The Ultrasonic Motor (USM) is an AF (auto focus) driving motor which Canon commercialized for the first time in the world. Conventional motors convert electromagnetic force into rotational force. Ultrasonic motors, on the other hand, are based on a completely new principle in which rotational force is generated from ultrasonic vibrational energy. The basic motor construction is very simple, consisting of an elastic stator and a rotating rotor. The stator has piezoelectric ceramic elements that, when energized by an AC voltage, make it vibrate to generate ultrasonic vibration energy. That vibrational energy is used to continuously rotate the rotor through the pressure contact between the rotor and stator. USMs make focusing faster, more accurate and quieter (almost silent) than conventional focusing drive by a motor and speed-reducing gear train. Most USM-equipped EF lenses allow full-time manual focus in the AF mode. The ring USMs are suitable for large aperture lenses and super telephoto lenses, and the Micro USMs for smaller lenses.
* The amplitude width of the elastic stator is only around 0.001mm.