Canon EF and RF Lenses – All Autofocus Motors

The type of stepper motor used in STM lenses is a 2-phase bipolar stepper motor with permanent magnet rotor. Unlike the AFD stepper motor, the STM version uses a rotationally symmetrical stator. First, the general principle of stepper motor operation is shown, and then the actual design of Canon's stepper motors is examined. Stepper Motor in General The following illustration shows the general principle of a 2-phase bipolar stepper motor. The term 2-phase refers to the use of two separate coils. The description bipolar indicates that each of these coils can be energized with current flowing in two different directions, allowing each coil to generate two opposite magnetic field polarities. The ferromagnetic metal parts of the stator pointing towards the rotor are called field poles. The coil windings of both phases are split into two portions and are wound around opposite field poles to ensure simultaneous magnetization during operation. Each phase is labeled with two letters, one of ... Read full article.