1. T. Sashida, Motor device utilizing ultrasonic oscillation, US Patent No. 4562374, 1985.
2. Piezoelectric ultrasonic motors;Wallaschek;Journal of Intelligent Material Systems and Structures,1995
3. Ultrasonic Motors Theory and Applications;Ueha,1993
4. Piezoelectric Actuator and Ultrasonic Motors;Uchino,1997