Patent Publication Number: US-7215063-B2

Title: Vibration element and vibration wave driving apparatus

Description:
This application is a divisional of U.S. patent application Ser. No. 10/044,936, filed on Jan. 15, 2002 now U.S. Pat. No. 6,930,436, now allowed. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a vibration wave driving apparatus, and more particularly, to a configuration of a vibration element used in a bar-shaped vibration wave driving apparatus. 
   2. Related Background Art 
   A bar-shaped vibration wave driving apparatus includes, as a basic structure, a vibration element composed of elastic members made of metal or the like and a piezoelectric element as an electro-mechanical energy conversion element. The bar-shaped vibration wave driving apparatus generates a driving vibration such as a traveling wave or the like through application of an alternating voltage as an alternating signal with different phases to the piezoelectric element. 
   A contact member is brought into pressure contact with a driving part of the elastic member by a pressurizing means and the contact member is frictionally driven by the driving vibration generated in the driving part of the elastic member to allow the vibration element and the contact member to be moved relative to each other. 
   There is a vibration wave motor as an example of such a vibration wave driving apparatus in which a vibration element is used as a stator and a contact member as a rotor. 
   Examples of the vibration element of the vibration wave motor include those with a configuration in which a ring-shaped piezoelectric element plate is attached to one surface of a ring- or disc-shaped elastic member and those of a type in which the rotation of the rotor is taken out through an output shaft or of a type in which the rotation of the rotor is taken out directly. 
   Such a vibration wave motor has been applied to products to be used for driving a camera lens and the like. There are annular type and bar-shaped type vibration wave motors. 
     FIG. 11A  is a structural view of a bar-shaped vibration element of a bar-shaped vibration wave motor used for driving a camera lens.  FIG. 11B  shows a vibration mode (with the z-axis assigned to the axial direction and the r-axis assigned to the radial direction) in an axis part of the bar-shaped vibration element. 
   Numeral  101  indicates a first elastic member; numeral  102 , a second elastic member; and numeral  103 , a piezoelectric element. Numeral  106  denotes a shaft member passing through the first elastic member  101 , the piezoelectric element  103 , and the second elastic member  102 . One end of the shaft member  106  located on the side of a rotor  108  is fixed to a fitting member  107  to be attached to a product and the other end is fixed to a nut  115 . A threaded portion is formed in the other end of the shaft member  106 . With the nut  115  tightened, the first elastic member  101 , the piezoelectric element  103 , and the second elastic member  102  disposed between a flange portion provided for the shaft member  106  and the nut  115  are sandwiched and fixed therebetween. Numeral  108  indicates the rotor as described above, and numeral  116  denotes a friction member fixed to the first elastic element  101  to be in contact with the rotor. 
   When a driving signal is applied to the piezoelectric element  103 , the bending vibration indicated in  FIG. 11B  is excited in the bar-shaped vibration element and thereby the bar-shaped vibration element makes a swing movement substantially about the z-axis. Accordingly, the friction member  116  makes a circular motion around the z-axis. 
   It seems that the vibration element of such a bar-shaped vibration wave driving apparatus has been reduced in size in its radial direction, but there is still room for reduction in size in its thrust direction, i.e. in length of its axis. 
   However, when the vibration element is simply shortened, there arise problems that the resonance frequency increases and the vibration displacement is reduced, which causes the deterioration in efficiency of friction drive, the increase in price of a driving circuit element due to the high frequency, or the increase in loss inside the element. 
   Hence, Japanese Patent Application Laid-open No. 4-91668 proposes one in which a vibration element is provided with a smaller diameter portion to reduce the resonance frequency. According to this proposal, however, when the vibration element is simply made thinner to lower the resonance frequency, the diameters of a piezoelectric element and a frictional surface also are reduced and thereby generating force of the piezoelectric element and the friction torque also decrease. 
   As a technique for shortening the axis of a bar-shaped vibration wave driving apparatus, for solving such matter, there is one disclosed in Japanese Patent Application Laid-open No. 2001-145376, which is shown in  FIG. 12 . 
   A vibration apparatus in such document is identical to a conventional product in that a piezoelectric element  203  is sandwiched and fixed between a first elastic member  201  and a second elastic member  202 . However, the apparatus is different from the conventional product in that the first elastic member  201  with a frictional surface is divided into two regions including an inner peripheral portion and an outer peripheral portion that are connected to each other through a thin connection part  210 . 
   According to this structure, even if the axis of the bar-shaped elastic member is shortened, a low resonance frequency can be obtained since the first elastic member has a sufficiently high mass. 
   According to this technique, however, when the connection part  210  is made thinner to allow the resonance frequency to be lowered and thereby its stiffness is deteriorated, the displacement generated in the piezoelectric element is absorbed by a soft spring of the connection part  210 . Consequently, it is difficult to transmit the driving force to a rotor efficiently. On the contrary, when the connection part  210  is made thick, the resonance frequency cannot be reduced effectively. Thus, it seems that there is still room for further improvement. 
   SUMMARY OF THE INVENTION 
   One aspect of this invention is to provide a vibration wave driving apparatus including a vibration element having an electro-mechanical energy conversion element that is disposed between a first elastic member and a second elastic member, in which the vibration element can have a plurality of vibration modes which are different in relative ratio between displacements of respective ends of the vibration element. Specifically, a third elastic member is disposed between the first elastic member and the second elastic member, and the vibration element is allowed to have two portions which are different in dynamic stiffness from each other and are arranged in the axial direction with the third elastic member interposed therebetween. According to this structure, the length in the axial direction of the vibration wave driving apparatus can be reduced and the internal loss of vibration energy can be suppressed to be small. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a cross-sectional view of a vibration wave motor showing a first embodiment of the present invention, and  FIGS. 1B and 1C  each show a vibration mode of its vibration element; 
       FIG. 2A  is a cross-sectional view of a vibration element showing a second embodiment of the present invention, and  FIGS. 2B and 2C  each show a vibration mode of the vibration element; 
       FIG. 3  is a cross-sectional view of a vibration element according to a third embodiment of the present invention; 
       FIG. 4  is a cross-sectional view of a vibration wave motor according to a fourth embodiment of the present invention; 
       FIG. 5  is a cross-sectional view of a vibration element of a vibration wave motor according to a fifth embodiment of the present invention; 
       FIG. 6  is a top view of a flange-shaped elastic member shown in  FIG. 5 ; 
       FIG. 7  is a cross-sectional view of a vibration element of a vibration wave motor according to a sixth embodiment of the present invention; 
       FIG. 8  is a cross-sectional view of a vibration element of a vibration wave motor, which is illustrated for explaining “distortion”; 
       FIG. 9  is a graph showing curves indicating distortions of the vibration element shown in  FIG. 8 ; 
       FIG. 10  is a cross-sectional view of a vibration wave motor according to a seventh embodiment of the present invention; 
       FIG. 11A  is a cross-sectional view of a conventional vibration wave motor and  FIG. 11B  shows a vibration mode of its vibration element; 
       FIG. 12  is a cross-sectional view of a conventional vibration wave motor; and 
       FIG. 13  is a drawing showing a vibration element whose upper and lower portions are substantially symmetrical to each other with respect to a flange-shaped elastic member interposed therebetween. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   The problem of the invention described in Japanese Patent Application Laid-open No. 2001-145376 is caused because the mass member attached to an end of a soft spring (connection part  210 ) serves as a frictional surface. Hence, it is conceivable that this problem can be solved through separation of a functional member for lowering the resonance frequency and a functional member for taking out driving force from each other. 
     FIGS. 1A to 1C  show a first embodiment of the present invention;  FIG. 1A  is a cross-sectional view of a vibration wave motor and  FIGS. 1B and 1C  show bending vibration modes of its vibration element. 
   Numeral  1  indicates a first elastic member with a hollow cylindrical shape that is made of a material with a low vibration damping loss such as brass. Numeral  2  denotes a second elastic member with a cylindrical shape that is also made of a material with a low vibration damping loss as in the case of the first elastic member  1 . Numeral  5  indicates a flange-shaped (disc-shaped) elastic member extending in the direction orthogonal to the axial direction of the vibration element. The first elastic member  1 , the second elastic member  2 , and the flange-shaped elastic member  5  are fixed together by a shaft  6  including threaded portions  6   a  and  6   b . This flange-shaped elastic member  5 , which is formed of a material with high abrasion resistance, is in contact with a rotor  8  by a portion in the vicinity of its outer periphery of its one surface, and rotates and drives the rotor  8 . As is apparent from  FIG. 1A , a frictional surface of the flange-shaped elastic member  5  that is in contact with the rotor  8  is located on the outer side with respect to the outer peripheries of the first elastic member  1  and the piezoelectric element  3  that are adjacent to the flange-shaped elastic member  5 . The shaft  6  is fixed to a mass member  7  to be attached to an unshown device by its one end portion and functions as a support pin for supporting the vibration element. The portion of the shaft that is not located inside the second elastic member  2 , the piezoelectric element  3 , and the flange-shaped elastic member  5  is formed to be sufficiently thin. Thus, the shaft  6  is configured so as to absorb the vibrations generated by the vibration element to prevent transmission of the vibrations to a device to be driven or the like. 
   Numeral  8  indicates the rotor as described above. The rotor  8  is subjected to pressuring force produced by a coil spring  4  and thereby is in pressure contact with the vicinity of the outer peripheral portion of the flange-shaped elastic member  5 . This rotor  8  can be disposed on the outer periphery of the elastic member  1  and therefore is advantageous in reducing the shaft length and the size of the motor accordingly. Numeral  9  denotes an output gear that rotates together with the rotor and transmits output to the device to be driven, and numeral  10  indicates a ball bearing for supporting the output gear. 
   For convenience, in the descriptions of the first embodiment and the second to tenth embodiments described later, the side on which the rotor and the first elastic member are located and the side on which the second elastic member is located in the axial direction of the vibration element are referred to as an “upper side” and a “lower side”, respectively, with the flange-shaped elastic member taken as a boundary therebetween. 
   In the present embodiment, with the flange-shaped elastic member  5  taken as a boundary between the upper and lower sides, the first elastic member  1  located on the upper side has a smaller diameter, and the piezoelectric element  3  and the second elastic member  2  that are located on the lower side have larger diameters. Thus, the vibration element is constructed such that with the flange-shaped elastic member  5  taken as a boundary between the upper and lower sides, the stiffness against vibration of a portion (a lower portion) of the vibration element provided with the second elastic member  2  that is located on the lower side is higher than that of a portion (an upper portion) of the vibration element provided with the first elastic member  1  that is located on the upper side, and thereby the upper and lower portions arranged with the flange-shaped elastic member  5  taken as a boundary therebetween are considerably different in dynamic stiffness from each other. 
   As described above, when the vibration element is formed to be asymmetrical with respect to the flange-shaped elastic member  5  with the upper and lower portions arranged in the axial direction by providing different outer diameters from each other, two kinds of primary bending vibration modes can be obtained, for example, on a plane parallel to the paper face as indicated in the vibration mode diagrams (showing a distribution of radial displacements in the axis part) shown in  FIGS. 1B and 1C . Specifically, with the flange-shaped elastic member  5  interposed between the upper and lower portions of the vibration element, the lower portion is displaced considerably large in the case of the vibration mode shown in  FIG. 1B , while the upper portion is displaced considerably large in the case of the vibration mode shown in  FIG. 1C . In other words, even when the bending vibrations are generated in the same displacement direction, it is possible to excite two vibration modes that are totally different in relative ratio between displacements of the respective ends of the vibration element. 
   The value of natural frequency of the vibration mode shown in  FIG. 1C  is smaller than that of the vibration mode shown in  FIG. 1B , and the natural frequencies in the vibration modes shown in  FIGS. 1B and 1C  are considerably different from each other. This is because the vibration mode shown in  FIG. 1B  mainly is adjusted for the second elastic member  2  having a larger outer diameter, while the vibration mode shown in  FIG. 1C  mainly is adjusted for the first elastic member  1  having a smaller outer diameter. 
   The increase in outer diameter of the second elastic member  2  allows sufficiently high mass to be secured even when the second elastic member  2  is shortened in the axial direction. In other words, the increase in outer diameter of the second elastic member  2  allows the second elastic member  2  to be shortened in the axial direction, and thereby a vibration element with a low natural frequency can be structured. 
   As can be seen from  FIG. 1B , when the upper and lower portions arranged in the axial direction with the flange-shaped elastic member  5  interposed therebetween are made considerably different in dynamic stiffness from each other, the displacement caused by bending vibrations generated in the vicinity of the flange-shaped elastic member  5  is small. Hence, when the piezoelectric element  3  is disposed in the vicinity of the flange-shaped elastic member, the distortion of the piezoelectric element  3  can be suppressed to a low level and thus a bar-shaped vibration element can be provided that has a small internal loss and high energy efficiency. 
   The bar-shaped vibration element shown in, for example,  FIG. 11A  or  FIG. 12  that is not provided with the flange-shaped elastic element  5  having a larger outer diameter than those of members adjacent thereto does not produce two different bending vibration modes. Furthermore, as shown in  FIG. 13 , even with the structure having a flange-shaped elastic element  305 , when a portion located on the upper side including a first elastic member  301  and a portion located on the lower side including a second elastic member  302  and a piezoelectric element  303 , which are arranged with the flange-shaped elastic element  305  interposed therebetween, are formed to be identical or similar in shape to each other, it is not possible to excite two vibration modes that are totally different in relative ratio between displacements of the respective ends of the vibration element. 
   The bar-shaped vibration element shown in  FIG. 13  can provide the same effect as that obtained by the bar-shaped vibration element shown in  FIG. 1A  in the point that a rotor  308  can be disposed on the outer periphery of the elastic member  301 . However, the bar-shaped vibration element shown in  FIG. 13  merely generates only one bending vibration mode or two bending vibration modes that have little difference from each other and thus produces a bending vibration mode causing great change in displacement of the vicinity of the piezoelectric element  303  and the flange-shaped elastic member  305 . As a result, an excessive distortion is caused in the piezoelectric element  303  and thereby the internal loss in the piezoelectric element  303  cannot be suppressed. Thus, a vibration element with low energy efficiency results. 
   On the contrary, in the case of the vibration element shown in  FIG. 1A  according to the present embodiment, the size of the vibration type driving apparatus is reduced with the rotor  8  disposed around the first elastic member  1  and a vibration element can be provided that has high driving efficiency and an energy loss thereinside is suppressed to be small. 
   Furthermore, if the protruding flange-shaped elastic member  5  is formed of a metal, even in the case where distortion concentrates on the protruding flange-shaped elastic member  5 , the increase in internal loss stays within a minimum range since damping characteristic of the metallic material is better than that of the piezoelectric element and thus a short vibration element with high efficiency can be obtained. 
   When the vibration element is configured to have a larger size, it is possible to augment the vibration displacement. In this case, therefore, torque required for driving can be produced relatively easily although the loss of vibration energy inside the vibration element may be somewhat greater. However, when the vibration element is configured to have a smaller size, the vibration displacement cannot be augmented much. Consequently, it is an important matter for securing sufficiently high driving torque that the loss of vibration energy inside the vibration element is suppressed to be small. 
   It is possible to selectively produce the vibration modes indicated in  FIGS. 1B and 1C  by selectively applying alternating signals substantially corresponding to the natural frequencies of the vibration modes indicated in  FIGS. 1B and 1C , respectively, to the piezoelectric element  3 . Hence, if the first elastic member  1  located on the upper side of the flange-shaped elastic member is formed of a metal causing less internal damping or the like, a high-efficiency vibration element causing less vibration damping can be obtained when the vibration mode indicated in  FIG. 1C  is produced in which vibration displacement of the first elastic member  1  is greater. 
   In addition, the natural frequency of the vibration mode indicated in  FIG. 1C  is affected by the dynamic stiffness of the first elastic member  1  considerably. Hence, when the first elastic member  1  is formed to be considerably thin, the drive frequency can be lowered, and it is not increased even if the shaft length is reduced. 
   Moreover, when the outer diameter of the flange-shaped elastic member, i.e. the diameter of its surface that is in contact with the rotor is increased in accordance with driving force that can be produced by the piezoelectric element  3  disposed on the lower side of the flange-shaped elastic member, the driving torque can be increased even when the shaft length of the vibration element is reduced. 
   In the present embodiment, the first and second elastic members  1  and  2  are made different in outer diameter from each other so that the vibration element is obtained with its upper and lower portions, which are arranged with the flange-shaped elastic member  5  interposed therebetween, being different in dynamic stiffness from each other. However, the vibration element may be constructed with the first and second elastic members  1  and  2  formed of different materials. When the second elastic member  2  is formed of a material with higher stiffness than that of the first elastic member  1 , similar vibration modes to those indicated in  FIGS. 1B and 1C  can be obtained. 
   Second Embodiment 
     FIGS. 2A to 2C  show a second embodiment of the present invention;  FIG. 2A  is a cross-sectional view of a vibration element, and  FIGS. 2B and 2C  show bending vibration modes of this vibration element. 
   The following description is centered on the respects that are different as compared to the vibration element shown in  FIG. 1A . 
   Numeral  11  indicates a first elastic member that is formed integrally with a flange-shaped (disc-shaped) elastic member  15  extending in the direction orthogonal to the axial direction of the vibration element. Numeral  12  denotes a second elastic member that passes through the center portions of a piezoelectric element  13  and the first elastic member  11  and also serves as a shaft member for supporting the vibration element. The end portion of the second elastic member  12  located on the upper side is fixed to a mass member  17 , and a threaded portion  12   b  provided in the shaft part is screwed together with the first elastic member  11 . Thus, the piezoelectric element  13  is sandwiched between and fixed with the first elastic member  11  and the second elastic member  12 . 
   In the vibration element shown in  FIG. 2A , the outer diameters of an upper end portion of the first elastic member  11  and a lower end portion  12   a  of the second elastic member  12  are increased and thereby mass of the free ends with higher bending vibration displacement is increased, and thus the natural frequency of the vibration element is reduced. Since the natural frequency can be reduced, a vibration element can be provided that is smaller in size when one has a natural frequency being equal to that of the vibration element. 
   Similarly in the present embodiment, the outer diameter of the portion of the second elastic member  12  located below the flange-shaped elastic portion  15  is set to be larger than that of the portion of the first elastic member  11  located above the flange-shaped elastic portion  15 , and thereby the above-mentioned portions are made different in dynamic stiffness from each other, so that two different bending vibration modes can be excited. 
   A rotor is disposed on the outer periphery side of the first elastic member  11  although it is not shown in the figure. In the present embodiment, a frictional member  51  with abrasion resistance is attached to the surface of the disc-shaped elastic portion  15  that comes into contact with the rotor. In the vibration element shown in  FIG. 1A , since the flange-shaped elastic member  5  comes into contact with the rotor  8 , the surface of the flange-shaped elastic member  5  is required to be subjected to surfacing process using a lap or the like. In the present embodiment, however, since the frictional member  51  is provided, it is no longer necessary to carry out the surfacing process with respect to the surface of the disc-shaped elastic portion  15 . 
   Third Embodiment 
     FIG. 3  is a cross-sectional view of a vibration element of a vibration wave motor showing a third embodiment. 
   Similarly, in the present embodiment, the description is centered on points differing from the vibration element shown in  FIG. 1A . 
   Numeral  21  denotes a first elastic member, numeral  22  a second elastic member, numeral  23  a piezoelectric element, and numeral  25  a flange-shaped (disc-shaped) elastic member extending in the direction orthogonal to the axial direction of the vibration element. The first elastic member  21  includes a shaft portion passing through the second elastic member  22 , the piezoelectric element  23 , and the flange-shaped elastic member  25 . The end of the shaft portion is fixed to a mass member  27  below the second elastic member  22 . Thus, the first elastic member  21  supports the whole vibration element. This shaft portion includes a threaded portion  21   b  and a flange portion  21   c . The second elastic member  22  is screwed together with the threaded portion  21   b  and thereby the flange-shaped elastic member  25  and the piezoelectric element  23  are sandwiched between and fixed with the second elastic member  22  and the flange portion  21   c.    
   A rotor is disposed on the outer periphery side of an end portion  21   a  and the flange portion  21   c  of the first elastic member  21  although it is not shown in the figure. 
   Similarly, in the present embodiment, the first elastic member  21  located on the upper side of the flange-shaped elastic member  25  is provided with a portion having a smaller outer diameter than that of the second elastic member  22  located on the lower side of the flange-shaped elastic member  25 , and thereby the two elastic members sandwiching the flange-shaped elastic member  25  therebetween are made different in dynamic stiffness. Thus, two different bending vibration modes can be excited as in the vibration elements shown in  FIGS. 1A and 2A  although they are not shown in the figure. 
   The first elastic member  21  includes the end portion  21   a  with an increased outer diameter as in the vibration element shown in  FIG. 2A  and thereby allows the natural frequency of the vibration element to be reduced. 
   The flange-shaped elastic member  25  is formed using a material having abrasion resistance and a low vibration damping loss caused by internal distortion, for example, ceramics or hardened stainless steel. In the flange-shaped elastic member  25 , a frictional surface  25   a  that comes into contact with the rotor and surfaces  25   b  and  25   c  by which the flange-shaped elastic member  25  is sandwiched are formed to protrude slightly to allow a period of time required for the surfacing process using a lap to be shorter. The frictional surface  25   a  and the surface  25   c  for sandwiching are present in the same plane so as to be lapped simultaneously. 
   Fourth Embodiment 
     FIG. 4  is a cross-sectional view of a vibration wave motor showing a fourth embodiment. 
   Numeral  31  indicates a first elastic member, numeral  32  a second elastic member, numeral  33  a piezoelectric element, and numeral  35  a flange-shaped elastic member extending in the direction orthogonal to the axial direction of a vibration element. 
   Numeral  36  denotes a shaft that includes a screw  36   a  for sandwiching and supporting the vibration element that is provided in its lower portion and a joint screw  36   c  to be joined with a mass member  37  that is provided in its upper portion. A contact spring  38   a  is secured to the outer periphery of a rotor  38  by adhesion or the like and a spring case  38   b  is joined with its inner periphery. Numeral  39  indicates an output gear that is engaged and joined with the spring case  38   b  so as to be prevented from being displaced relative to the spring case  38   b  in the radial direction. Numeral  34  indicates a coil spring for applying pressure force. A joint part  40  at which the mass member  37  and the gear  39  are joined together is constructed to be a sliding bearing. Numeral  44  denotes a flexible substrate for supplying electricity to the piezoelectric element  33 . 
   The coil spring  34  for applying pressure force is disposed between the lower end of the spring case  38   b  and the output gear  39 , and by the spring force of this spring  34 , the spring end of the contact spring  38   a  fixed to the outer peripheral portion of the rotor  38  is in pressure contact with the upper surface of the disc-shaped elastic member  35 . The mass member  37  prevents vibrations from leaking to the outside from the shaft  36 . 
   The first elastic member  31  is formed to have a smaller outer diameter than that of the second elastic member  32 . Similarly in the present embodiment, it is possible to excite two different bending vibration modes as in the vibration elements shown in  FIGS. 1A and 2A . 
   A driving signal is supplied to the flexible substrate  44  from a driving circuit not shown in the figure. This driving circuit selects a driving vibration for exciting one of the two different bending vibration modes to supply it to the flexible substrate  44 . 
   In the present embodiment, the vibration element is fixed and the rotor as a contact member that is in pressure contact with the vibration element is provided movably. However, the present invention is not limited to this. The contact member may be fixed and the vibration element may be provided movably, and the vibration element and the contact member may be frictionally driven relative to each other by the driving vibration generated in the disc-shaped elastic member protruding in a flange form of the vibration element. 
   Fifth Embodiment 
     FIG. 5  is a cross-sectional view of a vibration element of a vibration wave driving motor showing a fifth embodiment. 
   Numeral  61  indicates a first elastic member with a hollow cylindrical shape that is made of a material with a low vibration damping loss such as brass. Numeral  62  denotes a second elastic member with a cylindrical shape that is also made of a material with a low vibration damping loss as in the case of the first elastic member  61 . Numeral  65  indicates a flange-shaped (disc-shaped) elastic member extending in the direction orthogonal to the axial direction of the vibration element, and numeral  66  denotes a shaft. The shaft  66  is provided with a threaded portion in its one end. The second elastic member  62  is screwed together with the threaded portion of the shaft  66  and thereby the first elastic member  61 , the flange-shaped elastic member  65 , and a stacked piezoelectric element  63  are sandwiched between and fixed together with a flange portion of the shaft  66  and the second elastic member  62 . This flange-shaped elastic member  65  is formed of a material having abrasion resistance and comes into contact with an unshown rotor by its surface in the vicinity of its outer periphery to rotate and drive the rotor. The frictional surface of the flange-shaped elastic member  65  that comes into contact with the rotor is located on the outer side with respect to the outer peripheries of the first elastic member  61  and the piezoelectric element  63  that are adjacent to the flange-shaped elastic member  65 . 
   In the present embodiment, with the flange-shaped elastic member  65  taken as a boundary between the upper and lower sides, the first elastic member  61  located on the upper side is formed to have a smaller diameter and the piezoelectric element  63  and the second elastic member  62  that are located on the lower side are formed to have larger diameters. Thus, the vibration element is configured such that with the flange-shaped elastic member  65  taken as a boundary between the upper and lower sides, the stiffness against bending vibration of the portion (upper portion) of the vibration element located on the upper side is higher than that of the portion (lower portion) of the vibration element located on the lower side, and thereby the upper and lower portions are considerably different in dynamic Stiffness. Hence, even when the bending vibrations are generated in the same displacement direction, it is possible to excite two vibration modes that are totally different in the ratio of relative displacement between the both ends of the vibration element. 
   Similarly in the vibration element of the present embodiment, the upper and lower portions arranged in the axial direction with the flange-shaped elastic member  65  interposed therebetween are made considerably different in dynamic stiffness from each other, and thereby the displacement caused by bending vibrations generated in the vicinity of the flange-shaped elastic member  65  is reduced. Hence, when the piezoelectric element  63  is disposed in the vicinity of the flange-shaped elastic member, the distortion of the piezoelectric element  63  can be suppressed to a low level and thus a bar-shaped vibration element can be provided that has a small internal loss and high energy efficiency. 
   Furthermore, the size of a vibration type driving apparatus including the vibration element of the present embodiment can be reduced with a rotor disposed around the first elastic member  61 . If the first elastic member  61  is formed of a metal, even in the case where the distortion concentrates on the protruding flange-shaped elastic member  65 , the increase in internal loss stays within a minimum range since the damping characteristic of a metallic material is better than that of the piezoelectric element and thus a short vibration element with high efficiency can be obtained. 
   The vibration element in the present embodiment is provided with concave grooves  65   a  for augmenting the vibration displacement on the frictional surface of the flange-shaped elastic member  65  that comes into contact with the rotor and the opposite surface to the frictional surface so as to further increase driving torque. The concave groove  65   a  provided on the frictional surface side is provided on the inner peripheral side with respect to the frictional portion that comes into contact with the rotor. In the present embodiment, the identical grooves  65   a  are provided on both surfaces of the flange-shaped elastic member  65 . 
     FIG. 6  is a view of the flange-shaped elastic member  65  shown in  FIG. 5 , which is viewed from its top. In  FIG. 6 , the groove  65   a  is provided in a circular shape concentrically with the flange-shaped elastic member. The concave groove  65   a  augments out-of-plane displacement caused by the vibrations generated in the outer peripheral portion of the flange-shaped elastic member  65 . The groove  65   a  shown in  FIG. 6  has a circular shape. However, the shape of the groove is not limited to this and may be designed with consideration given to the regulation of the natural frequency of the vibration element. 
   Furthermore, it is necessary to carry out surfacing process by lapping process or the like with respect to the frictional surface of the vibration element on which the rotor frictionally slides. With the groove  65   a  provided for the frictional surface of the flange-shaped elastic member  65 , however, the area to be subjected to the lapping process can be reduced and the lapping process time period is reduced accordingly. 
   Sixth Embodiment 
     FIG. 7  is a cross-sectional view of a vibration element of a vibration wave driving motor showing a sixth embodiment. 
   Numeral  71  denotes a first elastic member, numeral  72  a second elastic member, numeral  73  a stacked piezoelectric element, and numeral  75  a flange-shaped elastic member. In this flange-shaped elastic member  75 , circular grooves  75   a  are engraved. 
   The vibration element is formed to be asymmetric with respect to the flange-shaped elastic member  75 . Hence, the distributions of absolute values of distortions caused on the respective surfaces located in the thickness direction of the flange-shaped elastic member  75  are asymmetrical to each other when a driving signal is applied to the vibration element. 
   In order to augment the out-of-plane displacement at the outer peripheral portion of the flange-shaped elastic member  75  maximally, the grooves  75   a  are provided at the sites where considerable distortions occur when the vibration mode used for driving is excited. In the vibration element according to the present embodiment, since the distortions occurring when the vibration mode used for driving is excited are caused at its upper and under surfaces asymmetrically to each other, the grooves  75   a  are also formed on the upper and under surfaces of the flange-shaped elastic member  75  asymmetrically to each other. 
   These circular grooves  75   a  also serve for augmenting the out-of-plane displacement of the outer peripheral portion of the flange-shaped elastic member  75  and can shorten the time for lapping process carried out with respect to the flange-shaped elastic member  75 . 
   A vibration element shown in  FIG. 8  is substantially identical to the vibration element shown in  FIG. 5  but is different from the one shown in  FIG. 5  in that a flange-shaped elastic member  85  is provided with no groove for augmenting out-of-plane displacement.  FIG. 9  shows distributions of absolute values of distortions of the flange-shaped elastic member  85  caused when a vibration mode used for driving is excited in the vibration element shown in  FIG. 8 . 
   The distributions of absolute values of distortions shown in  FIG. 9  are those between points A and B on the upper surface and between points C and D on the under surface of the flange-shaped elastic member  85 . As is apparent from  FIG. 9 , in the vibration element formed to have upper and lower portions arranged in the axial direction that are asymmetrical to each other with the flange-shaped elastic member  85  taken as a boundary therebetween, the sites where the maximum distortion occurs at the upper and under surfaces of the flange-shaped elastic member  85  are different from each other. The distributions of absolute values of distortions vary depending on the shape of the vibration element. Thus, in order to augment the vibration displacement of the frictional surface of the flange-shaped elastic member maximally, it is desirable to suitably provide grooves at locations where the maximum distortion occurs on a flange-shaped elastic member of each vibration element. 
   Seventh Embodiment 
     FIG. 10  is a structural view of a vibration wave motor according to a seventh embodiment of the present invention. 
   Numeral  91  indicates a first elastic member, numeral  92  a second elastic member, numeral  93  a piezoelectric element, and numeral  95  a flange-shaped elastic member extending in the direction orthogonal to the axial direction of a vibration element. 
   Numeral  96  denotes a shaft. The first elastic member  91 , the flange-shaped elastic member  95 , and the piezoelectric element  93  are sandwiched and fixed between the second elastic member  92  and a flange portion provided in the middle part of the shaft  96 . Numeral  98  indicates a rotor having an outer periphery to which a contact spring  98   a  is secured by adhesion or the like and an inner periphery with which a spring case  98   b  is joined. Numeral  99  indicates an output gear that is engaged and joined with the spring case  98   b  so as to be prevented from being displaced relative to the spring case  98   b  in the radial direction. Numeral  94  denotes a coil spring for applying pressure force. The spring  94  for applying pressure force is disposed between the output gear  99  and the lower end of the spring case  98   b . With the spring force of the spring  94 , the spring end of the contact spring  98   a  fixed to the outer peripheral portion of the rotor  98  is in pressure contact with the upper surface of the disc-shaped elastic member  95 . Numeral  100  indicates a flange for fixing a motor that is sandwiched between and fixed with a bolt screwed together with the shaft  96  and a flange portion of the shaft  96 . A joint part at which the gear  99  and the flange  100  for fixing a motor are joined together is constructed to be a sliding bearing. Numeral  97  denotes a flexible substrate for supplying electricity to the piezoelectric element  93 . 
   The first elastic member  91  is formed to have a smaller outer diameter than that of the second elastic member  92 . Similarly, in the present embodiment, two different bending vibration modes can be excited as in the vibration elements shown in  FIGS. 1A and 2A . Circular grooves  95   a  are provided for upper and under surfaces of the flange-shaped elastic member  95 , respectively, and augment the vibration displacement caused in the vicinity of the outer periphery of the flange-shaped elastic member  95 . 
   An unshown driving circuit is connected to the flexible substrate  97 . When an alternating voltage having a time phase difference of π/2 is applied to the piezoelectric element  93  from the driving circuit, the vibration element excites two types of bending vibrations in two directions orthogonal to each other. The composition of the vibrations allows a circular motion to be given to the upper surface of the outer peripheral portion of the flange-shaped elastic member  95  with which the rotor is in contact and thereby the rotor  98  pressed by the flange-shaped elastic member  95  having abrasion resistance is frictionally driven. 
   As described above, according to the present embodiment, it is possible to construct a vibration element having a shorter length in the axial direction and a low vibration energy loss thereinside and thus a small vibration wave driving apparatus with a low energy loss can be provided. 
   Since a diameter of the portion site where the vibration element and the rotor are in contact with each other can be set larger, the output characteristics of a low-speed high-torque vibration wave driving apparatus can be exhibited easily. In addition, since the piezoelectric element that is relatively expensive among the components is not required to have a large size, the cost may be reduced. 
   Furthermore, since the rotor can be disposed on the outer periphery of the protruding elastic member, the overall length of the vibration wave driving apparatus also may be reduced. 
   Moreover, when the grooves are provided for both surfaces of the flange-shaped elastic member and the out-of-plane displacement in the vicinity of the outer periphery of the flange-shaped elastic member is augmented, the output of the vibration wave motor whose size is reduced in the axial direction may be increased. Particularly, with concave grooves provided at the sites where a considerable distortion is caused by the vibration mode used for driving, it may be possible to increase the output effectively. 
   The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.