1. Piezoelectric ultrasonic motors: overview
2. An Introduction to Ultrasonic Motors;Sashida,1993
3. Motion Systemshttp://www.nanomotion.com/product-type/motion-systems/
4. Introduction to Piezoelectric Actuators and Transducers. Defense Technical Information Center Document ADA429659http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA429659