# A Novel Dual-Rotor Ultrasonic Motor for Underwater Propulsion
## 1 Introduction  
Underwater vehicles have developed rapidly in the last two decades and intelligent micro underwater vehicles (MUVs) have been emerging as an effective platform for scientists and engineers to explore oceans for environmental monitoring, animal protection, resources exploration, etc. [1, 2]. Concerning the propulsion force of MUVs, electromagnetic motors integrated with big mechanical transmission parts are normally selected to power the propellers. Furthermore, twin propeller engines were developed to increase the power of MUVs [3, 4]. However, utilizing electromagnetic motors to drive pairs of propellers increases the complexity and occupies more inside space of MUVs [5, 6, 7, 8], affecting its reliability concerning intensive electromagnetic interference, and limiting its versatility with less functional units [9, 10, 11]. Therefore, current MUVs still face the compromise between functionality and size miniaturization [12].  
Ultrasonic motors excited by piezoelectric materials can vibrate at ultrasound frequency and output mechanical forces via the contact friction between the stator and rotor [13; 14; 15]. Compared to electromagnetic motors, ultrasonic motors have the advantages of no electromagnetic interference, low noise, high torque to mass ratio and compact structure, as well as showing great potential for optical engineering, biomedical instruments and auto robots, etc. [16]. [PERSON] et al. proposed a high-speed rotary ultrasonic motor and discussed its capability for driving micro air vehicles [17; 18; 19]. [PERSON] summarized the features of piezoelectric motors and discussed their applications and prospective [20]. [PERSON] et al. proposed a piezoelectric actuator using different bending vibration modes to perform linear or rotary motions [21; 22; 23]. Most recently, [PERSON] et al. demonstrated a disk-type inertial rotary motor with a no-load speed of 2200 r/min and stalling torque of 0.5 mN\(\cdot\)m, which can be used for building a single shaft-type thruster to actuate a spherical underwater robot model moving at different operation patterns [24]. Considering the electromagnetic interference and size limitation, ultrasonic motors are competitive to traditional electromagnetic motors. This research gives basic guidance for developing novel thrusters powered by ultrasonic motors.  
In this paper, a new type of dual-rotor ultrasonic motor for underwater propulsion was designed, analyzed, manufactured, and tested. By means of torsion spring adjustment, the pre-pressure between the driving foot of the stator and the outer surface of the rotors were modulated, and the rotors, fixed at parallel shafts, were driven to rotate reversely and simultaneously via the contact friction when the stator vibrated at longitudinal mode. Based on the numerical simulation; critical size parameters and the working mode of the stator were determined. The vibration performance of the stator was measured and the mechanical characteristics of the prototype motor were tested. Finally, one pair of propellers were mounted to the shaft ends of the prototype motor to build a thruster for testing the propulsion behavior with an underwater robot model.