Abstract:
A piezoelectric actuator has a base and a movable body disposed over a surface of the base for undergoing movement relative thereto. The movable body has a frame and at least one cantilever having a first end integrally connected to the frame and a second free end. A piezoelectric element is disposed on the cantilever for undergoing expansion and contraction movement in response to application of an alternating voltage to bring the second free end of the cantilever into and out of contact with the surface of the base to thereby move the movable body relative to the surface of the base. Spring members are connected to the frame of the movable body for regulating a direction of movement of the movable body relative to the surface of the base.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a piezoelectric actuator in which at least one-directional movement of a moving body is facilitated, and which can insure high movement conversion efficiency even when miniaturized. 
     2. Background Information 
     FIG. 7 is a perspective drawing showing an example of a piezoelectric actuator of the related art. This piezoelectric actuator  300  is used as an XY stage for movement over a microscopic distance, and movement in each axial direction is realized by stacked piezoelectric elements. A metal plate is punched by wire discharge or die processing on an anchor block and a movable table  303  is placed inside the punched out part  302 . Respective support springs  304  are formed on the four edges of the movable table  303  and guide sections  305  are attached to the four edges of the movable table  303  through the support springs  304 . Also, stacked piezoelectric elements  306  are attached to ends of the guide sections at right angles to the movable table  306 . 
     An enlarged plan view of a support spring  304  is shown in FIG.  8 . This support spring  304  is formed as a rectangular frame, and a long edge  304   a  is thin while a short edge  304   b  is thick. This is because by making the long edge  304   a  long and thin, stress for a given amount of displacement is lowered exponentially or linearly so that the structure of the movable table  303  connected through the support springs  304  has a degree of freedom with respect to movement in an X axis direction in the drawings, which is the width direction of the long edge  304   a . As shown in FIG. 9, in the case where the width of the support spring is a and the thickness is b, an aspect ratio (b/a) is set to at least 1 and twisting displacement is suppressed when the movable table  303  is moved. 
     When this piezoelectric actuator  300  is moved in the X axis direction, a fixed voltage is applied to the stacked piezoelectric elements  306   a  for X direction movement to cause displacement. If the stacked piezoelectric elements  306   a  are displaced in the thickness direction, the movable table  303  connected by the guide sections  305   a  is joined and moves in the X direction. At this time, support springs  304  formed on the X direction edges move in the direction of the short edge. Also, X direction guide sections  305   a  function to guide the movable table  303  in the X direction. On the other hand, support springs  304  formed on the Y direction edges have difficulty moving in the direction of the long edge and so it is difficult to absorb displacement of the stacked piezoelectric elements  306   a . The same also applies to movement in the Y direction. 
     However, with the above described piezoelectric actuator  300  of the related art, since stacked piezoelectric elements  306  are generally used it is necessary to first of all form the stacked piezoelectric elements  306  so as to satisfy relevant standards. The stacked piezoelectric elements  306  are generally manufactured using a green sheet method employing a green sheet made from a piezoelectric slurry. After conductive paste for internal electrodes has been screen printed on green sheets, a specified number of the green sheets are laminated and baked. Characteristics of a stacked piezoelectric element formed in this way are that strong generated force is obtained, but displacement is microscopic. The thus formed stacked piezoelectric element is fitted between a guide section  305  and a fixed table  301 , and joining and complicated adjustment steps are necessary. This means that the manufacture of the XY stage for movement and the piezoelectric actuator  300  are difficult and complicated, and with increased amount of movement of the movable table  303  it is necessary to provide support points between the guide sections  305  and the fixed table  301  and to provided an enlarged cantilever. As a result, a correspondingly complicated structure results from further structural miniaturization. 
     SUMMARY OF THE INVENTION 
     The present invention has been conceived in view of the above described problems in the related art, and its object is to provide a novel approach to the problems. 
     In order to achieve this object, a piezoelectric actuator of a first aspect of the present invention comprises a base, a flat moving body constituting a cantilever with one end free and another end fixed and provided with piezoelectric elements, and support springs for supporting the moving body so as to track a surface of the base and for regulating direction of movement of the moving body. 
     If a alternating voltage is applied to the piezoelectric elements provided on the flat cantilever, the cantilever bends with oscillation and a free end of the cantilever comes into contact with the base. The free end of the cantilever undergoes elliptical motion which means that a sideways direction component excites movement of the moving body. Also, the contact state between the moving body and the base is kept stable by the support springs and the moving direction of the moving body is limited by the structure of the support springs, which means that motion and positioning of the moving body can be carried out easily. 
     A piezoelectric actuator of a second aspect of the present invention comprises a base, a flat moving body comprising a long-edged portion on which piezoelectric elements are provided and a short-edged section, and constituting a cantilever having the long-edged section free and the short-edged section fixed, and support springs for supporting the moving body so as to track a surface of the base and for regulating direction of move; of the moving body. 
     If the cantilever is formed having a long-edged section and a short-edged section, twisting vibration is excited at the same time as the cantilever bends with oscillation. As a result, the amplitude of oscillation at the free end becomes large and it is easy to obtain a larger motion of the moving body compared to the first piezoelectric actuator structure described above. 
     A third aspect of the present invention is the piezoelectric actuator of either the first or second aspects in which pressurization means is provided for causing the base and the moving body to be pressed into contact with each other. By pressing the moving body and the base with the pressurization means, not only is following further improved, and the state of contact with the cantilever made good, but also motion and locating of the moving body are made more stable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an assembly drawing showing a piezoelectric actuator relating to a first embodiment of the present invention. 
     FIG. 2 is an enlarged explanatory drawing showing a moving body shown in FIG.  1 . 
     FIG. 3 is an explanatory drawing showing a driving principle for the piezoelectric actuator of the present invention. 
     FIG. 4A is an assembly drawing of a piezoelectric actuator relating to a second embodiment of the present invention. 
     FIG. 4B is a cross-sectional drawing of a piezoelectric actuator relating to a second embodiment of the present invention. 
     FIG. 5A is an assembly drawing of a modified example of a piezoelectric actuator relating to a second embodiment of the present invention. 
     FIG. 5B is a cross sectional drawing of a modified example of a piezoelectric actuator relating to a second embodiment of the present invention. 
     FIG. 6 is an explanatory drawing showing a piezoelectric actuator according to the second embodiment of the present invention. 
     FIG. 7 is a perspective view showing an example of a conventional piezoelectric actuator. 
     FIG. 8 is an explanatory drawing showing a support spring of a conventional piezoelectric actuator. 
     FIG. 9 is an enlarged view showing a support spring of a conventional piezoelectric actuator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to the attached drawing. It is to be understood, however, that the present invention is not limited to these described embodiments. 
     FIG. 1 is an assembly drawing showing a piezoelectric actuator relating to a first embodiment of the present invention. This piezoelectric actuator  100  comprises a movable plate member  110  (hereinafter “movable plate”) having a movable body or moving body  121 , and a base  130 . The movable plate  110  is configured having the moving body  121  provided a frame  121   a  and four cantilevers  126   a ,  126   b  (hereinafter “cantilever  126 ”) connected to the frame which is supported to the movable plate  110  by four support biasing members or springs  112 . The support springs  112  are formed having a zigzag snaking shape, with a long-edged section  112   a  being thin, and a short-edged section  112   b  being thick, and such that it is possible to increase a distance between the moving body  121  and an external frame section of the movable plate  110  connected to the support springs  112 . Also, the thickness of the support springs  112  is such that with respect to the long-edged section  112   a , an aspect ratio (thickness/width) is at least 1. The support spring  112  structure enables easy bending deformation in the X axis direction, while deformation in the Y and Z axis directions is difficult, and twisting deformation is easy. At the same time as the moving body  121  supported on the support springs  112  has horizontal movement restructured to only the X axis direction, it is possible to cause vibration around the X axis and the Y axis. With the support spring  112  structure, the contact state between the base  130  and the moving body  121  can always be kept constant. The shape of the support springs  112  is not limited to the snaking shape shown in the drawings, and they can be any shape as long as it is easy to move the moving body  121 . 
     Stainless steel or aluminum material can be used for the movable plate  110 . It is also possible to use a metallic or non metallic elastic material such as beryllium copper, phosphor bronze, brass, duralumin, titanium or silicon. The movable plate  110  is preferably made using photolithography techniques. By using a non-mechanical manufacturing process, it is possible to eliminate deformation, stress and mechanical stress arising at the time of manufacture, which together are beneficial to mass production. Also, due to high precision of the components, it is possible to reduce assembly and adjustment processes for each of the constituent elements as much as possible, and this makes the function and reproducibility stable. The aspect ratio of the support springs  112  is at least one, but negative tone near ultra violet resist technology can also be employed to make the support springs  112 . 
     A sliding surface (omitted from the drawings) is preferably provided at a section of the base  130  contacting a lower surface of the moving body  121 . A material satisfying various conditions, such as a large frictional coefficient, excellent abrasion resistance, ability to maintain a stable frictional coefficient, etc. is preferably used on the sliding surface. For example, it is possible to make the sliding surface by carrying out oxidation film treatment. It is also possible for the sliding surface to use cellulose fiber, carbon fiber, a jointing material for whiskers and phenol resin, or a jointing material for polyimide resin and polyamide resin. 
     FIG. 2 is an enlarged drawing of the moving body  121  shown in FIG.  1 . The cantilever  126  shown here has a long-edged section and a short-edged section, with the long-edged section joined to the piezoelectric element  128  being a free end and the short-edged section connected to the moving body being a fixed end. The piezoelectric element  128  is also subjected to polarization processing in a specified direction. The piezoelectric element  128  is formed of a piezoelectric material, piezoelectric material being a material simultaneously provided with a strain generating function, a resonance function and a voltage generating function. Specifically, piezoelectric material is a material that exhibits displacement without stress in response to an applied voltage, causes a resonance phenomenon with the frequency of the applied voltage, and generates a voltage in response to applied force. A zircon lead titanate thin-film having high piezoelectric constant is used in the piezoelectric element  128  in this embodiment. It is also possible to use barium titanate, lithium niobate or zircon lead titanate. It is also possible to use an inclined function material or lithium niobate instead of the piezoelectric ceramics. A driver  140  for applying a alternating voltage is connected to electrodes  129  of the piezoelectric element, and the driver  140  controls behavior of the moving body by varying the frequency or voltage, or both the frequency and voltage, of the applied voltage. The number of cantilevers is not limited to four and can be determined according to the required driving force. 
     The cantilevers  126  are joined to the piezoelectric element  128  by adhesive. Conditions to be satisfied by the adhesive are that it is an extremely thin adhesive layer, that the adhesive layer is hard and tough, and that a resistance value in the vicinity of the resonant frequency is small after fastening the cantilevers  126  and the piezoelectric element  128 . A joining interface exists between the cantilevers  126  and the piezoelectric element  128  even if joined by direct joining or adhesive. This joining interface is a significant factor in determining propagation characteristics between the cantilevers  126  and the piezoelectric element  128 . Because of this, the characteristics of the adhesive or control of the film thickness of the adhesive is important. For example, polymer adhesive such as hot melt or epoxy resin is used as the adhesive. In this embodiment, epoxy type adhesive is used to obtain an optimum film thickness. It is also possible for the cantilevers  126  to directly make contact with the piezoelectric element  128  without using adhesive, and to provide a piezoelectric element using thin film formation and pressure membrane formation processing means. 
     It is possible to use either a unimorph type having a single plate, a bimorph type having two plates, or a multimorph type having four or more plates as the piezoelectric element. The material and method of fastening the piezoelectric element  128  and the cantilevers  126  is determined according to the amount of displacement, force, responsivity and structural limitations of the cantilevers required by the piezoelectric actuator. A unimorph structure is adopted for the cantilevers  126  of this embodiment, because it is difficult to cause hysterisis with respect to the displacement voltage characteristic in a unimorph structure. Furthermore, compared to a bimorph structure, the amount of displacement is small but the force generated force, and the load and applied force of the moving body are appropriate. It is possible to adopt a multimorph type structure and to increase the displacement and force by increasing the number of layers while maintaining a constant thickness, depending on the specifications of the piezoelectric element. With the moving body  121  having such a structure, bending and twisting displacement of the cantilevers  126  can be excited in an extremely stable manner. 
     The base  130  can be made using a similar material and method as for the movable plate  110 . It is also possible to use an inner surface of an equipment case as the base, without the need for a special component. 
     The principal of operation of the piezoelectric actuator  100  will now be described. FIG. 3 is an explanatory drawing showing the driving principal of the piezoelectric actuator of the present invention. If a positive or negative voltage is applied to an electrode  129 , the piezoelectric element  128  becomes elongated and the free ends of the cantilevers  126  bend towards the base  130 . If the voltage is then removed, the piezoelectric element  128  and the cantilevers  126  return to their original shape. The cantilevers  126  bend in accordance with displacement of the piezoelectric element  128  and the tips of the free ends of the cantilevers  126  generate elliptical motion and come into contact with the base  130 . As a result, force of a horizontal component (in a right to left direction in the drawing) of the tips of the free ends is transmitted by means of frictional force, and the moving body  121  is made to move a microscopic amount in the opposite direction (from left to right in the drawing). Accordingly, this microscopic movement is repeated by cyclically applying voltage to the piezoelectric element  128  and so it is possible to cause the moving body  121  to move continuously. In FIG. 3, contacting sections of the cantilevers  126  and the base  130  are shown as flat surfaces, but it is not necessary for the contacting sections to come in contact with each other in a flat manner, and point contact can also be made. 
     Next, operation of the piezoelectric actuator  100  will be described. If a alternating voltage is applied to the piezoelectric element  128  of a cantilever  126   a  facing in one direction because of the above described principal, the moving body  121  moves in a fixed direction (the X direction). Because the moving body  121  is supported by the support springs  112 , the support springs  112  bend in accordance with movement of the moving body  121 . Also, since the moving body  121  has a structure where direction of movement is regulated by the structure of the support springs  112  and is aligned with a direction of the free ends of the support springs  112  where the support springs  112  have a high degree of freedom, it is possible to reliably move the moving body  121  linearly as required. Also, if a alternating voltage is applied to the piezoelectric element  128  of a cantilever  126   b  facing in the opposite direction, the moving body will move in the opposite direction. 
     With respect to stable movement of the moving body  121 , it is necessary for opposite surfaces of the moving body  121  and the base  130  to maintain uniform contact. Since the moving body  121  is supported extremely flexibly in the X direction, it is possible to follow the surface of the moving body vibrating subtly around the X and Y axes. As a result, it is possible to control movement of the moving body in a stable manner. 
     A second embodiment of the present invention will now be described. FIG. 4A is an assembly drawing showing a piezoelectric actuator of the second embodiment of the present invention. The piezoelectric actuator  200  has a structure that enables pressurization of the moving body  221 , and comprises a movable plate  210  containing a moving body and a base  230 . 
     The moving plate  210  has the moving body  221  formed in the middle and has a structure such that the moving body  221  is fixedly supported with a degree of freedom in the X direction by support springs  212 . The support springs  212  are a rectangular frame shape consecutively connected at centers of a long-edged section. These support springs  212  have a width of the rectangular frame small and a thickness large, enabling movement of the moving body  221  in only the X axis direction, and also enabling twisting deformation following undulation of the base surface. A cantilever  226  having one end free and the other end fixed is arranged in the moving body  221 , and the moving body  221  is driven. The driving principal is the same as for the first embodiment. 
     A cross-section of the structure of pressurizing the moving body is shown in FIG.  4 ( b ). As a structure for generating preload, the moving body  221  is formed so that the movable plate  210  is thick towards the base  130  compared to the support springs  212 . The piezoelectric actuator  200  is assembled by gluing an outer frame of the movable plate  210  and the base  130  together. Because of the thickness of the moving body  221  at this time, the support springs  212  are deformed in a thickness direction (opposite to the base  230 ) at the same time as a sliding surface of the moving body  221  definitely comes into contact with a surface of the base  230 , which means that a constant contact pressure is always exerted on the moving body  221 . By adjusting the thickness of the moving body, it is possible to optimally control contact pressure and to provide a moving body  221  having high efficiency and precision. 
     The structure of generating preload is not limited to that described above. As shown in FIGS. 5A,  5 B, a similar function can be obtained by forming a protrusion  231  in the base  230  to come into contact with the moving body  221 . 
     This movable plate  210  can be formed of the same material and using the same methods as for the above described first embodiment. As a method of forming the moving body  221  in a protruding shape, there is a method of forming the movable plate  210  at a uniform thickness and then making parts other than the moving body  221  thinner by etching etc., or there is a method of adhering or joining the movable plate  210  having a joining section  222  and support springs  212  and the moving body  221  together after they have been individually processed (FIG.  6 ). Similarly, also in the case where there is a protruding section  231  on the base  230 , a method of adhering or joining only the protruding section  231  to the base  230  after it has been formed, or a method of forming the protruding section  231  and the base  230  together can be adopted. 
     Although not shown in the drawings, as an example of modification to the above described first and second embodiments, it is possible to provide the cantilevers  126  or  226  in two sizes. With this structure, coarse control can be carried out by the large cantilevers and fine control can be carried out by the small cantilevers. 
     As has been described above, with the piezoelectric actuator of the present invention, since piezoelectric elements are provided on flat plate-like cantilevers, it is possible to carry out machining processes and easily provide a high precision shape. At the same time, with respect to mass production, since the number of components is reduced it is easy to reduce the cost with large volume production. It is possible to realize support spring shape and movement distance of the moving body according to desired specifications for functions. Also, since the moving body faces and follows the base, it is possible to control movement in a stable manner. 
     According to the second embodiment of the present invention, by having the cantilevers comprising a long-edged section and a short-edged section, it is possible to obtain a large amplitude of vibration at the tips of free ends and operation of the moving body is improved. Also, since machining processes can be carried out, high precision shapes can be formed together and it is easy to reduce the cost for large scale production because the number of components is reduced. 
     The piezoelectric actuator of the present invention having the pressurization means enables high precision batch production and makes it possible to easily reduce costs with mass production, and at the same time since the pressurization means for causing pressurized contact between the moving body and the base is provided it is possible to improve locating precision and efficiency and to perform movement control in a stable manner.