@inproceedingsHemsel_Littmann_Wallaschek_2002, title=Piezoelectric transformers - state of the art and development trends, volume=1, DOI=10.1109/ULTSYM.2002.1193485, number=vol.1, booktitle=Ultrasonics Symposium, 2002. Proceedings. 2002 IEEE, author=Hemsel, Tobias and Littmann, Walter and Wallaschek, Jörg, year=2002, pages=645–648 
LibreCat: /record/8926
DOI: 10.1109/ULTSYM.2002.1193485

@inproceedingsHemsel_Wallaschek_2000, title=State of the art and development trends of ultrasonic linear motors, volume=1, DOI=10.1109/ULTSYM.2000.922635, booktitle=Ultrasonics Symposium, 2000 IEEE, author=Hemsel, Tobias and Wallaschek, Jörg, year=2000, pages=663–666 vol.1 
LibreCat: /record/8915
DOI: 10.1109/ULTSYM.2000.922635