Patent Publication Number: US-2007104043-A1

Title: Objective lens driving device for optical recording media and device having the objective lens driving device

Description:
FIELD OF THE INVENTION  
      The present invention relates to a pickup for optical recording media, in particular to a mechanical layout thereof, and to a device for accessing optical recording media having the pickup.  
     BACKGROUND OF THE INVENTION  
      In apparatus to read from or write to optical recording media having information tracks, a pickup is employed to access locations of the recording medium. For following the information tracks, the medium and the pickup are being moved relative to each other. In the pickup, typically, one or more light beams are generated, are focused onto a target spot on a layer within the medium, and when reading the medium, one or more readout beams coming from the target spot are projected onto a photodetector, where they are converted into electrical signals for further evaluation.  
      The medium mounted in the apparatus and being moved relative to the pickup may have position error in that the target spot varies relative to and around its nominal position. Such position error may occur within the layer in a direction at least substantially diagonal or perpendicular to the information tracks where it is generally called tracking error, the direction correspondingly being called tracking direction; it also may occur in a direction at least substantially perpendicular to the layer where it is generally called focus error, the direction correspondingly being called focus direction; or it occurs in the form that the orientation of the medium relative to the pickup deviates from a nominal orientation, which form is often called tilt error. A direction which is orthogonal both to the tracking direction as well as to the focusing direction will be called information direction in the following, because it is the direction of the tangent to the information track in the target spot.  
      To keep the light beam focused on the target spot and to keep the readout beam on the photodetector, optical media pickups typically have a so-called actuator or objective lens driving device. The actuator, typically, has a movable part which carries a lens and is positioned by magnets interacting with coils onto which electrical currents are imposed. A so-called tracking servo loop controls the position of the movable part in the tracking direction, a so-called focus servo loop controls the position of the movable part in the focus direction, and for some pickups a so-called tilt servo loop controls the angle of the readout beam which in turn influences the incidence of the readout beam onto the photodetector.  
      The tracking direction and the focus direction together span a plane of motion, in which the focussing, tracking and tilt motions take place. The movable part of the actuator is typically suspended by elastic suspension means like wires oriented in parallel and in the information direction, i.e. at right angle to the plane of motion.  
      Generally, the actuator carries on its movable part one or more focussing coils, one or more tracking coils, and optionally one or more tilt coils. These coils may be realized as printed coils or as conventional coils wound onto bobins. In each case, at least parts of the coils are facing magnets the surface of which typically is oriented in the plane of motion. These magnets, even if having several distinct poles, may consist of a single i.e. monolithic piece of magnetic material appropriately magnetized, or they may be compound magnet configurations comprizing but not restricted to one or more physically distinct poles and zero or more pieces of spacing meterial. All through the following description, “magnet” is always meant to denote both these forms.  
      In order to generate a strong tracking force with a closed loop tracking coil oriented in the plane of motion, the magnet will have a section where magnetic poles of opposing polarity adjoin along a separation line oriented in focus direction. This setup or an equivalent is needed in order that the upper branch of the tracking coil having a current in one direction, and the lower branch of the tracking coil having the return current flowing back in the opposite direction, will face magnetic fields of opposing polarity and hence will generate Lorentz forces both going in the same direction. According to the same principle, for a strong focus force, the magnet will have another section where magnetic poles of opposing polarity adjoin along a separation line oriented in tracking direction. The two separation lines of different orientation taken together, pickup magnets often have, on their surface, one or more boundaries or separation lines of opposing poles which are L-shaped, C-shaped or otherwise non-straight, buckled or curved. A magnetic pole confined by boundaries or separation lines like these will often be of unequal width.  
      In a coil facing a magnetic pole of non-straight circumference, the force generating coil branches, also called operative portions of the coil, may be facing parts of the magnetic pole that are of different width. As a consequence, the force generated by a branch will be unevenly distributed over the branch or between the branches, so that the resulting point of impact of the equivalent force or forces will deviate from the geometric center or centroid of the branch or branches involved. This in turn may lead to a torque being generated as an unwanted by-product when ideally a pure thrust force is aimed at.  
      US 2005/0185530 A1 discloses object lens driving devices where coils on a movable unit are facing magnets having two or more poles where in a part of the magnet that faces a coil two opposing magnetic poles adjoin along a first separation line, and where the first separation line has a first line part where at least one of the adjoining magnetic poles has a profile property of being wider, in a direction transversal to the first separation line, than at a second line part of the first separation line. L-shaped separation lines between other opposing poles are also disclosed. In all devices it discloses, a movable unit with a two-sided suspension is shown. In order to counteract unwanted torques, US 2005/0185530 proposes to place the focussing coils in such a way that their center is offset, in tracking direction, from the point where the smaller ones of the magnetic poles adjoining at the L-shaped boundary or separation line have maximum magnetic flux density. With such arrangement, when the movable unit is offset in tracking direction, the forces generated by the two focusing coils will not be equal, so that a counteracting torque is generated.  
     INVENTION  
      The prior art can be seen to have the drawbacks that the movable unit has a mechanically complicated suspension, that only those unwanted torques are neutralized that arise when focussing offset and tracking offset exist simultaneously and that, by the disclosed design, coils are being deliberately placed where the sensitivity of the coil-magnet interaction is sub-optimal. The invention aims to propose a pickup with an actuator that is improved in this regard.  
      According to the invention, an objective lens driving device or actuator is propsed, where in the vicinity of that part of a separation line between opposing magnetic poles where the poles are wider, an un-magnetized neutral zone between the poles is formed wider than where the poles are narrower.  
      An objective lens driving device or actuator according to the invention has a movable unit or movable part carrying coils that face fixed multipolar magnets. In the magnets, opposing magnetic poles adjoin along a first separation line having a first line part where one or both of the adjoining magnetic poles are wider than at a second line part of the first separation line. The invention then teaches that near the first line part of the first separation line, the magnet has an un-magnetized neutral zone that is wider than in the vicinity of the second line part. In that way, in light of the bigger area of the wider part of the poles, the magnetic flux is deliberately reduced there slighty and to such an extent that the force generated there is of same magnitude than the force generated in the narrower part of the poles adjoining at the second separation line. One advantage of this is that focusing coils facing magnetic poles adjoining at a separation line orthogonal to the first separation line can be freely positioned, for instance advantageously where the magnetic flux is maximum. Another advantage is that the scheme can be realized together with an actuator that has a one-sided suspension.  
      In the objective lens driving device, the property of a magnetic pole of being wider at the first line part may be due to the pole adjoining a third magnetic pole along a second separation line at least partially not parallel to the first separation line and the pole being confined by the first and second separation lines. In particular, the second separation line may be L-shaped with a first line segment being parallel to the first separation line and a second line segment being at right angles to the first separation line.  
      Alternatively, the property of a magnetic pole of being wider at the first line part than at a second line part may be due to the pole having a reduced space because of one or more other obstructing apparatus elements. Depending on design criteria like pickup dimensions, manufacturability or longevity, and on the specific pickup design resulting therefrom, the space for the magnetic poles in an actuator may be reduced or restricted. Depending on the material of any obstructing apparatus elements, it may be appropriate that a magnetic pole not only keeps out of the way of these elements, but even observes a certain distance or clearance therefrom, which further reduces the space where a magnetic pole may be situated. Space restrictions because of obstructing elements need not be symmetric with respect to the pole geometry, hence a pole may result having different width along different line parts of a separation line where it adjoins an opposing pole.  
      Advantageously, between the first and second line parts, the transition between the different widths of the un-magnetized neutral zone is formed in a stepped, linearly increasing, or steadily changing fashion, or in a combination thereof. The advantage is that account can be taken of the properties of any magnetizing equipment used for generating the magnetic poles.  
      When there are, besides the first separation line, other L-shaped separation lines where the poles adjoin with other, third poles, the transition between the parts of the un-magnetized neutral zone of different width is advantageously situated near where the L-shaped separation lines have their corner or buckling. In that way, the width of the force-generating interaction area between magnet pole and coil is substantially kept constant over the length of the first separation line.  
      Also, the width of the wider part of the un-magnetized neutral zone is advantageously dimensioned in such a way that the intersection area of the coil with the wider part of the magnetic pole is substantially equal to the intersection area of the coil with the narrower part of the magnetic pole. In that way, the Lorentz forces generated by the setup will be evenly distributed.  
      A device for accessing optical recording media according to the invention is a device for reading from such media or for writing to such media, or for reading and writing such media, which employs an objective lens driving device or actuator according to the invention as described above.  
      Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. They refer to the recording medium being an optical disk and to the readout beam being a reflected beam. Despite this, it is clear to those in the art that the invention can be employed on any kind of optical recording medium having information tracks in layers, like those in card or tape form; and regardless whether the readout principle is of a reflective or of a transductive type. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
      In the drawings, throughout which like parts are designated by like reference numerals,  
       FIG. 1  shows in perspective view a pickup for optical disks;  
       FIG. 2  shows in perspective view the assembly consisting of a printed circuit board carrying printed coils and magnets facing it, for a prior art actuator;  
       FIG. 3  shows in plan view the superposition of magnet contours and coil contours for a prior art actuator;  
       FIG. 4  shows in plan view the superposition of magnet contours and coil contours for an actuator according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1  shows in perspective view a pickup  1  for optical disks, optical disks being a common type of optical recording media. In optical disk media, information tracks are shaped as spirals or concentric circles. In pickups for optical disks, motion of the medium relative to the pickup is achieved by rotating the disk, for example by a so-called spindle motor, and by moving the pickup in a radial direction relative to the centre of the disk, for example by a step motor or a threaded spindle or an equivalent linear positioning device of the art. In a pickup with an optical disk, the tracking direction corresponds to the radial direction, and the focus direction corresponds to an axial direction parallel to the rotation axis of the disk. In the following description, a reflective optical disk is assumed, where the readout beam corresponds to the reflected light beam resulting from the incident light beam being reflected by an outer or inner surface or layer of the disk.  
      The pickup  1  of  FIG. 1  has a base plate  2  with two vertical yokes  3  carrying multipolar magnets  5 , and a suspension wire holder  6 , into which suspension wires  7  are fastened. On their other end, the elastic suspension wires  7  are fixed to a movable part  8  holding a lens  9  and a printed circuit board  10  having printed coils, occluded in this view. The printed circuit board  10  is oriented in the plane of motion spanned by the focus direction Y and the tracking direction X, the suspension wires are oriented in the information direction Z.  
       FIG. 2  shows in perspective view the assembly consisting of the printed circuit board  10  carrying printed coils occluded in this view, and the multipolar magnets  5  facing it, for a prior art actuator. Between the opposing poles of the magnet  5 , neutral zones  52 ,  54 ,  56  are shown in exaggerated width. Small neutral zones  52 ,  54 ,  56  result and are unavoidable whenever a magnetic material is magnetized into having opposing poles adjoining as in the magnets  5 . The Figure also shows that the neutral zone  52  between opposing poles  51  and  53  and the neutral zone  56  between opposing poles  55  and  57  are L-shaped or non-straight, and that the neutral zone  54  between opposing poles  53  and  55  has a first part  54 ″ where the adjoining magnetic poles  53 ,  55  are wider than at a second part  54 ′ thereof.  
       FIG. 3  shows in plan view the superposition of magnet contours  51 ,  53 ,  55 ,  57  and coil contours  121 ,  122 ,  123  for a prior art actuator. In the coil of contour  121 , which is a first focus or tilt coil, an upper horizontal branch  1211  interacts with the magnetic flux of magnet  51 , and a lower horizontal branch  1212  interacts with the magnetic flux of the lower part of magnet  53 . In the coil of contour  122 , which is the tracking coil, a left vertical branch  1221  interacts with the magnetic flux of magnet  53 , and a right vertical branch  1222  interacts with the magnetic flux of magnet  55 . In the coil of contour  123 , which is a second focus or tilt coil, an upper horizontal branch  1231  interacts with the magnetic flux of magnet  57 , and a lower horizontal branch  1232  interacts with the magnetic flux of the lower part of magnet  55 .  
      It can be seen that the part of the magnet  53  that is facing the top part of the left vertical branch  1221  of the coil of contour  122  is less wide than the part of the magnet  53  that is facing the bottom part thereof; and that the part of the magnet  55  that is facing the top part of the right vertical branch  1222  of the coil of contour  122  is less wide than the part of the magnet  55  that is facing the bottom part thereof. As a consequence of this, in the prior art actuator, the bottom parts of the vertical branches  1221 ,  1222  of the coil of contour  122  will produce a force that is slightly bigger than the force produced by the corresponding top parts. This in turn will result in an unwanted torque being produced, which, when no countermeasures are being taken, will rotate, i.e. tilt, the movable part around the axis of the information direction Z. Any tilting will deteriorate the readout signals from the disk. Also, keeping in mind that the disk is being rotated, and that tracking and focus forces are highly dynamic, such torque and rotation is likely to cause mechanical oscillations of the movable part and to de-stabilize the actuator and its servo control.  
       FIG. 4  shows in plan view the superposition of magnet contours  51 ,  53 ′,  55 ′,  57  and coil contours  121 ,  122 ,  123  for an actuator according to the invention. Here it is shown that in the magnet composed of poles  51 ,  53 ′,  55 ′,  57 , near a first, lower line part  13 ′ of a first separation line  13  the adjoining magnetic poles  53 ′ and  55 ′ are wider than at the top line part  13 ″ of the first separation line  13 . It is also shown that opposing magnetic poles  51  and  53 ′ adjoin along a non-straight, in this case L-shaped, second separation line  14 , and that opposing magnetic poles  55 ′ and  57  adjoin along a non-straight, L-shaped second separation line  15 . It is also shown that the magnet has an un-magnetized neutral zone  54 ″ in the vicinity of the first separation line  13  and in between the adjoining magnetic poles  53 ′,  55 ′ that is wider in the vicinity of the first, lower part  13 ′ than in the vicinity of the upper part  13 ″ of the first separation line  13 .  
      It is a matter of mechanical design, to dimension the widening of the neutral zone  54  such that the force generated in the wider parts of the magnetic poles equals that generated in the narrower parts, so that no torque is generated. As a first approximation, the dimensioning will be such that the area where the coil  122  faces the magnetic poles  53 ′,  55 ′ near the first line part  13 ′ is substantially equal to the area where the coil  122  faces the magnetic poles  53 ′,  55 ′ near the second line part  13 ″.  
      It has also been found that if the transition  540  between the narrower part  54 ′ and the wider part  54 ″ of the un-magnetized zone  54  is shaped in a sloped form  542 , this has the advantage of minimizing any unwanted focus force which may arise as an unwanted by-product of a tracking force being generated.  
      With other words, the invention refers to a pickup  1  for optical recording media having an actuator  8  for focus and tracking control where, in order to avoid unwanted torque components being generated when magnets  5  facing coils  121 ,  122 ,  123  are unsymmetric, the magnetic flux in broad parts of the magnet  53 ′,  55 ′ is deliberately reduced by broadening, in that region, the neutral zone  54  near a separation line  13  to which the broad magnet parts  53 ′,  55 ′ adjoin.