Ultrasonic motor

1. An ultrasonic motor, comprising:

a stator that includes a comb body with a plurality of circumferentially-arranged comb-like projections and a piezoelectric body integrally mounted on the comb body;

a rotor rotatable with respect to the stator, the rotor having a contact surface that establishes pressure contact with the stator; and

a driving circuit configured to apply a radio frequency voltage for driving the piezoelectric body to the piezoelectric body, and

wherein the driving circuit comprises a voltage generating unit configured to generate a first radio frequency voltage of which a frequency is randomly varied during a predetermined time period after start-up of the ultrasonic motor.

2. The ultrasonic motor according to claim 1 ,

wherein the voltage generating unit comprises:

a first voltage generating portion configured to generate the first radio frequency voltage;

a second voltage generating portion configured to generate a second radio frequency voltage with a constant frequency; and

a selecting portion configured to select the first radio frequency voltage generated by the first voltage generating portion during the predetermined time period after the start-up of the ultrasonic motor, and to select the second radio frequency voltage generated by the second voltage generating portion thereafter.

3. The ultrasonic motor according to claim 2 ,

wherein the first radio frequency voltage includes two kinds of radio frequency voltages, at least one of which has a frequency being randomly varied, and

wherein the second radio frequency voltage includes two kinds of radio frequency voltages, which have a same frequency, a same amplitude, and phases different from each other by π/2.

4. The ultrasonic motor according to claim 2 ,

wherein the voltage generating unit is configured to generate the first radio frequency voltage with at least one of a phase and amplitude thereof being randomly varied as well as the frequency thereof.

5. The ultrasonic motor according to claim 2 ,

wherein the predetermined time period is approximately 100 μsec.

6. The ultrasonic motor according to claim 1 ,

wherein the voltage generating unit is configured to generate a second radio frequency voltage with a constant frequency, and

wherein the driving circuit further comprises a control unit configured to control the voltage generating unit to generate the first radio frequency voltage during the predetermined time period after the start-up of the ultrasonic motor and to generate the second radio frequency voltage thereafter.

7. The ultrasonic motor according to claim 6 ,

wherein the first radio frequency voltage includes two kinds of radio frequency voltages, at least one of which has a frequency being randomly varied, and

wherein the second radio frequency voltage includes two kinds of radio frequency voltages, which have a same frequency, a same amplitude, and phases different from each other by π/2.

8. The ultrasonic motor according to claim 6 ,

wherein the voltage generating unit is configured to generate the first radio frequency voltage with at least one of a phase and amplitude thereof being randomly varied as well as the frequency thereof.

9. The ultrasonic motor according to claim 6 ,

wherein the predetermined time period is approximately 100 μsec.

10. The ultrasonic motor according to claim 1 ,

wherein the voltage generating unit is configured to generate the first radio frequency voltage with at least one of a phase and amplitude thereof being randomly varied as well as the frequency thereof.

11. The ultrasonic motor according to claim 1 ,

wherein the predetermined time period is approximately 100 μsec.