Abstract:
A microactuator providing an output force and displacement in response to an increase in thermal energy is displosed. The microactuator may have a substantially straight expansion member with a first and a second end. The first end may be coupled to a base member, and the second end may be coupled to a shuttle. The expansion member is capable of elongating in a elongation direction. Elongation of the expansion member may urge the shuttle to translate in an output direction substantially different than the elongation direction. In certain embodiments, multiple expansion members are arrayed along one side of the shuttle to drive the shuttle against a surface. Alternatively, expansion members may be disposed on both sides of the shuttle to provide balanced output force. If desired, multiple microactuators may be linked together to multiply the output displacement and/or output force.

Description:
This application claims the benefit of provisional application Ser. No. 60/212,249 filed Jun. 19, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present device relates to microelectromechanical systems. More particularly, the device relates to thermally activated microactuators. 
     TECHNICAL BACKGROUND 
     Many different transducers have been created to convert electricity and thermal energy into mechanical force or motion. For example, electric linear and rotary motors, relays, and the like are used for many applications. Relays, in particular, are used to carry out functions such as valving and switching when actuated by a current. 
     However, previously known transducers are typically ill-suited for use in microcircuits. Microcircuits are used in many different applications, from hearing aids to dog tags, many of which require small-scale mechanical operations. MEMS, or microelectromechanical systems, have been developed to provide mechanical operations in microscopic environments. 
     Nevertheless, known small-scale transducers, or microactuators, are in may respects limited. They are somewhat bulky with respect to the circuits in which they operate. They also require considerable voltage to operate, and provide only a relatively small amount of mechanical force or displacement in return. The high voltage requirements of most known transducers make them unusable in CMOS circuits, as found in personal computers, which typically operate at 5 Volts or less. In addition, known microactuators are often subject to failure due to contamination, which makes them useless in many exposed environments. Additionally, many known microactuators are inflexible in design, and thus cannot be readily adapted to suit different applications. Known devices also must often be manufactured through special processes that require entirely different equipment and procedures from those used to form a circuit. 
     One example of a known microactuator is a “U” shaped actuator, with a “hot” arm and a “cold” arm. Both arms have an anchored end and a free end. Each anchored end is fixed to a substrate and the free ends of the two arms are connected together by a thin member. The hot arm is a relatively thin member and the cold arm is a relatively thick member. Both arms have a thin flexure near the anchored end. The actuator is triggered by applying an electric current through the actuator, from anchor to anchor. The thin, hot arm has a higher current density than the thick, cold arm, due to its comparatively smaller cross-sectional area. The high current density causes the hot arm to heat and expand more than the cold arm. Because the arms are connected at the free end, the differences in expansion causes the actuator to bend such that the free end moves along an arc. This actuator functions in a manner similar to a bimetallic strip, in which the different expansion properties of the two metals cause the strip to curl. Multiple “U” shaped actuators may be connected to a common actuating structure form an array that compounds their output forces. This is accomplished by attaching a flexible yoke between the free end of the actuator and the common actuating structure. This flexible yoke is required to translate the arc-like motion into a linear actuation. 
     While this configuration does provide functional force and displacement characteristics, the “U” shaped actuator possesses multiple deficiencies. For example, arc incurred losses during conversion of the arcing output motion into linear translating motion. More specifically, the actuators in the array must expend a portion of their output energy to deform the flexible yokes so that the common actuating structure moves in a straight line. Additionally, the cold arm&#39;s bulky size resists deflection as the hot arm arcs towards the cold arm. The force required to bend the cold arm does not contribute to the ultimate output force at he “U” shaped microactuator. Furthermore, the cold arm requires material, volume, and energy but does not contribute to the actuating force. The noncontributing material, volume, and energy become even more burdensome when multiple “A” shaped actuators are connected to form an array. The flexible yoke members similarly require energy, material, and volume without contributing to the output force produced by the actuator. Thus, the bulk and energy requirements fo the “U” shaped actuator make such actuators unsuitable for certain applications. 
     Accordingly, a need exists for a microactuator that can provide a high output force and high displacements, while operating at a low input voltage. Furthermore, the actuator should be lightweight and small, and should continue to operate in the presence of contaminants common in microcircuit applications. The microactuator should have a flexible design that can be easily adapted to suit various input, output, size, and material specifications. Moreover, the microactuator should be simple and easy to manufacture, preferably through methods similar to those used to make the circuits in which they operate. 
     BRIEF SUMMARY OF THE INVENTION 
     The present micromechanism includes a microactuator that has advantageous size, displacement, and force characteristics. The micromechanism may comprise a generally long and thin expansion member that is coupled at a first end to a base member and at a second end to a displaceable shuttle. In one embodiment, the expansion member extends towards and the shuttle at an angle slightly offset from a perpendicular attachment to the base member. The expansion member may be configured to elongate in an elongation direction. The shuttle may be configured to travel in an output direction along a single axis. The displaceable shuttle may be constrained such that the lateral distance between the base member and the axis of shuttle&#39;s output direction is fixed. This output direction is substantially different from the elongation direction of the expansion member. In one embodiment, the shuttle travels in a direction nearly perpendicular to the elongation direction of the expansion member. The expansion member is comprised of a material that can be formed microscopically. The material and shape of the expansion member may be selected such that substantial elongation occurs when thermal energy increases in the expansion member. 
     Upon an increase of thermal energy within the expansion member, the expansion member elongates in a direction nearly perpendicular to the base member and shuttle. Since the lateral offset of the base member and shuttle is constant, the expansion member cannot expand perpendicular to the shuttle. The expansion member&#39;s movement at the base member coupling is limited to slight angular rotation and movement at the shuttle coupling is limited to the uniaxial travel of the shuttle. These limitations may force the expansion member to pivot near the base member end and drive the shuttle at the shuttle end. Relative motion between the base member and the shuttle permits pivoting of the expansion member such that the increased length of the expansion member can be accommodated. The result is that a relatively small elongation of the expansion member creates a large displacement of the shuttle. 
     The microactuators disclosed herein may function substantially in-plane, which entails operation of each component within a single plane. Thus, the microactuator may be made through film deposition methods similar to those used to construct flat circuits. In fact, a microactuator according to the invention may even be made simultaneously and unitarily with a circuit so that production can be econoznically and rapidly carried out. The low voltage requirement makes such microactuators operative for CMOS applications and the like, and their high force/displacement characteristics make them uniquely suited to other applications in which efficient motion is desirable. In addition, the simple design of the microactuators of the present invention enables them to continue operating even in the presence of small contaminants often found in circuit environments. 
     The purpose, function, and advantages of the present mechanism will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a plan view of one embodiment of a microactuator with having symmetrical sets of expansion members configured to drive a single shuttle; 
     FIG. 2 is the plan view of the microactuator of FIG. 1, depicting one possible shape of elongated expansion members and one manner in which the microactuator may be connected to receive an electrical input; 
     FIG. 3 is a plan view of an alternative embodiment of a microactuator in which only a single set of expansion members is used; 
     FIG. 4 is a plan view of another alternative embodiment of a microactuator, in which each expansion member has a variable width; 
     FIG. 5 is a plan view of another alternative embodiment of a microactuator, in which two symmetrical sets of expansion members are used, each of which contains two groups of expansion members; 
     FIG. 6 is a plan view of an embodiment of an array of microactuator in which two microactuators press inward to amplify the motion of a third microactuator; and 
     FIG. 7 is a plan view of another embodiment of an array of the microactuator, in which two microactuators press inward against an anchored microactuator and an unanchored microactuator to amplify the motion of the unanchored microactuator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present device will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present mechanism, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the apparatus and method, as represented in FIGS. 1 through 7, are not intended to limit the scope of the claimed mechanism, but are merely representative of present embodiments of the mechanism. 
     FIG. 1 depicts a mnicroactuator with enhanced force and displacement characteristics. A datum  4  has been established to provide orientation throughout the application. The datum shows negative  5  and positive  6  X directions (“lateral directions”) and negative  7  and positive  8  Y directions (“longitudinal directions”). The respective locations of the various elements of the mnicroactuator may be more precisely defined by referring to the datum  4 . The microactuator  10  has a first base member  12  anchored to a first surface  14  and a second base member  16  anchored to a second surface. In one embodiment, the surfaces  14 ,  18  may be parts of the substrate of a silicon chip or, in an alternative embodiment the surfaces  14 ,  18  may be parts of one or more other microelectromechanical mechanisms. Two sets of expansion members  20 ,  22  are coupled to the base members  12 ,  16  respectively. The expansion members  20 ,  22  may be generally elongated and are composed of a thermally expanding material. The expansion members  20 ,  22  extend from the base members  12 ,  16  and are coupled to a shuttle  24  to create a ladder shaped actuator. 
     The first expansion members  20  are attached to the first base member  12 . The first expansion members  20  extend in the positive X direction  6  and are coupled to a side of the shuttle  24 . Similarly, the second expansion members  22  extend from the second base member  16  in the negative Y direction  5  and are coupled to the shuttle  24  opposite the first expansion members  20 . The shuttle  24  may be generally stiff, and may be slidably disposed on a surface such as a semiconductor substrate. In FIG. 1, the shuttle  24  is only constrained by the expansion members  20 ,  22 . The tensile and compressive strengths of the expansion members  20 ,  22  substantially limit the movement of the shuttle to displace in the positive  8  and negative  7  Y directions. Thus, the lateral distance in the X directions  5 ,  6  between the base members  12 ,  16 , and the shuttle  24  does not change significantly during operation of the actuator  10 . FIG. 1 further depicts the shuttle  24  and base members  12 ,  16  as rectangular in shape, but one skilled in the art will recognize that these elements may be configured in any number of shapes to fit a particular design need. 
     While FIG. 1 demonstrates a preferred embodiment of present invention, an operable microactuator may be formed with a single expansion member  20  coupled to a single base member  12  and a shuttle  24 . The expansion member  20 , the base member  12 , and shuttle  24  form an “T” shaped actuator. Thus, any disclosure referring to multiple expansion members or groups of expansion members in the application may simply be replaced with a single expansion member to provide additional alternative embodiments of the invention. However, even though the micromechanism is operable with only a single expansion member, an increased number of expansion members will correspondingly increase the output force of the device. Thus, the microactuator of FIG. 1 will have a larger output force than an “T” shaped actuator with single expansion member. This relationship between the output force and the number of expansion members provides the microactuator of FIG. 1 with a large variety of versatile design options. The microactuator  10  can be simply optimized by adding or removing expansion members so that the microactuator  10  only outputs the required force for the actuating function. Thus, the overall size and energy consumption of the microactuator  10  can be minimalized. 
     The individual elements of the ladder shaped actuator  10  in FIG. 1 or the “I” shaped actuator described above, which is a subset of the microactuator  10 , may be attached to each other by multiple methods, such as chemical or adhesive bonding, integral formation, mechanical attachment, or the like. In one embodiment, the microactuator  10  is a compliant mechanism. In a compliant mechanism, the base members  12 ,  16 , the expansion members  20 ,  22 , and the shuttle  24  form a single continuous, unitary structure. Compliant mechanisms are a family of devices in which flexible and bendable members replace convention multi-part devices, such as pin joints. They provide several benefits including simple manufacturing, high strength, and flexibility. Moreover, a compliant mechanism is typically constructed in unitary fashion. For example, the various components of the embodiment of FIG. 1 may be formed from one or more planar layers of polysilicon. The motion of the microactuator  10  is determined by its geometry. Thick members, such as the base members  12 ,  16  and the shuttle  24 , will stay rigid. Conversely, thin or necked-down members, such as the expansion members  20 ,  22 , will flex. In FIG. 1, the expansion members  20 ,  22  are thin flexible members; however, where practical, they may be necked-down to form small length flexural pivots to obtain flexibility. Thus, the compliant nature of the expansion members  20 ,  22  provides the motion of the microactuator  10 . 
     The expansion members  20 ,  22  also supply actuating force for the mnicroactuator  10 . The expansion members  20 ,  22  are preferably made from a material with a high coefficient of thermal expansion (ratio of thermal expansion to temperature change) to obtain large displacements. However, a material with a lower coefficient or thermal expansion may be used when smaller displacements are desirable. The high coefficient allows for comparatively large elongation of the expansion members  20 ,  22  when the amount of thermal energy increases within the expansion members  20 ,  22 . 
     When thermal energy increases in the expansion members  20 ,  22 , they elongate. However, as stated above, the lateral distance between each of the base members  12 ,  16  and the shuttle  24  is fixed. Consequently, as the first expansion members  20  elongate in the positive X direction  6 , the compressive strength of the second expansion members  22  prevents the shuttle  24  from moving in the positive X direction  6 . Likewise, as the second expansion members  22  elongate in the negative X direction  5 , the compressive strength of the first expansion members  20  prevent the shuttle  24  from moving in the negative X direction  5 . The result of these constraints is that the expansion members  20 ,  22  drives the shuttle  24  along the Y-axis  7 ,  8 . This biasing occurs as the expansion members  20 ,  22  bend or pivot from a first attachment angle  23 , to a second attachment angle  21 , with respect to the base members  12 ,  16 . The first attachment angle  23  maybe substantially perpendicular, or close to 90°. Substantially parallel may be a ±15° offset from a true perpendicular attachment without diverging from the spirit of the embodiment. The second attachment angle  21  may be somewhat further from perpendicularity. The trigonometric effect of the decrease in the attachment angle displace the shuttle  24  in the positive Y direction  8  so that the expansion member can elongate. 
     Applying simple trigonometry to the actuator structure  10  in FIG. 1 demonstrates that if the first attachment angle  23  is assumed to be nearly perpendicular or close to 90°, then each of the expansion members  20 ,  22  at the second attachment angle  21  will form a hypothenuse of a right triangle in which the non-elongated expansion member  20  or  22  and the portion or the shuttle  24  between the couplings of the hypothenuse  26  or  28  and the non-elongated expansion members  20  or  22  form the other two sides of the triangle. The Pythagorean Theorem holds that the hypothenuse  26  or  28  is longer than the adjacent side  20  or  22 . Thus, as elongation of the expansion members  20 ,  22  forces the attachment angle to decrease, the expansion members  20 ,  22  then move from the adjacent position of the right triangle to the hypothenuse position  26 ,  28 . This angle decrease drives the attached shuttle  24  along the Y-axis  7 ,  8  to a displaced position  30 . Thus, elongation of the expansion members  20 ,  22  almost completely in the X direction  5 ,  6  produces a displacement  32  of the shuttle  24  in the Y direction  8 . Therefore, the elongation direction and the shuttle output direction are substantially different directions. It is important to understand, however, that a substantially different direction is not limited to a near 90° difference in directions. Various embodiments of the microactuator may only require small differences in direction. The difference between the elongation and output directions will depend on the desired force and displacement characteristics of the actuator. 
     An elongation direction that is completely perpendicular to the output direction may present some directional control problems; hence some departure from the perpendicular attachment may be needed. Thus, the microactuator  10  of FIG. 1 implements an initial angular offset of the expansion members  20 ,  22  to control the actuation direction. By offsetting the expansion members  20 ,  22  such that the first attachment angle  23  is slightly less than 90°, elongation of the expansion members  20 ,  22  will displace the shuttle  24  in the positive Y direction  8 . While comparatively large angle offsets will guarantee a predictable displacement direction, positioning the expansion members  20 ,  22  at a near perpendicular angle provides a greater output displacement  32 . 
     An advantage of the microactuator  10  over other designs is the ability to select a wide range of actuation force and displacement characteristics. The largest shuttle displacement occurs when the expansion members elongate in a direction nearly perpendicular to the direction of travel of the shuttle  24 . However, this displacement comes at the expense of force. The output force can be increased by offsetting the attachment of the expansion member from a perpendicular configuration. The output force will increase as the offset increases, but the displacement distance Will correspondingly decrease. Therefore, the first attachment angle  23  may be selected according to the force and displacement requirements for the application in which the microactuator  10  is to be used. Alternatively, additional expansion members  20 ,  22  can simply be added to a microactuator  10  to increase the output force while maintaining a desired output. 
     A beneficial feature of this design is that a relatively small elongation of the expansion members  20 ,  22  can produce a shuttle displacement over ten times larger than the elongation of the expansion members  20 ,  22 . The displacement of the shuttle  24  as a function of the elongation of expansion members  20 ,  22  can be derived from Pythagorean theorem, assuming the unelongated expansion members  20 ,  22  are nearly perpendicular to the base members  12 ,  16  and the shuttle  24 . As the expansion members  20 ,  22  elongate, they depart further from perpendicularity. The following equation may be used to obtain the output displacement of the shuttle  24  and variables are as follows: 
     
       
         Δ={square root over (( L   2 ) 2 −( L   1 ) 2 )} 
       
     
     Δ is the displacement of the shuttle  24  in the output direction; 
     L 1  is the unelongated length of the expansion members  20 ,  22 ; and 
     L 2  is the elongated length of the expansion members  20 ,  22 . 
     This equation measures the length of the unelongated and elongated expansion members as the distance from the base member attachment to the shuttle attachment. This measurement may vary somewhat from the actual length of the expansion member if bending or buckling occurs in the member. A ratio (R 1 ) of displacement to elongation can be obtained through the following equation.          R   1     =     (     Δ       L   2     -     L   1         )                            
     A more robust equation may also be employed to characterize the operation of the microactuator  10  without requiring unelongated expansion members  20 ,  22  to have a near perpendicular attachment. Such an equation may be obtained by referencing the unelongated and the elongated expansion members  20 ,  22  to theoretical expansion members (not shown) exactly perpendicularly fixed between the base members  12 ,  16  and the shuttle  25   24 . The length of this theoretical member is the fixed lateral distance between each of the base members  20 ,  22  and the shuttle  24 . This equation provides the output displacement of the shuttle  24  for any of a large range of values of the first attachment angle  23 . The equation and variables are as follows:        Δ   =       [         L   2     ·   sin                     (     arccos                   (       L   0       L   2       )       )       ]     -     [         L   1     ·   sin                     (     arccos                   (       L   0       L   1       )       )       ]                              
     Δ is the displacement of the shuttle  24  in the output direction; 
     L 0  is the fixed lateral distance between the base member  12  or  16  and the shuttle  24 ; 
     L 1  is the unelongated length of the expansion members  20 ,  22 ; and 
     L 2  is the elongated length of the expansion members  20 ,  22 . 
     This equation is derived from the trigonometric relationships of two right triangles that share the same base, the common base being the fixed lateral distance (L 0 ). The physical constraints must be considered in performing calculations with the above equation. The previous equation functions when the first attachment angle  23  is within 90° of the fixed lateral length. 
     A shuttle displacement to elongation ratio (R 2 ) may be calculated in the same manner as above:          R   2     =     (     Δ       L   2     -     L   1         )                            
     The previous two equations demonstrate that the displacement of the shuttle  12  is more than two times larger than the elongation of expansion members  20 ,  22 , even with a first attachment angle  23  less than 45°. Although, the highest displacement ratios occur when the first attachment angle  23  is near 90°, these equations demonstrate that high displacement to elongation ratios occur throughout a large range of values of the first attachment angle  23 . However, smaller displacement to elongation ratios can also be obtained through application of the present invention. Multiple applications may require such small displacements. This discussion is not intended to limit the invention to any displacement to elongation ratio. 
     The microactuator  10  may have comparatively low energy consumption which is due in part to the linear path of the expansion members  20 ,  22 . More specifically, each of the expansion members  20 ,  22  has one end coupled to the shuttle  24 ; these coupled ends move in a substantially linear path as the microactuator  10  operates. Because the most efficient path between to points is a straight line, the linear travel of the coupled ends is more efficient than any other path, such as an accurate, elliptical, or otherwise nonlinear path. 
     The substantially straight shape of the expansion members  20 ,  22  may also add to the efficiency of the microactuator  10 . Bending consumes energy; consequently, eliminating bending from a system will increase the efficiency of the system. Therefore, a generally stiff member is more efficient than an extremely flexible member. The stiffness (k) of a beam may be calculated using the linear spring constant:        k   =       3      EI       L   3                              
     k is the stiffness; 
     E is Young&#39;s modulus, which is a material property, 
     I is the moment of area of the cross-section of the beam; and 
     L is the beam length. 
     Assuming a given cross-sectional moment (I) and a given value of Young&#39;s modulus (E), the shortest member (L) will be the stiffest. Therefore, because the shortest distance between two points is a straight line, the substantially straight elongation member  20 ,  22  is the stiffest and consequently, consumes less energy than a non-straight member. Stiffness, as discussed above, does not require absolute rigidity, but simply entails sufficient rigidity to substantially avoid deflection that is not necessary for the motion of a microactuator. 
     While FIG. 1 depicts a substantially straight elongated expansion member  26 ,  28 , this embodiment represents an ideal elongation in which flexibility is limited to the points at which the expansion members  26 ,  28  are coupled to the base members  12 ,  16  and the shuttle  24 . This ideal elongation would require pin joints or necked-down cross-sections that permit flexing of the expansion members  20 ,  22  only at the ends of the expansion members  20 ,  22 . Maintaining a substantially straight expansion member may not be as simply accomplished when the expansion member is attached without necked-down section or pivot joints. In a compliant embodiment, in which the base members  12 ,  16 , the expansion members  20 ,  22 , and the shuttle  24  are a single continuous device, flexibility at the attachment points may be low. Therefore, bending at or near the attachment points will not occur as readily. In a member that is fixed on two ends, the most probable location for bending to occur is in the center of the member&#39;s length, assuming the member has a constant cross-section. 
     FIG. 2 shows the microactuator  10  of FIG. 1, with an alternative mode of expansion member elongation. More specifically, the expansion members  20 ,  22  may have elongated configurations  34 ,  36  respectively. The elongated expansion members  34 ,  36  way bend in the center during elongation to for an “S” shape. Despite this flexing, the elongated expansion members  34 ,  36  still remain substantially straight in some aspects. The expansion members  34 ,  36  remain substantially straight at the attachment points and only begin to flex near the center of their length. Even in the bent region of the “S” shape, the curvature remains relatively small. Consequently, the manner in which the expansion members  34 ,  36  bend is more efficient than other bending modes such as arcuate bending, in which a greater degree of bending is present over a greater length. Thus, the microactuator  10  disclosed herein remains efficient despite some bending. 
     The flexure of the “S” shaped expansion members  34 ,  36  can also be understood as an elastically buckling process. As the expansion members  20 ,  22  elongate in the positive X  6  and negative X  5  direction respectfully, the fixed distance between the base members  12 ,  16  and the shuttle  24  forces the expansion members  20 ,  22  to elastically buckle. Elastic buckling is not a permanent deformation of the member; rather, the elongated expansion member  34 ,  36  temporarily yield under axial loads, but return to their original substantially unaffected state  20 ,  22  when the load is removed. In the embodiment in FIG. 2, the direction of the buckling is controlled by the first attachment angle  23  of the expansion members  20 ,  22  with respect to the base members  12 ,  16 . The elongation and subsequent buckling drive the shuttle  24  in the positive Y direction  8 . 
     The elongation of the expansion members  20 ,  22  may be initiated in a variety of manners. FIG. 2 illustrates that the base members  12 ,  16 , have contact surfaces  38 ,  39 , respectively, by which the base members  12 ,  16  are electrically coupled to a current source  37 . In one embodiment, thermal energy in the expansion members  20 ,  22  is increased by an electrical current flowing through the expansion members  20 ,  22  from the current source  37 . As the current passes through the expansion members  20 ,  22 , the electrical resistance of the expansion members  20 ,  22  causes an increase in temperature. The temperature increase causes elongation of the expansion members  20 ,  22 . Thus, in one embodiment of the present invention, the shuttle  24  actuates when a current passes through the microactuator. Once the current is removed, the amount of thermal energy decreases to an equilibrium state and the shuttle  24  returns to its original position An increase in the ambient temperature surrounding the microactuator  10 , may also provide enough thermal energy in the expansion members  20 ,  22  to actuate the shuttle  24 . The ambient temperature may be increased by disposing a heat generating device near the microactuator  10 . Other methods of increasing thermal energy within the expansion members  20 ,  22 , such as conduction and radiation may also be used to actuate the shuttle  24 . Additionally, traditional methods of preventing heat loss maybe implemented in conjunction with the microactuator  10  to increase efficiency. An adequately insulated mechanism will require the addition of less thermal energy to provide actuation, and will remain in the actuated position with a lower steady state current input. 
     FIG. 3 demonstrates an alternative embodiment of a microactuator  40  according to the invention. The microactuator  40  may have only one set of expansion members  20 . The shuttle  24  directly abuts the second base member  42  to restrict motion of the shuttle  24  in the positive X direction  6 . The second base member  42  is anchored against a surface  44  (or another suitable anchoring feature) and in one embodiment, has a smooth surface  46  against which the shuttle  24  slides. The second base member  42  fixes the lateral distance between the first base member  12  and the shuttle  24 . Thus, elongation of the expansion members  20  drive the shuttle in the positive Y direction  8 . The microactuator  40  is more compact than other actuators, such as the microactuator  10 , that have two sets of expansion members  20 ,  22 . Therefore, the microactuator  40  requires less energy to actuate, but has a correspondingly lower output force. This embodiment  40  also has an “I” shape. Each of the four expansion members  20  forms an “I” shape in conjunction with the base member  12  and the shuttle  24 . 
     In another embodiment, the second base member  42 , need not be a fixed member, it simply must restrain movement of the shuttle  24  in the positive X direction  6 . Consequently, a variety of structures may be used in place of the second base member  42 . In one alternative embodiment, the abutting surfaces of the shuttle  24  and the second base  42  may be replaced with a rack-and-pinion type structure, so that the shuttle  24  can drive a gear or the like. Alternatively, the second base member  42  may be a member capable of sliding along the Y axis  7 ,  8 , such as another shuttle  24 . The contact surfaces between shuttle  24  and the second base member  42  need not be straight surfaces. Various shapes can be implemented that allow for travel in a combination of X  5 , 6  and Y  7 ,  8  directions. 
     FIG. 4 illustrates an alternative embodiment of expansion members suitable for use in any of the microactuator designs of the present invention. Each of the expansion members  49  shown here has a variable width. The rectangular shapes of the expansion members  49  of FIG. 4 represent only one of many possible geometric structures that could be incorporated into the expansion members  49 . The variable width geometry of the expansion members  49  may provide structural support so that the expansion members  49  can be made comparatively long. More specifically, the wide cross-section  48  prevents the expansion member  49  from plastically buckling, i.e. permanently deforming, when the mechanism is actuated. Additionally, various shaped cross-sections may also serve to control the manner in which the expansion member elastically buckles. 
     FIG. 5 shows yet another embodiment of the present invention. The microactuator  50  of this embodiment is generally similar to the microactuator  10  of FIG.  1 . However, in the microactuator  50 , the expansion members  20 ,  22  have been gathered to form groups  52 ,  54 . The effect of positioning the expansion members  20 ,  22  to form the groups  52 ,  54  is to reduce thermal energy losses because the expansion members  20 ,  22  thermally insulate each other. Actuation in a vacuum will also reduce energy requirements of a microactuator because thermal energy is not dissipated by convection. FIG. 5 also demonstrates an angle that has been shown in previous figures. The more accurate attachment clearly determines that the displacement direction of the shuttle  24  will be in the positive X direction  6 . 
     Referring to FIG. 6, an alternative embodiment shows multiple microactuators arrayed to form a single mnicroactuator  60 . This microactuator  60  is configured to provide a larger output displacement and/or force than a single microactuator would be capable of providing. FIG. 6 shows two transient microactuators  62   a ,  62   b  that are similar to the microactuator  10  depicted in FIG.  1 . The first transient microactuator  62   a  has two base members  64   a ,  65   a  that may be fixed to a substrate  66 . In the embodiment shown, multiple primary expansion members  67   a ,  68   a  are coupled to the base members  64   a ,  65   a  respectively. As in other embodiments, the actuation direction of the first transient microactuator  62  can be controlled by disposing the expansion members  67   a ,  68   a  at an offset angle from the base member  64   a ,  65   a . The embodiment of FIG. 6 shows the expansion members  67   a ,  68   a  coupled to a transient shuttle  69   a , with the expansion member  67   a ,  68   a  angling toward a biasing actuator  70 . The transient shuttle  69   a  is driven in a positive X direction  6  when the expansion members  67   a ,  68   a  elongate. The transient shuttle  69   a  is coupled to a first base member  71   a  of the biasing actuator  70 . 
     A second transient microactuator  62   b  is coupled to a second base member  71   b  of the biasing actuator  70 . Two sets of primary expansion members  67   b ,  68   b  are coupled between two base members  64   b ,  65   b  and a second transient shuttle  69   b  of the second transient actuator  62   b . Elongation of the primary expansion members  67   b ,  68   b  drives the second transient shuttle  69   b  in a negative X direction  8  (the direction opposite the travel direction of the first transient shuttle  69   a ). The displacement of the transient shuttles  69   a ,  69   b  decreases the lateral distance between the actuating shuttle  77  and base members  71   a ,  71   b  of the actuating shuttle. The decrease in lateral distance drives the actuating shuttle  77  in the Y direction  8 . More specifically, the motion of the two transient shuttles  69   a ,  69   b  forces two sets of secondary expansion members  72   a ,  72   b  to pivot or bend, driving the actuating shuttle  77  in the positive Y direction  8 . Alternatively, the secondary expansion members  72   a ,  72   b  can actuate the actuating shuttle  77  in a manner similar to the expansion members  20 ,  22  of the microactuator  10  of FIG. 1, even if none of the primary expansion members  67   a ,  67   b ,  68   a ,  68   b  have elongated. 
     The largest displacement occurs when all of the expansion members  67   a ,  67   b ,  68   a ,  68   b ,  72   a ,  72   b  are simultaneously elongated This will produce a larger output force and displacement than would be present if only the primary expansion members  67   a ,  67   b ,  68   a ,  68   b , or the secondary expansion members  72   a ,  72   b , were utilized. The microactuators  10 ,  40 ,  50  may be arrayed in various other combinations to amplify the displacement of a shuttle by using the shuttle of one microactuator  10 ,  40 ,  50  as a base member attachment for another microactuator  10 ,  40 ,  50 . The microactuator  50  of FIG. 6 also demonstrates that several advantages may be derived from affixing base members of a microactuator  10 ,  40 ,  50 . 
     The actuation of the microactuator  60  of FIG. 6 may be controlled through selectively applying electric current to the base members  64   a ,  64   b ,  65   a ,  65   b ,  71   a ,  71   b . More specifically, the primary expansion members  67   a ,  68   a  of the first actuator  62   a  may be elongated by applying a current source to the base members  64   a ,  65   a . The current then flows between the two base members  64   a ,  65   a  via the expansion member  67   a ,  68   a , to heat, and thereby elongate, the expansion members  67   a ,  68   a . A similar process may be used to elongate the expansion members  67   b ,  68   b  of the second actuator  62   b . A current source may be coupled to the base members  67   b ,  68   b  such that the current flows through, heats, and elongates the expansion members  67   b ,  68   b.    
     The secondary expansion members  72   a ,  72   b  may be elongated by coupling a current source to the lower base members  65   a , and  65   b . The current flowing between the base members  65   a ,  65   b  similarly flows through, heats, and elongates the expansion members  72   a ,  72   b . Thus, three different current sources may be coupled to the microactuator  60  to control the energy consumption and actuation sequence. However, a single current source may alternatively be used to actuate the microactuator  60  by electrically connecting the two lower base members  65   a ,  65   b  such that current can shunt between the base members  65   a ,  65   b . This may be accomplished by, for example, replacing the lower base members  65   a ,  65   b  with a single elongated base member  79 . A single current source may then be coupled to the upper base members  64   a ,  64   b  to actuate the entire microactuator  60 . Other current source coupling schemes may be implemented in the microactuator  60  or in other microactuator arrays to suit a large variety of control needs. 
     FIG. 7 illustrates an alternative embodiment of a microactuator  80  that implements multiple microactuators to obtain compound force and displacement characteristics. The microactuator  80  is based at least in part on the sliding microactuator  40  shown in FIG. 3. A first base member  82   a  is fixed to a surface  84   a . At least one primary expansion member  86   a  is coupled to the base member  82   a . The primary expansion members  86   a  are coupled to a first transient shuttle  88   a . A similar base member  82   b  and expansion member  86   b  structure is coupled to a second transient shuttle  88   b . In one embodiment, the expansion members  86   a ,  86   b  are angled from an orthogonal intersection with the base members  82   a ,  82   b . The two transient shuttles  88   a ,  88   b  serve as base members for a biasing actuator  89  and an anchoring actuator  90 . Secondary expansion members  91   a ,  91   b  are coupled between the transient shuttles  88   a ,  88   b  and a biasing shuttle  92 . Again, in one embodiment the secondary expansion members  91   a ,  91   b  are angled from an perpendicular intersection with the transient shuttles  88   a ,  88   b . Secondary, expansion members  94   a ,  94   b  are also coupled between the transient shuttles  88   a ,  88   b  and an anchoring shuttle  96 . 
     The microactuator  80  functions in a manner similar to the other microactuators  10 ,  40 ,  50 ,  60  previously discussed. In one embodiment, the expansion members  86   a ,  86   b ,  91   a ,  91   b ,  94   a ,  94   b  are simultaneous elongated. The primary expansion members  86   a ,  86   b  elongate, biasing the transient shuttles  88   a ,  88   b  toward the biasing shuttle  92  and the anchoring shuttle  96 . The motion of the transient shuttles  88   a ,  88   b  compresses the secondary expansion members  91   a ,  91   b ,  94   a ,  94   b  to transmit the biasing force to the actuating shuttle  92  and to the anchoring shuttle  96 . The secondary expansion members  91   a ,  91   b ,  94   a ,  94   b  may simultaneously elongate to partially or fully resist inward motion of the transient shuttles  88   a ,  88   b , and to create additional biasing force between the actuating shuttle  92  and the anchoring shuttle  96 . The secondary expansion members  91   a ,  91   b ,  94   a ,  94   b  may be made thinner or thicker than the primary expansion members  86   a ,  86   b  to control the proportions of biasing force provided by the expansion members  86   a ,  86   b ,  91   a ,  91   b ,  94   a ,  94   b.    
     As a result, the actuating shuttle receives force in the positive Y direction  8  and the anchoring shuttle  96  receives force in the negative Y direction  8 . These opposing forces tend to increase the distance  97  between the biasing shuttle  92  and the anchoring shuttle  96 . However, because the anchoring shuttle  96  is fixed in place, the actuating shuttle  92  receives all of the displacement  97 . Thus, the comparatively small force and displacements of the expansion members  86   a ,  86   b ,  91   a ,  91   b ,  94   a ,  94   b  in the microactuator  60  may be combined to create a larger output force and displacement in the actuating shuttle  92 . Similar results, but with a smaller force and displacement, will result if only the primary expansion members  86   a ,  86   b  are elongated. 
     The microactuators disclosed herein may be modified in many other ways to suit a wide variety of applications. The invention may be embodied in other specific forms without departing from its structures, methods, or other essential characteristics as broadly described herein and claimed hereinafter. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the intellectual property rights is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.