Vibration wave driven apparatus and image pickup apparatus including vibration wave driven apparatus

A vibration wave driven apparatus includes a vibrator configured to generate vibration, a rotor configured to be in frictional contact with the vibrator and to rotate about an axis of rotation, and a transmitting member configured to rotate about the axis and to transmit rotation of the rotor to an external component. A part of the transmitting member forms a worm portion of a worm gear.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vibration wave driven apparatus and an image pickup apparatus.

2. Description of the Related Art

Vibration wave driven apparatuses that are applied to products for driving camera lenses or the like include a bar-type vibration wave driven apparatus. Japanese Patent Laid-Open No. 2008-187805 discloses a bar-type vibration wave driven apparatus in which the output of a rotor is transmitted to an external device with a gear interposed therebetween. The mechanism of the known vibration wave driven apparatus will now be described with reference toFIGS. 10A and 10B.FIG. 10Ais a cross-sectional view of the known bar-type vibration wave driven apparatus.FIG. 10Bis a perspective view of a part where the rotor and the gear engage with each other.

InFIG. 10A, a first elastic body1, a second elastic body2, and a multilayer piezoelectric device3are fastened by a shaft4and a nut5with a specific clamping force. A rotor7has one surface (the lower end in the drawing) thereof being in contact with a wear-resistant member6. The wear-resistant member6is provided at the upper end of the first elastic body1. A gear8is provided so as to face the other surface of the rotor7. The gear8rotates with the rotor7and transmits the output of the vibration wave driven apparatus to an external device. The position of the gear8is fixed in a thrust direction along the shaft4by a flange (attaching portion)10provided for attachment of the vibration wave driven apparatus. A pressure spring15for applying a pressure to the rotor7is provided between the gear8and the rotor7.

As shown inFIG. 10B, the rotor7has a circular recess7cand in the top surface thereof a pair of grooves7aand7bextending radially and axisymmetrically with each other. The gear8has on the bottom surface thereof a cylindrical projection8cconfigured to engage with the circular recess7cof the rotor7, and a pair of projections8aand8b(the projection8bis not shown) configured to engage with the grooves7aand7b, respectively, of the rotor7. With such engagements, the rotation of the rotor7is transmitted to the gear8and is output.

In the known vibration wave driven apparatus, the gear8and a gear of the external device that receives the output from the gear8are spur gears. Therefore, the optical axis of a lens barrel of a camera and the axis of rotation of the vibration wave driven apparatus are to be arranged substantially parallel to each other.

On the other hand, the substantially parallel arrangement of the optical axis of the lens barrel and the axis of rotation of the vibration wave driven apparatus restricts the flexibility in the arrangement of the vibration wave driven apparatus. Therefore, further improvement has been demanded for efficient utilization of space.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a vibration wave driven apparatus includes a vibrator configured to generate vibration, a rotor configured to be in frictional contact with the vibrator and to rotate about an axis of rotation, and a transmitting member configured to rotate about the axis and to transmit rotation of the rotor to an external component. A part of the transmitting member forms a worm portion of a worm gear.

DESCRIPTION OF THE EMBODIMENTS

First Embodiment

FIG. 1is a cross-sectional view of a vibration wave driven apparatus according to a first embodiment of the present invention. Referring toFIG. 1, a first elastic body1, a second elastic body2, and a multilayer piezoelectric device3, functioning as an electro-mechanical energy conversion element, are fastened by a shaft4, corresponding to a shaft member, and a nut5with a specific clamping force. The first and second elastic bodies1and2are made of metal such as brass, stainless, or the like. The elastic bodies1and2and the multilayer piezoelectric device3together form a Langevin vibrator.

A rotor7has one surface (the bottom surface in the drawing) thereof being in contact with a wear-resistant member6. The wear-resistant member6is provided at the upper end of the first elastic body1. The one surface of the rotor7has a smaller contact area than the other surface (the upper surface in the drawing) of the rotor7and has an appropriate spring characteristic. A gear8, functioning as an output-transmitting member, is provided so as to face the other surface of the rotor7. The gear8coaxially rotates with the rotor7and transmits the output of the vibration wave driven apparatus (i.e., the rotation of the rotor7) to an external device. Details of the gear8will be described separately below.

The gear8is pressed against a flange10, functioning as an attaching portion at which the vibration wave driven apparatus is attached to the external device, by a pressure spring15, intended for application of a pressure to the rotor7, with a pressure the same as the pressure applied to the rotor7. The position of the gear8is fixed in the thrust direction along the shaft4by being in contact with an end surface of the flange10with the foregoing pressure.

As in the known bar-type vibration wave driven apparatus, when a high-frequency voltage is applied to the multilayer piezoelectric device3, a bending vibration having, for example, an ultrasonic frequency is generated on a vibrator device9. Consequently, an elliptic motion is produced on the top surface of the first elastic body1that is in contact with the rotor7, causing the rotor7that is pressed against the wear-resistant member6to be in frictional contact with the wear-resistant member6and to be driven to rotate. As in the known bar-type vibration wave driven apparatus, the rotor7and the gear8have on end surfaces thereof a pair of grooves and a pair of projections, respectively, such as the ones shown inFIG. 10B. With the engagement between the grooves and the projections, the rotation of the rotor7is transmitted to the gear8. Thus, the rotor7, the gear8, and the pressure spring15together rotate about the shaft4. Frictional contact refers to a state of contact accompanied by friction acting in a direction opposite to the direction of a relative motion occurring between two objects that are in contact with each other. A rotor refers to a component that is rotated by microvibration (an elliptic motion) of a stator and is configured so as to be pressed against a friction plate with a pressure applied by a pressure spring. The rotor is rotated with friction produced at the point of contact because of the elliptic motion occurring on the surface of the stator in contact therewith. A transmitting member is a component intended for transmission of the driving force of a motor to an external device, and is provided between a rotor and a flange. The transmitting member fits to one of the surfaces of the rotor opposite the one having the point of frictional contact. Thus, the transmitting member and the rotor rotate together.

Compared with the gear8in the known bar-type vibration wave driven apparatus shown inFIG. 10A, the gear8according to the first embodiment is longer in an axial direction, that is, a portion of the gear8extending along the shaft4is longer. Therefore, in the first embodiment, the diameter of the shaft4is larger than in the known device, whereby higher stiffness is obtained. Thus, the vibrator device9is supported stably during a driving operation.

Details of the gear8will now be described. The gear8shown inFIG. 1includes a worm81(a worm portion) functioning as an output-extracting portion. The worm81and a worm wheel11, which is an external gear (a first-stage gear), in combination form a worm gear. The worm gear produces a rotating motion about an axis substantially perpendicular to the axis of rotation of the rotor7. In general, a worm gear refers to a mechanism including a driving gear (worm) and a helical gear (worm wheel) that meshes therewith. The worm has a small diameter and produces thrust in the axial direction thereof at the meshing point. The worm gear is characterized by realizing a high speed-reduction ratio. The worm81according to the first embodiment is a cylindrical worm and is provided between the rotor7and the flange10. Needless to say, a double-enveloping worm whose diameter increases toward the ends thereof as shown inFIG. 2may alternatively be employed. If a double-enveloping worm is employed, a larger area of contact with the worm wheel11is provided, improving the wear resistance of the worm.

The output transmission diameter of the worm81(the diameter of the worm81at the position where the worm81meshes with the worm wheel11) is smaller than the outside diameter of the rotor7. This is because of the following reason. A thrust Fw produced by torque transmission by the worm gear acts as a moment that causes the gear8to tilt with respect to the shaft4. Therefore, with a smaller output transmission diameter, the moment causing the gear8to tilt becomes smaller.

The gear8fits to the bottom of the flange10at one end (the upper end inFIG. 1) thereof. The gear8also fits to the shaft4at a stopper82projecting inward from the inner periphery thereof. That is, the gear8fits to the shaft4at two points near the axial-direction ends thereof, the two points providing supports for suppressing changes in the orientation of the gear8caused by the thrust Fw. Thus, the gear8is prevented from tilting with respect to the shaft4. The stopper82is provided at such a position as to be in contact with a stepped portion of the shaft4. The thrust Fw is received by the stepped portion of the shaft4. Thus, the displacement of the gear8along the shaft4is suppressed. In the first embodiment, the position of the gear8in the direction in which the shaft4extends is fixed.

The gear8also has a flange portion83integrally formed thereon with a diameter larger than the diameter of the worm81. Radially extending slits as shown inFIG. 3may be provided in the flange portion83so that the flange portion83and a photointerrupter12provided separately therefrom together form a rotational sensor. In such a case, the high speed-reduction ratio of the worm gear is utilized in directly reading the rotation of the vibration wave driven apparatus operating at a high speed, whereby a highly precise driving operation is realized. Needless to say, a rotational sensor may alternatively be obtained by attaching separately prepared slits to the flange portion83.

In terms of spatial availability, it is beneficial that peripheral spaces about the shaft4other than the space occupied by the worm wheel can be used freely. Therefore, in the first embodiment, electrical components13are provided on the outer side of the periphery of the worm81but on the inner side of the outer periphery of the rotor7. Unlike typical electromagnetic motors, the vibration wave driven apparatus according to the first embodiment of the present invention is not affected by magnetic fields. Hence, there is no issue even if, for example, a coil that generates a magnetic field is provided and some fluxes of the magnetic field leak to the vibration wave driven apparatus.

To summarize, in the first embodiment, a part of the gear8, functioning as an output-transmitting member, rotating coaxially with the rotor7is configured as a worm of a worm gear. Therefore, the axis of rotation of the vibration wave driven apparatus can be provided substantially perpendicularly to the optical axis of a lens barrel without increasing the number of components.

Second Embodiment

FIGS. 4 and 5are cross-sectional views of a vibration wave driven apparatus according to a second embodiment of the present invention. Descriptions of elements having the same shapes as those in the first embodiment are omitted.

In the second embodiment, a space A (a clearance) is provided between the stopper82and the stepped portion of the shaft4(at a position where the stopper82and the shaft4fit to each other). Thus, the gear8subjected to the thrust Fw produced in torque transmission by the worm gear is movable along the shaft4.

FIG. 4shows a state of the vibration wave driven apparatus according to the second embodiment in which the thrust Fw acts toward the flange10. Hereinafter, the state shown inFIG. 4is referred to as a first state. The gear8is subjected to a pressure acting toward the flange10because of a force Fs of the pressure spring15that is compressed and the thrust Fw produced by the worm gear. With the pressure produced as the sum of the foregoing forces, the position of the gear8in the thrust direction along the shaft4is fixed such that the gear8is in contact with the flange10. A friction-driven portion of the rotor7(a portion of the rotor7that is in frictional contact) with respect to the wear-resistant member6is subjected to a pressure defined by the amount of compression of the pressure spring15in the above state.

FIG. 5shows a state of the vibration wave driven apparatus according to the second embodiment where the thrust Fw acts toward the rotor7. Hereinafter, the state shown inFIG. 5is referred to as a second state. When the thrust Fw produced by the worm gear is larger than the force Fs produced by the pressure spring15pushing up the gear8toward the flange10, the gear8moves along the shaft4toward the rotor7and stops at a position where the force Fs of the pressure spring15and the thrust Fw balance each other. When the difference between the force Fs of the pressure spring15and the thrust Fw is large, the gear8stops at a position where the stopper82comes into contact with the stepped portion of the shaft4. In this state, the gear8is in contact with the stepped portion of the shaft4with a pressure obtained by subtracting the force Fs of the pressure spring15from the thrust Fw. Thus, the position of the gear8in the direction in which the shaft4extends is fixed. The stepped portion of the shaft4is provided at such a position that the end of the gear8and the end of the rotor7do not come into contact with each other even if the gear8moves toward the rotor7and that the state where the gear8fits to the flange10is maintained. As in the first state shown inFIG. 4, the friction-driven portion of the rotor7with respect to the wear-resistant member6is subjected to a pressure defined by the amount of compression of the pressure spring15in the above state.

Accordingly, the amounts of compression of the pressure spring15in the first and second states are different from each other, and the pressures applied to the friction-driven portion in the first and second states are different from each other. Specifically, the pressing load applied to the friction-driven portion is larger in the second state where the thrust Fw acts toward the rotor7as shown inFIG. 5than in the first state where the thrust Fw acts toward the flange10as shown inFIG. 4.

Especially, the second embodiment of the present invention is designed such that, in a case where the vibration wave driven apparatus is applied to a camera lens, the direction of rotation of the rotor7, responsible for extension and retraction of a lens barrel, becomes the same as the direction of the thrust Fw. Extension refers to an operation in which the lens barrel moves in such a direction as to project, and retraction refers to an operation in which the lens barrel moves in the opposite direction (a direction in which the lens barrel is housed into the body of the camera).

To extend the lens barrel, the vibration wave driven apparatus is operated in the second state where the rotor7is rotated in such a direction that the pressing load applied to the friction-driven portion increases. To retract the lens barrel, the vibration wave driven apparatus is operated in the first state where the pressing load becomes smaller than in the second state. Thus, different driving modes are provided. Specifically, when the lens barrel is extended, the output is increased with a large torque, and when the lens barrel is retracted, the output is reduced with a relatively small torque. The known bar-type vibration wave driven apparatus is configured such that the thrusts produced in the extending operation and the retracting operation are substantially the same. Therefore, if the thrust in the retracting operation is reduced so as to prevent foreign substances from being caught by the lens barrel, the thrust in the extending operation has to be reduced inevitably. In the configuration according to the second embodiment, however, while foreign substances are prevented from being caught by the lens barrel during the retracting operation, a powerful extending operation is realized even if a force that tends to suppress the extension of the lens barrel is applied from outside.

Furthermore, the amplitude of alternating voltage, i.e., a drive signal of the vibration wave driven apparatus, is changed in accordance with the direction of rotation of the rotor7. Specifically, different widths of a pulse that generates an input signal are provided for the first and second states, respectively, whereby the amplitude of alternating voltage applied to the multilayer piezoelectric device3is variable. That is, in the second state, the pressing load applied to the friction-driven portion increases and the friction produced increases. Therefore, the width of the aforementioned pulse is increased and the amplitude of the voltage applied is increased, whereby a stable driving operation is realized.

Next, the behavior of the gear8in the second state will be described.FIG. 6shows the orientation of the gear8of the vibration wave driven apparatus according to the second embodiment of the present invention that has been operated for a long period of time. InFIG. 6, for easier recognition, the amount of change in the orientation is scaled larger than actual.

In the second state, the thrust Fw produces a lateral force Fw2acting radially at the portion of the gear8at which the gear8fits to the shaft4. The lateral force Fw2may lead to wear or deformation at the portion of the gear8at which the gear8fits to the shaft4. Furthermore, the thrust Fw acts as a moment that tends to rotate the gear8clockwise about the portion of the gear8at which the gear8fits to the flange10. Consequently, the gear8rotates in a steady orientation shown inFIG. 6.

When such a state is examined in a cross section focusing on peripheral regions about the shaft4, the pressure spring15is more compressed at a portion thereof near the worm wheel11. Accordingly, there is some nonuniformity in the load applied to press the rotor7against the wear-resistant member6, producing a pressure pattern at the friction-driven portion. Furthermore, the axis of the rotor7is pushed toward a side opposite to the side of the worm wheel11. Consequently, speed variations synchronous with the rotation tend to occur, resulting in an unstable driving operation.

In a situation where a pressure pattern occurs at the friction-driven portion and the axis of the rotor7is displaced as described above, an orientation of the rotor7allowing the rotor7to rotate stably with the axis thereof not being constrained by the gear8during the driving operation is determined on the basis of the entirety of the vibration wave driven apparatus. Specifically, as shown inFIG. 7, a clearance (gap) larger than the amount of change in the orientation of the gear8can be provided between a circular recess7cof the rotor7and a cylindrical projection8cof the gear8or between each of circular holes7hof the rotor7and a corresponding one of columnar projections8hof the gear8. The circular recess7cand the circular holes7hare fitting portions of the rotor7with respect to the gear8, and the cylindrical projection8cand the columnar projections8hare fitting portions of the gear8with respect to the rotor7.

FIGS. 8A and 8Bare perspective views showing the shapes of end surfaces of the rotor7and the gear8, respectively.FIG. 8Ashows the end surface of the gear8on which four columnar projections8hare provided on a virtual circle, concentric with the outline of the gear8, at intervals of 90 degrees.FIG. 8Bshows the rotor7in which the circular holes7hare provided in correspondence with the columnar projections8h, the circular holes7heach having such a diameter that a sufficient clearance is provided with respect to the corresponding columnar projection8h. With the circular holes7hand the columnar projections8hprovided at intervals of 90 degrees, torque is assuredly transmitted from the rotor7to the gear8through at least one of the columnar projections8h.

FIG. 8Cshows a state of the rotor7and the gear8fitting to each other seen in the axial direction. The rotor7is shown by solid lines, and the outline of the gear8is shown by broken lines. InFIG. 8C, the gear8is eccentric with respect to the rotor7with an upward displacement. The maximum allowable amount of eccentricity corresponds to the difference in radius between each circular hole7hand the corresponding columnar projection8h. Specifically, a clearance of about 0.1 to 0.3 mm is provided between the circular hole7hand the cylindrical projection8h. Torque is transmitted from the rotor7to the gear8through some of the columnar projections8h. When the rotor7rotates in the direction of the arrow shown inFIG. 8C, torque is transmitted through one of the columnar projections8hshown on the right inFIG. 8C. Needless to say, the number of columnar projections8hand the clearance between the columnar projection8hand the circular hole7hare to be adjusted appropriately.

To summarize, with a specific clearance provided between each fitting portion of the rotor7and the corresponding fitting portion of the gear8, the rotor7that is being driven is not subjected to a force that constrains the orientation thereof in the radial direction. Therefore, regardless of changes in the orientation of the gear8, the rotor7finds by itself such an orientation of the axis thereof as to be stably driven, whereby a smooth driving operation is maintained.

Third Embodiment

FIG. 9Ais a perspective view showing the appearance of a camera, corresponding to an image pickup apparatus, according to a third embodiment of the present invention. The camera includes a lens barrel40.FIG. 9Bis a schematic diagram of a mechanism that drives the lens barrel40.

InFIG. 9B, the camera according to the third embodiment includes, on one side of the lens barrel40, the vibration wave driven apparatus according to any of the above embodiments and speed reduction gears20ato20c. The output of the vibration wave driven apparatus is transmitted to a gear30provided on the lens barrel40through the speed reduction gears20ato20c, whereby the lens barrel40moves in the direction of the optical axis. The third embodiment employs three speed reduction gears20ato20cso as to reduce the rotational output of the worm wheel11and to transmit the reduced output to the gear30of the lens barrel40. The number of speed reduction gears may be changed in accordance with the speeds of extension and retraction of the lens barrel40. Alternatively, such speed reduction gears may be omitted, and the lens barrel40may be driven by configuring the worm wheel11to directly mesh with the gear30of the lens barrel40.

In the vibration wave driven apparatus according to any of the embodiments of the present invention, a part of the gear8, functioning as an output-transmitting member, forms the worm81of the worm gear and meshes with the worm wheel11. Therefore, a rotation about an axis extending in the vertical direction of a camera is converted into a rotation about an axis extending along an optical axis. Accordingly, as in the third embodiment, the vibration wave driven apparatus can be provided substantially perpendicularly to the optical axis (that is, the axis of rotation of the vibration wave driven apparatus can be provided substantially perpendicularly to the optical axis of the lens barrel).

Although the vibration wave driven apparatus shown inFIG. 9Bis provided on the side of the speed reduction gears20ato20c, if the vibration wave driven apparatus and the speed reduction gears20ato20care arranged in line along the optical axis, a compact arrangement is realized.

This application claims the benefit of Japanese Patent Application No. 2009-149055 filed Jun. 23, 2009, which is hereby incorporated by reference herein in its entirety.