lens or the storage medium will be avoided therefore.  
      While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.