Abstract:
A telescoping piezoelectric stack actuator, comprising a first expandable piezoelectric stack systems, a first sleeve supported by the first piezoelectric stack system for upward movement with said stack system, and a second, expandable piezoelectric stack system supported by said first sleeve for upward movement therewith. Preferably, the actuator further comprises a second sleeve supported by the second piezoelectric stack system for upward movement with said second stack system as the second stack system expands, and a third, expandable piezoelectric stack system supported by said second sleeve for upward movement therewith.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention generally relates to piezoelectric actuators. More specifically, the invention relates to a multi-stage, expandable piezoelectric actuator.  
           [0003]    2. Background Art  
           [0004]    Piezoelectric materials are frequently used as sensors and actuators. This is due to the electromechanical coupling present in the material.- For instance, a piezo electric actuator produces a force and displacement resulting from an applied electric field. Piezoelectric stack actuators are comprised of several layers of piezoelectric wafers. An electric field is applied to these actuators in the thickness direction, and the resulting force and displacement are also in the thickness direction. Actuators of this type provide sufficient force at the expense of displacement for many applications.  
           [0005]    There are some situations in which piezoelectric actuators have, heretofore, not been well suited. In particular, piezoelectric actuators are not well suited for applying a large displacement in very small spaces. In these situations, the height, width and length of the space that the actuator can occupy is a- significant limitation. Force and displacement requirements, given the limited space for the actuator, cannot be met with conventional piezoelectric actuators.  
           [0006]    Several types of materials and actuators are currently available to provide actuation in a limited space. These materials include piezoelectric, magnetostrictive, shape memory alloy, and common conducting materials such as steel or iron. Linear and rotary actuators using these materials are available. There are some demanding applications, however, where none of the known actuators are able to meet the necessary requirements. Obtaining sufficient displacement is particularly difficult.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of this invention is to provide an improved piezoelectric actutator.  
           [0008]    Another object of the invention is to provide a piezoelectric actuator with a unique telescoping mechanism.  
           [0009]    A further object of the present invention is to provide a piezoelectric actuator with a telescoping mechanism that allows the actuator to provide significant displacement in a very small area.  
           [0010]    Another object of the invention is to provide a piezoelectric actuator with a mechanism that allows the actuator to provide significantly more displacement, compared to conventions piezoelectric stack actuators, without a trade-off in force.  
           [0011]    These and other objects are attained with a telescoping piezoelectric stack actuator, comprising a first expandable piezoelectric stack system, a first sleeve supported by the first piezoelectric stack system for upward movement with said stack system, and a second, expandable piezoelectric stack system supported by said first sleeve for upward movement therewith. Preferably, the stack actuator further comprises a second sleeve supported by the second piezoelectric stack for upward movement with said second stack as the second stack expands, and a third, expandable piezoelectric stack system supported by said second sleeve for upward movement therewith.  
           [0012]    Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description, given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 illustrates a piezoelectric wafer.  
         [0014]    [0014]FIGS. 2 and 3 show a conventional piezoelectric stack actuator.  
         [0015]    [0015]FIGS. 4 and 5 show a piezoelectric stack actuator embodying this invention.  
         [0016]    [0016]FIG. 6 shows the elements of the stack actuator of FIG. 4  
         [0017]    [0017]FIG. 7 is an enlarged view of one of the sleeves of the stack actuator of FIGS. 4 and 5.  
         [0018]    [0018]FIG. 8 is a cross-sectional view of the stack actuator of FIG. 4 and  5 .  
         [0019]    [0019]FIG. 9 is a top view of an alternate actuator embodying this invention.  
         [0020]    [0020]FIG. 10 schematically illustrates an apparatus that may be used to measure the displacement of piezo actuators. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    Piezoelectric materials exhibit electromechanical coupling. With reference to FIG. 1, which shows piezoelectric material  10 , an applied electric field E 3 , through the thickness direction ( 3 ), causes the piezoelectric wafer to grow in all directions including in-plane transverse ( 1 ) and longitudinal ( 2 ). Piezoceramic and piezopolymeric materials grow in the thickness direction at a faster rate than either the transverse or longitudinal directions. Typically, the force generated by a piezoelectric wafer is large, but the displacement is very small. These properties of piezoelectric materials are well known.  
         [0022]    As illustrated in FIGS. 2 and 3, the displacement characteristics of piezoelectric materials can be amplified by stacking multiple (N) units  20  in series, as shown at  22 . Care must be taken to orient the poling direction of the piezoelectric wafers and the applied electric field correctly. This configuration of piezoelectric materials is well known and commercial piezoelectric stack actuators are available. However, these stack actuators are still limited in their displacement capabilities.  
         [0023]    The displacement characteristics of one and multiple wafers stacked in series are given more specifically below.  
         [0024]    Displacement of 1 wafer: 
         Δ u   1   =t×d   31   ×E   3 , where 
         [0025]    t—piezoelectric thickness  
         [0026]    d 31 —piezoelectric coefficient  
         [0027]    E 3 —electric field  
         [0028]    Displacement of N wafers: 
         Δ u   N   =N×Δu   1   
         [0029]    The present invention provides a telescoping design that allows the displacement to be amplified significantly. With reference to FIGS. 4-7, the actuator  30  comprises first, second and third stack systems  32 ,  34  and  36 , and first and second sleeves  40  and  42 . Each stack system, it may be noted, is comprised of one or more individual stacks. For example, stack system  32  is comprised of stacks  44  and  46 , system  34  is comprised of stacks  50  and  52 , and stack system  36  is comprised of stack  54 . Each of the stacks is comprised of a multitude of individual piezoelectric layers or wafers mounted one on top of another.  
         [0030]    Sleeve  40  has an elongated U-shape, and includes side members  40   a  and  40   b  and base member  40   c , and the sleeve forms an interior. The sleeve  40  also includes a pair of top flanges  40   e  and  40   f , with these flanges extending outward from the tops of side members  40   a  and  40   b  respectively. Sleeve  42 , likewise, has an elongated U-shape, and includes side members  42   a  and  42   b  and base member  42   c , and this sleeve forms an interior. The sleeve  42  also includes a pair of top flanges  42   e  and  42   f , with these flanges extending outward from the tops of side members  42   a  and  42   b  respectively.  
         [0031]    In actuator  30 , stacks  44  and  46  are positioned outside of and on opposite sides of sleeve  40 , with the tops of stacks  44  and  46  engaging flange s, 40   e  and  40   f . In this way, as stacks  44  and  46  expand, they push flanges  40   e  and  40   f , and the whole sleeve  40 , upwards.  
         [0032]    Sleeve  42  is disposed inside sleeve  40 , between side members  40   a  and  40   b , and preferably the sleeve  42  rests on base member  40   c . Stacks  50  and  52  are also disposed inside sleeve  42 , between side members  40   a  and  40   b  and on opposite sides of sleeve  40 . Also, the tops of stacks  50  and  52  engage flanges  42   e  and  42   f  so that, as stacks  50  and  52  expand, they push flanges  42   e  and  42   f , and the entire sleeve  42 , upwards.  
         [0033]    Stack  54  is positioned inside sleeve  42 , between side members  42   a  and  42   b , and preferably the stack  54  rests on base member  42   c , and stack  54  moves with sleeve  42  as that sleeve moves upward.  
         [0034]    With reference to FIG. 8, the telescoping design of the new actuator  30  allows the displacement to be amplified significantly. Stacks  44  and  46  push up on the sleeve  40  with a displacement of Δu N . Stacks  50  and  52  also push up with a displacement of Δu N . Due to the motion of the sleeve  40  connecting the stacks  44 ,  46 ,  50  and  52 , the total displacement is 2Δu N . Additional sleeves and stacks can be added to further increase the displacement.  
         [0035]    Also, it may be noted that actuators embodying this invention may have specific shapes and sizes. For instance, the actuator may have a square or rectangular shape. Alternatively, as another example, illustrated in FIG. 9, the actuator may have a round or circular shape. In this embodiment, expandable stacks  62  and  64  may have circular shapes, and the movable sleeves, one of which is shown at  66 , may also have circular shapes.  
         [0036]    In order to demonstrate the advantages of this invention, the displacement obtained with an actuator embodying the invention was compared to the displacement obtained with a prior art single stack piezo actuator.  
         [0037]    [0037]FIG. 10 schematically illustrates an apparatus  70  that was used to measure these displacements; and apparatus  70 , generally, comprises a rigid base  72 , a frame  74  and a suitable displacement measurement device  76 . In use, an actuator, such as actuator  80 , is placed on base  72 , directly below measurement device  76 , an electric voltage is applied to the actuator to expand that actuator, and the extent of this expansion, or displacement, is measured by device  76 . As will be understood by those of ordinary skill in the art, any other suitable apparatus may be used to measure the displacement of the actuator.  
         [0038]    To obtain a basis for comparison, the displacement of a single stack actuator, represented at  82  in FIG. 10, was measured. This actuator  82  had a height of 27 mm and a base of 6 mm by 7 mm. 100 Volts was applied to the actuator  82 , after being mounted on apparatus base  72 , and the measured elongation was  18  microns.  
         [0039]    The displacement of actuator  80 , embodying this invention, was also measured using apparatus  70 . Actuator  80  had a height of 31 mm and a base of 27 mm by 28 mm. The actuator  80  was placed on apparatus base  72  and 100 Volts was applied to the actuator, and the actuator elongation was 58 microns. This elongation of the actuator  80  of this invention was three times better than that of the single stack actuator  82 . Thus, 300% elongation was obtained with only a 15% increase in the actuator length. A 2.75 times, per unit length, improvement in performance was obtained compared to the single stack actuator.  
         [0040]    While it is apparent that the invention herein disclosed is well calculated to fulfill the objects stated above, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.