In the early days of autofocus photography (Canon's first SLR with AF was the T80 in 1985), the AF drive motor was frequently located in the camera body or attached to the lens and drove the lens mechanically. In 1987, with the introduction of the EF lens mount and its fully electronic connectors, Canon was able to miniaturise the autofocus motor to fit inside the lens itself. This raised the possibility that each AF motor could be optimised for the lens it was fitted into, thereby providing faster autofocus.
However, there was still a need to create a high-powered AF motor for fast aperture lenses with larger focusing groups, which could work efficiently and deliver fast, smooth and quiet autofocusing. The result was the EF 300mm f/2.8L USM lens, with a ring-type Ultra Sonic Motor (USM) that was both fast and near silent. In 1990, new manufacturing techniques made it possible to reduce the cost of manufacture, and ring-type USM motors found their way into Canon lenses at a consumer price level.
Two years later, in 1992, automated production lines led to the development of the Micro USM motor for use in consumer lenses. Ten years after that, in 2002, came the Micro USM II motor, which is only half the size of the original Micro USM motor.
A decade later, in 2012, a new type of focusing motor was introduced, STM, named after its use of stepper motors. This was developed with video particularly in mind because it enables very smooth, quiet focus changes.
In 2016 Canon introduced Nano USM focusing, which combines the speed of ring-type USM with the quietness and smoothness of STM focusing.
That makes four types of USM motor – the ring-type, Micro, Micro II and Nano types. Like all AF motors, they all aim to convert an electromagnetic force into a rotational force to drive the focusing elements in the lens. What is different about USM motors is that they use ultrasonic vibration energy which is converted into rotational force.