Abstract:
The present invention has been devised in order to raise driving efficiency of the vibration wave driving apparatus. A vibration member of a vibration wave driving apparatus of the present invention comprises the vibration member constituted of an elastic member and an electromechanical energy conversion element, and a rotor contacting the vibration member, and the vibration member generates a travelling wave in the elastic member when an alternating signal is applied to the electromechanical energy conversion element, in which an electrode film provided on a surface of the electromechanical energy conversion element of the vibration member is divided into a plurality of circular areas with different radiuses and each circular area is divided into a plurality of electrodes along its peripheral direction.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a vibration wave driving apparatus such as a vibration wave motor using a vibration, which an elastic member is caused to generate, as a driving force and, more specifically, to a structure of an electromechanical energy conversion element for causing the elastic member to generate a vibration.  
           [0003]    2. Related Background Art  
           [0004]    [0004]FIG. 13 is a sectional view of a vibration wave motor.  
           [0005]    In the figure, reference numeral  40  denotes a vibration member, which is constituted of an elastic member  10  made of metal or the like, a piezoelectric element  20  functioning as an electromechanical energy conversion element and a frictional member  30 . The piezoelectric element  20  is fixed to one side of the elastic member  10  and the frictional member  30  is fixed to the other side.  
           [0006]    The vibration member  40  is fixed to a housing  50  and a case  110  is fixed to the housing  50 . Bearings  120   a  and  120   b  are fixed to the case  110  and the housing  50 . The bearings  120   a  and  120   b  rotatably support a rotary shaft  100 .  
           [0007]    A rotor  60  having the rotary shaft  100  as a center is pressurized to contact the frictional member  30  of the vibration member  40 . The rotor  60  is pressurized toward the vibration member  40  by a pressurizing mechanism  90  consisting of a pressurizing spring  70  and a spring bracket  80 . The spring bracket  80  is fixed to the rotary shaft  100  and the rotor  60 , the pressurizing spring  70 , the spring bracket  80  and the rotary shaft  100  integrally rotate.  
           [0008]    The piezoelectric element  20  used for a vibration wave motor of a shape shown in FIG. 13 has a structure in which electrode films are provided on both sides of one piezoelectric ceramic of a circular plate shape. It is assumed that the electrode provided on one side is an electrode for applying a voltage from a power feeding substrate and the electrode provided on the other side is an electrode for ground. When an alternating voltage is applied to the piezoelectric element  20 , standing wave vibrations of different phases A and B are composited to generate a travelling wave on the surface of the elastic member  10 .  
           [0009]    [0009]FIG. 14A shows an example of a conventional electrode pattern for inputting an alternating voltage. Given that a wavelength of a travelling wave is λ, a plurality of electrodes for the A phase that are alternately polarized in opposite directions at a λ/2 pitch in their thickness direction and a plurality of electrodes for the B phase that are λ/4 apart from the electrodes for the A phase and are alternately polarized in opposite directions at a λ/2 pitch in their thickness direction are formed. FIG. 14B shows an example of an electrode pattern for ground, in which a circular electrode along a shape of a piezoelectric element is formed.  
           [0010]    In the electrode pattern shown in FIG. 14A, the electrodes for the A phase are formed on one side of the circle and the electrodes for the B phase are formed on the other side. A vibration generated in the electrodes for the A phase has a smaller amplitude as the vibration travels farther from the electrodes. A vibration generated in the electrodes for the B phase has a smaller amplitude as the vibration travels farther from the electrodes.  
           [0011]    This state is shown in FIGS. 15A to  15 C. In the figures, the horizontal axis indicates a distance in a peripheral direction of the piezoelectric element and the vertical axis indicates a magnitude of a vibration amplitude. FIG. 15A shows a standing wave vibration generated in the electrodes for the A phase in its left half and shows a standing wave vibration generated in the electrodes for the B phase in its right half.  
           [0012]    [0012]FIG. 15C shows a vibration amplitude of a travelling wave in which an A phase standing wave and a B phase standing wave are composited. Since the amplitude of the travelling wave is nonuniform in the peripheral direction, loci of rotational movements generated on the surface of the elastic member  10  are different as shown in FIG. 15B.  
           [0013]    When there is unevenness in the vibration amplitude generated by the piezoelectric element as described above, since slipping occurs between the frictional member  30  of the elastic member  40  and the rotor  60 , vibration energy cannot be used efficiently as driving energy. Therefore, unevenness in the vibration amplitude is not preferable.  
           [0014]    There might be other factors that cause unevenness in the vibration amplitude generated by the piezoelectric element. Polarization processing of the piezoelectric element is performed by applying a voltage to the parts between the electrode patterns formed on both the sides of the piezoelectric element. Since an electric field at this point does not act on non-electrode portions as shown in FIG. 16, if the electrode patterns on both the sides of the piezoelectric element are different, a direction of an electric field applied to the parts between the electrode patterns becomes nonuniform and unevenness also occurs in a polarization direction. If unevenness exists in the polarization direction, since an elastic modulus of the piezoelectric element varies, unevenness also occurs in a vibration generated when a vibration wave driving apparatus is driven.  
           [0015]    Therefore, in order to raise driving efficiency of the vibration wave driving apparatus, it is considered that there is still left room of improvement.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention has been devised in view of the above-mentioned drawbacks, and it is an object of the present invention to provide a vibration member of a vibration wave driving apparatus comprising: the vibration member constituted of an elastic member and an electromechanical energy conversion element; and a rotor contacting the vibration member, the vibration member generating a travelling wave in the elastic member when an alternating signal is applied to the electromechanical energy conversion element, in which an electrode film provided on a surface of the electromechanical energy conversion element of the vibration member is divided into a plurality of circular areas with different radiuses and each circular area is divided into a plurality of electrodes along its peripheral direction.  
           [0017]    It is another object of the present invention to provide a vibration member of a vibration wave driving apparatus comprising: the vibration member constituted of an elastic member and an electromechanical energy conversion element; and a rotor contacting the vibration member, the vibration member generating a travelling wave in the elastic member when an alternating signal is applied to the electromechanical energy conversion element, in which electrodes provided on opposing both sides of the electromechanical energy conversion element of the vibration member are formed in an identical shape and arranged in an identical phase. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a perspective view of an elastic member and a piezoelectric element in accordance with the present invention.  
         [0019]    [0019]FIG. 2 is a view showing an electrode pattern formed in a plurality of circular areas with different radiuses on a front surface of a piezoelectric element.  
         [0020]    [0020]FIG. 3 is a view showing an electrode film for applying an alternating voltage to the piezoelectric element shown in FIG. 2.  
         [0021]    [0021]FIG. 4 is a view showing a positional relationship of electrodes in the electrode pattern shown in FIG. 2.  
         [0022]    [0022]FIGS. 5A, 5B,  5 C and  5 D are views showing amplitudes of vibrations generated in the piezoelectric element shown in FIG. 2.  
         [0023]    [0023]FIG. 6 is a view showing an electrode pattern formed in a plurality of circular areas with different radiuses on a front surface of a piezoelectric element.  
         [0024]    [0024]FIG. 7 is a view showing an electrode pattern formed in a plurality of circular areas with different radiuses on a front surface of a piezoelectric element.  
         [0025]    [0025]FIG. 8 is a view showing an electrode pattern formed in a plurality of circular areas with different radiuses on a front surface of a piezoelectric element.  
         [0026]    [0026]FIG. 9 is a view showing an electrode pattern formed in a plurality of circular areas with different radiuses on a front surface of a piezoelectric element.  
         [0027]    [0027]FIGS. 10A and 10B are views showing electrode patterns that are formed on both sides of a piezoelectric element in an identical shape and an identical phase.  
         [0028]    [0028]FIGS. 8A and 11B are views showing electrode patterns that are formed on both sides of a piezoelectric element in an identical shape and an identical phase.  
         [0029]    [0029]FIG. 12 is a view showing a direction of an electric field at the time of polarization of the piezoelectric element of FIGS. 10A, 10B,  11 A and  11 B.  
         [0030]    [0030]FIG. 13 is a sectional view of a vibration wave motor.  
         [0031]    [0031]FIGS. 14A and 14B are views showing electrode patterns on both side of a conventional piezoelectric element.  
         [0032]    [0032]FIGS. 15A, 15B and  15 C are views showing amplitudes of vibrations generated in the piezoelectric element of FIGS. 14A and 14B.  
         [0033]    [0033]FIG. 16 is a view showing a direction of an electric field at the time of polarization of the conventional piezoelectric element. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]    The present invention is for raising a driving efficiency of a vibration wave driving apparatus by improving an electrode pattern of a piezoelectric element that is fixed to a vibration member  40  of the vibration wave driving apparatus.  
         [0035]    [0035]FIG. 1 shows a vibration member in accordance with an embodiment of the present invention.  
         [0036]    In the figure, reference numeral  10  denotes an elastic member, which is the same as a conventional elastic member. Reference numeral  21  denotes a piezoelectric element, which is different from the conventional piezoelectric element  20  in an electrode pattern for applying an alternating voltage when the vibration wave driving apparatus is driven.  
         [0037]    [0037]FIG. 2 shows a plan view of the piezoelectric element  21 . One side of the piezoelectric element  21  is divided into two circles with different radiuses. In each circle, positive electrodes (+) and negative electrodes (−) are formed which are polarized while being alternately reversed in their thickness direction. In FIG. 2, electrodes  201  to  206  on the left outer circumference side and electrodes  207  to  212  on the right inner circumference side form a first electrode group, and electrodes  301  to  306  on the right outer circumference side and electrodes  307  to  312  on the left inner circumference side form a second electrode group. Electrodes shown in FIG. 3 are formed on the piezoelectric element  21 . An alternating voltage is applied to the first electrode group from an electrode  22 A and an alternating voltage, whose phase is shifted by 90 degrees from the alternating voltage applied from the electrode  22 A, is applied to the second electrode group from an electrode  22 B. Wavelengths and amplitudes of these alternating voltages are equal. When these alternating voltages are applied, a fifth order standing wave vibration is generated in the first electrode group and the second electrode group. The electrodes  22 A and  22 B are formed on one side of the piezoelectric element  21  by, for example, screen printing and evaporation.  
         [0038]    [0038]FIG. 4 is a view showing a size and a positional relationship of the electrodes. The electrodes  202  to  205 , the electrodes  208  to  211 , the electrodes  302  to  305  and the electrodes  308  to  311  have a size that is ½ of a wavelength λ of the above-described standing wave in the peripheral direction of the piezoelectric element  21 . In addition, positions of the electrodes on the outer circumference side and positions of the electrodes on the inner circumference side deviate from each other in the peripheral direction by λ/4. However, in each electrode group, respective pairs of the electrodes  206  and  207 , the electrodes  306  and  307 , the electrodes  212  and  201  and the electrodes  312  and  301 , which are in positions where the inner circumference side and the outer circumference side change places, have the size of λ/2. Further, in order to equalize an area of the electrodes on the outer circumference side with an area of the electrodes on the inner circumference side, the electrodes are formed such that the electrodes located on the inner circumference side has a larger width in the radius direction of the piezoelectric element.  
         [0039]    Consequently, areas of the electrodes  202  to  205 , the electrodes  208  to  211 , the electrodes  302  to  305  and the electrodes  308  to  311  are equalized with each other. In addition, a total area of the electrodes  201  and  212 , a total area of the electrodes  206  and  207 , a total area of the electrodes  301  and  312  and a total area of the electrodes  306  and  307  are also equalized with each other. That is, the total areas of the first electrode group and the second electrode group are equal, and each electrode group can form an electrode having an equal area over the entire circumference.  
         [0040]    [0040]FIGS. 5A to  5 D show standing wave vibrations generated in the first and second electrode groups and a composited vibration of these standing wave vibrations. In the figure, the horizontal axis indicates a position in the peripheral direction of the piezoelectric element  21 .  
         [0041]    [0041]FIG. 5C shows an A phase standing wave vibration generated by the first electrode group and FIG. 5D shows a B phase standing wave vibration generated by the second electrode group. As described above, alternating voltages of an identical wavelength and an identical amplitude, whose phases are shifted by 90 degrees from each other, are applied to the first electrode group and the second electrode group, and the first electrode group and the second electrode group are arranged to deviate from each other in the peripheral direction by λ/4. When these standing wave vibrations are composited, a traveling wave vibration shown in FIG. 5B are obtained. This traveling wave vibration has a uniform wavelength and a uniform amplitude over the entire circumference of the piezoelectric element  21  and, as shown in FIG. 5A, loci of rotational movements generated on the surface of the elastic member  10  are uniform. Thus, slipping does not occur between the frictional member  30  of the elastic member  40  and the rotor  60 , and vibration energy can be efficiently utilized as driving energy. Therefore, a driving force is increased and, moreover, generation of noises due to unevenness of vibrations and deviated wear of a frictional member can be prevented.  
         [0042]    [0042]FIG. 6 shows a plan view of a piezoelectric element  22  that generates a third order standing wave vibration. Sixteen electrodes are formed on one side of the piezoelectric element  22 . The one side of the piezoelectric element  22  is divided into two circles with different radiuses. In each circle, positive electrodes (+) and negative electrodes (−) are formed which are polarized while being alternately reversed in their thickness direction. In FIG. 6, electrodes  400  to  403  on the left outer circumference side and electrodes  404  to  407  on the right inner circumference side form a first electrode group, and electrodes  500  to  503  on the right outer circumference side and electrodes  504  to  507  on the left inner circumference side form a second electrode group.  
         [0043]    Then, when alternating voltages of an identical wavelength and an identical amplitude, whose phases are shifted by 90 degrees from each other, are applied to the first electrode group and the second electrode group, two types of third order standing wave vibrations are generated in the piezoelectric element  22 . These two standing wave vibrations are composited to generate a traveling wave vibration on a surface of an elastic member.  
         [0044]    The electrodes  401 ,  402 ,  405  and  406  and the electrodes  501 ,  502 ,  505  and  506  have a size that is ½ of a wavelength X of the above-described standing wave in the peripheral direction of the piezoelectric element  22 . In addition, positions of the electrodes on the outer circumference side and positions of the electrodes on the inner circumference side deviate from each other in the peripheral direction by λ/4. However, in each electrode group, respective pairs of the electrodes  403  and  404 , the electrodes  503  and  504 , the electrodes  407  and  400  and the electrodes  507  and  500 , which are in positions where the inner circumference side and the outer circumference side change places, have the size of λ/2. Further, in order to equalize an area of the electrodes on the outer circumference side with an area of the electrodes on the inner circumference side, the electrodes are formed such that the electrodes located on the inner circumference side have a larger width in the radius direction of the piezoelectric element.  
         [0045]    Consequently, areas of the electrodes  401  and  402 , the electrodes  501  and  502 , the electrodes  405  and  406  and the electrodes  505  and  506  are equalized with each other. In addition, a total area of the electrodes  403  and  404 , a total area of the electrodes  503  and  504 , a total area of the electrodes  407  and  400  and a total area of the electrodes  507  and  500  are also equalized with each other. That is, the total areas of the first electrode group and the second electrode group are equal, and each electrode group can form an electrode having an equal area laterally.  
         [0046]    As a result, a traveling wave vibration generated on the surface of the elastic member  10  has a uniform wavelength and a uniform amplitude over the entire circumference of the piezoelectric element  22  and loci of rotational movements are uniform.  
         [0047]    [0047]FIG. 7 shows another embodiment of the present invention.  
         [0048]    A piezoelectric element  23  shown in FIG. 7 has an electrode pattern on one side that is different from that of the piezoelectric element  21  shown in FIG. 2.  
         [0049]    One side of the piezoelectric element  23  is also divided into two circles with different radiuses. In each circle, positive electrodes (+) and negative electrodes (−) are formed which are polarized while being alternately reversed in their thickness direction. However, the piezoelectric element  23  is different from the piezoelectric element  21  in that a first electrode group for generating an A phase standing wave vibration is arranged only on its outer circumference side and a second electrode group for generating a B phase standing wave vibration is arranged only on its inner circumference side.  
         [0050]    Each electrode has a size that is ½ of a wavelength λ of a standing wave vibration it generates in the peripheral direction. In addition, a phase of the electrodes on the outer circumference side and a phase of the electrodes on the inner circumference side deviate from each other by λ/4. Moreover, in order to equalize an area of the electrodes on the outer circumference side with an area of the electrodes on the inner circumference side, the electrodes are formed such that the electrodes located on the inner circumference side have a larger width in the radius direction of the piezoelectric element compared with the electrodes located on the outer circumference side.  
         [0051]    Then, when alternating voltages of an identical wavelength and an identical amplitude, whose phases are shifted by 90 degrees from each other, are applied to the first electrode group and the second electrode group, a traveling wave vibration is generated in the piezoelectric element  23 . Also, in this piezoelectric element  23 , this traveling wave vibration has a uniform wavelength and a uniform amplitude over the entire circumference of the piezoelectric element  23  and loci of rotational movements generated on the surface of the elastic member  10  are uniform.  
         [0052]    [0052]FIG. 8 shows yet another embodiment of the present invention.  
         [0053]    A piezoelectric element  24  shown in FIG. 8 has an electrode pattern on one side that is different from those of the piezoelectric elements  21  and  23  shown in FIGS. 2 and 7. Twenty-two electrodes are formed on the one side of the piezoelectric element  24 .  
         [0054]    The one side of the piezoelectric element  24  is also divided into two circles with different radiuses. In each circle, positive electrodes (+) and negative electrodes (−) are formed which are polarized while being alternately reversed in their thickness direction. In FIG. 8, electrodes  600  to  604  on the upper outer circumference side and electrodes  610  to  615  on the lower inner circumference side form a first electrode group, and electrodes  700  to  705  on the lower outer circumference side and electrodes  710  to  714  on the upper inner circumference side form a second electrode group. When alternating voltages of an identical wavelength and an identical amplitude, whose phases are shifted by 90 degrees from each other, are applied to the first electrode group and the second electrode group, a fifth order standing wave vibration is generated in each of the first electrode group and the second electrode group.  
         [0055]    The piezoelectric element  24  is different from the piezoelectric elements  21  and  23  in that positions where an inner circumference side and an outer circumference side on which the electrodes of each electrode group are formed change places (a position between the electrodes  600  and  700 , a position between the electrodes  610  and  710 , a position between the electrodes  604  and  705  and a position between the electrodes  614  and  715 ) are provided in positions to be nodes of a standing wave vibration such that it is possible to make a total area of electrodes large. Consequently, the number of slits between electrodes can be reduced and the total area of electrodes can be larger than that in the piezoelectric element  21  of FIG. 2. Further, in the piezoelectric element  24 , a total area of the first electrode group and a total area of the second electrode group are also identical. Also, in this piezoelectric element  24 , a traveling wave vibration generated on the surface of the elastic member  10  also has a uniform wavelength and a uniform amplitude over the entire circumference of the piezoelectric element  24  and loci of rotational movements are uniform.  
         [0056]    In addition, if an electrode pattern shown in FIG. 9 is formed as piezoelectric element for generating a third order standing wave vibration, a total area of electrodes can be larger than that in the electrode pattern shown in FIG. 6 due to the same reason as the electrode pattern shown in FIG. 8.  
         [0057]    Further, although a first electrode group and a second electrode group are formed on a piezoelectric element in the above-described embodiment, the present invention is not limited to this. For example, a front surface of a piezoelectric element may be divided into three circles with different radiuses to form a first electrode group, a second electrode group and a third electrode group or to form more electrode groups. In these cases, it is sufficient to form the electrode groups such that the number of electrodes in each electrode group, a total area of each electrode group and an arrangement pattern of each electrode group are equal.  
         [0058]    As described above, electrode groups for generating different standing wave vibrations are provided in a plurality of areas of a concentric circle shape with different radiuses, whereby unevenness of traveling wave vibrations generated in the elastic member  10  can be reduced.  
         [0059]    Next, another configuration of a piezoelectric element for reducing unevenness of traveling wave vibrations generated in an elastic member will be described.  
         [0060]    [0060]FIG. 10A shows an electrode pattern on a front surface of a piezoelectric element  26  and FIG. 10B shows an electrode pattern of a rear surface of the piezoelectric element  26 . The electrode pattern shown in FIG. 10A is identical with the electrode pattern shown in FIG. 14A. As it can be seen from FIGS. 10A and 10B, the electrode patterns on the front surface and the rear surface are formed such that their shapes and phases are completely identical. With this configuration, as shown in FIG. 12, all directions of electric fields applied to the part between the electrode pattern on the front surface and the electrode pattern on the rear surface at the time of polarization are in parallel with a thickness direction of the piezoelectric element  26 , and a polarization direction becomes uniform. Therefore, since an elastic modulus of the piezoelectric element  26  becomes uniform, unevenness of traveling wave vibrations generated when an alternating voltage is applied can be reduced.  
         [0061]    [0061]FIG. 11A shows an electrode pattern on a front surface of a piezoelectric element  27  and FIG. 11B shows an electrode pattern on a rear surface of the piezoelectric element  27 . The electrode pattern shown in FIG. 11A is for generating two standing wave vibrations of a wavelength λ and is formed at a pitch for one electrode of λ/4. In the electrode pattern, positive electrodes (+) for generating an A phase standing wave vibration, positive electrodes (+) for generating a B phase standing wave vibration, negative electrodes (−) for generating an A phase standing wave vibration and negative electrodes (−) for generating a B phase standing wave vibration are arranged in the peripheral direction in this order. When alternating voltages of an identical wavelength and an identical amplitude, whose phases are shifted by 90 degrees from each other, are applied to a first electrode group for generating an A phase standing wave vibration and a second electrode group for generating a B phase standing wave vibration, a traveling wave vibration is generated in the piezoelectric element  27 .  
         [0062]    The electrode pattern on the rear surface shown in FIG. 11B is formed such that its shape and phase are completely identical with those of the electrode pattern on the front surface. With this configuration, as shown in FIG. 12, all directions of electric fields applied to the part between the electrode pattern on the front surface and the electrode pattern on the rear surface at the time of polarization are in parallel with a thickness direction of the piezoelectric element  27 , and a polarization direction becomes uniform. Therefore, since an elastic modulus of the piezoelectric element  27  becomes uniform, unevenness of traveling wave vibrations generated when an alternating voltage is applied can be reduced.  
         [0063]    As described above, electrode patterns whose shapes and phases are both identical with each other are provided on a front surface and a rear surface of a piezoelectric element, whereby unevenness of traveling wave vibrations generated in the elastic member  10  can be reduced.  
         [0064]    In addition, electrode patterns are not limited to those shown in FIGS. 10A, 10B,  11 A and  11 B. For example, the electrode patterns shown in FIGS.  1  to  9  may be provided on both sides of a piezoelectric element.