A vibration driven actuator

Abstract: A vibration driven motor or actuator includes a vibrating member for generating a vibration therein in response to an electrical signal applied thereto, and a contact member frictionally engaged with the vibrating member to be driven by the vibration. The contact member has an element for, when the contact member is driven by the vibration, causing sin and cos modes of the vibration generated in the contact member to have different natural frequencies.

Description: The present invention relates to a vibration-driven motor or actuator and, more particularly, to a vibration driven motor or actuator, which vibrates an elastic member as a bar-shaped vibrating member by supplying electrical energy to electro-mechanical energy conversion elements provided to the bar-shaped elastic member so as to cause surface grains of the vibrating member to form a circular or elliptic motion, thereby frictionally driving a movable member urged against the vibrating member, and which motor or actuator is especially suitable for optical equipment such as cameras and OA equipment such as printers. A bar-shaped vibration driven motor excites surface grains of a bar-shaped vibrating member to form a circular or elliptic motion, thereby frictionally driving a movable member urged against the vibrating member. Since the magnitude of the circular or elliptic motion is at most about several µm, the vibrating member and a rotor must be finished to have a contact surface precision, which is high enough to cover the magnitude, so as to maintain a contact state over the entire peripheral surface of the vibrating member. However, in practice, the vibrating member and the rotor cannot have such a high precision in consideration of working cost. For this reason, spring characteristics are provided to the contact portion of the rotor so as to maintain the contact state over the entire peripheral surface of the vibrating member. A vibration driven motor driven according to the above-mentioned driving principle has motor characteristics, which exhibit a high torque at a low speed. Therefore, since no gears are required for decelerating the motor, the vibration driven motor is utilized in a product such as a camera, which must solve a gear noise problem, and requires quietness. However, the vibration driven motor often generates noise during driving. Such noise is caused by another vibration generated in the motor in addition to a vibration for driving the motor. The vibration is caused by a travelling wave. The present applicant proposed various countermeasures against this vibration. However, another slight noise still remains. Fig. 2 shows an observation result, using a microphone, of a vibration spectrum obtained when noise is generated during driving. In Fig. 2, 1f is the driving frequency of the vibrating member, 2f is the frequency component twice the frequency 1f, and 4f is the frquency in an audible frequency range (20 kHz or lower) for generating noise. These frequencies satisfy a relation 3f - 1f = 4f. Since the driving frequency is the frequency of an AC signal supplied to generate a driving vibration, it can be shifted by Δf. If the shifted frequency is represented by f s , it changes to satisfy 1f s = 1f + Δf, 2f s = 2f + 2Δf,.... On the other hand, the frequencies 4f and 3f change to satisfy 4f s = 4f - Δf and 3f s = 3f, respectively. Therefore, the vibration spectrum of noise corresponds to a differential frequency (4f s = 3f - 1f - Δf) between the frequency 3f and the driving frequency, and the frequency 3f can be considered as some natural vibration. Upon examination of this natural vibration, it was found that the natural vibration was a bending vibration of a rotor main ring 2 shown in Fig. 3. Fig. 4 is a perspective view of the rotor main ring 2. In Fig. 4, a solid line represents the rotor main ring 2 before deformation, and a dotted line represents a bending vibration mode. A contact spring 2a (see Fig. 3) also suffers from a bending deformation to follow the main ring portion 2 although it is not shown in Fig. 4. The object of the present invention is to prevent generation of such a vibration of the rotor main ring 2. In addition, another noise may often be generated in addition to the above-mentioned noise. Fig. 5 shows a vibration spectrum of this noise. Frequencies 1f, 2f, 3f, and 4f respectively represent the spectra of the driving vibration (1f), a vibration (2f) having a frequency twice the driving frequency, a natural vibration (3f) causing noise, and a vibration (Af) of noise like in Fig. 2. In Fig. 5, however, these frequencies satisfy 4f = 1f - (2f - 3f). Upon examination of the natural vibration 3f, it was found that the natural vibration was a bending vibration of the rotor contact spring 2a shown in Fig. 3. Fig. 6 shows the vibration mode of the contact spring 2a. In this mode, the deformation of the rotor main ring 2 is very small, and only the contact spring portion 2a suffers from a bending vibration unlike in the bending mode shown in Fig. 4. The Patent Abstracts of Japan, Vol. 14, No. 170 (E-913), 30 March 1990; & JP-A-20 26 283 disclose a torque control mechanism having a circular stator and a rotor provided with slits and facing the stator. In particular, the stator is provided with two rings which are constituted in the manner of generating a standing-wave vibration by the drive of a piezo-electric element. Due to the generation of a standing-wave vibration, a contact pressure between the rotor and stator or torque holding power, respectively, is decreased. However, this prior art arrangement suffers from a problem that noise vibration components or bending vibrations in the rotor may occur due to the generated vibration. It is therefore an object of the present invention to prevent the generation of the bending vibrations of the above mentioned rotor, rotor main ring portion and/or rotor spring portion, respectively, so as to prevent the generation of such noise components. This object is achieved with a vibration driven motor or actuator as set out in claim 1. Advantageous further developments of the invention are as set out in the dependent claims. Figs. 1A and 1B are respectively a plan view and a sectional view of a rotor of a vibration driven motor or actuator according to the first embodiment of the present invention, and Fig. lC is a plan view showing a vibration mode of the rotor; Fig. 2 is a graph showing a vibration spectrum of noise; Fig. 3 is a sectional view of a conventional rotor; Fig. 4 is a perspective view showing a vibration mode of a conventional rotor main ring, which causes noise; Fig. 5 is a graph showing a vibration spectrum of another noise; Fig. 6 is a view showing a bending vibration mode of a conventional rotor contact spring, which causes noise; Fig. 7 is a sectional view showing a rotor of a vibration driven motor according to the second embodiment of the present invention; Figs. 8A and 8B are respectively a plan view and a sectional view of a rotor of of a vibration driven motor according to the third embodiment of the present invention; Figs. 9A and 9B are respectively a plan view and a sectional view of a rotor of a vibration driven motor according to the fourth embodiment of the present invention; Figs. 10A and 10B are respectively a plan view and a sectional view of a rotor of a vibration driven motor according to the fifth embodiment of the present invention; Fig. 11 is a plan view showing a vibrating member of a vibration driven motor according to the sixth embodiment of the present invention; Fig. 12 is a plan view showing a vibrating member of a vibration driven motor according to the seventh embodiment of the present invention; and Fig. 13 is a schematic sectional view showing an apparatus, which adopts a vibration driven motor of the present invention as a driving source. Figs. 1A to 1C show a rotor of a vibration driven motor or actuator according to the first embodiment of the present invention. Fig. 1A is a plan view of the rotor, and Fig. 1B is a sectional view taken along a line A - A in Fig. 1A. In Figs. 1A and 1B, linear grooves 5 are formed on two upper peripheral portions of a rotor main ring 2. Note that a bar-shaped stator for driving the rotor shown in Figs. 1A to 1C by a travelling wave, and other elements for generating the travelling wave in the stator are substantially the same as those described in, e.g., U.S.P. No. 5,124,611 or 4,562,374, and a detailed description thereof will be omitted. When the grooves 5 are formed on the rotor (2, 2a) driven by the travelling wave generated in the stator, sin and cos modes are generated at positions illustrated in Fig. 1C. Since the grooves 5 are formed at positions corresponding to the anti-node positions of the vibration of the sin mode, the mode rigidity is considerably decreased, and hence, the natural frequency of the sin mode is lowered largely. However, since the grooves 5 are formed at positions corresponding to the node positions of the cos mode, a decrease in natural frequency of the cos mode is relatively small. As a result, the two natural frequencies have a difference therebetween. More specifically, the natural frequencies of the two modes coincide with each other at 54 kHz before formation of the grooves. However, after the grooves are formed, the natural frequency of the sin mode is lowered to 51 kHz, and that of the cos mode is lowered to 53 kHz. Thus, a frequency difference of 2 kHz is generated between the two natural frequencies. When the rotor (2, 2a, 5) was driven by the above-mentioned known stator (or vibrating member), noise caused by generation of a bending natural vibration of the rotor main ring was reduced. In this embodiment, the grooves 5 are formed at two positions. If four grooves are formed at four positions separated by equal angular pitches, the frequency difference between the two modes can be increased, and the noise prevention effect can be enhanced. Fig. 7 is a sectional view of main part of a ring-shaped rotor portion of a motor according to the second embodiment of the present invention. In Fig. 7, a member 6 receives a spring (not shown) for compressing a rotor (2, 2a) formed of a metal such as Fe, zinc, or the like. The member 6 is adhered to the rotor (2, 2a) to constitute an integrated structure. Note that the rotor (2, 2a, 6, 50) has a central axis CA. Linear grooves 50 as in Fig. 1A are formed on an upper portion of the spring receiving member 6, thus obtaining the same effect as in the first embodiment. In this manner, the same effect as in the first embodiment can be obtained if another member integrated with the rotor is utilized. Figs. 8A and 8B show the third embodiment of the present invention. This embodiment takes a countermeasure against generation of noise caused by generation of a bending vibration of a contact spring portion of a ring-shaped rotor. Fig. 8A is a plan view of a rotor when viewed from a side contacting a vibrating member, and Fig. 8B is a partial longitudinal sectional view of the rotor. Groove portions 51 are formed on corner portions of a rotor contact spring of the rotor, and noise can be prevented for the same reason as described above. Note that the ring-shaped rotor (2, 2a, 51) has a central axis CA. Figs. 9A and 9B show the fourth embodiment of the present invention. In this embodiment, a contact spring structure 8 is provided to a vibrating member 7, which is the same as that disclosed in Fig. 2 of U.S.P. No. 4,562,374 or in U.S.P. No. 5,124,611. Noise caused by a bending vibration generated in this portion is prevented by grooves 52. In each of the above embodiments, the two components, i.e., sin and cos modes of a vibration generated in the rotor are caused to have different natural frequencies by forming the grooves, thereby preventing generation of a travelling wave in the rotor, and reducing noise. As shown in Figs. 10A and 10B, a rotor may have an axially asymmetrical shape by forming a chamfer portion 9 on the side surface of a rotor (2, 2a), so that the sin and cos modes have different natural frequencies, thereby simultaneously taking countermeasures against noise for both the rotor main ring 2 and the contact spring 2a. Alternatively, as shown in Fig. 11, masses 10 such as screws may be added to the rotor (2, 2a). In each of the above embodiments, a countermeasure against noise is taken by forming grooves on two peripheral portions. However, when a mode causing noise is the third or fourth bending vibration, three or six grooves, or four or eight grooves are preferably formed at equal angular pitches. When noise components are generated due to the second and third vibrations, both two equal-pitch grooves and three equal-pitch grooves are formed, thereby simultaneously taking countermeasures against the noise components caused by the two vibrations. Eight equal-pitch grooves 11-1 to 11-8 need not always be formed for the fourth bending vibration. For example, as shown in Fig. 12, some grooves (11-4, 11-6, and 11-7) may be omitted to obtain the same effect as described above. Fig. 13 is a schematic sectional view showing an embodiment of an assembled state of a vibration driven motor according to the present invention, which motor has a linear-shaped groove 50A on an outer peripheral portion of a spring case i, and an apparatus (a camera in this embodiment), which adopts this vibration driven motor as a driving source. As disclosed in the above-mentioned U.S.P. No. 5,124,611, vibrating member structural bodies b 1 and b 2 are fastened by a bolt c to sandwich A- and B-phase piezoelectric elements a 1 and a 2 and an electrode plate therebetween, thereby constituting a bar-shaped vibrating member. When electrical signals having a temporal phase difference therebetween are supplied to the piezoelectric elements a 1 and a 2 , bending vibrations are generated in a plurality of different planes including the axis of the vibrating member, and a rotation is formed on the surface of the vibrating member. A rotor r is constituted by a rotor main ring 2 and a rotor contact spring 2a. The rotor r is urged against the driving surface of the vibrating member (b 1 , b 2 , a 1 , a 2 , c) via a compression spring h and a spring case i. A rotary output member f is formed with a gear portion on its outer circumferential surface, and is axially supported by a bearing e. The rotary output member f has two projections which are inserted loosely to linear-shaped grooves provided to a portion of spring case i, and is rotated integrally with the rotor r. A fixing member g is fixed to the distal end portion of the bolt c, and is attached to a stationary member (e.g., a stationary cylinder of a lens barrel) j by screws via a rubber sheet k. A gear 54 is constituted by large and small gear portions 54a and 54b. The large gear portion 54a is meshed with the gear portion of the rotary output member f, thus extracting the output of the bar-shaped vibration driven motor. A driven member (e.g., a lens barrel of a camera) 55 has a gear portion 55a, which is meshed with the small gear portion 54a, and is rotated by the vibration driven motor so as to attain, e.g., an auto-focus (AF) function. A slit plate 56 is rotated integrally with the gear 54. A photocoupler 57 detects rotation of the gear 54. As described above, noise generated in the bar-shaped vibration driven motor or actuator includes a noise component caused by a bending vibration generated in a movable member, e.g., the rotor main ring, and a noise component caused by a bending vibration generated in the contact spring portion of the rotor. Since the natural vibration modes (sin and cos modes) causing these noise components are caused to have a frequency difference therebetween by forming grooves or adding masses to the rotor main ring or the contact spring portion, generation of noise can be prevented. In each of the above embodiments, the vibrating member is fixed, and the rotor is moved. In contrast to this, the present invention as claimed can be applied to an actuator which has a movable vibrating member. Of course, in this case, grooves or the like for providing a natural frequency difference between the sin and cos modes are formed on a fixed member. A vibration driven motor or actuator includes a vibrating member for generating a vibration therein in response to an electrical signal applied thereto, and a contact member frictionally engaged with the vibrating member to be driven by the vibration. The contact member has an element for, when the contact member is driven by the vibration, causing sin and cos modes of the vibration generated in the contact member to have different natural frequencies.

Claims: A vibration driven motor or actuator comprising: a vibrating member (7), having a contact surface (b1, b2, a1, a2, c), for generating a travelling vibration therein in response to an electrical signal applied thereto; and a contact member (2, 2a) having a contact surface frictionally engaged with the contact surface of said vibrating member, said contact member further having a vibration noise suppressing means (5; 50; 51; 52; 9; 10; 11-1 to 11-8) for suppressing a travelling vibration generated in the contact member when said contact member is driven by the vibration generated in the vibration member, with said vibration noise suppressing means causing sin and cos modes of said travelling vibration generated in the contact member to have different natural frequencies when said contact member is driven by the vibration. A vibration driven motor or actuator according to claim 1, wherein said sin and cos modes of said travelling vibration generated in the contact member are bending vibrations. A vibration driven motor or actuator according to claim 1, further comprising an output transmission member (f), engaged with one of said contact member and said vibrating member, for moving a portion of a system. A vibration driven motor or actuator according to claim 1 or 3, wherein said contact member is a ring-shaped member. A vibration driven motor or actuator according to claim 4, wherein said vibration noise suppressing means is formed by a slit provided on a portion of said contact member. A vibration driven motor or actuator according to claim 1, wherein said contact member is a ring-shaped member and said vibration noise suppressing means causes the ring-shaped member to have an axially asymmetrical shape. A vibration driven motor or actuator according to claim 1, wherein said contact member is a ring-shaped member and said vibration noise suppressing means is formed by a ring-shaped groove provided on a portion of said contact member. A vibration driven motor or actuator according to claim 3, wherein said contact member is a ring-shaped member and said vibration noise suppressing means is formed by a linear-shaped groove provided on a portion of said contact member. A vibration driven motor or actuator according to claim 7 or 8, wherein said vibrating member includes a vibrating element and a pair of electro-mechanical energy conversion elements (a1, a2) which respond to respective electrical signals having a phase difference in time therebetween, so as to generate bending vibrations in different planes including an axis of the vibrating member. A vibration driven motor or actuator according to claim 1, wherein said contact member is a rotor member. A vibration driven motor or actuator according to claim 10, comprising a pair of electro-mechanical energy conversion elements (a1, a2) which respond to respective electrical signals having a phase difference in time therebetween, so as to generate bending vibrations in different planes including an axis of the vibrating member, whereby a combined vibration of the two bending vibrations is generated in said vibrating member and wherein said rotor has a support element (i) for supporting an urging member (h) for urging said rotor (r) against said vibrating member. A vibration driven motor or actuator according to claim 11, wherein said vibration noise suppressing means comprises a groove provided on a portion of said support element. A vibration driven motor or actuator according to claim 12, wherein said vibrating member is a bar-shaped member, and said rotor is a ring-shaped member which contacts an upper surface of the bar-shaped vibrating member. A vibration driven motor or actuator according to claim 11, wherein said rotor has a groove formed on a portion thereof and extending in a radial direction of the rotor. A vibration driven motor or actuator according to claim 1, wherein said contact member has a continuous contact surface frictionally engaged with the contact surface of said vibrating member. A vibration driven motor or actuator according to claim 15, further comprising an output transmission member, engaged with one of said contact member and said vibrating member, for moving a portion of a system. A vibration driven motor or actuator according to claim 10 or 14, wherein said rotor member has a continuous contact surface frictionally engaged with the contact surface of said vibrating member. A vibration driven system incorporating a vibration driven motor as claimed in any one of claims 1 to 17. A system as claimed in claim 18, characterized by said system being an optical system. A system as claimed in claim 18, characterized by said system being a camera system.

Inventors: KOREEDA SHINICHI, TSUKIMOTO TAKAYUKI, KOREEDA, SHINICHI, TSUKIMOTO, TAKAYUKI, KOREEDA, SHINICHI, 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