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 establishing pressure contact with the stator;

a resin film formed on at least one of pressure contact surfaces of the stator and rotor; and

a pressure contact force controlling portion that includes:

an elastic member configured to generate a pressure contact force between the stator and rotor with an elastic force thereof; and

a compressing portion configured to compress and shorten the elastic member,

wherein the pressure contact force controlling portion controls the pressure contact force between the stator and rotor by varying the elastic force of the elastic member depending on temperature of the motor.

2. The ultrasonic motor according to claim 1 ,

wherein the elastic member includes a compression coil spring provided between the stator and rotor, the compression coil spring being formed from shape-memory material that varies a natural length thereof depending on the temperature, and

wherein the compressing portion compresses and shortens the compression coil spring down to a predetermined length.

3. The ultrasonic motor according to claim 1 ,

wherein the elastic member includes a compression coil spring provided between the stator and rotor, and

wherein the compressing portion varies a compressed length of the compression coil spring depending on the temperature.

4. The ultrasonic motor according to claim 3 ,

wherein the compressing portion includes a bush provided to the stator so as to rotatably support the rotor,

wherein the compression coil spring is inserted into the bush, and

wherein the bush is formed from shape-memory material such that a length thereof in an axial direction thereof varies depending on the temperature.

5. The ultrasonic motor according to claim 3 ,

wherein the compressing portion includes:

a bush provided to the stator;

a ball bearing configured to rotatably support the rotor, the ball bearing being biased by the compression coil spring inside the bush; and

a locking member configured to lock the ball bearing biased by the compression coil spring so as to rotatably support the rotor, and

wherein the locking member is formed from shape-memory material to change a locking position where the ball bearing is locked depending on the temperature.

6. The ultrasonic motor according to claim 1 ,

wherein the resin film includes fluorocarbon resin.

7. The ultrasonic motor according to claim 6 ,

wherein the fluorocarbon resin included in the resin film includes at least one of polytetraflouroethylene (PTFE), perfluoroalkoxy polymer resin (PFA), and fluorinated ethylene-propylene (FEP).

8. 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 establishing pressure contact with the stator;

a resin film formed on at least one of pressure contact surfaces of the stator and rotor, a frictional coefficient on the resin film varying depending on temperature of the motor; and

a pressure contact force controlling portion that includes:

an elastic member configured to generate a pressure contact force between the stator and rotor with an elastic force thereof; and

a compressing portion configured to compress and shorten the elastic member,

wherein the pressure contact force controlling portion controls the pressure contact force between the stator and rotor by varying the elastic force of the elastic member depending on the temperature such that a frictional force on the resin film is maintained substantially constant.