# A High-Thrust Screw-Type Piezoelectric Ultrasonic Motor with Three-Wavelength Exciting Mode
## 1 Introduction  
Ultrasonic motors have attracted lots of attention in the past few decades due to their distinctive characteristics such as simple structure, low speed without a reducer, self-locking in the power-off state, anti-electromagnetic interference, rapid dynamic response, etc., which make them good candidates for precise positioning systems such as aerospace mechanisms, optical instruments, micro electromechanical systems, micro robots, and so on [1; 2; 3; 4]. The ultrasonic motor based on circumferential traveling wave vibrations is an important type of piezoelectric actuator that utilizes the inverse piezoelectric effect of piezoelectric materials to produce micro-amplitude vibrations at ultrasonic frequencies. This type of motor generally includes a stator and a rotor. The stator is composed of metal elastomer and piezoelectrics. The driving principle of the ultrasonic motor is that a combination vibration mode can generate the traveling wave and the elliptical motion at the surface, the continuous micro-deformation motion of piezoelectric materials is converted into the rotary motion of a rotor through an intermittent friction coupling [5; 6; 7; 8; 9; 10]. However, the traditional ultrasonic motor is generally designed with large preloads when applied in a precise positioning system that needs high thrust, which inevitably decreases the equipment lifespan [11; 12; 13]. Varioustypes of ultrasonic actuators have been proposed for particular positioning tasks that require the high thrust as well as a compact structure under specific conditions [14; 15; 16; 17]. As the most representative ultrasonic motors, the operating principle of the screw-type piezoelectric ultrasonic motors is similar to that of the ultrasonic motor. To date, screw-type piezoelectric ultrasonic motors have been widely researched. Some kinds of screw-type ultrasonic motors have been proposed for particular positioning tasks. [PERSON] et al. designed a screw-driven polyhedron linear ultrasonic motor (USM) of nut-type. The lens can be fixed in the rotor and realize an integrated design for an auto focus system that requires compact structures and low cost [18; 19]. [PERSON] et al. presented a new linear piezoelectric motor called the SQUIGGLE motor based on ultrasonic standing wave vibrations. Precision and low cost make SQUIGGLE motors ideal for emerging micro motion applications including mobile phone cameras, micro fluidic devices [20; 21]. [PERSON] et al. proposed a piezoelectric screw-driven motor operating in two shear vibration modes. This type of motor is expected to find applications in medical industry, precision machining, small size robots, and aerospace [22]. However, the output thrust of the above reported screw-type ultrasonic motors can generally achieve several Newtons. Thus, the reported motor has difficulty performing the particular positioning tasks that need high thrust [23; 24; 25; 26; 27]. In fact, some research work has been done to improve the output thrust of the motor. [PERSON] et al. presented a screw-thread-type ultrasonic actuator based on a Langevin piezoelectric vibrator. The actuator consisted of a threaded shaft and a transducer-formed stator. The stator based on the bolt-clamped Langevin vibrator can generate more energy and achieves high thrust at a relatively low speed. Besides, the actuator provides a long stroke corresponding to the length of the screw output shaft [28]. [PERSON] et al. proposed a linear actuator of screw-type with double piezoelectric vibrators based on \(d_{33}\) mode, and the double vibrators of \(d_{33}\) mode can effectively generate a circumferential traveling wave within a hollow cylinder and then drive a screw rod directly through the helical surface of a nut. A larger output torque and power can be obtained relative to \(d_{31}\) mode piezoelectric transducers [29]. Much study is still needed to improve the output thrust of a screw-type ultrasonic motor operating at an ultrasonic frequency.