Ultrasonic motor

An ultrasonic motor for driving a driven object with a multi-degree of freedom includes a vibrator configured to simultaneously excite two vibration modes to generate an elliptic vibration on an output surface thereof, a driven object configured to be driven by the elliptic vibration generated on the output surface, and a driving element interposed between the output surface and the driven object. The driven object includes a spherical portion to be brought into contact with at least the output surface via the driving element. The elliptic vibration exhibits different vibration amplitudes at different positions on the output surface. The output surface includes a plurality of regions which exhibit greater vibration amplitudes than other regions on the output surface. The driving element includes a contact portion with at least two regions exhibiting greater vibration amplitudes than other regions of the contact portion, and is provided on the output surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2010-001493, filed Jan. 6, 2010, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ultrasonic motor capable of driving a driven object with a multi-degree of freedom.

2. Description of the Related Art

An ultrasonic motor is known which applies a voltage to an ultrasonic vibrator to thereby friction drive a driven object kept in contact with the vibrator. In this case, if the driven object is made spherical to extract output therefrom, the ultrasonic motor can be used as a driving source of a multi-degree of freedom. Therefore, various applications of the ultrasonic motor, such as incorporation of the motor in a robot arm or a multi-joint arm, are expected.

As the ultrasonic motor capable of driving a driven object with a multi-degree of freedom, Jpn. Pat. Appln. KOKAI Publication No. 2005-143176, for example, has proposed a rotary driving device equipped with a driven object, rotary member support means, alternation power supply means, a vibrator, vibrator support means, rotation friction reduction means and pressing force adjusting means. The driven object has a substantially spherical surface. The rotary member support means supports the driven object so that the object can rotate about at least two axes. The alternation power supply means supplies electric energy to the vibrator. The vibrator converts, into mechanical energy, the electric energy supplied from the alternation power supply means, thereby generating a three-dimensional vibration. The vibrator support means supports the vibrator so that the vibrator will contact the substantially spherical surface of the driven object at a preset angle. The rotation friction reduction means reduces the friction that occurs when the driven object is rotated. The pressing force adjusting means adjusts, to a preset value, a pressing force with which the driven object and the vibrator contact each other.

In the rotary driving device disclosed in the above publication, the central portion of the upper surface (i.e., the output surface) of the vibrator is in point contact with the spherical surface of the driven object from the relationship between the output surface of the vibrator and the spherical surface of the object. Using the elliptic motion of the vibrator as a driving source, the driven object is friction driven about the contact point.

The vibrator will abrade at the contact point at which the vibrator and the spherical surface of the driven object directly contact each other. Further, the contact point will shift in accordance with the abrasion. Thus, the operation of the driving device is unstable.

To suppress the abrasion of the vibrator, the above publication has proposed to provide a sliding member on the upper portion of the vibrator. However, since the sliding member also abrades in accordance with the rotation of the vibrator, it does not fundamentally solve the abrasion problem.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of embodiments, there is provided an ultrasonic motor for driving a driven object with a multi-degree of freedom, comprising:

a vibrator having an output surface and configured to simultaneously excite two vibration modes to generate an elliptic vibration on the output surface;

a driven object configured to be driven by the elliptic vibration generated on the output surface; and

a driving element interposed between the output surface and the driven object, wherein

the driven object includes a spherical portion to be brought into contact with at least the output surface via the driving element;

the elliptic vibration exhibits different vibration amplitudes at different positions on the output surface;

the output surface includes a plurality of regions which exhibit greater vibration amplitudes than other regions on the output surface; and

the driving element includes a contact portion with at least two regions exhibiting greater vibration amplitudes than other regions of the contact portion, and is provided on the output surface.

DETAILED DESCRIPTION OF THE INVENTION

As shown inFIG. 1, an ultrasonic motor according to a first embodiment of the invention includes a cylindrical case10having a rectangular cross section and one opening. A vibrator12and a driven object14are contained in the case10. The opening of the case10is closed by a cap16. The cap16has an opening. An output shaft18is fixed to the driven object14. The output shaft18is extended through the opening of the cap16.

The case10and cap16are formed of, for example, member made of a resin with a high strength such as a peek, or metal.

The vibrator12is formed by stacking rectangular piezoelectric elements made of, for example, zirconate titanate. Using the stacked state of the piezoelectric elements, an X-axial bend portion20that bends along the X-axis, and a Y-axial bend portion22that bends along the Y-axis are formed. By supplying a predetermined alternating signal to the vibrator12, two different vibration modes, such as longitudinal and flexural vibrations are simultaneously excited. As a result of synthesis of these different vibration modes, an elliptic vibration will occur on the upper end surface or output surface24of the vibrator12. For particulars of the structure of the vibrator12and the elliptic vibration occurring on the output surface24thereof, see the afore-mentioned publication Jpn. Pat. Appln. KOKAI Publication No. 2005-143176.

The vibrator12is attached to the case10via an vibrator holder26made of, for example, a resin or metal material so that it can expand and contract. Further, for adjusting the press force of the vibrator12toward the driven object14to an appropriate value, a press mechanism28formed of springs and a press force adjusting member30formed rubber members are provided between the vibrator holder26and the case10.

The driven object14is made of a ceramic-based material such as zirconia or alumina, and formed spherical. The driven object14is friction-driven by the elliptic vibration that occurs on the output surface24of the vibrator12. In this case, by adjusting the X- and Y-directional driving forces, a driving force in an arbitrary direction can be acquired. For particulars of the driving method, also see Jpn. Pat. Appln. KOKAI Publication No. 2005-143176. Although the driven object14can be driven with a multi-degree of freedom, the movement of the output shaft18fixed to the driven object14is limited by the size and shape of the opening of the cap16.

In the ultrasonic motor of the first embodiment, a driving element32is adhered to the output surface24between the vibrator12and the driven object14. The driving element32is made of a resin material having a high abrasion resistance (such as a material obtained by containing potassium titanate whisker in polyphenylene sulfide (PPS), Teflon (trademark) resin, or fluorin resin). As shown inFIG. 2, the driving element32includes a base portion34and a contact portion36. The base portion34has a rectangular shape corresponding to the size of the output surface24. The contact portion36has a convex shape with a projection vertically projecting from the base portion34away from the vibrator12, namely, toward the driven object14.

One end surface of the contact portion36is processed to a shape of a bowl in accordance with the curvature of the spherical driven object14. The processed surface serves as a contact surface38to be brought into contact with the driven object14. In the lowest plan view ofFIG. 2, the contact surface38is shown hatched. It should be noted that this hatching does not indicate a cross section. Further, the contact portion36is formed symmetrical with respect to the orthogonal axes (X and Y axes) on the output surface24of the vibrator12. In the first embodiment, the contact portion36is formed annular so that it is symmetrical about an intersection point of the orthogonal axes.

The output surface24of the vibrator12performs an elliptic vibration as described above.FIGS. 3A and 3Bshow the measured vibration amplitudes of the respective portions of the output surface24. InFIGS. 3A and 3B, the thicker the hatching, the greater the vibration amplitude.

As is evident fromFIGS. 3A and 3B, the vibration amplitude of the output surface24varies at different positions on the output surface24, and there exist a plurality of regions which exhibit greater vibration amplitudes than the peripheral regions. Namely, the center of the output surface24has a smallest vibration amplitude, and portions of the output surface24diagonally away from the center exhibit greater amplitudes. Further, in positions away from the center by a predetermined distance or more, the vibration amplitude continues to increase in certain opposite diagonal directions, while decreasing in the diagonal directions perpendicular to the certain diagonal directions.

The directions, in which the vibration amplitude varies, are not limited to the diagonal directions, but can be set arbitrarily (e.g., can be changed to the axial directions) by changing the vibration mode and resonance frequency.

The annular contact portion36has its diameter and position determined to include at least two regions which exhibit great vibration amplitudes. In the elliptic vibration shown inFIGS. 3A and 3B, the first embodiment includes two regions which are located in certain opposite diagonal directions and exhibit great vibration amplitudes, and two regions which are located in the other opposite diagonal directions and exhibit vibration amplitudes slightly smaller than the above but much greater than the central region. In view of this, the contact surface38to be brought into contact with the driven object14is provided at positions having great vibration amplitudes in the cross section A-A′ of the driving element32, as is shown inFIG. 2.

In the ultrasonic motor constructed as the above, the driving element32incorporates the contact portion36that contacts the driven object14at several positions, and the contact surface38of the contact portion36is shaped like a bowl in accordance with the curvature of the driven object14so that it is kept in surface contact with the driven object14. By virtue of this structure, the driven object14can be reliably held, which stabilizes the contact surface of the driven object14. As a result, reliable multi-degree-freedom driving of the driven object14is realized. Further, the contact portion36is provided symmetrical with respect to the orthogonal axes on the output surface24of the vibrator12. For instance, the contact portion36is an annular member that is symmetrical about a point, which enables the driven object14to be held stably. Thus, in the ultrasonic motor of the first embodiment, the driving element32, which contacts the driven object14in at least two positions in which greater vibration amplitudes are observed, is provided between the driven object14and the vibrator12. As a result, the driven object14can be held in a reliable contact, which realizes reliable multi-degree-freedom driving of the driven object14.

Furthermore, since the driving element32is made of a material having a high abrasion resistance, the contact between itself and the driven object14is kept in a further reliable state, thereby further stabilizing the driving.

In addition, since the driving element32is formed by integrating the contact portion36with the base portion34having a rectangular bottom, it can be easily attached to the vibrator12.

Also, the contact portion36of the driving element32is positioned selectively to cover the regions which exhibit greater vibration amplitudes. Thus, the driven object14is not in contact with the regions which exhibit smaller vibration amplitudes, which enables the driven object14to be driven at the portions that vibrate greatly. As a result, the driving amount of the driven object14can be increased.

In the prior art shown inFIG. 4, the driven object14is friction driven at the center that has the smallest vibration amplitude as indicated by the thick arrow. At this time, the angle, through which the driven object14is rotated in one vibration cycle of the vibrator12, is θ1. The length of the thick white-arrow represents the level of the vibration amplitude measured as shown inFIGS. 3A and 3B.

In contrast, in the ultrasonic motor of the first embodiment shown inFIG. 5, when the vibration mode and resonance frequency of the vibrator12are set to the same values as in the prior art and the weight and surface roughness of the driven object14are also set to the same values as in the prior art, the portions of the driven object14, which have greater vibration amplitudes than the vibration amplitude in the prior art, are friction driven as indicated by the thick white-arrow. At this time, the angle, through which the driven object14is rotated in one vibration cycle of the vibrator12, is θ2 greater than θ1. Thus, the ultrasonic motor of the first embodiment can more greatly rotate the driven object14than in the prior art.

A second embodiment according to the present invention will now be described. In the second embodiment, elements similar to those of the first embodiment are denoted by corresponding reference numbers, and no detailed description will be given thereof. A description will be given only to different elements.

In the first embodiment, the contact portion36of the driving element32is formed annular. In contrast, in an ultrasonic motor according to the second embodiment, the contact portion36is formed axisymmetrical by eliminating parts of the annular contact portion36, as is shown inFIGS. 6 and 7. In the lowest plan views ofFIGS. 6 and 7, the contact surface38is shown hatched. It should be noted that this hatching does not indicate a cross section. As shown, in the ultrasonic motor of the second embodiment, the contact portion36includes four regions that exhibit greater vibration amplitudes than the other regions. The positions of the four regions can be determined in accordance with the desired sliding direction of the output shaft18.

By virtue of this structure, the second embodiment can provide the same advantage as the first embodiment. Further, in the second embodiment, since the contact portion36is formed axisymmetric, it can reliably hold the driven object14, although the degree of holding reliability is slightly lower than in the first embodiment.

Also, in the first embodiment, the contact portion36is annular, and hence the driving force acquired from regions of great vibration amplitudes cannot directly be utilized, but only a driving force resulting from the average vibration amplitude of all regions can be utilized. In contrast, in the second embodiment, only regions of great vibration amplitudes can be selected, namely, a great driving force acquired from the great vibration amplitude regions can be directly utilized. Thus, in the second embodiment, a greater driving force than in the first embodiment can be utilized.

It is a matter of course that the number of regions included in the contact portion36is not limited to four, but two or more regions, which are arranged symmetrical, may be included.

A third embodiment will be described. In the third embodiment, elements similar to those of the first or second embodiment are denoted by corresponding reference numbers, and no detailed description will be given thereof. A description will be given only to different elements.

In the ultrasonic motor according to the first or second embodiment, the contact portion36of the driving element32is formed as a projection vertically projecting from the base portion34away from the vibrator12, i.e., to the driven object14. In the ultrasonic motor of the third embodiment, the contact portion36is upwardly tapered toward the driven object14as shown inFIG. 8.

This structure can provide the same advantage as in the first and second embodiments.

Furthermore, the contact portion36of the third embodiment can concentrate, like an ultrasonic horn, vibration energy on the driven object14as indicated by the arrows inFIG. 8, thereby minimizing vibration energy loss. As a result, a greater driving force can be acquired.

Although in the first through third embodiments, the entire driven object14is spherical, the shape of the object14is not limited to this. It is sufficient if the portion of the driven object14that is brought into contact with the contact portion36of the driving element32has a spherical surface.

In addition, the X-axial bend portion20and Y-axial bend portion22of the vibrator12may be arranged upside down.