A vibration driven motor

Abstract: In a vibration driven motor or actuator, which has members for generating a first bending vibration and a second bending vibration in a direction different from the first bending vibration therein, and in which a vibration combined the first bending vibration with the second bending vibration is caused, rigidities in two directions of the vibration member are set to be equal to or substantially equal to each other.

Description: The present invention relates to a bar-shaped vibration driven motor or actuator and, more particularly, to a bar-shaped vibration driven motor suitably used for optical equipment such as cameras, and OA equipment such as printers. A bar-shaped vibration driven motor is basically constituted by a bar-shaped vibration member 1, and a rotor 2 contacting the end face of the vibration member 1, as shown in Fig. 2. When the positional phase difference among piezo-electric elements 1b11, 1b12, 1b21, and 1b22 of the vibration member 1, and the temporal phase difference of an AC voltage of an ultrasonic wave to be applied are properly selected, the surface grains of the end face serving as the driving surface of the vibration member 1 are caused to form a circular or elliptic motion, thereby rotating the rotor 2 contacting the driving surface. In the vibration member, the driving piezo-electric elements 1b11, 1b12, 1b21, and 1b22, and a vibration detection piezo-electric element 1b3 are arranged between columnar vibration member structural bodies 1a1 and 1a2, which are formed of a material such as metals (e.g., Bs, SUS, aluminum, and the like) causing less vibration attenuation. Electrode plates 1c3 to 1c6 are arranged between each two adjacent piezo-electric elements. A fastening bolt 3 inserted from the side of the vibration member structural body 1a2 is threadably engaged with a female screw portion of the vibration member structural body 1a2 to clamp and fix the piezo-electric elements, thus constituting an integrated vibration member. The rotor 2 contacts, under pressure, the driving surface of the vibration member 1 via a spring case 5a by the biasing force of a compression spring 5 so as to obtain a frictional force. A rotary output member 6 frictionally contacts the spring case 5a. The member 6 has a gear portion on its outer circumferential surface, and is meshed with a gear (not shown) to transmit the rotational force of the rotor 2 to an external mechanism. The rotary output member 6 has a bearing 7. Therefore, when the driving surface of the vibration member 1 makes a circular or elliptic motion, since the rotor 2 contacts near the peaks of the locus of, e.g., the elliptic motion, it is frictionally driven at a speed substantially proportional to the tangential speed. In order to increase the rotational speed of the motor, the vibration amplitude on the driving surface must be increased. Most of energy losses in the vibration member are internal frictional losses caused by the strain of the vibration member by the vibration, and depend on the total sum of strains. For this reason, in order to increase the rotational speed of the motor, and to reduce the energy losses, it is desirable to increase the vibration amplitude of only a portion near the contact portion. Thus, the present applicant proposed a vibration member in which a circumferential groove 1d is formed in the vibration member 1 so as to increase the vibration amplitude of only the portion near the contact portion. Figs. 3A and 3B show radial displacement distributions of the shaft portion of the vibration member depending on the presence/absence of the circumferential groove 1d of the vibration member 1. Fig. 3A shows a case of a vibration member having no circumferential groove 1d, and Fig. 3B shows a case of a vibration member having the circumferential groove 1d. As can be seen from Fig. 3B, in the vibration member having the circumferential groove 1d, the rigidity of the portion of the circumferential groove 1d is lowered, and a large displacement is obtained at the side of the contact portion with the rotor as the driving surface. From this fact, when the two vibration members are designed to have the same displacement at the contact portion with the rotor, the displacements in other portions of the vibration member shown in Fig. 3B are generally smaller than those of the vibration member shown in Fig. 3A. As a result, the total sum of strains, i.e., the internal loss in the vibration member can be reduced. A bar-shaped vibration driven motor utilizes orthogonal bending natural vibrations in two directions (x- and y-directions) as the driving force. Therefore, it is impossible to obtain large amplitudes in both the two directions unless the two natural frequencies are substantially equal to each other. In this case, the locus of the surface grains of the vibration member is considerably shifted from a circular motion, and undesirably becomes closer to a linear motion. As a result, a high rotational speed cannot be obtained as a motor output, resulting in poor efficiency. Note that the two natural frequencies can be originally matched with each other by design calculations. However, in practice, the two natural frequencies have a difference (to be referred to as Δf hereinafter) therebetween, and the difference varies depending on individual vibration members. As a result, the motor performance varies depending on individual motors. Upon examination of the cause for the variation, it is found that when a screw portion 4 for clamping and fixing the upper and lower vibration member structural bodies 1a1 and 1a2 (see Fig. 2) is present near the circumferential groove 1d, the variation Δf becomes large. It is considered that the above-mentioned fact is caused by the presence of strong and weak meshing portions due to machining errors of male and female screws. More specifically, this causes a nonuniform rigidity, and the natural frequencies have a difference therebetween depending on a rigidity difference in the x- and y-directions of the vibration member. As can be seen from Fig. 3B, at the position of the circumferential groove 1d having a low rigidity, a change ( 2 -  1 ) in inclination angle of a vibration mode is large, and a large strain occurs. Therefore, the rigidity difference at this position tends to appear as a difference between the bending natural frequencies. The prior art according to the US-A-4 965 482 discloses a piezoelectronic motor which works by means of an ultrasonic vibration. The motor comprises a first and a second vibration member. One end of the first vibration member has a driving portion which is in contact with a rotor member. An electro-mechanical energy conversion element for generating a first bending vibration in a first direction, namely a torsional vibration, and a second bending vibration in a second direction, namely a longitudinal vibration is clamped between the other end of the first vibration member and the second vibration member. A coupling member extends axially through the first and second vibration members, the electro-mechanical energy conversion structure and the rotor member and is fixed the first vibration member and the second vibration member. Further, the prior art according to the EP-A-0 469 883 shall be notified which constitutes a state of the art with regard to Art. 54(3) and (4) EPC. It is an object of the present invention to prevent the generation of undesired vibrations in the motor and the transmission of vibrations of the motor to a portion where the motor is fixed to. This object is achieved by a vibration driven motor comprising the features according to claim 1. Further advantageously features and improvements are the subject-matters of the appended dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing a vibration driven motor or actuator according to the first embodiment of the present invention; Fig. 2 is a sectional view showing a conventional vibration driven motor; Figs. 3A and 3B are views showing vibration modes of the motor shown in Fig. 2; Fig. 4 is a sectional view showing a vibration driven motor according to the second embodiment of the present invention; Fig. 5 is a schematic sectional view showing a vibration driven motor according to the third embodiment of the present invention; Fig. 6 is a schematic sectional view showing a vibration driven motor according to the fourth embodiment of the present invention; Fig. 7 is a schematic view showing a head portion structure of a fastening bolt; Fig. 8 is a schematic view showing a structure wherein a washer is arranged between a fastening bolt and a lower vibration member structural body; and Fig. 9 is a sectional view showing principal part of an embodiment of a camera system adopting the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a sectional view showing a vibration driven motor or actuator according to the first embodiment of the present invention. In Fig. 1, vibration member structural bodies 1a1 and 1a2 are formed of, e.g., a metal such as Bs, SUS, aluminum, or the like, which causes less attenuation. Electro-mechanical energy conversion elements 1b11, 1b12, 1b21, 1b22, and 1b3 adopt PZT in this embodiment. Electrode plates 1c1 to 1c6 are formed of a conductor such as Cu, and are used for applying an electric field to the PZT. These electrode plates are manufactured by press, etching, or the like. Note that the electrode plates 1c1, 1c3, and 1c5 serve as ground electrodes, and their inner-diameter side portions are in contact with and electrically connected to a bolt 3 formed of, e.g., Fe. Therefore, only the ground electrode 1c1 is used as a ground power supply port. The PZT elements 1b11 and 1b12 are paired, and excite one bending vibration. The PZT elements 1b21 and 1b22 excite a bending vibration in a direction perpendicular to that of the bending vibration excited by the PZT elements 1b11 and 1b12. The PZT element 1b3 is a sensor PZT element, and generates an electromotive voltage according to its strain between the electrodes 1c1 and 1c6. Therefore, an insulating sheet 1e formed of, e.g., polyimide, Teflon, or the like is inserted, so that the PZT element 1a2, which is in contact with and electrically connected to the electrode plate 1c6, is not electrically connected to the bolt 3 as a ground electrode. The above-mentioned components are integrally clamped and fixed using the bolt 3. When the above-mentioned components are clamped and fixed by fastening the bolt, in order to obtain a constant axial force, a lubricant is applied between the bolt 3 and the insulating sheet 1e and between the insulating sheet 1e and the lower structural body 1a2 so as to reduce and stabilize the friction coefficients therebetween, whereby the axial force can be controlled by the fastening torque of the bolt. At this time, a plurality of insulating sheets may be inserted, and a lubricant (e.g., grease) may be applied between these sheets, thus enhancing the effect. Furthermore, in order to uniform the contact pressure between the bolt 3 and the structural body 1a2, a circumferential groove 3a may be formed in the lower surface of the bolt 3 to obtain a spring structure, as shown in Fig. 7, or a washer 11 or a belleville spring may be inserted, as shown in Fig. 8. In order to uniform the friction coefficient and the meshing state of a screw portion 4, the screw portion 4 may be subjected to R tap or R dice machining. This machining is also effective for stabilizing the natural frequencies of the vibration member, as will be described later. Furthermore, for example, a resin having a proper viscosity may be coated on the screw portion, so as to prevent movement of a metal piece dropped from the screw portion, which may cause short-circuiting of the electrodes in the vibration member upon insertion of the screw. Moreover, for the purpose of preventing electrical leak and rust when a water component is attached to the outer side surfaces of the PZT elements, a coating agent of, e.g., a resin having a low water absorbency is preferably applied. A rotor 2 is formed of aluminum, and is anodized to improve the wear resistance. A spring case 5a formed of a metal such as Fe, Bs, aluminum, Zn, or the like is adhered to the rotor 2. Therefore, since the rotor 2 and the spring case 5a are integrated, the rigidity of the rotor 2 is improved, and the deformation of the rotor 2 due to the pressure of a spring 5, the torque from a gear 6, or the like, becomes small. As a result, the rotor can always be in smooth contact with the vibration member. A fixing flange 8 is formed by die-casting, e.g., Zn, and is coupled to the distal end of the bolt 3 by adhesion or press fitting. The flange 8 is fixed to a motor fixing member 14 by screws 13 via a rubber member 12. Note that the screws 13 are fixed to the flange 8 in a self-tapping manner. The rubber member 12 has a function of a shock absorber for preventing the flange 8 from being deformed under the influence of the low surface precision of the fixing member 14, and also has a vibration insulating function of preventing a small vibration of the flange 8 from being transmitted to the fixing member 14. In the bar-shaped vibration driven motor with the above-mentioned arrangement, the conventional arrangement undergoes the following improvements so as to solve the above-mentioned problems. The bolt for clamping and fixing the vibration member structural bodies 1a1 and 1a2 is located at the side of the PZT elements so as to be separated from a circumferential groove 1d formed in the upper vibration member structural body 1a1. Note that the threadable engaging portion between the bolt 3 and the vibration member structural body 1a1 need only be located even slightly on the PZT side of a lower surface 1a100 of the circumferential groove 1d. In other words, the threadable engaging portion need only be prevented from overlapping the lower surface 1a100. In the screw portion 4 in this case, as can be seen from the vibration mode shown in Fig. 3B, a change in inclination angle is small, and the influence of the nonuniform rigidity of the screw portion on Δf is also reduced. As a result, a variation Δf of the vibration member from a designed value (about zero) becomes small, and motor performance can be stabilized. Fig. 4 shows the second embodiment. In this embodiment, upper and lower vibration member structural bodies 1a1 and 1a2 are clamped and fixed by adhesion or press fitting. A fixing portion 9 such as an adhesion layer or a press fitting portion is arranged not to overlap a circumferential groove portion 1d. Since the fixing portion 9 such as the adhesion layer or the press fitting portion is a factor of a nonuniform rigidity as in screw coupling, the influence of the nonuniform rigidity on Δf can be eliminated by the structure of this embodiment. Fig. 5 shows the third embodiment. In this embodiment, a stepped portion 3b is formed on a bolt 3 for fastening upper and lower vibration member structural bodies 1a1 and 1a2, and a male screw portion is formed on the lowermost portion of the bolt 3. The structural bodies 1a1 and 1a2 are fastened by the bolt 3 using a nut 10 outside the lower vibration member structural body 1a2. More specifically, a screw portion 4 is constituted by the male screw portion and the nut 10, and the position of this screw portion 4 corresponds to the free end of a vibration, resulting in a small change in inclination angle of the vibration mode, and a small strain. As shown in Fig. 6, the bolt 3 may be formed integrally with the upper vibration member structural body 1a1, and the structural bodies may be fastened by threadably engaging the nut 10 with the male screw portion formed on the lower portion of the bolt 3 as in the embodiment shown in Fig. 5. Thus, the same effect as in the above embodiment can be obtained, and the number of parts can be reduced. In each of the above embodiments, in consideration of the influence of rigidity indefinite factors such as screw coupling, adhesion coupling, press fitting coupling, and the like on the natural frequencies, these factors are eliminated from a place where a strain is large. In general, since these factors also cause a large internal loss, they are preferably eliminated from the place where a strain is large, from this viewpoint as well. Fig. 9 is a schematic view of a driving apparatus (e.g., a camera system) which uses a bar-shaped vibration driven motor according to the present invention, e.g., a vibration driven motor of the first embodiment as a driving source. A coupling gear 54 has a large gear portion 54a and a small gear portion 54b. A gear portion formed on the outer circumferential portion of the rotary output member 6 of the vibration driven motor is meshed with the large gear portion 54a. The small gear portion 54b is meshed with a gear portion 55a of a driving portion 55 of, e.g., a lens barrel of a camera, and is rotated by the rotational force of the motor. An encoder slit plate 56 is fixed to the coupling gear 54, and the rotational speed and rotation angle of the rotor 2 are detected by a photocoupler 57. As described above, in a vibration member provided with a means for decreasing the rigidity near a contact portion, a coupling member such as a screw coupling portion, an adhesion coupling portion, or the like, which causes a nonuniform rigidity, is not arranged near the low-rigidity portion. For this reason, a variation in natural frequency of the vibration member can be reduced. As a result, a variation Δf can also be reduced, and motor performance can be stabilized. Since a loss at the coupling portion is also reduced, motor efficiency can be improved. In a vibration driven motor or actuator, which has members for generating a first bending vibration and a second bending vibration in a direction different from the first bending vibration therein, and in which a vibration combined the first bending vibration with the second bending vibration is caused, rigidities in two directions of the vibration member are set to be equal to or substantially equal to each other.

Claims: A vibration driven motor or actuator comprising: a first and a second vibration member structural bodies (1a1, 1a2) one end of said first vibration member structural body (1a1) having a driving portion which is contacted with a rotor member (2); electro-mechanical energy conversion elements (1b11, 1b12, 1b21, 1b22, 1b3) being clamped between the other end of said first vibration member structural body (1a1) and said second vibration member structural body (1a2) by a coupling member (3) extending axially through said first and second vibration member structural bodies (1a1, 1a2), said electromechanical energy conversion elements (1b11, 1b12, 1b21, 1b22, 1b3) and said rotor member (2) and being fixed to at least one of said first vibration member structural body (1a1) and said second vibration member structural body (1a2), a motor fixing member (14) being coupled to the distal end of said coupling member (3) said electro-mechanical energy conversion elements (1b11, 1b12, 1b21, 1b22, 1b3) are adapted to generate a first vibration in a first direction and a second vibration in a second direction different from the first direction in said first vibration member structural body (1a1), said first vibration member structural body (1a1) has an amplitude increasing portion (1d) for increasing a vibration amplitude on said driving portion; said coupling member (3) has a coupling portion (4) coupled to said first vibration member structural body (1a1) along an axial direction of said coupling member (3) at a position separated from said amplitude increasing portion (1d); characterized in that said coupling portion (4) is located at the side of said electromechanical energy conversion elements (1b11, 1b12, 1b21, 1b22, 1b3) so as to be separated from said amplitude increasing portion (1d); both said first and said second generated vibrations are bending vibrations, said distal end of said coupling member (3) is coupled to said motor fixing member (14), whereby a shock absorber (12) is provided therebetween, said shock absorber (12) preventing a vibration from being transmitted to said motor fixing member (14). A vibration driven motor or actuator according to claim 1, wherein said amplitude increasing portion (1d) has a shape for decreasing a rigidity of said first vibration member structural body (1a1). A vibration driven motor or actuator according to claim 1 or 2, wherein said coupling portion (4) of said coupling member (3) includes a screw portion coupled to a corresponding screw portion on said first vibration member structural body (1a1). A vibration driven motor or actuator according to claim 1, 2 or 3, wherein said first and second vibration member structural bodies (1a1, 1a2) have a bar shape, and said amplitude increasing portion (1d) is formed as a circumferential groove on said first vibration member structural body (1a1). A vibration driven motor or actuator according to claim 1, 2, 3 or 4, wherein a fixing flange (8) is coupled to said distal end of said coupling member (3) and is fixed to said motor fixing member (14). A vibration driven motor or actuator according to claim 1, 2, 3, 4 or 5, wherein said coupling portion (4) is arranged separated from said amplitude increasing portion (1d) at a side of said conversion element (1b). A system incorporating a vibration driven motor or actuator as claimed in any of claims 1 to 6.

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