Contents lists available at ScienceDirect Ultrasonics journal homepage: www.elsevier.com/locate/ultras A novel single-mode linear piezoelectric ultrasonic motor based on asymmetric structure Liang Wang, Junkao Liu, Yingxiang Liu ⁎ , Xinqi Tian, Jipeng Yan State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin 150001, China ARTICLE INFO Keywords: Ultrasonic motor Asymmetric structure Single-mode Longitudinal vibration ABSTRACT A novel single-mode linear piezoelectric ultrasonic motor based on asymmetric structure is proposed. The motor adopts the combination of the first longitudinal vibration and the asymmetric mechanical structure to produce the oblique movement on the driving foot; then, the linear output motion is obtained under the friction coupling between the driving foot and the runner. The motor is designed and its operation principle is illustrated in detail. The transient analysis is developed to verify the formed movement on the driving foot. A prototype is manu- factured and its vibration characteristics are measured by a scanning laser Doppler vibrometer. An experimental system is established and the output performances of the motor are tested and discussed. The results indicate that the maximal no-load velocity of the motor is about 127.31 mm/s under the voltage of 150 V p-p and the preload of 30 N. The maximum thrust force is about 2.8 N when the voltage and preload are 150 V p-p and 40 N. The related works in this paper has verified the feasibility of the proposed single-mode ultrasonic motor based on asym- metric structure. 1. Introduction As a new kind of micro actuators, the ultrasonic motors mainly take advantage of the inverse piezoelectric effect of the piezoelectric cera- mics to excite the resonant status of the stator, and the input electric energy is transformed into the output mechanical energy under the friction coupling effect between the stator and the runner [1–7]. In contrast to the traditional electromagnetic motors, the ultrasonic mo- tors exhibit a great many of unique features, such as low speed and large thrust force with simple construction, no bearing and lubrication, no electromagnetic interference, self-lock when power off, and so on [8–13]. Since the 1980s, the ultrasonic motors have long been in- vestigated extensively by the scholars all over the world. Due to their excellent superiorities, the ultrasonic motors have been applied suc- cessfully in lots of fields like digital cameras, biomedical therapies and aerospace apparatus [14–16]. According to the number of the used vibration mode, the ultrasonic motors are generally divided into the multi-mode type, the double- mode type and the single-mode type [17–20]. The multi-mode ultra- sonic motors usually make use of three or more vibration modes to achieve the output movements with multiple degrees of freedom like the multi-DOF motors proposed by Lu et al. [21], Shen et al. [22] and Yang et al [23]. For the double-mode type ultrasonic motors, two or- thogonal vibrations both in space and in time are composed to form the elliptical trajectory vibration on the driving foot, and then the linear or rotary motions are acquired under the effect of friction coupling. For instance, Wan et al. [24] developed a linear motor using the long- itudinal-bending mode and Liu et al. [25] presented a rotary ultrasonic motor utilized the composition of the third and fourth bending vibra- tion modes. It is well known that the resonance frequencies between the multiple vibration modes should be as close to each other as possible for the double-mode and multi-mode ultrasonic motors mentioned above. The frequency degeneration between the multiple modes frequently leads to the complicated designing and fabricating processes. The single-mode ultrasonic motors utilize the combination of only one vi- bration mode and special mechanical structures to form the oblique vibration trajectory on the driving foot [26,27]. Thus, the single-mode ultrasonic motors always have flexible constructions as they can avoid the frequency degeneration problem. In addition, the single-mode motors only employ single-phase exciting signal, which makes the drive circuit simpler. The frog-shaped linear piezoelectric actuator proposed by Zhang et al. [28] is a representative one, which successfully verify the principle of the single-mode ultrasonic motor mentioned above. However, the frog-shaped structure has more sensitive dimensions impacted on vibrations, which leads to the complicated structure and is not easy to be manufactured. In this paper, a novel single-mode linear piezoelectric ultrasonic motor based on asymmetric structure is designed and tested. The https://doi.org/10.1016/j.ultras.2018.05.010 Received 6 November 2017; Received in revised form 26 April 2018; Accepted 18 May 2018 ⁎ Corresponding author. E-mail address: liuyingxiang868@hit.edu.cn (Y. Liu). Ultrasonics 89 (2018) 137–142 Available online 26 May 2018 0041-624X/ © 2018 Elsevier B.V. All rights reserved. T