Abstract:
A stacked type electromechanical energy conversion element which is compact in size and capable of providing higher output power and enhanced operating efficiency. A plurality of electrode layers are formed, respectively, on one surfaces of a plurality of piezoelectric layers, and the material layers and said electrode layers are stacked one upon another. Electrodes are formed at least in the plurality of material layers for providing electrical connections between corresponding ones of the plurality of electrode layers. The plurality of electrode layers have a non-uniform configuration depending on a predetermined strain distribution that is to occur in the stacked type electromechanical energy conversion element.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a stacked type electromechanical energy conversion element and a vibration wave driving apparatus.  
         [0003]     2. Description of the Related Art  
         [0004]     Conventionally, a vibration wave motor such as one  90  shown in  FIG. 6  has been used for example, as an autofocusing motor for camera lenses.  
         [0005]     In  FIG. 6 , the vibration wave motor  90  is comprised of a bolt member  91 , a vibrator  92  and a rotating unit  93  into both of which a shaft of the bolt member  91  is inserted, and a nut member  94  screwed onto the bolt member  91  to clamp the rotating unit  93  in cooperation with the vibrator  92 .  
         [0006]     The vibrator  92  is comprised of a metal elastic body  95 , a printed circuit board (PCB)  96  connected to an external power supply (not shown), a stacked piezoelectric element  97 , and a metal elastic body  98  that cooperates with the elastic body  95  to hold the PCB  96  and the stacked piezoelectric element  97  therebetween. These component parts are arranged in the order mentioned as viewed from a head  91   a  of the bolt member  91 .  
         [0007]     The rotating unit  93  is comprised of a gear  99  rotatably supported on the nut member  94  via a ball bearing, a rotor  100  rotatable in unison with the gear  99  and disposed in contact with the elastic body  100 , a spring  101  urging the rotor  100  against the head  91   a  of the bolt member  91  in a direction away from the gear  99 , and a spring support  102 . These component parts are arranged in the order mentioned as viewed from the nut member  94 .  
         [0008]     The spring  101  serves to press the rotor  100  against the elastic body  98 .  
         [0009]     The stacked piezoelectric element  97  has a stacked structure formed of a plurality of ceramic layers (piezoelectric layers) that are stacked one upon another. These ceramic layers have an electromechanical energy conversion function and are formed at their respective one surface with electrode layers (hereinafter referred to as “the internal electrodes”) of an electrode material. With this stacked structure, application of low voltage may result in a high deformation strain and a large force. Such stacked piezoelectric elements are disclosed in, for example, Japanese Patent Publications Nos. 3,311,034, 3,313,782, and 3,432,321.  
         [0010]      FIGS. 7A and 7B  are views useful in explaining the stacked piezoelectric element  97  of  FIG. 6 , in which  FIG. 7A  is a perspective view of the device  97 , and  FIG. 7B  is an exploded perspective view thereof.  
         [0011]     Specifically,  FIGS. 7A and 7B  show a stacked piezoelectric element disclosed in Japanese Patent Publication No. 3,432,321.  
         [0012]     As illustrated in  FIG. 7A , the stacked piezoelectric element  97  has a circular or disk shape with a central opening formed therein. The dimensions are an outer diameter of 10 mm, an inner diameter of 2.8 mm and a thickness of approximately 2.3 mm. As shown in  FIG. 7B , the stacked piezoelectric element  97  is comprised of a first or uppermost piezoelectric layer  110 , and second to twenty-fifth layers in which piezoelectric layers  111  and  112  are alternately disposed. The piezoelectric layers  110 ,  111 , and  112  each have a thickness of approximately 90 micrometers (μm).  
         [0013]     Each piezoelectric layer  111  has a surface thereof facing toward the first or uppermost layer, which is formed with four segmented internal electrodes A+, B+, A−, and B−. Similarly, each piezoelectric layer  112  has a surface thereof facing the uppermost layer, which is formed with four segmented internal electrodes AG+, BG+, AG−, and BG−. The internal electrodes A+, B+, A−, B− of the piezoelectric layer  111  have the same inner and outer diameters as the internal electrodes AG+, BG+, AG−, BG− of the piezoelectric layer  112 . The thickness of each internal electrode is approximately 2 to 3 μm.  
         [0014]     The piezoelectric layers  111  and  112  are each formed therein with eight through electrodes  113  with an electrode material filled therein for causing corresponding internal electrodes to electrically conduct. Among these through electrodes, four through electrodes  113  are arranged to cause the respective corresponding segmented internal electrodes A+, B+, A−, and B− of the alternate piezoelectric layers  111  to electrically conduct independently. The other four through electrodes  113  are arranged to cause the respective corresponding segmented internal electrodes AG+, BG+, AG−, and BG− of the alternate piezoelectric layers  112  to electrically conduct independently. The twenty-fifth piezoelectric layer  112 , which is the lowermost layer, has no through electrodes.  
         [0015]     The through electrodes  113  extend through the stacked structure and have their respective one ends  113   a  exposed to an outer/upper surface of the first piezoelectric layer  110  where the one ends  113   a  are in direct contact with the PCB  96  and electrically connected to wiring conductors (not shown) on the PCB  96 .  
         [0016]     The stacked piezoelectric element  97  performs polarization by applying positive voltage to the internal electrodes A+ and B+ out of the four segmented internal electrodes of the piezoelectric layers  111  and negative voltage to the internal electrodes A− and B− of the same with the internal electrodes AG+, BG−, AG− and BG− grounded such that the paired internal electrodes A+, A− and the paired internal electrodes B+, B−, each pair being offset by 180 degrees, are opposite in polarity, i.e. one is positive, and the other is negative. The vibration wave motor  90  applies high-frequency voltage almost equal to the natural frequency of the vibrator  97  to an A phase to which the electrodes A+ and A− correspond, with the electrodes AG+, AG− corresponding to an AG phase and facing the A phase and the electrodes BG+, BG− corresponding to a BG phase and facing a B phase to which the electrodes B+ and B− correspond, the B phase being different by 90 degrees in spatial phase from the A phase, being grounded. Further, the vibration wave motor  90  applies high-frequency voltage equal in natural frequency to and electrically different in phase by 90 degrees from the high-frequency voltage applied to the A phase to the B phase. This causes the vibrator  92  to generate two bending vibrations intersecting with each other, whereby driving vibrations are obtained by synthesis of the generated two bending vibrations to thereby frictionally drive the rotor  100  disposed in urging contact with one end face of the elastic body  98  via the spring  101 .  
         [0017]     As stated above, the stacked piezoelectric element  97  of the conventional vibration wave motor  90  has the internal electrodes of uniform dimensions (inner and outer diameters) throughout the stacked structure. This is because the stacked piezoelectric element  97  will produce a relatively even strain distribution though there are some variations (a1), (b1) and (c1) in strain depending on locations in the device  97 , as shown in  FIG. 8 , and hence the internal electrodes can be uniform in dimensions.  FIG. 8  is a fragmentary vertical sectional view of a half of the piezoelectric element in the radial direction, useful in explaining strains over the piezoelectric element.  
         [0018]     Recently, there is a strong demand for a vibration wave motor of this type to be more compact in size, higher in efficiency, and higher in output so as to be applicable to various appliances, as disclosed in Japanese Laid-Open Patent Publications (Kokai) Nos. 2003-134858, 2003-199376, and 2003-209983.  
         [0019]     Vibration wave motors disclosed in the above-mentioned publications are still insufficient in efficiency and output, leaving room for improvement of the stacked piezoelectric element thereof. Specifically, analysis of the structure of the stacked piezoelectric element has revealed that in small-sized vibration wave motors, to produce efficient vibrations of the vibrator, strains generated in the stacked piezoelectric element should have such a distribution that the strains largely vary in the thickness or stacked direction and in the radial direction of the stacked piezoelectric element.  
         [0020]     That is, if all the internal electrodes of the stacked piezoelectric element are configured to have uniform dimensions as in the conventional stacked piezoelectric element, strains occur even in a region where inherently strains should not be generated or a region where inverse strains, i.e. strains caused by compression should be generated, which can cause loss of supplied electric power, and hence degraded operating efficiency and insufficient output of the vibration wave motor.  
       SUMMARY OF THE INVENTION  
       [0021]     It is an object of the invention to provide a stacked type electromechanical energy conversion element and a vibration wave driving apparatus which are compact in size and capable of providing higher output power and enhanced operating efficiency of a motor thereof.  
         [0022]     To attain the above object, in a first aspect of the present invention, there is provided a stacked type electromechanical energy conversion element comprising a plurality of stacked electromechanical energy conversion material layers having one surfaces, a plurality of electrode layers formed, respectively, on the one surfaces of the plurality of material layers, the material layers and the electrode layers being stacked one upon another, and electrodes formed at least in the plurality of material layers for providing electrical connections between corresponding ones of the plurality of electrode layers, wherein the plurality of electrode layers have a non-uniform configuration depending on a predetermined strain distribution that is to occur in the stacked type electromechanical energy conversion element.  
         [0023]     Preferably, the stacked type electromechanical energy conversion element has one surface thereof located on one side thereof in a direction in which the material layers and the electrode layers are stacked, and the plurality of electrode layers have outer diameters gradually decreasing toward the one surface of the device.  
         [0024]     Preferably, the stacked type electromechanical energy conversion element has one surface thereof located on one side thereof in a direction in which the material layers and the electrode layers are stacked, and the plurality of electrode layers have inner diameters gradually decreasing toward the one surface of the device.  
         [0025]     Preferably, each of the plurality of electrode layers has a circular shape.  
         [0026]     Alternatively, each of the plurality of electrode layers has a polygonal shape.  
         [0027]     More preferably, the polygonal shape is a square.  
         [0028]     To attain the above object, in a second aspect of the present invention, there is provided a vibration wave driving apparatus comprising a vibrator having a surface thereof, the vibrator including a stacked type electromechanical energy conversion element for inducing a vibration wave on the surface of the vibrator, when electrically driven, and a moving unit disposed in contact with the surface of the vibrator, the moving unit being driven by the vibration wave, wherein the stacked type electromechanical energy conversion element comprises a plurality of stacked electromechanical energy conversion material layers having one surfaces, a plurality of electrode layers formed, respectively, on the one surfaces of the plurality of material layers, the material layers and the electrode layers being stacked one upon another, and electrodes formed at least in the plurality of material layers for providing electrical connections between corresponding ones of the plurality of electrode layers, and wherein the plurality of electrode layers have a non-uniform configuration depending on a predetermined strain distribution that is to occur in the stacked type electromechanical energy conversion element.  
         [0029]     Preferably, the plurality of electrode layers have outer diameters gradually decreasing toward the moving unit.  
         [0030]     Preferably, the plurality of electrode layers have inner diameters gradually decreasing toward the moving unit.  
         [0031]     Preferably, the vibrator comprises first, second, and third elastic bodies, the first elastic body, the stacked type electromechanical energy conversion element, the second elastic body, and the third elastic body are arranged in an order mentioned, and the stacked type electromechanical energy conversion element has an outer diameter smaller than that of the second elastic body and larger than that of the third elastic body.  
         [0032]     The above and other features and advantages of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which like reference numerals or characters designate the same or similar parts throughout the figures thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  is a cross-sectional view of a vibration wave motor according to a first embodiment of the present invention;  
         [0034]      FIG. 2A  is a perspective view of a stacked piezoelectric element appearing in  FIG. 1 ;  
         [0035]      FIG. 2B  is an exploded perspective view of the stacked piezoelectric element in  FIG. 1 ;  
         [0036]      FIG. 3  is a fragmentary vertical cross-sectional view of the stacked piezoelectric element of  FIGS. 2A and 2B , useful in explaining a desired strain distribution that is to occur in the stacked piezoelectric element having internal electrodes A+ and AG+;  
         [0037]      FIG. 4  is a cross-sectional view of a vibration wave motor according to a second embodiment of the present invention;  
         [0038]      FIG. 5A  is a perspective view of a stacked piezoelectric element appearing in  FIG. 4 ;  
         [0039]      FIG. 5B  is an exploded perspective view of the stacked piezoelectric element in  FIG. 4 ;  
         [0040]      FIG. 6  is a cross-sectional view of a conventional vibration wave motor;  
         [0041]      FIG. 7A  is a perspective view of a stacked piezoelectric element appearing in  FIG. 6 ;  
         [0042]      FIG. 7B  is an exploded perspective view of the stacked piezoelectric element in  FIG. 6 ; and  
         [0043]      FIG. 8  is a fragmentary vertical cross-sectional view useful in explaining a strain distribution in a conventional stacked piezoelectric element appearing in  FIGS. 7A and 7B .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]     The present invention will now be described with reference to the drawings showing preferred embodiments thereof. In the drawings, like reference numerals designate the same or similar elements or parts throughout the figures thereof.  
         [0045]      FIG. 1  shows a cross-sectional view of a vibration wave motor according to a first embodiment of the present invention.  
         [0046]     As shown in  FIG. 1 , the vibration wave motor  50  is comprised of a shaft  51 , a vibrator  52 , a rotating unit  53 , the vibrator  52  and the rotating unit  53  being fitted around the shaft  51 , and a nut member  62  screwed onto the shaft  51  to clamp the rotating unit  53  in cooperation with the vibrator  52 . The vibration wave motor  50  can provide equivalent almost as large output torque as that provided by the motor  90 , although it is shorter in entire length and more compact in size than the conventional vibration wave motor  90  shown in  FIG. 6 .  
         [0047]     The vibrator  52  is comprised of a metal elastic body  54  screwed onto one end of the shaft  51 , a stacked piezoelectric element  55  ( FIG. 2 ) as a stacked type electromechanical energy conversion element, a printed circuit board (PCB)  56  connected to an external power supply (not shown), a metal elastic body  57 , and a metal elastic body  58  that cooperates with the elastic body  54  to hold the stacked piezoelectric element  55 , the PCB  56  and the elastic body  57  therebetween. These component parts are arranged in the order mentioned. The stacked piezoelectric element  55  is configured to have an outer diameter smaller than that of the elastic body  57  and greater than that of the elastic body  58 .  
         [0048]     The rotating unit  53  is comprised of a rotor  59  fitted on the shaft  51  in a manner being movable axially thereof and inhibited from rotating, a gear  60  as an output member rotatably fitted on the shaft  51  on the side close to an upper end of the rotor  59 , a disk member  61  rigidly fitted on an upper end of the shaft  51 , and a spring  63  disposed inside the rotor  59  between the rotor  59  and the gear  60  and urging the rotor  59  and the gear  60  in directions away from each other. A contact ring  59 A is fitted on a lower end of the rotor  59 , with a lower end thereof in contact with the elastic body  57 .  
         [0049]     The spring  63  urges the rotor  59  against the elastic body  57  to keep the same in contact with the elastic body  57 .  
         [0050]     The PCB  56  is comprised of patterned conductors for wiring (not shown) which are in tight mechanical contact with one end  14  of each of through electrodes exposed to a surface of the stacked piezoelectric element  55  ( FIG. 2 ), and conductors on the PCB  56  are electrically connected to the through electrodes.  
         [0051]      FIGS. 2A and 2B  illustrate the stacked piezoelectric element  55  of  FIG. 1 . Specifically,  FIG. 2A  is a perspective view of the piezoelectric element  55 , and  FIG. 2B  is an exploded perspective view of the piezoelectric element  55 .  
         [0052]     As illustrated in  FIG. 2A , the stacked piezoelectric element  55  has a generally circular or disk shape with a central opening formed therein. The stacked piezoelectric element  55  has an outer diameter of 6 mm, an inner diameter of 1.7 mm and a thickness of approximately 0.7 mm. As shown in  FIG. 2B , the stacked piezoelectric element  55  is comprised of a first or uppermost ceramic layer (hereinafter referred to as “the piezoelectric layer”)  1 , and second to eleventh piezoelectric ceramic layers  2  to  11 . Each of these layers  1  to  11  is circular in shape, serves as an electromechanical energy conversion layer, and has a thickness of approximately 60 micrometers (μm). It should be noted, however, that the above-mentioned dimensions are given only as an example. The dimensions may have other values depending on parameters required for a specific stacked piezoelectric element, such as output power and entire length.  
         [0053]     The even-numbered piezoelectric layers  2 ,  4 , - - - ,  10  each have an electrode layer of an electrode material formed in a surface thereof facing the uppermost layer  1 . The electrode layer includes four segmented electrodes (internal electrodes) A+, B+, A−, and B− that are separated from one another by a generally cross-shaped slit where no electrode is formed. Except for the first layer  1 , the odd-numbered piezoelectric layers  3 ,  5 , - - - ,  11  each also have four segmented internal electrodes AG+, BG+, AG−, and BG− formed in a surface thereof facing the uppermost layer  1 . The thickness of each internal electrode may be approximately 2 to 3 μm. While the internal electrodes of the conventional stacked piezoelectric element  97  in  FIGS. 7A and 7B  are uniform in dimensions (inner and outer diameters), the internal electrodes of the stacked piezoelectric element  55  of  FIGS. 2A and 2B  have non-uniform dimensions such that the internal electrodes of the piezoelectric layers  5  and  6  have an outer diameter smaller than that of the internal electrodes of the piezoelectric layers  7  to  11 , and the internal electrodes of the piezoelectric layers  2  to  4  have a further smaller outer diameter. The ground for setting such different dimensions will be described later.  
         [0054]     The piezoelectric layers  1  to  11  are each formed therein with eight through electrodes  13  with an electrode material filled therein for causing corresponding internal electrodes to electrically conduct. Among the through electrodes, four through electrodes  13  are arranged to cause the respective segmented internal electrodes A+, B+, A−, and B− of the even-numbered alternate piezoelectric layers  2 ,  4 ,  6 ,  8  and  10  to electrically conduct independently. The other four through electrodes  13  are arranged to cause the respective segmented internal electrodes AG+, BG+, AG−, and BG− of the odd-numbered alternate piezoelectric layers  3 ,  5 ,  7 ,  9  and  11  to electrically conduct independently. The eleventh piezoelectric layer  112  as the lowermost layer has no though-electrodes.  
         [0055]     The through electrodes  13  extend though the stacked structure and their respective ends  14  are exposed to an outer/upper surface of the first piezoelectric layer  1 , so that the ends  14  are in direct contact with the PCB  56  and electrically connected to wiring conductors (not shown) on the PCB  56 .  
         [0056]     The stacked piezoelectric element  55  performs polarization by applying positive voltage to the internal electrodes A+ and B+ out of the four segmented internal electrodes of the piezoelectric layers  2 ,  4 ,  6 ,  8 , and  10  and negative voltage to the internal electrodes A− and B− of the same with the internal electrodes AG+, BG−, AG− and BG− grounded such that the paired internal electrodes A+, A− and the paired internal electrodes B+, B−, each pair being offset by 180 degrees, are opposite in polarity, i.e. one is positive, and the other is negative. The vibration wave motor  50  applies high-frequency voltage almost equal to the natural frequency of the vibrator  52  to an A phase to which the electrodes A+ and A− correspond, with the electrodes AG+, AG− corresponding to an AG phase and facing the A phase and the electrodes BG+, BG− corresponding to a BG phase and facing a B phase to which the electrodes B+ and B− correspond, the B phase being different by 90 degrees in spatial phase from the A phase, being grounded. Further, the vibration wave motor  50  applies high-frequency voltage equal in natural frequency to and electrically different in phase by 90 degrees from the high-frequency voltage applied to the A phase to the B phase. This causes the vibrator  52  to generate two bending vibrations intersecting with each other, whereby driving vibrations are obtained by synthesis of the generated two bending vibrations to thereby frictionally drive the rotor  59  integrated with the contact ring  59 A disposed in contact with one end face of the elastic body  57  via the spring  63 .  
         [0057]     Next, a description will be given of the reason why the internal electrodes of the piezoelectric layers  5 ,  6  of the stacked piezoelectric element  55  are smaller in outer diameter than those of the piezoelectric layers  7  to  11  and those of the piezoelectric layers  2  to  4  are further smaller in outer diameter.  
         [0058]      FIG. 3  is a fragmentary vertical cross-sectional view of the stacked piezoelectric element in  FIGS. 2A and 2B , useful in explaining a desired strain distribution in the stacked piezoelectric element.  
         [0059]      FIG. 3  shows a distribution of strains caused by elongation of the stacked piezoelectric element  55  when elongation and contraction of the same occur in the thickness direction by vibrations of the internal electrodes A+ or AG+ for example. The strain distribution is such that in an upper part of the stacked piezoelectric element toward the rotor  59 , the strain is the largest at the inner periphery (region (a)); the strain becomes smaller toward the outer periphery of the stacked piezoelectric element and toward a lower side of the same (regions (b), (c), (d)); and the strain becomes almost zero in a region (e). In a region (f) at an upper and radially outer part of the piezoelectric element, a strain occurs, which is inverse in direction to the direction of strains in the regions (a) to (d), i.e. a strain caused by contraction. Such a strain distribution is determined by principal factors such as dimensions, shapes and materials of the piezoelectric element  55 , the elastic bodies  54 ,  57 , and  58 , and the shaft  51 , and is likely to occur in a compact vibrator, that is, a flat vibrator having a short length as shown in FIG.  1  in particular.  
         [0060]     The strain distribution in  FIG. 3  shows that no strain needs to occur in the region (e), and in the upper and radially outer region (f), a strain should occur, which is inverse in direction to strains that should inherently occur in the piezoelectric element. In particular, in this region, a strain caused by elongation of the piezoelectric element is of no use and can even cause loss of supplied electric power. Therefore, to eliminate the upper and radially outer region in which strains are generated, the electrode layers are configured such that they gradually decrease in outer diameter.  
         [0061]     As described above, according to the present embodiment, the internal electrodes of the stacked piezoelectric element  55  are different in dimensions in accordance with strains that are desired to occur in the stacked piezoelectric element  55 . This enables the vibrator to generate vibrations in an effective manner, to thereby make it possible to provide a more compact vibration wave motor with higher output power and enhanced operating efficiency.  
         [0062]     Although in the present embodiment, the internal electrodes shown in  FIG. 3  have three different dimensions or sizes, it may be configured such that all the electrode layers have respective different dimensions or sizes as appropriate. However, in an actual vibration wave motor, if the internal electrodes of the piezoelectric layers  5  and  6  are equal in dimensions to those of the piezoelectric layers  2  to  4  and/or to those of the piezoelectric layers  7  to  11 , advantageous effects can be provided. Using only such two different-sized internal electrodes can reduce the manufacturing cost, thus providing practically useful effects.  
         [0063]     The stacked piezoelectric element  55  is manufactured as follows: First, through holes for through electrodes are formed in green sheets of piezoelectric ceramic powder and an organic binder for forming piezoelectric layers of the piezoelectric element  55 . Then, patterns of silver-palladium powder paste for forming internal electrodes are formed by screen printing and the formed patterns are laid over the respective green sheets. Next, the green sheets are stacked one upon another, and are pressurized while being heated into a laminated structure. Before being sintered, a central opening is formed in the laminated structure by machining. The laminated structure is then sintered in a lead atmosphere at approximately 1100. After the sintering, polarization of the laminated structure is performed. Then the laminated structure is subjected to double-side lapping. Finally, the outer periphery of the laminated structure is machined or ground, thus completing a stacked piezoelectric element.  
         [0064]     In this way, the stacked piezoelectric element  55  according to the present embodiment has internal electrodes configured to have appropriately different dimensions in the stacked direction according to a strain distribution desired to occur in the piezoelectric element, whereby the vibrator can be vibrated with higher efficiency, thus resulting in higher output power (rotational speed×torque) and enhanced operating efficiency of the vibration wave motor, compared with the conventional stacked piezoelectric element having internal electrodes uniform in dimensions.  
         [0065]      FIG. 4  is a cross-sectional view of a vibration wave motor according to a second embodiment of the present invention.  
         [0066]     The second embodiment is basically identical or similar in configuration to the first embodiment described above. In  FIG. 4 , the same or similar elements or parts are designated by like reference numerals, and duplicate description thereof is omitted. In the following, only component elements different from the first embodiment will be described.  
         [0067]     As shown in  FIG. 4 , a vibration wave motor  70  has a stacked piezoelectric element  75  having a quadrangular or square cross section as shown in  FIGS. 5A and 5B . The vibration wave motor  70 , however, does not include a PCB  56  as shown in  FIG. 1 . The stacked piezoelectric element  75  has an outer peripheral surface thereof formed with eight external electrodes  42  at four corners thereof, two electrodes being formed at each corner. A PCB  86  is mounted on an outer periphery of the element  75  in a manner covering the external electrodes  42 , and is connected to the external electrodes  42  and an external power supply (not shown).  
         [0068]      FIGS. 5A and 5B  illustrate the stacked piezoelectric element  75  in  FIG. 4 . Specifically,  FIG. 5A  is a perspective view of the piezoelectric element  75 , and  FIG. 5B  is an exploded perspective view of the same.  
         [0069]     As shown in  FIG. 5A , the stacked piezoelectric element  75  has a square pole-like shape and has dimensions of a width and breadth of 5 mm, an inner diameter of 1.7 mm and a thickness (height) of approximately 0.7 mm. As shown in  FIG. 5B , the stacked piezoelectric element  75  is comprised of a first or uppermost quadrangular piezoelectric layer  21 , and lower, second to nineteenth quadrangular piezoelectric layers  22  to  39 . The quadrangular piezoelectric layers  21  to  39  each may have a thickness of approximately 35 micrometers (μm). It should be noted, however, that the above-mentioned dimensions are given only as an example. The dimensions may have other values depending on parameters required for a particular stacked piezoelectric element, such as output power and entire length.  
         [0070]     The even-numbered piezoelectric layers  22 ,  24 , - - - ,  38  each have four segmented electrodes A+, B+, A−, and B− formed in a surface thereof facing the first or uppermost layer  21  and separated from one another by a generally cross-shaped slit where no electrode is formed, and the odd-numbered piezoelectric layers  23 ,  25 , - - - ,  37 , - - -  39  each have four segmented internal electrodes AG+, BG+, AG−, and BG− similarly formed in a surface thereof facing the first layer  21 .  
         [0071]     In the stacked piezoelectric element  75  in  FIG. 4 , to realize the strain distribution desired to occur in the stacked piezoelectric element  75 , the piezoelectric layers  24  to  36  have the internal electrodes formed over substantially the entire surface thereof, the piezoelectric layers  22  and  23  have the internal electrodes formed only over a radially inner surface part thereof, and the piezoelectric layers  37  to  39  have the internal electrodes formed over only over a radially outer surface part thereof. That is, in the present embodiment, to eliminate an upper and radially outer region and a lower and radially inner region in which strains occur, the internal electrodes are configured such that the radially outer region and the radially inner region become smaller toward an upper end of the stacked piezoelectric element  75 . The internal electrodes each have a thickness of 2 to 3 μm, as is the same with the first embodiment.  
         [0072]     Out of the piezoelectric layers  22  to  39 , the even-numbered piezoelectric layers have connecting electrodes  41   a  for connection with the external electrodes  42 , formed at four locations, i.e. two electrodes at each of a pair of diametrically opposite corners of the stacked piezoelectric element  75 , and the odd-numbered piezoelectric layers have connecting electrodes  41   b  for connection with the external electrodes  42 , formed at four locations, i.e. two electrodes at each of the other pair of diametrically opposite corners of the stacked piezoelectric element  75 .  
         [0073]     The operation and manufacturing method of the vibration wave motor  70  according to the present embodiment are substantially the same as those of the first embodiment.  
         [0074]     Compared with the conventional vibration wave motor having a stacked piezoelectric element with uniform internal electrodes, the vibration wave motor  70  of the present embodiment having the non-uniform internal electrode configuration can be designed compact in size and provide enhanced operating efficiency.  
         [0075]     Although in the above described embodiments, stacked piezoelectric elements with a central opening are provided, the stacked piezoelectric element may be configured without any central opening.  
         [0076]     Although in the above described embodiments, stacked piezoelectric elements having circular and quadrangular shapes are provided, the stacked piezoelectric element may have any other polygonal shapes. Similarly, the internal electrodes may have any suitable shapes including a circle, quadrangle, and other polygons. In a stacked piezoelectric element having a circular cross section, internal electrode layers having different inner and/or outer diameters may be used. In a stacked piezoelectric element having a quadrangular or polygonal cross section, internal electrode layers having different inner and/or outer diameters may be used.  
         [0077]     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the claims.  
       CROSS REFERENCE TO RELATED APPLICATION  
       [0078]     This application claims priority from Japanese Patent Application No. 2004-176707 filed Jun. 15, 2004, which is hereby incorporated by reference herein.