A two-DOF linear ultrasonic motor utilizing the actuating approach of longitudinal-traveling-wave/bending-standing-wave hybrid excitation

Ultrasonic motors (USMs) have been extensively investigated for a few decades, but it is still required to improve their thrust force (or torque) density and/or power density for practical usage. According to conventional actuating methods, the dimensions of the transducers are difficult to greatly change after modal degeneration; this restricts the possibility of weight reduction. To tackle this problem, an actuating approach via longitudinal-traveling-wave (LTW) and bending-standing-wave (BSW) hybrid excitation is proposed and applied to a two-DOF linear ultrasonic motor in cross shape. Here, the driving force is generated with the LTW excited by modulating the phases according to the lengths of the transducers, while the elliptical motion shape is adjusted by tuning the BSW for optimal actuation. To assess the validity, a pair of Mason-circuit-based dynamic models were initially constructed to discuss three-dimensional vibration properties of the LTW and BSW on the cross-shaped transducer. Subsequently, a prototype with the size of 68 × 68 × 28 mm³ was designed and fabricated to form a motor capable of horizontally moving in the x and y axes. Finally, the movement and load characteristics were evaluated experimentally. At 250 V voltage and 27.4 kHz frequency, the motor yielded the maximal thrust force, no-load sliding speed, and maximal output power of 42.1 N, 833 mm/s, and 7.7 W, respectively, along the x axis; and 40.2 N, 877 mm/s, 8.1 W, respectively, along the y axis. Moreover, the thrust force density and power density reach respectively 188.3 N/kg and 36 W/kg, exceeding the values of most conventional two-DOF linear USMs. These results demonstrate high actuating capability induced by the LTW/BSW hybrid excitation and provide a new method to design powerful two-DOF linear motors.