Our previous studies showed that traveling-wave rotary ultrasonic motors with polymer-based vibrators yielded limited output power when operating in the commonly used low-order bending vibration modes. In this study, we employ a high-order bending mode in the polymer-based cylindrical ultrasonic motor, because this mode yields a relatively high electromechanical coupling factor, which may lead to high output power of the motor. Additionally, in contrast with the low-order modes with only vertical nodal lines, the high-order mode has both horizontal and vertical nodal lines on the circumferential outer surface of the polymer-based vibrator. With these attractive advantages, it is worth investigating basic vibration characteristics of polymer-based vibrators operating in high-order modes and their applications to ultrasonic motors. The vibrating body is made of poly phenylene sulfide, a functional polymer exhibiting low mechanical loss even under high-amplitude ultrasonic vibration and is activated by a piezoelectric ceramic element bonded on the back surface. The high-order modes are optimal for polymer-based vibrators with practical thicknesses, because their equivalent stiffnesses and masses are relatively low when operating in this mode. Polymer-based motors with the high-order modes exhibit relatively high output torques and powers, compared with not only polymer-based motors with low-order bending modes, but also metal-based motors operating in high-order vibration modes.

In this article, to achieve high torque density and high power density of rotary ultrasonic motors (USMs), we develop a rod-shaped transducer operating in torsional/bending (T/B) modes and excite two elliptical motions on its bilateral ends to drive the rotor orthogonally pressed onto the transducer. Here, the torsional vibration, the driving structure, and the rod shape may enable the T/B motor to provide high driving force, large driving distance, and low weight, which facilitate the realization of high torque density and high power density. To assess the validity, first, we construct a transducer 43 and 110 mm in diameter and length, respectively. Then, we investigate the load characteristics of the T/B motor: When it works at the frequency of 21.64 kHz, the maximal torque and the maximal output power reach 10.1 N·m and 38.1 W, respectively. In the meantime, it yields the torque density and the power density of, respectively, 20.0 N·m/kg and 75.7 W/kg, which are, as predicted, relatively high compared to the values of most conventional rotary motors. This article not only validates the effectiveness of our proposal but also provides a new approach to design powerful rotary USMs.

To enhance the torque and output power of the rod-shaped piezoelectric rotary motors, in this article, we employ alumina to substitute the metal parts of the transducers considering that the high Young's modulus of alumina may avoid preload-induced vibration reduction and enable the transducers to generate the high driving force. Besides, alumina exhibits a relatively low density compared to most of the materials commonly used as the vibrating bodies (except aluminum alloy); this probably enhances the power densities of the piezoelectric motors. To assess the validity of our proposal, we constructed a Langevin-type alumina/lead-zirconate-titanate (PZT) motor and investigated its load characteristics. The transducer of 35 mm in outer diameter operated in orthogonal bending (B ² ) modes and drove the rotor with its end surface. The alumina/PZT motor yields the maximal torque and maximal output power of 1.56 N·m and 26.0 W, respectively, both of which exceed those of the metal/PZT motors having identical structure and working in B ² modes. Additionally, its power density reaches 56.3 W/kg, relatively high among the rod-shaped rotary motors although some of which work in more powerful modes. This article validates the effectiveness of our proposal and provides a new approach to improve the performance of the rod-shaped piezoelectric rotary motors.

In this article, a new two-DOF linear ultrasonic motor operating in three vibration modes is developed and its capability to exhibit high thrust force density and high power density is tested. Here, the driving force is produced by synchronously exciting the torsional (T) and centrosymmetric-bending (B _c ) modes, whereas the normal force is generated with the symmetric-bending (B _s ) mode. Initially, the T-B _c /B _s hybrid transducer in cross shape is designed through modal degeneration and the working principle is verified by simulation. Subsequently, a 50-mm-high 130-mm-long transducer prototype is fabricated to make the slider movable along two horizontal axes ( x and y ). Finally, the performance is assessed via experiments. When the voltage is 250 V and the working frequency is ∼21.6 kHz, the motor yielded the maximal thrust force, no-load sliding speed, and maximal output power of, respectively, 84.3 N, 1562 mm/s, and 33.2 W in the x -axis; and 86.2 N, 1531 mm/s, and 34.1 W in the y -axis. Moreover, the thrust force density and power density reached, respectively, 78.4 N/kg and 31.0 W/kg, exceeding the values of most conventional two-DOF linear ones. This article validates the T-B _c /B _s motor's high actuating capability and also provides a new approach to design powerful two-DOF linear motors.