Abstract:
A storage device for retrieving data stored on a medium includes a lens having a focal axis. The storage device also includes a light source capable of generating light having a direction of propagation that is substantially parallel to the focal axis of the lens as the light passes through the lens. A support assembly supports the lens over the medium and a lens actuator coupled to the support assembly is capable of moving the lens relative to the light while maintaining the focal axis of the lens substantially parallel to the direction of propagation of the light through the lens.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application claims priority from U.S. Provisional Application Serial No. 60/059,488 entitled MICROACTUATOR FOR FINE TRACKING IN A MAGNETO-OPTICAL DRIVE, which was filed on Sep. 22, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to storage devices for computer systems. In particular, the present invention relates to optical and magneto-optical storage devices. 
     In optical and magneto-optical storage devices for computer systems, a beam of light is projected on to a disc surface that reflects the light in to a sensor. The surface of the dine is patterned to represent data that is typically stored in concentric tracks around the disc. The data is read from the disc by positioning the beam of light over a particular track on the disc and sensing the intensity and polarity of the reflected light from the disc. 
     To position the beam of light over a particular track, the art currently uses a course actuator in combination with a galvo-mirror assembly. The course actuator moves a lens assembly and a turning mirror over the disc. The light beam is projected toward the turning mirror, which reflects the light beam down into the lens assembly causing the light to focus on a track on the disc. The galvo-mirror assembly is used to direct the light toward the turning mirror. By applying an electrical current to the galvo-mirror assembly, the angle at which the light beam strikes the turning mirror and the lens assembly can be changed. By changing the angle at which the light beam is incident on the lens assembly, it is possible to move the light beam across one or more tracks while keeping the lens assembly fixed over a particular track. Thus, the galvo-mirror assembly provides fine control of the light beam. 
     Although the position of the light beam can be changed quickly using the galvo-mirror assembly, the light beam provided by the galvo-mirror system is less than ideal. In particular, at some angles of incidence of the light beam on the lens, the light beam can suffer from coma and astigmatism. Coma causes the spot of light on the disc to have a flare distribution and astigmatism causes different polarities of the light to focus at different distances relative to the surface of the disc. Thus, a fine control actuator is needed that can move the spot of light on the disc without causing as many imperfections in the spot of light. 
     SUMMARY OF THE INVENTION 
     A storage device for retrieving data stored on a medium includes a lens having a focal axis. The storage device also includes a light source capable of generating light having a direction of propagation that is substantially parallel to the focal axis of the lens as the light passes through the lens. A support assembly supports the lens over the medium and a lens actuator coupled to the support assembly is capable of moving the lens relative to the light while maintaining the focal axis of the lens substantially parallel to the direction of propagation of the light through the lens. 
     Under the present invention, a disc drive for a computer system includes a lens capable of directing light and a light beam production assembly capable of creating a light beam and directing the light beam toward the lens. A lens support structure supports the lens and a coarse actuator coupled to the light beam production assembly and the lens support structure moves the lens support structure and the light beam production assembly so that the lens moves to different positions over the disc while substantially maintaining a position of the light beam within the lens. A fine actuator coupled to the lens support structure is capable of moving the lens while substantially maintaining a position of the light beam relative to the disc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a magneto-optical disc drive. 
     FIG. 2 is a schematic diagram of a prior art magneto-optical disc drive. 
     FIG. 3 is a schematic diagram of a magneto-optical drive of the present invention. 
     FIG. 4 is a perspective view of another embodiment of a magneto-optical disc drive of the present invention. 
     FIG. 5 is a perspective view of a magneto-optical slider of one embodiment of the present invention. 
     FIG. 6 is a cross-section of the magneto-optical slider of FIG.  5 . 
     FIG. 7 is an exploded perspective view of a slider and a gimbal of one embodiment of the present invention. 
     FIG. 8 is a top view of one embodiment of the slider of FIG.  7 . 
     FIGS. 9-14 are perspective views of fabrication stages of a piezoelectric microactuator formed on the leading edge surface of a slider according to an embodiment of the present invention. 
     FIG. 15 is a top section view of a completed piezoelectric microactuator formed on a leading edge surface of the slider according to the embodiment of FIGS. 9-14. 
     FIGS. 16 and 17 show top and side views, respectively, of a fine control actuator positioned between a load beam and a gimbal in one embodiment of the present invention. 
     FIG. 18 is a perspective view of a lens support assembly with a bimorph piezoeolotric microactuator connected between an actuator arm and a load beam in accordance with one embodiment of the present invention. 
     FIG. 19 is a top view of the bimorph piezoelectric microactuator shown in FIG.  18 . 
     FIG. 20 is a top view of an alternative embodiment of the piezoelectric microactuator shown in FIG.  18 . 
     FIG. 21 is a top view of a lens support assembly showing piezoelectric elements embedded in opposite sides of an actuator arm in accordance with one embodiment of the present invention. 
     FIG. 22 is a top view of a lens support assembly having a piezoelectric microactuator located in an actuator arm according to a further embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a side view of an optical storage system  98  of one embodiment of the present invention. An optical module  108 , which includes a laser, creates a light beam  116  that is directed through an enclosed optical path  112  extending laterally from optical module  108 . Light beam  116  reflects off a mirror  114  toward an optical head  100 , which focuses the collimated beam into a small spot on a disc  118 . Together, optical module  108 , optical path  112  and mirror  114  provide a light beam production assembly. 
     Disc  118  spins about a central axis  120 , continuously bringing new data regions underneath the spot of light produced by optical head  100 . The light incident on disc  118  is reflected back through enclosed optical path  112  and is analyzed by a control module  126  and a servo controller  124  attached to optical module  108 . Through this process, optical storage system  98  retrieves data and servo information stored on disc  118 . Optical head  100  is supported by a support assembly  102  that includes an actuator arm  104 . Actuator arm  104 , optical module  108 , and enclosed optical path  112  are all supported by a spindle  106 , which rotates about a central axis  110 . As spindle  106  rotates, head  100  moves to different radial positions across disc  118  and enclosed optical path  112  rotates to remain aligned with optical head  100 . Servo controller  124  rotates spindle  106  by controlling a motor  128  connected to spindle  106  based on servo information read from the disc, and a desired position produced by control module  126 . Together, servo controller  124 , motor  128 , and spindle  106  form a coarse actuator for positioning the head over the disc. 
     FIG. 2 is a schematic diagram of optics in an optical system  150  of the prior art. Within an optical module  158 , a laser diode  160  generates a light that passes through a beam splitter  162  and a relay lens  164 , reflects off a galvo-mirror  166 , is collimated by an imaging lens  168 , reflects off tracking position sensor  170 , and is focused onto a disc  172  by optical head  174 . Based on the light incident on tracking position sensor  170 , portions of the sensor create electrical signals carried on electrical conductors  176 ,  178 ,  180 , and  182  to a servo controller  188 . Servo controller  188  uses the electrical signals to control the rotation of galvo-mirror  166  through electrical conductors  190 ,  192 . The rotation of galvo-mirror  166  changes the location of the focused light spot on disc  172  and changes the electrical signals produced by tracking position sensor  170 . Through galvo-mirror  166 , it is possible to move the focused spot across several tracks under the prior art. However, as galvo-mirror  166  shifts the light beam it changes the angle at which the light beam passes through optical head  174 . At some angles, this cause coma and/or astigmatism. 
     Some of the light incident on optical disc  172  reflects off optical disc  172 , returns through head  174 , reflects off tracking position sensor  170 , passes through imaging lens  168 , reflects off galvo-mirror  166 , passes through relay lens  164 , is reflected by beam splitter  162 , passes through a Wollaston prism  194 , and comes to focus either before or after a detector plane  196 , which generates an electrical signal on electrical conductors  198  and  200  indicative of the light that is incident on detector plane  196 . Conductors  198  and  200  carry the electrical signal to servo controller  188 , which uses the electrical signal to control galvo-mirror  166  and to position optical head  174 . Conductors  198  and  200  also carry the electrical signal produced by detector plane  196  to control module  202 . 
     FIG. 3 is a schematic diagram of optics  230  of an optical system of one embodiment of the present invention. Optics  230  includes a laser  232  that generates a beam of light  234 , which passes through a beam splitter  236 , and a lens  238  and is reflected off of a turning mirror  240  toward an optical head  242 . Optical head  242  focuses the beam into a spot on disc  244 . Light reflects off disc  244  back through optical head  242 , reflects off turning mirror  240 , passes through lens  238 , reflects off beam splitter  236  into Wollaston prism  246  and comes to focus either before or after a detector plane  248 . 
     Turning mirror  240  is supported by an arm  250  and head assembly  242  supported by a slider  252 , a load beam  254 , and an actuator arm  256 . Although not shown, an actuator motor moves actuator arm  256  and support  250  in unison over the disc under the direction of the signals received from servo control  260 . 
     Under the present invention, servo controller  260  is also connected to one or more microactuators located somewhere along actuator arm  256 , load beam  254 , and/or slider  252 . Embodiments showing specific locations for the microactuators are discussed below in connection with figures that better show the placement and operation of the microactuators. 
     FIG. 4 is a schematic diagram of an optical system under a further embodiment of the present invention. In FIG. 4, the optical system includes optics  280 , which has a laser diode  282  that generates a light beam  284 . Light beam  284  passes through beam splitter  286 , relay lens  288  and into optical fiber  290 . Optical fiber  290  is connected to slider  292  which also includes further optics that direct the light beam as a focused spot onto disc  294 . Slider  292  is connected to a load beam  296  through a gimbal. Load beam  296  is further connected to actuator arm  298 , which is moved by a coarse actuator that is not shown for clarity. The coarse actuator is controlled by servo controller  300 , which receives servo position information from detector plane  302  in optics  280 . Detector plane  302  detects a light beam that reflects from disc  294  through optical fiber  290 , relay lens  288 , beam splitter  286 , and a Wollaston prism  304 . As in FIG. 3, servo controller  300  of FIG. 4 is also connected to at least one microactuator positioned somewhere along actuator arm  298 , load beam  296 , and/or slider  292 . The specific locations for the microactuator are discussed below in connection with specific embodiments of the invention. 
     Note that in FIGS. 2,  3 , and  4 , the optic system and the slider are not shown to scale so that the details of the optic system may be shown more clearly. 
     FIG. 5 is a perspective view of slider  292  of FIG.  4  and its attachment to load beam  296 . Slider  292  includes zirconia oxide layer  320 , silicon layer  322 , electrostrictive polymer  324 , second silicon layer  326 , and electrosestrictive polymer pads  328  and  330 . A lateral conductive coating  332  extends along a side surface of electrostrictive polymer  324 . An end conductive coating  334  extends along the end of electrostrictive polymer  324  at the trailing end of slider  292 . Conductive pads  336  and  338  make electrical connections to conductive coatings  332  and  334 , respectively. Conductors  340  and  342  are bonded to pads  336  and  338 , respectively, and carry electrical signals to the respective pads. A conductor  344  connects to a pad similar to pad  336  on the side of slider  292  opposite the side containing pad  336 . The actual signals carried on conductors  340  and  344  cooperate to produce an electrical field across slider  292 . Similarly, a conductor  346  is connected to a pad (not shown) opposite pad  338  on slider  292  and provides an electrical signal that cooperates with the electrical signal provided on conductor  342  to generate an electrical field across slider  292  from the leading edge to the trailing edge. 
     A conductor  348  attaches to conductive films on electrostrictive polymer tabs  328  and  330 , and conductor  350  connects to conductive films on opposite ends of electrostrictive polymer tabs  328  and  330  from the conductive films connected to conductor  348 . Electrical signals carried on conductors  348  and  350  generate electric fields across electrostrictive polymer tabs  328  and  330 . The operation of electrostrictive polymer layer  324  and electrostrictive polymer tabs  328  and  330  is discussed below in connection with FIG.  6 . 
     FIG. 6 is a cross sectional view of slider  292  of FIG.  5 . Slider  292  is attached to load beam  296  through gimbal  370  that includes a bump  372 . 
     Within slider  292 , optical fiber  290  projects a light beam  374  that reflects off of a mirrored surface  376  made in second silicon layer  326 . The reflected light beam from mirrored surface  376  passes through an objective lens  378  which is mounted on a cylindrical support  380 . Objective lens  378  focuses light beam  374  toward a mesa  382  in zirconia oxide layer  320 . Mesa  382  further focuses the beam of light into a spot of light  384  at the bottom surface of zirconia oxide layer  320 . 
     Under the present invention, beam of light  374  is not moved by a galvo-mirror assembly to locate spot  384  at different track locations across the disc. Instead, the present invention uses a microactuator to shift objective lens  378  and mesa lens  382  relative to light beam  374 . 
     In FIGS. 5 and 6, this microactuator is formed by electrostrictive polymer layer  324  and conductive coatings  332 ,  334 ,  390 , and an additional conductive coating opposite conductive coating  332  (not shown). Specifically, when conductors  342  and  346  carry a differential voltage to pads  338  and  392 , respectively, conductive coatings  334  and  390  generate an electric field across electrostrictive polymer  324  that causes the polymer to move silicon layer  322 , zirconium oxide layer  320 , cylindrical support  380 , and objective lens  378  relative to beam  374  in a direction parallel to line  394  of FIG.  6 . Similarly, when conductors  340  and  344  carry a differential voltage, an electric field develops across electrostrictive polymer  324  such that electrostrictive polymer  324  moves objective lens  378  and mesa  382  in a direction parallel to a direction in and out of the page of FIG.  6 . 
     Note that with this microactuator movement, the angle of incidence of light beam  374  on objective lens  378  remains substantially constant, as does the direction of propagation of beam  374  toward the disc. However, because objective lens  378  and mesa  382  are moving within beam  374 , spot  384  moves to different locations as electrostrictive polymer  324  moves the lenses. Because the present invention does not change the angle of incidence of the light beam, the spot of light does not suffer as much from coma and astigmatism. 
     To enhance focusing of spot  384 , the present invention also provides a focusing microactuator comprised of electrostrictive polymer tabs  328  and  330  and conductive films  396 ,  398 ,  400 , and  402 . As noted above, conductive films  398  and  400  are connected to conductor  348  of FIG.  5  and conductive films  396  and  402  are connected to conductor  350  of FIG.  5 . Electrostrictive polymer  328  and  330  are connected to optical fiber  290  such that when an electric field is produced across the electrostrictive polymer tabs, optical fiber  290  moves laterally within slider  292 . Thus, differential voltage applied across conductors  348  and  350  will cause optical fiber  290  to move within slider  292  in response to movement of electrostrictive polymer tabs  328  and  330 . 
     FIG. 7 is an exploded perspective view of a portion of a disc drive system implementing another embodiment of a microactuator of the present invention. Specifically, FIG. 7 shows a gimbal and a slider of an optical drive, such as the drive of FIG. 3, with a leading edge slider microactuator system used to move a lens  456  within a fixed light beam. The disc drive system include a gimbal or flexure  422  mounted to the underside of a distal end of a load beam (not shown). Flexure  422  includes arms  422   a  and  422   b  forming aperture  444  therebetween to provide resilience or spring to flexure  422 . The distal ends of arms  422   a  and  422   b  are connected via cross beam  445 . Central tongue  448  extends from cross beam  445  into aperture  444  in a plane generally parallel to a plane defined by flexure arms  422   a  and  422   b . Tongue  448  extends beyond the leading edge surface of slider  424  and includes aperture  452 , through which the load beam applies a pre-load force to slider  424 . The portion of tongue  448  extending beyond the leading edge surface is bent downward into a right angle to form a flexure tab surface  450 , which is generally parallel to the leading edge surface of slider  424 . Slider  424  is adhesively attached to flexure tab surface  450 , preferably where microactuator  454  is formed on the leading edge surface of slider  424 . Optional shear layer  449  may be provided between tongue  448  and slider  424  to minimize the wear on slider  424 . Alternatively, a relatively small amount of lubricant may be employed between slider  424  and tongue  448  to achieve a similar result. 
     A coarse actuator is operated to move an actuator arm and load beam to coarsely position lens assembly  456 , which is supported by slider  424 , to various positions with respect to tracks on a surface of a disc. Microactuator  454  is formed at the leading edge of the slider, and cooperates with tab surface  450  to distortionally rotate slider  424  and thereby alter the position of lens assembly  456  located at the edge of slider  424 . In some embodiments, a second microactuator is formed near the opposite side of the leading edge surface of slider  424 , operating in cooperation with microactuator  454  to distortionally rotate slider  424 . The specific operation of microactuator  454  is discussed in detail below with respect to FIG.  8 . 
     FIG. 8 is a top view of slider  424  and tab surface  450  of gimbal  422  of FIG.  7 . Slider  424  includes two stacked piezoelectric microactuators  454   a  and  454   b . Microactuator  454   b  is identical to, or alternatively is a mirror image of, microactuator  454   a , so only microactuator  454   a  is shown in detail in FIG.  8 . Microactuator  454   a  is formed on insulating base coat  464  on the leading edge of slider substrate  424 . Lens assembly  456  is located at trailing edge  479  of slider  424 . Microactuator  454   a  includes right conductive terminal  466  having associated conductive teeth  467 , and left conductive terminal  468  having associated conductive teeth  469 . Teeth  467  and  469  are interdigitated between successive piezoelectric layers  470  of a stack of piezoelectric layers. Right bond pad  472  and left bond pad  474  are disposed on the outer surface of microactuator  454   a , on the same plane as the top surface of slider  424 , for example, for electrical connection to right conductive terminal  466  and left conductive terminal  468 , respectively. Alternatively, bond pads  472  and  474  may be disposed on overcoat layer  476  where it extends beyond flexure tab surface  450 , or on any other accessible surface of slider  424  or microactuators  454   a  and  454   b.    
     Piezoelectric layers  470  are initially polarized in the directions indicated by arrows  471  and  473 , with adjacent piezoelectric layers polarized in opposite directions. Each layer  467 ,  470 , and  469  is formed in succession from the leading edge surface of the slider. After formation of each piezoelectric layer  470 , the layer is polarized by applying a large electric field across the layer, thereby orienting the piezoelectric crystals to respond mechanically in a selected direction for an applied voltage across each layer. Alternatively, piezoelectric layers  470  may be poled in full sheet form before application to the previously formed structures. The region surrounding piezoelectric microactuators  454   a  and  454   b  is open space or is filled with a compliant material, to allow movement of the components of the microactuators. 
     It is preferred that piezoelectric microactuators  454   a  and  454   b  be formed on insulating base coat  464  and slider body  424  by thin-film wafer techniques. The process for forming piezoelectric layers are well known. An alternative is to form microactuators  454   a  and  454   b  separately, and then laminate them to slider  424 , but this process is less desirable because it introduces additional fabrication steps to the process and increases the risk of defective parts. While the apparatus hag been shown in FIG. 8 as employing a pair of complementary stacked piezoelectric microactuators  454   a  and  454   b , a single microactuator may be employed. 
     In operation, a first voltage is applied to right bond pad  472 , so that right conductor  466  and conductive teeth  467  are energized to a first electrical potential. A second voltage is applied to left bond pad  474  to energize left conductive terminal  468  and conductive teeth  469  to a second electrical potential. The difference between the first electrical potential and the second electrical potential across piezoelectric layers  470  causes all of the layers to either expand or contract. 
     Microactuator  454   a  is adhesively attached to flexure tab surface  450  of the disc drive system, so that expansion or contraction of piezoelectric layers  470  of microactuator  454   a  applies force against flexure tab surface  450  to cause distortional rotation of slider  424 . For example, when all piezoelectric layers  470  of microactuator  454   a  expand, the force pushing against flexure tab surface  450  causes clockwise distortional rotation of slider  424 , thereby moving lens assembly  456  at trailing edge  479  of slider  424  in a clockwise arc. In the embodiment where microactuator  454   b  is also provided, expansion of one microactuator and contraction of the other microactuator complement each other, thereby cooperating to cause distortional rotation of slider  424 . 
     Where optional shear layer  449  is included and bonded to both slider  424  and tongue portion  448  (see FIG.  7 ), the shear layer provides a compliant attachment of the slider to the gimbal, thus permitting the slider to move about the yaw axis (normal to the disc) and thereby permitting distortional rotation of the slider to finely position the lens assembly relative to a selected track on the disc. 
     While microactuators  454   a  and  454   b  are shown with open space or compliant material between the layers of the microactuators and overcoat  476  abutting flexure tab surface  450 , it will be understood that a design arranged such that an end layer of the conductive teeth layers abuts overcoat  476  is also feasible, to push directly against flexure tab surface  450 . Thus, selective positioning of lens assembly  456  at the trailing edge of slider  424  can be achieved by expanding and contracting piezoelectric layers  470  of microactuator  454   a  at the leading edge of slider  424 , by regulating the voltages applied to right bond pad  472  and left bond pad  474 . 
     The adhesive connection between microactuator  454   a  and flexure surface  450  at overcoat layer  476  shown in FIG. 8 is exemplary; flexure surface  450  may be oriented and positioned in any manner to achieve adhesive attachment to microactuator  454   a , so that expansion or contraction of piezoelectric layers  470  of microactuator  454   a  causes distortional rotation of slider  424  to position lens assembly  456 . 
     FIGS. 9-14 are perspective views illustrating fabrication of a microactuator on a leading edge of a slider  563  that is designed to contact tab portion  450  of FIG. 7 in accordance with another embodiment of the present invention. As shown in FIG. 9, hills  562 ,  564 , and  566  are formed on leading edge surface  560  of slider  567 . Hills  562 ,  564 , and  566  are preferably composed of a material that is electrically insulating and thermally compatible with the slider substrate and the ceramic member to be formed on the hills, such as a ceramic material. The regions  563  and  565  between hills  562 ,  564  and  566  are then filled with an easily removable material to form a structure having an exposed top surface coplanar with the exposed top surface of hills  562 ,  564  and  566 . 
     Subsequently, as shown in FIG. 10, ceramic member  570  formed of silica, alumina or zirconia, for example, is patterned on the planar surface formed by hills  562 ,  564  and  566  and regions of  563  and  565 . Ceramic member  570  extends the entire width of leading edge surface  560  of slider  567 , and includes beam  572  spanning region  563  between hills  562  and  564 , and beam  574  spanning region  565  hills  566  and  564 . After member  570  has been patterned, the material in regions  563  and  565  beneath structural beams  572  and  574  is removed, thereby leaving beams  572  and  574  to span the space between hills  562  and  564  and hills  564  and  566 , respectively. For example, the material in regions  563  and  565  may be a metal that is removed by chemical etching, or a polymer or salt that is dissolved. Member  570  will be the carrier for the microactuator to control the positioning of slider  567 , with structural beams  572  and  574  bending toward or away from slider  567  under the control of the microactuator. 
     As shown in FIG. 11, bottom electrical conductors  582  and  584  are formed on member  570 . The conductors preferably extend nearly to the lateral edges of slider  567  to permit connection to driving electronics (not shown) outside of the active area formed by beams  572  and  574  of the microactuator. As shown in FIG. 12, piezoelectric elements  592  and  594  are patterned on bottom electrical conductors  582  and  584 , respectively, directly over the respective beams  572  and  574 . Piezoelectric elements  592  and  594  are preferably patterned only over beams  572  and  574  to facilitate bending and reduce undesirable stresses on the materials of the microactuator. 
     As shown in FIG. 13, conductive via structures  602  and  604  are patterned on bottom electrical conductors  582  and  584  near the center of slider  567 , between the active microactuator beams  572  and  574 . An insulating planarization layer structure, composed of a material such as compliant epoxy is applied over the structure, creating a planar surface that includes the top surfaces of piezoelectric elements  592  and  594 , and via structures  602 ,  604 . The spaces under beams  572  and  574  are preferably masked off to prevent material from being deposited therein. After the insulating planarization layer is applied, top electrical conductors  612  and  614  are patterned as shown in FIG. 14, contacting via structures  602  and  604  and the top surfaces of piezoelectric elements  592  and  594 . Functionally, the configuration shown in FIG. 14 is a completed microactuator structure for slider  567 . 
     FIG. 15 is a section view of the layers and materials of the microactuator formed on leading edge surface  560  of slider  567 . In order to implement slider  567  in a disc drive system, encapsulating layer  620  is provided over the microactuator formed of a compliant epoxy material, for example, and is bonded (such as by adhesive) to flexure tab surface  450  (FIG.  7 ). Thus, the microactuator reacts against flexure tab surface  450  to position lens assembly  556  of slider  567  with respect to concentric tracks of a rotating disc. 
     In operation, a first voltage is applied to bottom electrical conductor  582 , and a second voltage is applied to bottom conductor  584 . Via structure  602  connects the first voltage to top electrical conductor  614 , and via structure  604  connects the second voltage to top electrical conductor  612 . Thus, the voltage differences across piezoelectric elements  592  and  594  are equal but opposite. In response to the voltage differences, one of the piezoelectric elements  592  and  594  longitudinally expands while the other longitudinally contracts. In the example shown in FIG. 15, piezoelectric element  592  expands in the direction of arrows  622 , while piezoelectric element  594  contracts in the direction of arrows  624 . 
     The expansion of piezoelectric element  592  causes structural beam  572  of member  570  to bend upward (away from slider  567 ) in the direction of arrow  626 . Conversely, the contraction of piezoelectric element  594  causes structural beam  574  of member  570  to bend downward (toward slider  567 ) in the direction of arrow  628 . These bending actions cause slider  567  to be rotationally displaced to the position  567 ′ shown in dashed lines. Lens assembly  556 , at the trailing edge of slider  567 , is displaced to the position  556 ′ shown in dashed line as well. Thus, application of voltages to bottom electrical conductors  582  and  584  produces controllable displacement of lens assembly  556  at the trailing edge of slider  567 . This displacement moves lens assembly  556  within a fixed beam of light, thus changing the location of a focused spot on the disc without changing the angle of incidence of the light beam on the lens assembly. 
     FIGS. 16 and 17 show an additional microactuator for moving a lens assembly in a fixed light beam according to another embodiment of the present invention. Linear motor  658  is attached between head suspension load beam  652  and gimbal  653 , which in turn carries slider  654  connected to tongue portion  657  of gimbal  653  near a distal end of slider  654 . Slider  654  carries optical lens assembly  655 . Gimbal  653  is preferably constructed with rectangular portion  653   a  at a proximal end, and arm portions  653   b  and  653   c  extending toward a distal end, forming aperture  661  between arm portions  653   b  and  653   c . Rectangular portion  653   a  of gimbal  653  is attached to rotor  658   b  of motor  658 , so that linear movement of rotor  658   b  in the X and/or Y directions effects rigid-body linear movement of gimbal  653  and slider  654 . Suspension load beam  652  applies pre-load force to slider  654  at pre-load tip  651  inside aperture  661  of gimbal  653 . Pre-load tip  651  slides across the top surface of slider  654  as movement of slider  654  occurs. Linear motor  658  is responsive to control signals from control circuitry  659  to linearly expand in the X and/or Y directions. Motor  658  includes stator  658   a  and rotor  658   b  (depicted symbolically in FIG.  17 ), and is preferably arranged with stator  658   a  attached to load beam  652  and rotor  658   b  attached to gimbal  653 , so that linear displacement of rotor  658   b  results in rigid-body motion of lens assembly  655 . Any two-dimensional movement of lens assembly  655  can be achieved by selective displacement (such as expansion or contraction) of motor  658  in the X and Y directions, controlled by signals from control circuitry  659 . 
     Again, because motor  658  is a discrete component separate from the design of head suspension  652 , gimbal  653 , and slider  654 , any small-scale linear motor technology (many of which are known in the art) may be used. Linear micromotors may operate via parallel plates attached to a fixed stator and a mobile rotor of the motor in opposing pairs. A control signal, such as a voltage, is applied to the pairs of plates to move the rotor with respect to the stator. The motor is preferably configured with parallel plates to cause linear motion in the X direction, and parallel plates configured to cause perpendicular linear motion in the Y direction. The design flexibility of the present invention, which permits the use of several existing micromotor designs, simplifies the design of high resolution lens assembly positioning mechanisms for optical and magneto-optical disc drives. 
     FIG. 18 is a perspective view of a slider  724 , a flexure  722 , and actuator arm  716  showing an additional microactuator configuration under an additional embodiment of the present invention. A bimorph piezoelectric microactuator  746  comprising piezoelectric layers  746   a  and  746   c  is clamped at one end by swaged flaps  745  of a swage plate  742 , and at its other end by swaged flaps  747 . Swaging is shown as an exemplary means for attaching microactuator  746 ; it will be apparent to one skilled in the art that other means of attachment may be employed. Swage plate  742  is connected to actuator arm  716  through aperture  744 , or by a similar connection mechanism. Slider  724  is attached to flexure  722  in a manner known in the art. In the exemplary embodiment shown in FIG. 18, the trailing edge of slider  724  is attached to tongue portion  749  at gimbal point  748 . Slider  724  includes lens assembly  725  near its center. 
     In operation, a voltage is applied to piezoelectric layers  746   a  and  746   c  of bimorph piezoelectric microactuator  746 , causing one of the piezoelectric layers to expand and the other to contract along the length between plate  742  and flexure  722 , thereby causing microactuator  746  to bend in the direction of arrows  751 . Movement of microactuator  746  results in corresponding movement of flexure  722  and slider  724 , thereby selectively changing the position of lens assembly  725  with respect to a light beam generated by a light source as discussed above in connection with FIG.  3 . 
     FIG. 19 is an enlarged view of piezoelectric microactuator  746  restrained at one end by swaged flaps  745  of plate  742 . Microactuator  746  includes first piezoelectric layer  746   a , second piezoelectric layer  746   c , and center metal shim  746   b  bonded between the piezoelectric layers by conductive adhesive. In the “parallel” configuration depicted in FIG. 19, piezoelectric layers  746   a  and  746   c  are poled in the direction of arrows  752   a  and  752   c . A first voltage is applied at terminal  754   a  to piezoelectric layer  746   a , and at terminal  754   c  to piezoelectric layer  746   c . A second voltage is applied at terminal  754   b  to metal shim  746   b . Thus, in the “parallel” configuration, piezoelectric layer  746   a  will contract, and piezoelectric layer  746   c  will expand, in response to the first and second voltages applied at the terminal. The result is a bending motion (shown in phantom) of piezoelectric microactuator  746 , since swage plate  742  restrains one end of microactuator  746 . The amount of bending of microactuator  746 , and thus the amount of displacement of slider  724  (FIG. 18) connected to microactuator by flexure  722 , is precisely controlled by the voltages applied to terminals  754   a ,  754   b  and  754   c . Thus, microactuator  746  is able to provide high resolution positioning of slider  724  over a selected track of a disc. 
     In an alternative embodiment depicted by FIG. 20, piezoelectric miroactuator  746  is constructed in a “series” configuration. Piezoelectric layer  746   a  is poled in the direction of arrow  756   a , and piezoelectric layer  746   c  is poled in the opposite direction, shown by arrow  756   c . A first voltage is applied at terminal  754   a  to piezoelectric layer  746   a , and a second voltage is applied at terminal  754   c  to piezoelectric layer  746   c . As a result, piezoelectric microactuator  746  bends as indicated in phantom, since one end of microactuator  746  is restrained by swage plate  742 . The “series” configuration is simpler and more economical than the “parallel” configuration, since it requires only two connections to the outside surfaces of piezoelectric layers  746   a  and  746   c . However, the “series” configuration yields less deflection per volt of applied potential than the “parallel” configuration shown in FIG.  20 . The “parallel” configuration is more complex, requiring three electrical connections, the additional connection being made to the center shim. Either of the configurations shown in FIGS. 19 and 20 art acceptable for effecting high resolution positioning of slider  724  over a selected track of a disc. 
     FIG. 21 is a top view of an actuation system  810  showing an additional alternative location for a microactuator under an additional embodiment of the present invention. Actuation system  810  includes a voice coil motor  812  operable to rotate actuator arm  816  and an optical light production system (not shown) about an axis  814  of a shaft  815 . A head suspension  818  is connected to a distal end of actuator arm  816  by head suspension mounting block  820 . Gimbal  822  is attached to a distal end of head suspension  818  and a slider  824  is mounted to gimbal  822  in a manner known in the art. Actuator arm  816  includes a space  819  forming arm side portions  821   a  and  821   b  on each side of a longitudinal axis  825 . 
     Side portions  821   a  and  821   b  are joined to a pivoting portion  850  of actuator arm  816  by two microactuators. The microactuator connected to side portion  821   a  is formed by a piezoelectric element  826  located between two conductive element  846  and  848 . The microactuator connected to side portion  821   b  is formed by a piezoelectric element  840  located between two conductive elements  842  and  844 . Piezoelectric elements  826  and  840  are preferably implemented with opposite polarities, so that an identical voltage introduced across terminals  846 ,  848  and  842 ,  844  of both piezoelectric elements induces expansion of one piezoelectric element and contraction of the other piezoelectric element. This complementary arrangement of piezoelectric elements allows a distortion of actuator arm  816  to be achieved, thereby enabling displacement of slider  824 . 
     As in the above embodiments, slider  824  supports a lens assembly that moves within a fixed beam of light when the piezoelectric elements are activated. This allows the location of a focused spot of light on the disc to change without changing the direction of propagation of the light beam toward the lens assembly and without activating voice coil motor  812 . 
     FIG. 22 is a top view of a disc drive actuation assembly  950  according to a further embodiment of the present invention. Disc drive actuation assembly  950  includes voice coil motor (VCM)  922 , body  925 , actuator arm  930  extending from body  925 , load beam  934  connected to actuator arm  930  at head mounting block  932 , and gimbal  936  connected at a distal end of load beam  934  to support slider  938 , which in turn carries a lens assembly. 
     Pivot cartridge  926  is provided in cavity  937  in body  925 , and is rigidly fastened to body  925  at one end, such as by one or more screws  928 . Piezoelectric element  952  is provided in body  925  and includes terminals  954  and  956 . Body is supported at three points to pivot cartridge  926 : at fastener  928  along the longitudinal axis of body  925 , at a point  945  adjacent to proximal end  943  of piezoelectric element  952 , and at hinge point  939 . 
     VCM  922  is operated in a manner known in the art to rotate body  925 , pivot cartridge  926 , and a light production system (not shown) around axis  924  and thereby coarsely position slider  938  over selected tracks of a disc. For more precise movements of slider  938  through a fixed beam of light produced by the light production system, piezoelectric element  952  is selectively expanded or contracted along its axis by applying a voltage to terminals  954  and  956 , distorting body  925  to alter the position of slider  938 . Relief  933  is preferably formed in body  925  adjacent to hinge point  939 , to facilitate distortion of body  925  in response to expansion or contraction of piezoelectric element  952 . Alternatively, a portion of body  925  near hinge point  939  may be composed of compliant material to achieve this result. 
     Through the movement caused by piezoelectric element  952 , the lens assembly on slider  938  is able to move to different locations within a beam of light that is directed into the page of FIG. 22 by the light production system that is moved by voice coil motor  922 . The movement of the lens assembly allows a spot of light on the disc to move across tracks without activating voice coil motor  922  and without changing the angle of incidence of the light beam into the lens assembly. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.