Abstract:
The present invention relates to a method and apparatus for dynamically positioning the objective lens in an optical disk drive to maintain focus despite loss of perpendicularity between the light beam and the information layer of the optical disk. Loss of perpendicularity may occur as a result of any number of factors, including irregularities in the manufacture of the disk, manufacturing tolerances and assembly of the disk drive components, bearing defect frequencies, shock and vibration. Failure to maintain perpendicularity may interference with the ability of the optical pick up unit of the drive to accurately read and write. The tilt focus mechanism of the present invention utilizes a rotary actuator that positions the objective lens in three dimensions relative to the surface of the optical disk. In one embodiment, a first voice coil motor positions the actuator generally in two dimensions parallel to the surface of the disk and a second voice coil motor positions the objective lens generally along an arcuate path orthogonal to the surface of the disk.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is related to U.S. patent application Ser. No. 09/315,398, filed May 20, 1999, entitled “Removable Optical Storage Device and System,” U.S. Provisional Application Ser. No. 60/140,633, filed Jun. 23, 1999, entitled “Combination Mastered and Writeable Medium and Use In Electronic Book Internet Appliance,” U.S. patent application Ser. No. 09/393,899, filed Sep. 10, 1999, entitled “Content Distribution Method and Apparatus,” U.S. patent application Ser. No. 09/393,150, filed Sep. 10, 1999, entitled “Writeable Medium Access Control Using a Medium Writeable Area,” U.S. patent application Ser. No. 09/548,128, filed Apr. 12, 2000, entitled “Low Profile and Medium Protecting Cartridge Assembly,” U.S. patent application Ser. No. 09/560,781, filed Apr. 28, 2000, entitled “Miniature Optical Disk for Data Storage,” and U.S. patent application Ser. No. 09/540,657, filed Mar. 31, 2000, entitled “Low Profile Optical Head,” all of which are incorporated herein by reference in their entireties. 

   FIELD OF THE INVENTION 
   The present invention is directed generally to an optical disk drive and more specifically to a method and mechanism for positioning an optical pickup element in three dimensions relative to an optical disk. The invention may utilize single or multiple optical disks. In the case of a single disk, the disk may be removable or non-removable. 
   BACKGROUND OF THE INVENTION 
   Optical disk drives are ideally suited for use in personal electronic devices (PEDs). By way of example, optical disk drives may be advantageously utilized in PEDs such as digital cameras, music reproduction equipment, MP3 players, cellular telephones, dictating equipment and personal digital assistants such as microcomputers. In particular, as compared to magnetic disk drives, optical disk drives are superior in terms of storage capacity, power consumption and data transfer speed. As a result, they can be smaller in size and cost. To be practical in PEDs, however, the optical drives need to be substantially pocket sized (e.g., no more than about 100 mm in the largest dimension, but preferably no more than about 50 mm, and preferably having at least one cross section no more than about 100 mm by about 50 mm, preferably no more than about 75 mm by about 25 mm) and have a mass of no greater than about ⅓ kg. 
   Much of the development of optical disk data storage has centered around apparatus in which the read/write mechanism was configured to position a read/write beam at a desired radial location on the disk in a substantially linear fashion (i.e., linear actuators). Typically, a sled carrying an objective lens moves radially along a pair of rails between the inside and outside diameter of a disk for course tracking purposes. A second mechanism or linkage is mounted in the sled and rotates the objective lens in an arcuate path for fine tracking purposes. Further structure also moves the objective lens orthogonally relative to the disk surface for purposes of adjusting the focus of the light beam on the data layer of the disk. While linear actuators have proved useful in a number of contexts, such as for reading/writing CDs and DVDs, the location and mass of the components in linear actuators has typically affected performance parameters such as access time, data transfer rates, and the like. In addition, linear actuators are relatively high-friction devices and require precise track alignment. Linear actuators typically add substantial thickness to a read/write or drive device and generally do not scale well toward miniaturization. Also, linear actuators are typically unbalanced systems in that the mass of the components, including the objective tens, is not evenly distributed relative to any pivot point. As a result, such actuators are highly susceptible to shock and vibration. Thus, linear actuators have, in general, found greatest use in applications where thickness, access time, bandwidth and power consumption are of less importance, and typically are used in larger stationary devices where space for moving the read/write head is available and the risk of shock or significant vibration is minimized. 
   Another factor affecting the size of an optical system is the size and shape of the light beam as it reaches the optical disk (the spot size and quality). Spot size and quality is, in turn, affected by a number of factors including, the size of the optical components, relative movement among the optical components, the distance the light beam must travel and the format of the optical disk. Although a wide variety of systems have been used or proposed, typical previous systems have used optical components (such as a laser source, lenses and/or turning mirrors) that were sufficiently large and/or massive that functions such as focus and/or tracking were performed by moving only some components of the system, such as moving the objective lens (e.g. for focus) relative to a fixed light source. However, relative movement between optical components, while perhaps useful for accommodating relatively large or massive components, presents certain disadvantages, including a relatively large form factor and the engineering and manufacturing associated with establishing and maintaining optical alignment between moveable components. Such alignment often involves manual and/or individual alignment or adjustment procedures which can undesirably increase manufacturing or fabrication costs for a reader/writer, as well as contributing to costs of design, maintenance, repair and the like. Accordingly, it would be useful to provide an optical head method, system and apparatus which can reduce or eliminate the need for relative movement between optical components during normal operation and/or can reduce or eliminate at least some alignment procedures, e.g., during reader/writer manufacturing. 
   In order to adequately miniaturize the mechanics associated with an optical disk drive for use in a commercially acceptable PEDs, the optical recording system&#39;s focus of the laser spot on the recording and playback surface must be maintained to assure acceptable recording and playback data integrity. In general terms, an objective lens directs a light beam to the optical disk and focuses the light beam into a conical shape with the apex or focal spot occurring at the data layer within the optical disk. Ideally, the conical beam is perpendicular to the surface of the disk, although, given irregularities in the manufacture of the disk and its component layers (i.e. disk flatness), bearing defect frequencies, and tolerances in the manufacture and assembly of the mechanical components, as well as shock and vibrations imparted into the disk drive during operation, perpendicularity between the disk surface and light beam is difficult to maintain. The distance between the objective lens and the data layer determines the particular characteristics which the objective lens must possess. For example, the farther the data layer of the disk is from the objective lens, the larger the objective lens must be in order to focus the light beam into the proper conical shape with the focal spot at or proximate to the data layer. In turn, as the objective lens increases in size in order to form the appropriately sized light beam, the other optical components must also increase in size in order to complement each other. Thus, for miniaturization purposes, it is critical to minimize this distance between the objective lens and the data layer on the disk. 
   A significant factor in reducing the distance between the objective lens and the data layer of the optical disk is the characteristics of the disk itself. Optical disks used in consumer products today typically utilize second surface optical media as opposed to first surface optical media. In the preferred embodiment of the present invention, the optical medium is first-surface media. Although it may be subject to more than one definition, first-surface optical media refers to media in which the read beam during a read operation is incident on or impinges on information content portions of the first-surface optical media before it impinges on a substrate of the first-surface optical media. The information content portions can be defined as portions of the optical media that store or contain servo data, address data, clock data, user data, system data, as well as any other information that is provided on the optical media. The information content portions can be integral with the substrate such as the case of a read-only media. The information content portions can also be separately provided. In such a case, the information content portions can be, for example, an information layer of a writeable media Stated conversely, second-surface media can refer to media in which the read beam is incident on the surface of the media or disk before it is incident on the information content portions. 
   A relatively thick and transparent outer layer or substrate of second-surface optical medium makes read-only or read-write operations relatively insensitive to dust particles, scratches and the like which are located more than 50 wavelengths from the information content portions. Considering the cone angle of the light beam after the light beam passes through the objective lens, there is also little detrimental change to the shape or power of the light spot by the time it reaches the information layer of this second-surface optical medium. On the other hand, the second-surface optical medium can be relatively sensitive to various optical aberrations. These optical aberrations include: (1) spherical aberrations—a phase error causing rays at different radii from the optic axis to be focused at different points; (2) coma—creating a “tail” on the recorded spot when the transparent layer is not perpendicular to the optical axis; (3) astigmatism—creating foci along two perpendicular lines, rather than a symmetric spot; and/or (4) birefringence—different polarizations of light behave differently because the read-only or read-write beam must propagate through a relatively longer distance before reaching the information layer, when an aberration is created at the air/transparent layer interface. This longer distance is attributable to the thickness of the relatively thick transparent substrate or layer. Compounding the unwanted birefringence is the requirement that the read-write beam must also traverse the transparent layer again after reflection. 
   Some or all of the aberrations arising from the presence of the thick transparent layer can, at least theoretically, be partially compensated for by using a suitable focus mechanism. However, such a focus mechanism, including the optical elements thereof, tends to be large in size and, concomitantly, increases the cost of the system. Additionally, such a focus mechanism typically can only provide compensation for a single, pre-defined thickness of the layer. Because there are likely be to spatial variations in the thickness or other properties of the transparent layer, such compensation may be less than desired at some locations of the medium. 
   Another drawback associated with second-surface optical media is that the optical requirements of such media are substantially inconsistent with the miniaturization of the disk drive and optical components for such media. As will be appreciated by reference to  FIG. 1A , a longer focal length “f” is required for an optical system that will read information from or write information onto second-surface media This is due to the relatively thick transparent layer “T” through which the radiation must pass to access the recording or data layer “D.” To provide the longer focal length a larger beam cone is required which, in turn, requires larger optical components (e.g., objective lens “O”). Moreover, the relatively long optical path through the thick transparent layer to the data layer and back through the transparent layer after reflection significantly decreases laser power efficiency in comparison to a medium without the transparent layer. In comparison, as shown in  FIG. 1B , a shorter focal length “f” can be achieved by utilizing first surface recording instead of second surface recording. Importantly, a smaller focal distance “f” allows use of a smaller objective lens “O.” This in turn allows the other optical components to be reduced in size thereby facilitating overall miniaturization. 
   To date, rotary actuators have not provided a solution to miniaturization in optical disk drives either. Like linear actuator systems, rotary actuator systems are subject to the same problems created by imperfections in the manufacture of disks, mechanical tolerances in the manufacture and assembly of the actuator arm and spindle, bearing defect frequencies, shock and vibration, among others. As a result, the data surface may be out of focus at any point in time, creating errors in reading from or writing to the disk. As stated earlier, optical drives have attempted to address this problem by moving the objective lens orthogonal to the ideal or presumed plane of the disk surface to change its focal length, and thereby attempt to maintain focus. This methodology has limited effectiveness. For example, in larger disks, such as DVDs and CDs, errors or fluctuations are compounded as the objective lens moves toward the outer diameter of the disk. Thus, in order to try to maintain focus, the objective lens is required to move a greater distance away from or toward the disk surface (in the Z direction). However, the necessary range of movement in a miniaturized system would likely be constrained by space limitations and/or physical limits purposefully placed in the drive to limit movement. In unbalanced systems in particular, such physical limits are required to prevent linkages from moving past their elastic limits, primarily due to external shock. 
   SUMMARY OF THE INVENTION 
   The focus mechanism of the present invention solves many of the miniaturization problems associated with previous optical disk drive systems. The present invention comprises a rotary actuator having a tracking arm for movement of an optical pick up unit generally parallel to the disk surface and a focus arm for movement generally perpendicular to the disk surface. The focus arm may be balanced or unbalanced, although a balanced system is preferred in order to best handle shock and vibration. The optical pick up unit is supported at the distal end of the focus arm. In the preferred embodiment, the optical pick up unit includes a light source, such as a laser, an objective lens for directing the light beam to the recording/playback surface of the disk and intermediate optical components such as turning mirrors and focusing lenses. The light beam is folded utilizing turning mirrors to achieve a length that is compatible with a chosen objective lens. The optical pickup unit achieves further miniaturization when used in combination with media utilizing first surface data, although it will also work with second surface media. In the context of first surface data, the objective lens can be smaller because the information containing portion or data layer is closer to the objective lens which allows use of a lens with a shorter focal length. 
   The tilt focus method of the present invention also introduces an out-of-perpendicular condition for the laser beam for purposes of maintaining the focus of the light beam on the data layer of the disk. Rotation of the focus arm relative to the tracking arm moves or pivots the focus arm which also moves the optical pick up unit, including the objective lens. In general terms, the optical pick up unit will move in an arcuate or curved path toward or away from the surface of the disk, although the directional component of movement orthogonal to the disk surface is substantially greater than the directional component of movement parallel to the disk surface. This is true for each of the embodiments described herein, except one, even though the magnitude of movement in each of the component directions may vary among embodiments. In the third principal embodiment described herein, the optical pick up unit does not move in an arcuate path. For purposes of this patent, however, the terms perpendicular or substantially perpendicular will be used to refer to movement of the optical pick up unit in each embodiment. 
   By dynamically adapting the position of the objective lens during operation of the drive, the system can respond to variations in the relative position of the data layer caused by imperfections in the manufacture of the disk, manufacture and assembly tolerances of component parts, bearing defects, spindle motor run out, shocks, vibrations and other conditions that cause misalignment of the light beam relative to data on the disk. In this manner, the present invention will overcome conditions that could otherwise result in read/write errors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a cross-sectional schematic view of a linear actuator and light beam focused on a non-first surface data layer. 
       FIG. 1B  is a cross-sectional schematic view of a linear actuator and light beam focused on a first surface data layer. 
       FIG. 2  is a three-quarter perspective view of an optical disk drive of the present invention. 
       FIG. 3  is a three-quarter perspective view of a first embodiment of a tilt focus mechanism of the present invention. 
       FIG. 4  is an exploded view of the component pieces of the tilt focus mechanism shown in FIG.  3 . 
       FIG. 5  is a three-quarter perspective view of the tracking arm of the tilt focus mechanism shown in FIG.  3 . 
       FIG. 6  is a three-quarter perspective view of the focus arm of the tilt focus mechanism shown in FIG.  3 . 
       FIG. 7  is a three-quarter perspective view of the tracking arm and focus arm of the tilt focus mechanism shown in FIG.  3 . 
       FIG. 8  is a cross-sectional view of the tilt focus mechanism shown in FIG.  3 . 
       FIG. 9  is a three-quarter perspective view of the flex circuit, optical pick up unit and heat sink of the tilt focus mechanism of FIG.  3 . 
       FIG. 10  is a cross-sectional view of a disk drive showing the tilt focus mechanism of  FIG. 3 , with the objective lens in a normal position relative to the optical disk. 
       FIG. 11  is a cross-sectional view of an optical disk drive containing the tilt focus mechanism of  FIG. 3 , further showing the objective lens pivoted 0.6 degrees closer to the optical disk. 
       FIG. 12  is a cross-sectional view of an optical disk drive containing the tilt focus mechanism of  FIG. 3 , further showing the objective lens pivoted 0.6 degrees away from the optical disk. 
       FIG. 13  is a three-quarter perspective view of a tracking arm and focus arm of a second embodiment of the tilt focus mechanism of the present invention. 
       FIG. 14  is an exploded perspective view of the tracking arm of the embodiment shown in FIG.  13 . 
       FIG. 15  is an exploded perspective view of the focus arm of the tilt focus mechanism shown in FIG.  13 . 
       FIG. 16  is a three-quarter perspective view of a third embodiment of the tilt focus mechanism of the present invention. 
       FIG. 17  is an exploded view of the tilt focus mechanism shown in FIG.  16 . 
       FIG. 18  is a three-quarter perspective view of the tracking arm of the tilt focus mechanism shown in FIG.  16 . 
       FIG. 19  is a three-quarter perspective view of the focus arm of the tilt focus mechanism shown in FIG.  16 . 
       FIG. 20  is a three-quarter perspective view of the fine actuator of the tilt focus mechanism shown in FIG.  16 . 
       FIG. 21  is a three-quarter perspective view of the flex circuit of the tilt focus mechanism shown in FIG.  16 . 
       FIG. 22  is a three-quarter perspective view of the suspension assembly for the tilt focus mechanism of the embodiment shown in FIG.  16 . 
       FIG. 23  is a three-quarter perspective view of a fourth embodiment of the tilt focus mechanism of the present invention. 
       FIG. 24  is an exploded view of the tilt focus mechanism shown in FIG.  23 . 
       FIG. 25  is a three-quarter perspective view of the tracking arm and focus arm of the tilt focus mechanism shown in FIG.  23 . 
       FIG. 26  is a three-quarter perspective view of the flex circuit, optical pick up unit and heat sink of the tilt focus mechanism shown in FIG.  23 . 
       FIG. 27  is an exploded perspective view of an alternative embodiment of the actuator arm of FIG.  23 . 
       FIG. 28  is an elevated plan view of the embodiment of FIG.  27 . 
       FIG. 29  is an elevated side view of the embodiment of FIG.  27 . 
       FIG. 30  is a three-quarter perspective view of a fifth embodiment of the tilt focus mechanism of the present invention. 
       FIG. 31  is a three-quarter exploded view of the components of the tilt focus mechanism shown in FIG.  30 . 
       FIG. 32  is a three-quarter perspective view of the tracking arm of the tilt focus mechanism shown in FIG.  30 . 
       FIG. 33  is a three-quarter perspective view of the focus arm of the tilt focus mechanism shown in FIG.  30 . 
       FIG. 34  is a three-quarter perspective view of the flex circuit, optical pick up unit and heat sink of the tilt focus mechanism shown in FIG.  30 . 
       FIG. 35  is a cross-sectional view of the tilt focus mechanism shown in FIG.  30 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Turning to  FIG. 2 , a first embodiment of the tilt focus mechanism  10  is shown within the housing  12  of an optical drive  14 . The housing  12  includes a base plate  16  having an aperture  18  for receiving a spin motor (not shown) and a slot  20  to receive a diskette containing an optical disk (not shown). The cover plate has been removed. A diskette is inserted into the slot  20  and engages the spin motor positioned in aperture  18 . An optical pick up unit  22  is positioned at the distal end of the tilt focus mechanism  10  and directs a light beam (not shown), such as a laser, to the optical disk which is spinning at a rapid rate. The light beam may be used to write information to the disk or may be used to read information resident on the disk. Because information is stored on the disk in tracks, typically concentrically arranged, the optical pick up unit (OPU)  22  must be able to traverse the surface of the disk from the inside to the outside diameter in order to access the information formatted on the disk, whether in tracks or not. To accomplish this, the tilt focus mechanism  10  moves in three directions relative to the surface of the optical disk. Generally, the tilt focus mechanism  10  moves laterally across the disk surface for tracking purposes, which can be defined as the X-Y plane for convenience purposes, and it also moves toward and away from the disk surface for focusing purposes, which can be defined as the Z direction for convenience purposes. In this manner, as explained in greater detail below, the tilt focus mechanism  10  can compensate for imperfections in the optical media and read and write data to and from the optical disk more accurately and faster than existing optical drives or magnetic drives. 
   As shown in  FIGS. 3-8 , a first embodiment of the tilt focus mechanism  10  comprises a tracking arm  24  and a focus arm  26  disposed on the distal end of the tracking arm  24 . Rotary motion is imparted to the tracking arm  24  by means of a voice coil motor (VCM)  28 . More specifically, the tracking arm  24 , shown separately in  FIGS. 4 and 5 , includes a central bearing mounting bore  30  which receives a bearing cartridge  32 . The bearing cartridge  32  pivots about a fixed shaft  34  mounted between the tracing VCM return plate  36  and a voice coil motor magnet plate  38 . The tracking arm  24  further includes a wire coil  40  wound around a bobbin  42  and adhered between a pair of rearwardly extending support arms  44 ,  46  of the tracking arm  24  with an adhesive  48 . By directing a current through the wire coil  40  a magnetic field is created which interacts with the magnetic fields surrounding a pair of permanent tracking magnets  50 ,  52  (shown in FIGS.  2  and  4 ), forcing the tracking arm  24  to pivot about the shaft  34 . It should be appreciated that the relative positions of the permanent tracking magnets  50 ,  52  and the wire coil  40  may be switched, with the coil  40  being stationary and the magnets  50 ,  52  affixed to and moving with the tracking arm  24 . 
   The focus arm  26  is mounted to the distal end of the tracking arm  24 . A counterweight  54  is typically affixed to the end of the tracking arm  24  for purposes of counterbalancing about the shaft  34  the weight of the focus arm  26  components on the opposite end of the tracking arm  24 . The OPU  22  is positioned on the distal end of the focus arm  26  between a pair of support arms  56 ,  58 . The purpose of the focus arm  26  is to move the OPU  22  toward and away from the disk surface, in the Z direction. A focus bearing assembly  60 , mounted in the tracking arm  24 , cooperates with a shaft  62  to allow the focus arm  26  to rotate relative to the tracking arm  24  and the disk surface (i.e., in the Z direction). The shaft  62  mounts in a pair of focus bearings  64  which, in turn, are mounted in a pair of pivot bearing supports  66 ,  68  in the focus arm  26 . 
   Movement of the focus arm  26  relative to the tracking arm  24  is created by a second voice coil motor (VCM)  70  (FIG.  4 ). As best seen in  FIGS. 5-8 , a voice coil motor frame  72  is disposed at the forward end of the tracking arm  24 . A pair of permanent magnets  74 ,  76  are mounted to the VCM frame  72 . A focus coil  78 , attached to the focus arm  26 , is positioned adjacent each of the permanent magnets  74 ,  76  with the center arm  75  of the VCM frame  72  positioned in the open center area of the focus coil  78 . A spacer  80  may be included to optimize the position of the focus coil within the magnetic field created by the magnets  74 ,  76 . By inducing a current in the focus coil  78 , the focus arm  24  will pivot in the Z direction about the bearing assembly  60  relative to the tracking arm  24  (perpendicular to the surface of the disk). A counterweight  82  is positioned at the distal end of the support arms  56 ,  58  to balance the weight of the focus arm  26  and its components about the shaft  62 . It should be appreciated, however, that the relative position of the counterweight  82  and VCM  70  can be switched and the same results achieved. As a result of the balanced nature of the focus arm  26 , the VCM  70  can more easily adjust the position of the focus arm  26  to focus the objective lens relative to the data surface of the disk. In addition, a key advantage of a balanced focus arm is its ability to withstand substantially larger shock and/or vibration forces than an unbalanced arm, without incurring a position error of the OPU  22  relative to the data track. 
   A flex assembly or flex circuit  84  is affixed to the tracking arm and focus arm to carry signals between the OPU  22  and appropriate processors mounted on a printed circuit board and maintained in the housing  14  of the optical drive. Two different embodiments of the flex assembly  84  are shown in the drawings with this embodiment. As shown in  FIGS. 3 and 9 , a first version of the flex circuit  84  is mounted to a bracket  86  affixed to the tracking arm  24  and is positioned along one side of the focus arm  26 , attaching to the underneath side of the focus arm  26  at its distal end. Alternatively, as shown in  FIGS. 4 and 13 , the flex circuit  84  includes a rectangular bracket  86  which is positioned along both sides of the focus arm  26 . Both flex circuits  84  are designed to pivot in all three directions of movement of the tilt focus mechanism  10  so as not to inhibit movement of the tilt focus mechanism. A heat sink  88  may be included in either version at the location where the OPU  22  attaches to the flex circuit  84  to facilitate dissipation of beat generated by the operation of the laser resident in the OPU  22 . 
   In operation, servo information embossed or otherwise residing in the data layer of the optical disk is monitored by the optical pick up unit  22  and sent to appropriate processors over the flex cable  84 . Based upon the servo information, a processor (not shown) directs current to flow through coil  40  thereby creating a magnet field which induces movement of the tracking arm  24 . The magnitude of the movement of the tracking arm is controlled by a processor. In this manner, the tracking arm  24  can move the OPU  22  across the entire disk surface to move from one track to another or can minimally adjust the position of the OPU  22  to maintain its position over a desired track. In other words, the tracking arm  24 , including VCM  28 , provides single stage tracking, i.e., both coarse and fine tracking. 
   In comparison, VCM  70  similarly adjusts the position of the OPU  22 , but in a direction substantially perpendicular to the disk surface. This orthogonal component of this movement repositions the OPU  22  and its objective lens  90  to accommodate for disk flatness, variations in thickness in the disk layers, vibrations imparted into the system by the various motors, bearing defects, spindle motor run out and any other imperfections that can lead to orthogonal misalignment of the OPU  22  relative to the data tracks. For example, if the disk is created in such a manner that the surface of the data layer fluctuates, the optical feed back to the processors can sense a change in the quality of the light beam and adjust the position of the OPU  22  using VCM  70  to correct for misalignment between the OPU  22  and the data layer. These adjustments are made dynamically to thereby decrease read/write errors and enhance performance. This adjustment is illustrated in  FIGS. 10-12  which provide a cross-sectional view of the tilt focus mechanism  10 .  FIG. 10  shows the optical drive with the OPU  22  in its normal position, with the objective lens  90  perpendicular to the surface of the disk “D.”  FIG. 11  shows the focus arm  26  repositioned such that the objective lens  90  is rotated 0.6 degrees closer to the disk drive surface. As a result, the spacing between the objective lens  90  and the surface of the disk “D” is decreased. Conversely,  FIG. 12  shows the focus arm  26  repositioned with the objective lens  90  0.6 degrees further away from the surface of the disk “D.” Thus, the range of movement of the focus arm allows the objective lens to maintain the light beam in a focused condition through a range of 1.2 degrees of movement thereby compensating for imperfections in the disk, the manufacture and assembly of the drive components and external shock or vibration. It should be appreciated that the range of motion can be increased or decreased and that the present invention is not limited to this particular embodiment or range of motion. The size of the objective lens and its focal length are an important factor in determining the amount of deviation from perpendicular that any system can accommodate. 
   A variation of the foregoing embodiment is illustrated in  FIGS. 13-15 . In this embodiment, the VCM  70  utilizes a single permanent magnet  74 . As a result, the VCM frame  72  is smaller and lighter in weight. Because of its lighter weight, the components of VCM  28  can be made smaller, as less torque is required to move the tilt focus mechanism  10 . More specifically, the coil  40  and bobbin  42  may be made smaller, as can the rear end of the tracking arm  24  supporting the VCM  28 . With less mass, the counterweight  54  may also be smaller. A lighter weight and smaller tilt focus mechanism  10  will achieve faster seek times and be more accurate. It will also be more compact, allowing further miniaturization. As with the previous embodiment, the general location of the counterweight  54  and VCM  70  may be switched. 
   A third embodiment of the present invention is shown in  FIGS. 16-22 . In general, this embodiment includes a tracking arm  100  for course movement in the X-Y direction (parallel to the surface of the optical disk) and a focus arm  102  for fine tracking and for focus movement in the Z direction (perpendicular to the disk surface). Thus, unlike the tracking arm  24  in the first two embodiments, tracking is accomplished by two stages rather than one. Like the tracking arm  24  in the first two embodiments, the tracking arm  100  includes a bearing mount bore  104  for receiving a bearing cartridge  106  which allows the tracking arm  100  to pivot about a shaft  108  mounted between a tracking VCM return plate and a VCM magnet plate (not shown) of the optical drive. As should be appreciated by one skilled in the art, as an alternative, the shaft  108 , in this embodiment or in any of the disclosed embodiments, may be fixed or stationary. A coil  110 , wound around bobbin  112 , is mounted between a pair of support members  114 ,  116  at the rearward end of the tracking arm  100 , together with the counterweight  118 . Magnets (not shown) are positioned adjacent the coil  110  to form a voice coil motor to provide a directional torque based upon the direction of current induced in the coil  110  to move the tracking arm  100  relative to the surface of the optical disk. It should be appreciated that the coil  110  may be stationary and the magnets may be positioned on the tracking arm  100  and move with the tracking arm. 
   As best seen in  FIG. 22 , a suspension member  1   16  for supporting and positioning the focus arm  102  comprises a cylindrical yoke  120  with two shoulders  122 ,  124  extending outwardly from the yoke  120  in opposite directions. The bearing assembly  106  fits inside the open center  126  of the yoke  120 . Two pair of parallel support wires  128 ,  130  extend forward from the suspension member  116  and terminate in a pair of front suspension mounts  132 ,  134 . The support wires are enlarged at location  136  (on the top and bottom surfaces of the yoke  120  and front suspension mounts  132 ,  134 ) to facilitate stability and mounting between the support wires  128 ,  130 , the yoke  120  and the forward suspension mounts  132 ,  134 . The focus arm  102  attaches to the front suspension mounts  132 ,  134  and moves relative to the tracking arm  100  by a flexing of the suspension wires  128 ,  130 . Unlike the previously discussed embodiments, the present embodiment of the focus arm is unbalanced and, therefore, the focus arm  102  is subject to constant adjustment in order to maintain proper position. Such constant adjustment can drain power, particularly if the weight of the components of the focus arm  102  is not minimized. Also, as an unbalanced member, it is more susceptible to misalignment errors created by shock or vibration. It may therefore be advisable to place physical limits on the range of movement of support wires  128 ,  130  to prevent them from moving past their elastic limit as a result of an external shock. 
   The focus arm  102  is moved relative to the tracking arm  100  by a hybrid pair of voice coil type motors for both fine tracking and focus of the OPU  138  disposed at the end of the focus arm  102 . A fine positioning actuator  140  is mounted between the front suspension mounts  132 ,  134 . The fine actuator  140  includes a forward portion  142  with a cutout  144  for housing the OPU  138 . Fine tracking coils  146 , 148  are positioned between a pair of rear suspension arms  150 ,  152  of the fine actuator  140 . A focus coil  154  is positioned perpendicular to and within the center cavity  156 ,  158  of the fine tracking coils  146 ,  148 . The fine tracking coils  146 ,  148  and focus coil  154  coact with a pair of permanent magnets  160 ,  162  mounted to the tracking arm  100  (FIG.  18 ). The tracking arm  100  also includes a pair of VCM end returns  164 ,  166 , a center return  168  and a top plate  170  to create a magnet flux path in association with the hybrid voice coil motor. It should be appreciated, as a further alternative, that the coils  146 ,  148  and  156  could be mounted on the tracking arm  100  and the magnets  160  and  162  positioned on the fine actuator  140 . 
   A flex circuit  172 , shown in  FIG. 21 , provides a communication path between the OPU  138  and the drive processors (not shown). In addition, a heat sink  174  may be added to facilitate removal of heat generated by the laser within the OPU  138 , as well as due to constant positioning of the fine actuator  140  for focusing, fine tracking and positioning of the objective lens  176 . 
   Applying a current to the fine tracking coils  146 ,  148  creates a force on the focus arm  102  generally parallel to the disk surface, i.e. in the X-Y plane. This causes the support wires  128 ,  130  to bend sideways or laterally, moving the OPU  138  and objective lens  176  generally parallel to the disk surface for fine tracking purposes. The flex circuit  172  includes flat portions  178 ,  180  which accommodate bending in the X-Y direction. Applying a current to the focus coil  154  will create a force which moves the OPU  138  in a direction generally perpendicular to the disk surface, i.e. in the Z direction. As compared to the other embodiments described herein, the four bar linkage created by support wires  128 ,  130  will tend to maintain the objective lens perpendicular to the surface of the disk, rather than move the objective lens through an arcuate path. Flat portions  182 ,  184  of the flex circuit  172  bend in response to the force created by the focus coil  154 . This movement allows the OPU  138  and objective lens  176  to move and maintain focus. 
   A fourth embodiment is disclosed in  FIGS. 23-29 . In general, this embodiment comprises a single actuator arm  200  having a bearing bore mount  202  which mounts to a bearing cartridge  204 . The bearing cartridge  204  is rotatably connected to a shaft  206  mounted between a tracking VCM return plate  208  and the cover or a similar cap structure (not shown). For coarse and fine tracking purposes, the actuator arm  200  moves in a conventional manner responsive to torque induced by VCM  210 . The VCM  210  comprises a coil  212  wound around a bobbin  214  placed within a pair of arms  216  and  218  at the rear end of the actuator arm  200 . Permanent magnets  220  and  222 , in cooperation with alternating current flowing in the coil  212  and the return path provided by tracking VCM return plate  208  and tracking VCM magnet plate  224 , create the necessary torque to pivot the actuator  200  about the shaft  206 . The tracking VCM magnet plate  224  further includes an aperture  226  to provide clearance for the shaft  206  and bearing cartridge  204  to be secured between the VCM return plate  208  and the cover. As will be appreciated, the components of the VCM  210  may be switched relative to each other such that the coil  212  is stationary and the magnets  220 ,  222  move with the actuator arm  200 . 
   This embodiment utilizes an unbalanced focus structure. The focus arm  228  of the actuator  200  includes a number of cutouts to lessen its weight. Additionally, a slot  230  at the distal end is adapted to receive OPU  232 . Movement of the focus arm  228  of the actuator  200  in the Z direction (perpendicular to the disk surface) is accomplished by an integral flexure pivot  234  in the actuator  200  adjacent the bearing bore mount  202 . It should be understood, however, that the flexure need not be integral to the actuator  200 , but may be a separate piece or layer in a laminated composite structure. For example, the laminate structure may comprise a carbon fiber composite upper layer  231 , a metal center layer which includes the flexure  233 , and a carbon fiber layer  235 , as shown in  FIGS. 27-29 . 
   A focus VCM  236  acts to move the focus arm  228  of the actuator  200  (the focus arm) in the Z direction. The VCM  236  comprises a coil  238  mounted to the focus arm  228 . The shape of the coil  238  forms a channel  240  which surrounds a permanent magnet  242  mounted within a VCM block  244 . More specifically, the permanent magnet  242  is positioned within a slot  246  formed in the VCM block  244 . However, it should be appreciated that the shape of the coil may vary without effecting operation. For example, the coil  238  may be flat, i.e. two dimensional, rather than the three dimensional structure shown. The outer walls  248  and  250  of the VCM block  244  create the return path for the magnetic flux, allowing the focus arm  228  to move perpendicular to the surface of the disk as the overall actuator arm  200  moves parallel to the surface of the optical disk. In addition, the coil  238  may be stationary and the magnet  242  moves in association with the focus arm  228 . 
   In this unbalanced embodiment, the voice coil motor  236  is positioned at the center of percussion for the focus arm  228 . It is advantageous to locate the voice coil motor of the focus arm at, or as near as possible to, the center of percussion for the overall focus arm in any unbalanced embodiments, if possible. In this manner, the force generated by the focus arm VCM will minimize, or preferably eliminate, any detrimental excitation or resonance at the pivot point (i.e., flexure  234 ) for the focus arm which could otherwise negatively affect focus. If the VCM  236  were not positioned at or near the center of percussion, the force placed on the focus arm  228  by the VCM  236  could generate forces at the pivot point  234  which would interfere with the positioning of the focus arm, thereby potentially creating focus errors and, therefore, inhibit the ability of the system to read and write. As used herein, the term center of percussion is understood to have the meaning set forth in  Mark&#39;s Standard Handbook for Mechanical Engineers  ( 8   th  ed.), which is incorporated by reference. 
   A flex circuit  252 , shown in  FIGS. 23 ,  24  and  25 , attaches along one side of the actuator  200 . A heat sink  254  is included to dissipate heat created by the laser (not shown) housed within the OPU  232 . Thus, as with the other embodiments, the objective lens  256  may be repositioned in the orthogonal direction relative to the disk surface in order to maintain focus. 
   As will be appreciated, the integral flexure pivot  234  is only one structure that allows for movement of the focus arm  228  in a direction perpendicular to the surface of the optical disk. First, the structure need not be a single piece of material, but may be multiple or separate pieces. Pivoting may be provided by any number of known mechanisms, including but not limited to a ball bearing pivot, a jewel bearing pivot, a knife edge pivot, or a torsional shear member pivot or any other type of pivot known by persons of skill in the art. While the various focus arms in the various embodiments illustrated herein can be lengthened to achieve a greater range of motion, the objective is to minimize the angular change of the objective lens for any given range of motion of the focus arm in the Z direction. This embodiment allows for the greatest range of movement of the objective lens with the least perpendicularity error. 
   A fifth embodiment of the tilt focus mechanism  10  of the present invention is shown in  FIGS. 30-35 . As can be seen in  FIG. 30 , the tilt focus mechanism includes a tracking arm  300  and a focus arm  302 . The tracking arm  300  is shown separately in FIG.  32  and the focus arm  302  is shown separately in  FIG. 33 , with the components of each shown in an exploded format in FIG.  31 . 
   With reference to the tracking arm  300 , a bearing bore mount  304  receives a bearing cartridge  306  which, in turn, mounts to a shaft  308 . The shaft  308  is seated between a tracking VCM return plate  310  and a tracking VCM magnet plate  312 . The rotational movement of the tracking arm  300  is provided by VCM  314 , which includes a coil  316  wound around a bobbin  318 . Permanent magnets  320  and  322 , in combination with the VCM magnet plate  312 , and return plate  310  and the coil  316 , cause the tracking arm  300  to pivot about the shaft  308  and move the focus arm  302  parallel to the surface of the disk for coarse and fine positioning of the OPU  324  relative to the tracks in the optical disk. 
   In this embodiment, the focus arm  302  is balanced. As can be appreciated from  FIG. 33 , the VCM block  326 , permanent magnet  328  and coil  330  are positioned on the opposite side of the pivot point  332  for the focus arm  302  than the OPU  324 . The focus arm  302  moves in a direction perpendicular to the surface of the optical disk by rotation about shaft  334 . The ends of shaft  334  are seated in cutout portions  336  and  338  formed in forward arms  340  and  342  of the tracking arm  300 . The shaft  334  passes through an aperture  344  formed in the VCM block  326 . Bearings  346  and  348  allow the focus arm  302  to pivot relative to the tracking arm  300 . Rotational movement of the focus arm  302  about the shaft  334  is caused by alternating the current path in coil  330  which creates a magnet field that interacts with the magnetic field of permanent magnet  328 . Depending upon the direction of the current in coil  330 , a torque is created relative to the field of the permanent magnet  328 , causing the focus arm  302  to move towards or away from the surface of the optical disk. 
   The forward end of the focus arm  302  includes a pair of support arms  350  and  352 , which hold and support the OPU  324  containing objective lens  356 . A flex circuit  358  provides control signals to the OPU from appropriate microprocessors (not shown). A heat sink  360  can be included to assist dissipating heat generated by the laser (not shown) within the OPU  324 . 
   While a few principal embodiments and certain alternative embodiments have been shown and described, it will be apparent that other modifications, alterations and variations may be made by and will occur to those skilled in the art to which this invention pertains, particularly upon consideration of the foregoing teachings. For example, the pivoting or rotation of the tracking arm and the focus arm may be provided by a ball bearing pivot, jewel bearing pivot, knife edge pivot, flexure pivot, bushing pivot, split band pivot or any type of torsional pivot such as a torsional shear member pivot or other type of structure known to persons of skill in the art for achieving the desired relative movement. In addition, it would be understood that the location of any pivot point of the focus arm could be changed, as could the location and arrangement of the voice coil motor components. For example, either the magnets or the coil could be stationary and the other move relative to the stationary components. Additionally, the respective VCM magnets and coils, on both the tracking arm and focus arm, can be alternatively positioned on the same side of the rotational axis as the optical pick up unit or on the opposite side of the rotational axis as the optical pick up unit for the respective arm. In doing so, however, it should be understood that this relative close proximity of multiple voice coil motors may lead to cross coupling between the VCMs which can affect the performance of the tracking arm and focus arm. In the present invention, this problem has been addressed by optimizing the various return path structures as shown in the illustrated embodiments. In particular, for the specific embodiments disclosed herein, the return paths have been selected, in part, to assist in directing the magnetic fields to the appropriate VCM and away from the other VCM. It is therefore contemplated that the present invention is not limited to the embodiments shown and described and that any such modifications and other embodiments as incorporate those features which constitute the essential features of the invention are considered equivalents and within the true spirit and scope of the present invention.