Abstract:
An ultrasonic motor includes a stator that includes a comb body with a plurality of circumferentially-arranged comb-like projections and a piezoelectric body integrally mounted on the comb body, a rotor rotatable with respect to the stator, the rotor establishing pressure contact with the stator, a resin film formed on at least one of pressure contact surfaces of the stator and rotor, and a pressure contact force controlling portion that includes, an elastic member configured to generate a pressure contact force between the stator and rotor with an elastic force thereof, and a compressing portion configured to compress and shorten the elastic member. The pressure contact force controlling portion controls the pressure contact force between the stator and rotor by varying the elastic force of the elastic member depending on temperature of the motor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The disclosure relates to an ultrasonic motor, particularly, to an ultrasonic motor configured to prevent variation of rotational efficiency thereof along with a temperature change. 
         [0002]    An ultrasonic motor is configured with a stator, which includes a piezoelectric body with a plurality of polarized piezoelectric segments circumferentially arranged, and a rotatable disc-shaped or annular rotor in contact with the stator under a predetermined pressure. In the ultrasonic motor, the piezoelectric body of the stator is vibrated with radio frequency voltage being applied thereto. The induced vibration of the piezoelectric body is enhanced in a circumferential direction of the stator by a comb body provided integrally to the piezoelectric body such that the comb body is driven to induce a traveling vibration wave in the circumferential direction. Thereby, the rotor, which frictionally engages with the piezoelectric body, can be rotated around an axis thereof. The comb body has a function of enlarging amplitude of the vibration of the piezoelectric body. However, since the amplitude is generally one micrometer to three micrometers, it is required that the comb body establishes close contact with the rotor evenly in the circumferential and radial directions of the stator, and that both close-contact surfaces between the comb body and the rotor are configured as pressure contact surfaces with a desired pressure contact force being applied thereto, so as to improve rotational efficiency of the rotor (i.e., rotational energy of the rotor to vibration energy of the stator). Therefore, when the ultrasonic motor has not been driven for a long time, the pressure contact surfaces between the comb body and the rotor come into a state of interfacial adhesion due to the pressure contact force, and it increases a static frictional force therebetween. Thereby, a large torque is needed for rotating the rotor at start-up of the ultrasonic motor, and the ultrasonic motor cannot begin smooth rotation. In the worst case, the motor cannot be rotated. 
         [0003]    In order to solve such a problem at the start-up of the motor, there has been proposed a technology in which a resin layer with a low frictional coefficient is formed on any of the pressure contact surfaces between the comb body and the rotor to reduce the static frictional force therebetween. Fluorocarbon resin such as polytetraflouroethylene (PTFE) can be cited as an example of the resin layer. In Japanese Patent Provisional Publication No. HEI 9-98587, there is proposed a technique in which a slider formed from polymer resin is attached onto a surface of the rotor. The technique is regarded as one of possible solutions that can prevent the interfacial adhesion between the pressure contact surfaces of the comb body and the rotor and reduce the static frictional force. 
         [0004]    Thus, the resin layer with a low frictional coefficient thereon that is formed on one of the pressure contact surfaces between the comb body and the rotor can reduce the static frictional force generated between both of the pressure contact surfaces. It means that the formation of the resin layer is so effective that the ultrasonic motor can smoothly be started up to a steady-state rotation state. However, this kind of resin has such a property that the frictional coefficient thereon varies depending on temperature. In particular, some kinds of resins have such a property that the frictional coefficient thereon is made lower than that at room temperature when the temperature rises. The aforementioned PTFE is provided with the property. In the ultrasonic motor with the resin layer being formed, when the temperature of the resin layer increases due to frictional heat generated between the comb body and the rotor along with the rotation of the motor, the frictional coefficient is made lower than necessary. Thereby, both the pressure contact surfaces between the comb body and the rotor come into a slippery state. Hence, the traveling vibration wave of the comb body cannot efficiently be transmitted to the rotor. Consequently, the rotational efficiency of the rotor, that is, the rotational efficiency of the ultrasonic motor gets worse, and thereby the rotational torque of the ultrasonic motor is reduced. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is advantageous in that there can be provided an improved ultrasonic motor that makes it possible to prevent rotational efficiency thereof from being worsened even though temperature of the motor increases along with rotation of the motor. 
         [0006]    According to an aspect of the present invention, there is provided an ultrasonic motor, which includes a stator that includes a comb body with a plurality of circumferentially-arranged comb-like projections and a piezoelectric body integrally mounted on the comb body, a rotor rotatable with respect to the stator, the rotor establishing pressure contact with the stator, a resin film formed on at least one of pressure contact surfaces of the stator and rotor, and a pressure contact force controlling portion that includes an elastic member configured to generate a pressure contact force between the stator and rotor with an elastic force thereof, and a compressing portion configured to compress and shorten the elastic member. The pressure contact force controlling portion controls the pressure contact force between the stator and rotor by varying the elastic force of the elastic member depending on temperature of the motor. 
         [0007]    Optionally, the elastic member may include a compression coil spring provided between the stator and rotor. In this case, the compression coil spring may be formed from shape-memory material that varies a natural length thereof depending on the temperature. Further, in this case, the compressing portion may compress and shorten the compression coil spring down to a predetermined length. 
         [0008]    Alternatively or optionally, the elastic member may include a compression coil spring provided between the stator and rotor. In this case, the compressing portion may vary a compressed length of the compression coil spring depending on the temperature. 
         [0009]    Still optionally, the compressing portion may include a bush provided to the stator so as to rotatably support the rotor, and the compression coil spring may be inserted into the bush. In this case, the bush may be formed from shape-memory material such that a length thereof in an axial direction thereof varies depending on the temperature. 
         [0010]    Alternatively or optionally, the compressing portion may include a bush provided to the stator, a ball bearing configured to rotatably support the rotor, the ball bearing being biased by the compression coil spring inside the bush, and a locking member configured to lock the ball bearing biased by the compression coil spring so as to rotatably support the rotor. In this case, the locking member may be formed from shape-memory material to change a locking position where the ball bearing is locked depending on the temperature. 
         [0011]    Optionally, the resin film may include fluorocarbon resin. 
         [0012]    Further optionally, the fluorocarbon resin included in the resin film may include at least one of polytetraflouroethylene (PTFE), perfluoroalkoxy polymer resin (PFA), and fluorinated ethylene-propylene (FEP). 
         [0013]    According to another aspect of the present invention, there is provided an ultrasonic motor, which includes a stator that includes a comb body with a plurality of circumferentially-arranged comb-like projections and a piezoelectric body integrally mounted on the comb body, a rotor rotatable with respect to the stator, the rotor establishing pressure contact with the stator, a resin film formed on at least one of pressure contact surfaces of the stator and rotor, a frictional coefficient on the resin film varying depending on temperature of the motor, and a pressure contact force controlling portion that includes, an elastic member configured to generate a pressure contact force between the stator and rotor with an elastic force thereof, and a compressing portion configured to compress and shorten the elastic member. The pressure contact force controlling portion controls the pressure contact force between the stator and rotor by varying the elastic force of the elastic member depending on the temperature such that a frictional force on the resin film is maintained substantially constant. 
     
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         [0014]      FIG. 1  is an external perspective view of an ultrasonic motor in a first embodiment according to the present invention. 
           [0015]      FIG. 2  is a cross-sectional view of the ultrasonic motor along a plane including a center axis of a rotating shaft thereof in the first embodiment according to the present invention. 
           [0016]      FIG. 3  partially shows an exploded perspective view of the ultrasonic motor in the first embodiment according to the present invention. 
           [0017]      FIGS. 4A ,  4 B, and  4 C are illustrations for explaining lengths of a compression coil spring in the first embodiment according to the present invention. 
           [0018]      FIG. 5  shows temperature dependency of a frictional coefficient on a resin film in the first embodiment according to the present invention. 
           [0019]      FIG. 6  is a cross-sectional view of an ultrasonic motor along a plane including a center axis of a rotating shaft thereof in a second embodiment according to the present invention. 
           [0020]      FIG. 7  is a cross-sectional view of an ultrasonic motor along a plane including a center axis of a rotating shaft thereof in a third embodiment according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0021]    Embodiments according to aspects of the present invention will be described with reference to the accompanying drawings.  FIG. 1  is an external perspective view of an ultrasonic motor in a first embodiment.  FIG. 2  is a cross-sectional view of the ultrasonic motor along a plane including a center axis of a rotating shaft  3  thereof.  FIG. 3  partially shows an exploded perspective view of the ultrasonic motor. As shown in  FIGS. 1 to 3 , there is integrally provided under an annular pedestal  11  having mounting holes  111  for mounting the motor, a short-cylinder-shaped comb body  12  that includes a plurality of comb-like projections  121  circumferentially arranged. In addition, there is integrally mounted on the comb body  12  an annular thin-plate-shaped piezoelectric body  13  that includes a plurality of polarized segments circumferentially arranged so as to correspond to the comb-like projections  121 , respectively. A stator  1  is configured with the pedestal  11 , piezoelectric body  13  and comb body  12 . Further, a radio frequency voltage can be applied to the piezoelectric body  13  via a flexible board  14 . A shaft hole  112  is opened at a center of the pedestal  11 , and a cylinder-shaped bush  15  is fixed on an inner circumferential surface of the shaft hole  112 . In addition, a ball bearing  17  is provided at an upper end portion inside the bush  15 , rotatably supporting the rotating shaft  3 . A washer  31  prevents the rotating shaft  3  from pulling out of the ball bearing  17 . A rotor  2  is attached to a lower end portion of the rotating shaft  3 . The rotor  2  has a resin film  4  formed on an upper end surface of a peripheral wall  21  thereof, i.e., a pressure contact surface  21   a . The resin film  4  is formed in a short cylinder shape to contact with a surface of the comb body  12 , namely, a surface  12   a  of the comb-like projections  121 . Furthermore, a compression coil spring  16  is provided between a lower end portion of the bush  15  and the ball bearing  17  in an axial direction. By an elastic force of the compression coil spring  16  in the axial direction, the ball bearing  17  and the rotating shaft  3  supported by the ball bearing  17  are biased in an upper direction, and the pressure contact surface  21   a  of the peripheral wall  21  of the rotor  2  (in this case, a pressure contact surface  4   a  of the resin film  4  formed on the pressure contact surface  21   a ) is biased toward the surface  12   a  of the comb body  12  of the stator  1 . 
         [0022]    The aforementioned resin film  4  is formed with fluorocarbon resin polytetraflouroethylene (PTFE) being applied on the pressure contact surface  21   a  of the rotor  2  to have a predetermined thickness. Lubricating property on the surface of the resin film  4  makes lower a static frictional coefficient between the pressure contact surface  12   a  of the comb body  12  and the pressure contact surface  4   a  of the resin film  4 . Further, in order to configure a pressure contact force controlling portion according to aspects of the invention, the compression coil spring  16  provided inside the cylinder-shaped bush  15  is formed from a wire rod made of shape-memory alloy. The compression coil spring  16  is inserted into the cylinder-shaped bush  15  in a state compressed and shortened so as to have an inserted coil length L 0  as shown in  FIG. 4A . The compression coil spring  16  is formed to have a length L 1  as shown in  FIG. 4B  as a natural length at a room temperature around 20° C. when removed from the cylinder-shaped bush  15 , and to have a natural length L 2  longer than L 1 , as shown in  FIG. 4C , at a high temperature (approximately 50 to 100° C.) over a critical temperature. Relationship between the natural lengths L 1  and L 2  is established such that (L 2 −L 0 ) is about twice as long as (L 1 −L 0 ). 
         [0023]    In the ultrasonic motor of the first embodiment, when a radio frequency voltage is applied to the piezoelectric body  13  via the flexible board  14 , the piezoelectric body  13  vibrates, and the comb body  12  configured integrally with the piezoelectric body  13  also vibrates such that the plurality of comb-like projections  121  circumferentially arranged are displaced in a circumferential direction. The pressure contact surface  21   a  of the rotor  2  establishes pressure contact with the surface  12   a  of the comb body  12  via the resin film  4  by the bias force of the compression coil spring  16 . Therefore, the above pressure contact generates a friction force between the pressure contact surface  4   a  of the resin film  4  of the rotor  2  and the surface  12   a  of the comb body  12 . By the friction force, the rotor  2  provided with the resin film  4  are moved in the circumferential direction, and the rotor  2  and the rotating shaft  3  supporting the rotor  2  are rotated. A rotating force of the rotating shaft  3  is conveyed outside via a gear (not shown) fixed to the rotating shaft  3 . 
         [0024]    Furthermore, the pressure contact surface  21   a  of the rotor  2  does not directly contact with the surface  12   a  of the comb body  12  of the stator  1 , and the resin film  4  exists therebetween. Hence, since metal surfaces of the rotor  2  and stator  1  do not directly contact with each other, a quiet rotational operation can be performed with a rotation noise being reduced. Further, there is the resin film  4  formed from PTFE between both the pressure contact surfaces of the comb body  12  and the rotor  2 . Since the frictional coefficient on the pressure contact surface  4   a  of the resin film  4  is lower than that on metal, the frictional coefficient between both the pressure contact surfaces  12   a  and  4   a  is more reduced than a case where the pressure contact surface  12   a  of the comb body  12  directly contacts the pressure contact surface  21   a  of the rotor  2 . Especially, the static frictional coefficient at the start-up of the ultrasonic motor is made lower, so that at the motor can smoothly be started up. 
         [0025]    In the meantime, when temperature of the rotor  2 , thereby, of the entire ultrasonic motor increases due to heat generated along with the rotation of the ultrasonic motor, temperature of the resin film  4  increases as well. As described above, the frictional coefficient on the resin film  4  has a dependency on the temperature. In the case of PTFE, as shown in  FIG. 5 , the frictional coefficient decreases as the temperature increases. Therefore, when the temperature of the ultrasonic motor increases, the frictional coefficient between the comb body  12  and the rotor  2  is made lower owing to the decrease of the frictional coefficient on the pressure contact surface  4   a  of the resin film  4 , and thereby both the pressure contact surfaces  12   a  and  4   a  come into a slippery state. For this reason, efficiency of transmitting the displacement from the comb body  12  toward the rotor  2  in the circumferential direction is made lower, and consequently the rotational efficiency of the ultrasonic motor is worsened. However, in the first embodiment, temperature of the compression coil spring  16  is increased due to increased temperature of the ultrasonic motor. Then, when the temperature of the compression coil spring  16  becomes over the critical temperature of the shape-memory alloy, the natural length of the compression coil spring  16  extends from L 1  to L 2 . In general, a spring constant of the compression coil spring  16  is constant regardless of change in the length thereof. Accordingly, when spring characteristics of the compression coil spring  16  are linear, a restoring force of the compression coil spring  16  becomes F times as large (here, F=(L 2 −L 0 )/(L 1 −L 0 )). In the present embodiment, F=2. Thereby, owing to the compression coil spring  16 , the pressure contact force between the pressure contact surface  4   a  of the resin film  4  provided to the rotor  2  and the pressure contact surface  12   a  of the comb body  12  becomes twice as large. Meanwhile, as shown in  FIG. 5 , the frictional coefficient on the resin film  4  is made approximately half as high from 0.3 to 0.15 due to the temperature elevation. Hence, the frictional force (frictional force=pressure contact force×frictional coefficient) between the comb body  12  and resin film  4  can be kept substantially constant. In other words, even though the frictional coefficient on the resin film  4  is made lower along with the temperature elevation of the ultrasonic motor, since the pressure contact force between the comb body  12  and the rotor  2  increases, the frictional force between the comb body  12  and resin film  4 , that is, between the comb body  12  and the rotor  2  can be kept substantially constant Consequently, the rotational efficiency of the ultrasonic motor is kept substantially constant and is not worsened. 
         [0026]    Meanwhile, when the drive of the ultrasonic motor is stopped, and the temperature of the ultrasonic motor decreases down to the room temperature, the frictional coefficient on the resin film  4  increases. Instead, the temperature of the compression coil spring  16  is made lower than the critical temperature, and the natural length of the compression coil spring  16  is compressed from L 2  to L 1 , depressing the restoring force of the compression coil spring  16 . Therefore, in this case as well, the frictional force between the comb body  12  and resin film  4  is kept substantially constant, and the rotational efficiency of the ultrasonic motor is not worsened. 
         [0027]    In the first embodiment, the pressure contact force controlling portion according to aspects of the invention is configured with the compression coil spring  16 , which is formed from the shape-memory alloy. In this configuration, the pressure contact force between the comb body  12  and the resin film  4  is changed by using the property that the natural length of the compression coil spring  16  varies depending on the temperature. However, the pressure contact force controlling portion may be configured such that the pressure contact force is controlled by changing the inserted coil length of the compression coil spring  16  inside the bush  15  depending on the temperature.  FIG. 6  exemplifies a structure of an ultrasonic motor in a second embodiment. The same portions as those in the first embodiment are provided with the same reference characters, and explanations thereof will be omitted. In the second embodiment, a bush  15  is formed from shape-memory alloy. By deforming a lower portion  15   a , which is located at a lower side of the bush  15  in  FIG. 6 , depending on the temperature, the inserted coil length of the compression coil spring  16  inserted between the lower portion  15   a  of the bush  15  and the ball bearing  17  fixed to an upper portion of the bush  15  is changed. At the room temperature lower than the critical temperature, the bush  15  has a longer length with the lower portion  15   a  thereof being extended as shown by a chain double-dashed line in  FIG. 6 . Namely, the bush  15  is configured to be in a shape with a longer inserted coil length L 11  at the room temperature. In addition, at higher than the critical temperature, as shown by a solid line in  FIG. 6 , the bush  15  has a shorter length with the lower portion  15   a  being bent to protrude outward. Namely, the bush  15  is configured to have a shape with a shorter inserted coil length L 12  at higher than the critical temperature. Here, the inserted coil lengths L 11  and L 12  are designed such that the restoring force of the compression coil spring  16  (i.e., the pressure contact force between the comb body  12  and the resin film  4 ) at the length L 12  is twice as large as that at the length L 11 . It is noted that the compression coil spring  16  is formed from a normal spring wire. 
         [0028]    In the second embodiment, when the ultrasonic motor starts to be driven, the lower portion  15   a  of the bush  15  is extended with the compression coil spring  16  having the longer inserted coil length L 11  as shown by the chain double-dashed line in  FIG. 6 . Therefore, the restoring force of the compression coil spring  16  is small, and the pressure contact force between the comb body  12  and resin film  4  due to the compression coil spring  16  is low. At this time, since the frictional coefficient on the resin film  4  is large, a predetermined frictional force is generated even though the pressure contact force is low, and predetermined rotational efficiency is achieved. Meanwhile, when the ultrasonic motor is driven, and the temperature rises, the frictional coefficient on the resin film  4  is made lower. However, at a moment when the temperature of the bush  15  reaches the critical temperature along with the temperature rise of the bush  15 , the lower portion  15   a  of the bush  15  is deformed so as to be compressed upward as indicated by the solid line in  FIG. 6 . Thereby, the compression coil spring  16  inserted into the bush  15  is compressed as well to have the inserted coil length L 12 , and the restoring force of the compression coil spring  16  is made higher, increasing the pressure contact force between the comb body  12  and resin film  4 . Thus, even though the frictional coefficient on the resin film  4  is decreased due to the temperature rise, the frictional force is made larger owing to the increase of the pressure contact force, and the predetermined rotational efficiency is maintained. 
         [0029]    In the same manner as the second embodiment, in a third embodiment, the pressure contact force is changed with the inserted coil length of the compression coil spring  16  in the bush  15  being changed depending on the temperature.  FIG. 7  exemplifies a structure of an ultrasonic motor in a third embodiment. The same reference characters are given to the same portions as those in the first and second embodiments. In the third embodiment, a washer  31 , which locks the ball bearing  17  located at an upper portion inside the bush  15  with respect to the rotation shaft  3 , is formed from shape-memory alloy as the pressure contact force controlling portion according to aspects of the invention. The washer  31  is formed to have a larger outer diameter than that in each of the first and second embodiments, and configured such that an outer portion  31   a  thereof is deformed in an axial direction of the rotation shaft  3  depending on the temperature. Namely, the washer  31  has the outer portion  31   a  extending straight in a direction perpendicular to the axial direction at lower than the critical temperature as indicated by a chain double-dashed line in  FIG. 7 . At this time, the ball bearing  17  is pushed upward along the axial direction by the restoring force of the compression coil spring  16 . Consequently, the compression coil spring  16  is made longer to have the inserted coil length L 11 . At higher than the critical temperature, the outer portion  31   a  is bent and arranged along the axial direction as indicated by a solid line in  FIG. 7 . At this time, the ball bearing  17  is pushed downward along the axial direction against the restoring force of the compression coil spring  16 , making the compression coil spring  16  shorter to have the inserted coil length L 12 . Thereby, the location of the ball bearing  17  in the axial direction is varied by the deformation of the outer portion of the washer  31 . Hence, the length of the compression coil spring  16  inserted in the bush  17  is changed, so that the restoring force of the compression coil spring is changed. In this case, the inserted coil lengths L 11  and L 12  are designed such that the restoring force of the compression coil spring  16  (i.e., the pressure contact force between the comb body  12  and the resin film  4 ) at the length L 12  is twice as large as that at the length L 11 . It is noted that the compression coil spring  16  is formed from a normal spring wire. 
         [0030]    In the third embodiment, when the ultrasonic motor begins to be driven, as indicated by the chain double-dashed line in  FIG. 7 , the washer  31  has the outer portion  31   a  extending horizontally, and the ball bearing  17  is pushed upward such that the compression coil spring  16  is made longer with the inserted coil length L 11 . Therefore, the pressure contact force between the comb body  12  and resin film  4  that is caused by compression coil spring  16  is lower. At this time, since the frictional coefficient on the resin film  4  is higher, the predetermined frictional force, that is, the predetermined rotational efficiency is achieved even though the pressure contact force is lower, Meanwhile, when the ultrasonic motor is driven, and the temperature rises, the frictional coefficient on the resin film  4  is made lower. However, as indicated by the solid line in  FIG. 7 , at a moment when the washer  31  reaches the critical temperature, the outer portion  31   a  is bent downward, pushing the ball bearing  17  downward such that the compression coil spring  16  is compressed to have the inserted coil length L 12 . Hence, the restoring force of the compression coil spring  16  is increased, and the pressure contact force between the comb body  12  and the resin film  4  is increased. Thus, even though the frictional coefficient on the resin film  4  is made lower along with the temperature elevation, the predetermined frictional force on the resin film  4  is achieved owing to the increase of the pressure contact force, and the predetermined rotational efficiency is maintained. 
         [0031]    In the first to third embodiments, the compression coil spring  16 , bush  15 , and washer  31  are formed from the shape-memory alloy, respectively. However, in the second and third embodiments, the bush  15  and washer  31  may be formed from bimetal continuously deformed depending on the temperature, respectively. By employing such configurations, the lower portion  15   a  of the bush  15  or the outer portion  31   a  of the washer  31 , which is formed from the bimetal, is continuously deformed. Therefore, the inserted coil length of the compression coil spring  16  can continuously be varied. Thereby, as shown in  FIG. 5 , it is possible to vary the pressure contact force on the resin film  4  by the compression coil spring  16  with following the continuous change of the frictional coefficient on the resin film  4  depending on the temperature, in order to attain an ultrasonic motor that can keep the rotational efficiency constant over a range within which the temperature may change. 
         [0032]    In addition, the present invention is not limited to the ultrasonic motor configured with the single structure in each of the first to third embodiments. The ultrasonic motor according to aspects of the invention may be configured with any appropriate combination from the structures in the first to third embodiments and the above structure using the bimetal so as to attain more excellent rotational efficiency that is less affected by the temperature change. Further, an elastic element for generating the pressure contact force is not limited to the compression coil spring. The elastic element according to aspects of the invention may include a plate spring configured such that a restoring force thereof varies depending on the temperature. 
         [0033]    Furthermore, the resin film  4  is not limited to PTFE, and may include resin material such as PFA (perfluoroalkoxy polymer resin) and FEP (fluorinated ethylene-propylene). 
         [0034]    Additionally, the ultrasonic motor according to aspects of the invention may be configured with a resin film having such a property that the frictional coefficient thereon increases as the temperature rises contrary to the first to third embodiments. In this case, the compression coil spring is required to generate a weaker pressure contact force as the temperature rises. 
         [0035]    The present disclosure relates to the subject matter contained in Japanese Patent Application No. P2006-342092, filed on Dec. 20, 2006, which is expressly incorporated herein by reference in its entirety.