Abstract:
A telescoping actuator assembly includes a plurality of cylindrical actuators in a concentric arrangement. Each cylindrical actuator is at least one piezoelectric fiber composite actuator having a plurality of piezoelectric fibers extending parallel to one another and to the concentric arrangement&#39;s longitudinal axis. Each cylindrical actuator is coupled to concentrically-adjacent ones of the cylindrical actuators such that the plurality of cylindrical actuators can experience telescopic movement. An electrical energy source coupled to the cylindrical actuators applies actuation energy thereto to generate the telescopic movement.

Description:
This invention was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to piezoelectric actuators. More specifically, the invention relates to cylindrically-shaped and telescopically-extending piezoelectric fiber composite actuator assemblies. 
     2. Description of the Related Art 
     Piezoelectric fiber composite actuators are flexible, planar actuators that can be bonded to a structure and then operated to generate and control or detect deflections/strain of the structure. A conventional piezoelectric fiber composite actuator has the following structural features: 
     (i) a layer of individual piezoelectric fibers (e.g., round, square, etc.) arrayed side-by-side and typically encased in a polymer matrix material; and 
     (ii) interdigitated electrodes etched or deposited onto one or two polymer film layers with the resulting layers sandwiching the layer of piezoelectric fibers. 
     The layer of individual piezoelectric fibers can be assembled from individually-extruded piezoelectric fibers or can be formed from a macro sheet of polymer-backed piezoelectric material that has been processed (e.g., piezoelectric material that has been mechanically diced or etched, laser etched, etc.) to yield parallel rows of piezoelectric material “fibers” attached to the polymer backing. A piezoelectric fiber composite actuator constructed in this fashion is also known as a macro-fiber composite actuator. A complete description of such an actuator is disclosed in U.S. Pat. No. 6,629,341, the contents of which are hereby incorporated by reference. 
     The piezoelectric fiber/macro-fiber composite actuator is a flat device that is lighter and smaller than hydraulic or gas piston-cylinder actuators/assemblies as well as piezoelectric “stack” actuators/assemblies. However, when not bonded to a structure, piezoelectric fiber/macro-fiber composite actuators tend to buckle when used to generate or detect a strain, displacement or force in the plane of the actuator. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an actuator assembly using piezoelectric fiber composite actuators. 
     Another object of the present invention is to provide a piezoelectric fiber composite actuator assembly that exhibits improved stiffness when generating/detecting strain in the plane of the actuator. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a telescoping actuator assembly includes a plurality of cylindrical actuators in a concentric arrangement that defines a common longitudinal axis. Each cylindrical actuator is at least one piezoelectric fiber composite actuator having a plurality of piezoelectric fibers extending parallel to one another and to the common longitudinal axis. Each cylindrical actuator is coupled to concentrically-adjacent ones of the cylindrical actuators such that the plurality of cylindrical actuators can experience telescopic movement along the common longitudinal axis of the concentric arrangement. An electrical energy source coupled to the cylindrical actuators applies actuation energy thereto to generate the telescopic movement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a conventional flat piezoelectric fiber composite actuator; 
         FIG. 2  is a perspective view of a cylindrical piezoelectric fiber composite actuator assembly in accordance with an embodiment of the present invention that utilizes a single piezoelectric fiber composite actuator; 
         FIG. 3  is a perspective view of a cylindrical piezoelectric fiber composite actuator assembly in accordance with another embodiment of the present invention that utilizes multiple piezoelectric fiber composite actuators; 
         FIG. 4  is a side view of an actuator assembly illustrating the layer of piezoelectric fibers in isolation to show their orientation in accordance with an embodiment of the present invention; 
         FIG. 5  is a side view of an actuator assembly illustrating the layer of piezoelectric fibers in isolation to show their orientation in accordance with another embodiment of the present invention; 
         FIG. 6  is a side view of an actuator assembly illustrating the layer of piezoelectric fibers in isolation to show their orientation in accordance with still another embodiment of the present invention; 
         FIG. 7  is a side view of another actuator assembly in accordance with the present invention where multiple, single-cylinder actuator assemblies are arranged in an end-to-end fashion; 
         FIG. 8  is a cross-sectional view of another actuator assembly in accordance with the present invention where multiple, single-cylinder actuator assemblies are arranged in a concentric fashion; 
         FIG. 9  is a cross-sectional view of the actuator assembly of  FIG. 8  after actuation energy has been applied to cause the assembly to telescopically lengthen in a first direction; 
         FIG. 10  is a cross-sectional view of the actuator assembly of  FIG. 8  after actuation energy has been applied to cause the assembly to telescopically lengthen in a second direction; 
         FIG. 11  is a cross-sectional view of another actuator assembly in accordance with the present invention where end caps are coupled to the assembly; 
         FIG. 12  is a perspective view of a multi-layer cylindrical piezoelectric fiber composite actuator assembly in accordance with another embodiment of the present invention; and 
         FIG. 13  is a perspective view of a cylindrical piezoelectric fiber composite actuator assembly having an embedded optical fiber sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Prior to describing the various cylindrical piezoelectric fiber composite actuator assemblies of the present invention, the basic and conventional piezoelectric fiber composite actuator will be explained with the aid of  FIG. 1  where the actuator is referenced generally by numeral  10 . Actuator  10  includes a planar layer  12  of individual piezoelectric fibers  14  (e.g., round, square, etc.) arrayed side-by-side and parallel to one another. Typically, layer  12  is encased in a polymer matrix material (not shown for clarity of illustration). Interdigitated electrodes  16 / 18  are etched or deposited onto one or two (e.g., usually two as in the illustrated example) planar polymer film layers  20  with the resulting layers sandwiching layer  12  of piezoelectric fibers  14 . Layer  12  of individual piezoelectric fibers  14  can be assembled from individually-extruded and laid up piezoelectric fibers or can be formed from a macro sheet of polymer-backed piezoelectric material that has been processed (e.g., piezoelectric material that has been mechanically diced or etched, laser etched, etc.) to yield parallel rows of piezoelectric material “fibers” attached to the polymer backing. The resulting piezoelectric fiber composite actuator constructed in this fashion is known as a macro-fiber composite actuator and is disclosed in U.S. Pat. No. 6,629,341, the contents of which are hereby incorporated by reference. Accordingly, it is to be understood that the phrase “piezoelectric fiber composite actuator” as used herein includes actuators fabricated using individually laid up fibers as well as those processed from a macro sheet of piezoelectric material. 
     The above-described conventional piezoelectric fiber composite actuator is a flat, flexible device that expands or contracts in the plane thereof with the application of an actuation voltage as is known in the art. In accordance with the present invention, one or more of the above-described actuators is used to form a cylindrical actuator assembly that is self-stiffening in the plane of the actuator(s). Several non-limiting examples of actuator assemblies will be described herein with the aid of  FIGS. 2-11 . For simplicity, the illustrated examples show circular cylinders. However, the present invention is not so limited as the term “cylindrical” referred to herein is meant to include any closed cylindrical shape such as oblique cylinders, elliptical cylinders, or other non-circular cylinders. 
     In  FIG. 2 , a single piezoelectric fiber composite actuator is formed into cylindrical actuator assembly  30  having a central longitudinal axis  30 A. For ease of description, it is assumed that the basic construction of previously-described actuator  10  is used in the fabrication of actuator assembly  30 . Accordingly, the reference numerals used to define the elements of actuator  10  are repeated in the illustration of actuator assembly  30 . The single piezoelectric fiber composite actuator used to fabricate assembly  30  is joined along the outboard edges of layers  20  as indicated by a join line  32 . Methods used to join layers  20  along line  32  would be well understood in the art. 
     Fabrication of actuator assembly  30  could be achieved by either (i) shaping a conventional flat piezoelectric fiber composite actuator into a cylinder (e.g., using a mandrel that is later removed), or (ii) building the cylindrical shape of assembly  30  from its constituent parts a layer at a time on a cylindrical form that would later be removed. The choice of fabrication methodology is not a limitation of the present invention. 
     The cylindrical shape of actuator assembly  30  is self-stiffened along its longitudinal planes parallel to longitudinal axis  30 A. In this way, when activated by actuation energy (e.g., voltage, current, etc.), actuator assembly  30  can extend/retract parallel to longitudinal axis  30 A and transmit force without buckling, especially while applying compressive force. 
     As mentioned above, there are many ways to construct an actuator assembly in accordance with the teachings of the present invention. For example, rather than using a single piezoelectric fiber composite actuator to form a cylindrical assembly, two or more piezoelectric fiber composite actuators could be used. In  FIG. 3 , an actuator assembly  40  uses two conventional piezoelectric fiber composite actuators  10 A and  10 B (i.e., each of which includes the elements previously described for actuator  10 ). Each of actuators  10 A and  10 B forms approximately one-half of the cylinder defined by actuator assembly  40 , where actuators  10 A and  10 B are joined to each other along join lines  42  and  44 . 
     In each actuation assembly of the present invention, (i.e., constructed from one or more “conventional” piezoelectric fiber composite actuators), the individual piezoelectric fibers  14  can extend in straight parallel lines along the length of the assembly&#39;s cylindrical shape such that they are parallel to the cylinder&#39;s longitudinal axis. This construction is illustrated in  FIG. 4  where a side view of actuator assembly  30  is shown with layers of piezoelectric&#39;s fibers  14  being exposed for purposes of illustration. This type of construction will produce the greatest amount of strain parallel to longitudinal axis  30 A. However, the present invention is not so limited as piezoelectric fibers  14  could be helically wrapped relative to longitudinal axis  30 A as shown in  FIG. 5 , or could be wrapped about the assembly&#39;s cylindrical shape such that they are perpendicularly oriented relative to longitudinal axis  30 A as shown in  FIG. 6 . The helical wrapping of piezoelectric fibers  14  shown in  FIG. 5  is useful for generating twisting strain, while the “perpendicular” wrapping of piezoelectric&#39;s fibers  14  is useful for generating radial compression/expansion forces. 
     Actuator assemblies of the present invention can also include multiples of the actuator assemblies described above. For example,  FIG. 7  illustrates an actuator assembly  50  that couples multiple single-cylinder actuator assemblies (e.g., actuator  30 ) of the present invention in an end-to-end fashion. Individual actuator assemblies  30  are coupled together at their respective ends by flexible or rigid couplings  52 . A voltage/current source  54  can be used to apply actuation energy to each of actuator assemblies  30 . The actuation energy can be applied independently to each assembly  30  to maximize control of the response of actuator assembly  50 . 
     The present invention also includes a telescoping type of actuator assembly made from multiple single-cylinder actuator assemblies of the present invention. An example of such an assembly and its operation will be described with the aid of  FIGS. 8-10 . In  FIG. 8 , an actuator assembly  60  has multiple single-cylinder actuator assemblies (e.g., actuator assembly  30 ) arranged concentrically. In the illustrated example, seven actuator assemblies  30  (i.e., assemblies  30 - 1  through  30 - 7 ) are shown, although more or less could be used without departing from the scope of the present invention. 
     Adjacent ones of actuator assemblies  30  are joined to one another at one end thereof by couplings  62  with the outermost actuator assembly  30 - 1  being fixed at one end thereof to a base  100 . Couplings  62  are provided at alternating ends of assembly  60  to provide for telescoping action. A voltage/current source  64  is provided and can be coupled to actuator assemblies  30 - 1  through  30 - 7  in an independent fashion for independent control of each actuator assembly. 
     In operation, actuator assembly  60  could be activated to telescope in one direction ( FIG. 9 ) or in an opposing direction ( FIG. 10 ). (For clarity of illustration, source  64  and its coupling to actuator assemblies  30 - 1  through  30 - 7  has been omitted from  FIGS. 9 and 10 .) In either case, actuation energy is applied in an alternating polarity fashion to actuator assemblies  30 - 1  through  30 - 7 . For example, if a positive polarity voltage/current is applied to actuator assemblies  30 - 1 ,  30 - 3 ,  30 - 5  and  30 - 7 , then a negative polarity voltage/current is applied to actuator assemblies  30 - 2 ,  30 - 4  and  30 - 6 . 
     The actuator assemblies described herein can be used in a wide variety of applications without departing from the scope of the present invention. The generated deflections/forces can be applied to devices/systems that lie outside the confines of the actuator assembly. However, the present invention can also be used to apply strain forces to a device/system maintained within the actuator assembly. One such actuator assembly and its application are illustrated in  FIG. 11  where an actuator assembly  70  is a concentric arrangement of actuator assemblies  30 - 1  through  30 - 7  similar to actuator assembly  60 . A first end cap  72  is provided at one axial end of assembly  70  and is fixedly coupled to the outermost actuator assembly  30 - 1  using, for example, couplings  74 . A second end cap  76  is provided at the opposite axial end of assembly  70  and is fixedly coupled to the innermost actuator assembly  30 - 7  using, for example, couplings  78 . A device to be strained (e.g., an optical fiber  200 ) is fixedly coupled to end caps  72  and  76  where it passes therethrough. (Note that end cap  76  could span and be coupled to more than one of inner ones of actuator assemblies  30 - 2  through  30 - 7 ) When optical fiber  200  (which can incorporate tunable optical elements such as Bragg gratings  202 ) is to be strained (e.g., for optical tuning thereof), actuation voltage/current is applied to actuator assemblies  30 - 1  through  30 - 7  to cause the assembly to telescopically grow in length as previously described. Since optical fiber  200  is fixed at end caps  72  and  76 , the telescopic lengthening of assembly  70  provides for the strain tuning of optical fiber  200 . 
     The advantages of the present invention are numerous. The cylindrically-shaped piezoelectric fiber composite actuator assemblies described herein possess all of the inherent advantages of planar piezoelectric fiber composite actuators, but will not buckle when applying forces in the plane of the actuator as the cylindrical shape provides self-stiffening properties. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. For example, the actuator assemblies of the present invention could be fabricated from multiple layers of actuator(s) bonded to one another. As shown in  FIG. 12 , actuator assembly  80  is made from a single actuator (e.g., actuator  10 ) wrapped around longitudinal axis  80 A two or more times with the wraps being bonded to one another. Obviously, this multi-layer construction could also be achieved with a multiplicity of individual actuators. As another example, the actuator assemblies could have embedded, sandwiched, or otherwise incorporated optical fiber sensors as shown in  FIG. 13  (illustrating a single optical fiber  90 ), such as Bragg gratings, which could be used in applications such as tunable fiber lasers or for measuring actuator strain. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.