Surface Acoustic Wave Motors and Actuators: Mechanism, Structure, Characteristic and Application

Ultrasonic motors, as one kind of actuators, have attracted a lot of attention since it was proposed more than 20 years ago. In such kind of motors, the sliders (for linear motors) or the rotors (for rotary motors) are driven by the frictional forces between the sliders (rotors) and the stators when ultrasonic waves are propagating on the stators. Since then, the ultrasonic motors have been developed and applied successfully in wide fields, such as mechanical, optic, electronic, and automatic, as well as aeronautic and astronautic industries and technologies because of their unique advantages over conventional electro-magnetic ones, such as high driving forces and torques, easy controllability, quiet operation, nonelectromagnetic induction, etc. (Sashida & Kenjo, 1993; Ueha & Tomikawa, 1993). Besides, with the rapid development of micro-electro-mechanical system (MEMS), miniature ultrasonic motors were developed (Dong et al., 2000; Zhang et al., 2006). However, the direct contact between the sliders (rotors) and the stators restricts the velocity and working lifetime of the motors, then a new kind of non-contact motors were presented, where a fluid is introduced between the stator and slider. Thus, instead of the frictional force, acoustic streaming excited by the acoustic wave on the stator and propagating in the fluid is used for driving the slider or rotor to move (Nakamura et al., 1990; Yamayoshi & Hirose, 1992; Hu et al., 1995; Cheng et al., 2007). On the basis of conventional ultrasonic motors, several studies on new types of ultrasonic motors (actuators) with the driving forces coming from surface acoustic waves (SAWs) were presented (Moroney et al., 1989; Kurosawa et al., 1994). For the SAW motors, the SAWs are excited by interdigital transducers (IDTs) deposited on surfaces of piezoelectric substrates or thin films, and the SAW energies are concentrated in the thin layers near the surfaces of the substrates (for Rayleigh waves) or in the thin films (for Lamb waves). In addition to the characteristics of conventional ultrasonic motors, the SAW motors have more advantages, such as the high operation frequency, high speed, high energy density around the surfaces, and higher output force/torque, etc. Meanwhile, since the SAWs are excited by IDTs, which can be fabricated with planar technologies of semiconductor industries, the new types of motors are suitable for miniaturizing and integrating with integrated circuits and MEMS devices, etc.