Vibration driven motor

Abstract: A vibration driven motor is arranged so that a vibration member and a movable member are moved relative to each other by utilizing non-travelling vibration. A supporting member for the motor is disposed at the position of the loop of the aforesaid vibration.

Description: The present invention relates to a vibration driven motor according to the preamble of claim 1, which rotates a driven member disposed coaxially to a vibration member by frictional driving obtained by utilizing the circular motion of a vibration member which is produced by a combination of vibrations due to expansion and contraction occurring in a plurality of axial directions. Conventional types of vibration driven motors are disclosed in, for example, JP-A-62-141980, JP-A-63-214381 and JP-A-59-96881. As shown in Fig. 9, such a motor includes a vibration member 100 made from a metallic round bar of horn shape whose root portion gradually decreases in outer diameter, a pressure member 101 made from an annular metallic member having the same outer diameter as the large-diameter portion of the vibration member 100, and two annular magnetostrictive plates 102 and 103 disposed between the vibration member 100 and the pressure member 101. The pressure member 101 is fixed to the vibration member 100 by a bolt 104, and the magnetostrictive plates 102 and 103 are pressed against each other. Each of the magnetostrictive plates 102 and 103 has two electrodes symmetrically divided on one face, and a common electrode is formed on the other face of each of the magnetostrictive plates 102 and 103. The polarization direction of the two divided electrodes of the magnetostrictive plate 102 differs from that of the two divided electrodes of the magnetostrictive plate 103. These plates 102 and 103 are disposed out of positional phase by 90 degrees with their respective divided electrode sides facing forward. Electrode plates 105 and 106 are disposed between the magnetostrictive plates 102 and 103. The electrode plate 105 is disposed in contact with the divided electrodes of the magnetostrictive plate 103 positioned on the second (or rear) side, while the electrode plate 106 is disposed in contact with the common electrode of the magnetostrictive plate 102 positioned on the first (or front) side. The divided electrodes of the first magnetostrictive plate 102 and the common electrode of the second magnetostrictive plate 103 are disposed in contact with the vibration member 100 and the common electrode plate 107, respectively. When AC voltages, which are equal in amplitude and frequency, are respectively applied across the first magnetostrictive plate 102 and the second magnetostrictive plate 103 with a phase difference in time, vibration in which the vibration of the magnetostrictive plate 102 is combined with that of the magnetostrictive plate 103 is generated in the vibration member, whereby the extending end of the vibration member is caused to make a circular motion. Fig. 10 shows a conventional vibration driven motor which utilizes such a vibration member as a driving source. The extending end of the vibration member is pressed in contact with one face of a disk 108 to frictionally drive the disk 108 by the circular motion of the extending end of the vibration member. Thus, rotational torque is transmitted from a rotary shaft 109 fixed to the center of the disk 108. In the above-described conventional arrangement, the vibration member does not vibrate at its extending end only, but the entire vibration assembly constitutes one vibration system. It is, therefore, necessary to support the vibration member while minimizing the influence of vibration on the same. The vibration driven motor utilizing such a vibration member has another difficulty. For example, although the motor makes use of the motion of the extending end of the vibration member, it has been found from the experiments conducted by the present inventor that the rotational torque of the extending end of the vibration member is weak and sufficient driving torque cannot be applied to the disk which is a driven member. A generic vibration driven motor for rotating a driven member is known from US-A-4 812 697. A vibration member consists of a rod-like member comprising an engagement portion which, on one end side, serves as an output end or driving portion to be engaged with the driven member. An electro-mechanical energy conversion member is maintained in contact with the other end side of the vibration member opposite the output end and generates a vibration in the vibration member to cause an oscillating motion thereof. Also US-A-4 728 834 shows a vibration member which consists of a rod-like member comprising an engagement portion which, on one end side, serves as an output end or driving portion to be engaged with the driven member. An electro-mechanical energy conversion member is maintained in contact with the other end side of the vibration member opposite the output end and generates a vibration in the vibration member to cause an oscillating motion thereof. US-A-4 091 805 shows a vibration driven artificial hand. An electro-mechanical energy conversion member is maintained in contact with a contact portion of a rod-like vibration member to be vibrated in longitudinal direction. At the free end of the rod-like vibration member, the artificial hand is fixedly mounted. It is an object of the present invention to further develop a vibration driven motor according to the preamble of claim 1 such that a vibration generated by the rod-like vibration member is large and the driving efficiency is high. According to the present invention, this object is achieved by the features of claim 1. Advantageous further developments are set out in the dependent claims. According to the present invention, the vibration driven motor is capable of supporting a vibration member while minimizing the influence of vibration on the same. Furthermore the vibration driven motor is capable of applying sufficient torque to a vibration member and efficiently creating rotational torque, and of supporting the vibration member while minimizing the influence of vibration on the same. With such a vibration driven motor there is no fear of damaging a device in which it is incorporated. The above object, as well as features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. Fig. 1 is an exploded perspective view showing a first comparative example of a vibration driven motor not according to the present invention; Fig. 2 is a waveform diagram of an AC source which is applied across a piezoelectric sheet; Fig. 3A is a side elevational view showing the oscillating motion of a vibration member for use in the first comparative example; Fig. 3B is a front elevational view showing the oscillating motion of the vibration member for use in the first comparative example; Fig. 4 is a cross-sectional side elevational view showing the manner in which the first comparative example of the vibration driven motor is mounted; Fig. 5 is a schematic view showing the manner in which the first comparative example of the vibration driven motor is driven; Fig. 6 is a cross-sectional, side elevational view showing a second comparative example of the vibration driven motor not according to the present invention; Fig. 7 is a cross-sectional, side elevational view showing a third comparative example of the vibration driven motor not according to the present invention; Fig. 8 is a cross-sectional view of, with a portion broken away, an embodiment of the present invention; Fig. 9 is a schematic cross-sectional view showing a conventional vibration member; and Fig. 10 is a schematic perspective view showing a vibration driven motor utilizing the conventional vibration member. A preferred embodiment of the present invention will be described below in detail with reference to the accompanying drawings. [First Comparative Example] Fig. 1 is an exploded perspective view showing a comparative example of a vibration driven motor not according to the present invention. A vibration assembly A includes a vibration member 1 made from a rod-like metallic member. The vibration member 1 is of a cylindrical configuration and includes a small-diameter portion 1a which constitutes a first end portion, a large-diameter portion 1b which constitutes a second end portion, and a horn-shaped portion 1c having a diameter which is gradually reduced along the axis toward the first end. The vibration assembly A also includes a pressure member 2 made from a rod-like metallic member having an outer diameter which is the same as the diameter of the large-diameter portion 1b, and plates 3 and 4 each of which is made from a disk-shaped piezoelectric element having the same outer diameter as the large-diameter portion 1b. The piezoelectric plates 3 and 4 serve as electrical-to-mechanical energy transducers, respectively, and are disposed between the vibration member 1 and the pressure member 2 in such a manner that they sandwich an electrode plate 5. The pressure member 2 is secured to the vibration member 1 by means a bolt 6 so that the piezoelectric plates 3 and 4 are secured in position between the vibration member 1 and the pressure member 2. The head of the bolt 6 is held against the pressure member 2 with an annular insulating member 7 interposed therebetween, while the shaft of the bolt 6 is held in non-contact with the piezoelectric plates 3 and 4 and the electrode plate 5. Each of the piezoelectric plates 3 and 4 is polarized on one side in the direction of the thickness thereof so as to have two opposite electrodes, i.e., a positive electrode a and a negative electrode b . These electrode electrodes a and b are symmetrically formed with respect to an insulating portion d which crosses the central axis of the vibration assembly A. As illustrated, the piezoelectric plates 3 and 4 are combined so that their directions of polarization differ from each other. Each of the piezoelectric plates 3 and 4 further has a common electrode c shared by the positive and negative electrodes a and b on the other side. The positive and negative electrodes a and b are disposed out of phase with each other by an angle of 90 degrees with respect to the axis of the vibration assembly A, and the common electrode c of the piezoelectric plate 3 and the polarized side of the piezoelectric plate 4 are disposed in contact with the respective sides of the electrode plate 5. The polarized side of the piezoelectric plate 3, i.e., the positive electrode a and the negative electrode b , are maintained in contact with the first end face of the vibration member 1 serving as a conductor, while the common electrode c of the piezoelectric plate 4 is maintained in contact with the first end face of the pressure member 2. An AC voltage V 1 is applied across the electrode plate 5 and the vibration member 1, while an AC voltage V 2 is applied across the electrode plate 5 and the pressure member 2, whereby the piezoelectric plate 3 and the piezoelectric plate 4 vibrate while expanding and contracting in the directions of their respective thicknesses. The resulting vibrations of the respective plates 3 and 5 in the direction of deflection are combined to vibrate the vibration assembly A. The above-described vibration is a vibration which results from a combination of bending vibrations which are generated in different directions in a plane perpendicular to an axis ℓ of the vibration assembly A. Such a combined vibration may be called a non-traveling vibration which does not propagate along the axis of the vibration assembly A. In another aspect, the combined vibration is a vibration whose node and loop are not displaced along the axis of the rod-like vibration assembly A. As shown in Fig. 2, the AC voltage V 1 and the AC voltage V 2 are equal to each other in amplitude and frequency, but are 90° out of phase with each other. It follows, therefore, that the vibration assembly A is eccentrically displaced in response to the applied electrical signal, and makes a circular motion about its axis. Since the principle of the circular motion is known, explanation is omitted. If the opposite ends of the vibration assembly A are assumed to be free ends, respectively, the loop of the vibration is positioned at each of the opposite ends. Accordingly, because of the configuration of the vibration assembly A, the diameter of the circular motion of the vibration member 1 at the first end of the vibration member 1 becomes even larger than the diameter of the second end of the pressure member 2, but torque caused by the circular motion at the first end of the vibration member 1 is made small. In the comparative example, the resonance frequency of the vibration assembly A is selected so that the loop of the vibration of the vibration assembly A is positioned at a first end B (hereinafter referred to as a "frictional moving portion") of the horn portion 1c of the vibration member 1. The piezoelectric plates 3 and 4 are actuated at that resonance frequency. More specifically, the vibration assembly A vibrates in the mode of three nodes or more with the loop of the vibration positioned at least at the opposite ends and frictional moving portion B of the vibration assembly A. Accordingly, the vibration member 1 makes an oscillating motion in such a manner that the horn portion 1c and the small-diameter portion 1a are oscillated about the axis ℓ, as shown in Figs. 3A and 3B, while a central point P at the first end of the small-diameter portion 1a is travelling around the circumference of a circle C, as shown in Fig. 3B. A rotor 8, which will be explained later, is rotated around the axis ℓ by utilizing the oscillating motion at the frictional moving portion B. The torque produced at the frictional moving portion B is even greater than that produced at the first end of the small-diameter portion 1a. The rotor 8 is engaged with the small-diameter portion 1a along the axis ℓ of the vibration assembly A, and a second end (hereinafter referred to as a "frictional contact portion") 8b of an inner-diameter portion 8a of the rotor 8 is located at a position corresponding to the frictional moving portion B, whereby the second end of the frictional contract portion 8b is engaged with the horn portion 1c. As shown in Fig. 4, the inner diameter of the inner-diameter portion 8a of the rotor 8 is selected to be slightly larger than the outer diameter of the small-diameter portion 1a of the vibration assembly A so as to accommodate the oscillating motion of the small-diameter portion 1a. The lengthwise central portion 8c of the inner diameter 8a is formed to have a far larger inner diameter so that it is prevented from coming into contact with the small-diameter portion 1a. The frictional contact portion 8b of the rotor 8 is formed to become gradually wider toward the first end so as to match the outer configuration of the frictional moving portion B. The frictional contact portion 8b comes into line contact with the frictional moving portion B during the oscillating motion of the vibration member 1. The rotor 8 is urged in the direction of the arrows shown in Fig. 4 through a thrust bearing (not shown) by a spring or the like (not shown), thereby generating a predetermined frictional force at the area of contact between the frictional contact portion 8b and the frictional moving portion B. The aforesaid thrust bearing allows the rotor 8 to rotate about the axis thereof. More specifically, when the vibration assembly A vibrates with the loop thereof positioned at the frictional moving portion B, that is, in the mode of three nodes, the vibration member 1 makes the above-described oscillating motion about the axis ℓ as shown in Fig. 3. Accordingly, as shown in Fig. 5, the frictional moving portion B makes a circular motion in contact with the frictional contact portion 8b of the rotor 8 while drawing a locus of predetermined radius r with respect to the axis ℓ in the clockwise or counterclockwise direction. In Fig. 5, the outer diameter of the frictional moving portion B is shown as being much smaller than the inner diameter of the frictional contact portion 8b of the rotor 8 for the purpose of illustrating the manner in which the frictional moving portion B makes the circular motion in frictional contact with the inner periphery of the frictional contact portion 8b of the rotor 8. However, in practice, since the radius of the oscillating motion of the frictional moving portion B is extremely small, the gap between the frictional moving portion B and the frictional contact portion 8b of the rotor 8 is extremely small. As described above, not only the vibration member 1 vibrate, but the vibration assembly A vibrates as a whole. Accordingly, when a vibration driven motor M including the above-described arrangement is to be mounted in a device such as a camera or a printer, it is necessary to consider a supporting method for the vibration assembly A and associated elements. A method of supporting the vibration assembly A at the nodal position of vibration seems to be optimum because the vibration is small in amplitude. However, since the vibration assembly A makes an oscillating motion which starts on the nodal position of vibration, a plane perpendicular to the axis ℓ at the nodal position of the vibration is made to oscillate in the direction of the axis ℓ. For this reason, if a motor mounting flange extends from the position on the vibration assembly A which corresponds to the nodal position of vibration, that is, the outer periphery of the vibration member 1 or the pressure member 2, that flange vibrates back and forth in the axial direction and a mounting portion for the aforesaid device such as a camera or a printer may be at worst broken due to vibrations. For this reason, it is inappropriate to support the vibration assembly A at the nodal position of vibration. From examinations of the state of vibration of the vibration assembly A in various aspects, it is found that the position of the loop of vibration, which may seem inappropriate, is appropriate for a supporting position for the vibration assembly A. More specifically, although the amplitude of vibration is large at the position of the loop thereof, the vibration assembly is only displaced in the radial direction thereof, whereby the above-described fluctuating motion can be prevented. As can be seen from the structure of the vibration driven motor M, the loop position of vibration which serves as the supporting position is selected at a location axially rearwardly from the frictional moving portion B. Accordingly, if the vibration assembly A is to be vibrated in the mode of three nodes, the supporting position corresponds to the second end of the vibration assembly, while if the vibration assembly A is to be vibrated in the mode of four nodes, the supporting position is located at the second end of the vibration assembly A or between the second end of the vibration assembly A and the frictional moving portion B. The amplitude of vibration of the vibration assembly A is extremely small, and the amplitude of variation at the loop position serving as the supporting position is far smaller than the amplitude of the variation of the frictional moving portion B for actuating the rotor 8. Accordingly, radial displacement can be substantially ignored, and the vibration assembly A can be stably mounted in a device by selecting the supporting position for the vibration assembly A as the loop position of vibration. In Fig. 1, reference numeral 2aa denotes a supporting member which is provided at the loop position of the vibration of the vibration driven motor M. In a case where the second end of the vibration assembly A is selected as the supporting position as shown in Fig. 1, if that second end is secured to the securing member of a device or the like, the vibration assembly A assumes a vibration mode at the nodal positions which correspond to the opposite ends of the vibration assembly A. As a result, since the loop is not be formed at the frictional moving portion B, it is necessary to provide support so as to allow radial displacement. Although it is desirable that the position where the aforesaid supporting member is located accurately corresponds to the position of the loop, the former position is not limited only to the position of the loop and, for example, it may be located in the vicinity of the same. [Second Comparative Example] Fig. 6 is a cross-sectional, side elevational view showing, with a portion broken away, a second comparative example. In the second comparative example, the vibration driven motor M, which includes a rotor different from that of the vibration driven motor M used in the first comparative example and which utilizes the same supporting structure as the vibration driven motor M, is used for a drive source for driving the platen roller of a printer. As shown in Fig. 6, the vibration driven motor M including the vibration assembly A and a rotor 8A is accommodated in one end portion of a platen roller 10. The vibration driven motor M also serves as a supporting shaft for one end of the platen roller 10. The vibration driven motor M has a securing flange 2a formed at the second end of the pressure member 2 to support the second end of the pressure member 2 of the vibration assembly A. The flange 2a is mounted on a printer mounting frame 11 with a low-friction sheet 12 made from a low-friction material such as teflon or the like interposed therebetween. The flange 2a has openings (not shown), and screws 13, whose shafts are smaller than the inner diameters of the respective openings, are inserted through the corresponding openings and threadedly fitted into the mounting frame 11. In this manner, the movement of the vibration driven motor M in the axial direction is restricted, but the expansion and contraction of the flange 2a in the radial direction is allowed. The platen roller 10 is rigidly fitted onto the rotor 8A of the incorporated vibration driven motor M so that the rotational force of the rotor 8A is directly transmitted to the platen roller 10. A supporting shaft 14 is fixed to the other end of the platen roller 10. The supporting shaft 14 is supported by a printer mounting frame 16 for rotation about its axis and for movement along its axis. A spring 17 is resiliently fitted onto the supporting shaft 14, and one end of the spring 17 is pressed against a thrust bearing 18, while the other end is pressed against a bearing 15. The spring force of the spring 17 acts to press the rotor 8A against the vibration assembly A and enable the roller 10 to rotate about its axis. [Third Comparative Example] Fig. 7 is a cross-sectional, side elevational view showing a third comparative example. In the third comparative example, the vibration assemblies A is respectively disposed at the opposite ends of a cylindrical rotor 20 so that the rotor 20 is rotated by means of the two vibration assemblies A. A supporting shaft 21 extends rearwardly from the rear end of each of the vibration assemblies A to be fitted into a mounting hole 23 formed in a adjacent mounting member 22. A spring 24 is, as shown, resiliently fitted onto the shaft 21 so that the horn portion 1c of the vibration assembly A is pressed against a corresponding end portion 26 of the axial bore 25 of the rotor 20. The end portion 26 of the axial bore 25 of the rotor 20 is formed in a manner similar to that of the friction contact portion 8b of the rotor shown in the first embodiment, and corresponds to the frictional moving portion B of the vibration assembly A. The rotor 20 is rotatably supported at the opposite ends of its outer periphery by bearing members 27, whereby the rotor 20 is prevented from being shifted in the radial direction owing to the oscillating motion of the vibration assemblies A and the oscillating motion of the vibration assemblies A can be transformed into rotational force. When the supporting shafts 21 are fixed in the respective mounting holes 23, vibration mode may be changed as described above. In the third comparative example, to assure the degree of freedom of the supporting shafts 21 in the radial direction, the open end of each mounting hole 23 is defined by a tapered periphery 28. The rotor 20 of the third comparative example may also be used as a platen roller for a printer such as that described in the second comparative example. [Embodiment] Fig. 8 is a schematic cross-sectional view showing an embodiment of the present invention. In each of the above-described comparative examples 2 and 3, the rotor (roller) is supported by the bearing members 27. However, in the embodiment of the present invention, supporting shafts 41 and 42 extend from the respective ends of the roller 40, and the supporting shafts 41 and 42 are rotatably secured to a mounting member 45 by means of bearings 43 and 44, respectively. An axial bore 46 is formed to extend through a vibration assembly AA of a vibration driven motor A' for driving the roller 40, and the supporting shaft 41 is inserted through the axial bore 46. The supporting shaft 41 and the axial bore 46 are axially disposed. The vibration assembly AA has a cylindrical vibration member 47 formed integrally with a small-diameter portion 1'a, and the cylindrical vibration member 47 has an engagement opening 48 of circular shape in cross section. The engagement opening 48 is engaged with a connecting shaft 49 formed at one end of the roller 40. The cylindrical vibration member 47 is oscillated by the motion of the vibration driven motor A' and the roller 40 is rotated by frictional contact with the connecting shaft 49. During this time, since the supporting shafts 41 and 42 extending from the respective opposite ends of the roller 40 are supported by the corresponding bearings 43 and 44, the roller 40 is not displaced in the radial direction during the driving of the vibration assembly AA and the connecting shaft 49 of the roller 40 is consistently maintained in frictional contact with the engagement opening 48 of the cylindrical vibration portion 47. As shown, a ring-shaped flange 50 is integrally formed on the portion of the vibration member 1' at which the loop of vibration is formed. A plurality of brackets 51 extend from the mounting member 45, and resilient members 52 made of, for example, rubber, are respectively fitted with the brackets 51. The vibration assembly AA is retained between the brackets 51 so that the radial displacement of the vibration assembly AA can be absorbed by the intervening resilient members 52. As is apparent from the foregoing, in accordance with the present invention, it is possible to stably support the vibration assembly by locating the loop of vibration of the vibration assembly at the position of the securing member of a device such as a printer. Since two vibration assemblies are used, it is possible to actuate a movable member by far larger torque. As described above, the small-diameter shafts of the respective two vibration assemblies may be connected. In this arrangement, if the movable member is driven by skipping motion with the loop of vibration located at the central portion of the connected small-diameter shafts, the movable member can be driven by large torque resulting from the sum of the torques of the two vibration assemblies, when compared to a conventional arrangement in which driving is performed by utilizing the first end of the vibration assembly. In the arrangement in which the large-diameter shaft of the vibration assembly is disposed in a hollow movable member so that the movable member is rotated by skipping motion, the large-diameter shaft of the vibration assembly can be accommodated in the movable member. Accordingly, space savings can be improved. In the arrangement in which both rotary shafts of a movable member are rotatably supported by a mounting member and a vibration assembly is inserted through the opening of one rotary shaft, the eccentric displacement or the like of the movable member can be prevented and rotational torque can be efficiently produced. In the above-described embodiment, the two piezoelectric elements are utilized to generate vibrations in two directions, and a rotor is rotated by force resulting from a combination of the vibrations. However, three or more piezoelectric elements may be offset in position so that vibrations in a plurality of directions can be generated, and the rotor may be driven by force resulting from a combination of the vibrations. While the present invention has been described with respect to what is presently considered to be the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims: A vibration driven motor for rotating a driven member, comprising: a vibration member made from a rod-like member (1', 47) having an engagement portion (48) at one end side serving as a driving portion to be engaged with said driven member; and an electro-mechanical energy conversion member (3, 4) maintained in contact with a contact portion of said vibration member (1', 47) provided at the other end side thereof, for generating, in response to an electrical signal applied thereto, a vibration in said vibration member (1', 47) to cause an oscillating motion thereof, characterized in that said rod-like member has an axial bore extending therethrough and comprises a reduced portion (1'a) of a reduced diameter between said engagement portion (48) and said contact portion, the outer diameter of the reduced portion (1'a) being smaller than that of the engagement portion (48). A vibration driven motor according to claim 1, characterized in that said vibration generated in said vibration member (1', 47) is a combined vibration of a first bending vibration and a second bending vibration. A vibration driven motor according to claim 2, characterized in that said first bending vibration is generated in a first plane by said conversion member (3, 4) and said second bending vibration is generated in a second plane different from said first plane by said conversion member (3, 4). A vibration driven motor according to claim 1, 2 or 3, characterized in that the outer diameter of said reduced portion (1'a) is smaller than said contact portion. A vibration driven motor according to claim 2, 3 or 4, characterized in that said first bending vibration and said second bending vibration are shifted in substantially 90 degrees in phase from each other. A vibration driven motor according to claim 1, 2, 3, 4 or 5, characterized in that a cross section of said reduced portion (1'a) is smaller than that of said engagement portion (48), whereby a rigidity is small. A vibration driven motor according to claim 1, 2, 3, 4 or 5, characterized in that a cross section of said reduced portion (1'a) is smaller than that of said engagement portion (48) and said contact portion, whereby a rigidity is small.

Inventors: JUN TAMAI, JUN, TAMAI, JUN, TAMAI, C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: H02N2
IPC codes: H01L41, H02N2