Ultrasonic motor

An ultrasonic motor utilizing flexural travelling wave components. A ball bearing is constituted by the inner ring, the balls, and an outer ring provided with a groove similar to the ball positioning groove of the inner ring, at a position where the ball are interposed. A rotor is provided inside the amplifying projections which are formed in the vibration member. A coil spring is provided in order to bring the rotor and the projections of the vibration member into pressure contact with each other. A spring pressing seat having a groove for receiving the other end of the coil spring is fixed to the support pin, with the coil spring and the support pin centered. The coil spring receiving groove of the inner ring and the coil spring are positioned so as not to share the same vertical plane with the ball. In this manner the thickness of the ultrasonic motor can be reduced.

BACKGROUND OF THE INVENTION 
This device relates to an ultrasonic motor. 
A structure of conventional ultrasonic motors is shown in FIG. 7. A support 
8 is provided with a vibrator portion 82 to which a piezoelectric element 
31 is adhered. A rotor 4 equipped with a bearing 9 is rotatably supported 
by a shaft portion 81 formed in the support 8 and which has balls 44. A 
coil spring 5 for bringing a bending plate portion 83 of the support 8 
into pressure contact with the rotor 4 is provided around the shaft 
portion 91 of the support 8 in such a manner as to share the same vertical 
plane with the bearing 9 and moreover, to come into pressure contact with 
it. The other end of the coil spring 5 is supported by a spring pressing 
seat 6. 
According to the conventional device, the coil spring 5 and a ball 
receiving sheet of the ball bearing are coaxially arranged so that they 
share the same vertical plane with each other. Therefore, this structure 
has a disadvantage for reduction of the thickness of ultrasonic motors. 
Since a means for restricting the ball position is not provided in the 
rotor or the ball receiving plate of the ball bearing, the balls are free 
to move to displaced positions, making the pressing force by the coil 
spring likely to be unstable. 
Since the bearing lower ring is fixed to the rotor, friction occurs in the 
balls of the bearing due to the rotation of the rotor, thereby rotating a 
bearing upper ring. Accordingly, the pressing spring is twisted, the 
pressing force varies, and the bearing upper ring and the support pin are 
worn out, making the rotation of the rotor unstable. Furthermore, the 
position of the bearing lower ring supported turnably, with the shaft 
portion of the support pin of the rotor centered, is positioned lower than 
the projections of the vibration member in the cross section. Therefore, 
creaks occur between the inner ring and the support pin because of the 
inclination of the rotor center portion in the pressing direction. As a 
result, noise results and the force of the coil spring cannot be 
transmitted sufficiently to the contact portion between the projections of 
the vibration member and the rotor. 
On the other hand, in a conventional travelling wave motor using a bearing 
for supporting the rotation of a rotor, the rotor is mounted rotatably on 
a central shaft via the bearing, In such a motor, the quantity of 
wriggling of the bearing is small and not set to a specific level. 
In a travelling wave motor, two kinds of alternating voltages of different 
phases are applied to a piezoelectric element adhered to a vibration 
member, to generate a flexural travelling wave in a vibration member in 
accordance with an expansion motion of the piezoelectric element and drive 
a rotor, which is engaged under pressure with the vibration member. The 
rotor is rotated by a frictional force in a direction opposite to the 
direction in which the travelling wave advances. The contacting condition 
of the vibration member 303 and a friction member 302 bonded to the rotor 
301 is as shown in FIG. 6, where A is the direction of rotor 301 and B is 
that of travelling wave. Since the rotor is driven by a frictional force, 
it is ideally desirable that all of the ridges of the travelling wave 
uniformly contact the sliding surface of the friction member. However, 
concerning the piezoelectric element, it is impossible to generate a 
consistently regular travelling wave due to the irregular shape of the 
electrode patterns and the irregular polarization condition thereof. Also, 
concerning the sliding surface of the friction member, it is impossible 
for the contacting condition thereof to be consistently uniform due to the 
lack of the manufacturing accuracy. Accordingly, in order to obtain a 
motor of a higher efficiency, it is necessary that the motor have a means 
for reducing the irregularities referred to above. In a motor using a 
bearing for supporting the rotation of a rotor, the wriggling portions of 
the bearing work to absorb such irregularities. Therefore, when the 
quantity of wriggling or play of a bearing is extremely small, such 
irregularity cannot be absorbed, so that the contacting condition becomes 
uneven. This causes a slip to occur, and the performance of the motor to 
be diminished. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a structure which 
solves the problems described above, can reduce the thickness of an 
ultrasonic motor, can apply a consistently stable pressing force, can 
prevent the following rotation of a bearing upper ring due to the rotation 
of a bearing lower ring, can prevent thrust between an inner ring and a 
support pin due to creaks, and can provide a motor of a high efficiency. 
In order to solve the problem described above, the present invention 
provides a support pin which is inserted into a support plate. A vibration 
member having a piezoelectric element adhered thereto is fixed to the 
support pin. Further, an inner ring having a groove for receiving a coil 
spring is rotatably fitted to the support pin. Balls are held by a ball 
positioning groove formed in the inner ring. A ball bearing is constituted 
by an outer ring having a groove, which is similar to the ball positioning 
groove formed in the inner ring, in a position spaced apart by the balls. 
A rotor is provided inside the amplifying projections which are formed in 
the vibration member. The rotor and the projections of the vibration 
member are brought into pressure contact with each other by a coil spring. 
There is provided a spring pressing seat having a groove for receiving the 
other end of the coil spring with the support pin centered. The coil 
spring is fixed to the support pin by a set screw through the spring 
support seat. The coil spring receiving groove of the inner ring and the 
coil spring are provided at positions where the balls of the ball bearings 
are positioned radially offset from and may be at least partially 
co-planar with the spring. A surface cut portion is formed at the upper 
part of the inner ring. An inner ring presser is fixed to the support pin 
by a set screw so as to prevent the following rotation of the inner ring 
due to the rotation of the outer ring. A protuberance is formed in either 
the inner ring or the support pin at the same level as the balls, so that 
creaks between the inner ring and the support pin due to the inclination 
of the rotor will hardly occur. 
Accordingly, the inner ring with which the coil spring is in contact 
applies a stable pressing force to the rotor disposed in the outer ring. 
The surface cut portion is formed at the upper part of the inner ring, and 
the inner ring presser is fixed to the support pin by the set screw. The 
inner ring does not rotate even when the ball rotates because of the 
rotation of the rotor through the outer ring fixed to the rotor, and 
friction occurs between the ball and the inner ring. Therefore, it is 
possible to fix the coil spring at a predetermined position and to 
stabilize the pressing force. Furthermore, it is also possible to prevent 
wear of the inner ring and the support pin and to stabilize the rotation 
of the rotor. The protuberance is formed either on the inner ring or the 
support pin at the same level as the balls. According to this arrangement, 
even when there exists an inclination of the vibration member fitted to 
the support pin within the fitting tolerance, there is a safety margin of 
a tilt angle of the inner ring with respect to the support pin before 
creaks or frictional noise develop. 
Furthermore, the present invention is constructed so that the quantity of 
wriggling of a bearing, which is used to support the rotation of a rotor, 
in the direction of the thrust thereof is larger than the quantity of 
amplitude of a travelling wave occurring in a vibration member, or larger 
than the largest of the quantity of amplitude of the same travelling wave 
and the quantity of waviness of a sliding surface of a friction member. 
The quantity of wriggling of a bearing in the direction of the thrust 
thereof mentioned above means a total quantity of vertical movement of the 
bearing occurring between the condition shown in FIG. 5(a), in which an 
outer ring 45 rises with respect to an inner ring 41 via balls 44, and the 
condition shown in FIG. 5(b), in which the outer ring 45 falls with 
respect to the inner ring 41 via the balls 44. 
When the quantity of wriggling of the bearing, which is used to support the 
rotation of the rotor, in the direction of the thrust thereof is set 
larger than the quantity of amplitude of a travelling wave occurring in 
the vibration member, the degree of freedom of vertical movement of the 
rotor increases. Accordingly, even when the travelling wave becomes 
uneven, the rotor is automatically driven so as to attain the most stable 
contacting condition. When the quantity of wriggling of the bearing in the 
direction of the thrust thereof is set larger than the quantity of 
waviness of the sliding surface of the friction member, the degree of 
freedom of vertical movement of the rotor increases. This can prevent the 
partial contacting of the contact surface with respect to the vibration 
member and enables the uniform contacting condition of the contact surface 
to be obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Hereinafter, an embodiment of the present invention will be explained with 
reference to the drawings. 
In FIG. 1, a support pin 2 is fixed to a support plate 1. A vibration 
member 3 having a piezoelectric element 31 adhered thereto is fixed to the 
support pin 2. An inner ring 41 having a coil spring receiving groove 46 
for receiving a coil spring 5 is fitted rotatably to the support pin 2. 
Balls 44 are held inside a ball positioning groove A 42 formed in the 
inner ring 41. Another ball positioning groove B 43 which is substantially 
the same as the ball positioning groove A 42 is formed in an outer ring 45 
at a position where the ball 44 are interposed between them. 
A ball bearing comprises the inner ring 41, the balls 44 and the outer ring 
45. A rotor 4 is provided inside projections 32 for displacement expansion 
which is formed in the vibration member 3. The rotor 4 and the projections 
32 of the vibration member 3 are brought into pressure contact with each 
other by the coil spring 5. This coil spring 5 is supported by a spring 
pressing seat 6 having a groove 61 for receiving the other end of the coil 
spring 5, with the support pin 2 centered and is fixed to the support pin 
2 by a set screw 7. The coil spring receiving groove 46 of the inner ring 
41 and the coil spring 5 are disposed at positions where they do not share 
the same vertical plane with the balls 44. FIG. 2 is a sectional view 
showing another embodiment of the present device, and a surface cut 
portion 47 is formed at the outer peripheral portion of the inner ring 41. 
An inner ring presser 10 is fitted between the support pin 2 and the 
spring pressing seat 6 in such a manner that the surface cut portion 47 of 
the inner ring 41 meshes with a recessed slit 101 of the inner ring 
presser 10. The inner ring presser 10 is fixed by the set screw 7 and 
restricts the rotation of the inner ring 41 caused by the rotation of the 
outer ring 45. FIGS. 3 and 4 are sectional views, each showing still 
another embodiment of the present device. Belt-like protuberance 48 or 21 
is provided either inside the hole of the inner ring 41 or at the outer 
periphery of the support pin 2 at the position of the same level of the 
balls 44. 
According to this structure, even when the inner ring 41 inclines to the 
ball 44, the variation of inclination is small because the belt width of 
the protuberance 48 or 21 existing on the sliding portion between the 
support pin 2 and the inner ring 41 becomes small. Accordingly, few creaks 
between the support pin 2 and the inner ring 41 occur. 
In accordance with the present invention described above, the ball bearing 
is supported by the spring pressing seat through the coil spring and is 
fixed to the support pin by the set screw. Since the coil spring receiving 
groove of the inner ring and the coil spring are disposed at the positions 
where they do not share the same vertical plane with the balls, the inner 
ring in contact with the coil spring supports the rotor which is provided 
on the outer ring through the balls. Therefore a stable pressing force can 
be applied to the rotor. Since the balls and the coil spring are provided 
at the positions where they do not share the same vertical plane, the 
thickness of the ultrasonic motor can be reduced simultaneously. 
Furthermore, since the surface cut portion is formed on the upper part of 
the inner ring and the inner ring presser is fixed to the support pin by 
the set screw, the following rotation of the inner ring because of the 
rotation of the outer ring can be prevented. The stable pressing force can 
be obtained between the rotor and the vibration member. Further, wear of 
the inner ring and the support pin can be prevented and the rotation of 
the rotor can be stabilized. Since the protuberance is disposed either on 
the inner ring or the support pin at the same level of the balls, creaks 
between the inner ring and the support pin due to the inclination of the 
rotor hardly occurs and the pressing force can be stabilized. 
A motor in which the outer diameter of the vibrator was 10 [mm] was 
practically produced. In this motor, the amplitude of the vibration of the 
vibration member in a no-load condition was around 1-2 [.mu.] and the 
waviness of the friction member around 5-10 [.mu.]. Experiments were 
conducted with this motor by varying the quantity of wriggling of the 
bearing in many ways to find out that, when the quantity of wriggling of 
the bearing was not more than 10 [.mu.], the rotating performance of the 
motor was inferior. It was also found out that the sliding sound was 
large, and that a slip occurred. Accordingly, it is desirable that the 
quantity of wriggling of the bearing be larger than that of waviness of 
the friction member. 
As described above, the present invention in which the quantity of 
wriggling of the bearing in the direction of the thrust thereof is set 
larger than the quantity of amplitude of a travelling wave occurring in 
the vibration member, or larger than the largest of the quantity of 
amplitude of the same travelling wave and the quantity of waviness of the 
sliding surface of the friction member, whereby the degree of freedom of 
the vertical movement of the rotor increases to enable the irregularities 
of the dimensions of the vibration member and friction member to be 
absorbed, a stable contacting condition of these parts can be obtained, 
and a travelling wave motor of a high efficiency can be provided.