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This paper reviews recent developments of micro ultrasonic rotary motors using piezoelectric resonant vibrations. Following the historical background, four ultrasonic motors recently developed at Penn State University are introduced; windmill, PZT tube, metal tube, and shear-type motors. Driving principles and motor characteristics are described in comparison with the conventional ultrasonic motors. Motors with 1.5 mm in diameter and 0.8 mN·m in torque have been actually developed.

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Unipoled piezoelectric transformers with different input and output area ratios were fabricated. The electrical properties in terms of voltage step-up ratio, output power, efficiency, and temperature rise were measured at various load resistance. The voltage step-up ratio increased proportionally with load resistance. The relative efficiency showed a maximum value of more than 98% in the transformer, which is 23.6 mm in diameter and 1.0 mm in thickness with input/out electrode area ratio of 0.62. With the driving voltage of 35 V rms , the 8.1 W output power was obtained in the transformer with an input/output electrodes area ratio of 6.47. In such conditions, the temperature rise was only 15 C from room temperature. This transformer, which has approximately ten times higher power density than a rectangular Rosen-type piezoelectric transformer, was successfully applied to operate an 8 W compact fluorescent lamp.

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