Abstract:
A piezoelectric actuator that can be operated in the d31 mode and which controls the potential energy of a spring is disclosed. The d31 mode of operation provides large actuator displacement and the potential energy of the spring significantly increases the force and work produced by the actuator. In a first embodiment, a single piezoelectric element, operating in the d31 mode, controls the potential energy of the spring. In another embodiment, two piezoelectric elements, both operating in the d31 mode, control the potential energy of the spring.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates, in general, to a piezoelectric actuator and, more particularly, to a piezoelectric actuator that controls the potential energy of a spring to increase the force and work provided by the actuator.  
       BACKGROUND ART  
       [0002]     There are various types of actuators that incorporate piezoelectric elements. These actuators utilize different modes of operation, referred to in the industry as the d33 or d31 operating modes, depending upon the direction of expansion or contraction of the piezoelectric material relative to the direction of the electric field that is applied to same. The relative displacement of the piezoelectric material in the d33 mode of operation is approximately two times greater than the displacement of same in the d31 operating mode. Displacement of piezoelectric material in the d33 operating mode is in the form of expansion in the same direction as the applied electrical field and poling direction, whereas displacement of such material in the d31 operating mode is in the form of contraction in a direction perpendicular to the applied electrical field and poling direction.  
         [0003]     Stack type piezoelectric actuators (d33 actuators) are solid-state linear devices. As such, these actuators utilize the expansion of piezoelectric material to produce a positive displacement. In general, the active part of these actuators consists of a stack of ceramic layers separated by thin metallic layers which act as electrodes. A typical stack type actuator may produce a deflection of about 0.002 inches, a force of about 200 lbs. and work of about 0.4 in-lbs. Thus, d33 mode actuators provide a large amount of work, however, they require a relatively complex assembly, a large package size, and a sophisticated, high cost power supply. In addition, these actuators possess excessive capacitances and hysteresis. Furthermore, these actuators are relatively expensive to produce and are heavy.  
         [0004]     Contraction type actuators (d31 actuators) utilize the contraction of piezoelectric material to produce a negative displacement. The piezoelectric material when bonded to a metallic strip exhibits a bending motion as it contracts. A bending type d31 mode actuator may consist of a single layer of piezoelectric material bonded to a metallic strip or several layers of bonded pairs. The displacement of such material provided by d31 actuators, which is perpendicular to the direction of the applied electrical field, is a function of the length of the actuator. The number of ceramic layers utilized determines the resulting stiffness and output force of the actuator. The layers or “bimorph” strips can produce a relatively large deflection in a relatively small, low cost package, however, these actuators are severely limited in their ability to produce a force. For example, a typical d31 mode actuator may produce a deflection of about 0.1 inches, a force of about 0.08 lbs. and work of about 0.008 in-lbs. Because these actuators are severely limited in their ability to produce force, they cannot be used in those applications that require a relatively large force, such as 1-100 pounds.  
         [0005]     In view of the foregoing, it has become desirable to develop a piezoelectric actuator that can be operated in the d31 mode to obtain the deflection advantages of this operating mode and which controls the potential energy of a spring to increase the force and work produced by the actuator.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention solves the problems associated with prior art piezoelectric actuators and other problems by providing a piezoelectric actuator that can be operated in the d31 mode and which controls the potential energy of a spring to increase the force and work produced by the actuator. The d31 mode of operation provides relatively large actuator displacement and the spring significantly increases the force and work provided by the actuator. In a first embodiment of the present invention, a single piezoelectric element, operating in the d31 mode, controls the potential energy of a spring. In another embodiment of the present invention, two piezoelectric elements, both operating in the d31 mode, control the potential energy of a spring.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a front elevational view of a first embodiment of the present invention showing the orientation of a single piezoelectric element and a spring when no voltage has been applied to the piezoelectric element.  
         [0008]      FIG. 2  is a front elevational view of the embodiment of the present invention shown in  FIG. 1  and illustrates the orientation of the piezoelectric element and the spring when a voltage has been applied to the piezoelectric element.  
         [0009]      FIG. 3  is a front elevational view of another embodiment of the present invention showing the orientation of two piezoelectric elements and a spring when no voltage has been applied to the piezoelectric elements.  
         [0010]      FIG. 4  is a front elevational view of the embodiment of the present invention shown in  FIG. 3  and illustrates the orientation of the two piezoelectric elements and the spring when a voltage has been applied to the piezoelectric elements.  
         [0011]      FIG. 5  is an electrical schematic illustrating the apparatus utilized to apply an electrical field to the piezoelectric element utilized in the embodiment of the present invention shown in  FIGS. 1 and 2 .  
         [0012]      FIG. 6  is an electrical schematic illustrating the apparatus utilized to apply an electrical field to the piezoelectric elements utilized in the embodiment of the present invention shown in  FIGS. 3 and 4 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     Referring now to the drawings where the illustrations are for the purpose of describing the preferred embodiment of the present invention and are not intended to limit the invention described herein,  FIG. 1  is a front elevational view of a first embodiment of the piezoelectric actuator  10  of the present invention. The piezoelectric actuator  10  is comprised of a piezoelectric element  12 , a spring  14 , a connecting arm  16  and a fulcrum  18 .  
         [0014]     The length of piezoelectric element  12  is greater than the width and/or thickness of element  12 . The width and thickness of piezoelectric element  12  may be the same or may be different. The piezoelectric element  12  may be comprised of two substantially concentric rings forming a hollow tubular structure. Alternatively, the piezoelectric element  12  may be comprised of two substantially concentric polygons forming a hollow structure having a generally uniform or non-uniform wall thickness.  
         [0015]     One end  20  of the piezoelectric element  12  is fixed whereas the oppositely disposed end  22  of piezoelectric element  12  is free. Similarly, one end  24  of spring  14  is fixed whereas the oppositely disposed end  26  of spring  14  is free. The spring  14  has an initial pre-load applied thereto. One end  28  of connecting arm  16  contacts end  22  of piezoelectric element  12  and the other end  30  of connecting arm  16  contacts end  26  of spring  14 . Fulcrum  18  contacts connecting arm  16  intermediate its ends  28 ,  30 . In  FIG. 1 , no voltage has been applied to piezoelectric element  12 .  
         [0016]      FIG. 2  is a front elevational view of the piezoelectric actuator  10  shown in  FIG. 1  and illustrates the orientation of the piezoelectric element  12 , spring  14  and connecting arm  16  after a voltage has been applied perpendicularly to the longitudinal axis of the piezoelectric element  12 . Application of a voltage perpendicularly to the longitudinal axis of the piezoelectric element  12 , i.e., in the d31 mode, causes the piezoelectric element  12  to contract which, in turn, causes the connecting arm  16  to rotate counterclockwise about fulcrum  18  resulting in spring  14  applying its pre-load to an object (not shown) via end  30  of connecting arm  16 . The connecting arm  16  acts as a lever about fulcrum  18  and, depending upon the position of the point of contact of fulcrum  18  on connecting arm  16 , effectively “multiplies” the pre-load force on spring  14 .  
         [0017]     Referring now to  FIG. 3 , a front elevational view of another embodiment of the piezoelectric actuator  40  of the present invention is illustrated. The piezoelectric actuator  40  is comprised of piezoelectric elements  42 ,  44 , a spring  46 , connecting arms  48 ,  50  and fulcrums  52 ,  54 . As in the process embodiment, the length of the piezoelectric elements  42 ,  44  is greater than the width and/or thickness of same. The width and thickness of each piezoelectric element  42 ,  44  may be the same or may be different. The piezoelectric elements  42 ,  44  may be comprised of two substantially concentric rings forming a hollow tubular structure. Alternatively, the piezoelectric elements  42 ,  44  may be comprised of two substantially concentric polygons forming a hollow structure having a generally uniform or non-uniform wall thickness.  
         [0018]     One end  56  of piezoelectric element  42  is fixed whereas the oppositely disposed end  58  of piezoelectric element  42  is free. With respect to piezoelectric element  44 , both ends  60 ,  62  are free. One end  64  of spring  46  is fixed whereas the oppositely disposed end  66  of spring  46  is free. The spring  46  has an initial pre-load applied thereto. One end  68  of connecting arm  48  contacts end  58  of piezoelectric element  42  and the other end  70  of connecting arm  48  contacts end  62  of piezoelectric element  44 . Fulcrum  52  contacts connecting arm  48  intermediate its ends  68 ,  70 . One end  72  of connecting arm  50  contacts end  60  of piezoelectric element  44  and the other end  74  of connecting arm  50  contacts end  66  of spring  46 . Fulcrum  54  contacts connecting arm  50  intermediate its ends  72 ,  74 . In  FIG. 3 , no voltage has been applied to piezoelectric elements  42 ,  44 .  
         [0019]      FIG. 4  is a front elevational view of the piezoelectric actuator  40  shown in  FIG. 3  and illustrates the orientation of the piezoelectric elements  42 ,  44 , spring  46 , and connecting arms  48 ,  50  after a voltage has been applied perpendicularly to the longitudinal axis of the piezoelectric elements  42 ,  44 , i.e., in the d31 mode. The application of such a voltage causes the piezoelectric elements  42 ,  44  to contract which, in turn, causes the connecting arms  48 ,  50  to rotate clockwise about their respective fulcrums  52 ,  54 , resulting in spring  46  applying its pre-load to an object (not shown) via end  74  of connecting arm  50 . The connecting arms  48 ,  50  act as levers about their respective fulcrums  52 ,  54  and, depending upon the position of the point of contact of fulcrums  52 ,  54  on their respective connecting arms  48 ,  50 , effectively “multiplies” the pre-load force on spring  46 .  
         [0020]     Referring now to  FIGS. 5 and 6 , an electrical schematic illustrating the apparatus, shown generally by the numeral  80 , utilized to apply an electrical field to the piezoelectric element(s)  12  or  42 ,  44 , in the d31 operating mode, is shown. As illustrated, electrodes  82 ,  84  are oppositely disposed along the longitudinal axis of the piezoelectric element(s)  12  or  42 ,  44 . Electrical conductors  86 ,  88  are connected to electrodes  82 ,  84 , respectively, permitting a voltage to be applied thereto. The voltage may be provided by an electrical transformer  90  having a primary coil  92  and a secondary coil  94  which is connected across the conductors  86 ,  88  to increase the voltage applied to the electrodes  82 ,  84 , and thus, to the piezoelectric element(s)  12  or  42 ,  44 .  
         [0021]     The present invention can be utilized in numerous diverse applications. For example, the piezoelectric actuator of the present invention can be utilized in precision robotic applications or applications that require precise alignment of various components or devices, such as the alignment of mirrors. Also, the present invention can be utilized where precise control of the operation of various devices is required, such as the operation of automotive fuel injectors, gas valves, fluid control valves, etc. The foregoing applications are not to be construed as being all inclusive, but are merely examples of the numerous applications in which the piezoelectric actuator of the present invention can be employed.  
         [0022]     Certain modifications and improvements will occur to those skilled in the art upon reading the foregoing. It is understood that all such modifications and improvements have been deleted herein for the sake on conciseness and readability, but are properly within the scope of the following claims.