Abstract:
A piezoelectric actuator has a vibrating member having a generally central portion and a vibratable cantilever extending from the central portion. The cantilever has a first major surface, a second major surface opposite the first major surface, a first end integrally connected to the central portion, and a second free end. A piezoelectric element is connected to the second major surface of the cantilever and is driven by a voltage signal applied thereto to undergo expansion and contraction to vibrationally drive the cantilever. A moving member is in contact with the first major surface of the cantilever to be frictionally driven by vibration of the cantilevers in response to expansion and contraction of the piezoelectric element.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a piezoelectric actuator, and more particularly to a piezoelectric actuator which can insure high efficiency in movement conversion even when miniaturized and has improved construction of a vibrating body as well as of a movable body to insure stable rotational movement. 
     Generally the principle of operation of an ultrasonic motor based on the conventional technology can largely be classified as the standing wave system and the traveling wave system. At first, description is made for an ultrasonic motor based on the standing wave system. FIG. 9 is a perspective explanatory view showing an ultrasonic motor based on the conventional type standing wave system. This ultrasonic motor  900  comprises a vibrating body  910  with a piezoelectric element  912  for generating vibration in the longitudinal direction held between metallic bodies  911 , and a movable body  920 , and a vibrating piece  913  to be struck against the movable body  920  is provided on and projects from an edge of said vibrating body  910 . The vibrating piece  913  is inclined at a specified angle against a direction of the normal line to the movable body  920 . When a voltage having a frequency close to a resonance frequency of the vibrating body  910  is loaded, the vibrating body  910  vibrates, and the vibrating piece  913  provided at a tip thereof hits and contacts the movable body  920 . As the vibrating piece  913  is inclined at a specified angle against the movable body  920 , when the vibrating piece  913  hits the movable body  920 , displacement of the vibrating piece  913  in the longitudinal direction is partially converted to that in the lateral direction. Namely, movement of a tip of the vibrating piece follows an elliptical orbit. With this operation, the movable body  920  moves in one direction. 
     FIG. 10 is an explanatory view showing a principle of operation of another type of ultrasonic motor based on the standing wave system. This ultrasonic motor  1000  makes use of distorted vibration, and has a plurality of projecting sections  1011  provided at a position having a displacement component of movement in the same direction on a vibrating body  1010  (Refer to (c) in the figure). Also, a vertex section of this projecting section  1011  is inclined at a specified angle. The vibrating body  1010  has a piezoelectric element (not shown) having a plurality of electric poles. With this ultrasonic motor  1000 , a moving direction of the movable body is decided according to a position where the projection  1010  is provided. When a standing wave is generated by loading a voltage to the piezoelectric element ((a) and (b) in the figure), the projecting section  1011  having a displacement component in one direction as described above contacts a movable body  1020  ((c) in the figure). For this reason, the movable body  1020  moves in one direction described above. The movable body  1020  can be moved in the opposite direction by changing a position of the projecting section  1011  ((c) in the figure). 
     Next, description is made for an ultrasonic motor based on the traveling wave system. FIG. 11 is a perspective view showing an ultrasonic motor  1100  based on the conventional type of traveling wave system. The ultrasonic motor  1100  comprises a vibrating body  1110  comprised of a ring-shaped metallic body  1102  with a plurality of projecting sections  1101  formed in the peripheral direction and a piezoelectric ceramic element  1103  adhered to a bottom surface of the metallic body  1102 , and a ring-shaped movable body  1120  pressure-fitted to a surface of this vibrating body  1110 . A frictional member with excellent wearing-resistance is spanned over a contact section  1121  of the movable body  1120 . Also, as shown in FIG. 12, a plurality of driving electrodes  1104  corresponding to the projecting sections  1101  are provided in the piezoelectric ceramics  1103 . This ring-shaped ultrasonic motor  1100  is used as a lens actuator for automatically focusing, for instance, a single-lens reflex camera. 
     In this ultrasonic motor  1100 , distorted vibration is loaded to the vibrating body  1110  by controlling an amplitude of the polarized piezoelectric ceramic element  1103 , and exciting a traveling wave of the distorted vibration according to a phase difference of the loaded voltage. The distorted vibration of the traveling wave converts the movement in the vertical direction to movement in the lateral direction according to a thickness of the vibrating body  1110 , and gives an elliptical movement as shown in FIG. 13 to the projecting section  1101  of the vibrating body  1110 . The movable body  1120  is driven by the frictional force with the projecting section  1101  and moves. As height of the projecting section  1101  gives influence to displacement in the lateral direction, it is required to set a moving speed of the movable body  1120 . 
     Further as shown in FIG. 14, the ultrasonic motor based on the traveling wave may have a disk-shaped configuration. This ultrasonic motor  1200  has a vibrating body  1210  comprising a disk-shaped metallic body  1203  with a plurality of projecting sections  1201  formed in the peripheral direction, and a shaft holder  1202  located in the center, a piezoelectric element  1204  adhered to a bottom surface of the metallic body  1203 , and a disk-shaped movable body  1220  press-fitted to a surface of this vibrating body  1210 . The remaining configuration and the operation are the same as those of the ring-shaped ultrasonic motor  1100  shown in FIG. 14, so that description thereof is omitted herein. 
     In the ultrasonic motors  900 ,  1000 ,  1100 , and  1200  based on the conventional technology as described above, the rotationally moving capability is dependent on the bending rigidity of the vibrating body, the mass of the projecting section, and the mechanical machining precision and homogeneity of a tip of the projecting section contacting the movable body as well as on friction with a contacting surface of the movable body, so that it is difficult to improve and stabilize the movement conversion efficiency. For instance, in the disk-shaped ultrasonic motor  1100  based on the traveling wave system, there have been problems that the metallic body  1102  is hardly distorted because the metallic body  1102  is disk-shaped, the movement conversion efficiency is low and the rotation is sometimes unstable because the mass of the projecting section  1101  is large and, as a result, the projecting section can hardly be vibrated. 
     Also, if a diameter of the ultrasonic motor is made smaller, an oscillatory wave generated by the vibrating body  1110  has no vibration node, so that the oscillatory wave is easily attenuated as the diameter is made smaller. For this reason, the efficiency in conversion of movement from the piezoelectric element  1103  to the movable body  1120  becomes remarkably low, which makes it extremely difficult to reduce size of the ultrasonic motor. Also as the ultrasonic motor becomes smaller, influence of bending rigidity of the vibrating body  1110  becomes larger, so that miniaturization of ultrasonic motor is limited. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a piezoelectric actuator which can insure high movement conversion efficiency and stable rotational movement even when miniaturized as described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an assembly diagram showing a rotational type of actuator according to Embodiment 1 of the present invention; 
     FIG. 2 is an explanatory view showing detailed construction of the vibrating body block shown in FIG. 1; 
     FIG. 3 is a graph showing vibrating behaviors of a section from a fixed edge of a oscillator and a free edge thereof when an input signal is loaded; 
     FIG. 4 is an assembly diagram showing a rotational type of actuator according to Embodiment 2 of the present invention; 
     FIG. 5 is an assembly diagram showing a rotational type of actuator according to Embodiment 3 of the present invention; 
     FIG. 6 is an explanatory view showing a variant of the oscillator; 
     FIGS. 7A-7B are explanatory views showing other variants of the oscillator; 
     FIGS. 8A-8B are explanatory views showing other variants of the vibrating body block; 
     FIG. 9 is a perspective explanatory view showing an ultrasonic motor based on the conventional type standing wave system; 
     FIG. 10 is an explanatory view showing the principle of operation of another ultrasonic motor using the standing wave system; 
     FIG. 11 is a perspective explanatory view showing an ultrasonic motor based on the conventional type traveling wave system; 
     FIG. 12 is an explanatory view showing a driving electrode corresponding to the projecting section shown in FIG. 11; 
     FIG. 13 is an explanatory view showing an example of movement of the projecting section shown in FIG. 11; and 
     FIG. 14 is a perspective explanatory view showing a variant of the ultrasonic motor shown in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The piezoelectric actuator according to one embodiment has a rotationally movable body, and a vibrating body comprising a supporting body having a plurality of cantilevers each extending in a direction of a tangential line for an internal circle of the rotationally movable body with one edge thereof fixed and another edge thereof kept free and a plurality of piezoelectric elements adhered to the cantilevers of the supporting body, respectively. 
     The piezoelectric actuator according to another embodiment has a rotationally movable body, and a vibrating body comprising a supporting body having a cantilever extending in a direction of a tangential line for an internal circle of this rotationally movable body with one edge thereof fixed and another edge thereof kept free and a piezoelectric element adhered to the cantilever of the supporting body. 
     The piezoelectric actuator according to another embodiment has a rotationally movable body, and a vibrating body comprising a supporting body having a plurality of cantilevers each comprising long edge sections and short edge sections with the long edge sections extending in a direction of a tangential line for an internal circle of the rotationally movable body to form a free edge and also with the short edge sections fixed, and a plurality of piezoelectric elements adhered to each cantilever of the supporting body, respectively. 
     The piezoelectric actuator according to another embodiment has a rotationally movable body, and a vibrating body comprising a supporting body having a cantilever comprising long edge sections and short edge sections with the long edge sections extending in a direction of a tangential line for an internal circle of the rotationally movable body to form a free edge and also with the short edge sections fixed, and a piezoelectric element adhered to the cantilever of the supporting body. 
     The piezoelectric actuator according to another embodiment has a plurality of cantilevers extending in the opposite directions in the piezoelectric actuator described above. 
     Next, a detailed description is made for this invention with reference to the related drawings. It should be noted that the present invention is not limited to the embodiments described below. 
     EMBODIMENT 1 
     FIG. 1 is an assembly view showing a rotational type actuator according to Embodiment 1 of the present invention. This rotational type of actuator  100  comprises a rotationally movable body  1  having a shaft-projecting section  11 , a vibrating member or body block  2  having L-shaped oscillating elements or oscillators  21 , and a support member or board chassis  3  for supporting the shaft-projecting section  11 . The vibrating body block  2  has a central section  25 , a first major surface A, a second major surface B opposite the first major surface, and side edges C extending between the first and second major surfaces. A magnetic material generating a magnetically attracting force is used for the rotationally movable body  1  as well as for the board chassis  3 , and the rotationally movable body  1  and the vibrating body block  2  are contacted to each other under a constant pressure. For instance, a stainless-based magnetic material is used for the rotationally movable body  1 , while a neodymium-based magnetic material is used for the board chassis. The board chassis  3  is attracted by this magnetic material to the side of the rotationally movable body, so that stable adhesion between a sliding section  12  of the rotationally movable body  1  and the oscillator  21  of the vibrating body block  2  is obtained. Also it is preferable to use a non-magnetic material for the vibrating body block  2 , especially for the oscillator  21  so that influence by magnetism generated by the magnetic material is prevented. 
     The shaft-projecting section  11  of the rotationally movable section  1  is pivotally supported in a hollow bearing hole  22  of the vibrating body block  2 . On the contrary, a hollow bearing hole may be provided in the side of the rotationally moving body with a shaft-projecting section provided in the side of the vibrating body block. The sliding section  12  of the rotationally movable body  1  is a section in which friction with the oscillator  22  is generated, so that it is preferable to use a material satisfying such requirements as a large frictional coefficient, high wearing-resistance, and the capability to maintain a stable frictional coefficient. For instance, a basic body of the rotationally movable body  1  is formed with a metal-based or resin-based material, and the sliding section  12  is coated with an oxide film. Also the sliding section may be made from such materials as cellulose-based fiber, a carbon-based fiber, a hybrid material between whisker and phenol resin, or a hybrid material between a polyimide resin and a polyamide resin. 
     FIG. 2 shows the detailed structure of the vibrating body block  2 . Three units of the oscillator  21  are provided at an even space, and each of the sections has a structure in which a piezoelectric element  23  generating extending and shrinking movement is adhered to a supporting body  24 . The oscillator  21  has an L-shaped form, and a short edge section of this L-shaped form is fixed to a central section  25  of the vibrating body block  2 . This oscillator  21  is positioned in an orientation identical to a direction of a tangential line for a circle included inside the sliding section  12 . A rotational direction of the rotationally movable body  1  is decided according to a direction of the elliptical movement drawn by a free edge section of this oscillator  21 . Herein, as the rotationally movable body  1  is pivotally supported, by matching an orbit of movement of the oscillator  21  to an orbit of movement of the rotationally movable body  1 , movement conversion from the oscillator  21  to the rotationally movable section  1  can efficiently be executed. 
     In the figure, the oscillator  21  is provided projecting clockwise, but if the oscillator  21  is provided counterclockwise, the rotationally movable section  1  is rotated in the contrary direction. A rotational torque and a rotational speed of the rotary type actuator are decided according to a distance from a rotational center of the oscillator  21  as well as to a form and the number of the oscillators  21  provided in the vibrating body block  2 . A position of the oscillator  21 , namely a distance from a rotational center of the rotationally movable section  1  to the oscillator  21  or a form and the number of oscillators  21  are set according to specifications of the required rotational type actuator. 
     The piezoelectric element  23  is a component having a distortion generating function, a resonating function, and a voltage generating function. Namely the component generates stress or displacement according to a loaded voltage, generates resonance to a frequency of the loaded voltage, and generates a voltage according to the loaded pressure. Piezoelectric ceramics as a representative example of the piezoelectric element  23  includes barium titanate, lithium niobate, lead zircontitanate. Also a material having a tilting function or lithium niobate may be used in place of the piezoelectric ceramics. For the vibrating body block, a metal-based or a non-metal-based elastic material such as stainless steel, bellium copper, phosphorous bronze, brass, duralumin, titanium, or silicon is used. 
     It is preferable to use the photo fabrication technology such as etching for forming the rotationally movable body  1  or the vibrating body block  2 . By using the non-mechanical machining process, it is possible to exclude deformation, stress, or mechanical stress generated when mechanically machined. Also by using high-precision parts, it is possible to miniaturize the number of processes for assembling and adjusting each component and to stabilize the functions and reproducibility. 
     Also the supporting body  24  and the piezoelectric element  23  are integrated to each other by means of adhesion. To realize the adhesion as described above, it is required that the adhesion layer be very thin, that the adhesion layer be very rigid and tough, and that a resistance value around the resonance frequency be very small after the piezoelectric element and the supporting body are adhered to each other. For instance, a high polymer adhesive as represented by hot-melt and epoxy resin is used for the adhesion. It should be noted that the piezoelectric element  23  is directed linked without using an adhesive. Also the piezoelectric element  23  may be provided by means of thin film formation or a pressurized-film formation. As the oscillator  21 , there are the uni-morph type oscillator comprising one sheet of piezoelectric element  23 , the bi-morph type oscillator comprising two sheets of piezoelectric elements, or the multi-morph type oscillator comprising four or more sheets of piezoelectric elements, and any type of the mechanism section may be used. A material for the piezoelectric element  23  or the supporting body  24  and a method of adhering the material is decided according to a displacement rate, a power, responsibility of the oscillator  21 , and structural requirements required for the rotational type of actuator  100 . 
     More specifically, in the vibrating body block  2  shown in FIG. 2, the oscillator  21  is formed with three units of piezoelectric elements  23 , and the oscillators  21  are positioned at an even space along an external periphery of the vibrating body block  2 . The diameter is around several millimeters. The supporting body  24  for the oscillator  21  is formed by etching a copper-based material having a thickness of around 100 microns. A thin film made from zircontitanate lead having a high piezoelectricity constant is used for the piezoelectric element  23 . Even in a case of direction junction or that with an adhesive, there is a junction interface between the supporting body  24  and the piezoelectric element  23 . This junction interface is an important factor for deciding the propagation characteristics between the supporting body  24  and the piezoelectric element  23 . For that purpose, control of the characteristics of the adhesive to be used and its film thickness is important. In this example, an epoxy-based adhesive is used to obtain the optimal film thickness. 
     The form of the oscillator  21  is set taking into account the fact that the effective length, especially a length from the fixed edge to the free edge, relates to a displacement rate in movement in the longitudinal direction as well as in the elliptical orbit. Also a frequency of vibration specified to each oscillator  21  is dependent on the form, so that the parameter is decided to match the specifications from a result of simulation modeling and experimental data. The form of the vibrating body block  2  in this embodiment was decided according to a diametrical dimension of the required rotational type actuator  100  and load conditions for the rotationally movable body  1 . 
     Also the uni-molf type oscillator  21  was employed in this embodiment 1. This type of oscillator has hardly hysteresis in the displacement voltage characteristics. The further reason is that, although a displacement rate in this type is smaller as compared to that in the bi-molf type, the generated power is larger and a load and a pressuring force to the rotationally movable body are appropriate. Also by employing the multi-molf type according to specifications of a rotary type actuator  100  to maintain the thickness at a constant level, the displacement and power can be increased by increasing the number of layers. Also the responsibility can be improved by providing a taper in a section from a fixed edge of the oscillator  21  to a free edge thereof. With the vibrating body block  2  having the configuration as described above, it is possible to excite bending and displacement by the oscillator  21  under extremely stable conditions. It should be noted that a position, a form, quantity, and configuration of the oscillator  21  of the vibrating body block  2  are not always limited to those shown in FIG.  2 . 
     FIG. 3 shows vibrating behaviors of a section from a fixed edge  26  of the oscillator  21  to a free edge  27  thereof when an input signal is loaded thereto. A distance from a right edge of the lateral axis to a left edge is an effective length from the fixed edge  26  of the oscillator  21  to the free edge  27  thereof. The vertical axis shows a displacement rate. A displacement rate of zero (0) indicates that vibration has not been excited. The oscillator  21  generates vibration in which minute displacement and power coexist according to load conditions of an input signal to excite movement in the longitudinal direction as well as that in the elliptical orbit. In the vibrating mode, displacement of the oscillator  21  is in the positive side, so that movement is delivered from the oscillator  21  to the sliding section  12  of the rotationally movable body  1 . A rotational direction of the rotationally movable body  1  is decided according to a lateral component of movement following the elliptical orbit. For this reason, the oscillator  21  of the vibrating body block  2  is positioned according to a rotational direction of the rotationally movable body  1 . Also whether the rotationally movable body  1  rotates clockwise or counterclockwise is decided according to an orientation of the oscillator  21 . 
     A loaded voltage and a frequency thereof to be inputted into the oscillator  21  are adjusted so that the parameters match dimensions and a form of the oscillator  21 . When a frequency of an input signal is set to a value close to the resonance frequency, the maximum frequency of the oscillator  21  can be obtained. For vibrating the vibrating body block  2 , a moving mechanism in either the primary vibration mode for enlarging displacement in which a longitudinal direction of the oscillator  21  is used or in the secondary or a higher-dimensional vibration mode can effectively be used. As a result of experiment, each vibration mode of the oscillator  21  could be confirmed in a voltage amplitude range of an input signal from 0.5 to several tens volts and in a frequency range from several tens to several hundreds K hertz. To excite rotational movement of the vibrating body block  2  under stable conditions, it is preferable to use the secondary or higher-dimensional vibration mode assuming that the voltage amplitude is in a range from 0.5 to 7 volts and the frequency is in a range from 2 to 3 hundreds K hertz. Further the rotationally movable body  1  can be rotated under stable conditions by making use of a phase difference of the input signal, by controlling a duty, or by making use of the multiplex vibration mode. 
     EMBODIMENT 2 
     FIG. 4 is an assembly diagram showing a rotational type actuator according to Embodiment 2 of the present invention. This rotational type actuator  200  comprises a rotationally movable body  201  and a vibrating body block  2 . Also provided on a board chassis  203  are J-shaped spring pressurizing section  204 , and the rotational movable body  201  and the vibrating body block  2  are pressurized and maintained by this spring pressurizing section  204 . In addition to a sheet spring, a plate spring or a coil spring may be used for the spring pressurizing section  204 . The rotationally movable body  201 , vibrating body block  2 , and board chassis  203  are pivotally supported by a supporting shaft  205 . The sliding section  212  between the supporting shaft  205  and the rotationally movable body  201  is subjected to high-precision milling and fluorine surface processing in order to make a frictional coefficient small. Also as a mechanical element for pivotally supporting the rotationally movable body  201 , a ball bearing or a sliding bearing may be used. It should be noted that the rotationally movable body  201  may be rotatably supported at the external periphery in place of pivotally supporting a center of the rotationally movable body  201 . It should be noted that a method of manufacturing this rotationally movable body  201  and a material thereof are the same as those of the rotational type actuator  100  according to Embodiment 1 and the description thereof is omitted herein. Also as operations this rotational type of actuator  200  are substantially the same as those of the rotational type actuator  100  according to Embodiment 1, description thereof is omitted herein. With this configuration, output from the actuator can be controlled by adjusting a pressuring force of the spring pressuring section  204 . 
     EMBODIMENT 3 
     FIG. 5 is an assembly diagram showing the rotational type actuator according to Embodiment 3 of the present invention. This rotational type actuator  300  comprises a disk-shaped vibrating body block  302  having one oscillator  321 , a cylindrical body  322  provided around this vibrating body block  302 , and a rotationally movable body  301  placed inside this cylindrical body  322  and on an upper surface of the vibrating body block  302 . A material and a manufacturing method of the vibrating body block  302  and the rotationally movable body  301  are substantially the same as those of the rotational type actuator  100  according to Embodiment 1, so that description thereof is omitted herein. The oscillator  321  has an L-shaped form and is fixed at the short edge section. A sliding section  312  is provided in the spanning state on a bottom surface of the rotationally movable body  301 . The oscillator  321  is positioned in a direction of a tangential line for a circle included in the sliding section  312 . The oscillator  321  comprises a supporting body  324  and a piezoelectric element  323  adhered to the bottom surface thereof. Also operations of this rotational type of actuator  300  are substantially the same as those the rotational type of actuator  100  according to Embodiment 1, so that description thereof is omitted herein. There is the advantage that this rotational type actuator  300  can easily be built. 
     EMBODIMENT 4 
     The oscillator  21  in Embodiment 1 described above may be formed into a T-shaped form. FIG. 6 shows the oscillator  21  according to Embodiment 1 formed into a T-shaped form. In this oscillator  421 , one edge section  426  of the T-shaped form functions as a fixed edge, and a piezoelectric element  23  is adhered to another edge section  427  thereof. 
     Also, the oscillator may be formed into a π-shaped form. The oscillator  21  according to Embodiment 1 formed into a π-shaped form is shown in FIG.  7 A. In this oscillator  521 , a supporting body  524  is supported with two leg sections  525 . A piezoelectric element  23  is adhered to the supporting body  524 . The oscillator  321  according to Embodiment 3 formed into a π-shaped form is shown in FIG.  7 B. Also in this oscillator  621 , the supporting body  624  is supported with two leg sections  625 . The piezoelectric element  23  is adhered to the supporting body  624 . 
     Also, extending directions of the oscillators may be located in the opposite directions. A case where the two units of oscillator  21  are provided in the opposite directions is shown in FIG. 8B. A case where oscillators  21  are provided in the directions opposite to each other is shown in FIG.  8 A. Other portions of the configuration are substantially the same as those in Embodiment 1. By providing extending directions of the oscillators  21  in the direction opposite to each other like those in the vibrating body blocks  802 ,  803 , a rotational type actuator can be rotated in a regular direction and in a reverse direction by vibrating either one of the oscillators  21 . 
     As described above, the piezoelectric actuator according to the present invention has a rotationally movable body, and a vibrating body comprising a supporting body having a cantilever extending in a direction of a tangential line for an internal circle of this rotationally movable section with one edge thereof fixed and another edge thereof kept free and a plurality of piezoelectric elements adhered to the cantilevers of the supporting section, so that the movement conversion efficiency between the vibrating body and the rotationally movable body is improved. Also it is possible to provide a smaller and thinner actuator. 
     The piezoelectric actuator according to the present invention has a rotationally movable body, and a vibrating body comprising a supporting body having a cantilever comprising long edge sections and short edge sections with the long edge section extending in a direction of a tangential line for an internal circle of the rotationally movable body and functioning as a free edge and also with the short edge section fixed and piezoelectric elements adhered to cantilevers of the supporting body, so that excellent movement conversion efficiency is insured even when made smaller and thinner and also stable rotational movement can be obtained. 
     In the piezoelectric actuator according to the present invention, a plurality of cantilevers extend in the opposite directions, so that it is possible to realize an actuator having a simple construction and capable of rotating in both regular and reverse directions. Also the excellent movement conversion efficiency is maintained even when made smaller and thinner and stable rotational movement can be obtained.