Autofocus Lens Stepping Motors Explained

Most modern autofocus lenses use stepping motors. Canon calls them “USM”, which is short for “piezoelectric ultrasonic motor”. Nikon calls them “STM” and “SWM”, while Pentax calls them “SDM”.

In 1961, Bulova watches invented the “tuning fork”, utilizing the piezoelectric effect for accurate timekeeping. Although interesting, nobody else had any ideas for using this microscopic vibration for anything useful.

H.V. Barth in 1973 proposed the theory of creating ultrasonic vibrations for motor applications, but didn’t do anything with the idea. In 1982, Toshiiku Sashida utilized the standing wave vibrations of ultrasonic piezoelectric motors for rotational motion. In 1985, Canon used Sashida’s technology to make their USM lenses, starting with their EF 300mm f/2.8. Good idea.

Piezoelectric motors are great for camera lenses, because they create high torque, they have great positional accuracy, and they’re energy-efficient and quiet. After Canon's 300mm lens came out, all of their competitors started paying attention.

The basis of the stepping motor operation is the piezoelectric crystal. This material expands when a voltage is applied across it. When the voltage is removed, the crystal returns to its original shape. The expansion is very tiny, but it’s enough to allow the crystal to push against another object and move it slightly.

Piezoelectric crystals can expand and contract very quickly, and they don’t waste much energy while doing so, either. This means that you can drive these crystals for a long time on a battery charge. It’s most efficient to drive these crystals at their “natural frequency”, where they expand to their maximum amount with the least energy. The rate of applying/removing voltage is about 30,000 times per second (30 KHz) at this natural frequency, or “resonant frequency”. The crystals only expand by something like 0.1% of their original dimensions, so they have to be very close to the part that they push against. Each little push is a ‘step’, which is where these stepping motors get their name.

The ultrasonic motors are ring-shaped, with the crystals placed side-by-side around the ring (typically called ‘teeth’). Voltage is progressively applied/removed moving around this ring of crystals, creating what’s called a “standing wave”. This wave can be envisioned like an ocean wave that pushes a surf board along. In this case, the surf board is replaced with the moving ring of the stepping motor, whose rotary motion causes lens elements to move back and forth very quickly and accurately. These motors can run about 480 degrees/second (Canon specifications) with high torque, so they can focus a lens very quickly.

Many piezoelectric ultrasonic motors are connected to gears, instead of the large and expensive ring motors. These motors are very tiny and cheaper to make than the ring type.

A Nikon SWM motor that drives gears for focus

In the shot above, the Nikon SWM autofocus motor uses the green stator to push against the silver-colored rotor to rotate a shaft connected to a gear train. The gears move a lens element group forward and backward along the lens axis to achieve autofocus. The green teeth in the stator of this motor are a piezoelectric material. This particular stator is 12 millimeters in diameter.

The Nikon SWM motor and gear module

How the motor drives the focus action in a Nikon SWM lens

Canon ring drive USM stator

Canon ring drive USM rotor

Canon USM piezoelectric motor

I read an article by Douglas Kerr here where he made a very useful diagram of the principle of how a USM motor works. The diagram below shows the expansion/contraction of the motor “stator” ring, which is the part that doesn’t actually rotate. The stator presses against the motor’s “rotor”, which is the part that spins around the stator. In the diagram, the rings are spread out flat, instead of being in a ring, or doughnut shape. Only the expanded part of the stator touches the rotor, while voltage is applied to that portion of the stator.

How a piezo stator ‘tooth’ moves in time

In Douglas’ diagram above, the voltage progresses around the motor ring through the stator in an “electrical wave”, moving left to right. The piezoelectric material expands and then retracts in a wave motion, moving from left-to-right following the voltage. If you look at the highlighted green “tooth” on the stator, it moves upward and towards the left (steps 1-3). As the voltage is reduced, the tooth retracts from the stator and back to the right until it points vertically at rest (steps 4-5). The next wave of voltage starts the next expansion.

Due to the tooth’s motion while pressing against the stator, the rotor moves to the left, or in the opposite direction from the wave induced in the stator.

The voltage can be applied in a wave in the opposite direction, which drives the motor motion in the opposite direction.

Each little tooth tip on the stator moves in basically an elliptical motion in response to the voltage wave. In the above diagram, it moves leftward while expanding and in contact with the rotor and then rightward while retracting and no longer in contact with the rotor. The net result is a leftward push on the rotor. Reversing the voltage wave direction will then reverse the motor’s rotor direction.

The motor’s rotor ring is a bit springy, so that the stator teeth don’t chew it up. As you might expect, there lots of little mechanical details that make the motor work reliably over many years without any maintenance required.

There are also several electrical details not covered here. For instance, the piezoelectric elements respond a little differently according to the ambient temperature and their variation in physical dimensions. Real motors have separate “reference” piezoelectric elements that are used in a feedback control system to adjust the controlling voltage frequency. Life’s complicated.

Toshiiku Sashida was one brilliant individual to have the vision to make these motors from the information he had to work with. Photography wouldn’t be where it is without him.