Abstract:
The present invention is to provide a stacked type electro-mechanical energy conversion element comprising a stack of superimposed layers with an electro-mechanical energy conversion function having an electrode film formed on a superimposed surface so as to improve the driving efficiency thereof. A part of the electrode film is connected to an electrode film formed on an edge portion of a layer, and on a side face of the stacked electro-mechanical energy conversion element, there are provided a connection terminal connectable to an external power supply, and a wiring portion connecting the connection terminal to the electrode film formed on the edge portion of the layer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a stacked type electro-mechanical energy conversion element and a vibration wave driving apparatus of stacked structure consisting of a stack of a plurality of piezoelectric bodies and, more particularly, to a configuration for connecting electrodes between layers in the stacked type electro-mechanical energy conversion element.  
           [0003]    2. Related Background Art  
           [0004]    Piezoelectric elements having the electro-mechanical energy conversion function are used in various use applications. The piezoelectric elements are generally classified into the structure comprised of a single piezoelectric body of plate shape and the stacked structure comprised of multiple piezoelectric bodies of plate shape. The piezoelectric elements of the stacked structure can generate greater distortion with supply of a lower applied voltage than the piezoelectric elements of the structure comprised of only one piezoelectric body of plate shape.  
           [0005]    A stacked type piezoelectric element is comprised of a plurality of piezoelectric layers of piezoelectric ceramics and electrode films (hereinafter referred to as internal electrodes) provided on surfaces of the respective piezoelectric layers. For connecting the internal electrodes on the respective piezoelectric layers to each other, it is common practice to provide electrode portions disposed on an outer peripheral surface or an inner peripheral surface of the stacked type piezoelectric element (hereinafter referred to as external electrodes), or to provide through holes along the stack direction in the piezoelectric layers and provide through electrodes (through holes) formed by burying an electrode material in the through holes.  
           [0006]    [0006]FIGS. 5 and 6 show configurations of stacked type piezoelectric elements used in a vibration body of a rodlike vibration wave motor disclosed in U.S. Pat. No. 5,770,916.  
           [0007]    The internal electrodes  103  indicated by hatching are formed on the surfaces of the second and lower piezoelectric layers  102  in the stacked type piezoelectric element  101  shown in FIG. 5. The internal electrodes  103  are not formed on the outer peripheral edges of the piezoelectric layers  102 . In other words, the internal electrodes  103  are formed inside the outside diameter of the piezoelectric layers  102 . Further, the internal electrodes  103  are out of contact with each other. Connection electrodes  103   a  (black solid portions in the drawing) are formed on the outer peripheral edges of the piezoelectric layers  102 , and the connection electrodes  103   a  are in contact with the internal electrodes  103 .  
           [0008]    The internal electrodes  103  on the respective piezoelectric layers  102  are stacked so as to be aligned in identical phases, and the connection electrodes  103   a  are formed at identical positions on every other layer. Then the external electrodes  104  are formed at positions to be superimposed on the connection electrodes  103   a , on the outer peripheral surface of the stacked piezoelectric element  101 , so as to connect the connection electrodes  103   a  on every other layer. Namely, the internal electrodes  103  located in identical phases are arranged as electrically conductible on every other layer.  
           [0009]    A plurality of surface electrodes  105  are provided along the circumferential direction and in the phases matched with those of the connection electrodes  103   a , on the outer peripheral edge of the surface of the uppermost piezoelectric layer forming the stacked piezoelectric element  101 . The surface electrodes  105  are connected to the external electrodes  104 .  
           [0010]    On the other hand, FIG. 6 shows another stacked piezoelectric element  201 , in which the internal electrodes  203  are formed in structure similar to that shown in FIG. 5, on the surfaces of the piezoelectric layers  202  and in which the internal electrodes  203  are connected by the through electrodes (through holes)  204 . The through electrodes  204  are exposed at their ends in the surface of the uppermost piezoelectric layer of the stacked piezoelectric element  201 , thereby forming the surface electrodes  205 .  
           [0011]    Further, FIG. 7 shows an application example in which the aforementioned stacked piezoelectric element  101  of FIG. 5 is applied to the vibration body of the rodlike vibration wave motor. A wiring board  111  is kept in contact with the surface electrodes  105  of the stacked piezoelectric element  101 , and the stacked piezoelectric element  101  and the wiring board  111  are placed between hollow metal members  21  and  22  of the vibration body. A bolt  23  penetrating a center hole of the stacked piezoelectric element  101  is inserted from the side of the metal member  22  to be screwed into the metal member  21 . By tightening this bolt  23 , the stacked piezoelectric element  101  and wiring board  111  are pinched and fixed between the two metal members  21  and  22 . The wiring board  111  is connected to an unrepresented drive circuit and the drive circuit applies alternating voltages for driving, to the stacked piezoelectric element  101 .  
           [0012]    Likewise, in the case where the stacked piezoelectric element  201  shown in FIG. 6 is applied, the stacked piezoelectric element  201  and the wiring board  211  in contact with the surface electrodes  205  are also pinched and fixed between the metal members  21  and  22 . The stacked piezoelectric element  201 , which uses all the through electrodes as means for connecting the internal electrodes to each other, is incorporated into the vibration wave motor of the structure shown in FIG. 7, which is now under practical use as a driving source for driving a camera lens to effect autofocus.  
           [0013]    The principle of driving of the rodlike vibration wave motor is as follows: a plurality of different bending vibrations with a temporal phase difference are generated in the vibration body equipped with the stacked piezoelectric element to force the distal end of the metal member  21  forming the vibration body, to perform a motion like a swinging motion. This motion rotates a rotor  24  kept in press contact with the metal member  21 , through friction.  
           [0014]    An example of the structure of the internal electrodes suitable for such driving is the quartered internal electrodes, as shown in FIGS. 5 and 6. Let us suppose that these internal electrodes are phase A, phase B, phase AG, and phase BG in the circumferential direction. Two internal electrodes located in the positional relation of 180° (phase A and phase AG; and phase B and phase BG) are polarized in directions different from each other. These phases A to BG are formed so as to be identical among the second and lower piezoelectric layers, and the internal electrodes of identical phases on the different layers are electrically connected to each other by the aforementioned external electrodes or through electrodes.  
           [0015]    When with the phases AG and BG being ground a high-frequency voltage (alternating signal) having a frequency approximately equal to the natural frequency of the vibration body is applied to phase A and to phase B different 90° from the phase of phase A with a temporal phase difference between them, two bending vibrations perpendicular to each other are generated in the vibration body.  
           [0016]    The method of interposing the stacked piezoelectric element  101 ,  201  and the wiring board  111 ,  211  between the metal members  21  and  22  as described above is high in reliability of electrical conduction between the stacked piezoelectric element  101 ,  201  and the wiring board  111 ,  211 , and easy to assemble.  
           [0017]    However, since the wiring board  111 ,  211  is interposed between the metal members forming the vibration body of the vibration wave motor, this wiring board  111 ,  211  causes damping of the vibrations. For this reason, there is conceivably plenty of scope for improvement in drive efficiency.  
           [0018]    In addition, in the case of the stacked piezoelectric element  101  shown in FIG. 5, where the surface electrodes  105  are formed, for example, by inexpensive screen printing, the heights of the surface electrodes tend to become uneven. Therefore, more careful processing was needed.  
           [0019]    The stacked piezoelectric element  201  shown in FIG. 6 required a system for forming the through electrodes and took a lot of processing time.  
         SUMMARY OF THE INVENTION  
         [0020]    One of the features of the present invention is to provide a stacked type electro-mechanical energy conversion element comprising a stack of superimposed layers with an electro-mechanical energy conversion function having an electrode film formed on a superimposed surface, wherein part of the electrode film is connected to an electrode film formed on an edge portion of a layer,  
           [0021]    wherein on a side face of the stacked electro-mechanical energy conversion element there are provided a connection terminal connectable to an external power supply, and a wiring portion connecting the connection terminal to the electrode film formed on the edge portion of the layer.  
           [0022]    When this stacked electro-mechanical energy conversion element is applied to a vibration wave driving apparatus, the alternating signals can be applied from the side to the stacked electro-mechanical energy conversion element, which can obviate the need for interposing a circuit board between an elastic member forming an vibration body and the stacked electro-mechanical energy conversion element.  
           [0023]    Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a perspective view showing the appearance of a stacked type piezoelectric element according to the present invention;  
         [0025]    [0025]FIG. 2 is an exploded perspective view and an appearance perspective view of a stacked piezoelectric body  5  of FIG. 1;  
         [0026]    [0026]FIG. 3A is a development of a circuit board  6  of FIG. 1;  
         [0027]    [0027]FIG. 3B is a sectional view along a line  3 B- 3 B and in the direction indicated by arrows, of the circuit board  6  of FIG. 3A;  
         [0028]    [0028]FIG. 3C is an enlargement of part  3 C in FIG. 3B;  
         [0029]    [0029]FIG. 4 is a sectional view of a vibration wave motor using the stacked piezoelectric element of FIG. 1;  
         [0030]    [0030]FIG. 5 is an exploded perspective view and an appearance perspective view of a conventional stacked type piezoelectric element;  
         [0031]    [0031]FIG. 6 is an exploded perspective view and an appearance perspective view of another conventional stacked type piezoelectric element; and  
         [0032]    [0032]FIG. 7 is a sectional view of a vibration wave motor using the stacked piezoelectric element shown in FIG. 5 or  6 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    [0033]FIG. 1 is a perspective view showing the appearance of the stacked piezoelectric element  1  which is a stacked type electro-mechanical energy conversion element according to the present invention.  
         [0034]    The stacked piezoelectric element  1  is comprised of a stacked piezoelectric body  5  of ring shape provided with a hole portion in the central region and a circuit board  6  constructed of a flexible printed board and as a wiring layer provided on the outer peripheral surface of the stacked piezoelectric body  5 .  
         [0035]    [0035]FIG. 2 is an exploded perspective view of the stacked piezoelectric body  5 . The second and lower piezoelectric layers  2  of the stacked piezoelectric body  5  are of the structure similar to that of the piezoelectric layers  102  shown in FIG. 5. The internal electrodes  3  of the quartered structure are formed on the surfaces of a plurality of piezoelectric layers  2 . In other words, the piezoelectric layers  2  and the electrode films  3  are alternately superimposed in a stacked state. The number and shape of internal electrodes  3  are determined according to the number and mode of bending vibrations generated in the vibration wave motor, and thus the shape of the internal electrodes  3  is not limited to the quartered shape as shown in FIG. 2.  
         [0036]    Further, the connection electrodes  3   a  (black solid portions in the drawing) connected to the respective internal electrodes  3  and extending to the outer peripheral edges of the piezoelectric layers  2  are formed on the surfaces of the piezoelectric layers  2 . The connection electrodes  3   a  are formed in identical phases on every other layer, for example, with respect to the internal electrodes  3  of identical phases.  
         [0037]    Then the connection electrodes  3   a  of identical phases are connected by the external electrodes  4  which are interlayer electrodes provided on the outer peripheral portion of the stacked piezoelectric body  5 . Namely, the external electrodes  4  connect the connection electrodes  3   a  on every other layer. The two upper and lower end faces of the stacked piezoelectric body  5  are comprised of piezoelectric layers without the internal electrodes  3 , and only the external electrodes  4  are in an electrically conductible state with the internal electrodes  3  through the connection electrodes  3   a.    
         [0038]    The external electrodes  4  extend in the stack direction of the stacked piezoelectric body  5 , and in the present embodiment the external electrodes  4  are eight electrodes on the outer periphery of the stacked piezoelectric body  5 .  
         [0039]    Here the stacked piezoelectric body  5  employed in the present embodiment is one wherein the outside diameter is 6 mm, the inside diameter: 1.8 mm, the thickness: about 1.4 mm, the thickness of the piezoelectric layers 2:55 μm, the thickness of the internal electrodes 3:2 to 3 μm, and the number of piezoelectric layers 2:25. The external electrodes  4  have the length of about 1.35 mm, the width of about 1.5 mm, and the thickness of about 0.2 mm.  
         [0040]    The following will describe a method of producing the stacked piezoelectric body  5 . A silver-palladium powder paste for formation of the internal electrodes  3  was printed by screen printing on green sheets of piezoelectric ceramic powder and an organic binder for formation of the piezoelectric layers, and the sheets were superimposed in order and stacked under heat and pressure. Thereafter, the stack was baked at 1100° C.-1200° C. in a lead atmosphere. After the baking, the outer peripheral portion was machined, electrodes of silver with an adhesive were printed by screen printing to form the external electrodes, and they were hardened at about 80° C. Finally, a polarizing process was conducted to polarize the internal electrodes  3  in specific polarization directions for the respective electrode patterns.  
         [0041]    [0041]FIG. 3A is a development of the circuit board  6  of FIG. 1, and FIG. 3B a sectional view thereof along the line  3 B- 3 B and in the direction indicated by arrows in FIG. 3A.  
         [0042]    A substrate  9  as a base of the circuit board  6  is made, for example, of a polyimide resin in the thickness of 30 μm. A plurality of wires  8  extending in the circumferential direction corresponding to the respective external electrodes  4  are formed on the outer peripheral side of the circuit board  6 . These wires  8  are made, for example, of copper foil in the thickness of 25 μm, and are electrically connected to the external electrodes  4  through respective through electrodes  8   a  (through holes) penetrating the substrate  9  to be exposed on the inner peripheral side of the circuit board  6 . These wires  8  are gathered to external terminals  7  provided on the outer peripheral side of the circuit board  6 . The external terminals  7  are provided in the same number as the number of external electrodes  4  formed on the outer peripheral surface of the stacked piezoelectric body  5  and are arranged in a line along the stack direction.  
         [0043]    One end of each of the wires  8  is electrically connected to an external terminal  7  different from those to which the other wires are connected, and the other end of each of the wires  8  is electrically connected to a through electrode  8   a  different from those to which the other wires are connected. The number of external terminals can be reduced by providing one wire  8  for each different alternating signal and connecting the external electrodes to which an identical alternating signal is applied, by an identical wire  8 .  
         [0044]    [0044]FIG. 3C is an enlargement of part  3 C in FIG. 3B, which shows a configuration in which a cover coat  10  covers the outer peripheral surface of the circuit board  6 . This cover coat  10  has electrically insulating nature and is provided over the entire outer peripheral surface except for the position of the external terminals  7 . This ensures the electrical insulation for the outer peripheral surface except for the external terminals  7 .  
         [0045]    The circuit board  6  is flexible enough to be bent. While the through electrodes  8   a  and the external electrodes  4  are aligned with each other, the circuit board  6  is wound around the outer periphery of the stacked piezoelectric body  5 , whereby electrical conduction is established between the external terminals  7  and the external electrodes  4 . On that occasion, the stacked piezoelectric body  5  is fixed to the circuit board  6  with an adhesive and an electrically conductive adhesive is used for adhesion between the copper-plated through electrodes  8   a  and the external electrodes  4 , which can ensure the electrical conduction between the external electrodes  4  and the external terminals  7 .  
         [0046]    As described above, in the case of the stacked piezoelectric element  1  shown in FIGS. 1, 2 and  3 A to  3 C, the circuit board  6  is fixed to the outer peripheral portion of the stacked piezoelectric body  5  and the connection between the unrepresented driving circuit and the stacked piezoelectric element  1  is allowed to be made only on the outer peripheral portion of the stacked piezoelectric element  1 . This eliminates the need for interposing the wiring board between the metal members on the occasion of incorporating the stacked piezoelectric element into the vibration body of the vibration wave motor, different from the conventional structure. Namely, there occurs no damping of the vibrations due to the configuration wherein the circuit board is interposed between the metal members of the vibration body. The configuration of the present embodiment also eliminates stress on the circuit board due to the interposed configuration between the metal members, thereby enhancing the reliability of electrical connection between the stacked piezoelectric element and the driving circuit.  
         [0047]    The above described the configuration in which the circuit board  6  preliminarily made of a polyimide sheet was wound around the outer peripheral portion of the stacked piezoelectric body  5 , but it is also possible to form the circuit board in structure similar to the above, by forming the insulating and conductive layers on the outer peripheral surface (or inner peripheral surface) of the stacked piezoelectric body  5  by screen printing or the like with a resin paste and a metal paste.  
         [0048]    The internal electrodes  3  shown in FIG. 2 are not in contact with the outer peripheral edges of the piezoelectric layers  2 , but it is noted that the shape of the internal electrodes  3  is not limited to this. The internal electrodes  3  may be formed in other shape as long as the electrodes (the internal electrodes  3  and the connection electrodes  3   a ) to which the different alternating signals are applied are electrically insulated from each other.  
         [0049]    It is, however, necessary to secure the electrical insulation at the outer peripheral edges except for the external electrodes  4 , for example, by the substrate  9  of polyimide.  
         [0050]    [0050]FIG. 4 is a sectional view of a rodlike vibration wave motor of structure wherein a rodlike vibration body is constructed using the stacked piezoelectric element  1  shown in FIGS. 1, 2 and  3 A to  3 C and a rotor is rotationally driven by the vibration body.  
         [0051]    Only the stacked piezoelectric element  1  is placed between cylindrical metal members. A bolt  23  with a distal end of smaller diameter is inserted through a metal member  22  of an elastic member, the stacked piezoelectric element  1 , and a metal member  21  of an elastic member as well in order, so that a screw part of the bolt  23  is screwed into a screw portion formed in the inside circumference of the metal member  21 . By tightening this bolt  23 , the stacked piezoelectric element  1  is pinched and fixed between the metal members  21  and  22 .  
         [0052]    Since the through electrodes and surface electrodes are not formed on the layers at the ends of the stacked piezoelectric element  1 , surface flatness is ensured there, so that the stacked piezoelectric element  1  is securely pinched and fixed between the metal members.  
         [0053]    A flat cable or flexible circuit board  11  is soldered to the external terminals  7  of this stacked piezoelectric element  1  to connect the circuit board  11  to the unrepresented driving circuit. The driving circuit generates alternating signals and applies the alternating signals through the external terminals  7  to the internal electrodes  3  of the stacked piezoelectric element  1 .  
         [0054]    One end face of the metal member  21  serves as a driving portion, and a contact portion of rotor (moving body)  24  is in press contact with the driving portion. This contact portion of the rotor  24  has elasticity both in the axial direction and in the radial direction. A spring retainer  25  is provided in the internal peripheral portion of the rotor  24  and a spring  26  is placed in the inner peripheral portion of the spring retainer  25 . The rotor  24  is placed so as to rotate around the center of rotation at the distal end portion of the bolt  23  formed in the smaller diameter.  
         [0055]    The spring retainer  25  rotates together with the rotor  24  and transmits the spring force of the spring  26  to the rotor  24  to keep the contact portion of the rotor  24  in press contact with the driving portion of the metal member  21 .  
         [0056]    The spring retainer  25  engages with an output member  27  which is a gear to rotate together with the rotor  24 . The spring retainer  25  and the output member  27  are arranged as juxtaposed in the axial direction, and their engaging portions are not regulated in the thrust direction, but are regulated in the rotational direction.  
         [0057]    The output member  27  has a projection for receiving the spring  26 , in an inside diameter portion and thus also serves as a member for receiving the reaction force of the spring  26 .  
         [0058]    A securing member  28  for securing the vibration wave motor to an unrepresented mount portion is coupled to the distal end of the bolt  23 . The output member  27  is rotatably attached to a bearing  29  mounted on the outer peripheral portion of this securing member  28 .  
         [0059]    As described above, the use of the stacked piezoelectric element  1  shown in FIGS. 1, 2 and  3 A to  3 C eliminates the need for interposing the circuit board for applying the driving signals to the stacked piezoelectric element, between the metal member and the stacked piezoelectric element. Namely, since the stacked piezoelectric element  1  is directly interposed between the two metal members  21  and  22 , damping is very little in the vibrations of the vibration body of the rodlike vibration wave motor, which can enhance the performance of the motor.  
         [0060]    The vibration wave motor may be any other motor than the rodlike vibration wave motor described above. For example, it can be the vibration wave motor with the known vibration body wherein the stacked piezoelectric element is bonded to one side of a metal elastic body of ring shape or disk shape. In the case of the stacked piezoelectric element used in this vibration wave motor, the external electrodes  4  may be provided on the inner peripheral portion of the hollow stacked piezoelectric body  5 . In this case, the circuit board  6  is also placed on the inner peripheral portion of the stacked piezoelectric body  5 .