Abstract:
An ultrasonic motor has a stator and a rotor which is press fit to the stator and rotates in accordance with the vibration of the stator. The stator includes a pair of metal blocks, a piezoelectric element located between the metal blocks, a tightening member and a positioning member. The tightening member is inserted through the metal blocks and the piezoelectric element to tighten the metal blocks and the piezoelectric element in the axial direction. The positioning member determines the radial position of the metal blocks. This reduces misalignment of metal blocks and the tightening member, and the stator for the ultrasonic motor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an ultrasonic motor and a stator for an ultrasonic motor. 
     As shown in FIG. 4, a typical progressive wave type (or bolted Langevin type) ultrasonic motor includes a stator  51  and a rotor  52 . The stator  51  includes first and second blocks  53 ,  54 , which are made of metal, first and second piezoelectric elements  55 ,  56 , first to third electrode plates  57  to  59 , and a tightening member, which is a bolt  60 . The first and second blocks  53 ,  54 , the first and second piezoelectric elements  55 ,  56 , and the first to third electrode plates  57  to  59  are piled in layer to form a substantially columnar shape. The first and second blocks  53 ,  54  are tightened by the bolt  60 , which is inserted through the first and second blocks  53 ,  54  in the axial direction. This couples the first and second blocks  53 ,  54 , the first and second piezoelectric elements  55 ,  56 , and the first to third electrode plates  57  to  59 . 
     A slit, which is not shown, is formed at the outer circumference of the lower portion of the stator  51 , or the outer circumference of the second block  54 . The slit generates torsional vibration based on the axial vibration. 
     The rotor  52  is substantially cylindrical and is rotatably pressed against the upper surface of the stator  51 , or the upper surface of the block  53 , by a pressing mechanism, which is not shown. 
     When high-frequency voltage is applied to the first to third electrode plates  57  to  59 , the first and second piezoelectric elements  55 ,  56  generate axial vibration. Then, the torsional vibration is generated at the slit of the second block  54 . The axial vibration of the stator  51  causes levitation force, and the torsional vibration causes driving force. The levitation force and the driving force cause the rotor  52  to rotate. 
     The first and second blocks  53 ,  54  are assembled by tightening the male screw of the bolt  60  to the female screws of the first and second blocks  53 ,  54 . This determines the positions of the first and second blocks  53 ,  54  in the radial direction. Since the positions of the first and second blocks  53 ,  54  and the bolt  60  are determined only by the male and female screws, the first and second blocks  53 ,  54  could be misaligned. Therefore, manufacturing deviations are caused per stator  51 , which causes variations in the rotational characteristics (such as frequency-rotational speed characteristic, voltage-torque characteristic, and the like) per product. Therefore, the rotor  52  cannot be used for purposes in which a high-accuracy rotational control is required (such as for rotating a drum in a copying machine). This limits the field of application of the ultrasonic motor. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide an ultrasonic motor that has a simple structure and reduces misalignment of metal blocks and a tightening member, and a stator for the ultrasonic motor. 
     To achieve the above objective, the present invention provides an ultrasonic motor, which includes a stator and a rotor. The stator includes a pair of metal blocks, a piezoelectric element, a tightening member, and a positioning member. The piezoelectric element is located between the metal blocks. When drive voltage having a predetermined frequency is applied to the piezoelectric element, the piezoelectric element vibrates the stator. The tightening member is inserted through the metal blocks and the piezoelectric element to tighten the metal blocks and the piezoelectric element in the axial direction. The positioning member determines the radial position of the metal blocks. The rotor is press fit to the stator and rotates in accordance with the vibration of the stator. 
     The present invention also provides a stator located in an ultrasonic motor, which includes a pair of metal blocks, a piezoelectric element, a tightening member, and a positioning member. The piezoelectric element is located between the metal blocks. When drive voltage having a predetermined frequency is applied to the piezoelectric element, the piezoelectric element vibrates the stator. The tightening member is inserted through the metal blocks and the piezoelectric element to axially tighten the metal blocks and the piezoelectric element. The positioning member determines the position of the metal blocks in the radial direction. 
    
    
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a partial cross-sectional view illustrating an actuator according to one embodiment of the present invention; 
     FIG.  2 ( a ) is a cross-sectional view illustrating metal blocks mounted to the actuator shown in FIG. 1; 
     FIG.  2 ( b ) is a cross-sectional view illustrating a tightening member and an insulated collar mounted to the actuator shown in FIG. 1; 
     FIG.  3 ( a ) is a cross-sectional view of metal blocks according to a modified embodiment; 
     FIG.  3 ( b ) is a cross-sectional view illustrating a tightening member and an insulated collar according to the modified embodiment; and 
     FIG. 4 is a partial cross-sectional view illustrating an ultrasonic motor according to a prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An actuator  1  according to a preferred embodiment of the present invention will now be described with reference to FIGS. 1 to  2 ( b ). The actuator  1  has a housing  2  and a progressive wave type ultrasonic motor  3 . 
     The housing  2  includes first and second housing members  4 ,  5 . The first housing member  4  is substantially cylindrical. First screw holes  4   a  (only one is shown in FIG. 1) are formed at the distal end (upper end as viewed in FIG. 1) of the first housing member  4 . Second screw holes  4   b  (only two are shown in FIG. 1) are formed at the proximal end (lower end as viewed in FIG. 1) of the first housing member  4 . 
     The second housing member  5  includes a cylindrical portion  5   a , an extended portion  5   b , which extends radially outward from the distal end (upper end as viewed in FIG. 1) of the cylindrical portion  5   a , and an annular projection  5   c , which projects inward from the axially middle portion of the cylindrical portion  5   a . The outer diameter of the cylindrical portion  5   a  is substantially the same as the inner diameter of the distal end of the first housing member  4 . Threaded through holes  5   d  are extend axially through the extended portion  5   b  at portions corresponding to the first screw holes  4   a  of the first housing member  4 . Screw holes  5   e  (only one is shown in FIG. 1) are formed in the extended portion  5   b  for securing the extended portion  5   b  to an external member. 
     The second housing member  5  is secured to the first housing member  4  by screws  6 , which are screwed to the first screw holes  4   a  through the threaded through holes  5   d.    
     First and second ball bearings  7 ,  8  are located inside the cylindrical portion  5   a  of the second housing member  5 . The first ball bearing  7  is located such that the outer ring of the first ball bearing  7  is located between the distal end of the cylindrical portion  5   a  and the annular projection  5   c . The second ball bearing  8  is located such that the outer ring of the second ball bearing  8  is located between the proximal end of the cylindrical portion  5   a  and the annular projection  5   c.    
     A rotary shaft  9  is supported by the first and second ball bearings  7 ,  8 . The rotary shaft  9  has a flange  9   a , which extends radially outward of the rotary shaft  9 . A protrusion  9   b , on which engaging grooves are formed, is located at the proximal end of the rotary shaft  9 . A rotor  10 , which is substantially columnar and forms part of the ultrasonic motor  3 , is secured to the protrusion  9   b . The rotor  10  does not rotate relative to the protrusion  9   b.    
     A first disk  11  is secured to the rotary shaft  9  with a nut  12 . An engaging projection  9   c , which has a rectangular cross-section as viewed in a direction perpendicular to the axis, is formed at the distal end of the rotary shaft  9 . The engaging projection  9   c  is coupled to a member S, which is located at the output side for an external device. A first conical spring  14  is located between the proximal end surface of the first disk  11  and the inner ring of the first ball bearing  7 . On the other hand, a second conical spring  15  is located between the proximal end surface of the flange  9   a  and the distal end surface of the rotor  10 . The first and second disk springs  14 ,  15  are compressed. The rotary shaft  9 , the rotor  10 , and the first disk  11  are axially movable in a predetermined range by the force of the first and second disk springs  14 ,  15 . The rotary shaft  9 , the rotor  10 , and the first disk  11  are located at a substantially middle position within the predetermined movable range. 
     A stator  21 , which constitutes the ultrasonic motor  3  with the rotor  10 , is secured to the first housing member  4 . 
     The stator  21  includes a first block  22  (see FIG.  2 ( a )), a second block  23  (see FIG.  2 ( a )), first and second piezoelectric elements  24 ,  25 , first to third electrode plates  26  to  28 , a tightening member  29  (see FIG.  2 ( b )), and an insulated collar  30 . 
     The first and second blocks  22 ,  23  are made of conductive metal, which is aluminum alloy in the preferred embodiment. 
     As shown in FIG.  2 ( a ), the first block  22  is substantially cylindrical. A horn  22   a  is formed at the upper portion of the first block  22  for amplifying vibration generated at the upper surface of the first block  22 . The inner diameter of the horn  22   a  is greater than the inner diameter of a portion of the first block  22  where the horn  22   a  is not formed. A female screw  22   b , which defines an insertion hole, is formed on the inner circumference of the first block  22 . 
     A first positioning fitting surface  22   c , which serves as a positioning surface, is formed at the lower end of the first block  22 . The diameter of the first positioning fitting surface  22   c  is greater than that of the female screw  22   b . A first collar fitting surface  22   d , which serves as a large diameter portion, is formed at the lower end of the first positioning fitting surface  22   c . The diameter of the first collar fitting surface  22   d  is greater than that of the first positioning fitting surface  22   c . A thin friction material  31  is attached to the upper surface of the first block  22 . 
     As shown in FIG.  2 ( a ), the outer diameter of the substantially cylindrical second block  23  is substantially the same as that of the first block  22 . An annular supporter  23   a , which extends radially outward, is formed on the outer circumferential surface of the second block  23 . A female screw  23   b , which defines an insertion hole, is formed on the inner circumference of the second block  23 . 
     A second positioning fitting surface  23   c , which serves as a positioning surface, is formed at the upper end of the inner circumference of the second block  23 . The diameter of the second positioning fitting surface  23   c  is greater than that of the female screw  23   b  and the same as that of the first positioning fitting surface  22   c  of the first block  22 . A second collar fitting surface  23   d , which serves as a large diameter portion, is formed at the upper end of the second positioning fitting surface  23   c . The diameter of the second collar fitting surface  23   d  is greater than that of the second positioning fitting surface  23   c.    
     Slits (recesses), which are not shown, are formed on the outer circumferential surface of the second block  23  above the supporter  23   a  for generating torsional vibration based on the axial vibration. Each slit is formed along the circumferential direction and are inclined with respect to the axial direction. 
     The first and second piezoelectric elements  24 ,  25  are disk-shaped. A through hole is formed at the center of each of the first and second piezoelectric elements  24 ,  25 . The inner diameters of the first and second piezoelectric elements  24 ,  25  are substantially the same as the diameter of the second collar fitting surface  23   d  (see FIG.  2 ( a )). 
     The first to third electrode plates  26  to  28  are disk-shaped. A through hole is formed at the center of each of the first to third electrode plates  26  to  28 . The inner diameters of the first to third electrode plates  26  to  28  are substantially the same as the diameter of the first collar fitting surface  22   d  of the first block  22  (see FIG.  2 ( a )). 
     As shown in FIG.  2 ( b ), male screws  29   a ,  29   b  are formed on the outer circumferential surface of the tightening member  29 , which is substantially columnar. The male screws  29   a ,  29   b  are screwed to the female screws  22   b ,  23   b , respectively. A columnar body  29   c , which serves as a positioning member, is formed at the middle of the tightening member  29 . The outer circumferential surface of the columnar body  29   c  engages with the first and second positioning fitting surfaces  22   c ,  23   c  in the radial direction to determine the position of the tightening member  29 . The diameter of the columnar body  29   c  is greater than the diameters of the male screws  29   a ,  29   b . The axial length H 1  of the columnar body  29   c  is slightly less than the distance H 2  between the first and second positioning fitting surfaces  22   c ,  23   c  when the first and second piezoelectric elements  24 ,  25  and the first to third electrode plates  26  to  28  are located between the first and second blocks  22 ,  23 . In FIG.  2 ( a ), the distance H 2  between the first block  22  and the second block  23  represents the actual distance when the first and second piezoelectric elements  24 ,  25  and the first to third electrode plates  26  to  28  are located between the first and second blocks  22 ,  23 . The columnar body  29   c  may be formed separately from one of the male screws  29   a ,  29   b.    
     The insulated collar  30  is cylindrical and is formed of insulating resin. The outer diameter of the insulated collar  30  is substantially the same as the diameters of the first and second collar fitting surfaces  22   d ,  23   d . The insulated collar  30  is fitted to the first and second collar fitting surfaces  22   d ,  23   d.    
     As shown in FIG. 1, the second block  23 , the third electrode plate  28 , the second piezoelectric element  25 , the second electrode plate  27 , the first piezoelectric element  24 , the first electrode plate  26 , and the first block  22  are piled on one another in this order and are tightened by the tightening member  29 , which extends through the piled components in the axial direction. More specifically, each of the first and second blocks  22 ,  23  is screwed to the fixed tightening member  29  from the corresponding end. The components are tightened together when the female screws  22   b ,  23   b  and the male screws  29   a ,  29   b  are screwed to each other. The columnar body  29   c  of the tightening member  29  is fitted to and radially engaged with the first and second positioning fitting surfaces  22   c ,  23   c  (see FIG.  2 ( a )) of the first and second blocks  22 ,  23 . This determines the radial position of the first and second blocks  22 ,  23  and the tightening member  29 . The insulated collar  30  is fitted to the middle portion of the columnar body  29   c  of the tightening member  29  and fitted in the first and second piezoelectric elements  24 ,  25  and the first to third electrode plates  26  to  28 . The ends of the insulated collar  30  are fitted in the first and second collar fitting surfaces  22   d ,  23   d  of the first and second blocks  22 ,  23  (see FIG.  2 ( a )), respectively. The first and second piezoelectric elements  24 ,  25  are piled on each other such that the polarizing directions are opposite to each other. 
     As shown in FIG. 1, a second disk  32  is secured to the supporter  23   a . Screws  33  are inserted through threaded through holes  32   a  formed in the second disk  32  and are threaded into the second screw holes  4   b  of the first housing member  4 . The second disk  32  is secured to the first housing member  4  by the screws  33 . When the stator  21  is secured, the friction material  31  at the upper surface of the stator  21  presses the proximal end surface of the rotor  10  upward. The first to third electrode plates  26  to  28  are electrically connected to a controller (not shown), which is located outside the housing  2 , by conductors (not shown). 
     The actuator  1  structured as described above generates axial vibration at the first and second piezoelectric elements  24 ,  25  when the controller applies high frequency voltage to the first to third electrode plates  26  to  28 . Then, torsional vibration is generated at the slit (not shown) of the stator  21  based on the axial vibration. Complex vibration of the torsional vibration and the axial vibration is generated on the upper surface of the first block  22  of the stator  21 . Levitation force generated by the axial vibration of the stator  21  and the driving force generated by the torsional vibration causes the rotor  10  to rotate, which rotates the rotary shaft  9 . 
     The preferred embodiment provides the following advantages. 
     The first and second positioning fitting surfaces  22   c ,  23   c  are formed in the first and second blocks  22 ,  23 , and the columnar body  29   c  is formed on the bolt  29  for determining the radial position of the first and second blocks  22 ,  23  and the columnar body  29   c . Therefore, the first and second blocks  22 ,  23  and the bolt  29  are aligned by a simple structure (simple shape) without increasing the number of components. Thus, the manufacturing deviations of the stator  21  are reduced. Accordingly, variations in the rotational characteristics (such as frequency-rotational speed characteristic, voltage-torque characteristic, and the like) are reduced. As a result, the ultrasonic motor  3  (actuator  1 ) is easily used for purposes in which a high-accuracy rotational control is required (such as for rotating a drum in a copying machine). This increases the field of application of the ultrasonic motor  3 . 
     The diameters of the first and second positioning fitting surfaces  22   c ,  23   c  are greater than the diameters of the female screws  22   b ,  23   b . The columnar body  29   c  corresponds to the first and second positioning fitting surfaces  22   c ,  23   c  and the diameter of the columnar body  29   c  is greater than the diameters of the male screws  29   a ,  29   b . In this case, the rigidity of the tightening member  29  is not reduced. Thus, the diameters of the male screws  29   a ,  29   b  need not be set greater than required for tightening the tightening member  29 . Thus, the vibration characteristic of the stator  21  is reliable. 
     The first positioning fitting surface  22   c  is formed on one of the ends of the first block  22  that faces the first piezoelectric element  24 . The second positioning fitting surface  23   c  is formed on one of the ends of the second block  23  that faces the second piezoelectric element  25 . The columnar body  29   c  of the tightening member  29  is located at the middle portion of the tightening member  29  and extends in the axial direction. The columnar body  29   c  engages with the first and second positioning fitting surfaces  22   c ,  23   c . In this case, the first and second blocks  22 ,  23  are easily assembled from the ends of the tightening member  29 . Also, the shape of the tightening member  29  is simplified as compared to a case in which separate columnar body is formed for each of the first and second positioning fitting surfaces  22   c ,  23   c.    
     The first and second piezoelectric elements  24 ,  25  are fitted to the middle portion of the columnar body  29   c  via the insulated collar  30 . The first collar fitting surface  22   d  is formed on the end of the first positioning fitting surface  22   c  facing the first piezoelectric elements  24 . The second collar fitting surface  23   d  is formed on the end of the second positioning fitting surface  23   c  facing the second piezoelectric element  25 . The diameters of the first and second collar fitting surfaces  22   d ,  23   d  are greater than the diameters of the first and second positioning fitting surfaces  22   c ,  23   c  and are fitted to the ends of the insulated collar  30 . Therefore, the axial length of the insulated collar  30  is longer than the axial length between the first and the second blocks  22 ,  23 , that is, the axial length when the first and second piezoelectric elements  24 ,  25  (more specifically, including the first to third electrode plates  26  to  28 ) are piled on each other. Thus, for example, although the insulated collar  30  is thin, the inner circumference of the first and second piezoelectric elements  24 ,  25  and the inner circumference of the first to third electrode plates  26  to  28  are reliably insulated from the columnar body  29   c  of the tightening member  29 . 
     The axial length H 1  of the columnar body  29   c  is slightly less than the distance H 2  between the first and second positioning fitting surfaces  22   c ,  23   c  when the first and second piezoelectric elements  24 ,  25  and the first to third electrode plates  26  to  28  are located between the first and second blocks  22 ,  23 . Therefore, the columnar body  29   c  does not limit the axial movement of the first and second blocks  22 ,  23 . That is, the first and second blocks  22 ,  23  are not prevented from moving toward each other. Thus, the columnar body  29   c  does not limit the tightening torque of the first and second piezoelectric elements  24 ,  25  by the first and second blocks  22 ,  23 . As a result, the vibration characteristic of the stator  21  is reliable. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     The first and second blocks  22 ,  23  may have no first and second collar fitting surfaces  22   d ,  23   d . For example, the first and second blocks  22 ,  23  may be modified as first and second blocks  41 ,  42  shown in FIG.  3 ( a ). A female screw  41   b  is formed on the inner circumference of the first block  41  at a portion other than where the horn  41   a  is formed. A first inner circumferential surface  41   c , the diameter of which is greater than that of the female screw  41   b , is formed at the lower end of the inner circumference of the first block  41 . A disk-like supporter  42   a  is formed on the outer circumference of the second block  42 . A female screw  42   b  is formed on the inner circumference of the second block  42 . A second inner circumferential surface  42   c  having larger diameter than the female screw  42   b  is formed at the upper end of the inner circumference of the second block  42 . The modified embodiment provides the same advantages as the preferred embodiment. In this case, the axial length of the insulated collar  43  (see FIG.  3 ( b )) needs to be less than or equal to the shortest distance between the first and second blocks  41 ,  42 , or the axial length between the first and second blocks  41 ,  42  when the first and second piezoelectric elements  24 ,  25  and the first to third electrode plates  26  to  28  are piled on one another. 
     Other positioning member may be formed on the first and second blocks  22 ,  23  and the tightening member  29  as long as the position in the radial direction is determined. 
     The diameters of the first and second positioning fitting surfaces  22   c ,  23   c  may be less than the diameters of the female screws  22   b ,  23   b , and the diameter of the columnar body  29   c  may be less than the diameters of the male screws  29   a ,  29   b  as long as the position in the radial direction is determined. 
     A separate columnar body, which serves as a positioning member, may be formed on the tightening member  29  corresponding to each of the first and second positioning fitting surfaces  22   c ,  23   c.    
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.