A vibration driven motor or actuator

Abstract: This invention has as its object to eliminate or reduce the speed difference between the contact surfaces of an oscillator and a rotor. In other words, a peripheral speed difference Δv between the contact surfaces of the rotor and the oscillator at arbitrary two points and a peripheral partial velocity difference ΔV of the oscillator main body at the two points satisfy ΔV > Δv.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a vibration driven motor or actuator. Related Background Art Fig. 2 shows polarization patterns and an arrangement of piezoelectric elements (PZT) used in an oscillator (or a vibration member) formed in a rod-like shape in a vibration driven motor (ultrasonic driven motor). Each piezoelectric element PZT is subjected to polarization processing, so that right and left portions on the central line as a boundary have opposite polarities. These piezoelectric elements are grouped into A and B phase groups each including two piezoelectric elements, and these groups are arranged to have a 90° phase difference therebetween. Note that the lowermost piezoelectric element is used for vibration detection (S phase), and although not shown, electrode plates are inserted between each two adjacent piezoelectric elements. The driving principle will be described below with reference to Fig. 3. When an AC voltage is applied to only the A phase, a primary flexural natural vibration is excited in an oscillator 1 in a direction parallel to the plane of drawing of Fig. 3 by expansion/shrinkage of the piezoelectric elements. When an AC voltage is applied to only the B phase, a vibration is generated in a direction perpendicular to the plane of drawing of Fig. 3. When a vibration in the horizontal direction excited by the A phase and a vibration in the vertical direction excited by the B phase are applied to temporally have a 90° phase difference therebetween, clockwise or counterclockwise motion with respect to the longitudinal axis is generated in the oscillator 1. Since the oscillator 1 has a circumferential groove 1q for amplifying a displacement, an oscillating motion shown in Fig. 3 is generated at the distal end of the oscillator 1. When viewed from the contact surface (the upper surface of the oscillator) with a rotor, this vibration corresponds to one travelling wave. When a rotor 2 having a contact spring is brought into press contact with the oscillator, the rotor contacts a stator at only one portion near the wavefront, and is rotated in the reverse direction. The output is extracted by a gear 4 attached to the outer circumferential surface of a ball bearing 3 at the upper portion of the rotor. In the rod-shaped vibration driven motor, a support pin rod 5 (shaft distal end) - flange 6 system is integrated, and the natural mode of the oscillator is FEM-analyzed, so that the vibration amplitude of the flange 6 is minimized. For this reason, the rod-like vibration driven motor has a very small support loss as compared to a ring-shaped oscillator. On the other hand, a contact spring 7 having a spring structure is formed on the lower portion of a rotor main ring of the rotor 2, and has a natural frequency sufficiently higher than the vibration application frequency of the oscillator as in a ring-shaped oscillator, so that it can follow a vibration. The rotor main ring has a sufficiently large inertial mass, so that no vibration is excited by a vibration applied from the oscillator. One of demerits of a vibration driven motor is low motor efficiency. In general, in a rod-shaped vibration driven motor, since an oscillator is supported by an elastic member using a metal pin consisting of, e.g., stainless steel, the support loss is small. Also, since the oscillator and the rotor consist of a material such as brass, aluminum, or the like, which causes less internal vibration attenuation, their internal losses are also small. Therefore, a loss due to a slip at the contact portion between a rotor and a stator is a dominant factor of energy losses in a motor, and the object of this invention is to improve motor efficiency by eliminating this loss factor. The slip at the contact portion can be classified into a radial slip and circumferential slip in a cylindrical coordinate system having the rotational axis of the rotor as the center. Methods of preventing the radial slip have been disclosed in Japanese Laid-Open Patent Application Nos. 61-224882, 63-174581, and the like. However, no countermeasures are conventionally taken against the circumferential slip. A mechanism of the circumferential slip will be described below. Fig. 4A shows the peripheral speed corresponding to the vibration displacement on the contact surface of an oscillator at a certain time by the lengths and directions of arrows, and Fig. 4B shows the peripheral speed distribution at that time. As can be seen from Fig. 4B, a vibration formed on the contact surface of the oscillator as a stator has a high speed near the wavefront. Fig. 5 shows the state of the contact portion of a movable member as a rotor contacting the oscillator. Referring to Fig. 5, a solid waveform represents the displacement of the oscillator, and a broken waveform represents the displacement of the rotor. Assume that the peripheral speeds at two points A and B on the contact surfaces of the oscillator and the rotor are represented by V as (the speed of the oscillator at the point A), V ar (the speed of the rotor at the point A), V bs (the speed of the oscillator at the point B), and V br (the speed of the rotor at the point B). Although the rotor is rotated at a certain peripheral speed v 0 by, e.g., an inertia, since the rotor contact surface contacts the vibration surface of the oscillator in a finite length, the rotor contact surface is fed at the respective contact points at different peripheral speeds v as to v bs . Therefore, contact portions corresponding to rotor peripheral speeds other than v 0 cause slips. For example, a slip of a relative speed v as - v 0 occurs at the point A, and a slip of a relative speed v 0 - v bs occurs at the point B. The product of the slip in the circumferential direction (driving direction) with the frictional force corresponds to loss power of the sliding portion, and the object of this invention is to reduce or eliminate this loss power. The reason why the above-mentioned speed difference occurs will be described below with reference to Figs. 7 to 10. Fig. 7 shows the radial flexure distribution on a neutral plane (indicated by a dotted curve) N in the contact spring formed on the rotor, and Fig. 8 shows the flexural deformation state in the circumferential direction at that time. As shown in Fig. 8, in the contact state with the oscillator, the circumferential distortion of the contact surface in the contact spring of the rotor is shrinkage (the speed is low) near the wavefront to have the neutral plane as the center, and expansion (the speed is high) near a contact release point. Fig. 9 shows temporal transitions of a point position on the contact surface upon travelling of a vibration. Assume that T represents the time required for a travelling wave to travel by one wavelength for one cycle, the vibration of the oscillator travels from the left to the right on the plane of drawing in Fig. 9, and the vibration of the rotor travels from the right to the left on the plane of drawing in Fig. 9. Referring to Fig. 9, a point P 6 moves from the left to the right at t = 1/10T , the entire rotor moves from the right to the left at a peripheral speed v r , and the central point P 6 of the deformation also has a speed Δvp 6 due to the deformation. Note that a speed Δv 4 by the deformation of a point P 4 at t = 2/10T has a direction opposite to that of Δp 6 . Fig. 10 shows the speed distribution when the deformation sequentially occurs in this manner. In Fig. 10, the direction of the rotor peripheral speed is defined to be the positive direction. On the other hand, the peripheral speed distribution on the contact surface of the oscillator has a sine wave state, as shown in Fig. 11, and a difference Δv between the peripheral speeds of the oscillator and the rotor has a distribution state shown in Fig. 12 in the contact region. In the document EP 0 473 423 a vibration driven motor of this kind is disclosed which has a vibration member and a contact member operatively connected to each other. A contact portion of the movable member is endowed with a spring property to thereby effect a uniform contact over the entire circumference of the movable member. With this construction an ultrasonic motor is realized which is good in efficiency and suffers little from the occurrence of sounds. The EP 0 469 881 discloses another vibration driven actuator comprising a bar-shaped elastic member and a piezo-electric element for causing a plurality of bending mode vibrations in a plurality of planes of the elastic member in order to drive a movable body. The EP 0 441 540 shows a vibration wave driven motor having an elastic member, which is in pressure contact with another member wherein provision is made of means for making the contact pressure on the surface of contact between the elastic member and the member which is in pressure contact with the elastic member non-uniform whereby the occurrence of noise is prevented. Finally the document EP 0 507 264 discloses an ultrasonic motor with a vibrating body and a moving body driven thereby. The moving body consists of a carbon fiber reinforced resin composite material and is constructed with a both-side supported or single-side beam construction so that an elasticity according to the pressure of the ultrasonic motor can be imparted in the direction of pressure of the moving body. Thus, the undulations in the contact surface of the vibration body and the moving body can be absorbed and a stable, uniform contact can be assured. SUMMARY OF THE INVENTION It is an object of the present invention to eliminate or reduce the above-mentioned sliding loss. This object is achieved by a vibration driven motor having the technical features according to patent claim 1. Other objects of the present invention will become apparent from the following detailed description of the present invention as claimed. One aspect of the present invention as claimed is to eliminate or reduce the speed difference between the contact surfaces of an oscillator and a rotor. In other words, a peripheral speed difference Δv between the contact surfaces of the rotor and the oscillator at arbitrary two points and a peripheral partial velocity difference ΔV of the oscillator main body at the two points satisfy ΔV > Δv. This will be described more easily with reference to Fig. 5. Δv = (v as - v ar ) - (v bs - v br ) ΔV = v as - v bs That is, paying attention to the rotor, the peripheral speed near the wavefront is set to be high like v ar > v br . Note that the peripheral speed of the oscillator main body represents the peripheral speed of the contact surface when no contact spring is provided to the oscillator, and represents the peripheral speed distribution of the contact surface obtained when the contact spring is not deformed by the frictional force (a non-contact state with the rotor), when the contact spring is provided. As means for satisfying the above-mentioned relationship: (1): a sliding surface is defined by a plurality of surfaces, and each divided surface is allowed to be independently displaceable, thereby preventing a circumferential slip; and (2): the peripheral speed distribution of the rotor contact surface is set to be close to that on the contact surface of the oscillator main body, thereby decreasing the speed difference therebetween. As an arrangement for realizing these means, the means (1) for dividing the sliding surface of the contact spring into a plurality of surfaces can be realized by providing circumferential flexibility to each divided contact surface. The contact spring preferably consists of a material such as a metal having a small internal attenuation although it can consist of a resin. For example, the above-mentioned means (1) may be realized by finely dividing the conventional contact spring into small portions in the circumferential direction. In this case, the finely divided contact surfaces are required to sufficiently respond to the driving frequency. The circumferential length of each divided contact surface is preferably as small as possible. This is because since the speed difference generated in the contact surface is decreased as the contact surface becomes shorter, a sliding loss in the surface is decreased. If possible, it is desirable that each divided contact surface has a size within a range in which the speed difference can be completely absorbed by shearing deformation in the material of the contact surface. This size varies depending on the contact portion material, frictional force, speed difference, and the like, and is very small for a metal material. If a resin or rubber is used, this range can be increased, and the size of each divided contact surface can be increased. However, since the resin or rubber is soft in the axial direction, it also causes a large deformation in this direction by a driving vibration, resulting in a decrease in motor rotational speed. It is desirable to provide anisotropy (i.e., stiff in the radial direction, and soft in the circumferential direction) by, e.g., adding fibers in the vibration displacement direction. It is not preferable that each two adjacent contact surfaces of segments constituting the divided contact surfaces be separated by a large distance. More specifically, as shown in Fig. 6, when each two adjacent contact surfaces are separated by a large distance, a non-contact time with a portion near the wavefront having a high peripheral speed is generated. Then, the motor rotational speed decreases, and energy to be extracted is decreased, thus impairing motor efficiency accordingly. Therefore, a large number of contact segments must be present in one wavelength of a driving vibration. The method of defining the rotor contact surface by a plurality of surfaces is disclosed in Japanese Laid-Open Patent Application No. 61-150677, and the method of dividing the contact spring in the circumferential direction is disclosed in Japanese Laid-Open Patent Application No. 4-91672. However, these methods utilize a surface deformation of the contact surface formed as a result of machining. For this reason, the number of divided surfaces in one wavelength is small, and since the surface deformation is utilized, divided spring members are required to have flexibility in a direction outside the contact surface, e.g., the axial direction in a rod-shaped vibration driven motor. Therefore, these methods have different objects and arrangements from those of the present invention. On the other hand, as the above-mentioned means (2) for decreasing the speed difference between the rotor and the oscillator by setting the peripheral speed distribution of the rotor contact surface to be close to that on the contact surface of the oscillator main body, if the neutral plane of a flexure of the contact spring is present outside the contact surface, the temporal transitions of a point position on the contact surface become as shown in Fig. 13, and the peripheral speed distribution caused by the flexural deformation at this time becomes as shown in Fig. 14. The peripheral speed distribution at this time becomes close to that shown in Fig. 11, and the speed difference between the contact surfaces of the rotor and the oscillator can be decreased. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing the first embodiment of the present invention; Fig. 2 is a perspective view showing polarization patterns of piezoelectric elements; Fig. 3 is a sectional view showing an arrangement of a rod-shaped vibration driven motor; Fig. 4A is a graph showing the relationship between the vibration deformation and the peripheral speed of an oscillator, and Fig. 4B is a graph showing the peripheral speed distribution in one wavelength or one cycle; Fig. 5 is a graph showing a contact state of a rotor; Fig. 6 is a graph showing a contact state of the rotor; Fig. 7 is a sectional view showing a neutral plane of the rotor; Fig. 8 is a circumferential exploded view of the neutral plane by contact; Fig. 9 is a view showing the moving state of points on the contact surface; Fig. 10 is a graph showing the peripheral speed distribution of the rotor contact surface; Fig. 11 is a graph showing the peripheral speed distribution of the oscillator contact surface; Fig. 12 is a graph showing the peripheral speed difference between the contact surfaces of the rotor and the oscillator; Fig. 13 is a view showing the moving state of points on the contact surface of a rotor according to the first embodiment; Fig. 14 is a graph showing the peripheral speed distribution on the rotor contact surface in the first embodiment; Fig. 15 is a sectional view showing a deformation state of a contact spring in the circumferential direction of an oscillator in the first embodiment; Fig. 16 is a perspective view of a rotor according to a modification of the first embodiment; Fig. 17 is a perspective view showing the second embodiment; Fig. 18 is an exploded view of a rotor contact spring shown in Fig. 17; Figs. 19A and 19B are respectively a partial perspective view and an enlarged sectional view of a rotor of a ring-shaped vibration driven motor according to the third embodiment; Figs. 20A and 20B are respectively a perspective view and a sectional view showing the fourth embodiment; Fig. 21 is a partial sectional view of a rotor according to another modification of the first embodiment; Fig. 22 is a sectional view showing a modification of the third embodiment; and Fig. 23 is a sectional view of a lens barrel using the vibration driven motor of the first embodiment as an AF motor. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows the embodiment of the present invention. This embodiment relates to a rod-shaped vibration driven motor (ultrasonic driven motor). A contact spring 2a formed on the lower portion of a rotor 2 has flexibility in the axial direction to absorb a surface deformation generated upon machining of a contact surface, and to properly deform by a vibration. A thin outer circumferential portion 2b is used as a contact portion, and a base portion is formed to project downward from the contact surface of the contact portion. Thus, as shown in Fig. 13, a neutral plane N of a flexure at the contact position of the rotor 2 is set at a level lower than the contact surface to set the peripheral speed distribution on the contact surface of an oscillator to be close to that on the rotor contact surface, thereby relaxing the speed difference therebetween. More specifically, upon comparison between Figs. 13 and 9, the speed of the contact surface near the wavefront is low in Fig. 9, while the speed of the contact surface near the wavefront is high in Fig. 13 of this embodiment. Therefore, since the peripheral speed distribution of the rotor becomes similar to that of the oscillator shown in Fig. 4B, the circumferential slip is reduced (Δv ≅ 0). In addition, this embodiment takes a countermeasure against the circumferential slip not only for the rotor but also for the oscillator. A contact spring 1a formed on a driving portion of an oscillator 1 is designed to have a structure having flexibility in the radial direction so as to prevent a radial slip. Also, the distal end portion of the contact spring 1a is divided in the circumferential direction to form a large number of projections 1b, and these projections 1b are designed to have a structure having flexibility in the circumferential direction, thereby preventing a circumferential slip. As a method of setting the neutral plane N of the flexure at the contact position of the rotor 2 at a level lower than the contact surface, as shown in Fig. 16, another member 2c consisting of a material having higher rigidity than that of the rotor material may be joined to serve as the thick base portion. Also, as shown in Fig. 21, the contact spring of the rotor may be temporarily bent upward from the axially downward extending end portion, and a bent end portion 2j may be used as the contact portion. In this manner, the neutral plane N of the flexure at the contact position of the rotor 2 can be set at a level lower than the contact surface. Fig. 17 shows the second embodiment of the present invention. A rotor 2 of this embodiment also serves as an output member since an outer circumferential portion 2A having a gear 4 consisting of a synthetic resin is joined to the outer circumferential surface of a metal inner circumferential base portion 2B for supporting a spring load. As shown in the exploded view in Fig. 18, a contact spring 2e having a trapezoidal section is formed on the lower portion of the outer circumferential portion 2A to obtain good frequency response. The contact spring 2e is formed to have resiliency in both the circumferential and radial directions. Since this contact spring 2e is formed on the outer peripheral edge of the rotor 2, a contact spring 2d of an oscillator 1 is formed on a flange so as to assure a contact state with the contact spring 2a of the rotor 2. Figs. 19A and 19B show the third embodiment of the present invention. This embodiment relates to a ring-shaped vibration driven motor. Fig. 19A is a partial perspective view of a rotor of the motor, and Fig. 19B is an enlarged sectional view of the rotor. In this embodiment, projections 21A for amplifying a circumferential displacement are formed in the circumferential direction on a contact spring 21 projecting downward from the inner peripheral portion of a rotor 20, thereby absorbing a speed difference in the circumferential direction. Since a large number of slits for amplifying a circumferential displacement are normally formed on the driving surface of an oscillator of the ring-shaped vibration driven motor, circular gaps 23 each having a narrow opening width 22 are formed, as shown in Fig. 19B, so as to prevent the projections 21A of the rotor 20 from being fitted in the grooves between the slits, and to assure proper resiliency in the circumferential direction. Furthermore, as shown in Fig. 22, springs 1B which can be displaced in the circumferential direction may be formed on the contact surface portion of each of projections 1A for amplifying a vibration of the oscillator of the ring-shaped vibration driven motor. As a ring-shaped vibration driven motor, a linear motor, in which an elliptic oscillator is formed to have a linear portion, and an object is pressed against the linear portion, has already been proposed. In this case, the structure of a contact spring of the object may be the same as described above. Figs. 20A and 20B show the fourth embodiment of the present invention. In this embodiment, a rod-shaped vibration driven motor is used as a linear motor. A flange 6 of an oscillator 1 is fixed to a mounting member (not shown) via a support pin 5, and a slide member 7 which is slidable along a guide shaft 9 via balls 8 is driven by a rotor 2 of the vibration driven motor. The rotor 2 has a contact spring 2g which projects radially, and has flexibility in the axial direction. Projections 2i are formed on the end portion of the contact spring by, e.g., a transfer method using a resin having a wear resistance. These projections 2i are formed to have a triangular section, and have flexibility in the moving direction of the slide member 7. Fig. 23 shows an AF driving system of a lens barrel which uses the vibration driven motor of the first embodiment of the above-mentioned embodiments as a driving source. A lens holding frame 30 is driven by the rotational force of the vibration driven motor via a reduction gear 31. The driving force of the vibration driven motor is transmitted to an output gear 31B via an input gear 31A, and rotates the lens holding frame. According to the present invention as claimed, the sliding loss can be reduced, and improvement of motor efficiency, improvement of wear resistance, prevention of noise generation, and the like can be realized. The above-mentioned invention as claimed can also be applied to a motor in which a rotor 2 is fixed, and a stator 1 rotates. This invention as claimed has as its object to eliminate or reduce the speed difference between the contact surfaces of an oscillator and a rotor. In other words, a peripheral speed difference Δv between the contact surfaces of the rotor and the oscillator at arbitrary two points and a peripheral partial velocity difference ΔV of the oscillator main body at the two points satisfy ΔV > Δv.

Claims: A vibration driven motor or actuator having a vibration member (1) and a contact member (2), said vibration member (1) being mutually coupleable with said contact member to achieve a driving reaction between them, such that a peripheral relative speed difference Δv between the peripheral speed differences between the contact surface of said contact member (2) and the contact surface of said vibration member (1) at two arbitrary points (A, B) of a contact portion between said contact surface of said contact member (2) and said contact surface of said vibration member (1), and a peripheral partial speed difference ΔV of the vibration member (1) between the peripheral speeds of said vibration member (1) at the two points (A, B) satisfy a relation ΔV > Δv, characterized in that, to achieve the above relation, the contact portion has a configuration wherein a neutral plane (N) of a vibration generated in said contact member (2) is located on a vibration member side of the contact surface of said contact member (2) to be brought in contact with the contact surface of said vibration member (1), for decreasing the peripheral relative speed difference Δv by setting the peripheral speed distribution of the contact surface of said contact member (2) to be close to that of the contact surface of said vibration member (1). A motor or actuator according to claim 1, wherein said contact member (2) is made of a first material, and a neutral plane adjustment member (2c) made of a second material having higher rigidity than the first material is connected to a surface of said contact member (2) facing said vibration member (1) such that the neutral plane (N) is set by said neutral plane adjustment member (2c). A motor or actuator according to claim 1, wherein a bent end portion (2j) of said contact member (2) is used as the contact portion such that the neutral plane (N) is set. A system driven using as a driving source a vibration driven motor or actuator according to claim 1, 2 or 3.

Inventors: OKUMURA ICHIRO, TSUKIMOTO TAKAYUKI, OKUMURA, ICHIRO, TSUKIMOTO, TAKAYUKI, OKUMURA, ICHIRO, C/O CANON KABUSHIKI KAISHA, TSUKIMOTO, TAKAYUKI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: H01L41
IPC codes: H02N2, H01L41