ABSTRACT - doug.kerr

The Canon USM (Ultrasonic Motor). Autofocus Drive System Douglas A. Kerr Issue 1. January 4, 2011. ABSTRACT . Many lenses in Canon's EF series have autofocus drive systems described as Ultrasonic Motor (USM) systems. There are actually a number of different systems in this classification. This article describes the construction and operating principles of the major varieties. Included is information on the mechanisms used to allow manual focusing even with the autofocus motor drive engaged (the so-called full time manual focus , or FTM, feature.). BACKGROUND. The Canon EOS camera line In 1987, Canon introduced a new line of single-lens reflex (SLR). cameras, the EOS series. The designation stands for electro-optical system , but it was also a clever allusion to Eos, the Greek goddess of light. The most prominent feature of the EOS line is that automatic focus (autofocus) is executed by a motor drive in the individual lens, controlled by a microprocessor in the lens, interacting with the microprocessor in the camera over an electrical interface passing across the lens mount (which is of an entirely new design).

The Canon USM Autofocus Drive System Page 4 Principle of operation The motor itself comprises two open metal rings, the stator and the rotor, which are held in contact by a wave-washer spring.(There are other rings in the “stack”, all held in mutual contact by that spring.)

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