Abstract:
A piezoelectric actuator with integrated features to provide linear displacement of a threaded rod is presented. One mechanism provides mechanically amplified piezo motion for high speed/short travel position scanning whereas the other provides a low speed/long travel piezo motorized position adjustment. Mechanical amplifier incorporates one or more piezo stacks in longitudinal axis with preload to translate an amplified motion in the order of a few times in the transverse axis, perpendicular to the piezo stack motion. The piezo amplified output travel is transmitted to the internally threaded features of the other mechanism where a screw with a ball at the end to push a desired surface for high speed scanning mode translation. The internally threaded feature of the other mechanism is also operated by a secondary piezo stack which produces slip-stick motion steps to rotate the screw in one direction or the other to produce a slow speed/long travel mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/050,814, filed Sep. 16, 2014, the contents of which are incorporated by reference herein. This application also relates to U.S. Provisional Application No. 62/037,997 filed on Aug. 15, 2014, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to piezo actuators. More particularly, the invention relates to amplified piezo actuators with a motorized adjustment screw. 
     BACKGROUND 
     Piezoelectric stacks provide a limited displacement upon excitation via a change of applied voltage. Flexure based mechanical structures have been developed to amplify the motion usually in a transverse direction to the piezo expansion in the order of a few time typically providing displacement in the order of up to 100s of microns. However, applications such as steering beams in mirror mounts, the possibility of long stroke position adjustment in the order of few millimeters as well as high frequency fine piezo driven adjustment in the order of sub-micron to a few hundreds of micros is not presented within one device. Therefore, there is a need to provide a slow/long stroke position adjustment in the same device as fast/short stroke scanning of a load. 
     SUMMARY 
     One embodiment of the invention provides an actuator, including: a threaded screw a piezo inertia driver; and a piezo amplifier; wherein the piezo inertia driver includes: a clamp having a first movable jaw for engagement with the threaded screw on a first side and a second jaw for engagement with the threaded screw on a second side opposite to said first side; and a first piezo stack mounted in said clamp for parallel movement of said first jaw element relative to said second jaw element; wherein when an alternating voltage is applied to the first piezo stack, the first piezo stack causes a parallel back-and-forth movement of the first movable jaw relative to the second jaw by expansion and contraction of the first piezo stack as a result of the applied alternating voltage; wherein the alternating voltage has a ramp up rate that is different from its ramp down rate, which causes the threaded screw to slip in the clamp more in one direction than in another direction of the back-and-forth movement, resulting in a net rotation of the threaded screw, and the rotation results in a first translation movement of the threaded screw; wherein the piezo amplifier includes: a top wall, a bottom wall, a first and second side walls, the walls being joined by flex hinges; and a second piezo stack; wherein one end of the second piezo stack is coupled to the first side wall and the other end of the second piezo stack is coupled to the second side wall; wherein the piezo inertia driver is coupled to the top wall of the piezo amplifier such that the thread screw engaged by the clamp is offset by a distance from a plane containing the first piezo stack; wherein when a second voltage is applied to the second piezo stack, the second piezo stack causes a horizontal movement of the side walls by expansion or contraction of the second piezo stack, the horizontal movement causes a perpendicular movement of the top wall via the flex hinges, and the perpendicular movement causes a second translation movement of the threaded screw by the top wall pulling or pushing the piezo inertia driver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of an amplified piezo actuator according to an embodiment. 
         FIG. 2  illustrates an amplified piezo actuator in a typical application for steering mirrors according to an embodiment. 
         FIG. 3  illustrates a first perspective view of the internal mechanism of an amplified piezo actuator according to an embodiment. 
         FIG. 4  illustrates a second perspective view of the internal mechanism of an amplified piezo actuator according to an embodiment. 
         FIG. 5  illustrates a perspective view of a piezo inertia driving mechanism for motorized adjustment of screw according to an embodiment. 
         FIG. 6  illustrates a top view of a piezo inertia driving mechanism for motorized adjustment of screw according to an embodiment. 
         FIG. 7  illustrates an enlarged top view of a piezo inertia driving mechanism for motorized adjustment of screw according to an embodiment. 
         FIG. 8  illustrates a front view of the internal mechanism of an amplified piezo actuator according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The description of illustrative embodiments according to principles of the present invention is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. In the description of embodiments of the invention disclosed herein, any reference to direction or orientation is merely intended for convenience of description and is not intended in any way to limit the scope of the present invention. Relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivative thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description only and do not require that the apparatus be constructed or operated in a particular orientation unless explicitly indicated as such. Terms such as “attached,” “affixed,” “connected,” “coupled,” “interconnected,” and similar refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. Moreover, the features and benefits of the invention are illustrated by reference to the exemplified embodiments. Accordingly, the invention expressly should not be limited to such exemplary embodiments illustrating some possible non-limiting combination of features that may exist alone or in other combinations of features; the scope of the invention being defined by the claims appended hereto. 
     This disclosure describes the best mode or modes of practicing the invention as presently contemplated. This description is not intended to be understood in a limiting sense, but provides an example of the invention presented solely for illustrative purposes by reference to the accompanying drawings to advise one of ordinary skill in the art of the advantages and construction of the invention. In the various views of the drawings, like reference characters designate like or similar parts. 
       FIG. 1  illustrates an amplified piezo actuator with a motorized adjustment screw according to an embodiment. Monolithic piezo flexure housing  101  is a hardened steel structure that accommodates two wire-eroded mechanisms, each driven by one or more piezo stacks that can be driven independently. The housing  101  is covered with plates  102  in both sides and also on the top and bottom sides. The housing also accommodates and holds a threaded screw  103  (with typically 100 threads per inch) to translate motion to the moving world. The screw  103  passes through a clearance hole within a mount bracket  104  to couple the monolithic flexure housing to the application. 
       FIG. 2  illustrates an example application in which a couple of amplified piezo actuators  100  are attached to the fixed world  220  of a kinematic mirror mount via the external thread  104   a . The threaded screw  103  is for actuating the moving world  230  of the kinematic mirror mount, so that the mirror  210  can be adjusted. The mount bracket  104  is to be set in the desired orientation and then be locked via a threaded nut  105 . While the screw is engaged with features of monolithic flexure housing for a motorized adjustment, a knob  106  is also attached to one end of the screw  103  for manual rotation and so linear translation of typically a steel ball  107  at the other end of the screw for coarse adjustments. Although a steel ball is illustrated for the screw tip, other shapes, forms and/or materials for the screw tip are also contemplated. By applying and controlling the voltage to the piezo stacks within the monolithic flexure housing  101  via a connector  108  or flying lead cable, micrometer/nanometer-linear adjustment of the moving world in the application is made. 
       FIGS. 3 and 4  illustrate the internal mechanism of the actuator. The mechanism of short travel/high speed scanning of the screw  103  is described as following. One of more piezo stacks  109  are fit into a mechanical amplifier flexure structure  110 . The piezo stack ends sit onto end cup features  112  of the amplifier mechanism and are coupled with thin coupling interface layers  113 . The coupling interface  113  at each end of the piezo stacks consists of layers of typically sub-micron to tens of micron of multiple materials including one or more aluminum, steel and also adhesive layers. These coupling layers have three functions. Aluminum layers provide a proper mechanical stress distribution on the mating surface between the ceramic piezo stack  109  and steel end cap  113 . Steel layers provide a means of preloading the mechanism by opening the distance between the two end caps  113  for increasing the stiffness of the system for high frequency operation. Adhesive layers are to provide rigidity to the mechanical coupling in case of a resonance. 
     The mechanical amplifier mechanism  110  has flex hinges  110   a ,  110   b  and ribs  110   c  within the monolithic flexure housing  101 . The geometries of flex hinges  110   b  within the amplifier mechanism  110  can be made with tight manufacturing tolerances and is crucial in providing a well maintained perpendicularity between the piezo stacks axis  109  and the screw direction. A straightness angular error between the piezo stack motion direction and the centre line between the two flex hinges  110   a  can be compensated via flexing the hinges  110   a  for both proper distribution of stress on the surface of the stack and also to avoid losing mechanical amplification factor. Anti-roll features in form of vertical and/or horizontal flex ribs  110   c  within the monolithic flexure housing control the planar cross-talk motion in relation to the piezo amplified displacement and also provide strength to the amplifier structure  110  when adjustment of the screw via the inertia driver  115  within the monolithic flexure housing  101  or manually via the knob  106 . 
     By excitation of the piezo stack  109  with a change in driving voltage, the expansion of piezo stacks results in displacement of the end caps  113  and consequently an amplified displacement in the perpendicular direction (in the order of typically 10 times) is obtained due to the chosen angular orientation of the tension members and the flex hinges  110   b . The amplified displacement is translated to the output feature of the amplifier  114 . On the other hand, the second piezo flexure mechanism acting as a piezo inertia (slip-stick) driver  115  within the monolithic flexure housing receives amplified displacement of the first mechanism and transfer that to the screw  103  via an internal threading feature  116 . The mechanical amplifier  110  can scan the screw  103  in providing a displacement pattern to typically drive a mirror in an externally closed loop optical circuit to stabilize a laser beam. 
     The monolithic flexure housing  101  features a second mechanism, a piezo inertia driver  115 , at the same time to provide a long travel adjustment of the screw  103  independent from the first mechanism, piezo amplifier feature  110 . This is made by making an offset  111  between mechanical amplifier mechanism  110  and the screw  103  engaged with the piezo inertia driver mechanism  115 . The screw  103  is engaged with the internal threads  116  and is preloaded by the application being under axial reaction force. 
       FIGS. 5 and 6  show the details of the piezo inertia driver mechanism  115  within the monolithic flexure housing  101 . The piezo inertia driver mechanism  115  is similar to that of described as a standalone mechanism in U.S. Pat. No. 5,410,206. The mechanism has an opening  117  to fit a piezo  118  within its flexure structure  115  and also uses a spring  119  to firmly preload the screw  103  in the radial directions. The inertia driver mechanism  115  is made by wire-eroding the structure in a perpendicular orientation to that of amplifier mechanism  110 . The wire-erosion produces a slot  120  and an opening  121  to separate two sections, fixed world  122  and moving world  123  of the inertia driver  115 . The two are connected via a flexure element  124  and pull towards each other by a compression spring  119  located in the features  125  within the mechanism. 
     Upon a change in its driving voltage, the piezo stack  118  which is resting onto the fixed world  122  from one side and onto the moving world  123  from the other side, induces a displacement of the moving world  123  as illustrated in  FIG. 6 . By a slow ramp up or down of voltage from a set value and given the preload onto the screw by the spring  119 , the screw rotates proportional to the translated piezo motion. This is due to the friction between the treads of screw  103  and internal threads  116  of the inertia drive mechanism  115 . By a sudden return of voltage (much faster change than that of slow ramp) back to the set value, there will be a difference between the reaction time of the screw and that of moving section  123 . Due to the inertia of the screw and the fast piezo return motion and a difference between static and dynamic friction coefficients in the engaged threads, a stick-slip effect is made. By this effect, there will be a small residual rotation between slow and fast ramp of the voltage profile. A well control over the voltage pulses via a waveform can produce a continuous stepping of rotational motion of the screw  103  in one direction or the other to produce linear adjustment of the application. This is independent but can be at the same time as the amplifier mechanism  110  motion. While stepping motion of the inertia drive piezo  118  can produce only a few 10s of nanometer per step and be operated at a few kilohertz, the motion translated to the screw is slow and typically a few 10s of micron per second for long travel position/hold adjustments (of 10s of millimeters) whereas the motion produced by the amplifier  110 , although limited to a few 100s of microns, can be 1000s of times faster (in the order of 10s of millimeters per second), making it function for fast laser beam scanning or stabilization applications. 
     Further details of an embodiment are shown in  FIGS. 7 and 8 .  FIG. 7  shows a clamp  700  having a first movable jaw  701  for engagement with the threaded screw  703  on a first side  704  and a second jaw  702  for engagement with the threaded screw  703  on a second side  705  opposite to the first side  704 .  FIG. 8  shows a piezo amplifier including: a top wall  801 , a bottom wall  802 , a first and second side walls  803 ,  804 , the walls being joined by flex hinges; and a second piezo stack  805 , with one end of the second piezo stack  807  is coupled to the first side wall  803  and the other end of the second piezo stack  808  is coupled to the second side wall  804 . 
     While the present invention has been described at some length and with some particularity with respect to the several described embodiments, it is not intended that it should be limited to any such particulars or embodiments or any particular embodiment, but it is to be construed with references to the appended claims so as to provide the broadest possible interpretation of such claims in view of the prior art and, therefore, to effectively encompass the intended scope of the invention. Furthermore, the foregoing describes the invention in terms of embodiments foreseen by the inventor for which an enabling description was available, notwithstanding that insubstantial modifications of the invention, not presently foreseen, may nonetheless represent equivalents thereto.