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View Full Version : Canon EF lenses - the ring USM drive


Doug Kerr
December 23rd, 2010, 05:45 PM
Many Canon EF-series lenses have what Canon calls the "ultrasonic motor" (USM) drive system. There are actually to dramatically different forms of this, the "ring USM" and the "micro USM motor" systems. The ring USM system is in general superior, and Canon has said that they are de-emphasizing the use of the micro USM motor in new EF lens designs.

In this note, I will give some insight into how the ring USM drive operates. More artistic and illustrative figures can be found in Canon's "EF Lens Work III" book, for those who have it.

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Figure 1 illustrates the central principle of the ring USM drive. The actual arrangement is, of course, circular, but where we have shown a part of it laid out flat for clarity (and for the sanity of the illustrator).

http://dougkerr.net/illustrations/usm_ring_01.gif
Figure 1. Principle of the ring USM drive

The stator is a metal part with numerous little pyramids on it, It is held in contact with the metal rotor under spring pressure. The rotor is the part that delivers the drive output; it can rotate (this its name). The stator does not rotate, but it does quite a little dance.

The stator lays on an array of piezoelectric elements, which deform when a voltage is applied to electrodes plated on them. They are excited with an AC signal with a frequency someplace in the area of 30,000 kHz (well in the ultrasonic range, thus the system name).

The elements are in two groups, and are excited in a "quadrature" phase relationship, so that their deformation pattern progresses (as a "wave") around the face of the element layer.

The deformation bends the stator to follow. The frequency of the excitation is made to match the natural resonant frequency of the stator in this mode of vibration. (It is related to the vibration mode of cymbals.)

There is in fact a feedback system that monitors the resonance and adjusts the frequency of the excitation to suit, thus keeping up with changes in the resonant frequency from temperature change and the like.

The figure shows the resulting undulation of the stator greatly emphasized; the actual motion is only a fraction of a millimeter. The wave in this example is propagating toward the right.

We see the situation in five successive instants, separated by the amount of time it takes the wave to propagate a distance equivalent to the tooth pitch. (The "wavelength" here is 8 tooth pitches.)

Keep in mind that the overall stator does not move, only the "wave" pattern of its deformation. We show a single tooth in green (with a fixed reference line) to emphasize that.

But note that the tooth "leans" back and forth as the wave moves. That is a key to the action of the motor (as we will see shortly).

At the right we can see (with the help of a reference line) that, as the wave propagates to the right, the rotor moves slowly to the left. (Of course, the rotor doesn't actually have an end, being a complete ring. This is just a convention I use to allow its motion to be "seen" on the figure.)

In figure 2 we see in more detail the behavior of a tooth tip.

http://dougkerr.net/illustrations/usm_ring_02.gif
Figure 1. Tooth tip motion

We see the situation of the tooth (in separate colors) for each of four stages in the passage of the wave. In the example, the "wavelength" is 8 tooth pitches. If we took a snapshot of our selected tooth as some point in the wave passed each successive tooth on the stator, we would see 8 different states. I have only shown every other one to avoid the figure becoming too cluttered.

We see that although the tooth overall doesn't move, its tip describes a sort of elliptical path. We can see that during the upper lobe of that path, when the tip is moving to the left, the tip engages the rotor; during the rest of the path (including when the tip is moving to the right) it does not engage the rotor. Thus, cilia-like, the collection of stator tooth tips cause a continuing movement (to the left in this case) of the rotor.

The tooth tip path I show is what it would be if there were no rotor. Obviously, the tooth tips do not dig into the rotor, as the picture would imply. In reality, the upward flexure caused by the piezoelectric elements is "thwarted" as the tooth tips contact the rotor.

But the effect is the same: during part of the left-moving part of the cycle of each tooth tip, it is pressed against the rotor and moves it to the left; during the rest of the cycle, regardless of which way the tip is moving, it is not in contact with the rotor and has no effect on it. And of course during those times, there are other tooth tips that are in contact with the rotor, keeping up its propulsion to the left.

Very clever.

Best regards,

Doug