Abstract:
A self-latching piezocomposite actuator includes a plurality of shape memory ceramic fibers. The actuator can be latched by applying an electrical field to the shape memory ceramic fibers. The actuator remains in a latched state/shape after the electrical field is no longer present. A reverse polarity electric field may be applied to reset the actuator to its unlatched state/shape. Applied electric fields may be utilized to provide a plurality of latch states between the latched and unlatched states of the actuator. The self-latching piezocomposite actuator can be used for active/adaptive airfoils having variable camber, trim tabs, active/deformable engine inlets, adaptive or adjustable vortex generators, active optical components such as mirrors that change shapes, and other morphing structures.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/916,432, titled “SELF-LATCHING PIEZOCOMPOSITE ACTUATOR.” filed on Dec. 16, 2013, the contents of which are hereby incorporated by reference in their entirety. The present application is also related to U.S. Pat. No. 6,629,341, titled “METHOD OF FABRICATING A PIEZOELECTRIC COMPOSITE APPARATUS,” the entire contents of which are incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The invention described herein was made in the performance of work under a NASA contract and by employees of the United States Government and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore. In accordance with 35 U.S.C. §202, the contractor elected not to retain title. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Piezoelectric actuators typically require constant control and management of electric fields to set and hold deflections. Without constant application of the controlling electrical field, for example, in the event of a power failure, the piezoelectric actuator will return to a neutral or near-neutral deflection state. For quasistatic deflection or shape control applications, electrical efficiency and fault tolerance of the piezoelectric system (integrated structure, actuators, and controls) could be improved by eliminating the need to maintain electrical power and active control on the piezoelectric actuator components once a desired deflection is achieved. 
       SUMMARY OF THE INVENTION 
       [0004]    One aspect of the present invention is a device that manipulates the remnant strain behavior present in certain ferroelectric ceramics to set or adjust quasistatic extensional or flexural deflections in a composite structure without the application of a persistent controlling electrical field. Potential aeronautics applications include adaptive-camber airfoils, trim tabs, deformable engine inlets, and adaptive or adjustable vortex generators. Space applications include active optics and reflector systems. 
         [0005]    One aspect of the present invention is a method of controlling a self-latching piezocomposite actuator having a layer of shape memory ceramic fibers and first and second layers that include conductive patterns. The first and second layers are disposed on opposite sides of the layer of shape memory ceramic fibers. The method includes causing the shape memory ceramic fibers to have a first strain state by at least partially poling the shape memory ceramic fibers utilizing a first electric field that is induced by causing a voltage difference in the conductive patterns of the first and second layers. The method farther includes removing the voltage difference whereby the shape memory ceramic fibers remain in the first strain state. The shape memory ceramic fibers are then at least partially de-poled utilizing a second electric field having a polarity that is substantially opposite a polarity of the first electric field to thereby cause the shape memory ceramic fibers to have a second strain state that is not equal to the first strain state. 
         [0006]    Another aspect of the present invention is a method of controlling the shape of a structure that is capable of defining at least first and second shapes. The method includes providing a self-latching piezocomposite actuator comprising a plurality of aligned shape memory ceramic fibers defining first and second strained states. The self-latching piezocomposite actuator is operably connected to the structure. The strain state of the shape memory ceramic fibers is changed from the first strain state to a second strain state by applying a first electric field to the shape memory ceramic fibers such that the shape of the structure changes from the first shape to the second shape. The first electrical field is removed after the fibers are in the second strain state, and wherein the actuator continues to maintain the structure in the second shape after the first electrical field is removed. A second electrical field is then applied to the shape memory ceramic fibers to cause the shape memory ceramic fibers to change from the second strain state to a third strain state that is between the first and second strain states or equal to the first strain state. The structure defines a third shape corresponding to the third strain state that is between the first and second shapes or is the same as the first shape. The structure is maintained in the third shape after the second electrical field is removed whereby the shape memory ceramic fibers of the actuator are maintained in the third strain state. 
         [0007]    Another aspect of the present invention is a method of controlling a self-latching piezocomposite actuator. The method includes providing a self-latching piezocomposite actuator comprising a plurality of aligned shape memory ceramic fibers defining first and second strain states and a plurality of intermediate strain states between the first and second strain states. The method includes determining a required intermediate strain state of the shape memory ceramic fibers corresponding to a required shape of a structure incorporating the actuator. The method further includes determining a present strain state of the shape memory ceramic fibers, and changing the strain state of the shape memory ceramic fibers from the present strain state to the required intermediate strain state by applying an electrical field to the shape memory ceramic fibers. The electrical field is removed after the fibers are in the required strain state, and the shape memory ceramic fibers remain in the required strain state. 
         [0008]    These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]      FIG. 1  is an exploded isometric view of a self-latching piezocomposite actuator according to one aspect of the present invention; 
           [0010]      FIG. 2  is an isometric view of a self-latching piezocomposite actuator according to another aspect of the present invention; 
           [0011]      FIG. 3  is a graph showing field-induced strain curves for a shape memory ceramic material that switches from an antiferroelectric to a ferroelectric state; 
           [0012]      FIG. 4  is partially schematic isometric view of active fiber composite plies that may be utilized in constructing an active structure accordingly to one aspect of the present invention; 
           [0013]      FIG. 5  is a partially fragmentary isometric view of a portion of a helicopter rotor having active twist control according to another aspect of the present invention; 
           [0014]      FIG. 6  is a partially schematic isometric view of a helicopter rotor having active blade twist control for vibration reduction according to another aspect of the present invention; 
           [0015]      FIG. 7  is a graph showing voltage that can be applied to a self-latching piezocomposite actuator to control remnant strain by partial poling/de-poling of the shape memory material; 
           [0016]      FIG. 8  is a graph showing longitudinal strain versus voltage; 
           [0017]      FIG. 9  is a graph showing longitudinal transverse strain versus voltage; 
           [0018]      FIG. 10  is a graph showing remnant strain versus hack voltage; 
           [0019]      FIG. 11  is a graph showing strain versus electric field for a 8/65/35 PLZT shape memory material according to another aspect of the present invention; 
           [0020]      FIG. 12  is a partially schematic end view of a wing having a variable camber according to another aspect of the present invention; 
           [0021]      FIG. 13  is a partially schematic cross sectional view showing the root of the wing of  FIG. 12 ; 
           [0022]      FIG. 14  is a partially schematic cross sectional view showing the tip of the wing of  FIG. 13 ; 
           [0023]      FIG. 15  is a partially schematic cross sectional view of the wing of  FIGS. 12-14  wherein the actuators are in an unlatched state; 
           [0024]      FIG. 16  is a partially schematic cross sectional view of the wing of  FIGS. 12-14  wherein the actuators are in a latched state to increase the camber of the wing; 
           [0025]      FIG. 17  is a partially schematic plan view of an aircraft including self-latching actuators and aerodynamic surfaces that change shape upon actuation of the self latching piezocomposite actuator; 
           [0026]      FIG. 18  is a side elevational view of the aircraft of FIG,  17 ; and 
           [0027]      FIG. 19  is a partially schematic cross sectional view of an aircraft engine having self-latching piezocomposite actuators that change the shape of the inlet of the engine; 
           [0028]      FIG. 20  is a perspective view of an active composite reflector including self-latching shape memory actuators according to another aspect of the present invention; and 
           [0029]      FIG. 21  is a perspective view of a solid reflector having a plurality of self-latching piezocomposite actuators according to another aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
         [0031]    With reference to  FIG. 1 , a self-latching piezocomposite actuator  10  according to one aspect of the present invention includes a first sheet  12  and second sheets  14 A and  14 B. The first sheet  12  comprises machined piezoceramic fibers  8  having rectangular cross sectional shapes, and the second sheets  14 A and  14 B comprise polyimide films  16 A and  16 B having interdigitated electrodes  18 A and  18 B. Structural epoxy matrix material  20 A and  20 B is disposed between the first sheet  12  and the second sheets  14 A and  14 B. Epoxy matrix material  20 C is also disposed between the piezoceramic fibers  8  of first sheet  12 . The actuator  10  may be fabricated utilizing the processes described in U.S. Pat. No. 6,629,341. 
         [0032]    A piezocomposite actuator  10 A according to another aspect of the present invention includes a first sheet or layer  22  including a plurality of cylindrical piezoceramic fibers  28 , and second sheets  24 A and  24 R that comprise epoxy material  30  and electrodes  32 . 
         [0033]    The fibers  8  ( FIG. 1 ) and fibers  28  ( FIG. 2 ) comprise a shape memory ceramic material. The shape memory ceramic material of fibers  8  and  28  changes shape when an electrical field is applied to the shape memory ceramic material, and the shape memory ceramic material remains in the changed shape even after the electrical field is no longer applied. A reverse electrical field can then be applied to return the shape memory ceramic to its initial state/shape. Fibers  8  and  28  may comprise a PZT 5H material defining a d 33  mode along the fibers whereby the fibers increase in length when actuated. The electrical fields can be selectively applied utilizing the interdigitated electrodes  18 A and  18 B to cause the actuator  10  to curve or bend due to increasing strain on one side of fibers  8  while simultaneously decreasing strain of the material of fibers  8  along an opposite side of first sheet  12 . Alternatively, the overall lengths of the fibers  8  may be increased and decreased by inducing substantially uniform strain states on opposite sides of fibers  8  by controlling the electrical fields generated as a result of electrical current traveling through the electrodes  18 A and  18 B. 
         [0034]    With further reference to  FIG. 3 , a shape memory ceramic material such as a lead zirconate stannate based Pb0.99Nb0.02((ZrxSn1-x)1-yTiy)0.98O3 system exhibits shape memory characteristics. The material begins at an unlatched state  34  wherein no electrical power (electric field) is applied to the material. If an electric field is applied to the material, the state of the material travels from the unlatched state  34  to an unpoling state  36  as shown by the arrow “A.” When the electric field is removed, the state of the material changes from the unpoling state  36  to the power-off, latched state  38  as shown by the arrow “B.” Significantly, the strain of the material changes in magnitude as shown by the dimension “S,” and the material remains in the power-off latched strain state. If a reverse electric field is applied, the state of the material changes from the power-off latched state  38  to the re-poling reset state  40  as shown by the arrow “C.” When the reverse electric field is removed, the material changes from the re-poling reset state  40  back to the original unlatched state  34  as shown by the arrow “D,” thereby causing a change in the magnitude of the strain as shown by the dimension “S 2 .” The shape memory ceramic fibers  8  and/or  28  comprise various a shape memory ceramic materials having field-induced strain characteristics. It will be understood that  FIG. 3  provides an example of a shape memory material, but the present invention is not limited to this specific material. As discussed in more detail below in connection with  FIGS. 8-12 , the fibers  8  and  28  may also comprise other types of shape memory ceramic materials. 
         [0035]    The actuators  10  and  10 A may be either partially or fully unlatched as required. Referring again to  FIG. 3 , if the piezoelectric ceramic material is in a latched state  38 , and if a weaker (i.e. between 0 and 40 kV/cm) reverse electric field is applied, the material will not change all the way to the re-poling reset state  40  when the reverse electric field is removed, and the ceramic material will instead return to a strain state  34 A that is between the unlatched state  34  and the power-off latched state  38 . Also, when the material is at the power-off unlatched state, a weaker electric field (i.e. between −20 and 0 kV/cm) can be applied and removed to shift the material to a power-off latched state  38 A that is between the power-off latched state  38  and the power-off unlatched state  34 . Thus, by controlling the electric field applied to the fibers  8  of actuator  10 , the actuator  10  can take on different states between the unlatched state  34  and the latched state  38  as required for a particular operating condition or application. 
         [0036]    Referring again to  FIGS. 1 and 2 , the characteristics of the piezocomposite actuators  10  and  10 A may be selected as required for a particular application. As discussed above, the fibers  8  and/or  28  may be fabricated such that the d 33  mode extends along the fibers, whereby the fibers decrease in length when changing from the power-off unlatched state  34  ( FIG. 3 ) to the power-off latched state  38 . Conversely, the fibers  8  and/or  28  may be fabricated with the d 31  mode extending along the length of the fibers whereby the fibers increase in length when shifting from a power-off unlatched state to a power-off latched state. Accordingly, it will be understood that the strain states (e.g.  FIG. 3 ) depend on the material selected, and the orientation of the mode of the fibers  8  and/or  28 , 
         [0037]    With further reference to  FIG. 4 , the actuators  10  and  10 A of  FIGS. 1 and 2 , respectively, may be utilized to form active fiber composite plies  42  by incorporating the actuators into conventional fiber composite plies. With further reference to  FIGS. 5 and 6 , a helicopter rotor blade  45  includes conventional fiber composite laminates  44 , and may include a core  48  comprising foam or other lightweight material. The fiber composite laminate  44  may comprise known materials such as carbon fibers and an epoxy matrix or other suitable materials. The active fiber plies  42  are disposed over at least a portion of the fiber composite laminate  44 . A flex circuit  50  extends between the upper side  52  of rotor blade  45  and lower side  54  of rotor blade  45 . An optional flex circuit  50  comprises piezoelectric material elements whereby the flex circuit  50  generates electricity as rotor blade  45  flexes. The electrical current from the flex circuit  50  may be applied to the actuator  10  of active fiber composite ply  42  to thereby latch and/or unlatch the actuator  10  to control the shape of the rotor blade  45 . It will be understood that the electrical power supplied to the active fiber composite plies  42  may come from a battery or other suitable electrical power source rather than flex circuit  50 . 
         [0038]    With further reference to  FIG. 6 , in use aerodynamic forces acting on rotor blade  45  generate a first moment “M 1 .” In  FIG. 6 , the moment M 1  is shown as acting at end  56  of rotor blade  45 . However, it will be understood that the moment M 1  actually acts along the length of the blade  45  due to the aerodynamic forces acting on the rotor blade  45 . A counter acting moment “M 2 ” at base end  58  of rotor blade  45  results from moment M 1 . Actuators  10  (or  10 A) and/or active fiber composite plies  42  can be oriented such that actuation of the actuators  10  generates threes within active fiber composite plies  42  tending to counteract the twist resulting from the applied moments M 1  and M 2 . Furthermore, the shape of rotor blade  45  can be varied utilizing actuators  10  to provide a desired rotor shape in use that optimizes lift, reduces noise, and/or provides other results as required for a particular application. 
         [0039]    The magnitude of the moments M 1  and M 2  may be related to helicopter operating conditions. For example, when the rotor blade  45  experiences a relatively large aerodynamic force, the moments M 1  and M 2  may tend to be larger. The amount of electric current and resulting electric field that is applied to the actuator  10  can be varied as required to compensate for the variation in the applied moment M 1 . For example, a plurality of strain sensors  60  may be imbedded in the fiber composite laminate  44  and/or the active fiber  42  on the upper side  52  and/or lower side  54  of rotor blade  45 . The strain data from strain sensors  60  may be utilized by a controller (not shown) to determine the magnitude of an electrical field to be applied to the actuator  10 . Referring again to  FIG. 3 , in the illustrated example, if an electric field of less than −20 kV/cm is applied to the fibers, the magnitude of the change in strain will be less than “S.” Thus, a variable electric field can be applied to the actuators  10  of rotor  45  ( FIGS. 4-6 ) to thereby control the twist of the rotor  45  as required for a particular operating condition. 
         [0040]    With further reference to  FIG. 7 , positive and negative voltages can be applied to the shape memory ceramic fibers  8 .  FIGS. 8-10  show the strain versus voltage characteristics of a PZT-5H shape memory ceramic material resulting from the voltages of  FIG. 7 . Specifically, application of the voltages of  FIG. 7  results in longitudinal strain as shown in  FIG. 8 , and transverse strain as shown in  FIG. 9 . As shown in  FIG. 10 , weaker back field voltages (i.e. weaker negative voltages in  FIG. 7 ) cause partial depoling which reduces remnant strain. However, as the back (negative) voltage is increased, the material re-poles and remnant strain increases as also shown in  FIG. 10 . 
         [0041]    With further reference to  FIG. 11 , a 8/65/35 PLZT material also exhibits self-latching characteristics. It will be understood that various shape memory ceramic materials may be utilized to form a self-latching piezocomposite actuator  10  according to the present invention. 
         [0042]    With further reference to  FIGS. 12-16 , an aircraft wing  62  defines a tip profile  64  ( FIG. 14 ) and a root profile  66  ( FIG. 13 ). Wing  62  also includes an internal spar structure  68 , an upper layer or sheet of material  70 , and a lower layer or sheet of material  72 . The layers/sheets  70  and  72  extend over the spar structure  68  from a leading edge “LE” of wing  62  to a trailing edge “TE” of wing  62 . The spar structure  68  is substantially rigid and defines a region having a fixed boundary “FB 1 ” The upper and lower layers/sheets  70  and  72 , respectively, are connected to the spar structure  68  in the fixed boundary region FB 1  such that the sheets  70  and  72  do not change shape in the fixed boundary region FB 1 . An internal space  74  is defined between the upper and lower sheets  70  and  72  in a free boundary region “FB 2 ” of wing  62 . The internal space  74  may be substantially empty, or it may comprise a flexible and/or compressible lightweight filler material. The layers/sheets  70  and  72  are at least somewhat flexible and capable of changing shape in the free boundary region FB 2  of wing  62 . 
         [0043]    A plurality of self-latching piezocomposite actuators  10  are disposed on or incorporated into, the upper and lower layers/sheets  70  and/or  72  in the free boundary region FB 2  of wing  62 . It will be understood that the thickness of the actuators  10  is exaggerated in  FIGS. 13-16  for purposes of showing the location of the actuators  10 . Sheets  70  and/or  72  may comprise carbon fiber/epoxy matrix material, and the actuators  10  may adhesively attached to inner or outer surfaces of the layers/sheets  70 ,  72 , or the actuators  10  may be embedded in the composite material. If the layers/sheets  70 ,  72  comprise metal, the actuators  10  may be adhesively bonded to the inner or outer surfaces of the layers/sheets  70 ,  72 . 
         [0044]    A shape-changing/morphing flexible region  76  is defined between lines “L 1 ” and “L 2 .” Actuators  10  may be configured to span across the region.  76  such that first ends  78  of actuators  10  are positioned in front of the line L 1 , and second ends  80  of actuators  10  are positioned to the rear of the line L 2 . In use, the actuators  10  on the upper and/or lower sides of wing  62  can be actuated to thereby vary the camber of the wing  62  in the free boundary region FB 2  to change the lift generated by the wing  62  as required for a particular operating condition. For example, lower sheet  72  may flex from the shape of  FIG. 15  to the shape of  FIG. 16  to provide increased concave curvature  76 A ( FIG. 16 ), and the upper sheet  70  may flex to provide increased convex curvature  76 B ( FIG. 16 ). By increasing the camber, the lift of the wing can be increased for takeoff and landing, and to provide increased lift if the aircraft is carrying a heavy cargo and/or has a relatively low airspeed. Conversely, the camber can be decreased to reduce lift and drag if the aircraft loading and/or flight conditions do not require increased lift. 
         [0045]    The actuators  10  may be actuated simultaneously or separately as required to provide a desired camber to optimize the lift of the wing  62  for a given flight condition/lift requirement. As discussed above, actuators  10  may be configured to shift from a flat (unlatched) configuration to a curved (latched) configuration. Actuators  10  on (or in) lower sheet  72  may be actuated to form a concave outer surface contour at the same time the actuators  10  on (or in) upper sheet  72  are actuated to provide increased convex curvature. By selectively actuating the actuators  10  to varying degrees (e.g. corresponding to strain states at or between unlatched state  34  and power-off latch state  38  of  FIG. 3 ) various camber and resulting lift/drag characteristics can be provided. 
         [0046]    Self-latching actuators  10  according to the present invention may be utilized in other types of active/morphing wing structures in addition to the active/variable camber wing  62  of  FIGS. 13-15 . For example, the self-latching actuators of the present invention may be utilized to provide an active leading wing edge that changes shape to prevent stalling when the wing is at a high angle of attack during takeoff and/or landing operations. Self-latching actuators may also be utilized to change the shape of the wing from a conventional airfoil to a supercritical airfoil to reduce the formation of shock waves at the surface of the wing during transonic flight conditions. 
         [0047]    With further reference to  FIGS. 17 and 18 , an aircraft  100  may include wings  62  having variable camber and/or other morphing features as discussed above in connection with  FIGS. 12-16 . Horizontal stabilizers  62 A of aircraft  100  may include elevators or other control surfaces that are controlled utilizing one or more self/latching piezocomposite actuators  10  according to the present invention. Similarly, aircraft  100  may include a vertical stabilizer  62 B having a rudder  84  that can be controlled utilizing actuators  10  on opposite sides of the rudder  84 . Self-latching actuators  10  may be utilized in connection with flexible aerodynamic surfaces to provide integrated elevators and/or flaps to thereby eliminate the gaps between the flaps and the primary wine structures that are formed by conventional control surfaces such as flaps. 
         [0048]    Aircraft  100  may also include one or more turbo fan or turbo jet engines  86  that provide thrust. With further reference to  FIG. 19 , engine  86  includes an inlet  88  that is defined by a forward portion  90  of engine nacelle structure  92 . Actuators  10  may be incorporated into engine structure  92  adjacent forward portion  90  to thereby change the shape of forward portion  90  to increase or decrease the size and shape of inlet  88 . For example, actuators  10  may be actuated to shift the forward portion  90  inwardly as shown by the dashed line  90 A, or outwardly as shown by the dashed line  90 B. It will be understood that the dashed lines  90 A and  90 B represent exaggerated movement/shape change for purposes of illustrating changes to the size and/or shape of the inlet  88 . 
         [0049]    With further reference to  FIG. 20 , a composite reflector  94  according to another aspect of the present invention includes a curved primary structure  96  that is fabricated from carbon fiber or other composite materials. A front reflective/mirror surface  98  of primary structure  96  is concave to provide predefined optical reflective properties (e.g. magnification). The reflector  94  may be utilized for space-based optical systems. For example, the composite reflector  94  may be utilized in a spacecraft  95  as a component of a telescope. 
         [0050]    The composite reflector  94  includes a plurality of self-latching piezocomposite actuators  10  that are disposed on a rear surface  102  of primary structure  96 . Actuators  10  may be adhesively bonded to rear surface  102 , or they may be integrally formed with the composite materials of the primary structure  96 . In the illustrated example, actuators  10  extend between junctions  104  to form a hexagonal pattern. However, the actuators  10  may be oriented in any suitable configuration. The actuators  10  may be operably connected to a power source and a controller (not shown) whereby the shape of the front surface  98  is changed/controlled by the actuators  10 . The actuators  10  thereby compensate for distortions in front surface  98  due to thermal effects, stress, or other environmental influences. The actuators  10  may also be utilized to compensate for imperfections in front surface  98  that may occur as a result of the fabrication process utilized to form main structure  96 . 
         [0051]    With further reference to  FIG. 21  an optical reflector  105  according to another aspect of the present invention comprises a main body  106  that is generally disc-shaped, and forms a reflective front surface  108 . The body  106  may comprise glass, ceramic, or other material, and the reflective front surface  108  may be coated with a reflective metal material or the like to form an optical mirror. A plurality of self-latching piezocomposite actuators  10  are disposed on rear surface  110  of body  106 . Actuators  10  may be operably connected to an electrical power source (not shown) by wires or other suitable conductors not shown), and a controller (not shown) may be utilized to control the electrical power supplied to the actuators  10 . One or more of the actuators  10  may be actuated to generate a force acting on the body  106  to thereby change the shape of reflective front surface  108 . In this way, distortions in the front surface  108  due to thermal effects, applied loads, or other environmental factors can be actively corrected utilizing the actuators 
         [0052]    The reflector  94  and reflector  105  of  FIGS. 20 and 21 , respectively, may be controlled utilizing open loop or closed loop control systems. For example, sensors may be utilized to measure the shape of the reflective surfaces, and the actuators  10  may be selectively actuated to compensate for the measured distortions in the reflective surfaces. Alternatively, empirical data and/or analytical calculations may be utilized to predict the shapes of the reflective surfaces under various thermal and other environmental conditions. The temperature can then be measured or estimated, and the actuators  10  can be actuated as required to compensate for the estimated distortions in the reflective surfaces.