How a High-Tech Motor
Moves the Lens
The ultrasonic motor is built on a phenomenon called the piezoelectric
effect. The effect turns up in certain substances, such as the ceramic
leadzirconium titanate(PZT). When an electrical voltage is applied
to a strip of PZT, the ceramic expands in one direction and compresses
in the other. If the voltage’s polarity—the plus or minus charge—is
reversed, the ceramic now compresses in the first direction and expands
in the second.1
To create a piezo bender, or bimorph, PZT is bonded to both sides of
a thin strip of steel spring metal. A positive charge is applied to one side
and a negative charge is applied to the other side. Now the only way the
ceramic can expand or contract is to bend the metal strip. The negatively
charged side bends out, and the other, positively charged side bends
inward. If the charges are reversed, the bimorph bends the opposite way.2
Any automatic focusing camera must have a motor to move the lens elements to bring
the subject into focus. That’s not a simple task when you consider how much speed and
precision the camera requires and how little space the lens provides to hide a motor. As
with the autofocus mechanisms devised to measure the distance from camera to subject,
camera makers have come up with several ingenious ways to slip motors into minuscule
spaces. We’ll look here at a motor designed by Canon that many other camera manu-
facturers have adapted: the ultrasonic motor (USM).
44 PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES