Abstract:
An apparatus and process for pre-loading an electrically stimulated smart material actuator product to obtain maximum work from the actuator. When a smart material actuator is optimally pre-loaded certain desirable characteristics become apparent, such as work, operational frequency, hysteresis, repeatability, and overall accuracy. When used in conjunction with a mechanically leveraged actuator structure the smart material actuator can be used to its greatest potential. Since the mechanically leveraged actuator can be based on the maximum work provided by the smart material actuator, certain attributes such as the force, and displacement of total system can be adjusted without loss to system efficiency. Pre-loading methods and a determination of the optimal pre-load force are disclosed. Each smart material actuator type has a unique work curve. In the design of an actuator assembly, the process of optimizing uses the unique work curve to optimize the design for the requirements of the particular application. The unique work curve is used by finding the place where the smart material actuator is capable of providing the most work in order to set the optimum pre-load point accordingly. Different mechanical pre-load techniques are provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of provisional patent application Ser. No. 60/460,548 filed on Apr. 4, 2003, which is incorporated by reference herein in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to an actuator assembly, and more specifically relates to optimization of work from a support structure moveable in response to electrical activation of a smart material actuator.  
         BACKGROUND OF THE INVENTION  
         [0003]    The invention is based on actuator technologies being developed for a wide range of applications including industry. One component used in this type of actuator is an electrically stimulated smart material actuator. These smart material actuators when electrically stimulated change shape. This shape change can be designed such that one axis predominantly changes. As this axis changes dimension it is magnified by a lever integral to the main support structure creating an actuator with a useful amount of displacement. This displacement is useful for general-purpose industrial applications such as grippers, linear motors, and consumer applications such as speakers. Presently, electromechanical devices are used such as motors, solenoids, and voice coils. In general these devices encompass many shortcomings, i.e. they are large and heavy, consume high amounts of power, and do not work in a proportional manner.  
           [0004]    Various types of smart material actuators are known to those skilled in the art. Traditionally the smart material actuator is used two ways, first direct acting and second in a mechanically leveraged system. Most of these systems have some sort of mechanical pre-load. This pre-load has largely been used to capture the smart material actuator within the main structure. It has not generally been recognized that the pre-load force applied to the smart material actuator can affect the performance of the actuator.  
           [0005]    In such known devices, when the smart material actuator is electrically activated, the geometry of the device expands predominantly along a predetermined axis. When the smart material device is deactivated, the geometry of the device contracts predominantly along the predetermined axis. This expansion and contraction of the smart material can be used to operate an apparatus, e.g. to open or close a gripper or vibrate a speaker cone.  
         SUMMARY OF THE INVENTION  
         [0006]    Heretofore, it has not generally been recognized that individual smart material actuator types have an optimal pre-load and/or range, where the smart material actuator provides optimal work. For the purpose of this discussion, work is defined as the force/displacement product, given that input energy is relatively constant. When using the smart material actuator within its peak work area, the smart material actuator is at its peak efficiency. Since the optimal pre-load for a large smart actuator can be greater than 100 pounds, the method used to create the pre-load force is critical.  
           [0007]    The smart material can be disposed between a main support structure with an integral hinge, spring, and at least one arm in a curvilinear path around the main support structure. The optimal pre-load force can be designed into the main support structure and provide for pre-load adjustment. The smart material actuator in most known configurations provides little opportunity to select different hinge axis locations, high pre-load forces and/or structural configurations to optimize performance. These objectives have been a difficult combination to achieve with inexpensive materials for high volume commercialization of smart material actuators.  
           [0008]    The present invention optimizes the performance of a smart material actuator, providing performance and flexibility never possible before. The present invention provides a process for determining optimal preload for a mechanically leveraged smart material actuator. Preferably, a smart material actuator can be captured in place between a rigid non-flexing portion and force transfer member, by way of example and not limitation, machined from a single block of material with integral preload mechanism. The apparatus can include a support having a rigid non-flexing portion, at least one arm portion extending forward from the rigid portion, at least one surface on each pivotable arm for movement relative to the support structure, and a force transfer member operably positioned with respect to the at least one arm. A rigid non-flexing portion can support the preload mechanism. An actuator can be operably engaged between the preload mechanism and the force transfer member to drive the force transfer member in movement along a fixed path causing the at least one pivotable arm portion to pivot in response to an electrical activation. The support, pivotable arm, and force transfer member of the structure can be designed to be rigid, non-flexing portions of a monolithic structure interconnected by flexible hinge portions allowing the at least one arm to move relative to the remaining support structure. Any unplanned flexing can reduce the effective life of the mechanism, and reduce the amount of force transferred through the hinge axis to the at least one pivot arm. The reduction in force limits the displacement and force of the pivoting arm. The selection of the hinge axis location and corresponding structural configuration can allow substantial capability to optimize the performance and size of the apparatus for the particular application.  
           [0009]    The smart material can be preloaded with a force when installed in the support element. For example, the smart material actuator can be clamped within the support structure with an adjustable screw supporting one end allowing the optimal force preloading. An adjustable screw configuration is easy to use and allows for a large adjustability. Depending on the preload force an acceptable screw configuration can be designed. Preloading the smart material actuator in a suitable fashion can contribute to the maximum efficiency of the force transfer during the actuation, and allows fine-tuning of the initial position of the apparatus prior to the actuation of the smart material element. Certain smart materials have an optimal preload, i.e. the actuator performs the largest amount of work at that preload. Preload can also ensure that the smart material actuator maintains contact with the apparatus at opposite ends throughout the range of expansion and contraction. The use of a threaded adjustment screw for preloading enables assembly without requiring adhesives or other means of securely connecting the smart material actuator at opposite ends to the apparatus, and avoids the possibility of damaging tension or torsional movements on the smart material actuator. The threaded adjustment screw allows simple compensation for dimensional variations in the smart material actuator during assembly to the support. The present invention optimizes the preload such that the smart material actuator can provide the optimal work, as well as several preload mechanisms suitable for the apparatus.  
           [0010]    Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein:  
         [0012]    [0012]FIG. 1 is a graph illustrating the performance of a smart material actuator showing deflection versus force for both energized and de-energized states;  
         [0013]    [0013]FIG. 2 is a graph illustrating the product of displacement times blocking force versus force for the values shown in FIG. 1;  
         [0014]    [0014]FIG. 3 is a graph illustrating performance of a smart material actuator at a predetermined pre-load;  
         [0015]    [0015]FIG. 4 is a perspective view of one embodiment of the present invention;  
         [0016]    [0016]FIG. 5 a  is a perspective view of another embodiment of the present invention;  
         [0017]    [0017]FIG. 5 b  is a detail view of FIG. 5 a  in accordance with the present invention;  
         [0018]    [0018]FIG. 6 a  is a side view of another embodiment of the present invention;  
         [0019]    [0019]FIG. 6 b  is a detail view of FIG. 8 a  in accordance with the present invention;  
         [0020]    [0020]FIG. 7 a  is a cutaway perspective view of another embodiment of the present invention;  
         [0021]    [0021]FIG. 7 b  is a detail view of FIG. 8 a  in accordance with the present invention;  
         [0022]    [0022]FIG. 8 is a cutaway perspective view of another embodiment of the present invention;  
         [0023]    [0023]FIG. 9 is a cutaway perspective view of another embodiment of the present invention;  
         [0024]    [0024]FIG. 10 is a side view of another embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Referring now to FIG. 1, a displacement-force graph for a smart material actuator both energized and de-energized is depicted. For the de-energized curve the smart material actuator was shorted or de-energized. The de-energized curve was taken starting at a force of 10 and an ending force of 100. The compressive deflection was noted at various points between the forces of 10 to 100.  
         [0026]    These points were then plotted as the line on the graph in FIG. 1 with diamonds to indicate the series. For the energized curve the smart material actuator was connected to a power supply delivering the correct actuation voltage. The energized curve was taken starting at a force of 10 and an ending force of 100. These points were then plotted as the line on the graph in FIG. 1 with squares to indicate the series. From the graph it can be seen that the energized and de-energized functions are not linear, nor are the lines parallel to each other. This means that the delta displacement between energized and de-energized states at a given force can be greater than or less than the delta displacement at another point.  
         [0027]    Now referring to FIG. 2, a delta displacement-force product graph derived from the graph in FIG. 1 is depicted. This graph indicates a peak work value at  40 . That is the product of the force times the delta displacement is its greatest value at  40  indicating the displacement and force are peaking.  
         [0028]    Now referring to FIG. 3, a displacement-force graph using the same data as in FIG. 1 and FIG. 2 is depicted. After looking at FIG. 2, it can be seen that the peak work value is located at  40 . A right angle triangle  10  is overlaid on the graph, three line segments are formed AB, BC, CA, where maximum displacement is line segment CA, blocking force line segment AB, and actuator working line segment BC. The displacement line segment CA is aligned with the peak in work value shown in FIG. 2. This is the point around which a smart material actuator can be optimally pre-loaded to preferably within at least 40% of the peak work value, more preferably to within at least 25% of the peak work value, and most preferably to within at least 10% of the peak work value or the approximate peak work value itself.  
         [0029]    If the smart material actuator were pre-loaded to 40 units the maximum displacement would be the line segment CA, or 13 units. If the smart material actuator is energized and the pre-load is increased to 60 units, blocking force would be achieved, and the line segment depicting the displacement to blocking force is segment AB. At this point the smart material actuator is back to its original height. Since it is impossible to build a spring with no resistance the line segment CA is impossible to achieve, so practical design rules prevail and a point on working segment BC can be used. This point can be optimized to be as close to the comer of triangle  10  at the intersections of line segments CA, BC. It should be noted that triangle  10  can be moved up or down slightly from the peak in work value, graphed in FIG. 2 in order to make subtle pre-loading optimizations by one skilled in the art. It should also be noted that because of the wide range of materials and geometries from which the actuators are made, each material and geometry combination can have a different set of graphs requiring each actuator to be evaluated within its particular application.  
         [0030]    Now referring to FIG. 4, an actual embodiment of a pre-load mechanism is depicted. Actuator assembly  10  includes a smart material actuator  20 , force transfer member  30 , rigid capture ratchet cap  40 , and ratchet teeth  50 . In this embodiment, the compliant mechanism of the actuator assembly  10  is press fit with the ratchet cap structure  40 , engaging the ratchet teeth  50 , trapping the smart material actuator  20  between the force transfer member  30  and ratchet cap structure  40 , causing the smart material actuator  20  to be pre-loaded by the amount of force that the ratchet cap structure  40  is forced against the force transfer member  30  and its compliant structure.  
         [0031]    Now referring to FIG. 5 a , a second embodiment of the present invention is depicted. Actuator assembly  10  includes a smart material actuator  20 , force transfer member  30 , floating plate  100 , back holding plate  110 , and fastener means  110 . In this embodiment, the compliant mechanism of the actuator assembly  10  is held together by the back holding plate  120  with two fasteners  110  trapping the smart material actuator  20  between the force transfer member  30  and floating plate  100  causing the smart material actuator  20  to be pre-loaded by the relationship of the back holding plate  120  to the force transfer member  30  and its compliant structure.  
         [0032]    Now referring to FIG. 5 b,  a close-up view of the floating plate  100  is depicted. As the two fasteners  110  are engaged, back plate  110  will not move in a parallel fashion to the force transfer member  30 . The smart material actuator  20  does not tolerate misalignment well. Misalignment can cause a failure of the smart material actuator  20  during assembly. Floating plate  100  is designed to allow misalignment between the two surfaces. It accomplishes this by creating a point contact with back holding plate  110  and a flat surface with smart material actuator  20 .  
         [0033]    Now referring to FIG. 6 a , a third embodiment of the present invention is depicted. Actuator assembly  10  includes a smart material actuator  20 , force transfer member  30 , rigid back plate  40 , lower cam block  210 , upper cam block  220 , and adjustable cam  210 . In this embodiment, the compliant mechanism of the actuator assembly  10  is of a single one-piece design, with two main features including the rigid rear support  40  and force transfer member  30 . A second subassembly including the cam block assembly  200 ,  210 ,  220  is designed as an adjustable spacer. The smart material actuator  20  is captured between the cam block assembly  200 ,  210 ,  220  and force transfer member  30 . The cam block assembly  200 ,  210 ,  220  is supported by the rigid rear support  40 . As the adjustment cam  210  is moved the dimensions of the adjustable spacer change, creating greater or less pre-load.  
         [0034]    Now referring to FIG. 6 b , an exploded view of the cam block assembly of FIG. 6 a  of the present invention is depicted. The lower cam block  200  acts as a bearing for cam screw  210 , and upper cam block  220  acts as the surface against which the cam screw  210  can act. As the cam screw  210  is rotated the upper cam block moves changing the overall dimension, and creating an adjustable spacer.  
         [0035]    Now referring to FIG. 7 a , a cutaway view of a fourth embodiment of the present invention is depicted. The actuator assembly  10  is shown cut at about the midpoint, such that the internal features are visible. Actuator assembly  10  includes a smart material actuator  20 , force transfer member  30 , rigid back support  40 , lower wedge  300 , upper wedge  310 , and floating plate  100 . In this embodiment, the compliant mechanism of the actuator assembly  10  is of a single one-piece design, with two main features including the rigid rear support  40  and force transfer member  30 . A second subassembly including the wedge block assembly  300 ,  310  is designed as an adjustable spacer. The smart material actuator  20  is captured between the wedge block assembly  300 ,  310  and floating plate  100 . The wedge block assembly  300 ,  310  is supported by the rigid rear support  40 . As the wedge assembly  300 ,  310  is moved with respect to one another the dimensions of the adjustable spacer change, creating greater or less pre-load. The smart material actuator  20  does not tolerate misalignment well. Misalignment could cause a failure of the smart material actuator  20  during assembly. Floating plate  100  is designed to allow misalignment between the two surfaces. It accomplishes this by creating a point contact with back holding plate  110  and a flat surface with a smart material actuator  20 .  
         [0036]    Referring now to FIG. 7 b , a close-up view of the wedge block assembly  300 ,  310  of FIG. 7 a  of the present invention is depicted. The lower wedge block  300  and upper wedge block  310  act as an adjustable spacer. As the upper and lower wedges  300  are driven together the spacer increases in dimension and as the upper and lower wedges are driven away from one another the spacer decreases in dimension. The wedges are held in place with a toothed arrangement. In this manner, an adjustable spacer is created.  
         [0037]    Referring now to FIG. 8, a cutaway view of a fifth embodiment of the present invention is presented. The actuator assembly  10  is shown cut at about the midpoint such that the internal features are visible. Actuator assembly  10  includes a smart material actuator  20 , force transfer member  30 , rigid back support  40 , ring spacer  410 , ring adjustment screw  400 , and floating plate  100 . In this embodiment, the compliant mechanism of the actuator assembly  10  is of a single one-piece design, with two main features including the rigid rear support  40  and force transfer member  30 . A second subassembly, the adjustable ring spacer assembly  400 ,  410  can be designed as an adjustable spacer. The smart material actuator  20  can be captured between the adjustable ring spacer assembly  400 ,  410 , and floating plate  100 . The adjustable ring spacer assembly  400 ,  410  can be supported by the rigid rear support  40 . As the ring adjustment screw  400  is rotated, the dimensions of the adjustable spacer change, creating greater or less pre-load. The smart material actuator  20  does not tolerate misalignment well. Misalignment could cause a failure of the smart material actuator  20  during assembly. Floating plate  100  is designed to allow misalignment between the two surfaces. It accomplishes this by creating a point contact with back holding plate  110  and a flat surface with smart material actuator  20 .  
         [0038]    Referring now to FIG. 9, a cutaway view of a sixth embodiment of the present invention is depicted. The actuator assembly  10  is shown cut at about the midpoint such that the internal features are visible. Actuator assembly  10  includes of a smart material actuator  20 , force transfer member  30 , rigid back support  40 , lower semicircle wedge  520 , upper semicircle wedge  540 , center wedge  530 , wedge adjustment screw  510 , and upper and lower bearings  500 ,  540 . In this embodiment, the compliant mechanism of the actuator assembly  10  is of a single one-piece design, with two main features including the rigid rear support  40  and force transfer member  30 . A second subassembly, the adjustable wedge assembly  500 ,  510 ,  520 ,  530 ,  540 ,  550  can be designed as an adjustable spacer. The smart material actuator  20  can be captured between the adjustable wedge assembly  500 ,  510 ,  520 ,  530 ,  540 ,  550 . The adjustable wedge assembly  500 ,  510 ,  520 ,  530 ,  540 ,  550  can be supported by the rigid rear support  40 . As the wedge adjustment screw  510  is rotated, the dimensions of the adjustable spacer change, creating greater or less pre-load. Bearing blocks  500 ,  540  can provide a surface for the upper and lower semicircle wedges to rotate. Upper and lower semicircle wedges  500 ,  540  have a second bearing surface that can interface with the center wedge  530  as the center wedge  530  is drawn toward the head of the wedge adjustment screw  510  driving the upper and lower semicircle wedges away from each other driving upper and lower bearing blocks creating more pre-load. As the center wedge  530  is drawn away from the head of the wedge adjustment screw  510 , driving the upper and lower semicircle wedges towards each other, driving upper and lower bearing blocks, and creating less pre-load.  
         [0039]    Now referring to FIG. 10, a seventh embodiment of the present invention is depicted. Actuator assembly  10  includes a smart material actuator  20 , force transfer member  30 , rigid back plate  40 , pre-load screw  600 , and floating plate  100 . In this embodiment, the compliant mechanism of the actuator assembly  10  is of a single one-piece design, with two main features including the rigid rear support  40  and force transfer member  30 . The pre-load screw  100  can be supported by the rigid back plate  40 , and the floating plate  100  can be positioned between smart material actuator  20  and pre-load screw  600 . Pre-load screw  600  can be threaded and as the screw rotates it can act as an adjustable spacer. As the pre-load screw  600  rotates, such that additional force is applied to the smart material actuator  20 , the pre-load value is increasing or greater, and as the screw rotates such that force is being removed from the smart material actuator  20 , the pre-load value is decreasing or less. The smart material actuator  20  does not tolerate misalignment well. Misalignment can cause a failure of the smart material actuator  20  during assembly. Floating plate  100  is designed to allow misalignment between the two surfaces. It accomplishes this by creating a point contact with the pre-load screw  600  and a flat surface with smart material actuator  20 .  
         [0040]    While the invention has been described in conjunction with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, it is intended to cover various modifications and equivalent arrangement included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as permitted under law.