Piezoelectric element and ultrasonic actuator

1. A piezoelectric element obtained by alternately stacking generally rectangular piezoelectric layers and internal electrode layers, wherein

the internal electrode layers include a common electrode layer and feed electrode layers alternately placed in a stacking direction with the piezoelectric layers interposed between the common electrode layer and the feed electrode layers,

the common electrode layer has a common electrode,

the feed electrode layers include a first feed electrode layer disposed on a principal surface of one of the piezoelectric layers and a second feed electrode layer disposed on a principal surface of one of the piezoelectric layers different from the piezoelectric layer on the principal surface of which the first feed electrode layer is disposed,

the first feed electrode layer has a pair of electrically continuous first electrodes disposed on two areas opposed along a first-diagonal-line direction of the principal surface of the associated piezoelectric layer among four areas defined by dividing the principal surface of the associated piezoelectric layer into two parts in the longitudinal and transverse directions,

the second feed electrode layer has a pair of electrically continuous second electrodes disposed on two areas opposed along a second-diagonal-line direction of the principal surface of the associated piezoelectric layer among the four areas, and

the common electrode, the first electrodes, and the second electrodes are connected to associated external electrodes disposed on outer surfaces of the piezoelectric element.

2. The piezoelectric element of claim 1 , wherein

the first feed electrode layer has, in addition to the pair of first electrodes, a first side electrode disposed on one of the two areas opposed along the second-diagonal-line direction,

the second feed electrode layer has, in addition to the pair of second electrodes, a second side electrode disposed on one of the two areas opposed along the first-diagonal-line direction,

the first electrodes are connected through an associated one of the external electrodes to the second side electrode, and

the second electrodes are connected through an associated one of the external electrodes to the first side electrode.

3. The piezoelectric element of claim 1 , wherein

the first feed electrode layer has, in addition to the pair of first electrodes, a pair of first side electrodes disposed on the two areas opposed along the second-diagonal-line direction, respectively,

the second feed electrode layer has, in addition to the pair of second electrodes, a pair of second side electrodes disposed on the two areas opposed along the first-diagonal-line direction, respectively,

the first electrodes are connected through associated ones of the external electrodes to the second side electrodes, and

the second electrodes are connected through associated ones of the external electrodes to the first side electrodes.

4. The piezoelectric element of claim 1 , wherein

the shape of each said feed electrode layer is generally point-symmetric with respect to the central point of a principal surface of an associated one of the piezo electric layers.

5. The piezoelectric element of claim 1 , wherein

the shape of the first feed electrode layer and the shape of the second feed electrode layer are identical with each other but inverse to each other with respect to the central line extending along a longitudinal direction of the principal surface of each said piezoelectric layer.

6. The piezoelectric element of claim 1 , wherein the number of first feed electrode layers is equal to that of second feed electrode layers.

7. The piezoelectric element of claim 1 , wherein

the feed electrode layers are configured such that the first feed electrode layer and the second feed electrode layer are alternately placed in a stacking direction.

8. The piezoelectric element of claim 1 , wherein

the outermost layers of the piezoelectric element are the piezoelectric layers.

9. The piezoelectric element of claim 1 , wherein

the external electrodes are disposed only on the surrounding surfaces of the piezoelectric element including the end and side surfaces of the piezoelectric element.

10. An ultrasonic actuator comprising:

the piezoelectric element of claim 1 ;

a driver disposed on an end or side surface of the piezoelectric element; and

a movable object supported by the driver,

wherein the power supply to the internal electrode layers allows the piezoelectric element to create a vibration produced by synthesizing a primary mode of a stretching vibration and a secondary mode of a bending vibration, and the vibration allows the driver to produce a generally elliptical motion, thereby moving the movable object relative to the piezoelectric element.

11. The ultrasonic actuator of claim 10 further comprising

a supporter having a supporting member for supporting a piezoelectric element,

the supporting member being made of conductive rubber.