Hemsel, Tobias, et al. “A Novel Approach for High Power Ultrasonic Linear Motors.”10.1109/ULTSYM.2004.1417988__Ultrasonics Symposium, 2004 IEEE, vol. 2, no. Vol.2, 2004, pp. 1161–64, doi:10.1109/ULTSYM.2004.1417988.