Comparison study of bending and three-wave vibration modes for micro ultrasonic motors

Bending and three-wave vibration modes are the fundamental principle of millimeter-scale ultrasonic motors. The question is that which vibration mode is suitable for smaller ultrasonic motors with a scale of 1–2 mm or less. In this study, we fabricate prototype stators capable of generating the two vibration modes in the same structure and compare them in an equitable manner. The stators comprise a bronze cuboid with sides of 1 mm and four piezoelectric plates adhered to the four side surfaces. At the stator's length ranging from 1 to 2 mm, the prototype motors change their electrical and mechanical characteristics, and examining these characteristics reveal the advantages of selecting the vibration modes. We evaluate the performance of micro ultrasonic motors using the two modes via FEM piezoelectric analyses, impedance measurements, and experiments for measuring motor outputs. The experimental results show that the bending mode is advantageous for generating a higher output at a longer stator with a length of 2 mm. At a shorter length of 1 mm, using the three-wave mode performs a higher efficiency, although the bending mode reduces the output.