Vibration driven motor

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 combined vibration of the first bending vibration and 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.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a bar-shaped vibration driven motor or 
actuator and, more particularly, to a bar-shaped vibration driven motor 
suitable for use in optical equipment such as cameras, and OA equipment 
such as printers. 
2. Related Background Art 
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 a positional phase 
difference among piezo-electric elements 1b11, 1b12, 1b21, and 1b22 of the 
vibration member 1, and a temporal phase difference of an applied AC 
voltage for an ultrasonic wave are properly selected, surface portions of 
the end face, which serves as the driving surface of the vibration member 
1, are caused to follow 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 pair of adjacent piezo-electric elements. A 
fastening bolt 3 having a male screw thread is inserted from the side of 
the vibration member structural body 1a2, and is threadably engaged with a 
female screw portion of the vibration member structural body 1a2 to clamp 
and fix the piezo-electric elements therebetween, thus constituting an 
integrated vibration member. 
The rotor 2 is press contacted to 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 the energy losses in the vibration member are internal frictional 
losses caused by strain in the vibration member caused by the vibration, 
and depend on the total sum of the 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 of the vibration member near the contact 
portion. 
Thus, the present applicant has proposed a vibration 10 member in which a 
circumferential groove 1d is formed in the vibration member 1 so as to 
increase the vibration amplitude of only a portion of the vibration member 
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 a 
circumferential groove 1d of the vibration member 1. FIG. 3A shows the 
case of a vibration member having no circumferential groove 1d, and FIG. 
3B shows the case of a vibration member having a circumferential groove 
1d. As can be seen from FIG. 3B, in the vibration member having a 
circumferential groove 1d, the rigidity of the portion of the vibration 
member at 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, if the vibration members of FIGS. 3A and 3B are designed to 
have the same displacement at the contact portion with the rotor, then the 
displacements in other portions of the vibration member shown in FIG. 3B 
are generally smaller than the corresponding portions 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 directions 
unless the two natural frequencies are substantially equal to each other. 
In this case, the locus of the surface portions 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 .DELTA.f 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 has been 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 .DELTA.f becomes large. 
It is believed that the above-mentioned fact is caused by the presence of 
strong and weak meshing portions due to machining errors of the male and 
female screw portions. 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 (.theta..sub.2 -.theta..sub.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. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a vibration driven 
motor or actuator, which can solve the above-mentioned problems. 
In order to achieve this object, according to one aspect of the present 
invention, there is provided a vibration driven motor, which comprises: a 
vibration member in which electro-mechanical energy conversion elements 
are clamped between elastic members, the elastic members are fastened and 
integrated by fastening means, and an AC voltage having a predetermined 
temporal phase difference is applied to the electro-mechanical energy 
conversion elements, thereby causing surface portions of a driving surface 
to follow a circular or elliptic motion upon synthesis of bending 
vibrations excited in different planes and a movable member press 
contacted to the driving surface of the vibration member, the motor 
frictionally driving the movable member by the circular or elliptic motion 
followed by the vibration member, wherein the elastic members of the 
vibration member are formed with an amplitude increasing portion for 
increasing a vibration amplitude by decreasing a rigidity, and the 
fastening means does not have a coupling member near the amplitude 
increasing portion. 
According to another aspect of the present invention, in a vibration driven 
motor or actuator, which has a vibration member, including a portion for 
increasing an amplitude in a driving surface, for generating a vibration 
obtained by combining vibrations in two directions, rigidities in two 
directions of the vibration member are set to be equal to or substantially 
equal to each other. 
According to still another aspect of the present invention, 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 combined vibration of the 
first bending vibration and 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 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 are paired, and 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 a strain between the electrodes 1c1 and 1c6. 
Therefore, an insulating sheet 1e formed of, e.g., polyimide, 
polytetrafluoroethylene (TEFLON), or the like is inserted between 
vibration member body 1a2 and bolt 3, 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 pressure 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 make the contact pressure between the bolt 3 and 
the structural body 1a2, uniform, 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 make the friction coefficient and the meshing state of a screw 
portion 4 uniform, 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 any 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 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 deformation of the rotor 2 due to the pressure of a spring 
5, the torque from a gear 6, or the like, is 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 functions as 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 .DELTA.f is 
also reduced. As a result, a variation .DELTA.f of the vibration member 
from a designed value (about zero) becomes small, and motor performance 
can be stabilized. 
FIG. 1A illustrates an alternative configuration of the embodiment of FIG. 
1, wherein a circumferencial groove 1d is located in vibration member 
structural body 1a2. 
FIG. 4 shows the second embodiment of the present invention. 
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 .DELTA.f can be eliminated by the structure of this 
embodiment. 
FIG. 5 shows the third embodiment of the present invention. 
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 on 
rigidity of indefinite factors such as screw coupling, adhesion coupling, 
press fitting coupling, and the like, and thus on the natural frequencies, 
these factors are eliminated from a place where the 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 .DELTA.f 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.